Newnes Electrical Engineer`s Handbook

Newnes Electrical Engineer`s Handbook
,.--
I
Newnes
Electrical Engineer’s
Handbook
Newnes
Electrical Engineer’s
Handbook
D.F. Warne
Newnes
OXFORD AUCKIAND BOSTON JOHANNESBURG MELBOURNE NEW DELHl
Newnes
An imprint of Butterworth-Heinemann
Linacre House, Jordan Hillt Oxford OX2 8DP
225 Wildwood Avenue, Woburn, MA 01801-2041
A division of Reed Educational and Professional Publishing Ltd
A member of the Reed Elsevier plc group
First published 2000
Q D.F. Warne 2000
All rights reserved. No part of this publication
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to some other use of this publication) without the
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Applications for the copyright holder's written permission
to reproduce any part of this publication should be
addressed to the publishers
British Library Cataloguing in Publication Data
A catalogue record for this book is available from the British Library
ISBN 0 7506 4879 1
Library of Congress Cataloguing in Publication Data
A catalogue record for this book is available from the
British Library of Congress
Typeset at Replika Press Pvt Ltd, 100% EOU,Delhi 110 040, India
Printed and bound in Great Britain by Biddles Ltd
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Contents
Acknowledgements
xv
1
Introduction
1
2
Principles of electrical engineering
3
2.1 Nomenclature and units
3
2.2 Electromagnetic fields
3
2.2.1
2.2.2
2.2.3
2.2.4
Electric fields
Electric currents
Magnetic fields
Electromagnetism
2.3 Circuits
2.3.1
2.3.2
2.3.3
DC circuits
AC circuits
Magnetic circuits
2.4 Energy and power
2.4.1
2.4.2
2.4.3
2.4.4
3
Mechanical energy
Electrical energy
Per-unit notation
Energy transformation effects
Materials For electrical engineering
3
5
7
10
11
11
14
18
20
20
21
24
24
29
Introduction
29
3.2 Magnetic materials
29
3.1
3.2.1
3.2.2
3.2.3
3.2.4
3.3
Soft (high-permeability) materials
3.2.1.1 Sheet steels
3.2.1.2 Amolphous alloys
3.2.1.3 Nickel-iron alloys
3.2.1.4 Ferrires and garnets
3.2.1.5 Sofl magnetic composites
Hard (permanent magnet) materials
3.2.2.1 Alnico alloys
3.2.2.2 Ferrites
3.2.2.3 Rare earth alloys
3.2.2.4 Bonded magnets
3.2.2.5 Applications
Other materials
Standards
Insulating materiaIs
3.3.1
Solid dielectrics
3.3.1.1 Inorganic ( c e m k and glass) materials
3.3.1.2 Plastic films
3.3.1.3 Flexible insulating slceving
3.3.1.4 Rigid fibrous reinforred laminates
32
32
33
34
34
35
35
35
35
36
37
37
37
38
39
41
41
42
42
42
vi
Contents
3.3.2
3.3.3
3.3.4
3.3.5
3.3.1.5 Resins and varnishes
3.3.1.6 Pressure-sensitive adhesive tapes
3.3.1.7 Cellulosic materials
3.3.1.8 Combined flexible materials
3.3.1.9 Mica products
3.3.1.10 Textile insulation
3.3.1.11 Elastomers and thermoplastics
Liquid dielectrics
Gas insulation
Vacuum insulation
Standards
3.4 Conducting materials
3.4.1
3.4.2
3.4.3
4
Conductors
3.4.1.1 Copper and its alloys
3.4.1.2 Aluminium and its alloys
3.4.1.3 Resistance alloys
Semiconductors
3.4.2.1 Impurity effects and doping
3.4.2.2 The transistor
3.4.2.3 Printed circuits and integrated circuias
3.4.2.4 The microprocessor
Superconductors
42
43
43
43
43
46
46
46
46
47
47
48
48
49
52
53
54
55
55
57
57
58
3.5 Standards
59
Measurement and instrumentation
61
4.1 Introduction
61
4.2 The oscilloscope
62
4.2.1
4.2.2
4.2.3
4.2.4
4.2.5
4.2.6
4.2.7
4.2.8
4.2.9
4.2.10
The cathode ray oscilloscope
The steady display of a repetitive signal
The display of one signal against another - Lissajous figures
The display of two signals against time
The acldc switch
The loading effect of the CRO
CRO probes
CRO bandwidth and the measurement of rise time
The high bandwidth CRO
The digital storage oscilloscope
4.3 Digital frequency counters, voltmeters and multimeters
4.3.1
4.3.2
4.3.3
4.4
The frequency counter
4.3.1.1 The measurement of low fizquencies
4.3.1.2 The accuracy of the frequency counter
The digital voltmeter
The digital multimeter
62
64
64
66
68
69
70
71
72
72
74
74
75
76
76
77
Sensors
78
4.4.1
4.4.2
4.4.3
79
80
81
The strain gauge
The piezoelectric accelerometer
The linear variable differential transformer
4.5 The international measurement system, traceability and
calibration
82
vii
Contents
5
5.1
Introduction
85
5.2
Main generator types
85
5.3
Principles of operation
87
87
92
93
5.3.1
5.3.2
5.3.3
5.3.4
5.4
5.5
Brushless excitation
97
98
5.5.3
5.7
94
95
5.5.2
5.6
No-load operation
The effect of load
Damping of transients
Voltage waveform
The Automatic Voltage Regulator (AVR)
5.5.1
6
85
Generators
Separate exciter
Capacitor excitation
Induction generator
100
100
Construction
101
5.6.1
5.6.2
5.6.3
101
102
102
Stator
Rotor
Cooling
Rating and specification
5.7. I Rated output
104
5.7.2
5.7.3
106
107
Reactances
Main items of specification
104
5.8
Testing
107
5.9
Standards
108
Transformers
109
Principles of operation
109
6.2 Main features of construction
113
6.1
6.2.1
6.2.2
6.2.3
6.2.4
6.2.5
6.2.6
6.3
The core
Windings
Winding connections
Bushings
Tapchangers
Cooling equipmnt
Main classes of transformer
6.3.1
6.3.2
6.3.3
6.3.4
6.3.5
6.3.6
6.3.7
6.3.8
6.3.9
6.3.10
6.3.11
6.3.12
Transformers for electronics
Small transformers
Distribution transformers
Supply transformers
Transmission (or intertie) u-ansformers
Generator (or stepup) transformers
Phase-shifting transformers
Converter transformers
Railway transformers
Rectifier and furnace transformers
Dry-typ transformers
Gas-fiLled transformers
113
116
117
122
122
127
127
127
127
128
129
130
130
131
131
133
133
133
133
viii
Contents
6.4
6.5
6.6
6.7
7
Rating principles
134
6.4.1
6.4.2
6.4.3
134
135
135
Test methods
135
6.5.1
6.5.2
135
138
Specification testing
In-service testing
Commissioning, maintenance and repair
139
6.6.1
6.6.2
6.6.3
139
140
140
Commissioning
Maintenance
Diagnostics and repair
Standards
140
Switchgear
143
7.1
Introduction
143
7.2
Principles of operation
144
7.3
Low-voltage switchgear
145
7.3.1
7.3.2
7.3.3
7.3.4
7.3.5
7.4
7.5
7.6
8
Rated power
Overloading
Parallel operation of transformers
Switches, disconnectors, switch disconnectors and
fuse combination units
Air circuit breakers and moulded case circuit breakers
Miniature circuit breakers
Residual current devices
Standards
145
149
154
156
157
Medium-voltage (distribution) switchgear
157
7.4.1
7.4.2
7.4.3
7.4.4
7.4.5
7.4.6
158
161
168
169
171
171
Types of circuit breaker
Main classes of equipment
Rating principles
Test methods
Commissioning and maintenance
Standards
High-voltage (transmission) switchgear
171
7.5.1
7.5.2
7.5.3
7.5.4
7.5.5
7.5.6
171
172
174
178
178
179
System considerations
Types of circuit breaker
Main classes of equipment
Rating principles
Test methods
Commissioning and maintenance
Standards
180
Fuses and protection d a y s
181
8.1
Protection and co-ordination
181
8.2
Fuses
183
8.2.1
8.2.2
8.2.3
Principles of design and operation
Rating principles and properties
Main classes of equipment
183
185
188
ix
Contents
8.2.4
8.2.5
Test methods
Standards
8.3 Protection relays
8.3.1
8.3.2
8.3.3
8.3.4
9
principles of design and operation
Rating principles and properties
Main classes of relay
Test methods
194
194
200
20 1
204
8.4 Standards
204
Wires and cables
207
9.1 scope
207
9.2 Principles of power cable design
9.2.1
9.2.2
9.2.3
9.2.4
9.2.5
Terminology
General considerations
Paper-insulated cables
Polymeric cables
Low Smoke and Fume (LSF) cables
9.3 Main classes of cable
9.3.1
9.3.2
9.3.3
Cables for the electricity supply industry
9.3.1.1 Paper-insulated cables
9.3.1.2 MV polymeric cables
9.3.1.3 LVpolyneric cables
Industrial cables
9.3.2.1 Paper-insulated cables
9.3.2.2 Polymeric cables f o r f i e d installations
9.3.2.3 Polymeric cables for flexible connections
Wiring cables
9.4 Parameters and test methods
9.4.1
9.4.2
9.4.3
9.4.4
9.4.5
Current rating
Capacitance
Inductance
Voltage drop
Symmetrical and earth fault capacity
9.5 Optical communication cables
9.5.1
9.5.2
9.5.3
9.5.4
10
193
194
Optical fibres
Optical cable design
Interconnections
Installation
207
201
208
209
210
212
212
212
212
213
215
217
218
219
219
220
221
22 1
222
223
223
223
224
225
227
23 1
232
9.6 Standards
233
9.6.1
9.6.2
233
235
Metallic wires and cables
Optical communication cables
Motors, motor control and drives
237
10.1 Introduction
237
238
10.2 General characteristics
10.2.1 The production of torque
10.2.2 Main components and construction
10.2.3 Losses and heat transfer
238
239
24 1
Contents
X
10.3
10.4
Main classes of machine
244
10.3.1 Induction motors
10.3.1.1 Single-phase induction motors
10.3.2 Synchronous motors
10.3.3 Commutator motors
10.3.3.1 AC commutator motors
10.3.3.2 DC motors
10.3.3.3 Universal motors
10.3.4 Permanent magnet motors
10.3.4.1 Permanent magnets on the stator
10.3.4.2 Permanent magnets on the rotor
10.3.5 Switched reluctance motors
10.3.6 Stepper motors
244
249
250
25 1
252
252
253
253
254
254
255
257
Variable speed drives
260
26 1
10.4.1
10.4.2
10.4.3
10.4.4
10.5
11
DC motor drives
AC motor drives
Switched reluctance drives
Commissioning of drives
Ratings, standards and testing
26 1
26 1
262
262
Static power supplies
265
11.1
Scope
265
11.2
General issues
265
11.2.1
11.2.2
11.2.3
11.2.4
11.2.5
265
265
266
266
267
11.3
Design optimization
Thermal management
Safety
Compatibility
Commissioning, testing, installation and maintenance
Supply-frequency diode rectifiers
267
11.3.1
11.3.2
11.3.3
11.3.4
267
268
268
270
Half-wave single-phase rectifiers
Full-wave single-phase rectifiers
Three-phase bridge rectifiers
Voltage multipliers
Phase-controlled power converters
27 1
11.4.1 AC power controllers
11.4.2 Phase-controlled rectifiers and inverters
272
11.5
Linear dc voltage regulators
274
11.6
Inverters
275
11.7
Switching dc voltage regulators (dc to dc converters)
276
11.8
AC supply harmonic filters
278
11.8.1
11.8.2
11.8.3
11.8.4
279
280
282
282
11.4
11.9
Supply current harmonics
The generation of current harmonics
Current harmonics and power factor
Reduction of current harmonics
Filters for high-frequency noise
11.10 Systems with cascaded converters
272
282
284
Contents
xi
11.10.1 The Uninterruptible Power Supply (UPS)
11.10.2 Regulated dc power supplies
11.10.3 Power conditioners for photovoltaic arrays
12
11.11 Standards
288
Batteries and fuel cells
291
12.1
Introduction
29 1
12.2
Primary cells
29 1
12.2.1
12.2.2
12.2.3
12.2.4
12.2.5
12.2.6
12.2.7
12.2.8
292
293
294
294
294
296
297
297
Zinc carbon
Alkaline manganese
Mercury oxide
Silver oxide
Zinc air
Lithium manganese dioxide
Lithium thionyl chloride
Lithium copper oxyphosphate
12.3
Secondary cells
12.3.1 Sealed nickel cadmium
12.3.2 Vented nickel cadmium
12.3.3 Nickel metal hydride
12.3.4 Lead acid - pasted plate
12.3.5 Lead acid - tubular
12.3.6 Lead acid - Plant6
13.3.7 Lead acid - valve regulated sealed (VRSLA)
298
299
300
300
301
303
304
305
12.4
Fuel cells
305
12.4.1 The Solid Polymer Fuel Cell (SPFC)
12.4.2 The Solid Oxide Fuel Cell (SOFC)
307
308
Battery charging
308
12.5.1 Small commercial batteries (up to 10 Ah)
12.5.2 Automotive batteries
12.5.3 Motive power
12.5.4 Standby power applications
308
309
311
316
Battery monitoring
3 19
12.6.1 Load-discharge testing
12.6.2 dV/dt-load testing
12.6.3 Computer monitoring of cell voltages
12.6.4 Conductance monitoring
3 19
319
3 19
322
12.7
Installation, testing and commissioning
324
12.8
Operation and maintenance
325
12.8.1 Primary cells
12.8.2 Secondary cells
325
326
Standards
327
12.5
12.6
12.9
13
284
285
288
Electroheat
329
13.1
329
Introduction
xii
Contents
13.2
13.3
13.4
Metal melting
330
13.2.1
13.2.2
13.2.3
13.2.4
13.2.5
13.2.6
13.2.7
The arc furnace
The submerged arc process
Electroslag refining
The electron-beam furnace
The coreless induction furnace
The channel induction furnace
The resistance furnace
331
332
332
332
333
333
335
Heat treatment by direct resistance heating
336
13.3.1
13.3.2
13.3.3
13.3.4
337
339
339
340
Metals
Glass
Salt baths
Foods and other fluids
Indirect resistance heating
341
13.4.1
13.4.2
13.4.3
13.4.4
34 1
34 1
34 1
342
Metallic elements
Sheathed elements
Ceramic elements
Electric ovens and furnaces
13.5
Infrared heating
343
13.6
Induction heating
345
13.6.1 Power sources
13.6.2 Through-heating of billets and slabs
13.6.3 Strip heating
347
347
347
Indirect induction heating
348
13.7.1 Semiconductor manufacture
13.7.2 Process heating and calcining
348
348
Dielectric heating
349
13.8.1 Radio frequency power sources and applicators
13.8.2 Microwave power sources and applicators
35 1
352
Ultraviolet processes
353
13.7
13.8
13.9
13.10 Plasma torches
13.10.1 Plasma furnaces or reactors
14
353
356
13.11 Glow discharge processes
356
13.12 Lasers
356
13.13 Standards
357
The power system
361
14.1 Introduction
361
14.2 Generation
361
14.3 Transmission
362
14.3.1 Principles of design
14.3.1.1 HVDC transmission
14.3.1.2 AC system compensation
14.3.2 System operation
364
366
368
369
xi11
Contents
14.4 Distribution
14.4.1 System design
14.4.2 System operation
15
372
373
14.5 Future trends
373
Electromagnetic compatiblity
375
15.1 Introduction
375
15.1.1 Sources
15.1.2 Coupling mechanisms
15.1.3 Equipment sensitivity
15.2 Simple source models
375
376
377
377
15.2.1 Receptor efficiency
378
15.3 Signal waveforms and spectra
378
15.4 EMC limits and test levels
381
15.4.1 Emissions
15.4.1.1 Conducted emissions
15.4.1.2 Radiated emissions
15.4.1.3 Powerfirquency harmonics
15.4.2 Immunity
15.4.2.I Electrostatic Discharge (ESD)
15.4.2.2 Elecm*calfast transients
15.4.2.3 Radio Frequency (RF)fields
15.4.2.4 Dips, surges and voltage interruptions
15.4.2.5 Magnetic fields
15.5 Design for EMC
15.5.1
15.5.2
15.5.3
15.5.4
15.5.5
15.5.6
381
381
381
382
382
382
383
384
384
384
384
Basic concepts
Shielding
Cable screens termination
PCB design and layout
384
386
386
387
Grounding
Systems and installations
388
15.6 Measurements
15.6.1 Emissions
15.6.2 Immunity
15.7 Standards
15.7.1 Generic standads
15.7.2 Important product standards
15.7.3 Basic standards
16
370
388
388
388
389
390
391
39 1
39 1
Hazardous area equipment
393
16.1 Introduction
393
16.2 Hazardous areas
393
16.2.1 Zone classification
16.2.2 Gas grouping temperature and classification
16.2.3 Area assessment procedure
394
395
396
xiv
Contents
16.3 Protection concepts for electrical equipment
16.3.1
16.3.2
16.3.3
16.3.4
16.3.5
16.3.6
16.3.7
16.3.8
16.3.9
Flameproof enclosure - EEx ‘d’
Increased safety - EEx ‘e’
Non-sparking - Ex ‘n’
Intrinsically safe - Ex 5’
Pressurized - EEx ‘p’
Oil immersion - EEx ‘0’
Quartz-sand filled - EEx ‘q’
Encapsulation - EEx ‘m’
Special protection - Ex ‘s’
397
397
398
399
40 1
40 1
402
402
402
16.4 Installation, inspection and maintenance
16.4.1 Flameproof EEx ‘d’ equipment
16.4.2 Increased safety EEx ‘e’ equipment
16.4.3 Intrinsically safe EEx ‘i’ equipment
403
405
405
405
16.5 Certification
406
16.5.1 ATEX Directive
16.6 Standards and codes of practice
Index
396
406
407
409
Acknowledgements
Preparation of this book seemed at the outset to be a fairly straightforward task. It
has, however, proved to be quite different. A structural change from the content of
the preceding Newnes Electrical Pocket Book was necessary because much of the
content and scope of this has been taken to other books in the new series such as the
Newnes Building Services Pocket Book and the Newnes Electronics Engineer’s Pocket
Book. At the same time, much of what is often regarded as ‘mature technology’ is
changing surprisingly rapidly, and much of the remaining material needed a complete
overhaul with some new areas introduced.
Inevitably the chapters have been developed at different paces and the task of
finalizing the content of each chapter has been quite difficult because of continual
development, especially in the technology and in standards. The forbearance of
specialist chapter authors during this process of ‘spinning plates’ is gratefully
acknowledged.
No project of this type goes without some personal and social sacrifice. Without
the patience and support of my wife Gill the completion of the book would have been
very much more difficult.
D.F. Warne
September 1999
Chapter 1
Introduction
There seems to be a trend in the public perception of engineering and technology that
to be able to operate a piece of equipment or a system is to understand how it works.
Nothing could be further from the truth. The gap between the ability to operate and
a genuine understanding is, if anything, widening because much of the complexity
added to modem electrical equipment has the specific aim of making it operable or
‘user-friendly’without special training or knowledge.
The need for a basic explanation of principles, leading to a simple description of
how various important and common classes of electrical equipment works, has never
been stronger. Perhaps more so than in its predecessor, Newnes Electrical Pucker
Book, an attempt is made to address fundamentals in this book, and the reader is
encouraged to follow through any areas of interest using the references at the end of
each chapter. More comprehensive coverage of all the subjects covered in this pocket
book is available in the Newnes Electrical Engineer’s Reference Book.
More so now than ever before, the specification and performance of electrical
equipment is governed by national and international standards. While it would be
inappropriate in a pocket book to cover standards in any detail, a summary of key
standards is included for reference purposes at the end of each chapter.
The structure of the book is based around three groups of chapters, which address:
0
fundamentals and general material
the design and operation of the main classes of electrical equipment
special technologies which apply to a range of equipment
The first group comprises three chapters which set out fundamentals and principles
running through all aspects of electrical technology.
The opening chapter deals with fundamentals of electric and magnetic fields and
circuits, with energy and power conversion principles.
This is followed by a review of the materials that are so crucial to the design of
electrical equipment, and these are grouped into sections on magnetic, insulating and
conducting materials. In each of these areas technology is moving ahead rapidly. The
great increases in the strength of permanent magnets in the past ten years has done
much to make possible the miniaturization of equipment such as the Sony Walkman,
and the introduction of so many small motors and actuators in our homes and motor
cars. Developments in insulating mataids mean that increased reliability and operation
at higher temperatures can now readily be achieved. Under the heading of conductors
there are continuing advances in superconductors, which are now able to operate in
liquid nitrogen, and of course semiconductor development has transformed the way
in which equipment can be controlled and the processing power in computers.
Finally in this opening group there is a chapter on measurement and instrumentation.
2
Introduction
A classical textbook on electrical measurement would in the past have included
sections on moving iron and moving coil instruments, but these have been omitted
here in favour of the oscilloscope and sensors which now dominate measurements in
most areas.
The following group of nine chapters make up the main core of the book and
cover the essential groups of electrical equipment found today in commerce and
industry.
The opening five chapters here cover generators, transformers, switchgear, fuses
and wire and cables. These are the main technologies for theprodzicfjion and handling
of electrical energy, from high power and high voltage levels down to the powers and
voltages found in the household. Exciting developments in this area include the
advances made in high voltage switchgear using SF6 as an insulating medium, the
extension of polymer insulation into high voltage cables and the continuing compaction
of miniature and moulded-case circuit breakers. A new section in the wire and cables
chapter addresses the growing technology of optical fibre cables; although the main
use for this technology is in telecommunications, which is outside the scope of the
book, a chapter on wires and cables would not be complete without it and optical
fibres have a growing number of applications in electrical engineering.
The following four chapters describe different groups of equipment that use or
srore electrical energy. Probably the most important here is electric motors, since
these use almost two-thirds of all electrical energy generated. Static power supplies
are also of growing importancein applicationssuch as emergency standby for computers;
this technology is now based on power electronics and the opportunity is taken in
this chapter to explain the fundamentals of power electronic design and technology.
Rotating converters were important for many of the duties now handled by power
electronics, but these are now in decline and are not covered here. The range of
batteries being developed and appearing in a variety of applications is now very large
and this is the subject of a special chapter, which also covers the techniques of
battery charging and the emerging related technology of fuel cells. If fuel cells fulfil
their promise and start to play a greater part in the generation of electricity in the
future then we can expect to see coverage of this area grow and perhaps move to the
generation section in future editions. Another major electricity consumer is the range
of technologiesgenerally known as electroheat.This covers a spectrum of technologies
from arc furnaces through microwave heating to ultraviolet drying techniques which
are described in a special chapter.
The final group of three chapters cover subjects that embrace a range of technologies
and equipment. There is a chapter on power systems which describes the way in
which generators,switchgear,transformers,lines and cables are connected and controlled
to transmit and distribute our electrical energy. The privatization of electricity supply
in countries across the world has brought great changes in the way in which power
systems are operated and these are touched upon here. The second chapter in this
group concerns electromagnetic compatibility (emc); with the growing amount of
electronic and high-frequency equipment in use today it is imperative that precautions
are taken to prevent interference and legislation has been introduced to enforce this
prevention. The techniques for tackling this are complex and influence a wide range
of equipment. Finally there is a chapter describing the design and use of equipment
for operation in hazardous and explosive environments; this again covers a wide
range of equipments and there are a number of different classifications of protection.
Chapter 2
Principles of electrical engineering
Dr D.W. Shimmin
University of Liverpool
2.1
Nomenclature and units
This book uses notation in accordance with the current British and International
Standards. Units for engineering quantities are printed in upright roman characters,
with a space between the numerical value and the unit, but no space between the
decimal prefix and the unit, e.g. 275 kV. Compound units have a space, dot or /
between the unit elements as appropriate, e.g. 1.5 N m, 300 d s , or 9.81 m-C2.
Variable symbolsare printed in italic typeface, e.g. V. For ac quantities,the instantaneous
value is printed in lower case italic, peak value in lower case italic with caret (*), and
rms value in upper case, e.g. i, i, I. Symbols for the important electrical quantities
with their units are given in Table 2.1, and decimal prefix symbols are shown in
Table 2.2. Graphical symbols for basic electrical engineering components are shown
on Fig. 2.1.
2.2
2.2.1
Electromagnetic fields
Electrlc fields
Any object can take an electric or electrostatic charge. When the object is charged
positively, it has a deficit of electrons, and when charged negatively it has an excess
of electrons. The electron has the smallest known charge, -1.602 x lO-” C.
Charged objects produce an electric field. The electricfield strength E (Vlm) at a
distance d(m) from an isolated point charge Q ( C ) in air or a vacuum is given by
E=- Q
4m0d2
where the permittivity offree space E, = 8.854 x lo-’’ Flm. If the charge is inside an
insulating material with relative permittivity q the electric field strength becomes
E=
Q
4rno&,d2
(2.2)
Any charged object or particle experiences a force when inside an electric field. The
force F (N) experienced by a charge Q (C) in an electric field strength E (Vim) is
given by
4
Principles of electrical engineering
Table 2.1 Symbols for standard quantities and units
-
Symbol
Quantity
Unit
Unit
symbol
A
B
C
E
geometric area
magnetic flux density
capacitance
electric field strength
mechanical force
magnetomotiveforce (mmf)
conductance
magnetic field strength
electric current
electric current density
moment of inertia
self-inductance
mutual inductance
number of turns
active or real power
electric charge
reactive power
electrical resistance
reluctance
apparent power
mechanical torque
electric potential or voltage
energy or work
reactance
admittance
impedance
square metre
tesla
farad
volt per metre
newton
ampere
siemens
ampere per metre
ampere
ampere per square metre
kilogram metre squared
henry
henry
m2
T
F
Fnl
G
H
I
J
J
L
M
N
P
Q
Q
R
Rm
S
T
V
W
X
Y
Z
f
i
/
rn
n
P
t
V
0
F
V/m
N
A0rA.t
S
Nm
A
Nm2
kg m2
H
H
-
watt
coulomb
volt ampere reactive
ohm
ampere per weber
volt ampere
newton metre
volt
joule
ohm
siemens
ohm
W
hertz
Hz
metre
kilogram
revolution per minute
m
kg
rPm
second
metre per second
S
permittivity
efficiency
angle
power factor
permeance
permeability
resistivity
conductivity
phase angle
magnetic flux
magnetic flux linkage
farad per metre
F/m
radian or degree
rad or
weber per ampere
henry per metre
ohm metre
siemens per metre
radian
weber
weber or weber-turn
angular velocity or angular
frequency
radian per second
WblA
Hlm
R-rn
Slm
rad
Wb
Wb
0rWb.t
rad/s
frequency
square root of -1
length
mass
rotational speed
number of machine pole pairs
time
linear velocity
C
VAR
R
ANVb
V-A
N.m
V
J
n
S
n
mls
5
Newnes Electrical Engineer's Handbook
'Wle 2.2 Standard decimal prefix symbols
Prefix
Name
Multiple
T
G
M
k
tera
gigs
mega
kilo
deci
centi
milli
micro
nano
pic0
femto
10"
d
C
rn
P
n
P
f
109
106
103
10-1
10-2
109
10-B
10-9
10-12
10-15
F=QE
(2.3)
Electric field strength is a vector quantity. The direction of the force on one charge
due to the electric field of another is repulsive or attractive. Charges with the same
polarity repel; charges with opposite polarities attract.
Work must be done to move charges of the same polarity together. The effort
required is described by a voltage or electrostatic potential. The voltage at a point is
defined as the work required to move a unit charge from infinity or from earth. (It is
normally assumed that the earth is at zero potential.) Positively charged objects have
a positive potential relative to the earth.
If a positively charged object is held some distance above the ground, then the
voltage at points between the earth and the object rises with distance from the
ground, so that there is apotenrinl gradient between the earth and the charged object.
There is also an electric field pointing away from the object, towards the ground. The
electric field strength is equal to the potential gradient, and opposite in direction.
E = - - dV
du
2.2.2
(2.4)
Electric currents
Electric charges are static if they are separated by an insulator. If charges are separated
by a conductor, they can move giving an electric current. A current of one ampere
flows if one coulomb passes along the conductor every second.
I = -Q
t
A given current flowing through a thin wire represents a greater density of current
than if it flowed through a thicker wire. The current densiq J (Nm2) in a wire with
cross-section area A (m2) carrying a current I (A) is given by
J = -I
A
(2.6)
For wires made from most conducting materials, the current flowing through the
wire is directly related to the difference in potential between the ends of the wire.
6
Principles of electrical engineering
p?JJ
.Q 0inductive
Fixed
Variable
Preset
Potentiometer
RESISTORS
Fixed Polarized Variable Preset
CAPACITORS
Fixed
Variable
INDUCTORS
2/
2/
______
&
acsuPPlY
%phase
acsupply
-
Tapped
Iron
cored
or
Fuse
dc supply
Earth
Current
transformer
d
TRANSFORMERS
Two-winding
Fig. 2.1 Standard graphical symbols
d
Auto
Newnes Electrical Engineer's Handbook
7
Ohm's law gives this relationship between the potential difference V ( V ) and the
current I (A) as
V = I R or I = V G
(2.7)
where R (a)is the resistance, and G ( S ) = 1/R is the conductance (Fig. 2.2). For a
wire of length I and cross-section area A, these quantities depend on the resistivity p
(a m) and conductivity Q (Wm) of the material
-
R
I
4
V = IR
F
i
g
.2.2
Ohm's law
R = p - 1 and G = o -A
(2.8)
A
I
For materials normally described as conducioi-sp is small, while for insulators p is
large. Semicondz6ctors have resistivity in between these extremes, and are usually
very dependent on purity and temperature.
In metal conductors, the resistivity increases with temperature approximately
linearly:
RT=RT,(l +a("- To))
(2.9)
for a conductor with resistance RT, at reference temperature To.This is explained in
more detail in section 3.4.1.
Charges can be stored on conducting objects if the charge is prevented from
moving by an insulator. The potential of the charged conductor depends on the
capacitance C (F) of the metallinsulator object, which is a function of its geometry.
The charge is related to the potential by
Q=CV
(2.10)
A simple parallel-plate capacitor, with plate area A, insulator thickness d and relative
permittivity E, has capacitance
C = -E o E , A
d
(2.11)
2.2.3 Magnetic fields
A flow of current through a wire produces a magnetic field in a circular path around
the wire. For a current flowing forwards, the magnetic field follows a clockwise path,
as given by the right-hand corkscrew rule (Fig. 2.3). The magnetic field strength
H (A m-I) is a vector quantity whose magnitude at a distance d from a current Z is
given by
H=- I
2nd
(2.12)
Principles of electrical engineering
8
Fig. 2.3 Right-hand corkscrew rule
For a more complicated geometry, Amptrek law relates the number of turns N in a
coil, each carrying a current I, to the magnetic field strength H and the distance
around the lines of magnetic field I .
HI = NI
= F,
(2.13)
where F, (ampere-turns) is the nzagnetomotive force (mmf). This only works for
situations where H is uniform along the lines of magnetic field.
The magnetic field produced by a current does not depend on the material surrounding
the wire. However, the magnetic force on other conductors is greatly affected by the
presence of ferromagneticmaterials, such as iron or steel. The magnetic field produces
a magneticflux density B (T) in air or vacuum
B = M
(2.14)
where the permeability offi-e space po = 4n x lo-’ Wm. In a ferromagnetic material
with relative permeability
B = p&H
(2.15)
A second conductor of length I carrying an electric current Z will experience a force
F in a magnetic flux density B
F = BZ1
(2.16)
The force is at right angles to both the wire and the magnetic field. Its direction is
given by Fleming’s left-hand rule (Fig. 2.4). If the magnetic field is not itself
perpendicular to the wire, then the force is reduced; only the component of B at right
angles to the wire should be used.
A flow of magnetic flux @ (Wb) is caused by the flux density in a given crosssection area A as
@ = BA
(2.17)
The mmf F, required to cause a magnetic flux @to flow through a region of length
I and cross-section area A is given by the reluctance R, ( A N b ) or the permeance
A (Wb/A) of the region
F, = @R,
or @ = AF,
(2.18)
where
1
R,= P O P 4
(2.19)
Newnes Electrical Engineer's Handbook
9
I
Current
(second finger)
Fig. 2.4 Fleming's left-hand rule
t
H
(a) Magnetizationcurve
(b) Hysteresis loop
Fig. 2.5 Magnetic characteristics
Principles of electrical engineering
10
In ideal materials, the flux density B is directly proportional to the magnetic field
strength H.In ferromagnetic materials the relation between B and H is non-linear
(Fig. 2.5(a)), and also depends on the previous magnetic history of the sample. The
magnetization or hysteresis or BH loop of the material is followed as the applied
magnetic field is changed (Fig. 2.5(b)). Energy is dissipated as heat in the material
as the operating point is forced around the loop, giving hysteresis loss in the material.
These concepts are developed further in section 3.2.
2.2.4
Electromagnetism
Any change in the magnetic field near a wire generates a voltage in the wire by
electromagnetic induction. The changing field can be caused by moving the wire in
the magnetic field. For a length 1 of wire moving sideways at speed v ( d s ) across a
magnetic flux density B, the induced voltage or electromotiveforce (emf)is given by
V = Bvl
(2.20)
The direction of the induced voltage is given by Fleming’s right-hand nile (Fig. 2.6).
An emf can also be produced by keeping the wire stationary and changing the
magnetic field. In either case the induced voltage can be found using Faraday’s law.
If a magnetic flux 0 passes through a coil of N turns, the magnetic flux linkage Y
(Wb t) is
-
Induced
current
(second finger)
Fig. 2.6 Flemings right-hand rule
Y = Ndi
(2.21)
Faraday’s law says that the induced emf is given by
v=-- ddtY
(2.22)
The direction of the induced emf is given by Lenz’s law, which says that the induced
voltage is in the direction such that, if the voltage caused a current to flow in the
wire, the magnetic field produced by this current would oppose the change in Y. The
negative sign indicates the opposing nature of the emf.
A current flowing in a simple coil produces a magnetic field. Any change in the
current will change the magnetic field, which will in turn induce a back-emfin the
coil. The self-inductance or just inductance L (H) of the coil relates the induced
voltage to the rate of change of current
Newnes Electrical Engineer’s Handbook
11
v = L -ddtl
(2.23)
Two coils placed close together will interact. The magnetic field of one coil will link
with the wire of the second. Changing the current in the primary coil will induce a
voltage in the secondary coil, given by the nzurual inductance M (H)
v, =M-a
dt
Placing the coils very close together, on the same former, gives close coupling of the
coils. The magnetic flux linking the primary coil nearly all links the secondary coil.
The voltages induced in the primary and secondary are each proportional to their
number of turns, so that
(2.25)
and by conservation of energy, approximately
(2.26)
A two-winding transformer consists of two coils wound on the same ferromagnetic
core. An autotransformer has only one coil with tapping points. The voltage across
each section is proportional to the number of turns in the section. Transformer action
is described more fully in section 6.1.
2.3 Circuits
2.3.1
DC circuits
DC power is supplied by a battery, dc generator or rectifying power supply from the
mains. The power flowing in a dc circuit is the product of the voltage and current
p = VI = i 2 R =
2
(2.27)
R
Power in a resistor is converted directly into heat.
When two or more resistors are connected in series, they carry the same current
but their voltages must be added together (Fig. 2.7)
v = vl+ m + v3
As a result, the total resistance is given by
V = M + \R+ v3=l(R1 + R 2 + R 3 )
Fig. 2.7 Series resistors
(2.28)
Principles of electrical engineering
12
(2.29)
R=Rl+R2+R3
When two or more resistors are connected in parallel, they have the same voltage but
their currents must be added together (Fig. 2.8)
Z=Il + n + n
c
7,
I = I1
(2.30)
+ 12 + 13
V = IR
I1
m
R2
R3
1=1+-+1
1
R
R1
R2
R3
Fig. 2.8 Parallel resistors
The total resistance is given by
(2.31)
A complicatedcircuit is made of several components of brunches connected together
at nodes forming one or more complete circuits, loops or meshes. At each node,
Kii-chhofS’scirrrent law (Fig. 2.9(a)) says that the total current flowing into the node
must be balanced by the total current flowing out of the node. In each loop, the sum
of all the voltages taken in order around the loop must add to zero, by KirchhofS’s
volrage law (Fig. 2.9(b)). Neither voltage nor current can be lost in a circuit.
DC circuits are made of resistors and voltage or current sources. A circuit with
only two connections to the outside world may be internally complicated. However,
to the outside world it will behave as if it contains some resistance and possibly a
source of voltage or current. The Tlzivvenin equivalent circuit consists of a voltage
source and a resistor (Fig. 2.10(a)), while the Norton equivalent circuit consists of a
current source and a resistor (Fig. 2.10(b)). The resistor equals the internal resistance
13
Newnes Electrical Engineer's Handbook
\
I
I1
+ 12 + 13 + 14 = 0
." .
\
-
(a) Current law
v4
v2
v3
P
V1+V2+V3+V4=0
(b) Voltage law
Fig. 2.9 Kirchhoff's laws
of the circuit, the Thevenin voltage source equals the open-circuit voltage. and the
Norton current source is equal to the short-circuit current.
Many circuits contain more than one source of voltage or current. The current
flowing in each branch, or the voltage at each node, can be found by considering
each source separately and adding the results. During this calculation by superposition,
all sources except the one being studied must be disabled: voltage sources are shortcircuited and current sources are open-circuited. In Fig. 2.11,each of the loop currents
Zl and 12 can be found by considering each voltage source separately and adding the
results, so that I1 = Zl, + Zlb,and 12 = 12, + 12,.
Principles of electrical engineering
14
R
+
V
(b) Norton equivalent circuit
(a) Thkvenin equivalent circuit
Fig. 2.10 Equivalent circuits
2.3.2 AC circuits
AC power is supplied by the mains from .-lrge ac generators or alternators, by a local
alternator, or by an electronic synthesis. AC supplies are normally sinusoidal, so that
at any instant the voltage is given by
u = V,,
=
sin(m - @)
5 sin(ot - $)
(2.32)
,V is the peak voltage or amplitude, o is the angularfrequency (rad s8) and @ the
phase angle (rad). The angular frequency is related to the ordinaryfreqzrencyf(Hz)
by
[email protected]
(2.33)
and the period is llf. The peak-to eak or pk-pk voltage is 2Vm, and the mot mean
square or rms voltage is V,, I$ It is conventional for the symbols V and Z in ac
circuits to refer to the rms values, unless indicated otherwise. AC voltages and
currents are shown diagrammatically on a phasor diagram (Fig. 2.12).
It is convenient to represent ac voltages using complex numbers. A sinusoidal
voltage can be written
v = vm,[email protected]
(2.34)
A resistor in an ac circuit behaves the same as in a dc circuit, with the current and
voltage in phase and related by the resistance or conductance (Fig. 2.13).
The current in an inductor lags the voltage across it by 90" ( d 2 rad) (Fig. 2.14).
The ac resistance or reactance X of an inductor increases with frequency
x, = CUL
(2.35)
The phase shift and reactance are combined in the complex impedance Z
1
= zL= j x , = j o L
I
(2.36)
Inductors in series behave as resistors in series
L, = L,+ Lz
+ L3
(2.37)
Newnes Electrical Engineer’s Handbook
15
+
cr;:
S
73
Fig. 2.11 Superposition
Principles of electrical engineering
16
4
c
m
a¶
a
Fig. 2.12 Sinusoidal ac quantities
V
I
Resistor
b
Fig. 2.13 Resistor in an ac circuit
and inductors in parallel behave as resistors in parallel
(2.38)
For a capacitor, the current leads the voltage across it by 90" (1d2 rad) (Fig. 2.15).
The reactance decreases with increasing frequency
x, = 1
WC
(2.39)
In a capacitor, the current leads the voltage, while in an inductor, the voltage leads
the current.
The impedance is given by
(2.40)
Capacitors in series behave as resistors in parallel (eqn 2.41) and capacitors in
parallel behave as resistors in series (eqn 2.42)
_
1 - -I + - +1cs
c1
c
2
1
c
3
(2.41)
Newnes Electrical Engineer's Handbook
r
'1
17
4
V
Ideal
Fig.
2.11 Inductor in an ac circuit
z
A
I
Ideal
V
Fig. 2.15 Capacitor in an ac circuit
+
V
Principles of electrical engineering
18
cp= c1 + c, + c,
(2.42)
The direction of the phase shift in inductors and capacitors is easily remembered by
the mnemonic CIVIL (i.e. C-IV, VI-L). Imperfect inductors and capacitors have
some inherent resistance, and the phase lead or lag is less than 90". The difference
between the ideal phase angle and the actual angle is called the loss angle 6. For a
component of reactance X having a series resistance R
R
tan(6) = X
The reciprocal of impedance is admittance
(2.43)
-Z= y = - 1
V
Z
Combinations of resistors, capacitors and inductors will have a variation of impedance
or admittance with frequency which can be used to select signals at certain frequencies
in preference to others. The circuit acts as aBltel; which can be low-pass, high-pass,
band-pass, or band-stop.
An important filter is the resonant circuir. A series combination of inductor and
capacitor has zero impedance (infinite admittance) at its resonant frequency
1
w o = 27cf0= -
(2.45)
m
A parallel combination of inductor and capacitor has infinite impedance (zero
admittance) at the same frequency.
In practice a circuit will have some resistance (Fig. 2.16), which makes the resonant
circuit imperfect. The q u a l i factor
~
Q of a series resonant circuit with series resistance
R is given by
=(%)
e=(+)
X
w=wo
(2.46)
e-0
and for a parallel circuit with shunt resistance R
Q=($)
=(e)
o=coo
(2.47)
-0
A filter with a high Q will have a sharper change of impedance with frequency than
one with a low Q,and reduced losses.
Mains electricity is generated and distributed using three phases. Three equal
voltages are generated at 120' phase intervals. The phase voltage V, is measured
with respect to a common star point or neutral point, and the line voltage V, is
measured between the separate phases. In magnitude,
v,= a v p
(2.48)
Any three-phase generator, transformer or load can be connected in a star or Y
configuration, or in a delta configuration (Fig. 2.17).
2.3.3 Magnetic circuits
A reluctance in a magnetic circuit behaves in the same way as a resistance in a dc
elecmc circuit. Reluctances in series add together
Newnes Electrical Engineer’s Handbook
19
Parallel
resonant circuit
0.1
1
Normalized frequency
L
Series
resonant circuit
i
10
t
-
R
Fig. 2.16 Resonant circuits
Star or Y
Fig. 2.17 Three-phase connections
Delta
20
Principles of electrical engineering
or
(2.49)
and for reluctances in parallel
or
(2.50)
Transformers and power reactors may contain no air gap in the magnetic circuit.
However, motors and generators always have a small air gap between the rotor and
stator. Many reactors also have an air gap to reduce saturation of the ferromagnetic
parts. The reluctance of the air gap is in series with the reluctance of the steel rotor
and stator. The high relative permeability of steel means that the reluctance of even
a small air gap can be much larger than the reluctance of the steel parts of the
machine. For a total air gap g (m) in a magnetic circuit, the magnetic flux density B
in the air gap is given approximately by
(2.51)
where Nfis the number of series turns on the field winding of the machine, and If is
the current in the field winding (Fig. 2.18).
't
I
Fig. 2.18 Air-gap magnetic circuit
2.4
2.4.1
Energy and power
Mechanical energy
According to Newton's third law of motion, mechanical force causes movement is a
straight line, such that the force F (N) accelerates a mass m (kg) with acceleration a
W S 2 )
Newnes Electrical Engineer's Handbook
21
F=ma
(2.52)
Rotational movement depends in the same way upon torque T (N m) accelerating a
moment of inertia J (kg . m2) with angular acceleration a (rds')
T = JCC
(2.53)
Movement is often opposed by friction. Friction forces and torques always work
against the movement. Friction between dry surfaces has a maximum value, depending
on the contact force. Once the driving force exceeds the limiting friction force, the
system will move and the friction force stays constant. Viscous damping gives a
restraining force which increases with the speed. Friction between lubricated surfaces
is mainly a viscous effect.
Objects store potential energy when they are lifted up. The stored energy W (J) of
a mass m is proportional to the height h (m) above ground level when the acceleration
due to gravity g = 9.81 m / s
W = mgh
(2.54)
Moving objects have kinetic energy depending on their linear speed v ( m / s ) or
angular speed o (rads)
1
W = -mu2
2
= -Jo'
1
(2.55)
2
Mechanical work is done whenever an object is moved a distance x (m) against an
opposing force, or through an angle 8 (rad) against an opposing torque
W = Fx
= T8
(2.56)
Mechanical power P (W)is the rate of doing work
P=Fv
= TO
2.4.2
(2.57)
Electrical energy
In electrical circuits, electrical potential energy is stored in a capacitance C charged
to a voltage V
1
w = -cv2
2
(2.58)
while kinetic energy is stored in an inductance L carrying a current 1
W = - L1 Z 2
(2.59)
2
Capacitors and inductors store electrical energy-Resistors dissipate energy and convert
it into heat. The power dissipated and lost to the electrical system in a resistor R has
already been shown in eqn 2.27.
Electrical and mechanical systems can convert and store energy, but overall the
total energy in a system is conserved.The overall energy balance in an electromechanical
system can be written
22
Principles of electrical engineering
electrical energy in + mechanical energy in
= electrical energy lost in resistance
+ mechanical energy lost in friction
+ magnetic energy lost in steel core
+ increase in stored mechanical energy
+ increase in stored electrical energy
(2.60)
The energy balance is sometimes illustrated in a power flow diagram (Fig. 2.19).
Electrical power input
------
i
Stator copper loss
Stator core loss
Air-gap power
i* _ - -
-----
Rotor core loss
Rotor copper loss
Mechanical power converted
t
i
Air resistance (windage) Mechanical shaft OUtpUt power
Mechanical friction loss
Fig. 2.19 Power flow diagram (example of a typical motor)
The overall efficiency of a system is the ratio of the useful output power to the
total input power, in whatever form
q = -Pout
en
(2.61)
In an ac circuit, the instantaneous power depends on the instantaneous product of
voltage and current. For sinusoidal voltage and current waveforms, the apparent
power S (VA) is the product of the r m s voltage and the rms current.
s = VI
(2.62)
When the voltage waveform leads the current waveform by an angle @, the average,
active or real power P(W) is
P = VI cos(@)
(2.63)
The factor relating the real power to the apparent power is the powerfactor A
a = cos(@)
(2.64)
The real power is converted into heat or mechanical power. In addition there is an
oscillating flow of instantaneous power, which is stored and then released each cycle
by the capacitance and inductance in the circuit. This imaginary or reactive power Q
(VAr)is given by
Q = VI sin($)
(2.65)
Newnes Electrical Engineer’s Handbook
23
By convention an inductive circuit (where the current lags the voltage) absorbs VAr,
while a capacitive circuit (where the current leads the voltage) acts as a source of
VAr. The relationship between S, P and Q is
S2= P2 + Q2
(2.66)
In a three-phasecircuit, the total real power is the sum of the power flowing into each
phase. For a balanced three-phase circuit with phase-neutral voltage Vp and phase
current Ip the total power is the sum of the powers in each phase
P = 3 vpzp cos($)
(2.67)
In terms of the line voltage VL, the real power is
P = fiVL
zp
(2.68)
cos($)
Similar relations hold for the reactive and apparent power.
The power in a three-phase circuit can be measured using three wattmeters, one
per phase, and the measurements added together (Fig. 2.20(a)). If it is known that the
(a) With three wattmeters
1
wl
Y
b
(b) With two wattmeters
Fig. 2.20 Three-phase power measurement
Principles of electrical engineering
24
load is balanced, with equal current and power in each phase, the measurement can
be made with just two wattmeters (Fig. 2.200)). The total power is then
w = WI + w,
(2.69)
The two wattmeter arrangement also yields the power factor angle
(2.70)
2.4.3
Per-unit notation
Power systems often involve transformers which step the voltage up or down.
Transformersare very efficient, so that the output power from the transformer is only
slightly less than the input power. Analysis and design of the circuit is made easier
if the circuit values are normalized, such that the normalized values are the same on
both sides of the transformers. This is acccomplished using the per-unit system. A
given section of the power system operates at a certain base voltage VB. Across
transformers the voltage steps up or down according to the turns ratio. The base
voltage will be different on each side of the transformer. A section of the system is
allocated a base apparent power rating or base VA V .AB. This will be the same on
both sides of the transformer.Combining these base quantities gives a base impedance
ZB and base current I,
(2.7 1)
The base impedance and base current change across a transformer.
All voltages and impedances in the system are normalized using the appropriate
base value. The resulting normalized quantities are per-unit values. Once the circuit
has been converted to per-unit values, the transformershave no effect on nominal tap
position, and can be replaced by their own equivalent per-unit impedance.
The per-unit values are sometimes quoted as per cent values, by multiplying by
100 per cent.
A particular advantage of the per-unit and per cent notation is that the resulting
impedances are very similar for equipment of very different sizes.
2.4.4
Energy transformation effects
Most electricalenergy is generated by electromagnetic induction. However, electricity
can be produced by other means than electromagnetic induction. Batteries use
electrochemistry to produce low voltages (typically 1-2 V). An electrolyte is a solution
of chemicals in water such that the chemical separates into positively and negatively
charged ions when dissolved. The charged ions react with the conducting electrodes
and release energy, as well as giving up their charge (Fig. 2.21). A fixed electrode
potential is associated with the reaction at each electrode; the difference between the
two electrode potentials drives a current around an external circuit. The electrolyte
must be sealed into a safe container to make a suitable battery. ‘Dry’ cells use an
electrolyte in the form of a gel or thick paste. Aprirnary cell releases electricity as
Newnes Electrical Engineer’s Handbook
25
the chemicals react, and the cell is discarded once all the active chemicals have been
used up, or the electrodeshave become contaminated.A secondary cell uses a reversible
chemical reaction, so that it can be recharged to regenerate the active chemicals. The
fuel cell is a primary cell which is constructed so that the active chemicals (fuel) pass
through the cell, and the cell can be used for long periods by replenishing the
chemicals. Large batteries consist of cells connected in series or parallel to increase
the output voltage or current. The main types of primary, secondary and fuel cell are
described in sections 12.2, 12.3 and 12.4, respectively.
Electricity can be generated directly from heat. When two different materials are
used in an electrical circuit, a small electrochemical voltage (contact potential) is
generated at the junction. In most circuits these contact potentials cancel out around
the circuit and no current flows. However, the junction potential varies with temperature,
so that if one junction is at a different temperature from the others, the contact
potentials will not cancel out and the net circuit voltage causes current to flow
(Seebeck effect). The available voltage is very small, but can be made more useful by
Acid
electrolyte
I
Fig. 2.21 Simple cell
d
Principles of electrical engineering
26
connecting many pairs of hot and cold junctions in series. The thermocouple is used
mostly for measurement of temperature by this effect, rather than for the generation
of electrical power (Fig. 2.22). The efficiency of energy conversion is greater with
semiconductor junctions, but metal junctions have a more consistent coefficient and
are preferred for accurate measurements. The effect can be reversed with suitable
materials, so that passing an electric current around the circuit makes one junction
hotter and the other colder (Peltier effect). Such miniature heat pumps are used for
cooling small components.
Copper wire
f,
...............
icke, wire...............)....
.......
Measurement
junction (hot)
Fig. 2.22 A thermocouple (Seebeck effect)
Certain crystalline chemicals are made from charged ions of different sizes. When
a voltage is applied across the crystal, the charged ions move slightly towards the
side of opposite polarity, causing a small distortion of the crystal. Conversely, applying
a force so as to distort the crystal moves the charged ions and generates a voltage.
This piezoelectric effect is used to generate high voltages from a small mechanical
force, but very little current is available. The use of piezoelectric sensors for the
measurement of mechanical pressure and force is described in section 4.42.
Ferromagnetic materials also distort slightly in a magnetic field. The magnetostrictive
effect produces low frequency vibration (hum) in ac machines and transformers.
Electricity can be produced directly from light. The photovoltaic effect occurs
when light falls on suitable materials, releasing electrons from the material and
generating electricity. The magnitude of the effect is greater with short wavelength
light (blue) than long wavelength light (red), and stops altogether beyond a wavelength
threshold. Light falling on small photovoltaic cells is used for light measurement,
communications and for proximity sensors. On a larger scale, semiconductor solar
cells are being made with usable efficiency for power generation.
Light is produced from electricity in incandescent filament bulbs, by heating a
wire to sufficiently high temperature that it glows. Fluorescent lights produce an
electrical discharge through a low-pressuregas. The discharge emits ultraviolet radiation,
which causes a fluorescent coating on the inside of the tube to glow.
References
2A.
2B.
2c.
2D.
Professional brief edited by Bums, R.W., Dellow, E and Forbes, R.G., Symbols andAbbreviations
f o r use in Electrical and Electronic Engineering, The Institution of Electrical Engineers, 1992.
BS 3939: 1985, Graphical symbols f o r electrical power, telecommunications and electronics
diagrams, BSI, 1985.
BS 5555:1993 (IS0 1000:1992), SI units and recommendations f o r the use of their multiples and
certain other units, BSI, 1993.
BS 5775: 1993 (IS0 32: 1992), Quantities, units and symbols, Part 5: electriciry and magnetism,
BSI, 1993.
Newnes Electrical Engineer’s Handbook
2E.
2F.
2G.
2H.
21.
27
Smith,R.J. and Dorf, R.C., Circuits. Devices and Systems, 5th edn. Wiley. 1992. ISBN 0-47155221-6.
Hughes, E. (levisedSmith,LM.), Hughes Electrical Technology, Longman Scientific and Technical,
1995, ISBN 0-582-22696-1.
Bird, J.O.,
Higher Electrical Technology, Butterworth-Heinemam, 1994. ISBN 0-7506-01019.
Breithaupt, J.. Understanding Physics for Advanced Level, S . Thornes, 1990, ISBN 0-748705 10-4.
Del Toro, V.,Elecrricnl Engineering Fundamentals, Prentice-Hall, 1986, ISBN 0-13-24713 1-0.
Chapter 3
Materials for electrical engineering
Professor A.G. Clegg
Magnet Centre, University of Sunderland
Mr A.G. Whitman
Jones Stroud Insulations
3.1
Introduction
The performance of most types of electrical equipment (as opposed to electronic
equipment) relies for their safe and efficient performance on an electrical circuit and
the means to keep this circuit isolated from the surroundingmaterials and environment.
Many types of equipment also have a magnetic circuit, which is linked to the electrical
circuit by the laws outlined in Chapter 2.
The main material characteristicsof relevance to electricalengineeringare therefore
those associated with conductors for the electrical circuit, with the insulation system
necessary to isolate this circuit, and with the specialized steels and permanent magnets
used for the magnetic circuit.
Although other properties, e.g. mechanical, thermal and chemical, are also relevant,
these are often important in specialized cases and coverage of these properties is
best left to other books which address these areas more broadly. The scope of this
chapter is restricted to the main types and characteristics of conductors, insulation
systems and magnetic materials which are used generally in electrical plant and
equipment.
3.2
Magnetic materials
All materials have magnetic properties. These characteristicproperties may be divided
into five groups:
0
diamagnetic
paramagnetic
ferromagnetic
antifemmagnetic
ferrimagnetic
Only the ferromagneticand ferrimagneticmaterials have properties that are useful in
practical applications.
Materials for electrical engineering
30
Ferromagnetic properties are confined almost entirely to iron, nickel and cobalt and
their alloys. The only exceptions are some alloys of manganese and some of the rare
earth elements.
Ferrimagnetism is the magnetism of the mixed oxides of the ferromagnetic elements.
These are variously called femtes and garnets. The basic ferrite is magnetite, or
Fe304, which can be written as Fe0.F%03. By substituting for the FeO with other
divalent oxides, a wide range of compounds with useful properties can be produced.
The main advantage of these materials is that they have high electrical resistivity
which minimizes eddy currents when they are used at high frequencies.
The important parameters in magnetic materials can be defined as:
permeabiliry - this is the flux density B per unit of magnetic field H,as
defined in eqs 2.14 and 2.15. It is usual and more convenient to quote the
value of relative permeability pr,which is B/poH.A curve showing the variation
of permeability with magnetic field for a ferromagnetic material is given in
Fig. 3.1. This is derived from the initial magnetization curve and it indicates
that the permeability is a variable which is dependent on the magnetic field.
The two important values are the initialpermeubility, which is the slope of the
magnetization curve at H = 0, and the maximum pemzeabiliiy, corresponding
to the knee of the magnetization curve.
Flux density B
or
permeability 1
Magnetic field H
Fig. 3.1 Magnetization and permeability curves
0
0
saturation - when sufficient field is applied to a magnetic material it becomes
saturated. Any further increase in the field will not increase the magnetization
and any increase in the flux density will be due to the added field. The saturation
magnetization is M, in A h and J, or B, in tesla.
remanence, B, and coercivity, H,- these are the points on the hysteresis loop
shown in Fig. 3.2 at which the field H is zero and the flux density B is zero,
respectively. It is assumed that in passing round this loop the material has been
saturated. If this is not the case, then an inner loop is traversed with lower
values of remanence and coercivity.
Ferromagnetic and ferrimagnetic materials are characterized by moderate to high
Newnes Electrical Engineer’s Handbook
31
B
Br
+H
-H
-Hc
Fig. 3.2 Magnetization and hysteresis curves
permeabilities, as shown in Table 3.1. The permea.,ility varies with the applied
magnetic field, rising to a maximum at the knee of the B-H curve and reducing to a
low value at very high fields. They also exhibit magnetic hysteresis whereby the
intensity of magnetization of the material varies according to whether the field is
being increased in a positive sense or decreased in a negative sense, as shown in
Fig. 3.2. When the magnetization is cycled continuously round a hysteresis loop, as
Table 3.1 Properties of soft magnetic materials
Maximum Saturation
Coercivity Curie
Resistivity
permeability magnetization
temperature
P
J*
Hc
(x10-7
(Nm)
(“C)
m x lo8)
(a-
m
Sheet steels
3% Si grain-oriented
2.5% Si non-oriented
~0.5%Si non-oriented
90
2.0
8
8
2.0
Low-carbon iron
3-1 0
50% Ni
Amorphous
I ron-based
2.1
2.1
6-7
40
50-100
50-120
745
745
770
770
48
44
12
12
250-400
100
0.8
1.5-1.6
1 .o
10
350
530
40
60
35-600
1.3-1.8
1.0-1.6
310-415
120-140
110-220
250-400
11-280
( a .m)
0.05-10
>lo4
>lo6
Nickel-mn
78% Ni
Ferrite
MnZn
NiZn
1.8-3.0
0.12-1.5
Garnet
-
0.45-0.50
0.20-0.35
0.3-1.7
15-27
30-300
-
Materials for electrical engineering
32
for example when the applied field arises from an alternating current, there is an
energy loss proportional to the area of the included loop. This is the hysteresis loss,
and it is measured in joules/m3. High hysteresis loss is associated with permanent
magnetic characteristics exhibited by materials commonly termed ‘hard‘ magnetic
materials, as these often have hard mechanical properties. Those materials with low
hysteresis loss are termed ‘soft and are difficult to magnetize permanently.
Ferromagnetic or ferrimagnetic properties disappear reversibly if the material is
heated above the Curie temperature, at which point it becomes paramagnetic, that is
effectively non-magnetic.
3.2.1
Soft (high-permeability) materials
There is a wide variety of soft magnetic materials for applications from constant dc
field through 50 Hz up to microwave frequencies. For the bulk of applications iron,
steel or cast iron are used. They have the advantage of low cost and strength, but they
should only be used for dc applications since they have low electrical resistivity
which would result in eddy currents if used in alternating fields.
In choosing a high-permeabilitymaterial for a particular application there may be
special considerations and the following is a guide to the choice. If the frequency of
the applied voltage is 10 kHz or above then femtes or garnets will normally be used.
For constant-field applications mild steel will be used, or low-carbon iron where the
highest permeability or lowest coercivity is needed. For 50 Hz power transformers,
grain-orientedsilicon steel is used but there is now serious competition from amorphous
strip and although this is more expensive the core loss is significantlylower than that
of silicon steel. For the highest permeability and lowest coercivity in specialist
applications up to about 10 kHz nickel iron would be the preferred choice although
its cost may be prohibitive.
3.2.1.1 Sheet steels
There are a number of grades of sheet steel and these comprise by far the greatest
part of the soft, high-permeability material which is used. These metallic materials
have a comparativelylow electrical resistivity and they are used in sheet (or lamination)
form because this limits the flow of eddy currents and the losses which result. The
range of sheet thickness used for 50 or 60 Hz applications is 0.35-0.65 mm for nonoriented materials and 0.13-0.35 mm for grain-oriented silicon steels. For higher
frequencies of 400-1000 Hz thicknesses of 0.05-0.20 mm are used.
In grain-oriented steel an increasing level of silicon content reduces the losses but
also reduces the permeability; for many applications a 3 per cent silicon content
represents a good balance. The effect of the silicon is to increase the electrical
resistivity of the steel; not only does this reduce eddy current losses but it also
improves the stability of the steel and aids the production of grain orientation. The
sheet is subject to cold rolling and a complex annealing treatment to produce the
grain orientation in the rolling direction and this also gives improved magnetic
permeability in that direction. The properties are further improved at the annealing
stage with a glass film on the surface which holds the steel in a state of tension and
provides electrical insulation between laminations in a core. A phosphate coating is
applied to complete the tensioning and insulation. The grain size in the resulting
sheets is comparatively large and the domain boundaries are quite widely spaced.
Artificial grain boundaries can be produced by laying down lines of ablated spots on
Newnes Electrical Engineer’s Handbook
33
the steel surface; this gives a stress and atomic disruption pattern which pins domain
walls and leads to a smaller wall spacing. Various methods have been used to produce
this ablating, including spark and laser techniques. All these processes are applied
within the steel manufacturer’s works and the resulting steel is often referred to as
fully processed. The main application of this grain-oriented material is in power
transformers where low power loss is important, since the transformer is always
connected even when its loading is at a minimum.
For rotating machinery, especially motors rated more than about 100 kW. nonoriented steels with a lower silicon content are used. Whilst efficiency remains
important in this application, high permeability is now also important in order to
minimize magnetizing current and to maximize torque. This material is also used in
smaller transformers, chokes for fluorescent tubes, meters and magnetic shielding.
For smaller motors silicon-free non-oriented steels are often used. These are
produced by the steel manufacturer with a relatively high carbon content. The
carbon makes the sheet sufficiently hard for punching into laminations and after
punching the material is decarburized and annealed to increase the grain size. Because
of the need for this secondary processing the materials are often known as semiprocessed. These materials are generally cheaper than silicon steels but in their
finished form they have a much higher permeability; in small motors particularly this
can be more important than efficiency. Other applications include relays and magnetic
clutches.
Silicon steels are also produced in the form of bars, rods or wires for relays,
stepping motors and gyroscope housings. The tensile strength of the silicon steels
may be improved by the addition of alloying elements such as manganese; this type
of material is used in highly stressed parts of the magnetic circuit in high-speed
motors or generators.
3.2.1.2 Amorphous alloys
This class of alloys, often called metallic glasses, is now an established group of soft
magnetic materials. The materials are produced by rapid solidification of the alloy at
cooling rates of about a million degrees centigrade per second. The alloys solidify
with a glass-like atomic structure which is a non-crystalline frozen liquid. The rapid
cooling is achieved by causing the molten alloy to flow through an orifice onto a
rapidly rotating water-cooled drum. This can produce sheets as thin as 10 pm and a
metre or more wide.
There are two main groups of amorphous alloys. The first is the iron-rich group
of magnetic alloys, which have the highest saturation magnetization among the
amorphous alloys and are based on inexpensive raw materials. Iron-rich alloys are
currently being used in long-term tests in power transformers in the USA. The
second is the cobalt-based group. which has very low or zero magnetostriction,
leading to the highest permeability and the lowest core loss. Cobalt-based alloys
are used for a variety of high-frequency applications including pulsing devices
and tape recorder heads, where their mechanical hardness provides excellent wear
resistance.
All of these alloys have a resistivity which is higher than that of conventional
crystalline electrical steels. Because of this, eddy current losses are minimized both
at 50 Hz and at higher frequencies. The alloys have other advantages including
flexibility without loss of hardness, high tensile strength and better corrosion resistance
than similar crystalline materials.
34
Materials for electrical engineering
3.2.1.3 Nickelkiron alloys
The very high magnetic permeability and low coercivity of nickel-iron alloys are
due to two fundamentalproperties, which are mugnetostriction and magnetic anisotropy.
At a nickel content of about 78 per cent both of these parameters are zero.
Magnetostriction has been briefly referred to in section 2.4.4; it is the change of
dimensions in a material due to magnetization, and when this is zero there are no
internal stresses induced during magnetizing. Magnetic anisotropy is the difference
between magnetic behaviour in different directions; when this is zero the magnetization
increases steeply under the influence of a magnetic field, independent of crystal
direction. The alloys with about 78 per cent nickel content are variously called
Mumetal or Permalloy.
The commercial alloys in this class have additions of chromium, copper and
molybdenum in order to increase the resistivity and improve the magnetic properties.
Applications include special transformers, circuit breakers, magnetic recording heads
and magnetic shielding.
Fifty per cent nickel-iron alloys have the highest saturation magnetization of this
class of materials, and this results in the best flux-carrying capacity. They have a
higher permeability and better corrosion resistance than silicon iron materials, but
they are more expensive. A wide range of properties can be produced by various
processing techniques. Severe cold reduction produces a cube texture and a square
hysteresis loop in annealed strip. The properties can also be tailored by annealing the
material below the Curie temperature in a magnetic field. Applications for this material
include chokes, relays and small motors.
3.2.1.4 Ferrites and garnets
Ferrites are iron oxide compounds containing one or more other metal oxide. The
important high magnetic permeability materials are manganese zinc ferrite and nickel
zinc ferrite. They are prepared from the constituent oxides in powder form, preferably
of the same particle size intimately mixed. The mixture is fired at about 1000°C and
this is followed by crushing, milling and then pressing of the powder in a die or
extrusion to the required shape. The resulting compact is a black brittle ceramic and
any subsequent machining must be by grinding. The materials may be prepared with
high permeability, and because their high electric resistivity limits eddy currents to
a negligible level they can be used at frequencies up to 20 MHz as the solid core of
inductors or transformers.
A combination of hysteresis, eddy current and residual losses occurs, and these
components may be separately controlled by composition and processing conditions,
taking into account the required permeability and the working frequency.
The saturation flux density of femtes is relatively low, making them unsuitable
for power applications. Their use is therefore almost entirely in the electronic and
telecommunications industry where they have now largely replaced laminated alloy
and powder cores.
Garnets are used for frequencies of 100 MHz and above. They have resistivities
in excess of lo8 S2 . m compared with ferrite resistivity which is up to lo3 &2 . m.
Since eddy current losses are limited by resistivity, they are greatly reduced in
garnets. The basic Yttrium-Iron Garnet (YIG) composition is 3Y203 5Fe203.This
is modified to obtain improved properties such as very low loss and greater temperature
stability by the addition of other elements including A1 and Gd. The materials are
-
Newnes Electrical Engineer's Handbook
35
prepared by heating of the mixed oxides under pressure at over 1300°C for up to 10
hours. Garnets are used in microwave circuits in filters, isolators, circulators and
mixers.
3.2.7.5 Soft magnetic composites
These composites (SMC) consist of iron or iron alloy powder mixed with a binder
and a small amount of lubricant. The composite is pressed into the final shape which
can be quite complex. The lubricant reduces the friction during pressing and aids the
ejection of the part from the die. After pressing the parts are either cured at 150 to
275°C or heat treated at 500°C.
The isolation of the particles within the binder minimizes the eddy currents and
makes the components very suitable for medium frequency applications up to 1 kHz
or more. They have the added advantages of being isotropic and can be used for
components with quite complex shapes.
3.2.2
Hard (permanent magnet) materials
The key properties of a permanent magnet material are given by the demagnetization
curve, which is the hysteresis curve in the second quadrant between B, and -H,. It
can be shown that when a piece of permanent magnet material is a part of a magnetic
circuit, the magnetic field generated in a gap in the circuit is proportional to B x H
x V, where B and H represent a point on the demagnetization curve and V is the
volume of permanent magnet. To obtain a given field with a minimum volume of
magnet the product B x H must therefore be a maximum, and the (BH),,, value is
useful in comparing material characteristics.
The original permanent magnet materials were steels, but these have now been
superseded by better and more stable materials including Alnico, ferrifes and rare
earth alloys. The magnetic properties of all the permanent magnet materials are
summarized in Table 3.2.
3.2.2.7 Alnico alloys
A wide range of alloys with magnetically useful properties is based on the A1-NiCo-Fe system. These alloys are characterized by high remanence, high available
energy and moderately high coercivity. They have a low and reversible temperature
coefficient of about -0.02 per cent/OC and the widest useful temperature range (up to
over 500°C) of any permanent magnet material.
The alloys are produced either by melting or sintering together the constituent
elements. Anisotropy is achieved by heating to a high temperature and allowing the
material to cool at a controlled rate in a magnetic field in the direction in which the
magnets are to be magnetized. The properties are much improved in this direction at
the expense of properties in the other directions. This is followed by a tempering
treatment in the range 650-550OC. A range of coercivities can be produced by varying
the cobalt content. The properties in the preferred direction may be further improved
by producing an alloy with columnar crystals.
3.2.2.2 Ferrites
The permanent magnet ferrites are also called ceramics and they are mixtures of
ferric oxide and an oxide of a divalent heavy metal, usually barium or strontium.
These ferrites are made by mixing together barium or strontium carbonate with iron
Materials for electrical engineering
36
Table 3.2 Characteristics of permanent magnet materials
-
Remanence
Alnico
Normal anisotropic
High coercivity
Columnar
U)
4
Energy
product
(BH),,,
(kJ/m3)
Coercivity Relative
recoil
Hc
permea(Wm)
bility
Curie
Resistivity
temperature
(“C)
(Q-m x 1
1 .l-1.3
0.8-0.9
1.35
36-43
32-46
60
46-60
95-150
60-130
80-50
50
800-850 50
80C-850 50
2.6-4.4
2.0-2.8
1.8
oe>
Ferrites (ceramics)
Barium isotropic
0.22
8
Barium anisotropic
0.39
28.5
Strontium anisotropic 0.360.43 24-34
130-1 55 (a) 1.2
1.05
150 (a)
240-300 (a) 1.05
450
450
450
Bonded ferrite
Isotropic
Anisotropic
0.14
3.2
0.23-0.27 10-14
90 (a)
180 (a)
450
450
Rare earth
SmCo5 sintered
SmCo5 bonded
SmpCo,, sintered
NdFeB sintered
NdFeB bonded
0.9
0.5-0.6
1 .l
0.9-1.4
0.25-0.8
600-660 (b) 1.05
400460 (b) 1.1
600-700 (c) 1.05
750-920 (d) 1.05
180460 (e) 1.05
Intrinsic coercivities:
160
56-64
150-240
160-360
10-95
(a) 160340 kA/m
(b) 800-1500 kA/m
(c) 600-1300 kAlm
(d) 950-3000 kA/m
(e) 460-1 300 kAlm
1.1
1.05
>700
>700
>700
310
310
10’2
10’2
10’2
1
50-60
*
75-85
140-1 60
*
* Dependent on the resistivity of the polymer bond.
oxide. The mixture is fired and the resulting material is milled to a particle size of
about 1 p.
The powder is then pressed to the required shape in a die and anisotropic
magnets are produced by applying a magnetic field in the pressing direction. The
resulting compact is then fired.
3.2.2.3 Rare earth alloys
The (BH), values that can be achieved with rare earth alloys are 4-6 times greater
than those for Alnico or ferrite.
There are three main permanent magnet rare earth alloys, two of which (SmCo5
and Sm2Col,) are based on samarium and cobalt, the other being neodymium iron
boron (NdFeB). These materials may be produced by alloying the constituent elements
together, or more usually by reducing a mixture of the oxides together in a hydrogen
atmosphere using calcium as the reducing agent. The alloy is then milled to a particle
size of about 10 pm, pressed in a magnetic field and sintered in vacuum.
SmCo5was the first alloy to be available, but this has gradually been replaced by
Sm2Col7because of its lower cost and better temperature stability. The more recently
developed NdFeB magnets have the advantage of higher remanence B, and higher
(BH)mau,and they are lower in cost because the raw materials are cheaper. The
disadvantage of NdFeB materials is that they are subject to corrosion and they suffer
from a rapid change of magnetic properties (particularlycoercivity) with temperature.
Corrosion can be prevented by coating the magnets and the properties at elevated
temperature may be improved by small additions of other elements.
Newnes Electrical Engineer’s Handbook
37
3.2.2.4 Bonded magnets
Ferrites and rare earth magnets are also produced in bonded forms. The magnet
powder particles are mixed with the bond and the resulting compact can be rolled,
pressed or injection moulded. For a flexible magnet the bonds may be rubber: or for
a rigid magnet they may be nylon, polypropylene, resin or other polymers. Although
the magnetic properties are reduced by the bond, they can be easily cut or sliced and
in contrast to the sintered magnets they are not subject to cracking or chipping.
Rolled or pressed magnets give the best properties as some anisotropy may be induced.
but injection moulding is sometimes preferred to produce complex shapes which
might even incorporate other components. Injection moulding also produces a precise
shape with no waste of material.
3.2.2.5 Applications
Permanent magnets have a very wide range of applications and virtually every part
of industry and commerce uses them to some extent. At one time Alnico was the only
available high-energy material, but it has gradually been replaced by ferrites and rare
earth alloys except in high-stability applications. Ferrites are much cheaper than
Alnico but because of their lower flux density and energy product a larger magnet is
often required. However, about 70 per cent of magnets used are ferrite. They find
bulk applications in loudspeakers, small motors and generators and a wide range of
electronic applications. The rare earth alloys are more expensive but despite this and
because of their much greater strength they are being used in increasing quantities.
They give the opportunity for miniaturization, the ‘Walkman’portable stereo player
being an example. Figures 3.3 and 3.4 show the range of applications for permanent
magnets in the home and in a car.
,
Telephone
Electricity
meter
Cordless tools
Drill, screwdriver
Cycle dynamo
Microwave
Washing machine motor
Fridge & freezer seals
Food mixer
Fig. 3.3 Magnets in the home
3.2.3
Other materials
In addition to the two main groups of soft and hard magnetic materials there are other
materials which meet special needs.
The feebly magnetic steels are austenitic, and their virtually non-magnetic properties
are achieved by additions of chromium and nickel to a low-carbon steel. To attain a
Materials for electrical engineering
38
Sun roof motor
Speedometer,
\
Starter motor
Throile and
crankshaft position
sensors
Fig. 3.4 Magnets in the cia
relative permeability of 1.05 or less the recommended composition is 18 per cent
chromium and 10 per cent nickel, or greater. These steels, which have minimum
strength requirements, are used for non-magnetic parts of machinery, for magnetic
measuring equipment and for minesweeping equipment, where magnetic flux can
actuate magnetic mines.
Magnetic recording makes use of fine magnetic particles which are embedded in
the tape or disc. These particles are of metal or of iron or chromium oxide and the
choice depends on a compromise between price and quality. The heads used for
magnetic recording are usually made from high-permeability ferrites, but amorphous
metal is now also being used. Magnetic storage is a rapidly expanding area with
higher and higher information densities being achieved.
3.2.4
Standards
The main international standard for magnetic materials is IEC 60404, to which BS
6404 is equivalent. This standard has many parts with specifications for the properties
of silicon steels, nickel irons and permanent magnets. Also included are measurement
standards for these materials.
Newnes Electrical Engineer’s Handbook
39
3.3 Insulating materials
The reason for using insulating materials is to separate electrically the conducting
parts of equipment from each other and from earthed components. Earthed components
may include the mechanical casing or structure that is necessary to enable the equipment
to be handled and to operate. Whereas the ‘active’ parts of the equipment play a
useful role in its operation, the insulation is in many ways a necessary evil. For
example. in an electric motor the copper of the winding and the steel core making up
the magnetic circuit are the active components and both contribute to the power
output of the motor; the insulation which keeps these two components apart contributes
nothing, in fact it takes up valuable space and it may be considered by the designer
as not much more than a nuisance.
For these reasons, insulating materials have become a design focus in many types
of electrical equipment, with many companies employing specialists in this field and
carrying out sophisticated life testing of insulation systems. Such is the importance
attached to this field that major international conferences on the subject are held
regularly, for instance by the IEEE in USA and by IEE and BEAMA in the UK.
The simplest way to define an insulating material is to state what it is not. It is not
a good conductor of electricity and it has a high electrical resistance that decreases
with rising temperature, unlike conductors. The following are the most important
properties of insulating materials:
0
0
0
volume resistivity, which is also known as specific resistance
re1ativepermirri~~it;v
(or dielectric constant), which is defined as the ratio of the
electric flux density produced in the material to that produced in a vacuum by
the same electric field strength. The definitions have been set down in eqs 2.1
and 2.2. Relative permittivity can be expressed as the ratio of the capacitance
of a capacitor made of that material to that of the same capacitor using a
vacuum as its dielectric (see eqn 2.11).
dielectric loss (or electrical dissipation factor), which is defined as the ratio of
the power loss in a dielectric material to the total power transmitted through
it. It is given by the tangent of the loss angle and is commonly known as tun
delta. Tan delta has been defined in eqn 2.43.
The volume resistivity, relative permittivity and tan delta values for a range of
insulating materials are shown in Table 3.3.
The most important characteristic of an insulating material is its ability to withstand
electric stress without breaking down. This ability is sometimes known as its dielectric
strength. and is usually quoted in kVlmm. Typical values may range from 5 to
100 kV/mm, but it is dependent on a number of other factors which include the speed
of applicationof the electric field, the length of time for which it is applied, temperature
and whether ac or dc voltage is used.
Another significant aspect of all insulating materials that dominates the way in
which they are categorized is the maximum temperature at which they will perform
satisfactorily. Generally speaking, insulating materials deteriorate more quickly at
higher temperatures and the deterioration can reach a point at which the insulation
ceases to perform its required function. This characteristic is known as ageing, and
for each material it has been usual to assign a maximum temperature beyond which
it is unwise to operate if a reasonable life is to be achieved. The main gradings or
40
Materials for electrical engineering
Table 3.3 Representative properties of typical insulating materials
Volume reslstivity
(Q m)
Relative permittivity
Tan delta
(at 50 Hz)
infinity
infinity
10~~-10~~
108
1.o
1.0006
2.0-2.5
3.1
1.9-2.9
2.8-4.0
5.0-7.0
6.1-6.7
2.8-4.1
3.5-4.5
4.5-7.0
5.0-8.7
3.3
2.5-3.5
4.5-4.7
424.0
2.6
2.3
2.8
5.0-7.0
3.9
0
0
0.0002
0.01 3
0.005
0.005
-
vacuum
air
mineral insulating oil
pressboard
d v paper
oiled paper
porcelain
E-glass
polyester resin
epoxy resin
mica
micapaper
PETP film
aramid paper
epoxy glass laminate
silicone glass laminate
polystyrene
polyethylene
methyl methacrylate
polyvinyl chloride
fused quartz
10‘0
-
1010-1 012
1016
1014-1 0l6
1o l e 1015
1oll-1 015
1013-1 017
1018
1016
-
1015
1015
IOl3
10”
1016
-
0.002-0.005
0.008-0.041
0.01
0.0003
0.0003
0.0025
0.005-0.020
0.008
0.003
0.0002
0.0001
0.06
0.1
-
classes of insulation as defined in IEC 60085 and its UK equivalent BS 2757 are
listed in Table 3.4. Where a thermal class is used to describe an item of electrical
equipment, it normally represents the maximum temperature found within that product
under rated load and other conditions. However, not all the insulation is necessarily
located at the point of maximum temperature, and insulation with a lower thermal
classification may be used in other parts of the equipment.
Table 3.4 Thermal classes for insulation
Thermal class
Operating temperature
(”c)
Y
A
90
105
120
130
155
180
200
220
250
~
E
B
F
H
200
220
250
The ageing of insulation depends not only on the physical and chemical properties
of the material and the thermal stress to which it is exposed, but also on the presence
and degree of influence of mechanical, electrical and environmental stresses. The
processing of the material during manufacture and the way in which it is used in the
complete equipment may also significantly affect the ageing process. The definition
of a useful lifetime will also vary according to the type and usage of equipment; for
Newnes Electrical Engineer’s Handbook
41
instance the running hours of a domestic appliance and a power station generator will
be very different over a 25-year period. All of these factors should therefore influence
the choice of insulating material for a particular application.
There is therefore a general movement in the development of standards and methods
of testing for insulating materials towards the consideration of combinations of materials
or insirluring systems, rather than focusing on individual materials. It is not uncommon
to consider life testing in which more than one form of stress is introduced; this is
known as miiCtifunctional or multifactor testing.
Primaiy insulation is often taken to mean the main insulation, as in the PVC
coaring on a live conductor or wire. Seconduq insulation refers to a second ‘line of
defence’ which ensures that even if the primary insulation is damaged, the exposed
live component does not cause an outer metal casing to become live. Sleeving is
frequently used as a secondary insulation.
Insulating materials may be divided into basic groups which are solid dielectrics,
liquid dielectrics, gas and vacuum. Each is covered separately in the following
sections.
3.3.1
Solid dielectrics
Solid dielectric insulating materials have in the past (for instance in BS 5691 part 2)
been subdivided into three general groups:
solid insulation of all forms not undergoing a transformation during application
solid sheet insulation for winding or stacking, obtained by bonding superimposed
layers
insulation which is solid in its final state, but is applied in the form of a liquid
or paste for filling, varnishing or bonding
A more convenient and up-to-date way to subdivide this very large group of materials
is used by the IEC Technical Committee 15: Insulating Materials:
inorganic (ceramic and glass) materials
plastic films
flexible insulating sleeving
rigid fibrous reinforced laminates
resins and varnishes
pressure-sensitive adhesive tapes
cellulosic materials
combined flexible materials
mica products
This subdivision is organized on the basis of application and is therefore more
helpful to the practising engineer. A brief description of each of these classes of
material is given in the following sections.
3.3.1.1 Inorganic (ceramic and glass) materials
A major application for materials in this category is in high-voltage overhead lines
as suspension insulators (see Figs 14.2 and 14.8), or as bushings on high-voltage
transformers and switchgear (see Fig. 6.14). In either case the material is formed into
Materials for electrical engineering
42
the well-known series of flanged discs to increase the creepage distance along the
surface of the complete insulator. Ceramic materials are used for a number of reasons
including:
e
ease of production of a wide range of shapes
good electrical breakdown strength
retention of insulating characteristics in the event of surface damage
3.3.1.2 Plastic films
Materials such as polyethylene terephthalate (PETP, more commonly referred to as
polyester), polycarbonate, polyimide and polyethylene naphthalate (PEN) have been
used as films in a variety of applications such as the insulation between foils in
capacitors, slot insulation in rotating electrical machines (either by themselves or as
a composite with other sheet materials) and more recently as a backing for micabased products used in the insulation of high-voltage equipment. Plastic films are
used in applications requiring dimensional stability, high dielectric strength, moisture
resistance and physical toughness.
3.3.1.3 Flexible insulating sleeving
This fulfils a number of requirements including the provision of primary or secondary
electrical insulation of component wiring, the protection of cables and components
from the deleterious effects of mechanical and thermal damage, and as a rapid and
low-cost method of bunching and containing cables. Sleevings may find application
in electrical machines, transformers, domestic and heating appliances, light fittings,
cable connections, switchgear and as wiring harnesses in domestic appliances and in
vehicles. They are used because of their ease of application, flexibility and high
dielectric strength; they lend themselves to an extremely wide range of formats
including shrink sleeving, expandable constructions and textile-reinforced grades for
low and high voltage and temperatures across the range -70°C to +450"C.
3.3.1.4 Rigid fibrous reinforced laminates
In the manufacture of most electrical equipment there is a need for items machined
out of solid board or in the form of tubes and rods. These items can take the form of
densified wood or laminates of paper, woven cotton, glass or polyester, or glass or
polyester random mats laminated together with a thermosetting resin which might be
phenolic, epoxy, polyester, melamine, silicone or polyimide, depending upon the
properties required. Rigid boards are used because they are capable of being machined
to size, and they retain their shape and properties during their service life, unlike
unseasoned timber which was used in early equipment.
3.3.1.5 Resins and varnishes
In addition to their use in the laminates outlined in section 3.3.1.4, a wide range of
varnishes and resins are used by themselves in the impregnation and coating of
electrical equipment in order to improve its resistance to working conditions, to
enhance its electrical characteristics and to increase its working life. At first many
resins and varnishes were based on naturally occurring materials such as bitumen,
shellac and vegetable oils, but now they are syntheticallyproduced in a comprehensive
Newnes Electrical Engineer’s Handbook
43
range of thermoplastic, thermosetting and elastomeric forms. The more common
types are phenolic, polyester, epoxy, silicone and polyimide, and these can be formulated
to provide the most suitable processing and the required final characteristics.Varnishes
and resins are used because of their ability to impregnate, coat and bond basic
insulating materials; this assists in the application of the insulating materials and it
significantly improves their service life and their ability to withstand dirt and moisture.
3.3. I . 6 Pressure-sensitive adhesive tapes
Certain types of pressure-sensitive adhesive (PSA)tapes have become so much a
pari of modern life that the trade names have been absorbed into everyday language.
The usefulness of PSA in short lengths for holding down, sealing or locating applies
equally in the field of insulation and a range of tapes has been developed which is
based on paper, film or woven glass cloth, coated with suitable adhesive such as
rubber, silicone or acrylic.
3.3.1.7 Cellulosic materials
Materials in the form of papers, pressboards and presspapers continue to play a vital
role in oil-filled power transformers. Included in this area are other materials which
are produced by paper-making techniques, but which use aramid fibres; these materials
have found wide application in high-temperature and dry-type transformers as well
as in other types of electrical equipment. Cellulose materials are mainly used in
conjunction with oil, and it is their porous nature that lends itself to successful use in
transformers and cables.
3.3.1.8 Combined flexible materials
In order to produce suitable materials with the required properties such as tear
strength, electric strength and thermal resistance at an acceptable price, a range of
laminated or combined flexible sheet products has been developed. These employ
cellulosic, aramid and glass fleeces as well as other materials, in combination with
many of the plastic films already referred to, in a range of forms to suit the application.
These products are used in large quantities in low-voltage electric motors.
3.3.1.9 Mica products
Materials based on mica, which is a naturally occurring mineral, play a central part
in the design and manufacture of high-voltage rotating machines. Originally the
material was in the form of mica splittings,but at present the industry uses predominantly
micapaper, which is produced by breaking down the mica into small platelets by
chemical or mechanical means, producing a slurry and then feeding this through a
traditional paper-making machine. The resulting micapaper, when suitably supported
by a woven glass or film backing and impregnated with epoxy or a similar resin, is
used to insulate the copper bars which make up the stator winding of the machine.
Micapaper is used in the ground insulation of the winding which is shown as part of
the stator slot section illustrated in Fig. 3.5.
Micapaper is available in a resin-rich form, in which all the necessary resin for
consolidation of the insulation around the winding is included within the material.
This consolidation is usually carried out in large steam or electrically heated processes
into which an insulated coil side or bar can be placed; heat and pressure are then
44
Materials for electrical engineering
Copper conductor
1
Conductor insulation
.
Turn insulation
-
Ground insulation
Slot liner
Slot wedge
Fig. 3.5 Stator coil section of a high-voltageelectrical machine (courtesy of
ALSTOM Electrical Machines Ltd)
Newnes Electrical Engineer’s Handbook
45
applied as necessary to cure fully the resin-rich micapaper insulation. Alternatively
it is available for use with vacuum pressure impregnation (VPI), in which most of the
resin is introduced after winding the machine. The use of VPI eliminates the need to
consolidate the insulation in a press; consolidation is achieved either by the use of
hydrostatic forces or by ensuring full impregnation of the bars or coils already placed
or wound into slots. A large electrical machine stator is shown being lowered into a
VPI tank in Fig. 3.6.
Fig. 3.6 A large vacuum and pressure impregnation facility capable of treating stators up to 100 MVA
(courtesy of ALSTOM Electrical Machines Ltd)
Mica-based products dominate high-voltage insulation systems because of their
unique combination of properties, which include:
high dielectric strength
low dielectric loss at high frequency
high surface and volume resistivity
excellent resistance to corona discharge and electric arc erosion
temperature capability from -273°C to 1000°C
flame resistance
excellent chemical resistance
high resistance to compressive forces
Materials for electrical engineering
46
3.3.1.10 Textile insulation
Although the use of fully varnished fabric is becoming less common, products using
glass and polyester-based yarn, and to a lesser extent cotton and rayon, are still in
use.
A much larger range of unvarnished narrow-fabric products, more commonly
called woven tapes, exist and these use a variety of combinations of different glass
and polyester yams tailored to meet specific applications. Primarily these tapes are
used for finishing on top of other insulation such as micapaper, in order to provide
a tough outer surface which can readily be coated with a final varnish or paint
finish. When manufactured on modern shuttle-less looms they are an economic
proposition.
Woven tapes are used because of their ease of application, good conformity and
bedding down. An example of the complex shapes that they can be used to cover is
shown in Fig. 3.7,which illustrates the endwinding of a high-voltageelectrical machine.
3.3.1.11 Elastomers and thermoplastics
There is a very wide range of polymeric and rubber-like insulation materials. These
have traditionally been dealt with in connection with electric cables and IEC and BSI
reflect this by dealing with them separately in Technical Committee 20: Electrical
Cables. Some elastomers such as silicone have found application in sleeving, traction
systems and increasingly as overhead line insulators, but the bulk of their application
continues to be related to cables. The leading materials such as PVC, MDPE, XLPE
and EPR are therefore referred to in Chapter 9.
3.3.2
Liquid dielectrics
A liquid dielectric remains in the liquid state throughout its working life, unlike
resins and varnishes which become solid after processing.
The principal uses of liquid dielectrics are as a filling and cooling medium for
transformers, capacitors and rheostats, as an arc-quenching medium in switchgear
and as an impregnant of absorbent insulation used mainly in transformers, switchgear,
capacitors and cables. The important properties of dielectric liquids are therefore
electric strength, viscosity, chemical stability and flashpoint.
Typical materials include highly refined hydrocarbon mineral oils obtained from
selected crude petroleum, silicone fluids, synthetic esters and hydrocarbons with
high molecular weight. A specially interesting material for cables has been the waxy
Mineral Insulating Non-Draining (MIND) compound which has been used in paperinsulated cables; this is described in section 9.2.3. A group of polychlorinated biphenyls
(PCBs) has been used in transformers, but these materials are now being replaced
and carefully disposed of because of their toxic nature and their resistance to biological
and chemical degradation.
3.3.3 Gas insulation
Two gases already in common use for insulation are nitrogen and sulphur hexafluoride
(SF6). Nitrogen is used as an insulating medium in some sealed transformers, while
SF6 is finding increasing use in transmission and distribution switchgear because of
its insulating properties and its arc-extinguishing capabilities; this is described further
in sections 7.4.1 and 7.5.3(b).
Newnes Electrical Engineer’s Handbook
47
Fig. 3.7 Stator endwinding bracing system for a 4-pole. 11 kV, 8 MW induction motor (courtesy of
ALSTOM Electrical Machines Ltd)
3.3.4
Vacuum insulation
Vacuum insulation is now used in a range of medium-voltage switchgear. Like SF6,
it has both insulating and arc-extinguishing properties. The action of the vacuum in
a circuit breaker is explained in section 7.4.1.
3.3.5
Standards
A selection of national and international standards covering the insulation field is
given in Table 3.5. The majority of the IEC standards with their EN and BSEN
equivalents consist of three parts:
Materials for electrical engineering
48
Table 3.5 National and international standards relating to insulating materials
____
IEC
ENMD
BS
60085
HD56681
2757
60216
60243
60371
60394
60454
60455
60464
60626
60641
60672
60674
60684
60819
60893
61067
61068
61212
~
_
_
_
Subject
_
_
_
N.
American
Thermal evaluation and classification of
electrical insulation
HD 611.1Sl 5691
Determination of thermal endurance
properties of electrical insulating materials
EN 60243 EN 60243 Methods of test for electric strength of solid
insulating materials
EN 60371 EN 60371 Specifications for insulating materials
based on mica
5689
Varnish fabrics for electrical purposes
EN 60454 EN 60454 Specifications for pressure-sensitive
adhesive tapes for electrical purposes
EN 60455 EN 60455 Specifications for solventless polymerizable
5664
resinous compounds used for electrical
insulation
EN 60464 5629
Specifications for insulating varnishes
containing solvent
EN 60626 EN 60626 Combined flexible materials for electrical
insulation
EN 60641 EN 60641 Specification for pressboard and presspaper
for electrical purposes
EN 60672 EN 60672 Specifications for ceramic and glass
insulating materials
EN 60674 EN 60674 Specifications for plastic films for electrical
purposes
EN 60684 EN 60684 Specifications for flexible insulating sleevings NEMA
vs-1
EN 60819 EN 60819-1 Non-cellulosic papers for electrical purposes
7824
EN 60893 EN 60893 Specifications for industrial rigid laminated
NEMA
LI-1
sheets based on thermosetting resins for
electrical purposes
EN 61067 EN 61067 Specifications for glass and glass polyester
fibre woven tapes
EN 61068 EN 61068 Polyester fibre woven tapes
EN 61212 EN 61212 Industrial rigid laminated tubes and rods based
6128
on thermosetting resins for electrical purposes
Part 1: Definitions, classification and general requirements
Part 2: Methods of test
Part 3: Specifications for individual materials
3.4
Conducting materials
Strictly, conducting materials fall into three groups, which are conductors,
semiconductors and imperfect insulators. Insulators have been covered in section
3.3, so the focus here is on conductors and semiconductors.
3.4.1
Conductors
In general, metals and alloys are conductors of electricity. The conductivity in metals
such as copper and aluminium is due to electrons which are attracted to the positive
49
Newnes Electrical Engineer's Handbook
terminal when a voltage is applied. The freedom with which the electrons can move
determines the conductivity and resistivity. The restraints on electron movements are
impurities, stresses and thermal lattice vibrations, so to obtain the highest conductivity
the metal must be very pure and in the annealed state. With increasing temperature
the thermal lattice vibrations increase and conductivity is therefore reduced.
The principal materials for commercial application as conductors are the pure
metals aluminium and copper, although very widely used are the alloys of these
metals, with small additions of other elements to improve their properties for particular
applications. Table 3.6 compares typical values of the key parameters for the two
metals.
Table 3.6 Comparison of the typical properties of aluminium and copper
Electrical conductivity
at 20°C
Electrical resistivity
at 20°C
Density at 20°C
Temperature coefficient
of resistance
Aluminium
Copper
37.2x 10'
58.1 x
2.69
1.72
p R . cm
8933
3.93 x 10-3
kglm3
per "C
238
599
Wlm "C
0.909
0.388
J1g"C
2689.8
4.2 x
lo3
lo6
Units
Slm
(0-l0O~C)
Thermal conductivity
(0-100°C)
Mean specific heat
(0-100%)
Coefficient of linear
expansion (0-1 00°C)
Melting point
23.5 x
660
lo4
16.8 x
1085
10-6
per "C
"C
3.4.1.1 Copper and its alloys
Copper has the highest electrical and thermal conductivity of the common industrial
metals. It has good mechanical properties, is easy to solder, is readily available and
has high scrap value. It is widely used in wire form, and Table 3.7 gives information
for the commonly used wire sizes.
The electrical resistance of copper, as of all other pure metals, varies with temperatm.
The variation is sufficient to reduce the conductivity of pure copper at 100°C to
about 76 per cent of its value at 20°C.
The resistance RtI at temperature rI is given by the relationship
R ~ =I Rr[1 + ar(tl - t)l
(3.1)
where CC, is the constant-mass temperature coefficient of resistance of copper at the
reference temperature t ("C). Although resistance may be regarded for practical
purposes as a linear function of temperature, the value of the temperature coefficient
is not constant, but depends upon the reference temperature according to the law in
eqn 3.2.
1
1
a,= t + l/ao
t + 234.45
(3.2)
At 20"C, the value of qowhich is given by eqn 3.2 is O.O0393/"C, and this is the
value which is adopted by IEC. Multiplier constants and their reciprocals correlating
Materials for electrical engineering
50
Table 3.7 IEC and BSI recommended sizes of annealed copper wires
Nominal
diameter
(mm)
5.000
4.750
4.500
4.000
3.750
3.550
3.350
3.150
3.000
2.800
2.650
2.500
2.360
2.240
2.120
2.000
1 .goo
1.800
1.700
1.600
1.500
1.400
1.320
1.250
1.180
1.120
1.060
1 .DO0
.950
.goo
.850
.800
.750
,710
.630
560
.500
.450
.400
.355
.315
.280
.250
.224
.200
.180
.160
,140
.125
.112
.loo
.090
.080
-071
.063
Cross-sectional
area
(mm2)
19.64
17.72
15.90
12.57
11.045
9.898
8.814
7.793
7.069
6.158
5.515
4.909
4.374
3.941
3.530
3.142
2.835
2.545
2.270
2.011
1.767
1.539
1.368
1.227
1.094
.9852
.a825
.7854
.7088
.6362
,5675
.5027
,4418
.3959
,3117
,2463
.1 964
.1 590
.1 257
.0990
.0779
.0616
.0491
.0394
-0314
.0255
.0201
,0154
.0123
.00985
.00785
.00636
.00503
.00396
.00312
Weight
(kgkm)
Resistance
(mm)
174.56
157.54
141.39
111.71
98.19
87.99
78.38
69.28
62.84
54.74
49.03
43.64
38.89
35.03
31.38
27.93
25.21
22.62
20.18
17.874
15.710
13.685
12.166
10.910
9.722
8.758
7.845
6.982
6.301
5.656
5.045
4.469
3.927
3.520
2.771
2.190
1.746
1.414
1.117
.880
,693
547
.436
,350
.279
.227
.179
.137
.lo9
.0876
.0698
.0566
.0447
.0352
.0277
.8703
.9646
1.0750
1.3602
1.5476
1.7269
1.9393
2.193
2.418
2.776
3.099
3.482
3.908
4.338
4.843
5.441
6.029
6.718
7.531
8.502
9.673
11.10
12.49
13.93
15.63
17.35
19.37
21.76
24.12
26.87
30.12
34.01
38.69
43.18
54.84
69.40
87.06
107.5
136.0
172.7
219.3
277.6
348.2
433.8
544.1
671.8
850.2
1110
1393
1735
2176
2687
3401
4318
5484
Current rating at
1000 M n 2
(A)
30.43
27.47
24.65
19.48
17.12
15.43
13.66
12.08
10.96
9.544
8.549
7.609
6.780
6.108
5.471
4.869
4.375
3.944
3.518
3.116
2.739
2.386
2.121
1.902
1.695
1.527
1.368
1.217
1.099
.9861
.8796
.7791
.e848
.6137
.a32
.3a1a
.3043
.2465
.1 948
.1534
.1208
.0954
.0761
.0611
.0487
.0396
.0312
,0239
.0190
.0153
.0122
.0099
.0078
.0061
.0048
Newnes Electrical Engineer's Handbook
51
the resistance of copper at a standard temperature with the resistance at other
temperatures may be obtained from tables which are included in BS 125, BS 14321434 and BS 4109.
Cadmium copper, chromium copper, silver copper, tellurium copper and sulphur
copper find wide application in the electrical industry where high conductivity is
required. The key physical properties of these alloys are shown in Table 3.8. It can
be seen that some of the alloys are deoxidized and some are 'tough pitch' (oxygen
containing) or deoxidized. Tough pitch coppers and alloys become embrittled at
elevated temperatures in a reducing atmosphere, and where such conditions are
likely to be met, oxygen-free or deoxidized materials should be used.
Cadiilium copper has greater strength than ordinary copper under both static and
alternating stresses and it has better resistance to wear. It is particularly suitable for
the contact wires in electric railways, tramways, trolley buses, gantry cranes and
similar equipment, and is also used in overhead telecommunications lines and
transmission lines of long span. It retains its hardness and strength in temperatures at
which high conductivity copper would soften, and is used in electrode holders for
spot and seam welding of steel; it has also been used in commutator bars for certain
types of motor. Because it has a comparatively high elastic limit in the work-hardened
condition, it is also used in small springs required to carry current, and is used as thin
hard-rolled strip for reinforcing the lead sheaths of cables which operate under internal
pressure. Castings of cadmium copper have some application in switchgear components
and in the secondaries of transformers for welding machines. Cadmium copper can
be soft soldered, silver soldered and brazed in the same way as ordinary copper,
although special fluxes are required under certain conditions, and these should contain
fluorides: being a deoxidized material there is no risk of embrittlement by reducing
gases during such processes.
Chmrnizm copper is particularly suitable for high-strength applications such as spot
and seam types of welding electrodes. Strip and, to a lesser extent, wire are used for
light springs which carry current. In its heat-treated state, the material can be used at
temperatures up to 350°C without risk of deterioration of properties, and it is used
Table 3.8 Selected physical properties of copper alloys
Property
Cadmium
copper
Content
0.7-1 .O%
0.4-0.870 0.0341%
cadmium
chromium silver
deoxidized deoxidized tough pitch
or
deoxidized
132
108
118
Tough pitch (oxygencontaining) or deoxidized
Modulus of elasticity
(1OQ N m-')
Resistivity at 20%
(IO-*R . m)
annealed
2.2-1.9
solution heat treated
precipitation hardened cold worked
2.3-2.0
*Tough pitch.
"Solution heat treated or annealed.
Chromium Silver
copper
copper
4.9
2.3-2.0
-
1.74-1.71
Tellurium
copper
Sulphur
copper
0.3-0.7%
tellurium
tough pitch
or
deoxidized
118-
0.3-0.670
sulphur
deoxidized
1.76**
118
1.81
-
-
-
-
1.78
1.80
1.85
52
Materials for electrical engineering
for commutator segments in rotating machines where the temperatures are higher
than normal. In the solution heat-treated condition, chromium copper is soft and can
be machined; in the hardened state it is not difficult to cut but it is not free-machining
like leaded brass or tellurium copper. Joining methods similar to cadmium copper are
applicable, and chromium copper can be welded using gas-shielded arcs.
Silver copper has the same electrical conductivity as ordinary high-conductivity
copper, but its softening temperature, after hardening by cold work, is much higher
and its resistance to creep at moderately elevated temperatures is enhanced. Because
its outstanding properties are in the work-hardened state it is rarely required in the
annealed condition. Its principal uses are in electrical machines which operate at
high temperatures or are exposed to high temperatures in manufacture. Examples of
the latter are soft soldering or stoving of insulating materials. Silver copper is available
in hard-drawn or rolled rods and sections, especially those designed for commutator
segments, rotor bars and similar applications. Silver copper can be soft soldered,
silver soldered, brazed or welded without difficulty but the temperatures involved in
all these processes are sufficient to anneal the material if in the cold-worked condition.
Because the tough pitch material contains oxygen as dispersed particles of cuprous
oxide, it is also important to avoid heating it to brazing and welding temperatures in
a reducing atmosphere. In the work-hardened state silver copper is not free-cutting,
but it is not difficult to machine.
Telliirium copper offers free-machining, high electrical conductivity, retention of
work hardening at moderately elevated temperatures and good corrosion resistance.
It is unsuitable for most types of welding, but gas-shielded arc welding and resistance
welding can be effected with care. A typical application is magnetron bodies, which
are often machined from solid. Tellurium copper can be soft soldered, silver soldered
and brazed without difficulty. For tough pitch, brazing should be done in an inert or
slightly oxidizing atmosphere since reducing atmospheres are conducive to embrittlement. Deoxidized tellurium copper is not subject to embrittlement.
Sulphur copper-is free-machiningand does not have the tendency of tellurium copper
to form coarse stringers in the structure which can affect accuracy and finish. It has
greater resistance to softening than high-conductivity copper at moderately high
temperatures and gives good corrosion resistance. Sulphur copper has applicationsin
all machinedparts requiring high electricalconductivity,such as contacts and connectors;
its joining characteristics are similar to those of tellurium copper. It is deoxidized
with a controlled amount of phosphorus and therefore does not suffer from hydrogen
embrittlement in normal torch brazing, but long exposure to a reducing atmosphere
can result in loss of sulphur and consequent embrittlement.
3.4.1.2 Aluminium and its alloys
For many years aluminium has been used as a conductor in most branches of electrical
engineering. Several aluminium alloys are also good conductors, combining strength
with acceptable conductivity. Aluminium is less dense and cheaper than copper, and
its price is not subject to the same wide fluctuations as copper. World production of
aluminium has steadily increased over recent years to overtake that of copper, which
it has replaced in many electrical applications.
There are two specifications for aluminium, one for pure metal grade ZE and the
other for a heat-treatable alloy 91E. Grade 1E is available in a number of forms
Newnes Electrical Engineer’s Handbook
53
which are extruded tube (ElE), solid conductor (ClE), wire (GlE) and rolled strip
(D 1E). The heat-treatable alloy, which has moderate strength and a conductivity
approaching that of aluminium, is available in tubes and sections (E91E).
The main application areas are:
Busburs. Although aluminium has been used as busbars for many years, only recently
has it been accepted generally. The electricity supply industry has now adopted
aluminium busbars as standard in 400 kV substations, and they are also used widely
in switchgear, plating shops, rising mains and in the UK aluminium smelting plants.
Sometimes the busbars are tin-plated in applications where joints have to be opened
and remade frequently.
Cable. The use of aluminium in wires and cables is described at length in Chapter
9. Aluminium is used extensively in cables rated up to 11 kV and house wiring cable
above 2.5 mm2 is also available with aluminium conductor.
Owrhend lines. The Aluminium Conductor Steel Reinforced (ACSR) conductor
referred to in section 14.3 and Fig. 14.3 is the standard adopted throughout the
world, although in the USA Aluminium Conductor Aluminium alloy wire Reinforced
(ACAR) is rapidly gaining acceptance; it offers freedom from bimetallic corrosion
and improved conductance for a given cross-section.
Motors. The use of aluminium in cage rotors of induction motors is described in
Chapter 10. Motor frames are often die-cast or extruded from aluminium, and shaftdriven cooling fans are sometimes of cast aluminium.
Foil windings are suitable for transformers, reactors and solenoids. They offer a
better space factor than a wire-wound copper coil, the aluminium conductor occupying
about 90 per cent of the space, compared to 60 per cent occupied by copper. Heat
transfer is aided by the improved space factor and the reduced insulation that is
needed in foil windings, and efficient radial heat transfer ensures an even temperature
gradient. Windings of transformers are described in more depth in section 6.2.2.
Hearing elements have been developed in aluminium but they are not widely used at
present. Applications include foil film wallpaper, curing concrete and possibly soil
warming.
Heat siriks are an ideal application for aluminium because of its high thermal
conductivity and the ease of extrusion or casting into solid or hollow shapes with
integral fins. They are used in a variety of applications such as semiconductor devices
and transformer tanks. The low weight of aluminium heat sinks make them ideal for
pole-mounted transformers and there is the added advantage that the material does
not react with transformer oil to form a sludge.
3.4. f . 3 Resistance alioys
Many alloys with high resistivity have been developed, the two main applications
being resistors and heating elements.
Alloys for standard resistors are required to have a low temperature coefficient of
resistivity in the region of room temperature. The traditionally used alloy is Manganin,
but this has increasingly been replaced by Ni-Cr-A1 alloys with the trade names
54
Materials for electrical engineering
Karma and Evanohm. The resistivity of these alloys is about 1.3 pa m and the
temperature coeficient is k 0.5 x 10-5/0C.For lower-precision applications coppernickel alloys are used, but these have a lower resistivity and a relatively high thermo
emf against copper.
For heating elements in electric fires, storage heaters and industrial and laboratory
furnaces there is a considerable range of alloys available. A considerable resistivity
is required from the alloy in order to limit the bulk of wire required, and the temperature
coefficient of resistivity must be small so that the current remains reasonably constant
with a constant applied voltage. Ni-Cr alloys are used for temperatures up to 1100°C,
and Cr-Fe-Al alloys are used up to 1400°C.Ceramic rods are used for higher
temperatures and silicon carbide may be used up to 1600°C. For even higher
temperatures, the cermets MoSiz and Zircothal are used. The maximum temperatures
at which the materials may be used depend on the type of atmosphere.
3.4.2 Semiconductors
A semiconductor is able at room temperature to conduct electricity more readily than
an insulator but less readily than a conductor. At low temperatures,pure semiconductors
behave like insulators. When the temperature of a semiconductor is increased, or
when it is illuminated, electrons are energized and these become conduction electrons.
Deficiencies or ‘holes’ are left behind: these are said to be camers of positive electricity.
The resulting conduction is called intrinsic conducrion.
The common semiconductors include elements such as silicon, germanium and
selenium and compounds such as indium arsenide and gallium antimonide. Germanium
was originally used for the manufacture of semiconductor devices, but because of
resistivity problems and difficulty of supply it was replaced by silicon which is now
the dominant material for production of all active devices such as diodes, bipolar
transistors, MOSFETs, thryistors and IGBTs.
Both silicon and germanium are group IV elements of the periodic table, having
four electrons in their outer orbit. This results in a diamond-type crystal giving a tight
bond of the electrons. Figure 3.8 shows the atoms in a silicon crystal. Each atom is
Fig. 3.8 Atoms in a silicon crystal
Newnes Electrical Engineer’s Handbook
55
surrounded by eight electrons, four its own and four from neighbouring atoms; this
is the maximum number in an orbit and it results in a strong equilibrium. It is for this
reason that pure crystals of silicon and germanium are not good conductors at low
temperature.
3.4.2.1 Impurity effects and doping
The conductivity of group IV semiconductors like silicon can be greatly increased by
the addition of small amounts of elements from group V (such as phosphorus, arsenic
or tin) or group 111 (such as boron, aluminium, gallium or indium).
Phosphorus has five electrons in its outer shell and when an atom of phosphorus
replaces an atom of silicon it generates afree electron, as shown in Fig. 3.9. This is
called doping. The extra electrons are very mobile; when a voltage is applied they
move very easily and a current passes. If 10l6phosphorus atoms/cm3 are added to a
pure crystal, the electron concentration is greatly increased and the conductivity is
increased by a factor of about a million. The impurities are called donor atoms and
the material is an impuriry semiconductor. This is called an n-wpe serniconductor,
and n represents the excess of free electron carriers.
If the material is doped with group III atoms such as indium, then a similar effect
occurs. This is shown in Fig. 3.10.The missing electron forms a ‘hole’in the structure
which acts as a positive carrier. This structure is known as a p - y p e sernicundcrcfuiand p represents the excess of positive carriers. The impurities are called acceptor
atoms.
A single crystal containing both n-type and p-type regions can be prepared by
introducing the donor and acceptor impurities into molten silicon at different stages
of the crystal formation. The resultant crystal has two distinct regions of p-type and
n-type material, and the boundary joining the two areas is known as a p-n junction.
Such a junction may also be produced by placing a piece of donor impurity material
against the surface of a p-type crystal or a piece of acceptor impurity material against
an n-type crystal, and applying heat to diffuse the impurity atoms through the outer
layer. When an external voltage is applied, the n-p junction acts as a rectifier, permitting
current to flow in only one direction. If the p-type region is connected to the positive
terminal of a battery and the n-type to the negative terminal. a large current flows
through the material across the junction, but when the battery is connected in the
opposite manner, no current flows. This characteristic is shown in Fig. 3.11.
3.4.2.2 The transistor
Many types of device can be built with quite elaborate combinations and constructions
based around the n-p and p n junction. Further information on these devices may be
found in reference 3F.
Possibly the most important single device is the transistor, in which a combination
of two or more junctions may be used to achieve amplification. One type, known as
the n-p-n junction transistor, consists of a very thin layer of p-type material between
two sections of n-type material, arranged in a circuit shown in Fig. 3.12.The n-type
material at the left of the diagram is the emitter element of the transistor, constituting
the electron source. To permit the flow of current across the n-p junction, the emitter
has a small negative voltage with respect to the p-type layer, or base component, that
controls the electron flow. The n-type material in the output circuit serves as the
collector element, which has a large positive voltage with respect to the base in order
to prevent reverse current flow. Electrons moving from the emitter enter the base, are
Materials for electrical engineering
56
electron
Fig. 3.9 Representation of an n-type semiconductor
'Hole'
created by
indium
atom
Fig. 3.10 Representation of a p-type semiconductor
I A
---ElCurrent
w when forward
biased
Anode
-bk-
Cathode
V
Reverse
leakage
current
Fig. 3.11 Construction, symbol and characteristic of a semiconductor diode
w
Forward
voltage
drop
Newnes Electrical Engineer’s Handbook
57
amacted to the positively charged collector, and flow through the output circuit. The
input impedance between the collector and base is low, whereas the output impedance
between collector and base is high. Therefore, small changes in the voltage of the
base cause large changes in the voltage drop across the collector resistance, making
this type of transistor an effective amplifier.
Similar in operation to the n-p-n type is the p-n-p junction transistor also shown
in Fig. 3.12. This also has two junctions and is equivalent to a triode vacuum tube.
Base
Fig. 3.12 Construction and symbols for n-p-n and p-n-p transistors
Other types, such as the n-pn-p junction transistor, provide greater amplification
than these two-junction transistors.
3.4.2.3 Printed circuits and integrated circuits
A printed circuit is an electrical circuit made by printing and bonding conducting
material as a network of fine threads on a thin ceramic or polymer insulating sheet.
This replaces the wiring used in conventional circuits. Other elements such as
transistors, resistors and capacitors can be deposited onto the same base as the
pri-ntedcircuit.
An integrated cirrcuit is effectively a combination of many printed circuits. It is
formed as a single unit by diffusing impurities into single-crystal silicon, which then
serves as a semiconductor material, or by etching the silicon by means of electron
beams. Several hundred Integrated Circuits (ICs) are made at a time on a thin wafer
several centimetres in diameter, and the wafer is subsequently sliced into individual
ICs called clzips.
In large-scale integration (LSI), several thousand circuit elements such as resistors
and transistors are combined in a 5 mm square area of silicon no more than 0.5 mm
thick. Over 200 such circuits can be arrayed on a silicon wafer 100 mm in diameter.
In veiy large-scale integration (VLSI), hundreds of thousands of circuit elements
fit onto a single silicon chip. Individual circuit elements on a chip are interconnected
by thin metal or semiconductor films which are insulated from the rest of the circuit
by thin dielectric layers. This is achieved by the formation of a silicon dioxide layer
on the silicon wafer surface, silicon dioxide being an excellent dielectric. Metal
oxide semiconductorfield effecttransistors (MOSFETs) are made using this technique.
These transistors are used for high-frequency switching applications and for random
access memories in computers. They have very high speed and low power consumption.
3.4.2.4 The microprocessor
The microprocessor is a single chip of silicon which has the ability to control processes.
Materials for electrical engineering
58
It can form the central processing unit (CPU) of a small computer and it can be used
in a wide range of other applications. A microprocessor may incorporate from a
thousand up to several hundred thousand elements. It typically contains a read-only
memory (ROM), that is a memory that can be read repeatedly but cannot be changed,
but it may also have some random-access memory (RAM) for holding transient data.
Also present in a microprocessor are registers for holding computing instructions, for
holding the ‘address’ of each instruction in turn and for holding data, and a logic unit.
Interfaces for connecting with external memories and other systems are included as
required.
The microprocessors used in personal computers (PCs) have been the subject of
intensive development during the last decade. The speed of operation is usually
defined as a frequency and chips with frequencies of 667 MHz or higher are now
available; this corresponds to an individual operation time of 1.5 nanoseconds. The
amount of information that can be transferred in parallel and held in registers is
known as a bit, and 64-bit processors are now available.
3.4.3
Superconductors
The ideal superconducting state is characterized by two fundamental properties,
which are the disappearance of resistance when the temperature is reduced to a
critical value, and the expulsion of any magnetic flux in the material when the critical
temperature (T,) is reached. Superconductivity was first discovered in mercury in
1911. Other elements have subsequently been found to exhibit superconductivity and
theories have been developed to explain the phenomenon. The critical temperatures
for these materials were typically about 10K (-263°C) which meant that they had to
be cooled with liquid helium at 4K. In general these materials have been of academic
interest only because they could only support a low current density in a low magnetic
field without losing their superconducting properties.
In the 1950s a new class of materials was discovered. These are metallic alloys,
the most important being niobium titanium and niobium tin. The highest critical
temperatures achieved by these materials is 23.2K and they can be used to produce
magnetic flux densities of over 15 T. The main commercial application for these lowT, superconductors is for magnets in medical imaging equipment which requires the
high fields to excite magnetic resonance in nuclei of hydrogen and other elements.
The magnet or solenoid of the magnetic resonance imaging (MRI) unit has an internal
diameter of about 1.2 m and the patient to be examined is put into this aperture. The
image from the resonance test shows unexpected concentrations of fluids or tissue
and enables a diagnosis. Superconducting magnets producing high magnetic fields
are also used in magnetic research and in high-energy physics research; other
applications such as dc motors and generators, levitated trains, cables and ac switches
have been explored but the complexity and high cost of providing liquid helium has
prevented commercial development.
In late 1986 a ceramic material LaBaCuO was discovered to be superconducting
at 35K and in 1987 the material Yl3aCuO was found to have a critical temperature of
92K. Since that time the critical temperatures of new materials has progressively
increased to over 130K. Examples of these are BiSrCaCuO (with a T, of 106K),
ThBaCaCuO (T, of 125K) and HgBaCaCuO (T, of 133K). The enormous significance
of these discoveries is that the materials will be superconducting in liquid nitrogen,
which has a boiling point of 77K and is much easier and cheaper to provide than
helium.
Newnes Electrical Engineer’s Handbook
59
The consequence of these high-temperature superconductors has been an
unprecedented upsurge of research activity throughout the world. Much of this work
is directed towards finding materials with even higher T, values and towards establishing
viable production methods. These new materials are brittle and unless they are in
the form of very thin films they will only operate up to current densities of about
0.1-1 N m 2 .The indications are, however, that they will operate in flux densities
exceeding 50 T.
Attention is now being turned to applications for the materials which are available.
Their brittleness rules out their direct use as conventional wires in magnets or cables,
although a prototype cable carrying 2300 A over 1 m has been made. In this case the
superconducting ceramic oxide was deposited onto flexible tape, and this technique
leads to the possible use of cable for winding solenoids to produce high magnetic
fields. Other applications which are already in use are superconducting quantum
interference device (SQUID) magnetometers which are used for detecting very small
currents in the human body to detect and pinpoint defects. The materials are also in
use in transmission aerials, in which low noise at low temperatures is an advantage.
3.5 Standards
Each country has in the past had its own standards for materials. Over the past 20
years or so there has been a movement towards international standards which for
electrical materials are produced by IEC (InternationalElectrotechnicalCommission).
When an IEC standard is produced the member countries copy this standard and
Table 3.9 Standards for conducting materials
Copper
BS
IEC
Subject of standard
1432
1433
1434
4109
60356
Specifications for copper for electrical purposes.
7884
Aluminium
215 ptsl&2
2627
2897
60207
2898
3242
60208
3988
60121
6360
60228
60228A
Semiconductors 4727
60050
ptl group 05 (521)
6493
60747
Specifications for copper for electrical purposes.
Wire for general electrical purposesand for insulated
cables and flexible cords.
Specifications for copper and copper-cadrnium
stranded conductors for overhead electric traction
and power transmission systems.
Aluminium conductors.
Wrought aluminium for electrical purposes -wire
Wrought aluminium for electrical purposes- strip
with drawn or rolled edges.
Wrought aluminium for electrical purposes - bars,
tubes and sections.
Aluminium alloy stranded conductorsfor overhead
power transmission.
Wrought aluminium for electrical purposes- solid
conductors for insulated cables.
Specifications for conductors in insulated cables
and cords.
Semiconductor terminology.
This has many parts with specificationsfor diodes,
transistors, thyristors and integrated circuits.
60
Materials for electrical engineering
issue it under their own covers. This now applies to BSI, ASTM, DIN and the French
standards organization. Where appropriate, standards are also issued as applying to
all the European Union.
Leading standards for electrical materials are shown in Table 3.9.
References
3A.
3B.
3c.
3D.
3E.
3F.
3G.
3H.
Jones, G.R., Laughton M A . and Say, M.G.(eds). Elecrricul Engineer's Refewnce Book (15th
edn), Butterworth-Heinemann, 1993.
Brandes, E.A. and Brook G.R. Srniihells Metals Reference Book (7thedn), Butterworth-Heinemam,
1992.
McCaig, M. and Clegg, A.G. Permanenr Magnets in Theoy and Practice (2nd edn). Pentech
and Wiley. 1987.
Sillars, R.W. Electrical Insulating Materials and Their Application, Peter Peregrinus, 1973.
Tillar Shugg, W. Handbook of Elecrrical and Electionic Insularing Materials, Van Nostrand,
1986.
Brindley, K. Newnes Electronics Engineer's Pocker Book, Butterworth-Heinemam, 1993.
Mayo, 'WilliamE. Piocessing andApplicafionsof High T,Superconductors, Metallurgical Society
Inc., 1988.
Wames, L.A.A. Electronic and Electrical Engineering, Macmillan, 1994 (good chapters on
semiconductors).
Chapter 4
Measurement and instrumentation
Dr M.J. Cunningham
Department of Computer Science, University of Manchester
4.1
Introduction
Measurement is an activity which pervades almost all human endeavours. In industrial
countries about 5 per cent of the gross national product is devoted to making
measurements and this figure is comparable with the expenditure on the health
services in the UK. Three major areas of the use of measurement and instrumentation
are in product manufacture,in the process industries and in the exchange of commodities.
Measurement by means of instruments takes place at all stages of the manufacturing
process from the validation of bought-in components, through the assessment of the
process, in safety tests and environmental tests and in the final check of conformance
with the customer’s specification. Measurement is the arbiter of quality - the ability
to meet the customer’s needs.
Instruments pervade the process industries where flow, temperature, pressure and
liquid level are very commonly measured quantities. The ability to control these
quantities depends on the ability to measure the quantities satisfactorily. Thus
measurement, often by means of a sensor, is found in every control loop of the
process.
When commodities change hands, from pears to petroleum, acceptable measurement
is the dispenser of fair play. A traditional role of government is to ensure the availability
of an accepted system of measurement for trade throughout the country.
When some property of an object is measured a number is assigned to the property
in terms of the agreed unit value in such a way that the number faithfully reflects the
characteristics of the property. This is a definition of measurement. In producing a
number which reflects the property, the quantity (how much?) of the quality (of what
sort?) is obtained. In order that the number does reflect the property in question, it is
essential that the influence of other qualities is kept to a minimum.An ideal measurement
would therefore be one for which the result is only affected by the size of the quantity
to be measured and by no other. Much effort in designing and using instruments goes
into meeting this condition of satisfactory measurement as closely as possible.
It is clear from the above that the subject of measurement and instrumentation is
a vast one. Restricting the subject to electrical engineering is in fact not much of a
restriction as very many non-electrical measurements are converted into electrical
measurements via suitable sensors, owing to the accuracy and convenience of electrical
measurement. It is therefore impossible to cover this subject completely, nor is it
necessary. By describing the major instruments- the Cathode Ray OsciUoscope(CRO),
the Digital Storage Oscilloscope (DSO),the frequency counter, the Digital Voltmeter
Measurement and instrumentation
62
(DVM), the Digital Multimeter (DMM) and some important sensors -the principles
and practice of measurement and instrumentation can be outlined.
4.2
The oscilloscope
The most commonly used instrument in electronic and electrical engineering is the
cathode ray oscilloscope or its digital counterpart, the digital storage osciuoscope. It
is difficult to envisage design, development, testing or maintenance without the
oscilloscope. The oscilloscope makes visible the waveforms in the circuits of interest
and is therefore the means whereby the engineer can bring his knowledge, experience
and creativity to bear on the situation.
4.2.1
The cathode ray oscilloscope
The CRO is an instrument which presents to the engineer a ‘graph’ of the voltage
signal at a point in a circuit against time, as shown in Fig. 4.1. This is a very helpful
form of presentation for the engineer since this is the way in which signals are
conceived. It is possible to measure both time and amplitude information with such
a display. The display can be used quantitatively to investigate, for example, waveform
distortion. How this is achieved can be seen by a consideration of the construction of
the CRO.
Voltage
Fig. 4.1 The CRO display
Figure 4.2 shows a simplified diagram of a Cathode Ray Tube (CRT). The CRT
is at the heart of the CRO. It consists of an indirectly heated cathode which is coated
with a material capable of giving a large number of electrons when heated. The
electrons are attracted away from the vicinity of the cathode by the anode which is
at a high positive voltage. By careful design the electrons pass along the tube and hit
the phosphor coating inside the CRT screen. Phosphor is the name of a family of
materials that possess fluorescence and phosphorescence.Fluorescence is the property
of a material to give off light when struck by the fast moving electrons. Phosphorescence
is the property of a material to keep giving off light for some time after having been
struck by the electrons. The time for which the light is given out is the persistence of
the phosphor. CROs are supplied with either general purpose or long persistence
phosphors. General purpose phosphors have persistences of the order of tens of
Newnes Electrical Engineer’s Handbook
-
63
- - - - - -F
Light
Fig. 4.2 Cathode ray tube construction
milliseconds and long persistence phosphors have persistences of the order of a few
seconds. A long persistence phosphor is advantageous for a very low frequency
waveform, perhaps less than 1 Hz, as the whole waveform can be seen by means of
the long period of phosphorescence. However, long persistence phosphors are easily
damaged by, for example, allowing the electron beam to be stationary for some time.
Most CROs therefore have general purpose phosphors.
The deflection of the electron beam in order to display the applied signal is
achieved very directly in the CRO by applying a scaled version of the signal to the
deflection plates. The relationship between the deflection and the applied voltage is
a linear one and so the interpretation of the resulting display is an easy matter.
In order for the display on the CRO screen to look like a graph of the signal it is
necessary to have uniformly increasing horizontal deflection as well as the vertical
signal deflection described. A linearly rising voltage applied to the X deflection plates
achieves this. To obtain an easily viewed display of a repetitive signal, it is usual for
internal circuitry to produce a repeated sweep from left to right. The waveform
which achieves this is called a saw-tooth waveform and is shown in Fig. 4.3.
/
Fig. 4.3 The saw-tooth waveform
Measurement and instrumentation
64
4.2.2
The steady display of a repetitive signal
To obtain a usable display, it is clearly necessary for the frequencies of the timebase
and the input frequency to be simply related, such as 1 : 1, 1 : 2, 1 : 3 etc. This
presents some difficulty since the frequency of the input waveform will probably be
unknown until viewed on the CRO. To overcome this, a variable timebasefrequency
control is provided on the CRO front panel. When this is adjusted to bring the
timebase frequency close to a simple relationship with that of the input frequency,
internal circuitry locks the timebase frequency to the exact relationship. The display
on the screen will therefore be that of one period of the input signal for a 1 : 1 ratio,
two periods of the input signal for a 2 : 1 ratio and so on. Thus a steady display of the
input signal is provided for signals over a very wide frequency range. Parameters of
interest such as period and amplitude can be measured conveniently. An example of
a 1 : 1 ratio is shown in Fig. 4.4. This is called the synchronized method of display.
It often happens, however, that the engineer would like to look at only a small
part of the period of a repetitive waveform. Figure 4.5 shows a sinusoidal type input,
but with a ripple on each maximum positive excursion. The synchronized method for
signal display shown earlier will not allow the detailed examination of the ripple area
of interest, but the trigger method will. In the internal trigger method, a variable dc
voltage is set by a control on the front panel. This trigger level voltage is compared
with the input signal and one sweep of the timebase is enabled when the input signal
exceeds the trigger level. The timebase runs for a time set by the front panel and is
in no way related to the value of the input signal. After the sweep, the spot is blanked,
flies back and waits for the next occasion when the input exceeds the trigger level.
At this time another sweep occurs and so the display is that of the repetition of the
same portion of the waveform again and again. By suitable adjustment of the trigger
level and the timebase sweep rate, a small detail can be investigated easily as shown
in Fig. 4.5.
4.2.3
-
The display of one signal against another Lissajous figures
A CRO can be configured to display one signal against another externally provided
signal rather than against the internally provided timebase signal. The ensuing displays
are called Lissajous figures after the nineteenth-century Frenchman who investigated
the effect of the combination of simple harmonic motions. The method can be very
useful for measuring phase shift in a circuit (the concept of phase angle in an ac
circuit has been explained in section 2.3.2). Figure 4.6 shows a linear circuit supplied
by a sinusoidal voltage source. The output must also be sinusoidal, but the amplitude
and phase will be different from that of the applied input. If the input signal is
displayed on the horizontal axis and the output signal is displayed on the vertical
axis, then a display such as Fig. 4.7 occurs. The display can be used to measure the
gain, which is the maximum to minimum vertical excursion divided by the maximum
to minimum horizontal excursion, yJxg, taking into account the scaling of the x and
y controls. More useful, however, is the phase measurement capability. If the horizontal
deflection signal is u, = V, sin wt and the vertical deflection signal is uy = Vv sin(m
+ 4) then the phase shift Q, can be found by the following:
at
t = 0,v, = 0
u,. = Vy sin(0 + Q,) = V,
65
Newnes Electrical Engineer's Handbook
Voltage
Time
Tirnebase
/
Voltage
Fig. 4.4 CRO display for 1 : 1 ratio of timebase and input frequency
therefore
Expressed in words the phase shift between two signals (such as the input and the
output in Fig. 4.6) can be measured by taking the ratio of the maximum y excursion
and the intercept on the x axis and then performing the inverse sine operation. In this
Measurement and instrumentation
66
Voltage
Trigger
level
voltage
Signal
vc
Time
Timebase
Fig. 4.5 Trigger method for display of part of a waveform
way it is easy to observe the phase shift in, for example, an amplifier and to explore
how the phase shift changes with frequency.
4.2.4
The display of two signals against time
Very often it is helpful to be able to observe the waveform and make measurements
at two points in a circuit simultaneously.This allows the assessment of the performance
of the circuit, for example the distortion introduced by an audio amplifier. In order
67
Newnes Electrical Engineer’s Handbook
J
Sinusoidal
input
Linear
circuit
the same
frequency as
the input but
with a changed
amplitude
and phase
Fig. 4.7 A Lissajous figure
to do this without significantly increasing the cost of the CRO, then one electron
beam must write both signals. Provided this is carefully done, the eye does not see
this, but sees two traces simultaneously.
There are two ways of achieving this and most CROs provide both, often
automatically switching between the two to suit the frequency of the signal. The two
ways are called the alternate method and the chop method. The effect of the alternate
method is shown in Fig. 4.8. In the alternate method, a sweep of the timebase is
traced with alternately one input then the other connected to the Y deflection plates.
The two inputs y1 and y2 have their own gain controls and Y deflection controls. This
method works well provided the input signal frequency is not too low. For low
frequencies, it becomes evident to the eye that the display consists of one trace
followed by another.
For low frequencies the chop method is appropriate, and the effect of this is
shown in Fig. 4.9. An internal oscillator running at about 500 lcHz is used to switch
the two inputs in small sections to the deflection plates. For low-frequency signals,
there are very many sections for a sweep, for example 5000 for a 100 Hz signal, and
the display appears continuous. At high frequencies, the sectional nature becomes
very evident to the eye and the alternate method is used.
Measurement and instrumentation
68
Alternate mode: alternate sweeps of
channel 1 and channel 2 input signals
t
Fig. 4.8 The alternate mode
t
4
I
4
4
I
Chop mode:
the display is
of small sections
of each signal
in turn
t
Fig. 4.9 Display of two ?: inputs in the chop mode
4.2.5
The addc switch
There are applications where it is desirable to be able to measure the characteristics
of a small varying quantity superimposed on a large de value. A typical case would
be the assessment of residual ripple on a dc power supply. The CRO has an ac/dc
switch on the front panel and in the dc position, the displayed waveform would look
like Fig. 4.10(a). If the sensitivity of the CRO is increased by changing the gain
switch, the trace would disappear from the screen. In order to be able to view an
Newnes Electrical Engineer's Handbook
69
enlarged version of the ac component, the switch should be set in the ac position.
This introduces a logical capacitor in series with the signal, blocking the dc component.
The display then looks like Fig. 4.10(b). It is possible in this position to increase the
sensitivity as desired to view the ac quantity. If the displayed signal is very low in
frequency, say a few Hz,much of the signal will also be effectively blocked by the
capacitor in the ac position and a much reduced amplitude of display will result. For
this reason the ac/dc switch should be kept in the dc position for general use.
VY $
dc mode
t
(a)
ac mode
Fig. 4.10 The effect of the ac/& switch
4.2.6
The loading effect of the CRO
In common with all instruments, the CRO affects the condition of the circuit to
which it is connected. This always happens and it is important to discover whether
this effect is significant or not. It is called the bading eflect of the instrument. A
manufacturer will state, usually by the Y input socket, the effective resistance and
capacitance of the CRO. The effect of the CRO on the circuit is as if it were a parallel
resistor and capacitor, shown in Fig. 4.11, with typical values for resistance and
capacitance shown. As an example a simple potential divider circuit is shown in Fig.
4.11. Before connecting the CRO, the voltage across the lower 1 Mi2 resistor is 6 V
pk-pk by symmetry. When the CRO is connected in the case of low frequencies for
which the capacitive effect of the CRO can be neglected, the 1 MQ of the CRO
reduces the lower resistance to a combined resistance of 1/2 MR. This means that the
voltage across the lower resistance will fall to 4 V pk-pk and this is what a perfectly
accurate CRO will read. Of course the voltage will return to 6 V pk-pk when the
CRO is removed. The effect of the CRO on the circuit is clearly very significant and
the effect becomes worse at high frequencies, where the effect of the capacitance of
the CRO is to lower further the impedance of the instrument.
If the characteristics of the circuit and the CRO are known, as in the situation
shown in Fig. 4.11, then it is possible to calculate the effect of the loading and correct
for it. Usually, however, the CRO is the means of investigating the circuit and no
such previous knowledge will be available. The loading effect will only be serious
where the impedance of the circuit approaches the impedance of the CRO and it can
70
Measurement and instrumentation
Effective resistance
and reactance of
the CRO
Circuit to be
measured
12 v
Crocodile
clip
Fig. 4.11 CRO loading effect
usually be neglected if the circuit impedance is not greater than one-tenth of the CRO
impedance. There are many cases where this condition is not met or it is not known
if it is met. The solution is to use a suitable probe with the CRO.
4.2.7
CRO probes
In the body of a typical CRO probe is mounted a switch labelled x l and x10. In the
x l position the signal is passed directly to the CRO. In the x10 position a 9 MQ
resistor and variable parallel capacitor are switched in series with the signal lead, as
shown in Fig. 4.12. At low frequencies, where the effect of the two capacitances can
be neglected, the effect of the x10 switch is to increase the effective CRO resistance
to 10 MR thus making the loading effect much less significant. At high frequency,
the effect of the probe capacitor reduces the effective capacitance of the instrument.
Signal
Fig. 4.12 The use of the x10 probe
Newnes Electrical Engineer's Handbook
71
Cable capacitance makes the effect less predictable than for resistance, but it will
also significantly reduce the loading effect at high frequencies.
The effect of the x10 probe will also be to drop nine-tenths of the signal voltage
across the probe and so only one-tenth will be presented to the CRO. The amplitude
readings of the CRO must be multiplied by 10 to give the signal amplitude, hence the
name of the probe.
Other more specialist probes are available to further reduce the loading effect of
the CRO, but it is found that for many cases the x10 probe gives a sufficient reduction.
4.2.8
CRO bandwidth and the measurement of rise time
Probably the most important parameter of the CRO is the bandwidth. The bandwidth
gives the maximum frequency for which a faithful display of amplitude takes palce.
If a sinusoidal signal above the bandwidth frequency is applied to the CRO, then the
displayed amplitude will be smaller than would be the case for the same size input
at a lower frequency. This is illustrated in Fig. 4.13, and it is clear that the instrument
calibration can only be used for sinusoids below the bandwidth frequency. This is not
the most significant effect of the bandwidth of the CRO. It has a direct bearing on the
fastest rising edge of a rapidly changing voltage which can faithfully be displayed on
the CRO. The display and measurement of such edges are very important in assessing
the digital signals that occur in, for example, computers. A CRO cannot respond
instantaneously to a very sudden change in voltage. If a very sudden change is
applied, the response of the CRO will look like Fig. 4.14. The response is usually
described by the rise time, the time taken for the response to change from 10 per cent
to 90 per cent of the final value. It is found that there is a simple relation between the
rise time of the CRO and the CRO bandwidth, which is given by
Bandwidth x Rise time = 0.35
~
~~
~~
~
Fig. 4.13 Effect of a CRO bandwidth of 20 MHz
For example, a 20 MHz CRO will have a rise time of 0.35420 x lo6)seconds, or
17.5 ns. This means that if the instrument is presented with a voltage waveform with
rise times faster than this value, the rise time that will be displayed is the CRO rise
time. In order for the effect of the properties of the CRO not to significantly intrude
on the display, the fastest rise time that can be displayed should be not less than eight
times the CRO rise time. From this relation, the required CRO bandwidth can be
calculated in order to display a given signal rise time. For example, if the signal rise
time is 80 ns, then the CRO rise time must be 10 ns in order not to change the display
significantly. A 10 ns rise time corresponds to a CRO bandwidth of 0.35/10-' Hz,or
35 MHz. This procedure allows the necessary bandwidth to be calculated for a given
Measurement and instrumentation
72
Fig. 4.14 Response of a CRO to a very fast change in voltage
rise time and checked against the instrument in use, and it can establish whether a
new instrument is needed.
Since fast rising voltages are commonly found in digital circuits, the need for high
bandwidth CROs is clear.
4.2.9
The high bandwidth CRO
A high bandwidth CRO is significantly more expensive than a general purpose CRO.
There are several reasons for this. In the first place, as the spot moves over the
surface very rapidly at high frequencies, the emitted light reduces. To compensate for
this the electron beam velocity is increased, but this tends to reduce the sensitivity of
the instrument and so extra care is required with the electronics. As the electron beam
passes through the deflection plates the forces on the electrons change significantly
at high frequencies. This again reduces the sensitivity. This is overcome by splitting
the Y &flection plates into a number of small sections arid introducing time delays
between each. This ensures that the signal on each plate remains closely the same as
the electron beam passes down the tube. It makes the CRT much more expensive,
however.
4.2.1 0 The digital storage oscilloscope
The digital storage oscilloscope solves some of the problems of the CRO at high
frequencies by means of storing the signal and then displaying at an effective lower
frequency. This allows a cheaper CRT to be used. The block diagram of a DSO Y
deflection system is shown in Fig. 4.15. The Analogue to Digital ( A D )converter
gives a digital version of the voltage input suitable for storing in memory. For the
DSO, the CRT ceases to be the critical component at high frequencies. The critical
component for the DSO is the AID converter. This element samples the input waveform
at regular intervals and converts to a digital equivalent. If the sampling rate is not
high enough or the levels of quantization not fine enough, then this will reflect on the
accuracy of the quantities measured. An illustration of what happens on a leading
voltage edge with a DSO is shown in Fig. 4.16. The samples are shown by crosses.
This means that the analogue voltage to be measured is converted into numbers at the
73
Newnes Electrical Engineer's Handbook
-
Analogue
to digital
-
Memory
-
Digital to
analogue
-
Amplifier
-
Low-frequency
CRT
Fig. 4.15 Y-deflection system in
a digital storage oscilloscope
times shown. The fastest rising edge which can be displayed is when there is one
sample on the rising edge. The effective rise time of the DSO is therefore given by
1.6 sample intervals, this is the time from 10 per cent to 90 per cent at the final value.
This represents the DSO rise time and a signal should have a slower rise time than
this, say six times slower. The fastest acceptable signal rise time is therefore 6 x 1.6
sampling intervals, or lO/sampling rate.
Fig. 4.16 DSO rise time
The sampling rate of the DSO is therefore a key parameter of the instrument and
is quoted as, for example, 500 M samples per second. Such an instrument is capable
of faithfully displaying a signal rise time of 20 ns, and this roughly corresponds to a
140 MHz bandwidth CRO.
The other key parameter of the A/D converter is the fineness of quantization, or
the number of bits of the A/D.If the number of bits is not sufficiently high, then
significant information is lost in the An> process. It is for this reason that a signal
displayed on a cheap DSO often appears to have a better signal to noise ratio than for
a CRO. The DSO is merely not responding to the small voltage differences of the
noise component.
If a good A D converter is used, then the DSO will give a faithful display of an
applied signal over a wide range of amplitudes sand frequencies. The DSO has other
advantages than cheaper CRT construction. The information in digital form can
easily be manipulated to find, for example, average values and rms values. The
information can be easily directly communicated to a computer and a hard copy of
the display is fairly easily made available.
74
4.3
Measurement and instrumentation
Digital frequency counters, voltmeters and
multimeters
Digital multimeters, voltmeters and frequency counters are the next most common
instruments used by the electronic and electrical engineer after the CRO and DSO.
Since they develop one from the other, the frequency counter will be described first,
then the digital voltmeter (DVM) and then the digital multimeter (DMM).
4.3.1
The frequency counter
The ability to measure accurately the frequency of a signal is a common requirement,
for example when designing a filter circuit. This can be achieved by counting the
number of periods of the signal that occur in one second, which is the frequency of
the signal. The basis of the instrument shown in Fig. 4.17.
-
Shaping
circuit
Signal
Crystal
oscillator
1/10
1 MHz
Gate
-
- 1/10 - 1/10
Decimal counter
and display
1/10
1k z
- 1/10 1 Hz
Fig. 4.17 A frequency counter
The signal is converted into voltage pulses at the same frequency by the shaping
circuits, the output from which passes through a gate, is converted into a decimal
number, and then shown on a suitable display such as liquid crystal or light emimng
diode. The gate is opened for 1 second by arranging that the signal which opens the
gate and the signal which closes the gate are derived from a quartz crystal oscillator.
This arrangement works very well for high-frequency signals, since the number of
pulses counted is very high, but not so well for a low-frequency signal, for example
one with a frequency of 10 Hz.
For such a signal, the +1 error, which occurs for all digital instruments, is a major
source of uncertainty in the measurement results. When the number of pulses from
the shaping circuit is measured, the gating time is not synchronized with the signal.
It is therefore possible to just miss pulses or just include pulses at either end of the
gating period, as shown in Fig. 4.18. The measured result is therefore 9 Hz and
11 Hz respectively, whereas 10 Hz would be a more representative value. Because of
the ever present +1 error in digital instruments, it is important to ensure that the
number of pulses counted is very high so that the effect of the f error becomes very
small.
Newnes Electrical Engineer's Handbook
r
I"
75
I
I
I
I
I
I
I
I
I
I
I
I
I
I
-b
,t
Gate open4I
I
Fig. 4.18 The f l error
4.3.1.1
The measurement of low frequencies
For a low-frequency signal, the above condition of ensuring many counted pulses is
difficult to achieve. It would be possible to wait for 10 seconds instead of 1 second
for the result, which would help a little in reducing the effect of the f l error, but this
would still not be acceptable, neither would waiting for 100 seconds. The preferred
solution is to swap the signal and crystal oscillator output around as shown in
Fig. 4.19. The signal now opens the gate for exactly one period. The decimal counter
will accumulate the number of 1 ps pulses from the crystal oscillator in one period
of the signal and the instrument therefore measures the period of the signal. It is clear
that the decimal counter will collect a large number of pulses for a low-frequency
signal and the f l error will have relatively little effect on the period measurement.
Gate
-
Decimal counter
and display
of signal
Crystal
-
Fig. 4.19 The period counter
Measurement and instrumentation
76
Therefore for a high-frequency signal, frequency measurement should be used
and for a low-frequency signal, period measurement should be used. In the case
illustrated, it makes no difference for a 1 lcHz signal which method is used.
4.3,1.2 The accuracy of the frequency counter
The potential accuracy of the frequency counter is very high since essentially counting
is involved. The key component for the accuracy of the instrument is the crystal
oscillator.
The frequency of a crystal depends on many things but usually the temperature,
the supply voltage and its age are the three dominant factors. By selecting a crystal
cut at a very carefully specified angle to the crystallographic axis, called the AT cut,
a quartz crystal with a very low temperature coefficient can be produced. For instruments
for which very high accuracy is specified, this crystal is also enclosed in a temperature
controlled box. This is more expensive, but gives a very stable frequency with
changes in ambient temperature.
The dc supply to the oscillator needs to be carefully controlled in order to avoid
frequency changes owing to this cause. A typical figure would be a change of 1 part
in lo9 for a 210 per cent change in power supply voltage.
Quartz crystals change their frequency with time after manufacture. In common
with many products, the change is most rapid initially and then settles down to a
smaller rate of change. A typical characteristic is shown in Fig. 4.20. From this it can
be seen that the initial rate of change might well be significant and recalibration is
required more frequently when the instrument is new. This is described in more
detail in section 4.5.
Fractional change
in frequency of
a quartz crystal
Fig. 4.20 Change in frequency of a quartz crystal with time
With reasonable care with these matters it is possible to produce a very accurate
frequency counter for a modest cost.
4.3.2
The digital voltmeter
As frequency and time interval can be measured accurately easily and cheaply, it is
Newnes Electrical Engineer's Handbook
77
attractive to make a digital voltmeter (DVM) which converts the dc voltage to be
measured to a corresponding time interval or frequency. There are several methods
for achieving this and, by way of illustration, the dual slope DVM will be described.
The block diagram for the dual slope DVM and the voltage waveforms are shown
in Figs 4.21 and 4.22 respectively. At the beginning of the cycle, the unknown
voltage is connected to the integrator. The output of an integrator for a dc input is a
ramp voltage the slope of which is proportional to the unknown voltage. After one
period of the mains supply (20 ms for 50 Hz) the integrator input is switched to an
internal stable reference voltage of opposite polarity from that of the voltage to be
measured. The output of the integrator then ramps down with a constant slope in all
cases. At the same moment as the integrator input is switched, the gate in a timing
set-up is opened. This allows pulses from the crystal oscillator to the decimal counter.
This continues until the instant at which the output of the integrator returns to the
common voltage, when the gate is closed. The number of pulses counted is directly
proportional to the unknown voltage and the display is suitably scaled in volts.
comp.
Unknown\,
voltage
Open
.\
\
display
Clock
Fig. 4.21 Dual slope DVM
This method for voltage measurement has several attractions. First, it can be
manufactured cheaply. Also mains frequency interference, which must always be
present, is eliminated by the integration over one period of the mains. The fact that
two slopes are involved means that parameters of the integrator, which change with
ambient conditions such as temperature, affect both slopes equally and cancel.
Reasonable accuracy and noise immunity can therefore be achieved by this method.
The accuracy of the digital voltmeter is that of the frequency counter plus the
contributions of the extra components, particularly the voltage source and range
defining resistors.
4.3.3
The digital multimeter
The digital multimeter (DMM) is an extension to the digital voltmeter. Just as the
digital voltmeter turns the voltage to be measured into a time interval, so the digital
multimeter turns the quantity to be measured into a voltage.
Since the digital voltmeter contains a frequency counter, it is possible to provide
frequency measurement capabilities directly with modest performance. DC current
measurement can be obtained by passing the current to be measured through a
resistor and then measuring the voltage developed by the digital voltmeter. The
digital multimeter contains a chain of stable resistors which can be selected for
78
Measurement and instrumentation
output of
integrator
Slope proportional
to internal source
- 20 ms -Gate -Gate
Open
I
closed
Gate open for a time proportional
to the size of the unknown dc voltage
Fig. 4.22 Timing diagram for the dual slope DVM
various current ranges. The accuracy on the current range will clearly not be as high
as for dc voltage. AC voltage can be measured by converting to an equivalent dc
voltage. The cheaper schemes rely on rectification and averaging and the calibrations
only hold for a sinusoidal signal. More generally used is the true rms converter which
gives the nns voltage of the measured quantity regardless of its waveform. The
instrument with true rms capability tends to be more expensive.
Resistance measurement can be added in the digital multimeter. The most elegant
way of doing this for a dual slope measurement is to pass the same current through
the resistor to be measured and a stable reference resistor. The dual slope method can
be involved and the voltage across the unknown resistor applied to the integrator of
Fig. 4.21 for the first of the slopes and the voltage across the reference resistor
applied to the integrator for the second of the slopes. The time for which the gate is
open will be proportional to the unknown resistor and the display can be scaled in
ohms. This method is insensitive to changes in the value of current flowing through
the resistors, but the stability of the reference resistor is clearly important for the
accuracy of the DMM in this mode.
4.4
Sensors
A sensor is a device which converts the quantity to be measured into another quantity
which is easier to present as a number, to manipulate or to display. The human body
of course employs sensors, the eye converting optical information, the ear sound
information and so on into internal information suitable for further processing. The
range of industrial sensors is vast since almost any physical effect relating two
quantities can potentially be used as the basis of a sensor. Most conveniently, the
Newnes Electrical Engineer's Handbook
79
output quantity is an electrical one since this makes available the array of tools of
signal conditioning and computation.
Sensors can be categorized in a number of ways. Sensors are described as active
when energy conversion takes place, such as the piezoelectric sensor converting
pressure information into electrical information. Sensors are described as passive
when external power is required, such as the rotary potentiometer converting angle
into slider voltage when an electrical current flows.
Sensors can be categorized according to the quantity which is changed by the
input quantity, such as resistive sensors for measuring temperature of pressure or
light intensity or strain or magnetic field.
As always in measurement and instrumentation, the acceptable sensor must give
information about the quantity to be measured and be as little sensitive as possible to
all other influence quantities. This is by no means easy, since the list of resistive
sensors above indicates that, for example, a strain gauge will also change its resistance
with temperature, pressure, light intensity, magnetic field and so on. Careful design
and intelligence in use is necessary for reliable measurement. The sensor must also
be reliable, cheap, not unduly disturb the system to be measured and preferably not
involve a major education programme on the part of the potential users.
To illustrate from among the vast list of sensors available, three common and
typical examples will be described - the strain gauge, the piezoelectric accelerometer
and the Linear Variable Differential Transformer (LVDT).
4.4.1
The strain gauge
A strain gauge is a sensor which converts change in length into a change in resistance
and hence voltage. By carefully attaching the strain gauge to the surface of an object,
dimensional changes in the object can be monitored and measured. A typical simple
strain gauge is shown in Fig. 4.23. It consists of a metal foil formed into a grid
structure by a photo-etching process situated on a resin film. The gauge is designed
to have much more conductor along the axis to be measured than at right angles. As
the gauge is strained, the resistance changes. The resistance is given by R = pWA
where p is the resistivity of the material, L is the length and A the cross-sectional
area. All three of these change with strain. For commonly used material the gauge
factor, the fractional change in resistance caused by a fractional change in length, is
close to 2. Alloys are selected with a low temperature coefficient of resistance and a
low temperature coefficient of linear expansion.
Fig. 4.23 The strain gauge
If the strain gauge is connected in the Wheatstone bridge circuit shown in Fig.
4.24, then the output will change as the strain changes. Influence quantities such as
temperature will also produce changes in the output. A useful method for significantly
reducing these effects is to mount a second gauge in a suitable position. For the
measurement of the strain of a cantilever, shown in Fig. 4.25, then the two gauges
will experience dimensional changes which are equal but of opposite sign (one
Measurement and instrumentation
80
Fig. 4.24 Strain gauge in a Wheatstone bridge circuit
Strain gauge 2 (compressed)
Fig. 4.25 Strain gauges mounted on a cantilever
stretched and the other compressed) but equal changes in resistance owing to influence
quantities such as temperature. By connecting the gauges to adjacent arms of the
Wheatstone bridge circuit, then the unwanted effects tend to cancel, whilst the effect
of the change in resistance with strain is doubled.
4.4.2
The piezoelectric accelerometer
Changes in the vibrations of machinery give information about likely future failure
of the equipment, and measurement of vibration using an accelerometer can therefore
be used to inform maintenance actions in a cost effective manner. A common way of
achieving this is by a piezoelectric accelerometer, shown in Fig. 4.26. As its name
implies, a piezoelectric element converts pressure into an electrical quantity and vice
versa. Usually in accelerometers a manufactured ceramic piezoelectric material such
as PZT (lead zirconate titanate) is used. Acceleration causes the piezoelectric element
to be compressed, thus giving an electrical output which can be used to monitor the
acceleration. In a commercial device, care is taken to avoid sensitivity to acceleration
in other axes. This is an active device and the output signal in electrical form is
generated from the input signal in mechanical form. These types of device are also
known as transducers. The piezoelectric accelerometer is usually operated well below
its resonance frequency where the output varies little with frequency. There is no dc
output from these devices, but when used with a charge amplifier good low-frequency
performance can be achieved.
81
Newnes Electrical Engineer's Handbook
A
Acceleration
Seismic mass
Piezoelectric
material
Fig. 4.26 The piezoelectric accelerometer
4.4.3
The linear variable differential transformer
The LVDT has found widespread acceptance as an industrial displacement sensor.
The construction of the LVDT is shown in Fig. 4.27. The sensor is a transformer
that has a primary and two identical secondary windings which are connected in
series opposition. The effect of this is that the output voltage is the difference between
the two second voltages. A ferromagnetic rod made from a nickel-iron alloy moves
inside the coils and as it moves the output voltage in one coil goes up and the other
down. About the centre position, the phase of the signal reverses. LVDTs are made
commercially with movement ranges from flOO pm to f25 cm.
The LVDT has some attractivefeatures for industrialuse. Since there is no mechanical
connection between the core and the rest of the instrument friction is low, tolerance
I
'1
Fig. 4.27 The LVDT
e
82
Measurement and instrumentation
to mechanical overload and shock is high and reliability is high. The windings can be
set in epoxy resin and the sensor housed in a metal shield thus enabling the LVDT to
be used in hostile environments. There is electrical isolation between the primary
and secondary windings thus allowing flexibility in the choice of the common
connection.
Because of these attractive features the LVDT is commonly the preferred method
for industrial displacement measurement.
4.5
The international measurement system,
traceability and calibration
Measurement is a comparison process and all useful measurements are ultimately
related to the International Measurement System, the SI system of units. The base
units of the SI system are the metre, kilogram, second, ampere, kelvin, candela and
mole. All other measurement units are derived from these, but they are often given
special names for ease in use. For example, the volt is the kg m2s”A-’ in terns of the
base units, but fortunately the name volt is used instead.
The international community establishes the base units for comparison purposes.
This provision is not well known by most of the people who benefit from it, but it is
very important that the provision is there.
The process of relating the reading used say in manufacture to assess the performance
of a product to the internationalmeasurementstandards is called traceability.Traceability
is the unbroken chain of comparisons performed in an acceptable way linking the
measurement with the national or internationalmeasurement standards. The comparison
with a more accurate instrument is termed calibmtion. The user gains traceability by
having his instrumentcalibratedby an accredited calibrationlaboratory.This calibration
laboratory is responsible for the other comparisons back to national or international
measurement standards. In the UK, calibration laboratories demonstrating traceability
are accredited by UKAS (United Kingdom Accreditation Service). The various
accredited calibration laboratories offer their services on a competitive basis related
to price, speed of service and a collect and delivery system.
Clearly a calibration is only valid at the instant of calibration and even then the
measurement will be subject to the uncertainty associated with the calibration laboratory
for that parameter and range. Following calibration, the stability of the instrument’s
readings is important. The reading of an instrument for the same quantity will change
with time or drift and will also be affected by a number of influence quantities. An
influence quantity is a quantity which is not the subject of the measurement but
nevertheless changes the result of the measurement. Common influence quantities
are temperature, humidity, power supply voltage and a range of radiations such as
electromagnetic interference. It is usually possible to correct for these influence
quantities. In some cases the manufacturer will give, for example, the temperature
coefficient of the instrument. By means of measuring the difference between the
temperature of the room in use from the temperature at which calibration was performed,
often 23°C the correction for temperature can be applied. These corrections are
never completely perfect and it is necessary to estimate the residual uncertainty in
the measurement from all such causes.
The change in the instrument reading for the same applied quantity with time
necessitates regular calibration. As noted when discussing frequency counters, the
change in an instrument reading is greatest when new. This means that recalibration
Newnes Electrical Engineer’s Handbook
83
is necessary more frequently initially. Depending on use, it might be necessary to
recalibrate after 90 days and then once a year.
Many manufacturing and other organizations are obtaining registration to the
international quality system standard I S 0 9000. In order to obtain such registration,
it is necessary to demonstrate the traceability of measurements affecting product
quality, that the uncertainty of measurements is suitable and that a systematicprocedure
is in place for the regular calibration of instruments used.
References
4.4. Gillies, R.B. Instrumentation and Measurements for Electronic Technicians, Merrill, 1988.
4B. Cooper, W.D. and Helfrick, A.D. Electronic Instrumentation and Measurement Techniques,
Prentice Hall, 1988.
4C. Bentley, J.P. Principles of Measrrrvment Sysrems, Longman, 1995.
4D. Pallas-Areny, R. and Webster, J.F. Sensors and Signal Conditioning, Wiley, 1991.
4E. Cunningham M.J. Measurement Errors and Instrument Inaccuracies, J. Phys. E.. 1981, Vol. 14,
pp. 901-8.
Chapter 5
Generators
Dr G.W. McLean
Generac Corporation
5.1
Introduction
Throughout the world, whether in underdeveloped areas or in highly developed
countries, there is a need for generators in many different applications.
In addition to the underlying need for a public supply of electricity, there are a
number of situations in which independent supplies are needed. The applications for
generators are categorized broadly:
0
0
0
5.2
public supply networks in which a number of high-power generator sets may
operate in parallel
private or independent generators which may run in parallel with the public
supply or isolated from it. Examples of this include:
- peak shaving to reduce the maximum demand of electricity by a user; this
can avoid large financial penalties during times of generally high demand
on the system
- stadby emergency generutors to protect the supply to critical circuits
such as hospitals or water supplies
- temporag supplies which are needed by the construction industry, or in
cases of breakdown
- combined heat andpower where the waste heat from the generator engine
is used for other purposes such as building heating
portable supplies, often trailer-mounted, where no alternative supply is available
Main generator types
The two main types of generator are 'turbo' or cylindrical-rotor and salient-pole
generators. Both these types are synchronous machines in which the rotor turns in
exact synchronism with the rotating magnetic field in the stator. Since most generators
are in this class, it forms the basis of most of the chapter.
The largest generators used in major power stations are usually turbo-generators.
They operate at high speeds and are usually directly coupled to a steam or gas turbine.
The general construction of a turbo-generator is shown in Fig. 5.1. The rotor is made
from solid steel for strength, and embedded in slots within the rotor are the field or
excitation windings. The outer stator also contains windings which are located in
slots, this is again for mechanical strength and so that the teeth between the slots form
a7
Newnes Electrical Engineer’s Handbook
Exciter
-,. Base frame
(c)
Fig. 5.1 Turbo-generator construction: (a) stator, (b) rotor, (c) assembly (courtesy of ABB ALSTOM
Power)
a good magnetic path. Most of the constructional features are very specialized, such
as hydrogen cooling instead of air, and direct water cooling inside the stator windings,
so only passing reference is made to this class of machine in the following descriptions.
More commonly used in smaller and medium power ranges is the salient-pole
generator. An example is shown in Fig. 5.2. Here, the rotor windings are wound
around the poles which project from the centre of the rotor. The stator construction
is similar in form to the turbo-generator stator shown in Fig. 5.1.
Less commonly used are induction generators and inductor generators.
Induction generators have a simple form of rotor construction shown in Fig. 5.3,
in which aluminium bars are cast into a stack of laminations. These aluminium bars
require no insulation and the rotor is therefore much cheaper to manufacture and
much more reliable than the generators shown in Figs 5.1 and 5.2. The machine has
characteristics which suit wind turbines very well, and they also provide a low-cost
alternative for small portable generators. The basic action of the induction generator
will be described later in this chapter, but both the construction and the operation of
the machine are very similar to the induction motor described in Chapter 10.
Inductor generators use solid steel rotors cut with slots to produce a flux pulsation
in the stator as the rotor turns. These machines are usually used at high speed for
specialized applications requiring high frequency.
5.3
5.3.1
Principles of operation
No-load operation
The basic operation of all these generator types can be explained using two simple
88
Generators
Fig. 5.2 Salient-polegenerator construction:Top: stator, bottom: rotor (courtesy of Generac Corporation)
Newnes Electrical Engineer’s Handbook
89
Fig. 5.3 Induction generator construction (courtesy of Invensys Brook Crompton)
rules, the first for magnetic circuits and the second for the voltage induced in a
conductor when subjected to a varying magnetic field.
The means of producing a magnetic field using a current in an electric circuit have
been explained in section 2.2.3, and eqs 2.13 and 2.18 have shown that the flux @
in a magnetic circuit which has a reluctance R, is the result of a magneto-motive
force (mmf) F,, which itself is the result of a current Z flowing in a coil of N turns.
@ = F,,,IR,
and
F,,, = IN
The main magnetic and electrical parts of a salient-pole generator are shown
diagrammatically in Fig. 5.4. In Fig. 5.4(a), dc current is supplied to the rotor coils
through brushes and sliprings. The product of the current Z and the coil turns N
results in mmf F, as in eqn 5.2, and this acts on the reluctance of the magnetic circuit
to produce a magnetic flux, the path of which is shown by the broken lines in Fig.
5.4(b). As the rotor turns, the flux pattern created by the mmf F, turns with it; this
is illustrated by the second plot of magnetic flux in Fig. 5.4(b). In section 2.2.3 it has
also been explained that when a magnetic flux @ passes through a magnetic circuit
with a cross-section A, the resulting flux density B is given by
B = @IA
(5.3)
Figure 5.4(a) also shows a stator with a single coil with an axial length 1. As the rotor
turns its magnetic flux crosses this stator coil with a velocity u. It has been explained
in section 2.2.4 that an electromotive force (emf) V will be generated, where
V = Bul
(5.4)
90
Generators
Lines of
magnetic flux
or and field at 0
'
I
Induced v
v
I
wIIuu,,Lor
I
I velocity
I
I
I
Bottom conductor
I
I
Induced voltage V
I
Velocity u
I
(a) Main parts, and direction
I- - _ - - -of- induced
_ - _ _voltages
-_B
Tan
.
A
.
.
.
.
.
.
.
"4
let
tor and field at 90"
(b) Rotation of magnetic field with the rotor
Fig. 5.4 Principles of generator operation
The direction of the voltage is given by Fleming's right-hand rule, as shown in
Fig. 2.6.
Figure 5.4(b) shows that as the magnetic field rotates, the flux density at the
stator coil changes. When the pole face is next to the coil, the air gap flux density B
is at its highest, and B falls to zero when the pole is 90" away from the coil. The
induced emf or voltage V therefore varies with time (Fig. 5.5) in the same pattern as
the flux density varies around the rotor periphery. The waveform is repeated for each
revolution of the rotor; if the rotor speed is 3000 rpm (or 50 reds) then the voltage
will pass through 50 cycleds (or 50 Hz).This is the way in which the frequency of
the electricity supply from the generator is established. The case shown in Fig. 5.4
is a 2-pole rotor, but if a 4-pole rotor were run at 1500 rpm, although the speed is
lower, the number of voltage alternations within a revolution is doubled, and a
frequency of 50 Hz would also result. The general rule relating the synchronous
speed n, (rpm), number of poles p and the generated frequencyf (Hz) is given by
f = a,p/120
(5.5)
The simple voltage output shown in Fig. 5.5 can be delivered to the point of use (the
'load') with a pair of wires and this form of supply is known as single phase. If more
Newnes Electrical Engineer’s Handbook
91
Fig. 5.5 Induced voltage waveform
coils are added to the stator shown in Fig. 5.4(a), and if these are equally spaced,
then a three-phase output as shown in Fig. 5.6 can be generated. The three phases are
‘V’ and ‘W’. The positive voltage peaks occur equally
conventionally labelled ‘U’,
spaced, one-third of a cycle apart from each other. The nature of single-phase and
three-phase circuits has been explained in Chapter 2. The three coils either supply
three separate loads, as shown in Fig. 5.7(a) for three electric heating elements, or
more usually they are arranged in either ‘star’ or ‘delta’in a conventional three-phase
circuit (Fig. 5.7(b)).
(4
Fig. 5.6 Three-phase generation
In a practical generator .--e stator windings are embedded in slots, the inuced
voltage remaining the same as if the winding is in the gap as shown in Fig. 5.4(b).
Also, in a practical machine there will be more than the six slots shown in Fig.
5.6(a). This is arranged by splitting the simple coils shown into several subcoils
which occupy separate slots, each phase still being connected together to form a
continuous winding. Figures 5.1 and 5.2 show the complexity that results in a
complete stator winding.
92
Generators
(a) Separate connection
I
I
U
I
I
I
I
I
I
I
I
I
I
I
I
Line
I
I
I
Star
I
(b) Star and delta connections
Fig. 5.7 Three-phase connections
5.3.2
The effect of load
In the circuits shown in Fig. 5.7 currents flow in each phase, and these currents will
have a waveform similar to the voltage waveform shown in Fig. 5.6(b). The concept
of a phase shift between voltage and current in ac circuits which contain inductance
or capacitance has been explained in section 2.3.2. If an inductive or capacitive load
is connected, then the current waveforms will respectively ‘lag’ or ‘lead’ the voltage
waveforms by 90”. For the inductive load case shown in Fig. 5.6(a), the current in
the U phase will be zero, but current will be flowing in V and W phases. It can be
seen that the lines of magnetic flux now enclose not only the rotor excitation current,
but also the stator currents in the V and W phases. Equations 5.1 and 5.2 show that
the flux is the result of the m f acting on a magnetic circuit, but it can now be seen
that the mmf is a combination of the ampere-turns from the rotor and the stator
winding. If I,., I,, N,, and N, are the currents and turns in the stator and rotor windings
respectively then eqns 5.1 and 5.2 combine to give
@ = (IrNr + IsNs)IRm
(5.6)
In Fig. 5.6 a cross is used to indicate a current flow into the page, and a dot shows
current flowing out of the page. It is seen that the stator currents oppose the excitation
current in the rotor and their effect is to reduce the flux, with a corresponding
reduction in the generated voltage. This demagnetizing effect is called ‘armature
reaction’; it is the way in which Lenz’s law (section 2.2.4) operates in a generator.
The underlined currents in eqn 5.6 indicate that these are vectors and a vector
addition is necessary. The armature reaction effect therefore depends on the extent to
Newnes Electrical Engineer’s Handbook
93
which the stator currents lag or lead the voltages (often called the ‘phase’ or ‘phase
angle’). If, for example, the generator load is capacitive, the currents will lead the
voltages by 90”.and they will be opposite in direction to that shown in Fig. 5.6 for
an inductive load. The ampere-turns of stator and rotor windings will add in this case
and the flux and the generated voltage will be higher. In the case of a resistive load,
the ampere-turns of the stator will act at 90” to the rotor poles, tending to concentrate
the flux towards the trailing edge of the pole and producing magnetic saturation here
when large stator currents flow; this reduces the flux and the output voltage, but not
so much as in the inductive load case.
The output voltage is influenced not only by armature reaction, but also by voltage
drop within the stator winding. This voltage drop is partly due to the internalresistance
of the winding, and partly due to flux which links the stator winding but not the rotor
winding; this flux is known as ‘Zenkugeflid and it appears in the stator electrical
circuit as a leakage inductance, which also creates a voltage drop. The phase angle
between stator currents and voltages will affect this voltage drop, producing a greater
drop at lagging currents, and a negative drop (an increase) in voltage at leading
currents.
In order to maintain a constant output voltage it is therefore necessary to change
the excitation current in the rotor to compensate for the load conditions. The variation
in rotor current to do this is shown in Fig. 5.8.
Inductive
A
Resistive
-Capacitor
c
c
P
L
2
0
Load current
>
Fig. 5.8 Variation of excitation current with load current to maintain constant output voltage
5.3.3
Damping of transients
Transient changes in stator load result in a change of flux in the rotor pole, and if it
can be arranged that this flux change induces a voltage and a flow of current in the
pole face, this current will oppose the change in stator flux. To achieve this, it is
normal to insert into the pole face a set of aluminium or copper ‘damper’ bars,
connected at either end by a ring or end plate to form a conducting cage in the pole
faces.
The damper cage has a considerable influence on the transient current flow in the
stator, particularly in the case of a short circuit. In addition, if the load in the three
Generators
94
phases is unbalanced, the induced currents in the damper cage will act to reduce
distortion of the waveform and to reduce asymmetry in the output phase voltages. A
single-phase generator represents a severe case of asymmetry, and this requires very
careful damper cage design because of the high induced currents.
The cage also helps to damp mechanical oscillations of the rotor speed about the
synchronous speed when the generator is connected in parallel with other machines.
These oscillations might otherwise become unstable, leading to the poles ‘slipping’
in relation to the frequency set by other generators,and resulting in a loss of synchronism.
Such a condition would be detected immediately by the generator protection circuits
and the generator would then be isolated from the network.
5.3.4
Voltage waveform
The specified voltage waveform for a generator is usually a sine wave with minimum
distortion. A sine wave supply has advantages for many loads because it minimizes
the losses in the equipment; this is especially the case with motors and transformers.
The voltage waveform of a practical generator usually contains some distortion or
harmonics, as shown in Fig. 5.9. The distorted waveform shown in Fig. 5.9(a) can be
represented as a series of harmonics, consisting of the fundamentalrequired frequency
and a series of higher frequencies which are multiples of the fundamental frequency.
Sine wave
fundaments;\
1
3
5
7
9
Harmonic
50 Hz 150 Hz 250 Hz 350 Hz 450 Hz
(a) Waveform
(b) Spectrum
Fig. 5.9 Harmonic distortion
The harmonic distortion is calculated using a Fourier analysis or other means
of obtaining the spectrum of harmonics. An example of a spectrum is shown in
Fig. 5.9(b). Distortion is defined by a ‘distortionfactor’,where
Distortion factor = (E V;)lnIV,
(5.7)
In eqn 5.7, V, is the magnitude of the nth harmonic, VIbeing the magnitude of the
fundamental.
There are several ways in which a generator can be designed to produce minimum
distortion factor.
The higher frequency ‘slot hamonics’ are due to distortions in the air gap flux
density wave, these being created by the stator slot openings. The distortions can be
reduced by skewing the stator slots so that they are no longer parallel to the rotor
shaft, but form part of a helix. The slots are often skewed by an amount close to the
pitch between one stator slot and the next.
A second step is to use more than one stator slot per pole for each phase winding;
Newnes Electrical Engineer’s Handbook
95
Fig. 5.10 shows a winding with three stator slots per pole for each phase. This
distributes the effect of the winding better and reduces the harmonics.
u
++
I
u
u
Coil pitch
Fig. 5.10 Slot and winding layout
Harmonics can also be reduced by ‘short-pitching’the stator winding as shown in
Fig. 5.10. Except for the smallest machines, most generators have a double layer
winding in which the one side of a coil is laid into a slot above the return side of a
different coil. The simplest (fully pitched) winding has all the coil sides of one phase
in slots above the return sides of coils in that same phase, but by displacing one layer
of the winding with respect to the other, the harmonics are reduced.
A fourth technique is to shape the face of the rotor pole so that the air gap between
rotor and stator is larger at the tips. This prevents a ‘flat-topped’ shape to the flux
wave in the air gap and therefore reduces voltage distortion.
Finally, correct spacing of the damper bars in the pole face and proper choice of
the arc length of the pole face reduces the high frequency harmonics which are
produced by currents induced in the bars by the passage of the stator slots.
5.4 The Automatic Voltage Regulator (AVR)
While some small generators have an inherent ability to produce reasonably constant
voltage as the load varies, it is clear from the previous explanations that some form
of automatic voltage control is required in the usual form of generator. This control
is referred to as an automatic voltage regulator, or AVR, and it is based on a closedloop control principle.
The basis of this closed-loop control is shown in Fig. 5.11. The output voltage is
converted, usually through a transformer or resistor network, to a low voltage dc
signal, and this ‘feedback’ signal is subtracted from a fixed ‘reference’ voltage to
produce an ‘error’ signal.
The error signal is processed by a ‘compensator’before being amplified to drive
the rotor excitation current. The change in rotor excitation current produces a variation
in output voltage, closing the control loop. If the ‘gain’ of the control loop is large
Generators
96
Reference
Compensator Amplifier
Error
Rotor
Stator
P
I
Load
I t
I
Output voltage feedback
Fig. 5.11 Closed-loop voltage control
enough then only a small error is required to produce the necessary change in excitation
current and the output current will therefore remain substantially constant.
A high gain can lead to instability in the circuit, with oscillations in the output
voltage; the purpose of the compensating circuit is to enable small errors to be
handled in a stable way. The most common form of compensator is a PID circuit in
which the error is amplified proportionately (P), integrated (I) and differentiated (D)
in three parallel circuits before being added together. Many AVRs have adjustment
potentiometers which allow the gains of each channel to be varied in order to achieve
the best performance. The integral term enables compensator output to be achieved
at zero error, and this produces the minimum error in output voltage.
The layout of a commercial AVR is shown in Fig. 5.12.
Fig. 5.12 Layout of an automatic voltage regulator (AVR) (courtesy of Newage International)
Many AVRs are now offered with digital circuitry. The principle of the feedback
loop remains the same, but the feedback signal is converted to digital form using an
analogue to digital converter. The calculations are performed digitally in a
microprocessor and the output is on or off, using pulse width modulation (PWM) to
vary the average level of dc supplied to the rotor excitation winding. Alternatively,
the phase angle of a thyristor bridge can be used to vary the output level; this is
known as phase-angle control.
Newnes Electrical Engineer’s Handbook
97
The continuous improvement in power electronic controls and processor power is
bringing further advances in voltage and speed control, with more flexible protection
of the generator and its connected circuits. An example of recent developments is the
variable-speed constant-frequency generator from Generac Corporation; this is
illustrated in Fig. 5.13. This consists of an variable-speed engine-driven permanentmagnet generator feeding a power electronic frequency-changer circuit, the output
being at constant 50 Hz or 60 Hz frequency. A microprocessor is used to control the
switching of the output devices and to regulate the engine speed depending upon the
load applied to the generator. At low power demand the engine speed is reduced to
minimize noise, increase efficiency and extend life expectancy. The result is a saving
in the volume and weight of the generator.
Fig. 5.13 Variable-speed constant-frequency generator (courtesy of Generac Corporation)
5.5
Brushless excitation
Although some generators are still produced with brushes and sliprings to provide
the rotor current as illustrated in Fig. 5.4, most now have a brushless excitation
system. The two main techniques for synchronous generators are the separate exciter
and capacitor excitation and these are described in the following sections. Also
included for convenience here is a brief description of induction generators, since
these also provide a brushless system.
98
5.5.1
Generators
Separate exciter
The most common way of supplying dc current to the rotor winding without brushes
and sliprings is shown in Fig. 5.14.
Stator
Exciter
stator
Exciter
rotor
Rotating
diode bridge
Main
field
Fig. 5.14 Separate brushless exciter
The output of the AVR drives a dc current Ifthrough the poles of the excirer, which
are mounted in a stator frame. The stator then produces a stationary flux which
induces a voltage in the rotor winding as it turns. Figure 5.15 shows that the exciter
rotor is mounted on the same shaft as the main generator. The ac voltage produced
by the rotor winding of the exciter is converted to dc by a bridge rectifier which is
also mounted on the rotor shaft. This rectifier unit is shown clearly at the end of the
shaft in Fig. 5.18. The dc output of the rectifier is connected to the main rotor
windings by conductors laid in a slot along the rotor shaft. The inductance of the
main generator rotor coils is usually sufficient to smooth out the ripple in the bridge
rectifier output.
The power supply to the AVR is either provided by a separate excitation winding
in the main generator stator, or by a small permanent-magnet generator mounted on
Newnes Electrical Engineer’s Handbook
99
Fig. 5.15 Cut-away section of an ac generator (courtesy of Newage International)
the shaft of the main generator. This is often referred to as a ‘pilot exciter’. The
advantage of the pilot exciter is that the generator has a source of power available
once the shaft is turning; the voltage supplied to the AVR is completely independent
of generator load and there is no reliance on residual magnetism to start the selfexcitation process. The first method relies on residual flux in the magnetic circuit of
the main generator to start self-excitation.
The pilot exciter also enables the generator to supply current to the connected
network even when a short-circuit occurs, enabling the high current to be detected by
protection relays which will then disconnect the faulty circuit. If the AVR is supplied
from an excitation winding in the main generator stator, the supply voltage is very
small when the stator windings experience a short-circuit, and the AVR is unable to
drive an adequate rotor excitation current.
One manufacturer uses two excitation windings to provide a voltage from the
AVR in short-circuit conditions, so that sufficient current is supplied into the fault to
trip the protection system. During a short circuit the air gap flux density in these
machines shows a pronounced third harmonic component. This component induces
voltage in coils of one of the excitation windings, which are one-third pitched and
this therefore delivers a voltage to the AVR under short-circuit conditions. The second
excitation winding is fundamental-pitched and provides the major drive for the AVR
under normal operating conditions. It is claimed that the performance of this system
is comparable to a machine using a permanent-magnet exciter.
Another method used to provide voltage to the AVR under short-circuit conditions
is a series transformer driven by the generator output current.
100
-
Generators
Load current
1
Stator
excitation
winding
Diode
Capacitor
shorting rotor
winding
Fig. 5.16 Capacitor excitation
5.5.2 Capacitor excitation
The use of this technique is usually restricted to single-phase generators with a rated
output less than 10 kW.
A separate excitation winding in the stator has a capacitor connected directly
across its output as shown in Fig. 5.16. The rotor is usually of salient-pole construction
as described previously, but in this case the rotor winding is shorted through a diode.
During starting, residual flux in the rotor body induces a s m a l l voltage in the stator
excitation winding and a current flows through the capacitor. This current produces
two waves of magnetic flux around the air gap of the generator. One wave travels in
the same direction as the rotor, to create the armature reaction described in section
5.3.2. The second wave travels in a direction opposite to the rotor, and induces a
voltage in the rotor windings at twice output frequency. The current circulated in the
rotor windings by this induced voltage is rectified by the diode to produce a dc
current. This dc current increases the magnetic flux in the machine, which in turn
drives more current through the stator excitation winding, which in turn produces
more rotor current. This self-excitation process continues until the flux reaches a
point at which the magnetic circuit is saturated, and a stable voltage results. The
process also produces an inherent AVR action, since any load current in the output
stator winding induces more rotor current to offset the armature reaction effect.
5.5.3
Induction generator
The principles and construction of the cage induction motor are explained in Chapter
10. If a three-phase motor of this type is energized, it will accelerate as a motor up
to a speed near its synchronous speed. As shown in eqn 5.5, the synchronous speed
for a 4-pole machine operated at 50 Hz would be 1500 rpm. If the machine is driven
faster than the synchronous speed by an engine or other prime mover, the machine
torque reverses and electrical power is delivered into the connected circuit.
A simple form of wind turbine generator uses an induction machine driven by the
wind turbine. The induction machine is first connected to the three-phase supply, and
acting as a motor it accelerates the turbine up to near the synchronous speed. At this
point, the torque delivered by the wind turbine is sufficient to accelerate the unit
Newnes Electrical Engineer’s Handbook
101
further, the speed exceeds the synchronous speed and the induction machine becomes
a generator.
It is also possible to operate an induction machine as a generator where there is no
separate mains supply available. It is necessary in this case to self-excite the machine,
and this is done by connecting capacitors across the stator winding as shown in Fig.
5.17(a). The leading current circulatingthrough the capacitor and the winding produces
a travelling wave of mmf acting on the magnetic circuit of the machine. This travelling
wave induces currents in the rotor cage which in turn produce the travelling flux
wave necessary to induce the stator voltage. For this purpose, some machines have
a excitation winding in the stator which is separate from the main stator output
winding. Figure 5.17(b) shows a single-phase version of the capacitor excitation
circuit.
Rotor
Stator winding
Capacitors
L Aluminium cage
Rotor
Stator
7
. ” :@
04
Fig. 5.17 Self-excitation of an induction generator: (a) three phase (b) single phase
In small sizes, the induction generator can provide a low-cost alternative to the
synchronous generator, but it has relatively poor performance when supplying a low
power factor load.
5.6
5.6.1
Construction
Stator
The stator core is constructed from a stack of thin steel sheets or laminations which
are stamped to shape and insulated electrically from each other. either by a thin
coating or by an oxide layer which is produced during heat treatment. The steel used
has a small silicon content as described in Chapter 3; this increases the resistivity of
the steel and therefore reduces the losses due to eddy currents. The steel is carefully
102
Generators
processed in order to minimize the hysteresis losses because the whole stator core is
subjected to alternating magnetic flux. In large turbo-generators the core is built up
in segments and grain-oriented steel is used to reduce the losses further.
The stator windings are located in axial slots in the stator core which are formed
by the shape of the laminations. Except in high voltage machines, the individual coils
of the winding are wound with copper wire covered with a layer of polyester/polyamide
enamel which is about 0.05 mm thick. The slots are lined with a tough insulating
sheet, usually about 0.25-0.5 mm thick; a popular material is a laminate of Mylar
and Nomex. The coils are impregnated when in place with a resin to give the winding
mechanical strength as well as to improve the heat transfer from the copper to the
cooling air. Windings operating at different voltages, such as the three phases, have
a further sheet of insulating material separating them in the end-winding region. A
general guide to the insulation materials and processes in use is given in Chapter 3.
The mounting of the stator core in its frame differs according to the size of the
machine, but in the majority of medium-sized generators the arrangement is as
shown in Fig. 5.15. At either end of the frame are bearing housings which locate the
rotor shaft. These housings or end-bells are cast in smaller and medium size machines,
and fabricated in larger sizes. The generator is often mounted directly onto the
engine, and in this case it is usual to eliminate the drive-end bearing and to use the
rear bearing of the engine to locate the generator shaft.
5.6.2
Rotor
It has already been noted that the construction of a turbo-generator is very specialized
and the rotor for these machines is not dealt with here. However, even within the
class of salient-pole generators, quite different forms of rotor construction are used,
depending upon size.
Generators rated up to about 500 kW use rotor laminations which are stamped in
one piece. In larger machines the poles are made separately from stacks of laminations,
and each pole is keyed using a dovetail arrangement onto a spider which is mounted
on the rotor shaft. In large high-speed machines the poles can be made from solid
steel for extra strength and to reduce mechanical distortion; these solid poles are
screwed to the shaft, as shown in the large 4-pole machine in Fig. 5.18.
The nature of the rotor coils also depends upon the size of the machine. Because
the ratio of surface area to volume is larger in the coils of small generators, these are
easier to cool. Generators rated above about 25 kW therefore use a ‘layer-wound’
coil in which each layer of the coil fits exactly into the grooves formed by the layer
below; this is illustrated in Fig. 5.19. Rectangular cross-section wire can be used to
minimize the coil cross-section. The simplest and cheapest way to make the coils,
often used in smaller machines, is to wind them in a semi-random way as shown in
Fig. 5.20. In either case, the coils are impregnated after winding like the stator
windings to give extra mechanical strength and to improve the heat transfer by
removing air voids within the coil.
The coils are under considerable centrifugal stress when the rotor turns at full
speed, and they are usually restrained at both ends of the pole by bars, and by wedges
in the interpole spaces, as shown in Fig. 5.18.
5.6.3
Cooling
Adequate cooling is a vital part of the design and performance of a generator. Forced
Newnes Electrical Engineer's Handbook
103
Fig. 5.18 Large salient-pole rotor (courtesy of Brush Electrical Machines)
cooling is needed because of the high loss densities that are necessary to make
economic use of the magnetic and electrical materials in the generator.
The most critical areas in the machine are the windings, and particularly the rotor
winding. It is explained in Chapter 3 that the life expectancy of an insulation system
decreases rapidly if its operating temperature exceeds recommended temperatures. It
is crucial for reliability therefore that the cooling system is designed to maintain the
winding temperatures within these recommended limits. As shown in Table 3.4,
insulation materials and systems are defined by a series of letters according to their
temperature capability. As improved insulation with higher temperature capability
has become available, this has been adopted in generator windings and so the usual
class of insulation has progressed from class A (40-50 years ago) through class E and
class B to class F and class H, the latter two being the systems generally in use at
present. Class H materials are available and proven, and this system is becoming
increasingly accepted. An important part of the generator testing process is to ensure
that the cooling system maintains the winding temperature within specified limits,
and this is explained later in section 5.8.
In turbo-generators it has already been noted that very complex systems using
hydrogen and de-ionized water within the stator coils are used. The cooling of small
and medium-sized machines is achieved by a flow of air driven around the machine
by a rotor-mounted fan.
A typical arrangement is shown in Fig. 5.15. In this case the cooling air is drawn
in through ducts; it is then drawn through the air gap of the machine and ducts around
the back of the stator core before reaching the centrifugal fan which then expels the
104
Generators
Fig. 5.19 Layer-wound rotor (courtesy of Generac Corporation)
air from the machine. There are many variations to the cooling system, particularly
for larger generators. In some machines there is a closed circulated air path cooled by
a secondary heat exchanger which rejects the heat to the outside atmosphere. This
results in a large and often rectangular generator enclosure as shown in Fig. 5.21.
5.7 Rating and specification
In order that a generator can be selected to suit a particular application, manufacturers
issue specification data. This can be used and interpreted according to the following
sections.
5.7.1
Rated output
The key aspect of the specification is the rated output of the generator, which is
normally expressed in terms of the apparent power (VA,kVA or MVA) when supplying
the maximum load at the rated power factor, assumed to be 0.8. The rated output is
usually based upon continuous operation in a maximum ambient temperature of
Newnes Electrical Engineer’s Handbook
Fig. 5.20 Random-wound rotors (courtesy of Generac Corporation)
Fig. 5.21 Closed air circuit cooled generator (courtesy of Brush Electrical Machines)
105
106
Generators
40°C. If the machine has a special short-time rating the nameplate should state the
time limits of operation.
The rated output from a given size of generator is related to the size and speed of
the machine:
Rated output power = K x D i x L, x n,
(5.8)
where Dgis the stator bore diameter, L, is the stator core length and n, is the speed.
K is a design constant which is proportional to the product of the air gap flux density
and the stator current density.
The rated output of a machine is reduced in ambient temperaturesexceeding 40°C
and at altitudes above sea level exceeding 1000 m. The latter is because the air
density is decreased, and its ability to cool the machine is reduced. Derating factors
are applied for these conditions and typical values are summarized in Tables 5.1 and
5.2.
lslble 5.1 Typical derating factors for class H insulation in
ambient temperatures above 40°C
Ambient temperature (“C)
Derating factor
40
45
50
55
60
1.oo
0.97
0.94
0.91
0.88
Table 5.2 Typical altitude derating factors
~~
Altitude (m)
Derating factor
1000
1500
1.oo
0.97
0.97
0.94
2000
2500
3000
3500
0.87
0.82
0.77
4000
5.7.2
Reactances
The generator can be characterizedby several reactances, each being useful in working
out the performance and protection requirements under different circumstances.These
include the synchronous reactance, the transient reactance,the subtransient reactance,
the Potier reactance and the negative- and zero-sequence reactances.
The subtransientreactance represents the output impedance of the generator within
the first few cycles after a short-circuit occurs at the generator terminals. It is used
for selecting protective relays for the connected load circuit. The lower the value of
the subtransient reactance, the more onerous is the protection requirement.
Transient reactance represents the impedance of the machine over a slightly longer
period, and is relevant to the performance of the generator and its AVR under changing
load conditions.A low transient reactance is beneficial in responding to load changes.
Associated with the subtransientand transient reactances are time constants which
define the rate of decay of these reactances.
Newnes Electrical Engineer’s Handbook
107
The negative and positive sequence reactances influence the performance of the
generator when supplying an unbalanced three-phase load.
5.7.3
Main items of specification
In summary, the following items are important when considering an application and
specifying the appropriate generator:
rated output, expressed as apparent power (VA)
cooling air requirement (m3/s)
moment of inertia of the rotor
generator weight
efficiency at full, 314, 1/2 and 1/4 load
stator winding resistance
reactances and time constants, as listed in section 5.7.2
maximum short-circuit current delivered by the generator
Typical values for a 200 k W generator are shown in Table 5.3.
Table 5.3 Typical parameters for a 200 kW generator
Line-line voltage, frequency
Cooling air flow
Rotor moment of inertia
Weight
Efficiency
Stator resistance
Rotor resistance
Reactances
Time constants
Short-circuit current
5.8
full load
314 full load
1/2full load
1/4full load
synchronous, X,
transient, XA
subtransient, X:
positive sequence
negative sequence
zero sequence
transient, Tdo
subtransient, Ti1
400 V, 50 Hz
0.4 m3/s
1.9 kg mp
600 kg
93%
94%
94%
939/0
0.025 P
1.9 P
1.9 pu
0.2 pu
0.1 pu
0.07 pu
0.1 pu
0.07 pu
0.8s
0.01 s
2.5 pu
Testing
Type tests are performed by manufacturers in order to confirm that a design meets its
specifications, and production tests are done in order to check that each machine as
it is manufactured conforms to performance and safety standards.
These tests usually include:
full-load tests to measure the temperature rise of the machine windings and
insulation. The temperature rise is calculated from the change in resistance of
the stator windings.
tests to determine the excitation current required to deliver a given output
Generators
108
0
0
0
voltage. These are done for open-circuit conditions and also for various load
currents and power factors. The resulting curves are usually known as ‘saturation’
curves.
short-circuit tests to determine the current that can be driven by the generator
into a short-circuit fault in the connected load
transient shm-ciruit rests to determinethe subtransientand transient reactances
and time constants
overspeed tests to confirm that the rotor does not distort or disintegrate. This
test is normally performed at 150per cent of rated speed, at full rated temperature.
The results from these tests are used to calculate the data described in section5.7, an
example of which has been shown in Table 5.3.
It is now necessary for any generator manufactured or imported into the European
Union that relevant EU Directives are met through certification. Strictly it is the
manufacturer of the generator set, including the engine and all controls, that is
responsible for this certification, but many manufacturers of generators and AVRs
will assist in the tests.
5.9 Standards
The leading international, regional and national standards adopted by users and
suppliers of generators for manufacturing and testing are shown in Table 5.4.
Table 5.4 International, regional and national standards relating to generators
lECl
IS0
ENMD
IEC 34-1
IEC 34-2A
IEC 34-4
IEC 34-6
IEC 34-8
EN 60034-1 4999-101
4999-102
EN 60034-4 4999-104
EN 60034-6 4999-106
HD 53.8
4999-108
4999-140
IEC 34-141
I S 0 2373
IEC529
EN60529
I S 0 8528-3
IS0 8528-8
IEC 335-1 EN 60335-1
EN 60742
BS
4999-142
5490
7698-3
7698-8
3456-1
3535-1
Subject of standard
N. American
Ratings and performance
Losses and efficiency
Synchronous machine quantities
Methods of cooling
Terminal markings
Voltage regulation, parallel operation
Vibration
Degrees of protection
Generators for generating sets
Low power generating sets
Safety of electrical appliances
Isolating transformers
Motors and generators
Cylindrical-rotor synchronous
generators
GT-driven cyl-rotor synch generators
NEMA MG 1
IEEE C50.13
IEEE C50.14
Chapter 6
Transformers
Professor D.J. AI Ian
ALSTOM T&D - Transformers
6.1
Principles of operation
In simple form, a transformer consists of two windings connected by a magnetic
core. One winding is connected to a power supply and the other to a load. The circuit
containing the load may operate at a voltage which differs widely from the supply
voltage, and the supply voltage is modified through the transformer to match the load
voltage.
In a practical transformer there may be more than two windings as well as the
magnetic core, and there is the need for an insulation system and leads and bushings
to allow connection to different circuits. Larger units are housed within a tank for
protection and to contain oil for insulation and cooling.
In the simplest case, with no load current flowing, the transformer can be represented
by two windings on a common core, as shown in Fig. 6.1. It has been explained in
Chapter 2 (eqns 2.25 and 2.26) that the input and output voltages and currents in a
transformer are related by the number of turns in these two windings, which are
usually called the primary and secondary windings. These equations are repeated
here for convenience.
Vi/V, = NlIN2
(6.1)
IilIz = N2INl
(6.2)
The magnetic flux density in the core is determined by the voltage per turn:
VllN1 = 4.44jBJ
(6.3)
In the no-load case, a small current Io flows to supply the magnetomotive force
which drives the magnetic flux around the transformer core; this current lags the
primary voltage by almost 90". Io is limited in magnitude by the effective resistance
(R,) and reactance (X,)of the magnetizingcircuit, as shown in Fig. 6.2. The magnetizing
current is typically 2 per cent to 5 per cent of the full load current and it has a power
factor in the range 0.1 to 0.2.
When the transformer is loaded, there is an internal voltage drop due to the current
flowing through each winding. The voltage drops due to the primary and secondary
winding resistances (R1and R2) are in phase with the winding voltage, and the
voltage drops due to the primary and secondary winding leakage reactances (XIand
Xz)lag the winding voltage by 90". The leakage reactances represent those parts of
the transformer flux which do not link both windings; they exist due to the flow of
Transformers
110
Primary winding
Secondary winding
Fig. 6.1 Simple transformer circuit
Fig. 6.2 Equivalent circuit of a transformer
opposing currents in each winding and they are affected strongly by the winding
geometry.
The current flow and voltage drops within the windings can be calculated using
the equivalent circuit shown in Fig. 6.2. This circuit is valid for frequencies up to 2
H z .A vectorial representation of the voltages and currents is shown in Fig. 6.3 for
the case of a load L with a power factor angle @.
The decrease in output voltage when a transformer is on load is known as reguZution.
The output voltage is less than the open-circuit voltage calculated according to eqn
6.1 because of voltage drops within the winding when load current flows through the
resistive and reactive components shown in Fig. 6.2. The resistive drops are usually
much smaller than the reactive voltage drops, especially in large transformers, so the
impedance Z of the transformer is predominantly reactive. Regulation is usually
Newnes Electrical Engineer's Handbook
111
11 = i*
Vl
-
L'
Transformers
112
+ R2(Nl/N*)'
x = x,+ X2(N1/N2)2
R = R1
and
The impedance of the transformer is given by:
z = [R2 -J-
(6.7)
x2y
Values for Rr and R2 can be established by measuring the resistance of the windings.
The value of X is determined by calculation or by derivation from the total impedance
Z, which can be measured with one winding of the transformer short-circuited. Z is
given by
z = VII
(6.8)
where V is the voltage necessary to circulate the full-load current I in the windings
under short-circuit conditions. When Vis expressed as a percentage of rated voltage,
this gives Z as a percentage value referred to rated power.
When the transformer is energized, but without a load applied, the no-load power
loss is due to the magnetic characteristics of the core material used and the flow of
eddy currents in the core laminations. The loss due to magnetizing current flowing
in the winding is small and can be ignored.
When the transformer is loaded, the no-load loss is combinedwith a larger component
of loss due to the flow of load current through the winding resistance. Additional
losses on load are due to eddy currents flowing within the conductors and to circulating
currents which flow in metallic structural parts of the transformer. The circulating
currents are induced by leakage flux which is generated by the load current flowing
in the windings and they are load dependent. These additional losses are known as
sway losses.
The efficiency of a transformer is expressed as:
Efficiency = (outputhput) x 100%
= (input - 1osses)linputx 100%
= [l - (lossedinput)] x 100%
(6.9)
where input, output and losses are all expressed in units of power.
The total losses consist of the no-load loss (or iron loss) which is constant with
voltage and the load loss (or copper loss) which is proportional to the square of load
current. Total losses are usually less than 2 per cent for distribution transformers, and
they may be as low as 0.5 per cent in very large transformers.
No-load and load losses are often specified as target values by the user for larger
transformers, or they m a y be evaluated by the capitalization of losses. The capitalization
formula is of the type:
C, = C T +A
X
Po + B
X
PK
where C, = capitalized cost
CT = tendered price
A = capitalization rate for no-load loss ( m W )
Po = guaranteed no-load loss (kW)
(6.10)
Newnes Electrical Engineer's Handbook
113
B = capitalization rate for load loss (EkW)
PK = guaranteed load loss (kW)
6.2 Main features of construction
6.2.1
The core
The magnetic circuit in a rransformer is built from sheets or laminations of electrical
steel. Hysteresis loss is controlled by selecting a material with appropriatecharacteristics,
often with large grain size. Eddy current loss is controlledby increasingthe resistivity
of the material using the silicon content, and by rolling it into a very thin sheet. The
power loss characteristics of available materials are shown in Fig. 6.5. Generally
speaking the steels with lower power loss are more expensive, so that the 0.3 mm
thick CGO steel shown is the cheapest of the range shown in Fig. 6.5, and the very
low loss 0.23 mm thick Hi-Bmaterial with domain control techniques used to modify
the apparent crystal size is the most expensive. New materials with thicknesses of
0.1 mm are available on a laboratory basis, but the use of such thin materials presents
some production problems. The 25 pm thick amorphous ribbon shown with very low
loss is made using a rapid coolingtechnique;it can be used in very low loss transformers
up to a few MVA and a core with losses of only one-sixth of the conventional 0.3 mm
CGO material is possible. The general principles applying to electrical steels are
discussed in Chapter 3.
I
1.64
-
0.30rnrn CGO
0.30 mrn Hi-B
1.20.23 rnrn Hi-B
domain refined
1.0Power loss. w m
0.8 0.6 -
0.1 rnrn Hi-B
domain refined
0.4 -
25 prn amorphous ribbon
0.2-
0
I
1.0 1.2
I
I
I
I
1.4
1.6
1.8
2.0
Magnetic induction, tesla
Fig. 6.5 Power loss characteristics of various electrical steels and amorphous materials at 50 Hz
The audible sound radiated by a transformer is generated by magnetostrictive
deformation of the core and by electromagnetic forces in the windings, tank walls
and magnetic shields. The dominant sound is generated by longitudinal vibrations in
the core laminations, which are induced by the magnetic field. The amplitude ofthe
vibrations depends upon the flux density in the core and on the magnetic properties
114
Transformers
of the core steel. In a large power transformer operating at high flux density the
audible sound level can exceed 100 dB(A) and it may be necessary to use high
quality core material, improved core-joint techniques and perhaps external cladding
or a sound enclosure to reduce the sound to an acceptable level.
In small power transformers, the core may take the form of a continuous strip of
steel wound into a coil. The windings may be formed directly onto this core using
toroidal winding machines, or the core may be cut to allow preformed windings to be
fitted, and reinterlaced with the windings in place.
Where transformer weight is critical, it may be advantageous to operate a local
power system at high frequency. Equation 6.3 relates voltage per turn to frequency
and core cross-section. If the system is designed to operate at 400 Hz (a typical
requirement for aircraft), it can be seen that the core cross-section will be only 12.5
per cent of that which would be necessary at 50 Hz. The clear advantage in cost and
weight must be balanced against higher core losses at the increased frequency (although
these can be reduced by using thinner laminations) and against higher load losses
caused by high-frequency currents in the windings.
A longer-established form of construction uses the stacked-core technique. For
transformers which are rated in hundreds of watts the laminations may be stamped in
E and I shapes, or in C and I shapes, then built into cores and assembled round the
windings. At higher ratings in kVA or MVA, the usual constructionis to cut laminations
to length, and to assemble them in a building berth which includes part of the coreclamping structure. When the core has been built, the remaining part of the clamping
structure is fitted and the core is turned upright. If it has been fitted during the
stacking stage, the top yoke is then removed, the windings are mounted on the core
and the top yoke is then reinterlaced.
Single-phase transformers usually have a three-leg core with high-voltage and
low-voltage windings mounted concenmcally on the centre leg to reduce leakage
flux and minimize the winding impedance. The outer legs form the return path for the
magnetizing flux. This type of construction is shown in Fig. 6.6(a). For very high
ratings in the region of 500 MVA, it may be economic to use a two-leg construction
with two sets of windings connected in parallel and mounted one on each leg, as
shown in Fig. 6.6(b). Where a transport height limit applies, a large single-phase
transformer may be mounted on a four-leg core where the outer legs are used to
returnremnant flux, in conjunction with smaller top and bottom yokes. This arrangement
is shown in Fig. 6.6(c).
Three-phase transformers are also based usually on a three-leg construction. In
this case the high-voltage and low-voltage windings of each phase are mounted
concentrically with one phase on each of the three legs. The phase fluxes sum to zero
in the top and bottom yokes and no physical return path is necessary. This arrangement
is shown in Fig. 6.6(d). Where transport heights are a limitation, a three-phase
transformer may be built using a five-leg construction as shown in Fig. 6.6(e). The
yoke areas in this case are reduced and the outer legs are included as return flux
paths.
Core laminationsare compacted and clamped together using yoke clamps connected
by flitch plates on the outer faces of the legs, or by tie-bars locking the top and
bottom yoke clamps together. Figure 6.7 shows a large three-phase, three-leg core in
which the yokes are clamped by rectangular-section steel frames and insulated straps;
flitch plates lock the two clamps together and the core-bands on the legs are temporary,
to be removed when the windings are fitted.
2
I
Transformers
116
Fig. 6.7 Three-leg core for a three-phase transmission transformer
6.2.2
Windings
Winding conductors may be of copper or aluminium, and they may be in foil or sheet
form, or of round or rectangular section. Foil or sheet conductors are insulated from
each other by paper or Nomex interleaves, whereas round or rectangular conductors
may be coated with enamel or wrapped with paper or some other solid insulation
covering.
Low-voltage windings for transformers with ratings up to about 4 MVA may use
foils with the full width of the winding, or round conductors. For higher-power
transformers a large cross-section may be necessary to carry the current in the lowvoltage winding, and it may be economic to use stranded or parallel conductors in
order to reduce eddy currents. It may also be necessary to transpose conductors
during the winding operation to reduce circulating currents within the winding. For
very high current applications, continuously transposed conductor (CTC) may be
used, where typically 40 or 50 conductors are machine transposed as shown in Fig.
6.8. In a CTC, each strand is enamel insulated and the cable is enclosed in a paper
covering. The conductors for low-voltage windings are usuaIly wound in layers, with
cooling ducts and interlayer insulation as part of the construction.
High-voltage windings have more turns and carry less current than the low-voltage
windings. They are usually formed from round enamelled wire or paper-wrapped
rectangular conductors. As with low-voltage windings, if the winding loss is to be
minimized, parallel conductors or CTC may be used. The conductors are wound in
layers or in discs, as shown in Figs 6.9(a) and 6.9(b) respectively. The labour cost in
a layer winding is high, and disc windings are usually considered to be more stable
mechanically under the effects of through-current faults.
Newnes Electrical Engineer's Handbook
(a) Transposed strip conductor (27 strips in parallel)
117
(b) Transposed strip conductor
in paper envelope
Fig. 6.8 Continuously transposed conductor
Although simple two-winding transformers are widely used, many transformers
have three or more windings. These may include regulating windings, tertiary windings
to balance harmonic currents or supply auxiliary loads, multiple secondary windings
to supply separate load circuits, or multiple primary circuits to connect to power
supplies at different voltages or frequencies. The constructional aspects are common
between multiple windings, but design aspects are more complicated.
6.2.3 Winding connections
Three-phase transformers are usually operated with the high-voltage and low-voltage
windings connected in Y (star), D (delta) or 2 (zigzag) connection. The three styles
are shown in Fig. 6.10.
In star connection, one end of each of the three-phase windings is joined together
at a neutral point N and line voltage is applied at the other end; this is shown in Fig.
6.10(a). The advantages of star connection are:
it is cheaper for a high-voltage winding
the neutral point is available
earthing is possible, either directly or though an impedance
a reduced insulation level (graded insulation) is possible at the neutral
winding tappings and tapchanger may be located at the neutral end of each
phase, with low voltages to earth and between phases
single-phase loading is possible, with a neutral current flowing
In delta connection, the ends of the three windings are connected across adjacent
phases of the supply as shown in Fig. 6.10(b). The advantages of a delta connection
are:
0
0
it is cheaper for a high-current low-voltage winding
in combination with a star winding, it reduces the zero-sequence impedance of
that winding
Fig. 6.9(a) Layer-type winding on a horizontal winding machine
Fig. 6.9(b) Disc-type winding on a vertical winding machine
Newnes Electrical Engineer's Handbook
npoint
e L
119
p1
j
voltages
1 (a) Star connection
E
, Line
voltages
(b) Delta connection
Line
voltages
I
N
neutral
point
(c) Zigzag connection
Fig. 6.10 Three-phase winding connections
The zigzag connection is used for special purposes where two windings are available
on each leg and are interconnected between phases as shown in Fig. 6.10(c). The
advantages of a zigzag connection are:
it peimits neutral current loading with an inherently low zero-sequence
impedance, and it is used in 'earthing transformers' to create an artificial
neutral terminal on the system
voltage imbalance is reduced in systems where the load is not evenly distributed
between phases
In order to define the range of possible connections, a designation has been adopted
by E C in which the letters Y, D, Z and N are assigned to the high-voltage windings
and y, d, z and n are assigned to the low-voltage windings. Clock-hour designations
1 to 12 are used to signify in 30" steps the phase displacement between primary and
secondary windings. YyO is the designation for a star-star connection where primary
and secondary voltages are in phase. Ydl is the designation for a star-delta connection
with a 30" phase shift between primary and secondary voltages. The common
connections for three-phase transformers are shown in Fig. 6.11.
Transformers
120
-
DdO
Yz1
Lu i
...
Yz5
-
Dd6
11
Ydll
MA
Dyl 1
-
Dz6
Yzl 1
Fig. 6.11 Clock-hour designation of three-phase winding connections (based on IEC 60076-1)
In many transmission transformers, an autotransformer connection is employed.
The connection is shown in Fig. 6.12, and unlike the two-winding transformer in
which primary and secondary are isolated, it involves a direct connection between
two electrical systems. This connection can have a cost advantage where the ratio of
input and output voltages is less than 5 : 1. Current flowing in the ‘series’ winding
corresponds to that in the higher-voltage system only. The ‘common’winding carries
Newnes Electrical Engineer’s Handbook
121
Primary current /,
D
Series
winding
Secondary current l2
s
Common
winding
Difference current /2 - I,
Fig. 6.12 Autotransformerconnection
a current which is the difference between the two systems, and being sized for this
lower current it can be significantly cheaper. The autotransformeris more susceptible
to damage from lightningimpulse voltages and it has a lower strength against throughfault currents; both of these weaknesses can be corrected, but at increased cost.
It may occasionallybe necessary to make a three-phase to two-phase transformation.
This might be in order to supply an existing two-phase system from a new threephase system, or to supply a two-phase load (such as a furnace) from a three-phase
system, or to supply a three-phase load (such as a motor) from a two-phase system.
In all these cases the usual method of making the transformation is by using two
single-phase transformers connected to each of the systems and to each other by the
Scott connection.
A Scott connection is shown in Fig. 6.13. On one transformer the turns ratio is
equal to the transformation ratio, and the mid-point of the winding connected to the
three-phase system is brought out for connection to the other single-phase transformer.
The second transformer has a turns ratio of 0.866 times the transformation ratio. The
primary winding is connected between the third phase of the three-phase supply and
the mid-point of the primary winding of the first transformer.The secondary windings
Transformers
122
/
Three-phase
supply
I
Fig. 6.13 Scott connectionof transformers
of the two transformers are connected to the two-phase supply (or load) using a
three- or four-wire connection.
6.2.4
Bushings
Where transformers are enclosed within tanks to contain the insulating and cooling
fluid, it is necessary to link the windings connections to the network using throughbushings which penetrate the tank.
Low-voltage bushings are generally solid. They are made of porcelain, ceramic or
epoxy insulation with sufficient electrical strength to withstand abnormal voltages
due to lightning activity or switching operations, and to withstand the service voltage
over the lifetime of the transformer. Some low-voltage bushings carry high current
and cooling is necessary. In such cases it is usual to employ a hollow porcelain
construction in which the internal connections are cooled by the oil in the tank.
High-voltage bushings must withstand much higher voltage transients. They are
usually of composite construction with a core of oil-impregnated or resin-bonded
paper in an outer porcelain or epoxy cylinder. This outer cylinder is ‘shedded’ at the
outside (air) end to increase its electrical strength under wet conditions. A typical
high-voltage bushing with an oil-paper core is shown in Fig. 6.14(a); its internal
construction detail is shown in Fig. 6.14(b).
6.2.5
Tapchangers
It has been explained in section 6.1 that when a transformer carries load current there
is a variation in output voltage from a transformer which is known as regulation. In
order to compensate for this, additional turns are often made available so that the
Newnes Electrical Engineer’s Handbook
123
voltage ratio can be changed using a switch mechanism known as a tapchanger. An
off-circuit tapchanger can only be adjusted to switch additional turns in or out of
circuit when the transformer is de-energized; it usually has between two and five
tapping positions. An on-load tapchanger (OLTC) is designed to increase or decrease
the voltage ratio when the load current is flowing, and the OLTC should switch the
transformer load current from the tapping in operation to the neighbouring tapping
without interruption. The voltage between tapping positions (the step voltage) is
normally between 0.8 per cent and 2.5 per cent of the rated voltage of the transformer.
OLTC mechanisms are based either on a slow-motion reactor principle or a highspeed resistor principle. The former is commonly used in North America on the lowvoltage winding, and the latter is normally used in Europe on the high-voltage
winding.
The usual design of OLTC in Europe employs a selector mechanism to make
connection to the winding tapping contacts and a diverter mechanism to control
current flows while the tapchanging takes place. The selector and diverter mechanisms
Fig. 6.14(a) High-voltage oil-air bushing
124
Transformers
Line connection
TOP cap
capacitance foils
'
Oil end ceramic
Stress distributor
Connection to winding
Fig. 6.14(b) Internal construction of high-voltage oil-air bushing
tank
Newnes Electrical Engineer’s Handbook
125
may be combined or separate, depending upon the power rating. In an OLTC which
comprises a diverter switch and a tap selector, the tap change occurs in two operations.
First, the next tap is selected by the tap switch but does not carry load current, then
the diverter switches the load current from the tap in operation to the selected tap.
The two operations are shown in seven stages in Fig. 6.15.
High-voltage
winding
Selector
switch
Tappings
\
ii
(a)
Switching sequence of tap selector
(dl
(e)
Switching sequence of diverter switch
Fig. 6.15 Operation of selector and diverter switches in an on-load tap changer of high-speed resistor type
The tap selector operates by gearing directly from a motor drive, and at the same
time a spring accumulator is tensioned. This spring operates the diverter switch in a
very short time (40-60 ms in modern designs), independently of the motion of the
motor drive. The gearing ensures that the diverter switch operation always occurs
after the tap selection has been completed. During the diverter switch operation
shown in Fig. 6.15(d), (e) and (f), transition resistors are inserted; these are loaded
for 20-30 m and since they have only a short-time loading the amount of material
required is very low.
The basic arrangement of tapping windings is shown in Fig. 6.16. The linear
arrangement in Fig. 6.16(a) is generally used on power transformers with moderate
regulating ranges up to 20 per cent. The reversing changeover selector shown in
Transformers
126
P
P
High-voltage
winding
-
Changeover
switch
Tappings
P
-- /
--o
Selector
A
0
(b) Reversing changeover selector
(a) Linear tapping winding
High-voltage
winding
\
Changeover switch
Coarse
tappings
Selector
6
(c) Coarsefine tapping windings
Fig. 6.16 Basic arrangements of tapping windings
Fig. 6.16(b) enables the voltage of the tapped winding to be added or subtracted from
the main winding so that the tapping range may be doubled or the number of taps
reduced. The greatest copper losses occur at the position with the minimum number
of effective turns. This reversing operation is achieved with a changeover selector
which is part of the tap selector of the OLTC.A two-part coarse-fine arrangement
shown in Fig. 6.16(c) may also be used. In this case the reversing changeover
Newnes Electrical Engineer’s Handbook
127
selector for the fine winding can be connected to the ‘plus’ or ‘minus’ tapping of the
coarse winding, and the copper losses are lowest at the position of the lowest number
of effective turns. The coarse changeover switch is part of the OLTC.
Regulation is mostly camed out at the neutral point in star windings, resulting in
a simple, low-cost, compact OLTC and tapping windings with low insulation strength
to earth. Regulation of delta windings requires a three-phase OLTC, in which the
three phases are insulated for the highest system voltage which appears between
them; alternatively three single-phase OLTCs may be used.
6.2.6
Cooling equipment
Transformers may be naturally cooled, in which case the cooling medium (oil or air)
circulates by thermosyphon forces, or they may be forced cooled, with fans or pumps
to circulate the air or oil over the core and through the windings.
In oil-cooled transformers a more economic solution is to use directed oil-flow
cooling in which oil is pumped directly into the windings. The oil is cooled by
passing (or pumping) it through plate radiators, which may be externally cooled by
fans, or by using forced air-cooled tubular construction, or by pumping it through
water coolers. Designations for these cooling systems which identify both the cooling
fluid and the type of cooling have been assigned by IEC and these are summarized
in Table 6.1.
6.3
Main classes of transformer
Transformers are used for a wide variety of purposes, with the complete range of
voltage and power ratings as well as many special features for particular applications.
The following covers the main types.
6.3.1
Transformers for electronics
Transformers for electronic circuits or for low-voltage power supplies are used to
match the supply voltage to the operating voltage of components or accessories, or
to match the impedance of a load to a supply in order to maximize power throughput.
The core is usually constructed in the low-power transformers from C- and Ilaminations or from E- and I-laminations. The windings are usually of round enamelled
wire, and the assembly may be varnished or encapsulated in resin for mechanical
consolidation and to prevent ingress of moisture.
Increasing numbers of this type operate at high frequencies in the kHz range and
use laminations of special steel often containing cobalt to reduce the iron losses.
6.3.2 Small transformers
These are used for stationary, portable or hand-held power supply units, as isolating
transformers and for special applications such as burner ignition, shavers, shower
heaters, bells and toys. They may be used to supply three-phase power up to 40 kVA
at frequencies up to 1 MHz. These transformers are usually air insulated, the smaller
units using enamelled windings wires and ring cores and the larger units using C- and
I- or E- and I-laminated cores.
Safety is a major concern for these transformers and they are identified as class I,
class I1 or class III. Class I units are insulated and protected by an earth terminal.
128
Transformers
Table 6.1 Cooling system designations
For oil immersed transformers a four-letter code is used:
First letter: internal cooling medium in contact with windings:
0
K
L
mineral oil or insulating liquid with fire point S3OO0C
insulating liquid with fire point >300%
insulating liquid with no measurable fire point
Second letter: circulation mechanism for internal cooling medium:
N
F
D
natural thermosyphon flow
forced oil circulation, but thermosyphon cooling in windings
forced oil circulation, with oil directed into the windings
Third letter: external cooling medium
A
W
air
water
Fourth letter: circulation mechanism for external cooling medium
N
F
natural convection
forced circulation
Examples: ONAN or OFAF
For dry-type transformers a four-letter code is used:
First letter: internal cooling medium in contact with the windings
A
G
air
gas
Second letter: circulation mechanism for internal cooling medium
N
F
natural convection
forced cooling
Third letter: external cooling medium
A
G
air
gas
Fourth letter: circulation mechanism for external cooling medium
N
F
natural convection
forced cooling
Examples: AN or GNAN
Class II transformers have double insulation or reinforced insulation. Class I11 transformers have outputs at Safety Extra-Low Voltages (SELV) below 50 V ac or 120 V dc.
6.3.3
Distribution transformers
These are used to distribute power to domestic or industrial premises. They may be
single phase or three phase, pole mounted or ground mounted, and they have ratings
ranging from 16 kVA up to 2500 kVA.
The windings and core are immersed in mineral oil, with natural cooling, and
there are two windings per phase. The primary (high-voltage) winding has a highest
voltage ranging from 3.6 kV to 36 kV; the secondary (low-voltage) winding voltage
Newnes Electrical Engineer’s Handbook
129
does not exceed 1.1 kV. The high-voltage winding is usually provided with offcircuit tappings of 2.5 per cent, or +2 x 2.5 per cent, -3 x 2.5 per cent.
The preferred values of rated output are 16,25,50, 100, 160,250,400,630, 1000,
1600 and 2500 kVA, and the preferred values of short-circuit impedance are 4 per
cent or 6 per cent. Losses are assigned from lists, for instance from BS 7821-1, or by
using a loss-capitalization formula.
The core and windings of a typical distribution transformer rated at 800 kVA,
11 0o0/440V are shown in Fig. 6.17.
*
Fig. 6.17 Core and windings of an 800 kVA, 11 000/440V distribution transformer
6.3.4
Supply transformers
These are used to supply larger industrial premises or distribution substations. Ratings
range from 4 M V A to 30 MVA, with primary windings rated up to 66 kV and
secondary windings up to 36 kV.
Transformers in this class are fluid cooled. Most supply transformers use mineral
oil, but for applications in residential buildings, oil rigs and some factories the
coolant may be synthetic esters, silicone fluid or some other fluid with a higher fire
point than mineral oil.
130
Transformers
6.3.5 Transmission (or intertie) transformers
These are among the largest and highest voltage transformers in use. They are used
to transmit power between high-voltage networks. Ratings range from 60 MVA to
1000 MVAand the windings are rated for the networks which they link, such as 33,
66, 132, 275 and 400 kV in the UK, or voltages up to 500 kV or 800 kV in other
countries. The impedance of a transmission transformer is usually 18 per cent in the
UK, or 8 per cent in continental Europe, but for some system conditions, an impedance
of up to 30 per cent is used.
Transmission transformers are oil filled, and are usually fitted with oil pumps and
radiator fans to assist cooling of the windings and cores. They are usually fitted with
OLTCs, but some networks at 400 kV and 275 kV are linked by transformers without
regulating windings. The core and windings of a five-legged transmission transformer
rated at 1000 MVA and 400 kVl275 kV111 kV are shown in Fig. 6.18.
Fig. 6.18 Core and windings of a 1000 MVA, 400/275 kV transmission transformer
6.3.6 Generator (or step-up) transformers
Power is usually generated in large power stations at typically 18-20 kV, and generator
transformers are used to step up this voltage to the system voltage level. These
transformers are usually rated at 400, 500, 630, 800 or 1000 MVA.
Base-load power stations usually operate continuously at full load, and the power
is transformed to the highest operating voltage in one step. Combined-cycle gas
turbine stations may be embedded in the transmission and distribution system at
lower operating voltages such as 132 kV.
Generator transformers are usually fitted with regulating windings and OLTCs.
Newnes Electrical Engineer's Handbook
6.3.7
131
Phase-shifting transformers
Where power is transmitted along two or more parallel transmission lines, the power
flow divides between the lines in inverse proportion to the line impedances. Higher
power is therefore transmitted through the line with lowest impedance and this can
result in overload on that line, when the parallel line is only partly loaded. Phaseshifting transformers are used to link two parallel lines and to control power flow by
injecting a voltage 90" out of phase (in quadrature) with the system voltage into one
line, at either leading or lagging power factor. Where the transformer controls the
phase angle but not the voltage, the unit is known as a quadrature booster. Where the
voltage is also controlled, the unit is known as aphase-shifting transformel: Fig. 6.19
shows a 2000 MVA, 400 kV quadrature booster transformer on site; the unit is split
between two tanks in order to meet construction limitations of size and weight.
Fig. 6.19 2000 MVA 400 kV quadrature booster transformer in two tanks on site
6.3.8 Converter transformers
Where power is transmitted through an HVDC system, a converter station is used to
change ac power to dc using multiple rectifier bridges. DC power is converted back
to ac using inverter bridges. Converter transformers handle ac power and power at
mixed acldc voltages by combining the power flow through 12 phases of rectifier/
inverter bridges through dc valve windings.
The insulation structure must withstand all normal and abnormal conditions when
ac voltage is mixed with dc voltage of differing polarities over the operating temperature
range. The presence of dc currents may also cause dc saturation of the core, leading
to abnormal magnetizing currents and variations in sound.
A phase of a three-phase converter transformer bank typically comprises a highvoltage primary winding and two secondary acldc valve windings. Three such
132
Transformers
+ 215 kV
I
DC system
3 x 1 phase
transformer
bank
Lower level
valve group
Lower level
valve group
3 x 1 phase
transformer
bank
IIL
- 215 kV
Fig. 6.20 Schematic diagram of acldc transmission system
Newnes Electrical Engineer’s Handbook
133
transformers together form the two secondary three-phase systems; one is connected
in delta and the other in star. Each secondary system feeds a six-pulse bridge and the
two bridges are connected in series to form a 12-pulse arrangement, as shown
schematically in Fig. 6.20. Two such transformer banks are used with the secondary
circuits connected in opposite polarity to form a +/-215 kV dc transmission system.
6.3.9
Railway transformers
Transformers for railway applications may be trackside units to supply power to the
track, or on-board transformers in the locomotive or under the coaches, to power the
drive motors.
Trackside transformersare subjected to uneven loading dependingupon the position
of the train in the railway system. On-board transformers are designed for the lowest
possible weight, resulting in a high-loss performance. Modem train control systems
using thyristors or GTOs subject the transformers to severe harmonic currents that
require special design consideration.
6.3.1 0 Rectifier and furnace transformers
Special consideration is needed for transformers in industrial applications involving
arc furnaces or heavy-current dc loads in electrochemicalplant. The primary windings
in such cases are usually rated at 33 kV or 132 kV in the UK, but the secondary
windings carry many thousands of amperes and ace rated at less than 1 kV. Current
sharing between parallel paths in the transformer becomes important because of the
magnetic fields created by the high currents. These strong magnetic fields can cause
excess heating in magnetic steels if these are used in the structure of the transformer,
because of the flow of proximity currents in the steel. To reduce this excess heating,
non-magnetic steel is often used to form part of the tank or the cover.
The OLTCs in furnace transformers are subject to a heavy duty; they may perform
hundreds of thousands of operating cycles a year, which is more than a lifetime’s
duty for many transmission transformers.
6.3.11
Dry-type transformers
A dry-type construction is possible where a higher-temperature class of insulation is
required than is offered by cellulose and a class ‘0’or class ‘ K fluid. Dry-type
transformers use non-cellulosic solid insulation and the windings may be varnish
dipped to provide a class ‘C’ capability, or vacuum encapsulated in epoxy resin to
form a class T‘ or class ‘H’ system. Ratings can be up to 20 MVA at voltages up to
36 kV. Overload performance can be provided by fans.
This type of transformer is more expensive than a fluid-filled equivalent, and
because of the reduced fire risk they are used in special applications where the public
are involved, such as underground tunnels, residential blocks of flats or oil rigs.
A typical cast-resin transformer rated 2500 kVA, 11 O00/440 V is shown in Fig.
6.21.
6.3.1 2 Gas-filled transformers
For applications where low flammability is paramount, designs have been developed
in which the transformer is insulated and cooled with SF6 gas. This provides an
alternative to dry-type construction, but it has only been used on a limited scale.
High-voltage SF6 transformers are available at high cost, but this may be justifiable
Transformers
134
Fig. 6.21 Dry-type 2500 kVA, 11 000/440V transformer with cast-resin encapsulation
in cases of high land values, where the overall ‘footprint’ of the unit can be reduced
by the elimination of fire-fighting equipment.
6.4
6.4.1
Rating principles
Rated power
The rated power of each winding in a transformer refers to a continuous loading and
it is a reference value for guarantees and tests concerning load losses and temperature
rise. Where a transformer winding has different values of apparent power under
different cooling conditions, the highest value is defined as the rated power. A twowinding transformer is given only one value of rated power, which is identical for
both windings, but a multi-winding transformer may have reduced levels of rated
power on auxiliary windings.
When a transformer has rated voltage applied to the primary winding and rated
current flows through the secondary winding, the transformer is carrying rated power
for that set of windings. This definition of rated power, used by IEC, implies that the
rated power is the value of power input to the transformer, including its own absorption
of active and reactive power. The secondary voltage under full load differs from the
rated voltage by the voltage drop in the transformer.
In North America a different definition is adopted in ANSUIEEE standard C57.1200.
Here, rated power is based on the output that can be delivered at rated secondary
voltage, and allowance must be made so that the necessary primary voltage can be
applied to the transformer. The difference between basing rated power on loaded
voltages (IEC) or open-circuit voltages (ANSUIEEE) is significant.
Newnes Electrical Engineer's Handbook
6.4.2
135
Overloading
Although transformers are rated for continuous operation, it is possible to supply
overloads for limited periods. The analysis of overloading profiles is based on the
deterioration of cellulose. At temperatures above the rated temperature, cellulose
degrades at a faster rate, and the load cycle should therefore include a period of
operation at lower temperature to balance effects of these faster ageing rates at
higher temperature. In general, the ageing rate doubles for every 8°C rise in temperature.
When the ambient temperature is below the rated ambient temperature it is also
permissible to overload the transformer, the limit of the overloading being the operating
temperature of the winding. Under severe operational pressures it is sometimes
necessary to overload a transformer well beyond its nameplate rating, but such an
operation will usually result in shorter life. IEC 354 provides a guide to overloading
of oil-filled transformers based on these principles.
A maximum winding temperature of 140°C should not be exceeded even under
emergency conditions, because free gas can be produced by cellulose at temperatures
above this, and gas bubbles may cause dielectric failure of the windings.
6.4.3
Parallel operation of transformers
For operational reasons it may be necessary to operate transformers connected directly
in parallel. For successful parallel operation, the transformers must have:
0
0
0
the same voltage ratings and ratio (with some tolerance)
the same phase-angle relationship (clock-hour number)
the same percentage impedance (with some tolerance)
It is not advisable to parallel operate transformers with widely different power ratings
as the natural impedance for optimal design varies with the rating of the transformer.
The power divides between parallel-connected transformers in a relationship which
is inversely proportional to their impedances; a low-impedance transformer operated
in parallel with a higher-impedance unit will pass the greater part of the power and
may be overloaded. A mismatch in loading of up to 10 per cent is normally acceptable.
6.5 Test methods
6.5.1
Specification testing
Transformers for power supply units and isolating transformers are subject to production
testing, where compliance is generally checked by inspection and electrical tests.
The elecmcal tests establish earthing continuity (for class I), no-load output voltage,
dielectric strength between live parts of the input circuit and accessible conducting
parts of the transformer, and dielectric strength between input and output circuits.
Tests on larger transformers are carried out as part of the manufacturing process
to ensure that transformers meet the characteristics specified by the purchaser. The
tests prescribed by IEC are grouped in three categories, these being routine tests,
type tests and special tests:
0
0
0
routine tests, to be carried out on all transformers
type rests, to be carried out on new designs or the first unit of a contract
special tests, to be carried out at the specific request of the purchaser
Transformers
136
(a) Routine rests
These are carried out on all transformers and include:
measurement of winding resistance using a dc measurement circuit or a bridge
measurement of voltage ratio and check of phase displacement. The voltage
ratio is checked on each tap position and the connection symbol of three-phase
transformers is confirmed.
measurement of short-circuit impedance and load loss. The short-circuit
impedance (including reactance and ac resistance in series) is determined by
measuring the primary voltage necessary to circulate full-load current with the
secondary terminals short-circuited. The load loss (copper loss) is measured
using the same circuit.
measurement of no-load loss and magnetizing current, at rated voltage and
frequency. This is measured on one set of windings with the other windings
open-circuit.Because the power factor at no-load may be between 0.1 and 0.2,
special low power factor wattmeters are necessary to ensure accuracy.A threewattmeter connection is used for load loss measurement of three-phase
transformers,because the previously used two-wattmeter method is inaccurate.
dielectrictesls, consisting of applied voltage and induced voltage tests, sometimes
linked with a switching surge test. The applied voltage test verifies the integrity
of the winding insulation to earth and between windings, and the induced
voltage test verifies the insulation between turns and between windings. The
general requirements set down by E C are shown in Table 6.2. For transformers
rated up to 300 kV, the induced voltage test is for one minute at an overvoltage
of between 2.5 and 3.5 times rated voltage, carried out at a higher frequency
to avoid core saturation. For rated voltages over 300 kV,the induced test is for
30 minutes at 1.3 or 1.5 times rated voltage, with an initial short period at a
higher voltage to initiate activity in any vulnerable partial discharge sites; this
is linked with a partial discharge test. This long-term test will not prove the
insulation against switching overvoltages, and so for this it must be linked to
a switching impulse test.
tests on OLTCs to confirm the timing of the tapchanger mechanism and to
prove the capability on full-voltage and full-current operation.
(b) Q p e tests
These are camed out on new designs, or on the first unit of a contract. They include:
a temperature rise test using the normal cooling equipment to verify the
temperature rise of the windings under full-load or overload conditions. For
smaller transformers it may be possible to carry out the temperature rise test
at full voltage by supplying a suitable load. For larger transformers it is more
usual to supply the total losses (load loss and no-load loss) under short-circuit
conditions and to establish the top oil temperature and the average winding
temperature at a lower voltage. Where a number of transformers are available,
it is possible to supply them in a ‘back-to-back’ connection and to make the
test at full voltage and full current. The test is carried out over a period of 8
to 24 hours to establish steady-state temperatures, and it may be associated
with Dissolved Gas Analysis (DGA) of oil samples taken during the test to
identify possible evidence of microdeteriorationin the insulation at high local
temperatures.
Newnes Electrical Engineer’s Handbook
137
Table 6.2 Dielectric test values for transformers
Rated withstand voltages for windings of transformers e300 kV
Highest voltage for
equlpment
kV (rms)
Rated short duration power
frequency voltage test
kV (rms)
51.1
3.6
7.2
12
24
36
72.5
145
245
3
10
20
28
50
70
140
230 or 275
325 or 360 or 395
Rated lightning impulse
voltage test
kV (peak)
20 or 40
40 or 60
60 or 75
95 or 125
145 or 170
325
550 or 650
750 or 850 or 950
Rated withstand voltages for windings of transformers 2300 kV, in conjunction with a 30 m test
at 1 . 3 ~or 1 . 5 ~rated voltage
________
~~
Highest voltage for
equipment
kV (rms)
Rated switching impulse
wlthstand voltage
kV (peak)
300
420
750 or 850
950 or 1050
525
765
1175
1425 or 1550
Rated lightning impulse
withstand voltage
kV (peak)
850 or 950 or 1050
1050 or 1175 or 1300
or 1425
1425 or 1550
1550 or 1800 or 1950
________~
~
Notes: 1 In general the switching impulse test level is 83% of the lightning impulse test level.
2 These test levels are not applicable in North America.
J
a lightning impulse voltage test to prove performanceunder atmosphericlightning
conditions. The specified impulse waveform has a 1 ps front time (time-tocrest) and a 50 ps tail time (time from crest to half-crest value). The winding
is tested at a level prescribed by IEC which is linked to the rated voltage. This
is set out in Table 6.2. One test application is made at 75 per cent of the test
value (Reduced Full Wave, or RFW) and this is followed by three test applications
at full test value (FW).The test is considered satisfactory if there are no
discrepanciesbetween the recorded oscillograms of applied voltage and between
the recorded oscillograms of neutral current.
(c) Special tests
These are carried out at the specific request of the purchaser. The more usual special
tests are:
chopped-impulse wave test. The purchaser may wish to simulate the situation
when a lightning surge propagated along a transmission line is chopped to
earth by a lightning protection rod gap mounted on the bushing. The Chop
Wave (CW) is specified as an F W waveform with a chop to earth occurring
between 3 and 7 seconds after the crest. The chop-wave test is combined with
the lightning impulse voltage test and the test sequence is 1-RFW. 1-FW, 2CW and 2-FW test applications, sometimes followed by a further RFW for
Transformers
138
0
0
e
6.5.2
comparison purposes. A successful test shows no discrepanciesbetween applied
voltage oscillograms or between neutral current oscillograms, although if rod
gaps are used to perform the chopped-wave test there may be acceptable
deviations between neutral current oscillograms after the time of chop. If a
triggered gap is used to control time of chop the oscillograms should be
identical.
determinationof zero-sequence impedance, measured between the line terminals
of a star-connected or zigzag-connected winding connected together, and its
neutral terminal. The impedance is expressed in ohms per phase, and it is a
measure of the impedance presented by the transformer to a three-phase fault
to earth.
short-circuit withstand test to verify the integrity of the transformer under
through-fault conditions. IEC recommendationsallow the withstand performance
of a winding to be demonstrated by calculation or by test. The resource needed
to test fully a large power transformer is substantial, and few test facilities are
available in the world.
determination of sound levels. The sound produced by a transformer is
predominantly due to magnetostriction in the core and is related to core flux
density. The sound level can be measured in terms of sound power, sound
pressure or sound intensity. The last method was developed especially for
measurement of low sound levels in the presence of high sound ambients. In
transformers which have been designed with low flux density to reduce the
magnetostrictioneffect, the predominant sound may be produced by the windings
as a result of movements caused by the load currents. This load-current generated
sound should be measured by the sound intensity method.
In-service testing
Two types of in-service testing are used. Surveillancetesting involves periodic checks,
and condition monitoring offers a continuous check.
(a) Swveillance testing - oil samples
When transformers are in operation many users carry out surveillance testing to
monitor operation. The most simple tests are carried out on oil samples taken on a
regular basis. Measurement of oil properties such as breakdown voltage, water content,
acidity, dielectric loss angle, volume resistivity and particle content all give valuable
information on the state of the transformer. DGA gives early warning of deterioration
due to electricalor thermal causes, particularly sparking, arcing and service overheating.
Analysis of the oil by High-Performance Liquid Chromatography (HPLC) may detect
the presence of furanes or furfuranes which will provide further information on
moderate overheating of the insulation.
(b) Condition monitoring
Sensors can be built into the transformer so that parameters can be monitored on a
continuous basis. The parameters which are typically monitored are winding
temperature, tank temperature, water content, dissolved hydrogen, load current and
voltage transients. The data collection system may simply gather and analyse the
information, or it may be arranged to operate alarms or actuate disconnections under
specified conditions and limits which represent an emergency.
Newnes Electrical Engineer’s Handbook
6.6
6.6.1
139
Commissioning, maintenance and repair
Commissioning
Power transformers can be transported to site complete with oil, bushings, tapchangers
and cooling equipment. It is then a relatively simple matter to lift them onto a pole
or plinth and connect them into the system.
Large transformers are subject to weight restrictions and size limitations when
they are moved by road or rail and it is necessary to remove the oil, bushings, cooling
equipment and other accessories to meet these limitations. Very large transformers
are usually carried on custom-built transporters, such as the 112-wheel transporter
shown in Fig. 6.22; this is carrying a 1000 MVA, 400 kV transmission transformer
that has been dismantled to meet a 400 tonne road weight restriction. Once a transformer
of this size arrives on site, it must be lifted or jacked onto its plinth for re-erection.
In some cases with restricted space it may be necessary to use special techniques
such as water skates to manoeuvre the transformer into position.
Fig. 6.22 Road transporter carrying a 1000 MVA, 400 kV transmission transformer
When the transformer has been erected and the oil filled and reprocessed, it is
necessary to carry out commissioning tests to check that all electrical connections
have been correctly made and that no deterioration has occurred in the insulation
system. These commissioning tests are selected from the routine tests and usually
include winding resistance and ratio, magnetizing current at 440 V, and analysis of
oil samples to establish breakdown strength, water content and total gas content.
Transformers
140
6.6.2
Maintenance
Transformersrequire little maintenance in service, apart from regular inspection and
servicing of the OLTC mechanism. The diverter contacts experience significant wear
due to arcing, and they must be replaced at regular intervals which are determined by
the operating regime. For furnace transformers it may be advisable to filter the oil
regularly in a diverter compartment in order to remove carbon particles and maintain
the electrical strength.
The usual method of protecting the oil breather system in small transformers is to
use silicone gel breathers to dry incoming air; in larger transformers refrigerated
breathers continuously dry the air in a conservator. Regular maintenance (at least
once a month) is necessary to maintain a silica gel breather in efficient working
order.
If oil samples indicate a high water content then it may be necessary to dry the oil
using a heating-vacuum process. This also indicates a high water content in the paper
insulation and it may be necessary to redry the windings by applying a heating and
vacuum cycle on site, or to return the transformer to the manufacturer for reprocessing
or refurbishment.
6.6.3
Diagnostics and repair
In the event of a failure, the user must first decide whether to repair or replace the
transformer. Where small transformers are involved, it is usually more economic to
replace the unit. In order to reach a decision, it is usually necessary to cany out
diagnostic tests to identify the number of faults and their location. Diagnostic tests
may include the surveillance tests referred to in section 6.5.2, and it may also be
decided to use acoustic location devices to identify a sparking site, low-voltage
impulse tests to identify a winding fault and fquency response analysis of a winding
to an applied square wave to detect winding conductor displacement.
If the fault is in a winding, it usually requires either replacement of the winding
in a repair workshop or rewinding by the manufacturer, but many faults external to
the windings such as connection or core faults can be corrected on site.
Where a repair can be undertaken on site it is essential to maintain dry conditions
in the transformer by continual purging using dry air. Any material taken into the
tank must be fully processed and a careful log should be maintained of all materials
taken into and brought out of the tank.
When a repair is completed, the transformer must be redried and reimpregnated,
and the necessary tests carried out to verify that the transformer can be returned to
service in good condition.
6.7 Standards
The performance requirements of power transformers are covered by a range of
international, regional and national standards. At the highest level are the
recommendationspublished by the InternationalElectrotechnicalCommission (IEC).
These are performance standards, but they are not mandatory unless referred to in a
contract.
Regional standards in Europe are Euro-Norms (ENS)
or Harmonized Documents
(HDs) published by the European Committee for Electrotechnical Standardization
(CENELEC).CENELEC standards are part of European law, ENSmust be transposed
Newnes Electrical Engineer's Handbook
141
into national standards and no national standard may conflict with an HD. Many ENS
and HDs are based on E C recommendations,but some have been specifically prepared
to match European legislation requirements such as EU Directives.
National standards in the UK are published by the British Standards Institution
(BSI). BSI standards are generally identical to IEC or CENELEC standards, but
some BS standards address issues not covered by IEC or CENELEC.
In North America the main regional standards are published by the American
National Standards Institute (ANSI) in conjunction with the Institute of Electrical
and Electronic Engineers (IEEE). ANSYIEEE standards are generally different from
E C recommendations, but the two are becoming closer as a result of international
harmonization following GATT treaties on international trade.
The main transformer standards in IEC, CENELEC and BSI are shown in Table
6.3, together with their interrelationship. Where appropriate the equivalent ANSI/
IEEE standard is also referenced.
Table 6.3 Comparison of internalionlil. regional and national standards for timisfomem
IEC
EWHD
BS
Subject of standard
ANSIAEEE
60076
60076-1
60076-2
60076-3
60076-4
60076-5
60076-6
60076-7
60076-8
60076-9
60076-10
60076-11
60076-12
60137
HD 398
EN 60076-1
EN 60076-2
HD 398.3
BS 171
BSEN 60076-1
BSEN 60076-2
BS 171-3
Power transformers
: general
: temperature rise
: dielectric tests
: guide for lightning impulse testing
: ability to withstand short-circuit
: reactors
: loading guide -oil filled
: application guide
: terminals and tapping markings
: determination of sound levels
: dry-type power transformers
: loading guide -dry type
Insulated bushings for ac voltages above 1 kV
Bushings for liquid-filled transformers 1 kV to 36 kV
Tap-changers
: for power transformers
: application guide
Converter transformers
: transformers for industrial applications
Transformers and inductors for electronics and telecommunications
Safety of power transformers, power supply units and similar
Oil-filled distribution transformers
: general to 24 kV
: transformers with cable boxes
: transformers for 36 kV
: harmonic currents
Dry-type distribution transformers
: general to 24 kV
: transformers for 36 kV
Safety requirements for transformers that may be stationary or portable, single
phase or polyphase
C.57.12.00
-
602 14
60214-1
60214-2
61378
61378-1
61248
61558
-
-
-
-
-
-
HD 398.5
EN 60289
BS 171-5
BSEN 60289
BS 7735
855953
-
-
EN 60551
HD 464
BSEN 60551
BS 7806
-
-
EN 60137
HD 506
BSEN 60137
BS 7616
EN 60214
BSEN 60214
-
-
HD 428
HD 428-1
HD 428-2
HD 428-3
HD 428-4
HD 538
HD 538.1
HD 538.2
BS 7281
BS 7281-1
BS 7281-2
BS 7281-3
BS 7281-4
BS 7844
BS 7844-1
BS 7844-2
BS 3535
C.57.12.90
C.57.12.90
C.57.98
C.57.12.90
C.57.21
C.57.92
C.57.12.90
C.57.12.90
C.57.12.01
C.57.96
C.57.19.00
C.57.131
C.57.12.22
C.57.12.51
Chapter 7
Switchgear
Mr N.P. Allen
GEC Alsthom Low Voltage Equipment Ltd
Mr T. Harris
ALSTOM T&D Distribution Switchgear
Mr C.J. Jones
VA TECH Reyrolle Ltd
7.1
Introduction
Switchgear is used to connect and disconnect electric power supplies and systems. It
is a general term which covers the switchingdevice and its combination with associated
control, measuring, protective and regulating equipment, together with accessories,
enclosures and supporting structures.
Switchgear is applied in electrical circuits and systems from low voltage, such as
domestic 220/240 V applications, right up to transmission networks up to 1100 kV.
To meet this range of voltages and powers, a wide range of technology is needed, and
this chapter has been split into the main three subdivisions of low voltage, medium
voltage (distribution) and high voltage (transmission) in order to deal with this wide
range.
The main classes of equipment are:
disconnectors, or isolators
switches
fuse-switch combinations
circuit breakers
earthing switches
A disconnecror is a mechanical switching device which in the open position provides
a safe working gap in the electrical system, to withstand normal working system
voltage and any overvoltages which m a y occur. It is able to open or close a circuit if
a negligible current is switched, or if no significant change occurs in the voltage
between the terminals of the poles. Currents can be carried for specified times in
normal operation and under abnormal conditions.
A w i t c h is a mechanical device which is able not only to make, carry and interrupt
current occurring under normal conditions in a system, but also to close a circuit
safely, even if a fault is present. It must therefore be able to close satisfactorily
Switchgear
144
carrying a peak current corresponding to the short-circuit fault level, and it must be
able to carry this fault current for a specified period, usually one or three seconds.
A fuse and a switch can be used in combination with ratings chosen so that the
fuse operates at currents in excess of the rated interrupting or breaking capacity of
the switch. Such a device is known as a yuse switch’ if the fuseholder is also used as
part of the main moving contact assembly, or a ‘switchfuse’ if the fuse is a separate
and static part of an assembly which includes the switch connected in series.
Figure 7.1 shows schematically the various combinations of fuse, switch and
disconnector that are available.
Switch
I Disconnector
Switch-disconnector
Switch-fuse
Disconnector-fuse
Switch-disconnector-fuse
Fuse-switch
I Fuse-disconnector
Fuse-switch-disconnector
Fig. 7.1 Summary of equipment definitions
A circuit breaker is a mechanical’switchingdevice which is not only able to make,
carry and interrupt currents occurring in the system under normal conditions, but
also to carry for a specified time and to make and interrupt currents arising in the
system under defined abnormal conditions, such as short-circuits. It experiences the
most onerous of all the switching duties and is a key device in many switching and
protection systems.
An earthing switch is a mechanical device for the earthing and short-circuiting of
circuits. It is able to withstand currents for a specified time under abnormal conditions,
but it is not required to carry normal service current. An earthing switch may also
have a short-circuit making capacity.
7.2
Principles of operation
Since they perform the most arduous duty, circuit breakers are used here for the
following brief description of switchgear operating principles.
The heart of a circuit breaker is the contact system which comprises a set of
moving contacts, a set of fixed contacts, their current carrying conductors or leads
and an opening mechanism which is often spring loaded.
The fixed and moving contacts are normally of copper, with sufficient size and
cross-section to carry the rated current continuously. Attached or plated onto the
copper are the contact tips or faces which are of silver or an alloy. The contact tip or
face is the point at which the fixed and moving contacts touch, enabling the current
to flow. It is therefore a requirement not only that a low contact junction resistance
is maintained, but also that the contacts are not welded, destroyed or unduly eroded
by the high thermal and dynamic stresses of a short-circuit.
Newnes Electrical Engineer's Handbook
145
As the device closes, the contact faces are forced together by spring or other
pressure generated from the mechanism. This pressure is required in order to reduce
the contact junction resistance and therefore the ohmic heating at the contacts; it also
assists with the destruction or compaction of foreign material such as oxides which
may contaminate the contact faces. The closing process is further assisted by arranging
the geometry to give a wiping action as the contacts come together; the wiping helps
to ensure that points of purely metallic contact are formed.
The breaking or interruption action is made particularly difficult by the formation
of an arc as the contacts separate. The arc is normally extinguished as the current
reaches a natural zero in the ac cycle; this mechanism is assisted by drawing the arc
out to maximum length, therefore increasing its resistance and limiting the arc current.
Various techniques are adopted to extend the arc; these differ according to size,
rating and application and this is covered in more detail in the following sections.
The interruption of a resistive load current is not usually a problem. In this case
the power factor is close to unity. When the circuit breaker contacts open, draw an
arc and interrupt the current, the voltage rises slowly from its zero to its peak following
its natural 50 Hz or 60 Hz shape. The build-up of voltage across the opening contacts
is therefore relatively gentle, and it can be sustained as the contact gap increases to
its fully open point.
However, in many circuits the inductive component of current is much higher
than the resistive component. If a short-circuit occurs near the circuit breaker, not
only is the fault impedance very low, and the fault current at its highest, but also the
power factor may be very low, often below 0.1, and the current and voltage are nearly
90" out of phase. As the contacts open and the current is extinguished at its zero
point, the voltage tends instantaneously to rise to its peak value. This is illustrated in
Fig. 7.2. This results in a high rate of rise of voltage across the contacts, aiming for
a peak transient recovery voltage which is considerably higher than the normal peak
system voltage. There is a risk under these circumstances that the arc will restrike
even though the contacts are separating, and the design of the circuit breaker must
take this into account.
7.3 Low-voltage switchgear
7.3.1
Switches, disconnectors, switch disconnectors and
fuse combination units
(a) Construction and operation
These switches are categorized by the ability to make and break current to particular
duties which are listed for ac applications in Table 7.1. The category depends upon
a multiple of the normal operating current and an associated power factor.
In all these devices, the contacts are usually of silver-plated copper; it is unusual
to find the alloy contact tips used in circuit breakers. The contact system, connecting
copperwork and terminals are usually housed within a housing of thermoset plastic
or of a thermoplastic with a high temperature capability. The mechanism of the
switch may be located in a number of positions, but it is usual to mount it at the side
of or below the contact system.
The switching devices may be fitted with auxiliaries which normally take the
form of small switches attached to the main device. These auxiliary switches provide
means of indication and monitoring the position of the switch.
Switchgear
146
L
kHz
superimposed onto 50 Hz supply voltage
Fig. 7.2 Circuit breaker recovery voltage in an inductive circuit
Table 7.1 Categories of duty and overload currentratings for switches, disconnectors, switch diSCOMeCtOrS
and fuse combination units
Category of duty
% of full-load current rating
making
AC21
AC22
AC23
150%
300%
1000%
Application
breaklng
150%
300%
800%
resistive loads
resistive and inductive loads
highly inductive loads, stalled
motor conditions
There are switching devices that are motor assisted to open and close the contacts,
and some have tripping devices incorporated into the mechanism to open the switch
under circuit conditions outside the protective characteristics of the fuse.
Many types of fuselink can be accommodated on a switch fuse or a fuse switch.
In the UK, the bolted type is probably the most common, followed by the 'clip-in'
type. There are many designs available to hold the clip-in fuselink within the switch
moulding. In continental Europe the DIN type fuselink is the most common; here the
fuse tags are formed as knife blades to fit into spring-tensioned clips.
The construction of afuse switch is illustrated generally in Fig. 7.3. It consists of
a moving carriage containing the fuselinks, which is switched into and out of a fixed
system containing the contacts and mechanism. The moving carriage is shown in
Fig. 7.4;it consists of a plastic and steel frameworkwith copper contacts and terminals
Newnes Electrical Engineer’s Handbook
Fig. 7.3 Main features of a fuse switch
Fig. 7.4 Bill Sovereign fuse switch disconnector (courtesy of Bill Switchgear)
147
Switchgear
148
onto which are mounted the fuselinks. This is normally a three-pole or four-pole
construction. It is usual to be able to extract the moving carriage to facilitate the
changing of fuselinks outside the switch. Removal of the carriage also ensures total
isolation of the load from the supply. The moving camage is moved in and out by a
spring-charged mechanism that ensures a quick make and break of the contacts.
In a switch-fuse the fuselinks remain static; the simplest form is a knife blade
contact in series with a fuselink. In the modem form of switch fuse the fuselinks are
mounted on a plastic enclosure which contains the contacts and the mechanism to
operate these contacts. The contacts are normally arranged on both load side and
supply side of the fuselink in order to ensure that the fuse is isolated when the switch
is in the off position. The contact system is driven by a spring-assisted mechanism
which is usually designed to provide quick make and break operation independentof
the speed at which the operator moves the on-off handle.
In its simplest form, the switch is a knife blade contact, but the modem switch
would follow the form of the fuse switch without the fuselinks. Many manufacturers
use a fuse switch or switch fuse with solid copper links in place of the fuselinks.
The disconnector also follows the constructionof the switch,but it must be capable
of providing completeisolation when the contacts are open. This usually requires the
provision of adequate clearance and creepage distances between live parts.
The switch disconnector combines the properties of a switch and a disconnector;
this is in most cases what a manufacturer would build for sale.
(b) Standards and testing
Switches, disconnectors, switch disconnectorsand fuse combinationunits are designed
to comply with the requirements of IEC 947-3 or EN 60947-3. This requires a
combination of type tests and routine tests.
A sequence of type tests is required to prove compliance with the standard. This
sequence includes the following key tests:
0
the making and breaking switching capacity test, which shows that the device
is suitable for extreme overload conditions of resistive, inductive or highly
inductive loads. The categories of duty and the current overload ratings have
been shown in Table 7.1. The test is carried out by operating the device a
number of times at the assigned rating.
temperature-rise verification. This is carried out at the highest current rating
of the device to prove that under full-load conditions in service the device will
not damage cables, terminals and insulating materials or put operators at risk
through contact with hot accessible parts. The limits of acceptabletemperature
rise are stipulated in the standard.
the operational performance test. This is conducted to prove the mechanical
and electrical durability of the device. A number of on-load and off-load
switching operations are made, depending upon the makebreak duty assigned
by the manufacturer.
dielectric verification to prove that the device has completed the sequence
without damage to its insulation system.
A fuse switch or switch fuse is also proven by a fuse-protectedshort-circuitwithstand
(breaking) test. In this test, the fuselinks used have to be of the maximum rating, and
with a breaking capacity assigned by the manufacturer. No damage such as welding
of the contacts must occur to the switch as a result of this test. A fuse-protected
Newnes Electrical Engineer’s Handbook
149
making test is also carried out in which the switch is closed onto the declared rated
short-circuit current.
A disconnector has to provide isolation properties and additional type tests are
performed to prove these. In particular, a leakage current test is conducted after the
main test sequence; maximum levels of acceptable leakage current are specified. In
addition, the isolator handle is subjected to a force of three times the normal operation
force necessary to switch the device off (within given minimum and maximum
limits), the contacts of one phase being artificially locked in the off position and the
test force being applied to open the switch. The on-off indicators of the disconnector
must not give a false indication during and after this test.
Routine tests to be applied to all switch devices include an operational check in
which each device is operated five times to check mechanical integrity. Also a
dielectric test is carried out at a voltage which depends upon the rated voltage of the
device. For a rated operating voltage of 380/415 V the dielectric test voltage is 2500 V.
7.3.2
Air circuit breakers and moulded case circuit breakers
(a) Construction and operation
Both the air circuit breaker and the moulded case circuit breaker (mccb) comprise the
following features:
a contact system with arc-quenching and current-limiting means
a mechanism to open and close the contacts
auxiliaries which provide additional means of protection and indication of the
switch positions
The modem air circuit breaker is generally used as an incoming device on the supply
side of a low voltage switchboard, and it represents the first line of protection on the
load side of the transformer. An example is shown in Fig. 7.5. In addition to the
above features, it also includes:
a tripping and protection system to open the circuit breaker under fault conditions
(if required)
a means of isolating the device from the busbars
usually an open construction, or the contact system housed in a plastic moulding
current ratings from 400 A to 6300 A
The mccb may be used as an incoming device, but it is more generally used as an
outgoing device on the load side of a switchboard. It is normally mounted into a lowvoltage switchboard or a purpose-designed panel board. In addition to the three
features listed at the start of this section, it also includes:
0
an electronic or thermaVelectromagnetictrip sensing system to operate through
the tripping mechanism and open the circuit breaker under overload or fault
conditions
all parts housed within a plastic moulded housing made in two halves
current ratings usually from 10 A to 1600 A
The basis of the main contact system has been explained in section 7.3.l(aj. The
fixed contacts are usually mounted on a back panel or within a plastic moulding, and
the moving contacts are usually supported on an insulated bar or within an insulated
carriage.
Switchgear
150
Fig. 7.5
Air circuit breaker (courtesy of Terasaki (Europe) Ltd
In addition to these main contacts, arcing contacts are provided. The arcing contacts
generally comprise a silver-tungsten alloy which provides high electrical conductivity
with excellent wear against arc erosion and good anti-welding properties. The arc
contacts are positioned in such a way that they make first and break last during
opening and closing of the contacts. Their purpose is to ensure that any arcing takes
place on the arcing contacts before the main contacts touch; the arcing contacts can
then break once the main contacts have been forced together. During opening, the
sequence is reversed. This action protects the main contacts from damage by arcing.
Extinguishing of the arc is an important feature in all low-voltage switchgear. All
air circuit breakers have devices to extinguish the arc as quickly as possible. It is well
known that an arc, once formed, will tend to move away from its point of origin; by
forming the contact system carefully, the magnetic field generated by the current
flow is used to move the arc into an arcing chamber where its extinction is aided.
Within the arcing chamber are number of metal arc-splitter plates which normally
have a slot or ‘V’ shape cut into them in order to encourage the arc to run into the
arcing chamber. A typical arrangement is shown in Fig. 7.6. The plates are used to
Newnes Electrical Engineer’s Handbook
151
Fig. 7.6 Contact system and arc chute in an air circuit breaker
split the arc into a number of smaller arcs, having the effect of lengthening and
cooling the arc. The arc is then extinguished when the voltage drop along it is equal
to the voltage across the open contacts.
If the arc can be extinguished very quickly before the full prospective fault current
is reached, the maximum current passed through the circuit breaker can be limited.
This current limiting is achieved if the arc is extinguished within 10 ms of the start
of the fault, before the peak of the sinusoidal current waveform is reached. Many
designs have been tried in an attempt to achieve current limiting. Many of these
designs are based on forming the current-carrying conductors within the circuit
breaker in such a way that the electromagnetic forces created by the currents force
the arc into the arcing chamber very quickly. A particular development of this is to
form ‘blow-out’ coils from the current-carrying conductors.
The opening and closing mechanism of an air circuit breaker generally uses the
energy developed in releasing a charged spring. The spring may be charged manually
or with the aid of a winding mechanism attached to a motor. The charge is usually
held until a release mechanism is activated, and once the spring energy is released to
the main mechanism, linkages are moved into an over-toggle mode to close the
moving contacts onto the fixed contacts. The force of the main spring is also used to
transfer energy into compressing the moving contact springs, and it is these springs
which provide the energy to open the contacts when required.
Attached to and interfacing with the main mechanism is the tripping mechanism
which is used to open the contacts. The means of tripping an air circuit breaker is a
coil, which may be a shunt trip coil, an undervoltage coil or a polarized trip coil. The
shunt trip coil is current operated and consists of a winding on a bobbin with a
moving core at its centre. When a current flows, the magnetic flux causes the core to
152
Switchgear
move and this core operates the trip mechanism. An undervoltage coil is similar in
construction, but the moving core is held in place by the magnetic flux against a
spring; if the voltage across the winding falls this allows the spring to release the
core and operate the trip mechanism. In the polarizing trip coil, the output of the coil
is used to nullify the magnetic field from a permanent magnet which is set into the
coil; when the coil is energized, the permanent magnetic field is overcome and using
a spring energy a moving core is released to operate the trip mechanism.
The release coil receives its signal from the overcurrent detection device, which
is usually an electronic tripping unit. This tripping unit has the capability to detect
overloads and short-circuits, and it may be able to detect earth fault currents. The
overload protection device detects an overload current and relates this to the time for
which the currenthas flowed before initiatinga trip signal; the time-current characteristic
which is followed here is determined by the manufacturer within the guidelines of
the standard. The general principles of time-current protection are explained more
fully in Chapter 8, but a typical time-current characteristicis shown for convenience
in Fig. 7.7.The short-circuit protection enables a high fault current to be detected
and the circuit breaker to be opened in less than 20 ms. The short-circuit protection
characteristics are usually adjustable in multiples of the full-load rated current, and
they may incorporate a time interval which would usually be less than a second.
Time-current characteristic-thermal magnetic
10 m
t(s)
1000
100
10
I
I
1
I
I
I
I
I
I
I
I
0.1
'-,
0.01
0.001
9
1
2
3 4 5
10
Notes: 1. Thermal release from cold
2. Thermal release from hot
Fig. 7.7 Typical time-current characteristic curves
20 30
50
100
1/1 r
Newnes Electrical Engineer’s Handbook
153
The mccb also incorporates an electronic trip device similar to that described
above, but this may not include all the sophistication of the air circuit breaker system.
Some mccbs will include an electromechanical thermal trip and electromagnetic trip
in place of the electronic trip unit. These trips are shown diagrammatically in Fig 7.8.
The electromechanical thermal trip consists of a heater and bimetal, the heater being
part of the main current-carrying circuit. In overload conditions the heater causes
deflection of the bimetal which is time related depending upon the current flow
through the heater. The bimetal deflection results in operation of the trip mechanism
and hence a time-current characteristic can be drawn for the circuit breaker. An
electromagnetic trip usually comprises a U-shaped coil around the current-carrying
circuit of the mccb, together with a moving pole piece. When a fault current passes,
the magnetic flux within the U-shaped coil pulls in the pole piece and this movement
is used to trip the circuit breaker.
(b) Testing and standards
Low-voltage air circuit breakers and mccbs are designed and tested according to the
requirements of IEC 947-2 or EN 60947-2.
A series of type tests is carried out to prove compliance with the standard. This
series of tests includes:
a short-circuit capability test. This determines the maximum short-circuit current
the device can withstand, while after the test retaining the ability to pass fullload current without overheating and allow the circuit breaker limited use.
This test is carried out in a short-circuit test laboratory able to deliver exactly
the current specified in the standard; large generators and transformers are
necessary for this work, making it expensive.
Fig. 7.8 Electromechanical and electromagnetic trips used in a mccb
154
Switchgear
a short-time withstand test. This establishes that the circuit breaker can carry
a short-circuit current for a period of time; in many cases this would be 50 kA
for 1 s. A circuit breaker with this capability can be used as an incoming
device to a distributionsystem. Based on the principles of co-ordination explained
in Chapter 8, this allows other devices within the distribution system to open
before the incoming circuit breaker, thereby not necessarily shutting off all of
that distribution system.
an endurance test. This determines the number of open-ciose operations the
circuit breaker can withstand before some failure occurs which means that the
device will no longer carry its rated current safely. The tests are carried out
with and without current flowing through the contacts.
opening during overload or fault conditions. This test verifies that the device
opens according to the parameters set by the standard and those given by the
manufacturer. It determines the tjme-current characteristic of the circuit breaker,
an example of which has been shown in Fig 7.7.
a temperature rise test. This is carried out after all the above type tests. The
standard specifies the maximum temperature rise permissible at the terminals
and ensures that the device will be safe to use after short-circuits have been
cleared and after a long period in service.
7.3.3
Miniature circuit breakers
Like the mccb, the miniature circuit breaker (mcb) has a contact system and means
of arc quenching, a mechanism and tripping and protection system to open the circuit
breaker under fault conditions.
The mcb has advanced considerably in the past 25 years. Early devices were
generally of the ‘zero-cutting’type, and during a short-circuit the current had to pass
through a zero before the arc was extinguished; this provided a short-circuit breaking
capacity of about 3 kA.Most of these early mcbs were housed in a bakelite moulding.
The modern mcb is a much smaller and more sophisticated device. All the recent
developments associated with moulded case circuit breakers have been incorporated
into mcbs to improve their performance, and with breaking capacities of 10 kA to
16 kA now availabIe, mcbs are used in all areas of commerce and industry as a
reliable means of protection.
Most mcbs are of single-pole construction for use in single-phase circuits. The
complete working system is housed within a plastic moulding, the typical external
appearance of which being shown in Fig. 7.9. A schematic showing the principal
parts of the mcb is shown in Fig. 7.10. The contact system comprises a fixed and a
moving contact, and attached to each is a contact tip which provides a low-resilience
contact junction to resist welding.
Modem mcbs are fitted with arc chutes consisting of metal plates which are held
in position by insulating material. The arc chute does not necessarily surround the
contact; in some designs arc runners are provided to pull the arc into the arc chute.
The tripping mechanism usually consists of a thermal-magnetic arrangement. The
thermal action is provided by a bimetal with, in some cases, a heater. For ratings in
the range 6-63 A the bimetal forms part of the current path, the heat generated within
the bimetal itself being sufficient to cause deflection. The deflection is then used to
activate the tripping mechanism. The characteristics of the bimetal are chosen to
provide particular delays under certain overload or fault currents according to the
required tirne-current characteristic.A high-resistance bimetal is used for low-current
Newnes Electrical Engineer’s Handbook
Fig. 7.9 Miniature circuit breaker, external view
Fig. 7.10 Principal parts of an mcb
155
Switchgear
156
devices and a lower-resistancebimetal for high-current devices. In very low-current
mcbs a heater may be incorporated around the bimetal in order to generate sufficient
heat to deflect it.
The magnetic tripping element usually consists of a coil which is wrapped around
a tube, there being a spring-loaded slug within the tube. Movement of the slug
operates the tripping mechanism to open the mcb. It can also be used to assist in
opening the contacts by locating the coil near to the moving contact. When a fault
current flows, the high magnetic field generated by the coil overcomes the spring
force holding the slug in position; the slug then moves to actuate the tripping mechanism
and forces the contacts apart by striking the moving contact arm,For low mcb ratings
the coil is formed from thin wire with many turns; for higher ratings the wire is
thicker, with fewer turns. The magnetic trip is set by the manufacturer according to
the required characteristics. These characteristics are defined in the standard and
form 'types' which are shown in Table 7.2.
MCBs are designed and tested according to the requirements of IEC 898.
Table 7.2 Magnetic trip settings for mcbs
MCB type
Mlnimum
Maximum
trip current trip current
B
31"
C
5
a
1or,
D
101"
201"
7.3.4
5 1"
Application
Domestic:
resistive or small inductive loads such as lighting
and socket outlets
Light industrial:
inductive loads such as fluorescent lighting and
motors
Very inductive loads such as welding machines
Residual current devices
The residual current device (rcd) is used to detect earth fault currents and to interrupt
supply if an earth current flows. The main application is to prevent electrocution but
rcds can also be used to protect equipment, especially against fire. The earth fault
currents that operate an rcd can range from 5 mA up to many amperes. For typical
domestic applications the typical trip current would be 30 mA.
The rcd can be opened and closed manually to switch normal load currents, and
it opens automatically when an earth fault current flows which is about 50 per cent
or more of the rated tripping current.
The main features of an rcd are shown in Fig 7.11. The key component is a
toroidal transformer, upon which the load current (live) and return current (neutral)
conductors are wound in opposite directions. The toroid also carries a detecting
winding. If no earth fault current is flowing then the load and return currents are
equal. In this case the mmfs generated by the load and return current windings are
equal; there is no resultant flux in the toroid and the detecting winding does not
generate any current. When a fault current flows there is a difference between the
load and return currents which generates a resultant flux in the toroid and induces a
current in the detecting winding. The current generated in the detecting winding
operates a relay which opens the main contacts of the rcd.
From a very small output the detecting winding has to produce sufficient power
to operate the tripping mechanism. Two alternative methods are used. In the fitst
method, the output signal from the detecting coil is electronically amplified and the
Newnes Electrical Engineer’s Handbook
157
Fig. 7.11 Residual current device
second method uses a polarized relay operating on a sensitive mechanical trip
mechanism. The operation of a polarized trip relay has already been described for
circuit breakers in section 7.3.2(a); it is based on the magnetic output of a small coil
nullifying the field from a permanent magnet, causing the release of an armature. The
basic operation is illustrated in Fig. 7.12.
The operation of an rcd has here been described for single-phase operation, but it
may also be applied in a three-phase application where typically it might be used in
a light industrial system for protection against fire. There are two arrangements of a
three-phase rcd. Either the three phases are wound around a current transformer, or
the three phases and the neutral are wound onto a balancing transformer.
The rcd has only limited breaking capacity and it is not a replacement for overcurrent
protection devices such as the mcb. The residual current breaker with overcurrent
(rcbo) is now available; this is an rcd with an overcurrent tripping mechanism and
enhanced contacts to cope with interruption of fault conditions.
RCDs are designed and tested according to the requirements of IEC 1008 and IEC
1009.
7.3.5
Standards
The leading international standards adopted for low-voltage switchgear are shown in
Table 7.3.
7.4
Medium-voltage (distribution) switchgear
Distribution switchgear, also commonly referred to as medium-voltage switchgear, is
generally acknowledged to cover the range 1 kV to 36 kV. 72.5 kV and even 132 kV
158
Switchgear
4
L
Armature
N
I
/
Flux from coil
Flux from magnet
L
N
Permanent
magnet
Fig. 7.12 Operation of polarized trip relay
Table 7.3 International standards Elating to low-voltage switchgear
IEC
ENMD
Subject of standard
IEC 898
IEC 347-2
IEC 947-3
IEC 1008
EN 60898
EN 60947-2
EN 60947-3
EN 61008
Miniature circuit breakers (mcbs)
Air circuit breakers(acbs) and moulded case circuit breakers(mccbs)
Switches - fuse combination switches
Residual current devices (rcds)
can be considered as distribution voltages rather than transmission voltages, and the
equipment overlaps with high-voltage or transmission switchgear in this range.
Switchgearin the dismbution-voltagerange differs considerably from low-voltage
switchgear. There are similarities with aspects of transmission switchgear,but many
functions and practices are different.
7.4.1 Types of circuit breaker
In the past, oil- and air-break devices have predominated, but now several alternative
methods of arc interruption are used in distribution-voltage circuit breakers.
Early oil designs featuredplain-break contactsin a tank of oil capable of withstanding
the considerablepressure built up from large quantities of gas generated by long arcs.
During the 1920s and 1930s various designs of arc control device were introduced to
improve performance. These were designed such that the arc created between the
contacts produces enough energy to break down the oil molecules, generating gases
and vapours which by the cooling and de-ionizing of the arc resulted in successful
clearance at current zero. During interruption, the arc control device encloses the
contacts, the arc, a gas bubble and oil. Carefully designed vents allow the gas to
escape as the arc is lengthened and cooled. Figure 7.13 shows an axial-blast arc
control device. The use of oil switchgear is reducing significantly in most areas of
the world because of the need for regular maintenance and the risk of fire in the event
of failure.
Newnes Electrical Engineer’s Handbook
159
Fixed contact
e-
Direction
of
blast
4 Direction of opening
Fig. 7.13 Axial-blast arc control device
Sulphur hexafli~oride{SF,) is the most effective gas for the provision of insulation
and arc interruption. It was initially introduced and used predominantly in transmission
switchgear, but it became popular at distribution voltages from the late 1970s.
The main early types of SF6 circuit breaker are known as ‘puffer’ types. These use
a piston and cylinder arrangement, driven by an operating mechanism. On opening,
the cylinder containing SF6 gas is compressed against the piston, increasing the gas
pressure; this forces the gas through an annular nozzle in the contact giving a ‘puff’
of gas in the arc area. The gas helps to cool and de-ionize the arc, resulting in arc
interruption at current zero. The dielectric properties of SF6 are quickly re-established
in the heat of the arc, giving a rapid increase in dielectric strength as the voltage
across the contacts begins to rise. The energy required to drive the puffer type of
circuit breaker is relatively high, resulting in the need for a powerful mechanism.
Other designs of SF6 circuit breaker started to be introduced in the 1980s. These
make more use of the arc energy to aid interruption, allowing the use of a lighter and
cheaper operating mechanism. Whilst in the puffer design the compressed SF6 is
made to flow over an arc which is basically stationary, these newer designs move the
arc through the SF6 to aid interruption. In the rotating-arc SF6 circuit breaker a coil
is wound around the fixed contact. As the moving contact withdraws from the fixed
contact the current is transferred to an arcing contact and passes through the coil. The
160
Switchgear
current in the coil sets up an axial magnetic field with the arc path at right angles to
the magnetic flux, and rotation of the arc is induced according to Fleming’s left-hand
rule (see Chapter 2, eqn 2.16). The arc rotation is proportional to the magnitude of
the current. Careful design is required in order to ensure an adequate phase shift
between the flux and the current to maintain adequate movement of the arc as the
current falls towards its zero crossing. The operation of a rotating-arc interrupter is
illustrated in Figs 7.14 and 7.15.
Fixed contact ring
The arc has been extended towards the coil axis by transverse movement of the
contact bar. It is shown after transfer to the arcing tube and consequent production of
the magnetic field. It is thus in the best plane to commence rotation
Fig. 7.14 Operation of a rotating-arc SF6 interrupter
Other types of self-extinguishing circuit breaker use an insulating nozzle which is
similar to that in a puffer breaker, but with a coil to produce a magnetic field for
moving the arc. The thermal energy in the arc is used to increase the local pressure.
This results in arc movement which again is proportional to the current, and successful
interruption follows. In some designs a small puffer is also used to ensure there is
enough gas flow to interrupt low levels of current, where the thermal energy in the
arc is relatively low.
The various types of SF6 high-voltage switchgear are referred to in section 7.5.2,
and Fig. 7.24 shows the main configurations.
The main alternative to the SF6 circuit breaker in the medium-voltage range is the
vucuunz interruptel: This is a simple device, comprising only a fixed and moving
contact located in a vacuum vessel, but it has proved by far the most difficult to
develop. Although early work started in the 1920s, it was not until the 1960s that the
first vacuum interrupters capable of breaking large currents were developed, and
commercial circuit breakers followed about a decade later.
The principle of operation of a vacuum interrupter is that the arc is not supported
by an ionized gas, but is a metallic vapour caused by vaporization of some of the
contact metal. At current zero the collapse of ionization and vapour condensation is
very fast, and the extremely high rate of recovery of dielectric strength in the vacuum
ensures a very effective interruptingperformance. The features of a vacuum interrupter
Newnes Electrical Engineer’s Handbook
161
Arrows show the
Iof elements
arc column
the influence
magnetic
Initial arc
produced radially
Arcing tube
within field c
--______---
~~~~~
~
~
Development of a spiral arc. The arc is motivated to move sideways but this
tendency is limited by the shape of the electrodes and it develops quickly into
a spiral with each element tending to move sideways as shown by the arrows
Fig. 7.15 Operation of a rotating-arc SFs interrupter
which are key to its performance are the contact material, the contact geometry and
ensuring that the envelope (a glass or ceramic tube with welded steel ends) remains
vacuum-tight throughout a working life in excess of 20 years. Even today, specialized
manufacturing techniques are necessary to achieve this. A typical vacuum interrupter
is shown in Fig. 7.16.
The circuit breaker is located within a switchgear housing, some types of which
are described later. The main insulation in the housing is usually air, although some
designs now have totally sealed units filled with SF6 gas. Structural isolation is
required to support current-carrying conductors; this is normally some type of cast
resin. Thermoplastic materials which can be injection moulded are often used for
smaller components, but larger items such as bushings which are insulation covered
are usually made from thermosetting materials such as polyurethane or epoxy resin
mixed with filler to improve its mechanical and dielectric properties.
7.4.2
Main classes of equipment
(a) Primary switchgear
Primary substations up to 36 kV are usually indoor, although they can be outdoor
where space is not a problem. Indoor equipment is usually metal enclosed, and it can
be subdivided into metalclad, compartmented and cubicle types. In the metalclad
type, the main switching device, the busbar section and cable terminations are segregated
by metal partitions. In compartmented types the components are housed in separate
compartments but the partitions are non-metallic. The third category covers any form
of metal-enclosed switchgear other than the two described.
The vertically isolated, horizontally withdrawn circuit breaker is the traditional
162
Switchgear
Fixed contact
Stainless
steel end
I
\
Sputter shield
Moving contact
/
Stainless steel
bellows
Fig. 7.16 Typical vacuum interrupter (courtesy Meidensha Corporation, Japan)
arrangement in the UK or UK-influenced areas. This originates from the bulk-oil
circuit breaker, for which there was a need for easy removal for frequent maintenance,
but manufacturers have utilized the already available housings to offer vacuum or
SF6 circuit breakers as replacements for oil circuit breakers, or even for new types to
this design. The design also provides convenient earthing of the circuit or busbars by
alternative positions of the circuit breaker in the housing; this is achieved by
disconnecting the circuit breaker, lowering it from its service position, moving it
horizontally and then raising and reconnecting the connections between the busbar or
cable and earth. A typical section is shown in Fig. 7.17.
Horizontally isolated, horizontally withdrawn equipment has been used extensively
in mainland Europe, with air-break, small oil volume and latterly with SF6 and
vacuum circuit breaker truck designs. A section of this type of switchgear is shown
in Fig. 7.18. Since the circuit breaker cannot be used as the earthing device in this
design, integral earthing switches or portable earthing devices are required.
Fixed-position circuit breakers were introduced in the 1970s; these depart from
the withdrawable arrangements and are based on a ‘sealed-for-life’concept. Due to
the early problems with vacuum interrupters, some users were reluctant to accept
fixed-position vacuum switchgear, but now that excellent long-term reliability of
vacuum interrupters has been established, newer designs have been introduced again
recently. Several arrangements in which the housing is completely sealed and
filled with SF6 as a dielectric medium are now available and an example is shown in
Fig. 7.19. Because of the compact dimensions that SF6 permits these are gaining
Newnes Electrical Engineer’s Handbook
163
Dry type epoxy resin
Relay panel
Current transformers
\
\
..
insulatedvoltage
transformer
contacts for
service position
Fig. 7.17 Section of vertically isolated, horizontally withdrawn switchgear
popularity, particularly at 36 kV, where there can be a cost advantage not only in the
price, but also in the reduced size of the equipment and the substation required.
(bj Secondary switchgear
Secondary distribution switchgearis that which is connected directly to the electrical
utility transformers which provide low-voltage supplies to customers. The systems
distributing power from the primary substations can be conveniently divided into
overhead circuits and underground cable networks.
Most faults on overhead systems are caused by lightning strikes, branches brushing
the conductors, clashing of conductors in high wind, or large birds bridging the lines;
they are usually transient in nature. In most cases the fault duration is short and the
circuit breakers used in these circuits, known as reclosers, are programmed to close
again a very short time after they have opened. This allows the fault to be cleared
with a minimum of disruption to consumers. It is normal to programme the recloser
to open then close up to four times in order to allow time for the fault to disappear.
If after this time the fault is still present, it is assumed that the fault is not transient;
the recloser then locks out and the faulty section of line is isolated. A recloser
Shutters drop down on removal of circuit breaker
for safety, to prevent access to potentially live
fixed isolating contacts
Isolating contacts, fixed side
locatedinside insulationspouts
mechanism
Current
transformers
Circuit earth
switch
service to isolated
circuit breaker
Fig. 7.18 Section of horizontully isolated. horizontrlly withdrawn switchgear
lnterpanel
earthbar
165
Newnes Electrical Engineer's Handbook
-7
-9
8
@ Circuit-breakerwith drive
@ Busbar 1
0 Cable connection compartment
@ LOW voltage cabinet
1 Drive for circuit-breaker and disconnector/
earthing switch with control panel
2 Circuit-breakercompartment
3 Vacuum interrupter
4 Disconnector/earthing switch blades
(in position 'Disconnector MADE)
5
6
7
8
9
Busbar compartment
Busbar
Current transformer
Connecting cable
Cabinet for control and auxiliary
devices (low-voltage cabinet)
Fig. 7.19 Section of 36 kV SF6 gas-insulated switchgear (courtesy ALSTOM TLD)
normally controls several circuits. An 11 kV pole-mounted auto-recloser is shown in
Fig. 7.20. Reclosers are available with SF6 and vacuum interrupters; the equipment
used in conjunction with reclosers are switches and sectionalizers, which can be
air-break switches, vacuum or SF6 devices, expulsion fuses and off-load switching
devices.
Most urban areas in the UK and Europe are supplied by underground cable. In this
case the step-down transformer (for example, 11000/415 V) which supplies a consumer
166
Switchgear
Fig. 7.20 11 kV pole-mounted auto-recloser (courtesy FKI plc -Engineering Group)
circuit is connected to a protective device such as a fuse switch or circuit breaker in
a ring. A simplified distribution network with the main components is shown in
Fig. 7.21. The ‘ring main unit’ usually has two load-break switches with the control
protective device combined into a single unit. In the UK this type of unit is ruggedly
designed for outdoor use, but in continental Europe indoor designs are more normal.
The traditional oil-filled unit has been replaced largely by SF6 equipment over recent
years. The ring main unit has normally been manually operated with a spring-operated
mechanism to ensure that its operation is not dependent on the force applied by the
operator. However, as utilities press to improve the reliability of supplies to consumers
and to reduce the time lost through outages caused by faults, the use of automatically
operated motor-driven spring mechanisms is now on the increase.
Secondary
Load break
ring switches
I
11 kV/415V
I
11 kV
Primary
132 kV
Circuit
breaker
132/33 kV
33 kV
bulk
supply
X
I
Ring
main unit
x-++
Motor
33/11 kV
Cable
supply
-
XXX-
Overhead
line supply
L
Recloser
Fig. 7.21 Simplified distribution network showing the main awitchgear components
Sectionalizer
or switch
T off transformer
protection device
Switchgear
168
7.4.3
Rating principles
The main consideration in the choice of switchgearfor a particular system voltage is
the current it is capable of carrying continuously without overheating (the rated
normal current) and the maximum current it can withstand, interrupt and make onto
under fault conditions (the rated short-circuitcurrent). Studies and calculationsbased
on the circuits of interest will quantify these parameters. The switchgear rating can
then be chosen from tables of preferred ratings in the appropriate standard, such as
IEC 694 and IEC 56. Table 7.4 shows figures taken from IEC 56 showing the coTable 7A co-ordination of rated values for circuit breakers, taken from IEC.56
Rated
voltage
U
(kV)
3.6
7.2
12
17.5
24
36
52
72.5
Rated
short-circuit
breaking
current
I*,
Rated normal current
1,
(A)
(W
10
16
25
40
8
12.5
16
25
40
8
12.5
16
25
40
50
8
12.5
16
25
40
8
12.5
16
25
40
8
12.5
16
25
40
8
12.5
20
12.5
16
20
31.5
400
630
400
400
400
400
400
400
630
630
630
630
630
630
1250
1250
1250
1600
1600
2500
2500
4000
1250
1250
1250
1250
1600
1600
1600
2500
2500
4000
1600
1600
1600
1600
2500
2500
2500
4000
4000
1600
2500
1600
1600
2500
2500
4000
1600
1600
1600
2500
2500
4000
1250
1250
1250
1250
1250
1250
1250
1250
1250
1250
630
630
630
630
630
630
1250
1250
1250
1250
630
630
630
1250
1250
1250
800
1250
800
800
1250
1250
1250
1250
1250
1600
2000
1600
1600
2000
2000
Newnes Electrical Engineer’s Handbook
169
ordination of rated values for circuit breakers. Ratings vary commonly from less than
400 A in a secondary system to 2000 A or more in a primary substation, with fault
currents of 6 kA in some overhead line systems up to 40 kA and above in some
primary circuits.
The standards also specify overvoltagesand impulse voltages which the switchgear
must withstand. Normal overvoltages which occur during switching or which may be
transferred due to lightning striking exposed circuits must be withstood. The switchgear
also needs to provide sufficient isolating distances to allow personnel to work safely
on a part of the system that has been disconnected. The lightning impulse withstand
voltage and the normal power frequency withstand voltage are specified relative to
the normal system voltage in the standards.
7.4.4
Test methods
Type tests are performed on a single unit of the particular type and rating in question.
They are performed to demonstrate that the equipment is capable of performing the
rated switching and withstand duties without damage, and that it will provide a
satisfactory service life within the limits of specified maintenance. The main type
tests are:
dielectric tests. Lightning impulse is simulated using a standard waveshape
having a very fast rise time of 1.2 ps and a fall to half value of 50 ps. Power
frequency tests are performed at 50 Hz or 60 Hz. The method of performing
the tests is detailed in standards, for instance IEC 60, ‘Guide to high voltage
testing techniques’. This requires the switchgear to be arranged as it would be
in service. If outdoor rated, the tests need to be carried out in dry and wet
conditions, the latter using water of specified conductivity which is sprayed
onto the equipment at a controlled rate during application of the high voltage.
temperature rise tests. Again the switchgear is arranged in its service condition,
and the normal continuous current is applied until the temperature of the main
components has stabilized. The final temperature, measured by thermocouples,
must not exceed values stated in the specifications; these values have been
chosen to ensure that no deterioration of metal or insulation is caused by
continuous operation.
short-circuit and switching tests. These are conducted over a range of currents
from 10 per cent to 100 per cent of the short-circuit rating. The tests are at low
power factor, which represents the most onerous switching conditions seen in
service. The different levels represent short-circuits at various points in the
system, from close to the circuit breaker terminals to a long distance along the
cable or line. Whilst the current to interrupt varies with these positions. another
major factor is the rate of rise of the transient recovery voltage appearing
across the circuit breaker contacts; this can have a significant influence on the
circuit breaker performance and it plays a key part in the design. The ability
to interrupt the short-circuit current with a decaying dc component superimposed
on the power frequency must also be demonstrated. The decrement or rate of
decay of the dc voltage is specified in the standards, but the dc component
which is present when the contacts open depends upon the opening time of the
circuit breaker (it being reduced if the opening time is long). Figure 7.22
shows the determination of short-circuit making and breaking currents and of
the dc component. Circuit breakers must also be tested when closing onto a
fault condition, and then carrying the full short-circuit current for a specified
period, usually one or three seconds.
Switchgear
170
I
E
I:
I
C
7 A - F
- - - - D'
B
---I
610/86
E'
envelope of current-wave
normal zero line
displacement of current-wave zero-line at any instant, the dc component
rms value of the ac component of current at any instant, measured from CC'
instant of contact separation (initiation of the arc), with dc component 4c
indicated
making current
peak value of ac component of current at instant EE'
rms value of the ac component of current at instant E E
dc component of current at contact separation point
percentage value of the dc component
Fig. 7.22
0
Determination of short-circuit making and breaking currents and of dc component
mechanical endurance test. To demonstrate that the equipmenthas a satisfactory
life, this test is performed over a specified number of operations which depends
upon the type of equipment and duty; the number of open-close operations
can vary from 1000 to 10 000. The tests are carried out without electrical load,
but at the rated output of the mechanism of the device.
Routine tests are performed by the manufacturer on every unit of switchgear to
ensure that the construction is satisfactory and that the operating parameters are
similar to the unit which was subjected to the type test. These tests are specified in
the standards, and the minimum performed will include the following:
0
power frequency voltage withstand dry tests on the main circuit
voltage tests on the control and auxiliary circuits
measurement of the resistance of the main circuit
mechanical operating tests
Newnes Electrical Engineer’s Handbook
7.4.5
171
Commissioning and maintenance
Switchgear is usually mansported in sections which need to be carefully erected and
connected together at the substation.
Commissioning tests are performed after installation;these are necessary to ensure
that the connections are sound and that the equipment is functioning satisfactorily.
These tests will normally include checks of current and voltage transformers,checking
of the protection scheme, circuit breaker operation and high-voltage withstand
tests.
With the demise of oil-filled switchgearthe requirement for maintenance has been
significantly reduced. SF6 and vacuum circuit breakers which are ‘sealed for life’
should require no servicing of the interrupting device. Manufacturers provide
instructions on simple inspection and servicing of other parts of the circuit breaker;
this is usually limited to cleaning, adjustment and lubrication of the mechanism.
7.4.6
Standards
Some of the commonly used standards and specificationsfor distribution switchgear
are given in Table 7.5.
Table 7.5 Standards and specifications for distribution switchgear
IEC
BS
56
129
265
298
5311
EN60129
EN60265
EN60298
420
694
EN60420
EN60694
Electrlcity
Association
41-26
41-27
Subject
High voltage ac circuit breakers
AC disconnectors and earth switches
High voltage switches
AC metal enclosed switchgear and
controlgear for rated voltages above 1 kV
up to and including 52 kV
High voltage ac switch fuse combinations
Common specifications for high voltage
switchgear and controlgear standards
Distribution switchgear for service up to
36 kV (cable connected)
Distribution switchgear for service up to
36 kV (overhead line connected)
Ovemead, pad mounted, dry vault, and
submersible automatic circuit reclosers and
fault interrupters for ac systems
Overhead, pad mounted, dry vault, and
submersible automatic line sectionalizers
for ac systems
N. Amerlcan
ANSIlIEEE
C37.60
ANSlllEEE
C37.63
7.5 High-voltage (transmission) switchgear
7.5.1
System considerations
The transmission of high powers over long distances necessitates the use of High
( H V ) , Extra High (EHV)or Ultra High (UHV) voltages. Historically these voltages
were classed as 72.5 kV and above. The lower voltages were introduced first, and as
the technologies have developed these have increased so that now the highest
transmission voltages being used are 1100 kV. For long transmission distances it is
Switchgear
172
also possible to use dc systems; these have particular advantages when two systems
are to be connected, but they are beyond the scope of this section, which considers
only ac transmission switchgear.
In the UK the 132kV system was first established in 1932, and subsequent growth
has required expansion of the system up to 275 kV (1955) and 420 kV (1965). Other
countries have developed similarly, so a multiple voltage transmission system exists
in virtually every major electricity-using nation.
In recent years there has been some harmonization of the rated voltages of
transmission systems, the agreed levels set down in IEC 694 being 72.5, 100, 123,
145, 170, 245,300, 362,420,550 and 800 kV. Systems will be operated at a variety
of n o d voltage levels such as 66, 132,275 and 400 kV, but the maximum system
operating voltage may be higher than these nominal ratings, so the next highest
standard value will be selected as the rated voltage of the system.
In order to connect, control and protect these transmission systems it is necessary
to use switchgear of various types and ratings.
A switchgear installation which allows the connection and disconnection of the
interconnectingparts of a transmission system is referred to as a substation.A substation
will include not only the switchgear, but also transformers and the connections to
overhead lines or cable circuits. The main functional elements of the switchgear
installation in a substation are circuit breakers, switches, disconnectors and earthing
switches. Some of the most common configurations for substations are illustrated in
Fig. 7.23.
Switching conditions on transmission systems are similar in principle to those on
distribution systems, with a requirement to interrupt normal load currents, fault
currents and to undertake off-load operations.Because of the different circuit parameters
in transmission and distribution networks, the switching duties impose particular
requirements which have to be taken into account during the design and type testing
of switchgear equipment. A particular requirement for transmission is the ability to
deal with the onerous transient recovery voltages which are generated during interruption
of faults on overhead lines when the position of the fault is close to the circuit
breaker terminals; this is defined in IEC 56 as the short line fault condition. A further
requirement is the energization of long overhead lines, which can generate large
transient overvoltages on the system unless countermeasures are taken.
7.5.2
Types of circuit breaker
The first designs of transmission circuit breaker used minimum-volume oil interrupter
technology similar to that which had been developed for distribution switchgear.
The 1940s saw the development and introduction throughout the world of airblast circuit breakers. These devices used compressed air at pressures up to 1200 psi,
not only to separate the contacts but also to cool and de-ionize the arc drawn between
the contacts. Early air-blast designs used interrupters that were not permanently
pressurized, and reclosed when the blast was shut off. Isolation was achieved separately
by a series-connectedswitch which interrupted any residual resistive and/or capacitive
grading current, and had the full rated fault-making capacity.
The increase of system voltages to 400 kV in the late 1950s coincided with the
discovery, investigation and specification of the kilometric or short line fault condition.
Air-blast interrupters are particularly susceptible to high rates of rise of recovery
voltage (RRRV), and parallel-connected resistors are needed to assist interruption.
At the increased short-circuit current levels of up to 60 kA which were now required,
Newnes Electrical Engineer’s Handbook
173
Single busbar system
Double busbar system
Q
Main and transfer busbar system
Double busbar system with bypass DS
Transfer busbar
Main busbar
VT
1
CB system
Key:
?
Ring busbar system
CB Circuit breaker
DS Disconnecting switch
1 1 r-/ +
Single bus with bypass busbar system
ES Earthing switch
4 CT Current transformer
% VT Voltage transformer
Fig. 7.23 Examples of busbar configurations for high-voltage substations
parallel resistors of up to a few hundred ohms per phase were required, and the duty
of breaking the resistor current could no longer be left to the series-connected air
switch. To deal with this, permanently pressurized interrupters incorporating parallelresistor switching contacts were used. The original pressurized-head circuit breakers
used multiple interrupters per phase in order to achieve the required fault-switching
Switchgear
174
capacity at high transmission voltages; 10 or 12 interrupters were used for 400 kV.
These were proved by direct unit testing at short-circuit testing stations, each having
a direct output of 3 GVA. It was in fact the ability to test which determined the
number of interrupters used in the design of a circuit breaker.
The introduction of synthetic testing methods increased the effective capacity of
short-circuit testing stations by an order of magnitude, and this allowed the proving
of circuit breakers having fewer interrupters. 400 kV air-blast circuit breakers with
only four or six interrupters per phase were being supplied and installed up to the late
1970s.
The next step-change in technology occurred with the introduction of SF, circuit
breakers. The merits of electronegative gases such as SF6 had long been recognized,
and freon had been used in the late 1930s to provide the primary insulating medium
in 33 kV metalclad switchgear installed in two UK power stations. The reliability of
seals was a problem, and it was only during the nuclear reactor programme that
sufficiently reliable gas seals were developed to enable gas-insulated equipment to
be reconsidered.
The first practical application of SF6 in switchgear was as the insulating medium
for instrument transformers in transmission substations in the late 1950s and early
1960s. These were quickly followed by the use of SF6 not only as an insulating
medium but also as the means of extinguishing arcs in circuit breakers. The first
generation of SF6 circuit breakers were double-pressure devices which were based
on air-blast designs, but using the advantages of SF6. These double-pressure designs
were quickly followed by puffer circuit breakers which have already been described
in section 7.4.1. Puffer circuit breakers, now in the form of second or third generation
of development, form the basis of most present-day transmission circuit breaker
designs. Figure 7.24 illustrates the various forms of SF6 interrupter.
7.5.3
Main classes of equipment
Transmission substations may be built indoors, or outdoors, but because of the space
required it is usually difficult to justify an indoor substation.The strongestjustification
for an indoor substation is the effect of harsh environments on the design, operation
and maintenance of the equipment.
The two main types of transmission switchgear currently available are conventional
or Air-Insulated Switchgear (AIS) and gas-insulated metal-enclosed switchgear.
1. Double pressure
2. Classical puffer
3 Auto expansion
Fig. 7.24 Various forms of SF, interrupter
Newnes Electrical Engineer’s Handbook
175
(a) Conventional or Air-Insulated Switchgear (AZS)
Here the primary insulating medium is atmospheric air, and in the majority of AIS
the insulating elements are of either solid or hollow porcelain.
In circuit elements such as disconnectorsand earthing switches,where the switching
is done in air, the insulators are solid, and provide not only physical support and
insulation, but also the mechanical drive for the elements.Examples of disconnectors
and earth switch arrangements are shown in Fig. 7.25. The choice of disconnector
design is dependent on the substation layout and performance requirements.
Centre break disconnector
Rotating centre post disconnector
Pantograph disconnector
Semi-pantograph disconnector
Vertical break disconnector
Knee joint disconnector
Earthing switch
Fig. 7.25 Examples of disconnector and earth switch arrangements
176
Switchgear
Equipment such as circuit breakers which need an additional insulating or interrupting
medium use hollow porcelain insulators that contain the active circuit elements in the
appropriate insulating medium. As previously outlined, initial AIS installations used
oil as the insulating and interrupting medium, which in many cases was replaced by
the use of compressed air, and modem designs almost always use SF, gas.
The layout of a substation is governed by the need to ensure that air clearances
meet the dielectric design requirements, including conditions when access is required
to the substation for maintenance. Rules for the minimum clearances and the
requirements for creepage distances on the external porcelain insulators are contained
in the standards. For locations with atmospheric pollution it is necessary to provide
extended creepage distances on insulators. Guidelines for the selection of creepage
distance are given in IEC 815.
A recent development in AIS technology is the use of composite insulators for
functional elements in place of the traditional porcelain. There is a risk of explosion
with porcelain insulators when they are filled with insulating gas if a mechanical
fault occurs with the equipment. A composite insulator consists of silicone rubber
sheds on a filament glass-fibre tube; it is not brittle and will not explode if the
insulator is ruptured. Composite insulators are now being used on instrument
transformers and circuit breakers for AIS, and their use is likely to become more
common.
An example of an outdoor AIS substation is shown in Fig. 7.26.
(b) Gas-insulated metal-enclosed switchgear
In metal-enclosed switchgear there is an external metal enclosure which is intended
to be earthed; it is complete except for external connections. The earliest transmission
I
.
Fig. 7.26 Example of an AIS substation
,
Newnes Electrical Engineer’s Handbook
177
metal-enclosed equipment used oil as the insulating medium, but since the introduction
of SF,, all transmission metal-enclosed switchgear is based on SF6 as the insulating
medium.
The main significance of the Gas-Insulated Substation (GIS) principle is the
dramatic reduction of insulating clearances made possible by SF,, resulting in much
more compact substation layouts. GIS are easier and more economic to accommodate
where land use is critical, such as in many urban areas. It also offers considerable
advantages in applications where environmentalpollution is severe, such as in coastal,
desert or heavily industrialized areas. Environmental conditions can also influence
the need for maintenance of the switchgear and the implementation of that maintenance;
GIs is so compact that transmission substations can be indoor, reducing the maintenance
problems considerably. The greater reliability of GIS is another factor which is
encouraging its increased application.
GIS switchgear is generally of modular design. All individual elements such as
busbars, disconnectors, instrument transformers and circuit breakers are contained in
interconnecting earthed enclosures filled with SF, at up to 7 bar. At the highest
voltage levels, the three phases are housed in separate phase-segregated enclosures.
At the lower voltages it is common for all three phases to be housed in the same
enclosure and with some designs this philosophy is extended up to 300 kV. Because
of the possibility of heating effects from induced currents it is normal for phasesegregated designs to use non-magnetic materials (typically aluminium) for the
enclosures, but for phase-integrated designs it is possible to use magnetic steels for
some ratings without overheating constraints.
An example of a GIS substation is shown in Fig. 7.27.
Fig. 7.27 Example of a GIS substation
Switchgear
178
7.5.4
Rating principles
As for distribution switchgear, the main consideration in the choice of rating for a
particular system is the rated normal current and the rated short-circuit current.
Power system studies using load flow and transient analysis techniques will establish
the values of the parameters for individual systems. It is also necessary to select
lightningimpulse rated values from the range of standard values specifiedin standards
(as for distribution switchgear) and to specify for transmission systems with a rated
voltage of 300 kV and above a rated switching impulse withstand voltage. The rated
switching surge voltage level is defined as the peak voltage of a unipolar standard
250/2500 ps waveform; this represents the transient voltages which are generated by
the operation of circuit breakers when switching overhead lines, shunt reactors and
other circuits.
An important consideration in the choice of equipment ratings for transmission
systems is insulation co-ordination. Insulation co-ordination is the selection of the
electric strength of equipment and its application in relation to the voltages which
can appear on the system taking into account the characteristics of any protective
devices, so as to reduce to an economically and operationally acceptable level the
probability that the resulting voltage stresses imposed on the equipment will cause
damage to the equipment or affect continuity of service. For system voltages up to
300 kV, experience has demonstrated that the most important factor in determining
system design is the stress due to lightning; for voltages over 300 kV the switching
overvoltage increases in importance. It is now common to use metal oxide surge
arrestors (MOA)to limit lightning overvoltage levels on transmission systems. In
addition, the MOA can in many cases provide adequate limitation of switching
overvoltages,but for longer overhead lines it may be required to use a circuit breaker
with controlled(or point-on-wave) switching in order to keep overvoltages at acceptable
levels. The traditional technique of using circuit breakers with parallel pre-insertion
resistors is now being replaced by the use of the MOA and controlled switching.
7.5.5
Test methods
It is a requirement of international standards that switchgear must be so designed and
manufactured that it satisfies the test specifications with regard to its insulating
capacity, switching performance, protection against contact, cumnt-carrying capacity
and mechanical function. Evidence of this is obtained by type testing a prototype or
sample of the switchgear. In addition, routine tests are performed on each individual
item of switchgear manufactured, either on completed or subassembled units in the
factory or on site.
Dpe tests are performed on transmission switchgear in a similar manner to those
applied to distribution switchgear (see section 7.4.4), but because of the difference
in rated values the necessary test values, the test equipment and techniques differ in
detail. Some tests which are specific to transmission switchgear are:
dielectric rests. In addition to lightning impulse and power frequency tests it
is necessary to perform switching impulse tests for equipment with a rated
voltage above 300 kV.
shorr-circuitand switching rests. The test parameters for transmissionequipment
involve much higher energy requirements than for distribution equipment, so
Newnes Electrical Engineer’s Handbook
179
special techniques have been developed over the years. To provide the high
voltage and high currents needed for transmission switchgear, it is usual to
provide a ‘synthetic’ test source where the high voltage and high currents are
provided in the same test from different sources.
Routine fesfs are performed on all switchgearunits manufactured, or on subassemblies,
to ensure that the performance of the factory-built unit will match that of the typetested unit. AIS is normally fully assembled in the factory and routine tested as a
complete functioning unit. Routine tests will include mechanical operations to check
operating characteristics, resistance checks of current-carrying paths, tests on control
and auxiliary circuits and power-frequency withstand tests to verify insulation quality.
For GIs, particularly at higher voltage levels, it is not practical to assemble fully a
complete installation in the factory. Therefore subassemblies of GIS or transportable
assemblies are individually tested prior to shipment; for GIS at 145 kV or 245 kV
this might involve a complete ‘bay’ or circuit of equipment, but at 420 kV or 550 kV
the size of the equipment dictates that only subassemblies can be transported to site
as a unit. Tests on transportable assemblies include power-frequency withstand with
partial discharge detection, verification of gas tightness, mechanical operations and
main circuit resistance.
7.5.6
Commissioning and maintenance
Due to the physical size of transmission switchgear it is necessary to transport the
equipment to site in subassemblies and then to reassemble to complete switchgear on
site. This procedure is more extensive than for distribution switchgear and it results
in more elaborate commissioning tests.
After routine testing at the factory, AIS is dismantled into subassemblies for
transport to site and then reassembled at the substation. A short series of operational
and functional checks are performed before the equipment is put into service.
Site commissioning procedures for GIS are more elaborate than for AIS, since
GIS can be particularly sensitive to assembly defects or to particulate contamination;
because of the small electrical clearances made possible by SF,, it is essential that all
particulate contamination down to a size of about 1mm is excluded from the equipment.
For these reasons extensive power-frequency withstand tests are performed on site
on the completed installation. The requirements for commissioningof GIs are covered
in IEC 517.
Modem designs of transmission switchgear are becoming simpler and inherently
more reliable, with reduced maintenance requirements. The traditional preventive
maintenance approach, with maintenance performed at prescribed intervals irrespective
of the condition of the switchgear, is being replaced by predictive maintenance in
which maintenance is needed only when the condition of the equipment warrants
interventionor where operationalduties are severe. Diagnostic techniques and condition
monitoring to support this are now becoming available; in particular, it is very difficult
to apply conventional partial discharge tests to a complete GIs installation and special
diagnostic techniques have been developed to monitor its structural integrity.
Many items of transmission switchgear are still in service after 30,40,or even 50
years of satisfactory service. A typical design life for modern transmission switchgear
is 40 years, but this is conservative in comparison with the figures being achieved on
the older equipment still in service today.
180
Switchgear
7.6 Standards
Some of the commonly used standards and specificationsfor transmission switchgear
are given in 'Pable 7.6.
Table 7.6 Standards and specifications for transmission switchgear
IEC
EWD
BS
Subject
60056
HD348
5311
High voltage ac circuit breakers
60071-1
EN60071-1
60129
60265-2
EN60129
EN60265-2
60427
EN60427
60517
EN60517
60694
EN60694
61166
EN61166
61259
EN61259
N.
American
-ANSI
C37.04
C37.06
Insulation co-ordination. Pt 1 - Definitions, C92.1
principles and rules
EN60129
AC disconnectors and earthing switches C37.32
EN60265-2 High-voltage switches for rated voltages
of 52 kV and above
EN60427
Synthetic testing of high-voltage ac circuit C37.081
breakers
Gas-insulated metal-enclosed switchgear C37.55
EN60517
5524
rated 72.5 kV and above
EN60694
Specifications for high-voltage switchgear C37.09
and controlgear standards
EN61166
Seismic qualification of high-voltage ac
C37.81
circuit breakers
Gas-insulated metal-enclosed switchgear
EN61259
rated 72.5 kV and above - switching of
bus-charging currents by disconnectors
References
7A. Ryan, H.M.(ed.), High Voltage Engineering and Testing, Peter Pereanus, 1994.
7B. Ryan, H.M.and Jones, G.R. SF, Switchgear, Peter Peregrinus, 1989.
7C.
Flurscheim, C.H. (ed.), Power Circuir Breaker Theory and Design, Peter Peregrinus, 1985.
Chapter 8
Fuses and protection relays
Dr D.J.A. Williams
8.1
Protection and co-ordination
Fuse and protection relays are specialized devices for ensuring the safety of personnel
working with electrical systems and for preventing damage due to various types of
faults. Common applications include protection against overcurrents, short-circuits,
overvoltage and undervoltage.
The main hazard arising from sustained overcurrent is damage to conductors,
equipment or the source of supply by overheating, possible leading to fire. A shortcircuit may melt a conductor, resulting in arcing and the possibility of fire;the high
electromechanicalforces associated with a short-circuit also cause mechanical stresses
which can result in severe damage. A heavy short-circuit may also cause an explosion.
Rapid disconnection of overcurrents and short-circuits is therefore vital. An important
parameter in the design and selection of protective devices is the prospective current;
this is the current which would flow at a particular point in an electrical system if a
short-circuit of negligible impedance were applied. The prospective current can be
determined by calculation if the system impedance orfault capacit)l at that point is
known.
In addition, personnel working with electrical equipment and systems must be
protected from electric shock. A shock hazard exists when a dangerous voltage
difference is sustained between two exposed conducting surfaces which could be
touched simultaneously by different parts of the body. This voltage normally arises
between earth and metalwork which is unexpectedly made live and if contact is made
between the two through the body, a current to earth is caused. Fuses can provide
protection where there is a low-resistance path to earth, because a high current flows
and blows the fuse rapidly. To detect a wide range of currents flowing to earth it is
necessary to use current transformers and core-balance systems: these operate a
protective device, for example a circuit breaker such as the rcd described in section
7.3.4 for low-voltage systems.
When designing an electrical protection system it is also necessary to consider coordination so that when a fault occurs, the minimum section of the system around the
fault is disconnected. This is particularly important where disconnection has safety
implications. for instance in a hospital. An illustration of co-ordination is shown in
Fig. 8.1.
Protective devices are described by a time-current characteristic. In order to
achieve co-ordination between protective devices, their time-current characteristics
must be sufficiently separated, as shown in Fig. 8.2, so that a fault downstream of
both of them operates only the device nearest to the fault. A variety of shapes of
Fuses and protection relays
182
,
Fault in this zone
causes M to operate
and all supplies are lost
Fig. 8.1 Example of co-ordination
timecurrent characteristic for both fuses and protection relays are available for
different applications.
1000
100
Characteristicof relay
breaker M (Fig. 8.1)
0 10
zI
U
Characteristicof 200 A fuse
shown as P4 (Fig. 8.1)
v
E
I=
1
0.1
0.01
100
1000
10000
100000
Fault current (amperes) 3.3 kV base
Time-current characteristics for a fuse and a protection relay controlling a circuit breaker
(reproduced with permission from GEC Measurements)
Fig. 8.2
Newnes Electrical Engineer's Handbook
183
Another consideration when designing electrical protection is back-up. In some
circuits it is desirable to have a device such as an mcb (see section 7.3.3) which
disconnects lower overcurrents and can be reset. Higher overcurrents, which the mcb
cannot disconnect without being damaged, are disconnected by an upstream fuse in
series with the mcb. This arrangement has the advantage that reclosure onto a severe
fault is less likely, because replacement of the fuse would be necessary in this case
as well as reclosure of the mcb. It is also possible for a protective device to fail, for
example because of mishandling. The effects of a failure can be minimized or avoided
by a back-up protective device which operates under such conditions. In Fig. 8.1,
protective device M backs up each of the protective devices P. However, if M operates
as back-up device, co-ordination is lost because all four branches of the circuit lose
supply, not just the faulty branch.
8-2 Fuses
8.2.1
Principles of design and operation
A fuse consists of a replaceable part (the fuselink) and a fuse holder. Examples of
fuse holders are shown in Fig. 8.3.
The simplest fuselink is a length of wire. It is mounted by screw connections in
a holder which partly encloses it. When an overcurrent or short-circuit current flows,
the wire starts to melt and arcing commences at various positions along it. The arc
voltage causes the current to fall and once it has fallen to zero, the arcs are extinguished.
The larger the wire cross-section, the larger is the current that the fuselink will carry
without operating. In the UK, fuses of this type are specified for use at voltages up
to 250 V and currents up to 100A. They are known as semi-enclosed or rewireable fuses.
The most common fuselink is the cartridge type. This consists of a barrel (usually
of ceramic) contaiaing one or more elements which are connected at each end to caps
fitted over the ends of the barrel. The arrangement is shown in Figs 8.4 and 8.5. If
a high current breaking capacity is required the cartridge is filled with sand of high
chemical purity and controlled grain size. The entire fuselink is replaced after the
fuse has operated and a fault has been disconnected. Cartridge fuses are used for a
much wider range of voltages and currents than semi-enclosed fuses.
Fuselinks can be divided into current-limiting and non-current-limiting types. A
sand-filled cartridge fuselink is of the current-limiting type; when it operates it limits
the peak current to a value which is substantially lower than the prospective current.
A non-current-limiting fuse, such as a semi-enclosed fuse, does not limit the current
significantly.
The element shown in Fig. 8.4 is a notched tape. Melting occurs first at the
notches when an overcurrent flows and this results in a number of controlled arcs in
series. The voltage across each arc contributes to the total voltage across the fuse,
and this total voltage results in the current falling to zero. Because the number of arcs
is limited, the fuselink voltage should not be high enough to cause damage elsewhere
in the circuit. The characteristic development of current and voltage during the
operation of a fuse is shown in Fig. 8.6.
The function of the sand is to absorb energy from the arcs and to assist in quenching
them; when a high current is disconnected, the sand around the arcs is melted.
The element is usually of silver because of its resistance to oxidation. Oxidation
of the element in service would affect the current that could be carried without
184
Fuses and protection relays
Fig. 8.3 Fuse holders for miniature and compact low-voltage fuses: (a) fuse holders for miniature fuselinks; (b) Miniature fuselink (reproduced with permission from Littelfuse); (c) Fuse holders for compact
low-voltage fuselinks; (d) Compact low-voltage fuselink; (e) Rewirable fuse (not to the same scale)
melting, because the effective cross-section of the element is changed. Silver-plated
copper elements are also used.
Many elements include an m-effect blob, which can be deposited on wire
(Fig. 8.3(b)) or notched tape. The blob is of solder-type alloy which has a much
lower melting point than the element. If a current flows which is large enough to melt
Newnes Electrical Engineer's Handbook
185
I
I
I
Barrel
I
I
I
I
Fig. 8.4 Cross-section through a low-voltage cartridge fuselink
Fig. 8.5 High-voltage and low-voltage cartridge fuselinks (high-voltage fuse (top) and two low-voltage
fuses showing a range of fuse sizes) (reproduced with permission from ERA Technology Ltd)
only the m-effect blob, the solder diffuses into the silver. This creates a higher local
resistance in the element and the fuse operates at a lower current than it would have
done in the absence of the blob.
Other types include the expulsion fuse which is used at high voltage, and the
Universal Modular Fuse (UMF) which is used on Printed Circuit Boards (PCBs).
Fuses offer long life without deterioration in their characteristics or performance,
and cartridge fuses have the particular advantage that they contain the arc products
completely.
8.2.2
Rating principles and properties
(a) Current ratings (ZEC)
The rated current of a fuse is the maximum current that a fuselink will carry indefinitely
without deterioration. In the case of ac ratings, an rms symmetrical value is given.
The current rating printed on a fuselink applies only at temperatures below a
particular value. Derating may be necessary at high ambient temperatures and where
fuses are mounted in hot locations such as an enclosure with other heat-generating
equipment.
Fuses and protection relays
186
Prospective
current
E2 current
cut-off - - - - - - - - - -
CI
0
I
/ '
*Pre-arcing- PI-
period
Arcing period
*
Time
-
Fig. 8.6 Current and voltage during the operation of a fuse (reproduced with permission from
ERA Technology Ltd)
(b) Voltage ratings (IEC)
The rated voltage of a fuse is the nominal voltage for which it was designed. Fuselinks
will perform satisfactorilyat lower voltages, but at much lower voltages, the reduction
in current caused by the resistance of the fuselink should be considered. In the case
of ac ratings, the rms symmetrical value is given, and for dc ratings the mean value,
including ripple, is given.
IEC recommendations are moving towards harmonized low-voltage ac supplies
of 230,400 and 690 V, but although the nominal voltage is being changed in many
counnies it will be possible for the voltage to remain at its previous non-harmonized
level for several years. In Europe, the nominal voltage is 230 V, and the permitted
variations will allow supplies to remain at 240 V and 220 V. FuseIinks marked 230
V may have been designed originally for use with higher or lower voltages, and
problems may therefore arise when replacing fuselinksbecause a device manufactured
for use at 220 V would not be safe to use on a 240 V system. A fuselink designed for
240 V could safely be used at 220 V. Similar considerationsapply where the voltage
is changed from 415 V to 400 V, or 660 V to 690 V.
(c) Variations in rating principles
The IEC rating principles are used worldwide, except in North America, where UL
(Underwriters Laboratory) standards apply.
The rated current to a UL standard is the minimum current required to operate the
fuse after many hours, and the current that it will carry indefinitely (the IEC rated
current) is approximately 80 per cent of this rating.
The voltage rating marked on a UL fuselink is the maximzmz voltage at which it
can be used, whereas that marked on an IEC fuselink is the nominal voltage.
These differences must be considered when replacing fuselinks, particularly in the
case of miniature cartridgefuselinkswhich are interchangeable.In generalit is preferable
to replace a fuselink with one of the same rating from the same manufacturer; this
Newnes Electrical Engineer’s Handbook
187
ensures that its characteristics are as similar as possible to those of the previous
fuselink.
The IEC and UL ratings of fuse holders also differ. The IEC rating is the highest
rated current of a fuselink with which it is intended to be used. A higher rating may
be given in North America, this being related to the maximum current that does not
cause overheating when a link of negligible resistance is used.
(d) Frequerzcy ratings
Fuses are most commonly used in ac circuits with frequencies of 50 Hz or 60 Hz and
a fuse designed for one of these frequencies will generally operate satisfactorily at
the other. If the arc extinguishes at current zero, then the maximum arcing time on a
symmetrical fault will be 10 ms at 50 Hz and 8 ms at 60 Hz.
Fuse manufacturers should be consulted about the suitability of fuses for other
frequencies, which may include 17.67 Hz for some railway supplies, 400 Hz for
aircraft and higher frequencies for some electronic circuits.
In dc circuits there is no current zero in the normal waveform and fuselinks
designed for ac may not operate satisfactorily. Separate current and voltage ratings
are given for fuselinks tested for use in dc circuits. DC circuits can be more inductive
for a given current than ac systems, and since the energy in the inductance is dissipated
in the fuse it is necessary for the dc voltage rating to be reduced as the time constant
(WR)of a circuit increases.
(e) Breaking cupaciv
The breaking capacity of a fuse is the current which can be interrupted at the rated
voltage. The required breaking capacity will depend upon the position of the fuse in
the supply system. For instance, 6 kA may be suitable for domestic and commercial
applications, but 80 kA is necessary at the secondary of a distribution transformer.
The power factor of a short-circuit affects the breaking capacity, and appropriate
values are used when testing fuses.
(f j Eme-current characteristics
The time-current characteristic of a fuse is a graph showing the dependence upon
current of the time before arcing starts (the pre-arcing time); an example has been
shown in Fig. 8.2. The total operating time of a fuse consists of the pre-arcing time
and the arcing time. When pre-arcing times are longer than 100 ms and the arc is then
extinguished at its first current zero (that is an arcing time of less than 10 ms on a 50
Hz supply) then the time-current characteristic can be taken to represent the total
operating time.
The conventional time, the conventionalfising czirrent and the conventional nonfirsing current are often shown on time-current characteristics. These values are
defined in the standards. All fuses must operate within the conventional time when
carrying the conventional fusing current; when carrying the conventional non-fusing
current they must not operate within the conventional time.
(g) f2r
ZZt is defined
as the integral of the square of the current let through by a fuse over a
period of time. Values are given by manufacturers for pre-arcing Z2t and total letthrough Z2t.
Table 8.1 shows typical values of Z’t for low-voltage cartridge fuses of selected
current ratings: values differ between manufacturers.
Fuses and protection relays
188
Table 8.1 Example of the variation of 12r with current rating
Current rating (A)
16
40
100
250
630
Pre-arcing Pt (W-s)
0.3
3.0
30
300
3000
Total /2t(kA2.s)
0.8
8.0
80
800
8000
The heat generated in a circuit during a short-circuit or fault condition before the
fuse disconnects is given by the product of Z2t and the circuit resistance. As the letthrough Z2t becomes constant above a particular level of fault current, the heat generated
does not increase for prospective currents above this value, unless the breaking
capacity is exceeded.
(h) Power dissipation
The resistance of a fuse will result in dissipation of power in the protected circuit
when normal currents are flowing. This should be considered when designing the
layout of a protection system.
(i) Cut-off current
A current-limiting fuse prevents a fault current from rising above a level known as
the cut-off current. This is illustrated in Fig. 8.6. The cut-off current is approximately
proportional to the cube root of the prospective current, and the maximum current is
therefore very much lower than it would be if a non-current-limiting protection
device were used.
8.2.3 Main classes of equipment
Fuses are produced in many shapes and sizes, and various types are illustrated in
Figs 8.5, 8.7, 8.8 and 8.9. The main three categories are:
0
0
0
miniature
(up to 250 V)
low voltage (up to 1000 V ac or 1500 V dc)
high voltage (greater than 1000 V ac)
All three categories include current-limiting and non-current-limiting types.
(a) Miniature fuses
Cartridge fuses have in the past been the most common form of miniature fuse, but the
UMF (see Fig. 8.7) is becoming increasingly used on PCBs. A UMF is much smaller
than a cartridge fuse, and it is mounted directly on the PCB, whereas a cartridge fuse
is mounted in a holder. Subminiature fuses have pins for mounting on PCBs.
Miniature cartridge fuses and subminiature fuses are rated for use at 125 V or
250 V. UMFs have additional voltage ratings of 32 V and 63 V which make them
more suitable for many types of electronic circuit. Miniature fuses are available with
current ratings from 2 mA to 10A. The maximum sustained power dissipation which
is permitted in cartridge fuses ranges from 1.6 W to 4 W.
Miniature fuses may have a low, intermediate or high breaking capacity. All three
ranges are available for UMFs, and these are shown in Table 8.2.
Newnes Electrical Engineer’s Handbook
189
Fig. 8.7 Examples of UMFs (photograph printed with the permission of Littelfuse)
current rating
Insulating housing
Blades for connections, joined by U-shaped element
Fig. 8.8 Blade-type automotive fuselink (reproduced with permission from Littelfuse)
190
Fuses and protection relays
Fig. 8.9 A range of low-voltage fuselink types: (2) general purpose industrial fuselinks, (3) fuselinks for
domestic purposes, (4) fuselinks for protecting semiconductors, (5) fuselinks for use in UK electricity
supply networks, and (6) compact fuselinks for industrial applications (reproduced with permission from
ERA Technology Ltd)
Table 8.2 Breaking capacity of UMFs
Voltage rating (V)
Breaking capacity (A)
Maximum overvoltage (kV)
32
63
125
250
250
250
low
low
low
low
intermediate
high
0.33
0.5
0.8
1.5
2.5
4.0
35'
35'
35'
100
500
1500
*or 10 x rated current, whichever is the greater.
Cartridge fuses are available with low or high breaking capacity. Low breaking
capacity types have glass barrels without sand filler and a visual check can therefore
be made on whether or not the fuse has operated. High breaking capacity cartridge
fuselinks are generally sand filled and have ceramic barrels; they can interrupt currents
of up to 1500 A.
A range of speeds of operation is available. Time-lag (surge-proof) fuses are
required in circuits where there is an inrush current pulse, for instance when capacitors
are charged or when motors or transformers are magnetized. The fuse must not be
operated by these normal-operation surges, which must not cause deterioration of the
fuse. The five categories of time-lag are medium time-lag (M), time-lag (T), long
time-lag (TT), very quick-acting (FF) and quick-acting (F). They are available as
cartridge fuses and the last two are used in the protection of electronic circuits. The
letters shown in brackets are marked on the end caps.
UMFs are available in similar categories, which are super quick-acting (R), quickacting (F), time-lag (T) and super time-lag ( S ) .
Newnes Electrical Engineer’s Handbook
191
The time-current characteristics of miniature fuses of the same type but with
different ratings are similar in shape. Time can therefore be plotted against multiples
of rated current and it is unnecessary to show separate characteristics for each current
rating. Examples of time-current characteristics are shown in Fig. 8.10.
1ooc
100
10
A
m
U
E
\ \
v1
v
Quick-acting (F)characteristic
E
I=
1
A
Time-lag (T) characteristic
0.1
0.01
2
3
4
5 6
Times rated current
7 8 9 1 0
Fig. 8.10 Time-current characteristics of time-lag and quick-acting miniature fuses (reproduced with
permission from ERA Technology Ltd)
Cartridge fuses have various types of elements. Fast-acting types have a straight
wire, and time-delay types use wire with an m-effect blob (Fig. 8.3(b)), helical
elements on a heat-absorbing former or short elements with springs connecting them
to the end caps.
In addition to the most common types which have been described, miniature fuses
are produced in a wide range of shapes and sizes. As an example of this, a blade-type
automotive fuselink is shown in Fig. 8.8; the element in this fuse is visible through
the plastic casing.
Fuses and protection relays
192
(b) Low-voltagefuses
A wide range of low-voltagefuses is available for industrial and domestic applications.
These fuses have ratings appropriate for national or international single-phase or
three-phase supplies, for example 220, 230, 240,400,415,660 and 690 V.
Widely differing systems for domestic protection are used in different countries
and these cannot be described separately here. As an example, in the UK currentlimiting cartridge fuses are used in plugs which supply appliances,the consumer unit
supplyingan entire property may have current-limitingcartridge fuses, semi-enclosed
fuses or miniature circuit breakers and another fuse is installed by the supply authority
on the incoming supply.
Industrial fuses may have general-purpose (type ‘g’) fuselinks which will operate
correctly at any current between 1.6 times the rated current (the conventional fusing
current) and the breaking capacity. Such fuses must not be replaced by type ‘a’ backup fuselinks, which have a higher minimum breaking current and do not necessarily
operate safely below this current; this type ‘a’ back-up fuselink is used to save space.
Type ‘gG’ fuses are used for general application. These have a full breaking
capacity range and provide protection for cables and transformersand back-up protection
for circuit breakers. Specialized fuses are available for the protection of motors,
semiconductors, street lighting, pole-mounted transformers and other purposes.
Reference SA provides further detail. Common applications are motor protection
and semiconductorprotection and these are described briefly below.
Fuselinks for motor-starter protection must be able to withstand starting pulses
without deterioration. ‘gM’ fuselinks are designed for this purpose and they have a
dual rating. A designation 100M160, for example, means that the fuselink has a
continuous rating of 100A and the general-purposecharacteristicsof a fuselink rated
at 160 A.
Fuselinks for semiconductor protection are designed to operate with an arc voltage
which does not damage the semiconductor device; this voltage is therefore lower
than for other types of fuselink. Arc voltages at several supply voltages are shown in
Table 8.3 for typical semiconductor fuselinks.
Table 8.3 Peak arc voltages for semiconductor fuselinks
Supply voltage
Peak arc voltage
(VI
(VI
230
700
400
900
690
1400
Semiconductorfuselinks also have lower let-throughZ*rand cut-off current because
semiconductors are susceptible to damage by heat and overcurrents. These fuses
operate at higher temperatures than normal to achieve the necessary protection, and
forced air cooling may be used to increase their current rating.
(c) High-voltagefuses
High-voltage fuses can be of current-limitingor non-current-limitingtype. The latter
are expulsion fuses which do not contain the arc products when they operate; they
can be very noisy and are therefore normally used outdoors.
Current-limiting high-voltage fuses are enclosed (as already shown in Fig. 8.5)
and they may be used for the protection of motors, transformers and shunt power
Newnes Electrical Engineer’s Handbook
193
capacitors. The rated current of the fuselink is normally higher than the expected
current. These fuses are normally used in three-phase systems and are tested at 87 per
cent of their rated voltage. In a three-phase earthed neutral system the voltage rating
should be at least 100 per cent of the line-to-line voltage, and in a single-phase
system it should be at least 115 per cent of the circuit voltage.
Further information can be found in references 8A, 8B and 8C.
8.2.4
Test methods
(a) n p e tests
Before production of a type of fuselink commences, type tests are performed to
ensurethat preproductionf u s e l i samples comply with relevant nationalor international
standards. Measurements of power dissipation, time-current characteristic, overload
withstand capability, breaking capacity and resistance are included in these type
tests.
(b) Production tests
Routine testing of many important fuse characteristics is not possible because tests
such as breaking capacity are destructive. Extensive testing in production would also
be very costly. Fuse manufacturers therefore make production fuselinks as identical
as possible to the samples used for type testing.
The quality of fuselinks depends upon the quality of the components supplied to
the fuse manufacturer. Key items such as barrels, filling material, element material
and end caps are therefore regularly inspected and tested when received.
The dimensionsand sbaightness of barrels are checked and their ability to withstand
mechanical and thermal shock and internal pressure is tested. End cap dimensions
are checked to ensure that they fit closely over the barrel. The moisture content,
chemical composition and grain size of the filler are measured. The diameter or
thickness of the element wire or tape is checked and its resistance per metre is
measured. Where elements are produced from tape and notched, the dimensions and
pitch of the notches are tightly controlled.
During assembly checks are made to ensure that the fuselink is completely filled
with sand and that the element resistance is correct. After assembly the overall
dimensions are checked and the resistance is once more measured. A visual check
including the markings is then made.
Other tests are made in the case of specialized fuselinks. For example, the condition
of the elements in a high-voltage fuselink is examined using X-ray photography.
In addition to these routine tests, manufacturers may also occasionally take sample
fuselinks from production and subject them to some or all of the type tests.
(c) Site checks
Before use, every fuselink should be checked visually for cracks and tightness of end
caps and the resistance should be checked. It should also be checked that the ratings.
especially current, voltage, breaking capacity and time-current characteristic, are
correct for the application. In the case of semi-enclosed, rewireable fuses care should
be taken to use the appropriate diameter of fuse wire. Fuse holders should be checked
to ensure that the clips or means of connection are secure and correctly aligned.
If a fuselink has been dropped onto a hard surface or subjected to other mechanical
stress it should not be used, damage may not be visible but it could cause the fuse to
malfunction with potentially serious results.
Fuses and protection relays
194
If a fault occurs and the fuselink is overloaded, it should be replaced even if it has
not operated. This situation arises especially in three-phase systems where one or
two of the three fuses may operate to clear the fault.
Fuselinks (as opposed to rewireable fuses) cannot be safely repaired; they must
always be replaced.
8.2.5
Standards
Many national and international standards exist because of the number of different
fuse types. Tables 8.4,8.5 and 8.6 summarize the position for miniature, low-voltage
and high-voltage fuses respectively. IEC recommendations are listed, together with
related EN and BS standards and North American standards covering the same field.
Table 8.4 Comparison of international, regional and national standards for miniature fuses
IEC
127
127-1
127-2
127-3
127-4
127-5
127-6
EN
BS
60127-1
60127-2
60127-3
EN60127-1
EN60127-2
EN60127-3
60127-5
60127-6
EN60127-5
EN60127-6
127-9
127-10
Subject
Miniature fuses
: definitions and general requirements
: cartridge fuselinks
: subminiature fuselinks
: universal modular fuses (UMFs)
: quality assurance
: fuse holders for miniature cartridge
fuselinks
: test holders and test circuits
: user guide
Blade-type electric fuses
Miniature blade-type electric fuses
Fuses for supplementary overcurrent
protection
Automotive glass tube fuse (32 V)
N. American
SAE J 1284
SAE J 2077
UL 198G
UL 275
In order to comply with the EMC directive, the following statement has been
added to most of the UK fuse standards:
Fuses within the scope of this standard are not sensitiveto normal electromagnetic
disturbances, and therefore no immunity tests are required. Significant electromagnetic disturbance generated by a fuse is limited to the instant of its operation.
Provided that the maximum arc voltages during operation in the type test comply
with the requirements of the clause in the standard specifying maximum arc
voltage, the requirements for electromagnetic compatibility are deemed to be
satisfied.
8.3 Protection relays
8.3.1
Principles of design and operation
Systems incorporating protection relays can disconnect high currents in high-voltage
circuits which are beyond the scope of fuse systems.
In general, relays operate in the event of a fault by closing a set of contacts or by
triggering a thyristor. This results in the closure of a trip-coil circuit in the circuit
breaker which then disconnects the fault. The presence of the fault is detected by
current transformers, voltage transformers or bimetal strips.
Newnes Electrical Engineer's Handbook
195
Table 8.5 Comparison of international, regional and national standards for low-voltage fuses
IEC
EN
BS
241
269
269-1
269-2
269-2-1
EN60269 88-1
(related to
1361 and
1362)
88 secn 2.1
related to
88 secn 2.2
and 88-6
269-3
269-3A
269-3-1
263-4
related to
1361 and
1362
88-4
Subject
N. American
Fuses for domestic and similar purposes
(report)
Low voltage fuses
: general applications
: supplementary requirements for fuses for
use by authorized persons (fuses mainly
for industrial applications)
: examples of standardized fuses for
industrial applications
: supplementary requirements for fuses for
use by unskilled persons (fuses mainly
for household and similar applications)
: examples of standardized fuses mainly
for household and similar applications
: supplementary requirements for fuses for
use by unskilled persons (fuses mainly
for household and similar applications)
: supplementary requirements for fuselinks
for the protection of semiconductor
devices
Class H fuses
High-interrupting capacity fuses,
current-limiting types
Class K fuses
Class R fuses
Plug fuses
Class T fuses
DC fuses for industrial use
Fuseholders
Low-voltage cartridge fuses
Fuses (125,250, 600 V)
UL 1986
UL 198C
UL 198D
UL 198E
UL 198F
UL 198H
UL 198L
UL 512
NEMA FU
CSA C22-2
No 59
ANSVIEEE
Specification for low-voltage cartridge
fuses (600 V or less)
C97.1
Application guide for low-voltage ac
ANSlllEEE
non-integrally fused power circuit breakers C37.27
(using separately mounted current-limiting
fuses)
Electromechanical and solid-state relays are both widely used, but the latter are
becoming more widespread because of their bounce-fixe operation, long life, high
switching speed and additional facilities that can be incorporated into the relay.
Additional facilities can, for instance, include measurement of circuit conditions and
transmission of the data to a central control system by a microprocessor relay. This
type of relay can also monitor its own function and diagnose any problems that are
found. Solid-state relays can perform any of the functions of an electromechanical
relay whilst occupying less space, but electromechanicalrelays are less susceptible
to interference and transients. Electromechanical relays also have the advantage of
providing complete isolation and they are generally cheaper than solid-state devices.
Fuses and protection relays
196
Table 8.6 Comparison of international. regional and national standards for high-voltage fuses
IEC
EN
BS
282
282-1
60282-1
EN60282-1
2692-1
2692-2
related to
2692-3
282-2
282-3
549
644
787
60644-1
related to
5564
EN60644-1
5907
6553
Subject
N. American
High-voltage fuses
: current-limiting fuses
: expulsion and similar fuses
: determination of short-circuit power factor
for testing current-limiting fuses, expulsion
fuses and similar fuses
High-voltage fuses for the protection of
shunt capacitors
Specification for high-voltage fuselinks for
motor circuit applications
Guide for the selection of fuselinks of
high-voltage fuses for transformer circuit
applications
ANSIhEEE
Service conditions and definitions for
(237.40
high-voltage fuses, distribution enclosed
single-pole air switches, fuse disconnecting
switches, and accessories
ANSlllEEE
Design tests for high-voltage fuses,
distribution enclosed single-pole air
C37.41
switches, and accessories
Specifications for distribution cut-outs and ANSVIEEE
fuselinks
C37.42
Specifications for distribution oil cut-outs
ANSIAEEE
and fuselinks
c37.44
Specifications for power fuses and fuse
ANSIAEEE
disconnecting switches
C37.46
Specifications for distribution fuse
ANSVIEEE
disconnecting switches, fuse supports and C37.47
current-limiting fuses
ANSIAEEE
Guide for application, operation and
maintenance of high-voltage distribution
C37.48
cut-outs and fuselinks, secondary fuses,
distribution enclosed single-pole air
switches, power fuses, fuse disconnecting
switches and accessories
NEMA SG2
High-voltage fuses
Contacts in electromechanical relays may have to close in the event of a fault after
years of inactivity and twin sets of contacts can be used to improve reliability. The
contact material must be chosen to withstand corrosive effects of a local environment
because a film of corrosion would prevent effective contact being made. Dust in the
atmosphere can also increase the contact resistance and result in failure. Both corrosion
and dust contamination can be avoided by complete enclosure and sealing of the
relay.
The contacts must also withstand arcing during bounce on closure and when
opening, but this is usually less important than resisting corrosion. Because of the
need for high reliability the contacts are usually made of or plated with gold, platinum,
rhodium, palladium, silver or various alloys of these metals.
Solid-state relays are not affected directly by corrosion or dust, but temperature
and humidity may effect them if conditions are severe enough.
Electromechanical relays operate by induction, attraction or thermally where a
Newnes Electrical Engineer’s Handbook
197
bimetallic strip is used to detect overcurrent. The first two types are most common
and their principles are described, along with those of solid-state relays, in the
following sections.Further informationon protection relays can be found in references
8C, 8D and 8E.
(a) Induction relays
An induction relay has two electromagnets, labelled El and E2 in Fig. 8.11. Winding
A of electromagnet El is fed by a current transformer which detects the current in the
protected circuit. Winding E in electromagnet El is a secondary, and it supplies the
winding on E2. The phases of the currents supplied to El and E2 differ and therefore
the magnetic fluxes produced by the two electromagnets have different phases. This
results in a torque on the disc mounted between the electromagnets, but the disc can
only move when a certain torque level is reached because it is restricted by a hair
spring or a stop. Normal currents in the protected circuit do not therefore cause
movement of the disc.
Current transformer
I
1
1
I
Fig. 8.11 Principle of operation of an induction relay
When the disc does turn, its speed depends upon the current supplied by the
current transformer and the eddy current braking effected by a permanent magnet
located near the edge of the disc. When the disc rotates through a certain angle, the
relay contacts close and the time for this to occur can be adjusted by the position of
the stop or the angle through which the disc has to rotate. This adjustment allows
protection eo-ordination to be achieved by means of ‘time grading’. For example, in
the radial feeder shown in Fig. 8.12, the minimum rime taken for the protection relay
to operate would be set higher at points closer to the supply. A fault at point X, a
considerable distance from the supply, would cause operation of the relay set at a
minimum time of 0.4 s and the fault would be disconnected before it caused operation
of relays nearer the supply, thus preventing the unnecessary tripping of healthy
circuits.
Fuses and protection relays
198
I
Instantaneous
Fig. 8.12 Time grading
Induction relays have Inverse Definite Minimum Time (IDMT) tirne-current
characteristics in which the time varies inversely with current at lower fault currents,
but attains a constant minimum value at higher currents. This constant minimum
value depends upon the adjustments previously described.
Further adjustment is possible by means of tappings on the relay winding A in Fig.
8.11. For example, if a current transformer has a secondary winding rated at 1 A,
tappings could be provided in the range 50 per cent to 200 per cent in 25 per cent
steps, corresponding to currents of 0.5 A to 2 A in 0.25 A steps. If the circuit is
uprated, it may then be possible to adjust the relay rather than replace it. For example,
if the 100per cent setting is used when the maximum current expected in the protected
circuit is 400 A, the 150 per cent setting could be used if the maximum current is
increased to 600 A.
(b) Attracted-armature relays
The basis of operation of an attracted-armature relay is shown in Fig. 8.13. The
electromagnet pulls in the armature when the coil current exceeds a certain value and
the armature is linked to the contacts and when it moves it opens normally closed
contacts and closes normally open contacts. The time required for operation is only
a few seconds, and it depends upon the size of the current flowing in the coil.
These devices are called instantaneousrelays. They have a range of current settings
which are provided by changing the tapping of the coil or by varying the air gap
between the electromagnet and the armature.
(c) Solid-state relays
The first solid-state relays were based on transistors and performed straightforward
switching, but now they can often perform much more complicated functions by
means of digital logic circuits, microprocessors and memories.
Currents and voltages are measured by sampling incoming analogue signals, and
the results are stored in digital form. Logic operations such as comparison are then
performed on the data to determine whether the relay should operate to give an alarm
Newnes Electrical Engineer’s Handbook
199
Movement of
I armature
Air gap
I
I
1 1 1 1 1 1 1
I I I I I I I
I I I I I I I
I I
c
I 1
I I
0
j
I I
i
I I
I 1
I
I
I
/
1 1 1 1 1 1 1
A
M
a
9
n
e
1 1 1 1 1 1 1
c
I I
0
I 1
0
I I
t
i
I
t
i
I I
C
1 1 1 1 1 1 1
I
1 1 1 1 1 1 1
v
f
I
I
I I
Fixed contar&
t/ 4
I
t
U
r
e
I I
4
Contact
movement
I 1 1 1 1 1 1
1 1 1 1 1 1 1
C
1 1 1 1 1 1 1
1 1 1 1 1 1 1
0
1 1 1 1 1 1 1
1 1 1 1 1 1 1
r
1 1 1 1 1 1 1
1 1 1 1 1 1 1
e
1 1 1 1 1 1 1
Moving contacts
Fig. 8.13 Principle of operation of an attracted-armature relay
or trip a circuit breaker. Digital data can also be stored on computer €or subsequent
analysis, for instance after a fault has occurred.
Solid-staterelays may incorporate a variety of additionalcircuits. Figure 8.14 shows,
€or example, a circuit which imposes a time delay on the output signal from a relay.
Such circuits may also be used to shape the time-current characteristicof a protection
relay in various ways. A comparison of typical time-current characteristics from
electromechanicaland solid-state relays is shown in Fig. 8.15. There are many other
possible functions such as power supervision to minimize power use, an arc sensor
to override time delays and measurement of true rms values in the presence of
harmonics.
Setting
Timer
instantaneous
Setting
I
7
(XW
Fig. 8.14 Components of a solid-statetime-delayed overcwent relay (reproduced with permission from
GEC)
Fuses and protection relays
200
-
Current
(a) Electromechanical relay
Current
-
(b) Solid-state relay
Fig. 8.15 Comparison of time-current characteristicsof electromechanical and solid-state relays (reproduced
with permission from ERA Technology Ltd)
Solid-staterelays m y require shielding against electromagneticinterference arising
from electrostatic dischargesor high-voltage switching. Optical transmission of signals
is sometimes used to reduce the effects of this interference.
The electronics in solid-state relays can be damaged by moisture, and the relays
are usually encapsulated to prevent this.
8.3.2
Rating principles and properties
All components of the protection system including current transformers, relays and
circuit breakers must have the correct current, voltage and frequency rating, and the
f't let-through and interrupting capacity of the entire system depends upon the circuit
breaker. A protection relay must have a minimum operating current which is greater
than the rated current of the protected circuit, and other properties of the relay must
be chosen correctly in order for it to operate the circuit breaker as required by the
application.
Manufacturers publish information regarding the selection, installation and use of
relays. The following points in particular will need to be considered.
Heat is generated within a relay in use and if several relays are grouped together
in an enclosed space provision should be made to ensure that temperature rises are
not excessive.
Protection levels, time delays and other characteristics of both electromechanical
and solid-state relays can be changed on site. For example, overvoltage protection
may be set to operate at levels between 110 per cent and 130per cent, and Fig. 8.16
shows the effect of adjusting the operating time of an overcurrent relay.
In general, electromechanical relays can be adjusted continuously and solid-state
relays are adjusted in steps. Many other adjustments can be made. For example, DIP
switches in solid-state relays can be used to set the rated voltage and frequency and
to enable phase-sequence supervision.
The adjustment of operating times is used to provide time grading and eo-ordination,
which have already been explained with reference to Fig. 8.12. An alternative system
uses protection relays which operate only when a fault is in a clearly defined zone;
this is illustrated in Fig. 8.17. To achieve this, a relay compares quantities at the
boundaries of a zone. Protection based on this principle can be quicker than with
time-graded systems because no time delays are required.
Newnes Electrical Engineer’s Handbook
0
2
4
201
6
10
8
12
1
Current x full load
Fig. 8.16 Adjustment of timecurrent characteristics for an overcurrent relay
.x-x-x-x-x-x-x-x-x-x
1
X
I
-
I
X
I
Fig. 8.17 Overlapping zones of protection. The protection zone marked -%-x-% overlaps the zones
marked -. X marks the positions of circuit breakers with their associated protection relays
8.3.3 Main classes of relay
Protection relays may be ‘all-or-nothing’types, such as overcurrent tripping relays,
or they may be measuring types which compare one quantity with another. An
example of the latter is in synchronization, when connecting together two sources of
power.
Fuses and protection relays
202
A protection relay may be classified according to its function. Various functions
are noted below, and details for a wide range of functions are give in reference 8F.
Comrnon applications are:
0
undervoltage and overvoltage detection
overcurrent detection
overfrequency and underfrequency detection
These functions may be combined in a single relay. For exampIe, the relay shown in
Fig. 8.18 is used in power stations and provides overvoltage and undervoltage,
overfrequency and underfrequencyprotection, and it rapidly disconnects the generator
in the case of a failure in the connected power system.
L3
L1
N
A1
A2
6
A3
XN
U
0
24
21
22
14
11
12
8.18 Mains &coupling relay showing adjustments for voltage and frequency protection
(reproduced with permission from SEG)
Fig.
Another application in power systems is the protection of transmission lines by
distance relays. Current and voltage inputs to a distance relay allow detection of a
fault within a predetermined distance from the relay and within a defined zone. The
fault impedance is measured and if it is less than a particular value, then the fault is
within a particular distance. This is illustrated in the R-X diagrams shown in case (a)
Newnes Electrical Engineer’s Handbook
203
of Fig. 8.19; if the R and X values derived from the measured fault impedance R +
iX result in a point within the circle, then the relay operates. If directional fault
detection is required, the area of operation is moved, as shown in case (b).
fn
7
(a) Non-directional
(a) Directional I
Fig. 8.19 Operating areas for distance protection relays
Another form of protection for lengths of conductor is the pilot wire system.
Current transformers are placed at each end of a conductor and are connected by pilot
wires. Relays determine whether the currents at the two ends of the conductor are the
same, and they operate if there is an excessive difference. In the balanced voltage
system shown in Fig. 8.20(a), no current flows in the pilot wire unless there is a fault.
In a balanced current system, current does flow in the pilot wire in normal conditions,
and faults are detected by differences in voltage at the relays which are connected
between the pilot wires; this is shown in Fig. 8.20(b).
CT
CT
Protected conductor
---------
uuuu
uuuu
Pilot wires
- - - - - - - _>
Relay
Relay
--- - - -- (b) Balanced current system
Fig. 8.20 Pilot wire protection systems
Other applications involving relays include:
the protection of motor starters against overload
checking phase balance
the protection of generators from loss of field
the supervision of electrical conditions in circuits
Fuses and protection relays
204
8.3.4 Test methods
(a) Production tests
Manufacturers of electromechanical relays often produce their own components and
inspect them before assembly. Components for solid-state relays are generally bought
in from specialist manufacturers and there is an incoming check to eliminate those
that would be subject to early failure or excessive drift. Other characteristics such as
memory are checked as appropriate.
After assembly, manufacturers test all protection relays to ensure that they comply
with relevant national and internationalstandards.The calibration of adjustable settings
is checked.
Fault conditions that the relays are designed to protect against can be simulated,
and typical inputs to relays from current or voltage transformers can be duplicated by
test sets; these can be used to check, for example, the correct functioning of relays
for phase comparison and the proper disconnection of overcurrent and earth faults of
various impedances. Such tests can be performed by setting up a test circuit or by
means of a computer-based power system simulator which controls the inputs to the
relays. The latter method allows the effect of high-frequency transients and generator
faults to be investigated, and it is independent of an actual power supply; it may, for
example, be used for the testing of selectable protection schemes in distance relays.
Other tests include:
0
0
0
environmental tests are performed to check, for example, the effects on
performance of temperature and humidity
impact, vibration and seismic tests are performed on both solid-state and
electromechanical relays, although the latter are more prone to damage from
such effects
voltage transients are potentially damaging to solid-state relays, and the relays
are tested to ensure they can withstand a peak voltage of 5 kV with a rise time
of 1.2 ps and a decay time of 50 ps.
(b) Site tests
Primary injection tests are applied during initial commissioning and these should
show up any malfunction associated with protection relays. Secondary injection tests
should be performed if there is a maloperation which may be related to the protection
relay. Details of these injections tests are given in reference 8E.
Periodic inspection and testing is necessary throughout the lifetime of a protection
relay and computerized equipment is available for this purpose. Some relatively
complicated protection schemes incorporate automatic checking systems which send
test signals at regular intervals; the test signals can also be sent manually. Digital
relays may include continuous self-checking facilities.
8.4
Standards
There are many standards covering various types of relay and aspects of their use.
Some standards which apply to relays in general are also relevant to protection relays
and some apply specifically to protection relays. The key IEC recommendations
together with equivalent BS and EN standards and related North American standards
are summarized in Table 8.7.
Newnes Electrical Engineer’s Handbook
205
Table 8.7 Comparison of international. regional and national standards for protection relays
~~~
IEC
~~~~
ENlCECC
255
255-0-20
255-1-00
255-3
5992-1
5992-2
related to
142 secns
3.1 and 3.2
255-4
255-5
255-6
255-7
255-8
255-9
255-10
255-11
255-12
255-13
255-14
255-15
255-16
255-17
255-18
255-19
255-20
255-21
255-22
BS
N. American
Subject
Electrical relays
: contact performance
: all-or-nothing electrical relays
: single input energizing quantity
measuring relays with dependent or
independent time
: single input energizing quantity
measuring relays with dependent
specified time
: insulation tests
: measuring relays and protection
equipment
5992-3
EN60255-6
related to
142 secn 2.2
related to
related to
: test and measurement procedures for
CECC 16000, CECC 16000, all-or-nothing
- relays
EN116000-2 EN116000-2
142 secn 2.3 : thermal electrical relays
: dty reed make contact units
: application of the IEC QA system for
electronic components to all-or-nothing
relays
: interruptions to and alternating
component (ripple) in dc auxiliary
energizing quantity of measuring relays
: directional relays and power relays
related to
142 secn 4.2 with two input energizing quantities
: biased (percentage) differential relays
related to
142 secn 4.1
: endurance test for electrical relay
5992-4
contacts - preferred values for contact
loads
: endurance test for electrical relay
5992-5
contacts characteristics of test
equipment
: impedance measuring relays
: thermal electrical relays for motor
protection
: dimensions for general purpose
5992-6
all-or-nothing relays
CECC
QC 160100 : electromechanical relays of assessed
160100
quality
: protection (protective) systems
142 secns 1.4 : vibration, shock, bump and seismic
and 1.5
tests on measuring relays and
protection equipment
142 secn 1.4 : electrical disturbance tests
Relays and relay systems associated
with electric power systems
Surge withstand capability tests for
protective relays and relay systems
Guide for protective relay applications
to power
Guide for power system protective relay
applications of audio tones over
telephone channels
EN60255-6
-
ANSlllEEE
C37.90
ANSlilEEE
C37.90.1
ANSMEEE
C37.91
ANSVIEEE
c37.93
(contd)
206
Fuses and protection relays
Table 8.7 (contd)
IEC
ENKECC
BS
Subject
N. Amerlcan
Guide for protective relaying of utilityconsumer interconnections
Guide for protective relay applications
to power system buses
Seismic testing of relays
ANSVIEEE
c37.95
ANSVIEEE
c37.97
ANSI/IEEE
C37.98
ANSl/IEEE
C37.103
ANSI/IEEE
C37.105
Guide for differential and polarizing
relay circuit testing
Qualifying class 1E protective relays
and auxiliaries for nuclear power
generating stations
Testing procedures for relays and
electrical and electronic equipment
Solid-state relays
ANSVEIA
407-A
ANSI/EIA
407-A
References
8A.
Williams, D.J.A., Turner, H.W.
and Turner, C., User’sGuide m Fuses, 2nd edn, ERATechnology
Ltd, UK, 1993.
8B. Wright, A. and Newbery, P.G.,Eleclric Fuses, Peter Peregrinus Ltd, London & New York, 1982.
8C. Wright, A. and Christopoulos, C., Electric Power System Protection, Chapman & Hall, London,
1993.
8D. GEC Measurements, ‘Protectiverelays -Applications guide’, 3rd edn. The General Electric Co.,
UK, 1987.
8E. Jones, G.R., Laughton, M.A. and Say. M.G., Electrical Engineer’s Reference Book, ButterworthHeinemann, Oxford, UK, 1993.
8F. Recommended practice for protection and co-ordination of industrial and commercial power
systems, ZEEE, Wiley Interscience, 1986.
Chapter 9
Wires and cables
Mr V.A.A. Banks and Mr P.H. Fraser
BlCCGeneral
Mr A.J. Willis
Brand-Rex Ltd
9.1 Scope
Thousands of cable types are used throughout the world. They are found in applications
ranging from fibre-optic links for data and telecommunication purposes through to
EHV underground power transmission at 275 kV or higher. The scope here is limited
to cover those types of cable which fit within the general subject matter of the pocket
book.
This chapter therefore covers cables rated between 300/500 V and 19/33 kV for
use in the public supply network, in general industrial systems and in domestic and
commercial wiring. Optical communication cables are included in a special section.
Overhead wires and cables, submarine cables and flexible appliance cords are not
included.
Even within this relatively limited scope, it has been necessary to restrict the
coverage of the major metallic cable and wire types to those used in the UK in order
to give a cursory appreciation.
9.2
Principles of power cable design
9.2.1
Terminology
The voltage designation used by the cable industry does not always align with that
adopted by users and other equipment manufacturers, so clarification may be helpful.
A cable is given a voltage rating which indicates the maximum circuit voltage for
which it is designed, not necessarily the voltage at which it will be used. For example,
a cable designated 0.6/1 kV is suitable for a circuit operating at 600 V phase-to-earth
and 1000 V phase-to-phase. However, it would be normal to use such a cable on
distributionand industrial circuits operating at 2301400 V in order to provide improved
safety and increased service life. For light industrial circuits operating at 230/400 V
it would be normal to use cables rated at 450/750 V, and for domestic circuits
operating at 230/400 V, cable rated at 300/500V would often be used. Guidance on
the cables that are suitable for use in different locations is given in BS 7540.
The terms LV (Low Voltage), MV (MediumVoltage) and HV (High Voltage) have
Wires and cables
208
different meanings in different sectors of the electrical industry. In the power cable
industry the following bands are generally accepted and these are used in this chapter:
LV - cable rated from 3001500 V to 1.913.3 kV
MV- cable rated from 3.816.6 kV to 19/33 kV
HV- cable rated at greater than 19/33 kV
Mdticore cable is used in this chapter to describe power cable having two to five
cores. Control cable having seven to 48 cores is referred to as rnulricore control
cable.
Cable insulation and sheaths are variously &scribed as rhemoplastic, thermosetting,
vulcanized, cross-linked, polymeric or elastomeric. All extruded plastic materials
applied to cable are polymeric. Those which would remelt if the temperature during
use is sufficiently high are termed rlzerrnoplastic. Those which are modified chemically
to prevent them from remelting are termed thermosetting, cross-linkedor vulcanized.
Although these materials will not remelt, they will soften and deform at elevated
temperatures if subjected to excessive pressure. The main materials within the two
groups are as follows:
thermoplastic
- polyethylene
- medium-density polyethylene
- polyvinyl chloride
(PE)
(MDPE)
(PVC)
themiosem'ng, cross-linked or vulcanized
- cross-linked polyethylene
- ethylene-propylene rubber
(XLPE)
WR)
Elastomeric materials are polymeric. They are rubbery in nature, giving a flexible
and resilient extrusion. Elastomers are normally cross-linked, such as EPR.
9.2.2 General considerations
Certain design principles are common to power cables, whether they be used in the
industrial sector or by the electricity supply industry.
For many cable types the conductors may be of copper or aluminium. The initial
decision made by a purchaser will be based on price, weight, cable diameter, availability,
the expertise of the jointers available, cable flexibility and the risk of theft. Once a
decision has been made, however, that type of conductor will generally then be
retained by that user, without being influenced by the regular changes in relative
price which arise from the volatile metals market.
For most power cables, the form of conductor will be solid aluminium, stranded
aluminium or stranded copper, although the choice may be limited in certain cable
standards. Solid conductors provide for easier fitting of connectors and setting of the
cores at joints and terminations. Cables with stranded conductors are easier to install
because of their greater flexibility, and for some industrial applications a highly
flexible conductor is necessary.
Where cable route lengths are relatively short a multicore cable is generally
cheaper and more convenient to install than single-core cable. Single-core cables are
sometimes used in circuits where high load currents require the use of large conductor
sizes, between 500 mmz and 1200mm2. In these circumstances the parallel connection
of two or more multicore cables would be necessary in order to achieve the required
Newnes Electrical Engineer's Handbook
209
rating and this presents installation difficulties, especially at terminationboxes. Singlecore cable might also be preferred where duct sizes are small, where longer cable
runs are needed between joint bays or where jointing and termination requirements
dictate their use. It is sometimes preferable to use 3-core cable in the main part of the
route length, and to use single-corecable to enter the restricted space of a termination
box. In this case, a transition from one cable type to the other is achieved using
trifurcating joints which are positioned several metres from the termination box.
Armoured cables are available for applications where the rigours of installation
are severe and where a high degree of external protection against impact during
service is required. Steel Wim Annour (SWA)
or Steel Tape Armour (STA) cables are
available. Generally SWA is preferred because it enables the cable to be drawn into
an installation using a pulling stocking which grips the outside of the oversheath and
transfers all the pulling tension to the SWA.This cannot normally be done with STA
cables because of the risk of dislocating the annour tapes during the pull. Glanding
arrangements for SWA are simpler and they allow full usage of its excellent earth
fault capability. In STA the earth fault capability is much reduced and the retention
of this capability at glands is more difficult. The protection offered against a range
of real-life impacts is similar for the two types.
9.2.3 Paper-insulated cables
Until the mid- 1960s, paper-insulated cables were used worldwide for MV power
circuits. There were at that time very few alternatives apart from the occasional trial
installation or special application using PE or W C insulation.
The position is now quite different. There is a worldwide trend towards XLPE
cable and, in the UK, the industrial sector has adopted XLPE-insulated or EPRinsulated cable for the majority of MV applications, paper-insulated cable now being
restricted to minor uses such as extensions to older circuits or in special industrial
locations. The use of paper-insulated cables for LV has been superseded completely
by polymeric cables in all sectors throughout the world.
The success of polymeric-insulated cables has been due to the much easier, cleaner
and more reliable jointing and termination methods that they allow. However, because
of the large amount of paper-insulatedcable still in service and its continued specification
in some sectors for MV circuits, its coverage here is still appropriate.
Paper-insulated cables comprise copper or aluminium phase conductors which are
insulated with lapped paper tapes, impregnatedwith insulating compound and sheathed
with lead, lead alloy or corrugated aluminium. For mechanical protection, lead or
lead alloy sheathed cables are finished off with an armouring of steel tapes or steel
wire and a covering of either bitumenized hessian tapes or an extrudedPVC oversheath.
Cables that are sheathed with corrugatedaluminium need no further metallic protection,
but they are finished off with a coating of bitumen and an extruded PVC oversheath;
the purpose of the bitumen in this case is to provide additional corrosion protection
should water penetrate the PVC sheath at joints, in damaged areas or by long-term
permeation.
There are, therefore, several basic types of paper-insulated cable, and these are
specified according to existing custom and practice as much as to meet specific
needs and budgets. Particular features of paper-insulated cables used in the electricity
supply industry and in industrial applicationsare given in sections 9.3.1.1 and 9.3.2.1
respectively.
The common element is the paper insulation itself. This is made up of many layers
Wires and cables
210
of paper tape, each applied with a slight gap between the turns. The purity and grade
of the paper is selected for best electrical properties and the thickness of the tape is
chosen to provide the required electrical strength.
In order to achieve acceptable dielectric strength all moisture and air is removed
from the insulation and replaced by Mineral Insulating Non-Draining (MZND)
compound. Its waxy nature prevents any significant migration of the compound
during the lifetime of the cable, even at full operating temperature. This is in contrast
to oil-filled HV cables, which must be pressurized throughout their service life to
keep the insulation fully impregnated. Precautionsare taken atjoints and terminations
to ensure that there is no local displacement of MIND compound which might cause
premature failure at these locations. The paper insulation is impregnated with MIND
compound during the manufacture of the cable, immediately before the lead or
aluminium sheath is applied.
3-core construction is preferred in most MV paper-insulated cables. The three
cores are used for the three phases of the supply and no neutral conductor is included
in the design. The parallel combination of lead or aluminium sheath and amour can
be used as an earth continuity conductor, provided that circuit calculations prove its
adequacy for this purpose. Conductors of 95 mm2 cross-section and greater are
sector shaped so that when insulated they can be laid up in a compact cable construction.
Sector-shaped conductors are also used in lower cross-sections, down to 35, 50 and
70 mm2 for cables rated at 6, 10 and 15 kV respectively.
3-core 6.6 kV cables and most 3-core 11 kV cables are of the belted design. The
cores are insulated and laid up such that the insulation between conductors is adequate
for the full line-to-line voltage (6.6 kV or 11 kV). The laid-up cores then have an
additional layer of insulating paper, known as the belt layer; applied and the assembly
is then lead sheathed. The combination of core insulation and belt insulation is
sufficient for phase-to-earth voltage between core and sheath (3.8 kV or 6.35 kV).
15, 22 and 33 kV 3-core cables and some 11 kV 3-core cables are of screened
design. Here each core has a metallic screening tape and the core insulation is
adequate for the full phase-to-earth voltage. The screened cores are laid up and the
lead or aluminium sheath is then applied so that the screens make contact with each
other and with the sheath.
The bitumenized hessian serving or PVC oversheath is primarily to protect the
armour from corrosion in service and from dislocation during installation. The PVC
oversheath is now preferred because of the facility to mark cable details, and its clean
surface gives a better appearance when installed. It also provides a smooth firm
surface for glanding and for sealing at joints.
9.2.4
Polymeric cables
PVC and PE cables were being used for LV circuits in the 1950s and they started to
gain wider acceptance in the 1960s because they were cleaner, lighter, smaller and
easier to install than paper-insulated types. During the 1970s the particular benefits
of XLPE and HEPR insulations were being recognized for LV circuits and today it
is these cross-linked insulations, mainly XLPE, which dominate the LV market with
PVC usage in decline. LV XLPE cables are more standardized than MV polymeric
types, but even so there is a choice of copper or aluminium conductor, single core or
multicore, SWA or unarmoured, and PVC or Low Smoke and Fume (LSF) sheathed.
A further option is available for LV in which the neutral and/or earth conductor is a
layer of wires applied concentrically around the laid-up cores rather than as an
Newnes Electrical Engineer’s Handbook
21 1
insulated core within the cable. In this case the concentric each conductor can
replace the armour layer as the protective metal layer for the cable.
For MV cables, polyethylene and PVC were shown to be unacceptable for use as
general cable insulation in the years following the 1960s because their thermoplastic
nature resulted in significant temperature limitations. XLPE and EPR were required
in order to give the required properties. They allowed higher operating and shortcircuit temperatures within the cable as well as the advantages of easier jointing and
terminating than for paper-insulated cables. This meant that in some applications a
smaller conductor size could be considered than had previously been possible in the
paper-insulated case.
MV polymeric cables comprise copper or aluminium conductors insulated with
XLPE or EPR and covered with a thermoplastic sheath of MDPE, PVC or LSF
material. Within this general construction there are options of single-core or 3-core
types, individual or collective screens of different sizes and armoured or unarmoured
construction. Single-core polymeric cables are more widely used than single-core
paper-insulated cables, particularly for electricity supply industry circuits. Unlike
paper-insulated cables, polymeric 3-core cables normally have circular-section cores.
This is mainly because the increases in price and cable diameter are usually outweighed
in the polymeric case by simplicity and flexibility of jointing and termination methods
using circular cores.
Screening of the cores in MV polymeric cables is necessary for a number of
reasons, which combine to result in a two-level screening arrangement.This comprises
extruded semiconducting layers immediately under and outside the individual XLPE
or EPR insulation layer, and a metallic layer in contact with the outer semiconducting
layer. The semiconductinglayers are polymeric materials containing a high proportion
of carbon, giving them an electricalconductivity well below that of a metallic conductor,
but well above that required for an insulating material. The term should not be
confused with the semiconductor materials used in electronic components.
The two semiconducting layers must be in intimate contact in order to avoid
partial discharge activity at the interfaces, where any minute air cavity in the insulation
would cause a pulse of charge to transfer to and from the surface of the insulation in
each half-cycle of applied voltage. These charge transfers result in erosion of the
insulation surface and premature breakdown. In order to achieve intimate contact,
the insulation and screens are extruded during manufacture as an integral triple layer
and this is applied to the individual conductor in the same operation. The inner layer
is known as the conductor screen and the outer layer is known as the core screen or
dielectric screen.
When the cable is energized, the insulation acts as a capacitor and the core screen
has to transfer the associated charging current to the insulation on every half-cycle of
the voltage. It is therefore necessary to provide a metallic element in contact with the
core screen so that this charging current can be delivered from the supply. Without
this metallic element, the core screen at the supply end of the cable would have to
carry a substantial longitudinal current to charge the capacitance which is distributed
along the complete cable length, and the screen at the supply end would rapidly
overheat as a result of excessive current density. However, the core screen is able to
carry the current densities relating to the charging of a cable length of say 200 nun.
and this allows the use of a metallic element having an intermittent contact with the
core screen, or applied as a collective element over three laid-up cores. A 0.08 mm
thick copper tape is adequate for this purpose.
The normal form of armoirriitg is a single layer of wire laid over an inner sheath
Wires and cables
212
of PVC or LSF material. The wire is of galvanized steel for 3-core cables and
aluminium for single-core cables. Aluminium wire is necessary for single-core cables
to avoid magnetizing or eddy-current losses within the amour layer. In unarmoured
cable the screen is required to carry the earth fault current resulting from the failure
of any equipment being supplied by the cable or from failure of the cable itself. In
this case, the copper tape referred to previously is replaced by a screen of copper
wires of cross-section between say 6 mm2and 95 mm2, depending on the earth fault
capacity of the system.
9.2.5
Low Smoke and Fume (LSF) cables
Following a number of recent fire disasters, there has been a strong demand for
cables which behave more safely in a fire. Cables have been developed to provide the
following key areas of improvement:
improved resistance to ignition
reduced fire propagation
reduced smoke emission
reduced acid gas emission
An optimized combination of these properties is achieved in LSF cables.
The original concept of LSF cables was identified through the requirements of
underground railways in the 1970s. At that time, the main concern was to maintain
sufficient visibility that orderly evacuation of passengers through a tunnel could be
managed if the power to their train were intempted by a fire involving the supply
cables. This led to the development of a smoke test known as the ‘3-metre cube’, this
being based on the cross-section of a London Underground tunnel.
The demand for LSF performance has since spread to a wide range of products
and applications, and the term LSF now represents.a generic family of cables. Each
LSF cable will meet the 3-metre cube test, but ignition resistance, acid gas emission
and fire propagation performance is specified as appropriate to a particular product
and application. For instance, a power cable used in large arrays in a power station
has very severe fire propagation requirements, while a cable used in individual short
links to equipment would have only modest propagation requirements.
9.3
9.3.1
Main classes of cable
Cables for the electricity supply industry
9.3.1.1 Paper-insulated cables
Until the late 1970s, the large quantities of paper-insulated cables used in the UK
electricity supply industry for MV distribution circuits were manufactured according
to BS 6480. These cables incorporated lead sheaths, steel wire armour and bitumenized
textile beddings or servings. An example is illustrated in Fig. 9.1. The lead sheath
provided an impermeable barrier to moisture and a return path for earth fault currents,
and the layer of steel wire armour gave mechanical protection and an improved earth
fault capacity.
Following successful trials and extensive installation in the early 1970s, a new
standard (ESI 09-12) was issued in 1979 for Paper-Insulated Corrugated Aluminium
Newnes Electrical Engineer’s Handbook
213
Fig.9.1 Lead-sheathed,paper-insulatedMV cablefor the electricity supply industry (courtesy of BICCGeneral)
Sheathed (PICAS) cable. This enabled the electricity supply industry to replace
expensive lead sheath and steel-wire armour with a corrugated aluminium sheath
which offered a high degree of mechanical protection and earth fault capability,
while retaining the proven reliability of paper insulation. The standard was limited to
three conductor cross-sections (95, 185 and 300 mm2), using stranded aluminium
conductors with belted paper insulation. An example of PICAS cable is shown in
Fig. 9.2. PICAS cable was easier and lighter to install than its predecessor and it
found almost universal acceptance in the UK electricity supply sector.
9.3.1.2 MV polymeric cables
High-quality XLPE cable has been manufactured for over 25 years. IEC 502 (revised
in 1998 as IEC 60502) covered this type of cable and was first issued in 1975. A
comparable UK standard BS 6622 was issued in 1986 and revised in 1999.
The following features are now available in MV XLPE cables, and these are
accepted by the majority of users in the electrical utility sector:
0
copper or aluminium conductors
semiconducting conductor screen and core screen
individual copper tape or copper wire screens
PVC or MDPE bedding
copper wire collective screens
steel wire or aluminium armour
PVC or MDPE oversheaths
Early experience in North America in the 1960sresulted in a large number of premature
failures, mainly because of poor cable construction and insufficient care in avoiding
214
Wires and cables
Fig. 9.2 Paper-Insulated Corrugated Aluminium Sheathed (PICAS) cable for the electricity supply industry
(courtesy of BICCGeneral)
contamination of the insulation. The failures were due to ‘water treeing’, which is
illustrated in Fig. 9.3. In the presence of water, ionic contaminants and oxidation
products, electric stress gives rise to the formation of tree-like channels in the XLPE
insulation. These channels start either from defects in the bulk insulation (forming
‘bow-tie’ trees) or at the interfaces between the semiconducting screens and the
insulation (causing ‘vented’ trees). Both forms of tree cause a reduction in electrical
strength of the insulation and can eventually lead to breakdown. Water treeing has
largely been overcome by better materials in the semiconducting screen and by
improvements in the quality of the insulating materials and manufacturing techniques,
and reliable service performance has now been established.
The UK electricity supply industry is to adopt XLPE-insulated or EPR-insulated
cable for MV distribution circuits. Each distribution company is actively assessing
the best construction for its particular needs. An example of the variation between
companies is the difference in practice between solidly bonded systems and the use
of earthing resistors to limit earth fault currents. In the former case the requirement
might typically be to withstand an earth fault current of 13 kA for 3 seconds. In the
latter case only 1 kA for 1 second might be specified, and the use of copper-wire
collective screens in place of steel-wire armour is viable. The specific designs being
used by the UK electricity supply industry are now incorporated into BS 7870-4.
Examples of XLPE cable designs being considered by UK distribution companies
are shown in Figs 9.4, 9.5 and 9.6.
Newnes Electrical Engineer’s Handbook
215
Fig. 9.3 Example of water treeing in a polymeric cable (courtesy of Dr J.C. Fothergill, University of
Leicester)
9.3.1.3 LV polymeric cables
Protective Multiple Earthed (PME) systems which use Combined Neutral and Earth
(CNE) cables have become the preferred choice in the UK public supply network,
both for new installations and for extensions to existing circuits. This is primarily
because of the elimination of one conductor by the use of a common concentric
neutral and earth, together with the introduction of new designs which use aluminium
for all phase conductors.
Before CNE types became established, 4-core paper-insulated sheathed and armoured
cable was commonly used. The four conductors were the three phases and neutral,
and the lead sheath provided the path to the substation earth. The incentive for PME
was the need to retain good earthing for the protection of consumers. With the paper
cables, while the lead sheath itself could adequately carry prospective earth fault
currents back to the supply transformer, the integrity of the circuit was often jeopardized
by poor and vulnerable connections in joints and at terminations. By using the
216
Fig. 9.4 Example of lead-sheathed XLPE-insulated
cable for use in the UK electricity supply industry
(courtesy of BICCGeneral)
Wires and cables
Fig. 9.5 Example of 3-core SWA XLPE-insulated
cable for use in the UK electricity supply industry
(courtesy of BICCGeneral)
neutral conductor of the supply cable for this purpose, the need for a separate earth
conductor was avoided.
The adoption of 0.6/1 kV cables with extruded insulation for underground public
supply in the UK awaited the development of cross-linked insulation systems with a
performance similar to paper-insulated systems in overload conditions. An example
of the cables which have been developed is the ‘Waveform’ CNE type which is
XLPE-insulated and has the neutral/earth conductor applied concentrically in a
sinusoidal form. Insulated solid aluminium phase conductors are laid up to form a
three-phase cable, and the CNE conductor consists of a concentric layer of either
aluminium or copper wires.
If the wires in the CNE conductor are of aluminium, they are sandwiched between
layers of unvulcanized synthetic rubber compound to give maximum protection
against corrosion. This construction is known as ‘Waveconal’ and is illustrated in
Fig. 9.7. Where the CNE conductors are of copper, they are partially embedded in the
rubber compound without a rubber layer over the wires. This is termed ‘Wavecon’
and is illustrated in Fig. 9.8. Some electricity companies have adopted Wavecon
types because of concern over excessive corrosion in the aluminium CNE conductor.
Waveform cables are manufactured in accordance with BS 7870-3.
Both waveform types are compact, with cost benefits. The aluminium conductors
and synthetic insulation result in a cable that is light and easy to handle. In addition,
Newnes Electrical Engineer’s Handbook
Fig. 9.6 Example of single-core copper-wire screen
XLPE-insulated cable for use in the UK electricity
supply industry (courtesy of BICCGeneral)
217
Fig. 9.7 Construction of a ‘Waveform’ XLPEinsulated CNE cable; ‘Waveconal’ (courtesy of
BICCGeneral)
the waveform lay of the CNE conductors enables service joints to be readily made
without cutting the neutral wires, as they can be formed into a bunch on each side of
the phase conductors.
9.3.2
Industrial cables
‘Industrial cables’ are defined as those power circuit cables which are installed on the
customer’s side of the electricity supply point, but which do not fall into the category
of ‘wiring cables’.
Generally these cables are rated 0.6/1 kV or above. They are robust in construction
and are available in a wide range of sizes. They can be used for distribution of power
around a large industrial site, or for final radial feeders to individual items of plant.
Feeder cables might be fixed, or in cases such as coal-face cutting machines and
mobile cranes they may be flexible trailing or reeling cables.
Many industrial cables are supplied to customers’ individual specifications and
since these are not of general interest they are not described here. The following
sections focus on those types of cable which are manufactured to national standards
and which are supplied through cable distributors and wholesalers for general
use.
218
Wires and cables
Fig. 9.8 Construction of a ‘Waveform’XLPE-insulatedCNE cable; ‘Wavecon’ (courtesy of BICCGeneral)
9.3.2.1 Paper-insulated cables
For ratings between 0.6/1 kV and 19/33kV, paper-insulatedcables for fixed installations
were supplied in the UK to BS 480, and then to BS 6480 following metrication in
1969. These cables comprise copper or aluminium phase conductors insulated with
lapped paper tapes, impregnated with MIND compound and sheathed with lead or
lead alloy. For mechanical protection they were finished with an armouring of steel
tapes or steel wire and a covering of bitumenized hessian tapes or an extruded PVC
oversheath.
3-core cables of this type with steel-wire armour have been preferred for most
applications and these have become known as Paper-Insulated Lead-Covered Steel
Wire Armoured (PILCSWA) cables. Single-core cables to BS 6480 do not have
armouring; this is partly because the special installation conditions leading to the
selection of single core do not demand such protection, and partly because a nonmagnetic armouring such as aluminium would be needed to avoid eddy current
losses in the armour. These single-core cables are known as Paper-Insulated Lead
Covered (PILC).
It has already been observed that paper-insulated cables are now seldom specified
for industrial use, but BS 6480 remains an active standard.
Newnes Electrical Engineer’s Handbook
219
9.3.2.2 Polymeric cables for fixed installations
XLPE-insulated cables manufactured to BS 5467 are generally specified for 2301
400 V and 1.9/3.3kV LV industrial distributioncircuits. These cables have superseded
the equivalent PVC-insulated cables to BS 6346 because of their higher current
rating, higher short-circuit rating and better availability.
For M V applications in the range 3.816.6 kV to 19/33 kV, XLPE-insulated wirearmoured cables to BS 6622 are usually specified.
Multicore LV and MV cables are normally steel-wire armoured. This armouring
not only provides protection against impact damage for these generally bulky and
exposed cables, but it is also capable of carrying very large earth fault currents and
provides a very effective earth connection.
Single-core cables are generally unarmoured, although aluminium wire-armoured
versions are available. Single-core cables are usually installed where high currents
are present (for instance in power stationsj and where special precautions will be
taken to avoid impact damage. For LV circuits of this type, the most economic
approach is to use unarmoured cable together with a separate earth conductor, rather
than to connect in parallel the aluminium wire armour of several single-core cables.
For Mv applications each unarmoured cable has a screen of copper wires which
would, together, provide an effective earth connection.
Even in the harsh environment of coal mines, XLPE-insulated types are now
offered as an alternative to the traditional PVC- and EPR-insulated cables used at LV
and MV respectively.In this application the cables are always multicore types having
a single or double layer of steel-wire armour. The armour has to have a specified
minimum conductance because of the special safety requirements associated with
earth faults and this demands the substitution of some steel wires by copper wires for
certain cable sizes.
Where LSF fire performance is needed, LV wire-armoured cables to BS 6724 are
the established choice. These cables are identical in construction and properties to
those made to BS 5467 except for the LSF grade of sheathing material and the
associated fii performance. Cables meeting all the requirements of BS 6724 and, in
addition, having a measure of fire resistance such that they continue to function in a
fire are standardized in BS 7846, further details of which are given in section 9.3.3.
Similarly, BS 7835 for MV wire-armoured cable, which is identical to BS 6622 apart
from the LSF sheath and fire performance, was issued in 1996 and is currently under
revision.
The only other type of standardized cable used for fixed industrial circuits is
multicore control cable, often referred to as auxiliary cable. Such cable is used to
control industrial plant, including equipment in power stations. It is generally wire
armoured and rated 0.6/1 kV. Cables of this type are available with between 5 and 48
cores. The constructions are similar to 0.6/1 kV power cables and they are manufactured
and supplied to the same standards (BS 5467, BS 6346 and BS 6724, as appropriate).
9.3.2.3 Polymeric cables for flexible connections
Flexible connections for both multicore power cables and multicore control cables
are often required in industrial locations. The flexing duty varies substantially from
application to application. At one extreme, a cable may need to be only flexible
enough to allow the connected equipment to be moved occasionally for maintenance.
At the other extreme, the cable may be needed to supply a mobile crane from a cable
reel, or a coal-face cutter from cable-handling gear.
220
Wires and cables
Elastomeric-insulated and sheathed cable is used for all such applications. This
may have flexible stranded conductors (known as ‘class 5 ’ ) or highly flexible stranded
conductors (known as ‘class 6’).Where metallic protection or screening is needed,
this comprises a braid of fine steel or copper wires. For many flexible applications
the cable is required to have a resistance to various chemicals and oils.
Although flexible cables will normally be operated on a 2301400 V supply, it is
normal to use 450/750V rated cables for maximum safety and integrity.
A number of cable types have been standardized in order to meet the range of
performance requirements and the specification for these is incorporated into BS
6500. Guidance on the use of the cables is provided in this standard, and further
information is available in BS 7540.
9.3.3 Wiring cables
The standard cable used in domestic and commercial wiring in the UK since the
1960s is a flat PVC twin-and-earth type, alternatively known as 6242Y cable. This
comprises a flat formation of PVC-insulated live and neutral cores separated by a
bare earth conductor, the whole assembly being PVC-sheathed to produce a flat
cable rated at 300/500V. Cable is also available with three insulated cores and a bare
earth, for use on double-switched lighting circuits. These forms of cable are ideal for
installation under cladding in standard-depth plaster. They are defined in BS 6004,
which covers a large size range, only the smaller sizes of which are used in domestic
and commercial circuits.
There are other cable types included in BS 6004 which have more relevance to
non-domestic installations. These include cables in both flat and circular form, similar
to the 6242‘11type but with an insulated earth conductor. Circular cables designated
6183Y are widely used in commercial or light industrial areas, especially where
many circuits are mounted together on cable trays. Also included in BS 6004 are
insulated conductors designated 6491Y which are pulled into conduit or trunking, in
those circuits where mechanical protection or the facility to rewire are key factors.
An alternative type of cable with outstanding impact and crush strength is MineralInsirluted Cable (MZCC)manufactured to BS 6207.This is often known by its trade
name, Pyrotenax. In an MICC cable, the copper line and neutral conductors are
positioned inside a copper sheath, the spaces between the copper components being
filled with heavily compacted mineral powder of insulating grade. Pressure or impact
applied to the cable merely compresses the powder in such a way that the insulation
integrity is maintained. The copper sheath often acts as the circuit earth conductor.
An oversheath is not necessary, but is often provided for reasons of appearance or for
external marking. MICC cable has a relatively small cross-section and is easy to
install.
In shopping and office complexes or in blocks of flats there may be a need for a
distribution submain to feed individual supply points or meters. If this submain is to
be installed and operated by the owner of the premises, then a 0.6/1 kV split-concentric
service cable to BS 4533 may be used. This comprises a phase conductor insulated
in PVC or XLPE, around which are a layer of copper wires and an oversheath. Some
of the copper wires are bare and these are used as the earth conductor. The remainder
are polymer covered and they make up the neutral conductor. For larger installations,
3-core versions of this cable are available to manufacturers’ specification.
LSF versions of the above cable types are available from several manufacturers.
Some of these have already been standardized according to BS 7211. In circuits
Newnes Electrical Engineer’s Handbook
221
supplyingequipmentfor fire detection and alarm, emergency lighting and emergency
supplies, regulations dictate that the cables will continue to operate during a fire.
This continued operation can be ensured by measures such as embedding the cable
in masonry, but there is a growing demand for cablesthat are fireresistant in themselves.
Fire-resistance categories are set down in BS 6387.
The best fire resistance is offered by MICC cables, since the mineral insulation is
unaffected by fire. MICC cable will only fail when the copper conductor or sheath
melts and where such severe fires might occur the cable can be sheathed in LSF
material to delay the onset of melting. MTCC cable is categorized CWZ in BS 6387.
Alternatives to MICC cables for fire resistance have been developed by individual
manufacturers. Some rely on a filled silicone rubber insulation which degrades during
a fire but continues to provide separation between the conductors so that circuit
integrity is maintained. Other types supplement standard insulation with layers of
mica tape so that even if the primary insulation bums completely the mica tape
provides essential insulation to maintain supplies during the fire. These cable types
are assigned minimum fire-resistance categories BWX and SWX respectively and
manufacturers may claim fire resistance up to CWZ for their own products. Both
cable types are standardized in BS 7629.
Some circuits requiring an equivalent level of fire resistance need to be designed
with larger cables than are found in BS 7629. Such circuits might be for the main
emergency supply, fire-fighting Lifts, sprinkler systems and water pumps, smoke
extraction fans, fire shutters or smoke dampers. These larger cables are standardized
in BS 7846, which includes the size range and LSF performance of BS 6724. These
cables can be supplied to the CWZ performance level in BS 6387. However, there is
an additional fire test category in BS 7846, called F3. which is considered to be more
appropriate for applications where the cable might be subject to fire, impact and
water spray in combination during the fire.
9.4
Parameters and test methods
There are a large number of cable and material properties which are controlledby the
manufacturer in order to ensure fitness for purpose and reliable long-term service
performance. However, it is the operating parameters of the finished installed cable
which are of most importance to the user in cable selection. The major parameters of
interest are as follows:
current rating
capacitance
inductance
voltage drop
earth loop impedance
symmetrical fault capacity
earth fault capacity
These are dealt with in turn in the following sections.
9.4.1
Current rating
The current rating of each individual type of cable could be measured by subjecting
a sample to a controlled environment and by increasing the load current passing
Wires and cables
222
through the cable until the steady-state temperature of the limiting cable component
reached its maximum permissible continuous level. This would be a very costly way
of establishing current ratings for all types of cable in all sizes, in all environments
and in all ambient temperatures. Current ratings are therefore obtained using an
internationally accepted calculation method, published in IEC 60287. The formulae
and reference material properties presented in JEC 60287 have been validated by
correlation with data produced from laboratory experiments.
Current ratings are quoted in manufacturers’literature and are listed in IEEWiring
Regulations (BS 7671) for some industrial, commercial and domestic cables. The
ratings are quoted for each cable type and size in air, in masonry, direct-in-ground
and in underground ducts. Derating factors are given so that these quoted ratings can
be adjusted for different environmental conditions such as ambient temperature, soil
resistivity or depth of burial.
Information is given in BS 7671 and in the IEE Guidance Notes on the selection
of the appropriatefuse or mcb to protect the cable from overload and fault conditions,
and general background is given in sections 8.2 and 8.3.
9.4.2
Capacitance
The capacitancedata in manufacturers’literatureis calculated from the cable dimensions
‘and the permittivity of the insulation.
For example, the star capacitance of a 3-core belted armoured cable to BS 6346
is the effective capacitance between a phase conductor and the neutral star point. It
is calculated using the following formula:
C=
EO
18 h [ ( d + tl
+ t2)/d] [Wflunl
where 6 = relative permittivity of the cable insulation (8.0 for PVC)
d = diameter of the conductor [rnm]
tl = thickness of insulation between the conductors [mm]
t2 = thickness of insulation between conductor and armour [mm]
Equation 9.1 assumes that the conductors are circular in section. For those cables
having shaped conductors, the value of capacitance is obtained by multiplying the
figure obtained using eqn 9.1 by an empirical factor of 1.08.
The calculated capacitancetends to be conservative, that is the actual capacitance
will always be lower than the calculated value. However, if an unusual situation
arises in which the cable capacitanceis critical, then the manufactureris able to make
a measurement using a capacitance bridge.
If the measured capacitancebetween cores and between core and armour is quoted,
then the star capacitance can be calculated using eqn 9.2:
where C, = measuredcapacitancebetween one conductor and the other two connected
together to the armour
C,p= measured capacitance between three conductors connected together
and the amour
Newnes Electrical Engineer’s Handbook
9.4.3
223
Inductance
The calculation of cable inductance L for the same example of a 3-core armoured
cable to BS 6346 is given by eqn 9.3 as follows:
L = 1.02 x I0.2 x ln[2Y/dJ + k} [mwkm]
where
(9.3)
d = diameter of the conductor [mm]
Y = axial spacing between conductors [mm]
k = a factor which depends on the conductor make-up
(k =0.064 for 7-wire stranded
0.055 for 19-wire stranded
0.053 for 37-wire stranded
0.050 for solid)
The same value of cable inductance L is used for cables with circular- or sectorshaped conductors.
9.4.4
Voltage drop
BS 7671 specifies that within customer premises the voltage drop in cables is to be
a maximum value of 4 per cent. It is therefore necessary to calculate the voltage drop
along a cable.
The cable manufacturer calculates voltage drop assuming that the cable will be
loaded with the maximum allowable current which results in the maximum allowable
operating temperature of the conductor. The cable impedance used for calculating
the voltage drop is given by eqn 9.4:
z = { R +~ (27cj~- 1 / 2 7 t f ~ ~ [}a ~m~]
(9.4)
where R = ac resistance of the conductor at maximum conductor
temperature [Wm]
L = inductance [Wm]
C = capacitance [F/m]
f = supply frequency [Hz]
The voltage drop is then given by eqns 9.5 and 9.6:
For single-phase circuits:
voltage drop = 22 [V/A/m]
and for three-phase circuits:
voltage drop = d 32 [V/A/m]
9.4.5
Symmetrical and earth fault capacity
It is necessary that cables used for power circuits are capable of carrying any fault
currents that may flow, without damage to the cable; the requirements are specified
in BS 7671. This assessment demands a knowledge of the maximum prospective
fault currents on the circuit, the clearance characteristics of the protective device (as
explained in Chapter 8) and the fault capacity of the relevant elements in the cable.
For most installations it is necessary to establish the let-through energy of the protective
device and to compare this with the adiabatic heating capacity of the conductor (in
the case of symmetrical and earth faults) or of the steel amour (in the case of earth
faults).
Wires and cables
224
The maximum let-through energy (Z’t) of the protective device is explained in
Chapter 8. It can be obtained from the protective device manufacturer’s data. In
practice the value will be less than that shown by the manufacturer’s information
because of the reduction in current during the fault which results from the significant
rise in temperature and resistance of the cable conductors.
The fault capacity of the cable conductor and amour can be obtained from
information given in BS 7671 and the appropriate BS cable standard, as follows:
k2S2 = adiabatic fault capacity of the cable element
(9.7)
where S = the nominal cross-section of the conductor or the nominal
cross-section of, say, the armour [mm’]
k = a factor reflecting the resistivity, temperature coefficient,
allowable temperature rise and specific heat of the metallic
cable element
(k = 115 for a PVC-insulated copper conductor within the cable
= 176 for an XLPE-insulated copper earth conductor external
to the cable
= 46 for the steel armour of an XLPE-insulated cable)
In practice it will be found that provided the cable rating is at least equal to the
nominal rating of the protective device and the maximum fault duration is less than
5 seconds, the conductors and armour of cables to BS will easily accommodate the
let-through energy of the protective device.
It is also important that the impedance of the supply cable is not so high that the
protective device takes too long to operate during a zero-sequence earth fault on
connected equipment. This is important because of the need to protect any person in
contact with the equipment, by limiting the time that the earthed casing of the
equipment, say, can become energized during an earth fault.
This requirement, which is stated in BS 7671, places restrictions on the length of
cable that can be used on the load side of a protective device, and it therefore
demands a knowledge of the earth fault loop impedance of the cable. Some cable
manufacturers have calculated the earth fault impedance for certain cable types and
the data are presented in specialized literature. These calculations take account of the
average temperature of each conductor and the reactance of the cable during the
fault. The values are supported by independent experimental results. BS 7671 allows
the use of such manufacturer’s data or direct measurement of earth fault impedance
on a completed installation.
9.5
Optical communication cables
The concept of using light to convey information is not new. There is historical
evidence that Aztecs used flashing mirrors to communicate, and in 1880 Alexander
Graham Bell first demonstrated his photophone. in which a mirror mounted on the
end of a megaphone was vibrated by the voice to modulate a beam of sunlight,
thereby transmitting speech over distances up to 200 m.
Solid-state photodiode technology has its roots in the discovery of the lightsensitive properties of selenium in 1873, used as the detector in Bell’s photophone.
The Light Emitting Diode (LED) stems from the discovery in 1907 of the
electroluminescent properties of silicon, and when the laser was developed in 1959,
Newnes Electrical Engineer’s Handbook
225
Transmitter
Information input
(electrical)
Receiver
Connector
J
Connector
Light
Information
output
(optical)
2
Optical fibre
Fig. 9.9 Basic fibre optic system
9.5.1
Optical fibres
An optical fibre is a dielectric waveguide for the transmission of light, in the form of
a thin filament of very transparent silica glass. As shown in Fig. 9.10, a typical fibre
comprises a core, the cladding, a primary coating and sometimes a secondary coating
or buffer. Within this basic construction, fibres are further categorized as multimode
or single-mode fibres with a step or graded index.
7
Primary coating
J
Fig. 9.10 Basic optical fibre
The core is the part of the fibre that transmits light, and it is surrounded by a glass
cladding of lower refractive index. In early fibres, the homogeneous core had a
constant refractive index across its diameter, and with the refractive index of the
cladding also constant (at a lower value) the profile across the whole fibre diameter,
as shown in Fig 9.11(a), became known as a step index. In this type of fibre the light
rays can be envisaged as travelling along a zig-zag path of straight lines, kept within
the core by total reflection at the inner surface of the cladding. Depending on the
angle of the rays to the fibre axis, the path length will differ so that a narrow pulse
of light entering the fibre will become broader as it travels. This sets a limit to the
5
8
'md
cn
.-c
--a
B
a
5
8
8
0
T-
E
- 0
u
Newnes Electrical Engineer’s Handbook
227
rate at which pulses can be transmitted without overlapping and hence a limit to the
operating bandwidth.
To minimize this effect, which is known as mode dispersion, fibres have been
developed in which the homogeneous core is replaced by one in which the refractive
index varies progressively from a maximum at the centre to a lower value at the
interface with the cladding. Figure 9.11(b) shows such a graded indexfibre, in
which the rays no longer follow straight lines. When they approach the outer parts of
the core, travelling temporarily faster, they are bent back towards the centre where
they travel more slowly. Thus the more oblique rays travel faster and keep pace with
the slower rays travelling nearer the fibre centre. This significantly reduces the pulse
broadening effect of step index fibres.
The mode dispersion of step index fibres has also been minimized by the more
recent development of single-rnodefibres. As shown in Fig. 9.11(c), although it is a
step index fibre, the core is so small (of the order of 8 pm in diameter) that only one
mode can propagate.
Fibre manufacture involves drawing down a preform into a long thin filament.
The preform comprises both core and cladding, and for graded index fibres the core
contains many layers with dopants being used to achieve the varying refractive
index. Although the virgin fibre has a tensile strength comparable to that of steel, its
strength is determined by its surface quality. Micro-cracks develop on the surface of
a virgin fibre in the atmosphere, and the lightest touch or scratch makes the fibre
impractically fragile. Thus it must be protected, in line with the glass drawing before
it touches any solid object such as pulleys or dnxms, by a protective coating of resin,
acetate or plastic material, called the primal?; coating.
Typically the primary coating has a thiclaess of about 60 pm, and in some cases
a further layer of material called the bufler is added to increase the mechanical
protection.
9.5.2
Optical cable design
The basic aim of a transmission cable is to protect the transmission medium from its
environment and the rigours of installation. Conventional cables with metallic
conductors are designed to function effectively in a wide range of environments, as
shown in sections 9.2 and 9.3. However, optical fibres differ significantly from
copper wires to an extent that has a considerable bearing on cable designs and
manufacturing techniques. The transmission characteristics and lifetime of fibres are
adversely affected by quite low levels of elongation, and lateral compressions can
produce small kinks or sharp bends which create an increase in attenuation loss
known as microbending loss. This means that cables must protect the fibre from
strain during installation and service, and they must cater for longitudinal compression
that occurs, for example, with a change in cable temperature.
Fibre life in service is influenced by the presence of moisture as well as stress.
The minute cracks which cover the surface of all fibres can grow if the fibre is
stressed in the presence of water, so that the fibre could break after a number of years
in service. Cables must be able to provide a long service life in such environments
as tightly packed ducts which are filled with water.
The initial application of optical cables was the trunk routes of large
telecommunications networks, where cables were directly buried or laid in ducts in
very long lengths, and successful cable designs evolved to take into account the
constraints referred to above. The advantages of fibre optics soon led to interest in
228
Wires and cables
other applications such as computer and data systems, military systems and industrial
control. This meant that cable designs had to cater for tortuous routes of installation
in buildings, the flexibility of patch cords and the arduous environments of military
and industrial applications.
Nevertheless. many of the conventional approaches to cable design can be used
for optical cables, with modification to take into account the optical and mechanical
characteristics of fibres and their fracture mechanics.
Cables generally comprise several elements or individualtransmission components
such as copper pairs, or one or more optical fibres. The different types of element
used in optical cables are shown in Fig. 9.12.
The primary-coated fibre can be protected by a buffer of one or more layers of
plastic material as shown in Fig. 9.12(a). Typically for a two-layer buffer, the inner
layer is of a soft material acting as a cushion with a hard outer layer for mechanical
protection, the overall diameter being around 850 pm. In other cases the buffer can
be applied with a sliding fit to allow easy stripping over long lengths.
In ruggedizedfibres further protectionfor a buffered fibre is provided by surrounding
it with a layer of non-metallic synthetic yarns and an overall plastic sheath. This type
of arrangement is shown in Fig. 9.12(b).
When one or more fibres are run loosely inside a plastic tube, as shown in Fig.
9.12(c), they can move freely and will automaticallyadjust to a position of minimum
bending strain to prevent undue stress being applied when the cable is bent. If the
fibre is slightly longer than the tube, a strain margin is achieved when the cable is
stretched say during installation, and for underground and duct cables the tube can be
filled with a gel to prevent ingress of moisture. Correct choice of material and
manufacturingtechnique can ensure that the tube has a coefficientof thermal expansion
similar to that of the fibre, so that microbending losses are minimized with temperature
excursions.
Optical fibres can be assembled into a linear array as a ribbon, as shown in Fig.
9.lZ(d). Up to 12 fibres may be bonded together in this way or further encapsulated
if added protection is required.
In order to prevent undue cable elongation which could stress the fibres, optical
cables generally incorporate a strength member. This may be a central steel wire or
strand, or non-metallic fibreglassrods or syntheticyarns. The strength member should
be strong, light and usually flexible, although in some cases a stiff strength member
can be used to prevent cable buckling which would induce microbending losses in the
fibres. Strength members are shown in the cable layouts in Figs 9.13(b) and 9.13(c).
The strength member can be incorporated in a structural member which is used as
a foundation for accommodating the cable elements. An example is shown in Fig.
9.13(c). where a plastic section with slots is extruded over the strength member with
ribbons inserted into the slots to provide high fibre count cables.
A moisture barrier can be provided either by a continuous metal sheath or by a
metallic tape with a longitudinal overlap, bonded to the sheath. Moisture barriers can
be of aluminium, copper or steel and they may be flat or corrugated. In addition,
other cable interstices may be filled with gel or water-swellable filaments to prevent
the longitudinal ingress of moisture.
Where protection from external damage is required, or where additional tensile
strength is necessary, armouring can be provided; this may be metallic or nonmetallic. For outdoor cables, an overall sheath of polyethylene is applied. For indoor
cables the sheath is often of low-smoke zero-halogen materials for added safety in
the event of fire.
S
Fibre
(a) Buffered fibre
Fig. 9.12 Optical cable elements
(b) Ruggedized fibre
(c) Tube
(d) Ribbon
Armour
Sheath
Strength
member
Sheath
/ \
Strength
member
\
I
Strength
member
Primary
/fibres in
Buffe
fibre
(a) Duplex buffered fibre indoor cable
Fig. 9.13 Examples of optical cables
(b) 48 fire duct cable with tubed fibres
(c) 240 fire slotted-core cable for direct burial
Newnes Electrical Engineer’s Handbook
231
Although the same basic principles of cable construction are used, the wide range
of applications results in a variety of cable designs, from simplex indoor patch cords
to cables containing several thousand fibres for arduous environments, to suboceanic
cables. Figure 9.13 shows just a few examples.
9.5.3
Interconnections
The satisfactory operation of a fibre optic system requires effective jointing and
termination of the transmission medium, in the form of fibre-to-fibre splices and
fibre connections to repeaters and end equipment. This is particularly important
because with very low loss fibres the attenuation due to interconnections can be
greater than that due to a considerable length of cable.
For all types of interconnection there is a loss, known as the insertion loss, which
is caused by Fresnel reflection and by misalignment of the fibres.
Fresnel reflection is caused by the changes in refractive index at the fibre-airfibre interface, but it can be minimized by inserting into the air gap an indexmatching fluid with the same refractive index as the core.
Misalignrnent losses arise from three main sources as shown in Fig. 9.14.
Interconnection designs aim to minimize these losses. End-face separation (Fig.
9.14(a)) allows light from the launch fibre to spread so that only a fraction is captured
by the receive fibre; this should therefore be minimized. Normally the fibre cladding
is used as the reference surface for aligning fibres, and the fibre geometry is therefore
important,even when claddmgs are perfectly aligned. Losses due to lateral misalignment
(Fig. 9.14(b)) will therefore depend on the core diameter, non-circularity of the core,
cladding diameter, non-circularity of the cladding and the concentricity of the core
and cladding in the fibres to be jointed. Angular misalignment can result in light
entering the receive fibre at such an angle that it cannot be accepted. It follows that
very close tolerances are required for the geometry of the joint components and the
fibres to be jointed, especially with single-mode fibres with core diameters of 8 pm
and cladding diameters of 125 pm.
The main types of interconnections are fibre splices and demountable connectors.
Fibre splices are permanent joints made between fibres or between fibres and
device pigtails. They are made by fusion splicing or mechanical alignment. Infusion
splicing, prepared fibres are brought together, aligned and welded by local heating
combined with axial pressure. Sophisticated portable equipment is used for fusion
splicing in the field. This accurately aligns the fibres by local light injection and
carries out the electric arc welding process automatically. Nevertheless, a level of
skill is required in the preparation of the fibres, stripping the buffers and coatings and
cleaving the fibres to achieve a proper end face. There are a number of mechanical
techniques for splicing fibres which involve fibre alignment by close tolerance tubes,
ferrules and v-grooves, and fixing by crimps, glues or resins. Both fusion and mechanical
splicing techniques have been developed to allow simultaneous splicing of fibres
which are particularly suitable for fibre ribbons. For a complete joint, the splices
must be incorporated into an enclosure which is suitable for a variety of environments
such as underground chambers or pole tops. The enclosure must also terminate the
cables and organize the fibres and splices, and cassettes are often used where several
hundred splices are to be accommodated.
Demountable connectors provide system flexibility, particularly at and within the
transmission equipment and distribution panels, and they are widely used on patch
cords in certain data systems. As with splices, the connector must minimize Fresnel
Wires and cables
232
............................................
i
r
I
1-
1
I
I
(a) End face separation
(b) Lateral misalignment
...........................................
..........................................
(c)Angular misalignment
Fig. 9.14 Sources of misalignment loss
and misalignmentloss, but it must also allow for repeated connection and disconnection,
it must protect the fibre end face and it must cater for mechanical stress such as
tension, torsion and bending. Many designs have been developed, but in general the
tolerances that are achievable on the dimensions of the various components result in
a higher optical loss than in a splice. Demountable connectorshave also been developed
for multiple-fibre simultaneous connection, with array designs being particularly
suitable for fibre ribbons. For connector-intensive systems such as ofice data systems,
use is made of factory-predetermined cables and patch cords to reduce the need for
on-site termination.
9.5.4
Installation
Optical fibre cables are designed so that normal installation practices and equipment
can be used wherever possible, but as they generally have a lower strain limit than
metallic cables special care may be needed in certain circumstances and manufacturer’s
recommendations regarding tensile loads and bending radii should be followed.
Special care may be needed in the following circumstances:
because of their light weight, optical cables can be installed in greater lengths
than metallic cables. For long underground ducts access may be needed at
intermediate points for additional winching effort, and space should be allowed
for larger ‘figure 8’ cable deployment.
Newnes Electrical Engineer’s Handbook
233
mechanicalfuses and controlled winching may be necessary to ensure that the
rated tensile load is not exceeded
guiding eqzripinent may be necessary to avoid subjecting optical cables to
unacceptable bending stresses, particularly when the cable is also under tension
when installing cables in trenches the footing should be free from stones.
These could cause microbending losses.
in buildings, and particularly in risers, cleats and fixings should not be
overtightened, or appropriate designs should be used to prevent compression
and the resulting microbending losses
indoor cable routes should provide turning points if a large number of bends
is involved. Routes should be as straight as possible.
excess lengths for jointing and testing of optical cables are normally greater
than those required for metallic cables
where non-metallic optical cables are buried, consideration of the subsequent
location may be necessary. Marker posts and the incorporation of a location
wire may be advisable.
BlownJibre systems have been developed as a means of avoiding fibre overstrain
for complex route installation and of allowing easy system upgrading and future
proofing. It results in low initial capital costs and provides for the distribution of
subsequent costs. Initially developed by British Telecom, the network infrastructure
is created by the most appropriate cabling method, being one or a group of empty
plastic tubes. As and when circuit provision is required, one or more fibres can be
blown by compressed air into the tubes. Individual tubes can, by means of connectors,
be extended within buildings up to the fibre terminating equipment. The efficient
installation of fibres into the tube network often requires the use of specially designed
fibres and equipment such as air supply modules, fibre insertion tools and fibre payoffs. For installation it is necessary to follow the instructions provided by the supplier,
taking into account the requirements for the use of portable electrical equipment and
compressed air, and the handling, cutting and disposal of optical fibres. A novel
variation of this system is a data cable used for structural wiring systems. In a
‘figure-8’ configuration one unit comprises a 4-pair data cable and the other an
empty tube, so that when an upgrade is required to an optical system the appropriate
fibre can be blown in without the need for recabling.
9.6
9.6.1
Standards
Metallic wires and cables
Most generally available cables are manufactured to recognized standards which
may be national, European or international. Each defines the construction, the type
and quality of constituent materials, the performance requirementsand the test methods
for the completed cable.
IEC standards cover those cables which need to be standardized to facilitate world
trade, but this often requires a compromise by the parties involved in the preparation
and acceptance of a standard. Where cables are to be used in a particular country, the
practices and regulations in that country tend to encourage the more specific cable
types defined in the national standards for that country. BS remains the most appropriate
for use in the UK, and for the main cable types described in this chapter reference has
therefore been made mainly to the relevant BS.
Wires and cables
234
Some cables rated at 450/750 V or less have through trade become standard
throughout the EU, and these have been incorporatedinto HarmonizationDocuments
(HDs). Each EU country must then publish these requirements within a national
standard. A harmonized cable type in the UK for instance would still be specified to
the relevant BS and the cable would, if appropriate, bear the <HAR>mark.
The key standards for metallic wires and cables which have been referred to in the
chapter are listed in Table 9.1.
Table 9.1 International and national standards for metallic wires and cables
IEC
HD
BS
603
4553
60502-1
60227
21
60245
22
60055
621
621
60227 L%60245
21 L% 22
60502-2
620
22
60364
60287
603
620
626
604
622
Subject
600/1000 V PVC-insulated single-phase split
concentric cables with copper conductors
5467
Cables with thermosetting insulation up to 600/
1000 V and up to 1900/3300V
Non-armoured PVC-insulatedcables rated up to
6004
450/750 V
Non-armoured rubber-insulated cables rated up
6007
to 450/750 V
PVC-insulated cables
6346
Performance requirementsfor cables required to
6387
maintain integrity under fire conditions
Impregnatedpaper-insulatedlead sheathedcables
6480
up to 33 000 v
(EA 09-12) Paper-insulated corrugated aluminium sheathed
6350/11000 V cable
Insulated flexible cords and cables rated up to
6500
450/750 V
Cables with XLPE or EPR insulation from 3800/
6622
6600 V up to 19 000/33000 V
600/1000V and 1900/3300V armoured cables
6724
having thermosettinginsulationwith low emission
of smoke and corrosive gases in fire
Non-armoured cables having thermosetting
7211
insulationrated up to 450/750 V with low emission
of smoke and corrosive gases in fire
Fire-resistantthermosetting-insulated cables rated
7629
at 300/500V with limited circuit integrity in fire
7671
Requirementsfor electrical installations: IEE Wiring
Regulations (16th edition)
7769
Electric cables - calculation of current rating
Cables with XLPE or EPR insulation from 3800/
7835
6600 V up to 19 000/33000 V with low emission
of smoke and corrosive gases in fire
Polymeric-insulated cables for distribution rated
7870-3
at 600/1000V
Polymeric-insulatedcables for distribution rated
7870-4
from 380016600 V up to 19 000133 000 V
Polymeric-insulated aerial-bundled cables rated
7870-5
600/1000V for overhead distribution
Polymeric-insulated cables for generation rated
7870-6
at 600/1000V and 1900/3300V
Polymeric-insulated cables for generation rated
7870-7
from 3800/6600V UD to 19 000/33000 V
Newnes Electrical Engineer’s Handbook
9.6.2
235
Optical communication cables
For communicationsystems and their evolution to be effective, standardizationmust
be at an international level. Optical fibre and cable standardizationin IEC started in
1979. In Europe ENS have been published; these generally use the IEC standards as
a starting point but they incorporate any special requirements for sale within the EU.
Table 9.2 summarizes the main standards in the areas of optical fibres, optical
cables, connectors, connector interfaces and test and measurement procedures for
interconnecting devices.
Table 9.2 International and national standards for optical fibres, optical cables, connectors, connector
interfaces and test and measurement procedures for interconnecting devices
~~
IEC
EN
~
BS
Subject
7718
Code of practice for installation
of fibre optic cabling
60793
60794
60794-3
EN60794-3
60874-17
60874-19
61300
61754
186000 to 186290
187000
187100 to 187102
188000
188100 to 188102
188200 to 188202
EN60874-17
EN60874-19
EN61300
EN61754
EN186000 to
EN187000
EN187100 to
EN188000
EN188100 to
EN188200 to
60874
61300
61754
EN 186290
EN 187102
EN 188102
EN 188202
References
9A.
9B.
Moore. G.EI Electric Cables Haizdbook, 3rd edn, Blackwell Scientific Publications Ltd, 1997.
Heinhold, L., Power Cables mid Their Application, 3rd edn, Siemens AG, 1990.
Chapter 10
Motors, motor control and drives
Dr Norman N. Fulton
Swifched Reluctance Drives Ltd
(A subsidiary of Emerson Electric Go.)
10.1 Introduction
Electric motors are used throughout industry, commerce and in the home in a wide
variety of power ratings, speeds and duties. A typical home will easily have 4.0
motors in domestic appliances, hand tools and audio equipment; a modem car is
unlikely to have fewer than 15 motors, ranging from the starter motor to adjusters for
windows, seats and mirrors. Electric motors were invented well over 150 years ago
and a number of distinct types of machine have developed, as described later in this
chapter. Although usually regarded as a mature technology, the changing needs for
motors and the emergence of new associated technologies, such as power
semiconductors, continue to lead to changes in the types of machine that are used, in
their applications and also in the way that motors are designed and built.
For instance, the field of adjustable speed drives was at one time dominated by dc
machines because speed control could be readily managed without complicated highspeed switching of the power. With the emergence of power semiconductors, first in
the form of thyristors and bipolar transistors, then MOSFETs and IGBTs, power
switching technology has become commercially viable across a wide power range
from watts to megawatts. One application of that technology has been in variablefrequency inverters, which enable the cheaper, more reliable ac induction motor to be
used in many adjustable speed drives in place of the more complicated and expensive
dc motor.
Advances in magnet technology have made permanent magnet motors more costcompetitive, especially in adjustable speed drives, and the so-called ‘brushless dc
drive’ has already found widespread acceptance in applications in a wide variety of
domestic and commercial products where a low-cost solution is required. In the
industrial market, these drives are more commonly found in high-cost, high-precision
drive systems, often where a positioning function is required in addition to speed
control.
More recently, the switched reluctance motor has re-emerged. It was one of the
earliest forms of electric motor but fell into disuse because of a lack of suitable
control devices. Now incorporated into a switched reluctance drive using modem
semiconductors, the switched reluctance motor is arguably cheaper and simpler than
the induction motor, and it threatens to take over variable speed applications currently
served by induction motors, as well as competing with dc motors and ac universal
motors.
Motors, motor control and drives
238
These types of motor and drive are described later in the chapter, but each of them
relies on common basic principles and has to operate within certain constraints.
These principles and constraints are explained before describing the various types of
motor and drive.
10.2 General characteristics
This section covers the principles of torque production, the main components found
in all motors and the related topics of losses, efficiency and heat dissipation.
10.2.1 The production of torque
The power delivered by a rotating shaft is the product of torque and speed. The
mechanisms for the production of torque are therefore a critical part of motor operation
and, as shown below, are sometimes used to distinguish the different types of motor.
There are two basic principles for torque production in electric motors; these are
known as electromagnetic torque and reluctance torque. Virtually all motors use one
or the other, or a combination of both.
Figure 10.1 shows a conductor carrying a current. At right angles to the conductor
there is a uniform magnetic field having a density of B webers per square metre or
3 tesla. If the conductor has a length 1 metres and the current is I amps, it can be
shown that the force on the conductor in the direction shown [in newtons), is given
by eqn 10.1:
F=BxIxl
(10.1)
This is termed 'electmmagnetic'force since it is the product of an electric current
with a magnetic flux density. If this force acts on a conductor mounted on the rotor
Force
I
Current
I'
/
l
Fig. 10.1 Electromagnetic force production
l Magnetic Field
Newnes Electrical Engineer’s Handbook
239
of a motor at a radius r metres, then the torque acting on the rotor is F x r (in newton
metres). This means of electromagnetic torque production is the most commonly
used in electric motors, although different types of motors use different methods of
passing the current I through the conductor and different ways of producing the flux
density B. It is used in, for example, induction, synchronous and dc motors.
The second torque production method used in electric motors makes use of the
fact that any magnetic circuit tends to move to a position at which its reluctance is
a minimum (i.e. where the inductance of the exciting coil producing the mmf is a
maximum). This is exploited by having a magnetic structure which has saliency.
Figure 10.2 shows a machine with a stator and a rotor both having salient poles: as
the rotor turns, the air gap presented to the stator poles changes, resulting in a change
in the reluctance of the magnetic circuit. By passing current into the winding around
the stator poles, magnetic flux is set up in the magnetic circuit. The path of the flux
is shown very approximately in Fig. 10.2. The flux produces a force which acts to
reduce the reluctance of the magnetic circuit and the rotor will experience a reluctance
torque as it attempts to align itself in a position of minimum reluctance (or maximum
inductancej when the stator and rotor saliencies are aligned to give the minimum air
gap between rotor and stator. This torque production method is used in stepper
motors and switched reluctance motors; the torque can be produced by exciting a
winding on only one member and it is not necessary to have a permanent magnet on
either member.
Fig. 10.2 Reluctance torque production
10.2.2
Main components and construction
Generally, motors comprise a stationary part, the stator, and a rotating part, the rotor.
mically, the stator is on the outside and the rotor rotates within it, although inverted
machines, typically found in small fan drives, have the rotor on the outside. The main
240
Motors, motor control and drives
part of both stator and rotor is a core which is usually built from a stack of electrical
steel laminations and has slots or salient poles to accommodate and support the
windings. The laminations are normally 0.2 to 1.0 mm thick. In small and medium
power motors they are punched from coils of electrical steel by a multi-stage press
tool; in larger sizes they are produced on single-stage presses from sheet.
The general construction of small and medium power motors is illustrated by the
sectional drawing of a cage induction motor in Fig. 10.3. The stator windings are
embedded in slots around the inner bore of the stator core and the winding overhang
outside the core is securely laced together. The outer surface of the rotor core is
accurately machined to be concentric with the bearing journals and to have a diameter
which, in conjunction with the stator bore, will produce the air gap of the machine.
Newnes Electrical Engineer’s Handbook
241
The air gap will have a radial length of the order of 0.2 mm for very small machines
and up to several cm for machines developing megawatts.
In some motors, the stator is contained in an external frame and the rotor is
supportedin bearings mounted in the endshields or endbrackets, which are located in
the ends of the stator frame by spigots which ensure the rotor runs centrally within
the stator. These are shown in Fig. 10.3 and also more clearly in the exploded
diagram Fig. 10.4. In many cases, the frame is finned to increase the surface area and
improve dissipation of the heat generated by the electrical losses in the motor. The
complete motor can be supported by feet on the stator (foot mounted) or by providing
a flange on the drive-end endshield (flange mounted), or occasionally by both. By
contrast, motors which are intended for incorporation into an enclosed appliance,
such as a dishwasher pump, generally use a so-calledframeless construction, where
there is no separate frame for the core and skeletal bearing housings are mounted
directly to the ends of the stator core. Larger high-voltage motors are generally
mounted within fabricated steel frames and endshields.
One method of classification of machines, particularly for industrial applications,
is to describe the degree of protection offered to the user and the mechanical protection
afforded to the windings of the machine by the frame. Various levels of protection are
used and these are defined in, for example, BS 4999 Part 105 (IEC60034-5) in the
form of various ZP classifications. IP22, for instance, is a basic type of enclosure
often referred to as drip-proof or open ventilated and this is commonly used for
induction motors driving compressors. Amuch greater degree of protection is offered
by an IP55 enclosure which is often known as totally enclosed. Special forms of
enclosure are required for, for example, flameproof (EEx ‘d’) motors for hazardous
areas and these requirements are described further in Chapter 16. An induction
motor with an EEx ‘d‘ enclosure is illustrated in Fig. 16.3.
As noted above, the rotor is normally mounted in bearings housed in the endshields.
The rating of these bearings is dictated by, among other things, the speed of the
motor and the side and end loads on the shaft. Simple sleeve bearings are suitable at
low powers and loads, standard deep groove ball bearings are commonly used in the
1-200 kW range, roller bearings are often used for motors with high radial shaft
loads (such as a belt drive with high tension in the belt) and sophisticatedlubrication
systems are used on large machines.
With the advent of adjustable speed drives which require a speed feedback signal,
it is becoming more common to find some form of speed or position transducer
mounted on the motor, typically at the non-drive end. The devices used range from
simple, low-cost optical switches in switched reluctance motors through Hall-effect
transducers in brushless dc machines to high-cost, high-resolution resolvers in dc
servo drives. The devices may have their own terminal box and wiring loom or they
may be connected through the main terminal box of the motor.
10.2.3
Losses and heat transfer
Losses in electrical machines are important first because they determine the efficiency
of the machine and secondly because they generate heat in the machine. In many
cases efficiency is not a primary concern to the user but it should be noted that a
motor running continuously on full load will have an energy consumption which
could equal its capital cost in a matter of months. The heat generated by the losses
has to be dissipated effectively in order to ensure that the insulation in the machine
does not operate at temperatures above its rated capability and that the surface
Newnes Electrical Engineer’s Handbook
243
temperature of the enclosure does not present a hazard to the user. It follows that the
ability of the machine to dissipate its losses will directly affect its output rating.
The power loss in most electrical machines can be classified under four headings:
0
a
e
e
resistive loss in the windings (copper loss)
loss in the stator and rotor cores due to hysteresis and eddy current losses (iron
loss)
friction loss in the bearings
windage loss associated with the rotor causing turbulence in the air surrounding
it
Many machines operate at fixed speed and fixed voltage, and for these machines
some components of the loss are independent of the power level at which they are
operating; the friction and windage loss and many of the components of iron loss fall
into this category. However, the copper loss in the windings generally varies as the
square of the machine load. So although the losses increase with load, they decrease
with load when taken as a proportion of power output; this means that the efficiency
of an electrical machine generally increases as its power output increases and is
likely to peak near full load.
The balance of the components of the loss varies with both the type and size of
machine. In general, small machines have a much greater proportion of copper loss
than iron loss, whereas the opposite holds for larger machines. Since the copper loss
is generally load dependent, it follows that the efficiency of a small machine varies
much more with load than a larger machine. In terms of absolute values of efficiency
for different sizes of machine, induction machines of the same rated speed have fullload efficiencies of approximately:
0.1 kW
1.0 kW
10 kW
100 kW
1 MW
55%
75%
90%
95%
97%
The benefit of scale is clearly shown in these figures.
Because many of the power losses occur in the interior of the machine, arrangements
have to be madie to ensure that the heat resulting from these losses is conducted away
effectively. The heat from the windings is normally dissipated into air flowing around
exposed parts of the winding and/or into the core supporting the winding. To assist
in the latter, the winding is thermally bonded to the core by impregnating the assembly
with a varnish or epoxy compound which consolidates the winding, improves its
electrical insulation and greatly improves the heat transfer to the core. Methods of
impregnation include:
* dipping the wound core in varnish, draining the excess and baking it in an
e
0
oven
trickling a viscous gel (normally polyester based) onto the winding and allowing
it to be drawn in by capillary action before setting
evacuatiing the air from the winding in a vacuum chamber, flooding the chamber
with varnish (normally an epoxy compound), increasing the pressure in the
Motors, motor control and drives
244
chamber above ambient to force the impregnant into the winding, draining the
excess and baking in an oven. This process is known as Vacuum Pressure
Impregnation (VPI) and produces a very rugged assembly with good heat
transfer from the winding to the core. A large VPI plant is shown in Fig. 3.6.
Heat from the magnetic losses in the cores can be removed either by direct cooling
with an airstream or by conduction to the frame. In the latter case, the interference
fit between the core and the frame needs to be carefully controlled to ensure good
thermal conduction.
In small machines of less than a kilowatt rating, the heat transfer paths are short
and dissipating the losses is relatively straightforward, particularly since the machine
has a relatively high surface to volume ratio. In the medium range of power output
up to several hundred kilowatts, many machines are of the totally enclosed type
referred to in the previous section; these generally have a shaft-mounted fan outside
the frame at the non-drive end, which is arranged to drive air over the outside surface
of the frame. These are known as Totally Enclosed Fan Ventilated (TEFV) machines
(see Fig. 10.3). Larger machines in the megawatt range present greater problems for
heat transfer because of the absolute size of the losses involved (even though the
efficiency is higher) and the fact that the surface to volume ratio falls as the machine
size increases. Special arrangements have to be adopted, including dividing the cores
into short packets with cooling air flowing between them, providing water cooling
for the stator core, using closed air circulation within the machine in conjunction
with a water-cooled heat exchanger etc. A large closed air circuit generator is shown
in Fig. 5.21 and large motors have very similar forms of construction. In the largest
sizes, hollow conductors with de-ionized water flowing through them are used.
10.3 Main classes of machine
Electrical machines can be classified in many different ways, such as by:
0
0
0
type of supply (ac, dc or switched)
method of torque production (electromagnetic, reluctance or both)
method of providing the ‘working’ flux (electromagnetsor permanent magnets)
fixed or variable speed
number of phases
No one classificationis entirely satisfactory,not least because the marriage of machines
and electronic control systems has produced variable speed drives which do not fall
easily into any of the main groups. In this chapter six main groups are identified
which cover the most commonly found motors. These main groups, induction,
synchronous, commutator, permanent magnet, switched reluctance machines and
steppers, are shown in Fig. 10.5. Together, these cover the vast majority of all motors
driving equipment in domestic, commercial and industrial applications.Each is described
briefly in turn in the following sections.
10.3.1
Induction motors
Invented in the 188Os, the induction machine is by far the most common motor in
domestic, commercial and industrial use, driving fans, pumps, compressors, conveyors,
pb
E
Motors, motor control and drives
246
machine tools and a wide range of other loads. Motors ranging from a few watts to
several megawatts can be found, single-phase motors being commonly used up to
around a kilowatt and three-phase machines for higher powers. Two main subdivisions
are immediately apparent: the cage rotor induction motor and the wound rotor induction
motor.
The simplest, commonest, most rugged and reliable type is the cage rotor induction
motor, in which the rotor has a winding in the form of conducting bars embedded in
slots and connected together at the ends of the rotor core by short-circuiting rings
called endrings. In larger machines the cage is usually fabricated from copper bar
and care has to be exercised to ensure the integrity of all the joints in the fabrication.
In smaller motors, the cage is usually formed by casting aluminium into the slots of
the rotor core and around the ends to form the cage. In these machines, the cage is
often referred to as a ‘squirrel cage’ because of its shape. Sometimes the endrings
have cooling fins cast onto them to act as rudimentary fans when the rotor is rotating,
as shown in Figs 10.3 and 10.4. There is virtually no insulation between the cage and
the rotor core, unless (exceptionally) particular steps are taken to insulate the cage
during fabrication.
The wound rotor type, as its name implies, has a conventional insulated winding
in the slots of the rotor core. Almost invariably it will be a three-phase winding, with
its ends brought to shaft-mounted sliprings to provide a sliding connection. The
winding can be connected either to a resistance (usually variable) or to a supply to
modify the performance during starting or running, as will be described below.
Regardless of the type of rotor used, the stator carries the principal winding
which, with the exception of some very small motors, is embedded in the slots of the
stator core. The coils of the winding are connected so that, when fed from a balanced
supply system (normally three phase, although two phase, six phase and 12 phase are
also found in specialized applications) they produce a magnetic field of constant
magnitude which rotates around the bore of the stator. It can be shown that this
rotates at a speed of N, given by
N, = 120flp
(10.2)
where N, is in rev/min, f is the supply frequency in Hz and p is an even integer
describing the number of poles in the field pattern of the winding. For a 50 Hz supply
and a 4-pole winding (the most commonly found type), the speed of the field is
therefore 1500 rev/min.
This rotating field cuts the rotor conductors and induces (hence the name induction
motor) voltages which drive currents around the cage. The magnetic field of the
stator interacts with these currents to produce electromagnetic torque, as previously
described in section 10.2.1. Note that the field only cuts the conductors if the speed
of the rotating field is not equal to the rotor speed, so that when the rotor speed is
synchronous with the magnetic field, the torque will be zero. The torque is therefore
a function of the velocity of the rotor relative to the synchronous speed of the field.
This relative speed is referred to as the slip, defined as the ratio:
s = (N, - N,)IN,
(10.3)
where N , is the synchronous speed of the field and N , is the rotor speed. The ratio is
often expressed as a percentage, and full-load slip ranges from 10 per cent in small
motors to less than 0.5 per cent on large motors. Because the induction motor cannot
run at synchronous speed, it is sometimes (and particularly in continental Europe)
called the asynchronous motor.
Newnes Electrical Engineer’s Handbook
247
As the motor accelerates, the torque changes and a characteristic of the form
shown in Fig. 10.6 results. This is the torque-speed curve which is obtained by
supplying the motor from a fixed voltage. At standstill the motor is connected to the
supply and allowed to run up to full speed, and this torque-speed curve is followed
regardless of the applied load. If the load has a characteristic shown by the broken
line in Fig. 10.6 then the difference between the motor torque and the load torque at
any speed is the accelerating torque which is available to accelerate the load. If the
torque-speed curve were to have dips in it which intercepted the load line, the motor
would not accelerate through that point to reach full speed.
/ Pull-out torque
c *
\
operating point
0
1
Fig. 10.6 Torque-speed characteristic of an induction motor
The shape of the torque-speed characteristic can be influenced considerably by,
among other things, the depth and shape of the rotor conductors. To achieve high
efficiency and power factor, the rotor resistance and reactance are minimized but
unless the mainy design parameters are carefully chosen, the starting torque and
current may be adversely affected. At starting, the slip is 100 per cent and the
frequency of current in the rotor is the main supply frequency; under these conditions
skin effect forces current to the top of the rotor bars, increasing their effective
resistance and limiting the starting current. Special designs of rotor slot such as the
double cage design have been developed to maximize this effect. Even when these
designs are used, the starting current is typically 6 to 7 times the full-load rated
current. This can create difficulties when a motor is to be started from a ‘weak’
supply with high internal impedance, causing a dip in the supply voltage which may
affect other equipment and result in an unacceptably long run-up time for the motor.
The most common means of overcoming this problem for three-phase motors is
to use a star-delta start, in which the stator windings are connected in slur (sometimes
called ‘wye’) for starting, and a timed contactor reconnects the windings to delta
(sometimes called ‘mesh’) during run-up. The star and delta connections are shown
in Fig. 10.7. Tlhe voltage appearing across each winding when in star is only 58 per
cent of the full delta-connected voltage; the motor presents a higher impedance to the
supply and the starting current (and, unfortunately, the torque) is limited to one-third
of what it would have been in the delta connection. A second means of reducing
starting current is a soft starter, which uses a simple device such as a triac or pairs
248
Motors. motor control and drives
Fig. 10.7 Star and delta connections
of thyristors to delay the switching on of the voltage every cycle (see section 11.4),
and this reduces the effective voltage applied to the motor during starting. Highvoltage motors, which are usually connected in star for normal running in order to
reduce the voltage across each winding and the level of insulation required, are often
started through an autotransformer to reduce the motor voltage at starting.
When the motor has run up to speed on no load, the slip is very small, the torque
produced being only just enough to overcome friction and windage losses. As the
load on the motor is increased, the slip increases, the rotor current is increased and
the torque is greater, reaching a peak at the pull-out torque. The pull-out torque is
important because it determines the maximum temporary overload that the motor can
withstand. The torque of an induction motor varies approximately as the square of
voltage, so if the voltage drops to 90 per cent of its nominal value, the torque would
be reduced to 81 per cent. For this reason it is important to ensure that the pull-out
torque is adequate to cope with any short-term overloads even on lowest supply
voltage.
The wound rotor variant was mentioned earlier, although it is now becoming
relatively rare. It can be operated simply as a cage rotor, but the presence of a
winding accessible to the user can be exploited in two main ways. First, a resistance
can be added in series with each phase of the rotor winding. This resistance can
comprise a group of fan-cooled resistors or a liquid resistor in the form of a tank of
electrolyte into which electrodes are lowered. By using the maximum added resistance
at standstill, the starting current is reduced to a minimum, and by selecting a particular
value of resistance, pull-out torque can be achieved at standstill, with a good value
of torque per amp. As the motor runs up, the resistance can be gradually reduced to
zero, giving high efficiency at full speed. Secondly, the winding can be connected to
a second supply which is able to inject currents to alter the torque-speed curve.
Motors operated in this way are known as doubly fed motors and they were commonly
used in speed-controlled drives. In recent times they have been supplanted by inverters
feeding a standard cage rotor motor (see section 10.4.4).
It was noted above that the induction motor has to run at some level of slip even
Newnes Electrical Engineer's Handbook
249
on no-load, since it has to supply losses to the rotor. If the rotor is driven faster, the
slip will decrease to zero then become negative as the speed rises above synchronous
speed and the machine will then naturally generate power back into the supply. This
is a convenient way of braking an overhauling load, although the machine has a
negative pull-out torque beyond which it cannot increase its braking torque and load
control would then be lost.
10.3.1.1 Single-phase induction motors
Single-phaseinduction motors are common in domestic appliances such as refrigerators,
freezers, fans and air conditioners. While they are necessary in situations where a
three-phase supply is not available, they are typically limited to around 1 kW because
of supply current limits and because of their inherent low efficiency and high torque
ripple.
In section 10.3.1, reference was made to three-phase distributed windings in the
stator and the way in which these windings produce a rotating field. In a single-phase
motor the field pulsates, rather than rotates and it can be mathematically represented
by two contra-rotating fields, each producing its own torque-speed curve, but in
opposite directions, as shown in Fig. 10.8. The resultant field gives no starting torque
so special arrangements have to be made for starting, and the motor has a much
higher full-load slip (and hence lower efficiency) than the three-phase version.
Torque
Fig. 10.8 Torque-speed characteristic of a single-phase induction motor
Single-phase motors are normally started by adapting them to be an approximation
to a two-phase motor, in which two windings 90" apart in space around the bore of
the machine have balanced emfs 90" apart in time applied to them. The 90" spacing
between the windings is achieved by inserting the second winding (the starting or
Motors, motor control and drives
250
auxiliary winding) at the correct places in the stator, but providing balanced emfs 90"
apart in time generally involves a greater approximation. There are four common
arrangements, each having varying degrees of effectiveness.
In the split-phase or resistance-split motor, the starting winding uses fewer turns
of a finer wire and so has a higher resistance and a lower reactance than the main
winding. This results in the starting winding current leading the main winding current.
The consequent phase difference is sufficient to provide reasonable starting torque.
The starting winding is rated only for short periods because of its high current
density and it is switched out when the motor reaches 60 to 70 per cent of full speed.
Switching is usually done by a shaft-mounted centrifugal switch, although currentoperated relays are sometimes used. The split-phase motor is best suited to infrequent
starting with low-inertia loads, since its starting current is relatively high.
The capacitor start motor has a capacitor connected in series with the starting
winding. The result is that the starting winding current leads the main winding
current by a larger angle than in the split-phase case; this angle may approach 90" if
a sufficiently large capacitance is used. A short-time rated electrolytic capacitor is
normally used, and this is switched out of circuit when the motor reaches about 75
per cent of full speed, The capacitor start motor can deal with more frequent starting
and higher inertia loads with higher starting torque such as pumps and compressors,
and its starting current is lower.
In a capacitor start and run motor, a paper capacitor is connected permanently in
series with the starting winding. The starting torque is low and this type is generally
confined to fan drives, but running performance can approach that of a balanced twophase motor if the capacitor is correctly chosen and it is generally quieter than a
split-phase or capacitor start motor, with higher efficiency and power factor. These
are also known as Permanent Split Capacitor (PSC) motors.
In the capacitor start, capacitor run motor, a large electrolytic capacitor is used
for starting, but this is switched out before the motor reaches full speed and a smaller
paper capacitor is in circuit for normal operation. There are alternative switching
methods in which the paper capacitor is either permanently connected or switched
into circuit when the electrolytic starting capacitor is switched out. In this way the
good starting performance achieved with the large short-time rated electrolyticcapacitor
is combined with the good running performance achieved with the smaller paper
capacitor.
Another variant of the single-phase induction motor is the shaded pole motor.
These are to be found in sizes up to about 300 W output and comprise a standard cage
rotor and a stator with a small number of salient poles, typically 2,4 or 6. The poles
each carry a coil, with the coils connected together to form the single main winding.
At one side of each pole, near the air gap, a conducting ring is set into the lamination,
the function of which is to distort the pulsating field and produce a crude approximation
to a rotating field. Constructional variants abound, particularly in the so-called unicoil motors using a single bobbin-wound coil to excite the magnetic circuit. Although
very low in efficiency, these motors are often used for driving fans and pumps where
an ac supply is readily available.
10.3.2
Synchronous motors
The construction of synchronous machines has been described in Chapter 5 in
relation to generators, so it is sufficient to note here that wound field motors are
typically only found in the larger sizes of synchronous motor. However, where the
Newnes Electrical Engineer’s Handbook
25 1
wound field is replaced by a permanent magnet, the machine is (somewhat confusingly!)
often called a brushless dc machine (see section 10.3.4.2). These abound in sizes
below about 20 kW, particularly in small sizes where they are used, for example, in
audio equipment and computer fan drives. Synchronous motors have higher efficiency
than induction motors and for this reason are particularly found in the MW sizes in
petrochemical and other pumping applications where operation is almost continuous.
All of these motors, whatever their size, share the characteristic that the rotor
locks or synchronizes to the speed of the rotating field in the motor, so there is no
variation of speed with load, unlike the induction motor. Various methods are used
for starting and many of them use induction motor action to bring the motor to near
synchronous speed, allowing it to lock onto the field. The synchronous running is
exploited in applications where precise, constant speed is required, for instance in
paper or textile making equipment.
10.3.3
Commutator motors
This category covers a variety of motors which share the feature of having a commutator
mounted on the rotor shaft, to which the coils on the rotor are connected. Figure 10.9
shows a typical commutator; it consists of copper bars (segments) set in insulation,
with the ends of one or more coils connected to the riser portion. The surface of the
commutator is accurately machined for concentricity and surface finish and carbon
brushes provide sliding electrical connections. In all cases, the function of the
commutator is to switch the polarity of one or more coils as the rotor rotates so that
the direction of current in the coil is always correct with respect to the direction of
the magnetic field, enabling electromagnetic torque to be produced in the desired
direction.
Fig. 10.9 Commutator on a dc machine (courtesy of Invensys Brook Crompton)
252
Motors, motor control and drives
10.3.3.1 AC commutator motors
These are now seldom produced, generally being variants of three-phase induction
motors with a third or tertiary winding on the rotor. They were used in speedcontrolled applications, particularly lifts and hoists, but are now superseded by inverter
drives (see section 10.4).
10.3.3.2 DC motors
The dc machine, shown schematically in Fig. 10.10, is the classical commutator
motor. The winding on the rotor is generally referred to as the armature and carries
the main current. The magnetic field can be produced by a conventional winding, as
shown in Fig. 10.10, which can either be supplied from a separate source (separately
excited), connected in parallel across the armature supply (shunt connected), or
connected in series with the armature to carry the same current (series connected).
Alternatively, the field can be produced by permanent magnets housed in the stator,
in which case the magnetic field strength cannot be easily varied.
Field
/
Armature
L conductors
/
Commutator
Fig. 10.10 DC motor in schematic form
The operation and control of the dc machine is, in principle, very simple. As
explained in section 10.2.1, varying either the strength of the magnetic field or the
magnitude of the armature current will directly vary the torque; varying the direction
of either one will alter the direction of the torque. The ease of controlling the dc
motor made it the obvious choice for controlled-speed drives before inverter-fed
induction motors were available. It remains common in rail traction, steel rolling
mills, winders, hoists and cranes, despite frequent forecasts of its demise.
The main disadvantage is the high cost of the machine and the life and reliability
of the commutator and brushes, which also limit its operating speed. As the brushes
wear, carbon debris is deposited on the winding insulation, potentially shortening its
life. Overloads or fault currents can cause flashover on the commutator, often resulting
in permanent damage to the surface of the commutator segments, necessitating a
major overhaul.
Many of the developments in dc motors have concentrated on improving the
Newnes Electrical Engineer’s Handbook
253
commutation action, the best known being the introduction of interpoles. These are
narrow poles situated between the main stator poles and carrying a winding with a
few turns connected in series with the armature. The field of these interpoles is
arranged to induce a motional emf in the coils undergoing commutation, thus enabling
faster current reversal and preventing sparking.
Through-ventilated machines are the most common, with a fan driven either by
the armature shaft or, more commonly, by a small auxiliary motor. Not only does this
allow at least some of the carbon dust from brushes to be swept clear of the machine,
it allows direct cooling of the armature winding and yields a higher output from a
given motor size.
Before solid-state control became economic, series-connected dc motors were to
be found in virtually every traction application. They exhibit high starting torque,
with a falling torque which approaches zero at high speeds. By contrast, the shuntconnected machine operates substantially at constant speed, the speed being broadly
set by armature voltage and its drop with load being relatively small. However, with
the availability of modern controllers the separately excited machine is normally
used; this can be programmed to give a range of torque-speed curves.
In small sizes, typically in automotive auxiliary drives for radiator fans and
windshield wipers, the wound field is replaced by a cheaper permanent magnet, often
using simple ferrite magnets. The dc supply is connected directly to the brushes, and
reversal of direction is simply achieved by reversing this connection. Speed control
is achieved by reducing the armature voltage by a variety of means.
10.3.3.3 Universal motors
These motors are so called because they will operate on either dc or single-phase ac
supplies.This is because they are series connected, so the ac current reverses direction
in the field and armature at the same time, leaving the direction of torque unchanged.
Typically they are rated under 1.5 k W at several thousand revdmin. The highest
speeds are normally found in vacuum cleaners (up to 25 000 rev/min is common) and
the highest torques are in washing machines (up to 5 N m in horizontal axis models).
They are normally of frameless constructionto reduce cost. Being high-speed machines,
the life of the carbon brushes is a major limitation (it may be only a few hundred
hours in some cases) and commutator noise is often obtrusive.
Because of the high speeds, the specific power output of these machines is much
higher than, say, induction motors running at 1500 or 3000 redmin, so they are often
used where space andor weight is at a premium, with the final drive geared down
from the motor shaft.
10.3.4
Permanent magnet motors
Motors with permanent magnets have been mentioned in two of the previous sections,
illustrating the difficulty of using a simple classification method. Nevertheless it is
worth summarizing here the types of motors with permanent magnets that are likely
to be encountered.
Great care must always be exercised in working near or dismantling any permanent
magnet machine. If the magnets are allowed to adhere to a surface suddenly there is
a risk of the brittle magnet material chipping and firing out debris; the speed with
which articles are attracted together often catches the user unaware and traps ends of
fingers. In some cases removing the rotor from a machine can partially demagnetize
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Motors, motor control and drives
the magnets. During manufacture or maintenance,metallic swarftends to stick to the
magnets, often causing a hazard.
The last two decades have seen huge strides in the quality and performance of
magnetic materials (see section 3.2.2). The energy product of magnets now varies by
an order of magnitude from the cheaper ferrites (typically used on cheap domestic
appliance motors), throughAlnico (traditionally used on loudspeakers)and samariumcobalt to neodymium-iron-boron (see Table 3.2 for a generalcomparison of properties).
The range of materials now available to motor designers has enabled the performance
boundaries of permanent magnet motors to be considerably extended.
10.3.4.1 Permanent magnets on the stator
In section 10.3.3 it was noted that small dc motors sometimes use a permanent
magnet instead of a winding to produce the field. These are usually referred to as
brushedpennanent magnet motors since the armature current is supplied through the
carbon brushes as before. The motivation for adopting this constructionis cost; in the
smaller sizes it is more economical to use a magnet than a wound field. The stator
normally comprises a steel shell, into which the blocks of magnet material (usually
unmagnetized) are assembled. The blocks may be held in place mechanically or they
may be bonded in place with a suitable adhesive. The armature and endframes are
assembled and the motor is placed in a magnetizing fixture, where a very high pulse
of magnetic field is applied to the motor to ‘charge’ the magnets. This system is
amenable to volume production, but it can produce variable results in the motor
performance due to differences in the field strength of the magnets.
Since the magnet field strength is approximately constant, the performance of
these motors is similar to that of a separately excited motor supplied with constant
field current and the speed regulation with load is relatively small. The speed is
varied by controlling the armature voltage: this is done using external resistor(s) or
by using a pair of brushes not diametrically opposite each other, or more commonly
now by electronic control.
10.3.4.2 Permanent magnets on the rotor
In section 10.3.2 it was noted that a synchronous motor may have the wound field on
the rotor replaced by a permanent magnet system. This gives rise to two groups of
motors, although in principle there is little difference between them.
The first group has a distributed stator winding embedded in a large number of
slots, like an induction motor, and often has the rotor magnets embedded in the rotor
core. When the stator is supplied with balanced polyphase voltages, the rotor field
locks onto the resultant rotating field and synchronousrunning is achieved. Sometimes
a rudimentary cage winding is also provided for starting the motor, or the frequency
of the supply voltages can be linked to shaft position by providing rotor position
feedback to the controller. These motors have a very low rotor loss and an overall
efficiency which is significantly higher than induction motors. They have attracted
much academic interest over the past few years but, in spite of their apparent promise,
usage is limited and shows little sign of growth.
The second group is much more significant. Here the magnets are usually bonded
to the surface of the rotor core and, particularly in smaller sizes, the stator has only
a few slots. This is an inverted form of the brushed dc commutator motor described
in section 10.3.4.1, since we now have stationary windings and a rotating magnet.
Newnes Electrical Engineer's Handbook
255
Instead of the mechanical commutator, the currents are electronically switched
(commutated) at the correct moments by using position feedback from the rotor
(often from the rotor magnets themselves). This class of motor is almost always
described as the bnsshless dc motor. It is to be found in small sizes in medical and
computer equipment and in larger sizes (typically up to 10 kw) in servo drives where
absolute shaft position and shaft speed are of interest, for instance in weaving machines
where many shafts have to move in precise relationship to each other.
10.3.5
Switched reluctance motors
These were known in primitive form from around the 1830s. However, since there
was no convenient method of switching the currents in the inductive windings, they
were overtaken by the advent of good quality commutators for dc motors and by the
invention of the induction motor. In the 1970s the maturing capability of the power
semiconductor rekindled interest in this separate class of machine which produces
torque purely by reluctance action (see section 10.2.1).After intensive development
at the academic level, products have been developed commercially and drives based
on these motors are considered by many commentators to be at least the equal of
drives based on motors producing electromagnetic torque.
The motors are characterized by having clearly visible poles on both stator and
rotor (hence they are often described as doubly salient), but they only have windings
on the stator. They are supplied not from a sinusoidal supply but from a dc voltage
which is electronically switched to the appropriate winding, giving an essentially
triangular phase flux.While it is possible to operate without rotor position feedback
(so-called open loop),operation with simple position feedback (rotorposition switched)
is normal, and it is not possible to operate without an electronic drive system. Since
the motor is supplied from a dc source, the number of independent phase circuits is
a choice of the designer; systems with one to four phases are common and five
phases or more have been seen in specialist applications. Similarly, the number of
stator poles is flexible; 2 poles per phase gives a 2-pole field pattern (Fig. lO.ll(a))
and 4 poles per phase gives a 4-pole pattern (Fig. lO.ll(b)). The number of rotor
poles is also variable. Using a number of rotor poles two different from the stator
poles gives vernier action; two less makes the rotor move against the rotation of the
field and two more makes it move with it. A common choice is the machine with 6
stator poles and 4 rotor poles shown in Fig lO.ll(a).
Torque is developed by the tendency for the magnetic circuit to adopt a configuration
of minimum reluctance, that is for a pair of rotor poles to be pulled into alignment
with an excited pair of stator poles, maximizing the inductance of the exciting coils.
Continuous rotation in either direction is assured by switching the phases in the
appropriate sequence, so that torque is developed continuously. In simple terms, the
larger the current supplied to the coils, the greater the torque, although the design and
analysis of these machines is complex because the magnetic circuit is generally
operated above its linear region. The torque is independent of the direction of current
flow, so unidirectional currents can be used. This permits a simplification of the
electronic switching circuits compared with those required for most other forms of
motor.
The motors are generally operated in chopping mode or in single-pulse mode. For
the motor shown in Fig. 10.11(a) with each phase supplied by a switching circuit as
shown in Fig. 10.12, current is established in a phase winding by connecting it to the
dc supply by closing the switches S1 and S2 when the rotor poles are not aligned with
256
Motors, motor control and drives
(a) 2-pole field pattern
(b) 4-pole field pattern
Fig. 10.11 2-pole and 4-pole field patterns in a switched reluctance motor
that phase. The current rises rapidly to the desired level as shown in Fig. 10.13(a).
SI and S2 are now opened and the stored energy in the magnetic field ensures that the
current continues to flow through D1 and D,. The voltage now impressed on the
winding is negative, driving the current down. During this time some magnetic
energy is being returned to the supply and, while the inductance continues to rise,
some is being converted into mechanical output. When a lower specified current
level is reached, SI and S2are closed and the current rises again. At the end of the
required phase conduction period when the rotor poles are aligned with the stator
poles, both switches are turned off and the current falls to zero. Torque is controlled
by varying the level at which the current is chopped. This is only one of a number of
methods of chopping control.
Newnes Electrical Engineer's Handbook
257
+
Fig. 10.12 Power converter for a switched reluctance motor
At higher speeds, the rise and fall times for the current will be such that the
current is switched on and off only once in each conduction period and is never
chopped, as shown in Fig. 10.13(b). This is the single-pulse mode of operation in
which torque is controlled through the switching angles.
Because of the flexibility of control of switched reluctance machines, their
performance characteristics can be tailored to suit a wide range of applications.
Figure 10.14 shows motors developed for automatic door openers, rotary screw
compressors and mining conveyors. Their operation is characterized by an ability to
operate over very wide speed ranges, developing high efficiency over a wide range
of both torque and speed. They can also be arranged to give extremely high overload
torques in both motoring and braking.
The switched reluctance motor and controller are extremely robust. The motor has
simple coils, with small endwindings and no overlapping of phase windings, and the
rotor has no coils or magnets. The power converter has to supply only unidirectional
currents and the maximum rate of rise of switch current is limited by the stator coils,
thus avoiding the possibility of 'shoot-through' faults. More details on the design and
control of these machines can be found in Reference 10B.
10.3.6 Stepper motors
Stepper motors are often considered as a separate class of machines because of the
way in which they are operated, altbough constructionally they are similar to the
other types discussed above and they produce torque by reluctance action. However,
they are supplied from a source of discrete pulses, in response to which they move
or 'step' to a new angular position which is retained until the next pulse is sent. They
are positioning devices rather than variable speed motors, although they are sometimes
run in variable speed mode by simply increasing the pulse frequency so that they
appear to move continuously.They are entirely dependent on the driving electronics,
so they must be considered as part of a system, as shown in Fig. 10.15.
Two types are commonly encountered:
a variable reluctance stepper produces torque purely by reluctance action and
normally is constructed with a relatively small number of poles giving relatively
large step angles. For instance a 6-statod4-rotor pole machine will have a step
angle of 30" giving 12 steps per rev. Switchedreluctance machines are sometimes
258
Motors, motor control and drives
(a) Chopping mode current
I
Current
90"
9ort
(b) Single-pulse current
Fig. 10.13 'l)Tical chopping and single-pulse currents in a switched reluctance motor
thought of as large variable reluctance steppers because the construction is the
same. These are normally found in the power range 0.1 k W to 2 kW.
a hybrid stepper has on the rotor an axially magnetized permanent magnet,
and it normally has a large number of rotor teeth. The stator poles are divided
at the air gap to have several teeth per pole. These machines have relatively
small step angles. A typical small hybrid stepper might have 8 stator poles
each with 5 teeth, and a rotor with 50 teeth, giving a step angle of 1.8" or 200
steps/rev. The permanent magnet gives a detent or holding torque in the absence
of any excitation on the stator poles. Hybrid steppers are found in disc drives,
processing machinery and handling equipment.
Newnes Electrical Engineer’s Handbook
Fig. 10.14 Selection of SR Drives (photo by courtesy of Switched Reluctance Drives Ltd)
Direction
Fig. 10.15 Stepper motor system
259
Motors, motor control and drives
260
There are many ways of operating steppers. The simplest is the single-step mode
where a pulse is sent to a phase winding and the machine takes one step to the new
detent position. By equally exciting two adjacent phases, half-steppingcan be achieved,
where the rotor takes up a position midway between the single step positions of the
two phases. For very fine resolution such as for driving a prht head in a printer, the
currents in the phases are carefully controlled to be unequal, giving a mode known
as step division, mini stepping or micro stepping. More complex schemes exist where
a position transducer is used to give feedback, rather than relying on the motor to
move to the correct position on demand. The transient behaviour of the rotor is of
great importance during stepping and further details on this can be found in Chapter
8 of reference 1OA. Reference 1OD covers the entire subject in some detail.
10.4 Variable speed drives
Chapter 11 covers the details of different types of power electronic circuits which
can be used in conjunction with the motors described in section 10.3 to provide
speed control and position control of the rotor shaft. A more detailed treatment can
be found in Chapter 36 of reference 1OD.
The concept of quudranrs in the torque-speed plane is crucial to the specification
for a variable-speed drive. A drive operating with positive torque and positive speed
(or speed in the direction which the user defines as forwards) is said to be operating
in thefirst quadrant of the torque-speed plane, as shown in Fig. 10.16. If the torque
is reversed, braking or generating action takes place and power is extracted from the
load. This is second quadrant operation. Many applications require the drive to
operate in both motoring and braking modes; these are divided into drives that
I
+ Torque
4
Generating
1
Motoring
+ Speed
- Spied
Motoring
Generating
2
3
7
Fig. 10.16 Quadrants of operation
- Toque
Newnes Electrical Engineer’s Handbook
26 1
simply dissipate the regenerated power in resistors and drives that return power to
the ac supply by a dedicated inverter. If the speed falls to zero under the braking
action and the torque is maintained, the third quadrant will be entered in which both
speed and torque are negative and the drive is motoring in the reverse direction. By
reversing the torque once more, braking is achieved as the drive moves into the
fourth quadrant.
Basic drive systems only operate in the first and third quadrants; the more
sophisticated systems will operate in all four. The speed and smoothness of the
transition between quadrant is a significant test of the quality of the drive; dc motor
drives have excelled in this respect and much of the development of ac motor drives
has been targeted at improving this aspect.
10.4.1 DC motor drives
These are probably the simplest motors to control, since separate control over field
excitation and armature current is normally available. Typically the field is supplied
through a single-phase controlled rectifier and the armature through a three-phase
controlled rectifier. Reference 10A (especially Chapter 4)gives a full description of
the operation and also discusses the control aspects of these drives.
10.4.2 AC motor drives
Section 10.3.1 explained the dependence of the speed of the induction motor on
supply frequency (see eqn 10.2). It follows that by supplying the motor from a
suitable source of variable frequency, the synchronous speed of the motor can be
varied. For correct operation, the ratio of supply voltage to frequency also has to be
kept sensibly constant (see Chapter 7 of reference 10A) so the voltage also has to
vary with the flux. This gives rise to the use of a frequency inverter (see section 11.6)
for controlling an ac induction motor. PWM inverters are now by far the most
common in small and medium sizes, using the high switching speeds of IGBTs or
MOSFETs; older designs of 6-step inverters are still found in very large sizes. A full
discussion of different circuit topologies is given in Chapter 36 of reference 1OC.
In recent years, much effort has gone into improving the transient performance of
inverter drives in an attempt to take market share from dc drives where high control
bandwidth is required. This has led to the development of vector control, fieldoriented control and direct torque control by different manufacturers, all of which
control the position of the rotor current in relation to the position of the rotating flux
wave in the air gap. Such systems require rotor position feedback, which can be
derived by hardware such as a shaft encoder, or software such as an observer.
In larger sizes, and commonly above 1 M W , synchronous machines are preferred
to induction motors. These are normally driven by a different style of thyristor
inverter, which uses the back emf of the machine itself to commutate the thyristors.
10.4.3
Switched reluctance drives
The switched reluctance motor cannot be operated without its power converter.
While it is possible to operate it from a variable frequency inverter, with appropriate
changes to the control, such complexity is not required. The most common configuration
for the power converter is shown in Fig. 10.12, although many other variants exist,
many of them offering a reduced number of switches at the expense of reduced
control flexibility. In all cases, the power switches are in series with a phase winding,
262
Motors, motor control and drives
offering a number of advantages to the converter designer, not least the possibility of
reduced switch size. Switching speeds are normally lower than in ac inverter drives,
since there is no requirement for PWM at high speeds to synthesize a sinusoidal
voltage.
10.4.4
Commissioning of drives
Most commercially available drives have control systems which allow the drive to be
tailored to suit a variety of applications. This is especially true of general purpose
drives sold from a catalogue; these generally have to be tuned to suit the parameters
of the load, particularly if transient performance is an issue. Some systems have a
degree of self-tuition and simply require the execution of a set-up program to allow
the system to test the load so that correct responses can be stored for future operation.
With the improvements accruing from cheap on-board signal processing, most systems
now have comprehensive and intelligible fault signalling ability, and setting up new
drives is now much simpler than with first-generation drives.
10.5
Ratings, standards and testing
National and international standards exist for specifying the rating and performance
of several types of motor. Many of these also specify the test methods to be used,
although these tests are often complex and can be done only by the manufacturer or
a specialized independent test site. Contract testing can be undertaken by universities
or consulting organizations. In the UK, Nottingham University Electrical Drives
Centre is one of the best academic centres for testing machines and drives of all types
on a contract basis, having testing capacity up to 750 k W and up to 15 000 revhin
at low power. Where the motor is a component in a drive system, the situation is
much less clear, since there are few standards, other than EMC and Harmonic Limits
(see Chapters 11 and 15) which apply to complete systems. For free circulation
within the European market, CE marking under the Low Voltage and EMC Directives
is required.
Table 10.1 indicates the most commonly encountered standards, with approximate
national and international equivalents where appropriate. In spite of ongoing attempts
at harmonization, there remains in some cases no simple correspondence between
these standards and further advice (for instance from catalogues of national standards)
should be taken for details of the equivalence of individual parts.
For motors other than standard induction motors, and particularly for drives, it is
usually difficult to find a standard which is entirely relevant. In these cases, a detailed
specification drawn up between the supplier and the customer is generally preferred,
referring where appropriate to particular parts of other standards to cover specific
aspects of construction and performance.
Newnes Electrical Engineer's Handbook
263
Table 10.1 National and international standards for electric motors and drives
Construction and dimensions:
6s:
IEC:
Performance and rating:
NEMA
BS:
IEC:
Noise and vibration:
NEMA:
6s:
IEC:
NEMA:
BS:
Safety:
IEC:
EMC and harmonics:
UL
NEMA:
BS:
IEC:
4999, especially parts 105, 141 and 147
5000 part 10
EN 60035-6 and EN 60035-7
EN 60529
60034-5
60072
601 36
MG-13
4999 part 102
5000 part 10
EN 60034-1 and EN 60034-12
60034-2
60072
MG-10
EN 60034-9 and EN 60034-13
EN 60704
60034-2
60072
MG-3
3456 (many parts now superseded)
EN 60335
5345
5501 (potentially explosive atmospheres)
60335
60079
1004
MG-2
EN 50081
EN 50082
EN 55014
EN 55104
EN 61800
1000
50081
50082
55014
55104
61800
References
1OX.
1OB.
1OC.
10D.
Hughes, A. Electric Motors and Drives (2nd edn), Butterworth-Heinemann, 1993.
Miller. TJ.E. Switched Reluctance Motors and Their Conml, OxfordUniversity Press,Monogmphs
in Electrical and Electronic Engineering, No. 3 1, 0-19-859387-2.
Jones. G.R., Laughton, M.A. and Say, M.G.(eds), Electrical Engineer's Reference Book (15th
edn), Buttem-orth-Heineman, 1993.
Acarnley, P.P.Stepping Motors: A Guide to Modern Theory and Practice (2nd edn). Peter
Peregrinus, 1984.
Chapter 11
~~~~
Static power supplies
Dr C.D. Manning
Loughborough University
11.1 Scope
The primary function of a power supply is to draw electrical power from an existing
electrical system and convert this power into a form suitable for a particular load. A
wide variety of power supplies is used to meet the range of loads presented by
common electrical equipment. This chapter covers the common power conversion
circuits that use semiconductor switches. These are commonly known as static power
supplies, and the overall requirements which influence their design and construction
are described here. Systems that use rotating machines or which control the flow of
power using metal-contact switches have been common in the past but are increasingly
restricted to specialized applications; they are not covered.
The power that is handled by static power supplies can range from a few milliwatts,
supplying low-voltage integrated circuits, to gigawatts in high-voltage dc links. This
chapter concentrates on equipment of less than a few tens of kilowatts drawing ac
power from a standard utility supply; this represents the majority of static power
supplies manufactured and in use today.
11.2 General issues
11.2.1
Design optimization
The majority of static power supplies are manufactured to compete in markets where
the purchase price is often the primary concern of the customer, given a minimum
electrical specification and a basic level of reliability.
Other features such as size, weight, efficiency and acoustic noise are often secondary
issues, although in some battery-charging applications efficiency may be critical and
in aircraft weight is of paramount imp0rtan.e.
11.2.2 Thermal management
In static power supplies a significant proportion of the input power is dissipated as
heat, and adequate cooling must be provided in order to limit the temperatures
reached by the circuit components and to avoid damaging them. A maximum ambient
temperature of 40°C to 60°C is often specified, and safe component temperatures
must be maintained in these conditions. Component temperatures may be up to 70°C
for resistors, 85°C or 105OC for capacitors, 90°C to 110°C for heat sinks, 100°C
Static power supplies
266
for wound components (inductors and transformers) and 120°C for power
semiconductors.
At the lowest power levels, heat is removed from the components by natural
convection and radiation. At higher power levels fan cooling is used, air speeds being
typically in the range 0.5 to 5 m/s. Liquid cooling is normally used at much higher
power levels than 10 to 20 kW.
11.2.3
Safety
Voltages in static power supplies are often at lethal levels (in excess of 55 V relative
to earth). Prevention from electrocution is therefore necessary. Measures which are
used to meet the requirements set down in standards are:
the use of earthed metal enclosures in which any openings are below a specified
size
the referencing of output voltages to earth, which often requires the use of an
isolation transformer
design so that the insulation materials used to achieve safety isolation are
maintained within their rated maximum temperatures
Protection against fire is also an important consideration and an input fuse is used to
prevent sustained overheating and fire in the case of component failure.
11.2.4
Compatibility
Static power supplies must deal with various levels of disturbance in the utility
supply (or other supplies such as standby generators and systems powered by renewable
energy sources) and still maintain the required output to the load. The main disturbances
to the utility supply which must be dealt with are:
blackout (outage)- absence of utility supply for a cycle or more; this is often
due to faults on the electrical supply system, and the operation of system
protection equipment
brownout (sags)- the utility supply voltage falls below the normal minimum
rms level for more than one cycle. This is often due to the switching on of
large loads on the utility system
overvoltage - the utility supply voltage rises above the normal maximum rms
level for more than one cycle; this is often due to the switching off of large
loads on the utility system
voltage spikes - high voltages of short duration, superimposed on the utility
supply waveform; this is often caused by switching of inductive loads or by
lightning strikes in the utility system
hamzonic currents - distortion of the sinusoidal supply waveform; this is due
to harmonic currents (for instance from rectifiers, dc-dc converters and arc
welding) flowing through the utility system impedances
Radio Frequeiicy Interference (RFI) - these arise typically from high-speed
switching transients and from arc welding
Filters are commonly used at the input stage of static power supplies to protect
against or limit the effect of these disturbances, and unintermptible power supplies
are used to protect against blackouts and brownouts.
Newnes Electrical Engineer’s Handbook
267
11.2.5 Commissioning, testing, installation and maintenance
Commissioning of static power supplies is usually necessary only for equipment
which is rated at tens of kilowatts or more. Below this power level, in the range
which this chapter addresses, equipment is normally fully tested and set up by the
manufacturer before despatch to the customer.
11.3 Supply-frequency diode rectifiers
Rectifiers convert ac to dc. Full-wave and half-wave rectifier circuits are available
for both single-phaseand three-phase utility supplies, and the choice usually depends
upon the power to be delivered to the dc load.
The diodes used in rectifiers are often integrated into one component, which may
be mounted on a heat sink. The need for a suitable safety margin often results in the
use of 800 V-rated diodes for 230 V single-phase direct off-line power supplies, and
1200 V rating for 415 V three-phase direct off-line applications.
High-frequency filter capacitors are often connected in parallel with rectifier
diodes. These capacitors protect the diodes from transient voltage spikes by acting as
a low-impedance path to high-frequency voltages.
It is common to obtain the required output voltage using a transformer which is
connected between the ac supply input and the rectifier. The use of an isolating
transformer not only allows the output voltage to be set, but by isolating the rectifier
circuit from the ac supply it also enables one dc output voltage rail to be referenced
to earth. The earthing of a low-voltage output connection is necessary in many
applications in order to prevent injury by electrocution.
11.3.1
Half-wave single-phase rectifiers
For low power levels, a single-phasehalf-wave circuit with a large dc output capacitor
to reduce voltage ripple is normally used. The circuit is shown in Fig. 11.1.When the
ac supply voltage Vs(t) exceeds the voltage V,(t) at the output of the rectifier, the
voltage difference causes a supply current id(t) to flow. The rate of rise of this current
is limited mainly by the supply inductance Ls. As the capacitance C is charged, VL(t)
rises. When the supply voltage Vs(t) decreases after passing its peak, it falls below
VL(r)and the supply current id(r) falls to zero. During the period when idt) is zero,
VL(t)falls as the capacitor discharges. The decay in this voltage is approximately
linear. but it will be exponential if the load is resistive and falls with an increasing
rate when supplying constant power to a switching regulator.
When the circuit is first energized, the capacitor is initially fully discharged and
there is a large surge of supply current as it is charged. To limit this surge to an
acceptablelevel, thermistors with a negativetemperature coefficient are often connected
in the ac circuit. The resistance of the thermistor is initially high and the current is
limited by this to the required level, but after a short period of conduction the
thermistor temperature rises and its resistance drops, reducing the power loss and
voltage drop in normal operation. This thermal characteristic requires a minimum
time between the ac supply being switched off and reconnected in order that the
thermistor temperature can fall sufficiently to limit the surge current on reconnection
to an acceptablelevel. For applicationsrequiring maximum efficiency, the thermistor
may be replaced by a resistor connected in parallel with a relay contact or a
semiconductor switch which closes after the initial charging surge.
Static power supplies
h
Fig. 11.1 Half-wave single-phaserectifier
11.3.2
Full-wave single-phase rectifiers
The three main circuits used are illustrated in Fig. 11.2.
The bridge rectifier circuit in Fig. 11,2(a) allows c m n t to flow through diodes
D1 and D3 when the supply voltage V&) exceeds the load voltage, and through D2
and D4 when V,(t) is more negative than the load voltage is positive. Supply current
therefore flows in both polarities. Ideally this avoids a dc component of supply
current and reduces the peak supply current levels. The rectifier delivers power
during each half-cycle of the ac supply and this reduces the size of the capacitor
required to limit the output voltage ripple.
The voltage-doubler rectifier shown in Fig. l l 2 ( b ) can be regarded as two halfwave rectifiers, with D1 conducting during positive half-cycles of the ac supply and
D2 conducting during negative half-cycles.
The dual-voltage rectifier of Fig. 11.2(c) has a link S which can be used to
configure the rectifier as a bridge or as a voltage doubler. This arrangement is
commonly used to deliver approximately the same dc voltage using either bridge
rectification of 230 V ac or voltage-doubler rectification of 110 V ac. In this way, by
changing the link S, the rectifier can be configured for use in most countries.
11.3.3 Three-phase bridge rectifiers
Three-phase bridge rectifiers are typically used above 1 k W to provide a single dc
Newnes Electrical Engineer's Handbook
269
_--
Load
(a) Bridge rectifier
D1
D4
(c) Dual voltage rectifier
Fig. 11.2 Full-wave single-phase rectifiers
supply by rectifying the line-to-line voltage. The scheme is illustrated in Fig. 11.3(b).
For completeness Fig. 11.3(a) shows the half-wave circuit, but this is not commonly
used. The bridge rectifier produces six pulses of power near the peaks of the line-toline voltages. It is common for the load of a three-phase rectifier to include a large
inductance (shown as LJ) to create a dc current source. Figure 11.3(c) shows typical
waveforms and it illustrates the time required for the supply current to rise and fall
from the level of rectifier output current.
For high power outputs, two three-phase supplies are used. These have a phase
difference of 30" and the suppIy is rectified to provide 12 pulses per cycle. Acentretapped inductor is connected between the negative dc outputs of the two six-pulse
rectifiers, and the negative supply to the load is connected from the centre tap of this
inductor in order to limit sixth harmonic (300 Hz)currents. The main advantage of
12-pulse, compared with six-pulse, rectifiers is that the filtering requirements are
reduced; the primary current waveforms have no fifth or seventhharmonic components,
Static power supplies
270
(a) Half-wave circuit
(b) Full-wave circuit
V
(c) Full-wave circuit waveforms
Fig. 11.3 Three-phase diode rectifiers
which reduce the filtering required in the ac supply, and the dc output has more
pulses, which reduce the ripple contained in the dc output.
11-3.4 Voltage multipliers
These are rectifier circuits in which diodes and capacitors are used to deliver an
output voltage which is an integral multiple of the peak ac input voltage. Different
circuits are possible and a voltage doubler has already been shown in Fig. 11.2(b).
A common circuit is shown in Fig. 11.4 and the operation is:
C 1 charges to the negative peak supply voltage through D 1
when the ac source reverses, the voltage across C1 adds to the source voltage,
and C2 is charged through D2 to twice the peak source voltage
C3 and C4 charge in a similar way to C1 and C2, except that both these
capacitors charge to twice the peak ac voltage
Newnes Electrical Engineer’s Handbook
271
4v
Fig. 11.4 Voltage multiplier
Further stages can be added to increase the output voltage, although the output
voltage will not be the full multiple of the input voltage because of voltage drops in
the components.
Voltage multipliers are suited to low-current high-voltage supplies, and they are
rarely used for supplying more than a few milliamps.
11.4 Phase-controlled power converters
Phase-controlled power converters use thyristors, which begin to conduct when turned
on by a gate signal. If the load current exceeds a *latchingcurrent’ level, a thyristor
will latch in the conducting state and it will continue to conduct, irrespective of the
gate signal. When the load current falls below a relatively small ‘holding current’ the
latching action will stop and the thyristor will turn off provided the gate signal is in
the off state. Power flow is controlled by timing the instant of turn-on of the thyristor
to a set phase angle in the ac waveform.
Detection of ac supply zero-voltage crossings is used to ensure that the thyristor
control signals are timed in correct phase with the supply voltage waveform. A ramp
and a dc control voltage are taken as input to a comparator, which delivers an
‘enable’ signal to an oscillator. The oscillator output is amplified and passed through
a pulse transformer (for electrical isolation) and through a current-limiting resistor to
the thyristor terminals. A train of pulses is applied between gate and cathode terminals
of the thyristor to minimize the size of the isolation transformer whilst ensuring that
the thyristor cannot turn off for long during the conduction period set by the control
circuit.
Static power supplies
272
The switching of thyristors often requires snubbers. These prevent damage to the
thyristors by limiting overvoltages, the rate of rise of voltage across the thyristor and
the rate of change of thyristor current.A wide variety of snubber circuits are commonly
used.
11.4.1 AC power controllers
Circuits for the control of single-phase ac power using phase-controlled thyristors
are iIIustrated in Fig. 11.5. Anti-parallel thyristors (Fig. 11.5(c}}, or a thyristor with
an anti-parallel diode (Fig. 11.5(b)), or simply a triac (Fig. ll.S(a)) can be used.
PP
(a) Triac controller
(b) Half controlled
(c) Fully controlled
Fig. 11.5 Single-phase phase-controlled ac regulators
The resulting current waveform depends upon the load impedance, which is often
resistive, for instance in the case of dimmers used for incandescent lights.
11-4.2 Phase-controlled rectifiers and inverters
A phase-controlled inverter uses the same circuitry as a phase-controlled rectifier,
but with a different timing for the thyristor drive signal. The circuit is shown in Fig.
11.6(a). The basic three-phase bridge arrangementis similar to that used in the diode
rectifier circuit already seen in Fig. 11.3(b); the main difference is that using the
thyristors, the start of current conduction can be delayed for a controlled period after
the zero-voltage crossing of the anode-to-cathode voltage.
273
Newnes Electrical Engineer’s Handbook
I
Neutral
point
(at zero
volts)
l
l
-
1
Load
(a) Circuit
.
-1
.
(c) Waveforms during inversion
Fig. 11.6 Three-phase phase-controlled rectifiedinverter
Static power supplies
274
Phase-controlled rectifiers and inverters are normally used at high power levels
with three-phase supplies. A large inductance Lf is connected in series with the dc
output in order to ensure a substantially ripple-free dc current.
The waveforms in a three-phase rectifier with constant-current output are shown
in Fig. 11.6(b). The conduction of each thyristor is delayed beyond the instant that
a diode would conduct by a delay angle A. When conduction has begun, the rectifier
output takes a time, representedby angle B to commutatefrom the previously conducting
phase. This is illustrated by the gradual decay in the outgoing phase current i&) and
the corresponding rise in the incoming phase current [email protected]).
11.5 Linear dc voltage regulators
Linear regulators use a bipolar transistor connected in series with the load, and
operating in the linear mode, in order to regulate the load voltage by controlling the
transistor voltage drop. Basic schemes are shown in Fig. 11.7.
Pass
transistor
output
voltage
vOUT
(a) Linear regulator
Over-voltage
crowbar
I LOAD
(b) Foldback characteristic
Adjustable
regulator
(c) Adjustable regulator
(d) Current source regulator
Fig. 11.7 Linear regulator schemes
Linear regulators are often implemented in a single three-terminal Integrated
Circuit (IC), which requires only the connection of input and output decoupling
capacitors and mounting on a heat sink. Fixed output voltages at standard values are
delivered by regulators such as the 7 8 M series of positive voltage regulators and the
79XX series of negative voltage regulators. These regulator ICs usually contain
additional features such as overtemperature and overcurrent protection circuits.
In cases where the heat dissipation in the internal pass transistor is high, a separate
Newnes Electrical Engineer’s Handbook
275
transistor is often used instead of an IC. This enables the control circuits to operate
at an acceptably lower temperature while allowing a high temperature in the pass
transistor itself. In such applications, the dissipation in the pass transistor, which is
proportional to its voltage drop, is often minimized by the use o€ a ‘low dropout’
regulator. Standard linear regulators require the input voltage to be a minimum of
3 V higher than the output voltage for correct operation of the control circuitry. ‘Low
dropout’ regulators operate satisfactorily with minimum voltage drop which may be
less than 1 volt.
Output overvoltage protection is often used to safeguard the load. This protection
works by short-circuiting the regulator output in the event of an overvoltage; the
‘crowbar’ for achieving this is shown in Fig. 11.7(a). The resulting short-circuit
current operates either the current-limit protection within the regulator or the input
fuse. If the load current were limited by the regulator to its maximum value, then the
pass transistor would dissipate maximum power with the output short-circuited,
when the voltage across the regulator is also a maximum. Such high power dissipation
would usually exceed the maximum acceptable for normal operation, and it would
increase the required cooling. To avoid the need for this increased cooling, the output
current is often reduced (or ‘folded back’) in cases where the load impedance is less
than the full load impedance. A foldback characteristic is shown in Fig. [email protected]).
Adjustable regulators include an external resistive voltage divider to set the output
voltage, as shown in Fig. 11.7(c). A common IC used in adjustable regulators is the
LM317.
Micropower linear regulators for use in battery-powered systems are optimized
for very low quiescent current (the current that supplies the regulator control circuitry).
Dual-tracking regulators deliver equal positive and negative dc voltages. This is
achieved by arranging that the pass transistor on the negative rail tracks the magnitude
of the regulated output of the positive regulator.
11.6 Inverters
Although phase-controlled inverters require an existing ac system into which their
output is delivered, a variety of other inverter types are used in order to create an ac
supply-frequency power source.
In some applications, a dc source may be inverted to a square wave output by the
use of four transistors which connect the dc input to the load in alternating polarity
at the required frequency. This type of system is simple, efficient and has low noise;
it also enables the use of slow switches for low-frequency applications.
Resonant schemes apply a square wave at the required output frequency to a
resonant circuit which may be connected in series, in parallel or in series-parallel.
The resonant circuit has a natural frequency which is equal to the required output
frequency; it generates an approximate sine wave from the square wave, and acts as
a low-impedance source.
Linear amplifiers are used to amplify a reference sine wave to the required ac
output voltage, the output oscillating between positive and negative dc voltage rails.
The linear amplifier has a low source impedance, and it offers low noise generation
because of the avoidance of switching transients, but there are high power losses in
the amplifier transistors.
Pulse- Width Modulated (PWM)inverters use high-frequency switching of the dc
source voltage, in which the pulse width is proportional to the ac waveform amplitude.
276
Static power supplies
mically, a full bridge of transistors is controlled in such a way that a sinusoidal
output voltage is achieved once switching frequency components have been filtered
out. In comparison with linear inverters, PWM types are small, have higher efficiency
and cope well with non-linear loads, but they generate substantial high-frequency
noise.
11.7 Switching dc voltage regulators
(dc to dc converters)
DC to dc converters are widely used for the regulation of dc voltage, and they are
used in Switched-Mode Power Supplies (SMPS). They are commonly treated as a
single power converter, although isolated versions use a controlled inverter and a
rectifier. The circuitry differs significantly from that used in AC supply-frequency
power conversion.
Switching regulators use the duty cycle of a switch to regulate a voltage. The
switch has low power loss because it alternates between an off-state, during which a
negligible leakage current flows through the switch, and an on-state, during which
the switch conduction voltage is low. The switched waveforms are averaged using
low-pass filters which consist of capacitors and inductors. The size of these filter
components reduces with increasing frequency, but the switching loss increases in
proportion with frequency; typical commercial equipment operates at a switching
frequency in the range 100-500 W z .
The two types which are commonly used without isolation are the bzrck converter
and the boosr converter. These are shown in Figs 11.8 and 11.9 respectively. The
converters are said to operate in ‘continuous mode’ when the induction current iL(t)
is continuous, and in the ‘discontinuousmode’ when the iL(t)falls to zero for a part
of each cycle.
Buck coizverters deliver an output voltage which is less than the input voltage. The
dc input is switched using S with a pulse width that is controlled in order that the
average output voltage (filteredusing the inductor L and capacitor C) has an accurately
controlled dc value. SuitableICs are available.These incorporatemost of the components
and simplify the design.
Boost conveyfers deliver an output voltage which exceeds the input voltage. In
this case the switch S connects inductor L across the supply voltage, ramping up the
inductor current; when S is opened, the current is diverted through the diode D to the
output, and the current is reduced linearly.
If electrical isolation between input and output is required, a wide variety of
converters are possible. The introductionof a transformer into the circuit for electrical
isolation creates a need to limit the dc magnetizing current in the transformer in order
to minimize core saturation and avoid the consequent damage to components because
of excessive currents. The most common types of circuit are the flyback conveiler
and the buck converter: The flyback converter is used for low power levels and the
buck converter is used generally for outputs higher than about 200 W, where the size
and cost of the output capacitor in the fly back converter become prohibitive.
Circuits and waveforms for a typical flyback converter and a typical buck-derived
forward converter are shown in Figs 11.10 and 11.11 respectively.
The flyback converter uses a coupled inductor and operates in a similar way to the
boost converter. The inductor current is built up in the primary winding (is(t)) when
the switch S conducts, and is forced to flow in the secondary winding (iD(t))through
the diode D when the switch is off.
Newnes Electrical Engineer’s Handbook
277
1
V0(0= constant
Fig. 11.8 Buck converter
The primary switches in the buck-derived converter invert the dc input voltage,
and a PWM rectangular ac waveform V,(t)is applied to the primary winding of the
isolation transformer. The secondary voltage is rectified by diodes D1and D2before
being applied to the averaging filter which comprises inductor L and capacitor C.
The output from the filter is a smoothed dc waveform with very little switching
frequency ripple.
Stray inductances (mainly transformer leakage inductance) and stray capacitances
(mainly in the switch and diodes) form resonant circuits which oscillate in response
to switching transitions. These oscillations are damped in practical circuits by R-C
snubbers which are connected across transformer windings and in parallel with diodes;
the energy stored in stray and leakage inductances may be recovered using voltage
clamps.
Auxiliary circuitry is necessary for the supply of power to the control circuit, for
control circuit functions, to provide isolation in the feedback loop, and to drive
power transistors.
Static power supplies
278
S
Fig. 11.9 Boost converter
11.8 AC supply harmonic filters
Filters are used to block andor bypass selected frequency components of power
circuit waveforms. Power circuit waveforms are usually composed of a dc component
and an alternating component that is repeated each cycle. It can be shown that any
repetitive waveformmay be represented by the sum of a series of sinusoidal components,
each of which has a frequency which is an integral multiple of the repetition frequency
of the waveform. This series is known as the Fourier series. The component at the
basic repetition frequency is known as the fundumentuZ component, and other
components are known as hamzonics,the Nth harmonic having Ntimes the frequency
of the fundamental. The amplitudes of the harmonics generally decrease as their
frequency increases. The representation of a square wave is shown in Fig. 11.12.
A filter incorporates inductors and capacitors, the impedanceof which is PrOpOrtiOMl
to frequency and inversely proportional to frequency respectively. These components
are combined in a filter circuit to attenuate selectively the frequency components of
the input waveform.
Newnes Electrical Engineer’s Handbook
279
I
VA?)= constant
F
i
g
.11.10 Flyback converter
11.8.1 Supply current harmonics
Loads which are connected to the utility supply may disrupt the operation of other
loads by drawing harmonic currents from the network. Power supply rectifiers, which
charge large capacitors, are a main source of harmonic current flow in the utility
distribution system.
Distorted current waveforms affect other loads connected to the utility system
because of the harmonic voltages which are developed across common impedances
in the distribution network. Return currents flowing in single-phase neutral wires are
added at the three-phase supply neutral point. If the load in the three phases is
balanced, with unity power factor, then the neutral currents cancel exactly and zero
neutral current flows. Typical neutral currents in rectifiers do not cancel, and the
Static power supplies
280
1:l:l
S
Fig. 11.11 Forward converter
resulting neutral current, being the sum of the line currents, requires a higher-rated
cable.
Three-phase rectifiers usually deliver dc output. Load current does not flow through
the neutral wire and the input current waveform is usually acceptable without special
measures to reduce harmonics.
The generation of current harmonics
In diode rectifiers, which are used to convert ac mains to a dc voltage source, the dc
11.8.2
Newnes Electrical Engineer's Handbook
261
Second harmonic at 6 = 2fl
Third harmonic at & = 3fl
lstharmonic
lst + 2ndharmonics
18' + 2"6 + 3'(1 harmonics
0)
(b) Summation of harmonic components
s +2 s {sin(wt) + 113 sin(3ot) + 115 sin (5wt) + ...}
f(f) = 2
r
Fig.
11.12 Frequency components of a square wave
voltage ripple at the rectifier output is limited by a large capacitor. In the half-wave
rectifier shown in Fig. 11.1, the dc capacitor C draws a pulse of current during the
mains voltage peak. A full-wave rectifier as shown in Fig. 11.2 draws positive and
negative pulses of current when the supply voltage is near its peak positive and
negative values respectively.
The resulting supply current waveform is ac,and at the same fundamentalfrequency
as the utility power supply, but it is not a pure sine wave. As previously explained,
the waveform can be split into a Fourier series of pure sine wave frequencies which
are harmonics of the fundamental frequency. The amplitudes of the harmonic
components increase in magnitude if the pulse of current drawn by the capacitor is
shorter; the pulse width depends upon the ac circuit impedance and the magnitude of
the dc voltage ripple.
For a given dc voltage ripple requirement, which is set by the de capacitance
value and the dc load current, the width of the supply current pulse can be increased
by adding series inductance in the rectifier circuit, either in the ac connections or on
the supply side of the dc capacitance. This method is considered to be acceptable at
low power levels, but above about 400 W output the inductor becomes prohibitively
large and preference is given to active methods which allow the use of a smaller
inductor.
Static power supplies
282
11.8.3
Current harmonics and power factor
Power factor has been defined in Chapter 2. Simply restated here, it is the total
power taken from the supply divided by the ac supply voltage and the ac supply
current. Reactive power flow and current waveform distortion reduce the power
factor from its ideal value of unity.
Rectifiers delivering a dc voltage output usually draw little reactive current, but as
pointed out above, they draw significant harmonic currents. The amount of distortion
in the supply current waveform is usually quantified using the ac supply power
factor, since reactive power flow can be neglected. Typical single-phase full-wave
rectifiers of the type described above have a power factor in the region of 0.65 due
to distortion of the current waveform.
11.8.4
Reduction of current harmonics
In order to attenuatethe harmonic currentsdrawn by anon-linearload, an inductor may
be connectedin series with the load. This has the effect of reducing the load impedance
variation, this variation being responsible for generating the harmonic currents.
Alternatively, a circuit orfilter which provides a low-impedancepath at a harmonic
frequency may be connected into the supply side of the load in order to bypass most
of the harmonic current. At high power levels, a series resonant circuit is used; this
comprises an inductor and a capacitor which are tuned to &heharmonic frequency
which is to be bypassed. In a 12-pulseinverter, for instance, a tuned filter is generally
used to bypass the 11th and 13th harmonics and an LC filter is used for higher
harmonics. Care is taken in the design of these filters to avoid high power loss at the
fundamental fiequency. Filters of this type are generally used in very high power
applications such as HVDC transmission schemes.
Active powerfactor correction is often used in single-phasesystems with a power
output above about 400W, where a passive system as described above would require
an inductor of impractical size. Active power factor correction is usually based on a
boost converter switching at a constant high frequency of 16 H-Xz or above, and using
PWM control to achieve approximately unity power factor. The high-frequency
switching enables a relatively small inductance value to be used. A typical scheme is
shown in Fig. 11.13. In comparison with a non-corrected system, active power factor
correctionprovides the benefits of reduced dc voltage ripple, reduced dc bulk capacitance
and reduced rms supply current. It can also enable a universal input by working with
low-voltage ac supplies at higher line currents. These benefits do not compensate for
the increased cost, complexity and power losses associated with active power factor
correction, but supply harmonic limits may be enforced by law, and this would lead
to its widespread use.
A PWM-controlled inverter, when connected to the supply system as a load, can
also be controlled to bypass current harmonics. In order to avoid high-frequency
switching noise passing into the ac supply the PWM waveforms are filtered, and the
cut-off frequency of the filter limits the maximum frequency of the harmonic currents
which can be bypassed using this technique.
11.9
Filters for high-frequency noise
Switching converters generate high-frequency electrical noise which is conducted
NeJvnes Electrical Engineer's Handbook
283
V,(t) is approximately constant
Fig. 11.13 Power factor correction boost converter
out of the equipment along electrical connections and emitted by the equipment in
the form of electromagnetic radiation.
Conducted emissions are attenuated by an electromagnetic interference (EMI)
filter as shown in Fig. 11.13. The EM1 filter is a combination of LC filters for
common-mode and direreiitinl-inodenoise. In Fig. 11.13, C 1 and C4 are X capacitors,
which act with L2 and L3 to filter the differential-mode noise which flows between
line and neutral connections. C2 and C3 are Y capacitors, which act with L1, L2 and
L3 to filter the common-mode noise which flows through the earth connection and
splits equally between line and neutral circuits. X capacitors have typical values in
the range I O nF to 2 pF: these are used in cases where failure o f the capacitor would
not lead to a risk of electric shock. Y capacitors have typical values in the range
0.5 nF to 35 nF. These are used where failure of the capacitor could lead to risk of
electric shock. The mutual inductance of L1 typically ranges from 1.8 mH at 25 A
to 47 rnH at 0.3 A, and L2 and L3 may be provided by the leakage inductance of the
windings of L1. The resistance R is required in order to discharge the X capacitors
with a time constant of typically 1 second.
Power conditioizers limit the disturbances which enter a load from the ac supply.
They attenuate the high-frequency components of these disturbances using filters
and by clipping voltage spikes. A typical single-phase power conditioner circuit is
shown in Fig. 11.14. This circuit is similar to the filter circuit already shown in
284
Static power supplies
Fig. 11.14 Single-phase high-frequency power conditioner
Fig. 11.13. but it has non-linear voltage clamping components added; these are
shown as the metal oxide varistors MOV1, MOV2 and MOV3 and suppressor diodes
ZDI, ZD2 and ZD3.
Radiated emissions may be suppressed by various forms of screens, including
earthed enclosures, screens between transformer windings, screens between components
and their heat sinks, and screens around the joints between the halves of ferrite cores.
The origin of high-frequency noise in switching power converters is the high rate of
change of voltage and current during switching transitions. These high-frequency
components in the waveform can be limited by minimizing the areas of the highfrequency current loops, and by using slow switching and snubbers. Components
experiencing high rates of change of voltage propagate noise through capacitive
coupling, and current loops with high rates of change of current propagate noise
through radiated emissions.
The limits on Radio Frequency Inteference (RFI) are usually required by law.
This matter is dealt with in more depth in Chapter 15.
11.I 0 Systems with cascaded converters
The basic power conversion circuits which have been described are often combined
in more complex applications. Some of the more common combinations are described
in this section, and the merits of the different possibilities for some applications are
set out.
11.10.1 The Uninterruptible Power Supply (UPS)
The UPS is used to provide a continuous stable supply in the event of a utility supply
failure, this failure being typically defined as the ac voltage falling below 85 per cent
of its nominal value. Typical applications are in medical and communication systems,
in which the expense of a UPS is justified by the reduced chances of a system failure.
The UPS therefore incorporates a back-up power source, which is usually a battery.
A typical scheme is outlined in Fig. 11.15. Such equipment is usually designed to
provide 10-15 minutes’ supply at full load, although to minimize the battery size
some systems for computer supplies deliver the full load for just long enough for the
hardware to shut down automatically and to save data. Diesel generators are often
used where it is required to supply full load for longer periods,
Newnes Electrical Engineer's Handbook
s u p p ~ _ ( Filter
Rectifier1
dc bus
285
4
I n v e r t e r H Filter
Fig. 11.15 Typical on-line U P S block diagram
Most UPS systems are either on-line or standby types.
An on-line UPS continuously rectifies the utility supply, charges the back-up
battery as necessary and supplies power to the load by continuously inverting the dc.
If the rectifier or inverter fails, a bypass switch is used to connect the load directly
to the supply. When the utility supply fails, the back-up battery delivers dc power to
the inverter and the ac supply to the load is maintained.
In a standby UPS,under normal operation the load is supplied directly from the
utility network and the back-up battery is trickle charged. When a failure in the
utility supply occurs, a changeover switch connects the output of an inverter to the
load, and the inverter draws power from the battery. Standby systems are normally
cheaper than on-line systems.
The line-interactive UPS is an off-line type which includes a system that boosts
the supply voltage during line voltage sags. The voltage may be boosted by use of a
tap-changing transformer. This system reduces the number of occasions when battery
power is used; this helps to maintain a full charge in the battery for the event of a full
loss of supply.
The inverter is the heart of a battery-powered UPS, and different types are used
for different applications. The cheapest systems deliver a square wave, but these
require severe derating of power when supplying such loads as switched-mode power
supplies. Inverters delivering a sinusoidal output are generally used. The ac output of
the inverter is stepped up to the required output voltage using a transformer.
11.102 Regulated dc power supplies
There is a large demand for power supplies that convert the utility ac supply to an
isolated low-voltagedc output. This is mainly due to the near-universal use of transistor
circuits for control, communication and computing equipment, these transistor circuits
being powered by low-voltage dc.
For applications that require an accurately controlled output voltage, a switchedmode power supply or a linear regulator is necessary in order to compensate for
fluctuations in the ac input voltage and to limit the variation of output voltage with
load current. Switched-mode power supplies become economical above a few tens of
watts output, but below this level linear regulators are often prefenred, especially in
Static power supplies
286
lownoise, high-reliability applications. The dc output voltage is accurately controlled
by comparing it with a voltage reference and by using negative feedback. The amplified
voltage difference is used to control the regulator. This scheme requires an accurate
voltage reference, which is usually temperature compensated, and an error amplifier;
these are usually integrated with other components in an IC.
In a linear regulator a supply-frequencytransformer is used at the input for isolation
and to reduce the voltage level to that required by the load. The transformer output
is rectified into a capacitor, and load current flows through the linear regulator to the
output. A typical linear power supply scheme is shown in Fig. 11.16. These supplies
require a large, heavy transformer and their efficiency is typically only 30-60 per
cent. A large proportion of the losses occur in the regulator transistor, and they are
dissipated using a large heat sink.
rectifier
Power
capacitor
-
-+
Isolation
c
=
l
I
I
DC
==outPut
N
Fig. 11.16 Linear power supply scheme
Switched-mode power supplies use dc to dc converters. The ac supply is rectified
into a capacitor, which results in some voltage ripple. The substantial dc power is
passed through a dc to dc converter which regulates the output voltage using the
switch duty cycle. The supply is isolated using a small high-frequency transformer.
This avoids the high power dissipation and the large transformer of the linear power
supply, and efficiencies of 70-90 per cent are usual. The circuit of a typical 1 kW to
10 kW off-line switched-mode power supply is shown in Fig. 11.17, and Fig. 11.18
shows a typical 100W off-line power supply.A switched-mode power supply requires
more components than a linear power supply; it also produces significantly more
electrical noise because of the high-frequency switching, and this requires careful
layout of the power circuit with filters and screens to limit the noise to acceptable
levels.
Commercial dc power supplies may incorporate the following control and monitoring
functions:
remote regulation of output voltage
shutdowns for thermal overload, transformer primary overcurrent and output
overvoltage
external warning of power failure and external inhibit
output current foldback
parallel operation with forced current sharing
I
I
Main
output
Fig. 11.17 A typical off-line switched-mode power supply power circuit
Static power supplies
288
Fig. 11.18 A typical 100 W off-line power supply (courtesy of Coutant Lambda)
11.10.3
Power conditioners for photovoltaic arrays
A typical photovoltaic array consists of relatively large-area silicon semiconductor
diodes which are exposed to light that passes through a window. The diode is designed
such that photons absorbed within the semiconductor often produce charge carriers
in the form of electrons and holes. The increased carrier densities give rise to a
voltage, and the ability to deliver electrical power. Many diodes are connected in
series in order to provide sufficient voltage. A photovoltaic array delivers maximum
power at a given voltage and current, but this optimum voltage and current varies
with light intensity and temperature.
Photovoltaic cells may be used without power control circuitry to supply families
of integrated circuits that have wide operating voltage ranges. However, most large
systems use a peak power tracking converter which delivers the maximum power
available at any instant, at a controlled dc voltage; this controlled power may be
supplied to a static load, a battery or to an inverter.
Power converters which deliver ac output power to the utility network using a
PWM inverter have been proposed for widespread use, but such systems are rarely
practical at present because of the special requirements for connection of power
generating equipment.
11.11 Standards
The leading international, regional and national standards adopted by users and
suppliers of power supplies for manufacturing and testing are shown in Table 11.1.
Newnes Electrical Engineer’s Handbook
Table 11.1 International, regional and national standards relating to power supplies
~~
~~~~
~
IEC
EN
BS
60065
EN 60065
EN 60065
~~~~
~
Subject of standard
~~
N. American
Safety requirements for electronic
apparatus for household use
60335
EN 60335 EN 60335 Safety of household and similar
apparatus
60601-1 EN 60601-1 EN 60601-1 Medical electrical equipment pt 1:
general requirements for safety
60950
EN 60950 EN 60950 Safety of IT equipment
-
61010-1 EN 61010-1 EN 61010-1 Safety requirements for electrical
equipment for measurement, control
and laboratory use
Pt 1: general requirements
61204
LV power supply devices, dc output performance and safety
Industrial control equipment
Medical and dental equipment
Power units other than class 2
LV video products without CRT
displays
Medical electrical equipment - pt 1:
general requirements for safety
Electrical equipment for laboratoty
use - pt 1: general requirements
CSA 22.2 no
950-95
UL 1950
UL 508
UL 544
UL 1012
UL 1409
UL 2601-1
UL 3101-1
References
The first two references given below are academic treatments, and the third gives a design engineer’s
approach to low-power dc power supplies. Useful information can also be obtained from the catalogues
of companies that distribute power supplies: these catalogues often include general technical information
as well as information about specific products.
11A.
IIB.
11C.
Mohan, N., Undeland T.M. and Robbins, W.P. Power Electrotjics: Converters, AppIicariotrr and
Design, John Wiley & Sons Inc., 1995.
Williams, B.W. Power Electronics. Macmillan, 1992.
Billings, K.H.Switchmode Power Supply Handbook. McGraw-Hill, 1989.
Chapter 12
Batteries and fuel cells
Mr R.I. Deakin
Deakin Davenset Rectifiers
12.1 Introduction
The battery is at present the most practical and widely used means of storing electrical
energy. The storage capacity of a battery is usually defined in ampere-hours (Ah);
energy is strictly defined in kWh or joules, but since the voltage of the battery is
nominally fixed and known, the Ah definition is more convenient. The terms battery
and cell are often interchanged, although strictly a battery is a group of cells built
together in a single unit.
Batteries can be classified into primary and secondary types.
Aprimary battery stores electrical energy in a chemical form which is introduced
at the manufacturing stage. When it is discharged and this chemically stored energy
is depleted, the battery is no longer serviceable. Applications for primary batteries
are generally in the low-cost domestic environment, in portable equipment such as
torches, calculators, radios and hearing aids.
A Secondary (rechargeable) battery will absorb electrical energy, store this in a
chemical form and then release electrical energy when required. Once the battery has
been discharged and the chemical energy depleted, it can be recharged with a further
intake of electrical energy. Many cycles of charging and discharging can be repeated
in a secondary battery. Applications cover a wide range. In the domestic environment
secondary batteries are used in portable hand tools, laptop computers and portable
telephones. Higher-power applications in industry include use in road and rail vehicles
and in standby power applications, the latter having already been described in section
11.10.1. The capacity of secondary battery systems ranges from 100 mAh to
2000 Ah. Their useful life ranges from two to 20 years; this will depend, among other
things, upon the number of change-discharge cycles and the type and construction of
battery used.
Thefuel cell is a relatively new method of energy storage which is increasing in
acceptance for certain applications. A fuel cell converts the energy of a chemical
reaction into electrical energy in a way which is similar, in principle, to the battery,
but a fuel cell is ‘recharged’ by continuous replenishment of the chemical materials.
12.2 Primary cells
The majority of primary cells now in use fall into one of eight types:
Batteries and fuel cells
zinc carbon
alkaline manganese
mercury oxide
silver oxide
zinc air
lithium manganese dioxide
lithium thionyl chloride
lithium copper oxyphosphate
Each of these is briefly reviewed in turn in the following sections.
12.2.1 Zinc carbon
In its latest form, this has been in use since 1950. The basic cell operates with a
voltage range of 1.2 V to 1.6 V, and cells are connected in series to form batteries
such as M A , AA, C, D, PP3 and PP9. Typical construction of a PP9 battery is shown
in Fig. 12.1.
D Wax coating. This seals any capillary
passages between cells and the
atmosphere, so preventing the loss of
moisture.
E Plastic cell container. This plastic band
holds together all the components of a
single cell.
F Positive electrode. This is a flat cake
containing a mixture of manganese
dioxide and carbon black or graphite for
conductivity. Ammonium chloride and
zinc chloride are other necessary
ingredients.
G Paper tray. This acts as a separator
between mix cake and the zinc electrode.
H Carbon coated zinc electrode. Known as
a duplex electrode, this is a zinc plate to
which is adhered a thin layer of highly
conductive carbon which is impervious to
electrolyte.
I Electrolyte impregnated paper. This
contains electrolyte and is an additional
separator between the mix cake and the
A Protector card. This protects the terminals
zinc.
and is torn away before use.
J Bottom plate. This plastic plate closes
B Top plate. This plastic plate carries the snap
the bottom of the battery.
fastener connectors and closes the top of
K Conducting strip. This makes contact
the battery.
with the negative zinc plate at the base
C Metal jacket. This is crimped on to the outside
of the stack and is connected to the
of the battery and carries the printed design.
negative socket at the other end.
The jacket helps to resist bulging, breakage
and leakage and holds all components firmly
together.
Fig. 12.1 Zinc carbon PP9 battery construction (courtesy of Enegiser)
Newnes Electrical Engineer's Handbook
293
Zinc carbon batteries are low in cost, they have an operating temperature range
from -10°C to 50"C,and the shelf life at a temperature of 20°C is up to three years.
They are ideally suited to applications with intermittent loads, such as radios, torches,
toys, clocks and flashing warning lamps.
12.2.2 Alkaline manganese
The alkaline manganese battery became commercially available in its present form
in the late 1950s. The basic cell operates with a voltage range of 1.35 V to 1.55 V, and
cells are connected in series to form the standard battery sizes N, AAA, AA, C, D and
PP3. Typical construction of a single-cell battery is shown in Fig. 12.2.
A Cathode pellet. This is of synthetic manganese dioxide, with graphite for conductivity, pressed
into cylindrical pellets. The pellets make an interference fit with the can, ensuring good electrical contact. The graphite is the cathode current collector.
B Steel can. This acts as the cell container. The preformed stud at the closed end of the can
functions as the cell positive terminal.
C Closure. The hard plastic cell closure forms the seal at the open end of the can and carries the
bottom coverkurrent collector assembly.
D Bottom cover. This metal plate acts as the negative terminal of the cell. It is welded to the
current collector to ensure good electrical contact.
E Separator. This is of non-woven synthetic material.
F Anode current collector. This is a metal 'nail'.
G Zinc paste anode. The anode is of amalgamated high purity zinc powder made into a paste
with the electrolyte.
H Shrink label. This covers the outside of the cell and carries printed design, and provides
insulation of the side of cell.
Fig. 12.2 Construction of an alkaline manganese battery (courtesy of Energiser)
Cost is in the medium-low range. The operating temperature range is -15°C to
50°C. and the shelf life at 20°C is up to four years. Alkaline manganese batteries
are suited to a current drain of 5 mA to 2 A: this makes them ideally suited to
Batteries and fuel cells
294
continuous duty applications which can include radios, torches, cassette players, toys
and cameras.
12.2.3
Mercury oxide
This has been in commercial use since the 1930s. The single cell operates with a
narrow voltage range of 1.3 V to 1.4 V. This type is only available as a single button
cell, the construction of which is illustrated in Eig. 12.3.
Cell cap. The plated steel cap functions as the negative terminal
of the cell. The inside of the cap is laminated with copper.
Zinc anode. This is of high purity amalgamated zinc powder.
Absorbent pad. This is of a non-woven material and holds the
alkaline electrolyte.
Separator. This is a synthetic ion-permeable material.
Mercuric oxide cathode. The cathode is a pellet of mercuric
oxide plus graphite for conductivity and sometimes
manganese dioxide. It is compressed into the can.
Sealing grommet. This plastic grommet both seals the cell
and insulates the positive and negative terminals.
Cell can. The can is of nickel plated steel and functions as
the positive terminal of the cell.
Fig. 12.3 Mercury oxide button cell (courtesy of Energiser)
Cost is in the medium range. The operating temperature range is 0°C to 50"C, and
the shelf life at 20°C is up to four years. Mercury oxide button cells are suitable for
low current drains of 0.1 mA to 5 mA, with either continuous or intermittent loads.
Their small size and narrow voltage range are ideally suited for use in hearing aids,
cameras and electrical instruments.
12.2.4 Silver oxide
This has been in commercial use since the 1970s. The single cell operates with a very
narrow range from 1.50 V to 1.55 V. Construction of a silver oxide button cell is
shown in Fig. 12.4.
Cost is relatively high. They have an operating temperature range from 0°C to
50°C and a shelf life of up to three years. In button cell form, they are suitable for a
current drain of 0.1 mA to 5 mA, either continuous or intermittent. Their small size
and very narrow voltage range makes them ideal for use in watches, calculators and
electrical instruments.
12.2.5 Zinc air
This battery has been in commercial use since the mid-1970s. A single cell operates
Newnes Electrical Engineer’s Handbook
295
Cell cap. The plated steel cap functions as the negative terminal
of the cell. The inside of the cap is laminated with copper.
Zinc anode. This is a high purity amalgamated zinc powder.
Absorbent pad. This pad is of a non-woven material and holds
the alkaline electrolyte.
Separator. This is a synthetic ion-permeable membrane.
Cathode. This is a pellet of silver oxide plus graphite for
conductivity. It is compressed into the can.
Sealing grommet. This plastic grommet both seals the cell and
insulates the positive and negative terminals.
Cell can. The nickel plated steel can acts as a cell container
and as the positive terminal of the cell.
Fig. 12.4 Silver oxide button cell (courtesy of Energiser)
with a voltage range of 1.2 V to 1.4 V. The construction of a zinc air button cell is
shown in Fig. 12.5.
Cell cap. The plated steel cap functions as the negative terminal
of the cell.
Zinc anode. This is a high purity amalgamated zinc powder,
which also retains the alkaline electrolyte.
Separator. A synthetic ion-permeable membrane.
Cathode. This is a carbon/catalyst mixture with a wetproofing
agent on a mesh support, and with an outer layer of gaspermeable hydrophobic PTFE.
Sealing grommet. This plastic grommet seals and insulates the
positive and negative terminals, and seals the cathode to the base.
Diffusion membrane. This permeable layer distributes air from
the access holes uniformly across the cathode surface.
Can. This plated steel can forms a support for the cathode, acts
as a cell container and the positive terminal of the cell. The
holes in the can permit air access to the catalyst cathode.
Fig. 12.5 Zinc air cell (courtesy of Energiser)
296
Batteries and fuel cells
Because one of the reactants is air, the battery has a sealing tab which must be
removed before it is put into service. Following removal of the tab, the cell voltage
will rise to 1.4 V.
Cost is in the medium range. Operating temperature is from -10°C to 50°C and
the shelf life is almost unlimited while the seal remains unbroken. They are produced
as a button cell, and are suitable for a current drain from 1 mA to 10 mA, with either
intermittent or continuous loads. The service life is relatively short due to high selfdischarge characteristics. but end of life can be accurately estimated because of the
flat discharge current. The general performance and the life predictability makes zinc
air batteries ideal for use in hearing aids.
12.2.6
Lithium manganese dioxide
This type of banery has been available since 1975,but it has only become commercially
available in the past few years. The single cell operates with a relatively high voltage,
in the range 2.8 V to 3.0 V. A typical construction is shown in Fig. 12.6.
Current collector. This is a sheet of perforated stainless steel.
Stainless steel top cap. This functions as the negative
terminal of the cell.
Stainless steel cell can. This functions as the positive
terminal of the cell.
Polypropylene closure. This material is highly impermeable to
water vapour and prevents moisture entering the cell after it
has been sealed.
Lithium negative electrode. This is punched from sheet
lithium.
Separator containing electrolyte. The separator is of nonwoven polypropylene cloth and contains electrolyte, a
solution of lithium perchlorate in a mixture of propylene
carbonate and dimethoxyethane.
Manganese dioxide positive electrode. The cathode is made
from a highly active electrolytic oxide.
Fig. 12.6 Lithium manganese dioxide cell (courtesy of Energiser)
Cost is relatively high. The operating temperature covers a wide range, from
to 5OoC, and the shelf life is four to five years. Lithium manganese dioxide
batteries are suitable for continuous or intermittent loads; in the form of a button cell
they are suitable for 0.1 mA to 10 mA current drain, or as a cylindrical cell they can
deliver from 0.1 mA up to 1 A. They have a high discharge efficiency down to -20°C,
and are suitable for applications in instruments, watches and cameras.
-30°C
Newnes Electrical Engineer's Handbook
297
12.2.7 Lithium thionyl chloride
This type of battery has only recently become commercially available. The single
cell has a high voltage in the range 3.2 V to 3.5 V, with an open-circuit voltage of
3.67 V., A cr&s-secti& of an example is shown in Fig. 12.7. -
GlasdMetal seal
Stainless steel can
Bottom insulator
0
Fig. 12.7 Lithium thionyl chloride cell (courtesy of SAFT Ltd)
Cost is relatively high. The operating temperature range is one of the highest
available, from -55°C to 85"C, and the shelf life is up to 10years. These batteries m
suitable for intermittent or continuous duties, with a current drain from 15 mA up to
1.5 A. Their high energy density, wide temperaturerange and high dischargeefficiency
makes these batteries particularly well suited to memory back-up applications in
programmable logic controllers, personal computers and alarm systems.
12.2.8
Lithium copper oxyphosphate
This has only become commercially available recently. The single cell has a voltage
range 2.1 V to 2.5 V, with an open-circuit voltage of 2.7 V. A sectional view of a
typical unit is shown in Fig. 12.8.
These batteries are expensive because of their high energy density. They operate
over a temperature range -40°C to 60°C and they have a shelf life of up to 10 years.
They are suitable for continuous loads from 37 pA up to 25 mA. The high energy
density and wide temperature range make these batteries specially suitable for
applications in metering computers.
298
Batteries and fuel cells
0
Negative terminal
Insulating gasket
Nickel plated steel can
Electrolyte
Copper oxyphosphate cathode
Separator
Shrink + label
Lithium anode
Anode current collector
Bottom insulator
Insulating disk
Positive terminal
I
0
Fig. 12.8 Lithium copper oxyphosphate cell (courtesy of SAFT Ltd)
12.3 Secondary cells
The main forms of secondary or rechargeable battery are lead acid and nickel cadmium,
both of which have been in use for about 100 years.
Nickel cadmium and lead acid batteries are formed by connecting a number of
cells in series. Each cell consists of vertical plates which are connected in parallel
and are divided into a positive group and a negative group. Adjacent positive and
negative plates are insulated from each other by separators or rod insulators which
have to provide the following functions:
obstruct the transfer of ions between the plates as little as possible
keep in place the active material
enable the escape of charging gases from the electrolyte
All the plates and separators together form a plategmup or element.
The insulators in nickel cadmium batteries are vertical plastic rods, which may be
separate or in the form of grids which are inserted between the plates. Corrugated or
perforated PVC is also used as a separator. The active material is kept in place by
steel pockets.
In lead acid batteries, the separators are normally microporous sheets of plastic,
which are usually combined with corrugated and perforated spacers; porous rubber
separators are also used. In some designs the separator completely surrounds the
positive or negative plate. The selection of separator is very important, and it depends
upon the plate design and the use for which the battery is intended. In batteries
intended for short discharges, such as diesel engine starting, thin separators and
spacers are used, whereas batteries designed for long discharges, as in standby power
or emergency lighting, have thick separators and spacers..Batteries that require extra
Newnes Electrical Engineer’s Handbook
299
strength to withstand mechanical stresses often have a glass fibre separator against
the positive plates in order to minimize the shedding of active material from these
plates.
The outermost plates are of the same polarity on both sides. In lead acid batteries
the outside plates are normally negative; in nickel cadmium batteries they are normally
positive.
The plategroups are mounted in containers which are filled with electrolyte. The
electrolyte is dilute sulphuric acid in lead acid batteries, and potassium hydroxide in
nickel cadmium batteries. In a fully charged lead acid battery the electrolyte density
is between 1.20 and 1.29. In nickel cadmium batteries it is between 1.18 and 1.30
irrespective of the state of charge.
The containers may house a single cell or they may be of a rnonoblock type,
housing several cells. The top (or lid) may be glued or welded to the container, or it
may be formed integrally with the container. In the lid is a hole for the vent, which
is normally flame arresting, and the cells are topped up with electrolyte through the
vent hole.
Below the plates there is an empty space which is known as the sludge space. This
is provided so that shredded active material and corrosion products from the supporting
structure can be deposited without creating any short-circuits between the plates. The
sludge space in nickel cadmium batteries is smaller than in lead acid units because
there is no corrosion of the supporting steel structure and no shredding of active
material; it is usually created by hanging the plategroup in the lid. In lead acid
batteries the sludge space may be created by hanging the plates from the lid or
container walls (this is common in stationary batteries), by standing the plates on
supports from the bottom (this is common for locomotive starting batteries) or by
hanging the positive plates with the negative plates standing on supports (this is also
used for stationary batteries).
From the plates, the current is carried through plate lugs, a connecting strap or
bridge and a polebolt. The bolts pass through the lid and are sealed against the lid
with gaskets. Above the lid, the cells are connected to form the battery. Monoblocks
may have connectors arranged directly through the side of the partition cell wall, in
which case the polebolts do not pass through the lid, or the connectors may be
located between the cell lid and an outer lid. The monoblocks made in this way are
completely insulated and easy to keep clean.
The main forms of secondary cell now commercially available are:
0
0
0
0
0
0
nickel cadmium - sealed
nickel cadmium - vented
nickel metal hydride
lead acid - pasted plate
lead acid - tubular
lead acid - Plant6
lead acid - valve-regulated sealed (VRSLA)
Each of these is described separately in sections 12.3.1 to 12.3.7, and a summary of
the main features of the four lead acid types and nickel cadmium is given in Table 12.1.
12.3.1
Sealed nickel cadmium
This type of cell has been commercially available since the early 1950s. The single
Batteries and fuel cells
300
Table 12.1 Comparison of features and performance of the nmin types of secondary battery
Battery type
Lead acid
Nlckel
cadmium
Pasted plate
Tubular
Plante
VRSLA
Suited to
shallow
discharge
duty
40°C
Lead
antimony
Lead
calcium
Cycle duty
Good
Poor
Very good
Suited to
shallow
discharge
duty
Maximum
temperature
Gas
generation and
maintenance
Volume/Ah
indicator
Relative cost
45°C
35°C
45°C
45°C
Low
Verylow
Low
Very low
Negligible
No topping
up needed
Low to
moderate
50
50
45
100
35
55
70
70
90
100
ao
200-300
Suited to
shallow
discharge
duty
45°C
cell operates across a voltage range 1.0 V to 1.25 V and is ideally suited to a heavy
continuous current drain of up to eight times the nominal ampere-hour capacity of
the battery. The low internal resistance of the cell makes it ideal for heavy current
discharge applications in motor-driven appliances such as portable drills, vacuum
cleaners, toys and emergency systems. Batteries of this type are capable of being
recharged hundreds of times using a simple constant-current charging method.
The cells tolerate a wide temperature range from -30°C to 50"C, and they have a
shelf life of up to eight years at 20°C.
Typical construction of a sealed nickel cadmium cell is shown in Fig. 12.9.The
cell is available commercially in a variety of standard sizes including AAA, AA, C,
D, PP3 and PP9.
12.3.2 Vented nickel cadmium
Using the electrochemistry of nickel and cadmium compounds with a potassium
hydroxide electrolyte, vented cells use 'pocket' plates which are made from finely
perforated nickel-plated steel strip filled with active material, The pocket plates are
crimped together to produce a homogeneous plate. Translucent plastic or stainless
steel containers are used. An example is shown in Fig. 12.10.
These batteries are robust and they have the benefits of all-round reliability; they
are resistant to shock and extremes of temperature and electrical loading. They can
be left discharged without damage and require very little maintenance. Their cycling
ability is excellent and can offer a service life of up to 25 years.
Vented nickel cadmium batteries are used typically in long-life applications where
reliability through temperature extremes in required and where physical or electrical
abuse is likely. These applications include railway rolling stock, off-shore use, power
system switch tripping and closing, telecommunications,unintermptiblepower supplies
(see section 11.10.1), security and emergency systems and engine starting.
12.3.3 Nickel metal hydride
The sealed nickel metal hydride battery has become commercially available in the
301
Newnes Electrical Engineer's Handbook
A
B
C
D
E
Nylon sealing gasket
Reseallng safety vent
Nickel plated steel top plate (positive)
Positiveconnector
Nickel plated steel can (negative)
F
G
H
I
J
Sintered positive electrode
Separator
Support strip
Negativeelectrode
Negativeconnector
Fig. 12.9 Sealed nickel cadmium cell (courtesy of Energiser)
past few years. A single cell operates with a voltage range of 1.1 V to 1.45 V, and a
nominal voltage of 1.2 V. The construction of a metal hydride cell is illustrated in
Fig. 12.11.
These batteries are relatively expensive. Their energy density is in the region 4446 Wh/kg, which is higher than the sealed nickel cadmium battery. The operating
temperature range is -2OOC to 60°C and the shelf life is up to five years. Nickel metal
hydride batteries are suitable for applications with mA cumnr drainsfrom 0.2 to 3
times the nominal ampere-hour capacio of the battery. The high energy density
makes this type of battery ideally suited to applications for memory back-up and
portable communications.
Nickel metal hydride batteries will endure recharging up to 500 times, although
the method of charging is slightly more complicated than for sealed nickel cadmium
systems.
12.3.4
Lead add
- pasted plate
The positive plates of pasted plate cells are made by impressing an oxide paste into
a current-collectinglead alloy grid, in which the paste then forms the active material
of the plate. The paste is held in position by a long interlocking grid section. In
addition to the long-life microporous plastic separator, a glass-fibre mat is used. This
302
Batteries and fuel cells
Terminal seal
This is mechanically clipped and
Flame arresting vent
serves as a current collector
Fig. 12.10 Vented nickel cadmium battery (courtesy of ALCAD)
becomes embedded in the face of the positive plate holding in place the active
material and so prolonging the life of the battery.
Cells can be manufactured with lead antimony selenium alloy plates. These require
very little maintenance, offer good cycling performance and are tolerant to elevated
temperatures. Alternatively, lead calcium tin alloy plates may be used; these offer
even lower maintenance levels.
Pasted-plate lead acid batteries are the lowest in purchase cost; their service life
is in the range two to 20 years and the maintenance requirements are low, with
extended intervals between watering. They are compact and provide high power
density with excellent power output for short durations. Typical applications are in
medium-to-long duration uses where initial capital cost is a primary consideration.
These include telecommunications, UPS, power generation transmission, switch
tripping and closing, emergency lighting and engine starting.
Newnes Electrical Engineer’s Handbook
303
Fig. 12.11 Nickel metal hydride battery (courtesy of SAFT Ltd)
12.3.5 Lead acid
- tubular
In a tubular cell, an example of which is shown in Fig. 12.12, the positive plate is
constructed from a series of vertical lead alloy spines or fingers which resemble a
comb. The active material is lead oxide; this is packed around each spine and is
retained by tubes of woven glass fibre which are protected by an outer sleeve of
woven polyester or perforated PVC. This plate design enables the cell to withstand
the frequent charge-discharge cycles which cause rapid deterioration in other types
Vent plugs
Easy clean cell lids
Low-resistance cell
pillars and connectors
Clear-view plastic
containers
High-porosity
separators
Rugged positive plates
Tough, pasted negative
plates
Fig. 12.12 Tubular cell lead acid battery (courtesy of Invensys)
The advantages of the tubular cell are excellent deep cycling characteristics,
service life up to 15 years and a compact layout which gives a high power per unit
volume. The low antimony types also offer extended watering intervals.
304
Batteries and fuel cells
Typical applications are where the power supply is unreliable, and where discharges
are likely to be both frequent and deep. These include telecommunications, UPS,
emergency lighting and solar energy.
12.3.6
Lead acid - Plante
The distinguishing feature of the Plant6 cell is the single pure lead casting of the
Plant6 positive plate. The active material is formed from the plate surface, eliminating
the need for mechanical bonding of a separately applied active material to a current
collector. This makes the Plant6 cell the most reliable of all lead acid types. An
example of the construction of Plant6 batteries is shown in Fig. 12.13.
1. Vent plugs
2. Cell lids
3. Cell pillars and connectors
4. Bar guard
5. Negative plates
6. Separators
7. Plante positive plates
8. Plastic containers
Fig. 12.13 Lead acid Plant6 cells (courtesy of Invensys)
Ultra High Performance Plant6 cells have a thinner plate. This gives the highest
standards of reliability for UPS and other high-rate applications.
The advantages of the Plant6 cell are extremely long life (which can be in excess
of 20 years for High Performance cells and 15 years for Ultra High Performance
Newnes Electrical Engineer’s Handbook
305
cells) and the highest levels of reliability and integrity. Constant capacity is available
throughout the service life, and an assessment of condition and residual life can
readily be made by visual inspection. Usage at high temperatures can be tolerated
without significantlycompromisingthe life expectancy, and maintenancerequirements
are very low.
Typical applications are long-life float duties in which the ultimate in reliability
is required. These include critical power station systems, telecommunications, UPS,
switch tripping and closing. emergency lighting and engine starting.
12.3.7 Lead acid
- valve regulated sealed (VRSLA)
In sealed designs, lead calcium or pure lead is used in the grids. The separator is a
vital part of the design because of its influence on gas recombination, and the amount
of electrolyte that it retains; it often consists of a highly porous sheet of microfibre.
Examples of sealed cells are shown in Fig. 12.14.
Beside these sealed units, there is a large variety of so-called maintenance-free
batteries which rely more upon carefully controlled charging than upon the gas
recombination mechanism within the cells. Non-antimonial grids and pasted plates
are used, and in most units the electrolyte is immobilized or gelled. The batteries are
provided with vents which open at a relatively low overpressure.
The benefits are long life, no topping up, no acid fumes and no requirement for
forced ventilation. There is no need for a separate battery room and the units can be
located within the enclosure of an electronic system. In addition these batteries are
lighter and smaller than traditional vented cells.
Typical applicationsinclude mainexchanges for telecommunications,PABX systems,
cellular radio, microwave links, UPS,switch tripping and closing, emergency lighting
and engine starting.
12.4
Fuel cells
A fuel cell converts the energy of a chemical reaction directly into electrical energy.
The process involves the oxidation of a fuel, which is normally a hydrogen-rich gas,
and the reduction of an oxidant, which is usually atmospheric oxygen. Electrons are
passed from the fuel electrode to the oxidant electrode through the externally connected
load, and the electrical circuit is completedby ionized particles that cross an electrolyte
to recombine as water.
The fuel cell has a number of advantages over the conventional heat engine and
shaft-driven generator for the production of electrical power. The generation efficiency
is higher in a fuel cell, it has a higher power density, lower emission and vibration
characteristics, and reduced emission of pollutants.
An individual fuel cell operates at a dc voltage of about 1 V. Cells must therefore
be connected in stacks of series and parallel connection in order to deliver the
voltage and power required for many applications.
Another complication is that many fuel cells cannot operate effectively using a
raw hydrocarbon fuel; most types require a reformer, which converts the hydrocarbon
into a hydrogen-rich gas suitable for passing directly into the fuel cell.
The two main types of fuel cell at present being considered are the SoIid Polymer
Fuel Cell (SPFC) and the Solid Oxide Fuel Cell (SOFC).These two main classes are
distinguishedby the type of electrolyte they use. This in turn determines their operating
temperature.
Batteries and fuel cells
306
Battery construction (under 100 Ah)
Negative plate
Container
Battery construction (over 200 Ah)
legative
Sealing Vent
)late Separator compound plug
Cover Terminal
Fig. 12.14 Examples of sealed lead acid batteries (courtesy of Yuasa)
Positive
plate
Newnes Electrical Engineer’s Handbook
307
12.4.1 The Solid Polymer Fuel Cell (SPFC)
An example of an SPFC is shown in Figs 12.15 and 12.16.
The SPFC operates at temperatures below 100°C. It is a potentially clean method
of generating electricity which is silent, robust and efficient. Applications for the
Fuel cell structure
Fig. 12.15 The solid polymer fuel cell (courtesy of ETSU)
Fig. 12.16 The solid polymer fuel cell (courtesy of Loughborough University)
Batteries and fuel cells
308
SPFC cover the domestic, commercial and industrial range, but it will probably be
best suited to small-scale Combined Heat and Power (CHP) applications, and to
transport duties.
The SPFC is often described by reference to power density, which ranges from
0.25-1.0 kW/litre. Expected life is in the range 8-20 years.
12.4.2 The Solid Oxide Fuel Cell (SOFC)
The SOFC is illustrated in Fig. 12.17.
Fig. 12.17 The solid oxide fuel cell (courtesy of ETSU)
The SOFC operates at much higher temperatures than the SPFC, in the region
850°C to 1OOO"C. It offers the possibility of high electrical efficiency together with
high-grade exhaust heat; applications for the SOFC are therefore in the combined
heat and power field and in other generation applications where a significant heat
load is present.
Advantages of the SOFC are a high electrical efficiency, and the facility to produce
steam or hot water from the high temperatures that are available. Typical power
ratings are in the range 150-250 kW and life expectancy is 5-20 years, with adequate
maintenance.
12.5
Battery charging
12.5.1
Small commercial batteries (up to 10 Ah)
The rechargeable batteries used in portable equipment are mainly nickel cadmium or
nickel metal hydride. Recharging of these batteries can be carried out by the following
methods:
transformer-rectifier
switched-mode power supply unit
capacitive chargers
Newnes Electrical Engineer’s Handbook
309
The transformer-rectifier circuit is used to reduce ac mains voltage to a lower dc
voltage. The charge delivered to the battery can be regulated using a constant current,
a constant voltage, constant temperature or a combination of these. The transformer
in this system provides the inherent advantage of isolation between the mains and
low-voltage circuits.
In the switched-mode power supply unit, the mains voltage is rectified, switched
at a high frequency (between 20 kHz and 1 mHz), converted to low voltage through
a high-frequency transformer, rectified back to dc which is then regulated to charge
the battery in a controlled manner. Although more complex than the transformerrectifier circuit, this technique results in a smaller and more efficient charging unit.
In a capacitive charger the mains is rectified and then series coupled with a mainsvoltage capacitor with cumnt regulation directly to the battery pack. This makes for
a small and cheap charging unit, but it has to be treated with care, since the capacitor
is not isolated from the mains.
In addition to these three methods, several manufacturers have responded to the
need for shorter recharge periods and better battery condition monitoring by designing
specific integrated circuits that provide both the charging control and the monitoring.
12.5.2
Automotive batteries
The main types of requirement for recharging automotive batteries are:
single battery, out of the vehicle
multiple batteries, out of the vehicle
starter charging, battery in the vehicle
Transformer/reactance is predominantly used in each of these applications, with
either a ballast in the form of resistance, or resistance to control the charging current.
More sophisticated chargers have constant voltage and current-limiting facilities
to suit the charging of ‘maintenance-free’ batteries. A typical circuit is shown in
Fig. 12.18.
Vented automotive batteries are usually delivered to an agent in the dry condition.
They have to be filled and charged by the agent. A multiple set of batteries connected
in series is usually charged for a preset time in this case, and this requires the use of
a bench-type charger. Bench chargers are rated from 2 V to 72 V, with a current
capability ranging from 10-20 A dc (mean). Examples are illustrated in Fig. 12.19.
The bench-type charger may also be used for charging a single battery out of a
vehicle, or this battery may alternativelybe left on charge indefinitely using a constantvoltage, current-limited charger.
Starter charging is used to start a vehicle which has a discharged battery. A large
current is delivered for a short period, and starter chargers usually have a short-term
rating. They can be rated at 6, 12 or 24 V, delivering a short-circuit rated starting
current from 150 A to 500 A dc (rms) and a steady-state output of 10 A to 100 A dc.
Output voltage is from 1.8 V to 3.0 V per cell. Voltage ripple may be typically up
to 47 per cent with a simple single-phase transformer-rectifier, but the ripple will
depend upon output voltage, battery capacity and the state of discharge. Voltage
regulation is important in starter charging applications since high-voltage excursions
can damage sensitive electrical equipment in the vehicle.
Since these chargers are usually short-time rated, some derating may be required
if they have to operate in a high ambient temperature.
&
31 1
Newnes Electrical Engineer’s Handbook
lic
Fig. 12.19 Examples of bench-type battery chargers (courtesy of Deakin Davenset Rectifiers)
j- [-----pq
Basic charger circuit
input
Mains
Transformer
Rectifier
-
Battery
Transformer to match battery voltage and provide isolation
Rectifier converts acto dc
Ballast resistor to limit current
I=
Vrect
- Vbat
Ballast
Fig. 12.20 Modified constant potential, or taper charger
Safety features are built into automotive battery chargers in order to avoid the risk
of damage to the battery, to vehicle wiring or to the operator. The normal safety
features include:
0
reverse polarity protection
no battery - short-circuit protection
thermal trip - abuse protection
12.5.3
Motive power
Motive power or traction battery chargers are used in applications where the batteries
Batteries and fuel cells
312
provide the main propulsion for the vehicle. These applications include fork lift
trucks, milk floats, electrical guided vehicles, wheelchairs and golf trolleys. The
requirement is to recharge batteries which have been discharged to varying degrees,
within a short period (7-14 hours). Both the battery and the charger may be subject
to wide temperature variations. A well-designed charger will be simple to operate,
will automatically compensate for fluctuations in mains voltage and for differences
between batteries arising from such factors as manufacture,age and temperature, and
will even tolerate connection to abused batteries which may have some cells shortcircuited.
The most common type of charger is the modified constant potential or taper
charger shown in Fig. 12.18. In all but the smallest chargers, the ballast resistor
shown in this circuit is replaced by a reactance; this reactance may be in the form of
a choke connected in series with the primary or secondarywindings of the transformer,
or more usually it is built into the transformer as leakage reactance.
While the battery is on charge, the voltage rises steadily from 2.1 V per cell to
2.35 V per cell, at which point the battery is approximately 80 per cent charged and
gassing begins. Gassing is the result of breakdown and dissipation of the water
content in the electrolyte. Charging beyond this point is accompanied by a sharp rise
in voltage, and when the battery is fully charged the voltage settles to a constant
voltage, the value of which depends upon a number of factors, including battery
construction, age and temperature.
During the gassing phase (above 2.35 V per cell) the charging current must be
limited in order to prevent excessive overheating and loss of electrolyte. The purpose
of the ballast resistor in the charger circuit is to reduce the charging current as the
battery voltage rises, hence the name raper charger. By convention, the current
output from the charger is quoted at a voltage of 2.0 V per cell, and the proportion
of this current which is delivered at 2.6 V per cell is defined as the taper. A typical
current limit recommended by battery manufacturers is one-twelfth of the battery
capacity (defined as the 5-hour rate in ampere-hours) at the mean gassing voltage of
2.5 V per cell. A disadvantage of the high-reactancetaper charging system is that the
output may be very sensitive to changes in the input voltage; the charge termination
method and the required recharge time must therefore be taken into account in sizing
the charger.
Because of the inefficiencies of energy conversion, particularly due to the heating
and electrolysis during the gassing phase, the energy delivered by the motive power
charger during recharging is 12-15 per cent higher than the energy delivered by the
battery during discharge.
To recharge a battery fully in less than 14 hours a high rate of charge is necessary
and the termination of charging when the battery is fully charged must be controlled.
The two types of device for termination of charge are:
voltagetime termination
rate-of-charge termination
A voltage-time controller detects the point at which the battery voltage reaches
2.35 V per cell, and then allows a fixed ‘gassing’ time for further charging, which is
usually 4 hours. This method is not suitable for simple taper chargers with nominal
recharge times of less than 10 hours because of the variation in charge returned
during the time period as a result of mains supply fluctuations. If a short recharge
time is required with a voltage-time controller then a two-step taper charger is used,
Newnes Electrical Engineer’s Handbook
313
in which a higher charging current is used for the first part of the recharge cycle.
When the voltage reaches 2.35 V per cell, the timer is started as above, and the
current during the further charging period is reduced by introducing more ballast in
the circuit.
The rate-of-charge method of termination has predominated in large chargers
during the past decade, offering benefits to both the manufacturer and the user. In the
rate-of-charge system the battery voltage is continuously monitored by an electronic
circuit. When the battery voltage exceeds 2.35 V per cell, the rate of rise of the
battery voltage is calculated and charging is terminated when this rate of rise is zero,
that is when the battery voltage is constant. This method can be used with single-rate
taper chargers with recharge periods as short as 7 hours because of the higher precision
of termination. In order for a rate-of-charge termination system to operate satisfactorily,
there must be compensation for the effects of fluctuations in mains supply. A change
in mains voltage results in a proportional change in the secondary output voltage
from the charger transformer; if uncompensated this will cause a change in charging
current and therefore in battery voltage. For a 6 per cent change in mains voltage, the
charging current may change as much as 20 per cent and the battery voltage may
change by 3 per cent.
Many chargers have the facility for freshening or equalizing.
Freshening charge is supplied to the battery after the termination of normal charge
in order to compensate for the normal tendency of a battery to discharge itself. A
freshening charge may be a continuous low current, or trickle charging, or it may be
a burst of higher current applied at regular intervals.
Equalizing charge is supplied to the battery in addition to the normal charge to
ensure that those cells which have been more deeply discharged than others (due, for
instance, to tapping off a low-voltage supply) are restored to a fully charged state.
A controlled charger is a programmable power supply based on either thyristor
phase angle control or high-frequency switch mode techniques. The main part of the
recharge cycle is usually at constant current and the power taken by the charger is
therefore constant until the battery voltage reaches 2.35 V per cell. Many options are
available for the current-voltage profile during the gassing part of the recharging
cycle, but all of these profiles deliver a current which is lower than the first part of
the cycle.
Voltage dmp in the cable between the charger and the battery is important because
the charge control and termination circuitry relies upon accurate measurement of the
battery voltage. It is not normally practical to measure the voltage at the battery
terminals because the measuring leads would be either too costly or too susceptible
to damage, and it is common practice to sense the voltage at the output of the charger
and to make an allowance for the voltage drop in the cables. Alteration to the length
or cross-section of these cables will therefore cause errors, especially with lowvoltage batteries.
Motive power chargers are typically available from 6 V (three cells) to 160 V (80
cells) with mean dc output currents from 10 A to 200 A. A typical circuit is shown in
Fig. 12.21.
The dc output current is rated at 2.0 V per cell; the taper characteristic set by the
transformer reactance then results in 25 per cent output current at typically 2.65 V
per cell. Rating is not continuous, and derating to 80 per cent is typical to take into
account the taper characteristic. Consideration must be given to this if a multiple
shift working pattern is to be adopted. Typical charger units, together with their
ratings. are shown in Fig. 12.22.
5
Newnes Electrical Engineer’s Handbook
315
Motive power chargers
Charger selection table (based on 80% discharge at 2.0V per cell)
Charger
rating
(amps)
15
20
25
30
35
40
45
50
60
70
80
90
100
110
120
1 30
140
1 60
180
200
Recharge times (hours)
7.5
84
112
139
167
195
222
250
278
334
390
445
501
556
617
668
723
778
890
1001
1112
8
88
118
147
176
205
235
264
293
352
410
469
527
586
644
703
761
820
937
1054
1171
9
10
11
12
13
96
128
159
191
223
255
286
31 8
382
445
509
572
636
699
763
826
890
1017
1144
1271
103
138
172
206
240
275
309
343
412
480
549
617
686
754
821
891
960
1097
1234
1371
112
149
186
223
260
297
334
371
445
519
593
668
742
81 6
890
964
1038
1186
1334
1483
120
160
200
240
280
320
360
400
480
560
640
720
800
880
960
1040
1120
1280
1440
1600
130
173
21 6
259
302
345
389
431
517
603
690
776
862
948
1034
1120
1206
1378
1550
1723
Fig. 12.22 Examples of motive power chargers, with their ratings (courtesy of Deakin Davenset Rectifiers)
Batteries and fuel cells
316
Voltage ripple is typically 15-25 per cent for single-phase chargers and 5-15 per
cent for three-phase chargers. The precise level of ripple will vary with time. It will
depend upon mains voltage, battery capacity and the depth of discharge of the battery.
12.5.4
Standby power applications
Typical standby power applications include emergency lighting, switch tripping,
switch closing and telecommunications. The main functional requirements for the
battery charger in these cases are:
to ensure that the state of charge of the battery is maintained at an adequate
level, without reducing battery life or necessitating undue maintenance
to ensure that the output voltage and current of the complete system are
compatible with the connected electrical load
to ensure after a discharge that the battery is sufficiently recharged within a
specified time to perform the required discharge duty
to provide adequate condition monitoring, to the appropriateinterface standards
Assuming initially that the battery is fully charged. the simple option is to do nothing.
A charged battery will discharge if left disconnected from a load and from charging
equipment, but if the battery is kept clean and dry this discharge will be quite slow.
For some applications, open-circuit storage is therefore acceptable.
For most applications,however, there is a need for battery charge to be maintained.
The current-voltage characteristic is not linear, and a small increase in charging
voltage will result in a large increase in current. Nevertheless, it is always possible
to define a voltage which, when applied to a standby power battery, will maintain
charge without excessive current, and the charging current flowing into the battery
has only to replace the open-circuit losses in the battery, which are usually small.
Once these open-circuit losses have been made up, any additional current flowing
is unnecessary for charging purposes and is normally undesirable. In vented cells it
causes overheating and gassing and eventually, if not checked, damage and loss of
capacity of the cell. In sealed cells there can be overheating,in extreme cases expulsion
of gases through the pressure vent, and ultimately a loss of capacity.
On the other hand, if the battery voltage is allowed to fall too much, the opencircuit losses will not be replaced and the battery will slowly discharge.
The charging voltage has therefore to be controlled carefully for best battery
maintenance. The usual limits are within +1 per cent of the ideal voltage, which is
normally termed the float voltage. The float voltage has a negative temperature
coefficient which must be accounted for when batteries are to operate in exceptionally
hot or cold environments. Float voltages for the major types of standby power cell
are shown in Table 12.2.
For vented cells there are, however, circumstances under which the float voltage
should be exceeded. Batteries which are new or have suffered abuse will benefit
from a vigorous gassing up to the boost voltage shown in Table 12.2.Batteries which
have stood on float charge with no discharge-charge cycle for many months will
benefit from a refresh charge with gassing, at the refresh voltages which are also
shown in Table 12.2.
Few dc standby power systems can be designed without taking account of limits
on the load voltage. For good battery operation it is necessary to charge at the float
voltage (or sometimes a higher level), but it is also necessary to discharge the battery
Newnes Electrical Engineer’s Handbook
317
Table 12.2 Typical cell voltages for systems with limited load voltage excursions
Cell type
Cell voltage (V)
Vented nickel cadmium
Vented lead acid
Seaied lead acid
End of dlscharge
Float
Refresh
Boost
1.1
1.8
1.8
1.45
2.25
2.27
1.55
2.45
1.7
2.7
NIA
NIA
to a sufficiently low voltage if the full capacity is to be released from the cells. These
considerations impose fundamental limits which define the maximum excursion of
the system output voltage. Table 12.3 shows the minimum and maximum voltages
which are reasonable in a 50 V standby power system for each for the three major
battery types. Systems with other voltages will require excursions which are in direct
proportion.
Table 12.3 Load voltage ranges in a 50 V standby power system
Function Minimum Maximum
volffcell volffcell
(VI
(VI
50 V system
No. of
Minimum
cells
voltage (V)
Maximum
voltage (V)
rt: voltage
excursion (%)
Vented lead acid cells
Float
1.80
2.25
Refresh 1.80
2.45
Boost
I .ao
2.70
25
24
22
45.0
43.2
39.6
56.3
58.8
59.4
11.3
15.6
19.8
Vented nickel cadmium cells
Float
1.10
1.45
Refresh 1.10
1.55
39
38
42.9
41.8
56.6
58.9
13.7
17.1
Sealed lead acid
Float
1 .ao
25
45.0
56.8
11.8
2.27
It can be seen from Table 12.3 that, allowing for the discrete steps in voltage
when changing the number of cells, it is not possible to achieve a voltage excursion
of less than about k 12 per cent under conditions of float alone; the excursion limits
are larger in refresh operation, and still larger with boost.
Table 12.3 shows that the sealed lead acid cell offers minimum overall voltage
variation because of the absence of the larger excursions due to refresh and boost
charges. If it is not possible to use a sealed lead acid system, another alternative is to
disconnect the load for the full boosting operation; this should normally be necessary
only at the time of system installation in any case.
A voltage regulator should be included in the system if closer limits of voltage
variation are required. Diode regulators are now reliable and widely used; they
operate by switching banks of series-connected diodes in and out as the battery
voltage slowly vanes. It is important to ensure that switching in the regulator occurs
only as a result of changes in battery voltage, and not as a result of load changes; if
the regulator responds to changes which occur as a result of a load change, excursions
outside the specified voltage limits m a y occur because of the delays in the process of
switching the diode bank. Ensuring that this distinction is made normally requires a
computer simulation for all but the simplest regulators. Control of the switching of
the diode bank is best achieved by a programmable controller, especially if additional
complicated relay-type logic is required in the system. Diode regulators are large and
Batteries and fuel cells
318
their heat dissipationis substantial.Higher-efficiencyregulatorsusing actively switched
devices are becoming available, but they are at present limited to relatively low
power applications.
Condition monitoring is now included in most dc systems in order to warn of
excessive battery voltage excursions. The applications are diverse, but the main
features are:
high voltage detection: this is necessary in order to prevent a fault on the
supply system from damaging the battery or load circuit
low voltage detection: this warns of load failure due to insufficient voltage,
and it is also needed to trigger the disconnectionof sealed batteries which may
be damaged by excessive discharging
charge failrire: this is needed in order to stimulate action to restore the ac
supply or to prepare for disconnection of the load
earth leakage: this is needed where an unearthed load system is used, for
safety and for the avoidance of ‘double faults’
Communication of a fault signal is through a ‘volt-free’ contact on a relay, which
usually signals to the monitoring centre using a 110 V or other voltage supply.
For charging ofthe battery, it has been seen in Table 12.3 that there are restrictions
on the choice of charging voltage. The preference for many applications is to limit
the voltage to the float voltage, and while it may be necessary to incEase the charging
voltage to speed up the recharging, the voltage should be returned to the float voltage
as soon as possible. At this float voltage level, all the cell types discussed will be
recharged to about 80 per cent of their nominal capacity. Assuming the charger
current is at the adequate level shown in Table 12.4, recharge to 80 per cent capacity
will be achieved in the times indicated in the table.
Table 12.4 Charging time and current necessary to recharge to 80 per cent of capacity using
float voltage
Cell type
Charging current
Charging time
Vented lead acid
Vented nickel cadmium
Sealed lead acid
7% of capacity
2Ooh of capacity
10% of capacity
14 hours
6-8 hours {depending on type)
9 hours
To recover the remaining 20 per cent of the charge is more difficult, and different
techniques are necessary for the three cell types. A vented lead acid battery will be
fully recharged at float voltage in about 72 hours, but if the charging voltage is
boosted to 2.7 V per cell, a full recharge will take about 14 hours. For a vented nickel
cadmium cell full charge will never be achieved without increasing the voltage
above the float level. Exact times vary between cell types, but typically a refresh
charge at 1.55 V per cell will give full charge in about 200 hours on the highestperformance cells, and boosting to 1.7 V per cell will reduce this time to 9-10 hours.
Sealed lead acid cells will reach full charge after about 72 hours at float voltage; an
increase of charge voltage to 2.4 V per cell will reduce the charging time to about 48
hours, but there is no way in which rhis can be significantly reduced further.
If a fast recharge is essential, an alternative is to oversize the battery; for instance
if 100Ah capacity is required with a full recharge within 8 hours, then a battery with
125 Ah capacity could be installed.
Newnes Electrical Engineer’s Handbook
319
Most cells will protect themselves from excess charging current, provided that the
voltage is limited to the float voltage, but above this level excessive charging current
can damage the cell. For vented lead acid cells the 7 per cent of capacity shown in
Table 12.4 is recommended as an upper limit. For sealed lead acid cells, the
recommended upper limit is 50 per cent of capacity. Nickel cadmium cells normally
require a minimum charge current of 20 per cent of capacity, as indicated in Table
12.4, but a lower limit of 10 per cent of capacity is recommended when boosting in
order to avoid excessive gassing and electrolyte spray.
Standby power chargers are available in a wide range of capacities to suit many
applications. Typical dc outputs are 6, 12, 24, 30,48,60, 110,220 and 440 V. with
dc mean output current ranging from 1A to 1OOOA. DC output current is rated at 100
per cent of the output current at the full specified voltage. Regulation of the output
is generally within f l per cent for an input voltage change of f10 per cent and a load
current change of 0-100 per cent. Typical circuits are shown in Figs 12.23 and 12.24.
Although standby power systems are continuously rated, some derating may be
necessary for operation in tropical climates if this was not originally specified.
Differing levels of output smoothing can be incorporated into the charging system,
depending upon the application. General applications require a maximum of 5 per cent
ripple, but for telecommunications supplies, specifications are based on CCIlT
telecommunication smoothing, which requires 2 mV phospometrically weighted at
800 Hz. The key components of an installation are shown schematically in Fig. 12.25.
12.6 Battery monitoring
The monitoring of battery condition is becoming more important as remote operation
and reduced maintenance requirements are increasingly specified in standby power
systems. The options which are available are summarized in the following sections.
12.6.1
Load-discharge testing
The basis of this test is to discharge the battery into a selected load for a preset time,
after which the battery voltage is checked. In the majority of cases this will require
that the battery is taken off line.
The load-discharge test is very reliable, but it requires a special resistance load
bank, and it is labour intensive and time consuming. Figure 12.26 shows an example
of the type of equipment that is required.
12.6.2
dVldt-load testing
The technique here is to connect a fixed known load to the battery for a short period
and to record the battery voltage over this period.
By comparing the recorded load voltage with data from the battery manufacturer,
estimations can be made regarding the condition of the battery and its ability to perform.
12.6.3
Computer monitoring of cell voltages
In this case. measurement leads are taken from each cell to a central monitoiing
computer.
The voltage of each cell is recorded through charge and discharge of the battery.
Although cell voltage is not a direct indication of residual capacity, trends can be
observed and weak cells can be identified.
__
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322
Batteries and fuel cells
The rectifiedbattery
charger
/
The rectifierkharger converts
incoming ac mains into dc to provide
a stable, constant voltage output with
automatic current limiting. The
rectifier-charger automatically floats
and boost charges the battery
simultaneously providing the required
dc output to the load.
\r Control and distribution
Distribution panels typically provide
several fused outlets, monitoring
of busbar voltage and current,
earth fault status together with busbar
and interbusbar controls. Depending
upon application, the control and
distribution equipment may be
contained within the charger cubicle
(as shown) or may be housed in a
separate matching enclosure.
The battery
The battery is a key element in a secure power supply and must be selected with care. The
type of battery selected will depend on many factors including: reliability, operating temperature, cost, life, standby time required, maintenance parameters, ventilation and available
space. Depending on the type of battery, it can be supplied on a stand or in a ventilated
cabinet. The battery should always be located as close as possible to the dc power supply to
minimize line voltage drop.
The main types of battery used are:
valve regulated lead acid; minimal gassing so can be used in electronics enclosure,
maintenance free, relative installed cost medium, typical life 10 yrs
flat plate lead acid: gas, with adequate ventilation and periodic maintenance required.
Relative installed cost low, typical life 10-15 yrs.
Plante lead acid: gas, with adequate ventilation and periodic maintenance required.
Highest levels of reliablility. Relative installed cost high, typical life 20-25 yrs.
nickel cadmium: gas, with adequate ventilation and periodic maintenance required. Ideal
for arduous conditions. Relative installed cost very high, typical life 20-25 yrs.
Fig. 12.25 Key elements of a standby power battery unit
An example of this type of system is illustrated in Fig. 12.27, which includes a
typical screen display.
12.6.4
Conductance monitoring
This method has increased in popularity in recent years and several manufacturers
now offer a standard monitoring product based upon the technique. Figure 12.28
shows typical equipment.
Monitoring the conductance of individual cells over their working life can give an
indication of impending failure, allowing preventive maintenance to be carried out.
A particular advantage is that the readings can be reliably used whatever the state of
charge of the battery.
Newnes Electrical Engineer’s Handbook
Fig. 12.26 Equipment for load-discharge testing (courtesy of Deakin Davenset Rectifiers)
Fig. 12.27 Computer monitoring of cell voltages (courtesy of Invensys)
323
Batteries and fuel cells
324
Fig. 12.28 Conductance monitoring equipment
12.7
Installation, testing and commissioning
All battery equipment should be installed according to manufacturers’ recommendations.
In particular, larger lead acid or nickel cadmium installations should be sited in a
correctly constructed room, vehicle, cubicle or rack which allows ready access for
maintenance and ensures adequate ventilation of batteries and battery chargers. The
battery should not be located close to a source of heat such as a transformer or heater.
Battery racks should be assembled to manufacturers’ instructions, adhering to
recommendations regarding spacing between cells, and drip trays and insulators
should be fitted where applicable. All cell connections should be cleaned, coated
with no-oxide grease and tightened to the suppliers’ specification. A typical installation
is shown in Fig. 12.29.
The rating of the ac mains input cable to the battery charger should be based on
the worst-case condition, which is the peak current that will be drawn by the charger
at the lowest mains voltage. The rating of the dc cables from the battery charger and
in the battery should be no lower than the maximum protection fuse rating, and
attention should be paid to the overall voltage drop in the system, especially on lower
voltage dc systems. The cables should be adequately protected, and segregation may
be necessary on systems with a large dc current. All equipment should be earthed
with a conductor of the correct cross-sectional area.
Newnes Electrical Engineer’s Handbook
325
Fig. 12.29 Typical battery installation (courtesy of Invensys)
Battery systems should be commissioned in accordance with procedures
recommended by the manufacturer. It may be necessary to charge batteries for several
days before their full capacity is reached, and in some cases a special initial
commissioning charge level may be required.
Tests should be set up so that the results enable the operation of the system to be
compared with the original specification. For larger systems, the manufacturer should
propose a set of witness tests to be performed at the works, followed by final tests to
prove capacity and duty on site. A typical test schedule should be designed to establish:
the ability of the battery charger to support the standing load, where applicable
the ability of the battery charger to recharge a fully discharged battery in the
required time, whilst simultaneously time supporting the standing load, again
where applicable
the ability of the battery charger to maintain regulation within specified limits
throughout the required range of load and input conditions
correct operation of the monitoring features and instrumentation
the ability of the battery and charger system to supply the load for the required
period during a power failure
12.8 Operation and maintenance
12.8.1 Primary cells
Primary cells cannot be recharged and once their energy is depleted they should be
Batteries and fuel cells
326
disposed of in the manner recommended by the manufacturer. The cells should be
examined occasionally during their life for signs of leakage or physical deformation.
Chemical hazards can arise if batteries are misused or abused and in extreme cases
if there is a risk of fire or explosion.
12.8.2
Secondary cells
Small secondary cells should be examined periodically under both charging and
discharging conditions. Their cycle life is not indefinite, and as the battery ages its
ability to accept recharge and hold its capacity will decrease. This ageing is usually
shown by the dissipation of heat during charging, as a result of the inability to accept
charge, whilst on discharge a reduction of capacity will be observed. Excessive
overcharging will cause leakage of electrolyte or deformation of the cell.
Cells should be handled carefully and according to manufacturers’ instructions,
with the operator wearing the correct protective clothing.When handling vented cells,
suppliesof salinesolution(for eye washing)and clean water shouldbe readily available.
If a cell is to be filled, only purified water should be used when mixing the electrolyte.
The maximum storage time for cells which are filled and nominally charged is 6
months for nickel cadmium, 12 months for valve-regulated lead acid batteries (at
ZOOC) and 8 weeks for vented lead acid batteries. Cells which are stored for longer
than this should be periodically charged and the electrolyte level should be checked,
where applicable.
Whether filled or empty, the battery may contain explosive gases, and no smoking,
sparks or flames should be permitted in the vicinity of the installation.
Although the voltage at any point in a battery system can be reduced by the
removal of inter-cell connectors, the cells are electrically live at all times and cannot
be de-energized or isolated in the conventional sense. When connecting cells together,
insulated tools should be used wherever possible to avoid accidental short-circuits
and sparks. Tools should be cleaned before use if they are to be used on both lead
acid and nickel cadmium batteries, since acid will destroy a nickel cadmium battery.
On large installations it is good practice to operate a system of ‘cell log sheets’.
Such sheets or books are normally supplied by the cell manufacturer. Completion of
these sheets will require the measurement of voltage, ambient temperature and, if
applicable, the specific gravity of each cell at regular intervals. Cell electrolyte level
(where applicable) and the tightness of cell connections should also be checked at
these intervals.
Other regular operational and maintenance checks on the charger system should
include:
0
ac mains input voltage is correct and within tolerance
dc charger output voltage and current are correct and within tolerance, after
compensation for ambient temperature
fuses are intact
protection devices operate correctly
all connections are tight
no components are operating at excessive temperature
Faults to be watched for are:
loose connections in the cells or in the charger system
Newnes Electrical Engineer’s Handbook
0
e
0
e
0
0
0
0
327
low or high electrolyte level (where applicable)
debris or electrolyte spillage on the top of cells which may lead to shortcircuits
excessive loss of electrolyte (where applicable) due to overcharging or battery
ageing
cells overheating because of their inability to accept charge
loss of capacity due to undercharging (specific gravity should be checked
where applicable)
loss of charger regulation due to a control circuit fault
ac or dc fuses operated
excessive loading on the battery or on the charger
12.9 Standards
There are many standards covering various types of battery and battery charging
system. The key IEC recommendationstogether with equivalent BS and EN standards
and related North American standards are summarized in Table 12.5.
Table 12.5 Comparisonof international, regional and national standards for batteries and battery chargers
IEC
EN
BS
N. American
Subject
Specification for standard cell
General specification for lead acid cells
Plante cells
IEEE 4501484
Pasted cells
IEEE 4501484
Valve-regulated sealed cells
IEEE 1188
Portable valve regulated lead acid cells
Sulphuric acid for use in lead acid batteries
Specification of water for lead acid batteries
Specification for lead acid batteries in light
vehicles
254
60254
2550
Specification for lead acid traction batteries
Code of practice for safe operation of
6287
traction cells
95
60095
3911
Lead acid starter batteries
Code of practice for safe operation of lead
6133
acid stationary batteries
623
60623
6260
Nickel cadmium single cells
IEEE 1106
Nickel cadmium cylindrical cells
285
60285
5932
622
60622
6115
Nickel cadmium single cells (prismatic)
ANSI C 18.2 M
6132
Code of practice for safe operation of
alkaline secondary cells and batteries
1044
61044
EN61044 Operation charging of lead acid traction
batteries
DIN 41774
Traction battery chargers - taper
characteristics
DIN 41773
Traction battery chargers characteristics
952
60952
EN60952 Aircraft batteries
335-2-29 60335-2-29 3456-2-29 Domestic battery chargers
UL 1564
ANSI 1564
Specification for converters
UL 458
146
60146
4417
UL 1012
UL 1236
UL 1310
950
60950
7002
Information technology equipment
UL 1950
428
896
896
896
896
1056
60896-1
60896-2
60896-3
60896-4
61056-1
5142
6290-1
6290-2
6290-3
6290-4
6745
3031
4974
7483
-
328
Batteries and fuel cells
References
12A. May, G.J., Journal of Power Sources,42. 1993,pp. 147-153.
12B. May. G.J., Journal of Power Sources,53, 1995,pp. 111-117.
12C. Rechargeable Batteries Handbook, Butternorth-Hehemann, Oxford. UK, 1992.
Chapter 13
Electroheat
Dr Peter L. Jones
Pealjay Consultants
Mr I. Harvey
13.1 Introduction
Much of industry depends on heat processes to refine, shape, modify, dry, bake or
carry out any one of a number of operations. Traditionally, much of this heat has been
provided by fossil fuel; in earlier days this was coal and its derivatives and more
recently it has been oil and gas. Electricity was used to provide drives for machinery
and auxiliaries such as blowers, or for very specialized applications. The large-scale
use of electricity was limited to such processes as electrochemical extraction of the
chemicals originating from salt, or the extraction of aluminium from bauxite. In part,
the explanation for the limited use of electroheat has been an economic one, and in
some areas this continues to be the case. At the point of delivery, electricity in most
countries is much more expensive than fossil fuels. This is inevitable because most
electricity has been generated using fossil fuel, with a conversion efficiency that
cannot be better than that of the Carnot cycle. Furthermore, the equipment necessary
for the generation and distribution of electricity requires very significant capital
investment. As a result, the case for the use of electricity in process heating needs to
take into account other aspects of manufacturing costs and process benefits, such as
improvements in product quality, reduced manning levels and increased throughput
for a given capital investment.
Nevertheless. electroheat technologies are now widely established in processing
and manufacturing industries ranging from metals, glass and ceramics to textiles,
paper, food and drink. The heating techniques use frequencies which cover a significant
part of the electromagnetic spectrum, from dc to terahertz, and at powers which
range from a few watts to many megawatts. The different processes which have been
developed, and the frequency and power ranges which they use, are summarized in
Table 13.1.
The relative importance of the different techniques has changed as industry moves
away from heavy engineering operations. In particular, some electroheat processes
related to the iron and steel industry have decreased in relative importance whilst
others, aimed at materials with higher added value, are becomkg more widely accepted.
It is also apparent that even in the heavy industries such as metals and ceramics,
certain traditional and novel electroheat techniques are of great importance since
they bring with them better process control, leading to higher product quality and
330
Electroheat
Table 13.1 Electric heating processes
Technique
Frequency range
Power range
Direct resistance
Indirect resistance
Arc melting
Induction heating
Dielectric heating
Microwave heating
Plasma torch
Laser C02
Ultraviolet
0-50 Hz
50 Hz
50 Hz
50 Hz-I MHz
5-100 MHz
02-25 GHz
4 MHz
30 THz
750-1 500 THz
e1 kW-30 MW
0.5 kW-1 MW
1-100 MW
2 kW-30 MW
1 kW-5 MW
1-100 kW
1 kW-1 MW
0.140 kW
1 kW
reduced manufacturing costs. Further details of most of these processes can be found
in reference 13A.
It is possible to categorize electroheat techniques in a variety of ways. The following
are examples:
0
by the part of the frequency spectrum in which they operate, for example
infrared, microwave or ultraviolet
by the material being processed, such as metal melting or metal heat treatment
by the generic name of the process, such as induction heating, dielectric heating
or laser
No single categorization is generally accepted and a mixture of these is adopted in
the structure of this chapter. Metal melting by various methods is dealt with in
section 13.2. The route to a final component may also involve heat treatment, either
for size reduction, to modify properties or to work into final form. A number of
electroheat techniques are used and these are grouped under direct resistance heating,
in section 13.3, and indirect resistance heating, in section 13.4. The later sections
deal with more recently accepted technologies.
13.2 Metal melting
Electroheat is used in a number of ways in the metals industry, from the initial
refining through to the formation into finished shape. In the case of refining, perhaps
the greatest use is for the extraction of aluminium from bauxite. Because this is a
very energy-intensive process, it is normally carried out in places where cheap
hydroelectric power is available, such as Canada. There are relatively few installations
in Europe.
Metal melting may take place for secondary refining, for recycling of scrap or for
the production of castings. It is normally considered separately from metal heating
for hot working, because the implementation of the methods for liquid metals is
usually quite different.
The electroheat furnaces which are established for the melting of metals include
arc, electron beam and coreless induction furnaces. The choice is usually based on the
demand schedule for liquid metal which is required by the casting process. The
requirements of a small foundry producing a few specialist castings each day are quite
different from those of a large automated foundry. The seven main types of furnaces
commonly used in the melting of metals are described in the following sections.
Newnes Electrical Engineer’s Handbook
331
13.2.1 The arc furnace
This is mainly used for the production of liquid steel by the melting of scrap. It
consists of a squat, refractory-lined cylindrical vessel with a movable domed refractory
roof, as shown in Fig. 13.1. The roof is swung open to allow the furnace to be
charged with scrap metal, and is closed prior to the start of the melting process. The
three graphite electrodes pass through holes in the roof, and each is supplied through
flexible cables and bus tubes from one of the three phases of a transformer. The metal
charge forms the star point of the three-phase load circuit.
To three-phase ac
t t t
Door for charging
Fig. 13.1 Arc furnace
A variant of this conventional scrap melter is the vacuum arc furnace, which is
used for the refining of metals. A partial vacuum is maintained in the vessel to
minimize oxidation during melting.
The substation for a large arc furnace is normally adjacent to the furnace itself,
and it contains the furnace transformer, which normally has a star-connected primary
and (in the UK) an input voltage of 33 kV.The transformer must withstand the very
large electromechanical forces which are produced by the high short-circuit currents.
It is oil cooled and the power is varied using on-load tapchangers. Further comments
on this class of transformer can be found in section 6.3.10.
As an electrical load, the arc furnace is less than ideal. The current can vary from
zero, when the arc is extinguished at current zero, to a short-circuit level when scrap
contacts the electrodes. The physical movement of the pieces of solid charge and of
the melt causes variation in the arc length which can fluctuate many times within a
second. In addition, the arc length varies due to mechanical vibration of the electrode
and its supporting structure. These effects combine to prevent a constant power level
being delivered by each of the phases, and this results in a number of problems which
Electroheat
332
affect the performance of the furnace, the local electricity supply and the acoustic
noise level.
13.2.2
The submerged arc process
This is not essentially an arc process, since heating occurs mainly by direct resistance
effects, with perhaps some limited heating from arcs and sparks during interruption
of the current path. The general arrangement is shown in Fig. 13.2.
/
Metal '
Fig. 13.2 Submerged arc furnace
The principal application is in the reduction of ferroalloys such as ferromanganese,
nickel, chrome, silicon, tungsten and molybdenum, which have a high melting point.
These metals are subsequently remelted in an arc furnace to produce special alloys.
Fused oxides may also be produced by the submerged arc process.
13.2.3 Electroslag refining
This is an alternative to vacuum arc processes for materials which are not unduly
reactive in air. A high degree of refining can be obtained, since the droplets of molten
metal penetrate the molten slags, enabling desuphurization to be carried out and
oxide inclusions to be removed. As in the vacuum arc furnace, a molten pool is
formed on the solidifying ingot, and there are similar advantages.
The general arrangement of an electroslag refining furnace is shown in Fig. 13.3.
The current path vanes according to the construction of the furnace and varies with
the state of the melt. In general terms, current passes from the electrodes, through the
slag and from the slag to the baseplate, through the vessel walls or the ingot, or both.
13.2.4
The electron-beam furnace
Electron beams can be used for melting, as well as for welding and the production of
evaporated coatings. The beam is obtained from a heated filament or plate, and is
accelerated in an electron gun by a high electric field which is produced by one or
Newnes Electrical Engineer's Handbook
333
Water in
Fig. 13.3 Electroslag refining furnace
more annular electrodes. Electrons on the axis of the gun pass through the final
anode at very high speeds, typically 85 x lo6 m / s at 20 kV. The electron gun and
chamber are kept at a low pressure of around 0.001 Pa. Since little energy is lost from
scatter or from the production of secondary electrons, practically all of the kinetic
energy of the beam is converted into heat at the workpiece. The conversion from
electrical energy input to thermal energy in the workpiece is therefore very high.
The electron-beam furnace is shown diagrammatically in Fig. 13.4. It utilizes a
cooled ingot mould in the same way as the vacuum and electroslag furnaces.
13.2.5 The coreless induction furnace
This consists of a refractory crucible encircled by a solenoid coil which is excited
from a single-phase ac supply. A diagrammatic view is shown in Fig. 13.5.
The fluctuating axial magnetic field links the charge within the crucible and
causes Z2R heating in the charge. The power induced in the charge and the efficiency
of the furnace can be calculated according to the equations given later in section
13.6. using appropriate material property values. Depending upon the resistivity of
the material in the charge, the coreless furnace converts the electrical input power to
heat in the charge with an effciency of 65-80 per cent. This efficiency is largely
independent of frequency, but is improved by the high charge permeability found in
ferromagnetic materials such as steel scrap.
13.2.6
The channel induction furnace
During the 1970s the channel furnace became accepted as a melting unit for cast
iron, and largely replaced the coke-fired cupola. Its acceptance was due mainly to
334
Electroheat
Electrode
Electron beam guns
Molten zone
Water-cooled
ingot mould
Fig. 13.4 Electron-beam melting furnace
spout
Lid
5-,
Refractory
Furnace
coil
-
n
Fig. 13.5 Diagrammatic view of a coreless induction furnace
improved refractory technology, which allowed useful levels of power to be applied.
Until that time the furnace had mainly found application in iron foundries as a
holding unit, in which previously melted liquid metal is maintained at a temperature
suitable for pouring into the casting moulds.
Newnes Electrical Engineer's Handbook
335
In the channel induction furnace, a mains-frequency solenoid coil encircles one
leg of a lamination pack, and the resultant alternating magnetic field induces joule
heating in a loop of molten metal surrounding the coil, this metal loop acting as a
single-turn secondary of the transformer. The arrangement is illustrated in Fig. 13.6.
......................
. . . . . . . .. ..
..... ....
..........
Fig. 13.6 Diagrammatic view of a channel induction furnace
The close coupling between the induction coil and the metal loop results in a
higher power factor than is achieved with the coreless induction coil, and the efficiency
of converting electrical power into heat is therefore also higher, being typically
above 95 per cent. The molten metal loop must be continuous, and it is not allowed
to solidify while energized in case the continuity is broken during contraction.
The use of channel furnaces for aluminium melting has been constrained by
problems of oxide build-up in the metal loop. This necessitates special designs to
allow easy cleaning. For bulk melting applications, European furnace companies and
researchers have in recent years produced high-power designs of up to 1.3 MW per
inductor, with freedom from loop blockage. This enables the high efficiency of this
type of furnace to minimize melting costs.
13.2.7 The resistance furnace
Electric radiant heating techniques for metal melting are most widely used in the zinc
and aluminium foundry sectors. There is much less use in the copper base alloy
market which requires higher melting temperatures. In general, resistance heating is
used where induction furnaces are not suited to the metal demand patterns. In the
case of aluminium the application of resistance heating ranges from crucible bale-out
furnaces of 25 kg capacity, where metal is scooped out of the container as required
for pouring into moulds, to well-insulated box-like receivers of over 10 tomes capacity.
336
Electroheat
Crucible furnace designs comprise a steel shell which contains insulating material,
heating elements and a carbon-based crucible which is located in the central chamber.
The general arrangement is shown in Fig. 13.7. The highest outputs are around
120 kW, which gives a maximum melting rate for aluminium of 230 kghour in a
600 kg capacity furnace. This can be compared with a coreless induction furnace of
similar capacity which, with its higher power density, can achieve melting rates
about three times greater.
Fig. 13.7 Diagrammatic view of a resistance crucible furnace
Crucible furnaces exist in both tilting and bale-out configurations, and great strides
have been made to improve performance figures in the light of competition between
manufacturers and from other fuels. As an example, a typical 180kg bale-out furnace
has a holding requirement of about 4 kW, with advantages of automatic start-up,
good working environment and close temperature control. Typically, the metal content
is about eight times the optimum melt rate achieved when these large structures have
reached thermal equilibrium.
An alternative high-efficiency heating mode for metals with a low melting point
is provided by immersion heating, using silicon carbide or wire elements in silicon
carbidebased tubes.
13.3 Heat treatment by direct resistance heating
This is used in the iron and steel industry for heating rods, wire and billets prior to
rolling, forging or other hot working. It is also used for annealing of ferrous and nonferrous materials. The process works through direct contact with the material, and a
schematic of the general arrangement is shown in Fig. 13.8.
Direct resistance heating is also used for melting glass, in electrode boilers for
water heating or steam raising, and in salt baths for the surface heat treatment of
metallic components. In these applications it m a y be used alone or in combination
with other fuels.
Newnes Electrical Engineer’s Handbook
337
E3
Power factor
correction
Transformer
,
13Stock
p...
mechanism
Lrl
Subsequent
processing
Fig. 13.8 Direct resistance heating
13.3.1
Metals
The resistivity of several common metals is shown in Table 13.2, and the variation
of some of these resistivities with temperature is illustrated in Fig. 13.9.
Because of its relatively high resistivity, steel can be heated efficiently in billets
of up to 200 mm2 section, provided that the length of the billet is several times
greater than its diameter. Heating time is from seconds to a few minutes, so heat
losses (for instance radiation from the surface and thermal conduction through the
contacts) are small. The efficiency of the process can be 90 per cent or better. The
Electroheat
338
Table 13.2 Electrical resistivity of typical metals
Metal
Electrical reslstivityat 20°C (p2 cm)
Copper
Aluminium
Nickel
Iron
Mild steel
Stainless steel
1.6
2.5
6.1
8.9
16.0
69.0
12
10
x 8
B
.-2
-m
CI
9
2 6
5
.c
.-
u)
u)
d
4
2
1
0
200
400
600
Temperature (“C)
800
1000
Fig. 13.9 Variation of resistivity with temperature for aluminium, copper, nickel and iron
workpiece resistance is low, and for these high efficiencies to be achieved the supply
resistance must be much lower.
For copper and other low-resistivity metals, the length-to-diameter ratio of the
billet should be considerably greater than 6 if the process is to be successful, and
consequently it is usually applied to the annealing of wire and strip.
In all cases the cross-sectional area of the current path must be uniform, otherwise
excessive heating will occur at the narrower sections, with the possibility of melting.
In billets of larger cross-section, a non-uniform current density can arise as a result
of skin effect. This leads to higher heating rates at the surface, but these are counteracted
by increased heat loss at the surface.
The direct resistance heating of bar and billets is a single-phase load that is
switched at frequent intervals. This results in transient voltage disturbances and in
voltage unbalance at the point of common coupling. The load can be phase balanced
by inductive and capacitive components, and in large units the switch-on disturbances
may be compensated by a soft-start arrangement.
Newnes Electrical Engineer's Handbook
339
13.3.2 Glass
At temperatures above llOO°C, glass has a low viscosity and a resistivity which is
low enough for direct resistance heating to be considered at acceptable voltages. The
variation of the resistivity of glass with temperature is shown in Fig. 13.10. In the
UK, electricity is used in mixed melting units where electrodes are typically added
to a fuel-fired furnace to increase the output for a relatively low additional capital
investment.
0.20
0.15
h
E
C
v
-
.- 0.1
.-v1
2
a
0.05
0
1000
1100
1200
Temperature ("C)
1300
1400
Fig. 13.10 Variation of resistivity with temperature of glasses
Current is passed between electrodes which are immersed in the molten glass. The
electrodes must withstand the high temperatures and forces which arise from the
movement of molten glass across their surface. It is also necessary to protect the
electrodes from exposure to the atmosphere. Molybdenum or tin oxide electrodes are
used in order to avoid contamination of the glass by pick-up of electrode material,
and the current densities are of the order of 1500Nm2. Single-, two- or three-phase
electrode systems may be employed. Scott-connected transformers (as described in
section 6.2.3) are used for the two-phase connection. The three-phase arrangement
produces electromagnetic forces in the glass which, together with the thermal forces,
lead to a significant movement in the melt; this improves quality and melt rate
provided it is not excessive. Three phase is also preferred for phase balance and low
cost.
Existing electric furnaces have usually followed a traditional design pattern with
a rectangular tank, but circular designs have now been adopted and it is claimed that
these give improved stirring in the melting zone.
The power input to the furnace, and hence the melting rate, is controlled by
varying the input voltage with tapped transformers or saturable reactors: an alternative
method is to change the effective surface area of the electrodes by raising them from
the melt.
13.3.3 Salt baths
These can be used for the heat treatment of metal components. The heated salt reacts
340
Electroheat
chemically with the surface layer of the workpiece to give the required surface
properties. At temperatures above about 8OO0C,direct resistance heating is the only
practical method.
Although the salt is a good conductor when molten, the bath must be started up
from cold by using an auxiliary starting electrode to draw a localized arc. The
electrodes have to withstand the corrosive effects of the salt, and are manufactured
from graphite or a corrosion-resistantsteel alloy. Currents of up to 3 kA at 30 V are
required, and both single-phase and three-phase units are available.
13.3.4
Foods and other fluids
In recent years the principles of direct resistance heating have been applied to a range
of other pumpable fluids, typically food and aggressive liquids such as zinc phosphate.
The equipment in these applications consists of a pipeline interspaced with four
electrodes which are each built into an insulated housing and connected to a threephase supply in delta. The general arrangement is shown in Fig. 13.11. The pipeline
itself is made from stainless steel and is insulated internally by a polymeric coating
to ensure that the current flows through the fluid itself.
General arrangement
of ohmic heater
t Out
Electrode
(earthed)
I
I
I
Electrodes
(live)
Plastic-lined
stainless steel
Pipe
Electrodes
(earthed)
-
t
Feed in
Fig. 13.11 Direct resistance heating for pumpable fluids (courtesy of EA Technology Ltd)
Newnes Electrical Engineer’s Handbook
341
In the food industry the technique is known as ‘ohmic heating’. Equipment rated
up to 100 kW and 3.3 kV has been used in this industry to heat up to 120°C liquids
and pastes which often contain solid particulates with characteristic dimensions up to
25 mm. This results in the cooking and sterilizing of the food in a single, very rapid
operation of a few tens of seconds. When linked into an aseptic packaging line it
gives an almost indefinite shelf life in ambient temperature storage.
13.4 Indirect resistance heating
In this method, electricity is passed through a suitable conducting material which
forms a heating element, and heat is transferred to the workpiece or the medium to
be heated by conduction, convection or radiation, or a combination of these. The
element is the hottest component of the system, and the factors which determine the
choice of element materials will depend on the nature of the heat transfer process as
well as the physical and chemical characteristics of the process environment.
Infrared heating falls under the generic heading of indirect resistance heating, but
for clarity it is covered separately in section 13.5.
13.4.1
Metallic elements
Metallic elements traditionally take the form of wire, strip or tape. For a given
operating temperature, the shortest operating life tends to be found with the lowest
cross-section of wire or tape, because failure occurs through progressive oxidation of
the surface and the consequent reduction in mechanical strength.
The choice of metal composition for the element will depend upon the required
operating temperature, the material resistivity, the temperaturecoefficientof resistance,
corrosion resistance at high temperature, mechanical strength, formability and cost.
Many metals and alloys are used, but the most common for industrial applications
are based on nickel-chromium, iron-nickel-chromium or iron-chromium-aluminium
alloys.
13.4.2 Sheathed elements
The elements may be protected from the working environment by an insulation layer
and an outer sheath.
In the heating elementsof many domestic appliances (such as cooker rings, immersion
heaters and kettle elements) a purified magnesium oxide powder separates the helical
element coil from a sheath which may be of copper, stainless steel or a nickel-base
alloy. These elements are often rated in watts per square centimetre of sheath.
Such mineral-insulated elements are also used in cartridge heaters, radiant panels
and immersion heaters for industrial applications. Also available are thin-strip or
band heaters, which have mica insulation between the element and the sheath.
13.4.3
Ceramic elements
A range of ceramic materials exist which have a sufficiently high electrical conductivity
to act as element material. These materials include silicon carbide, molybdenum
disilicide, lanthanum chromite and hot zirconia. Graphite is another recognized nonmetallic element material, but this can only be used in inert atmospheres such as
steam and carbon dioxide.
342
Electroheat
Ceramic elements usually have a hot zone which is created by a thin section or a
spiral cut; this hot zone is supported by two or more cold ends which are of a thicker
section or have been impregnated with a metallic phase to lower the resistance
locally.
13.4.4 Electric ovens and furnaces
These are used for a great variety of purposes, ranging from sintering ceramic materials
at temperatures up to 1800°C to drying processes which are close to ambient temperature.
Power ratings may be from a few kilowatts up to more than a megawatt. An oven
usually has an upper temperature limit of about 450°C. and afurnace is usually used
for higher temperature processes. Most modem ovens and furnaces incorporate
lightweight insulation with low thermal conductivity to reduce thermal inertia and
surface losses.
Metal resistance-heating elements for furnaces are usually in the form of wire,
strip or tube. Elements for low voltage and high current have a heavy-section
construction in an alloy casting or with corrugated or welded tube. Helically wound
wire heating elements are made with a mandrel-to-wire diameter ratio of between
3 : 1 and 8 : 1. This ratio is limited by the tendency of the helical winding to collapse
under its own weight at high operating temperatures. Coiled-wire or strip elements
may be inserted in ledges or grooves, supported at intervals by pegs of nickel alloy
or ceramic. The end connections of the elements are normally of a different material
in order to reduce attack from oxidation and chemical reaction with the refractories,
and they have a lower resistance in order to reduce the heat dissipation where the
leads pass through the furnace wall.
Ovens may use natural or forced convection, and are widely employed in a range
of applications including curing, baking, the annealing of glass and aluminium, and
the drying and preheating of plastics prior to forming. Coiled nickel-chrome wire or
mineral-insulated metal-sheathed elements are distributed around the oven in order
to obtain a uniform temperature distribution. The heat transfer rate may be increased
by using a fan to circulate air over the heating elements and onto the workpiece, the
air being recirculated through ducts. An important advantage of convective ovens is
that the operating temperature is normally the element temperature, and the maximum
temperature is never exceeded since the process is self-limiting. This prevents
overheating if the material is left in the oven too long, and it is particularly important
for temperature-sensitive materials such as plastics.
High heating rates can be achieved by direct radiation from heating elements in
infrared ovens. Here the oven walls are made of sheet metal which reflects or reemits the radiation. Infrared processes are usually associated with surface heating
which is applied, for example, to paint and other coatings. The efficiency in these
applications may be very high in comparison with other methods which also heat the
substrate.
Forced-convectionfurnaces allow high heating rates and with careful design good
temperature uniformity can be achieved, but they are normally limited to a maximum
temperature of 700-900°C.
Higher operating temperatures can be achieved with the pit furnace, in which
radiation is the dominant mode of heat transfer and, by using a retort, the process can
be carried out in a controlled atmosphere.
The beZZfuinace may be used as a hot-retort vacuum furnace. The layout of a bell
furnace is shown in Fig. 13.12. By reducing the pressure inside the bell retort,
Newnes Electrical Engineer’s Handbook
Bell
343
-
,
Retort
i
I
4
Fig. 13.12 Bell furnace
oxidation of the product is minimized. Since heat losses by convection are greatly
reduced at low pressure, the bell can be raised when the required temperature is
reached, and one bell may be used to heat several retorts.
Further information on furnace types and rating methods is available in reference
13A.
13.5 Infrared heating
Infrared is one of the most widely used electrical process heating techniques in
industry, finding application in all sectors. Using electromagnetic energy as a source,
it provides the potential for high input power densities and non-contact heating.
The peak wavelength of an infrared source depends upon its temperature. As the
temperature is increased, the peak wavelength is shorter, proportionately more of the
power is radiated at shorter wavelengths and the power densities are higher. This
effect is illustrated in Fig. 13.13.
Typical input power densities for different types of infrared sources are given in
Table 13.3.
Lower-temperature infrared sources (long and medium wave) are normally used
for surface heating applications such as the drying of coatings, curing of paints and
powder finishes, reflow soldering and browning of foods. Short-wave heaters are
used where higher product temperatures are required (for instance in metal treatment)
or where high intensities are needed; they may also be used for the through-heating
of certain materials, for example in the moisture profiling of paper and board.
The efficient use of infrared radiation for heating is assisted by matching the
spectral output of the infrared source to the absorptive properties of the product in
the infrared range. The optimum choice may not always be apparent at first. For
example, in the drying of paper, the first choice would be medium wave due to the
very strong absorption, but this would provide surface drying only; by using a short-
Electroheat
344
5 103
U
J
C
P)
P)
5F!
c
E
.- Id
E
&
Q
E
c
E
2
!I
P10'
i2
-8
iii
5
E
u)
1100
B
d
O
2
4
6
8
10
12
14
16
Wavelength (pm)
Fig. 13.13 Black-body radiation characteristics
Table 13.3 Characteristics of infrared heaters
Heater type
Embedded ceramic
Mineral-insulated element
Tubular quartz-sheathed element
Circular heat lamp (375 W)
Linear heat lamp (1 kW)
parabolic reflector
elliptical reflector
Quartz-halogen linear heat lamp
(12kW)
parabolic reflector
elliptical reflector
Maximum surface
temperature
("C)
Maximum power Wavelength
denslty
(kWlm2)
600
800
900
68'
40*
2100
30*
long
long
medium
short
2100
21 00
50*
90""
short
short
2700
200*
short
2700
3000*'
short
50"
* Average power density.
** Power density at focus.
wave source, some of which is transmitted and reflected by a polished surface behind
the paper, penetration is achieved and through-drying is obtained.
Control of infrared heating may be effected in a number of ways. Infrared elements
are generally of less than 10 kW rating, and an installation will often use many
heaters. These can be switched on and off individually or in groups, or the voltage
and current to each heater or group can be separately adjusted. One aspect of altering
the power input to an infrared heater is that its temperature and hence its spectral
output will be changed. This can have implications for the efficient heating of a few
materials which show a variation in infrared absorption with wavelength. Recent
Newnes Electrical Engineer's Handbook
345
improvements in the speed of response of lower-temperature infrared sources have
extended the previous limitations in control of infrared heating.
The sensing of product parameters such as temperature or moisture content can
provide information for control systems.
13.6 Induction heating
When an electrically conducting body is placed in an alternating magnetic field,
eddy currents are induced in it and it is heated as a result of 12R losses. The induced
current density is greatest at the surface of the workpiece and decreases through its
thickness. This phenomenon is known as the skin effect. The skin depth, 6,is defined
as
6 = (2p/yr)'"
-
6 is given in metres, where p is the electrical resistivity of the workpiece (Q m), o
is the angular frequency [email protected] the coil current (rads) and p is the absolute magnetic
permeability of the workpiece (Wm). p is given by the product cr, x b,where po is
4a x
and pr is known as the relative permeability. The relative permeability is
a function of the applied magnetic field strength for magnetic materials, and it has
the value of 1for nonmagnetic materials such as copper and aluminium. If 'thickness'
is defined as the diameter of a round billet or the depth of a plate or slab, the ratio of
thickness to skin depth is an important yardstick for the perfomance of an induction
heating system. As this ratio increases, a greater proportion of the total power is
dissipated near the surface of the workpiece, and the efficiency of the conversion of
electrical energy into heat also increases.
The distribution of heating in the body of the workpiece can be controlled by the
choice of frequency and the effect this has on the skin depth. If the frequency is high,
most of the heat is dissipated in a thin surface layer, while lower frequencies give a
more uniform distribution.
In ferrous metals, additional heating arises from hysteresis loss. This is normally
small in comparison with the eddy current loss, but it is exploited in the heating of
metal powders at high frequencies.
Induction techniques are used for both through heating and surface heating of
metallic materials at frequencies across the range from 50 Hz to 1 MHz. They are
used for metal melting and at very high frequencies they are applied in the manufacture
of semiconductor materials and in the hot working of glass.
For regular-shaped workpieces such as cylindrical rods and wide rectangular
slabs, the power generated in the workpiece and the induction heating efficiency can
be calculated from analytical solutions to the diffusion equation for the induced
current, supplemented by empirical factors. Details are given in reference 13B.The
analytical solutions assume constant material properties thoughout the workpiece,
where in practice the resistivity and specific heat vary with temperature and the
relative permeability of magnetic materials is a function of both field strength and
temperature, reducing to unity above the Curie temperature, which is about 750°C
for steel. Computer-basednumerical solutions are now commonly used to take account
of these variations.
The power induced in a round solid billet, illustrated in Fig. 13.14, is given
approximately by:
Electroheat
346
\
ietic field lines
(Bottomhalf section omitted)
\
\
current
-wL
9' 1d
LA'
Resistivity &
Permeability p = pr, p,
W
HHHHHHEIHHEIaHHHHH
-L"-
Coil current lmSat frequency, C Hz
Coil copper space factor KA= N . h/Lc
Fig. 13.14 Diagrammatic section of induction heater for round billet
where the parameters are as shown in Figs 13.14 and 13.15. In particular, Qd is
shown in Fig. 13.15 as a function of db, and Kc,the coil shortness factor, is dependent
on the ratios dD,dlS and LbVl&. Reference 13C gives empirical values for Kc,
which tend to unity as d D and Lw/Lc approach unity.
1.4
1.2
1
Z i 0.8
d
v)
J
0.6
0.4
0.2
0
0
1
2
3
4
5
6
Diameter or thicknesdskin depth
7
8
9
1
0
Fig. 13.15 Loss factors Q and P for rods and slabs
The power induced in a hollow cylinder of wall thickness t is calculated with Qrd
in the above expression replaced by an equivalent factor Qcyl,which is a function of
tld, dlband pr.Reference 13B shows graphs of Qcylfor a range of these parameters,
although different terminology is used.
Similarly, for a rectangular slab of length L which has a width W much greater
than its thickness, the induced power is:
Pw = (Nx I x Kc)' x (2Wx p)l(b x L) x (U&)' x Qslab (watt)
It is necessary to know the reactive power in the workpiece in order to evaluate
the power factor of the coil. This can be calculated by multiplying the induced power
by the appropriate P/Qratio obtained from Fig. 13.15.
Newnes Electrical Engineer's Handbook
347
The efficiency of conversion of the electrical power supplied to the coil into
thermal power in the workpiece is known as the coil or electrical eficienq, VC. This
is given by :
qc = (1 + (l/K:) x (l/KA) x (Sc/Sw) X
(pc/pwclr)'" x
(l/Q))-'
(per unit)
In this formula, Q is the relevant loss factor, KA is the space factor of the coil system
and Sc/Sw is the ratio of the coil perimeter to that of the workpiece in the same plane.
This efficiency can be significantly increased by the use of multilayer windings
instead of a single-layer coil. Such high-efficiency coils are now commonly used for
the heating of non-ferrous billets at mains frequency.
The overall efficiency of induction heating is the product of the supply eficiency?
the thennal eficiency and the coil eficiency. Of these, the supply efficiency is
typically 0.8-0.9 (per unit); it accounts for losses in cables, power factor correction
capacitors and frequency conversion equipment. The thermal efficiency has typical
values in the range 0.7-0.9 (per unit). This represents the thermal losses from the
workpiece, and is critically dependent on the operating temperature, the thermal
insulation and the method of operation of the heater.
13.6.1
Power sources
Loads which can effectively be heated at 50 Hz include slabs, large billets, cylinders
and process vessels. Depending on the load rating, the power input is controlled by
either an off-load tap changing transformer, or an autotransformer. Power factor
correction is usually provided on the primary side of the heater supply transformer,
and phase-balancing networks are used to correct the imbalance of large single-phase
loads. Voltage transients on the supply network are minimized by the use of soft-start
arrangements when switching large loads.
Apart from mains-frequency installations, power supplies for modern induction
heaters are derived from solid-state frequency converters. Unit sizes of up to 7 MW
have been installed for metal melting at 1-3 kHz, and 1 M W units are now suitable
for frequencies up to 500 kHz, previously the domain of power vacuum tube triodes.
13.6.2 Through-heating of billets and slabs
Induction heating is used extensively for the through-heating of both ferrous and
non-ferrous billets prior to rolling, extrusion or forging. The billets, which may be of
circular or rectangular section, are either heated individually or passed in line through
a series of induction coils. The frequency of the currents, the power input and the
length of time in the coil are chosen to provide the right throughput rate and an
acceptable temperature distribution over the cross-section of the workpiece.
Metal slabs are also heated by induction processes. One of the largest recent
installations is in Sweden; this is rated at 37 MW and heats 15 tonne slabs for rolling
at the rate of 85 tonne/hour. Thin slabs, which may be from continuous casting
machines or at an intermediate rolling state, are heated at medium frequency.
13.6.3 Strip heating
The heating of continuous strip metals by the conventional induction method requires
the use of frequencies above 10 kHz, and the resulting efficiencies are low for nonferrous metals. An alternative technique is the transverse flux method by which, for
example, an efficiency over 70 per cent can be achieved when heating aluminium
Electroheat
348
strip at 250 kHz.The strip is passed between two flat inductors comprising windings
in a laminated iron or femte core which forms a series of magnetic poles. The flux
passes transversely through the sheet and currents are induced in the plane of the
sheet. The arrangement of the windings and poles must induce a current distribution
which results in a uniform temperature distribution over the sheet as the strip passes
through the inductor.
Installations rated at 1.8 MW and 2.8 M W are operating for the heat treatment of
aluminium strip in Japan and Belgium respectively.
13.7 Indirect induction heating
Although induction heating can only be directly effective in electrically conducting
materials, it can also provide a means of heating other materials by using a metal as
an intermediary, in a way that is analogous to indirect resistance heating. Examples
are found in the processing of semiconductorsand in the calcining of ceramic materials.
13.7.1 Semiconductor manufacture
Since the energy from the heating coil can be generated in the workpiece without any
heat transfer medium, indirect induction heating can be carried out in a vacuum. This
is particularly useful in the processing of semiconductor materials.
In one technique, the material is placed in an electrically conducting crucible in
the vacuum space. The crucible is directly heated by induction and its heat is transferred
to the semiconductorby radiation and conduction.
Semiconductors may also be heated directly by induction without the need for a
conducting crucible. In this case very high frequencies up to 4 MHz are used.
13.7.2
Process heating and calcining
There is an important and growing series of applications in which induction heating
is used to heat a vessel or other metal component, from which heat is transferred by
conduction to a product.
Examples include:
0
0
0
0
vessels containing a solid or liquid
pipes through which a liquid flows
chemical reactor vessels
extruders
screw conveyors
rotary kilns
mixer paddles
These applications often employ the coil arrangements already described elsewhere,
but there are recent developments using novel techniques. Examples are the ‘ROTEK
rotary kiln, the heated mixer paddle and the heated screw.
The ‘ROTEK kiln is heated by induction and is used for the drying and calcining
of granular solids which flow continuouslythrough a revolving drum. The kiln lining
and flights are heated directly, and the flights also have the function of lifting or
agitating the product as the vessel rotates. The heat is then transferred to the material
by conduction. All internal components are made of stainless steel or Inconel, and
Newnes Electrical Engineer’s Handbook
349
are heated by the passage of a very large current from end to end. This current is
perhaps greater than 20 kA,and cannot be fed through sliding contacts; the drum
therefore forms the single-turn secondary of a ring transformer, with a bar primary
which passes through its centre and returns through the outer copper conductors.
Figure 13.16 shows the general arrangement. The bar primary is fed from a lowvoltage transformer and the efficiency of the device as a heater is very high. Because
the ring core can be thermally insulated the operating temperature can be well above
the Curie point, and with an Inconel drum, temperatures exceeding 1000°C can be
achieved.
;
I
I
Double-walled drum
L_
Centre conductor
.
Current
\NsII riirrnnt
/
(a)
Busbar
t
Transformer
Flight
(only 4 shown
for clarity)
\
Centre conductor-
Inner drum
Thermal insulation
Fig. 13.16 Operating principle of a ROTEK kiln
13.8 Dielectric heating
The processing of non-metals by the conventional heating techniques of conduction,
convection and radiation is often limited by the physical characteristics of the materials.
Electmheat
350
Thermal conductivity is a particular problem and since many of these methods depend
on the transfer of heat through the product surfaces there is inevitably a temperature
gradient between the surface and the centre. If the heat flux is raised above a certain
level in such circumstances there is a risk that the surface will be damaged by
overheating. The choice in many operations is between process inefficiency and
product degradation.
Dielectric hearing is a generic term covering radio frequency (RF) and microwave
processing. RF and microwave occupy adjacent sections of the electromagnetic
spectrum, with microwaves having higher frequency than radio waves. However, the
distinction between the two frequency bands is very blurred; for example, some
applications at about 900MHz are described as radio frequency (cellular telephones)
and some as microwave (dielectric heating). Nevertheless, RF and microwave heating
can be distinguished by the technology that is used to produce the required electric
fields. RF heating systems use high-power electrical valves, transmission lines and
applicators in the form of capacitors, whereas microwave systems are based on
magnetrons, waveguides and resonant (or non-resonant) cavities.
There are internationally agreed and recognized frequency bands which can be
used for RF and microwave heating; these are known as the Industrial, Scientific and
Medical (ISM) bands. At radio frequencies, the ISM bands are:
0
0
13.56 MHz f 0.05% (f0.00678 MHz)
27.12 MHz f 0.6% (f0.16272 MHz)
40.68 MHz f 0.05% (f0.02034 MHz)
and at microwave frequencies the ISM bands are:
0
0
the so-called 900 band (896 MHz in the UK, 915 MHz in the USA and Japan,
but not permitted in Europe)
2450 MHZ f 50 MHz
Electromagnetic Compatibility (EMC) requirements impose severe limits on any
emissions outside these bands. These limits are much lower than health and safety
limits and are typically equivalent to microwatts of power at any frequency outside
the ISM bands. In most countries, compliance with the relevant EMC regulations is
a legal requirement, and this is covered in more detail in Chapter 15. Often it is the
higher harmonics which present the greatest containment problems.
Industrial applications are many and varied, ranging from the drying of textiles to
the welding of plastics, the tempering and thawing of meat, the heating of rubber
extrusions and the firing of ceramics. Details are given in reference 13D.
The heating effect arises from a number of polarizations, the most commonly
described being orientation polarization or oscillating dipole, as shown in Fig. 13.17.
Although this is the principal mode at microwave frequencies, it is of relatively little
significance at radio frequencies. The dominant mode in the RF range is space charge
polarization, which in turn is dependent on the ionic conductivity of the material
being processed. RF can therefore be regarded as a special case of direct resistance
heating.
It is possible in theory to choose for any particular material the most appropriate
frequency from those available in the industrial, scientific and medical bands. In
reality, many products can be processed by either, and the choice between them can
Newnes Electrical Engineer’s Handbook
Space charge polarization
35 1
Orientation polarization
Fig. 13.17 Orientation polarization and space charge polarization in dielectric heating
be based on other considerations such as the engineering needed to make a satisfactory
applicator which is compatible with process line requirements.
The heat transferred per unit volume of product, P , is given by:
P = 2zfq.5,E2
[W/m2]
is the permittivity of free space and E is the
where f is the frequency (in Hz),
applied electric field strength (in V/m). q is the loss factor, which is a dimensionless
property of the material indicating its susceptibility to heating at the given frequency;
the loss factor varies with moisture content, temperature and other factors.
13.8.1 Radio frequency power sources and applicators
Most RF power supplies in the 10 MHz to 100 MHz range are based on a class C
oscillator/amplifier. The oscillator valve is most often a triode, which is built into a
circuit of inductors and capacitors. When the components are rationalized this circuit
can take on the appearance of a tank, which is normally fabricated from aluminium.
With the increasing importance being attached to the need to avoid electromagnetic
interference with other equipment, alternative generator types such as crystal-driven
oscillators followed by amplifiers are being used. These so-called ‘50 Q’ systems
have many other advantages such as controllability and the simplicity of the applicator
construction.
RF applicators are essentially capacitors in which the product forms all or part of
the dielectric. Figure 13.18 shows three types in which the electric field, for the
unloaded condition, is shown by the broken lines.
The simplest and most widely used of these three is the through-field or pamllelplate electrode. In addition to the product there may be in some cases a clearance
which forms a series air-space capacitor. Such systems can be used in conjunction
with pressure, as in the case of plastic welding, which is still the biggest use of
dielectric heating. In this case there is no air gap and the field strength in the load can
therefore be relatively high for a given electrode voltage. When plate electrodes are
used for operations like drying, an air space is required above the dielectric to allow
for the movement of product through the machine and for ventilation of the water
vapour. This leads to an increase in voltage between the plates in order to maintain
an adequate field strength in the product. It is therefore important to consider the
relative dimensions of the dielectric and air-space capacitors so that heating is provided
without the risk of an electrical discharge.
For very thin materials it may be necessary to use an alternative electrode
configuration which gives a more suitable distribution. For example, to dry the
Electroheat
352
Q m
...........
..........
...........
..........
...........
.........
...........
...........
..........
(a) Through-field electrode
Al
U
w
(c) Staggered through-field
(b) Fringe-field array
Fig. 13.18 Radio frequency applicators
adhesive on a paper web in the manufacture of envelopes, it is necessary to use the
fringe or struyfield nrruy shown in Fig. 13.13, in which the electric field is arranged
to be in the plane of the paper.
For intermediate thickness applications such as post-baking of biscuits it is usual
to use a staggered through-field as shown in Fig. 13.18.
13.8.2 Microwave power sources and applicators
For industrial microwave heating applicationsthe usual power source is the magnetron.
This is a thermionic device which will launch electrons at a specific frequency when
connected to a high-voltage dc source. At the permitted operating frequency of 2450
MHz, the highest output from a magnetron until recently was 5 kW, but 15 kW units
are now becoming available. For the ‘900’ band high-efficiency magnetrons of up to
60 kW now exist. Where higher powers are needed, a number of magnetrons may be
fed into one applicator.
The most common form of industrial heating oven is the multimode oven, which
is essentially an enlarged version of the familiar domestic microwave oven; in the
case of continuous processing this will have appropriate product ports to allow the
passage of product but confine the microwave within the oven. In such applicators
the antenna of the magnetron may be mounted directly into the oven, but more often
the microwave power is transmitted from the power supply via waveguides to the
oven cavity, where it is launched into the chamber. Other options for microwave
applicators include directional horns and ‘leaky waveguides
Industrial microwave heating has been used extensively in the rubber industry for
curing and preheating prior to moulding. In the food industry it has been used for
tempering, melting, cooking and drying. Microwave vacuum dryers have recently
been developed for drying expensive, high-quality pharmaceuticals which are
temperature sensitive.
I.
Newnes Electrical Engineer’s Handbook
353
13.9 Ultraviolet processes
Inks and surface coatings can be cured at high rates with ultraviolet sources. The
coatings are specially formulated using monomers with photo-initiators, so that a
very rapid polymerization is brought about on exposure to ultraviolet radiation.
Although this is not strictly a heating process, it has much in common with infrared
heating for drying or crosslinking and is very often in direct competition with infrared.
The energy usage in ultraviolet is much lower than in infrared because the process
requires only the stimulation of a reaction which is overall exothermic.
The active spectral region covers the wavelengthrange 250400 nm in the ultraviolet,
and visible wavelengths in the range 400-500 nm may also be used. The distribution
can be confined to a few intense and narrow bands of wavelength. A typical output
spectrum from a medium-pressure (sometimes called high-pressure) mercury vapour
discharge lamp is shown in Fig. 13.19.Additional spectral bands can be generated by
incorporating metal halide dopants in the lamp fill. The power ratings of these lamps
are in the range 2-20 kW, over active lengths of 250-1800 mm.
1
600
200
300
400
Wavelength (nm)
500
Fig. 13.19 Spectral output from a medium-pressure mercury lamp
13.10 Plasma torches
The plasma torch uses an electric arc discharge to generate a thermal plasma, which
is a partially ionized gas in which the degree of ionization is linked to the temperature
of the gas. Here the temperature of the gas rises above 6000°C; the gas becomes a
reasonably good conductor of electricity. Temperatures in the core of an arc may
reach 20 000°C or higher.
The torches that are used in electroheat applications may be broadly classified
into the three families:
0
0
rod and nozzle electrode opes. An arc burns between the end of a rod and the
internal surface of a nozzle, and gas is blown around the arc and through the
nozzle. Figure 13.20 shows the general layout. The supply is usually dc, but
torches designed for ac use are also available.
linear coaxidtube Qpes. The arc burns between the internal surfaces of
tube electrodes, and gas is blown through the electrodes around the arc.
Figure 13.21 shows the arrangement. The power handling capacity is improved
as the arc root motion is induced by the imposition of axial magnetic fields.
electrodeless types. These include particularly induction-coupled and microwave
types. An induction-coupled arc burns in a ring-shaped electric field which is
Electroheat
354
ff
Plasma gas
Fig. 13.20 Rod and nozzle type of plasma torch
induced within a coil-carrying current typically at a frequency of a few MHz;
the arrangement is shown in Fig. 13.22. The arc plasma is effectively the
workpiece in an RF induction heater. In microwave torches the arc is maintained
by currents which are driven by microwave fields in a resonant cavity. Peak
plasma temperatures in electrodeless torches are typically within the range
7000-1 1 OOO"C, which is rather lower than for the electrode types.
Transferred torches are available for industrial applications at power ratings up to
about 7 MW in a single torch. Non-transferred torches are available in power ratings
up to about 100 Mw, but for industrial applications a maximum rating of about
10 MW is imposed by practical considerations of electrode life. The efficiency of
non-transferred torches may reach 90 per cent for coaxial-tube electrode types used
with high gas-flow rates, corresponding to relatively low gas temperatures of 3OO&
4000°C.Rod and nozzle types are generally used for lower power ratings up to about
100 kW, and their efficiency tends to be lower, being about 60 per cent at best.
n
Magnetic
field coil
Magnetic
field coil
(a)
(b)
(c)
Fig. 13.21 Linear coaxial tubular electrode types of plasma torch construction: (a) The Hiils type developed from Schoenherr’s original enrly design: (b) A typical 2 MW nontransferred torch (c) A typical stretched or segmented torch rated at 6 MW or higher
Electroheat
356
Water-cooled
insulating
tube
--
Plasma
gas
Powder
feed
Plasma
A
I
Fig. 13.22 An induction-coupled plasma torch
13.10.1 Plasma furnaces or reactors
In principle, the simplest type of furnace or reactor s the ‘in-flight’ reactor in whic I
theinput materials ak injected directly into the plasma stream or even into the torch,
and the required processes occur with all the reactants suspended in the plasma
stream. In-flight reactors generally require the input materials to be gaseous or finely
divided because the time available for the process is short, typically of the order of
milliseconds.
Transferred plasma torches are commonly used in open-bath furnaces. A return
electrode is then usually built into the hearth refractories, but three-phase ac and
bipolar dc multiple-torch systems have been developed to eliminate the need for the
hearth connection.
Non-transferred torches are often used in shaft-type furnaces where a hot reaction
zone is created close to the point of injection of the reactive plasma gases in the base
of a packed shaft.
Furnaces with rotating shells are also available, the layout being shown in
Fig. 13.23.These are used particularly for the fusion of pure refractory compounds,
or when a long residence time is required, as in the treatment of some waste materials.
13.11 Glow discharge processes
Glow discharges are being increasinglyused for industrial surface coating and surface
modification processes.
The characteristic of a glow discharge, from which it derives its name, is that
some large portion of the discharge vessel should be filled with a weakly ionized and
luminous plasma. The two types of glow discharge are the dc and the low-pressure
RF discharge.
13.12 Lasers
Since its discovery in 1960, the laser has found extensive application in industry for
cutting, welding, material removal and heat treatment processes which require power
densities above 100 W/mm2. The principal feature of a laser as a heat treatment
source is its monochromatic output, low divergence and high intensity. This enables
Newnes Electrical Engineer's Handbook
/
357
' I
Solid feed material
Fig. 13.23 Rotating shell furnace using a non-transferredtorch
the parallel monochromatic beam to be focused to a smaller diameter than is possible
with other light sources.
Industrial applications for C02 lasers include the dividing and perforation of
substrates for integrated circuits and electronic components,perforation of elastomeric
materials, the manufacture of dies for cutting cartons, ready-to-wear suits and thin
glass tubes for fluorescent lamps, quartz tubes and borosilicate glass, and for the
manufacture of flexigravure plates. Deep-penetration welds can be obtained at power
levels above about 1 kW, and the ratio of the weld depth to the heat-affected zone
using a laser can be ten times greater than that obtainable with electron-beam welding.
Selective heat treatment is also possible.
13.13 Standards
Table 13.4 gives a selection of standards covering the electroheat field, with special
emphasis on RF and microwave equipment and methods.
Table 13.4 Standards relating to electroheat
IEC
EN
0s
Subject
2316
2316-1
2316-3
3041
3041-1
3056
3056-4
6656
Radio frequency cables
- general requirements and tests
- cable data sheets
RF connectors
- general requirements and measuring methods
Sizes of refractory bricks
- bricks for electric arc furnace roofs
Prevention of inadvertent ignition of flammable atmospheres by
RF radiation
Assessment for electronic components’ sectional specification:
microwave modular electronic units of assessed quality
- index of test methods
Limits and methods of measurement of radio disturbance
characteristics of ISM RF equipment
Specification for radio interference measuring apparatus
and measuring methods
Determination of limits for ISM equipment
Electric infrared emitters for industrial heating
- short-wave infrared emitters
Safety in electroheat installations
- resistance heating equipment
- induction and conduction heating and induction melting
installations
- arc furnace installations
- industrial microwave installations
- high-frequencydielectric heating installations
Industrial microwave heating installations - test methods for
determination of power output
160200
EN160200
160200-2
55011
EN160200-2
EN55011
240
240-1
519
519-2
519-3
60240
60240-1
60519
60519-2
60519-3
EN60240
EN60240-1
EN60519
EN60519-2
EN605194
519-4
519-6
519-9
1307
60519-4
60519-6
60519-9
61307
EN60519-4
EN60519-6
EN60519-9
EN61307
ClSPR (International
Special Committee
on Radlo Interference)
11
16
23
N.
American
n b l e 13.4 (contd)
~
IEC
EN
BS
Subject
1308
61308
EN61308
HF dielectric heating installations - test methods for
determination of power output
Guidelines on limits of exposure to RF electromagnetic fields:
- recommendation (protection guide 1982)
techniques and instrumentation for the measurement of potentially
hazardous electromagnetic radiation
- recommended practice for measurement of potentially hazardous
electromagnetic fields
Guidance as to restrictions on exposures to timevarying electromagnetic fields
NRPB GS 11
~
ClSPR (International
Speclal Committee
on Radlo Interference)
N.
American
ANSI C95
ANSI C95.1
ANSI C95.2
ANSI C95.5
Electroheat
13A.
13B.
13C.
13D.
Electric resistance furnaces for metal heat treatment, EA Technology Ltd, Technical Note No.
EATL 1124.
Davies, E.J. Conduction and Induction Hearing, Peter Peregrinus Ltd, London, 1990.
orfeuil. M. Elecrric Process Heating, Battelle Press, Columbus, Richmond, Ohio, 1987.
Jones, P.L. and Rowley. A.T. Dielectric Drying, in Mujundra and Kudra (eds), Advances in
Drying, Marcel Dekker, New York, 1996.
Chapter 14
The power system
Dr B.J. Cory
Imperial College of Science, Technology and Medicine
14.1 Introduction
All countries now have a power system which transports electrical energy from
generators to consumers. tn some countries several separate systems may exist, but
it is preferable to interconnect small systems and to operate the combination as one,
so that economy of operation and security of supply to consumers is maximized. This
integrated system (often known as the ‘grid’) has become dominant in most areas
and it is usually considered as a major factor in the well-being and level of economic
activity in a country.
All systems are based on alternating current, usually at a frequency of either
50 Hz or 60 Hz. 50 Hz is used in Europe, India, Africa and Australia, and 60 Hz is
used in North and South America and parts of Japan.
Systems are traditionally designed and operated in the following three groupings:
0
0
0
the source of energy - generation
bulk transfer - transmission
supply to individual customers - distribution
14.2 Generation
Generators are required to convert fuels (such as coal, gas, oil and nuclear) and other
energy sources (such as water, wind and solar radiation) into electrical power. Nearly
all generators are rotating machines, which are controlled to provide a steady output
at a given voltage; the main types of generator and the means of control are described
in Chapter 5.
The total power output of all operating generators connected to the same integrated
system must at every instant be equal to the sum of the consumer demand and the
losses in the system. This implies careful and co-ordinated control such that the
system frequency is maintained, because all generators in an ac power system must
run in synchronism, that is their rotors, which produce a magnetic field, must lock
into the rotating magnetic field produced by alternating currents in the stator winding.
Any excess of generated power over the absorbed power causes the frequency to rise,
and a deficit causes the frequency to fall. As the demand of domestic, commercial
and industrial consumers varies, so the generated power must also vary, and this is
normally managed by transmission system control which instructs some generators
362
The power system
to maintain a steady output and others (particularly hydro and gas turbine plant) to
‘follow’ the load; load ‘following’ is usually achieved by sensitive control of the
input, dependent upon frequency. The aim of the system controller is to run the
generating plant such that the overall cost of supplying the consumer at all times is
a minimum, subject to the various constraints which are imposed by individual
generator characteristics.
14.3 Transmission
Many large generators require easy access to their fuel supply and cooling water, so
they cannot necessarily be sited close to areas of major consumption. Environmental
constraints may also preclude siting close to areas of consumption. A bulk power
transmission system is therefore needed between the generators and the consumers.
Large generating plant produces output ranging from 100 MW to 2000 MW and
for economic reasons this normally operates with phase-to-phase voltages in the
range 10 kV to 26 kV. In order to reduce transmission losses and so that transmission
circuits are economic and environmentally acceptable, a higher voltage is necessary.
Phase-to-phase transmission voltages of up to 765 kV are used in sparsely populated
large countries such as Brazil, the USA and Canada, but 380-400 kV is more prevalent
in Europe. The standard voltages recommended by IEC are 765, 500, 380-400, 345,
275,230-220, 185-138 and 66-69 kV.
Most transmission circuits are carried overhead on steel pylons. An example is
shown in Fig. 14.1. They are suspended from insulators which provide sufficient
insulation and air clearance to earth to prevent flashovers and danger to the public.
A typical suspension-type insulator is shown in Fig. 14.2. Each country has tended
to have its own acceptable tower and conductor design. At higher voltages,Aluminium
Conductor Steel Reinforced (ACSR) conductoris used, a core of steel strands providing
the required strength. A typical cross-section for an ACSR conductor is shown in Fig.
14.3. For voltages over 200 kV two or more conductors per phase are used. This
results in lower losses because of the large conductor cross-section, and lower radio
interference and corona because of the lower voltage stress at the conductor surface.
Where an overhead line route is impossible because of congestion in an urban
area or for environmental amenity reasons, buried cables may be employed, but the
cost is 15-20 times higher than an equivalent overhead line. On sea crossings an
underwater cable is the only solution, but these are often dc, for reasons explained in
section 14.3.1.1.
A high-voltage transmission system interconnects many large generators with
high areas of electricity demand; its reliability is paramount, since a failure could result
in loss of supply to many people and to vital industry and services. The system is
therefore arranged as a network so that the loss of one circuit can be tolerated. This
is shown in Fig. 14.4. In many countries, three-phase lines are duplicated on one tower,
in which case a tower failure might still result in a partial blackout. Mixed-voltage
systems are often carried on a single tower, but this is not the practice in the UK.
In order to achieve flexibility of operation, circuits are marshalled at substations.
The substations may include transformers to convert from one voltage level to another,
and switchgearto switch circuits and interrupt faults. Substationsare normally outdoors
and they occupy an extensive secure area, although compact indoor substations using
SF6 (as described in section 7.5.3(b)) have become more prevalent recently because
of their improved reliability in adverse weather.
363
Newnes Electrical Engineer’s Handbook
Fig. 14.1 Transmission line tower - a 400 kV double-circuit line
I
I
Sheds
Pin
I
Fig. 14.2 Suspension-type insulator
The power system
364
Fig. 14.3 Typical cross-section of ACSR overhead line conductor
An interconnected transmission network can comprise many substations which
are all remotely controlled and monitored to ensure rapid reconnection after a disturbance
or to enable maintenance.
14.3.1
Principles of design
The two main requirements of transmission systems are:
0
0
the interconnection of plant and neighbouring systems to provide security,
economic operation and the exchange of energy on a buy-sell basis
the transport of electrical energy from remote generation to load centres
These objectives are met by selection of the most economical overhead line design,
commensurate with the various constraints imposed by environmental and national
considerations. The design and approval process for new lines can take many years,
following public enquiries and judicial proceedings before planning permission is
granted and line construction begins. Trpical objections to new overhead lines,
particularly in industrialized countries, are:
0
0
0
0
the visual deterioration of open country areas
the possibility that electromagnetic field propagation may cause interference
with television, radio and telecommunications, with an increasing awareness
of carcinogenic risks
emission of noise by corona discharges, particularly at deteriorating conductor
surfaces, joints and insulation surfaces
the danger to low-flying aircraft and helicopters
a preference for alternative energy supply such as gas and local environmentfriendly generation including solar cells and wind power, or measures for
reducing electricity demand such as better thermal insulation in houses and
commercial premises, lower energy lighting, natural ventilation in place of air
conditioning, and even changes of lifestyle
Power system planners are required to show that extensive studies using a range of
r---------
Interconnected
-!-I-+-.
Fig. 14.4 Generation and transmission network (UK voltages and practice)
The power system
366
scenarios and sensitivities have been carried out, and that the most economical and
least environmentally damaging design has been chosen. Impact statements are also
required in many countries to address the concerns regarding the many issues raised
by local groups, planning authorities and others.
Technically, the key issues which have to be decided are:
whether to use an overhead line or underground cable
the siting of substations and the size of substation required to contain the
necessary equipment for control of voltage and power flow
provision for future expansion, for an increase in demand, and in particular for
the likelihood of tee-off connections to new load centres
the availability of services and access to substation sites, including secure
communications for control and monitoring
14.3.1.1 HVDC transmission
Direct current is being used increasingly for the high-voltage transport of electrical
energy. The main reasons for using dc in preference to ac are:
dc provides an asynchronousconnection between two ac systems which operate
at different frequencies, or which are not in phase with each other. It allows
real power to be dispatched economically, independently of differences in
voltage or phase angle between the two ends of the link.
in the case of underground cables or undersea crossings, the charging current
for ac cables would exceed the thermal capacity of the cable when the length
is over about 50 km, leaving no capacity for the transfer of real power. A dc
link overcomes this difficulty and a cable with a lower cross-section can be
used for a given power transfer.
Where a line is several hundred kilometres long, savings on cost and improvements
in appearance can be gained in the dc case by using just two conductors (positive and
negative) instead of the three conductors needed in an ac system. Some security is
provided should one dc conductor fail, since an emergency earth return can provide
half power. The insulation required in a dc line is equivalent only to that required for
the peak voltage in an ac system, and lower towers can therefore be used with
considerable cost savings.
Against these reductions in cost with a dc line must be set the extra cost of solidstate conversion equipment at the interfaces between the ac and dc systems, and the
corresponding harmonic correction and reactive compensation equipment which is
required in the substations. It is normally accepted that the break-even distance is 50
km for cable routes and 300 km for overhead lines; above these distances dc is more
economical than ac.
Because many ac systems already exist, and because the trading of energy across
national and international boundaries is becoming more prevalent, dc transmission is
being increasingly chosen as the appropriate link. An added advantage is that a dc
infeed to a system allows fast control of transients and rapid balancing of power in
the case of loss of a generator or other supply, and it does not contribute to the fault
level of the receiving system; it is important here that when a short-circuit occurs on
the ac system side, the current that flows can be safely interrupted by the ac-side
circuit breaker.
Newnes Electrical Engineer’s Handbook
367
The accepted disadvantages of a dc infeed, apart from the already-mentioned
extra cost of conversion equipment, are the lack of an acceptable circuit breaker for
flexible circuit operation, and the slightly higher power losses in the conversion
equipment compared with an equivalent ac infeed.
With careful design of the transmission and conversioncomponents, the reliability
of dc and ac systems is comparable.
A typical HVDC scheme providing two-way power flow is shown schematically
in Fig. 14.5. Each converter comprises rectifying components in the three-phase
bridge connection, and each of the rectifying components consists of a number of
thyristors connectedis series and parallel. Increasingly, Gate Turn Off (GTO) thyristors
are being used because of the greater control they allow. The current rating of each
bridge component can be up to 200 A at 200 kV and bridges may be connected in
series for higher voltages up to 400 kV or even 600 kV +/- to earth, each bridge
being supplied from a three-phase converter transformer. By triggering the thyristors
the current flow through the system can be controlled every few electrical degrees,
hence rapid isolation can be achieved in the event of a fault on the system. Similarly,
by delay triggering the current can be easily controlled, the direct voltage being best
set by tap change on the converter supply transformers. The triggering of the thyristors
may be by a light pulse which provides voltage isolation. Inversion, which enables
power flow from the dc system to the ac system, depends on the ac back-emf being
available with a minimum fault level in the receiving systems, so inversion into an
isolated system is not possible unless devices with turn-off capability are available.
Converter
Smoothing
Overhead line
________--__
Positive pole
Bridge
[email protected]
Emergency earth return
z-+
% Bridge
I-
- - - - - - _,I- - - - ---as24
/
Negative pole
iI
I
I
I
I
I
I
I
-_
I
Each
module
Each bridge
Fig. 14.5 Complete HVDC scheme: showing converters and dc link
A further feature of dc converter substations is the need for ac transmission filters
to produce an acceptable ac sinusoidal waveform following the infeed or outfeed of
The power system
36%
almost square-wave blocks of current. Such filters for 6n f 1 harmonics (where n is
the number of bridges and substations) can add 25 per cent to the cost of a substation,
although they can also be used to provide some of the VAr generation which is
necessary to control the power factor of the inverter.
14.3.1.2 AC system compensation
As ac power systems become more extensive at transmission voltages, it is desirable
to make provision for flexible operation with compensationequipment. Such equipment
not only enables larger power flows to be accommodated in a given rating of ac
circuit, but also provides a means of routing flows over the interconnected system for
economic or trading purposes.
Compensation equipment consisting of fixed or variable inductance and capacitance
can be connected in series with the circuit, in which case it must be rated to carry the
circuit current, or it may be connected in shunt, and used to inject or absorb reactive
power (VArs) depending upon requirements. In the same way that real power injected
into the system must alwaysjust balance the load on the system and the system losses
at that instant. so too must the reactive power achieve a balance over regions of the
system.
Transmission circuits absorb VArs because their conductors are inductive, but
they also generate VArs because they have a stray capacitance between phases and
between phase and earth. The latter can be particularly important with high-voltage
cables. The absorption is proportional to Z2X, where Z is the current and X is the
reactance of the circuit, and the generation is proportional to V'B, where V is the
voltage and B is the susceptance of the circuit. When Z2X is equal to V2B at all parts
of the circuit. it is found that the system voltage is close to the rated value. If V2B is
greater than Z'X, then the system voltage will be higher than the rated value, and vice
versa. Designers therefore need to maintain a balance over the foreseeable range of
current as loads vary from minimum to maximum during the day, and over the season
and the year.
Compensation may be provided by the following three main methods:
series capacitors connected in each phase to cancel the series inductance of
the circuit. Up to 70 per cent compensation is possible in this way.
shunt inductance to absorb excessive VArs generated by the circuit stray
capacitance or (exceptionally) to compensate for the leading power factor of
a load
shunt capacitance to generate VArs for the compensation of load power factor
or excessive VArs absorbed under heavy current flow conditions on short
overhead line circuits
Combinationsof these arrangements are possible, especially in transmission substations
where no generators are connected; generators are able to generate or absorb VArs
through excitation control (see section 5.3.2). Recently, Flexible AC Transmission
Systems (FACTS)devices have become available. In these, the amount of VAr absorption
by inductors is varied through the control of thyristors connected in series with the
inductor limbs. A typical shunt controllable unit is shown in Fig. 14.6. This is known
as a Shunt Variable Compensator (SVC). Other FACTS devices are variable series
capacitors, variable phase shifters and Universal Power Controllers (UPC), in which
energy is drawn from the system in shunt and injected back into the system in series
at a controlled phase angle by means of GTO thyristors.
Newnes Electrical Engineer’s Handbook
369
To other phases
Mechanical
switches v
Filter
bank
One phase of
variable
compensation
(VArs leading to
VArs lagging)
to thyristors
Fig. 14.6 Schematic of a Shunt Variable Compensator (SVC)
14.3.2
System operation
A transmission system may be vertically integrated, in which case the generating
plant belongs to the same utility, or it may be unbundled, in which case it has only
transmission capacity, with no generation plant. In either case the main tasks are to
maintain a constant frequency and voltage for all consumers, and to operate the
system economically and securely. Security in this context means maintaining voltage
within limits, staying within a prescribed stability margin and operating all circuits
within their thermal rating. This requires adequate monitoring of all the transmission
components, with sufficient communication and control facilities to achieve these
desired goals. Most transmission systems will, therefore, have a co-ordinating room
and possibly a number of manned outstations for local or regional devolvement of
responsibility.
For frequency control, some of the synchronized generators are equipped with
sensitive governors which use a frequency signal rather than a speed signal. The
output of these generators is dependent upon the balancing power required to achieve
a steady frequency over the whole system. The control engineer, backed up by
computer forecasts of load variations and knowledge of the available plant and their
costs, can then instruct generators to start up or shut down (unit commitment) and set
their output (loading or dispatching) so that over a prescribed hourly, daily or weekly
period they generate energy to meet the consumer demand at the minimum overall
cost. In the UK the use of a Generating Ordering And Loading (GOAL) program
attempts to ensure that over a 24-hour period the total cost of generation commensurate
with maintaining a secure system is the minimum that can possibly be provided. This
includes the trading of energy with Scotland and France, in addition to any surplus
power available from co-generation within industry and from renewable sources
such as wind, hydro, waste incineration and landfill gas.
There is considerable scope for minimization of the losses in an interconnected
system through the control of the compensation devices described in section 14.3.1.2.
This control is guided by the use of optimal load flow programs, security assessments
The power system
370
and calculations of transient stability margin. One of the main concerns is to arrange
patterns of generation, including some plant which is out-of-merit, to maintain voltage
despite outages of circuits and other components for maintenance, extension and
repair. Safety of utility personnel and the operation of the system to avoid risk to the
public are at all times paramount.
14.4
Distribution
An example of a three-phase distribution system is illustrated in Fig. 14.7.In the UK,
voltages of 132,110,66,33and 11kV are typically used to provide primary distribution,
with a 380-415 V three-phase and neutral low-voltage supply to smaller consumers
such as residential or smaller commercial premises, where 220-240 V single-phase
to neutral is taken off the three-phase supply. Distribution voltages in continental
Europe are typically 110,69 and 20 kV, but practice varies from country to country.
In the USA, voltages of 138, 115, 69, 34.5, 13.2 and 4.16 kV are employed.
The transformer stepping down from the primary distribution to the low-voltage
supply may be pole mounted or in a substation, and is close to the consumers in order
to limit the length of the low-voltage connection and the power losses in the lowvoltage circuit. In a national power system, many thousands of transformersand their
associated circuit breakers or fuses are required for distribution to low-voltage circuits,
in contrast to high-voltage transmission and primary distribution systems, where the
number of substations is in the hundreds.
It will be noted from Fig. 14.7 that the primary and low-voltage distribution
systems are connected in a radial configuration. Circuit loops between adjacent
substationsare avoided because these can lead to circulating currents, which increase
the power losses and create difficulty in protection schemes. However, tie circuits
between adjacent lines and cables are available to reconfigure the network when a
portion of the low-voltage circuit is out of service for maintenance or because of
failure. These tie circuits are controlled by a normally open switch which can be
closed manually within a few minutes, although an increasing trend is for automation
of this operation by radio or teleswitching.
In urban and suburban areas, much of the primary and Iow-voltage distribution
system is underground, with readily accessible substations sited in cellars or on small
secure plots. Industrial sites may also have a number of substationsincorporated into
buildings or secure areas; these may be controlled by the works engineer or they may
be operated and maintained by an electricity distribution company.
In rural areas and in more dispersed suburban areas, three-phase overhead lines
operating at 10-15 kV or 27-33 kV are supported for many miles on poles which
may be of wood, concrete or steel lattice. The 380-415 V three-phase supply is taken
from these lines through a small pole-mounted fused input/output transformer. If the
maximum load to be taken is below about 50 kW,the supplies for homes or farmsteads
may be derived from a single-phase 10-15 kV supply. Typically, a rural primary
distribution system supplies up to 50 step-down transformers spread over a wide
region. The lines in such a system are vulnerable to damage by tree branches, snow
and ice accumulation and lightning strikes and it therefore has lower reliability than
underground systems in urban areas. Considerable ingenuity has been applied to
protection of this type of system with the use of auto-reclosingsupply circuit breakers
and automatic reconnection switches, which are described in section 7.4.2(b).
It is now common practice in developedcountries to monitor the primary distribution
From grid
I
11 kV
Light
industry
-I
--
cable
&Disconnect
Villages and farms
11 kV
Step to
V
down 415
and
neutral
r
r
r
O D 00 U O D U
00 000 0 ~ 0
Town houses
Fig. 14.7 Distribution network
r
A AA
132 or 33 kV
from another substation
415 V underground single
phase connection to houses
The power system
372
system down to 10-15 kV and to display alarm, voltage and power-flow conditions
in a control room, and in the event of an incident repair crews are dispatched quickly.
Repairs to the low-voltage systemare still dependent,however, on consumersnotifying
a loss of supply.
The proper earthing of distribution systems is of prime importance in order that
excessive voltages do not appear on connections to individual consumers. It is the
practice in the UK and some other countries to connect to earth the neutral conductor
of the 4-wire system and the star point of the low-voltage winding on the step-down
transformer, not only at the transformer secondary output but also at every load point
with a local meter and protective fuse. This is known as the Protective Multiple Earth
(PME) system, which is described in relation to cable technology in section 9.3.1.3.
It is designed to ensure that all metallic covers and equipment fed from the supply are
bonded so that dangerously high voltages do not hazard lives.
14.4.1
System design
It is essential that a distribution system is economical in operation and easy to repair,
and its design should enable reconnection of a consumer through adjacent feeders to
supply substations in the event of failure or outage of part of the system.
Copper or aluminium conductors with a cross-section of 150 mm2to 250 mm2are
typically used at the lowest voltages, and these are arranged so that the maximum
voltage drop under the heaviest load conditions is no more than 6 per cent; alternatively
the voltage at the connection to every consumer must not rise more than 6 per cent
under light load conditions. Local adjustment is achieved by off-load tap changing
(usually f 2.5 per cent or f 5 per cent), and voltage in the primary circuits (usually
11 kV and 33 kV in the UK) is controlled by on-load tapchangers under automatic
voltage control. The construction and operation of tapchangers is described in more
detail in section 6.2.5. Reinforcement or extension on the LV network is usually
arranged through the installation of a new primary feed point with a transformer,
rather than by upgrading the LV network.
Primary underground networks now employ cross-linked polyethylene (XLPE)
three-phase cables, as described in sections 9.2.4 and 9.3.1.3. The latest designs
incorporate fibre-optic strands for communication purposes. Cables require careful
routing and physical protection to minimize the risk of inadvertent damage from road
and building works in the vicinity.
Overhead lines are usually of a simple, flat, three-phase configuration which
avoids conductor clashing in high winds and ice precipitation.An example is shown
in Fig. 7.20. Impregnated wood poles are normally used. These provide some degree
of insulation to ground, and they can be quickly replaced in the event of collapse or
decay. Insulators are usually of the cap-and-pintype, an example of which is illustrated
in Fig. 14.8. Surge diverters and arcing horns provide a considerable measure of
overvoltage protection, particularly where many kilometres of exposed line are fed
from a primary substation. Arcing horns consist of a carefully positioned air gap
between each conductor and earth. They are designed to flash over at a particular
voltage when a potentially dangerous surge occurs on the system. Since there is no
provision for extinguishing the resulting arc in this event, it is preferable, although
more expensive, to provide a surge diverter at strategic positions in the overhead
system. A surge diverter consists of a zinc oxide resistance between the live conductor
and earth; this presents a very high resistance at normal voltages, but a low value at
overvoltages, thereby conducting surge current safely to earth.
Newnes Electrical Engineer’s Handbook
Conductor tied down
with soft copper wire
373
I
Sheds
Porcelain
or glass
4j
Steel pin
Fig. 14.8 Pin-type insulator
Primary substations consist usually of two or three step-down tapchanging
transformers, with common busbars which may be duplicated for reliability and
circuit separation. Remotely controlled interconnecting circuit breakers are installed
to provide operating flexibility. Such substations are continuously monitored and
metered, the data being brought back to a distribution control centre for display and
archiving. Amodern tendency is to reduce the number of control centres and increase
the geographical area covered by each through the use of computers and clever alarm
handling and by analysis with powerful software which will in the future incorporate
artificial intelligence.
14.4.2
System operation
The most important requirement in distribution control is good communication with
district personnel and maintenance crews in order to ensure that maintenance and
repair is efficiently, safely and swiftly carried out. Ready telephone access by the
public and other consumers is also necessary so that dangerous situations or supply
failures can be easily reported to engineers and technicians in the field.
The main task of the distribution controller is, therefore, to monitor equipment
alarms and ensure that rapid and effective action is taken. Schedules of equipment
outages and maintenance have to be planned and effected with the minimum of
disruption to consumers.
14.5
Future trends
Power systems are continually evolving and with the increasing capability of computers
and software, systems are becoming more centraIly conuolled for economy of operation
and security.
There is a trend towards the separation of the ownership, management and control
of generating plant from that of the transmission and distribution system. This is
being accompanied by considerable economies in operating and technical personnel
and is leading to more competitive trading.
374
The power system
Economists have constantly urged utilities to operate at marginal costs in order to
achieve economic efficiency with the consequent downward pressure on prices, and
this has led to the setting up of electrical energy markets in many countries. In such
systems, entrepreneurial generators can enter with the expectation of making a profit
by supplying electrical energy to any consumer over the transmission and distribution
system. A charge has to be made for transport of energy over this system, but there
is no reason to believe that this process will be more expensive than the previous
vertically integrated system. Experience is beginning to show that such a market in
electrical energy can work, as in the UK, and can produce cost savings not only in the
supply of energy but also in its transportation.
References
14A. Weedy, B.M.and Cory BJ.Electric Power Systenrp (4thedn), Wiley, 1987.
14B. EHV Transmission Line Reference Book, Edison Electric Institute,USA, 1968.
1K. Modem Power Starion Practice, volume L, 3rd edn, Pergamon, 1991.
Chapter 15
Electromagnetic compatibility
Mr A.J. Maddocks
ERA Technology Ltd
15.1 Introduction
Electromagnetic Compatibility (EMC) is achieved when co-located equipment and
systems operate satisfactorily,without malfunction, in the presence of electromagnetic
disturbances. For example, the electrical noise generated by motor-driven household
appliances, if not properly controlled, is capable of causing interference to domestic
radio and TV broadcast reception. Equally, microprocessor-based electronic control
systems need to be designed to be immune to the electromagneticfields from handheld radio communication transmitters, if the system is to be reliable in service. The
issues covered by EMC are quality of life, spectrum utilization, and operational
reliability, through to safety of life, where safety-related systems are involved.
The electromagnetic environment in which a system is intended to operate may
comprise a large number of different disturbancetypes, emanating from a wide range
of sources including:
0
e
mains transients due to switching
radio frequency fields due to fixed, portable and mobile radio transmitters
electrostatic discharges from human body charging
powerline surges, dips and interruption
power frequency magnetic fields from power lines and transformers
In addition to having adequate immunity to all these disturbances, equipment and
systems should not adversely add electromagneticenergy to the environment above
the level that would permit interference-free radio communication and reception.
15.1.I Sources
The essence of all EMC situations is contained in the simple source-path-receptor
model shown in Fig. 15.1.
Path
Receptor
Fig. 15.1 Source, path, receptor model
Sources comprise electromechanical switches, commutator motors, power
semiconductordevices, digital logic circuits and intentional radio frequency generators.
376
Electromagneticcompatibility
The electromagnetic disturbances they create can be propagated via the path to the
receptor such as a radio receiver, which contains a semiconductordevice capable of
responding to the disturbance, and causing an unwanted response, i.e. interference.
For many equipments and systems, EMC requirements now form part of the
overall technical performance specification.The EU’s EMC Directive, 89/336/EEC,
was published in 1992 and came into full implementation on 1 January 1996. All
apparatus placed on the market or taken into service must, by law, comply with the
Directive’s essential requirements, that is it must be immune to electromagnetic
disturbance representative of the intended environment and must generate its own
disturbance at no greater than a set level that will permit interference-free radio
communication. The Directive refers to relevant standards which themselves define
the appropriate immunity levels and emission limits. More information on the EMC
Directive and its ramifications is available in Marshman (reference 15A).
15.1.2 Coupling mechanisms
The path by which electromagnetic disturbance propagates from source to receptor
comprises one or more of the following:
conduction
capacitive or inductive coupling
radiation
These paths are outlined in Fig. 15.2.
A
B
Capacitive and inductive coupling path
Radiation path
Fig. 15.2 Coupling mechanisms for electromagneticdisturbance
I
I
Newnes Electrical Engineer's Handbook
377
Coupling by conduction can occur where there is a galvanic link between the two
circuits, and dominates at low frequencies where the conductor impedances are low.
Capacitance and inductive coupling takes place usually between reasonably long colocated parallel cable runs. Radiation dominates where conductor dimensions are
comparable with a wavelength at the frequency of interest, and-efficient radiation
occurs.
For example, with a personal computer, the radiation path is more important for
both emission and immunity at frequencies above 30 MHz, where total cable lengths
are of the order of several metres. Designers and installers of electrical and electronic
equipment need to be aware that all three coupling methods exist so that the equipment
can be properly configured for compatibility.
15.1.3
Equipment sensitivity
Analogue circuits may respond adversely to unwanted signals in the order of millivolts.
Digital circuits may require only a few 100 millivolts of disturbance to change state.
Given the high levels of transient disturbance present in the environment, which may
be in the order of several kilovolts, good design is essential for compatibility to
occur.
15.2 Simple source models
For many EMC situations such as coupling by radiation, effective prediction and
analysis are achieved by reference to simple mathematical expressions. For nearly all
products experiencing EMC problems, the equipment will work perfectly in the
development laboratory and only when it is subjected to external electromagnetic
disturbance do other facets of its characteristics emerge. Under these circumstances,
circuit conductors are considered as antennas capable of both transmitting and receiving
radio energy. The circuit can usually be assessed as either a short monopole antenna,
for instance where one end of the conductor is terminated in a high impedance, or as
a loop where both ends are terminated in low impedance.
For the monopole equivalent at low frequencies and at distances greater than a
wavelength, the field strength E, at distance d (in metres) is given by:
E=
Axd
IZ [ v o ~ m e t r e ]
where Z is the current in amps, h is the length of the conductor and A is the wavelength
(= 300 + frequency in MHz), both in metres. It can be seen that the field strength is
greater for shorter distances and higher frequencies (shorter wavelengths).
For loop radiators at low frequencies, the field is given by:
where n is the number of turns and A the area of the loop. The field strength is greater
at shorter distances and higher frequencies.
At high frequencies where the conductor lengths are comparable with a wavelength,
a good approximation of the field at a distance can be taken if the source is considered
as a half-wave dipole. The field is then given by:
Electromagneticcompatibility
378
E =7 4 F [voltdmem]
d
where P is the power in watts available in the circuit.
15.2.1
Receptor efficiency
To estimate the degree of coupling in the radiated path, empirical data give values of
induced currents of about 3 mA for an incident electromagnetic wave of 1 voltlmetre.
This relationship can be used to good effect in converting the immunity test levels in
the standards into an engineering specification for induced currents impressed at an
input port due to coupling via an attached cable.
15.3 Signal waveforms and spectra
For many digital electronic systems, the main concerns in emission control at low
frequencies are associated with power line disturbance generated by switch mode
power supplies. These devices switch at a relatively high rate, in the order of 30-100
kHz, and produce a line spectrum of harmonics spreading over a wide frequency
band as shown in Fig. 15.3.
yul
80
70
60
QP
I - - - - - - - - - - -
________-------
AV
dB (pV) 50
40
30
20
.15 .2 .3 .4 .5 .6.7.8.91.0
2
3 4 5 6 7 8 910
Frequency (MHz)
20 30
Fig. 15.3 Typical conducted emission spectrum of a switch mode power supply
With no mains filtering, the emission levels from the individual harmonics are
considerably in excess of the common emission limits. Care is needed in the sourcing
of these subassemblies to ensure that they are compliant with the relevant standards.
At higher frequencies, noise from digital circuits switching at very high rates
(clock frequencies in excess of 30 MHz are not uncommon) couples via external
cables and radiates in the VHF band (30-300 MHz) or it may radiate directly from
circuit boards in the UHF band (300-1000 MHz). A clock oscillator has a waveform
and a spectrum of the type shown in Fig. 15.4.
Newnes Electrical Engineer’s Handbook
’
379
Time
T
Waveform
1/nt,
1lnt,
Frequency spectrum
Fig. 15.4 Waveform and frequency spectrum of a digital signal
It can be seen that the turning points in the spectrum are ( l h ) x the pulse width
t,,,,above which frequency the spectrum decreases inversely proportionallyto frequency,
and ( l h ) x the rise time tr. At frequencies greater than l/mr the spectrum decays
rapidly, in inverse proportion to the square of the frequency. Thus for longer pulse
durations the high-frequency content is reduced; if the rise time is slow then the
content is further reduced. For good emission control, slower clock speeds and
slower edges are better for EMC. This is contrary to the current trends where there
are strong performance demands for faster edges and higher-frequency clocks.
Many equipment malfunction problems in the field are caused either by transient
disturbances, usually coupled onto an interface cable, or by radar transmitters if
close to an airfield. The disturbance generated by a pair of relay contacts opening
comprises a series of short duration impulses at high repetition rate, as illustrated in
Fig. 15.5. As the contacts separate the energy stored in the circuit inductance is
released, causing a high voltage to occur across the contacts. The voltage is often
sufficient to cause a discharge and a spark jumps across the gap. This is repeated
until the gap is too wide. When coupled into a digital electroniccircuit, the disturbance
can change the state of a device and interferencein the form of an unwanted operation
or circuit ‘lock-up’ occurs. Control is exercised by ensuring that the receptor circuit
has adequate immunity to this type of disturbance.
Time
Fig. 15.5 Voltage across opening relay contacts
-
380
Electromagneticcompatibility
The radar transmission is one example of a modulated radio frequency signal, and
this can often cause interference even in low-frequency electronic circuits. The radio
frequency energy is rectified at the first semiconductor junction encountered in its
propagation path through the equipment, effectively acting as a diode rectifier or
demodulator. The rectified signal, an impulse wave in the case of radar, is thus
processed by the circuit electronics and interference may result if the induced signal
is of sufficient amplitude, see Fig. 15.6.
I
RF
Radar signal pulse modulated RF
-l-Ku-L
___,
Time
Radar signal after circuit rectification
Fig. 15.6 Radar signal
Most equipment is designed and constructed to be immune to radio frequency
fields of 3 volts per metre (or 10 volts per metre for industrial environments) at
frequencies up to 1 GHz. Many radars operate at frequencies above I GHz (for
example, 1.2,3 and 9.5 GHz) and equipmentclose to an airfield m a y suffer interference
because it is not designed to be immune. Under these circumstances, architectural
shielding is required, often comprising the use of glass with a thin metallic coating
which provides adequate light transmittanceand, more importantly,effective shielding
at these high radio frequencies.
Newnes Electrical Engineer’s Handbook
381
15.4 EMC limits and test levels
15.4.1
Emissions
The radio frequency emission limits quoted in EMC standards are usually based on
the level of disturbance that can be generated by an apparatus or system such that
radio or TV reception in a co-located receiver is interference free.
15.4.1.1 Conducfed emissions
Conducted emission limits are set to control the disturbance voltage that can be
impressed on the voltage supply shared by the source and receptor where coupling
by conduction occurs. Limits currently applied in European emission standards are
shown in Fig. 15.7. The Class A limits are appropriate for a commercial environment
where coupling between source and receptor is weaker than in the residential
environment. where the Class B limits apply. The difference in the limits reflects the
difference in attenuation in the respective propagation paths.
66 -
EN55022 Class A
;;I
dB (PV)
50
46
EN55022 Class B
-
0.15
0.5
5
Frequency, MHz
30
Fig. 15.7 Conducted emission limits (average detector)
The limits shown apply when using the ‘average’ detector of the measuring
instrument, and are appropriate for measuring discrete frequency harmonic spectral
line emissions. A ‘quasi-peak’ detector is employed to measure impulsive noise for
which the limits are up to about 10 dB higher, i.e. more relaxed. The standards often
require that the ‘average’ limit is met when using the ‘average’ detector, and the
‘quasi-peak’ limit is met using this ‘quasi-peak’ detector. Nearly all modern EMC
measuring instrumentation contains provision for measuring using both detectors to
the required degree of accuracy.
Typical limit levels are in the order of 1-2 mV for Class A and 0.2-0.6 mV for
Class B. These are quite onerous requirements given that some devices such as triacs
for motor speed control may be switching a few hundred volts peak.
15.4.1.2 Radiated emissions
The radiated emission limits are derived from a knowledge of the field strength at the
fringe of the service area, from typical signal to noise ratios for acceptable reception,
and also from applying probability factors where appropriate. Typical limits are
shown in Fig. 15.8.
382
,
47 40
dB (pV/m)
-
Electromagnetic compatibility
EN55022 Class A
I
EN55022 Class B
37 -
30 -
I
30
230
Frequency, MHz
1000
Rg.15.8 Radiated emission limits
Below 230 MHz the Class B limits are equivalent to a field strength of 30 pV/
metre at a distance of 10 metres. Many items of information technology equipment
have clock frequencies in the range 10-30 MHz, for which the harmonics in the
ranges up to a hundred MHz or more are effectively wanted signals, required to
preserve the sharp edges of the digital waveform. This harmonic energy has to be
contained within the apparatus by careful PCB design and layout andlor shielding
and filtering.
15.4.1.3 Power frequency harmonics
Low-frequency limits for harmonic content are derived from the levels of disturbance
that the supply networks can tolerate, and are expressed either as a percentage of the
fundamental voltage or as a maximum current. Even harmonics are more strongly
controlled than odd harmonics, since the even harmonics indicate the presence of a
dc component. Similarly, flicker limits are set by assessing the effects on lighting of
switching on and off heavy power loads such as shower heaters.
15.4.2
Immunity
Immunity test levels are set to be representative of the electromagnetic environment
in which the equipment is intended to operate. In European standards, two environments
are considered, the residential, commercial and light industrial environment, and the
heavy industrial environment. The distinction between the two is not always clear,
but most equipment suppliers and manufacturers are knowledgeable on the range of
disturbances that must be considered for their product to operate reliably in the field,
and can make the appropriate choice without difficulty. The key factor is whether
heavy current switching andor high-power radio frequency sources are present. If
they are, the more severe ‘industry’ levels should be selected.
15.4.2.I
Electrostatic Discharge (ESD)
Electrostatic charge is built up on a person walking across a carpet or by other
actions where electric charge separation can occur. The charge voltage is much
higher for synthetic materials in dry atmosphereswith low relative humidity. Although
charge potentials in the order of 10-15 kV may be encountered in some environments
such as hotels, the standards bodies have selected an air discharge level of 8 kV as
Newnes Electrical Engineer’s Handbook
383
being representative of a broad range of circumstances. The ESD event is very fast
with a sharp edge having a rise time of about 1 nanosecond and a duration of about
60 nanoseconds. This generates a spectrum which extends into the UHF bands and
therefore presents a formidable test for many types of equipment.
15.4.2.2 Electrical fast transients
This is the disturbancetype adopted by the standardsbodies to be representativeof the
showeringarc discharge encounteredacross opening relay contacts.Figure 15.9 shows
the generalwaveformof the disturbanceappliedinthe harmonizedEuropeanstandards.
o.5r0.1
I
I
I
b
Time
5 ns f 30%
Individual pulse
115 msl
I
I
Burst period 300 ms
I
Wavetrain
Fig. 15.9 Waveform of fault transient
These transients are applied directly to the mains power conductor or to interface
and input/output cables via a capacitive coupling method to simulate the effects of
co-located noisy power conductors. The voltage levels applied on the mains supply
are 1 kV for the residential environment and 2 kV for the industrial environment.
384
Electromagneticcompatibility
15.4.2.3 Radio Frequency (RF) fields
At low frequencies (below 80 MHz) the interaction of incident electromagnetic
waves with receptor systems can be simulated effectively by a simple induced voltage
impressed either with respect to ground via a network known as a CDN,or longitudinally
via a transformer (bulk current injection). At higher frequencies (80-1000 MHz) the
field is applied directly to the equipment under test, usually by setting up a calibrated
transmitting antenna situated at a separation distance of about 3 m. The tests are
performed in shielded enclosures in order that the RF energy can be controlled and
no external interference occurs.
In the more recent standards the walls of the screened rooms are lined with
absorbing material to provide a reasonably uniform field within the chamber. The
applied waveform is modulated with either a 1H z tone to simulate speech or a pulse
train to represent digital cellular radio transmission.
15.4.2.4 Dips, surges and voltage interruptions
Many other types of disturbances are present in the environment. These are the slowspeed types which are usually associated with power switching, lightning pulses and
power failure. They are usually simulated by specialist disturbance generators. In
many cases, immunity to these disturbances is achieved by good design of the mains
power supply in the equipment under test.
15.4.2.5 Magnetic fields
Equipment containing devices sensitive to magnetic fields should be subjected to
power frequency (50 Hz) magnetic fields. Typical levels are 3 ampdmetre for the
residential, commercial and light industrial environment and 30 ampdmetre in the
more severe industrial environment.
15.5
Design for EMC
15.5.1
Basic concepts
The preferred and most cost-effective approach to the achievement of electromagnetic
compatibility is to incorporate the control measures into the design. At the design
inception stage some thought should be given to the basic principles of the EMC
control philosophy to be applied in the design and construction of the product. The
overall EMC design parameters can be derived directly or determined from the EMC
standards to be applied, either as part of the procurement specification or as part of
the legal requirements for market entry. There are usually two fundamental options
for product EMC control:
0
shielding and filtering (see Fig. lS.lO(a))
board level control (see Fig. 15.10(b))
For the shielding andPlter solution, all external cables are either screened leads,
with the screens bonded to the enclosure shield, or unscreened leads connected via
a filter. The basic principle is to provide a well-defined barrier between the inner
surface of the shield facing the emissions from the PCBs and the outer surface of the
Newnes Electrical Engineer’s Handbook
385
Shielded
enclosures
/
Screened
lead
/
Unscreened
lead
Filter
I
I
I
I
I
I
I
I
Filters
i
Unscreened
lead
*Can be shielded
(b) Board level control solution
Fig. 15.10 Options for product EMC control
shield interacting with the external environment, This solution requires the use of a
metal or metal-coated enclosure.
Where this is not possible, or not preferred, then the board level control option is
appropriate, where the PCB design and layout provides inherent barriers to the
transfer of electromagnetic energy. Filters are required at all the cable interfaces,
except where an effective screened interface can be utilized.
Electromagnetic compatibility
386
15.5.2
Shielding
For design purposes, effective shielding of about 3040 dB can be provided at high
frequencies by relatively thin metal sheets or metal coatings on plastic. The maximum
shielding achievable is associated with the apertures, slots and discontinuities in the
surface of the shield enclosure. These may be excited by electromagnetic energy and
can resonate where their physical length is comparable with a wavelength, significantly
degrading the performance of the shield. The following basic rules apply:
0
0
0
0
the maximum length of a slot should be no greater than 1/40of the wavelength
at the highest frequency of concern
a large number of small holes in the shield gives better performance than a
small number of large holes
the number of points of contact between two mating halves of an enclosure
should be maximized
mains or signal line filters should be bonded to the enclosure at the point of
entry of the cable
15.5.3 Cable screens termination
Ideally for maximum performance, cable shields should be terminated at both ends
with a 360" peripheral (i.e. glanded) bond. This is not always possible and in some
cases it is undesirable because of the associated ground loop problem, Fig. 15.11.
Noise currents, I, in the ground generate a voltage V between the two circuits A and
B which can drive a high current on the outer surface of the interconnecting screen,
thus permitting energy to be coupled into the internal system conductors.
Fig. 15.11 The ground loop problem for screen terminated at both ends
In many applications involving the use of long conductors in noisy environments,
the simplest solution is to break the ground loop by bonding the screen at one end
only, as shown in Fig. 15.12. Here the ground noise voltage is eliminated but the
shield protects only against electric fields and capacitive coupling. Additional or
Newnes Electrical Engineer’s Handbook
387
No screen termination
Fig. 15.12 Ground loop broken by termination of the screen at one end onty
alternative measures such as opto-isolation are required if intense magnetic fields are
present.
15.5.4
PCB design and layout
Control at board level can be achieved by careful design of the board involving
device selection and track layout. As discussed in section 15.2, emissions from the
PCB tracks may be reduced at high frequencies if the devices are chosen to have
slow switching rates, and slow transition (i.e. long rise and fall) times.
Device selection can also improve immunity by bandwidth control. The smaller
the bandwidth, the less likely it is that high frequency disturbances will be encountered
within the passband of the circuit. Although rectification of the disturbance may
occur in the out-of-band region, the conversion process is inefficient and higher
immunity usually results.
The tracks on PCBs can act as antennas and the control methods involve reducing
their efficiency. The following methods can be applied to good effect:
0
0
0
0
reduce the area of all track loops
minimize the length of all high-frequency signal paths
terminate lines in resistors equal to the characteristic impedance
ensure that the signal return track is adjacent to the signal track
remove the minimum amount of coppei on the board, Le. maximize the surface
area of the OV (zero-volt ground) and VCC (power) planes
The latter two points are generally achieved where a multi-layer board configuration
is employed. These measures are highly effective at reducing board emissions and
improving circuit immunity to external disturbances. Multi-layerboards are sometimes
considered relatively expensive but the extra cost must be compared with the total
388
Electromagneticcompatibility
costs of other measures that may be required with single or double-sided boards,
such as shielding, filtering and additional development and production costs.
15.5.5 Grounding
Grounding is the method whereby signal returns are managed, and it should not be
confused with earthing, which deals with protection from electrical hazards. Grounding
is important at both the PCB level and circuit interconnection level. The three main
schemes are shown in Fig. 15.13.
EqY
Single point ground
Series ground
Multipoint ground
Fig. 15.13 Three main grounding schemes
In the series ground scheme, noise from circuit A can couple into circuit C by the
common impedance Z. This problem is overcome in the single point ground, but the
scheme is wasteful of conductors and not particularly effective at high frequencies
where the impedance of the grounding conductors may vary, and potential differences
can be set up.
The ideal scheme is the multipoint ground. Generally, single point grounding is
used to separate digital, analogue and power circuits. Multipoint grounding is then
used whenever possible within each category of analogue or digital. Some series
ground techniques may be employed where the coupled noise levels can be tolerated.
Usually the overall optimum solution is derived from good basic design and successive
experimentation.
15.5.6
Systems and installations
The general principles discussed above may be applied at a system or installation
level. Guidance on this topic is available in references 15B and 15C.
15.6 Measurements
15.6.1
Emissions
Conducted emission tests comprise measurements of voltage across a defined network
which simulates the RF impedance of a typical mains supply. These Line Impedance
Stabilizing Networks (LISNs) also provide filtering of the supply to the Equipment
Under Test (EUT) and are also known as artificial mains networks or isolating
networks. The EUT is connected to the LISN in a manner which is representative of
its installation and use in its intended environment. Figure 15.14 shows the general
arrangement.
Newnes Electrical Engineer’s Handbook
389
f
40 cm
1.
Ground plane
Fig. 15.14 Measurement of conducted emissions
The EUT is configured in a typical manner with peripherals and inputs/outputs
attached, and operated in a representative way which maximizes emissions.
Radiated emissions are made by measuring the field strength produced by the
EUT at a defined distance, usually 3 m or 10 m. The measurements are made on an
open area test site which comprises a metallic ground plane, over a flat surface with
no reflecting objects and within a defined ellipse.
The ground plane should cover a larger area than the test range, for example a 6
x 9 m area would be ideal for a 3 m range, and a 10 x 20 m area for a 10 m range.
The EUT is situated 1 m above the ground plane on an insulating support (unless it
is floorstanding equipment) and a calibrated antenna is placed at the required test
distance from the EUT. At any emission frequency, such as the harmonic of the clock
oscillator in a PC, the receiver is tuned to the frequency and the antenna height is
raised between 1 and 4 metres in order to observe the maximum field strength
radi\ated by the product. (The net field strength is the sum of the direct and groundreflected waves and it varies with height.) The EUT is also rotated about a vertical
axis in order to measure the maximum radiation in the horizontal plane.
Measuring instruments for both conducted and radiated emission measurements
comprise spectrum analysers or dedicated measuring receivers. The spectrum analyser
usually has to be modified to have a stage for preselection which prevents overload
and damage in the presence of impulsive noise, and it may require additional external
pulse-limitiug protection when performing conducted emissions measurements with
a LISN. Both instruments usually have facilities of computer control by the IEEE
bus, avoiding the necessity for manual operation. When using spectrum analysers it
is important to check for overload or spurious emissions by ensuring that the observed
indication on the display reduces by 10 dB when an additional 10 dB RF attenuation
is introduced at the front end of the analyser.
EMC measuring receivers are designed to meet the stringent requirements of
CISPR (Committee International Special Perturbations RadioClectrique),a subdivision
of the IEC. This sets out specifications for input impedance, sensitivity, bandwidth,
detector function and meter response, such that the reproducibility of tests can be
guaranteed.
15.6.2 Immunity
ESD tests are made with an ESD ‘gun’, set to the desired voltage which is equivalent
to the human charge potential, and having well-defined charge and discharge
Electromagneticcompatibility
390
characteristics.The ESD discharge is applied to all user-accessibleparts of the EUT.
The operation of the equipment is thus observed for any malfunction. Immunity to
the ESD event is improved by minimizing the ESD energy that can enter the enclosure
containing the electronics.The ideal solution is either a good shielded enclosure with
s m a l l apertures and good bonding between sections, or a totally non-conducting
surface. Generally it is difficult to design a product which completely satisfies either
solution, but designers should attempt to steer towards one or the other.
Measurements of immunity to RF fields are made in a shielded enclosure, the
modern types being lined with absorbing materials, such as ferrite tiles on at least
five of the six inner surfaces. The EUT is subjected to radiation from an antenna
situated in the near vicinity as shown in Fig. 15.15. The field is precalibrated to the
required level of field strength specified in the appropriate standard, prior to the
introduction of the EUT into the chamber. The RF frequency is swept slowly from
80-1000 MHz and any equipment misoperationsnoted, the performance level of the
EUT having been defined prior to the start of the test.
Ferrite tiles
I
___---------------I
I
I
Screened room
I
I
_ -__
I
I
Fig. 15.15 RF field immunity-test arrangement
Transients, surges, dips and interruptions tests are performed with dedicated test
instrumentation which fully satisfies the requirements of the relevant standards.
Generally the tests are much quicker to perform than the RF field test, and information
on the EMC performance of the EUT can be gathered rapidly.
For ESD tests and fast transient tests the equipment should carry on working after
the application of the disturbance without any loss of data. For the RF field test there
should be no loss of performance outside that specified by the manufacturer at any
timeduring the test. For dips and surges etc., provided the equipmentworks satisfactorily,
after a manual reset, it will be deemed to have passed the test.
15.7 Standards
The three types of EMC standards in current use in Europe are basic standards,
generic standards and product-specific standards. Basic standards contain the test
methods and test levels at limits but do not specify a product type. Product standards
contain comprehensivedetails on how the product should be configured and operated
during the test and what parameters should be observed. Generic standards apply in
the absence of a product standard and are relevant to all products which may be
Newnes Electrical Engineer’s Handbook
391
operated within a defined environment. Both product standards and generic standards
may refer to basic standards for their test methods. The important standards in
current use in Europe are listed below.
15.7.1 Generic standards
EN 50 08 1-1 :Emissions; Residential, Commercial and Light Industrial Environment
EN 50 08 1-2 : Emissions; Industry Environment
EN 50 081-1 : Immunity; Residential, Commercial and Light Industrial Environment
EN 50 081-3 : Immunity; Industrial Environment
15.7.2
Important product standards
EN 50011 : Emissions, Industrial, Scientific and Medical Equipment
EN 50013 : Emissions, Radio and TV Equipment
EN 50014-1 : Emissions, Household Appliances and Portable Tools
EN 50015 : Emissions, Lighting Equipment
EN 50022 : Emissions, Information Technology Equipment
EN 55014-2 : Immunity,Household Appliances
EN 55020 : Immunity, Radio and TV Equipment
EN 55024 : Immunity, Information Technology Equipment
15.7.3 Basic standards
EN 61000-4-2 : Immunity: ESD
EN 61000-4-3 : Immunity: R F fields
EN 61000-4-4 : Immunity: fast transients
EN 61000-4-5 : Immunity: surges
EN 61000-4-6 : Immunity: conducted RF voltages
EN 61000-4-8 : Immunity: magnetic fields
EN 61000-4-11 : Immunity: dips and interruptions
Other product standards are emerging at a high rate which will mean that reliance on
generic standards will diminish significantIy in the future.
References
Marshman, C . The Guide to the EMCDiivctive 89/336/EEC! EPA Press, 1992, ISBN 095173623X.
308 pp.
l5B. IbIaddocks, A.J.. Dum, J.H. and Hicks. G.P. Designing for Elecrromagnetic Conipntibilig: A
Practical Gciide, ERA Report 95-0030, Leatherhead, ERA Technology Ltd, March 1995. ISBN
0700805842, 147 pp.
15C. Goedbloed, J. Electromagnetic Compatibiliry, Prentice Hall, 1992, ISBN 0132492938,400 pp.
15D. Ott, H.W.Noise Reduction Techniques in Electronic Systems (2nd edn),John Wiley. USA, 1988,
ISBN 0471850683.426 pp.
15A.
Chapter 16
Hazardous area equipment
Mr K. Morris
CEAG Crouse-Hinds
16.1 Introduction
In areas where explosive mixtures of gas and air can be present, sparks given off by
electrical equipment or hot surfaces on this equipment constitute a potential hazard,
and the consequences of an explosion can be disastrous.
Combustible materials can form explosive atmospheres and can under certain
circumstances cause an explosion. Such materials are used widely in the chemical,
mining and other industries and even in everyday life. The concept of ‘explosion
protection’ of electrical equipment has been developed and formalized in order to
prevent explosive accidents in hazardous areas during the normal operation of the
equipment.
The coal mining industry has provided one of the main pressures for the development
of special equipment, procedures, standards and codes because of its especially
hazardous working environments. Davy’s safety lamp for miners is an example of a
piece of equipment developed specifically for use in hazardous areas. The legacy of
the importance of this industry remains today through the difference in the regulations
which apply to mining and to other hazardous areas. Apart from mining, hazardous
areas are found especially in offshore gas and oil installations and in petrochemical
complexes.
There are numerous standards and codes of practice governing the manufacture,
selection, installation and maintenance of electricalequipment in potentially hazardous
areas. These tend to differ around the world and despite harmonization across the
European Union, the complexity can be intimidating. A summary is presented in
section 16.6. Because of the special implications for safety, equipment and systems
for use in hazardous areas must be tested and certified by approved authorities;
certification is covered in section 16.5.
16.2 Hazardous areas
Hazaxdous areas are classified into zones according to the nature of the gases present
in the potentially explosive atmosphere, and the likelihood of that atmosphere being
present. The nature of the atmosphere is characterized by the chemical composition
of the gas and its ignition temperature, and the notions of gas grouping and temperature
classificationhave been developed in order to formalize this.
A useful concept in the consideration of how explosions occur is the hazard
Hazardous area equipment
394
triangle shown in Fig. 16.1. The sides of the triangle represent fuel, oxygen and a
source of ignition, all of which are required in order to create an explosion. For the
purposes of this chapter, a fuel is considered as a flammable gas, vapour or liquid,
although dust may also be a potential fuel. Oxygen is of course present in air at a
concentration of about 21 per cent. The ignition source could be a spark or a high
temperature. Given that a hazardous area may contain fuel and oxygen, the basis for
preventing explosion is to ensure that any ignition source is either eliminated or
prevented from coming into contact with the fuel-oxygen mixture.
Ignitionsource
Fig. 16.1 The ‘hazard triangle’
16.2.1
Zone classification
The zone classification defined in IEC 79 is used in Europe and most other parts of
the world; it is summarized in Table 16.1. Various types of explosion protection are
available, and their suitability for the different zones is shown in the table.
Table 16.1 IEC 79 classification of hazardous area zones
Suitable protection
Zone 0
Areas in which hazardous explosive gas
atmospheres are present constantly or
for long periods, for example in pipes
or containers
Ex ‘ia’
Ex ‘s’(where specially
certified Zone 0)
Zone 1
Areas in which hazardous explosive gas
atmospheres are occasionally present,
for example in areas close to pipes or
draining stations
Ex Id‘; Ex ‘ib’;
Ex ‘p’; Ex ‘e’;
Ex ‘s’;
Ex ‘0’;
Ex ‘q’; Ex ‘m’;
Equipment suitable for
Zone 0
Zone 2
Areas in which hazardous explosive gas
atmospheres are rare or only exist for a
short time, for example areas close to
Zones 0 and 1
Ex ‘N’/Ex ‘n’;
Equipment suitable for Zones
1& O
Newnes Electrical Engineer’s Handbook
395
In the USA hazardous areas are classified in a slightly different way, according to
the National Electrical Code. In brief, hazardous areas are classified either as Division
I! where ignitable concentrations of flammable gases or vapours may be present
during normal operation, or as Division 2, where flammable gases or vapours occur
in ignitable concentrations only in the event of an accident or a failure of a ventilation
system.
16.2.2
Gas grouping and temperature classification
The energy required for ignition differs from gas to gas, and the grouping of gases
together with their classification by temperature is used in Europe to describe the
suitability of a piece of electrical equipment for use with explosive atmospheres of
particular gases.
Table 16.2 lists common industrial gases within their appropriate groups. Gas
group I is reserved for the classification of equipment suitable for use in coal mines.
Gas group I1 contains those gases found in other industrial applications, and it is
subdivided according to the relative flammability of the most explosive mixture of
the gas with air.
Table 16.2 CENELEC/IEC gas grouping
Group
Representative gases
I
IIA
Methane
Acetone, ethane, ethyl acetate, ammonia, benzol,
acetic acid, carbon monoxide, methanol, propane,
toluene, ethyl alcohol, I-amyl acetate, N-hexane,
N-butane,N-butyl alcohol, petrol, diesel, aviation fuel,
heating oils, acetaldehyde, ethyle ether
Town gas, ethylene (ethene)
Hydrogen, acetylene (ethyne), hydrogen disulphide
IIB
IIC
Temperatures are classified from T1 to T6, as shown in Table 16.3. The levels
show the maximum surface temperature permitted for an equipment which has been
assigned that temperature class, and the common gases for which each class is
appropriate are also shown.
North American practice is to define hazardous materials in classes. Flammable
gases and vapours are Class 1 materials, combustible dusts are Class 2 materials and
Table 16.3 CENELECDEC temperature classification
Class
Highest permissible
surface temperature
(“C)
Representative gases
T1
450
T2
300
T3
T4
T5
T6
200
135
100
85
Acetone, ethane, ethyle acetate, ammonia, benzol,
acetic acid, carbon monoxide, methanol, propane,
toulene, town gas, hydrogen
Ethyl alcohol, (-amyl acetate, N-hexane, N-butane,
N-butyl alcohol, ethylene
Petrol, diesel, aviation fuel, heating oils
Acetaldehyde, ethyl ether
Hydrogen disulphide
Hazardous area equipment
396
‘flyings’, such as sawdust, are Class 3 materials. Class 1 is subdivided into four
groups depending on their flammability: A (including acetylene), B (including
hydrogen), C (including ethylene) and D (including propane and methane). The
subgroup letters are in the opposite order of flammability to the IEC groupings
shown in Table 16.2. The North American temperature classification is similar to the
IEC system shown in Table 16.3, but the classes are further subdivided to give more
specific temperature data.
16.2.3
Area assessment procedure
Companies using flammable materials carry out an area assessment exercise in
accordance with national standards such as BS 5345, Pt 2 in the UK or relevant
industry codes such as those existing in the petroleum and chemical industries. In
general, this assessment procedure results in a written report which identifies and
lists the flammable materials used, records all the potential hazards with their source
and type, identifies the extent of the zone taking into account for instance the type of
potential release of flammable material and ventilation systems, and includes other
relevant data. This procedure is complicated, and there are specialist companies
which offer a commercial hazardous area assessment service.
16.3 Protection concepts for electrical equipment
There is a need for electrical power in hazardous areas to supply motors, lighting,
control equipment and instrumentation, and a range of equipment is available which
has been tested and certified to the appropriate standards. Equipment that has been
designed and tested in Europe usually carries a string of codes which gives information
about its suitability for use, and also carries the mark of the certifying body.
The definitions of gas group and temperature classification have already been
explained. The following sections describe the basis of the protection code.
According to the harmonized European standard EN 50014, electrical equipment
for use in explosive atmospheres can be designed with various protection concepts;
these are listed in Table 16.4. Also listed in the table are the Ex ‘N’ and Ex ‘s’
concepts which have not been the subject of European harmonization. They are not
covered in EN 50014 and do not attract the EEx mark signifying certification to a
harmonized European standard.
The engineer designing an electrical system has to make a choice regarding the
method of protection to be used, and has to select apparatusand components accordingly.
Table 16.4 Types of protection
Protection concept
Designation
Oil immersion
Pressurized
Quartz-sand filled
Flameproof enclosure
Increased safety
Intrinsic safety
Encapsulation
Non-sparkinglincendive
Special protection
EEx ‘0’
EEx ‘p’
EEx ‘q’
EEx ‘d’
EEx ‘e’
EEx ‘i’
EEx ‘m‘
EXn rN’l‘n’
Ex ‘s’
Newnes Electrical Engineer’s Handbook
397
(Ex)
Fig. 161 The EN conformity mark for certified explosion-protectedequipment
In practice, the majority of electrical equipment for use in hazardous areas will be
designated according to the EEx ‘d‘, EEx ‘e’, Ex ‘N/‘n’ or EEx ‘i’ concepts and
these four are therefore highlighted in the following sections.
16.3.1
Flameproof enclosure
- EEx ‘d’
In this concept, those parts of the electrical equipment that can ignite an explosive
air-gas mixture are contained in an enclosure. The enclosure can withstand the
pressure created in the event of ignition of explosive gases inside it, and can prevent
the communication of the explosion to the atmosphere surrounding the enclosure.
The rationale is the containment of any explosion which may be created by the
equipment. The concept is therefore applicable to virtually all types of electrical
apparatus, provided that the potential sparking or hot elements can be contained in a
suitably sized and sufficiently strong enclosure.
The factors taken into account by equipment manufacturers and system designers
include:
0
arc and flame path lengths and types
surface temperature
internal temperature with regard to temperature classification
distance between components in the enclosure and the enclosure wall (12 mm
minimum is specified by the standard)
Some types of component are unsuitable for use in a flameproof enclosure. These
include rewireable fuses and components containing flammable liquids.
A major consideration in the use of EEx ‘d’ enclosures is the making of flameproof
joints, which must be flanged, spigotted or screwed. The maximum gaps and minimum
lengths of any possible flame path through ajoint are defined by the standard, which
also lays down requirements for the pitch, quality and length of screw threads. The
nature of the construction required is shown in Fig. 16.3 which illustrates an EEx ‘d’
induction motor.
The certification for EEx ‘d’ equipment involves examination of the mechanical
strength of the enclosure, and explosion and ignition tests under controlled conditions.
16.3.2
Increased safety
- EEx ‘e’
Measures are taken in this concept to prevent sparks or hazardous temperatures in
internal or external parts of the electrical equipment during normal operation. The
guiding philosophy here is the prevention of explosion from normally non-sparking/
arcing or hot equipment.
Hazardous area equipment
398
Fig. 16.3 Sectional illustration of an EEx ‘d’ induction motor (courtesy of Invensys Brook Crompton)
No sparking devices can be used, and various electrical, mechanical and thermal
methods are used to increase the level of safety to meet the certificationtest requirements.
One advantage of increased safety protection compared with flameproof protection
is that boxes and enclosures can be made of plastics and other materials that are
easier to work with. Examples of equipment commonly designed and constructed to
EEx ‘e’ protection include luminaires, terminal boxes and motors. Examples of an
EEx ‘e’junction box, emergency luminaire and cable gland are shown in Figs 16.4,
16.5 and 16.6 respectively.
Key design considerations for EEx ‘e’ equipment are the electrical, physical and
thermal stability of the materials and the compatibility of different materials which
might be used for items such as terminations. Specialist manufacturers have been
ingenious in the design of electrical terminations for EEx ‘e’ equipment to ensure
firm, positive and maintenance-free connection of conductors.
16.3.3 Non-sparking
- Ex ‘n’
This is a British designation for electrical equipment, commonly known as ‘Type N’,
which is suitable for use in Zone 2 applications, but not in Zone 1. It is not yet
incorporated into harmonized European standards and does not therefore carry the
EEx designation.
The concept combines certain aspects of EEx ‘e’ and EEx ‘i’ protection in its use
of ‘non-incendive’circuit elements and in the design for increased safety. Equipment
manufactured with Type N protection includes terminal boxes, luminaires and motors.
Because of the less hazardous nature of locations assessed as Zone 2 and the
Newnes Electrical Engineer’s Handbook
399
Fig. 16.4 EEx ‘e’ junction box and mounting plate (courtesy of CEAG Crouse-Hinds)
consequently less demanding requirements of the standard, this equipment can be
manufactured more simply and at less cost than other types of explosion-protected
equipment for Zone 1 use.
16.3.4
Intrinsically safe
- Ex ‘i’
In an Intrinsically Safe (IS) equipment, under normal operation and certain specified
400
Hazardous area equipment
Fig. 16.5 EEx‘e’ Zone 1 emergency luminaire (courtesy of CEAG Crouse-Hinds)
Fig. 16.6 Deluge-protectedcable gland with integral entry thread ‘0’
ring seal (courtesy of CMP Products)
Newnes Electrical Engineer’s Handbook
401
fault conditions, no sparks or thermal effects are produced which can cause ignition
of a specified gas atmosphere. Sparks or thermal effects are not produced because the
energy in the IS circuits is very low. IS circuits are control and instrumentation
circuits rather than power circuits.
According to the Code of Practice in BS 5345, Pt 4, in order to be defined as a
‘simple apparatus’ the maximum stipulated voltage that a field device can generate
is 1.2 V, with current, energy and power not exceeding 0.1 A, 20 mJ and 25 mW
respectively. IS circuitry exceeding these ratings may still meet the requirements of
EEx ‘i’, but it requires certification.
There are two types of EEx *i’protection, these being EEx ‘ia‘ and EEx ‘ib’.
EEx ‘ia’ equipment will not cause ignition in normal operation, with a single fault
and with any two faults. A safety factor of 1.5 applies in normal operation and with
one fault, and a safety factor of 1.0 applies with two faults. EEx ‘ia’ equipment is
suitable for use in all zones, including Zone 0. An example of EEx ‘ia’ equipment is
the pressure transmitter shown in Fig. 16.7.
EEx ‘ib’ equipment is incapable of causing ignition in normal operation and with
a single fault. A safety factor of 1.5 applies in normal operation and with one fault.
A safety factor of 1.0 applies with one fault if the apparatus contains no unprotected
switch contacts in parts which are likely to be exposed to the potentially explosive
atmosphere. and the fault is self-revealing. EEx ‘ib’ equipment is suitable for use in
all zones except Zone 0.
Components for IS circuits contain barriers to prevent excessive electrical energy
from entering the circuit. The two principal types are Zener barriers, which are used
when an IS earth is available, and galvanically isolated barriers, which are used when
an IS earth is not available. An IS earth must be provided by a clearly marked
conductor of not less than 4 mm* cross-section and with an impedance no greater
than 1 R from the barrier earth to the earth on the main power supply.
Energy can be stored in the inductance and capacitance of a cable, and this must
be taken into account when designing an IS circuit. This is achieved by strict control
of capacitance and LIR ratios in conjunction with Zener barriers or galvanic isolators.
BS 5308 covers polyethylene-insulated cables for use in petroleum refineries and
related applications and the PVC-insulated cables which are widely used in chemical
and industrial applications, and cables meeting this specification may be suitable for
Group 11 IS systems.
16.3.5
-
Pressurized EEx ‘p’
This type of protection uses air or inert gas to maintain a positive pressure within the
enclosure or room. The positive pressure prevents the entry of flammable gas or
vapour into the protected area.
An alternative method is to reduce the volume of gas or vapour within the enclosure
or room below the explosive level of the gas-air mixture by dilution from a clean
external source. The main features of a pressurization installation are shown in
Fig. 16.8, and Fig. 16.9 shows the layout of a leakage compensation unit.
16.3.6
Oil immersion
- EEx ‘0’
This rarely encountered protection refers to apparatus in which the ignition of a gasair mixture is prevented by immersing the live or sparking equipment in a specified
minimum depth of oil. The necessary depth is determined by testing.
Hazardous area equipment
402
Fig. 16.7 EEx ‘ia’ field-cased pressure transmitter (courtesy of WIKA Instruments Ltd)
16.3.7 Quartz-sand filled
- EEx ‘q’
EEx ‘q’ protected apparatus has the live or sparking elements immersed in granular
quartz or other similar material.
16.3.8
Encapsulation - EEx ‘m’
In this form of protection, potential sources of ignition are encapsulated to prevent
them from coming into contact with explosive atmospheres.
16.3.9 Special protection - Ex ‘s’
Ex ‘s’ equipment has been shown by test to be suitable for use in the appropriate
Newnes Electrical Engineer's Handbook
403
Purge air or inert gas supply
Explosion-proof
H I
I
'
Purging inlet
fa
I
11
Enclosure pressure
sensing connection
1power and alarm box
(direct rear entry to PE)
*Alarm signal
Power supply
Purge flow switch signal tube
Relief valve and
-
Pneumatic
connections
Electrical
connections
Fig. 16.8 Main features of a pressurization installation (courtesy of Expo Safety Systems Ltd)
zone, althoughthe apparatus does not comply with the standard of any of the established
concepts previously described.
16.4 Installation, inspection and maintenance
In addition to giving information on the selection of electrical equipment for use in
hazardous areas, BS 5345 also contains guidance on the installation, inspection and
maintenance of equipment. The code is divided into nine parts. Part 1 gives general
guidance and part 2 covers the classification of hazardous areas. Each of the remaining
parts is specific to one of the types of protection concept. It contains useful general
information concerning electrical work in hazardous areas, but it does not cover
work in mines or areas where explosive dusts may be present, and it is not intended
to replace recommendations which have been produced for specific industries or
particular applications.
In general, operation and maintenance should be taken into account when designing
process equipmentand systems in order that the release of flammablegases is minimized.
For example, the requirement for routine opening and closing of parts of a system
should be borne in mind at the design stage.
No modifications should be made to plant without reference to those responsible
for the classification of hazardous areas, who should be knowledgeable in such
matters. Whenever equipment is reassembled, it should be carefully examined. The
code gives recommended inspection schedulesfor equipmentof each type of protection
concept. These schedules set out what should be inspected on commissioning and at
periodic intervals. For all equipment, the protection type, surface temperature class
Purge time control
Air supply filter
Logic pressure
\
Pressurized enclosure
pressure test point
Power, alam and
‘manual override’
terminals (EEx i)
‘Action on pressure
failure’ selection
High purge flow ,
sensor
‘Pressurized’ switch
(EEx i)
\
‘Purge complete’
switch (EEX i)
\
High purge flow
control
Leakage compensation
valve
Fig. 16.9 Internal view of a leakage compensation unit (courtesy of Expo Safety Systems Ltd)
Minimum pressure sensor
Newnes Electrical Engineer’s Handbook
405
and gas group should be checked to ensure that the equipment is suitable for its zone
of use, and circuit identification should also be checked.
Some of the areas of recommendation of the code are presented below, in particular
from those parts relating to EEx ‘d’, EEx ‘e’ and EEx ‘i’ equipment, but these
highlights can in no way replace the code itself. The electrical engineer requiring
detailed and specific guidance on the installation, inspection and maintenance of
electrical equipment in hazardous areas should consult BS 5345 or other codes
relevant to specific industries or applications.
16.4.1
Flameproof EEx ‘d’ equipment
For installationsmaking use of flameproof enclosures, the code recommends ensuring
that solid obstacles such as steelwork, walls or other electrical equipment cannot be
close to flanged joints or openings. Minimum clearance distances of up to 40 mm are
given.
Gaps should be protected against the ingress of moisture with approved nonsetting agents, and extreme care should be taken in the selection of these agents to
avoid the potential separation of joint surfaces.
The code specifies the type of threads to be used for entry tappings into flameproof
enclosures, and it is stressed that directions contained in the certification documents
for cable systems and terminations should be followed. The types of cables suitable
for use and their appropriate methods of entry are also specified. EEx ‘d’ equipment
with integral cables where the cable terminations are encapsulated must be returned
to the manufacturer if maintenance is required.
16.4.2
Increased safety EEx ‘e’ equipment
The section of the code dealing with EEx ‘e’ equipment includes a recommendation
that the ratings of lamps are correct, since these may have been replaced. An appendix
gives guidance on the chemical influence of combustible gases on certain mechanical
and electrical properties of any insulating materials such as panels, gaskets or
encapsulation.
An appendix covering cage induction motors and their associated protection
equipment gives recommendations which are intended to ensure that all parts of the
motor do not rise above a safe temperature.
16.4.3 Intrinsically safe EEx ‘i’ equipment
The part of the code dealing with EEx ‘i’ equipment pays particular attention to the
interconnecting cables used in IS systems. It recommends, for instance:
0
0
0
minimum conductor sizes to ensure temperature compliance in fault conditions
specific separations between individual IS circuits and earth
insulation thicknesses
screening and mechanical properties of cables
The use of multicore cable is considered, as is the siting of cables to avoid potential
induction problems. In general, cable entries should be designedto minimize mechanical
damage to cables.
With all IS equipment,the need to follow the specific requirementsof the certification
documents is emphasized and it is recommended that during inspection attention is
Hazardous area equipment
406
paid to lamps, fuses, earthing and screens, barriers and cabling. Certain specified onsite testing and maintenance of energized IS circuits is permitted inside the hazardous
area, provided that the test equipment is certified as inmnsically safe in itself, and
that conditions on the certification documents are followed.
16.5 Certification
The certification process involves an assessment of the equipment with regard to its
conformity to the specific standard sought, an examination of a prototype to ensure
that it complies with the design documents, and testing. The certified design is
defined in a set of approved drawings listed in the certificate.
Within Europe there are national authoritiesor test houses which issue certification
documents for electrical equipment in order to prove that the equipment meets a
specific standard for explosion protection. Table 16.5 lists these authorities by country.
In the UK BASEEFA and SlRA are the accredited authorities. As well as being
accepted throughout the European Union, certification by BASEEFA and SIRA is
also recognized in other parts of the world, especially the Middle East and Far East.
Table 16.5 Certifying authorities
Country
Certifying authority
Belgium
Canada
Denmark
France
INIEX
CSA
DEMKO
CERCHAR
LClE
PTB
CESl
NEMKO
LOM
SEMKO
BASEEFA
SlRA
MECS (for mining applications)
FM
Germany
Italy
Norway
Spain
Sweden
UK
USA
Certification to the relevant standard for use in mines is carried out in the UK by
MECS. The procedure is similar to the certification to standards for other hazardous
areas, but in addition to explosion protection requirements, electrical equipment
must also provide the high degree of electrical, mechanical and operational safety
(pitworthiness requirements) demanded by the Mines Inspectorate.
Once a product has been tested and certified to a specific standard by an authorized
test house, the equipment manufacturer may then certify that product under licence
from the test authority. Equipment certified as meeting the European Norms carries
the distinctivehexagonal conformity mark shown in Fig. 16.2. Individual components
usually have conditions for safe use attached to their certificate. A ‘certificate of
conformity’for a completepiece of electrical apparatus allows installation in hazardous
areas without further verification.
16.5.1
ATEX Directive
The current procedure for manufacturersintroducing products to the market has been
Newnes Electrical Engineer's Handbook
407
outlined in the previous section. In future, however, manufacturerswill follow Directive
94/9/EC, dated 23.3.1994. This came into effect on 1.3.1996, and will become mandatory
from 1.7.2003. This new Directive has a wider scope and differs from the current
procedure in the following respects:
CE marking must be applied with explosion-protected marking
mining and surface gas groups are addressed
electrical and mechanical equipment is covered
equipment categories (1-3) are included
the issue of dust is addressed
The Directive places more emphasis on continued compliance and does not allow for
different interpretations.
Installers and operators will not see a great change in equipment, as most electrical
equipment currently on the market will comply with the new Directive. They will see
different equipment markings, with more information being included on equipment
labels.
16.6 Standards and codes of practice
As in many areas of industry, European Norms (EN) exist alongside equivalent
British Standards. The ENS,with equivalent BS and IEC references are shown for the
different explosion protection concepts in Table 16.6. The origins of the ENS are in
the European Directives published in 1975 and 1979concerning electrical equipment
Table 16.6 International, regional and national standards relating to electrical equipment for hazardous
areas
IEC
IEC 79-0
IEC 79-0
IEC 79-6
IEC 79-2
IEC 79-5
IEC 79-1
IEC 79-7
IEC 79-11
EN
EN 50014
EN 50015
EN 50016
EN 50017
EN 50018
EN 50019
EN 50020
EN 50039
EN 50028
BS
Subiect of standard
5345-2
5501-1
5501-2
5501 -3
5501-4
5501 -5
5501 -6
5501 -7
5501-9
5501-8
4683-3
SFA 3009
Zone classification
General requirements
Oil immersion
Pressurized
Quartz-sand filled
Flameproof enclosure
Increased safety
Intrinsic safety
Intrinsic safety
Encapsulation
Non-sparking/incendive
Special protection
Of special relevance to cables:
IEC 92-3
6883
IEC 754-1
IEC 331
IEC 332
6425
6387
4066
6207
Wiring for ships and
offshore topside installation
Smoke and halogen emission
Fire resistance
Flame retardance
Mineral-insulatedcables
N. American
408
Hazardous area equipment
for use in potentially explosive atmospheres.A separate Directive relating to mines
was published in 1982.
It is important for the electrical engineer to be aware of the ENSand their equivalents
in their latest editions because they determine the types of equipment available for
use, and also impinge on installation procedure.
References
There are a few reference texts 011this subjectand the reader is r e f e d for furtherdetail to the comprehensive
literature which is produced by reputable manufacturers. This is produced in much greater depth than
ordinary commercial literature because of the complexity of specifying, installing and using equipment
for hazardous areas.
Index
AC power controller, 272
AC supply harmonic filter. 278
AC system compensation, 368
Active power factor correction, 282
Admittance, 18
Alternate mode, 67
Aluminium. 52
Aluminium conductor steel reinforced (ACSR), 362
Amorphous alloy. 33
Ampere's law, 8
Amplifier. linear, 275
Angular frequency, 14
Arc splitter plate, 150
Arcing horn, 372
Armature, 252
Armature reaction, 92
ATEX Directive. 406
Atoms:
acceptor, 55
donor, 55
Actraeted-armature relay, 198
Austenitic steel. 37
Automatic voltage regulator (AVR). 95
Auto-recloser. 165. 370
Autotransformer. 120
Back-emf, 10
Bandwidth. 71
Base. 55
Battery, 291
charging, 308
freshening charge, 313
lead acid, 298
monitoring, 319
nickel cadmium, 298
nickel metal hydride, 300
primary, 291
Plante lead acid. 304
secondary, 291. 298
traction. 3I1
tubular lead acid, 303
valve regulated sealed lead acid (VRLSA), 305
BH loop, 10, 31
Black body radiation characteristic. 344
Bl.wkout. 266
Blown fibre system, 233
Brownout. 266
Brushless excitation, 97
Busbar, 53
Busbar configuration, 173
Bushings, 122
Ceble. 207
belted. 210
capacitance, 222
combined neutral and earth (CNE), 2 15
cross-linked polyethylene (XLPE), 208, 372
ethylene propylene rubber (EPR), 208
inductance, 223
industrial, 2 17
low smoke and fume (LSF), 210, 112
medium density polyethylene (MDPE), 208
mineral insulated (MICC). 220
multicore, 208
multicore control, 208
optical communication, 224
paper-insulated, 209, 212. 218
paper-insulated corrugated aluminium sheath
(PICAS), 212
paper-insulated lead-covered (PILC), 218
paper-insulated lead-covered steel wire armoud
(PILSCWA), 218
polethylene (PE), 208
polymeric, 210, 219
polyvinyl chloride (PVC), 208
steel tape armour (STA), 209
steel wire armour (SWA),209
wavecon, 2 16
waveconal, 216
waveform, 216
wiring, 220
Calibration, 82
Capacitance, 7, 16, 21
Capacitive coupling, 376
Capacitor excitation, 100
Cathode ray oscilloscope (CRO), 62
chop mode, 67
probe, 70
Cell:
alkaline manganese. 292, 293
dry. 24
fuel. 25
lithium copper oxyphosphate, 292,297
lithium manganese dioxide, 292, 296
lithium thionyl chloride, 292, 297
mercury oxide, 292, 294
pasted-plate lead acid, 301
primary! 24,291
secondary, 25
silver oxide, 292. 294
zinc air, 292, 294
zinc carbon, 292
Chopping mode, 255
Circuit breaker, 144
air. 149
air-blast, 172
miniature (mcb), 154, 183
moulded-case (mccb), 149
puffer-type, 159
Coercivity, 30
410
Collector, 55
Combustible dust, 395
Common-mode noise, 283
Conductance, 7
Conductivity, 7
Conductor, 48
Connection:
asynchronous, 366
delta, 18, 91, 117, 247
parallel, 12
series, 11
star, 18,91, 117,247
zigzag, 117
Connector, demountable, 232
Contact, arcing, 150
Converter:
boost, 276
buck, 276
cascaded, 284
DC to DC converters, 276
fly-back, 276
phase-controlled power. 271
Copper, 49
cadmium, 5 1
chromium, 51
silver. 52
sulphur, 52
tellurium, 52
wires, 50
Curie temperature, 32
Current, 5
harmonic, 266, 279
holding, 271
latching, 271
magnetizing, 109
prospective, 181
current density, 5
DC voltage ripple, 281
Demagnetization curve, 35
Derating factor, 106
Dielectric:
constant, 39
heating, 349
liquid, 41, 46
loss, 39
solid, 41
strength, 39
Differential-mode noise, 283
Digital storage oscilloscope (DSO),61
Digital multimeter (DMM), 77
Digital voltmeter (DVM), 76
Diode, 56
anti-parallel, 272
rectifier, 267
Direct resistance heating, 336
food, 340
glass, 339
Index
metal, 337
salt bath, 339
Direct torque control, 261
Directional horn, 352
Disconnector, 143, 148
Distortion factor, 94
Distribution network, 167, 370
Doping, 55
Drive:
AC motor?261
brushless DC, 237, 251, 255
DC motor, 261
switched reluctance, 261
variable speed, 260
Earthing, 372
Earthing switch, 144
Elastomer, 46, 208
Electric charge, 3
Electic field strength, 3
Electrical steel, 32
fully-processed, 33
grain-oriented, 32
semi-processed, 33
Electrode:
potential, 24
through-field, 351
Electroheat, 329
Electrolyte, 24
Electromagnetic compatibility (EMC), 350, 375
Electromagnetic induction, 10
Electromagnetic torque, 238
Electromotive force, 10
Electroslag refining, 332
Electrostatic charge, 3
Electrostatic discharge (ESD), 382
Electrostatic potential, 5
immunity, 382
limits, 381
shielding, 386
source, 375
Emission, 38 1
conducted, 38 1
radiated, 381
Emitter, 55
Enclosure, flameproof, 397,405
End-face separation, 23 1
Energy product, BH, 35
Equalizing charge, 3 13
Evanohm, 54
Exciter. 98
pilot, 99
Faraday’s law, 10
Fault capacity, 181
Ferrimagnetism, 30
Ferrites, 34, 35
Index
Ferromagnetism. 30
Field-oriented control, 261
Filter, 18, 282
Fleming's left-hand rule, 8
Fleming's right-hand rule, IO, 90
Flexible AC transmission systems (FACTS), 368
Fluorescence, 62
Flux:
leakage, 93
magnetic, 8
Fourier series, 278
Free electron, 55
Frequency counter, 74
Fresnel reflection, 23 1
Fringe-field array. 352
Fuel cell, 25, 291. 305
solid oxide (SOFC), 305, 308
solid polymer (SPFC), 305, 307
Fundamental component. 278
Furnace:
arc, 331
k l l , 342
channel induction, 333
coreless induction. 333
crucible, 336
electric, 342
electron beam, 332
forced-cooled. 342
pit. 342
plasma. 356
resistance. 335
Fuse. 181
cartridge, 183, 190
current-limiting,183
expulsion, 185
high-voltage, 192
low-voltage. 192
non-current-limiting, 183
rewireable. 183
semi-enclosed, 183
surge-proof, 190
time-lag, 190
universal modular (UMF), 185, 188
Fuselink 183
semiconductor, 192
Fuse switch, 143, 146
Fusion splicing. 23 1
Gamet: 34
Gas insulation, 46
Generation, 36 1
three-phase, 91
Generation and transmission network, 362
dispatching, 369
unit commitmenf 369
Geaerator, 85
cylindrical-rotor, 85
damper cage, 93
41 1
induction, 87, 100
inductor, 87
salient-pole, 85
turbo-, 85
Germanium, 54
GIS, 177
Glow discharge process, 356
Graded-index fibre, 227
Grounding, 388
Half-stepping, 260
Harmonic, 278
Harmonic distortion, 94
Hard magnetic material, 32,35
Hazardous area, 393
gas grouping, 395
protection code, 396
temperature classification, 395
zone classification, 394
Hazard triangle, 394
Heating:
dielectric, 349
indirect induction, 348
indirect resistance, 341
induction, 345
infrared, 343
Heat sink, 53
Hole, 54
Hysteresis loop, 10, 31
Impedance, 14
Impurity effect, 55
Impurity semiconductor, 55
Increased safety, 397,405
Inductance. 10, 16, 21
leakage, 93
mutual. I1
self, 10
Lnduction relay, 197
Inductive coupling, 376
Influence quantity, 82
Insulation:
primary! 41
secondary, 41
vacuum, 47
Insulator, cap-and-pin, 372
Integrated circuit (IC), 274
Interpole, 253
Intrinsic conduction, 54
Intrinsically safe, 399,405
Inverse definite minimum time (IDMT)
characteristic, 198
Inverter, 275
phase-controlled, 272
Karma,54
412
Kirchhoff's current law, 12
Kirchhoff's voltage law. 12
Large-scale integration (LSI), 57
Laser, 356
Leaky waveguide, 352
Lenz's law, 10, 92
Light emitting diode (LED), 224
Lissajous figure, 64
Loading effect, 69
Loss:
angle, 19
capitalization of, 112
dielectric, 39
hysteresis, 10, 32
insertion, 231
microbending, 227
misalignment, 23 1
stray, 112
Magnetic anisotropy. 34
Magnetic field strength, 7
Magnetic dux linkage, 10
Magnetization loop, 10, 31
Magnetomotive force (mmf), 8
Magnetostrictive effect, 26, 113
Manganin, 53
Measurement, 61
emission, 388
immunity to RF fields, 389
M-effect blob, 184
Metal melting, 330
Metal oxide surge arrestor, 178,372
Metallic glass, 33
Mica, 43
Micapaper, 46
Microprocessor, 57
Micro stepping, 260
kficrowave applicators, 352
Microwave power source, 352
Mineral insulated non-draining (MnvD) compound,
210
Mini stepping, 260
Mode dispersion, 227
Modified constant potential charger, 312
Motor, 237
AC commutator, 252
brushed permanent magnet, 254
cage rotor induction, 246
capacitor start, 250
control, 251
DC, 237
doubly-fed, 252
drives, 248
hybrid stepper, 237
induction, 237,244
permanent magnet, 253
Index
permanent split capacitor (PSC), 250
resistance split, 250
shaded pole, 250
single-phase induction, 249
split-phase, 250
stepper, 257
switched reluctance, 237, 255
synchronous, 250
totally enclosed fan ventilated (TEFV), 244
universal, 253
variable reluctance stepper, 257
wound rotor induction, 246
Murnetal, 34
Mylar,102
Nickel-iron alloy. 34
Nomex, 102, 116
Non-sparking, 398
Norton equivalent circuit, 12
Ohm's law, 7
Oil immersion, 401
On-load tapchanger, 123, 331
Optical fibre, 225
Oscillating dipole, 350
Oven:
elecmc, 342
multimode, 352
Overvoltage, 266
Parallel-plate electrode, 351
P-n junction, 55
Paramagnetic, 32
Peltier effect, 26
Permalloy, 34
Permanent magnet, 35
alnico, 35
bonded, 37
neodymium iron boron, 36
samarium cobalt, 36
Permeability, 30
free space, 8
initial, 30
maximum, 30
relative, 8
Permeance, 8
Permitivity:
free space, 3
relative, 3, 39
Persistence, 62
Per-unit system, 24
Per cent value. 24
Phase angle, 14, 93
Phasor diagram, 14
Phorjphorescence,62
Photovoltaic effect, 26
Index
Piezoelectric effect, 26
Piezoelectric accelerometer, 80
Pilot wire protection system, 203
Plasma torch, 353
Polarization:
orientation, 350
space charge, 350
Polycarbonate. 42
Polyester, 42
Polyethylene naphthalate (PEN), 42
Polyethylene terephthalate (PETP),42
Polyimide. 42
Potential gradient, 4
Power:
active, 22
apparent, 22
measurement, 23
reactive, 22
real, 22
Power factor, 22
Power loss characteristic, 113
Power conditioner, 283, 288
Power system, 36 1
Pressure-sensitive adhesive (PSA) tape,43
Pressurization installation. 401
Printed circuit, 57
Printed circuit board (PCB) layout, 387
Protected multiple earth (PME) system, 215
Protection. 181
co-ordination, 181: 197
explosion, 393
relays, 181. 194
Pulse width modulation (PWM), 275, 282
Pyrotenax. 220
413
Rectifier
half-wave single-phase, 267
full-wave single-phase, 268
phase-controlled, 272
three-phase bridge, 268
voltage doubler, 268
Regulator:
linear DC voltage, 274
low dropout, 275
switching DC voltage, 276
Regulated DC power supply, 285
Regulation, 110
Relay:
distance. 202
solid state. 198
Reluctance. 8, 239
Remanence, 30
Residual current breaker with overcurrent (rcbo).
157
Residual current device (rcd), 156
Resin, 42
Resistance, 7, 49
alloy. 53
temperature coefficient, 49
Resistivity, 7, 338
volume, 39
Resonant circuit, 18, 275
Right-hand corkscrew rule, 8
Rise time, 71
Rotating-arc SF, interrupter, 160
ROTEK kiln, 348
Saliency, 339
Sag 266
Quadrant, 260
Quadrature booster. 131
Quality factor. 18
Radio frequency applicator, 35 1
Radio frequency interference (RFI), 266, 284
Radio frequency power source, 35 1
Random-access memory (RAM), 58
Rare earth alloy, 36
Reactance, 14
leakage, 109
negative-sequence. 106
positive-sequence, 106
Potier, 106
subtransient reactance, 106
synchronous. 106
transient, 106
Reactor,
in-flight, 356
plasma, 356
Read-only memory (ROM). 58
Recloser, 163
Sampling rate, 73
Saturation, 30
Saturation magnetization, 30
Seebeck effect, 5
Semiconductor. 54
n-type, 55
p-type, 55
Sensor, 78
Screened cable, 210
termination for EMC 386
Shunt variable compensator (SVC), 368
Silicon. 54
Silicone, 46
Single-mode fibre, 227
Single-pulse mode, 255
Single-step mode, 260
Skin depth, 315
Skin effect, 345
Slewing, flexible insulated, 42
Slip, 246
Slot harmonic, 94
Slot skew, 94
Snubber. 272
Soft magnetic material, 32
414
Soft starter, 247
Soft magnetic composite, 35
Source-path-receptor model, 375
Staggered through-field, 352
Star-data start, 247
Static power supply, 265
Stator core, 101
Step index, 225
Strain gauge, 79
Submerged are process, 332
Substation, gas-insulated, 177
Sulphur hexafluoride (SF), 159, 174
Superconductor, 58
high-temperature, 59
Superposition, 13
Switch, 143
diverter, 125
Switch disconnector, 148
Switched mode power supply (SMPS), 276, 286
Switch fuse, 144, 148
Switchgear, 143
air-insulated (AIS),174
disrribution, 157
gas-insulated metal-enclosed, 176
high-voltage, 171
medium-voltage, 157
primary, 161
secondary, 163
transmission, 171
Symbols:
decimal prefix, 5
standard graphical, 6
standard quantities and units, 4
Synchronism, 361
Tan delta, 39
Tapchanger, 122
Taper charger, 3 12
Test:
chopped impulse wave, 137
lightning impulse voltage, 137. 169
Thermistor, 267
Thermocouple, 26
Thermoplastic, 208
Thermosetting, 208
Thevenin equivalent circuit, 12
Thyristor. 272
anti-parallel, 272
gate turn off (GTO),367
Timebase, 64
Time-current characteristic, 152, 181, 187, 191
Time grading, 197
Torque:
detent, 258
pull-out, 248
reluctance, 238
Traceability, 82
Transducer. 80
Index
Transformer. 109
clock-hour designations, 119
converter, 131
distribution, 128
dry-type, 128, 133
foil windings, 53
furnace, 133, 331
gas-filled, 133
generator, 130
linear variable differential (LVDT), 81
oil-immeeed, 128
phase-shifting, 131
pole-mounted, 370
railway, 133
rectifier, 133
Scott connection, 121
supply, 130
transmission, 130
Transistor, 55
bipolar, 274
Transmission, 362
HVDC,366
unbundled, 369
vertically integrated, 369
voltage, 171, 362
Transverse flux method, 347
Triac, 272
Trigger, 64
Ultraviolet process, 353
Uninterruptible power supply (UPS),284
line-iteractive, 285
on-line, 285
standby, 285
Universal power controller, 368
Vacuum interrupter, 160
Vacuum pressure impregnation ( W I ) , 45
Varnish, 42
Vector control, 261
Very large-scale integration (VLSI), 57
Voltage, 5
base, 24
electrochemical, 25
float, 316
line 18
multiplier, 270
peak-to-peak, 14
phase, 18
mot mean square (rms), 14
spikes. 266
transmission, 171, 362
Vulcanized, 208
Water treeing, 2 14
Wattmeter, 23
Index
Winding:
disc-type, 1 18
layer-type, 118
Primary, 109
secondary, 109
short-pitched, 95
415
transposed, 116
Wire, 207
copper, 50
Yttrium-iron garnet (YIG),34
EU
1
- 1
5
HANDBOOK
D. F. WARNE
0
A unique, cancise reference book with each chapter written by
leading pdessmmIs or academii currently working in the field
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Easy to use: clearly presented and logically arranged.
provides all the
-
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technicians and s t d e n t s on a -bask.
0
Summary of key standards included at the end of each cha)ler.
The gap between operating electrical equipment and having a genuine
understanding of how it works is widening nowadays. Much of the complexity
added to modern electrical equipment has the specific aim of making it operable or
'user-friendly' without specialist training or knowledge. The need for basic
explanation of principles, leading to a simple description of how various important
and common classes of electrical equipment works, has therefore never been
stronger. This handbook addresses the fundamentals as well as more practical
aspects of equipment specification and use.
Three groups of chapters:
Principles of electrical engineering and characteristics of the key groups of
materials used in electrical equipment.
Design and operation of the main classes of electrical equipment, also covering
the operation of such equipment within a power system.
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Special technologies applying to a range of equipment, particularly EMC and
hazardous area principles.
For managers and non-specialists, or specialists seeking knowledge outside their
field, Newnes Electrical Engineer's Handbook is an essential tool.
Doug Warne provides consultancy and engineering support in the design, testing
and performance of rotating electrical machinery, and is highly qualified on the
subject matter.
ISBN 0 - 7 5 0 6 - 4 8 7 9 - 1
Newnes
An imprint of Butterworth-Heinemann
http://www.newnespress.com
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