ARC PROTECTION AS INTEGRATED PART OF LINE
ARC PROTECTION AS INTEGRATED PART OF LINE PROTECTION RELAYS
O. J. Vähämäki
VAMP Ltd
ABSTRACT
This paper first presents general characteristics of an
electric arc. Possible damages and reasons which may
cause arcs in a switchgear are discussed. Ways to limit
the damages caused by the arc short-circuit are presented.
Different ways to implement arc protection is discussed.
New type of relays with integrated arc protection function
are finally introduced.
INTRODUCTION
An electric arc short-circuit is an infrequent switchgear
fault where an explosion-like heat and pressure-effect may
cause large material damage and jeopardize the job safety
of the opeation staff. The goal of the arc protection is to
detect the arc and minimize its burning time thus
protecting people and property. The burning time is
usually minimized by cutting off the current path feeding
the arc.
Generally arc protection is implemented using a separate
arc protection system. In this system arc detectors are
connected to specialized arc protection units.
New type of line protection relays include sensor inputs
and the arc sensors can be directly connected to them. In
this way arc protection can easily be integrated as part of
the total protection concept with minimal additional cost.
An overcurrent relay with integrated arc sensor inputs can
be used to build short-circuit and arc protection of one
feeder in a switchgear. The relay will selectively trip the
breaker of the feeder in case of arc fault is detected in the
feeder cubicle. The information about the activation of
the sensor can additionally be wired to the incoming
feeder’s overcurrent relay or arc protection unit. The
incoming feeder can thus be tripped if there is an arc
detected anywhere inside the switchgear.
The following general information about the arc shortcircuit's features, damages, reasons and ways to limit its
damages is mainly based on references [1] and [2].
FEATURES OF AN ELECTRIC ARC
An electric arc is formed if current flows from one
electrode to another via a channel of ionized gas. The arc
phenomena can be intentionally caused like in an arc
furnace or in arc welding. Arcing also regularly happens
in normal switching operations.
Electric arcing is
especially harmful and dangerous, when it happens
unintentionally and irregularly. Lightning and corona
effects are examples of this kind of harmful electrical arcs.
An electrical arc which causes a short-circuit is the worst
possible fault that can happen in a power distribution
system.
High temperature
The electric arc is formed when current flows through
isolation material - like air or gas - with no galvanic
contact. The air becomes conductive when its temperature
rises to about 3000°C. To make the air this hot and to
start an arc some kind of ignition is required. This may
be e.g. a thin wire which burns away when current flows
through it.
The temperature of a burning arc is depending on the fault
current, arc voltage and cooling conditions. In shortcircuit situations the temperature in the centre of the arc
may be up to 10.000 …20.000 K. In the contact points
the temperature is smaller, about 3000...4500 K.
Ionisation
In high temperature air and other gas molecules are
broken to atoms and further to ions and electrons. This
causes the gas to conduct electricity.
The total power and energy of an arc short-circuit depends
on the number of separate electric arcs burning in the
system. There may be one arc burning between two
phases or there may be three or more arcs burning
between all the phases, this depends on the mechanical
construction of the switchgear. The power of the arc may
be somewhere between 8...60 MW depending on the
short-circuit power of the switchgear.
Light and other radiation
Part of the arc energy flows to the environment as
infrared, ultraviolet and visible light radiation. Also
radiation in the radio frequencies is generated. The
radiation is transmitted by the arc itself and the materials
heated up by the arc.
At the brightest the light will be between 100..200 ms
after the ignition of the arc until smoke and metal steam
will reduce the sight. Some measurements have indicated
9000 lux brightness from 6..7 meters.
The total radiation may be 1....10 W/cm2 at a distance of
1,5 meters. This is rather high value compared e.g. to the
radiation from the sun on the surface of the earth, which is
about 0,1 W/ cm2.
The energy of the arc is consumed to (the percentage
figures are only approximate values):
- heating of the air (40...65%)
- heating of the materials (~10%)
- forming of the pressure (~3%)
- melting and evaporation of electrode materials (~7%)
- melting and evaporation of other materials (~5%)
- electrode thermal losses and radiation (~8%)
- radiation from the arc itself (~7%).
DAMAGES CAUSED BY ARC SHORT-CIRCUIT
The energy released from the electric arc can cause both
material and personal damages. In most cases the arc
fault causes also economical losses due to interruption of
energy distribution.
Movement of the arc
The thermal force tries to lift the centre of the arc
upwards, because the air in the centre is warmer. The
lifting of the hot air upwards causes air to move, this
convection force further pushes the centre of the arc
upwards. These forces make the electric arc to bend to its
characteristic arc form. The current flowing due to the
shortcircuit creates elctrodynamic force which makes the
arc to move to the direction of the energy flow in the
circuit. In swichgears the arc moves to the end of the
conducting bars or close to through-holes.
Voltage and resistance
In medium voltage switchgear the arc voltage is about
500...1000 V. In low voltage circuits the arc voltage is
about 300 V. The resistance of the arc is usually less than
0,1 Ω. In short-circuit calculations this resistance can
often be neglected.
Power and energy
FIGURE 1: A cubicle damaged by arc short-circuit.
The series of events related to an electrical arc can be
divided into 4 steps, compression, expansion, emission
and thermal phase. First the gas will be compressed as the
pressure rises in closed space. When the pressure relief
valves opens the gas expand and it will flow to the
ambient room or outside. In case this works properly the
pressure will decrease inside the cubicle and in the
switchgear room.
contain e.g. carbon monoxide and copper or aluminium
steam.
The compression and expansion time totally is approx.
5...15ms. After this the radiation and thermal phase will
follow. The radiation phase lasts for some hundreds of
milliseconds and the thermal phase will continue until the
arc is extinguished
Pressure effect
The burning arc will heat up the ambient air, causing
expansion of the air and create a huge pressure inside the
cubicle of a switchgear.
In the worst case the doors of the switchgear may open
and weak walls or ceilings of the switchgear or the
building may be broken due to the pressure. Additionally
the pressure may throw out loose or detached parts from
the cubicle.
FIGURE 2: Arc energy as function of burning time.
Points where metal parts start to burn are indicated.
Arc protection will function before cable fire starts.
Effects of radiation and sound
New switchgear is planned to withstand high pressure. In
tests, over 120 kPa (1200kg/m2) pressure has been
measured. This can be achieved by pressure relief valves
or openings.
Direct radiation from the arc usually does not cause direct
danger to persons. This is because the radiation level is
below the dangerous level (25 W/cm2). The bright light
can anyhow cause temporary blindness which combined
with loud sound effect might cause the worker to go into
shock and this will increase the risk of injury.
Burning effect
REASONS TO ARC SHORT-CIRCUITS
The burning effect of arc will be on the electrodes, cubicle
doors, walls and on the busbar. There will be burning
gases and hot particles exploding out to the environment.
The most common reasons to arc short-circuits can be
classified into two main groups 1) human and operational
errors and 2) technical reasons.
The arc will melt and evaporate electrode material. Part
of melting material is splashed around and part of it will
be mixed with the air. The electrodes will suffer most in
those spots where the arc will stay for a longer time. The
burning effect may cut busbars and wires, holes may be
burned to doors, walls or ceilings and equipment may be
destroyed. In addition to the burning effect of the arc
itself the arc may set up fires in the materials of the
building or in the cables.
Typical human and operational errors are
- work in a wrong cubicle
- operation of a wrong isolator
- forgetting to ground the working area
- forgetting to test the presense of voltage in the working
area
Hot or even burning gases may explode out from the
cubicles causing damages to the property or humans. The
clothes of a person standing by may catch fire due to the
hot gases. The gases may also be toxic because they may
Technical reasons to arc short-circuits:
- faults in equipment and false operation of equipment
- ageing of insulation and mechanical wear
- overvoltage
- overheating
- moisture, dirt
- equipment wear
- corrosion
- foreign objects (e.g. tools) in the switchgear
- small animals
- installation errors
- bad wire and busbar connections
WAYS TO LIMIT THE DAMAGES CAUSED BY
ARC SHORT-CIRCUITS
- 35 ms: no significant damage to persons or switchgear,
which can often be taken in use after checking the
insulation resistances
- 100ms: small damage, requires cleaning and possibly
some minor repair before swithchgear is taken in use
again
- 500ms: large damage both for persons and the
switchgear, which must be partly replaced.
The arc burning time is the sum of the time to detect the
arc and the time to open the correct breaker.
The damages caused by arc short-circuits can be limited in
principle in three ways:
- improving the construction of the equipment
- limiting the arc short-circuit power
- limiting the arc burning time
ARC DETECTION
The goal of arc protection
Improvements to the construction
The construction of the switchgear can be improved so
that they are able to withstand the pressure and burning
effects of the arc. The pressure withstand can be
improved with proper desing of the pressure vents and
channels. The damaged area can be limited by dividing
the swithgear to smaller cells. The arc withstanding of a
certain switchgear type must be checked by testing.
The goal of the arc protection is to protect property and
people in case of an electric arc fault by limiting the arc
burning time. To do this the arc protection must first
detect an arc and then cut the flow of current. The
flow of arc current is usually cut by opening a cicuit
breaker. The other possibility - especially in low voltage
systems - is to cause an intentional galvanic short-circuit,
which will drain off the current flow from the arc thus
extinguishing it.
The costruction must be such that it limits the possible
injuries to working personel. Poisonous gases must be
conducted to a safe direction and it should be easy to
escape from the vicinity of the switchgear.
Limiting the short-circuit power
This is usually not possible because the use of electricity
is increasing all the time and the switchgears must handle
larger currents.
Limiting the arc burning time
The best way to limit damages of an arc short-circuit is to
limit the arc burning time. The burning time can be
limited by tripping off the breaker, which is feeding the
arc current. This will effectively limit both direct and
indirect damages. Especially when arc is developed
during work in the switchgear the short arc burning time
has a great value.
Rules of tumb for different arc burning times:
FIGURE 3: Light sensor for detecting electric arc.
Arc detection methods
Arching is accompanied with radiation in the form of
ligth, sound, heat and electromagnetic waves. The
presence of of the arc can be detected by analysing visible
light, acoustic waves, infrared or radio frequency
radiation [3]. Even the pressure wave caused by the arc
has been used for detecting it. The most commonly used
metod is detecting visible light.
Some manufacturers use electromechanical relays and
some semiconductors as tripping contacts.
Some
manufacturers implement the arc sensors with photo
transistors and some support fiber optic loop as light
sensor. In addition to light sensors the system may
include e.g. termperature sensors.
To avoid errorneous trips the presence of short-circuit
current together with one of the above mentioned arc
indications is usually used as condition before making an
arc short-circuit trip.
Partial discharge detection
Damaged inslution can lead to partial discharges (PD's)
which bridge only part of the insulation clearance. To
begin with, these PD's are not dangerous, but over time
they cause ever greater damage to the insulation and their
intensity and frequency both increase. If these occurences
are not detected in good time and the cause eliminated, the
progressive PD's will invevitably lead to a disruptive
discharge, causing a complete failure of the operating
equipment [4].
Partial discharges can in the end damage the isulators so
that an arc short-circuit will follow.
Partial discharges may be detected by using search coils
for detecting high-frequecy electrical fields produced by
partial discharges.
FIGURE 4: Structure of an arc protection system.
The arc protection system opens the main breaker
(incoming feeder) of the switchgear when arc short-circuit
is detected. If the switchgear contain two or more zones
separated by breakers then the system can be built to open
only the breaker of the faulty zone and the breaker(s)
separating the zones.
Detection of partial discharge is an early warning of an arc
and an alarm from detected partial discharge is usually
given.
ARC PROTECTION WITH SPECIALIZED
DEVICES
Generally arc protection is implemented using a separate
arc protection system. In this system arc detectors are
connected to specialized arc protection units, which take
care of the arc short-circuit protection. Line protection is
implemented with standard protective relaying.
These devices are usually composed of:
- arc sensors for detecting the arc
(both fibre loop or photo transistor sensors are used)
- slave units collecting data from many arc sensors
- master unit for finally collecting all the arc sensor data,
measuring the current and making the final decision about
tripping the breaker if both over current and light is
detected.
FIGURE 5: Arc protection system with two protection
zones implemented with two master units (VAMP) and
multiple slave units (VAM). If fault is detected e.g. in
zone 1 then breakers CB1 and CB5 will be opened.
In multi-zone arrangements the master units must be able
to transfer data to each other. Information about an
activated arc sensor can be distributed to all the master
units in the system then all master units which have
detected overcurrent can open the circuit breakers in their
own zone. In the same way information about the locally
detected overcurrent can be passed to the other units to be
used as trip condition if the other unit only detects ligth
but no overcurrent.
INTEGRATED ARC PROTECTION
New type of protection relays can be equipped with arc
sensor inputs and the arc sensors can be directly
connected to them. In this way arc protection can easily
be integrated as part of the total protection concept with
minimal additional cost. The arc sensor input may include
e.g. two inputs for arc sensors and one binary input and
one output for receiving/sending arc sensor data to other
relays. The relays include high-speed overcurrent and
earthfault stages dedicated to operate with the arc sensors.
of the outgoing feeders. Normally the the overcurrent
relays take care of the arc protection of the feeders and the
arc protection relay protects the busbars section. In case
power is fed in through one of the normally outgoing
feeders the overcurrent relay of the feeder in question is
also used to make arc protection of the switchgear part.
This is possible because the information about the
activation of the arc sensors of the arc protection system
can be brought to the overcurrent relay using the binary
outputs and inputs. The arc protection function of the
overcurrent relay will trip the breaker whenever it
measures arc short-circuit current and either its own or
one of the sensors of the switchgear part (sensors Arc 3
and Arc4 in figure 6) detects an arc.
The protection relay with an arc sensor input can be use to
1) increase selectivity of the arc protection
2) make arc protection more adaptive to feeding point
changes
3) implement cost effective arc protection
4) increase the coverage of the arc protection
Increasing selectivity of the arc protection system
The selectivity of the arc protection can be increased, if
the arc sensors of an outgoing feeder are connected to the
overcurrent relay of the feeder (see figure 6.) If arc fault
happens in the feeder area then only the breaker of the
feeder in question is opened and the rest of the substation
remains operational. The main arc protection system
opens the incoming breaker only if there is an arc shortcircuit in an area where the outgoing feeders can not
measure short-circuit current.
Arc sensors Arc1 and Arc2 in figure 6 should in practice
be installed to the cable chambers of the outgoing feeder
cubicles.
Arc sensors connected to the main arc
protection system (e.g. Arc3 and Arc4) would be installed
to the truck and busbar chambers.
Adaptive arc protection
The example in figure 6 shows a part of a switchgear
where power is normally fed through the transformer but
in special situation power may also be brought in via one
FIGURE 6: Overcurrent relays with arc sensor option
and specialised arc protection system (VAMP and
VAM). Information about activation of arc sensors
Arc3 and Arc4 is also brought to the overcurrent
relays via the binary inputs (BI).
Cost effective arc protection system
A low cost arc protection system can be built using
protection relays with arc sensors (see figure 7). The
information about activated arc sensor can be transferred
between the relays using binary inputs and outputs. The
relay makes arc short-circuit trip if it detects overcurrent
and at the same time its own arc sensor or one of the arc
sensors of the other relays is activated. It must anyhow be
noticed that the coverage of this kind of system is limited
compared to a specialized arc protection system, because
the number of arc sensors is limited.
information about the activation of arc sensors is brought
from the VAMP 220 relay to VAMP 265 relay using the
binary output and input. The VAMP 265 relay opens the
breaker when it detects fault current caused by the arc and
its own or VAMP 220 relay's arc sensors detects the arc.
CONCLUSIONS
FIGURE 7: Arc protection system using overcurrent
relays with arc sensor option. Information about the
activation of the arc sensors may be carried between
the relays using the binary inputs and outputs (La>).
Increasing the coverage of the arc protection
The protection relays with arc sensor option can be used
to measure arc short-circuit in such places where the
specialized arc protection system can not measure the arc
short-circuit current. An example of this is shown in
figure 8.
FIGURE 8: Transformer differential relay VAMP 265
with arc option used to extend the coverage of the arc
protection system VAMP 220.
If arc short-circuit happens in the indicated place (figure
8) the short-circuit current can not be measured by the
VAMP 220 relay. The current can anyhow be measured
by the transformer differential relay VAMP 265. The
Arc protection is a vital part of switchgear protection due
to the possibly drastic damages caused by arc shortcircuits. Adding an arc sensor option to standard
protection relays provides new possibilities to implement
arc protection. The protection can either be implemented
with lower than before cost or the selectivity, flexibility
and coverage of the arc protection can be increased.
REFERENCIES
1. Martikainen S, "660 V:n kennokeskuksen valokaari- ja
oikosulkukestoisuuden tutkiminen", 1.4.1975.
2. Verkkonen V, "Kytkinlaitosten valokaarikestoisuus ja turvallisuus", INSKO 25-91 XIII, 1991.
3. Sidhu T.S, Gurdeep S, Sachdew M.S, "Microprocessor
Based Instrument for Detecting and Locating Electric
Arcs", IEEE Transactions on Power Delivery, Vol. 13,
No. 4, October 1998.
4. INDIPARD, Partial Discharge Indicator, Product
brochure, May Elektronik Gmbh, 2001.
5. VAMP 220 Arc Protection System, User's Manual,
Vaasa Electronics Ltd, 9.3.2001.
6. VPJ 140 Combined overcurrent and earth-fault relay,
Technical description, Vaasa Electronics Ltd, 2002.
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