chapter one introduction
1.1 Overview
Rosaries hydropower plant located on the Blue Nile in Rosaries city, 500km
southeast of the capital Khartoum .it consists of 7 unit’s 40MW for each .and
the total capacity 280MW. There are two types of water turbines impulse
turbines and reaction turbines the type of the hydro turbine use in plant is
Kaplan turbine.
The basic function of an excitation system is to provide
necessary direct current to the field winding of the synchronous generator. The
excitation system must be able to automatically adjust the field current to
maintain the required terminal voltage. The DC field current is obtained from a
separate source called an exciter. The excitation systems have taken many forms
over the years of their evolution. The following are the different types of
excitation systems [1].The first type is DC excitation systems utilize direct
current generators. In such systems direct current is provided to the rotor of the
synchronous generator through the slip rings. The exciter maybe placed on the
same shaft with power generator or is separately driven by a motor. Exciter may
be self–excited or with separate excitation, with permanent magnet generator
applied [1].The second type is AC excitation systems utilize AC machines for
generator excitation. Exciter is typically placed on the same shaft with the
turbine. AC is rectified by controlled or non-controlled rectifiers, to provide DC
to the generator field winding. Also AC excitation systems may differ by output
control method and source of excitation for the exciter. Presently stationary and
rotating AC rectifier systems are in use. In stationary rectifiers the DC output is
fed to the field winding of the generator through the slip rings. On the contrary,
in rotating rectifiers there is no need in slip rings and brushes and DC is directly
fed to the generator field as the armature of the exciter and rectifiers rotate with
the generator field. Such systems are known as brushless systems and were
developed to avoid the problems with brushes when extremely high field
currents of large generators are applied [1]. The third type is Brushless AC
excitation systems Brushless systems are used for excitation of larger generators
(power over 600 MVA) and in flammable and explosive environments.
Brushless system consists of AC exciter, rotating Diode Bridge and auxiliary
AC generator realized with permanent magnet excitation. Attempts to build
brushless system with Thyristor Bridge were not successful because of
problems with thyristor control reliability. The result of this problem is
significant disadvantage of these systems, inability of generator deexcitation.
Another disadvantage is slower response of system, especially in case of low
excitation [1]. The fourth type which is Static excitation system will be
discussed in details since it is the type that used in Rosiers Hydropower Plant.
In static excitation system, aportion of the AC from each phase of synchronous
generator output is fed back to the field windings, as DC excitations, through a
system of transformers, and rectifiers. An external source of DC is necessary for
initial excitation of the field windings. On engine driven generators, the initial
excitation may be obtained from the storage batteries used to start the engine. In
our plant this is achieved by afield flashing unit [1].
1.2 Problem statement:
- There is no accurate model for the static excitation system in rosaries hydro
power plant for stability studies, and the existing system is single channel type.
- There is no way to estimate the stability bandwidth and to determine the linear
model of the excitation system in rosaries hydro power plant.
1.3 Problem solution
- A computerized model for rosaries static excitation system is created
depending on the system parameters which are available in the plant
- A closed loop frequency response is carried out using sine sweep signal (chirp
signal) as input to estimate the stability bandwidth and to determine the linear
model of the excitation system
1.4 Methodology
- The modeling process based on matlab/Simulink software, which is commonly
used in many scientific problems solutions.
- To control the firing angle required to trigger a three phase thyristor converter.
Nickolas-Ziegler method is used for tuning the PID gains necessary to reach the
desirable stability band.
1.5Thesis outlines
The following provides an outline of this thesis.
In Chapter 2 Rosaries excitation system: focused on a general excitation system
and Roseirs Hydro Power Plant excitation system will be discussed in details,
since it is the type that used in the power Plant. This chapter illustrates in details
the Convertor unit and also the control unit (AVR) which adjusts the firing
angle of the converter. And discharge unit which perform tow functions
overvoltage protection and field discharge during de excitation. The initial
excitation may be obtained from the storage batteries used to start the engine. In
this study initial excitation is achieved by afield flashing unit.
Chapter 3 presents the modeling process based on matlab/Simulink software,
which is commonly used in many scientific problems solutions.
Chapter4 presents the computer simulations of the existing static excitation
system. This chapter illustrates the performance of the model compared to the
real system performance.
Chapter 5 concludes the thesis and future work.
2.1 Functions and Performance Requirements of Excitation Systems:
The functions of an excitation system are:
 to provide direct current to the synchronous generator field winding, and
 to perform control and protective functions essential to the satisfactory
operation of the power system
The performance requirements of the excitation system are determined by
 Generator considerations:
 supply and adjust field current as the generator output varies within its
continuous capability
 respond to transient disturbances with field forcing consistent with the
generator short term capabilities:
 rotor insulation failure due to high field voltage
 rotor heating due to high field current
 stator heating due to high VAR loading
 heating due to excess flux (volts/Hz)
 Power system considerations:
 Contribute to effective control of system voltage and improvement of
system stability [3].
Elements of an Excitation System:
The elements of excitation system are shown in figure 2.1.
 Exciter: provides dc power to the generator field winding
 Regulator: processes and amplifies input control signals to a level
and form appropriate for control of the exciter
 Terminal voltage transducer and load compensator: senses generator
terminal voltage, rectifies and filters it to dc quantity and compares with a
reference; load comp may be provided if desired to hold voltage at a remote
 Power system stabilizer: provides additional input signal to the regulator to
damp power system oscillations
 Limiters and protective circuits: ensure that the capability limits of exciter
and generator are not exceeded [3].
Figure 2.1: Elements of excitation system
Types of Excitation Systems:
Excitation Systems are classified into three broad categories based on the
excitation power source:
• DC excitation systems
• AC excitation systems
• Static excitation systems
2.3.1. DC Excitation Systems:
DC Excitation Systems utilize dc generators as source of power; driven by a
motor or the shaft of main generator andself or separately excited. They
represent early systems (1920s to 1960s) and lost favor in the mid-1960s
because of large size; superseded by ac exciters.
The voltage regulators range from the early non-continuous rheostatic type to
the later system using magnetic rotating amplifiers.
Figure 2.2 shows a simplified schematic of a typical dc excitation system with
an amplidyne voltage regulator
• Self-excited dc exciter supplies current to the main generator field through
slip rings
• Exciter field controlled by an amplidyne which provides incremental
changes to the field in a buck-boost scheme
• The exciter output provides rest of its own field by self-excitation [3].
Figure 2.2: DC excitation system with amplidyne voltage regulators
2.3.2. AC Excitation Systems:
The AC Excitation Systems use ac machines (alternators) as source of power.
Usually, the exciter is on the same shaft as the turbine-generator. The ac output
of exciter is rectified by either controlled or non-controlled rectifiers. The
rectifiers may be stationary or rotating.
The early systems used a combination of magnetic and rotating amplifiers as
regulators; most new systems use electronic amplifier regulators [3].
2.3.3. Static Excitation Systems:
All components in the Static Excitation Systems are static or stationary. The dc
is supplied directly to the field of the main generator through slip rings. The
power supply to the rectifiers is from the main generator or the station auxiliary
bus [3].
Potential-source controlled rectifier system:
The excitation power in this type is supplied through a transformer from the
main generator terminals and regulated by a controlled rectifier as shown in
figure 2.3. It is commonly known as bus-fed or transformer-fed static excitation
system and it has very small inherent time constant. The maximum exciter
output voltage is dependent on input ac voltage; during system faults the
available ceiling voltage is reduced [3].
Figure 2.3: Potential-source controlled-rectifier excitation system
7 Compound-source rectifier system:
The power to the exciter in this type is formed by utilizing current as well as
voltage of the main generator and this is achieved through a power potential
transformer (PPT) and a saturable current transformer (SCT). The regulator
controls the exciter output through controlled saturation of excitation
transformer (see figure 2.4). During a system fault, with depressed generator
voltage, the current input enables the exciter to provide high field forcing
capability [3].
Figure 2.4: Compound-source rectifier excitation system Compound-controlled rectifier system:
This type of static excitation system utilizes controlled rectifiers in the exciter
output circuits and the compounding of voltage and current within the generator
stator as in figure 2.5. The result is a high initial response static system with full
"fault-on" forcing capability [3].
Figure 2.5: Compound-controlled rectifier excitation system
Overall System Description
A static excitation system regulates the terminal voltage and the reactive power
flow of the synchronous machine by direct control of the field current using
thyristor converters. According to the block diagram of an UNITROL 5000
Excitation System for approximately3200 A, which is shown on figure 2.6.
Block diagram of a typical UNITROL 5000 Excitation System, the entire
system can be divided into four major function groups:
· Excitation transformer -T02
· Excitation modules with control electronics (-A10, -A20)
· Thyristors converter units -G31 to -G34
· Field flashing (-R03, -V03, -Q03) and field suppression equipment (-Q02, F02,-R02) [4].
In static excitation systems (so-called shunt excitation or self-excitation), the
excitation power is taken from the machines terminals. The field current of the
synchronous machine flows through the excitation transformer -T02, the field
circuit-breaker -Q02 and the power converter G31 to G34 (thyristor converter).
The excitation transformer reduces the generator terminal voltage to the
required input voltage for the thyristor converter, provides the galvanic isolation
between the machine terminals and the field winding and acts at the same time
as the commutating reactance for the thyristor converter. The power converter
G31 to G34 converts the AC current into a controlled DC current If. At the
beginning of the starting sequence the field flashing energy is derived from the
residual machine terminal voltage. As soon as 10 to 20 V at the input of the
thyristor converter are reached, the thyristor converter and control electronics
are ready for the normal operation and a soft-start sequence takes place. The
new start up facilities and design of the field flashing equipment (-R03, -V03, Q03), which have been developed for UNITROL 5000, are more described in
chapter 7 of this document. After synchronizing with the network the excitation
system can operate in AVR mode regulating the generator terminal voltage and
reactive power or can operate in one of the superimposed mode. That is,
machine’s Cos-phi control or MVAr (reactive power) control. In addition, it can
be included in an overall joint voltage and reactive control of the power plant
The purpose of the field suppression equipment is to disconnect the excitation
system from the excitation transformer and to discharge the field winding
energy as fast as possible. The field suppression circuit consists basically of the
field circuit breaker -Q02,the field suppression resistor -R02 and the
CROWBAR thyristors -F02 with their associated triggering electronics. The
field suppression with field circuit breaker on the AC side of the thyristor
converter is described in greater detail in chapter 6 of this document. If
explicitly requested in the technical specifications and based on the ratings of
the excitation system, the field circuit breaker may be connected at the DC side
of the thyristor converter. This solution mainly takes place for the field currents
of more than 3500 A [4].
Based on the system requirements the control electronics is configured as a
single AVR channel (-A10) or double AVR-channel (-A20). One channel
comprises basically one Excitation Module with a Control Board (COB) and a
Measuring Unit Board (MUB), forming an individual processing system. Each
channel contains the Software for the machine terminal voltage regulation, field
current regulation, excitation monitoring / protection functions and a
programmable logic control. In a single AVR-channel configuration a separate
controller so called Extended Gate Controller (EGC) is employed as a back-up
channel that is a MANUAL controller [4].
In addition to control electronics interface cards such as Fast Input / Output
(FIO) and Power Signal Interface (PSI) are employed to provide the galvanic
separation of the measuring and control signals. Further, each thyristor bridge is
equipped with a set of the converter interface cards including Converter
Interface (CIN), Gate Driver Interface (GDI) and Converter Display (CDP).
UNITROL 5000 has a facility, by extension, for connection to a serial
communication link. For example a Modbus type serial interface can be applied.
In addition, TCPIP type serial interface with Ethernet protocol can be provided
optionally. Within the excitation system the exchange of the control and status
signals is performed serially over the ARC net Field bus. The field-breaker
tripping circuits are additionally hard-wired [4].
Figure 2.6: Block diagram of a typical UNITROL 5000 Excitation System.
2.4.1. UNITROL 5000 Control Electronics
UNITROL 5000 is the microprocessor based control system. The platform for
the control electronics is an enhanced version of DCS 500 System mainly
intended for the control and regulation of the large static drive systems. By
development of the control electronics several elegant solutions and facilities
for of the excitation systems were introduced, for example dynamic current
distribution mechanism for parallel operation of thyristor bridges and start up
from the residual machine terminal voltage. Further, the communication and
diagnostic features were also improved. In its special versions UNITROL 5000
has facilities to be supplied from the network frequency of 16 2/3 Hz or from
the high frequencies exciter machines up to 500 Hz [4]. Configuration and Mechanical Design
The core of the control electronics is a Control Board (COB) for all the
regulation and control functions as well as for the pulse generation. In addition,
a Measuring Unit Board(MUB) with a digital signal processor is applied for the
fast processing of the actual measuring values. These boards are attached
together in a way to reassemble a double layer board and are assembled in a
metallic case, making up an individual processing channel. In such a
configuration an Extended Gate Controller (EGC), which is mechanically
separated, is used as a back-up channel for the field current regulation. In a fully
redundant system a second double layer board assembled in a separate metallic
case is provided (second channel). That is, both channels are mechanically
separated, enabling an easy maintenance on-line. Each channel of the control
electronics can control one or more parallel connected thyristor converters with
the system rated current up to10’000 A [4].
The interface devices such as Fast I/O card and Converter Interface (CIN) are
applied for the galvanic separation and matching of the control signals and are
placed where the signals are originated, e.g. one converter interface is placed
into each thyristor unit. The communication within the excitation system, if not
time critical, is performed serially over the ARC net Field Bus [4].Possible
configurations with UNITROL 5000 System are shown on the Figure 1 to
Figure9. The abbreviations to the figures are as follows:
AVR - Automatic Voltage Regulator
FCR - Field Current Regulator
BFCR - Backup Field Regulator
UG - Measuring signal of the generator terminal voltage
IG - Measuring signal of the generator terminal current
IF - Measuring signal of the generator field current
The two controller states are “on-line” and “stand-by”. The latter is a backup for
the “online “controller.
Figure 2.7 shows a minimum configuration of UNITROL 5000 System which is
used in Roseirs Hydro Power Plant. The main Control Board (COB) provides
all regulation and control functions including a field current regulator. The
Backup Regulator, which function is included in the Extended Gate Controller
(EGC), follows the main control and takes over automatically in case of failure
of the main control. One thyristor bridge without redundancy is employed in
this configuration [4].
Figure 2.7: Type D5E – Single Channel and Back up Control with a Single Converter.
Figure 2.8 shows another configuration, called Twin Configuration where two
fully redundant converters are employed. Any of these can be chosen as the
“on-line” converter, whereas the other is in “stand by”. This configuration is
usually applied for the rated currents up to 1800 A [4].
Figure 2.8: Type D5T – Single Channel and Back up Control with fully Redundant Twin
In the followings configurations, Figure 2.9 to Figure 2.11, a full redundant dual
channel control is employed. Each of the control channels can be “on line” or in a
“stand by” mode. In addition to the automatic voltage regulation each channel also
includes software functions for PSS, limiters, protections and monitoring as well as
the Manual Control [4].
Figure 2.9: Type A5E – Full redundant dual channel control with Single Converter.
Figure 2.10: Type A5T – Full redundant dual channel control with Twin Converters.
Figure 2.11: Type A5S – Full Redundant Dual Channel Control with Redundant n-1
16 Main Control Boards
Implementation of the automatic voltage regulation, limiters, protection and
control functions into a single processing board was possible due to its high
performance and computing power of the Control Board (COB). It basically
contains an enhanced microprocessor, which operates with 32 MHz clock. An
ASIC (Application Specific Integrated Circuit) takes care of exchange and
storage the data, control pulse generation, A/D and D/A conversion and
interfacing with other devices within excitation system(ARC net Field bus
coupler). It supports the communication with the local control, service panels
and CMT-Tools. Further, it provides serial ports and has a self-diagnostic
function (watchdog). For the rapid diagnostics and fault tracing purposes
Control Board is equipped with a seven-segment alarm display [4].
A typical configuration of the double channel system is shown on Figure 2.12.
Further, for the diagnostics purposes the control board is equipped with a
transient recorder and fault logger. These are handled with the CMT-Tools
(Commissioning and maintenance tools). The fault logger and transient recorder
can also be synchronized with the real time clock [4].
By the extensions with sub-prints or adapters the serial communication links to
the Digital Control System can be provided. The particular functions and merits
of the system are described in separate items of this document [4].
Figure 2.12: Typical configuration of the double channel system. Measuring Unit Board (MUB)
Measuring Unit Board (MUB) consists of a Digital Signal Processor (DSP).
This provides fast processing of the actual measuring values, galvanic
separation and level matching.
The following functions are implemented in the Measuring Unit Board (MUB):
- Filtering and digitizing.
- Calculation of synchronous machine’s voltages and currents from the PT’s and
CT’s signals as well as the calculation of the active and reactive power; Cos-phi
and machine’s frequency.
- Power System Stabilizer (PSS) with accelerating power and frequency input
signals, in which the control algorithm is based on IEEE Std. 421-Type 2A.
- Adaptable Power System Stabilizer (an alternative to the PSS) [4].
18 Extended Gate Controller (EGC)
Extended Gate Controller (EGC) is employed as a back-up controller in single
automatic channel configurations and for the pulse generation of the system
rated frequencies different from 50/60 Hz. In latter case, its typical application
is for the automatic voltage regulators supplied from the high frequency pilot
exciters, whose rated frequency can be up to 500 Hz. Further, it is applied in
excitation systems for the generators of 16 2/3 rated frequency for railway
networks. This controller is assembled in the same case as the above-mentioned
boards but is mechanically separated. The following functions are also
implemented in this board:
- Field current regulation.
- Follow up control for the smooth change over in case the control board (COB)
- Back-up over current relay, instantaneous (ANSI 50).
- Back-up over current relay, inverse-time (ANSI 51).
- DC short circuit protection.
- Thyristor converter conduction monitoring based on the ripple monitoring
- Inherent power supply [4]. Power Signal Interface (PSI)
Power Signal Interface (PSI) is used for the galvanic separation and matching of
the field measurement signals, before being sent to the Measuring Unit Board
(MUB) [4]. Power Supply of the Control Electronics
All electronic devices are supplied from a 24 VDC bus. The 24 VDC bus is
derived from two fully redundant power packs; a DC/DC-power pack which is
supplied from a DC source (e.g. station battery) and an AC/DC-power pack
which is usually supplied from the secondary side of the excitation transformer
19 Communication within the Excitation System
The communication within the excitation system is performed serially via the
so-called ARC net field bus. For example, this internal communication link is
employed for the exchange of the control and status signals to/from the thyristor
converter. Further, the measured values and the alarms for the Local Control
Panel (LCP) as well as the local control commands are sent through this link
[4]. Human-Machine Interface
A user-friendly Local Control Panel (LCP) can be used for the local control and
supervision of the excitation system. It can be located in the Power Station
Control Room, serving for the remote control (see figure 2.13).
The panel offers following facilities:
 Display of measuring and processing signals on 8 display lines, each
containing 40 characters (240 x 64).
Either 8 analogue signals or 4 with analogue – bar display in scale from 0 to
120% can be displayed simultaneously. A maximum of 32 pre-defined signals
can be selected. Display mode is enabled with the function keys. Signal
selection is performed by means of the scroll key or page keys. A display line is
chosen with the cursor key [4].
Figure 2.13: Local Control Panel (LCP) in a typical operation.
 Alarm annunciation
In case of their appearance the excitation system alarms take precedence over
the measuring signal display.
The displayed alarm includes its alarm number and description text of 40
characters. 8alarm messages can be displayed simultaneously. The first occurred
alarm appears on the first line and following alarms appear in order of their
alarm numbers. If more than 8alarms appear, they can be displayed by the scroll
key. Maximum 80 alarms can be displayed. On the alarm function key there is
an alarm LED, that blinks by each alarm occurring.
After pressing the confirmation key the alarm LED changes to a steady light as
long as an alarm exists. When the alarm disappears, the alarm LED switches
 Printing of the signals and alarm messages
The information saved by the control panel can be printed out over its serial
interface portRS-232.
 Indication of the selected mode
Each of selected mode keys is equipped with a LED for mode indication.
 Local operation control
16 function keys with LED’s for status indication are available for local
operation control of the excitation system [4]. Control Interface to the Power Station Control System
The digital and analogue command and status signals are interfaced via the Fast
Input /Output boards (FIO). Each fast Input / Output board (FIO) is equipped
- 16 Digital inputs with opto-couplers, rated for 24 VDC. The control inputs are
activated using a local 24 VDC from the internal power packs
- 18 output relays with change over contacts for status indications and alarms
- 4 multipurpose analogue inputs + 10 V or + 20 mA
- 4 multipurpose analogue outputs 4 to 20 mA
- 3 analogue inputs for excitation transformer temperature measuring PTC or
A maximum number of two Fast Input / Output boards can be implemented per
system, which is sufficient for most system requirements. In case that more
digital inputs and outputs are requested, the system can be supplemented with
Digital Input Interface (DII)and Relay Output Interface (ROI). These are
connected to the ARC net Field Bus over a Field Bus Coupler (FBC).
Standard interface signals are assigned according to Table 1.
Two separate hard-wired signals of the internal trip are available for the
generator protection system. Two trip signals from the generator protection are
foreseen to act directly on the excitation trip circuits [4].
Table 2.1: List of standard remote control, status and alarm signals.
2.4.2. Voltage Regulation, Monitoring and Protection Functions
An overview of the main control functions implemented into the UNITROL
5000 is provided in the following items. Functions covered by the Control Electronics
The main objective of an automatic voltage regulator (AVR) is the accurate
control and regulation of the terminal voltage and the reactive power of a
synchronous machine. In order to fulfill this requirement, the field voltage must
react quickly to changes of the operating conditions, i.e. with a response time
that does not exceed a few milliseconds.
To accomplish this, a high-speed controller is required. It shall compare
continuously the actual values with the set-point values and vary the final
control element (firing angle for the thyristor converter) with an insignificant
The UNITROL 5000 digital voltage regulator calculates the controlled variable
from the measured and the set-point value in very short time intervals. The
result is a quasi continuous behavior with a negligible time delay (comparable
with an analogue regulator).
The calculations are fully digital. Analogue measurement signals such as
terminal voltage and current are converted into digital signals by
analogue/digital converters, which are parts of the Measuring Unit Board
(MUB). The set points and limit values are already defined in digital form.
Figure 2.14 shows the overall software’s functions of an AVR, which are
implemented into standard UNITROL 5000 static excitation system. For a
better understanding of those software’s functions, they are split into functional
blocks followed by the short functional description. The numbers (X) within the
functional blocks correspond to those mentioned in the descriptions [4].
Figure 2.14: Internal structure of UNITROL 5000 Automatic Voltage Regulator. Set point Generation and Voltage Regulation
(1) AVR set point generator can be raised, lowered or reset to a pre-set value
using digital input commands or analog input signals or through the serial
communication link. The excursion time from minimum to maximum limits can
be adjusted independently from the set point range [4].
 Active and reactive power compensation
(2) P-static and (3) Q-static, are intended to add additional signals
proportionally to the active or the reactive power to the set-point value. The
reason is the compensation of the voltage drop caused by the active or reactive
power across the unit transformer and/or the transmission line. The reactive
power signal is also necessary for parallel operation of two and more generators
connected to the same bus. In this case, the Q-Static signal shall reduce the
AVR set point proportionally to the increase of reactive power. The rate of
change of the set point as function of the active and/or reactive power can be
adjusted in the range of -20% to +20% [4].
 V/Hz Limiter
(4) V/Hz limiter is provided in order to avoid over fluxing of the transformers.
If the set point of the AVR is too high for a certain frequency, the set point will
reduce smoothly according to a pre-adjusted V/Hz characteristic. The limiter
becomes active after an adjustable delay time.
 Soft-start
(5) Soft start facility prevents overshoots of the terminal voltage when building
up the excitation (field flashing). As soon as the excitation is switched on and
after the initial field flashing is complete (approx. 10% of generator voltage),
the soft-start-signal increases the machine terminal voltage with an adjustable
gradient. This signal is given priority until its value exceeds the signal from the
set point generator.
 Automatic follow-up control
In each Control Board (COB), including an Automatic Voltage Regulator
(AVR) and a
Field Current Regulator (FCR), the automatic follow-up control ensures a
bump-less changeover from automatic voltage control mode (AUTO mode) to
the field current regulation (MANUAL mode). The change over may be
initiated either by the loss of the PT’s measuring signal or by the operator (e.g.
from the Local Control Panel (LCP)). The difference signal derived from the
control signals of the AVR and the FCR (Follow up block (6)) is used for the
follow-up control of the regulator, which is not active. The follow upis
guaranteed for both the AVR and FCR [4].
In a double automatic channel configuration the changeover is normally from
AUTO mode of the active channel to the AUTO Mode of the stand-by channel.
Indeed, any of channels may be active or stand-by. In case the change over to
the AUTO Mode of standby channel is not possible, a change over to the
MANUAL mode will be initiated. Only incase that both of channels are
inoperable a trip command will be given. In this configuration the follow-up
signal to the inactive channel is derived from the difference of the control
signals of the active and inactive channels (Follow up block (7)) [4].
In a Single Channel and Back up control configuration the Backup Field
Current Regulator (BFCR) automatically follows up the Control Board (COB).
In case the Control Board (COB) fails the automatic follow-up ensures a bumpless changeover from the Control Board (COB) to the BFCR, i.e. to the field
current regulation (MANUAL mode). As already mentioned the BFCR is a
software function of the Extended Gate Controller (EGC) [4].
 Limiter gates
The limiter gates (8) and (9) determine the priority of the over excitation or
under excitation limiters over the terminal voltage control. In order to avoid that
two limiters are active at the same time (e.g. in case of system fault), a priority
flag can be set choosing which of the limiter groups (over excitation or under
excitation) acts first.
 PID Controller
The input voltage of the PID controller (11) represents the voltage error that is
the difference between an actual value and a set point. The output of the PID
controller, which is so called control voltage Uc, is required as an input signal
for the gate control unit (12).
The feedback parameters of the PID controller are automatically adjusted in a
way to achieve an optimum control performance of the synchronous machine.
Depending on that, which limiter becomes active, the Parameter Selector (10)
activates the appropriate set of PID parameters (SET1/SET2). This contributes
to the transient stability of the synchronous machine [4]. Limiter Functions
The purpose of the limiters is to maintain the operation point of the machine
within permissible limits and therefore to avoid its undesired shut-down by
operation of the protective relays.
Figure 2.15 shows a typical power chart of a salient pole synchronous machine
with the corresponding operation limits in steady state condition for 1 p.u.
terminal voltage.
Figure 2.15: Power chart of a salient pole synchronous machine.
UNITROL 5000 excitation system provides the following limiter functions:
 Over excitation limiters
 Maximum field current limiter (Figure 2.16)
 Stator current limiter over excited (Figure 2.17)
 Under excitation limiters
 P/Q limiter (Figure 2.18)
 Stator current limiter under excited (Figure 2.17)
 Minimum field current limiter (Figure 2.16)
Figure 2.16: Maximum and Minimum field current limiters.
Figure 2.17: Stator current limiter for over and under excited mode of operation
Figure 2.18: P/Q limiter.
A physical model of the Excitation system has been created with Matlab/
Simulink/ SimPowerSystems toolbox. The simulation results from the model
will be compared against measurements from the real system to see if they will
give the same results. SimPowerSystems is a more powerful tool in modeling
power systems .Instead of deriving, programming and solving dynamics
equations of the system components, SimPowerSystems provides more detailed
component blocks for modeling power systems components in the Simulink
environment. In SimPowerSystems library, there are over 100 different blocks
which including various power systems and electrical components. One of the
most important advantages of this toolbox is that a SimPower systems model
can be connected to a hydraulic or mechanical system for a multidomain
simulation. Moreover, a SimPower Systems model closely resembles the
excitation system schematic, which lets the user to understand and analyze the
model much more efficiently.The SimPowerSystemsmodel is shown in figure
(3.1) Most of the parameters of the SimPowerSystemsmodel blocks were
extracted from the components datasheets.
Figure 3.1: SimPowerSystems Model
3.1. Three phase Source model
The three phase source model represents the three phase output of the generator
and it was modeled using the AC Voltage source block (see figure 3.2).
The AC Voltage Source block implements an ideal AC voltage source. The
generated voltage U is described by the following relationship:
+ ∅)……………………………………………………...…. (3.1)
Negative values are allowed for amplitude and phase. A frequency of 0 and
phase equal to 90 degrees specify a DC voltage source. Negative frequency is
not allowed; otherwise the software signals an error, and the block displays a
question mark in the block icon. Figure 3.3 shows the parameters used in AC
voltage source block [5].
Voltage measurement Model
The Voltage measurement model represents the VT at the output of the
generator three phase and it was modeled using the Voltage Measurement block
Voltage Measurement block. The Voltage Measurement block measures the
instantaneous voltage between two electric nodes. The output provides a
Simulink® signal that can be used by other Simulink blocks [5]. The Output
signal specifies the format of the output signal when the block is used in a
phasor simulation. The Output signal parameter is disabled when the block is
not used in a phasor simulation. The phasor simulation is activated by a
powergui block placed in the model. It can be set many parameters to get the
desired output. Set to Complex to output the measured current as a complex
value. The output is a complex signal. Set to Real-Image to output the real and
imaginary parts of the measured current. The output is a vector of two elements.
Set to Magnitude-Angle to output the magnitude and angle of the measured
current. The output is a vector of two elements.Set to Magnitude to output the
magnitude of the measured current. The output is a scalar value [5].
Excitation Transformer Model
The excitation transformer was modeled using the Three-Phase Transformer
(Two Windings) block which implements a three-phase transformer with
configurable winding connections (see figure 3.6).
Figure 3.2: Three-Phase Transformer (Two Windings) block
The Three-Phase Transformer (Two Windings) block implements a three-phase
transformer using three single-phase transformers. You can simulate the
saturable core or not simply by setting the appropriate check box in the
parameter menu of the block. See the Linear Transformer block and Saturable
Transformer block sections for a detailed description of the electrical model of a
single-phase transformer. The two windings of the transformer can be connected
in the following manner:
 Y
 Y with accessible neutral
 Grounded Y
 Delta (D1), delta lagging Y by 30 degrees
 Delta (D11), delta leading Y by 30 degrees
The two windings of the transformer was connected using Delta (D1) as in the
real system.
The block takes into account the connection type you have selected, and the
icon of the block is automatically updated. An input port labeled N is added to
the block if you select the Y connection with accessible neutral for winding 1. If
you ask for an accessible neutral on winding 2, an extra output port labeled n is
generated. The saturation characteristic, when activated, is the same as the one
described for the Saturable Transformer block, and the icon of the block is
automatically updated. If the fluxes are not specified, the initial values are
automatically adjusted so that the simulation starts in steady state. The leakage
inductance and resistance of each winding are given in p.u. based on the
transformer nominal power Pn and on the nominal voltage of the winding (V1
or V2). For an explanation of per units, refer to the Linear Transformer and
Saturable Transformer block reference pages [5].
Parameters of Three-Phase Transformer (Two Windings) block are shown in
figure 3.7.
Figure 3.3: Parameters of Three-Phase Transformer (Two Windings) block
3.3. Convertor Model
The Converter was modeled using the universal bridge Block which can
implement a universal three-phase power converter that consists of up to six
power switches connected in a bridge configuration. The type of power switch
and converter configuration is selectable from the dialog box.
The Universal Bridge block allows simulation of converters using both naturally
commutated (and line-commutated) power electronic devices (diodes or
thyristors) and forced-commutated devices (GTO, IGBT, and MOSFET).
The Universal Bridge block is the basic block for building two-level voltagesourced converters (VSC) [5]. The type of Universal Bridge used in this model
is the thyristors as in the real system. Number of bridge arms: Set to 1 or 2 to
get a single-phase converter (two or four switching devices). Set to 3 to get a
three-phase converter connected in Graetz bridge configuration (six switching
devices). Snubber resistance Rs: The snubber resistance, in ohms (). Set the
Snubber resistance Rs parameter to inf to eliminate the snubbers from the
model. Snubber capacitance Cs: The snubber capacitance, in farads (F). Set the
Snubber capacitance Cs parameter to 0 to eliminate the snubbers, or to inf to get
a resistive snubber. In order to avoid numerical oscillations when your system is
discretized, you need to specify Rs and Cs snubber values for diode and
thyristor bridges. For forced-commutated devices (GTO, IGBT, or MOSFET),
the bridge operates satisfactorily with purely resistive snubbers as long as firing
pulses are sent to switching devices. If firing pulses to forced-commutated
devices are blocked, only antiparallel diodes operate, and the bridge operates as
a diode rectifier. In this condition appropriate values of Rs and Cs must also be
used [5]. When the system is discretized, the following formulas can be used to
compute approximate values of Rs and Cs:
………………………………………………………………… (3.2)
………………….…………………………………….. (3.3)
These Rs and Cs values are derived from the following two criteria:
 The snubber leakage current at fundamental frequency is less than 0.1% of
nominal current when power electronic devices are not conducting.
 The RC time constant of snubbers is higher than two times the sample time
Ts. These Rs and Cs values that guarantee numerical stability of the
discretized bridge can be different from actual values used in a physical
3.4. Rotor DC current measurement
Rotor DC current measurement was modeled using the current measurement
block and it parallels the CT in the real system .The Current Measurement block
is used to measure the instantaneous current flowing in any electrical block or
connection line. The Simulink output provides a Simulink signal that can be
used by other Simulink blocks In this model the output parameter was set to
complex .
Pulse Generator Model
The pulse Generator is a part of the COB which output pulses for the CIN and
then to the GDI of the thyristor converter. The pulse Generator was modeled
using Synchronized 6-Pulse Generator which implements a synchronized pulse
generator to fire the thyristors of a six-pulse converter.
The Synchronized 6-Pulse Generator block can be used to fire the six thyristors
of a six-pulse converter. The output of the block is a vector of six pulses
individually synchronized on the six thyristor voltages. The pulses are generated
alpha degrees after the increasing zero crossings of the thyristor commutation
voltages [5].
Figure 3.12: Synchronization of the six pulses for an alpha angle of 0 degrees
The Synchronized 6-Pulse Generator block can be configured to work in
double-pulsing mode. In this mode two pulses are sent to each thyristor: a first
pulse when the alpha angle is reached, then a second pulse 60 degrees later,
when the next thyristor is fired. Figure 3.13 below display the synchronization
of the six pulses for an alpha angle of 30 degrees and with double-pulsing
mode. Notice that the pulses are generated 30 degrees after the zero crossings of
the line-to-line [5].
3.6 The PID Controller Model
The PID Controller Model was modeled using the PID Controller subsystem as
in figure 3.16.
Figure 3.16: PID Controller subsystem
The range of firing angle is specified to be up to (150 deg) same as built in
the existing system, and in order to control the (DC) output voltage,
Proportional integral derivative method was used, which is also incorporated in
the control algorithm of the existing ABB unitrol5000, to control the firing
angle required to trigger a three phase thyristor converter. Nickolas-Ziegler
method is used for tuning the PID gains necessary to reach the desirable
stability band.
3.7 Exciter Mathematical Model
the establishing the missing dynamic link between the field voltage vf and the
synchronous generator terminal voltage |V|. Considering the field of the
synchronous generator, giving:
Δvf = Rf Δif + Lf f dt/d* (Δif) ...…………………………..........….……. (3.4)
Taking Laplace transform
( ) =
_ +
(s).................................................................. (3.5)
As the terminal voltage equals to internal emf minus the voltage drop across the
internal impedance, it is clear that the relationship between vf and |V| depends
on the generator loading. The simplest possible relationship exists at low or zero
loading in which case V approximately equals to internal emf E. In the
generator, internal emf and the field currents are related as [6]
∗ )/√2 ................................................................................... (3.6)
is the mutual inductance coefficient between rotor field and stator
Δif =√2 ∗
............................................................................................. (3.7)
Laplace transform of above eq. gives
Δif (s)= √2 ∗
(S)................................................................................... (3.8)
( )=
Substituting the above in eq. (3.7),(3.8),
( ) = √2/
( ) = (
)/√2 ∗ 1/(
( ) results in
( ) ...................................................(3.9)
( )...........................(3.10)
From the above equation. the field voltage transfer ratio can be written as
∆ ( )
( )
∆ ( )
∆ ( )
.................................................. (3.11)
√ ∗
In Chapter 3, excitation system model was developed. The objective of this
chapter is to verify the model experimentally. For verification, measured
variables on the real system will be compared with simulation results.
In this study, measurements are done directly on the real excitation system and
measured variables are:
 Excitation voltage at no load in Volts.
 Excitation current at no load in Amps.
Furthermore, the frequency response of the model will be analyzed and the
linear model is obtained using the linearized frequency response data.
The first order system transfer function parameter is compared to the real plant
Step response comparison
Figures 4.1 and 4.2 show the step response of voltage and current at no load
respectively. As mentioned before, Rosiers Power Plant consists of seven units,
so unit 4 was taken as sample and its step response was obtained from the trends
of POS clients in the CCR.
Table 4.1: Comparison between Simulation and Experimental Results
Field Voltage (Volts)
Field Current (Amps)
Simulation Error Experimental
Figure 4.1: Field Voltage step response
It can be seen from table 4.1 that the model step response conforms to the real
system step response which is found in Appendix A.
Figure 4.2: Field Current step response
4.2. Closed loop Frequency response:
In order to identify the excitation model characteristics, a closed loop frequency
response has been carried out, using sine sweep chirp signal start frequency
sweep from 0.1 Hz up to 100 Hz. The model response data has been sampled
over a period of 2 sec, using sampling frequency of (1000Hz). The resulting
data of the inputs and out puts are saved to MATLAB work-space.
In order to get the magnitude and phase of the model response, FFT method is
used with the same sampling frequency, sweep frequency range and time
interval of 2 sec an M-file containing the code for calculations of (FFT) is
attached in appendix (B).
The resulting plots are shown in figure 4.3.
Figure 4.3: Closed Loop Frequency response
The built in basic fitting function in matlab is used to linearize the frequency
response characteristics [see fig 4.4], from the it is clear that frequency
corresponding to -3 dB point is about (11 Hz), which approximately matches
the cut off frequency given by (R/L) about 12.3 Hz for the rotor winding, this
fact verifies that the band width of the exciter is dominated to some extent by
the characteristics of the generator rotor winding. The deviation in band limit is
a bout (9.7%), which is due to commutation impedance which affects the
thyristors switching speed, and thus the overall performance of the system.
However, the non-linear model response is not consistent with the linearize
model response at higher frequencies. This is the result of linearization, with the
increasing excitation frequency the operating points where the linearization is
performed changes.
To obtain the transfer function, we proposed a second order approximation for
the system; the linearized model response shown in [fig 4.4] is used as follow:
From the figure maximum overshoot is about 6.31 dB giving amplitude ratio
of M = 2.06. The cut off frequency is read from the -3dB which is 11 Hz, so the
transfer function H(s) is:
H(s) =
..... (4.1)
ωn: is the system cutoff frequency =11 Hz
K: is the system gain.
ξ: is the damping factor, which can be calculated using the relation between it
and the maximum overshoot (M) as follow:
M = e [-2ξ/root (1- ξ 2]
ξ = root (M2/ [4+M2]) ................................................................................. (4.2)
And it is found to be 0.47 .The transfer function gain (K) is taken to be equal to
the system gain of (2.06/ ωn2) so the resulting transfer function is:
H(s) =
.................................................... (4.3)
(0.0083S2 +0.0855S+1)
From the transfer function it is clear that the system tends to behave as
integrator, because the second order term has very small effect on the transfer
characteristics, so the system is reduced to a first order with the following
transfer function:
H(s) 1st-order = 0.02/0.0855S+1. ........................................................................ 4.4
Fig 4.4: Linearized Frequency Response of the Model
We can see from fig 4.4, the time constant of the first order approximation
(τ =0.0855), is an important parameter, because at time t= τ, the response of the
system reaches 63.2% of its total change. This can be verified from the
linearized system response shown in fig 4.5, at time 0.0855 sec the system
response is (85.884 volt) about 63.15% of 136 volt. On the other hand the
parameter (1/ τ) equal 11.7 approximates the band width of the linear system
mentioned above.
Time [s]
Fig4.5: Equivalent first order system response
This research, a detailed description for the following topics is presented:
- The functions and performance requirements of excitation systems in
- The structure and type of excitation systems is presented,
- A description of the existing excitation system in Rosaires Hydro Power
Plant presented in chapter two.
- In chapter three, a detailed description of a three phase source, voltage
measurement, and excitation transformer models.
- Also, structure of models for the converter block, rotor current
measurement, pulse generator and firing angle controller is detailed.
- In chapter four, a step response is carried out to determine system
stability characteristics.
- Also in this chapter, a closed loop frequency response is carried out
using sine sweep signal(chirp signal) as input to estimate the stability
bandwidth and to determine the linear model of the excitation system.
We recommend a double channel excitation system, which consists of two
converter units operating in parallel. Each converter supplying 50% of the rotor
field current and capable of supplying the rotor rated field current compared to
the single converter this type will facilitate the following advantages:
- If one converter tripped the other will take the load without any
disturbance to the generator terminal voltage.
- No need for complete unit shutdown to maintain excitation system.
- Increase of converters life time due to partial loading.
- Enhancement of system overall response.
[1] Chan-Ki Kim, Hong-Woo Rhew and Yoon Ho Kim, “Stability performance
of new static excitation system with boost-buck converter” Korea Electric
Power Research Institute (KEPRI) 103-12 Munji-Dong, Yusung-Gu, Daejon,
[2] K.N. Shubhanga, “Transient Stability-Constrained Generation Rescheduling
Sensitivities" Ph.D. Thesis submitted to IIT Bombay, 2003.
[3] Kundur, “P. Power System Stability and Control” McGraw-Hill. Fourth
edition, 1994.
[4] The Mathworks Inc, “Sim power system Reference” 2014.
[6] IEEE Recommended Practice for “Excitation Systems Model for Power
System Stability Studies” IEEE Standard 421.5-1992.
Figure A-1: Field voltage Peak overshoot value from POS client for real system
Figure A-2: Field voltage steady state value from POS client for real system
Figure A-3: Field Current Peak overshoot value from POS client for real system
Figure A-4: Field current steady state value from POS client for real system
MATALB m-files
Pos(:,1)= vout;
Sp(:,1)= vin; % Reference Position
fs=1000; % Sampling Rate [Hz]
tstart=0.1; % Start Time [s]
tend=2.1; % End Time [s]
FreqMin=0.1; % Minimum Frequency [Hz]
FreqMax=3.1; % Maximum Frequncy [Hz]
Freq_Inc=0.049975012; % Frequency Increment [Hz]
for i=1:1
out(:,i)= Pos(:,i);
% Remove the 'linear' trend of the output
% Calculate the FFT of the input and the Output
% Take the Avarage FFT
for i=1:length(out_fft)
% Frequency Array
%% Bode Plot
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