Project
-
Upload
mounika-reddy -
Category
Documents
-
view
979 -
download
1
Transcript of Project
CHAPTER 1
INTRODUCTION
11 Brief Description of the Excitation System
The static excitation equipment regulates the voltage (andor the flow of reactive
power during parallel operation) from the synchronous machine (generator) by direct
control of the rotor current (field current) using (static) Thyristor converters
The entire unit can be broken down into four major groups
bull The excitation transformer T01
bull The control unit REG
bull Thyristor converter TY
bull The field breaker field flashing and de-excitation equipment FF amp F
In excitations with shunt-connected supply there is no enough remnant voltage in
the rotating generator to build up the generator voltage autonomously via the converter
To accomplish this special field flashing equipment is needed When the field flashing
equipment is being supplied with power from a DC power source (power station battery)
resistor is used to limit the field flashing current When it is being supplied from an AC
power grid a transformer serves as the adapter needed
Excitation of the generator is started by closing the field circuit-breaker and the field
flashing breaker This supplies current to the field which excites the generator up to 15
30 of Generator voltage The generator then supplies voltage to the converter voltage
the firing electronics and the converters are able to continue the voltage build-up so that
the field flashing circuit is relieved of current Once the voltage exceeds approx 70 of
Generator voltage the field flashing breaker is opened having no current The diode
bridge at the input to the field flashing breaker prevents a backflow of current to the field
flashing source
1
To accomplish this special field flashing equipment is needed When the field
flashing equipment is being supplied with power from a DC power source (power station
battery) a resistor is used to limit the field flashing current When it is being supplied
from an AC power grid a transformer serves as the adapter needed Excitation of the
generator is started by closing the field circuit-breaker Q1 and the field flashing breaker
Q2 This supplies current to the field which excites the generator up to 15 30 U The
generator then supplies voltage to the converter via the excitation transformer Starting
from approx10 of the generator voltage the firing electronics and the converter are
able to continue the voltage build-up so that the field flashing circuit is relieved of
current Once the voltage exceeds approx 25 of U the field flashing breaker is finally
opened having no current The diode bridge at the input to the field flashing breaker
prevents a back-flow of current to the field flashing source The converter TY has Final
Pulse Stage cooling and monitoring of the elements Redundancy in the regulator section
is ensured by means of two fully separate channels with independent measuring inputs
and extensive monitoring (ldquoSUPERVISIONrdquo)
Channel 1 (AUTOMATIC channel) is built as voltage regulators and is ON during
normal operation In addition to the voltage regulator which has a PID control algorithm
2
the AUTOMATIC channel also contains various limiters and corrective control circuits
to ensure the use and stable operation of the synchronous machine up to its operating
limits This channel possesses a Gate Control Unit with a subsequent Intermediate Pulse
Stage to generate the firing pulses for the Thyristor converter During normal operation
the Intermediate Pulse Stage of AUTOMATIC Channel is active and transmits the firing
pulses galvanically separated to the common pulse bus at the input to the Final Pulse
Stage Various monitoring functions of the AUTOMATIC channel and pulse monitoring
on the common pulse bus initiate an automatic switch-over to stand by Channel
(MANUAL)in case of a malfunction
Channel 2 (the MANUAL channel) is built as a simple field-current regulator with a PI
control algorithm It serves as a back-up channel in case of a malfunction on the
AUTOMATIC channel Manual channel performs valuable service for testing
commissioning and preventive maintenance The MANUAL channel has its own Gate
Control Unit (the software for the If regulator is also implemented therein) and its own
intermediate pulse Stage During normal operation (AUTOMATIC) the output pulses
from Intermediate Pulse Stage are blocked from reaching the pulse bus Various
monitoring on the MANUAL channel initiate an alarm in case of a malfunction while the
MANUAL channel is on stand-by If the MANUAL channel suffers a malfunction while
it is in operation the excitation is switched off (TRIP) Both channels are equipped with
tracking equipment so that the inactive channel always generates the same control
variable as the active channel during steady-state operation
This ensures smooth switch-over from Automatic to Manual channel and vice
versa To ensure that the MANUAL channel will in a switch-over initiated by a
malfunction take over the operating point of the machine as it was prior to the problem
the response of the tracking for the MANUAL channel is set relatively slow In addition
to the pulse monitors (ldquoSUPERVISIONrdquo) shown in the basic circuit diagram the
excitation system has an autonomous Excitation Monitoring As one of its functions this
equipment monitors for field currents that exceed acceptable maximum limits It initiates
an emergency switch-over to the MANUAL channel whenever the field current exceeds
the preset limit If even after such a switch-over the field current does not drop back to
3
the permissible level the excitation is switched off by Excitation Protection The most
important measuring inputs for the excitation system (If Ug Usyn) are redundant (2-
fold) The Excitation Monitoring checks these measuring inputs for discrepancy and
plausibility An alarm is always initiated in case of malfunction In certain cases a
switch-over to MANUAL channel is also initiated The excitation system contains an
Excitation Protection to protect the excitation transformer the converters and the
synchronous machine The protection system can detect short-circuits in the excitation
circuit and keep secondary damage within acceptable limits by a quick tripping of the
excitation and an opening of the generator breaker An overheating of the excitation
transformer first sets off an alarm (at a given preset limit) and then likewise initiates a
protective shut-down at an even higher limit The over voltage protection in the de-
excitation equipment provides an autonomous protective function for the rotor and the
rectifier This protection system monitors the field voltage in both polarities for over
voltage and if necessary de-energizes the field via the de-excitation resistor
12 Principle of primary power supply
In the shunt excitation system the excitation transformer also provides
the power supply for the electronic equipment and the converter fans So a failure of the
auxiliary supply to the converter fans does not cause a shutdown of the excitation When
the auxiliary supply fails the supply to converter fans is switched over to Excitation
transformer OP with a contactor A station battery supply is absolutely necessary for the
control of the field circuit breaker It is the power source for the electronic devices till the
generator is able to supply voltage Auxiliary power to the field flashing equipment must
be present in order to build up the generator excitation The power supply for standstill
heating and the cubicle lighting is also from Station Auxiliary Power Supply and is of
secondary importance for operation of the plant Power supply to rotor earth fault
detection circuit too is from Station Auxiliary Supply The two synchronous voltages
Usyn are each supplied to the AUTOMATIC channel and the MANUAL channel
separately across transformers The Gate Control Units need these voltages to enable
4
them to issue the pulses at a given firing angle relative to the input voltage of the
converter
5
CHAPTER 2
Digital Automatic Voltage Regulator (DAVR)
21 Principle of Operation of the Regulator (DAVR)
To regulate the voltage and the reactive power of a synchronous machine the
field voltage must be adjusted quickly to the changes in the operating conditions (with a
response time that does not exceed a few ms) To accomplish this analog control systems
include amplifiers which make continuous comparison of the actual values against the
reference values and vary the control variable to the converter with almost no delay Most
of the delay that occurs originates in the converter since the firing pulses for changing
the rectifier phase angle are only issued periodically (every 33 ms)
The DVR digital voltage regulator calculates the control variable from the
measured and reference data in very short time intervals This results outwardly in a
quasi-continuous behavior with a negligible delay time (as in an analog regulator) The
calculations are made in the binary number system Analog measurement signals such as
those for generator voltage and generator current are converted into binary signals in
analogdigital converters The set-points and limit values have already been defined in
digital (binary) form An understanding of the actual computation processes in the digital
voltage regulator is not necessary for operation preventive maintenance or
troubleshooting Like the operator of a pocket calculator or a personal computer all the
operator needs is to know how to operate the instrument and the programming for this
working tool For that reason we will explain below only the principle division of work
among the various modules and the flow of data processing The purpose is above all to
make clear how the processor system has been integrated into the rest of the power
electronics system
6
22 Basic Structure of the Processor Systems
7
The signal processors 25 analog inputoutput modules Each of these processor
systems has a common bus circuit and output and the control lines There is a specific
range of addresses assigned to each assignment Board including the power supply bus
the address lines the two data lines to the input calculates the reactive current (I sin φ)
and the active current ( Icos φ) With these two channel processor Synchronized with
these interrupts (ie with the phase positions of current Ig the field current If and the
synchronous voltage Usyn From the exchange data with the microprocessor card across
the two data lines generator voltage Ug) this processor measures the generator current
Ig and then hardwired connections or multi-conductor cables Binary and analog
inputoutput modules ie for galvanic isolation and adaptation to the electronics level
The most important input interrupts per period to trigger the cycles for processing actual
values in the AUTOMATIC module on the processor bus) for filtering and further
processing
Monitoring each consist of the central microprocessor module and binary and
parameters to the AUTOMATIC channel are the generator voltage Ug the generator
peripheral unit Ug Ig and Usyn are sent to the Interrupt Generator (plug-in peripheral
units (wall-mounted units) peripheral units are used for preprocessing signals from
external measurement circuits power supply units Signals are exchanged among these
processor systems via processed across separate peripheral units for each channel These
processor working on the bus (a house address that can be adjusted using a switch)
systems The AUTOMATIC channel the programmable controls and the Excitation The
actual values measured from AUTOMATIC channel and MANUAL channel are The
AUTOMATIC channel and the MANUAL channel each have their own The digital
voltage regulator is broken down into several autonomous microprocessor The inputs and
outputs of the processor systems are directed across voltage-isolating The Interrupt
Generator also uses the 3-phase Ug signal to generate the 12 themselves contain a limited
number of hardware inputs and outputs with fixed equipment Whenever addresses from
this range are called up the signal processing module can results the processor is then
able to derive further operating parameters such as the load angle the active power etc
The functions of all microprocessor systems other than the programmable controls
have been accomplished in firmware The non-varying standard function modules can be
configured to the design desired for plant-specific purposes using software switches
(KFlags) Thus for example the stored status of a K-Flag determines whether or not a
Limiter is active and whether the de-excitation or the excitation limiters take precedence
Because these K-flags determine the software Scope of Supply for the installation they
cannot be changed permanently via the Micro-Terminal In this way they differ from
such setting data as the values of the parameters for the PID filter of the voltage regulator
or the set-points for the limiters These values can be permanently changed using the
Micro-Terminal Communication is possible with each of the processor systems via the
Micro-Terminal by plugging on the connecting cable In this way signals within the
processor and setting parameters can be viewed analog signals can be issued and the set
parameters can be altered temporarily (F range) or permanently (C range) Unlike the
other processor systems the programmable controls do not include any firmware for
realization of the functions They have been designed so that the designer can adapt and
change their functions easily using the ldquoFunctional Block Programming Language P10
Digital and analog functions can be implemented in practically any degree of complexity
desired using the P10 functional blocks The control variable of the voltage regulator
(AUTOMATIC channel) and the control variable of the field current regulator
(MANUAL channel) are each processed in separate Gate Control Unit and formed into a
chain of pulses at the appropriate firing angle The pulses of the active channel are
directed to the pulse bus via the associated Intermediate Pulse Stage The pulses for each
converter block are amplified sufficiently in Final Pulse Stage to fire the Thyristor
231 General Information
The functions of the automatic voltage regulator AVR are
1 to regulate the generator voltage
2 to regulate the effect of the reactive andor active current on the voltage
3 to limit VoltHz
4 to limit max and min field current
5 to limit inductive stator current
6 to limit capacitive stator current
7 to limit the load angle
8 to stabilize the power system
Block Diagram shows the software structure of AUTOMATIC channel The
generator limiters not provided for the installation in question (optional equipment) are
identified in this overview as ldquoNot Suppliedrdquo The parameter values signal values and
software switches (flags) marked with addresses (hexadecimal numbers) can be viewed
and altered via the Micro-Terminal The values selected are displayed in sec pu Hz
etc and can where necessary be changed directly in these formats The plant-specific
settings of the variables and the flags can be obtained from the Test and Commissioning
Report This block diagram provides information about the important functions and
possible settings of the AUTOMATIC channel For the sake of clarity no detailed
presentation has been given of special functions such as tracking circuits initializations
etc The page heading cross-refers this overview to the various sheets of the schematic
diagram Binary signals are shown in broken lines analog signals in solid lines The
corresponding text designations in the schematic diagram can be used for identification of
the input signals (hardware inputs) The only analog output signal from the automatic
voltage regulator control variable Ucontr is sent via the data bus (CRU bus) to the Gate
Control Unit Most of the binary messages (outputs) from the AVR are of no interest
functionally and they have been omitted for the sake of clarity The basic structure of the
digital voltage regulator and the limiters is simple This is necessary in order that the
behavior of the regulatorslimiters will remain calculable and understandable in all
operating situations and that there will be no problem in adjusting and optimizing them
The central PID filter in the digital voltage regulator defines the dynamic response of the
closed-loop controls both in the voltage regulator mode and after limiters have
intervened The ldquocontrol deviationrdquo at the input to the PID filter is either the control
deviation for voltage the control deviation of a de-excitation limiter (the value
determined by minimum value selection) or the control deviation of an excitation limiter
(the value determined by maximum value selection) Flag F730 (ldquoPRIORrdquo) is used to
determine whether the exciting (Min value) or the de-exciting signal takes precedence on
the minmax value limiter (normally F730 = 1111 ie the de-exciting signal takes
precedence) With the exception of the Minimum Field Current Limiter all other limiters
have variable factoring multipliers of the signal outputs so that they can be adjusted
individually together with the common PID filter which has been optimized for voltage
regulation The setting parameters for this PID filter are as follows
Vo = KR Static amplification
1
Ta = ---- Integration time constant
Tc1
Vp Proportional amplification
1
Tb = ---- Differential time constant
Tc2
Vinfin Amplification of high frequencies
The BODE diagram below shows the assignment of settings in accordance with
DINIEC standards based on a typical example
The PID filter amplifications Vo Vp and Vinfin can be adjusted in pu values But
the ldquoceiling factorrdquo pl+ must be adjusted correctly with parameter F310 if the total
amplification (circuit amplification) of the control circuit is actually to conform to the
pu settings This factor must agree with the ldquoexternalrdquo amplification ie with the
ceiling value of the transformer- converter circuit
Ceiling factor(pl+) = Ufmax Ufo
in which Ufmax = ceiling field voltage
Ufo = no-load field voltage
To attain a suitable response of the AVR when starting excitation
(ldquoEXCITATION ONrdquo) it may be necessary to change the proportional amplification of
the regulator during this phase Vp2 (transiently activated) and Vp1 (permanently
activated) can be adjusted for this purpose For example the value of Vp2 takes effect
immediately once the excitation is switched on and remains effective for a period as set at
F30C Once the period F30C (eg 5 sec) has expired Vp shifts over to Vp1 (becomes
the steady-state Vp) at the rate of change set The standard operating mode for the PID
filter is voltage regulation for which the discrepancy between the voltage set-point and
the current value for generator voltage Ug (the control deviation) is supplied at the input
To compensate for the voltage drop in the block transformer or whenever several
generators are operating to the same distributing bus the generator voltage must be
varied in proportion to the measured generator current (droop influence) To accomplish
this the voltage set-point is varied as a function of the measured reactive current IX
andor active current IR Flag F712 enables the IX droop Flag F710 the IR droop The
desired compensation is set in F282 and F286 respectively Flags F284 and F288 are used
to select whether this droop influence is to increase the voltage or to reduce it
(compensation) Combined influence of the active and reactive currents is attained by
enabling both droops IX and IR Flag F716 activates a so-called ldquoSoft-Startrdquo at the
starting of excitation This ldquoSoft-Startrdquo ensures that the voltage set-point integrates from
0 to 100 within the time set on F290 when the excitation is switched on
(ldquoEXCITATION ONrdquo) A ldquosmoothrdquo excitation of the generator can be achieved in this
way whenever there is no demand for a quick excitation
232 Voltage Set-Point
Various signals and settings control and limit the voltage set-point F270 For
example the values of F254 and F252 define the normal operating range possible for set-
point adjustment (eg 90 110) using external control commands (control room local
operatorrsquos panel superposed control system) The effective set-point adjustment rate is
governed The set-point can be set at the values of F250 and F256 by activating
appropriate control commands for ldquoSETrdquo input Enabling Flag F71A and activating a
binary input prior to switching on the excitation (ldquoEXCITATION OFFrdquo) sets the Ug set-
point at the value of UAUX This makes it possible for example to ensure that the
generator voltage will agree exactly with the network voltage after the voltage build-up
An external value with variable amplification can be added to the Ug set point by
enabling F724 (for example for stability tests)
233 Regulator Tracking in MANUAL Operation
Whenever the AUTOMATIC channel is not in operation (the MANUAL channel
is ON) a follow-up equipment ensures a smooth switch-back to the AUTOMATIC
mode will always be possible To track the voltage set-point is shifted by means of
RAISELOWER pulses from the Gate Control Unit so that control variable Ucontr at the
output from the PID filter is held steady and identical to the control variable Ucontr from
the MANUAL channel Because this tracking must react slowly resultant transient
control deviations resulting from the amplification in the PID filter might cause severe
interference with control variable Ucontr
To prevent this the follow-up equipment intervenes on the regulators mixing
point with a corresponding compensation signal
234 Ugf Limiter
At under frequency the Ugf Limiter reduces the generator voltage so as to
prevent saturation effects in the supply and measuring transformers To adjust this
limiter the max permissible generator voltage at rated frequency is defined and set
When any under-frequency occurs the generator voltage is thus reduced in proportion to
that setting
235 Field Current Maximum Limiter
The Field Current Maximum Limiter is provided to protect the generator rotor
from s occurring in steady-state and transient operation High field currents are normally
the result of a sharp drop in network voltage or of an improper raising of the voltage set-
point by the operating staff The field current is held steady at the value TH1 ie at the
maximum thermal value permissible for the excitation circuit and the rotor In order that
the generator can support the power network with its transient overload capacity during
brief collapses in voltage a temporary switch-over is made to the transient limit MAX1
(a higher setting) When the generator or the converter is operating at a reduced capacity
These limits TH1MAX1 can be switched over to the lower settings TH2MAX2 by
activating the corresponding binary signals The switch-over from the thermal limit
TH12 to the transient limit MAX12 can be configured in one of three ways
a) Depending on the over current with -dUdt ENABLE
Flag programming F418 = any setting desired F41A = 0000
This variant enables the transient value MAX12 whenever a collapse of voltage
in the network is detected The ENABLE time is fixed and can be set The example
below shows the typical behavior of the limiter configured in this way
b) Dependent on the time integral with -dUdt ENABLE
Flag programming F418 = inactive F41A = 1111
This variant likewise enables the transient value only when a collapse of network
voltage has been detected However the switch-back to the thermal limit is not made
dependent upon the time itself but on the calculated time integral intisup2dt of the The setting
on Parameter F414 in spu takes into account the time the rotor needs to cool down ie
the rate of temperature change in the case of intermittent operation The example below
shows how the timing of the switch-back to the thermal limit depends on the present
value for intisup2dt
The time integral is based on the formula
Example The setting of =isup2dt equivalent to Version a (F416) at a constant 16 times the
nominal field current for 10 seconds (with TH12 = 105) is
c) Dependent on the time integral without any preconditions
Flag programming F418 = 1111 F41A = 1111
In this variant the transient becomes available without any prior conditions
(without a -dUdt ENABLE) with the time integral intisup2dt
237 Inductive Stator Current Limiter
The Inductive Stator Current Limiter holds the stator current Ig within permissible
limits while the generator is in the ldquoover-excitedrdquo operating range by reducing the field
current accordingly The setting TH (thermal limit) provides the limit against stationary s
that might occur To take advantage of the generatorrsquos transient overload capacity a
switch-over is made to the higher setting MAX The principle of operation of this switch-
over to the value MAX permissible only transiently is identical to that employed for the
field current limiter (refer to the description above) When the drive output from the
turbine is very high stator current may exceed permissible limits even while inductive
loading of the generator is low In this case if the stator current limiter is not kept from
influencing the field current the control circuit will oscillate back and forth between the
Inductive Stator Current Limiter (de-
exciting) and the Capacitive Stator Current Limiter (exciting)The output signal of that
function then dominates the control variable of the Ig-dependent limiter via a maximum
value selection
238 Capacitive Stator Current Limiter
239 Load Angle Limiter
The Load Angle Limiter prevents the synchronous machine from slipping out of
phase due to slippage of the rotor The load angle δ the difference in phase between the
rotor and the stator rotating field results mainly from the driving torque (active power P)
acting on the generator and the level of rotor current (field current) If the driving torque
remains constant a increase in the field current reduces the load angle δ The current load
angle δ at any moment is obtained from the generator current and generator voltage based
on a simplified model of the generator Whenever this calculated load angle δ exceeds the
preset limit angle the limiter increases the field current until the load angle has dropped
back to its permissible value The quadrature reactance Xq of the generator and the
network reactance Xe during normal operation must be adjusted on the regulator in order
to obtain the load angle δ The graph below shows the Power Chart for a salient-pole
machine with typical limiter characteristics
The purpose of a Power System Stabilizer is to use the generator excitation to
damp electromechanical oscillations between the network and the generator Depending
on the design of the generator and the requirements imposed for network stability its
main function will be either to damp the oscillations originating in the machine or those
from the network A synchronous generator working in a combined power network is in
principle an oscillating structure In order to produce a torque the magnetic field of the
rotor and the stator must form a given angle (referred to as the rotor displacement or load
angle δ) The electrical torque ME increases as the angle δ increases just as with a
torsion spring Because the ME of the generator and the mechanical driving torque MA
from the turbine are in equilibrium during steady-state operation the angle δ remains in a
given position Whenever this state of equilibrium between MA and ME is disturbed the
load angle slips of this rest position and change thereby the electrical torque ME The
torque attempts to restore the load angle to a stationary position Due to the mass inertia
of the turbinegenerator rotor however this can only take place aperiodically It does so
in the form of more or less effectively damped oscillations (again similar to the effect of
mass inertia on a torsion spring) In order to damp the oscillations there must be a
damping torque produced depending not on the electrical torque ME associated with the
angle but on the difference in frequency (Df) between the rotor and the stator rotating
field ie on the slippage This torque is produced mainly by the so-called damper
winding in the rotor but the dimensioning of this is subject to limits imposed by
considerations of design and economy Some further action is therefore needed to
increase the damping effect The following drastically simplified formula shows the
parameters upon which the amount of active power PE supplied by the generator
depends
PE = active power
It can be seen from the above relationship that the active power that the generator
transfers depends not only on the load angle δ but also on the field current If That means
that a transient change can be made in the active power PE and with that in the effective
electrical torque ME by varying the field current The principle of operation of the DVR
Power System Stabilizer becomes clear from a consideration of the oscillations in power
output and frequency (ΔPE Δf) and the vector diagram If it is assumed that oscillations
in the network frequency generates load oscillations with the mass inertia of the rotor
then the active load of the generator (eg MW-measured) is influenced with a sinusoidal
value -ΔPE (ME-MA = -ΔPE) By inversion of -ΔPE one obtains the fluctuation in
power provided by the rotor +ΔPE As is known the slip signal Δf follows +ΔPE with a
phase delayed by 90deg The +ME produced by the periodic changes in the load angle δ is
in phase with +ΔPE A good damping is attained if ME is varied in phase with the slip
Δf However this signal must also be advanced somewhat to compensate for the time
constants in the excitation circuit and the generator
As mentioned above the electrical torque ME can be influenced by varying the
field current To accomplish this a suitable control signal referred to as variable
disturbance compensation must be imposed upon the voltage set-point or the converter
control variable Ucontr As can be seen from the vector diagram by applying proper
weighting factors (K1 K2) and then adding together the signals -ΔPE and Δf an overall
stabilization signal can be produced that rotates in advance of the Df signal by any angle
desired between 0deg and 90deg Because the amplitude of -DPE remains proportional to the
amplitude of Δf a constant angle in advance of Δf results for the compensation of the
time constants referred to above The optimum weighting factors K1 and K2 for a
synchronous generator working to a power network depend on its operating point at any
moment and the external reactance of the network Normally the selection of a
compromise setting is good enough to attain stability in all operating points and for all
external reactance For special demands these settings must be parameterized as a
function of the external reactance (which means optional equipment Xe-Identification)
The Power System Stabilizer PSS is a section of the AVR computer program and is
processed once per network cycle The voltage at the generator terminals and the
generator current are measured in order to define the signals ΔPE and Δf The calculated
signals for _P_ (=PE) and Δf are then sent across DC filters ldquoDrdquo (real differentiators) that
transmit only the dynamic portion of the signals The ΔPE and Δf signals obtained in this
way are then weighted (multiplied by) with the factors K1 and K2 and sent to the
summing point of the voltage regulator
The PSS stabilization signal is imposed on the automatic voltage regulator only if
the following prerequisites are met
bull Generator on line
bull Generator power output gt the value F338
bull Generator voltage in a range between F33C and F33A
The stabilization signal is limited at the output from the PSS to the lower and
upper limits Flag defines whether the stabilization signal is introduced before or after the
PID filter (usually before the filter) Because the PID filter as noted above already takes
the ceiling factor Vp1 into account the PSS signal needs to be multiplied by Vp1 if it is
added to the voltage regulator following the PID filter (divider at the input to the
minmax limiter) This precaution prevents the DC filter ldquoDrdquo in the P-channel from
producing an unnecessary ldquostabilizationrdquo effect in the case of rapid changes in turbine
load As an alternative for the AVRrsquos Power System Stabilizer a stabilization signal from
an outside system can be imposed by activating the binary input ldquoPSS-SIGNEXTrdquo Flag
F340 can be used to select between an analog and a 12-bit signal and F33E to select the
polarity desired for that signal
24 The MANUAL Channel
241 Summary
The MANUAL channel (Channel 2) has been built as a simple field current
regulator
without additional limiters Its main function is to maintain the excitation of the generator
even if the AUTOMATIC channel becomes non-operational The MANUAL channel
also performs valuable service for purposes of testing commissioning and preventive
maintenance Its measurements regulator generation of firing pulses and power supply
are physically separate from those on the AUTOMATIC channel
242 Principle of Operation
All the functions of the MANUAL channel including the generation of firing
pulses have been implemented in a single electronic module the Gate Control Unit The
control variable Ucontr of voltage regulator is used as the reference value for generating
firing pulses on the principle known as ldquoramp controlrdquo (Comparison of Ucontr with
Usynsynchronous sawtooth signal) For further processing in the UN 0096 Intermediate
Pulse Stage the Gate Control Unit supplies six firing pulses at its output whose phase
position with respect to the synchronous voltage Usyn is in accordance with control
variable Ucontr An internal linearization ensures that the field voltage produced via the
firing pulses remains proportional to the control variable Ucontr throughout the entire
range As a result the circuit amplification of the control remains constant over the entire
range Whenever excitation is switched ON the set-point for Generator Voltage is set
automatically at the preset - ref Value This provision ensures that the generator voltage
always attains approximately its nominal value after the field flashing The Gate Control
Unit can be refunctioned ( by pre-selection with a switch ) for purposes of testing to act
as a purely firing pulse control In this case the control variable Ucontr is adjusted
directly using the RAISELOWER push buttons on the front of the module In this way
for example the relationship between the phase position of the firing pulses and the
control variable Ucontr can be checked easily
CHAPTER 3
PULSE SECTION
31 Pulse Generation and Amplification
The Gate Control Units of both AUTOMATIC channel and MANUAL channel
each supply six firing pulses for operating the 6-pulse thyristor bridges The low-power
pulse signals from these Gate Control Units are then amplified in the Intermediate Pulse
Stage galvanically isolated and then sent to the common pulse bus On the output end
the Intermediate Pulse Stage of the non-active channel) is always blocked The Gate
Control Units generate the pulses based on microprocessor control The reference voltage
used for the firing pulse phase location is the output voltage from the excitation
transformer (Usyn1 Usyn2) The commutation spikes of the synchronous voltage caused
by the converter are calculated prior to use of the voltage as a reference value and are
deliberately filtered out The lower limit for the firing pulses (double pulses) which are
offset from one another by 60deg is defined by the limit rectifier position (αmin) and the
upper limit by the limit inverter position (αmax) for the firing angle αmin and αmax can
be adjusted on the Gate Control Units using BCD (Binary Coded Decimal) switches
αmin ensures that the firing pulses will not be issued (premature firing) until there is
sufficient positive phase voltage on the thyristor involved αmax prevents a dangerous
ldquotippingrdquo of the thyristor bridge into the rectifier mode if the firing angle α is too large
(ldquolate firingrdquo) The critical factors determining αmax are the overlap time uumlmax (max
commutation time) and the ldquorecovery timerdquo of the thyristors (αmax lt 180deg - uumlmax - γ )
An external control signal can force the firing pulses into their inverter limit position
Other binary inputs can block or direct the firing pulses of the Gate Control Units so as to
produce freewheeling on the thyristor bridge During freewheeling the firing pulses for
the thyristor pair R and S are blocked and the pulse signals T+T- are engaged with
chains of pulses Both Gate Control Units (for the MANUAL amp AUTOMATIC channels)
contain a field current monitor that blocks the firing pulses immediately whenever the
current exceeds a preset threshold level In this case the field circuit-breaker is also
tripped via an output contact The purpose of these provisions is to prevent damage to
thyristors and thyristor fuses in case of a slip-ring short-circuit or to keep any damage
that does occur to a minimum The pulse signals are galvanically separated at the outputs
from the Intermediate Pulse Stage (with pulse transmitters) and are then directed to the
common pulse bus This transmission of the pulse signals to the pulse bus via passive
transmitters ensures a high degree of active channel autonomy Practically no possible
malfunctions on the inactive channel (including for example sustained pulses) affect the
active channel
32 Pulse Monitoring
The ldquoPulse Busrdquo and the pulse signals of the AUTOMATIC channel are
monitored This monitoring device consists of potential isolating stages and the common
monitor If the pulse monitoring of the ldquoPulse Busrdquo responds a switch-over is made to
MANUAL channel The function of the potential isolating stages is to couple the pulse
monitoring device to the pulse circuits without any feedback effect The pulse monitoring
checks the six pulse lines for the following malfunctions continuous or periodic failure
of one or more pulses Periodic occurrence of synchronous or asynchronous false pulses
Continuous pulses the pulse monitoring device can be tested while the machine is in
operation
CHAPTER 4
CONVERTER
Thyristor
The term thyristor usually refers to a family of four layer solid state device having
turn on characteristics that can be externally controlled by either current or voltage They
are also referred to as breakdown device because their working depends on avalanche
breakdown Thyristors have only two stages OFF and ON Thyristors have a similar
function to Uni-junctions they act as switches Thyristors use current flow as a switch
Thyristors have three states
1 Reverse blocking mode mdash Voltage is applied in the direction that would be
blocked by a diode
2 Forward blocking mode mdash Voltage is applied in the direction that would cause
a diode to conduct but the thyristor has not yet been triggered into conduction
3 Forward conducting mode mdash The thyristor has been triggered into conduction
and will remain conducting until the forward current drops below a threshold value
known as the holding current Converter is a semiconductor device which converts ac
input voltage into a constant dc output voltage In present excitation system three phase
fully controlled thyristor converter is used
Because of the following advantages thyristor converters are used
a) Thyristors are used for high power applications ie up to 10Kv3500A1KHz
b) Having high reliability and low losses
c) Uni-directional device like diode
d) Itrsquos operation as a rectifier which are low resistance in forward conduction
mode and high resistance in reverse conduction mode
PROTECTION OF THYRISTORS
For reliable operation of a thyristor demands that its specified ratings are not
exceeded When Subjected to or over voltages During the turn - on of SCR didt
prohibitively large False triggering of SCR by high value of dvdt andSpurious signals
between gate and cathode may leads to unwanted turn ndash on
DIDT AND PROTECTION
When thyristor starts conducting in forward conduction mode and is turned on by
gate pulse The anode current increases rapidly whole area of the gate to Cathode
junction then hot spots will be formed near the gate connection this locality of heating
destroys the thyristor Thyristor thermal time is constant The causes due to faults and
short circuits or surge currents Electronic crowbar protection is used against the over
voltages The rate rise of anode current must be kept at the time of turn on below the
rated or specified limiting value The didt value maintained below limited value by using
a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating
is avoided by applying gate current but not greater the maximum gate current
DVDT AND OVER VOLTAGE PROTECTION
With forward voltage across the anode and cathode of a thyristor the two outer
junctions are forward biased but the inner junction is reverse biased This reverse biased
junction J2 has the characteristics of a capacitor due to charges existing across the
junction In other words space-charges exist in the depletion region around junction J2
and therefore junction J2 behaves like a capacitance If the entire anode to cathode
forward voltage Va appears across J2 junction and the charge is denoted by Q then a
charging current i given by Eq (46) follows
i = dQdt =d(Cj Va )dt
= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)
As Cj the capacitance of junction J2 is almost constant the current is given by
i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)
If the rate of rise of forward voltage dVadt is high the charging current i will be
more This charging current plays the role of gate current and turns on the SCR even
when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on
must be avoided as it leads to false operation of the thyristor circuit
For controllable operation of the thyristor the rate of rise of forward anode to
cathode voltage dVadt must be kept below the specified rated limit Typical values of
dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by
using a snubber circuit in parallel with the device thyristor are very sensitive for over
voltage than the semiconductor devices
Over voltage transients are perhaps the main cause of thyristor failure
In thyristor there are mainly two types
1 Internal over voltages
Due to the commutation of the thyristors large voltages are generated internally
Because of the series inductance of the SCR circuit the large transient voltages L didt
produced This voltage several times the break over voltage of the device then thyristor
destroys permanently
2 External over voltages
External over voltages are caused due to the interruptions of current flow in an
inductive circuit and also due to the lightening strokes on the lines feeding the thyristor
system For the reliable operation of thyristor the over voltages must be suppressed by
adopting suitable techniques
Suppression of over voltages
The RC circuit called snubber circuit is connected across the device to protect In
order to keep the protective components to a minimum the thyristors are chosen with
their peak voltages ratings are 25 to 3 times of the normal peak working voltage
ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are
commonly employed for protecting the thyristor circuit against the over voltages
Gate protection
Gate circuit should also be protected against the over voltages and surges Over
voltage at gate circuit can cause false triggering of the SCR may rises the junction
temperature behind specified limit leading to its damage Protection against over
voltage can be achieved by connecting a ZD across the gate circuit and a resister is
connected in series with gate circuit to protect against the s A capacitor and resister are
connected across gate to cathode to by pass the noise
41 Final Pulse Stages
The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage
(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor
bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided
with a power supply module The amplified output pulses from the Final Pulse Stages
start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main
pulse This initial pulse edge assures proper firing of the thyristors being triggered
Subsequently the weaker part of main pulse keeps firing conditions steady As already
mentioned the Final Pulse Stages and their associated thyristor bridges form single units
All six pulse outputs from a Final Pulse Stage can be blocked by an external control
signal so that all thyristors in the associated thyristor bridge will block the current A
blocking of the pulses is initiated whenever there is a malfunction in the associated
thyristor bridge
42 Converter Power Section
The thyristor converter consists of three independent parallel rectifier blocks TY1
to TY3 which are all in service Even if one block fails the remaining blocks take over
automatically the full design current of the excitation circuit During normal operation
(with ideal current share) and all three bridges in operation each of these blocks has to
carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation
is limited Only when all three bridges fail the excitation is switched off Each thyristor
bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any
arm is identified by a Current flow monitoring module
43 Converter Cooling
A cooling system is needed to dissipate heat losses in the converter blocks and
electronics Each converter block has therefore been equipped with a fan supplied with
power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing
cubicle) The fans are protected with motor protection circuit breakers An air flow
monitoring unit is provided for monitoring the air flow through the thyristor bridge If a
circuit breaker failure is detected or if the air flow monitor drops off at one of the
thyristor bridges the bridge involved is immediately set out of operation by blocking its
firing pulses
44 Thyristor Converter Monitoring
A thyristor bridge in which defects occur that could threaten the safety of
operation or cause secondary damage is switched off automatically ie its firing pulses
are blocked This happens whenever A thyristor fuse is blown The fuses are monitored
individually with micro switches The Final Pulse Stage fails which is detected by
internal monitors (supply voltage sustained pulse short-circuit on the output end) The
power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or
is insufficient Isolator on ACDC side is open
CHAPTER 5
Field Current Circuit Excitation Transformer
51 Field Circuit Breaker
The circuit-breaker in the field circuit is used to isolate the field circuit from the
converter It is capable of switching off the synchronous machine from full load under the
maximum conditions of a 3-phase short-circuit In addition to its main contacts the field
circuit-breaker also has a de-excitation contact with which the field energy stored in the
field can be dissipated across the de-excitation resistor The de-excitation contact closes
shortly before the main contacts open so as to ensure proper commutation of the field
current from the main contacts to the de-excitation contact when the breaker is switched
off The field circuit-breaker is switched on by electromagnetic force and is kept switched
on by a mechanical latch When the latch is released by a trip coil the circuit-breaker
opens The circuit-breaker also has auxiliary contacts that report its status
52 Field Flashing
In shunt supplied excitation circuits (excitation transformer connected to the
generator terminals) the generator does not have enough remnant voltage for a generator
voltage build-up via the converter In this case a field flashing circuit is provided It
consists of the field flashing contactor the diode bridge and a transformer used to adapt
the auxiliary input voltage to the voltage needed for field flashing when power is
supplied from the auxiliaries network
Fig Field Flashing
Because the field flashing contactor is not able to switch off the energy stored in
the field the control ensures that the contactor can only reopen if the field circuit breaker
has already been opened (generating the TRIP order) or in a normal field flashing
sequence when the converter has taken over the field current Field flashing occurs in the
following stages
1048729The excitation is switched on closing the field flashing contactor ( Field
Circuit Breaker is already closed )
1048729The start-up excitation current flows through the rotor driving the generator
voltage up to approx 15 U
1048729After about 10 U the firing pulses to the converter are released and it begins
to excite the generator to its rated voltage
1048729After about 30 U the field flashing contactor opens (with no current since
the converter is now supplying the current)
The diode bridge at the input to the field flashing contactor prevents a feed-back
from the converter to the source of field flashing while the contactor is still closed
53 De-excitation
When malfunctions occur the stored field energy must be dissipated as quickly
and safely as possible to protect the generator This is done by the converter the field
circuit-breaker and the de-excitation (discharge) resistor
De-excitation (with opening of the field circuit-breaker) takes place in the following
stages
1048729The converter drives to its inverter limit position (negative ceiling voltage)
recovers a portion of the field energy into the network A trip command is given to the
field circuit breaker
1048729The de-excitation contact closes diverting the field voltage to the de-excitation
resistor
1048729Then immediately the main contacts open building voltage The field voltage
commutates to the de-excitation resistor
1048729The current diminishes at a given time constant TE
(With linear resistance TE = Lf (Rf + Re))
Due to the reversal of the field voltage by the converter the field current
commutates from the main contacts of the field circuit-breaker to the de-excitation
resistor in a very early phase This reversal of the field voltage prevents burn-off on the
main contacts and provides effective protection for the field circuit-breaker Depending
on the operating policy an operational shut-down of the excitation can also be effected
with the field circuit-breaker closed This method is useful mainly when the excitation is
switched on and off frequently In this case the converter is merely driven into the
inverter limit position so that the field energy is recovered into the network The
converter then blocks since it is supplying positive current only
54 Excitation Transformer
The excitation transformer matches the generator voltage to the field voltage
(required ceiling voltage) It also serves as a commutation reactance for the thyristor
converter and as a potential isolator between the network and the excitation circuit In
addition the transformer functions as a current limiter in that it makes it possible to keep
any short circuits in the excitation circuit under better control The excitation transformer
is equipped with temperature monitoring probes which set off an alarm when the
temperature exceeds a first max limit and then trips the excitation if the temperature
continues rising to a second (higher)limit
CHAPTER 6
Monitoring and Protection
61 Excitation Monitoring
611 General Information
The main goal of Excitation Monitoring is to make optimum use of the
redundancies provided in the excitation system and to give alarm whenever a malfunction
makes these redundancies unavailable The field current is monitored to see that it does
not exceed a maximum level and if necessary a switch-over to the MANUAL channel is
initiated In addition the criterion for switching off the field flashing is generated The
excitation Monitoring consists of an autonomous processor system
612 over current Alarms
In the Excitation Monitoring the limits for are set at higher levels than the
settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of
the nominal field current contact R1 and the binary output associated with it are
activated immediately If field current remains gt 110 then after a preset inverse-time
has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs
associated with them are activated Parameters match the measurements for If1 and If2 to
the nominal value for field current so that the internal values can be processed and read as
pu values It can be used to falsify the actual value of the field current If (to raise it) so
as to cause a response from the alarm limits for purposes of testing The processed If
signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As
long as the field current If is above the threshold value 11 Ifn its peak value is
measured This is stored (until RESET) and can be read at any time on the Micro-
Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever
the integrated time-current value (intisup2dt) exceeds the preselected reference value the
alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously
issued to switch over to the stand by AUTO channel Software switch F758 enables the
three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-
time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the
desired limit curve for response is set using the factor F216
613 Switch-Off Criterion for Field Flashing
The Excitation Monitoring supplies the criterion for switching off the field
flashing Whether this criterion is activated based on the actual value for generator voltage
Ug or for field current If or both depends on the settings of the two threshold values
F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements
Ug12 and If12 are switched over depending on the present status of the channels
(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is
fixed at ldquological 1rdquo
614 Storage of Alarm Status
The outputs of the over current alarms (R1 R2 R0) and the messages NO
FAILURE MONITORING PARAMETERS CHANGED are stored messages can be
erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the
front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the
situation causing the alarm or the malfunction is no longer present Whenever the self-
diagnosis equipment in the processor detects a malfunction the output NO FAILURE of
MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo
is activated whenever parameters or settings of software switches have been changed via
the Micro-Terminal
615 Actual Value Monitoring
The actual values for generator voltage Ug synchronous voltage Usyn and field
current If are monitored for malfunctions This monitoring is active regardless of whether
or not the generator is in operation Essentially when the generator is in operation the
measurements are monitored by comparing the signals (the smaller signal reading is
detected as incorrect) When the generator is not in operation the measured data are
monitored for extreme values The percentage of deviation permissible in the
measurement signals being compared
(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and
F20AIf the excitation transformer is being supplied from an auxiliary power source (no
shunt operation) the values of Ug and Usyn will be different in some operational
conditions
In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1
and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the
messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2
and the binary message from CH1 reports no malfunction a malfunction on
Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is
also present whenever the binary message CH1 DISTURBANCE is reported and a
discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2
FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares
Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the
comparison channel reports a malfunction or whenever both binary messages report no
malfunction - but both secondary monitors report a malfunction As long as the secondary
monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction
signals for the measurement channel involved (suspicion that there is a corresponding
error in Usyn) The measurement channel malfunctions are enabled operationally
whenever after excitation has been switched on generator voltage Ug exceeds the value
set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is
switched off to see that they do not exceed the limit value F210 that applies to both of
them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt
the operating range) Monitoring for extreme values is likewise enabled during normal
operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the
malfunction signals to the binary outputs Basically the monitoring of the actual values
for If1If2 functions like that of the Ug1Ug2 monitoring
62 Excitation Protection
621 General Information
The Excitation Protection switches off the excitation (and de-excites the machine
rapidly) whenever a danger arises that threatens the excitation transformer the converter
or the generator Generally limiter or monitoring functions precede the emergency trips
and these normally respond before the Excitation Protection must initiate a trip
Protective trip commands are issued directly to the field circuit-breaker from potential
free contacts of the board via the trip relays They are directed redundantly to the
operative field circuit-breaker ldquoOFFrdquo command
622 Protection against Excitation Transformer Overheating
This equipment monitors the excitation transformer for overheating in the
windings that could result from over current short-circuits or inadequate cooling The
monitoring uses temperature monitoring modules in conjunction with temperature
sensors built into the windings Normally the temperature is monitored in two stages the
first stage sets off an alarm the second causes a trip of the excitation
623 Rotor Over voltage Protection
Malfunctions in the generator circuit (eg terminal short-circuit failed
synchronization asynchronous operation) cause induced negative field currents that
produce high voltages in the field circuit These must be restricted to a level with a
sufficient safety margin below the insulation capacity of the field winding (test voltage)
and also below the peak blocking voltage of the converter thyristors The crow bar
employs spark gap elements to detect over voltages in the field circuit Whenever they
respond the associated thyristors are fired immediately switching the de-excitation
resistor parallel to the field The de-excitation current generated thereby initiates an
excitation trip via a supervision circuit causing an immediate opening of the field circuit-
breaker The malfunction isets off an alarm and an internal malfunction is indicated at the
cubicle
TEST VALUES OBTAINED WHEN EXCITATION IS RAISED
TEST
SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE
1 VREF 996 100
2 VACT 997 1003
3 IFACT 735 765
4 IGACT 703 707
5 ACTIVE
POWER
703 705
6 REACTIVE
POWER
101 142
7 POWER
FACTOR
099 IND 098 IND
8 ACTIVE
CURRENT(IR)
705 703
9 REACTIVE
CURRENT(IX)
102 138
10 POWER
ANGLE
566 546
11 FIRING
ANGLE
640 632
VALUES OBTAINED WHEN EXCITATION IS LOWERED
SNO PARTICULARS ACTUAL
VALUE
OBTAINED VALUE
1 VREF 100 997
2 VACT 100 997
3 IFACT 787 761
4 IGACT 839 837
5 ACTIVE
POWER
839 830
6 REACTIVE
POWER
155 110
7 POWER
FACTOR
098 IND 099 IND
8 ACTIVE
CURRENT(IR)
835 830
9 REACTIVE
CURRENT(IX)
140 94
10 POWER
ANGLE
613 DEG 631 DEG
11 FIRING
ANGLE
601 DEG 599 DEG
CONCLUSION
For generating the EMF in stator winding excitation is required to the rotor of a
generator There are two types of excitation
1 Static excitation system
2 Brushless excitation system
A certain disadvantage in brushless excitation system is the slow response time of
the field in case of fast load changes specified No slip-rings and brushes direct
measurements of the field parameters not possible
To avoid all loses static excitation is used Since it does not have any rotating
parts mechanical loses and windage loses This system has fast response and speed
control While preferring this excitation system there are no limitations for the
redundancy of Thyristor bridge circuits
Static excitation has fast field discharge by resistor and inverter operation direct
measurement of field quantity is possible The meaning of excitation is nothing but
continuous supply of DC current (ie field current) to the rotor to buildup required
output voltage in the stator
Field current is changed with respect to the change of load so the digital
automatic voltage regulator (DAVR) is used to regulate the output voltage according to
the load variations
So we conclude that static excitation system with DAVR is preferred since it is
having excellent dynamic performance and better options for R amp M
To accomplish this special field flashing equipment is needed When the field
flashing equipment is being supplied with power from a DC power source (power station
battery) a resistor is used to limit the field flashing current When it is being supplied
from an AC power grid a transformer serves as the adapter needed Excitation of the
generator is started by closing the field circuit-breaker Q1 and the field flashing breaker
Q2 This supplies current to the field which excites the generator up to 15 30 U The
generator then supplies voltage to the converter via the excitation transformer Starting
from approx10 of the generator voltage the firing electronics and the converter are
able to continue the voltage build-up so that the field flashing circuit is relieved of
current Once the voltage exceeds approx 25 of U the field flashing breaker is finally
opened having no current The diode bridge at the input to the field flashing breaker
prevents a back-flow of current to the field flashing source The converter TY has Final
Pulse Stage cooling and monitoring of the elements Redundancy in the regulator section
is ensured by means of two fully separate channels with independent measuring inputs
and extensive monitoring (ldquoSUPERVISIONrdquo)
Channel 1 (AUTOMATIC channel) is built as voltage regulators and is ON during
normal operation In addition to the voltage regulator which has a PID control algorithm
2
the AUTOMATIC channel also contains various limiters and corrective control circuits
to ensure the use and stable operation of the synchronous machine up to its operating
limits This channel possesses a Gate Control Unit with a subsequent Intermediate Pulse
Stage to generate the firing pulses for the Thyristor converter During normal operation
the Intermediate Pulse Stage of AUTOMATIC Channel is active and transmits the firing
pulses galvanically separated to the common pulse bus at the input to the Final Pulse
Stage Various monitoring functions of the AUTOMATIC channel and pulse monitoring
on the common pulse bus initiate an automatic switch-over to stand by Channel
(MANUAL)in case of a malfunction
Channel 2 (the MANUAL channel) is built as a simple field-current regulator with a PI
control algorithm It serves as a back-up channel in case of a malfunction on the
AUTOMATIC channel Manual channel performs valuable service for testing
commissioning and preventive maintenance The MANUAL channel has its own Gate
Control Unit (the software for the If regulator is also implemented therein) and its own
intermediate pulse Stage During normal operation (AUTOMATIC) the output pulses
from Intermediate Pulse Stage are blocked from reaching the pulse bus Various
monitoring on the MANUAL channel initiate an alarm in case of a malfunction while the
MANUAL channel is on stand-by If the MANUAL channel suffers a malfunction while
it is in operation the excitation is switched off (TRIP) Both channels are equipped with
tracking equipment so that the inactive channel always generates the same control
variable as the active channel during steady-state operation
This ensures smooth switch-over from Automatic to Manual channel and vice
versa To ensure that the MANUAL channel will in a switch-over initiated by a
malfunction take over the operating point of the machine as it was prior to the problem
the response of the tracking for the MANUAL channel is set relatively slow In addition
to the pulse monitors (ldquoSUPERVISIONrdquo) shown in the basic circuit diagram the
excitation system has an autonomous Excitation Monitoring As one of its functions this
equipment monitors for field currents that exceed acceptable maximum limits It initiates
an emergency switch-over to the MANUAL channel whenever the field current exceeds
the preset limit If even after such a switch-over the field current does not drop back to
3
the permissible level the excitation is switched off by Excitation Protection The most
important measuring inputs for the excitation system (If Ug Usyn) are redundant (2-
fold) The Excitation Monitoring checks these measuring inputs for discrepancy and
plausibility An alarm is always initiated in case of malfunction In certain cases a
switch-over to MANUAL channel is also initiated The excitation system contains an
Excitation Protection to protect the excitation transformer the converters and the
synchronous machine The protection system can detect short-circuits in the excitation
circuit and keep secondary damage within acceptable limits by a quick tripping of the
excitation and an opening of the generator breaker An overheating of the excitation
transformer first sets off an alarm (at a given preset limit) and then likewise initiates a
protective shut-down at an even higher limit The over voltage protection in the de-
excitation equipment provides an autonomous protective function for the rotor and the
rectifier This protection system monitors the field voltage in both polarities for over
voltage and if necessary de-energizes the field via the de-excitation resistor
12 Principle of primary power supply
In the shunt excitation system the excitation transformer also provides
the power supply for the electronic equipment and the converter fans So a failure of the
auxiliary supply to the converter fans does not cause a shutdown of the excitation When
the auxiliary supply fails the supply to converter fans is switched over to Excitation
transformer OP with a contactor A station battery supply is absolutely necessary for the
control of the field circuit breaker It is the power source for the electronic devices till the
generator is able to supply voltage Auxiliary power to the field flashing equipment must
be present in order to build up the generator excitation The power supply for standstill
heating and the cubicle lighting is also from Station Auxiliary Power Supply and is of
secondary importance for operation of the plant Power supply to rotor earth fault
detection circuit too is from Station Auxiliary Supply The two synchronous voltages
Usyn are each supplied to the AUTOMATIC channel and the MANUAL channel
separately across transformers The Gate Control Units need these voltages to enable
4
them to issue the pulses at a given firing angle relative to the input voltage of the
converter
5
CHAPTER 2
Digital Automatic Voltage Regulator (DAVR)
21 Principle of Operation of the Regulator (DAVR)
To regulate the voltage and the reactive power of a synchronous machine the
field voltage must be adjusted quickly to the changes in the operating conditions (with a
response time that does not exceed a few ms) To accomplish this analog control systems
include amplifiers which make continuous comparison of the actual values against the
reference values and vary the control variable to the converter with almost no delay Most
of the delay that occurs originates in the converter since the firing pulses for changing
the rectifier phase angle are only issued periodically (every 33 ms)
The DVR digital voltage regulator calculates the control variable from the
measured and reference data in very short time intervals This results outwardly in a
quasi-continuous behavior with a negligible delay time (as in an analog regulator) The
calculations are made in the binary number system Analog measurement signals such as
those for generator voltage and generator current are converted into binary signals in
analogdigital converters The set-points and limit values have already been defined in
digital (binary) form An understanding of the actual computation processes in the digital
voltage regulator is not necessary for operation preventive maintenance or
troubleshooting Like the operator of a pocket calculator or a personal computer all the
operator needs is to know how to operate the instrument and the programming for this
working tool For that reason we will explain below only the principle division of work
among the various modules and the flow of data processing The purpose is above all to
make clear how the processor system has been integrated into the rest of the power
electronics system
6
22 Basic Structure of the Processor Systems
7
The signal processors 25 analog inputoutput modules Each of these processor
systems has a common bus circuit and output and the control lines There is a specific
range of addresses assigned to each assignment Board including the power supply bus
the address lines the two data lines to the input calculates the reactive current (I sin φ)
and the active current ( Icos φ) With these two channel processor Synchronized with
these interrupts (ie with the phase positions of current Ig the field current If and the
synchronous voltage Usyn From the exchange data with the microprocessor card across
the two data lines generator voltage Ug) this processor measures the generator current
Ig and then hardwired connections or multi-conductor cables Binary and analog
inputoutput modules ie for galvanic isolation and adaptation to the electronics level
The most important input interrupts per period to trigger the cycles for processing actual
values in the AUTOMATIC module on the processor bus) for filtering and further
processing
Monitoring each consist of the central microprocessor module and binary and
parameters to the AUTOMATIC channel are the generator voltage Ug the generator
peripheral unit Ug Ig and Usyn are sent to the Interrupt Generator (plug-in peripheral
units (wall-mounted units) peripheral units are used for preprocessing signals from
external measurement circuits power supply units Signals are exchanged among these
processor systems via processed across separate peripheral units for each channel These
processor working on the bus (a house address that can be adjusted using a switch)
systems The AUTOMATIC channel the programmable controls and the Excitation The
actual values measured from AUTOMATIC channel and MANUAL channel are The
AUTOMATIC channel and the MANUAL channel each have their own The digital
voltage regulator is broken down into several autonomous microprocessor The inputs and
outputs of the processor systems are directed across voltage-isolating The Interrupt
Generator also uses the 3-phase Ug signal to generate the 12 themselves contain a limited
number of hardware inputs and outputs with fixed equipment Whenever addresses from
this range are called up the signal processing module can results the processor is then
able to derive further operating parameters such as the load angle the active power etc
The functions of all microprocessor systems other than the programmable controls
have been accomplished in firmware The non-varying standard function modules can be
configured to the design desired for plant-specific purposes using software switches
(KFlags) Thus for example the stored status of a K-Flag determines whether or not a
Limiter is active and whether the de-excitation or the excitation limiters take precedence
Because these K-flags determine the software Scope of Supply for the installation they
cannot be changed permanently via the Micro-Terminal In this way they differ from
such setting data as the values of the parameters for the PID filter of the voltage regulator
or the set-points for the limiters These values can be permanently changed using the
Micro-Terminal Communication is possible with each of the processor systems via the
Micro-Terminal by plugging on the connecting cable In this way signals within the
processor and setting parameters can be viewed analog signals can be issued and the set
parameters can be altered temporarily (F range) or permanently (C range) Unlike the
other processor systems the programmable controls do not include any firmware for
realization of the functions They have been designed so that the designer can adapt and
change their functions easily using the ldquoFunctional Block Programming Language P10
Digital and analog functions can be implemented in practically any degree of complexity
desired using the P10 functional blocks The control variable of the voltage regulator
(AUTOMATIC channel) and the control variable of the field current regulator
(MANUAL channel) are each processed in separate Gate Control Unit and formed into a
chain of pulses at the appropriate firing angle The pulses of the active channel are
directed to the pulse bus via the associated Intermediate Pulse Stage The pulses for each
converter block are amplified sufficiently in Final Pulse Stage to fire the Thyristor
231 General Information
The functions of the automatic voltage regulator AVR are
1 to regulate the generator voltage
2 to regulate the effect of the reactive andor active current on the voltage
3 to limit VoltHz
4 to limit max and min field current
5 to limit inductive stator current
6 to limit capacitive stator current
7 to limit the load angle
8 to stabilize the power system
Block Diagram shows the software structure of AUTOMATIC channel The
generator limiters not provided for the installation in question (optional equipment) are
identified in this overview as ldquoNot Suppliedrdquo The parameter values signal values and
software switches (flags) marked with addresses (hexadecimal numbers) can be viewed
and altered via the Micro-Terminal The values selected are displayed in sec pu Hz
etc and can where necessary be changed directly in these formats The plant-specific
settings of the variables and the flags can be obtained from the Test and Commissioning
Report This block diagram provides information about the important functions and
possible settings of the AUTOMATIC channel For the sake of clarity no detailed
presentation has been given of special functions such as tracking circuits initializations
etc The page heading cross-refers this overview to the various sheets of the schematic
diagram Binary signals are shown in broken lines analog signals in solid lines The
corresponding text designations in the schematic diagram can be used for identification of
the input signals (hardware inputs) The only analog output signal from the automatic
voltage regulator control variable Ucontr is sent via the data bus (CRU bus) to the Gate
Control Unit Most of the binary messages (outputs) from the AVR are of no interest
functionally and they have been omitted for the sake of clarity The basic structure of the
digital voltage regulator and the limiters is simple This is necessary in order that the
behavior of the regulatorslimiters will remain calculable and understandable in all
operating situations and that there will be no problem in adjusting and optimizing them
The central PID filter in the digital voltage regulator defines the dynamic response of the
closed-loop controls both in the voltage regulator mode and after limiters have
intervened The ldquocontrol deviationrdquo at the input to the PID filter is either the control
deviation for voltage the control deviation of a de-excitation limiter (the value
determined by minimum value selection) or the control deviation of an excitation limiter
(the value determined by maximum value selection) Flag F730 (ldquoPRIORrdquo) is used to
determine whether the exciting (Min value) or the de-exciting signal takes precedence on
the minmax value limiter (normally F730 = 1111 ie the de-exciting signal takes
precedence) With the exception of the Minimum Field Current Limiter all other limiters
have variable factoring multipliers of the signal outputs so that they can be adjusted
individually together with the common PID filter which has been optimized for voltage
regulation The setting parameters for this PID filter are as follows
Vo = KR Static amplification
1
Ta = ---- Integration time constant
Tc1
Vp Proportional amplification
1
Tb = ---- Differential time constant
Tc2
Vinfin Amplification of high frequencies
The BODE diagram below shows the assignment of settings in accordance with
DINIEC standards based on a typical example
The PID filter amplifications Vo Vp and Vinfin can be adjusted in pu values But
the ldquoceiling factorrdquo pl+ must be adjusted correctly with parameter F310 if the total
amplification (circuit amplification) of the control circuit is actually to conform to the
pu settings This factor must agree with the ldquoexternalrdquo amplification ie with the
ceiling value of the transformer- converter circuit
Ceiling factor(pl+) = Ufmax Ufo
in which Ufmax = ceiling field voltage
Ufo = no-load field voltage
To attain a suitable response of the AVR when starting excitation
(ldquoEXCITATION ONrdquo) it may be necessary to change the proportional amplification of
the regulator during this phase Vp2 (transiently activated) and Vp1 (permanently
activated) can be adjusted for this purpose For example the value of Vp2 takes effect
immediately once the excitation is switched on and remains effective for a period as set at
F30C Once the period F30C (eg 5 sec) has expired Vp shifts over to Vp1 (becomes
the steady-state Vp) at the rate of change set The standard operating mode for the PID
filter is voltage regulation for which the discrepancy between the voltage set-point and
the current value for generator voltage Ug (the control deviation) is supplied at the input
To compensate for the voltage drop in the block transformer or whenever several
generators are operating to the same distributing bus the generator voltage must be
varied in proportion to the measured generator current (droop influence) To accomplish
this the voltage set-point is varied as a function of the measured reactive current IX
andor active current IR Flag F712 enables the IX droop Flag F710 the IR droop The
desired compensation is set in F282 and F286 respectively Flags F284 and F288 are used
to select whether this droop influence is to increase the voltage or to reduce it
(compensation) Combined influence of the active and reactive currents is attained by
enabling both droops IX and IR Flag F716 activates a so-called ldquoSoft-Startrdquo at the
starting of excitation This ldquoSoft-Startrdquo ensures that the voltage set-point integrates from
0 to 100 within the time set on F290 when the excitation is switched on
(ldquoEXCITATION ONrdquo) A ldquosmoothrdquo excitation of the generator can be achieved in this
way whenever there is no demand for a quick excitation
232 Voltage Set-Point
Various signals and settings control and limit the voltage set-point F270 For
example the values of F254 and F252 define the normal operating range possible for set-
point adjustment (eg 90 110) using external control commands (control room local
operatorrsquos panel superposed control system) The effective set-point adjustment rate is
governed The set-point can be set at the values of F250 and F256 by activating
appropriate control commands for ldquoSETrdquo input Enabling Flag F71A and activating a
binary input prior to switching on the excitation (ldquoEXCITATION OFFrdquo) sets the Ug set-
point at the value of UAUX This makes it possible for example to ensure that the
generator voltage will agree exactly with the network voltage after the voltage build-up
An external value with variable amplification can be added to the Ug set point by
enabling F724 (for example for stability tests)
233 Regulator Tracking in MANUAL Operation
Whenever the AUTOMATIC channel is not in operation (the MANUAL channel
is ON) a follow-up equipment ensures a smooth switch-back to the AUTOMATIC
mode will always be possible To track the voltage set-point is shifted by means of
RAISELOWER pulses from the Gate Control Unit so that control variable Ucontr at the
output from the PID filter is held steady and identical to the control variable Ucontr from
the MANUAL channel Because this tracking must react slowly resultant transient
control deviations resulting from the amplification in the PID filter might cause severe
interference with control variable Ucontr
To prevent this the follow-up equipment intervenes on the regulators mixing
point with a corresponding compensation signal
234 Ugf Limiter
At under frequency the Ugf Limiter reduces the generator voltage so as to
prevent saturation effects in the supply and measuring transformers To adjust this
limiter the max permissible generator voltage at rated frequency is defined and set
When any under-frequency occurs the generator voltage is thus reduced in proportion to
that setting
235 Field Current Maximum Limiter
The Field Current Maximum Limiter is provided to protect the generator rotor
from s occurring in steady-state and transient operation High field currents are normally
the result of a sharp drop in network voltage or of an improper raising of the voltage set-
point by the operating staff The field current is held steady at the value TH1 ie at the
maximum thermal value permissible for the excitation circuit and the rotor In order that
the generator can support the power network with its transient overload capacity during
brief collapses in voltage a temporary switch-over is made to the transient limit MAX1
(a higher setting) When the generator or the converter is operating at a reduced capacity
These limits TH1MAX1 can be switched over to the lower settings TH2MAX2 by
activating the corresponding binary signals The switch-over from the thermal limit
TH12 to the transient limit MAX12 can be configured in one of three ways
a) Depending on the over current with -dUdt ENABLE
Flag programming F418 = any setting desired F41A = 0000
This variant enables the transient value MAX12 whenever a collapse of voltage
in the network is detected The ENABLE time is fixed and can be set The example
below shows the typical behavior of the limiter configured in this way
b) Dependent on the time integral with -dUdt ENABLE
Flag programming F418 = inactive F41A = 1111
This variant likewise enables the transient value only when a collapse of network
voltage has been detected However the switch-back to the thermal limit is not made
dependent upon the time itself but on the calculated time integral intisup2dt of the The setting
on Parameter F414 in spu takes into account the time the rotor needs to cool down ie
the rate of temperature change in the case of intermittent operation The example below
shows how the timing of the switch-back to the thermal limit depends on the present
value for intisup2dt
The time integral is based on the formula
Example The setting of =isup2dt equivalent to Version a (F416) at a constant 16 times the
nominal field current for 10 seconds (with TH12 = 105) is
c) Dependent on the time integral without any preconditions
Flag programming F418 = 1111 F41A = 1111
In this variant the transient becomes available without any prior conditions
(without a -dUdt ENABLE) with the time integral intisup2dt
237 Inductive Stator Current Limiter
The Inductive Stator Current Limiter holds the stator current Ig within permissible
limits while the generator is in the ldquoover-excitedrdquo operating range by reducing the field
current accordingly The setting TH (thermal limit) provides the limit against stationary s
that might occur To take advantage of the generatorrsquos transient overload capacity a
switch-over is made to the higher setting MAX The principle of operation of this switch-
over to the value MAX permissible only transiently is identical to that employed for the
field current limiter (refer to the description above) When the drive output from the
turbine is very high stator current may exceed permissible limits even while inductive
loading of the generator is low In this case if the stator current limiter is not kept from
influencing the field current the control circuit will oscillate back and forth between the
Inductive Stator Current Limiter (de-
exciting) and the Capacitive Stator Current Limiter (exciting)The output signal of that
function then dominates the control variable of the Ig-dependent limiter via a maximum
value selection
238 Capacitive Stator Current Limiter
239 Load Angle Limiter
The Load Angle Limiter prevents the synchronous machine from slipping out of
phase due to slippage of the rotor The load angle δ the difference in phase between the
rotor and the stator rotating field results mainly from the driving torque (active power P)
acting on the generator and the level of rotor current (field current) If the driving torque
remains constant a increase in the field current reduces the load angle δ The current load
angle δ at any moment is obtained from the generator current and generator voltage based
on a simplified model of the generator Whenever this calculated load angle δ exceeds the
preset limit angle the limiter increases the field current until the load angle has dropped
back to its permissible value The quadrature reactance Xq of the generator and the
network reactance Xe during normal operation must be adjusted on the regulator in order
to obtain the load angle δ The graph below shows the Power Chart for a salient-pole
machine with typical limiter characteristics
The purpose of a Power System Stabilizer is to use the generator excitation to
damp electromechanical oscillations between the network and the generator Depending
on the design of the generator and the requirements imposed for network stability its
main function will be either to damp the oscillations originating in the machine or those
from the network A synchronous generator working in a combined power network is in
principle an oscillating structure In order to produce a torque the magnetic field of the
rotor and the stator must form a given angle (referred to as the rotor displacement or load
angle δ) The electrical torque ME increases as the angle δ increases just as with a
torsion spring Because the ME of the generator and the mechanical driving torque MA
from the turbine are in equilibrium during steady-state operation the angle δ remains in a
given position Whenever this state of equilibrium between MA and ME is disturbed the
load angle slips of this rest position and change thereby the electrical torque ME The
torque attempts to restore the load angle to a stationary position Due to the mass inertia
of the turbinegenerator rotor however this can only take place aperiodically It does so
in the form of more or less effectively damped oscillations (again similar to the effect of
mass inertia on a torsion spring) In order to damp the oscillations there must be a
damping torque produced depending not on the electrical torque ME associated with the
angle but on the difference in frequency (Df) between the rotor and the stator rotating
field ie on the slippage This torque is produced mainly by the so-called damper
winding in the rotor but the dimensioning of this is subject to limits imposed by
considerations of design and economy Some further action is therefore needed to
increase the damping effect The following drastically simplified formula shows the
parameters upon which the amount of active power PE supplied by the generator
depends
PE = active power
It can be seen from the above relationship that the active power that the generator
transfers depends not only on the load angle δ but also on the field current If That means
that a transient change can be made in the active power PE and with that in the effective
electrical torque ME by varying the field current The principle of operation of the DVR
Power System Stabilizer becomes clear from a consideration of the oscillations in power
output and frequency (ΔPE Δf) and the vector diagram If it is assumed that oscillations
in the network frequency generates load oscillations with the mass inertia of the rotor
then the active load of the generator (eg MW-measured) is influenced with a sinusoidal
value -ΔPE (ME-MA = -ΔPE) By inversion of -ΔPE one obtains the fluctuation in
power provided by the rotor +ΔPE As is known the slip signal Δf follows +ΔPE with a
phase delayed by 90deg The +ME produced by the periodic changes in the load angle δ is
in phase with +ΔPE A good damping is attained if ME is varied in phase with the slip
Δf However this signal must also be advanced somewhat to compensate for the time
constants in the excitation circuit and the generator
As mentioned above the electrical torque ME can be influenced by varying the
field current To accomplish this a suitable control signal referred to as variable
disturbance compensation must be imposed upon the voltage set-point or the converter
control variable Ucontr As can be seen from the vector diagram by applying proper
weighting factors (K1 K2) and then adding together the signals -ΔPE and Δf an overall
stabilization signal can be produced that rotates in advance of the Df signal by any angle
desired between 0deg and 90deg Because the amplitude of -DPE remains proportional to the
amplitude of Δf a constant angle in advance of Δf results for the compensation of the
time constants referred to above The optimum weighting factors K1 and K2 for a
synchronous generator working to a power network depend on its operating point at any
moment and the external reactance of the network Normally the selection of a
compromise setting is good enough to attain stability in all operating points and for all
external reactance For special demands these settings must be parameterized as a
function of the external reactance (which means optional equipment Xe-Identification)
The Power System Stabilizer PSS is a section of the AVR computer program and is
processed once per network cycle The voltage at the generator terminals and the
generator current are measured in order to define the signals ΔPE and Δf The calculated
signals for _P_ (=PE) and Δf are then sent across DC filters ldquoDrdquo (real differentiators) that
transmit only the dynamic portion of the signals The ΔPE and Δf signals obtained in this
way are then weighted (multiplied by) with the factors K1 and K2 and sent to the
summing point of the voltage regulator
The PSS stabilization signal is imposed on the automatic voltage regulator only if
the following prerequisites are met
bull Generator on line
bull Generator power output gt the value F338
bull Generator voltage in a range between F33C and F33A
The stabilization signal is limited at the output from the PSS to the lower and
upper limits Flag defines whether the stabilization signal is introduced before or after the
PID filter (usually before the filter) Because the PID filter as noted above already takes
the ceiling factor Vp1 into account the PSS signal needs to be multiplied by Vp1 if it is
added to the voltage regulator following the PID filter (divider at the input to the
minmax limiter) This precaution prevents the DC filter ldquoDrdquo in the P-channel from
producing an unnecessary ldquostabilizationrdquo effect in the case of rapid changes in turbine
load As an alternative for the AVRrsquos Power System Stabilizer a stabilization signal from
an outside system can be imposed by activating the binary input ldquoPSS-SIGNEXTrdquo Flag
F340 can be used to select between an analog and a 12-bit signal and F33E to select the
polarity desired for that signal
24 The MANUAL Channel
241 Summary
The MANUAL channel (Channel 2) has been built as a simple field current
regulator
without additional limiters Its main function is to maintain the excitation of the generator
even if the AUTOMATIC channel becomes non-operational The MANUAL channel
also performs valuable service for purposes of testing commissioning and preventive
maintenance Its measurements regulator generation of firing pulses and power supply
are physically separate from those on the AUTOMATIC channel
242 Principle of Operation
All the functions of the MANUAL channel including the generation of firing
pulses have been implemented in a single electronic module the Gate Control Unit The
control variable Ucontr of voltage regulator is used as the reference value for generating
firing pulses on the principle known as ldquoramp controlrdquo (Comparison of Ucontr with
Usynsynchronous sawtooth signal) For further processing in the UN 0096 Intermediate
Pulse Stage the Gate Control Unit supplies six firing pulses at its output whose phase
position with respect to the synchronous voltage Usyn is in accordance with control
variable Ucontr An internal linearization ensures that the field voltage produced via the
firing pulses remains proportional to the control variable Ucontr throughout the entire
range As a result the circuit amplification of the control remains constant over the entire
range Whenever excitation is switched ON the set-point for Generator Voltage is set
automatically at the preset - ref Value This provision ensures that the generator voltage
always attains approximately its nominal value after the field flashing The Gate Control
Unit can be refunctioned ( by pre-selection with a switch ) for purposes of testing to act
as a purely firing pulse control In this case the control variable Ucontr is adjusted
directly using the RAISELOWER push buttons on the front of the module In this way
for example the relationship between the phase position of the firing pulses and the
control variable Ucontr can be checked easily
CHAPTER 3
PULSE SECTION
31 Pulse Generation and Amplification
The Gate Control Units of both AUTOMATIC channel and MANUAL channel
each supply six firing pulses for operating the 6-pulse thyristor bridges The low-power
pulse signals from these Gate Control Units are then amplified in the Intermediate Pulse
Stage galvanically isolated and then sent to the common pulse bus On the output end
the Intermediate Pulse Stage of the non-active channel) is always blocked The Gate
Control Units generate the pulses based on microprocessor control The reference voltage
used for the firing pulse phase location is the output voltage from the excitation
transformer (Usyn1 Usyn2) The commutation spikes of the synchronous voltage caused
by the converter are calculated prior to use of the voltage as a reference value and are
deliberately filtered out The lower limit for the firing pulses (double pulses) which are
offset from one another by 60deg is defined by the limit rectifier position (αmin) and the
upper limit by the limit inverter position (αmax) for the firing angle αmin and αmax can
be adjusted on the Gate Control Units using BCD (Binary Coded Decimal) switches
αmin ensures that the firing pulses will not be issued (premature firing) until there is
sufficient positive phase voltage on the thyristor involved αmax prevents a dangerous
ldquotippingrdquo of the thyristor bridge into the rectifier mode if the firing angle α is too large
(ldquolate firingrdquo) The critical factors determining αmax are the overlap time uumlmax (max
commutation time) and the ldquorecovery timerdquo of the thyristors (αmax lt 180deg - uumlmax - γ )
An external control signal can force the firing pulses into their inverter limit position
Other binary inputs can block or direct the firing pulses of the Gate Control Units so as to
produce freewheeling on the thyristor bridge During freewheeling the firing pulses for
the thyristor pair R and S are blocked and the pulse signals T+T- are engaged with
chains of pulses Both Gate Control Units (for the MANUAL amp AUTOMATIC channels)
contain a field current monitor that blocks the firing pulses immediately whenever the
current exceeds a preset threshold level In this case the field circuit-breaker is also
tripped via an output contact The purpose of these provisions is to prevent damage to
thyristors and thyristor fuses in case of a slip-ring short-circuit or to keep any damage
that does occur to a minimum The pulse signals are galvanically separated at the outputs
from the Intermediate Pulse Stage (with pulse transmitters) and are then directed to the
common pulse bus This transmission of the pulse signals to the pulse bus via passive
transmitters ensures a high degree of active channel autonomy Practically no possible
malfunctions on the inactive channel (including for example sustained pulses) affect the
active channel
32 Pulse Monitoring
The ldquoPulse Busrdquo and the pulse signals of the AUTOMATIC channel are
monitored This monitoring device consists of potential isolating stages and the common
monitor If the pulse monitoring of the ldquoPulse Busrdquo responds a switch-over is made to
MANUAL channel The function of the potential isolating stages is to couple the pulse
monitoring device to the pulse circuits without any feedback effect The pulse monitoring
checks the six pulse lines for the following malfunctions continuous or periodic failure
of one or more pulses Periodic occurrence of synchronous or asynchronous false pulses
Continuous pulses the pulse monitoring device can be tested while the machine is in
operation
CHAPTER 4
CONVERTER
Thyristor
The term thyristor usually refers to a family of four layer solid state device having
turn on characteristics that can be externally controlled by either current or voltage They
are also referred to as breakdown device because their working depends on avalanche
breakdown Thyristors have only two stages OFF and ON Thyristors have a similar
function to Uni-junctions they act as switches Thyristors use current flow as a switch
Thyristors have three states
1 Reverse blocking mode mdash Voltage is applied in the direction that would be
blocked by a diode
2 Forward blocking mode mdash Voltage is applied in the direction that would cause
a diode to conduct but the thyristor has not yet been triggered into conduction
3 Forward conducting mode mdash The thyristor has been triggered into conduction
and will remain conducting until the forward current drops below a threshold value
known as the holding current Converter is a semiconductor device which converts ac
input voltage into a constant dc output voltage In present excitation system three phase
fully controlled thyristor converter is used
Because of the following advantages thyristor converters are used
a) Thyristors are used for high power applications ie up to 10Kv3500A1KHz
b) Having high reliability and low losses
c) Uni-directional device like diode
d) Itrsquos operation as a rectifier which are low resistance in forward conduction
mode and high resistance in reverse conduction mode
PROTECTION OF THYRISTORS
For reliable operation of a thyristor demands that its specified ratings are not
exceeded When Subjected to or over voltages During the turn - on of SCR didt
prohibitively large False triggering of SCR by high value of dvdt andSpurious signals
between gate and cathode may leads to unwanted turn ndash on
DIDT AND PROTECTION
When thyristor starts conducting in forward conduction mode and is turned on by
gate pulse The anode current increases rapidly whole area of the gate to Cathode
junction then hot spots will be formed near the gate connection this locality of heating
destroys the thyristor Thyristor thermal time is constant The causes due to faults and
short circuits or surge currents Electronic crowbar protection is used against the over
voltages The rate rise of anode current must be kept at the time of turn on below the
rated or specified limiting value The didt value maintained below limited value by using
a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating
is avoided by applying gate current but not greater the maximum gate current
DVDT AND OVER VOLTAGE PROTECTION
With forward voltage across the anode and cathode of a thyristor the two outer
junctions are forward biased but the inner junction is reverse biased This reverse biased
junction J2 has the characteristics of a capacitor due to charges existing across the
junction In other words space-charges exist in the depletion region around junction J2
and therefore junction J2 behaves like a capacitance If the entire anode to cathode
forward voltage Va appears across J2 junction and the charge is denoted by Q then a
charging current i given by Eq (46) follows
i = dQdt =d(Cj Va )dt
= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)
As Cj the capacitance of junction J2 is almost constant the current is given by
i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)
If the rate of rise of forward voltage dVadt is high the charging current i will be
more This charging current plays the role of gate current and turns on the SCR even
when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on
must be avoided as it leads to false operation of the thyristor circuit
For controllable operation of the thyristor the rate of rise of forward anode to
cathode voltage dVadt must be kept below the specified rated limit Typical values of
dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by
using a snubber circuit in parallel with the device thyristor are very sensitive for over
voltage than the semiconductor devices
Over voltage transients are perhaps the main cause of thyristor failure
In thyristor there are mainly two types
1 Internal over voltages
Due to the commutation of the thyristors large voltages are generated internally
Because of the series inductance of the SCR circuit the large transient voltages L didt
produced This voltage several times the break over voltage of the device then thyristor
destroys permanently
2 External over voltages
External over voltages are caused due to the interruptions of current flow in an
inductive circuit and also due to the lightening strokes on the lines feeding the thyristor
system For the reliable operation of thyristor the over voltages must be suppressed by
adopting suitable techniques
Suppression of over voltages
The RC circuit called snubber circuit is connected across the device to protect In
order to keep the protective components to a minimum the thyristors are chosen with
their peak voltages ratings are 25 to 3 times of the normal peak working voltage
ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are
commonly employed for protecting the thyristor circuit against the over voltages
Gate protection
Gate circuit should also be protected against the over voltages and surges Over
voltage at gate circuit can cause false triggering of the SCR may rises the junction
temperature behind specified limit leading to its damage Protection against over
voltage can be achieved by connecting a ZD across the gate circuit and a resister is
connected in series with gate circuit to protect against the s A capacitor and resister are
connected across gate to cathode to by pass the noise
41 Final Pulse Stages
The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage
(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor
bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided
with a power supply module The amplified output pulses from the Final Pulse Stages
start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main
pulse This initial pulse edge assures proper firing of the thyristors being triggered
Subsequently the weaker part of main pulse keeps firing conditions steady As already
mentioned the Final Pulse Stages and their associated thyristor bridges form single units
All six pulse outputs from a Final Pulse Stage can be blocked by an external control
signal so that all thyristors in the associated thyristor bridge will block the current A
blocking of the pulses is initiated whenever there is a malfunction in the associated
thyristor bridge
42 Converter Power Section
The thyristor converter consists of three independent parallel rectifier blocks TY1
to TY3 which are all in service Even if one block fails the remaining blocks take over
automatically the full design current of the excitation circuit During normal operation
(with ideal current share) and all three bridges in operation each of these blocks has to
carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation
is limited Only when all three bridges fail the excitation is switched off Each thyristor
bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any
arm is identified by a Current flow monitoring module
43 Converter Cooling
A cooling system is needed to dissipate heat losses in the converter blocks and
electronics Each converter block has therefore been equipped with a fan supplied with
power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing
cubicle) The fans are protected with motor protection circuit breakers An air flow
monitoring unit is provided for monitoring the air flow through the thyristor bridge If a
circuit breaker failure is detected or if the air flow monitor drops off at one of the
thyristor bridges the bridge involved is immediately set out of operation by blocking its
firing pulses
44 Thyristor Converter Monitoring
A thyristor bridge in which defects occur that could threaten the safety of
operation or cause secondary damage is switched off automatically ie its firing pulses
are blocked This happens whenever A thyristor fuse is blown The fuses are monitored
individually with micro switches The Final Pulse Stage fails which is detected by
internal monitors (supply voltage sustained pulse short-circuit on the output end) The
power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or
is insufficient Isolator on ACDC side is open
CHAPTER 5
Field Current Circuit Excitation Transformer
51 Field Circuit Breaker
The circuit-breaker in the field circuit is used to isolate the field circuit from the
converter It is capable of switching off the synchronous machine from full load under the
maximum conditions of a 3-phase short-circuit In addition to its main contacts the field
circuit-breaker also has a de-excitation contact with which the field energy stored in the
field can be dissipated across the de-excitation resistor The de-excitation contact closes
shortly before the main contacts open so as to ensure proper commutation of the field
current from the main contacts to the de-excitation contact when the breaker is switched
off The field circuit-breaker is switched on by electromagnetic force and is kept switched
on by a mechanical latch When the latch is released by a trip coil the circuit-breaker
opens The circuit-breaker also has auxiliary contacts that report its status
52 Field Flashing
In shunt supplied excitation circuits (excitation transformer connected to the
generator terminals) the generator does not have enough remnant voltage for a generator
voltage build-up via the converter In this case a field flashing circuit is provided It
consists of the field flashing contactor the diode bridge and a transformer used to adapt
the auxiliary input voltage to the voltage needed for field flashing when power is
supplied from the auxiliaries network
Fig Field Flashing
Because the field flashing contactor is not able to switch off the energy stored in
the field the control ensures that the contactor can only reopen if the field circuit breaker
has already been opened (generating the TRIP order) or in a normal field flashing
sequence when the converter has taken over the field current Field flashing occurs in the
following stages
1048729The excitation is switched on closing the field flashing contactor ( Field
Circuit Breaker is already closed )
1048729The start-up excitation current flows through the rotor driving the generator
voltage up to approx 15 U
1048729After about 10 U the firing pulses to the converter are released and it begins
to excite the generator to its rated voltage
1048729After about 30 U the field flashing contactor opens (with no current since
the converter is now supplying the current)
The diode bridge at the input to the field flashing contactor prevents a feed-back
from the converter to the source of field flashing while the contactor is still closed
53 De-excitation
When malfunctions occur the stored field energy must be dissipated as quickly
and safely as possible to protect the generator This is done by the converter the field
circuit-breaker and the de-excitation (discharge) resistor
De-excitation (with opening of the field circuit-breaker) takes place in the following
stages
1048729The converter drives to its inverter limit position (negative ceiling voltage)
recovers a portion of the field energy into the network A trip command is given to the
field circuit breaker
1048729The de-excitation contact closes diverting the field voltage to the de-excitation
resistor
1048729Then immediately the main contacts open building voltage The field voltage
commutates to the de-excitation resistor
1048729The current diminishes at a given time constant TE
(With linear resistance TE = Lf (Rf + Re))
Due to the reversal of the field voltage by the converter the field current
commutates from the main contacts of the field circuit-breaker to the de-excitation
resistor in a very early phase This reversal of the field voltage prevents burn-off on the
main contacts and provides effective protection for the field circuit-breaker Depending
on the operating policy an operational shut-down of the excitation can also be effected
with the field circuit-breaker closed This method is useful mainly when the excitation is
switched on and off frequently In this case the converter is merely driven into the
inverter limit position so that the field energy is recovered into the network The
converter then blocks since it is supplying positive current only
54 Excitation Transformer
The excitation transformer matches the generator voltage to the field voltage
(required ceiling voltage) It also serves as a commutation reactance for the thyristor
converter and as a potential isolator between the network and the excitation circuit In
addition the transformer functions as a current limiter in that it makes it possible to keep
any short circuits in the excitation circuit under better control The excitation transformer
is equipped with temperature monitoring probes which set off an alarm when the
temperature exceeds a first max limit and then trips the excitation if the temperature
continues rising to a second (higher)limit
CHAPTER 6
Monitoring and Protection
61 Excitation Monitoring
611 General Information
The main goal of Excitation Monitoring is to make optimum use of the
redundancies provided in the excitation system and to give alarm whenever a malfunction
makes these redundancies unavailable The field current is monitored to see that it does
not exceed a maximum level and if necessary a switch-over to the MANUAL channel is
initiated In addition the criterion for switching off the field flashing is generated The
excitation Monitoring consists of an autonomous processor system
612 over current Alarms
In the Excitation Monitoring the limits for are set at higher levels than the
settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of
the nominal field current contact R1 and the binary output associated with it are
activated immediately If field current remains gt 110 then after a preset inverse-time
has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs
associated with them are activated Parameters match the measurements for If1 and If2 to
the nominal value for field current so that the internal values can be processed and read as
pu values It can be used to falsify the actual value of the field current If (to raise it) so
as to cause a response from the alarm limits for purposes of testing The processed If
signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As
long as the field current If is above the threshold value 11 Ifn its peak value is
measured This is stored (until RESET) and can be read at any time on the Micro-
Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever
the integrated time-current value (intisup2dt) exceeds the preselected reference value the
alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously
issued to switch over to the stand by AUTO channel Software switch F758 enables the
three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-
time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the
desired limit curve for response is set using the factor F216
613 Switch-Off Criterion for Field Flashing
The Excitation Monitoring supplies the criterion for switching off the field
flashing Whether this criterion is activated based on the actual value for generator voltage
Ug or for field current If or both depends on the settings of the two threshold values
F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements
Ug12 and If12 are switched over depending on the present status of the channels
(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is
fixed at ldquological 1rdquo
614 Storage of Alarm Status
The outputs of the over current alarms (R1 R2 R0) and the messages NO
FAILURE MONITORING PARAMETERS CHANGED are stored messages can be
erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the
front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the
situation causing the alarm or the malfunction is no longer present Whenever the self-
diagnosis equipment in the processor detects a malfunction the output NO FAILURE of
MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo
is activated whenever parameters or settings of software switches have been changed via
the Micro-Terminal
615 Actual Value Monitoring
The actual values for generator voltage Ug synchronous voltage Usyn and field
current If are monitored for malfunctions This monitoring is active regardless of whether
or not the generator is in operation Essentially when the generator is in operation the
measurements are monitored by comparing the signals (the smaller signal reading is
detected as incorrect) When the generator is not in operation the measured data are
monitored for extreme values The percentage of deviation permissible in the
measurement signals being compared
(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and
F20AIf the excitation transformer is being supplied from an auxiliary power source (no
shunt operation) the values of Ug and Usyn will be different in some operational
conditions
In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1
and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the
messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2
and the binary message from CH1 reports no malfunction a malfunction on
Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is
also present whenever the binary message CH1 DISTURBANCE is reported and a
discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2
FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares
Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the
comparison channel reports a malfunction or whenever both binary messages report no
malfunction - but both secondary monitors report a malfunction As long as the secondary
monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction
signals for the measurement channel involved (suspicion that there is a corresponding
error in Usyn) The measurement channel malfunctions are enabled operationally
whenever after excitation has been switched on generator voltage Ug exceeds the value
set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is
switched off to see that they do not exceed the limit value F210 that applies to both of
them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt
the operating range) Monitoring for extreme values is likewise enabled during normal
operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the
malfunction signals to the binary outputs Basically the monitoring of the actual values
for If1If2 functions like that of the Ug1Ug2 monitoring
62 Excitation Protection
621 General Information
The Excitation Protection switches off the excitation (and de-excites the machine
rapidly) whenever a danger arises that threatens the excitation transformer the converter
or the generator Generally limiter or monitoring functions precede the emergency trips
and these normally respond before the Excitation Protection must initiate a trip
Protective trip commands are issued directly to the field circuit-breaker from potential
free contacts of the board via the trip relays They are directed redundantly to the
operative field circuit-breaker ldquoOFFrdquo command
622 Protection against Excitation Transformer Overheating
This equipment monitors the excitation transformer for overheating in the
windings that could result from over current short-circuits or inadequate cooling The
monitoring uses temperature monitoring modules in conjunction with temperature
sensors built into the windings Normally the temperature is monitored in two stages the
first stage sets off an alarm the second causes a trip of the excitation
623 Rotor Over voltage Protection
Malfunctions in the generator circuit (eg terminal short-circuit failed
synchronization asynchronous operation) cause induced negative field currents that
produce high voltages in the field circuit These must be restricted to a level with a
sufficient safety margin below the insulation capacity of the field winding (test voltage)
and also below the peak blocking voltage of the converter thyristors The crow bar
employs spark gap elements to detect over voltages in the field circuit Whenever they
respond the associated thyristors are fired immediately switching the de-excitation
resistor parallel to the field The de-excitation current generated thereby initiates an
excitation trip via a supervision circuit causing an immediate opening of the field circuit-
breaker The malfunction isets off an alarm and an internal malfunction is indicated at the
cubicle
TEST VALUES OBTAINED WHEN EXCITATION IS RAISED
TEST
SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE
1 VREF 996 100
2 VACT 997 1003
3 IFACT 735 765
4 IGACT 703 707
5 ACTIVE
POWER
703 705
6 REACTIVE
POWER
101 142
7 POWER
FACTOR
099 IND 098 IND
8 ACTIVE
CURRENT(IR)
705 703
9 REACTIVE
CURRENT(IX)
102 138
10 POWER
ANGLE
566 546
11 FIRING
ANGLE
640 632
VALUES OBTAINED WHEN EXCITATION IS LOWERED
SNO PARTICULARS ACTUAL
VALUE
OBTAINED VALUE
1 VREF 100 997
2 VACT 100 997
3 IFACT 787 761
4 IGACT 839 837
5 ACTIVE
POWER
839 830
6 REACTIVE
POWER
155 110
7 POWER
FACTOR
098 IND 099 IND
8 ACTIVE
CURRENT(IR)
835 830
9 REACTIVE
CURRENT(IX)
140 94
10 POWER
ANGLE
613 DEG 631 DEG
11 FIRING
ANGLE
601 DEG 599 DEG
CONCLUSION
For generating the EMF in stator winding excitation is required to the rotor of a
generator There are two types of excitation
1 Static excitation system
2 Brushless excitation system
A certain disadvantage in brushless excitation system is the slow response time of
the field in case of fast load changes specified No slip-rings and brushes direct
measurements of the field parameters not possible
To avoid all loses static excitation is used Since it does not have any rotating
parts mechanical loses and windage loses This system has fast response and speed
control While preferring this excitation system there are no limitations for the
redundancy of Thyristor bridge circuits
Static excitation has fast field discharge by resistor and inverter operation direct
measurement of field quantity is possible The meaning of excitation is nothing but
continuous supply of DC current (ie field current) to the rotor to buildup required
output voltage in the stator
Field current is changed with respect to the change of load so the digital
automatic voltage regulator (DAVR) is used to regulate the output voltage according to
the load variations
So we conclude that static excitation system with DAVR is preferred since it is
having excellent dynamic performance and better options for R amp M
the AUTOMATIC channel also contains various limiters and corrective control circuits
to ensure the use and stable operation of the synchronous machine up to its operating
limits This channel possesses a Gate Control Unit with a subsequent Intermediate Pulse
Stage to generate the firing pulses for the Thyristor converter During normal operation
the Intermediate Pulse Stage of AUTOMATIC Channel is active and transmits the firing
pulses galvanically separated to the common pulse bus at the input to the Final Pulse
Stage Various monitoring functions of the AUTOMATIC channel and pulse monitoring
on the common pulse bus initiate an automatic switch-over to stand by Channel
(MANUAL)in case of a malfunction
Channel 2 (the MANUAL channel) is built as a simple field-current regulator with a PI
control algorithm It serves as a back-up channel in case of a malfunction on the
AUTOMATIC channel Manual channel performs valuable service for testing
commissioning and preventive maintenance The MANUAL channel has its own Gate
Control Unit (the software for the If regulator is also implemented therein) and its own
intermediate pulse Stage During normal operation (AUTOMATIC) the output pulses
from Intermediate Pulse Stage are blocked from reaching the pulse bus Various
monitoring on the MANUAL channel initiate an alarm in case of a malfunction while the
MANUAL channel is on stand-by If the MANUAL channel suffers a malfunction while
it is in operation the excitation is switched off (TRIP) Both channels are equipped with
tracking equipment so that the inactive channel always generates the same control
variable as the active channel during steady-state operation
This ensures smooth switch-over from Automatic to Manual channel and vice
versa To ensure that the MANUAL channel will in a switch-over initiated by a
malfunction take over the operating point of the machine as it was prior to the problem
the response of the tracking for the MANUAL channel is set relatively slow In addition
to the pulse monitors (ldquoSUPERVISIONrdquo) shown in the basic circuit diagram the
excitation system has an autonomous Excitation Monitoring As one of its functions this
equipment monitors for field currents that exceed acceptable maximum limits It initiates
an emergency switch-over to the MANUAL channel whenever the field current exceeds
the preset limit If even after such a switch-over the field current does not drop back to
3
the permissible level the excitation is switched off by Excitation Protection The most
important measuring inputs for the excitation system (If Ug Usyn) are redundant (2-
fold) The Excitation Monitoring checks these measuring inputs for discrepancy and
plausibility An alarm is always initiated in case of malfunction In certain cases a
switch-over to MANUAL channel is also initiated The excitation system contains an
Excitation Protection to protect the excitation transformer the converters and the
synchronous machine The protection system can detect short-circuits in the excitation
circuit and keep secondary damage within acceptable limits by a quick tripping of the
excitation and an opening of the generator breaker An overheating of the excitation
transformer first sets off an alarm (at a given preset limit) and then likewise initiates a
protective shut-down at an even higher limit The over voltage protection in the de-
excitation equipment provides an autonomous protective function for the rotor and the
rectifier This protection system monitors the field voltage in both polarities for over
voltage and if necessary de-energizes the field via the de-excitation resistor
12 Principle of primary power supply
In the shunt excitation system the excitation transformer also provides
the power supply for the electronic equipment and the converter fans So a failure of the
auxiliary supply to the converter fans does not cause a shutdown of the excitation When
the auxiliary supply fails the supply to converter fans is switched over to Excitation
transformer OP with a contactor A station battery supply is absolutely necessary for the
control of the field circuit breaker It is the power source for the electronic devices till the
generator is able to supply voltage Auxiliary power to the field flashing equipment must
be present in order to build up the generator excitation The power supply for standstill
heating and the cubicle lighting is also from Station Auxiliary Power Supply and is of
secondary importance for operation of the plant Power supply to rotor earth fault
detection circuit too is from Station Auxiliary Supply The two synchronous voltages
Usyn are each supplied to the AUTOMATIC channel and the MANUAL channel
separately across transformers The Gate Control Units need these voltages to enable
4
them to issue the pulses at a given firing angle relative to the input voltage of the
converter
5
CHAPTER 2
Digital Automatic Voltage Regulator (DAVR)
21 Principle of Operation of the Regulator (DAVR)
To regulate the voltage and the reactive power of a synchronous machine the
field voltage must be adjusted quickly to the changes in the operating conditions (with a
response time that does not exceed a few ms) To accomplish this analog control systems
include amplifiers which make continuous comparison of the actual values against the
reference values and vary the control variable to the converter with almost no delay Most
of the delay that occurs originates in the converter since the firing pulses for changing
the rectifier phase angle are only issued periodically (every 33 ms)
The DVR digital voltage regulator calculates the control variable from the
measured and reference data in very short time intervals This results outwardly in a
quasi-continuous behavior with a negligible delay time (as in an analog regulator) The
calculations are made in the binary number system Analog measurement signals such as
those for generator voltage and generator current are converted into binary signals in
analogdigital converters The set-points and limit values have already been defined in
digital (binary) form An understanding of the actual computation processes in the digital
voltage regulator is not necessary for operation preventive maintenance or
troubleshooting Like the operator of a pocket calculator or a personal computer all the
operator needs is to know how to operate the instrument and the programming for this
working tool For that reason we will explain below only the principle division of work
among the various modules and the flow of data processing The purpose is above all to
make clear how the processor system has been integrated into the rest of the power
electronics system
6
22 Basic Structure of the Processor Systems
7
The signal processors 25 analog inputoutput modules Each of these processor
systems has a common bus circuit and output and the control lines There is a specific
range of addresses assigned to each assignment Board including the power supply bus
the address lines the two data lines to the input calculates the reactive current (I sin φ)
and the active current ( Icos φ) With these two channel processor Synchronized with
these interrupts (ie with the phase positions of current Ig the field current If and the
synchronous voltage Usyn From the exchange data with the microprocessor card across
the two data lines generator voltage Ug) this processor measures the generator current
Ig and then hardwired connections or multi-conductor cables Binary and analog
inputoutput modules ie for galvanic isolation and adaptation to the electronics level
The most important input interrupts per period to trigger the cycles for processing actual
values in the AUTOMATIC module on the processor bus) for filtering and further
processing
Monitoring each consist of the central microprocessor module and binary and
parameters to the AUTOMATIC channel are the generator voltage Ug the generator
peripheral unit Ug Ig and Usyn are sent to the Interrupt Generator (plug-in peripheral
units (wall-mounted units) peripheral units are used for preprocessing signals from
external measurement circuits power supply units Signals are exchanged among these
processor systems via processed across separate peripheral units for each channel These
processor working on the bus (a house address that can be adjusted using a switch)
systems The AUTOMATIC channel the programmable controls and the Excitation The
actual values measured from AUTOMATIC channel and MANUAL channel are The
AUTOMATIC channel and the MANUAL channel each have their own The digital
voltage regulator is broken down into several autonomous microprocessor The inputs and
outputs of the processor systems are directed across voltage-isolating The Interrupt
Generator also uses the 3-phase Ug signal to generate the 12 themselves contain a limited
number of hardware inputs and outputs with fixed equipment Whenever addresses from
this range are called up the signal processing module can results the processor is then
able to derive further operating parameters such as the load angle the active power etc
The functions of all microprocessor systems other than the programmable controls
have been accomplished in firmware The non-varying standard function modules can be
configured to the design desired for plant-specific purposes using software switches
(KFlags) Thus for example the stored status of a K-Flag determines whether or not a
Limiter is active and whether the de-excitation or the excitation limiters take precedence
Because these K-flags determine the software Scope of Supply for the installation they
cannot be changed permanently via the Micro-Terminal In this way they differ from
such setting data as the values of the parameters for the PID filter of the voltage regulator
or the set-points for the limiters These values can be permanently changed using the
Micro-Terminal Communication is possible with each of the processor systems via the
Micro-Terminal by plugging on the connecting cable In this way signals within the
processor and setting parameters can be viewed analog signals can be issued and the set
parameters can be altered temporarily (F range) or permanently (C range) Unlike the
other processor systems the programmable controls do not include any firmware for
realization of the functions They have been designed so that the designer can adapt and
change their functions easily using the ldquoFunctional Block Programming Language P10
Digital and analog functions can be implemented in practically any degree of complexity
desired using the P10 functional blocks The control variable of the voltage regulator
(AUTOMATIC channel) and the control variable of the field current regulator
(MANUAL channel) are each processed in separate Gate Control Unit and formed into a
chain of pulses at the appropriate firing angle The pulses of the active channel are
directed to the pulse bus via the associated Intermediate Pulse Stage The pulses for each
converter block are amplified sufficiently in Final Pulse Stage to fire the Thyristor
231 General Information
The functions of the automatic voltage regulator AVR are
1 to regulate the generator voltage
2 to regulate the effect of the reactive andor active current on the voltage
3 to limit VoltHz
4 to limit max and min field current
5 to limit inductive stator current
6 to limit capacitive stator current
7 to limit the load angle
8 to stabilize the power system
Block Diagram shows the software structure of AUTOMATIC channel The
generator limiters not provided for the installation in question (optional equipment) are
identified in this overview as ldquoNot Suppliedrdquo The parameter values signal values and
software switches (flags) marked with addresses (hexadecimal numbers) can be viewed
and altered via the Micro-Terminal The values selected are displayed in sec pu Hz
etc and can where necessary be changed directly in these formats The plant-specific
settings of the variables and the flags can be obtained from the Test and Commissioning
Report This block diagram provides information about the important functions and
possible settings of the AUTOMATIC channel For the sake of clarity no detailed
presentation has been given of special functions such as tracking circuits initializations
etc The page heading cross-refers this overview to the various sheets of the schematic
diagram Binary signals are shown in broken lines analog signals in solid lines The
corresponding text designations in the schematic diagram can be used for identification of
the input signals (hardware inputs) The only analog output signal from the automatic
voltage regulator control variable Ucontr is sent via the data bus (CRU bus) to the Gate
Control Unit Most of the binary messages (outputs) from the AVR are of no interest
functionally and they have been omitted for the sake of clarity The basic structure of the
digital voltage regulator and the limiters is simple This is necessary in order that the
behavior of the regulatorslimiters will remain calculable and understandable in all
operating situations and that there will be no problem in adjusting and optimizing them
The central PID filter in the digital voltage regulator defines the dynamic response of the
closed-loop controls both in the voltage regulator mode and after limiters have
intervened The ldquocontrol deviationrdquo at the input to the PID filter is either the control
deviation for voltage the control deviation of a de-excitation limiter (the value
determined by minimum value selection) or the control deviation of an excitation limiter
(the value determined by maximum value selection) Flag F730 (ldquoPRIORrdquo) is used to
determine whether the exciting (Min value) or the de-exciting signal takes precedence on
the minmax value limiter (normally F730 = 1111 ie the de-exciting signal takes
precedence) With the exception of the Minimum Field Current Limiter all other limiters
have variable factoring multipliers of the signal outputs so that they can be adjusted
individually together with the common PID filter which has been optimized for voltage
regulation The setting parameters for this PID filter are as follows
Vo = KR Static amplification
1
Ta = ---- Integration time constant
Tc1
Vp Proportional amplification
1
Tb = ---- Differential time constant
Tc2
Vinfin Amplification of high frequencies
The BODE diagram below shows the assignment of settings in accordance with
DINIEC standards based on a typical example
The PID filter amplifications Vo Vp and Vinfin can be adjusted in pu values But
the ldquoceiling factorrdquo pl+ must be adjusted correctly with parameter F310 if the total
amplification (circuit amplification) of the control circuit is actually to conform to the
pu settings This factor must agree with the ldquoexternalrdquo amplification ie with the
ceiling value of the transformer- converter circuit
Ceiling factor(pl+) = Ufmax Ufo
in which Ufmax = ceiling field voltage
Ufo = no-load field voltage
To attain a suitable response of the AVR when starting excitation
(ldquoEXCITATION ONrdquo) it may be necessary to change the proportional amplification of
the regulator during this phase Vp2 (transiently activated) and Vp1 (permanently
activated) can be adjusted for this purpose For example the value of Vp2 takes effect
immediately once the excitation is switched on and remains effective for a period as set at
F30C Once the period F30C (eg 5 sec) has expired Vp shifts over to Vp1 (becomes
the steady-state Vp) at the rate of change set The standard operating mode for the PID
filter is voltage regulation for which the discrepancy between the voltage set-point and
the current value for generator voltage Ug (the control deviation) is supplied at the input
To compensate for the voltage drop in the block transformer or whenever several
generators are operating to the same distributing bus the generator voltage must be
varied in proportion to the measured generator current (droop influence) To accomplish
this the voltage set-point is varied as a function of the measured reactive current IX
andor active current IR Flag F712 enables the IX droop Flag F710 the IR droop The
desired compensation is set in F282 and F286 respectively Flags F284 and F288 are used
to select whether this droop influence is to increase the voltage or to reduce it
(compensation) Combined influence of the active and reactive currents is attained by
enabling both droops IX and IR Flag F716 activates a so-called ldquoSoft-Startrdquo at the
starting of excitation This ldquoSoft-Startrdquo ensures that the voltage set-point integrates from
0 to 100 within the time set on F290 when the excitation is switched on
(ldquoEXCITATION ONrdquo) A ldquosmoothrdquo excitation of the generator can be achieved in this
way whenever there is no demand for a quick excitation
232 Voltage Set-Point
Various signals and settings control and limit the voltage set-point F270 For
example the values of F254 and F252 define the normal operating range possible for set-
point adjustment (eg 90 110) using external control commands (control room local
operatorrsquos panel superposed control system) The effective set-point adjustment rate is
governed The set-point can be set at the values of F250 and F256 by activating
appropriate control commands for ldquoSETrdquo input Enabling Flag F71A and activating a
binary input prior to switching on the excitation (ldquoEXCITATION OFFrdquo) sets the Ug set-
point at the value of UAUX This makes it possible for example to ensure that the
generator voltage will agree exactly with the network voltage after the voltage build-up
An external value with variable amplification can be added to the Ug set point by
enabling F724 (for example for stability tests)
233 Regulator Tracking in MANUAL Operation
Whenever the AUTOMATIC channel is not in operation (the MANUAL channel
is ON) a follow-up equipment ensures a smooth switch-back to the AUTOMATIC
mode will always be possible To track the voltage set-point is shifted by means of
RAISELOWER pulses from the Gate Control Unit so that control variable Ucontr at the
output from the PID filter is held steady and identical to the control variable Ucontr from
the MANUAL channel Because this tracking must react slowly resultant transient
control deviations resulting from the amplification in the PID filter might cause severe
interference with control variable Ucontr
To prevent this the follow-up equipment intervenes on the regulators mixing
point with a corresponding compensation signal
234 Ugf Limiter
At under frequency the Ugf Limiter reduces the generator voltage so as to
prevent saturation effects in the supply and measuring transformers To adjust this
limiter the max permissible generator voltage at rated frequency is defined and set
When any under-frequency occurs the generator voltage is thus reduced in proportion to
that setting
235 Field Current Maximum Limiter
The Field Current Maximum Limiter is provided to protect the generator rotor
from s occurring in steady-state and transient operation High field currents are normally
the result of a sharp drop in network voltage or of an improper raising of the voltage set-
point by the operating staff The field current is held steady at the value TH1 ie at the
maximum thermal value permissible for the excitation circuit and the rotor In order that
the generator can support the power network with its transient overload capacity during
brief collapses in voltage a temporary switch-over is made to the transient limit MAX1
(a higher setting) When the generator or the converter is operating at a reduced capacity
These limits TH1MAX1 can be switched over to the lower settings TH2MAX2 by
activating the corresponding binary signals The switch-over from the thermal limit
TH12 to the transient limit MAX12 can be configured in one of three ways
a) Depending on the over current with -dUdt ENABLE
Flag programming F418 = any setting desired F41A = 0000
This variant enables the transient value MAX12 whenever a collapse of voltage
in the network is detected The ENABLE time is fixed and can be set The example
below shows the typical behavior of the limiter configured in this way
b) Dependent on the time integral with -dUdt ENABLE
Flag programming F418 = inactive F41A = 1111
This variant likewise enables the transient value only when a collapse of network
voltage has been detected However the switch-back to the thermal limit is not made
dependent upon the time itself but on the calculated time integral intisup2dt of the The setting
on Parameter F414 in spu takes into account the time the rotor needs to cool down ie
the rate of temperature change in the case of intermittent operation The example below
shows how the timing of the switch-back to the thermal limit depends on the present
value for intisup2dt
The time integral is based on the formula
Example The setting of =isup2dt equivalent to Version a (F416) at a constant 16 times the
nominal field current for 10 seconds (with TH12 = 105) is
c) Dependent on the time integral without any preconditions
Flag programming F418 = 1111 F41A = 1111
In this variant the transient becomes available without any prior conditions
(without a -dUdt ENABLE) with the time integral intisup2dt
237 Inductive Stator Current Limiter
The Inductive Stator Current Limiter holds the stator current Ig within permissible
limits while the generator is in the ldquoover-excitedrdquo operating range by reducing the field
current accordingly The setting TH (thermal limit) provides the limit against stationary s
that might occur To take advantage of the generatorrsquos transient overload capacity a
switch-over is made to the higher setting MAX The principle of operation of this switch-
over to the value MAX permissible only transiently is identical to that employed for the
field current limiter (refer to the description above) When the drive output from the
turbine is very high stator current may exceed permissible limits even while inductive
loading of the generator is low In this case if the stator current limiter is not kept from
influencing the field current the control circuit will oscillate back and forth between the
Inductive Stator Current Limiter (de-
exciting) and the Capacitive Stator Current Limiter (exciting)The output signal of that
function then dominates the control variable of the Ig-dependent limiter via a maximum
value selection
238 Capacitive Stator Current Limiter
239 Load Angle Limiter
The Load Angle Limiter prevents the synchronous machine from slipping out of
phase due to slippage of the rotor The load angle δ the difference in phase between the
rotor and the stator rotating field results mainly from the driving torque (active power P)
acting on the generator and the level of rotor current (field current) If the driving torque
remains constant a increase in the field current reduces the load angle δ The current load
angle δ at any moment is obtained from the generator current and generator voltage based
on a simplified model of the generator Whenever this calculated load angle δ exceeds the
preset limit angle the limiter increases the field current until the load angle has dropped
back to its permissible value The quadrature reactance Xq of the generator and the
network reactance Xe during normal operation must be adjusted on the regulator in order
to obtain the load angle δ The graph below shows the Power Chart for a salient-pole
machine with typical limiter characteristics
The purpose of a Power System Stabilizer is to use the generator excitation to
damp electromechanical oscillations between the network and the generator Depending
on the design of the generator and the requirements imposed for network stability its
main function will be either to damp the oscillations originating in the machine or those
from the network A synchronous generator working in a combined power network is in
principle an oscillating structure In order to produce a torque the magnetic field of the
rotor and the stator must form a given angle (referred to as the rotor displacement or load
angle δ) The electrical torque ME increases as the angle δ increases just as with a
torsion spring Because the ME of the generator and the mechanical driving torque MA
from the turbine are in equilibrium during steady-state operation the angle δ remains in a
given position Whenever this state of equilibrium between MA and ME is disturbed the
load angle slips of this rest position and change thereby the electrical torque ME The
torque attempts to restore the load angle to a stationary position Due to the mass inertia
of the turbinegenerator rotor however this can only take place aperiodically It does so
in the form of more or less effectively damped oscillations (again similar to the effect of
mass inertia on a torsion spring) In order to damp the oscillations there must be a
damping torque produced depending not on the electrical torque ME associated with the
angle but on the difference in frequency (Df) between the rotor and the stator rotating
field ie on the slippage This torque is produced mainly by the so-called damper
winding in the rotor but the dimensioning of this is subject to limits imposed by
considerations of design and economy Some further action is therefore needed to
increase the damping effect The following drastically simplified formula shows the
parameters upon which the amount of active power PE supplied by the generator
depends
PE = active power
It can be seen from the above relationship that the active power that the generator
transfers depends not only on the load angle δ but also on the field current If That means
that a transient change can be made in the active power PE and with that in the effective
electrical torque ME by varying the field current The principle of operation of the DVR
Power System Stabilizer becomes clear from a consideration of the oscillations in power
output and frequency (ΔPE Δf) and the vector diagram If it is assumed that oscillations
in the network frequency generates load oscillations with the mass inertia of the rotor
then the active load of the generator (eg MW-measured) is influenced with a sinusoidal
value -ΔPE (ME-MA = -ΔPE) By inversion of -ΔPE one obtains the fluctuation in
power provided by the rotor +ΔPE As is known the slip signal Δf follows +ΔPE with a
phase delayed by 90deg The +ME produced by the periodic changes in the load angle δ is
in phase with +ΔPE A good damping is attained if ME is varied in phase with the slip
Δf However this signal must also be advanced somewhat to compensate for the time
constants in the excitation circuit and the generator
As mentioned above the electrical torque ME can be influenced by varying the
field current To accomplish this a suitable control signal referred to as variable
disturbance compensation must be imposed upon the voltage set-point or the converter
control variable Ucontr As can be seen from the vector diagram by applying proper
weighting factors (K1 K2) and then adding together the signals -ΔPE and Δf an overall
stabilization signal can be produced that rotates in advance of the Df signal by any angle
desired between 0deg and 90deg Because the amplitude of -DPE remains proportional to the
amplitude of Δf a constant angle in advance of Δf results for the compensation of the
time constants referred to above The optimum weighting factors K1 and K2 for a
synchronous generator working to a power network depend on its operating point at any
moment and the external reactance of the network Normally the selection of a
compromise setting is good enough to attain stability in all operating points and for all
external reactance For special demands these settings must be parameterized as a
function of the external reactance (which means optional equipment Xe-Identification)
The Power System Stabilizer PSS is a section of the AVR computer program and is
processed once per network cycle The voltage at the generator terminals and the
generator current are measured in order to define the signals ΔPE and Δf The calculated
signals for _P_ (=PE) and Δf are then sent across DC filters ldquoDrdquo (real differentiators) that
transmit only the dynamic portion of the signals The ΔPE and Δf signals obtained in this
way are then weighted (multiplied by) with the factors K1 and K2 and sent to the
summing point of the voltage regulator
The PSS stabilization signal is imposed on the automatic voltage regulator only if
the following prerequisites are met
bull Generator on line
bull Generator power output gt the value F338
bull Generator voltage in a range between F33C and F33A
The stabilization signal is limited at the output from the PSS to the lower and
upper limits Flag defines whether the stabilization signal is introduced before or after the
PID filter (usually before the filter) Because the PID filter as noted above already takes
the ceiling factor Vp1 into account the PSS signal needs to be multiplied by Vp1 if it is
added to the voltage regulator following the PID filter (divider at the input to the
minmax limiter) This precaution prevents the DC filter ldquoDrdquo in the P-channel from
producing an unnecessary ldquostabilizationrdquo effect in the case of rapid changes in turbine
load As an alternative for the AVRrsquos Power System Stabilizer a stabilization signal from
an outside system can be imposed by activating the binary input ldquoPSS-SIGNEXTrdquo Flag
F340 can be used to select between an analog and a 12-bit signal and F33E to select the
polarity desired for that signal
24 The MANUAL Channel
241 Summary
The MANUAL channel (Channel 2) has been built as a simple field current
regulator
without additional limiters Its main function is to maintain the excitation of the generator
even if the AUTOMATIC channel becomes non-operational The MANUAL channel
also performs valuable service for purposes of testing commissioning and preventive
maintenance Its measurements regulator generation of firing pulses and power supply
are physically separate from those on the AUTOMATIC channel
242 Principle of Operation
All the functions of the MANUAL channel including the generation of firing
pulses have been implemented in a single electronic module the Gate Control Unit The
control variable Ucontr of voltage regulator is used as the reference value for generating
firing pulses on the principle known as ldquoramp controlrdquo (Comparison of Ucontr with
Usynsynchronous sawtooth signal) For further processing in the UN 0096 Intermediate
Pulse Stage the Gate Control Unit supplies six firing pulses at its output whose phase
position with respect to the synchronous voltage Usyn is in accordance with control
variable Ucontr An internal linearization ensures that the field voltage produced via the
firing pulses remains proportional to the control variable Ucontr throughout the entire
range As a result the circuit amplification of the control remains constant over the entire
range Whenever excitation is switched ON the set-point for Generator Voltage is set
automatically at the preset - ref Value This provision ensures that the generator voltage
always attains approximately its nominal value after the field flashing The Gate Control
Unit can be refunctioned ( by pre-selection with a switch ) for purposes of testing to act
as a purely firing pulse control In this case the control variable Ucontr is adjusted
directly using the RAISELOWER push buttons on the front of the module In this way
for example the relationship between the phase position of the firing pulses and the
control variable Ucontr can be checked easily
CHAPTER 3
PULSE SECTION
31 Pulse Generation and Amplification
The Gate Control Units of both AUTOMATIC channel and MANUAL channel
each supply six firing pulses for operating the 6-pulse thyristor bridges The low-power
pulse signals from these Gate Control Units are then amplified in the Intermediate Pulse
Stage galvanically isolated and then sent to the common pulse bus On the output end
the Intermediate Pulse Stage of the non-active channel) is always blocked The Gate
Control Units generate the pulses based on microprocessor control The reference voltage
used for the firing pulse phase location is the output voltage from the excitation
transformer (Usyn1 Usyn2) The commutation spikes of the synchronous voltage caused
by the converter are calculated prior to use of the voltage as a reference value and are
deliberately filtered out The lower limit for the firing pulses (double pulses) which are
offset from one another by 60deg is defined by the limit rectifier position (αmin) and the
upper limit by the limit inverter position (αmax) for the firing angle αmin and αmax can
be adjusted on the Gate Control Units using BCD (Binary Coded Decimal) switches
αmin ensures that the firing pulses will not be issued (premature firing) until there is
sufficient positive phase voltage on the thyristor involved αmax prevents a dangerous
ldquotippingrdquo of the thyristor bridge into the rectifier mode if the firing angle α is too large
(ldquolate firingrdquo) The critical factors determining αmax are the overlap time uumlmax (max
commutation time) and the ldquorecovery timerdquo of the thyristors (αmax lt 180deg - uumlmax - γ )
An external control signal can force the firing pulses into their inverter limit position
Other binary inputs can block or direct the firing pulses of the Gate Control Units so as to
produce freewheeling on the thyristor bridge During freewheeling the firing pulses for
the thyristor pair R and S are blocked and the pulse signals T+T- are engaged with
chains of pulses Both Gate Control Units (for the MANUAL amp AUTOMATIC channels)
contain a field current monitor that blocks the firing pulses immediately whenever the
current exceeds a preset threshold level In this case the field circuit-breaker is also
tripped via an output contact The purpose of these provisions is to prevent damage to
thyristors and thyristor fuses in case of a slip-ring short-circuit or to keep any damage
that does occur to a minimum The pulse signals are galvanically separated at the outputs
from the Intermediate Pulse Stage (with pulse transmitters) and are then directed to the
common pulse bus This transmission of the pulse signals to the pulse bus via passive
transmitters ensures a high degree of active channel autonomy Practically no possible
malfunctions on the inactive channel (including for example sustained pulses) affect the
active channel
32 Pulse Monitoring
The ldquoPulse Busrdquo and the pulse signals of the AUTOMATIC channel are
monitored This monitoring device consists of potential isolating stages and the common
monitor If the pulse monitoring of the ldquoPulse Busrdquo responds a switch-over is made to
MANUAL channel The function of the potential isolating stages is to couple the pulse
monitoring device to the pulse circuits without any feedback effect The pulse monitoring
checks the six pulse lines for the following malfunctions continuous or periodic failure
of one or more pulses Periodic occurrence of synchronous or asynchronous false pulses
Continuous pulses the pulse monitoring device can be tested while the machine is in
operation
CHAPTER 4
CONVERTER
Thyristor
The term thyristor usually refers to a family of four layer solid state device having
turn on characteristics that can be externally controlled by either current or voltage They
are also referred to as breakdown device because their working depends on avalanche
breakdown Thyristors have only two stages OFF and ON Thyristors have a similar
function to Uni-junctions they act as switches Thyristors use current flow as a switch
Thyristors have three states
1 Reverse blocking mode mdash Voltage is applied in the direction that would be
blocked by a diode
2 Forward blocking mode mdash Voltage is applied in the direction that would cause
a diode to conduct but the thyristor has not yet been triggered into conduction
3 Forward conducting mode mdash The thyristor has been triggered into conduction
and will remain conducting until the forward current drops below a threshold value
known as the holding current Converter is a semiconductor device which converts ac
input voltage into a constant dc output voltage In present excitation system three phase
fully controlled thyristor converter is used
Because of the following advantages thyristor converters are used
a) Thyristors are used for high power applications ie up to 10Kv3500A1KHz
b) Having high reliability and low losses
c) Uni-directional device like diode
d) Itrsquos operation as a rectifier which are low resistance in forward conduction
mode and high resistance in reverse conduction mode
PROTECTION OF THYRISTORS
For reliable operation of a thyristor demands that its specified ratings are not
exceeded When Subjected to or over voltages During the turn - on of SCR didt
prohibitively large False triggering of SCR by high value of dvdt andSpurious signals
between gate and cathode may leads to unwanted turn ndash on
DIDT AND PROTECTION
When thyristor starts conducting in forward conduction mode and is turned on by
gate pulse The anode current increases rapidly whole area of the gate to Cathode
junction then hot spots will be formed near the gate connection this locality of heating
destroys the thyristor Thyristor thermal time is constant The causes due to faults and
short circuits or surge currents Electronic crowbar protection is used against the over
voltages The rate rise of anode current must be kept at the time of turn on below the
rated or specified limiting value The didt value maintained below limited value by using
a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating
is avoided by applying gate current but not greater the maximum gate current
DVDT AND OVER VOLTAGE PROTECTION
With forward voltage across the anode and cathode of a thyristor the two outer
junctions are forward biased but the inner junction is reverse biased This reverse biased
junction J2 has the characteristics of a capacitor due to charges existing across the
junction In other words space-charges exist in the depletion region around junction J2
and therefore junction J2 behaves like a capacitance If the entire anode to cathode
forward voltage Va appears across J2 junction and the charge is denoted by Q then a
charging current i given by Eq (46) follows
i = dQdt =d(Cj Va )dt
= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)
As Cj the capacitance of junction J2 is almost constant the current is given by
i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)
If the rate of rise of forward voltage dVadt is high the charging current i will be
more This charging current plays the role of gate current and turns on the SCR even
when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on
must be avoided as it leads to false operation of the thyristor circuit
For controllable operation of the thyristor the rate of rise of forward anode to
cathode voltage dVadt must be kept below the specified rated limit Typical values of
dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by
using a snubber circuit in parallel with the device thyristor are very sensitive for over
voltage than the semiconductor devices
Over voltage transients are perhaps the main cause of thyristor failure
In thyristor there are mainly two types
1 Internal over voltages
Due to the commutation of the thyristors large voltages are generated internally
Because of the series inductance of the SCR circuit the large transient voltages L didt
produced This voltage several times the break over voltage of the device then thyristor
destroys permanently
2 External over voltages
External over voltages are caused due to the interruptions of current flow in an
inductive circuit and also due to the lightening strokes on the lines feeding the thyristor
system For the reliable operation of thyristor the over voltages must be suppressed by
adopting suitable techniques
Suppression of over voltages
The RC circuit called snubber circuit is connected across the device to protect In
order to keep the protective components to a minimum the thyristors are chosen with
their peak voltages ratings are 25 to 3 times of the normal peak working voltage
ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are
commonly employed for protecting the thyristor circuit against the over voltages
Gate protection
Gate circuit should also be protected against the over voltages and surges Over
voltage at gate circuit can cause false triggering of the SCR may rises the junction
temperature behind specified limit leading to its damage Protection against over
voltage can be achieved by connecting a ZD across the gate circuit and a resister is
connected in series with gate circuit to protect against the s A capacitor and resister are
connected across gate to cathode to by pass the noise
41 Final Pulse Stages
The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage
(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor
bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided
with a power supply module The amplified output pulses from the Final Pulse Stages
start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main
pulse This initial pulse edge assures proper firing of the thyristors being triggered
Subsequently the weaker part of main pulse keeps firing conditions steady As already
mentioned the Final Pulse Stages and their associated thyristor bridges form single units
All six pulse outputs from a Final Pulse Stage can be blocked by an external control
signal so that all thyristors in the associated thyristor bridge will block the current A
blocking of the pulses is initiated whenever there is a malfunction in the associated
thyristor bridge
42 Converter Power Section
The thyristor converter consists of three independent parallel rectifier blocks TY1
to TY3 which are all in service Even if one block fails the remaining blocks take over
automatically the full design current of the excitation circuit During normal operation
(with ideal current share) and all three bridges in operation each of these blocks has to
carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation
is limited Only when all three bridges fail the excitation is switched off Each thyristor
bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any
arm is identified by a Current flow monitoring module
43 Converter Cooling
A cooling system is needed to dissipate heat losses in the converter blocks and
electronics Each converter block has therefore been equipped with a fan supplied with
power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing
cubicle) The fans are protected with motor protection circuit breakers An air flow
monitoring unit is provided for monitoring the air flow through the thyristor bridge If a
circuit breaker failure is detected or if the air flow monitor drops off at one of the
thyristor bridges the bridge involved is immediately set out of operation by blocking its
firing pulses
44 Thyristor Converter Monitoring
A thyristor bridge in which defects occur that could threaten the safety of
operation or cause secondary damage is switched off automatically ie its firing pulses
are blocked This happens whenever A thyristor fuse is blown The fuses are monitored
individually with micro switches The Final Pulse Stage fails which is detected by
internal monitors (supply voltage sustained pulse short-circuit on the output end) The
power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or
is insufficient Isolator on ACDC side is open
CHAPTER 5
Field Current Circuit Excitation Transformer
51 Field Circuit Breaker
The circuit-breaker in the field circuit is used to isolate the field circuit from the
converter It is capable of switching off the synchronous machine from full load under the
maximum conditions of a 3-phase short-circuit In addition to its main contacts the field
circuit-breaker also has a de-excitation contact with which the field energy stored in the
field can be dissipated across the de-excitation resistor The de-excitation contact closes
shortly before the main contacts open so as to ensure proper commutation of the field
current from the main contacts to the de-excitation contact when the breaker is switched
off The field circuit-breaker is switched on by electromagnetic force and is kept switched
on by a mechanical latch When the latch is released by a trip coil the circuit-breaker
opens The circuit-breaker also has auxiliary contacts that report its status
52 Field Flashing
In shunt supplied excitation circuits (excitation transformer connected to the
generator terminals) the generator does not have enough remnant voltage for a generator
voltage build-up via the converter In this case a field flashing circuit is provided It
consists of the field flashing contactor the diode bridge and a transformer used to adapt
the auxiliary input voltage to the voltage needed for field flashing when power is
supplied from the auxiliaries network
Fig Field Flashing
Because the field flashing contactor is not able to switch off the energy stored in
the field the control ensures that the contactor can only reopen if the field circuit breaker
has already been opened (generating the TRIP order) or in a normal field flashing
sequence when the converter has taken over the field current Field flashing occurs in the
following stages
1048729The excitation is switched on closing the field flashing contactor ( Field
Circuit Breaker is already closed )
1048729The start-up excitation current flows through the rotor driving the generator
voltage up to approx 15 U
1048729After about 10 U the firing pulses to the converter are released and it begins
to excite the generator to its rated voltage
1048729After about 30 U the field flashing contactor opens (with no current since
the converter is now supplying the current)
The diode bridge at the input to the field flashing contactor prevents a feed-back
from the converter to the source of field flashing while the contactor is still closed
53 De-excitation
When malfunctions occur the stored field energy must be dissipated as quickly
and safely as possible to protect the generator This is done by the converter the field
circuit-breaker and the de-excitation (discharge) resistor
De-excitation (with opening of the field circuit-breaker) takes place in the following
stages
1048729The converter drives to its inverter limit position (negative ceiling voltage)
recovers a portion of the field energy into the network A trip command is given to the
field circuit breaker
1048729The de-excitation contact closes diverting the field voltage to the de-excitation
resistor
1048729Then immediately the main contacts open building voltage The field voltage
commutates to the de-excitation resistor
1048729The current diminishes at a given time constant TE
(With linear resistance TE = Lf (Rf + Re))
Due to the reversal of the field voltage by the converter the field current
commutates from the main contacts of the field circuit-breaker to the de-excitation
resistor in a very early phase This reversal of the field voltage prevents burn-off on the
main contacts and provides effective protection for the field circuit-breaker Depending
on the operating policy an operational shut-down of the excitation can also be effected
with the field circuit-breaker closed This method is useful mainly when the excitation is
switched on and off frequently In this case the converter is merely driven into the
inverter limit position so that the field energy is recovered into the network The
converter then blocks since it is supplying positive current only
54 Excitation Transformer
The excitation transformer matches the generator voltage to the field voltage
(required ceiling voltage) It also serves as a commutation reactance for the thyristor
converter and as a potential isolator between the network and the excitation circuit In
addition the transformer functions as a current limiter in that it makes it possible to keep
any short circuits in the excitation circuit under better control The excitation transformer
is equipped with temperature monitoring probes which set off an alarm when the
temperature exceeds a first max limit and then trips the excitation if the temperature
continues rising to a second (higher)limit
CHAPTER 6
Monitoring and Protection
61 Excitation Monitoring
611 General Information
The main goal of Excitation Monitoring is to make optimum use of the
redundancies provided in the excitation system and to give alarm whenever a malfunction
makes these redundancies unavailable The field current is monitored to see that it does
not exceed a maximum level and if necessary a switch-over to the MANUAL channel is
initiated In addition the criterion for switching off the field flashing is generated The
excitation Monitoring consists of an autonomous processor system
612 over current Alarms
In the Excitation Monitoring the limits for are set at higher levels than the
settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of
the nominal field current contact R1 and the binary output associated with it are
activated immediately If field current remains gt 110 then after a preset inverse-time
has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs
associated with them are activated Parameters match the measurements for If1 and If2 to
the nominal value for field current so that the internal values can be processed and read as
pu values It can be used to falsify the actual value of the field current If (to raise it) so
as to cause a response from the alarm limits for purposes of testing The processed If
signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As
long as the field current If is above the threshold value 11 Ifn its peak value is
measured This is stored (until RESET) and can be read at any time on the Micro-
Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever
the integrated time-current value (intisup2dt) exceeds the preselected reference value the
alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously
issued to switch over to the stand by AUTO channel Software switch F758 enables the
three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-
time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the
desired limit curve for response is set using the factor F216
613 Switch-Off Criterion for Field Flashing
The Excitation Monitoring supplies the criterion for switching off the field
flashing Whether this criterion is activated based on the actual value for generator voltage
Ug or for field current If or both depends on the settings of the two threshold values
F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements
Ug12 and If12 are switched over depending on the present status of the channels
(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is
fixed at ldquological 1rdquo
614 Storage of Alarm Status
The outputs of the over current alarms (R1 R2 R0) and the messages NO
FAILURE MONITORING PARAMETERS CHANGED are stored messages can be
erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the
front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the
situation causing the alarm or the malfunction is no longer present Whenever the self-
diagnosis equipment in the processor detects a malfunction the output NO FAILURE of
MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo
is activated whenever parameters or settings of software switches have been changed via
the Micro-Terminal
615 Actual Value Monitoring
The actual values for generator voltage Ug synchronous voltage Usyn and field
current If are monitored for malfunctions This monitoring is active regardless of whether
or not the generator is in operation Essentially when the generator is in operation the
measurements are monitored by comparing the signals (the smaller signal reading is
detected as incorrect) When the generator is not in operation the measured data are
monitored for extreme values The percentage of deviation permissible in the
measurement signals being compared
(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and
F20AIf the excitation transformer is being supplied from an auxiliary power source (no
shunt operation) the values of Ug and Usyn will be different in some operational
conditions
In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1
and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the
messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2
and the binary message from CH1 reports no malfunction a malfunction on
Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is
also present whenever the binary message CH1 DISTURBANCE is reported and a
discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2
FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares
Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the
comparison channel reports a malfunction or whenever both binary messages report no
malfunction - but both secondary monitors report a malfunction As long as the secondary
monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction
signals for the measurement channel involved (suspicion that there is a corresponding
error in Usyn) The measurement channel malfunctions are enabled operationally
whenever after excitation has been switched on generator voltage Ug exceeds the value
set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is
switched off to see that they do not exceed the limit value F210 that applies to both of
them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt
the operating range) Monitoring for extreme values is likewise enabled during normal
operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the
malfunction signals to the binary outputs Basically the monitoring of the actual values
for If1If2 functions like that of the Ug1Ug2 monitoring
62 Excitation Protection
621 General Information
The Excitation Protection switches off the excitation (and de-excites the machine
rapidly) whenever a danger arises that threatens the excitation transformer the converter
or the generator Generally limiter or monitoring functions precede the emergency trips
and these normally respond before the Excitation Protection must initiate a trip
Protective trip commands are issued directly to the field circuit-breaker from potential
free contacts of the board via the trip relays They are directed redundantly to the
operative field circuit-breaker ldquoOFFrdquo command
622 Protection against Excitation Transformer Overheating
This equipment monitors the excitation transformer for overheating in the
windings that could result from over current short-circuits or inadequate cooling The
monitoring uses temperature monitoring modules in conjunction with temperature
sensors built into the windings Normally the temperature is monitored in two stages the
first stage sets off an alarm the second causes a trip of the excitation
623 Rotor Over voltage Protection
Malfunctions in the generator circuit (eg terminal short-circuit failed
synchronization asynchronous operation) cause induced negative field currents that
produce high voltages in the field circuit These must be restricted to a level with a
sufficient safety margin below the insulation capacity of the field winding (test voltage)
and also below the peak blocking voltage of the converter thyristors The crow bar
employs spark gap elements to detect over voltages in the field circuit Whenever they
respond the associated thyristors are fired immediately switching the de-excitation
resistor parallel to the field The de-excitation current generated thereby initiates an
excitation trip via a supervision circuit causing an immediate opening of the field circuit-
breaker The malfunction isets off an alarm and an internal malfunction is indicated at the
cubicle
TEST VALUES OBTAINED WHEN EXCITATION IS RAISED
TEST
SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE
1 VREF 996 100
2 VACT 997 1003
3 IFACT 735 765
4 IGACT 703 707
5 ACTIVE
POWER
703 705
6 REACTIVE
POWER
101 142
7 POWER
FACTOR
099 IND 098 IND
8 ACTIVE
CURRENT(IR)
705 703
9 REACTIVE
CURRENT(IX)
102 138
10 POWER
ANGLE
566 546
11 FIRING
ANGLE
640 632
VALUES OBTAINED WHEN EXCITATION IS LOWERED
SNO PARTICULARS ACTUAL
VALUE
OBTAINED VALUE
1 VREF 100 997
2 VACT 100 997
3 IFACT 787 761
4 IGACT 839 837
5 ACTIVE
POWER
839 830
6 REACTIVE
POWER
155 110
7 POWER
FACTOR
098 IND 099 IND
8 ACTIVE
CURRENT(IR)
835 830
9 REACTIVE
CURRENT(IX)
140 94
10 POWER
ANGLE
613 DEG 631 DEG
11 FIRING
ANGLE
601 DEG 599 DEG
CONCLUSION
For generating the EMF in stator winding excitation is required to the rotor of a
generator There are two types of excitation
1 Static excitation system
2 Brushless excitation system
A certain disadvantage in brushless excitation system is the slow response time of
the field in case of fast load changes specified No slip-rings and brushes direct
measurements of the field parameters not possible
To avoid all loses static excitation is used Since it does not have any rotating
parts mechanical loses and windage loses This system has fast response and speed
control While preferring this excitation system there are no limitations for the
redundancy of Thyristor bridge circuits
Static excitation has fast field discharge by resistor and inverter operation direct
measurement of field quantity is possible The meaning of excitation is nothing but
continuous supply of DC current (ie field current) to the rotor to buildup required
output voltage in the stator
Field current is changed with respect to the change of load so the digital
automatic voltage regulator (DAVR) is used to regulate the output voltage according to
the load variations
So we conclude that static excitation system with DAVR is preferred since it is
having excellent dynamic performance and better options for R amp M
the permissible level the excitation is switched off by Excitation Protection The most
important measuring inputs for the excitation system (If Ug Usyn) are redundant (2-
fold) The Excitation Monitoring checks these measuring inputs for discrepancy and
plausibility An alarm is always initiated in case of malfunction In certain cases a
switch-over to MANUAL channel is also initiated The excitation system contains an
Excitation Protection to protect the excitation transformer the converters and the
synchronous machine The protection system can detect short-circuits in the excitation
circuit and keep secondary damage within acceptable limits by a quick tripping of the
excitation and an opening of the generator breaker An overheating of the excitation
transformer first sets off an alarm (at a given preset limit) and then likewise initiates a
protective shut-down at an even higher limit The over voltage protection in the de-
excitation equipment provides an autonomous protective function for the rotor and the
rectifier This protection system monitors the field voltage in both polarities for over
voltage and if necessary de-energizes the field via the de-excitation resistor
12 Principle of primary power supply
In the shunt excitation system the excitation transformer also provides
the power supply for the electronic equipment and the converter fans So a failure of the
auxiliary supply to the converter fans does not cause a shutdown of the excitation When
the auxiliary supply fails the supply to converter fans is switched over to Excitation
transformer OP with a contactor A station battery supply is absolutely necessary for the
control of the field circuit breaker It is the power source for the electronic devices till the
generator is able to supply voltage Auxiliary power to the field flashing equipment must
be present in order to build up the generator excitation The power supply for standstill
heating and the cubicle lighting is also from Station Auxiliary Power Supply and is of
secondary importance for operation of the plant Power supply to rotor earth fault
detection circuit too is from Station Auxiliary Supply The two synchronous voltages
Usyn are each supplied to the AUTOMATIC channel and the MANUAL channel
separately across transformers The Gate Control Units need these voltages to enable
4
them to issue the pulses at a given firing angle relative to the input voltage of the
converter
5
CHAPTER 2
Digital Automatic Voltage Regulator (DAVR)
21 Principle of Operation of the Regulator (DAVR)
To regulate the voltage and the reactive power of a synchronous machine the
field voltage must be adjusted quickly to the changes in the operating conditions (with a
response time that does not exceed a few ms) To accomplish this analog control systems
include amplifiers which make continuous comparison of the actual values against the
reference values and vary the control variable to the converter with almost no delay Most
of the delay that occurs originates in the converter since the firing pulses for changing
the rectifier phase angle are only issued periodically (every 33 ms)
The DVR digital voltage regulator calculates the control variable from the
measured and reference data in very short time intervals This results outwardly in a
quasi-continuous behavior with a negligible delay time (as in an analog regulator) The
calculations are made in the binary number system Analog measurement signals such as
those for generator voltage and generator current are converted into binary signals in
analogdigital converters The set-points and limit values have already been defined in
digital (binary) form An understanding of the actual computation processes in the digital
voltage regulator is not necessary for operation preventive maintenance or
troubleshooting Like the operator of a pocket calculator or a personal computer all the
operator needs is to know how to operate the instrument and the programming for this
working tool For that reason we will explain below only the principle division of work
among the various modules and the flow of data processing The purpose is above all to
make clear how the processor system has been integrated into the rest of the power
electronics system
6
22 Basic Structure of the Processor Systems
7
The signal processors 25 analog inputoutput modules Each of these processor
systems has a common bus circuit and output and the control lines There is a specific
range of addresses assigned to each assignment Board including the power supply bus
the address lines the two data lines to the input calculates the reactive current (I sin φ)
and the active current ( Icos φ) With these two channel processor Synchronized with
these interrupts (ie with the phase positions of current Ig the field current If and the
synchronous voltage Usyn From the exchange data with the microprocessor card across
the two data lines generator voltage Ug) this processor measures the generator current
Ig and then hardwired connections or multi-conductor cables Binary and analog
inputoutput modules ie for galvanic isolation and adaptation to the electronics level
The most important input interrupts per period to trigger the cycles for processing actual
values in the AUTOMATIC module on the processor bus) for filtering and further
processing
Monitoring each consist of the central microprocessor module and binary and
parameters to the AUTOMATIC channel are the generator voltage Ug the generator
peripheral unit Ug Ig and Usyn are sent to the Interrupt Generator (plug-in peripheral
units (wall-mounted units) peripheral units are used for preprocessing signals from
external measurement circuits power supply units Signals are exchanged among these
processor systems via processed across separate peripheral units for each channel These
processor working on the bus (a house address that can be adjusted using a switch)
systems The AUTOMATIC channel the programmable controls and the Excitation The
actual values measured from AUTOMATIC channel and MANUAL channel are The
AUTOMATIC channel and the MANUAL channel each have their own The digital
voltage regulator is broken down into several autonomous microprocessor The inputs and
outputs of the processor systems are directed across voltage-isolating The Interrupt
Generator also uses the 3-phase Ug signal to generate the 12 themselves contain a limited
number of hardware inputs and outputs with fixed equipment Whenever addresses from
this range are called up the signal processing module can results the processor is then
able to derive further operating parameters such as the load angle the active power etc
The functions of all microprocessor systems other than the programmable controls
have been accomplished in firmware The non-varying standard function modules can be
configured to the design desired for plant-specific purposes using software switches
(KFlags) Thus for example the stored status of a K-Flag determines whether or not a
Limiter is active and whether the de-excitation or the excitation limiters take precedence
Because these K-flags determine the software Scope of Supply for the installation they
cannot be changed permanently via the Micro-Terminal In this way they differ from
such setting data as the values of the parameters for the PID filter of the voltage regulator
or the set-points for the limiters These values can be permanently changed using the
Micro-Terminal Communication is possible with each of the processor systems via the
Micro-Terminal by plugging on the connecting cable In this way signals within the
processor and setting parameters can be viewed analog signals can be issued and the set
parameters can be altered temporarily (F range) or permanently (C range) Unlike the
other processor systems the programmable controls do not include any firmware for
realization of the functions They have been designed so that the designer can adapt and
change their functions easily using the ldquoFunctional Block Programming Language P10
Digital and analog functions can be implemented in practically any degree of complexity
desired using the P10 functional blocks The control variable of the voltage regulator
(AUTOMATIC channel) and the control variable of the field current regulator
(MANUAL channel) are each processed in separate Gate Control Unit and formed into a
chain of pulses at the appropriate firing angle The pulses of the active channel are
directed to the pulse bus via the associated Intermediate Pulse Stage The pulses for each
converter block are amplified sufficiently in Final Pulse Stage to fire the Thyristor
231 General Information
The functions of the automatic voltage regulator AVR are
1 to regulate the generator voltage
2 to regulate the effect of the reactive andor active current on the voltage
3 to limit VoltHz
4 to limit max and min field current
5 to limit inductive stator current
6 to limit capacitive stator current
7 to limit the load angle
8 to stabilize the power system
Block Diagram shows the software structure of AUTOMATIC channel The
generator limiters not provided for the installation in question (optional equipment) are
identified in this overview as ldquoNot Suppliedrdquo The parameter values signal values and
software switches (flags) marked with addresses (hexadecimal numbers) can be viewed
and altered via the Micro-Terminal The values selected are displayed in sec pu Hz
etc and can where necessary be changed directly in these formats The plant-specific
settings of the variables and the flags can be obtained from the Test and Commissioning
Report This block diagram provides information about the important functions and
possible settings of the AUTOMATIC channel For the sake of clarity no detailed
presentation has been given of special functions such as tracking circuits initializations
etc The page heading cross-refers this overview to the various sheets of the schematic
diagram Binary signals are shown in broken lines analog signals in solid lines The
corresponding text designations in the schematic diagram can be used for identification of
the input signals (hardware inputs) The only analog output signal from the automatic
voltage regulator control variable Ucontr is sent via the data bus (CRU bus) to the Gate
Control Unit Most of the binary messages (outputs) from the AVR are of no interest
functionally and they have been omitted for the sake of clarity The basic structure of the
digital voltage regulator and the limiters is simple This is necessary in order that the
behavior of the regulatorslimiters will remain calculable and understandable in all
operating situations and that there will be no problem in adjusting and optimizing them
The central PID filter in the digital voltage regulator defines the dynamic response of the
closed-loop controls both in the voltage regulator mode and after limiters have
intervened The ldquocontrol deviationrdquo at the input to the PID filter is either the control
deviation for voltage the control deviation of a de-excitation limiter (the value
determined by minimum value selection) or the control deviation of an excitation limiter
(the value determined by maximum value selection) Flag F730 (ldquoPRIORrdquo) is used to
determine whether the exciting (Min value) or the de-exciting signal takes precedence on
the minmax value limiter (normally F730 = 1111 ie the de-exciting signal takes
precedence) With the exception of the Minimum Field Current Limiter all other limiters
have variable factoring multipliers of the signal outputs so that they can be adjusted
individually together with the common PID filter which has been optimized for voltage
regulation The setting parameters for this PID filter are as follows
Vo = KR Static amplification
1
Ta = ---- Integration time constant
Tc1
Vp Proportional amplification
1
Tb = ---- Differential time constant
Tc2
Vinfin Amplification of high frequencies
The BODE diagram below shows the assignment of settings in accordance with
DINIEC standards based on a typical example
The PID filter amplifications Vo Vp and Vinfin can be adjusted in pu values But
the ldquoceiling factorrdquo pl+ must be adjusted correctly with parameter F310 if the total
amplification (circuit amplification) of the control circuit is actually to conform to the
pu settings This factor must agree with the ldquoexternalrdquo amplification ie with the
ceiling value of the transformer- converter circuit
Ceiling factor(pl+) = Ufmax Ufo
in which Ufmax = ceiling field voltage
Ufo = no-load field voltage
To attain a suitable response of the AVR when starting excitation
(ldquoEXCITATION ONrdquo) it may be necessary to change the proportional amplification of
the regulator during this phase Vp2 (transiently activated) and Vp1 (permanently
activated) can be adjusted for this purpose For example the value of Vp2 takes effect
immediately once the excitation is switched on and remains effective for a period as set at
F30C Once the period F30C (eg 5 sec) has expired Vp shifts over to Vp1 (becomes
the steady-state Vp) at the rate of change set The standard operating mode for the PID
filter is voltage regulation for which the discrepancy between the voltage set-point and
the current value for generator voltage Ug (the control deviation) is supplied at the input
To compensate for the voltage drop in the block transformer or whenever several
generators are operating to the same distributing bus the generator voltage must be
varied in proportion to the measured generator current (droop influence) To accomplish
this the voltage set-point is varied as a function of the measured reactive current IX
andor active current IR Flag F712 enables the IX droop Flag F710 the IR droop The
desired compensation is set in F282 and F286 respectively Flags F284 and F288 are used
to select whether this droop influence is to increase the voltage or to reduce it
(compensation) Combined influence of the active and reactive currents is attained by
enabling both droops IX and IR Flag F716 activates a so-called ldquoSoft-Startrdquo at the
starting of excitation This ldquoSoft-Startrdquo ensures that the voltage set-point integrates from
0 to 100 within the time set on F290 when the excitation is switched on
(ldquoEXCITATION ONrdquo) A ldquosmoothrdquo excitation of the generator can be achieved in this
way whenever there is no demand for a quick excitation
232 Voltage Set-Point
Various signals and settings control and limit the voltage set-point F270 For
example the values of F254 and F252 define the normal operating range possible for set-
point adjustment (eg 90 110) using external control commands (control room local
operatorrsquos panel superposed control system) The effective set-point adjustment rate is
governed The set-point can be set at the values of F250 and F256 by activating
appropriate control commands for ldquoSETrdquo input Enabling Flag F71A and activating a
binary input prior to switching on the excitation (ldquoEXCITATION OFFrdquo) sets the Ug set-
point at the value of UAUX This makes it possible for example to ensure that the
generator voltage will agree exactly with the network voltage after the voltage build-up
An external value with variable amplification can be added to the Ug set point by
enabling F724 (for example for stability tests)
233 Regulator Tracking in MANUAL Operation
Whenever the AUTOMATIC channel is not in operation (the MANUAL channel
is ON) a follow-up equipment ensures a smooth switch-back to the AUTOMATIC
mode will always be possible To track the voltage set-point is shifted by means of
RAISELOWER pulses from the Gate Control Unit so that control variable Ucontr at the
output from the PID filter is held steady and identical to the control variable Ucontr from
the MANUAL channel Because this tracking must react slowly resultant transient
control deviations resulting from the amplification in the PID filter might cause severe
interference with control variable Ucontr
To prevent this the follow-up equipment intervenes on the regulators mixing
point with a corresponding compensation signal
234 Ugf Limiter
At under frequency the Ugf Limiter reduces the generator voltage so as to
prevent saturation effects in the supply and measuring transformers To adjust this
limiter the max permissible generator voltage at rated frequency is defined and set
When any under-frequency occurs the generator voltage is thus reduced in proportion to
that setting
235 Field Current Maximum Limiter
The Field Current Maximum Limiter is provided to protect the generator rotor
from s occurring in steady-state and transient operation High field currents are normally
the result of a sharp drop in network voltage or of an improper raising of the voltage set-
point by the operating staff The field current is held steady at the value TH1 ie at the
maximum thermal value permissible for the excitation circuit and the rotor In order that
the generator can support the power network with its transient overload capacity during
brief collapses in voltage a temporary switch-over is made to the transient limit MAX1
(a higher setting) When the generator or the converter is operating at a reduced capacity
These limits TH1MAX1 can be switched over to the lower settings TH2MAX2 by
activating the corresponding binary signals The switch-over from the thermal limit
TH12 to the transient limit MAX12 can be configured in one of three ways
a) Depending on the over current with -dUdt ENABLE
Flag programming F418 = any setting desired F41A = 0000
This variant enables the transient value MAX12 whenever a collapse of voltage
in the network is detected The ENABLE time is fixed and can be set The example
below shows the typical behavior of the limiter configured in this way
b) Dependent on the time integral with -dUdt ENABLE
Flag programming F418 = inactive F41A = 1111
This variant likewise enables the transient value only when a collapse of network
voltage has been detected However the switch-back to the thermal limit is not made
dependent upon the time itself but on the calculated time integral intisup2dt of the The setting
on Parameter F414 in spu takes into account the time the rotor needs to cool down ie
the rate of temperature change in the case of intermittent operation The example below
shows how the timing of the switch-back to the thermal limit depends on the present
value for intisup2dt
The time integral is based on the formula
Example The setting of =isup2dt equivalent to Version a (F416) at a constant 16 times the
nominal field current for 10 seconds (with TH12 = 105) is
c) Dependent on the time integral without any preconditions
Flag programming F418 = 1111 F41A = 1111
In this variant the transient becomes available without any prior conditions
(without a -dUdt ENABLE) with the time integral intisup2dt
237 Inductive Stator Current Limiter
The Inductive Stator Current Limiter holds the stator current Ig within permissible
limits while the generator is in the ldquoover-excitedrdquo operating range by reducing the field
current accordingly The setting TH (thermal limit) provides the limit against stationary s
that might occur To take advantage of the generatorrsquos transient overload capacity a
switch-over is made to the higher setting MAX The principle of operation of this switch-
over to the value MAX permissible only transiently is identical to that employed for the
field current limiter (refer to the description above) When the drive output from the
turbine is very high stator current may exceed permissible limits even while inductive
loading of the generator is low In this case if the stator current limiter is not kept from
influencing the field current the control circuit will oscillate back and forth between the
Inductive Stator Current Limiter (de-
exciting) and the Capacitive Stator Current Limiter (exciting)The output signal of that
function then dominates the control variable of the Ig-dependent limiter via a maximum
value selection
238 Capacitive Stator Current Limiter
239 Load Angle Limiter
The Load Angle Limiter prevents the synchronous machine from slipping out of
phase due to slippage of the rotor The load angle δ the difference in phase between the
rotor and the stator rotating field results mainly from the driving torque (active power P)
acting on the generator and the level of rotor current (field current) If the driving torque
remains constant a increase in the field current reduces the load angle δ The current load
angle δ at any moment is obtained from the generator current and generator voltage based
on a simplified model of the generator Whenever this calculated load angle δ exceeds the
preset limit angle the limiter increases the field current until the load angle has dropped
back to its permissible value The quadrature reactance Xq of the generator and the
network reactance Xe during normal operation must be adjusted on the regulator in order
to obtain the load angle δ The graph below shows the Power Chart for a salient-pole
machine with typical limiter characteristics
The purpose of a Power System Stabilizer is to use the generator excitation to
damp electromechanical oscillations between the network and the generator Depending
on the design of the generator and the requirements imposed for network stability its
main function will be either to damp the oscillations originating in the machine or those
from the network A synchronous generator working in a combined power network is in
principle an oscillating structure In order to produce a torque the magnetic field of the
rotor and the stator must form a given angle (referred to as the rotor displacement or load
angle δ) The electrical torque ME increases as the angle δ increases just as with a
torsion spring Because the ME of the generator and the mechanical driving torque MA
from the turbine are in equilibrium during steady-state operation the angle δ remains in a
given position Whenever this state of equilibrium between MA and ME is disturbed the
load angle slips of this rest position and change thereby the electrical torque ME The
torque attempts to restore the load angle to a stationary position Due to the mass inertia
of the turbinegenerator rotor however this can only take place aperiodically It does so
in the form of more or less effectively damped oscillations (again similar to the effect of
mass inertia on a torsion spring) In order to damp the oscillations there must be a
damping torque produced depending not on the electrical torque ME associated with the
angle but on the difference in frequency (Df) between the rotor and the stator rotating
field ie on the slippage This torque is produced mainly by the so-called damper
winding in the rotor but the dimensioning of this is subject to limits imposed by
considerations of design and economy Some further action is therefore needed to
increase the damping effect The following drastically simplified formula shows the
parameters upon which the amount of active power PE supplied by the generator
depends
PE = active power
It can be seen from the above relationship that the active power that the generator
transfers depends not only on the load angle δ but also on the field current If That means
that a transient change can be made in the active power PE and with that in the effective
electrical torque ME by varying the field current The principle of operation of the DVR
Power System Stabilizer becomes clear from a consideration of the oscillations in power
output and frequency (ΔPE Δf) and the vector diagram If it is assumed that oscillations
in the network frequency generates load oscillations with the mass inertia of the rotor
then the active load of the generator (eg MW-measured) is influenced with a sinusoidal
value -ΔPE (ME-MA = -ΔPE) By inversion of -ΔPE one obtains the fluctuation in
power provided by the rotor +ΔPE As is known the slip signal Δf follows +ΔPE with a
phase delayed by 90deg The +ME produced by the periodic changes in the load angle δ is
in phase with +ΔPE A good damping is attained if ME is varied in phase with the slip
Δf However this signal must also be advanced somewhat to compensate for the time
constants in the excitation circuit and the generator
As mentioned above the electrical torque ME can be influenced by varying the
field current To accomplish this a suitable control signal referred to as variable
disturbance compensation must be imposed upon the voltage set-point or the converter
control variable Ucontr As can be seen from the vector diagram by applying proper
weighting factors (K1 K2) and then adding together the signals -ΔPE and Δf an overall
stabilization signal can be produced that rotates in advance of the Df signal by any angle
desired between 0deg and 90deg Because the amplitude of -DPE remains proportional to the
amplitude of Δf a constant angle in advance of Δf results for the compensation of the
time constants referred to above The optimum weighting factors K1 and K2 for a
synchronous generator working to a power network depend on its operating point at any
moment and the external reactance of the network Normally the selection of a
compromise setting is good enough to attain stability in all operating points and for all
external reactance For special demands these settings must be parameterized as a
function of the external reactance (which means optional equipment Xe-Identification)
The Power System Stabilizer PSS is a section of the AVR computer program and is
processed once per network cycle The voltage at the generator terminals and the
generator current are measured in order to define the signals ΔPE and Δf The calculated
signals for _P_ (=PE) and Δf are then sent across DC filters ldquoDrdquo (real differentiators) that
transmit only the dynamic portion of the signals The ΔPE and Δf signals obtained in this
way are then weighted (multiplied by) with the factors K1 and K2 and sent to the
summing point of the voltage regulator
The PSS stabilization signal is imposed on the automatic voltage regulator only if
the following prerequisites are met
bull Generator on line
bull Generator power output gt the value F338
bull Generator voltage in a range between F33C and F33A
The stabilization signal is limited at the output from the PSS to the lower and
upper limits Flag defines whether the stabilization signal is introduced before or after the
PID filter (usually before the filter) Because the PID filter as noted above already takes
the ceiling factor Vp1 into account the PSS signal needs to be multiplied by Vp1 if it is
added to the voltage regulator following the PID filter (divider at the input to the
minmax limiter) This precaution prevents the DC filter ldquoDrdquo in the P-channel from
producing an unnecessary ldquostabilizationrdquo effect in the case of rapid changes in turbine
load As an alternative for the AVRrsquos Power System Stabilizer a stabilization signal from
an outside system can be imposed by activating the binary input ldquoPSS-SIGNEXTrdquo Flag
F340 can be used to select between an analog and a 12-bit signal and F33E to select the
polarity desired for that signal
24 The MANUAL Channel
241 Summary
The MANUAL channel (Channel 2) has been built as a simple field current
regulator
without additional limiters Its main function is to maintain the excitation of the generator
even if the AUTOMATIC channel becomes non-operational The MANUAL channel
also performs valuable service for purposes of testing commissioning and preventive
maintenance Its measurements regulator generation of firing pulses and power supply
are physically separate from those on the AUTOMATIC channel
242 Principle of Operation
All the functions of the MANUAL channel including the generation of firing
pulses have been implemented in a single electronic module the Gate Control Unit The
control variable Ucontr of voltage regulator is used as the reference value for generating
firing pulses on the principle known as ldquoramp controlrdquo (Comparison of Ucontr with
Usynsynchronous sawtooth signal) For further processing in the UN 0096 Intermediate
Pulse Stage the Gate Control Unit supplies six firing pulses at its output whose phase
position with respect to the synchronous voltage Usyn is in accordance with control
variable Ucontr An internal linearization ensures that the field voltage produced via the
firing pulses remains proportional to the control variable Ucontr throughout the entire
range As a result the circuit amplification of the control remains constant over the entire
range Whenever excitation is switched ON the set-point for Generator Voltage is set
automatically at the preset - ref Value This provision ensures that the generator voltage
always attains approximately its nominal value after the field flashing The Gate Control
Unit can be refunctioned ( by pre-selection with a switch ) for purposes of testing to act
as a purely firing pulse control In this case the control variable Ucontr is adjusted
directly using the RAISELOWER push buttons on the front of the module In this way
for example the relationship between the phase position of the firing pulses and the
control variable Ucontr can be checked easily
CHAPTER 3
PULSE SECTION
31 Pulse Generation and Amplification
The Gate Control Units of both AUTOMATIC channel and MANUAL channel
each supply six firing pulses for operating the 6-pulse thyristor bridges The low-power
pulse signals from these Gate Control Units are then amplified in the Intermediate Pulse
Stage galvanically isolated and then sent to the common pulse bus On the output end
the Intermediate Pulse Stage of the non-active channel) is always blocked The Gate
Control Units generate the pulses based on microprocessor control The reference voltage
used for the firing pulse phase location is the output voltage from the excitation
transformer (Usyn1 Usyn2) The commutation spikes of the synchronous voltage caused
by the converter are calculated prior to use of the voltage as a reference value and are
deliberately filtered out The lower limit for the firing pulses (double pulses) which are
offset from one another by 60deg is defined by the limit rectifier position (αmin) and the
upper limit by the limit inverter position (αmax) for the firing angle αmin and αmax can
be adjusted on the Gate Control Units using BCD (Binary Coded Decimal) switches
αmin ensures that the firing pulses will not be issued (premature firing) until there is
sufficient positive phase voltage on the thyristor involved αmax prevents a dangerous
ldquotippingrdquo of the thyristor bridge into the rectifier mode if the firing angle α is too large
(ldquolate firingrdquo) The critical factors determining αmax are the overlap time uumlmax (max
commutation time) and the ldquorecovery timerdquo of the thyristors (αmax lt 180deg - uumlmax - γ )
An external control signal can force the firing pulses into their inverter limit position
Other binary inputs can block or direct the firing pulses of the Gate Control Units so as to
produce freewheeling on the thyristor bridge During freewheeling the firing pulses for
the thyristor pair R and S are blocked and the pulse signals T+T- are engaged with
chains of pulses Both Gate Control Units (for the MANUAL amp AUTOMATIC channels)
contain a field current monitor that blocks the firing pulses immediately whenever the
current exceeds a preset threshold level In this case the field circuit-breaker is also
tripped via an output contact The purpose of these provisions is to prevent damage to
thyristors and thyristor fuses in case of a slip-ring short-circuit or to keep any damage
that does occur to a minimum The pulse signals are galvanically separated at the outputs
from the Intermediate Pulse Stage (with pulse transmitters) and are then directed to the
common pulse bus This transmission of the pulse signals to the pulse bus via passive
transmitters ensures a high degree of active channel autonomy Practically no possible
malfunctions on the inactive channel (including for example sustained pulses) affect the
active channel
32 Pulse Monitoring
The ldquoPulse Busrdquo and the pulse signals of the AUTOMATIC channel are
monitored This monitoring device consists of potential isolating stages and the common
monitor If the pulse monitoring of the ldquoPulse Busrdquo responds a switch-over is made to
MANUAL channel The function of the potential isolating stages is to couple the pulse
monitoring device to the pulse circuits without any feedback effect The pulse monitoring
checks the six pulse lines for the following malfunctions continuous or periodic failure
of one or more pulses Periodic occurrence of synchronous or asynchronous false pulses
Continuous pulses the pulse monitoring device can be tested while the machine is in
operation
CHAPTER 4
CONVERTER
Thyristor
The term thyristor usually refers to a family of four layer solid state device having
turn on characteristics that can be externally controlled by either current or voltage They
are also referred to as breakdown device because their working depends on avalanche
breakdown Thyristors have only two stages OFF and ON Thyristors have a similar
function to Uni-junctions they act as switches Thyristors use current flow as a switch
Thyristors have three states
1 Reverse blocking mode mdash Voltage is applied in the direction that would be
blocked by a diode
2 Forward blocking mode mdash Voltage is applied in the direction that would cause
a diode to conduct but the thyristor has not yet been triggered into conduction
3 Forward conducting mode mdash The thyristor has been triggered into conduction
and will remain conducting until the forward current drops below a threshold value
known as the holding current Converter is a semiconductor device which converts ac
input voltage into a constant dc output voltage In present excitation system three phase
fully controlled thyristor converter is used
Because of the following advantages thyristor converters are used
a) Thyristors are used for high power applications ie up to 10Kv3500A1KHz
b) Having high reliability and low losses
c) Uni-directional device like diode
d) Itrsquos operation as a rectifier which are low resistance in forward conduction
mode and high resistance in reverse conduction mode
PROTECTION OF THYRISTORS
For reliable operation of a thyristor demands that its specified ratings are not
exceeded When Subjected to or over voltages During the turn - on of SCR didt
prohibitively large False triggering of SCR by high value of dvdt andSpurious signals
between gate and cathode may leads to unwanted turn ndash on
DIDT AND PROTECTION
When thyristor starts conducting in forward conduction mode and is turned on by
gate pulse The anode current increases rapidly whole area of the gate to Cathode
junction then hot spots will be formed near the gate connection this locality of heating
destroys the thyristor Thyristor thermal time is constant The causes due to faults and
short circuits or surge currents Electronic crowbar protection is used against the over
voltages The rate rise of anode current must be kept at the time of turn on below the
rated or specified limiting value The didt value maintained below limited value by using
a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating
is avoided by applying gate current but not greater the maximum gate current
DVDT AND OVER VOLTAGE PROTECTION
With forward voltage across the anode and cathode of a thyristor the two outer
junctions are forward biased but the inner junction is reverse biased This reverse biased
junction J2 has the characteristics of a capacitor due to charges existing across the
junction In other words space-charges exist in the depletion region around junction J2
and therefore junction J2 behaves like a capacitance If the entire anode to cathode
forward voltage Va appears across J2 junction and the charge is denoted by Q then a
charging current i given by Eq (46) follows
i = dQdt =d(Cj Va )dt
= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)
As Cj the capacitance of junction J2 is almost constant the current is given by
i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)
If the rate of rise of forward voltage dVadt is high the charging current i will be
more This charging current plays the role of gate current and turns on the SCR even
when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on
must be avoided as it leads to false operation of the thyristor circuit
For controllable operation of the thyristor the rate of rise of forward anode to
cathode voltage dVadt must be kept below the specified rated limit Typical values of
dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by
using a snubber circuit in parallel with the device thyristor are very sensitive for over
voltage than the semiconductor devices
Over voltage transients are perhaps the main cause of thyristor failure
In thyristor there are mainly two types
1 Internal over voltages
Due to the commutation of the thyristors large voltages are generated internally
Because of the series inductance of the SCR circuit the large transient voltages L didt
produced This voltage several times the break over voltage of the device then thyristor
destroys permanently
2 External over voltages
External over voltages are caused due to the interruptions of current flow in an
inductive circuit and also due to the lightening strokes on the lines feeding the thyristor
system For the reliable operation of thyristor the over voltages must be suppressed by
adopting suitable techniques
Suppression of over voltages
The RC circuit called snubber circuit is connected across the device to protect In
order to keep the protective components to a minimum the thyristors are chosen with
their peak voltages ratings are 25 to 3 times of the normal peak working voltage
ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are
commonly employed for protecting the thyristor circuit against the over voltages
Gate protection
Gate circuit should also be protected against the over voltages and surges Over
voltage at gate circuit can cause false triggering of the SCR may rises the junction
temperature behind specified limit leading to its damage Protection against over
voltage can be achieved by connecting a ZD across the gate circuit and a resister is
connected in series with gate circuit to protect against the s A capacitor and resister are
connected across gate to cathode to by pass the noise
41 Final Pulse Stages
The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage
(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor
bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided
with a power supply module The amplified output pulses from the Final Pulse Stages
start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main
pulse This initial pulse edge assures proper firing of the thyristors being triggered
Subsequently the weaker part of main pulse keeps firing conditions steady As already
mentioned the Final Pulse Stages and their associated thyristor bridges form single units
All six pulse outputs from a Final Pulse Stage can be blocked by an external control
signal so that all thyristors in the associated thyristor bridge will block the current A
blocking of the pulses is initiated whenever there is a malfunction in the associated
thyristor bridge
42 Converter Power Section
The thyristor converter consists of three independent parallel rectifier blocks TY1
to TY3 which are all in service Even if one block fails the remaining blocks take over
automatically the full design current of the excitation circuit During normal operation
(with ideal current share) and all three bridges in operation each of these blocks has to
carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation
is limited Only when all three bridges fail the excitation is switched off Each thyristor
bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any
arm is identified by a Current flow monitoring module
43 Converter Cooling
A cooling system is needed to dissipate heat losses in the converter blocks and
electronics Each converter block has therefore been equipped with a fan supplied with
power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing
cubicle) The fans are protected with motor protection circuit breakers An air flow
monitoring unit is provided for monitoring the air flow through the thyristor bridge If a
circuit breaker failure is detected or if the air flow monitor drops off at one of the
thyristor bridges the bridge involved is immediately set out of operation by blocking its
firing pulses
44 Thyristor Converter Monitoring
A thyristor bridge in which defects occur that could threaten the safety of
operation or cause secondary damage is switched off automatically ie its firing pulses
are blocked This happens whenever A thyristor fuse is blown The fuses are monitored
individually with micro switches The Final Pulse Stage fails which is detected by
internal monitors (supply voltage sustained pulse short-circuit on the output end) The
power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or
is insufficient Isolator on ACDC side is open
CHAPTER 5
Field Current Circuit Excitation Transformer
51 Field Circuit Breaker
The circuit-breaker in the field circuit is used to isolate the field circuit from the
converter It is capable of switching off the synchronous machine from full load under the
maximum conditions of a 3-phase short-circuit In addition to its main contacts the field
circuit-breaker also has a de-excitation contact with which the field energy stored in the
field can be dissipated across the de-excitation resistor The de-excitation contact closes
shortly before the main contacts open so as to ensure proper commutation of the field
current from the main contacts to the de-excitation contact when the breaker is switched
off The field circuit-breaker is switched on by electromagnetic force and is kept switched
on by a mechanical latch When the latch is released by a trip coil the circuit-breaker
opens The circuit-breaker also has auxiliary contacts that report its status
52 Field Flashing
In shunt supplied excitation circuits (excitation transformer connected to the
generator terminals) the generator does not have enough remnant voltage for a generator
voltage build-up via the converter In this case a field flashing circuit is provided It
consists of the field flashing contactor the diode bridge and a transformer used to adapt
the auxiliary input voltage to the voltage needed for field flashing when power is
supplied from the auxiliaries network
Fig Field Flashing
Because the field flashing contactor is not able to switch off the energy stored in
the field the control ensures that the contactor can only reopen if the field circuit breaker
has already been opened (generating the TRIP order) or in a normal field flashing
sequence when the converter has taken over the field current Field flashing occurs in the
following stages
1048729The excitation is switched on closing the field flashing contactor ( Field
Circuit Breaker is already closed )
1048729The start-up excitation current flows through the rotor driving the generator
voltage up to approx 15 U
1048729After about 10 U the firing pulses to the converter are released and it begins
to excite the generator to its rated voltage
1048729After about 30 U the field flashing contactor opens (with no current since
the converter is now supplying the current)
The diode bridge at the input to the field flashing contactor prevents a feed-back
from the converter to the source of field flashing while the contactor is still closed
53 De-excitation
When malfunctions occur the stored field energy must be dissipated as quickly
and safely as possible to protect the generator This is done by the converter the field
circuit-breaker and the de-excitation (discharge) resistor
De-excitation (with opening of the field circuit-breaker) takes place in the following
stages
1048729The converter drives to its inverter limit position (negative ceiling voltage)
recovers a portion of the field energy into the network A trip command is given to the
field circuit breaker
1048729The de-excitation contact closes diverting the field voltage to the de-excitation
resistor
1048729Then immediately the main contacts open building voltage The field voltage
commutates to the de-excitation resistor
1048729The current diminishes at a given time constant TE
(With linear resistance TE = Lf (Rf + Re))
Due to the reversal of the field voltage by the converter the field current
commutates from the main contacts of the field circuit-breaker to the de-excitation
resistor in a very early phase This reversal of the field voltage prevents burn-off on the
main contacts and provides effective protection for the field circuit-breaker Depending
on the operating policy an operational shut-down of the excitation can also be effected
with the field circuit-breaker closed This method is useful mainly when the excitation is
switched on and off frequently In this case the converter is merely driven into the
inverter limit position so that the field energy is recovered into the network The
converter then blocks since it is supplying positive current only
54 Excitation Transformer
The excitation transformer matches the generator voltage to the field voltage
(required ceiling voltage) It also serves as a commutation reactance for the thyristor
converter and as a potential isolator between the network and the excitation circuit In
addition the transformer functions as a current limiter in that it makes it possible to keep
any short circuits in the excitation circuit under better control The excitation transformer
is equipped with temperature monitoring probes which set off an alarm when the
temperature exceeds a first max limit and then trips the excitation if the temperature
continues rising to a second (higher)limit
CHAPTER 6
Monitoring and Protection
61 Excitation Monitoring
611 General Information
The main goal of Excitation Monitoring is to make optimum use of the
redundancies provided in the excitation system and to give alarm whenever a malfunction
makes these redundancies unavailable The field current is monitored to see that it does
not exceed a maximum level and if necessary a switch-over to the MANUAL channel is
initiated In addition the criterion for switching off the field flashing is generated The
excitation Monitoring consists of an autonomous processor system
612 over current Alarms
In the Excitation Monitoring the limits for are set at higher levels than the
settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of
the nominal field current contact R1 and the binary output associated with it are
activated immediately If field current remains gt 110 then after a preset inverse-time
has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs
associated with them are activated Parameters match the measurements for If1 and If2 to
the nominal value for field current so that the internal values can be processed and read as
pu values It can be used to falsify the actual value of the field current If (to raise it) so
as to cause a response from the alarm limits for purposes of testing The processed If
signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As
long as the field current If is above the threshold value 11 Ifn its peak value is
measured This is stored (until RESET) and can be read at any time on the Micro-
Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever
the integrated time-current value (intisup2dt) exceeds the preselected reference value the
alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously
issued to switch over to the stand by AUTO channel Software switch F758 enables the
three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-
time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the
desired limit curve for response is set using the factor F216
613 Switch-Off Criterion for Field Flashing
The Excitation Monitoring supplies the criterion for switching off the field
flashing Whether this criterion is activated based on the actual value for generator voltage
Ug or for field current If or both depends on the settings of the two threshold values
F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements
Ug12 and If12 are switched over depending on the present status of the channels
(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is
fixed at ldquological 1rdquo
614 Storage of Alarm Status
The outputs of the over current alarms (R1 R2 R0) and the messages NO
FAILURE MONITORING PARAMETERS CHANGED are stored messages can be
erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the
front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the
situation causing the alarm or the malfunction is no longer present Whenever the self-
diagnosis equipment in the processor detects a malfunction the output NO FAILURE of
MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo
is activated whenever parameters or settings of software switches have been changed via
the Micro-Terminal
615 Actual Value Monitoring
The actual values for generator voltage Ug synchronous voltage Usyn and field
current If are monitored for malfunctions This monitoring is active regardless of whether
or not the generator is in operation Essentially when the generator is in operation the
measurements are monitored by comparing the signals (the smaller signal reading is
detected as incorrect) When the generator is not in operation the measured data are
monitored for extreme values The percentage of deviation permissible in the
measurement signals being compared
(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and
F20AIf the excitation transformer is being supplied from an auxiliary power source (no
shunt operation) the values of Ug and Usyn will be different in some operational
conditions
In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1
and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the
messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2
and the binary message from CH1 reports no malfunction a malfunction on
Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is
also present whenever the binary message CH1 DISTURBANCE is reported and a
discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2
FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares
Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the
comparison channel reports a malfunction or whenever both binary messages report no
malfunction - but both secondary monitors report a malfunction As long as the secondary
monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction
signals for the measurement channel involved (suspicion that there is a corresponding
error in Usyn) The measurement channel malfunctions are enabled operationally
whenever after excitation has been switched on generator voltage Ug exceeds the value
set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is
switched off to see that they do not exceed the limit value F210 that applies to both of
them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt
the operating range) Monitoring for extreme values is likewise enabled during normal
operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the
malfunction signals to the binary outputs Basically the monitoring of the actual values
for If1If2 functions like that of the Ug1Ug2 monitoring
62 Excitation Protection
621 General Information
The Excitation Protection switches off the excitation (and de-excites the machine
rapidly) whenever a danger arises that threatens the excitation transformer the converter
or the generator Generally limiter or monitoring functions precede the emergency trips
and these normally respond before the Excitation Protection must initiate a trip
Protective trip commands are issued directly to the field circuit-breaker from potential
free contacts of the board via the trip relays They are directed redundantly to the
operative field circuit-breaker ldquoOFFrdquo command
622 Protection against Excitation Transformer Overheating
This equipment monitors the excitation transformer for overheating in the
windings that could result from over current short-circuits or inadequate cooling The
monitoring uses temperature monitoring modules in conjunction with temperature
sensors built into the windings Normally the temperature is monitored in two stages the
first stage sets off an alarm the second causes a trip of the excitation
623 Rotor Over voltage Protection
Malfunctions in the generator circuit (eg terminal short-circuit failed
synchronization asynchronous operation) cause induced negative field currents that
produce high voltages in the field circuit These must be restricted to a level with a
sufficient safety margin below the insulation capacity of the field winding (test voltage)
and also below the peak blocking voltage of the converter thyristors The crow bar
employs spark gap elements to detect over voltages in the field circuit Whenever they
respond the associated thyristors are fired immediately switching the de-excitation
resistor parallel to the field The de-excitation current generated thereby initiates an
excitation trip via a supervision circuit causing an immediate opening of the field circuit-
breaker The malfunction isets off an alarm and an internal malfunction is indicated at the
cubicle
TEST VALUES OBTAINED WHEN EXCITATION IS RAISED
TEST
SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE
1 VREF 996 100
2 VACT 997 1003
3 IFACT 735 765
4 IGACT 703 707
5 ACTIVE
POWER
703 705
6 REACTIVE
POWER
101 142
7 POWER
FACTOR
099 IND 098 IND
8 ACTIVE
CURRENT(IR)
705 703
9 REACTIVE
CURRENT(IX)
102 138
10 POWER
ANGLE
566 546
11 FIRING
ANGLE
640 632
VALUES OBTAINED WHEN EXCITATION IS LOWERED
SNO PARTICULARS ACTUAL
VALUE
OBTAINED VALUE
1 VREF 100 997
2 VACT 100 997
3 IFACT 787 761
4 IGACT 839 837
5 ACTIVE
POWER
839 830
6 REACTIVE
POWER
155 110
7 POWER
FACTOR
098 IND 099 IND
8 ACTIVE
CURRENT(IR)
835 830
9 REACTIVE
CURRENT(IX)
140 94
10 POWER
ANGLE
613 DEG 631 DEG
11 FIRING
ANGLE
601 DEG 599 DEG
CONCLUSION
For generating the EMF in stator winding excitation is required to the rotor of a
generator There are two types of excitation
1 Static excitation system
2 Brushless excitation system
A certain disadvantage in brushless excitation system is the slow response time of
the field in case of fast load changes specified No slip-rings and brushes direct
measurements of the field parameters not possible
To avoid all loses static excitation is used Since it does not have any rotating
parts mechanical loses and windage loses This system has fast response and speed
control While preferring this excitation system there are no limitations for the
redundancy of Thyristor bridge circuits
Static excitation has fast field discharge by resistor and inverter operation direct
measurement of field quantity is possible The meaning of excitation is nothing but
continuous supply of DC current (ie field current) to the rotor to buildup required
output voltage in the stator
Field current is changed with respect to the change of load so the digital
automatic voltage regulator (DAVR) is used to regulate the output voltage according to
the load variations
So we conclude that static excitation system with DAVR is preferred since it is
having excellent dynamic performance and better options for R amp M
them to issue the pulses at a given firing angle relative to the input voltage of the
converter
5
CHAPTER 2
Digital Automatic Voltage Regulator (DAVR)
21 Principle of Operation of the Regulator (DAVR)
To regulate the voltage and the reactive power of a synchronous machine the
field voltage must be adjusted quickly to the changes in the operating conditions (with a
response time that does not exceed a few ms) To accomplish this analog control systems
include amplifiers which make continuous comparison of the actual values against the
reference values and vary the control variable to the converter with almost no delay Most
of the delay that occurs originates in the converter since the firing pulses for changing
the rectifier phase angle are only issued periodically (every 33 ms)
The DVR digital voltage regulator calculates the control variable from the
measured and reference data in very short time intervals This results outwardly in a
quasi-continuous behavior with a negligible delay time (as in an analog regulator) The
calculations are made in the binary number system Analog measurement signals such as
those for generator voltage and generator current are converted into binary signals in
analogdigital converters The set-points and limit values have already been defined in
digital (binary) form An understanding of the actual computation processes in the digital
voltage regulator is not necessary for operation preventive maintenance or
troubleshooting Like the operator of a pocket calculator or a personal computer all the
operator needs is to know how to operate the instrument and the programming for this
working tool For that reason we will explain below only the principle division of work
among the various modules and the flow of data processing The purpose is above all to
make clear how the processor system has been integrated into the rest of the power
electronics system
6
22 Basic Structure of the Processor Systems
7
The signal processors 25 analog inputoutput modules Each of these processor
systems has a common bus circuit and output and the control lines There is a specific
range of addresses assigned to each assignment Board including the power supply bus
the address lines the two data lines to the input calculates the reactive current (I sin φ)
and the active current ( Icos φ) With these two channel processor Synchronized with
these interrupts (ie with the phase positions of current Ig the field current If and the
synchronous voltage Usyn From the exchange data with the microprocessor card across
the two data lines generator voltage Ug) this processor measures the generator current
Ig and then hardwired connections or multi-conductor cables Binary and analog
inputoutput modules ie for galvanic isolation and adaptation to the electronics level
The most important input interrupts per period to trigger the cycles for processing actual
values in the AUTOMATIC module on the processor bus) for filtering and further
processing
Monitoring each consist of the central microprocessor module and binary and
parameters to the AUTOMATIC channel are the generator voltage Ug the generator
peripheral unit Ug Ig and Usyn are sent to the Interrupt Generator (plug-in peripheral
units (wall-mounted units) peripheral units are used for preprocessing signals from
external measurement circuits power supply units Signals are exchanged among these
processor systems via processed across separate peripheral units for each channel These
processor working on the bus (a house address that can be adjusted using a switch)
systems The AUTOMATIC channel the programmable controls and the Excitation The
actual values measured from AUTOMATIC channel and MANUAL channel are The
AUTOMATIC channel and the MANUAL channel each have their own The digital
voltage regulator is broken down into several autonomous microprocessor The inputs and
outputs of the processor systems are directed across voltage-isolating The Interrupt
Generator also uses the 3-phase Ug signal to generate the 12 themselves contain a limited
number of hardware inputs and outputs with fixed equipment Whenever addresses from
this range are called up the signal processing module can results the processor is then
able to derive further operating parameters such as the load angle the active power etc
The functions of all microprocessor systems other than the programmable controls
have been accomplished in firmware The non-varying standard function modules can be
configured to the design desired for plant-specific purposes using software switches
(KFlags) Thus for example the stored status of a K-Flag determines whether or not a
Limiter is active and whether the de-excitation or the excitation limiters take precedence
Because these K-flags determine the software Scope of Supply for the installation they
cannot be changed permanently via the Micro-Terminal In this way they differ from
such setting data as the values of the parameters for the PID filter of the voltage regulator
or the set-points for the limiters These values can be permanently changed using the
Micro-Terminal Communication is possible with each of the processor systems via the
Micro-Terminal by plugging on the connecting cable In this way signals within the
processor and setting parameters can be viewed analog signals can be issued and the set
parameters can be altered temporarily (F range) or permanently (C range) Unlike the
other processor systems the programmable controls do not include any firmware for
realization of the functions They have been designed so that the designer can adapt and
change their functions easily using the ldquoFunctional Block Programming Language P10
Digital and analog functions can be implemented in practically any degree of complexity
desired using the P10 functional blocks The control variable of the voltage regulator
(AUTOMATIC channel) and the control variable of the field current regulator
(MANUAL channel) are each processed in separate Gate Control Unit and formed into a
chain of pulses at the appropriate firing angle The pulses of the active channel are
directed to the pulse bus via the associated Intermediate Pulse Stage The pulses for each
converter block are amplified sufficiently in Final Pulse Stage to fire the Thyristor
231 General Information
The functions of the automatic voltage regulator AVR are
1 to regulate the generator voltage
2 to regulate the effect of the reactive andor active current on the voltage
3 to limit VoltHz
4 to limit max and min field current
5 to limit inductive stator current
6 to limit capacitive stator current
7 to limit the load angle
8 to stabilize the power system
Block Diagram shows the software structure of AUTOMATIC channel The
generator limiters not provided for the installation in question (optional equipment) are
identified in this overview as ldquoNot Suppliedrdquo The parameter values signal values and
software switches (flags) marked with addresses (hexadecimal numbers) can be viewed
and altered via the Micro-Terminal The values selected are displayed in sec pu Hz
etc and can where necessary be changed directly in these formats The plant-specific
settings of the variables and the flags can be obtained from the Test and Commissioning
Report This block diagram provides information about the important functions and
possible settings of the AUTOMATIC channel For the sake of clarity no detailed
presentation has been given of special functions such as tracking circuits initializations
etc The page heading cross-refers this overview to the various sheets of the schematic
diagram Binary signals are shown in broken lines analog signals in solid lines The
corresponding text designations in the schematic diagram can be used for identification of
the input signals (hardware inputs) The only analog output signal from the automatic
voltage regulator control variable Ucontr is sent via the data bus (CRU bus) to the Gate
Control Unit Most of the binary messages (outputs) from the AVR are of no interest
functionally and they have been omitted for the sake of clarity The basic structure of the
digital voltage regulator and the limiters is simple This is necessary in order that the
behavior of the regulatorslimiters will remain calculable and understandable in all
operating situations and that there will be no problem in adjusting and optimizing them
The central PID filter in the digital voltage regulator defines the dynamic response of the
closed-loop controls both in the voltage regulator mode and after limiters have
intervened The ldquocontrol deviationrdquo at the input to the PID filter is either the control
deviation for voltage the control deviation of a de-excitation limiter (the value
determined by minimum value selection) or the control deviation of an excitation limiter
(the value determined by maximum value selection) Flag F730 (ldquoPRIORrdquo) is used to
determine whether the exciting (Min value) or the de-exciting signal takes precedence on
the minmax value limiter (normally F730 = 1111 ie the de-exciting signal takes
precedence) With the exception of the Minimum Field Current Limiter all other limiters
have variable factoring multipliers of the signal outputs so that they can be adjusted
individually together with the common PID filter which has been optimized for voltage
regulation The setting parameters for this PID filter are as follows
Vo = KR Static amplification
1
Ta = ---- Integration time constant
Tc1
Vp Proportional amplification
1
Tb = ---- Differential time constant
Tc2
Vinfin Amplification of high frequencies
The BODE diagram below shows the assignment of settings in accordance with
DINIEC standards based on a typical example
The PID filter amplifications Vo Vp and Vinfin can be adjusted in pu values But
the ldquoceiling factorrdquo pl+ must be adjusted correctly with parameter F310 if the total
amplification (circuit amplification) of the control circuit is actually to conform to the
pu settings This factor must agree with the ldquoexternalrdquo amplification ie with the
ceiling value of the transformer- converter circuit
Ceiling factor(pl+) = Ufmax Ufo
in which Ufmax = ceiling field voltage
Ufo = no-load field voltage
To attain a suitable response of the AVR when starting excitation
(ldquoEXCITATION ONrdquo) it may be necessary to change the proportional amplification of
the regulator during this phase Vp2 (transiently activated) and Vp1 (permanently
activated) can be adjusted for this purpose For example the value of Vp2 takes effect
immediately once the excitation is switched on and remains effective for a period as set at
F30C Once the period F30C (eg 5 sec) has expired Vp shifts over to Vp1 (becomes
the steady-state Vp) at the rate of change set The standard operating mode for the PID
filter is voltage regulation for which the discrepancy between the voltage set-point and
the current value for generator voltage Ug (the control deviation) is supplied at the input
To compensate for the voltage drop in the block transformer or whenever several
generators are operating to the same distributing bus the generator voltage must be
varied in proportion to the measured generator current (droop influence) To accomplish
this the voltage set-point is varied as a function of the measured reactive current IX
andor active current IR Flag F712 enables the IX droop Flag F710 the IR droop The
desired compensation is set in F282 and F286 respectively Flags F284 and F288 are used
to select whether this droop influence is to increase the voltage or to reduce it
(compensation) Combined influence of the active and reactive currents is attained by
enabling both droops IX and IR Flag F716 activates a so-called ldquoSoft-Startrdquo at the
starting of excitation This ldquoSoft-Startrdquo ensures that the voltage set-point integrates from
0 to 100 within the time set on F290 when the excitation is switched on
(ldquoEXCITATION ONrdquo) A ldquosmoothrdquo excitation of the generator can be achieved in this
way whenever there is no demand for a quick excitation
232 Voltage Set-Point
Various signals and settings control and limit the voltage set-point F270 For
example the values of F254 and F252 define the normal operating range possible for set-
point adjustment (eg 90 110) using external control commands (control room local
operatorrsquos panel superposed control system) The effective set-point adjustment rate is
governed The set-point can be set at the values of F250 and F256 by activating
appropriate control commands for ldquoSETrdquo input Enabling Flag F71A and activating a
binary input prior to switching on the excitation (ldquoEXCITATION OFFrdquo) sets the Ug set-
point at the value of UAUX This makes it possible for example to ensure that the
generator voltage will agree exactly with the network voltage after the voltage build-up
An external value with variable amplification can be added to the Ug set point by
enabling F724 (for example for stability tests)
233 Regulator Tracking in MANUAL Operation
Whenever the AUTOMATIC channel is not in operation (the MANUAL channel
is ON) a follow-up equipment ensures a smooth switch-back to the AUTOMATIC
mode will always be possible To track the voltage set-point is shifted by means of
RAISELOWER pulses from the Gate Control Unit so that control variable Ucontr at the
output from the PID filter is held steady and identical to the control variable Ucontr from
the MANUAL channel Because this tracking must react slowly resultant transient
control deviations resulting from the amplification in the PID filter might cause severe
interference with control variable Ucontr
To prevent this the follow-up equipment intervenes on the regulators mixing
point with a corresponding compensation signal
234 Ugf Limiter
At under frequency the Ugf Limiter reduces the generator voltage so as to
prevent saturation effects in the supply and measuring transformers To adjust this
limiter the max permissible generator voltage at rated frequency is defined and set
When any under-frequency occurs the generator voltage is thus reduced in proportion to
that setting
235 Field Current Maximum Limiter
The Field Current Maximum Limiter is provided to protect the generator rotor
from s occurring in steady-state and transient operation High field currents are normally
the result of a sharp drop in network voltage or of an improper raising of the voltage set-
point by the operating staff The field current is held steady at the value TH1 ie at the
maximum thermal value permissible for the excitation circuit and the rotor In order that
the generator can support the power network with its transient overload capacity during
brief collapses in voltage a temporary switch-over is made to the transient limit MAX1
(a higher setting) When the generator or the converter is operating at a reduced capacity
These limits TH1MAX1 can be switched over to the lower settings TH2MAX2 by
activating the corresponding binary signals The switch-over from the thermal limit
TH12 to the transient limit MAX12 can be configured in one of three ways
a) Depending on the over current with -dUdt ENABLE
Flag programming F418 = any setting desired F41A = 0000
This variant enables the transient value MAX12 whenever a collapse of voltage
in the network is detected The ENABLE time is fixed and can be set The example
below shows the typical behavior of the limiter configured in this way
b) Dependent on the time integral with -dUdt ENABLE
Flag programming F418 = inactive F41A = 1111
This variant likewise enables the transient value only when a collapse of network
voltage has been detected However the switch-back to the thermal limit is not made
dependent upon the time itself but on the calculated time integral intisup2dt of the The setting
on Parameter F414 in spu takes into account the time the rotor needs to cool down ie
the rate of temperature change in the case of intermittent operation The example below
shows how the timing of the switch-back to the thermal limit depends on the present
value for intisup2dt
The time integral is based on the formula
Example The setting of =isup2dt equivalent to Version a (F416) at a constant 16 times the
nominal field current for 10 seconds (with TH12 = 105) is
c) Dependent on the time integral without any preconditions
Flag programming F418 = 1111 F41A = 1111
In this variant the transient becomes available without any prior conditions
(without a -dUdt ENABLE) with the time integral intisup2dt
237 Inductive Stator Current Limiter
The Inductive Stator Current Limiter holds the stator current Ig within permissible
limits while the generator is in the ldquoover-excitedrdquo operating range by reducing the field
current accordingly The setting TH (thermal limit) provides the limit against stationary s
that might occur To take advantage of the generatorrsquos transient overload capacity a
switch-over is made to the higher setting MAX The principle of operation of this switch-
over to the value MAX permissible only transiently is identical to that employed for the
field current limiter (refer to the description above) When the drive output from the
turbine is very high stator current may exceed permissible limits even while inductive
loading of the generator is low In this case if the stator current limiter is not kept from
influencing the field current the control circuit will oscillate back and forth between the
Inductive Stator Current Limiter (de-
exciting) and the Capacitive Stator Current Limiter (exciting)The output signal of that
function then dominates the control variable of the Ig-dependent limiter via a maximum
value selection
238 Capacitive Stator Current Limiter
239 Load Angle Limiter
The Load Angle Limiter prevents the synchronous machine from slipping out of
phase due to slippage of the rotor The load angle δ the difference in phase between the
rotor and the stator rotating field results mainly from the driving torque (active power P)
acting on the generator and the level of rotor current (field current) If the driving torque
remains constant a increase in the field current reduces the load angle δ The current load
angle δ at any moment is obtained from the generator current and generator voltage based
on a simplified model of the generator Whenever this calculated load angle δ exceeds the
preset limit angle the limiter increases the field current until the load angle has dropped
back to its permissible value The quadrature reactance Xq of the generator and the
network reactance Xe during normal operation must be adjusted on the regulator in order
to obtain the load angle δ The graph below shows the Power Chart for a salient-pole
machine with typical limiter characteristics
The purpose of a Power System Stabilizer is to use the generator excitation to
damp electromechanical oscillations between the network and the generator Depending
on the design of the generator and the requirements imposed for network stability its
main function will be either to damp the oscillations originating in the machine or those
from the network A synchronous generator working in a combined power network is in
principle an oscillating structure In order to produce a torque the magnetic field of the
rotor and the stator must form a given angle (referred to as the rotor displacement or load
angle δ) The electrical torque ME increases as the angle δ increases just as with a
torsion spring Because the ME of the generator and the mechanical driving torque MA
from the turbine are in equilibrium during steady-state operation the angle δ remains in a
given position Whenever this state of equilibrium between MA and ME is disturbed the
load angle slips of this rest position and change thereby the electrical torque ME The
torque attempts to restore the load angle to a stationary position Due to the mass inertia
of the turbinegenerator rotor however this can only take place aperiodically It does so
in the form of more or less effectively damped oscillations (again similar to the effect of
mass inertia on a torsion spring) In order to damp the oscillations there must be a
damping torque produced depending not on the electrical torque ME associated with the
angle but on the difference in frequency (Df) between the rotor and the stator rotating
field ie on the slippage This torque is produced mainly by the so-called damper
winding in the rotor but the dimensioning of this is subject to limits imposed by
considerations of design and economy Some further action is therefore needed to
increase the damping effect The following drastically simplified formula shows the
parameters upon which the amount of active power PE supplied by the generator
depends
PE = active power
It can be seen from the above relationship that the active power that the generator
transfers depends not only on the load angle δ but also on the field current If That means
that a transient change can be made in the active power PE and with that in the effective
electrical torque ME by varying the field current The principle of operation of the DVR
Power System Stabilizer becomes clear from a consideration of the oscillations in power
output and frequency (ΔPE Δf) and the vector diagram If it is assumed that oscillations
in the network frequency generates load oscillations with the mass inertia of the rotor
then the active load of the generator (eg MW-measured) is influenced with a sinusoidal
value -ΔPE (ME-MA = -ΔPE) By inversion of -ΔPE one obtains the fluctuation in
power provided by the rotor +ΔPE As is known the slip signal Δf follows +ΔPE with a
phase delayed by 90deg The +ME produced by the periodic changes in the load angle δ is
in phase with +ΔPE A good damping is attained if ME is varied in phase with the slip
Δf However this signal must also be advanced somewhat to compensate for the time
constants in the excitation circuit and the generator
As mentioned above the electrical torque ME can be influenced by varying the
field current To accomplish this a suitable control signal referred to as variable
disturbance compensation must be imposed upon the voltage set-point or the converter
control variable Ucontr As can be seen from the vector diagram by applying proper
weighting factors (K1 K2) and then adding together the signals -ΔPE and Δf an overall
stabilization signal can be produced that rotates in advance of the Df signal by any angle
desired between 0deg and 90deg Because the amplitude of -DPE remains proportional to the
amplitude of Δf a constant angle in advance of Δf results for the compensation of the
time constants referred to above The optimum weighting factors K1 and K2 for a
synchronous generator working to a power network depend on its operating point at any
moment and the external reactance of the network Normally the selection of a
compromise setting is good enough to attain stability in all operating points and for all
external reactance For special demands these settings must be parameterized as a
function of the external reactance (which means optional equipment Xe-Identification)
The Power System Stabilizer PSS is a section of the AVR computer program and is
processed once per network cycle The voltage at the generator terminals and the
generator current are measured in order to define the signals ΔPE and Δf The calculated
signals for _P_ (=PE) and Δf are then sent across DC filters ldquoDrdquo (real differentiators) that
transmit only the dynamic portion of the signals The ΔPE and Δf signals obtained in this
way are then weighted (multiplied by) with the factors K1 and K2 and sent to the
summing point of the voltage regulator
The PSS stabilization signal is imposed on the automatic voltage regulator only if
the following prerequisites are met
bull Generator on line
bull Generator power output gt the value F338
bull Generator voltage in a range between F33C and F33A
The stabilization signal is limited at the output from the PSS to the lower and
upper limits Flag defines whether the stabilization signal is introduced before or after the
PID filter (usually before the filter) Because the PID filter as noted above already takes
the ceiling factor Vp1 into account the PSS signal needs to be multiplied by Vp1 if it is
added to the voltage regulator following the PID filter (divider at the input to the
minmax limiter) This precaution prevents the DC filter ldquoDrdquo in the P-channel from
producing an unnecessary ldquostabilizationrdquo effect in the case of rapid changes in turbine
load As an alternative for the AVRrsquos Power System Stabilizer a stabilization signal from
an outside system can be imposed by activating the binary input ldquoPSS-SIGNEXTrdquo Flag
F340 can be used to select between an analog and a 12-bit signal and F33E to select the
polarity desired for that signal
24 The MANUAL Channel
241 Summary
The MANUAL channel (Channel 2) has been built as a simple field current
regulator
without additional limiters Its main function is to maintain the excitation of the generator
even if the AUTOMATIC channel becomes non-operational The MANUAL channel
also performs valuable service for purposes of testing commissioning and preventive
maintenance Its measurements regulator generation of firing pulses and power supply
are physically separate from those on the AUTOMATIC channel
242 Principle of Operation
All the functions of the MANUAL channel including the generation of firing
pulses have been implemented in a single electronic module the Gate Control Unit The
control variable Ucontr of voltage regulator is used as the reference value for generating
firing pulses on the principle known as ldquoramp controlrdquo (Comparison of Ucontr with
Usynsynchronous sawtooth signal) For further processing in the UN 0096 Intermediate
Pulse Stage the Gate Control Unit supplies six firing pulses at its output whose phase
position with respect to the synchronous voltage Usyn is in accordance with control
variable Ucontr An internal linearization ensures that the field voltage produced via the
firing pulses remains proportional to the control variable Ucontr throughout the entire
range As a result the circuit amplification of the control remains constant over the entire
range Whenever excitation is switched ON the set-point for Generator Voltage is set
automatically at the preset - ref Value This provision ensures that the generator voltage
always attains approximately its nominal value after the field flashing The Gate Control
Unit can be refunctioned ( by pre-selection with a switch ) for purposes of testing to act
as a purely firing pulse control In this case the control variable Ucontr is adjusted
directly using the RAISELOWER push buttons on the front of the module In this way
for example the relationship between the phase position of the firing pulses and the
control variable Ucontr can be checked easily
CHAPTER 3
PULSE SECTION
31 Pulse Generation and Amplification
The Gate Control Units of both AUTOMATIC channel and MANUAL channel
each supply six firing pulses for operating the 6-pulse thyristor bridges The low-power
pulse signals from these Gate Control Units are then amplified in the Intermediate Pulse
Stage galvanically isolated and then sent to the common pulse bus On the output end
the Intermediate Pulse Stage of the non-active channel) is always blocked The Gate
Control Units generate the pulses based on microprocessor control The reference voltage
used for the firing pulse phase location is the output voltage from the excitation
transformer (Usyn1 Usyn2) The commutation spikes of the synchronous voltage caused
by the converter are calculated prior to use of the voltage as a reference value and are
deliberately filtered out The lower limit for the firing pulses (double pulses) which are
offset from one another by 60deg is defined by the limit rectifier position (αmin) and the
upper limit by the limit inverter position (αmax) for the firing angle αmin and αmax can
be adjusted on the Gate Control Units using BCD (Binary Coded Decimal) switches
αmin ensures that the firing pulses will not be issued (premature firing) until there is
sufficient positive phase voltage on the thyristor involved αmax prevents a dangerous
ldquotippingrdquo of the thyristor bridge into the rectifier mode if the firing angle α is too large
(ldquolate firingrdquo) The critical factors determining αmax are the overlap time uumlmax (max
commutation time) and the ldquorecovery timerdquo of the thyristors (αmax lt 180deg - uumlmax - γ )
An external control signal can force the firing pulses into their inverter limit position
Other binary inputs can block or direct the firing pulses of the Gate Control Units so as to
produce freewheeling on the thyristor bridge During freewheeling the firing pulses for
the thyristor pair R and S are blocked and the pulse signals T+T- are engaged with
chains of pulses Both Gate Control Units (for the MANUAL amp AUTOMATIC channels)
contain a field current monitor that blocks the firing pulses immediately whenever the
current exceeds a preset threshold level In this case the field circuit-breaker is also
tripped via an output contact The purpose of these provisions is to prevent damage to
thyristors and thyristor fuses in case of a slip-ring short-circuit or to keep any damage
that does occur to a minimum The pulse signals are galvanically separated at the outputs
from the Intermediate Pulse Stage (with pulse transmitters) and are then directed to the
common pulse bus This transmission of the pulse signals to the pulse bus via passive
transmitters ensures a high degree of active channel autonomy Practically no possible
malfunctions on the inactive channel (including for example sustained pulses) affect the
active channel
32 Pulse Monitoring
The ldquoPulse Busrdquo and the pulse signals of the AUTOMATIC channel are
monitored This monitoring device consists of potential isolating stages and the common
monitor If the pulse monitoring of the ldquoPulse Busrdquo responds a switch-over is made to
MANUAL channel The function of the potential isolating stages is to couple the pulse
monitoring device to the pulse circuits without any feedback effect The pulse monitoring
checks the six pulse lines for the following malfunctions continuous or periodic failure
of one or more pulses Periodic occurrence of synchronous or asynchronous false pulses
Continuous pulses the pulse monitoring device can be tested while the machine is in
operation
CHAPTER 4
CONVERTER
Thyristor
The term thyristor usually refers to a family of four layer solid state device having
turn on characteristics that can be externally controlled by either current or voltage They
are also referred to as breakdown device because their working depends on avalanche
breakdown Thyristors have only two stages OFF and ON Thyristors have a similar
function to Uni-junctions they act as switches Thyristors use current flow as a switch
Thyristors have three states
1 Reverse blocking mode mdash Voltage is applied in the direction that would be
blocked by a diode
2 Forward blocking mode mdash Voltage is applied in the direction that would cause
a diode to conduct but the thyristor has not yet been triggered into conduction
3 Forward conducting mode mdash The thyristor has been triggered into conduction
and will remain conducting until the forward current drops below a threshold value
known as the holding current Converter is a semiconductor device which converts ac
input voltage into a constant dc output voltage In present excitation system three phase
fully controlled thyristor converter is used
Because of the following advantages thyristor converters are used
a) Thyristors are used for high power applications ie up to 10Kv3500A1KHz
b) Having high reliability and low losses
c) Uni-directional device like diode
d) Itrsquos operation as a rectifier which are low resistance in forward conduction
mode and high resistance in reverse conduction mode
PROTECTION OF THYRISTORS
For reliable operation of a thyristor demands that its specified ratings are not
exceeded When Subjected to or over voltages During the turn - on of SCR didt
prohibitively large False triggering of SCR by high value of dvdt andSpurious signals
between gate and cathode may leads to unwanted turn ndash on
DIDT AND PROTECTION
When thyristor starts conducting in forward conduction mode and is turned on by
gate pulse The anode current increases rapidly whole area of the gate to Cathode
junction then hot spots will be formed near the gate connection this locality of heating
destroys the thyristor Thyristor thermal time is constant The causes due to faults and
short circuits or surge currents Electronic crowbar protection is used against the over
voltages The rate rise of anode current must be kept at the time of turn on below the
rated or specified limiting value The didt value maintained below limited value by using
a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating
is avoided by applying gate current but not greater the maximum gate current
DVDT AND OVER VOLTAGE PROTECTION
With forward voltage across the anode and cathode of a thyristor the two outer
junctions are forward biased but the inner junction is reverse biased This reverse biased
junction J2 has the characteristics of a capacitor due to charges existing across the
junction In other words space-charges exist in the depletion region around junction J2
and therefore junction J2 behaves like a capacitance If the entire anode to cathode
forward voltage Va appears across J2 junction and the charge is denoted by Q then a
charging current i given by Eq (46) follows
i = dQdt =d(Cj Va )dt
= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)
As Cj the capacitance of junction J2 is almost constant the current is given by
i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)
If the rate of rise of forward voltage dVadt is high the charging current i will be
more This charging current plays the role of gate current and turns on the SCR even
when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on
must be avoided as it leads to false operation of the thyristor circuit
For controllable operation of the thyristor the rate of rise of forward anode to
cathode voltage dVadt must be kept below the specified rated limit Typical values of
dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by
using a snubber circuit in parallel with the device thyristor are very sensitive for over
voltage than the semiconductor devices
Over voltage transients are perhaps the main cause of thyristor failure
In thyristor there are mainly two types
1 Internal over voltages
Due to the commutation of the thyristors large voltages are generated internally
Because of the series inductance of the SCR circuit the large transient voltages L didt
produced This voltage several times the break over voltage of the device then thyristor
destroys permanently
2 External over voltages
External over voltages are caused due to the interruptions of current flow in an
inductive circuit and also due to the lightening strokes on the lines feeding the thyristor
system For the reliable operation of thyristor the over voltages must be suppressed by
adopting suitable techniques
Suppression of over voltages
The RC circuit called snubber circuit is connected across the device to protect In
order to keep the protective components to a minimum the thyristors are chosen with
their peak voltages ratings are 25 to 3 times of the normal peak working voltage
ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are
commonly employed for protecting the thyristor circuit against the over voltages
Gate protection
Gate circuit should also be protected against the over voltages and surges Over
voltage at gate circuit can cause false triggering of the SCR may rises the junction
temperature behind specified limit leading to its damage Protection against over
voltage can be achieved by connecting a ZD across the gate circuit and a resister is
connected in series with gate circuit to protect against the s A capacitor and resister are
connected across gate to cathode to by pass the noise
41 Final Pulse Stages
The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage
(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor
bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided
with a power supply module The amplified output pulses from the Final Pulse Stages
start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main
pulse This initial pulse edge assures proper firing of the thyristors being triggered
Subsequently the weaker part of main pulse keeps firing conditions steady As already
mentioned the Final Pulse Stages and their associated thyristor bridges form single units
All six pulse outputs from a Final Pulse Stage can be blocked by an external control
signal so that all thyristors in the associated thyristor bridge will block the current A
blocking of the pulses is initiated whenever there is a malfunction in the associated
thyristor bridge
42 Converter Power Section
The thyristor converter consists of three independent parallel rectifier blocks TY1
to TY3 which are all in service Even if one block fails the remaining blocks take over
automatically the full design current of the excitation circuit During normal operation
(with ideal current share) and all three bridges in operation each of these blocks has to
carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation
is limited Only when all three bridges fail the excitation is switched off Each thyristor
bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any
arm is identified by a Current flow monitoring module
43 Converter Cooling
A cooling system is needed to dissipate heat losses in the converter blocks and
electronics Each converter block has therefore been equipped with a fan supplied with
power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing
cubicle) The fans are protected with motor protection circuit breakers An air flow
monitoring unit is provided for monitoring the air flow through the thyristor bridge If a
circuit breaker failure is detected or if the air flow monitor drops off at one of the
thyristor bridges the bridge involved is immediately set out of operation by blocking its
firing pulses
44 Thyristor Converter Monitoring
A thyristor bridge in which defects occur that could threaten the safety of
operation or cause secondary damage is switched off automatically ie its firing pulses
are blocked This happens whenever A thyristor fuse is blown The fuses are monitored
individually with micro switches The Final Pulse Stage fails which is detected by
internal monitors (supply voltage sustained pulse short-circuit on the output end) The
power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or
is insufficient Isolator on ACDC side is open
CHAPTER 5
Field Current Circuit Excitation Transformer
51 Field Circuit Breaker
The circuit-breaker in the field circuit is used to isolate the field circuit from the
converter It is capable of switching off the synchronous machine from full load under the
maximum conditions of a 3-phase short-circuit In addition to its main contacts the field
circuit-breaker also has a de-excitation contact with which the field energy stored in the
field can be dissipated across the de-excitation resistor The de-excitation contact closes
shortly before the main contacts open so as to ensure proper commutation of the field
current from the main contacts to the de-excitation contact when the breaker is switched
off The field circuit-breaker is switched on by electromagnetic force and is kept switched
on by a mechanical latch When the latch is released by a trip coil the circuit-breaker
opens The circuit-breaker also has auxiliary contacts that report its status
52 Field Flashing
In shunt supplied excitation circuits (excitation transformer connected to the
generator terminals) the generator does not have enough remnant voltage for a generator
voltage build-up via the converter In this case a field flashing circuit is provided It
consists of the field flashing contactor the diode bridge and a transformer used to adapt
the auxiliary input voltage to the voltage needed for field flashing when power is
supplied from the auxiliaries network
Fig Field Flashing
Because the field flashing contactor is not able to switch off the energy stored in
the field the control ensures that the contactor can only reopen if the field circuit breaker
has already been opened (generating the TRIP order) or in a normal field flashing
sequence when the converter has taken over the field current Field flashing occurs in the
following stages
1048729The excitation is switched on closing the field flashing contactor ( Field
Circuit Breaker is already closed )
1048729The start-up excitation current flows through the rotor driving the generator
voltage up to approx 15 U
1048729After about 10 U the firing pulses to the converter are released and it begins
to excite the generator to its rated voltage
1048729After about 30 U the field flashing contactor opens (with no current since
the converter is now supplying the current)
The diode bridge at the input to the field flashing contactor prevents a feed-back
from the converter to the source of field flashing while the contactor is still closed
53 De-excitation
When malfunctions occur the stored field energy must be dissipated as quickly
and safely as possible to protect the generator This is done by the converter the field
circuit-breaker and the de-excitation (discharge) resistor
De-excitation (with opening of the field circuit-breaker) takes place in the following
stages
1048729The converter drives to its inverter limit position (negative ceiling voltage)
recovers a portion of the field energy into the network A trip command is given to the
field circuit breaker
1048729The de-excitation contact closes diverting the field voltage to the de-excitation
resistor
1048729Then immediately the main contacts open building voltage The field voltage
commutates to the de-excitation resistor
1048729The current diminishes at a given time constant TE
(With linear resistance TE = Lf (Rf + Re))
Due to the reversal of the field voltage by the converter the field current
commutates from the main contacts of the field circuit-breaker to the de-excitation
resistor in a very early phase This reversal of the field voltage prevents burn-off on the
main contacts and provides effective protection for the field circuit-breaker Depending
on the operating policy an operational shut-down of the excitation can also be effected
with the field circuit-breaker closed This method is useful mainly when the excitation is
switched on and off frequently In this case the converter is merely driven into the
inverter limit position so that the field energy is recovered into the network The
converter then blocks since it is supplying positive current only
54 Excitation Transformer
The excitation transformer matches the generator voltage to the field voltage
(required ceiling voltage) It also serves as a commutation reactance for the thyristor
converter and as a potential isolator between the network and the excitation circuit In
addition the transformer functions as a current limiter in that it makes it possible to keep
any short circuits in the excitation circuit under better control The excitation transformer
is equipped with temperature monitoring probes which set off an alarm when the
temperature exceeds a first max limit and then trips the excitation if the temperature
continues rising to a second (higher)limit
CHAPTER 6
Monitoring and Protection
61 Excitation Monitoring
611 General Information
The main goal of Excitation Monitoring is to make optimum use of the
redundancies provided in the excitation system and to give alarm whenever a malfunction
makes these redundancies unavailable The field current is monitored to see that it does
not exceed a maximum level and if necessary a switch-over to the MANUAL channel is
initiated In addition the criterion for switching off the field flashing is generated The
excitation Monitoring consists of an autonomous processor system
612 over current Alarms
In the Excitation Monitoring the limits for are set at higher levels than the
settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of
the nominal field current contact R1 and the binary output associated with it are
activated immediately If field current remains gt 110 then after a preset inverse-time
has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs
associated with them are activated Parameters match the measurements for If1 and If2 to
the nominal value for field current so that the internal values can be processed and read as
pu values It can be used to falsify the actual value of the field current If (to raise it) so
as to cause a response from the alarm limits for purposes of testing The processed If
signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As
long as the field current If is above the threshold value 11 Ifn its peak value is
measured This is stored (until RESET) and can be read at any time on the Micro-
Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever
the integrated time-current value (intisup2dt) exceeds the preselected reference value the
alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously
issued to switch over to the stand by AUTO channel Software switch F758 enables the
three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-
time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the
desired limit curve for response is set using the factor F216
613 Switch-Off Criterion for Field Flashing
The Excitation Monitoring supplies the criterion for switching off the field
flashing Whether this criterion is activated based on the actual value for generator voltage
Ug or for field current If or both depends on the settings of the two threshold values
F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements
Ug12 and If12 are switched over depending on the present status of the channels
(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is
fixed at ldquological 1rdquo
614 Storage of Alarm Status
The outputs of the over current alarms (R1 R2 R0) and the messages NO
FAILURE MONITORING PARAMETERS CHANGED are stored messages can be
erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the
front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the
situation causing the alarm or the malfunction is no longer present Whenever the self-
diagnosis equipment in the processor detects a malfunction the output NO FAILURE of
MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo
is activated whenever parameters or settings of software switches have been changed via
the Micro-Terminal
615 Actual Value Monitoring
The actual values for generator voltage Ug synchronous voltage Usyn and field
current If are monitored for malfunctions This monitoring is active regardless of whether
or not the generator is in operation Essentially when the generator is in operation the
measurements are monitored by comparing the signals (the smaller signal reading is
detected as incorrect) When the generator is not in operation the measured data are
monitored for extreme values The percentage of deviation permissible in the
measurement signals being compared
(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and
F20AIf the excitation transformer is being supplied from an auxiliary power source (no
shunt operation) the values of Ug and Usyn will be different in some operational
conditions
In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1
and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the
messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2
and the binary message from CH1 reports no malfunction a malfunction on
Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is
also present whenever the binary message CH1 DISTURBANCE is reported and a
discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2
FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares
Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the
comparison channel reports a malfunction or whenever both binary messages report no
malfunction - but both secondary monitors report a malfunction As long as the secondary
monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction
signals for the measurement channel involved (suspicion that there is a corresponding
error in Usyn) The measurement channel malfunctions are enabled operationally
whenever after excitation has been switched on generator voltage Ug exceeds the value
set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is
switched off to see that they do not exceed the limit value F210 that applies to both of
them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt
the operating range) Monitoring for extreme values is likewise enabled during normal
operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the
malfunction signals to the binary outputs Basically the monitoring of the actual values
for If1If2 functions like that of the Ug1Ug2 monitoring
62 Excitation Protection
621 General Information
The Excitation Protection switches off the excitation (and de-excites the machine
rapidly) whenever a danger arises that threatens the excitation transformer the converter
or the generator Generally limiter or monitoring functions precede the emergency trips
and these normally respond before the Excitation Protection must initiate a trip
Protective trip commands are issued directly to the field circuit-breaker from potential
free contacts of the board via the trip relays They are directed redundantly to the
operative field circuit-breaker ldquoOFFrdquo command
622 Protection against Excitation Transformer Overheating
This equipment monitors the excitation transformer for overheating in the
windings that could result from over current short-circuits or inadequate cooling The
monitoring uses temperature monitoring modules in conjunction with temperature
sensors built into the windings Normally the temperature is monitored in two stages the
first stage sets off an alarm the second causes a trip of the excitation
623 Rotor Over voltage Protection
Malfunctions in the generator circuit (eg terminal short-circuit failed
synchronization asynchronous operation) cause induced negative field currents that
produce high voltages in the field circuit These must be restricted to a level with a
sufficient safety margin below the insulation capacity of the field winding (test voltage)
and also below the peak blocking voltage of the converter thyristors The crow bar
employs spark gap elements to detect over voltages in the field circuit Whenever they
respond the associated thyristors are fired immediately switching the de-excitation
resistor parallel to the field The de-excitation current generated thereby initiates an
excitation trip via a supervision circuit causing an immediate opening of the field circuit-
breaker The malfunction isets off an alarm and an internal malfunction is indicated at the
cubicle
TEST VALUES OBTAINED WHEN EXCITATION IS RAISED
TEST
SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE
1 VREF 996 100
2 VACT 997 1003
3 IFACT 735 765
4 IGACT 703 707
5 ACTIVE
POWER
703 705
6 REACTIVE
POWER
101 142
7 POWER
FACTOR
099 IND 098 IND
8 ACTIVE
CURRENT(IR)
705 703
9 REACTIVE
CURRENT(IX)
102 138
10 POWER
ANGLE
566 546
11 FIRING
ANGLE
640 632
VALUES OBTAINED WHEN EXCITATION IS LOWERED
SNO PARTICULARS ACTUAL
VALUE
OBTAINED VALUE
1 VREF 100 997
2 VACT 100 997
3 IFACT 787 761
4 IGACT 839 837
5 ACTIVE
POWER
839 830
6 REACTIVE
POWER
155 110
7 POWER
FACTOR
098 IND 099 IND
8 ACTIVE
CURRENT(IR)
835 830
9 REACTIVE
CURRENT(IX)
140 94
10 POWER
ANGLE
613 DEG 631 DEG
11 FIRING
ANGLE
601 DEG 599 DEG
CONCLUSION
For generating the EMF in stator winding excitation is required to the rotor of a
generator There are two types of excitation
1 Static excitation system
2 Brushless excitation system
A certain disadvantage in brushless excitation system is the slow response time of
the field in case of fast load changes specified No slip-rings and brushes direct
measurements of the field parameters not possible
To avoid all loses static excitation is used Since it does not have any rotating
parts mechanical loses and windage loses This system has fast response and speed
control While preferring this excitation system there are no limitations for the
redundancy of Thyristor bridge circuits
Static excitation has fast field discharge by resistor and inverter operation direct
measurement of field quantity is possible The meaning of excitation is nothing but
continuous supply of DC current (ie field current) to the rotor to buildup required
output voltage in the stator
Field current is changed with respect to the change of load so the digital
automatic voltage regulator (DAVR) is used to regulate the output voltage according to
the load variations
So we conclude that static excitation system with DAVR is preferred since it is
having excellent dynamic performance and better options for R amp M
CHAPTER 2
Digital Automatic Voltage Regulator (DAVR)
21 Principle of Operation of the Regulator (DAVR)
To regulate the voltage and the reactive power of a synchronous machine the
field voltage must be adjusted quickly to the changes in the operating conditions (with a
response time that does not exceed a few ms) To accomplish this analog control systems
include amplifiers which make continuous comparison of the actual values against the
reference values and vary the control variable to the converter with almost no delay Most
of the delay that occurs originates in the converter since the firing pulses for changing
the rectifier phase angle are only issued periodically (every 33 ms)
The DVR digital voltage regulator calculates the control variable from the
measured and reference data in very short time intervals This results outwardly in a
quasi-continuous behavior with a negligible delay time (as in an analog regulator) The
calculations are made in the binary number system Analog measurement signals such as
those for generator voltage and generator current are converted into binary signals in
analogdigital converters The set-points and limit values have already been defined in
digital (binary) form An understanding of the actual computation processes in the digital
voltage regulator is not necessary for operation preventive maintenance or
troubleshooting Like the operator of a pocket calculator or a personal computer all the
operator needs is to know how to operate the instrument and the programming for this
working tool For that reason we will explain below only the principle division of work
among the various modules and the flow of data processing The purpose is above all to
make clear how the processor system has been integrated into the rest of the power
electronics system
6
22 Basic Structure of the Processor Systems
7
The signal processors 25 analog inputoutput modules Each of these processor
systems has a common bus circuit and output and the control lines There is a specific
range of addresses assigned to each assignment Board including the power supply bus
the address lines the two data lines to the input calculates the reactive current (I sin φ)
and the active current ( Icos φ) With these two channel processor Synchronized with
these interrupts (ie with the phase positions of current Ig the field current If and the
synchronous voltage Usyn From the exchange data with the microprocessor card across
the two data lines generator voltage Ug) this processor measures the generator current
Ig and then hardwired connections or multi-conductor cables Binary and analog
inputoutput modules ie for galvanic isolation and adaptation to the electronics level
The most important input interrupts per period to trigger the cycles for processing actual
values in the AUTOMATIC module on the processor bus) for filtering and further
processing
Monitoring each consist of the central microprocessor module and binary and
parameters to the AUTOMATIC channel are the generator voltage Ug the generator
peripheral unit Ug Ig and Usyn are sent to the Interrupt Generator (plug-in peripheral
units (wall-mounted units) peripheral units are used for preprocessing signals from
external measurement circuits power supply units Signals are exchanged among these
processor systems via processed across separate peripheral units for each channel These
processor working on the bus (a house address that can be adjusted using a switch)
systems The AUTOMATIC channel the programmable controls and the Excitation The
actual values measured from AUTOMATIC channel and MANUAL channel are The
AUTOMATIC channel and the MANUAL channel each have their own The digital
voltage regulator is broken down into several autonomous microprocessor The inputs and
outputs of the processor systems are directed across voltage-isolating The Interrupt
Generator also uses the 3-phase Ug signal to generate the 12 themselves contain a limited
number of hardware inputs and outputs with fixed equipment Whenever addresses from
this range are called up the signal processing module can results the processor is then
able to derive further operating parameters such as the load angle the active power etc
The functions of all microprocessor systems other than the programmable controls
have been accomplished in firmware The non-varying standard function modules can be
configured to the design desired for plant-specific purposes using software switches
(KFlags) Thus for example the stored status of a K-Flag determines whether or not a
Limiter is active and whether the de-excitation or the excitation limiters take precedence
Because these K-flags determine the software Scope of Supply for the installation they
cannot be changed permanently via the Micro-Terminal In this way they differ from
such setting data as the values of the parameters for the PID filter of the voltage regulator
or the set-points for the limiters These values can be permanently changed using the
Micro-Terminal Communication is possible with each of the processor systems via the
Micro-Terminal by plugging on the connecting cable In this way signals within the
processor and setting parameters can be viewed analog signals can be issued and the set
parameters can be altered temporarily (F range) or permanently (C range) Unlike the
other processor systems the programmable controls do not include any firmware for
realization of the functions They have been designed so that the designer can adapt and
change their functions easily using the ldquoFunctional Block Programming Language P10
Digital and analog functions can be implemented in practically any degree of complexity
desired using the P10 functional blocks The control variable of the voltage regulator
(AUTOMATIC channel) and the control variable of the field current regulator
(MANUAL channel) are each processed in separate Gate Control Unit and formed into a
chain of pulses at the appropriate firing angle The pulses of the active channel are
directed to the pulse bus via the associated Intermediate Pulse Stage The pulses for each
converter block are amplified sufficiently in Final Pulse Stage to fire the Thyristor
231 General Information
The functions of the automatic voltage regulator AVR are
1 to regulate the generator voltage
2 to regulate the effect of the reactive andor active current on the voltage
3 to limit VoltHz
4 to limit max and min field current
5 to limit inductive stator current
6 to limit capacitive stator current
7 to limit the load angle
8 to stabilize the power system
Block Diagram shows the software structure of AUTOMATIC channel The
generator limiters not provided for the installation in question (optional equipment) are
identified in this overview as ldquoNot Suppliedrdquo The parameter values signal values and
software switches (flags) marked with addresses (hexadecimal numbers) can be viewed
and altered via the Micro-Terminal The values selected are displayed in sec pu Hz
etc and can where necessary be changed directly in these formats The plant-specific
settings of the variables and the flags can be obtained from the Test and Commissioning
Report This block diagram provides information about the important functions and
possible settings of the AUTOMATIC channel For the sake of clarity no detailed
presentation has been given of special functions such as tracking circuits initializations
etc The page heading cross-refers this overview to the various sheets of the schematic
diagram Binary signals are shown in broken lines analog signals in solid lines The
corresponding text designations in the schematic diagram can be used for identification of
the input signals (hardware inputs) The only analog output signal from the automatic
voltage regulator control variable Ucontr is sent via the data bus (CRU bus) to the Gate
Control Unit Most of the binary messages (outputs) from the AVR are of no interest
functionally and they have been omitted for the sake of clarity The basic structure of the
digital voltage regulator and the limiters is simple This is necessary in order that the
behavior of the regulatorslimiters will remain calculable and understandable in all
operating situations and that there will be no problem in adjusting and optimizing them
The central PID filter in the digital voltage regulator defines the dynamic response of the
closed-loop controls both in the voltage regulator mode and after limiters have
intervened The ldquocontrol deviationrdquo at the input to the PID filter is either the control
deviation for voltage the control deviation of a de-excitation limiter (the value
determined by minimum value selection) or the control deviation of an excitation limiter
(the value determined by maximum value selection) Flag F730 (ldquoPRIORrdquo) is used to
determine whether the exciting (Min value) or the de-exciting signal takes precedence on
the minmax value limiter (normally F730 = 1111 ie the de-exciting signal takes
precedence) With the exception of the Minimum Field Current Limiter all other limiters
have variable factoring multipliers of the signal outputs so that they can be adjusted
individually together with the common PID filter which has been optimized for voltage
regulation The setting parameters for this PID filter are as follows
Vo = KR Static amplification
1
Ta = ---- Integration time constant
Tc1
Vp Proportional amplification
1
Tb = ---- Differential time constant
Tc2
Vinfin Amplification of high frequencies
The BODE diagram below shows the assignment of settings in accordance with
DINIEC standards based on a typical example
The PID filter amplifications Vo Vp and Vinfin can be adjusted in pu values But
the ldquoceiling factorrdquo pl+ must be adjusted correctly with parameter F310 if the total
amplification (circuit amplification) of the control circuit is actually to conform to the
pu settings This factor must agree with the ldquoexternalrdquo amplification ie with the
ceiling value of the transformer- converter circuit
Ceiling factor(pl+) = Ufmax Ufo
in which Ufmax = ceiling field voltage
Ufo = no-load field voltage
To attain a suitable response of the AVR when starting excitation
(ldquoEXCITATION ONrdquo) it may be necessary to change the proportional amplification of
the regulator during this phase Vp2 (transiently activated) and Vp1 (permanently
activated) can be adjusted for this purpose For example the value of Vp2 takes effect
immediately once the excitation is switched on and remains effective for a period as set at
F30C Once the period F30C (eg 5 sec) has expired Vp shifts over to Vp1 (becomes
the steady-state Vp) at the rate of change set The standard operating mode for the PID
filter is voltage regulation for which the discrepancy between the voltage set-point and
the current value for generator voltage Ug (the control deviation) is supplied at the input
To compensate for the voltage drop in the block transformer or whenever several
generators are operating to the same distributing bus the generator voltage must be
varied in proportion to the measured generator current (droop influence) To accomplish
this the voltage set-point is varied as a function of the measured reactive current IX
andor active current IR Flag F712 enables the IX droop Flag F710 the IR droop The
desired compensation is set in F282 and F286 respectively Flags F284 and F288 are used
to select whether this droop influence is to increase the voltage or to reduce it
(compensation) Combined influence of the active and reactive currents is attained by
enabling both droops IX and IR Flag F716 activates a so-called ldquoSoft-Startrdquo at the
starting of excitation This ldquoSoft-Startrdquo ensures that the voltage set-point integrates from
0 to 100 within the time set on F290 when the excitation is switched on
(ldquoEXCITATION ONrdquo) A ldquosmoothrdquo excitation of the generator can be achieved in this
way whenever there is no demand for a quick excitation
232 Voltage Set-Point
Various signals and settings control and limit the voltage set-point F270 For
example the values of F254 and F252 define the normal operating range possible for set-
point adjustment (eg 90 110) using external control commands (control room local
operatorrsquos panel superposed control system) The effective set-point adjustment rate is
governed The set-point can be set at the values of F250 and F256 by activating
appropriate control commands for ldquoSETrdquo input Enabling Flag F71A and activating a
binary input prior to switching on the excitation (ldquoEXCITATION OFFrdquo) sets the Ug set-
point at the value of UAUX This makes it possible for example to ensure that the
generator voltage will agree exactly with the network voltage after the voltage build-up
An external value with variable amplification can be added to the Ug set point by
enabling F724 (for example for stability tests)
233 Regulator Tracking in MANUAL Operation
Whenever the AUTOMATIC channel is not in operation (the MANUAL channel
is ON) a follow-up equipment ensures a smooth switch-back to the AUTOMATIC
mode will always be possible To track the voltage set-point is shifted by means of
RAISELOWER pulses from the Gate Control Unit so that control variable Ucontr at the
output from the PID filter is held steady and identical to the control variable Ucontr from
the MANUAL channel Because this tracking must react slowly resultant transient
control deviations resulting from the amplification in the PID filter might cause severe
interference with control variable Ucontr
To prevent this the follow-up equipment intervenes on the regulators mixing
point with a corresponding compensation signal
234 Ugf Limiter
At under frequency the Ugf Limiter reduces the generator voltage so as to
prevent saturation effects in the supply and measuring transformers To adjust this
limiter the max permissible generator voltage at rated frequency is defined and set
When any under-frequency occurs the generator voltage is thus reduced in proportion to
that setting
235 Field Current Maximum Limiter
The Field Current Maximum Limiter is provided to protect the generator rotor
from s occurring in steady-state and transient operation High field currents are normally
the result of a sharp drop in network voltage or of an improper raising of the voltage set-
point by the operating staff The field current is held steady at the value TH1 ie at the
maximum thermal value permissible for the excitation circuit and the rotor In order that
the generator can support the power network with its transient overload capacity during
brief collapses in voltage a temporary switch-over is made to the transient limit MAX1
(a higher setting) When the generator or the converter is operating at a reduced capacity
These limits TH1MAX1 can be switched over to the lower settings TH2MAX2 by
activating the corresponding binary signals The switch-over from the thermal limit
TH12 to the transient limit MAX12 can be configured in one of three ways
a) Depending on the over current with -dUdt ENABLE
Flag programming F418 = any setting desired F41A = 0000
This variant enables the transient value MAX12 whenever a collapse of voltage
in the network is detected The ENABLE time is fixed and can be set The example
below shows the typical behavior of the limiter configured in this way
b) Dependent on the time integral with -dUdt ENABLE
Flag programming F418 = inactive F41A = 1111
This variant likewise enables the transient value only when a collapse of network
voltage has been detected However the switch-back to the thermal limit is not made
dependent upon the time itself but on the calculated time integral intisup2dt of the The setting
on Parameter F414 in spu takes into account the time the rotor needs to cool down ie
the rate of temperature change in the case of intermittent operation The example below
shows how the timing of the switch-back to the thermal limit depends on the present
value for intisup2dt
The time integral is based on the formula
Example The setting of =isup2dt equivalent to Version a (F416) at a constant 16 times the
nominal field current for 10 seconds (with TH12 = 105) is
c) Dependent on the time integral without any preconditions
Flag programming F418 = 1111 F41A = 1111
In this variant the transient becomes available without any prior conditions
(without a -dUdt ENABLE) with the time integral intisup2dt
237 Inductive Stator Current Limiter
The Inductive Stator Current Limiter holds the stator current Ig within permissible
limits while the generator is in the ldquoover-excitedrdquo operating range by reducing the field
current accordingly The setting TH (thermal limit) provides the limit against stationary s
that might occur To take advantage of the generatorrsquos transient overload capacity a
switch-over is made to the higher setting MAX The principle of operation of this switch-
over to the value MAX permissible only transiently is identical to that employed for the
field current limiter (refer to the description above) When the drive output from the
turbine is very high stator current may exceed permissible limits even while inductive
loading of the generator is low In this case if the stator current limiter is not kept from
influencing the field current the control circuit will oscillate back and forth between the
Inductive Stator Current Limiter (de-
exciting) and the Capacitive Stator Current Limiter (exciting)The output signal of that
function then dominates the control variable of the Ig-dependent limiter via a maximum
value selection
238 Capacitive Stator Current Limiter
239 Load Angle Limiter
The Load Angle Limiter prevents the synchronous machine from slipping out of
phase due to slippage of the rotor The load angle δ the difference in phase between the
rotor and the stator rotating field results mainly from the driving torque (active power P)
acting on the generator and the level of rotor current (field current) If the driving torque
remains constant a increase in the field current reduces the load angle δ The current load
angle δ at any moment is obtained from the generator current and generator voltage based
on a simplified model of the generator Whenever this calculated load angle δ exceeds the
preset limit angle the limiter increases the field current until the load angle has dropped
back to its permissible value The quadrature reactance Xq of the generator and the
network reactance Xe during normal operation must be adjusted on the regulator in order
to obtain the load angle δ The graph below shows the Power Chart for a salient-pole
machine with typical limiter characteristics
The purpose of a Power System Stabilizer is to use the generator excitation to
damp electromechanical oscillations between the network and the generator Depending
on the design of the generator and the requirements imposed for network stability its
main function will be either to damp the oscillations originating in the machine or those
from the network A synchronous generator working in a combined power network is in
principle an oscillating structure In order to produce a torque the magnetic field of the
rotor and the stator must form a given angle (referred to as the rotor displacement or load
angle δ) The electrical torque ME increases as the angle δ increases just as with a
torsion spring Because the ME of the generator and the mechanical driving torque MA
from the turbine are in equilibrium during steady-state operation the angle δ remains in a
given position Whenever this state of equilibrium between MA and ME is disturbed the
load angle slips of this rest position and change thereby the electrical torque ME The
torque attempts to restore the load angle to a stationary position Due to the mass inertia
of the turbinegenerator rotor however this can only take place aperiodically It does so
in the form of more or less effectively damped oscillations (again similar to the effect of
mass inertia on a torsion spring) In order to damp the oscillations there must be a
damping torque produced depending not on the electrical torque ME associated with the
angle but on the difference in frequency (Df) between the rotor and the stator rotating
field ie on the slippage This torque is produced mainly by the so-called damper
winding in the rotor but the dimensioning of this is subject to limits imposed by
considerations of design and economy Some further action is therefore needed to
increase the damping effect The following drastically simplified formula shows the
parameters upon which the amount of active power PE supplied by the generator
depends
PE = active power
It can be seen from the above relationship that the active power that the generator
transfers depends not only on the load angle δ but also on the field current If That means
that a transient change can be made in the active power PE and with that in the effective
electrical torque ME by varying the field current The principle of operation of the DVR
Power System Stabilizer becomes clear from a consideration of the oscillations in power
output and frequency (ΔPE Δf) and the vector diagram If it is assumed that oscillations
in the network frequency generates load oscillations with the mass inertia of the rotor
then the active load of the generator (eg MW-measured) is influenced with a sinusoidal
value -ΔPE (ME-MA = -ΔPE) By inversion of -ΔPE one obtains the fluctuation in
power provided by the rotor +ΔPE As is known the slip signal Δf follows +ΔPE with a
phase delayed by 90deg The +ME produced by the periodic changes in the load angle δ is
in phase with +ΔPE A good damping is attained if ME is varied in phase with the slip
Δf However this signal must also be advanced somewhat to compensate for the time
constants in the excitation circuit and the generator
As mentioned above the electrical torque ME can be influenced by varying the
field current To accomplish this a suitable control signal referred to as variable
disturbance compensation must be imposed upon the voltage set-point or the converter
control variable Ucontr As can be seen from the vector diagram by applying proper
weighting factors (K1 K2) and then adding together the signals -ΔPE and Δf an overall
stabilization signal can be produced that rotates in advance of the Df signal by any angle
desired between 0deg and 90deg Because the amplitude of -DPE remains proportional to the
amplitude of Δf a constant angle in advance of Δf results for the compensation of the
time constants referred to above The optimum weighting factors K1 and K2 for a
synchronous generator working to a power network depend on its operating point at any
moment and the external reactance of the network Normally the selection of a
compromise setting is good enough to attain stability in all operating points and for all
external reactance For special demands these settings must be parameterized as a
function of the external reactance (which means optional equipment Xe-Identification)
The Power System Stabilizer PSS is a section of the AVR computer program and is
processed once per network cycle The voltage at the generator terminals and the
generator current are measured in order to define the signals ΔPE and Δf The calculated
signals for _P_ (=PE) and Δf are then sent across DC filters ldquoDrdquo (real differentiators) that
transmit only the dynamic portion of the signals The ΔPE and Δf signals obtained in this
way are then weighted (multiplied by) with the factors K1 and K2 and sent to the
summing point of the voltage regulator
The PSS stabilization signal is imposed on the automatic voltage regulator only if
the following prerequisites are met
bull Generator on line
bull Generator power output gt the value F338
bull Generator voltage in a range between F33C and F33A
The stabilization signal is limited at the output from the PSS to the lower and
upper limits Flag defines whether the stabilization signal is introduced before or after the
PID filter (usually before the filter) Because the PID filter as noted above already takes
the ceiling factor Vp1 into account the PSS signal needs to be multiplied by Vp1 if it is
added to the voltage regulator following the PID filter (divider at the input to the
minmax limiter) This precaution prevents the DC filter ldquoDrdquo in the P-channel from
producing an unnecessary ldquostabilizationrdquo effect in the case of rapid changes in turbine
load As an alternative for the AVRrsquos Power System Stabilizer a stabilization signal from
an outside system can be imposed by activating the binary input ldquoPSS-SIGNEXTrdquo Flag
F340 can be used to select between an analog and a 12-bit signal and F33E to select the
polarity desired for that signal
24 The MANUAL Channel
241 Summary
The MANUAL channel (Channel 2) has been built as a simple field current
regulator
without additional limiters Its main function is to maintain the excitation of the generator
even if the AUTOMATIC channel becomes non-operational The MANUAL channel
also performs valuable service for purposes of testing commissioning and preventive
maintenance Its measurements regulator generation of firing pulses and power supply
are physically separate from those on the AUTOMATIC channel
242 Principle of Operation
All the functions of the MANUAL channel including the generation of firing
pulses have been implemented in a single electronic module the Gate Control Unit The
control variable Ucontr of voltage regulator is used as the reference value for generating
firing pulses on the principle known as ldquoramp controlrdquo (Comparison of Ucontr with
Usynsynchronous sawtooth signal) For further processing in the UN 0096 Intermediate
Pulse Stage the Gate Control Unit supplies six firing pulses at its output whose phase
position with respect to the synchronous voltage Usyn is in accordance with control
variable Ucontr An internal linearization ensures that the field voltage produced via the
firing pulses remains proportional to the control variable Ucontr throughout the entire
range As a result the circuit amplification of the control remains constant over the entire
range Whenever excitation is switched ON the set-point for Generator Voltage is set
automatically at the preset - ref Value This provision ensures that the generator voltage
always attains approximately its nominal value after the field flashing The Gate Control
Unit can be refunctioned ( by pre-selection with a switch ) for purposes of testing to act
as a purely firing pulse control In this case the control variable Ucontr is adjusted
directly using the RAISELOWER push buttons on the front of the module In this way
for example the relationship between the phase position of the firing pulses and the
control variable Ucontr can be checked easily
CHAPTER 3
PULSE SECTION
31 Pulse Generation and Amplification
The Gate Control Units of both AUTOMATIC channel and MANUAL channel
each supply six firing pulses for operating the 6-pulse thyristor bridges The low-power
pulse signals from these Gate Control Units are then amplified in the Intermediate Pulse
Stage galvanically isolated and then sent to the common pulse bus On the output end
the Intermediate Pulse Stage of the non-active channel) is always blocked The Gate
Control Units generate the pulses based on microprocessor control The reference voltage
used for the firing pulse phase location is the output voltage from the excitation
transformer (Usyn1 Usyn2) The commutation spikes of the synchronous voltage caused
by the converter are calculated prior to use of the voltage as a reference value and are
deliberately filtered out The lower limit for the firing pulses (double pulses) which are
offset from one another by 60deg is defined by the limit rectifier position (αmin) and the
upper limit by the limit inverter position (αmax) for the firing angle αmin and αmax can
be adjusted on the Gate Control Units using BCD (Binary Coded Decimal) switches
αmin ensures that the firing pulses will not be issued (premature firing) until there is
sufficient positive phase voltage on the thyristor involved αmax prevents a dangerous
ldquotippingrdquo of the thyristor bridge into the rectifier mode if the firing angle α is too large
(ldquolate firingrdquo) The critical factors determining αmax are the overlap time uumlmax (max
commutation time) and the ldquorecovery timerdquo of the thyristors (αmax lt 180deg - uumlmax - γ )
An external control signal can force the firing pulses into their inverter limit position
Other binary inputs can block or direct the firing pulses of the Gate Control Units so as to
produce freewheeling on the thyristor bridge During freewheeling the firing pulses for
the thyristor pair R and S are blocked and the pulse signals T+T- are engaged with
chains of pulses Both Gate Control Units (for the MANUAL amp AUTOMATIC channels)
contain a field current monitor that blocks the firing pulses immediately whenever the
current exceeds a preset threshold level In this case the field circuit-breaker is also
tripped via an output contact The purpose of these provisions is to prevent damage to
thyristors and thyristor fuses in case of a slip-ring short-circuit or to keep any damage
that does occur to a minimum The pulse signals are galvanically separated at the outputs
from the Intermediate Pulse Stage (with pulse transmitters) and are then directed to the
common pulse bus This transmission of the pulse signals to the pulse bus via passive
transmitters ensures a high degree of active channel autonomy Practically no possible
malfunctions on the inactive channel (including for example sustained pulses) affect the
active channel
32 Pulse Monitoring
The ldquoPulse Busrdquo and the pulse signals of the AUTOMATIC channel are
monitored This monitoring device consists of potential isolating stages and the common
monitor If the pulse monitoring of the ldquoPulse Busrdquo responds a switch-over is made to
MANUAL channel The function of the potential isolating stages is to couple the pulse
monitoring device to the pulse circuits without any feedback effect The pulse monitoring
checks the six pulse lines for the following malfunctions continuous or periodic failure
of one or more pulses Periodic occurrence of synchronous or asynchronous false pulses
Continuous pulses the pulse monitoring device can be tested while the machine is in
operation
CHAPTER 4
CONVERTER
Thyristor
The term thyristor usually refers to a family of four layer solid state device having
turn on characteristics that can be externally controlled by either current or voltage They
are also referred to as breakdown device because their working depends on avalanche
breakdown Thyristors have only two stages OFF and ON Thyristors have a similar
function to Uni-junctions they act as switches Thyristors use current flow as a switch
Thyristors have three states
1 Reverse blocking mode mdash Voltage is applied in the direction that would be
blocked by a diode
2 Forward blocking mode mdash Voltage is applied in the direction that would cause
a diode to conduct but the thyristor has not yet been triggered into conduction
3 Forward conducting mode mdash The thyristor has been triggered into conduction
and will remain conducting until the forward current drops below a threshold value
known as the holding current Converter is a semiconductor device which converts ac
input voltage into a constant dc output voltage In present excitation system three phase
fully controlled thyristor converter is used
Because of the following advantages thyristor converters are used
a) Thyristors are used for high power applications ie up to 10Kv3500A1KHz
b) Having high reliability and low losses
c) Uni-directional device like diode
d) Itrsquos operation as a rectifier which are low resistance in forward conduction
mode and high resistance in reverse conduction mode
PROTECTION OF THYRISTORS
For reliable operation of a thyristor demands that its specified ratings are not
exceeded When Subjected to or over voltages During the turn - on of SCR didt
prohibitively large False triggering of SCR by high value of dvdt andSpurious signals
between gate and cathode may leads to unwanted turn ndash on
DIDT AND PROTECTION
When thyristor starts conducting in forward conduction mode and is turned on by
gate pulse The anode current increases rapidly whole area of the gate to Cathode
junction then hot spots will be formed near the gate connection this locality of heating
destroys the thyristor Thyristor thermal time is constant The causes due to faults and
short circuits or surge currents Electronic crowbar protection is used against the over
voltages The rate rise of anode current must be kept at the time of turn on below the
rated or specified limiting value The didt value maintained below limited value by using
a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating
is avoided by applying gate current but not greater the maximum gate current
DVDT AND OVER VOLTAGE PROTECTION
With forward voltage across the anode and cathode of a thyristor the two outer
junctions are forward biased but the inner junction is reverse biased This reverse biased
junction J2 has the characteristics of a capacitor due to charges existing across the
junction In other words space-charges exist in the depletion region around junction J2
and therefore junction J2 behaves like a capacitance If the entire anode to cathode
forward voltage Va appears across J2 junction and the charge is denoted by Q then a
charging current i given by Eq (46) follows
i = dQdt =d(Cj Va )dt
= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)
As Cj the capacitance of junction J2 is almost constant the current is given by
i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)
If the rate of rise of forward voltage dVadt is high the charging current i will be
more This charging current plays the role of gate current and turns on the SCR even
when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on
must be avoided as it leads to false operation of the thyristor circuit
For controllable operation of the thyristor the rate of rise of forward anode to
cathode voltage dVadt must be kept below the specified rated limit Typical values of
dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by
using a snubber circuit in parallel with the device thyristor are very sensitive for over
voltage than the semiconductor devices
Over voltage transients are perhaps the main cause of thyristor failure
In thyristor there are mainly two types
1 Internal over voltages
Due to the commutation of the thyristors large voltages are generated internally
Because of the series inductance of the SCR circuit the large transient voltages L didt
produced This voltage several times the break over voltage of the device then thyristor
destroys permanently
2 External over voltages
External over voltages are caused due to the interruptions of current flow in an
inductive circuit and also due to the lightening strokes on the lines feeding the thyristor
system For the reliable operation of thyristor the over voltages must be suppressed by
adopting suitable techniques
Suppression of over voltages
The RC circuit called snubber circuit is connected across the device to protect In
order to keep the protective components to a minimum the thyristors are chosen with
their peak voltages ratings are 25 to 3 times of the normal peak working voltage
ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are
commonly employed for protecting the thyristor circuit against the over voltages
Gate protection
Gate circuit should also be protected against the over voltages and surges Over
voltage at gate circuit can cause false triggering of the SCR may rises the junction
temperature behind specified limit leading to its damage Protection against over
voltage can be achieved by connecting a ZD across the gate circuit and a resister is
connected in series with gate circuit to protect against the s A capacitor and resister are
connected across gate to cathode to by pass the noise
41 Final Pulse Stages
The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage
(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor
bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided
with a power supply module The amplified output pulses from the Final Pulse Stages
start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main
pulse This initial pulse edge assures proper firing of the thyristors being triggered
Subsequently the weaker part of main pulse keeps firing conditions steady As already
mentioned the Final Pulse Stages and their associated thyristor bridges form single units
All six pulse outputs from a Final Pulse Stage can be blocked by an external control
signal so that all thyristors in the associated thyristor bridge will block the current A
blocking of the pulses is initiated whenever there is a malfunction in the associated
thyristor bridge
42 Converter Power Section
The thyristor converter consists of three independent parallel rectifier blocks TY1
to TY3 which are all in service Even if one block fails the remaining blocks take over
automatically the full design current of the excitation circuit During normal operation
(with ideal current share) and all three bridges in operation each of these blocks has to
carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation
is limited Only when all three bridges fail the excitation is switched off Each thyristor
bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any
arm is identified by a Current flow monitoring module
43 Converter Cooling
A cooling system is needed to dissipate heat losses in the converter blocks and
electronics Each converter block has therefore been equipped with a fan supplied with
power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing
cubicle) The fans are protected with motor protection circuit breakers An air flow
monitoring unit is provided for monitoring the air flow through the thyristor bridge If a
circuit breaker failure is detected or if the air flow monitor drops off at one of the
thyristor bridges the bridge involved is immediately set out of operation by blocking its
firing pulses
44 Thyristor Converter Monitoring
A thyristor bridge in which defects occur that could threaten the safety of
operation or cause secondary damage is switched off automatically ie its firing pulses
are blocked This happens whenever A thyristor fuse is blown The fuses are monitored
individually with micro switches The Final Pulse Stage fails which is detected by
internal monitors (supply voltage sustained pulse short-circuit on the output end) The
power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or
is insufficient Isolator on ACDC side is open
CHAPTER 5
Field Current Circuit Excitation Transformer
51 Field Circuit Breaker
The circuit-breaker in the field circuit is used to isolate the field circuit from the
converter It is capable of switching off the synchronous machine from full load under the
maximum conditions of a 3-phase short-circuit In addition to its main contacts the field
circuit-breaker also has a de-excitation contact with which the field energy stored in the
field can be dissipated across the de-excitation resistor The de-excitation contact closes
shortly before the main contacts open so as to ensure proper commutation of the field
current from the main contacts to the de-excitation contact when the breaker is switched
off The field circuit-breaker is switched on by electromagnetic force and is kept switched
on by a mechanical latch When the latch is released by a trip coil the circuit-breaker
opens The circuit-breaker also has auxiliary contacts that report its status
52 Field Flashing
In shunt supplied excitation circuits (excitation transformer connected to the
generator terminals) the generator does not have enough remnant voltage for a generator
voltage build-up via the converter In this case a field flashing circuit is provided It
consists of the field flashing contactor the diode bridge and a transformer used to adapt
the auxiliary input voltage to the voltage needed for field flashing when power is
supplied from the auxiliaries network
Fig Field Flashing
Because the field flashing contactor is not able to switch off the energy stored in
the field the control ensures that the contactor can only reopen if the field circuit breaker
has already been opened (generating the TRIP order) or in a normal field flashing
sequence when the converter has taken over the field current Field flashing occurs in the
following stages
1048729The excitation is switched on closing the field flashing contactor ( Field
Circuit Breaker is already closed )
1048729The start-up excitation current flows through the rotor driving the generator
voltage up to approx 15 U
1048729After about 10 U the firing pulses to the converter are released and it begins
to excite the generator to its rated voltage
1048729After about 30 U the field flashing contactor opens (with no current since
the converter is now supplying the current)
The diode bridge at the input to the field flashing contactor prevents a feed-back
from the converter to the source of field flashing while the contactor is still closed
53 De-excitation
When malfunctions occur the stored field energy must be dissipated as quickly
and safely as possible to protect the generator This is done by the converter the field
circuit-breaker and the de-excitation (discharge) resistor
De-excitation (with opening of the field circuit-breaker) takes place in the following
stages
1048729The converter drives to its inverter limit position (negative ceiling voltage)
recovers a portion of the field energy into the network A trip command is given to the
field circuit breaker
1048729The de-excitation contact closes diverting the field voltage to the de-excitation
resistor
1048729Then immediately the main contacts open building voltage The field voltage
commutates to the de-excitation resistor
1048729The current diminishes at a given time constant TE
(With linear resistance TE = Lf (Rf + Re))
Due to the reversal of the field voltage by the converter the field current
commutates from the main contacts of the field circuit-breaker to the de-excitation
resistor in a very early phase This reversal of the field voltage prevents burn-off on the
main contacts and provides effective protection for the field circuit-breaker Depending
on the operating policy an operational shut-down of the excitation can also be effected
with the field circuit-breaker closed This method is useful mainly when the excitation is
switched on and off frequently In this case the converter is merely driven into the
inverter limit position so that the field energy is recovered into the network The
converter then blocks since it is supplying positive current only
54 Excitation Transformer
The excitation transformer matches the generator voltage to the field voltage
(required ceiling voltage) It also serves as a commutation reactance for the thyristor
converter and as a potential isolator between the network and the excitation circuit In
addition the transformer functions as a current limiter in that it makes it possible to keep
any short circuits in the excitation circuit under better control The excitation transformer
is equipped with temperature monitoring probes which set off an alarm when the
temperature exceeds a first max limit and then trips the excitation if the temperature
continues rising to a second (higher)limit
CHAPTER 6
Monitoring and Protection
61 Excitation Monitoring
611 General Information
The main goal of Excitation Monitoring is to make optimum use of the
redundancies provided in the excitation system and to give alarm whenever a malfunction
makes these redundancies unavailable The field current is monitored to see that it does
not exceed a maximum level and if necessary a switch-over to the MANUAL channel is
initiated In addition the criterion for switching off the field flashing is generated The
excitation Monitoring consists of an autonomous processor system
612 over current Alarms
In the Excitation Monitoring the limits for are set at higher levels than the
settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of
the nominal field current contact R1 and the binary output associated with it are
activated immediately If field current remains gt 110 then after a preset inverse-time
has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs
associated with them are activated Parameters match the measurements for If1 and If2 to
the nominal value for field current so that the internal values can be processed and read as
pu values It can be used to falsify the actual value of the field current If (to raise it) so
as to cause a response from the alarm limits for purposes of testing The processed If
signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As
long as the field current If is above the threshold value 11 Ifn its peak value is
measured This is stored (until RESET) and can be read at any time on the Micro-
Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever
the integrated time-current value (intisup2dt) exceeds the preselected reference value the
alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously
issued to switch over to the stand by AUTO channel Software switch F758 enables the
three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-
time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the
desired limit curve for response is set using the factor F216
613 Switch-Off Criterion for Field Flashing
The Excitation Monitoring supplies the criterion for switching off the field
flashing Whether this criterion is activated based on the actual value for generator voltage
Ug or for field current If or both depends on the settings of the two threshold values
F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements
Ug12 and If12 are switched over depending on the present status of the channels
(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is
fixed at ldquological 1rdquo
614 Storage of Alarm Status
The outputs of the over current alarms (R1 R2 R0) and the messages NO
FAILURE MONITORING PARAMETERS CHANGED are stored messages can be
erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the
front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the
situation causing the alarm or the malfunction is no longer present Whenever the self-
diagnosis equipment in the processor detects a malfunction the output NO FAILURE of
MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo
is activated whenever parameters or settings of software switches have been changed via
the Micro-Terminal
615 Actual Value Monitoring
The actual values for generator voltage Ug synchronous voltage Usyn and field
current If are monitored for malfunctions This monitoring is active regardless of whether
or not the generator is in operation Essentially when the generator is in operation the
measurements are monitored by comparing the signals (the smaller signal reading is
detected as incorrect) When the generator is not in operation the measured data are
monitored for extreme values The percentage of deviation permissible in the
measurement signals being compared
(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and
F20AIf the excitation transformer is being supplied from an auxiliary power source (no
shunt operation) the values of Ug and Usyn will be different in some operational
conditions
In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1
and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the
messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2
and the binary message from CH1 reports no malfunction a malfunction on
Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is
also present whenever the binary message CH1 DISTURBANCE is reported and a
discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2
FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares
Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the
comparison channel reports a malfunction or whenever both binary messages report no
malfunction - but both secondary monitors report a malfunction As long as the secondary
monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction
signals for the measurement channel involved (suspicion that there is a corresponding
error in Usyn) The measurement channel malfunctions are enabled operationally
whenever after excitation has been switched on generator voltage Ug exceeds the value
set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is
switched off to see that they do not exceed the limit value F210 that applies to both of
them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt
the operating range) Monitoring for extreme values is likewise enabled during normal
operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the
malfunction signals to the binary outputs Basically the monitoring of the actual values
for If1If2 functions like that of the Ug1Ug2 monitoring
62 Excitation Protection
621 General Information
The Excitation Protection switches off the excitation (and de-excites the machine
rapidly) whenever a danger arises that threatens the excitation transformer the converter
or the generator Generally limiter or monitoring functions precede the emergency trips
and these normally respond before the Excitation Protection must initiate a trip
Protective trip commands are issued directly to the field circuit-breaker from potential
free contacts of the board via the trip relays They are directed redundantly to the
operative field circuit-breaker ldquoOFFrdquo command
622 Protection against Excitation Transformer Overheating
This equipment monitors the excitation transformer for overheating in the
windings that could result from over current short-circuits or inadequate cooling The
monitoring uses temperature monitoring modules in conjunction with temperature
sensors built into the windings Normally the temperature is monitored in two stages the
first stage sets off an alarm the second causes a trip of the excitation
623 Rotor Over voltage Protection
Malfunctions in the generator circuit (eg terminal short-circuit failed
synchronization asynchronous operation) cause induced negative field currents that
produce high voltages in the field circuit These must be restricted to a level with a
sufficient safety margin below the insulation capacity of the field winding (test voltage)
and also below the peak blocking voltage of the converter thyristors The crow bar
employs spark gap elements to detect over voltages in the field circuit Whenever they
respond the associated thyristors are fired immediately switching the de-excitation
resistor parallel to the field The de-excitation current generated thereby initiates an
excitation trip via a supervision circuit causing an immediate opening of the field circuit-
breaker The malfunction isets off an alarm and an internal malfunction is indicated at the
cubicle
TEST VALUES OBTAINED WHEN EXCITATION IS RAISED
TEST
SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE
1 VREF 996 100
2 VACT 997 1003
3 IFACT 735 765
4 IGACT 703 707
5 ACTIVE
POWER
703 705
6 REACTIVE
POWER
101 142
7 POWER
FACTOR
099 IND 098 IND
8 ACTIVE
CURRENT(IR)
705 703
9 REACTIVE
CURRENT(IX)
102 138
10 POWER
ANGLE
566 546
11 FIRING
ANGLE
640 632
VALUES OBTAINED WHEN EXCITATION IS LOWERED
SNO PARTICULARS ACTUAL
VALUE
OBTAINED VALUE
1 VREF 100 997
2 VACT 100 997
3 IFACT 787 761
4 IGACT 839 837
5 ACTIVE
POWER
839 830
6 REACTIVE
POWER
155 110
7 POWER
FACTOR
098 IND 099 IND
8 ACTIVE
CURRENT(IR)
835 830
9 REACTIVE
CURRENT(IX)
140 94
10 POWER
ANGLE
613 DEG 631 DEG
11 FIRING
ANGLE
601 DEG 599 DEG
CONCLUSION
For generating the EMF in stator winding excitation is required to the rotor of a
generator There are two types of excitation
1 Static excitation system
2 Brushless excitation system
A certain disadvantage in brushless excitation system is the slow response time of
the field in case of fast load changes specified No slip-rings and brushes direct
measurements of the field parameters not possible
To avoid all loses static excitation is used Since it does not have any rotating
parts mechanical loses and windage loses This system has fast response and speed
control While preferring this excitation system there are no limitations for the
redundancy of Thyristor bridge circuits
Static excitation has fast field discharge by resistor and inverter operation direct
measurement of field quantity is possible The meaning of excitation is nothing but
continuous supply of DC current (ie field current) to the rotor to buildup required
output voltage in the stator
Field current is changed with respect to the change of load so the digital
automatic voltage regulator (DAVR) is used to regulate the output voltage according to
the load variations
So we conclude that static excitation system with DAVR is preferred since it is
having excellent dynamic performance and better options for R amp M
22 Basic Structure of the Processor Systems
7
The signal processors 25 analog inputoutput modules Each of these processor
systems has a common bus circuit and output and the control lines There is a specific
range of addresses assigned to each assignment Board including the power supply bus
the address lines the two data lines to the input calculates the reactive current (I sin φ)
and the active current ( Icos φ) With these two channel processor Synchronized with
these interrupts (ie with the phase positions of current Ig the field current If and the
synchronous voltage Usyn From the exchange data with the microprocessor card across
the two data lines generator voltage Ug) this processor measures the generator current
Ig and then hardwired connections or multi-conductor cables Binary and analog
inputoutput modules ie for galvanic isolation and adaptation to the electronics level
The most important input interrupts per period to trigger the cycles for processing actual
values in the AUTOMATIC module on the processor bus) for filtering and further
processing
Monitoring each consist of the central microprocessor module and binary and
parameters to the AUTOMATIC channel are the generator voltage Ug the generator
peripheral unit Ug Ig and Usyn are sent to the Interrupt Generator (plug-in peripheral
units (wall-mounted units) peripheral units are used for preprocessing signals from
external measurement circuits power supply units Signals are exchanged among these
processor systems via processed across separate peripheral units for each channel These
processor working on the bus (a house address that can be adjusted using a switch)
systems The AUTOMATIC channel the programmable controls and the Excitation The
actual values measured from AUTOMATIC channel and MANUAL channel are The
AUTOMATIC channel and the MANUAL channel each have their own The digital
voltage regulator is broken down into several autonomous microprocessor The inputs and
outputs of the processor systems are directed across voltage-isolating The Interrupt
Generator also uses the 3-phase Ug signal to generate the 12 themselves contain a limited
number of hardware inputs and outputs with fixed equipment Whenever addresses from
this range are called up the signal processing module can results the processor is then
able to derive further operating parameters such as the load angle the active power etc
The functions of all microprocessor systems other than the programmable controls
have been accomplished in firmware The non-varying standard function modules can be
configured to the design desired for plant-specific purposes using software switches
(KFlags) Thus for example the stored status of a K-Flag determines whether or not a
Limiter is active and whether the de-excitation or the excitation limiters take precedence
Because these K-flags determine the software Scope of Supply for the installation they
cannot be changed permanently via the Micro-Terminal In this way they differ from
such setting data as the values of the parameters for the PID filter of the voltage regulator
or the set-points for the limiters These values can be permanently changed using the
Micro-Terminal Communication is possible with each of the processor systems via the
Micro-Terminal by plugging on the connecting cable In this way signals within the
processor and setting parameters can be viewed analog signals can be issued and the set
parameters can be altered temporarily (F range) or permanently (C range) Unlike the
other processor systems the programmable controls do not include any firmware for
realization of the functions They have been designed so that the designer can adapt and
change their functions easily using the ldquoFunctional Block Programming Language P10
Digital and analog functions can be implemented in practically any degree of complexity
desired using the P10 functional blocks The control variable of the voltage regulator
(AUTOMATIC channel) and the control variable of the field current regulator
(MANUAL channel) are each processed in separate Gate Control Unit and formed into a
chain of pulses at the appropriate firing angle The pulses of the active channel are
directed to the pulse bus via the associated Intermediate Pulse Stage The pulses for each
converter block are amplified sufficiently in Final Pulse Stage to fire the Thyristor
231 General Information
The functions of the automatic voltage regulator AVR are
1 to regulate the generator voltage
2 to regulate the effect of the reactive andor active current on the voltage
3 to limit VoltHz
4 to limit max and min field current
5 to limit inductive stator current
6 to limit capacitive stator current
7 to limit the load angle
8 to stabilize the power system
Block Diagram shows the software structure of AUTOMATIC channel The
generator limiters not provided for the installation in question (optional equipment) are
identified in this overview as ldquoNot Suppliedrdquo The parameter values signal values and
software switches (flags) marked with addresses (hexadecimal numbers) can be viewed
and altered via the Micro-Terminal The values selected are displayed in sec pu Hz
etc and can where necessary be changed directly in these formats The plant-specific
settings of the variables and the flags can be obtained from the Test and Commissioning
Report This block diagram provides information about the important functions and
possible settings of the AUTOMATIC channel For the sake of clarity no detailed
presentation has been given of special functions such as tracking circuits initializations
etc The page heading cross-refers this overview to the various sheets of the schematic
diagram Binary signals are shown in broken lines analog signals in solid lines The
corresponding text designations in the schematic diagram can be used for identification of
the input signals (hardware inputs) The only analog output signal from the automatic
voltage regulator control variable Ucontr is sent via the data bus (CRU bus) to the Gate
Control Unit Most of the binary messages (outputs) from the AVR are of no interest
functionally and they have been omitted for the sake of clarity The basic structure of the
digital voltage regulator and the limiters is simple This is necessary in order that the
behavior of the regulatorslimiters will remain calculable and understandable in all
operating situations and that there will be no problem in adjusting and optimizing them
The central PID filter in the digital voltage regulator defines the dynamic response of the
closed-loop controls both in the voltage regulator mode and after limiters have
intervened The ldquocontrol deviationrdquo at the input to the PID filter is either the control
deviation for voltage the control deviation of a de-excitation limiter (the value
determined by minimum value selection) or the control deviation of an excitation limiter
(the value determined by maximum value selection) Flag F730 (ldquoPRIORrdquo) is used to
determine whether the exciting (Min value) or the de-exciting signal takes precedence on
the minmax value limiter (normally F730 = 1111 ie the de-exciting signal takes
precedence) With the exception of the Minimum Field Current Limiter all other limiters
have variable factoring multipliers of the signal outputs so that they can be adjusted
individually together with the common PID filter which has been optimized for voltage
regulation The setting parameters for this PID filter are as follows
Vo = KR Static amplification
1
Ta = ---- Integration time constant
Tc1
Vp Proportional amplification
1
Tb = ---- Differential time constant
Tc2
Vinfin Amplification of high frequencies
The BODE diagram below shows the assignment of settings in accordance with
DINIEC standards based on a typical example
The PID filter amplifications Vo Vp and Vinfin can be adjusted in pu values But
the ldquoceiling factorrdquo pl+ must be adjusted correctly with parameter F310 if the total
amplification (circuit amplification) of the control circuit is actually to conform to the
pu settings This factor must agree with the ldquoexternalrdquo amplification ie with the
ceiling value of the transformer- converter circuit
Ceiling factor(pl+) = Ufmax Ufo
in which Ufmax = ceiling field voltage
Ufo = no-load field voltage
To attain a suitable response of the AVR when starting excitation
(ldquoEXCITATION ONrdquo) it may be necessary to change the proportional amplification of
the regulator during this phase Vp2 (transiently activated) and Vp1 (permanently
activated) can be adjusted for this purpose For example the value of Vp2 takes effect
immediately once the excitation is switched on and remains effective for a period as set at
F30C Once the period F30C (eg 5 sec) has expired Vp shifts over to Vp1 (becomes
the steady-state Vp) at the rate of change set The standard operating mode for the PID
filter is voltage regulation for which the discrepancy between the voltage set-point and
the current value for generator voltage Ug (the control deviation) is supplied at the input
To compensate for the voltage drop in the block transformer or whenever several
generators are operating to the same distributing bus the generator voltage must be
varied in proportion to the measured generator current (droop influence) To accomplish
this the voltage set-point is varied as a function of the measured reactive current IX
andor active current IR Flag F712 enables the IX droop Flag F710 the IR droop The
desired compensation is set in F282 and F286 respectively Flags F284 and F288 are used
to select whether this droop influence is to increase the voltage or to reduce it
(compensation) Combined influence of the active and reactive currents is attained by
enabling both droops IX and IR Flag F716 activates a so-called ldquoSoft-Startrdquo at the
starting of excitation This ldquoSoft-Startrdquo ensures that the voltage set-point integrates from
0 to 100 within the time set on F290 when the excitation is switched on
(ldquoEXCITATION ONrdquo) A ldquosmoothrdquo excitation of the generator can be achieved in this
way whenever there is no demand for a quick excitation
232 Voltage Set-Point
Various signals and settings control and limit the voltage set-point F270 For
example the values of F254 and F252 define the normal operating range possible for set-
point adjustment (eg 90 110) using external control commands (control room local
operatorrsquos panel superposed control system) The effective set-point adjustment rate is
governed The set-point can be set at the values of F250 and F256 by activating
appropriate control commands for ldquoSETrdquo input Enabling Flag F71A and activating a
binary input prior to switching on the excitation (ldquoEXCITATION OFFrdquo) sets the Ug set-
point at the value of UAUX This makes it possible for example to ensure that the
generator voltage will agree exactly with the network voltage after the voltage build-up
An external value with variable amplification can be added to the Ug set point by
enabling F724 (for example for stability tests)
233 Regulator Tracking in MANUAL Operation
Whenever the AUTOMATIC channel is not in operation (the MANUAL channel
is ON) a follow-up equipment ensures a smooth switch-back to the AUTOMATIC
mode will always be possible To track the voltage set-point is shifted by means of
RAISELOWER pulses from the Gate Control Unit so that control variable Ucontr at the
output from the PID filter is held steady and identical to the control variable Ucontr from
the MANUAL channel Because this tracking must react slowly resultant transient
control deviations resulting from the amplification in the PID filter might cause severe
interference with control variable Ucontr
To prevent this the follow-up equipment intervenes on the regulators mixing
point with a corresponding compensation signal
234 Ugf Limiter
At under frequency the Ugf Limiter reduces the generator voltage so as to
prevent saturation effects in the supply and measuring transformers To adjust this
limiter the max permissible generator voltage at rated frequency is defined and set
When any under-frequency occurs the generator voltage is thus reduced in proportion to
that setting
235 Field Current Maximum Limiter
The Field Current Maximum Limiter is provided to protect the generator rotor
from s occurring in steady-state and transient operation High field currents are normally
the result of a sharp drop in network voltage or of an improper raising of the voltage set-
point by the operating staff The field current is held steady at the value TH1 ie at the
maximum thermal value permissible for the excitation circuit and the rotor In order that
the generator can support the power network with its transient overload capacity during
brief collapses in voltage a temporary switch-over is made to the transient limit MAX1
(a higher setting) When the generator or the converter is operating at a reduced capacity
These limits TH1MAX1 can be switched over to the lower settings TH2MAX2 by
activating the corresponding binary signals The switch-over from the thermal limit
TH12 to the transient limit MAX12 can be configured in one of three ways
a) Depending on the over current with -dUdt ENABLE
Flag programming F418 = any setting desired F41A = 0000
This variant enables the transient value MAX12 whenever a collapse of voltage
in the network is detected The ENABLE time is fixed and can be set The example
below shows the typical behavior of the limiter configured in this way
b) Dependent on the time integral with -dUdt ENABLE
Flag programming F418 = inactive F41A = 1111
This variant likewise enables the transient value only when a collapse of network
voltage has been detected However the switch-back to the thermal limit is not made
dependent upon the time itself but on the calculated time integral intisup2dt of the The setting
on Parameter F414 in spu takes into account the time the rotor needs to cool down ie
the rate of temperature change in the case of intermittent operation The example below
shows how the timing of the switch-back to the thermal limit depends on the present
value for intisup2dt
The time integral is based on the formula
Example The setting of =isup2dt equivalent to Version a (F416) at a constant 16 times the
nominal field current for 10 seconds (with TH12 = 105) is
c) Dependent on the time integral without any preconditions
Flag programming F418 = 1111 F41A = 1111
In this variant the transient becomes available without any prior conditions
(without a -dUdt ENABLE) with the time integral intisup2dt
237 Inductive Stator Current Limiter
The Inductive Stator Current Limiter holds the stator current Ig within permissible
limits while the generator is in the ldquoover-excitedrdquo operating range by reducing the field
current accordingly The setting TH (thermal limit) provides the limit against stationary s
that might occur To take advantage of the generatorrsquos transient overload capacity a
switch-over is made to the higher setting MAX The principle of operation of this switch-
over to the value MAX permissible only transiently is identical to that employed for the
field current limiter (refer to the description above) When the drive output from the
turbine is very high stator current may exceed permissible limits even while inductive
loading of the generator is low In this case if the stator current limiter is not kept from
influencing the field current the control circuit will oscillate back and forth between the
Inductive Stator Current Limiter (de-
exciting) and the Capacitive Stator Current Limiter (exciting)The output signal of that
function then dominates the control variable of the Ig-dependent limiter via a maximum
value selection
238 Capacitive Stator Current Limiter
239 Load Angle Limiter
The Load Angle Limiter prevents the synchronous machine from slipping out of
phase due to slippage of the rotor The load angle δ the difference in phase between the
rotor and the stator rotating field results mainly from the driving torque (active power P)
acting on the generator and the level of rotor current (field current) If the driving torque
remains constant a increase in the field current reduces the load angle δ The current load
angle δ at any moment is obtained from the generator current and generator voltage based
on a simplified model of the generator Whenever this calculated load angle δ exceeds the
preset limit angle the limiter increases the field current until the load angle has dropped
back to its permissible value The quadrature reactance Xq of the generator and the
network reactance Xe during normal operation must be adjusted on the regulator in order
to obtain the load angle δ The graph below shows the Power Chart for a salient-pole
machine with typical limiter characteristics
The purpose of a Power System Stabilizer is to use the generator excitation to
damp electromechanical oscillations between the network and the generator Depending
on the design of the generator and the requirements imposed for network stability its
main function will be either to damp the oscillations originating in the machine or those
from the network A synchronous generator working in a combined power network is in
principle an oscillating structure In order to produce a torque the magnetic field of the
rotor and the stator must form a given angle (referred to as the rotor displacement or load
angle δ) The electrical torque ME increases as the angle δ increases just as with a
torsion spring Because the ME of the generator and the mechanical driving torque MA
from the turbine are in equilibrium during steady-state operation the angle δ remains in a
given position Whenever this state of equilibrium between MA and ME is disturbed the
load angle slips of this rest position and change thereby the electrical torque ME The
torque attempts to restore the load angle to a stationary position Due to the mass inertia
of the turbinegenerator rotor however this can only take place aperiodically It does so
in the form of more or less effectively damped oscillations (again similar to the effect of
mass inertia on a torsion spring) In order to damp the oscillations there must be a
damping torque produced depending not on the electrical torque ME associated with the
angle but on the difference in frequency (Df) between the rotor and the stator rotating
field ie on the slippage This torque is produced mainly by the so-called damper
winding in the rotor but the dimensioning of this is subject to limits imposed by
considerations of design and economy Some further action is therefore needed to
increase the damping effect The following drastically simplified formula shows the
parameters upon which the amount of active power PE supplied by the generator
depends
PE = active power
It can be seen from the above relationship that the active power that the generator
transfers depends not only on the load angle δ but also on the field current If That means
that a transient change can be made in the active power PE and with that in the effective
electrical torque ME by varying the field current The principle of operation of the DVR
Power System Stabilizer becomes clear from a consideration of the oscillations in power
output and frequency (ΔPE Δf) and the vector diagram If it is assumed that oscillations
in the network frequency generates load oscillations with the mass inertia of the rotor
then the active load of the generator (eg MW-measured) is influenced with a sinusoidal
value -ΔPE (ME-MA = -ΔPE) By inversion of -ΔPE one obtains the fluctuation in
power provided by the rotor +ΔPE As is known the slip signal Δf follows +ΔPE with a
phase delayed by 90deg The +ME produced by the periodic changes in the load angle δ is
in phase with +ΔPE A good damping is attained if ME is varied in phase with the slip
Δf However this signal must also be advanced somewhat to compensate for the time
constants in the excitation circuit and the generator
As mentioned above the electrical torque ME can be influenced by varying the
field current To accomplish this a suitable control signal referred to as variable
disturbance compensation must be imposed upon the voltage set-point or the converter
control variable Ucontr As can be seen from the vector diagram by applying proper
weighting factors (K1 K2) and then adding together the signals -ΔPE and Δf an overall
stabilization signal can be produced that rotates in advance of the Df signal by any angle
desired between 0deg and 90deg Because the amplitude of -DPE remains proportional to the
amplitude of Δf a constant angle in advance of Δf results for the compensation of the
time constants referred to above The optimum weighting factors K1 and K2 for a
synchronous generator working to a power network depend on its operating point at any
moment and the external reactance of the network Normally the selection of a
compromise setting is good enough to attain stability in all operating points and for all
external reactance For special demands these settings must be parameterized as a
function of the external reactance (which means optional equipment Xe-Identification)
The Power System Stabilizer PSS is a section of the AVR computer program and is
processed once per network cycle The voltage at the generator terminals and the
generator current are measured in order to define the signals ΔPE and Δf The calculated
signals for _P_ (=PE) and Δf are then sent across DC filters ldquoDrdquo (real differentiators) that
transmit only the dynamic portion of the signals The ΔPE and Δf signals obtained in this
way are then weighted (multiplied by) with the factors K1 and K2 and sent to the
summing point of the voltage regulator
The PSS stabilization signal is imposed on the automatic voltage regulator only if
the following prerequisites are met
bull Generator on line
bull Generator power output gt the value F338
bull Generator voltage in a range between F33C and F33A
The stabilization signal is limited at the output from the PSS to the lower and
upper limits Flag defines whether the stabilization signal is introduced before or after the
PID filter (usually before the filter) Because the PID filter as noted above already takes
the ceiling factor Vp1 into account the PSS signal needs to be multiplied by Vp1 if it is
added to the voltage regulator following the PID filter (divider at the input to the
minmax limiter) This precaution prevents the DC filter ldquoDrdquo in the P-channel from
producing an unnecessary ldquostabilizationrdquo effect in the case of rapid changes in turbine
load As an alternative for the AVRrsquos Power System Stabilizer a stabilization signal from
an outside system can be imposed by activating the binary input ldquoPSS-SIGNEXTrdquo Flag
F340 can be used to select between an analog and a 12-bit signal and F33E to select the
polarity desired for that signal
24 The MANUAL Channel
241 Summary
The MANUAL channel (Channel 2) has been built as a simple field current
regulator
without additional limiters Its main function is to maintain the excitation of the generator
even if the AUTOMATIC channel becomes non-operational The MANUAL channel
also performs valuable service for purposes of testing commissioning and preventive
maintenance Its measurements regulator generation of firing pulses and power supply
are physically separate from those on the AUTOMATIC channel
242 Principle of Operation
All the functions of the MANUAL channel including the generation of firing
pulses have been implemented in a single electronic module the Gate Control Unit The
control variable Ucontr of voltage regulator is used as the reference value for generating
firing pulses on the principle known as ldquoramp controlrdquo (Comparison of Ucontr with
Usynsynchronous sawtooth signal) For further processing in the UN 0096 Intermediate
Pulse Stage the Gate Control Unit supplies six firing pulses at its output whose phase
position with respect to the synchronous voltage Usyn is in accordance with control
variable Ucontr An internal linearization ensures that the field voltage produced via the
firing pulses remains proportional to the control variable Ucontr throughout the entire
range As a result the circuit amplification of the control remains constant over the entire
range Whenever excitation is switched ON the set-point for Generator Voltage is set
automatically at the preset - ref Value This provision ensures that the generator voltage
always attains approximately its nominal value after the field flashing The Gate Control
Unit can be refunctioned ( by pre-selection with a switch ) for purposes of testing to act
as a purely firing pulse control In this case the control variable Ucontr is adjusted
directly using the RAISELOWER push buttons on the front of the module In this way
for example the relationship between the phase position of the firing pulses and the
control variable Ucontr can be checked easily
CHAPTER 3
PULSE SECTION
31 Pulse Generation and Amplification
The Gate Control Units of both AUTOMATIC channel and MANUAL channel
each supply six firing pulses for operating the 6-pulse thyristor bridges The low-power
pulse signals from these Gate Control Units are then amplified in the Intermediate Pulse
Stage galvanically isolated and then sent to the common pulse bus On the output end
the Intermediate Pulse Stage of the non-active channel) is always blocked The Gate
Control Units generate the pulses based on microprocessor control The reference voltage
used for the firing pulse phase location is the output voltage from the excitation
transformer (Usyn1 Usyn2) The commutation spikes of the synchronous voltage caused
by the converter are calculated prior to use of the voltage as a reference value and are
deliberately filtered out The lower limit for the firing pulses (double pulses) which are
offset from one another by 60deg is defined by the limit rectifier position (αmin) and the
upper limit by the limit inverter position (αmax) for the firing angle αmin and αmax can
be adjusted on the Gate Control Units using BCD (Binary Coded Decimal) switches
αmin ensures that the firing pulses will not be issued (premature firing) until there is
sufficient positive phase voltage on the thyristor involved αmax prevents a dangerous
ldquotippingrdquo of the thyristor bridge into the rectifier mode if the firing angle α is too large
(ldquolate firingrdquo) The critical factors determining αmax are the overlap time uumlmax (max
commutation time) and the ldquorecovery timerdquo of the thyristors (αmax lt 180deg - uumlmax - γ )
An external control signal can force the firing pulses into their inverter limit position
Other binary inputs can block or direct the firing pulses of the Gate Control Units so as to
produce freewheeling on the thyristor bridge During freewheeling the firing pulses for
the thyristor pair R and S are blocked and the pulse signals T+T- are engaged with
chains of pulses Both Gate Control Units (for the MANUAL amp AUTOMATIC channels)
contain a field current monitor that blocks the firing pulses immediately whenever the
current exceeds a preset threshold level In this case the field circuit-breaker is also
tripped via an output contact The purpose of these provisions is to prevent damage to
thyristors and thyristor fuses in case of a slip-ring short-circuit or to keep any damage
that does occur to a minimum The pulse signals are galvanically separated at the outputs
from the Intermediate Pulse Stage (with pulse transmitters) and are then directed to the
common pulse bus This transmission of the pulse signals to the pulse bus via passive
transmitters ensures a high degree of active channel autonomy Practically no possible
malfunctions on the inactive channel (including for example sustained pulses) affect the
active channel
32 Pulse Monitoring
The ldquoPulse Busrdquo and the pulse signals of the AUTOMATIC channel are
monitored This monitoring device consists of potential isolating stages and the common
monitor If the pulse monitoring of the ldquoPulse Busrdquo responds a switch-over is made to
MANUAL channel The function of the potential isolating stages is to couple the pulse
monitoring device to the pulse circuits without any feedback effect The pulse monitoring
checks the six pulse lines for the following malfunctions continuous or periodic failure
of one or more pulses Periodic occurrence of synchronous or asynchronous false pulses
Continuous pulses the pulse monitoring device can be tested while the machine is in
operation
CHAPTER 4
CONVERTER
Thyristor
The term thyristor usually refers to a family of four layer solid state device having
turn on characteristics that can be externally controlled by either current or voltage They
are also referred to as breakdown device because their working depends on avalanche
breakdown Thyristors have only two stages OFF and ON Thyristors have a similar
function to Uni-junctions they act as switches Thyristors use current flow as a switch
Thyristors have three states
1 Reverse blocking mode mdash Voltage is applied in the direction that would be
blocked by a diode
2 Forward blocking mode mdash Voltage is applied in the direction that would cause
a diode to conduct but the thyristor has not yet been triggered into conduction
3 Forward conducting mode mdash The thyristor has been triggered into conduction
and will remain conducting until the forward current drops below a threshold value
known as the holding current Converter is a semiconductor device which converts ac
input voltage into a constant dc output voltage In present excitation system three phase
fully controlled thyristor converter is used
Because of the following advantages thyristor converters are used
a) Thyristors are used for high power applications ie up to 10Kv3500A1KHz
b) Having high reliability and low losses
c) Uni-directional device like diode
d) Itrsquos operation as a rectifier which are low resistance in forward conduction
mode and high resistance in reverse conduction mode
PROTECTION OF THYRISTORS
For reliable operation of a thyristor demands that its specified ratings are not
exceeded When Subjected to or over voltages During the turn - on of SCR didt
prohibitively large False triggering of SCR by high value of dvdt andSpurious signals
between gate and cathode may leads to unwanted turn ndash on
DIDT AND PROTECTION
When thyristor starts conducting in forward conduction mode and is turned on by
gate pulse The anode current increases rapidly whole area of the gate to Cathode
junction then hot spots will be formed near the gate connection this locality of heating
destroys the thyristor Thyristor thermal time is constant The causes due to faults and
short circuits or surge currents Electronic crowbar protection is used against the over
voltages The rate rise of anode current must be kept at the time of turn on below the
rated or specified limiting value The didt value maintained below limited value by using
a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating
is avoided by applying gate current but not greater the maximum gate current
DVDT AND OVER VOLTAGE PROTECTION
With forward voltage across the anode and cathode of a thyristor the two outer
junctions are forward biased but the inner junction is reverse biased This reverse biased
junction J2 has the characteristics of a capacitor due to charges existing across the
junction In other words space-charges exist in the depletion region around junction J2
and therefore junction J2 behaves like a capacitance If the entire anode to cathode
forward voltage Va appears across J2 junction and the charge is denoted by Q then a
charging current i given by Eq (46) follows
i = dQdt =d(Cj Va )dt
= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)
As Cj the capacitance of junction J2 is almost constant the current is given by
i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)
If the rate of rise of forward voltage dVadt is high the charging current i will be
more This charging current plays the role of gate current and turns on the SCR even
when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on
must be avoided as it leads to false operation of the thyristor circuit
For controllable operation of the thyristor the rate of rise of forward anode to
cathode voltage dVadt must be kept below the specified rated limit Typical values of
dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by
using a snubber circuit in parallel with the device thyristor are very sensitive for over
voltage than the semiconductor devices
Over voltage transients are perhaps the main cause of thyristor failure
In thyristor there are mainly two types
1 Internal over voltages
Due to the commutation of the thyristors large voltages are generated internally
Because of the series inductance of the SCR circuit the large transient voltages L didt
produced This voltage several times the break over voltage of the device then thyristor
destroys permanently
2 External over voltages
External over voltages are caused due to the interruptions of current flow in an
inductive circuit and also due to the lightening strokes on the lines feeding the thyristor
system For the reliable operation of thyristor the over voltages must be suppressed by
adopting suitable techniques
Suppression of over voltages
The RC circuit called snubber circuit is connected across the device to protect In
order to keep the protective components to a minimum the thyristors are chosen with
their peak voltages ratings are 25 to 3 times of the normal peak working voltage
ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are
commonly employed for protecting the thyristor circuit against the over voltages
Gate protection
Gate circuit should also be protected against the over voltages and surges Over
voltage at gate circuit can cause false triggering of the SCR may rises the junction
temperature behind specified limit leading to its damage Protection against over
voltage can be achieved by connecting a ZD across the gate circuit and a resister is
connected in series with gate circuit to protect against the s A capacitor and resister are
connected across gate to cathode to by pass the noise
41 Final Pulse Stages
The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage
(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor
bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided
with a power supply module The amplified output pulses from the Final Pulse Stages
start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main
pulse This initial pulse edge assures proper firing of the thyristors being triggered
Subsequently the weaker part of main pulse keeps firing conditions steady As already
mentioned the Final Pulse Stages and their associated thyristor bridges form single units
All six pulse outputs from a Final Pulse Stage can be blocked by an external control
signal so that all thyristors in the associated thyristor bridge will block the current A
blocking of the pulses is initiated whenever there is a malfunction in the associated
thyristor bridge
42 Converter Power Section
The thyristor converter consists of three independent parallel rectifier blocks TY1
to TY3 which are all in service Even if one block fails the remaining blocks take over
automatically the full design current of the excitation circuit During normal operation
(with ideal current share) and all three bridges in operation each of these blocks has to
carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation
is limited Only when all three bridges fail the excitation is switched off Each thyristor
bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any
arm is identified by a Current flow monitoring module
43 Converter Cooling
A cooling system is needed to dissipate heat losses in the converter blocks and
electronics Each converter block has therefore been equipped with a fan supplied with
power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing
cubicle) The fans are protected with motor protection circuit breakers An air flow
monitoring unit is provided for monitoring the air flow through the thyristor bridge If a
circuit breaker failure is detected or if the air flow monitor drops off at one of the
thyristor bridges the bridge involved is immediately set out of operation by blocking its
firing pulses
44 Thyristor Converter Monitoring
A thyristor bridge in which defects occur that could threaten the safety of
operation or cause secondary damage is switched off automatically ie its firing pulses
are blocked This happens whenever A thyristor fuse is blown The fuses are monitored
individually with micro switches The Final Pulse Stage fails which is detected by
internal monitors (supply voltage sustained pulse short-circuit on the output end) The
power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or
is insufficient Isolator on ACDC side is open
CHAPTER 5
Field Current Circuit Excitation Transformer
51 Field Circuit Breaker
The circuit-breaker in the field circuit is used to isolate the field circuit from the
converter It is capable of switching off the synchronous machine from full load under the
maximum conditions of a 3-phase short-circuit In addition to its main contacts the field
circuit-breaker also has a de-excitation contact with which the field energy stored in the
field can be dissipated across the de-excitation resistor The de-excitation contact closes
shortly before the main contacts open so as to ensure proper commutation of the field
current from the main contacts to the de-excitation contact when the breaker is switched
off The field circuit-breaker is switched on by electromagnetic force and is kept switched
on by a mechanical latch When the latch is released by a trip coil the circuit-breaker
opens The circuit-breaker also has auxiliary contacts that report its status
52 Field Flashing
In shunt supplied excitation circuits (excitation transformer connected to the
generator terminals) the generator does not have enough remnant voltage for a generator
voltage build-up via the converter In this case a field flashing circuit is provided It
consists of the field flashing contactor the diode bridge and a transformer used to adapt
the auxiliary input voltage to the voltage needed for field flashing when power is
supplied from the auxiliaries network
Fig Field Flashing
Because the field flashing contactor is not able to switch off the energy stored in
the field the control ensures that the contactor can only reopen if the field circuit breaker
has already been opened (generating the TRIP order) or in a normal field flashing
sequence when the converter has taken over the field current Field flashing occurs in the
following stages
1048729The excitation is switched on closing the field flashing contactor ( Field
Circuit Breaker is already closed )
1048729The start-up excitation current flows through the rotor driving the generator
voltage up to approx 15 U
1048729After about 10 U the firing pulses to the converter are released and it begins
to excite the generator to its rated voltage
1048729After about 30 U the field flashing contactor opens (with no current since
the converter is now supplying the current)
The diode bridge at the input to the field flashing contactor prevents a feed-back
from the converter to the source of field flashing while the contactor is still closed
53 De-excitation
When malfunctions occur the stored field energy must be dissipated as quickly
and safely as possible to protect the generator This is done by the converter the field
circuit-breaker and the de-excitation (discharge) resistor
De-excitation (with opening of the field circuit-breaker) takes place in the following
stages
1048729The converter drives to its inverter limit position (negative ceiling voltage)
recovers a portion of the field energy into the network A trip command is given to the
field circuit breaker
1048729The de-excitation contact closes diverting the field voltage to the de-excitation
resistor
1048729Then immediately the main contacts open building voltage The field voltage
commutates to the de-excitation resistor
1048729The current diminishes at a given time constant TE
(With linear resistance TE = Lf (Rf + Re))
Due to the reversal of the field voltage by the converter the field current
commutates from the main contacts of the field circuit-breaker to the de-excitation
resistor in a very early phase This reversal of the field voltage prevents burn-off on the
main contacts and provides effective protection for the field circuit-breaker Depending
on the operating policy an operational shut-down of the excitation can also be effected
with the field circuit-breaker closed This method is useful mainly when the excitation is
switched on and off frequently In this case the converter is merely driven into the
inverter limit position so that the field energy is recovered into the network The
converter then blocks since it is supplying positive current only
54 Excitation Transformer
The excitation transformer matches the generator voltage to the field voltage
(required ceiling voltage) It also serves as a commutation reactance for the thyristor
converter and as a potential isolator between the network and the excitation circuit In
addition the transformer functions as a current limiter in that it makes it possible to keep
any short circuits in the excitation circuit under better control The excitation transformer
is equipped with temperature monitoring probes which set off an alarm when the
temperature exceeds a first max limit and then trips the excitation if the temperature
continues rising to a second (higher)limit
CHAPTER 6
Monitoring and Protection
61 Excitation Monitoring
611 General Information
The main goal of Excitation Monitoring is to make optimum use of the
redundancies provided in the excitation system and to give alarm whenever a malfunction
makes these redundancies unavailable The field current is monitored to see that it does
not exceed a maximum level and if necessary a switch-over to the MANUAL channel is
initiated In addition the criterion for switching off the field flashing is generated The
excitation Monitoring consists of an autonomous processor system
612 over current Alarms
In the Excitation Monitoring the limits for are set at higher levels than the
settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of
the nominal field current contact R1 and the binary output associated with it are
activated immediately If field current remains gt 110 then after a preset inverse-time
has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs
associated with them are activated Parameters match the measurements for If1 and If2 to
the nominal value for field current so that the internal values can be processed and read as
pu values It can be used to falsify the actual value of the field current If (to raise it) so
as to cause a response from the alarm limits for purposes of testing The processed If
signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As
long as the field current If is above the threshold value 11 Ifn its peak value is
measured This is stored (until RESET) and can be read at any time on the Micro-
Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever
the integrated time-current value (intisup2dt) exceeds the preselected reference value the
alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously
issued to switch over to the stand by AUTO channel Software switch F758 enables the
three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-
time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the
desired limit curve for response is set using the factor F216
613 Switch-Off Criterion for Field Flashing
The Excitation Monitoring supplies the criterion for switching off the field
flashing Whether this criterion is activated based on the actual value for generator voltage
Ug or for field current If or both depends on the settings of the two threshold values
F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements
Ug12 and If12 are switched over depending on the present status of the channels
(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is
fixed at ldquological 1rdquo
614 Storage of Alarm Status
The outputs of the over current alarms (R1 R2 R0) and the messages NO
FAILURE MONITORING PARAMETERS CHANGED are stored messages can be
erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the
front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the
situation causing the alarm or the malfunction is no longer present Whenever the self-
diagnosis equipment in the processor detects a malfunction the output NO FAILURE of
MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo
is activated whenever parameters or settings of software switches have been changed via
the Micro-Terminal
615 Actual Value Monitoring
The actual values for generator voltage Ug synchronous voltage Usyn and field
current If are monitored for malfunctions This monitoring is active regardless of whether
or not the generator is in operation Essentially when the generator is in operation the
measurements are monitored by comparing the signals (the smaller signal reading is
detected as incorrect) When the generator is not in operation the measured data are
monitored for extreme values The percentage of deviation permissible in the
measurement signals being compared
(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and
F20AIf the excitation transformer is being supplied from an auxiliary power source (no
shunt operation) the values of Ug and Usyn will be different in some operational
conditions
In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1
and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the
messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2
and the binary message from CH1 reports no malfunction a malfunction on
Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is
also present whenever the binary message CH1 DISTURBANCE is reported and a
discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2
FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares
Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the
comparison channel reports a malfunction or whenever both binary messages report no
malfunction - but both secondary monitors report a malfunction As long as the secondary
monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction
signals for the measurement channel involved (suspicion that there is a corresponding
error in Usyn) The measurement channel malfunctions are enabled operationally
whenever after excitation has been switched on generator voltage Ug exceeds the value
set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is
switched off to see that they do not exceed the limit value F210 that applies to both of
them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt
the operating range) Monitoring for extreme values is likewise enabled during normal
operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the
malfunction signals to the binary outputs Basically the monitoring of the actual values
for If1If2 functions like that of the Ug1Ug2 monitoring
62 Excitation Protection
621 General Information
The Excitation Protection switches off the excitation (and de-excites the machine
rapidly) whenever a danger arises that threatens the excitation transformer the converter
or the generator Generally limiter or monitoring functions precede the emergency trips
and these normally respond before the Excitation Protection must initiate a trip
Protective trip commands are issued directly to the field circuit-breaker from potential
free contacts of the board via the trip relays They are directed redundantly to the
operative field circuit-breaker ldquoOFFrdquo command
622 Protection against Excitation Transformer Overheating
This equipment monitors the excitation transformer for overheating in the
windings that could result from over current short-circuits or inadequate cooling The
monitoring uses temperature monitoring modules in conjunction with temperature
sensors built into the windings Normally the temperature is monitored in two stages the
first stage sets off an alarm the second causes a trip of the excitation
623 Rotor Over voltage Protection
Malfunctions in the generator circuit (eg terminal short-circuit failed
synchronization asynchronous operation) cause induced negative field currents that
produce high voltages in the field circuit These must be restricted to a level with a
sufficient safety margin below the insulation capacity of the field winding (test voltage)
and also below the peak blocking voltage of the converter thyristors The crow bar
employs spark gap elements to detect over voltages in the field circuit Whenever they
respond the associated thyristors are fired immediately switching the de-excitation
resistor parallel to the field The de-excitation current generated thereby initiates an
excitation trip via a supervision circuit causing an immediate opening of the field circuit-
breaker The malfunction isets off an alarm and an internal malfunction is indicated at the
cubicle
TEST VALUES OBTAINED WHEN EXCITATION IS RAISED
TEST
SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE
1 VREF 996 100
2 VACT 997 1003
3 IFACT 735 765
4 IGACT 703 707
5 ACTIVE
POWER
703 705
6 REACTIVE
POWER
101 142
7 POWER
FACTOR
099 IND 098 IND
8 ACTIVE
CURRENT(IR)
705 703
9 REACTIVE
CURRENT(IX)
102 138
10 POWER
ANGLE
566 546
11 FIRING
ANGLE
640 632
VALUES OBTAINED WHEN EXCITATION IS LOWERED
SNO PARTICULARS ACTUAL
VALUE
OBTAINED VALUE
1 VREF 100 997
2 VACT 100 997
3 IFACT 787 761
4 IGACT 839 837
5 ACTIVE
POWER
839 830
6 REACTIVE
POWER
155 110
7 POWER
FACTOR
098 IND 099 IND
8 ACTIVE
CURRENT(IR)
835 830
9 REACTIVE
CURRENT(IX)
140 94
10 POWER
ANGLE
613 DEG 631 DEG
11 FIRING
ANGLE
601 DEG 599 DEG
CONCLUSION
For generating the EMF in stator winding excitation is required to the rotor of a
generator There are two types of excitation
1 Static excitation system
2 Brushless excitation system
A certain disadvantage in brushless excitation system is the slow response time of
the field in case of fast load changes specified No slip-rings and brushes direct
measurements of the field parameters not possible
To avoid all loses static excitation is used Since it does not have any rotating
parts mechanical loses and windage loses This system has fast response and speed
control While preferring this excitation system there are no limitations for the
redundancy of Thyristor bridge circuits
Static excitation has fast field discharge by resistor and inverter operation direct
measurement of field quantity is possible The meaning of excitation is nothing but
continuous supply of DC current (ie field current) to the rotor to buildup required
output voltage in the stator
Field current is changed with respect to the change of load so the digital
automatic voltage regulator (DAVR) is used to regulate the output voltage according to
the load variations
So we conclude that static excitation system with DAVR is preferred since it is
having excellent dynamic performance and better options for R amp M
The signal processors 25 analog inputoutput modules Each of these processor
systems has a common bus circuit and output and the control lines There is a specific
range of addresses assigned to each assignment Board including the power supply bus
the address lines the two data lines to the input calculates the reactive current (I sin φ)
and the active current ( Icos φ) With these two channel processor Synchronized with
these interrupts (ie with the phase positions of current Ig the field current If and the
synchronous voltage Usyn From the exchange data with the microprocessor card across
the two data lines generator voltage Ug) this processor measures the generator current
Ig and then hardwired connections or multi-conductor cables Binary and analog
inputoutput modules ie for galvanic isolation and adaptation to the electronics level
The most important input interrupts per period to trigger the cycles for processing actual
values in the AUTOMATIC module on the processor bus) for filtering and further
processing
Monitoring each consist of the central microprocessor module and binary and
parameters to the AUTOMATIC channel are the generator voltage Ug the generator
peripheral unit Ug Ig and Usyn are sent to the Interrupt Generator (plug-in peripheral
units (wall-mounted units) peripheral units are used for preprocessing signals from
external measurement circuits power supply units Signals are exchanged among these
processor systems via processed across separate peripheral units for each channel These
processor working on the bus (a house address that can be adjusted using a switch)
systems The AUTOMATIC channel the programmable controls and the Excitation The
actual values measured from AUTOMATIC channel and MANUAL channel are The
AUTOMATIC channel and the MANUAL channel each have their own The digital
voltage regulator is broken down into several autonomous microprocessor The inputs and
outputs of the processor systems are directed across voltage-isolating The Interrupt
Generator also uses the 3-phase Ug signal to generate the 12 themselves contain a limited
number of hardware inputs and outputs with fixed equipment Whenever addresses from
this range are called up the signal processing module can results the processor is then
able to derive further operating parameters such as the load angle the active power etc
The functions of all microprocessor systems other than the programmable controls
have been accomplished in firmware The non-varying standard function modules can be
configured to the design desired for plant-specific purposes using software switches
(KFlags) Thus for example the stored status of a K-Flag determines whether or not a
Limiter is active and whether the de-excitation or the excitation limiters take precedence
Because these K-flags determine the software Scope of Supply for the installation they
cannot be changed permanently via the Micro-Terminal In this way they differ from
such setting data as the values of the parameters for the PID filter of the voltage regulator
or the set-points for the limiters These values can be permanently changed using the
Micro-Terminal Communication is possible with each of the processor systems via the
Micro-Terminal by plugging on the connecting cable In this way signals within the
processor and setting parameters can be viewed analog signals can be issued and the set
parameters can be altered temporarily (F range) or permanently (C range) Unlike the
other processor systems the programmable controls do not include any firmware for
realization of the functions They have been designed so that the designer can adapt and
change their functions easily using the ldquoFunctional Block Programming Language P10
Digital and analog functions can be implemented in practically any degree of complexity
desired using the P10 functional blocks The control variable of the voltage regulator
(AUTOMATIC channel) and the control variable of the field current regulator
(MANUAL channel) are each processed in separate Gate Control Unit and formed into a
chain of pulses at the appropriate firing angle The pulses of the active channel are
directed to the pulse bus via the associated Intermediate Pulse Stage The pulses for each
converter block are amplified sufficiently in Final Pulse Stage to fire the Thyristor
231 General Information
The functions of the automatic voltage regulator AVR are
1 to regulate the generator voltage
2 to regulate the effect of the reactive andor active current on the voltage
3 to limit VoltHz
4 to limit max and min field current
5 to limit inductive stator current
6 to limit capacitive stator current
7 to limit the load angle
8 to stabilize the power system
Block Diagram shows the software structure of AUTOMATIC channel The
generator limiters not provided for the installation in question (optional equipment) are
identified in this overview as ldquoNot Suppliedrdquo The parameter values signal values and
software switches (flags) marked with addresses (hexadecimal numbers) can be viewed
and altered via the Micro-Terminal The values selected are displayed in sec pu Hz
etc and can where necessary be changed directly in these formats The plant-specific
settings of the variables and the flags can be obtained from the Test and Commissioning
Report This block diagram provides information about the important functions and
possible settings of the AUTOMATIC channel For the sake of clarity no detailed
presentation has been given of special functions such as tracking circuits initializations
etc The page heading cross-refers this overview to the various sheets of the schematic
diagram Binary signals are shown in broken lines analog signals in solid lines The
corresponding text designations in the schematic diagram can be used for identification of
the input signals (hardware inputs) The only analog output signal from the automatic
voltage regulator control variable Ucontr is sent via the data bus (CRU bus) to the Gate
Control Unit Most of the binary messages (outputs) from the AVR are of no interest
functionally and they have been omitted for the sake of clarity The basic structure of the
digital voltage regulator and the limiters is simple This is necessary in order that the
behavior of the regulatorslimiters will remain calculable and understandable in all
operating situations and that there will be no problem in adjusting and optimizing them
The central PID filter in the digital voltage regulator defines the dynamic response of the
closed-loop controls both in the voltage regulator mode and after limiters have
intervened The ldquocontrol deviationrdquo at the input to the PID filter is either the control
deviation for voltage the control deviation of a de-excitation limiter (the value
determined by minimum value selection) or the control deviation of an excitation limiter
(the value determined by maximum value selection) Flag F730 (ldquoPRIORrdquo) is used to
determine whether the exciting (Min value) or the de-exciting signal takes precedence on
the minmax value limiter (normally F730 = 1111 ie the de-exciting signal takes
precedence) With the exception of the Minimum Field Current Limiter all other limiters
have variable factoring multipliers of the signal outputs so that they can be adjusted
individually together with the common PID filter which has been optimized for voltage
regulation The setting parameters for this PID filter are as follows
Vo = KR Static amplification
1
Ta = ---- Integration time constant
Tc1
Vp Proportional amplification
1
Tb = ---- Differential time constant
Tc2
Vinfin Amplification of high frequencies
The BODE diagram below shows the assignment of settings in accordance with
DINIEC standards based on a typical example
The PID filter amplifications Vo Vp and Vinfin can be adjusted in pu values But
the ldquoceiling factorrdquo pl+ must be adjusted correctly with parameter F310 if the total
amplification (circuit amplification) of the control circuit is actually to conform to the
pu settings This factor must agree with the ldquoexternalrdquo amplification ie with the
ceiling value of the transformer- converter circuit
Ceiling factor(pl+) = Ufmax Ufo
in which Ufmax = ceiling field voltage
Ufo = no-load field voltage
To attain a suitable response of the AVR when starting excitation
(ldquoEXCITATION ONrdquo) it may be necessary to change the proportional amplification of
the regulator during this phase Vp2 (transiently activated) and Vp1 (permanently
activated) can be adjusted for this purpose For example the value of Vp2 takes effect
immediately once the excitation is switched on and remains effective for a period as set at
F30C Once the period F30C (eg 5 sec) has expired Vp shifts over to Vp1 (becomes
the steady-state Vp) at the rate of change set The standard operating mode for the PID
filter is voltage regulation for which the discrepancy between the voltage set-point and
the current value for generator voltage Ug (the control deviation) is supplied at the input
To compensate for the voltage drop in the block transformer or whenever several
generators are operating to the same distributing bus the generator voltage must be
varied in proportion to the measured generator current (droop influence) To accomplish
this the voltage set-point is varied as a function of the measured reactive current IX
andor active current IR Flag F712 enables the IX droop Flag F710 the IR droop The
desired compensation is set in F282 and F286 respectively Flags F284 and F288 are used
to select whether this droop influence is to increase the voltage or to reduce it
(compensation) Combined influence of the active and reactive currents is attained by
enabling both droops IX and IR Flag F716 activates a so-called ldquoSoft-Startrdquo at the
starting of excitation This ldquoSoft-Startrdquo ensures that the voltage set-point integrates from
0 to 100 within the time set on F290 when the excitation is switched on
(ldquoEXCITATION ONrdquo) A ldquosmoothrdquo excitation of the generator can be achieved in this
way whenever there is no demand for a quick excitation
232 Voltage Set-Point
Various signals and settings control and limit the voltage set-point F270 For
example the values of F254 and F252 define the normal operating range possible for set-
point adjustment (eg 90 110) using external control commands (control room local
operatorrsquos panel superposed control system) The effective set-point adjustment rate is
governed The set-point can be set at the values of F250 and F256 by activating
appropriate control commands for ldquoSETrdquo input Enabling Flag F71A and activating a
binary input prior to switching on the excitation (ldquoEXCITATION OFFrdquo) sets the Ug set-
point at the value of UAUX This makes it possible for example to ensure that the
generator voltage will agree exactly with the network voltage after the voltage build-up
An external value with variable amplification can be added to the Ug set point by
enabling F724 (for example for stability tests)
233 Regulator Tracking in MANUAL Operation
Whenever the AUTOMATIC channel is not in operation (the MANUAL channel
is ON) a follow-up equipment ensures a smooth switch-back to the AUTOMATIC
mode will always be possible To track the voltage set-point is shifted by means of
RAISELOWER pulses from the Gate Control Unit so that control variable Ucontr at the
output from the PID filter is held steady and identical to the control variable Ucontr from
the MANUAL channel Because this tracking must react slowly resultant transient
control deviations resulting from the amplification in the PID filter might cause severe
interference with control variable Ucontr
To prevent this the follow-up equipment intervenes on the regulators mixing
point with a corresponding compensation signal
234 Ugf Limiter
At under frequency the Ugf Limiter reduces the generator voltage so as to
prevent saturation effects in the supply and measuring transformers To adjust this
limiter the max permissible generator voltage at rated frequency is defined and set
When any under-frequency occurs the generator voltage is thus reduced in proportion to
that setting
235 Field Current Maximum Limiter
The Field Current Maximum Limiter is provided to protect the generator rotor
from s occurring in steady-state and transient operation High field currents are normally
the result of a sharp drop in network voltage or of an improper raising of the voltage set-
point by the operating staff The field current is held steady at the value TH1 ie at the
maximum thermal value permissible for the excitation circuit and the rotor In order that
the generator can support the power network with its transient overload capacity during
brief collapses in voltage a temporary switch-over is made to the transient limit MAX1
(a higher setting) When the generator or the converter is operating at a reduced capacity
These limits TH1MAX1 can be switched over to the lower settings TH2MAX2 by
activating the corresponding binary signals The switch-over from the thermal limit
TH12 to the transient limit MAX12 can be configured in one of three ways
a) Depending on the over current with -dUdt ENABLE
Flag programming F418 = any setting desired F41A = 0000
This variant enables the transient value MAX12 whenever a collapse of voltage
in the network is detected The ENABLE time is fixed and can be set The example
below shows the typical behavior of the limiter configured in this way
b) Dependent on the time integral with -dUdt ENABLE
Flag programming F418 = inactive F41A = 1111
This variant likewise enables the transient value only when a collapse of network
voltage has been detected However the switch-back to the thermal limit is not made
dependent upon the time itself but on the calculated time integral intisup2dt of the The setting
on Parameter F414 in spu takes into account the time the rotor needs to cool down ie
the rate of temperature change in the case of intermittent operation The example below
shows how the timing of the switch-back to the thermal limit depends on the present
value for intisup2dt
The time integral is based on the formula
Example The setting of =isup2dt equivalent to Version a (F416) at a constant 16 times the
nominal field current for 10 seconds (with TH12 = 105) is
c) Dependent on the time integral without any preconditions
Flag programming F418 = 1111 F41A = 1111
In this variant the transient becomes available without any prior conditions
(without a -dUdt ENABLE) with the time integral intisup2dt
237 Inductive Stator Current Limiter
The Inductive Stator Current Limiter holds the stator current Ig within permissible
limits while the generator is in the ldquoover-excitedrdquo operating range by reducing the field
current accordingly The setting TH (thermal limit) provides the limit against stationary s
that might occur To take advantage of the generatorrsquos transient overload capacity a
switch-over is made to the higher setting MAX The principle of operation of this switch-
over to the value MAX permissible only transiently is identical to that employed for the
field current limiter (refer to the description above) When the drive output from the
turbine is very high stator current may exceed permissible limits even while inductive
loading of the generator is low In this case if the stator current limiter is not kept from
influencing the field current the control circuit will oscillate back and forth between the
Inductive Stator Current Limiter (de-
exciting) and the Capacitive Stator Current Limiter (exciting)The output signal of that
function then dominates the control variable of the Ig-dependent limiter via a maximum
value selection
238 Capacitive Stator Current Limiter
239 Load Angle Limiter
The Load Angle Limiter prevents the synchronous machine from slipping out of
phase due to slippage of the rotor The load angle δ the difference in phase between the
rotor and the stator rotating field results mainly from the driving torque (active power P)
acting on the generator and the level of rotor current (field current) If the driving torque
remains constant a increase in the field current reduces the load angle δ The current load
angle δ at any moment is obtained from the generator current and generator voltage based
on a simplified model of the generator Whenever this calculated load angle δ exceeds the
preset limit angle the limiter increases the field current until the load angle has dropped
back to its permissible value The quadrature reactance Xq of the generator and the
network reactance Xe during normal operation must be adjusted on the regulator in order
to obtain the load angle δ The graph below shows the Power Chart for a salient-pole
machine with typical limiter characteristics
The purpose of a Power System Stabilizer is to use the generator excitation to
damp electromechanical oscillations between the network and the generator Depending
on the design of the generator and the requirements imposed for network stability its
main function will be either to damp the oscillations originating in the machine or those
from the network A synchronous generator working in a combined power network is in
principle an oscillating structure In order to produce a torque the magnetic field of the
rotor and the stator must form a given angle (referred to as the rotor displacement or load
angle δ) The electrical torque ME increases as the angle δ increases just as with a
torsion spring Because the ME of the generator and the mechanical driving torque MA
from the turbine are in equilibrium during steady-state operation the angle δ remains in a
given position Whenever this state of equilibrium between MA and ME is disturbed the
load angle slips of this rest position and change thereby the electrical torque ME The
torque attempts to restore the load angle to a stationary position Due to the mass inertia
of the turbinegenerator rotor however this can only take place aperiodically It does so
in the form of more or less effectively damped oscillations (again similar to the effect of
mass inertia on a torsion spring) In order to damp the oscillations there must be a
damping torque produced depending not on the electrical torque ME associated with the
angle but on the difference in frequency (Df) between the rotor and the stator rotating
field ie on the slippage This torque is produced mainly by the so-called damper
winding in the rotor but the dimensioning of this is subject to limits imposed by
considerations of design and economy Some further action is therefore needed to
increase the damping effect The following drastically simplified formula shows the
parameters upon which the amount of active power PE supplied by the generator
depends
PE = active power
It can be seen from the above relationship that the active power that the generator
transfers depends not only on the load angle δ but also on the field current If That means
that a transient change can be made in the active power PE and with that in the effective
electrical torque ME by varying the field current The principle of operation of the DVR
Power System Stabilizer becomes clear from a consideration of the oscillations in power
output and frequency (ΔPE Δf) and the vector diagram If it is assumed that oscillations
in the network frequency generates load oscillations with the mass inertia of the rotor
then the active load of the generator (eg MW-measured) is influenced with a sinusoidal
value -ΔPE (ME-MA = -ΔPE) By inversion of -ΔPE one obtains the fluctuation in
power provided by the rotor +ΔPE As is known the slip signal Δf follows +ΔPE with a
phase delayed by 90deg The +ME produced by the periodic changes in the load angle δ is
in phase with +ΔPE A good damping is attained if ME is varied in phase with the slip
Δf However this signal must also be advanced somewhat to compensate for the time
constants in the excitation circuit and the generator
As mentioned above the electrical torque ME can be influenced by varying the
field current To accomplish this a suitable control signal referred to as variable
disturbance compensation must be imposed upon the voltage set-point or the converter
control variable Ucontr As can be seen from the vector diagram by applying proper
weighting factors (K1 K2) and then adding together the signals -ΔPE and Δf an overall
stabilization signal can be produced that rotates in advance of the Df signal by any angle
desired between 0deg and 90deg Because the amplitude of -DPE remains proportional to the
amplitude of Δf a constant angle in advance of Δf results for the compensation of the
time constants referred to above The optimum weighting factors K1 and K2 for a
synchronous generator working to a power network depend on its operating point at any
moment and the external reactance of the network Normally the selection of a
compromise setting is good enough to attain stability in all operating points and for all
external reactance For special demands these settings must be parameterized as a
function of the external reactance (which means optional equipment Xe-Identification)
The Power System Stabilizer PSS is a section of the AVR computer program and is
processed once per network cycle The voltage at the generator terminals and the
generator current are measured in order to define the signals ΔPE and Δf The calculated
signals for _P_ (=PE) and Δf are then sent across DC filters ldquoDrdquo (real differentiators) that
transmit only the dynamic portion of the signals The ΔPE and Δf signals obtained in this
way are then weighted (multiplied by) with the factors K1 and K2 and sent to the
summing point of the voltage regulator
The PSS stabilization signal is imposed on the automatic voltage regulator only if
the following prerequisites are met
bull Generator on line
bull Generator power output gt the value F338
bull Generator voltage in a range between F33C and F33A
The stabilization signal is limited at the output from the PSS to the lower and
upper limits Flag defines whether the stabilization signal is introduced before or after the
PID filter (usually before the filter) Because the PID filter as noted above already takes
the ceiling factor Vp1 into account the PSS signal needs to be multiplied by Vp1 if it is
added to the voltage regulator following the PID filter (divider at the input to the
minmax limiter) This precaution prevents the DC filter ldquoDrdquo in the P-channel from
producing an unnecessary ldquostabilizationrdquo effect in the case of rapid changes in turbine
load As an alternative for the AVRrsquos Power System Stabilizer a stabilization signal from
an outside system can be imposed by activating the binary input ldquoPSS-SIGNEXTrdquo Flag
F340 can be used to select between an analog and a 12-bit signal and F33E to select the
polarity desired for that signal
24 The MANUAL Channel
241 Summary
The MANUAL channel (Channel 2) has been built as a simple field current
regulator
without additional limiters Its main function is to maintain the excitation of the generator
even if the AUTOMATIC channel becomes non-operational The MANUAL channel
also performs valuable service for purposes of testing commissioning and preventive
maintenance Its measurements regulator generation of firing pulses and power supply
are physically separate from those on the AUTOMATIC channel
242 Principle of Operation
All the functions of the MANUAL channel including the generation of firing
pulses have been implemented in a single electronic module the Gate Control Unit The
control variable Ucontr of voltage regulator is used as the reference value for generating
firing pulses on the principle known as ldquoramp controlrdquo (Comparison of Ucontr with
Usynsynchronous sawtooth signal) For further processing in the UN 0096 Intermediate
Pulse Stage the Gate Control Unit supplies six firing pulses at its output whose phase
position with respect to the synchronous voltage Usyn is in accordance with control
variable Ucontr An internal linearization ensures that the field voltage produced via the
firing pulses remains proportional to the control variable Ucontr throughout the entire
range As a result the circuit amplification of the control remains constant over the entire
range Whenever excitation is switched ON the set-point for Generator Voltage is set
automatically at the preset - ref Value This provision ensures that the generator voltage
always attains approximately its nominal value after the field flashing The Gate Control
Unit can be refunctioned ( by pre-selection with a switch ) for purposes of testing to act
as a purely firing pulse control In this case the control variable Ucontr is adjusted
directly using the RAISELOWER push buttons on the front of the module In this way
for example the relationship between the phase position of the firing pulses and the
control variable Ucontr can be checked easily
CHAPTER 3
PULSE SECTION
31 Pulse Generation and Amplification
The Gate Control Units of both AUTOMATIC channel and MANUAL channel
each supply six firing pulses for operating the 6-pulse thyristor bridges The low-power
pulse signals from these Gate Control Units are then amplified in the Intermediate Pulse
Stage galvanically isolated and then sent to the common pulse bus On the output end
the Intermediate Pulse Stage of the non-active channel) is always blocked The Gate
Control Units generate the pulses based on microprocessor control The reference voltage
used for the firing pulse phase location is the output voltage from the excitation
transformer (Usyn1 Usyn2) The commutation spikes of the synchronous voltage caused
by the converter are calculated prior to use of the voltage as a reference value and are
deliberately filtered out The lower limit for the firing pulses (double pulses) which are
offset from one another by 60deg is defined by the limit rectifier position (αmin) and the
upper limit by the limit inverter position (αmax) for the firing angle αmin and αmax can
be adjusted on the Gate Control Units using BCD (Binary Coded Decimal) switches
αmin ensures that the firing pulses will not be issued (premature firing) until there is
sufficient positive phase voltage on the thyristor involved αmax prevents a dangerous
ldquotippingrdquo of the thyristor bridge into the rectifier mode if the firing angle α is too large
(ldquolate firingrdquo) The critical factors determining αmax are the overlap time uumlmax (max
commutation time) and the ldquorecovery timerdquo of the thyristors (αmax lt 180deg - uumlmax - γ )
An external control signal can force the firing pulses into their inverter limit position
Other binary inputs can block or direct the firing pulses of the Gate Control Units so as to
produce freewheeling on the thyristor bridge During freewheeling the firing pulses for
the thyristor pair R and S are blocked and the pulse signals T+T- are engaged with
chains of pulses Both Gate Control Units (for the MANUAL amp AUTOMATIC channels)
contain a field current monitor that blocks the firing pulses immediately whenever the
current exceeds a preset threshold level In this case the field circuit-breaker is also
tripped via an output contact The purpose of these provisions is to prevent damage to
thyristors and thyristor fuses in case of a slip-ring short-circuit or to keep any damage
that does occur to a minimum The pulse signals are galvanically separated at the outputs
from the Intermediate Pulse Stage (with pulse transmitters) and are then directed to the
common pulse bus This transmission of the pulse signals to the pulse bus via passive
transmitters ensures a high degree of active channel autonomy Practically no possible
malfunctions on the inactive channel (including for example sustained pulses) affect the
active channel
32 Pulse Monitoring
The ldquoPulse Busrdquo and the pulse signals of the AUTOMATIC channel are
monitored This monitoring device consists of potential isolating stages and the common
monitor If the pulse monitoring of the ldquoPulse Busrdquo responds a switch-over is made to
MANUAL channel The function of the potential isolating stages is to couple the pulse
monitoring device to the pulse circuits without any feedback effect The pulse monitoring
checks the six pulse lines for the following malfunctions continuous or periodic failure
of one or more pulses Periodic occurrence of synchronous or asynchronous false pulses
Continuous pulses the pulse monitoring device can be tested while the machine is in
operation
CHAPTER 4
CONVERTER
Thyristor
The term thyristor usually refers to a family of four layer solid state device having
turn on characteristics that can be externally controlled by either current or voltage They
are also referred to as breakdown device because their working depends on avalanche
breakdown Thyristors have only two stages OFF and ON Thyristors have a similar
function to Uni-junctions they act as switches Thyristors use current flow as a switch
Thyristors have three states
1 Reverse blocking mode mdash Voltage is applied in the direction that would be
blocked by a diode
2 Forward blocking mode mdash Voltage is applied in the direction that would cause
a diode to conduct but the thyristor has not yet been triggered into conduction
3 Forward conducting mode mdash The thyristor has been triggered into conduction
and will remain conducting until the forward current drops below a threshold value
known as the holding current Converter is a semiconductor device which converts ac
input voltage into a constant dc output voltage In present excitation system three phase
fully controlled thyristor converter is used
Because of the following advantages thyristor converters are used
a) Thyristors are used for high power applications ie up to 10Kv3500A1KHz
b) Having high reliability and low losses
c) Uni-directional device like diode
d) Itrsquos operation as a rectifier which are low resistance in forward conduction
mode and high resistance in reverse conduction mode
PROTECTION OF THYRISTORS
For reliable operation of a thyristor demands that its specified ratings are not
exceeded When Subjected to or over voltages During the turn - on of SCR didt
prohibitively large False triggering of SCR by high value of dvdt andSpurious signals
between gate and cathode may leads to unwanted turn ndash on
DIDT AND PROTECTION
When thyristor starts conducting in forward conduction mode and is turned on by
gate pulse The anode current increases rapidly whole area of the gate to Cathode
junction then hot spots will be formed near the gate connection this locality of heating
destroys the thyristor Thyristor thermal time is constant The causes due to faults and
short circuits or surge currents Electronic crowbar protection is used against the over
voltages The rate rise of anode current must be kept at the time of turn on below the
rated or specified limiting value The didt value maintained below limited value by using
a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating
is avoided by applying gate current but not greater the maximum gate current
DVDT AND OVER VOLTAGE PROTECTION
With forward voltage across the anode and cathode of a thyristor the two outer
junctions are forward biased but the inner junction is reverse biased This reverse biased
junction J2 has the characteristics of a capacitor due to charges existing across the
junction In other words space-charges exist in the depletion region around junction J2
and therefore junction J2 behaves like a capacitance If the entire anode to cathode
forward voltage Va appears across J2 junction and the charge is denoted by Q then a
charging current i given by Eq (46) follows
i = dQdt =d(Cj Va )dt
= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)
As Cj the capacitance of junction J2 is almost constant the current is given by
i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)
If the rate of rise of forward voltage dVadt is high the charging current i will be
more This charging current plays the role of gate current and turns on the SCR even
when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on
must be avoided as it leads to false operation of the thyristor circuit
For controllable operation of the thyristor the rate of rise of forward anode to
cathode voltage dVadt must be kept below the specified rated limit Typical values of
dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by
using a snubber circuit in parallel with the device thyristor are very sensitive for over
voltage than the semiconductor devices
Over voltage transients are perhaps the main cause of thyristor failure
In thyristor there are mainly two types
1 Internal over voltages
Due to the commutation of the thyristors large voltages are generated internally
Because of the series inductance of the SCR circuit the large transient voltages L didt
produced This voltage several times the break over voltage of the device then thyristor
destroys permanently
2 External over voltages
External over voltages are caused due to the interruptions of current flow in an
inductive circuit and also due to the lightening strokes on the lines feeding the thyristor
system For the reliable operation of thyristor the over voltages must be suppressed by
adopting suitable techniques
Suppression of over voltages
The RC circuit called snubber circuit is connected across the device to protect In
order to keep the protective components to a minimum the thyristors are chosen with
their peak voltages ratings are 25 to 3 times of the normal peak working voltage
ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are
commonly employed for protecting the thyristor circuit against the over voltages
Gate protection
Gate circuit should also be protected against the over voltages and surges Over
voltage at gate circuit can cause false triggering of the SCR may rises the junction
temperature behind specified limit leading to its damage Protection against over
voltage can be achieved by connecting a ZD across the gate circuit and a resister is
connected in series with gate circuit to protect against the s A capacitor and resister are
connected across gate to cathode to by pass the noise
41 Final Pulse Stages
The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage
(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor
bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided
with a power supply module The amplified output pulses from the Final Pulse Stages
start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main
pulse This initial pulse edge assures proper firing of the thyristors being triggered
Subsequently the weaker part of main pulse keeps firing conditions steady As already
mentioned the Final Pulse Stages and their associated thyristor bridges form single units
All six pulse outputs from a Final Pulse Stage can be blocked by an external control
signal so that all thyristors in the associated thyristor bridge will block the current A
blocking of the pulses is initiated whenever there is a malfunction in the associated
thyristor bridge
42 Converter Power Section
The thyristor converter consists of three independent parallel rectifier blocks TY1
to TY3 which are all in service Even if one block fails the remaining blocks take over
automatically the full design current of the excitation circuit During normal operation
(with ideal current share) and all three bridges in operation each of these blocks has to
carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation
is limited Only when all three bridges fail the excitation is switched off Each thyristor
bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any
arm is identified by a Current flow monitoring module
43 Converter Cooling
A cooling system is needed to dissipate heat losses in the converter blocks and
electronics Each converter block has therefore been equipped with a fan supplied with
power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing
cubicle) The fans are protected with motor protection circuit breakers An air flow
monitoring unit is provided for monitoring the air flow through the thyristor bridge If a
circuit breaker failure is detected or if the air flow monitor drops off at one of the
thyristor bridges the bridge involved is immediately set out of operation by blocking its
firing pulses
44 Thyristor Converter Monitoring
A thyristor bridge in which defects occur that could threaten the safety of
operation or cause secondary damage is switched off automatically ie its firing pulses
are blocked This happens whenever A thyristor fuse is blown The fuses are monitored
individually with micro switches The Final Pulse Stage fails which is detected by
internal monitors (supply voltage sustained pulse short-circuit on the output end) The
power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or
is insufficient Isolator on ACDC side is open
CHAPTER 5
Field Current Circuit Excitation Transformer
51 Field Circuit Breaker
The circuit-breaker in the field circuit is used to isolate the field circuit from the
converter It is capable of switching off the synchronous machine from full load under the
maximum conditions of a 3-phase short-circuit In addition to its main contacts the field
circuit-breaker also has a de-excitation contact with which the field energy stored in the
field can be dissipated across the de-excitation resistor The de-excitation contact closes
shortly before the main contacts open so as to ensure proper commutation of the field
current from the main contacts to the de-excitation contact when the breaker is switched
off The field circuit-breaker is switched on by electromagnetic force and is kept switched
on by a mechanical latch When the latch is released by a trip coil the circuit-breaker
opens The circuit-breaker also has auxiliary contacts that report its status
52 Field Flashing
In shunt supplied excitation circuits (excitation transformer connected to the
generator terminals) the generator does not have enough remnant voltage for a generator
voltage build-up via the converter In this case a field flashing circuit is provided It
consists of the field flashing contactor the diode bridge and a transformer used to adapt
the auxiliary input voltage to the voltage needed for field flashing when power is
supplied from the auxiliaries network
Fig Field Flashing
Because the field flashing contactor is not able to switch off the energy stored in
the field the control ensures that the contactor can only reopen if the field circuit breaker
has already been opened (generating the TRIP order) or in a normal field flashing
sequence when the converter has taken over the field current Field flashing occurs in the
following stages
1048729The excitation is switched on closing the field flashing contactor ( Field
Circuit Breaker is already closed )
1048729The start-up excitation current flows through the rotor driving the generator
voltage up to approx 15 U
1048729After about 10 U the firing pulses to the converter are released and it begins
to excite the generator to its rated voltage
1048729After about 30 U the field flashing contactor opens (with no current since
the converter is now supplying the current)
The diode bridge at the input to the field flashing contactor prevents a feed-back
from the converter to the source of field flashing while the contactor is still closed
53 De-excitation
When malfunctions occur the stored field energy must be dissipated as quickly
and safely as possible to protect the generator This is done by the converter the field
circuit-breaker and the de-excitation (discharge) resistor
De-excitation (with opening of the field circuit-breaker) takes place in the following
stages
1048729The converter drives to its inverter limit position (negative ceiling voltage)
recovers a portion of the field energy into the network A trip command is given to the
field circuit breaker
1048729The de-excitation contact closes diverting the field voltage to the de-excitation
resistor
1048729Then immediately the main contacts open building voltage The field voltage
commutates to the de-excitation resistor
1048729The current diminishes at a given time constant TE
(With linear resistance TE = Lf (Rf + Re))
Due to the reversal of the field voltage by the converter the field current
commutates from the main contacts of the field circuit-breaker to the de-excitation
resistor in a very early phase This reversal of the field voltage prevents burn-off on the
main contacts and provides effective protection for the field circuit-breaker Depending
on the operating policy an operational shut-down of the excitation can also be effected
with the field circuit-breaker closed This method is useful mainly when the excitation is
switched on and off frequently In this case the converter is merely driven into the
inverter limit position so that the field energy is recovered into the network The
converter then blocks since it is supplying positive current only
54 Excitation Transformer
The excitation transformer matches the generator voltage to the field voltage
(required ceiling voltage) It also serves as a commutation reactance for the thyristor
converter and as a potential isolator between the network and the excitation circuit In
addition the transformer functions as a current limiter in that it makes it possible to keep
any short circuits in the excitation circuit under better control The excitation transformer
is equipped with temperature monitoring probes which set off an alarm when the
temperature exceeds a first max limit and then trips the excitation if the temperature
continues rising to a second (higher)limit
CHAPTER 6
Monitoring and Protection
61 Excitation Monitoring
611 General Information
The main goal of Excitation Monitoring is to make optimum use of the
redundancies provided in the excitation system and to give alarm whenever a malfunction
makes these redundancies unavailable The field current is monitored to see that it does
not exceed a maximum level and if necessary a switch-over to the MANUAL channel is
initiated In addition the criterion for switching off the field flashing is generated The
excitation Monitoring consists of an autonomous processor system
612 over current Alarms
In the Excitation Monitoring the limits for are set at higher levels than the
settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of
the nominal field current contact R1 and the binary output associated with it are
activated immediately If field current remains gt 110 then after a preset inverse-time
has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs
associated with them are activated Parameters match the measurements for If1 and If2 to
the nominal value for field current so that the internal values can be processed and read as
pu values It can be used to falsify the actual value of the field current If (to raise it) so
as to cause a response from the alarm limits for purposes of testing The processed If
signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As
long as the field current If is above the threshold value 11 Ifn its peak value is
measured This is stored (until RESET) and can be read at any time on the Micro-
Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever
the integrated time-current value (intisup2dt) exceeds the preselected reference value the
alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously
issued to switch over to the stand by AUTO channel Software switch F758 enables the
three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-
time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the
desired limit curve for response is set using the factor F216
613 Switch-Off Criterion for Field Flashing
The Excitation Monitoring supplies the criterion for switching off the field
flashing Whether this criterion is activated based on the actual value for generator voltage
Ug or for field current If or both depends on the settings of the two threshold values
F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements
Ug12 and If12 are switched over depending on the present status of the channels
(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is
fixed at ldquological 1rdquo
614 Storage of Alarm Status
The outputs of the over current alarms (R1 R2 R0) and the messages NO
FAILURE MONITORING PARAMETERS CHANGED are stored messages can be
erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the
front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the
situation causing the alarm or the malfunction is no longer present Whenever the self-
diagnosis equipment in the processor detects a malfunction the output NO FAILURE of
MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo
is activated whenever parameters or settings of software switches have been changed via
the Micro-Terminal
615 Actual Value Monitoring
The actual values for generator voltage Ug synchronous voltage Usyn and field
current If are monitored for malfunctions This monitoring is active regardless of whether
or not the generator is in operation Essentially when the generator is in operation the
measurements are monitored by comparing the signals (the smaller signal reading is
detected as incorrect) When the generator is not in operation the measured data are
monitored for extreme values The percentage of deviation permissible in the
measurement signals being compared
(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and
F20AIf the excitation transformer is being supplied from an auxiliary power source (no
shunt operation) the values of Ug and Usyn will be different in some operational
conditions
In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1
and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the
messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2
and the binary message from CH1 reports no malfunction a malfunction on
Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is
also present whenever the binary message CH1 DISTURBANCE is reported and a
discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2
FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares
Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the
comparison channel reports a malfunction or whenever both binary messages report no
malfunction - but both secondary monitors report a malfunction As long as the secondary
monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction
signals for the measurement channel involved (suspicion that there is a corresponding
error in Usyn) The measurement channel malfunctions are enabled operationally
whenever after excitation has been switched on generator voltage Ug exceeds the value
set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is
switched off to see that they do not exceed the limit value F210 that applies to both of
them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt
the operating range) Monitoring for extreme values is likewise enabled during normal
operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the
malfunction signals to the binary outputs Basically the monitoring of the actual values
for If1If2 functions like that of the Ug1Ug2 monitoring
62 Excitation Protection
621 General Information
The Excitation Protection switches off the excitation (and de-excites the machine
rapidly) whenever a danger arises that threatens the excitation transformer the converter
or the generator Generally limiter or monitoring functions precede the emergency trips
and these normally respond before the Excitation Protection must initiate a trip
Protective trip commands are issued directly to the field circuit-breaker from potential
free contacts of the board via the trip relays They are directed redundantly to the
operative field circuit-breaker ldquoOFFrdquo command
622 Protection against Excitation Transformer Overheating
This equipment monitors the excitation transformer for overheating in the
windings that could result from over current short-circuits or inadequate cooling The
monitoring uses temperature monitoring modules in conjunction with temperature
sensors built into the windings Normally the temperature is monitored in two stages the
first stage sets off an alarm the second causes a trip of the excitation
623 Rotor Over voltage Protection
Malfunctions in the generator circuit (eg terminal short-circuit failed
synchronization asynchronous operation) cause induced negative field currents that
produce high voltages in the field circuit These must be restricted to a level with a
sufficient safety margin below the insulation capacity of the field winding (test voltage)
and also below the peak blocking voltage of the converter thyristors The crow bar
employs spark gap elements to detect over voltages in the field circuit Whenever they
respond the associated thyristors are fired immediately switching the de-excitation
resistor parallel to the field The de-excitation current generated thereby initiates an
excitation trip via a supervision circuit causing an immediate opening of the field circuit-
breaker The malfunction isets off an alarm and an internal malfunction is indicated at the
cubicle
TEST VALUES OBTAINED WHEN EXCITATION IS RAISED
TEST
SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE
1 VREF 996 100
2 VACT 997 1003
3 IFACT 735 765
4 IGACT 703 707
5 ACTIVE
POWER
703 705
6 REACTIVE
POWER
101 142
7 POWER
FACTOR
099 IND 098 IND
8 ACTIVE
CURRENT(IR)
705 703
9 REACTIVE
CURRENT(IX)
102 138
10 POWER
ANGLE
566 546
11 FIRING
ANGLE
640 632
VALUES OBTAINED WHEN EXCITATION IS LOWERED
SNO PARTICULARS ACTUAL
VALUE
OBTAINED VALUE
1 VREF 100 997
2 VACT 100 997
3 IFACT 787 761
4 IGACT 839 837
5 ACTIVE
POWER
839 830
6 REACTIVE
POWER
155 110
7 POWER
FACTOR
098 IND 099 IND
8 ACTIVE
CURRENT(IR)
835 830
9 REACTIVE
CURRENT(IX)
140 94
10 POWER
ANGLE
613 DEG 631 DEG
11 FIRING
ANGLE
601 DEG 599 DEG
CONCLUSION
For generating the EMF in stator winding excitation is required to the rotor of a
generator There are two types of excitation
1 Static excitation system
2 Brushless excitation system
A certain disadvantage in brushless excitation system is the slow response time of
the field in case of fast load changes specified No slip-rings and brushes direct
measurements of the field parameters not possible
To avoid all loses static excitation is used Since it does not have any rotating
parts mechanical loses and windage loses This system has fast response and speed
control While preferring this excitation system there are no limitations for the
redundancy of Thyristor bridge circuits
Static excitation has fast field discharge by resistor and inverter operation direct
measurement of field quantity is possible The meaning of excitation is nothing but
continuous supply of DC current (ie field current) to the rotor to buildup required
output voltage in the stator
Field current is changed with respect to the change of load so the digital
automatic voltage regulator (DAVR) is used to regulate the output voltage according to
the load variations
So we conclude that static excitation system with DAVR is preferred since it is
having excellent dynamic performance and better options for R amp M
systems The AUTOMATIC channel the programmable controls and the Excitation The
actual values measured from AUTOMATIC channel and MANUAL channel are The
AUTOMATIC channel and the MANUAL channel each have their own The digital
voltage regulator is broken down into several autonomous microprocessor The inputs and
outputs of the processor systems are directed across voltage-isolating The Interrupt
Generator also uses the 3-phase Ug signal to generate the 12 themselves contain a limited
number of hardware inputs and outputs with fixed equipment Whenever addresses from
this range are called up the signal processing module can results the processor is then
able to derive further operating parameters such as the load angle the active power etc
The functions of all microprocessor systems other than the programmable controls
have been accomplished in firmware The non-varying standard function modules can be
configured to the design desired for plant-specific purposes using software switches
(KFlags) Thus for example the stored status of a K-Flag determines whether or not a
Limiter is active and whether the de-excitation or the excitation limiters take precedence
Because these K-flags determine the software Scope of Supply for the installation they
cannot be changed permanently via the Micro-Terminal In this way they differ from
such setting data as the values of the parameters for the PID filter of the voltage regulator
or the set-points for the limiters These values can be permanently changed using the
Micro-Terminal Communication is possible with each of the processor systems via the
Micro-Terminal by plugging on the connecting cable In this way signals within the
processor and setting parameters can be viewed analog signals can be issued and the set
parameters can be altered temporarily (F range) or permanently (C range) Unlike the
other processor systems the programmable controls do not include any firmware for
realization of the functions They have been designed so that the designer can adapt and
change their functions easily using the ldquoFunctional Block Programming Language P10
Digital and analog functions can be implemented in practically any degree of complexity
desired using the P10 functional blocks The control variable of the voltage regulator
(AUTOMATIC channel) and the control variable of the field current regulator
(MANUAL channel) are each processed in separate Gate Control Unit and formed into a
chain of pulses at the appropriate firing angle The pulses of the active channel are
directed to the pulse bus via the associated Intermediate Pulse Stage The pulses for each
converter block are amplified sufficiently in Final Pulse Stage to fire the Thyristor
231 General Information
The functions of the automatic voltage regulator AVR are
1 to regulate the generator voltage
2 to regulate the effect of the reactive andor active current on the voltage
3 to limit VoltHz
4 to limit max and min field current
5 to limit inductive stator current
6 to limit capacitive stator current
7 to limit the load angle
8 to stabilize the power system
Block Diagram shows the software structure of AUTOMATIC channel The
generator limiters not provided for the installation in question (optional equipment) are
identified in this overview as ldquoNot Suppliedrdquo The parameter values signal values and
software switches (flags) marked with addresses (hexadecimal numbers) can be viewed
and altered via the Micro-Terminal The values selected are displayed in sec pu Hz
etc and can where necessary be changed directly in these formats The plant-specific
settings of the variables and the flags can be obtained from the Test and Commissioning
Report This block diagram provides information about the important functions and
possible settings of the AUTOMATIC channel For the sake of clarity no detailed
presentation has been given of special functions such as tracking circuits initializations
etc The page heading cross-refers this overview to the various sheets of the schematic
diagram Binary signals are shown in broken lines analog signals in solid lines The
corresponding text designations in the schematic diagram can be used for identification of
the input signals (hardware inputs) The only analog output signal from the automatic
voltage regulator control variable Ucontr is sent via the data bus (CRU bus) to the Gate
Control Unit Most of the binary messages (outputs) from the AVR are of no interest
functionally and they have been omitted for the sake of clarity The basic structure of the
digital voltage regulator and the limiters is simple This is necessary in order that the
behavior of the regulatorslimiters will remain calculable and understandable in all
operating situations and that there will be no problem in adjusting and optimizing them
The central PID filter in the digital voltage regulator defines the dynamic response of the
closed-loop controls both in the voltage regulator mode and after limiters have
intervened The ldquocontrol deviationrdquo at the input to the PID filter is either the control
deviation for voltage the control deviation of a de-excitation limiter (the value
determined by minimum value selection) or the control deviation of an excitation limiter
(the value determined by maximum value selection) Flag F730 (ldquoPRIORrdquo) is used to
determine whether the exciting (Min value) or the de-exciting signal takes precedence on
the minmax value limiter (normally F730 = 1111 ie the de-exciting signal takes
precedence) With the exception of the Minimum Field Current Limiter all other limiters
have variable factoring multipliers of the signal outputs so that they can be adjusted
individually together with the common PID filter which has been optimized for voltage
regulation The setting parameters for this PID filter are as follows
Vo = KR Static amplification
1
Ta = ---- Integration time constant
Tc1
Vp Proportional amplification
1
Tb = ---- Differential time constant
Tc2
Vinfin Amplification of high frequencies
The BODE diagram below shows the assignment of settings in accordance with
DINIEC standards based on a typical example
The PID filter amplifications Vo Vp and Vinfin can be adjusted in pu values But
the ldquoceiling factorrdquo pl+ must be adjusted correctly with parameter F310 if the total
amplification (circuit amplification) of the control circuit is actually to conform to the
pu settings This factor must agree with the ldquoexternalrdquo amplification ie with the
ceiling value of the transformer- converter circuit
Ceiling factor(pl+) = Ufmax Ufo
in which Ufmax = ceiling field voltage
Ufo = no-load field voltage
To attain a suitable response of the AVR when starting excitation
(ldquoEXCITATION ONrdquo) it may be necessary to change the proportional amplification of
the regulator during this phase Vp2 (transiently activated) and Vp1 (permanently
activated) can be adjusted for this purpose For example the value of Vp2 takes effect
immediately once the excitation is switched on and remains effective for a period as set at
F30C Once the period F30C (eg 5 sec) has expired Vp shifts over to Vp1 (becomes
the steady-state Vp) at the rate of change set The standard operating mode for the PID
filter is voltage regulation for which the discrepancy between the voltage set-point and
the current value for generator voltage Ug (the control deviation) is supplied at the input
To compensate for the voltage drop in the block transformer or whenever several
generators are operating to the same distributing bus the generator voltage must be
varied in proportion to the measured generator current (droop influence) To accomplish
this the voltage set-point is varied as a function of the measured reactive current IX
andor active current IR Flag F712 enables the IX droop Flag F710 the IR droop The
desired compensation is set in F282 and F286 respectively Flags F284 and F288 are used
to select whether this droop influence is to increase the voltage or to reduce it
(compensation) Combined influence of the active and reactive currents is attained by
enabling both droops IX and IR Flag F716 activates a so-called ldquoSoft-Startrdquo at the
starting of excitation This ldquoSoft-Startrdquo ensures that the voltage set-point integrates from
0 to 100 within the time set on F290 when the excitation is switched on
(ldquoEXCITATION ONrdquo) A ldquosmoothrdquo excitation of the generator can be achieved in this
way whenever there is no demand for a quick excitation
232 Voltage Set-Point
Various signals and settings control and limit the voltage set-point F270 For
example the values of F254 and F252 define the normal operating range possible for set-
point adjustment (eg 90 110) using external control commands (control room local
operatorrsquos panel superposed control system) The effective set-point adjustment rate is
governed The set-point can be set at the values of F250 and F256 by activating
appropriate control commands for ldquoSETrdquo input Enabling Flag F71A and activating a
binary input prior to switching on the excitation (ldquoEXCITATION OFFrdquo) sets the Ug set-
point at the value of UAUX This makes it possible for example to ensure that the
generator voltage will agree exactly with the network voltage after the voltage build-up
An external value with variable amplification can be added to the Ug set point by
enabling F724 (for example for stability tests)
233 Regulator Tracking in MANUAL Operation
Whenever the AUTOMATIC channel is not in operation (the MANUAL channel
is ON) a follow-up equipment ensures a smooth switch-back to the AUTOMATIC
mode will always be possible To track the voltage set-point is shifted by means of
RAISELOWER pulses from the Gate Control Unit so that control variable Ucontr at the
output from the PID filter is held steady and identical to the control variable Ucontr from
the MANUAL channel Because this tracking must react slowly resultant transient
control deviations resulting from the amplification in the PID filter might cause severe
interference with control variable Ucontr
To prevent this the follow-up equipment intervenes on the regulators mixing
point with a corresponding compensation signal
234 Ugf Limiter
At under frequency the Ugf Limiter reduces the generator voltage so as to
prevent saturation effects in the supply and measuring transformers To adjust this
limiter the max permissible generator voltage at rated frequency is defined and set
When any under-frequency occurs the generator voltage is thus reduced in proportion to
that setting
235 Field Current Maximum Limiter
The Field Current Maximum Limiter is provided to protect the generator rotor
from s occurring in steady-state and transient operation High field currents are normally
the result of a sharp drop in network voltage or of an improper raising of the voltage set-
point by the operating staff The field current is held steady at the value TH1 ie at the
maximum thermal value permissible for the excitation circuit and the rotor In order that
the generator can support the power network with its transient overload capacity during
brief collapses in voltage a temporary switch-over is made to the transient limit MAX1
(a higher setting) When the generator or the converter is operating at a reduced capacity
These limits TH1MAX1 can be switched over to the lower settings TH2MAX2 by
activating the corresponding binary signals The switch-over from the thermal limit
TH12 to the transient limit MAX12 can be configured in one of three ways
a) Depending on the over current with -dUdt ENABLE
Flag programming F418 = any setting desired F41A = 0000
This variant enables the transient value MAX12 whenever a collapse of voltage
in the network is detected The ENABLE time is fixed and can be set The example
below shows the typical behavior of the limiter configured in this way
b) Dependent on the time integral with -dUdt ENABLE
Flag programming F418 = inactive F41A = 1111
This variant likewise enables the transient value only when a collapse of network
voltage has been detected However the switch-back to the thermal limit is not made
dependent upon the time itself but on the calculated time integral intisup2dt of the The setting
on Parameter F414 in spu takes into account the time the rotor needs to cool down ie
the rate of temperature change in the case of intermittent operation The example below
shows how the timing of the switch-back to the thermal limit depends on the present
value for intisup2dt
The time integral is based on the formula
Example The setting of =isup2dt equivalent to Version a (F416) at a constant 16 times the
nominal field current for 10 seconds (with TH12 = 105) is
c) Dependent on the time integral without any preconditions
Flag programming F418 = 1111 F41A = 1111
In this variant the transient becomes available without any prior conditions
(without a -dUdt ENABLE) with the time integral intisup2dt
237 Inductive Stator Current Limiter
The Inductive Stator Current Limiter holds the stator current Ig within permissible
limits while the generator is in the ldquoover-excitedrdquo operating range by reducing the field
current accordingly The setting TH (thermal limit) provides the limit against stationary s
that might occur To take advantage of the generatorrsquos transient overload capacity a
switch-over is made to the higher setting MAX The principle of operation of this switch-
over to the value MAX permissible only transiently is identical to that employed for the
field current limiter (refer to the description above) When the drive output from the
turbine is very high stator current may exceed permissible limits even while inductive
loading of the generator is low In this case if the stator current limiter is not kept from
influencing the field current the control circuit will oscillate back and forth between the
Inductive Stator Current Limiter (de-
exciting) and the Capacitive Stator Current Limiter (exciting)The output signal of that
function then dominates the control variable of the Ig-dependent limiter via a maximum
value selection
238 Capacitive Stator Current Limiter
239 Load Angle Limiter
The Load Angle Limiter prevents the synchronous machine from slipping out of
phase due to slippage of the rotor The load angle δ the difference in phase between the
rotor and the stator rotating field results mainly from the driving torque (active power P)
acting on the generator and the level of rotor current (field current) If the driving torque
remains constant a increase in the field current reduces the load angle δ The current load
angle δ at any moment is obtained from the generator current and generator voltage based
on a simplified model of the generator Whenever this calculated load angle δ exceeds the
preset limit angle the limiter increases the field current until the load angle has dropped
back to its permissible value The quadrature reactance Xq of the generator and the
network reactance Xe during normal operation must be adjusted on the regulator in order
to obtain the load angle δ The graph below shows the Power Chart for a salient-pole
machine with typical limiter characteristics
The purpose of a Power System Stabilizer is to use the generator excitation to
damp electromechanical oscillations between the network and the generator Depending
on the design of the generator and the requirements imposed for network stability its
main function will be either to damp the oscillations originating in the machine or those
from the network A synchronous generator working in a combined power network is in
principle an oscillating structure In order to produce a torque the magnetic field of the
rotor and the stator must form a given angle (referred to as the rotor displacement or load
angle δ) The electrical torque ME increases as the angle δ increases just as with a
torsion spring Because the ME of the generator and the mechanical driving torque MA
from the turbine are in equilibrium during steady-state operation the angle δ remains in a
given position Whenever this state of equilibrium between MA and ME is disturbed the
load angle slips of this rest position and change thereby the electrical torque ME The
torque attempts to restore the load angle to a stationary position Due to the mass inertia
of the turbinegenerator rotor however this can only take place aperiodically It does so
in the form of more or less effectively damped oscillations (again similar to the effect of
mass inertia on a torsion spring) In order to damp the oscillations there must be a
damping torque produced depending not on the electrical torque ME associated with the
angle but on the difference in frequency (Df) between the rotor and the stator rotating
field ie on the slippage This torque is produced mainly by the so-called damper
winding in the rotor but the dimensioning of this is subject to limits imposed by
considerations of design and economy Some further action is therefore needed to
increase the damping effect The following drastically simplified formula shows the
parameters upon which the amount of active power PE supplied by the generator
depends
PE = active power
It can be seen from the above relationship that the active power that the generator
transfers depends not only on the load angle δ but also on the field current If That means
that a transient change can be made in the active power PE and with that in the effective
electrical torque ME by varying the field current The principle of operation of the DVR
Power System Stabilizer becomes clear from a consideration of the oscillations in power
output and frequency (ΔPE Δf) and the vector diagram If it is assumed that oscillations
in the network frequency generates load oscillations with the mass inertia of the rotor
then the active load of the generator (eg MW-measured) is influenced with a sinusoidal
value -ΔPE (ME-MA = -ΔPE) By inversion of -ΔPE one obtains the fluctuation in
power provided by the rotor +ΔPE As is known the slip signal Δf follows +ΔPE with a
phase delayed by 90deg The +ME produced by the periodic changes in the load angle δ is
in phase with +ΔPE A good damping is attained if ME is varied in phase with the slip
Δf However this signal must also be advanced somewhat to compensate for the time
constants in the excitation circuit and the generator
As mentioned above the electrical torque ME can be influenced by varying the
field current To accomplish this a suitable control signal referred to as variable
disturbance compensation must be imposed upon the voltage set-point or the converter
control variable Ucontr As can be seen from the vector diagram by applying proper
weighting factors (K1 K2) and then adding together the signals -ΔPE and Δf an overall
stabilization signal can be produced that rotates in advance of the Df signal by any angle
desired between 0deg and 90deg Because the amplitude of -DPE remains proportional to the
amplitude of Δf a constant angle in advance of Δf results for the compensation of the
time constants referred to above The optimum weighting factors K1 and K2 for a
synchronous generator working to a power network depend on its operating point at any
moment and the external reactance of the network Normally the selection of a
compromise setting is good enough to attain stability in all operating points and for all
external reactance For special demands these settings must be parameterized as a
function of the external reactance (which means optional equipment Xe-Identification)
The Power System Stabilizer PSS is a section of the AVR computer program and is
processed once per network cycle The voltage at the generator terminals and the
generator current are measured in order to define the signals ΔPE and Δf The calculated
signals for _P_ (=PE) and Δf are then sent across DC filters ldquoDrdquo (real differentiators) that
transmit only the dynamic portion of the signals The ΔPE and Δf signals obtained in this
way are then weighted (multiplied by) with the factors K1 and K2 and sent to the
summing point of the voltage regulator
The PSS stabilization signal is imposed on the automatic voltage regulator only if
the following prerequisites are met
bull Generator on line
bull Generator power output gt the value F338
bull Generator voltage in a range between F33C and F33A
The stabilization signal is limited at the output from the PSS to the lower and
upper limits Flag defines whether the stabilization signal is introduced before or after the
PID filter (usually before the filter) Because the PID filter as noted above already takes
the ceiling factor Vp1 into account the PSS signal needs to be multiplied by Vp1 if it is
added to the voltage regulator following the PID filter (divider at the input to the
minmax limiter) This precaution prevents the DC filter ldquoDrdquo in the P-channel from
producing an unnecessary ldquostabilizationrdquo effect in the case of rapid changes in turbine
load As an alternative for the AVRrsquos Power System Stabilizer a stabilization signal from
an outside system can be imposed by activating the binary input ldquoPSS-SIGNEXTrdquo Flag
F340 can be used to select between an analog and a 12-bit signal and F33E to select the
polarity desired for that signal
24 The MANUAL Channel
241 Summary
The MANUAL channel (Channel 2) has been built as a simple field current
regulator
without additional limiters Its main function is to maintain the excitation of the generator
even if the AUTOMATIC channel becomes non-operational The MANUAL channel
also performs valuable service for purposes of testing commissioning and preventive
maintenance Its measurements regulator generation of firing pulses and power supply
are physically separate from those on the AUTOMATIC channel
242 Principle of Operation
All the functions of the MANUAL channel including the generation of firing
pulses have been implemented in a single electronic module the Gate Control Unit The
control variable Ucontr of voltage regulator is used as the reference value for generating
firing pulses on the principle known as ldquoramp controlrdquo (Comparison of Ucontr with
Usynsynchronous sawtooth signal) For further processing in the UN 0096 Intermediate
Pulse Stage the Gate Control Unit supplies six firing pulses at its output whose phase
position with respect to the synchronous voltage Usyn is in accordance with control
variable Ucontr An internal linearization ensures that the field voltage produced via the
firing pulses remains proportional to the control variable Ucontr throughout the entire
range As a result the circuit amplification of the control remains constant over the entire
range Whenever excitation is switched ON the set-point for Generator Voltage is set
automatically at the preset - ref Value This provision ensures that the generator voltage
always attains approximately its nominal value after the field flashing The Gate Control
Unit can be refunctioned ( by pre-selection with a switch ) for purposes of testing to act
as a purely firing pulse control In this case the control variable Ucontr is adjusted
directly using the RAISELOWER push buttons on the front of the module In this way
for example the relationship between the phase position of the firing pulses and the
control variable Ucontr can be checked easily
CHAPTER 3
PULSE SECTION
31 Pulse Generation and Amplification
The Gate Control Units of both AUTOMATIC channel and MANUAL channel
each supply six firing pulses for operating the 6-pulse thyristor bridges The low-power
pulse signals from these Gate Control Units are then amplified in the Intermediate Pulse
Stage galvanically isolated and then sent to the common pulse bus On the output end
the Intermediate Pulse Stage of the non-active channel) is always blocked The Gate
Control Units generate the pulses based on microprocessor control The reference voltage
used for the firing pulse phase location is the output voltage from the excitation
transformer (Usyn1 Usyn2) The commutation spikes of the synchronous voltage caused
by the converter are calculated prior to use of the voltage as a reference value and are
deliberately filtered out The lower limit for the firing pulses (double pulses) which are
offset from one another by 60deg is defined by the limit rectifier position (αmin) and the
upper limit by the limit inverter position (αmax) for the firing angle αmin and αmax can
be adjusted on the Gate Control Units using BCD (Binary Coded Decimal) switches
αmin ensures that the firing pulses will not be issued (premature firing) until there is
sufficient positive phase voltage on the thyristor involved αmax prevents a dangerous
ldquotippingrdquo of the thyristor bridge into the rectifier mode if the firing angle α is too large
(ldquolate firingrdquo) The critical factors determining αmax are the overlap time uumlmax (max
commutation time) and the ldquorecovery timerdquo of the thyristors (αmax lt 180deg - uumlmax - γ )
An external control signal can force the firing pulses into their inverter limit position
Other binary inputs can block or direct the firing pulses of the Gate Control Units so as to
produce freewheeling on the thyristor bridge During freewheeling the firing pulses for
the thyristor pair R and S are blocked and the pulse signals T+T- are engaged with
chains of pulses Both Gate Control Units (for the MANUAL amp AUTOMATIC channels)
contain a field current monitor that blocks the firing pulses immediately whenever the
current exceeds a preset threshold level In this case the field circuit-breaker is also
tripped via an output contact The purpose of these provisions is to prevent damage to
thyristors and thyristor fuses in case of a slip-ring short-circuit or to keep any damage
that does occur to a minimum The pulse signals are galvanically separated at the outputs
from the Intermediate Pulse Stage (with pulse transmitters) and are then directed to the
common pulse bus This transmission of the pulse signals to the pulse bus via passive
transmitters ensures a high degree of active channel autonomy Practically no possible
malfunctions on the inactive channel (including for example sustained pulses) affect the
active channel
32 Pulse Monitoring
The ldquoPulse Busrdquo and the pulse signals of the AUTOMATIC channel are
monitored This monitoring device consists of potential isolating stages and the common
monitor If the pulse monitoring of the ldquoPulse Busrdquo responds a switch-over is made to
MANUAL channel The function of the potential isolating stages is to couple the pulse
monitoring device to the pulse circuits without any feedback effect The pulse monitoring
checks the six pulse lines for the following malfunctions continuous or periodic failure
of one or more pulses Periodic occurrence of synchronous or asynchronous false pulses
Continuous pulses the pulse monitoring device can be tested while the machine is in
operation
CHAPTER 4
CONVERTER
Thyristor
The term thyristor usually refers to a family of four layer solid state device having
turn on characteristics that can be externally controlled by either current or voltage They
are also referred to as breakdown device because their working depends on avalanche
breakdown Thyristors have only two stages OFF and ON Thyristors have a similar
function to Uni-junctions they act as switches Thyristors use current flow as a switch
Thyristors have three states
1 Reverse blocking mode mdash Voltage is applied in the direction that would be
blocked by a diode
2 Forward blocking mode mdash Voltage is applied in the direction that would cause
a diode to conduct but the thyristor has not yet been triggered into conduction
3 Forward conducting mode mdash The thyristor has been triggered into conduction
and will remain conducting until the forward current drops below a threshold value
known as the holding current Converter is a semiconductor device which converts ac
input voltage into a constant dc output voltage In present excitation system three phase
fully controlled thyristor converter is used
Because of the following advantages thyristor converters are used
a) Thyristors are used for high power applications ie up to 10Kv3500A1KHz
b) Having high reliability and low losses
c) Uni-directional device like diode
d) Itrsquos operation as a rectifier which are low resistance in forward conduction
mode and high resistance in reverse conduction mode
PROTECTION OF THYRISTORS
For reliable operation of a thyristor demands that its specified ratings are not
exceeded When Subjected to or over voltages During the turn - on of SCR didt
prohibitively large False triggering of SCR by high value of dvdt andSpurious signals
between gate and cathode may leads to unwanted turn ndash on
DIDT AND PROTECTION
When thyristor starts conducting in forward conduction mode and is turned on by
gate pulse The anode current increases rapidly whole area of the gate to Cathode
junction then hot spots will be formed near the gate connection this locality of heating
destroys the thyristor Thyristor thermal time is constant The causes due to faults and
short circuits or surge currents Electronic crowbar protection is used against the over
voltages The rate rise of anode current must be kept at the time of turn on below the
rated or specified limiting value The didt value maintained below limited value by using
a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating
is avoided by applying gate current but not greater the maximum gate current
DVDT AND OVER VOLTAGE PROTECTION
With forward voltage across the anode and cathode of a thyristor the two outer
junctions are forward biased but the inner junction is reverse biased This reverse biased
junction J2 has the characteristics of a capacitor due to charges existing across the
junction In other words space-charges exist in the depletion region around junction J2
and therefore junction J2 behaves like a capacitance If the entire anode to cathode
forward voltage Va appears across J2 junction and the charge is denoted by Q then a
charging current i given by Eq (46) follows
i = dQdt =d(Cj Va )dt
= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)
As Cj the capacitance of junction J2 is almost constant the current is given by
i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)
If the rate of rise of forward voltage dVadt is high the charging current i will be
more This charging current plays the role of gate current and turns on the SCR even
when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on
must be avoided as it leads to false operation of the thyristor circuit
For controllable operation of the thyristor the rate of rise of forward anode to
cathode voltage dVadt must be kept below the specified rated limit Typical values of
dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by
using a snubber circuit in parallel with the device thyristor are very sensitive for over
voltage than the semiconductor devices
Over voltage transients are perhaps the main cause of thyristor failure
In thyristor there are mainly two types
1 Internal over voltages
Due to the commutation of the thyristors large voltages are generated internally
Because of the series inductance of the SCR circuit the large transient voltages L didt
produced This voltage several times the break over voltage of the device then thyristor
destroys permanently
2 External over voltages
External over voltages are caused due to the interruptions of current flow in an
inductive circuit and also due to the lightening strokes on the lines feeding the thyristor
system For the reliable operation of thyristor the over voltages must be suppressed by
adopting suitable techniques
Suppression of over voltages
The RC circuit called snubber circuit is connected across the device to protect In
order to keep the protective components to a minimum the thyristors are chosen with
their peak voltages ratings are 25 to 3 times of the normal peak working voltage
ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are
commonly employed for protecting the thyristor circuit against the over voltages
Gate protection
Gate circuit should also be protected against the over voltages and surges Over
voltage at gate circuit can cause false triggering of the SCR may rises the junction
temperature behind specified limit leading to its damage Protection against over
voltage can be achieved by connecting a ZD across the gate circuit and a resister is
connected in series with gate circuit to protect against the s A capacitor and resister are
connected across gate to cathode to by pass the noise
41 Final Pulse Stages
The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage
(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor
bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided
with a power supply module The amplified output pulses from the Final Pulse Stages
start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main
pulse This initial pulse edge assures proper firing of the thyristors being triggered
Subsequently the weaker part of main pulse keeps firing conditions steady As already
mentioned the Final Pulse Stages and their associated thyristor bridges form single units
All six pulse outputs from a Final Pulse Stage can be blocked by an external control
signal so that all thyristors in the associated thyristor bridge will block the current A
blocking of the pulses is initiated whenever there is a malfunction in the associated
thyristor bridge
42 Converter Power Section
The thyristor converter consists of three independent parallel rectifier blocks TY1
to TY3 which are all in service Even if one block fails the remaining blocks take over
automatically the full design current of the excitation circuit During normal operation
(with ideal current share) and all three bridges in operation each of these blocks has to
carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation
is limited Only when all three bridges fail the excitation is switched off Each thyristor
bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any
arm is identified by a Current flow monitoring module
43 Converter Cooling
A cooling system is needed to dissipate heat losses in the converter blocks and
electronics Each converter block has therefore been equipped with a fan supplied with
power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing
cubicle) The fans are protected with motor protection circuit breakers An air flow
monitoring unit is provided for monitoring the air flow through the thyristor bridge If a
circuit breaker failure is detected or if the air flow monitor drops off at one of the
thyristor bridges the bridge involved is immediately set out of operation by blocking its
firing pulses
44 Thyristor Converter Monitoring
A thyristor bridge in which defects occur that could threaten the safety of
operation or cause secondary damage is switched off automatically ie its firing pulses
are blocked This happens whenever A thyristor fuse is blown The fuses are monitored
individually with micro switches The Final Pulse Stage fails which is detected by
internal monitors (supply voltage sustained pulse short-circuit on the output end) The
power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or
is insufficient Isolator on ACDC side is open
CHAPTER 5
Field Current Circuit Excitation Transformer
51 Field Circuit Breaker
The circuit-breaker in the field circuit is used to isolate the field circuit from the
converter It is capable of switching off the synchronous machine from full load under the
maximum conditions of a 3-phase short-circuit In addition to its main contacts the field
circuit-breaker also has a de-excitation contact with which the field energy stored in the
field can be dissipated across the de-excitation resistor The de-excitation contact closes
shortly before the main contacts open so as to ensure proper commutation of the field
current from the main contacts to the de-excitation contact when the breaker is switched
off The field circuit-breaker is switched on by electromagnetic force and is kept switched
on by a mechanical latch When the latch is released by a trip coil the circuit-breaker
opens The circuit-breaker also has auxiliary contacts that report its status
52 Field Flashing
In shunt supplied excitation circuits (excitation transformer connected to the
generator terminals) the generator does not have enough remnant voltage for a generator
voltage build-up via the converter In this case a field flashing circuit is provided It
consists of the field flashing contactor the diode bridge and a transformer used to adapt
the auxiliary input voltage to the voltage needed for field flashing when power is
supplied from the auxiliaries network
Fig Field Flashing
Because the field flashing contactor is not able to switch off the energy stored in
the field the control ensures that the contactor can only reopen if the field circuit breaker
has already been opened (generating the TRIP order) or in a normal field flashing
sequence when the converter has taken over the field current Field flashing occurs in the
following stages
1048729The excitation is switched on closing the field flashing contactor ( Field
Circuit Breaker is already closed )
1048729The start-up excitation current flows through the rotor driving the generator
voltage up to approx 15 U
1048729After about 10 U the firing pulses to the converter are released and it begins
to excite the generator to its rated voltage
1048729After about 30 U the field flashing contactor opens (with no current since
the converter is now supplying the current)
The diode bridge at the input to the field flashing contactor prevents a feed-back
from the converter to the source of field flashing while the contactor is still closed
53 De-excitation
When malfunctions occur the stored field energy must be dissipated as quickly
and safely as possible to protect the generator This is done by the converter the field
circuit-breaker and the de-excitation (discharge) resistor
De-excitation (with opening of the field circuit-breaker) takes place in the following
stages
1048729The converter drives to its inverter limit position (negative ceiling voltage)
recovers a portion of the field energy into the network A trip command is given to the
field circuit breaker
1048729The de-excitation contact closes diverting the field voltage to the de-excitation
resistor
1048729Then immediately the main contacts open building voltage The field voltage
commutates to the de-excitation resistor
1048729The current diminishes at a given time constant TE
(With linear resistance TE = Lf (Rf + Re))
Due to the reversal of the field voltage by the converter the field current
commutates from the main contacts of the field circuit-breaker to the de-excitation
resistor in a very early phase This reversal of the field voltage prevents burn-off on the
main contacts and provides effective protection for the field circuit-breaker Depending
on the operating policy an operational shut-down of the excitation can also be effected
with the field circuit-breaker closed This method is useful mainly when the excitation is
switched on and off frequently In this case the converter is merely driven into the
inverter limit position so that the field energy is recovered into the network The
converter then blocks since it is supplying positive current only
54 Excitation Transformer
The excitation transformer matches the generator voltage to the field voltage
(required ceiling voltage) It also serves as a commutation reactance for the thyristor
converter and as a potential isolator between the network and the excitation circuit In
addition the transformer functions as a current limiter in that it makes it possible to keep
any short circuits in the excitation circuit under better control The excitation transformer
is equipped with temperature monitoring probes which set off an alarm when the
temperature exceeds a first max limit and then trips the excitation if the temperature
continues rising to a second (higher)limit
CHAPTER 6
Monitoring and Protection
61 Excitation Monitoring
611 General Information
The main goal of Excitation Monitoring is to make optimum use of the
redundancies provided in the excitation system and to give alarm whenever a malfunction
makes these redundancies unavailable The field current is monitored to see that it does
not exceed a maximum level and if necessary a switch-over to the MANUAL channel is
initiated In addition the criterion for switching off the field flashing is generated The
excitation Monitoring consists of an autonomous processor system
612 over current Alarms
In the Excitation Monitoring the limits for are set at higher levels than the
settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of
the nominal field current contact R1 and the binary output associated with it are
activated immediately If field current remains gt 110 then after a preset inverse-time
has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs
associated with them are activated Parameters match the measurements for If1 and If2 to
the nominal value for field current so that the internal values can be processed and read as
pu values It can be used to falsify the actual value of the field current If (to raise it) so
as to cause a response from the alarm limits for purposes of testing The processed If
signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As
long as the field current If is above the threshold value 11 Ifn its peak value is
measured This is stored (until RESET) and can be read at any time on the Micro-
Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever
the integrated time-current value (intisup2dt) exceeds the preselected reference value the
alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously
issued to switch over to the stand by AUTO channel Software switch F758 enables the
three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-
time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the
desired limit curve for response is set using the factor F216
613 Switch-Off Criterion for Field Flashing
The Excitation Monitoring supplies the criterion for switching off the field
flashing Whether this criterion is activated based on the actual value for generator voltage
Ug or for field current If or both depends on the settings of the two threshold values
F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements
Ug12 and If12 are switched over depending on the present status of the channels
(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is
fixed at ldquological 1rdquo
614 Storage of Alarm Status
The outputs of the over current alarms (R1 R2 R0) and the messages NO
FAILURE MONITORING PARAMETERS CHANGED are stored messages can be
erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the
front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the
situation causing the alarm or the malfunction is no longer present Whenever the self-
diagnosis equipment in the processor detects a malfunction the output NO FAILURE of
MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo
is activated whenever parameters or settings of software switches have been changed via
the Micro-Terminal
615 Actual Value Monitoring
The actual values for generator voltage Ug synchronous voltage Usyn and field
current If are monitored for malfunctions This monitoring is active regardless of whether
or not the generator is in operation Essentially when the generator is in operation the
measurements are monitored by comparing the signals (the smaller signal reading is
detected as incorrect) When the generator is not in operation the measured data are
monitored for extreme values The percentage of deviation permissible in the
measurement signals being compared
(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and
F20AIf the excitation transformer is being supplied from an auxiliary power source (no
shunt operation) the values of Ug and Usyn will be different in some operational
conditions
In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1
and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the
messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2
and the binary message from CH1 reports no malfunction a malfunction on
Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is
also present whenever the binary message CH1 DISTURBANCE is reported and a
discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2
FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares
Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the
comparison channel reports a malfunction or whenever both binary messages report no
malfunction - but both secondary monitors report a malfunction As long as the secondary
monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction
signals for the measurement channel involved (suspicion that there is a corresponding
error in Usyn) The measurement channel malfunctions are enabled operationally
whenever after excitation has been switched on generator voltage Ug exceeds the value
set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is
switched off to see that they do not exceed the limit value F210 that applies to both of
them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt
the operating range) Monitoring for extreme values is likewise enabled during normal
operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the
malfunction signals to the binary outputs Basically the monitoring of the actual values
for If1If2 functions like that of the Ug1Ug2 monitoring
62 Excitation Protection
621 General Information
The Excitation Protection switches off the excitation (and de-excites the machine
rapidly) whenever a danger arises that threatens the excitation transformer the converter
or the generator Generally limiter or monitoring functions precede the emergency trips
and these normally respond before the Excitation Protection must initiate a trip
Protective trip commands are issued directly to the field circuit-breaker from potential
free contacts of the board via the trip relays They are directed redundantly to the
operative field circuit-breaker ldquoOFFrdquo command
622 Protection against Excitation Transformer Overheating
This equipment monitors the excitation transformer for overheating in the
windings that could result from over current short-circuits or inadequate cooling The
monitoring uses temperature monitoring modules in conjunction with temperature
sensors built into the windings Normally the temperature is monitored in two stages the
first stage sets off an alarm the second causes a trip of the excitation
623 Rotor Over voltage Protection
Malfunctions in the generator circuit (eg terminal short-circuit failed
synchronization asynchronous operation) cause induced negative field currents that
produce high voltages in the field circuit These must be restricted to a level with a
sufficient safety margin below the insulation capacity of the field winding (test voltage)
and also below the peak blocking voltage of the converter thyristors The crow bar
employs spark gap elements to detect over voltages in the field circuit Whenever they
respond the associated thyristors are fired immediately switching the de-excitation
resistor parallel to the field The de-excitation current generated thereby initiates an
excitation trip via a supervision circuit causing an immediate opening of the field circuit-
breaker The malfunction isets off an alarm and an internal malfunction is indicated at the
cubicle
TEST VALUES OBTAINED WHEN EXCITATION IS RAISED
TEST
SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE
1 VREF 996 100
2 VACT 997 1003
3 IFACT 735 765
4 IGACT 703 707
5 ACTIVE
POWER
703 705
6 REACTIVE
POWER
101 142
7 POWER
FACTOR
099 IND 098 IND
8 ACTIVE
CURRENT(IR)
705 703
9 REACTIVE
CURRENT(IX)
102 138
10 POWER
ANGLE
566 546
11 FIRING
ANGLE
640 632
VALUES OBTAINED WHEN EXCITATION IS LOWERED
SNO PARTICULARS ACTUAL
VALUE
OBTAINED VALUE
1 VREF 100 997
2 VACT 100 997
3 IFACT 787 761
4 IGACT 839 837
5 ACTIVE
POWER
839 830
6 REACTIVE
POWER
155 110
7 POWER
FACTOR
098 IND 099 IND
8 ACTIVE
CURRENT(IR)
835 830
9 REACTIVE
CURRENT(IX)
140 94
10 POWER
ANGLE
613 DEG 631 DEG
11 FIRING
ANGLE
601 DEG 599 DEG
CONCLUSION
For generating the EMF in stator winding excitation is required to the rotor of a
generator There are two types of excitation
1 Static excitation system
2 Brushless excitation system
A certain disadvantage in brushless excitation system is the slow response time of
the field in case of fast load changes specified No slip-rings and brushes direct
measurements of the field parameters not possible
To avoid all loses static excitation is used Since it does not have any rotating
parts mechanical loses and windage loses This system has fast response and speed
control While preferring this excitation system there are no limitations for the
redundancy of Thyristor bridge circuits
Static excitation has fast field discharge by resistor and inverter operation direct
measurement of field quantity is possible The meaning of excitation is nothing but
continuous supply of DC current (ie field current) to the rotor to buildup required
output voltage in the stator
Field current is changed with respect to the change of load so the digital
automatic voltage regulator (DAVR) is used to regulate the output voltage according to
the load variations
So we conclude that static excitation system with DAVR is preferred since it is
having excellent dynamic performance and better options for R amp M
231 General Information
The functions of the automatic voltage regulator AVR are
1 to regulate the generator voltage
2 to regulate the effect of the reactive andor active current on the voltage
3 to limit VoltHz
4 to limit max and min field current
5 to limit inductive stator current
6 to limit capacitive stator current
7 to limit the load angle
8 to stabilize the power system
Block Diagram shows the software structure of AUTOMATIC channel The
generator limiters not provided for the installation in question (optional equipment) are
identified in this overview as ldquoNot Suppliedrdquo The parameter values signal values and
software switches (flags) marked with addresses (hexadecimal numbers) can be viewed
and altered via the Micro-Terminal The values selected are displayed in sec pu Hz
etc and can where necessary be changed directly in these formats The plant-specific
settings of the variables and the flags can be obtained from the Test and Commissioning
Report This block diagram provides information about the important functions and
possible settings of the AUTOMATIC channel For the sake of clarity no detailed
presentation has been given of special functions such as tracking circuits initializations
etc The page heading cross-refers this overview to the various sheets of the schematic
diagram Binary signals are shown in broken lines analog signals in solid lines The
corresponding text designations in the schematic diagram can be used for identification of
the input signals (hardware inputs) The only analog output signal from the automatic
voltage regulator control variable Ucontr is sent via the data bus (CRU bus) to the Gate
Control Unit Most of the binary messages (outputs) from the AVR are of no interest
functionally and they have been omitted for the sake of clarity The basic structure of the
digital voltage regulator and the limiters is simple This is necessary in order that the
behavior of the regulatorslimiters will remain calculable and understandable in all
operating situations and that there will be no problem in adjusting and optimizing them
The central PID filter in the digital voltage regulator defines the dynamic response of the
closed-loop controls both in the voltage regulator mode and after limiters have
intervened The ldquocontrol deviationrdquo at the input to the PID filter is either the control
deviation for voltage the control deviation of a de-excitation limiter (the value
determined by minimum value selection) or the control deviation of an excitation limiter
(the value determined by maximum value selection) Flag F730 (ldquoPRIORrdquo) is used to
determine whether the exciting (Min value) or the de-exciting signal takes precedence on
the minmax value limiter (normally F730 = 1111 ie the de-exciting signal takes
precedence) With the exception of the Minimum Field Current Limiter all other limiters
have variable factoring multipliers of the signal outputs so that they can be adjusted
individually together with the common PID filter which has been optimized for voltage
regulation The setting parameters for this PID filter are as follows
Vo = KR Static amplification
1
Ta = ---- Integration time constant
Tc1
Vp Proportional amplification
1
Tb = ---- Differential time constant
Tc2
Vinfin Amplification of high frequencies
The BODE diagram below shows the assignment of settings in accordance with
DINIEC standards based on a typical example
The PID filter amplifications Vo Vp and Vinfin can be adjusted in pu values But
the ldquoceiling factorrdquo pl+ must be adjusted correctly with parameter F310 if the total
amplification (circuit amplification) of the control circuit is actually to conform to the
pu settings This factor must agree with the ldquoexternalrdquo amplification ie with the
ceiling value of the transformer- converter circuit
Ceiling factor(pl+) = Ufmax Ufo
in which Ufmax = ceiling field voltage
Ufo = no-load field voltage
To attain a suitable response of the AVR when starting excitation
(ldquoEXCITATION ONrdquo) it may be necessary to change the proportional amplification of
the regulator during this phase Vp2 (transiently activated) and Vp1 (permanently
activated) can be adjusted for this purpose For example the value of Vp2 takes effect
immediately once the excitation is switched on and remains effective for a period as set at
F30C Once the period F30C (eg 5 sec) has expired Vp shifts over to Vp1 (becomes
the steady-state Vp) at the rate of change set The standard operating mode for the PID
filter is voltage regulation for which the discrepancy between the voltage set-point and
the current value for generator voltage Ug (the control deviation) is supplied at the input
To compensate for the voltage drop in the block transformer or whenever several
generators are operating to the same distributing bus the generator voltage must be
varied in proportion to the measured generator current (droop influence) To accomplish
this the voltage set-point is varied as a function of the measured reactive current IX
andor active current IR Flag F712 enables the IX droop Flag F710 the IR droop The
desired compensation is set in F282 and F286 respectively Flags F284 and F288 are used
to select whether this droop influence is to increase the voltage or to reduce it
(compensation) Combined influence of the active and reactive currents is attained by
enabling both droops IX and IR Flag F716 activates a so-called ldquoSoft-Startrdquo at the
starting of excitation This ldquoSoft-Startrdquo ensures that the voltage set-point integrates from
0 to 100 within the time set on F290 when the excitation is switched on
(ldquoEXCITATION ONrdquo) A ldquosmoothrdquo excitation of the generator can be achieved in this
way whenever there is no demand for a quick excitation
232 Voltage Set-Point
Various signals and settings control and limit the voltage set-point F270 For
example the values of F254 and F252 define the normal operating range possible for set-
point adjustment (eg 90 110) using external control commands (control room local
operatorrsquos panel superposed control system) The effective set-point adjustment rate is
governed The set-point can be set at the values of F250 and F256 by activating
appropriate control commands for ldquoSETrdquo input Enabling Flag F71A and activating a
binary input prior to switching on the excitation (ldquoEXCITATION OFFrdquo) sets the Ug set-
point at the value of UAUX This makes it possible for example to ensure that the
generator voltage will agree exactly with the network voltage after the voltage build-up
An external value with variable amplification can be added to the Ug set point by
enabling F724 (for example for stability tests)
233 Regulator Tracking in MANUAL Operation
Whenever the AUTOMATIC channel is not in operation (the MANUAL channel
is ON) a follow-up equipment ensures a smooth switch-back to the AUTOMATIC
mode will always be possible To track the voltage set-point is shifted by means of
RAISELOWER pulses from the Gate Control Unit so that control variable Ucontr at the
output from the PID filter is held steady and identical to the control variable Ucontr from
the MANUAL channel Because this tracking must react slowly resultant transient
control deviations resulting from the amplification in the PID filter might cause severe
interference with control variable Ucontr
To prevent this the follow-up equipment intervenes on the regulators mixing
point with a corresponding compensation signal
234 Ugf Limiter
At under frequency the Ugf Limiter reduces the generator voltage so as to
prevent saturation effects in the supply and measuring transformers To adjust this
limiter the max permissible generator voltage at rated frequency is defined and set
When any under-frequency occurs the generator voltage is thus reduced in proportion to
that setting
235 Field Current Maximum Limiter
The Field Current Maximum Limiter is provided to protect the generator rotor
from s occurring in steady-state and transient operation High field currents are normally
the result of a sharp drop in network voltage or of an improper raising of the voltage set-
point by the operating staff The field current is held steady at the value TH1 ie at the
maximum thermal value permissible for the excitation circuit and the rotor In order that
the generator can support the power network with its transient overload capacity during
brief collapses in voltage a temporary switch-over is made to the transient limit MAX1
(a higher setting) When the generator or the converter is operating at a reduced capacity
These limits TH1MAX1 can be switched over to the lower settings TH2MAX2 by
activating the corresponding binary signals The switch-over from the thermal limit
TH12 to the transient limit MAX12 can be configured in one of three ways
a) Depending on the over current with -dUdt ENABLE
Flag programming F418 = any setting desired F41A = 0000
This variant enables the transient value MAX12 whenever a collapse of voltage
in the network is detected The ENABLE time is fixed and can be set The example
below shows the typical behavior of the limiter configured in this way
b) Dependent on the time integral with -dUdt ENABLE
Flag programming F418 = inactive F41A = 1111
This variant likewise enables the transient value only when a collapse of network
voltage has been detected However the switch-back to the thermal limit is not made
dependent upon the time itself but on the calculated time integral intisup2dt of the The setting
on Parameter F414 in spu takes into account the time the rotor needs to cool down ie
the rate of temperature change in the case of intermittent operation The example below
shows how the timing of the switch-back to the thermal limit depends on the present
value for intisup2dt
The time integral is based on the formula
Example The setting of =isup2dt equivalent to Version a (F416) at a constant 16 times the
nominal field current for 10 seconds (with TH12 = 105) is
c) Dependent on the time integral without any preconditions
Flag programming F418 = 1111 F41A = 1111
In this variant the transient becomes available without any prior conditions
(without a -dUdt ENABLE) with the time integral intisup2dt
237 Inductive Stator Current Limiter
The Inductive Stator Current Limiter holds the stator current Ig within permissible
limits while the generator is in the ldquoover-excitedrdquo operating range by reducing the field
current accordingly The setting TH (thermal limit) provides the limit against stationary s
that might occur To take advantage of the generatorrsquos transient overload capacity a
switch-over is made to the higher setting MAX The principle of operation of this switch-
over to the value MAX permissible only transiently is identical to that employed for the
field current limiter (refer to the description above) When the drive output from the
turbine is very high stator current may exceed permissible limits even while inductive
loading of the generator is low In this case if the stator current limiter is not kept from
influencing the field current the control circuit will oscillate back and forth between the
Inductive Stator Current Limiter (de-
exciting) and the Capacitive Stator Current Limiter (exciting)The output signal of that
function then dominates the control variable of the Ig-dependent limiter via a maximum
value selection
238 Capacitive Stator Current Limiter
239 Load Angle Limiter
The Load Angle Limiter prevents the synchronous machine from slipping out of
phase due to slippage of the rotor The load angle δ the difference in phase between the
rotor and the stator rotating field results mainly from the driving torque (active power P)
acting on the generator and the level of rotor current (field current) If the driving torque
remains constant a increase in the field current reduces the load angle δ The current load
angle δ at any moment is obtained from the generator current and generator voltage based
on a simplified model of the generator Whenever this calculated load angle δ exceeds the
preset limit angle the limiter increases the field current until the load angle has dropped
back to its permissible value The quadrature reactance Xq of the generator and the
network reactance Xe during normal operation must be adjusted on the regulator in order
to obtain the load angle δ The graph below shows the Power Chart for a salient-pole
machine with typical limiter characteristics
The purpose of a Power System Stabilizer is to use the generator excitation to
damp electromechanical oscillations between the network and the generator Depending
on the design of the generator and the requirements imposed for network stability its
main function will be either to damp the oscillations originating in the machine or those
from the network A synchronous generator working in a combined power network is in
principle an oscillating structure In order to produce a torque the magnetic field of the
rotor and the stator must form a given angle (referred to as the rotor displacement or load
angle δ) The electrical torque ME increases as the angle δ increases just as with a
torsion spring Because the ME of the generator and the mechanical driving torque MA
from the turbine are in equilibrium during steady-state operation the angle δ remains in a
given position Whenever this state of equilibrium between MA and ME is disturbed the
load angle slips of this rest position and change thereby the electrical torque ME The
torque attempts to restore the load angle to a stationary position Due to the mass inertia
of the turbinegenerator rotor however this can only take place aperiodically It does so
in the form of more or less effectively damped oscillations (again similar to the effect of
mass inertia on a torsion spring) In order to damp the oscillations there must be a
damping torque produced depending not on the electrical torque ME associated with the
angle but on the difference in frequency (Df) between the rotor and the stator rotating
field ie on the slippage This torque is produced mainly by the so-called damper
winding in the rotor but the dimensioning of this is subject to limits imposed by
considerations of design and economy Some further action is therefore needed to
increase the damping effect The following drastically simplified formula shows the
parameters upon which the amount of active power PE supplied by the generator
depends
PE = active power
It can be seen from the above relationship that the active power that the generator
transfers depends not only on the load angle δ but also on the field current If That means
that a transient change can be made in the active power PE and with that in the effective
electrical torque ME by varying the field current The principle of operation of the DVR
Power System Stabilizer becomes clear from a consideration of the oscillations in power
output and frequency (ΔPE Δf) and the vector diagram If it is assumed that oscillations
in the network frequency generates load oscillations with the mass inertia of the rotor
then the active load of the generator (eg MW-measured) is influenced with a sinusoidal
value -ΔPE (ME-MA = -ΔPE) By inversion of -ΔPE one obtains the fluctuation in
power provided by the rotor +ΔPE As is known the slip signal Δf follows +ΔPE with a
phase delayed by 90deg The +ME produced by the periodic changes in the load angle δ is
in phase with +ΔPE A good damping is attained if ME is varied in phase with the slip
Δf However this signal must also be advanced somewhat to compensate for the time
constants in the excitation circuit and the generator
As mentioned above the electrical torque ME can be influenced by varying the
field current To accomplish this a suitable control signal referred to as variable
disturbance compensation must be imposed upon the voltage set-point or the converter
control variable Ucontr As can be seen from the vector diagram by applying proper
weighting factors (K1 K2) and then adding together the signals -ΔPE and Δf an overall
stabilization signal can be produced that rotates in advance of the Df signal by any angle
desired between 0deg and 90deg Because the amplitude of -DPE remains proportional to the
amplitude of Δf a constant angle in advance of Δf results for the compensation of the
time constants referred to above The optimum weighting factors K1 and K2 for a
synchronous generator working to a power network depend on its operating point at any
moment and the external reactance of the network Normally the selection of a
compromise setting is good enough to attain stability in all operating points and for all
external reactance For special demands these settings must be parameterized as a
function of the external reactance (which means optional equipment Xe-Identification)
The Power System Stabilizer PSS is a section of the AVR computer program and is
processed once per network cycle The voltage at the generator terminals and the
generator current are measured in order to define the signals ΔPE and Δf The calculated
signals for _P_ (=PE) and Δf are then sent across DC filters ldquoDrdquo (real differentiators) that
transmit only the dynamic portion of the signals The ΔPE and Δf signals obtained in this
way are then weighted (multiplied by) with the factors K1 and K2 and sent to the
summing point of the voltage regulator
The PSS stabilization signal is imposed on the automatic voltage regulator only if
the following prerequisites are met
bull Generator on line
bull Generator power output gt the value F338
bull Generator voltage in a range between F33C and F33A
The stabilization signal is limited at the output from the PSS to the lower and
upper limits Flag defines whether the stabilization signal is introduced before or after the
PID filter (usually before the filter) Because the PID filter as noted above already takes
the ceiling factor Vp1 into account the PSS signal needs to be multiplied by Vp1 if it is
added to the voltage regulator following the PID filter (divider at the input to the
minmax limiter) This precaution prevents the DC filter ldquoDrdquo in the P-channel from
producing an unnecessary ldquostabilizationrdquo effect in the case of rapid changes in turbine
load As an alternative for the AVRrsquos Power System Stabilizer a stabilization signal from
an outside system can be imposed by activating the binary input ldquoPSS-SIGNEXTrdquo Flag
F340 can be used to select between an analog and a 12-bit signal and F33E to select the
polarity desired for that signal
24 The MANUAL Channel
241 Summary
The MANUAL channel (Channel 2) has been built as a simple field current
regulator
without additional limiters Its main function is to maintain the excitation of the generator
even if the AUTOMATIC channel becomes non-operational The MANUAL channel
also performs valuable service for purposes of testing commissioning and preventive
maintenance Its measurements regulator generation of firing pulses and power supply
are physically separate from those on the AUTOMATIC channel
242 Principle of Operation
All the functions of the MANUAL channel including the generation of firing
pulses have been implemented in a single electronic module the Gate Control Unit The
control variable Ucontr of voltage regulator is used as the reference value for generating
firing pulses on the principle known as ldquoramp controlrdquo (Comparison of Ucontr with
Usynsynchronous sawtooth signal) For further processing in the UN 0096 Intermediate
Pulse Stage the Gate Control Unit supplies six firing pulses at its output whose phase
position with respect to the synchronous voltage Usyn is in accordance with control
variable Ucontr An internal linearization ensures that the field voltage produced via the
firing pulses remains proportional to the control variable Ucontr throughout the entire
range As a result the circuit amplification of the control remains constant over the entire
range Whenever excitation is switched ON the set-point for Generator Voltage is set
automatically at the preset - ref Value This provision ensures that the generator voltage
always attains approximately its nominal value after the field flashing The Gate Control
Unit can be refunctioned ( by pre-selection with a switch ) for purposes of testing to act
as a purely firing pulse control In this case the control variable Ucontr is adjusted
directly using the RAISELOWER push buttons on the front of the module In this way
for example the relationship between the phase position of the firing pulses and the
control variable Ucontr can be checked easily
CHAPTER 3
PULSE SECTION
31 Pulse Generation and Amplification
The Gate Control Units of both AUTOMATIC channel and MANUAL channel
each supply six firing pulses for operating the 6-pulse thyristor bridges The low-power
pulse signals from these Gate Control Units are then amplified in the Intermediate Pulse
Stage galvanically isolated and then sent to the common pulse bus On the output end
the Intermediate Pulse Stage of the non-active channel) is always blocked The Gate
Control Units generate the pulses based on microprocessor control The reference voltage
used for the firing pulse phase location is the output voltage from the excitation
transformer (Usyn1 Usyn2) The commutation spikes of the synchronous voltage caused
by the converter are calculated prior to use of the voltage as a reference value and are
deliberately filtered out The lower limit for the firing pulses (double pulses) which are
offset from one another by 60deg is defined by the limit rectifier position (αmin) and the
upper limit by the limit inverter position (αmax) for the firing angle αmin and αmax can
be adjusted on the Gate Control Units using BCD (Binary Coded Decimal) switches
αmin ensures that the firing pulses will not be issued (premature firing) until there is
sufficient positive phase voltage on the thyristor involved αmax prevents a dangerous
ldquotippingrdquo of the thyristor bridge into the rectifier mode if the firing angle α is too large
(ldquolate firingrdquo) The critical factors determining αmax are the overlap time uumlmax (max
commutation time) and the ldquorecovery timerdquo of the thyristors (αmax lt 180deg - uumlmax - γ )
An external control signal can force the firing pulses into their inverter limit position
Other binary inputs can block or direct the firing pulses of the Gate Control Units so as to
produce freewheeling on the thyristor bridge During freewheeling the firing pulses for
the thyristor pair R and S are blocked and the pulse signals T+T- are engaged with
chains of pulses Both Gate Control Units (for the MANUAL amp AUTOMATIC channels)
contain a field current monitor that blocks the firing pulses immediately whenever the
current exceeds a preset threshold level In this case the field circuit-breaker is also
tripped via an output contact The purpose of these provisions is to prevent damage to
thyristors and thyristor fuses in case of a slip-ring short-circuit or to keep any damage
that does occur to a minimum The pulse signals are galvanically separated at the outputs
from the Intermediate Pulse Stage (with pulse transmitters) and are then directed to the
common pulse bus This transmission of the pulse signals to the pulse bus via passive
transmitters ensures a high degree of active channel autonomy Practically no possible
malfunctions on the inactive channel (including for example sustained pulses) affect the
active channel
32 Pulse Monitoring
The ldquoPulse Busrdquo and the pulse signals of the AUTOMATIC channel are
monitored This monitoring device consists of potential isolating stages and the common
monitor If the pulse monitoring of the ldquoPulse Busrdquo responds a switch-over is made to
MANUAL channel The function of the potential isolating stages is to couple the pulse
monitoring device to the pulse circuits without any feedback effect The pulse monitoring
checks the six pulse lines for the following malfunctions continuous or periodic failure
of one or more pulses Periodic occurrence of synchronous or asynchronous false pulses
Continuous pulses the pulse monitoring device can be tested while the machine is in
operation
CHAPTER 4
CONVERTER
Thyristor
The term thyristor usually refers to a family of four layer solid state device having
turn on characteristics that can be externally controlled by either current or voltage They
are also referred to as breakdown device because their working depends on avalanche
breakdown Thyristors have only two stages OFF and ON Thyristors have a similar
function to Uni-junctions they act as switches Thyristors use current flow as a switch
Thyristors have three states
1 Reverse blocking mode mdash Voltage is applied in the direction that would be
blocked by a diode
2 Forward blocking mode mdash Voltage is applied in the direction that would cause
a diode to conduct but the thyristor has not yet been triggered into conduction
3 Forward conducting mode mdash The thyristor has been triggered into conduction
and will remain conducting until the forward current drops below a threshold value
known as the holding current Converter is a semiconductor device which converts ac
input voltage into a constant dc output voltage In present excitation system three phase
fully controlled thyristor converter is used
Because of the following advantages thyristor converters are used
a) Thyristors are used for high power applications ie up to 10Kv3500A1KHz
b) Having high reliability and low losses
c) Uni-directional device like diode
d) Itrsquos operation as a rectifier which are low resistance in forward conduction
mode and high resistance in reverse conduction mode
PROTECTION OF THYRISTORS
For reliable operation of a thyristor demands that its specified ratings are not
exceeded When Subjected to or over voltages During the turn - on of SCR didt
prohibitively large False triggering of SCR by high value of dvdt andSpurious signals
between gate and cathode may leads to unwanted turn ndash on
DIDT AND PROTECTION
When thyristor starts conducting in forward conduction mode and is turned on by
gate pulse The anode current increases rapidly whole area of the gate to Cathode
junction then hot spots will be formed near the gate connection this locality of heating
destroys the thyristor Thyristor thermal time is constant The causes due to faults and
short circuits or surge currents Electronic crowbar protection is used against the over
voltages The rate rise of anode current must be kept at the time of turn on below the
rated or specified limiting value The didt value maintained below limited value by using
a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating
is avoided by applying gate current but not greater the maximum gate current
DVDT AND OVER VOLTAGE PROTECTION
With forward voltage across the anode and cathode of a thyristor the two outer
junctions are forward biased but the inner junction is reverse biased This reverse biased
junction J2 has the characteristics of a capacitor due to charges existing across the
junction In other words space-charges exist in the depletion region around junction J2
and therefore junction J2 behaves like a capacitance If the entire anode to cathode
forward voltage Va appears across J2 junction and the charge is denoted by Q then a
charging current i given by Eq (46) follows
i = dQdt =d(Cj Va )dt
= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)
As Cj the capacitance of junction J2 is almost constant the current is given by
i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)
If the rate of rise of forward voltage dVadt is high the charging current i will be
more This charging current plays the role of gate current and turns on the SCR even
when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on
must be avoided as it leads to false operation of the thyristor circuit
For controllable operation of the thyristor the rate of rise of forward anode to
cathode voltage dVadt must be kept below the specified rated limit Typical values of
dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by
using a snubber circuit in parallel with the device thyristor are very sensitive for over
voltage than the semiconductor devices
Over voltage transients are perhaps the main cause of thyristor failure
In thyristor there are mainly two types
1 Internal over voltages
Due to the commutation of the thyristors large voltages are generated internally
Because of the series inductance of the SCR circuit the large transient voltages L didt
produced This voltage several times the break over voltage of the device then thyristor
destroys permanently
2 External over voltages
External over voltages are caused due to the interruptions of current flow in an
inductive circuit and also due to the lightening strokes on the lines feeding the thyristor
system For the reliable operation of thyristor the over voltages must be suppressed by
adopting suitable techniques
Suppression of over voltages
The RC circuit called snubber circuit is connected across the device to protect In
order to keep the protective components to a minimum the thyristors are chosen with
their peak voltages ratings are 25 to 3 times of the normal peak working voltage
ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are
commonly employed for protecting the thyristor circuit against the over voltages
Gate protection
Gate circuit should also be protected against the over voltages and surges Over
voltage at gate circuit can cause false triggering of the SCR may rises the junction
temperature behind specified limit leading to its damage Protection against over
voltage can be achieved by connecting a ZD across the gate circuit and a resister is
connected in series with gate circuit to protect against the s A capacitor and resister are
connected across gate to cathode to by pass the noise
41 Final Pulse Stages
The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage
(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor
bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided
with a power supply module The amplified output pulses from the Final Pulse Stages
start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main
pulse This initial pulse edge assures proper firing of the thyristors being triggered
Subsequently the weaker part of main pulse keeps firing conditions steady As already
mentioned the Final Pulse Stages and their associated thyristor bridges form single units
All six pulse outputs from a Final Pulse Stage can be blocked by an external control
signal so that all thyristors in the associated thyristor bridge will block the current A
blocking of the pulses is initiated whenever there is a malfunction in the associated
thyristor bridge
42 Converter Power Section
The thyristor converter consists of three independent parallel rectifier blocks TY1
to TY3 which are all in service Even if one block fails the remaining blocks take over
automatically the full design current of the excitation circuit During normal operation
(with ideal current share) and all three bridges in operation each of these blocks has to
carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation
is limited Only when all three bridges fail the excitation is switched off Each thyristor
bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any
arm is identified by a Current flow monitoring module
43 Converter Cooling
A cooling system is needed to dissipate heat losses in the converter blocks and
electronics Each converter block has therefore been equipped with a fan supplied with
power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing
cubicle) The fans are protected with motor protection circuit breakers An air flow
monitoring unit is provided for monitoring the air flow through the thyristor bridge If a
circuit breaker failure is detected or if the air flow monitor drops off at one of the
thyristor bridges the bridge involved is immediately set out of operation by blocking its
firing pulses
44 Thyristor Converter Monitoring
A thyristor bridge in which defects occur that could threaten the safety of
operation or cause secondary damage is switched off automatically ie its firing pulses
are blocked This happens whenever A thyristor fuse is blown The fuses are monitored
individually with micro switches The Final Pulse Stage fails which is detected by
internal monitors (supply voltage sustained pulse short-circuit on the output end) The
power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or
is insufficient Isolator on ACDC side is open
CHAPTER 5
Field Current Circuit Excitation Transformer
51 Field Circuit Breaker
The circuit-breaker in the field circuit is used to isolate the field circuit from the
converter It is capable of switching off the synchronous machine from full load under the
maximum conditions of a 3-phase short-circuit In addition to its main contacts the field
circuit-breaker also has a de-excitation contact with which the field energy stored in the
field can be dissipated across the de-excitation resistor The de-excitation contact closes
shortly before the main contacts open so as to ensure proper commutation of the field
current from the main contacts to the de-excitation contact when the breaker is switched
off The field circuit-breaker is switched on by electromagnetic force and is kept switched
on by a mechanical latch When the latch is released by a trip coil the circuit-breaker
opens The circuit-breaker also has auxiliary contacts that report its status
52 Field Flashing
In shunt supplied excitation circuits (excitation transformer connected to the
generator terminals) the generator does not have enough remnant voltage for a generator
voltage build-up via the converter In this case a field flashing circuit is provided It
consists of the field flashing contactor the diode bridge and a transformer used to adapt
the auxiliary input voltage to the voltage needed for field flashing when power is
supplied from the auxiliaries network
Fig Field Flashing
Because the field flashing contactor is not able to switch off the energy stored in
the field the control ensures that the contactor can only reopen if the field circuit breaker
has already been opened (generating the TRIP order) or in a normal field flashing
sequence when the converter has taken over the field current Field flashing occurs in the
following stages
1048729The excitation is switched on closing the field flashing contactor ( Field
Circuit Breaker is already closed )
1048729The start-up excitation current flows through the rotor driving the generator
voltage up to approx 15 U
1048729After about 10 U the firing pulses to the converter are released and it begins
to excite the generator to its rated voltage
1048729After about 30 U the field flashing contactor opens (with no current since
the converter is now supplying the current)
The diode bridge at the input to the field flashing contactor prevents a feed-back
from the converter to the source of field flashing while the contactor is still closed
53 De-excitation
When malfunctions occur the stored field energy must be dissipated as quickly
and safely as possible to protect the generator This is done by the converter the field
circuit-breaker and the de-excitation (discharge) resistor
De-excitation (with opening of the field circuit-breaker) takes place in the following
stages
1048729The converter drives to its inverter limit position (negative ceiling voltage)
recovers a portion of the field energy into the network A trip command is given to the
field circuit breaker
1048729The de-excitation contact closes diverting the field voltage to the de-excitation
resistor
1048729Then immediately the main contacts open building voltage The field voltage
commutates to the de-excitation resistor
1048729The current diminishes at a given time constant TE
(With linear resistance TE = Lf (Rf + Re))
Due to the reversal of the field voltage by the converter the field current
commutates from the main contacts of the field circuit-breaker to the de-excitation
resistor in a very early phase This reversal of the field voltage prevents burn-off on the
main contacts and provides effective protection for the field circuit-breaker Depending
on the operating policy an operational shut-down of the excitation can also be effected
with the field circuit-breaker closed This method is useful mainly when the excitation is
switched on and off frequently In this case the converter is merely driven into the
inverter limit position so that the field energy is recovered into the network The
converter then blocks since it is supplying positive current only
54 Excitation Transformer
The excitation transformer matches the generator voltage to the field voltage
(required ceiling voltage) It also serves as a commutation reactance for the thyristor
converter and as a potential isolator between the network and the excitation circuit In
addition the transformer functions as a current limiter in that it makes it possible to keep
any short circuits in the excitation circuit under better control The excitation transformer
is equipped with temperature monitoring probes which set off an alarm when the
temperature exceeds a first max limit and then trips the excitation if the temperature
continues rising to a second (higher)limit
CHAPTER 6
Monitoring and Protection
61 Excitation Monitoring
611 General Information
The main goal of Excitation Monitoring is to make optimum use of the
redundancies provided in the excitation system and to give alarm whenever a malfunction
makes these redundancies unavailable The field current is monitored to see that it does
not exceed a maximum level and if necessary a switch-over to the MANUAL channel is
initiated In addition the criterion for switching off the field flashing is generated The
excitation Monitoring consists of an autonomous processor system
612 over current Alarms
In the Excitation Monitoring the limits for are set at higher levels than the
settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of
the nominal field current contact R1 and the binary output associated with it are
activated immediately If field current remains gt 110 then after a preset inverse-time
has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs
associated with them are activated Parameters match the measurements for If1 and If2 to
the nominal value for field current so that the internal values can be processed and read as
pu values It can be used to falsify the actual value of the field current If (to raise it) so
as to cause a response from the alarm limits for purposes of testing The processed If
signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As
long as the field current If is above the threshold value 11 Ifn its peak value is
measured This is stored (until RESET) and can be read at any time on the Micro-
Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever
the integrated time-current value (intisup2dt) exceeds the preselected reference value the
alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously
issued to switch over to the stand by AUTO channel Software switch F758 enables the
three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-
time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the
desired limit curve for response is set using the factor F216
613 Switch-Off Criterion for Field Flashing
The Excitation Monitoring supplies the criterion for switching off the field
flashing Whether this criterion is activated based on the actual value for generator voltage
Ug or for field current If or both depends on the settings of the two threshold values
F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements
Ug12 and If12 are switched over depending on the present status of the channels
(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is
fixed at ldquological 1rdquo
614 Storage of Alarm Status
The outputs of the over current alarms (R1 R2 R0) and the messages NO
FAILURE MONITORING PARAMETERS CHANGED are stored messages can be
erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the
front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the
situation causing the alarm or the malfunction is no longer present Whenever the self-
diagnosis equipment in the processor detects a malfunction the output NO FAILURE of
MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo
is activated whenever parameters or settings of software switches have been changed via
the Micro-Terminal
615 Actual Value Monitoring
The actual values for generator voltage Ug synchronous voltage Usyn and field
current If are monitored for malfunctions This monitoring is active regardless of whether
or not the generator is in operation Essentially when the generator is in operation the
measurements are monitored by comparing the signals (the smaller signal reading is
detected as incorrect) When the generator is not in operation the measured data are
monitored for extreme values The percentage of deviation permissible in the
measurement signals being compared
(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and
F20AIf the excitation transformer is being supplied from an auxiliary power source (no
shunt operation) the values of Ug and Usyn will be different in some operational
conditions
In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1
and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the
messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2
and the binary message from CH1 reports no malfunction a malfunction on
Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is
also present whenever the binary message CH1 DISTURBANCE is reported and a
discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2
FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares
Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the
comparison channel reports a malfunction or whenever both binary messages report no
malfunction - but both secondary monitors report a malfunction As long as the secondary
monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction
signals for the measurement channel involved (suspicion that there is a corresponding
error in Usyn) The measurement channel malfunctions are enabled operationally
whenever after excitation has been switched on generator voltage Ug exceeds the value
set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is
switched off to see that they do not exceed the limit value F210 that applies to both of
them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt
the operating range) Monitoring for extreme values is likewise enabled during normal
operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the
malfunction signals to the binary outputs Basically the monitoring of the actual values
for If1If2 functions like that of the Ug1Ug2 monitoring
62 Excitation Protection
621 General Information
The Excitation Protection switches off the excitation (and de-excites the machine
rapidly) whenever a danger arises that threatens the excitation transformer the converter
or the generator Generally limiter or monitoring functions precede the emergency trips
and these normally respond before the Excitation Protection must initiate a trip
Protective trip commands are issued directly to the field circuit-breaker from potential
free contacts of the board via the trip relays They are directed redundantly to the
operative field circuit-breaker ldquoOFFrdquo command
622 Protection against Excitation Transformer Overheating
This equipment monitors the excitation transformer for overheating in the
windings that could result from over current short-circuits or inadequate cooling The
monitoring uses temperature monitoring modules in conjunction with temperature
sensors built into the windings Normally the temperature is monitored in two stages the
first stage sets off an alarm the second causes a trip of the excitation
623 Rotor Over voltage Protection
Malfunctions in the generator circuit (eg terminal short-circuit failed
synchronization asynchronous operation) cause induced negative field currents that
produce high voltages in the field circuit These must be restricted to a level with a
sufficient safety margin below the insulation capacity of the field winding (test voltage)
and also below the peak blocking voltage of the converter thyristors The crow bar
employs spark gap elements to detect over voltages in the field circuit Whenever they
respond the associated thyristors are fired immediately switching the de-excitation
resistor parallel to the field The de-excitation current generated thereby initiates an
excitation trip via a supervision circuit causing an immediate opening of the field circuit-
breaker The malfunction isets off an alarm and an internal malfunction is indicated at the
cubicle
TEST VALUES OBTAINED WHEN EXCITATION IS RAISED
TEST
SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE
1 VREF 996 100
2 VACT 997 1003
3 IFACT 735 765
4 IGACT 703 707
5 ACTIVE
POWER
703 705
6 REACTIVE
POWER
101 142
7 POWER
FACTOR
099 IND 098 IND
8 ACTIVE
CURRENT(IR)
705 703
9 REACTIVE
CURRENT(IX)
102 138
10 POWER
ANGLE
566 546
11 FIRING
ANGLE
640 632
VALUES OBTAINED WHEN EXCITATION IS LOWERED
SNO PARTICULARS ACTUAL
VALUE
OBTAINED VALUE
1 VREF 100 997
2 VACT 100 997
3 IFACT 787 761
4 IGACT 839 837
5 ACTIVE
POWER
839 830
6 REACTIVE
POWER
155 110
7 POWER
FACTOR
098 IND 099 IND
8 ACTIVE
CURRENT(IR)
835 830
9 REACTIVE
CURRENT(IX)
140 94
10 POWER
ANGLE
613 DEG 631 DEG
11 FIRING
ANGLE
601 DEG 599 DEG
CONCLUSION
For generating the EMF in stator winding excitation is required to the rotor of a
generator There are two types of excitation
1 Static excitation system
2 Brushless excitation system
A certain disadvantage in brushless excitation system is the slow response time of
the field in case of fast load changes specified No slip-rings and brushes direct
measurements of the field parameters not possible
To avoid all loses static excitation is used Since it does not have any rotating
parts mechanical loses and windage loses This system has fast response and speed
control While preferring this excitation system there are no limitations for the
redundancy of Thyristor bridge circuits
Static excitation has fast field discharge by resistor and inverter operation direct
measurement of field quantity is possible The meaning of excitation is nothing but
continuous supply of DC current (ie field current) to the rotor to buildup required
output voltage in the stator
Field current is changed with respect to the change of load so the digital
automatic voltage regulator (DAVR) is used to regulate the output voltage according to
the load variations
So we conclude that static excitation system with DAVR is preferred since it is
having excellent dynamic performance and better options for R amp M
intervened The ldquocontrol deviationrdquo at the input to the PID filter is either the control
deviation for voltage the control deviation of a de-excitation limiter (the value
determined by minimum value selection) or the control deviation of an excitation limiter
(the value determined by maximum value selection) Flag F730 (ldquoPRIORrdquo) is used to
determine whether the exciting (Min value) or the de-exciting signal takes precedence on
the minmax value limiter (normally F730 = 1111 ie the de-exciting signal takes
precedence) With the exception of the Minimum Field Current Limiter all other limiters
have variable factoring multipliers of the signal outputs so that they can be adjusted
individually together with the common PID filter which has been optimized for voltage
regulation The setting parameters for this PID filter are as follows
Vo = KR Static amplification
1
Ta = ---- Integration time constant
Tc1
Vp Proportional amplification
1
Tb = ---- Differential time constant
Tc2
Vinfin Amplification of high frequencies
The BODE diagram below shows the assignment of settings in accordance with
DINIEC standards based on a typical example
The PID filter amplifications Vo Vp and Vinfin can be adjusted in pu values But
the ldquoceiling factorrdquo pl+ must be adjusted correctly with parameter F310 if the total
amplification (circuit amplification) of the control circuit is actually to conform to the
pu settings This factor must agree with the ldquoexternalrdquo amplification ie with the
ceiling value of the transformer- converter circuit
Ceiling factor(pl+) = Ufmax Ufo
in which Ufmax = ceiling field voltage
Ufo = no-load field voltage
To attain a suitable response of the AVR when starting excitation
(ldquoEXCITATION ONrdquo) it may be necessary to change the proportional amplification of
the regulator during this phase Vp2 (transiently activated) and Vp1 (permanently
activated) can be adjusted for this purpose For example the value of Vp2 takes effect
immediately once the excitation is switched on and remains effective for a period as set at
F30C Once the period F30C (eg 5 sec) has expired Vp shifts over to Vp1 (becomes
the steady-state Vp) at the rate of change set The standard operating mode for the PID
filter is voltage regulation for which the discrepancy between the voltage set-point and
the current value for generator voltage Ug (the control deviation) is supplied at the input
To compensate for the voltage drop in the block transformer or whenever several
generators are operating to the same distributing bus the generator voltage must be
varied in proportion to the measured generator current (droop influence) To accomplish
this the voltage set-point is varied as a function of the measured reactive current IX
andor active current IR Flag F712 enables the IX droop Flag F710 the IR droop The
desired compensation is set in F282 and F286 respectively Flags F284 and F288 are used
to select whether this droop influence is to increase the voltage or to reduce it
(compensation) Combined influence of the active and reactive currents is attained by
enabling both droops IX and IR Flag F716 activates a so-called ldquoSoft-Startrdquo at the
starting of excitation This ldquoSoft-Startrdquo ensures that the voltage set-point integrates from
0 to 100 within the time set on F290 when the excitation is switched on
(ldquoEXCITATION ONrdquo) A ldquosmoothrdquo excitation of the generator can be achieved in this
way whenever there is no demand for a quick excitation
232 Voltage Set-Point
Various signals and settings control and limit the voltage set-point F270 For
example the values of F254 and F252 define the normal operating range possible for set-
point adjustment (eg 90 110) using external control commands (control room local
operatorrsquos panel superposed control system) The effective set-point adjustment rate is
governed The set-point can be set at the values of F250 and F256 by activating
appropriate control commands for ldquoSETrdquo input Enabling Flag F71A and activating a
binary input prior to switching on the excitation (ldquoEXCITATION OFFrdquo) sets the Ug set-
point at the value of UAUX This makes it possible for example to ensure that the
generator voltage will agree exactly with the network voltage after the voltage build-up
An external value with variable amplification can be added to the Ug set point by
enabling F724 (for example for stability tests)
233 Regulator Tracking in MANUAL Operation
Whenever the AUTOMATIC channel is not in operation (the MANUAL channel
is ON) a follow-up equipment ensures a smooth switch-back to the AUTOMATIC
mode will always be possible To track the voltage set-point is shifted by means of
RAISELOWER pulses from the Gate Control Unit so that control variable Ucontr at the
output from the PID filter is held steady and identical to the control variable Ucontr from
the MANUAL channel Because this tracking must react slowly resultant transient
control deviations resulting from the amplification in the PID filter might cause severe
interference with control variable Ucontr
To prevent this the follow-up equipment intervenes on the regulators mixing
point with a corresponding compensation signal
234 Ugf Limiter
At under frequency the Ugf Limiter reduces the generator voltage so as to
prevent saturation effects in the supply and measuring transformers To adjust this
limiter the max permissible generator voltage at rated frequency is defined and set
When any under-frequency occurs the generator voltage is thus reduced in proportion to
that setting
235 Field Current Maximum Limiter
The Field Current Maximum Limiter is provided to protect the generator rotor
from s occurring in steady-state and transient operation High field currents are normally
the result of a sharp drop in network voltage or of an improper raising of the voltage set-
point by the operating staff The field current is held steady at the value TH1 ie at the
maximum thermal value permissible for the excitation circuit and the rotor In order that
the generator can support the power network with its transient overload capacity during
brief collapses in voltage a temporary switch-over is made to the transient limit MAX1
(a higher setting) When the generator or the converter is operating at a reduced capacity
These limits TH1MAX1 can be switched over to the lower settings TH2MAX2 by
activating the corresponding binary signals The switch-over from the thermal limit
TH12 to the transient limit MAX12 can be configured in one of three ways
a) Depending on the over current with -dUdt ENABLE
Flag programming F418 = any setting desired F41A = 0000
This variant enables the transient value MAX12 whenever a collapse of voltage
in the network is detected The ENABLE time is fixed and can be set The example
below shows the typical behavior of the limiter configured in this way
b) Dependent on the time integral with -dUdt ENABLE
Flag programming F418 = inactive F41A = 1111
This variant likewise enables the transient value only when a collapse of network
voltage has been detected However the switch-back to the thermal limit is not made
dependent upon the time itself but on the calculated time integral intisup2dt of the The setting
on Parameter F414 in spu takes into account the time the rotor needs to cool down ie
the rate of temperature change in the case of intermittent operation The example below
shows how the timing of the switch-back to the thermal limit depends on the present
value for intisup2dt
The time integral is based on the formula
Example The setting of =isup2dt equivalent to Version a (F416) at a constant 16 times the
nominal field current for 10 seconds (with TH12 = 105) is
c) Dependent on the time integral without any preconditions
Flag programming F418 = 1111 F41A = 1111
In this variant the transient becomes available without any prior conditions
(without a -dUdt ENABLE) with the time integral intisup2dt
237 Inductive Stator Current Limiter
The Inductive Stator Current Limiter holds the stator current Ig within permissible
limits while the generator is in the ldquoover-excitedrdquo operating range by reducing the field
current accordingly The setting TH (thermal limit) provides the limit against stationary s
that might occur To take advantage of the generatorrsquos transient overload capacity a
switch-over is made to the higher setting MAX The principle of operation of this switch-
over to the value MAX permissible only transiently is identical to that employed for the
field current limiter (refer to the description above) When the drive output from the
turbine is very high stator current may exceed permissible limits even while inductive
loading of the generator is low In this case if the stator current limiter is not kept from
influencing the field current the control circuit will oscillate back and forth between the
Inductive Stator Current Limiter (de-
exciting) and the Capacitive Stator Current Limiter (exciting)The output signal of that
function then dominates the control variable of the Ig-dependent limiter via a maximum
value selection
238 Capacitive Stator Current Limiter
239 Load Angle Limiter
The Load Angle Limiter prevents the synchronous machine from slipping out of
phase due to slippage of the rotor The load angle δ the difference in phase between the
rotor and the stator rotating field results mainly from the driving torque (active power P)
acting on the generator and the level of rotor current (field current) If the driving torque
remains constant a increase in the field current reduces the load angle δ The current load
angle δ at any moment is obtained from the generator current and generator voltage based
on a simplified model of the generator Whenever this calculated load angle δ exceeds the
preset limit angle the limiter increases the field current until the load angle has dropped
back to its permissible value The quadrature reactance Xq of the generator and the
network reactance Xe during normal operation must be adjusted on the regulator in order
to obtain the load angle δ The graph below shows the Power Chart for a salient-pole
machine with typical limiter characteristics
The purpose of a Power System Stabilizer is to use the generator excitation to
damp electromechanical oscillations between the network and the generator Depending
on the design of the generator and the requirements imposed for network stability its
main function will be either to damp the oscillations originating in the machine or those
from the network A synchronous generator working in a combined power network is in
principle an oscillating structure In order to produce a torque the magnetic field of the
rotor and the stator must form a given angle (referred to as the rotor displacement or load
angle δ) The electrical torque ME increases as the angle δ increases just as with a
torsion spring Because the ME of the generator and the mechanical driving torque MA
from the turbine are in equilibrium during steady-state operation the angle δ remains in a
given position Whenever this state of equilibrium between MA and ME is disturbed the
load angle slips of this rest position and change thereby the electrical torque ME The
torque attempts to restore the load angle to a stationary position Due to the mass inertia
of the turbinegenerator rotor however this can only take place aperiodically It does so
in the form of more or less effectively damped oscillations (again similar to the effect of
mass inertia on a torsion spring) In order to damp the oscillations there must be a
damping torque produced depending not on the electrical torque ME associated with the
angle but on the difference in frequency (Df) between the rotor and the stator rotating
field ie on the slippage This torque is produced mainly by the so-called damper
winding in the rotor but the dimensioning of this is subject to limits imposed by
considerations of design and economy Some further action is therefore needed to
increase the damping effect The following drastically simplified formula shows the
parameters upon which the amount of active power PE supplied by the generator
depends
PE = active power
It can be seen from the above relationship that the active power that the generator
transfers depends not only on the load angle δ but also on the field current If That means
that a transient change can be made in the active power PE and with that in the effective
electrical torque ME by varying the field current The principle of operation of the DVR
Power System Stabilizer becomes clear from a consideration of the oscillations in power
output and frequency (ΔPE Δf) and the vector diagram If it is assumed that oscillations
in the network frequency generates load oscillations with the mass inertia of the rotor
then the active load of the generator (eg MW-measured) is influenced with a sinusoidal
value -ΔPE (ME-MA = -ΔPE) By inversion of -ΔPE one obtains the fluctuation in
power provided by the rotor +ΔPE As is known the slip signal Δf follows +ΔPE with a
phase delayed by 90deg The +ME produced by the periodic changes in the load angle δ is
in phase with +ΔPE A good damping is attained if ME is varied in phase with the slip
Δf However this signal must also be advanced somewhat to compensate for the time
constants in the excitation circuit and the generator
As mentioned above the electrical torque ME can be influenced by varying the
field current To accomplish this a suitable control signal referred to as variable
disturbance compensation must be imposed upon the voltage set-point or the converter
control variable Ucontr As can be seen from the vector diagram by applying proper
weighting factors (K1 K2) and then adding together the signals -ΔPE and Δf an overall
stabilization signal can be produced that rotates in advance of the Df signal by any angle
desired between 0deg and 90deg Because the amplitude of -DPE remains proportional to the
amplitude of Δf a constant angle in advance of Δf results for the compensation of the
time constants referred to above The optimum weighting factors K1 and K2 for a
synchronous generator working to a power network depend on its operating point at any
moment and the external reactance of the network Normally the selection of a
compromise setting is good enough to attain stability in all operating points and for all
external reactance For special demands these settings must be parameterized as a
function of the external reactance (which means optional equipment Xe-Identification)
The Power System Stabilizer PSS is a section of the AVR computer program and is
processed once per network cycle The voltage at the generator terminals and the
generator current are measured in order to define the signals ΔPE and Δf The calculated
signals for _P_ (=PE) and Δf are then sent across DC filters ldquoDrdquo (real differentiators) that
transmit only the dynamic portion of the signals The ΔPE and Δf signals obtained in this
way are then weighted (multiplied by) with the factors K1 and K2 and sent to the
summing point of the voltage regulator
The PSS stabilization signal is imposed on the automatic voltage regulator only if
the following prerequisites are met
bull Generator on line
bull Generator power output gt the value F338
bull Generator voltage in a range between F33C and F33A
The stabilization signal is limited at the output from the PSS to the lower and
upper limits Flag defines whether the stabilization signal is introduced before or after the
PID filter (usually before the filter) Because the PID filter as noted above already takes
the ceiling factor Vp1 into account the PSS signal needs to be multiplied by Vp1 if it is
added to the voltage regulator following the PID filter (divider at the input to the
minmax limiter) This precaution prevents the DC filter ldquoDrdquo in the P-channel from
producing an unnecessary ldquostabilizationrdquo effect in the case of rapid changes in turbine
load As an alternative for the AVRrsquos Power System Stabilizer a stabilization signal from
an outside system can be imposed by activating the binary input ldquoPSS-SIGNEXTrdquo Flag
F340 can be used to select between an analog and a 12-bit signal and F33E to select the
polarity desired for that signal
24 The MANUAL Channel
241 Summary
The MANUAL channel (Channel 2) has been built as a simple field current
regulator
without additional limiters Its main function is to maintain the excitation of the generator
even if the AUTOMATIC channel becomes non-operational The MANUAL channel
also performs valuable service for purposes of testing commissioning and preventive
maintenance Its measurements regulator generation of firing pulses and power supply
are physically separate from those on the AUTOMATIC channel
242 Principle of Operation
All the functions of the MANUAL channel including the generation of firing
pulses have been implemented in a single electronic module the Gate Control Unit The
control variable Ucontr of voltage regulator is used as the reference value for generating
firing pulses on the principle known as ldquoramp controlrdquo (Comparison of Ucontr with
Usynsynchronous sawtooth signal) For further processing in the UN 0096 Intermediate
Pulse Stage the Gate Control Unit supplies six firing pulses at its output whose phase
position with respect to the synchronous voltage Usyn is in accordance with control
variable Ucontr An internal linearization ensures that the field voltage produced via the
firing pulses remains proportional to the control variable Ucontr throughout the entire
range As a result the circuit amplification of the control remains constant over the entire
range Whenever excitation is switched ON the set-point for Generator Voltage is set
automatically at the preset - ref Value This provision ensures that the generator voltage
always attains approximately its nominal value after the field flashing The Gate Control
Unit can be refunctioned ( by pre-selection with a switch ) for purposes of testing to act
as a purely firing pulse control In this case the control variable Ucontr is adjusted
directly using the RAISELOWER push buttons on the front of the module In this way
for example the relationship between the phase position of the firing pulses and the
control variable Ucontr can be checked easily
CHAPTER 3
PULSE SECTION
31 Pulse Generation and Amplification
The Gate Control Units of both AUTOMATIC channel and MANUAL channel
each supply six firing pulses for operating the 6-pulse thyristor bridges The low-power
pulse signals from these Gate Control Units are then amplified in the Intermediate Pulse
Stage galvanically isolated and then sent to the common pulse bus On the output end
the Intermediate Pulse Stage of the non-active channel) is always blocked The Gate
Control Units generate the pulses based on microprocessor control The reference voltage
used for the firing pulse phase location is the output voltage from the excitation
transformer (Usyn1 Usyn2) The commutation spikes of the synchronous voltage caused
by the converter are calculated prior to use of the voltage as a reference value and are
deliberately filtered out The lower limit for the firing pulses (double pulses) which are
offset from one another by 60deg is defined by the limit rectifier position (αmin) and the
upper limit by the limit inverter position (αmax) for the firing angle αmin and αmax can
be adjusted on the Gate Control Units using BCD (Binary Coded Decimal) switches
αmin ensures that the firing pulses will not be issued (premature firing) until there is
sufficient positive phase voltage on the thyristor involved αmax prevents a dangerous
ldquotippingrdquo of the thyristor bridge into the rectifier mode if the firing angle α is too large
(ldquolate firingrdquo) The critical factors determining αmax are the overlap time uumlmax (max
commutation time) and the ldquorecovery timerdquo of the thyristors (αmax lt 180deg - uumlmax - γ )
An external control signal can force the firing pulses into their inverter limit position
Other binary inputs can block or direct the firing pulses of the Gate Control Units so as to
produce freewheeling on the thyristor bridge During freewheeling the firing pulses for
the thyristor pair R and S are blocked and the pulse signals T+T- are engaged with
chains of pulses Both Gate Control Units (for the MANUAL amp AUTOMATIC channels)
contain a field current monitor that blocks the firing pulses immediately whenever the
current exceeds a preset threshold level In this case the field circuit-breaker is also
tripped via an output contact The purpose of these provisions is to prevent damage to
thyristors and thyristor fuses in case of a slip-ring short-circuit or to keep any damage
that does occur to a minimum The pulse signals are galvanically separated at the outputs
from the Intermediate Pulse Stage (with pulse transmitters) and are then directed to the
common pulse bus This transmission of the pulse signals to the pulse bus via passive
transmitters ensures a high degree of active channel autonomy Practically no possible
malfunctions on the inactive channel (including for example sustained pulses) affect the
active channel
32 Pulse Monitoring
The ldquoPulse Busrdquo and the pulse signals of the AUTOMATIC channel are
monitored This monitoring device consists of potential isolating stages and the common
monitor If the pulse monitoring of the ldquoPulse Busrdquo responds a switch-over is made to
MANUAL channel The function of the potential isolating stages is to couple the pulse
monitoring device to the pulse circuits without any feedback effect The pulse monitoring
checks the six pulse lines for the following malfunctions continuous or periodic failure
of one or more pulses Periodic occurrence of synchronous or asynchronous false pulses
Continuous pulses the pulse monitoring device can be tested while the machine is in
operation
CHAPTER 4
CONVERTER
Thyristor
The term thyristor usually refers to a family of four layer solid state device having
turn on characteristics that can be externally controlled by either current or voltage They
are also referred to as breakdown device because their working depends on avalanche
breakdown Thyristors have only two stages OFF and ON Thyristors have a similar
function to Uni-junctions they act as switches Thyristors use current flow as a switch
Thyristors have three states
1 Reverse blocking mode mdash Voltage is applied in the direction that would be
blocked by a diode
2 Forward blocking mode mdash Voltage is applied in the direction that would cause
a diode to conduct but the thyristor has not yet been triggered into conduction
3 Forward conducting mode mdash The thyristor has been triggered into conduction
and will remain conducting until the forward current drops below a threshold value
known as the holding current Converter is a semiconductor device which converts ac
input voltage into a constant dc output voltage In present excitation system three phase
fully controlled thyristor converter is used
Because of the following advantages thyristor converters are used
a) Thyristors are used for high power applications ie up to 10Kv3500A1KHz
b) Having high reliability and low losses
c) Uni-directional device like diode
d) Itrsquos operation as a rectifier which are low resistance in forward conduction
mode and high resistance in reverse conduction mode
PROTECTION OF THYRISTORS
For reliable operation of a thyristor demands that its specified ratings are not
exceeded When Subjected to or over voltages During the turn - on of SCR didt
prohibitively large False triggering of SCR by high value of dvdt andSpurious signals
between gate and cathode may leads to unwanted turn ndash on
DIDT AND PROTECTION
When thyristor starts conducting in forward conduction mode and is turned on by
gate pulse The anode current increases rapidly whole area of the gate to Cathode
junction then hot spots will be formed near the gate connection this locality of heating
destroys the thyristor Thyristor thermal time is constant The causes due to faults and
short circuits or surge currents Electronic crowbar protection is used against the over
voltages The rate rise of anode current must be kept at the time of turn on below the
rated or specified limiting value The didt value maintained below limited value by using
a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating
is avoided by applying gate current but not greater the maximum gate current
DVDT AND OVER VOLTAGE PROTECTION
With forward voltage across the anode and cathode of a thyristor the two outer
junctions are forward biased but the inner junction is reverse biased This reverse biased
junction J2 has the characteristics of a capacitor due to charges existing across the
junction In other words space-charges exist in the depletion region around junction J2
and therefore junction J2 behaves like a capacitance If the entire anode to cathode
forward voltage Va appears across J2 junction and the charge is denoted by Q then a
charging current i given by Eq (46) follows
i = dQdt =d(Cj Va )dt
= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)
As Cj the capacitance of junction J2 is almost constant the current is given by
i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)
If the rate of rise of forward voltage dVadt is high the charging current i will be
more This charging current plays the role of gate current and turns on the SCR even
when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on
must be avoided as it leads to false operation of the thyristor circuit
For controllable operation of the thyristor the rate of rise of forward anode to
cathode voltage dVadt must be kept below the specified rated limit Typical values of
dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by
using a snubber circuit in parallel with the device thyristor are very sensitive for over
voltage than the semiconductor devices
Over voltage transients are perhaps the main cause of thyristor failure
In thyristor there are mainly two types
1 Internal over voltages
Due to the commutation of the thyristors large voltages are generated internally
Because of the series inductance of the SCR circuit the large transient voltages L didt
produced This voltage several times the break over voltage of the device then thyristor
destroys permanently
2 External over voltages
External over voltages are caused due to the interruptions of current flow in an
inductive circuit and also due to the lightening strokes on the lines feeding the thyristor
system For the reliable operation of thyristor the over voltages must be suppressed by
adopting suitable techniques
Suppression of over voltages
The RC circuit called snubber circuit is connected across the device to protect In
order to keep the protective components to a minimum the thyristors are chosen with
their peak voltages ratings are 25 to 3 times of the normal peak working voltage
ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are
commonly employed for protecting the thyristor circuit against the over voltages
Gate protection
Gate circuit should also be protected against the over voltages and surges Over
voltage at gate circuit can cause false triggering of the SCR may rises the junction
temperature behind specified limit leading to its damage Protection against over
voltage can be achieved by connecting a ZD across the gate circuit and a resister is
connected in series with gate circuit to protect against the s A capacitor and resister are
connected across gate to cathode to by pass the noise
41 Final Pulse Stages
The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage
(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor
bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided
with a power supply module The amplified output pulses from the Final Pulse Stages
start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main
pulse This initial pulse edge assures proper firing of the thyristors being triggered
Subsequently the weaker part of main pulse keeps firing conditions steady As already
mentioned the Final Pulse Stages and their associated thyristor bridges form single units
All six pulse outputs from a Final Pulse Stage can be blocked by an external control
signal so that all thyristors in the associated thyristor bridge will block the current A
blocking of the pulses is initiated whenever there is a malfunction in the associated
thyristor bridge
42 Converter Power Section
The thyristor converter consists of three independent parallel rectifier blocks TY1
to TY3 which are all in service Even if one block fails the remaining blocks take over
automatically the full design current of the excitation circuit During normal operation
(with ideal current share) and all three bridges in operation each of these blocks has to
carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation
is limited Only when all three bridges fail the excitation is switched off Each thyristor
bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any
arm is identified by a Current flow monitoring module
43 Converter Cooling
A cooling system is needed to dissipate heat losses in the converter blocks and
electronics Each converter block has therefore been equipped with a fan supplied with
power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing
cubicle) The fans are protected with motor protection circuit breakers An air flow
monitoring unit is provided for monitoring the air flow through the thyristor bridge If a
circuit breaker failure is detected or if the air flow monitor drops off at one of the
thyristor bridges the bridge involved is immediately set out of operation by blocking its
firing pulses
44 Thyristor Converter Monitoring
A thyristor bridge in which defects occur that could threaten the safety of
operation or cause secondary damage is switched off automatically ie its firing pulses
are blocked This happens whenever A thyristor fuse is blown The fuses are monitored
individually with micro switches The Final Pulse Stage fails which is detected by
internal monitors (supply voltage sustained pulse short-circuit on the output end) The
power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or
is insufficient Isolator on ACDC side is open
CHAPTER 5
Field Current Circuit Excitation Transformer
51 Field Circuit Breaker
The circuit-breaker in the field circuit is used to isolate the field circuit from the
converter It is capable of switching off the synchronous machine from full load under the
maximum conditions of a 3-phase short-circuit In addition to its main contacts the field
circuit-breaker also has a de-excitation contact with which the field energy stored in the
field can be dissipated across the de-excitation resistor The de-excitation contact closes
shortly before the main contacts open so as to ensure proper commutation of the field
current from the main contacts to the de-excitation contact when the breaker is switched
off The field circuit-breaker is switched on by electromagnetic force and is kept switched
on by a mechanical latch When the latch is released by a trip coil the circuit-breaker
opens The circuit-breaker also has auxiliary contacts that report its status
52 Field Flashing
In shunt supplied excitation circuits (excitation transformer connected to the
generator terminals) the generator does not have enough remnant voltage for a generator
voltage build-up via the converter In this case a field flashing circuit is provided It
consists of the field flashing contactor the diode bridge and a transformer used to adapt
the auxiliary input voltage to the voltage needed for field flashing when power is
supplied from the auxiliaries network
Fig Field Flashing
Because the field flashing contactor is not able to switch off the energy stored in
the field the control ensures that the contactor can only reopen if the field circuit breaker
has already been opened (generating the TRIP order) or in a normal field flashing
sequence when the converter has taken over the field current Field flashing occurs in the
following stages
1048729The excitation is switched on closing the field flashing contactor ( Field
Circuit Breaker is already closed )
1048729The start-up excitation current flows through the rotor driving the generator
voltage up to approx 15 U
1048729After about 10 U the firing pulses to the converter are released and it begins
to excite the generator to its rated voltage
1048729After about 30 U the field flashing contactor opens (with no current since
the converter is now supplying the current)
The diode bridge at the input to the field flashing contactor prevents a feed-back
from the converter to the source of field flashing while the contactor is still closed
53 De-excitation
When malfunctions occur the stored field energy must be dissipated as quickly
and safely as possible to protect the generator This is done by the converter the field
circuit-breaker and the de-excitation (discharge) resistor
De-excitation (with opening of the field circuit-breaker) takes place in the following
stages
1048729The converter drives to its inverter limit position (negative ceiling voltage)
recovers a portion of the field energy into the network A trip command is given to the
field circuit breaker
1048729The de-excitation contact closes diverting the field voltage to the de-excitation
resistor
1048729Then immediately the main contacts open building voltage The field voltage
commutates to the de-excitation resistor
1048729The current diminishes at a given time constant TE
(With linear resistance TE = Lf (Rf + Re))
Due to the reversal of the field voltage by the converter the field current
commutates from the main contacts of the field circuit-breaker to the de-excitation
resistor in a very early phase This reversal of the field voltage prevents burn-off on the
main contacts and provides effective protection for the field circuit-breaker Depending
on the operating policy an operational shut-down of the excitation can also be effected
with the field circuit-breaker closed This method is useful mainly when the excitation is
switched on and off frequently In this case the converter is merely driven into the
inverter limit position so that the field energy is recovered into the network The
converter then blocks since it is supplying positive current only
54 Excitation Transformer
The excitation transformer matches the generator voltage to the field voltage
(required ceiling voltage) It also serves as a commutation reactance for the thyristor
converter and as a potential isolator between the network and the excitation circuit In
addition the transformer functions as a current limiter in that it makes it possible to keep
any short circuits in the excitation circuit under better control The excitation transformer
is equipped with temperature monitoring probes which set off an alarm when the
temperature exceeds a first max limit and then trips the excitation if the temperature
continues rising to a second (higher)limit
CHAPTER 6
Monitoring and Protection
61 Excitation Monitoring
611 General Information
The main goal of Excitation Monitoring is to make optimum use of the
redundancies provided in the excitation system and to give alarm whenever a malfunction
makes these redundancies unavailable The field current is monitored to see that it does
not exceed a maximum level and if necessary a switch-over to the MANUAL channel is
initiated In addition the criterion for switching off the field flashing is generated The
excitation Monitoring consists of an autonomous processor system
612 over current Alarms
In the Excitation Monitoring the limits for are set at higher levels than the
settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of
the nominal field current contact R1 and the binary output associated with it are
activated immediately If field current remains gt 110 then after a preset inverse-time
has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs
associated with them are activated Parameters match the measurements for If1 and If2 to
the nominal value for field current so that the internal values can be processed and read as
pu values It can be used to falsify the actual value of the field current If (to raise it) so
as to cause a response from the alarm limits for purposes of testing The processed If
signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As
long as the field current If is above the threshold value 11 Ifn its peak value is
measured This is stored (until RESET) and can be read at any time on the Micro-
Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever
the integrated time-current value (intisup2dt) exceeds the preselected reference value the
alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously
issued to switch over to the stand by AUTO channel Software switch F758 enables the
three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-
time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the
desired limit curve for response is set using the factor F216
613 Switch-Off Criterion for Field Flashing
The Excitation Monitoring supplies the criterion for switching off the field
flashing Whether this criterion is activated based on the actual value for generator voltage
Ug or for field current If or both depends on the settings of the two threshold values
F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements
Ug12 and If12 are switched over depending on the present status of the channels
(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is
fixed at ldquological 1rdquo
614 Storage of Alarm Status
The outputs of the over current alarms (R1 R2 R0) and the messages NO
FAILURE MONITORING PARAMETERS CHANGED are stored messages can be
erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the
front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the
situation causing the alarm or the malfunction is no longer present Whenever the self-
diagnosis equipment in the processor detects a malfunction the output NO FAILURE of
MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo
is activated whenever parameters or settings of software switches have been changed via
the Micro-Terminal
615 Actual Value Monitoring
The actual values for generator voltage Ug synchronous voltage Usyn and field
current If are monitored for malfunctions This monitoring is active regardless of whether
or not the generator is in operation Essentially when the generator is in operation the
measurements are monitored by comparing the signals (the smaller signal reading is
detected as incorrect) When the generator is not in operation the measured data are
monitored for extreme values The percentage of deviation permissible in the
measurement signals being compared
(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and
F20AIf the excitation transformer is being supplied from an auxiliary power source (no
shunt operation) the values of Ug and Usyn will be different in some operational
conditions
In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1
and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the
messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2
and the binary message from CH1 reports no malfunction a malfunction on
Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is
also present whenever the binary message CH1 DISTURBANCE is reported and a
discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2
FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares
Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the
comparison channel reports a malfunction or whenever both binary messages report no
malfunction - but both secondary monitors report a malfunction As long as the secondary
monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction
signals for the measurement channel involved (suspicion that there is a corresponding
error in Usyn) The measurement channel malfunctions are enabled operationally
whenever after excitation has been switched on generator voltage Ug exceeds the value
set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is
switched off to see that they do not exceed the limit value F210 that applies to both of
them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt
the operating range) Monitoring for extreme values is likewise enabled during normal
operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the
malfunction signals to the binary outputs Basically the monitoring of the actual values
for If1If2 functions like that of the Ug1Ug2 monitoring
62 Excitation Protection
621 General Information
The Excitation Protection switches off the excitation (and de-excites the machine
rapidly) whenever a danger arises that threatens the excitation transformer the converter
or the generator Generally limiter or monitoring functions precede the emergency trips
and these normally respond before the Excitation Protection must initiate a trip
Protective trip commands are issued directly to the field circuit-breaker from potential
free contacts of the board via the trip relays They are directed redundantly to the
operative field circuit-breaker ldquoOFFrdquo command
622 Protection against Excitation Transformer Overheating
This equipment monitors the excitation transformer for overheating in the
windings that could result from over current short-circuits or inadequate cooling The
monitoring uses temperature monitoring modules in conjunction with temperature
sensors built into the windings Normally the temperature is monitored in two stages the
first stage sets off an alarm the second causes a trip of the excitation
623 Rotor Over voltage Protection
Malfunctions in the generator circuit (eg terminal short-circuit failed
synchronization asynchronous operation) cause induced negative field currents that
produce high voltages in the field circuit These must be restricted to a level with a
sufficient safety margin below the insulation capacity of the field winding (test voltage)
and also below the peak blocking voltage of the converter thyristors The crow bar
employs spark gap elements to detect over voltages in the field circuit Whenever they
respond the associated thyristors are fired immediately switching the de-excitation
resistor parallel to the field The de-excitation current generated thereby initiates an
excitation trip via a supervision circuit causing an immediate opening of the field circuit-
breaker The malfunction isets off an alarm and an internal malfunction is indicated at the
cubicle
TEST VALUES OBTAINED WHEN EXCITATION IS RAISED
TEST
SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE
1 VREF 996 100
2 VACT 997 1003
3 IFACT 735 765
4 IGACT 703 707
5 ACTIVE
POWER
703 705
6 REACTIVE
POWER
101 142
7 POWER
FACTOR
099 IND 098 IND
8 ACTIVE
CURRENT(IR)
705 703
9 REACTIVE
CURRENT(IX)
102 138
10 POWER
ANGLE
566 546
11 FIRING
ANGLE
640 632
VALUES OBTAINED WHEN EXCITATION IS LOWERED
SNO PARTICULARS ACTUAL
VALUE
OBTAINED VALUE
1 VREF 100 997
2 VACT 100 997
3 IFACT 787 761
4 IGACT 839 837
5 ACTIVE
POWER
839 830
6 REACTIVE
POWER
155 110
7 POWER
FACTOR
098 IND 099 IND
8 ACTIVE
CURRENT(IR)
835 830
9 REACTIVE
CURRENT(IX)
140 94
10 POWER
ANGLE
613 DEG 631 DEG
11 FIRING
ANGLE
601 DEG 599 DEG
CONCLUSION
For generating the EMF in stator winding excitation is required to the rotor of a
generator There are two types of excitation
1 Static excitation system
2 Brushless excitation system
A certain disadvantage in brushless excitation system is the slow response time of
the field in case of fast load changes specified No slip-rings and brushes direct
measurements of the field parameters not possible
To avoid all loses static excitation is used Since it does not have any rotating
parts mechanical loses and windage loses This system has fast response and speed
control While preferring this excitation system there are no limitations for the
redundancy of Thyristor bridge circuits
Static excitation has fast field discharge by resistor and inverter operation direct
measurement of field quantity is possible The meaning of excitation is nothing but
continuous supply of DC current (ie field current) to the rotor to buildup required
output voltage in the stator
Field current is changed with respect to the change of load so the digital
automatic voltage regulator (DAVR) is used to regulate the output voltage according to
the load variations
So we conclude that static excitation system with DAVR is preferred since it is
having excellent dynamic performance and better options for R amp M
The BODE diagram below shows the assignment of settings in accordance with
DINIEC standards based on a typical example
The PID filter amplifications Vo Vp and Vinfin can be adjusted in pu values But
the ldquoceiling factorrdquo pl+ must be adjusted correctly with parameter F310 if the total
amplification (circuit amplification) of the control circuit is actually to conform to the
pu settings This factor must agree with the ldquoexternalrdquo amplification ie with the
ceiling value of the transformer- converter circuit
Ceiling factor(pl+) = Ufmax Ufo
in which Ufmax = ceiling field voltage
Ufo = no-load field voltage
To attain a suitable response of the AVR when starting excitation
(ldquoEXCITATION ONrdquo) it may be necessary to change the proportional amplification of
the regulator during this phase Vp2 (transiently activated) and Vp1 (permanently
activated) can be adjusted for this purpose For example the value of Vp2 takes effect
immediately once the excitation is switched on and remains effective for a period as set at
F30C Once the period F30C (eg 5 sec) has expired Vp shifts over to Vp1 (becomes
the steady-state Vp) at the rate of change set The standard operating mode for the PID
filter is voltage regulation for which the discrepancy between the voltage set-point and
the current value for generator voltage Ug (the control deviation) is supplied at the input
To compensate for the voltage drop in the block transformer or whenever several
generators are operating to the same distributing bus the generator voltage must be
varied in proportion to the measured generator current (droop influence) To accomplish
this the voltage set-point is varied as a function of the measured reactive current IX
andor active current IR Flag F712 enables the IX droop Flag F710 the IR droop The
desired compensation is set in F282 and F286 respectively Flags F284 and F288 are used
to select whether this droop influence is to increase the voltage or to reduce it
(compensation) Combined influence of the active and reactive currents is attained by
enabling both droops IX and IR Flag F716 activates a so-called ldquoSoft-Startrdquo at the
starting of excitation This ldquoSoft-Startrdquo ensures that the voltage set-point integrates from
0 to 100 within the time set on F290 when the excitation is switched on
(ldquoEXCITATION ONrdquo) A ldquosmoothrdquo excitation of the generator can be achieved in this
way whenever there is no demand for a quick excitation
232 Voltage Set-Point
Various signals and settings control and limit the voltage set-point F270 For
example the values of F254 and F252 define the normal operating range possible for set-
point adjustment (eg 90 110) using external control commands (control room local
operatorrsquos panel superposed control system) The effective set-point adjustment rate is
governed The set-point can be set at the values of F250 and F256 by activating
appropriate control commands for ldquoSETrdquo input Enabling Flag F71A and activating a
binary input prior to switching on the excitation (ldquoEXCITATION OFFrdquo) sets the Ug set-
point at the value of UAUX This makes it possible for example to ensure that the
generator voltage will agree exactly with the network voltage after the voltage build-up
An external value with variable amplification can be added to the Ug set point by
enabling F724 (for example for stability tests)
233 Regulator Tracking in MANUAL Operation
Whenever the AUTOMATIC channel is not in operation (the MANUAL channel
is ON) a follow-up equipment ensures a smooth switch-back to the AUTOMATIC
mode will always be possible To track the voltage set-point is shifted by means of
RAISELOWER pulses from the Gate Control Unit so that control variable Ucontr at the
output from the PID filter is held steady and identical to the control variable Ucontr from
the MANUAL channel Because this tracking must react slowly resultant transient
control deviations resulting from the amplification in the PID filter might cause severe
interference with control variable Ucontr
To prevent this the follow-up equipment intervenes on the regulators mixing
point with a corresponding compensation signal
234 Ugf Limiter
At under frequency the Ugf Limiter reduces the generator voltage so as to
prevent saturation effects in the supply and measuring transformers To adjust this
limiter the max permissible generator voltage at rated frequency is defined and set
When any under-frequency occurs the generator voltage is thus reduced in proportion to
that setting
235 Field Current Maximum Limiter
The Field Current Maximum Limiter is provided to protect the generator rotor
from s occurring in steady-state and transient operation High field currents are normally
the result of a sharp drop in network voltage or of an improper raising of the voltage set-
point by the operating staff The field current is held steady at the value TH1 ie at the
maximum thermal value permissible for the excitation circuit and the rotor In order that
the generator can support the power network with its transient overload capacity during
brief collapses in voltage a temporary switch-over is made to the transient limit MAX1
(a higher setting) When the generator or the converter is operating at a reduced capacity
These limits TH1MAX1 can be switched over to the lower settings TH2MAX2 by
activating the corresponding binary signals The switch-over from the thermal limit
TH12 to the transient limit MAX12 can be configured in one of three ways
a) Depending on the over current with -dUdt ENABLE
Flag programming F418 = any setting desired F41A = 0000
This variant enables the transient value MAX12 whenever a collapse of voltage
in the network is detected The ENABLE time is fixed and can be set The example
below shows the typical behavior of the limiter configured in this way
b) Dependent on the time integral with -dUdt ENABLE
Flag programming F418 = inactive F41A = 1111
This variant likewise enables the transient value only when a collapse of network
voltage has been detected However the switch-back to the thermal limit is not made
dependent upon the time itself but on the calculated time integral intisup2dt of the The setting
on Parameter F414 in spu takes into account the time the rotor needs to cool down ie
the rate of temperature change in the case of intermittent operation The example below
shows how the timing of the switch-back to the thermal limit depends on the present
value for intisup2dt
The time integral is based on the formula
Example The setting of =isup2dt equivalent to Version a (F416) at a constant 16 times the
nominal field current for 10 seconds (with TH12 = 105) is
c) Dependent on the time integral without any preconditions
Flag programming F418 = 1111 F41A = 1111
In this variant the transient becomes available without any prior conditions
(without a -dUdt ENABLE) with the time integral intisup2dt
237 Inductive Stator Current Limiter
The Inductive Stator Current Limiter holds the stator current Ig within permissible
limits while the generator is in the ldquoover-excitedrdquo operating range by reducing the field
current accordingly The setting TH (thermal limit) provides the limit against stationary s
that might occur To take advantage of the generatorrsquos transient overload capacity a
switch-over is made to the higher setting MAX The principle of operation of this switch-
over to the value MAX permissible only transiently is identical to that employed for the
field current limiter (refer to the description above) When the drive output from the
turbine is very high stator current may exceed permissible limits even while inductive
loading of the generator is low In this case if the stator current limiter is not kept from
influencing the field current the control circuit will oscillate back and forth between the
Inductive Stator Current Limiter (de-
exciting) and the Capacitive Stator Current Limiter (exciting)The output signal of that
function then dominates the control variable of the Ig-dependent limiter via a maximum
value selection
238 Capacitive Stator Current Limiter
239 Load Angle Limiter
The Load Angle Limiter prevents the synchronous machine from slipping out of
phase due to slippage of the rotor The load angle δ the difference in phase between the
rotor and the stator rotating field results mainly from the driving torque (active power P)
acting on the generator and the level of rotor current (field current) If the driving torque
remains constant a increase in the field current reduces the load angle δ The current load
angle δ at any moment is obtained from the generator current and generator voltage based
on a simplified model of the generator Whenever this calculated load angle δ exceeds the
preset limit angle the limiter increases the field current until the load angle has dropped
back to its permissible value The quadrature reactance Xq of the generator and the
network reactance Xe during normal operation must be adjusted on the regulator in order
to obtain the load angle δ The graph below shows the Power Chart for a salient-pole
machine with typical limiter characteristics
The purpose of a Power System Stabilizer is to use the generator excitation to
damp electromechanical oscillations between the network and the generator Depending
on the design of the generator and the requirements imposed for network stability its
main function will be either to damp the oscillations originating in the machine or those
from the network A synchronous generator working in a combined power network is in
principle an oscillating structure In order to produce a torque the magnetic field of the
rotor and the stator must form a given angle (referred to as the rotor displacement or load
angle δ) The electrical torque ME increases as the angle δ increases just as with a
torsion spring Because the ME of the generator and the mechanical driving torque MA
from the turbine are in equilibrium during steady-state operation the angle δ remains in a
given position Whenever this state of equilibrium between MA and ME is disturbed the
load angle slips of this rest position and change thereby the electrical torque ME The
torque attempts to restore the load angle to a stationary position Due to the mass inertia
of the turbinegenerator rotor however this can only take place aperiodically It does so
in the form of more or less effectively damped oscillations (again similar to the effect of
mass inertia on a torsion spring) In order to damp the oscillations there must be a
damping torque produced depending not on the electrical torque ME associated with the
angle but on the difference in frequency (Df) between the rotor and the stator rotating
field ie on the slippage This torque is produced mainly by the so-called damper
winding in the rotor but the dimensioning of this is subject to limits imposed by
considerations of design and economy Some further action is therefore needed to
increase the damping effect The following drastically simplified formula shows the
parameters upon which the amount of active power PE supplied by the generator
depends
PE = active power
It can be seen from the above relationship that the active power that the generator
transfers depends not only on the load angle δ but also on the field current If That means
that a transient change can be made in the active power PE and with that in the effective
electrical torque ME by varying the field current The principle of operation of the DVR
Power System Stabilizer becomes clear from a consideration of the oscillations in power
output and frequency (ΔPE Δf) and the vector diagram If it is assumed that oscillations
in the network frequency generates load oscillations with the mass inertia of the rotor
then the active load of the generator (eg MW-measured) is influenced with a sinusoidal
value -ΔPE (ME-MA = -ΔPE) By inversion of -ΔPE one obtains the fluctuation in
power provided by the rotor +ΔPE As is known the slip signal Δf follows +ΔPE with a
phase delayed by 90deg The +ME produced by the periodic changes in the load angle δ is
in phase with +ΔPE A good damping is attained if ME is varied in phase with the slip
Δf However this signal must also be advanced somewhat to compensate for the time
constants in the excitation circuit and the generator
As mentioned above the electrical torque ME can be influenced by varying the
field current To accomplish this a suitable control signal referred to as variable
disturbance compensation must be imposed upon the voltage set-point or the converter
control variable Ucontr As can be seen from the vector diagram by applying proper
weighting factors (K1 K2) and then adding together the signals -ΔPE and Δf an overall
stabilization signal can be produced that rotates in advance of the Df signal by any angle
desired between 0deg and 90deg Because the amplitude of -DPE remains proportional to the
amplitude of Δf a constant angle in advance of Δf results for the compensation of the
time constants referred to above The optimum weighting factors K1 and K2 for a
synchronous generator working to a power network depend on its operating point at any
moment and the external reactance of the network Normally the selection of a
compromise setting is good enough to attain stability in all operating points and for all
external reactance For special demands these settings must be parameterized as a
function of the external reactance (which means optional equipment Xe-Identification)
The Power System Stabilizer PSS is a section of the AVR computer program and is
processed once per network cycle The voltage at the generator terminals and the
generator current are measured in order to define the signals ΔPE and Δf The calculated
signals for _P_ (=PE) and Δf are then sent across DC filters ldquoDrdquo (real differentiators) that
transmit only the dynamic portion of the signals The ΔPE and Δf signals obtained in this
way are then weighted (multiplied by) with the factors K1 and K2 and sent to the
summing point of the voltage regulator
The PSS stabilization signal is imposed on the automatic voltage regulator only if
the following prerequisites are met
bull Generator on line
bull Generator power output gt the value F338
bull Generator voltage in a range between F33C and F33A
The stabilization signal is limited at the output from the PSS to the lower and
upper limits Flag defines whether the stabilization signal is introduced before or after the
PID filter (usually before the filter) Because the PID filter as noted above already takes
the ceiling factor Vp1 into account the PSS signal needs to be multiplied by Vp1 if it is
added to the voltage regulator following the PID filter (divider at the input to the
minmax limiter) This precaution prevents the DC filter ldquoDrdquo in the P-channel from
producing an unnecessary ldquostabilizationrdquo effect in the case of rapid changes in turbine
load As an alternative for the AVRrsquos Power System Stabilizer a stabilization signal from
an outside system can be imposed by activating the binary input ldquoPSS-SIGNEXTrdquo Flag
F340 can be used to select between an analog and a 12-bit signal and F33E to select the
polarity desired for that signal
24 The MANUAL Channel
241 Summary
The MANUAL channel (Channel 2) has been built as a simple field current
regulator
without additional limiters Its main function is to maintain the excitation of the generator
even if the AUTOMATIC channel becomes non-operational The MANUAL channel
also performs valuable service for purposes of testing commissioning and preventive
maintenance Its measurements regulator generation of firing pulses and power supply
are physically separate from those on the AUTOMATIC channel
242 Principle of Operation
All the functions of the MANUAL channel including the generation of firing
pulses have been implemented in a single electronic module the Gate Control Unit The
control variable Ucontr of voltage regulator is used as the reference value for generating
firing pulses on the principle known as ldquoramp controlrdquo (Comparison of Ucontr with
Usynsynchronous sawtooth signal) For further processing in the UN 0096 Intermediate
Pulse Stage the Gate Control Unit supplies six firing pulses at its output whose phase
position with respect to the synchronous voltage Usyn is in accordance with control
variable Ucontr An internal linearization ensures that the field voltage produced via the
firing pulses remains proportional to the control variable Ucontr throughout the entire
range As a result the circuit amplification of the control remains constant over the entire
range Whenever excitation is switched ON the set-point for Generator Voltage is set
automatically at the preset - ref Value This provision ensures that the generator voltage
always attains approximately its nominal value after the field flashing The Gate Control
Unit can be refunctioned ( by pre-selection with a switch ) for purposes of testing to act
as a purely firing pulse control In this case the control variable Ucontr is adjusted
directly using the RAISELOWER push buttons on the front of the module In this way
for example the relationship between the phase position of the firing pulses and the
control variable Ucontr can be checked easily
CHAPTER 3
PULSE SECTION
31 Pulse Generation and Amplification
The Gate Control Units of both AUTOMATIC channel and MANUAL channel
each supply six firing pulses for operating the 6-pulse thyristor bridges The low-power
pulse signals from these Gate Control Units are then amplified in the Intermediate Pulse
Stage galvanically isolated and then sent to the common pulse bus On the output end
the Intermediate Pulse Stage of the non-active channel) is always blocked The Gate
Control Units generate the pulses based on microprocessor control The reference voltage
used for the firing pulse phase location is the output voltage from the excitation
transformer (Usyn1 Usyn2) The commutation spikes of the synchronous voltage caused
by the converter are calculated prior to use of the voltage as a reference value and are
deliberately filtered out The lower limit for the firing pulses (double pulses) which are
offset from one another by 60deg is defined by the limit rectifier position (αmin) and the
upper limit by the limit inverter position (αmax) for the firing angle αmin and αmax can
be adjusted on the Gate Control Units using BCD (Binary Coded Decimal) switches
αmin ensures that the firing pulses will not be issued (premature firing) until there is
sufficient positive phase voltage on the thyristor involved αmax prevents a dangerous
ldquotippingrdquo of the thyristor bridge into the rectifier mode if the firing angle α is too large
(ldquolate firingrdquo) The critical factors determining αmax are the overlap time uumlmax (max
commutation time) and the ldquorecovery timerdquo of the thyristors (αmax lt 180deg - uumlmax - γ )
An external control signal can force the firing pulses into their inverter limit position
Other binary inputs can block or direct the firing pulses of the Gate Control Units so as to
produce freewheeling on the thyristor bridge During freewheeling the firing pulses for
the thyristor pair R and S are blocked and the pulse signals T+T- are engaged with
chains of pulses Both Gate Control Units (for the MANUAL amp AUTOMATIC channels)
contain a field current monitor that blocks the firing pulses immediately whenever the
current exceeds a preset threshold level In this case the field circuit-breaker is also
tripped via an output contact The purpose of these provisions is to prevent damage to
thyristors and thyristor fuses in case of a slip-ring short-circuit or to keep any damage
that does occur to a minimum The pulse signals are galvanically separated at the outputs
from the Intermediate Pulse Stage (with pulse transmitters) and are then directed to the
common pulse bus This transmission of the pulse signals to the pulse bus via passive
transmitters ensures a high degree of active channel autonomy Practically no possible
malfunctions on the inactive channel (including for example sustained pulses) affect the
active channel
32 Pulse Monitoring
The ldquoPulse Busrdquo and the pulse signals of the AUTOMATIC channel are
monitored This monitoring device consists of potential isolating stages and the common
monitor If the pulse monitoring of the ldquoPulse Busrdquo responds a switch-over is made to
MANUAL channel The function of the potential isolating stages is to couple the pulse
monitoring device to the pulse circuits without any feedback effect The pulse monitoring
checks the six pulse lines for the following malfunctions continuous or periodic failure
of one or more pulses Periodic occurrence of synchronous or asynchronous false pulses
Continuous pulses the pulse monitoring device can be tested while the machine is in
operation
CHAPTER 4
CONVERTER
Thyristor
The term thyristor usually refers to a family of four layer solid state device having
turn on characteristics that can be externally controlled by either current or voltage They
are also referred to as breakdown device because their working depends on avalanche
breakdown Thyristors have only two stages OFF and ON Thyristors have a similar
function to Uni-junctions they act as switches Thyristors use current flow as a switch
Thyristors have three states
1 Reverse blocking mode mdash Voltage is applied in the direction that would be
blocked by a diode
2 Forward blocking mode mdash Voltage is applied in the direction that would cause
a diode to conduct but the thyristor has not yet been triggered into conduction
3 Forward conducting mode mdash The thyristor has been triggered into conduction
and will remain conducting until the forward current drops below a threshold value
known as the holding current Converter is a semiconductor device which converts ac
input voltage into a constant dc output voltage In present excitation system three phase
fully controlled thyristor converter is used
Because of the following advantages thyristor converters are used
a) Thyristors are used for high power applications ie up to 10Kv3500A1KHz
b) Having high reliability and low losses
c) Uni-directional device like diode
d) Itrsquos operation as a rectifier which are low resistance in forward conduction
mode and high resistance in reverse conduction mode
PROTECTION OF THYRISTORS
For reliable operation of a thyristor demands that its specified ratings are not
exceeded When Subjected to or over voltages During the turn - on of SCR didt
prohibitively large False triggering of SCR by high value of dvdt andSpurious signals
between gate and cathode may leads to unwanted turn ndash on
DIDT AND PROTECTION
When thyristor starts conducting in forward conduction mode and is turned on by
gate pulse The anode current increases rapidly whole area of the gate to Cathode
junction then hot spots will be formed near the gate connection this locality of heating
destroys the thyristor Thyristor thermal time is constant The causes due to faults and
short circuits or surge currents Electronic crowbar protection is used against the over
voltages The rate rise of anode current must be kept at the time of turn on below the
rated or specified limiting value The didt value maintained below limited value by using
a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating
is avoided by applying gate current but not greater the maximum gate current
DVDT AND OVER VOLTAGE PROTECTION
With forward voltage across the anode and cathode of a thyristor the two outer
junctions are forward biased but the inner junction is reverse biased This reverse biased
junction J2 has the characteristics of a capacitor due to charges existing across the
junction In other words space-charges exist in the depletion region around junction J2
and therefore junction J2 behaves like a capacitance If the entire anode to cathode
forward voltage Va appears across J2 junction and the charge is denoted by Q then a
charging current i given by Eq (46) follows
i = dQdt =d(Cj Va )dt
= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)
As Cj the capacitance of junction J2 is almost constant the current is given by
i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)
If the rate of rise of forward voltage dVadt is high the charging current i will be
more This charging current plays the role of gate current and turns on the SCR even
when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on
must be avoided as it leads to false operation of the thyristor circuit
For controllable operation of the thyristor the rate of rise of forward anode to
cathode voltage dVadt must be kept below the specified rated limit Typical values of
dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by
using a snubber circuit in parallel with the device thyristor are very sensitive for over
voltage than the semiconductor devices
Over voltage transients are perhaps the main cause of thyristor failure
In thyristor there are mainly two types
1 Internal over voltages
Due to the commutation of the thyristors large voltages are generated internally
Because of the series inductance of the SCR circuit the large transient voltages L didt
produced This voltage several times the break over voltage of the device then thyristor
destroys permanently
2 External over voltages
External over voltages are caused due to the interruptions of current flow in an
inductive circuit and also due to the lightening strokes on the lines feeding the thyristor
system For the reliable operation of thyristor the over voltages must be suppressed by
adopting suitable techniques
Suppression of over voltages
The RC circuit called snubber circuit is connected across the device to protect In
order to keep the protective components to a minimum the thyristors are chosen with
their peak voltages ratings are 25 to 3 times of the normal peak working voltage
ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are
commonly employed for protecting the thyristor circuit against the over voltages
Gate protection
Gate circuit should also be protected against the over voltages and surges Over
voltage at gate circuit can cause false triggering of the SCR may rises the junction
temperature behind specified limit leading to its damage Protection against over
voltage can be achieved by connecting a ZD across the gate circuit and a resister is
connected in series with gate circuit to protect against the s A capacitor and resister are
connected across gate to cathode to by pass the noise
41 Final Pulse Stages
The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage
(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor
bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided
with a power supply module The amplified output pulses from the Final Pulse Stages
start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main
pulse This initial pulse edge assures proper firing of the thyristors being triggered
Subsequently the weaker part of main pulse keeps firing conditions steady As already
mentioned the Final Pulse Stages and their associated thyristor bridges form single units
All six pulse outputs from a Final Pulse Stage can be blocked by an external control
signal so that all thyristors in the associated thyristor bridge will block the current A
blocking of the pulses is initiated whenever there is a malfunction in the associated
thyristor bridge
42 Converter Power Section
The thyristor converter consists of three independent parallel rectifier blocks TY1
to TY3 which are all in service Even if one block fails the remaining blocks take over
automatically the full design current of the excitation circuit During normal operation
(with ideal current share) and all three bridges in operation each of these blocks has to
carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation
is limited Only when all three bridges fail the excitation is switched off Each thyristor
bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any
arm is identified by a Current flow monitoring module
43 Converter Cooling
A cooling system is needed to dissipate heat losses in the converter blocks and
electronics Each converter block has therefore been equipped with a fan supplied with
power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing
cubicle) The fans are protected with motor protection circuit breakers An air flow
monitoring unit is provided for monitoring the air flow through the thyristor bridge If a
circuit breaker failure is detected or if the air flow monitor drops off at one of the
thyristor bridges the bridge involved is immediately set out of operation by blocking its
firing pulses
44 Thyristor Converter Monitoring
A thyristor bridge in which defects occur that could threaten the safety of
operation or cause secondary damage is switched off automatically ie its firing pulses
are blocked This happens whenever A thyristor fuse is blown The fuses are monitored
individually with micro switches The Final Pulse Stage fails which is detected by
internal monitors (supply voltage sustained pulse short-circuit on the output end) The
power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or
is insufficient Isolator on ACDC side is open
CHAPTER 5
Field Current Circuit Excitation Transformer
51 Field Circuit Breaker
The circuit-breaker in the field circuit is used to isolate the field circuit from the
converter It is capable of switching off the synchronous machine from full load under the
maximum conditions of a 3-phase short-circuit In addition to its main contacts the field
circuit-breaker also has a de-excitation contact with which the field energy stored in the
field can be dissipated across the de-excitation resistor The de-excitation contact closes
shortly before the main contacts open so as to ensure proper commutation of the field
current from the main contacts to the de-excitation contact when the breaker is switched
off The field circuit-breaker is switched on by electromagnetic force and is kept switched
on by a mechanical latch When the latch is released by a trip coil the circuit-breaker
opens The circuit-breaker also has auxiliary contacts that report its status
52 Field Flashing
In shunt supplied excitation circuits (excitation transformer connected to the
generator terminals) the generator does not have enough remnant voltage for a generator
voltage build-up via the converter In this case a field flashing circuit is provided It
consists of the field flashing contactor the diode bridge and a transformer used to adapt
the auxiliary input voltage to the voltage needed for field flashing when power is
supplied from the auxiliaries network
Fig Field Flashing
Because the field flashing contactor is not able to switch off the energy stored in
the field the control ensures that the contactor can only reopen if the field circuit breaker
has already been opened (generating the TRIP order) or in a normal field flashing
sequence when the converter has taken over the field current Field flashing occurs in the
following stages
1048729The excitation is switched on closing the field flashing contactor ( Field
Circuit Breaker is already closed )
1048729The start-up excitation current flows through the rotor driving the generator
voltage up to approx 15 U
1048729After about 10 U the firing pulses to the converter are released and it begins
to excite the generator to its rated voltage
1048729After about 30 U the field flashing contactor opens (with no current since
the converter is now supplying the current)
The diode bridge at the input to the field flashing contactor prevents a feed-back
from the converter to the source of field flashing while the contactor is still closed
53 De-excitation
When malfunctions occur the stored field energy must be dissipated as quickly
and safely as possible to protect the generator This is done by the converter the field
circuit-breaker and the de-excitation (discharge) resistor
De-excitation (with opening of the field circuit-breaker) takes place in the following
stages
1048729The converter drives to its inverter limit position (negative ceiling voltage)
recovers a portion of the field energy into the network A trip command is given to the
field circuit breaker
1048729The de-excitation contact closes diverting the field voltage to the de-excitation
resistor
1048729Then immediately the main contacts open building voltage The field voltage
commutates to the de-excitation resistor
1048729The current diminishes at a given time constant TE
(With linear resistance TE = Lf (Rf + Re))
Due to the reversal of the field voltage by the converter the field current
commutates from the main contacts of the field circuit-breaker to the de-excitation
resistor in a very early phase This reversal of the field voltage prevents burn-off on the
main contacts and provides effective protection for the field circuit-breaker Depending
on the operating policy an operational shut-down of the excitation can also be effected
with the field circuit-breaker closed This method is useful mainly when the excitation is
switched on and off frequently In this case the converter is merely driven into the
inverter limit position so that the field energy is recovered into the network The
converter then blocks since it is supplying positive current only
54 Excitation Transformer
The excitation transformer matches the generator voltage to the field voltage
(required ceiling voltage) It also serves as a commutation reactance for the thyristor
converter and as a potential isolator between the network and the excitation circuit In
addition the transformer functions as a current limiter in that it makes it possible to keep
any short circuits in the excitation circuit under better control The excitation transformer
is equipped with temperature monitoring probes which set off an alarm when the
temperature exceeds a first max limit and then trips the excitation if the temperature
continues rising to a second (higher)limit
CHAPTER 6
Monitoring and Protection
61 Excitation Monitoring
611 General Information
The main goal of Excitation Monitoring is to make optimum use of the
redundancies provided in the excitation system and to give alarm whenever a malfunction
makes these redundancies unavailable The field current is monitored to see that it does
not exceed a maximum level and if necessary a switch-over to the MANUAL channel is
initiated In addition the criterion for switching off the field flashing is generated The
excitation Monitoring consists of an autonomous processor system
612 over current Alarms
In the Excitation Monitoring the limits for are set at higher levels than the
settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of
the nominal field current contact R1 and the binary output associated with it are
activated immediately If field current remains gt 110 then after a preset inverse-time
has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs
associated with them are activated Parameters match the measurements for If1 and If2 to
the nominal value for field current so that the internal values can be processed and read as
pu values It can be used to falsify the actual value of the field current If (to raise it) so
as to cause a response from the alarm limits for purposes of testing The processed If
signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As
long as the field current If is above the threshold value 11 Ifn its peak value is
measured This is stored (until RESET) and can be read at any time on the Micro-
Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever
the integrated time-current value (intisup2dt) exceeds the preselected reference value the
alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously
issued to switch over to the stand by AUTO channel Software switch F758 enables the
three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-
time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the
desired limit curve for response is set using the factor F216
613 Switch-Off Criterion for Field Flashing
The Excitation Monitoring supplies the criterion for switching off the field
flashing Whether this criterion is activated based on the actual value for generator voltage
Ug or for field current If or both depends on the settings of the two threshold values
F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements
Ug12 and If12 are switched over depending on the present status of the channels
(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is
fixed at ldquological 1rdquo
614 Storage of Alarm Status
The outputs of the over current alarms (R1 R2 R0) and the messages NO
FAILURE MONITORING PARAMETERS CHANGED are stored messages can be
erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the
front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the
situation causing the alarm or the malfunction is no longer present Whenever the self-
diagnosis equipment in the processor detects a malfunction the output NO FAILURE of
MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo
is activated whenever parameters or settings of software switches have been changed via
the Micro-Terminal
615 Actual Value Monitoring
The actual values for generator voltage Ug synchronous voltage Usyn and field
current If are monitored for malfunctions This monitoring is active regardless of whether
or not the generator is in operation Essentially when the generator is in operation the
measurements are monitored by comparing the signals (the smaller signal reading is
detected as incorrect) When the generator is not in operation the measured data are
monitored for extreme values The percentage of deviation permissible in the
measurement signals being compared
(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and
F20AIf the excitation transformer is being supplied from an auxiliary power source (no
shunt operation) the values of Ug and Usyn will be different in some operational
conditions
In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1
and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the
messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2
and the binary message from CH1 reports no malfunction a malfunction on
Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is
also present whenever the binary message CH1 DISTURBANCE is reported and a
discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2
FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares
Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the
comparison channel reports a malfunction or whenever both binary messages report no
malfunction - but both secondary monitors report a malfunction As long as the secondary
monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction
signals for the measurement channel involved (suspicion that there is a corresponding
error in Usyn) The measurement channel malfunctions are enabled operationally
whenever after excitation has been switched on generator voltage Ug exceeds the value
set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is
switched off to see that they do not exceed the limit value F210 that applies to both of
them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt
the operating range) Monitoring for extreme values is likewise enabled during normal
operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the
malfunction signals to the binary outputs Basically the monitoring of the actual values
for If1If2 functions like that of the Ug1Ug2 monitoring
62 Excitation Protection
621 General Information
The Excitation Protection switches off the excitation (and de-excites the machine
rapidly) whenever a danger arises that threatens the excitation transformer the converter
or the generator Generally limiter or monitoring functions precede the emergency trips
and these normally respond before the Excitation Protection must initiate a trip
Protective trip commands are issued directly to the field circuit-breaker from potential
free contacts of the board via the trip relays They are directed redundantly to the
operative field circuit-breaker ldquoOFFrdquo command
622 Protection against Excitation Transformer Overheating
This equipment monitors the excitation transformer for overheating in the
windings that could result from over current short-circuits or inadequate cooling The
monitoring uses temperature monitoring modules in conjunction with temperature
sensors built into the windings Normally the temperature is monitored in two stages the
first stage sets off an alarm the second causes a trip of the excitation
623 Rotor Over voltage Protection
Malfunctions in the generator circuit (eg terminal short-circuit failed
synchronization asynchronous operation) cause induced negative field currents that
produce high voltages in the field circuit These must be restricted to a level with a
sufficient safety margin below the insulation capacity of the field winding (test voltage)
and also below the peak blocking voltage of the converter thyristors The crow bar
employs spark gap elements to detect over voltages in the field circuit Whenever they
respond the associated thyristors are fired immediately switching the de-excitation
resistor parallel to the field The de-excitation current generated thereby initiates an
excitation trip via a supervision circuit causing an immediate opening of the field circuit-
breaker The malfunction isets off an alarm and an internal malfunction is indicated at the
cubicle
TEST VALUES OBTAINED WHEN EXCITATION IS RAISED
TEST
SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE
1 VREF 996 100
2 VACT 997 1003
3 IFACT 735 765
4 IGACT 703 707
5 ACTIVE
POWER
703 705
6 REACTIVE
POWER
101 142
7 POWER
FACTOR
099 IND 098 IND
8 ACTIVE
CURRENT(IR)
705 703
9 REACTIVE
CURRENT(IX)
102 138
10 POWER
ANGLE
566 546
11 FIRING
ANGLE
640 632
VALUES OBTAINED WHEN EXCITATION IS LOWERED
SNO PARTICULARS ACTUAL
VALUE
OBTAINED VALUE
1 VREF 100 997
2 VACT 100 997
3 IFACT 787 761
4 IGACT 839 837
5 ACTIVE
POWER
839 830
6 REACTIVE
POWER
155 110
7 POWER
FACTOR
098 IND 099 IND
8 ACTIVE
CURRENT(IR)
835 830
9 REACTIVE
CURRENT(IX)
140 94
10 POWER
ANGLE
613 DEG 631 DEG
11 FIRING
ANGLE
601 DEG 599 DEG
CONCLUSION
For generating the EMF in stator winding excitation is required to the rotor of a
generator There are two types of excitation
1 Static excitation system
2 Brushless excitation system
A certain disadvantage in brushless excitation system is the slow response time of
the field in case of fast load changes specified No slip-rings and brushes direct
measurements of the field parameters not possible
To avoid all loses static excitation is used Since it does not have any rotating
parts mechanical loses and windage loses This system has fast response and speed
control While preferring this excitation system there are no limitations for the
redundancy of Thyristor bridge circuits
Static excitation has fast field discharge by resistor and inverter operation direct
measurement of field quantity is possible The meaning of excitation is nothing but
continuous supply of DC current (ie field current) to the rotor to buildup required
output voltage in the stator
Field current is changed with respect to the change of load so the digital
automatic voltage regulator (DAVR) is used to regulate the output voltage according to
the load variations
So we conclude that static excitation system with DAVR is preferred since it is
having excellent dynamic performance and better options for R amp M
to select whether this droop influence is to increase the voltage or to reduce it
(compensation) Combined influence of the active and reactive currents is attained by
enabling both droops IX and IR Flag F716 activates a so-called ldquoSoft-Startrdquo at the
starting of excitation This ldquoSoft-Startrdquo ensures that the voltage set-point integrates from
0 to 100 within the time set on F290 when the excitation is switched on
(ldquoEXCITATION ONrdquo) A ldquosmoothrdquo excitation of the generator can be achieved in this
way whenever there is no demand for a quick excitation
232 Voltage Set-Point
Various signals and settings control and limit the voltage set-point F270 For
example the values of F254 and F252 define the normal operating range possible for set-
point adjustment (eg 90 110) using external control commands (control room local
operatorrsquos panel superposed control system) The effective set-point adjustment rate is
governed The set-point can be set at the values of F250 and F256 by activating
appropriate control commands for ldquoSETrdquo input Enabling Flag F71A and activating a
binary input prior to switching on the excitation (ldquoEXCITATION OFFrdquo) sets the Ug set-
point at the value of UAUX This makes it possible for example to ensure that the
generator voltage will agree exactly with the network voltage after the voltage build-up
An external value with variable amplification can be added to the Ug set point by
enabling F724 (for example for stability tests)
233 Regulator Tracking in MANUAL Operation
Whenever the AUTOMATIC channel is not in operation (the MANUAL channel
is ON) a follow-up equipment ensures a smooth switch-back to the AUTOMATIC
mode will always be possible To track the voltage set-point is shifted by means of
RAISELOWER pulses from the Gate Control Unit so that control variable Ucontr at the
output from the PID filter is held steady and identical to the control variable Ucontr from
the MANUAL channel Because this tracking must react slowly resultant transient
control deviations resulting from the amplification in the PID filter might cause severe
interference with control variable Ucontr
To prevent this the follow-up equipment intervenes on the regulators mixing
point with a corresponding compensation signal
234 Ugf Limiter
At under frequency the Ugf Limiter reduces the generator voltage so as to
prevent saturation effects in the supply and measuring transformers To adjust this
limiter the max permissible generator voltage at rated frequency is defined and set
When any under-frequency occurs the generator voltage is thus reduced in proportion to
that setting
235 Field Current Maximum Limiter
The Field Current Maximum Limiter is provided to protect the generator rotor
from s occurring in steady-state and transient operation High field currents are normally
the result of a sharp drop in network voltage or of an improper raising of the voltage set-
point by the operating staff The field current is held steady at the value TH1 ie at the
maximum thermal value permissible for the excitation circuit and the rotor In order that
the generator can support the power network with its transient overload capacity during
brief collapses in voltage a temporary switch-over is made to the transient limit MAX1
(a higher setting) When the generator or the converter is operating at a reduced capacity
These limits TH1MAX1 can be switched over to the lower settings TH2MAX2 by
activating the corresponding binary signals The switch-over from the thermal limit
TH12 to the transient limit MAX12 can be configured in one of three ways
a) Depending on the over current with -dUdt ENABLE
Flag programming F418 = any setting desired F41A = 0000
This variant enables the transient value MAX12 whenever a collapse of voltage
in the network is detected The ENABLE time is fixed and can be set The example
below shows the typical behavior of the limiter configured in this way
b) Dependent on the time integral with -dUdt ENABLE
Flag programming F418 = inactive F41A = 1111
This variant likewise enables the transient value only when a collapse of network
voltage has been detected However the switch-back to the thermal limit is not made
dependent upon the time itself but on the calculated time integral intisup2dt of the The setting
on Parameter F414 in spu takes into account the time the rotor needs to cool down ie
the rate of temperature change in the case of intermittent operation The example below
shows how the timing of the switch-back to the thermal limit depends on the present
value for intisup2dt
The time integral is based on the formula
Example The setting of =isup2dt equivalent to Version a (F416) at a constant 16 times the
nominal field current for 10 seconds (with TH12 = 105) is
c) Dependent on the time integral without any preconditions
Flag programming F418 = 1111 F41A = 1111
In this variant the transient becomes available without any prior conditions
(without a -dUdt ENABLE) with the time integral intisup2dt
237 Inductive Stator Current Limiter
The Inductive Stator Current Limiter holds the stator current Ig within permissible
limits while the generator is in the ldquoover-excitedrdquo operating range by reducing the field
current accordingly The setting TH (thermal limit) provides the limit against stationary s
that might occur To take advantage of the generatorrsquos transient overload capacity a
switch-over is made to the higher setting MAX The principle of operation of this switch-
over to the value MAX permissible only transiently is identical to that employed for the
field current limiter (refer to the description above) When the drive output from the
turbine is very high stator current may exceed permissible limits even while inductive
loading of the generator is low In this case if the stator current limiter is not kept from
influencing the field current the control circuit will oscillate back and forth between the
Inductive Stator Current Limiter (de-
exciting) and the Capacitive Stator Current Limiter (exciting)The output signal of that
function then dominates the control variable of the Ig-dependent limiter via a maximum
value selection
238 Capacitive Stator Current Limiter
239 Load Angle Limiter
The Load Angle Limiter prevents the synchronous machine from slipping out of
phase due to slippage of the rotor The load angle δ the difference in phase between the
rotor and the stator rotating field results mainly from the driving torque (active power P)
acting on the generator and the level of rotor current (field current) If the driving torque
remains constant a increase in the field current reduces the load angle δ The current load
angle δ at any moment is obtained from the generator current and generator voltage based
on a simplified model of the generator Whenever this calculated load angle δ exceeds the
preset limit angle the limiter increases the field current until the load angle has dropped
back to its permissible value The quadrature reactance Xq of the generator and the
network reactance Xe during normal operation must be adjusted on the regulator in order
to obtain the load angle δ The graph below shows the Power Chart for a salient-pole
machine with typical limiter characteristics
The purpose of a Power System Stabilizer is to use the generator excitation to
damp electromechanical oscillations between the network and the generator Depending
on the design of the generator and the requirements imposed for network stability its
main function will be either to damp the oscillations originating in the machine or those
from the network A synchronous generator working in a combined power network is in
principle an oscillating structure In order to produce a torque the magnetic field of the
rotor and the stator must form a given angle (referred to as the rotor displacement or load
angle δ) The electrical torque ME increases as the angle δ increases just as with a
torsion spring Because the ME of the generator and the mechanical driving torque MA
from the turbine are in equilibrium during steady-state operation the angle δ remains in a
given position Whenever this state of equilibrium between MA and ME is disturbed the
load angle slips of this rest position and change thereby the electrical torque ME The
torque attempts to restore the load angle to a stationary position Due to the mass inertia
of the turbinegenerator rotor however this can only take place aperiodically It does so
in the form of more or less effectively damped oscillations (again similar to the effect of
mass inertia on a torsion spring) In order to damp the oscillations there must be a
damping torque produced depending not on the electrical torque ME associated with the
angle but on the difference in frequency (Df) between the rotor and the stator rotating
field ie on the slippage This torque is produced mainly by the so-called damper
winding in the rotor but the dimensioning of this is subject to limits imposed by
considerations of design and economy Some further action is therefore needed to
increase the damping effect The following drastically simplified formula shows the
parameters upon which the amount of active power PE supplied by the generator
depends
PE = active power
It can be seen from the above relationship that the active power that the generator
transfers depends not only on the load angle δ but also on the field current If That means
that a transient change can be made in the active power PE and with that in the effective
electrical torque ME by varying the field current The principle of operation of the DVR
Power System Stabilizer becomes clear from a consideration of the oscillations in power
output and frequency (ΔPE Δf) and the vector diagram If it is assumed that oscillations
in the network frequency generates load oscillations with the mass inertia of the rotor
then the active load of the generator (eg MW-measured) is influenced with a sinusoidal
value -ΔPE (ME-MA = -ΔPE) By inversion of -ΔPE one obtains the fluctuation in
power provided by the rotor +ΔPE As is known the slip signal Δf follows +ΔPE with a
phase delayed by 90deg The +ME produced by the periodic changes in the load angle δ is
in phase with +ΔPE A good damping is attained if ME is varied in phase with the slip
Δf However this signal must also be advanced somewhat to compensate for the time
constants in the excitation circuit and the generator
As mentioned above the electrical torque ME can be influenced by varying the
field current To accomplish this a suitable control signal referred to as variable
disturbance compensation must be imposed upon the voltage set-point or the converter
control variable Ucontr As can be seen from the vector diagram by applying proper
weighting factors (K1 K2) and then adding together the signals -ΔPE and Δf an overall
stabilization signal can be produced that rotates in advance of the Df signal by any angle
desired between 0deg and 90deg Because the amplitude of -DPE remains proportional to the
amplitude of Δf a constant angle in advance of Δf results for the compensation of the
time constants referred to above The optimum weighting factors K1 and K2 for a
synchronous generator working to a power network depend on its operating point at any
moment and the external reactance of the network Normally the selection of a
compromise setting is good enough to attain stability in all operating points and for all
external reactance For special demands these settings must be parameterized as a
function of the external reactance (which means optional equipment Xe-Identification)
The Power System Stabilizer PSS is a section of the AVR computer program and is
processed once per network cycle The voltage at the generator terminals and the
generator current are measured in order to define the signals ΔPE and Δf The calculated
signals for _P_ (=PE) and Δf are then sent across DC filters ldquoDrdquo (real differentiators) that
transmit only the dynamic portion of the signals The ΔPE and Δf signals obtained in this
way are then weighted (multiplied by) with the factors K1 and K2 and sent to the
summing point of the voltage regulator
The PSS stabilization signal is imposed on the automatic voltage regulator only if
the following prerequisites are met
bull Generator on line
bull Generator power output gt the value F338
bull Generator voltage in a range between F33C and F33A
The stabilization signal is limited at the output from the PSS to the lower and
upper limits Flag defines whether the stabilization signal is introduced before or after the
PID filter (usually before the filter) Because the PID filter as noted above already takes
the ceiling factor Vp1 into account the PSS signal needs to be multiplied by Vp1 if it is
added to the voltage regulator following the PID filter (divider at the input to the
minmax limiter) This precaution prevents the DC filter ldquoDrdquo in the P-channel from
producing an unnecessary ldquostabilizationrdquo effect in the case of rapid changes in turbine
load As an alternative for the AVRrsquos Power System Stabilizer a stabilization signal from
an outside system can be imposed by activating the binary input ldquoPSS-SIGNEXTrdquo Flag
F340 can be used to select between an analog and a 12-bit signal and F33E to select the
polarity desired for that signal
24 The MANUAL Channel
241 Summary
The MANUAL channel (Channel 2) has been built as a simple field current
regulator
without additional limiters Its main function is to maintain the excitation of the generator
even if the AUTOMATIC channel becomes non-operational The MANUAL channel
also performs valuable service for purposes of testing commissioning and preventive
maintenance Its measurements regulator generation of firing pulses and power supply
are physically separate from those on the AUTOMATIC channel
242 Principle of Operation
All the functions of the MANUAL channel including the generation of firing
pulses have been implemented in a single electronic module the Gate Control Unit The
control variable Ucontr of voltage regulator is used as the reference value for generating
firing pulses on the principle known as ldquoramp controlrdquo (Comparison of Ucontr with
Usynsynchronous sawtooth signal) For further processing in the UN 0096 Intermediate
Pulse Stage the Gate Control Unit supplies six firing pulses at its output whose phase
position with respect to the synchronous voltage Usyn is in accordance with control
variable Ucontr An internal linearization ensures that the field voltage produced via the
firing pulses remains proportional to the control variable Ucontr throughout the entire
range As a result the circuit amplification of the control remains constant over the entire
range Whenever excitation is switched ON the set-point for Generator Voltage is set
automatically at the preset - ref Value This provision ensures that the generator voltage
always attains approximately its nominal value after the field flashing The Gate Control
Unit can be refunctioned ( by pre-selection with a switch ) for purposes of testing to act
as a purely firing pulse control In this case the control variable Ucontr is adjusted
directly using the RAISELOWER push buttons on the front of the module In this way
for example the relationship between the phase position of the firing pulses and the
control variable Ucontr can be checked easily
CHAPTER 3
PULSE SECTION
31 Pulse Generation and Amplification
The Gate Control Units of both AUTOMATIC channel and MANUAL channel
each supply six firing pulses for operating the 6-pulse thyristor bridges The low-power
pulse signals from these Gate Control Units are then amplified in the Intermediate Pulse
Stage galvanically isolated and then sent to the common pulse bus On the output end
the Intermediate Pulse Stage of the non-active channel) is always blocked The Gate
Control Units generate the pulses based on microprocessor control The reference voltage
used for the firing pulse phase location is the output voltage from the excitation
transformer (Usyn1 Usyn2) The commutation spikes of the synchronous voltage caused
by the converter are calculated prior to use of the voltage as a reference value and are
deliberately filtered out The lower limit for the firing pulses (double pulses) which are
offset from one another by 60deg is defined by the limit rectifier position (αmin) and the
upper limit by the limit inverter position (αmax) for the firing angle αmin and αmax can
be adjusted on the Gate Control Units using BCD (Binary Coded Decimal) switches
αmin ensures that the firing pulses will not be issued (premature firing) until there is
sufficient positive phase voltage on the thyristor involved αmax prevents a dangerous
ldquotippingrdquo of the thyristor bridge into the rectifier mode if the firing angle α is too large
(ldquolate firingrdquo) The critical factors determining αmax are the overlap time uumlmax (max
commutation time) and the ldquorecovery timerdquo of the thyristors (αmax lt 180deg - uumlmax - γ )
An external control signal can force the firing pulses into their inverter limit position
Other binary inputs can block or direct the firing pulses of the Gate Control Units so as to
produce freewheeling on the thyristor bridge During freewheeling the firing pulses for
the thyristor pair R and S are blocked and the pulse signals T+T- are engaged with
chains of pulses Both Gate Control Units (for the MANUAL amp AUTOMATIC channels)
contain a field current monitor that blocks the firing pulses immediately whenever the
current exceeds a preset threshold level In this case the field circuit-breaker is also
tripped via an output contact The purpose of these provisions is to prevent damage to
thyristors and thyristor fuses in case of a slip-ring short-circuit or to keep any damage
that does occur to a minimum The pulse signals are galvanically separated at the outputs
from the Intermediate Pulse Stage (with pulse transmitters) and are then directed to the
common pulse bus This transmission of the pulse signals to the pulse bus via passive
transmitters ensures a high degree of active channel autonomy Practically no possible
malfunctions on the inactive channel (including for example sustained pulses) affect the
active channel
32 Pulse Monitoring
The ldquoPulse Busrdquo and the pulse signals of the AUTOMATIC channel are
monitored This monitoring device consists of potential isolating stages and the common
monitor If the pulse monitoring of the ldquoPulse Busrdquo responds a switch-over is made to
MANUAL channel The function of the potential isolating stages is to couple the pulse
monitoring device to the pulse circuits without any feedback effect The pulse monitoring
checks the six pulse lines for the following malfunctions continuous or periodic failure
of one or more pulses Periodic occurrence of synchronous or asynchronous false pulses
Continuous pulses the pulse monitoring device can be tested while the machine is in
operation
CHAPTER 4
CONVERTER
Thyristor
The term thyristor usually refers to a family of four layer solid state device having
turn on characteristics that can be externally controlled by either current or voltage They
are also referred to as breakdown device because their working depends on avalanche
breakdown Thyristors have only two stages OFF and ON Thyristors have a similar
function to Uni-junctions they act as switches Thyristors use current flow as a switch
Thyristors have three states
1 Reverse blocking mode mdash Voltage is applied in the direction that would be
blocked by a diode
2 Forward blocking mode mdash Voltage is applied in the direction that would cause
a diode to conduct but the thyristor has not yet been triggered into conduction
3 Forward conducting mode mdash The thyristor has been triggered into conduction
and will remain conducting until the forward current drops below a threshold value
known as the holding current Converter is a semiconductor device which converts ac
input voltage into a constant dc output voltage In present excitation system three phase
fully controlled thyristor converter is used
Because of the following advantages thyristor converters are used
a) Thyristors are used for high power applications ie up to 10Kv3500A1KHz
b) Having high reliability and low losses
c) Uni-directional device like diode
d) Itrsquos operation as a rectifier which are low resistance in forward conduction
mode and high resistance in reverse conduction mode
PROTECTION OF THYRISTORS
For reliable operation of a thyristor demands that its specified ratings are not
exceeded When Subjected to or over voltages During the turn - on of SCR didt
prohibitively large False triggering of SCR by high value of dvdt andSpurious signals
between gate and cathode may leads to unwanted turn ndash on
DIDT AND PROTECTION
When thyristor starts conducting in forward conduction mode and is turned on by
gate pulse The anode current increases rapidly whole area of the gate to Cathode
junction then hot spots will be formed near the gate connection this locality of heating
destroys the thyristor Thyristor thermal time is constant The causes due to faults and
short circuits or surge currents Electronic crowbar protection is used against the over
voltages The rate rise of anode current must be kept at the time of turn on below the
rated or specified limiting value The didt value maintained below limited value by using
a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating
is avoided by applying gate current but not greater the maximum gate current
DVDT AND OVER VOLTAGE PROTECTION
With forward voltage across the anode and cathode of a thyristor the two outer
junctions are forward biased but the inner junction is reverse biased This reverse biased
junction J2 has the characteristics of a capacitor due to charges existing across the
junction In other words space-charges exist in the depletion region around junction J2
and therefore junction J2 behaves like a capacitance If the entire anode to cathode
forward voltage Va appears across J2 junction and the charge is denoted by Q then a
charging current i given by Eq (46) follows
i = dQdt =d(Cj Va )dt
= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)
As Cj the capacitance of junction J2 is almost constant the current is given by
i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)
If the rate of rise of forward voltage dVadt is high the charging current i will be
more This charging current plays the role of gate current and turns on the SCR even
when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on
must be avoided as it leads to false operation of the thyristor circuit
For controllable operation of the thyristor the rate of rise of forward anode to
cathode voltage dVadt must be kept below the specified rated limit Typical values of
dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by
using a snubber circuit in parallel with the device thyristor are very sensitive for over
voltage than the semiconductor devices
Over voltage transients are perhaps the main cause of thyristor failure
In thyristor there are mainly two types
1 Internal over voltages
Due to the commutation of the thyristors large voltages are generated internally
Because of the series inductance of the SCR circuit the large transient voltages L didt
produced This voltage several times the break over voltage of the device then thyristor
destroys permanently
2 External over voltages
External over voltages are caused due to the interruptions of current flow in an
inductive circuit and also due to the lightening strokes on the lines feeding the thyristor
system For the reliable operation of thyristor the over voltages must be suppressed by
adopting suitable techniques
Suppression of over voltages
The RC circuit called snubber circuit is connected across the device to protect In
order to keep the protective components to a minimum the thyristors are chosen with
their peak voltages ratings are 25 to 3 times of the normal peak working voltage
ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are
commonly employed for protecting the thyristor circuit against the over voltages
Gate protection
Gate circuit should also be protected against the over voltages and surges Over
voltage at gate circuit can cause false triggering of the SCR may rises the junction
temperature behind specified limit leading to its damage Protection against over
voltage can be achieved by connecting a ZD across the gate circuit and a resister is
connected in series with gate circuit to protect against the s A capacitor and resister are
connected across gate to cathode to by pass the noise
41 Final Pulse Stages
The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage
(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor
bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided
with a power supply module The amplified output pulses from the Final Pulse Stages
start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main
pulse This initial pulse edge assures proper firing of the thyristors being triggered
Subsequently the weaker part of main pulse keeps firing conditions steady As already
mentioned the Final Pulse Stages and their associated thyristor bridges form single units
All six pulse outputs from a Final Pulse Stage can be blocked by an external control
signal so that all thyristors in the associated thyristor bridge will block the current A
blocking of the pulses is initiated whenever there is a malfunction in the associated
thyristor bridge
42 Converter Power Section
The thyristor converter consists of three independent parallel rectifier blocks TY1
to TY3 which are all in service Even if one block fails the remaining blocks take over
automatically the full design current of the excitation circuit During normal operation
(with ideal current share) and all three bridges in operation each of these blocks has to
carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation
is limited Only when all three bridges fail the excitation is switched off Each thyristor
bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any
arm is identified by a Current flow monitoring module
43 Converter Cooling
A cooling system is needed to dissipate heat losses in the converter blocks and
electronics Each converter block has therefore been equipped with a fan supplied with
power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing
cubicle) The fans are protected with motor protection circuit breakers An air flow
monitoring unit is provided for monitoring the air flow through the thyristor bridge If a
circuit breaker failure is detected or if the air flow monitor drops off at one of the
thyristor bridges the bridge involved is immediately set out of operation by blocking its
firing pulses
44 Thyristor Converter Monitoring
A thyristor bridge in which defects occur that could threaten the safety of
operation or cause secondary damage is switched off automatically ie its firing pulses
are blocked This happens whenever A thyristor fuse is blown The fuses are monitored
individually with micro switches The Final Pulse Stage fails which is detected by
internal monitors (supply voltage sustained pulse short-circuit on the output end) The
power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or
is insufficient Isolator on ACDC side is open
CHAPTER 5
Field Current Circuit Excitation Transformer
51 Field Circuit Breaker
The circuit-breaker in the field circuit is used to isolate the field circuit from the
converter It is capable of switching off the synchronous machine from full load under the
maximum conditions of a 3-phase short-circuit In addition to its main contacts the field
circuit-breaker also has a de-excitation contact with which the field energy stored in the
field can be dissipated across the de-excitation resistor The de-excitation contact closes
shortly before the main contacts open so as to ensure proper commutation of the field
current from the main contacts to the de-excitation contact when the breaker is switched
off The field circuit-breaker is switched on by electromagnetic force and is kept switched
on by a mechanical latch When the latch is released by a trip coil the circuit-breaker
opens The circuit-breaker also has auxiliary contacts that report its status
52 Field Flashing
In shunt supplied excitation circuits (excitation transformer connected to the
generator terminals) the generator does not have enough remnant voltage for a generator
voltage build-up via the converter In this case a field flashing circuit is provided It
consists of the field flashing contactor the diode bridge and a transformer used to adapt
the auxiliary input voltage to the voltage needed for field flashing when power is
supplied from the auxiliaries network
Fig Field Flashing
Because the field flashing contactor is not able to switch off the energy stored in
the field the control ensures that the contactor can only reopen if the field circuit breaker
has already been opened (generating the TRIP order) or in a normal field flashing
sequence when the converter has taken over the field current Field flashing occurs in the
following stages
1048729The excitation is switched on closing the field flashing contactor ( Field
Circuit Breaker is already closed )
1048729The start-up excitation current flows through the rotor driving the generator
voltage up to approx 15 U
1048729After about 10 U the firing pulses to the converter are released and it begins
to excite the generator to its rated voltage
1048729After about 30 U the field flashing contactor opens (with no current since
the converter is now supplying the current)
The diode bridge at the input to the field flashing contactor prevents a feed-back
from the converter to the source of field flashing while the contactor is still closed
53 De-excitation
When malfunctions occur the stored field energy must be dissipated as quickly
and safely as possible to protect the generator This is done by the converter the field
circuit-breaker and the de-excitation (discharge) resistor
De-excitation (with opening of the field circuit-breaker) takes place in the following
stages
1048729The converter drives to its inverter limit position (negative ceiling voltage)
recovers a portion of the field energy into the network A trip command is given to the
field circuit breaker
1048729The de-excitation contact closes diverting the field voltage to the de-excitation
resistor
1048729Then immediately the main contacts open building voltage The field voltage
commutates to the de-excitation resistor
1048729The current diminishes at a given time constant TE
(With linear resistance TE = Lf (Rf + Re))
Due to the reversal of the field voltage by the converter the field current
commutates from the main contacts of the field circuit-breaker to the de-excitation
resistor in a very early phase This reversal of the field voltage prevents burn-off on the
main contacts and provides effective protection for the field circuit-breaker Depending
on the operating policy an operational shut-down of the excitation can also be effected
with the field circuit-breaker closed This method is useful mainly when the excitation is
switched on and off frequently In this case the converter is merely driven into the
inverter limit position so that the field energy is recovered into the network The
converter then blocks since it is supplying positive current only
54 Excitation Transformer
The excitation transformer matches the generator voltage to the field voltage
(required ceiling voltage) It also serves as a commutation reactance for the thyristor
converter and as a potential isolator between the network and the excitation circuit In
addition the transformer functions as a current limiter in that it makes it possible to keep
any short circuits in the excitation circuit under better control The excitation transformer
is equipped with temperature monitoring probes which set off an alarm when the
temperature exceeds a first max limit and then trips the excitation if the temperature
continues rising to a second (higher)limit
CHAPTER 6
Monitoring and Protection
61 Excitation Monitoring
611 General Information
The main goal of Excitation Monitoring is to make optimum use of the
redundancies provided in the excitation system and to give alarm whenever a malfunction
makes these redundancies unavailable The field current is monitored to see that it does
not exceed a maximum level and if necessary a switch-over to the MANUAL channel is
initiated In addition the criterion for switching off the field flashing is generated The
excitation Monitoring consists of an autonomous processor system
612 over current Alarms
In the Excitation Monitoring the limits for are set at higher levels than the
settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of
the nominal field current contact R1 and the binary output associated with it are
activated immediately If field current remains gt 110 then after a preset inverse-time
has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs
associated with them are activated Parameters match the measurements for If1 and If2 to
the nominal value for field current so that the internal values can be processed and read as
pu values It can be used to falsify the actual value of the field current If (to raise it) so
as to cause a response from the alarm limits for purposes of testing The processed If
signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As
long as the field current If is above the threshold value 11 Ifn its peak value is
measured This is stored (until RESET) and can be read at any time on the Micro-
Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever
the integrated time-current value (intisup2dt) exceeds the preselected reference value the
alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously
issued to switch over to the stand by AUTO channel Software switch F758 enables the
three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-
time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the
desired limit curve for response is set using the factor F216
613 Switch-Off Criterion for Field Flashing
The Excitation Monitoring supplies the criterion for switching off the field
flashing Whether this criterion is activated based on the actual value for generator voltage
Ug or for field current If or both depends on the settings of the two threshold values
F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements
Ug12 and If12 are switched over depending on the present status of the channels
(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is
fixed at ldquological 1rdquo
614 Storage of Alarm Status
The outputs of the over current alarms (R1 R2 R0) and the messages NO
FAILURE MONITORING PARAMETERS CHANGED are stored messages can be
erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the
front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the
situation causing the alarm or the malfunction is no longer present Whenever the self-
diagnosis equipment in the processor detects a malfunction the output NO FAILURE of
MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo
is activated whenever parameters or settings of software switches have been changed via
the Micro-Terminal
615 Actual Value Monitoring
The actual values for generator voltage Ug synchronous voltage Usyn and field
current If are monitored for malfunctions This monitoring is active regardless of whether
or not the generator is in operation Essentially when the generator is in operation the
measurements are monitored by comparing the signals (the smaller signal reading is
detected as incorrect) When the generator is not in operation the measured data are
monitored for extreme values The percentage of deviation permissible in the
measurement signals being compared
(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and
F20AIf the excitation transformer is being supplied from an auxiliary power source (no
shunt operation) the values of Ug and Usyn will be different in some operational
conditions
In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1
and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the
messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2
and the binary message from CH1 reports no malfunction a malfunction on
Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is
also present whenever the binary message CH1 DISTURBANCE is reported and a
discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2
FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares
Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the
comparison channel reports a malfunction or whenever both binary messages report no
malfunction - but both secondary monitors report a malfunction As long as the secondary
monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction
signals for the measurement channel involved (suspicion that there is a corresponding
error in Usyn) The measurement channel malfunctions are enabled operationally
whenever after excitation has been switched on generator voltage Ug exceeds the value
set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is
switched off to see that they do not exceed the limit value F210 that applies to both of
them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt
the operating range) Monitoring for extreme values is likewise enabled during normal
operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the
malfunction signals to the binary outputs Basically the monitoring of the actual values
for If1If2 functions like that of the Ug1Ug2 monitoring
62 Excitation Protection
621 General Information
The Excitation Protection switches off the excitation (and de-excites the machine
rapidly) whenever a danger arises that threatens the excitation transformer the converter
or the generator Generally limiter or monitoring functions precede the emergency trips
and these normally respond before the Excitation Protection must initiate a trip
Protective trip commands are issued directly to the field circuit-breaker from potential
free contacts of the board via the trip relays They are directed redundantly to the
operative field circuit-breaker ldquoOFFrdquo command
622 Protection against Excitation Transformer Overheating
This equipment monitors the excitation transformer for overheating in the
windings that could result from over current short-circuits or inadequate cooling The
monitoring uses temperature monitoring modules in conjunction with temperature
sensors built into the windings Normally the temperature is monitored in two stages the
first stage sets off an alarm the second causes a trip of the excitation
623 Rotor Over voltage Protection
Malfunctions in the generator circuit (eg terminal short-circuit failed
synchronization asynchronous operation) cause induced negative field currents that
produce high voltages in the field circuit These must be restricted to a level with a
sufficient safety margin below the insulation capacity of the field winding (test voltage)
and also below the peak blocking voltage of the converter thyristors The crow bar
employs spark gap elements to detect over voltages in the field circuit Whenever they
respond the associated thyristors are fired immediately switching the de-excitation
resistor parallel to the field The de-excitation current generated thereby initiates an
excitation trip via a supervision circuit causing an immediate opening of the field circuit-
breaker The malfunction isets off an alarm and an internal malfunction is indicated at the
cubicle
TEST VALUES OBTAINED WHEN EXCITATION IS RAISED
TEST
SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE
1 VREF 996 100
2 VACT 997 1003
3 IFACT 735 765
4 IGACT 703 707
5 ACTIVE
POWER
703 705
6 REACTIVE
POWER
101 142
7 POWER
FACTOR
099 IND 098 IND
8 ACTIVE
CURRENT(IR)
705 703
9 REACTIVE
CURRENT(IX)
102 138
10 POWER
ANGLE
566 546
11 FIRING
ANGLE
640 632
VALUES OBTAINED WHEN EXCITATION IS LOWERED
SNO PARTICULARS ACTUAL
VALUE
OBTAINED VALUE
1 VREF 100 997
2 VACT 100 997
3 IFACT 787 761
4 IGACT 839 837
5 ACTIVE
POWER
839 830
6 REACTIVE
POWER
155 110
7 POWER
FACTOR
098 IND 099 IND
8 ACTIVE
CURRENT(IR)
835 830
9 REACTIVE
CURRENT(IX)
140 94
10 POWER
ANGLE
613 DEG 631 DEG
11 FIRING
ANGLE
601 DEG 599 DEG
CONCLUSION
For generating the EMF in stator winding excitation is required to the rotor of a
generator There are two types of excitation
1 Static excitation system
2 Brushless excitation system
A certain disadvantage in brushless excitation system is the slow response time of
the field in case of fast load changes specified No slip-rings and brushes direct
measurements of the field parameters not possible
To avoid all loses static excitation is used Since it does not have any rotating
parts mechanical loses and windage loses This system has fast response and speed
control While preferring this excitation system there are no limitations for the
redundancy of Thyristor bridge circuits
Static excitation has fast field discharge by resistor and inverter operation direct
measurement of field quantity is possible The meaning of excitation is nothing but
continuous supply of DC current (ie field current) to the rotor to buildup required
output voltage in the stator
Field current is changed with respect to the change of load so the digital
automatic voltage regulator (DAVR) is used to regulate the output voltage according to
the load variations
So we conclude that static excitation system with DAVR is preferred since it is
having excellent dynamic performance and better options for R amp M
234 Ugf Limiter
At under frequency the Ugf Limiter reduces the generator voltage so as to
prevent saturation effects in the supply and measuring transformers To adjust this
limiter the max permissible generator voltage at rated frequency is defined and set
When any under-frequency occurs the generator voltage is thus reduced in proportion to
that setting
235 Field Current Maximum Limiter
The Field Current Maximum Limiter is provided to protect the generator rotor
from s occurring in steady-state and transient operation High field currents are normally
the result of a sharp drop in network voltage or of an improper raising of the voltage set-
point by the operating staff The field current is held steady at the value TH1 ie at the
maximum thermal value permissible for the excitation circuit and the rotor In order that
the generator can support the power network with its transient overload capacity during
brief collapses in voltage a temporary switch-over is made to the transient limit MAX1
(a higher setting) When the generator or the converter is operating at a reduced capacity
These limits TH1MAX1 can be switched over to the lower settings TH2MAX2 by
activating the corresponding binary signals The switch-over from the thermal limit
TH12 to the transient limit MAX12 can be configured in one of three ways
a) Depending on the over current with -dUdt ENABLE
Flag programming F418 = any setting desired F41A = 0000
This variant enables the transient value MAX12 whenever a collapse of voltage
in the network is detected The ENABLE time is fixed and can be set The example
below shows the typical behavior of the limiter configured in this way
b) Dependent on the time integral with -dUdt ENABLE
Flag programming F418 = inactive F41A = 1111
This variant likewise enables the transient value only when a collapse of network
voltage has been detected However the switch-back to the thermal limit is not made
dependent upon the time itself but on the calculated time integral intisup2dt of the The setting
on Parameter F414 in spu takes into account the time the rotor needs to cool down ie
the rate of temperature change in the case of intermittent operation The example below
shows how the timing of the switch-back to the thermal limit depends on the present
value for intisup2dt
The time integral is based on the formula
Example The setting of =isup2dt equivalent to Version a (F416) at a constant 16 times the
nominal field current for 10 seconds (with TH12 = 105) is
c) Dependent on the time integral without any preconditions
Flag programming F418 = 1111 F41A = 1111
In this variant the transient becomes available without any prior conditions
(without a -dUdt ENABLE) with the time integral intisup2dt
237 Inductive Stator Current Limiter
The Inductive Stator Current Limiter holds the stator current Ig within permissible
limits while the generator is in the ldquoover-excitedrdquo operating range by reducing the field
current accordingly The setting TH (thermal limit) provides the limit against stationary s
that might occur To take advantage of the generatorrsquos transient overload capacity a
switch-over is made to the higher setting MAX The principle of operation of this switch-
over to the value MAX permissible only transiently is identical to that employed for the
field current limiter (refer to the description above) When the drive output from the
turbine is very high stator current may exceed permissible limits even while inductive
loading of the generator is low In this case if the stator current limiter is not kept from
influencing the field current the control circuit will oscillate back and forth between the
Inductive Stator Current Limiter (de-
exciting) and the Capacitive Stator Current Limiter (exciting)The output signal of that
function then dominates the control variable of the Ig-dependent limiter via a maximum
value selection
238 Capacitive Stator Current Limiter
239 Load Angle Limiter
The Load Angle Limiter prevents the synchronous machine from slipping out of
phase due to slippage of the rotor The load angle δ the difference in phase between the
rotor and the stator rotating field results mainly from the driving torque (active power P)
acting on the generator and the level of rotor current (field current) If the driving torque
remains constant a increase in the field current reduces the load angle δ The current load
angle δ at any moment is obtained from the generator current and generator voltage based
on a simplified model of the generator Whenever this calculated load angle δ exceeds the
preset limit angle the limiter increases the field current until the load angle has dropped
back to its permissible value The quadrature reactance Xq of the generator and the
network reactance Xe during normal operation must be adjusted on the regulator in order
to obtain the load angle δ The graph below shows the Power Chart for a salient-pole
machine with typical limiter characteristics
The purpose of a Power System Stabilizer is to use the generator excitation to
damp electromechanical oscillations between the network and the generator Depending
on the design of the generator and the requirements imposed for network stability its
main function will be either to damp the oscillations originating in the machine or those
from the network A synchronous generator working in a combined power network is in
principle an oscillating structure In order to produce a torque the magnetic field of the
rotor and the stator must form a given angle (referred to as the rotor displacement or load
angle δ) The electrical torque ME increases as the angle δ increases just as with a
torsion spring Because the ME of the generator and the mechanical driving torque MA
from the turbine are in equilibrium during steady-state operation the angle δ remains in a
given position Whenever this state of equilibrium between MA and ME is disturbed the
load angle slips of this rest position and change thereby the electrical torque ME The
torque attempts to restore the load angle to a stationary position Due to the mass inertia
of the turbinegenerator rotor however this can only take place aperiodically It does so
in the form of more or less effectively damped oscillations (again similar to the effect of
mass inertia on a torsion spring) In order to damp the oscillations there must be a
damping torque produced depending not on the electrical torque ME associated with the
angle but on the difference in frequency (Df) between the rotor and the stator rotating
field ie on the slippage This torque is produced mainly by the so-called damper
winding in the rotor but the dimensioning of this is subject to limits imposed by
considerations of design and economy Some further action is therefore needed to
increase the damping effect The following drastically simplified formula shows the
parameters upon which the amount of active power PE supplied by the generator
depends
PE = active power
It can be seen from the above relationship that the active power that the generator
transfers depends not only on the load angle δ but also on the field current If That means
that a transient change can be made in the active power PE and with that in the effective
electrical torque ME by varying the field current The principle of operation of the DVR
Power System Stabilizer becomes clear from a consideration of the oscillations in power
output and frequency (ΔPE Δf) and the vector diagram If it is assumed that oscillations
in the network frequency generates load oscillations with the mass inertia of the rotor
then the active load of the generator (eg MW-measured) is influenced with a sinusoidal
value -ΔPE (ME-MA = -ΔPE) By inversion of -ΔPE one obtains the fluctuation in
power provided by the rotor +ΔPE As is known the slip signal Δf follows +ΔPE with a
phase delayed by 90deg The +ME produced by the periodic changes in the load angle δ is
in phase with +ΔPE A good damping is attained if ME is varied in phase with the slip
Δf However this signal must also be advanced somewhat to compensate for the time
constants in the excitation circuit and the generator
As mentioned above the electrical torque ME can be influenced by varying the
field current To accomplish this a suitable control signal referred to as variable
disturbance compensation must be imposed upon the voltage set-point or the converter
control variable Ucontr As can be seen from the vector diagram by applying proper
weighting factors (K1 K2) and then adding together the signals -ΔPE and Δf an overall
stabilization signal can be produced that rotates in advance of the Df signal by any angle
desired between 0deg and 90deg Because the amplitude of -DPE remains proportional to the
amplitude of Δf a constant angle in advance of Δf results for the compensation of the
time constants referred to above The optimum weighting factors K1 and K2 for a
synchronous generator working to a power network depend on its operating point at any
moment and the external reactance of the network Normally the selection of a
compromise setting is good enough to attain stability in all operating points and for all
external reactance For special demands these settings must be parameterized as a
function of the external reactance (which means optional equipment Xe-Identification)
The Power System Stabilizer PSS is a section of the AVR computer program and is
processed once per network cycle The voltage at the generator terminals and the
generator current are measured in order to define the signals ΔPE and Δf The calculated
signals for _P_ (=PE) and Δf are then sent across DC filters ldquoDrdquo (real differentiators) that
transmit only the dynamic portion of the signals The ΔPE and Δf signals obtained in this
way are then weighted (multiplied by) with the factors K1 and K2 and sent to the
summing point of the voltage regulator
The PSS stabilization signal is imposed on the automatic voltage regulator only if
the following prerequisites are met
bull Generator on line
bull Generator power output gt the value F338
bull Generator voltage in a range between F33C and F33A
The stabilization signal is limited at the output from the PSS to the lower and
upper limits Flag defines whether the stabilization signal is introduced before or after the
PID filter (usually before the filter) Because the PID filter as noted above already takes
the ceiling factor Vp1 into account the PSS signal needs to be multiplied by Vp1 if it is
added to the voltage regulator following the PID filter (divider at the input to the
minmax limiter) This precaution prevents the DC filter ldquoDrdquo in the P-channel from
producing an unnecessary ldquostabilizationrdquo effect in the case of rapid changes in turbine
load As an alternative for the AVRrsquos Power System Stabilizer a stabilization signal from
an outside system can be imposed by activating the binary input ldquoPSS-SIGNEXTrdquo Flag
F340 can be used to select between an analog and a 12-bit signal and F33E to select the
polarity desired for that signal
24 The MANUAL Channel
241 Summary
The MANUAL channel (Channel 2) has been built as a simple field current
regulator
without additional limiters Its main function is to maintain the excitation of the generator
even if the AUTOMATIC channel becomes non-operational The MANUAL channel
also performs valuable service for purposes of testing commissioning and preventive
maintenance Its measurements regulator generation of firing pulses and power supply
are physically separate from those on the AUTOMATIC channel
242 Principle of Operation
All the functions of the MANUAL channel including the generation of firing
pulses have been implemented in a single electronic module the Gate Control Unit The
control variable Ucontr of voltage regulator is used as the reference value for generating
firing pulses on the principle known as ldquoramp controlrdquo (Comparison of Ucontr with
Usynsynchronous sawtooth signal) For further processing in the UN 0096 Intermediate
Pulse Stage the Gate Control Unit supplies six firing pulses at its output whose phase
position with respect to the synchronous voltage Usyn is in accordance with control
variable Ucontr An internal linearization ensures that the field voltage produced via the
firing pulses remains proportional to the control variable Ucontr throughout the entire
range As a result the circuit amplification of the control remains constant over the entire
range Whenever excitation is switched ON the set-point for Generator Voltage is set
automatically at the preset - ref Value This provision ensures that the generator voltage
always attains approximately its nominal value after the field flashing The Gate Control
Unit can be refunctioned ( by pre-selection with a switch ) for purposes of testing to act
as a purely firing pulse control In this case the control variable Ucontr is adjusted
directly using the RAISELOWER push buttons on the front of the module In this way
for example the relationship between the phase position of the firing pulses and the
control variable Ucontr can be checked easily
CHAPTER 3
PULSE SECTION
31 Pulse Generation and Amplification
The Gate Control Units of both AUTOMATIC channel and MANUAL channel
each supply six firing pulses for operating the 6-pulse thyristor bridges The low-power
pulse signals from these Gate Control Units are then amplified in the Intermediate Pulse
Stage galvanically isolated and then sent to the common pulse bus On the output end
the Intermediate Pulse Stage of the non-active channel) is always blocked The Gate
Control Units generate the pulses based on microprocessor control The reference voltage
used for the firing pulse phase location is the output voltage from the excitation
transformer (Usyn1 Usyn2) The commutation spikes of the synchronous voltage caused
by the converter are calculated prior to use of the voltage as a reference value and are
deliberately filtered out The lower limit for the firing pulses (double pulses) which are
offset from one another by 60deg is defined by the limit rectifier position (αmin) and the
upper limit by the limit inverter position (αmax) for the firing angle αmin and αmax can
be adjusted on the Gate Control Units using BCD (Binary Coded Decimal) switches
αmin ensures that the firing pulses will not be issued (premature firing) until there is
sufficient positive phase voltage on the thyristor involved αmax prevents a dangerous
ldquotippingrdquo of the thyristor bridge into the rectifier mode if the firing angle α is too large
(ldquolate firingrdquo) The critical factors determining αmax are the overlap time uumlmax (max
commutation time) and the ldquorecovery timerdquo of the thyristors (αmax lt 180deg - uumlmax - γ )
An external control signal can force the firing pulses into their inverter limit position
Other binary inputs can block or direct the firing pulses of the Gate Control Units so as to
produce freewheeling on the thyristor bridge During freewheeling the firing pulses for
the thyristor pair R and S are blocked and the pulse signals T+T- are engaged with
chains of pulses Both Gate Control Units (for the MANUAL amp AUTOMATIC channels)
contain a field current monitor that blocks the firing pulses immediately whenever the
current exceeds a preset threshold level In this case the field circuit-breaker is also
tripped via an output contact The purpose of these provisions is to prevent damage to
thyristors and thyristor fuses in case of a slip-ring short-circuit or to keep any damage
that does occur to a minimum The pulse signals are galvanically separated at the outputs
from the Intermediate Pulse Stage (with pulse transmitters) and are then directed to the
common pulse bus This transmission of the pulse signals to the pulse bus via passive
transmitters ensures a high degree of active channel autonomy Practically no possible
malfunctions on the inactive channel (including for example sustained pulses) affect the
active channel
32 Pulse Monitoring
The ldquoPulse Busrdquo and the pulse signals of the AUTOMATIC channel are
monitored This monitoring device consists of potential isolating stages and the common
monitor If the pulse monitoring of the ldquoPulse Busrdquo responds a switch-over is made to
MANUAL channel The function of the potential isolating stages is to couple the pulse
monitoring device to the pulse circuits without any feedback effect The pulse monitoring
checks the six pulse lines for the following malfunctions continuous or periodic failure
of one or more pulses Periodic occurrence of synchronous or asynchronous false pulses
Continuous pulses the pulse monitoring device can be tested while the machine is in
operation
CHAPTER 4
CONVERTER
Thyristor
The term thyristor usually refers to a family of four layer solid state device having
turn on characteristics that can be externally controlled by either current or voltage They
are also referred to as breakdown device because their working depends on avalanche
breakdown Thyristors have only two stages OFF and ON Thyristors have a similar
function to Uni-junctions they act as switches Thyristors use current flow as a switch
Thyristors have three states
1 Reverse blocking mode mdash Voltage is applied in the direction that would be
blocked by a diode
2 Forward blocking mode mdash Voltage is applied in the direction that would cause
a diode to conduct but the thyristor has not yet been triggered into conduction
3 Forward conducting mode mdash The thyristor has been triggered into conduction
and will remain conducting until the forward current drops below a threshold value
known as the holding current Converter is a semiconductor device which converts ac
input voltage into a constant dc output voltage In present excitation system three phase
fully controlled thyristor converter is used
Because of the following advantages thyristor converters are used
a) Thyristors are used for high power applications ie up to 10Kv3500A1KHz
b) Having high reliability and low losses
c) Uni-directional device like diode
d) Itrsquos operation as a rectifier which are low resistance in forward conduction
mode and high resistance in reverse conduction mode
PROTECTION OF THYRISTORS
For reliable operation of a thyristor demands that its specified ratings are not
exceeded When Subjected to or over voltages During the turn - on of SCR didt
prohibitively large False triggering of SCR by high value of dvdt andSpurious signals
between gate and cathode may leads to unwanted turn ndash on
DIDT AND PROTECTION
When thyristor starts conducting in forward conduction mode and is turned on by
gate pulse The anode current increases rapidly whole area of the gate to Cathode
junction then hot spots will be formed near the gate connection this locality of heating
destroys the thyristor Thyristor thermal time is constant The causes due to faults and
short circuits or surge currents Electronic crowbar protection is used against the over
voltages The rate rise of anode current must be kept at the time of turn on below the
rated or specified limiting value The didt value maintained below limited value by using
a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating
is avoided by applying gate current but not greater the maximum gate current
DVDT AND OVER VOLTAGE PROTECTION
With forward voltage across the anode and cathode of a thyristor the two outer
junctions are forward biased but the inner junction is reverse biased This reverse biased
junction J2 has the characteristics of a capacitor due to charges existing across the
junction In other words space-charges exist in the depletion region around junction J2
and therefore junction J2 behaves like a capacitance If the entire anode to cathode
forward voltage Va appears across J2 junction and the charge is denoted by Q then a
charging current i given by Eq (46) follows
i = dQdt =d(Cj Va )dt
= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)
As Cj the capacitance of junction J2 is almost constant the current is given by
i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)
If the rate of rise of forward voltage dVadt is high the charging current i will be
more This charging current plays the role of gate current and turns on the SCR even
when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on
must be avoided as it leads to false operation of the thyristor circuit
For controllable operation of the thyristor the rate of rise of forward anode to
cathode voltage dVadt must be kept below the specified rated limit Typical values of
dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by
using a snubber circuit in parallel with the device thyristor are very sensitive for over
voltage than the semiconductor devices
Over voltage transients are perhaps the main cause of thyristor failure
In thyristor there are mainly two types
1 Internal over voltages
Due to the commutation of the thyristors large voltages are generated internally
Because of the series inductance of the SCR circuit the large transient voltages L didt
produced This voltage several times the break over voltage of the device then thyristor
destroys permanently
2 External over voltages
External over voltages are caused due to the interruptions of current flow in an
inductive circuit and also due to the lightening strokes on the lines feeding the thyristor
system For the reliable operation of thyristor the over voltages must be suppressed by
adopting suitable techniques
Suppression of over voltages
The RC circuit called snubber circuit is connected across the device to protect In
order to keep the protective components to a minimum the thyristors are chosen with
their peak voltages ratings are 25 to 3 times of the normal peak working voltage
ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are
commonly employed for protecting the thyristor circuit against the over voltages
Gate protection
Gate circuit should also be protected against the over voltages and surges Over
voltage at gate circuit can cause false triggering of the SCR may rises the junction
temperature behind specified limit leading to its damage Protection against over
voltage can be achieved by connecting a ZD across the gate circuit and a resister is
connected in series with gate circuit to protect against the s A capacitor and resister are
connected across gate to cathode to by pass the noise
41 Final Pulse Stages
The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage
(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor
bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided
with a power supply module The amplified output pulses from the Final Pulse Stages
start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main
pulse This initial pulse edge assures proper firing of the thyristors being triggered
Subsequently the weaker part of main pulse keeps firing conditions steady As already
mentioned the Final Pulse Stages and their associated thyristor bridges form single units
All six pulse outputs from a Final Pulse Stage can be blocked by an external control
signal so that all thyristors in the associated thyristor bridge will block the current A
blocking of the pulses is initiated whenever there is a malfunction in the associated
thyristor bridge
42 Converter Power Section
The thyristor converter consists of three independent parallel rectifier blocks TY1
to TY3 which are all in service Even if one block fails the remaining blocks take over
automatically the full design current of the excitation circuit During normal operation
(with ideal current share) and all three bridges in operation each of these blocks has to
carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation
is limited Only when all three bridges fail the excitation is switched off Each thyristor
bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any
arm is identified by a Current flow monitoring module
43 Converter Cooling
A cooling system is needed to dissipate heat losses in the converter blocks and
electronics Each converter block has therefore been equipped with a fan supplied with
power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing
cubicle) The fans are protected with motor protection circuit breakers An air flow
monitoring unit is provided for monitoring the air flow through the thyristor bridge If a
circuit breaker failure is detected or if the air flow monitor drops off at one of the
thyristor bridges the bridge involved is immediately set out of operation by blocking its
firing pulses
44 Thyristor Converter Monitoring
A thyristor bridge in which defects occur that could threaten the safety of
operation or cause secondary damage is switched off automatically ie its firing pulses
are blocked This happens whenever A thyristor fuse is blown The fuses are monitored
individually with micro switches The Final Pulse Stage fails which is detected by
internal monitors (supply voltage sustained pulse short-circuit on the output end) The
power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or
is insufficient Isolator on ACDC side is open
CHAPTER 5
Field Current Circuit Excitation Transformer
51 Field Circuit Breaker
The circuit-breaker in the field circuit is used to isolate the field circuit from the
converter It is capable of switching off the synchronous machine from full load under the
maximum conditions of a 3-phase short-circuit In addition to its main contacts the field
circuit-breaker also has a de-excitation contact with which the field energy stored in the
field can be dissipated across the de-excitation resistor The de-excitation contact closes
shortly before the main contacts open so as to ensure proper commutation of the field
current from the main contacts to the de-excitation contact when the breaker is switched
off The field circuit-breaker is switched on by electromagnetic force and is kept switched
on by a mechanical latch When the latch is released by a trip coil the circuit-breaker
opens The circuit-breaker also has auxiliary contacts that report its status
52 Field Flashing
In shunt supplied excitation circuits (excitation transformer connected to the
generator terminals) the generator does not have enough remnant voltage for a generator
voltage build-up via the converter In this case a field flashing circuit is provided It
consists of the field flashing contactor the diode bridge and a transformer used to adapt
the auxiliary input voltage to the voltage needed for field flashing when power is
supplied from the auxiliaries network
Fig Field Flashing
Because the field flashing contactor is not able to switch off the energy stored in
the field the control ensures that the contactor can only reopen if the field circuit breaker
has already been opened (generating the TRIP order) or in a normal field flashing
sequence when the converter has taken over the field current Field flashing occurs in the
following stages
1048729The excitation is switched on closing the field flashing contactor ( Field
Circuit Breaker is already closed )
1048729The start-up excitation current flows through the rotor driving the generator
voltage up to approx 15 U
1048729After about 10 U the firing pulses to the converter are released and it begins
to excite the generator to its rated voltage
1048729After about 30 U the field flashing contactor opens (with no current since
the converter is now supplying the current)
The diode bridge at the input to the field flashing contactor prevents a feed-back
from the converter to the source of field flashing while the contactor is still closed
53 De-excitation
When malfunctions occur the stored field energy must be dissipated as quickly
and safely as possible to protect the generator This is done by the converter the field
circuit-breaker and the de-excitation (discharge) resistor
De-excitation (with opening of the field circuit-breaker) takes place in the following
stages
1048729The converter drives to its inverter limit position (negative ceiling voltage)
recovers a portion of the field energy into the network A trip command is given to the
field circuit breaker
1048729The de-excitation contact closes diverting the field voltage to the de-excitation
resistor
1048729Then immediately the main contacts open building voltage The field voltage
commutates to the de-excitation resistor
1048729The current diminishes at a given time constant TE
(With linear resistance TE = Lf (Rf + Re))
Due to the reversal of the field voltage by the converter the field current
commutates from the main contacts of the field circuit-breaker to the de-excitation
resistor in a very early phase This reversal of the field voltage prevents burn-off on the
main contacts and provides effective protection for the field circuit-breaker Depending
on the operating policy an operational shut-down of the excitation can also be effected
with the field circuit-breaker closed This method is useful mainly when the excitation is
switched on and off frequently In this case the converter is merely driven into the
inverter limit position so that the field energy is recovered into the network The
converter then blocks since it is supplying positive current only
54 Excitation Transformer
The excitation transformer matches the generator voltage to the field voltage
(required ceiling voltage) It also serves as a commutation reactance for the thyristor
converter and as a potential isolator between the network and the excitation circuit In
addition the transformer functions as a current limiter in that it makes it possible to keep
any short circuits in the excitation circuit under better control The excitation transformer
is equipped with temperature monitoring probes which set off an alarm when the
temperature exceeds a first max limit and then trips the excitation if the temperature
continues rising to a second (higher)limit
CHAPTER 6
Monitoring and Protection
61 Excitation Monitoring
611 General Information
The main goal of Excitation Monitoring is to make optimum use of the
redundancies provided in the excitation system and to give alarm whenever a malfunction
makes these redundancies unavailable The field current is monitored to see that it does
not exceed a maximum level and if necessary a switch-over to the MANUAL channel is
initiated In addition the criterion for switching off the field flashing is generated The
excitation Monitoring consists of an autonomous processor system
612 over current Alarms
In the Excitation Monitoring the limits for are set at higher levels than the
settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of
the nominal field current contact R1 and the binary output associated with it are
activated immediately If field current remains gt 110 then after a preset inverse-time
has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs
associated with them are activated Parameters match the measurements for If1 and If2 to
the nominal value for field current so that the internal values can be processed and read as
pu values It can be used to falsify the actual value of the field current If (to raise it) so
as to cause a response from the alarm limits for purposes of testing The processed If
signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As
long as the field current If is above the threshold value 11 Ifn its peak value is
measured This is stored (until RESET) and can be read at any time on the Micro-
Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever
the integrated time-current value (intisup2dt) exceeds the preselected reference value the
alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously
issued to switch over to the stand by AUTO channel Software switch F758 enables the
three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-
time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the
desired limit curve for response is set using the factor F216
613 Switch-Off Criterion for Field Flashing
The Excitation Monitoring supplies the criterion for switching off the field
flashing Whether this criterion is activated based on the actual value for generator voltage
Ug or for field current If or both depends on the settings of the two threshold values
F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements
Ug12 and If12 are switched over depending on the present status of the channels
(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is
fixed at ldquological 1rdquo
614 Storage of Alarm Status
The outputs of the over current alarms (R1 R2 R0) and the messages NO
FAILURE MONITORING PARAMETERS CHANGED are stored messages can be
erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the
front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the
situation causing the alarm or the malfunction is no longer present Whenever the self-
diagnosis equipment in the processor detects a malfunction the output NO FAILURE of
MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo
is activated whenever parameters or settings of software switches have been changed via
the Micro-Terminal
615 Actual Value Monitoring
The actual values for generator voltage Ug synchronous voltage Usyn and field
current If are monitored for malfunctions This monitoring is active regardless of whether
or not the generator is in operation Essentially when the generator is in operation the
measurements are monitored by comparing the signals (the smaller signal reading is
detected as incorrect) When the generator is not in operation the measured data are
monitored for extreme values The percentage of deviation permissible in the
measurement signals being compared
(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and
F20AIf the excitation transformer is being supplied from an auxiliary power source (no
shunt operation) the values of Ug and Usyn will be different in some operational
conditions
In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1
and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the
messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2
and the binary message from CH1 reports no malfunction a malfunction on
Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is
also present whenever the binary message CH1 DISTURBANCE is reported and a
discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2
FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares
Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the
comparison channel reports a malfunction or whenever both binary messages report no
malfunction - but both secondary monitors report a malfunction As long as the secondary
monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction
signals for the measurement channel involved (suspicion that there is a corresponding
error in Usyn) The measurement channel malfunctions are enabled operationally
whenever after excitation has been switched on generator voltage Ug exceeds the value
set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is
switched off to see that they do not exceed the limit value F210 that applies to both of
them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt
the operating range) Monitoring for extreme values is likewise enabled during normal
operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the
malfunction signals to the binary outputs Basically the monitoring of the actual values
for If1If2 functions like that of the Ug1Ug2 monitoring
62 Excitation Protection
621 General Information
The Excitation Protection switches off the excitation (and de-excites the machine
rapidly) whenever a danger arises that threatens the excitation transformer the converter
or the generator Generally limiter or monitoring functions precede the emergency trips
and these normally respond before the Excitation Protection must initiate a trip
Protective trip commands are issued directly to the field circuit-breaker from potential
free contacts of the board via the trip relays They are directed redundantly to the
operative field circuit-breaker ldquoOFFrdquo command
622 Protection against Excitation Transformer Overheating
This equipment monitors the excitation transformer for overheating in the
windings that could result from over current short-circuits or inadequate cooling The
monitoring uses temperature monitoring modules in conjunction with temperature
sensors built into the windings Normally the temperature is monitored in two stages the
first stage sets off an alarm the second causes a trip of the excitation
623 Rotor Over voltage Protection
Malfunctions in the generator circuit (eg terminal short-circuit failed
synchronization asynchronous operation) cause induced negative field currents that
produce high voltages in the field circuit These must be restricted to a level with a
sufficient safety margin below the insulation capacity of the field winding (test voltage)
and also below the peak blocking voltage of the converter thyristors The crow bar
employs spark gap elements to detect over voltages in the field circuit Whenever they
respond the associated thyristors are fired immediately switching the de-excitation
resistor parallel to the field The de-excitation current generated thereby initiates an
excitation trip via a supervision circuit causing an immediate opening of the field circuit-
breaker The malfunction isets off an alarm and an internal malfunction is indicated at the
cubicle
TEST VALUES OBTAINED WHEN EXCITATION IS RAISED
TEST
SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE
1 VREF 996 100
2 VACT 997 1003
3 IFACT 735 765
4 IGACT 703 707
5 ACTIVE
POWER
703 705
6 REACTIVE
POWER
101 142
7 POWER
FACTOR
099 IND 098 IND
8 ACTIVE
CURRENT(IR)
705 703
9 REACTIVE
CURRENT(IX)
102 138
10 POWER
ANGLE
566 546
11 FIRING
ANGLE
640 632
VALUES OBTAINED WHEN EXCITATION IS LOWERED
SNO PARTICULARS ACTUAL
VALUE
OBTAINED VALUE
1 VREF 100 997
2 VACT 100 997
3 IFACT 787 761
4 IGACT 839 837
5 ACTIVE
POWER
839 830
6 REACTIVE
POWER
155 110
7 POWER
FACTOR
098 IND 099 IND
8 ACTIVE
CURRENT(IR)
835 830
9 REACTIVE
CURRENT(IX)
140 94
10 POWER
ANGLE
613 DEG 631 DEG
11 FIRING
ANGLE
601 DEG 599 DEG
CONCLUSION
For generating the EMF in stator winding excitation is required to the rotor of a
generator There are two types of excitation
1 Static excitation system
2 Brushless excitation system
A certain disadvantage in brushless excitation system is the slow response time of
the field in case of fast load changes specified No slip-rings and brushes direct
measurements of the field parameters not possible
To avoid all loses static excitation is used Since it does not have any rotating
parts mechanical loses and windage loses This system has fast response and speed
control While preferring this excitation system there are no limitations for the
redundancy of Thyristor bridge circuits
Static excitation has fast field discharge by resistor and inverter operation direct
measurement of field quantity is possible The meaning of excitation is nothing but
continuous supply of DC current (ie field current) to the rotor to buildup required
output voltage in the stator
Field current is changed with respect to the change of load so the digital
automatic voltage regulator (DAVR) is used to regulate the output voltage according to
the load variations
So we conclude that static excitation system with DAVR is preferred since it is
having excellent dynamic performance and better options for R amp M
b) Dependent on the time integral with -dUdt ENABLE
Flag programming F418 = inactive F41A = 1111
This variant likewise enables the transient value only when a collapse of network
voltage has been detected However the switch-back to the thermal limit is not made
dependent upon the time itself but on the calculated time integral intisup2dt of the The setting
on Parameter F414 in spu takes into account the time the rotor needs to cool down ie
the rate of temperature change in the case of intermittent operation The example below
shows how the timing of the switch-back to the thermal limit depends on the present
value for intisup2dt
The time integral is based on the formula
Example The setting of =isup2dt equivalent to Version a (F416) at a constant 16 times the
nominal field current for 10 seconds (with TH12 = 105) is
c) Dependent on the time integral without any preconditions
Flag programming F418 = 1111 F41A = 1111
In this variant the transient becomes available without any prior conditions
(without a -dUdt ENABLE) with the time integral intisup2dt
237 Inductive Stator Current Limiter
The Inductive Stator Current Limiter holds the stator current Ig within permissible
limits while the generator is in the ldquoover-excitedrdquo operating range by reducing the field
current accordingly The setting TH (thermal limit) provides the limit against stationary s
that might occur To take advantage of the generatorrsquos transient overload capacity a
switch-over is made to the higher setting MAX The principle of operation of this switch-
over to the value MAX permissible only transiently is identical to that employed for the
field current limiter (refer to the description above) When the drive output from the
turbine is very high stator current may exceed permissible limits even while inductive
loading of the generator is low In this case if the stator current limiter is not kept from
influencing the field current the control circuit will oscillate back and forth between the
Inductive Stator Current Limiter (de-
exciting) and the Capacitive Stator Current Limiter (exciting)The output signal of that
function then dominates the control variable of the Ig-dependent limiter via a maximum
value selection
238 Capacitive Stator Current Limiter
239 Load Angle Limiter
The Load Angle Limiter prevents the synchronous machine from slipping out of
phase due to slippage of the rotor The load angle δ the difference in phase between the
rotor and the stator rotating field results mainly from the driving torque (active power P)
acting on the generator and the level of rotor current (field current) If the driving torque
remains constant a increase in the field current reduces the load angle δ The current load
angle δ at any moment is obtained from the generator current and generator voltage based
on a simplified model of the generator Whenever this calculated load angle δ exceeds the
preset limit angle the limiter increases the field current until the load angle has dropped
back to its permissible value The quadrature reactance Xq of the generator and the
network reactance Xe during normal operation must be adjusted on the regulator in order
to obtain the load angle δ The graph below shows the Power Chart for a salient-pole
machine with typical limiter characteristics
The purpose of a Power System Stabilizer is to use the generator excitation to
damp electromechanical oscillations between the network and the generator Depending
on the design of the generator and the requirements imposed for network stability its
main function will be either to damp the oscillations originating in the machine or those
from the network A synchronous generator working in a combined power network is in
principle an oscillating structure In order to produce a torque the magnetic field of the
rotor and the stator must form a given angle (referred to as the rotor displacement or load
angle δ) The electrical torque ME increases as the angle δ increases just as with a
torsion spring Because the ME of the generator and the mechanical driving torque MA
from the turbine are in equilibrium during steady-state operation the angle δ remains in a
given position Whenever this state of equilibrium between MA and ME is disturbed the
load angle slips of this rest position and change thereby the electrical torque ME The
torque attempts to restore the load angle to a stationary position Due to the mass inertia
of the turbinegenerator rotor however this can only take place aperiodically It does so
in the form of more or less effectively damped oscillations (again similar to the effect of
mass inertia on a torsion spring) In order to damp the oscillations there must be a
damping torque produced depending not on the electrical torque ME associated with the
angle but on the difference in frequency (Df) between the rotor and the stator rotating
field ie on the slippage This torque is produced mainly by the so-called damper
winding in the rotor but the dimensioning of this is subject to limits imposed by
considerations of design and economy Some further action is therefore needed to
increase the damping effect The following drastically simplified formula shows the
parameters upon which the amount of active power PE supplied by the generator
depends
PE = active power
It can be seen from the above relationship that the active power that the generator
transfers depends not only on the load angle δ but also on the field current If That means
that a transient change can be made in the active power PE and with that in the effective
electrical torque ME by varying the field current The principle of operation of the DVR
Power System Stabilizer becomes clear from a consideration of the oscillations in power
output and frequency (ΔPE Δf) and the vector diagram If it is assumed that oscillations
in the network frequency generates load oscillations with the mass inertia of the rotor
then the active load of the generator (eg MW-measured) is influenced with a sinusoidal
value -ΔPE (ME-MA = -ΔPE) By inversion of -ΔPE one obtains the fluctuation in
power provided by the rotor +ΔPE As is known the slip signal Δf follows +ΔPE with a
phase delayed by 90deg The +ME produced by the periodic changes in the load angle δ is
in phase with +ΔPE A good damping is attained if ME is varied in phase with the slip
Δf However this signal must also be advanced somewhat to compensate for the time
constants in the excitation circuit and the generator
As mentioned above the electrical torque ME can be influenced by varying the
field current To accomplish this a suitable control signal referred to as variable
disturbance compensation must be imposed upon the voltage set-point or the converter
control variable Ucontr As can be seen from the vector diagram by applying proper
weighting factors (K1 K2) and then adding together the signals -ΔPE and Δf an overall
stabilization signal can be produced that rotates in advance of the Df signal by any angle
desired between 0deg and 90deg Because the amplitude of -DPE remains proportional to the
amplitude of Δf a constant angle in advance of Δf results for the compensation of the
time constants referred to above The optimum weighting factors K1 and K2 for a
synchronous generator working to a power network depend on its operating point at any
moment and the external reactance of the network Normally the selection of a
compromise setting is good enough to attain stability in all operating points and for all
external reactance For special demands these settings must be parameterized as a
function of the external reactance (which means optional equipment Xe-Identification)
The Power System Stabilizer PSS is a section of the AVR computer program and is
processed once per network cycle The voltage at the generator terminals and the
generator current are measured in order to define the signals ΔPE and Δf The calculated
signals for _P_ (=PE) and Δf are then sent across DC filters ldquoDrdquo (real differentiators) that
transmit only the dynamic portion of the signals The ΔPE and Δf signals obtained in this
way are then weighted (multiplied by) with the factors K1 and K2 and sent to the
summing point of the voltage regulator
The PSS stabilization signal is imposed on the automatic voltage regulator only if
the following prerequisites are met
bull Generator on line
bull Generator power output gt the value F338
bull Generator voltage in a range between F33C and F33A
The stabilization signal is limited at the output from the PSS to the lower and
upper limits Flag defines whether the stabilization signal is introduced before or after the
PID filter (usually before the filter) Because the PID filter as noted above already takes
the ceiling factor Vp1 into account the PSS signal needs to be multiplied by Vp1 if it is
added to the voltage regulator following the PID filter (divider at the input to the
minmax limiter) This precaution prevents the DC filter ldquoDrdquo in the P-channel from
producing an unnecessary ldquostabilizationrdquo effect in the case of rapid changes in turbine
load As an alternative for the AVRrsquos Power System Stabilizer a stabilization signal from
an outside system can be imposed by activating the binary input ldquoPSS-SIGNEXTrdquo Flag
F340 can be used to select between an analog and a 12-bit signal and F33E to select the
polarity desired for that signal
24 The MANUAL Channel
241 Summary
The MANUAL channel (Channel 2) has been built as a simple field current
regulator
without additional limiters Its main function is to maintain the excitation of the generator
even if the AUTOMATIC channel becomes non-operational The MANUAL channel
also performs valuable service for purposes of testing commissioning and preventive
maintenance Its measurements regulator generation of firing pulses and power supply
are physically separate from those on the AUTOMATIC channel
242 Principle of Operation
All the functions of the MANUAL channel including the generation of firing
pulses have been implemented in a single electronic module the Gate Control Unit The
control variable Ucontr of voltage regulator is used as the reference value for generating
firing pulses on the principle known as ldquoramp controlrdquo (Comparison of Ucontr with
Usynsynchronous sawtooth signal) For further processing in the UN 0096 Intermediate
Pulse Stage the Gate Control Unit supplies six firing pulses at its output whose phase
position with respect to the synchronous voltage Usyn is in accordance with control
variable Ucontr An internal linearization ensures that the field voltage produced via the
firing pulses remains proportional to the control variable Ucontr throughout the entire
range As a result the circuit amplification of the control remains constant over the entire
range Whenever excitation is switched ON the set-point for Generator Voltage is set
automatically at the preset - ref Value This provision ensures that the generator voltage
always attains approximately its nominal value after the field flashing The Gate Control
Unit can be refunctioned ( by pre-selection with a switch ) for purposes of testing to act
as a purely firing pulse control In this case the control variable Ucontr is adjusted
directly using the RAISELOWER push buttons on the front of the module In this way
for example the relationship between the phase position of the firing pulses and the
control variable Ucontr can be checked easily
CHAPTER 3
PULSE SECTION
31 Pulse Generation and Amplification
The Gate Control Units of both AUTOMATIC channel and MANUAL channel
each supply six firing pulses for operating the 6-pulse thyristor bridges The low-power
pulse signals from these Gate Control Units are then amplified in the Intermediate Pulse
Stage galvanically isolated and then sent to the common pulse bus On the output end
the Intermediate Pulse Stage of the non-active channel) is always blocked The Gate
Control Units generate the pulses based on microprocessor control The reference voltage
used for the firing pulse phase location is the output voltage from the excitation
transformer (Usyn1 Usyn2) The commutation spikes of the synchronous voltage caused
by the converter are calculated prior to use of the voltage as a reference value and are
deliberately filtered out The lower limit for the firing pulses (double pulses) which are
offset from one another by 60deg is defined by the limit rectifier position (αmin) and the
upper limit by the limit inverter position (αmax) for the firing angle αmin and αmax can
be adjusted on the Gate Control Units using BCD (Binary Coded Decimal) switches
αmin ensures that the firing pulses will not be issued (premature firing) until there is
sufficient positive phase voltage on the thyristor involved αmax prevents a dangerous
ldquotippingrdquo of the thyristor bridge into the rectifier mode if the firing angle α is too large
(ldquolate firingrdquo) The critical factors determining αmax are the overlap time uumlmax (max
commutation time) and the ldquorecovery timerdquo of the thyristors (αmax lt 180deg - uumlmax - γ )
An external control signal can force the firing pulses into their inverter limit position
Other binary inputs can block or direct the firing pulses of the Gate Control Units so as to
produce freewheeling on the thyristor bridge During freewheeling the firing pulses for
the thyristor pair R and S are blocked and the pulse signals T+T- are engaged with
chains of pulses Both Gate Control Units (for the MANUAL amp AUTOMATIC channels)
contain a field current monitor that blocks the firing pulses immediately whenever the
current exceeds a preset threshold level In this case the field circuit-breaker is also
tripped via an output contact The purpose of these provisions is to prevent damage to
thyristors and thyristor fuses in case of a slip-ring short-circuit or to keep any damage
that does occur to a minimum The pulse signals are galvanically separated at the outputs
from the Intermediate Pulse Stage (with pulse transmitters) and are then directed to the
common pulse bus This transmission of the pulse signals to the pulse bus via passive
transmitters ensures a high degree of active channel autonomy Practically no possible
malfunctions on the inactive channel (including for example sustained pulses) affect the
active channel
32 Pulse Monitoring
The ldquoPulse Busrdquo and the pulse signals of the AUTOMATIC channel are
monitored This monitoring device consists of potential isolating stages and the common
monitor If the pulse monitoring of the ldquoPulse Busrdquo responds a switch-over is made to
MANUAL channel The function of the potential isolating stages is to couple the pulse
monitoring device to the pulse circuits without any feedback effect The pulse monitoring
checks the six pulse lines for the following malfunctions continuous or periodic failure
of one or more pulses Periodic occurrence of synchronous or asynchronous false pulses
Continuous pulses the pulse monitoring device can be tested while the machine is in
operation
CHAPTER 4
CONVERTER
Thyristor
The term thyristor usually refers to a family of four layer solid state device having
turn on characteristics that can be externally controlled by either current or voltage They
are also referred to as breakdown device because their working depends on avalanche
breakdown Thyristors have only two stages OFF and ON Thyristors have a similar
function to Uni-junctions they act as switches Thyristors use current flow as a switch
Thyristors have three states
1 Reverse blocking mode mdash Voltage is applied in the direction that would be
blocked by a diode
2 Forward blocking mode mdash Voltage is applied in the direction that would cause
a diode to conduct but the thyristor has not yet been triggered into conduction
3 Forward conducting mode mdash The thyristor has been triggered into conduction
and will remain conducting until the forward current drops below a threshold value
known as the holding current Converter is a semiconductor device which converts ac
input voltage into a constant dc output voltage In present excitation system three phase
fully controlled thyristor converter is used
Because of the following advantages thyristor converters are used
a) Thyristors are used for high power applications ie up to 10Kv3500A1KHz
b) Having high reliability and low losses
c) Uni-directional device like diode
d) Itrsquos operation as a rectifier which are low resistance in forward conduction
mode and high resistance in reverse conduction mode
PROTECTION OF THYRISTORS
For reliable operation of a thyristor demands that its specified ratings are not
exceeded When Subjected to or over voltages During the turn - on of SCR didt
prohibitively large False triggering of SCR by high value of dvdt andSpurious signals
between gate and cathode may leads to unwanted turn ndash on
DIDT AND PROTECTION
When thyristor starts conducting in forward conduction mode and is turned on by
gate pulse The anode current increases rapidly whole area of the gate to Cathode
junction then hot spots will be formed near the gate connection this locality of heating
destroys the thyristor Thyristor thermal time is constant The causes due to faults and
short circuits or surge currents Electronic crowbar protection is used against the over
voltages The rate rise of anode current must be kept at the time of turn on below the
rated or specified limiting value The didt value maintained below limited value by using
a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating
is avoided by applying gate current but not greater the maximum gate current
DVDT AND OVER VOLTAGE PROTECTION
With forward voltage across the anode and cathode of a thyristor the two outer
junctions are forward biased but the inner junction is reverse biased This reverse biased
junction J2 has the characteristics of a capacitor due to charges existing across the
junction In other words space-charges exist in the depletion region around junction J2
and therefore junction J2 behaves like a capacitance If the entire anode to cathode
forward voltage Va appears across J2 junction and the charge is denoted by Q then a
charging current i given by Eq (46) follows
i = dQdt =d(Cj Va )dt
= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)
As Cj the capacitance of junction J2 is almost constant the current is given by
i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)
If the rate of rise of forward voltage dVadt is high the charging current i will be
more This charging current plays the role of gate current and turns on the SCR even
when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on
must be avoided as it leads to false operation of the thyristor circuit
For controllable operation of the thyristor the rate of rise of forward anode to
cathode voltage dVadt must be kept below the specified rated limit Typical values of
dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by
using a snubber circuit in parallel with the device thyristor are very sensitive for over
voltage than the semiconductor devices
Over voltage transients are perhaps the main cause of thyristor failure
In thyristor there are mainly two types
1 Internal over voltages
Due to the commutation of the thyristors large voltages are generated internally
Because of the series inductance of the SCR circuit the large transient voltages L didt
produced This voltage several times the break over voltage of the device then thyristor
destroys permanently
2 External over voltages
External over voltages are caused due to the interruptions of current flow in an
inductive circuit and also due to the lightening strokes on the lines feeding the thyristor
system For the reliable operation of thyristor the over voltages must be suppressed by
adopting suitable techniques
Suppression of over voltages
The RC circuit called snubber circuit is connected across the device to protect In
order to keep the protective components to a minimum the thyristors are chosen with
their peak voltages ratings are 25 to 3 times of the normal peak working voltage
ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are
commonly employed for protecting the thyristor circuit against the over voltages
Gate protection
Gate circuit should also be protected against the over voltages and surges Over
voltage at gate circuit can cause false triggering of the SCR may rises the junction
temperature behind specified limit leading to its damage Protection against over
voltage can be achieved by connecting a ZD across the gate circuit and a resister is
connected in series with gate circuit to protect against the s A capacitor and resister are
connected across gate to cathode to by pass the noise
41 Final Pulse Stages
The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage
(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor
bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided
with a power supply module The amplified output pulses from the Final Pulse Stages
start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main
pulse This initial pulse edge assures proper firing of the thyristors being triggered
Subsequently the weaker part of main pulse keeps firing conditions steady As already
mentioned the Final Pulse Stages and their associated thyristor bridges form single units
All six pulse outputs from a Final Pulse Stage can be blocked by an external control
signal so that all thyristors in the associated thyristor bridge will block the current A
blocking of the pulses is initiated whenever there is a malfunction in the associated
thyristor bridge
42 Converter Power Section
The thyristor converter consists of three independent parallel rectifier blocks TY1
to TY3 which are all in service Even if one block fails the remaining blocks take over
automatically the full design current of the excitation circuit During normal operation
(with ideal current share) and all three bridges in operation each of these blocks has to
carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation
is limited Only when all three bridges fail the excitation is switched off Each thyristor
bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any
arm is identified by a Current flow monitoring module
43 Converter Cooling
A cooling system is needed to dissipate heat losses in the converter blocks and
electronics Each converter block has therefore been equipped with a fan supplied with
power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing
cubicle) The fans are protected with motor protection circuit breakers An air flow
monitoring unit is provided for monitoring the air flow through the thyristor bridge If a
circuit breaker failure is detected or if the air flow monitor drops off at one of the
thyristor bridges the bridge involved is immediately set out of operation by blocking its
firing pulses
44 Thyristor Converter Monitoring
A thyristor bridge in which defects occur that could threaten the safety of
operation or cause secondary damage is switched off automatically ie its firing pulses
are blocked This happens whenever A thyristor fuse is blown The fuses are monitored
individually with micro switches The Final Pulse Stage fails which is detected by
internal monitors (supply voltage sustained pulse short-circuit on the output end) The
power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or
is insufficient Isolator on ACDC side is open
CHAPTER 5
Field Current Circuit Excitation Transformer
51 Field Circuit Breaker
The circuit-breaker in the field circuit is used to isolate the field circuit from the
converter It is capable of switching off the synchronous machine from full load under the
maximum conditions of a 3-phase short-circuit In addition to its main contacts the field
circuit-breaker also has a de-excitation contact with which the field energy stored in the
field can be dissipated across the de-excitation resistor The de-excitation contact closes
shortly before the main contacts open so as to ensure proper commutation of the field
current from the main contacts to the de-excitation contact when the breaker is switched
off The field circuit-breaker is switched on by electromagnetic force and is kept switched
on by a mechanical latch When the latch is released by a trip coil the circuit-breaker
opens The circuit-breaker also has auxiliary contacts that report its status
52 Field Flashing
In shunt supplied excitation circuits (excitation transformer connected to the
generator terminals) the generator does not have enough remnant voltage for a generator
voltage build-up via the converter In this case a field flashing circuit is provided It
consists of the field flashing contactor the diode bridge and a transformer used to adapt
the auxiliary input voltage to the voltage needed for field flashing when power is
supplied from the auxiliaries network
Fig Field Flashing
Because the field flashing contactor is not able to switch off the energy stored in
the field the control ensures that the contactor can only reopen if the field circuit breaker
has already been opened (generating the TRIP order) or in a normal field flashing
sequence when the converter has taken over the field current Field flashing occurs in the
following stages
1048729The excitation is switched on closing the field flashing contactor ( Field
Circuit Breaker is already closed )
1048729The start-up excitation current flows through the rotor driving the generator
voltage up to approx 15 U
1048729After about 10 U the firing pulses to the converter are released and it begins
to excite the generator to its rated voltage
1048729After about 30 U the field flashing contactor opens (with no current since
the converter is now supplying the current)
The diode bridge at the input to the field flashing contactor prevents a feed-back
from the converter to the source of field flashing while the contactor is still closed
53 De-excitation
When malfunctions occur the stored field energy must be dissipated as quickly
and safely as possible to protect the generator This is done by the converter the field
circuit-breaker and the de-excitation (discharge) resistor
De-excitation (with opening of the field circuit-breaker) takes place in the following
stages
1048729The converter drives to its inverter limit position (negative ceiling voltage)
recovers a portion of the field energy into the network A trip command is given to the
field circuit breaker
1048729The de-excitation contact closes diverting the field voltage to the de-excitation
resistor
1048729Then immediately the main contacts open building voltage The field voltage
commutates to the de-excitation resistor
1048729The current diminishes at a given time constant TE
(With linear resistance TE = Lf (Rf + Re))
Due to the reversal of the field voltage by the converter the field current
commutates from the main contacts of the field circuit-breaker to the de-excitation
resistor in a very early phase This reversal of the field voltage prevents burn-off on the
main contacts and provides effective protection for the field circuit-breaker Depending
on the operating policy an operational shut-down of the excitation can also be effected
with the field circuit-breaker closed This method is useful mainly when the excitation is
switched on and off frequently In this case the converter is merely driven into the
inverter limit position so that the field energy is recovered into the network The
converter then blocks since it is supplying positive current only
54 Excitation Transformer
The excitation transformer matches the generator voltage to the field voltage
(required ceiling voltage) It also serves as a commutation reactance for the thyristor
converter and as a potential isolator between the network and the excitation circuit In
addition the transformer functions as a current limiter in that it makes it possible to keep
any short circuits in the excitation circuit under better control The excitation transformer
is equipped with temperature monitoring probes which set off an alarm when the
temperature exceeds a first max limit and then trips the excitation if the temperature
continues rising to a second (higher)limit
CHAPTER 6
Monitoring and Protection
61 Excitation Monitoring
611 General Information
The main goal of Excitation Monitoring is to make optimum use of the
redundancies provided in the excitation system and to give alarm whenever a malfunction
makes these redundancies unavailable The field current is monitored to see that it does
not exceed a maximum level and if necessary a switch-over to the MANUAL channel is
initiated In addition the criterion for switching off the field flashing is generated The
excitation Monitoring consists of an autonomous processor system
612 over current Alarms
In the Excitation Monitoring the limits for are set at higher levels than the
settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of
the nominal field current contact R1 and the binary output associated with it are
activated immediately If field current remains gt 110 then after a preset inverse-time
has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs
associated with them are activated Parameters match the measurements for If1 and If2 to
the nominal value for field current so that the internal values can be processed and read as
pu values It can be used to falsify the actual value of the field current If (to raise it) so
as to cause a response from the alarm limits for purposes of testing The processed If
signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As
long as the field current If is above the threshold value 11 Ifn its peak value is
measured This is stored (until RESET) and can be read at any time on the Micro-
Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever
the integrated time-current value (intisup2dt) exceeds the preselected reference value the
alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously
issued to switch over to the stand by AUTO channel Software switch F758 enables the
three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-
time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the
desired limit curve for response is set using the factor F216
613 Switch-Off Criterion for Field Flashing
The Excitation Monitoring supplies the criterion for switching off the field
flashing Whether this criterion is activated based on the actual value for generator voltage
Ug or for field current If or both depends on the settings of the two threshold values
F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements
Ug12 and If12 are switched over depending on the present status of the channels
(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is
fixed at ldquological 1rdquo
614 Storage of Alarm Status
The outputs of the over current alarms (R1 R2 R0) and the messages NO
FAILURE MONITORING PARAMETERS CHANGED are stored messages can be
erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the
front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the
situation causing the alarm or the malfunction is no longer present Whenever the self-
diagnosis equipment in the processor detects a malfunction the output NO FAILURE of
MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo
is activated whenever parameters or settings of software switches have been changed via
the Micro-Terminal
615 Actual Value Monitoring
The actual values for generator voltage Ug synchronous voltage Usyn and field
current If are monitored for malfunctions This monitoring is active regardless of whether
or not the generator is in operation Essentially when the generator is in operation the
measurements are monitored by comparing the signals (the smaller signal reading is
detected as incorrect) When the generator is not in operation the measured data are
monitored for extreme values The percentage of deviation permissible in the
measurement signals being compared
(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and
F20AIf the excitation transformer is being supplied from an auxiliary power source (no
shunt operation) the values of Ug and Usyn will be different in some operational
conditions
In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1
and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the
messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2
and the binary message from CH1 reports no malfunction a malfunction on
Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is
also present whenever the binary message CH1 DISTURBANCE is reported and a
discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2
FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares
Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the
comparison channel reports a malfunction or whenever both binary messages report no
malfunction - but both secondary monitors report a malfunction As long as the secondary
monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction
signals for the measurement channel involved (suspicion that there is a corresponding
error in Usyn) The measurement channel malfunctions are enabled operationally
whenever after excitation has been switched on generator voltage Ug exceeds the value
set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is
switched off to see that they do not exceed the limit value F210 that applies to both of
them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt
the operating range) Monitoring for extreme values is likewise enabled during normal
operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the
malfunction signals to the binary outputs Basically the monitoring of the actual values
for If1If2 functions like that of the Ug1Ug2 monitoring
62 Excitation Protection
621 General Information
The Excitation Protection switches off the excitation (and de-excites the machine
rapidly) whenever a danger arises that threatens the excitation transformer the converter
or the generator Generally limiter or monitoring functions precede the emergency trips
and these normally respond before the Excitation Protection must initiate a trip
Protective trip commands are issued directly to the field circuit-breaker from potential
free contacts of the board via the trip relays They are directed redundantly to the
operative field circuit-breaker ldquoOFFrdquo command
622 Protection against Excitation Transformer Overheating
This equipment monitors the excitation transformer for overheating in the
windings that could result from over current short-circuits or inadequate cooling The
monitoring uses temperature monitoring modules in conjunction with temperature
sensors built into the windings Normally the temperature is monitored in two stages the
first stage sets off an alarm the second causes a trip of the excitation
623 Rotor Over voltage Protection
Malfunctions in the generator circuit (eg terminal short-circuit failed
synchronization asynchronous operation) cause induced negative field currents that
produce high voltages in the field circuit These must be restricted to a level with a
sufficient safety margin below the insulation capacity of the field winding (test voltage)
and also below the peak blocking voltage of the converter thyristors The crow bar
employs spark gap elements to detect over voltages in the field circuit Whenever they
respond the associated thyristors are fired immediately switching the de-excitation
resistor parallel to the field The de-excitation current generated thereby initiates an
excitation trip via a supervision circuit causing an immediate opening of the field circuit-
breaker The malfunction isets off an alarm and an internal malfunction is indicated at the
cubicle
TEST VALUES OBTAINED WHEN EXCITATION IS RAISED
TEST
SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE
1 VREF 996 100
2 VACT 997 1003
3 IFACT 735 765
4 IGACT 703 707
5 ACTIVE
POWER
703 705
6 REACTIVE
POWER
101 142
7 POWER
FACTOR
099 IND 098 IND
8 ACTIVE
CURRENT(IR)
705 703
9 REACTIVE
CURRENT(IX)
102 138
10 POWER
ANGLE
566 546
11 FIRING
ANGLE
640 632
VALUES OBTAINED WHEN EXCITATION IS LOWERED
SNO PARTICULARS ACTUAL
VALUE
OBTAINED VALUE
1 VREF 100 997
2 VACT 100 997
3 IFACT 787 761
4 IGACT 839 837
5 ACTIVE
POWER
839 830
6 REACTIVE
POWER
155 110
7 POWER
FACTOR
098 IND 099 IND
8 ACTIVE
CURRENT(IR)
835 830
9 REACTIVE
CURRENT(IX)
140 94
10 POWER
ANGLE
613 DEG 631 DEG
11 FIRING
ANGLE
601 DEG 599 DEG
CONCLUSION
For generating the EMF in stator winding excitation is required to the rotor of a
generator There are two types of excitation
1 Static excitation system
2 Brushless excitation system
A certain disadvantage in brushless excitation system is the slow response time of
the field in case of fast load changes specified No slip-rings and brushes direct
measurements of the field parameters not possible
To avoid all loses static excitation is used Since it does not have any rotating
parts mechanical loses and windage loses This system has fast response and speed
control While preferring this excitation system there are no limitations for the
redundancy of Thyristor bridge circuits
Static excitation has fast field discharge by resistor and inverter operation direct
measurement of field quantity is possible The meaning of excitation is nothing but
continuous supply of DC current (ie field current) to the rotor to buildup required
output voltage in the stator
Field current is changed with respect to the change of load so the digital
automatic voltage regulator (DAVR) is used to regulate the output voltage according to
the load variations
So we conclude that static excitation system with DAVR is preferred since it is
having excellent dynamic performance and better options for R amp M
loading of the generator is low In this case if the stator current limiter is not kept from
influencing the field current the control circuit will oscillate back and forth between the
Inductive Stator Current Limiter (de-
exciting) and the Capacitive Stator Current Limiter (exciting)The output signal of that
function then dominates the control variable of the Ig-dependent limiter via a maximum
value selection
238 Capacitive Stator Current Limiter
239 Load Angle Limiter
The Load Angle Limiter prevents the synchronous machine from slipping out of
phase due to slippage of the rotor The load angle δ the difference in phase between the
rotor and the stator rotating field results mainly from the driving torque (active power P)
acting on the generator and the level of rotor current (field current) If the driving torque
remains constant a increase in the field current reduces the load angle δ The current load
angle δ at any moment is obtained from the generator current and generator voltage based
on a simplified model of the generator Whenever this calculated load angle δ exceeds the
preset limit angle the limiter increases the field current until the load angle has dropped
back to its permissible value The quadrature reactance Xq of the generator and the
network reactance Xe during normal operation must be adjusted on the regulator in order
to obtain the load angle δ The graph below shows the Power Chart for a salient-pole
machine with typical limiter characteristics
The purpose of a Power System Stabilizer is to use the generator excitation to
damp electromechanical oscillations between the network and the generator Depending
on the design of the generator and the requirements imposed for network stability its
main function will be either to damp the oscillations originating in the machine or those
from the network A synchronous generator working in a combined power network is in
principle an oscillating structure In order to produce a torque the magnetic field of the
rotor and the stator must form a given angle (referred to as the rotor displacement or load
angle δ) The electrical torque ME increases as the angle δ increases just as with a
torsion spring Because the ME of the generator and the mechanical driving torque MA
from the turbine are in equilibrium during steady-state operation the angle δ remains in a
given position Whenever this state of equilibrium between MA and ME is disturbed the
load angle slips of this rest position and change thereby the electrical torque ME The
torque attempts to restore the load angle to a stationary position Due to the mass inertia
of the turbinegenerator rotor however this can only take place aperiodically It does so
in the form of more or less effectively damped oscillations (again similar to the effect of
mass inertia on a torsion spring) In order to damp the oscillations there must be a
damping torque produced depending not on the electrical torque ME associated with the
angle but on the difference in frequency (Df) between the rotor and the stator rotating
field ie on the slippage This torque is produced mainly by the so-called damper
winding in the rotor but the dimensioning of this is subject to limits imposed by
considerations of design and economy Some further action is therefore needed to
increase the damping effect The following drastically simplified formula shows the
parameters upon which the amount of active power PE supplied by the generator
depends
PE = active power
It can be seen from the above relationship that the active power that the generator
transfers depends not only on the load angle δ but also on the field current If That means
that a transient change can be made in the active power PE and with that in the effective
electrical torque ME by varying the field current The principle of operation of the DVR
Power System Stabilizer becomes clear from a consideration of the oscillations in power
output and frequency (ΔPE Δf) and the vector diagram If it is assumed that oscillations
in the network frequency generates load oscillations with the mass inertia of the rotor
then the active load of the generator (eg MW-measured) is influenced with a sinusoidal
value -ΔPE (ME-MA = -ΔPE) By inversion of -ΔPE one obtains the fluctuation in
power provided by the rotor +ΔPE As is known the slip signal Δf follows +ΔPE with a
phase delayed by 90deg The +ME produced by the periodic changes in the load angle δ is
in phase with +ΔPE A good damping is attained if ME is varied in phase with the slip
Δf However this signal must also be advanced somewhat to compensate for the time
constants in the excitation circuit and the generator
As mentioned above the electrical torque ME can be influenced by varying the
field current To accomplish this a suitable control signal referred to as variable
disturbance compensation must be imposed upon the voltage set-point or the converter
control variable Ucontr As can be seen from the vector diagram by applying proper
weighting factors (K1 K2) and then adding together the signals -ΔPE and Δf an overall
stabilization signal can be produced that rotates in advance of the Df signal by any angle
desired between 0deg and 90deg Because the amplitude of -DPE remains proportional to the
amplitude of Δf a constant angle in advance of Δf results for the compensation of the
time constants referred to above The optimum weighting factors K1 and K2 for a
synchronous generator working to a power network depend on its operating point at any
moment and the external reactance of the network Normally the selection of a
compromise setting is good enough to attain stability in all operating points and for all
external reactance For special demands these settings must be parameterized as a
function of the external reactance (which means optional equipment Xe-Identification)
The Power System Stabilizer PSS is a section of the AVR computer program and is
processed once per network cycle The voltage at the generator terminals and the
generator current are measured in order to define the signals ΔPE and Δf The calculated
signals for _P_ (=PE) and Δf are then sent across DC filters ldquoDrdquo (real differentiators) that
transmit only the dynamic portion of the signals The ΔPE and Δf signals obtained in this
way are then weighted (multiplied by) with the factors K1 and K2 and sent to the
summing point of the voltage regulator
The PSS stabilization signal is imposed on the automatic voltage regulator only if
the following prerequisites are met
bull Generator on line
bull Generator power output gt the value F338
bull Generator voltage in a range between F33C and F33A
The stabilization signal is limited at the output from the PSS to the lower and
upper limits Flag defines whether the stabilization signal is introduced before or after the
PID filter (usually before the filter) Because the PID filter as noted above already takes
the ceiling factor Vp1 into account the PSS signal needs to be multiplied by Vp1 if it is
added to the voltage regulator following the PID filter (divider at the input to the
minmax limiter) This precaution prevents the DC filter ldquoDrdquo in the P-channel from
producing an unnecessary ldquostabilizationrdquo effect in the case of rapid changes in turbine
load As an alternative for the AVRrsquos Power System Stabilizer a stabilization signal from
an outside system can be imposed by activating the binary input ldquoPSS-SIGNEXTrdquo Flag
F340 can be used to select between an analog and a 12-bit signal and F33E to select the
polarity desired for that signal
24 The MANUAL Channel
241 Summary
The MANUAL channel (Channel 2) has been built as a simple field current
regulator
without additional limiters Its main function is to maintain the excitation of the generator
even if the AUTOMATIC channel becomes non-operational The MANUAL channel
also performs valuable service for purposes of testing commissioning and preventive
maintenance Its measurements regulator generation of firing pulses and power supply
are physically separate from those on the AUTOMATIC channel
242 Principle of Operation
All the functions of the MANUAL channel including the generation of firing
pulses have been implemented in a single electronic module the Gate Control Unit The
control variable Ucontr of voltage regulator is used as the reference value for generating
firing pulses on the principle known as ldquoramp controlrdquo (Comparison of Ucontr with
Usynsynchronous sawtooth signal) For further processing in the UN 0096 Intermediate
Pulse Stage the Gate Control Unit supplies six firing pulses at its output whose phase
position with respect to the synchronous voltage Usyn is in accordance with control
variable Ucontr An internal linearization ensures that the field voltage produced via the
firing pulses remains proportional to the control variable Ucontr throughout the entire
range As a result the circuit amplification of the control remains constant over the entire
range Whenever excitation is switched ON the set-point for Generator Voltage is set
automatically at the preset - ref Value This provision ensures that the generator voltage
always attains approximately its nominal value after the field flashing The Gate Control
Unit can be refunctioned ( by pre-selection with a switch ) for purposes of testing to act
as a purely firing pulse control In this case the control variable Ucontr is adjusted
directly using the RAISELOWER push buttons on the front of the module In this way
for example the relationship between the phase position of the firing pulses and the
control variable Ucontr can be checked easily
CHAPTER 3
PULSE SECTION
31 Pulse Generation and Amplification
The Gate Control Units of both AUTOMATIC channel and MANUAL channel
each supply six firing pulses for operating the 6-pulse thyristor bridges The low-power
pulse signals from these Gate Control Units are then amplified in the Intermediate Pulse
Stage galvanically isolated and then sent to the common pulse bus On the output end
the Intermediate Pulse Stage of the non-active channel) is always blocked The Gate
Control Units generate the pulses based on microprocessor control The reference voltage
used for the firing pulse phase location is the output voltage from the excitation
transformer (Usyn1 Usyn2) The commutation spikes of the synchronous voltage caused
by the converter are calculated prior to use of the voltage as a reference value and are
deliberately filtered out The lower limit for the firing pulses (double pulses) which are
offset from one another by 60deg is defined by the limit rectifier position (αmin) and the
upper limit by the limit inverter position (αmax) for the firing angle αmin and αmax can
be adjusted on the Gate Control Units using BCD (Binary Coded Decimal) switches
αmin ensures that the firing pulses will not be issued (premature firing) until there is
sufficient positive phase voltage on the thyristor involved αmax prevents a dangerous
ldquotippingrdquo of the thyristor bridge into the rectifier mode if the firing angle α is too large
(ldquolate firingrdquo) The critical factors determining αmax are the overlap time uumlmax (max
commutation time) and the ldquorecovery timerdquo of the thyristors (αmax lt 180deg - uumlmax - γ )
An external control signal can force the firing pulses into their inverter limit position
Other binary inputs can block or direct the firing pulses of the Gate Control Units so as to
produce freewheeling on the thyristor bridge During freewheeling the firing pulses for
the thyristor pair R and S are blocked and the pulse signals T+T- are engaged with
chains of pulses Both Gate Control Units (for the MANUAL amp AUTOMATIC channels)
contain a field current monitor that blocks the firing pulses immediately whenever the
current exceeds a preset threshold level In this case the field circuit-breaker is also
tripped via an output contact The purpose of these provisions is to prevent damage to
thyristors and thyristor fuses in case of a slip-ring short-circuit or to keep any damage
that does occur to a minimum The pulse signals are galvanically separated at the outputs
from the Intermediate Pulse Stage (with pulse transmitters) and are then directed to the
common pulse bus This transmission of the pulse signals to the pulse bus via passive
transmitters ensures a high degree of active channel autonomy Practically no possible
malfunctions on the inactive channel (including for example sustained pulses) affect the
active channel
32 Pulse Monitoring
The ldquoPulse Busrdquo and the pulse signals of the AUTOMATIC channel are
monitored This monitoring device consists of potential isolating stages and the common
monitor If the pulse monitoring of the ldquoPulse Busrdquo responds a switch-over is made to
MANUAL channel The function of the potential isolating stages is to couple the pulse
monitoring device to the pulse circuits without any feedback effect The pulse monitoring
checks the six pulse lines for the following malfunctions continuous or periodic failure
of one or more pulses Periodic occurrence of synchronous or asynchronous false pulses
Continuous pulses the pulse monitoring device can be tested while the machine is in
operation
CHAPTER 4
CONVERTER
Thyristor
The term thyristor usually refers to a family of four layer solid state device having
turn on characteristics that can be externally controlled by either current or voltage They
are also referred to as breakdown device because their working depends on avalanche
breakdown Thyristors have only two stages OFF and ON Thyristors have a similar
function to Uni-junctions they act as switches Thyristors use current flow as a switch
Thyristors have three states
1 Reverse blocking mode mdash Voltage is applied in the direction that would be
blocked by a diode
2 Forward blocking mode mdash Voltage is applied in the direction that would cause
a diode to conduct but the thyristor has not yet been triggered into conduction
3 Forward conducting mode mdash The thyristor has been triggered into conduction
and will remain conducting until the forward current drops below a threshold value
known as the holding current Converter is a semiconductor device which converts ac
input voltage into a constant dc output voltage In present excitation system three phase
fully controlled thyristor converter is used
Because of the following advantages thyristor converters are used
a) Thyristors are used for high power applications ie up to 10Kv3500A1KHz
b) Having high reliability and low losses
c) Uni-directional device like diode
d) Itrsquos operation as a rectifier which are low resistance in forward conduction
mode and high resistance in reverse conduction mode
PROTECTION OF THYRISTORS
For reliable operation of a thyristor demands that its specified ratings are not
exceeded When Subjected to or over voltages During the turn - on of SCR didt
prohibitively large False triggering of SCR by high value of dvdt andSpurious signals
between gate and cathode may leads to unwanted turn ndash on
DIDT AND PROTECTION
When thyristor starts conducting in forward conduction mode and is turned on by
gate pulse The anode current increases rapidly whole area of the gate to Cathode
junction then hot spots will be formed near the gate connection this locality of heating
destroys the thyristor Thyristor thermal time is constant The causes due to faults and
short circuits or surge currents Electronic crowbar protection is used against the over
voltages The rate rise of anode current must be kept at the time of turn on below the
rated or specified limiting value The didt value maintained below limited value by using
a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating
is avoided by applying gate current but not greater the maximum gate current
DVDT AND OVER VOLTAGE PROTECTION
With forward voltage across the anode and cathode of a thyristor the two outer
junctions are forward biased but the inner junction is reverse biased This reverse biased
junction J2 has the characteristics of a capacitor due to charges existing across the
junction In other words space-charges exist in the depletion region around junction J2
and therefore junction J2 behaves like a capacitance If the entire anode to cathode
forward voltage Va appears across J2 junction and the charge is denoted by Q then a
charging current i given by Eq (46) follows
i = dQdt =d(Cj Va )dt
= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)
As Cj the capacitance of junction J2 is almost constant the current is given by
i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)
If the rate of rise of forward voltage dVadt is high the charging current i will be
more This charging current plays the role of gate current and turns on the SCR even
when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on
must be avoided as it leads to false operation of the thyristor circuit
For controllable operation of the thyristor the rate of rise of forward anode to
cathode voltage dVadt must be kept below the specified rated limit Typical values of
dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by
using a snubber circuit in parallel with the device thyristor are very sensitive for over
voltage than the semiconductor devices
Over voltage transients are perhaps the main cause of thyristor failure
In thyristor there are mainly two types
1 Internal over voltages
Due to the commutation of the thyristors large voltages are generated internally
Because of the series inductance of the SCR circuit the large transient voltages L didt
produced This voltage several times the break over voltage of the device then thyristor
destroys permanently
2 External over voltages
External over voltages are caused due to the interruptions of current flow in an
inductive circuit and also due to the lightening strokes on the lines feeding the thyristor
system For the reliable operation of thyristor the over voltages must be suppressed by
adopting suitable techniques
Suppression of over voltages
The RC circuit called snubber circuit is connected across the device to protect In
order to keep the protective components to a minimum the thyristors are chosen with
their peak voltages ratings are 25 to 3 times of the normal peak working voltage
ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are
commonly employed for protecting the thyristor circuit against the over voltages
Gate protection
Gate circuit should also be protected against the over voltages and surges Over
voltage at gate circuit can cause false triggering of the SCR may rises the junction
temperature behind specified limit leading to its damage Protection against over
voltage can be achieved by connecting a ZD across the gate circuit and a resister is
connected in series with gate circuit to protect against the s A capacitor and resister are
connected across gate to cathode to by pass the noise
41 Final Pulse Stages
The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage
(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor
bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided
with a power supply module The amplified output pulses from the Final Pulse Stages
start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main
pulse This initial pulse edge assures proper firing of the thyristors being triggered
Subsequently the weaker part of main pulse keeps firing conditions steady As already
mentioned the Final Pulse Stages and their associated thyristor bridges form single units
All six pulse outputs from a Final Pulse Stage can be blocked by an external control
signal so that all thyristors in the associated thyristor bridge will block the current A
blocking of the pulses is initiated whenever there is a malfunction in the associated
thyristor bridge
42 Converter Power Section
The thyristor converter consists of three independent parallel rectifier blocks TY1
to TY3 which are all in service Even if one block fails the remaining blocks take over
automatically the full design current of the excitation circuit During normal operation
(with ideal current share) and all three bridges in operation each of these blocks has to
carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation
is limited Only when all three bridges fail the excitation is switched off Each thyristor
bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any
arm is identified by a Current flow monitoring module
43 Converter Cooling
A cooling system is needed to dissipate heat losses in the converter blocks and
electronics Each converter block has therefore been equipped with a fan supplied with
power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing
cubicle) The fans are protected with motor protection circuit breakers An air flow
monitoring unit is provided for monitoring the air flow through the thyristor bridge If a
circuit breaker failure is detected or if the air flow monitor drops off at one of the
thyristor bridges the bridge involved is immediately set out of operation by blocking its
firing pulses
44 Thyristor Converter Monitoring
A thyristor bridge in which defects occur that could threaten the safety of
operation or cause secondary damage is switched off automatically ie its firing pulses
are blocked This happens whenever A thyristor fuse is blown The fuses are monitored
individually with micro switches The Final Pulse Stage fails which is detected by
internal monitors (supply voltage sustained pulse short-circuit on the output end) The
power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or
is insufficient Isolator on ACDC side is open
CHAPTER 5
Field Current Circuit Excitation Transformer
51 Field Circuit Breaker
The circuit-breaker in the field circuit is used to isolate the field circuit from the
converter It is capable of switching off the synchronous machine from full load under the
maximum conditions of a 3-phase short-circuit In addition to its main contacts the field
circuit-breaker also has a de-excitation contact with which the field energy stored in the
field can be dissipated across the de-excitation resistor The de-excitation contact closes
shortly before the main contacts open so as to ensure proper commutation of the field
current from the main contacts to the de-excitation contact when the breaker is switched
off The field circuit-breaker is switched on by electromagnetic force and is kept switched
on by a mechanical latch When the latch is released by a trip coil the circuit-breaker
opens The circuit-breaker also has auxiliary contacts that report its status
52 Field Flashing
In shunt supplied excitation circuits (excitation transformer connected to the
generator terminals) the generator does not have enough remnant voltage for a generator
voltage build-up via the converter In this case a field flashing circuit is provided It
consists of the field flashing contactor the diode bridge and a transformer used to adapt
the auxiliary input voltage to the voltage needed for field flashing when power is
supplied from the auxiliaries network
Fig Field Flashing
Because the field flashing contactor is not able to switch off the energy stored in
the field the control ensures that the contactor can only reopen if the field circuit breaker
has already been opened (generating the TRIP order) or in a normal field flashing
sequence when the converter has taken over the field current Field flashing occurs in the
following stages
1048729The excitation is switched on closing the field flashing contactor ( Field
Circuit Breaker is already closed )
1048729The start-up excitation current flows through the rotor driving the generator
voltage up to approx 15 U
1048729After about 10 U the firing pulses to the converter are released and it begins
to excite the generator to its rated voltage
1048729After about 30 U the field flashing contactor opens (with no current since
the converter is now supplying the current)
The diode bridge at the input to the field flashing contactor prevents a feed-back
from the converter to the source of field flashing while the contactor is still closed
53 De-excitation
When malfunctions occur the stored field energy must be dissipated as quickly
and safely as possible to protect the generator This is done by the converter the field
circuit-breaker and the de-excitation (discharge) resistor
De-excitation (with opening of the field circuit-breaker) takes place in the following
stages
1048729The converter drives to its inverter limit position (negative ceiling voltage)
recovers a portion of the field energy into the network A trip command is given to the
field circuit breaker
1048729The de-excitation contact closes diverting the field voltage to the de-excitation
resistor
1048729Then immediately the main contacts open building voltage The field voltage
commutates to the de-excitation resistor
1048729The current diminishes at a given time constant TE
(With linear resistance TE = Lf (Rf + Re))
Due to the reversal of the field voltage by the converter the field current
commutates from the main contacts of the field circuit-breaker to the de-excitation
resistor in a very early phase This reversal of the field voltage prevents burn-off on the
main contacts and provides effective protection for the field circuit-breaker Depending
on the operating policy an operational shut-down of the excitation can also be effected
with the field circuit-breaker closed This method is useful mainly when the excitation is
switched on and off frequently In this case the converter is merely driven into the
inverter limit position so that the field energy is recovered into the network The
converter then blocks since it is supplying positive current only
54 Excitation Transformer
The excitation transformer matches the generator voltage to the field voltage
(required ceiling voltage) It also serves as a commutation reactance for the thyristor
converter and as a potential isolator between the network and the excitation circuit In
addition the transformer functions as a current limiter in that it makes it possible to keep
any short circuits in the excitation circuit under better control The excitation transformer
is equipped with temperature monitoring probes which set off an alarm when the
temperature exceeds a first max limit and then trips the excitation if the temperature
continues rising to a second (higher)limit
CHAPTER 6
Monitoring and Protection
61 Excitation Monitoring
611 General Information
The main goal of Excitation Monitoring is to make optimum use of the
redundancies provided in the excitation system and to give alarm whenever a malfunction
makes these redundancies unavailable The field current is monitored to see that it does
not exceed a maximum level and if necessary a switch-over to the MANUAL channel is
initiated In addition the criterion for switching off the field flashing is generated The
excitation Monitoring consists of an autonomous processor system
612 over current Alarms
In the Excitation Monitoring the limits for are set at higher levels than the
settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of
the nominal field current contact R1 and the binary output associated with it are
activated immediately If field current remains gt 110 then after a preset inverse-time
has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs
associated with them are activated Parameters match the measurements for If1 and If2 to
the nominal value for field current so that the internal values can be processed and read as
pu values It can be used to falsify the actual value of the field current If (to raise it) so
as to cause a response from the alarm limits for purposes of testing The processed If
signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As
long as the field current If is above the threshold value 11 Ifn its peak value is
measured This is stored (until RESET) and can be read at any time on the Micro-
Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever
the integrated time-current value (intisup2dt) exceeds the preselected reference value the
alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously
issued to switch over to the stand by AUTO channel Software switch F758 enables the
three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-
time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the
desired limit curve for response is set using the factor F216
613 Switch-Off Criterion for Field Flashing
The Excitation Monitoring supplies the criterion for switching off the field
flashing Whether this criterion is activated based on the actual value for generator voltage
Ug or for field current If or both depends on the settings of the two threshold values
F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements
Ug12 and If12 are switched over depending on the present status of the channels
(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is
fixed at ldquological 1rdquo
614 Storage of Alarm Status
The outputs of the over current alarms (R1 R2 R0) and the messages NO
FAILURE MONITORING PARAMETERS CHANGED are stored messages can be
erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the
front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the
situation causing the alarm or the malfunction is no longer present Whenever the self-
diagnosis equipment in the processor detects a malfunction the output NO FAILURE of
MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo
is activated whenever parameters or settings of software switches have been changed via
the Micro-Terminal
615 Actual Value Monitoring
The actual values for generator voltage Ug synchronous voltage Usyn and field
current If are monitored for malfunctions This monitoring is active regardless of whether
or not the generator is in operation Essentially when the generator is in operation the
measurements are monitored by comparing the signals (the smaller signal reading is
detected as incorrect) When the generator is not in operation the measured data are
monitored for extreme values The percentage of deviation permissible in the
measurement signals being compared
(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and
F20AIf the excitation transformer is being supplied from an auxiliary power source (no
shunt operation) the values of Ug and Usyn will be different in some operational
conditions
In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1
and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the
messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2
and the binary message from CH1 reports no malfunction a malfunction on
Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is
also present whenever the binary message CH1 DISTURBANCE is reported and a
discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2
FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares
Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the
comparison channel reports a malfunction or whenever both binary messages report no
malfunction - but both secondary monitors report a malfunction As long as the secondary
monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction
signals for the measurement channel involved (suspicion that there is a corresponding
error in Usyn) The measurement channel malfunctions are enabled operationally
whenever after excitation has been switched on generator voltage Ug exceeds the value
set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is
switched off to see that they do not exceed the limit value F210 that applies to both of
them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt
the operating range) Monitoring for extreme values is likewise enabled during normal
operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the
malfunction signals to the binary outputs Basically the monitoring of the actual values
for If1If2 functions like that of the Ug1Ug2 monitoring
62 Excitation Protection
621 General Information
The Excitation Protection switches off the excitation (and de-excites the machine
rapidly) whenever a danger arises that threatens the excitation transformer the converter
or the generator Generally limiter or monitoring functions precede the emergency trips
and these normally respond before the Excitation Protection must initiate a trip
Protective trip commands are issued directly to the field circuit-breaker from potential
free contacts of the board via the trip relays They are directed redundantly to the
operative field circuit-breaker ldquoOFFrdquo command
622 Protection against Excitation Transformer Overheating
This equipment monitors the excitation transformer for overheating in the
windings that could result from over current short-circuits or inadequate cooling The
monitoring uses temperature monitoring modules in conjunction with temperature
sensors built into the windings Normally the temperature is monitored in two stages the
first stage sets off an alarm the second causes a trip of the excitation
623 Rotor Over voltage Protection
Malfunctions in the generator circuit (eg terminal short-circuit failed
synchronization asynchronous operation) cause induced negative field currents that
produce high voltages in the field circuit These must be restricted to a level with a
sufficient safety margin below the insulation capacity of the field winding (test voltage)
and also below the peak blocking voltage of the converter thyristors The crow bar
employs spark gap elements to detect over voltages in the field circuit Whenever they
respond the associated thyristors are fired immediately switching the de-excitation
resistor parallel to the field The de-excitation current generated thereby initiates an
excitation trip via a supervision circuit causing an immediate opening of the field circuit-
breaker The malfunction isets off an alarm and an internal malfunction is indicated at the
cubicle
TEST VALUES OBTAINED WHEN EXCITATION IS RAISED
TEST
SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE
1 VREF 996 100
2 VACT 997 1003
3 IFACT 735 765
4 IGACT 703 707
5 ACTIVE
POWER
703 705
6 REACTIVE
POWER
101 142
7 POWER
FACTOR
099 IND 098 IND
8 ACTIVE
CURRENT(IR)
705 703
9 REACTIVE
CURRENT(IX)
102 138
10 POWER
ANGLE
566 546
11 FIRING
ANGLE
640 632
VALUES OBTAINED WHEN EXCITATION IS LOWERED
SNO PARTICULARS ACTUAL
VALUE
OBTAINED VALUE
1 VREF 100 997
2 VACT 100 997
3 IFACT 787 761
4 IGACT 839 837
5 ACTIVE
POWER
839 830
6 REACTIVE
POWER
155 110
7 POWER
FACTOR
098 IND 099 IND
8 ACTIVE
CURRENT(IR)
835 830
9 REACTIVE
CURRENT(IX)
140 94
10 POWER
ANGLE
613 DEG 631 DEG
11 FIRING
ANGLE
601 DEG 599 DEG
CONCLUSION
For generating the EMF in stator winding excitation is required to the rotor of a
generator There are two types of excitation
1 Static excitation system
2 Brushless excitation system
A certain disadvantage in brushless excitation system is the slow response time of
the field in case of fast load changes specified No slip-rings and brushes direct
measurements of the field parameters not possible
To avoid all loses static excitation is used Since it does not have any rotating
parts mechanical loses and windage loses This system has fast response and speed
control While preferring this excitation system there are no limitations for the
redundancy of Thyristor bridge circuits
Static excitation has fast field discharge by resistor and inverter operation direct
measurement of field quantity is possible The meaning of excitation is nothing but
continuous supply of DC current (ie field current) to the rotor to buildup required
output voltage in the stator
Field current is changed with respect to the change of load so the digital
automatic voltage regulator (DAVR) is used to regulate the output voltage according to
the load variations
So we conclude that static excitation system with DAVR is preferred since it is
having excellent dynamic performance and better options for R amp M
The purpose of a Power System Stabilizer is to use the generator excitation to
damp electromechanical oscillations between the network and the generator Depending
on the design of the generator and the requirements imposed for network stability its
main function will be either to damp the oscillations originating in the machine or those
from the network A synchronous generator working in a combined power network is in
principle an oscillating structure In order to produce a torque the magnetic field of the
rotor and the stator must form a given angle (referred to as the rotor displacement or load
angle δ) The electrical torque ME increases as the angle δ increases just as with a
torsion spring Because the ME of the generator and the mechanical driving torque MA
from the turbine are in equilibrium during steady-state operation the angle δ remains in a
given position Whenever this state of equilibrium between MA and ME is disturbed the
load angle slips of this rest position and change thereby the electrical torque ME The
torque attempts to restore the load angle to a stationary position Due to the mass inertia
of the turbinegenerator rotor however this can only take place aperiodically It does so
in the form of more or less effectively damped oscillations (again similar to the effect of
mass inertia on a torsion spring) In order to damp the oscillations there must be a
damping torque produced depending not on the electrical torque ME associated with the
angle but on the difference in frequency (Df) between the rotor and the stator rotating
field ie on the slippage This torque is produced mainly by the so-called damper
winding in the rotor but the dimensioning of this is subject to limits imposed by
considerations of design and economy Some further action is therefore needed to
increase the damping effect The following drastically simplified formula shows the
parameters upon which the amount of active power PE supplied by the generator
depends
PE = active power
It can be seen from the above relationship that the active power that the generator
transfers depends not only on the load angle δ but also on the field current If That means
that a transient change can be made in the active power PE and with that in the effective
electrical torque ME by varying the field current The principle of operation of the DVR
Power System Stabilizer becomes clear from a consideration of the oscillations in power
output and frequency (ΔPE Δf) and the vector diagram If it is assumed that oscillations
in the network frequency generates load oscillations with the mass inertia of the rotor
then the active load of the generator (eg MW-measured) is influenced with a sinusoidal
value -ΔPE (ME-MA = -ΔPE) By inversion of -ΔPE one obtains the fluctuation in
power provided by the rotor +ΔPE As is known the slip signal Δf follows +ΔPE with a
phase delayed by 90deg The +ME produced by the periodic changes in the load angle δ is
in phase with +ΔPE A good damping is attained if ME is varied in phase with the slip
Δf However this signal must also be advanced somewhat to compensate for the time
constants in the excitation circuit and the generator
As mentioned above the electrical torque ME can be influenced by varying the
field current To accomplish this a suitable control signal referred to as variable
disturbance compensation must be imposed upon the voltage set-point or the converter
control variable Ucontr As can be seen from the vector diagram by applying proper
weighting factors (K1 K2) and then adding together the signals -ΔPE and Δf an overall
stabilization signal can be produced that rotates in advance of the Df signal by any angle
desired between 0deg and 90deg Because the amplitude of -DPE remains proportional to the
amplitude of Δf a constant angle in advance of Δf results for the compensation of the
time constants referred to above The optimum weighting factors K1 and K2 for a
synchronous generator working to a power network depend on its operating point at any
moment and the external reactance of the network Normally the selection of a
compromise setting is good enough to attain stability in all operating points and for all
external reactance For special demands these settings must be parameterized as a
function of the external reactance (which means optional equipment Xe-Identification)
The Power System Stabilizer PSS is a section of the AVR computer program and is
processed once per network cycle The voltage at the generator terminals and the
generator current are measured in order to define the signals ΔPE and Δf The calculated
signals for _P_ (=PE) and Δf are then sent across DC filters ldquoDrdquo (real differentiators) that
transmit only the dynamic portion of the signals The ΔPE and Δf signals obtained in this
way are then weighted (multiplied by) with the factors K1 and K2 and sent to the
summing point of the voltage regulator
The PSS stabilization signal is imposed on the automatic voltage regulator only if
the following prerequisites are met
bull Generator on line
bull Generator power output gt the value F338
bull Generator voltage in a range between F33C and F33A
The stabilization signal is limited at the output from the PSS to the lower and
upper limits Flag defines whether the stabilization signal is introduced before or after the
PID filter (usually before the filter) Because the PID filter as noted above already takes
the ceiling factor Vp1 into account the PSS signal needs to be multiplied by Vp1 if it is
added to the voltage regulator following the PID filter (divider at the input to the
minmax limiter) This precaution prevents the DC filter ldquoDrdquo in the P-channel from
producing an unnecessary ldquostabilizationrdquo effect in the case of rapid changes in turbine
load As an alternative for the AVRrsquos Power System Stabilizer a stabilization signal from
an outside system can be imposed by activating the binary input ldquoPSS-SIGNEXTrdquo Flag
F340 can be used to select between an analog and a 12-bit signal and F33E to select the
polarity desired for that signal
24 The MANUAL Channel
241 Summary
The MANUAL channel (Channel 2) has been built as a simple field current
regulator
without additional limiters Its main function is to maintain the excitation of the generator
even if the AUTOMATIC channel becomes non-operational The MANUAL channel
also performs valuable service for purposes of testing commissioning and preventive
maintenance Its measurements regulator generation of firing pulses and power supply
are physically separate from those on the AUTOMATIC channel
242 Principle of Operation
All the functions of the MANUAL channel including the generation of firing
pulses have been implemented in a single electronic module the Gate Control Unit The
control variable Ucontr of voltage regulator is used as the reference value for generating
firing pulses on the principle known as ldquoramp controlrdquo (Comparison of Ucontr with
Usynsynchronous sawtooth signal) For further processing in the UN 0096 Intermediate
Pulse Stage the Gate Control Unit supplies six firing pulses at its output whose phase
position with respect to the synchronous voltage Usyn is in accordance with control
variable Ucontr An internal linearization ensures that the field voltage produced via the
firing pulses remains proportional to the control variable Ucontr throughout the entire
range As a result the circuit amplification of the control remains constant over the entire
range Whenever excitation is switched ON the set-point for Generator Voltage is set
automatically at the preset - ref Value This provision ensures that the generator voltage
always attains approximately its nominal value after the field flashing The Gate Control
Unit can be refunctioned ( by pre-selection with a switch ) for purposes of testing to act
as a purely firing pulse control In this case the control variable Ucontr is adjusted
directly using the RAISELOWER push buttons on the front of the module In this way
for example the relationship between the phase position of the firing pulses and the
control variable Ucontr can be checked easily
CHAPTER 3
PULSE SECTION
31 Pulse Generation and Amplification
The Gate Control Units of both AUTOMATIC channel and MANUAL channel
each supply six firing pulses for operating the 6-pulse thyristor bridges The low-power
pulse signals from these Gate Control Units are then amplified in the Intermediate Pulse
Stage galvanically isolated and then sent to the common pulse bus On the output end
the Intermediate Pulse Stage of the non-active channel) is always blocked The Gate
Control Units generate the pulses based on microprocessor control The reference voltage
used for the firing pulse phase location is the output voltage from the excitation
transformer (Usyn1 Usyn2) The commutation spikes of the synchronous voltage caused
by the converter are calculated prior to use of the voltage as a reference value and are
deliberately filtered out The lower limit for the firing pulses (double pulses) which are
offset from one another by 60deg is defined by the limit rectifier position (αmin) and the
upper limit by the limit inverter position (αmax) for the firing angle αmin and αmax can
be adjusted on the Gate Control Units using BCD (Binary Coded Decimal) switches
αmin ensures that the firing pulses will not be issued (premature firing) until there is
sufficient positive phase voltage on the thyristor involved αmax prevents a dangerous
ldquotippingrdquo of the thyristor bridge into the rectifier mode if the firing angle α is too large
(ldquolate firingrdquo) The critical factors determining αmax are the overlap time uumlmax (max
commutation time) and the ldquorecovery timerdquo of the thyristors (αmax lt 180deg - uumlmax - γ )
An external control signal can force the firing pulses into their inverter limit position
Other binary inputs can block or direct the firing pulses of the Gate Control Units so as to
produce freewheeling on the thyristor bridge During freewheeling the firing pulses for
the thyristor pair R and S are blocked and the pulse signals T+T- are engaged with
chains of pulses Both Gate Control Units (for the MANUAL amp AUTOMATIC channels)
contain a field current monitor that blocks the firing pulses immediately whenever the
current exceeds a preset threshold level In this case the field circuit-breaker is also
tripped via an output contact The purpose of these provisions is to prevent damage to
thyristors and thyristor fuses in case of a slip-ring short-circuit or to keep any damage
that does occur to a minimum The pulse signals are galvanically separated at the outputs
from the Intermediate Pulse Stage (with pulse transmitters) and are then directed to the
common pulse bus This transmission of the pulse signals to the pulse bus via passive
transmitters ensures a high degree of active channel autonomy Practically no possible
malfunctions on the inactive channel (including for example sustained pulses) affect the
active channel
32 Pulse Monitoring
The ldquoPulse Busrdquo and the pulse signals of the AUTOMATIC channel are
monitored This monitoring device consists of potential isolating stages and the common
monitor If the pulse monitoring of the ldquoPulse Busrdquo responds a switch-over is made to
MANUAL channel The function of the potential isolating stages is to couple the pulse
monitoring device to the pulse circuits without any feedback effect The pulse monitoring
checks the six pulse lines for the following malfunctions continuous or periodic failure
of one or more pulses Periodic occurrence of synchronous or asynchronous false pulses
Continuous pulses the pulse monitoring device can be tested while the machine is in
operation
CHAPTER 4
CONVERTER
Thyristor
The term thyristor usually refers to a family of four layer solid state device having
turn on characteristics that can be externally controlled by either current or voltage They
are also referred to as breakdown device because their working depends on avalanche
breakdown Thyristors have only two stages OFF and ON Thyristors have a similar
function to Uni-junctions they act as switches Thyristors use current flow as a switch
Thyristors have three states
1 Reverse blocking mode mdash Voltage is applied in the direction that would be
blocked by a diode
2 Forward blocking mode mdash Voltage is applied in the direction that would cause
a diode to conduct but the thyristor has not yet been triggered into conduction
3 Forward conducting mode mdash The thyristor has been triggered into conduction
and will remain conducting until the forward current drops below a threshold value
known as the holding current Converter is a semiconductor device which converts ac
input voltage into a constant dc output voltage In present excitation system three phase
fully controlled thyristor converter is used
Because of the following advantages thyristor converters are used
a) Thyristors are used for high power applications ie up to 10Kv3500A1KHz
b) Having high reliability and low losses
c) Uni-directional device like diode
d) Itrsquos operation as a rectifier which are low resistance in forward conduction
mode and high resistance in reverse conduction mode
PROTECTION OF THYRISTORS
For reliable operation of a thyristor demands that its specified ratings are not
exceeded When Subjected to or over voltages During the turn - on of SCR didt
prohibitively large False triggering of SCR by high value of dvdt andSpurious signals
between gate and cathode may leads to unwanted turn ndash on
DIDT AND PROTECTION
When thyristor starts conducting in forward conduction mode and is turned on by
gate pulse The anode current increases rapidly whole area of the gate to Cathode
junction then hot spots will be formed near the gate connection this locality of heating
destroys the thyristor Thyristor thermal time is constant The causes due to faults and
short circuits or surge currents Electronic crowbar protection is used against the over
voltages The rate rise of anode current must be kept at the time of turn on below the
rated or specified limiting value The didt value maintained below limited value by using
a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating
is avoided by applying gate current but not greater the maximum gate current
DVDT AND OVER VOLTAGE PROTECTION
With forward voltage across the anode and cathode of a thyristor the two outer
junctions are forward biased but the inner junction is reverse biased This reverse biased
junction J2 has the characteristics of a capacitor due to charges existing across the
junction In other words space-charges exist in the depletion region around junction J2
and therefore junction J2 behaves like a capacitance If the entire anode to cathode
forward voltage Va appears across J2 junction and the charge is denoted by Q then a
charging current i given by Eq (46) follows
i = dQdt =d(Cj Va )dt
= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)
As Cj the capacitance of junction J2 is almost constant the current is given by
i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)
If the rate of rise of forward voltage dVadt is high the charging current i will be
more This charging current plays the role of gate current and turns on the SCR even
when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on
must be avoided as it leads to false operation of the thyristor circuit
For controllable operation of the thyristor the rate of rise of forward anode to
cathode voltage dVadt must be kept below the specified rated limit Typical values of
dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by
using a snubber circuit in parallel with the device thyristor are very sensitive for over
voltage than the semiconductor devices
Over voltage transients are perhaps the main cause of thyristor failure
In thyristor there are mainly two types
1 Internal over voltages
Due to the commutation of the thyristors large voltages are generated internally
Because of the series inductance of the SCR circuit the large transient voltages L didt
produced This voltage several times the break over voltage of the device then thyristor
destroys permanently
2 External over voltages
External over voltages are caused due to the interruptions of current flow in an
inductive circuit and also due to the lightening strokes on the lines feeding the thyristor
system For the reliable operation of thyristor the over voltages must be suppressed by
adopting suitable techniques
Suppression of over voltages
The RC circuit called snubber circuit is connected across the device to protect In
order to keep the protective components to a minimum the thyristors are chosen with
their peak voltages ratings are 25 to 3 times of the normal peak working voltage
ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are
commonly employed for protecting the thyristor circuit against the over voltages
Gate protection
Gate circuit should also be protected against the over voltages and surges Over
voltage at gate circuit can cause false triggering of the SCR may rises the junction
temperature behind specified limit leading to its damage Protection against over
voltage can be achieved by connecting a ZD across the gate circuit and a resister is
connected in series with gate circuit to protect against the s A capacitor and resister are
connected across gate to cathode to by pass the noise
41 Final Pulse Stages
The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage
(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor
bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided
with a power supply module The amplified output pulses from the Final Pulse Stages
start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main
pulse This initial pulse edge assures proper firing of the thyristors being triggered
Subsequently the weaker part of main pulse keeps firing conditions steady As already
mentioned the Final Pulse Stages and their associated thyristor bridges form single units
All six pulse outputs from a Final Pulse Stage can be blocked by an external control
signal so that all thyristors in the associated thyristor bridge will block the current A
blocking of the pulses is initiated whenever there is a malfunction in the associated
thyristor bridge
42 Converter Power Section
The thyristor converter consists of three independent parallel rectifier blocks TY1
to TY3 which are all in service Even if one block fails the remaining blocks take over
automatically the full design current of the excitation circuit During normal operation
(with ideal current share) and all three bridges in operation each of these blocks has to
carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation
is limited Only when all three bridges fail the excitation is switched off Each thyristor
bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any
arm is identified by a Current flow monitoring module
43 Converter Cooling
A cooling system is needed to dissipate heat losses in the converter blocks and
electronics Each converter block has therefore been equipped with a fan supplied with
power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing
cubicle) The fans are protected with motor protection circuit breakers An air flow
monitoring unit is provided for monitoring the air flow through the thyristor bridge If a
circuit breaker failure is detected or if the air flow monitor drops off at one of the
thyristor bridges the bridge involved is immediately set out of operation by blocking its
firing pulses
44 Thyristor Converter Monitoring
A thyristor bridge in which defects occur that could threaten the safety of
operation or cause secondary damage is switched off automatically ie its firing pulses
are blocked This happens whenever A thyristor fuse is blown The fuses are monitored
individually with micro switches The Final Pulse Stage fails which is detected by
internal monitors (supply voltage sustained pulse short-circuit on the output end) The
power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or
is insufficient Isolator on ACDC side is open
CHAPTER 5
Field Current Circuit Excitation Transformer
51 Field Circuit Breaker
The circuit-breaker in the field circuit is used to isolate the field circuit from the
converter It is capable of switching off the synchronous machine from full load under the
maximum conditions of a 3-phase short-circuit In addition to its main contacts the field
circuit-breaker also has a de-excitation contact with which the field energy stored in the
field can be dissipated across the de-excitation resistor The de-excitation contact closes
shortly before the main contacts open so as to ensure proper commutation of the field
current from the main contacts to the de-excitation contact when the breaker is switched
off The field circuit-breaker is switched on by electromagnetic force and is kept switched
on by a mechanical latch When the latch is released by a trip coil the circuit-breaker
opens The circuit-breaker also has auxiliary contacts that report its status
52 Field Flashing
In shunt supplied excitation circuits (excitation transformer connected to the
generator terminals) the generator does not have enough remnant voltage for a generator
voltage build-up via the converter In this case a field flashing circuit is provided It
consists of the field flashing contactor the diode bridge and a transformer used to adapt
the auxiliary input voltage to the voltage needed for field flashing when power is
supplied from the auxiliaries network
Fig Field Flashing
Because the field flashing contactor is not able to switch off the energy stored in
the field the control ensures that the contactor can only reopen if the field circuit breaker
has already been opened (generating the TRIP order) or in a normal field flashing
sequence when the converter has taken over the field current Field flashing occurs in the
following stages
1048729The excitation is switched on closing the field flashing contactor ( Field
Circuit Breaker is already closed )
1048729The start-up excitation current flows through the rotor driving the generator
voltage up to approx 15 U
1048729After about 10 U the firing pulses to the converter are released and it begins
to excite the generator to its rated voltage
1048729After about 30 U the field flashing contactor opens (with no current since
the converter is now supplying the current)
The diode bridge at the input to the field flashing contactor prevents a feed-back
from the converter to the source of field flashing while the contactor is still closed
53 De-excitation
When malfunctions occur the stored field energy must be dissipated as quickly
and safely as possible to protect the generator This is done by the converter the field
circuit-breaker and the de-excitation (discharge) resistor
De-excitation (with opening of the field circuit-breaker) takes place in the following
stages
1048729The converter drives to its inverter limit position (negative ceiling voltage)
recovers a portion of the field energy into the network A trip command is given to the
field circuit breaker
1048729The de-excitation contact closes diverting the field voltage to the de-excitation
resistor
1048729Then immediately the main contacts open building voltage The field voltage
commutates to the de-excitation resistor
1048729The current diminishes at a given time constant TE
(With linear resistance TE = Lf (Rf + Re))
Due to the reversal of the field voltage by the converter the field current
commutates from the main contacts of the field circuit-breaker to the de-excitation
resistor in a very early phase This reversal of the field voltage prevents burn-off on the
main contacts and provides effective protection for the field circuit-breaker Depending
on the operating policy an operational shut-down of the excitation can also be effected
with the field circuit-breaker closed This method is useful mainly when the excitation is
switched on and off frequently In this case the converter is merely driven into the
inverter limit position so that the field energy is recovered into the network The
converter then blocks since it is supplying positive current only
54 Excitation Transformer
The excitation transformer matches the generator voltage to the field voltage
(required ceiling voltage) It also serves as a commutation reactance for the thyristor
converter and as a potential isolator between the network and the excitation circuit In
addition the transformer functions as a current limiter in that it makes it possible to keep
any short circuits in the excitation circuit under better control The excitation transformer
is equipped with temperature monitoring probes which set off an alarm when the
temperature exceeds a first max limit and then trips the excitation if the temperature
continues rising to a second (higher)limit
CHAPTER 6
Monitoring and Protection
61 Excitation Monitoring
611 General Information
The main goal of Excitation Monitoring is to make optimum use of the
redundancies provided in the excitation system and to give alarm whenever a malfunction
makes these redundancies unavailable The field current is monitored to see that it does
not exceed a maximum level and if necessary a switch-over to the MANUAL channel is
initiated In addition the criterion for switching off the field flashing is generated The
excitation Monitoring consists of an autonomous processor system
612 over current Alarms
In the Excitation Monitoring the limits for are set at higher levels than the
settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of
the nominal field current contact R1 and the binary output associated with it are
activated immediately If field current remains gt 110 then after a preset inverse-time
has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs
associated with them are activated Parameters match the measurements for If1 and If2 to
the nominal value for field current so that the internal values can be processed and read as
pu values It can be used to falsify the actual value of the field current If (to raise it) so
as to cause a response from the alarm limits for purposes of testing The processed If
signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As
long as the field current If is above the threshold value 11 Ifn its peak value is
measured This is stored (until RESET) and can be read at any time on the Micro-
Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever
the integrated time-current value (intisup2dt) exceeds the preselected reference value the
alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously
issued to switch over to the stand by AUTO channel Software switch F758 enables the
three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-
time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the
desired limit curve for response is set using the factor F216
613 Switch-Off Criterion for Field Flashing
The Excitation Monitoring supplies the criterion for switching off the field
flashing Whether this criterion is activated based on the actual value for generator voltage
Ug or for field current If or both depends on the settings of the two threshold values
F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements
Ug12 and If12 are switched over depending on the present status of the channels
(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is
fixed at ldquological 1rdquo
614 Storage of Alarm Status
The outputs of the over current alarms (R1 R2 R0) and the messages NO
FAILURE MONITORING PARAMETERS CHANGED are stored messages can be
erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the
front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the
situation causing the alarm or the malfunction is no longer present Whenever the self-
diagnosis equipment in the processor detects a malfunction the output NO FAILURE of
MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo
is activated whenever parameters or settings of software switches have been changed via
the Micro-Terminal
615 Actual Value Monitoring
The actual values for generator voltage Ug synchronous voltage Usyn and field
current If are monitored for malfunctions This monitoring is active regardless of whether
or not the generator is in operation Essentially when the generator is in operation the
measurements are monitored by comparing the signals (the smaller signal reading is
detected as incorrect) When the generator is not in operation the measured data are
monitored for extreme values The percentage of deviation permissible in the
measurement signals being compared
(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and
F20AIf the excitation transformer is being supplied from an auxiliary power source (no
shunt operation) the values of Ug and Usyn will be different in some operational
conditions
In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1
and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the
messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2
and the binary message from CH1 reports no malfunction a malfunction on
Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is
also present whenever the binary message CH1 DISTURBANCE is reported and a
discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2
FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares
Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the
comparison channel reports a malfunction or whenever both binary messages report no
malfunction - but both secondary monitors report a malfunction As long as the secondary
monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction
signals for the measurement channel involved (suspicion that there is a corresponding
error in Usyn) The measurement channel malfunctions are enabled operationally
whenever after excitation has been switched on generator voltage Ug exceeds the value
set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is
switched off to see that they do not exceed the limit value F210 that applies to both of
them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt
the operating range) Monitoring for extreme values is likewise enabled during normal
operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the
malfunction signals to the binary outputs Basically the monitoring of the actual values
for If1If2 functions like that of the Ug1Ug2 monitoring
62 Excitation Protection
621 General Information
The Excitation Protection switches off the excitation (and de-excites the machine
rapidly) whenever a danger arises that threatens the excitation transformer the converter
or the generator Generally limiter or monitoring functions precede the emergency trips
and these normally respond before the Excitation Protection must initiate a trip
Protective trip commands are issued directly to the field circuit-breaker from potential
free contacts of the board via the trip relays They are directed redundantly to the
operative field circuit-breaker ldquoOFFrdquo command
622 Protection against Excitation Transformer Overheating
This equipment monitors the excitation transformer for overheating in the
windings that could result from over current short-circuits or inadequate cooling The
monitoring uses temperature monitoring modules in conjunction with temperature
sensors built into the windings Normally the temperature is monitored in two stages the
first stage sets off an alarm the second causes a trip of the excitation
623 Rotor Over voltage Protection
Malfunctions in the generator circuit (eg terminal short-circuit failed
synchronization asynchronous operation) cause induced negative field currents that
produce high voltages in the field circuit These must be restricted to a level with a
sufficient safety margin below the insulation capacity of the field winding (test voltage)
and also below the peak blocking voltage of the converter thyristors The crow bar
employs spark gap elements to detect over voltages in the field circuit Whenever they
respond the associated thyristors are fired immediately switching the de-excitation
resistor parallel to the field The de-excitation current generated thereby initiates an
excitation trip via a supervision circuit causing an immediate opening of the field circuit-
breaker The malfunction isets off an alarm and an internal malfunction is indicated at the
cubicle
TEST VALUES OBTAINED WHEN EXCITATION IS RAISED
TEST
SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE
1 VREF 996 100
2 VACT 997 1003
3 IFACT 735 765
4 IGACT 703 707
5 ACTIVE
POWER
703 705
6 REACTIVE
POWER
101 142
7 POWER
FACTOR
099 IND 098 IND
8 ACTIVE
CURRENT(IR)
705 703
9 REACTIVE
CURRENT(IX)
102 138
10 POWER
ANGLE
566 546
11 FIRING
ANGLE
640 632
VALUES OBTAINED WHEN EXCITATION IS LOWERED
SNO PARTICULARS ACTUAL
VALUE
OBTAINED VALUE
1 VREF 100 997
2 VACT 100 997
3 IFACT 787 761
4 IGACT 839 837
5 ACTIVE
POWER
839 830
6 REACTIVE
POWER
155 110
7 POWER
FACTOR
098 IND 099 IND
8 ACTIVE
CURRENT(IR)
835 830
9 REACTIVE
CURRENT(IX)
140 94
10 POWER
ANGLE
613 DEG 631 DEG
11 FIRING
ANGLE
601 DEG 599 DEG
CONCLUSION
For generating the EMF in stator winding excitation is required to the rotor of a
generator There are two types of excitation
1 Static excitation system
2 Brushless excitation system
A certain disadvantage in brushless excitation system is the slow response time of
the field in case of fast load changes specified No slip-rings and brushes direct
measurements of the field parameters not possible
To avoid all loses static excitation is used Since it does not have any rotating
parts mechanical loses and windage loses This system has fast response and speed
control While preferring this excitation system there are no limitations for the
redundancy of Thyristor bridge circuits
Static excitation has fast field discharge by resistor and inverter operation direct
measurement of field quantity is possible The meaning of excitation is nothing but
continuous supply of DC current (ie field current) to the rotor to buildup required
output voltage in the stator
Field current is changed with respect to the change of load so the digital
automatic voltage regulator (DAVR) is used to regulate the output voltage according to
the load variations
So we conclude that static excitation system with DAVR is preferred since it is
having excellent dynamic performance and better options for R amp M
that a transient change can be made in the active power PE and with that in the effective
electrical torque ME by varying the field current The principle of operation of the DVR
Power System Stabilizer becomes clear from a consideration of the oscillations in power
output and frequency (ΔPE Δf) and the vector diagram If it is assumed that oscillations
in the network frequency generates load oscillations with the mass inertia of the rotor
then the active load of the generator (eg MW-measured) is influenced with a sinusoidal
value -ΔPE (ME-MA = -ΔPE) By inversion of -ΔPE one obtains the fluctuation in
power provided by the rotor +ΔPE As is known the slip signal Δf follows +ΔPE with a
phase delayed by 90deg The +ME produced by the periodic changes in the load angle δ is
in phase with +ΔPE A good damping is attained if ME is varied in phase with the slip
Δf However this signal must also be advanced somewhat to compensate for the time
constants in the excitation circuit and the generator
As mentioned above the electrical torque ME can be influenced by varying the
field current To accomplish this a suitable control signal referred to as variable
disturbance compensation must be imposed upon the voltage set-point or the converter
control variable Ucontr As can be seen from the vector diagram by applying proper
weighting factors (K1 K2) and then adding together the signals -ΔPE and Δf an overall
stabilization signal can be produced that rotates in advance of the Df signal by any angle
desired between 0deg and 90deg Because the amplitude of -DPE remains proportional to the
amplitude of Δf a constant angle in advance of Δf results for the compensation of the
time constants referred to above The optimum weighting factors K1 and K2 for a
synchronous generator working to a power network depend on its operating point at any
moment and the external reactance of the network Normally the selection of a
compromise setting is good enough to attain stability in all operating points and for all
external reactance For special demands these settings must be parameterized as a
function of the external reactance (which means optional equipment Xe-Identification)
The Power System Stabilizer PSS is a section of the AVR computer program and is
processed once per network cycle The voltage at the generator terminals and the
generator current are measured in order to define the signals ΔPE and Δf The calculated
signals for _P_ (=PE) and Δf are then sent across DC filters ldquoDrdquo (real differentiators) that
transmit only the dynamic portion of the signals The ΔPE and Δf signals obtained in this
way are then weighted (multiplied by) with the factors K1 and K2 and sent to the
summing point of the voltage regulator
The PSS stabilization signal is imposed on the automatic voltage regulator only if
the following prerequisites are met
bull Generator on line
bull Generator power output gt the value F338
bull Generator voltage in a range between F33C and F33A
The stabilization signal is limited at the output from the PSS to the lower and
upper limits Flag defines whether the stabilization signal is introduced before or after the
PID filter (usually before the filter) Because the PID filter as noted above already takes
the ceiling factor Vp1 into account the PSS signal needs to be multiplied by Vp1 if it is
added to the voltage regulator following the PID filter (divider at the input to the
minmax limiter) This precaution prevents the DC filter ldquoDrdquo in the P-channel from
producing an unnecessary ldquostabilizationrdquo effect in the case of rapid changes in turbine
load As an alternative for the AVRrsquos Power System Stabilizer a stabilization signal from
an outside system can be imposed by activating the binary input ldquoPSS-SIGNEXTrdquo Flag
F340 can be used to select between an analog and a 12-bit signal and F33E to select the
polarity desired for that signal
24 The MANUAL Channel
241 Summary
The MANUAL channel (Channel 2) has been built as a simple field current
regulator
without additional limiters Its main function is to maintain the excitation of the generator
even if the AUTOMATIC channel becomes non-operational The MANUAL channel
also performs valuable service for purposes of testing commissioning and preventive
maintenance Its measurements regulator generation of firing pulses and power supply
are physically separate from those on the AUTOMATIC channel
242 Principle of Operation
All the functions of the MANUAL channel including the generation of firing
pulses have been implemented in a single electronic module the Gate Control Unit The
control variable Ucontr of voltage regulator is used as the reference value for generating
firing pulses on the principle known as ldquoramp controlrdquo (Comparison of Ucontr with
Usynsynchronous sawtooth signal) For further processing in the UN 0096 Intermediate
Pulse Stage the Gate Control Unit supplies six firing pulses at its output whose phase
position with respect to the synchronous voltage Usyn is in accordance with control
variable Ucontr An internal linearization ensures that the field voltage produced via the
firing pulses remains proportional to the control variable Ucontr throughout the entire
range As a result the circuit amplification of the control remains constant over the entire
range Whenever excitation is switched ON the set-point for Generator Voltage is set
automatically at the preset - ref Value This provision ensures that the generator voltage
always attains approximately its nominal value after the field flashing The Gate Control
Unit can be refunctioned ( by pre-selection with a switch ) for purposes of testing to act
as a purely firing pulse control In this case the control variable Ucontr is adjusted
directly using the RAISELOWER push buttons on the front of the module In this way
for example the relationship between the phase position of the firing pulses and the
control variable Ucontr can be checked easily
CHAPTER 3
PULSE SECTION
31 Pulse Generation and Amplification
The Gate Control Units of both AUTOMATIC channel and MANUAL channel
each supply six firing pulses for operating the 6-pulse thyristor bridges The low-power
pulse signals from these Gate Control Units are then amplified in the Intermediate Pulse
Stage galvanically isolated and then sent to the common pulse bus On the output end
the Intermediate Pulse Stage of the non-active channel) is always blocked The Gate
Control Units generate the pulses based on microprocessor control The reference voltage
used for the firing pulse phase location is the output voltage from the excitation
transformer (Usyn1 Usyn2) The commutation spikes of the synchronous voltage caused
by the converter are calculated prior to use of the voltage as a reference value and are
deliberately filtered out The lower limit for the firing pulses (double pulses) which are
offset from one another by 60deg is defined by the limit rectifier position (αmin) and the
upper limit by the limit inverter position (αmax) for the firing angle αmin and αmax can
be adjusted on the Gate Control Units using BCD (Binary Coded Decimal) switches
αmin ensures that the firing pulses will not be issued (premature firing) until there is
sufficient positive phase voltage on the thyristor involved αmax prevents a dangerous
ldquotippingrdquo of the thyristor bridge into the rectifier mode if the firing angle α is too large
(ldquolate firingrdquo) The critical factors determining αmax are the overlap time uumlmax (max
commutation time) and the ldquorecovery timerdquo of the thyristors (αmax lt 180deg - uumlmax - γ )
An external control signal can force the firing pulses into their inverter limit position
Other binary inputs can block or direct the firing pulses of the Gate Control Units so as to
produce freewheeling on the thyristor bridge During freewheeling the firing pulses for
the thyristor pair R and S are blocked and the pulse signals T+T- are engaged with
chains of pulses Both Gate Control Units (for the MANUAL amp AUTOMATIC channels)
contain a field current monitor that blocks the firing pulses immediately whenever the
current exceeds a preset threshold level In this case the field circuit-breaker is also
tripped via an output contact The purpose of these provisions is to prevent damage to
thyristors and thyristor fuses in case of a slip-ring short-circuit or to keep any damage
that does occur to a minimum The pulse signals are galvanically separated at the outputs
from the Intermediate Pulse Stage (with pulse transmitters) and are then directed to the
common pulse bus This transmission of the pulse signals to the pulse bus via passive
transmitters ensures a high degree of active channel autonomy Practically no possible
malfunctions on the inactive channel (including for example sustained pulses) affect the
active channel
32 Pulse Monitoring
The ldquoPulse Busrdquo and the pulse signals of the AUTOMATIC channel are
monitored This monitoring device consists of potential isolating stages and the common
monitor If the pulse monitoring of the ldquoPulse Busrdquo responds a switch-over is made to
MANUAL channel The function of the potential isolating stages is to couple the pulse
monitoring device to the pulse circuits without any feedback effect The pulse monitoring
checks the six pulse lines for the following malfunctions continuous or periodic failure
of one or more pulses Periodic occurrence of synchronous or asynchronous false pulses
Continuous pulses the pulse monitoring device can be tested while the machine is in
operation
CHAPTER 4
CONVERTER
Thyristor
The term thyristor usually refers to a family of four layer solid state device having
turn on characteristics that can be externally controlled by either current or voltage They
are also referred to as breakdown device because their working depends on avalanche
breakdown Thyristors have only two stages OFF and ON Thyristors have a similar
function to Uni-junctions they act as switches Thyristors use current flow as a switch
Thyristors have three states
1 Reverse blocking mode mdash Voltage is applied in the direction that would be
blocked by a diode
2 Forward blocking mode mdash Voltage is applied in the direction that would cause
a diode to conduct but the thyristor has not yet been triggered into conduction
3 Forward conducting mode mdash The thyristor has been triggered into conduction
and will remain conducting until the forward current drops below a threshold value
known as the holding current Converter is a semiconductor device which converts ac
input voltage into a constant dc output voltage In present excitation system three phase
fully controlled thyristor converter is used
Because of the following advantages thyristor converters are used
a) Thyristors are used for high power applications ie up to 10Kv3500A1KHz
b) Having high reliability and low losses
c) Uni-directional device like diode
d) Itrsquos operation as a rectifier which are low resistance in forward conduction
mode and high resistance in reverse conduction mode
PROTECTION OF THYRISTORS
For reliable operation of a thyristor demands that its specified ratings are not
exceeded When Subjected to or over voltages During the turn - on of SCR didt
prohibitively large False triggering of SCR by high value of dvdt andSpurious signals
between gate and cathode may leads to unwanted turn ndash on
DIDT AND PROTECTION
When thyristor starts conducting in forward conduction mode and is turned on by
gate pulse The anode current increases rapidly whole area of the gate to Cathode
junction then hot spots will be formed near the gate connection this locality of heating
destroys the thyristor Thyristor thermal time is constant The causes due to faults and
short circuits or surge currents Electronic crowbar protection is used against the over
voltages The rate rise of anode current must be kept at the time of turn on below the
rated or specified limiting value The didt value maintained below limited value by using
a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating
is avoided by applying gate current but not greater the maximum gate current
DVDT AND OVER VOLTAGE PROTECTION
With forward voltage across the anode and cathode of a thyristor the two outer
junctions are forward biased but the inner junction is reverse biased This reverse biased
junction J2 has the characteristics of a capacitor due to charges existing across the
junction In other words space-charges exist in the depletion region around junction J2
and therefore junction J2 behaves like a capacitance If the entire anode to cathode
forward voltage Va appears across J2 junction and the charge is denoted by Q then a
charging current i given by Eq (46) follows
i = dQdt =d(Cj Va )dt
= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)
As Cj the capacitance of junction J2 is almost constant the current is given by
i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)
If the rate of rise of forward voltage dVadt is high the charging current i will be
more This charging current plays the role of gate current and turns on the SCR even
when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on
must be avoided as it leads to false operation of the thyristor circuit
For controllable operation of the thyristor the rate of rise of forward anode to
cathode voltage dVadt must be kept below the specified rated limit Typical values of
dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by
using a snubber circuit in parallel with the device thyristor are very sensitive for over
voltage than the semiconductor devices
Over voltage transients are perhaps the main cause of thyristor failure
In thyristor there are mainly two types
1 Internal over voltages
Due to the commutation of the thyristors large voltages are generated internally
Because of the series inductance of the SCR circuit the large transient voltages L didt
produced This voltage several times the break over voltage of the device then thyristor
destroys permanently
2 External over voltages
External over voltages are caused due to the interruptions of current flow in an
inductive circuit and also due to the lightening strokes on the lines feeding the thyristor
system For the reliable operation of thyristor the over voltages must be suppressed by
adopting suitable techniques
Suppression of over voltages
The RC circuit called snubber circuit is connected across the device to protect In
order to keep the protective components to a minimum the thyristors are chosen with
their peak voltages ratings are 25 to 3 times of the normal peak working voltage
ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are
commonly employed for protecting the thyristor circuit against the over voltages
Gate protection
Gate circuit should also be protected against the over voltages and surges Over
voltage at gate circuit can cause false triggering of the SCR may rises the junction
temperature behind specified limit leading to its damage Protection against over
voltage can be achieved by connecting a ZD across the gate circuit and a resister is
connected in series with gate circuit to protect against the s A capacitor and resister are
connected across gate to cathode to by pass the noise
41 Final Pulse Stages
The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage
(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor
bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided
with a power supply module The amplified output pulses from the Final Pulse Stages
start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main
pulse This initial pulse edge assures proper firing of the thyristors being triggered
Subsequently the weaker part of main pulse keeps firing conditions steady As already
mentioned the Final Pulse Stages and their associated thyristor bridges form single units
All six pulse outputs from a Final Pulse Stage can be blocked by an external control
signal so that all thyristors in the associated thyristor bridge will block the current A
blocking of the pulses is initiated whenever there is a malfunction in the associated
thyristor bridge
42 Converter Power Section
The thyristor converter consists of three independent parallel rectifier blocks TY1
to TY3 which are all in service Even if one block fails the remaining blocks take over
automatically the full design current of the excitation circuit During normal operation
(with ideal current share) and all three bridges in operation each of these blocks has to
carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation
is limited Only when all three bridges fail the excitation is switched off Each thyristor
bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any
arm is identified by a Current flow monitoring module
43 Converter Cooling
A cooling system is needed to dissipate heat losses in the converter blocks and
electronics Each converter block has therefore been equipped with a fan supplied with
power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing
cubicle) The fans are protected with motor protection circuit breakers An air flow
monitoring unit is provided for monitoring the air flow through the thyristor bridge If a
circuit breaker failure is detected or if the air flow monitor drops off at one of the
thyristor bridges the bridge involved is immediately set out of operation by blocking its
firing pulses
44 Thyristor Converter Monitoring
A thyristor bridge in which defects occur that could threaten the safety of
operation or cause secondary damage is switched off automatically ie its firing pulses
are blocked This happens whenever A thyristor fuse is blown The fuses are monitored
individually with micro switches The Final Pulse Stage fails which is detected by
internal monitors (supply voltage sustained pulse short-circuit on the output end) The
power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or
is insufficient Isolator on ACDC side is open
CHAPTER 5
Field Current Circuit Excitation Transformer
51 Field Circuit Breaker
The circuit-breaker in the field circuit is used to isolate the field circuit from the
converter It is capable of switching off the synchronous machine from full load under the
maximum conditions of a 3-phase short-circuit In addition to its main contacts the field
circuit-breaker also has a de-excitation contact with which the field energy stored in the
field can be dissipated across the de-excitation resistor The de-excitation contact closes
shortly before the main contacts open so as to ensure proper commutation of the field
current from the main contacts to the de-excitation contact when the breaker is switched
off The field circuit-breaker is switched on by electromagnetic force and is kept switched
on by a mechanical latch When the latch is released by a trip coil the circuit-breaker
opens The circuit-breaker also has auxiliary contacts that report its status
52 Field Flashing
In shunt supplied excitation circuits (excitation transformer connected to the
generator terminals) the generator does not have enough remnant voltage for a generator
voltage build-up via the converter In this case a field flashing circuit is provided It
consists of the field flashing contactor the diode bridge and a transformer used to adapt
the auxiliary input voltage to the voltage needed for field flashing when power is
supplied from the auxiliaries network
Fig Field Flashing
Because the field flashing contactor is not able to switch off the energy stored in
the field the control ensures that the contactor can only reopen if the field circuit breaker
has already been opened (generating the TRIP order) or in a normal field flashing
sequence when the converter has taken over the field current Field flashing occurs in the
following stages
1048729The excitation is switched on closing the field flashing contactor ( Field
Circuit Breaker is already closed )
1048729The start-up excitation current flows through the rotor driving the generator
voltage up to approx 15 U
1048729After about 10 U the firing pulses to the converter are released and it begins
to excite the generator to its rated voltage
1048729After about 30 U the field flashing contactor opens (with no current since
the converter is now supplying the current)
The diode bridge at the input to the field flashing contactor prevents a feed-back
from the converter to the source of field flashing while the contactor is still closed
53 De-excitation
When malfunctions occur the stored field energy must be dissipated as quickly
and safely as possible to protect the generator This is done by the converter the field
circuit-breaker and the de-excitation (discharge) resistor
De-excitation (with opening of the field circuit-breaker) takes place in the following
stages
1048729The converter drives to its inverter limit position (negative ceiling voltage)
recovers a portion of the field energy into the network A trip command is given to the
field circuit breaker
1048729The de-excitation contact closes diverting the field voltage to the de-excitation
resistor
1048729Then immediately the main contacts open building voltage The field voltage
commutates to the de-excitation resistor
1048729The current diminishes at a given time constant TE
(With linear resistance TE = Lf (Rf + Re))
Due to the reversal of the field voltage by the converter the field current
commutates from the main contacts of the field circuit-breaker to the de-excitation
resistor in a very early phase This reversal of the field voltage prevents burn-off on the
main contacts and provides effective protection for the field circuit-breaker Depending
on the operating policy an operational shut-down of the excitation can also be effected
with the field circuit-breaker closed This method is useful mainly when the excitation is
switched on and off frequently In this case the converter is merely driven into the
inverter limit position so that the field energy is recovered into the network The
converter then blocks since it is supplying positive current only
54 Excitation Transformer
The excitation transformer matches the generator voltage to the field voltage
(required ceiling voltage) It also serves as a commutation reactance for the thyristor
converter and as a potential isolator between the network and the excitation circuit In
addition the transformer functions as a current limiter in that it makes it possible to keep
any short circuits in the excitation circuit under better control The excitation transformer
is equipped with temperature monitoring probes which set off an alarm when the
temperature exceeds a first max limit and then trips the excitation if the temperature
continues rising to a second (higher)limit
CHAPTER 6
Monitoring and Protection
61 Excitation Monitoring
611 General Information
The main goal of Excitation Monitoring is to make optimum use of the
redundancies provided in the excitation system and to give alarm whenever a malfunction
makes these redundancies unavailable The field current is monitored to see that it does
not exceed a maximum level and if necessary a switch-over to the MANUAL channel is
initiated In addition the criterion for switching off the field flashing is generated The
excitation Monitoring consists of an autonomous processor system
612 over current Alarms
In the Excitation Monitoring the limits for are set at higher levels than the
settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of
the nominal field current contact R1 and the binary output associated with it are
activated immediately If field current remains gt 110 then after a preset inverse-time
has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs
associated with them are activated Parameters match the measurements for If1 and If2 to
the nominal value for field current so that the internal values can be processed and read as
pu values It can be used to falsify the actual value of the field current If (to raise it) so
as to cause a response from the alarm limits for purposes of testing The processed If
signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As
long as the field current If is above the threshold value 11 Ifn its peak value is
measured This is stored (until RESET) and can be read at any time on the Micro-
Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever
the integrated time-current value (intisup2dt) exceeds the preselected reference value the
alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously
issued to switch over to the stand by AUTO channel Software switch F758 enables the
three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-
time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the
desired limit curve for response is set using the factor F216
613 Switch-Off Criterion for Field Flashing
The Excitation Monitoring supplies the criterion for switching off the field
flashing Whether this criterion is activated based on the actual value for generator voltage
Ug or for field current If or both depends on the settings of the two threshold values
F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements
Ug12 and If12 are switched over depending on the present status of the channels
(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is
fixed at ldquological 1rdquo
614 Storage of Alarm Status
The outputs of the over current alarms (R1 R2 R0) and the messages NO
FAILURE MONITORING PARAMETERS CHANGED are stored messages can be
erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the
front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the
situation causing the alarm or the malfunction is no longer present Whenever the self-
diagnosis equipment in the processor detects a malfunction the output NO FAILURE of
MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo
is activated whenever parameters or settings of software switches have been changed via
the Micro-Terminal
615 Actual Value Monitoring
The actual values for generator voltage Ug synchronous voltage Usyn and field
current If are monitored for malfunctions This monitoring is active regardless of whether
or not the generator is in operation Essentially when the generator is in operation the
measurements are monitored by comparing the signals (the smaller signal reading is
detected as incorrect) When the generator is not in operation the measured data are
monitored for extreme values The percentage of deviation permissible in the
measurement signals being compared
(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and
F20AIf the excitation transformer is being supplied from an auxiliary power source (no
shunt operation) the values of Ug and Usyn will be different in some operational
conditions
In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1
and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the
messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2
and the binary message from CH1 reports no malfunction a malfunction on
Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is
also present whenever the binary message CH1 DISTURBANCE is reported and a
discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2
FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares
Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the
comparison channel reports a malfunction or whenever both binary messages report no
malfunction - but both secondary monitors report a malfunction As long as the secondary
monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction
signals for the measurement channel involved (suspicion that there is a corresponding
error in Usyn) The measurement channel malfunctions are enabled operationally
whenever after excitation has been switched on generator voltage Ug exceeds the value
set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is
switched off to see that they do not exceed the limit value F210 that applies to both of
them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt
the operating range) Monitoring for extreme values is likewise enabled during normal
operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the
malfunction signals to the binary outputs Basically the monitoring of the actual values
for If1If2 functions like that of the Ug1Ug2 monitoring
62 Excitation Protection
621 General Information
The Excitation Protection switches off the excitation (and de-excites the machine
rapidly) whenever a danger arises that threatens the excitation transformer the converter
or the generator Generally limiter or monitoring functions precede the emergency trips
and these normally respond before the Excitation Protection must initiate a trip
Protective trip commands are issued directly to the field circuit-breaker from potential
free contacts of the board via the trip relays They are directed redundantly to the
operative field circuit-breaker ldquoOFFrdquo command
622 Protection against Excitation Transformer Overheating
This equipment monitors the excitation transformer for overheating in the
windings that could result from over current short-circuits or inadequate cooling The
monitoring uses temperature monitoring modules in conjunction with temperature
sensors built into the windings Normally the temperature is monitored in two stages the
first stage sets off an alarm the second causes a trip of the excitation
623 Rotor Over voltage Protection
Malfunctions in the generator circuit (eg terminal short-circuit failed
synchronization asynchronous operation) cause induced negative field currents that
produce high voltages in the field circuit These must be restricted to a level with a
sufficient safety margin below the insulation capacity of the field winding (test voltage)
and also below the peak blocking voltage of the converter thyristors The crow bar
employs spark gap elements to detect over voltages in the field circuit Whenever they
respond the associated thyristors are fired immediately switching the de-excitation
resistor parallel to the field The de-excitation current generated thereby initiates an
excitation trip via a supervision circuit causing an immediate opening of the field circuit-
breaker The malfunction isets off an alarm and an internal malfunction is indicated at the
cubicle
TEST VALUES OBTAINED WHEN EXCITATION IS RAISED
TEST
SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE
1 VREF 996 100
2 VACT 997 1003
3 IFACT 735 765
4 IGACT 703 707
5 ACTIVE
POWER
703 705
6 REACTIVE
POWER
101 142
7 POWER
FACTOR
099 IND 098 IND
8 ACTIVE
CURRENT(IR)
705 703
9 REACTIVE
CURRENT(IX)
102 138
10 POWER
ANGLE
566 546
11 FIRING
ANGLE
640 632
VALUES OBTAINED WHEN EXCITATION IS LOWERED
SNO PARTICULARS ACTUAL
VALUE
OBTAINED VALUE
1 VREF 100 997
2 VACT 100 997
3 IFACT 787 761
4 IGACT 839 837
5 ACTIVE
POWER
839 830
6 REACTIVE
POWER
155 110
7 POWER
FACTOR
098 IND 099 IND
8 ACTIVE
CURRENT(IR)
835 830
9 REACTIVE
CURRENT(IX)
140 94
10 POWER
ANGLE
613 DEG 631 DEG
11 FIRING
ANGLE
601 DEG 599 DEG
CONCLUSION
For generating the EMF in stator winding excitation is required to the rotor of a
generator There are two types of excitation
1 Static excitation system
2 Brushless excitation system
A certain disadvantage in brushless excitation system is the slow response time of
the field in case of fast load changes specified No slip-rings and brushes direct
measurements of the field parameters not possible
To avoid all loses static excitation is used Since it does not have any rotating
parts mechanical loses and windage loses This system has fast response and speed
control While preferring this excitation system there are no limitations for the
redundancy of Thyristor bridge circuits
Static excitation has fast field discharge by resistor and inverter operation direct
measurement of field quantity is possible The meaning of excitation is nothing but
continuous supply of DC current (ie field current) to the rotor to buildup required
output voltage in the stator
Field current is changed with respect to the change of load so the digital
automatic voltage regulator (DAVR) is used to regulate the output voltage according to
the load variations
So we conclude that static excitation system with DAVR is preferred since it is
having excellent dynamic performance and better options for R amp M
way are then weighted (multiplied by) with the factors K1 and K2 and sent to the
summing point of the voltage regulator
The PSS stabilization signal is imposed on the automatic voltage regulator only if
the following prerequisites are met
bull Generator on line
bull Generator power output gt the value F338
bull Generator voltage in a range between F33C and F33A
The stabilization signal is limited at the output from the PSS to the lower and
upper limits Flag defines whether the stabilization signal is introduced before or after the
PID filter (usually before the filter) Because the PID filter as noted above already takes
the ceiling factor Vp1 into account the PSS signal needs to be multiplied by Vp1 if it is
added to the voltage regulator following the PID filter (divider at the input to the
minmax limiter) This precaution prevents the DC filter ldquoDrdquo in the P-channel from
producing an unnecessary ldquostabilizationrdquo effect in the case of rapid changes in turbine
load As an alternative for the AVRrsquos Power System Stabilizer a stabilization signal from
an outside system can be imposed by activating the binary input ldquoPSS-SIGNEXTrdquo Flag
F340 can be used to select between an analog and a 12-bit signal and F33E to select the
polarity desired for that signal
24 The MANUAL Channel
241 Summary
The MANUAL channel (Channel 2) has been built as a simple field current
regulator
without additional limiters Its main function is to maintain the excitation of the generator
even if the AUTOMATIC channel becomes non-operational The MANUAL channel
also performs valuable service for purposes of testing commissioning and preventive
maintenance Its measurements regulator generation of firing pulses and power supply
are physically separate from those on the AUTOMATIC channel
242 Principle of Operation
All the functions of the MANUAL channel including the generation of firing
pulses have been implemented in a single electronic module the Gate Control Unit The
control variable Ucontr of voltage regulator is used as the reference value for generating
firing pulses on the principle known as ldquoramp controlrdquo (Comparison of Ucontr with
Usynsynchronous sawtooth signal) For further processing in the UN 0096 Intermediate
Pulse Stage the Gate Control Unit supplies six firing pulses at its output whose phase
position with respect to the synchronous voltage Usyn is in accordance with control
variable Ucontr An internal linearization ensures that the field voltage produced via the
firing pulses remains proportional to the control variable Ucontr throughout the entire
range As a result the circuit amplification of the control remains constant over the entire
range Whenever excitation is switched ON the set-point for Generator Voltage is set
automatically at the preset - ref Value This provision ensures that the generator voltage
always attains approximately its nominal value after the field flashing The Gate Control
Unit can be refunctioned ( by pre-selection with a switch ) for purposes of testing to act
as a purely firing pulse control In this case the control variable Ucontr is adjusted
directly using the RAISELOWER push buttons on the front of the module In this way
for example the relationship between the phase position of the firing pulses and the
control variable Ucontr can be checked easily
CHAPTER 3
PULSE SECTION
31 Pulse Generation and Amplification
The Gate Control Units of both AUTOMATIC channel and MANUAL channel
each supply six firing pulses for operating the 6-pulse thyristor bridges The low-power
pulse signals from these Gate Control Units are then amplified in the Intermediate Pulse
Stage galvanically isolated and then sent to the common pulse bus On the output end
the Intermediate Pulse Stage of the non-active channel) is always blocked The Gate
Control Units generate the pulses based on microprocessor control The reference voltage
used for the firing pulse phase location is the output voltage from the excitation
transformer (Usyn1 Usyn2) The commutation spikes of the synchronous voltage caused
by the converter are calculated prior to use of the voltage as a reference value and are
deliberately filtered out The lower limit for the firing pulses (double pulses) which are
offset from one another by 60deg is defined by the limit rectifier position (αmin) and the
upper limit by the limit inverter position (αmax) for the firing angle αmin and αmax can
be adjusted on the Gate Control Units using BCD (Binary Coded Decimal) switches
αmin ensures that the firing pulses will not be issued (premature firing) until there is
sufficient positive phase voltage on the thyristor involved αmax prevents a dangerous
ldquotippingrdquo of the thyristor bridge into the rectifier mode if the firing angle α is too large
(ldquolate firingrdquo) The critical factors determining αmax are the overlap time uumlmax (max
commutation time) and the ldquorecovery timerdquo of the thyristors (αmax lt 180deg - uumlmax - γ )
An external control signal can force the firing pulses into their inverter limit position
Other binary inputs can block or direct the firing pulses of the Gate Control Units so as to
produce freewheeling on the thyristor bridge During freewheeling the firing pulses for
the thyristor pair R and S are blocked and the pulse signals T+T- are engaged with
chains of pulses Both Gate Control Units (for the MANUAL amp AUTOMATIC channels)
contain a field current monitor that blocks the firing pulses immediately whenever the
current exceeds a preset threshold level In this case the field circuit-breaker is also
tripped via an output contact The purpose of these provisions is to prevent damage to
thyristors and thyristor fuses in case of a slip-ring short-circuit or to keep any damage
that does occur to a minimum The pulse signals are galvanically separated at the outputs
from the Intermediate Pulse Stage (with pulse transmitters) and are then directed to the
common pulse bus This transmission of the pulse signals to the pulse bus via passive
transmitters ensures a high degree of active channel autonomy Practically no possible
malfunctions on the inactive channel (including for example sustained pulses) affect the
active channel
32 Pulse Monitoring
The ldquoPulse Busrdquo and the pulse signals of the AUTOMATIC channel are
monitored This monitoring device consists of potential isolating stages and the common
monitor If the pulse monitoring of the ldquoPulse Busrdquo responds a switch-over is made to
MANUAL channel The function of the potential isolating stages is to couple the pulse
monitoring device to the pulse circuits without any feedback effect The pulse monitoring
checks the six pulse lines for the following malfunctions continuous or periodic failure
of one or more pulses Periodic occurrence of synchronous or asynchronous false pulses
Continuous pulses the pulse monitoring device can be tested while the machine is in
operation
CHAPTER 4
CONVERTER
Thyristor
The term thyristor usually refers to a family of four layer solid state device having
turn on characteristics that can be externally controlled by either current or voltage They
are also referred to as breakdown device because their working depends on avalanche
breakdown Thyristors have only two stages OFF and ON Thyristors have a similar
function to Uni-junctions they act as switches Thyristors use current flow as a switch
Thyristors have three states
1 Reverse blocking mode mdash Voltage is applied in the direction that would be
blocked by a diode
2 Forward blocking mode mdash Voltage is applied in the direction that would cause
a diode to conduct but the thyristor has not yet been triggered into conduction
3 Forward conducting mode mdash The thyristor has been triggered into conduction
and will remain conducting until the forward current drops below a threshold value
known as the holding current Converter is a semiconductor device which converts ac
input voltage into a constant dc output voltage In present excitation system three phase
fully controlled thyristor converter is used
Because of the following advantages thyristor converters are used
a) Thyristors are used for high power applications ie up to 10Kv3500A1KHz
b) Having high reliability and low losses
c) Uni-directional device like diode
d) Itrsquos operation as a rectifier which are low resistance in forward conduction
mode and high resistance in reverse conduction mode
PROTECTION OF THYRISTORS
For reliable operation of a thyristor demands that its specified ratings are not
exceeded When Subjected to or over voltages During the turn - on of SCR didt
prohibitively large False triggering of SCR by high value of dvdt andSpurious signals
between gate and cathode may leads to unwanted turn ndash on
DIDT AND PROTECTION
When thyristor starts conducting in forward conduction mode and is turned on by
gate pulse The anode current increases rapidly whole area of the gate to Cathode
junction then hot spots will be formed near the gate connection this locality of heating
destroys the thyristor Thyristor thermal time is constant The causes due to faults and
short circuits or surge currents Electronic crowbar protection is used against the over
voltages The rate rise of anode current must be kept at the time of turn on below the
rated or specified limiting value The didt value maintained below limited value by using
a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating
is avoided by applying gate current but not greater the maximum gate current
DVDT AND OVER VOLTAGE PROTECTION
With forward voltage across the anode and cathode of a thyristor the two outer
junctions are forward biased but the inner junction is reverse biased This reverse biased
junction J2 has the characteristics of a capacitor due to charges existing across the
junction In other words space-charges exist in the depletion region around junction J2
and therefore junction J2 behaves like a capacitance If the entire anode to cathode
forward voltage Va appears across J2 junction and the charge is denoted by Q then a
charging current i given by Eq (46) follows
i = dQdt =d(Cj Va )dt
= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)
As Cj the capacitance of junction J2 is almost constant the current is given by
i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)
If the rate of rise of forward voltage dVadt is high the charging current i will be
more This charging current plays the role of gate current and turns on the SCR even
when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on
must be avoided as it leads to false operation of the thyristor circuit
For controllable operation of the thyristor the rate of rise of forward anode to
cathode voltage dVadt must be kept below the specified rated limit Typical values of
dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by
using a snubber circuit in parallel with the device thyristor are very sensitive for over
voltage than the semiconductor devices
Over voltage transients are perhaps the main cause of thyristor failure
In thyristor there are mainly two types
1 Internal over voltages
Due to the commutation of the thyristors large voltages are generated internally
Because of the series inductance of the SCR circuit the large transient voltages L didt
produced This voltage several times the break over voltage of the device then thyristor
destroys permanently
2 External over voltages
External over voltages are caused due to the interruptions of current flow in an
inductive circuit and also due to the lightening strokes on the lines feeding the thyristor
system For the reliable operation of thyristor the over voltages must be suppressed by
adopting suitable techniques
Suppression of over voltages
The RC circuit called snubber circuit is connected across the device to protect In
order to keep the protective components to a minimum the thyristors are chosen with
their peak voltages ratings are 25 to 3 times of the normal peak working voltage
ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are
commonly employed for protecting the thyristor circuit against the over voltages
Gate protection
Gate circuit should also be protected against the over voltages and surges Over
voltage at gate circuit can cause false triggering of the SCR may rises the junction
temperature behind specified limit leading to its damage Protection against over
voltage can be achieved by connecting a ZD across the gate circuit and a resister is
connected in series with gate circuit to protect against the s A capacitor and resister are
connected across gate to cathode to by pass the noise
41 Final Pulse Stages
The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage
(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor
bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided
with a power supply module The amplified output pulses from the Final Pulse Stages
start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main
pulse This initial pulse edge assures proper firing of the thyristors being triggered
Subsequently the weaker part of main pulse keeps firing conditions steady As already
mentioned the Final Pulse Stages and their associated thyristor bridges form single units
All six pulse outputs from a Final Pulse Stage can be blocked by an external control
signal so that all thyristors in the associated thyristor bridge will block the current A
blocking of the pulses is initiated whenever there is a malfunction in the associated
thyristor bridge
42 Converter Power Section
The thyristor converter consists of three independent parallel rectifier blocks TY1
to TY3 which are all in service Even if one block fails the remaining blocks take over
automatically the full design current of the excitation circuit During normal operation
(with ideal current share) and all three bridges in operation each of these blocks has to
carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation
is limited Only when all three bridges fail the excitation is switched off Each thyristor
bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any
arm is identified by a Current flow monitoring module
43 Converter Cooling
A cooling system is needed to dissipate heat losses in the converter blocks and
electronics Each converter block has therefore been equipped with a fan supplied with
power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing
cubicle) The fans are protected with motor protection circuit breakers An air flow
monitoring unit is provided for monitoring the air flow through the thyristor bridge If a
circuit breaker failure is detected or if the air flow monitor drops off at one of the
thyristor bridges the bridge involved is immediately set out of operation by blocking its
firing pulses
44 Thyristor Converter Monitoring
A thyristor bridge in which defects occur that could threaten the safety of
operation or cause secondary damage is switched off automatically ie its firing pulses
are blocked This happens whenever A thyristor fuse is blown The fuses are monitored
individually with micro switches The Final Pulse Stage fails which is detected by
internal monitors (supply voltage sustained pulse short-circuit on the output end) The
power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or
is insufficient Isolator on ACDC side is open
CHAPTER 5
Field Current Circuit Excitation Transformer
51 Field Circuit Breaker
The circuit-breaker in the field circuit is used to isolate the field circuit from the
converter It is capable of switching off the synchronous machine from full load under the
maximum conditions of a 3-phase short-circuit In addition to its main contacts the field
circuit-breaker also has a de-excitation contact with which the field energy stored in the
field can be dissipated across the de-excitation resistor The de-excitation contact closes
shortly before the main contacts open so as to ensure proper commutation of the field
current from the main contacts to the de-excitation contact when the breaker is switched
off The field circuit-breaker is switched on by electromagnetic force and is kept switched
on by a mechanical latch When the latch is released by a trip coil the circuit-breaker
opens The circuit-breaker also has auxiliary contacts that report its status
52 Field Flashing
In shunt supplied excitation circuits (excitation transformer connected to the
generator terminals) the generator does not have enough remnant voltage for a generator
voltage build-up via the converter In this case a field flashing circuit is provided It
consists of the field flashing contactor the diode bridge and a transformer used to adapt
the auxiliary input voltage to the voltage needed for field flashing when power is
supplied from the auxiliaries network
Fig Field Flashing
Because the field flashing contactor is not able to switch off the energy stored in
the field the control ensures that the contactor can only reopen if the field circuit breaker
has already been opened (generating the TRIP order) or in a normal field flashing
sequence when the converter has taken over the field current Field flashing occurs in the
following stages
1048729The excitation is switched on closing the field flashing contactor ( Field
Circuit Breaker is already closed )
1048729The start-up excitation current flows through the rotor driving the generator
voltage up to approx 15 U
1048729After about 10 U the firing pulses to the converter are released and it begins
to excite the generator to its rated voltage
1048729After about 30 U the field flashing contactor opens (with no current since
the converter is now supplying the current)
The diode bridge at the input to the field flashing contactor prevents a feed-back
from the converter to the source of field flashing while the contactor is still closed
53 De-excitation
When malfunctions occur the stored field energy must be dissipated as quickly
and safely as possible to protect the generator This is done by the converter the field
circuit-breaker and the de-excitation (discharge) resistor
De-excitation (with opening of the field circuit-breaker) takes place in the following
stages
1048729The converter drives to its inverter limit position (negative ceiling voltage)
recovers a portion of the field energy into the network A trip command is given to the
field circuit breaker
1048729The de-excitation contact closes diverting the field voltage to the de-excitation
resistor
1048729Then immediately the main contacts open building voltage The field voltage
commutates to the de-excitation resistor
1048729The current diminishes at a given time constant TE
(With linear resistance TE = Lf (Rf + Re))
Due to the reversal of the field voltage by the converter the field current
commutates from the main contacts of the field circuit-breaker to the de-excitation
resistor in a very early phase This reversal of the field voltage prevents burn-off on the
main contacts and provides effective protection for the field circuit-breaker Depending
on the operating policy an operational shut-down of the excitation can also be effected
with the field circuit-breaker closed This method is useful mainly when the excitation is
switched on and off frequently In this case the converter is merely driven into the
inverter limit position so that the field energy is recovered into the network The
converter then blocks since it is supplying positive current only
54 Excitation Transformer
The excitation transformer matches the generator voltage to the field voltage
(required ceiling voltage) It also serves as a commutation reactance for the thyristor
converter and as a potential isolator between the network and the excitation circuit In
addition the transformer functions as a current limiter in that it makes it possible to keep
any short circuits in the excitation circuit under better control The excitation transformer
is equipped with temperature monitoring probes which set off an alarm when the
temperature exceeds a first max limit and then trips the excitation if the temperature
continues rising to a second (higher)limit
CHAPTER 6
Monitoring and Protection
61 Excitation Monitoring
611 General Information
The main goal of Excitation Monitoring is to make optimum use of the
redundancies provided in the excitation system and to give alarm whenever a malfunction
makes these redundancies unavailable The field current is monitored to see that it does
not exceed a maximum level and if necessary a switch-over to the MANUAL channel is
initiated In addition the criterion for switching off the field flashing is generated The
excitation Monitoring consists of an autonomous processor system
612 over current Alarms
In the Excitation Monitoring the limits for are set at higher levels than the
settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of
the nominal field current contact R1 and the binary output associated with it are
activated immediately If field current remains gt 110 then after a preset inverse-time
has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs
associated with them are activated Parameters match the measurements for If1 and If2 to
the nominal value for field current so that the internal values can be processed and read as
pu values It can be used to falsify the actual value of the field current If (to raise it) so
as to cause a response from the alarm limits for purposes of testing The processed If
signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As
long as the field current If is above the threshold value 11 Ifn its peak value is
measured This is stored (until RESET) and can be read at any time on the Micro-
Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever
the integrated time-current value (intisup2dt) exceeds the preselected reference value the
alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously
issued to switch over to the stand by AUTO channel Software switch F758 enables the
three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-
time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the
desired limit curve for response is set using the factor F216
613 Switch-Off Criterion for Field Flashing
The Excitation Monitoring supplies the criterion for switching off the field
flashing Whether this criterion is activated based on the actual value for generator voltage
Ug or for field current If or both depends on the settings of the two threshold values
F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements
Ug12 and If12 are switched over depending on the present status of the channels
(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is
fixed at ldquological 1rdquo
614 Storage of Alarm Status
The outputs of the over current alarms (R1 R2 R0) and the messages NO
FAILURE MONITORING PARAMETERS CHANGED are stored messages can be
erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the
front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the
situation causing the alarm or the malfunction is no longer present Whenever the self-
diagnosis equipment in the processor detects a malfunction the output NO FAILURE of
MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo
is activated whenever parameters or settings of software switches have been changed via
the Micro-Terminal
615 Actual Value Monitoring
The actual values for generator voltage Ug synchronous voltage Usyn and field
current If are monitored for malfunctions This monitoring is active regardless of whether
or not the generator is in operation Essentially when the generator is in operation the
measurements are monitored by comparing the signals (the smaller signal reading is
detected as incorrect) When the generator is not in operation the measured data are
monitored for extreme values The percentage of deviation permissible in the
measurement signals being compared
(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and
F20AIf the excitation transformer is being supplied from an auxiliary power source (no
shunt operation) the values of Ug and Usyn will be different in some operational
conditions
In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1
and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the
messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2
and the binary message from CH1 reports no malfunction a malfunction on
Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is
also present whenever the binary message CH1 DISTURBANCE is reported and a
discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2
FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares
Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the
comparison channel reports a malfunction or whenever both binary messages report no
malfunction - but both secondary monitors report a malfunction As long as the secondary
monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction
signals for the measurement channel involved (suspicion that there is a corresponding
error in Usyn) The measurement channel malfunctions are enabled operationally
whenever after excitation has been switched on generator voltage Ug exceeds the value
set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is
switched off to see that they do not exceed the limit value F210 that applies to both of
them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt
the operating range) Monitoring for extreme values is likewise enabled during normal
operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the
malfunction signals to the binary outputs Basically the monitoring of the actual values
for If1If2 functions like that of the Ug1Ug2 monitoring
62 Excitation Protection
621 General Information
The Excitation Protection switches off the excitation (and de-excites the machine
rapidly) whenever a danger arises that threatens the excitation transformer the converter
or the generator Generally limiter or monitoring functions precede the emergency trips
and these normally respond before the Excitation Protection must initiate a trip
Protective trip commands are issued directly to the field circuit-breaker from potential
free contacts of the board via the trip relays They are directed redundantly to the
operative field circuit-breaker ldquoOFFrdquo command
622 Protection against Excitation Transformer Overheating
This equipment monitors the excitation transformer for overheating in the
windings that could result from over current short-circuits or inadequate cooling The
monitoring uses temperature monitoring modules in conjunction with temperature
sensors built into the windings Normally the temperature is monitored in two stages the
first stage sets off an alarm the second causes a trip of the excitation
623 Rotor Over voltage Protection
Malfunctions in the generator circuit (eg terminal short-circuit failed
synchronization asynchronous operation) cause induced negative field currents that
produce high voltages in the field circuit These must be restricted to a level with a
sufficient safety margin below the insulation capacity of the field winding (test voltage)
and also below the peak blocking voltage of the converter thyristors The crow bar
employs spark gap elements to detect over voltages in the field circuit Whenever they
respond the associated thyristors are fired immediately switching the de-excitation
resistor parallel to the field The de-excitation current generated thereby initiates an
excitation trip via a supervision circuit causing an immediate opening of the field circuit-
breaker The malfunction isets off an alarm and an internal malfunction is indicated at the
cubicle
TEST VALUES OBTAINED WHEN EXCITATION IS RAISED
TEST
SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE
1 VREF 996 100
2 VACT 997 1003
3 IFACT 735 765
4 IGACT 703 707
5 ACTIVE
POWER
703 705
6 REACTIVE
POWER
101 142
7 POWER
FACTOR
099 IND 098 IND
8 ACTIVE
CURRENT(IR)
705 703
9 REACTIVE
CURRENT(IX)
102 138
10 POWER
ANGLE
566 546
11 FIRING
ANGLE
640 632
VALUES OBTAINED WHEN EXCITATION IS LOWERED
SNO PARTICULARS ACTUAL
VALUE
OBTAINED VALUE
1 VREF 100 997
2 VACT 100 997
3 IFACT 787 761
4 IGACT 839 837
5 ACTIVE
POWER
839 830
6 REACTIVE
POWER
155 110
7 POWER
FACTOR
098 IND 099 IND
8 ACTIVE
CURRENT(IR)
835 830
9 REACTIVE
CURRENT(IX)
140 94
10 POWER
ANGLE
613 DEG 631 DEG
11 FIRING
ANGLE
601 DEG 599 DEG
CONCLUSION
For generating the EMF in stator winding excitation is required to the rotor of a
generator There are two types of excitation
1 Static excitation system
2 Brushless excitation system
A certain disadvantage in brushless excitation system is the slow response time of
the field in case of fast load changes specified No slip-rings and brushes direct
measurements of the field parameters not possible
To avoid all loses static excitation is used Since it does not have any rotating
parts mechanical loses and windage loses This system has fast response and speed
control While preferring this excitation system there are no limitations for the
redundancy of Thyristor bridge circuits
Static excitation has fast field discharge by resistor and inverter operation direct
measurement of field quantity is possible The meaning of excitation is nothing but
continuous supply of DC current (ie field current) to the rotor to buildup required
output voltage in the stator
Field current is changed with respect to the change of load so the digital
automatic voltage regulator (DAVR) is used to regulate the output voltage according to
the load variations
So we conclude that static excitation system with DAVR is preferred since it is
having excellent dynamic performance and better options for R amp M
without additional limiters Its main function is to maintain the excitation of the generator
even if the AUTOMATIC channel becomes non-operational The MANUAL channel
also performs valuable service for purposes of testing commissioning and preventive
maintenance Its measurements regulator generation of firing pulses and power supply
are physically separate from those on the AUTOMATIC channel
242 Principle of Operation
All the functions of the MANUAL channel including the generation of firing
pulses have been implemented in a single electronic module the Gate Control Unit The
control variable Ucontr of voltage regulator is used as the reference value for generating
firing pulses on the principle known as ldquoramp controlrdquo (Comparison of Ucontr with
Usynsynchronous sawtooth signal) For further processing in the UN 0096 Intermediate
Pulse Stage the Gate Control Unit supplies six firing pulses at its output whose phase
position with respect to the synchronous voltage Usyn is in accordance with control
variable Ucontr An internal linearization ensures that the field voltage produced via the
firing pulses remains proportional to the control variable Ucontr throughout the entire
range As a result the circuit amplification of the control remains constant over the entire
range Whenever excitation is switched ON the set-point for Generator Voltage is set
automatically at the preset - ref Value This provision ensures that the generator voltage
always attains approximately its nominal value after the field flashing The Gate Control
Unit can be refunctioned ( by pre-selection with a switch ) for purposes of testing to act
as a purely firing pulse control In this case the control variable Ucontr is adjusted
directly using the RAISELOWER push buttons on the front of the module In this way
for example the relationship between the phase position of the firing pulses and the
control variable Ucontr can be checked easily
CHAPTER 3
PULSE SECTION
31 Pulse Generation and Amplification
The Gate Control Units of both AUTOMATIC channel and MANUAL channel
each supply six firing pulses for operating the 6-pulse thyristor bridges The low-power
pulse signals from these Gate Control Units are then amplified in the Intermediate Pulse
Stage galvanically isolated and then sent to the common pulse bus On the output end
the Intermediate Pulse Stage of the non-active channel) is always blocked The Gate
Control Units generate the pulses based on microprocessor control The reference voltage
used for the firing pulse phase location is the output voltage from the excitation
transformer (Usyn1 Usyn2) The commutation spikes of the synchronous voltage caused
by the converter are calculated prior to use of the voltage as a reference value and are
deliberately filtered out The lower limit for the firing pulses (double pulses) which are
offset from one another by 60deg is defined by the limit rectifier position (αmin) and the
upper limit by the limit inverter position (αmax) for the firing angle αmin and αmax can
be adjusted on the Gate Control Units using BCD (Binary Coded Decimal) switches
αmin ensures that the firing pulses will not be issued (premature firing) until there is
sufficient positive phase voltage on the thyristor involved αmax prevents a dangerous
ldquotippingrdquo of the thyristor bridge into the rectifier mode if the firing angle α is too large
(ldquolate firingrdquo) The critical factors determining αmax are the overlap time uumlmax (max
commutation time) and the ldquorecovery timerdquo of the thyristors (αmax lt 180deg - uumlmax - γ )
An external control signal can force the firing pulses into their inverter limit position
Other binary inputs can block or direct the firing pulses of the Gate Control Units so as to
produce freewheeling on the thyristor bridge During freewheeling the firing pulses for
the thyristor pair R and S are blocked and the pulse signals T+T- are engaged with
chains of pulses Both Gate Control Units (for the MANUAL amp AUTOMATIC channels)
contain a field current monitor that blocks the firing pulses immediately whenever the
current exceeds a preset threshold level In this case the field circuit-breaker is also
tripped via an output contact The purpose of these provisions is to prevent damage to
thyristors and thyristor fuses in case of a slip-ring short-circuit or to keep any damage
that does occur to a minimum The pulse signals are galvanically separated at the outputs
from the Intermediate Pulse Stage (with pulse transmitters) and are then directed to the
common pulse bus This transmission of the pulse signals to the pulse bus via passive
transmitters ensures a high degree of active channel autonomy Practically no possible
malfunctions on the inactive channel (including for example sustained pulses) affect the
active channel
32 Pulse Monitoring
The ldquoPulse Busrdquo and the pulse signals of the AUTOMATIC channel are
monitored This monitoring device consists of potential isolating stages and the common
monitor If the pulse monitoring of the ldquoPulse Busrdquo responds a switch-over is made to
MANUAL channel The function of the potential isolating stages is to couple the pulse
monitoring device to the pulse circuits without any feedback effect The pulse monitoring
checks the six pulse lines for the following malfunctions continuous or periodic failure
of one or more pulses Periodic occurrence of synchronous or asynchronous false pulses
Continuous pulses the pulse monitoring device can be tested while the machine is in
operation
CHAPTER 4
CONVERTER
Thyristor
The term thyristor usually refers to a family of four layer solid state device having
turn on characteristics that can be externally controlled by either current or voltage They
are also referred to as breakdown device because their working depends on avalanche
breakdown Thyristors have only two stages OFF and ON Thyristors have a similar
function to Uni-junctions they act as switches Thyristors use current flow as a switch
Thyristors have three states
1 Reverse blocking mode mdash Voltage is applied in the direction that would be
blocked by a diode
2 Forward blocking mode mdash Voltage is applied in the direction that would cause
a diode to conduct but the thyristor has not yet been triggered into conduction
3 Forward conducting mode mdash The thyristor has been triggered into conduction
and will remain conducting until the forward current drops below a threshold value
known as the holding current Converter is a semiconductor device which converts ac
input voltage into a constant dc output voltage In present excitation system three phase
fully controlled thyristor converter is used
Because of the following advantages thyristor converters are used
a) Thyristors are used for high power applications ie up to 10Kv3500A1KHz
b) Having high reliability and low losses
c) Uni-directional device like diode
d) Itrsquos operation as a rectifier which are low resistance in forward conduction
mode and high resistance in reverse conduction mode
PROTECTION OF THYRISTORS
For reliable operation of a thyristor demands that its specified ratings are not
exceeded When Subjected to or over voltages During the turn - on of SCR didt
prohibitively large False triggering of SCR by high value of dvdt andSpurious signals
between gate and cathode may leads to unwanted turn ndash on
DIDT AND PROTECTION
When thyristor starts conducting in forward conduction mode and is turned on by
gate pulse The anode current increases rapidly whole area of the gate to Cathode
junction then hot spots will be formed near the gate connection this locality of heating
destroys the thyristor Thyristor thermal time is constant The causes due to faults and
short circuits or surge currents Electronic crowbar protection is used against the over
voltages The rate rise of anode current must be kept at the time of turn on below the
rated or specified limiting value The didt value maintained below limited value by using
a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating
is avoided by applying gate current but not greater the maximum gate current
DVDT AND OVER VOLTAGE PROTECTION
With forward voltage across the anode and cathode of a thyristor the two outer
junctions are forward biased but the inner junction is reverse biased This reverse biased
junction J2 has the characteristics of a capacitor due to charges existing across the
junction In other words space-charges exist in the depletion region around junction J2
and therefore junction J2 behaves like a capacitance If the entire anode to cathode
forward voltage Va appears across J2 junction and the charge is denoted by Q then a
charging current i given by Eq (46) follows
i = dQdt =d(Cj Va )dt
= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)
As Cj the capacitance of junction J2 is almost constant the current is given by
i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)
If the rate of rise of forward voltage dVadt is high the charging current i will be
more This charging current plays the role of gate current and turns on the SCR even
when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on
must be avoided as it leads to false operation of the thyristor circuit
For controllable operation of the thyristor the rate of rise of forward anode to
cathode voltage dVadt must be kept below the specified rated limit Typical values of
dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by
using a snubber circuit in parallel with the device thyristor are very sensitive for over
voltage than the semiconductor devices
Over voltage transients are perhaps the main cause of thyristor failure
In thyristor there are mainly two types
1 Internal over voltages
Due to the commutation of the thyristors large voltages are generated internally
Because of the series inductance of the SCR circuit the large transient voltages L didt
produced This voltage several times the break over voltage of the device then thyristor
destroys permanently
2 External over voltages
External over voltages are caused due to the interruptions of current flow in an
inductive circuit and also due to the lightening strokes on the lines feeding the thyristor
system For the reliable operation of thyristor the over voltages must be suppressed by
adopting suitable techniques
Suppression of over voltages
The RC circuit called snubber circuit is connected across the device to protect In
order to keep the protective components to a minimum the thyristors are chosen with
their peak voltages ratings are 25 to 3 times of the normal peak working voltage
ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are
commonly employed for protecting the thyristor circuit against the over voltages
Gate protection
Gate circuit should also be protected against the over voltages and surges Over
voltage at gate circuit can cause false triggering of the SCR may rises the junction
temperature behind specified limit leading to its damage Protection against over
voltage can be achieved by connecting a ZD across the gate circuit and a resister is
connected in series with gate circuit to protect against the s A capacitor and resister are
connected across gate to cathode to by pass the noise
41 Final Pulse Stages
The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage
(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor
bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided
with a power supply module The amplified output pulses from the Final Pulse Stages
start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main
pulse This initial pulse edge assures proper firing of the thyristors being triggered
Subsequently the weaker part of main pulse keeps firing conditions steady As already
mentioned the Final Pulse Stages and their associated thyristor bridges form single units
All six pulse outputs from a Final Pulse Stage can be blocked by an external control
signal so that all thyristors in the associated thyristor bridge will block the current A
blocking of the pulses is initiated whenever there is a malfunction in the associated
thyristor bridge
42 Converter Power Section
The thyristor converter consists of three independent parallel rectifier blocks TY1
to TY3 which are all in service Even if one block fails the remaining blocks take over
automatically the full design current of the excitation circuit During normal operation
(with ideal current share) and all three bridges in operation each of these blocks has to
carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation
is limited Only when all three bridges fail the excitation is switched off Each thyristor
bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any
arm is identified by a Current flow monitoring module
43 Converter Cooling
A cooling system is needed to dissipate heat losses in the converter blocks and
electronics Each converter block has therefore been equipped with a fan supplied with
power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing
cubicle) The fans are protected with motor protection circuit breakers An air flow
monitoring unit is provided for monitoring the air flow through the thyristor bridge If a
circuit breaker failure is detected or if the air flow monitor drops off at one of the
thyristor bridges the bridge involved is immediately set out of operation by blocking its
firing pulses
44 Thyristor Converter Monitoring
A thyristor bridge in which defects occur that could threaten the safety of
operation or cause secondary damage is switched off automatically ie its firing pulses
are blocked This happens whenever A thyristor fuse is blown The fuses are monitored
individually with micro switches The Final Pulse Stage fails which is detected by
internal monitors (supply voltage sustained pulse short-circuit on the output end) The
power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or
is insufficient Isolator on ACDC side is open
CHAPTER 5
Field Current Circuit Excitation Transformer
51 Field Circuit Breaker
The circuit-breaker in the field circuit is used to isolate the field circuit from the
converter It is capable of switching off the synchronous machine from full load under the
maximum conditions of a 3-phase short-circuit In addition to its main contacts the field
circuit-breaker also has a de-excitation contact with which the field energy stored in the
field can be dissipated across the de-excitation resistor The de-excitation contact closes
shortly before the main contacts open so as to ensure proper commutation of the field
current from the main contacts to the de-excitation contact when the breaker is switched
off The field circuit-breaker is switched on by electromagnetic force and is kept switched
on by a mechanical latch When the latch is released by a trip coil the circuit-breaker
opens The circuit-breaker also has auxiliary contacts that report its status
52 Field Flashing
In shunt supplied excitation circuits (excitation transformer connected to the
generator terminals) the generator does not have enough remnant voltage for a generator
voltage build-up via the converter In this case a field flashing circuit is provided It
consists of the field flashing contactor the diode bridge and a transformer used to adapt
the auxiliary input voltage to the voltage needed for field flashing when power is
supplied from the auxiliaries network
Fig Field Flashing
Because the field flashing contactor is not able to switch off the energy stored in
the field the control ensures that the contactor can only reopen if the field circuit breaker
has already been opened (generating the TRIP order) or in a normal field flashing
sequence when the converter has taken over the field current Field flashing occurs in the
following stages
1048729The excitation is switched on closing the field flashing contactor ( Field
Circuit Breaker is already closed )
1048729The start-up excitation current flows through the rotor driving the generator
voltage up to approx 15 U
1048729After about 10 U the firing pulses to the converter are released and it begins
to excite the generator to its rated voltage
1048729After about 30 U the field flashing contactor opens (with no current since
the converter is now supplying the current)
The diode bridge at the input to the field flashing contactor prevents a feed-back
from the converter to the source of field flashing while the contactor is still closed
53 De-excitation
When malfunctions occur the stored field energy must be dissipated as quickly
and safely as possible to protect the generator This is done by the converter the field
circuit-breaker and the de-excitation (discharge) resistor
De-excitation (with opening of the field circuit-breaker) takes place in the following
stages
1048729The converter drives to its inverter limit position (negative ceiling voltage)
recovers a portion of the field energy into the network A trip command is given to the
field circuit breaker
1048729The de-excitation contact closes diverting the field voltage to the de-excitation
resistor
1048729Then immediately the main contacts open building voltage The field voltage
commutates to the de-excitation resistor
1048729The current diminishes at a given time constant TE
(With linear resistance TE = Lf (Rf + Re))
Due to the reversal of the field voltage by the converter the field current
commutates from the main contacts of the field circuit-breaker to the de-excitation
resistor in a very early phase This reversal of the field voltage prevents burn-off on the
main contacts and provides effective protection for the field circuit-breaker Depending
on the operating policy an operational shut-down of the excitation can also be effected
with the field circuit-breaker closed This method is useful mainly when the excitation is
switched on and off frequently In this case the converter is merely driven into the
inverter limit position so that the field energy is recovered into the network The
converter then blocks since it is supplying positive current only
54 Excitation Transformer
The excitation transformer matches the generator voltage to the field voltage
(required ceiling voltage) It also serves as a commutation reactance for the thyristor
converter and as a potential isolator between the network and the excitation circuit In
addition the transformer functions as a current limiter in that it makes it possible to keep
any short circuits in the excitation circuit under better control The excitation transformer
is equipped with temperature monitoring probes which set off an alarm when the
temperature exceeds a first max limit and then trips the excitation if the temperature
continues rising to a second (higher)limit
CHAPTER 6
Monitoring and Protection
61 Excitation Monitoring
611 General Information
The main goal of Excitation Monitoring is to make optimum use of the
redundancies provided in the excitation system and to give alarm whenever a malfunction
makes these redundancies unavailable The field current is monitored to see that it does
not exceed a maximum level and if necessary a switch-over to the MANUAL channel is
initiated In addition the criterion for switching off the field flashing is generated The
excitation Monitoring consists of an autonomous processor system
612 over current Alarms
In the Excitation Monitoring the limits for are set at higher levels than the
settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of
the nominal field current contact R1 and the binary output associated with it are
activated immediately If field current remains gt 110 then after a preset inverse-time
has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs
associated with them are activated Parameters match the measurements for If1 and If2 to
the nominal value for field current so that the internal values can be processed and read as
pu values It can be used to falsify the actual value of the field current If (to raise it) so
as to cause a response from the alarm limits for purposes of testing The processed If
signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As
long as the field current If is above the threshold value 11 Ifn its peak value is
measured This is stored (until RESET) and can be read at any time on the Micro-
Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever
the integrated time-current value (intisup2dt) exceeds the preselected reference value the
alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously
issued to switch over to the stand by AUTO channel Software switch F758 enables the
three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-
time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the
desired limit curve for response is set using the factor F216
613 Switch-Off Criterion for Field Flashing
The Excitation Monitoring supplies the criterion for switching off the field
flashing Whether this criterion is activated based on the actual value for generator voltage
Ug or for field current If or both depends on the settings of the two threshold values
F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements
Ug12 and If12 are switched over depending on the present status of the channels
(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is
fixed at ldquological 1rdquo
614 Storage of Alarm Status
The outputs of the over current alarms (R1 R2 R0) and the messages NO
FAILURE MONITORING PARAMETERS CHANGED are stored messages can be
erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the
front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the
situation causing the alarm or the malfunction is no longer present Whenever the self-
diagnosis equipment in the processor detects a malfunction the output NO FAILURE of
MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo
is activated whenever parameters or settings of software switches have been changed via
the Micro-Terminal
615 Actual Value Monitoring
The actual values for generator voltage Ug synchronous voltage Usyn and field
current If are monitored for malfunctions This monitoring is active regardless of whether
or not the generator is in operation Essentially when the generator is in operation the
measurements are monitored by comparing the signals (the smaller signal reading is
detected as incorrect) When the generator is not in operation the measured data are
monitored for extreme values The percentage of deviation permissible in the
measurement signals being compared
(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and
F20AIf the excitation transformer is being supplied from an auxiliary power source (no
shunt operation) the values of Ug and Usyn will be different in some operational
conditions
In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1
and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the
messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2
and the binary message from CH1 reports no malfunction a malfunction on
Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is
also present whenever the binary message CH1 DISTURBANCE is reported and a
discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2
FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares
Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the
comparison channel reports a malfunction or whenever both binary messages report no
malfunction - but both secondary monitors report a malfunction As long as the secondary
monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction
signals for the measurement channel involved (suspicion that there is a corresponding
error in Usyn) The measurement channel malfunctions are enabled operationally
whenever after excitation has been switched on generator voltage Ug exceeds the value
set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is
switched off to see that they do not exceed the limit value F210 that applies to both of
them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt
the operating range) Monitoring for extreme values is likewise enabled during normal
operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the
malfunction signals to the binary outputs Basically the monitoring of the actual values
for If1If2 functions like that of the Ug1Ug2 monitoring
62 Excitation Protection
621 General Information
The Excitation Protection switches off the excitation (and de-excites the machine
rapidly) whenever a danger arises that threatens the excitation transformer the converter
or the generator Generally limiter or monitoring functions precede the emergency trips
and these normally respond before the Excitation Protection must initiate a trip
Protective trip commands are issued directly to the field circuit-breaker from potential
free contacts of the board via the trip relays They are directed redundantly to the
operative field circuit-breaker ldquoOFFrdquo command
622 Protection against Excitation Transformer Overheating
This equipment monitors the excitation transformer for overheating in the
windings that could result from over current short-circuits or inadequate cooling The
monitoring uses temperature monitoring modules in conjunction with temperature
sensors built into the windings Normally the temperature is monitored in two stages the
first stage sets off an alarm the second causes a trip of the excitation
623 Rotor Over voltage Protection
Malfunctions in the generator circuit (eg terminal short-circuit failed
synchronization asynchronous operation) cause induced negative field currents that
produce high voltages in the field circuit These must be restricted to a level with a
sufficient safety margin below the insulation capacity of the field winding (test voltage)
and also below the peak blocking voltage of the converter thyristors The crow bar
employs spark gap elements to detect over voltages in the field circuit Whenever they
respond the associated thyristors are fired immediately switching the de-excitation
resistor parallel to the field The de-excitation current generated thereby initiates an
excitation trip via a supervision circuit causing an immediate opening of the field circuit-
breaker The malfunction isets off an alarm and an internal malfunction is indicated at the
cubicle
TEST VALUES OBTAINED WHEN EXCITATION IS RAISED
TEST
SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE
1 VREF 996 100
2 VACT 997 1003
3 IFACT 735 765
4 IGACT 703 707
5 ACTIVE
POWER
703 705
6 REACTIVE
POWER
101 142
7 POWER
FACTOR
099 IND 098 IND
8 ACTIVE
CURRENT(IR)
705 703
9 REACTIVE
CURRENT(IX)
102 138
10 POWER
ANGLE
566 546
11 FIRING
ANGLE
640 632
VALUES OBTAINED WHEN EXCITATION IS LOWERED
SNO PARTICULARS ACTUAL
VALUE
OBTAINED VALUE
1 VREF 100 997
2 VACT 100 997
3 IFACT 787 761
4 IGACT 839 837
5 ACTIVE
POWER
839 830
6 REACTIVE
POWER
155 110
7 POWER
FACTOR
098 IND 099 IND
8 ACTIVE
CURRENT(IR)
835 830
9 REACTIVE
CURRENT(IX)
140 94
10 POWER
ANGLE
613 DEG 631 DEG
11 FIRING
ANGLE
601 DEG 599 DEG
CONCLUSION
For generating the EMF in stator winding excitation is required to the rotor of a
generator There are two types of excitation
1 Static excitation system
2 Brushless excitation system
A certain disadvantage in brushless excitation system is the slow response time of
the field in case of fast load changes specified No slip-rings and brushes direct
measurements of the field parameters not possible
To avoid all loses static excitation is used Since it does not have any rotating
parts mechanical loses and windage loses This system has fast response and speed
control While preferring this excitation system there are no limitations for the
redundancy of Thyristor bridge circuits
Static excitation has fast field discharge by resistor and inverter operation direct
measurement of field quantity is possible The meaning of excitation is nothing but
continuous supply of DC current (ie field current) to the rotor to buildup required
output voltage in the stator
Field current is changed with respect to the change of load so the digital
automatic voltage regulator (DAVR) is used to regulate the output voltage according to
the load variations
So we conclude that static excitation system with DAVR is preferred since it is
having excellent dynamic performance and better options for R amp M
CHAPTER 3
PULSE SECTION
31 Pulse Generation and Amplification
The Gate Control Units of both AUTOMATIC channel and MANUAL channel
each supply six firing pulses for operating the 6-pulse thyristor bridges The low-power
pulse signals from these Gate Control Units are then amplified in the Intermediate Pulse
Stage galvanically isolated and then sent to the common pulse bus On the output end
the Intermediate Pulse Stage of the non-active channel) is always blocked The Gate
Control Units generate the pulses based on microprocessor control The reference voltage
used for the firing pulse phase location is the output voltage from the excitation
transformer (Usyn1 Usyn2) The commutation spikes of the synchronous voltage caused
by the converter are calculated prior to use of the voltage as a reference value and are
deliberately filtered out The lower limit for the firing pulses (double pulses) which are
offset from one another by 60deg is defined by the limit rectifier position (αmin) and the
upper limit by the limit inverter position (αmax) for the firing angle αmin and αmax can
be adjusted on the Gate Control Units using BCD (Binary Coded Decimal) switches
αmin ensures that the firing pulses will not be issued (premature firing) until there is
sufficient positive phase voltage on the thyristor involved αmax prevents a dangerous
ldquotippingrdquo of the thyristor bridge into the rectifier mode if the firing angle α is too large
(ldquolate firingrdquo) The critical factors determining αmax are the overlap time uumlmax (max
commutation time) and the ldquorecovery timerdquo of the thyristors (αmax lt 180deg - uumlmax - γ )
An external control signal can force the firing pulses into their inverter limit position
Other binary inputs can block or direct the firing pulses of the Gate Control Units so as to
produce freewheeling on the thyristor bridge During freewheeling the firing pulses for
the thyristor pair R and S are blocked and the pulse signals T+T- are engaged with
chains of pulses Both Gate Control Units (for the MANUAL amp AUTOMATIC channels)
contain a field current monitor that blocks the firing pulses immediately whenever the
current exceeds a preset threshold level In this case the field circuit-breaker is also
tripped via an output contact The purpose of these provisions is to prevent damage to
thyristors and thyristor fuses in case of a slip-ring short-circuit or to keep any damage
that does occur to a minimum The pulse signals are galvanically separated at the outputs
from the Intermediate Pulse Stage (with pulse transmitters) and are then directed to the
common pulse bus This transmission of the pulse signals to the pulse bus via passive
transmitters ensures a high degree of active channel autonomy Practically no possible
malfunctions on the inactive channel (including for example sustained pulses) affect the
active channel
32 Pulse Monitoring
The ldquoPulse Busrdquo and the pulse signals of the AUTOMATIC channel are
monitored This monitoring device consists of potential isolating stages and the common
monitor If the pulse monitoring of the ldquoPulse Busrdquo responds a switch-over is made to
MANUAL channel The function of the potential isolating stages is to couple the pulse
monitoring device to the pulse circuits without any feedback effect The pulse monitoring
checks the six pulse lines for the following malfunctions continuous or periodic failure
of one or more pulses Periodic occurrence of synchronous or asynchronous false pulses
Continuous pulses the pulse monitoring device can be tested while the machine is in
operation
CHAPTER 4
CONVERTER
Thyristor
The term thyristor usually refers to a family of four layer solid state device having
turn on characteristics that can be externally controlled by either current or voltage They
are also referred to as breakdown device because their working depends on avalanche
breakdown Thyristors have only two stages OFF and ON Thyristors have a similar
function to Uni-junctions they act as switches Thyristors use current flow as a switch
Thyristors have three states
1 Reverse blocking mode mdash Voltage is applied in the direction that would be
blocked by a diode
2 Forward blocking mode mdash Voltage is applied in the direction that would cause
a diode to conduct but the thyristor has not yet been triggered into conduction
3 Forward conducting mode mdash The thyristor has been triggered into conduction
and will remain conducting until the forward current drops below a threshold value
known as the holding current Converter is a semiconductor device which converts ac
input voltage into a constant dc output voltage In present excitation system three phase
fully controlled thyristor converter is used
Because of the following advantages thyristor converters are used
a) Thyristors are used for high power applications ie up to 10Kv3500A1KHz
b) Having high reliability and low losses
c) Uni-directional device like diode
d) Itrsquos operation as a rectifier which are low resistance in forward conduction
mode and high resistance in reverse conduction mode
PROTECTION OF THYRISTORS
For reliable operation of a thyristor demands that its specified ratings are not
exceeded When Subjected to or over voltages During the turn - on of SCR didt
prohibitively large False triggering of SCR by high value of dvdt andSpurious signals
between gate and cathode may leads to unwanted turn ndash on
DIDT AND PROTECTION
When thyristor starts conducting in forward conduction mode and is turned on by
gate pulse The anode current increases rapidly whole area of the gate to Cathode
junction then hot spots will be formed near the gate connection this locality of heating
destroys the thyristor Thyristor thermal time is constant The causes due to faults and
short circuits or surge currents Electronic crowbar protection is used against the over
voltages The rate rise of anode current must be kept at the time of turn on below the
rated or specified limiting value The didt value maintained below limited value by using
a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating
is avoided by applying gate current but not greater the maximum gate current
DVDT AND OVER VOLTAGE PROTECTION
With forward voltage across the anode and cathode of a thyristor the two outer
junctions are forward biased but the inner junction is reverse biased This reverse biased
junction J2 has the characteristics of a capacitor due to charges existing across the
junction In other words space-charges exist in the depletion region around junction J2
and therefore junction J2 behaves like a capacitance If the entire anode to cathode
forward voltage Va appears across J2 junction and the charge is denoted by Q then a
charging current i given by Eq (46) follows
i = dQdt =d(Cj Va )dt
= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)
As Cj the capacitance of junction J2 is almost constant the current is given by
i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)
If the rate of rise of forward voltage dVadt is high the charging current i will be
more This charging current plays the role of gate current and turns on the SCR even
when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on
must be avoided as it leads to false operation of the thyristor circuit
For controllable operation of the thyristor the rate of rise of forward anode to
cathode voltage dVadt must be kept below the specified rated limit Typical values of
dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by
using a snubber circuit in parallel with the device thyristor are very sensitive for over
voltage than the semiconductor devices
Over voltage transients are perhaps the main cause of thyristor failure
In thyristor there are mainly two types
1 Internal over voltages
Due to the commutation of the thyristors large voltages are generated internally
Because of the series inductance of the SCR circuit the large transient voltages L didt
produced This voltage several times the break over voltage of the device then thyristor
destroys permanently
2 External over voltages
External over voltages are caused due to the interruptions of current flow in an
inductive circuit and also due to the lightening strokes on the lines feeding the thyristor
system For the reliable operation of thyristor the over voltages must be suppressed by
adopting suitable techniques
Suppression of over voltages
The RC circuit called snubber circuit is connected across the device to protect In
order to keep the protective components to a minimum the thyristors are chosen with
their peak voltages ratings are 25 to 3 times of the normal peak working voltage
ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are
commonly employed for protecting the thyristor circuit against the over voltages
Gate protection
Gate circuit should also be protected against the over voltages and surges Over
voltage at gate circuit can cause false triggering of the SCR may rises the junction
temperature behind specified limit leading to its damage Protection against over
voltage can be achieved by connecting a ZD across the gate circuit and a resister is
connected in series with gate circuit to protect against the s A capacitor and resister are
connected across gate to cathode to by pass the noise
41 Final Pulse Stages
The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage
(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor
bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided
with a power supply module The amplified output pulses from the Final Pulse Stages
start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main
pulse This initial pulse edge assures proper firing of the thyristors being triggered
Subsequently the weaker part of main pulse keeps firing conditions steady As already
mentioned the Final Pulse Stages and their associated thyristor bridges form single units
All six pulse outputs from a Final Pulse Stage can be blocked by an external control
signal so that all thyristors in the associated thyristor bridge will block the current A
blocking of the pulses is initiated whenever there is a malfunction in the associated
thyristor bridge
42 Converter Power Section
The thyristor converter consists of three independent parallel rectifier blocks TY1
to TY3 which are all in service Even if one block fails the remaining blocks take over
automatically the full design current of the excitation circuit During normal operation
(with ideal current share) and all three bridges in operation each of these blocks has to
carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation
is limited Only when all three bridges fail the excitation is switched off Each thyristor
bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any
arm is identified by a Current flow monitoring module
43 Converter Cooling
A cooling system is needed to dissipate heat losses in the converter blocks and
electronics Each converter block has therefore been equipped with a fan supplied with
power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing
cubicle) The fans are protected with motor protection circuit breakers An air flow
monitoring unit is provided for monitoring the air flow through the thyristor bridge If a
circuit breaker failure is detected or if the air flow monitor drops off at one of the
thyristor bridges the bridge involved is immediately set out of operation by blocking its
firing pulses
44 Thyristor Converter Monitoring
A thyristor bridge in which defects occur that could threaten the safety of
operation or cause secondary damage is switched off automatically ie its firing pulses
are blocked This happens whenever A thyristor fuse is blown The fuses are monitored
individually with micro switches The Final Pulse Stage fails which is detected by
internal monitors (supply voltage sustained pulse short-circuit on the output end) The
power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or
is insufficient Isolator on ACDC side is open
CHAPTER 5
Field Current Circuit Excitation Transformer
51 Field Circuit Breaker
The circuit-breaker in the field circuit is used to isolate the field circuit from the
converter It is capable of switching off the synchronous machine from full load under the
maximum conditions of a 3-phase short-circuit In addition to its main contacts the field
circuit-breaker also has a de-excitation contact with which the field energy stored in the
field can be dissipated across the de-excitation resistor The de-excitation contact closes
shortly before the main contacts open so as to ensure proper commutation of the field
current from the main contacts to the de-excitation contact when the breaker is switched
off The field circuit-breaker is switched on by electromagnetic force and is kept switched
on by a mechanical latch When the latch is released by a trip coil the circuit-breaker
opens The circuit-breaker also has auxiliary contacts that report its status
52 Field Flashing
In shunt supplied excitation circuits (excitation transformer connected to the
generator terminals) the generator does not have enough remnant voltage for a generator
voltage build-up via the converter In this case a field flashing circuit is provided It
consists of the field flashing contactor the diode bridge and a transformer used to adapt
the auxiliary input voltage to the voltage needed for field flashing when power is
supplied from the auxiliaries network
Fig Field Flashing
Because the field flashing contactor is not able to switch off the energy stored in
the field the control ensures that the contactor can only reopen if the field circuit breaker
has already been opened (generating the TRIP order) or in a normal field flashing
sequence when the converter has taken over the field current Field flashing occurs in the
following stages
1048729The excitation is switched on closing the field flashing contactor ( Field
Circuit Breaker is already closed )
1048729The start-up excitation current flows through the rotor driving the generator
voltage up to approx 15 U
1048729After about 10 U the firing pulses to the converter are released and it begins
to excite the generator to its rated voltage
1048729After about 30 U the field flashing contactor opens (with no current since
the converter is now supplying the current)
The diode bridge at the input to the field flashing contactor prevents a feed-back
from the converter to the source of field flashing while the contactor is still closed
53 De-excitation
When malfunctions occur the stored field energy must be dissipated as quickly
and safely as possible to protect the generator This is done by the converter the field
circuit-breaker and the de-excitation (discharge) resistor
De-excitation (with opening of the field circuit-breaker) takes place in the following
stages
1048729The converter drives to its inverter limit position (negative ceiling voltage)
recovers a portion of the field energy into the network A trip command is given to the
field circuit breaker
1048729The de-excitation contact closes diverting the field voltage to the de-excitation
resistor
1048729Then immediately the main contacts open building voltage The field voltage
commutates to the de-excitation resistor
1048729The current diminishes at a given time constant TE
(With linear resistance TE = Lf (Rf + Re))
Due to the reversal of the field voltage by the converter the field current
commutates from the main contacts of the field circuit-breaker to the de-excitation
resistor in a very early phase This reversal of the field voltage prevents burn-off on the
main contacts and provides effective protection for the field circuit-breaker Depending
on the operating policy an operational shut-down of the excitation can also be effected
with the field circuit-breaker closed This method is useful mainly when the excitation is
switched on and off frequently In this case the converter is merely driven into the
inverter limit position so that the field energy is recovered into the network The
converter then blocks since it is supplying positive current only
54 Excitation Transformer
The excitation transformer matches the generator voltage to the field voltage
(required ceiling voltage) It also serves as a commutation reactance for the thyristor
converter and as a potential isolator between the network and the excitation circuit In
addition the transformer functions as a current limiter in that it makes it possible to keep
any short circuits in the excitation circuit under better control The excitation transformer
is equipped with temperature monitoring probes which set off an alarm when the
temperature exceeds a first max limit and then trips the excitation if the temperature
continues rising to a second (higher)limit
CHAPTER 6
Monitoring and Protection
61 Excitation Monitoring
611 General Information
The main goal of Excitation Monitoring is to make optimum use of the
redundancies provided in the excitation system and to give alarm whenever a malfunction
makes these redundancies unavailable The field current is monitored to see that it does
not exceed a maximum level and if necessary a switch-over to the MANUAL channel is
initiated In addition the criterion for switching off the field flashing is generated The
excitation Monitoring consists of an autonomous processor system
612 over current Alarms
In the Excitation Monitoring the limits for are set at higher levels than the
settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of
the nominal field current contact R1 and the binary output associated with it are
activated immediately If field current remains gt 110 then after a preset inverse-time
has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs
associated with them are activated Parameters match the measurements for If1 and If2 to
the nominal value for field current so that the internal values can be processed and read as
pu values It can be used to falsify the actual value of the field current If (to raise it) so
as to cause a response from the alarm limits for purposes of testing The processed If
signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As
long as the field current If is above the threshold value 11 Ifn its peak value is
measured This is stored (until RESET) and can be read at any time on the Micro-
Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever
the integrated time-current value (intisup2dt) exceeds the preselected reference value the
alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously
issued to switch over to the stand by AUTO channel Software switch F758 enables the
three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-
time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the
desired limit curve for response is set using the factor F216
613 Switch-Off Criterion for Field Flashing
The Excitation Monitoring supplies the criterion for switching off the field
flashing Whether this criterion is activated based on the actual value for generator voltage
Ug or for field current If or both depends on the settings of the two threshold values
F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements
Ug12 and If12 are switched over depending on the present status of the channels
(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is
fixed at ldquological 1rdquo
614 Storage of Alarm Status
The outputs of the over current alarms (R1 R2 R0) and the messages NO
FAILURE MONITORING PARAMETERS CHANGED are stored messages can be
erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the
front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the
situation causing the alarm or the malfunction is no longer present Whenever the self-
diagnosis equipment in the processor detects a malfunction the output NO FAILURE of
MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo
is activated whenever parameters or settings of software switches have been changed via
the Micro-Terminal
615 Actual Value Monitoring
The actual values for generator voltage Ug synchronous voltage Usyn and field
current If are monitored for malfunctions This monitoring is active regardless of whether
or not the generator is in operation Essentially when the generator is in operation the
measurements are monitored by comparing the signals (the smaller signal reading is
detected as incorrect) When the generator is not in operation the measured data are
monitored for extreme values The percentage of deviation permissible in the
measurement signals being compared
(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and
F20AIf the excitation transformer is being supplied from an auxiliary power source (no
shunt operation) the values of Ug and Usyn will be different in some operational
conditions
In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1
and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the
messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2
and the binary message from CH1 reports no malfunction a malfunction on
Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is
also present whenever the binary message CH1 DISTURBANCE is reported and a
discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2
FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares
Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the
comparison channel reports a malfunction or whenever both binary messages report no
malfunction - but both secondary monitors report a malfunction As long as the secondary
monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction
signals for the measurement channel involved (suspicion that there is a corresponding
error in Usyn) The measurement channel malfunctions are enabled operationally
whenever after excitation has been switched on generator voltage Ug exceeds the value
set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is
switched off to see that they do not exceed the limit value F210 that applies to both of
them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt
the operating range) Monitoring for extreme values is likewise enabled during normal
operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the
malfunction signals to the binary outputs Basically the monitoring of the actual values
for If1If2 functions like that of the Ug1Ug2 monitoring
62 Excitation Protection
621 General Information
The Excitation Protection switches off the excitation (and de-excites the machine
rapidly) whenever a danger arises that threatens the excitation transformer the converter
or the generator Generally limiter or monitoring functions precede the emergency trips
and these normally respond before the Excitation Protection must initiate a trip
Protective trip commands are issued directly to the field circuit-breaker from potential
free contacts of the board via the trip relays They are directed redundantly to the
operative field circuit-breaker ldquoOFFrdquo command
622 Protection against Excitation Transformer Overheating
This equipment monitors the excitation transformer for overheating in the
windings that could result from over current short-circuits or inadequate cooling The
monitoring uses temperature monitoring modules in conjunction with temperature
sensors built into the windings Normally the temperature is monitored in two stages the
first stage sets off an alarm the second causes a trip of the excitation
623 Rotor Over voltage Protection
Malfunctions in the generator circuit (eg terminal short-circuit failed
synchronization asynchronous operation) cause induced negative field currents that
produce high voltages in the field circuit These must be restricted to a level with a
sufficient safety margin below the insulation capacity of the field winding (test voltage)
and also below the peak blocking voltage of the converter thyristors The crow bar
employs spark gap elements to detect over voltages in the field circuit Whenever they
respond the associated thyristors are fired immediately switching the de-excitation
resistor parallel to the field The de-excitation current generated thereby initiates an
excitation trip via a supervision circuit causing an immediate opening of the field circuit-
breaker The malfunction isets off an alarm and an internal malfunction is indicated at the
cubicle
TEST VALUES OBTAINED WHEN EXCITATION IS RAISED
TEST
SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE
1 VREF 996 100
2 VACT 997 1003
3 IFACT 735 765
4 IGACT 703 707
5 ACTIVE
POWER
703 705
6 REACTIVE
POWER
101 142
7 POWER
FACTOR
099 IND 098 IND
8 ACTIVE
CURRENT(IR)
705 703
9 REACTIVE
CURRENT(IX)
102 138
10 POWER
ANGLE
566 546
11 FIRING
ANGLE
640 632
VALUES OBTAINED WHEN EXCITATION IS LOWERED
SNO PARTICULARS ACTUAL
VALUE
OBTAINED VALUE
1 VREF 100 997
2 VACT 100 997
3 IFACT 787 761
4 IGACT 839 837
5 ACTIVE
POWER
839 830
6 REACTIVE
POWER
155 110
7 POWER
FACTOR
098 IND 099 IND
8 ACTIVE
CURRENT(IR)
835 830
9 REACTIVE
CURRENT(IX)
140 94
10 POWER
ANGLE
613 DEG 631 DEG
11 FIRING
ANGLE
601 DEG 599 DEG
CONCLUSION
For generating the EMF in stator winding excitation is required to the rotor of a
generator There are two types of excitation
1 Static excitation system
2 Brushless excitation system
A certain disadvantage in brushless excitation system is the slow response time of
the field in case of fast load changes specified No slip-rings and brushes direct
measurements of the field parameters not possible
To avoid all loses static excitation is used Since it does not have any rotating
parts mechanical loses and windage loses This system has fast response and speed
control While preferring this excitation system there are no limitations for the
redundancy of Thyristor bridge circuits
Static excitation has fast field discharge by resistor and inverter operation direct
measurement of field quantity is possible The meaning of excitation is nothing but
continuous supply of DC current (ie field current) to the rotor to buildup required
output voltage in the stator
Field current is changed with respect to the change of load so the digital
automatic voltage regulator (DAVR) is used to regulate the output voltage according to
the load variations
So we conclude that static excitation system with DAVR is preferred since it is
having excellent dynamic performance and better options for R amp M
from the Intermediate Pulse Stage (with pulse transmitters) and are then directed to the
common pulse bus This transmission of the pulse signals to the pulse bus via passive
transmitters ensures a high degree of active channel autonomy Practically no possible
malfunctions on the inactive channel (including for example sustained pulses) affect the
active channel
32 Pulse Monitoring
The ldquoPulse Busrdquo and the pulse signals of the AUTOMATIC channel are
monitored This monitoring device consists of potential isolating stages and the common
monitor If the pulse monitoring of the ldquoPulse Busrdquo responds a switch-over is made to
MANUAL channel The function of the potential isolating stages is to couple the pulse
monitoring device to the pulse circuits without any feedback effect The pulse monitoring
checks the six pulse lines for the following malfunctions continuous or periodic failure
of one or more pulses Periodic occurrence of synchronous or asynchronous false pulses
Continuous pulses the pulse monitoring device can be tested while the machine is in
operation
CHAPTER 4
CONVERTER
Thyristor
The term thyristor usually refers to a family of four layer solid state device having
turn on characteristics that can be externally controlled by either current or voltage They
are also referred to as breakdown device because their working depends on avalanche
breakdown Thyristors have only two stages OFF and ON Thyristors have a similar
function to Uni-junctions they act as switches Thyristors use current flow as a switch
Thyristors have three states
1 Reverse blocking mode mdash Voltage is applied in the direction that would be
blocked by a diode
2 Forward blocking mode mdash Voltage is applied in the direction that would cause
a diode to conduct but the thyristor has not yet been triggered into conduction
3 Forward conducting mode mdash The thyristor has been triggered into conduction
and will remain conducting until the forward current drops below a threshold value
known as the holding current Converter is a semiconductor device which converts ac
input voltage into a constant dc output voltage In present excitation system three phase
fully controlled thyristor converter is used
Because of the following advantages thyristor converters are used
a) Thyristors are used for high power applications ie up to 10Kv3500A1KHz
b) Having high reliability and low losses
c) Uni-directional device like diode
d) Itrsquos operation as a rectifier which are low resistance in forward conduction
mode and high resistance in reverse conduction mode
PROTECTION OF THYRISTORS
For reliable operation of a thyristor demands that its specified ratings are not
exceeded When Subjected to or over voltages During the turn - on of SCR didt
prohibitively large False triggering of SCR by high value of dvdt andSpurious signals
between gate and cathode may leads to unwanted turn ndash on
DIDT AND PROTECTION
When thyristor starts conducting in forward conduction mode and is turned on by
gate pulse The anode current increases rapidly whole area of the gate to Cathode
junction then hot spots will be formed near the gate connection this locality of heating
destroys the thyristor Thyristor thermal time is constant The causes due to faults and
short circuits or surge currents Electronic crowbar protection is used against the over
voltages The rate rise of anode current must be kept at the time of turn on below the
rated or specified limiting value The didt value maintained below limited value by using
a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating
is avoided by applying gate current but not greater the maximum gate current
DVDT AND OVER VOLTAGE PROTECTION
With forward voltage across the anode and cathode of a thyristor the two outer
junctions are forward biased but the inner junction is reverse biased This reverse biased
junction J2 has the characteristics of a capacitor due to charges existing across the
junction In other words space-charges exist in the depletion region around junction J2
and therefore junction J2 behaves like a capacitance If the entire anode to cathode
forward voltage Va appears across J2 junction and the charge is denoted by Q then a
charging current i given by Eq (46) follows
i = dQdt =d(Cj Va )dt
= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)
As Cj the capacitance of junction J2 is almost constant the current is given by
i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)
If the rate of rise of forward voltage dVadt is high the charging current i will be
more This charging current plays the role of gate current and turns on the SCR even
when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on
must be avoided as it leads to false operation of the thyristor circuit
For controllable operation of the thyristor the rate of rise of forward anode to
cathode voltage dVadt must be kept below the specified rated limit Typical values of
dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by
using a snubber circuit in parallel with the device thyristor are very sensitive for over
voltage than the semiconductor devices
Over voltage transients are perhaps the main cause of thyristor failure
In thyristor there are mainly two types
1 Internal over voltages
Due to the commutation of the thyristors large voltages are generated internally
Because of the series inductance of the SCR circuit the large transient voltages L didt
produced This voltage several times the break over voltage of the device then thyristor
destroys permanently
2 External over voltages
External over voltages are caused due to the interruptions of current flow in an
inductive circuit and also due to the lightening strokes on the lines feeding the thyristor
system For the reliable operation of thyristor the over voltages must be suppressed by
adopting suitable techniques
Suppression of over voltages
The RC circuit called snubber circuit is connected across the device to protect In
order to keep the protective components to a minimum the thyristors are chosen with
their peak voltages ratings are 25 to 3 times of the normal peak working voltage
ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are
commonly employed for protecting the thyristor circuit against the over voltages
Gate protection
Gate circuit should also be protected against the over voltages and surges Over
voltage at gate circuit can cause false triggering of the SCR may rises the junction
temperature behind specified limit leading to its damage Protection against over
voltage can be achieved by connecting a ZD across the gate circuit and a resister is
connected in series with gate circuit to protect against the s A capacitor and resister are
connected across gate to cathode to by pass the noise
41 Final Pulse Stages
The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage
(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor
bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided
with a power supply module The amplified output pulses from the Final Pulse Stages
start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main
pulse This initial pulse edge assures proper firing of the thyristors being triggered
Subsequently the weaker part of main pulse keeps firing conditions steady As already
mentioned the Final Pulse Stages and their associated thyristor bridges form single units
All six pulse outputs from a Final Pulse Stage can be blocked by an external control
signal so that all thyristors in the associated thyristor bridge will block the current A
blocking of the pulses is initiated whenever there is a malfunction in the associated
thyristor bridge
42 Converter Power Section
The thyristor converter consists of three independent parallel rectifier blocks TY1
to TY3 which are all in service Even if one block fails the remaining blocks take over
automatically the full design current of the excitation circuit During normal operation
(with ideal current share) and all three bridges in operation each of these blocks has to
carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation
is limited Only when all three bridges fail the excitation is switched off Each thyristor
bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any
arm is identified by a Current flow monitoring module
43 Converter Cooling
A cooling system is needed to dissipate heat losses in the converter blocks and
electronics Each converter block has therefore been equipped with a fan supplied with
power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing
cubicle) The fans are protected with motor protection circuit breakers An air flow
monitoring unit is provided for monitoring the air flow through the thyristor bridge If a
circuit breaker failure is detected or if the air flow monitor drops off at one of the
thyristor bridges the bridge involved is immediately set out of operation by blocking its
firing pulses
44 Thyristor Converter Monitoring
A thyristor bridge in which defects occur that could threaten the safety of
operation or cause secondary damage is switched off automatically ie its firing pulses
are blocked This happens whenever A thyristor fuse is blown The fuses are monitored
individually with micro switches The Final Pulse Stage fails which is detected by
internal monitors (supply voltage sustained pulse short-circuit on the output end) The
power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or
is insufficient Isolator on ACDC side is open
CHAPTER 5
Field Current Circuit Excitation Transformer
51 Field Circuit Breaker
The circuit-breaker in the field circuit is used to isolate the field circuit from the
converter It is capable of switching off the synchronous machine from full load under the
maximum conditions of a 3-phase short-circuit In addition to its main contacts the field
circuit-breaker also has a de-excitation contact with which the field energy stored in the
field can be dissipated across the de-excitation resistor The de-excitation contact closes
shortly before the main contacts open so as to ensure proper commutation of the field
current from the main contacts to the de-excitation contact when the breaker is switched
off The field circuit-breaker is switched on by electromagnetic force and is kept switched
on by a mechanical latch When the latch is released by a trip coil the circuit-breaker
opens The circuit-breaker also has auxiliary contacts that report its status
52 Field Flashing
In shunt supplied excitation circuits (excitation transformer connected to the
generator terminals) the generator does not have enough remnant voltage for a generator
voltage build-up via the converter In this case a field flashing circuit is provided It
consists of the field flashing contactor the diode bridge and a transformer used to adapt
the auxiliary input voltage to the voltage needed for field flashing when power is
supplied from the auxiliaries network
Fig Field Flashing
Because the field flashing contactor is not able to switch off the energy stored in
the field the control ensures that the contactor can only reopen if the field circuit breaker
has already been opened (generating the TRIP order) or in a normal field flashing
sequence when the converter has taken over the field current Field flashing occurs in the
following stages
1048729The excitation is switched on closing the field flashing contactor ( Field
Circuit Breaker is already closed )
1048729The start-up excitation current flows through the rotor driving the generator
voltage up to approx 15 U
1048729After about 10 U the firing pulses to the converter are released and it begins
to excite the generator to its rated voltage
1048729After about 30 U the field flashing contactor opens (with no current since
the converter is now supplying the current)
The diode bridge at the input to the field flashing contactor prevents a feed-back
from the converter to the source of field flashing while the contactor is still closed
53 De-excitation
When malfunctions occur the stored field energy must be dissipated as quickly
and safely as possible to protect the generator This is done by the converter the field
circuit-breaker and the de-excitation (discharge) resistor
De-excitation (with opening of the field circuit-breaker) takes place in the following
stages
1048729The converter drives to its inverter limit position (negative ceiling voltage)
recovers a portion of the field energy into the network A trip command is given to the
field circuit breaker
1048729The de-excitation contact closes diverting the field voltage to the de-excitation
resistor
1048729Then immediately the main contacts open building voltage The field voltage
commutates to the de-excitation resistor
1048729The current diminishes at a given time constant TE
(With linear resistance TE = Lf (Rf + Re))
Due to the reversal of the field voltage by the converter the field current
commutates from the main contacts of the field circuit-breaker to the de-excitation
resistor in a very early phase This reversal of the field voltage prevents burn-off on the
main contacts and provides effective protection for the field circuit-breaker Depending
on the operating policy an operational shut-down of the excitation can also be effected
with the field circuit-breaker closed This method is useful mainly when the excitation is
switched on and off frequently In this case the converter is merely driven into the
inverter limit position so that the field energy is recovered into the network The
converter then blocks since it is supplying positive current only
54 Excitation Transformer
The excitation transformer matches the generator voltage to the field voltage
(required ceiling voltage) It also serves as a commutation reactance for the thyristor
converter and as a potential isolator between the network and the excitation circuit In
addition the transformer functions as a current limiter in that it makes it possible to keep
any short circuits in the excitation circuit under better control The excitation transformer
is equipped with temperature monitoring probes which set off an alarm when the
temperature exceeds a first max limit and then trips the excitation if the temperature
continues rising to a second (higher)limit
CHAPTER 6
Monitoring and Protection
61 Excitation Monitoring
611 General Information
The main goal of Excitation Monitoring is to make optimum use of the
redundancies provided in the excitation system and to give alarm whenever a malfunction
makes these redundancies unavailable The field current is monitored to see that it does
not exceed a maximum level and if necessary a switch-over to the MANUAL channel is
initiated In addition the criterion for switching off the field flashing is generated The
excitation Monitoring consists of an autonomous processor system
612 over current Alarms
In the Excitation Monitoring the limits for are set at higher levels than the
settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of
the nominal field current contact R1 and the binary output associated with it are
activated immediately If field current remains gt 110 then after a preset inverse-time
has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs
associated with them are activated Parameters match the measurements for If1 and If2 to
the nominal value for field current so that the internal values can be processed and read as
pu values It can be used to falsify the actual value of the field current If (to raise it) so
as to cause a response from the alarm limits for purposes of testing The processed If
signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As
long as the field current If is above the threshold value 11 Ifn its peak value is
measured This is stored (until RESET) and can be read at any time on the Micro-
Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever
the integrated time-current value (intisup2dt) exceeds the preselected reference value the
alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously
issued to switch over to the stand by AUTO channel Software switch F758 enables the
three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-
time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the
desired limit curve for response is set using the factor F216
613 Switch-Off Criterion for Field Flashing
The Excitation Monitoring supplies the criterion for switching off the field
flashing Whether this criterion is activated based on the actual value for generator voltage
Ug or for field current If or both depends on the settings of the two threshold values
F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements
Ug12 and If12 are switched over depending on the present status of the channels
(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is
fixed at ldquological 1rdquo
614 Storage of Alarm Status
The outputs of the over current alarms (R1 R2 R0) and the messages NO
FAILURE MONITORING PARAMETERS CHANGED are stored messages can be
erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the
front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the
situation causing the alarm or the malfunction is no longer present Whenever the self-
diagnosis equipment in the processor detects a malfunction the output NO FAILURE of
MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo
is activated whenever parameters or settings of software switches have been changed via
the Micro-Terminal
615 Actual Value Monitoring
The actual values for generator voltage Ug synchronous voltage Usyn and field
current If are monitored for malfunctions This monitoring is active regardless of whether
or not the generator is in operation Essentially when the generator is in operation the
measurements are monitored by comparing the signals (the smaller signal reading is
detected as incorrect) When the generator is not in operation the measured data are
monitored for extreme values The percentage of deviation permissible in the
measurement signals being compared
(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and
F20AIf the excitation transformer is being supplied from an auxiliary power source (no
shunt operation) the values of Ug and Usyn will be different in some operational
conditions
In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1
and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the
messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2
and the binary message from CH1 reports no malfunction a malfunction on
Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is
also present whenever the binary message CH1 DISTURBANCE is reported and a
discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2
FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares
Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the
comparison channel reports a malfunction or whenever both binary messages report no
malfunction - but both secondary monitors report a malfunction As long as the secondary
monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction
signals for the measurement channel involved (suspicion that there is a corresponding
error in Usyn) The measurement channel malfunctions are enabled operationally
whenever after excitation has been switched on generator voltage Ug exceeds the value
set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is
switched off to see that they do not exceed the limit value F210 that applies to both of
them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt
the operating range) Monitoring for extreme values is likewise enabled during normal
operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the
malfunction signals to the binary outputs Basically the monitoring of the actual values
for If1If2 functions like that of the Ug1Ug2 monitoring
62 Excitation Protection
621 General Information
The Excitation Protection switches off the excitation (and de-excites the machine
rapidly) whenever a danger arises that threatens the excitation transformer the converter
or the generator Generally limiter or monitoring functions precede the emergency trips
and these normally respond before the Excitation Protection must initiate a trip
Protective trip commands are issued directly to the field circuit-breaker from potential
free contacts of the board via the trip relays They are directed redundantly to the
operative field circuit-breaker ldquoOFFrdquo command
622 Protection against Excitation Transformer Overheating
This equipment monitors the excitation transformer for overheating in the
windings that could result from over current short-circuits or inadequate cooling The
monitoring uses temperature monitoring modules in conjunction with temperature
sensors built into the windings Normally the temperature is monitored in two stages the
first stage sets off an alarm the second causes a trip of the excitation
623 Rotor Over voltage Protection
Malfunctions in the generator circuit (eg terminal short-circuit failed
synchronization asynchronous operation) cause induced negative field currents that
produce high voltages in the field circuit These must be restricted to a level with a
sufficient safety margin below the insulation capacity of the field winding (test voltage)
and also below the peak blocking voltage of the converter thyristors The crow bar
employs spark gap elements to detect over voltages in the field circuit Whenever they
respond the associated thyristors are fired immediately switching the de-excitation
resistor parallel to the field The de-excitation current generated thereby initiates an
excitation trip via a supervision circuit causing an immediate opening of the field circuit-
breaker The malfunction isets off an alarm and an internal malfunction is indicated at the
cubicle
TEST VALUES OBTAINED WHEN EXCITATION IS RAISED
TEST
SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE
1 VREF 996 100
2 VACT 997 1003
3 IFACT 735 765
4 IGACT 703 707
5 ACTIVE
POWER
703 705
6 REACTIVE
POWER
101 142
7 POWER
FACTOR
099 IND 098 IND
8 ACTIVE
CURRENT(IR)
705 703
9 REACTIVE
CURRENT(IX)
102 138
10 POWER
ANGLE
566 546
11 FIRING
ANGLE
640 632
VALUES OBTAINED WHEN EXCITATION IS LOWERED
SNO PARTICULARS ACTUAL
VALUE
OBTAINED VALUE
1 VREF 100 997
2 VACT 100 997
3 IFACT 787 761
4 IGACT 839 837
5 ACTIVE
POWER
839 830
6 REACTIVE
POWER
155 110
7 POWER
FACTOR
098 IND 099 IND
8 ACTIVE
CURRENT(IR)
835 830
9 REACTIVE
CURRENT(IX)
140 94
10 POWER
ANGLE
613 DEG 631 DEG
11 FIRING
ANGLE
601 DEG 599 DEG
CONCLUSION
For generating the EMF in stator winding excitation is required to the rotor of a
generator There are two types of excitation
1 Static excitation system
2 Brushless excitation system
A certain disadvantage in brushless excitation system is the slow response time of
the field in case of fast load changes specified No slip-rings and brushes direct
measurements of the field parameters not possible
To avoid all loses static excitation is used Since it does not have any rotating
parts mechanical loses and windage loses This system has fast response and speed
control While preferring this excitation system there are no limitations for the
redundancy of Thyristor bridge circuits
Static excitation has fast field discharge by resistor and inverter operation direct
measurement of field quantity is possible The meaning of excitation is nothing but
continuous supply of DC current (ie field current) to the rotor to buildup required
output voltage in the stator
Field current is changed with respect to the change of load so the digital
automatic voltage regulator (DAVR) is used to regulate the output voltage according to
the load variations
So we conclude that static excitation system with DAVR is preferred since it is
having excellent dynamic performance and better options for R amp M
CHAPTER 4
CONVERTER
Thyristor
The term thyristor usually refers to a family of four layer solid state device having
turn on characteristics that can be externally controlled by either current or voltage They
are also referred to as breakdown device because their working depends on avalanche
breakdown Thyristors have only two stages OFF and ON Thyristors have a similar
function to Uni-junctions they act as switches Thyristors use current flow as a switch
Thyristors have three states
1 Reverse blocking mode mdash Voltage is applied in the direction that would be
blocked by a diode
2 Forward blocking mode mdash Voltage is applied in the direction that would cause
a diode to conduct but the thyristor has not yet been triggered into conduction
3 Forward conducting mode mdash The thyristor has been triggered into conduction
and will remain conducting until the forward current drops below a threshold value
known as the holding current Converter is a semiconductor device which converts ac
input voltage into a constant dc output voltage In present excitation system three phase
fully controlled thyristor converter is used
Because of the following advantages thyristor converters are used
a) Thyristors are used for high power applications ie up to 10Kv3500A1KHz
b) Having high reliability and low losses
c) Uni-directional device like diode
d) Itrsquos operation as a rectifier which are low resistance in forward conduction
mode and high resistance in reverse conduction mode
PROTECTION OF THYRISTORS
For reliable operation of a thyristor demands that its specified ratings are not
exceeded When Subjected to or over voltages During the turn - on of SCR didt
prohibitively large False triggering of SCR by high value of dvdt andSpurious signals
between gate and cathode may leads to unwanted turn ndash on
DIDT AND PROTECTION
When thyristor starts conducting in forward conduction mode and is turned on by
gate pulse The anode current increases rapidly whole area of the gate to Cathode
junction then hot spots will be formed near the gate connection this locality of heating
destroys the thyristor Thyristor thermal time is constant The causes due to faults and
short circuits or surge currents Electronic crowbar protection is used against the over
voltages The rate rise of anode current must be kept at the time of turn on below the
rated or specified limiting value The didt value maintained below limited value by using
a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating
is avoided by applying gate current but not greater the maximum gate current
DVDT AND OVER VOLTAGE PROTECTION
With forward voltage across the anode and cathode of a thyristor the two outer
junctions are forward biased but the inner junction is reverse biased This reverse biased
junction J2 has the characteristics of a capacitor due to charges existing across the
junction In other words space-charges exist in the depletion region around junction J2
and therefore junction J2 behaves like a capacitance If the entire anode to cathode
forward voltage Va appears across J2 junction and the charge is denoted by Q then a
charging current i given by Eq (46) follows
i = dQdt =d(Cj Va )dt
= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)
As Cj the capacitance of junction J2 is almost constant the current is given by
i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)
If the rate of rise of forward voltage dVadt is high the charging current i will be
more This charging current plays the role of gate current and turns on the SCR even
when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on
must be avoided as it leads to false operation of the thyristor circuit
For controllable operation of the thyristor the rate of rise of forward anode to
cathode voltage dVadt must be kept below the specified rated limit Typical values of
dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by
using a snubber circuit in parallel with the device thyristor are very sensitive for over
voltage than the semiconductor devices
Over voltage transients are perhaps the main cause of thyristor failure
In thyristor there are mainly two types
1 Internal over voltages
Due to the commutation of the thyristors large voltages are generated internally
Because of the series inductance of the SCR circuit the large transient voltages L didt
produced This voltage several times the break over voltage of the device then thyristor
destroys permanently
2 External over voltages
External over voltages are caused due to the interruptions of current flow in an
inductive circuit and also due to the lightening strokes on the lines feeding the thyristor
system For the reliable operation of thyristor the over voltages must be suppressed by
adopting suitable techniques
Suppression of over voltages
The RC circuit called snubber circuit is connected across the device to protect In
order to keep the protective components to a minimum the thyristors are chosen with
their peak voltages ratings are 25 to 3 times of the normal peak working voltage
ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are
commonly employed for protecting the thyristor circuit against the over voltages
Gate protection
Gate circuit should also be protected against the over voltages and surges Over
voltage at gate circuit can cause false triggering of the SCR may rises the junction
temperature behind specified limit leading to its damage Protection against over
voltage can be achieved by connecting a ZD across the gate circuit and a resister is
connected in series with gate circuit to protect against the s A capacitor and resister are
connected across gate to cathode to by pass the noise
41 Final Pulse Stages
The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage
(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor
bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided
with a power supply module The amplified output pulses from the Final Pulse Stages
start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main
pulse This initial pulse edge assures proper firing of the thyristors being triggered
Subsequently the weaker part of main pulse keeps firing conditions steady As already
mentioned the Final Pulse Stages and their associated thyristor bridges form single units
All six pulse outputs from a Final Pulse Stage can be blocked by an external control
signal so that all thyristors in the associated thyristor bridge will block the current A
blocking of the pulses is initiated whenever there is a malfunction in the associated
thyristor bridge
42 Converter Power Section
The thyristor converter consists of three independent parallel rectifier blocks TY1
to TY3 which are all in service Even if one block fails the remaining blocks take over
automatically the full design current of the excitation circuit During normal operation
(with ideal current share) and all three bridges in operation each of these blocks has to
carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation
is limited Only when all three bridges fail the excitation is switched off Each thyristor
bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any
arm is identified by a Current flow monitoring module
43 Converter Cooling
A cooling system is needed to dissipate heat losses in the converter blocks and
electronics Each converter block has therefore been equipped with a fan supplied with
power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing
cubicle) The fans are protected with motor protection circuit breakers An air flow
monitoring unit is provided for monitoring the air flow through the thyristor bridge If a
circuit breaker failure is detected or if the air flow monitor drops off at one of the
thyristor bridges the bridge involved is immediately set out of operation by blocking its
firing pulses
44 Thyristor Converter Monitoring
A thyristor bridge in which defects occur that could threaten the safety of
operation or cause secondary damage is switched off automatically ie its firing pulses
are blocked This happens whenever A thyristor fuse is blown The fuses are monitored
individually with micro switches The Final Pulse Stage fails which is detected by
internal monitors (supply voltage sustained pulse short-circuit on the output end) The
power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or
is insufficient Isolator on ACDC side is open
CHAPTER 5
Field Current Circuit Excitation Transformer
51 Field Circuit Breaker
The circuit-breaker in the field circuit is used to isolate the field circuit from the
converter It is capable of switching off the synchronous machine from full load under the
maximum conditions of a 3-phase short-circuit In addition to its main contacts the field
circuit-breaker also has a de-excitation contact with which the field energy stored in the
field can be dissipated across the de-excitation resistor The de-excitation contact closes
shortly before the main contacts open so as to ensure proper commutation of the field
current from the main contacts to the de-excitation contact when the breaker is switched
off The field circuit-breaker is switched on by electromagnetic force and is kept switched
on by a mechanical latch When the latch is released by a trip coil the circuit-breaker
opens The circuit-breaker also has auxiliary contacts that report its status
52 Field Flashing
In shunt supplied excitation circuits (excitation transformer connected to the
generator terminals) the generator does not have enough remnant voltage for a generator
voltage build-up via the converter In this case a field flashing circuit is provided It
consists of the field flashing contactor the diode bridge and a transformer used to adapt
the auxiliary input voltage to the voltage needed for field flashing when power is
supplied from the auxiliaries network
Fig Field Flashing
Because the field flashing contactor is not able to switch off the energy stored in
the field the control ensures that the contactor can only reopen if the field circuit breaker
has already been opened (generating the TRIP order) or in a normal field flashing
sequence when the converter has taken over the field current Field flashing occurs in the
following stages
1048729The excitation is switched on closing the field flashing contactor ( Field
Circuit Breaker is already closed )
1048729The start-up excitation current flows through the rotor driving the generator
voltage up to approx 15 U
1048729After about 10 U the firing pulses to the converter are released and it begins
to excite the generator to its rated voltage
1048729After about 30 U the field flashing contactor opens (with no current since
the converter is now supplying the current)
The diode bridge at the input to the field flashing contactor prevents a feed-back
from the converter to the source of field flashing while the contactor is still closed
53 De-excitation
When malfunctions occur the stored field energy must be dissipated as quickly
and safely as possible to protect the generator This is done by the converter the field
circuit-breaker and the de-excitation (discharge) resistor
De-excitation (with opening of the field circuit-breaker) takes place in the following
stages
1048729The converter drives to its inverter limit position (negative ceiling voltage)
recovers a portion of the field energy into the network A trip command is given to the
field circuit breaker
1048729The de-excitation contact closes diverting the field voltage to the de-excitation
resistor
1048729Then immediately the main contacts open building voltage The field voltage
commutates to the de-excitation resistor
1048729The current diminishes at a given time constant TE
(With linear resistance TE = Lf (Rf + Re))
Due to the reversal of the field voltage by the converter the field current
commutates from the main contacts of the field circuit-breaker to the de-excitation
resistor in a very early phase This reversal of the field voltage prevents burn-off on the
main contacts and provides effective protection for the field circuit-breaker Depending
on the operating policy an operational shut-down of the excitation can also be effected
with the field circuit-breaker closed This method is useful mainly when the excitation is
switched on and off frequently In this case the converter is merely driven into the
inverter limit position so that the field energy is recovered into the network The
converter then blocks since it is supplying positive current only
54 Excitation Transformer
The excitation transformer matches the generator voltage to the field voltage
(required ceiling voltage) It also serves as a commutation reactance for the thyristor
converter and as a potential isolator between the network and the excitation circuit In
addition the transformer functions as a current limiter in that it makes it possible to keep
any short circuits in the excitation circuit under better control The excitation transformer
is equipped with temperature monitoring probes which set off an alarm when the
temperature exceeds a first max limit and then trips the excitation if the temperature
continues rising to a second (higher)limit
CHAPTER 6
Monitoring and Protection
61 Excitation Monitoring
611 General Information
The main goal of Excitation Monitoring is to make optimum use of the
redundancies provided in the excitation system and to give alarm whenever a malfunction
makes these redundancies unavailable The field current is monitored to see that it does
not exceed a maximum level and if necessary a switch-over to the MANUAL channel is
initiated In addition the criterion for switching off the field flashing is generated The
excitation Monitoring consists of an autonomous processor system
612 over current Alarms
In the Excitation Monitoring the limits for are set at higher levels than the
settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of
the nominal field current contact R1 and the binary output associated with it are
activated immediately If field current remains gt 110 then after a preset inverse-time
has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs
associated with them are activated Parameters match the measurements for If1 and If2 to
the nominal value for field current so that the internal values can be processed and read as
pu values It can be used to falsify the actual value of the field current If (to raise it) so
as to cause a response from the alarm limits for purposes of testing The processed If
signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As
long as the field current If is above the threshold value 11 Ifn its peak value is
measured This is stored (until RESET) and can be read at any time on the Micro-
Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever
the integrated time-current value (intisup2dt) exceeds the preselected reference value the
alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously
issued to switch over to the stand by AUTO channel Software switch F758 enables the
three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-
time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the
desired limit curve for response is set using the factor F216
613 Switch-Off Criterion for Field Flashing
The Excitation Monitoring supplies the criterion for switching off the field
flashing Whether this criterion is activated based on the actual value for generator voltage
Ug or for field current If or both depends on the settings of the two threshold values
F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements
Ug12 and If12 are switched over depending on the present status of the channels
(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is
fixed at ldquological 1rdquo
614 Storage of Alarm Status
The outputs of the over current alarms (R1 R2 R0) and the messages NO
FAILURE MONITORING PARAMETERS CHANGED are stored messages can be
erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the
front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the
situation causing the alarm or the malfunction is no longer present Whenever the self-
diagnosis equipment in the processor detects a malfunction the output NO FAILURE of
MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo
is activated whenever parameters or settings of software switches have been changed via
the Micro-Terminal
615 Actual Value Monitoring
The actual values for generator voltage Ug synchronous voltage Usyn and field
current If are monitored for malfunctions This monitoring is active regardless of whether
or not the generator is in operation Essentially when the generator is in operation the
measurements are monitored by comparing the signals (the smaller signal reading is
detected as incorrect) When the generator is not in operation the measured data are
monitored for extreme values The percentage of deviation permissible in the
measurement signals being compared
(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and
F20AIf the excitation transformer is being supplied from an auxiliary power source (no
shunt operation) the values of Ug and Usyn will be different in some operational
conditions
In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1
and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the
messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2
and the binary message from CH1 reports no malfunction a malfunction on
Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is
also present whenever the binary message CH1 DISTURBANCE is reported and a
discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2
FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares
Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the
comparison channel reports a malfunction or whenever both binary messages report no
malfunction - but both secondary monitors report a malfunction As long as the secondary
monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction
signals for the measurement channel involved (suspicion that there is a corresponding
error in Usyn) The measurement channel malfunctions are enabled operationally
whenever after excitation has been switched on generator voltage Ug exceeds the value
set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is
switched off to see that they do not exceed the limit value F210 that applies to both of
them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt
the operating range) Monitoring for extreme values is likewise enabled during normal
operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the
malfunction signals to the binary outputs Basically the monitoring of the actual values
for If1If2 functions like that of the Ug1Ug2 monitoring
62 Excitation Protection
621 General Information
The Excitation Protection switches off the excitation (and de-excites the machine
rapidly) whenever a danger arises that threatens the excitation transformer the converter
or the generator Generally limiter or monitoring functions precede the emergency trips
and these normally respond before the Excitation Protection must initiate a trip
Protective trip commands are issued directly to the field circuit-breaker from potential
free contacts of the board via the trip relays They are directed redundantly to the
operative field circuit-breaker ldquoOFFrdquo command
622 Protection against Excitation Transformer Overheating
This equipment monitors the excitation transformer for overheating in the
windings that could result from over current short-circuits or inadequate cooling The
monitoring uses temperature monitoring modules in conjunction with temperature
sensors built into the windings Normally the temperature is monitored in two stages the
first stage sets off an alarm the second causes a trip of the excitation
623 Rotor Over voltage Protection
Malfunctions in the generator circuit (eg terminal short-circuit failed
synchronization asynchronous operation) cause induced negative field currents that
produce high voltages in the field circuit These must be restricted to a level with a
sufficient safety margin below the insulation capacity of the field winding (test voltage)
and also below the peak blocking voltage of the converter thyristors The crow bar
employs spark gap elements to detect over voltages in the field circuit Whenever they
respond the associated thyristors are fired immediately switching the de-excitation
resistor parallel to the field The de-excitation current generated thereby initiates an
excitation trip via a supervision circuit causing an immediate opening of the field circuit-
breaker The malfunction isets off an alarm and an internal malfunction is indicated at the
cubicle
TEST VALUES OBTAINED WHEN EXCITATION IS RAISED
TEST
SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE
1 VREF 996 100
2 VACT 997 1003
3 IFACT 735 765
4 IGACT 703 707
5 ACTIVE
POWER
703 705
6 REACTIVE
POWER
101 142
7 POWER
FACTOR
099 IND 098 IND
8 ACTIVE
CURRENT(IR)
705 703
9 REACTIVE
CURRENT(IX)
102 138
10 POWER
ANGLE
566 546
11 FIRING
ANGLE
640 632
VALUES OBTAINED WHEN EXCITATION IS LOWERED
SNO PARTICULARS ACTUAL
VALUE
OBTAINED VALUE
1 VREF 100 997
2 VACT 100 997
3 IFACT 787 761
4 IGACT 839 837
5 ACTIVE
POWER
839 830
6 REACTIVE
POWER
155 110
7 POWER
FACTOR
098 IND 099 IND
8 ACTIVE
CURRENT(IR)
835 830
9 REACTIVE
CURRENT(IX)
140 94
10 POWER
ANGLE
613 DEG 631 DEG
11 FIRING
ANGLE
601 DEG 599 DEG
CONCLUSION
For generating the EMF in stator winding excitation is required to the rotor of a
generator There are two types of excitation
1 Static excitation system
2 Brushless excitation system
A certain disadvantage in brushless excitation system is the slow response time of
the field in case of fast load changes specified No slip-rings and brushes direct
measurements of the field parameters not possible
To avoid all loses static excitation is used Since it does not have any rotating
parts mechanical loses and windage loses This system has fast response and speed
control While preferring this excitation system there are no limitations for the
redundancy of Thyristor bridge circuits
Static excitation has fast field discharge by resistor and inverter operation direct
measurement of field quantity is possible The meaning of excitation is nothing but
continuous supply of DC current (ie field current) to the rotor to buildup required
output voltage in the stator
Field current is changed with respect to the change of load so the digital
automatic voltage regulator (DAVR) is used to regulate the output voltage according to
the load variations
So we conclude that static excitation system with DAVR is preferred since it is
having excellent dynamic performance and better options for R amp M
Because of the following advantages thyristor converters are used
a) Thyristors are used for high power applications ie up to 10Kv3500A1KHz
b) Having high reliability and low losses
c) Uni-directional device like diode
d) Itrsquos operation as a rectifier which are low resistance in forward conduction
mode and high resistance in reverse conduction mode
PROTECTION OF THYRISTORS
For reliable operation of a thyristor demands that its specified ratings are not
exceeded When Subjected to or over voltages During the turn - on of SCR didt
prohibitively large False triggering of SCR by high value of dvdt andSpurious signals
between gate and cathode may leads to unwanted turn ndash on
DIDT AND PROTECTION
When thyristor starts conducting in forward conduction mode and is turned on by
gate pulse The anode current increases rapidly whole area of the gate to Cathode
junction then hot spots will be formed near the gate connection this locality of heating
destroys the thyristor Thyristor thermal time is constant The causes due to faults and
short circuits or surge currents Electronic crowbar protection is used against the over
voltages The rate rise of anode current must be kept at the time of turn on below the
rated or specified limiting value The didt value maintained below limited value by using
a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating
is avoided by applying gate current but not greater the maximum gate current
DVDT AND OVER VOLTAGE PROTECTION
With forward voltage across the anode and cathode of a thyristor the two outer
junctions are forward biased but the inner junction is reverse biased This reverse biased
junction J2 has the characteristics of a capacitor due to charges existing across the
junction In other words space-charges exist in the depletion region around junction J2
and therefore junction J2 behaves like a capacitance If the entire anode to cathode
forward voltage Va appears across J2 junction and the charge is denoted by Q then a
charging current i given by Eq (46) follows
i = dQdt =d(Cj Va )dt
= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)
As Cj the capacitance of junction J2 is almost constant the current is given by
i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)
If the rate of rise of forward voltage dVadt is high the charging current i will be
more This charging current plays the role of gate current and turns on the SCR even
when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on
must be avoided as it leads to false operation of the thyristor circuit
For controllable operation of the thyristor the rate of rise of forward anode to
cathode voltage dVadt must be kept below the specified rated limit Typical values of
dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by
using a snubber circuit in parallel with the device thyristor are very sensitive for over
voltage than the semiconductor devices
Over voltage transients are perhaps the main cause of thyristor failure
In thyristor there are mainly two types
1 Internal over voltages
Due to the commutation of the thyristors large voltages are generated internally
Because of the series inductance of the SCR circuit the large transient voltages L didt
produced This voltage several times the break over voltage of the device then thyristor
destroys permanently
2 External over voltages
External over voltages are caused due to the interruptions of current flow in an
inductive circuit and also due to the lightening strokes on the lines feeding the thyristor
system For the reliable operation of thyristor the over voltages must be suppressed by
adopting suitable techniques
Suppression of over voltages
The RC circuit called snubber circuit is connected across the device to protect In
order to keep the protective components to a minimum the thyristors are chosen with
their peak voltages ratings are 25 to 3 times of the normal peak working voltage
ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are
commonly employed for protecting the thyristor circuit against the over voltages
Gate protection
Gate circuit should also be protected against the over voltages and surges Over
voltage at gate circuit can cause false triggering of the SCR may rises the junction
temperature behind specified limit leading to its damage Protection against over
voltage can be achieved by connecting a ZD across the gate circuit and a resister is
connected in series with gate circuit to protect against the s A capacitor and resister are
connected across gate to cathode to by pass the noise
41 Final Pulse Stages
The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage
(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor
bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided
with a power supply module The amplified output pulses from the Final Pulse Stages
start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main
pulse This initial pulse edge assures proper firing of the thyristors being triggered
Subsequently the weaker part of main pulse keeps firing conditions steady As already
mentioned the Final Pulse Stages and their associated thyristor bridges form single units
All six pulse outputs from a Final Pulse Stage can be blocked by an external control
signal so that all thyristors in the associated thyristor bridge will block the current A
blocking of the pulses is initiated whenever there is a malfunction in the associated
thyristor bridge
42 Converter Power Section
The thyristor converter consists of three independent parallel rectifier blocks TY1
to TY3 which are all in service Even if one block fails the remaining blocks take over
automatically the full design current of the excitation circuit During normal operation
(with ideal current share) and all three bridges in operation each of these blocks has to
carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation
is limited Only when all three bridges fail the excitation is switched off Each thyristor
bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any
arm is identified by a Current flow monitoring module
43 Converter Cooling
A cooling system is needed to dissipate heat losses in the converter blocks and
electronics Each converter block has therefore been equipped with a fan supplied with
power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing
cubicle) The fans are protected with motor protection circuit breakers An air flow
monitoring unit is provided for monitoring the air flow through the thyristor bridge If a
circuit breaker failure is detected or if the air flow monitor drops off at one of the
thyristor bridges the bridge involved is immediately set out of operation by blocking its
firing pulses
44 Thyristor Converter Monitoring
A thyristor bridge in which defects occur that could threaten the safety of
operation or cause secondary damage is switched off automatically ie its firing pulses
are blocked This happens whenever A thyristor fuse is blown The fuses are monitored
individually with micro switches The Final Pulse Stage fails which is detected by
internal monitors (supply voltage sustained pulse short-circuit on the output end) The
power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or
is insufficient Isolator on ACDC side is open
CHAPTER 5
Field Current Circuit Excitation Transformer
51 Field Circuit Breaker
The circuit-breaker in the field circuit is used to isolate the field circuit from the
converter It is capable of switching off the synchronous machine from full load under the
maximum conditions of a 3-phase short-circuit In addition to its main contacts the field
circuit-breaker also has a de-excitation contact with which the field energy stored in the
field can be dissipated across the de-excitation resistor The de-excitation contact closes
shortly before the main contacts open so as to ensure proper commutation of the field
current from the main contacts to the de-excitation contact when the breaker is switched
off The field circuit-breaker is switched on by electromagnetic force and is kept switched
on by a mechanical latch When the latch is released by a trip coil the circuit-breaker
opens The circuit-breaker also has auxiliary contacts that report its status
52 Field Flashing
In shunt supplied excitation circuits (excitation transformer connected to the
generator terminals) the generator does not have enough remnant voltage for a generator
voltage build-up via the converter In this case a field flashing circuit is provided It
consists of the field flashing contactor the diode bridge and a transformer used to adapt
the auxiliary input voltage to the voltage needed for field flashing when power is
supplied from the auxiliaries network
Fig Field Flashing
Because the field flashing contactor is not able to switch off the energy stored in
the field the control ensures that the contactor can only reopen if the field circuit breaker
has already been opened (generating the TRIP order) or in a normal field flashing
sequence when the converter has taken over the field current Field flashing occurs in the
following stages
1048729The excitation is switched on closing the field flashing contactor ( Field
Circuit Breaker is already closed )
1048729The start-up excitation current flows through the rotor driving the generator
voltage up to approx 15 U
1048729After about 10 U the firing pulses to the converter are released and it begins
to excite the generator to its rated voltage
1048729After about 30 U the field flashing contactor opens (with no current since
the converter is now supplying the current)
The diode bridge at the input to the field flashing contactor prevents a feed-back
from the converter to the source of field flashing while the contactor is still closed
53 De-excitation
When malfunctions occur the stored field energy must be dissipated as quickly
and safely as possible to protect the generator This is done by the converter the field
circuit-breaker and the de-excitation (discharge) resistor
De-excitation (with opening of the field circuit-breaker) takes place in the following
stages
1048729The converter drives to its inverter limit position (negative ceiling voltage)
recovers a portion of the field energy into the network A trip command is given to the
field circuit breaker
1048729The de-excitation contact closes diverting the field voltage to the de-excitation
resistor
1048729Then immediately the main contacts open building voltage The field voltage
commutates to the de-excitation resistor
1048729The current diminishes at a given time constant TE
(With linear resistance TE = Lf (Rf + Re))
Due to the reversal of the field voltage by the converter the field current
commutates from the main contacts of the field circuit-breaker to the de-excitation
resistor in a very early phase This reversal of the field voltage prevents burn-off on the
main contacts and provides effective protection for the field circuit-breaker Depending
on the operating policy an operational shut-down of the excitation can also be effected
with the field circuit-breaker closed This method is useful mainly when the excitation is
switched on and off frequently In this case the converter is merely driven into the
inverter limit position so that the field energy is recovered into the network The
converter then blocks since it is supplying positive current only
54 Excitation Transformer
The excitation transformer matches the generator voltage to the field voltage
(required ceiling voltage) It also serves as a commutation reactance for the thyristor
converter and as a potential isolator between the network and the excitation circuit In
addition the transformer functions as a current limiter in that it makes it possible to keep
any short circuits in the excitation circuit under better control The excitation transformer
is equipped with temperature monitoring probes which set off an alarm when the
temperature exceeds a first max limit and then trips the excitation if the temperature
continues rising to a second (higher)limit
CHAPTER 6
Monitoring and Protection
61 Excitation Monitoring
611 General Information
The main goal of Excitation Monitoring is to make optimum use of the
redundancies provided in the excitation system and to give alarm whenever a malfunction
makes these redundancies unavailable The field current is monitored to see that it does
not exceed a maximum level and if necessary a switch-over to the MANUAL channel is
initiated In addition the criterion for switching off the field flashing is generated The
excitation Monitoring consists of an autonomous processor system
612 over current Alarms
In the Excitation Monitoring the limits for are set at higher levels than the
settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of
the nominal field current contact R1 and the binary output associated with it are
activated immediately If field current remains gt 110 then after a preset inverse-time
has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs
associated with them are activated Parameters match the measurements for If1 and If2 to
the nominal value for field current so that the internal values can be processed and read as
pu values It can be used to falsify the actual value of the field current If (to raise it) so
as to cause a response from the alarm limits for purposes of testing The processed If
signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As
long as the field current If is above the threshold value 11 Ifn its peak value is
measured This is stored (until RESET) and can be read at any time on the Micro-
Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever
the integrated time-current value (intisup2dt) exceeds the preselected reference value the
alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously
issued to switch over to the stand by AUTO channel Software switch F758 enables the
three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-
time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the
desired limit curve for response is set using the factor F216
613 Switch-Off Criterion for Field Flashing
The Excitation Monitoring supplies the criterion for switching off the field
flashing Whether this criterion is activated based on the actual value for generator voltage
Ug or for field current If or both depends on the settings of the two threshold values
F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements
Ug12 and If12 are switched over depending on the present status of the channels
(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is
fixed at ldquological 1rdquo
614 Storage of Alarm Status
The outputs of the over current alarms (R1 R2 R0) and the messages NO
FAILURE MONITORING PARAMETERS CHANGED are stored messages can be
erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the
front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the
situation causing the alarm or the malfunction is no longer present Whenever the self-
diagnosis equipment in the processor detects a malfunction the output NO FAILURE of
MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo
is activated whenever parameters or settings of software switches have been changed via
the Micro-Terminal
615 Actual Value Monitoring
The actual values for generator voltage Ug synchronous voltage Usyn and field
current If are monitored for malfunctions This monitoring is active regardless of whether
or not the generator is in operation Essentially when the generator is in operation the
measurements are monitored by comparing the signals (the smaller signal reading is
detected as incorrect) When the generator is not in operation the measured data are
monitored for extreme values The percentage of deviation permissible in the
measurement signals being compared
(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and
F20AIf the excitation transformer is being supplied from an auxiliary power source (no
shunt operation) the values of Ug and Usyn will be different in some operational
conditions
In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1
and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the
messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2
and the binary message from CH1 reports no malfunction a malfunction on
Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is
also present whenever the binary message CH1 DISTURBANCE is reported and a
discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2
FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares
Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the
comparison channel reports a malfunction or whenever both binary messages report no
malfunction - but both secondary monitors report a malfunction As long as the secondary
monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction
signals for the measurement channel involved (suspicion that there is a corresponding
error in Usyn) The measurement channel malfunctions are enabled operationally
whenever after excitation has been switched on generator voltage Ug exceeds the value
set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is
switched off to see that they do not exceed the limit value F210 that applies to both of
them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt
the operating range) Monitoring for extreme values is likewise enabled during normal
operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the
malfunction signals to the binary outputs Basically the monitoring of the actual values
for If1If2 functions like that of the Ug1Ug2 monitoring
62 Excitation Protection
621 General Information
The Excitation Protection switches off the excitation (and de-excites the machine
rapidly) whenever a danger arises that threatens the excitation transformer the converter
or the generator Generally limiter or monitoring functions precede the emergency trips
and these normally respond before the Excitation Protection must initiate a trip
Protective trip commands are issued directly to the field circuit-breaker from potential
free contacts of the board via the trip relays They are directed redundantly to the
operative field circuit-breaker ldquoOFFrdquo command
622 Protection against Excitation Transformer Overheating
This equipment monitors the excitation transformer for overheating in the
windings that could result from over current short-circuits or inadequate cooling The
monitoring uses temperature monitoring modules in conjunction with temperature
sensors built into the windings Normally the temperature is monitored in two stages the
first stage sets off an alarm the second causes a trip of the excitation
623 Rotor Over voltage Protection
Malfunctions in the generator circuit (eg terminal short-circuit failed
synchronization asynchronous operation) cause induced negative field currents that
produce high voltages in the field circuit These must be restricted to a level with a
sufficient safety margin below the insulation capacity of the field winding (test voltage)
and also below the peak blocking voltage of the converter thyristors The crow bar
employs spark gap elements to detect over voltages in the field circuit Whenever they
respond the associated thyristors are fired immediately switching the de-excitation
resistor parallel to the field The de-excitation current generated thereby initiates an
excitation trip via a supervision circuit causing an immediate opening of the field circuit-
breaker The malfunction isets off an alarm and an internal malfunction is indicated at the
cubicle
TEST VALUES OBTAINED WHEN EXCITATION IS RAISED
TEST
SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE
1 VREF 996 100
2 VACT 997 1003
3 IFACT 735 765
4 IGACT 703 707
5 ACTIVE
POWER
703 705
6 REACTIVE
POWER
101 142
7 POWER
FACTOR
099 IND 098 IND
8 ACTIVE
CURRENT(IR)
705 703
9 REACTIVE
CURRENT(IX)
102 138
10 POWER
ANGLE
566 546
11 FIRING
ANGLE
640 632
VALUES OBTAINED WHEN EXCITATION IS LOWERED
SNO PARTICULARS ACTUAL
VALUE
OBTAINED VALUE
1 VREF 100 997
2 VACT 100 997
3 IFACT 787 761
4 IGACT 839 837
5 ACTIVE
POWER
839 830
6 REACTIVE
POWER
155 110
7 POWER
FACTOR
098 IND 099 IND
8 ACTIVE
CURRENT(IR)
835 830
9 REACTIVE
CURRENT(IX)
140 94
10 POWER
ANGLE
613 DEG 631 DEG
11 FIRING
ANGLE
601 DEG 599 DEG
CONCLUSION
For generating the EMF in stator winding excitation is required to the rotor of a
generator There are two types of excitation
1 Static excitation system
2 Brushless excitation system
A certain disadvantage in brushless excitation system is the slow response time of
the field in case of fast load changes specified No slip-rings and brushes direct
measurements of the field parameters not possible
To avoid all loses static excitation is used Since it does not have any rotating
parts mechanical loses and windage loses This system has fast response and speed
control While preferring this excitation system there are no limitations for the
redundancy of Thyristor bridge circuits
Static excitation has fast field discharge by resistor and inverter operation direct
measurement of field quantity is possible The meaning of excitation is nothing but
continuous supply of DC current (ie field current) to the rotor to buildup required
output voltage in the stator
Field current is changed with respect to the change of load so the digital
automatic voltage regulator (DAVR) is used to regulate the output voltage according to
the load variations
So we conclude that static excitation system with DAVR is preferred since it is
having excellent dynamic performance and better options for R amp M
destroys the thyristor Thyristor thermal time is constant The causes due to faults and
short circuits or surge currents Electronic crowbar protection is used against the over
voltages The rate rise of anode current must be kept at the time of turn on below the
rated or specified limiting value The didt value maintained below limited value by using
a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating
is avoided by applying gate current but not greater the maximum gate current
DVDT AND OVER VOLTAGE PROTECTION
With forward voltage across the anode and cathode of a thyristor the two outer
junctions are forward biased but the inner junction is reverse biased This reverse biased
junction J2 has the characteristics of a capacitor due to charges existing across the
junction In other words space-charges exist in the depletion region around junction J2
and therefore junction J2 behaves like a capacitance If the entire anode to cathode
forward voltage Va appears across J2 junction and the charge is denoted by Q then a
charging current i given by Eq (46) follows
i = dQdt =d(Cj Va )dt
= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)
As Cj the capacitance of junction J2 is almost constant the current is given by
i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)
If the rate of rise of forward voltage dVadt is high the charging current i will be
more This charging current plays the role of gate current and turns on the SCR even
when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on
must be avoided as it leads to false operation of the thyristor circuit
For controllable operation of the thyristor the rate of rise of forward anode to
cathode voltage dVadt must be kept below the specified rated limit Typical values of
dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by
using a snubber circuit in parallel with the device thyristor are very sensitive for over
voltage than the semiconductor devices
Over voltage transients are perhaps the main cause of thyristor failure
In thyristor there are mainly two types
1 Internal over voltages
Due to the commutation of the thyristors large voltages are generated internally
Because of the series inductance of the SCR circuit the large transient voltages L didt
produced This voltage several times the break over voltage of the device then thyristor
destroys permanently
2 External over voltages
External over voltages are caused due to the interruptions of current flow in an
inductive circuit and also due to the lightening strokes on the lines feeding the thyristor
system For the reliable operation of thyristor the over voltages must be suppressed by
adopting suitable techniques
Suppression of over voltages
The RC circuit called snubber circuit is connected across the device to protect In
order to keep the protective components to a minimum the thyristors are chosen with
their peak voltages ratings are 25 to 3 times of the normal peak working voltage
ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are
commonly employed for protecting the thyristor circuit against the over voltages
Gate protection
Gate circuit should also be protected against the over voltages and surges Over
voltage at gate circuit can cause false triggering of the SCR may rises the junction
temperature behind specified limit leading to its damage Protection against over
voltage can be achieved by connecting a ZD across the gate circuit and a resister is
connected in series with gate circuit to protect against the s A capacitor and resister are
connected across gate to cathode to by pass the noise
41 Final Pulse Stages
The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage
(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor
bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided
with a power supply module The amplified output pulses from the Final Pulse Stages
start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main
pulse This initial pulse edge assures proper firing of the thyristors being triggered
Subsequently the weaker part of main pulse keeps firing conditions steady As already
mentioned the Final Pulse Stages and their associated thyristor bridges form single units
All six pulse outputs from a Final Pulse Stage can be blocked by an external control
signal so that all thyristors in the associated thyristor bridge will block the current A
blocking of the pulses is initiated whenever there is a malfunction in the associated
thyristor bridge
42 Converter Power Section
The thyristor converter consists of three independent parallel rectifier blocks TY1
to TY3 which are all in service Even if one block fails the remaining blocks take over
automatically the full design current of the excitation circuit During normal operation
(with ideal current share) and all three bridges in operation each of these blocks has to
carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation
is limited Only when all three bridges fail the excitation is switched off Each thyristor
bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any
arm is identified by a Current flow monitoring module
43 Converter Cooling
A cooling system is needed to dissipate heat losses in the converter blocks and
electronics Each converter block has therefore been equipped with a fan supplied with
power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing
cubicle) The fans are protected with motor protection circuit breakers An air flow
monitoring unit is provided for monitoring the air flow through the thyristor bridge If a
circuit breaker failure is detected or if the air flow monitor drops off at one of the
thyristor bridges the bridge involved is immediately set out of operation by blocking its
firing pulses
44 Thyristor Converter Monitoring
A thyristor bridge in which defects occur that could threaten the safety of
operation or cause secondary damage is switched off automatically ie its firing pulses
are blocked This happens whenever A thyristor fuse is blown The fuses are monitored
individually with micro switches The Final Pulse Stage fails which is detected by
internal monitors (supply voltage sustained pulse short-circuit on the output end) The
power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or
is insufficient Isolator on ACDC side is open
CHAPTER 5
Field Current Circuit Excitation Transformer
51 Field Circuit Breaker
The circuit-breaker in the field circuit is used to isolate the field circuit from the
converter It is capable of switching off the synchronous machine from full load under the
maximum conditions of a 3-phase short-circuit In addition to its main contacts the field
circuit-breaker also has a de-excitation contact with which the field energy stored in the
field can be dissipated across the de-excitation resistor The de-excitation contact closes
shortly before the main contacts open so as to ensure proper commutation of the field
current from the main contacts to the de-excitation contact when the breaker is switched
off The field circuit-breaker is switched on by electromagnetic force and is kept switched
on by a mechanical latch When the latch is released by a trip coil the circuit-breaker
opens The circuit-breaker also has auxiliary contacts that report its status
52 Field Flashing
In shunt supplied excitation circuits (excitation transformer connected to the
generator terminals) the generator does not have enough remnant voltage for a generator
voltage build-up via the converter In this case a field flashing circuit is provided It
consists of the field flashing contactor the diode bridge and a transformer used to adapt
the auxiliary input voltage to the voltage needed for field flashing when power is
supplied from the auxiliaries network
Fig Field Flashing
Because the field flashing contactor is not able to switch off the energy stored in
the field the control ensures that the contactor can only reopen if the field circuit breaker
has already been opened (generating the TRIP order) or in a normal field flashing
sequence when the converter has taken over the field current Field flashing occurs in the
following stages
1048729The excitation is switched on closing the field flashing contactor ( Field
Circuit Breaker is already closed )
1048729The start-up excitation current flows through the rotor driving the generator
voltage up to approx 15 U
1048729After about 10 U the firing pulses to the converter are released and it begins
to excite the generator to its rated voltage
1048729After about 30 U the field flashing contactor opens (with no current since
the converter is now supplying the current)
The diode bridge at the input to the field flashing contactor prevents a feed-back
from the converter to the source of field flashing while the contactor is still closed
53 De-excitation
When malfunctions occur the stored field energy must be dissipated as quickly
and safely as possible to protect the generator This is done by the converter the field
circuit-breaker and the de-excitation (discharge) resistor
De-excitation (with opening of the field circuit-breaker) takes place in the following
stages
1048729The converter drives to its inverter limit position (negative ceiling voltage)
recovers a portion of the field energy into the network A trip command is given to the
field circuit breaker
1048729The de-excitation contact closes diverting the field voltage to the de-excitation
resistor
1048729Then immediately the main contacts open building voltage The field voltage
commutates to the de-excitation resistor
1048729The current diminishes at a given time constant TE
(With linear resistance TE = Lf (Rf + Re))
Due to the reversal of the field voltage by the converter the field current
commutates from the main contacts of the field circuit-breaker to the de-excitation
resistor in a very early phase This reversal of the field voltage prevents burn-off on the
main contacts and provides effective protection for the field circuit-breaker Depending
on the operating policy an operational shut-down of the excitation can also be effected
with the field circuit-breaker closed This method is useful mainly when the excitation is
switched on and off frequently In this case the converter is merely driven into the
inverter limit position so that the field energy is recovered into the network The
converter then blocks since it is supplying positive current only
54 Excitation Transformer
The excitation transformer matches the generator voltage to the field voltage
(required ceiling voltage) It also serves as a commutation reactance for the thyristor
converter and as a potential isolator between the network and the excitation circuit In
addition the transformer functions as a current limiter in that it makes it possible to keep
any short circuits in the excitation circuit under better control The excitation transformer
is equipped with temperature monitoring probes which set off an alarm when the
temperature exceeds a first max limit and then trips the excitation if the temperature
continues rising to a second (higher)limit
CHAPTER 6
Monitoring and Protection
61 Excitation Monitoring
611 General Information
The main goal of Excitation Monitoring is to make optimum use of the
redundancies provided in the excitation system and to give alarm whenever a malfunction
makes these redundancies unavailable The field current is monitored to see that it does
not exceed a maximum level and if necessary a switch-over to the MANUAL channel is
initiated In addition the criterion for switching off the field flashing is generated The
excitation Monitoring consists of an autonomous processor system
612 over current Alarms
In the Excitation Monitoring the limits for are set at higher levels than the
settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of
the nominal field current contact R1 and the binary output associated with it are
activated immediately If field current remains gt 110 then after a preset inverse-time
has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs
associated with them are activated Parameters match the measurements for If1 and If2 to
the nominal value for field current so that the internal values can be processed and read as
pu values It can be used to falsify the actual value of the field current If (to raise it) so
as to cause a response from the alarm limits for purposes of testing The processed If
signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As
long as the field current If is above the threshold value 11 Ifn its peak value is
measured This is stored (until RESET) and can be read at any time on the Micro-
Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever
the integrated time-current value (intisup2dt) exceeds the preselected reference value the
alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously
issued to switch over to the stand by AUTO channel Software switch F758 enables the
three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-
time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the
desired limit curve for response is set using the factor F216
613 Switch-Off Criterion for Field Flashing
The Excitation Monitoring supplies the criterion for switching off the field
flashing Whether this criterion is activated based on the actual value for generator voltage
Ug or for field current If or both depends on the settings of the two threshold values
F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements
Ug12 and If12 are switched over depending on the present status of the channels
(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is
fixed at ldquological 1rdquo
614 Storage of Alarm Status
The outputs of the over current alarms (R1 R2 R0) and the messages NO
FAILURE MONITORING PARAMETERS CHANGED are stored messages can be
erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the
front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the
situation causing the alarm or the malfunction is no longer present Whenever the self-
diagnosis equipment in the processor detects a malfunction the output NO FAILURE of
MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo
is activated whenever parameters or settings of software switches have been changed via
the Micro-Terminal
615 Actual Value Monitoring
The actual values for generator voltage Ug synchronous voltage Usyn and field
current If are monitored for malfunctions This monitoring is active regardless of whether
or not the generator is in operation Essentially when the generator is in operation the
measurements are monitored by comparing the signals (the smaller signal reading is
detected as incorrect) When the generator is not in operation the measured data are
monitored for extreme values The percentage of deviation permissible in the
measurement signals being compared
(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and
F20AIf the excitation transformer is being supplied from an auxiliary power source (no
shunt operation) the values of Ug and Usyn will be different in some operational
conditions
In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1
and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the
messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2
and the binary message from CH1 reports no malfunction a malfunction on
Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is
also present whenever the binary message CH1 DISTURBANCE is reported and a
discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2
FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares
Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the
comparison channel reports a malfunction or whenever both binary messages report no
malfunction - but both secondary monitors report a malfunction As long as the secondary
monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction
signals for the measurement channel involved (suspicion that there is a corresponding
error in Usyn) The measurement channel malfunctions are enabled operationally
whenever after excitation has been switched on generator voltage Ug exceeds the value
set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is
switched off to see that they do not exceed the limit value F210 that applies to both of
them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt
the operating range) Monitoring for extreme values is likewise enabled during normal
operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the
malfunction signals to the binary outputs Basically the monitoring of the actual values
for If1If2 functions like that of the Ug1Ug2 monitoring
62 Excitation Protection
621 General Information
The Excitation Protection switches off the excitation (and de-excites the machine
rapidly) whenever a danger arises that threatens the excitation transformer the converter
or the generator Generally limiter or monitoring functions precede the emergency trips
and these normally respond before the Excitation Protection must initiate a trip
Protective trip commands are issued directly to the field circuit-breaker from potential
free contacts of the board via the trip relays They are directed redundantly to the
operative field circuit-breaker ldquoOFFrdquo command
622 Protection against Excitation Transformer Overheating
This equipment monitors the excitation transformer for overheating in the
windings that could result from over current short-circuits or inadequate cooling The
monitoring uses temperature monitoring modules in conjunction with temperature
sensors built into the windings Normally the temperature is monitored in two stages the
first stage sets off an alarm the second causes a trip of the excitation
623 Rotor Over voltage Protection
Malfunctions in the generator circuit (eg terminal short-circuit failed
synchronization asynchronous operation) cause induced negative field currents that
produce high voltages in the field circuit These must be restricted to a level with a
sufficient safety margin below the insulation capacity of the field winding (test voltage)
and also below the peak blocking voltage of the converter thyristors The crow bar
employs spark gap elements to detect over voltages in the field circuit Whenever they
respond the associated thyristors are fired immediately switching the de-excitation
resistor parallel to the field The de-excitation current generated thereby initiates an
excitation trip via a supervision circuit causing an immediate opening of the field circuit-
breaker The malfunction isets off an alarm and an internal malfunction is indicated at the
cubicle
TEST VALUES OBTAINED WHEN EXCITATION IS RAISED
TEST
SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE
1 VREF 996 100
2 VACT 997 1003
3 IFACT 735 765
4 IGACT 703 707
5 ACTIVE
POWER
703 705
6 REACTIVE
POWER
101 142
7 POWER
FACTOR
099 IND 098 IND
8 ACTIVE
CURRENT(IR)
705 703
9 REACTIVE
CURRENT(IX)
102 138
10 POWER
ANGLE
566 546
11 FIRING
ANGLE
640 632
VALUES OBTAINED WHEN EXCITATION IS LOWERED
SNO PARTICULARS ACTUAL
VALUE
OBTAINED VALUE
1 VREF 100 997
2 VACT 100 997
3 IFACT 787 761
4 IGACT 839 837
5 ACTIVE
POWER
839 830
6 REACTIVE
POWER
155 110
7 POWER
FACTOR
098 IND 099 IND
8 ACTIVE
CURRENT(IR)
835 830
9 REACTIVE
CURRENT(IX)
140 94
10 POWER
ANGLE
613 DEG 631 DEG
11 FIRING
ANGLE
601 DEG 599 DEG
CONCLUSION
For generating the EMF in stator winding excitation is required to the rotor of a
generator There are two types of excitation
1 Static excitation system
2 Brushless excitation system
A certain disadvantage in brushless excitation system is the slow response time of
the field in case of fast load changes specified No slip-rings and brushes direct
measurements of the field parameters not possible
To avoid all loses static excitation is used Since it does not have any rotating
parts mechanical loses and windage loses This system has fast response and speed
control While preferring this excitation system there are no limitations for the
redundancy of Thyristor bridge circuits
Static excitation has fast field discharge by resistor and inverter operation direct
measurement of field quantity is possible The meaning of excitation is nothing but
continuous supply of DC current (ie field current) to the rotor to buildup required
output voltage in the stator
Field current is changed with respect to the change of load so the digital
automatic voltage regulator (DAVR) is used to regulate the output voltage according to
the load variations
So we conclude that static excitation system with DAVR is preferred since it is
having excellent dynamic performance and better options for R amp M
produced This voltage several times the break over voltage of the device then thyristor
destroys permanently
2 External over voltages
External over voltages are caused due to the interruptions of current flow in an
inductive circuit and also due to the lightening strokes on the lines feeding the thyristor
system For the reliable operation of thyristor the over voltages must be suppressed by
adopting suitable techniques
Suppression of over voltages
The RC circuit called snubber circuit is connected across the device to protect In
order to keep the protective components to a minimum the thyristors are chosen with
their peak voltages ratings are 25 to 3 times of the normal peak working voltage
ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are
commonly employed for protecting the thyristor circuit against the over voltages
Gate protection
Gate circuit should also be protected against the over voltages and surges Over
voltage at gate circuit can cause false triggering of the SCR may rises the junction
temperature behind specified limit leading to its damage Protection against over
voltage can be achieved by connecting a ZD across the gate circuit and a resister is
connected in series with gate circuit to protect against the s A capacitor and resister are
connected across gate to cathode to by pass the noise
41 Final Pulse Stages
The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage
(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor
bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided
with a power supply module The amplified output pulses from the Final Pulse Stages
start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main
pulse This initial pulse edge assures proper firing of the thyristors being triggered
Subsequently the weaker part of main pulse keeps firing conditions steady As already
mentioned the Final Pulse Stages and their associated thyristor bridges form single units
All six pulse outputs from a Final Pulse Stage can be blocked by an external control
signal so that all thyristors in the associated thyristor bridge will block the current A
blocking of the pulses is initiated whenever there is a malfunction in the associated
thyristor bridge
42 Converter Power Section
The thyristor converter consists of three independent parallel rectifier blocks TY1
to TY3 which are all in service Even if one block fails the remaining blocks take over
automatically the full design current of the excitation circuit During normal operation
(with ideal current share) and all three bridges in operation each of these blocks has to
carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation
is limited Only when all three bridges fail the excitation is switched off Each thyristor
bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any
arm is identified by a Current flow monitoring module
43 Converter Cooling
A cooling system is needed to dissipate heat losses in the converter blocks and
electronics Each converter block has therefore been equipped with a fan supplied with
power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing
cubicle) The fans are protected with motor protection circuit breakers An air flow
monitoring unit is provided for monitoring the air flow through the thyristor bridge If a
circuit breaker failure is detected or if the air flow monitor drops off at one of the
thyristor bridges the bridge involved is immediately set out of operation by blocking its
firing pulses
44 Thyristor Converter Monitoring
A thyristor bridge in which defects occur that could threaten the safety of
operation or cause secondary damage is switched off automatically ie its firing pulses
are blocked This happens whenever A thyristor fuse is blown The fuses are monitored
individually with micro switches The Final Pulse Stage fails which is detected by
internal monitors (supply voltage sustained pulse short-circuit on the output end) The
power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or
is insufficient Isolator on ACDC side is open
CHAPTER 5
Field Current Circuit Excitation Transformer
51 Field Circuit Breaker
The circuit-breaker in the field circuit is used to isolate the field circuit from the
converter It is capable of switching off the synchronous machine from full load under the
maximum conditions of a 3-phase short-circuit In addition to its main contacts the field
circuit-breaker also has a de-excitation contact with which the field energy stored in the
field can be dissipated across the de-excitation resistor The de-excitation contact closes
shortly before the main contacts open so as to ensure proper commutation of the field
current from the main contacts to the de-excitation contact when the breaker is switched
off The field circuit-breaker is switched on by electromagnetic force and is kept switched
on by a mechanical latch When the latch is released by a trip coil the circuit-breaker
opens The circuit-breaker also has auxiliary contacts that report its status
52 Field Flashing
In shunt supplied excitation circuits (excitation transformer connected to the
generator terminals) the generator does not have enough remnant voltage for a generator
voltage build-up via the converter In this case a field flashing circuit is provided It
consists of the field flashing contactor the diode bridge and a transformer used to adapt
the auxiliary input voltage to the voltage needed for field flashing when power is
supplied from the auxiliaries network
Fig Field Flashing
Because the field flashing contactor is not able to switch off the energy stored in
the field the control ensures that the contactor can only reopen if the field circuit breaker
has already been opened (generating the TRIP order) or in a normal field flashing
sequence when the converter has taken over the field current Field flashing occurs in the
following stages
1048729The excitation is switched on closing the field flashing contactor ( Field
Circuit Breaker is already closed )
1048729The start-up excitation current flows through the rotor driving the generator
voltage up to approx 15 U
1048729After about 10 U the firing pulses to the converter are released and it begins
to excite the generator to its rated voltage
1048729After about 30 U the field flashing contactor opens (with no current since
the converter is now supplying the current)
The diode bridge at the input to the field flashing contactor prevents a feed-back
from the converter to the source of field flashing while the contactor is still closed
53 De-excitation
When malfunctions occur the stored field energy must be dissipated as quickly
and safely as possible to protect the generator This is done by the converter the field
circuit-breaker and the de-excitation (discharge) resistor
De-excitation (with opening of the field circuit-breaker) takes place in the following
stages
1048729The converter drives to its inverter limit position (negative ceiling voltage)
recovers a portion of the field energy into the network A trip command is given to the
field circuit breaker
1048729The de-excitation contact closes diverting the field voltage to the de-excitation
resistor
1048729Then immediately the main contacts open building voltage The field voltage
commutates to the de-excitation resistor
1048729The current diminishes at a given time constant TE
(With linear resistance TE = Lf (Rf + Re))
Due to the reversal of the field voltage by the converter the field current
commutates from the main contacts of the field circuit-breaker to the de-excitation
resistor in a very early phase This reversal of the field voltage prevents burn-off on the
main contacts and provides effective protection for the field circuit-breaker Depending
on the operating policy an operational shut-down of the excitation can also be effected
with the field circuit-breaker closed This method is useful mainly when the excitation is
switched on and off frequently In this case the converter is merely driven into the
inverter limit position so that the field energy is recovered into the network The
converter then blocks since it is supplying positive current only
54 Excitation Transformer
The excitation transformer matches the generator voltage to the field voltage
(required ceiling voltage) It also serves as a commutation reactance for the thyristor
converter and as a potential isolator between the network and the excitation circuit In
addition the transformer functions as a current limiter in that it makes it possible to keep
any short circuits in the excitation circuit under better control The excitation transformer
is equipped with temperature monitoring probes which set off an alarm when the
temperature exceeds a first max limit and then trips the excitation if the temperature
continues rising to a second (higher)limit
CHAPTER 6
Monitoring and Protection
61 Excitation Monitoring
611 General Information
The main goal of Excitation Monitoring is to make optimum use of the
redundancies provided in the excitation system and to give alarm whenever a malfunction
makes these redundancies unavailable The field current is monitored to see that it does
not exceed a maximum level and if necessary a switch-over to the MANUAL channel is
initiated In addition the criterion for switching off the field flashing is generated The
excitation Monitoring consists of an autonomous processor system
612 over current Alarms
In the Excitation Monitoring the limits for are set at higher levels than the
settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of
the nominal field current contact R1 and the binary output associated with it are
activated immediately If field current remains gt 110 then after a preset inverse-time
has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs
associated with them are activated Parameters match the measurements for If1 and If2 to
the nominal value for field current so that the internal values can be processed and read as
pu values It can be used to falsify the actual value of the field current If (to raise it) so
as to cause a response from the alarm limits for purposes of testing The processed If
signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As
long as the field current If is above the threshold value 11 Ifn its peak value is
measured This is stored (until RESET) and can be read at any time on the Micro-
Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever
the integrated time-current value (intisup2dt) exceeds the preselected reference value the
alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously
issued to switch over to the stand by AUTO channel Software switch F758 enables the
three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-
time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the
desired limit curve for response is set using the factor F216
613 Switch-Off Criterion for Field Flashing
The Excitation Monitoring supplies the criterion for switching off the field
flashing Whether this criterion is activated based on the actual value for generator voltage
Ug or for field current If or both depends on the settings of the two threshold values
F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements
Ug12 and If12 are switched over depending on the present status of the channels
(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is
fixed at ldquological 1rdquo
614 Storage of Alarm Status
The outputs of the over current alarms (R1 R2 R0) and the messages NO
FAILURE MONITORING PARAMETERS CHANGED are stored messages can be
erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the
front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the
situation causing the alarm or the malfunction is no longer present Whenever the self-
diagnosis equipment in the processor detects a malfunction the output NO FAILURE of
MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo
is activated whenever parameters or settings of software switches have been changed via
the Micro-Terminal
615 Actual Value Monitoring
The actual values for generator voltage Ug synchronous voltage Usyn and field
current If are monitored for malfunctions This monitoring is active regardless of whether
or not the generator is in operation Essentially when the generator is in operation the
measurements are monitored by comparing the signals (the smaller signal reading is
detected as incorrect) When the generator is not in operation the measured data are
monitored for extreme values The percentage of deviation permissible in the
measurement signals being compared
(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and
F20AIf the excitation transformer is being supplied from an auxiliary power source (no
shunt operation) the values of Ug and Usyn will be different in some operational
conditions
In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1
and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the
messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2
and the binary message from CH1 reports no malfunction a malfunction on
Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is
also present whenever the binary message CH1 DISTURBANCE is reported and a
discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2
FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares
Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the
comparison channel reports a malfunction or whenever both binary messages report no
malfunction - but both secondary monitors report a malfunction As long as the secondary
monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction
signals for the measurement channel involved (suspicion that there is a corresponding
error in Usyn) The measurement channel malfunctions are enabled operationally
whenever after excitation has been switched on generator voltage Ug exceeds the value
set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is
switched off to see that they do not exceed the limit value F210 that applies to both of
them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt
the operating range) Monitoring for extreme values is likewise enabled during normal
operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the
malfunction signals to the binary outputs Basically the monitoring of the actual values
for If1If2 functions like that of the Ug1Ug2 monitoring
62 Excitation Protection
621 General Information
The Excitation Protection switches off the excitation (and de-excites the machine
rapidly) whenever a danger arises that threatens the excitation transformer the converter
or the generator Generally limiter or monitoring functions precede the emergency trips
and these normally respond before the Excitation Protection must initiate a trip
Protective trip commands are issued directly to the field circuit-breaker from potential
free contacts of the board via the trip relays They are directed redundantly to the
operative field circuit-breaker ldquoOFFrdquo command
622 Protection against Excitation Transformer Overheating
This equipment monitors the excitation transformer for overheating in the
windings that could result from over current short-circuits or inadequate cooling The
monitoring uses temperature monitoring modules in conjunction with temperature
sensors built into the windings Normally the temperature is monitored in two stages the
first stage sets off an alarm the second causes a trip of the excitation
623 Rotor Over voltage Protection
Malfunctions in the generator circuit (eg terminal short-circuit failed
synchronization asynchronous operation) cause induced negative field currents that
produce high voltages in the field circuit These must be restricted to a level with a
sufficient safety margin below the insulation capacity of the field winding (test voltage)
and also below the peak blocking voltage of the converter thyristors The crow bar
employs spark gap elements to detect over voltages in the field circuit Whenever they
respond the associated thyristors are fired immediately switching the de-excitation
resistor parallel to the field The de-excitation current generated thereby initiates an
excitation trip via a supervision circuit causing an immediate opening of the field circuit-
breaker The malfunction isets off an alarm and an internal malfunction is indicated at the
cubicle
TEST VALUES OBTAINED WHEN EXCITATION IS RAISED
TEST
SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE
1 VREF 996 100
2 VACT 997 1003
3 IFACT 735 765
4 IGACT 703 707
5 ACTIVE
POWER
703 705
6 REACTIVE
POWER
101 142
7 POWER
FACTOR
099 IND 098 IND
8 ACTIVE
CURRENT(IR)
705 703
9 REACTIVE
CURRENT(IX)
102 138
10 POWER
ANGLE
566 546
11 FIRING
ANGLE
640 632
VALUES OBTAINED WHEN EXCITATION IS LOWERED
SNO PARTICULARS ACTUAL
VALUE
OBTAINED VALUE
1 VREF 100 997
2 VACT 100 997
3 IFACT 787 761
4 IGACT 839 837
5 ACTIVE
POWER
839 830
6 REACTIVE
POWER
155 110
7 POWER
FACTOR
098 IND 099 IND
8 ACTIVE
CURRENT(IR)
835 830
9 REACTIVE
CURRENT(IX)
140 94
10 POWER
ANGLE
613 DEG 631 DEG
11 FIRING
ANGLE
601 DEG 599 DEG
CONCLUSION
For generating the EMF in stator winding excitation is required to the rotor of a
generator There are two types of excitation
1 Static excitation system
2 Brushless excitation system
A certain disadvantage in brushless excitation system is the slow response time of
the field in case of fast load changes specified No slip-rings and brushes direct
measurements of the field parameters not possible
To avoid all loses static excitation is used Since it does not have any rotating
parts mechanical loses and windage loses This system has fast response and speed
control While preferring this excitation system there are no limitations for the
redundancy of Thyristor bridge circuits
Static excitation has fast field discharge by resistor and inverter operation direct
measurement of field quantity is possible The meaning of excitation is nothing but
continuous supply of DC current (ie field current) to the rotor to buildup required
output voltage in the stator
Field current is changed with respect to the change of load so the digital
automatic voltage regulator (DAVR) is used to regulate the output voltage according to
the load variations
So we conclude that static excitation system with DAVR is preferred since it is
having excellent dynamic performance and better options for R amp M
41 Final Pulse Stages
The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage
(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor
bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided
with a power supply module The amplified output pulses from the Final Pulse Stages
start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main
pulse This initial pulse edge assures proper firing of the thyristors being triggered
Subsequently the weaker part of main pulse keeps firing conditions steady As already
mentioned the Final Pulse Stages and their associated thyristor bridges form single units
All six pulse outputs from a Final Pulse Stage can be blocked by an external control
signal so that all thyristors in the associated thyristor bridge will block the current A
blocking of the pulses is initiated whenever there is a malfunction in the associated
thyristor bridge
42 Converter Power Section
The thyristor converter consists of three independent parallel rectifier blocks TY1
to TY3 which are all in service Even if one block fails the remaining blocks take over
automatically the full design current of the excitation circuit During normal operation
(with ideal current share) and all three bridges in operation each of these blocks has to
carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation
is limited Only when all three bridges fail the excitation is switched off Each thyristor
bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any
arm is identified by a Current flow monitoring module
43 Converter Cooling
A cooling system is needed to dissipate heat losses in the converter blocks and
electronics Each converter block has therefore been equipped with a fan supplied with
power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing
cubicle) The fans are protected with motor protection circuit breakers An air flow
monitoring unit is provided for monitoring the air flow through the thyristor bridge If a
circuit breaker failure is detected or if the air flow monitor drops off at one of the
thyristor bridges the bridge involved is immediately set out of operation by blocking its
firing pulses
44 Thyristor Converter Monitoring
A thyristor bridge in which defects occur that could threaten the safety of
operation or cause secondary damage is switched off automatically ie its firing pulses
are blocked This happens whenever A thyristor fuse is blown The fuses are monitored
individually with micro switches The Final Pulse Stage fails which is detected by
internal monitors (supply voltage sustained pulse short-circuit on the output end) The
power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or
is insufficient Isolator on ACDC side is open
CHAPTER 5
Field Current Circuit Excitation Transformer
51 Field Circuit Breaker
The circuit-breaker in the field circuit is used to isolate the field circuit from the
converter It is capable of switching off the synchronous machine from full load under the
maximum conditions of a 3-phase short-circuit In addition to its main contacts the field
circuit-breaker also has a de-excitation contact with which the field energy stored in the
field can be dissipated across the de-excitation resistor The de-excitation contact closes
shortly before the main contacts open so as to ensure proper commutation of the field
current from the main contacts to the de-excitation contact when the breaker is switched
off The field circuit-breaker is switched on by electromagnetic force and is kept switched
on by a mechanical latch When the latch is released by a trip coil the circuit-breaker
opens The circuit-breaker also has auxiliary contacts that report its status
52 Field Flashing
In shunt supplied excitation circuits (excitation transformer connected to the
generator terminals) the generator does not have enough remnant voltage for a generator
voltage build-up via the converter In this case a field flashing circuit is provided It
consists of the field flashing contactor the diode bridge and a transformer used to adapt
the auxiliary input voltage to the voltage needed for field flashing when power is
supplied from the auxiliaries network
Fig Field Flashing
Because the field flashing contactor is not able to switch off the energy stored in
the field the control ensures that the contactor can only reopen if the field circuit breaker
has already been opened (generating the TRIP order) or in a normal field flashing
sequence when the converter has taken over the field current Field flashing occurs in the
following stages
1048729The excitation is switched on closing the field flashing contactor ( Field
Circuit Breaker is already closed )
1048729The start-up excitation current flows through the rotor driving the generator
voltage up to approx 15 U
1048729After about 10 U the firing pulses to the converter are released and it begins
to excite the generator to its rated voltage
1048729After about 30 U the field flashing contactor opens (with no current since
the converter is now supplying the current)
The diode bridge at the input to the field flashing contactor prevents a feed-back
from the converter to the source of field flashing while the contactor is still closed
53 De-excitation
When malfunctions occur the stored field energy must be dissipated as quickly
and safely as possible to protect the generator This is done by the converter the field
circuit-breaker and the de-excitation (discharge) resistor
De-excitation (with opening of the field circuit-breaker) takes place in the following
stages
1048729The converter drives to its inverter limit position (negative ceiling voltage)
recovers a portion of the field energy into the network A trip command is given to the
field circuit breaker
1048729The de-excitation contact closes diverting the field voltage to the de-excitation
resistor
1048729Then immediately the main contacts open building voltage The field voltage
commutates to the de-excitation resistor
1048729The current diminishes at a given time constant TE
(With linear resistance TE = Lf (Rf + Re))
Due to the reversal of the field voltage by the converter the field current
commutates from the main contacts of the field circuit-breaker to the de-excitation
resistor in a very early phase This reversal of the field voltage prevents burn-off on the
main contacts and provides effective protection for the field circuit-breaker Depending
on the operating policy an operational shut-down of the excitation can also be effected
with the field circuit-breaker closed This method is useful mainly when the excitation is
switched on and off frequently In this case the converter is merely driven into the
inverter limit position so that the field energy is recovered into the network The
converter then blocks since it is supplying positive current only
54 Excitation Transformer
The excitation transformer matches the generator voltage to the field voltage
(required ceiling voltage) It also serves as a commutation reactance for the thyristor
converter and as a potential isolator between the network and the excitation circuit In
addition the transformer functions as a current limiter in that it makes it possible to keep
any short circuits in the excitation circuit under better control The excitation transformer
is equipped with temperature monitoring probes which set off an alarm when the
temperature exceeds a first max limit and then trips the excitation if the temperature
continues rising to a second (higher)limit
CHAPTER 6
Monitoring and Protection
61 Excitation Monitoring
611 General Information
The main goal of Excitation Monitoring is to make optimum use of the
redundancies provided in the excitation system and to give alarm whenever a malfunction
makes these redundancies unavailable The field current is monitored to see that it does
not exceed a maximum level and if necessary a switch-over to the MANUAL channel is
initiated In addition the criterion for switching off the field flashing is generated The
excitation Monitoring consists of an autonomous processor system
612 over current Alarms
In the Excitation Monitoring the limits for are set at higher levels than the
settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of
the nominal field current contact R1 and the binary output associated with it are
activated immediately If field current remains gt 110 then after a preset inverse-time
has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs
associated with them are activated Parameters match the measurements for If1 and If2 to
the nominal value for field current so that the internal values can be processed and read as
pu values It can be used to falsify the actual value of the field current If (to raise it) so
as to cause a response from the alarm limits for purposes of testing The processed If
signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As
long as the field current If is above the threshold value 11 Ifn its peak value is
measured This is stored (until RESET) and can be read at any time on the Micro-
Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever
the integrated time-current value (intisup2dt) exceeds the preselected reference value the
alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously
issued to switch over to the stand by AUTO channel Software switch F758 enables the
three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-
time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the
desired limit curve for response is set using the factor F216
613 Switch-Off Criterion for Field Flashing
The Excitation Monitoring supplies the criterion for switching off the field
flashing Whether this criterion is activated based on the actual value for generator voltage
Ug or for field current If or both depends on the settings of the two threshold values
F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements
Ug12 and If12 are switched over depending on the present status of the channels
(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is
fixed at ldquological 1rdquo
614 Storage of Alarm Status
The outputs of the over current alarms (R1 R2 R0) and the messages NO
FAILURE MONITORING PARAMETERS CHANGED are stored messages can be
erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the
front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the
situation causing the alarm or the malfunction is no longer present Whenever the self-
diagnosis equipment in the processor detects a malfunction the output NO FAILURE of
MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo
is activated whenever parameters or settings of software switches have been changed via
the Micro-Terminal
615 Actual Value Monitoring
The actual values for generator voltage Ug synchronous voltage Usyn and field
current If are monitored for malfunctions This monitoring is active regardless of whether
or not the generator is in operation Essentially when the generator is in operation the
measurements are monitored by comparing the signals (the smaller signal reading is
detected as incorrect) When the generator is not in operation the measured data are
monitored for extreme values The percentage of deviation permissible in the
measurement signals being compared
(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and
F20AIf the excitation transformer is being supplied from an auxiliary power source (no
shunt operation) the values of Ug and Usyn will be different in some operational
conditions
In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1
and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the
messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2
and the binary message from CH1 reports no malfunction a malfunction on
Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is
also present whenever the binary message CH1 DISTURBANCE is reported and a
discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2
FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares
Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the
comparison channel reports a malfunction or whenever both binary messages report no
malfunction - but both secondary monitors report a malfunction As long as the secondary
monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction
signals for the measurement channel involved (suspicion that there is a corresponding
error in Usyn) The measurement channel malfunctions are enabled operationally
whenever after excitation has been switched on generator voltage Ug exceeds the value
set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is
switched off to see that they do not exceed the limit value F210 that applies to both of
them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt
the operating range) Monitoring for extreme values is likewise enabled during normal
operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the
malfunction signals to the binary outputs Basically the monitoring of the actual values
for If1If2 functions like that of the Ug1Ug2 monitoring
62 Excitation Protection
621 General Information
The Excitation Protection switches off the excitation (and de-excites the machine
rapidly) whenever a danger arises that threatens the excitation transformer the converter
or the generator Generally limiter or monitoring functions precede the emergency trips
and these normally respond before the Excitation Protection must initiate a trip
Protective trip commands are issued directly to the field circuit-breaker from potential
free contacts of the board via the trip relays They are directed redundantly to the
operative field circuit-breaker ldquoOFFrdquo command
622 Protection against Excitation Transformer Overheating
This equipment monitors the excitation transformer for overheating in the
windings that could result from over current short-circuits or inadequate cooling The
monitoring uses temperature monitoring modules in conjunction with temperature
sensors built into the windings Normally the temperature is monitored in two stages the
first stage sets off an alarm the second causes a trip of the excitation
623 Rotor Over voltage Protection
Malfunctions in the generator circuit (eg terminal short-circuit failed
synchronization asynchronous operation) cause induced negative field currents that
produce high voltages in the field circuit These must be restricted to a level with a
sufficient safety margin below the insulation capacity of the field winding (test voltage)
and also below the peak blocking voltage of the converter thyristors The crow bar
employs spark gap elements to detect over voltages in the field circuit Whenever they
respond the associated thyristors are fired immediately switching the de-excitation
resistor parallel to the field The de-excitation current generated thereby initiates an
excitation trip via a supervision circuit causing an immediate opening of the field circuit-
breaker The malfunction isets off an alarm and an internal malfunction is indicated at the
cubicle
TEST VALUES OBTAINED WHEN EXCITATION IS RAISED
TEST
SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE
1 VREF 996 100
2 VACT 997 1003
3 IFACT 735 765
4 IGACT 703 707
5 ACTIVE
POWER
703 705
6 REACTIVE
POWER
101 142
7 POWER
FACTOR
099 IND 098 IND
8 ACTIVE
CURRENT(IR)
705 703
9 REACTIVE
CURRENT(IX)
102 138
10 POWER
ANGLE
566 546
11 FIRING
ANGLE
640 632
VALUES OBTAINED WHEN EXCITATION IS LOWERED
SNO PARTICULARS ACTUAL
VALUE
OBTAINED VALUE
1 VREF 100 997
2 VACT 100 997
3 IFACT 787 761
4 IGACT 839 837
5 ACTIVE
POWER
839 830
6 REACTIVE
POWER
155 110
7 POWER
FACTOR
098 IND 099 IND
8 ACTIVE
CURRENT(IR)
835 830
9 REACTIVE
CURRENT(IX)
140 94
10 POWER
ANGLE
613 DEG 631 DEG
11 FIRING
ANGLE
601 DEG 599 DEG
CONCLUSION
For generating the EMF in stator winding excitation is required to the rotor of a
generator There are two types of excitation
1 Static excitation system
2 Brushless excitation system
A certain disadvantage in brushless excitation system is the slow response time of
the field in case of fast load changes specified No slip-rings and brushes direct
measurements of the field parameters not possible
To avoid all loses static excitation is used Since it does not have any rotating
parts mechanical loses and windage loses This system has fast response and speed
control While preferring this excitation system there are no limitations for the
redundancy of Thyristor bridge circuits
Static excitation has fast field discharge by resistor and inverter operation direct
measurement of field quantity is possible The meaning of excitation is nothing but
continuous supply of DC current (ie field current) to the rotor to buildup required
output voltage in the stator
Field current is changed with respect to the change of load so the digital
automatic voltage regulator (DAVR) is used to regulate the output voltage according to
the load variations
So we conclude that static excitation system with DAVR is preferred since it is
having excellent dynamic performance and better options for R amp M
44 Thyristor Converter Monitoring
A thyristor bridge in which defects occur that could threaten the safety of
operation or cause secondary damage is switched off automatically ie its firing pulses
are blocked This happens whenever A thyristor fuse is blown The fuses are monitored
individually with micro switches The Final Pulse Stage fails which is detected by
internal monitors (supply voltage sustained pulse short-circuit on the output end) The
power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or
is insufficient Isolator on ACDC side is open
CHAPTER 5
Field Current Circuit Excitation Transformer
51 Field Circuit Breaker
The circuit-breaker in the field circuit is used to isolate the field circuit from the
converter It is capable of switching off the synchronous machine from full load under the
maximum conditions of a 3-phase short-circuit In addition to its main contacts the field
circuit-breaker also has a de-excitation contact with which the field energy stored in the
field can be dissipated across the de-excitation resistor The de-excitation contact closes
shortly before the main contacts open so as to ensure proper commutation of the field
current from the main contacts to the de-excitation contact when the breaker is switched
off The field circuit-breaker is switched on by electromagnetic force and is kept switched
on by a mechanical latch When the latch is released by a trip coil the circuit-breaker
opens The circuit-breaker also has auxiliary contacts that report its status
52 Field Flashing
In shunt supplied excitation circuits (excitation transformer connected to the
generator terminals) the generator does not have enough remnant voltage for a generator
voltage build-up via the converter In this case a field flashing circuit is provided It
consists of the field flashing contactor the diode bridge and a transformer used to adapt
the auxiliary input voltage to the voltage needed for field flashing when power is
supplied from the auxiliaries network
Fig Field Flashing
Because the field flashing contactor is not able to switch off the energy stored in
the field the control ensures that the contactor can only reopen if the field circuit breaker
has already been opened (generating the TRIP order) or in a normal field flashing
sequence when the converter has taken over the field current Field flashing occurs in the
following stages
1048729The excitation is switched on closing the field flashing contactor ( Field
Circuit Breaker is already closed )
1048729The start-up excitation current flows through the rotor driving the generator
voltage up to approx 15 U
1048729After about 10 U the firing pulses to the converter are released and it begins
to excite the generator to its rated voltage
1048729After about 30 U the field flashing contactor opens (with no current since
the converter is now supplying the current)
The diode bridge at the input to the field flashing contactor prevents a feed-back
from the converter to the source of field flashing while the contactor is still closed
53 De-excitation
When malfunctions occur the stored field energy must be dissipated as quickly
and safely as possible to protect the generator This is done by the converter the field
circuit-breaker and the de-excitation (discharge) resistor
De-excitation (with opening of the field circuit-breaker) takes place in the following
stages
1048729The converter drives to its inverter limit position (negative ceiling voltage)
recovers a portion of the field energy into the network A trip command is given to the
field circuit breaker
1048729The de-excitation contact closes diverting the field voltage to the de-excitation
resistor
1048729Then immediately the main contacts open building voltage The field voltage
commutates to the de-excitation resistor
1048729The current diminishes at a given time constant TE
(With linear resistance TE = Lf (Rf + Re))
Due to the reversal of the field voltage by the converter the field current
commutates from the main contacts of the field circuit-breaker to the de-excitation
resistor in a very early phase This reversal of the field voltage prevents burn-off on the
main contacts and provides effective protection for the field circuit-breaker Depending
on the operating policy an operational shut-down of the excitation can also be effected
with the field circuit-breaker closed This method is useful mainly when the excitation is
switched on and off frequently In this case the converter is merely driven into the
inverter limit position so that the field energy is recovered into the network The
converter then blocks since it is supplying positive current only
54 Excitation Transformer
The excitation transformer matches the generator voltage to the field voltage
(required ceiling voltage) It also serves as a commutation reactance for the thyristor
converter and as a potential isolator between the network and the excitation circuit In
addition the transformer functions as a current limiter in that it makes it possible to keep
any short circuits in the excitation circuit under better control The excitation transformer
is equipped with temperature monitoring probes which set off an alarm when the
temperature exceeds a first max limit and then trips the excitation if the temperature
continues rising to a second (higher)limit
CHAPTER 6
Monitoring and Protection
61 Excitation Monitoring
611 General Information
The main goal of Excitation Monitoring is to make optimum use of the
redundancies provided in the excitation system and to give alarm whenever a malfunction
makes these redundancies unavailable The field current is monitored to see that it does
not exceed a maximum level and if necessary a switch-over to the MANUAL channel is
initiated In addition the criterion for switching off the field flashing is generated The
excitation Monitoring consists of an autonomous processor system
612 over current Alarms
In the Excitation Monitoring the limits for are set at higher levels than the
settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of
the nominal field current contact R1 and the binary output associated with it are
activated immediately If field current remains gt 110 then after a preset inverse-time
has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs
associated with them are activated Parameters match the measurements for If1 and If2 to
the nominal value for field current so that the internal values can be processed and read as
pu values It can be used to falsify the actual value of the field current If (to raise it) so
as to cause a response from the alarm limits for purposes of testing The processed If
signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As
long as the field current If is above the threshold value 11 Ifn its peak value is
measured This is stored (until RESET) and can be read at any time on the Micro-
Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever
the integrated time-current value (intisup2dt) exceeds the preselected reference value the
alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously
issued to switch over to the stand by AUTO channel Software switch F758 enables the
three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-
time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the
desired limit curve for response is set using the factor F216
613 Switch-Off Criterion for Field Flashing
The Excitation Monitoring supplies the criterion for switching off the field
flashing Whether this criterion is activated based on the actual value for generator voltage
Ug or for field current If or both depends on the settings of the two threshold values
F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements
Ug12 and If12 are switched over depending on the present status of the channels
(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is
fixed at ldquological 1rdquo
614 Storage of Alarm Status
The outputs of the over current alarms (R1 R2 R0) and the messages NO
FAILURE MONITORING PARAMETERS CHANGED are stored messages can be
erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the
front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the
situation causing the alarm or the malfunction is no longer present Whenever the self-
diagnosis equipment in the processor detects a malfunction the output NO FAILURE of
MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo
is activated whenever parameters or settings of software switches have been changed via
the Micro-Terminal
615 Actual Value Monitoring
The actual values for generator voltage Ug synchronous voltage Usyn and field
current If are monitored for malfunctions This monitoring is active regardless of whether
or not the generator is in operation Essentially when the generator is in operation the
measurements are monitored by comparing the signals (the smaller signal reading is
detected as incorrect) When the generator is not in operation the measured data are
monitored for extreme values The percentage of deviation permissible in the
measurement signals being compared
(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and
F20AIf the excitation transformer is being supplied from an auxiliary power source (no
shunt operation) the values of Ug and Usyn will be different in some operational
conditions
In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1
and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the
messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2
and the binary message from CH1 reports no malfunction a malfunction on
Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is
also present whenever the binary message CH1 DISTURBANCE is reported and a
discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2
FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares
Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the
comparison channel reports a malfunction or whenever both binary messages report no
malfunction - but both secondary monitors report a malfunction As long as the secondary
monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction
signals for the measurement channel involved (suspicion that there is a corresponding
error in Usyn) The measurement channel malfunctions are enabled operationally
whenever after excitation has been switched on generator voltage Ug exceeds the value
set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is
switched off to see that they do not exceed the limit value F210 that applies to both of
them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt
the operating range) Monitoring for extreme values is likewise enabled during normal
operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the
malfunction signals to the binary outputs Basically the monitoring of the actual values
for If1If2 functions like that of the Ug1Ug2 monitoring
62 Excitation Protection
621 General Information
The Excitation Protection switches off the excitation (and de-excites the machine
rapidly) whenever a danger arises that threatens the excitation transformer the converter
or the generator Generally limiter or monitoring functions precede the emergency trips
and these normally respond before the Excitation Protection must initiate a trip
Protective trip commands are issued directly to the field circuit-breaker from potential
free contacts of the board via the trip relays They are directed redundantly to the
operative field circuit-breaker ldquoOFFrdquo command
622 Protection against Excitation Transformer Overheating
This equipment monitors the excitation transformer for overheating in the
windings that could result from over current short-circuits or inadequate cooling The
monitoring uses temperature monitoring modules in conjunction with temperature
sensors built into the windings Normally the temperature is monitored in two stages the
first stage sets off an alarm the second causes a trip of the excitation
623 Rotor Over voltage Protection
Malfunctions in the generator circuit (eg terminal short-circuit failed
synchronization asynchronous operation) cause induced negative field currents that
produce high voltages in the field circuit These must be restricted to a level with a
sufficient safety margin below the insulation capacity of the field winding (test voltage)
and also below the peak blocking voltage of the converter thyristors The crow bar
employs spark gap elements to detect over voltages in the field circuit Whenever they
respond the associated thyristors are fired immediately switching the de-excitation
resistor parallel to the field The de-excitation current generated thereby initiates an
excitation trip via a supervision circuit causing an immediate opening of the field circuit-
breaker The malfunction isets off an alarm and an internal malfunction is indicated at the
cubicle
TEST VALUES OBTAINED WHEN EXCITATION IS RAISED
TEST
SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE
1 VREF 996 100
2 VACT 997 1003
3 IFACT 735 765
4 IGACT 703 707
5 ACTIVE
POWER
703 705
6 REACTIVE
POWER
101 142
7 POWER
FACTOR
099 IND 098 IND
8 ACTIVE
CURRENT(IR)
705 703
9 REACTIVE
CURRENT(IX)
102 138
10 POWER
ANGLE
566 546
11 FIRING
ANGLE
640 632
VALUES OBTAINED WHEN EXCITATION IS LOWERED
SNO PARTICULARS ACTUAL
VALUE
OBTAINED VALUE
1 VREF 100 997
2 VACT 100 997
3 IFACT 787 761
4 IGACT 839 837
5 ACTIVE
POWER
839 830
6 REACTIVE
POWER
155 110
7 POWER
FACTOR
098 IND 099 IND
8 ACTIVE
CURRENT(IR)
835 830
9 REACTIVE
CURRENT(IX)
140 94
10 POWER
ANGLE
613 DEG 631 DEG
11 FIRING
ANGLE
601 DEG 599 DEG
CONCLUSION
For generating the EMF in stator winding excitation is required to the rotor of a
generator There are two types of excitation
1 Static excitation system
2 Brushless excitation system
A certain disadvantage in brushless excitation system is the slow response time of
the field in case of fast load changes specified No slip-rings and brushes direct
measurements of the field parameters not possible
To avoid all loses static excitation is used Since it does not have any rotating
parts mechanical loses and windage loses This system has fast response and speed
control While preferring this excitation system there are no limitations for the
redundancy of Thyristor bridge circuits
Static excitation has fast field discharge by resistor and inverter operation direct
measurement of field quantity is possible The meaning of excitation is nothing but
continuous supply of DC current (ie field current) to the rotor to buildup required
output voltage in the stator
Field current is changed with respect to the change of load so the digital
automatic voltage regulator (DAVR) is used to regulate the output voltage according to
the load variations
So we conclude that static excitation system with DAVR is preferred since it is
having excellent dynamic performance and better options for R amp M
CHAPTER 5
Field Current Circuit Excitation Transformer
51 Field Circuit Breaker
The circuit-breaker in the field circuit is used to isolate the field circuit from the
converter It is capable of switching off the synchronous machine from full load under the
maximum conditions of a 3-phase short-circuit In addition to its main contacts the field
circuit-breaker also has a de-excitation contact with which the field energy stored in the
field can be dissipated across the de-excitation resistor The de-excitation contact closes
shortly before the main contacts open so as to ensure proper commutation of the field
current from the main contacts to the de-excitation contact when the breaker is switched
off The field circuit-breaker is switched on by electromagnetic force and is kept switched
on by a mechanical latch When the latch is released by a trip coil the circuit-breaker
opens The circuit-breaker also has auxiliary contacts that report its status
52 Field Flashing
In shunt supplied excitation circuits (excitation transformer connected to the
generator terminals) the generator does not have enough remnant voltage for a generator
voltage build-up via the converter In this case a field flashing circuit is provided It
consists of the field flashing contactor the diode bridge and a transformer used to adapt
the auxiliary input voltage to the voltage needed for field flashing when power is
supplied from the auxiliaries network
Fig Field Flashing
Because the field flashing contactor is not able to switch off the energy stored in
the field the control ensures that the contactor can only reopen if the field circuit breaker
has already been opened (generating the TRIP order) or in a normal field flashing
sequence when the converter has taken over the field current Field flashing occurs in the
following stages
1048729The excitation is switched on closing the field flashing contactor ( Field
Circuit Breaker is already closed )
1048729The start-up excitation current flows through the rotor driving the generator
voltage up to approx 15 U
1048729After about 10 U the firing pulses to the converter are released and it begins
to excite the generator to its rated voltage
1048729After about 30 U the field flashing contactor opens (with no current since
the converter is now supplying the current)
The diode bridge at the input to the field flashing contactor prevents a feed-back
from the converter to the source of field flashing while the contactor is still closed
53 De-excitation
When malfunctions occur the stored field energy must be dissipated as quickly
and safely as possible to protect the generator This is done by the converter the field
circuit-breaker and the de-excitation (discharge) resistor
De-excitation (with opening of the field circuit-breaker) takes place in the following
stages
1048729The converter drives to its inverter limit position (negative ceiling voltage)
recovers a portion of the field energy into the network A trip command is given to the
field circuit breaker
1048729The de-excitation contact closes diverting the field voltage to the de-excitation
resistor
1048729Then immediately the main contacts open building voltage The field voltage
commutates to the de-excitation resistor
1048729The current diminishes at a given time constant TE
(With linear resistance TE = Lf (Rf + Re))
Due to the reversal of the field voltage by the converter the field current
commutates from the main contacts of the field circuit-breaker to the de-excitation
resistor in a very early phase This reversal of the field voltage prevents burn-off on the
main contacts and provides effective protection for the field circuit-breaker Depending
on the operating policy an operational shut-down of the excitation can also be effected
with the field circuit-breaker closed This method is useful mainly when the excitation is
switched on and off frequently In this case the converter is merely driven into the
inverter limit position so that the field energy is recovered into the network The
converter then blocks since it is supplying positive current only
54 Excitation Transformer
The excitation transformer matches the generator voltage to the field voltage
(required ceiling voltage) It also serves as a commutation reactance for the thyristor
converter and as a potential isolator between the network and the excitation circuit In
addition the transformer functions as a current limiter in that it makes it possible to keep
any short circuits in the excitation circuit under better control The excitation transformer
is equipped with temperature monitoring probes which set off an alarm when the
temperature exceeds a first max limit and then trips the excitation if the temperature
continues rising to a second (higher)limit
CHAPTER 6
Monitoring and Protection
61 Excitation Monitoring
611 General Information
The main goal of Excitation Monitoring is to make optimum use of the
redundancies provided in the excitation system and to give alarm whenever a malfunction
makes these redundancies unavailable The field current is monitored to see that it does
not exceed a maximum level and if necessary a switch-over to the MANUAL channel is
initiated In addition the criterion for switching off the field flashing is generated The
excitation Monitoring consists of an autonomous processor system
612 over current Alarms
In the Excitation Monitoring the limits for are set at higher levels than the
settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of
the nominal field current contact R1 and the binary output associated with it are
activated immediately If field current remains gt 110 then after a preset inverse-time
has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs
associated with them are activated Parameters match the measurements for If1 and If2 to
the nominal value for field current so that the internal values can be processed and read as
pu values It can be used to falsify the actual value of the field current If (to raise it) so
as to cause a response from the alarm limits for purposes of testing The processed If
signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As
long as the field current If is above the threshold value 11 Ifn its peak value is
measured This is stored (until RESET) and can be read at any time on the Micro-
Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever
the integrated time-current value (intisup2dt) exceeds the preselected reference value the
alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously
issued to switch over to the stand by AUTO channel Software switch F758 enables the
three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-
time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the
desired limit curve for response is set using the factor F216
613 Switch-Off Criterion for Field Flashing
The Excitation Monitoring supplies the criterion for switching off the field
flashing Whether this criterion is activated based on the actual value for generator voltage
Ug or for field current If or both depends on the settings of the two threshold values
F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements
Ug12 and If12 are switched over depending on the present status of the channels
(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is
fixed at ldquological 1rdquo
614 Storage of Alarm Status
The outputs of the over current alarms (R1 R2 R0) and the messages NO
FAILURE MONITORING PARAMETERS CHANGED are stored messages can be
erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the
front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the
situation causing the alarm or the malfunction is no longer present Whenever the self-
diagnosis equipment in the processor detects a malfunction the output NO FAILURE of
MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo
is activated whenever parameters or settings of software switches have been changed via
the Micro-Terminal
615 Actual Value Monitoring
The actual values for generator voltage Ug synchronous voltage Usyn and field
current If are monitored for malfunctions This monitoring is active regardless of whether
or not the generator is in operation Essentially when the generator is in operation the
measurements are monitored by comparing the signals (the smaller signal reading is
detected as incorrect) When the generator is not in operation the measured data are
monitored for extreme values The percentage of deviation permissible in the
measurement signals being compared
(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and
F20AIf the excitation transformer is being supplied from an auxiliary power source (no
shunt operation) the values of Ug and Usyn will be different in some operational
conditions
In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1
and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the
messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2
and the binary message from CH1 reports no malfunction a malfunction on
Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is
also present whenever the binary message CH1 DISTURBANCE is reported and a
discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2
FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares
Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the
comparison channel reports a malfunction or whenever both binary messages report no
malfunction - but both secondary monitors report a malfunction As long as the secondary
monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction
signals for the measurement channel involved (suspicion that there is a corresponding
error in Usyn) The measurement channel malfunctions are enabled operationally
whenever after excitation has been switched on generator voltage Ug exceeds the value
set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is
switched off to see that they do not exceed the limit value F210 that applies to both of
them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt
the operating range) Monitoring for extreme values is likewise enabled during normal
operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the
malfunction signals to the binary outputs Basically the monitoring of the actual values
for If1If2 functions like that of the Ug1Ug2 monitoring
62 Excitation Protection
621 General Information
The Excitation Protection switches off the excitation (and de-excites the machine
rapidly) whenever a danger arises that threatens the excitation transformer the converter
or the generator Generally limiter or monitoring functions precede the emergency trips
and these normally respond before the Excitation Protection must initiate a trip
Protective trip commands are issued directly to the field circuit-breaker from potential
free contacts of the board via the trip relays They are directed redundantly to the
operative field circuit-breaker ldquoOFFrdquo command
622 Protection against Excitation Transformer Overheating
This equipment monitors the excitation transformer for overheating in the
windings that could result from over current short-circuits or inadequate cooling The
monitoring uses temperature monitoring modules in conjunction with temperature
sensors built into the windings Normally the temperature is monitored in two stages the
first stage sets off an alarm the second causes a trip of the excitation
623 Rotor Over voltage Protection
Malfunctions in the generator circuit (eg terminal short-circuit failed
synchronization asynchronous operation) cause induced negative field currents that
produce high voltages in the field circuit These must be restricted to a level with a
sufficient safety margin below the insulation capacity of the field winding (test voltage)
and also below the peak blocking voltage of the converter thyristors The crow bar
employs spark gap elements to detect over voltages in the field circuit Whenever they
respond the associated thyristors are fired immediately switching the de-excitation
resistor parallel to the field The de-excitation current generated thereby initiates an
excitation trip via a supervision circuit causing an immediate opening of the field circuit-
breaker The malfunction isets off an alarm and an internal malfunction is indicated at the
cubicle
TEST VALUES OBTAINED WHEN EXCITATION IS RAISED
TEST
SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE
1 VREF 996 100
2 VACT 997 1003
3 IFACT 735 765
4 IGACT 703 707
5 ACTIVE
POWER
703 705
6 REACTIVE
POWER
101 142
7 POWER
FACTOR
099 IND 098 IND
8 ACTIVE
CURRENT(IR)
705 703
9 REACTIVE
CURRENT(IX)
102 138
10 POWER
ANGLE
566 546
11 FIRING
ANGLE
640 632
VALUES OBTAINED WHEN EXCITATION IS LOWERED
SNO PARTICULARS ACTUAL
VALUE
OBTAINED VALUE
1 VREF 100 997
2 VACT 100 997
3 IFACT 787 761
4 IGACT 839 837
5 ACTIVE
POWER
839 830
6 REACTIVE
POWER
155 110
7 POWER
FACTOR
098 IND 099 IND
8 ACTIVE
CURRENT(IR)
835 830
9 REACTIVE
CURRENT(IX)
140 94
10 POWER
ANGLE
613 DEG 631 DEG
11 FIRING
ANGLE
601 DEG 599 DEG
CONCLUSION
For generating the EMF in stator winding excitation is required to the rotor of a
generator There are two types of excitation
1 Static excitation system
2 Brushless excitation system
A certain disadvantage in brushless excitation system is the slow response time of
the field in case of fast load changes specified No slip-rings and brushes direct
measurements of the field parameters not possible
To avoid all loses static excitation is used Since it does not have any rotating
parts mechanical loses and windage loses This system has fast response and speed
control While preferring this excitation system there are no limitations for the
redundancy of Thyristor bridge circuits
Static excitation has fast field discharge by resistor and inverter operation direct
measurement of field quantity is possible The meaning of excitation is nothing but
continuous supply of DC current (ie field current) to the rotor to buildup required
output voltage in the stator
Field current is changed with respect to the change of load so the digital
automatic voltage regulator (DAVR) is used to regulate the output voltage according to
the load variations
So we conclude that static excitation system with DAVR is preferred since it is
having excellent dynamic performance and better options for R amp M
52 Field Flashing
In shunt supplied excitation circuits (excitation transformer connected to the
generator terminals) the generator does not have enough remnant voltage for a generator
voltage build-up via the converter In this case a field flashing circuit is provided It
consists of the field flashing contactor the diode bridge and a transformer used to adapt
the auxiliary input voltage to the voltage needed for field flashing when power is
supplied from the auxiliaries network
Fig Field Flashing
Because the field flashing contactor is not able to switch off the energy stored in
the field the control ensures that the contactor can only reopen if the field circuit breaker
has already been opened (generating the TRIP order) or in a normal field flashing
sequence when the converter has taken over the field current Field flashing occurs in the
following stages
1048729The excitation is switched on closing the field flashing contactor ( Field
Circuit Breaker is already closed )
1048729The start-up excitation current flows through the rotor driving the generator
voltage up to approx 15 U
1048729After about 10 U the firing pulses to the converter are released and it begins
to excite the generator to its rated voltage
1048729After about 30 U the field flashing contactor opens (with no current since
the converter is now supplying the current)
The diode bridge at the input to the field flashing contactor prevents a feed-back
from the converter to the source of field flashing while the contactor is still closed
53 De-excitation
When malfunctions occur the stored field energy must be dissipated as quickly
and safely as possible to protect the generator This is done by the converter the field
circuit-breaker and the de-excitation (discharge) resistor
De-excitation (with opening of the field circuit-breaker) takes place in the following
stages
1048729The converter drives to its inverter limit position (negative ceiling voltage)
recovers a portion of the field energy into the network A trip command is given to the
field circuit breaker
1048729The de-excitation contact closes diverting the field voltage to the de-excitation
resistor
1048729Then immediately the main contacts open building voltage The field voltage
commutates to the de-excitation resistor
1048729The current diminishes at a given time constant TE
(With linear resistance TE = Lf (Rf + Re))
Due to the reversal of the field voltage by the converter the field current
commutates from the main contacts of the field circuit-breaker to the de-excitation
resistor in a very early phase This reversal of the field voltage prevents burn-off on the
main contacts and provides effective protection for the field circuit-breaker Depending
on the operating policy an operational shut-down of the excitation can also be effected
with the field circuit-breaker closed This method is useful mainly when the excitation is
switched on and off frequently In this case the converter is merely driven into the
inverter limit position so that the field energy is recovered into the network The
converter then blocks since it is supplying positive current only
54 Excitation Transformer
The excitation transformer matches the generator voltage to the field voltage
(required ceiling voltage) It also serves as a commutation reactance for the thyristor
converter and as a potential isolator between the network and the excitation circuit In
addition the transformer functions as a current limiter in that it makes it possible to keep
any short circuits in the excitation circuit under better control The excitation transformer
is equipped with temperature monitoring probes which set off an alarm when the
temperature exceeds a first max limit and then trips the excitation if the temperature
continues rising to a second (higher)limit
CHAPTER 6
Monitoring and Protection
61 Excitation Monitoring
611 General Information
The main goal of Excitation Monitoring is to make optimum use of the
redundancies provided in the excitation system and to give alarm whenever a malfunction
makes these redundancies unavailable The field current is monitored to see that it does
not exceed a maximum level and if necessary a switch-over to the MANUAL channel is
initiated In addition the criterion for switching off the field flashing is generated The
excitation Monitoring consists of an autonomous processor system
612 over current Alarms
In the Excitation Monitoring the limits for are set at higher levels than the
settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of
the nominal field current contact R1 and the binary output associated with it are
activated immediately If field current remains gt 110 then after a preset inverse-time
has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs
associated with them are activated Parameters match the measurements for If1 and If2 to
the nominal value for field current so that the internal values can be processed and read as
pu values It can be used to falsify the actual value of the field current If (to raise it) so
as to cause a response from the alarm limits for purposes of testing The processed If
signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As
long as the field current If is above the threshold value 11 Ifn its peak value is
measured This is stored (until RESET) and can be read at any time on the Micro-
Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever
the integrated time-current value (intisup2dt) exceeds the preselected reference value the
alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously
issued to switch over to the stand by AUTO channel Software switch F758 enables the
three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-
time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the
desired limit curve for response is set using the factor F216
613 Switch-Off Criterion for Field Flashing
The Excitation Monitoring supplies the criterion for switching off the field
flashing Whether this criterion is activated based on the actual value for generator voltage
Ug or for field current If or both depends on the settings of the two threshold values
F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements
Ug12 and If12 are switched over depending on the present status of the channels
(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is
fixed at ldquological 1rdquo
614 Storage of Alarm Status
The outputs of the over current alarms (R1 R2 R0) and the messages NO
FAILURE MONITORING PARAMETERS CHANGED are stored messages can be
erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the
front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the
situation causing the alarm or the malfunction is no longer present Whenever the self-
diagnosis equipment in the processor detects a malfunction the output NO FAILURE of
MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo
is activated whenever parameters or settings of software switches have been changed via
the Micro-Terminal
615 Actual Value Monitoring
The actual values for generator voltage Ug synchronous voltage Usyn and field
current If are monitored for malfunctions This monitoring is active regardless of whether
or not the generator is in operation Essentially when the generator is in operation the
measurements are monitored by comparing the signals (the smaller signal reading is
detected as incorrect) When the generator is not in operation the measured data are
monitored for extreme values The percentage of deviation permissible in the
measurement signals being compared
(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and
F20AIf the excitation transformer is being supplied from an auxiliary power source (no
shunt operation) the values of Ug and Usyn will be different in some operational
conditions
In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1
and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the
messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2
and the binary message from CH1 reports no malfunction a malfunction on
Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is
also present whenever the binary message CH1 DISTURBANCE is reported and a
discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2
FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares
Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the
comparison channel reports a malfunction or whenever both binary messages report no
malfunction - but both secondary monitors report a malfunction As long as the secondary
monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction
signals for the measurement channel involved (suspicion that there is a corresponding
error in Usyn) The measurement channel malfunctions are enabled operationally
whenever after excitation has been switched on generator voltage Ug exceeds the value
set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is
switched off to see that they do not exceed the limit value F210 that applies to both of
them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt
the operating range) Monitoring for extreme values is likewise enabled during normal
operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the
malfunction signals to the binary outputs Basically the monitoring of the actual values
for If1If2 functions like that of the Ug1Ug2 monitoring
62 Excitation Protection
621 General Information
The Excitation Protection switches off the excitation (and de-excites the machine
rapidly) whenever a danger arises that threatens the excitation transformer the converter
or the generator Generally limiter or monitoring functions precede the emergency trips
and these normally respond before the Excitation Protection must initiate a trip
Protective trip commands are issued directly to the field circuit-breaker from potential
free contacts of the board via the trip relays They are directed redundantly to the
operative field circuit-breaker ldquoOFFrdquo command
622 Protection against Excitation Transformer Overheating
This equipment monitors the excitation transformer for overheating in the
windings that could result from over current short-circuits or inadequate cooling The
monitoring uses temperature monitoring modules in conjunction with temperature
sensors built into the windings Normally the temperature is monitored in two stages the
first stage sets off an alarm the second causes a trip of the excitation
623 Rotor Over voltage Protection
Malfunctions in the generator circuit (eg terminal short-circuit failed
synchronization asynchronous operation) cause induced negative field currents that
produce high voltages in the field circuit These must be restricted to a level with a
sufficient safety margin below the insulation capacity of the field winding (test voltage)
and also below the peak blocking voltage of the converter thyristors The crow bar
employs spark gap elements to detect over voltages in the field circuit Whenever they
respond the associated thyristors are fired immediately switching the de-excitation
resistor parallel to the field The de-excitation current generated thereby initiates an
excitation trip via a supervision circuit causing an immediate opening of the field circuit-
breaker The malfunction isets off an alarm and an internal malfunction is indicated at the
cubicle
TEST VALUES OBTAINED WHEN EXCITATION IS RAISED
TEST
SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE
1 VREF 996 100
2 VACT 997 1003
3 IFACT 735 765
4 IGACT 703 707
5 ACTIVE
POWER
703 705
6 REACTIVE
POWER
101 142
7 POWER
FACTOR
099 IND 098 IND
8 ACTIVE
CURRENT(IR)
705 703
9 REACTIVE
CURRENT(IX)
102 138
10 POWER
ANGLE
566 546
11 FIRING
ANGLE
640 632
VALUES OBTAINED WHEN EXCITATION IS LOWERED
SNO PARTICULARS ACTUAL
VALUE
OBTAINED VALUE
1 VREF 100 997
2 VACT 100 997
3 IFACT 787 761
4 IGACT 839 837
5 ACTIVE
POWER
839 830
6 REACTIVE
POWER
155 110
7 POWER
FACTOR
098 IND 099 IND
8 ACTIVE
CURRENT(IR)
835 830
9 REACTIVE
CURRENT(IX)
140 94
10 POWER
ANGLE
613 DEG 631 DEG
11 FIRING
ANGLE
601 DEG 599 DEG
CONCLUSION
For generating the EMF in stator winding excitation is required to the rotor of a
generator There are two types of excitation
1 Static excitation system
2 Brushless excitation system
A certain disadvantage in brushless excitation system is the slow response time of
the field in case of fast load changes specified No slip-rings and brushes direct
measurements of the field parameters not possible
To avoid all loses static excitation is used Since it does not have any rotating
parts mechanical loses and windage loses This system has fast response and speed
control While preferring this excitation system there are no limitations for the
redundancy of Thyristor bridge circuits
Static excitation has fast field discharge by resistor and inverter operation direct
measurement of field quantity is possible The meaning of excitation is nothing but
continuous supply of DC current (ie field current) to the rotor to buildup required
output voltage in the stator
Field current is changed with respect to the change of load so the digital
automatic voltage regulator (DAVR) is used to regulate the output voltage according to
the load variations
So we conclude that static excitation system with DAVR is preferred since it is
having excellent dynamic performance and better options for R amp M
Fig Field Flashing
Because the field flashing contactor is not able to switch off the energy stored in
the field the control ensures that the contactor can only reopen if the field circuit breaker
has already been opened (generating the TRIP order) or in a normal field flashing
sequence when the converter has taken over the field current Field flashing occurs in the
following stages
1048729The excitation is switched on closing the field flashing contactor ( Field
Circuit Breaker is already closed )
1048729The start-up excitation current flows through the rotor driving the generator
voltage up to approx 15 U
1048729After about 10 U the firing pulses to the converter are released and it begins
to excite the generator to its rated voltage
1048729After about 30 U the field flashing contactor opens (with no current since
the converter is now supplying the current)
The diode bridge at the input to the field flashing contactor prevents a feed-back
from the converter to the source of field flashing while the contactor is still closed
53 De-excitation
When malfunctions occur the stored field energy must be dissipated as quickly
and safely as possible to protect the generator This is done by the converter the field
circuit-breaker and the de-excitation (discharge) resistor
De-excitation (with opening of the field circuit-breaker) takes place in the following
stages
1048729The converter drives to its inverter limit position (negative ceiling voltage)
recovers a portion of the field energy into the network A trip command is given to the
field circuit breaker
1048729The de-excitation contact closes diverting the field voltage to the de-excitation
resistor
1048729Then immediately the main contacts open building voltage The field voltage
commutates to the de-excitation resistor
1048729The current diminishes at a given time constant TE
(With linear resistance TE = Lf (Rf + Re))
Due to the reversal of the field voltage by the converter the field current
commutates from the main contacts of the field circuit-breaker to the de-excitation
resistor in a very early phase This reversal of the field voltage prevents burn-off on the
main contacts and provides effective protection for the field circuit-breaker Depending
on the operating policy an operational shut-down of the excitation can also be effected
with the field circuit-breaker closed This method is useful mainly when the excitation is
switched on and off frequently In this case the converter is merely driven into the
inverter limit position so that the field energy is recovered into the network The
converter then blocks since it is supplying positive current only
54 Excitation Transformer
The excitation transformer matches the generator voltage to the field voltage
(required ceiling voltage) It also serves as a commutation reactance for the thyristor
converter and as a potential isolator between the network and the excitation circuit In
addition the transformer functions as a current limiter in that it makes it possible to keep
any short circuits in the excitation circuit under better control The excitation transformer
is equipped with temperature monitoring probes which set off an alarm when the
temperature exceeds a first max limit and then trips the excitation if the temperature
continues rising to a second (higher)limit
CHAPTER 6
Monitoring and Protection
61 Excitation Monitoring
611 General Information
The main goal of Excitation Monitoring is to make optimum use of the
redundancies provided in the excitation system and to give alarm whenever a malfunction
makes these redundancies unavailable The field current is monitored to see that it does
not exceed a maximum level and if necessary a switch-over to the MANUAL channel is
initiated In addition the criterion for switching off the field flashing is generated The
excitation Monitoring consists of an autonomous processor system
612 over current Alarms
In the Excitation Monitoring the limits for are set at higher levels than the
settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of
the nominal field current contact R1 and the binary output associated with it are
activated immediately If field current remains gt 110 then after a preset inverse-time
has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs
associated with them are activated Parameters match the measurements for If1 and If2 to
the nominal value for field current so that the internal values can be processed and read as
pu values It can be used to falsify the actual value of the field current If (to raise it) so
as to cause a response from the alarm limits for purposes of testing The processed If
signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As
long as the field current If is above the threshold value 11 Ifn its peak value is
measured This is stored (until RESET) and can be read at any time on the Micro-
Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever
the integrated time-current value (intisup2dt) exceeds the preselected reference value the
alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously
issued to switch over to the stand by AUTO channel Software switch F758 enables the
three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-
time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the
desired limit curve for response is set using the factor F216
613 Switch-Off Criterion for Field Flashing
The Excitation Monitoring supplies the criterion for switching off the field
flashing Whether this criterion is activated based on the actual value for generator voltage
Ug or for field current If or both depends on the settings of the two threshold values
F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements
Ug12 and If12 are switched over depending on the present status of the channels
(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is
fixed at ldquological 1rdquo
614 Storage of Alarm Status
The outputs of the over current alarms (R1 R2 R0) and the messages NO
FAILURE MONITORING PARAMETERS CHANGED are stored messages can be
erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the
front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the
situation causing the alarm or the malfunction is no longer present Whenever the self-
diagnosis equipment in the processor detects a malfunction the output NO FAILURE of
MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo
is activated whenever parameters or settings of software switches have been changed via
the Micro-Terminal
615 Actual Value Monitoring
The actual values for generator voltage Ug synchronous voltage Usyn and field
current If are monitored for malfunctions This monitoring is active regardless of whether
or not the generator is in operation Essentially when the generator is in operation the
measurements are monitored by comparing the signals (the smaller signal reading is
detected as incorrect) When the generator is not in operation the measured data are
monitored for extreme values The percentage of deviation permissible in the
measurement signals being compared
(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and
F20AIf the excitation transformer is being supplied from an auxiliary power source (no
shunt operation) the values of Ug and Usyn will be different in some operational
conditions
In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1
and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the
messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2
and the binary message from CH1 reports no malfunction a malfunction on
Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is
also present whenever the binary message CH1 DISTURBANCE is reported and a
discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2
FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares
Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the
comparison channel reports a malfunction or whenever both binary messages report no
malfunction - but both secondary monitors report a malfunction As long as the secondary
monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction
signals for the measurement channel involved (suspicion that there is a corresponding
error in Usyn) The measurement channel malfunctions are enabled operationally
whenever after excitation has been switched on generator voltage Ug exceeds the value
set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is
switched off to see that they do not exceed the limit value F210 that applies to both of
them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt
the operating range) Monitoring for extreme values is likewise enabled during normal
operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the
malfunction signals to the binary outputs Basically the monitoring of the actual values
for If1If2 functions like that of the Ug1Ug2 monitoring
62 Excitation Protection
621 General Information
The Excitation Protection switches off the excitation (and de-excites the machine
rapidly) whenever a danger arises that threatens the excitation transformer the converter
or the generator Generally limiter or monitoring functions precede the emergency trips
and these normally respond before the Excitation Protection must initiate a trip
Protective trip commands are issued directly to the field circuit-breaker from potential
free contacts of the board via the trip relays They are directed redundantly to the
operative field circuit-breaker ldquoOFFrdquo command
622 Protection against Excitation Transformer Overheating
This equipment monitors the excitation transformer for overheating in the
windings that could result from over current short-circuits or inadequate cooling The
monitoring uses temperature monitoring modules in conjunction with temperature
sensors built into the windings Normally the temperature is monitored in two stages the
first stage sets off an alarm the second causes a trip of the excitation
623 Rotor Over voltage Protection
Malfunctions in the generator circuit (eg terminal short-circuit failed
synchronization asynchronous operation) cause induced negative field currents that
produce high voltages in the field circuit These must be restricted to a level with a
sufficient safety margin below the insulation capacity of the field winding (test voltage)
and also below the peak blocking voltage of the converter thyristors The crow bar
employs spark gap elements to detect over voltages in the field circuit Whenever they
respond the associated thyristors are fired immediately switching the de-excitation
resistor parallel to the field The de-excitation current generated thereby initiates an
excitation trip via a supervision circuit causing an immediate opening of the field circuit-
breaker The malfunction isets off an alarm and an internal malfunction is indicated at the
cubicle
TEST VALUES OBTAINED WHEN EXCITATION IS RAISED
TEST
SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE
1 VREF 996 100
2 VACT 997 1003
3 IFACT 735 765
4 IGACT 703 707
5 ACTIVE
POWER
703 705
6 REACTIVE
POWER
101 142
7 POWER
FACTOR
099 IND 098 IND
8 ACTIVE
CURRENT(IR)
705 703
9 REACTIVE
CURRENT(IX)
102 138
10 POWER
ANGLE
566 546
11 FIRING
ANGLE
640 632
VALUES OBTAINED WHEN EXCITATION IS LOWERED
SNO PARTICULARS ACTUAL
VALUE
OBTAINED VALUE
1 VREF 100 997
2 VACT 100 997
3 IFACT 787 761
4 IGACT 839 837
5 ACTIVE
POWER
839 830
6 REACTIVE
POWER
155 110
7 POWER
FACTOR
098 IND 099 IND
8 ACTIVE
CURRENT(IR)
835 830
9 REACTIVE
CURRENT(IX)
140 94
10 POWER
ANGLE
613 DEG 631 DEG
11 FIRING
ANGLE
601 DEG 599 DEG
CONCLUSION
For generating the EMF in stator winding excitation is required to the rotor of a
generator There are two types of excitation
1 Static excitation system
2 Brushless excitation system
A certain disadvantage in brushless excitation system is the slow response time of
the field in case of fast load changes specified No slip-rings and brushes direct
measurements of the field parameters not possible
To avoid all loses static excitation is used Since it does not have any rotating
parts mechanical loses and windage loses This system has fast response and speed
control While preferring this excitation system there are no limitations for the
redundancy of Thyristor bridge circuits
Static excitation has fast field discharge by resistor and inverter operation direct
measurement of field quantity is possible The meaning of excitation is nothing but
continuous supply of DC current (ie field current) to the rotor to buildup required
output voltage in the stator
Field current is changed with respect to the change of load so the digital
automatic voltage regulator (DAVR) is used to regulate the output voltage according to
the load variations
So we conclude that static excitation system with DAVR is preferred since it is
having excellent dynamic performance and better options for R amp M
1048729The converter drives to its inverter limit position (negative ceiling voltage)
recovers a portion of the field energy into the network A trip command is given to the
field circuit breaker
1048729The de-excitation contact closes diverting the field voltage to the de-excitation
resistor
1048729Then immediately the main contacts open building voltage The field voltage
commutates to the de-excitation resistor
1048729The current diminishes at a given time constant TE
(With linear resistance TE = Lf (Rf + Re))
Due to the reversal of the field voltage by the converter the field current
commutates from the main contacts of the field circuit-breaker to the de-excitation
resistor in a very early phase This reversal of the field voltage prevents burn-off on the
main contacts and provides effective protection for the field circuit-breaker Depending
on the operating policy an operational shut-down of the excitation can also be effected
with the field circuit-breaker closed This method is useful mainly when the excitation is
switched on and off frequently In this case the converter is merely driven into the
inverter limit position so that the field energy is recovered into the network The
converter then blocks since it is supplying positive current only
54 Excitation Transformer
The excitation transformer matches the generator voltage to the field voltage
(required ceiling voltage) It also serves as a commutation reactance for the thyristor
converter and as a potential isolator between the network and the excitation circuit In
addition the transformer functions as a current limiter in that it makes it possible to keep
any short circuits in the excitation circuit under better control The excitation transformer
is equipped with temperature monitoring probes which set off an alarm when the
temperature exceeds a first max limit and then trips the excitation if the temperature
continues rising to a second (higher)limit
CHAPTER 6
Monitoring and Protection
61 Excitation Monitoring
611 General Information
The main goal of Excitation Monitoring is to make optimum use of the
redundancies provided in the excitation system and to give alarm whenever a malfunction
makes these redundancies unavailable The field current is monitored to see that it does
not exceed a maximum level and if necessary a switch-over to the MANUAL channel is
initiated In addition the criterion for switching off the field flashing is generated The
excitation Monitoring consists of an autonomous processor system
612 over current Alarms
In the Excitation Monitoring the limits for are set at higher levels than the
settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of
the nominal field current contact R1 and the binary output associated with it are
activated immediately If field current remains gt 110 then after a preset inverse-time
has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs
associated with them are activated Parameters match the measurements for If1 and If2 to
the nominal value for field current so that the internal values can be processed and read as
pu values It can be used to falsify the actual value of the field current If (to raise it) so
as to cause a response from the alarm limits for purposes of testing The processed If
signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As
long as the field current If is above the threshold value 11 Ifn its peak value is
measured This is stored (until RESET) and can be read at any time on the Micro-
Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever
the integrated time-current value (intisup2dt) exceeds the preselected reference value the
alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously
issued to switch over to the stand by AUTO channel Software switch F758 enables the
three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-
time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the
desired limit curve for response is set using the factor F216
613 Switch-Off Criterion for Field Flashing
The Excitation Monitoring supplies the criterion for switching off the field
flashing Whether this criterion is activated based on the actual value for generator voltage
Ug or for field current If or both depends on the settings of the two threshold values
F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements
Ug12 and If12 are switched over depending on the present status of the channels
(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is
fixed at ldquological 1rdquo
614 Storage of Alarm Status
The outputs of the over current alarms (R1 R2 R0) and the messages NO
FAILURE MONITORING PARAMETERS CHANGED are stored messages can be
erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the
front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the
situation causing the alarm or the malfunction is no longer present Whenever the self-
diagnosis equipment in the processor detects a malfunction the output NO FAILURE of
MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo
is activated whenever parameters or settings of software switches have been changed via
the Micro-Terminal
615 Actual Value Monitoring
The actual values for generator voltage Ug synchronous voltage Usyn and field
current If are monitored for malfunctions This monitoring is active regardless of whether
or not the generator is in operation Essentially when the generator is in operation the
measurements are monitored by comparing the signals (the smaller signal reading is
detected as incorrect) When the generator is not in operation the measured data are
monitored for extreme values The percentage of deviation permissible in the
measurement signals being compared
(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and
F20AIf the excitation transformer is being supplied from an auxiliary power source (no
shunt operation) the values of Ug and Usyn will be different in some operational
conditions
In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1
and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the
messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2
and the binary message from CH1 reports no malfunction a malfunction on
Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is
also present whenever the binary message CH1 DISTURBANCE is reported and a
discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2
FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares
Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the
comparison channel reports a malfunction or whenever both binary messages report no
malfunction - but both secondary monitors report a malfunction As long as the secondary
monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction
signals for the measurement channel involved (suspicion that there is a corresponding
error in Usyn) The measurement channel malfunctions are enabled operationally
whenever after excitation has been switched on generator voltage Ug exceeds the value
set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is
switched off to see that they do not exceed the limit value F210 that applies to both of
them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt
the operating range) Monitoring for extreme values is likewise enabled during normal
operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the
malfunction signals to the binary outputs Basically the monitoring of the actual values
for If1If2 functions like that of the Ug1Ug2 monitoring
62 Excitation Protection
621 General Information
The Excitation Protection switches off the excitation (and de-excites the machine
rapidly) whenever a danger arises that threatens the excitation transformer the converter
or the generator Generally limiter or monitoring functions precede the emergency trips
and these normally respond before the Excitation Protection must initiate a trip
Protective trip commands are issued directly to the field circuit-breaker from potential
free contacts of the board via the trip relays They are directed redundantly to the
operative field circuit-breaker ldquoOFFrdquo command
622 Protection against Excitation Transformer Overheating
This equipment monitors the excitation transformer for overheating in the
windings that could result from over current short-circuits or inadequate cooling The
monitoring uses temperature monitoring modules in conjunction with temperature
sensors built into the windings Normally the temperature is monitored in two stages the
first stage sets off an alarm the second causes a trip of the excitation
623 Rotor Over voltage Protection
Malfunctions in the generator circuit (eg terminal short-circuit failed
synchronization asynchronous operation) cause induced negative field currents that
produce high voltages in the field circuit These must be restricted to a level with a
sufficient safety margin below the insulation capacity of the field winding (test voltage)
and also below the peak blocking voltage of the converter thyristors The crow bar
employs spark gap elements to detect over voltages in the field circuit Whenever they
respond the associated thyristors are fired immediately switching the de-excitation
resistor parallel to the field The de-excitation current generated thereby initiates an
excitation trip via a supervision circuit causing an immediate opening of the field circuit-
breaker The malfunction isets off an alarm and an internal malfunction is indicated at the
cubicle
TEST VALUES OBTAINED WHEN EXCITATION IS RAISED
TEST
SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE
1 VREF 996 100
2 VACT 997 1003
3 IFACT 735 765
4 IGACT 703 707
5 ACTIVE
POWER
703 705
6 REACTIVE
POWER
101 142
7 POWER
FACTOR
099 IND 098 IND
8 ACTIVE
CURRENT(IR)
705 703
9 REACTIVE
CURRENT(IX)
102 138
10 POWER
ANGLE
566 546
11 FIRING
ANGLE
640 632
VALUES OBTAINED WHEN EXCITATION IS LOWERED
SNO PARTICULARS ACTUAL
VALUE
OBTAINED VALUE
1 VREF 100 997
2 VACT 100 997
3 IFACT 787 761
4 IGACT 839 837
5 ACTIVE
POWER
839 830
6 REACTIVE
POWER
155 110
7 POWER
FACTOR
098 IND 099 IND
8 ACTIVE
CURRENT(IR)
835 830
9 REACTIVE
CURRENT(IX)
140 94
10 POWER
ANGLE
613 DEG 631 DEG
11 FIRING
ANGLE
601 DEG 599 DEG
CONCLUSION
For generating the EMF in stator winding excitation is required to the rotor of a
generator There are two types of excitation
1 Static excitation system
2 Brushless excitation system
A certain disadvantage in brushless excitation system is the slow response time of
the field in case of fast load changes specified No slip-rings and brushes direct
measurements of the field parameters not possible
To avoid all loses static excitation is used Since it does not have any rotating
parts mechanical loses and windage loses This system has fast response and speed
control While preferring this excitation system there are no limitations for the
redundancy of Thyristor bridge circuits
Static excitation has fast field discharge by resistor and inverter operation direct
measurement of field quantity is possible The meaning of excitation is nothing but
continuous supply of DC current (ie field current) to the rotor to buildup required
output voltage in the stator
Field current is changed with respect to the change of load so the digital
automatic voltage regulator (DAVR) is used to regulate the output voltage according to
the load variations
So we conclude that static excitation system with DAVR is preferred since it is
having excellent dynamic performance and better options for R amp M
Monitoring and Protection
61 Excitation Monitoring
611 General Information
The main goal of Excitation Monitoring is to make optimum use of the
redundancies provided in the excitation system and to give alarm whenever a malfunction
makes these redundancies unavailable The field current is monitored to see that it does
not exceed a maximum level and if necessary a switch-over to the MANUAL channel is
initiated In addition the criterion for switching off the field flashing is generated The
excitation Monitoring consists of an autonomous processor system
612 over current Alarms
In the Excitation Monitoring the limits for are set at higher levels than the
settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of
the nominal field current contact R1 and the binary output associated with it are
activated immediately If field current remains gt 110 then after a preset inverse-time
has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs
associated with them are activated Parameters match the measurements for If1 and If2 to
the nominal value for field current so that the internal values can be processed and read as
pu values It can be used to falsify the actual value of the field current If (to raise it) so
as to cause a response from the alarm limits for purposes of testing The processed If
signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As
long as the field current If is above the threshold value 11 Ifn its peak value is
measured This is stored (until RESET) and can be read at any time on the Micro-
Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever
the integrated time-current value (intisup2dt) exceeds the preselected reference value the
alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously
issued to switch over to the stand by AUTO channel Software switch F758 enables the
three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-
time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the
desired limit curve for response is set using the factor F216
613 Switch-Off Criterion for Field Flashing
The Excitation Monitoring supplies the criterion for switching off the field
flashing Whether this criterion is activated based on the actual value for generator voltage
Ug or for field current If or both depends on the settings of the two threshold values
F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements
Ug12 and If12 are switched over depending on the present status of the channels
(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is
fixed at ldquological 1rdquo
614 Storage of Alarm Status
The outputs of the over current alarms (R1 R2 R0) and the messages NO
FAILURE MONITORING PARAMETERS CHANGED are stored messages can be
erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the
front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the
situation causing the alarm or the malfunction is no longer present Whenever the self-
diagnosis equipment in the processor detects a malfunction the output NO FAILURE of
MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo
is activated whenever parameters or settings of software switches have been changed via
the Micro-Terminal
615 Actual Value Monitoring
The actual values for generator voltage Ug synchronous voltage Usyn and field
current If are monitored for malfunctions This monitoring is active regardless of whether
or not the generator is in operation Essentially when the generator is in operation the
measurements are monitored by comparing the signals (the smaller signal reading is
detected as incorrect) When the generator is not in operation the measured data are
monitored for extreme values The percentage of deviation permissible in the
measurement signals being compared
(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and
F20AIf the excitation transformer is being supplied from an auxiliary power source (no
shunt operation) the values of Ug and Usyn will be different in some operational
conditions
In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1
and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the
messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2
and the binary message from CH1 reports no malfunction a malfunction on
Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is
also present whenever the binary message CH1 DISTURBANCE is reported and a
discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2
FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares
Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the
comparison channel reports a malfunction or whenever both binary messages report no
malfunction - but both secondary monitors report a malfunction As long as the secondary
monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction
signals for the measurement channel involved (suspicion that there is a corresponding
error in Usyn) The measurement channel malfunctions are enabled operationally
whenever after excitation has been switched on generator voltage Ug exceeds the value
set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is
switched off to see that they do not exceed the limit value F210 that applies to both of
them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt
the operating range) Monitoring for extreme values is likewise enabled during normal
operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the
malfunction signals to the binary outputs Basically the monitoring of the actual values
for If1If2 functions like that of the Ug1Ug2 monitoring
62 Excitation Protection
621 General Information
The Excitation Protection switches off the excitation (and de-excites the machine
rapidly) whenever a danger arises that threatens the excitation transformer the converter
or the generator Generally limiter or monitoring functions precede the emergency trips
and these normally respond before the Excitation Protection must initiate a trip
Protective trip commands are issued directly to the field circuit-breaker from potential
free contacts of the board via the trip relays They are directed redundantly to the
operative field circuit-breaker ldquoOFFrdquo command
622 Protection against Excitation Transformer Overheating
This equipment monitors the excitation transformer for overheating in the
windings that could result from over current short-circuits or inadequate cooling The
monitoring uses temperature monitoring modules in conjunction with temperature
sensors built into the windings Normally the temperature is monitored in two stages the
first stage sets off an alarm the second causes a trip of the excitation
623 Rotor Over voltage Protection
Malfunctions in the generator circuit (eg terminal short-circuit failed
synchronization asynchronous operation) cause induced negative field currents that
produce high voltages in the field circuit These must be restricted to a level with a
sufficient safety margin below the insulation capacity of the field winding (test voltage)
and also below the peak blocking voltage of the converter thyristors The crow bar
employs spark gap elements to detect over voltages in the field circuit Whenever they
respond the associated thyristors are fired immediately switching the de-excitation
resistor parallel to the field The de-excitation current generated thereby initiates an
excitation trip via a supervision circuit causing an immediate opening of the field circuit-
breaker The malfunction isets off an alarm and an internal malfunction is indicated at the
cubicle
TEST VALUES OBTAINED WHEN EXCITATION IS RAISED
TEST
SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE
1 VREF 996 100
2 VACT 997 1003
3 IFACT 735 765
4 IGACT 703 707
5 ACTIVE
POWER
703 705
6 REACTIVE
POWER
101 142
7 POWER
FACTOR
099 IND 098 IND
8 ACTIVE
CURRENT(IR)
705 703
9 REACTIVE
CURRENT(IX)
102 138
10 POWER
ANGLE
566 546
11 FIRING
ANGLE
640 632
VALUES OBTAINED WHEN EXCITATION IS LOWERED
SNO PARTICULARS ACTUAL
VALUE
OBTAINED VALUE
1 VREF 100 997
2 VACT 100 997
3 IFACT 787 761
4 IGACT 839 837
5 ACTIVE
POWER
839 830
6 REACTIVE
POWER
155 110
7 POWER
FACTOR
098 IND 099 IND
8 ACTIVE
CURRENT(IR)
835 830
9 REACTIVE
CURRENT(IX)
140 94
10 POWER
ANGLE
613 DEG 631 DEG
11 FIRING
ANGLE
601 DEG 599 DEG
CONCLUSION
For generating the EMF in stator winding excitation is required to the rotor of a
generator There are two types of excitation
1 Static excitation system
2 Brushless excitation system
A certain disadvantage in brushless excitation system is the slow response time of
the field in case of fast load changes specified No slip-rings and brushes direct
measurements of the field parameters not possible
To avoid all loses static excitation is used Since it does not have any rotating
parts mechanical loses and windage loses This system has fast response and speed
control While preferring this excitation system there are no limitations for the
redundancy of Thyristor bridge circuits
Static excitation has fast field discharge by resistor and inverter operation direct
measurement of field quantity is possible The meaning of excitation is nothing but
continuous supply of DC current (ie field current) to the rotor to buildup required
output voltage in the stator
Field current is changed with respect to the change of load so the digital
automatic voltage regulator (DAVR) is used to regulate the output voltage according to
the load variations
So we conclude that static excitation system with DAVR is preferred since it is
having excellent dynamic performance and better options for R amp M
The Excitation Monitoring supplies the criterion for switching off the field
flashing Whether this criterion is activated based on the actual value for generator voltage
Ug or for field current If or both depends on the settings of the two threshold values
F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements
Ug12 and If12 are switched over depending on the present status of the channels
(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is
fixed at ldquological 1rdquo
614 Storage of Alarm Status
The outputs of the over current alarms (R1 R2 R0) and the messages NO
FAILURE MONITORING PARAMETERS CHANGED are stored messages can be
erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the
front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the
situation causing the alarm or the malfunction is no longer present Whenever the self-
diagnosis equipment in the processor detects a malfunction the output NO FAILURE of
MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo
is activated whenever parameters or settings of software switches have been changed via
the Micro-Terminal
615 Actual Value Monitoring
The actual values for generator voltage Ug synchronous voltage Usyn and field
current If are monitored for malfunctions This monitoring is active regardless of whether
or not the generator is in operation Essentially when the generator is in operation the
measurements are monitored by comparing the signals (the smaller signal reading is
detected as incorrect) When the generator is not in operation the measured data are
monitored for extreme values The percentage of deviation permissible in the
measurement signals being compared
(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and
F20AIf the excitation transformer is being supplied from an auxiliary power source (no
shunt operation) the values of Ug and Usyn will be different in some operational
conditions
In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1
and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the
messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2
and the binary message from CH1 reports no malfunction a malfunction on
Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is
also present whenever the binary message CH1 DISTURBANCE is reported and a
discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2
FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares
Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the
comparison channel reports a malfunction or whenever both binary messages report no
malfunction - but both secondary monitors report a malfunction As long as the secondary
monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction
signals for the measurement channel involved (suspicion that there is a corresponding
error in Usyn) The measurement channel malfunctions are enabled operationally
whenever after excitation has been switched on generator voltage Ug exceeds the value
set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is
switched off to see that they do not exceed the limit value F210 that applies to both of
them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt
the operating range) Monitoring for extreme values is likewise enabled during normal
operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the
malfunction signals to the binary outputs Basically the monitoring of the actual values
for If1If2 functions like that of the Ug1Ug2 monitoring
62 Excitation Protection
621 General Information
The Excitation Protection switches off the excitation (and de-excites the machine
rapidly) whenever a danger arises that threatens the excitation transformer the converter
or the generator Generally limiter or monitoring functions precede the emergency trips
and these normally respond before the Excitation Protection must initiate a trip
Protective trip commands are issued directly to the field circuit-breaker from potential
free contacts of the board via the trip relays They are directed redundantly to the
operative field circuit-breaker ldquoOFFrdquo command
622 Protection against Excitation Transformer Overheating
This equipment monitors the excitation transformer for overheating in the
windings that could result from over current short-circuits or inadequate cooling The
monitoring uses temperature monitoring modules in conjunction with temperature
sensors built into the windings Normally the temperature is monitored in two stages the
first stage sets off an alarm the second causes a trip of the excitation
623 Rotor Over voltage Protection
Malfunctions in the generator circuit (eg terminal short-circuit failed
synchronization asynchronous operation) cause induced negative field currents that
produce high voltages in the field circuit These must be restricted to a level with a
sufficient safety margin below the insulation capacity of the field winding (test voltage)
and also below the peak blocking voltage of the converter thyristors The crow bar
employs spark gap elements to detect over voltages in the field circuit Whenever they
respond the associated thyristors are fired immediately switching the de-excitation
resistor parallel to the field The de-excitation current generated thereby initiates an
excitation trip via a supervision circuit causing an immediate opening of the field circuit-
breaker The malfunction isets off an alarm and an internal malfunction is indicated at the
cubicle
TEST VALUES OBTAINED WHEN EXCITATION IS RAISED
TEST
SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE
1 VREF 996 100
2 VACT 997 1003
3 IFACT 735 765
4 IGACT 703 707
5 ACTIVE
POWER
703 705
6 REACTIVE
POWER
101 142
7 POWER
FACTOR
099 IND 098 IND
8 ACTIVE
CURRENT(IR)
705 703
9 REACTIVE
CURRENT(IX)
102 138
10 POWER
ANGLE
566 546
11 FIRING
ANGLE
640 632
VALUES OBTAINED WHEN EXCITATION IS LOWERED
SNO PARTICULARS ACTUAL
VALUE
OBTAINED VALUE
1 VREF 100 997
2 VACT 100 997
3 IFACT 787 761
4 IGACT 839 837
5 ACTIVE
POWER
839 830
6 REACTIVE
POWER
155 110
7 POWER
FACTOR
098 IND 099 IND
8 ACTIVE
CURRENT(IR)
835 830
9 REACTIVE
CURRENT(IX)
140 94
10 POWER
ANGLE
613 DEG 631 DEG
11 FIRING
ANGLE
601 DEG 599 DEG
CONCLUSION
For generating the EMF in stator winding excitation is required to the rotor of a
generator There are two types of excitation
1 Static excitation system
2 Brushless excitation system
A certain disadvantage in brushless excitation system is the slow response time of
the field in case of fast load changes specified No slip-rings and brushes direct
measurements of the field parameters not possible
To avoid all loses static excitation is used Since it does not have any rotating
parts mechanical loses and windage loses This system has fast response and speed
control While preferring this excitation system there are no limitations for the
redundancy of Thyristor bridge circuits
Static excitation has fast field discharge by resistor and inverter operation direct
measurement of field quantity is possible The meaning of excitation is nothing but
continuous supply of DC current (ie field current) to the rotor to buildup required
output voltage in the stator
Field current is changed with respect to the change of load so the digital
automatic voltage regulator (DAVR) is used to regulate the output voltage according to
the load variations
So we conclude that static excitation system with DAVR is preferred since it is
having excellent dynamic performance and better options for R amp M
In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1
and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the
messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2
and the binary message from CH1 reports no malfunction a malfunction on
Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is
also present whenever the binary message CH1 DISTURBANCE is reported and a
discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2
FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares
Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the
comparison channel reports a malfunction or whenever both binary messages report no
malfunction - but both secondary monitors report a malfunction As long as the secondary
monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction
signals for the measurement channel involved (suspicion that there is a corresponding
error in Usyn) The measurement channel malfunctions are enabled operationally
whenever after excitation has been switched on generator voltage Ug exceeds the value
set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is
switched off to see that they do not exceed the limit value F210 that applies to both of
them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt
the operating range) Monitoring for extreme values is likewise enabled during normal
operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the
malfunction signals to the binary outputs Basically the monitoring of the actual values
for If1If2 functions like that of the Ug1Ug2 monitoring
62 Excitation Protection
621 General Information
The Excitation Protection switches off the excitation (and de-excites the machine
rapidly) whenever a danger arises that threatens the excitation transformer the converter
or the generator Generally limiter or monitoring functions precede the emergency trips
and these normally respond before the Excitation Protection must initiate a trip
Protective trip commands are issued directly to the field circuit-breaker from potential
free contacts of the board via the trip relays They are directed redundantly to the
operative field circuit-breaker ldquoOFFrdquo command
622 Protection against Excitation Transformer Overheating
This equipment monitors the excitation transformer for overheating in the
windings that could result from over current short-circuits or inadequate cooling The
monitoring uses temperature monitoring modules in conjunction with temperature
sensors built into the windings Normally the temperature is monitored in two stages the
first stage sets off an alarm the second causes a trip of the excitation
623 Rotor Over voltage Protection
Malfunctions in the generator circuit (eg terminal short-circuit failed
synchronization asynchronous operation) cause induced negative field currents that
produce high voltages in the field circuit These must be restricted to a level with a
sufficient safety margin below the insulation capacity of the field winding (test voltage)
and also below the peak blocking voltage of the converter thyristors The crow bar
employs spark gap elements to detect over voltages in the field circuit Whenever they
respond the associated thyristors are fired immediately switching the de-excitation
resistor parallel to the field The de-excitation current generated thereby initiates an
excitation trip via a supervision circuit causing an immediate opening of the field circuit-
breaker The malfunction isets off an alarm and an internal malfunction is indicated at the
cubicle
TEST VALUES OBTAINED WHEN EXCITATION IS RAISED
TEST
SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE
1 VREF 996 100
2 VACT 997 1003
3 IFACT 735 765
4 IGACT 703 707
5 ACTIVE
POWER
703 705
6 REACTIVE
POWER
101 142
7 POWER
FACTOR
099 IND 098 IND
8 ACTIVE
CURRENT(IR)
705 703
9 REACTIVE
CURRENT(IX)
102 138
10 POWER
ANGLE
566 546
11 FIRING
ANGLE
640 632
VALUES OBTAINED WHEN EXCITATION IS LOWERED
SNO PARTICULARS ACTUAL
VALUE
OBTAINED VALUE
1 VREF 100 997
2 VACT 100 997
3 IFACT 787 761
4 IGACT 839 837
5 ACTIVE
POWER
839 830
6 REACTIVE
POWER
155 110
7 POWER
FACTOR
098 IND 099 IND
8 ACTIVE
CURRENT(IR)
835 830
9 REACTIVE
CURRENT(IX)
140 94
10 POWER
ANGLE
613 DEG 631 DEG
11 FIRING
ANGLE
601 DEG 599 DEG
CONCLUSION
For generating the EMF in stator winding excitation is required to the rotor of a
generator There are two types of excitation
1 Static excitation system
2 Brushless excitation system
A certain disadvantage in brushless excitation system is the slow response time of
the field in case of fast load changes specified No slip-rings and brushes direct
measurements of the field parameters not possible
To avoid all loses static excitation is used Since it does not have any rotating
parts mechanical loses and windage loses This system has fast response and speed
control While preferring this excitation system there are no limitations for the
redundancy of Thyristor bridge circuits
Static excitation has fast field discharge by resistor and inverter operation direct
measurement of field quantity is possible The meaning of excitation is nothing but
continuous supply of DC current (ie field current) to the rotor to buildup required
output voltage in the stator
Field current is changed with respect to the change of load so the digital
automatic voltage regulator (DAVR) is used to regulate the output voltage according to
the load variations
So we conclude that static excitation system with DAVR is preferred since it is
having excellent dynamic performance and better options for R amp M
622 Protection against Excitation Transformer Overheating
This equipment monitors the excitation transformer for overheating in the
windings that could result from over current short-circuits or inadequate cooling The
monitoring uses temperature monitoring modules in conjunction with temperature
sensors built into the windings Normally the temperature is monitored in two stages the
first stage sets off an alarm the second causes a trip of the excitation
623 Rotor Over voltage Protection
Malfunctions in the generator circuit (eg terminal short-circuit failed
synchronization asynchronous operation) cause induced negative field currents that
produce high voltages in the field circuit These must be restricted to a level with a
sufficient safety margin below the insulation capacity of the field winding (test voltage)
and also below the peak blocking voltage of the converter thyristors The crow bar
employs spark gap elements to detect over voltages in the field circuit Whenever they
respond the associated thyristors are fired immediately switching the de-excitation
resistor parallel to the field The de-excitation current generated thereby initiates an
excitation trip via a supervision circuit causing an immediate opening of the field circuit-
breaker The malfunction isets off an alarm and an internal malfunction is indicated at the
cubicle
TEST VALUES OBTAINED WHEN EXCITATION IS RAISED
TEST
SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE
1 VREF 996 100
2 VACT 997 1003
3 IFACT 735 765
4 IGACT 703 707
5 ACTIVE
POWER
703 705
6 REACTIVE
POWER
101 142
7 POWER
FACTOR
099 IND 098 IND
8 ACTIVE
CURRENT(IR)
705 703
9 REACTIVE
CURRENT(IX)
102 138
10 POWER
ANGLE
566 546
11 FIRING
ANGLE
640 632
VALUES OBTAINED WHEN EXCITATION IS LOWERED
SNO PARTICULARS ACTUAL
VALUE
OBTAINED VALUE
1 VREF 100 997
2 VACT 100 997
3 IFACT 787 761
4 IGACT 839 837
5 ACTIVE
POWER
839 830
6 REACTIVE
POWER
155 110
7 POWER
FACTOR
098 IND 099 IND
8 ACTIVE
CURRENT(IR)
835 830
9 REACTIVE
CURRENT(IX)
140 94
10 POWER
ANGLE
613 DEG 631 DEG
11 FIRING
ANGLE
601 DEG 599 DEG
CONCLUSION
For generating the EMF in stator winding excitation is required to the rotor of a
generator There are two types of excitation
1 Static excitation system
2 Brushless excitation system
A certain disadvantage in brushless excitation system is the slow response time of
the field in case of fast load changes specified No slip-rings and brushes direct
measurements of the field parameters not possible
To avoid all loses static excitation is used Since it does not have any rotating
parts mechanical loses and windage loses This system has fast response and speed
control While preferring this excitation system there are no limitations for the
redundancy of Thyristor bridge circuits
Static excitation has fast field discharge by resistor and inverter operation direct
measurement of field quantity is possible The meaning of excitation is nothing but
continuous supply of DC current (ie field current) to the rotor to buildup required
output voltage in the stator
Field current is changed with respect to the change of load so the digital
automatic voltage regulator (DAVR) is used to regulate the output voltage according to
the load variations
So we conclude that static excitation system with DAVR is preferred since it is
having excellent dynamic performance and better options for R amp M
TEST VALUES OBTAINED WHEN EXCITATION IS RAISED
TEST
SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE
1 VREF 996 100
2 VACT 997 1003
3 IFACT 735 765
4 IGACT 703 707
5 ACTIVE
POWER
703 705
6 REACTIVE
POWER
101 142
7 POWER
FACTOR
099 IND 098 IND
8 ACTIVE
CURRENT(IR)
705 703
9 REACTIVE
CURRENT(IX)
102 138
10 POWER
ANGLE
566 546
11 FIRING
ANGLE
640 632
VALUES OBTAINED WHEN EXCITATION IS LOWERED
SNO PARTICULARS ACTUAL
VALUE
OBTAINED VALUE
1 VREF 100 997
2 VACT 100 997
3 IFACT 787 761
4 IGACT 839 837
5 ACTIVE
POWER
839 830
6 REACTIVE
POWER
155 110
7 POWER
FACTOR
098 IND 099 IND
8 ACTIVE
CURRENT(IR)
835 830
9 REACTIVE
CURRENT(IX)
140 94
10 POWER
ANGLE
613 DEG 631 DEG
11 FIRING
ANGLE
601 DEG 599 DEG
CONCLUSION
For generating the EMF in stator winding excitation is required to the rotor of a
generator There are two types of excitation
1 Static excitation system
2 Brushless excitation system
A certain disadvantage in brushless excitation system is the slow response time of
the field in case of fast load changes specified No slip-rings and brushes direct
measurements of the field parameters not possible
To avoid all loses static excitation is used Since it does not have any rotating
parts mechanical loses and windage loses This system has fast response and speed
control While preferring this excitation system there are no limitations for the
redundancy of Thyristor bridge circuits
Static excitation has fast field discharge by resistor and inverter operation direct
measurement of field quantity is possible The meaning of excitation is nothing but
continuous supply of DC current (ie field current) to the rotor to buildup required
output voltage in the stator
Field current is changed with respect to the change of load so the digital
automatic voltage regulator (DAVR) is used to regulate the output voltage according to
the load variations
So we conclude that static excitation system with DAVR is preferred since it is
having excellent dynamic performance and better options for R amp M
VALUES OBTAINED WHEN EXCITATION IS LOWERED
SNO PARTICULARS ACTUAL
VALUE
OBTAINED VALUE
1 VREF 100 997
2 VACT 100 997
3 IFACT 787 761
4 IGACT 839 837
5 ACTIVE
POWER
839 830
6 REACTIVE
POWER
155 110
7 POWER
FACTOR
098 IND 099 IND
8 ACTIVE
CURRENT(IR)
835 830
9 REACTIVE
CURRENT(IX)
140 94
10 POWER
ANGLE
613 DEG 631 DEG
11 FIRING
ANGLE
601 DEG 599 DEG
CONCLUSION
For generating the EMF in stator winding excitation is required to the rotor of a
generator There are two types of excitation
1 Static excitation system
2 Brushless excitation system
A certain disadvantage in brushless excitation system is the slow response time of
the field in case of fast load changes specified No slip-rings and brushes direct
measurements of the field parameters not possible
To avoid all loses static excitation is used Since it does not have any rotating
parts mechanical loses and windage loses This system has fast response and speed
control While preferring this excitation system there are no limitations for the
redundancy of Thyristor bridge circuits
Static excitation has fast field discharge by resistor and inverter operation direct
measurement of field quantity is possible The meaning of excitation is nothing but
continuous supply of DC current (ie field current) to the rotor to buildup required
output voltage in the stator
Field current is changed with respect to the change of load so the digital
automatic voltage regulator (DAVR) is used to regulate the output voltage according to
the load variations
So we conclude that static excitation system with DAVR is preferred since it is
having excellent dynamic performance and better options for R amp M
CONCLUSION
For generating the EMF in stator winding excitation is required to the rotor of a
generator There are two types of excitation
1 Static excitation system
2 Brushless excitation system
A certain disadvantage in brushless excitation system is the slow response time of
the field in case of fast load changes specified No slip-rings and brushes direct
measurements of the field parameters not possible
To avoid all loses static excitation is used Since it does not have any rotating
parts mechanical loses and windage loses This system has fast response and speed
control While preferring this excitation system there are no limitations for the
redundancy of Thyristor bridge circuits
Static excitation has fast field discharge by resistor and inverter operation direct
measurement of field quantity is possible The meaning of excitation is nothing but
continuous supply of DC current (ie field current) to the rotor to buildup required
output voltage in the stator
Field current is changed with respect to the change of load so the digital
automatic voltage regulator (DAVR) is used to regulate the output voltage according to
the load variations
So we conclude that static excitation system with DAVR is preferred since it is
having excellent dynamic performance and better options for R amp M