January 15, 2022 PMI Revision 00 1 Generator Excitation System & AVR
April 9, 2023 PMI Revision 00 1
Generator
Excitation System
& AVR
April 9, 2023 PMI Revision 00 2
Presentation outline Understanding basic principle
Types of excitation
Components of excitation system
Brief Description of most commonly used Excitation systems in power generating plants:
Static Excitation system
Brushless Excitation System
AVR
Experience sharing
Conclusion
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What is Excitation system?
• Creating and strengthening the magnetic field of the generator by passing DC through the filed winding.
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Why Excitation system?
• With large alternators in the power system, excitation plays a vital role in the management of voltage profile and reactive power in the grid thus ensuring ‘Stability’
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Purpose of excitation system
• Basic function is to provide the direct current to the synchronous machine field winding
• Regulate the terminal voltage of the machine• Meet the excitation power, regulates under all normal
operating conditions• Control reactive power flow and facilitates the sharing of
reactive load between the machines operated parallel in the grid
• Enable max utilization of machine capability• Guard the machine against inadvertent tripping during
transients• Improve dynamic and transient stability there by increasing
availability
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STATOR
ROTOR
EXCITATION PRINCIPLE
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STATOR
EXCITATION PRINCIPLE
ROTORN S
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Stator induced Voltage
E = K. L. dΦ/ dt
K = constant
L = length exposed to flux
dΦ/ dt = rate of change of flux
Frequency of induced Voltage
F = NP / 120
Magnitude of flux decides generated voltage and speed of rotation decides frequency of generated voltage
EXCITATION PRINCIPLE
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0 180
360
90
270
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The Equipment for supply, control and monitoring of this DC supply is called the Excitation system
G
Flux in the generator rotor is produced by feeding DC supply in the field coils, thus forming a 2 pole magnet of rotor
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TYPES OF EXCITATION
• Different types of excitation systems1.DC excitation system upto 100/110 MW
units2.Static excitation system3.Brush less excitation system
• Static excitation system is used in 200 MW units
• Brush less excitation system is used in 500 MW units
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EXCITATION SYSTEM REQUIREMENT
• Reliability
• Sensitivity and fast response
• Stability
• Ability to meet abnormal conditions
• Monitoring and annunciation of parameters
• User friendliness
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COMPONENTS OF TYPICAL EXCITATION SYSTEM
• Input and output interface , Aux. power supply, FB
• AVR: At least two independent channels
• Follow up control and changeover
• Excitation build up and Field Discharging system
• Cooling / heat dissipation components
•Limiters
• Protective relays
• Testing , Monitoring and alarm / trip initiation• Specific requirements :
Field Flashing, Stroboscope, PSS,
April 9, 2023 PMI Revision 00 14
Field
CT
PT
Voltage RegulatorManual Control
Rectifier Transformer
Thyristor Converter
Slip Ring
3 Ph AC
DC
Armature
Generator
G
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AVRAUTO
MAN
FDR
FF
415 v AC
STATIC EXCITATION SYSTEM ( 200 MW)
F B15.7
5 kV
575 v
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Static Excitation system
• Supply tapped from generator terminals• Quick response time• Brushes are used to supply field current• Thyristor bridge is used with full wave rectification to supply DC
voltage• At the time of starting field is supplied from external source
called field flashing• 1500 kva transformer• Thyristors are cooled by air• Max field current 1326 Amps
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Static Excitation system Contd..
• Crowbar is used across field terminals to prevent over voltages
• A resistor will be connected across the terminals of field at the time of tripping to de excite the machine
• Search coils are used to check the healthiness of the thysristors
April 9, 2023 PMI Revision 00 18
Static excitation system
voltage regulator
GT
EXC TRFR18KV/700V1500KVA
THYRISOR BRIDGE
GENERATOR
FIELD
From TGMCC- C
415/40V,10KVA
Pre Excitation
Non linear resistor
Field Breaker
Field discharge Resistor
Crow Bar
April 9, 2023 PMI Revision 00 19
Static excitation system
• Excitation power from generator via excitation transformer. Protective relays for excitation transformer
• Field forcing provided through 415 v aux supply
• Converter divided in to no of parallel (typically4 ) paths. Each one having separate pulse output stage and air flow monitoring.
• Two channels : Auto & manual, provision for change over from Auto to Manual Limiters : Stator current limiter, Rotor current limiter, Load angle limiter etc.
• Alternate supply for testing
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Field flashing • It is required for initial excitation as no power is available to excitation
system
• For start up DC excitation is fed to the field from external source like station battery or rectified AC from station Ac supply .
• Filed flashing is used to build up voltage up to 30 %.
• From 30 to 70 % both flashing and regulation remains in circuit.
• 70 % above flashing gets cut-off
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BRUSH GEAR
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Brushless excitation
PILOT EXCITER
MAIN EXCITER
GENERATOR
FIELD BREAKER
FIELD
(PM)
ARMATURE
ROTATING DIODES
R
Y
B
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N
S
Armature FieldArmature
Field
Rotating Rectifier
Voltage RegulatorManual Control
FieldArmature
GeneratorPilot Exciter Main Exciter
CT
PT
3 Ph AC
DCThyristor Rectifier
Rotor
April 9, 2023 PMI Revision 00 24
Components of Brush less Excitation System
•Three Phase Main Exciter.•Three Phase Pilot Exciter.•Regulation cubicle•Rectifier Wheels•Exciter Coolers•Metering and supervisory equipment.
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Brushless excitation• Brush less excitor consists of a 3-phase
permanent magnet pilot exciter , the output of which is rectified and controlled by Thyristor voltage regulator to provide variable d.c. current for the main exciter.
• The 3-phase are induced in the rotor of the main exciter and is rectified by the rotating diodes and to the field winding of generator through the d.c. leads fed in the rotor shaft.
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Brushless excitation• Since the rotating rectifier bridge is mounted on the
rotor, the slip rings are not required and the output of the rectifier is connected directly to the field winding through the generator rotor shaft.
• A common shaft carries the rectifier wheels, the rotor of the main exciter and permanent magnet rotor of the pilot exciter.
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Pilot exciter
• The three phase pilot exciter is a 16 pole revolving-field
Permanent magnet generator
• Each pole consists of 12 separate permanent magnets which
are housed in a non-magnetic metallic enclosure
• It supplies 220 v 400 hz supply to main exciter
• AVR is connected at the output of this pilot exciter
• Field breaker is also provided at the output only
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Pilot Exciter:
• Apparent power : 65 KVA• Current : 195 A• Voltage : 220 V• Speed : 3000 rpm• Poles : 16• Frequency : 400 HZ
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PMG
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Three phase main exciter
The three-phase main exciter is a six-pole revolving armature unit.
Stator consists of field Rotor produces three phase supply to feed to
rectifier wheels quadrature-axis coil is fitted for inductive measurement of
the exciter current. Rotating diode assembly on the shaft rectifies this voltage
and supplies to field winding through the shaft Stroboscope technique is used to check the healthiness of
the diodes 60 diodes per wheel and two wheels will be there.
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Main Exciter :
• Active power : 3780 KW• Current : 6300 A• Voltage : 600 V• Speed : 3000 rpm• Poles : 6• Frequency : 150 HZ
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MAIN EXCITER
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April 9, 2023 PMI Revision 00 34Pilot Exciter RotorFan
Main Exciter Armature
Rectifier Wheels Multi Contact Connector
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Excitation Power Requirement
Unit capacity MW
Excitation Current at Full Load
Excitation Voltage at full load
Ceiling Volts
200/ 210 2600 310 610
500 6300 600 1000
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AVR
BRUSHLESS EXCITATION SYSTEM (500 MW)
21 KV
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Brushless Excitation System
•Eliminates Slip Rings, Brushgear and all problems associated with transfer of current via sliding contacts
•Simple, Reliable and increasingly popular system the world over, Ideally suited for large sets
•Minimum operating and maintenance cost
•Self generating excitation unaffected by system fault/disturbances because of shaft mounted pilot exciter
Multi contact electrical connections between exciter and generator field
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• Rotor E/F monitoring system • alarm 80 KΏ, Trip 5 KΏ
• Stroboscope for thyristor fuse monitoring (one fuse for each pair of diodes, )
• Auto channel thyristor current monitor • For monitoring of thyristor bridge current , and initiating
change over to manual.
• ‘Auto’ to ‘Manual’ changeover in case of Auto channel power supply, thyristor set problem, or generator volts actual value problem
Brushless Excitation system
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DIFFERENCES BETWEEN BRUSHLESS AND STATIC EXCITATION SYSTEMS
More since slip rings and brushes are required. Also over hang vibrations are very high resulting in faster wear and tear.
Less since slip rings and brushes are avoided.
Maintenance. 5
No additional bearing and increase in shaft length are required.
One additional bearing and an increase in the shaft length are required.
Requirement of additional bearing and increase of turbo generator shaft length.
4
Very fast response in the order of 40 ms. due to the direct control and solid state devices employed.
Slower than static type since control is indirect (on the field of main exciter) and magnetic components involved.
Response of the excitation system.
3
Field flashing supply required for excitation build up.
No external source requirement since pilot exciter has permanent magnet field.
Dependency on external supply.
2
Static excitation system uses thyristors & taking supply from output of the generator
Brushless system gets activated with pilot exciter, main exciter and rotating diodes.
Type of system. 1
Static ExcitationBrushless ExcitationDescriptionS.NO
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EXCITER COOLINGVAPOUR EXHAUST
COOLER
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XG
EF VT
GENERATOR
Equivalent circuit of Generator
I
EF = I . XG + VT
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GENERATOR
VT
IL
IL.Xd
Ef
Phasor diagram of the Generator
ф
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GVbusVT
XTXd
Ef
GENERATOR
Generator + Generator Transformer Eq. Ckt.
G
GTGCB
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Vbus
VT
EF
IL
ф
Vector Diagram of Generator and GT connected to an infinite bus
GENERATOR
IL.XT
IL.Xd
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In the equivalent Circuit and Phasor diagram, the notations used have the following description:
Vbus : Infinite bus voltage
VT : Generator Terminal Voltage
EF : Induced Voltage (behind synchronous Impedance) of Generator, proportional to excitation.
Xd : Direct axis sync. Reactance assumed same as quadrature axis sync. Reactance
XT : Transformer reactance
IL : Load Current
Ф : Phase angle
: Torque Angle (rotor/load angle)
GENERATOR
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Referring to the phasor diagram on slide no.14;
Sin / IL.{Xd+XT} = Sin (90+ Ф) / EF
Putting Xd+XT =X, and multiplying both sides by VIL,
V Sin /X = VIL Cos Ф / EF
{Sin (90+ Ф) = Cos Ф}
or,
(EF . V / X) Sin = VIL Cos Ф = P
Pmax = EF . V / X
Note that the Electrical Power Output varies as the Sin of Load angle
GENERATORPOWER ANGLE EQUATION
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Torque angle diagram
0
0.2
0.4
0.6
0.8
1
1.2
0 30 60 90 120 150 180
Angle in degrees
Sin
de
lta
Torque angle diagram
0
0.2
0.4
0.6
0.8
1
1.2
Angle in degrees
Po
we
r in
pu
April 9, 2023 PMI Revision 00 50
ROTOR
STATOR
δ
Rotor mag. axis
Stator mag. axis
N
S
S
N
red
yellow
blue
Physical significance of load angle
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O Vbus
EF1
EF2 P1
P2
Locus of Constant Excitation
I2
I1
ф1
ф212
•Excitation constant;
•Steam flow increased
•Power output P1 to P2
ACTIVE POWER CHANGE
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O Vbus
EF1
EF2
Locus of P = const.
Locus of Constant Excitation
I2
I1
ф1
ф212
•Steam Flow constant;
•Excitation increased
•Power output Constant
I Cos ф = Constant
EXCITATION CHANGE
April 9, 2023 PMI Revision 00 53
Excitation Control
Power Angle Diagrams for Different Excitation Levels
00.20.40.60.8
11.21.4
0 30 60 90 120 150 180Power Angle (delta), in degrees
Pow
er in
per
un
it
P1
P2
P3
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AVR
April 9, 2023 PMI Revision 00 55
TYPES OF AVR SYSTEMS
• Single channel AVR system
• Dual channel AVR system
• Twin channel AVR system
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Single channel AVR systemHere we have two controllers one is automatic and the other is manual and both the controllers are fed from the same supply
The AVR senses the circuit parameters through current transformers and voltage transformers and initiates the control action by initiating control pulses , which are amplified and sent to the circuit components
The gate controller is used to vary the firing angle in order to control the field current for excitation
In case of any fault in the automatic voltage regulator the control can be switched on to the manual controller.
April 9, 2023 PMI Revision 00 57
Dual channel AVR systemHere also we have two controllers in the same manner as the previous case i.e. one automatic voltage controller and one manual controller
But here in contrary to the previous case we have different power supply, gate control and pulse amplifier units for each of the controllers
Reliability is more in this case than previous one since a fault in either gate control unit or pulse amplifier or power supply in single channel AVR will cause failure of whole unit, but in dual channel AVR this can be avoided by switching to another channel.
April 9, 2023 PMI Revision 00 58
Twin channel AVR system
This system almost resembles the dual channel AVR but the only difference is that here we have two automatic voltage regulators instead of one automatic voltage regulator and one manual Voltage regulator
This system has an edge over the previous one in the fact that in case of failure in the AVR of the Dual voltage regulator the manual system is switched on and it should be adjusted manually for the required change in the system and if the fault in AVR is not rectified in reasonable time it will be tedious to adjust the manual voltage regulator
April 9, 2023 PMI Revision 00 59
Twin channel AVR system
In Twin channel AVR both the AVRs sense the circuit parameters separately and switching to other regulator incase of fault is much easier and hence the system is more flexible than the other types.
Generally switching to manual regulator is only exceptional cases like faulty operation of AVR or commissioning and maintenance work and hence we can easily manage with one AVR and one manual regulator than two AVRs. So Twin channel AVR is only used in very few cases and generally Dual channel AVR is preferred.
April 9, 2023 PMI Revision 00 60
AVR
The feedback of voltage and current output of the generator is fed to avr where it is compared with the set point generator volts se from the control room
There are two independent control systems1. Auto control2. Manual control
The control is effected on the 3 phase output of the pilot exciter and provides a variable d.c. input to the main exciter
April 9, 2023 PMI Revision 00 61
AVR
The main components of the voltage Regulator are two closed –loop control systems each followed by separate gate control unit and thyristor set and de excitation equipment
Control system 1 for automatic generator voltage control
(AUTO) comprises the following
April 9, 2023 PMI Revision 00 62
AVR
Excitation current regulator, controlling the field current of
the main exciter
Circuits for automatic excitation build-up during start –up and field suppression during shut-down
Generator voltage control
The output quantity of this control is the set point for a following.
April 9, 2023 PMI Revision 00 63
AVR
This equipment acts on to the output of the generator voltage, control, limiting the set point for the above excitation current regulator. The stationary value of this limitation determines the maximum possible excitation current set-point (field forcing limitation);
Limiter for the under-excited range (under excitation limiter),
Delayed limiter for the overexcited range (over excitation limiter)
April 9, 2023 PMI Revision 00 64
AVR
In the under excitation range, the under excitation ensures that the minimum excitation required for stable parallel operation of the generator with the system is available and that the under -excited reactive power is limited accordingly
April 9, 2023 PMI Revision 00 65
AVR• Control system 2(manual) mainly comprises a
second excitation current regulator with separate sensing for the actual value this control system is also called manual control system, because for constant generator voltage manual re-adjusting of the excitation current set-point is required
April 9, 2023 PMI Revision 00 66
AVR
The set-point adjuster of the excitation current regulator for manual is tracked automatically (follow-up control) so that, in the event of faults, change over to the manual control system is possible without delay
Automatic change over is initiated by some special fault condition. Correct operation of the follow-up control circuit is monitored and can be observed on a matching instrument in the control room. This instrument can also be used for manual matching.
April 9, 2023 PMI Revision 00 67
AVR• The manual change over command is normally
issued from the control room. • Push buttons AUTO, MATCH, MANUAL are
provided for manual change over. • The MATCH push button must be actuated prior to
manual change over.
April 9, 2023 PMI Revision 00 68
AVR• Following this the RAISE, LOWER push buttons
must be actuated for matching the o/p value of set point adjuster for MANUAL or the set point adjuster for AUTO to actual excitation state.
• When matched state is reached the matching instrument in the control room indicates zero.
• Since different controlled variables are associated to the MANUAL and AUTO modes of operation, matching must not be effected by balancing of the set point adjuster position, which are also indicated in the control room.
April 9, 2023 PMI Revision 00 69
AVR• Change over to MANUAL or AUTO is only possible
after the MATCH condition has been selected and is done by remnant relay module in gate control set.
• When all the conditions for change over are fulfilled, change over is initiated by actuating pushbutton MANUAL or AUTO.
• The stored commands MATCH or AUTO are cancelled by check back signal “gate control set MANUAL ON” or “gate control set AUTO ON”.
April 9, 2023 PMI Revision 00 70
AVR
FAULT INDICATIONS
The following alarms are issued from the voltage regulator to the control room.
•AVR fault
•AVR automatic change over to MANUAL
•AVR loss of voltage alarm
April 9, 2023 PMI Revision 00 71
AVR
There are 3 limiters
1.Under excitation limiter
2.Over excitation limiter
3. V/F limiter
The current feedback is utilized for active and reactive power compensation and for limiters
April 9, 2023 PMI Revision 00 72
Excitation Interlocks
5s delay
Excitation ON command
N>90%
Protection Off
FCB Off feedback
External trip
GCB is OFF
ExcitationON
Preconditions for Excitation Preconditions for Excitation ONON
April 9, 2023 PMI Revision 00 73
Excitation OFF Interlocks
Delay 1sec
Exc. OFF from Field flashing
Exc OFF command
GCB OFF
N>90%
External trip
Exc OFF
GCB OFF
April 9, 2023 PMI Revision 00 74
Capability Curve• Capability Curve relates to the limits in which a generator can Operate safely.• Boundaries of the Curve within with the machine will operate safelyLagging Power Factor/Overexcited region Top Section Relates to Field Heating in Rotor Winding• Right Section Relates to Stator current Limit • Straight line relates to Prime Mover Output
Leading Power Factor/ Underexicted region
• Lower Side relates to Stator end ring Limit• Further down relates to Pole slipping
April 9, 2023 PMI Revision 00 75
Limiters• The limiters of the excitation system ensure an
operation within the generator capability curve• The characteristics of the limiters are within the power
characteristics
Field
heatin
g lim
it
Q,Reactive power
P,Active power
Max Turbine power limit
Under
ExcitedOver excited
O
N
M
P
Core
end h
eatin
g lim
it
Armature heating limit
QI
If
April 9, 2023 PMI Revision 00 76
• Points within the M-N-O-P-Q are allowed in the generator characteristics.
• Section M-O regards UEL stability limit• Section O-P regards the stator current limit- maintain the
stator temp raise within the limits• Section P-Q regards the generator rotor current limit – limit
the rotor temp raise • Section O-P-Q allow dynamic overshooting with an
adjustable PI characteristicTypes• Over excitation limiter• Under excitation limiter• Rotor angle limiter• Stator current limiter• V/F limiter
April 9, 2023 PMI Revision 00 77
April 9, 2023 PMI Revision 00 78
Excitation presentation04/09/23 Excitation presentation 78 of 5204/09/23
Generator Operation & Capability diagram
Rated MVA of the machine
Rated Megawatts
Rated synchronous impedance
Rated Short circuit ratio
Rated Power factor
Rated Hydrogen pressure
Minimum Boiler load
Key inputs required for the capability diagram to be drawn for the Turbo
generator
April 9, 2023 PMI Revision 00 79
Excitation presentation04/09/23 Excitation presentation 79 of 5204/09/23
Generator Operation & Capability diagram
April 9, 2023 PMI Revision 00 80
Excitation presentation04/09/23 Excitation presentation 80 of 5204/09/23
Generator Operation & Capability diagram
Normal Overexcited Operation
Under excited Operation
April 9, 2023 PMI Revision 00 81
LIMITERS
• Over excitation limiter• Under excitation limiter• Rotor angle limiter• Stator current limiter• V/F limiter
April 9, 2023 PMI Revision 00 82
Over excitation limiter
• Line voltage drops due to more reactive power requirement , switching operations or faults
• AVR increases generator excitation to hold the voltage constant
• Line voltage drops , thermal over loading of generator can result
• OEL is automatic limitation of generator excitation by lowering the generator voltage (otherwise the set point of generator voltage is reduced in time or the transformation ratio of the GT is to be adjusted )
• OEL permits excitation values above the normal excitation and extended to max excitation (for field forcing) for a limited time, so as to permit the generator to perform the grid stabilization in response to short drops in line voltage
• When IF >110% of Ifn , the OEL and Field forcing limiter are active
April 9, 2023 PMI Revision 00 83
Under Excitation limiter
• Function is to correct the reactive power when the excitation current falls below minimum excitation current value required for stable operation of generator
• Activation of UEL takes over the control from the closed loop voltage control, acting via a max selection
• The limit characteristic is adjustable (shifted parallel)• I reactive ref is compared with the measured I reactive , the
error is fed to P- amplifier. When the value drops below the characteristic the amplified diff signal causes the field current to increase
• For commissioning purpose provision is made to mirror the characteristic in the inductive range, this allowing both the direction in which the control signal acts and the blocking of the set point generators is to be changed
April 9, 2023 PMI Revision 00 84
Rotor Angle Limiter
• Stable operation rotor angle <900, for higher degree of stability a further margin of 10-12% is normally provided
• RAL gives the o/p as
permissible I reactive =F ( I active)
• Characteristic is shifted linearly as a function of generator voltage
• Permissible I reactive is compared with the measured value and is fed to the limit controller when the I reactive achieved value drops below the permissible value then the limiter comes in action and I reactive
April 9, 2023 PMI Revision 00 85
Stator current limiter• During operation at high active power P and / low voltage the
stator current of the generator tends to rise beyond its rated value and can cause the thermal overloading of stator, in spite of the action of the UEL
• An additional stator current limiting controller acting on the generator excitation is provided as a safe guard against such states of operation
• SCL always monitors the stator current measured value for crossing the rated stator current
• SCL permits small time over load but comes in action thereafter and influences the effective generator voltage set point- to reduce the Q till the stator current is brought down below the rated value
• Change in generator voltage set point is not blocked when SCL active
• SCL does not operate near the unity PF because near this value any limiter would cause oscillations
April 9, 2023 PMI Revision 00 86
V/F limiter• Also known as over fluxing limiter• It is the protection function for the GT• V/F ratio , eddy current , the local eddy current causes
thermal over loading of GT• In DVR mode V/F ratio is continuously monitors the limit
violation• In case V/F ratio crosses the limit characteristic, the upper limit
as the effective AVR set point is reduced as a function of V/F ratio
• This limiter is used when it is required to keep the unit operating even in case of substantial frequency drops , for instance in order to prevent complete breakdown of the system, a V/F limiter is used to lower the voltage proportional with frequency drop
April 9, 2023 PMI Revision 00 87
PRIORITY STRUCTURE OF AVR
Voltage regulator
UN-2010
3 rd priority
Stator current limiter
Capacitive
UN0027
Load angle limiter
UN1043
2 nd priority
Stator current li miter inductive
UN0027
Rotor current limiter
UN1024
1st priority
April 9, 2023 PMI Revision 00 88
Field failure protection
• Loss of generator field excitation under normal running conditions may arise due to any of the following condition.1. Failure of brush gear.2.unintentional opening of the field circuit breaker.3. Failure of AVR.
When generator on load loses it’s excitation , it starts to operate as an induction generator, running above synchronous speed.cylindrical rotor generators are not suited to such operation , because they don't have damper windings able to carry the induced currents, consequently this type of rotor will overheat rather quickly.
April 9, 2023 PMI Revision 00 89
Important alarms & actions1. Emergency C/O to ECR
----- Monitor terminal voltage2. Stator current limiter active
----- Reduce excitation by decreasing AVR set point3. Over excitation limiter active
------ Reduce excitation AVR set point4. Under excitation limiter active
----- Increase excitation Increase AVR set point5. V/F limiter active
----- Reduce excitation AVR set point6. Rotor angle limiter active
----- Increase excitation Increase aVR set point7. Fans on aux supply ----- Switch over to main supply
April 9, 2023 PMI Revision 00 90
THANK YOU