Generator Excitation System & AVR 4 August 2011
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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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EXCITATION PRINCIPLE
ROTOR
STATOR 4 August 2011
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ROTOR
S
N
EXCITATION PRINCIPLE
STATOR
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EXCITATION PRINCIPLE 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 4 August 2011
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270 0
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90
360
180
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G
Flux in the generator rotor is produced by feeding DC supply in the field coils, thus forming a 2 pole magnet of rotor
The Equipment for supply, control and monitoring of this DC supply is called the Excitation system
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EXCITATION SYSTEM REQUIREMENT • Regulate terminal voltage of the machine
•Meet excitation power requirements under all normal operating conditions •Enable maximum utilisation of machine capability
•Guards the machine against inadvertent tripping during transients •Improve dynamic & transient stability thereby increasing availability
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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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TYPES OF EXCITATION EXCITATION SYSTEM ROTATING SYSTEM
Conventional Rotating machines
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STATIC SYSTEM
High frequency excitation
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Brushless Excitation System
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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, 4 August 2011
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STATIC EXCITATION SYSTEM ( 200 MW) 575 v
AVR AUTO
15.75 kV
MAN
FB FF FDR
415 v AC 4 August 2011
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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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Static excitation system GT
EXC TRFR 18KV/700V 1500KVA
Field Breaker THYRISOR BRIDGE
FIELD
voltage regulator
GENERATOR
Crow Bar
From TGMCC- C
Non linear resistor
Field discharge Resistor
Pre Excitation
415/40V,10KVA 4 August 2011
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Field flashing • 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 FIELD BREAKER
R Y
ARMATURE
B
ROTATING DIODES
FIELD (PM) PILOT EXCITER
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MAIN EXCITER
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GENERATOR
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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 SYSTEM (500 MW)
AVR
21 KV 4 August 2011
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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 Stroboscope for fuse failure detection Rotor Earth fault monitoring system 4 August 2011
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Brushless Excitation system • 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 4 August 2011
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Excitation Power Requirement
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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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PMG
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DIFFERENCES BETWEEN BRUSHLESS AND STATIC EXCITATION SYSTEMS S.NO
Description
Brushless Excitation
1
Type of system.
Brushless system gets activated with pilot exciter, main exciter and rotating diodes.
Static excitation system uses thyristors & taking supply from output of the generator
2
Dependency on external supply.
No external source requirement since pilot exciter has permanent magnet field.
Field flashing supply required for excitation build up.
3
Response of the excitation system.
Slower than static type since control is indirect (on the field of main exciter) and magnetic components involved.
Very fast response in the order of 40 ms. due to the direct control and solid state devices employed.
4
Requirement of additional bearing and increase of turbo generator shaft length.
One additional bearing and an increase in the shaft length are required.
No additional bearing and increase in shaft length are required.
5
Maintenance.
Less since slip rings and brushes are avoided.
More since slip rings and brushes are required. Also over hang vibrations are very high resulting in faster wear and tear.
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Static Excitation
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MAIN EXCITER
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EXCITER ROTOR
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EXCITER COOLING VAPOUR EXHAUST
COOLER
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GENERATOR I
XG EF =
EF
I . XG + VT
VT
Equivalent circuit of Generator 4 August 2011
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GENERATOR Phasor diagram of the Generator
Ef
IL.Xd VT
ф IL
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GCB
GENERATOR
GT
G
Xd
G
XT
VT
Ef
Vbus
Generator + Generator Transformer Eq. Ckt. 4 August 2011
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GENERATOR Vector Diagram of Generator and GT connected to an infinite bus
EF
IL.Xd
VT
IL.XT Vbus
ф
IL
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GENERATOR 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)
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GENERATOR POWER ANGLE EQUATION 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
Pmax
=
EF . V / X
=
P
Note that the Electrical Power Output varies as the Sin of Load angle 4 August 2011
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1.2 1
Torque angle diagram
0.81.2 1 0.6 0.8 0.40.6 0.20.4 0.2 0 0
0
0
180
18
Angle in degrees
150
15
120
0
90
12
60
90
30
60
0
30
0
Sin delta
Power in pu
Torque angle diagram
Angle in degrees 4 August 2011
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Physical significance of load angle
Stator mag. axis
red N ROTOR
δ S
Rotor mag. axis yellow
N
S
STATOR
blue 4 August 2011
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P2
•Excitation constant; EF2
•Steam flow increased
P1
•Power output P1 to P2 EF1
O
1
2
ф2 I2
ф1
Vbus
Locus of Constant Excitation
I1 ACTIVE POWER CHANGE 4 August 2011
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Locus of P = const. •Steam Flow constant; •Excitation increased
EF2
•Power output Constant
O
EF1
1
2
ф1
ф2 I2
Vbus
Locus of Constant Excitation
I1 I Cos ф = Constant 4 August 2011
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EXCITATION CHANGE
Power in per unit
Excitation Control
1.4 1.2 1 0.8 0.6 0.4 0.2 0
Power Angle Diagrams for Different Excitation Levels
P1 P2 P3 0 30 60 90 120 150 180 Power Angle (delta), in degrees
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AVR
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TYPES OF AVR SYSTEMS • Single channel AVR system • Dual channel AVR system • Twin channel AVR system
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Single channel AVR system Here 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. 4 August 2011
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Dual channel AVR system Here 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. 4 August 2011
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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
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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.
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AVR There are two independent control systems 1. Auto control 2. 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
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 4 August 2011
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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
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AVR Generator voltage control The output quantity of this control is the set point for a following.
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 4 August 2011
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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)
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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
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AVR The set-point adjuster of the excitation current regulator for manual is tracked automatically (followup 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. 4 August 2011
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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
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AVR The current feedback is utilized for active and reactive power compensation and for limiters
There are 3 limiters 1.Under excitation limiter 2.Over excitation limiter
3. V/F limiter 4 August 2011
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Excitation Interlocks Preconditions for Excitation ON Excitation ON command N>90% Protection Off FCB Off feedback
5s delay
Excitation ON
External trip GCB is OFF
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Excitation OFF Interlocks Exc. OFF from Field flashing
Exc OFF command
GCB OFF N>90%
Exc OFF
Delay 1sec
GCB OFF External trip
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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 safely Lagging 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 4 August 2011
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LIMITERS • • • • •
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Over excitation limiter Under excitation limiter Rotor angle limiter Stator current limiter V/F limiter
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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 4 August 2011
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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 4 August 2011
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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
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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 4 August 2011
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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 4drop August 2011 PMI Revision 00 67
PRIORITY STRUCTURE OF AVR 1st priority
Rotor current limiter UN1024
Stator current li miter inductive UN0027
2 nd priority
Load angle limiter UN1043
Stator current limiter Capacitive UN0027
3 rd priority
Voltage regulator UN-2010
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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. 4 August 2011
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THANK YOU
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