generator excitation
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4 August 2011 PMI Revision 00 1
Generator
Excitation System
&
AVR
4 August 2011 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
4 August 2011 PMI Revision 00 3
What is Excitation system?
• Creating and strengthening the magnetic field of
the generator by passing DC through the filed
winding.
4 August 2011 PMI Revision 00 4
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’
4 August 2011 PMI Revision 00 5
STATOR
ROTOR
EXCITATION PRINCIPLE
4 August 2011 PMI Revision 00 6
STATOR
EXCITATION PRINCIPLE
ROTOR N
S
4 August 2011 PMI Revision 00 7
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
4 August 2011 PMI Revision 00 10
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
4 August 2011 PMI Revision 00 11
EXCITATION SYSTEM
REQUIREMENT
• Reliability
• Sensitivity and fast response
• Stability
• Ability to meet abnormal conditions
• Monitoring and annunciation of parameters
• User friendliness
4 August 2011 PMI Revision 00 12
TYPES OF EXCITATION
EXCITATION SYSTEM
ROTATING SYSTEM
STATIC SYSTEM
Conventional Rotating machines
High frequency excitation
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 PMI Revision 00 14
AVR
AUTO
MAN
FDR
FF
415 v AC
STATIC EXCITATION SYSTEM ( 200 MW)
F B 15.7
5 k
V
575 v
4 August 2011 PMI Revision 00 15
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
4 August 2011 PMI Revision 00 16
Static excitation system
voltage regulator
GT
EXC TRFR
18KV/700V
1500KVA
THYRISOR
BRIDGE
GENERATOR
FIELD
From TGMCC- C
415/40V,10KVA
Pre Excitation
Non linear resistor
Field Breaker
Field discharge Resistor
Crow Bar
4 August 2011 PMI Revision 00 17
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
4 August 2011 PMI Revision 00 18
BRUSH GEAR
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4 August 2011 PMI Revision 00 20
Brushless excitation
PILOT EXCITER
MAIN EXCITER
GENERATOR
FIELD BREAKER
FIELD
(PM)
ARMATURE
ROTATING DIODES
R
Y
B
4 August 2011 PMI Revision 00 21
Components of Brush less
Excitation System
•Three Phase Main Exciter.
•Three Phase Pilot Exciter.
•Regulation cubicle
•Rectifier Wheels
•Exciter Coolers
•Metering and supervisory equipment.
4 August 2011 PMI Revision 00 22
AVR
BRUSHLESS EXCITATION SYSTEM (500 MW)
21 KV
4 August 2011 PMI Revision 00 23
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 PMI Revision 00 26
• 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
4 August 2011 PMI Revision 00 27
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
4 August 2011 PMI Revision 00 28
PMG
4 August 2011 PMI Revision 00 29
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 Excitation Brushless Excitation Description S.NO
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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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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
4 August 2011 PMI Revision 00 35
G Vbus VT
XT Xd
Ef
GENERATOR
Generator + Generator Transformer Eq. Ckt.
G
GT GCB
4 August 2011 PMI Revision 00 36
Vbus
VT
EF
IL
ф
Vector Diagram of Generator and GT
connected to an infinite bus
GENERATOR
IL.XT
IL.Xd
4 August 2011 PMI Revision 00 37
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
4 August 2011 PMI Revision 00 38
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
GENERATOR
POWER ANGLE EQUATION
4 August 2011 PMI Revision 00 39
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
del
ta
Torque angle diagram
0
0.2
0.4
0.6
0.8
1
1.2
0 30 60 90120
150180
Angle in degrees
Po
we
r in
pu
4 August 2011 PMI Revision 00 40
ROTOR
STATOR
δ
Rotor
mag.
axis
Stator
mag.
axis
N
S
S
N
red
yellow
blue
Physical
significance
of load angle
4 August 2011 PMI Revision 00 41
O Vbus
EF1
EF2 P1
P2
Locus of
Constant
Excitation I2
I1
ф1
ф2 1
2
•Excitation constant;
•Steam flow increased
•Power output P1 to P2
ACTIVE POWER CHANGE
4 August 2011 PMI Revision 00 42
O Vbus
EF1
EF2
Locus of P = const.
Locus of
Constant
Excitation I2
I1
ф1
ф2 1
2
•Steam Flow constant;
•Excitation increased
•Power output Constant
I Cos ф = Constant
EXCITATION CHANGE
4 August 2011 PMI Revision 00 43
Excitation Control
Power Angle Diagrams for Different
Excitation Levels
0
0.2
0.4
0.60.8
1
1.2
1.4
0 30 60 90 120 150 180Power Angle (delta), in degrees
Po
wer
in p
er
un
it
P1
P2
P3
4 August 2011 PMI Revision 00 44
AVR
4 August 2011 PMI Revision 00 45
TYPES OF AVR SYSTEMS
• Single channel AVR system
• Dual channel AVR system
• Twin channel AVR system
4 August 2011 PMI Revision 00 46
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 PMI Revision 00 47
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 PMI Revision 00 48
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
4 August 2011 PMI Revision 00 49
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.
4 August 2011 PMI Revision 00 50
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 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
4 August 2011 PMI Revision 00 51
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
4 August 2011 PMI Revision 00 52
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.
4 August 2011 PMI Revision 00 53
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)
4 August 2011 PMI Revision 00 54
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
4 August 2011 PMI Revision 00 55
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.
4 August 2011 PMI Revision 00 56
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
4 August 2011 PMI Revision 00 57
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
4 August 2011 PMI Revision 00 58
Excitation Interlocks
5s delay
Excitation ON command
N>90%
Protection Off
FCB Off feedback
External trip
GCB is OFF
Excitation ON
Preconditions for Excitation ON
4 August 2011 PMI Revision 00 59
Excitation OFF Interlocks
Delay 1sec
Exc. OFF from Field flashing
Exc OFF command
GCB OFF
N>90%
External trip
Exc OFF
GCB OFF
4 August 2011 PMI Revision 00 60
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 PMI Revision 00 61
4 August 2011 PMI Revision 00 62
LIMITERS
• Over excitation limiter
• Under excitation limiter
• Rotor angle limiter
• Stator current limiter
• V/F limiter
4 August 2011 PMI Revision 00 63
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 PMI Revision 00 64
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 PMI Revision 00 65
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
4 August 2011 PMI Revision 00 66
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 PMI Revision 00 67
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
4 August 2011 PMI Revision 00 68
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
4 August 2011 PMI Revision 00 69
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 PMI Revision 00 70
THANK YOU
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