This document discusses automatic voltage regulators (AVRs) and excitation systems for synchronous generators. It provides information on: 1) The basic concepts of excitation systems including synchronous generator operating zones, synchronous condenser and motor zones, and stability requirements. 2) The response characteristics of excitation systems including steady state and transient stability requirements. 3) The components and operation of different excitation system types including DC, AC, brushless, and static excitation systems. 4) The components, controls, and protection features of AVRs including voltage regulators, current regulators, set point controls, and under/over excitation limiters.

1. INTRODUCTION
Syn React
XS
GT
XT
Infinit
e Bus
V
T
AVR
Ref Volt
level
E
x
t
GEN

2. BASIC CONCEPT
+R
-R
+X
-X
Syn Gen
over-Excited zone
Syn Gen under-Excited zone /
Ind. Gen No Excitation (FB open)
Syn Condencer
Over-excited zone
Syn Motor
under-Excited

3. OPERATIONAL REQUIREMENT
* FAST & HIGH RESPONSE
* HIGH DEGREE OF RELIABILITY
* STABLE ARREANGEMENT OF FIELD DISCHARGE

4. RESPONSE CHARECTERISTICS
P

0
180°
90°
Pmax
UNSTABLE
ZONE
STABLE
ZONE
P=
E x V
X
Sin 
1 2
P1
P2
INCREASED
EXCITATION
I
V Infinite
Bus Voltage
E
1
1 I at Unity PF
2
2
E
I
E
Lag
Load
STEADY STATE
I.Xe
1 pu
0.9 pu
TIME
Rise Time
SETTLING
TIME
OVER SHOOT
Steady State Error <  0.5%
Ideal Performance One Over
shoot & One Under Shoot
TRANSIENT STABILITY
Severe disturbance during FAULT for a short time
dip in the M/C terminal voltage & power transfer
–resulting acceleration of m/c.
Response
Characteristics
Time
0.5Sec
Rated
Ceiling
DYNAMIC STABILITY
Following a disturbance , the group of m/c operating in the sane control group
Experience smaller oscillation reacting with each other , Hence change of Excitation
May not result in a stable operation for a slow acting exciter having a inherent time delay
Depending on the oscillation a separate excitation requirement to be meet. Faste excitation
System meet this requirement rather avoid this problem to certain extent by PSS to damp out
The smaller oscillation

5. CAPABILITY & LIMITATION
1pu 247MW
220 MW
210MW
118 100
LEADING LAGGING
130
δ Φ
Stator
Current
Limiter
Field
Current
Limiter
Turbine Limit
70MW
β
γ
Under Excitation
Limiter (Auto)
Under Excitation
Limiter (Man)

6. Type of Excitation System
* DC EXCITATION SYSTEM : Using DC Exciter & Voltage Regulator
* AC EXCITATION SYSTEN: Using AC exciter, static rectifier & volt. regulator
* BRUSHLESS EXCITATION SYSTEM: AC exciter, rotating diode & volt. regulator
* SEE : static excitation system without any rotating machine, Thyristor used for
Controlled DC of main generator

7. COVENTIONAL DC EXCITER
DC
Exciter
GEN FDR
FB
FF
FB
Additional Fld
Wndg
Controller
3 ph
AC
Mag Ampl
Gen Volt Feedback
From Pilot Exciter
DC
Exciter
Shunt Fld
Amplidyne
Buck Fld
Boost Fld
Fig-1A
Fig-1B

8. INDIRECT HIGH FREQ. AC EXCITER WITH BRUSH &
WITHOUT BRUSH
AC
Exciter
FDR
FB
FF
PMG
GEN
AC
Exciter
Rotating Diode
With Brush
Without Brush
GEN

9. BASIC DIA. OF BRUSHLESS EXCITATION SYSTEM
10
no
10
no
10
no GEN
2080A
235V
AVR
(AUTOMATIC VOLTAGE REGULATOR)
PMG
MAIN
EXCITER
3200A
420V 150Hz
220V
400Hz
+ve Wheel
-ve Wheel
CT
PT
FDR
FB
For
Rotor E/F
Q
a
x
i
s
Generator
Control Desk
ON/OFF &
STATUS
GRP(L/O) &
ATRS (speed>
& < 2790 rpm
Q-Axis Coil
mv
6 50A 1sec,15KWS

10. BASIC DIA. OF STATIC EXCITATION SYSTEM
CT
PT
Excitation
Transformer
Aux
Tr
+
_
FB
FF 3 ph
415v ac
1
Shunt
Amp
Extn Tr. O/C Prot
CT
2. 5MVA
FDR
R
220v DC
Gen.
Control
Desk
AVR
(AUTOMATI VOLTAGE
REGULATOR)
Shunt
ON/OFF &
STATUS
575 volt
Six Pulse

11. BLOCK DIA. AVR (BRUSHLESS)
SET POINT
GEN VOLT (AUTO)
PSS
V/Hz
LIMITER
ACTUAL
GEN VOLT
REACTIVE POWER >
ACTIVE POWER >
UNDER-EXCITATION
LIMITER
OVER-EXCITATION
LIMITER
mv
V
~
_
~
_
CT
VT1
_
-  +
-
/
+
+
_
+  -
-  +
Reactive
Power
Compensation
+/-
Volt
Reg
Current
Reg (A)
FOLLOW
UP
CONTROL
v(man)
v
+
-
EXCITATION
BUILD UP
& FLD FOCING
LIMITATION
(Rectifier Limit)
Ust (-)
FIRING
CARD (A)
FIELD
SUPPRESSION
A / M
CHANGE OVER

GEN –PROT & ATRS
FROM GCD
AUTO / MAN
CHANGEOVER
FB ON /OFF & STATUS
Ust (A)
mv
V
FIELD
AMP
FIELD
AMP
SET POINT
EIELD AMP (MAN)
-
+
CURRENT
REGULATOR
(MAN)
_
FIRING
CARD (M)
Ust (-)
Ust (M)
M
GCD
UNDER EXCITATION
LIMITER (MAN)
RAISE COMMAND &
BLOCK LOWER
FROM GCD
RAISE / LOWER
RAISE / LOWER
M
~ _
VT3
PULSE BLOCK
SYN.
VOLT
FIRING PULSE

12. SET POINT CONTROL COMMAND (AUTO & MAN)
AUTO SYN
GCD
AVR
LOWER COMMAND
RAISE COMMAND
AUTO SYN
GCD
AVR
AVR in AUTO
MATCH
MATCH
AVR in MAN
AVR in AUTO
GCB is ON
MATCH
OR
OR
OR
LOWER
RAISE
AUTO LOWER
AUTO RAISE
MAN LOWER
MAN RAISE
OR
UNDER EXCT.
LIMITER OPTD.
DE-Exct(FB OFF)
OVER EXCT
LIMITER OPTD
SET POINT LOWER
SET POINT RAISE
EXCT. LOW
GEN VOLT <
REF. MIN.
OR
OR
LOWER FROM
FOLLOW UP
REF. MAX.
RAISE FROM FOLLOW UP
RAISE FROM UNDER EXCT (MAN)
SET POINT LOWER
SET POINT RAISE
A
U
T
O
M
A
N
OR

13. GENERATOR VOLTAGE SET POINT (AUTO & MAN)
CONTROL
& LOGIC
CONTROL
& LOGIC
SET PT. LOWER (A)
FB CLOSE COMMAND
95% VOLT (256 COUNT)
SET PT. RAISE (A)
SET PT. LOWER (M)
SET PT. RAISE (M)
UP / DOWN
COUNTER
0-1024 for 30Sec
D / A
CONVERTER
UP / DOWN
COUNTER
0-1024 for 30 Sec
D/A
CONVERTER
+
_
_
+
_
_
VOLT
REG.
(A)
CURRENT
REG (M)
+7.2--+8..8 V
(90% --110%)
0—10V (0% --100%)
0—10V (0% ---100%)
AUTO
MAN
0-+5.7V
(0-110%)

14. VOLTAGR REGULATOR (AUTO)
_
220K
47K
100K
22M
100K
47K
_
RAMP GEN LOW
(PERMISSIVE OF
FB CLOSE)
-10 V
1--RUN UP
0—RUN BACK
INV. LIMIT
RECT. LIMIT
-4
+10
-
+
-
+
-
+
-1
-1
_
_
_
FOLLOW UP INT
MAN
_
T
O
C
U
R
R
E
N
T
R
E
G
U
L
A
T
O
R
FOLLOW
UP INT
FEED
BACK
LIMITER
REFERENCE
SET POINT
DEVIATION
MAN
+7.2 –
+8.8V
-8.0V (100%)
_
+
+10v
-10V (>150% of 23A)
_
DYN. P.
GAIN
INT. TIME
STATIC GAIN
PI O/P
110%
(23A)
OR
GCB OFF
FF LIMITED
AVR PNL
Sel Sw
_
RAMP FUNC
GENERATOR
UP  20Sec
DN  5Sec
(SPEED > & FB ON)
100
+1.23V
1sec
A/M Balance
Integrator.
O/P
90%…110%
Initially OV at RAMP UP it
reaches +10 V Ceiling level
46.5A ,135V for 10 sec
_
RAMP GEN FAULTY
<+0.8V
<+5.2V
AVR FAULTY
A / M C/O (AVR in AUTO & GCB ON
+5.5v
33K
33K
100K


15. FOLLOW UP INTEGRATOR & MATCHING
-
-
_
_
_
I 23A-5V
MAN
_
FOLLOW UP
INT
-10V + 10V
_
_
OR
BLOCKING
OF AUTO
FB ON
INT >I f
INT < I f
TO VOLT
REGULATOR


VOLT
INT > I f
VOLT
INT <I f
Ust (AUTO)
Ust
(MAN)
OR
FB OFF /
DE-EXCT
MAN
_
_
MATCH
AUTO
MAN N/O
GCB OFF
MAN SET VALUE
FB OFF
OR
OR
SET PT.
NO-LOAD
RAISE FROM
FOLLOW UP
LOW FROM
FOLLOW UP
SET POINT RAISE (M)
SET POINT
LOWER
(M)
-10V
+10V
_
+
_
+
MATCH
_
_
OR
AUTO
FB ON
F/UP
FAULTY
DEV + HIGH
DEV -- HIGH
AUTO
MAN
DEVIATION
A/M
Balance
_

16. CURRENT REGULATOR (AUTO)
_
1.1 X 200% I f (23A)
+
Fld. Forcing Limiter Faulty (AM)
I f (50A)
ACTUAL +10V _
-5V (23A)
SET POINT FROM
VOLT. REG (PI)
+
_
33K
33K
MATCHING
UST AUTO
+
-15V
GCB OFF
-0.57V
(110% UGN)
+15V
DE-EXCT
(FB OFF)
-ve  <90 Rectifier
-5v32.8
+ ve  >90 Inverter
+5v 148
0 V   90
High Gain
+VE for RECTIFIER
-VE for INVERTER

17. UNDER EXCITATION LIMITER (MAN)

IS
UTR
UST
URS
_
(+)
(-)
GEN VOLT
(ACTUAL)
I Sin 
(-) Under Excited
(+) Over Excited
I Cos 
(+)
A
B

Point- A
Point-B
Angle- 
-
-
-
+ ve ON
RESPONSE.
>+0.2V BLOCKING
OF SET POINT.(M) LOW
>+0.5V RAISE TO
MAN SET PT. HIGH
-
-ve on
Response
MAN
GEN VOLT > 85%

18. CURRENT REGULATOR (MAN)
100K
-
-
FB
TRIP
Gr-1 & -2
-4 +10
-4
+10
-
-
+
+
RECT
INV
+10
(-)
GEN VOLT
(ACTUAL)
GCB
OPEN
Ust
Matching
FIRING
CKT
-
+1
-5.5V
(110%)
+5..3V
-2.18V
GEN VOLT 106%
FROM GEN VOLT SET POINT
+5.7 (110%)
EXCT. CURRENT
(ACTUAL)

19. AUTO / MANNUAL CHANGE OVER
AUTO
CHANNEL
MAN
CHANNEL
By Fault OR By Desire
By Desire only
Fault: (1) Thyristor Set Auto Faulty
(24 MCB OFF,400Hz Power Source OFF,
Main Exct Fld Current Low, Conduction Monitor
with time delay 100msec)
(2) Field forcing Limiter ( 200% Ifn / Ramp Gen at GCB OFF)Faulty Td 0.3 sec.
( Gen Reactive Current >)
( Gen Voltage >)/ (O/P Volt Tyhr.Set >)
(3) Auto Channel Power Supply Faulty
(400Hz Gate Control MCB
( 24V ,15V MCB)
(4) Gen actual Voltage Faulty (PT sensing voltage Faulty)
By Desire must be enable through MATCH Push Button
Prior to change over Channel to be healthy

20. OVER EXCT. LIMITER (FIELD CURRENT)
+
_
+1
_ +1
IF2N=23A
50A +10V
23A(+6V)
-10V
-6.3V (105%)
-15V
-9.49V
Gain
Gain
adjusted so
as –10V at
140%
+ve
MAN
+4.58V
Integrator Reset on Manual
Integrator
_
-ve
GEN VOLT SET POINT
(AUTO)
+7.2V (90%) to 8.8V
(110%)
-10V
-1.67V
+5V at VGN
-10V
-8.49V
LIMIT
VLUE
-10V to +5V
On Response
>0.1V
ALARM
CLOSE
ON AUTO
SEL SW
V
O
L
T
R
E
G
STATOR
CURRENT
LIMITER
BLOCKING
SET POINT
(RAISE)

 From Stator Current Limiter
SET POINT
OF
RESPONSE
-
Response Time 10Sec
Res. Time
30s
20s
10s

1.4
1.2
FLD CURRENT
Setting
Integrator
RESET

21. OVER EXCT. LIMITER (STATOR CURRENT)
_
Stator Current
+VE
+6V
(9054A)
GAIN
+15V
Gain adjusted
140% Ign=-10V
-10V
-6.3V (105%)
-
- +1
-
-6V (247MVAR)
Over Excitation
>130MVAR
+5V
At Vgn
-10V
-9V
LIMITE
VALUE
Resp. Time
+10…-10V
10sec
_
ALARM
STATOR CURRENT
HIGH
PERMISSION
FOR STATOR CURRENT LIMITER
V
O
L
T
R
E
G
CLOSE
ON AUTO
SEL SW

 FROM ROTOR CURRENT LIMITER
LED IND
REACTIVE CURRENT HIGH
Td
750ms

22. UNDER EXCITATION LIMITER Characteristics (AUTO)
B
D
_
-1
_
+1
_
GEN VOLT (-8V dc)
+2.88V (118MVAR)
VBR ~
IY ~
MVAR
+ Lag
- Lead
MW
 6V (247MVAR))
+ 6V
(247MW)
- 10V
VY-R
VY-B
IY
-5.31V
(218MW)
_
-6.0V (247MW)
-2.43V
(100MVAR)
A


-ve on
Response
+ve on
Response


A
B
C
D
100MVAR 0
118MVAR
LEAD
LAG
70MW
218MW
-1.71V (70MW)

23. UNDER EXCITATION LIMITER Dynamic Performance (AUTO)
1K
475K -1
_
+1
_
+1
_
+
_
+10V
_
_
+ VE
BLOCKING
GEN VOLT < 85%
+VE ON
RESPONSE
GEN VOLT REF.
+7.2V ---+8.8V
+10V +5V
+
-VE ON RESPONSE
-VE ON RESPONSE
_
ALARM OF
RESPONSE
>2V
INTEGRATOR
PROPORTIONAL
SEL SW
SEL SW
VOLT REG
V
O
L
T
R
E
G
Min. to
Respond
(-0.7V)
LIMITER OF FAST CORRECTING
ACTION OF PROP. AMPLIFIER
1F
AUTO
AUTO
P Gain
Match with
exciter Time
Const.
INTEGRATOR RESET
Gen Volt
> 90%

24. V/Hz LIMITER (AUTO)
AUTO
GEN VOLT >0.5 pu
+10V
INTEGRATOR
VOLT REG
SEL SW
-1
_
+1
_
_
V/Hz HIGH
ALARM
GEN VOLT
VYB
GEN VOLT
VY-N
˜
+8V =50Hz
V  f
-7.2 to –8.0V
1 pu to 1.1 pu
-7.44V
(1.07 pu)
V  VG
+ ve to –ve
ON RESPONSE
-ve to +ve
ON ACTION
V/Hz TIME
1.2 18sec
1.3 10sec
_

Response Value
(Setting)
Setting V /Hz ( p.u.)
Res. Time
10s
20s
30s
1.2 1.3

25. REACTIVE POWER COMPENSATION (AUTO)
VOLTAGE
REGULATOR
+/-6V 247MVAR
 Control the Generator voltage to partly compensate for the voltage drop due to Unit Transformer
or voltage rise due to long Transmission line charging on Generator voltage build up specially for
Hydro set
 Effect of this reactive current adjustable between 0..0.1% of rated voltage
 AVR of the large generators are preferably operated without compensation

26. POWER SYSTEM STABILISER (AUTO) PART-1
_
+
+
+
+
F
U
X
_
+
V  P
(Change of Power)
V  F (Change of Freq.
i.e. Slip Signal)
U
I
SIGNAL CONVERTER ACTIVE POWER SIGNAL CONVERTER
3Volt for 1pu of Voltage & Current
Signal
0.61Vac
210MW
+5.5V
0..-10V
0V for P> Pmin (60%)
Power Monitor
>60%
VOLTAGE MONITOR
>90% <110%
ACCELARATING SIGNAL
2V / HZ
< > 50HZ
0V at 50HZ
SENSING OF ROTOR SIGNAL
V-I.Xq (Xq=1.89)
-10..+10V
-10…+10V
Tc=10sec
Tc=10sec
0
0
0
0

27. POWER SYSTEM STABILISER (AUTO) PART-2
K+
K+
K-
For FAULT P/ t >0.15pu / 100 msec
C 5..10%
Diff.
PROPORTION REGULATOR
X
X
MONITORING
(BLOCKING)
+1.6V
-1.6V
STAB. SIGNAL
EXCCED LIMIT
>20 SEC
ACTIVE POWER < 60% (BLOCK)
VOLTAGE >90% <110% (BLOCK)
P
F
SUMMING & MONITORING
STABILISING SIGNALADAPTER
Kp
PROP. WT.
With Z.Diode
(Kp+C) (I/P)²
Gain Km at
NETWARK
FAULT
Weighted P
Weighted F
Weighting of Stab. Signal during
Power Swing have a square
characteristics i.e. for small swing
influence of Slip Stab. be very week
& for large swing influence of Slip
Stab be very strong.
V
.
R
E
G
Sel Sw
AUTO
&
PSS ON
**
** Mixing Resistor determine the
weighting of Stab Signal w.r.t. Signal
of Voltage Regulator
10k820K (#1,2)/
560K(#3) done during tuning
of PSS through Step Test
S/H Peak value Detector
_
C
Km
LIMITATION

28. FIRING CARDS (AUTO)
R
S
T
220V 400HZ
From PMG


30V 400HZ
R
S
T
+
9V
0
ZERO CROSSING
DETECTOR
L
O
G
I
C
(DOUBLE
PULSE)
180- / 20
6 PULSE
5.6V
0=90°
At Ust =0V
W (MAX ANGLE)
INVERTER LIMIT
G (MIN ANGLE)
RECTIFIER LIMIT
Ust
DOUBLE PULSE BLOCKING 1 BLOCK ;0 ENABLE
PULSE
FINAL
STAGE
AMPLIFIER
0 ENABLE
1 (N/C)  BLOCKING
M
BLOCKING AT "MAN” BY
SELECTION
MONITOR “M”
POTENTIAL
HEALTHYNESS
0 BLOCKING
1 (N/C) ENABLE
BLOCKING AT
RUNBACK MODE (GCB
OFF & SPEED <) OR FB
TRIP ON PROT. OPTD.
EXCT. BLOCKED AT
AVR PANEL
SWITH OFF POWER
ISOLATOR THY(AUTO
ONLY)
AUTO NOT SELECTED
b6
d28
z28 d24
0BLOCKING
1(N/C)ENABLE
R+
R-
S+
S-
T+
T-
+
+
+
_
A32
M75
148
9V
0
9V
0

29. FIELD BREAKER CONTROL DURING START-UP & SHUTDOWN
500
CC
OR
AUTO & RAMP MIN
AUTO SET MIN
AUTO CH. HEALTHY
MAN & SET MIN
MAN CH. HEALTHY
PERMISSION
FB ON
FB ON
COMMAND
FB TRIP VOLT
(220V) HEALTHY
EXCT NOT BLOCKED
FROM AVR PANEL
FB ON COMMAND
FROM ATRS / GCD
OR
OR
GEN PROT.
CL-A or B Group-1
GEN PROT.
CL-A or B Group-2
OR
OR
GCB OPEN & SPEED <93%
GCB OPEN & FB OFF COMMAND FROM GCD / ATRS
EMERGENCY DE-EXCITE FROM AVR PANEL
FB OFF COMMAND
TC
INVERTER
OPERATION
INVERTER
OPERATION
FB ON/OFF
STATUS FOR
RAMP-UP &
RUN-BACK

30. FIELD BREAKER & FIELD SUPRRESSION
FB CLOSING PERMISSIVE : TRIP VOLTAGE HEALTHY & EXCT. NOT BLOCKED FROM AVR PANEL
AUTO RAMP GEN MIN & SET POINT MIN.
MAN SET POINT MIN.
FB CLOSE COMMAND: From GCD OR from ATRS
FB OFF COMMAND: From GCD when GCB in OFF condition (Generator Not Loaded)
GCB OFF & Speed <
Generator Protection Class-A & B
ACTION: FB will OPEN through Inverter Operation
Pulse Blocking Signal to Gate Control Unit
Run_Down of Ramp Function generator
UG
IF
UF
t
DE-EXCITATION OF UNLOADED GEN BY INVERTER
BY INVERTER OPERATION OF THYRISTOR SET
GEN VOLT
FB
FROM
AVR TO MAIN
EXCITER
ROTOR
+VE
-VE
FDR
6, 50A
1Sec,15KWS

31. EXCITATION BUILD UP & FIELD FORCING
Automatic Voltage Regulator initially calling for maximum excitation (as no feedback), the Thyristor set
supplies a higher voltage (upto ceiling level) to the Main Exciter compare to actual requirement under
Steady State condition until the actual current reaches the set point coming from Current Regulator.
Overdriving the voltage at the Field winding of Main Exciter reduce the Voltage Build Up time. This over
driving function referred to below as Field Forcing .
IF
UF
Utmax
Itmax
Response
with Fld
Forcing
Limitation
Response W/O
Overdriving
function
t
For Increasing Excitation
UG
IF
UF
No-Load (GCB open)
For Decreasing Excitation
For smaller change / Control , overdrive is achieved by proportional Current Regulator
The current & Voltage Limit i.e. (Maximum ) determined by Gate Control Unit

32. THYRISTOR & POWER CIRCUIT
+
_
CONDUCTION
MONITORING
UNIT
FIELD
CURRENT
>4A
THYRISTOR
AUTO
FAULTY
ISOLATOR
SWITCH
220V 400Hz 3Phase
MCB
0.1..0.16A
+
_
MAIN EXCITER FIELD
220V 400Hz 3Phase
OR
THY SET MAN FAULTY
R S T
R S T
R+ S+ T+
R- S- T-
R+ S+ T+
R- S- T-
MAN CHANNEL
AUTO CHANNEL
Pulse
Transf.
1K
150W
R+
S+
T+
R-
S-
CT

33. THYRISTOR POWER CONTROL
S-R+ T-S+ R-T+
S- T- R-
R-T+
S+
T+
S-
R+
T-
S+
R-

S-
S+R-
R+T-
T+S- T+S-
 = Firing
Angle w.r.t.
Natural
Commutation
 <30
=60
T+ T- S+ R- T+
=90
S- R+
=120
0
t
S-
S-R+
R-
RECTIFIER ZONE ZERO VOLT
INVERTER
ZONE