by
Michael J. Basler
With great assistance by the IEEE/PES
Excitation System Sub-Committee

Introduction
„
Excitation systems can be broken down into three basic types:
„
„
„
Type DC - dc commutator type exciters
Type AC - Alternator supplied rectifier excitation systems
Type ST - Static excitation systems

Control Techniques
„
Modern Control Techniques
„
„
Proportional, Integral and Derivative
„
PID for type DC and AC excitation systems
„
PI for type ST excitation systems
Rate Feedback

Control Techniques
Type DC
V
OEL
ALTERNATE
OEL INPUTS
V
UEL
V
S
V
C
V
REF
+
_
Σ
_
+
ALTERNATE
UEL INPUTS
V
UEL
V
RMAX
/K
A
K
P
+
K
I
sK
D
+ s 1+sT
D
V
RMIN
/K
A
HV
GATE
V
OEL
LV
GATE
V
T
Π
V
T
*V
RMAX
K
A
1+sT
A
V
R
+
-
Σ
V
T
*V
RMIN
Σ
+
+
V
X
V
1 sT
E
EMIN
K
E
V
X
=V
E
S
E
(E
FD
)
E
FD
V
F sK
F
(1 + sT
F
)

Control Techniques
Type AC
K
P
V
T
V
S
V
UEL
V
C
V
RMAX
+ +
-
+
Σ
-
K
PR
+
K s
IR + sK
+
DR
DR
V
RMIN
+
-
Σ K
PA
+
K
IA s
V
AMIN
V
AMAX
V
A π
-K
L
V
FE
+
Σ
V
EMIN
V
FEMAX
K
E
+ S
E
− K
D
I
FD
[ ]
1
1 + sT
E
V
E
V
Ref
+
Σ
+
K
F2
V
FE V
X
V
X
= V
E
S
E
[V
E
] sK
F3
1 + sT
F
+
Σ
+
+
Σ
+
K
E
π
F
EX
F
EX
= f [I
N
]
I
N
I
N
=
K
C
I
FD
V
E
K
D
I
FD
K
F1
E
FD

V
C
V
U E L
Control Techniques
Type ST
K
G
V
S
V
O E L
-
+
+
Σ
+
V
R m a x
K
P R
+
K
IR s
V
R 1
1 + s T
A
+
-
Σ
V
M m a x
K
P M
+
K
IM s V
M
L V
G a te
V
R m in
V
R E F
V
M m in
π
E fd
V
T
I
T
V
E
= | K
P
V
T
+ j(K
I
+ K
P
X
L
) I
T
|
V
E π
I
F D
I
N
= K c
I
F D
V
E
I
N
F
E X
= f(I
N
)
F
E X
V
B m a x
V
B

Control Techniques
„
Adaptive controls
„
„
„
„
Gain modification based on PSS status
Artificial Neural Networks
Fuzzy Logic
Optimal Control

Control Techniques
„
Intelligent Techniques
„
„
„
No known “production” excitation systems based on intelligent techniques
Controller gains typically tuned at no load, plant changes gain / time constant with load – non-optimal settings
Industry concerns about adaptive controls

Control Techniques
„
Power System Stabilizers
„
Single Input Stabilizers
„
„
„
„
Speed
Frequency
Power
Dual Input Stabilizers
„
„
Integral of Accelerating Power type PSS
Multi-band PSS
„
Adaptive PSS
„
Intelligent Techniques

Control Techniques
„
V
SI1MAX
V
SI1 sT
W1
1 + sT
W1
Power System Stabilizers sT
W2
1 + sT
W2
1
1 + sT
6
+
+
∑ 1 + sT
8
(1 + sT
9
) M
N +
-
∑
K
S1
1 + sT
1
1 + sT
2
V
SI1MIN
K
S3
V
SI2
V
SI2MAX sT
W3
1 + sT
W3 sT
W4
1 + sT
W4
K
S2
1 + sT
7
V
SI2MIN
1 + sT
3
1 + sT
4
1 + sT
10
1 + sT
11
V
STMAX
V
STMIN

Control Techniques
„
Supplemental Controls
„
Reactive Current Compensators
„
„
„
„
„
„ var/PF Controllers
Over / Under Excitation Limiters
V/Hz Limiters
Stator Current Limiters
Field Temperature Limiters
Limiter set points may be a function of cooling gas temperature / pressure

Redundancy
„
Manual Backup
„
„
„
„
„
Direct Control of Power Stage
Field Voltage / Current Regulator
Excitation Limiters
Auto-tracking
Auto-transfer

Redundancy
„
Redundant AVRs
„
„
Dual AVRs
Automatic Transfer
„
„
Watchdog
Autonomous External Module
„
„
Automatic Tracking
Best 2 of 3 voting

Redundancy

Redundancy
„
Power Converter Redundancy
„
„
„
Twin Excitation Systems
N+1 Redundancy
Current Sharing
„
Paralleling Inductors
„
Electronic Current Sharing Control

Redundancy
Dual Redundant
Power Bridges

Protection
„
Excitation System Protection
„
Converter temperature
„
„
„
„
„
„
Thyristor conduction monitoring
Converter fuse failure detection
Fan monitoring
Power potential transformer overload and fuse failure
Loss of sensing pt detection
Exciter phase unbalance

Protection
„
Generator Protection
„
Generator terminal over and under voltage
„
„
„
„
„
„
Field over voltage and over current
Volts per hertz
Field over temperature
Loss of excitation
Rotating diode monitoring
Field ground detection

Communications
„
Intra-unit Communications
„
„
Many digital excitation systems implement some form of communications within the controller
For information exchange among modules within the excitation system

Communications
„
Intra-unit Communications
„
Modern digital excitation systems use serial or parallel data format
„
„
„
„
Provide data exchange between the main processor and other circuits or modules in the excitation system
Communications channel is designed for a high degree of noise immunity
Fiber optics may be used to avoid influence from electrical noise
Communications protocols are typically proprietary to the equipment manufacturer

Communications
„
Inter-unit Communications
„
There are a number of situations where communications between separate excitation system controllers is necessary
„
Redundant controllers
„
To facilitate bumpless transfer
„
Sharing of reactive current among machines in parallel
„
To eliminate need for cross connection of CTs

Communications
„
External Communications
„
Most digital excitation systems provide a means to communicate to external devices
„
„
„
Various communications protocols used
„
„
„
USB
RS-232 / RS-485
Ethernet TCP/IP
Used to interface with
„
„
„
Overall plant control computer
Prime mover controls
User interface via a PC or PLC
Most external communications protocols include some form of password protection for security

Diagnostics
„
„
Self-Test
„
Most modern digital excitation systems provide some form of self-test
„
May include test of the memory elements and various power supply levels
Watchdog
„
„
Provided on most digital systems
Operates autonomous of the processor
„
„
Requires the processor to periodically service the watchdog function
If a problem exists, then the processor is reset and an alarm is issued

Diagnostics
„
Oscillographic Records
„
Many systems provide this feature
„
„
„
„
„
„
Records store many data points
Triggered by a variety of parameters
Variable number of data points per record
Variable time interval between data points
User specified amount of pre-trigger
Records may be stored in a non-volatile memory for recall and display at a later time

Diagnostics

Diagnostics

Diagnostics

Diagnostics
„
Event Logging
„
Parameters monitored by the excitation system can be set up to trigger an event log
„
„
„
„
Logs are typically date and time stamped and identifies what parameters changed when the event occurred
Time stamp may be synchronized with a system clock or using an IRIG-B signal
Input/output status changes, alarms and system status changes can be used to trigger an event log
These logs may also be stored in non-volatile memory for recall at a later time

Diagnostics
DECS-200 SEQUENCE OF EVENTS REPORT ( New Events ) Qty = 123
REPORT DATE : 24-05-06
REPORT TIME : 11:24A25
STATION ID : STATION ID
DEVICE ID : DECS 200
USER1 ID : USER 1 ID
USER2 ID : USER 2 ID
--DATE-- ----TIME---- --------POINT DESCRIPTION-------- --STATUS--
--------------------------------------------------------------------------------
24-05-06 11:07A53.973 STOP CONTACT DISABLED
24-05-06 11:07A52.822 SOFT START MODE STARTUP
24-05-06 11:07A52.772 ON/OFF RELAY DISABLED
24-05-06 11:07A52.672 UNDERFREQUENCY ALARM OFF
SYSTEM BELOW 10 HZ ALARM OFF
STOP/START STOP
24-05-06 11:07A52.622 STOP CONTACT ENABLED
24-05-06 11:07A52.221 SOFT START MODE OFF
24-05-06 11:07A47.617 START CONTACT DISABLED
24-05-06 11:07A47.266 ON/OFF RELAY ENABLED
24-05-06 11:07A47.216 SYSTEM BELOW 10 HZ ALARM ON
24-05-06 11:07A47.166 UNDERFREQUENCY ALARM ON
STOP/START START
24-05-06 11:07A47.066 START CONTACT ENABLED

Summary
„
„
„
„
„
Provide a general overview of the state of the art in excitation systems
Advent of digital-based excitation systems resulted in significant advancement in control and protective functions
Many more details available to the interested reader
Review the IEEE421 series of standards or
Contact the Excitation System Sub-Committee

Acknowledgements
„
The author would like to acknowledge the contribution of the members of the
IEEE/PES Excitation System Sub-
Committee, its chairman Om Malik, its working groups and task forces.

Questions?