Functions and Performance Requirements
Elements of an Excitation System
Types of Excitation Systems
Control and Protection Functions
Modeling of Excitation Systems
The functions of an excitation system are
to provide direct current to the synchronous generator field winding, and
to perform control and protective functions essential to the satisfactory operation of the power system
The performance requirements of the excitation system are determined by
Generator considerations:
supply and adjust field current as the generator output varies within its continuous capability
respond to transient disturbances with field forcing consistent with the generator short term capabilities:
rotor insulation failure due to high field voltage
rotor heating due to high field current
stator heating due to high VAR loading
heating due to excess flux (volts/Hz)
Power system considerations:
contribute to effective control of system voltage and improvement of system stability
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Excitation Systems
1. Functions and Performance
Requirements
2. Elements of an Excitation System
3. Types of Excitation Systems
4. Control and Protection Functions
5. Modeling of Excitation Systems
Outline
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Functions and Performance
Requirements of Excitation Systems
The functions of an excitation system are
to provide direct current to the synchronous
generator field winding, and
to perform control and protective functions
essential to the satisfactory operation of the
power system
The performance requirements of the excitation
system are determined by
a) Generator considerations:
supply and adjust field current as the generator
output varies within its continuous capability
respond to transient disturbances with field forcing
consistent with the generator short term capabilities:
- rotor insulation failure due to high field voltage
- rotor heating due to high field current
- stator heating due to high VAR loading
- heating due to excess flux (volts/Hz)
b) Power system considerations:
contribute to effective control of system voltage and
improvement of system stability
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Elements of an Excitation System
Exciter: provides dc power to the generator field winding
Regulator: processes and amplifies input control signals
to a level and form appropriate for control of the exciter
Terminal voltage transducer and load compensator:
senses generator terminal voltage, rectifies and filters it
to dc quantity and compares with a reference; load comp
may be provided if desired to hold voltage at a remote
point
Power system stabilizer: provides additional input signal
to the regulator to damp power system oscillations
Limiters and protective circuits: ensure that the
capability limits of exciter and generator are not
exceeded
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Types of Excitation Systems
Classified into three broad categories based on the
excitation power source:
• DC excitation systems
• AC excitation systems
• Static excitation systems
1. DC Excitation Systems:
• utilize dc generators as source of power;
driven by a motor or the shaft of main generator;
self or separately excited
• represent early systems (1920s to 1960s);
lost favor in the mid-1960s because of large size;
superseded by ac exciters
• voltage regulators range from the early non-
continuous rheostatic type to the later system
using magnetic rotating amplifiers
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Figure 8-2 shows a simplified schematic of a typical
dc excitation system with an amplidyne voltage
regulator
• self-excited dc exciter supplies current to the
main generator field through slip rings
• exciter field controlled by an amplidyne which
provides incremental changes to the field in a
buck-boost scheme
• the exciter output provides rest of its own field by
self-excitation
2. AC Excitation Systems:
• use ac machines (alternators) as source of power
• usually, the exciter is on the same shaft as the
turbine-generator
• the ac output of exciter is rectified by either
controlled or non-controlled rectifiers
• rectifiers may be stationary or rotating
• early systems used a combination of magnetic
and rotating amplifiers as regulators; most new
systems use electronic amplifier regulators
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2.1 Stationary rectifier systems:
• dc output to the main generator field supplied
through slip rings
• when non-controlled rectifiers are used, the
regulator controls the field of the ac exciter; Fig.
8.3 shows such a system which is representative
of GE-ALTERREX system
• When controlled rectifiers are used, the regulator
directly controls the dc output voltage of the
exciter; Fig. 8.4 shows such a system which is
representative of GE-ALTHYREX system
2.2 Rotating rectifier systems:
• the need for slip rings and brushes is eliminated;
such systems are called brushless excitation
systems
• they were developed to avoid problems with the
use of brushes perceived to exist when supplying
the high field currents of large generators
• they do not allow direct measurement of
generator field current or voltage
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Figure 8.3: Field controlled alternator rectifier excitation
system
Figure 8.4: Alternator supplied controlled-rectifier
excitation system
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3. Static Excitation Systems:
• all components are static or stationary
• supply dc directly to the field of the main
generator through slip rings
• the power supply to the rectifiers is from the main
generator or the station auxiliary bus
3.1 Potential-source controlled rectifier system:
• excitation power is supplied through a
transformer from the main generator terminals
• regulated by a controlled rectifier
• commonly known as bus-fed or transformer-fed
static excitation system
• very small inherent time constant
• maximum exciter output voltage is dependent on
input ac voltage; during system faults the
available ceiling voltage is reduced
Figure 8.6: Potential-source controlled-rectifier excitation system
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3.2 Compound-source rectifier system:
• power to the exciter is formed by utilizing current
as well as voltage of the main generator
• achieved through a power potential transformer
(PPT) and a saturable current transformer (SCT)
• the regulator controls the exciter output through
controlled saturation of excitation transformer
• during a system fault, with depressed generator
voltage, the current input enables the exciter to
provide high field forcing capability
An example is the GE SCT-PPT.
3.3 Compound-controlled rectifier system:
• utilizes controlled rectifiers in the exciter output
circuits and the compounding of voltage and
current within the generator stator
• result is a high initial response static system with
full "fault-on" forcing capability
An example is the GE GENERREX system.
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Control and Protective Functions
A modern excitation control system is much more
than a simple voltage regulator
It includes a number of control, limiting and
protective functions which assist in fulfilling the
performance requirements identified earlier
Figure 8.14 illustrates the nature of these functions
and the manner in which they interface with each
other
any given system may include only some or all of
these functions depending on the specific
application and the type of exciter
control functions regulate specific quantities at
the desired level
limiting functions prevent certain quantities from
exceeding set limits
if any of the limiters fail, then protective functions
remove appropriate components or the unit from
service
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AC Regulator:
basic function is to maintain generator stator voltage
in addition, other auxiliaries act through the ac
regulator
DC Regulator:
holds constant generator field voltage (manual
control)
used for testing and startup, and when ac regulator is
faulty
Excitation System Stabilizing Circuits:
excitation systems with significant time delays have
poor inherent dynamic performance
unless very low steady-state regulator gain is used,
the control action is unstable when generator is on
open-circuit
series or feedback compensation is used to improve
the dynamic response
most commonly used form of compensation is a
derivative feedback (Figure 8.15)
Figure 8.15: Derivative feedback excitation control system
stabilization
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Power System Stabilizer (PSS):
uses auxiliary stabilizing signals (such as shaft
speed, frequency, power) to modulate the
generator field voltage so as to damp system
oscillations
Load Compensator:
used to regulate a voltage at a point either within
or external to the generator
achieved by building additional circuitry into the
AVR loop (see Fig. 8.16)
with RC and XC positive, the compensator
regulates a voltage at a point within the
generator;
used to ensure proper sharing VARs between
generators bussed together at their terminals
commonly used with hydro units and cross-compound
thermal units
with RC and XC negative, the compensator
regulates voltage at a point beyond the generator
terminals
commonly used to compensate for voltage drop across
step-up transformer when generators are connected
through individual transformers
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Figure 8.16: Schematic diagram of a load compensator
The magnitude of the resulting compensated voltage (Vc), which is fed
to the AVR, is given by
tcctc IjXREV
~~
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Underexcitation Limiter (UEL):
intended to prevent reduction of generator
excitation to a level where steady-state (small-
signal) stability limit or stator core end-region
heating limit is exceeded
control signal derived from a combination of
either voltage and current or active and reactive
power of the generator
a wide variety of forms used for implementation
should be coordinated with the loss-of-excitation
protection (see Figure 8.17)
Overexcitation Limiter (OXL)
purpose is to protect the generator from
overheating due to prolonged field overcurrent
Fig. 8.18 shows thermal overload capability of
the field winding
OXL detects the high field current condition and,
after a time delay, acts through the ac regulator
to ramp down the excitation to about 110% of
rated field current; if unsuccessful, trips the ac
regulator, transfers to dc regulator, and
repositions the set point corresponding to rated
value
two types of time delays used: (a) fixed time, and
(b) inverse time
with inverse time, the delay matches the thermal
capability as shown in Figure 8.18
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Figure 8.17: Coordination between UEL, LOE relay and
stability limit
Figure 8.18: Coordination of over-excitation limiting with
field thermal capability
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Volts per Hertz Limiter and Protection:
used to protect generator and step-up
transformer from damage due to excessive
magnetic flux resulting from low frequency
and/or overvoltage
excessive magnetic flux, if sustained, can cause
overheating and damage the unit transformer and
the generator core
Typical V/Hz limitations:
V/Hz limiter (or regulator) controls the field
voltage so as to limit the generator voltage when
V/Hz exceeds a preset value
V/Hz protection trips the generator when V/Hz
exceeds the preset value for a specified time
Note: The unit step-up transformer low voltage
rating is frequently 5% below the generator
voltage rating
V/Hz (p.u.) 1.25 1.2 1.15 1.10 1.05
Damage Time in
Minutes
GEN 0.2 1.0 6.0 20.0
XFMR 1.0 5.0 20.0
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Modeling of Excitation Systems
Detail of the model required depends on the
purpose of study:
the control and protective features that impact
on transient and small-signal stability studies
are the voltage regulator, PSS and excitation
control stabilization
the limiter and protective circuits normally need
to be considered only for long-term and voltage
stability studies
Per Unit System:
Several choices available:
a) per unit system used for the main generator field
circuit
chosen to simplify machine equations but not
considered suitable for exciter quantities; under
normal operating conditions field voltage in the order
of 0.001 (too small)
b) per unit system used for excitation system
specifications
rated load filed voltage as one per unit
not convenient for system studies
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c) Preferred per unit system for system studies:
one per unit exciter output voltage equal to the
generator field voltage required to produce rated
armature voltage on the air-gap line; one per unit
exciter output current is the corresponding
generator field current (see Figure 8.21)
referred to as the non-reciprocal per unit system
to distinguish it from the reciprocal per unit
system used for modelling the generator
Figure 8.22: Per unit conversion at the interface between
excitation system and synchronous machine field circuit
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Figure 8.21: Synchronous machine open circuit characteristics
Referring to Figure 8.21, the field current required to produce 1.0
per unit stator terminal voltage on the air-gap line (slope = Ladu) is
determined by
Therefore, in the reciprocal per unit system, the field current ifd and
field voltage efd required to generate rated stator terminal voltage on
the air-gap line are given by
By definition, the corresponding values of exciter output voltage Efd
and current Ifd are each equal to 1.0 per unit. Therefore,
.u.p0.1iLeE fdaduqt
.u.p
L
R
iRe
.u.p
L
1
i
adu
fd
fdfdfd
adu
fd
fdadufd
fd
fd
adu
fd
iLI
e
R
L
E
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8.6.2 Modeling of Excitation System Components
The basic elements which form different types of
excitation systems are the dc exciters (self or separately
excited); ac exciters; rectifiers (controlled or non-
controlled); magnetic, rotating, or electronic amplifiers;
excitation system stabilizing feedback circuits; signal
sensing and processing circuits
Separately excited dc exciter
Figure 8.26: Block diagram of a dc exciter
Self-excited dc exciter
The block diagram of Fig. 8.26 also applies to the self-
excited dc exciter. The value of KE, however, is now equal
to Ref/Rg-1 as compared to Ref/Rg for the separately excited
case.
The station operators usually track the voltage regulator
by periodically adjusting the rheostat setpoint so as to
make the voltage regulator output zero. This is accounted
for by selecting the value of KE so that the initial value of
VR is equal to zero. The parameter KE is therefore not
fixed, but varies with the operating condition.
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Figure 8.28: Block diagram of an ac exciter
Figure 8.30: Rectifier regulation model
AC Exciter and Rectifier
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Figure 8.34: (a) Integrator with windup limits
Figure 8.34: (b) Integrator with non-windup limits
Representation:
System equation:
Limiting action:
Representation:
System equation:
Limiting action:
Windup and Non-Windup Limits
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8.6.3 Modeling of Complete Excitation Systems
Figure 8.39 depicts the general structure of a detailed
excitation system model having a one-to-one
correspondence with the physical equipment. While this
model structure has the advantage of retaining a direct
relationship between model parameters and physical
parameters, such detail is considered too great for general
system studies. Therefore, model reduction techniques are
used to simplify and obtain a practical model appropriate
for the type of study for which it is intended.
The parameters of the reduced model are selected such that
the gain and phase characteristics of the reduced model
match those of the detailed model over the frequency range
of 0 to 3 Hz. In addition, all significant nonlinearities that
impact on system stability are accounted for. With a
reduced model, however, direct correspondence between
the model parameters and the actual system parameters is
generally lost.
Figure 8.39: Structure of a detailed excitation system model
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Standard IEEE Models
IEEE has standardized 12 model structures for
representing the wide variety of excitation systems
currently in use (see IEEE Standard 421.5-1992):
these models are intended for use in transient
and small-signal stability studies
Figures 8.40 to 8.43 show four examples
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Modeling of Limiters
Standard models do not include limiting circuits;
these do not come into play under normal
conditions
These are, however, important for long-term and
voltage stability studies
Implementation of these circuits varies widely
models have to be established on a case by case
basis
Figure 8.47 shows as an example the model of a
field current limiter