Transcript excitation systems

EXCITATION SYSTEMS
Copyright © P. Kundur
This material should not be used without the author's consent
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Excitation Systems
Outline

Functions and Performance
Requirements

Elements of an Excitation System

Types of Excitation Systems

Control and Protection Functions
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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:
1.
•
DC excitation systems
•
AC excitation systems
•
Static excitation systems
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superseded by ac exciters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 noncontinuous 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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Figure 8.2: DC excitation system with amplidyne voltage
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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Figure 8.5: Brushless 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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Fig. 8.7: Compound-source rectifier excitation system
Figure 8.8: GENERREX compound-controlled rectifier
excitation system ©IEEE1976 [16]
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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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Figure 8.14: Excitation system control and protective
circuits
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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
~
~


V

E

R

jX
c
t
c
cI
t
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
Underexcitation Limiter (UEL):
 intended to prevent reduction of generator
excitation to a level where steady-state (smallsignal) 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 (p.u.)
Damage Time in
Minutes
1.25
1.2
1.15
1.10
1.05
GEN
0.2
1.0
6.0
20.0

XFMR
1.0
5.0
20.0


 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
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