EXCITATION SYSTEMS
EXCITATION SYSTEMS
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Excitation Systems
Excitation Systems
1.
Functions and Performance
Requirements
2.
Elements of an Excitation System
3.
Types of Excitation Systems
Outline
3.
Types of Excitation Systems
4.
Control and Protection Functions
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Functions and Performance
Functions and Performance
Requirements of Excitation Systems
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
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
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Types of Excitation Systems
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;
• 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lost favor in the mid-1960s because of large size; superseded by ac exciters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:
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
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:
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
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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
• very small inherent time constant
• maximum exciter output voltage is dependent on
input ac voltage; during system faults the available ceiling voltage is reduced
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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
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Fig. 8.7: Compound-source rectifier excitation system
Figure 8.8: GENERREX compound-controlled rectifier
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Control and Protective Functions
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
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Figure 8.14: Excitation system control and protective circuits
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, 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)
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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 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
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
(
c c)
t tc
E
R
jX
I
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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 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
Figure 8.17: Coordination between UEL, LOE relay and stability limit
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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.) 1.25 1.2 1.15 1.10 1.05
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
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
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)
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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
AC Exciter and Rectifier
Figure 8.28: Block diagram of an ac exciter
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Figure 8.34: (a) Integrator with windup limits
Representation:
System equation:
Limiting action:
Windup and Non-Windup Limits
Figure 8.34: (b) Integrator with non-windup limits
Representation:
System equation:
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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 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.
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Standard IEEE Models
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
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1. Type DC1A Exciter model
Figure 8.40: IEEE type DC1A excitation system model. ©IEEE 1991[8] The type DC1A exciter model represents field controlled dc
communtator exciters, with continuously acting voltage regulators. The exciter may be separately excited or self excited, the latter type being more common. When self excited, KE is selected so that initially
VR=0, representing operator action of tracking the voltage regulator by
periodically trimming the shunt field rheostat set point.
2. Type AC1A Exciter model
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3. Type AC4A exciter model
The type AC4A exciter model represents an alternator supplied controlled rectifier excitation system - a high initial response excitation system utilizing full wave thyristor bridge circuit. Excitation system stabilization is usually provided in the form of a series lag-lead network (transient gain reduction). The time constant associated with the regulator and firing of thyristors is represented by TA. The overall gain is represented by KA. The
rectifier operation is confined to mode 1 region. Rectifier regulation effects on exciter output limits are accounted for by constant KC.
Figure 8.42: IEEE type AC4A excitation system model © IEEE 1991 [8]
4. Type ST1A exciter model
effects on exciter output limits are accounted for by constant KC.
The type ST1A exciter model represents potential-source controlled-rectifier systems. The excitation power is supplied through a transformer from
generator terminals; therefore, the exciter ceiling voltage is directly
proportional to generator terminal voltage. The effect of rectifier regulation on ceiling voltage is represented by KC. The model provides flexibility to represent series lag-lead or rate feedback stabilization. Because of very high field forcing capability of the system, a field current limiter is
sometimes employed; the limit is defined by lLR and the gain by KLR.
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Modeling of Limiters
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
basis
Figure 8.47 shows as an example the model of a
field current limiter
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(a) Block diagram representation
(b) Limiting characteristics
