excitation

1
EXCITATION SYSTEMS
Copyright P. KundurThis material should not
be used without the author's consent
2
Excitation Systems
Outline

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

3
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
• 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

4
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
• 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  superseded by ac exciters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
• 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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• 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
14
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
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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
  timeNote The unit step-up transformer low
  voltage rating is frequently 5 below the
  generator voltage rating

V/Hz (p.u.) 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 ?
Damage Time in Minutes 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 SystemSeveral choices available
• 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)
• 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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• 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 circuitsSeparately
  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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• AC Exciter and Rectifier

Figure 8.28 Block diagram of an ac exciter
Figure 8.30 Rectifier regulation model
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Windup and Non-Windup Limits
RepresentationSystem equationLimiting
action
Figure 8.34 (a) Integrator with windup limits
RepresentationSystem equationLimiting
action
Figure 8.34 (b) Integrator with 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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• Type DC1A Exciter model
• Type AC1A Exciter model

Figure 8.40 IEEE type DC1A excitation system
model. IEEE 19918
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
VR0, representing operator action of tracking
the voltage regulator by periodically trimming
the shunt field rheostat set point.
Figure 8.41 IEEE type AC1A excitation system
model. IEEE 19918
The type AC1A exciter model represents a field
controlled alternator excitation system with
non-controlled rectifiers, applicable to a
brushless excitation system. The diode rectifier
characteristic imposes a lower limit of zero on
the exciter output voltage. The exciter field
supplied by a pilot exciter, and the voltage
regulator power supply is not affected by
external transients.
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1. Type AC4A exciter model
2. Type ST1A 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.
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

• 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

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(a) Block diagram representation
(b) Limiting characteristics
Figure 8.47 Field-current limiter model