EXCITATION SYSTEMSEXCITATION SYSTEMS

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Copyright © P. KundurThis material should not be used without the author's consent

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Excitation SystemsExcitation Systems

1. Functions and Performance

Requirements

2. Elements of an Excitation System

3. Types of Excitation Systems

Outline

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3. Types of Excitation Systems

4. Control and Protection Functions

5. Modeling of Excitation Systems

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Functions and Performance Functions and Performance

Requirements of Excitation SystemsRequirements 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

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a) Generator considerations:

� supply and adjust field current as the generator output varies within its continuous capability

� respond to transient disturbances with field forcingconsistent 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 SystemElements of an Excitation System

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� 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 SystemsTypes 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;

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• 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:

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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:

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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

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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

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• 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.

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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

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Figure 8.8: GENERREX compound-controlled rectifier excitation system ©IEEE1976 [16]

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Control and Protective FunctionsControl 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

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� 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

� 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,

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� 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

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� 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~~

++=

)

� 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

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� 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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Figure 8.18: Coordination of over-excitation limiting with field thermal capability

�

� 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

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� 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 ∞ ∞

Modeling of Excitation SystemsModeling 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

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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

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

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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.

Figure 8.28: Block diagram of an ac exciter

AC Exciter and Rectifier

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Figure 8.28: Block diagram of an ac exciter

Figure 8.30: Rectifier regulation model

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Figure 8.34: (a) Integrator with windup limits

Representation:

System equation:

Limiting action:

Windup and Non-Windup Limits

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Figure 8.34: (b) Integrator with non-windup limits

Representation:

System equation:

Limiting action:

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

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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 ModelsStandard 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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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.

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2. Type AC1A Exciter model

Figure 8.41: IEEE type AC1A excitation system model. ©IEEE 1991[8]

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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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]

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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.

Figure 8.43: IEEE type ST1A excitation system model © IEEE 1991 [8]

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Modeling of LimitersModeling 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

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basis

� Figure 8.47 shows as an example the model of a

field current limiter

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(a) Block diagram representation

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(b) Limiting characteristics

Figure 8.47: Field-current limiter model