3. Excitation Systems

EEPM523 POWER SYSTEM DYNAMICS

([email protected])

Aznan Ezraie AriffinUNITEN

Semester 1, May Sept 2014

Presentations: 3 July 2014

Voltage Control in Power Systems

Reactive power must be balanced so as that the voltages are within acceptable limits

Improper reactive power balance will result in deviations of the voltages

Normally the power system is operated such that the voltage drops along the lines are smaller and the node

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voltage drops along the lines are smaller and the node voltages of the system are almost equal

Voltage magnitudes can be controlled to desired values by control of the reactive power

There are several sources of reactive power but it cannot be transported over long distances in the system

Voltage Control in Power Systems (cont) Important generators of reactive power:

Overexcited synchronous machines Capacitor banks Capacitance of overhead lines and cables FACTs devices

Important consumers of reactive power:

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Important consumers of reactive power: Inductive static loads Underexcited synchronous machines Induction motors Inductance of overhead lines and cables Transformer inductances FACTs devices

Topics on Excitation System

Excitation requirements Types of excitation systems Elements of an excitation system Control and corrective functions

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Control and corrective functions Dynamic performances Modeling of excitation systems Step Responses Tests Review of Classical Control Techniques

The functions of an excitation system are:

To provide direct current to the synchronous generator field winding

To perform control and protective functions essential to the satisfactory

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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 generator short term capabilities: Rotor insulation failure due to high field voltage

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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: Contribution to effective control system voltage and improvement

of system stability

Performance requirements of an excitation system

- meet specified response criteria- provide limiting and protection to prevent damage to

To fulfill the above roles satisfactorily, the excitation system must satisfy the following requirements:

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- provide limiting and protection to prevent damage to itself, the generator, and other equipment

- meet specified requirements for operating flexibility- meet the desired reliability and availability

Functional block diagram of a synchronous generator excitation control system

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Evolution of excitation systems

- Early exciters were controlled manually

- In the 1920s, continuous and fast acting regulators contributed to improvements in steady-state and transient stability

- In the 1960s,the role of excitation systems expanded by use

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- In the 1960s,the role of excitation systems expanded by use of power system stabilizer

- Modern exciters are capable of practically instantaneous response with high ceiling voltages with a wide array of control and protective circuits

- Digital excitation system are widely utilized.



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Types of Excitation Systems

Classified into 3 broad categories based on the excitation power source:

DC excitation systemsAC excitation systemsStatic excitation systems

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Static excitation systems

DC Excitation Systems

Utilise DC generators as source of power; driven by a motor or the shaft of main generator

Represents early systems (1920s to 1960s); lost favour 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 systems using magnetic and rotating amplifiers

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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

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An example of direct current excitation system

Self-excited (main field connected across the terminals of the exciter armature). Two control fields, one assists the main field, the other reduces the main field, referred as a

boost-buck scheme. The power of control fields is supplied by a pilot exciter (permanent magnet generator),

through the AVR.

DC excitation system with an amplidyne voltage regulator

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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 the exciter is rectified by either controlled or non-controlled rectifiers

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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

AC Excitation Systems: stationary type

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 (GE-ALTERREX)

When controlled rectifiers are used, the regulator directly controls the DC output voltage of the exciter

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directly controls the DC output voltage of the exciter (GE-ALTHYREX)

Field controlled alternator rectifier excitation system (GE-ALTERREX)

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Alternator supplied controller-rectifier excitation system (GE-ALTHYREX)

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AC Excitation Systems: rotating type

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

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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

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

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main generator or the station auxiliary bus

Static Excitation Systems: potential-source controlled

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

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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

Potential-source controlled rectifier excitation system

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Static Excitation Systems: compound-source

Power to the exciter is formed by utilising 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

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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

Compound-source rectifier excitation system (GE SCT-PPT)

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Static Excitation Systems: compound-controlled

Utilizes controlled rectifiers in the exciter output circuits and the compounding of voltage and current within the generator stator

Result in a high initial response static system with full fault-on forcing capability (GE

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with full fault-on forcing capability (GE GENERREX)

Compound-controlled rectifier excitation 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

The following figure illustrates the nature of these functions and the manner in which they interface with each other

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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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Elements of an Excitation System

1. Exciter: provides DC power to the generator field winding2. Regulator: processes and amplifies input control signals to a

level and form appropriate for control of the exciter3. Terminal voltage transducer and load compensator: senses

generator terminal voltage, rectifies and filters it to a DC quantity and compares with a reference;load compensator may be provided if desired to hold voltage at remote point

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be provided if desired to hold voltage at remote point4. Power system stabiliser: provides additional input signal to

the regulator to damp power system oscillations5. Limiters and protective circuits: ensure that the capability

limits of exciter and generator are not exceeded

Voltage Sensing and Load Compensation

Voltage Sensing

Exciter GeneratorField Shorting

DC Regulator

AC Regulator

Exc. Sys. Stab. circuits

Adjuster

Adjuster

Excitation system control and protective circuits

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Note: Field shorting circuits are applicable to ac and static exciters only Some systems have open loop dc regulator Max. exc. limiter may also be used with dc regulator

Power System Stabilizer

Max. Exc.Limiter

Under Exc. Limiter

V/Hz Limiter Protection

Var and/or PF Controller

Components of an Excitation System: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)

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Holds constant generator field voltage (manual control) Used for testing and startup, and when AC regulator is faulty

Excitation system 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 the generator is on open-circuit

Components of an Excitation System (cont): Series or feedback compensation is used to improve the dynamic

response Most commonly used form of compensation is a derivative

feedback Static excitation systems have negligible inherent time delays and

do not require stabilization

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lh}y+

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Transient Gain Reduction

Stabilization of Excitation Control System

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The derivative feedback scheme is often used for rotating exciters, while the transient gain reduction scheme is for ac and static exciters Its not common for both schemes to be employed at the same time.

Derivative Feedback

F

F

sTsK+1

Components of an Excitation System (cont):Power System Stabilizer: 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

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the generator Achieved by building additional circuitry into the AVR loop 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

Components of an Excitation System (cont): With Rc and Xc negative, the compensator regulates a voltage at

a point beyond the generator commonly used to compensate for voltage drop across step-up

transformer when generators are connected through individual transformers

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Components of an Excitation System (cont):Underexcitation Limiter (UEL): Intended to prevent reduction of generator excitation to a level where steady-state stability limit or stator core end-region heating limit is exceeded

Control signal derived from a combination of either voltage or current or active and reactive power of the

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voltage or 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



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Coordination between UEL, LOE relay and stability limit

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Components of an Excitation System (cont):Overexcitation Limiter (OEL): Purpose is to protect the generator from overheating due to

prolonged field over-current The figure next shows thermal overload capability of the field

winding OEL detects the high field current condition and, after a time

delay, acts through the AC regulator to ramp down the excitation

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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 fixed time and inverse time With inverse time, the delay matches the thermal capability (as

shown in the figure next)

Coordination of over-excitation limiting with field thermal capability

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The measure of volts per hertz is an indication of the flux conditions in the generator stator core, which can be seen from the following equation:

Volts per Hertz Limiter and Protection

pi = tdr NkfE 2

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pi = tdr NkfE 2Where

E = armature phase voltage (volts,rms),kd = distribution factor , fr = rotor speed (Hz) = flux in the machine core (megalines),Nt = effective number of series turn per armature phase per

circuit, linking the flux

Components of an Excitation System (cont):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:

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Typical V/Hz limitations:V/Hz (p.u.) 1.25 1.2 1.15 1.1 1.05Damage Time in Minutes

GEN 0.2 1.0 6.0 20.0 inf

XFMR 1.0 5.0 20.0 inf

Components of an Excitation System (cont):

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

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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

Modelling 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

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stabilization The limiter and protective circuits normally need to be

considered only for long-term and voltage stability studies.



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Dynamic Performance Measures

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Representation of the overall excitation system in the classical form describing feedback control system

Performance of excitation control system depends on the characteristics of Excitation system Generator Power system

Large Signal Performance Measures

Provide a means of assessing the excitation system performance for severe transients

Performance measures are defined under specified conditions Ceiling voltage max direct voltage indicative of field forcing

capability Ceiling current max direct current

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Voltage time response output voltage as function of time Voltage response time time in secs to attain 95% between

ceiling voltage and rated load field voltage High initial response response time of 0.1 secs or less Nominal reponse -

( )( )oeaocd

Large Signal Performance Measures

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Excitation system nominal response

AB

C

D

Actual response

Nominal Response Ratio = CD /AO / 0.5

Note: The basis for considering a nominal time span of 0.5 seconds in

Large-signal Performance Measures

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A D

EO Time in seconds

Area ABD = Area ACDOE = 0.5 secondsAO = Rated field voltage

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nominal time span of 0.5 seconds in the definition is that, following a severe disturbance, the generator rotor angle swing normally peaks between 0.4 s and 0.75 s. The excitation system must act within this time period to be effective in enhancing transient stability. Accordingly 0.5 s was chosen for the definition.

Generally Accepted Values of SmallGenerally Accepted Values of Small--signal signal Indexes Characterizing Good Feedback Control Indexes Characterizing Good Feedback Control

System PerformanceSystem Performance

Gain Margin Gm : 6 dB (open-loop)Phase Margin m : 40 (open-loop)Overshoot: 0 ~ 15% (time step)Peak Value MP : 1.1 ~ 1.6 dB (closed loop)Damping Ratio: 0.6 (S-plane)

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Small Signal Performance Measures

52Typical time response to step input


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Typical open-loop frequency response with generator open circuited


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Typical closed-loop frequency response with generator open circuited



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Modelling of Excitation System Components

The basic elements which form different types of excitation systems are:

DC exciters (self or separately excited) AC exciters Rectifiers (controlled or non-controlled) Amplifiers (magnetic, rotating or electronic)

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Amplifiers (magnetic, rotating or electronic) Excitation system stabilizing feedback circuits Signal sensing and processing circuits

Block diagram of a DC exciter (page 351 PB)

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For separately excited DC exciter, the value of KE is Ref/Rg For self-excited DC exciter, the value of KE is Ref/Rg - 1

Block diagram of AC exciter

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Rectifier regulation model

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Amplifiers

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Amplidyne model (rotating amplifier)

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Integrator with windup limits

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Integrator with non-windup limits

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Single time constant block with windup limits

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Single time constant block with non-windup limits

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Lead-lag function with non-windup units

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Gating functions

u

v

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Structure of a detailed excitation system model

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IEEE Standard Exciter 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.

Modeling of Limiters

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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.

Type DC1A exciter model

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Type AC1A exciter model

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Type AC4A excitation system model

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Type ST1A exciter model

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Field current or over-excitation limiter

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Field current limiter model

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Digital Excitation Systems

There is a growing trend toward using the digital technology to perform control and protection functions of modern excitation systems

They are not just digital version of their analog

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They are not just digital version of their analog counterparts, but contain sophisticated control functions not readily available in analog excitation systems

The use of digital systems are economically feasible Examples: GE EX2000, ABB Unitrol-F, Basler Decs

ExciterController

Scaling Circuitry

Set Point nnnn+-

Block Diagram of an Analog Excitation System

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Block Diagram of a Digital Excitation System

ExciterController

Scaling Circuitry

Set Point nnnn+-

D/A

Microprocessor based digital systemA/D

Features of Digital Excitation Systems

Extremely sophisticated control strategy and algorithms can be readily implemented. They can be nonlinear, fuzzy logic, adaptive or any type of control.

The control and protection functions can be incorporated in the microprocessor codes, eliminating the need of using separate hardware

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microprocessor codes, eliminating the need of using separate hardware devices:

- Power system stabilizer (PSS)- Var or power factor control (Var/PF)- Under and over excitation limiters (UEL/OEL)- Stator current limiter (SCL)- Volts per Hertz limiter (V/Hz)


Communication capability - Digital systems typically have some form of communications available to users, from the simplest local key pad and display, to more complex scheme such as local serial link, remote serial link, modem, local area network etc. The communication capability may be used to change controller parameter settings or exchange

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used to change controller parameter settings or exchange data with other controllers in the system such as the speed governor or a supervisory controller.

Data recording The ability to record various parameters associated with the excitation system. It may output data to an external data recorder via D/A converters, or directly do the recording internally.


Metering Digital systems can provide the display of various generator system parameters that are not normally available on analog systems without including some additional transducers. It may be linked to a main computer to provide metering quantities.

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computer to provide metering quantities.

Self test and system test many contain on-board test features.

Cost per function is typically lower for the digital system.

Off-line setup means reduced commissioning time.



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AVR Step Response Test

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IEEE Type ST1 Exciter Model Validated for the Static Excitation System

TR VIMAX VIMIN TC TB KA TA VRMAX VRMIN KC KF TF

0.02 0.1 -0.1 0.0 0.0 120 0.02 6.4 0.0 0.0 0.01 2.5

step applied here

step applied here


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IEEET1 DC Exciter Model Validated for the Excitation System

Test Procedure:

Operate unit at full speed no load (off-line) Set exciter in AVR control Apply a step change (5% typical ) to the AVR set point


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Apply a step change (5% typical ) to the AVR set point Repeat with a 10% step change, trying to hit the field

voltage ceiling and floor limits

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Measured-Vt Simulated-Vt

Measured-Vf Simulated-Vf


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AVR Step Test Results for a Potential-Source Static Excitation System

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AVR Step Test Results for a 80MVA Hydro Generator with a dc Exciter (Sample)

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Time in seconds

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Measured Vt Simulated VtMeasured Vf Simulated-Vf

Excitation Systems Need for Accurate Model

Since excitation systems play such an important rule in the characteristics of oscillations, their modelling is also critical

Appropriate excitation system models must be

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Appropriate excitation system models must be developed

Typical model and data should not be usedThe least is to use the manufacturer recommended modelsIf possible, the models should be field tested and validated

Generator terminal voltage (pu)

1.040

1.060

1.080Generator terminal voltage (pu)

1.040

1.060

1.080

Sustained oscillations

IEEE AC5A type exciter

Rate feedback gain K (at

Exciter Step Response Examples (Contd)

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Time (sec)0.000 2.000 4.000 6.000 8.000 10.000

0.980

1.000

1.020

Time (sec)0.000 2.000 4.000 6.000 8.000 10.000

0.980

1.000

1.020

Rate feedback gain KF (at 0.005) is too small

Setting KF to 0.05 makes response very reasonable


1.040

1.060


1.040

1.060

1.080

Large swings


Rate feedback time constant TF(at 0.017) is too small


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Time (sec)0.000 2.000 4.000 6.000 8.000 10.000

0.980

1.000

1.020

Time (sec)0.000 2.000 4.000 6.000 8.000 10.000

0.980

1.000

1.020 Setting TF = 1.0 makes response very reasonable


1.040

1.060


1.040

1.060

1.080

Very slow response


Rate feedback time constant TFand gain KF are both at 2.9; apparently set incorrectly


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Time (sec)0.000 2.000 4.000 6.000 8.000 10.000

0.980

1.000

1.020

Time (sec)0.000 2.000 4.000 6.000 8.000 10.000

0.980

1.000

1.020

Fapparently set incorrectly

Setting TF = 1.0 and KF = 0.05 makes response more reasonable


1.060

1.080


1.060

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1.100

Large swings

The AVR has a PI controller

The proportional and integral gains (at 0.0357 and 3.57 respectively) are not appropriately


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Time (sec)0.000 2.000 4.000 6.000 8.000 10.000

0.980

1.000

1.020

1.040

Time (sec)0.000 2.000 4.000 6.000 8.000 10.000

0.980

1.000

1.020

1.040respectively) are not appropriately coordinated

Setting both of proportional and integral gains to 3.57 makes response more reasonable


1.100

1.130

1.160

Large overshoot


The AVR gain KA is set at 2894 with a slow exciter time constant T (at 1.2 seconds). This


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Time (sec)0.000 2.000 4.000 6.000 8.000 10.000

0.980

1.010

1.040

1.070TE (at 1.2 seconds). This apparently is poorly coordinated

A transient gain reduction would possibly make the response better

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Field testing is an effective way to validate exciter model as shown in the example with exciter step test


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Measured responseSimulated (old model)Simulated (new model)

Exciter Step Response Examples Field test

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Exciter Step Response Examples Vt and Q response due to step input

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Classical Control Technique

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Open Circuit Response

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Documents

3. Excitation Systems