Power system analyses(Dynamic modeling)

Using PSS/E for power system analyses- Excitation system modeling -

Istanbul, May 2011

IntroductionIt is important to model excitation system of a generator due to the crucial influence to the transient processes in a power plant during the disturbance (first 10 seconds)Largest influence is present in:

maintaining steady voltage setpoint of generator terminalelectromagnetic processes of synchronous machine, capability of machine to dampen the small signal oscillations (small signal stability)

Least influence is present in Active Power-Frequency loop (Pf control)

Principle scheme of excitation

Rotating excitation systems

Non rotating excitation systems

Main Objectives

To identify the types of excitation systems in Black Sea Region

To collect as much data as possible for these excitation systems

Select the proper model from PSS/E dynamic library

DC excitation systems

DC excitation system

It uses DC machines (with parallel or independent self excitation) for electromechanical amplification of control signals and generation of field current for rotor circuit of synchronous generatorSpecial DC machines (amplidynes) are commonUsed widely during 50’s and 60’s and now considered as obsolete

DC excitation system

Advantages:Independency toward the voltage occurrences in grid in some cases

Backdraws:Decreased reliability due to the usage of many partsSlow response during the transient processesLow amplification of control signals inside the excitation system

DC excitation system

There are several models for DC excitation systems in PSS/E library:

generic models: IEEET1, IEEEX1, IEEEX4, IEEET5

more detailed models: ESDC1A, ESDC2A, EXDC2, IEET1B, IEEET4 , IEEET5A, DC4B

DC excitation system

Recommendation is to use IEEET1 and ESDC1A as a representation in PSS/E is with (it is the simplest one and it covers all the necessary parts)

DC excitation system

Parameter ranges for various DC excitation systems

KA usually less than

150

AC excitation systems

AC excitation systems

It uses AC machine to generate AC excitation voltage which is then rectified over a diode or thyristor bridgeRotor of synchronous generator may have slide contact rings or it could be brushlessVarious types of controllers for regulator could be used (PI, PID, Fuzzy, Neural etc.)In case of brushless excitation system, special type of excitation generator is usedWidely used excitation system

Advantages:Independency toward the voltage occurrences in grid in some casesExpanded range for excitation voltages and currents with high amplificationVery fast response

Backdraws:Decreased reliability due to the usage of many partsCommutation drop in brushless systemsDependency toward the voltage occurrences in grid in some cases

AC excitation systems

AC excitation systems

There are several models for AC excitation systems in PSS/E library:

alternators with non controlled rectifiers: EXAC1/2/3, ESAC1/2/3A, EXAC1A, ESAC5A, ESAC6A, ESAC8B, EXPIC, AC7B, AC8B

alternators with thyristor bridges: EXAC4, ESAC4A, EX2000, EXBAS

AC excitation systems

Fundamental difference between “western”and “eastern” control logic

GVoltage controller

Excitation limiters

Power system

stabilizer

V, deltaV

P, f, dV/dt, df/dt

V, deltaV, Pel, Qel

GVoltage

controller

V, deltaV

Ifd, dV/dt, df/dt, (d2V/dt2)

Pel, Qel, f, I

Σ

AC excitation systems

Recommendation is to use EXAC4 or ESAC4A as a representation in PSS/E (but not alone)

AC excitation systems

In case of brushless excitation, recommendation is to use EXAC1 or ESAC1A as a representation in PSS/E (but not alone)

AC excitation systems

Recommendation is to combine excitation systems with PSS’s (and excitation limiters)

AC excitation systems

Input signals for all power system stabilizers

ωΔfΔ

V

n

el

SP

dtdfdt

dV

AC excitation systems

Parameter ranges for AC1 excitation systems

“Shape” of response

Saturation of exciter machine

AC excitation systems

Parameter ranges for AC4 excitation systems

Limitation of control inputs

“Shape” of response

AC excitation systems

Parameter ranges for power system stabilizers

Limitation of control outputs

Static AC excitation system

Static AC excitation system

Uses generator terminals voltage, self excitation transformer and a thyristor bridge for rectification of AC signalThere are no rotating partsVarious types of controllers for regulator could be used (PI, PID, Fuzzy, Neural etc.)Widely used concept for excitation systems nowadays

Static AC excitation system

Advantages:Expanded range for excitation voltages and currents with high amplificationVery fast responseSimplicity of design (occupies small space)

Backdraws:Dependency toward the voltage occurrences Current compaundation is necessary

Static AC excitation system

There are several models for static AC excitation systems in PSS/E library:

Generic static excitation systems: EXST1/2/3, EXST2A, SCRX

Detailed static excitation systems: ESST1/2/3A ESST4B, EXELI, URST5T, ST6B

Static AC excitation system

Recommendation is to use EXST1 or ESST1A as a representation in PSS/E (with PSS and limiters)

Static AC excitation system

In case of some new excitation system installed, usually there are PI or PID controllers implemented. In such cases use ESST4B (PI controllers) or ST6B (PID controller)

Static AC excitation system

During the adjustment of the model, keep in mind:

Proportional part determines the amplification of the control signalIntegral part adjusts the time appearance of the control signal (lagging) and its rate of changeDifferential part amplifies and corrects all fast changing input signals (very sensitive value which can accelerate the change of control signal)

Static AC excitation system

Parameter ranges for ST1 excitation systems

Limitation of control inputs

“Shape” of response

Generic excitation systems

In case of absence of any knowledge about the excitation system in one power plant, use SEXS

Generic excitation systems

Avoid using SCRX, which is generic model, but used for special static excitations and it imposes “negative current logic”

Generic excitation systems

Typical values and ranges for SEXS parameters

Free to increase K up to 400

Excitation limiters

Supplementary equipment to the excitation systems

Main purpose is to readjust the operation of excitation system in “difficult regimes” of generator

Their action is restricted to maintaining of upper and lower limits for field voltage applied to rectifier unit (or to rotor winding of synchronous machine)

Excitation limiters

Recommendation for Maximum Excitation Limiters (OEL) is MAXEX1:

Excitation limiters

Recommendation for Minimum Excitation Limiters (UEL) is MNLEX1:

Excitation limiters

Minimum excitation limiter sends it’s signal to High Value Gate for comparison with the regular voltage signal. In case it is higher than the actual voltage it will superimpose and maintain the limit of low voltage

Maximum excitation limiter sends it’s signal to Low Value Gate for comparison with the regular voltage signal. In case it is lower than the actual voltage it will superimpose and maintain the limit of maximum voltage

Excitation limiters

Parameter ranges for excitation limiters

Adjustment of excitation systems

It is done according to the IEEE standards

There are two standard tests which are used in practice and modeled in PSS/E:

Open circuit testResponse Ratio Test

These tests are activated by ESTR function in PSS/E

Adjustment of excitation systems

Open Circuit test

UT

TT

nV

nEfd

EfdΔ

1V1Efd

1V

fdnfd EE ⋅<Δ 8.0

sTT 5.2≤

sT 5=

sTs U 11.0 ≤≤

After reaching the steady state, if the excitation voltage is too high compared to the terminal voltage, check the saturation factors of generator

Adjustment of excitation systems

Response Ratio test shows the capability of forcing the maximum excitation voltage on nominally loaded generator. Rising period should be over for less than 0.5 s and maximum field voltage should be at least twice the nominal value.

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Thank youThank you