THE EFFECT OF EXCITATION ON STABILITY:

TUNING OF AVR PARAMETERS

THE EFFECT OF EXCITATION ON STABILITY:

TUNING OF AVR PARAMETERS

ByProf. C. Radhakrishna

ByProf. C. Radhakrishna

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CONTENTS

THE EFFECT OF EXCITATION ON STABILITY:

TUNING OF AVR PARAMETERS

Effect of Excitation on Generator Power Limits

Effect of the Excitation System on Transient Stability

Effect of Excitation on Dynamic Stability

Further considerations of the regulator gain and time constant

Approximate excitation system representation

Some General Comments on the Effect of Excitation on Stability

Tuning of AVR Parameters

Excitation system tuning

Exciter tuning objectives

Other Tuning Approaches

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Effect of Excitation on Generator Power Limits

• With the ideal regulation there is no stability limit.• Operation in the region where δ > 90˚ is possible.• Assumed physical system is not realizable since

there is always a lag in the excitation response evenif the voltage regulator is ideal.

Effect of the Excitation System on Transient Stability

• The concern is whether the system is able to maintainsynchronism during and following the disturbances.

• The period of interest is relatively short (at most a fewseconds), with the first swing being of primary importance.

• Main factors that affect the performance during severetransients.1. The disturbing influence of the impact. This includes

the type of disturbance, its location, and its duration.2. The ability of the transmission system to maintain

strong synchronizing forces during the transientinitiated by a disturbance.

3. The turbine-generator parameters.

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The system parameters influencing these factors1) The synchronous machine parameters. (a) the inertia

constant, (b) the direct axis transient reactance, (c) thedirect axis open circuit time constant, and (d) the ability ofthe excitation system to hold the flux level of thesynchronous machine and increase the output powerduring the transient.

(2) The transmission system impedances under normal,faulted, and postfault conditions.

(3) The protective relaying scheme and equipment.

Effect of Excitation on Dynamic Stability• Fast excitation-systems are usually acknowledged to be

beneficial to transient stability• These fast excitation changes are not necessarily

beneficial in damping the oscillations that follow the firstswing.

• They sometimes contribute growing oscillations severalseconds after the occurrence of a large disturbance.

• With proper design and compensation, a fast exciter canbe an effective means of enhancing stability in thedynamic range as well as in the first few cycles after adisturbance.

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The transfer function for Vt/VREF can be obtainedVt /VREF = Kt /[(1+Ke) + s(τe+τ’d0) + τ’d0τes2]Vt/VREF = K/(s2 + 2ζωns + ωn

2)whereK = Ke/τ’d0τe , ωn

2 = (1 + Ke) / τ’d0τe , 2ζωn = (1/τe + 1/τ’d0)

• For good dynamic performance, i.e.. for good dampingcharacteristics, a reasonable value of ζ is 1/ √2.

• For typical values of the gains and time constants infast exciters we usually have τ’d0 >> τe, and Ke >> 1.

• We can show then that for good performanceKe≅τ’d0/2τe.

• This is usually lower than the value of gain required forsteady-state performance.

Some considerations of the regulator gain and time constant

Fig. 1: Block diagram representing the machine terminal voltage at no load

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• For less severe transients, the effect of modern fastexcitation systems on first swing transients is marginal.• For more severe transients or for transients initiated byfaults of longer duration, these modern exciters can have amore pronounced effect.• Their effects on damping torques are small; but in thecases where the system exhibits negative dampingcharacteristics, the voltage regulator usually aggravates thesituation by increasing the negative damping.• Supplementary signals to introduce artificial dampingtorques and to reduce intermachine and intersystemoscillations have been used with great success.• Large interconnected power systems experience negativedamping at very low frequencies of oscillations. Theparameters of the PSS for a particular generator must beadjusted after careful study of the power system dynamicperformance.• Use of a signal derived from speed or frequency deviationprocessed through a PSS network to give the desired dampingcharacteristic.

Some General Comments on the Effect of Excitation on Stability

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• Whether the stabilizing signal derived from speedprovides the best answer is an open question.

• Signals derived from the various “states" of the systemare fed back with different gains to optimize the systemdynamic performance.

Tuning of AVR Parameters

• Procedure is based on emulation of the open-circuit stepresponse test, a standard control tuning practice for severaldecades.

• Guidelines are provided for tuning models until well-behaved, and realistic, responses are achieved.

• Limits and other nonlinear parameters in exciter modelsare important.

• Determine the exciter response following a largeperturbation to the system.

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Frequency response techniques are basic to control system tuning.

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Excitation system tuning• The unit is brought to nominal voltage with open-circuit

generator terminals, and a small step is applied to thevoltage reference.

• Field and terminal voltages responses are recorded.

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• If the response of the voltage-regulating loop isinadequate (too slow or too oscillatory), the fieldengineer retunes whatever tunable parameters theexcitation system has, so that the response is withinexpected performance.

• If the responses of the model to the step test arejudged unacceptable, and no additional test data isavailable, the system engineer modifies parameters ofthe excitation system model that can be tuned by thefield engineer.

Exciter tuning objectives• Typical values of field and modern static exciter time

constants are 5 and 0.05 seconds respectively.

Figure 3: Block diagram of simplified voltage-regulating loop with machine on open circuit.

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Steady-state gain KA is limited to:

• Steady-state gain values are limited to less than 50 for atypical static exciter.

• This imposes a restriction on exciter performancebecause steady-state gain is directly related to exciterregulation.

• Under steady-state (s=0) and open-circuit orinterconnected conditions, voltage error (VT-VREF) will beequal to the change in EFD/KA.

• Let us assume a 2.7 pu full load EFD , a no-load EFD of 1pu and a KA of 50. This means that with costantreference voltage , terminal voltage will change by100 (2.7 – 1 )/ 50 = 3.4%, from zero to full-loadconditions, such regulation values are unacceptable innormal practice.

• Regulation of less than 1 % is usually required.

A

dA T

TK2

'0

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Transient gain reduction is one widely used method theindustry has used to resolve this conflict of objectivesbetween a stable and well-damped voltage-regulatingloop, and a low value of exciter regulation.

Other Tuning Approaches• There is a discussion in the industry whether transient

gain reduction is necessary or not, particularly in thecase of high-response, very low time constant exciters.

• The relatively high transient gains will generallyrequire the use of Power System Stabilizers.

• The actual transient gain may be lower than the valuerequired by open-circuit regulator constraints.

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REFERENCES :[ 1 ] P.M. Anderson & A.A. Fouad : “Power System Control and Stability” ,2nd edition, IEEE Press Power Engineering Series, Wiley-Interscience,2003.[ 2 ] Rodolfo J. Koessler, : “Techniques for Tunning Excitation SystemParametres”, IEEE Transactions on Energy Conversion, Vol.3, No.4,December 1988, pp 785-791.

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CONCLUSIONS

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THANK YOU