Tuning AVR for Transient Stability

THE EFFECT OF EXCITATION ON STABILITY: TUNING OF AVR PARAMETERS By Prof. C. Radhakrishna

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 even if the voltage regulator is ideal. Effect of the Excitation System on Transient Stability • • •

The concern is whether the system is able to maintain synchronism during and following the disturbances. The period of interest is relatively short (at most a few seconds), with the first swing being of primary importance. Main factors that affect the performance during severe transients. 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 transient initiated by a disturbance. 3. The turbine-generator parameters.

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The system parameters influencing these factors 1) The synchronous machine parameters. (a) the inertia constant, (b) the direct axis transient reactance, (c) the direct axis open circuit time constant, and (d) the ability of the excitation system to hold the flux level of the synchronous machine and increase the output power during 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 first swing. • They sometimes contribute growing oscillations several seconds after the occurrence of a large disturbance. • With proper design and compensation, a fast exciter can be an effective means of enhancing stability in the dynamic range as well as in the first few cycles after a disturbance. 4 10/17/2010 10:23 AM

Some considerations of the regulator gain and time constant The transfer function for Vt/VREF can be obtained Vt /VREF = Kt /[(1+Ke) + s(τe+τ’d0) + τ’d0τes2] Vt/VREF = K/(s2 + 2ζωns + ωn2) where K = Ke/τ’d0τe , ωn2 = (1 + Ke) / τ’d0τe , 2ζωn = (1/τe + 1/τ’d0) • For good dynamic performance, i.e.. for good damping characteristics, a reasonable value of ζ is 1/ √2. • For typical values of the gains and time constants in fast exciters we usually have τ’d0 >> τe, and Ke >> 1. • We can show then that for good performance Ke≅τ’d0/2τe. • This is usually lower than the value of gain required for steady-state performance.

Fig. 1: Block diagram representing the machine terminal voltage at no load 10/17/2010 10:23 AM

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Some General Comments on the Effect of Excitation on Stability • For less severe transients, the effect of modern fast excitation systems on first swing transients is marginal. • For more severe transients or for transients initiated by faults of longer duration, these modern exciters can have a more pronounced effect. • Their effects on damping torques are small; but in the cases where the system exhibits negative damping characteristics, the voltage regulator usually aggravates the situation by increasing the negative damping. • Supplementary signals to introduce artificial damping torques and to reduce intermachine and intersystem oscillations have been used with great success. • Large interconnected power systems experience negative damping at very low frequencies of oscillations. The parameters of the PSS for a particular generator must be adjusted after careful study of the power system dynamic performance. • Use of a signal derived from speed or frequency deviation processed through a PSS network to give the desired damping characteristic. 10/17/2010 10:23 AM

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Whether the stabilizing signal derived from speed provides the best answer is an open question. • Signals derived from the various “states" of the system are fed back with different gains to optimize the system dynamic performance.

Tuning of AVR Parameters •

Procedure is based on emulation of the open-circuit step response test, a standard control tuning practice for several decades. • Guidelines are provided for tuning models until wellbehaved, and realistic, responses are achieved. • Limits and other nonlinear parameters in exciter models are important. • Determine the exciter response following a large perturbation 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 the voltage reference. • Field and terminal voltages responses are recorded.

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If the response of the voltage-regulating loop is inadequate (too slow or too oscillatory), the field engineer retunes whatever tunable parameters the excitation system has, so that the response is within expected performance. • If the responses of the model to the step test are judged unacceptable, and no additional test data is available, the system engineer modifies parameters of the excitation system model that can be tuned by the field 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:

KA

T d' 0  2T A

• Steady-state gain values are limited to less than 50 for a typical static exciter. • This imposes a restriction on exciter performance because steady-state gain is directly related to exciter regulation. • Under steady-state (s=0) and open-circuit or interconnected conditions, voltage error (VT-VREF) will be equal to the change in EFD/KA. • Let us assume a 2.7 pu full load EFD , a no-load EFD of 1 pu and a KA of 50. This means that with costant reference voltage , terminal voltage will change by 100 (2.7 – 1 )/ 50 = 3.4%, from zero to full-load conditions, such regulation values are unacceptable in normal practice. •10/17/2010 Regulation 18 10:23 AM of less than 1 % is usually required.

Transient gain reduction is one widely used method the industry has used to resolve this conflict of objectives between a stable and well-damped voltage-regulating loop, 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 the case of high-response, very low time constant exciters. • The relatively high transient gains will generally require the use of Power System Stabilizers. • The actual transient gain may be lower than the value required 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 System Parametres”, IEEE Transactions on Energy Conversion, Vol.3, No.4, December 1988, pp 785-791. 10/17/2010 10:23 AM

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CONCLUSIONS

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THANK YOU 10/17/2010 10:23 AM

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