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TURBOGENERATOR SELF-TUNING AUTOMATIC VOLTAGE REGULATOR

J.W. Finch* Senior Member, IEEE K.J. Zachariah** Member, IEEE, M. Farsi* Member, IEEE

*Department of Electrical and Electronic Engineering **was with Parsons Power Generation SystemsUniversity of Newcastle upon Tyne now with Merz & McLellan,Newcastle upon Tyne, NE1 7RU, UK Newcastle upon Tyne, NE4 7YQ, UK

Abstract Results are presented from new self-tuningcontrollers for turbogenerators. Simulations and micro machinetests have been successful, with a prototype self-tuning controllerbeing manufactured. This is intended as a direct replacement fora conventional AVR, so a more conventional fixed parametercontroller is also considered, serving as a comparison ofperformance. Typical results are given for normal set pointchanges and fault dynamics. An additional modification whichcould be introduced if a power signal were available, a form ofpower system stabiliser, is also covered. These new controllersare shown to have good potential for practical application.

Key words: turbogenerator control, self-tuning control, AVR

I. INTRODUCTION

The Automatic Voltage Regulator (AVR) of theturbogenerator (TG) attempts to control the terminal voltageand reactive power whilst also ensuring proper sharing of thereactive power amongst parallel connected generators. TGs arenonlinear systems which are continuously subjected to loadvariations. The AVR design must cope with both normal loadand fault conditions of operation. Such operating conditionvariations cause considerable changes in the system dynamics.When conventional linear fixed-gain AVRs are used thisdegrades the performance. Response variations can in somecircumstances cause the AVR to introduce negative damping,degrading system stability [1]. The development philosophywas to overcome these problems by developing an exactfunctional replacement for a conventional AVR, with machinevoltage the only feedback signal. No extra transducers areneeded over those for a conventional AVR.

Previous development of a digital AVR (DGAVR) had usedthe same concept of a direct replacement for a conventionalanalogue AVR [2]. Computer technology progress gives thepossibility of more sophisticated digital control, with similarhardware. A new self-tuning AVR (STAVR) would adapt tothe changing operational conditions of the TG. This has thepotential to yield the best range of responses from thecombined machine/AVR system. The option of adding anexternal power system stabiliser (PSS) for furtherimprovement, if needed, is applicable to any of these schemes.This PSS option has also been studied. An additional benefitof the STAVR lies in the reduction in the commissioningprocess for the AVR, often done on open-circuit.

These possible benefits has meant that much attention hasbeen devoted to using modern control techniques for TGs,often centred around various types of self-tuning (ST) control.These include Minimum Variance (MV), GeneralisedMinimum Variance (GMV), optimal predictor, and PoleAssignment (PA) [1,3,4]. In much ST AVR work, additionalsignals are used to improve robustness, which is avoided herefor compatibility with existing AVR designs. MV generallygives very lively control and can be highly sensitive to non-minimum phase plant. GMV, although more robust andgeneralised, is vulnerable to unknown or varying plant deadtime and can have difficulty with dc offsets. PA aims to locatethe closed-loop poles of the system at pre-specified locationsleading to 'smooth' controllers, but the algorithm can shownumerical sensitivity when the plant model is over-parameterised.

A self-tuning (ST) controller algorithm using GeneralisedPredictive Control (GPC) was selected. The algorithm hasbeen described in the literature [5-7], so only a very briefreview of the aspects relevant to this TG controller is givenhere.

In this STAVR on-line recursive parameter estimation isemployed to evaluate the time-varying or unknown parametersof a discrete time model of the system. In the TG changes inthe system dynamics are slow and the estimator should be andis able to track parameter variations well. The requiredperformance would then be given over the complete operatingrange, rather than the compromise of using a fixed parametercontroller chosen at a particular operating point. Any conernsover the ST controller ranging too far can be met in thesoftware by imposing limits or 'jacketing' the control.

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II SELF-TUNING CONTROL

A. Generalised Predictive Control

Advances in microcomputer technology have made moresophisticated algorithms feasible. GPC [5] was chosen here,for these practical reasons:

1) ‘Capable’: can control difficult systems such as a TGwithout special adjustments.

2) ‘Multi-step predictor’: control signal can be influencedby future system output bounds, giving robustness.

3) ‘Tuning knobs’: enable customised performance andgive flexibility.

4) ‘Future target reference’: can be set helping controlduring scheduled changes.

5) ‘Structure’: simple or complex controllers as needed.

B. AVR Function Replacement

An important requirement of the new controllers was thatthey do not require extra transducers or feed-back signals,compared to a conventional AVR. Both the new STAVR andthe fixed parameter DGAVR operate with a single controllerinput (voltage) and supply a single output (excitation signal).Both can be direct functional replacements for a conventionalAVR. In each case auxiliary software calculates power,VARs, and frequency. This is not part of the main controlleralgorithm and is only for monitoring and protection.

Fig. 1 Block diagram for ST and DG AVR system

Fig. 1 shows a block diagram, and Fig. 2 a software flowdiagram of the ST and DG AVRs. The GPC algorithm isdescribed elsewhere [5-7].

C. Implementation

Compatibility with existing hardware, and ease of upgradingto more powerful processors were requirements.

VMEbus compatible microcomputer modules centredaround a MC68030 processor were chosen for implementationtrials. A production model only requires software engineering,trials on a full size TG will then be possible.

In the developed system analogue and digital I/O areavailable as required, and the terminal voltage reference can bechanged via the interface. A user can also scroll through thevarious parameters of the controller for checking or modifyingpurposes. 'C' was used to code the control algorithm whichuses a recursive least-squares parameter estimator with avariable forgetting factor [8].

In practice the controller would be fed with good initialmodel parameter values obtained during previous runs. Thesetrials used zero initial estimates, a more arduous test. Randomvalues of control input were therefore applied for the first 10sample periods before initiating the control law calculation.

Fig. 2 Contrasting flow diagram for ST and DG AVRs

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Fig. 3 Open-circuit terminal voltage step response (micro-alternator),STAVR-F solid line; DGAVR-F dotted line.

III. TESTING METHODS

A 3-phase, 4 pole micro-alternator system was used forpractical tests. The micro-alternator field is driven through atime constant regulator; a setting of 6 seconds was used inthese tests. The DC motor drive to the micro-alternator canalso be electronically controlled to represent the turbine and itsgovernor, if needed. All the major system variables areaccessible for testing. Initial tests probed controllerperformance during normal operation, these were laterextended to cover behaviour with power system faults.

A specially written ‘C’ code standard two-axis theory fluxlinkage based state space simulation [9] allowed tests beyondthe capability of the micro-alternator system, including wideranging fault simulation studies. A 10th order model withconstant reactance values was used for much of the work, withsingle damper coils on each d-q axis, and lumped rotor inertia.Other model complexities are possible.

IV. STEP RESPONSE TESTS

Often the specification on desired behaviour includes TGopen circuit response, this was certainly the case here.Frequently an AVR is site tuned on open circuit.Consequently the first tests used the micro-alternator in thiscondition, at rated voltage and speed. Each controller designwas evaluated by standard tests, including applying a 3%positive step. A small number of specifications were set ascontroller design goals. The 'fast' design specification istypical where rapid action is required for dynamic responsecontrol. This set aims of: overshoot 4.3%, rise time 130ms,settling time 230ms, and closed-loop system bandwidth 4.0Hz.

A conventional digital AVR which attempts the designspecification for the 'fast' excitation control system wasproduced for comparisons. Such AVRs are the digitalequivalents of the sort of controller in use for many years, andoffer a good standard of performance [2]. An approximatedesign using simulation studies fine-tuned by trial and error onthe micro-alternator gave the parameters of this digital AVRas: loop gain with generator on open circuit = 325; lag timeconstants = 9.0 and 0.025 seconds; lead time constant = 3.0seconds. For future reference, this design is termed DGAVR-F. The frequency response of the micro-alternator system wasobtained by a Dynamic Signal Analyser using Fast FourierTransforms. These tests used a small (3%) set point change inoutput; the eventual field demand settles to a new steady statevalue also close to a 3% change showing operation is close tomagnetically linear here. The terminal voltage response givenhad overshoot 4%,, rise time 175ms, settling time 350ms andbandwidth 2.8Hz, considered acceptably close to the designaims.

GPC has various parameters or 'tuning knobs' which can bechosen to vary the behaviour [5]. One such is the controlhorizon Nu which specifies the number of steps over which thedemand increments are varied. Initial trials used values of 1-3,with large values causing a faster response. Nu = 2 is a goodcompromise giving a terminal voltage step response similar butslightly improved over the previous test under identicalconditions. Values given were: overshoot 1%, rise time180ms, settling time 300ms and bandwidth 3.0Hz. This andthe previous DGAVR-F result are shown in Fig. 3. As Nuapproaches the prediction horizon Ny (the number of stepsover which the output directly influences the controller,typically set to 10), the step response gets closer to the designvalues. The chosen value yields a reasonably 'fast' responsewhich is not very different from the design values, without thepossible reduction in the controller robustness and additionalcomputational burden imposed by higher values. For futurereference, this 'fast' design using Nu = 2 is termed STAVR-F.

B: Step response: generator on load

As mentioned earlier, it is the response of the excitationcontrol system when the turbine generator is on load that isreally important since the system operates in this mode most ofits life. The responses of the different types of AVR obtainedon open circuit in the previous section cannot normally beachieved when the generator is on load. This is due to thesignificant changes that the generator characteristics undergowhen the operating mode is changed from open circuit to theloaded state. It was observed during an evaluation of the STparameter estimator that the steady state gain and dominanttime constant with load are considerably lower than their opencircuit values and the system can exhibit some degree ofoscillatory behaviour at high load conditions.

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.u.

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Fig. 4 Loaded step response (micro-alternator),a) terminal voltage, b) real power .

STAVR-F solid line; DGAVR-F dotted line.

The step response obtained when using the different AVRson the TG simulator, representing a typical 660MW set, hasalso been investigated. A positive step of 3% was used with anoperating point of P = 0.8pu; Q = 0pu. These tests confirmedthat performance similar to that obtained with the micro-alternator can be repeated with the TG simulator.

Fig. 4 shows the terminal voltage step response with theSTAVR-F, values given were: overshoot 3.5%, rise time500ms, settling time 500ms; also shown is the variation in realpower. The rise time differs markedly from OC conditions,since the alternator system’s steady state gain has changed byabout 5. The bandwidth of 3.6Hz is similar to the OC case.The corresponding step response with the conventionalDGAVR-F (also in Fig. 4) gave overshoot 3%, rise time840ms, settling time 840ms, bandwidth 1.7Hz, showingconsiderable changes from the OC values. These resultsclearly indicate that the STAVR is able to maintain itsresponse characteristics under changing system conditions,while a fixed AVR fails to do so. These responses arecomparable to those on the simulator, a useful confirmation.

V. RESPONSE TO POWER SYSTEM FAULTS

Major disturbances that occur in the power system fromtime to time can seriously affect the smooth operation of theexcitation control system. These disturbances which are

transient in nature are classed as abnormal operatingconditions of the generator. Although the occurrence of theseabnormal operating conditions is very infrequent, theperformance of an AVR during these events should beevaluated to assess whether the controller is able to cope withsuch situations satisfactorily.

In the case of the STAVR, the GPC cost function considersonly the deviations of the terminal voltage from its set pointand the liveliness of the control signal. However, during majordisturbances the rotor angle of the generator with respect to theinfinite busbar of the power system is disturbed significantlyand can take some time to settle down following the event. Itis generally well known that a fast acting AVR such as the STcontroller can reduce the damping torque of the generator if ituses only the terminal voltage as its feedback signal. Theconsequence of this is the reduction in the damping of rotoroscillations following a major disturbance. This aspect shouldtherefore be examined in detail to ensure that sufficientdamping of rotor oscillations is provided.

The response of a turbine generator to severe disturbancesdepends very much on its severity as well as the conditions ofthe power system at which the disturbance occurs. A severedisturbance, regarded as a standard test, is a 3-phase short-circuit. The performance of the new AVR is now examinedunder these conditions using the simulator.

A. Three phase short circuit

During this test, a sudden short circuit is applied to thestator terminals of the generator and is removed after a periodof 100 ms The operating point of the generator has beenchosen as P = 0.8 pu, Q = 0 pu to obtain a large rotor loadangle. The greater the rotor angle the more severe is the testsince the stability margin of the rotor is lesser in that case. Forcomparisons on the damping available to the rotor during thedisturbance, a factor called the 'Effective Damping Ratio'(EDR) has been used. This factor is widely used in theindustry and is defined as the ratio of the peak-to-peakamplitude between the first undershoot of a signal following adisturbance and the second over-shoot to the peak-to-peakamplitude between the first undershoot and the first overshoot.A lower value of the EDR indicates higher damping.

Fig. 5 gives the response with STAVR-F. The EDR of therotor angle signal is 0.64 and its settling time to within 2% isfound to be 1.25 seconds; the terminal voltage settles down in0.39 seconds, a satisfactory performance. The test wasrepeated with the conventional AVR, DGAVR-F, and a rathersimilar response was obtained (also in Fig. 5). The EDR andthe settling time of the rotor angle found were 0.73 and 2.14seconds respectively and the settling time of the terminalvoltage is 0.6 seconds. This performance indicates that theSTAVR has improved the rotor damping in this case.

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Fig. 5 3-phase SC (simulation), a) terminal voltage; b) rotor angle; ,a) terminal voltage, b) real power .

STAVR-F solid line; DGAVR-F dotted line..

VI. POWER SIGNAL TESTS

Fast acting AVRs have the potential to improve transientbehaviour of the system. A key element in the designphilosophy here was to retain the input/output replacement foran earlier AVR, rejecting use of extra signals or transducers.Using additional devices to improve stability has becomepopular, such “Power System Stabilisers” (PSS) [10] may useseveral auxiliary feedback signals, but power is common.

To allow comparisons, a version of the new AVR wasdesigned using terminal voltage, as before, and electricalpower. Step response tests on this PSS were again used tocompare behaviour.

A number of trials were done to establish an effectivebalance between the terminal voltage and real power signals.The modified step response showing both terminal voltage andreal power is shown in Fig. 6, an equivalent result withDGAVR is included for comparison. A good standard ofbehaviour is again given with this PSS STAVR, with improveddamping of power (Fig. 6b compared with Fig. 5b), butslightly degraded voltage response (Fig. 6a compared with Fig.5a). This is an expected consequence of the controllerattempting to control both voltage and power in the transient.

time, s

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Fig. 6 Loaded step response with power signal (PSS) (micro-alternator),a) terminal voltage, b) real power .

STAVR-F solid line; DGAVR-F dotted line.

VI. CONCLUSIONS

Use of GPC for the adaptive control of a turbogeneratorexcitation system has been shown to offer considerablepromise. A laboratory micro-alternator and a versatilecomputer simulation model were used in these studies. Boththe conventional DGAVR and the STAVR were designedusing similar performance criteria, including a requirement fora direct input/output replacement for an existing AVR(terminal voltage/field excitation signal). The STAVRperforms considerably better where there is a significantchange in the system, for example between OC and loadedconditions, but also during a severe disturbance. The goodbehaviour of the new ST controller stems from it tracking thevarying dynamics of the plant and its cost function minimisingcontrol strategy, enabling the rapid regulation of terminalvoltage. Tested disturbances used 3-phase short circuits buthave also included full load rejection, line switching, etc.

Tests have also explored the possible behavioural changes ifextra signals were employed. Use of such a power systemsstabiliser has shown improved damping of power and rotorangle at the expense of terminal voltage. Such developments,and the use of governor control, provide possibilities forfurther developments. Meanwhile, good performance for theuse of self-tuning control for turbine generator excitationsystems has been proven.

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VII. ACKNOWLEDGEMENTS

The authors thank Parsons Power Generation Systems Ltd,the University of Newcastle upon Tyne and SERC/ERCOS forsupport, the use of facilities, and for permission to publish.

VII. REFERENCES

[1] B.W. Hogg, “Representation and control ofturbogenerators in electric power systems”, Chapter 5 inModelling of dynamical systems, Vol: 2 (H. Nicholson,Ed), Peter Peregrinus, 1981, pp. 112-149

[2] R.S. Hingston, P.A.L. Ham, and N.J. Green,“Development of a digital excitation control system”,IEE EMDA'89 Conference, London, pp. 125-129, 1989

[3] A.S. Ibrahim, B.W. Hogg, and M.M. Sharaf, “Self-tuningautomatic voltage regulator for a synchronous generator”,IEE Proc. D, 1989, 136, (5), pp. 252-260

[4] A. Chandra, K.K. Wong, O.P. Malik, and G.S. Hope,“Implementation and test results of a generalised self-tuning excitation controller”, IEEE Trans. En. Conv.,1991, 6, (1), pp. 186-192

[5] D.W. Clarke, C. Mohtadi, and P.S. Tuffs, 1987,“Generalised predictive control, the basic algorithm, andextensions and interpretations”, Automatica, 23, (2), pp.137-160

[6] M. Farsi, K.J. Zachariah, J.W. Finch, and P.A.L. Ham,“A self-tuning regulator for turbogenerators”, ACC'91,Boston, pp. 1026-31, 1991

[7] J.W. Finch, K.J. Zachariah, and M. Farsi, “Self-tuningcontrol applied to turbogenerator AVRs”, IEE Proc.GTD, Sept. 1996, 143, (5), pp. 492-499

[8] T.R. Fortescue., L.S. Kershenbaum, B.E. Ydstie,“Implementation of self-tuning regulators with variableforgetting factors”, Automatica, 17, 6, pp 831-835, 1981

[9] B. Adkins, and R.G. Harley, The general theory ofalternating current machines, Chapman and Hall,London, 1975

[10] N.C. Pahalawathatha, G.S. Hope, and O.P. Malik,“MIMO self-tuning power system stabiliser”, Int. J.Control, 1991, 54, (4), pp. 815-829

John W. Finch (M'90, SM'92) wasborn in Co. Durham, England. Hereceived the BSc(Eng) degree fromUniversity College London, where hegraduated with First Class Honours inElectrical Engineering, and the Ph.D.from the University of Leeds. He has aconsultancy activity with manyNational and International firms, andhas over 100 publications. Areascovered include CAD, applied control,

simulation, electrical machines and drives, robotics. He isReader in Electrical Control Engineering at the University ofNewcastle upon Tyne, and is an IEE Fellow, and a CharteredEngineer. Dr Finch is a winner of the Goldsmid Medal andPrize (UCL Faculty prize), the Carter Prize (Leeds Universitypost-graduate thesis prize), and the IEE's Heaviside Premium.He serves on the IEE Professional Group P1 'ElectricalMachines and Drives', and B2 'Applied Control Techniques'.

K. J. Zachariah obtained hisBSc(Eng) in Electronics &Communications from the Universityof Kerala, India in 1975, his MSc andPhD by research in Self-tuning Controlfrom the University of Newcastle uponTyne in 1986 and 1994. From 1975 to1984 he worked in the Kerala StateElectronics Development CorporationLtd., India where he specialised in thedesign and engineering of industrial

systems and controllers for power plant. He joined ParsonsPower Generation Systems Ltd in 1986 where his workinvolved the development of self-tuning control systems forturbine generators; and is now with Merz & McLellan,Newcastle upon Tyne, UK

Mohammad Farsi was born inShahriar, Tehran, Iran in 1940. Hereceived his BSc degree in ElectricalEngineering from the University ofIdaho in 1968. He worked as WeaponElectrical Officer in the Iranian Navyup to 1982. He received his PhD inSelf-tuning Control and ModelReduction in Robotics from theUniversity of Newcastle upon Tyne in1986; where he is now Senior Lecturer.

His research interests lie in optimal, adaptive, intelligent,neural and genetic algorithm controllers, with applications inrobotics and automation. He has over 80 publications andserves on the IEE Professional Group 'Applied ControlTechniques', C9.