IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS AND CONTROL INSTRUMENTATION, VOL. IECI-23, NO. 3, AUGUST 1976

MICROPROCESSOR ASSISTED SOLID STATE EXCITATIONSYSTEM FOR SYNCHRONOUS MACHINES

Vilayil I. John Douglas J. ClarkeDepartment of Electrical Engineering

Queen's UniversityKingston, Canada

Abstract

A digitally controlled cycloconverterexcitation system for a laboratory size two-axis synchronous machine is described. Thecontrol strategies for start up, steady syn-chronous and asynchronous operations areprovided. The possibility of the use of amicroprocessor to replace the hard wiredlogic and further increase the excitationsystem flexibility is detailed.

INTRODUCTION

The rapid increase in the popularity ofmicroprocessors make them the most importantinnovation of recent times in the field ofelectronics. The growing use of micropro-cessors is making power electronic controlsystems more reliable and less expensive.Conventional forms of control using electro-mechanical components for large motors andgenerators are increasingly being replacedby power electronic control systems usingmicroprocessors.

Excitation systems of large synchronousmachines have undergone a steady evolutionin the past three decades. The unit capa-cities of such machines are continuing toincrease from the present-day sizes of 1000MVA to 2000 MVA and even 3000 MVA. With theanticipated growth in electrical energydemand in the coming decades, the dimensionsand complexity of interconnected power sys-tems are bound to increase. From the pointof view of stability, synchronous machinesconnected to the system form the crucial com-ponents. Being vulnerable, all possibleimprovements should be incorporated in thesemachines for greater reliability. Thusthere is a real and urgent need for morereliable excitation control with greaterflexibility and versatility for these largesynchronous machines.

This paper deals with the control andperformance of a cycloconverter excitationsystem for a laboratory size dual excited(two-axis) synchronous machine. A combina-tion of digital and analog feedback isemployed. The analog feedback is obtainedfrom a conventional proportional voltageregulator while the digital feedback isaccomplished through the use of hard wiredintegrated circuit components with feedbacksignals obtained from an optical speed trans-ducer. The added flexibility obtained throughthe use of a microprocessor (F-8) is alsodetailed. It is hoped that the principle ofthis excitation system will be readily adapt-able to large central station synchronousmachines.

EXCITATION CONTROL OFSYNCHRONOUS MACHINES

It is well-known that for synchronousmachines operating in parallel with a largepower system appreciable improvements intransient and dynamic stability limits can beachieved by the use of high speed excitationsystems. The block diagram in Fig l repre-sents a typical excitation system (IEEEType 1)1 using a continuously acting voltageregulator. Improvement in performance isobtained through the control of the varioustime constants and gains in the system as wellas by the introduction of a variety of otherfeedback signals2 3

I _NFIIONI

+SST

VREF VR MAX .+:e > H 2 > g | 1 ~~~~~~~~EFCFig. 1. Block diagram of IEEE Type 1 Excita-

tion system (IEEE Trans . PAS Vol .PAS-87, June 1968, p. 1461).

It has also been shown'-8 that improveddynamic stability limits, especially in theleading power factor region and continuousasynchronous operation for short periods oftime, are possible in the case of multiexcitedsynchronous machines. Nevertheless, the needfor changes in the basic synchronous machinedesign and the increased complexity in theexcitation system have prevented the use ofmultiexcited synchronous machines. But withthe advent of more reliable and less expensivesolid state excitation systems, the overalladvantages of multi-excited synchronousmachines might outweigh the disadvantages.

A schematic diagram of a two-axis synch-ronous machine is given in Fig. 2. Undernormal synchronous steady state operation dcis supplied to the two-field windings. Opera-tion with only one winding is possible (incase of faults in one of the excitation supplysource). But for equal field heating and fullutilization of the windings equal excitationis maintained in both the windings. Theresultant field axis is midway between the'd' and 'q' axis. It may be noted that bycontrolling the magnitude and direction of thecurrents in the windings it is possible tomove the axis of the rotor field. In particu-lar, by supplying the rotor field windingswith balanced two phase slip frequency

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0to

-J0.

a.

1--

REF VOLTAGE

.ig. 3. Two-axis synchronous machine connec-

ted to large system through a trans-mission line.

Fig. 2. Coupled circuit model of a two-axissynchronous machines (two fieldwindings and two equivalent damperwindings).

currents, it is possible to produce a circu-larly rotating field rotating at slip speedwith respect to the rotor. It is this fea-ture which helps the machine to run in elec-trical synchronism even when the mechanicalspeed of the rotor is different from synch-ronous. Moreover, by independent feedbackcontrol for the two windings, say voltagefeedback and an angle feedback, it is possibleto extend the range of stable operation ofthe machine.

DESCRIPTION OF THE ACTUAL SYSTEM

Fig. 4. Block diagram of the excitationsystem for the two-axis machine.

The block diagram for the actual setupis given in Fig. 3. The figure representsa two-axis (dual-excited) synchronous machineoperating in parallel with a large systemthrough a transmission line. The excitationsystem consists of a two-phase cycloconverterwith analog and digital feedback. The machineis a special G.E. (General Electric) designwith two symmetrical two-phase windings onthe stator and a balanced three-phase windingon the rotor, with uniform airgap. Thus themachine is operated as an inverted synchronousmachine with the field excitation provided tothe stator windings. The cycloconverter isthe logical choice in providing dc and slipfrequency ac to the field windings as itperforms admirably even in the case of highlyinductive loads. The block diagram in Fig. 4gives further details in the use of voltageand speed feedback for the gate control ofthe cycloconverters.

DIGITAL CONTROL STRATEGY

Three modes of operation are possiblefor the synchronous machine. The first isthe starting up mode where the machine is notsynchronized to the system. The second modeis the normal synchronous. steady operationof the machine connected to the external

system. The third mode of operation is theasynchronous steadystate operation when therotor mechanical speed is not synchronous.

Information on three basic aspects ofthe machine are required to control the two-axis excitation system. These are:

1. a continuous knowledge of angle S

2. knowledge of the direction of changeof S

3. knowledge whether the machine is inmode two or mode three.

This information can be derived from twosignals. In this design, the two signals arethe power system voltage and the rotor mecha-nical position. The system voltage is used togenerate the first required pulse train. Thepulse train indicates the system frequencyand, more importantly, the phase of the statormagnetic field, %s. The rotor position is

obtained as a pulse train using an opticaltransducer. The transducer provides twopulses per mechanical revolution for the fourpole machine. With proper calibration thetime interval between a pulse from the systemvoltage and the rotor position pulse is ameasure of angle 0a

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q- AX

N

TRANSMISSION oLINE iD

If 6 becomes greater than 90 degrees, 256 fssynchronism is lost under close manual con-trol. A continuous examination of 6 allows Psimmediate detection of loss of synchronism. COUNT UP NThe direction of 6 movement can be obtained Pr CONTROL CONTROLby comparison of successive values of S.This indicates whether the machine is speed- _ing up or slowing down.

For convenience, all angle measurements Iare made in electrical degrees. The mecha- 224 MULTIPLEXER P e aCOUNTERnical movement of the rotor is converted to t BUFFERelectrical degrees by multiplying by the 1number of stator pole pairs. Since S is the counte Putquantity of concern absolute measurement of LAST OUTPUT to proms-the rotor movement is unimportant. The maxi- COUNT MULTIPLEXERmum relative movement of S is 360 electrical lUlFER3UFFERdegrees. The power system frequency may notremain constant during a system disturbance.Thus the 6 measurement system must use boththe frequency and phase information. The DIRECTION SYNCcomplete control system diagram is given in COMPARATOR_ COMPARATOR ---160-AFig. 5.

To minimize noise problems an all-digitaldesign philosophy has been used. The neces- ECTIONsary analog signals are generated only as the DECISION SYNCfinal outputs. STORE

The S measurement is done in the DigitalProcessing Unit (DPU). The other blocks in DIRECTION SYNC SYNC SETthe top half of Fig. 5 are the necessary DECISION ENABLEinterface and auxiliary circuits for the DPU.The lower half of the diagram depicts thecontrol circuit for the cycloconverter. Adetailed block diagram of the DPU is givenin Fig. 6.

Fig. 6. Detailed block diagram of theDigital Processing Unit.

System -- - ooVoltage |Pulse |PS | Pulse Pr

R

us ostiorShoper | Gen. tX |Shoper Pulse

DIGITAL PROCESSING UNIT (DPU)

i The essential element in the DPU is anPhase256fs ||up/down counter. The counter is started by a

Loop DIGITAL pulse (P ) from the system voltage pulseVolloge PROCESSING generator. It continues to count until aReference 1olag UNIT pulse (P ) from the rotor transducer isVoltage _ Regulator| received. The clock pulses for the counterSense are generated by a phase-locked loop. The

Reference _ _ 1 +_.ll clock frequency is exactly 256 times theVoltoge t [tl 1+ ; power system frequency. An eight-bit counter

rDigital l l 0 l Digital 1 is used and a count of 255 represents theto i__Sine Cosine to maximum rotor movement of 360 electrical

lC onverter Alconverterl degrees. If the rotor moves more than 360Cosine Cosine[^ - rvelectrical degrees, the counter rolls over toReferences References zero and continues counting. Thus the counter

Firing _ Gate Gate Firing output is always the principal value of theGPlen._ Enable Enable Pulse rotor angular position. Since the phase-

locked loop ensures a clock frequency of 256IA T 1;1 1;1 f IB times the system frequency, the count is

always an angle measurement in electricalCYCLO- CONVERTOR degrees.I POWER ClIRCUIT |

Rather than give the circuitry details,OutputA u put B we will give a functional description of theOutput a Output B design. In the starting up mode, external

synchronization controls are used. To ensureFig. 5. More detailed block diagram of the proper and reliable synchronization, it is

excitation system. important that only the external controls haveany effect on the excitation of the generator.

Thus, during this mode both Sync and- 235 -

Sync Enable are negated. The counter circuitis operating but its outputs are unused. TheOutput Multiplexer outputs a constant presetvalue to the Programmable Read Only Memories(PROMs).

When the external circuitry has synch-ronized the generator to the power system,mode two operation begins. Both Sync andSync Enable are now asserted. The OutputMultiplexer continues to output the same pre-set value to the PROMs. This provides aconstant value of 6 to the generator field.The system power angle and hence 0 adjustautomatically to the proper values withoutany action by the DPU. This mode continuesas long as 0 remains less than 90 degrees.

During this mode 0 is continuously moni-tored. To determine the direction of 0 move-ment the Counter, the Last Count Buffer andthe Direction Comparator are used. Everytime a new value of 0 is stored in the CounterBuffer, it is compared with the value storedin the Last Counter Buffer. The result ofthis magnitude comparison and the knowledgeof the present count direction give an indi-cation of the correct counter direction.Table I gives the possible combinations anddecisions. Since a synchronous machine

TABLE I

Present Magnitude NewDirection Comparison Direction

Up CB > LCB Up

Up CB < LCB DownDown CB > LCB Up

Down CB < LCB Down

LCB = Last Count BufferCB = Count Buffer

can oscillate about its operating point,incorrect direction decision can be made.This problem is avoided by storing eachdirection decision. Only when the last threedecisions agree can the counter mode bechanged. The Last Count Buffer is updatedwith the new count value after each compari-son.

The Sync Comparator is used to checkthat the machine continues to operate synch-ronously. If the count indicates 0 isgreater than 90 degrees, synchronism has beenlost. Since 0 movement is limited by themechanical time constant of the generator,the counter direction circuitry has ampletime to establish the correct direction.With the loss of synchronism, DPU begins modethree operation.

Since in mode three,the rotor speed isnot equal to synchronous,Sync Comparator

direction to the rotor movement. The resul-tant axis of the rotor field is 01 given by:a

el = em + AO + e - AGe = 6a m m f f a (1)

Since e and 0 are constant under modea 5

three control 0 remains constant at 90degrees. As 0 remains constant the counterdirection can no longer be determined bycomparing successive counts. This is unim-portant as the machine must revert to modetwo operation before the direction becomesincorrect.

The Sync Comparator continues to operate.When the rotor mechanical position has moved360 electrical degrees synchronous operationmay be possible again. Mode two operation isrestarted as the rotor position approaches360 degrees. If synchronism is regained,mode two continues. If the machine does notreach synchronous speed, Sync Comparatorkeeps the DPU to mode three operation.

External speed governors act indepen-dently to return the generator to mechanicalsynchronism. When they succeed, Sync Compa-rator will ensure normal mode two operation.During the entire mode three operation, 0 ismaintained constant at 90 degrees and thegenerator continues to maintain power trans-fer.

Circuitry Details

During mode one and mode two, operationOf is set equal to 45 degrees. This assures

equal currents in the two field windings.The effect of this initial offset is removedby the calibration of the rotor positiontransducer. For generator operation synch-ronism is lost with 0 equal to plus 90degrees. The counter preset is adjusted sothat at the time of losing synchronism, thecounter output is equal to the constant valuefed to the PROMs during modes one and two.This minimizes any transient between modestwo and three. Since the counter is bi-directional, a separate preset is used foreach count direction. The Input Multiplexeris controlled by the Count mode control toprovide the correct preset value. A furtheroffset of 180 degrees is provided in thecircuit to simplify the comparator design.All preset values are modified by thisadditional offset.

External Circuitry

The remainder of the control circuits arestandard designs. The cycloconverter phaseis controlled by the DPU. The sine and cosinePROMs provide the digital signals for the twodigital to analog converters (DAC). Theamplitude of the DACs is controlled by thevoltage feedback.

negates Sync. The coun-.r output is directlyfed to the PROMs by the Output Multiplexer.As the rotor continues to move the countchanges. The new counts generate new valuesof ef. The new value of ef is in the opposite

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MICROPROCESSOR ASSISTED CONTROL

The most obvious use of a dedicatedmicroprocessor is the direct replacement ofthe DPU. This replaces hard wired logic witheasily modified computer software. Thisprovides an important benefit in extendingthe range of standard synchronous machines(Mode two) operation also. With a continu-ously acting voltage regulator,stable opera-tion with 0 greater than 90 degrees ispossible. The exact value depends on thegenerator operating point. With a micropro-cessor, it will be possible to continuouslymonitor the machine's operating point. Thisinformation is used to ensure that mode threeoperation starts only at the actual stabilitylimit.

Other control strategies have been triedfor two-axis machines. The microprocessorwill allow the comparison of various controlsby a simple change of programs. If it isbeneficial, the microprocessor can use twoseparate control schemes for modes two andthree.

Also the microprocessor can be used toreplace the external circuitry. The synch-ronization control needed in mode one usesthe same control signals needed in modes twoand three. The microprocessor can be pro-grammed to handle initial synchronizationand further reduce external circuitry.

CONCLUS ION

The feasibility of a digitally control-led cycloconverter excitation system for alaboratory-size two-axis excitation systemis demonstrated. The controls needed forthree modes of operation are provided. Thetransition from mode two to three and vice-versa can be made smooth by the appropriatechoice of presets on the counter.

An F-8 Evaluation Kit is being tried toreplace the hard-wired DPU. Also, controlstrategies for improving the dynamic stabi-lity limit of single-axis (conventional)synchronous machines with voltage and anglefeedback are being tried using the micropro-cessor. The results of these efforts shouldbe of interest in the evolution of excitationsystems of synchronous machines.

0 - the angle between stator field androtor field. Equals e - e.

a s

REFERENCES

1. IEEE Committee Report, "Computer Repre-sentation of Excitation Systems",IEEE Trans. on PAS, Vol. PAS-87,pp. 1460-63, June 1968.

2. F.P. deMello and C. Concordia, "Conceptsof synchronous machine stability asaffected by excitation control",IEEE Trans. on PAS, Vol. PAS-88,pp. 316-329, April 1969.

3. W. Watson and G. Manchur, "Experience withSupplementary Damping Signals forGenerator Static Excitation Systems",IEEE Trans. on PAS, Vol. PAS-92,pp. 199-204, Jan.-Feb. 1973.

4. J.A. Soper and A.R. Fagg, "Divided-winding synchronous generator", Proc.IEE, Vol. 116, pp. 113-126, January1969.

5. S.C. Kapoor, S.S. Kalsi and B. Adkins,"Improvement of alternator stabilityby controlled quadrature axis exci-tation", Proc. IEE, Vol. 116, pp.771-780, May 1969.

6. R.G. Harleyand B. Adkins, "Stability ofsynchronous machines with divided-winding rotor", Proc. IEE, Vol. 117,pp. 933-947, May 1970.

7. P.C. Krause and J.N. Towle, "Synchronousmachine damping by excitation controlwith direct and quadrature axis fieldwindings", IEEE Trans. on PAS, Vol.PAS-88, pp. 1266-74, August 1969.

8. V.I. John and J.A. Bennett, "Optimal fieldcontrol of a two-axis synchronousmachine", Engineering Institute ofCanada, Transactions, Vol. 14, No.C-3, pp. 1-4, March 1971.

NOMENCLATURE

e - phase angle of stator field measuredrelative to an arbitrary axis.

- the angle of the rotor 'd' axis withrespect to the stator reference.

ef - the angle of the rotor magnetic fieldwith respect to 'd' axis.

0 - the angle of the rotor magnetic fieldwith respect to the stator referenceequals em +±f-

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