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I 1

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

SIMULATION OF ALTERNATOR RECTIFIER EXCITATION SYSTEM USING MICROMACHINE AND POWER TRANSISTOR ANPLIFIERS

K. V. N. Rao, Member, IEEE, Donald C. Macdonald and Bernard Adkins

ABSTRACT

This paper describes the modeling of an alter-nator with rectifier excitation system on a microalternator using a novel scheme of control with powertransistor amplifiers. The theory of simulation isexplained and the salient features of the scheme areoutlined. Tests performed on the system have shownthe accuracy and reliability of the scheme.

NOMENCLATURE

E

Gl, G2,Gfb Gr

irrr

rVr

VdVrt

Tfp

if,Aif

exciter phase voltage behind the commu-tating reactance

voltage gains of amplifiersrectifier current in the model

resistor, the drop in which equals lossof rectifier voltage arising fromcommutation

feedback resistorvoltage behind the source impedance of

the rectifierforward voltage drop of the rectifier

rectifier output voltage

field flux linkages

dtheavyside operator, dt

field circuit currents during forwardand reverse bias, respectively

INTRODUCTION

In modern practice, ac exciter-rectifier systemsare used in place of conventional dc exciters to pro-vide the excitation power of large alternators. Thebehavior of such alternators, under abnormal operatingconditions, has been the subject of investigation insite tests, analog and digital simulations and micro-machine studies but the last of these, because of theease and flexibility with which testing can be done,has been extremely useful in identifying the onerousconditions under which the system components arestresses to the maximum. At Imperial College of Sci-ence & Technology, this problem has been studied [1]in detail, both theoretically and experimentally,using micromachines. The theoretical studies pertain-ing to some of the abnormal conditions are describedin a paper [2] published recently. The present paperdescribes the modeling of an alternator with rectifierexcitation system on a micro alternator using a novelmethod of control with power transistor amplifiers.

A micro alternator is an important constituent ofthe model power system. It is a small specially de-signed alternator of 3 to 5 kVA capacity, which hasmost of its parameters similar to those of large al-ternators and therefore gives a similar behavior undertransient or abnormal operating conditions. The perunit field resistance of the micro alternator, how-ever, is usually 6 to 8 times larger than that of apractical machine but this is "effectively" reducedwith the use of a high gain dc amplifier A1 and a

Manuscript received August 12, 1975. The work de-scribed in this paper was done under a grant from SRC.

Dr. Rao is with the M. A. College of Technology,Bhopal, India.

Drs. Macdonald and Adkins are with Imperial Collegeof Science & Technology, London, England.

closed loop control as shown in Fig. l(a).AF is a "shadow" winding with the same number of

turns as the field winding F but with a much smallercross section and wound in the same slots. With nearlyperfect coupling between the two, the voltage appearingacross AF is equal to the induced voltage p'Yf in thefield winding. This is added to a voltage dropGfb r-if proportional to the field current and is fed

back to the input of A1.

If a voltage Vf is applied to the input of Al'it can be shown that for a high gain G1, of the ampli-fier

Vf = pVf + Gfb r . if (1)

Thus whatever be the value of the actual resist-ance of the field winding any desired value of "effec-tive" resistance can be simulated by adjusting the gainGfb of the operational amplifier Afb. For all prac-tical purposes, this closed loop control called theTime Constant Regulator (t.c.r.) is regarded as an in-tegral part of the excitation system and the equivalentcircuit is shown in Fig. l(b).

HIGH GAINy DC. POWER

ANlPLIFIER L F AF

Al

(PLf +Gfj) f R

(Q)

Lg; j ~~LFr G fb r

0--

(b)

Fig. 1. Arrangement for timeconstant regulation.(a) Schematic diagram.(b) Equivalent circuit.

If the voltage Vf is derived from a dc exciter,the above scheme then represents a conventional dc ex-citation system.

Model schemes similar to the one described aboveare extensively used in U.K., France and U.S.S.R. forthe study of generator and power system problems.These, however, need to be modified, if rectifier ex-citation systems are to be simulated on micro-alternators.

DESCRIPTION OF RECTIFIER EXCITATION SYSTEM

Fig. 2 shows a typical rectifier excitation sys-tem of a large turbo-alternator. The ac exciter is adirect coupled three-phase alternator connected to thesilicon diode bridge rectifier which in turn suppliesthe excitation power of the main alternator. A volt-age regulator controls the field power to the ac ex-citer depending upon the terminal conditions of themain alternator.

It is important to note the differences in thebehavior of this system and a conventional dc excita-

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A. C. EXCITER RECTIFIERBRIDGE

GENERATOR BUSBAR& LINE REACRANCE

FIELD

AV,R X

A. C. EXCITER

Fig. 2. Typical rectifier excitation system oflarge alternators.

tion system for a proper modeling on the micro-alternator:

1) The rectifier permits current in the fieldwinding to flow in one direction only. It "open cir-cuits" the alternator field when "reverse biased" andlarge induced field voltages will appear as an inversevoltage across the tectifier. Such situations arisein abnormal operating conditions like poleslipping orasynchronous operation.

2) Unlike in a dc exciter, the armature reactionin ac exciter is quite predominant and affects themagnitude of E under changing load conditions.

3) The operation of the rectifier bridge isgoverned by the commutating reactance offered by theac exciter and the dynamic impedance of the alternatorfield winding. The output voltage of the rectifierbridge is significantly affected by the mode of opera-tion of the rectifier and is given by

rt =T r r dr

SIMULATION OF RECTIFIER EXCITATION SYSTEM

Fig. 3. Schematic diagram of arrangement fortime constant regulation and current simu-lation.

tional to the field current, if * r, and the other

proportional to the rectifier current - G * i * r.r r

The output of the amplifier A2 is

VA2 = -G2('f - Gr r)

that

(3)

This is applied to the output of the rectifier so

rt VA2 + ir r

Combining (3) and (4)

(4)

(2)r Vrt + G2 * if * r

Ir =

r(l + G2Gr) (5)

The scheme of Fig. l(a) cannot simulate a recti-fier excitation system by merely connecting the acexciter-rectifier to the input of A1. This is be-cause, in this case, the rectifier does not directlysupply the field current and is therefore unable toprevent its reversal in any abnormal condition. More-over, as the rectifier feeds only a few mA into A1,the effects of the ac excitet and of the mode of oper-ation of the rectifier (both of which are current de-pendent) cannot be taken into account.

A diode put at S may prevent reversal of thefield current but its use is not desirable as theclosed loop operation of the t.c.r. is lost and timeconstant regulation is not possible.

For a simulation of rectifier excitation systemof micro-alternators it is thus necessary

a) to simulate rectifier action in the field cir-cuit without really using a unidirectional device sothat time constant regulation may be maintained.

b) to obtain in the rectifier circuit a currentequal to the field current so that the exciter andrectifier effects are reflected in the voltage Vrtor Vf applied.

THEORY OF CURRENT SIMULATION

and if G2 >> 1

ir= if/Gr (6)

which shows that the rectifier current follows thefield current but is reduced by a factor lIGr.

The input to the amplifier A1 is now VA2 andhence

VA2 = PTf + Gfb r if (7)

Combining (1), (4) and (7)

r -Tr E Vdr = PVf + [Gfb *r + (rr + r)/Gr] if

(8)

It may be seen that for a value of G = 1, the ac ex-r

citer rectifier effects have been fully taken into ac-count.

Under such abnormal operating conditions whenpTf becomes large, if is reduced and ir becomeszero.From (5), when i = 0r

To perform the above functions, an additionalfeedback circuit called Current Simulator circuit wasintroduced. As the name implies, its main purpose isto simulate in the rectifier circuit a current equalto the field current.

Fig. 3 shows the current simulator acting in con-junction with the time constant regulator.

The current simulator has two inputs, one propor-

if = Vrt/G2r

and if G2 >> 1

Also

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I IrI I--

_ Sl-A =,V2t v

(9a)

if 0 (9b)

VA2. =V Pf Gfb *r if (10)

If p f increases further tending to increase the

field circuit current to a higher negative value - Aif

such that Aif > Vrt/G2r, then the two input voltages

to A1 namely, G2 * r * Aif and pPf - Gfb * r * Aif9act to hold the field current close to zero. The rec-tifier action is thus simulated in the field circuitwhen i is constrained at zero by the rectifierbridge.r

The inverse voltage presented to the rectifier is

VRt = VA2 = G2 * r - Aif pPf Gfb r Af

G2

G2 + Gfb f

|' -FIRST STAGE S-ECOND STAE-

Fig. 4. Amplifier A1.

(11)

and if G >> G2fb

Vt = pf (12)

i.e., the rectifier receives the correct value of in-verse voltage from the field.

PRACTICAL REALIZATION OF THE SCHEME

Eqs. (1), (6) and (12) show that it is necessaryto build A1 and A2 as high gain dc power ampli-

fiers for developing a workable scheme for the simula-tion of the rectifier excitation system. A1 should

be able to supply the ceiling excitation power re-quirements. A2 on the other hand, has a much less

power requirement because of the level of operatingvoltage Vrt' It also depends on the value of Gr.

If a correct simulation of the ac exciter rectifiersystem is desired, Gr must have a value equal to one.

If inverse voltage across the rectifier is the onlyquantity of interest, even higher values of Gr up to

5 can be chosen, thus reducing further the power re-quirements of A2.

Both the amplifiers should have a large bandwidth as far as possible so that the sharp cutoffaction of the rectifier can be simulated. It wastherefore decided to develop both the power amplifiersusing power transistors.

AMPLIFIER A1

This was designed as a class A-B amplifier tosupply ±80V and 7A and the details are shown inFig. 4. The power supply was ±lOOV, obtained from anexisting general laboratory rectifier used in conjunc-tion with the stabilizing and smoothing circuit ofFig. 5.

The output obtained.between a "float rail" (whosepotential can be altered between ±80V, depending onthe input signal) and earth, is applied across thefield winding of the main alternator. There was someinitial difficulty from "secondary breakdown" of thepower transistors owing to the highly inductive natureof the load.

The amplifier has three stages of amplification.The first stage comprises OPl and OP2 and a differen-tial amplifier which, by virtue of a feedback from the"float rail" additionally serves to remove any ampli-tude or phase distortion that may occur in the posi-

Fig. 5. Stabilized power supply.

tive and negative outputs.This stage is followed by a voltage amplifier

which alters the potential of the float rail dependingupon a few volts variation of- F about zero. Thisalso involves a change in dc level which is broughtabout by ZD3, T3 and T4. The potential at J de-pends upon the V of T4 and in quiescent operat-

ceing conditions is held slightly above earth potential.

In the third stage, the power amplification isobtained by two independent units of positive and neg-ative current amplifiers, only one of which is "ON" atany time depending on whether the potential of J isabove or below earth potential. Each of these unitsincorporates a predriver and driver stages driving anumber of power transistors connected in parallel. Aclamping circuit incorporating T9, D2 and D3 pro-tects the amplifier against simultaneous conduction ofboth the units.

One of the significant features in the design ofthe above amplifier is that no PNP transistors areused.

A large bandwidth (20 kHz) was obtained for theamplifier alone, but when connected for time constantregulation, see Fig. l(a), this had to be limited to300 Hz because of the resonance-characteristics of thefield windings at high frequencies.

AMPLIFIER, A2This amplifier, see Fig. 6, is similar to A1 ex-

cept for the following differences:

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3) Only one operational amplifier is used in thefirst stage of amplification, to get the appropriatepolarity for the output.

4) The amplifier has an overall gain 1000 whichis sufficient to make ir = if/G.r

TESTING OF THE SCHEME

In addition to the tests performed to ensure thatA1 functioned properly as a time constant regulator,

the following tests were performed to prove the over-all scheme of simulation of Fig. 3.

1) Short circuit tests: Fig. 7 gives the oscillo-

Fig. 6. Amplifier A2.

TABLE I

LIST OF COMPONENTS FOR THE STABILIZEDPOWER SUPPLY (FIG. 5)

Resistances: 10 Watts

Rl,Rlo170 ohm

R2'R11 1 ohm

R3,R12 4.7 ohm

Resistances: 1 Watt2

R4'R13 100 ohm

R5,R14 2.2 K

R6,R15 100 ohm

R7,R16 18 K

R.,Ri, 0 to 2 K Pot

R9 18 12 K

Fig. 7. Field current and simulator cur-

rent on three phase fault.(a) Field current, if.

(b) Simulator current, ir.

Capacitors

Cj,C7c2 1c8 Ic6 ICillC3 ,C9

C,C4'I10

c5'11

T1 ,T2

T3T 7

T4,T8

T5 I6

D1 ID3D2 OD4

ZD1, ZD2

7500 pF

2500 pF

0.1 uF

250 piF

1 oF

Transistors

BFY50

DT6305

MJ423

BFX29 or 2N2904A

Diodes

SX 752 diode

SX 631 XD diode

Two BZY95C39Zeners in series

gram of the field circuit current if and the recti-

fier circuit current i (for a value of G = 1) taken

on a sudden three-phase short circuit at the alternatorterminals. The currents were measured and plotted sep-

arately in Fig. 8.

1) The output stage consists of the positive cur-

rent unit only. The negative current unit and theclamping circuit become unnecessary because of therectifier in the circuit.

2) The stabilized power supply used is +30,-150 V, the larger negative voltage being provided tosimulate the large field induced inverse voltages.

Fig. 8. Field currentthree phase fault.

and simulator current on

A close agreement can be seen in the magnitude andphase of these currents which are in fact varying at50 Hz.

2) Poleslipping tests: The combined action of thetime constant regulator and the current simulator in

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OUr PUT

1L

TABLE II

LIST OF COMPONENTS FOR AMPLIFIERS A1, A2 (FIGS. 4 & 6)

High Stability Resistances: 1/2 Watt

60 K

54 K

120 K

13.3 K

20 K

100 K

11.1 K

10 K

1 m

R

R44 R45R46

Resistances: 1/4 Watt

R11 R47R12 ,R14 ,R25 ,R28 I R34$R49'R51' 60' 64

R13 R48

R5,R50R16'R18'R20'R52'R54 R56

10 K

1 K

100 K

1.5 K

R17' R29 1R35'R41 R62 2R65R19 I R55

R23 R59

100 Ohm

O to 1 K Pot

500 Ohm +1 K Pot

100 +250 Ohm Pot

15 Ohm5.1 K

High Watt Resistances: 6 Watts

R21' 22

R24 R63R27R30,R31 IR36, R39 IR66 R68R32 R33 R37 R38 R67 R69

C1c2c3 cC4Cc81c9

ZD1,ZD22 ZD4, ZD5 ZD6

ZD3

ZD7

DJDD4D2

D3

T,T2 T14 T15T1' T9' 16'1T4 T6'Til1T93' 9' 16

4' 6' 1'1'1

T T TT5'T11-18T7 ,T8 ,T12 'T13 sT20 'T21

OP1,2OP2,0P4,AFB,AR

15 V Zener diodes (BZY 95C15)Two 39 V Zener diodes (BZY 61039)

Zener diodes (two nos. BZX 61C30 in series with oneno. BZX 61075)

2 A silicon diodes (SX 632)

Germanium diode (OA5)

Two 3A, 400 V p.t.v. diodes (DD3076) in series

Transistors

2N3702 (PNP)

BFY50 (NPN)

36 W power transistor DT4305 (NPN)

3 W power transistor DT 1003 (NPN)125 W power transistor MJ423 (NPN)

Integrated circuits

Operational amplifier, NEXUS type - SQ lOa 410 V, 5 mA

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R3,R4

R5R6R7R

R57R58R61R70

1.25 K

4.7 K

3.9 K

5 Ohm

2.3 Ohm

1.5 K

68 K

9 K

1 K (22 W)

Capacitors

c70.002 iiF

.00239 PF

.1 -PF

400 pF

Diodes

Fig. 9. Currents and voltages during poleslipping conditionresulting from loss of excitation.

(i) X = 0.288 p.u.(ii) X = 0.288 p.u. (enlarged time scale).

(iii) X = 0.12 p.u.

a Voltage across the field windingb Inverse voltage across the rectifierc Field current, ifd Simulator circuit current, ir

the simulation of rectifier action is tested by induc-ing a poleslipping condition of the alternator con-nected to the infinite bus through an external react-ance of 0.288 p.u., Fig. 9(i). This is done byswitching off the ac exciter from the rectifierbridge. Fig. 9(ii) gives an enlarged time scale os-cillogram for the same test. One more such test wasperformed with a smaller value of external reactanceso that the alternator is stiffly coupled.Fig. 9(iii) gives the oscillogram under this test.

For the case of Fig. 9(ii), the various quanti-ties Vf, Vrt, if and ir were measured and plotted

for comparison in Fig. 10. A small error current(Aif) of 100 mA is present in the field circuit dur-

ing the period of rectifier cutoff. This is a meas-ure of the accuracy of simulation of the rectifierexcitation system. It is, nevertheless, necessaryfor the operation of the current simulator. The errorcan be reduced if higher values of gain are chosen forthe simulator circuit, but stability considerations

within the amplifier and the system usually limit thevalue used.

There is a good agreement between VRt and ac-

tual voltage across the field winding during theDeriod of cut off, the difference of 3 to 4 percentbeing attributed to the value of Aif present. Both

these voltages will, however, be smaller than thevoltage that will be induced in the field, had thefield current been ideally constrained at zero. Thisdiscrepancy between the ideal and measured values isfound to be not more than 5 to 6 percent, a tolerancewhich is reasonable considering the accuracy withwhich some of the machine parameters can be determined.

CONCLUSIONS

The above scheme is operating satisfactorily atImperial College where it was first developed fouryears ago. The measured results show a close agree-ment with the calculations based on accurate methods

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MI

I: 0

Fig. 10. Currents and voltages during poleslipping.a Voltage across the field winding, measuredb Inverse voltage across the rectifier,

measuredc Field current, ifd Simulator circuit current, ir

and reported in a recent paper.

K. V. N. Rao (M '70) was born inMadras, India on October 1, 1938,He received the B. Tech. and M.Tech. degrees from Indian Instituteof Technology, Kharagpur in 1958and 1959 respectively. He wasawarded the UNESCO fellowship in1967 for higher studies in U.K.where he received his Ph.D. fromUniversity of London.

From 1961 onwards, he has beenwith M.A. College of Technology,Bhopal, where he is presently

Professor in Electrical Engineering. His current re-search interests include: Synchronous machine and itscontrollers, machine transients, system stabilitystudies.

Donald C. Macdonald was born in London in 1933. Hegraduated from Imperial College, London University in1956. He held apprenticeship and Research Fellowshipwith A.E.E. Ltd., Rugby and subsequently became a

development engineer. In 1964 he was appointed to hispresent position of Lecturer in Imperial College andearned his Ph.D in 1969. He became a M.I.E.E. in 1963.Photo not available.

ACKNOWLEDGMENT

Dr. Rao acknowledges the receipt of a UNESCO Re-search Fellowship and leave from M. A. College ofTechnology, Bhopal which enabled him to study atImperial College. Thanks are due to S R C for a grantunder which the work was done.

REFERENCES

[1] K. V. N. Rao, Fault Studies on Synchronous Gener-ators with Rectifier Excitation, Ph.D., Thesis,University of London, 1970.

[2] K. V. N. Rao, D. C. Macdonald and B. Adkins, "Peakinverse voltages in the rectifier excitation sys-tem of synchronous machines," IEEE Trans. PowerApp. Syst., PAS - 5, Oct./Nov. 1974.

Bernard Adkins was born in Sulgrave,England on December 26, 1903. Hereceived the M.A. degree in Mechan-ical Sciences from Cambridge Uni-versity in 1925 and the D.Sc. degreeof London University in 1958.He was a design engineer with the

B.T.H. Co., Rugby, England from1928 to 1951, when he became Reader

in Electrical Engineering at Imperial College, LondonUniversity. He is the author of many papers relatingto electrical machines and of books on "The generaltheory of electrccal machines" and "Polyphase commuta-tor machines.

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