Self-excited induction generator/force-commutatedrectifier system operating as a DC power supply

C.H. Watanabe, DEng, MScA.N. Barreto, ElectEng

Indexing terms: Power conversion, Generators, Electrical machines

Abstract: The paper presents a new DC energygeneration system based on a 3-phase self-excitedinduction generator coupled to a force-commutated rectifier. This rectifier is switched insuch a way that its input current fundamentalcomponent is advanced with respect to the corre-sponding voltage, producing a capacitive charac-teristic on its terminals. This characteristic helpsthe self-excitation and improves the direct outputvoltage regulation, with respect to load variation,to the range of 1 to 2%, for some firing angle.Also, this capacitive effect greatly increases theoutput power limit, and allows the system tosupply a constant direct output voltage over a rea-sonably wide range of rotor speed variation. Thesteady-state performance of the system is analysedbased on a simplified mathematical model, whichis validated experimentally. A study on the influ-ence of the smoothing reactor size is presentedand it is shown that the system can operate prop-erly even without this reactor, if there is no limitfor current ripple in the load.

Resumo: Este trabalho apresenta um novosistema de geracao CC baseado em um gerador deinduc.ao trifasico auto-excitado acoplado a umretificador de comutacao forcada. Este retificadore chaveado de tal modo que a componente funda-mental de sua corrente de entrada fica avanc,adaem relacao a tensao correspondente, produzindouma caracteristica capacitiva nos seus terminais.Esta caracteristica auxilia a auto-excitacao emelhora a regulac.ao da tensao CC, com relacao avariacao da carga, ate uma faixa de 1 a 2%, paraalguns angulos de disparo. Tambem, este efeitocapacitivo aumenta em muito o limite de potenciade saida, e permite que o sistema gere tensao CCconstante em uma faixa razoavelmente larga devariagao de velocidade do rotor. O desempenhodo sistema em regime permanente e analisadobaseado em um modelo matematico simplificado,o qual e validado experimentalmente. Um estudosobre a influencia do tamanho do reator de alisa-mento e apresentado e e mostrado que o sistemapode operar devidamente mesmo sem este reator,se nao existir limite de 'ripple' de corrente nacarga.

Paper 5504B (PI), first received 13th February and in revised form 29thMay 1987The authors are with the Electrical Engineering Department, COPPE/Universidade Federal do Rio de Janeiro, Caixa Postal 68504, 21945 Riode Janeiro-RJ, Brazil

1 Introduction

The induction generator self-excitation phenomenon hasbeen well known since the beginning of this century.Bassett and Potter [1] and Wagner [2] published theirwork on this subject explaining the self-excitation processand suggested some applications. The main advantagesof this kind of generator are its ruggedness, low main-tenance, low cost, low weight and small size, when com-pared with a synchronous or DC generator. Also, thisgenerator is self-protected against short-circuits.However, the generator has a relatively poor voltage andfrequency regulation.

Doxey [3] proposed the use of saturated reactors tocontrol the voltage, which was shown to be efficient forsmall load variations. Brennen and Abbondanti [4] pro-posed the use of a static exciter to control the outputvoltage. All these works were dealing with the possibilityof using the generated AC voltage.

Two papers by Watson et al. [5] and Arrillaga andWatson [6] showed the possibility of obtaining control-lable DC power from variable-speed self-excited induc-tion machines (wind driven systems). Hayashi et al. [7]and Donker et al. [8] presented studies of an inductiongenerator connected to a rectifier.

The technique of obtaining controllable DC powerfrom a self-excited induction generator is especially inter-esting in applications such as in a windmill of smallhydroelectric power plant without speed control. The DCpower obtained in these systems can be used directly inDC equipment, charging batteries or it can be connectedto an AC network through a DC link [7].

This paper is the continuation of work done in thefield of induction generator control [9, 10]. Rocha et al.[9] presented the steady-state characteristics of a 3-phaseself-excited induction generator connected to a diode rec-tifier in series with a synchronous chopper. Carneiro etal. [10] presented a similar study for a single-phaseinduction generator. In both cases the chopper isswitched on and off in such a way as to produce a capac-itive current at the rectifier input terminals, helping theself-excitation of the induction generator. The 3-phasesystem was shown to be very good for operation with avariable speed. However, the equivalent capacitorsused at the rectifier terminals were of relatively smallvalues.

The work presented here, which is a revised version ofthe work presented by Barreto and Watanabe [11], wasdeveloped based on the same principle, but changing thediode rectifier/chopper system for a force-commutatedcontrolled rectifier, which is a simplified 3-phase versionof that proposed by Kataoka et al. [12]. The same circuitof this rectifier is normally used as an inverter (e.g.

IEE PROCEEDINGS, Vol. 134, Pt. B, No. 5, SEPTEMBER 1987 255

Yavarajan et al. [13]). Therefore, the system to be studiedhere comprises a squirrel-cage induction machine, excit-ing capacitors and force-commutated rectifier. A simpli-fied steady-state model of the system is presented andvalidated experimentally. As a result it is shown that it ispossible to use the proposed system as a constant directvoltage power supply, even when it is driven at a variablespeed.

and that the commutation time is negligible. The effectsof smoothing reactor size will be studied afterwards.

The firing angle a is measured from the voltage cross-ing points and in this rectifier it can be positive (as in aconventional line commutated rectifier) or negative.When the firing angle a is negative, the current funda-mental component ial leads the voltage va, giving acapacitive characteristic to the rectifier. When a is posi-tive its characteristic becomes inductive.

2 Proposed system

The system proposed here is shown in Fig. 1. The induc-tion generator is obtained by connecting three capacitorsto the squirrel-cage induction machine terminals. The

2.1 Rectifier input equivalent circuitAs the magnitude of current ia is equal to Id (directcurrent), the RMS value of ia is given by

h = y/ii)h (1)

A-inductiongenerator

k

c ' c

A

V

f fexcitingcapacitors force commutated

rectifier

Fig. 1 Circuit diagram of the proposed system

force-commutated rectifier comprises six thyristors, sixdiodes and six small force-commutated auxiliary capac-itors. For this study the load will be formed by a resist-ance R in series with a smoothing reactor L.

Fig. 2a shows the induction generator phase-to-neutralvoltages va, vb, vc and the rectifier output voltage ed. Fig.

oc< ocapacitive inductive

Fig. 2 Alternating input voltage, direct output voltage, and currentwaveforms {for L = oo)

2b shows the a-phase current ia and its fundamental com-ponent ial. Of course, these waveforms have been drawnon the assumption that the smoothing reactor is verylarge, that is, there is no ripple in the output current Id,

On the other hand, the RMS value of the fundamentalcomponent is

01 ~V2(2)

From eqns. 1 and 2 it is possible to calculate the distor-tion factor given by

= ^ - = 0.955 (3)

which shows that the harmonic content of the rectifierinput current is relatively low.

Based on this result, the rectifier input current will berepresented only by the fundamental component ial,which is given by

, -htl,d sin {cat — a) (4)

The validity of this assumption will be confirmed experi-mentally.

The rectifier average output voltage Vd is given by

Vd = V cos a (5)

where V is the RMS value of the phase-to-phase voltageat the rectifier input.

256 IEE PROCEEDINGS, Vol. 134, Pt. B, No. 5, SEPTEMBER 1987

From eqn. 5 it is possible to calculate the steady-stateoutput current Id:

nRV cos a (6)

The a-phase voltage (phase-neutral), as shown in Fig. 2a,is defined as:

and the rectifier input current fundamental componential, given in eqn. 4, can be rewritten as the sum of twocurrents:

ial = -^— Id cos a sin cot — -*— Id sin a cos cot (8)

The first term on the right-hand side of eqn. 8 is a currentin phase with va and the second term is phase shifted by— 90° from va. Therefore, they represent the active andreactive currents, respectively. Dividing the RMS value ofthe voltage va by the RMS values of these active andreactive currents, it is possible to calculate the rectifierphase-to-neural equivalent inputreactance Xeql given by:

resistance Reqi and

R Vn

eqi 3 ^ 2 / , cos a

xeql

Vn

Substituting Id in eqn. 9 as given in eqn. 6 and trans-forming these Y-connected equivalent resistances andreactances to their delta-connected equivalents, theybecome:

R "2R

eq 6 cos2 a

X = ^ -eq 3 sin 2a

(10)

When the firing angle a < 0, the Xeq becomes capacitiveand the equivalent capacitor Ceq is given by

and for a > 0 the equivalent reactor is given by

q 3co sin 2a

The equivalent resistance Req, given in eqn. 10, for agiven firing angle a, is directly proportional to the loadresistance R. On the other hand, the equivalent capacitorCeq in eqn. 11 is inversely proportional to the load resist-ance. This fact is very important because, in an induc-tion generator, when the load current is increased (loadresistance decreased), the exciting capacitor should alsobe increased to keep the voltage constant and it happensautomatically.

Under normal operation, it is supposed that the firingangle a will be negative. However, it is possible to makeit positive and produce an equivalent inductance, whichis interesting when it is desirable to decrease the excitingcapacitances. This situation may occur when the rotorspeed is too high or the load is too small. Here, emphasiswill be put on the case of the system operating only withnegative a.

2.2 Induction generator equivalent circuitThe equivalent circuit used to represent the inductiongenerator is the conventional one, and is shown in Fig. 3.In this Figure the symbols are:s = slipri> r2 = stator and rotor winding resistances/l5 l2 = stator and rotor leakage inductancesC = exciting capacitanceRp = hysteresis and eddy current losses equivalent

resistanceLM = magnetising inductanceReq > Xeq

= equivalent resistance and reactance as definedin eqn. 10

r2

Fig. 3 Equivalent circuit of the proposed system

The magnetising inductance LM was measured, for theexperimental system, as a function of the relationbetween the induced voltage and the angular frequency(V/co). The hysteresis and eddy current losses equivalent

(m resistance Rp was represented as a function of theinduced voltage and frequency.

The current Iab represented in Fig. 3 is the phasecurrent in the delta connection.

2.3 Influence of the smoothing reactorIn the analysis described in Section 2.1, the smoothingreactor was considered as being infinite; however, in realcases this is not true.

In this Section, the other extreme case of L = 0 will beanalysed, with the objective of studying the influence ofthe smoothing reactor in the modelling. In real cases,the smoothing reactance L will be between L = 0 andL = oo.

The rectifier input current for the case of L = 0 isshown in Fig. 4. In this analysis it will be assumed thatthe magnitude of the firing angle | a | is smaller than 60°,

uot

Fig. 4(L = 0)

Input current waveform in the case of no smoothing reactance

to ensure that direct current will be always flowing.From Fig. 4 it is possible to calculate the rectifier inputcurrent fundamental component RMS value I'al. Therelation raJlaX is given by:

!kcos a

(11)

Fig. 5 shows the relation rai/Ial as a function of a. Fromthis Figure it is easy to see that the difference between Ial

and I'al is less than 1% for 0 < a < 58°, therefore, it ispossible to conclude that the influence of the smoothingreactor on the magnitude of the input current fundamen-tal component RMS value is negligible.

IEE PROCEEDINGS, Vol. 134, Pt. B, No. 5, SEPTEMBER 1987 257

When the smoothing reactance L is infinite, the phaseangle between voltage and current fundamental com-ponent is given by the firing angle a. However, for L = 0

1.01

1.00

L = 0

10 20 30 40 50

(X, degrees

60

Fig. 5 Influence of the smoothing reactance on the input current fun-damental component RMS value

this phase becomes

= arctan (tan a) cos a

cos a +

(14)

Fig. 6 shows the phase angle 0 as a function of a for bothL = 0 and L = 00. This Figure has been drawn for posi-

6 0 r

50

40

20

10

L = oo

10 20 30 40<x, degrees

50 60

Fig. 6 Influence of the smoothing reactance on the input current fun-damental component phase angle

tive a (inductive mode), but it is the same for negative aas well, putting the minus sign in both a and <f>.

Using the same methodology as in Section 2.1, it ispossible to derive the equivalent resistance R'eq and equiv-alent capacitance C'eq for L = 0. Expressing R'eq and Ceq

in terms of Req and Ceq they become:

cos aeq K cos

e

K s in (f>q sin a q

where K = I'aJIal, as given in eqn. 13.

258

(15)

(16)

Fig. 7 shows the relation R'eq/Req and C'eq/Ceq as func-tions of a. From this Figure it becomes clear that forL = 0 and 0 < a < 50° the difference between R'eq, Ceq

and Rea, Cea is smaller than 10%.

30 40Oc, degrees

50 60

Fig. 7 Influence of the smoothing reactance on the equivalent inputresistance and capacitance

This analysis shows that there is no need to make thesmoothing reactance too large, because even when it isequal to zero, the system can operate properly. The sizeof this reactance should be chosen considering the per-missible ripple at the load, interference problems etc.

3 Experimental system and its steady-statecharacteristics

An induction generator based on a 1.5 kW/220 V squirrelcage induction machine and three 31 JIF exciting capac-itors were used for the laboratory test. The force-commutated rectifier has been designed with six 12 A/1100 V thyristors, six diodes and six 6 /iF/1000 V forcecommutation capacitors. The driving system was basedon a 3 kW DC motor. The parameters of the system aregiven in Table 1. The smoothing reactance used was

Table 1: Parameters of the system

=3.08 0= 2.85 Q

/, =/2 = 0.016 HC = 31 /vF

L = 10 mH, but measurements made with L = 0 andL = 20 mH confirmed the validity of the analysis present-ed in the preceding Section, that the main effect of thisreactance is in the direct current ripple content.

Fig. 8 shows the validation of eqn. 11 through com-

30

20

10

= 20H

-30 -60Of, degrees

- 9 0

Fig. 8 Calculated and experimental equivalent capacitancecalculatedexperimental

parison between experimental and calculated results. Inthis case the rotor speed n has been kept constant at 2000rev/min and the rectifier load resistance has been variedfrom 20 Q to 100 fi. At a = -45° the equivalent capac-itance is a maximum, and for R = 20 Q its value is 37 fiF,which is even greater than the exciting capacitances used.

IEE PROCEEDINGS, Vol. 134, Pt. B, No. 5, SEPTEMBER 1987

Fig. 9 shows the induction generator terminal alternat-ing voltage/current characteristics when it is connected tothe rectifier and a is varying. The relatively small errors

300 r

200 -

100 -

0 1 2 3 4 5 6 7 8 9 10

Fig. 9 Voltage/current characteristics at the rectifier input terminalsfor various a— # — experimental

calculatedC = 31 /iF, n = 1800 rev/min

between the calculated and measured results confirm thevalidity of the model of the system. In Fig. 9 the outputvoltages for Ia = 0 are the same, because in this situationthere is no equivalent capacitor or reactor at the rectifierinput, that is, the voltage is given only by the excitingcapacitors and the induction machine magnetisationcharacteristic. For Ia > 0, the smaller the firing angle a isthe larger the induction generator operating range is. Forfiring angle a equal to —30° the alternating outputvoltage characteristic becomes almost flat, even for Iagreater than the rated value (the rated current is 3.92 A).When the line current Ia is increased, under normal con-ditions, the AC terminal voltage would decrease;however, as shown in eqn. 11, the increase in Ia(equivalent to a decrease in load resistance R) increasesthe equivalent capacitance Ceq at the rectifier input,increasing the alternating voltage.

Fig. 10a shows the voltage/current characteristics atthe rectifier output for firing angle a equal to zero and

0 1

300,

^ 200

OC=-20°

-30°

0 1 2 3 4 5 6 7 8!d< A

b

Fig. 10 Voltage/current characteristics at the rectifier output termin-alsa a = 0 ' and a = — 10'b a = - 2 0 ' and a = - 30°- - O - -

I

— 10°. Fig. 10b shows the same characteristic fora = — 20° and — 30°. In both Figures the induction gen-eration was driven at 1800 rev/min. For a = 0 the equiva-lent reactance at the rectifier input Xeq is zero, i.e. therectifier works as a purely resistive load. In this case thevoltage regulation is poor and the maximum current islimited to 5.5 A. On the other hand, when a increases, theequivalent capacitance Ceq also increases, the voltageregulation is improved and current limit is increased.

In the case of a = — 30° it is clear that the increase inthe equivalent capacitance Ceq, due to the increase in theload current, corresponds to the capacitor that should beadded to the excitation bank, to compensate the voltagedrop due to this same load current. As the rated outputpower of the induction machine is 1500 W, the ratedoutput direct current is 5.7 A, for a = — 30°. The voltageregulation is better than 2% over the whole range andfor short periods it is possible to have the current greaterthan the rated value and still have good voltage regula-tion.

The present system is proposed to work as a DCpower supply with constant output voltage even atvarying rotor speed. Fig. 11 shows the rectifier output

-90 -60 -30 0 30cx, degrees

calculated

Fig. 11 Output direct voltage against firing angle for various values ofrotor speed and constant load resistance

- - O - - experimentalcalculated

voltage Kdasa function of the firing angle a, for variousrotor speeds. From this Figure it can be seen that we canhave 200 V direct output voltage for a rotor speed assmall as 1550 rev/min up to 2000 rev/min, only varyingthe firing angle a. Also, from this Figure it is clear thatthere is no need to work with inductive and capacitivecharacteristics, that is, the capacitive characteristic only isenough for the system to operate properly over the wholerotor speed range.

4 Conclusion

A new DC power supply system based on a 3-phase self-excited induction generator coupled to a force-commutated rectifier has been proposed. A steady-stateperformance analysis of the system was introduced based

1EE PROCEEDINGS, Vol. 134, Pt. B, No. 5, SEPTEMBER 1987 259

on a simplified mathematical model, which was validatedexperimentally.

It was shown that the proposed system has a verygood voltage regulation with respect to load variation,and also a relatively good voltage regulation with respectto induction generator rotor speed variation. Theseresults make this system very interesting for applicationsin wind energy systems and in small hydroelectric powerplants. In the case of a small hydroelectric power plant itis also possible to connect this generator through a DClink to an existing AC network.

The effect of the smoothing reactance was studied andit was shown that the system can operate properlywithout this smoothing reactance, if the load has nocurrent ripple limit.

5 Acknowledgments

The authors would like to express their gratitude toCNPq-Conselho Nacional de DesenvolvimentoCientifico e Tecnologico and to Eletrobras for theirsupport of this research.

6 References

1 BASSETT, E.D., and POTTER, F.W.: 'Capacitive excitation forinduction generators', Trans. Amer. Inst. Electr. Eng., 1935, 34, pp.540-545

2 WAGNER, C.F.: 'Self-excitation of induction motors', ibid., 1939,38, pp. 47-51

3 DOXEY, B.C.: 'Theory and applications of capacitors excitedinduction generators', The Engineer, Nov. 1963, pp. 893-897

4 BRENNEN, M.B., and ABBONDANTI, A.: 'Static exciters forinduction generators', IEEE Trans., 1977, IA-13, (5), pp. 422-428

5 WATSON, D.B., ARRILLAGA, J., and DENSEM, T.: 'Controlla-ble DC power supply from wind-driven self-excited inductionmachines', Proc. IEE, 1979,126, (12), pp. 1245-1248

6 ARRILLAGA, J., and WATSON, D.B.: 'Static power conversionfrom self-excited induction generators', Proc. IEE, 1978, 125, (8), pp.743-746

7 HAYASHI, Y., SATO, N., and FUNAKI, S.: 'Self-excitated induc-tion generators paralleled to the AC power line through DC linkconverters'. IEEE International Power Electronics Conference(IPEC), Tokyo, Japan, 1983, pp. 1229-1240

8 DONKER, R., GEYSEN, W, VANDENPUT, A., and BELMANS,R.: 'A three-phase self-excited induction generator loaded by a con-trolled rectifiers bridge'. IEEE International Power Electronics Con-ference (IPEC), Tokyo, Japan, 1983, pp. 1252-1265

9 ROCHA, C.L., WATANABE, E.H., and CARNEIRO, S.: 'Estudode um sistema gerador de inducao auto-excitado acoplado a umretificador/"chopper"'. Anais do 50 Congresso Brasileiro deAutomatica/10 Congresso Latino-Americano de Automatica, 1984,pp. 293-297 (in Portuguese)

10 CARNEIRO, S., WATANABE, E.H., and NASCIMENTO, J.L.:'Application of the single-phase self-excited induction generatorcoupled to a rectifier-chopper'. Proceedings of International Con-ference on Evolution and modern aspects of induction machines,Torino, Italy, 1986, pp. 544-548

11 BARRETO, A.N., and WATANABE, E.H.: 'Self-excited inductiongenerator/force-commutated rectifier system operating as a DCpower supply'. First European Conference on Electrical Machines,Brussels, Belgium, 1985

12 KATAOKA, T., MIZUMACHI, K., and MIYAIRI, S.: 'A pulsewidth controlled AC-to-DC converter to improve power factor andwaveform of AC line current', IEEE Trans., 1980, IA-15, (6), pp.670-675

13 YAVARAJAN, S., RAMASWAMI, V., and SUBRAHAMANYAN,V.: 'Analysis of a current-controlled inverter — induction motordrive using digital simulation', ibid., 1980, IECI-27, (2), pp. 67-76

260 IEE PROCEEDINGS, Vol. 134, Pt. B, No. 5, SEPTEMBER 1987