modeling of standalone hydro parallel operated synchronous and induction generator

Voltage and frequency control of parallel operatedsynchronous generator and induction generatorwith STATCOM in micro hydro scheme

I. Tamrakar, L.B. Shilpakar, B.G. Fernandes and R. Nilsen

Abstract: Parallel operation of synchronous and induction generators in micro hydro scheme ispresented. The synchronous generator has an exciter, which provides a fixed excitation toproduce normal rated terminal voltage at full resistive load. On the other hand, the induction gen-erator has neither exciter nor speed controller. Static compensator (STATCOM) is connected to thecommon bus for terminal voltage and frequency control. A resistive dump load is connected acrossthe DC link capacitor of STATCOM through a chopper to control active power. Simulink model isdeveloped to perform transient analysis of the proposed scheme. Experimental results are presentedto compare with the simulation results. It is found that connection of an induction generator in par-allel with the synchronous is much simpler than connecting two synchronous generators in parallel.

1 Introduction

Micro hydro plant (MHP) is one of the popular renewableenergy sources in the developing countries. Most of theMHP plants operate in isolated mode supplying the electri-city in the local rural area where the population is very smalland sparsely distributed and the extension of grid system isnot financially feasible because of high-cost investmentrequired for transmission line. The MHP designers havemade their efforts to reduce the construction cost of MHPby adopting the following strategies: using electronic loadcontroller (ELC) instead of conventional oil pressure mech-anical governor, allowing larger variation of voltage andfrequency to reduce the cost of control component andusing induction generator instead of synchronous generator.Frequency variation of+2% and terminal voltage variationof +5% from their nominal rated values are generallyacceptable in MHP schemes.Use of induction generator is increasingly becoming

more popular in MHP application because of its simplerexcitation system, lower fault level, lower capital cost andless maintenance requirement [1–3]. However, one of itsmajor drawbacks is that it cannot generate the reactivepower as demanded by the load. Most of the early stagesMHP plants are equipped with synchronous generators. Infuture, many of the existing MHP plants with synchronousgenerator may have to install an add-on plant and connectit in parallel with the existing MHP plant to fulfill the

# The Institution of Engineering and Technology 2007

doi:10.1049/iet-gtd:20060385

Paper first received 26th September 2006 and in revised form 4th February 2007

I. Tamrakar is with the Department of Electrical Engineering, Institute ofEngineering, TU, Nepal

L.B. Shilpakar is with the Nepal Electricity Authority and also with the Instituteof Engineering, TU, Nepal

B.G. Fernandes is with the Department of Electrical Engineering, IndianInstitute of Technology Bombay, Mumbai, India

R. Nilsen is with the Department of Electrical Power Engineering, NTNU,Norway, and currently with the Wartsila Automation, Norway

E-mail: [email protected]

IET Gener. Transm. Distrib., 2007, 1, (5), pp. 743–750

increasing load demand. In such a situation, the plant costcan be reduced further if induction generator could beused as the add-on plant to the MHP with synchronousgenerator.Analysis of grid connected induction generators has been

reported in the literature [4, 5]. Parallel operation of mul-tiple number of induction generators is also reported inthe literature [6, 7]. Parallel operation of synchronous andinduction generators in isolated MHP scheme has becomethe interesting topic of research. In such a scheme, control-lers are required to control the terminal voltage and fre-quency within the acceptable range. STATCOM forterminal voltage control has been discussed in the literature[3, 8–11] and ELC for frequency control has been discussedin the literature [11]. This paper deals with the transientanalysis of parallel operation of synchronous and inductiongenerators in MHP scheme. In the proposed scheme,STATCOM is used for terminal voltage control as well asspeed control. It does not need a separate ELC. Thescheme is simulated using MatLab-Simulink and exper-imental study is carried out to validate the simulationresults.

2 Proposed scheme

Fig. 1 shows the schematic diagram of the proposedscheme. The synchronous generator has an exciter, whichprovides a constant excitation to produce normal rated term-inal voltage at full resistive load and it is capable of gener-ating some reactive power too. It is driven by constantmechanical power input of 1 pu. The induction generatorhas neither speed controller nor excitation controller, andit is also driven by constant mechanical power input of1 pu. The STATCOM is connected to the common bus,which controls the terminal voltage as well as frequencyof the scheme.In the absence of STATCOM, the synchronous generator

is required to generate the reactive power demanded by theload. The STATCOM supplies the reactive powerdemanded by the load so that the reactive power generationof the synchronous generator does not exceed its capablity

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limit. The induction generator is driven by constant mech-anical power input of 1 pu. Since it does not have speed con-troller, it is bound to follow the synchronous generator andruns with a constant speed above the synchronous speedwith a negative slip corresponding to its full load. The syn-chronous generator is also driven by constant mechanicalpower input of 1 pu. When the consumers load changes,the chopper on the DC side of the STATCOM controlsthe active power consumed by the dump load so that thetotal load on the synchronous generator remains constantand equal to its full load capacity thus by resulting in con-stant speed operation.

3 Modelling of the proposed scheme

3.1 Modelling of synchronous and inductionmachines

The synchronous and induction machine models availablein the MatLab-Simulink [12] are used for performing thetransient analysis of the proposed scheme. The d–q equiv-alent circuit models of the synchronous and inductionmachines are used in the simulation model, which takescare of dynamics of stator, field and damper windings.Both the models have considered the effect of magnetic sat-uration. Stator windings of synchronous and induction gen-erators are assumed to be connected in star with groundedneutral.

3.2 Modelling of STATCOM

STATCOM is widely used for reactive power compen-sation, because it has several advantages over the conven-tional shunt capacitor compensation [8, 9, 13]. Basically,STATCOM is an inverter connected to the system busand controlled to draw leading current in order to

Fig. 1 Schematic diagram of the proposed scheme

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compensate the lagging current drawn by the load fromthe bus. STATCOM proposed in the scheme also drawsthe in-phase component of the current and the activepower flow through the STATCOM branch is dissipatedinto the heat energy through the dump load. The volt–amp capacity of this type of STATCOM is equal to thesum of active power to be dissipated in the dump loadand the reactive power to be injected to the bus. Simulinkmodel of STATCOM is developed as a current-controlledinverter with the hysteresis band current control principle.Fig. 2 shows the basic circuit diagram and control strategyof the STATCOM with hysteresis band current controlpulse width modulation (PWM) inverter which cancontrol reactive power as well as active power.The bus voltage is sensed and compared with the refer-

ence value and the error thus obtained is passed through aproportional integral (PI) controller to obtain the magnitudeof the q-axis component of the reference current iabc (ref).The frequency is sensed and compared with the referencefrequency and the error thus obtained is passed through aPI controller to obtain the duty cycle of the chopper tocontrol the power dissipation in the dump load. Similarly,the magnitude of the d-axis component of the referencecurrent is determined by comparing the actual DC-linkvoltage with the reference value. The d–q axes referencecurrents are then transformed to stationary a-b-c referenceframe to obtain the three-phase reference current iabc (ref).The hysteresis band current controller compares the actualcurrents through the STATCOM branch with the referencecurrents and generates the gate signals to turn on and off theswitch pairs T1-T2, T3-T4 and T5-T6 several times in a cycleso that the actual inverter current i0 (actual) tracks the refer-ence current iabc (ref ) within a limited hysteresis band. Theactual current through the STATCOM branch current isgiven by the following equation [3]

i0a ¼ �R0

L0

ði0a dt �

1

L0

ð(Vsa � V0a) dt (1)

i0b ¼ �R0

L0

ði0b dt �

1

L0

ð(Vsb � V0b) dt (2)

i0c ¼ �R0

L0

ði0c dt �

1

L0

ð(Vsc � V0c) dt (3)

Fig. 3 shows the Simulink model developed to simulatethe hysteresis band current controller, which generatesgate signals Sa, Sb and Sc. The inverter model shown inFig. 4 computes the phase voltages of inverter output as

Fig. 2 STATCOM with hysteresis band current control PWM inverter

IET Gener. Transm. Distrib., Vol. 1, No. 5, September 2007

follows

Voa ¼Vdc

3(2Sa � Sb � Sc) (4)

Vob ¼Vdc

3(2Sb � Sa � Sc) (5)

Voc ¼Vdc

3(2Sc � Sb � Sa) (6)

Sa, Sb and Sc are the switching functions of switch pairsT1-T2, T3-T4 and T5-T6, respectively. The switching func-tion takes the value of 1 if the upper switch of the inverterleg is on and lower switch is off. It is 0 if the lower switch inthe same leg is on and upper switch is off. The modelling ofDC side of the inverter is based on the instantaneous powerbalance between AC side and DC side of the inverter and

Fig. 3 Simulink model of hysteresis band current controller

Fig. 4 Simulink model of inverter


the following equations [3, 14]

vDC iDC ¼ vaia þ vbib þ vcic (7)

iDC ¼vaia þ vbib þ vcic

vDC(8)

vDC ¼1

C

ðiDC dt (9)

icap ¼ iDC � id (10)

id ¼Sd � VDC

Rd

(11)

where Sd is the switching function of the chopper.

4 Simulation results

The complete simulation model of parallel operation of syn-chronous and induction generators with STATCOM isshown in Fig. 5. The scheme consists of a 16 kVA synchro-nous generator and 4 kW induction generator. The ratings ofthe machines are selected to match with the machines usedin the experimental study. The excitation voltage Vf of syn-chronous generator is limited to 2 pu, which is just sufficientto produce 1 pu of stator terminal voltage at full resistiveload. In such a situation, STATCOM is responsible for gen-erating the reactive power demanded by the load. A fixedexcitation capacitor of 1.5 kVar is connected across theinduction generator terminals. It is the minimum capaci-tance required for self-excitation of the induction generatorat no-load. The ratings and parameters of the synchronousgenerator, induction generator and STATCOM are givenin the Appendix.The model is simulated with initial load of (12þ j8) kVA

on the system bus. The induction generator is connectedafter 3 s of initial load switching and additional load of

Fig. 5 Simulation model of parallel operation of synchronous and induction generators

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Fig. 6 Simulation results of the program run for 10 s

a Terminal voltage of synchronous generatorb Stator current of synchronous generatorc Speed response of synchronous generatord Active power output of synchronous generatore Reactive power output of synchronous generatorf Terminal voltage of induction generatorg Stator current of induction generatorh Speed response of induction generatori Active power output of induction generatorj Reactive power consumed by induction generatork Active power consumed by STATCOM branchl Reactive power generated by STATCOMm Magnified view of inverter output voltagen STATCOM currento Magnified view of statcom current and reference currentp Active power consumed by loadq Reactive power consumed by load


Fig. 6 Continued.

4 kW is connected after 6 s. The simulation results of theprogram run for 10 s are shown in Fig. 6.Table 1 shows the balance of active and reactive power

between generation and consumption in the scheme andTable 2 shows the other performance variables of thesystem during the simulation period. These tables showthe data obtained from the responses shown in Fig. 6. Thepositive sign refers to the generation and the negative signrefers to the consumption. The simulation results showthat there are some transients because of switching on ofinduction generator at t ¼ 3 s and load perturbation att ¼ 6 s. Initially, synchronous generator alone is supplyingactive power demanded by the load whereas synchronousgenerator together with STATCOM is supplying reactivepower demanded by the load. There is a balance betweengeneration and consumption of active and reactive powerin the scheme resulting constant speed operation at0.996 pu and constant terminal voltage of 1 pu from 0 to

Table 1: Balance of active and reactive power betweengeneration and consumption

Sources/sinks 0–3, s 3–6, s 6–10, s

PSG þ15.4 kW þ15.4 kW þ15.4 kW

QSG þ1.0 kVar þ1.0 kVar þ1.0 kVar

PIG 0 þ3.7 kW þ3.7 kW

QIG 0 0 0

PLoad 211.7 kW 212 kW 216 kW

QLoad 27.8 kVar 28 kVar 28 kVar

PSTAT 23.8 kW 27.0 kW 23.2 kW

QSTAT þ6.8 kVar þ7.0 kVar þ7.0 kVar

unbalance of ‘P ’ 20.1 Kw þ0.1 kW 20.1 Kw

Unbalance of ‘Q ’ 0 kVar þ0 kVar 0 kVar


3 s as shown in Tables 1 and 2. When the induction genera-tor freely running at the speed of 1.25 pu is connected att ¼ 3 s, it suddenly draws the reactive power from the syn-chronous generator resulting in a voltage dip in the statorterminal voltage. A spike transient of 2.2 pu appeared inthe stator current of synchronous generator, because theSTATCOM, synchronous generator and the excitationcapacitor cannot instantly generate the additional reactivepower demanded by the induction generator. These transi-ents die out in a short span of time and the terminalvoltage and speed settles down to 1 pu each as shown inTable 2. Here the induction generator does not go throughthe voltage build-up process, but when it is suddenlyswitched on to the synchronous generator bus, it catchesup the system voltage within 0.25 s. The induction genera-tor draws a high transient current of 7.5 pu at starting toestablish the air gap flux and then runs in generatingmode. The speed of the induction generator drops downand becomes stable at 1.035 pu within 0.25 s. Here theinduction generator has injected an additional activepower of 3.7 kW to the system and at the same time,

Table 2: Other performance variables of the systemduring the simulation period

Variables 0–3, s 3–6, s 6–10, s

speed (Syn Gen) 0.996 pu 1.0 pu 0.995 pu

VSG (Syn Gen) 1.0 pu 1.0 pu 1.0 pu

iSG (Syn Gen) 1.0 pu 1.0 pu 1.0 pu

speed (Ind Gen) 0–1.25 pu 1.035 pu 1.035 pu

VIG (Ind Gen) 0 1.0 pu 1.0 pu

IIG (Ind Gen) 0 1.0 pu 1.0 pu

V0(inverter voltage) 1.23 pu 1.23 pu 1.23 pu

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Table 3: Steady-state data

Observation no. Readings on synchronous

generator

Readings on induction

generator

Readings on STATCOM Readings on load

VLL, V Freq, Hz ISG, A VLL, V IIG, A Speed, rpm Istat, A IL, A

1 380 50 21.5 0 0 2150 4.2 17

2 380 50.5 21.5 380 5.6 1553 9.5 17

3 380 50.5 21.5 380 5.6 1553 4.8 22

STATCOM has responded to draw this additional activepower to make active power balance as shown in Table 1resulting constant speed operation of synchronous generatorat 1.001 pu. When 4 kW of consumer’s load is switched onat t ¼ 6 s, there are some transients in the system. Theinduction generator does not respond to this change in con-sumer’s load. The terminal voltage, stator current, speed,

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active power generation and reactive power consumptionof the induction generator remain constant at their previousvalues with a small transient at the switching instant. Thesynchronous generator and STATCOM have responded tothis change in consumer’s load to make the active powerbalance as shown in Table 1, thus maintaining a constantspeed operation of synchronous generator at 0.995 pu.

Fig. 7 Transient and load perturbation are recorded and are compared with the simulated results

a Transient in stator terminal voltage and current of synchronous generator because of switching of IGb Transient in stator terminal voltage and current of induction generator because of switching of IGc Transient in stator terminal voltage and STATCOM current because of resistive load perturbation


Fig. 6n shows the response of STATCOM current and itsmagnified view along with the reference current is shownin Fig. 6o. It is found that the STATCOM current is trackingthe reference current within the set values of hysteresisband.

5 Experimental results

Experimental study is carried out on the laboratory set upwhich consists of a 16 kVA synchronous generator withfixed excitation and 4 kW induction generator with exci-tation capacitor. The experiment is aimed to validate transi-ents because of switching on of induction generator andresistive load perturbation. Synchronous generator is firststarted and loaded it near to the full load. Its turbine withpartial gate opening then drives the induction generator,but it is not connected yet to the synchronous generatorbus. The induction generator is freely running without gen-erating any voltage and its speed is found to be 2150 rpm.The induction generator stator terminal is then connectedto the synchronous generator bus. It is observed that theinduction generator catches up with the synchronous gen-erator and stabilises at a speed of 1553 rpm within a fractionof a second and delivers power to the load thus by increas-ing the active power flow through the STATCOM branch.The consumer load is suddenly switched on in order toobserve the effect of sudden load perturbation on thesystem terminal condition. The steady-state data recordedare tabulated in Table 3.It is observed from the experimental study that connect-

ing an induction generator in parallel with the synchronousgenerator is much simpler than connecting two synchronousgenerators in parallel. It does not require synchronisingpanel hardware. A simple commercially available inductionmotor can be used as generator without the turbine control-ler and excitation controller. It is also observed fromTable 3 that the change in consumers load is respondedby the synchronous generator and STATCOM to keep thefrequency nearly constant to 50 Hz. The induction generatordoes not respond to the change in consumer’s load. Italways operates at its full rating. The transient during theswitching of induction generator and load perturbation arerecorded and measured they are compared with the simu-lated results as shown in Fig. 7. The simulated resultsshown in Fig. 7 are the magnified views of the waveformsshown in Fig. 6.Fig. 7a shows the transient in stator terminal voltage and

current of synchronous generator because of the switchingon of induction generator. Fig. 7b shows the transient instator terminal voltage and current of induction generatorbecause of the switching on of induction generator andFig. 7c shows the transient in stator terminal voltage of syn-chronous and STATCOM current because of the resistiveload perturbation. The measured responses are matchingwith the corresponding simulated responses.

6 Conclusions

The simulation results show that when an induction genera-tor driven by constant mechanical power input is connectedin parallel with the synchronous generator, the inductiongenerator is bound to follow the synchronous generatorwith a speed little above the speed of synchronous generatorwith a negative slip of about 0.035 pu. The transients instator terminal voltage, stator current of synchronous gen-erator and induction generator at the switching instantsare found to be acceptable for practical implementation.The speed deviation of synchronous generator because of


switching on of induction generator is also found to bevery small and settle down to steady-state speed within frac-tion of a second. The Simulink model developed forSTATCOM has shown the perfect control of system busvoltage to 1 pu and the frequency control within the rangeof 49–51 Hz which is good enough for isolated plant sup-plying the rural area. The experimental results are matchingwith the simulation results, which validate the perfection ofthe simulation model. The experimental results also showthat connecting an induction generator in parallel with thesynchronous is much simpler than connecting two synchro-nous generators in parallel. It does not require synchronis-ing panel hardware and a simple commercially availableinduction motor can be used as generator without governor.Any change in consumers load is responded by the synchro-nous generator and STATCOM to keep the speed nearlyconstant to 1 pu. The induction generator does notrespond to the change in consumer’s load and it alwaysoperates at its full rating.

7 References

1 Al-Bahrani, A.H., and Malik, N.H.: ‘Steady state analysis andperformance characteristics of a three phase induction generator selfexcited with a single capacitor’, IEEE Trans. Energy Conversion,1990, 5, (4), pp 725–732

2 Henderson, D.S.: ‘Synchronous or induction generators? - The choicefor small scale generation’. Opportunities and Advances in Int. PowerGeneration, IEE Conf., March 1996, (Publication No. 419),pp. 146–149

3 Singh, B., and Shilpakar, L.B.: ‘Analysis of a novel solid state voltageregulator for self-exited induction generator’, IEE Proc. Gener.Transm. Distrib., 1998, 145, (6), pp. 647–655

4 Wang, L., Yang, Y.-F., and Kuo, S.-C.: ‘Analysis of grid-connectedinduction generators under three-phase balanced conditions’. Proc.of the Int. Conf. on Energy Conversion 2002, 2002, pp. 413–416

5 Murthy, S.S., Jha, C.S., Ghorashi, A.H., and P. S. Nagendra Roa:‘Performance analysis of grid connected induction generators drivenby hydro/wind turbines including grid abnormalities’. Proc. of the24th Int. Conf. on Energy Conversion, 1989, vol. 4, pp. 2045–2050

6 Wang, L., and Lee, C.H.: ‘Dynamic analyses of parallel operatedself-excited induction generators feeding an induction motor load’,IEEE Trans. Energy Conversion, 1999, 14, (3), pp. 479–485

7 Chakraborty, C., Das, S.P., and Bhadra, S.N.: ‘Some studies on theparallel operation of self excited induction generators’. Proc. of theInt. Conf. on Energy Conversion, 1993, pp. 361–366

8 Marra, E.G., and Pomilio, J.A.: ‘Self-excited induction generatorcontrolled by a VS-PWM bi-directional converter for ruralapplications’, IEEE Trans. Ind. Electron., 2000, 47, (4), pp. 908–914

9 Freitas, W., Asada, E., Morelato, A., and Xu, W.: ‘Dynamicimprovement of induction generator connected to distributionsystem using a DSTATCOM’, Power System Technology, 2202.Proc. Power Con. 2002, Int. Conf., October 2002, vol. 1,pp. 173–177

10 Jayaramaiah, G.V., and Fernandes, B.G.: ‘Analysis of voltageregulator for a 3-phase self-excited induction generator usingcurrent controlled voltage source inverter’. Proc. on First Int. Conf.on Power Electronics System and Application 2004, November2004, pp. 102–106

11 Singh, B., Murthy, S.S., and Gupta, S.: ‘Analysis and design ofelectronic load controller for self-excited induction generators’,IEEE Trans. Energy Conversion, 2006, 21, (1), pp. 285–293

12 MathLab version-7.1, Release-14, 200513 Tamrakar, I., and Malik, O.P.: ‘Power factor correction of induction

motors using PWM inverter fed auxiliary stator winding’, IEEETrans. Energy Conversion, 1999, 14, (3), pp. 426–432

14 Giroux, P., Sybille, G., and Le-Huy, H.: ‘Modeling and simulation of adistribution STATCOM using Simulink’s power system blockset’.Proc. of IECON’01: the 27th annual Conf. of the IEEE industrialelectronics society, 2001, pp. 990–994

8 Appendix

Ratings and parameters of synchronous generator, inductiongenerator and STATCOM used in the simulation are asfollows:

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Synchronous generator:

16 kVA, 400 V, 50 Hz, 1500 rpmXd ¼ 1.734 pu, Xd

0 ¼ 0.177 pu, Xd00 ¼ 0.112 pu

Xq ¼ 0.861 pu, Xq00 ¼ 0.199 pu, Xl ¼ 0.07 pu

Td0 ¼ 0.018 s, Td

00 ¼ 0.0045 s, Tq00 ¼ 0.0045 s

RS ¼ 0.02 pu, H ¼ 6 s

Induction generator:

4 kW, 400 V, 50 Hz

750

RS ¼ 0.035 pu, Lls ¼ 0.045 pu, Rr ¼ 0.034 pu, Llr ¼0.045 pu, Lm ¼ 2.8 pu, H ¼ 1.2 s, P ¼ 4Excitation capacitor ¼ 1.5 kVar, 400 V

STATCOM parameters:

25 kVar, 400 VVDC ¼ 600 V, DC capacitor C ¼ 600 mFCoupling inductor: R0 ¼ 2.5 V, L0 ¼ 0.008HDump load resistance: Rd ¼ 25 V


modeling of standalone hydro parallel operated synchronous and induction generator

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