Voltage and frequency contdy

ilse

nd inn exsistitaticconhopopod thconn

Use of induction generator is increasingly becomingmore popular in MHP application because of its simplerexcitation system, lower fault level, lower capital cost and

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 simulation

osedhicherm-ner-stantratorandt ofbus,ency

ratorthewertionblityless maintenance requirement [13]. 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

results.

2 Proposed scheme

Fig. 1 shows the schematic diagram of the propscheme. The synchronous generator has an exciter, wprovides a constant excitation to produce normal rated tinal voltage at full resistive load and it is capable of geating some reactive power too. It is driven by conmechanical power input of 1 pu. The induction genehas neither speed controller nor excitation controller,it is also driven by constant mechanical power inpu1 pu. The STATCOM is connected to the commonwhich controls the terminal voltage as well as frequof the scheme.In the absence of STATCOM, the synchronous gene

is required to generate the reactive power demanded byload. The STATCOM supplies the reactive podemanded by the load so that the reactive power generaof the synchronous generator does not exceed its capa

# 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: imtamrakar@ioe.edu.npsynchronous generator anwith STATCOM in micro h

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

Abstract: Parallel operation of synchronous apresented. The synchronous generator has aproduce normal rated terminal voltage at full reerator has neither exciter nor speed controller. Scommon bus for terminal voltage and frequencythe DC link capacitor of STATCOM through a cdeveloped to perform transient analysis of the prto compare with the simulation results. It is founallel with the synchronous is much simpler than

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.IET Gener. Transm. Distrib., 2007, 1, (5), pp. 743750rol of parallel operatedinduction generatordro scheme

n

duction generators in micro hydro scheme isciter, which provides a fixed excitation tove load. On the other hand, the induction gen-compensator (STATCOM) is connected to thetrol. A resistive dump load is connected acrossper to control active power. Simulink model issed scheme. Experimental results are presentedat connection of an induction generator in par-ecting two synchronous generators in parallel.

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, 811] 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,743

power input of 1 pu. When the consumers load changes,

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 active

branch is dissipatedp load. The voltOM is equal to thein the dump load

o the bus. Simulinka current-controllednt control principle.and control strategyand current controlverter which cane power.ared with the refer-is passed through abtain the magnitudece current iabc (ref).with the reference

is passed through aPI controller to obtain the duty cycle of the chopper to

erterthe 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 dq 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. 2 STATCOM with hysteresis band current control PWM inv

744control 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 dq 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 R0L0

i0a dt

1

L0

(Vsa V0a) dt (1)

i0b R0L0

i0b dt

1

L0

(Vsb V0b) dt (2)

i0c R0L0

i0c dt

1

L0

(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

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

power flow through the STATCOMinto the heat energy through the dumamp capacity of this type of STATCsum of active power to be dissipatedand the reactive power to be injected tmodel of STATCOM is developed asinverter with the hysteresis band curreFig. 2 shows the basic circuit diagramof the STATCOM with hysteresis bpulse width modulation (PWM) incontrol reactive power as well as activThe bus voltage is sensed and comp

ence value and the error thus obtainedproportional integral (PI) controller to oof the q-axis component of the referenThe frequency is sensed and comparedfrequency and the error thus obtained

Fig. 1 Schematic diagram of the proposed scheme

Vob 3 (2Sb Sa Sc) (5)

the following equations [3, 14]

vDC iDC vaia vbib vcic (7)

)

)

)

)

-is-fd-nte

load. In such a situation, STATCOM is responsible for gen-

d indVoc Vdc3

(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. 5 Simulation model of parallel operation of synchronous an

IET Gener. Transm. Distrib., Vol. 1, No. 5, September 2007erating 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 (12 j8) kVA

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

uction generators

745follows

Voa Vdc3

(2Sa Sb Sc) (4)Vdc

iDC vaia vbib vcic

vDC(8

vDC 1

C

iDC dt (9

icap iDC id (10

id Sd VDC

Rd(11

where Sd is the switching function of the chopper.

4 Simulation results

The complete simulation model of parallel operation of synchronous and induction generators with STATCOMshown in Fig. 5. The scheme consists of a 16 kVA synchronous generator and 4 kW induction generator. The ratings othe machines are selected to match with the machines usein the experimental study. The excitation voltage Vf of synchronous generator is limited to 2 pu, which is just sufficieto produce 1 pu of stator terminal voltage at full resistiv

Fig. 3 Simulink model of hysteresis band current controller

Fig. 4 Simulink model of inverter

r 2007Fig. 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

IET Gener. Transm. Distrib., Vol. 1, No. 5, Septembe746

refers to the consumption. The simulation results showthat there are some transients because of switching on of

power demanded by the induction generator. These transi-ents die out in a short span of time and the terminalinduction generator at t 3 s and load perturbation att 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

voltage 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 1: Balance of active and reactive power between

generation and consumption

Sources/sinks 03, s 36, s 610, s

PSG 15.4 kW 15.4 kW 15.4 kWQSG 1.0 kVar 1.0 kVar 1.0 kVarPIG 0 3.7 kW 3.7 kWQIG 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 6.8 kVar 7.0 kVar 7.0 kVarunbalance of P 20.1 Kw 0.1 kW 20.1 KwUnbalance of Q 0 kVar 0 kVar 0 kVar

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

Variables 03, s 36, s 610, 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) 01.25 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

IET Gener. Transm. Distrib., Vol. 1, No. 5, September 2007 7474 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 sign

Fig. 6 Continued.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 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 reactive

ionousTheto

wertantpu.

Table 3: Steady-state data

Observation no. Readings on synchronous Readings on induction Readings on STATCOM Readings on loadFig. 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

IET Gener. Transm. Distrib., Vol. 1, No. 5, September 2007748STATCOM 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 consumers load is switched onat t 6 s, there are some transients in the system. Theinduction generator does not respond to this change in con-sumers load. The terminal voltage, stator current, speed,

active power generation and reactive power consumptof the induction generator remain constant at their previvalues with a small transient at the switching instant.synchronous generator and STATCOM have respondedthis change in consumers load to make the active pobalance as shown in Table 1, thus maintaining a consspeed operation of synchronous generator at 0.995

generator generator

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

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 tracking

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 forthe 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 consumers 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

IET Gener. Transm. Distrib., Vol. 1, No. 5, September 2007STATCOM has shown the perfect control of system busvoltage to 1 pu and the frequency control within the rangeof 4951 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 consumers 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 725732

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. 146149

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. 647655

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. 413416

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. 20452050

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. 479485

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. 361366

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. 908914

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. 173177

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. 102106

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. 285293

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. 426432

14 Giroux, P., Sybille, G., and Le-Huy, H.: Modeling and simulation of adistribution STATCOM using Simulinks power system blockset.Proc. of IECON01: the 27th annual Conf. of the IEEE industrialelectronics society, 2001, pp. 990994

8 Appendix

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

749

Synchronous generator:

16 kVA, 400 V, 50 Hz, 1500 rpmXd 1.734 pu, Xd0 0.177 pu, Xd00 0.112 puXq 0.861 pu, Xq00 0.199 pu, Xl 0.07 puTd0 0.018 s, Td00 0.0045 s, Tq00 0.0045 s

RS 0.02 pu, H 6 sInduction generator:

4 kW, 400 V, 50 Hz

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

STATCOM parameters:

25 kVar, 400 VVDC 600 V, DC capacitor C 600 mFCoupling inductor: R0 2.5 V, L0 0.008HDump load resistance: Rd 25 V750 IET Gener. Transm. Distrib., Vol. 1, No. 5, September 2007