PERFORMANCE ANALYSIS OF A PV POWERED DC MOTOR DRIVING A 3-PHASE SELF-EXCITED INDUCTION GENERATOR

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EEE Trnsctions on Energy Conversion, Vol. 11, No. 1, Mrch 1996 PERFORMANCE ANALYSS OF A PV POERED DC MOTOR DRVNG A 3-PHASE SELF-EXCTED NDUCTON GENERATOR 155 S. M. Alghuwinem EX.

1 EEE Trnsctions on Energy Conversion, Vol. 11, No. 1, Mrch 1996 PERFORMANCE ANALYSS OF A PV POERED DC MOTOR DRVNG A 3-PHASE SELF-EXCTED NDUCTON GENERATOR 155 S. M. Alghuwinem EX. Deprtment, King Sud University Riydh, Sudi Arbi ABSTRACT Photovoltic ( PV ) powered dc motors driving dedicted lods ( e.g. wter pumps ) re incresingly used in the remote rurl res of mny developing countries. The key to their success is simplicity ( direct coupling, no dc-c inversion, no storge btteries, etc. ). n this pper PV powered dc motor is used to drive n isolted three-phse self-excited induction genertor ( SEG ). t is found tht due to the unique torquespeed chrcteristics of the SEG, utiliztionefficiency is close to mximum t ll insoltion levels with no pek-power trcking. Theproposedrrngementisusefulsprtof nintegrted renewble energy system ( RES ), which tkes dvntge of the inherent diversity of wind nd insoltion in most developing countries to improve power qulity. The SEG is driven by wind turbine, dc motor, or both. Performnce of the system under different insoltion conditions is nlyzed. Keywords: - Photovoltic genertors; solr cells; dc motors; induction genertors; integrted renewble energy systems. NTRODUCTON Utilities in mny developing countries re finding it difficult to estblish nd mintin remote rurl re electrifiction. The cost of delivering power to such res is becoming excessively lrge due to lrge investments in trnsmission lines or loclly instlled cpcities, nd lrge trnsmission line losses. For these resons, mny utilitiesre seriously considering locl renewble energy resources, minly wind nd PV, s lterntives for supplying energy to rurl remote res. n mny developing countries, wind nd insoltion re complementry over the nnul cycle [ 1 1. t is therefore desirble to tke dvntge of this inherent diversity by combining PV nd wind systems into one integrted renewble energy system ( RES ). n ddition to minimizing the overll system cost by shring some of the 95 SM EC A pper recommended nd pproved by the EEE Energy Development nd Power Genertion Committee of the EEE Power Engineering Society for presenttion t the 1995 EEE/PES Summer Meeting, July 23-21, 1995, Portlnd, OR. Mnuscript submitted December 29, 1993; mde vilble for printing June 5, 1995 equipment, the lres improves power qulity nd minimizes the need for energy storge. The squirrel-cge self-excited three-phse induction genertor ( SEG ) hs been used for mny yers to convert wind power to electricl power. Due to its simplicity, ruggedness, nd low cost, it provides relible nd reltively inexpensive mens to convert mechnicl wind power to three-phse electricl power over wide rnge of rotor speeds for lods where frequency nd voltge need not be regulted. Over the pst decde, mny reserchers hve investigted the stedy-stte performnce of the SEG [ 2-4. References [5-71 evlute the excittion cpcitnce requirements of the SEG under different lod nd speed conditions. PV genertors convert solr energy to dc electricl power, which my be used directly by some lods where energy is stored in btteries or converted to c power using dc-c inverter. nverter control equipment, pek power trckers, hrmonic filters, nd protection devices re ll incorported with the inverter in one unit clledpower conditioningunit (PCU). The cost of XU, efficiency, nd relibility, re importnt fctors to consider when opting for dc-c inversion. The simplest nd lest expensive mens to convert PV power to mechnicl power is to use it to drive dc motor. n order to void dditionl costs, the motor is directly coupled to the PV genertor without storge btteries. This rrngement is typiclly used on noncriticl lods such s wter pumps, which need not operte continuously nd wter cn be used directly or stored esily. ith the incresed use of these systems, more ttention is pid to their design nd optimum utiliztion in order to chieve the most relible nd economicl opertion. Becuse of the reltively high cost of PV genertor, the system designer is interested in its full utiliztion by optimum mtching of the dc motor nd its mechnicl lod to the PV genertor so tht mximum utiliztion efficiency is chieved. n this pper dc motor is used to drive three-phse squirrel-cge self excited induction genertor ( SEG ). The system consists of three different devices; the PV genertor, the dc motor, nd the induction genertor. Ech device hs its own operting chrcteristics which is the volt-mpere chrcteristics for the PV genertor nd the dc motor, nd the torque-speed chrcteristics for the induction genertor. The dc motor drives the induction genertor whose torque requirements depend on its electricl lod, exciting cpcitor, nd /96/$ EEE

2 156. The dc motor is supplied from the PV genertor whose volt-mpere chrcteristics depend on the solr insoltion vritions nd on the current drwn by the dc motor. There is unique point on the volt-mpere chrcteristics t which power output from the PV genertor is mximum, nd for optimum utiliztion the equilibrium operting point must coincide with this point. However, since the mximum power point vries with insoltion it is, in generl, difficult to mintin optimum mtching t ll insoltion levels. n order to correct this problem, two options re generlly vilble to the system designer. A ) Crefully select the dc motor type nd its mechnicl lod so tht they mtch s closely s possible the mximum power line of the PV genertor, or B ) Use n electronic control device known s pek-power trcker ( PPT ), which continuously mtches the output chrcteristics of thepv genertor to theinput chrcteristics of the dc motor. Option ( A) offers compromise mtching which is vlid only for some solr insoltion levels due to drifting of the mximum power point with insoltion vritions. Reference [ 8 ] is comprehensive study of the strting nd stedy-stte performnce of severl types of PVpowered dc motors driving severl types of wter pumps. n [ 9 1, mtching of dc motors to PV genertors for mximum dily gross mechnicl energy is reported. Reference [ 10 ] dels with the opertion of lods powered by seprte or common PV source. n [ 11, the opertion of permnent-mgnet dc motors driving different types of wter pumps nd powered by common PV genertor is investigted. Pek-power trcking ( option B ) is chieved either by discretely interchnging the series-prllel connections of solr cell modules within the PV rry [ 12 1, or by using controlled dc-dc converter to djust the voltge nd current levels [ 13-16]. OPERATON TH DRECT COUPLNG CHARACTERSTCS OF THE SEG The SEG is ctully n induction motor tht is driven by prime mover while its sttor excittion is provided by n externl cpcitor connected to the sttor. Becuse the SEG is isolted, its sttor frequency is free to vry with the rotor speed nd the operting slip remins smll nd negtive. Fig. 2 shows the per-phse equivlent circuit commonly used for the stedy-stte nlysis of the three-phse self-excited induction genertor [ 2. b Fig. 2 Per-phse equivlent circuit of self-excited induction genertor & f --y The circuit hs been trnsformed to the bse frequency (60 z. ) by introducing the prmeters f nd v, where f is the per-unit frequency nd v is the speed in per-unit of the synchronous speed. The totl current t node c my be written s Therefore, for successful voltge build-up the totl dmittnce must be zero, since E1 # 0. d The motor is connected directly to the terminls of the PV genertor s shown in Fig. 1. Ech device hs its own chrcteristics with unique operting point t which performnce is optimum. Therefore for optimum performnce of the overll system, the equilibrium operting point must coincide with the point of optimum performnce for ech device. n order to nlyze the performnce of the whole system, it is necessry determine the chrcteristics of the individul devices. 00 t VPV J 1- PV Genertor DC Motor nduction Gen. Equting the rel nd imginry prts of eqution ( 2 ) to zero, we hve (7) Fig. 1 Opertion ith Direct Coupling

3 obtin X,. The ir-gp voltge E1 is determined from the mgnetiztion curve which is plot of E1 versus Xm n thispper, theinduction mchineusedis3-phse, 4-pole, 60 Hz, 380 V, 1.0 k, str-connected, squirrel-cge whoseper phse equivlent circuit prmeters in per unit re s follows: R =O.l,X1=0.2,Rz=O.06,X2=0.2.Theirgpvolt- h 0.7- ge E1 is expressed in terms of X, by piecewise linerized eqution of the form: G c=1.5 P.U., R=2 p.u. " c-2.0 P.U.. RE2 p.u. C=1.5 P.u., &=3 P.U. 0.5 E1 = xm Xm<2 El = Xm 25Xm E1 = 0 Xm A stble operting point exists, provided tht X, is less thn the unsturted vlue ( 3 P.u.). Hving determined ll constnts including X, nd El, the equivlent circuit of Fig. 2 is completely solved for the stedy-stte performnce of the induction genertor. The electricl torque is given by 31z2Rz TG = - f-v (10) Fig. 4 Frequency Versus Speed Becuse the SEG is isolted, its sttor frequency is free to vry with the rotor speed. This is indicted by Fig. 4 which shows tht the vlues of the exciting cpcitor nd lod resistnce hve negligible effects on the system frequency. where E1 2 = Rz/(f - v) + jxz (11) =i w C=1.5 P.u., &=2 p.u. c g P.U.. k=2 p.u. c-1.5 P.U.. &-3 p.u. > ti w C=1.5 P.u., k=2 P.U meee c02.0 P.U.. &=2 p.u. * C=1.5 p.. k=3 p.u Fig. 5 Electricl Torque Versus Speed Fig. 3 Air-Gp Voltge Versus Speed The electricl torque increses with speed s shown in Fig. 5. This reltionship is of the form T = kwn, k is constnt, ndn % 3 (12) Figs. 3 is plots of the ir-gp voltge, which shows n increses in voltge with increse in speed. Strting with smll vlue of the lod resistnce RL, the ir-gp voltge first increses rpidly nd then slowly s the speed increses. The electricl torque increses s the vlue of the exciting cpcitor is incresed, but it lso increses the negtive slip t the sme speed ( Fig. 6 ) which degrdes efficiency of opertion.

4 158 3 v) C=1.5 p.~., &=2 P.U. P c-2.0 p, &=2 p. Cz1.5 P.u., k=3 P.U. C MOTOR CURRENT (A) Fig. 6 Negtive Slip Versus Speed The negtive slip vrition with speed is shown in Fig. 6. The slip remins smll which results in high efficiency. n order to keep frequency t full lod smll, the exciting cpcitor must be kept close to the minimum requirement for self excittion. CHARACTERSTCS OF THE DC MOTOR The dc motor used in this study hve the followingprmeters: Motor type Rted voltge Rted current Rted speed Armture resistnce Mutul nductnce Armture rection ron losses Rottionl losses Permnent Mgnet 120 v A. w = rd./sec. RA = 1.9. MAF = H. Neglected. Neglected. TL = w N-m. The stedy-stte voltge, current, speed, nd torque rc given by Fig. 7 DC Motor V- Chrcteristics CHARACTERSTCS OF THE PV GENERATOR A PV genertor consists of n rry of PV modules connected in series-prllel combintions to provide the desired dc voltge nd current. The overll volt-mpere chrcteristics of the rry depend on the number of cells in series nd the number of prllel strings. The PV genertor used in this study consists of 18 prllel strings, 324 cells in series per string, such tht the overll volt-mpere chrcteristic is given by PH - PV VPV = n ( )- 0.9 pv (15) where VPV nd PV re the terminl voltge nd current. The photocurrent ~ is H directlyproportionl to insoltion, ~ = H 14.4 A., t 100 % insoltion ( 1 k/m2 ). Fig 8 is plot of eqution (15) for vrious insoltion levels. where TG is the electricl torque of the induction genertor, nd TL ccounts for totl system rottionl losses. The torque pplied to the shft of the dc motor is equl to the electricl torque of the induction genertor plus the torque necessry to overcome rottionl losses. The electricl torque of induction genertor s function of shft speed is shown in Fig. 5. The corresponding dc motor current nd voltge re evluted t ech speed. Fig. 7 is plot of the volt-mpere chrcteristics of the dc motor. L oo N 0, o 0 p r(l P 0 ; 1 O Gen. Current (A) Fig. 8 V- Chrcteristic of the PV Genertor

5 The locus of ll mximum power points is indicted by the dshed line in Fig. 8. For mximum utiliztion efficiency of the PV genertor, the operting point must follow this locus s the insoltion vries. Most lods, however, hve their own V- chrcteristics nd the common operting point my follow different locus which py be fr wy from the mximum power locus. n this cse mtching is necessry to move the operting line s close s possible to the mximum power locus. 159 The shft speed increses with insoltion, s shown in Fig. 10, which lso shows the effect of the exciting cpcitor C, nd the lod resistnce R; on the system speed s the insoltion vries e O" 30 h o l e n fn PV Gen. & Motor Current (A) Fig. 9 Common V- Chrcteristics ith direct coupling of the dc motor to the PV genertor, voltge nd current of the two devices re equl, hence the equilibrium operting point is given by the intersection of the two V - chrcteristics s shown in Fig. 9. The operting line is close to the mximum power locus line t lmost ll insoltion levels. n order to void dditionl costs, no mtching devices or pek-power trckers re necessry. t is lso noted tht the operting line pproches the mximum power line s the insoltion increses SOLAR NSOLATON P.U * U LL c=1.5 P.U.. RL=2 p.u. OS6O 1 - c=2.0 P.U.. h=2 p.u. Cx1.5 P.u.. R~z3 P.U. 0.50, SOLAR NSOLATON P.U Fig. 11 Air-Gp Voltge Versus nsoltion Fig. 11 is plot of the ir-gp voltge vrition with insoltion. The voltge builds up rpidly s the insoltion increses form low vlues nd then slowly s the insoltion reches higher levels. t is lso seen from Fig. 11 tht the rte of the ir-gp build up is lmost independent of C nd RL. C=15 P.u., RL=2 P.U. h=2 p.u. C=1.5 P.u., &=3 P.U. "e Cd.0 P.U., SOLAR NSOLATON P.U. Fig. 10 Speed Versus nsoltion. Fig. 12 Frequency Versus nsoltion

6 n B w e J v) $! w Z kl w -J SOLAR NSOLATON P.U. Fig. 13 Slip Versus nsoltion P - C-1.5 PA.. k=2 P.U. 1992). c-2.0 P.U., k=2 p.u. C=1.5 P.u., h=3 p.~. dc motor, or both. n mny developing countries wind energy nd solr energy re complementry to ech other over the nnul cycle. The lres cn be designed to tke dvntge of this inherent diversity to improve power qudity nd minimize energy storge requirements. REJ3ERENCES [ 1 f R. Rmhmr, Renewble energy sources nd developing countries, EEE Trns. Power Apprtus Syst., vol. PAS-102, no. 2, pp ,Feb : 21 S.S. Murthy, O.P. Mlik, nd A.K Tndon, Anlysis of self-excited induction genertors, EE Proceedings, vol. 129, Pt. C, no. 6, pp ,Nov [ 3 ] L. OuzenendG. McPherson, Jr., Anlysisof theisolted induction genertor, EEE Trns. Power Apprtus Syst., vol. PAS-102, no. 8,pp , Aug [ 4 ] N.H. ik nd S.E. Hgue, Stedy stte nlysis nd performnce of n isolted self-excited induction genertor, EEE Trns. on Energy Conversion, vol. EC-1, no. 3, pp , Sep [ 5 ] N.H. Mlik nd A.N. A-Bhrni, nfluence of the terminl cpcitor on the performnce chrcteristics of self excited induction genertor, EE Proceedings, vol. 137, Pt. C, no. 2,pp ,Mr [ 6 ] A.K. Al Jbri nd A.. Alolh, Limits on the performnce of of three-phse induction genertors, ZEEE Trns. on Energy Conversion, vol. EC-5, no. 2, pp , June [ 7 1 T.F. Chn, Cpcitnce requirements of self-excited in duction genertors, EEE Trns. on Energy Conversion,

7 161 [ 121 Z. Zinger, nd A. Brunstein, Dynmic mtching of solr-electricl ( photovoltic ) system - An estimtion of the minimum requirements on the mtching system, EEE Trns. Power Apprtus nd System, vol. PAS-100, no. 3,pp ,M~~h [ 13 ] S. M. Alghuwinem, Appliction of dc chopper to mximize utiliztion of solr-cell genertors, Pper 91 M EC, 1991 EEEPES 1991 inter Meeting, New York, New York, Feb. 3-7,1991. [ 14 ] M. M. Sied, A. A. Hnfy, et l., Optiml design prmeters for PV rry coupled to dc motor vi dc-dc trnsformer, EEE Trns. on Energy Conversion, vol. EC-6, no. 4, pp, , December [ 15 ] S. M. Alghuwinem, Stedy-stte performnce of dc motors supplied from photovoltic genertors with step-up converter, EEE Trns. on Energy Conversion, vol. EC-7, no. 2, pp , June [ 16 ] S. M. Alghuwinem, Mtching of dc motor to photovoltic genertor using step-up converter with currentlocked loop, Pper 93 SM EC, presented t the ZEEEPES 1993 Summer Meeting, Vncouver, B.C., Cnd, July 18-22,1993. Sd M. Alghuwinem received B.Sc. in Electricl Engineering from University of Riydh in From 1974 to 1976 he worked s teching ssistnt in the EE Deprtment, Riydh University. He received M.Sc. degree in Electricl Engineering ( Power ) from University of Colordo, Boulder in From 1979 to 1981 he worked in the EE Deprtment, King Sud University s reserch ssistnt. From 1982 to 1986 he ttended The University of Michign, Ann Arbor where he received Ph.D. in Electricl Engineering. Since 1986 he hs been with the Deprtment of Electricl Engineering, King Sud University, where he is currently n ssocite professor. His interests include renewble energy sources, energy conversion systems, power system protection, nd electromgnetic trnsients.

CATALYST GROUP) B"sic Mech"nic"l Engineering

CATALYST GROUP) Bsic Mechnicl Engineering CATALYST GROUP) B"sic Mech"nic"l Engineering 1. In " split ph"se motor, the running winding should h"ve (") high resist"nce "nd low induct"nce (b) low resist"nce "nd high induct"nce (c) high resist"nce