Power Sharing Strategy for Photovoltaic based Distributed Generators Operating in Parallel

Transcription

1 ower harng trategy for hotooltac based Dstrbuted Generators Operatng n arallel Ur N. atel and Hren H. atel Abstract In many countres, the grd-code or standards do not allow the hotooltac (V) nerters to exchange reacte power wth the grd. Recently, some countres hae relaxed the standards. Hence, capacty of the nerters to control reacte power must be utlzed. Howeer, the reacte power that a V nerter can supply s constraned by the maxmum power that a V array generates and changes wth the enronmental condtons. A reacte power sharng algorthm s proposed that not only ensures proper dstrbuton of reacte power amongst the nerters, but also ensures that the maxmum power generated by V s suppled to the grd. In case of dentcal V nerters, the algorthm operates all nerters at nearly equal apparent power leadng to nearly equal percentage utlzaton of the nerters, thereby acheng unform heatng of the smlar deces of the nerters. The algorthms are further nestgated for power sharng amongst V nerters of unequal ratngs. It s hghlghted that the proposed algorthm results nto the least change n the utlzaton factor of a V nerter, whose power changes due to the change n enronmental condtons. The effecteness of the algorthm oer other algorthms n sharng power amongst nerters s dsplayed through MATLAB/mulnk smulatons. Keywords hotooltac, Reacte power, ower sharng. I. INTRODUCTION Last couple of decades hae experenced sgnfcant rse n the electrcty generaton from non-conentonal energy sources lke wnd and solar. It s attrbuted manly by the ncreased enronmental concern, fast depleton of conentonal energy sources, ncrease n cost of conentonal energy sources, and decrease n the cost of renewable based energy generaton. In recent years, one of the renewable sources that has seen the fastest growth and penetraton n the electrcal grd s the solar photooltac (V). The reason for the ncrease n penetraton s the reduced cost of V system and the encouragng feed-n-tarff polces by the goernments. Howeer, ncreased penetraton of V sources has also gen rse to seeral challenges. The challenges are manly due to the dependence of V source s performance on the enronment, whch makes t ntermttent and uncertan n nature. V source s connected to the grd through the statc power conerters [1]. Thus, t s nerta-less source of energy unlke the conentonal rotatonal generators. Hence, f the energy generaton n the grd s hghly domnated by nerta-less V (.e. n a weak grd), the sudden change n output power of V resultng from the sudden change n rradaton, may affect the stablty of grd and the systems connected wth the utlty. Also, f the power electronc U. N. atel s wth Department of Electrcal Engneerng, C. K. thawala College of Engneerng and Technology, urat, Inda. (emal: ur.patel@ckpcet.ac.n). conerters are not controlled approprately n such weak electrcal grd or a mcrogrd (MG), they are lkely to create ssues lke harmonc njecton, change n oltage leels and power flow, flcker, resonance, mal-operaton of protecton scheme etc. On the contrary, f the power electronc conerters are properly controlled []-[], they can mproe the oltage profle and performance of the MG. Ths can be acheed f V systems, whch are usually commssoned to supply acte power, are allowed to nject desred reacte power nto the grd. V systems are usually desgned wth reasonable margns, and most of the tmes operate under lghtly loaded condtons (n fact nacte at nght tme). Thus, there s a room for reacte power njecton to keep the oltage at a desrable leel. Ths objecte, along wth the transfer of maxmum power generated by V, can be acheed by controllng the ampltude and phase angle of the output oltage of the nerter. The task becomes challengng when seeral such V based dstrbuted energy generators are operatng n a MG, whch een comprses of other types of renewable energy sources. V nerters are commonly controlled as current controlled source usng - control strategy to exchange acte and reacte and respectely, wth the mcrogrd [4]. In slandng mode.e. when man grd s dsconnected, the oltage V and frequency ω are controlled, usng -ω and -V droop control methods to share acte and reacte power amongst the dstrbuted generators (DG) [5]-[7]. Battery storage s essental n such system when slanded, n order to mantan power balance n the system. Lasseter et al., hae presented flexble control and proper coordnaton amongst DG sources to oercome some problems assocated wth V and other non-conentonal sources operatng smultaneously n a MG []. Local power management system for coordnaton of arous DG sources to manage acte and reacte power successfully s addressed n [8]-[1]. In [8], fundamental algorthm employng herarchcal droop control of power management s presented, where nerter control s consdered as prmary control whereas Mcrogrd Central Controller (MGCC) s under secondary control. econdary control focuses on power management and optmzaton algorthm to optmze performance of MG. ower management system plays ery mportant role when MG s hang many V connected nerters, as rapdly aryng rradaton condton may cause oltage sags and swells that result n degradaton of power qualty [16]-[]. To regulate oltage under such transent condton, V nerters must hae H. H. atel s wth the Department of Electrcal Engneerng, arajank College of Engneerng & Technology, urat, Inda. (e-mal: hren.patel@scet.ac.n). TRANACTION ON ENVIRONMENT AND ELECTRICAL ENGINEERING IN Vol 1, No 4 (16) Ur N. atel and Hren H. atel

2 the capablty to match-up the VAR requrement quckly [11]. As acte power delered by nerter depends on maxmum power that V can generate under gen (enronmental) condtons, t s necessary to allocate reacte power amongst nerters n a proper way to hae unform loadng of the nerters and to also aod oer loadng of nerters [1]. An accurate reacte power sharng control that shares reacte power equally amongst nerters s presented [1]. Total reacte power of the system s calculated by MGCC and the nformaton s passed to all nerters through communcaton lnk. Though ths method shares reacte power accurately amongst the nerters, n case when acte power ares wth the change n rradaton, t fals to accurately share the reacte power amongst the V nerters. It may also cause nerter to work beyond ts nomnal apparent power transfer capablty. In [14], reacte power algorthm s presented whch takes nto account apparent power lmt of each V connected nerter as well as acte power delered by each V nerter. Optmal reacte power strategy [15] assgns reacte power to each nerter such that entre system can achee maxmum reacte power transfer capablty. Howeer, these algorthms are unable to unformly utlze apparent power capablty of each nerter. The paper proposes an approach to oercome these drawbacks. The proposed reacte power algorthm frst determnes the acte power that V nerters are supplyng under gen condtons and based on the aalable margn t assgns the reacte powers to the nerters. ecton II ntroduces system confguraton and control scheme employed for operatng V nerters whle the secondary control algorthm mplemented n MGCC for accurate reacte power sharng s presented n secton III. The results of the smulaton study performed n MATLAB/mulnk are ncluded n secton IV to demonstrate the performance of algorthm for V nerters operatng n parallel for two dfferent cases: () all nerters wth equal ratngs and () nerters wth unequal ratngs. II. YTEM DECRITION AND CONTROL Fg.1 shows the system confguraton consdered for ealuaton of the proposed algorthm. The mcrogrd comprses of four dentcal dstrbuted energy generators that along wth the man grd (or a relately stff source) supply the local loads. Each DG unt conssts of V as a prmary energy source, a three phase nerter and an LC flter. The nerters not only extract the maxmum power from the V but also supply snusodal current to the load and grd. V shown n Fg. 1 represents a V array wth ts dc-dc conerter operated wth maxmum power pont trackng control. The DGs are connected to the CC through a transformer, whch for the sake of smplcty, s not shown n Fg.1. 1=1+j1 MGCC Z 1 V1 =+j A.c Grd tatc wtch =+j CC Z Z 4=4+j4 Z 4 V V V 4 Fg.1. ystem confguraton of a Mcrogrd hang four DGs The mpedances Z o, where represents th DG, takes nto account the mpedance of nterfacng nductor, the mpedance of cable and solaton transformer. Acte and reacte power management task s performed by MGCC unt usng low bandwdth communcaton lnks. Mcrogrd herarchcal structure conssts of manly prmary, secondary and tertary control [1], [11]. rmary control coers nerters control present n mcrogrd whereas secondary control conssts of MGCC unt. Tertary control prodes nteracton between multple mcrogrd and utlty grd. rmary and secondary controls are used n ths paper whle tertary control s not requred at ths stage. Inerter control s acheed by acte-reacte power (-) control method [4]. - method s used to operate nerter as a controlled current source for desred acte and reacte power transfer wth grd. Inerter output current s tghtly regulated by nner current control loop. Reference currents for current control loop are proded by outer power control loop accordng to power references proded by MGCC. hase locked loop (LL) used for grd synchronzaton prodes desred angle (ρ) for abc to dq frame transformaton. Fg. shows control crcut dagram for one of the nerters. Fg. shows the detals of the power and current control loops shown n Fg.. The oltage V dc, across capactor C s mantaned at a desred oltage, V dcref by a oltage control loop. V In-1 CC abc R L + C Vsabc V dc - C f WM abc/ ρ ρ abc/ Gate dq dq Dre ρ ower Control Loop ref dref qref ref Current Control Loop DC Bus Voltage Control dq V dcref V dc V V dq ω Fg.. Control scheme of V nerter ρ LL L g =tatc wtch Load Grd

3 V dcref V dc + I + - DC-Bus Voltage control V Fg.. Acte-reacte power control ref +_ ref + _ I ower control dref d qref +_ Lω Lω V d + + _ / m d I + _ I + + / q I V q V DC Current control loop To mantan ths oltage constant t s ensured that the power obtaned from V array, V s entrely transferred to the grd sde. Ths s done through the power control loop, whch compares actual DG output power () wth reference power ( ref). The reacte power reference ( ref) s obtaned usng the algorthm presented n the next secton. ref and ref are used to generate requred current references dref and qref for the current control loop. The drect and quadrature axes components of the nerter output currents d and q, respectely, are obtaned through d-q transformaton. The current control loop fnally determnes the drect and quadrature components of the reference waeform from the drect and quadrature axes modulaton ndces, m d and m q, respectely. III. ROOED REACTIVE OWER HARING ALGORITHM As the acte power that V nerters supply s drectly dependent on the enronmental condtons (manly rradaton), most of the tmes the nerters do not operate at ther ratng and hence, ther capacty s not utlzed fully. The aalable margn ares wth the rradaton, wth maxmum at nght or when rradaton s the least. The reacte power sharng algorthm shown n Fg. 4 reles on assgnng the reacte power algorthm amongst the nerters based on the margn aalable wth each of them. The algorthm starts wth ntalzng the number of nerters (m) and the apparent ratngs of the nerters ( N), where stands for th nerter. The output power of the V systems ( ) s obtaned from the maxmum power pont tracker (MT), whch ensures that the V system operates at ts maxmum (acte) power pont. As the apparent power ratngs ( N) of the nerters are known and as the nerter must be operated to deler acte power ( ) to the grd sde, the aalable reacte power ( ) s expressed as (1) N The nerter s capable of supplyng and drawng reacte power and t must match the load and grd requrements. Accordngly () and (), assgns the reacte power lmts for laggng and leadng type of reacte demand, respectely. max () max () Hence, at a gen nstant, the total acte power ( T), reacte power ( T) and apparent power ( T) capabltes that the m q nerters possess to match the reacte power demand of load and to supply the acte power of V systems to grd are represented by (4), (5) and (6), respectely. m T 1 m T 1 T T T (6) If output currents of all the nerters are equal, temperature of smlar deces of the dfferent nerters can be made equal. Ths can be realzed f all the nerters operate wth the same apparent power. Hence, the nerters are made to operate wth the reference apparent power ( Tnew) to hae unform utlzaton and heatng. [( m 1) ] Tnew T (7) The algorthm ealuates the condton expressed by (8), and f Tnew exceeds N, the reference apparent and reacte powers are set to alues N and max (or mn), respectely. Tnew N The algorthm then assgns the reference reacte power ref and for each nerter, where the acte power references ( ) for the nerters are obtaned from the MT. Once any nerter s assgned the reference acte and reacte powers, the total unassgned acte and reacte powers to be suppled by the remanng nerters are updated by subtractng the ref and assgned to the earler nerters from T and D, where D s the reacte power demand of the load. The remanng acte power ( Tn) to be suppled and reacte power demand to be met ( Tn) s calculated as shown n (9), and (1), respectely. 1 Tn T 1 Tn D ref (4) (5) (8) where (9) where (1) ref Accordngly, the apparent power ( ) that th nerter must supply s obtaned by (11) / ( m 1) (11) Tn Tn Hence, the reference reacte power for the th nerter s (1) ref IV. IMULATION REULT To demonstrate the effecteness of the aboe control strategy, mcrogrd system shown n Fg.1 s smulated n MATLAB/mulnk. In addton to the proposed control algorthm, two more control approaches: (optmal reacte power [15] and equal reacte power sharng [1]) are also ealuated and the results are compared wth that obtaned wth the proposed control algorthm. Two dfferent cases are consdered for comparng the performance of ths algorthm.

4 tart Intalze N =Nom. Apparent power m=no. of V nerters ref Measure D =Reacte power demand NO =+1 Calculate T & T usng(4) and (5) respectely T Tnew =1 Measure =Acte power of th nerter B N T T Is Tnew N? Is =m? YE T =1 [( m1) ] A YE NO Calculate Tn, Tn usng (9) and (1) ref Fg. 4. roposed reacte power sharng algorthm C Calculate usng (11) A =+1 Is =m? END new N ref C YE NO In case (), all the nerters are consdered to hae the equal ratngs whle n case (), nerters of unequal ratngs are consdered. Case (): Equal DG ratngs The parameters consdered for ealuatng the performance of the algorthms usng the system of Fg. 1 are mentoned n TABLE I. As shown, all DGs are consdered to hae the equal nomnal apparent power ratng of 5 kva. TABLE I RATING AND ARAMETER FOR THE YTEM OF FIG.1 Nomnal power ratng of DG1 ( 1N) Nomnal power ratng of DG ( N) Nomnal power ratng of DG ( N) Nomnal power ratng of DG4 ( 4N) Grd oltage(v g), Frequency(f) Lne parameter (Z 1=Z =Z =Z 4) 5 kva 5 kva 5 kva 5 kva B 415V, 5 Hz L=1µH,R=.7mΩ,C f=5µf Load 1.9 MVA,.78 power factor (lag) No of V nerters (m) 4 Fg. 5 shows the results wth the optmal reacte power sharng (OR) algorthm. Reacte power references for nerters shown n TABLE II are calculated usng the OR algorthm of [15], whle the acte power references for the nerters are set at the alue equal to the maxmum power that the correspondng V system generates at a gen nstant. The acte power generated by V arrays V 1, V, V and V 4 tll t=.5s are 4kW, kw, 5kW and 45kW, respectely. A step change n rradaton on V array V 1 occurs at t=.5s, whch results n the output of V 1 to decrease to kw. At t=1s, step change n rradaton on V array V occurs, resultng nto the change n the output power from 5kW to 4kW. Reacte power references for the nerters obtaned wth optmum reacte power control are mentoned n TABLE II. Fg. 5, shows acte, reacte and apparent power of nerters 1 through Tme (s) Fg. 5. Results wth OR algorthm: acte power fed by DGs, reacte power shared by the nerters, apparent power of each nerter. TABLE II UTILIZATION FACTOR OF EACH DG FOR OR ALGORITHM Ut. Fac. Tme Interal (s) t=-.5 t=.5-1 t=1- ref / N

5 It s obsered from Fgs. 5 and that, when 1 s decreased from 4kW to kw at t=.5s, 1 changes from kvar to 6kVAR. Not only 1, but through 4 also changes. mlarly at t=1s, when ncreases to 4kW, 1 through changes. Thus, f power generated by any one of the V array changes, the reacte power references and hence, the reacte power suppled by all the nerters change (except those whch are operatng at ther lmts N). Fg. 5 shows that nerters 1 and 4 operate at ther maxmum apparent power lmts ( 1N and 4N, respectely) tll t=.5s. At t=.5s, when 1 reduces, 1 also reduces smultaneously and hence, from t=.5s to t=.1s only nerter 4 operates at ts full capacty. It s obsered that the change n and s such that the rato / remans equal for all the nerters that do not reach the rated capacty. An ndex defned as utlzaton factor ( / N) s used to ndcate the extent to whch the capacty of the nerter s utlzed. It s also obsered from the TABLE II that all the nerters are operatng at dfferent utlzaton factors. The utlzaton factors ary greatly showng that some of the nerters operate much below ther rated capacty when some others hae already ht ther lmts. For example, nerter-1 operates wth the lowest utlzaton factor (.65 from t=.5s tll 1s and.69 from t=1s tll s) whle nerter-4 s operatng at ts lmt. The unequal utlzaton of the nerters, not only results nto unequal losses, effcency and heatng of dfferent nerters, but may damage the nerters that contnuously operate at ther apparent power lmts. Fg. 6 shows the results obtaned wth equal reacte power sharng (ER) algorthm [1], accordng to whch reacte power demand s equally shared amongst the nerters. The Fg. 6. Results wth ER algorthm: acte power fed by DGs, reacte power shared by the nerters, apparent power of each nerter Tme (s) rradaton pattern on the V array s consdered the same as that consdered for the ealuaton of OR approach. Fg Tme (s) Fg. 7. Results wth roposed algorthm: acte power fed by DGs, eacte power shared by the nerters, apparent power of each nerter shows acte, reacte and apparent powers respectely, of nerters 1 through 4. If t s ntended to meet the total reacte power demand of the load mentoned n TABLE I (1kVAR) through the nerters 1 through 4 usng ER control, each nerter must output kvar. Hence, the reference reacte power for nerter 1, and are set equal to kvar (reacte power demand of load = 1kVAR) whle for nerter- 4 whch hts ts apparent power lmt, t s restrcted to 18 kvar. It s obsered from Fgs. 6-, and TABLE III that, een f the acte power suppled by the V array changes, the effect s not obsered n the reacte power sharng. It s also edent from Fg. 6 that nerter-4 contnuously operates at ts rated capacty of 5kVA. Inerters 1 and also operate at ther rated capactes for some tme. It s also obsered that (for =1, and 4) remans almost constant for t=.5s to s nspte of the change n at t=1s. The reason beng no change n and (for =1, and 4) for ths perod. Unlke OR the reacte power demand of the load s not met fully nspte of the fact that many nerters stll operate below ther rated lmts. Thus, the nerters are not utlzed optmally and also the percentage utlzaton of all the nerters ares greatly. Fg.7 shows performance wth proposed algorthm when same pattern of rradaton on the V array as that consdered for OR and ER s mantaned. At t=.5s, when the rradaton of V 1 decreases resultng nto the decrease n the

6 TABLE III UTILIZATION FACTOR OF EACH DG FOR ER ALGORITHM Tme Interal(s) ref Ut. Fac. / N t= t= t= output power of nerter 1, the reacte power of nerter 1 ncreases. multaneously, the reacte powers of all other nerters decrease n spte of the fact that there s no change n the power output from V arrays V, V and V 4. Ths results nto mnmzng the gap of percentage utlzaton of dfferent nerters. mlarly, at t=1s when changes from 5kW to 4kW, reacte power of all the nerters changes to achee better sharng of the acte and reacte power amongst them. TABLE IV shows the acte, reacte and apparent powers shared by the nerters oer the dfferent perods. Unlke OR and ER, the utlzaton factors ary lttle for all the DGs ndcatng unform loadng of the nerters. The three algorthms are tested een wth a dfferent load hang a leadng power factor (F). TABLE V shows the results obtaned wth a load of 1.16 MVA,.86 power factor (lead). It s obsered that een wth leadng power factor, proposed algorthm performance s superor. tandard deatons of the TABLE IV UTILIZATION FACTOR OF EACH DG FOR ROOED ALGORITHM Tme Interal(s) ref Ut. Fac. / N t= t= t= TABLE V COMARION OF THE VARIOU ALGORITHM FOR LEADING F LOAD Algorthms ref Ut. Fac. / N OR ER roposed utlzaton factors of the arous nerters are calculated, to quantfy the effecteness of the algorthm to dstrbute the apparent power equally amongst the nerters. tandard deatons of the utlzaton factors for the three schemes for the case represented by TABLE V are.4,.147 and.55. The least the standard deaton better s the performance. Case (): Unequal DG ratngs The three algorthms are also ealuated for the case when all DGs of the system shown n Fg. 1 hae unequal ratngs. The nomnal ratngs for the DGs are mentoned n TABLE VI. The load, lne parameters, capactance C and the grd oltage are consdered same as that of case (). In ths case the acte power generated by V arrays V 1, V, V and V 4 are kw, kw, 4kW and 5kW, respectely. A step ncrease n rradaton on V array V 1 occurs at t=.5s, whch results n the output of V 1 to ncrease to kw. At t=1s, rradaton on V array V decreases suddenly, resultng nto the change n ts output power from 4kW to kw. The acte, reacte and apparent power sharng by nerters 1 through 4 wth OR control are dsplayed n Fg. 8 and the results are quantfed n TABLE VII. Fgs. 8 and shows that when V 1 s ncreased from kw to kw at t=.5s, 1 also ncreases from 171kVAR to 4kVAR. Hence ts apparent power ncreases, leadng to ts utlzaton factor of.96. The reacte powers of nerters through 4 decrease wth ther acte powers stll at the same alues. Thus, through 4 decrease lowerng the utlzaton of nerters through 4. Ths ncreases the mss-match n the utlzaton factors. The mss-match further ncreases after t=1s, when the output power of V decreases from 4kW to kw. The decrease n at t=1s s assocated wth the smultaneous decrease n. Hence, to meet the reacte power demand of the load, more reacte power needs to be suppled by nerters 1, and 4. Hence, whle the utlzaton factor of nerter- decreases, utlzaton factor of other nerter ncreases.thus, nerter- s the least utlzed nerter wth utlzaton factor of.45 whle nerter-1 s fully utlzed wth the utlzaton factor of 1.. Fg. 5 also hghlghts that after t=1s, nerter-1 operates at ts apparent power lmt ( 1N). It s obsered from TABLE VII that the percentage change (decrease) n utlzaton factor of nerter- n response to the decrease n output power of nerter- s -47%. Fg. 9 shows the power shared by DGs (hang ratngs mentoned n TABLE VI) when operated wth ER algorthm. The same shadng pattern, adopted earler for OR of case (), s consdered. The reference reacte power for all the nerters s set equal to kvar to meet the load s reacte power demand (TABLE VIII). Fg. 9 shows that after t=.5s, nerter-1 contnuously operates at ts rated capacty 4kVA and hence, s unable to meet ts desred reacte power share of kvar. Lke earler case wth ER control, the reacte TABLE VI Ratngs Of Dg Of The ystem Of Fg.1 For Case () Nomnal power ratng of DG1 ( 1N) 4 kva Nomnal power ratng of DG ( N) 5 kva Nomnal power ratng of DG ( N) Nomnal power ratng of DG4 ( 4N) 6 kva 7 kva

7 Tme (s) Fg. 8. Results wth OR algorthm: acte power fed by DGs, reacte power shared by the nerters, apparent power of each nerter power demand of the load s once agan not met fully. Thus, the nerters are not utlzed optmally. gnfcant araton n utlzaton factors s obsered. Also the percentage change n the utlzaton factor of nerter- due to change n at t=1s s -7.7%. The power sharng, the utlzaton factors and the araton n the utlzaton factors are hghly dependent on the nomnal ratngs of the nerters and the load. Fg. 1 shows performance of proposed algorthm wth same pattern of rradaton on the V array as consdered earler for ER and OR algorthm of case (). It s obsered from TABLE IX that durng t=s to t=.5s, the proposed algorthm tres to share the apparent power equally amongst all the nerters. Hence, as the nerter-1 reaches ts lmt, t s operated at 4kVA (1% capacty), whle nerters, and 4 are operated around 5kVA demonstratng the tendency of equalzng the reacte power sharng. At t=.5s, when the rradaton of V 1 ncreases resultng nto the ncrease n the TABLE VII UTILIZATION FACTOR OF EACH DG FOR OR ALGORITHM Tme Interal(s) ref Ut. Fac. / N t= t= t= output power of nerter 1, the output reacte power of nerter 1 decreases. multaneously the reacte powers of all other nerters ncrease. TABLE VIII UTILIZATION FACTOR OF EACH DG FOR ER ALGORITHM Tme Interal(s) t=-.5 t=.5-1 t= ref Ut. Fac. / N Tme (s) Fg.9. Results wth ER algorthm acte power fed by DGs, reacte power shared by the nerters, apparent power of each nerter Ths results nto mnmzng the mss-match n the reacte powers of the nerters and hence, reduces the gap of percentage utlzaton of dfferent nerters. Thus, the algorthm nherently has the feature of mnmzng the msmatch. But stll the msmatch s relately large. Ths s due to the equal apparent power sharng prncple of the algorthm, whch nspte of the unequal nomnal kva ratng of the nerters, tres to allocate the apparent power equally amongst the DG nerters. Hence, t results nto the unequal utlzaton factor of the DGs. At t=1s when changes from 4 kw to kw, ncreases and 4 and decrease to achee better power sharng amongst the nerters. The least utlzaton factor of.6 s obsered for nerter-. It s obsered from TABLE IX that percentage decrease n utlzaton factor for nerter- (due to

8 change n at t=1s) s -1.8%, whch s relately smaller than that obsered wth OR (-47%) and ER (-7.7%) Tme (s) Fg.1. Results wth proposed algorthm acte power fed by DGs, ) reacte power shared by the nerters, apparent power of each nerter TABLE IX UTILIZATION FACTOR OF EACH DG FOR ROOED ALGORITHM Tme Interal(s) ref Ut. Fac. / N t= t= t= V. CONCLUION In case of renewable energy source (V or wnd) based DG, the reacte power that t can supply ares as the acte power suppled by t changes. The conentonal algorthm, whch reles on the sharng of equal reacte power amongst the nerters, fals under such case. Not only the nerter gets oerloaded but also the dstrbuton of the total apparent power amongst the nerters ary greatly leadng to uneen percentage utlzaton of the nerters. The optmal reacte algorthm also suffers from smlar drawbacks. It s obsered that the proposed algorthm mantans operaton of all nerters wthn ther nomnal ratngs and yet they are able to match the total reacte power demand of the load. As the reacte power assgned to the nerters s lnked wth the aalable reacte power capabltes, the nerter that supples lesser acte power s controlled to share a greater amount of reacte power. If the DG nerters hae equal kva ratngs, then wth the proposed algorthm, not only the apparent power sharng s better than other algorthms but the utlzaton factors of the nerters are also nearly smlar. Howeer, as the algorthm tres to share the apparent power equally amongst the nerters, the utlzaton factors are not the same for the nerters of unequal kva ratngs. But the algorthm always operates to mnmze the mss-match n the utlzaton factors. Hence, wth the proposed approach, comparately better apparent power sharng s obsered leadng to reducton n the araton of percentage utlzaton of the nerters. REFERENCE [1] Y. Huang, F. Z. eng, J. Wang and D. Yoo urey of power condtonng system for V power generaton, ower Electroncs pecalst Conference, EC 6, pp.1-6, June 6. [] Y. Rffonneau,. Bacha, F. Barruel, and. lox Optmal power flow management for grd connected V systems wth batteres IEEE Trans. ustanable energy, ol., no., pp.9- July 11. [] R. H. Lasseter and. ag, Mcrogrd: A conceptual soluton, presented at the IEEE ower Electron. pec. Conf. Aachen, Germany, 4. [4] N. Eghtedarpour and E. Farjah, ower control and management na hybrd AC/DC mcrogrd, IEEE Trans. mart Grd, ol. 5, no.,pp , May 14. [5] C.T. Lee, C.C. Chu, and.t. Cheng, A new droop control method for the autonomous operaton of dstrbuted energy resource s nterface conerters, IEEE Trans. ower Electron., ol. 8, no. 4, pp , Aprl 1. [6]. Zhong, Robust Droop Controller for Accurate roportonal Load harng Among Inerters Operated n arallel, IEEE Trans. Ind. Electroncs, ol. 6, no. 4, pp , Aprl 1. [7] I. U. Nutkan,. C. Loh, and F. Blaabjerg, Droop scheme wth consderng of operatng cost, IEEE Trans. ower Electron., ol. 9, no.,pp , March 14. [8] A. Bdram and A. Daoud, Herarchcal structure of mcrogrds control system, IEEE Trans. mart Grd, ol., no. 4, pp , December 1. [9] J. He, and Y L An accurate reacte power sharng control strategy for DG unts n a mcrogrd 8th Internatonal Conference on ower Electroncs - ECCE Asa,the hlla Jeju, Korea. May - June, 11. [1] A. Mlczarek, M. Malnowsk and J. M. Guerrero Reacte power management n slanded mcrogrd proportonal power sharng n herarchcal droop control IEEE Trans. mart grd, ol. 6, no. 4, pp , July 15. [11] K. Turtsyn, etrˇulc,. Backhaus, and M. Chertko Optons for control of reacte power by dstrbuted photooltac generators roceedngs of the IEEE, ol. 99, No. 6,pp 16-17, June 11. [1] F. Oler,. Arstdou, D. Ernst, and T. V. Cutsem, Acte management of low-oltage networks for mtgatng oer oltages due to photooltac unts IEEE Trans. mart grd, ol. 7, NO., pp. 96-9, March 16. [1] A. Mcallef, M. Apap, J. M. Guerrero, and J. C. Vasquez, Reacte power sharng and oltage harmonc dstorton compensaton of droop controlled sngle phase slanded mcrogrds IEEE Trans. mart grd, ol. 5, no., pp , May 14. [14]. Duan, Y. Meng, J. Xong, Y. Kang and J. Chen. arallel operatonc techqnque of oltage source nerters n U, IEEE Internatonal Conference on ower Electroncs and Dre ystems,, Hong Kong, , ED 99. [15] Z.Wang. K.M.,assno, J.Wang, Optmal reacte power allocaton n large scale grd connected photooltac system, Journal of optmzaton theory and applcaton, Noember 15. [16] A. Mcallef, M. Apap and C. ptertanes, J. M. Guerrero Zapata econdary control for reacte power sharng and oltage ampltude restoraton n droop-controlled slanded mcrogrds rd IEEE Internatonal ymposum on ower Electroncs for Dstrbuted Generaton ystems (EDG) 1.

9 [17] T. L. Vandoorn, B. Renders, L. Degroote, B. Meersman, and L. Vandeelde, Controllable harmonc current sharng n slanded mcrogrds: DG unts wth programmable resste behaor toward harmoncs, IEEE Trans. ower Del., ol. 7, no., pp ,July 1. [18] Adhkar and Fangxng L, Coordnated V-f and - control of solar photooltac generators wth MT and battery storage n mcrogrds IEEE trans. on smart grd, ol. 5, no.,pp ,May 14. [19] H. Mahmood,, D. Mchaelson, and J. Jang, Accurate reacte power sharng n an slanded mcrogrd usng adapte rtual mpedances IEEE Tran. ower electron.ol., no., pp ,March 15. [] J. W. mth, W. underman, R. Dugan, Bran eal, mart nerter Volt/Var control functons for hgh penetraton of V on dstrbuton ystems ower system conference and Exposton (ACE) 11.. Ur atel receed B.E degree n electrcal engneerng from the.v. Regonal College of Engneerng and Technology (now.v. Natonal Insttute of Technology), outh Gujarat Unersty, urat, Inda, n, and the M.E. degree n Electrcal engneerng n 9 from the M.. Unersty, Baroda, Inda he s currently workng as an Assstant rofessor n the Department of Electrcal Engneerng at C. K. thawalla College of Engneerng and Technology, urat and pursung h.d. n electrcal engneerng. Her current research nterests nclude dstrbuted generaton, renewable energy and mcrogrd ssues. he s a Lfe Member of the Indan ocety for Techncal Educaton and a member of IEEE. Hren atel receed the B.E. degree n electrcal engneerng from the.v. Regonal College of Engneerng and Technology (now.v. Natonal Insttute of Technology), outh Gujarat Unersty, urat, Inda, n 1996, and M. Tech. n energy systems and hd n Electrcal Engneerng degree from the Indan Insttute of Technology Bombay (IITB), Mumba, Inda n and 9, respectely. He s workng as a rofessor and Dean, R&D at the arajank College of Engneerng and Technology, urat. Hs current research nterests nclude computer aded smulaton technques, dstrbuted generaton, and renewable energy, especally energy extracton from photooltac arrays. He has to hs name seeral publcatons n nternatonal and natonal journals and conferences. He s a Lfe Member of the Indan ocety for Techncal Educaton and the Insttute of Engneers.