CHAPTER 6 DESIGN OF THE PV-UPQC SYSTEM FOR LONG VOLTAGE INTERRUPTION COMPENSATION

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1 125 CHAPTER 6 DESIGN OF THE PV-UPQC SYSTEM FOR LONG VOLTAGE INTERRUPTION COMPENSATION INTRODUCTION Cntralzd powr gnraton systms ar facng th dual constrants of shortag of fossl ful and th nd to rduc hazardous nronmnt mpact. Long transmsson systms ar on of th prm causs for lctrcal powr losss. Thrfor, much mphass s gn to DG ntworks wth ntgraton of rnwabl nrgy systms nto th grd, whch ntually lad to nrgy ffcncy and rducton n hazardous nronmnt mpact. Wth th ncras of th Rnwabl Enrgy (RE) pntraton n to th grd, powr qualty of th mdum to low oltag powr dstrbuton systm s bcomng a major ara of ntrst. Majorty of th cumulat work don n rnwabl nrgy systms was prsntd by Hassmann t al (1993) and Marcar t al (2003). Rnwabl nrgy sourcs such as sunlght, wnd, flowng watr, and bomass offrs promsng clan and abundant nrgy. Among th RE sourcs, solar nrgy s th most accptabl opton n practc. Ths usful nrgy s suppld n th form of DC powr from solar PV array. Th DC powr s conrtd nto mor connnt AC powr usng an nrtr systm. It was th concpt of Han t al (2006) that th UPQC has th promnnt capablty of mprong th qualty of oltag and currnt at th pont of nstallaton on powr dstrbuton systms or ndustral powr systms. Thrfor, UPQC can b consdrd as th most promsng soluton to th powr qualty problm. Howr, t cannot supply larg act

2 126 powr to customrs stadly du to th lmtaton of powr storag n th DC lnk. Th UPQC cannot compnsat for th long oltag ntrrupton bcaus t has no nrgy storag n th dc lnk. Th abo lmtaton can b aodd usng solar nrgy (PV array). Th adantag of th proposd PV-UPQC s that t can b usd to compnsat for th long oltag ntrrupton and t s sutabl for act powr supply. Th dsgn of combnd opraton of UPQC and PV array s proposd. Th proposd systm as shown n fgur 6.1 s composd of srs and shunt nrtrs and PV array connctd to DC lnk by DC-DC conrtr. Fgur 6.1 Proposd PV-UPQC control systms

3 127 Th proposd systm wll b abl to compnsat for long oltag ntrruptons. Also t can njct th act powr n to th PCC, n addton to ts ablty to mpro of powr qualty n th PCC. Thr ar two mods of opraton of th PV-UPQC systm: Intrconnctd mod- In ths mod, th PV array transfrs powr to load and sourc. Islandng mod- In ths mod, th sourc oltag s ntrruptd and th PV array suppls th load powr sparatly. Th Islandng mod wll b consdrd whn a thr phas fault occurs at th sourc sd, rsultng load oltag of PCC as zro and ntat xtrnal supply of powr to load from PV array. Th proposd controllr dsgnng of PV-UPQC s composd of two controllrs. Thy ar shunt nrtr controllr and PV array DC-DC conrtr controllr. 6.1 INTERRUPTION MODE OPERATION Th ntr procdur usd n PV-UPQC opraton n ntrrupton mod s shown n fgur 6.1. Th powr crcut s modld as a thr-phas systm. Th control modl s dsgnd usng control blocks of Smulnk softwar. Th oltag dsturbanc such as sag and swll can b compnsatd wthout th nrgy storag lmnt as dscrbd by Han t al (2006). Howr durng ntrrupton, th supply oltag s zro whch nds xtrnal sourc for compnsaton of ntrrupton. PV array act as an xtrnal sourc and supply th act powr durng ntrrupton tm. Whnr th fault occurs n sourc sd th sourc oltag s zro n that nstant th shunt nrtr control systm chang or to paralll mod opraton to slandng mod opraton. Th srs nrtr s swtchd off and shunt nrtr starts and PV array supply th powr to

4 128 load though shunt nrtr and load oltag mantan constant up to fault clarng. Aftr fault clarng, th srs nrtr swtchs on and th shunt nrtr opratng n th normal mod. Fgur 6.2 wll hghlght th abo dscrpton. Fgur 6.2 Flow chart of PV-UPQC ntrrupton mod opraton 6.2 PV-UPQC CONTROL STRATEGY DURING INTERRUPTION MODE Th functons of th shunt nrtr ar to compnsat th currnt harmoncs and to supply th act powr to th load durng long ntrrupton. Whn th oltag ntrrupton occurs, th shunt controllr opraton s changd from normal compnsaton mod to ntrrupton mod. Th PV array prods th act powr to load and t s mantan th load oltag constant. Th nstantanous powr thory s usd to control th proposd systm. Ths schm ncluds PV nrgy rsourc to dlr PV powr to

5 129 th load and for mantanng DC lnk oltag. Th thory s basd on conrtng thr axs paramtrs nto two axs by dfnng wll-known transfr matrx. Th act and ract nstantanous powr can b dcomposd n to DC componnt and AC harmonc componnts, whch consst of ngat squnc componnt and harmonc componnt. In chaptr 3, thr phas nstantanous powr p-q thory of control systms, wth or wthout nutral conductor, s dscussd n th paralll mod opraton of th proposd systm. In ths chaptr, th proposd systm oprats n ntrrupt mod opraton. Th rfrnc currnt calculatd by usng th axs transformaton s gn by th followng quaton: = p - q control contoral (6.1) ca cb cc 0 = [C] T (6.2) whr 1/ [C] T 2 1/ 2 1/ 2 3 / 2 3 1/ 2 1/ 2 3 / 2 Th shunt nrtr of th proposd confguraton s usd for th control of DC lnk capactor and PV array. Th proposd systm stll satsfs th compnsaton dmand of load such as ngat and harmonc compnsaton, consdrng that th act powr control s th man ssu. Thn, th axs currnt rfrnc s gn by quaton (6.3). = p control q contoral p PV (6.3)

6 130 whr V,V - Transformd rfrnc oltags, - compnsatng currnts P PV - act powr dlrs to local load wth photooltac array Th PV powr dlrs to th local loads through th shunt conrtr. It should b notd that, t s dffcult to compnsat ract powr and harmonc currnt usng srs conrtr only. Ths s du to th fact that th sgnals from th conrtr output trmnals must b passd through th fltrs. In that cas, th fltr dsgn strongly dpnds on th systm paramtr lk load sz and transformr turns rato. Th shunt nrtr controls th load oltag and load currnt usng th PI controllr. Th dynamc quatons can b drd usng th qualnt crcut of shunt nrtr. Equatons (6.4) and (6.5) thy dfn th currnt control. Th control block dagram, shown n fgur 6.3, llustrats th mplmntaton of control quatons (6.6) and (6.7) drd from th shunt nrtr qualnt crcut. I K (V V ) C V I (6.4) PFp PI Tp Tp PF Tp IPFq K PI (VTq VTq ) C PFVTp I Lq (6.5) Lp V K (I I ) L I V (6.6) 1p 1q PI PI PFp PFq PFp PFq PF PFq V K (I I ) L I V (6.7) PF PFp Tp Tq whr I PFp -Fltr currnt p axs, I PFq - Fltr currnt q axs V1 p, Vl q load oltags wth p and q axs V Tp, V Tq masurd load oltags wth p and q axs kp, k I - proportonal and ntgral constant

7 131 Fgur 6.3 shunt nrtr controllr durng ntrrupton Fgur 6.4 Voltag control mod of shunt nrtr durng ntrrupton

8 PV ARRAY DC-DC CONVERTER CONTROLLER Th PV s usd to prod a DC sourc for th UPQC by njctng act powr nto a dstrbuton systm. Th DC-DC s th boost conrtr whch s dsgnd to boost th powr that s gnratd from th PV and snt to th DC lnk capactor. It conssts of nductors, dods and capactors. A contnuous oltag of th conrtr output can b obtand by connctng a larg capactor btwn th cathod and ground such that whn th capactor alu ncrass, th output oltag ncrass. Th output oltag of th conrtr must b gratr than th sourc and stp up th oltag across th DC lnk capactor by aryng th duty cycl D. Whn D=0, th mnmum output oltag bcoms th sourc oltag DESIGN OF PHOTOVOLTAIC DC-DC CONVERTER PV systms ar nonlnar powr sourcs whos output powr s dpndd upon th ffct of radaton and nronmntal tmpratur. Th dsadantag of PV systm s low ffcncy, bcaus solar clls rarly oprat at thr maxmum powr pont. So, n ordr to ncras th ffcncy, as much as possbl powr should b xtractd from th array. Th MPPT s obtand by controllng th duty cycl of DC-DC conrtr swtchs. Th PV array and DC conrtr block dagram as shown n Fgur 6.5. Th DC oltag gnratd by a photooltac array ars and s low n magntud. A stp-up DC-DC conrtr s ssntal to gnrat a rgulatd hghr DC oltag. Th DC-DC conrtr s rsponsbl for absorbng powr from th photooltac array. Thrfor, t should b dsgnd to match photooltac array rppl currnt spcfcatons whch should not conduct any ngat currnt nto th photooltac array. Th

9 133 low oltag nputs of photooltac array unts ha bn connctd to th DC bus by srs connctd boost conrtrs. Fgur 6.5 Schmatc block dagram of PV array and control Th output of DC-DC conrtr s fd to th DC-AC oltag sourc nrtr to produc th AC output for AC dstrbuton systm. Th powr flow and output oltag of photooltac array unts ha bn mantand constant by controllng th duty cycls of IGBT swtchs and th control systm of DC-DC conrtr. Th output oltag of conrtr has bn compard wth a rfrnc alu and th rror sgnal s appld to PIcontrollr. Th output sgnal of ths controllr wll act as an nput of PWM swtchng for adjustng th duty cycl. 6.4 THE PV ARRAY MAXIMUM POWER POINT TRACKER (MPPT) Th maxmum powr pont of a solar array s th pont along th P- V cur that corrsponds to th maxmum output powr possbl for th array, as shown n fgur 6.6. Th goal of MPPT algorthms s to xtract th maxmum powr from th PV array. Usually, n th condton dp/d = 0

10 134 s adoptd to locat ths opratng pont, snc PV array show a unqu global maxmum pont. Fgur 6.6 Powr Voltag charactrstc cur of PV array Fgur 6.7 Voltag Currnt charactrstc cur of PV array Th followng mthod s usd for obtanng MPPT form PV array. Prturb and Obsr (P and O) s on of th most usd MPPT mthods. Th man adantag of ths tchnqu s that th sarch for th Maxmum Powr Pont (MPP) wll b don ndpndntly of th nronmntal condtons. Th P and O algorthm calculats th output powr of th PV array. 6.5 DESIGN OF PV ARRAY PARAMETERS Th photooltac array s usd to prod a DC sourc for th UPQC by njctng act powr nto a dstrbuton systm. It ncluds nductors,

11 135 dods and capactors. A contnuous oltag of th conrtr output can b obtand by connctng a larg capactor btwn th DC lnks. A boost conrtr to charg th DC lnk capactor wth maxmum ffcncy s dsgnd and modld. Th capactor s connctd n paralll wth th PV panl to lmt th oltag. Th powr s gnratd by PV array and s snt to th DC lnk capactor. Th output oltag of th conrtr s rgulatd by aryng th duty cycl (D). (6.8) whr, Vo - output oltag of DC conrtr Vp - photooltac array oltag Th oltag across th capactor can b acclratd by aryng th duty cycl D of th conrtr. Whn D = 0, th output oltag s qual to th sourc oltag. Whn th conrtr swtch s closd, th nductor currnt rss and nrgy s stord n th nductor. Th nductor oltag s gn by d 1 VL L (6.9) dt whr V L - nductor oltag In ths stuaton, th sourc nrgy s transfrrd to th nductor and th pak-to-pak rppl nductor currnt s obtand as I V s t L Th nductor currnt n ths mod bcoms V I t I L s 1( t) 1 (6.10) (6.11) Whn th dc (IGBT) s swtchd off, th stord nrgy n th nductor s transfrrd to th capactor and th nductor currnt falls. Lt us consdr th nrgy suppld to th capactor, whch s gn by th quaton

12 C( VMAX VMIN ) L( I MAX I MIN ) (6.12) 2 2 Th capactanc s gn by LI NOM. I C V. V (6.13) NOM whr I NOM- Nomnal currnt of th boost conrtr nput V NOM- Nomnal oltag of th boost conrtr nput 6.6 DESIGN OF DC LINK CAPACITOR SIZE Capactor sz plays a sgnfcant rol n th proposd PV-UPQC as t acts as a DC sourc to prod ract powr to th load durng oltag sag and ntrrupton condton. Equaton (6.14) s usd to dtrmn th szng of th capactor (C DC ) CDC ( VC MAX VDC ) VS ILT (6.14) 2 2 From th abo quaton th capactor alu for a thr phas systm can b drd and gn as follows: C dc whr V dc V 3 V s L 2 2 C MAX V dc 3 3 V. S I T (6.15) Th prcntag of sag s calculatd by usng th formula gn blow: V pr-sag pr(pu) %Sag (6.16) V pr V sag(pu)

13 RESULTS AND DISCUSSION Usng Smulnk tools, th proposd PV-UPQC modl s aluatd and smulaton s carrd out. Th smulatd prod s 0 to 0.5s. Hr th load rfrnc oltag s consdrd as th bas oltag. Th shunt nrtr starts opraton at 100ms, whl th srs nrtr starts opraton at 200ms. In th ntrrupton mod, th thr-phas fault taks plac at 200ms. Fgur 6.8 shows th output oltag across th load wthout PV- UPQC. Fgur 6.9 shows th njctd oltag by shunt nrtr. Fgur 6.10 shows th compnsatd output oltag across th load wth PV- UPQC. Fgur 6.11 shows that th DC lnk oltag s mantand at constant alu durng ntrrupton. Fgur 6.12 dpcts th output currnt suppld by th PV array durng th oltag ntrrupton. Fgur 6.13 shows th act powr suppld by th PV array Intrrupton Tm(s) Fgur 6.8 Sourc Voltag durng ntrrupton

14 Injctd oltag by Inrtr Tm(s) Fgur 6.9 Injctd Voltag by shunt nrtr Tm(s) Fgur 6.10 Load oltags

15 139 Fgur 6.11 DC lnk Voltag durng ntrrupton Fgur 6.12 DC lnk capactor currnt durng ntrrupton Fgur 6.13 Act powr suppld by PV array durng ntrrupton

16 140 For analyzng th smulaton rsults, load oltag paramtr s consdrd. Normally, n UPQC approach, t has bn obsrd that load oltag compnsaton durng ntrrupton s sconds. Th proposd systm, usng th PV array, prods powr supply for a longr duraton about 0.2 sc, as shown n tabl 6.1 and fgur Tabl 6.1 Comparson of load oltag compnsaton durng ntrrupton by UPQC and proposd PV-UPQC systms Load oltag compnsaton duraton tm for UPQC systm Load oltag compnsaton duraton tm for PV-UPQC systm Load oltag compnsaton durng ntrrupton 0.02 to 0.06sc 0.2 to 0.4 sc Th prformanc analyss of th PV-UPQC systm shows that t s bttr than UPQC. Th UPQC suppls act powr to th PCC for only 3 cycls (0.06sc) durng oltag ntrrupton. On th othr hand, th PV- UPQC systm suppls act powr to th PCC for mor than 10 cycls (0.2sc) durng long oltag ntrrupton. On th bass of th rsarch work prsntd n ths chaptr, a papr nttld Photooltac Unfd Powr Qualty Condtonr Sag and Intrrupton Mtgaton has bn publshd n th Australan Journal of Elctrcal and Elctroncs Engnrng, Vol.9 No.2, pp , 2012.

17 141 Fgur 6.14 Intrrupton duraton for UPQC and PV-UPQC systm 6.8 CONCLUSION In ths chaptr, th photooltac array, whch s usd n powr qualty mpromnt n ntgratd powr dstrbuton systm usng PV- UPQC, s dscrbd. Th dsgn of th proposd systm s rfd through th smulaton usng Matlab/Smulnk. Th proposd PV-UPQC systm has th capablty of mprong th powr qualty at th pont of nstallaton n dstrbuton systms. Th maxmum powr s xtractd from PV arrays usng P and O mthods. Th smulaton rsults of th combnd opraton of PV and UPQC shows that th proposd systm s capabl of compnsatng long oltag ntrrupton of th sourc and njctng of act powr to th load. Th adantag of th proposd PV UPQC systm s ts drct ntrfac to th PCC. Th powr supply form PV array s gn to th DC lnk, whch compnsats long oltag ntrrupton. Th smulaton rsults pro that th proposd systm oprats ffcntly.