Rcibido: 11 d sptimbr d 2013 Acptado: 13 d novimbr 2013 PHOTOVOLTAIC MODULE-INTEGRATED CONVERTER (MIC) BASED ON SEMI-Z-SOURCE INVERTERS (SZSI): A NEW SLIDING MODE CONTROL SCHEME CONVERTIDORES FOTOVOLTAICOS EN MÓDULOS INTEGRADOS BASADOS EN INVERSORES DE FUENTE-SEMI-CUASI Z: UN NUEVO ESQUEMA DE CONTROL POR MODOS DESLIZANTES PhD(c). Ramón A. Álvarz Lópz, MSc. Maritza A. García Angarita MSc. Angélica M. Torrgroza Espinosa Univrsidad d Sucr - Grupo d Automatización y lctrónica industrial. Tcnológico d Antioquia - Programa d Ingniría n Softwar. E-mail: ramon.alvarz@unisucr.du.co, magangarita@gmail.com Abstract: A control schm for powr conditioning in a photovoltaic (PV) systm basd on smi-quasi-z-sourc invrtrs (SqZSI) is proposd. Th adoptd invrtr topology allows th indpndnt control of ach lg currnt, nabling th tracking of th maximum powr point (MPP). Th schm is an implmntation of a hystrtic currnt mod controllr that controls quasi-z-sourc singl-phas invrtr. Th good agrmnt btwn th analytical study and simulation rsults confirms th validity of th proposd control systm. Kywords: Modul intgratd convrtr (MIC), smi-quasi-z-sourc invrtr (SqZSI), sliding-mod control. Rsumn: S propon un squma d acondicionaminto d potncia para un sistma d gnración fotovoltaica (FV) basado n un invrsor d funt-smi-cuasi -Z. La topología d control slccionada facilita l control indpndint d corrint por rama, a la vz qu prmit sguir l punto d máxima potncia. El squma s basa n un control por histérsis n modo d corrint qu actúa sobr l invrsor d funt-smi-cuasi Z. Los bunos rsultados obtnidos ntr l studio analítico y las simulacions, confirman la valids dl sistma d control propusto. Palabras clav: Módulos d convrtidors intgrados, invrsor d funt-smi-cuasi Z, publicación, control n modo dslizant. 1. INTRODUCTION Th us of distributd gnration systms (DGSs) has facilitatd th dvlopmnt of modulintgratd convrtr (MIC), which allow to optimiz th xtractd powr from a grid connctd photovoltaic gnration systm (PV). Additionally, th MICs hav an xcllnt rlationship btwn cost, rliability, flxibility and asy installation. Howvr, compard with th string invrtrs, MICs hav lowr fficincis Th availabl commrcial MICs hav rachd 96.3% pak fficincy (Zhou t al., 2013). In most xisting MIC topologis, its low fficincy is du to th switching loss of th smiconductor dvics and transformr loss. Th rsarch and dvlopmnt of MICs hav rsultd in invrtrs ranging from 100 to 300 W. 61
In rcnt yar, MICs basd on Z-sourc invrtrs hav bn applid in PV systms (Araqu t al., 2013; Cao t al., 2011). Th Z-sourc invrtrs, show in Fig. 1, which by mploying fiv activ switchs (MOSFETs with thir anti-paralll diods) four of ths convrtrs can raliz bidirctional and bipolar opration. This family owns both buck and boost charactristics dpnding on control of th duty cycl. With on polar input voltag sourc this convrtr can produc ngativ and positiv voltag only by controlling duty cycl, which maks th invrtr simplr and mor conomical than othr invrtrs. This papr prsnts a MIC basd on smi-quasi-zsourc invrtr proposd in (Cao t al., 2011), as shown in Fig. 2. Th rason of using sqzsi is to rduc th numbr of th smiconductor dvics and thrby improv th invrtr fficincy. Howvr, compard with th H invrtr, two control loops ar ncssary for th sqzs invrtr. Th control schm proposd uss two slidingmod control loops, which will b xplaind in dtail in Sction III. A control loop to track th grid voltag is usd and anothr to xtract th maximum powr from th PV panls. Th applid maximum powr point trackr (MPPT) algorithm uss a typ of currnt control by sliding-mod control, which in turn allowing th sqzsi invrtr to xtract maximum powr from th PV array (Cabal t al., 2013). 2. ANALYSIS OF SqZSI CONVERTER Th circuit diagram of a convrtr is show in Fig. 3. A smi-quasi-z-sourc is a DC-DC convrtr, whos output voltag can b controlld for synthsis of sinusoidal wavforms. Fig. 3. Singl-phas Smi-quasi-Z-sourc invrtr. 2.1 Stat I (S1 is ON) Whn th switch S1 is ON (S2 is OFF), th inductor L1 absorbs nrgy from th sourc and th nrgy stord in capacitor C1 is transfrrd to inductor L2. Th stord nrgy in capacitor C2 is dlivrd to load R. Th inductor currnts i L1 and i L2 incras linarly during this mod and th input currnt i IN is th sum of i L1 and i L2. Th dynamic quations during a Stat I opration ar givn by: dil1 vin dil2 vc1 vc2 = = dt L1 dt L2 L2 dvc1 il2 dvc2 il2 io = = + dt C1 dt C2 C2 (1) Fig. 1. Singl-phas grid connctd PV MIC basd on ZSI. (a) (b) Fig. 4. Smi-quasi-Z-sourc invrtr opration mod. (a) Stat I S1is ON. (b) Stat II S2 is ON. 2.2 Stat II (S2 is ON). Fig. 2. Singl-phas grid connctd PV MIC basd on sqzsi. Whn switch S2 is turnd ON. Th inductor L1 transfrs th nrgy to capacitor C1. At th sam tim, i L2 flows through C2 R. This nrgy supplis th load currnt and rplnishs th charg draind away from th output capacitor C2 whn it alon was supplying th load currnt during th ON tim. Both currnts il1 and il2 dcras during this 62
mod of opration. Th dynamic quations during this mod: dil1 vc1 dil2 vin vc2 = = dt L1 dt L2 L2 dvc1 il2 dvc2 il2 io = = + dt C1 dt C2 C2 3. PROPOSED CONTROL SCHEME FOR MIC BASED ON Sqzsi INVERTER (2) In this sction, w aim at dsigning a controllr that will b abl to nsur: (i) a prfct MPPT (whatvr th powr of th PV panl), (ii) synthsis of th grid voltag (Álvarz y Arango, 2013). A control schm proposd for Singl-phas grid connctd PV MIC basd on sqzsi is shown in Fig. 5. Th structur is composd of two smiquasi-z-sourc invrtr (sqzsi). Each sqzsi consists of two powr switchs with anti-paralll diods and th sam DC-bus (PV panl). Morovr, ach sqzsi invrtr has its own control loop. 3.1 MPPT basd on sliding mod control for SqZSI convrtr Du to its potntial to significantly rduc th rror signal and high prformanc in DC/DC convrtrs for photovoltaic systms, th hystrtic currnt mod controllrs has rcivd an incrasing attntion rcntly (Alaa t al., 2009; Fadil t al., 2012; Farjah t al., 2012; Kavitha & Uma, 2012). In th continuous condition mod th SqZSI convrtr can b rprsntd by th following st of diffrntial quations: di v v L1 C1 IN = (1 u) + u dt L1 L1 dv i i C1 L1 L2 = (1 u) + u dt C1 C1 di v v v L2 C1 C2 IN = u u+ (1 u) dt L2 L2 L2 dv i i C2 L2 O = + dt C2 C2 Whr u=1 during T ON (i.. S 1 ON and S 1 OFF) (3) Rcall that th control objctiv is to nforc th currnt i PV to track th optimal point i PV = I PV +Asin wt. To this nd, currnt sliding-mod control dsign principls ar invokd (Bil t al., 2004; Cao t al., 2011; Carpita & Marchsoni, 1996; Gupta & Ghosh, 2006; Lopz t al., 2013). Lt us introduc th following tracking rror: x = i i i L1 L2 PV Th switching function slctd is: S(x ) = i i i S(x ) = il1 il2 i L1 L2 PV Th control law is: L1 PV L1 PV PV ut () = 1 if i < i or quivalntly ut () = 1 if S(x )<0 (4) ut () = 1 if i > i or quivalntly ut () = 0 if S(x )>0 Achiving th tracking objctiv amounts to nforcing th rror x to vanish. To this nd, th invarianc condition S(x )=0 and S(x )=0 is imposd. It follows from (3) and (4) that, th xprssion of th quivalnt control is: u q L2vC1 L1vIN + L1L2Awcos wt = (L1-L2) v + L1 v + (L2-L1) v C1 C2 IN (5) Fig. 5. Control schm proposd for Singl-phas grid connctd PV MIC basd on sqzsi. Assuming L1=L2=L, lad to th following xprssion of u u q v v + LAwcos wt C1 IN q = (6) vc2 63
From (6) w conclud that a sliding rgim will xist sinc v C2 0. Th constraints imposd on th lowr and uppr bounds of th quivalnt u q control 0< < 1rsult in th condition: LAw v v v v < min 1 +, vc2 vc2 vc2 IN C1 IN C1 (7) 3.2 Slid-mod control for synthsis of grid voltag, Vg As in th prcding sction, th control slidingmod control stratgy is dsignd. Th slctd sliding surfac is: S(x ) = v v v S(x ) = v v v g C2A C2B g C2A C2B 4. SIMULATION RESULTS (8) Th proposd control schm has bn implmntd in PSIM softwar for a PV grid connctd systm with paramtrs givn in Tabl I. Tabl I: simulation paramtrs Paramtr Valu C1 1 µf C2 200 µf L1=L2 47 mh PV Panl KC200GT Fig. 6 dpicts th transint bhavior of th systm input currnt and output voltag for positiv suddn chang of PV charactristics, which is carrid out by th incras th input currnt rfrnc. (b) (c) Fig. 6. Input currnts, inductor currnt and output voltag wavforms for stp load variations. (a) Input currnts for positiv variation of PV powr, (b) inductor currnt for positiv variation of PV powr, (c) output voltag for positiv variation of PV powr. Not that thr is a vry small stat-stat rror btwn currnt ipv and iin (i PV and i PV rspctivly), showing th fficacy of th tracking by mans of th sliding-mod control. As prdictd by th thory, th sliding surfac is a highfrquncy signal of vry small amplitud oscillating around zro. A similar intrprtation can b drivd from Fig. 7 whn th systm undrgos a ngativ suddn chang of PV powr, which is conductd by a chang of th radiation conditions. It can b obsrvd that th nw stady stat is rachd aftr 0.08 ms approximatly, and that th currnt and powr ar in phas opposition during th transintstat. Hnc, th sarch starts at th right of th MPP yilding a dcrmnt of th inductor currnt rfrnc and thus vrifying th thortical prdiction of th MPPT algorithm. (a) 64
(a) Fig. 8. Bhaviour of u q for sqzsi invrtr proposd. 5. CONCLUSIONS (b) (c) Fig. 7. Input currnts, inductor currnt and output voltag wavforms for stp load variations. (a) Input currnts for ngativ variation of PV powr, (b) inductor currnt for ngativ variation of PV powr, (c) output voltag for ngativ variation of PV powr. Fig. 8 dpicts th Bhaviour of u q for sqzsi invrtr proposd. Th voltags vc2a and vc2b of th systm to b controlld is asily masurd and compard with th grid voltag. Th lattr is only affctd by th input quantity v g and provids th rquird trajctoris that th controlld systm will hav to follow, indpndntly of paramtr variations or disturbancs rlatd to th load currnt. 65 A nw sliding mod control dsign of SqZSI invrtr was dvlopd in this papr. A control schm that improvs th prformancs of th invrtr PV systm was proposd. Th influnc of th control paramtrs on th prformancs of th systm was studid. An optimization algorithm was dvlopd in ordr to calculat th optimal valus of paramtrs basd on a prdfind spcification. Th importanc of th application of this typ of controllrs for th high fficincy PV invrtr is highlightd. Rsults show xcllnt dynamic rspons of controllr and robustnss to load and input voltag with larg variations around nominal valus. Furthr rsarch may focus to th study of th obsrvation of load currnt, and on th stability of th controllr-obsrvr closd loop. REFERENCES Alaa, H., Di Lorto, M., Eric, B., Pascal, V., Guy, C., & Grard, R. (2009). Sliding mod control of boost convrtr: Application to nrgy storag systm via suprcapacitors. In Powr Elctronics and Applications, 2009. EPE 09. 13th Europan Confrnc on (pp. 1 10). Álvarz Lópz, R. A., y Arango Manriqu, A. (2013). Aplicación d gnración fotovoltaica como apoyo a la falla n rds d distribución. Tcnologías Avanzadas, 2(22). Rtrivd from http://rvistas.unipamplona.du.co/ojs_vicinv s/indx.php/rcta/indx Arango Manriqu, A., y Álvarz Lópz, R. A. (2013). Microgrids: nuvo paradigma n las rds léctricas.
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