Simplified Reactive Power Control for Grid-connected Photovoltaic Inverters

Transcription

1 Indan Journal of Scence and Technology, Vol 8(13), DOI: /jst/2015/v813/57487, July 2015 ISSN (Prnt) : ISSN (Onlne) : Splfed Reactve Power Control for Grd-connected Photovoltac Inverters R Geetha 1 * and V Jayachtra 2 1 Departent of Electrcal Engneerng, Sr Krshna College of Technology, Cobatore, , Inda; geethakathr26@galco 2 Sr Krshna College of Technology, Cobatore, , Inda Abstract Reactve power plays an enent role for the power syste to operate effcently as well as securely In order to control the reactve power by the proposed Splfed Reactve Power Control (SRPC) strategy for sngle-phase grd-ted Photovoltac (PV) nverters In ths proposed SRPC strategy, the reactve power control s acheve effectvely by usng the Current-Mode Asynchronous Sga-Delta Modulaton (CASDM) Ths CASDM s adopted to enhance the current control s dynac response, reduce both the current haronc dstorton and electroagnetc nterference In ths paper, the proposed PV nverter s to extract the axu power fro the PV panels and to nject the correspondng actve power nto the utlty grd In the eante, the put reactve power Q calculated by the splfed power calculaton Wth both the SRPC and CASDM, the sngle-phase PV nverter can acheve the desred RPC wth low current haronc dstorton Keywords: Current Mode Asynchronous Sga-Delta Modulaton (CASDM), Current Reference Generaton (CRG), Photovoltac (PV) Inverter, Reactve Power Control (RPC), Sngle-Phase 1 Introducton Nowadays the energy deand s ncreased and the contnuous reducton n exstng sources of fossl fuels The growng concern regardng envronent polluton, have pushed anknd to explore new technologes for the producton of electrcal energy usng clean, renewable sources Because of ts quetness and polluton free nature, the Photovoltac (PV) power generaton syste has becoe the ost prosng renewable energy source for resdental applcatons Snce the best way to utlze the PV power s to nject t nto the ac ans wth usng energy storage facltes, the grd-connected PV nverter s always necessary for the PV power syste Moreover, the PV nverter s a potental canddate to provde reactve power for the utlty grd to prove ts power qualty In order to accurately control the reactve put power, the Current Reference Generaton (CRG), whch can generate the proper current reference wth deanded power coponents, should be establshed n advance In dfferent lterature, any Reactve Power Control (RPC) strateges for the sngle-phase nverter have been proposed 1,3 10 Aong the, the drect-quadrature (d-q) transforaton 3,4 and the nstantaneous reactve power theory (the p-q theory) 5 7 are the ost addressed control schees to generate two orthogonal current references Furtherore, soe odfed ethods were presented to prove the controlled perforance The authors n 8 adopted a snusodal sgnal ntegrator along wth the p-q theory to reduce the controlled senstvty to the grd voltage dstorton Whle the authors n 9 used the dscrete Fourer transfor Phase Lock Loop (PLL) ethod to acheve ore precse RPC In order to reduce the coputatonal burden and uses a splfed power calculaton, where the actve power and the reactve power can be easly calculated by two sapled current values wthn one ac ans cycle, s proposed n ths paper The splfed power calculaton along wth a sooth power adjustent fors a Splfed RPC (SRPC) strategy Moreover, n order to reduce the controlled senstvty to the grd current dstorton And by usng of Current-Mode Asynchronous Sga-Delta Modulaton (CASDM), whch has the ert of low current haronc dstorton, fast dynac response and *Author for correspondence

2 Splfed Reactve Power Control for Grd-connected Photovoltac Inverters low electroagnetc nterference s adopted for the PV nverter s put current control strategy 2 Splfed Power Calculaton The splfed power calculaton, n whch the actve power and the reactve power can be easly calculated by two sapled current values wthn one ac ans cycle, s proposed n ths paper Frst, the grd voltage s assued to be a purely snusodal wavefor, so the put power of nverter s s deterned drectly by ts njected snusodal current The typcal phasor dagra of the nverter s put current ac s shown n Fgure 1 The actve power s produced by the current coponent p whch s n phase wth grd voltage v ac, and the reactve power s controlled by the current q whch s 90 o of phase wth the voltage v ac The apltudes of the currents p and q can be expressed p q = I cosθ (1) = I snθ (2) where I and θ are the apltude and phase angle of the current ac, respectvely By controllng the I and the θ of the current ac, the actve and reactve power control can be acheved The prncple of the proposed splfed power calculaton ethod can be explaned by usng the typcal grd voltage and current wavefors shown n Fgure 2 The voltage and current wavefors equaton can be Fgure 2 ans v ()= t V snω t (3) ac t I sn ωt-θ (4) ac ()= ( ) Typcal voltage and current wavefors of the ac where ω s the lne frequency of the ac ans n radans per second At te t p, the apltude of the njected ac current s ac ( p ) = ( )= t I sn π 2 θ I cosθ (5) It should be entoned that the current p can be easured by delayng a quarter of the ac lne perod when the zero crossng pont of the ac ans voltage v ac s detected Equaton (5) says that the sapled quantty at te t p s equal to the apltude of the actve current coponent p Thus, the actve put power of the nverter can be deterned as P 12 I cos θ 12V t (5a) = V = ac ( p ) Slarly, the apltude of the njected ac current at te t q s Fgure 1 Phasor dagra of the ac put current ( )= ( )= t I sn π θ I sn θ (6) ac q Ths current value can be easured when the zero crossng pont of the ac ans voltage v ac s detected It s the apltude of the reactve current coponent q and the reactve put power of the nverter becoes 2 Vol 8 (13) July 2015 wwwndjstorg Indan Journal of Scence and Technology

3 R Geetha and V Jayachtra Q 12V 12V ac t q = = ( ) I sn θ (6a) Fro the above equatons, the actve and the reactve power of the nverter can be easly calculated by two sapled current values wthn one ac lne cycle However, ths ethod senstve because of ether current rpple or the current dstorton, by addng a band pass flter n the current feedback loop the pact of the unavodable current rpple to the power calculaton can be easly elnated Thus, t s necessary to desgn a good current control strategy for reduce the current dstorton 3 Sooth Power Adjustent After the actve power and the reactve power are obtaned, a proper controller s needed to copensate the power error In general, the proportonal-ntegral-dervatve controller has advantage of fast transent response, but the sudden change of the ac put current t ay cause large current dstorton whch wll lead to naccurate power calculaton Therefore, acheve sooth power adjustent s needed to control the actve and the reactve power of the grd-connected nverter, the power phasor should ove aong these operaton ponts By adjustng the apltude and phase angle of the reference current cycle by cycle, the desred actve and reactve put powers can be acheved soothly Snce the actve put power P s regulated by changng the apltude of the njected ac current, the easured power P s copared wth the hysteress coparator wth hgh bound P ref,h and low bound P ref,l The postve or negatve apltude adjustent ΔI s deterned by the followng algorth: (a) The current apltude generator f P > P then k=-1; else f P < P then k=+1; else k=0; ref,h ref,l ( )= ( )+ I n+ 1 I n k I (7) where I (n) and I (n+1) are the apltudes of the njected current reference for the nth and (n+1) th ac ans cycles On the other hand, the reactve put power Q s regulated by adjustng the phase angle of the njected current Slarly, the easured reactve power s copared wth the hysteress coparator wth hgh bound Q ref,h and low bound Q ref,l The postve or negatve phase shft adjustent Δθ s deterned by the followng algorth: (b) The current phase angle generator f Q > Q then =-1; ref,h else f Q < Q then =+1; θ ref,l else =0; ( n+ 1)= θ ( n)+ θ (8) Fgure 3 Conceptual dagra of the proposed sooth power adjustent where θ(n) and θ(n+1) are the phase angles of the njected current reference for the n th and (n+1) th of ac ans cycles It should be entoned that the actve power P and the reactve power Q vary cosne and sne functons, respectvely Therefore, for θ close to 90 o and P - Q, a change n current agntude wll cause a sgnfcant change n Q, and a change n phase angle wll cause a relatvely larger change n P The power transton wll take a long te f the ΔI and Δθ are too sall Thus, an adaptve adjustent, whch has larger ΔI and Δθ values durng large power change transent and saller ones durng steady state, s adopted to prove the overall perforance The block dagra of the sngle-phase PV nverter wth the proposed control strategy s shown n Fgure 5 The power stage of the PV nverter conssts of a boost Vol 8 (13) July 2015 wwwndjstorg Indan Journal of Scence and Technology 3

4 Splfed Reactve Power Control for Grd-connected Photovoltac Inverters slopes, the lnear odulaton s caused and also nfnte power supply rejecton rato For the PV nverter s actve power control, the voltage V should be regulated by the current apltude generator wth a hysteress coparator to generate the current reference s apltude I The algorth of the current apltude generator to regulate the voltage V can be Fgure 4 Graphc coparson of coputatonal burden Fgure 5 Block dagra of the sngle-phase grd-connected PV nverter converter wth the Maxu Power Pont Trackng (MPPT) functon and a full brdge nverter wth both actve and reactve power put The control algorth conssts of the SRPC ethod and the CASDM control strategy The objectve of the proposed PV nverter s to extract the axu power fro the PV panels and to nject the correspondng actve power nto the utlty grd In the eante, the put reactve power Q calculated by the splfed power calculaton and the reactve power coand Q ref are copared to deterne the phase shft of the current reference, θ The apltude and the phase angle of the grd voltage are captured by usng the Peak Value Detector (PVD) and the Zero-Crossng Detector (ZCD), respectvely It should be entoned that the ZCD along wth the PVD s an easy-to-pleent approach On the other hand, the perforance of the proposed SRPC ethod can be further proved by usng a PLL crcut f V > V H then k=+1; else f V < V L then k=-1; else k=0; I ( n+ 1 ) = I ( n)+ k I (9) Eventually, the current reference ac,ref s obtaned and the current apltude s ultplyng I wth the sgnal sn(θ) For the put current control, the easured put current ac s copared wth the current reference ac, ref to generate the current error sgnal ac, err whch s sent nto a current controller to generate the voltage reference v ref 5 Asynchronous Sga-Delta Modulaton Delta odulaton s based on quantzng the change n the sgnal fro saple to saple rather than the absolute value of the sgnal at each saple The ost strkng feature of a sga delta odulator, whch s the an 4 Current Control Strategy The hysteress controller can be ade wth ether a current- or a voltage-loop In ths hysteress controllers the saw tooth-shaped carrer wth deally havng straght Fgure 6 Functonal dagra of sngle-bt sga delta odulator 4 Vol 8 (13) July 2015 wwwndjstorg Indan Journal of Scence and Technology

5 R Geetha and V Jayachtra 8 References coponent of sga delta As, s that these odulators use a very hgh saplng rate The saplng rate used s n the range of MHz, whch s uch hgher than the Nyqust rate, generally n the range of khz Hence, the over saplng rato s very hgh, because of whch these are soetes referred to as over saplng As Then, the ASDM, whch s an analog-to-dgtal sgnal converter wth varous frequency features, s adopted because to generate the gate sgnals, as vgs1 vgs4 6 Sulaton and Results A Splfed Reactve Power Control (SRPC) strategy for sngle-phase grd-ted photovoltac (PV) nverter, where the actve and reactve power control s acheved by the followng sulaton results s obtaned fro the MAT- LAB/SIMULINK Wth both the SRPC and CASDM, the sngle-phase PV nverter can acheve the desred RPC wth low current haronc dstorton 7 Concluson Ths paper proposed a SRPC ethod along wth the CASDM for the sngle-phase grd-connected PV nverter wth RPC The proposed SRPC ethod can reduce the coputatonal burden of the processor Along wth the sall sgnal analyss for the CASDM s presented to prove the control stablty Wth both the SRPC and CASDM, So ths the PV nverter can acheve the desred RPC 1 Lu C, Sun P, La J-S, J Y, Wang M, Chen C-L, Ca G Cascade dual-boost/buck actve-front-end converter for ntellgent unversal transforer IEEE Trans Ind Electron Dec 2012; 59(12): Snsukthavorn W, Ortjohann E, Mohd A, Hasc N IEEE Trans Ind Electron 2012 Oct; 59(10): Lu J, Yang J, Wang Z A new approach for sngle-phase haronc current detectng and ts applcaton n a hybrd actve power flter Proceedngs IEEE IECON/IECON; 1999 p Satou M, Matsu N, Shzu T A control strategy of sngle-phase actve flter usng a novel d-q transforaton Conference Record IEEE 38th IAS Annual Meetng; 2003 p Satou M, Shzu T Generalzed theory of nstantaneous actve and reactve powers n sngle-phase crcuts based on Hlbert transfor Proceedngs IEEE PESC; 2002 p Zhang Q, Sun X-D, Zhong Y-R, Matsu M, Ren B-Y Analyss and desgn of a dgtal phase-locked loop for sngle-phase grd-connected power converson systes IEEE Trans Ind Electron 2011 Aug; 58(8): Xu S, Wang J, Xu J A current decouplng parallel control strategy of sngle-phase nverter wth voltage and current dual closed-loop feedback IEEE Trans Ind Electron 2013 Apr; 60(4): Bojo RI, Long LR, Rou D, Tencon A Enhanced power qualty control strategy for sngle-phase nverters n dstrbuted generaton systes IEEE Trans Power Electron 2011 Mar; 26(3): Lu L, L H, Wu Z, Zhou Y A cascaded photovoltac syste ntegratng segented energy storages wth self-regulatng power allocaton control and wde range reactve power copensaton IEEE Trans Power Electron 2011 Dec; 26(12): Sun P, Lu C, La J-S, Chen C-L Grd-te control of cascade dual buck nverter wth wde-range power flow capablty for renewable energy applcatons IEEE Trans Power Electron 2012 Apr; 27(4): Chen Y-M, Chang C-H, Lu K-Y Grd-ted nverter wth current ode asynchronous sga-delta odulaton Proceedngs IEEE ECCE; 2009 p Chang C-H, Wu F-Y, Chen Y-M Modularzed bdrectonal grd connected nverter wth constant-frequency asynchronous sga-delta odulaton IEEE Trans Ind Electron 2012 Nov; 59(11): Paraesh J, von Jouanne A Use of sga delta odulaton to control EMI fro swtch-ode power supples IEEE Trans Ind Electron 2001 Feb; 48(1):111 7 Vol 8 (13) July 2015 wwwndjstorg Indan Journal of Scence and Technology 5