A Grid Connected Three-Port Solar Micro Inverter

Transcription

1 Bonfrg Internatonal Journal of Power ystems and Integrated Crcuts, ol. 4, No., May 04 5 A Grd Connected Three-Port olar Mcro Inverter. Rsha, Ju Mathew arghese and Dr. M. hah Abstract--- In the present scenaro of creasg energy crss Interest ac module strategy for P based mcroverter has grown recent years because of ts mproved energy harvest, mproved system effcency, lower stallaton costs, plug-n-play operaton, and enhanced flexblty and modularty.mcroverters avod the drastc reducton power produced by P panels due to shadg effect. In ths paper a three-port mcroverter topology s beg studed whch s connected to sgle phase grd. The proposed topology s bascally a flyback converter wth an extra swtch and dode. Ths arrangement allows the use of a low capactance flm capactor, wth long lfespan, as decouplg capactor stead of E-caps, wth short lfespan. Thus proposed three-port mcroverter has longer lfespan.the operaton prcples and desgn consderatons are explaed ths paper. The MATLAB/IMULINK based smulaton of the mcroverter s also performed. Keywords--- AC Modules, Flyback Converter, Maxmum Power Pot Trackg (MPPT), Power Decouplg, Three- Port Mcro Inverter, Utlty Interactve Inverter T I. INTRODUCTION HE renewable energy sources, such as wd turbe, photovoltac (P) panels, etc. become more and more popular dustral and resdental applcatons, because of the energy crss. Due to ther promse of clear and seemgly lmtless generated energy the photovoltac (P) has wtnessed the unprecedented growth, among renewable sources []-[3]. Wth dfferent radance and temperature, the dc output voltage and the maxmum output power of the P panel wll change. o, an verter terface wth maxmum power pot trackg (MPPT) s requred for the P panel to connect to the grd. There are three popular technologes for the P panels connected to the grd through an verter: centralzed threephase verter for matrx connected P panels, sgle-phase strg verter for seres connected P panels and module. Rsha, M.Tech, tudent, Electrcal and Electroncs Department, Government College of Engeerg, Kannur, Inda. E-mal: rshabrahm@gmal.com Ju Mathew arghese, M.Tech, tudent, Electrcal and Electroncs Department, Government College of Engeerg, Kannur, Inda. E-mal: jumathew7@gmal.com Dr.M. hah, Assocate Professor, Electrcal and Electroncs Department, Government College of Engeerg, Kannur, Inda. E-mal: shahmtly@gmal.com tegrated mcro-verter for a sgle P panel [4]-[7].The centralzed verter structure s shown Fg. (a). Due to low cost and hgh relablty, module tegrated mcro-verter whch s shown Fg. (b), s preferred for the future applcaton. It removes the msmatch losses between P modules sce there s only one P module, and also supports optmal adjustment between the P module and the verter and, hence, the dvdual MPPT. Fg. : (a) Centralzed Inverter (b) Mcro Inverter tructure It cludes the possblty of easy enlargg of the system, due to the modular structure. The mcroverter can be used as a plug and play devce, whch can be used by persons wthout any knowledge of electrcal stallatons, s also an herent feature of mcroverter based ac module. Based on the galvanc solaton, the P verter topologes can be dvded to transformer-less topologes and transformer solated topologes. ce the output voltage of a sgle P panel s as low as 0~50, a hgh voltage ga verter s requred for the P panel to connect to the sglephase grd. Although the transformer-less verter topologes have low cost and hgh effcency features, they usually do not have enough voltage ga to boost the put voltage. Therefore, the transformer solated verter topologes wth hgh voltage ga are preferred for a sgle P panel grdconnected applcaton. For the transformer solated verter topologes, there are two popular approaches. The frst approach utlzes the sgle stage flyback verter or solated buck-boost verter, whch s able to replace the electrolytc capactor wth hgh voltage low capactance flm capactor for the energy storage. Ths approach usually uses less swtches wth low cost, the hgh frequency transformer s the major desgn ssue for these topologes to acheve hgh effcency. The second approach s two-stage approach wth a transformer solated dc-dc converter as the frst stage and a full-brdge verter as the second stage [8]. A hgh effcency dc-dc converter wth soft swtchg s usually desgned for the frst stage. A full brdge verter wth le frequency swtches or PWM s usually used for the second stage. The dc-dc converter the frst stage s DOI : /BIJPIC.4873

2 Bonfrg Internatonal Journal of Power ystems and Integrated Crcuts, ol. 4, No., May 04 6 the major desgn ssue to acheve low cost and hgh effcency. Fg. shows the block dagram of conventonal two stage solated solar mcroverter. The dc dc converter frst stage releases the maxmum solar energy from a P panel and provdes a hgh voltage dc bus for the second stage dc ac verter that converts the dc voltage to a susodal ac output voltage and connects wth the ac grd system. The converson effcency of the conventonal solar mcroverter s poor due to the two-stage power converson. Hgh-voltage electrolytc capactors (E-Caps) are usually necessary on the dc bus between the dc dc and dc ac stages. They are expensve and serously shorten the lfespan of the solar mcroverter that s cyclcally exposed to a hgh temperature envronment under solar rradaton. The commercal success of the module-tegrated mcroverter depends on ts relablty and effcency and long lfespan and low power loss must be ensured. other case when put power s less than output power (P o > P ) then the defct power s beg suppled from the decouplg capactor by turng on swtch.hence the operatg mode of ths topology s dvded to two modes as shown Fg. 4. Fg. 3: Proposed Three-Port Mcro Inverter [4] Fg. : Block Dagram of Conventonal olar Mcro Inverter A three-port converter s a best choce for power decouplg a sgle stage verter [9]-[3].Here one port s dedcated for MPPT task and other s dedcated for power decouplg. In ths paper, a three-port mcroverter based on flyback topology whch can perform both dc- ac converson and power decouplg has been proposed [4]. mple topology, less power loss and decent effcency are the advantages of the proposed topology. In ths topology conventonal snubber crcut s also not requred as the decouplg capactor act as snubber crcut. The operaton prcples and desgn consderatons wll be analyzed and descrbed the followg sectons. Ths paper s organzed as follows. The system descrpton and operaton states analyss s gven secton II. The desgn consderatons are gven secton III. The results and dscusson are gven secton I and secton respectvely, followed by concluson secton I. II. YTEM DECRIPTION AND OPERATION The proposed topology as shown Fg. 3 s a basc flyback topology wth an extra swtch and a dode D. A hgh frequency flyback transformer s used here. The secondary of transformer have two wdgs to produce the ac output voltage usg the two swtches 3 and 4 seres wth dodes D 3 and D 4 whch are swtchg at le frequency. The power decouplg s performed by decouplg capactance C D. When the Input power s hgher than the output power (P >P o ) surplus power s stored the decouplg capactor and the Fg. 4: Waveforms When the put power from P, P P s larger than output power P o, the verter operates mode I and when P o >P P verter operates Mode II. A. Mode I In ths mode the extra energy from the P panel wll be stored the decouplg capactor C D and swtches, 3 ( 4 ) are operatg whle and 4 ( 3 ) are always OFF. The operaton ths mode s dvded to four crcut states each swtchg cycle, as shown Fg.5. Fg. 5: wtchg Waveforms for and 3 Each state, as shown Fg. 6, wll be dscussed detal. tate [t 0 t ]: Durg ths state, shown Fg. 6(a), s turned ON, and the magnetzg ductance starts storg energy from the P panel. The magnetzg current keeps

3 Bonfrg Internatonal Journal of Power ystems and Integrated Crcuts, ol. 4, No., May 04 7 rampg up and once t reaches ts peak value ( Lm peak ), s turned OFF, and the next stage starts. D T Lm_peak Lm tate [t t ]: All swtches are OFF durg ths mode,as Fg. 5(b). The transformer magnetzg energy wll dscharge to the decouplg capactor. The current wll keep dschargg untl t reaches the current, Lm peak, whch s gven as (). Lm _ peak IT 0 0 L m s () t (a) tate (t 0 -t ) (b) tate (t -t ) (c) tate 3(t -t 3 ) (d) tate 4(t 3 -t 4 ) () Fg. 6: Operaton tates durg Mode I [4] The duty cycles D and D can be calculated by equaton (3) and (4) L m Lm _ peak (3) The total power from the P panel from ths two stages s gven as (5) P Lm _ peakd Lm _ peak Lm _ peak D (5) ubsttutg (3) and (4) (5) Lm_peak s gven as: Lm _ peak Lm _ peak C L D m CD P T (6) Equaton () and (6) can be used to calculate D and D. tate 3 [t t 3 ]: Ths state starts wth turng on of swtch 3 ( 4 ).As shown Fg. 5(c), durg ths state, the current s released through one of the secondary wdgs, and the correspondg ac swtch, ether 3 or 4. The grd voltage durg one swtchg perod v ac can be assumed to be constant; therefore, the current s expressed as n v () t t t Lm _ peak ac 3 n Lm The current reaches zero at t 3 and tme duraton D 3 T can be calculated by (8) 3 L n Lm _ peak m n tate4 [t 3 t 4 ]: Ths state starts wth current reachg zero. Durg ths state, all the swtches are turned OFF, as shown Fg. 5(d). The capactor C r and ductor L r keep pumpg energy to the grd, and the flux the flyback transformer s reset. B. Mode II Ths Mode s also dvded to four states as shown Fg.7. In Mode II, swtch 3 ( 4 ) s always ON. The frst and thrd state of Fg. 8 s smlar to Mode I. However the Lm peak s kept at the same level ths mode and hence accordg to equaton (3) D T remas constant ths mode. In the second state the decouplg capactance dscharges ts energy through swtch and thus the tme duraton D T can be calculated as: v ac L m Lm _ peak Lm _ peak CD (8) The calculaton of D 3 T s same as Mode I. (9) (7) L m Lm _ peak Lm _ peak CD (4) Fg. 7: wtchg Waveforms for, and 3

4 Bonfrg Internatonal Journal of Power ystems and Integrated Crcuts, ol. 4, No., May 04 8 transformer causes the wdg voltage correspondg to 4 to be v ac.hence, the voltage stress on 4 s double grd voltage, and s gven as: v () 4 ac _ peak (a) tate (t 0 -t ) (b) tate (t -t ) (c) tate 3(t -t 3 ) (d) tate 4(t 3 -t 4 ) Fg. 8: Operaton tates Mode II [4] III. DEIGN CONIDERATION ome desgn gudeles necessary for power devce selecton and decouplg capactance value selecton are gven ths secton. A. Power Devce oltage tress The voltage across decouplg capactance s pulsatg at double-le frequency wth a peak value of peak_ cd whose value s determed by the selected capactance and voltage rpple. Therefore, the voltage stress across and D s peak_cd and the voltage stress on and D s peak_cd. When the output ac voltage s at ts peak value, v ac_peak and when s turned ON, D 3 or D 4 s subjected to peak voltages and the voltage stress s gven as follows: n peak cd D4_ reverse ac _ peak n v (0) uppose stage 3 3 s ON and 4 s OFF, the wdg voltage correspondg to swtch 3 s clamped to grd voltage v ac. In ths case, the symmetrc secondary wdg B. Decouplg Capactance The mmal voltage across the decouplg capactor must be greater than the voltage stress across, whch gven as follows: n s( t) () 0 n Otherwse, the energy stored the prmary sde would damp to the decouplg capactor. I. REULT A 00W P panel supplyg power to grd (30) through proposed three-port mcroverter was desgned and smulated usg MATLAB/IMULINK. pecfcatons used for smulaton s gven Table I. Frst the P panel was modelled and smulated [5] usg MATLAB/IMULINK and then proposed three-port mcroverter model was connected to the P panel that converted the dc voltage of P to ac whch s to be suppled to the grd. A. P Characterstcs Table I: Desgn pecfcatons P Panel pecfcatons Maxmum Power (P max ) 00W oltage at Pmax ( mp ) 6.3 Current at P max (I mp ) 7.6A Open-crcut voltage ( oc ) 3.7 hort-crcut current (I sc ) 8.5A Other Desgn Parameters wtchg frequency 50kHz Grd voltage 30 Grd frequency 50Hz The MATLAB functon code was wrtten to obta the module current under TC (5 0 C,.5AM, 000W/m ).The I curve and P curve obtaed from smulaton of P panel s shown Fg.9 and Fg. 0 respectvely. Fg. 9: I- Characterstcs of P Panel

5 Bonfrg Internatonal Journal of Power ystems and Integrated Crcuts, ol. 4, No., May 04 9 The grd voltage and current waveform are shown Fg. 3.The grd current s not purely susodal as seen from Fg.3. Fg. 0: P- Characterstcs of P Panel From Fg. 9 t s clear that the short crcut current and open crcut voltage obtaed by smulaton s same as gven Table I. The maxmum power obtaed by smulaton s nearly 00W as gven Table I. B. Crcut mulaton The smulaton of the 00W three-port mcroverter was done usg MATLAB/IMULINK. The smulaton model of three-port mcroverter connected to 30 grd s shown Fg. Fg. 3: Grd oltage and Current The three-port mcroverter was smulated for dfferent load condtons of R load other than connecton to grd. The studed solar mcroverter was found to be workg under R load. The correspondg output waveforms are shown Fg. 4. Fg. : mulaton Dagram of Three-Port Mcroverter The swtchg pulses for,,3 and 4 where generated and are shown as Fg.. Fg. 4: oltage and Current Output for R Load From Fg. 4 t s clear that three-port solar mcroverter works satsfactorly gvg requred output voltage rrespectve of whether the load s R or grd. Fg. : Gate Pulses for wtches. DICUION Maxmum power obtaed from central verter and strg verter based P system reduce even when any one of the P panel connected to them has been partally shaded.to avod ths problem a mcroverter s used whch s connected to a sgle P panel. The ma consderaton whle desgng mcroverter s ts lfespan whch s to be equal to the lfespan of P panel. But the E-caps used conventonal mcroverter have short lfespan. The three-port mcroverter

6 Bonfrg Internatonal Journal of Power ystems and Integrated Crcuts, ol. 4, No., May avods ths problem by usg small capactance for power decouplg and the same capactance act as snubber crcut also. Also ths mcroverter s suted for R load and can be connected to grd.in case of grd connected three-port mcroverter the output current was found to be non susodal whch can be mtgated by modfyg the control methods. [4] H. Hu,,. Harb, N. H. Kutkut, Z. J. hen, and I. Batarseh,, A gle- tage Mcroverter Wthout Usg Eletrolytc Capactors, IEEE Transactons on Power Electroncs, volume 8, no. 6, June 03 [5]. hek Mohammed, Modellg and mulaton of Photovoltac module usg MATLAB/mulk, Internatonal Journal of Chemcal and Envronmental Engeerg, olume, No.5, October 0 I. CONCLUION A new mcroverter topology has been presented ths paper. The lfetme of the three-port mcroverter has been prolonged because of the replacement of hgh voltage E-caps wth a new method of power decouplg wth small flm capactance. The power decouplg capactance also handles the transformer leakage energy and so avods the need for extra dsspatve crcuts,whch leads to reduced power losses and thus mproved effcency. REFERENCE [] R. Mastromauro and M. Dell Lserre, A control ssues sgle-stage photovoltac systems: MPPT, current and voltage control, IEEE Transactons on Industral Informaton, olume 8, No., pp. 4 54, May 0 [] Al Nabuls and R. Dhaouad, Effcency optmzaton of a DP-based standalone P system usg fuzzy logc and dual-mppt control, IEEE Transactons on Industral Informaton, olume 8, No. 3, pp , August 0 [3] T. Hrose and H. Matsuo, tandalone hybrd wd solar power generaton system applyg dump power control wthout dump load, IEEE Transactons on Industral Electroncs, olume 59, No., pp , February 0. [4]. B. Kjaer, J. H. Pedersen, and F. Blaabjerg, A revew of sgle-phase grd-connected verters for photovoltac modules, IEEE Transactons on Industral Applcatons, volume 4, no. 5, pp , eptember 005 [5] Q. L and P. Wolfs, A revew of the sgle phase photovoltac module tegrated converter topologes wth three dfferent dc lk confguratons, IEEE Transactons on Power Electroncs, volume 3, no. 3, pp , May 008 [6] M. Calas, J. Myrzk, T. pooner, and. G. Agelds, Inverters for sgle-phase grd connected photovoltac systems An overvew, Proceedgs IEEE PEC 0, volume, 00, pp [7] M. Mehardt and G. Cramer, Past, present and future of grd connected photovoltac- and hybrd-power-systems, Proceedgs IEEE-PE ummer Meetg, volume, 000, pp [8] Cao,. Jang, and F. Z. Peng, Y. L, Low Cost Transformer Isolated Boost Half brdge Mcro-verter for gle-phase Grd-connected Photovoltac ystem, proceedgs of Appled Power Electroncs Conference and Exposton (APEC), Orlando, pp.7-78, February 0. [9] Z. Qan, O. Abdel-Rahman, H. Al-Atrash, and I. Batarseh, Modelg and control of three-port dc/dc converter terface for satellte applcatons, IEEE Transactons on Power Electroncs, volume 5, no. 3, pp , March 00 [0] Lohner, T. Meyer, and A. Nagel, A new panel-tegratable verter concept for grd-connected photovoltac systems, Proceedgs IEEE IIE 96, volume, 996, pp [] Z. Qan, O. Abdel-Rahman, H. Hu, and I. Batarseh, An tegrated three-port verter for stand-alone pv applcatons, Proc. IEEE Energy Converson Congress and Exposton, pp , eptember 6, 00 [] H. Tao, A. Kotsopoulos, J. L. Duarte, and M. A. M. Hendrx, Transformer-coupled multport Z bdrectonal dc dc converter wth wde put range, IEEE Transactons on Power Electroncs, volume 3, no., pp , March 008 [3] Z. Chuanhong,. D. Round, and J. W. Kolar, An solated three-port bdrectonal dc dc converter wth decoupled power flow management, IEEE Transactons on Power Electroncs, volume, pp , eptember 008