grid-connected photovoltaic module integrated converter system with high-speed communication interfa

High-efficiency grid-connected photovoltaic module integrated converter system with high-speed communication interfaces for small-scale distribution power generationWoo-Young Choi *,Jih-Sheng (Jason)LaiFuture Energy Electronics Center,Department of Electrical and Computer Engineering,Virginia Polytechnic Institute and State University,Blacksburg,VA,USAReceived 19June 2009;received in revised form 13December 2009;accepted 3January 2010Available online 21February 2010Communicated by:Associate Editor Igor TyukhovAbstractThis paper presents a high-efficiency grid-connected photovoltaic (PV)module integrated converter (MIC)system with reduced PV current variation.The proposed PV MIC system consists of a high-efficiency step-up DC–DC converter and a single-phase full-bridge DC–AC inverter.An active-clamping flyback converter with a voltage-doubler rectifier is proposed for the step-up DC–DC converter.The proposed step-up DC–DC converter reduces the switching losses by eliminating the reverse-recovery current of the output rectifying diodes.To reduce the PV current variation introduced by the grid-connected inverter,a PV current variation reduction method is also suggested.The suggested PV current variation reduction method reduces the PV current variation without any additional components.Moreover,for centralized power control of distributed PV MIC systems,a PV power control scheme with both a central control level and a local control level is presented.The central PV power control level controls the whole power production by sending out reference power signals to each individual PV MIC system.The proposed step-up DC–DC converter achieves a high-efficiency of 97.5%at 260W output power to generate the DC-link voltage of 350V from the PV voltage of 36.1V.The PV MIC system including the DC–DC converter and the DC–AC inverter achieves a high-efficiency of 95%with the PV current ripple less than 3%variation of the rated PV current.Ó2010Elsevier Ltd.All rights reserved.Keywords:Photovoltaic (PV);Module integrated converter (MIC);Step-up DC–DC converter;Full-bridge inverter;High-efficiency1.IntroductionThe grid-connected photovoltaic (PV)power condition-ing system (PCS)is the key technology in the future distrib-uted production of electricity using solar energy (Roman et al.,2008).There are basically three types of grid-con-nected PV PCSs:the centralized PCS,the string PCS,and the module integrated converter (MIC)PCS (Kjaer et al.,2005).Among these,the PV MIC system is the most recent approach for the grid-connected PV PCS.The PV MIC system offers “plug and play ”concept,greatly opti-mizing the energy yield from the PV module.Each PV module has its own DC–AC inverter,performing the max-imum power point tracking (MPPT)function (Enrique et al.,2007).To make the PV MIC system commercially viable,a low-cost and high-efficiency power conversion scheme should be developed for delivering electrical power to the grid with a high power factor (Meinhardt et al.,1999).Generally,the PV MIC system,as shown in Fig.1,con-sists of a DC–DC converter and a single-phase DC–AC inverter (Li and Wolfs,2008).The PV module voltage has a low voltage characteristic.Its output voltage typically ranges from 20to 45V.In order for the low PV module voltage to generate 60Hz,220V AC output voltage,the0038-092X/$-see front matter Ó2010Elsevier Ltd.All rights reserved.doi:10.1016/j.solener.2010.01.004*Corresponding author.E-mail address:wychoi@ (W.-Y.Choi)./locate/solenerAvailable online at Solar Energy 84(2010)636–649output of the DC–DC converter should be about 350V.Thus,a step-up DC–DC converter with a high voltage gain should be required in order to boost the low PV module voltage into high DC-link voltage.Up to now,various step-up DC–DC converters have been studied for PV MIC system applications.As a current-fed step-up DC–DC converter,the current-fed two-inductor boost convert-ers (Li and Wolfs,2006,2007)ware proposed.The conduc-tion losses of the power switches at the PV module side are reduced by using two input inductors.However,current-fed two-inductor boost converters are difficult to imple-ment in the PV MIC system.This is predominantly due to the large inductor size required to meet the high output current of the PV module,which increases the overall size and cost of the PV MIC system.To reduce the size and cost of the PV MIC system,the flyback converter has been uti-lized for the step-up DC–DC converter (Martins and Demonti,2002).As a simple power circuit topology,the flyback converter is suitable for a wide voltage variation of the PV module.However,the conventional flyback converter suffers from high switching losses of the power switches.To reduce the switching losses,the active-clamp-ing flyback converters (Boeke et al.,2006;Tseng et al.,2007)have been studied.The active-clamping flyback con-verters reduce the switching losses by clamping the volt-age stress of the power switches.However,the output rectifying diode in the active-clamping flyback converter still has the reverse-recovery problems.The output rectify-ing diode operating in high DC-link voltage produces large reverse-recovery current during its turn-offinstant (Karadzinov and Hamill,2000).The diode reverse-recov-ery current causes a significant increase of the switching losses at the output rectifying diode,including a large amount of electromagnetic interference (EMI)noises and additional thermal management problems in the PV MIC system.As shown in Fig.1,since the DC–AC inverter injects the active power into a single-phase grid,the power variation at double grid-frequency is superimposed to the DC-link voltage.Thus,the DC-link voltage variation occurs,which introduces the PV current variation (Casadei et al.,2006).Because the PV current variation decreases the MPPT effi-ciency,the PV current variation should be as small as pos-sible in order to maximize the power extracted from the PV module.The PV current variation can be reduced through the addition of the electrolytic capacitor to the input side of the step-up DC–DC converter (Schenck et al.,2005).Also,a large DC-link capacitor can be used to reduce the PV cur-rent variation (Brekken et al.,2002).However,the addition of the electrolytic capacitor (Schenck et al.,2005)or the large DC-link capacitor (Brekken et al.,2002)increases the overall size and cost of the PV MIC system.As different alternatives,the active ripple compensation methods with external DC–DC converter have been suggested:a ripple power port (Krein and Balog,2009),an active power filter (Mazumder et al.,2007),and a DC power smoothing cir-cuit (Shimizu et al.,2002).However,all these methods require additional power conversion circuits,which increase the manufacturing cost and decrease the power conversion efficiency of the PV MIC system.To cope with all these problems,this paper presents a high-efficiency grid-connected PV MIC system with reduced PV current variation.For a high-efficiency step-up DC–DC converter,an active-clamping flyback con-verter with a voltage-doubler rectifier is proposed.As shown in Fig.2,a boost type of active-clamping circuit is used at the PV module side.At the DC-link voltage side,a voltage-doubler rectifier using one resonant capacitor is used.The voltage-doubler rectifier provides a series-reso-nant circuit consisting of the transformer leakage inductor and the resonant capacitor.Zero-current switching of the output rectifying diodes is achieved (Choi et al.,2007),Fig.1.Block diagram of the single-phase grid-connected PV MIC system.W.-Y.Choi,Jih-Sheng (Jason)Lai /Solar Energy 84(2010)636–649637which can effectively eliminate the reverse-recovery current of the output rectifying diodes.In order to reduce the PV current variation,a practical PV current variation reduc-tion method is also suggested.By adding the PV current variation cancellation duty ratio to the control duty ratio of the proposed step-up DC–DC converter,the low fre-quency PV current variation can be reduced without any additional components.The overall system configuration is briefly described.The operation of the proposed step-up DC–DC converter is presented with the PV current var-iation reduction control.Moreover,for centralized power control of distributed PV MIC systems,a PV power con-trol scheme with both a central control level and a local control level is presented.The central PV power control level controls the whole power production by sending out reference power signals to each individual PV MIC system.The local PV power control level ensures that the reference power signal from the central control level is reached.Experimental results based on 260W prototype of the pro-posed PV MIC system are discussed.The proposed step-up DC–DC converter achieves an efficiency of 97.5%at 260W power to generate the DC-link voltage of 350V from the PV voltage of 36.1V.The overall PV MIC system includ-ing the DC–DC converter and the DC–AC inverter achieves a high efficiency of 95%with the PV current ripple less than 3%variation of the rated PV current.The central-ized PV power control scheme is also evaluated through two sets of proposed PV MIC systems.2.Overall system configurationFig.2shows the circuit diagram of the proposed PV MIC system.The PV MIC system consists of an active-clamping flyback converter with a voltage-doubler rectifier and a sin-gle-phase full-bridge DC–AC inverter.The step-up DC–DC converter consists of a boost type of an active-clamping cir-cuit (C c ,S 1,S 2),a transformer (T ),and a voltage-doubler rectifier (L lk ,C r ,D o 1,D o 2).The switches S 1and S 2are the metal–oxide-semiconductor field effect transistors (MOS-FETs),which are driven complementarily.The full-bridgeinverter (S 3$S 6,L o )is controlled to supply the electrical power to the grid with a unity power factor,performing the MPPT function (Kim,2007).The switches S 3$S 6are the insulated gate bipolar transistors (IGBTs).3.Step-up DC–DC converterFig.3shows the simplified circuit model of the proposed step-up DC–DC converter,which shows the reference directions of currents and voltages.The capacitors C pv and C c are the input capacitor and clamping capacitor,respectively.The capacitor C d is the DC-link capacitor.The capacitors C pv ,C c ,and C d are large enough so that the PV module voltage V pv ,clamping capacitor voltage V c ,and DC-link capacitor voltage V d are constant.The active-clamping circuit is of boost type so that V c >V pv .The diodes D S 1and D S 2are the body diodes of S 1and S 2,respectively.The capacitors C S 1and C S 2are the output capacitors of S 1and S 2,respectively.The turns ratio n of the transformer T is defined as n =N s /N p .L m is the magne-tizing inductor of T at the primary side.L lk is the leakage inductor of T at the secondary side.C r is the resonant capacitor in the voltage-doubler rectifier.D o 1and D o 2are the output rectifying diodes.The proposed step-up DC–DC converter has six operating modes during one switch-ing period T s .Fig.4shows the corresponding circuit dia-gram of the proposed step-up DC–DC converter for each operating mode.Fig.5shows the theoretical waveforms of the proposed step-up DC–DC converter during T s .The duty ratio D is referred to S 1turn-on duty.Mode 1[t 0,t 1]:At t =t 0,S 1is turned on.Since v Lm =V pv ,the magnetizing inductor current i Lm increases linearly as i Lm ðt Þ¼i p ðt 0ÞþV pvL mðt Àt 0Þ:ð1ÞWhen nV pv is applied across the secondary winding of T ,the output rectifying diode D o 1is turned on.A series-reso-nant circuit consisting of L lk and C r is formed at the sec-ondary side.By the series-resonance between L lk and C r,Fig.2.Circuit diagram of the proposed PV MIC system.638W.-Y.Choi,Jih-Sheng (Jason)Lai /Solar Energy 84(2010)636–649the energy stored in the resonant capacitor C r is transferred to the DC-link capacitor C d .The angular resonant fre-quency x r of the series-resonant circuit is x r ¼2p f r ¼1ffiffiffiffiffiffiffiffiffiffiffiL lk C rp ;ð2Þwhere f r is the resonant frequency.By referring the output rectifying diode current i Do 1to the primary side,the pri-mary current i p is expressed as i p ðt Þ¼i p ðt 0ÞþV pvL mðt Àt 0Þþni Do 1;peak sin x r ðt Àt 0Þ;ð3Þwhere i Do 1,peak is the peak value of the output rectifying diode current i Do 1.Mode 2[t 1,t 2]:At t =t 1,the half-resonant period of the output rectifying diode current i Do 1is finished.The output rectifying diode current i Do 1is zero before D o 1is turned off.Zero-current switching of D o 1is achieved without any diode reverse-recovery current at the end of Mode 2.Mode 3[t 2,t 3]:At t =t 2,S 1is turned off.The primary current i p charges C S 1and discharges C S 2.The voltage v S 1across S 1increases from zero to the clamping capacitor voltage V c .Since the capacitor C S (C S =C S 1=C S 2)is very small,the time interval during this mode is considered neg-ligible compared to T s .Mode 4[t 3,t 4]:At t =t 3,the voltage v S 2across S 2is zero.The voltage v S 1across S 1is clamped at V c .The pri-mary current i p begins to flow the body diode D S 2of S 2.S 2is turned on at zero-voltage.Since v Lm =À(V c ÀV pv ),the magnetizing inductor current i Lm decreases linearly as i Lm ðt Þ¼i p ðt 3ÞþV pv ÀV cL mðt Àt 3Þ:ð4ÞWhen n (V pv ÀV c )is applied reversely across the second-ary winding of T ,the output rectifying diode D o 2is turned on.The series-resonant circuit consisting of L lk and C r is formed again.The input power is transferred to the reso-nant capacitor C r by the series-resonance between L lk and C r .By referring the output rectifying diode current i Do 2to the primary side,the primary current i p is expressed asi P ðt Þ¼i p ðt 3ÞþV pv ÀV cL mðt Àt 3ÞÀni Do 2;peak sin x r ðt Àt 3Þ;ð5Þwhere i Do 2,peak is the peak value of the output rectifying diode current i Do 2.Mode 5[t 4,t 5]:At t =t 4,the half-resonant period of the output rectifying diode current i Do 2is finished.The output rectifying diode current i Do 2is zero before D o 2is turned off.Zero-current switching of D o 2is achieved without any diode reverse-recovery current at the end of Mode 5.Mode 6[t 5,t 6]:At t =t 5,S 2is turned off.The current i p discharges C S 1and charges C S 2.The voltage v S 1across S 1decreases from the clamping voltage V c to zero.The time interval during this mode is considered negligible as Mode 3.The next switching cycle begins when S 1is turned on again.From the volt-second balance on the primary winding of T ,the clamping capacitor voltage V c is obtained as V c =V pv /(1ÀD ).From the volt-second balance on the sec-ondary winding of T ,the resonant capacitor voltage V Cr is obtained as V Cr =(1ÀD )V d .During T s ,the PV module power is directly transferred to the DC-link capacitor in two steps.The power P d 1transferred by the first series-res-onance during Mode 1is expressed asP d 1¼1T s Z t 1t 0nV pv i Do 1;peak sin ½x r ðt Àt 0Þ dt ¼nV pv x s i Do 1;peakpx r ;ð6Þwhere x s is the angular switching frequency as x s =2p f s ,and f s is the switching frequency as f s =1/T s .ThepowerFig.3.Simplified circuit model of the proposed step-up DC–DC converter.W.-Y.Choi,Jih-Sheng (Jason)Lai /Solar Energy 84(2010)636–649639P d 2transferred by the second series-resonance during Mode 4is expressed asP d 2¼1T s Z t 4t 3n ðV c ÀV pv Þi Do 2;peak sin ½x r ðt Àt 3Þ dt¼nV pv D x s i Do 2;peakpx r ð1ÀD Þ:ð7ÞSince the DC-link power P d is supplied by the sum of P d 1and P d 2during T s ,we have the following relation:P d ¼nV pv x s i Do 1;peak r þnV pv D x s i Do 2;peakr ð8ÞSince the same components using L lk and C r are usedfor both series-resonant circuits,we assume that i S ,peak =i Do 1,peak =i Do 2,peak .By rearranging (8),we have i S ;peak ¼px r ð1ÀD ÞP dn x s V pvð9ÞThe average DC-link current I d during T s is expressed asI d ¼1T s Z t 1t 0i S ;peak sin ½x r ðt Àt o Þ dt ð10ÞBy simplifying (10),wehaveFig.4.Corresponding circuit diagram of the proposed step-up DC–DC converter for each operating mode during T s .640W.-Y.Choi,Jih-Sheng (Jason)Lai /Solar Energy 84(2010)636–649i S ;peak ¼px r I dx s:ð11ÞFrom (9),(11),and the relation of P d =V d I d ,the volt-age conversion ratio of the proposed step-up DC–DC con-verter is obtained as V d V pv ¼nð1ÀD Þ:ð12ÞFig.6shows the relationship between the PV module voltage V pv and the duty ratio D to generate the DC-link voltage of 350V for different turns ratios of T .The lower PV voltage is used for generating the DC-link voltage for a higher turns ratio of T .To eliminate the reverse-recovery current of the output rectifying diode,the output rectifying diode current should be zero before the output rectifying diode is turned off.For zero-current switching of each output rectifying diode,the half-resonant period of each output rectifying diode cur-rent should be finished before the output rectifying diode is turned off.Then,the following condition should be sat-isfied assin ½x cr D max T s ¼0;ifD max <0:5ð13Þsin ½x cr ð1ÀD max ÞT s ¼0;ifD max >0:5ð14Þwhere D max is the maximum duty ratio,x cr is the critical angular resonant frequency as x cr =2p f cr ,and f cr is the critical resonant frequency.For zero-current switching of each output rectifying diode,the resonant frequency f r should be higher than the critical resonant frequency f cr .Then,the resonant capacitor C r should be determined asC r <1x 2cr L lk¼D 2max T 2sp 2L lk ;if D max <0:5ð15Þ¼ð1ÀD max Þ2T 2sp 2L lk;ifD max >0:5:ð16Þ4.PV current variation reduction controlWhen the full-bridge DC–AC inverter injects the outputpower P o into a single-phase grid,the instantaneous output power p o operates at the double frequency of the output voltage v o .If the output current i o is in phase with the out-put voltage v o ,the instantaneous output power p o injected into the single-phase grid is expressed asp o ¼v o i o ¼ffiffiffi2p V o sin x t Áffiffiffi2p I o sin x t ¼V o I o ð1Àcos 2x t Þð17Þwhere V o and I o are the root-mean-squared (rms)values of the output voltage and output current,respectively.x is the angular frequency of the output voltage as x =2p f ,and f is the grid-frequency.The power ripple at the angular fre-quency of 2x is superimposed to the average DC-link volt-age V d .It is reflected on the instantaneous DC-link voltage v d as a voltage ripple,which introduces the PV current var-iation.To avoid the PV current variation,the step-up DC–DC converter should supply only DC component current I d .The instantaneous DC-link power p d is expressed as p d ¼V d i d ¼V d ðI d þi Cd Þð18Þwhere i Cd is the instantaneous current of the DC-link capacitor.The DC component current I d is expressed as I d ¼V o I o V dð19ÞFig.5.Theoretical waveforms of the proposed step-up DC–DC converter during T s .W.-Y.Choi,Jih-Sheng (Jason)Lai /Solar Energy 84(2010)636–649641From (17)–(19),the instantaneous current i Cd of the DC-link capacitor is expressed as i Cd ¼V o I oV dcos 2x t :ð20ÞFrom the instantaneous current i Cd of the DC-linkcapacitor,the instantaneous DC-link voltage v d can be obtained as follows:v d ¼V d þ1d Z i Cd dt ¼V d þV o I od d sin 2x t :ð21ÞThe instantaneous DC-link voltage v d consists of theaverage DC-link voltage V d and its voltage variation.The DC-link voltage variation introduces the PV current varia-tion followed by the step-up DC–DC converter.In order to reduce the PV current variation for the instantaneous DC-link voltage v d ,the duty ratio in (12)should be v d V pv ¼n1ÀðD ctrl þD var Þð22Þwhere D ctrl is the control duty ratio to regulate the average DC-link voltage V d ,and D var is the PV current variation cancellation duty ing (21),the relation between v d and V pv in (22)can be rewritten as V pv d¼V pvV d 1þV o I o 2x C d V 2dsin 2x tffiV pv V d 1ÀV o I o 2x C d V dsin 2x t :ð23ÞUsing the relation of V d /V pv =n /(1–D ctrl )in (12),(23)can be rewritten asV pv v d ¼1n ð1ÀD ctrl ÀnV pv V o I o2x C d V 3dsin 2x t Þ:ð24ÞFrom (22)and (24),the PV current variation cancella-tion duty ratio D var is given by D var ¼k var V pv I o sin 2x t ð25Þwhere k var ¼nV o2x C d V 3d:ð26ÞFig.7shows the block diagram of the PV current vari-ation reduction controller where V d,ref is the reference valuefor the average DC-link voltage V d .5.Simulation resultsThe suggested PV current variation reduction control is simulated by using PSIM for the proposed system.PSIM is a simulation software package especially designed for power electronics and motor control (Veerachary,2006).The electrical system parameters for the simulation are the DC-link voltage V d =350V,output voltage V o =60Hz/220V,and the output power P o =260W.Acircuit-Fig.6.Relationship between the PV voltage and the duty ratio to generate the DC-link voltage of 350V for different turns ratios of T.Fig.7.Block diagram of the PV current variation reduction controller.642W.-Y.Choi,Jih-Sheng (Jason)Lai /Solar Energy 84(2010)636–649based solar module model is formed,using the photovol-taic array model in Villalva et al.(2009).The electrical characteristics of the PV module are shown in Fig.8.The variable parameters such as solar insolation and module surface temperature are included in the external dynamic linked library(DLL)block,which allows users to write code in C/C++and link it with PSIM.The PV module produces the maximum power of260W at the PV voltage of36.1V.Fig.9shows the overall simulation schematic diagram of the proposed system.The controllers for the proposed step-up DC–DC converter and the full-bridge DC–AC inverter are implemented using two DLL blocks, respectively.In the DC–DC converter controller,the sug-gested PV current variation reduction control in Fig.7is implemented.The PV module voltage V pv,the DC-link voltage V d,and the output current I o are measured in order to generate the DC-link voltage of350V and to reduce the PV current variation.In the DC–AC inverter controller, the maximum power point tracking(MPPT)control and unity power factor control are implemented.The conven-tional incremental and conductance method(Liu et al., 2008)is adopted for MPPT control,which can track the maximum power point of the PV module and generate the current reference for the power factor correction of the DC–AC inverter.The PV module voltage V pv,the PV module current I pv,the output voltage V o,and the output current I o are measured for MPPT and grid-connection of the PV MIC system.Fig.10shows the simulated results of the PV MIC sys-tem when it supplies260W power to the grid.Fig.10(a) shows the PV module current and grid voltage when the suggested PV current variation reduction control is not used in the DC–DC converter.A large ripple current of the double grid-frequency of120Hz appears on the PV module side.Fig.10(b)shows the PV module current and grid voltage when the suggested PV current variation reduction control is implemented in the DC–DC converter controller.The simulation results shows that the low fre-quency PV current variation in the grid-connected PV MIC system can be reduced by the suggested PV current variation reduction control.By adding only the PV current variation cancellation duty ratio to the steady-state control duty ratio of the DC–DC converter,the low frequency PV current is reduced without using any additional circuit components.6.Experimental resultsTo evaluate the performance of the proposed PV MIC system,a260W prototype system was built and tested. Table1shows the major components and parameters of the PV MIC system.For the minimum PV voltage of V pv=22V and the selected turns ratio of n=6,the maxi-mum duty ratio D max is determined as about D max=0.62. For f s=50kHz,the critical resonant frequency f cr is deter-mined as f cr=65kHz by(14).For the leakage inductor of L lk=0.8l H,the resonant capacitor C r is selected as C r=7 l F in order to ensure zero-current switching of the output rectifying diodes by(16).The resonant frequency f r is determined as about f r=67kHz,which is slightly higher than f cr.The full-bridge DC–AC inverter is implemented with a sinusoidal pulse-width modulation(SPWM)method (Lee et al.,2008)at20kHz switching frequency to generate 60Hz/220V output voltage.The control function is imple-mented by using a single-chip microcontroller, dsPIC30F4011(Microchip),which has9-channel10-bit A/D converter,one controller area network(CAN),and 6-PWM channels.Fig.11shows the experimental waveforms of the step-up DC–DC converter at the maximum PV power of 260W for the PV voltage of36.1V.Fig.11(a)shows the voltage and current waveforms of S2.When S2isturned Fig.8.Electrical characteristics of the PV module.W.-Y.Choi,Jih-Sheng(Jason)Lai/Solar Energy84(2010)636–649643644W.-Y.Choi,Jih-Sheng(Jason)Lai/Solar Energy84(2010)636–649system.Fig.9.Overall simulation schematic diagram of the proposed Array Fig.10.Simulation results:(a)PV module current and grid voltage when the suggested PV current variation reduction control is not used.(b)PV module current and grid voltage when the suggested PV current variation reduction control is used.on,the voltage across S2is zero before the switch current i S2changes its direction.Thus,zero-voltage switching of S2is achieved during its turn-on instant.When S2is turned off,the voltage across S2is almost constant without any significant voltage stress.Fig.11(b)and(c)show the volt-age and current waveforms of D o1and D o2,respectively. When the output rectifying diode is turned on,the series-resonance between the leakage inductor L lk and the reso-nant capacitor C r occurs.Before the output rectifying diode is turned off,the half-resonant period of the output rectifying diode current isfinished.Thus,zero-current switching of each output rectifying diode is achieved during its turn-offinstant without any diode reverse-recovery current.Fig.12shows the experimental waveforms of the PV MIC system when it supplies260W power to the grid. Fig.12(a)shows the grid voltage and current waveforms. The grid current is sinusoidal and in phase with the 60Hz/220V grid voltage,which implies that the PV MIC system feeds only real power to the grid with an almost unity power factor.Fig.12(b)shows the PV current and grid voltage without the PV current variation reduction control.The average input current is7.2A,and the peak-to-peak current ripple is1.8A(25.0%).It is observed that a large ripple current of the double grid-frequency of 120Hz appears on the input side of the DC–DC converter without the PV current variation reduction control. Fig.12(c)shows the PV current and grid voltage with the PV current variation reduction control.The peak-to-peak current ripple is less than0.2A(2.7%),which shows thatthe PV current variation is significantly reduced by the sug-gested PV current variation reduction control.Fig.13shows the measured efficiencies of the proposed PV MIC system for the PV module voltage of36.1V. Fig.13(a)shows the instrument to measure the power effi-ciency of the experimental system.YOKOGAWA WT210, as a digital power meter,was utilized for precisely measur-ing the input and output power of the PV MIC system and calculating the efficiency.It has such precise accuracy of the measurement as very low current measurements within 5mA range,DC measurement from0.5Hz to100kHz fre-quency range,and high-speed data update as fast as10 readings per second.Fig.13(b)shows the measured effi-ciencies of the proposed converter,the conventionalfly-back converter(Martins and Demonti,2002),and the conventional active-clampingflyback converter(BoekeTable1Major components and parameters of the PV MIC system.System parameters ValueMajor components and system parametersDC-link voltage V d350VOutput voltage V o60Hz/220VSwitching frequency f s50kHzInputfilter capacitor C pv1200l F/100VDC-link capacitor C d220l F/450VClamping capacitor C c680l F/100VResonant capacitor C r7l F/450VOutput capacitor C s1,C S2540pF/100VTrans fomier turns ratio n=6(N=4,N s=24) Magnetizing inductor10l HLeakage inductor0.8l HOutputfilter inductor1mHMOSFET S1,S2IRFB4310{100V/4A)IGBT S3$S6FGP7N60RUFD(600V/7A) Diode D o l D o2FFPF15U40S(400V/15A)Fig.11.Experimental waveforms of the step-up DC–DC converter at themaximum PV power of260W for the PV voltage of36.1V:(a)voltageand current waveforms of S2.(b)Voltage and current waveforms of D o1.(c)Voltage and current waveforms of D o2.W.-Y.Choi,Jih-Sheng(Jason)Lai/Solar Energy84(2010)636–649645。