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A Novel Single-Stage Solar Inverter using HybridActive Filter with Power Quality ImprovementB. Mariappan, B. G. Fernandes, M.RamamoortyDepartment of Electrical Engineering,Indian Institute of Technology Bombay,Powai, Mumbai-400076, India.E-mail: [email protected], [email protected], [email protected] photovoltaic(PV)systemswith powerelectronic interfaces are becoming popular since they do notcontribute to environmental pollution. However, one of theissues with grid feeding inverter is the requirement of highdc-link voltage. In viewof this, single stage solar invertersusing conventional inverters may not be suitable, since theyrequire input dc voltage higher than the peak of line-linevoltage. Therefore, two-stagetopologies whichtypicallyconsistof onedc-dcpower stage toboost thedc voltage, inadditionto a Current Source Inverter (CSI) for dc-ac conversion arereportedforapplicationswheretheinputvoltageislowerthanthe peak of the output voltage. However, this increases thecircuitry complexity. In addition, CSI requires bulky inductanceinDCside,whichincreaseslosses.Hence, Inthispaper anovelsingle stage solar inverter using shunt active hybridlter ispresented. Theinverterfeaturesasinglepowerstage, withdclink voltage less than the peak line-line voltage, which willreducethepowerlossesandcircuitcomplexity. Inaddition, theproposedsolarinverter canalsoprovideharmoniclteringtoimprove the power qualityof the system. The operationandcontrol ofthenovel singlestagesolarinverterforactivepowercontrol andharmoniccontrol isdescribed. Adetailedanalysis,simulationalong withthe hardware results for the proposedsingle stagesolar inverterispresented. Experiments arecarriedout on a 1.5kWlaboratory prototype which demonstratedthe performance of the inverter for active power control andharmonic compensation. The proposed inverter has an efciencyof 94%, comparedtoanconventional active lterbasedsolarinvertersefciencyof 90%. Moreover, it hasbeenshownthattheswitchingrippleinjectedbytheproposedsolarinverterisjusthalfoftheconventionalactivelterbasedsolarinverter.Index TermsHybrid Active Filter, Active Filter, solarphotovoltaic, D-QControl,HarmonicCompensation.I. INTRODUCTIONPower generation from renewable sources is increasing due toseveralreasons includingenergy securityandenvironmentalconcerns. Solar photovoltaic is one of the major contributorsto renewable power generation. Power electronic invertersareusedas aninterfacewhileconnectingthesesourcestogrid. Since PV modules have relatively low power conversionefciency, the overall system cost can be reduced using highefciencypower conditioners [1]. Ingeneral, theDClinkvoltage of the PV source is lower than the peak grid voltageand their output voltage varies in a wide range according tooperatingconditions [2]. ForboostingPVoutputvoltageinorder to accommodate the buck-type grid connected inverter,atwo-stagetopologythat boosts thePVvoltagebyadc-dc converter in the rst stage andtheninverts it intoacvoltagesinthesecondstagewas reportedin[3], [4]. But,this increases the number of stages and component count andthus reduces the overall system efciency. Hence, single-stageinverter topologies are gaining interest. In this inverter, serialconnection of several PV modules is necessary, so that the PVvoltageismaintainedhigherthanthepeakofinputvoltage[5]. These long strings of panels (and hence cells) bring withthem many complications like large size and poor efciency,when individual panels are running under different conditions[6]. In [7], a current source inverter (CSI) based single stagesolar inverter has been presented. This requires bulky inductorinDCside, whichincreases losses. Further, inCSIlteringswitching ripple at grid side becomes difcult [7].The last several decades have seen a rapid increase of powerelectronics-based loads connected to the utility systeminindustries. However, the proliferation of these non-linear loadshas raised the resulting harmonic distortion levels of thesupplycurrent on the power system. Hybridactive ltersare developed to mitigate the harmonics and provide reactivepower compensation. They consists of passive lter in serieswith active lter. Since passive lter provides high impedanceat fundamental frequency thehybridlterdoesnot needtosupport grid voltage for harmonic compensation. Thus, itrequires very less dc link voltage for harmonic compensation[8-10]. However, ahybridactivelter isusuallyonlyusedfor harmoniccompensation[11]. Sincehybridactiveltersrequire less dc link voltage, they can be preferred for singlestage solar inverters, along with power quality improvement.Asinglephasehybridactivelter for PVapplicationhasbeen presented in [12]. However this method uses a high passlterinparallel with activelterto lterout low frequencyswitching harmonics effectively. Hence, this method requireshighDClinkvoltage. In[13], hybridlterapplicationsforpower quality improvement utilizing renewable energy sourceshas beenpresented. However, this methoduses distributedpassive lters in parallel with active lters. Hence it does notuse the advantage of hybrid active lter. A three phase hybridactive lter with photovoltaic generation and hysteresis currentcontrol has been reported in [14]. This method still requireshigher DC link voltage, as in this conguration, capacitor isnot connected in series with active lter.Inthispaper, anovelsinglestagesolarinverterusingpure978-1-4799-4032-5/14/$31.00 2014 54432Fig. 1. Power Circuit Diagram of Proposed Single Stage Solar Inverterhybridactive lter is proposedfor reducingthe DClinkvoltage requirement and ensuring a single-stage system. D-Qcurrent control is used for active current control. A modiedwide band current control has been used for harmonic com-pensation. Theproposedsolar inverter, lters theharmoniccurrents fromthesourceeffectivelyandat the same timesupplies power from PV arrays to utilities.The power stage and control strategy is described in section II.In section III, control system analysis, mathematical modelingandcontroller designispresented. SectionIVpresentsthesimulationandexperimental resultsalongwithperformancecomparisonoftheproposedsinglestagesolarinverterwithconventional active lter based solar inverter.II. POWER STAGE AND CONTROL STRATEGYThe proposed, single stage solar inverter consists of a passivelterinseries withanactivelteralong withathree phasefull bridge Voltage Source Inverter (VSI) connected to a DCbus capacitor and PV array. Fig.1 shows the proposed systemfeeding a non-linear load of 6kW. In order to reduce the size ofthe passive lter (LC), it is tuned for 7thharmonic frequency(L = 1mH, and C= 240uF). The capacitor has been selectedto supply 7kVAR of reactive power(at 300V(L-L)), which cancompensate for lagging load. Since the series capacitance ofLClter, bears most of fundamental voltage, the requiredDClinkvoltageratingfor thehybridactivelter is muchsmaller thanthat of aconventional pureactivelter [11].Sincehybridactivelter requiresless DCLinkvoltageascompared to active lter, it can be preferred for single stagesolar inverters. Alongwithpowerqualityimprovement, theproposedsystemprovidessignicant advantageintermsofless switching ripple as compared to pure active lter basedsolar inverterandreducedinstallationspace. Moreover, theefciency of the proposed single stage solar inverter is higheras compared to pure active lter based solar inverter.Thecontrol strategyfor theproposedsingle-stagesolar in-verter is shown in Fig.2. The control system has two controlloops. Oneistheactivepowercontrol loop, whichisusedto inject the power fromsolar panel to grid. The activepower control is done using D-Q control method. The othercontrol loop is harmonic current control loop, which is usedtocompensate theharmonic current produced bynon linearFig. 2. Control Strategy for Proposed Single Stage Solar Inverterloads. For harmoniccurrent control, amodiedwide-bandcontrol method hasbeenusedtoeffectively compensate theharmonic currents from source.A. Active PowerControlThe single-phase equivalent circuit andvector diagramofhybrid active lter at fundamental frequency for active powerinjection are shown in Fig.3. Here, Vsis the supply voltage.IFqis the lter current, when lter voltage (VF) is zero. Thelter current (IFq) leads the supply voltage by900, whenVFis zero. Equation (1) shows the relationship betweenIFqandsupply voltage (VS).IFq=

VSXC(1)When the lter voltage (VF) is generated which is 900laggingwith source voltage (VS), the net voltage (VNET)appearingacross the passive lterbranch is the vector sumof VSandVF. This is shown in equation (2).

VNET=

VS

VF= VS+ jVF(2)Nowthecurrent owingthroughthelter (IF), leads thevoltage (VNET) by 900. This is shown in equation (3), and inthe vector diagram.IF=j

VNETXC= j

VS+ jVFXC

= jVSXCVFXC= IFd + jIFqHere IFd= VFXC, IFq=VSXC(3) 54443Fig. 3. Power Circuit and Vector Diagram for active current controlFromequation(3), it canbeobserved that thecurrent (IF)hasactive(IFd)andreactive(IFq)components. Theactivecomponent is directly proportional to the inverter voltage(VF), which is900lagging with source voltage. The reactivecomponent is directly proportional to source voltage (VS).The control strategy of hybrid lter for active power controlis shown in Fig.2. The three phase currents are transformed intotheD-QreferenceframeandtheD-axiscurrent (ActiveCurrent) is controlledbygeneratingQ-axis voltage, whichis proportional to PI Controller output. This is multipliedby coswt, which isaunit vector 900lagging withsourcevoltage derived from PLL. The PI controller has been designedfor20Hzbandwidth, sincethephotovoltaicsystemisveryslow response system.B. Harmonic current controlForharmoniccurrent control, amodiedwide-bandcontrolmethod has been used to effectively compensate the harmoniccurrents from source. Proportional (P) controllers are widelyused for wide-band harmonic current control [16]. However,they cannot track the reference signals composed by substan-tial harmonics without any steady-state error. In addition, theproportional coefcient of P controller, for harmonic compen-sation cannot be very large to guarantee stability of the systemandenoughattenuationforswitchingripples. In[8]-[10], acomposite control strategy with grid current feedback and fthharmonic current feed-forward for improved compensation isproposed. However, both load and grid current are required tobe sensed and the passive impedance is brought into controlloop. Inthispaper, aneffectiveclosedloopPIDcontrollerhasbeenimplementedwithgridcurrentfeedback. ThePIDcontroller has been designed to reduce the steady state errorand the bandwidth of closed loop control is designed at 5 kHz.III. CONTROL SYSTEM ANALYSISThe control system analysis is presented in two sub-sections.Initially, the D-Q control method used for active power controlis presented. Later thewide-bandharmoniccurrent controlloop used for harmonic compensation is presented.A. ActivePowerControl LoopThe active power control has beenimplementedusingD-Qcontrolmethod. ThecontrollersareimplementedinD-Qreference frame where the presence of dc quantities allows theuseofProportional Integral(PI)controllersfortheseloops.Since, thecontrol isdoneusingD-Qmethod, themodelofhybrid active lter in D-Q reference frame needs to be derived.In this sub-section, mathematical model of hybrid active lterin D-Q reference frame is derived. The design of PI controllersinD-Qreferenceframeis explainedandstepresponseofthe system is examined. The power circuit and active currentcontrol loop of proposed solar inverter are shown in Fig.4. Themathematical model of the hybrid active lter is given by:[vS]abc= [iF]abc R+d [iF]abcdt L1C

[iF]abc dt + [vF]abc(4)Differentiating Equation 4;d [vS]abcdt=d [iF]abcdt R+d2[iF]abcdt2 L +1C [iF]abc +d [vF]abcdt(5)Using D-Q transformation,iFaiFbiFc =sinwt coswtsin(wt 120) cosw(wt 120)sin(wt 240) cosw(wt 240)

iFdiFq

(6)Similarly, other parameterscanalsobederivedusingD-Qtransformation. Using D-Q transformation, Equation 5 can bewritten as follows.dvSddt= R(diFddtwiFq)+L(d2iFddt22wdiFqdtw2iFd)+iFdC+dvFddtwvFq(7)Usingsmallsignal analysisandtaking laplacetransform on(a)Single Line DiagramP VDC /2(w/L) s2 + s(R/L) + (1/LC)FdrefMqVfqG(s)fd(b)Closed Loop Control ModelFig. 4. Power Circuit and Active Current Control Loop of Proposed SingleStage Solar Inverter54454(a)Bode Plot without controller (b) Bode Plot with controllerFig. 5. Bode Plot of Open Loop System of Active Current Control LoopFig. 6. Closed Loop Step Response of Active Current Controlboth sides of equation (7),svSdw= iFd[s2Lw+sRw+(1wCwL)]+iFq(s2L+R)vFq + svFdw(8)From Equation (8),iFd can be written as followsiFd=vFqwLs2+ sRL+1LCiFqwL(s2L + R)s2+ sRL+1LCsvFdLs2+ sRL+1LC+vSds2+ sRL+1LC(9)Neglectingthelast threeterms(disturbances), iFdcanbewritten as follows:iFd=vFqwLs2+ sRL+1LC(10)From equation (10), the closed loop control system for activecurrent control canbedrawnas showninFig.4(b). FromFig.4(b) the open loop transfer function of the system is givenas follows.G(s) =iFd(s)Mq(s)=VDC2wLs2+ sRL+1LC(11)Thefollowingvaluesofparametersareconsidered, VDC=400V, L=1mH, C=240uF, R=0.1whichgivestheopen loop transfer function as:iFd(s)Mq(s)=62.8 106s2+ 100s + 4.1 106(12)The Bode plot of above open loop transfer function is shown inFig.5(a). Designing PI Controller (Kp= 0.0189, Ki = 10) for20 Hz bandwidth, the Bode plot of the open loop system withPI controller is given in 5(b). The closed loop step responseoftheabovesystemfromtheabovemathematicalmodelisshowninFromFig.6. Fromthisgure, thecalculatedstepresponse of the system is 50 ms.B. Harmonic CurrentControlLoopThe voltage-mode control isused tocontrol thepower con-verter for harmonic compensation. The power converter gener-ates a compensating voltage that is converted into a compen-satingcurrentinordertolterharmoniccurrentsgeneratedbynonlinearloads. Here, thegridharmoniccurrentsundertheconditionof ideal lteringareregardedas thecontrolreference and the real-time harmonic currents of the grid areconsideredas the feedback. The control systemof hybridactivelter for harmoniccompensationis showninFig.7.When the characteristics of hybrid active lter are ideal, theharmonic currents of the grid are equal to zero, so the referencecurrent is set as zero. FromFig.7, the openlooptransferfunction of harmonic current control loop is given as follows.G(s) =1Zfh(s)=sLs2+ sRL+1LC(13)The following values of parameters are considered, L =1mH, C=240uF, R=0.1whichgives theopenlooptransfer function as follows.G(s) =1Zfh(s)=1000ss2+ 100s + 4.1 106(14)The Bode plot of above open loop transfer function is shownin Fig.8(a). From the above Bode plot the resonant frequencyisfound tobe350Hz. Thephaseshiftofthesystembelowresonant frequencyis +90o, andphaseshift of thesystemaboveresonant frequencyis90o. Designingcontrollerfor5kHz bandwidth, the Bode plot of the open loop system withcontroller is shown in Fig.8(b). Here the controller has beendesigned for phase margin of45o. The controller is designedtoreducethesteadystateerrorbyprovidinglaggingphaseangle before resonant frequency and leading phase angle afterresonant frequency. The transfer function of the controller isgiven as follows.Gc(s) = Kp(1 +swz)(1 +swp) (1 + wl)s(15)Here Kp= 12, wz= 12500rad/sec, wp=62800rad/sec, andwl = 500rad/secIV. SIMULATION AND EXPERIMENTALVERIFICATIONThe simulations based onPSIMsoftware are executedtovalidatetheperformanceoftheproposedsinglestagesolarinverter and the above analysis of proposed solar inverter. Thesimulation parameters considered are given as follows:ACVoltage = 300V (L L), 50Hz, Filter Active Power=1.5kW, (IFdRef: 4A), Filter fund. Reactive Power =Fig. 7. Harmonic Current Control Loop54465(a)Bode Plot without controller (b) Bode Plot with controllerFig. 8. BodePlot of HarmonicCurrent Control LoopwithandwithoutController7kV AR, Filter HarmonickVAR=1.5kV AR, LoadPower=5.2kW, DC Voltage=300V , L=1mH, C=240uF, R= 0.1PWM frequency used is 12.8kHz. Fig.9, shows thesteady state simulation results for 1.5kWpower injec-tion and 1.5 kVARharmonic compensation. The currentreference(IFdRef) given is 4 A. FromFig.9, it can beobserved that theproposed control regulates thelteractivecurrent and effectively controls the harmonic current, sothat thesourcecurrent is freefromharmonics. Theactivecurrent(IFd)injectedtogridis4A, whichcorresponds theactivepower of 1.5kW. Theinverter reactivecurrent (IFq)is19A, whichcorrespondstoreactivepower of 7kVAR.Here, wecanobservethat thesourcecurrent is freefromharmonics and its THD is 4%, while the load current THDis 27%. Fig.10, shows thesimulatedtransient responseofthe system, whenIFdRefchanges from 4A to 6A, with thecontrollers designedasgiveninSection-III. FromFig.10, itcan be observed that the transient response of the simulatedsystem is around 50 ms. Comparing Fig.6 and Fig.10, it canbe observed that the transient response of mathematical modeland simulation model are in close agreement.Experiments are performed forIFdRef= 4A, 1.5kVAR har-moniccompensationand7kVARreactivepower compensa-tion, with300V(L-L) ACVoltage. Theexperimental setupparametersaresimilar tothesimulationcircuit parameters,except for the inductive loading (Load Reactive Power) beingabsent. Thecontrol algorithmis implementedbyusingTIFig. 9. Steady State Simulation Results for Proposed Solar InverterFig. 10. Step Response Simulation of Proposed Solar InverterTMS320F2812 DSP Processor toperform signal processing,such as harmonic calculation, frame transformation, impleme-nation of PI controllers, lters, and PWM algorithm. The DCPower Supply used is Chroma DC Power Supply of 3kW.Fig.11, shows the experimental results of the proposed single-stagesolarinverter. FromFig.11(d), itcanbeobserved thatthe source power before compensation is 5.2kWand thesource power after compensation for proposed solar inverter is3.72kW. So, the power injected to grid (Pg) is around 1.48kW.So the active current (IFd), injected to grid isIFd= (Pg/3VS/3)2 = 4.02AIt can be seen from above that, that active current injected isalmost equal toIFdRef.Fig.11(f), shows theexperimental transient responseof thesystem when IFdRef changes from 4A to 6A. From Fig.11(f),it can be observed that the transient response of the system isaround 50ms. Fig.11(f), shows the DC current value becausedirect measurement ofIFdcomponent in lter current is notpossible. Since, change inIFd component will directly affecttheDCcurrent its responsecanbetakenas theresponseofIFdcomponent. Comparing Fig.6, Fig.10 and Fig.11(f), itcanbeobserved thatthetransient response ofmathematicalmodel, simulation model and experimental system are nearlythe same. Also, the close agreement between these three resultsis clear.FromFig.11(e), it canbeobservedthat theDCpower forproposedsolar inverter is equal to 309 5.10 =1.57kW.Hence, theefciency oftheproposed converter isevaluatedto be 94%. To compare the performance of the proposed solarinverter withconventional activelter basedsolar inverter,the conventional solar inverter is operated for the same valueof compensationas proposedsolar inverter. Fig.12 showsthe performance ofthe conventional active lterbasedsolarinverter for IFdRef= 4A, along with 7 kVAR reactive powerand1.5kVARharmoniccompensation. Note that the DCvoltageintheproposedsolar inverter is 300V, whilethatin the conventional active lter based solar inverter is 500 V.54476(a) SourceVoltage andCurrent Be-fore Compensation(b) Source Voltage and Current AfterCompensation(c) SourcePowerParametersBeforeCompensation(d) Source Power Parameters withProposed Single Stage Solar Inverter(e) DCVoltageandCurrent ofPro-posed Single Stage Solar Inverter(f) Step Response of Proposed SingleStage Solar InverterFig. 11. Experimental Results for Proposed Single Stage Solar Inverter(a) Source Power Parameters Aftercompensation for Conventional Ac-tive Filter based Solar Inverter(b) DCVoltage and current, Aftercompensation for Conventional Ac-tive Filter based Solar InverterFig. 12. Experimental ResultsforConventional ActiveFilterbasedSolarInverterThis distinct feature of the extremely low DC voltage allowsthe proposed solar inverter to result in higher efciency andless switchingripples. From12(a) the source power aftercompensation for conventional active lter based solar inverteris 3.73kW. Sothe power injected togrid is around 1.48kW.FromFig.12(b), theDCpower for activelter basedsolarinverter is equal to 509 3.20 = 1.62kW. So the efciency ofthe conventional active lter based solar inverter is evaluatedto be 90.8%. Moreover, by comparing, Fig.13(a) and Fig.13(b)it can be observed that the conventional active lter based solarinverter produces twice the ripple current as compared to theproposed solar inverter.(a)Harmonic Spectrum of Proposed Solar Inverter(b) Harmonic Spectrumof Conventional Active Filter basedSolar InverterFig. 13. ComparisonofHarmonicSpectrumof conventional activelterbased and proposed solar inverter.V. CONCLUSIONIn thispaper, anovel single-stagesolar inverter isproposedfor athree-phasegrid-connectedinverter whichemploys asingle power stage for power conversion froma lowdcvoltage source to the ac grid system, along with power qualityimprovement. A control method for combined active currentcontrol andharmoniccurrent control ispresented. Analysisof active current control and harmonic current control loop isalso presented. The modeling and control system analysis areexplained and some design guidelines are presented. Simula-tion and experiments are carried out with 1.5kW active power,7 kVAR reactive power and 1.5 kVAR harmonic kVAR. Stepresponseof thesystemis presentedandis shownthat theexperimental andsimulationresultsareincloseagreementwith the results predicted by the model. Further, the efciencyof the proposed solar inverter with the conventional active lterbased solar inverter is compared. It is shown that, the efciencyof the proposed solar inverter is 94%, while the conventionalactive lter based solar inverter efciency is 90.8%. In additionit isshownthat theripplecurrentinjectedbytheproposedsolar inverterishalf of theconventional activelter basedsolar inverter.REFERENCES[1] E. Koutroulis, and K. 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