DECOUPLED CONTROL STRATEGY OF GRID INTERACTIVE INVERTER SYSTEM WITH OPTIMAL LCL FILTER DESIGN

Transcription

1 DECOUPLED CONTROL STRATEGY OF GRID INTERACTIVE INVERTER SYSTEM WITH OPTIMAL LCL FILTER DESIGN ANUP ANURAG (9EE5) Department of Eletrial Engineering, National Institute of Tehnology, Rourkela

2 DECOUPLED CONTROL STRATEGY OF GRID INTERACTIVE INVERTER SYSTEM WITH OPTIMAL LCL FILTER DESIGN A Thesis submitte in partial fulfillment of the reuirements for the egree of Bahelor of Tehnology in Eletrial Engineering By ANUP ANURAG (9EE5) Uner the guiane of Prof. B.CHITTI BABU Department of Eletrial Engineering National Institute of Tehnology Rourkela-7698 (ODISHA) May- - - P a g e

3 DEPARTMENT OF ELECTRICAL ENGINEERING NATIONAL INSTITUTE OF TECHNOLOGY, ROURKELA ODISHA, INDIA-7698 CERTIFICATE This is to ertify that the thesis title Deouple Control Strategy of Gri Interative Inverter System with Optimal LCL Filter Design, submitte to the National Institute of Tehnology, Rourkela by Mr. Anup Anurag, Roll No: 9EE5 for the awar of Bahelor of Tehnology in Eletrial Engineering is a bonafie reor of researh work arrie out by him uner my supervision an guiane. The aniate has fulfille all the presribe reuirements. The thesis whih is base on aniate s own work, has not submitte elsewhere for a egree/iploma. In my opinion, the thesis is of stanar reuire for the awar of a Bahelor of Tehnology in Eletrial Engineering. Prof. B. Chitti Babu Supervisor Department of Eletrial Engineering National Institute of Tehnology Rourkela (ODISHA) - - P a g e

4 ACKNOWLEDGEMENTS I have been highly inebte in the preparation of this report to my supervisor, Prof. B. Chitti Babu, whose patiene an kinness, as well as his aaemi experiene, has been invaluable to me. I oul not have aske for a better role moel, inspirational, supportive, an patient guie. I oul not be prouer of my aaemi roots an hope that I an in turn pass on the researh values an the reams that he has given to me. The informal support an enouragement of many friens has been inispensable, an I woul like partiularly to aknowlege the ontribution of all the stuents working uner Prof. B. Chitti Babu. I woul not have ontemplate this roa if not for my parents, who instille within me a love of reative pursuits, siene an language, all of whih fins a plae in this report. AnupAnurag 9EE5 i P a g e

5 Deiate to My Parents an My Brother ii P a g e

6 ABSTRACT This thesis presents a ontrol strategy for a three-phase gri interative voltage soure inverter (VSI) that links a renewable energy soure to the utility gri through a LCL-type filter. An optimize LCL-type filter has been esigne an implemente so as to reue the urrent harmonis in the gri onsiering the onution an swithing losses at onstant moulation inex (M a ). The ontrol strategy aopte here eouples the ative an reative power loops, thus ahieving esirable performane with inepenent ontrol of ative an reative power injete to the gri. The startup transients are also ontrolle by the implementation of this partiular ontrol metho. Another ontrol tehniue has also been stuie so as to overome the hithes fae by the onventional ontrollers. It ounterbalanes the istortions in gri voltage by injeting sinusoial urrent into the gri. The filter has been esigne onsiering the onution an swithing opper losses as well as ore losses. A traeoff has been mae between the total losses ue to the inutor an the THD% of the gri urrent, an the filter inutor has been esigne aoringly. In orer to stuy the ynami performane of the system an to onfirm the analytial results, the moels are simulate in the MATLAB/Simulink environment an the results are analyze. iii P a g e

7 CONTENTS Abstrat Contents List of Figures List of Tables Abbreviations an Aronyms i ii iv vi vii CHAPTER INTRODUCTION. Motivation. Bakgroun an Literature Review 4. Thesis Objetives 5.4 Organization of Thesis 5 CHAPTER CONTROL OF GRID CONNECTED INVERTER FOR SINUSOIDAL CURRENT INJECTION WITH IMPROVED PERFORMANCE. Introution 8. Mathematial Moeling of Gri Connete Inverter System 8. Control Algorithm of Gri Connete Inverter.. Analysis of Current Controller.. Filter Design.. Ative an Reative Power Control.4 Results an Disussions 5.4. Ative an Reative Power Control 6.4. Current Control Strategy 8 A. Uner Voltage Sag 8 B. Uner line to groun fault C. Uner presene of harmonis using onventional ontroller iv P a g e

8 D. Uner presene of harmonis using MRF PI ontroller.5 Conlusions 4 CHAPTER- DECOUPLED CONTROL STRATEGY OF GRID INTERACTIVE INVERTER SYSTEM. Introution 6. Mathematial Moeling of Three Phase Gri Connete Inverter 6. Control Strategy of the Three Phase Gri Connete Inverter 7.4 Results an Disussions.5 Conlusions CHAPTER-4 FILTER DESIGN USING POWER LOSS CALCULATIONS 4. Introution 5 4. Filter Design Inutor Power Loss Damping Ciruit an Capaitor 7 A. Funamental Freueny Power Loss 7 B. Swithing Freueny Power Loss 7 4. Results an Disussions Conlusions 4 CHAPTER-5 CONCLUSIONS AND FUTURE WORK 5. Conlusions 44 Referenes 45 Publiations 49 v P a g e

9 LIST OF FIGURES Fig. No Name of the Figure Page No.. Shemati iagram of Gri Connete PWM Inverter 8. MRF PI Current Controller struture. Shemati iagram for the Gri Connete Inverter with its ontrol 4.4 (a) Distorte gri voltages 7 (b) omponents of istorte gri voltages. 7 () Converter urrent to the gri 7 () -urrent omponent onverter response 8 (e) Harmoni spetrum of the gri urrent 8.5 (a) Distorte gri voltages 9 (b) Converter urrent to the gri 9 () -urrent omponent onverter response 9 () -urrent omponent onverter response 9 (e) Harmoni spetrum of the gri urrent.6 (a) Distorte gri voltages (b) Converter urrent to the gri () -urrent omponent onverter response () -urrent omponent onverter response (e) Harmoni spetrum of the gri urrent.7 (a) Distorte gri voltages (b) Converter urrent to the gri () -urrent omponent onverter response () -urrent omponent onverter response vi P a g e

10 (e) Harmoni spetrum of the gri urrent..8 (a) Distorte gri voltages (b) Converter urrent to the gri () -urrent omponent onverter response () -urrent omponent onverter response (e) Harmoni spetrum of the gri urrent 4. Shemati Diagram of Three Phase GCI with LCL-type Filter 6. Control System blok iagram of the LCL -type onverter. (a) -link voltage aross the apaitor (b) an axis urrent () Gri urrent waveforms 4. LCL-type filter with passive amping sheme 5 4. Total Power loss vs Inutane value 9 4. Plot between THD% an Power Losses vspu value of inutane Harmoni waveform of the gri urrent at optimum inutor values 4 vii P a g e

11 LIST OF TABLES Table. No. Name of the Table Page No.. Parameters use in Simulation 6. Design Parameters for Control Algorithm 5. Losses at ifferent inutane values 8 4. Design Parameter of LCL type filter 4 viii P a g e

12 ABBREVIATIONS AND ACRONYMS GCI - Gri Connete Inverter DG - Distribute Generation MRF - Multiple Referene Frame PI - Proportional Integral AC - Alternating Current DC - Diret Current MATLAB - MATrix LABoratory VSI - Voltage Soure Inverter PID - Proportional, Integral an Derivative IGBT - Insulate Gate Bipolar Transistor SRF - Synhronous Referene Frame ix P a g e

13 CHAPTER Introution P a g e

14 . MOTIVATION With the impening eregulate environment, it is imperative that the new tehnologies provie aeptable power uality an reliability. It has been prophesize that Distribution Generation (DG) System woul play a very vital role in the oming years for meeting the future energy eman with lean an green environment. Various DG tehnologies are present inluing the photovoltai systems, win turbines, gas turbines, iesel engines an fuel ell systems []. At present, win energy an photovoltai have beome the most ompetitive among all. The avent of power eletroni evies has also ushere a new era for the maintenane of power uality as they effiiently interfae these renewable energy systems to the gri. The integration of these renewable energy to the gri an result is several benefits inluing line loss reution, inrease overall energy effiieny, reue environmental impats, relieve transmission an istribution ongestion et. However, the asynhronous interfae provie by the power eletroni onverter raises issues regaring power uality [].The power onversion unit basially onsists of the soure sie an the gri sie onverters. Due to the persistent narrowing of the stanars in the gri oe omplianes, the gri onnete inverter ontrol has taken the entre stage whenever we onsier the improvement of DGS. The gri oe omplianes inlue ontrol of ative an reative power between the DGS an the gri, balaning the power flow between the input sie onverter an the gri, proviing high output power uality an maintaining synhronization with the gri [].The main funtions of the ontrollers are to maintain the power uality an to ontrol the ative an the reative power of the gri inepenently. Optimal filter esign is neessary so as to reue the urrent harmonis in the gri onsiering the onution an swithing losses at onstant moulation inex (M a ) leaing to better power uality. P a g e

15 . BACKGROUND AND LITREATURE REVIEW A number of ontrol strategies have been propose an are extensively stuie in the available literature [-5]. Proper power flow regulation using vetor ontrol priniple has also been propose in [6]. Dual Vetor Current ontrol whih was first propose in [7] uses two VCC s for positive an negative seuene omponents along with DC link voltage ontrol. Synhronous PI urrent ontrol has also been propose in [8] whih onvert the three phase gri voltages to synhronously rotating () frame for proper eoupling. The gri urrents beome DC variables an thus no steay state-state error ajustment is reuire. A metho for ative an reative power ontrol has been mentione in [9]. It ontrols the DC link voltage by esigning a Voltage Control Loop. However, the transient time has not been taken into onsieration. Harmonis mitigation tehniues have also been propose using high-pass an low-pass filters in [6] whih on t take into aount the effet of the filters on the ontrol loop. The MRF PI ontroller evelope in [] oesn t nee urrent signal high pass an low pass filtering. However, it also oesn t take the filters into onsieration while eriving the ontrol strategy for inepenent ative an reative power ontrol. For stuy purposes, the L-type eoupling terms have been applie to LCL -type filters for esigning the ontrollers [,]. This is just an approximate solution whih auses performane an stability problems for higher values of swithing freuenies []. However, this problem has been mitigate in [] by further aition of seon-orer transfer funtions in the ontrol loops but they o not guarantee the amping of the resonane moe. An alternative approah to takle the problem has been introue in [] whih is base on a linear full state feebak an eabeat ontrol. However, this oesn t onsier the nonlinear ynamis of the link voltage. P a g e

16 Also, for the reution of the urrent harmonis in the gri an optimal LCL- type filter esign is imperative. LCL-type filters installe at onverter outputs offer higher harmoni mitigation, but areful esign is reuire to amp LCL resonane. Out of the two amping proesses available we prefer passive amping for gri onnete appliations [8].. THESIS OBJECTIVES The following objetives are hopefully ahieve at the en of the projet.. To ontrol the three phase gri onnete inverter system for sinusoial urrent injetion with improve performane. To analyse a ontrol strategy for the three-phase gri onnete inverter. To provie an optimal filter esign whih inlues the power loss alulations 4. To analyse the results obtaine through simulation in the MATLAB/Simulink environment..4 ORGANISATION OF THESIS The thesis has been organise into six hapters inluing the introution. Eah hapter is ifferent from the other an is esribe along with the neessary theory reuire to omprehen it. Chapter eals with a ontrol strategy for a three-phase Gri Connete Inverter (GCI) uner abnormal onitions of the gri like voltage sag, line to groun fault an presene of harmonis. This strategy intens to overome the hithes fae by the onventional ontroller working uner abnormal onitions of the gri. The ontrol strategy aopte here ounterbalanes the istortions in gri voltage by injeting sinusoial urrent into the gri. In aition to this, ative an reative power ontrol uring the step hanges of the loa has also been stuie. In orer to stuy the ynami performane of the system, the moels are simulate an the results are analyse. P a g e

17 Chapter presents a ontrol strategy for a three-phase gri interative voltage soure inverter (VSI) that links a renewable energy soure to the utility gri through a LCL-type filter. The ontrol strategy aopte here eouples the ative an reative power loops, thus ahieving esirable performane with inepenent ontrol of ative an reative power injete to the gri. The start-up transients are also ontrolle by the implementation of this partiular ontrol metho. In orer to stuy the ynami performane of the system an to onfirm the analytial results, the moels are simulate in the MATLAB/Simulink environment an the results are analyse. Chapter 4 presents the optimal esign of the LCL-type filter whih links a renewable energy soure to the utility gri through a LCL-type filter. The optimal esign has been mae so as to reue the urrent harmonis in the gri onsiering the onution an swithing losses at onstant moulation inex (M a ). It takes into effet the onution an swithing opper losses as well as ore losses. A trae-off has been mae between the total losses ue to the inutor an the THD% of the gri urrent, an the filter inutor has been mae aoringly. In orer to stuy the ynami performane of the system an to onfirm the analytial results, the moels have been simulate in the MATLAB/Simulink environment an the results are analyse. Chapter 5 onlues the work an esribes the future sope of the same. Some possible limitations in the work have also been provie. The future work that an be one in improving the urrent senario is mentione. 4 P a g e

18 CHAPTER Control of Gri Connete Inverter System for Sinusoial Current Injetion with Improve Performane 5 P a g e

19 . INTRODUCTION A esign of a urrent ontroller along with ative an reative power ontrol in orer to ameliorate the power uality in the gri is shown. This tehniue intens to overome the hithes fae by the onventional ontroller working uner unbalane onitions of the gri. The ontrol strategy aopte ounterbalanes the istortions in gri voltage by injeting sinusoial urrent to the gri through a MRF PI ontroller strategy [9]. For the ative an reative power ontrol, ross oupling is one in orer to eouple the - omponents an thus helping in effetive ontrol of the ative an reative power inepenently.. MATHEMATICAL MODELING OF GRID CONNECTED INVERTER SYSTEM The shemati iagram of three phase gri-onnete PWM inverter is shown in Fig.., whih onsists of link apaitor, IGBT swithes an filter iruit in the gri sie. RENEWABLE ENERGY SOURCE - LEVEL PWM VSI Pin L FILTER UTILITY GRID PHOTO- VOLTAIC (PV) PANEL C + v - L L L Va Vb V DC LINK Fig... Shemati iagram of Gri Connete PWM Inverter Assume that the three phase loop resistane R an L are of the same value, affetion of istribution parameters are negligible an swithing loss an on state voltage rop is negligible. 6 P a g e

20 Base on the topology, the ynami euation of the output sie of the gri onnete inverter an be eue as follows: e ab iab( t) vab( t) L Riab( t) t (.) We an write the ynami euation for the input sie of the gri onnete inverter as: v( t) i i( t) il C t (.) The line voltages an the phase urrents an be transforme into synhronous referene frame ( oorinates) by the use of the transformation matrix v T. v ab (.) where T os( ) os( /) os( /) sin( ) sin( /) sin( /) is the transformation matrix an is the rotating angle of transformation Now we an transform the ynami euation iretly into synhronous referene frame ( oorinates) e i( t) v( t) Ri( t) L LMi( t) t (.4) M By assuming that the three phase voltage soure is balane without the zero seuene omponents we an write i v e L Ri Li t (.5) 7 P a g e

21 i v e L Ri Li t (.6) The s-omain representation an be shown as follows: i ( s) [ e ( s) v ( s) Li ( s)] Ls R i ( s) [ e ( s) v ( s) Li ( s)] Ls R (.7) (.8) For the input sie of the onverter v R L i (.9) CRLs Now for the instantaneous power euations we an write S E. I* P( t) jq( t) (.) where * represents the omplex onjugate P(t) represents the instantaneous ative power an Q(t) represents the instantaneous reative power P Re[( E je )( I ji )*] P( t) [ e i e i ] Q Im[( E je )( I ji )*] Q( t) [ e i e i ] (.) (.). CONTROL ALGORITHM OF GRID CONNECTED INVERTER.. Analysis of Current Controller Conventional urrent ontrollers use only ross oupling an feeforwar terms. It has been seen that even with ertain moifiations in the PLL struture, there is little improvement in the power uality, the main ulprit being the lower orer harmonis. 8 P a g e

22 It is seen that the istribution gri voltage is onsierably istorte ue to the presene of nonlinear loas. This leas to the presene of signifiant lower orer harmoni ontent espeially of 5 th, 7 th, th an th orer. v a V os( ) V5 os(5 ) V7 os(7 ) v b V os( ) V5 os5( ) V7 os7( ) (.) v V os( ) V5 os5( ) V7 os7( ) V, V5, V7,... enotes the magnitue of the harmonis. Transforming these into synhronously rotating referene frame we get v v V ( V ( V 5 5 V V 7 7 ).os(6 ) ( V ).sin(6 ) ( V V V )os( )... )sin( )... (.4) In aition to these, there are other higher orer harmonis present. However, we o not take them into onsieration ue to their low magnitues in the system. Here it is seen that the - gri voltage omponents have basially freueny omponents whih are multiples of six of the freueny of the gri. During unbalane onitions suh as voltage sags, presene of harmonis an fault onitions the onventional ontroller fails to provie the reuire sinusoial gri urrent an voltage whih egraes the power uality of the system. In orer to mitigate lower orer harmonis in the gri urrent, we reuire a moifie urrent ontroller to ensure proper operation of the gri uner istorte onitions. The use of multiple referene frame (MRF) PI ontroller is use here as shown in Fig... The MRF PI ontroller oesn t extrat harmonis from the urrent by the appliation of high-pass an low pass filter but injets a omponent antagonisti to the harmonis present in the same [6]: 9 P a g e

23 Fig... MRF PI Current Controller struture.. Filter Design It is seen that the onverter urrent is mostly plague by the lower orer harmonis, the major ones being the lower orer harmonis. Appropriate L filters are esigne in orer to filter out the harmoni omponents from the urrent an thus making them sinusoial. The esign of the L filters an be one aoring to the formula [7]: V L ( ma ) 4. I rate ripple f SW m a (.5) where I rate is the rate utility urrent, is the maximum ripple magnitue perentage (5 ripple % - 5 %), V is the DC link voltage, L is the total filter inutane, f sw is the swithing freueny (in Hz) an ma is the moulation inex... Ative an Reative Power Control Moreover the ative an reative power ontrol isn t possible in ase of the onventional ontroller. We use the feebak from the gri in orer to generate the urrent referene whih in turn ontrols the ative an reative power inepenently. The avantage of ynami VAR P a g e

24 systems is that they etet an instantaneously ompensate for voltage isturbanes by injeting leaing or lagging reative power at ruial juntures to the power transmission gris. Through the ynami VAR ontrol system, reative power is supplie in the gri with fast ynamis. Thus it helps in regulating the system voltage an stabilizing the gri [8]. The instantaneous power flowing into the gri an be written as S P( t) jq( t) (.6) The instantaneous ative an reative power euations are given by: P( t) [ e Q( t) [ e. i. i e e. i. i ] ] (.7) The - link power is known by: V P V( t). I ( t) V( t). C t (.8) Assuming that the inverter losses are neglete i.e. the swithing losses are zero we an euate the input power with the output power an the power balane an be given by V VC t [ u. i u. i ] (.9) Assuming balane ase an taking the -omponents to be zero we get P e i (.) From (.5) an (.6) it an be seen that the -omponent an the -omponent are highly ouple whih leas to the egraation of the ynami performane. The vetor ontroller eouples these terms an thus proviing the ability to ontrol eah urrent omponent. P a g e

25 Fig... Shemati iagram for the Gri Connete Inverter with its ontrol The elementary priniple of the vetor oriente ontrol metho is to ontrol the instantaneous ative an reative gri power whih an be one by ontrolling the gri urrents, by separate ontrollers inepenent of eah other. Two urrent ontrollers are employe namely -urrent ontroller an -urrent ontroller. The gri voltages an urrents are first sense an with the help of Synhronous Referene Frame (SRF) Phase Loke Loop, the gri phase angle is etete in orer to synhronize the GCI output with gri. The emane amount of urrent an voltage are then estimate from the gri at the esire power fator an the referene urrents in a synhronous frame are alulate. Conseuently, the urrent ontrollers try to reue the error an make the loa urrents follow the referene urrent vetor. Thus we try to ontrol the urrent an therefore ontrolling the inherent power flow. So by this proess, the ative an reative powers are metiulously ontrolle. The onverter DC voltage is etermine by: V [ I ( t) Il ] t C. (.) P a g e

26 The DC voltage an be regulate using PI ontroller by hoosing the urrent referene v I ref K [ Vref V( t)] Ki [ Vref V( t)]. t p (.) wherev ref is the referene DC link voltage, K an p ontroller. The referene urrents an be given by: K are the onstant gains of the PI i i i ref ref [ U [ U. P. P ref ref U U. Q. Q ref ref ] ] (.) u where u, P ref (t) an (t) are the ative an reative power referenes. Now, Q ref v v ref ref e e U U Ri Li Ri Li (.4) Where v ref (t) an v ref (t)are the an voltage referenes. U an U are the effetive voltage referenes. U U K K p p.[ i.[ i ref ref i i ] K ] K i i [ i [ i ref ref i i ]. t ]. t (.5) Where K p an K i are the onstant gains of the PI ontroller..4 RESULTS AND DISCUSSIONS To valiate the MRF PI urrent ontroller for Gri Connete Inverter (GCI), extensive omputer simulations were performe using MATLAB/Simulink. Uner istorte an unistorte onitions, the etaile swithing moel of the spae vetor moulate (SVPWM) gri onnete inverter has been evelope. The system parameters use in the simulations are given in the Table-. P a g e

27 TABLE. PARAMETERS USED IN SIMULATION Parameter Label Value Coupling inutane L 7.5 [mh] Coupling resistane R.6 [Ω] Gri voltage V 4 [V] DC link voltage V 58 [V] Swithing freueny f SW 5 [Hz] Base voltage V b 964 [V] Base urrent I b 7.84 [A] The values of the gains K p an K i are alulate aoring to the formulae [8]: K p. f V SW K i SW. f. V (.6) where L/ R is gri time onstant ; L is the oupling inutane between the GCI an the gri, R is the oupling resistane V is the -link voltage in p.u. f SW is the swithing freueny. The values of the gains K p an K i of the MRF PI ontroller are alulate aoring to the formulae [9] K p 4. T V SW an K i K p where =. s (.7) i i.4. Ative an Reative Power Control For the stuy of ynami VAR ompensation, a single ase is onsiere i.e. a step hange of the loa. A lagging reative loa is ae at. se. as shown in the Fig 4. The referene urrents are generate from the gri sie aoring to the (.) an the atual values are ompare with the referene values through PI ontrollers in orer to bring the urrents an thus the power to their respetive referene values. 4 P a g e

28 Gri phase urrents Iab [p.u.] Gri voltages V [p.u.] Gri voltages vab [p.u.] In orer to estimate the power injetion harateristis we arry out the test of inreasing the reative loa omponent. The step hange to the loa is mae at. se.the response of the ontroller is shown in Fig.4. With the hange in loa at. s, the ative urrent ereases in orer to supply the lagging power to the loa (Fig..4()). The reative urrent first ereases an then inreases to go above the referene value to ompensate for the reative nature of the loa. The onverter phase urrent ereases ue to the hange in the loa as shown in Fig..4(e). The gri voltages show a hange ue to the step hange in the loa. The harmoni spetrum using Fast Fourier Analysis for the urrent is realize in Fig..4(f). We analyze the onverter urrent whih shows a THD of.66 % whih is very muh within the permitte istortion levels as per IEEE speifiations..4. Va Vb V Time [se] (a)..5 V V Time [se] (b).5. Ia Ib I Time [se] () 5 P a g e

29 Mag (% of Funamental) -urrent atual [p.u.].5.4 i urrent atual Time [se] () Funamental (5Hz) =., THD=.66% Freueny (Hz) (e) Fig..4. (a) Distorte gri voltages (b) omponents of istorte gri voltages. () Converter urrent to the gri () -urrent omponent onverter response (e) Harmoni spetrum of the gri urrent.4. Current Control Strategy In orer to authentiate the MRF PI ontroller we take three ifferent onitions of the gri. The first ase investigates the operation uner voltage sag onitions. Next the gri voltages an urrents uner line to groun fault onitions were analyse an finally we observe the GCI operation uner injetion of non-sinusoial gri voltages. The MRF PI urrent ontroller is basially fouse on the mitigation of harmonis in the onverter phase urrents. However, it also shows onsierable improvement in other unbalane onitions. The omparative stuy regaring the presene of harmonis is analyse in etail here. Thorough omparisons regaring the other ases are avoie for brevity. A. Uner Voltage Sag The voltage sag onitions are employe with the help of a three phase programmable AC soure in Simulink environment as shown in Fig..5. The freueny was maintaine at 5 6 P a g e

30 -urrent atual I [p.u.] -urrent atual [p.u.] Converter Phase Current Iab [p.u.] Gri Voltage Vab [p.u.] Hz. Voltage amplitue was maintaine at.468 p.u. The sag is fore at. se an maintaine up to.5 se. (Fig.5(a)). The referene -urrent is taken as a step input an the referene urrent was maintaine at zero. SRF PLL is use for the etetion of phase suh that the -omponent of the gri voltage is maintaine at phase voltage an the omponent is maintaine at zero Va Vb V Time [se] (a).5..5 Ia Ib I Time [se] (b).4.5 urrent atual Time [se] ().5 urrent atual Time [se] () 7 P a g e

31 -urrent atual I [p.u.] Converter Phase Current Iab [p.u.] Gri Phase Voltages Vab [p.u.] (e) Fig..5. (a) Distorte gri voltages (b) Converter urrent to the gri () -urrent omponent onverter response () -urrent omponent onverter response (e) Harmoni spetrum of the gri urrent B. Uner line to groun fault The gri is assume to be uner fault from. se to.5 se. whih was reate with the help of a programmable AC soure in Simulink environment. Referene urrent an referene urrent are taken the same as before. SRF PLL is use for etetion of the phase an to transform the ab to referene frame in orer to maintain proper eoupling between the an voltages of the gri. The gri voltages an gri urrents were then examine Va Vb V Time [se] (a).5..5 Ia Ib I Time [se]. (b) Time [se] () 8 P a g e

32 Converter urrent Iab [p.u.] Gri Voltages Vab [p.u.] -urrent atual [p.u.].5 I Time [se] () (e) Fig..6. (a) Distorte gri voltages (b) Converter urrent to the gri () -urrent omponent onverter response () -urrent omponent onverter response (e) Harmoni spetrum of the gri urrent C. Uner presene of harmonis using onventional ontroller Due to the presene of huge proportions of non-linear loa, the gri voltages generally eviate from its sinusoial nature. A robust ontroller is reuire in orer to maintain reliability an regulate uality power flow to the loa. Fig..7 gives simulation results for GCI operation onnete to the istorte gri when the onventional ontroller is use. The response of the urrent ontroller was observe. All the other parameters remaine the same as in the previous ases Va Vb V Time [se] (a).. Ia Ib I Time [se] (b) 9 P a g e

33 -urrent atual I [p.u.] -urrent atual I [p.u.].5 urrent Time [se].5 () Time [se] () (e) Fig..7. (a) Distorte gri voltages (b) Converter urrent to the gri () -urrent omponent onverter response () -urrent omponent onverter response (e) Harmoni spetrum of the gri urrent. Fig..7 (e) shows that the harmonis ompensation apability is very poor onsiering the use of the onventional ontroller. The THD level of 6. % expresses it all. From the Fig. 7() an 7() it is obvious that the 6 th harmoni an the th harmoni omponents present aroun the referene values aren t eliminate. D. Uner presene of harmonis using MRF PI ontroller In orer to satisfy this rawbak the MRF PI urrent ontroller is use where mitigation tehniues for the 6 th an the th harmonis are employe. Fig..8 shows the simulation results for the GCI operation onnete to istorte gri when the MRF PI ontroller was use. P a g e

34 -urrent atual I [p.u.] -urrent atual [p.u.] Converter Currents Iab [p.u.] Gri Voltages Vab [p.u.].5..5 Va Vb V Time [se] (a).5..5 Ia Ib I Time [se] (b).4.5 urrent atual Time [se] ().5 urrent atual Time [se] () (e) Fig..8. (a) Distorte gri voltages (b) Converter urrent to the gri () -urrent omponent onverter response () -urrent omponent onverter response (e) Harmoni spetrum of the gri urrent It is seen from the Fig..8(e) that, the THD of the gri urrent is reue to.68 % in omparison to the THD of 6. % of onventional ontroller with sinusoial urrent P a g e

35 injetion into gri. The - urrent omponents of gri also show the faster response with proper eoupling over onventional ontroller to improve the gri ynamis..5 CONCLUSIONS This provies an analytial bakgroun of the ative an reative power ontrol strategy for a gri onnete inverter system. In this metho inepenent ontrol of the ative an reative power was realize in an effetive way. It also inlues an improve urrent harmonis ompensation apability through the use of MRF PI ontroller. Comparative stuies mae in the text have shown that the MRF PI ontroller provies better performane in terms of improvement of gri ynamis as ompare to the onventional ontroller. The agreement with the simulation results to a proigious extent with a goo sinusoial gri urrent, a lesser harmoni omponent was also obtaine by MRF PI ontroller. P a g e

36 CHAPTER Deouple Control Strategy of Gri interative Inverter System P a g e

37 . INTRODUCTION A ontrol strategy for a three-phase gri onnete voltage soure inverter that links a renewable energy soure to the gri through an optimize LCL-type filter so as to attenuate the urrent harmonis in the gri is shown. This approah uses a moifie set of variables that removes the non-linearity of the original set of euations. This linear system has been eouple into two ifferent ontrol loops orresponing to the ative an reative power ontrol. The ative power loop is ontrolle by the link energy rather than link voltage an the reative power loop is ontrolle by the reative omponent of the gri urrent. Perfet eoupling an be obtaine by the proper esign of the ontrol struture. The major avantage of having two eouple loops is the neutralization of unesirable transients of reative urrent ause by hanges of ative power an vie versa. Also, the start-up transients are minimize by the use of aitional terms in the ontrol struture.. MATHEMATICAL MODELING OF THE THREE PHASE GRID CONNECTED INVERTER The shemati iagram of three phase gri-onnete PWM inverter is shown in Fig.., whih onsists of link apaitor, IGBT swithes an filter iruit in the gri sie. RENEWABLE ENERGY SOURCE - LEVEL PWM VSI Pin LCL FILTER UTILITY GRID PHOTO- VOLTAIC (PV) PANEL C + v - L L L L L L Va Vb V R R R DC LINK V + - C C C Fig... Shemati Diagram of Three Phase Gri Connete Inverter with LCL-type Filter 4 P a g e

38 Assume that the three phase loop resistane R an L are of the same value an affetion of istribution parameters are negligible. Base on the topology, the ynami euation of the output sie of the gri onnete inverter in the frame an be eue as follows: L i t L i u v v L i t L i u v v C v t C v i i g C v C v i i (.) g t L L i i g t t g L i L i g g v v v g C v v t v g i g p in The transformation has been mae taking the gri voltage angle as the referene. The system is a multivariable nonlinear system of orer seven.. CONTROL STRATEGY OF THE THREE PHASE GRID CONNECTED INVERTER The main objetives of implementing this ontrol strategy are to ontrol the ative an reative power injetion to the gri inepenently an to maintain pure sinusoial form of gri urrent. As seen, the system is a multivariable nonlinear system an the reative power is ontrolle by the reative part of gri urrent whereas the ative power ontrol is one taking the -link energy. In this paper, the eoupling terms an ontroller esign are aomplishe 5 P a g e

39 without any approximation. As a result of whih the performane of the system is not affete by the swithing freueny. The eoupling is one by the introution of a new ontrol variable η whih follows the euation L = v + L i + i L (.) From () we an fin the values of ig i, t t g an i g as t i g t v ig A( ig, v ) (.) L i g t i g ( v L v g ) v L C L ( i i g ) ( ) igp v vg i B( ig, v, i, vg) (.4) L L i g t ( ). i g v L v L ( i C i g ) i ( uv v L ) C( ig, v, i ) uv (.5) L a B a A a i (.6) g where LC an η is the input signal. We get that from η to the reative urrent i g the system is eouple an linearize. The open loop transfer funtion is given by I g( s) ( s) s a s a s a (.7) Taking a =, a = an a = the lose loop transfer funtion results in I I g ref g ( s) ( s) k s( s ) k (.8) wherek is the ontroller gain. 6 P a g e

40 The ontrol law an thus be erive from the above: u L i g ig i ref g ( i i ) t u v i g v t L t t (.9) Similar euations an be erive for the ative power ontrol loop. Basing on feebak w w w tehniue we fin the values of,, t t t an 4 w from (.). 4 t w t v i p D i, p ) (.) g g in ( g in t w v ( i g g L ( v v g )) vg v gig v vg E( ig, v, vg) (.) L L t w v g ( i g L v v ) L g ( v C ( i i g )) v g ( ) vgig v vg i F( ig, v, i ) (.) L 4 4 t w ( ) v g ( i g L ( v v g v )) L g ( v C ( i i g )) v g ( i ( v L v )) v g ( ) Lv g Lvg ( ) ig v v L L g i 7 P a g e

41 8 P a g e g g v u L v v L ) ( g g g v u L v v i v G i ),,, ( w a D E a F a (.) Here it has been assume that the input power p in remains onstant an thus the erivative of the same has been neglete. The open loop transfer funtion an thus be given by a s a s a s a s s s W s G 4 ) ( ) ( ) ( (.4) With the seletion of 4, 4, 6, 4 a a a a this an be simplifie to 4 ) ( ) ( ) ( ) ( s s s W s G (.5) With the help of an integral ontroller s k s C ) (, the lose loop transfer funtion results in ref C k s s k s W s W 4 ) ( ) ( ) ( (.6) The oeffiients are selete to plae the poles at esire loation an thus obtaining esirable transient response. The ontrol law an hene be written as: g ref g w t w t w t w v u L v t w t w w v v L u ) ( (.7)

42 The start-up ontrol is one by some moifiations to the existing ontroller whih isn t shown here for brevity. The ontrol system blok iagram for the LCL-type onverter has been shown in Fig... It an be seen that the terms above are a linear ombination of the system state variables an there are no time erivatives involve. M=L /ω I g A B C Mβ Mβ Mβ M I o v i ref _ + k /s η M _ + _ u w ref + _ η k /s -N _ + u BLACKBOX OF THE CONVERTER AS WELL AS THE FILTER i g /C(.) w N=L /V gω Nβ 4 4Nβ 6Nβ 4Nβ N I o v w D E F G Fig... Control System blok iagram of the LCL -type onverter.4 RESULTS AND DISCUSSIONS A kw solar PV moel onnete to the three phase gri onnete inverter system is onsiere for the stuy an is simulate in MATLAB/Simulink environment.table. gives the esign parameters for the ontrol metho. TABLE. DESIGN PARAMETERS FOR CONTROL ALGORITHM Parameters Rate power P Value kw Rate voltage V V Gri freueny f o 5 Hz Swithing freueny f sw khz DC Link Voltage V 5 V Resonant freueny f r khz Moulation Inex M a.8 9 P a g e

43 Iab( in A) I (in A) Vapaitor (in V) Vapaitor (in V) Vapaitor (in V) The simulation senario has been efine as follows: the system has been given a full irraiation of kw at the beginning whih rops own to 6 W at t=.5 se an the reative urrent step hange from A to A is one at t=. se. Fig. 4 shows the graphs for the gri urrent omponents i an i, the -link apaitor voltage V an the gri urrent waveforms, I ab. It is seen that the -link voltage is regulate to 5 V with small transients at the hange 5 instants Time (in se) Time (in se) Time (in se) 5 (a) Time (in se) (b) I I Ia Ib I Time (in se) () Fig... (a) -link voltage aross the apaitor (b) an axis urrent () Gri urrent waveforms 4 P a g e

44 It is seen that the ontroller shows esirable performane with eouple ative an reative power flow. The reative urrent follows the esire response when a step hange is introue with fewer transients in the -link voltage..5 CONCLUSIONS Here, a ontrol sheme for a three-phase gri-onnete inverter with an LCL-type filter has been investigate. For the inverter system, the ontrol metho has been given to aurately formulate the eoupling terms for LCL-type filter. The eoupling metho obviates the nee for inorporating aitional filters to ompensate for instabilities whih are ause by approximate eoupling one in high swithing freuenies. The agreement with the simulation results to a proigious extent with a goo sinusoial gri urrent, a lesser harmoni omponent was also obtaine. 4 P a g e

45 CHAPTER 4 Filter Design using Power Loss Calulations 4 P a g e

46 4. INTRODUCTION An inverter generally introues higher orer harmonis into the gri system ue to nonlinear harateristis of the inverter swithes. In orer to attenuate these introue harmonis an LCL-type filter is onnete in series with the inverter on the gri sie. It makes sure that the harmoni ontent is below a speifie limit. However, in aition to it the filter auses extra power losses whih are unesirable. Here, the LCL-type filter is hosen over L- type an LC-type ue to its better harmoni attenuation properties for the same value of inutanes [5]. However, a resonane problem is present whih an be mitigate by using ifferent amping methos. Here a passive amping metho has been aopte to eliminate the resonane problem in here, a resistor an a apaitor has been ae in parallel to the original apaitor as shown in fig.. It is known that for gri onnete appliations a passive metho is more appropriate, as when the inverter is swithe off while gri is still on the ON onition, the resistor that is use helps in proteting the system. L L i I g C C U i I U U g R Fig. 4..LCL-type filter with passive amping sheme 4. FILTER DESIGN Here, the filter has been esigne onsiering the THD% of the urrent waveform an losses. It is known that the THD% woul be higher for lower values of L an onution 4 P a g e

47 losses woul be higher for higher value of filter. Hene, the trae-off between them gives the optimal value of inutor to be use in the filter esign. Power losses are aopte from []. The optimal value of inutane obtaine is ivie into L an L in suh a way that the apaitane obtaine shoul be minimal an it ours when L is a little higher than L, sine the harmonis in the urrent entering L is higher than that in L. We an say, L L L (4.) where L L L (4.) 4 We know, r (4.) LC The resonant freueny (ω r ) shoul be in the range of ω o <ω res < (ω sw /) in orer to reue resonane problems an a onstant value in this range is selete. Capaitor value an be obtaine by 4 C (4.4) L r wherec C C (4.5) When uality fator an power losses are onsiere, the effetive amping ours when C an C are eual. The resistane (R ) of amping iruit is obtaine as follows L R (4.6) C 4.. Inutor Power Loss: Power losses in an inutor are ivie into ore loss an opper loss. There are some aitional losses whih are negligible. So both ore an opper losses at funamental an swithing freuenies are alulate using software alle Magneti Design. As input we give inutor value, peak to peak urrent an rms urrent. 44 P a g e

48 4.. Damping Ciruit an Capaitor: Power losses of the amping iruit an be alulate as follows: A. Funamental freueny power loss: It is alulate at the funamental freueny of 5 Hz. At funamental freueny, thevoltage aross the amping iruit (U ) is same as that of gri voltage. Therefore, U U Power loss an be alulate from P Real [ U * ] (4.7) (5) I where, I is urrent flowing through amping iruit an an be erive as follows, g I I sc U (4.8) sc R j C ( j C R ) 5 5 U (4.9) 5C R P (5) U C R (4.) 5 5C R B. Swithing freueny power loss: Gri is onsiere as a short iruit at the swithing freueny but it is an ieal voltage at funamental freueny. So at swithing freueny U g =, U U i.5 j.5 swc R sw sw ( ) j ( swc R r r a ) (4.) I swc ( C R j) U (4.) C R sw sw P [ U * ] (4.) (sw) I Representing U an I as omplex frations, 45 P a g e

49 U a jb Ui (4.4) j I U swc ( x jy) C R sw (4.5) U I * U i a jb U j a jb C * sw i ( ) ( x jy j swc R ) * (4.6) * a b swc P (sw) Real [ U I ] U i x (4.7) C R sw sw sw Here, a. 5, b.5 sw C R,, swc R ( ), x C R a sw, y r r 4. RESULTS AND DISCUSSIONS The funamental opper loss, iron loss an the swithing ore loss an the opper loss were all alulate an are shown in Table 4.. L Copper Loss Swithing Losses TABLE 4. LOSSES AT DIFFERENT INDUCTANCE VALUES Core Loss Funamental Losses Copper Core Loss Loss At funamental freueny At swithing freueny Total losses P a g e

50 THD % Total power losses (in watt) The plot between the total losses an the value of inutor is given in Fig L(pu) Fig. 4..Total Power loss vs Inutane value It is seen that both the ore loss an the opper loss of the inutor at the swithing freueny has a very high value at lower inutane an it ereases with inrease in the inutor values; whereas the losses at funamental freueny inrease with the rise in inutor value. Hene, the total losses ue to the inutor inrease with an inrease in inutane value as seen from the Fig. 5.an a lower inutane is to be selete so that the losses are optimize. THD% Power Loss Inutane (in pu) Fig. 4.. Plot between THD% an Power Losses vs pu value of inutane When the amping iruit losses are stuie, it is observe that at the lower inutor values, losses are higher than that present, at higher inutor values. Fig. 6 shows the plot of 47 P a g e

51 Mag (% of Funamental) total losses an the THD of urrent waveformvs the inutor value. It is seen that at lower value of inutane the total losses off whole filter is very high. The point of intersetion provies the optimal value of the inutor where a traeoff has been mae between the losses an the THD% of urrent waveform 6 Funamental (5Hz) = 74.55, THD=.99% Harmoni orer Fig Harmoni waveform of the gri urrent at optimum inutor values The optimize value of inutane an be foun from the above figure whih omes out to be.5 p.u. whih omes out to be.6 mh. Fig. 7 shows the value of THD% of the gri urrent. It is seen that the THD% is.99% whih is well within the limit of 5% from L =.68 mh. L is hosen to be a bit less than L an it omes out to be.988 mh. The optimal values of the apaitor is then alulate from () by taking the effet of both the losses an the apaity of filter an have been shown in table. At the same time, the one with lower THD% is given prime onsieration. TABLE 4. DESIGN PARAMETERS OF LCL-TYPE FILTER Parameters Value L.8 mh L.988 mh C.7 uf C.7 uf R 7.5 ohms Losses 5.98 W THD%.99 % 48 P a g e

52 4.4 CONCLUSIONS Here, it is seen that the filter esign is one by a trae-off between the THD% an the total losses, an thus provies the optimal values for the filter esign. Simulation results verify the effetiveness of the system. The agreement with the simulation results to a proigious extent with a goo sinusoial gri urrent, a lesser harmoni omponent was also obtaine by the optimization one by the filter esign. 49 P a g e

53 CHAPTER 5 Conlusions 5 P a g e

54 5.. CONCLUSIONS Here, a ontrol sheme for a three-phase gri-onnete inverter with an LCL-type filter has been investigate. For the inverter system, the ontrol metho has been given to aurately formulate the eoupling terms for LCL-type filter. An analytial bakgroun of the ative an reative power ontrol strategy for a gri onnete inverter system is provie. In this metho inepenent ontrol of the ative an reative power was realize in an effetive way. It also inlues an improve urrent harmonis ompensation apability through the use of MRF PI ontroller. Comparative stuies mae in the text have shown that the MRF PI ontroller provies better performane in terms of improvement of gri ynamis as ompare to the onventional ontroller. The agreement with the simulation results to a proigious extent with a goo sinusoial gri urrent, a lesser harmoni omponent was also obtaine by MRF PI ontroller. For the inverter system, the ontrol metho has been given to aurately formulate the eoupling terms for LCL-type filter. It is also seen that the filter esign is one by a trae-off between the THD% of gri urrent an the total losses, an thus provies the optimal values for the filter esign. Simulation results verify the effetiveness of the system. The agreement with the simulation results to a proigious extent with a goo sinusoial gri urrent, a lesser harmoni omponent was also obtaine by the optimization one by the filter esign. 5 P a g e

55 Referenes [] F.Blaabjerg, R. Teooresu, M. Liserre, A.V.Timbus,, "Overview of Control an Gri Synhronization for Distribute Power Generation Systems," IEEE Transations on Inustrial Eletronis,, vol.5, no.5, pp.98-49, Ot. 6 [] Anurag, A.; Babu, B.C.;, "Control of Gri Connete Inverter system for sinusoial urrent injetion with improve performane," In Pro. IEEE Stuents Conferene on Engineering an Systems,, pp.-6, 6-8 Marh [] J. Dannehl, C.Wessels, an F. Fuhs, Limitations of voltage-oriente PI urrent ontrol of gri-onnete pwm retifiers with filters, IEEE Trans. In. Eletron., vol. 56, no., pp. 8 88, Feb. 9. [4] T.CY.Wang, Z.Ye, G.Sinha, X.Yuan, "Output Filter Design for a Gri-interonnete Three-Phase Inverter, "Conferene Reors on PESC', vol., pp , June. [5] Jae-Seok Noh, Jaeho Choi, "Design of Voltage Soure PWM Converter with AC Input LCL Filter", Journal of Power Eletronis, The Korean Institute of Power Eletronis, vol.7, no.5, pp , Otober (Korean) [6] E.M.Azi, D.P.Mareti, V.A.Kati, V.A.; M.S. Dzi, "Gri-onnete Voltage Soure Converter operation uner istorte gri voltage," 4th International Power Eletronis an Motion Control Conferene (EPE/PEMC), vol., no., pp.t-44-t-5, 6-8 Sept. 5 P a g e

56 [7] Hong-Seok Song; Kwanghee Nam;, "Dual urrent ontrol sheme for PWM onverter uner unbalane input voltage onitions," IEEE Transations on Inustrial Eletronis, vol.46, no.5, pp , Ot 999. [8] Ma Liang, T.Q. Zheng, "Synhronous PI ontrol for three-phase gri-onnete photovoltai inverter," Chinese Control an Deision Conferene (CCDC), pp.-7, 6-8 May. [9] Ranjan Ku. Behera, WenzhongGao, A Novel Controller Design for Gri-sie Converter of Doubly Fe Inution Generator for Win Power Interfae: An Experimental Investigation, International Journal of Eletri Power Components an Systems Vol. 8, Issue. 4,. [] M. Liserre, F. Blaabjerg, S. Hansen, an D. i Elettrotenia, Design an ontrol of an ll-filter-base three-phase ative retifier, IEEE Trans. In. Appl., vol. 4, no. 5, pp. 8 9, Sep./Ot. 5. [] E. Wu an P. Lehn, Digital urrent ontrol of a voltage soure onverter with ative amping of LCL resonane, IEEE Trans. Power Eletron., vol., no. 5, pp. 64 7, Sep. 6. [] Khajehoin, S.A.; Karimi-Ghartemani, M.; Jain, P.K.; Bakhshai, A.;, "A Control Design Approah for Three-Phase Gri-Connete Renewable Energy Resoures", IEEE Transations onsustainable Energy, vol., no.4, pp.4-4, Ot. [] Channegowa, P.; John, V.;, "Filter Optimization for Gri Interative Voltage Soure Inverters," Inustrial Eletronis, IEEE Transations on, vol.57, no., pp.46-44, De. [4] Xiyou Chen, DianguoXu, Fenghun Liu, Jianiu Zhang, "A Novel Inverter-Output Passive Filter for Reuing Both Differential- an Common-Moe v/t at the Motor 5 P a g e

57 Terminals in PWM Drive Systems," Trans. on Inustrial Eletronis, vol. 54, no., pp , Feb. 7. [5] Debati Marani, Naga Sowmya Tontepu, B.Chitti Babu, Comparative Stuy between Unipolar an Bipolar Swithing Sheme with LCL Filter for Single-Phase Gri Connete Inverter System, Pro. of IEEE Stuents Conferene on Eletrial, Eletronis an Computer Sienes (SCEECS ), MANIT, Bhopal, Mar/. [6] F.A. Maguee, ; A.Sannino, J. Svensson, J.;, "Transient performane of voltage soure onverter uner unbalane voltage ips", 4 IEEE 5th Annual Power Eletronis Speialists Conferene, PESC 4, vol., no., pp Vol., - 5 June 4. [7] Hanju Cha; T.K.Vu, "Comparative analysis of low-pass output filter for single-phase gri-onnete Photovoltai inverter," Pro, of Twenty-Fifth Annual IEEE Applie Power Eletronis Conferene an Exposition (APEC),, vol., no., pp , -5 Feb.. [8] M.N.N.Amin, O.A.Mohamme, O.A., "Vetor oriente ontrol of voltage soure PWM inverter as ynami VAR ompensator for win energy onversion system onnete to utility gri, Pro. of 5 th IEEE Applie Power Eletronis Conferene (APEC), pp.64-65, Feb-. [9] T.C.Y. Wang, Xiaoming Yuan;, "Design of multiple-referene-frame PI ontroller for power onverters,", Pro. of 4 IEEE 5th Annual Power Eletronis Speialists Conferene, Vol.5, no., pp. - 5, -5 June P a g e

58 PAPERS PUBLISHED. Anup Anurag, T.Naga Sowmya, Debati Marani, B.Chitti Babu, Deouple Control Strategy of Gri Interative Inverter System with Optimal LCL Filter Design, International Journal of Emerging Eletri Power Systems (IJEEPS), UC Berkeley Press, (Paper ID: IJEEPS..5)- Uner review. Anup Anurag, B.Chitti Babu, Control of Gri Connete Inverter System for Sinusoial Current Injetion with Improve Performane, In Pro. IEEE Stuents Conferene on Engineering & Systems (SCES ), MNNIT, Allahaba, Mar/. 55 P a g e