New Control Strategy for Three-Phase Grid-Connected LCL Inverters without a Phase-Locked Loop

Transcription

1 Jornl of Power Electronics, Vol. 3, No. 3, My JPE New Control Strtegy for ThreePhse GridConnected LCL Inverters withot PhseLocked Loop Lin Zho, Ming Yng, Qing Li, nd Ke Go Stte Key Lortory of Power Trnsmission Eqipment & System Secrity nd New Technology, Chongqing University, Chongqing, Chin Astrct The threephse synchronos reference frme phselocked loop (SRFPLL is widely sed for synchroniztion pplictions in power systems. In this pper, new control strtegy for threephse gridconnected LCL inverters withot PLL is presented. According to the new strtegy, crrent reference cn e generted y sing the instntneos power control scheme nd the proposed positiveseqence voltge detector. Throgh theoreticl nlysis, it is indicted tht highqlity grid crrent cn e prodced y introdcing the new control strtegy. In ddition, kind of independent control for rective power cn e chieved nder nlnced nd distorted grid conditions. Finlly, the excellent performnce of the proposed control strtegy is vlidted y mens of simltion nd experimentl reslts. Key words: Control strtegy, Gridconnected inverters, Instntneos power control scheme, LCL filter, Positiveseqence voltge detector I. INTRODUCTION In recent yers, environmentl concerns hve focsed worldwide ttention on solr power technology nd there is strong spport to promote nd develop solr power technology. In photovoltic (PV technology, nd other renewle energy sorces like wind power, power electronic inverters ply key role in controlling, mnging nd delivering power to the power grid []. In sch grid connected mode, the inverter is typiclly controlled s crrent sorce [] to inject certin mont of crrent into the grid. Generlly, the control strtegy sed in gridconnected inverters is cscdedloop control. The internl crrent loop regltes the grid crrent while the externl voltge loop controls the dclink voltge [3], [4]. A control strtegy nmed direct power control (DPC, which is sed on dclink voltge loop cscded with n inner power loop insted of crrent loop, hs een reported in the litertre [5]. In this wy, the crrent injected into the tility network is indirectly Mnscript received Nov. 9, ; revised Fe. 8, 3 Recommended for pliction y Associte Editor JnKen Ji. Corresponding Athor: zholin@cq.ed.cn Tel: , Fx: , Chongqing University Stte Key Lortory of Power Trnsmission Eqipment & System Secrity nd New Technology, Chongqing University, Chin controlled. Moreover, control strtegy employing n oter power loop nd n inner loop hs lso een reported [6]. Nonliner control strtegies like hysteresis control or ded et control [7] re preferred de to their high dynmics. As n lterntive to these control methods, other control strtegies hve een proposed in recent plictions, sch s predictive control [8], [9] nd constntfreqency hysteresis control []. However, in highpower pplictions, n lterntive filter implementtion is chieved with thirdorder otpt filter, sch s n LCL filter, which cn chieve redced levels of hrmonics distortion t lower switching freqency nd with less totl indctnce []. Ths the cost nd weight of the inverter re redced. On the other hnd, de to the need to dmp resonnces, the filter nd the crrent control design re more complex. Active dmping [] is preferred to pssive dmping in order to improve the efficiency of conversion. In this cse, some control strtegies sch s the trditionl hysteresis control cnnot e considered ecse of ctive dmping nd the lower switching freqency. Likewise, in the cse of DPC, this pproch cnnot e sed since the switching hrmonic spectrm is not clerly defined t spred de to the vrile switching freqency. A modified DPC strtegy hs een proposed llowing the connection of threephse inverters to the grid throgh LCL filters [3].

2 488 Jornl of Power Electronics, Vol. 3, No. 3, My 3 r r rc i i i c sl sl c i c c i c cc i cc sc i i i c sl g c r r sl sl i i ic ic s L L g sc c ( c sc s( L L g i i Fig.. Gridconnected inverter with n LCL otpt filter. In this pper, new control strtegy for threephse gridconnected LCL inverters withot PLL, which is sed on the positiveseqence voltge detector, is proposed. This pper is orgnized s follows. First, specil ttention is pid to the description of the threephse gridconnected LCL power converter, nd the choice of the reference frme is nlyzed in detil. Second, n instntneos power control scheme sed on the sttionry reference frme nd the proposed positiveseqence voltge detector for threephse threewire systems re presented, giving rise to new control strctre. Next, the new control strctre is descried nd the design prmeters of the control system re nlyzed. Finlly, the theoreticl stdy is vlidted throgh simltion nd experimentl reslts. II. THREEPHASE GRIDCONNECTED LCL INVERTERS A. System Configrtion In this section, the modeling nd nlysis of single threephse gridconnected inverter with n LCL filter re descried. Althogh LCL filter resonnce hs lredy een discssed in the ville litertre [][3], the im is to choose the right reference frme in threephse gridconnected power converters when thirdorder LCL filter is sed. The circit of threephse gridconnected LCL inverter is shown in Fig.. The series prsitic elements re neglected in oth the power components nd the grid model for the ske of simplifiction, where rc re the inverter otpt voltges, i c re the inverter side crrents, i c re the grid side crrents, cc re the cpcitor voltges, i cc re the cpcitor crrents, L g is the grid impednce, nd c re the voltge sorces. It is worth noticing tht if the threephse system is symmetricl nd lnced, the electric potentils of the M point nd O point re the sme. By sing the Kirchhoff voltge nd crrent lws, the stte eqtions of n LCL filter in the ntrl frme cn e modeled s: ì di k ïl = rk ck dt ï ï dck íc = i k ik ï dt ï dik ï( L Lg = ck k î dt where k=,, c represents the threephse ntrl frme. ( Fig.. Mthemticl model of LCL filter in sttionry reference frme. B. The Choice of the Reference Frme Nowdys, high proportion of the threephse power converters re connected to threephse threewire networks. Hence only two crrent controllers re necessry since the third crrent is given y the Kirchhoff crrent lw. In order to redce the complexity of the control system, the control loops cn e strctred in the sttionry reference frme sing the Clrke trnsformtion [T αβ ] or in the synchronos reference frme sing the Prk trnsformtion [T dq ], where [T αβ ] nd [T dq ] re given y: T é é cosq sinq é = û, ét 3 dq 3 3 û = û sinq cosq. ( û By sing the Clrke trnsformtion [T αβ ], the stte eqtions of the LCL filter in the sttionry frme cn e written s: ìdi di ï = = ï dt L L dt L L ïdc d í = = ï dt C C dt C C ïdi di r c, r c c i i, i i ï = ( c, = ( c dt L Lg dt L Lg ïî. (3 According to (3, the mthemticl model of the LCL filter in the sttionry reference frme is shown in Fig.. Another possile wy to strctre the control loops is to se the synchronos reference frme. In this cse, y sing the Prk trnsformtion [T dq ], trnsforming (3 into the synchronos reference frme gives rise to the following stte eqtions: ìdi di d ï = = ï dt L L dt L L ïdcd d í = = ï dt C C dt C C ïdi di q rd cd wi q, rq cq wi d cq i d id wcq, i q iq wcd d q ï = ( cd d wiq, = ( cq q wid dt L Lg dt L Lg ïî. (4 According to (4, the mthemticl model of the LCL filter in the synchronos reference frme is shown in Fig. 3.

3 Jornl of Power Electronics, Vol. 3, No. 3, My rd rq sl wl wl sl i d i q i cd i cq d sc cd s( L L g wc w( L L g wc w( L L g cq sc s( L L g Fig. 3. Mthemticl model of LCL filter in synchronos reference frme. By compring the mthemticl models shown in Fig. nd Fig.3, it cn e seen tht there re six crosscopling terms, nd tht decopling control is the mjor drwck in the synchronos reference frme. Moreover, the phse ngle of the grid voltge is necessry in this implementtion. However, in the cse of the mthemticl model shown in Fig., things re different. Ech phse cn e independent controlled. Additionlly, if PR controllers re sed for crrent regltion, the complexity of the control ecomes lower when compred to the model shown in the synchronos reference frme. III. INSTANTANEOUS POWER CONTROL SCHEME BASED ON THE STATIONARY REFERENCE FRAME By pplying the instntneos power theory, the ctive nd rective powers in the sttionry reference frme cn e written s: q ér 3 é éi Q = i û û û i d i q. (5 From (5, the crrent reference cn e expressed s: éi é ér 3 é ér = =. (6 i 3 Q Q û û û û û Most of the PV inverters designed for gridinterconnected service operte normlly t nity power fctor. In this cse, the rective power control is not llowed, e.g., y mking Q=, the crrent reference in the αβ reference frme will e given y: éi R é = i 3 û û. (7 However, in cse of grid flt, the inverters cn deliver rective power nd control the ctive nd rective power ccording to the lowvoltge ridethrogh reqirements given y the grid codes. In ddition, in the cse of PV implementtion, the inpt power of the inverters is decided y the mximm power point trcking of the PV rry, nd energy cn only flow onewy. As conseqence, independent control of the rective power cn e chieved y the trditionl instntneos power theory. By sing (6, the instntneos power control scheme sed on the sttionry reference frme cn e expressed s in Fig. 4. In this cse, the crrent reference cn e generted y sing the instntneos power control scheme sed on the instntneos power theory. Moreover, working in the αβ reference frme, the phse ngle informtion is not necessity, nd the filtered grid voltges cn e sed s templte for the reference crrent clcltion. In ddition, the power control scheme will provide indctive rective power when Q ref >, nd it will provide cpcitive rective power when Q ref <. Althogh the grid voltge wveforms re sinsoidl nd lnced nder reglr operting conditions, they cn esily ecome nlnced nd distorted de to the effects of grid flts or nonliner lods. Under these conditions, precise nd fst detection of the instntneos positiveseqence voltge component for the reference crrent clcltion is necessry in order to sty ctively connected, spport the grid services nd keep the genertion p nd rnning [4][6]. IV. DESIGNED POSITIVESEQUENCE VOLTAGE DETECTOR Precise chrcteriztion of the grid voltge is crcil isse in order to ensre the qlity of the power tht is delivered from the PV systems to the grid. In this pper, sch chrcteriztion is performed y mens of positiveseqence voltge detector sed on dole resonnt filter. A. Detection Principle of the PositiveSeqence Component In nlnced grid operting conditions (withot voltge hrmonics, the threephse voltge vector cn e split into its instntneos positive, negtive, nd zeroseqence components, which cn e expressed s: i = cos( wt k p 3 cos( wt k p f cos( wt f (8 3 where the sperscripts, nd define the coefficients for the positive, negtive, nd zeroseqence components, nd k tkes the vles k=,, for i=,, c, respectively. Most of the threephse gridconnected power converters employ threewire connection. Therefore, sing the Clrke trnsformtion, the tility voltge cn e given y: R i inpt Q ref Fig. 4. Instntneos power control scheme sed on the sttionry reference frme. i

4 49 Jornl of Power Electronics, Vol. 3, No. 3, My 3 é é cos( t cos( t T é w é w f = é û = sin( wt. (9 û û sin( wt f û c û From (9, it cn e seen tht the zeroseqence component ws eliminted on the αβ reference frme, which cnnot e controlled in threephse threewire power converters. Moreover, (9 cn e rerrnged in the following form: é cos( wt é cos( wt f p p cos( w cos( w f é =. ( û t t û û Then, the 9 shifted versions of α nd β in ( cn e expressed s follows: é p é p é cos( t cos( t w w f =. ( û cos( wt û cos( wt f û Finlly, ccording to ( nd (, the instntneos positiveseqence voltge component in the αβ reference frme cn e clclted y: é cos( é wt é = = sin( t. ( w û û û B. The Design of the PositiveSeqence Voltge Detector Althogh 9 lgging phseshifting of sinsoidl signl cn e chieve y sing n integrtor or differentitor, the initil vle of the integrtor is ncertin, nd the differentitor is difficlt to implement. In this pper, firstorder llpss filter ws sed to chieve the 9 phseshifting of the α nd β signls s follows: w s H ( s = w s. (3 Fig. 5 shows the Bode plot of the trnsfer fnction of (3. As Fig. 5 clerly shows, the signl is lwys 9 lgging when the inpt freqency ω=ω. If the inpt freqency vries ω=±.5 πrd/s when flt occrs, the phse error is only H =.7. From the reslts, it cn e conclded tht the 9 phse shifter cn esily chieve the 9 shifted versions when the grid freqency vries within permissile rnge, e.g., f=5±.5hz. It is worth noting tht the premise of detecting the positiveseqence component y ( is possile since the grid voltge does not consider the hrmonic components. Therefore, in order to filter the hrmonic components, new filter clled dole resonnt filter ws proposed in this pper. The proposed filter digrm is shown in Fig. 6. The dole resonnt filter is forthorder ndpss filter, nd its trnsfer fnction is given y: k s D( s = ( s ks k w s kw s w ( where ω nd k re the resonnce freqency nd the dmping fctor of the filter Freqency(Hz Fig. 5. Bode plots of the 9 phse shifter. ks s w s w Fig. 6. Strctre of the dole resonnt filter ks 8 Fig. 7. Bode plots of the dole resonnt filter for different vles of k. Bode plots from the trnsfer fnction of (4 re shown in Fig. 7 for severl vles of k. The plot shows tht the higher the vle of k, the fster the response of the filter. However, the gin k lso ffects the ndwidth of the filter. A very high vle for k wold redce the immnity of the filter in terms of the hrmonics in the inpt. On the other hnd, very low vle for k gives rise to very long trnsient response of the filter. Therefore, resonle vle for k shold e selected etween the dynmic response nd the hrmonic rejection sed on the ctl sittion. From the plots shown in Fig. 7, nd considering tht the ctl grid voltge minly contins the 5 th nd 7 th hrmonics in threephse threewire power converters, etter trdeoff etween the dynmic response nd the hrmonic rejection cn e chieved with k=5. In this cse, the response time of the filter is t s 3ms, nd the ttention of the filter t 5Hz is 34dB. In ddition, the phse error is only D = when the inpt freqency vries ω=±.5 πrd/s. Therefore, this vle of k=5 reslts in n interesting selection in terms of hrmonic rejection nd response time.

5 Jornl of Power Electronics, Vol. 3, No. 3, My 3 49 c é T û w ks s w ks s w ks s w ks s w s w s w s w s Fig. 8. Block digrm of the positiveseqence voltge detector. DC power sorce dc dc PI R Q ref inverter PWM αβ c Gc ( s Gc ( s i i P Q control k d i i c αβ k d LCL icc Grid side crrent c αβ i c PCC positiveseqence detection gc Grid impednce Fig. 9. Block digrm of the control system with LCL filter. c Finlly, ccording to (, (3, nd Fig.6, lock digrm of the proposed positiveseqence voltge detector in this pper is shown in Fig. 8. As shown in Fig. 8, y sing the Clrke trnsformtion, only two dole resonnt filters re necessry to filter the hrmonic components in threephse pplictions, one for α nd the other for β. Moreover, ecse of the sper hrmonic ttention fnction of the dole resonnt filter, the proposed positiveseqence voltge detector cn e considered in n nlnced nd highly distorted threephse grid voltge. V. PROPOSED CONTROL STRATEGY Either the inverter side crrent or the grid side crrent of the LCL filter cn e controlled. Ech lterntive hs its own dvntges nd drwcks. Depending on the controlled crrent, specific ctive dmping techniqes hve een proposed. A. The Strctre of the Control System Considering tht the instntneos power control scheme sed on the sttionry reference frme, expressed in Fig. 4, nd the proposed positiveseqence voltge detector, shown in Fig.8, it cn e seen tht if the grid side crrent is controlled, nd the corresponding cpcitor crrent ctive dmping techniqe is chosen, the complete lock digrm tht is shown in Fig. 9 shows the concept of the proposed control pproch. i e i Gc ( s pwm k d k r Y ic s ( L Lg C i Fig.. Control loop for the grid side crrent with feedck of the cpcitor crrent for ctive dmping filter. TABLE I INVERTER SYSTEM PARAMETER VALUES Qntity Rted otpt power P Switching freqency f sw DClink voltge U dc Inverter side indctnce L Filter cpcitnce C Grid side indctnce L Grid impednce L g Nominl grid voltge g Vle 5 [kw] [khz] 68 [V] 4.58 [mh] 4.7 [µf].9 [mh]. [mh] [V rms] As shown in Fig. 9, k d is the dmping fctor of the cpcitor crrent, gc re the ctl grid voltges in the connection point, nd G c (s is the crrent controller of the control system. The sic ide of the proposed control pproch is tht the crrent reference cn e generted y sing the instntneos power control scheme nd the proposed positiveseqence voltge detector. In ddition, the compttionl cost of the control system cn e redced since no trigonometric trnsformtions re performed. B. Prmeters Design of the Control System The design procedre of the LCL filter is eyond of the scope of this pper. For frther reserch, the stdy in [] my e conslted. The following nlysis is sed on the fllridge threephse inverter prmeter vles listed in Tle Ι. It is worth mentioning tht these prmeters hve een extrcted from lortory prototype. As shown in Fig. 9, the control strctre etween the α phse nd the β phse does not contin crosscopling terms. Therefore, in the following prmeters design of the control system cn only e discssed in the α phse. In this cse, the control digrm shown in Fig. 9 cn e depicted in Fig.. As cn e oserved in Fig., the feedck pth of the cpcitor crrent for ctive dmping prposes is implemented. The trnsfer fnction Y etween the grid side crrent i α nd the inverter otpt voltge rα is esily otined from Fig.. i Y = =. (5 3 s L ( L L C s( L L L r g g In Fig., the trnsfer fnction k pwm is the gin of the fllridge threephse inverter, nd cn e pproximted y:

6 49 Jornl of Power Electronics, Vol. 3, No. 3, My 3 Udc k pwm =. (6 Since the control vriles re sinsoidl in this sittion nd de to the known drwck of PI controllers in filing to remove stedystte errors when controlling sinsoidl wveforms, employment of proportionl resonnt (PR controller is necessry, nd the PR controller is defined s: kiwcs GPR ( s = k p (7 s w s w where k p is the proportionl gin, k i is the integrl gin, ω is the resonnce freqency of the controller, nd the ndwidth of the controller is decided y ω c. In ddition, hrmonic compenstion (HC cn e chieved y cscding severl generlized integrtors tned to resonte t desired freqency. Therefore, the crrent controller shown in Fig. cn e expressed y: kihwcs Gc ( s = k p å s w s ( hw. (8 h=,5,7 c As reslt, ccording to the control loop shown in Fig., the loop gin of the control system cn e written s: ì k pwm T = Gc ( s ï 3 s L L C s k pwmkd L C s( L L í. (9 ï L ïî = L Lg By ptting the openloop controltootpt trnsfer fnction into the stndrd normlized qdrtic form, the dmping rtio ζ cn e given y: kd k pwm ( L Lg C z = L ( L L L c g. ( Althogh the higher the vle of ζ, the etter the dmping effect, very high vle for ζ will redce the stility mrgin of the system. Ths etter trdeoff cn e chieved with ζ=.77, nd k d cn e indirectly clclted y (. Finlly, ccording to the inverter prmeter listed in Tle Ι nd the ove nlysis, the control prmeters of Tle II cn e sed, nd Bode plot of the loop gin T is shown in Fig.. From the Bode plot depicted in Fig., it cn e oserved tht the phse mrgin (PM is pproximtely eql to 57.7 t crossover freqency (CF of 58Hz, indicting high stility nd dynmic response. C. The Effect of Grid Impednce An incresingly importnt concern for gridconnected inverters is the effect of grid impednce on inverter control performnce nd stility. Uslly, the vle of the grid impednce is fr less thn tht of the grid side indctnce nder stiff grid conditions (see Tle Ι, nd the effect of the grid impednce on the inverter control performnce cn e ignored. However, the grid impednce vle my ecome high nder wek grid conditions. In this cse, Bode plot of the loop gin Fig.. Bode plot of the loop gin T with PRHC controller. Mgnitde(dB Phse(deg the 5 th hrmonic compenstion the 7 th hrmonic compenstion Lg=.mH Lg=.mH Freqency(Hz Fig.. Bode plot of the loop gin T when the grid impednce increses. TABLE II CONTROL SYSTEM PARAMETER VALUES Prmeter Vle k d.3 k p.55 k i 5 k i5 k i7 ω c T considering different vles of L g hs een depicted in Fig.. From the Bode plot depicted in Fig., it cn e oserved tht the PM nd the CF hve een redced with n increse of the grid impednce, nd tht the system cn ecome nstle de to the redced ndwidth or the chnge in the resonnce freqency. On the other hnd, s the power rnge of the PV inverters increses, the effect of grid impednce ecomes more nd more ovios. According to the lock digrm shown in Fig. 9, the reltionship etween the ctl grid voltge g nd the voltge sorce is given y: di Lg = g. ( dt π

7 Jornl of Power Electronics, Vol. 3, No. 3, My i α i β 3 i (5A/div ( t/s ( / (% (V/div 3 i (A/div ( P(kW/div ( (c Fndmentl(5.5Hz=3.9, THD=.4% Freqency(Hz (d Fig. 3. Response of the positiveseqence voltge detector nder nlnced nd distorted conditions. ( Utility voltge. ( Hrmonic spectrm of the tility voltge. (c Detected positiveseqence signls. (d Hrmonic spectrm of the detected positiveseqence signls. In (, it cn e seen tht there is phse difference etween i nd. It cn lso e seen tht the grid crrent phse is indirectly ffected y the grid impednce nd the grid power. Tht is why the control of the rective power ecomes more importnt in highpower pplictions. VI. SIMULATION RESULTS In order to confirm the effectiveness of the proposed system, MATLAB/SIMULINK model hs een implemented. The electricl prmeters for the simltion model re given in Tle Ι nd Tle II. Q(kvr/div (c Fig. 4. Simltion reslts of the proposed control system. ( Generted crrent reference signls. ( Aphse grid voltge nd threephse crrent. (c Actl ctive nd rective powers. First, n nlnced nd highly distorted threephse grid voltge ws considered in the simltion for demonstrting the excellent performnce of the proposed positiveseqence voltge detector with k=5. In this cse, the fndmentl freqency ws set to 5.5Hz, nd the positive nd negtiveseqence voltge t the fndmentl freqency were set to =3V, = nd =V, =6. In terms of the hrmonics, they were set to 5 =V, 5 =45 nd 7 =V, 7 =3 for the 5 th nd 7 th hrmonics, respectively. In Fig. 3(, the wveforms of the nlnced nd distorted grid voltge considered in this simltion re depicted. Fig. 3(c shows the positiveseqence component detected y the proposed positiveseqence voltge detector, nd the freqency error is only f =.Hz. Likewise, the respective hrmonic spectrms re drwn in Fig. 3( nd (d. From the reslts shown in Fig. 3, it cn e conclded tht the proposed positiveseqence voltge detector is very precise nd fst synchroniztion system. The detected instntneos positiveseqence component cn e sed to clclte the crrent reference throgh (6.

8 494 Jornl of Power Electronics, Vol. 3, No. 3, My 3 ( ( ( ( Fig. 6. Experimentl reslts of step in the rective power reference. ( Aphse grid voltge nd threephse crrent. ( Aphse grid voltge nd Aphse crrent. VII. EXPERIMENTAL RESULTS (c Fig. 5. Experimentl reslts of the positiveseqence voltge detector nder ctl grid operting conditions. ( Utility voltge nd the detected positiveseqence signls. ( Hrmonic spectrm of the tility voltge. (c Hrmonic spectrm of the detected positiveseqence signls. Second, the control system shown in Fig. 9 ws tested nder the forementioned nlnced nd pollted grid conditions. In this simltion, step chnge from kvr to kvr occrs in the indctive rective power reference vle Q ref t t=.4s, nd nother step chnge from 5kw to 3.5kW occrs in the ctive power reference vle P inpt t t=.8s. Fig. 4 shows the simltion reslts of the proposed control system with n LCL filter. The plot in Fig. 4( shows the crrent reference generted y sing the instntneos power control scheme. In trn, Fig. 4( shows the grid crrent nd the voltge wveforms. From the reslts, it cn e seen tht the grid crrent lgs ehind the voltge when the rective power reference chnges t t=.4s, nd the mplitde of the grid crrent increses slightly. Additionlly, in order to mintin constnt rective power, the grid crrent frther lgs ehind the voltge when the ctive power reference redced t t=.8s. Finlly, Fig. 4(c shows the trcking reslts of the ctl ctive nd rective power, which mtch the power reference vle. To experimentlly vlidte the performnce of the proposed control system, prototype of the system, depicted in Fig. 9, hs een constrcted on DG tested. Additionlly, the lgorithm of the control system is implemented y RTLAB, nd the smpling period is set to μs. In the first experiment, the cpility of the proposed positiveseqence voltge detector with k=5 online detecting the positiveseqence component ws tested. Considering tht the pek vle signls of the RTLAB nlog otpt chnnel shown on digitl oscilloscope re 5V, the mgnitde of the detected positiveseqence component shown in Fig. 5( hs een ttented ( gα nd gβ represent the ttented signls. As cn e noted in Fig. 5( nd (c, lthogh the ctl grid voltge wveforms re distorted, the hrmonic nlysis shows tht the detected positiveseqence voltge wveforms re nerly sine wve in shpe, which indictes precise estimtion. In the second experiment, the cpility of the control system shown in Fig. 9 ws tested nder ctl grid conditions. The prcticl mesred wveforms re shown in Fig. 6 nd Fig. 7. Fig. 6 shows tht the experimentl reslts of the rective power reference experienced sdden jmp from kvr to.5kvr. From the reslts, it cn e seen tht the crrent wveforms re inphse with the grid voltge initilly. However, t the end, the crrent wveforms lg ehind the voltge de to the step chnge of the rective power reference.

9 Jornl of Power Electronics, Vol. 3, No. 3, My reference step is not ffected y the inclsion of dditionl control loops. Good performnce is grnteed even nder nlnced nd distorted grid voltges. ACKNOWLEDGMENT This reserch work ws spported y the Ntionl High Technology nd Development Progrm (863 Progrm of Chin nder Project AA5A3. ( ( (c Fig. 7. Experimentl reslts of step in the ctive power reference. ( Aphse grid voltge nd threephse crrent. ( Aphse grid voltge nd Aphse crrent. (c Hrmonic spectrm of the grid crrent. In ddition, s shown in Fig. 7, in order to mintin constnt rective power, the grid crrent frther lgs ehind the voltge when the ctive power reference is redced, which demonstrtes its excellent performnce. VIII. CONCLUSIONS A new control strctre for threephse gridconnected voltge sorce inverters (VSI with n LCLfilter is proposed. By sing the instntneos power control scheme nd the proposed positiveseqence voltge detector, the crrent reference cn e indirectly generted, which voids the complex PLL. The effectiveness of the proposed system for threephse gridconnected VSIs is demonstrted vi simltion reslts, which show significnt improvement in oth the stedy stte nd trnsient ehvior. The sme ehvior is experimentlly verified. The fst dynmic response to REFERENCES [] X. Wng, J. M. Gerrero, F. Bljerg, nd Z. Chen, A review of power electronics sed microgrids, Jornl of Power Electronics, Vol., No., pp. 89, Jn.. [] S. Peng, A. Lo, Y. Chen, nd Z. Lv, Dlloop power control for singlephse gridconnected converters with LCL filters, Jornl of Power Electronics, Vol., No. 4, pp , Jly.. [3] F. Bljerg, R. Teodoresc, M. Liserre, nd A. V. Tims, Overview of control nd grid synchroniztion for distrited power genertion systems, IEEE Trns. Ind. Electron., Vol. 53, No. 5, pp , Oct. 6. [4] R. Inznz, T. Smiy, Y. Fjii, nd E. Ikw, Prllel connection of gridconnected LCL inverters for MWscled photovoltic systems, in Proc. IEEE IPEC, pp ,. [5] T. Nogchi, H. Tomiki, S. Kondo, nd I. Tkhshi, Direct power control of PWM converter withot powersorce voltge sensors, IEEE Trns. Ind. Appl., Vol. 34, No. 3, pp , Mr./ Jn [6] D. Cndsso, L. Vlero, nd A. Wlter, Modeling, control nd simltion of fel cell sed power spply system with energy mngement, in Proc. IEEE IECON, pp. 9499,. [7] P. Mttvelli, G. Spizzi, nd P. Tenti, Predictive digitl control of power fctor preregltors with inpt voltge estimtion sing distrnce oservers, IEEE Trns. Power Electron., Vol., No., pp. 447, Jn. 5. [8] E. Twining nd D. G. Holmes, Grid crrent regltion of threephse voltge inverter with n LCL inpt filter, IEEE Trns. Power Electron., Vol. 8, No. 3, pp , My 3. [9] R. W, S. B. Dewn, nd G. R. Slemon, Anlysis of PWM AC to DC voltge sorce converter nder the predicted crrent control with fixed switching freqency, IEEE Trns. Ind. Appl., Vol. 7, No. 4, pp , Jl./Ag. 99. [] L. Mlesni, P. Mttvelli, nd P. Tomsin, Improved constntfreqency hysteresis crrent control of VSI inverters with simple feedforwrd ndwidth prediction, IEEE Trns. Ind. Appl., Vol. 33, No. 5, pp. 94, Sep./Oct [] K. Jlili nd S. Bernet, Design of LCL filters of ctivefrontend twolevel voltgesorce converters, IEEE Trns. Ind. Electron., Vol. 56, No. 5, pp , My. 9. [] I. J. Ge, V. F. Montgner, nd H. Pinheiro, Design, nd implementtion of rost crrent controller for VSI connected to the grid throgh n LCL filter, IEEE Trns. Power Electron., Vol. 4, No. 6, pp , Jn. 9.

10 496 Jornl of Power Electronics, Vol. 3, No. 3, My 3 [3] L. A. Serp, S. Ponnlri, P. M. Bros, nd J. W. Kolr, A modified direct power control strtegy llowing the connection of threephse inverters to the grid throgh LCL filters, IEEE Trns. Ind. Appl., Vol. 43, No. 5, pp. 3884, Sep./Oct. 7. [4] P. Rodrigez, A. V. Tims, R. Teodoresc, M. Liserre, nd F. Bljerg, Flexile ctive power control of distrited power genertion systems dring grid flts, IEEE Trns. Ind. Electron., Vol. 54, No. 5, pp , Oct. 7. [5] P. Rodrigez, A. Ln, I. EtxeerriOtdi, R. Teodoresc, nd F. Bljerg, A sttionry reference frme grid synchroniztion system for threephse gridconnected power converters nder dverse grid conditions, IEEE Trns. Power Electron., Vol. 7, No., pp. 99, Jn.. [6] P. Rodrigez, J. Po, J. Bergs, J. I. Cndel, R. P. Brgos, nd D. Boroyevich, Decopled dole synchronos reference frme PLL for power converters control, IEEE Trns. Power Electron., Vol., No., pp , Mr. 7. Lin Zho ws orn in Sichn, Chin, in 96. He received his B.S., M.S., nd Ph.D. in Electricl Engineering from Chongqing University, Chongqing, Chin, in 984, 988, nd 4, respectively. Since 984, he hs een engged in teching nd scientific reserch in the fields of power electronic technology nd power systems. From Decemer 8 to Decemer 9, he worked t Drhm University s Visiting Scholr, condcting reserch on microgrids nd renewle energy genertion. His crrently reserch interests inclde key technologies of distrited genertion nd power qlity control. Ming Yng ws orn in Henn, Chin, in 98. He received his B.S. nd M.S. in Electricl Engineering nd Atomtion from Henn Polytechnic University, Henn, Chin, in 5 nd 8, respectively. From Septemer 8 to Agst, he worked s Teching Assistnt t Henn Polytechnic University. He is crrently working towrd his Ph.D. in Electricl Engineering t Chongqing University, Chongqing, Chin. His crrent reserch interests inclde modeling nd control of photovoltic inverters, nd distrited genertion technologies. Qing Li ws orn in Lioning, Chin, in 976. He received his B.S. nd M.S. in Electricl Engineering from Chongqing University, Chongqing, Chin, in 999 nd 6, respectively. Since 999, he hs een engged in teching nd scientific reserch in the fields of power electronic technology nd power systems. He is crrently working towrd his Ph.D. in Electricl Engineering t Chongqing University. His crrent reserch interests inclde power converter control techniqes, renewle energy nd power qlity. Ke Go ws orn in Sichn, Chin, in 973. He received his B.S. nd M.S. in Electricl Engineering from Chongqing University, Chongqing, Chin, in 996 nd 8, respectively. Since 996, he hs een engged in teching nd scientific reserch in the fields of power electronic technology nd power systems. His crrently reserch interests inclde the ppliction of emedded systems nd distrited genertion.