High-Gain Resonant Switched-Capacitor Cell-Based DC/DC Converter for Offshore Wind Energy Systems

Transcription

1 This aricle has been acceped fr publicain in a fuure issue f his jurnal, bu has n been fully edied. Cnen may change prir final publicain. Ciain infrmain: DOI 1.119/TEL , IEEE Transacins n wer Elecrnics 1 HighGain Resnan SwichedCapacir CellBased DC/DC Cnverer fr Offshre Wind Energy Sysems Amir arasar, Suden Member, IEEE, and JulKi Sek, Senir Member, IEEE 1 Absrac Wih he increasing inegrain f renewable energy generain in high pwer grids, ransmissin a he DC level is becming increasingly mre useful han AC ransmissin. In his regard, emerging applicains, such as ffshre wind farms, require a high vlage gain DC/DC cnversin sysem inerface wih high pwer ransmissin newrks. This paper presens a new high vlage gain resnan swichedcapacir (RSC) DC/DC cnverer fr high pwer ffshre wind energy sysems. The prpsed DC/DC cnverer is characerized by he resnan swiching ransiins achieve minimal swiching lsses and maximum sysem efficiency. Therefre, a higher swiching frequency is cnceivable aain a higher pwer densiy. The duble sage upu vlage f he prpsed cnverer peraes a seven imes as high as he inpu vlage wih a small device cun. The upu capacirs are charged and discharged cninuusly by a 18º phase shif wih respec each her eliminae he upu vlage ripples wih he lw capaciance requiremens. The prpsed seriesmdular and cascade cnfigurains shw he inrinsic advanage f being readily applicable mulisage pwer swiching cnverers. The develped plgy has been implemened n a 5kW prype cnverer es is feasibiliy. Index Terms High pwer densiy, high vlage gain, ffshre wind energy, resnan swiching ransiins, resnan swichedcapacir (RSC) cnverer. I. INTRODUCTION ffshre wind farms are grwing rapidly because f heir cmparaively mre sable wind cndiins han nshre Oand landbased wind farms [1]. Offshre 51 MW marine urbines are becming mre aracive fr he wind pwer indusry [34]. In paricular, hey increase he efficiency and reduce generain cs, cmpared previus wind urbine echnlgies [5]. The pwer capaciies f hese ffshre behemhs resul in an increase in he size f each cmpnen [56]. Therefre, ffshre wind urbine manufacurers are aemping creae an pimal design fr large marine urbines. The pimized design f ffshre wind urbines shuld cpe wih he fllwing challenges make high pwer cnversin sysems a feasible alernaive. Bulky and huge elecrical cmpnens have high invesmen css because f he mre This wrk was suppred by he Nainal Research Fundain f Krea(NRF) gran funded by he Krea gvernmen(msi) (1859). The auhrs are wih he Schl f Elecrical Engineering, Yeungnam Universiy, Gyengsan, Gyengbuk 71749, Krea ( a_parasar@ynu.ac.kr; dljk@ynu.ac.kr). difficul erecin and he equipmen ransprain frm he shre he insallain sies [7]. In addiin, here is a greaer need fr high reliabiliy due he inheren lack f urbine access a sea, which makes perain and mainenance mre difficul [8]. Therefre, an pimal pwer cnversin sysem shuld feaure high pwer densiy, high efficiency, high reliabiliy, and lw css fr high pwer ffshre wind energy applicains. On he her hand, he lnger ransmissin disances frm ffshre wind urbines he lad cener lead higher energy lsses due he lw upu vlage f wind urbines. In his regard, high vlage DC (HDC) ransmissin prmises a very flexible and efficien echnlgy fr ffshre wind farms ha requires pwer cnversin sysems sepup and cnrl he wind urbine upu. A cnveninal HDC sysem uses an AC line frequency (5/6 Hz) ransfrmer bs he vlage and AC/DC cnverers fr recificain and pwer flw cnrl [911]. This echnlgy is rbus and reliable, bu i causes a cnsiderable increase in weigh and vlume, which leads higher insallain cs. A high pwer densiy can be bained by replacing he bulky 5/6 Hz ransfrmers wih high frequency ransfrmers [114]. Unfrunaely, high frequency ransfrmers wih large urn rais are difficul design a high vlages and mega pwer levels because f he enrmus expense f he magneic maerial, cre and dialecic lsses. One f he keyenabling cmpnens fr HDC is he high pwer DC/DC cnversin sysem because i has a rigid srucure, is easy cnrl sysem and mre cmpac [1516]. T vercme he increasing pwer lsses and mainain a high pwer densiy, i is expeced ha large marine urbines will require a higher vlage wih high vlage gain DC/DC cnversin sysems inerface wih he pwer ransmissin newrks. Singlemdule DC/DC bs cnverers can hereically achieve infinie vlage cnversin rais bu pracically, he maximum gain is limied by circui imperfecins, such as parasiic elemens and swich cmmuain imes [1718]. Muliplemdule bs cnverers have been prpsed achieve high cnversin rais fr applicains ffshre wind farms [16]. Neverheless, because he duy rai f he main swich is large achieve highvlage gain, he swiching frequency is relaively lw reduce he lsses and als allws sufficien urnff ime fr he swiches. Therefre, increasing he size f passive elemens, such as bs inducrs and filer capacirs, is ineviable due (c) 13 IEEE. ersnal use is permied, bu republicain/redisribuin requires IEEE permissin. See hp:// fr mre infrmain.

2 This aricle has been acceped fr publicain in a fuure issue f his jurnal, bu has n been fully edied. Cnen may change prir final publicain. Ciain infrmain: DOI 1.119/TEL , IEEE Transacins n wer Elecrnics s Lad Fig. 1. rpsed RSC plgy. Fig.. 7level RSC cnverer. he lw swiching frequency. Recenly, he cmmn ypes f swichedcapacir (SC) cnverers are cnsidered as an aracive sluin fr meeing he requiremens, such as high pwer densiy and cnrl simpliciy. In [19], a resnan SC (RSC) cnverer was invesigaed, where an exra inducr was added frm a sinusidal manner wih he capacirs perfrm a sf swiching. In [], a mulilevel RSC plgy was prpsed wih significan benefis, including a mdular srucure, lw vlage sress f he swiches, and reduced swiching lss. On he her hand, he large number f capacirs, high passive cmpnen lsses, and ineviably large physical size f he cnverers have limied he use f hese plgies in high vlage gain ffshre wind energy sysems. A 55 kw 3 (he upu vlage is hree imes he inpu vlage) flyingcapacir DC/DC cnverer was inrduced fr hybrid elecric vehicles [1]. The majr drawbacks are he nnmdular srucure, cmplicaed swiching scheme, and lw vlage gain. An RSC vlage ripler wih inerleaving capabiliy and high efficiency was presened in []. Neverheless, i sill has several prblems including he passive cmpnen cuns when a high vlage gain is required fr high pwer applicains due he lw vlage cnversin rai f he circui. T slve he prblems lised previusly, his paper presens a new highgain RSC DC/DC cnverer fr ffshre wind energy sysems. The prpsed cnverer cmbines he upu f w mdular cells reduce he device cun, upu capaciance requiremens, and al capacir pwer raing. The principle f a sfswiching perain and upu vlage analysis f he prpsed cnverer are described in deail. The upu capacirs are charged and discharged cninuusly by a 18º phase shif wih respec each her eliminae he upu vlage ripples wihu adding exra cmpnens. In his paper, he seriesmdular and cascade RSC cnfigurains are inrduced increase he reliabiliy and reduce he cnrl cmplexiy. These cnfigurains are verified by a simulain and heir efficiency, vlume, weigh, and device cun are cmpared wih a cunerpar highligh is advanages fr high vlage and high pwer ffshre winds applicains. A cmprehensive cllecin f he experimens are carried u evaluae he feasibiliy f he prpsed cnverer. II. ROOSED RSC CONFIGURATION A. General Tplgy Fig. 1 shws he general cnfigurain wih a hreephase generaed AC vlage and an AC/DC cnverer in he frnend f he prpsed RSC cnverer. A large capacir is assumed be used fr energy srage a he upu f he AC/DC cnverer. The RSC cnverer cnsiss f w mdular cells which use a new arrangemen f he slidsae swiches, dides, capacirs, and inducrs. B. rinciple f he rpsed RSC Cnverer Operain Fig. presens a 7level RSC cnverer wih w sages. The RSC cnverer is cmpsed f fur resnan capacirs (C r1, C r, C rb1, and C rb ), w upu filer capacirs (C and C b ), fur resnan inducrs (L r1, L r, L rb1, and L rb ), w upu resnan inducrs (L and L b ), six dides (D 1, D, D, D b1, D b, and D b ), and fur swiches (S 1, S, S b1, and S b ). In his paper, subscrips and b represen he crrespnding variables he circui cmpnens a he p and bm cells, respecively. The swiches (S 1, S ) and (S b1, S b ) are cnrlled cmplemenarily wih a 5% duy cycle minimize he cnducin lsses in he pwer devices and passive cmpnens []. Here, he fllwing assumpins are made simplify he analysis: 1) All he swiches, dides, capacirs, and inducrs are ideal; ) All he capaciances are equal and he inducrs have he same values; 3) The swiching frequency is less han he resnan frequency achieve a zercurren swiching (ZCS) [17]; 4) s is an ideal DC vlage surce and he lad is mdeled by a pure resisr (R lad ). 1) Mde I [, 1 ] [Fig. 3(a)] In he beginning f his mde ( ), S b1 and S b are ON, whereas S 1 and S are OFF (see Fig. 4(a) and (b)). The dides D b1 and D b are reverse biased. The charging currens flw hrugh (D 1, S b1 ) and (D, S b ), as shwn in Fig. 3(a). In he p cell, C r1 and C r are charged, whereas C rb1 and C rb are discharged C b in he bm cell (C rb1 and C rb were previusly charged a ne and w imes he inpu vlage level in Mde III, respecively) hrugh a resnan phenmenn (see he red ded, red dashed, and blue ded dashed lines in Fig. 3(a)). Fig. 5 shws he simplified equivalen circuis in his mde. The resnan inducr currens rise and hen fall in a (c) 13 IEEE. ersnal use is permied, bu republicain/redisribuin requires IEEE permissin. See hp:// fr mre infrmain.

3 This aricle has been acceped fr publicain in a fuure issue f his jurnal, bu has n been fully edied. Cnen may change prir final publicain. Ciain infrmain: DOI 1.119/TEL , IEEE Transacins n wer Elecrnics 3 Sb1 & Sb S1 & S i in i Lb i L i L r1 i Lr i Lrb1 i Lrb i C b i C i S b1 i Sb i S 1 i S Fig. 3. Operaing mdes f he 7level RSC cnverer. (a) Mde I [, 1 ]. (b) Mde II [ 1, ] and Mde I [ 3, 4]. (c) Mde III [, 3]. sinusidal manner, as shwn in Fig. 4(e) and (f). S b1 and S b can hen be OFF under he zercurren cndiin. Here, ppsie energy ransmissin is n allwed because D 1, D, and D b make hree unidirecinal pahs in he resnan circui and blck he reverse curren flws (see Fig. 5). ) Mde II [ 1, ] [Fig. 3(b)] In his mde, all he swiches and dides are urned OFF. The resnances sp a hree lps as shwn in Fig. 3(b). Therefre, he inducr currens are equal zer. The resnan capacir vlages f C r1, C r, C rb1, and C rb are unchanged. The upu capacir vlages f C and C b (C b and C were charged up hree imes he inpu vlage in Mdes I and III, respecively) are discharged he lad, as shwn in Fig. 6. 3) Mde III [, 3 ] [Fig. 3(c)] A he insan, S 1 and S are urned ON, while S b1 and S b are OFF. I can be seen frm Fig. 4(j) ha he currens hrugh S 1 and S are increased by a sfswiching perain Fig. 4. Key wavefrms f he 7level RSC cnverer a he seadysae. (a) and (b) Swiching paerns. (c) Inpu curren. (d) Oupu inducr currens. (e) and (f) Resnan inducr currens. (g) Oupu capacir currens. (h) and (j) Swich currens. wih he halfcycle resnan shape. In his mde, C r1 and C r are discharged C, whereas C rb1 and C rb are charged hrugh a resnan phenmenn as shwn in Figs. 3(c) and 7. In his mde, D b1, D b, and D make hree unidirecinal pahs in he resnan circui avid ppsie energy ransmissin. The curren hrugh L is decreased zer afer he halfresnan perid (refer Fig. 4(d)). A he ime f 3, S 1 and S becme OFF under he zercurren cndiin, as illusraed in Fig. 4(j). 4) Mde I [ 3, 4 ] [Fig. 3(b)] The perain f his mde is similar ha f Mde II. Therefre, all he swiches and dides are urned OFF. The resnances sp a hree lps and he inducr currens are equal zer (see Fig. 4(d), (e), and (f)). Applying he charge balance principle C leads d (i ( ) )d 3 d L 4. (1) 3 where and are he upu pwer and upu vlage, respecively. Frm Fig. 4, by ignring he impac f he shr (c) 13 IEEE. ersnal use is permied, bu republicain/redisribuin requires IEEE permissin. See hp:// fr mre infrmain.

4 This aricle has been acceped fr publicain in a fuure issue f his jurnal, bu has n been fully edied. Cnen may change prir final publicain. Ciain infrmain: DOI 1.119/TEL , IEEE Transacins n wer Elecrnics 4 s D 1 C r1 L r1 Sb1 s S b D L rb1 imes 1 and 3 4, i can be assumed ha 1 Ts /, and 3 4 Ts, hen (1) can be rewrien as i ( ) L sin( r ) () where T s and ω r ( r / Ts ) are he swiching perid and he resnan frequency ( 1 C r1 L r1 By applying he principle f charge balance C r, Ts i ( )d Ts C r Ts il ( )d. (3) Therefre, he resnan capacir and inducr currens, and he upu filer capacir currens f C and C b are i ( ) i ( ) C L sin( r ), r r (4) i ( ) i ( ) C L sin( r ), r1 r1 (5) Ts i C ( ) sin( r ) Ts Ts sin( r ) Ts i C ( ) b. (7) Ts Ts Fr he ksage f he prpsed RSC cnverer as shwn in Fig. 1, he resnan inducr r capacir currens, inpu curren, swich currens, dide currens, and upu vlage can be expressed, respecively, as fllws: k h il rh ( ) sin( r), (8) C rb1 C r L r Sb1 L rb C rb Fig. 5. Simplified equivalen circuis f Mde I [, 1 ]. Fig. 6. Simplified equivalen circui f Mde II [ 1, ] and I [ 3, 4 ]. Fig. 7. Simplified equivalen circuis f Mde III [, 3 ]. S b L rb1 L C rb1 D C b S b1 k h il rbh ( ) sin( r), (9) i ( ) ( k ) in 1 sin( r ), (1) Ts i kh S h ( ) 1, (11) sin( r ) Ts Ts k h1 sin( r) Ts is bh ( ), (1) Ts Ts kh i D ( ) sin( r ) Ts h Ts Ts, (13) Ts i k h D ( ) bh sin( r ) Ts Ts, (14) k ( 1 1) s. (15) where h 1,, 3,... k. Deailed derivains f he mahemaical equains fr he prpsed ksage cnverer are presened in he Appendix. I can be bserved frm (15) ha he prpsed ksage RSC cnverer inherenly ffers he penial fr he large gains by he lwes number f passive cmpnens and swiches due he expnenial effec. C. Oupu lage Analysis f he rpsed RSC Cnverer As explained in Secin IIB, when C is discharged he lad in he p cell, C b is charged hrugh C rb1 and C rb in Mde I (see Fig. 4(g)). Therefre, he p and bm cells perae 18º u f phase. Fig. 8 shws key seadysae vlage wavefrms f he upu and filer capacirs fr he 7level RSC cnverer. The vlage f C during he discharging prcess can be analyzed frm he RC circui wih he lad and during he charging sae can be expressed in a sinusidal frm frm (6). If he iniial vlage f C a is assumed be C, hen he capacir vlage funcin f C can be expressed as / C e RC Ts / C e RC (cs( r ) 1) C Cr ( ) Ts Ts (16) Cr where RC ( RC RladC ) is he ime cnsan f he RC circui. I can be ned ha bh upu capacirs C and C b are discharged he lad wih he RC Rlad CCb /(C Cb ) fr 1 and 3 4 (see Fig. 6). Hwever, he shr imes can be negleced in his analysis. Because RC is much larger han T s, (c) 13 IEEE. ersnal use is permied, bu republicain/redisribuin requires IEEE permissin. See hp:// fr mre infrmain.

5 This aricle has been acceped fr publicain in a fuure issue f his jurnal, bu has n been fully edied. Cnen may change prir final publicain. Ciain infrmain: DOI 1.119/TEL , IEEE Transacins n wer Elecrnics 5 he discharge frm C he lad can be apprximaed linearly and he upu filer capacir vlages are bained as fllws: C Ts Cr C cs( r ) C C r Cr Ts T, (17) s Cr C b Sb1 & Sb C S1 & S Cb Fig. 8. Key seadysae vlage wavefrms f he upu and filer capacirs. Cb cs( r ) C br Cbr Cbr Cb Cbr Cbr Ts Ts Ts. (18) The upu vlage is he sum f he upu filer capacir vlages and inpu vlage. The resuling vlage has a frequency ha is w imes ha f he swiching frequency. The inheren inerleaving prpery f he prpsed circui effecively reduces he upu vlage ripple value wihu adding exra cmpnens. Therefre, he upu capaciance requiremen is ne hird f he capaciance in he cnveninal RSC cnverers fr he same upu vlage ripple. Mrever, he vlage raing f C and C b are always ( s ) / fr he prpsed circui, whereas, in ms SC and RSC cnverers, he upu capacir has he vlage raing value f [35]. D. Cmpnen Sress Analysis and Cmparisn Based n afremenined analysis f he prpsed RSC cnverer in Secin IIB and C, all he cmpnens in he p cell have idenical vlage and curren raings wih he cunerpars in he bm cell. C r1 and C r need susain vlages f s and s, respecively. Therefre, he vlage sresses f S 1 and S are he same as he capacir vlages f C r1 and C r (see Fig. 3 (a) and (c)). Frm (6), he rms currens f C and C b can be bained as fllws: IrmsC 1 T / T s d s T / sin( r ) d T s s, I IrmsC b (19) 1 T / T s sin( r ) d s T / d T s s I () Table I liss he vlage and curren raings f he cmpnens fr he prpsed cnverer and ZCSRSC cnverer in [] fr a vlage gain f 7. The curren raings f he cmpnen are he peak values excep fr he upu filer capacirs. I can be seen frm Table I ha he cmpnens have he differen vlage and curren raings a he differen sages. The number f resnan capacirs and upu filer capacirs are smaller in he prpsed circui han in he ZCSRSC cnverer. The vlampere raing (AR) f a resnan capacir is defined as he prduc f he average vlage raing f he resnan capacir and average charging/discharging curren flwing hrugh i. The al AR (TAR) requiremen f he resnan capacirs fr a vlage gain f 7 can be bained frm Table I. The nrmalized TARs are 8 and 1 fr he prpsed RSC and ZCSRSC circuis, TABLE I lage and curren raings f he cmpnens fr he prpsed and ZCSRSC cnverers Tplgy rpsed cnverer ZCSRSC cnverer[] N. f resnan capacirs 4 6 N. f resnan inducrs 6 6 N f upu filer capacirs 6 lage raing f resnan capacirs s and s s Curren raing f resnan capacirs and inducrs πi, πi 3πI, πi, πi lage raing f filer capacirs 3 s s Curren raing f filer capacirs 4.81I 1.1I,.7I, and (RMS curren) 1.1I N. f swiches 4 lage raing f swiches s and s s Curren raing f swiches 4πI, πi 6πI N. f dides 6 1 lage raing f dides s, 3 s s Curren raing f dides πi, πi πi (c) 13 IEEE. ersnal use is permied, bu republicain/redisribuin requires IEEE permissin. See hp:// fr mre infrmain.

6 This aricle has been acceped fr publicain in a fuure issue f his jurnal, bu has n been fully edied. Cnen may change prir final publicain. Ciain infrmain: DOI 1.119/TEL , IEEE Transacins n wer Elecrnics 6 respecively (see Fig. 9). The general TARC r equain fr he prpsed ksage cnverer can be expressed as k TARC r sik. (1) Deailed prf f (1) is given in he Appendix. A base cmparisn is cnsidered due he differen vlage and curren sresses f he cmpnens. Table II liss he vlage and curren raings f he base cmpnens. Here, fr he ZCSRSC cnverer, he filer capacirs are cnsidered be 5 imes larger han he resnan capacirs achieve nearly same resnan frequency in all he resnan lps. Therefre, a filer capacir requires 5 resnan capacirs in a parallel cnnecin gain he required capaciance. The al number f swiches and dides can be bained wih parallel and serial cnnecins f he base swich and base dide, respecively. Fr example, realize he swich wih a vlage raing f s and a curren raing f 6πI, 6 base swiches are needed in parallel verify he curren raing f he swich in he ZCSRSC circuis (see rws 1 and 11 in Table I). Fig. 1(a) shws he al number f cmpnens fr bh plgies. The prpsed circui emplys far fewer capacirs han hse f he ZCSRSC circui. Tha is because he upu capacirs have larger capaciance and higher curren raings in he ZCSRSC circui. On he her hand, he al number f dides fr he prpsed circui is larger han ha f he ZCSRSC cnverer because f he higher vlage and curren raings f he dides. Bh cnverers have he same swich numbers because he prpsed circui has a lwer curren and higher vlage raing TABLE II lage and curren raings f base cmpnens Base cmpnen lage Curren Base capacir (C) s I Base inducr (L) πi Base swich s πi Base dide s πi Fig. 9. Nrmalized TARs f he resnan capacirs versus vlage gain. f he swiches, whereas he ZCSRSC cnverer requires swiches wih he inpu vlage and curren raings. Finally, cmparaive analysis reveals ha he ZCSRSC circui requires mre passive cmpnen, which resuls in a larger verall size and cnverer cs, paricularly in he higher vlage gains (see Fig.1 (b)). Highpwerdensiy pwer cnversin sysems ffer he ppruniy insall lighweigh cmpnens such as wer and plafrm sysems fr ffshre wind energy sysems. Base inducr number 6 4 ZCSRSC cnverer rpsed cnverer lage gain (b) Fig. 1. Cmpnen cun cmparisns. (a) Cmpnen number fr a vlage gain f 7. (b) Base inducr number versus vlage gain. E. wer Lss Analysis Cnducin lss f pwer devices: wer device cnducin lss can be calculaed using a device apprximain wih a series cnnecin f a DC vlage surce (U ), represening device nsae zercurren vlage and nsae resisance (R c ) [6]. The average and rms currens f he pwer devices can be bained frm (11) and (1), as fllws: k h1 I dcs I h dcs, bh () k h I rmss I h rmss. bh (3) Therefre, he cnducin lss equains f he pwer devices can be derived as kh1 kh cs US R h h cs h, (4) kh 1 kh cd UD RcD, h h h (5) c D U D R c D. (6) Resnan capacir and inducr lsses: Assume ha he resnan capacir ESR is R ESR, and he resnan inducr resisance is R ESL, he resnan capacir and inducr lsses can be bained as fllws: kh C RESRC, rh rh (7) (a) (c) 13 IEEE. ersnal use is permied, bu republicain/redisribuin requires IEEE permissin. See hp:// fr mre infrmain.

7 This aricle has been acceped fr publicain in a fuure issue f his jurnal, bu has n been fully edied. Cnen may change prir final publicain. Ciain infrmain: DOI 1.119/TEL , IEEE Transacins n wer Elecrnics 7 kh L RESLL. (8) rh rh Oupu inducr and capacir lsses: Assume ha he upu capacir ESR is R ESR, and he upu inducr resisance is R ESL, he upu capacir and inducr lsses can be expressed as 4 C RESRC I R rmsc ESRC, (9) L RESLL I R rmsl ESL L. (3) S b1a, S ba S 1A, S A S 1B, S B S b1b, S bb I in [A] I ina [A] I inb [A] III. SIMULATION RESULTS AND EALUATION OF THE ROOSED RSC CONERTER Offshre wind farms in he MW range are needed inerface he high vlage pwer newrks. T cpe wih his siuain, he prpsed seriesmdular and cascade RSC cnfigurains can be inrduced achieve a high vlage gain and high raed pwer. A 1MW wind urbine wih an upu f 6.6 k LL,rms is cnsidered as an inpu surce. This vlage will be bsed 14 k fr HDC ransmissin hrugh a hreephase AC/DC cnverer and he prpsed seriesmdular RSC cnverer. The RSC cnverer by iself has a pr regulain prpery; i nly realizes a designed high vlage gain and high efficiency wih a fixed 5% duy cycle. The AC/DC inerface cnverer supplies a regulaed inpu DC vlage fr he RSC cnverer agains he generaed vlage variain, as shwn in Fig. 1. Hwever, his is n suiable mehd regulae inpu DC vlage f he RSC cnverer under a wide range f he lad variain. In rder regulae he upu vlage agains he lad variain, a lw pwer buckbs cnverer can be cnneced in series wih he prpsed RSC cnverer [7]. The 14level RSC cnverer was develped using MATLAB/Simulink wih he LECS Blckse [8]. The resnan frequency is se 3 khz. All he capacirs and inducrs seleced are 1 µf and 8 µh, respecively. The seriesmdular pwer cnversin sysem has an advanage regarding he reliabiliy fr ffshre wind energy sysems, where mainenance and exchange f pars are he main issue. I Sb1A [A] I S1A [A] Fig. 1. Curren wavefrms f he 14level RSC cnverer. (a) and (b) Swiching paerns. (c) Inpu curren. (d) Inpu currens f cells A and B. (e) Swich currens f cell A. CbA [] CA [] CbB [] [] CrbA [] A[] CB [] CrA [] i in i incell A Tp mdular cell A 1 k 7 k i incell B Bm mdular cell A Tp mdular cell B Bm mdular cell B 7 k (a) Fig. 11. (a) rpsed seriesmdular RSC cnfigurain. (b) Tp mdular cell. (c) Bm mdular cell. 14 k (b) (c) Fig. 13. lage wavefrms f he 14level RSC cnverer. (a) and (b) Oupu filer capacir vlages. (c) Oupu vlage f cell A. (d). Oupu vlage. (e) Resnan capacir vlages f cell A. Fr example, if a single mdule fails, he cnverer can sill funcin, albei a a reduced pwer level. The semicnducr devices in series cnverer experience nly half he al vlage sress, which is an advanage in high vlage cnverers, where several swiches mus be cnneced in series wihsand he raed vlage. Tw mdular cells are designed fr equal (c) 13 IEEE. ersnal use is permied, bu republicain/redisribuin requires IEEE permissin. See hp:// fr mre infrmain.

8 This aricle has been acceped fr publicain in a fuure issue f his jurnal, bu has n been fully edied. Cnen may change prir final publicain. Ciain infrmain: DOI 1.119/TEL , IEEE Transacins n wer Elecrnics 8 filer capacirs are cnsidered larger han he resnan capacirs in he ZCSRSC cnfigurains as discussed in Secin IID. The suiable high pwer capacirs and inducrs wih heir weighs and sizes are seleced frm AX, GSESI, and REO [931]. These cnverers are cmpared based n he fllwing feaures: 1) al number f devices; ) passive cmpnen weighs and vlumes; and 3) lsses. Fig. 14. (a) rpsed cascade RSC cnfigurain. (b) Basic cell. vlage gains f 7 and idenical inpu pwers f 5 MW. Fig. 11(a) shws he prpsed seriesmdular cnfigurain. Each cell A r B includes w p and bm mdular cells, as shwn in Fig. 11(b) and (c). Bh cells perae frm equal inpu vlages and draw he same inpu currens. Fig. 1 shws he simulain wavefrms fr he swiching paerns, inpu curren, inpu currens f cells A and B, and swich currens f cell A. Frm Fig. 1(a) and (b), wih a 9º phase shif beween cells A and B, he inpu curren ripple can be reduced significanly he prper value f A (see Fig. 1(c)). Therefre, he inpu filer size can be lwered cnsiderably by he inerleaving perain. If he cnverer wrks wihu inerleaving funcinaliy and he w cells, A and B, perae wih n phase shif, he inpu curren ripple f he cnverer is wice he inpu curren f cell A. In Fig. 1(d), he inpu curren f each cell has a frequency w imes he swiching frequency, whereas he inpu curren f he seriesmdular RSC cnverer has fur imes he swiching frequency due he inerleaving funcinaliy. Fig. 1(e) shws ha all he swiches can be ON and OFF under he zercurren cndiin. The simulain resuls clsely mach he analysis and perain f he prpsed RSC cnverer in Secin II. Fig. 13 shws he wavefrms f he upu filer capacir vlages fr he cells A and B, upu vlage f cell A. Seriesmdular Cnfigurains The seriesmdular cnfigurains are designed wih he specificains such as inpu vlage, upu vlage, and inpu pwer, used in simulain. Therefre, w mdular cells A and B are designed fr equal vlage gains f 7 as shwn in Fig. 11(a). The vlage and curren sresses f he cmpnens can be bained based n Table I fr he prpsed RSC and ZCSRSC cnverers. B. Cascade Cnfigurains Fr he cascade cnfigurains, an inpu f 1 k DC will be bsed he 15 k DC fr HDC ransmissin. Therefre, hree sages f he prpsed RCS cnverer mus prvide a vlage gain f 15 ( 4 1), as indicaed in Fig. 14(a) and (b). The vlage and curren raings f he cmpnens fr he prpsed cnverer can be bained frm (8) (14) and (44) in he Appendix. Fr he cascade ZCSRSC cnfigurain, 7 sages are required achieve a vlage gain f 15. Each sage cnsiss f w resnan inducrs, w resnan capacirs, w upu filer capacirs, and fur dides []. Fr all he cnfigurains, each swich r dide is cmprised f several series and parallelcnneced devices wihsand he raed curren and vlage. The swiching frequency, inducances, and resnan capacir values are he same as hse in he simulain fr bh RSC and ZCSRSC cnverers. The upu C. Discussin Fig. 15 shws he device cun cmparisns beween seriesmdular and cascade cnfigurains f he prpsed RSC and ZCSRSC cnverers. I can be seen ha he cascade RSC cnverer requires a larger swich cun cmpared he seriesmdular RSC cnverer due he higher vlage and curren raings f he swiches. Fr he ZCSRSC cnverer, he vlage and curren sresses f he dides are he inpu vlage and upu curren ha resuls in a decrease in he dide cun and cnducin lsses. In Figs. 16(a) and 17(a), he passive cmpnen lsses are alleviaed fr he prpsed seriesmdular and cascade cnfigurains due reduced number and lwer curren raing f he passive cmpnens. The nrmalized al lss values sugges ha he prpsed seriesmdular RSC cnverer yields apprximaely similar sysem lsses ha f he ZCSRSC cnverer. The prpsed seriesmdular RSC cnverer achieves an efficiency f higher han 98%. On he her hand, he cascade RSC cnfigurain has higher lsses because an increase in he number f pwer devices in series will increase he swich and dide cnducin lsses. I can be bserved frm Figs. 16(b) and 17(b) ha he passive cmpnen vlumes and weighs are reduced significanly in he cases f he prpsed seriesmdular and cascade RSC cnfigurains. This is because he fac ha he passive cmpnen cuns in he ZCSRSC cnverer are much larger han hse f he prpsed RSC cnfigurains. Fr example, he cascade ZCSRSC cnverer uses 14 inducrs and 8 capacirs, whereas he prpsed cascade RSC circui requires 8 inducrs and 8 capacirs (see Fig. 14) wih lwer curren raings in is circui. The nrmalized TAR Fig. 15. wer device cun cmparisns. (a) Seriesmdular cnfigurains. (b) Cascade cnfigurains (c) 13 IEEE. ersnal use is permied, bu republicain/redisribuin requires IEEE permissin. See hp:// fr mre infrmain.

9 This aricle has been acceped fr publicain in a fuure issue f his jurnal, bu has n been fully edied. Cnen may change prir final publicain. Ciain infrmain: DOI 1.119/TEL , IEEE Transacins n wer Elecrnics 9 requiremens fr he resnan capacirs are 4 and 56 in he cascade RSC and ZCSRSC cnfigurains, respecively (see Fig. 9). Alhugh he RSC cascade cnfigurain has w filer capacirs wih he vlage sresses f 7 s, he ZCSRSC cnverer requires 14 filer capacirs wih he vlage raings f s, larger capaciances, and higher curren raings. Fr example, a ripling f he capaciance requires an increase in he vlume and weigh f he capacir by a facr f apprximaely [3]. If he same lw vlage ripple is cnsidered fr all resnan capacirs, he ZCSRSC cnverer requires larger resnan capaciances due he higher curren raings f he capacirs. Therefre, he al weigh f he capacirs in he ZCSRSC cnfigurains will be fur imes heavier han hse f he prpsed RSC cnfigurains. Cnsequenly, he nrmalized pwer densiy (pu/kg) f he prpsed cnfigurains can be enhanced mre han w and hree imes hse f he ZCSRSC cnverer pwer densiy in hese cmparisns. The physical size f he pwer cnversin sysems is an impran issue because hey mus be aken he places where ffshre wind urbines are lcaed. Bulky and huge elecrical cmpnens incur high invesmen css due he mre difficul erecin, large cranes, lifing vessels, and he equipmen ransprain frm he shre he insallain sies. In her wrds, if he highpwerdensiy elecrical cmpnens are uilized in ffshre wind farms, ffshre flaing plafrms can be fully assembled and insalled wih a reduced burden fr large vessels. Cncepual cmparisns f he prpsed cnfigurains he ZCSRSC cnfigurains shw ha he prpsed cnverer is superir he ZCSRSC cnverer in erms f he passive cmpnen cun, curren and al pwer raings f he passive cmpnens, vlume and weigh. Wih he develpmen f SiC echnlgy in high vlage applicains, he devices wih a higher swiching speed, higher emperaure perain, and greaer vlage blcking capabiliy will reduce he size f he passive cmpnens and increase he cnverer efficiency in he near fuure. is abu.8 khz. Table III liss he specificains f he capacirs and inducrs. Fig. 18 shws he vlage and curren wavefrms f he swich S b. GE is he gaeemier vlage f S b, CE is he cllecremier vlage f S b, and I CE is he cllecremier curren f S b. Frm Fig. 18, i can be seen ha I CE drps zer befre he swiching ransiin realize he zercurren. Cnsequenly, all swiches can be urned ON and OFF under he zercurren cndiin. Fig. 19(b), (c), (d), and (e) shws he experimenal curren wavefrms f he upu capacirs and upu inducrs. I can be bserved ha he curren wavefrms clsely mach he analysis f () and (6) in Secin II and he simulain resuls f Fig. 4(d) and (g). The peak value f he upu inducr curren is.5 A, which is cnsisen wih he analysis f (). The upu vlage and upu filer capacir vlages C and Cb, are shwn in Fig. 19(f), (g), and (h). These vlages have peak peak vlage ripples f 4 and 1, respecively. The upu vlage has a frequency wice ha f he swiching frequency due he 18 phase shif beween he p and bm upu filer capacir vlages. The empirical vlages are in a clse agreemen wih he analyical and simulain resuls in Figs. 8 and 13, respecively. The TABLE III Specificains f he capacirs and inducrs Cmpnens Symbls alue Uni Resnan inducr L r1, L rb1 14 µh Resnan inducr L r, L rb 14 µh Oupu inducr L r, L rb 14 µh Resnan capacir C r1, C rb1 µf Resnan capacir C r, C rb µf Oupu capacir C, C b µf I. EXERIMENTAL RESULTS The prpsed 7level RSC cnverr was implemened n a 5kW labrary prype cnverer verify he hereical develpmens presened abve. The prpsed cnverer was designed bs a 1 inpu he maximum upu vlage f 7. The swiching frequency Fig. 18. lage and curren wavefrms f he swich S b. Fig. 16. Nrmalized cmparisns beween seriesmdular cnfigurains. (a) Lss cmparisns. (b) Weigh and vlume cmparisns f he passive cmpnens. Fig. 17. Nrmalized cmparisns beween cascade cnfigurains. (a) Lss cmparisns. (b) Weigh and vlume cmparisns f he passive cmpnens (c) 13 IEEE. ersnal use is permied, bu republicain/redisribuin requires IEEE permissin. See hp:// fr mre infrmain.

10 This aricle has been acceped fr publicain in a fuure issue f his jurnal, bu has n been fully edied. Cnen may change prir final publicain. Ciain infrmain: DOI 1.119/TEL , IEEE Transacins n wer Elecrnics 1 GE [5/div] (a) I C [5A/div] (b) (c) I Cb [5A/div] I L [3.75A/div] (d) I Lb [3.75A/div] (e) (f) (g) (h) Time[1μs/div] Fig. 19. Operaing wavefrms f he prpsed cnverer. (a) Gaeemier vlage f swich S b. (b) and (c) Oupu capacir currens. (d) and (e) Oupu inducr currens. (f) Oupu vlage. (g) and (h) Oupu filer capacir vlages. Efficiency (%) [5/div] C [1/div] Cb [1/div] Oupu pwer (w) Fig.. Efficiency curve f he prpsed 7level RSC cnverer. efficiency f he 7level RSC cnverer fr a differen lad was esed and he resul is shwn in Fig.. The efficiency reaches mre han 95%, which is gd cnsidering ha he cnverer is a prype. The perfrmance shuld be imprved a higher pwer and vlage levels due he lwer swich cnducin and passive cmpnen lsses. The es resuls clearly shw ha he prpsed circui can enhance he pwer densiy and efficiency wing he minimum swiching lsses.. CONCLUSION A new resnan swichedcapacir (RSC) cellbased DC/DC cnverer wih a high vlage gain is prpsed fr ffshre wind energy applicains. The sfswiching acin is prvided by he resnan cndiin f he circui. Therefre, he swiching lsses are minimal in bh ON and OFF insans, and he pwer densiy f he sysem can be enhanced by increasing he swiching frequency. Oupu filer capacir vlages are phase shifed by 18º wih respec each her eliminae he upu vlage ripples wihu adding exra cmpnens. The prpsed seriesmdular and cascade RSC cnfigurains have he inheren advanage f being readily applicable mulisage pwer swiching cnverers. Cncepual cmparisns f he prpsed cnverer a cunerpar shw ha he prpsed cnverer is well suied fr high vlage and high pwer ffshre wind applicains requiring a high pwer densiy and high efficiency. The simulain and experimenal resuls cnfirm verify he feasibiliy f he prpsed cnverer. AENDIX Generalized equains are represened fr he prpsed ksage RSC cnverer a he seadysae as fllws: Frm () and (3), he resnan capacir curren f he k h sage can be direcly bained as i ( ) C sin( r ). (31) r( k ) Since he p resnan capacirs and bm resnan capacirs are charged and discharged by a 18º phase shif wih respec each her, he bm resnan capacir can be expressed as i ( ) C sin( r ). (3) rb( k ) In Mde III, all he p swiches are ON, whereas all he bm swiches are OFF. Therefre, he p resnan capacir currens in his mde ( Ts / Ts ) can be represened as i ( ) i ( ) i ( ) C C C sin( r ) r( k1 ) r( k ) rb( k ) (33) 4 i ( ) i ( ) i ( ) C C C sin( r ) r( k ) r( k1 ) rb( k1 ) (34) 8 i ( ) i ( ) i ( ) C C C sin( r r( k 3 ) r( k ) rb( k ) (35) k 1 i ( ) i ( ) i ( ) C C C sin( r ). r( 1) r( ) rb( ) (36) Similarly, he p and bm resnan capacir currens f he h h sage during ne swiching perid can be bained as k h i ( ) C rh sin( r ), ( h 1,,...,k ), (37) k h i ( ) C rbh sin( r ), ( h 1,,...,k ). (38) In Mde I and Mde III, he inpu curren can be defined as k id ( ) ic ( ) Ts iin ( ) i i1. (39) i S ( ) ic ( ) Ts T 1 s ) (c) 13 IEEE. ersnal use is permied, bu republicain/redisribuin requires IEEE permissin. See hp:// fr mre infrmain.

11 This aricle has been acceped fr publicain in a fuure issue f his jurnal, bu has n been fully edied. Cnen may change prir final publicain. Ciain infrmain: DOI 1.119/TEL , IEEE Transacins n wer Elecrnics 11 Subsiuing (6), (11), and (13) in (39) yields i ( ) k ( ) in 1 sin( r ). (4) Frm (4), he average inpu curren can be calculaed as k I 1 T in s i in ( )d ( 1 1) I T s. (41) Frm he afremenined assumpins in Secin IIB, siin I. (4) Cmbining (41) and (4), he upu vlage can be represened by k ( 1 1) s. (43) Frm (43) and he prpsed cnverer perain described in Secin II, he resnan capacir vlage f C rh can be expressed as h 1 C rh s. (44) Because he resnan capacirs in he p cell have he same pwer raings wih he cunerpars in he bm cell, he erm TARC r can be defined as k TARC r C I rh C. (45) rh h1 Subsiuing (37) and (44) in (45) gives k 1 k k 3 TARC r (s I s I 4s I. (46) k 1 k... si ) sik REFERENCES [1] J. M. Carrasc, L. G. Franquel, J. T. Bialasiewicz, E. Galvan, R.. Guisad, M. A. ras, J. I. Len, and N. M. Alfns, werelecrnic sysems fr he grid inegrain f renewable energy surces: a survey, IEEE Trans. Ind. Elecrn., vl. 53, n. 4, pp , Aug. 6. [] T. Kawaguchi, T. Sakazaki, T. Isbe, and R. Shimada, Offshre wind farm cnfigurain using dide recifier wih MERS in curren link plgy, IEEE Trans. Ind. Elecrn., vl. 6, n. 7, pp , Jul. 13. [3] M. Liserre, R. Cardenas, M. Mlinas, and J. Rdriguez, Overview f mulimw wind urbines and wind parks, IEEE Trans. Ind. Elecrn., vl. 58, n. 4, pp , Apr. 11. [4] F. Blaabjerg, K. Ma, Fuure n pwer elecrnics fr wind urbine sysems, IEEE J. Emerg. Sel. Tpics wer Elecrn., vl. 1, n. 3, pp , Sep. 13. [5] S. Gjerde,. K. Olsen, K. Ljkelsy, and T. Undeland, Cnrl and faul handling in a mdular series cnneced cnverer fr a ransfrmerless 1 k lw weigh ffshre wind urbine, IEEE Trans. Ind. Appl., vl., n. 99, pp. 1, 13. [6] W. Musial and B. Ram, Largescale ffshre wind pwer in he Unied Saes, NREL/T54745, Sep. 1. [Online]. Available: hp:// [7] A. Garces and M. Mlinas, A sudy f efficiency in a reduced marix cnverer fr ffshre wind farms, IEEE Trans. Ind. Elecrn., vl. 59, n. 1, pp , Jan. 1. [8] A. arasar and J. K. Sek, Highpwerdensiy pwer cnversin sysems fr HDCcnneced ffshre wind farms, Jurnal f wer Elecrnics, vl. 13, n. 5, pp , Sep. 13. [9] C. Meyer, M. Hing, A. eersn, and R. W. De Dncker, Cnrl and design f DC grids fr ffshre wind farms, IEEE Trans. Ind. Appl., vl. 43, n. 6, pp , Nv./Dec. 7. [1] T. Nguyen, D. Lee, and C. Kim, A seriescnneced plgy f dide recifier and vlagesurce cnverer fr HDC ransmissin sysem, IEEE Trans. wer Elecrn., vl. 9, n. 4, pp , Apr. 14. [11] R. Li, S. Bzhk, and G. Asher, Frequency cnrl design fr ffshre wind farm grid wih LCCHDC link cnnecin, IEEE Trans. wer Elecrn., vl. 3, n. 3, pp , May 8. [1] G. Oriz, J. Biela, and J. W. Klar, Opimized design f medium frequency ransfrmers wih high islain requiremens, in rc. IEEE IECON, 1, pp [13] G. Oriz, J. Biela, and J. W. Klar, 1 megawa, khz, islaed, bidirecinal 1k 1.k DCDC cnverer fr renewable energy applicains, in rc. IEEEICE, 1, pp [14] K. Haua, S. Dua, A. Tripahi, S. Baek, G. Karimi, and S. Bhaacharya, Transfrmer less inelligen pwer subsain design wih 15k SiC IGBT fr grid inercnnecin, in rc. IEEE ECCE, 11, pp [15] S.. Engel, N. slau, H. Sagge, and R. W. De Dncker, Dynamic and balanced cnrl f hreephase highpwer dualacive bridge DC/DC cnverers in DCgrid applicains, IEEE Trans. wer Elecrn., vl. 8, n. 4, pp , Dec. 1. [16] N. Dennisn, A. Massud, S. Ahmed, and. Enjei, Muliplemdule highgain highvlage DC DC ransfrmers fr ffshre wind energy sysems, IEEE Trans. wer Elecrn., vl. 58, n. 5, pp , May 11. [17] N. Mhan, T. M. Undeland, and W.. Rbbin, wer elecrnics cnverers, applicain and design, Wiley, [18] I. Laird and D. D. C. Lu, High sepup DC/DC plgy and MT algrihm fr use wih a hermelecric generar, IEEE Trans. wer Elecrn., vl. 8, n. 7, pp , Jul. 13. [19] K. K. Law, K. W. E. Cheng, and Y.. B. Yeung, Design and analysis f swichedcapacirbased sepup resnan cnverers, IEEE Trans. Circuis Sys. I, Reg. apers, vl. 5, n. 5, pp , May 5. [] W. Chen, A. Q. Huang, C. Li, G. Wang, and W. Gu, Analysis and cmparisn f medium vlage high pwer DC/DC cnverers fr ffshre wind energy sysems, IEEE Trans. wer Elecrn., vl. 8, n. 4, pp. 14 3, Apr. 13. [1] W. Qian, H. Cha, F. Z. eng, and L. M. Tlber, 55kW variable 3X DC DC cnverer fr plugin hybrid elecric vehicles, IEEE Trans. wer Elecrn, vl. 7, n. 4, pp , Apr. 1. [] K. Zu, M. J. Sc, and J. Wang, A swichedcapacir vlage ripler wih aumaic inerleaving capabiliy, IEEE Trans. wer Elecrn., vl. 7, n. 6, pp , Jun. 1. [3] D. Ca and F. Z. eng, Zercurrenswiching mulilevel mdular swichedcapacir DC DC cnverer, IEEE Trans. Ind. Appl., vl. 46, n. 6, pp , Nv./Dec. 1. [4] F. H. Khan and L. M. Tlber, A mulilevel mdular capacirclamped DCDC cnverer, IEEE Trans. Ind. Appl., vl. 43, n. 6, pp , Nv./Dec. 7. [5] F. Zhang, F. Z. eng, and, Z. Qian Sudy f he mulilevel cnverers in DC DC applicains, in rc. IEEE ESC, 4, pp [6] D. Gravc and M. urschel, IGBT pwer lsses calculain using he daashee parameers Applicain ne, vl. 1.1, Jan. 9. [Online]. Available: hp:// [7] Z. Liang, A. Q. Huang, and R. Gu, High efficiency swiched capacir buckbs cnverer fr applicain, in rc. IEEE AEC, 1, pp [8] [Online]. Available: hp:// [9] [Online]. Available: hp:// [3] [Online]. Available: hp:// [31] [Online]. Available: hp:// Amir arasar (S 11) received he B.S. and M.S. degrees frm he Deparmen f Elecrical Engineering, Islamic Azad Universiy, Science and Research branch, Tehran, Iran, in 4 and 8, respecively. He is currenly wrking ward he h.d. degree in he wer Cnversin Labrary, Deparmen f Elecrical Engineering, Yeungnam Universiy, Gyengsan, Krea. His curren research ineress include highpwer DC/DC cnversin sysems and pwer cnverer cnrl f ffshre wind farms (c) 13 IEEE. ersnal use is permied, bu republicain/redisribuin requires IEEE permissin. See hp:// fr mre infrmain.

12 This aricle has been acceped fr publicain in a fuure issue f his jurnal, bu has n been fully edied. Cnen may change prir final publicain. Ciain infrmain: DOI 1.119/TEL , IEEE Transacins n wer Elecrnics 1 JulKi Sek (S 94 M 98 SM 9) received he B. S., M.S., and h.d. degrees frm Seul Nainal Universiy, Seul, Krea, in 199, 1994, and 1998, respecively, all in elecrical engineering. Frm , he was a Senir Engineer wih he rducin Engineering Cener, Samsung Elecrnics, Suwn, Krea. Since 1, he has been a member f he faculy f he Schl f Elecrical Engineering, Yeungnam Universiy, Gyengsan, Krea, where he is currenly a rfessr. His specific research areas are mr drives, pwer cnverer cnrl f ffshre wind farms, and nnlinear sysem idenificain relaed he pwer elecrnics field. Dr. Sek serves as an Assciae Edir f he IEEE Transacins n Indusry Applicains (c) 13 IEEE. ersnal use is permied, bu republicain/redisribuin requires IEEE permissin. See hp:// fr mre infrmain.