Evaluation of an Alternate Soft Charge Circuit for Diode Front End Variable Frequency Drives

Transcription

1 Evaluaon of an Alernae Sof Charge Crcu for Dode Fron End arable Frequency Drves Mahesh Swamy Member, IEEE Yaskawa Elecrc Amerca Waukegan, I 60085, USA mahesh_swamy@yaskawa.com Tsuneo J. Kume Fellow, IEEE Yaskawa Elecrc Corporaon Kokura, Fukuoka, , Japan jkume@yaskawa.co.jp Noryuk Takada Yaskawa Elecrc Amerca Waukegan, I 60085, USA nory_akada@yaskawa.com Absrac -- arable Frequency Drves (FDs wh dode recfer fron end are ypcally equpped wh a ressorconacor arrangemen o lm he nrush curren no he dc bus capacors, hereby provdng a means for sof chargng he dc bus capacors. Because of he mechancal naure of he magnec conacor ypcally used n FDs, here exss a concern for fague. In adon, durng a brown ou conon, ypcally he conacor remans closed and when he volage recovers, he ensung ransen s ofen large enough o possbly cause unfavorable nfluence o surroundng componens n he FD. Many researchers and applcaon engneers have hough abou hs ssue and many are acvely seekng non-mechancal soluons n a cos effecve manner. In hs paper, a new opology o sof charge he dc bus capacor s proposed. Oher echnques ha have been evaluaed are also nroduced. The relave advanages and dsadvanages are dscussed. Expermenal ess o show he feasbly of he proposed dea s also provded. conon, he dfference beween he peak value of he npu volage and he dc bus volage s large enough o force he rggerng angle o ncrease and hereby reduce he hgh nrush curren. Ths echnque s well esablshed and s used by some FD manufacurers, bu has he followng ssues: Needs sx (or hree pack hyrsor modules can be expensve, especally for small szes. Needs sx (or hree gae-rgger crcus along wh sensng and decson makng logc. The rgger and logc crcus occupy space and are expensve. The hyrsors cause volage nochng effec due o overlap phenomenon durng commuaon. Ths requres he use of npu ac nducor o reduce he nochng effec on oher equpmen. The adon of AC nducor occupes space and wll be an added cos. Index Terms Sof Charge Crcu, Inrush Curren mer, Surge Curren mer afer Brown Ou, Brown Ou Conon. I. INTRODUCTION Typcally, n mos arable Frequency Drves (FDs ha have a large bus capacor fler, here exss a need for a crcu smlar o a ressor-conacor arrangemen o lm he nrush curren no he capacors, and hereby provde a means o sof-charge he dc bus capacors (Fg.. Because of he mechancal naure of he conacor, he relably of he FD s adversely affeced. Moreover, nerrupng curren under ceran crcumsances can deerorae he negry of conacs. Gven hese facs, he sof-charge crcu s perhaps one par of he drve ha s due for some aenon and mprovemen. There have been suggesons of replacng he magnec conacor (MC n Fg. (a wh a semconducor swch, as shown n Fg. (b. However, he semconducor swch wll requre nellgen conrol and s assocaed wh seady-sae power loss. Thyrsor conrolled recfers [] have been used n FDs as shown n Fg. (c. In hs opology, he npu recfers are replaced by hyrsors. The rggerng angle of he hyrsors are conrolled n such a manner ha he dc bus capacor charges up smoohly wh no nrush. When a brown ou occurs, he hyrsor angle s such ha provdes he maxmum oupu volage possble, smlar o a ypcal dode brdge. When he volage recovers afer a brown ou (c Fg.. Sof-charge crcu confguraon, a. Wh Magnec Conacor (MC, b. Wh Thyrsor, c. Thyrsor based recfer []. (a (b

2 Ths paper nvesgaes alernae echnques o sof charge he dc bus capacor. The arge feaures are: No mechancal conacors; Should be able o handle brown ou conons n an effcen manner; Auonomous operaon (whou any conrol logc o handle varous power supply conons; Un should be compac and economcal. II. STUDY OF ATERNATE SOFT CHARGE CIRCUITS In hs secon, wo alernae sof charge crcus ha mee que a few pons n he arge feaures are dscussed. They are: a. Use of Magneo Ressve (MR elemen; and b. Semconverer ype sof charge crcu. A. Magneo Ressve (MR Elemen based Sof Charger One of he opology suded here ncludes he use of Magneo Ressve devce (MR ha shows hgh ressance under he nfluence of large magnec feld and low ressance when he magnec feld reses o a lower level []. The MR elemen could be conneced n seres wh he dc bus capacor o sof charge a sar up or durng he recovery me afer a brown ou conon. The crcu confguraon s shown n Fg. (a and he concepual schemac n Fg. (b. In place of an E-I core based nducor, one can use a orod and embed he MR elemen n he ar-gap of he orodal nducor. However, some mporan ssues o consder whle mplemenng an MR elemen based soluon are as follows: Behavor of he MR elemen a elevaed operang emperaure should be consdered. Snce, mos of he hea s produced n he ar-gap of an nducor, placng an MR elemen n he ar-gap needs careful engneerng. Snce raed load curren has o pass hrough he MR elemen, hs dea may be lmed o small power due o he lmaon of presenly avalable maeral. When he raed curren ncreases, he MR elemen can become large and placng n he ar gap may pose a problem. (a (b Fg.. Sof charge crcu wh MR elemen. a. Schemac, b. Concepual mplemenaon. B. Sem-converer Topology based Chargng [3] In he sem-converer opology based chargng, as he name suggess, a sx-pulse recfer s delberaely operaed as a sem-converer by employng a wye conneced auxlary chargng crcu as shown n Fg. 3.. The opology proposed here borrows he dea of a ypcal sar-dela sared used n convenonal 3-phase ac moors. The dc bus capacor s charged as a sem-converer a sar and once he dc bus volage reaches he seady sae volage dcaed by he semconverer, he full converer confguraon s engaged, resulng n a second charge up perod. Snce he chargng s carred ou n wo sages, he nrush curren hrough he nducor, capacor, and dode s well conrolled wh almos no sress. The swchng from he sem-converer confguraon o he full brdge confguraon can eher be dcaed by level of dc bus volage or by a mer. Boh hese mehods have been smulaed and found o yeld accepable resuls as shown n Fgs 4 and 5. Fg. 3. Half-brdge o full brdge changeover opology ha faclaes wosage chargng [3]. The value of he dummy nducor n Fg. 3 s chosen o be such ha he resulng crculang curren s abou 0.0pu of he raed curren. In he frs of wo sraeges smulaed here, he changeover from half-brdge confguraon o full-brdge confguraon s faclaed by deecng he level of he dc bus volage across he man dc bus capacors. The operang conons assumed for smulaon are as follows: The npu lne-lne volage s assumed o be 58, 60Hz correspondng o 0% hgher han nomnal 480; The dc lnk nducor s assumed o be 0% lower han s nomnal value based on he manufacurer s specfcaon; The dc bus capacor s assumed o be 0% hgher han s nomnal value based on he manufacurer s specfcaon; The equvalen seres ressance (ESR of he dc lnk nducor and he dc bus capacor s deermned from he manufacurer s daashee; The parasc ressance of he cable and he bus bars are assumed o be zero; The source s consdered o have a shor crcu capacy of 3,000 Amps (rms, wh an X/R rao of 5. For a 480 sysem, he value of he sysem nducance and ressance s compued o be.5μh and.7m-ω;

3 Power-up conon s smulaed assumng no load s appled o he drve durng power-up; The drve smulaed s raed a 0-kW and he value of he dummy nducor chosen s 4mH. The dc bus volage a whch he half-brdge opology s changed over o he full-brdge confguraon s seleced o be 600dc. Resuls of he smulaon are gven n Fg. 4. ol (Amp In Fg. 5, he observed peak surge curren s 689A for a drve ha has raed curren of 55Arms. The maxmum value of he dc bus volage s seen o be 734, whch s well below he over volage rp level of 850. Snce he basc operaon requres a logc crcu, hs mehod was consdered no o mee he arge desred feaures. Moreover, a semconducor swch has more losses han a mechancal conacor and hs mehod can resul n hgher overall losses n he drve durng normal operaon. Gven hese mporan shor comngs, hs echnque was no pursued furher. The need of a dummy nducor was ye one more drawback of hs opology (Fg. 3. III. PROPOSED SOFT CHARGE CIRCUIT The proposed opology, shown n Fg. 6, mees mos of he arge desred feaures. A dc lnk nducor wh a ressor asss crcu s employed o sof charge he dc bus capacor [4]. The asss ressor also has a seres hyrsor m 0m 30m 40m 50m 60m 70m 80m Fg. 4. Smulaed resuls for he opology shown n Fg. 3. SW s acvaed based on level of dc bus volage. From he resul shown n Fg. 4, s neresng o see ha even afer swch SW s commanded o be closed, he large value of he dummy nducor keeps curren flowng hrough he exernal dode D. Durng hs perod, wo chargng pahs exs one hrough D and he oher hrough he lower secon of he full-brdge recfer dodes. In Fg. 4, he waveform for I D has been shfed by 00A o dsngush from I. In Fg. 4, he observed peak surge curren s 987A for a drve ha has raed curren of 55Arms. The maxmum value of he dc bus volage s seen o be 773, whch s well below he over volage rp level of 850. Smulaon resuls for he case when swch SW s closed afer a pre-deermned and fxed me s shown n Fg. 5. ol (Amp m 0m 30m 40m 50m 60m 70m 80m 95m Fg. 5. Smulaed resuls for he opology shown n Fg. 3. Here SW s shown acvaed afer a pre-deermned me of 65ms. Fg. 6. Proposed crcu for sof chargng he dc bus capacor, employng wo hyrsors [4]. When AC power s appled o he crcu shown n Fg. 6, an nrush curren begns o flow, assumng ha he dc bus capacor has no nal sored volage. The nrush curren s dvded no wo dsnc pahs. The frs pah s hrough he ressor-hyrsor (TH combnaon and he second pah s hrough he dc lnk nducor,. The curren hrough he ressor-hyrsor pah s nally hgher and qucker han ha hrough snce he nducor delays he buld up of curren hrough. The dc bus capacor sars o charge, wh he ressor-hyrsor combnaon provdng as much chargng as possble. The second chargng pah, hrough, creaes a resonan crcu. Due o he naure of C crcu, he volage across he dc bus capacor ends o ncrease over and above he peak value of he appled npu AC volage. A hs me, he hyrsor across, TH, urns ON because s forward based and he gae volage gven by values of R and R exceeds he hreshold gae volage for TH. In he meanme, TH urns OFF because he curren hrough falls below he holdng curren for TH. In adon, he urnng ON of TH causes he volage across he nducor o sar fallng and evenually reverse bases hyrsor TH and assures ha TH s OFF. The nducor volage lnearly ramps o zero and ges clamped by TH. The volage across he bus capacor sops ncreasng and evenually dscharges no s dscharge ressor o a level dcaed by he npu volage conon. The mporan aspec of he ressor-asss crcu canno be

4 overlooked snce he chargng curren flowng hrough s reduced due o he parallel ressor asss crcu. Ths reduces he sored energy n. I also lowers he sauraon curren requremen and makes he nducor physcally smaller. Due o he C naure of he crcu, he volage across he capacor s sll hgher han he peak value of he npu volage. The clampng crcu conssng of TH assures ha he dc bus volage s clamped o a safe value. I. THEORY OF OPERATION The chargng process, as dscussed n he precedng paragraphs, s dvded no wo nervals nerval I and nerval II. In he frs nerval, he ressor asss crcu wh hyrsor TH s prmarly nvolved n chargng up he dc bus capacor. A chargng curren also flows hrough he man dc lnk nducor. When he volage across he dc bus capacor reaches he peak of he npu appled volage, he curren hrough TH falls o zero and shus off. Ths marks he end of nerval and begnnng of nerval II. Dependng on he relave value of R and used, he sar of nerval II could see only curren hrough he nducor. Evenually, when he volage across he dc bus capacor exceeds he gae hreshold volage for TH, he clampng hyrsor TH urns ON. Turnng ON of THshors ou he dc lnk nducor and clamps he volage across o a value dcaed by he parasc ressance of he nducor and he forward volage drop of hyrsor TH. Ths phenomenon also clamps he dc bus capacor volage o some safe level dcaed by he forward volage drop of hyrsor TH. Expressons for he chargng curren, fnal value of dc bus volage and curren hrough he wo hyrsors are derved nex. A. Inerval I Referrng o Fg. 7(a, nerval I of operaon begns when he power s urned ON and he peak lne-lne volage s appled o he nducor-ressor-capacor combnaon and lass ll he volage across he dc capacor, v C reaches he peak npu volage and curren hrough TH falls o zero. Durng nerval I, wo curren pahs exs one hrough he ass ressor and he oher hrough he dc lnk nducor. A end of nerval I, curren hrough R s zero snce curren hrough TH goes o zero. R dc dc (a Inerval I (b Inerval II Fg. 7. (a. Equvalen crcu for nerval I; (b. Equvalen crcu for nerval II. The expresson for capacor curren ( + R for zero nal capacor volage s: dc dc( d + ( C R + d R R ( Dfferenang ( once and subsung ( no he resul yelds he followng equaon: d d (3 R ( D D, D + R D + R ± 4R 0 As can be seen from (5, here are hree possble soluons for he nducor curren. They are:. under-damped,. overdamped and. crcally-damped. Operaon of he un ha resuls n an over-damped sysem s no advsable snce he curren hrough he ressor would no have ded before nerval II sars and hs can creae hgher curren o crculae hrough TH n nerval II. Such a suaon s delberaely avoded and so wll no be analyzed here. Under-damped Case In hs case, R > /( 4 C. The expresson for can hen be derved as follows: ( α+ j ( α j Ae + A e (6 α ; R e {( A + A cos( + ( A A jsn( } 4R A 0 +, 0 ; Subsung hs conon yelds: + ( 0 ( A + A 0 (9 A e j sn( (0 A 0 +, snce he nal volage across he capacor C s assumed o be zero, all he appled volage appears across he dc lnk nducor-charge asss ressor combnaon. Usng hs conon, he value of A and are derved nex: d Ae j ( α sn( + cos( ( A j ( ( A j ( (7 (8 (3 ( sn( (4 Snce he dc lnk nducor and he asss ressor are n parallel when hyrsor TH s conducng durng nerval I, he curren hrough he asss ressor can be esmaed by equang he volage drop across he nducor o be same as ha across he ressor. Once he curren hrough he asss ( (4 (5

5 ressor s esmaed, he sum of he currens hrough he asss ressor and he dc lnk nducor can hen be esmaed, whch wll equal he acual chargng curren no he capacor. Such an exercse s underaken nex: d R R (5 ( α R e ( α sn( + cos( (6 R C + R (7 ( e sn( + ( α sn( + cos( (8 R Inerval I ends when he curren hrough he asss ressor decays o zero. Equang 6 o zero yelds he end me ( for nerval I as follows: 0 α sn( + cos( (9 R π φ ; φ an α (0 Crcally Damped Case Crcally damped case happens when R /(4 C. The physcal nerpreaon s ha n a crcally damped sysem, he urnng ON of TH concdes wh he urnng OFF of TH. The expresson for can be derved by assumng he followng characersc for nducor curren: A + A ( ω ( C ( + A 0, 0; A 0 (3 Agan, a 0 +, he nal volage across he capacor C s zero and all he appled volage appears across he dc lnk nducor-charge asss ressor combnaon. Usng hs conon, he value of A and are derved nex: d ( A ( (4 + d ( A 0, ( A (5 ( (6 The curren hrough he asss ressor can now be evaluaed as done earler by equang he volage drops across he nducor and ha across he asss ressor. Ths yelds: d ( ( R R (7 ( R ( (8 R The sum of he currens hrough he dc lnk nducor and he asss ressor wll yeld he oal chargng curren no he dc bus capacor. For crcally damped sysem, hs curren s esmaed as follows: C + R (9 ( C ( + (30 R Agan, nerval I ends when he curren hrough he asss ressor decays o zero. Equang 8 o zero yelds he end me ( for nerval I as follows: 0 (3 R ; ω ( (3 B. Inerval II Referrng o Fg. 7(b, nerval II of operaon begns when he curren hrough hyrsor TH falls o zero. Ths happens when he volage across he dc bus capacor C reaches he nsananeous maxmum value of he appled lne-lne volage. Durng hs nerval, dependng on he relave value of and R, hyrsor TH evenually urns ON and clamps he volage across he nducor o a value gven by he forward volage drop of hyrsor TH. The energy sored n he nducor s dsspaed n he lumped parasc ressance, R par, comprsng of he ressance of he wndngs of he dc lnk nducor, and he on-sae ressance of TH. The equaon descrbng he curren decay hrough he nducor s derved nex. ddc 0 dc + dc Rpar; (33 I I dc dc (a endof nervali ( e (a endof nervali (a endof nervali For a crcallydampedsysem, dc I ( ( α + / C ( e α( π φ Rpar dc ( ( e Rpar dc Rpar dc α( π φ ( α + sn( φ α ( π φ (34 (35 (36 (37 (38. TEST OF PROPOSED SOFT CHARGE CIRCUIT Power up and load ess were conduced usng he crcu shown n Fg. 6. The deals of he es seup are as follows:. arspeed F7 drve F7U , 30kW, 60A. nk Inducor 0.9mH; I raed 60Arms 3. R -Ω 4. Exernal SCRs Seres SCR (SKKT 50/6; I raed 50Arms; 600

6 5. Gae Trgger Crcu R R 4 900Ω. R 6Ω; R3 5.5Ω 6. Operang Conon. Power-Up usng 3-phase 480, 60Hz AC supply;. oad operaon o 50hp Fg. 8. Performance of proposed sof charge crcu a Power Up. The varous relevan parameers measured afer power up are as follows: Peak value of dc bus volage: 745dc Maxmum dc nducor curren:,59a Maxmum hyrsor module curren: 76A The maxmum value of he chargng curren no he capacor was measured o be,30a, whch s well below he maxmum surge rang of,000a for he dode used. The peak hyrsor curren s well below 7,000A, whch s he rang of he module used. The peak nducor curren s,59a and he nducor does no show any sgn of sauraon snce he core used has a very low relave permeably of 0. Under raed load conon, he volage across he dc lnk nducor s small enough so as no o urn ON hyrsors TH and TH. Ths s clearly vsble n Fg. 9 and shows ha he hyrsors do no nerfere durng normal operaon. Brown-ou ess were conduced nex. The es plan was: a. Power up he un wh 3-phase 480ac (lne-lne wh he sof charge crcu. b. oad he drve o half raed load conon. c. Supply he drve wh auxlary power n parallel usng a 3-phase auo ransformer se a 380ac (lne-lne. d. Remove man supply power so ha he load s suppled va he auxlary power source a 380ac. e. Afer a bref nerval of me, reapply power from he man AC source raed a 480ac (lne-lne. The schemac of he es seup s shown n Fg. 0 and he resuls are gven n Fg.. Fg. 0. Schemac of es se up o sudy performance durng brown ou conon. A. Wh proposed crcu, and B. Wh sandard crcu ha has magnec-conacor and sof-charge ressor un. Fg. 9. Operaon of proposed crcu a raed load conon of 50hp. From he es resuls shown n Fgs. 8 and 9, he followng nferences can be made: The over volage level for he drve used n he es s 850dc and snce he maxmum dc volage observed was 745, he un dd NOT faul ou on over-volage.

7 355A 400 (0 675 I TH (00A/dv I (00A/dv (50/dv (50/dv 0ms 630 Fg.. Resuls obaned wh proposed sof charge crcu n operaon. From he resuls shown n Fg., he followng nferences can be made:. The surge curren resulng from he reapplcaon of raed volage o he drve afer a brown ou conon s very well lmed by he proposed sof charge crcu. Ths acheves he mos mporan goal of he proposed crcu.. The over-volage resulng from he reapplcaon of he raed volage o he drve afer a brown ou conon s low and well below he over-volage rp level for he drve. I s also well whn he volage olerance level of he capacors and IGBTs used. Ths shows ha he clampng mechansm helps clamp he volage o a safe level. 3. Durng a brown ou conon, he curren hrough he nducor ncreases sgnfcanly for he supplyng he same load a lower dc bus volage. The ncreased volage drop across he nducor urns ON TH durng he brown ou perod. Ths feaure helps o keep he clamp crcu acve and ready for he case when he volage recovers as observed n Fg. and clamps he capacor volage o a safe level. lower n cos. The proposed crcu does no need he adon of any exernal volage or chargng curren sensng and decson makng crcus. Ths makes he crcu very smple. Tess have shown ha he proposed crcu performs sasfacorly durng a volage dp conon, as well. The oher advanage of he proposed crcu s ha does away wh mechancal conacor and assocaed me delay. The power loss aspec assocaed wh power semconducor replacemen of mechancal conacor crcus s no presen n he proposed mehod. An added advanage of employng an opmally szed dc lnk nducor as par of he sof charge crcu s ha under normal operang conon, he npu curren THD s seen o be near 35%, and he npu rue power facor s close o Employng he dc lnk nducor also helps reduce he oal capacance needed n he dc bus, snce he curren rpple s sgnfcanly reduced. Though hs aspec has no been confrmed here, s expeced o resul n savng of space and hence cos of he enre drve sysem. REFERENCES [] Ajh Wjenayake, Tom Glmore, Rch ukaszewsk, Dale Anderson, and Gary Walersdorf, Modelng and Analyss of Shared /Common Bus Operaon of AC Drves, Par, n proceedngs of 997, IEEE Indusry Applcaons Socey Annual Meeng, New Orleans, ousana. [] Tsuneo J. Kume, Use of Magneo Ressve Elemen o Sof Charge Bus Capacors, Japanese Paen, 6 January 005. [3] Mahesh Swamy, Performance Evaluaon of a New Type of Sof Charge Crcu Appled for US Paen, January 009. [4] Mahesh Swamy, T. J. Kume, Alernae Technques for Sof Chargng bus Capacors Appled for US Paen, January 009. I. CONCUSIONS Ths paper presens an alernae sof charge crcu ha s shown o be effecve n sof chargng he dc bus capacor of a hree-phase AC o recfer crcu. Followng are he mporan feaures of he proposed crcu: The volage across he bus capacor s lmed by a clamp crcu ha s acve only when he volage dfferenal beween he npu of a dc lnk nducor and he dc bus capacor exceeds a pre-deermned value. A ressor asss crcu s employed o asss n sof chargng he dc bus capacor. The ressor asss par of he crcu s mporan snce prevens large curren flowng hrough he dc lnk nducor ha could poenally saurae he nducor. I also makes possble he realzaon of an nducor ha s smaller n sze and