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1 Florda State Unverty Lbrare Electronc Thee, Treate and Dertaton The Graduate School 3 Advanced Iolated B-Drectonal DC- DC Converter Technology for Smart Grd Applcaton Xaohu Lu Follow th and addtonal work at the FSU Dgtal Lbrary. For more nformaton, pleae contact lb-r@fu.edu

2 FLORIDA STATE UNIVERSITY COLLEGE OF ENGINEERING ADVANCED ISOLATED BI-DIRECTIONAL DC-DC CONVERTER TECHNOLOGY FOR SMART GRID APPLICATIONS By XIAOHU LIU A Dertaton ubmtted to the Department of Electrcal and Computer Engneerng n partal fulfllment of the requrement for the degree of Doctor of Phloophy Degree Awarded: Fall Semeter, 3

3 Xaohu Lu defended th dertaton on September 3, 3 The member of the upervory commttee were: Hu L Profeor Drectng Dertaton Juan Ordonez Unverty Repreentatve Uwe H. Meyer-Baee Commttee Member Jm P. Zheng Commttee Member Chr S. Edrngton Commttee Member The Graduate School ha verfed and approved the above-named commttee member, and certfe that the dertaton ha been approved n accordance wth unverty requrement.

4 To my famly: My parent: Yongln Lu, Cazhen Zhan My ter and brother-n-law: Xaoqng Lu, Zhoulang Zhu My grlfrend: Jyao Zhang

5 ACKNOWLEDGMENTS Frt of all I want to thank my advor, Dr. Hu L, for gvng me the opportunty to do my Ph. D program n Center for Advance Power Sytem CAPS at FSU, provdng me valuable dea, uggeton on the feld of power electronc. I alo learned the rgorou atttude toward reearch from her, whch wll be ueful for my future tudy and lfe. I want to thank my commttee: Dr. Juan Ordonez, Dr. Jm P. Zheng, Dr. Chr S. Edrngton and Dr. Uwe H. Meyer-Baee for gvng me ther valuable advce and broadenng my horzon on the reearch. I would alo thank CAPS to provde me o convenent reearch envronment, and thank the taff, centt and techncan n CAPS for ther upport durng my Ph.D tudy. Many thank to our drector, Dr. Stenar Dale, and Mr. Steve McClellan, Mr. Mchael Coleman, M. Nancy Raney, M. Joann Jrak, Mr. John Hauer, Mr. Steve Ranner, Mr. Ted Wllam, M. Banca Trocewtz, and Mr. Mchael Sloan. I would thank my group colleague, Dr. Lmng Lu, Dr. Jn Sh, Dr. Zhan Wang, Dr. Le Wang, Dr. Hafeng Fan, Yan Zhou, Panam Tatcho, Ye Yang, Yuxang Sh, Ran Mo, Guoxng Zhang and other tudent at CAPS, for ther valuable uggeton, comment, dcuon and upport. Fnally, I wh to expre my ncere grattude to my famly for ther love, upport and undertandng durng hard tme. v

6 TABLE OF CONTENTS Lt of Fgure... v Abtract...x. INTRODUCTION.... Reearch Background..... Future Smart Grd Sytem..... Advanced B-drectonal DC-DC Converter Applcaton n Smart Grd Sytem...3. Reearch Objectve and Dertaton Outlne...4. DC-DC CONVERTER DEVELOPMENT FOR THREE-STAGE SOLID-STATE TRANSFORMER...6. Technology Revew...6. Propoed Start-up Scheme Hgh Frequency Tranformer Current Analy Start-up Scheme Decrpton and Comparon....3 Expermental Verfcaton Summary DC-DC CONVERTER DEVELOPMENT FOR FUEL CELL POWER CONDITIONING SYSTEM Technology Revew Propoed Sngle-phae Lower Power Fuel Cell Power Condtonng Sytem Rpple Energy Analy of Sngle-phae Fuel Cell Power Condtonng Sytem Propoed CF-DHB Converter Operaton Analy and Small-gnal Modelng Tranformer current analy of CF-DHB converter wth vared d ZVS analy of CF-DHB converter wth vared d and mall-gnal modelng Propoed Pulaton Power Decouplng Control Sytem and Degn Expermental Verfcaton Propoed Sngle-phae Hgher Power Fuel Cell Power Condtonng Sytem Propoed Hgher Power Fuel Cell Converter Propoed fuel cell ytem decrpton Equvalent rpple crcut modelng of propoed fuel cell ytem Drect Double-frequency Rpple Current Control Sytem and Degn ZVS operaton analy wth large wng bu voltage Control degn prncple wth large wng bu voltage Expermental Verfcaton Summary CONCLUSIONS AND FUTURE WORK Concluon Future Work...8 v

7 APPENDICES...8 A. CF-DHB CONVERTER SMALL-SIGNAL MODEL...8 B. THREE-PHASE HFL CONVERTER SMALL-SIGNAL MODEL...83 REFERENCES...85 BIOGRAPHICAL SKETCH...9 v

8 LIST OF FIGURES. Electrc grd dagram conceptualzng Future Renewable Electrc Energy Delvery and Management FREEDM Sytem.... FREEDM three-tage SST: a Gen-I modular tructure baed SST, b Gen-II 5kV SC Mofet baed SST...7. Three-tage SST topologe wth dfferent - tage: a DAB baed three-tage SST, b Modular three-tage SST baed on a Four-level Rectfer and three - full-brdge converter Three-tage old tart tranformer crcut topology DHB DC-DC converter. a. prmary-referred equvalent crcut; b the mplfed fundamental model; c. The deal operaton waveform....5 Start-up cheme of a three-tage old tate tranformer: a rectfer tart-up; b tart-up cheme I; c tart-up cheme II; d propoed tart-up cheme r and r max of dfferent tart-up cheme Propoed tart-up control block dagram kw SST expermental prototype Start-up expermental reult wth rectfer output 4VDC. a tart-up cheme I; and b propoed tart-up cheme...9. Start-up expermental reult wth rectfer output 5VDC of the propoed tart-up cheme 3. Conventonal olated full-brdge - converter baed fuel cell ytem Converonal olated full-brdge - converter baed fuel cell ytem wth the actve flter Iolated V6 converter baed fuel cell ytem Iolated full-brdge - converter baed fuel cell ytem wth capactor connected to the center tap of the tranformer Tradtonal full-brdge - converter baed fuel cell power condtonng ytem Propoed CF-DHB converter baed lower power fuel cell power condtonng ytem...6 v

9 3.7 C v. V wth dfferent bu voltage for kw fuel cell ytem Prmary referred equvalent crcut of CF-DHB converter ervng nverter load Idealzed CF-DHB converter key operaton waveform r_peak and r_rm v. P o_dhb of dfferent duty cycle wth the defned d range: a r_peak, and b r_rm Scaled ZVS of S 4 at D.5 where Φ.6π, L /L Propoed fuel cell ytem power pulaton decouplng control dagram The open-loop tranfer functon G o bode dagram wth dfferent value of C Fuel cell current mall-gnal model block dagram The bode dagram of compenated G o wth C and the PR controller G cr Control-to-output tranfer functon G v root locu: a. open-loop, b cloed-loop kw CF-DHB converter baed fuel cell power condtonng ytem tet bed Expermental reult: a C 3 mf wthout the propoed method, b C wthout the propoed method, c C wth the propoed method The caled FFT analy reult of I fc hown n the Fg. 4: a I fc n Fg. 4b, b I fc n Fg. 4c CF-DHB converter four wtche ZVS waveform wth the propoed method: a S and S, b S 3 and S Propoed two-tage hgh-frequency-lnk baed hgh power fuel cell power condtonng ytem Idealzed three-phae HFL converter key operaton waveform Equvalent rpple crcut model of propoed fuel cell ytem Propoed drect double-frequency rpple current control ytem dagram The caled power curve and ZVS boundare of S a, S a, S r and S r at D.5 wth L /L The open-loop tranfer functon bode dagram wth the dfferent value of C : a G o, b G D...63 v

10 3.7 Phae A nput current mall-gnal model block dagram Bode dagram of PR controller G r, compenated G o and G D wth C F Bode dagram of open-loop and cloed-loop G v wth C F The propoed three-phae HFL converter baed fuel cell ytem tet bed Baelne cae I: expermental reult wthout propoed control method, C p F, C 3.8mF Baelne cae II: expermental reult wthout propoed control method, C p F, C 8F. a ytem performance, b I fc FFT analy reult Expermental reult wth the propoed control method, C p F, C 8F. a ytem performance, b I fc FFT analy reult Swtch S a wtchng waveform: a fxed D.5 control, b d vared duty cycle control Power lo break down analy reult of the three-phae HFL converter wth the rated output power Effcency data comparon between the tradtonal method wth large electrolytc capactor and the propoed method wth mall flm capactor...75 B. Bode dagram of compenated G v, G vo and G vd wth C F x

11 ABSTRACT The growng concern for envronment, ncreang demand growth and threat of energy hortage contnue to accentuate the requet of upgradng the tradtonal power grd. Smart grd envoned to take advantage of all avalable modern technologe n tranformng the current grd to one that functon more ntellgently to facltate ntegraton of renewable energy ource and energy torage, to provde hgher qualty of ervce and o on. Sold State Tranformer SST an eental technology for ntegraton of the dtrbuted energy reource, dtrbuted energy torage, and ntellgent load. It ha the advantage of t reduced ze and weght/volume wth hgh frequency tranformer, hgh power denty, and good power qualty. Comparng to other SST topologe, the three-tage SST more promng due to t advantage to provde power factor correcton, reactve power compenaton and an addtonal regulated DC bu The b-drectonal - converter the key tage n the three-tage SST topology confguraton nce t provde not only the hgh frequency galvanc olaton, but alo determne the ytem overall effcency and power denty. Therefore, the - converter promng dynamc performance the key requrement for the three-tage SST. Currently, few lterature reearch on the three-tage SST ytem dynamc, epecally for the oft tart-up ue. Three-tage SST requre a delcate tart-up control cheme becaue of three converon tage and hgh frequency tranformer. The major challenge of reearchng tart-up ue how to develop a theoretcal analy trategy to tudy dfferent cheme. The the reearche th ue by formulatng the tranformer ntaneou peak current wth repect to all the key mpact factor. A a reult, dfferent cheme can be nvetgated by ung a unfed formula. Moreover, a new oft tart-up cheme propoed wth the mnmzed tranformer x

12 current repone. The thorough analy of the DC-DC converter tranformer current durng the three-tage SST tart-up and the propoed tart-up cheme decrbed n detal n chapter. Fuel cell an mportant renewable energy ource of dtrbuted energy torage devce for new moble applcaton and power generaton ytem nce t offer hgh effcency, low emon of regulated pollutant and excellent part-load performance. The major challenge for fuel cell power condtonng ytem to lmt the fuel cell low-frequency current rpple reulted from the nverter load. The tradtonal oluton to adopt the large electrolytc capactor a the energy buffer. However, the large-zed electrolytc capactor wll decreae the ytem lfetme a well a ncreae the ytem volume and cot. A new current-fed phae-hft controlled - converter baed fuel cell power condtonng ytem propoed n th the wth low-frequency rpple free nput current ung a control-orented power pulaton decouplng control cheme. Wthout addng any extra component, the propoed fuel cell converter realze the power pulaton decouplng functon o a to reduce the bu capactor, thu allowng for choong long lfetme flm capactor to replace the bulky electrolytc capactor. The propoed decouplng control degn baed on the ytem mall-gnal average model. The detaled ytem operaton analy and propoed decouplng control degn gudelne preented n chapter 3. The chapter 4 ummarze the dertaton work. x

13 CHAPTER ONE INTRODUCTION. Reearch Background.. Future Smart Grd Sytem Today power grd wa bult a the centralzed undrectonal ytem of electrc power tranmon, electrcty dtrbuton, and demand-drven control. However, ncreang penetraton level of renewable energy, growng concern for envronment, energy utanablty and demand growth contnue to accentuate the need for a quantum leap n applcaton of uch technologe. Smart grd envoned to take advantage of all avalable modern technologe n tranformng the current grd to one that functon more ntellgently to facltate []: ntegraton of renewable energy ource uch a the olar, wnd and o on; ntegraton of energy torage uch a fuel cell power converon to counter the varablty of renewable reource and demand; Several ongong ntatve project are targetng on realzng the mart grd von [-5]. Future Renewable Electrc Energy Delvery and Management FREEDM Sytem one example of developng mart grd ytem. Fg.. how the Electrc grd dagram conceptualzng Future Renewable Electrc Energy Delvery and Management FREEDM Sytem. The FREEDM mart grd ytem merge advanced power electronc technology and nformaton technology to form a future dtrbuted power grd wth the followng capablte [6]:

14 Allow plug and play of any dtrbuted renewable energy reource DRER or dtrbuted energy torage devce DESD, anywhere and anytme; Manage load, DRER and DESD through Dtrbuted Grd Intellgence DGI oftware; Interface the load, DRER and DESD through a revolutonary nterface old tate tranform; Have a revolutonary protecton devce called Fault Iolaton Devce FID; Have perfect power qualty and guaranteed ytem tablty and have mproved ytem effcency, operatng the alternatng current ytem wth unty power factor. A hghlghted n Fg..[6], the Sold-tate Tranformer SST and Dtrbuted Energy Storage Devce DESD are the FREEDM mart grd key element. Th dertaton work wll be focued on the SST and fuel cell baed DESD. Fg.. Electrc grd dagram conceptualzng Future Renewable Electrc Energy Delvery and Management FREEDM Sytem.

15 .. Advanced B-drectonal DC-DC Converter Applcaton n Smart Grd Sytem SST an eental technology for ntegraton of the dtrbuted energy reource, dtrbuted energy torage, and ntellgent load. It ha the advantage of t reduced ze and weght/volume wth hgh frequency tranformer, hgh power denty, and good power qualty compared wth the conventonal lne-frequency 5/6 Hz tranformer. Dfferent topologe of SST have been preented n [6-]. A three-tage old tate tranformer SST nclude a rectfer, a DC-DC converter and an nverter. Comparng to other SST topologe, the three-tage SST more promng due to t advantage to provde power factor correcton, reactve power compenaton and an addtonal regulated DC bu [9-]. In partcular, the regulated DC bu can be nterfaced wth the renewable and energy torage ource for the DC dtrbuton applcaton. The olated b-drectonal - converter the key tage n the three-tage SST topology confguraton, nce t provde not only the hgh frequency galvanc olaton, but alo determne the ytem overall effcency and power denty. Therefore, the - converter dynamc performance the key charactertc for the three-tage SST. Fuel cell an mportant renewable energy ource of dtrbuted energy torage devce for new moble applcaton and power generaton ytem becaue t offer hgh effcency, low emon of regulated pollutant and excellent part-load performance [-4]. Currently, mot of the preent fuel cell tack module produce an output voltage rangng from 4-5V [4]. Therefore, the fuel cell tack need a hgh-gan converter to boot the voltage up to /4V n order to generate 5/6 Hz, V/4V ac voltage for redental applcaton. A a reult, two-tage fuel cell power condtonng ytem wth hgh gan - converter preferred. The electrcal olaton between the low-voltage output of the fuel cell tack and the hgh-voltage bu neceary for protecton, partcularly when the dfference n voltage ubtantal. 3

16 Tranformer are often ncorporated n the - converter for th reaon. Therefore, the olated hgh-gan - converter the key tage of the fuel cell power condtonng ytem. Fuel cell ytem prefer a pure load nce t lfe tme decreaed by the lowfrequency rpple current. The tudy performed n [5] ugget that the fuel cell rpple current wth frequency below 4 Hz mpar the fuel cell tack f the rpple factor bgger than 4% of the fuel cell current. However, when a ngle-phae nverter adopted, the fuel cell current wll uffer a double-frequency rpple current. Therefore, n conventonal fuel cell ytem, the large electrolytc capactor adopted a the energy buffer to reduce the fuel cell double-frequency rpple current. However, the ue of large-zed electrolytc capactor degrade the ytem relablty and ncreae the ytem volume and cot.. Reearch Objectve and Dertaton Outlne Baed on the applcaton dcued above, the frt objectve to develop the three-tage SST wth the olated b-drectonal - converter and nvetgate the olated bdrectonal converter dynamc charactertc. For the dynamc charactertc, one of the key ue the oft tart-up tranent performance. The three-tage SST requre a delcate tart-up control cheme becaue of three converon tage and hgh frequency tranformer. However, the optmzed tart-up cheme of three-tage SST ha not been preented n the recent publcaton. A traght-forward tart-up cheme to enable each tage one by one. However, th cheme may lead to relatvely hgh tranformer current and large nput nruh current. Th the wll delcate to analyze the - converter charactertc durng the threetage SST tart-up tranent and propoe a new oft-tart-up cheme to further mprove the ytem tart-up performance. 4

17 The econd objectve to develop an olated hgh-gan - converter for fuel cell power condtonng ytem to olve the fuel cell low-frequency rpple current ue. The requrement of fuel cell - converter nclude: Fuel cell low-frequency rpple current mnmzaton due to t negatve affecton on the fuel conumpton and fuel cell lfe pan Galvanc olaton for fuel cell protecton Voltage boot functon due to the low-voltage hgh-current energy ource Soft-wtchng operaton due to hgh-effcency operaton requrement The ret of the the organzed a follow. Chapter II tude the frt topc of the the: three-tage SST. The detaled analy of the - converter charactertc durng the SST tart-up tranent preented and a new ynchronzaton tart-up cheme propoed. Chapter III nvetgate the econd topc of the the: fuel cell power condtonng ytem. The fuel cell ytem wth low-frequency rpple free nput current ung a control-orented power pulaton decouplng trategy propoed. Chapter IV ummare the work preented n the the. 5

18 CHAPTER TWO DC-DC CONVERTER DEVELOPMENT FOR THREE-STAGE SOLID-STATE TRANSFORMER. Technology Revew Fg.. how the FREEDM mart grd ytem Gen-I and Gen II three-tage old tate tranformer topologe, repectvely. A hown n Fg.. a, Gen-I SST rated a ngle phae nput voltage 7.kV, 6Hz, output voltage 4/V, 6Hz, ngle-phae/3 wre []. The SST cont of three tage, an AC/DC rectfer, a Dual Actve Brdge converter DAB wth a hgh frequency tranformer and a DC/AC nverter. The SST cont of a cacaded hgh voltage hgh frequency AC/DC rectfer that convert 6Hz, 7. kv AC to three 3.8 kv DC bue, three hgh voltage hgh frequency DC-DC b-drectonal converter that convert 3.8 kv to 4V DC bu and a voltage ource nverter VSI that nvert 4V DC to 6Hz, 4/ V, ngle-phae/3 wre. Fg.. b how the Gen-II SST 5kV SC Mofet-baed kva ngle phae SST crcut topology []. Snce the new generaton SC Mofet adopted, the prevou Gen-I SST mplfed wthout modular tructure, whch reult n gnfcantly ze reducton. The control mplementaton alo become eaer compared to Gen-I SST. Reference [3] alo preent other three-tage SST topologe wth dfferent - converter a hown n Fg... Fg.. a how the ngle DAB baed SST. Fg.. b how the modular three-tage SST baed on a Four-level Rectfer and three - full-brdge converter. 6

19 a b Fg... FREEDM three-tage SST: a Gen-I modular tructure baed SST, b Gen-II 5kV SC Mofet baed SST. 7

20 a b Fg... Three-tage SST topologe wth dfferent - tage: a DAB baed three-tage SST, b Modular three-tage SST baed on a Four-level Rectfer and three - full-brdge converter. The olated b-drectonal - converter the key tage n the three-tage SST topology confguraton, nce t provde not only the hgh frequency galvanc olaton, but alo determne the ytem overall effcency and power denty. A hown n Fg.. and Fg.., DAB converter and DHB converter are the mot popular - topologe for SST applcaton nce they are able to acheve ZVS for all wtche wthout auxlary crcut, reultng hgh effcency operaton. 8

21 . Propoed Start-up Scheme.. Hgh Frequency Tranformer Current Analy Fg..3 how the elected three-tage SST where a dual-half-brdge DHB DC-DC converter adopted to be the b-drectonal DC-DC converon tage. The prmary-referred equvalent crcut of the DHB DC-DC converter, the mplfed fundamental model and the deal operaton waveform are llutrated n Fg..4. The DHB converter operaton prncple can be found n [7-] and wll not be repeated here. Fg..3. Three-tage old-tate tranformer crcut topology. The tranformer current r durng tart-up tranent may reach large value whch would reult n magnetc aturaton of the hgh frequency tranformer, leadng to an even larger current. In addton, the large tranformer current would requre the over-zed wtchng devce to prevent the damage. In order to analyze tranformer current durng tranent, the peak value of r durng one wtchng cycle derved baed on Fg..4: 9

22 V3V π V+ V4 r ωlr V3V π + V+ V4 r ωlr r π r r π + r. where the phae hft angle; L r the tranformer leakage nductor; ω the wtchng angular frequency; V, V, V 3 and V 4 are the voltage acro capactor C, C, C 3 and C 4 repectvely. If V n and V out defned a the nput and output voltage of DC-DC converter repectvely, V out the DC-DC output voltage n the prmary-referred crcut, V out can be aumed to be maller than V n durng the tranent under the condton that th DC-DC converter acheve oft tart-up wth no voltage overhoot. Therefore, the magntude of r and r can be further expreed n.-.3. ' ' Vn Vout π+ V out r. 4ωL r π+ π, > 4ωL r V r π π, < < 4ωL r ' ' Vout Vn V n Vn Vout ' ' Vn Vout V n Vn Vout n V n.3 where V V.5V n, V 3 V 4.5V out. A a reult, r - r gven n.4. [ π ] >, > 4ωL r V r r ' ' Vn + Vout π Vn Vout >, < < 4ωLr Vn ' ' Vn Vout π Vn Vout n.4

23 a Fg..4. DHB DC-DC converter. a. prmary-referred equvalent crcut; b the mplfed fundamental model; c. The deal operaton waveform. By obervng.4, t can be concluded that r > r durng the tart-up tranent nce the phae hft angle alway potve and maller than π durng the tart-up. Therefore, the peak value of r envelop equal to r and the maxmum tranformer current, defned a r max, equal to r max. To further tudy the r max durng the tart-up tranent, a varable k defned n.5:

24 r can be expreed n.6 n term of k: ' Vn Vout k, k.5 V n r [ k π + ] kπvn + k V V n n.6 4ω L 4ωL r r Durng tart-up tranent, V n of the DC-DC converter determned by the rectfer control. The phae hft angle the output of the DC-DC converter controller, whch uually mplemented ung a PI controller. If the PI controller degned approprately, the tranent of wll be fat enough to be neglgble, therefore wll reach and mantan a maxmum value m durng tart-up. In addton, V out controlled to ncreae monotoncally, k wll be monotonc decreang accordng to.5 and t wll be decreaed to zero after fnhng the tart-up. A a reult, r max manly dependent on the ntal value of k, whch defned a k n th the... Start-up Scheme Decrpton and Comparon Fg..5a preent the rectfer oft tart-up. The rectfer frt charge C n through the body dode of wtchng devce. A chargng retor ued to lmt the chargng current. t R and t R repreent the begnnng and endng pont of the dode rectfcaton tage. After the dode rectfcaton tage end, the relay wll wtch off the chargng retor and the cloed-loop control of the rectfer enabled at t R to regulate the capactor voltage V n and the voltage reache the dered level at t R3. Fg..5b how that the tart-up of DC-DC converter enabled after rectfer tart-up fnhe and V out ncreang lnearly. Therefore th cheme can be treated a the tage-by-tage tart-up method, and defned a tart-up cheme I. t -I and t -I repreent the begnnng and

25 endng pont of the DC-DC converter tart-up perod of cheme I. Accordng to.5, k can be calculated and equal to for th cheme. Fg..5c llutrate tart-up cheme II where V out frtly ncreaed to a elected value by pre-chargng C out. t -II and t -II repreent the begnnng and endng pont of the pre-charge tage of cheme II. Then the DC-DC converter tart-up enabled at t -II and fnhed at t 3-II. Th tart-up method of DC-DC converter wa decrbed n []. Accordng to.5, the value of k between and baed on the dfferent pre-charged V out. For example, f V out pre-charged to 5% of the rated value, then k.5. Fg..5d preent the propoed tart-up method. Intead of enablng the DC-DC converter after the rectfer tart-up, the propoed cheme ynchronze the rectfer tart-up and the DC-DC converter tart-up n a controlled manner, whch can acheve k durng the whole tart-up tranent n order to mnmze the r dynamc. t -III repreent the begnnng pont of the cloed-loop control n cheme III and t -III denote the pont that V out of cheme III reache the rated value whle Φ m. wll be quckly decreaed to the teady tate value durng the nterval from t -III to t 3-III. The per unt value of r of three tart-up cheme can be derved baed on 6 and compared n Fg..6, repectvely. The calng unt of current Φ m V n_r /ω Lr. V n_r the rectfer rated output voltage. t and t 3 repreent the begnnng and endng pont of DC-DC converter tart-up cloed-loop control. t denote the pont that V out reache the rated value whle Φ m for the propoed tart-up cheme. A typcal cae of tart-up cheme II where k.5 elected n Fg.4 to compare wth cheme I and propoed cheme. 3

26 Fg..5. Start-up cheme of a three-tage old tate tranformer: a rectfer tart-up; b tartup cheme I; c tart-up cheme II; d propoed tart-up cheme.. Fg..6. r and r max of dfferent tart-up cheme. 4

27 After r derved, r max p.u. can be found accordngly. Fg..6 how r max of cheme I, II, and propoed cheme are π/φ m,.5π+φ m /Φ m,, repectvely. In the real applcaton, n order to reduce the current tre and the crculatng energy, the phae hft angle cannot be degned too bg [-]. Φ m et to be º. Therefore, r max of cheme I 4.5 tme bgger than that of propoed method. r max of cheme II le than that of cheme I but alway bgger than that of propoed method, where r max of cheme II.75 tme bgger than that of propoed method f k.5. Fg..6 alo how that r alway monotoncally decreang f DC-DC tage tart-up enabled after V n tablzed. A a reult, r max of cheme I and II are the value of r at t when DC-DC converter ntally enabled. Therefore, for the tart-up cheme I and II, the large ntal tranformer current may lead to a relatvely large nput nruh current I n_. Th becaue the r at t would requre a large ntal nruh power. Snce V n controlled to be the contant by the rectfer tage after the rectfer tart-up end and there alo a large DC-lnk capactor C n whch prevent the fat voltage change. The DC-DC converter would uffer a relatvely large I n_ n order to provde uch large nruh power. However, r max of the propoed method happened at t and r at t zero, therefore, the nput nruh current I n_ of the propoed tart-up cheme theoretcally zero becaue t ntal tranformer current zero. The teady tate value of r of three cheme are the ame, whch a relatvely mall value becaue of the no load tart-up condton..3 Expermental Verfcaton The experment were conducted n the laboratory to verfy the aforementoned theoretcally analy and the propoed tart-up performance. Fg..7 decrbe the propoed 5

28 tart-up control block dagram. The rectfer tage adopt the ngle-phae dual-loop d-q vector control ytem. To realze the d-q tranformaton, a vrtual voltage v ` and current ` are obtaned by delayng 9 degree of the nput voltage v and current, repectvely. The q-ax current utlzed to control V n and the zero reference gven to the d-ax current to acheve the unty power factor control. The nner current loop d-q control tructure can be referred to the tradtonal d-q control ytem [3]. The DC-DC converter utlze the phae-hft control wth fxed 5% duty cycle and the phae hft angle the control varable to regulate V out. The control reference for V out gven a N /N p V n n order to realze the ynchronzed tart-up wth the rectfer. N p and N are prmary and econdary de tranformer turn, repectvely. Fg..8 how the kw SST expermental prototype, whch compoed of a nglephae AC/DC rectfer and a b-drectonal DC-DC converter wth the hgh frequency tranformer. The nverter tage not ncluded here nce t wll not affect the hgh frequency tranformer current. The dgtal control board employ DSP TMS3F8335.The wtchng frequence of the AC/DC rectfer and DC-DC converter are khz and 5 khz, repectvely. The prmary and econdary de devce of DC-DC converter are mplemented wth power MOSFET IXFT4N8P 4A, 8V and IXFT 36N6P 36A, 6V, repectvely. The rectfer rated nput AC voltage 8V rm. and the rated output DC voltage 5V. The DC-DC converter tranformer degn adopt the planar tranformer wth col encapulated wthn multlayer prnted-crcut-board PCB whch can acheve lower profle and hgher power denty than conventonal wre-wound tranformer. A par of PC4 PQ7/87/7 ferrte core modfed to lower profle whle keepng the dered cro ectonal area. The tranformer turn rato 5:. The leakage nductor 9 H and the rated nput and output voltage for the DC-DC converter 5V and 4V, repectvely. 6

29 Fg..7. Propoed tart-up control block dagram. Fg..8. kw SST expermental prototype. Fg..9a-b how the expermental reult of tart-up cheme I and propoed method when rectfer output 4VDC. Scheme I and propoed method can be treated a two extreme 7

30 cae of cheme II when k and k repectvely, expermental reult of cheme II therefore not elected here for comparon. Fve trace, n_rec, V n, I n_, V out and r repreent the rectfer nput current, rectfer output voltage and the DC-DC converter nput current, DC-DC converter output voltage and tranformer current repectvely. Fg..9a how that the r max of cheme I about -4A. The theoretcally value πv n_r /4ωL r, whch 3.9A. Th large r max may be caued by the magnetc aturaton of the tranformer. The mlar cenaro ha been oberved n []. The nput nruh current I n_ 7A whch 3.5 tme of the rated nput current. Fg..9b how that the ntal value of r for the propoed tart-up cheme around 4A and the theoretcally value zero. Th nonzero ntal current caued by the nonzero power tranfer for zero phae hft angle. The detaled analy for th phenomenon preented n [4]. The nput nruh current I n_ neglgble compared to Fg..9a. The r max of the propoed tart-up cheme about 4A whch cloe to t theoretcally value. The theoretcally value Φ m V n_r /ωl r, whch 3.A. The waveform of V o and V n have the ynchronzed hape whch verfe the propoed tart-up control. Comparng Fg..9a wth Fg. 7b, the r max of method I about tme larger than that of propoed method. Therefore, the propoed method ha the gnfcantly mproved performance compared to the tage-by-tage method. Fg.. how the tart-up expermental reult at rated rectfer output voltage of 5 V wth the propoed tart-up cheme. The reult mlar to Fg..9b that the DC-DC converter ha the mnmzed r repone and the neglgble I n_. The r max tll around 4A. Th reult further verfe the theoretcal analy and the dered control performance of the propoed SST tart-up control. 8

31 a b Fg..9. Start-up expermental reult wth rectfer output 4VDC. a tart-up cheme I; and b propoed tart-up cheme. 9

32 Fg... Start-up expermental reult wth rectfer output 5VDC of propoed tart-up cheme. The experment of method I under 5 V ha been conducted but the expermental reult are not provded. Th becaue the prmary de devce of DC-DC converter IXFT4N8P 4A, 8V, the r max of method I already 4A when rectfer output 4 V, a a reult, the r max of method I under 5 V damaged the MOSFET..4 Summary A new tart-up cheme for the three-tage old tate tranformer wth mnmzed tranformer current repone wa propoed. The thorough analy of the DC-DC converter tranformer current durng the SST tart-up llutrated that the key oluton to uppre the tranformer current wa to mnmze the voltage dfference between the tranformer prmary and econdary de voltage durng the tranent. Therefore, ntead of enablng the DC-DC converter

33 after the rectfer tart-up, the propoed cheme ynchronze the tart-up of the rectfer and the DC-DC converter n a controlled manner, whch dynamcally balance the tranformer prmary and econdary voltage. The theoretcal analy wa verfed by the expermental reult. The propoed tart-up cheme can be ued n other applcaton. Generally, th tart-up cheme can be appled to the mult-tage topology that ha the olated DC-DC converter wth hgh frequency tranformer. The propoed technology can alo be appled to the cacaded SST topology n [4]. But the tart-up cheme need to be modfed wth repect to the voltage and power balance control for th cacaded modular tructure.

34 CHAPTER THREE DC-DC CONVERTER DEVELOPMENT FOR FUEL CELL POWR CONDITIONING SYSTEM 3. Technology Revew Fuel cell an mportant renewable energy ource for new moble applcaton and power generaton ytem nce t beneft hgh effcency, low emon of regulated pollutant and excellent part-load performance [-4]. Currently, mot of the preent fuel cell tack module output voltage rangng from 4-5 V [4]. Therefore, a hgh-gan converter to boot the voltage up to /4 V for generatng /4 V, 5/6 Hz ac voltage requred for fuel cell redental applcaton. A a reult, two-tage fuel cell power condtonng ytem wth hgh gan - converter preferred [5-]. The electrcal olaton between the low-voltage output of the fuel cell tack and the hgh-voltage bu neceary for protecton, partcularly when the voltage dfference ubtantal. The - converter wth olated tranformer preferred for th reaon. Fuel cell gan a better performance when a load connected. Th becaue the load current doe not have low-frequency current rpple whch exhbt a hytere behavor wth fuel cell and reult a thermal problem among tack. The tudy performed n [] ugget that the fuel cell rpple current wth frequency below 4 Hz mpar the fuel cell tack f the rpple factor bgger than 4% of the fuel cell current. However, when a ngle-phae nverter connected to a fuel cell tack, the fuel cell current wll have a double-frequency rpple current generated by the nverter load. Therefore, a bulky electrolytc capactor uually appled a the

35 energy buffer to reduce the fuel cell double-frequency rpple current [4]. Fg. 3. how the conventonal olated full-brdge - converter baed fuel cell ytem [4]. However, the electrolytc capactor wll decreae the ytem lfetme a well a ncreae the ytem volume and cot. To reduce the fuel cell low-frequency rpple current and avod ung the large electrolytc capactor, the actve flter ha been propoed n [-4]. Th method apple the chopper to nject the rpple current n order to uppre the nput rpple current. Fg. 3. how the converonal olated full-brdge - converter baed fuel cell ytem wth actve flter [4]. The dadvantage of th method that t requre an auxlary chopper wth extra wtchng devce, reultng n a large volume devce. Reference [5] ha preented another rpple current mtgaton method by ung a current-loop control on a propoed three-phae - converter [6]. Fg. 3.3 how the olated V6 converter baed fuel cell power ytem [5]. However, a large capactor tll requred a an energy buffer. Study performed n [7] ha realzed the actve flter functon wthout addng extra actve devce. The crcut topology llutrated n Fg It connect the energy buffer capactor to the center tap of the olaton tranformer and ue the common-mode voltage to decouple the pulaton power. However, th method ncreae the current ratng of tranformer. Reference [8] ha preented the advantage of ung CF-DHB converter for fuel cell applcaton. Compared to the dual full-brdge topologe, t ha half the component count for the ame power ratng wth no total devce ratng penalty, zero voltage wtchng ZVS mplementaton wthout extra component, hgh effcency and mple control. However, the method to reduce the nput rpple current caued by the nverter load ha not been dcued. 3

36 Fg. 3. Conventonal olated full-brdge - converter baed fuel cell ytem. Fg. 3. Converonal olated full-brdge - converter baed fuel cell ytem wth the actve flter. Fg. 3.3 Iolated V6 converter baed fuel cell ytem. 4

37 Fg Iolated full-brdge - converter baed fuel cell ytem wth capactor connected to the center tap of the tranformer. 3. Propoed Sngle-phae Lower Power Fuel Cell Power Condtonng Sytem 3... Rpple Energy Analy of Sngle-phae Fuel Cell Power Condtonng Sytem Fg. 3.5 how the ngle-phae tradtonal olated full-brdge - converter baed fuel cell power condtonng ytem [7]. The ytem cont of a frt-tage nverter, a hgh frequency tranformer, a dode rectfer and a econd-tage nverter to generate /4 V, 5/6 Hz ac voltage for nverter load. The functon of the frt-tage nverter to generate hgh frequency ac voltage. Fg. 3.6 how a CF-DHB converter baed fuel cell power condtonng ytem that propoed n th the to acheve a new power decouplng control. The ytem cont of a CF-DHB converter and an nverter. The half brdge of low voltage de LVS of CF-DHB converter provde boot functon and produce a hgh frequency ac voltage. The boot functon acheved by the nductor L and the LVS half brdge. The hgh frequency tranformer provde the olaton and further voltage boot functon. L the tranformer leakage nductor whch utlzed a the energy tranfer element. 5

38 Fg Tradtonal full-brdge - converter baed fuel cell power condtonng ytem. Fg Propoed CF-DHB converter baed lower power fuel cell power condtonng ytem. The rpple energy calculaton for the ngle-phae fuel cell power condtonng ytem can be found n [7] and wll not be repeated here. The requred bu capactor C to provde the rpple energy can be calculated by 3.. C ω load P V load V 3. where P load the load real power, ω load the load angular frequency, V and V are the bu average voltage and voltage varatonpeak-to-peak of C where V V + V. 6

39 Fg C v. V wth dfferent bu voltage for kw fuel cell ytem. Fg. 3.7 derved baed on 3. and llutrate the relatonhp between C and V wth dfferent value of V. A kw fuel cell ytem for redental applcaton [9] elected a the calng bae n th the. The p.u. for each varable defned a hown n Fg V calculated baed on V bae. C oberved that can be gnfcantly reduced by ncreang V. In addton, C alo decreaed by ncreang V. The tradtonal olated full brdge - converter baed fuel cell ytem hown n Fg. 3.5 ha to lmt the V n order to uppre the nput rpple current nce t cannot decouple the rpple energy between the fuel cell tack and nverter load [7]. Therefore, a relatvely large C and mall V ha to be elected to provde the requred rpple energy accordng to Fg However, a relatvely mall C and large V able to provde the requred rpple energy for the ytem of Fg Th becaue f a propoed proportonal-reonant PR 7

40 controller appled to the above ytem to acheve an extra hgh control gan at degned reonant frequency, a a reult, the large V dturbance to the CF-DHB converter nput current can be elmnated. Therefore, flm capactor wth mall capactance and longer lfetme can be employed for C ntead of bulky electrolytc capactor Propoed CF-DHB Converter Operaton Analy and Small-gnal Modelng Although the operaton prncple of CF-DHB converter have been dcued n [8, ], they were baed on the aumpton of a contant V. The analy n ecton II how that V of propoed CF-DHB baed fuel cell ytem wll preent a relatvely large double-frequency rpple nce a much maller C can be appled ung propoed control technology. Th change may lead to dfferent converter operaton charactertc and mall gnal model, whch wll affect the controller degn. Therefore, the operaton analy of CF-DHB converter wth mall capactor and t mall-gnal model derved n th ecton. Fg. 3.8 how the prmary referred equvalent crcut of CF-DHB converter ervng nverter load. Snce the operaton charactertc of the propoed CF-DHB converter dffer from the prevou tudy due to the vared V, a varable d defned n 3. to llutrate th dfference more clearly. d the prmary-referred voltage gan. Reference [] frtly defne the voltage gan whch apple to t tuded topologe. ` V DN p d V+ V 3. V V N d n where V` the voltage of C n prmary referred equvalent crcut; V d the prmary de voltage; D the duty cycle; N p and N are prmary and econdary de tranformer turn, repectvely; and V n the nput voltage. V`, V d, and V n can be found n Fg. 3.8 where the defnton of V and V can be referred from 3. and found n Fg

41 Fg Prmary referred equvalent crcut of CF-DHB converter ervng nverter load The CF-DHB converter operaton analy performed n th the can be condered for a more general cae where d treated a a varable o that the tranformer current, output power, ZVS condton derved are all related to d Tranformer current analy of CF-DHB converter wth vared d. Fg. 3.9 how the dealzed key operaton waveform of CF-DHB converter crcut hown n Fg.3.8. S S 4 are the drver gnal for wtch S S 4, repectvely. V V 4 are voltage of four capactor n the crcut of Fg.3.8 whch are derved n 3.3 where V 3 and V 4 are not only related to duty cycle D but alo dependng on d. D V Vn D V Vn D V3 dv D V4 dvn n 3.3 9

42 3 Fg Idealzed CF-DHB converter key operaton waveform. The phae hft angle between prmary-de and econdary-de Φ. Therefore, the ntantaneou tranformer current r of each mode can be expreed n : : : : π θ ω π π θ ω π θ ω θ ω D L V V D IV Mode D L V V D III Mode L V V II Mode L V V I Mode r r r r r r r r 3.4 where r, r Φ, r Dπ, r Dπ+Φ are four ntal tate of r n each mode and can be derved n 3.5 baed on the tranformer current average value over one wtchng perod zero.

43 r r r r K [ d Dπ + d π d] K[ d D π + d Dπ D+ ] D Dπ K[ d D π + d Dπ dd+ d] Dπ + K[ d Dπ + d π ] where K V n /ω L and ω the wtchng angular frequency. D 3.5 Baed on , the tranformer current peak value r_peak can be obtaned n 3.6 wth the aumpton that Φ< mn {Dπ, -Dπ}. The tranformer rm current r_rm can be calculated from 3.6 and hown n demontrate that r_rm and r_peak are all related to d. The output power equaton calculated accordngly n 3.8. Compared to the tudy performed n [8], the power equaton developed n [8] the pecal cae of 3.8 wth d. Fg. 3. compare r_peak and r_rm per unt value wth dfferent duty cycle by ncreang output power. The caled p.u defnton are gven n the appendx A-. The range of d of a pecfed D calculated from 3. by aumng the V 5% of V. Th becaue wth th value, the propoed technology can allow a relatvely mall capactor and mantan the ZVS operaton a well. r _ peak K[ d D π + d Dπ D+ ] D, d > K[ d D π + d Dπ dd+ d] D, d K[ d π + ], d > K[ d π + d], d K[ d Dπ + d π + ], d > [ ] K d Dπ + d π + d, d when when when D<.5 D.5 D> V π n 3 3 r _ rm D D d + 6 dd D d 3.7 6πωL D π dvn Po _ DHB [ 4πD D ] 3.8 4πωL D 3

44 If V elected a a larger value, the capactance can be reduced further but the ZVS operaton cannot be guaranteed. Th wll be explaned n the ZVS analy of ecton B. Baed on the above condton, r_peak of D from.3.7 wth each pecfed d range ha been calculated from 3.6 where D.5 wth d between.875 and.5 found to have the mnmum r_peak. In addton, D.4, D.5, D.6 are elected a three typcal cae to be compared n Fg.3. a. Smlarly, r_rm of D from.3.7 wth each pecfed d range ha been calculated from 3.7. Snce d not a contant value, the average r_rm value of one double-frequency cycle calculated where D.5 wth d between.875 and.5 alo found to have the mnmum r_rm. Fg. 3.b how the r_rm of D.4, D.5, and D.6. Therefore, D.5 wth.875 d.5 the optmzed duty cycle control to acheve the mnmzed r_rm and r_peak for CF-DHB converter baed fuel cell ytem wth mall capactor ZVS analy of CF-DHB converter wth vared d and mall-gnal modelng. The bac prncple of ZVS operaton condton of CF-DHB converter to gate on the ncomng devce whle the ant-parallel dode conductng and the ZVS condton for each wtch are gven n 3.9 [8]. In order to fnd out the ZVS condton when d vared, 3. derved below by ubttutng 3.5 nto 3.9. The duty cycle D.5 n 3.. Fg. 3. derved baed on 3.8 and 3. and caled to p.u. baed on appendx A- to demontrate the ZVS range of four wtche when power and phae hft degree change. S : r fc < S : r Dπ fc Dπ > S3 : r > S4 : r Dπ + < 3.9 3

45 a b Fg. 3.. r_peak and r_rm v. P o_dhb of dfferent duty cycle wth the defned d range: a r_peak, and b r_rm. 33

46 S S S S 3 4 π + π + πl : d < π π + π + πl : d < π π : d > π π : d > π L L 3. Snce D.5 wth.875 d.5 the optmzed duty cycle control operaton and elected for propoed converter, the three power curve of D.5, d.85; D.5, d.; and D.5, d.5 are drawn n Fg.3. repectvely. In addton, the ZVS boundare of S 4 are alo labeled n Fg. 3.. Therefore, the haded area are ZVS range of four wtche. A llutrated, the ZVS well mantaned for all the wtche. Fg. 3. alo explan why V cannot be too large. Accordng to 3., V 3% of V when d.85 and.5. If V > 3% of V, d wll be away from.85 and.5, the ZVS operaton therefore cannot be guaranteed. A a reult, V 5% of V elected to mantan a afe ZVS range for all the wtche. The average model of CF-DHB converter wth mall capactor derved n 3. baed on The tate varable are choen to be the fc, v and v 34. v v + v, v 34 v 3 + v 4. v v 4 are voltage of four capactor n the crcut of Fg d dt dv dt dv34 dt fcavg avg avg v L D C p n C t Dv L fcavg avg + 4πD 4πD + v πc ωl + 4πD 4πD v πcωl t p avg 34avg 3. where C t C + C. 34

47 Fg. 3.. Scaled ZVS of S 4 at D.5 where Φ.6π, L /L.5. Equaton 3. how that the developed averaged model not related to d and mlar to the one derved n []. In addton, the mall-gnal model can be further developed baed on 3. by ntroducng the mall perturbaton around the nomnal operaton pont, whch D.5. The developed mall-gnal model gven n the appendx A- A-4. Baed on the mall-gnal model, the tranfer functon matrx from the nput factor to the fuel cell current can be obtaned n 3.. Therefore, the nverter load current dturbance to fuel cell current tranfer functon G o and control-to-fuel cell current tranfer functon G can be calculated and hown n 3.3 and 3.4, repectvely. 35

48 C fc G ϕ I A Bu + G v + G v n o fc π Cpcon G o C m C L con con fc _ ref + π + v n t p fc C p π [ V34con+ π Vn ] G ϕ m + C L π con con 3.4 p + vn where con πc P ωl, con πc t ωl, m C P L concon The tranfer functon matrx and control-to-output tranfer functon G v for output voltage can alo be calculated and hown n 3.5 and 3.6, repectvely. v C I A Bu G + G v + G vϕ vv n vo 3.5 v π [ π C plv 34+ C plv n con+ V con] n G v m + C L π con con 3.6 p + n v 3..3 Propoed Pulaton Power Decouplng Control Sytem and Degn The propoed power pulaton decouplng control dagram of CF-DHB converter baed fuel cell ytem decrbed n Fg. 3., whch can be dvded nto CF-DHB converter control ytem and nverter control ytem. The CF-DHB converter control ytem nclude duty cycle control and phae hft control. Snce the optmzed duty cycle ha been found to be.5, D et to be.5 n Fg. 3.. The phae hft control ytem can acheve two functon. Frtly, t regulate V to a certan level o that the nverter can generate the requred ac voltage. Secondly, t uppree the fuel cell double-frequency rpple current that caued by the nverter load. A proportonal-ntegral PI controller G c ued to regulate V. The voltage reference 36

49 V * a contant value. The feedback V cont of a contant value V and a doublefrequency rpple V. The control objectve of G c to let the component of V to follow V *. Therefore, a PI controller can be elected for G c. A a reult, a relatvely large V wll generate the double-frequency component n the controller output Φ. In addton, th doublefrequency component wll be reflected on the fuel cell double-frequency rpple current. Therefore, another proportonal-reonant PR controller G cr developed n th the to elmnate the fuel cell current double-frequency rpple. The current rpple reference I * fc_r. The real-tme I fc whch cont of a component and a double-frequency rpple ued drectly for feedback becaue the controller G cr degned to be mmune to the component and only repone to double-frequency component. A a reult, Φ r, the output of G cr, a doublefrequency component whch can offet the double-frequency component of Φ. Therefore, the fnal output Φ a component to regulate I fc to be a pure current. The degn gudelne of G c and G cr wll be explaned later. The nverter control adopt the tradtonal ngle-phae dual-loop d-q vector control [3-4]. However, nce V ha the relatvely large double-frequency rpple, the nverter control need to feedback the real-tme V for PWM modulaton, whch hghlghted n Fg. 3.. In order to degn the G cr, the fuel cell current repone to the nverter load dturbance frtly analyzed. Fg. 3.3 how the open-loop nverter load dturbance to the fuel cell current tranfer functon G o bode dagram baed on 3.3. The crcut parameter value of 3.3 are L 8 H, L. H, C p.7 mf, C.64 mf, f khz. The teady-tate Φ.6π at rated power. The low-frequency component of only the Hz rpple current nce the nverter output 6 Hz ac voltage. Therefore, the Hz repone of G o hghlghted n Fg A hown, the repone.3 db for C whch mean the fuel cell current rpple 37

50 38 magntude 3.73 tme of magntude. The repone for C 3 mf only db whch mean the fuel cell current rpple magntude only.38 tme of magntude. Th explan why the large capactor able to uppre the fuel cell double-frequency rpple. Baed on the CF-DHB converter mall-gnal model derved n the lat ecton, the fuel cell current mallgnal model block dagram llutrated n Fg The cloed-loop nverter load dturbance to the fuel cell current tranfer functon can be obtaned and gven n 3.7. A hown, the compenated ytem ha the loop gan /+T r. Th loop gan repone at Hz found to be very mall f the propoed controller G cr degned to have a relatvely large repone at Hz. A a reult, the fuel cell current repone to nverter load dturbance can be mnmzed nce the low-frequency component of th dturbance current only cont of Hz component. The PR controller therefore elected becaue t can be degned to have a relatvely large gan only at t reonant frequency. And th reonant frequency et to be Hz for the propoed technology. The PR controller G cr can be expreed by p f r K π + where f r. The prncple of choong K p to enure that G cr ha neglgble component repone and relatvely large reonant frequency repone. It a trade-off degn. Fg. 3.5 how the bode dagram of compenated G o wth C and the PR controller G cr. K p elected to be.. _ H G G G T G r cr o r o v fc n ref fc ϕ _ H G G H G G G G v v v c v c v c v ref n

51 Fg. 3.. Propoed fuel cell ytem power pulaton decouplng control dagram. Fg The open-loop tranfer functon G o bode dagram wth dfferent value of C. 39

52 Th value approprate nce the reonant frequency repone 54.4 db and component repone maller than - db a llutrated n Fg A a reult, the real-tme I fc can be drectly ued for feedback wth repect of zero reference nce t component repone neglgble. The compenated G o repone to Hz dturbance current -4 db whch theoretcally elmnate the double-frequency rpple. In order to degn the controller G c, the open-loop control-to-output tranfer functon G v root locu frtly tuded. The root locu obtaned baed on 3.6 and hown n Fg. 3.6 a. The crcut parameter of 3.6 are the ame a thoe ued n 3.3. A hown, t ha two zero n rght-half plan RHP and two pole are cloe to the jω-ax. Therefore, the openloop converter doe not have much tablty margn. However, a explaned n [], all the loy component add more nerta to the ytem o the real converter actually more table. Baed on the mall-gnal model, the cloed-loop tranfer functon obtaned and hown n 3.8. Fg Fuel cell current mall-gnal model block dagram. 4

53 Fg The bode dagram of compenated G o wth C and the PR controller G cr. G c degn prncple to enure the ytem tablty. Fg. 3.6 b how the cloedloop ytem root locu where G c.3+e-3/. The pole are p and p Therefore, the compenated ytem pole are n the left-half-plan LHP and t ncreae the ytem tablty Expermental Verfcaton The experment were conducted n the laboratory to verfy the aforementoned theoretcal analy and the propoed pulaton power decouplng control performance. A kw fuel cell ytem tet bed wa developed and hown n Fg The voltage ource wa ued to emulate the teady tate charactertc of the commercal kw fuel cell module. 4

54 a b Fg Control-to-output tranfer functon G v root locu: a. open-loop, b cloed-loop. 4

55 Th commercal module cont of 36 ere connected ngle 5 cm Proton Exchange Membrane PEM fuel cell developed by Bng Energy [5] and t rated operaton pont around 5 V wth 4 A output. The CF-DHB converter crcut parameter are the ame a thoe ued for the theoretcal analy: L 8 µh, L. µh, C p.7 mf, C.64 mf, f khz. The teady-tate Φ.6π at rated power. An nput LC flter µh and mf wth 5 khz bandwdth added to flter the hgh frequency current. The tranformer turn rato 4:6. The prmary and econdary de devce are mplemented wth power MOSFET FB8SA 4 paralleled and FA38SA-5LCP, repectvely. The nverter wtchng frequency khz and the LC fler parameter are 5 mh and 6 µf. The nverter rated output V rm., 6 Hz ac voltage and the load 6.9. The dgtal control board employ DSP TMS3F8335. Fg. 3.8 how the expermental reult wth and wthout propoed pulaton power decouplng control. Fve trace, V, v o, I fc, Φ, and r repreent the - converter output voltage, nverter output voltage, fuel cell current, phae hft angle and tranformer current, repectvely. Φ obtaned by ung the DSP controller board dgtal-to-analog D/A functon. Fg kw CF-DHB converter baed fuel cell power condtonng ytem tet bed. 43

56 Fg. 3.8 a how the expermental reult of C 3 mf wthout the propoed method. The equvalent bu capactor C eq C + C / / 3.3 mf. A hown, V almot a contant value due to the relatvely large C eq. r_rm 37.7 A and r_peak 59. A. v o V rm.. The load 6.9. Therefore, the magntude of can be calculated and equal to.44/6.9. A. From the theoretcal analy reult hown n Fg. 3.3, the nverter load current dturbance repone wth C 3 mf db whch.38. A a reult, the theoretcal fuel cell current rpple I fc peak-to-peak A. The expermental reult of I fc peak-to-peak 6 A whch very cloe to the theoretcal analy. Therefore, the prevou mall-gnal modelng baed theoretcal analy valdated. Fg. 3.8 b how the expermental reult of C wthout the propoed method. The equvalent bu capactor C eq.3 mf. A hown, V 3V, r_rm 48.5A and r_peak 9.5A. I fc contan the relatvely large Hz rpple current due to the relatvely mall C eq. From Fg. 3.3, the nverter load current dturbance repone wth C.3 db whch Therefore, the theoretcal fuel cell current rpple I fc peak-to-peak A. The expermental reult I fc peak-to-peak 68 A whch alo very cloe to the theoretcal analy. Th further valdate the prevou analy. Φ contan the relatvely large Hz rpple whch content wth the prevou analy reult. Fg. 3.8 c how the expermental reult of C wth the propoed method. The equvalent bu capactor C eq tll.3 mf. A hown, V 5 V. The fuel cell current ha the neglgble low-frequency rpple. Therefore, t prove that the propoed method can almot elmnate the fuel cell low-frequency rpple wth relatvely mall capactor. Φ almot the component ncludng very mall rpple whch valdate the analy of waveform Φ hown n Fg. 3.. r_rm 37.8 A and r_peak 63. A. Compared to the reult hown n Fg. 3.8 a, 44

57 thoe two r_rm value are the ame and r_peak value are very cloe. In other word, the propoed method wth the much maller capactor can acheve the ame performance a the tradtonal control wth the much larger capactor. The bu capactance reducton up to / %. Actually, the propoed method performance even better wth repect to the neglgble low-frequency rpple. Fg. 3.9 how the caled FFT analy reult of I fc hown n Fg Fg. 3.9 a how the FFT analy reult of I fc n Fg. 3.8 b. A hown, the Hz component 58.6% of the component wthout the propoed technology. In contrat, the Hz component only.% of the component wth the propoed technology a llutrated n Fg. 3.9b. a Fg Expermental reult: a C 3 mf wthout the propoed method, b C wthout the propoed method, c C wth the propoed method. 45

58 b c Fg contnued. 46

59 a b Fg The caled FFT analy reult of I fc hown n the Fg. 4: a I fc n Fg. 4b, b I fc n Fg. 4c. 47

60 a b Fg. 3.. CF-DHB converter four wtche ZVS waveform wth the propoed method: a S and S, b S 3 and S 4. 48

61 Therefore, the I fc FFT analy reult proved that the propoed technology can elmnate I fc double-frequency rpple current. Fg. 3. llutrate the CF-DHB converter four wtche ZVS waveform wth propoed power pulaton decouplng control. From prevou ZVS analy reult hown n Fg. 3., the ZVS can be alway mantaned f V < 3% of V. From Fg c, V 5 V and V V. Therefore, V 5% of V and the ZVS can be acheved theoretcally. The expermental reult hown n Fg. 3. alo valdate th concluon. A hown n Fg. 3.a, the wtch S turn-off tranent ha the overhoot cenaro and no uch cenaro oberved from other three. Th becaue the S turn-off current relatvely larger compared to other. The mlar cenaro ha been oberved n [8, ]. 3.3 Propoed Sngle-phae Hgher Power Fuel Cell Power Condtonng Sytem Propoed Hgher Power Fuel Cell Converter The CF-DHB converter baed fuel cell ytem lmtaton are that the plt-capactor have to take full load current. Therefore, t not utable for hgher power applcaton. Moreover, the voltage wng range lmted by the ZVS operaton condton. To olve thoe ue, th the preent a new three-phae - converter baed hgher power fuel cell ytem. In fuel cell hgh-power applcaton, reearch ha been focued on the three-phae - converter baed power condtonng ytem nce t offer better performance over t nglephae counterpart n term of hgher power denty, lower wtchng devce current tre, maller ze of pave component and o on [6], [6]-[8]. The type of three-phae - converter could be ether current-fed or voltage-fed. Baed on the tudy performed n [6]-[8], current-fed topology better uted to low-voltage hgh-current fuel cell applcaton where hgh 49

62 voltage tep-up rato requred. Moreover, the current-fed topology beneft from the drect and prece nput current control. Prevou reearch of three-phae - converter for fuel cell manly focued on the hgh effcency and hgh power denty. The method to reduce the fuel cell low-frequency rpple current ha been eldom dcued. The voltage-fed V6 hgh-power fuel cell converter [6] preent the rpple current mtgaton method by ung a current-loop control wthn the extng voltage loop to mtgate the fuel cell low-frequency rpple. However, the large electrolytc capactor tll requred a an energy buffer. Up to date, no lterature reearch on the fuel cell low-frequency rpple current reducton method baed on current-fed three-phae - converter. Th the propoe a three-phae current-fed nterleaved-tructure baed HFL fuel cell ytem. Compared to other three-phae - converter baed fuel cell ytem, the unque advantage of our propoal fall nto the followng three apect. Frtly, a drect doublefrequency rpple current control baed on the current-fed three-phae HFL converter propoed to acheve the low-frequency rpple-free nput current. Secondly, the propoed method approach to apply a control-orented rpple current mtgaton trategy wthout addng any extra crcut component. The mall flm capactor can be adopted to replace the bulky electrolytc capactor. Thrdly, the control ytem can realze the full utlzaton of capactve rpple energy ource n the propoed fuel cell ytem whch beneft a further reducton of -bu capactance. Moreover, wth all thoe contrbuton lted above, the zero-voltage-wtchng ZVS operaton of all wtchng devce n the - tage can tll be mantaned wthout addng any extra crcut. 5

63 Fg. 3.. Propoed two-tage hgh-frequency-lnk baed hgh power fuel cell power condtonng ytem Propoed fuel cell ytem decrpton. Fg. 3. how the propoed two-tage hgh-frequency-lnk baed hgh power fuel cell ytem. The ytem cont of a current-fed three-phae HFL converter wth olated Y-Y connected hgh-frequency HF tranformer and an nverter. The three-phae HFL converter power flow controlled by the phae hft angle between the actve wtche on low voltage de LVS and hgh voltage de HVS. The converter can be operated ether n the boot mode or n the buck mode. In th the, the converter operated n boot mode for fuel cell applcaton. The boot functon acheved by the nductor L, L and L 3 and three half brdge on LVS. The leakage nductor L, L and L 3 are the energy tranfer element for each phae. The major feature of th three-phae HFL converter nclude: ncreaed converter power ratng by parallelng phae, not by parallelng multple devce; reduced ze of nput nductor and -bu capactor wth nterleavng tructure; 3 mantaned oft wtchng operaton and hgh effcency wthout any extra crcut component. The above feature have been tuded n [9]. However, the method to reduce the nput double-frequency rpple current caued by the nverter load ha not been dcued. Th the reearch focu to tudy the 5

64 drect double-frequency rpple current control of th three-phae HFL converter when connectng a ngle-phae nverter load. Fg. 3.. Idealzed three-phae HFL converter key operaton waveform. 5

65 Fg. 3. how the dealzed three-phae HFL converter key operaton waveform. G x the gatng gnal of wtch S x hown n Fg. 3. accordngly. A hown n Fg. 3., the gate gnal for upper and lower wtche on each phae are complementary, wth the phae angle π/3 between phae leg on one de. Benefttng from current-fed topology, the duty cycle D controllable. Therefore, the LVS bu voltage can be controlled n a way to alway mantan the oft-wtchng operaton. Th key charactertc wll be further explaned later. ra and I a the phae A tranformer current and nductor current, repectvely. V an and V rm the phae A tranformer prmary-de and econdary-de voltage, repectvely. Fg Equvalent rpple crcut model of propoed fuel cell ytem Equvalent rpple crcut modelng of propoed fuel cell ytem. Fg. 3.3 how the equvalent rpple crcut model of propoed fuel cell ytem. V fc the fuel cell tack voltage. If fuel cell current ha neglgble low-frequency rpple current, V fc the contant 53

66 voltage. The - converter can be mplfed a an deal tranformer nce t wtchng frequency much larger than the ytem rpple frequency [6]. The nverter load modeled by a double-frequency pulaton current ource. A hown, both the LVS -bu voltage V d and HVS -bu voltage V ha the relatvely large voltage wng. Three reaon for th control degn. Frtly, large voltage varaton of V lead to mall HVS bu capactor C whch make t vable to replace the electrolytc capactor wth flm capactor. Th ha already been explaned n [5]. Secondly, f real-tme balancng of tranformer prmary-de and econdaryde voltage can be mantaned by ynchronzng V d wth prmary-referred V, the three-phae HFL converter can alway mantan the ZVS operaton. Th wll be further explaned n the next ecton. Thrdly, voltage varaton on both LVS and HVS bue make both the prmary-de and econdary-de capactve energy ource C p and C a hown n the crcut of Fg. 3 to provde the rpple energy requred by the nverter load. Accordng to the fuel cell ytem rpple energy calculaton performed n [5], the crcut rpple energy balancng hown n Fg. 3.3 can be expreed n 3.9. C V p V C V V P load d d+ 3.9 ωload where V d and V d are the LVS bu average voltage and voltage varaton peak-to-peak, repectvely. V and V are the HVS bu average voltage and voltage varaton peak-topeak, repectvely. P load the load real power, ω load the load angular frequency. A llutrated n 3.9, the propoed fuel cell ytem can make full untlzaton of ytem capactve rpple erngy. Th a unque advantage compared to the voltage-fed fuel cell converter. Snce the voltage-fed fuel cell ytem connecte the LVS bu capactor C p drectly to the fuel cell tack [6]. Therefore, the capactve rpple energy of C p not able to be utlzed. 54

67 3.3.. Drect Double-frequency Rpple Current Control Sytem and Degn The propoed drect double-frequency rpple current control ytem dagram decrbed n Fg The propoed control ytem nclude duty cycle control and phae hft control. The duty cycle D D + D r a llutrated. Frtly, the component of D, D et to be.5. Th becaue the three-phae HFL converter ha the optmzed operaton effcency at 5% duty cycle [9]. Secondly, the rpple component of D, D r generated by ynchronzng the LVS bu voltage V d wth the prmary-referred HVS bu voltage V. The purpoe to real-tme balance the tranformer prmary-de and econdary-de voltage n order to enure the ZVS operaton of all the three-phae HFL converter wtchng devce. The detaled analy of ZVS operaton wll be explaned n the next ubecton. The proportonal-reonant PR controller adopted for G Dr to regulate the wng of V d. The degn gudelne of controller G Dr wll be explaned later. A hown n Fg. 3.4, the real-tme prmary-referred V employed a the voltage reference. V d V fc /D due to the LVS half brdge boot functon. V fc a contant value f I fc ha no low-frequency rpple. Therefore, D wll contan a double-frequency rpple n order to keep V d ynchronzed wth prmary-referred V whch ha the double-frequency varaton. The phae hft angle + r a llutrated. Frtly, the component of, generated by regulatng V to meet the nverter modulaton requrement o the nverter can generate the requred ac voltage. A proportonal-ntegral PI controller G adopted to regulate V. The voltage reference V * a contant value. The feedback V cont of a contant value V and a double-frequency rpple V. The control objectve of G only to regulate the component of V to follow V *. Therefore, a PI controller can be elected. Due to the relatvely large V n the feedback, the double-frequency component wll be generated n the controller output. And th double-frequency component wll be reflected on 55

68 the fuel cell current f the nverter load rpple energy propagated nto the fuel cell tack through the HFL converter. In order to block the rpple energy propagaton from nverter load to fuel cell tack, a drect double-frequency rpple current controller G r developed. The PR controller employed for G r nce the PR controller can generate an extra hgh control gan at t reonant frequency. Th hgh gan can be vewed a the vrtual hgh mpedance for blockng the rpple energy. A llutrated, the current rpple reference I * fc_r. The real-tme I fc whch cont of a component and a double-frequency rpple ued drectly for feedback becaue the controller G r degned to be mmune to the component and only repone to double-frequency component. A a reult, r, the output of G r, a double-frequency component. Fg Propoed drect double-frequency rpple current control ytem dagram. 56

69 The nverter control adopt the tradtonal ngle-phae dual-loop d-q vector control [3]. However, nce V ha the relatvely large double-frequency rpple, the nverter control need to feedback the real-tme V for PWM modulaton ZVS operaton analy wth large wng bu voltage. The tuded three-phae HFL converter ZVS operaton analy ha been performed n [9]. The bac prncple of ZVS operaton to gate on the ncomng devce whle the ant-parallel dode conductng. To analyze the ZVS condton, the varable d defned n [9] whch lted n the Appendx B-. d mean that V d matche wth prmary-referred V. If d, the ZVS condton are alway atfed for HVS wtche and LVS upper wtche S a, S b, S c a hown n the crcut of Fg For LVS lower wtche S a, S b, S c a hown n the crcut of Fg. 3.4 ZVS condton, the rato of nductor dvded by leakage nductor another key factor. Small rato lead to large nductor current rpple whch reult n large oft wtchng operaton regon for LVS lower wtche. However, the large current rpple lead to large nductor core lo and conducton lo. It therefore a trade-off degn. In th the, th rato elected to be 3.3. Due to the converter ymmetrc property, each phae wtche on the ame poton have the ame ZVS condton. The phae A wtche are elected for tudy. Accordng to [9], the oft wtchng condton for phae A LVS and HVS wtche are lted n the appendx B-. The three-phae HFL output power and p.u. bae power are gven n appendx B-3 and B-4, repectvely. Baed on B--B-4, the caled power curve and ZVS boundare of phae A wtche at D.5 wth L /L 3.3 hown n Fg A llutrated, the haded area oft wtchng regon. If d control can be acheved, the operaton can be alway mantaned wthn the oft wtchng regon. Th explan why V d real-tme ynchronzed wth prmaryreferred V n the propoed control ytem. 57

70 3.3.. Controller degn prncple wth large wng bu voltage. In order to degn the controller G r, G and G Dr, the three-phae HFL converter mall-gnal model need to be tuded. Reference [9]-[3] have developed the average model of the tuded three-phae converter. Due to the ymmetrc property, the modelng of three-phae HFL converter can be treated a the model of ngle-phae half-brdge converter. The phae A elected to llutrate the average model whch gven n the appendx B-5. However, the mall-gnal model wth repect to the nverter load current dturbance ha not been tuded n [9]-[3]. Therefore, the mall-gnal model conderng the nverter load current dturbance gven below. By ntroducng the mall perturbaton around the nomnal operaton pont n the model gven n B- 5, the mall-gnal model of phae A half-brdge converter could be developed a follow: L C C p da dt dv d dt dv dt v n D + I Dv a a d V D 4π 3 D v 8πωL 4π 3 + v 8πωL d d V 4π 6 ϕ 8πωL Vd 4π 6 + ϕ 8πωL 3. where the nverter double-frequency pulaton current a hown n the crcut of Fg Therefore, the tate-pace equaton can be expreed n Baed on the mall-gnal model hown n , the tranfer functon matrx from nput factor to the nput current can be obtaned n 3.4. The nverter load current dturbance to nput current tranfer functon G o, the control-to-nput current tranfer functon G and the duty cycle dturbance to nput current tranfer functon G D can be calculated and hown n 3.5, 3.6 and 3.7, repectvely. 58

71 Fg The caled power curve and ZVS boundare of S a, S a, S r and S r at D.5 wth L /L 3.3. Fg. 3.6 how the open-loop G o and G D bode dagram baed on 3.5 and 3.7 wth dfferent value of C. The crcut parameter value of 3.5 and 3.7 are: L 4.6 H, L. H, C p F, f 4 khz. The nomnal operaton pont.56π, V d V, V V, I a 6A. The low-frequency component of only the Hz rpple current nce the nverter output 6 Hz ac voltage. Therefore, the Hz repone of G o hghlghted. A hown n Fg. 3.6a, the repone 3. db wth C F whch mean the nput current Hz rpple magntude 4.57 tme of magntude. The repone wth C mf only -6. db whch mean the nput current Hz rpple magntude only.5 tme of magntude. Th explan why the large capactor able to uppre the nput current double-frequency rpple. 59

72 6 [ ] [ ] [ ] [ ] + T v d v a a T v d v a v D n v C con d V p C a I con V L d V L v d v a con con p C D L D v d v a ϕ π π π π 3. + Cx y Bu Ax x, con con C D L D A p π π, B d p a d C con V C I con V L V L π π 3. ] [, ] [, ] [, 3 3 C v y C v y C y d a 3.3 where con 8πωL C p, con 8πωL C.

73 6 Snce the duty cycle D alo ha the double-frequency rpple a explaned n ubecton B, t dturbance to the nput current alo need to be tuded. A hown n Fg. 3.6b, the nput current alway ha a relatvely large repone to the duty cycle dturbance regardle of large C value. The repone 39.dB wth C mf whch mean the nput current Hz rpple magntude tme of D rpple magntude. Therefore, th dturbance ha to be mtgated f mplementng the propoed d vared duty cycle control. G D G G v G Bu A I C o D n v a ϕ p D v a o L C D con con C con L D G n + + π π D con con L C con con L C V con D V C G p p d p D v a n π π π π ϕ D con con L C con con L C C V con con DI C V con con D G p p d p a d p v a D n π π 3.7 _ G G G T G r o D v a r o n ref a ϕ _ G G G D T G r D v a r D n ref a ϕ Baed on the developed mall-gnal model, the phae A nput current mall-gnal model block dagram llutrated n Fg Therefore, the cloed-loop nverter load dturbance and duty cycle dturbance to the fuel cell current tranfer functon can be obtaned and gven n 3.8 and 3.9, repectvely. A hown, the compenated ytem ha the loop gan /+T r.

74 Th loop gan repone at Hz found to be very mall f the propoed controller G r degned to have a relatvely large repone at Hz. A a reult, the fuel cell current repone to nverter load dturbance and duty cycle dturbance can be both mnmzed. The PR controller therefore elected becaue t can be degned to have an extra hgh gan only at t reonant frequency. And th reonant frequency et to be Hz for the propoed technology. The PR controller G r can be expreed by K + π where f r. The p f r prncple of choong K p to enure that G r ha neglgble component repone and relatvely large reonant frequency repone. It a trade-off degn. Fg. 3.8 how the bode dagram of PR controller G r, compenated G o and G D wth C F. K p elected to be.. Th value approprate nce the Hz dturbance repone 85.8 db and component repone maller than - db a llutrated n Fg A a reult, the real-tme I fc can be drectly ued for feedback wth repect of zero reference nce t component repone neglgble. The compenated G o and G D repone to Hz dturbance -5 db and -78.3dB, repectvely. Therefore, the compenated ytem can theoretcally elmnate the fuel cell double-frequency rpple current caued by the nverter load current and duty cycle. v C3 I A Bu G v + G + G vv n D + G vϕ vd vo 3.3 G 4π 6 C pvd Lcon C pv L 4π 3 + VdD con C L concon + C L 4π 3 + D concon v p p 3.3 Baed on the mall-gnal model hown n , the tranfer functon matrx from nput factor to the output voltage V gven n 3.3. In order to degn the controller G, control-to-output tranfer functon G v calculated and hown n 3.3 baed on the developed mall-gnal model. 6

75 a b Fg The open-loop tranfer functon bode dagram wth dfferent value of C : a G o, b G D. 63

76 Fg Phae A nput current mall-gnal model block dagram. Fg Bode dagram of PR controller G r, compenated G o and G D wth C F. 64

77 A hown n 3.3, t ha a par of conjugated rght half plane zero. Therefore, the ytem tablty crteron that the phae margn Φ m 8º - 8º+G v jπf c >, where f c the cro-over frequency. Fg. 3.9 how the G v bode dagram of open-loop and cloedloop wth C F. The parameter of 3.3 are the ame a thoe ued n 3.6. A hown, the ytem open-loop phae margn Φ m -7º <. Therefore, the tablty of open-loop ytem theoretcally very poor. However, a explaned n [], all the loy component add more nerta to the ytem o the real converter actually more table. G degn prncple to enure the ytem tablty. A hown n Fg. 3.9 where G +. /, the compenated ytem phae margn Φ m 9º >. Therefore, the compenated ytem more table. The degn procedure of controller G Dr very mlar to the degn of G r. Therefore, the detaled proce wll not be repeated here. The G Dr degn prncple to enure the V d ha the relatvely large repone at Hz o t can be ynchronzed wth prmary-referred V d to mantan d. In addton, the dturbance caued by the phae hft angle and nverter load hould be mtgated. The tranfer functon matrx from nput factor to the output voltage V d gven n the Appendx B-6. All the key tranformer functon are lted n appendx B-7 - B-9. The compenated G v, G vo and G vd bode dagram alo gven n appendx Fg. B wth G Dr. + π. A llutrated n Fg. B, V d repone at Hz 46.5dB and t repone to nverter load and phae hft angle dturbance are -4dB and - 8.8dB, repectvely. Therefore, the compenated ytem can theoretcally mantan d control and elmnate the nverter load current and phae hft angle dturbance. 65

78 Expermental Verfcaton The experment were conducted n the laboratory to verfy the aforementoned theoretcal analy and the propoed drect double-frequency rpple current control performance. A 3.5 kw fuel cell ytem tet bed wa developed and hown n Fg The DC nductor and tranformer degn n th the adopt the planar core wth col encapulated wthn multlayer prnted crcut board PCB. Fg Bode dagram of open-loop and cloed-loop G vϕ wth C F. 66

79 Both the LVS and HVS bu capactor adopt the Epco flm capactor a the energy buffer. The voltage ource wa ued to emulate the teady tate charactertc of the fuel cell module. The three-phae HFL converter baed fuel cell ytem tet bed key crcut parameter are lted n the Table 3.. The dgtal control board employ DSP TMS3F8335. Fg. 3.3 Fg how the expermental reult wth and wthout propoed drect double-frequency rpple current control method. ra, I fc, V d, V, v o repreent the phae A econdary-de tranformer current, fuel cell current, LVS bu voltage, HVS bu voltage and nverter output voltage, repectvely. Snce the propoed fuel cell ytem ha the hghfrequency tranformer, the ynchronzed oft tart-up cheme preented n [33]-[34] adopted to acheve the mnmzed tranformer current repone durng the tart-up tranent. Leakage nductor H Tranformer magnetzng nductor mh Prmary-de Mofet Prmary-de flm capactor Table 3.. The three-phae HFL converter baed fuel cell ytem tet bed key crcut parameter Three-phae HFL converter Phae A:./Phae B:./Phae C:. Phae A:./Phae B:.9/Phae C:. IXFN3N7T Epco F/45V DC nductor H Tranformer turn rato Secondaryde Mofet Secondaryde flm capactor Phae A: 4.64/Phae B: 4.65/Phae C: 4.7 H-brdge nverter L flter nductor mh. 4:6 Mofet Cree SC CMFD Epco 6 F/8V 3 Ant-parallel Dode Load Cree SC CMF D Cree SC C3D 6A 6. Swtchng frequency 4kHz Swtchng frequency khz 67

80 Fg The propoed three-phae HFL converter baed fuel cell ytem tet bed. Fg Baelne cae I: expermental reult wthout propoed control method, C p F, C 3.8mF. 68

81 Fg. 3.3 how the baelne cae I expermental reult wthout the propoed method. In order to uppre the fuel cell current double-frequency rpple, the large electrolytc capactor are connected to the HVS -bu. The adopted capactor for th cae are: C p F, C 3.8mF. A hown, V d 5V, V V and they both have relatvely very mall rpple due to the relatvely large capactor. ra_rm 5.3 A and ra_peak.4 A. v o V rm.. I fc average value around 4A. Fg. 3.3 how the baelne cae II expermental reult wthout the propoed method. The adopted capactor for th cae are: C p F, C 8F. A hown n Fg. 3.3 a, I fc contan the relatvely large Hz rpple current due to the relatvely mall capactor. I fc peak-to-peak 86.5A. V 6V, v o V rm.. ra_rm 7.A and r_peak.a. The tranformer current ha much larger rm. and peak value compared to Fg. 3.3 nce the double-frequency rpple current propagated nto the fuel cell tack through the tranformer. Fg. 3.3 b how the FFT analy reult of I fc n Fg. 3.3 a. A llutrated, the Hz component 7.A whch 7./466.3% of the component. It further valdate that I fc ha the relatvely large double-frequency component. Fg how the expermental reult wth the propoed method. The adopted capactor for th cae are: C p F, C 8F. A hown n Fg a, I fc ha the neglgble low-frequency rpple. Therefore, t prove that the propoed method can almot elmnate the fuel cell double-frequency rpple wth relatvely mall flm capactor. v o V rm.. V 86V, V d V. A hown, the LVS -bu voltage V d ha been ynchronzed wth V. And the voltage wng rato V /V d 4.95 whch very cloe to the tranformer turn rato 4.. Accordng to appendx B-, the propoed vared duty cycle control wth d therefore valdated. 69

82 a b Fg Baelne cae II: expermental reult wthout propoed control method, C p F, C 8F.a ytem performance, b I fc FFT analy reult 7

83 In addton, the reult hown n Fg. 3.33a alo demontrate that the propoed drectrpple current control and d control decoupled perfectly a degned. ra_rm 5.9A and ra_peak 3.7A. Compared to the reult hown n Fg. 3.3 a, the tranformer current ha much maller rm. and peak value nce the double-frequency rpple current propagaton path ha been blocked. Compared to the reult hown n Fg. 3.3, thoe two ra_rm value are very cloe. The ra_peak value about % bgger wth propoed method. The bu capactance reducton up to / % wth the propoed method. Fg b how the FFT analy reult of I fc n Fg a. A llutrated, the Hz component almot zero whch further valdate the performance of propoed drect doublefrequency rpple current control. The larget low-frequency rpple component 4Hz and the value.a whch only./45.% of the component. Fg llutrate the phae A LVS lower wtch S a wtchng waveform wth fxed D.5 control and d vared duty cycle control. The LVS lower wtch wtchng waveform are elected nce ther ZVS condton are more crtcal compared to the LVS upper wtche and HVS wtche. Th caued by the effect of nductor current [9]. For the tet cae hown n Fg. 3.34,.8π and L /L 3.3. Therefore, by calculaton baed on the ZVS condton gven n B-, the ZVS operaton of S a requre d <.7. Fg. 3.34a how S a wtchng waveform wth fxed D.5 control. V d contant due to fxed D and V ha the Hz rpple due to the mall bu capactance. A llutrated, two cae wtchng waveform wth maxmum V and mnmum V are hghlghted. S a wa hard-wtchng wth d. at maxmum V and wa oft-wtchng wth d.88 at mnmum V. Th reult valdated the ZVS condton analy. Fg. 3.34b how S a wtchng waveform wth d vared duty cycle control. 7

84 a b Fg Expermental reult wth the propoed control method, C p F, C 8F. a ytem performance, b I fc FFT analy reult. 7

85 A hown n Fg. 3.4b, V d wa controlled to be ynchronzed wth prmary-referred V for mantanng d.two cae wtchng waveform wth maxmum V and mnmum V are alo hghlghted. S a wa oft-wtchng n both cae. Therefore, t prove that the d vared duty cycle control can mantan ZVS operaton of LVS lower wtche. Fg how the three-phae HFL converter power lo break down analy at rated load wth aumed ZVS operaton. A hown, the wtchng devce conducton lo, turn-off lo and nductor core lo are the three man loe. By comparng the reult hown n Fg. 3.3 and Fg. 3.33, the I fc and ra rm. and peak value are very cloe. Therefore, compared to the tradtonal method wth large electrolytc capactor, the wtchng devce rm. current and turnoff current wll not be ncreaed too much f applyng the propoed method wth mall flm capactor. The nductor core lo manly determned by the nductor current hgh-frequency rpple whch not affected by the propoed method. In addton, the expermental reult hown n Fg valdated that the ZVS operaton can be mantaned wth the propoed d vared duty cycle control. In concluon, the propoed drect rpple current control method wll not degrade the three-phae HFL converter operaton effcency. Fg how the propoed fuel cell ytem effcency data comparon between the tradtonal method wth large electrolytc capactor C p F, C 3.8mF. and the propoed method wth mall flm capactor C p F, C 8F. A hown, the ytem effcency wth propoed method very cloe to the one wth tradtonal method. The peak effcency for tradtonal method and propoed method 94.% and 93.7%, repectvely. The dfference only.4%. For rated load operaton, the effcency decreaed by.9%. Th reult content wth the reult hown n Fg. 3.3 and Fg nce the propoed method ha about % larger tranformer peak current. 73

86 a b Fg Swtch S a wtchng waveform: a fxed D.5 control, b d vared duty cycle control. 74

87 Fg Power lo break down analy reult of three-phae HFL converter wth the rated output power. Fg Effcency data comparon between the tradtonal method wth large electrolytc capactor and the propoed method wth mall flm capactor. 75

88 3.4 Summary A ngle-phae fuel cell power condtonng ytem baed on CF-DHB converter that can acheve low-frequency rpple free nput current ung a control-orented power pulaton decouplng trategy ha been propoed. The operaton prncple and oft wtchng condton of CF-DHB converter wth mall capactor and vared bu voltage have been analyzed n detal. The duty cycle D.5 ha been derved to acheve mnmzed tranformer rm. current and peak current wth a lmted vared d n fuel cell applcaton. The mplementaton of propoed pulaton power decouplng control manly acheved by the CF-DHB converter phae hft control. Epecally, a PR controller developed to acheve an extra hgh control gan at Hz reonant frequency to elmnate fuel cell double-frequency rpple current. The controller degn gudelne ha been derved baed on the developed converter mallgnal model. The performance of the propoed technology verfed by the kw fuel cell ytem expermental reult. However, the CF-DHB converter baed fuel cell ytem lmtaton are that the plt-capactor have to take full load current. Therefore, t not utable for hgher power applcaton. Moreover, the voltage wng range lmted by the ZVS operaton condton. To olve thoe ue, th the preent a new three-phae - converter baed hgher power fuel cell ytem. The propoed three-phae HFL baed fuel cell power condtonng ytem can acheve low-frequency rpple free nput current by ung a drect double-frequency rpple current control. The fuel cell ytem rpple crcut modelng preented to llutrate that the propoed method can make full utlzaton of capactve rpple energy ource. The control ytem degn baed on the mall-gnal model ha been analyzed n detal. To drectly elmnate the fuel cell current double-frequency rpple, a PR controller 76

89 developed to acheve an extra hgh control gan at Hz reonant frequency. Th controller generate the vrtual hgh mpedance that can block the rpple energy propagaton from nverter load to fuel cell tack and t alo elmnate the dturbance from vared duty cycle. The preented oft-wtchng analy how that the propoed d vared duty cycle control can mantan all wtchng devce ZVS operaton wth large bu voltage wng. The PR controller adopted for duty cycle control n order to acheve the d operaton and elmnate the nverter load current and phae hft varaton dturbance. The expermental reult valdate the propoed technology performance and the bu capactance reducton up to 94.3% compared to the tradtonal method. 77

90 CHAPTER FOUR CONCLUSIONS AND FUTURE WORK 4. Concluon In the preented work, a new tart-up cheme for the three-tage old tate tranformer wth mnmzed tranformer current repone wa propoed. The thorough analy of the DC- DC converter tranformer current durng the SST tart-up llutrated that the key oluton to uppre the tranformer current wa to mnmze the voltage dfference between the tranformer prmary and econdary de voltage durng the tranent. Therefore, ntead of enablng the DC- DC converter after the rectfer tart-up, the propoed cheme ynchronze the tart-up of the rectfer and the DC-DC converter n a controlled manner, whch dynamcally balance the tranformer prmary and econdary voltage. The theoretcal analy wa verfed by the expermental reult and t can be concluded that the propoed method can acheve the tranformer current mnmzaton and the nput nruh current elmnaton control of the DC-DC converter at no extra cot. A ngle-phae fuel cell power condtonng ytem baed on CF-DHB converter that can acheve low-frequency rpple free nput current ung a control-orented power pulaton decouplng trategy ha been propoed. The operaton prncple and oft wtchng condton of CF-DHB converter wth mall capactor and vared bu voltage have been analyzed n detal. The duty cycle D.5 ha been derved to acheve mnmzed tranformer rm. current and peak current wth a lmted vared d n fuel cell applcaton. The mplementaton of propoed pulaton power decouplng control manly acheved by the CF-DHB converter phae hft control. Epecally, a PR controller developed to acheve an extra hgh control gan at Hz 78

91 reonant frequency to elmnate fuel cell double-frequency rpple current. The controller degn gudelne ha been derved baed on the developed converter mall-gnal model. The performance of the propoed technology verfed by the kw fuel cell ytem expermental reult. However, the CF-DHB converter baed fuel cell ytem lmtaton are that the pltcapactor have to take full load current. Therefore, t not utable for hgher power applcaton. Moreover, the voltage wng range lmted by the ZVS operaton condton. To olve thoe ue, th the preent a new three-phae - converter baed hgher power fuel cell ytem. The propoed three-phae HFL baed fuel cell power condtonng ytem can acheve low-frequency rpple free nput current by ung a drect double-frequency rpple current control. The fuel cell ytem rpple crcut modelng preented to llutrate that the propoed method can make full utlzaton of capactve rpple energy ource. The control ytem degn baed on the mall-gnal model ha been analyzed n detal. To drectly elmnate the fuel cell current double-frequency rpple, a PR controller developed to acheve an extra hgh control gan at Hz reonant frequency. Th controller generate the vrtual hgh mpedance that can block the rpple energy propagaton from nverter load to fuel cell tack and t alo elmnate the dturbance from vared duty cycle. The preented oft-wtchng analy how that the propoed d vared duty cycle control can mantan all wtchng devce ZVS operaton wth large bu voltage wng. The PR controller adopted for duty cycle control n order to acheve the d operaton and elmnate the nverter load current and phae hft varaton dturbance. The expermental reult valdate the propoed technology performance and the bu capactance reducton up to 94.3% compared to the tradtonal method. 79

92 4. Future Work Future reearch on three-tage SST oft tart-up can be focued on applyng the propoed method to cacaded SST topology. A preented n the the, the propoed tart-up cheme can be appled to the mult-tage topology that ha the olated DC-DC converter wth hgh frequency tranformer. However, when applyng to the cacaded SST topology, the tart-up cheme need to be modfed wth repect to the voltage and power balance control for the cacaded modular tructure. For the fuel cell power condtonng ytem, future reearch can be focued on the threephae - converter baed ytem hardware optmzaton. Wth the propoed technology, mall flm capactor can be appled. Therefore, a hgher power denty orented degn wth ntegrated tranformer and nductor can be reearched. The fuel cell eentally a low dynamc ource. If the load requre a pulaton power, t better to have a hybrd energy torage ource to provde uch hgh ntantaneou power. The uper capactor or battery can be therefore combned wth fuel cell to acheve an optmzed power oluton. An advanced power crcut and control degn can be reearched to enable the hybrd energy torage power upply. 8

93 8 APPENDIX A CF-DHB CONVERTER SMALL-SIGNAL MODEL o n o rm r o peak r n o o DHB o L V K V L P defnton p u π πω ω π.5 6 :.. _ A- Chooe 34,, v v fc a tate varable, v n,, a control nput and fc v, a controlled output. v v v +, v v v v +. The lnearzed mall-gnal model can be gven a follow: [ ] [ ] [ ] [ ] Φ Φ + Φ Φ Φ Φ T fc fc T fc n t n fc p fc v v v v v v C con V con V L v v con con C L v v π π π π A- + Cx y Bu Ax x, Φ Φ Φ Φ con con C L A p π π, A-3

94 8 Φ Φ t n C con V con V L B 34 π π ] [, ] [, C y C v y fc A-4

95 APPENDIX B THREE-PHASE HFL CONVERTER SMALL-SIGNAL MODEL N pv d B- N V Where N p and N are the tranformer prmary-de and econdary-de turn rato, repectvely. V d and V are the LVS and HSV -bu voltage, repectvely. The ZVS condton at D.5 for phae A wtche can be expreed a follow: d S S S S a a r r : : : : 8π + 9π L L d > 47πϕ π 3ϕ 8π + 9π L L d< 4 πϕ+ π 3ϕ π d > π 3ϕ π d > π 3ϕ B- P o dvdϕ4π 3ϕ B-3 6πωL p.u. bae power: Vd Po _ bae B-4 ωl Where P o the three-phae HFL converter output power. the phae hft angle. ω the wtchng angular frequency. L the leakage nductor. Chooe a, vd, v a tate varable, v n, D, ϕ, a control nput and a, v a controlled output. The average model tate equaton can be gven a follow: 83

96 d a d p d n a L V dt dv C L V D dt dv C Dv v dt d L πω ϕ π ϕ πω ϕ π ϕ B-5 G D G G v G Bu A I C v vo vd v n vv d ϕ B con con D L C con con L C V con V L C G p p d p v π π π B con con D L C con con L C DV L I con con G p p d a vd + + π B con con D L C con con L C C C con L G p p p vo + + π π B-9 Fg. B.. Bode dagram of compenated G v, G vo and G vd wth C F.