Analysis and Design of Bi-Directional DC-DC Converter in the Extended Run Time DC UPS System Based on Fuel Cell and Supercapacitor

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1 Downloaded from orbt.dtu.dk on: Oct 8, 08 Analyss and Desgn of B-Drectonal DC-DC Converter n the Extended Run Tme DC UPS System Based on Fuel Cell and Suercaactor Zhang, Zhe; Thomsen, Ole Cornelus; Andersen, Mchael A. E.; Schmdt, Jacob D.; Nelsen, Hennng R. Publshed n: Twenty-Fourth Annual EEE Aled Power Electroncs Conference and Exoston, 009. APEC 009 Lnk to artcle, DO: 0.09/APEC Publcaton date: 009 Document Verson Publsher's PDF, also known as Verson of record Lnk back to DTU Orbt Ctaton (APA): Zhang, Z., Thomsen, O. C., Andersen, M. A. E., Schmdt, J. D., & Nelsen, H. R. (009). Analyss and Desgn of B-Drectonal DC-DC Converter n the Extended Run Tme DC UPS System Based on Fuel Cell and Suercaactor. n Twenty-Fourth Annual EEE Aled Power Electroncs Conference and Exoston, 009. APEC 009 ( ). EEE. DO: 0.09/APEC General rghts Coyrght and moral rghts for the ublcatons made accessble n the ublc ortal are retaned by the authors and/or other coyrght owners and t s a condton of accessng ublcatons that users recognse and abde by the legal requrements assocated wth these rghts. Users may download and rnt one coy of any ublcaton from the ublc ortal for the urose of rvate study or research. You may not further dstrbute the materal or use t for any roft-makng actvty or commercal gan You may freely dstrbute the URL dentfyng the ublcaton n the ublc ortal f you beleve that ths document breaches coyrght lease contact us rovdng detals, and we wll remove access to the work mmedately and nvestgate your clam.

2 Analyss and Desgn of B-drectonal DC-DC Converter n Extended Run Tme DC UPS System Based on Fuel Cell and Suercaactor Zhe Zhang, Ole C. Thomsen, Mchael A. E. Andersen, Jacob D. Schmdt and Hennng R. Nelsen. Deartment of Electrcal Engneerng, Techncal Unversty of Denmark Kgs. Lyngby 800, Denmark. APC Denmark AS., Slcon Alle, Koldng, Denmark, DK-6000 Abstract-n ths aer, an extended run tme DC UPS system structure wth fuel cell and suercaactor s nvestgated. A wde nut range b-drectonal dc-dc converter s descrbed along wth the hase-shft modulaton scheme and hase-shft wth duty cycle control, n dfferent modes. The delvered ower and eak current are analyzed and calculated. The key arameters of the b-drectonal dc-dc converter, the relatonshs between the nut voltage, hase-shft angle, rato of the transformer and leakage nductance are analyzed and otmzed. Buld the system mathematc model and a novel nut voltage combned wth load current feedback usng P controller wth ant-wndu scheme to realze closed-loo control of the whole system, and verfy the feasblty of the control scheme roosed by smulaton. A kw rototye controlled by TMS30F808 DSP s mlemented and tested. Exermental results show the valdty of desgn. Fuel Cell DC-DC Converter Grd Suer Ca Varable Low Voltage Bus AC-DC-PFC Converter B-DC-DC Converter Constant Hgh Voltage Bus Fgure. Block dagram of the whole system structure. DC load or nverter. NTRODUCTON Nowadays, wth the growth of electrc demand and the regulaton of energy suly, the occurrence of outage s ncreasng n many countres. Extended run tme UPS (Unnterrutble Power Suly) systems could be used to overcome these roblems []. There are several ways of rovdng the extended run tme, but most common solutons today are stll the use of desel generator and batteres. Comarng wth desel generators and batteres, fuel cells and suercaactors hybrd UPS system can convert the chemcal otental of the hydrogen nto electrc ower drectly, wth consequent hgh converson effcency and ossblty to obtan the extended range wth the combustble feed from the outsde []. Durng the fuel cell startng u or load requrement transent, the suercaactor can be used as the bufferng ower source. The UPS block dagram researched n ths aer s shown n Fg.. We could fnd that the bdrectonal DC-DC converter s to control the ower flow from the energy sources and load and s key stage n the system. For hgh voltage rato alcaton, transformer couled and solated b-drectonal DC converters have been roosed such as dual actve brdge (DAB) [3], [4], [5] [6], dual actve half brdge [7], [8], [9], full brdge current fed [0], [] and ther dervatves [], [3], [4]. And the ower flow could be controlled under the hase-shft modulaton. To mrove the system effcency as well as ncrease the range of oeraton for the large-range-changeable nut voltage, some SCs L f S S 4 Fgure. Voltage-fed full brdge wth voltage doubler dc-dc converter modulaton schemes such as trangular modulaton, traezodal modulaton and PWM lus hase-shft modulaton are nvestgated n [5], [6], [7] and [8], wth addng the duty rato as a modulated arameter. n ths aer, the voltage-fed full brdge wth voltage doubler b-drectonal dc-dc converter, shown n Fg., wth hase-shft modulaton and hase-shft wth duty cycle control s nvestgated. All the swtches n the converter can mlement turn-on under ZVS. Methods n choosng and desgnng crcut arameters such as leakage nductance, swtchng frequency and turn rato to mrove the effcency and to ncrease the ower range are dscussed and hybrd modulaton scheme for extended ZVS range and system startu method n secton. n secton, a novel load current feed back P controller wth ant-wndu scheme are desgned. Fnally, n secton V, exermental results verfy the effectveness of the analyss and desgn. Q :N C f A L B T S 3 s D S S D S3 D S D S4 Q C D Q C N D Q C L f C f Z /09/$ EEE 74 Authorzed lcensed use lmted to: Danmarks Teknske nformatonscenter. Downloaded on August 0,00 at 0:09:53 UTC from EEE Xlore. Restrctons aly.

3 v L - V -V L s :N n n T Phase shft V / -V / v L 0 t 0 Phase-shft angle S t t t 3 t 4 Fgure 3. Smlfed toology and theoretcal waveforms δ t t δ V o /N V Mode - M -V t V t Mode Mode Fgure 4. Theoretcal waveforms n Mode Fgure 5. Mode selecton Modes TABLE OPERATNG PRNCPLE LV sde HV sde Gated Conductng Gated Conductng t 0 <t<t S, S 4 D S, D S4 Q D Q t 0 <t<t S, S 4 S, S 4 Q Q t <t<t S, S 4 S, S 4 Q D Q t <t<t 3 S, S 3 D S, D S3 Q D Q t <t<t 3 S, S 3 S, S 3 Q Q t 3 <t<t 4 S, S 3 S, S 3 Q D Q. B-DRECTONAL CONVERTER A. Stable oeraton mode When the fuel cell has been started u, the system works n the stable oeraton mode (Mode ) under V V o /N. The gated and conductng swtches durng the varous oeratng ntervals of the b-drectonal dc-dc converter shown n Fg. are lsted n the Table. And the tycal waveforms are shown n Fg. 3. Durng the nterval t 0 <t<t and t <t<t 3 the resultant mmf exceeds the value needed to establsh the rated core flux, unlke normal transformer oeraton, and the wndng current changes radly and causes hgh RMS value. Wth reference to Fg. 3, the current values as: 4NV δ ( V NV ) π o Δ. () 4ωNL V δ (NV V ) π o o Δ. () 4ωNL Vo ( π δ ) Δ ( V ). (3) N ωl where V s voltage on the suercaactor, V o s outut voltage, ω s the swtchng angular frequency, N s the turn rato, L s the sum of the transformer leakage nductance and that of the auxlary nductor. t s evdent that when the 0, the current waveform s traezodal. Under the condton that the rmary mressed voltage s not equal to the secondary voltage referred to the rmary caused by the load dsturbance, the nstantaneous nductor current, nut voltage v AB and the average ower, P AVG are gven: 4 Vo ( δ, t) cosnδ V cosnωt n πωl N,3,5...,3,5... V sn nδ sn nωt. (4) o n πωln v AB ( t),3,5...,3,5... 4V nωt nπ sn. (5) 4V Vo P sn nδ. (6) AVG 3 n π ωln Equaton (4), (5) and (6) are very useful to analyze the harmoncs n the system and extend the analyss results n [8], and then we can get the equvalent equaton blow: V V δ ( π δ ) o PAVG Po o Vo. (7) π ω N L Under the constrant condton as: 8 ω N L P. (8) π V V o B. Dynamc oeraton mode Durng the fuel cell start-u erod, suercaactor tank s dscharged to suly the energy that load requred. The Vc wll decrease from V cnormal to V cstable, and V cnormal V cstable. To extend the ZVS range, n ths mode, hase-shft wth duty rato control s used (Mode ). The defnton of the duty rato and tycal oeratng waveforms are shown n Fg. 4. Because of the close-loo control, the outut voltage s assume to be constant; thus the HV sde oerates n the D0.5, square-wave mode. nut voltage vares, so we add the duty rato n the rmary sde, and calculate the duty cycle as: D V V cstable o d. (9) π V 4NV Accordng to (9), the duty rato s nversely roortonal to the voltage. f the V equals to V cstable, then D0.5 and crcut translates to Mode. Wth reference to Fg. 4, the ower flow and eak current n ths mode are calculated, resectvely, under 0.5<D<0.5, to be as: [ δ ( π δ ) ( D δ ) ( π δ D) ] VV o P. (0) 4πNωL V δ V π D( NV V ) o o o eak >. () eak 4ωNL From (0), t s shown that due to the duty rato control, under δ < πd, more ower transferred at gven hase shft and crcut arameters comarng wth that under only hase shft. On other hand, the eak current value s hgher that that n Mode, and t causes the hgh RMS value to lead bad /09/$ EEE 75 Authorzed lcensed use lmted to: Danmarks Teknske nformatonscenter. Downloaded on August 0,00 at 0:09:53 UTC from EEE Xlore. Restrctons aly.

4 v v o /4Nv T f v ref K /T - dt δ δ c - K a Mode d M or δ G (M) G t - load G c /Z v o ß Fgure 7. Block dagram of the closed-loo control scheme effcency, so the system s not sutable to work n Mode for long erod. The system can realze the seamless converson between the dfferent modes. But as mentoned above, the load dsturbance wll cause v changed and lead system to Mode where eak current s hgh. To avod the chatterng between the two modes, a mode selecton method wth the hysteress comarator s aled, shown n Fg. 5. C. Parameters From (7), the control varable, δ, s non-lnear relatonsh wth the outut ower or the outut voltage. So t s more comlcated to choose the arameters. Accordng to the aer [9], the hase shft should be restrcted to less than 0.75.u. (.u. reresents 80 degrees) to avod large reactve ower crculatng wthn ower stage whch causes large loss. That wll lmt the ower could be delvered as well as the arameters: the nductance of L, swtchng frequency and turns rato n. The ZVS regon and boundares are analyzed n [4], [6]. At the rated load, the all swtches could be oerated under the ZVS condton. But the hgh RMS value and eak currents on the rmary sde of the ower converter reresented the bg challenge and lmted the effcency n system desgn. The RMS value of the current n the swtch n rmary sde s as: δ ( 3( ) 4 ). () s 3 3π To otmzed desgn to lmt the value n (), the nductor and turns rato are carefully selected. nductor value drectly affects the ower delvery caablty and turns rato affects the haseshft angle, whch affects the reactve ower and ower losses. For gven outut voltage and ower, the Fg. 6 shows the relatonsh of the s, RMS value of the current n swtch n rmary sde, N and L. n the range of N and L beng both smaller, the s changes dramatcally. Select the arameters as: n 4.5, L 9uH at V 50V, V o 400V, P o kw. Because the V s changed durng the fuel cell stack start-u erod and nstantaneous load dsturbance, the relatonsh of nut voltage varaton and s s shown n Fg. 6. We can fnd that when oerate the converter based on the arameters we have chosen, the s s not very senstve to nut voltage varaton when the nut voltage s 50V around, but when the nut voltage s near to 00V, the s wll be bgger and we should change the oeraton mode there. s (A) N L (uh) L (0-5 H) v (V) Fgure 6. Relatonshs between the key arameters Fgure 8. Smulaton results: control wth load current feedback (red one) and wthout load current feedback (blue one). Deendng on the L, the values of C and C, and the rle voltage on the outut caactors s exressed as:. (3) 00ω L C, 4ω δmax ω max ΔU t dt c, C 0 δ. (4) 4ωC, max, δ D. Startng rocedure At the system startng stage, at that tme the V o 0, from () and (3), an nrush would flow and cause the nductor saturaton. Soft-start or re-charge has been dscussed n many aers [5]. A smle one s roosed based on the modulaton s (A) Outut voltage Voltage Load Current current Wth feedforward Wthout feedforward Tme (S) /09/$ EEE 76 Authorzed lcensed use lmted to: Danmarks Teknske nformatonscenter. Downloaded on August 0,00 at 0:09:53 UTC from EEE Xlore. Restrctons aly.

5 scheme used n ths aer. n (), when hase-shft angle equals to zero, the LV sde swtches are under duty cycle control and HV sde swtches are under synchronous rectfyng oeraton, so at the frst ulse of V wth the duty cycle of d, the eak value of, f, s exressed as: V π f d. (5) ω L t s evdent that we can lmt the nrush current through suresson of the duty cycle.. SYSTEM CONTROL A. Closed loo controller desgn Equaton (7) and () ndcate that V o s not roortonal to the hase-shft angle drectly, but under δ < u. t s lnear control regon of the hase-shft for the converter, aroxmately. Snce the voltage on the suercaactor vares very slow comarng wth the swtchng frequency, so t a very slow dsturbance to the whole system and under the closed-loo oeraton, n the Mode, the d s nversely roortonal to the suercaactor voltage, so d can also be vewed as a very slow dsturbance. The system s control dagram s shown n Fg. 7. A load current redctve PD control wth ant-wndu rotecton s used n the system. n the scheme, the v and o are measured. t s necessary to measure v to realze the mode selecton and avod over-voltage and under-voltage. For the load sensor, t s useful to montor over-current and outut ower, to lmt the ower from fuel cells. As o load, so the load s consdered as the frst erturbaton value of the system to the outut of the P controller through the T f, and v, the voltage on the suercaactor stacks, s consdered as a very slow dsturbance to the system, where T f contans the equaton (7) but n ths aer the functon obtaned through the measured results on the rototye because of loss n the system. The ant-wndu rotecton s helful to revent the saturaton of the ntegrator to mrove the stablty of the system, where K a should be n the same order wth K /τ. Added the small-sgnal dsturbance n (9), the small-sgnal transfer functons are: ˆ V ( π δ ) o G ( ) ˆ δ 4πωNL G t std Rc G c sr C c where T d Tsamle Tm T, defned n [4], and T r samle s the samlng tme of the rocessor system. The tme delay T m s the delay tme caused by the modulator and equal toπ / ω, and T contans the remanng delay tmes. We can get the R r c by aralleled dschargng resstor n C and the resstor of vrtual load. Wth the same method, G () s derved from (0), by addng the small-sgnal dsturbance wth on-lne calculated duty cycle. The smulaton results based on Matlab are shown n Fg. 8. The smulaton arameters are: V 50V, V o 400V, and the outut ower set u from kw to.5kw at 0.5 second wth the 3 3 K rad / V, K 5 0 rad / V. Addng the load current feedback control wll mrove the system dynamc erformance. V. PROTOTYPE AND EXPERMENT n order to verfy the feasblty and erformance of the converter, a laboratory rototye s bult and tested. Crcut arameters and comonents used of the converter are lsted n Table. n Fg. 9 and Fg. 0, the waveforms comarson shows the valdty of the arameters otmzaton desgn. Under the rated nut and outut voltages, the nductor current s flat after choosng reasonable nductance and transformer turn rato. n Fg. and Fg., the gate drve sgnals and the corresondng dran-source voltage waveforms show that the all swtches n rmary sde or secondary sde can realze ZVS on under rated ower. Waveforms n Mode, are shown n Fg. 3, under V o 400V, δ8.8 o and d0.43. n ths mode, v AB shows the waveform wth three voltage-levels. Under the lght-load condton, the ZVS on s hard to be realzed for the swtches, esecally for the hgh voltage sde swtches. We could fnd that n Fg. 4, when Q turn on, the secondary sde current s about zero, so the Q s under hardswtchng. TABLE PROTOTYPE PARAMETERS AND COMPONENTS Swtches of the rmary sde STW34NB0 (00V/34A) Swtches of secondary sde RFP450LC (500V/4A) Outut DC caactor (C, C ) 470uF/350V Transformer core materal Transformer turns 6:6 Transformer leakage nductance Auxlary nductor Auxlary transformer core materal Snubber flm caactor Swtchng frequency Dgtal controller Ferrte PC40 uh 9uH Ferrte N7 nf 0kHz T TMS30F808 DSP Fgure 9. Waveforms before otmzaton desgn. v L /09/$ EEE 77 Authorzed lcensed use lmted to: Danmarks Teknske nformatonscenter. Downloaded on August 0,00 at 0:09:53 UTC from EEE Xlore. Restrctons aly.

6 v AB v GSQ v L v CN v DSQ Fgure 0. Waveforms after otmzaton desgn. Fgure 5. Waveforms of the Q under ZVS n Mode. v GSS v DSS Fgure. Waveforms of S under ZVS v GSQ Fgure 6. The system dynamc resonses under load sudden change: from 0.A to.8a; from.8a to 0.A v DSQ 00% 90% 80% 70% effcency % 60% 50% 40% 30% Fgure. Waveforms of Q under ZVS. 0% 0% v AB 0% 30W 480W 640W 800W 900W 000W outut ow er (W) Fgure 7. The effcency curve. v CN Fgure 3. Waveforms of converter oeratng n Mode. v GSQ v DSQ Then add the duty control, at same outut ower, t can realze ZVS agan, shown n Fg. 5. n Fg. 6, the dynamc resonse shows the effect of the closed-loo control scheme s recevable. The maxmum effcency of the converter shown n Fg. 7 s about 9%. At dle mode, the dle loss s smaller than 5W. Because of the lmtaton of hase-shft angle by ractcal crcuts, under the rated nut and outut voltages, converter can not oerate wthout load. Under rated outut ower condton, the loss manly causes by the hgh RMS current value n the rmary sde, so the aralleled Mosfet to decrease the on-resstance of the swtch should be used to mrove the effcency. V. CONCLUSON Fgure 4. Waveforms of the Q under hard-swtchng n Mode. Ths aer has addressed the b-drectonal solated DC/DC converter to be used n the fuel cell and suercaactor hybrd /09/$ EEE 78 Authorzed lcensed use lmted to: Danmarks Teknske nformatonscenter. Downloaded on August 0,00 at 0:09:53 UTC from EEE Xlore. Restrctons aly.

7 system. Wth the analyss of the characterstcs of the converter, mode converson scheme should be used n the system to extend the ZVS range and mrove the system effcency n terms of the dfferent nut voltage levels. And the hase-shft wth duty control can also to be used to lmt the nrush current durng the start-u stage. Based on the smulaton and exermental results, the closed-loo control scheme can realze the converter outut stable and dynamc resonse well, but the arameters n controller should be analyzed and desgned n deth to mrove the system erformance and stablty n the future. n general, the results from the rototye n lab controlled by DSP clarfy the valdty of the analyss and desgn. REFERENCES [] Jose M. Guerrero, Unnterrutble ower suly systems rovde rotecton, n: EEE ndustral Electroncs Magazne, 007, (), [] Yu X., Starke, M.R., Tolbert, L.M., Oznec B., Fuel cell ower condtonng for electrc ower alcatons: a summary, ET Electrc Power Alcatons, vol., , 007. [3] R. W. De Doncker, D. M. Dvan, and M. H. Kheraluwala, A three- hase soft-swtched hgh-ower desty dc/dc converter for hgh ower alcatons, EEE Transactons on ndustry Alcaton, vol. 7, no.,.63-67, 99. [4] M. H. Kheraluwala, R. W. Gascogne, D. M. Dvan and E. D. Baumann, Performance characterzaton of a hgh-ower dual actve brdge dc-dc converter, EEE Transactons on ndustry Alcaton, vol. 8, no.6, , Nov./Dec. 99. [5] H. L. Chan, K. W. E. Cheng and D. Sutanto, Phase-shft controlled DC- DC converter wth b-drectonal ower flow, EE Proc.-Electr. Power Al., vol. 48, no., Mar. 00, [6] S. noue and H. Akag, A Bdrectonal DC DC Converter for an Energy Storage System Wth Galvanc solaton, EEE Transactons on Power Electroncs, vol., no. 6, , 007. [7] F. Z. Peng, H. L, G. J. Su, and J. S. Lawler, A new ZVS b-drectonal dc-dc converter for fuel cell and battery alcatons, EEE Transactons on Power Electroncs, vol. 9, no., , 004. [8] H. L and F. Z. Peng, Modelng of a new zvs b-drectonal dc-dc converter, EEE Transactons on Aerosace and Electronc Systems, vol. 40,. 7 83, January 004. [9] D. Lu and H. L, Desgn and mlementaton of a DSP based dgtal controller for a dual half brdge solated b-drectonal dc-dc converter, Twenty-Frst Annual EEE Aled Power Electroncs Conference and Exoston, APEC006,. [0] K. Wang, C. Y. Ln, L. Zhu, D. Qu, F. C. Lee, and J. S. La, Bdrectonal dc-dc converters for fuel cell systems, EEE Transactons On Transortaton,. 47 5, Oct [] L. Zhu, A novel soft-commutatng solated boost full-brdge ZVS- PWM dc-dc converter for bdrectonal hgh ower alcaton, EEE Transacton on Power Electroncs, vol., no.,. 4-49, Mar [] J. Wnag, F. Z. Peng, J. Anderson, A. Joseh and R. Buffenbarger, Low cost fuel cell converter system for resdental ower generaton, EEE Transactons on Power Electroncs, vol. 9, no. 5,. 35-3, 004. [3] H. Tao, Kotsooulos A., Duarte J.L., Hendrx M.A.M., Transformercouled multort ZVS bdrectonal DC DC converter wth wde nut range, EEE Transactons on Power Electroncs, vol. 3, no., , 008. [4] H. Xao and S. Xe, A Zvs bdrectonal dc-dc converter wth hasedshft lus PWM control scheme, EEE Transactons on Power Electroncs, vol. 3, no., , Mar [5] N. Schbl, Symmetrcal multlevel converters wth two quadrant DC- DC feedng. EPFL, PhD Thess, no.0, 000. [6] F. Krsmer, S. Round, and J. W. Kolar, Performance otmzaton of a hgh current dual actve brdge wth a wde oeratng voltage range, The 37th EEE Power Electroncs Secalsts Conference, PESC006,. 7, June 006. [7] D. Xu, C. Zhao and H. Fan, A PWM lus hase-shft control bdrectonal DC-DC converter EEE Transactons on Power Electroncs, vol. 9, no. 3, , Mar [8] G. D. Demetrades, On small-sgnal analyss and control of the sngle and the dual actve brdge toologes, KTH, Doctoral thess, 005. [9] M. N. Gtaua, G. Ebersohna and J. G. Kettleboroughb, Power rocessor for nterfacng battery storage system to 75v dc bus, Energy Converson and Management, vol. 48, , Mar /09/$ EEE 79 Authorzed lcensed use lmted to: Danmarks Teknske nformatonscenter. Downloaded on August 0,00 at 0:09:53 UTC from EEE Xlore. Restrctons aly.