Line-Interactive Transformerless Uninterruptible Power Supply (UPS)

19 Pages·2017·6.92 MB·English

energies
Article
Line-Interactive Transformerless Uninterruptible
Power Supply (UPS) with a Fuel Cell as the
Primary Source
MuhammadIftikhar1,MuhammadAamir2,*,AsadWaqar2 ID,Naila2,FahadBinMuslim3
andImtiazAlam2 ID
1 DepartmentofElectricalEngineering,UniversityofEngineeringandTechnology,25000Peshawar,Pakistan;
[email protected]
2 DepartmentofElectricalEngineering,BahriaUniversity,44000Islamabad,Pakistan;
[email protected](A.W.);[email protected](N.);
[email protected](I.A.)
3 DepartmentofElectricalEngineering,IqraUniversity,44000Islamabad,Pakistan;[email protected]
* Correspondence:[email protected];Tel.:[phone]
Received:8December2017;Accepted:7February2018;Published:2March2018
Abstract: Thispaperpresentsline-interactivetransformerlessUninterruptiblePowerSupply(UPS)
with a fuel cell as the prime energy source. The proposed UPS consists of three major parts
(i.e., an output inverter, a unidirectional DC–DC converter, and a battery charger/discharger).
Non-isolatedtopologiesofboththeunidirectionalconverterandbatterycharger/dischargerensure
transformerlessoperationoftheUPSsystem. Anewtopologyofhighgainconverterisemployedfor
boostingthelowvoltageofthefuelcelltoahigherDClinkvoltage,withminimumsemiconductor
count,andhighefficiency.Ahigh-gainbatterycharger/dischargerrealizesthebidirectionaloperation
between the DC link and the battery bank. Besides, it regulates the DC link voltage during the
cold start of fuel cells and keeps the battery bank voltage to only 24 V. A new inverter control
schemeisintroducedthatregulatestheoutputvoltageandminimizesthetotalharmonicdistortion
for non-linear loading condition. The proposed control scheme integrates proportional-resonant
controlwithslidemodecontrol,whichimprovesthecontroller’sperformanceintransientconditions.
TheproposedUPSsystemisvalidatedbydevelopinga1-kVAexperimentalprototype.
Keywords:fuelcell;line-interactive;uninterruptiblepowersupply;transformerless;DC–DCconverter
1. Introduction
Uninterruptiblepowersupplies(UPS)areextensivelydeliveringbackuppowertosensitiveloads
(e.g., dataservers, medicalequipment, communicationdevices, etc.)[1,2]. NormallyUPSsystems
provide high efficiency and high reliability, with an additional important feature of fast response
duringchangeofmodes[3,4].
Generally, a lead acid or Ni–Cad battery is used to provide backup power in a UPS system.
However,thebattery-basedUPShasdisadvantagesofshortlifespan,lowreliability,highmaintenance
costofthebatteries[5],etc. Ontheotherhand,fuelcellshavealongerbackuptime,higherpower
density,lowermaintenancecost,andlongerlifetime,andhenceareanexcellentalternativetolead
acidbatteries[6–9].
Line-interactiveUPSisasuitablechoiceforhybridenergystoragesystem. Inline-interactive
UPS, the main grid and local load are connected in parallel with the UPS system. They are more
reliable,withhighefficiency,simplestructure,andaremuchcheaperascomparedtotheonlineUPS.
However,owingtotheslowerfuelcellresponse,ahybridstoragesystemisintroducedtoensurethe
Energies 2018,11,542;doi:10.3390/en11030542 www.mdpi.com/journal/energies
Energies 2018,11,542 2of19
appropriateUPSoperationunderdifferentloadingconditions. DifferenthybridenergystorageUPS
systemsarepresentedin[10–16]. Mostofthesesystemsemployhigh-frequencyisolatedconvertersfor
theparalleloperationofdifferentenergystoragesources. However,anextranumberofactiveswitches
andgalvanicisolationaffecttheoverallefficiencyofthesystemaswellasitsreliability. Ontheother
hand,atransformerlesssystemcanreplaceisolatedsystemsbyprovidinghigherefficiency,higher
powerdensity,lowercost,andsmallervolumeandweight.
Fuel-cell-basedtransformerlessUPSsystemshavenotbeenextensivelyinvestigatedyet. Inthis
paper,atransformerless,line-interactivehybridpowersourceUPSsystemhasbeenproposedwitha
fuelcellasthemajorsourceofpower. Ahigh-gainbidirectionalboostconverterhasbeenemployed
betweenthebatterybankandtheDClink. Atthesametime, itworksasaunidirectionalDC–DC
converter(boostmodeonly),steppingupthelowerfuelcellvoltagetoamuchhigherDClinkvoltage.
Sincethebatteryprovidespowerforaveryshorttime,i.e.,coldstartoffuelcellsandtransients,asmall
battery bank is more appropriate for this purpose. A high-gain bidirectional charger/discharger
ensurestheuseofalowbatterybankofmerely24V(singlebattery),whicheliminatestheproblems
associatedwiththecombinationofseries-connectedbatteries. SincetheconnectedloadintheUPS
system is non-linear, a novel robust inverter control scheme has been introduced by integrating
proportional-resonant(PR)withslidemodeandcontrol. Thecontrollerreducesthetotalharmonics
distortion(THD)foranon-linearloadandregulatestheoutputinbothimpulsiveandstepchangein
load. A1kVAexperimentalprototypeoftheUPSsystemhasbeendevelopedinthelaboratory,which
validatesitsperformanceindynamicandsteadystatecondition. Theproposedsystemoffersseveral
benefitssuchas:
1. Ahigh-gainDC–DCconverterisused,whichbooststhelowvoltageofthefuelcelltoahigher
DClinkvoltage.
2. Anovelbatterycharging/dischargingcircuithasbeenintroduced,whichcausesasignificant
reductioninthebatterybanksize.
3. Arobustinvertercontrolschemehasbeenproposedforbothimpulseandnonlinearloads.
4. Theproposedsystemhashigherefficiencyandlowercostascomparedtoitscontemporaries.
The remainder of the paper is organized in the following way. In Section 2, the proposed
line-interactiveUPSsystemisexplained,includingtheconvertertopology. InSection3,anexplanation
of the control of different subparts of the UPS system is presented. Experimental results from a
1-kWlaboratoryprototypearepresentedinSection4duringloadvariations. Finally,conclusionsare
presentedinSection5.
2. CircuitDescription
The proposed system consists of a unidirectional DC–DC converter connected to a fuel cell,
H-bridgeinverteratthebackend,andhigh-gainboostconverterconnectedwiththebatterybankof
theUPSsystem.Figure1showsaschematicoftheproposedfuel-cell-basedline-interactiveUPSsystem.
TheUPSsystemiscomposedofaunidirectionalconverter,abidirectionalbatterycharging/discharging
unit,andanH-bridgeinverter. Theprimarysourceofenergyisafuelcell,whilethebatterybankis
utilizedasthetransientenergystorage. Theunidirectionalconverteractsasaninterfacebetweenthe
low-voltagefuelcellandthehigh-voltageDClinkoftheinverter. Similarly,abidirectionalconverter
isemployedasabatterycharger/discharger,withhighvoltageconversionratio,whichshrinksthe
batterybankconsiderably. AnewcontrolschemehasbeenproposedfortheH-bridgeinverterthat
accomplishesoutputregulationforbothlinearandnon-linearloadingconditions.
Energies 2018,11,542 3of19
Energies 2018, 11, x FOR PEER REVIEW 3 of 19
Energies 2018, 11, x FOR PEER REVIEW 3 of 19

FiguFrigeu1r.e P1r. oPproopsoesdedci cricrucuititd diaiaggrraamm ooff lliinnee--ininteterarcatcivtiev teratrnasnfosrfmoremrleesrsl eUsPsSU sPySstesmys. tem.

2.1. M2.1o.d MesoodfesO opf eOFraipgteuiroarneti o1n. P roposed circuit diagram of line-interactive transformerless UPS system.
The proposed UPS system has two distinct operational modes, the grid mode and the fuel
2T.h1.e Mpordoesp oofs OedperUatPioSn system has two distinct operational modes, the grid mode and the fuel
cell/battery-powered mode, as depicted in Figure 2.
cell/battery-poweredmode,asdepictedinFigure2.
The proposed UPS system has two distinct operational modes, the grid mode and the fuel
Grid mode:
Gridcmello/bdaet:tery-powered mode, as depicted in Figure 2.
In case of a stable grid voltage and no power failure, the grid mode of the UPS is active. The grid
Incaseofastablegridvoltageandnopowerfailure,thegridmodeoftheUPSisactive. Thegrid
dGirreidct lmy oddriev: es the load. Moreover, the fuel cell feeds the unidirectional converter for regulating the DC
directlydrivestheload. Moreover,thefuelcellfeedstheunidirectionalconverterforregulatingthe
link aInnd c achsea rogfi na gs ttahbel eb agtrtiedr yv.o Tlthaeg eb iadnirde cntoio pnoawl ceorn fvaeilruterer ,c thhaer ggerisd a mndo ddeis cohf athrgee Us PthSe i bs aatctteirvye .b Tahnek .g rid
DClinkandchargingthebattery. Thebidirectionalconverterchargesanddischargesthebatterybank.
Fduireelc ctleyl ld/briavtetes rtyh-ep loowade.r Medo mreoovdeer: , the fuel cell feeds the unidirectional converter for regulating the DC
Fuellcinekll /aIbnna dtt hcteeh rasyrcg-epinnoagwr tiohe ero ebfd apttmoewroyed. rTe bh:ree baikddiroewctnio onra la cnoyn vveorlttearg ceh saargge as ta tnhde diinspchuat,r gthese tmhea bgantetetircy cboanntka.c tor
(FIMnueCthl) ceoespllce/ebnnsa,at trceiarouyso-pifnogpw otehwreee dgr rmbidroe tdaoek :dd ioscwonnnoercta fnryomvo tlhtea gloeasda.g Naotwth, ethien pfuuetl, ctehlel amndag tnhee tbicatctoernyt abcatnokr (MC)
openssu,pcpalIuyns pitnhoewg setchre etnoag trrhiioed lootfoa dpd.o iHwsceeorre nb tnrheeeac ektndferorowgmyn stothor erainlnogya ddveo.vlNtiacego,ew i .s,ea.t,gh t heaetf ubthaeetlt ecinreypll ubaat,nn tdkh,e tp hmleayabsga nttwetteoirc iy mcobpnaotnratkcatnostru pply
power(oMrletCos). tFohiperesltnolsya,, dctah.ueH sbienargtet ettrhhieee sg ehrniadne rdtogle yd tishstceoo srnhinnoegrctt d tfiemrovemi cl eot,haide. e lm.o,aitsdhm.e Nabtocahwt tt,e otr hyoev befuarcenolk mc,eepl ll taahnye dss lttowhweo bdiaymtnteparmoyr ibctasa nnotkf roles.
Firstltsyhu,ept fhpuleeylb pcaeotlwtl.e eSrrie etcoso nthhdaelny ld,o talhede.t bhHaeetrtseehr tyoh rbet aetnnimke roegvyleo rsactoodrmimnegsi s dtmheeva iitnccesh,t ait.neot.,ao tnvheeeo rbucaso tpmteorewy tebhra efnlsukl,co tpuwlaaytdisoy ntnwsa obm eicmiacpusosoerf ttahtnhete fuel
ProFlCes s. tFacirksst lmy,a tyh eta bkaet steormiees thimaned ilne otrhdee srh too ratt ttaimine t hloea rde qmuiisrmeda tocuht ptou to vvoelrtcaogme ea st hthee s hloywdr dogyenna mcaincsn ootf
cell. Secondly, the battery bank overcomes the instantaneous power fluctuations because the PFC
bthee f efude fla cset lel.n Soeucgohn.d ly, the battery bank overcomes the instantaneous power fluctuations because the
stacksmaytakesometimeinordertoattaintherequiredoutputvoltageasthehydrogencannotbe
PFC stacks may take some time in order to attain the required output voltage as the hydrogen cannot
fedfastenough.
be fed fast enough.

Figure 2. Operational modes of the line-interactive UPS system.

Figure 2. Op erational modes of the line-interactive UPS system.
Figure2.Operationalmodesoftheline-interactiveUPSsystem.

2.2. ModelofFuelCell

A1kW30Vpolymer-electrolytemembranefuelcell(PEMFC)modelbasedon[17]isusedinthe
prop osedsystem. ThetotalvoltageacrosstheFCisthesumoftheactivationovervoltage,Nernst’s
Energies 2018, 11, x FOR PEER REVIEW 4 of 19
Energies 2018,11,542 4of19
2.2. Model of Fuel Cell
voltage,cAon 1c eknWtr 3a0t iVon poovlyemrveor-letlaegcter,oalyntde mohemmbicraonvee fruveol lctaelgl e(.PETMheFCN)e mrnosdte’sl ibnassteadn otann [e1o7]u iss vuosletda gine V
cell
the proposed system. The total voltage across the FC is the sum of the activation overvoltage,
andtheirreversiblevoltageV givetheoutputvoltagefromthePEMFC,asgiveninEquation(1):
irrev
Nernst’s voltage, concentration overvoltage, and ohmic overvoltage. The Nernst’s instantaneous
voltage Vcell and the irreversible voltagVe Virre=v gVive th−e oVutput voltage from the PEMFC, as given in (1)
out cell irrev
Equation (1):
(cid:32) (cid:33)
RV.T =V pH−VO (1)
V =Vo − (cid:2925)(cid:2931)(cid:2930)ln (cid:2913)(cid:2915)(cid:2922)(cid:2922) 2 (cid:2919)(cid:2928)(cid:2928)(cid:2915)(cid:2932) −k .(T−T ) (2)
cell cell 2R.F.T pp0H.5.pO e ref
V(cid:2913)(cid:2915)(cid:2922)(cid:2922)=V(cid:2913)(cid:2925)(cid:2915)(cid:2922)(cid:2922)−2.Fln(cid:4678)p(cid:2868)O.(cid:2873)2.(cid:2870)pH(cid:4679)2 −k(cid:2915).(T−T(cid:2928)(cid:2915)(cid:2916)) (2)
Virrev =Vact+(cid:2899)(cid:3118)Vco(cid:2892)n(cid:3118)+Vohm, (3)
V =V +V +V , (3)
where Vo is the open circuit voltage ((cid:2919)(cid:2928)O(cid:2928)(cid:2915)C(cid:2932)V) a(cid:2911)(cid:2913)t(cid:2930)stan(cid:2913)(cid:2925)d(cid:2924)ard(cid:2925)t(cid:2918)e(cid:2923)mperature and pressure, R is the ideal
cell
gas cwonhesrtae nVt(cid:2913)(cid:2925),(cid:2915)(cid:2922)T(cid:2922) iiss tthhee ospteanc kcirtceumit pveorlatatguer e(O(kC)V,)F ati sstFanadraadrda yte’smCpeornatsutaren atn(dC /prmesoslu)r,ek, eR iiss athfeu indcetailo n of
gas constant, T is the stack temperature (k), F is Faraday’s Constant (C/mol), k is a function of the
the entropy change, pH O, p , and p is the partial pressure of water,(cid:2915)oxygen, and hydrogen
2 O2 H2
respeecntitvroeplyy. Scihmanilgaer,l ypVH(cid:2870)O,, VpO2, aanndd pVH2 is atrhee thpearaticatli vparteiossnu,rceo nocf ewntartaetri,o nox,yagnedn,o hamndi chvyodltraoggeend rops
act conc ohm
oftherefsupeelctcievlell.y. Similarly Vact, Vconc and Vohm are the activation, concentration, and ohmic voltage drops
of the fuel cell.
The equivalent electric model of PEMFC is shown in Figure 3, where C is the equivalent
The equivalent electric model of PEMFC is shown in Figure 3, where C is the equivalent
capacitancebecauseofthedouble-layerchargingeffect,Vcrepresentscapacitorvoltage,R indicates
capacitance because of the double-layer charging effect, Vc represents capacitor voltagacet, Ract
theactivationresistance,andR istheconcentrationresistance.
indicates the activation resistcaonncce, and Rconc is the concentration resistance.

Figure 3. Equivalent electric model of fuel cell.
Figure3.Equivalentelectricmodeloffuelcell.
The hydrogen partial pressure can be computed as
Thehydrogenpartialpressurecanbecomputedas
(cid:2914)(cid:2926)(cid:2914)(cid:2892)(cid:2930)(cid:3118)=(cid:2902)(cid:2906)(cid:3140)(cid:3159)(cid:3118)(cid:3172).(cid:2904)(q(cid:2892)(cid:2870)(cid:2879)(cid:2919)(cid:2924)−q(cid:2892)(cid:2870)(cid:2879)(cid:2928)(cid:2915)(cid:2911)(cid:2913)−q(cid:2892)(cid:2870)(cid:2879)(cid:2925)(cid:2931)(cid:2930)), (4)
where R(cid:2892)(cid:3118) represents gdaspd Hcto2ns=tanRtV Hf2o.rT h(cid:0)yqdHr2o−gienn−, Tq His2 −trheeac t−emqpHe2r−atouurte(cid:1) ,and Van is the anode’s (4)
volume. Similarly q and q a nare input flow and reacted flow of the hydrogen gas in the
(cid:2892)(cid:2870)(cid:2879)(cid:2919)(cid:2924) (cid:2892)(cid:2870)(cid:2879)(cid:2925)(cid:2931)(cid:2930)
anode, respectively.
whereR representsgasconstantforhydrogen,TisthetemperatureandV istheanode’svolume.
H2 an
Similarly qH2−in and qH2−out are input floqw(cid:2892)(cid:2870)(cid:2879)a(cid:2928)n(cid:2915)(cid:2911)d(cid:2913)=re(cid:2898)a(cid:2870).c(cid:2893).(cid:3138)(cid:2890)t(cid:3135)e, d flow of the hydrogen gas in th(5e) anode,
respectively.
where N is the total number of cells required to attain the required output voltage from the fuel cell,
N.I
and the fuel cell supplied current is repreqsHe2n−terdea cby= IFC2. .FFC, (5)
The hydrogen output flow can be computed using the expression
whereNisthetotalnumberofcellsrequiredtoattaintherequiredoutputvoltagefromthefuelcell,
q =k .q , (6)
andthefuelcellsuppliedcurrentisrepres(cid:2892)e(cid:2870)n(cid:2879)(cid:2925)te(cid:2931)(cid:2930)dby(cid:2925)I(cid:2931)F(cid:2930)(cid:2879)C(cid:2892).(cid:2870) (cid:2892)(cid:2870)
Twhheerhey kdrogen coaunt pbue tcfloomwpuctaend buesicnogm thpeu treedlautiosinnsghitph ebeetxwpereens stihoen molar flow and its partial
(cid:2925)(cid:2931)(cid:2930)(cid:2879)(cid:2892)(cid:2870)
pressure of the hydrogen gas inside the channel as represented as follows:
qkH(cid:2925)2(cid:2931)−(cid:2930)(cid:2879)o(cid:2892)u(cid:2870)t(cid:3493)=M(cid:2923)k(cid:2925)o(cid:2922)u(cid:2911)(cid:2928)t−=H(cid:2900)(cid:2927)2(cid:3171)(cid:3171).q(cid:3173)(cid:3173)H(cid:3170)(cid:3170)(cid:3159)(cid:3159)(cid:3176)(cid:3176)2., (7) (6)
wherekouSti−mHi2lacrlayn, tbhee cooxmygpeunt peadrtuiasli npgretshsuerree lisa tgiiovnesnh aips betweenthemolarflowanditspartialpressure
ofthehydrogengasinsidethechannelasrepresentedasfollows:

(cid:112) q
kout−H2 Mmolar = molar. (7)
P
molar
Energies 2018,11,542 5of19
Similarly,theoxygenpartialpressureisgivenas
Energies 2018, 11, x FOR PEER REdVpIEW = RO2.T(cid:0)q −q −q (cid:1) 5 of 19 (8)
O2 V O2−in O2−rec O2−out
cat
R .T
dp(cid:2899)(cid:2870)= V(cid:2899)(cid:2870)q (q(cid:2899)(cid:2870)(cid:2879)(cid:2919)(cid:2924)=−qN(cid:2899).(cid:2870)I(cid:2879)F(cid:2928)C(cid:2915)(cid:2913)−q(cid:2899)(cid:2870)(cid:2879)(cid:2925)(cid:2931)(cid:2930)) (8) (9)
(cid:2913)(cid:2911)(cid:2930)O2−reac 4.F
N.I
qO2−qo(cid:2899)u(cid:2870)t(cid:2879)=(cid:2928)(cid:2915)(cid:2911)(cid:2913)ko=ut_4O.2F(cid:2890).(cid:2887)p O2, (9) (10)
whereVcatisthevolumeofcathode,RO2qis(cid:2899)(cid:2870)t(cid:2879)h(cid:2925)e(cid:2931)(cid:2930)o=xykg(cid:2925)e(cid:2931)(cid:2930)n_(cid:2899)g(cid:2870).aps(cid:2899)c(cid:2870)o, nstant,andqO2−in,qO2−out,an(1d0q) O2−rec
aretheinput,theoutput,andthereactedoxygeninthecathode,respectively. TheH-1000HorizonFC
where Vcat is the volume of cathode, RO2 is the oxygen gas constant, and q(cid:2899)(cid:2870)(cid:2879)(cid:2919)(cid:2924), q(cid:2899)(cid:2870)(cid:2879)(cid:2925)(cid:2931)(cid:2930), and q(cid:2899)(cid:2870)(cid:2879)(cid:2928)(cid:2915)(cid:2913)
datahavebeenusedintheproposedmodel,whichispresentedinTable1.
are the input, the output, and the reacted oxygen in the cathode, respectively. The H-1000 Horizon
FC data have been used in the proposed model, which is presented in Table 1.
Table1.PEMFCspecifications.
Table 1. PEMFC specifications.
Parameters Value
Parameters Value
RatedPower 1000W
Rated Power 1000 W
RatedPerformance 28.8V@35A
Rated Performance 28.8 V @ 35 A
No.ofcells 48
No. of cells 48
Max.StackTemp 338K
OxMygaexn. SPtraecsks uTreemp 0.23638B aKr
HydOroxgyegnenP rPersessusruere 0.450–.02.65 5BaBra r
Hydrogen Pressure 0.45–0.55 Bar
2.3. BatteryModel
2.3. Battery Model:
Figure 4 shows the equivalent electric circuit of the lead acid battery, which represents the
Figure 4 shows the equivalent electric circuit of the lead acid battery, which represents the
Theveninbatterymodel[18]. Equation(11)representsthebatteryterminalvoltage:
Thevenin battery model [18]. Equation (11) represents the battery terminal voltage:
(cid:0) (cid:1)
VBVa(cid:2886)t(cid:2911)(cid:2930)==NNs(cid:2929)(EEO(cid:2899)−−II(cid:2886)B(cid:2911)a(cid:2930)tRR(cid:2919)i−−VV(cid:2913)(cid:2926)cp), , (11) (11)
wherwehEeOre =EO I=d eIdaelavl ovlotlataggee ooff tthhee bbaatttetre, r,Ri R=i I=nteIrnntael rrneaslistraenscies,t aCnPc e= ,PColPari=zatPioonla craizpaatciiotonr, cRaPp =a citor,
RP=PPoollaarrizizaatitoion nrerseissitsatnacne,c eV,CVP =C PPo=laPriozlaatrioizna vtioolntagveo, lNtaSg =e ,NNuSm=beNr oufm ceblelsr coofncneelcltsedc oinn nseerciteesd, NinP =s eries,
NP=NNuummbbere rofo cfecllesl lcsonconnecnteedct iend pianrapllaerla, lIl(cid:2886)e(cid:2911)l(cid:2930),=IBa(cid:2893)(cid:3144)t(cid:2898)(cid:3173)(cid:3174)(cid:3159)=(cid:3162) =ILN Bopaadtt=erBy actutrerreyntc u rrent.
Allthemodelcomponentsarefunctionsofthebatterystateofcharge(SoC).Modelparameters
All the model components are functions of the battery state of charge (SoC). Model parameters
correcsoprorensdpionngdtinogt htoe tbhaet btearttyerayr eargei gvievnenin inT Taabblele2 2..

Figure 4. Thevenin’s model of a lead acid battery.
Figure4.Thevenin’smodelofaleadacidbattery.
Table 2. Battery specifications.
Table2.Batteryspecifications.
Parameters Value
PRaraatmede tCearspacity V2a1l uAeh
Nominal Voltage 24 V
RatedCapacity 21Ah
Min. Voltage 16 V
NominalVoltage 24V
Max. Charging Current limit 9.9 A
Min.Voltage 16V
Max.MChaaxr. gDinisgchCaurrgree nCtulirmreintt 91.095A A
Max.DiscIhnairtgiael CSouCrr ent 10750%A
InInteitrinaallS RoCesistance 780 m%Ω
InternalResistance 8mΩ
2.4. Bidirectional DC–DC Converter
Energies 2018,11,542 6of19
2.4. BidirectionalDC–DCConverter
Anovelnon-isolatedDC–DCconverter(basedonacoupledinductor)withbidirectionaloperation
hasbeenproposed,whichisseparatelyconnectedtothefuelcellalongwiththebatterybank. Firstly,
itperformstheunidirectionaloperationandbooststhelow-levelfuelcellvoltagetoahigh-levelDClink
voltage(boostmodeofoperation). Secondly,itactsasabatterycharging/dischargingunitoperating
betweenthebatterybankandtheDClink(bothbuckandboostmodesofoperation). Theconverter
offersthefollowingbenefits:
1. Highvoltagediversityinbothmodesofoperation,i.e.,buckandboostmodes.
2. Reducedpassivecomponentsareemployedintheproposedconverter.
3. Bidirectionaloperationisperformedusingonlythreeactiveswitches.
4. ZeroVoltageSwitching(ZVS),voltageclampingcircuit,andsynchronousrectificationareusedto
minimizetheconductionandswitchinglosses.
Intheproposedbidirectionalconverter,thecoupledinductorisusedwithprimaryandsecondary
windings,representedbyL andL ,respectively. TheboostpumpcapacitorC provideshighvoltage
P S b2
diversity, with continuous current flow and reduced current stress in the primary winding of the
coupledinductor. ThevoltagestressofcapacitorC isverylowatthispointinthecircuit.
b2
ThereareseverevoltageandcurrentrippleswithtwicetheoutputACfrequencyintheDCbus.
Inthiscase,thecurrentrippleswillreactwiththeoutputterminaloftheFCsthroughtheDC/DC
convertersothatthepowersourceswillsustainextraloads. Thelow-frequencycurrentrippleand
transientloadwillcauseadecreaseinFCefficiency[19,20].However,apassivecompensationapproach
hasbeenusedinwhichthecapacitorparalleltotheFCandbatterybankareselectedwithslightly
largervalues,whichlimitsthecurrentandvoltageripplestolessthan10%.
2.4.1. BatteryCharging/BuckOperation
Figure5showsthecharacteristicwaveformsduringbuckmode. D isthedutycycleofS and
1 3
S ,whileD isthedutycycleofswitchS . D andD arerelatedtoeachotherbytherelationship
ax 3 4 1 3
D (=1−D ). L representsthecoupledinductormagnetizinginductancehavingaturnsratioof
1 3 m
N=N /N ,whereN indicatesthenumberofturnsinprimarywindingandN indicatesthenumber
2 1 1 2
ofturnsforsecondarywinding. V isthevoltageacrossDClink,V isthebatteryvoltage,V and
d Bat LP
V aretheprimaryandsecondarywindingvoltages, andV isthevoltageacrosscapacitorC .
LS cb2 b2
Figure6showstheoperationoftopologyinintervalsthroughoutthebatterychargingmode.
Interval1(t ~t ): TheswitchS remainsONwhiletheswitchesS &S areturnedOFFduringthis
0 1 4 3 ax
interval. Thedirectionofcurrenti isfromtheDClinktowardsthebatterybankviathecapacitorC
LS b2
andthecoupledinductor’swindings. EmployingKVL,wegetEquation(12):
V =V +V +V +V (12)
d LS Cb2 LP Bat
V =V (1+N)+V +V . (13)
d LP Cb2 Bat
Thecontinuousinductorcurrenti flowsthroughthediodeD towardsthebatterybank. V
Lb b3 Bat
representsthevoltageacrosstheinductorL .
b
Interval 2 (t ~t ): When interval 2 starts, switch S turns OFF and the polarities of primary and
1 2 4
secondarycoils(L &L )ofcoupledinductorarereversedbecauseoftheenergystoredintheleakage
P S
inductor. InthismodeofoperationSwitchS isOFF,butthesecondarycurrenti isstillflowing,
4 LS
hence the body diode of the switch Sax is forward-biased to ensure that the current i keeps on
LS
flowing. ThismodekeepsthediodeD inforwardbias. SwitchS bodydiodeisforward-biasedwith
b3 3
thereductionofthesecondarycurrenti ;however,theprimarycurrenti remainsunchanged.
LS LP
Energies 2018,11,542 7of19
Interval 3 (t ~t ): Both S and S turn ON following zero voltage switching (ZVS) in this mode.
2 3 3 ax
ThecapacitorC beginstodischargeacrossthebatterybankthroughtheswitchS andtheinductor
b2 ax
L . ThedischargingcapacitorC causesthesecondarycurrenti toinduceinthereversedirection.
b b2 LS
The body diode D allows clamp capacitor C to discharge and adds small current i into the
b2 b1 3
secondarycurrentthatfurtherflowstowardsthebattery.
Byemployingthevoltagesecondbalance,V canbedeterminedasfollows:
Cb2
V =V +V +V . (14)
Cb2 Lb Bat LS
Theprimarycurrentreleasesthestoredenergyincoupledinductorintothebatterybankthrough
Energies 2018, 11, x FOR PEER REVIEW 7 of 19
switchS . Utilizingthevoltage-secondbalance,theV isgivenby
3 Lb
V(cid:2887)D(cid:2912)(cid:2870)1V=LbV(cid:2896)=(cid:2912)+DV3V(cid:2886)(cid:2911)B(cid:2930)a+t.V(cid:2896)(cid:2903). (14) (15)
TheVTLhPe cparnimbaeryd ecuterrrmenitn reedleaassefso ltlhoew sst:ored energy in coupled inductor into the battery bank
through switch S3. Utilizing the voltage-second balance, the VLb is given by
D V =D V . (16)
3D LVP =D1VBat. (15)
(cid:2869) (cid:2896)(cid:2912) (cid:2871) (cid:2886)(cid:2911)(cid:2930)
PuttinTgheE VqLuPa tciaonn b(e1 5d)etaenrmditnheed vaas lfuoellsowofs:V LbandVLPintoEquation(13),thevoltagegainduring
thebuckoperationmodeisgivenbytheequation
D V =D V . (16)
(cid:2871) (cid:2896)(cid:2900) (cid:2869) (cid:2886)(cid:2911)(cid:2930)
Putting Equation (G1b5u) cakn=d tVhBe avt/alVudes= of[ DV3L(b 1a−ndD V3)LP] /in[2tNo (E1q−uaDti3o)n2 (+131)], .the voltage gain during (17)
the buck operation mode is given by the equation
Interval4(t ~t ): BothS andS turnOFFatthebeginningofthismode. Thecurrentscorresponding
3 4 3 G ax=V ⁄V =[D (1−D )]⁄[2N(1−D )(cid:2870)+1]. (17)
(cid:2912)(cid:2931)(cid:2913)(cid:2921) (cid:2886)(cid:2911)(cid:2930) (cid:2914) (cid:2871) (cid:2871) (cid:2871)
totheprimaryandsecondarywindingsi andi continuetoflowowingtothecoupledinductor
LP LS
leakagInetienrvdaulc 4t a(tn3~cte4.):T Bhoeths Se3c oanndd aSraxy tucrunr rOeFnFt caht tahreg ebsegtihnenipnagr aofs itthicis cmapodace.i tTahnec ecucrorerrnetss pcoornrdesipnogntdoinSg and
3
to the primary and secondary windings iLP and iLS continue to flow owing to the coupled inductor
S ,anddischargestheparasiticcapacitancecorrespondingtoS . WhenthevoltageacrossS equals
ax leakage inductance. The secondary current charges the parasitic 4capacitance corresponding to S3a axnd
V ,theS bodydiodeturnsON.Asthismodeends,theprimarycurrenti goesondecreasinguntilit
d Sax, 4and discharges the parasitic capacitance corresponding to S4. When thLeP voltage across Sax equals
becomesequaltothesecondarycurrenti .
Vd, the S4 body diode turns ON. As thisL mS ode ends, the primary current iLP goes on decreasing until
Intervita ble5co(mt e~st e)q:uSal ttou trhnes seOcoNnddauryri cnugrrtehnits iLiSn. terval under ZVS conditions. The capacitor C gets
4 5 4 b1
chargIendtebrvyalt h5e (tc4~lat5m): pSe4 dturdniso dOeND durianng dthtihs einpterrivmala uryndaenr dZVseSc coonnddaitriyoncs.u Trrheen ctsapbaecigtoinr Ctob1 ignectsr ease.
b1
charged by the clamped diode Db1 and the primary and secondary currents begin to increase. The
Thecircuitrepeatsfrominterval1attheendofthisinterval.
circuit repeats from interval 1 at the end of this interval.

Figure 5. Characteristic waveforms of a bidirectional DC–DC converter.
Figure5.CharacteristicwaveformsofabidirectionalDC–DCconverter.
Energies 2018,11,542 8of19
Energies 2018, 11, x FOR PEER REVIEW 8 of 19
Energies 2018, 11, x FOR PEER REVIEW 8 of 19

Figure 6. Topological stages of battery charger/discharger during buck mode of operation: (a) Mode
Figure6.Topologicalstagesofbatterycharger/dischargerduringbuckmodeofoperation:(a)Mode1;
Figure 6. Topological stages of battery charger/discharger during buck mode of operation: (a) Mode
1; (b) Mode 2; (c) Mode 3; (d) Mode 4; (e) Mode 5.
(b)Mode2;(c)Mode3;(d)Mode4;(e)Mode5.
1; (b) Mode 2; (c) Mode 3; (d) Mode 4; (e) Mode 5.
2.4.2. Battery Discharging/Boost Operation
2.4.2. BatteryDischarging/BoostOperation
2.4.2. Battery Discharging/Boost Operation
Figure 7 shows the bidirectional converter operational waveform in the battery discharging
Figure 7 shows the bidirectional converter operational waveform in the battery discharging
Figure 7 shows the bidirectional converter operational waveform in the battery discharging
mode of operation. The circuits for both battery discharging and the fuel cell operation are identical.
modeofoperation. Thecircuitsforbothbatterydischargingandthefuelcelloperationareidentical.
mode of operation. The circuits for both battery discharging and the fuel cell operation are identical.
The switch Sax remains OFF during boost operation. The bidirectional converter boosts the low level
TheswitchS remainsOFFduringboostoperation. Thebidirectionalconverterbooststhelowlevel
Tbhaet tsewryit cbha nSkaax x vroemltaagine st oO aF Fh idguhreirn lge vbeolo DstC o plinerka tvioolnta. gTeh. eS bimidiilraerclyti,o tnhael lcoown vveorlttearg eb ooof sftus etlh cee llol wis sletevpe l
batterybankvoltagetoahigherlevelDClinkvoltage. Similarly,thelowvoltageoffuelcellisstepup
buatpte troy hbiagnhk DvCol tlaingke tvoo lat ahgieg hbeyr ulenviedli rDecCti olinnakl voopletaragtei.o Sni mofi ltahrely c, othnve elrotwer .v Tohltea gbea totfe rfyu edl icsechlla irsg sintegp
tohighDClinkvoltagebyunidirectionaloperationoftheconverter. Thebatterydischargingoperation
uopp teor ahtiiognh dDuCri nlign vka vrioolutasg ien tberyv aulns iidsi dreecptiicotneadl ionp Feirgautrioe n8 . of the converter. The battery discharging
duringvariousintervalsisdepictedinFigure8.
operation during various intervals is depicted in Figure 8.

Figure 7. Characteristic waveforms during boost mode.
FFiigguurree 77.. CChhaarraacctteerriissttiicc wwaavveeffoorrmmss dduurriinngg bboooosstt mmooddee..
Energies 2018,11,542 9of19
Energies 2018, 11, x FOR PEER REVIEW 9 of 19

Figure 8. topological stages during boost mode: (a) Mode 1; (b) Mode 2; (c) Mode 3; (d) Mode 4; (e)
Figure8. Topologicalstagesduringboostmode: (a)Mode1; (b)Mode2; (c)Mode3; (d)Mode4;
Mode 5; (f) Mode 6.
(e)Mode5;(f)Mode6.
Interval 1 (t0~t1): In interval 1, S3 is ON, and S4 remains OFF. Fuel cell/battery bank voltage is
pInrotevrivdaeld1 to(t th~et s)i:dIen oifn ttheer vbaildi1r,ecStioinsaOl Nco,navnedrteSr wreitmh aloinws vOoFlFta.gFeu. eBlecfoelrle/ ibnattetrevrayl b1a, nthke vcoalptaagcietoirs
0 1 3 4
Cprb2o vreidmeadintso cthhaersgiedde aonfdth tehbe icdoiurepclteiodn ianldcuocntovre rmteargwneitthizlionwg cvuorlrteangte i.LBM eifnocrreeainsetesr ivna al 1li,ntheaerc mapaancniteorr,
aCs derpemictaeidns inch Fairgguerde a7n. dthecoupledinductormagnetizingcurrenti increasesinalinearmanner,
b2 LM
asdeBpyic utetidliizninFgi gKuVreL7, .we can determine
ByutilizingKVL,wecandetermineV =V =V ⁄N. (18)
(cid:2886)(cid:2911)(cid:2930) (cid:2896)(cid:2926) (cid:2896)(cid:2903)
Using voltage second balance, thVe vo=ltaVge a=croVss /pNri.mary winding VLP can be derived( 1a8s)
Bat Lp LS
follows:
Usingvoltagesecondbalance,thevoltaVgeDacr=osVsprDim.a rywindingVLPcanbederivedasfol(l1o9w) s:
(cid:2896)(cid:2900) (cid:2871) (cid:2886)(cid:2911)(cid:2930) (cid:2869)
Interval 2 (t1~t2): In interval 2, S3 turns OVFF. DS3 p=arVasitiDc c.apacitance is charged by primary curr(e1n9t)
LP 3 Bat 1
iLP, while S4 parasitic capacitance is discharged by secondary current iLS. This interval ends when the
voltage across S3 and the capacitor voltage VCb1 become equal.
Interval2(t ~t ): Ininterval2,S turnsOFF.S parasiticcapacitanceischargedbyprimarycurrent
1 2 3 3
Interval 3 (t2~t3): S3 is turned OFF in this interval. The primary current iLP decreases, while the
i ,whileS parasiticcapacitanceisdischargedbysecondarycurrenti . Thisintervalendswhenthe
LP 4 LS
secondary current iLS is enhanced due to the coupled inductor leakage inductance. Consequently, the
voltageacrossS andthecapacitorvoltageV becomeequal.
3 Cb1
S4 body diode turns ON. Since the S3 voltage becomes higher than that across the capacitor Cb1, the
cIanptearcvitaolr3 C(bt12 ~get3t)s: cSh3arigsetdu rvniead dOioFdFe Dinb1t.h Tisheinreteforvrea,l .thTeh veolptarigme asrtyrecsus rarcernotssiL tPhed escwreitacshe sis, wkehpitl elotwhe.
Tsehceo cnadpaarcyitcour rvroenlttagiLeS VisC1e insh gainvecend byd uetothecoupledinductorleakageinductance. Consequently,
theS bodydiodeturnsON.SincetheS voltagebecomeshigherthanthatacrossthecapacitorC ,
4 3 V =V +V . (20b) 1
thecapacitorC getschargedviadiodeD(cid:2887)(cid:2869). The(cid:2886)r(cid:2911)e(cid:2930)fore(cid:2896),(cid:2900)thevoltagestressacrosstheswitchiskept
b1 b1
low.UThsiencga Epqacuiatotirovno (l1t8a)g,e V isgivenby
C1
V = V ⁄D . (21)
V (cid:2887)(cid:2869)=V (cid:2886)(cid:2911)+(cid:2930) V(cid:2871) . (20)
C1 Bat LP
Interval 4 (t3~t4): In this interval, S4 turns ON under the condition of zero voltage switching (ZVS).
Both Uthsein cgouEpquleadt ioinnd(u1c8t)o,r windings LP and LS, and the boost pump capacitor Cb2 are in series and
provide energy to the DC link. The iLS beginVsC t1o= inVcrBeaat/seD u3.ntil it approaches iLP, after which it follo(w21s)
the iLP till the interval 4 ends. Hence, the energy stored in both the coupled inductor windings
Interval4(t ~t ): Inthisinterval,S turnsONundertheconditionofzerovoltageswitching(ZVS).
discharges a3cro4ss the DC link. Both4 Db1 and Db2 are reverse-biased in this interval, as depicted in
BoththecoupledinductorwindingsL andL ,andtheboostpumpcapacitorC areinseriesand
Figure 8d. Applying voltage second balPance, wSe obtain Equation (22): b2
provideenergytotheDClink. Thei beginstoincreaseuntilitapproachesi ,afterwhichitfollows
LS LP
the i till the interval 4 ends. Hen cVe(cid:2914),=thVe(cid:2886)(cid:2911)e(cid:2930)n+erVg(cid:2896)y(cid:2903)s+toVr(cid:2887)e(cid:2870)d+inV(cid:2896)b(cid:2926)o th the coupled inductor wind(2i2n)g s
LP
V =V +V +(N+1)V . (23)
(cid:2914) (cid:2886)(cid:2911)(cid:2930) (cid:2887)(cid:2870) (cid:2896)(cid:2900)
Interval 5 (t4~t5): S4 turns OFF during this interval. The current iLS charges the S4 parasitic
capacitance. Cb1 begins to discharge across Cb2 via the diode Db2.
Energies 2018,11,542 10of19
dischargesacrosstheDClink. BothDb1andDb2arereverse-biasedinthisinterval,asdepictedin
Figure8d. Applyingvoltagesecondbalance,weobtainEquation(22):
V =V +V +V +V (22)
d Bat LS C2 Lp
V =V +V +(N+1)V . (23)
d Bat C2 LP
Interval5(t ~t ):S turnsOFFduringthisinterval.Thecurrenti chargestheS parasiticcapacitance.
4 5 4 LS 4
C beginstodischargeacrossC viathediodeD .
b1 b2 b2
V =V =V /D (24)
Cb2 Cb1 Bat 3
ByputtingEquations(19)and(24)intoEquation(23),thevoltagegainofthecircuitis
V =V +V /D +(N+1)D /D V (25)
d Bat Bat 3 1 3 Bat
G =V /V = (2+ND )/(1−D ). (26)
boost d Bat 1 1
TheS bodydiodeisforward-biasedduetothepolaritiesofC andinductorL .
3 b2 P
Interval6(t ~t ): Ininterval6,S turnsONfollowingZVSconditions. Hencetheswitchinglossesare
5 6 3
reducedandtheefficiencyofthesystemisconsiderablyimproved. Thenextswitchingcyclestarts
whenthevoltageacrosscapacitorsC andC becomesequivalent.
b1 b2
AturnratioofN=2satisfiestheoperationofunidirectionalconverterwhenthefuelcellsare
connected,andN=4forbidirectionalconversionbetweenthedesiredDClinkandthebatterybank.
Table 3 shows a comparison of proposed bidirectional converter with other state-of-the-art works.
Theproposedconvertershowsahighconversionratioascomparedto[20,21]. Also,thenumberof
switchesisreducedascomparedtootherconverters. In[21]theauthorshaveshownhighvoltage
diversity;however,fiveswitchesareusedinthecircuit,whichincreasesthesizeandcostofthesystem.
Table3.Comparisonofproposedbidirectionalconverter.
Features [19] [22] [21] [20] ProposedTopology
Switches 4 5 4 4 3
AuxiliaryCapacitors 2 3 2 2 2
Coupled-Inductor 1 1 1 0 1
AuxiliaryInductor 1 0 0 1 1
MBOOST N 1+N +N 2+N 2 2+ND
1−D (1−D) D 1−D 1−D
MBUCK D D D D D(1−D)
N 1+N+DN N+2 2 2N(1−D)2+1
Efficiency 97% 96% 95% 94% 96%
Size Large Large Medium Medium Small
EstimatedCost(USD) ~130 ~172 ~118 ~136 ~116
3. ControlStrategy
Thecontrolschemefordifferentpartsoftheline-interactiveUPSsystemisshowninFigure9.
The inverter operates only during the fuel cell/battery-powered mode. However, the battery
charger/dischargerswitchesdependonthemodechange. Ingridmode,thebidirectionalconverter
follows the battery discharger control, while in fuel cell/battery-powered mode it follows battery
charging control, as shown in Figure 9. Similarly, thecontrolschemefortheunidirectionalDC–DC
converterregulatestheinverterDClinkvoltageduringthefuelcell/battery-poweredmodeofoperation.