Control strategy of fuel cell supercapacitors hybr

Journal of Power Sources158(2006)

806–814

Control strategy of fuel cell/supercapacitors hybrid power sources for

electric vehicle

Phatiphat Thounthong∗,St´e phane Ra¨e l,Bernard Davat

Institut National Polytechnique de Lorraine(INPL),GREEN,CNRS(UMR7037)2,Avenue de la Forˆe t de Haye,54516Vandœuvre-l`e s-Nancy,France

Received15May2005;accepted6September2005

Available online22November2005

Abstract

This paper presents a control principle for utilizing PEM fuel cell as main power source and supercapacitors as auxiliary power source for electric vehicle applications.The strategy is based on dc link voltage regulation,and fuel cell is simply operating in almost steady state conditions in order to minimize the mechanical stresses of fuel cell and to ensure a good synchronization between fuelflow and fuel cell current.Supercapacitors are functioning during transient energy delivery or transient energy recovery.To authenticate control algorithms,the system structure is realized by analogical current loops and digital voltage loops(dSPACE).The experimental results with a500W PEM fuel cell point out the fuel cell starvation problem when operating with dynamic load,and also confirm that the supercapacitor can improve system performance for hybrid power sources.©2005Elsevier B.V.All rights reserved.

Keywords:Electric vehicle;Hybrid electrical system;Polymer electrolyte membrane fuel cell;Power electronics;Supercapacitor

1.Introduction

In this day and age,fuel cell(FC)power generation sys-tems are expected to be used in more and more applications. For portable power,a fuel cell coupled with a fuel container can offer a higher energy storage density and more convenience than conventional battery systems[1].In transportation applica-tions,fuel cells offer higher efficiency than conventional engines coupled with electrical generators,but the main task is still to develop a cost-effective,reliable and safe method of storing suf-ficient hydrogen on-board the vehicle[2].In stationary power applications,low emissions permit fuel cells to be located in high power density areas where they can supplement the exist-ing electrical network.Furthermore,fuel cell systems can be directly connected to a building to provide both power and heat with higher overall efficiencies[3].

There are several types of fuel cells,which are characterized by the employed electrolyte.The chemical makeup of each type determines its operating characteristics.Scientists are develop-ing many different types of fuel cells employing different fuels

∗Corresponding author.Tel.:+33383595654;fax:+33383595653.

E-mail address:Phatiphat.Thounthong@ensem.inpl-nancy.fr

(P.Thounthong).and electrolytes.One of the most promising for electric vehicles is the lightweight,relatively easy to build and small polymer electrolyte membrane fuel cell(PEMFC)[4].

Different previous works have already pointed out the pos-sibility to use fuel cell in distributed power generation systems. Chandler et al.[5]reported experimental results carried out on a Fuel Cell Transit Bus,known as hybrid fuel cell/battery bus prepared for the U.S.Department of Energy(DOE).This bus has a rated power of60kW,and its energy source is composed of a UTC PEM fuel cell(60kW,160–250V)as main power source,and of48Panasonic lead-acid12-V batteries associated in series as auxiliary source.In addition,Rodatz et al.[6]pre-sented thefield tests,in urban and highway sections,of a Fuel Cell/Supercapacitor-Powered Hybrid Vehicle with a PEM fuel cell(40kW,150A),a supercapacitor module(5.67F,250A, 360V),and an ac motor(45kW).This work also explained one of the key week points of fuel cell,which is dynamic limitation. In fact,fuel cell voltage is highest when no current isflowing, and drops with increasing current because of activation over-voltage and ohmic resistance losses in the membrane.At high currents,the voltage drops sharply as the transport of reactant gases is not able to follow the amount used in the reaction. Consequently,reactant starvation occurs at the reaction side and the cell fails.Since a current is drawn through the stack,the failed cell may start to operate as an electrolytic cell and cause

0378-7753/$–see front matter©2005Elsevier B.V.All rights reserved. doi:10.1016/j.jpowsour.2005.09.014P.Thounthong et al./Journal of Power Sources158(2006)806–814807

irreversible damage.Therefore,the current that can be supplied by the fuel cell needs to be limited.Moreover,a lag between fuel cell load current and response of the reactant supply system results in an undersupply of reactants to the fuel cell.This leads to a breakdown of the chemical reaction and to a rapid loss in voltage.This phenomenon may be avoided by restricting the dynamics of the load.Therefore,in P.Rodatz’s tests,the dynamic of the fuel cell system was limited to a conservative2.5kW s−1.

Pukrushpan et al.[7,8],furthermore,attempted to improve fuel cell dynamics by controlling fuel cell processor in order to avoid fuel starvation,when current is rapidly drawn from fuel cell.In addition,Schenck et al.[9],working with a Bal-lard Nexa TM PEM fuel cell(1.2kW),and Thounthong et al.

[10],working with a ZSW PEM fuel cell(0.5kW),evidently demonstrated the fuel cell starvation phenomenon when fuel cell worked with step electrical loads.They demonstrated that its dynamics was limited by the hydrogen and oxygen deliv-ery system,which contains pumps,valves,and in some cases, a hydrogen reformer.They recommended to utilize fuel cell,as main power source,with at least a fast auxiliary power source to improve the system performances.Additionally,fuel cell has slow dynamics by nature.If it is operated in nearly steady state condition in order to circumvent fast change of fuel cell current, mechanical stresses are avoided,and lifetime of fuel cell stack will increase[11].

Besides,in electrical network of electric vehicle,the dc bus voltage control has problem when electrical loads demand or recover high energy in short time(for example,while wheel motors start or brake).Consequently,in order to answer these problems,the system must have a rapid supplementary power source to deliver or to absorb high transient energy.Choi et al.

[12]and Burke[13]reported that the innovative high current supercapacitor technology has been developed for this reason. Then,the very fast power response of supercapacitors can be used to match the slower power output of the fuel cell to pro-duce the compatibility and performance characteristics needed by distributed system.

Precisely,Rufer et al.[14]already proposed a solution by utilizing a supercapacitive-storage-based substation with60V, 200A,75F supercapacitor tank(series association of24super-capacitors1800F)for the compensation of resistive voltage drops in transportation networks.Dixon et al.[15]also pre-sented a hybrid battery/supercapacitor vehicle with a32kW wheel motor,and a supercapacitor bank(300V,200A,20F). J.W.Dixon’s work was designed and implemented to manage powerflow between a supercapacitor bank and lead-acid batter-ies.Supercapacitors are controlled for higher accelerations and decelerations of the vehicle with minimal loss of energy,and minimal degradation of the main battery pack.

This paper presents a small-scale test bench of42V,500W dc bus having a PEM fuel cell as main source and supercapacitors as auxiliary source as proposed in Fig.1[16].It especially details a new control strategy based on a sharing of energy between the two sources taking into account the low dynamics of the fuel cell.The experimental results are divided in two parts.Thefirst one shows only PEM fuel cell characteristics when connecting with converter to the dc bus,and the second one shows hybrid source characteristics for different situations while connecting to dc bus in order to confirm system operation.

2.Hybrid power sources

In general,dc bus voltage v Bus(for example,500V for Toy-ota Prius Hybrid Vehicle)is higher than fuel cell voltage v FC.As a result,fuel cell converter,which delivers a directional current, has to boost the fuel cell voltage to the dc bus level.Supercapac-itor converter controls a bidirectional current in order to store or generate energy as presented in Fig.1.The studied hybrid source comprises a small-scale42V dc bus supplied by a500W PEM fuel cell and a supercapacitive storage device.The function of the fuel cell is to supply mean power to the load,whereas the storage device is used as a power source:it supplies transient power demand,and peak loads required during acceleration and deceleration.

2.1.Fuel cell converter[10]

Fuel cell operates giving direct current at a low voltage; thereby,the boost converter,presented in Fig.2,is selected to adapt the low dc voltage delivered by the fuel cell,which is around12.5V at its rated power,to the42V dc bus.The fuel cell converter,designed for a25kHz PWM is composed of a high frequency inductor L1,a total dc bus capacitor C,a diode D1(STPS80H100TV:100V,40A)and a main switch S1.Switch S2is a shutdown device for test security to prevent the fuel cell stack from short circuits in case of accidental destruction of S1

, Fig.1.Fuel cell/supercapacitor hybrid power sources.

808P .Thounthong et al./Journal of Power Sources 158(2006)

806–814

Fig.2.Fuel cell boost converter.

or faulty operation of the regulator.S 1and S 2are STE180NE10(100V ,180A).

The fuel cell converter is driven,through MOSFET S 1gate signal,by means of a pulse width modulation (PWM)for average current control,in order to obtain constant switching frequency for fuel cell current.The used PWM generator is a high-speed PWM generator UC28025B (Texas Instruments Inc.).The spe-cial features of this generator are suitability for average current control,soft start and pulse-by-pulse current limitation.2.2.Supercapacitor converter

The supercapacitors are connected to the dc bus by means of a 2-quadrant dc/dc converter,as shown in Fig.3.L 2represents the inductor used for energy transfer and filtering.The inductor size is classically defined by switching frequency and current ripple [17].Rufer et al.[18]already presented that supercapacitor size is defined by dc bus energy requirements deduced from hybrid power profile.The supercapacitor current,which flows across the storage device,can be positive or negative,allowing energy to be transferred in both directions.Finally,the converter is driven by means of complementary pulses,generated by a

hysteresis

Fig.3.2-quadrant supercapacitor converter.

comparator,and applied on the gates of the two MOSFET S 3and S 4(in one module SKM200GB 123D:200A,1200V).2.3.Hybrid control structure

The proposed hybrid control is depicted in Fig.4.For rea-sons of safety and dynamics,the fuel cell and supercapacitor converters are primary operated by inner current loop controls which are realized by analogical circuits.Therefore,current commands of fuel cell and supercapacitors depend on condi-tions of hybrid operation.Moreover,classical PID controller is selected for “Fuel Cell Current Controller”,which generates duty ratio signal for the PWM generator.A hysteresis controller is selected for “Supercapacitor Current Controller”.

Because fuel cell is supplied with gas through pumps,valves and compressors,it has large time constants (several seconds).As a result,it cannot accurately reply to fast increasing or decreasing power loads,and may be damaged by repetitive stepped power loads.For this reason,the fuel cell in the hybrid system is only operating in nearly steady state conditions,and supercapacitors are functioning during transient energy delivery or transient energy

recovery.

Fig.4.Proposed hybrid system control structure.

The main objective of the control is to regulate dc bus voltage v Bus.Taking into account of fuel cell dynamics,hybrid control system generates two current reference signals i FCREF for fuel cell and i SuperCREF for supercapacitors with the following con-straints:

(1)Fuel cell current slope must be limited to a maximum abso-

lute value(for example,4A s−1[19])in order to guarantee matching the reactant delivery rate and the usage rate. (2)Fuel cell current must be kept within an interval[I FCRated

(rated value),I FCMin(minimum value)or zero].

(3)Supercapacitive storage device voltage must be kept within

an interval[V SuperCMin(minimum value),V SuperCMax(max-imum value)].Normally,the system attempts to reach the normal voltage V SuperCNormal.

Note that fuel cell current reference i FCREF is synchronously sent to fuel cell processor,in order to adjust hydrogen and oxygen flows to the desired electrical current.

In previous work,Rufer et al.[14]already proposed to employ supercapacitors-based substation for compensating voltage drop on a700V dc bus in which the control of supercapacitor con-verter is independent from dc main source rectifier.With a fuel cell as main source,Thounthong et al.[20]have tried in hybrid sources built with a PEM fuel cell as main source and supercapacitors as secondary source to regulate the dc bus voltage through a power control(load power=fuel cell power+supercapacitor power),with a limitation of the fuel cell current slope.One of the major problems,which appear in such a control,is the presence of mode definitions(motoring or gen-erative braking,for example)with difficulties to avoid instability when operating at the border of two different modes.

For this reason,the novel conception is that the hybrid system control proposes a regulation of the dc bus voltage through the power only delivered by the fast auxiliary source, here the supercapacitors.And supercapacitor voltage is reg-ulated to V SuperCNormal through the slow main source,the fuel cell.

More precisely,the“dc bus voltage controller”(PI controller) generates a power reference,called p SuperCREF.By power law conservation and no loss assumption,the supercapacitor current reference is a consequence of the power demand.It is calculated by dividing the power reference p SuperCREF by the measured supercapacitor voltage.Because this voltage contains harmonics due to the converter switching and the series resistance of device (ESR),the measured signal must befiltered by low passfilter with low cut-off frequency(for instance,1Hz).

Besides,the supercapacitor voltage controller(I controller) generates a fuel cell current reference i FCREF.This signal is lim-ited in level(I FCRated and I FCMin)and in slope,thefirst and second constraints.By this strategy,system can visibly guaran-tee that fuel cell current will gradually increase and decrease, and over rated or lower minimum fuel cell current will not hap-pen.Note that minimum value for fuel cell current is set in order to maintain continuous conducting mode of fuel cell current. Fuel cell current,in addition,is functioned at slow dynamics, and in this case,integral controller(I controller)is enough

for

Fig.5.Supercapacitors current limitation functions. supercapacitor voltage controller,and it also ensures no static error.

Finally,to keep the supercapacitor voltage within its defined limit[V SuperCMin,V SuperCMax],the last constraint,the superca-pacitor current has to be limited.For example,during regenera-tive breaking,fuel cell current is limited at its minimum value. As a consequence,the dc bus voltage will exceed its reference value,and hence the voltage controller will reduce power refer-ence to a negative value.It means that over energy will be stored in supercapacitors,and supercapacitor voltage will increase over V SuperCNormal.While charging,system must limit supercapacitor voltage to V SuperCMax.Hybrid system,for this reason,ensures supercapacitor voltage limitation by limiting current as proposed in Fig.5.

The minimum negative current during charging can be revealed by:

I SuperCMin=−I SuperCRated×min

1,

v SuperCMax−v SuperCMea

v

(1)

For example,with V Super C Normal(V Super CREF)=13V, V Super C Max=15V,I SuperCRated=200A,and v=0.5V(which corresponds to an average value of the voltage drop across the ESRfixed once for the considered work),if v SuperCMea=14V, one obtains I SuperCMin=−200A.In this case,where superca-pacitor voltage is far from V SuperCMax,the charging current limit I SuperCMin is equal to its negative rated value−I SuperCRated. In the case,when the supercapacitor voltage is closed to its maximum limit,the charging current limit I SuperCMin becomes greater than its negative rated value−I SuperCRated.For instance, when v SuperCMea is equal to14.8V,the charging current limit becomes−80A.

With the same principle,one defines the upper boundary of the discharging supercapacitor current as:

I SuperCMax=I SuperCRated×min

1,

v SuperCMea−v SuperCMin

v

(2)

3.System equations

3.1.Fuel cell current loop

To obtain the transfer function of the fuel cell current loop, the linearized differential equations are defined as follows

810P .Thounthong et al./Journal of Power Sources 158(2006)806–814

[21,22]:L 1d ˜i FC (t )d t

=˜v FC (t )−R L 1˜i FC (t )−(1−D )˜v Bus (t )+V Bus ˜d (t )(3)C

d˜v Bus

d t

=(1−D )˜i FC −I FC ˜d

−˜i load (4)

where D is the nominal duty ratio of the PWM fuel cell converter,˜d

is the duty ratio variation,V Bus is the nominal dc bus voltage,˜v

Bus is the dc bus voltage variation,I FC is the nominal fuel cell current,˜i FC is the fuel cell current variation,˜i load is the load current variation and R L 1is the series resistance of L 1.Note that series resistance of C is ignored.

From (3)and (4),the fuel cell current loop can be modeled by the following transfer function:˜i FC Mea (s )˜i FC REF (s )

OL

=PID Controller

G C (T Ci s +1)T Ci s (T Cd s +1)

(T cd s +1)×PWM

1V P

×

˜i FC (s )/˜d

(s )

G i (T z s +1)(s/ωn )2+(2ζ/ωn )s +1×filter

K 1

T fc s +1

(5)

where V P is the amplitude of the PWM saw tooth carrier signal,and

G i =I FC (1−D ),T z =

V Bus C (1−D )I FC ωn = (1−D )2L 1C ,ζ=R L 1C (1−D )2ωn 2⎫

⎪

⎪⎪

⎬⎪⎪⎪⎭(6)3.2.DC bus voltage loop

To obtain the transfer function of the dc bus voltage loop,one

can write power conservation equation with the assumption that system has no losses:p SuperC (t )=Cv Bus (t )

d v Bus (t )

d t

+v Bus (t )i load (t )(7)

where p SuperC is the power supplied by supercapacitors,i load is the load current,which can be considered as a disturbance of the system.

The linearized differential equation can be written as follows:˜p SuperC =CV Bus

d˜v Bus

d t

+I load ˜v

Bus +V Bus ˜i load (8)

where ˜p SuperC is the variation of power supplied by supercapac-itors,I load is the nominal load current and ˜i load is the load current variation.

From (8),the dc bus voltage loop can be expressed by the following transfer function,written in the worst case (no load,which leads to a pure integrator behavior):˜v Bus Mea (s )˜v

Bus REF (s ) OL =PI Controller

G V

(T Vi s +1)

T Vi s ·˜v Bus (s )/˜p SuperC (s )

1CV Bus s ·

filter

K 2T fv1s +1

(9)

Fig.6.Hybrid system context diagram.

3.3.Supercapacitor voltage loop

At last,to obtain the transfer function of the supercapacitor

voltage loop,one can write the relationship of the powers as:v FC (t )i FC (t )=v SuperC (t )i SuperC (t )+p load (t )

(10)

where v FC is the fuel cell voltage,v SuperC and i SuperC are the supercapacitor voltage and current and p load is the load power.While considering that v FC (t )and p load (t )are disturbance of the system,Eq.(10)can be linearized under the form:

C SuperC

d˜v SuperC d t =V FC ˜

i FC V SuperC (11)where V FC is the nominal fuel cell voltage,V SuperC is the nominal

supercapacitor voltage,and ˜v

SuperC is the supercapacitor voltage variation and C SuperC is the supercapacitor capacitance.

Therefore,the supercapacitor voltage loop can be presented by the following transfer function:˜v SuperC Mea (s )˜v

SuperC REF (s )

OL =I Controller

1T i s טv SuperC (s )/˜i FC (s )

V FC /V SuperC C SuperC s

×

filter

K 3T fv2s +1(12)

4.Hybrid system implementation

Fig.6additionally depicts the realization for hybrid system

algorithm in Matlab/Simulink environment for dSPACE inter-facing card (real time card DS1104).The hybrid system com-municates with the operator by means of ControlDesk software,and is linked with converters (acquisition of v SuperC and v Bus ,generation of current reference signals i SuperCREF and i FCREF )by Digital to Analog Conversion (DAC)and Analog to Digital Conversion (ADC)of dSPACE interfacing card.5.Experimental results and discussions

The hybrid test bench is presented in Fig.7.The consid-ered fuel cell is a 40A,500W PEMFC stack,constructed by Zentrum f¨u r Sonnenenergie und Wasserstoff-Forschung (ZSW),Ulm,Germany,installed in a test bench provided by gas sup-ply and control.Air is supplied from a compressor through a humidification unit,and pure dry hydrogen comes from bottles.

811

Fig.7.Hybrid system test bench.

A cooling system permits to impose a constant stack temperature by heating or cooling of the water circuit.A programmable con-trolling unit contains all necessary control functions as reference setting,measurement conditioning,and security shutdown.The stack contains23cells with an active surface of100cm2.The experiment is carried out under a stack temperature of55◦C,a humidifier temperature of45◦[23].Besides,its simplified dia-gram is illustrated in Fig.8.

5.1.Fuel cell converter testing with a PEM fuel cell

In a practical system,when fuel cell operates,its fuelflow is controlled by fuel cell processor,which receives current demand from current reference as shown in Fig.4.The fuelflow must be adjusted to match the reactant delivery rate to the usage rate.

Nonetheless,to present the fuel cell characteristics,this test bench is operated in two different ways for fuelflow.Firstly,fuel cell works at constant fuelflow corresponding to the maximum available current of50A(40A is the rated current).In this case, the fuel cell has always enough hydrogen and oxygen.Secondly, the fuelflow varies depending on fuel cell current reference.

Figs.9and10present the dynamic response of fuel cell cur-

rent loop to a step reference,in the case of constant fuel

flow

Fig.8.Simplified diagram of the ZSW500W PEM fuel cell

system.

Fig.9.Fuel cell current response to a10–40A step reference at constant fuel

flow(set for50A).

for Fig.9,and in the case of adjusted fuelflow for Fig.10.

One can observe on voltage curve in Fig.10,compared with

Fig.9,the effect of mechanical delay which implies that fuel

supply and delivered electrical current do not coincide.Fuel

flow has difficulties to follow the current step,and this condition

of operating is evidently hazardous for the fuel cell stack and

will essentially lead to degrade the stack lifetime.It is essen-

tial to note that Schenck et al.[9]obtained the same kind of

response,and similar time response,with a1.2kW Ballard fuel

cell.Therefore,to utilize fuel cell in dynamic application,its

current slope must be limited in order to insure a good synchro-

nization between fuelflow and fuel cell current.In addition,the

steady-state characteristics of the PEM fuel cell when connect-

ing with converter,with switching frequency of25kHz,are also

illustrated in Figs.11and12.One can observe that the PEM fuel

cell contains complicated impedance component[23].

5.2.Hybrid system test bench

Technical specifications of the test bench are as follows: L1=72␮H,L2=54␮H,C=0.702F.The supercapacitive

stor-

Fig.10.Fuel cell current response to a10–40A step reference at variable fuel

flow.

812P .Thounthong et al./Journal of Power Sources 158(2006)

806–814

Fig.11.Steady-state characteristics of the PEM fuel cell at 10

A.

Fig.12.Steady-state characteristics of the PEM fuel cell at 40A (rated).

age device is obtained by means of six SAFT supercapacitors (3500F,2.5V ,400A,ESR:0.8m )connected in series.•The fuel cell current loop parameters are D =0.7,V Bus =42V ,R L 1=10m ,V P =10V ,fuel cell current slope limitation set to 4A s −1,I FCRated =40A,I FCMin =5A,K 1=8V/40A,T fc =1ms,T Cd =1ms,T Cd =2␮s,T Ci =0.381ms and G C =0.02.

•The dc bus voltage loop parameters are K 2=1V/1V ,T fv1=20ms,T Vi =0.245s and G V =800.

•The supercapacitor voltage loop parameters are v =0.5V ,I SuperCRated =200A,V SuperCNormal =13V ,V SuperCMax =15V ,V SuperC Min =8V ,K 3=1V/1V ,T fv2=0.16s and T i =10ms.

Note that the rated power of the main source is 500W,but system has some losses.Then,dc bus can absorb about 440W from fuel cell power.This means that rated power at dc bus is 440W.From now,the fuel flow is adapted to the value of the delivered current to improve the efficiency of the system.

The experimental tests presented hereafter have been carried out by connecting to the dc bus a resistance,and an active load composed of a current reversible chopper loaded by an induc-tance in series with two batteries.The current through batteries is controlled by means of a hysteresis corrector.

Fig.13presents hybrid characteristics during supercapacitor charge from 11V to 13V .At the beginning of charge (from t =1to 10s),the fuel cell current reaches its rated value with a slope of 4A s −1.At t =60s,v SuperC is equal to 13V ,and then charging current is reduced.Because of the sloped limitation of fuel cell current,v SuperC reaches over 13V .As a result,

system

Fig.13.Charging supercapacitors from 11to 13V .

Fig.14.Hybrid system response when overloading by177%.

Fig.15.Hybrid system response when recovering.814P.Thounthong et al./Journal of Power Sources158(2006)806–814

tries to discharge supercapacitors at t=68s.At the end of charge (t=96s),fuel cell current comes back to beginning value for constant load at dc bus,and i SuperC comes back to nearly zero.

Fig.14presents transient responses of the hybrid system to an excessive load of177%(created by the active load)during20s. It shows that the supercapacitors compensate the main source during both transient state and steady state.During the transient state,beginning at t=5s,the current delivered by the fuel cell slowly increases(with a controlled slope of4A s−1)up to its rated value.During the steady state,beginning at t=12s,fuel cell current is equal to its rated value.In both cases,fuel cell power is not enough to supply the load power,and thus the dc bus voltage regulation leads supercapacitors to deliver the lacking energy.Then,the sudden decrease of the load power at t=24s gives rise to charge supercapacitors because v SuperC is lower than V SuperCNormal.Fuel cell current i FC still stays at its rated value.Clearly,control system can excellently regulate the dc bus voltage,even though the high peak load.Furthermore, the fuel cell problems of fuelflow are noticeably avoided by controlling its current slope.

As afinal test,Fig.15corresponds to a sudden recovery of energy on the dc bus from t=10s to28s.This energy is recovered by the supercapacitors,while fuel cell current slowly decreases(−4A s−1)down to its minimum value.When the recovery period isfinished at t=28s,i SuperC changes in order to discharge the supercapacitors,because v SuperC is over than V SuperCNormal.Therefore,fuel cell current still keeps its min-imum value until v SuperC is equal to V SuperCNormal.After that fuel cell current rises back to its initial value,to supply constant power load(about240W)at dc bus.

6.Conclusion

This work presents a new method of regulating dc bus for electric vehicle applications supplied by hybrid sources using a PEM fuel cell,as main power source,and supercapacitors as auxiliary power source.The essential constrain is to avoid rapid transition of fuel cell current in order to prevent fuel(hydrogen and oxygen)starvation problem by controlling fuel cell current slope,and then to reduce mechanical stresses on the system(fuel pressure,water pressure in tubes and stack).The control strategy is that dc bus voltage controller operates on a fast power source, supercapacitors,and the supercapacitor voltage being control by the fuel cell delivered current.

The experimental results on a small-scale dc bus(42V, 500W),with a12.5V,500W PEM fuel cell,have confirmed the fuel starvation phenomena.Results carried out by means of a hybrid system test bench,which employs a storage device com-posed of six SAFT3500F supercapacitors connected in series, have evidently revealed the excellent performances of the pro-posed control principle in conditions of overload and energy recovery in a short time.

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