Control of Fuel Cell/Supercapacitor Hybrid Sources for Electric Vehicle ...

Control of Fuel Cell/Supercapacitor Hybrid Sources for Electric Vehicle Applications

Thus, it is recommended, when utilizing fuel cell to associate it with at least a fast source to improve dynamic performances of the whole system. Moreover, one can take advantage of this auxiliary source to achieve an actual hybrid source to reduce mean power sizing from peak transient power, to condense in volume and weight, and to be able of regenerative braking. For example, some authors have explained the employ of supercapacitors for power assistance in transportation network [4] or in electrical vehicle [5].

Abstract This paper presents a novel control strategy for the dc bus voltage regulation employing a PEM fuel cell as main source and supercapacitors as supplementary source for electric vehicle applications. The proposed control is clearly simpler than usual state machines used for hybrid source control, and free of chattering problems. The essential goal is to avoid fast operation of fuel cell current in order to ensure a good synchronization between fuel flow and cell current. To authenticate control algorithms, hardware system is realized by analogical current loops and digital voltage loops (dSPACE). Experimental results with small-scale devices, a PEM fuel cell and SAFT supercapacitor modules, show the excellent control principle.

Keywords:

Stéphane Raël*, Bernard Davat** Institut National Polytechnique de Lorraine (INPL), 2, Avenue de la Forêt de Haye, Vandoeuvre-lès-Nancy, 54516 France [email protected]* [email protected]**

CH3: Fuel Cell Voltage 2.5 V/Div

(a)

Voltage undershoot (1.5 V) due to FC mechanical delays 600 W

Electric Vehicle, Hybrid Power Sources, Fuel Cell, Supercapacitor

CH2: Fuel Cell Current 10 A/Div

1. Introduction

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Fuel cell (FC) power generation systems are expected to be utilized in more and more applications: in portable, in transportation, and in power stationary for which fuel cell systems can provide both power and heat with high efficiency of cogeneration [1]. Scientists are developing many different types of fuel cell employing different fuels and electrolytes. One of the most promising for automotive applications is the lightweight, relatively easy to build and small Polymer Electrolyte Membrane Fuel Cell (PEMFC). According to M. E. Schenck et al. [2], working with a 1.2 kW fuel cell (Ballard System Power Inc.), and P. Thounghong et al. [3], working with a 0.5 kW fuel cell (ZSW Inc.), it is commonly accepted that one of the major weak points of fuel cell is its time constants (several hundred of milliseconds) dominated by temperature, fuel delivery system, (pumps, valves, and in some cases a hydrogen reformer). Therefore, fast load demanding will cause high voltage drop in a short time, known as fuel starvation. To be clarified, Fig. 1 presents the 0.5 kW PEM fuel cell voltage response to a current profile obtained by means of PWM boost converter operated by a PID current corrector, which will be presented later in this paper. The tests operate in two different ways: current step, and current slope. One can observe the drop of voltage curve in Fig. 1(a), compared with Fig. 1(b), because fuel flow has difficulties to follow the current step, and this condition of operating is evidently harmful for fuel cell stack [3].

Time: 0.2 s/Div CH3: Fuel Cell Voltage 5 V/Div

(b)

CH2: Fuel Cell Current 10 A/Div Time: 4 s/Div

Fig. 1 Fuel cell dynamic characteristics to (a) current step (b) current slope: 4 A.s-1. In this paper, the work deals with the conception and the achievement of the regulation of a dc bus voltage supplied by a PEM fuel cell as main energy source, and supercapacitors as secondary source. The experimental results substantiate the proposed control algorithms.

2. Hybrid Power Sources In general, dc bus voltage vBus (500 V for Toyota Prius Hybrid Vehicle, for example) is higher than fuel cell voltage vFC. Hence, fuel cell converter delivering a directional current must boost vFC to the dc bus level. Supercapacitor converter must be bidirectional in current in order to store or generate energy. To manage energy exchanges among dc bus and two sources, one may define three operating modes (states):

การประชุมวิชาการทางวิศวกรรมไฟฟ้า ครั้งที่ 28 (EECON-28) 20-21 ตุลาคม 2548 มหาวิทยาลัยธรรมศาสตร์

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Phatiphat Thounthong Department of Teacher Training in Electrical Engineering, King Mongkut’s Institute of Technology North Bangkok, 1518, Piboolsongkram Rd, Bangkok, 10800 Thailand E-mail: [email protected] [email protected]

•

charge mode, in which the main source supplies energy to the storage device and to the load, • discharge mode, in which both main source and storage device supply energy to the load, • recovery mode (regenerative braking), in which the load supplies energy to the storage device. This method has been investigated before, for example by P. Thounthong et al. [5] on fuel cell/supercapacitor hybrid sources. The problem of such a control is well known: the chattering phenomenon when the system is operating near a border between two states. For this reason, the proposed control here regulates the dc bus voltage through the power delivered by the fast source, the supercapacitors. The fuel cell, as main slow source, is used to maintain the supercapacitor voltage vSuperC. For reasons of safety and dynamics, the converters are primary controlled by inner current loops (Fig. 2). A classical PID and hysteresis correctors are selected for FC and supercapacitor current controls, respectively. These loops are supplied by two reference signals, supercapacitor current reference iSuperCREF and FC current reference iFCREF (also supplies to the FC processor to adjust fuel flow to the desired current) generated by the dc bus and the supercapacitor voltage regulation loops, respectively, with the following constraints: 1). Supercapator voltage must be kept within an interval [VSuperCMin (minimum value), VSuperCMax (maximum value)]. 2). FC current slope must be limited to a maximum absolute value (for example, 4 A.s-1). 3). FC current must be kept within an interval [IFCRated (rated value), IFCMin (minimum value) or zero]. In previous work [5], vBus was directly considered as control variable, and dc bus voltage controller demands power from hybrid sources. One of the difficulties comes from a non-linear transfer function. Then, to obtain a linear system, the energy stored in the dc bus capacitor C has been considered as control variable in place of the dc bus voltage (Fig. 3). By neglecting the losses in converters, the dc bus energy EBus can be written as,

dE Bus (t ) = p SuperC (t ) + p FC (t ) − p Load (t ) dt

6Controller 47 4 48 4 KV (TV s + 1) = ⋅ TV s

E BusMea (s ) E BusREF (s ) OL

E Bus PSuperC

} 1 s

Filter 6 78 K1 ⋅ (2) T1s + 1

Simply, classical controller design called “Symmetrical Optimum” will be used to obtain phase margin. Fuel Cell Processor

I

H 2O

H2

Fuel Cell Heat

(1 / 2 )O2

F

iFCMea

-

FC Current Controller

iFC Fuel Cell Converter

PWM

=

d

iFCREF

Bus

=

vBus

SuperC Converter

SuperC Current Controller

iSuperCREF

-

=

1 0

iSuperC

v SuperC

=

Supercapacitors

Fig. 2 Inner current control loop of hybrid sources. E BusMea DC Bus Energy Controller

v BusREF E Bus vBus

E BusREF

-

E Bus

EBus v Bus

v Bus

SuperC Limitation Function

pSuperCREF

÷

iSuperCREF

vSuperCMea

Fig. 3 DC bus voltage control loop. For the first constraint which consists in keeping the supercapacitor voltage within its defined limits [VSuperCMin, VSuperCMax], iSuperCREF must be limited. The supercapacitor current saturation function (Fig. 4) aims to limit iSuperCREF to the interval [ISuperCMin, ISuperCMax] defined versus vSuperCMea as follows, æ v SuperCMax − vSuperCMea ö ÷ I SuperCMin = − I SuperCRated × minçç 1, ÷ ∆v è ø æ vSuperCMea − vSuperCMin ö ÷ I SuperCMax = I SuperCRated × minçç 1, ÷ ∆v è ø

(1)

where pSuperC is the supercapacitor power, pFC the FC power, pLoad the load power. The “DC Bus Energy Controller” generates supercapacitor power reference pSuperCREF. By power conservative law, iSuperCREF is a consequence of pSuperCREF divided by the measured supercapacitor voltage vSuperCMea. The function called “EBus/vBus” is a voltage-to-energy conversion, where the energy is proportional to the total dc bus capacitance C and to the square of the dc bus voltage (EBus=0.5CVBus2). Because (1) is composed of a pure integrator and of a low-pass filter (Fig. 3), PI controller is enough for obtaining system performances. As pload and pFC can be considered as perturbation terms, the open loop can be described as,

(3) where ISuperCRated and ∆v are regulation parameters. Supercapacitors

i SuperC (t )

i SuperCREF

+

ESR

+ I SuperCRated

+

∆v

+

v SuperCMea

vSuperC

-

VSuperCMax − ∆v VSuperCMax V + ∆v VSuperCMin SuperCMin

VSuperCMea

− I SuperCRated

-

Fig. 4 Supercapacitors current limitation function.


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To charge supercapacitors to the reference value VSuperCREF (VSuperCNormal), VSuperC is controlled through FC by “Supercapacitor Voltage Controller” demanding iFC. This signal must be limited by the second and third constraints before becoming iFCREF (Fig. 5). As a consequence, system can ensure that iFC will gradually increase and decrease, and over rated or under minimum fuel cell current will not happen. In order to obtain the transfer function of the supercapacitor voltage regulation loop, one considers as,

SAFT Supercapacitor Module FC Current Controller FC Converter SuperC Current Controller

500 W PEMFC

(4)

PE006

v FC (t ) ⋅ iFC (t ) = −vSuperC (t ) ⋅ iSuperC (t ) + p Load (t )

frequency of 25 kHz. Moreover, ControlDesk software enable to change control parameters.

Control Panel

while considering that vFC and pLoad are disturbances of the system. Then, the supercapacitor voltage open loop, linearized form at operating point, can be presented as, V SuperC Controller

vSuperCMea (s )

vSuperCREF (s )

= OL

v

dSPACE Interfacing Card

Fig. 6 Hybrid system test bench.

i

6SuperC 4 474FC 4 8 6filter 78 } VFC VSuperC K2 (5) KVC ⋅ ⋅ CSuperC s T2 s + 1

Fuel Cell

Z FC

VSuperCREF

-

vFC (t )

+ -

-

D1

DC Bus +

S2

vBus (t )

Security

C

S1

Main Switch

-

DC Bus

+

S3 vBus (t )

i FCREF

C

-

v SuperC

Supercapacitors L2

iSuperC (t ) +

S4

0A

vSuperCMea

iS (t )

iD1 (t )

Fig. 7 Fuel cell boost converter [3].

FC Sloped Limitation

I FCRated

iFC (t )

+

where VFC is the nominal fuel cell voltage, VSuperC the nominal supercapacitor voltage, and CSuperC the total supercapacitor capacitance. Because of the large capacitance of the storage device, using an integral (I) action will lead to quite unstable loop. A proportional (P) controller is sufficient for “Supercapacitor Voltage Controller” as far as the gain is high enough to introduce only little static error. SuperC Voltage Controller

L1

vSuperC (t )

CSuperC

-

Fig. 8 2-quadrant supercapacitor converter [5]. Fig. 5 SuperC voltage control loop and FC limitation.

The tests have been carried out by connecting the dc bus to an active load composed of a 2-quadrant dc/dc converter loaded by a dc motor. Fig. 9 presents motor start up to 600 rpm and a break to stop. The initial state is zero for both iFC and iSuperC, and 25 V for vSuperC. During motor start, one can observe that the supercapacitors deliver 500 W peak load power required during motor acceleration. Fuel cell current increases with limited slope up to a level lower than 40 A. Steady state load power is about 350 W, entirely supplied by the fuel cell. Concurrently, after a sharp increase during motor acceleration, supercapacitor current decreases down to zero, and final vSuperC is lower than 25 V, because of the static error introduced by the proportional controller of supercapacitor voltage regulation loop. During motor break, the supercapacitors first recover the over energy supplied by the fuel cell and by the regenerative breaking (motor current is negative) to the dc bus, and then is slightly charged by the fuel cell up to 25 V. iFC immediately decreases with slope, and in a second phase (supercapacitor final charge) slowly decreases down to zero. Peak load power during motor

3. Experimental Validation The studied small-scale hardware (Fig. 6-8) is realized with dc bus: 42 V, 500 W; ZSW PEMFC [5]: 500 W, 40 A, around 12.5 V; SAFT supercapacitor modules: VSuperCMax = 30 V, VSuperCNormal = 25 V, VSuperCMin = 15 V, 400 A. Note that, for the fuel cell converter, switch S2 is a shutdown device for test security to prevent the stack from short circuits in case of accidental destruction of S1, or faulty operation of the regulator [3], and the total dc bus capacitance C is equal to 720 mF. Designed control parameters are as follows: K1 = 1, T1 = 20 ms, TV = 250 ms, KV = 14 (58° of phase margin), K2 = 1, T2 = 159 ms, ISuperCRated = 200 A, ∆v = 0.5 V, VSuperCREF = 25 V, and KVC = 100 (so that the maximum static error on supercapacitor voltage is 400 mV (IFCRated/KVC). FC current slope limitation is set to 4 A.s-1. These two loops, which generate current references iSuperCREF and iFCREF, have been implemented in the real time card dSPACE DS1104, through the mathematic environment of Matlab/Simulink, with a sampling


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Voltage [V] Power [W] Motor Speed [rpm] Current [A]

Load Power [W]

SuperC

200 0

25

20

30

40

50 60 Time [s]

Motor Speed [rpm] Current [A]

10

20 30 Time [s]

40

0 120

SuperC

80

Fuel Cell

40

Motor

0 10

20 30 Time [s]

40

Phatiphat Thounthong received the B.S. and M.E. degree in Electrical Engineering both from King Mongkut’s Institute of Technology North Bangkok (KMITNB), Bangkok, Thailand, in 1996 and 2000, respectively. Since 2003, he has been preparing a Ph.D thesis under Thai-French Joint Research of Power Electronics and Drive Technology at INPL.

Fuel Cell SuperC

10

0

200

[1] D.A.J. Rand, R.M. Dell, “The Hydrogen Economy: a Threat or an Opportunity for Lead–Acid Batteries?,” J. Power Sources, Available: http://www.elsevier.com. [2] M.E. Schenck, J.S. Lai, K. Stanton, “Fuel Cell and Power Conditioning System Interactions,” in IEEEAPEC2005 Conf., USA, March 6-10, 2005, pp. 114-120. [3] P. Thounthong, S. Raël, B. Davat, “Test of a PEM Fuel Cell with Low Voltage Static Converter,” J. Power Sources, 2005, Available: http://www.elsevier.com. [4] A. Rufer, D. Hotellier, P. Barrade, “A Supercapacitor-Based Energy-Storage Substation for VoltageCompensation in Weak Transportation Networks,” IEEE Trans. Power Delivery, vol. 19, no. 2, April 2004, pp. 629-636. [5] P. Thounthong, S. Raël, B. Davat, “Utilizing Fuel Cell and Supercapacitors for Automotive Hybrid Electrical System,” in IEEE-APEC2005 Conf., Texas-USA, March 6-10, 2005, pp. 90-96.

400

24 0

500

400

References

Fuel Cell

Motor

Fuel Cell

600

Fig. 10 System response to a high load step.

600

60 40 20 0 -20 -40

SuperC

1000

Fuel Cell

Load

DC Bus

42 35 28 21 14 7

1500

DC Bus

42 35 28 21 14

600 400 200 0 -200 -400 -600

SuperC Voltage [V]

Voltage [V]

break is about -150 W, recovered by the supercapacitors thank to the dc bus voltage regulation which imposes negative current reference. The final state is zero for both iFC and iSuperC, and 25 V for supercapacitor voltage. Finally, Fig. 10 presents waveforms during a load step. While overloading, load power reaches a constant level of 1500 W, three times higher than fuel cell rated power. One can observe the fast increase of iSuperC, so that vBus is slightly disturbed by the load step. iFC increases slowly up to 40 A, and then remains constant. Load step duration (20 s) leads, of course, to storage device discharge, to vSuperC decrease and to iSuperC increase (because of constant load power). After the overload, FC current remains at its rated value in order to provide power to the motor (no-load power: 380 W) and to the supercapacitors (charging power: around 120 W).

70

80

Stéphane Raël received the Engineer degree at ENSIEG, Grenoble, France in 1992, and the Ph.D degree from INP Grenoble in 1996. Since 1998, he has been working as assistant Professor at INPL in the field of power electronics components, supercapacitor, battery and fuel cell.

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Fig. 9 System response to start and break motor.

4. Conclusion This work proposes a new method of regulating dc bus voltage supplied by hybrid sources employing a PEM fuel cell, as main source, and supercapacitors as supplementary source. The key constraint is to avoid fast transition of fuel cell current in order to prevent fuel starvation problem by controlling fuel cell current slope and then to reduce mechanical stresses in the system (fuel pressure, water pressure in tubes and stack). The experimental results have confirmed the excellent performances of the proposed control during starting, breaking and high loading motor.

Bernard Davat received the Engineer degree at ENSEEIHT, Toulouse, France in 1975, the Ph.D degree in 1978 and the “Docteur d’Etat” degree in 1984, both from INPT. Since 1988, he has been Professor at INPL. His main research interests deal with power electronics, drives and new electrical devices (fuel cell, and supercapacitor).


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