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Performance Evaluation of Modified 4-Phase Interleaved Fuel Cell Converter for High-Gain High-Power Applications Phatiphat Thounthong, Nichamon Poonnoi King Mongkut’s University of Technology North Bangkok 1518, Piboolsongkram Road Bangsue, Bangkok, Thailand [email protected]

Panarit Sethakul

Bernard Davat

Thai-French Innovation Institute King Mongkut’s University of Technology North Bangkok 1518, Piboolsongkram Road Bangsue, Bangkok, Thailand [email protected]

Nancy Université GREEN (UMR 7037)- ENSEM-INPL 2 avenue de la Forêt de Haye 54516, Vandœuvre-lés-Nancy, Lorraine, France [email protected]

Abstract -- This paper presents a modified 4-phase paralleled step-up converter for fuel cell generator. In high power applications, the interleaving technique for paralleling input of the converter system has been studied for many years. The modified 4-phase interleaved fuel cell converter is studied for higher voltage conversion ratio. By virtue of paralleling the converters, the input current can be shared among the cells or phases, so that high reliability and efficiency in power electronic systems can be obtained. In addition, it is possible to improve the system characteristics such as maintenance, repair, fault tolerance, and low heat dissipation. The average-current-control in continuous conduction mode for the proposed converter is analyzed in details. The prototype fuel cell converter of 1.2-kW connected with a NexaTM fuel cell (1.2-kW, 26 V) developed by Ballard Power System Company is implemented for highvoltage applications. Experimental results of the power converter authenticate the excellent performance. The power converter can boost a low dc fuel cell voltage of 26 V up to a dc bus voltage of 200 V. Index Terms--Converters, current control, energy conversion, energy management, hydrogen, oxygen, polymer electrolyte membrane fuel cell (PEMFC).

I. vFC iFC iFCREF pFC vBus iLoad pLoad Ca Cb D iL

NOMENCLATURE

Fuel cell voltage [V]. Fuel cell current [A]. Fuel cell current reference [A]. Fuel cell power, = vFC× iFC [W]. DC bus voltage [V]. DC bus load current [A]. DC bus load power, = vbus× iLoad [W]. Top output capacitance [Farad, F]. Bottom output capacitance [Farad, F]. Duty Cycle [per-unit or percentage] Inductor current [A]. II.

INTRODUCTION

Fuel cells (FCs) have received a great deal of interest in recent years for their prospective to solve several key challenges facing our World today: dependence on petroleum imports, poor air quality, and greenhouse gas emissions. An FC is defined as an electrochemical device that continuously converts chemical energy into electric energy (and some

heat) for as long as fuel and oxidant are supplied. The electrical energy obtained by FCs can be utilized in residential, stationary, adventure, distant communication and transport applications. The biggest advantage of FC energy compared to traditional energy is its high efficiency. Unlike internal combustion and steam engines, heat exchange and mechanical work are no longer the major energy conversion methods. The electrons from the chemical reactions themselves are collected and conveyed directly to supply power. With the proper selection of fuel such as pure hydrogen, the fuel cell energy is fairly clean, showing great potential of mitigating the environmental pollution problem of modern industrial world [1], [2]. In spite of the potential, fuel cells have not been commercialized to a large extent after its first invention almost two hundred years ago. The state-of-the-art fuel cells on the market exist in three formats: as network or uninterrupted power supply (UPS) to certain residential and industrial applications, as major power system for adventure and telecommunication at distant locations, and as demonstration units in universities and automobile companies [3]. Proton exchange membrane fuel cells (PEMFCs) are being seriously considered by the automotive industry as vehicle primary or secondary engine for their high efficiency, clean side product (H2O) and employment of solid state electrolyte. Somewhat a few prototypes have been established to demonstrate the capability of PEMFCs to power not only vehicles but also residential areas. Nonetheless, the source of pure hydrogen remains an unanswered issue for PEMFC applications [4]. Although hydrogen can be obtained by water electrolysis, methanol/natural gas/gasoline reformation and bacterial production, the cost of hydrogen generation, storage and building hydrogen fueling infrastructure is high. In particular, reformation of hydrocarbon compounds cannot eliminate the dependence of power generation on naturally conserved fossil resources and has carbon dioxide (CO2) as one of the end products, causing environmental issues by itself [5]. While the cost problem of fuel cells can be reduced by mass sale/production or mitigated due to certain circumstances such as remote locations, durability and

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reliability of fuel cells are essential for the goal of fuel cell commercialization. Fuel cells must last long enough in order to serve their duties and compete with the conventional energy devices. Fuel cells are subject to high temperature, high humidity, flow of fuel and oxidant and strong acid or alkaline environment. There are a number of components in the system, including the electrolytes, catalyst layers, gas diffusion layers (which comprise the membrane-electrodeassemblies, MEAs), bipolar plates and current collector plates [6], [7]. Fuel cells produce low dc voltage, and they are always connected to electric power networks through a step-up (boost) converter [8], [9]. In high power application (for example: 50-kW in fuel cell vehicle [10]), an interleaving parallel technique of converter modules is studied recently. However, because component parasitic elements limit the practically realizable voltage conversion ratio of the power converter, this becomes a critical issue in the case of the classical interleaved boost converter [11], [12]. Furthermore, in many applications, the use of an isolation transformer can provide increased output/input voltage conversion ratio, as required and full-bridge topologies can be used. However, there are applications where transformerless energy converter systems could potentially offer significant advantages, including simplicity, cost, and converter size reduction, particularly in high power applications [11]. Different power converter topologies can be used for the power electronic interface between the fuel cell and the utility dc bus. For the dc link voltage level, it is depending on its applications [11]: • 270 V or 350 V for the standard on the all-electric aircraft, • 48 V; 120 V; or 400 V to 480 V for stand-alone or parallel grid connections, • 42 V (PowerNet) a new standard voltage for automobile systems, • 270 V to 540 V for electric (fuel cell) vehicles, • 350 V (transit bus systems) to 750V (tramway and locomotive systems). This paper presents a study of modified 4-phase interleaved fuel cell converter for high power, high voltage conversion ratio, and transformer-less applications. A prototype FC converter of 1.2-kW is implemented. The power circuit, the proposed FC current control loop, and the dynamic system equations are detailed. Experimental results will illustrate the system performances. III. PROPOSED HIGH-GAIN HIGH VOLTAGE CONVERTER CIRCUIT The paralleled interleaving technique for power electronic converter has been studied for many years. Converter modules are controlled by interleaved switching signals, which have the same switching frequency and the same phase

shift. It is possible to improve the system characteristics such as maintenance, repair, fault tolerance, and low heat dissipation. As a consequence of the interleaving operation, the interleaved boost converter exhibits both lower current ripple at the input side and lower voltage ripple at the output side. Therefore, the size and losses of the filtering stages can be reduced, and the switching and conduction losses can be significantly decreased [13]-[16].

Fig. 1. NexaTM PEM fuel cell system (1.2-kW, 46-A) developed by Ballard Power Systems Inc.

Fig. 2. Switching characteristics of a classical 4-phase interleaved fuel cell boost converter at the fuel cell current of 30 A (VFC = 28 A, VBus = 60 V).

For clarity, Fig. 2. shows the steady-state waveforms of a 4-phase interleaved boost converter (1.2-kW, 25-kHz) connected with our PEMFC system in our laboratory (Fig. 1) when operating at a FC current of 30 A. It illustrates the FC current, the first, the second and the third inductor currents. It can be observed that the inductor ripple current is around 2 A and, due to the interleaving technique, the FC ripple current is absolutely reduced to nearly zero [11].

A.

Power Circuit One proposes the modified interleaved boost converter (named here “Dual Converter”) as depicted in Fig. 3, to order to obtain a higher voltage conversion ratio of the classical interleaved boost converter. The interleaving concept is guaranteed by the parallel connection of the four modules at the input and the phase shifted control of the four switches (S1a, S2a, S1b, and S2b). High voltage-gain is obtained by the series connection of the top (A) and bottom (B) modules at the output. The input source is also connected in series with the two output capacitors (Ca and Cb) at all times. The series connection at the output results in reduced overall output voltage ripple. To reduce a voltage ripple in the two output capacitors Ca and Cb, the top module (A) and bottom modules (B) are operated as a 2-phase interleaved converter. Then, the power switches operate in an interleaved fashion in a cyclic sequence of S1a, S1b, S2a, S2b; S1a, S1b, S2a, S2b; S1a, S1b, …. . Then, one may write:

V V VBus + VFC ⎧ , D = 1 − FC = 1 − FC ⎨Va = Vb = 2 Va Vb ⎩

(1)

V − VFC ⎧ 1 ⋅ VFC , D = Bus ⎨Va = Vb = (1 − D ) VBus + VFC ⎩

(2)

⎧ (1 + D ) ⋅ V ⎨VBus = (1 − D ) FC ⎩

(3)

where, VBus is the dc bus voltage, VFC the FC voltage, D the duty cycle. From equations above, it is clear that the proposed converter has conversion ratio higher than a classical boost converter. To obtain the transfer function, an averaged circuit model of the dual step-up converter developed from the switching model by time averaging of high frequency periodical waveforms is portrayed in Fig. 4 [17]-[21]. The low frequency dynamics of the averaged model is described by continuous functions, which can be linearized and employed for control design.

Fig. 3. Proposed high-gain transformer-less converter for FC high power applications

Fig. 4. Averaged circuit model of the proposed fuel cell step-up converter.

Then, the linearized differential equations (which are developed from the state-space averaged model of a boost converter) are defined as follows [22]: L1a L2a L1b

~ d iL1a (t ) dt ~ d iL2a (t ) dt ~ d iL1b (t ) dt

~ ~ = v~FC (t ) − RL1a iL1a (t ) − (1 − D1 )v~a (t ) + Va d1 (t ) ~ ~ = v~FC (t ) − RL2a iL 2a (t ) − (1 − D2 )v~a (t ) + Va d 2 (t ) ~ ~ = v~FC (t ) − RL1b iL1b (t ) − (1 − D3 )~ vb (t ) + Vb d 3 (t )

L2b

~ d iL2b (t ) dt

~ ~ = v~FC (t ) − RL2b iL2b (t ) − (1 − D4 )~ vb (t ) + Vb d 4 (t )

H i (s ) =

T f s +1 ~ K id (Tz s + 1) i (s ) Gid (s ) = L~ = 2 d ⎛ s ⎞ 2ζ ⎜⎜ ⎟⎟ + s +1 ωn ⎝ ωn ⎠

(4)

~ d v~ (t ) ~ ~ Ca a = (1 − D1 )iL1a (t ) − I L1a d1 (t ) + (1 − D2 )iL 2a (t ) dt (5) ~ ~ − I L 2a d 2 (t ) − iLoad (t ) Cb

~ d v~b (t ) ~ ~ = (1 − D3 )iL1b (t ) − I L1b d 3 (t ) + (1 − D4 )iL 2b (t ) dt (6) ~ ~ − I L 2b d 2 (t ) − iLoad (t ) ~ vBus = 2~ va − ~ vFC = 2~ vb − ~ vFC

(7)

where, X is the nominal parameter, ~x is the parameter variations. Note that RL is the series resistance of inductors L and series resistance of Ca and C b is ignored.

Kf

(11) (12)

and, ⎧ 2 IL ⎧ ⎪ω = (1 − D ) ⎪ K id = (1 − D ) n ⎪ L ⋅ Ca ⎪ and ⎨ ⎨ V C a a RL ⋅ C a ω n ⎪ ⎪T = ⎪⎩ z (1 − D )I L ⎪ζ = (1 − D )2 2 ⎩

(13)

The proportional gain Ki and the integral time constant TCi of the PI controller are set to obtain the desired phase margin. For the equations above, we consider that the FC power source is an ideal source (no FC complex impedances taken into account) [24], [25].

B.

Current Control Loops of the Parallel Converter For the 4-phase pulse width modulation (PWM) generators, the four saw tooth generators (90 degree out of phase) are implemented by using a synchronization clock. The clock frequency is a switching frequency of power switches. Each saw tooth signal is sent to compare with the output of the inductor current controller (here PI-controller). Then, the system obtains the four signals of gate drives (vGS), refer to Fig. 3. As depicted in Fig. 5, for current measurements of each inductor current, a first order filter H iN (s ) is used to reduce harmonics due to high switching frequency. A classical proportional–integral (PI) controller CiN (s ) associated to a PWM generator GPN (s ) is selected for inductor current control. If we consider that the parameters in each converter are ideally the same values, the inductor current control loops can be modeled from (4)–(7), taking into account VP, the amplitude of the PWM saw tooth carrier signals, as the following closed-loop transfer function [23]: ~ Ci (s ) ⋅ GP (s ) ⋅ Gid (s ) iL (s ) = ~ iLREF (s ) 1 + Ci (s ) ⋅ GP (s ) ⋅ Gid (s ) ⋅ H i (s )

with: Ci (s ) =

K i (TCi s + 1)

G P (s ) =

TCi s

1 VP

(8)

(9) (10)

Fig. 5. Proposed current control loops for the high-gain fuel cell converter.

TABLE II POWER CONVERTER SPECIFICATION

IV. EXPERIMENTAL VALIDATION A.

Test Bench Description The system test bench is presented in Fig. 6. The NexaTM PEMFC system (1.2-kW, 46 A, around 26 V) was developed and commercialized by the Ballard Power Systems Inc. It is supplied using hydrogen from bottles under pressure, and with clean and dry air from a compressor. The design requirements and specification of implemented power converter are detailed in Tables. I and II. The implemented power converter is illustrated in Fig. 7. Some guidelines of a boost power converter design can be seen in [24]. Measurement of each inductor current i LN  is performed by means of a zero-flux Hall effect current sensor. The inductor current reference iLREF is generated by the real time card dSPACE DS1104, through the mathematical environment of Matlab–Simulink and the ControlDesk software.

B.

Fig. 6. System test bench.

Fig. 7. Implemented FC power converter of 1.2-kW. TABLE I FUEL CELL CONVERTER DESIGN REQUIREMENT

Experimental Results The experimental tests have been carried out by connecting the dc link to an adjustable resistor and an active load composed of a current reversible chopper, loaded by a servo motor coupled with a powder brake. Then, the load at dc bus can be varied to the desired operating point by adjusting the load resistor and the powder motor brake. The oscilloscope waveforms of the inductor currents (iL1a, iL2a, iL1b, and iL2b) in Figs. 8 and 9 depict the results of a converter operating at a rated dc bus voltage of 200 V and the inductor current references of 2 A and 3 A, respectively. It can be observed that the phase-shift of each inductor current is 90 degree, consecutively. Fig. 10 illustrates the inductor currents (iL1a and iL2a), the fuel cell current, and the dc bus voltage. It depicts the results of a converter operating at a rated dc bus voltage of 200 V and the inductor current references of 5 A. One can see that the conversion ratio of the proposed converter is 6 times and the FC ripple current is nearly zero by the interleaved switching technique. It means that the FC mean current is close to the FC rms current. In addition, it can be scrutinized that the FC ripple frequency of 100-kHz is four-times the switching frequency of 25-kHz PWM. As a final test, the oscilloscope waveforms of the inductor current iL1a, the fuel cell current, and the output capacitor voltages va and vb in Fig. 11 depict the results of a converter operating at a rated dc bus voltage of 200 V and the inductor current references of 7 A. It can be observed that the output capacitor voltage va is equal to the output capacitor voltage vb.

Fig. 8. Switching characteristics of the FC power converter at an inductor current reference of 2 A (IFC = 6.90 A, VFC = 35.4 V, VBus = 200 V).

Fig. 10. Switching characteristics of the FC power converter at an inductor current reference of 5 A (IFC = 17.70 A, VFC = 33.5 V, VBus = 200 V). Fig. 9. Switching characteristics of the FC power converter at an inductor current reference of 3 A (VFC = 34.4 V, VBus = 200 V).

V.

CONCLUSION

The main objective of this paper is to present the highgain transformer-less power converter with interleaving technique for a FC generator for high power applications. By the proposed topology, one obtains a higher voltage conversion ratio compared with a classical interleaved stepup converter. High power dc distributed power system supplied by FC invokes the need to parallel power modules with interleaving technique. By method of the parallel converter modules with interleaving algorithm for a FC generator for high power applications, inductor size (ferrite core and Litz-wire) are simple to design and fabricate; and the FC ripple current can be virtually reduced to zero.

The prototype high-gain power converter has been designed and implemented as a FC converter of 1.2-kW. So, the inductor average current control loops realized by analog circuits of paralleled converters are inevitable to share the same currents, and to control the FC current. Experimental results of implemented converter connected with a NexaTM PEMFC system (1.2-kW, 46-A) authenticate the excellent performances. Future works are to use the implemented parallel converter with interleaving algorithm to function in hybrid sources: FC/Supercapacitor [26] - [28] or FC/Li-Ion Battery [29], [30] or FC/Super-capacitor/Li-Ion Battery [31] for a high power dc distributed generation system [32], [33].

[5]

[6] [7] [8] [9] [10]

[11] [12]

[13] [14] [15]

[16]

[17] [18] Fig. 11. Switching characteristics of the FC power converter at an inductor current reference of 7 A (IFC = 25.6 A,VFC = 33.5 V, VBus = 200 V).

[19]

ACKNOWLEDGMENT [20]

The research work was supported in part by the Thai-French Innovation Institute (TFII), King Mongkut’s University of Technology North Bangkok (KMUTNB), and the Thailand Research Fund (TRF) under Grant MRG5180348.

[21]

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