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Three-Port Bidirectional Converter
for Hybrid Fuel Cell Systems
Jorge L. Duarte, Marcel Hendrix, and Marcelo Godoy Simões, Senior Member, IEEE
Abstract—The implementation of a hybrid fuel cell/battery
system is proposed to improve the slow transient response of a fuel
cell stack. This system can be used for an autonomous device with
quick load variations. A suitable three-port, galvanic isolated,
bidirectional power converter is proposed to control the power
flow. An energy management method for the proposed three-port
circuit is analyzed and implemented. Measurements from a 500-W
laboratory prototype are presented to demonstrate the validity of
the approach.
Index Terms—Battery, fuel cells.
I. INTRODUCTION
F
UEL cells have very slow response due to the natural electrochemical reactions required for the balance of enthalpy
[1]–[4]. Therefore, electrical output load power is not matched
during transients, and the deficiency or surplus must be managed
by an external leveling system. A fuel cell generator will shut
down or collapse when more current is taken than it can supply;
so, current demand should never exceed the available current.
Current demand may be less than available current, but this results in unused fuel and decrease of efficiency from the fuel cell.
For these two reasons bidirectional energy storage is required to
sink/source the power difference. Lead acid batteries provide a
suitable choice for storage because they show fast response time
to load changes, being therefore capable of handling the power
difference between the load demand and the available fuel cell
generation. Moreover, lead acid batteries are not expensive, and
widely available.
The subject of this paper is the design and the implementation of a suitable interface circuit for a hybrid fuel cell/battery system, aiming at feeding a small autonomous load. An
overview of the complete system is shown in Fig. 1, where a
converter controls the power flow between a 25–39 V, 500-W
PEM fuel cell stack and 48-V lead acid batteries. As soon as
power deficiency or excess occurs because of load variations,
the converter regulates this extra power flow from or to the
energy storage element. Furthermore, since the possibility to
supply ac loads through a 400-Vdc inverter output should also
be available, a three-port bidirectional topology has to be chosen
Manuscript received August 25, 2004; revised May 23, 2006. This paper was
presented at the IEEE PESC’04. Recommended for publication by Associate
Editor J. D. van Wyk.
J. L. Duarte and M. Hendrix are with the Group of Electromechanics and
Power Electronics, Technische Universiteit Eindhoven, Eindhoven 5600 MB,
The Netherlands (e-mail: j.l.duarte@tue.nl).
M. G. Simões is with the Engineering Division, Colorado School of Mines,
Golden, CO 80401 USA.
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TPEL.2006.889928
Fig. 1. System overview: a power electronic converter regulates the energy flow
between the fuel cell generator, an energy storage device, and the load.
Fig. 2. (a) Proposed dc–ac–dc converter topology that matches sources and
sinks of energy in Fig. 1 through a three-winding transformer and bidirectional
high-frequency switching bridges. Full H-bridges are shown at each port;
however, it would be also possible to implement this converter by using half
bridges. (b) Fundamental system model: three square-wave voltage sources
that exchange energy through a grid of inductors, as a consequence of the phase
shift angle between the switching patterns. The network of inductors is derived
from the transformer in (a) based on a -model representation.
in view of the characteristic behavior of the fuel cells, batteries,
and load. Of course, there should be no compromising in reliability and battery lifetime.
Multiple-port, bidirectional converter topologies that may be
suitable for the system requirements in Fig. 1 can be found in the
literature [5], [6]. The main drawback of the existing concepts
is that they cannot handle a wide variety of voltage range inputs.
A resonant converter topology is presented in [7], but it is very
hard to implement. Since the system under consideration combines a 25–39 V fuel cell stack and 48-V batteries with a 400-V
inverter output, the use of magnetic transformers may facilitate
0885-8993/$25.00 © 2007 IEEE
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Fig. 3. Simulation results of the three-port converter in Fig. 2: square wave voltages across the transformer terminals and corresponding current waveforms at a
time base of 2 s=div . Left frame: fuel-cell (Trace 1: 50 V/div, Trace 3: 10 A/div) and load terminals (Trace 2: 500 V/div, Trace 4: 1 A/div.); right frame: battery
terminals (Trace 1: 50 V/div, Trace 2: 2 A/div.)
matching the different voltage levels. The dual active bridge described in [8], proposed to control the power flow between two
ports, can be expanded to three ports in order to satisfy the needs
of the complete system in Fig. 1. The advantages of a magnetically coupled, multiple-port topology aiming at UPS applications have also been recognized in [9].
This paper is a reviewed version of our previous work
[10]. A three-port concept for the converter, which employs
a single high-frequency isolation transformer, is introduced
in Section II. Then, a control-oriented modeling approach
is presented in Section III. Short-term and long-term power
management strategies are discussed in Section IV. Theoretical
considerations are verified by simulation results in Section V,
and by measurement results in Section VI. Finally, concluding
remarks are placed in Section VII.
Fig. 4. Equivalent dynamic electric circuit model of a PEM fuel-cell generator
[1].
According to the definitions in Fig. 2, the relationship between the bridge phase shift angles and the power flow in the
system is found to be
(1)
(2)
II. TRANSFORMER-COUPLED CONVERTER
(3)
Fig. 2(a) shows a three-port converter, as an extension of the
ideas in [8], which may support the bidirectional energy flow
requirements in Fig. 1. The full-bridge modules are coupled by
means of a three-winding transformer, eventually with the addition of external inductors. Each full-bridge operates at fixed
switching frequency (100 kHz in the current application) and
fixed 50% duty cycle. The power flow between sources and
sinks can be controlled by shifting the switching patterns with
respect to the master module, i.e., the fuel cell bridge.
(4)
(5)
(6)
III. SYSTEM MODELING
Conceptually, the circuit in Fig. 2(a) can be viewed as a grid
of inductors (the transformer magnetizing inductance, leakages and external inductors) driven by controlled square-wave
voltage sources. The voltage sources are phase shifted from
each other by controlled angles, and these displacements impose the power flow between the sources. Fig. 2(b) illustrates
this fundamental modeling approach, based on the -equivalent
transformer representation with the magnetizing inductance
and the leakages referred to the fuel cell side. The transformer
-model in Fig. 2(b) facilitates the system analysis, in addition simple formulas allow to convert the parameters from a
conventional T-model to the -description (see Appendix).
(7)
where
and
and
and
switching frequency;
load and battery voltages, respectively,
referred to the fuel cell side;
phase shifts (in radians) of the load
bridge and battery bridge with reference
to the fuel cell bridge, respectively;
power delivered by the fuel cell
generator;
power consumed by the load (a negative
means energy injection into the grid of
inductors from the dc buffer capacitor at
the load side);
power stored into the battery (negative
sign means that the energy is drawn from
the battery).
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Fig. 5. Simulation results showing (a) step change in the load while the energy delivered by the fuel cells remains constant (load variation from 370 W to 320 W),
and (b) battery charging (step of 40 W) under constant output load (300 W). Traces from top to button: power delivered by the fuel cells, battery and load; lower
picture frames are zoomed views of the upper ones.
Fig. 3 illustrates some simulation results of the triple active
bridge system under open-loop control of the bridge phase shift
angles. The simulation parameters are given in the next section.
denote the desired electric power to be delivered
Let now
, the nominal
by the fuel-cell generator (normally
power of the fuel cells.) Then, in view of (2), (4), and (8), the
phase shift to be adjusted in the battery bridge should obey
IV. POWER FLOW CONTROL
On the basis of (1)–(7) different control strategies can be realis imposed by a classic analog
ized. For instance, if
is kept constant. If
in(PID) compensator the voltage
will be decreased accreases due to some load variation,
cordingly such that less power will be delivered to the dc buffer
capacitor at the load side. As a consequence of (1), phase shift
will impose the power flow through
in Fig. 2(b), that is
(8)
In order to avoid multiple solutions in (8), the absolute value of
the load bridge phase shift must be bounded to
2.
(9)
where
(10)
After some algebraic manipulations, the solution of (9) is found
to be
sign
(11)
Another possible energy flow situation would be to charge the
battery when the power delivered to the load, denoted as , is
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483
TABLE I
CIRCUIT QUANTITIES
less than
. Again,
is imposed by a compenis known (for instance by measuring the dc
sator, and now
in the range
,
load current). By choosing
as given by (11), the battery will be charged
and by adjusting
with an average current level , where
Fig. 6. Experimental set-up, showing (a) the fuel-cell generator (right) together
with the DSpace system (left) and (b) details of the three-port converter.
TABLE II
TRANSFORMER DESIGN PARAMETERS
Eventually, by making
the charging process will be
stopped.
In the power management policies described above, it should
is obtained by means of closed-loop control, as
be clear that
implemented by an analog compensator that compares continand a desired reference value. Howually the error between
is obtained by means of feed-forward conever, the value of
trol according to (11) on the basis of measured values of a few
, ,
and ). The calculations in (11)
circuit variables (
can be easily performed by a digital signal processor.
V. SIMULATION RESULTS
A Spice-based model was developed to investigate the performance of the system. Parameters for the simulations are as
follows.
• Fuel-cell generator: modeled according to [1]. Fig. 4
shows the equivalent circuit, where
with
57 V,
0.3 V,
30 mA,
4.6 A,
0.23 ,
270 mF, and
is the current drawn
from the terminals.
48 V).
• Battery: modeled as a constant current source (
• Load: modeled as simple resistances in parallel with a bus
4.7 F ; desired voltage level:
capacitance
400 V.
7.66,
0.96,
350 H,
• Transformer:
26 H,
230 H,
230 H.
100 kHz.
• Switching frequency:
In all situations the same PI-compensator was applied to control
the output voltage, implemented as
(12)
, and
6.2 rad/V,
50 krad/s.
with
A variety of operating conditions were studied to verify the
effectiveness of the power control algorithm. Fig. 5(a) shows
the response of the system to step changes in the load, assumed
in this case to be resistors suddenly switched in parallel with
the output dc capacitor. The results in Fig. 5(a) illustrate the
output voltage is regulated to a constant value, while the power
delivered by the fuel cells remains unchanged at its nominal
value. Fig. 5(b) also shows a charging cycle for the battery while
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IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 22, NO. 2, MARCH 2007
Fig. 7. Measurement results: square wave voltages across the transformer terminals and corresponding current waveforms at a time base of 2 s/div. Left frames:
fuel-cell (Ch.1: 50 V/div, Ch. 2: 10 A/div) and load terminals (Ch. 3: 500 V/div, Ch. 4: 1 A/div.); right frames: battery terminals (Ch. 1: 50 V/div, Ch. 2: 2 A/div.):
(a) '
34 , '
34 and (b) '
28 , '
40 .
=
=0
=
=
keeping constant the power delivered to the load. Simulation
results have also shown that, if the circuit parameters are adequately designed, it is possible to assure soft-switching for all
bridges over the whole phase shift range.
VI. EXPERIMENTAL RESULTS
An experimental set-up was assembled using Mosfets as
switching devices, the test circuit being rated at 500 W for
100-kHz switching frequency. A PEM 500-W fuel cell set from
Avista Labs [1] was used as generator in combination with
48 V–12 A lead-acid batteries. A dSpace DS1104 controller
board has been chosen to implement the energy management
strategies. The experimental circuit parameters are shown in
Table I, together with the ones used for numerical simulations in
the previous section. Fig. 6 gives an overview of the laboratory
set-up.
The overall system control is based on prescribed phase-shift
of a three-port magnetic coupled structure. This control
strategy works really well during steady-state conditions.
However, during transients there is always a dc-offset that may
build up in one of the ports, which can lead the transformer
towards saturation. Although a programmable slew-rate on the
phase-shift control helps to mitigate the dc-current build-up,
with an eventual decay based on the time constant of the magnetic structure’s Thevenin equivalent circuit, the transformer
needs to incorporate an airgap to store any remaining energy.
The worst-case scenario was simulated in order to determine the
required magnetizing inductance of the transformer. In addition
to the phase-shift transients under rated power conditions, the
effects of harmonics were also considered. For this particular
structure a value of 350 H was determined that would lead to
a maximum of 10% of dc-current flow (based on rated power).
The (Philips’ proprietary) software programs Magtool and
Conv were used to design and optimized the transformer core,
windings, interleaving and half-winding effects. A combination
of Litz wire (for primary) and solid wire (for secondary) was
determined. The transformer parameters are given in Table II
and the experimental setup corroborated the successful operation of this transformer under practical conditions.
Fig. 7 shows measurement results illustrating characteristic
voltage and current waveforms. Operating conditions are chosen
to be equivalent to the ones as for the simulation results in Fig. 3.
A comparison between both figures reveals that the simulated
DUARTE et al.: THREE-PORT BIDIRECTIONAL CONVERTER
485
Fig. 8. Measurement results; in all pictures the traces from top to bottom correspond to: i) trigger event (Ch. 2), ii) current drawn from the fuel cell generator
(ground reference at Ch.1, 1 A/div), iii) dc load current (ground ref at Ch.3, 0.2 A/div), and iv) battery current (ground ref at Ch.4, 0.5 A/div; note that, to fit the
screen, this current is shown as a negative value): (a) step reduction of 50 W in the load; (b) injection of 50 W by the fuel cells; and (c) step increase of 50 W in
the fuel cells and no power is stored in the battery. All traces are shown at a time scale of 20 s/div.
and measured results are consistent. Also, it is possible to recognize the soft-switching operation of the topology from the
voltage and current waveforms in Fig. 7.
Fig. 8 illustrates the response for a pulsating load demand
while keeping the power drawn from the fuel cells constant, and
the ability to charge the battery according to an arbitrary profile.
In Fig. 8(a), a step reduction of about 50 W in the load takes
place; it can be seen that after a transient the power delivered
by the fuel cells returns to its nominal value while the deficit
is covered by the battery. In Fig. 8(b), the output load is kept
constant and an increase of around 50 W is injected into the
system by the fuel cells. Therefore the power delivered by the
battery decreases. In Fig. 8(c), the fuel cell generator feeds the
load (step increase of 50 W) while the energy from the battery is
kept constant. In Fig. 8, the current variations are directly related
to energy changes because during the time period shown the
voltage changes are not significant.
VII. CONCLUSION
A power electronic system capable of interfacing battery energy storage to a fuel cell generator and a generic load was de-
scribed. A three-port galvanically isolated topology was developed based on full bridge converters that allow bidirectional
power flow in each port. Such a configuration facilitates the
matching of different voltage levels in the overall system. The
transformer design was optimally performed in order to incorporate the leakage inductances as required by the topology. The
power flow control has a closed-loop strategy to keep output
voltage constant during transients, with a feedforward strategy
to distribute the energy. The fundamental behavior of the proposed converter system was verified on a 500-W Avista fuel cell
system.
APPENDIX
Considering the three-port transformer in Fig. 2, parameter
conversion from the T-model to the -model representation is
as follows:
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ACKNOWLEDGMENT
The authors wish to thank M. Michon and H. Tao for their
help in the experimental work.
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Jorge L. Duarte received the M.Sc. degree from the University of Rio de
Janeiro, Rio de Janeiro, Brazil, in 1980 and the Dr.-Ing. degree from the Institut
National Polytechnique de Lorraine (INPL), Nancy, France, in 1985.
He has been with the Electromechanics and Power Electronics Group, Technical University of Eindhoven, Eindhoven, The Netherlands, as a member of the
scientific staff, since 1990. During 1989, he was appointed a Research Engineer
at Philips Lighting Central Development Laboratory, and since October 2000
he has also been a consultant Engineer at Philips Power Solutions, Eindhoven.
His teaching and research interests include modeling, simulation and design optimization of power electronic systems.
Marcel Hendrix received the M.S. degree in electronic circuit design from the
Eindhoven University of Technology (TU Eindhoven), Eindhoven, The Netherlands, in 1981.
He is a Senior Principal Engineer at Philips Lighting, Eindhoven. In 1983, he
joined Philips Lighting, Eindhoven, and started to work in the Pre-Development
Laboratory, Business Group Lighting Electronics and Gear (BGLE&G). Since
that time he has been involved in the design and specification of switched power
supplies for both low and high pressure gas-discharge lamps. This work has a
strong relation to lamp physics. BGLE&G’s Pre-Development Laboratory has
its own analog/digital IC design facility and works in close cooperation with
Philips Research Labs, Aachen, Germany, and Briarcliff Manor, NY. In July
1998, he was appointed a part-time Professor (UHD) with the Electromechanics
and Power Electronics Group, TU Eindhoven, where he teaches design-oriented
courses in power electronics below 2000 W. His professional interests are with
cost function based simulation and sampled-data, nonlinear modeling, real-time
programming, and embedded control.
DUARTE et al.: THREE-PORT BIDIRECTIONAL CONVERTER
Marcelo Godoy Simões (S’89–M’95–SM’98) received the B.S. and M.Sc. degrees in electrical engineering from the University of São Paulo, São Paulo,
Brazil, in 1985 and 1990, respectively, the Ph.D. degree in electrical engineering
from the University of Tennessee, Knoxville, in 1995, and the D.Sc. degree in
mechanical engineering from the University of São Paulo, São Paulo, Brazil, in
1998.
He joined the faculty of the Colorado School of Mines, Golden, in 2000 and
has been working to establish research and education activities in the development of intelligent control for high-power electronics applications in renewable
and distributed energy systems. He authored Renewable Energy Systems: Design and Analysis with Induction Generators (Orlando, FL: CRC Press) and
Integration of Alternative Sources of Energy (New York: Wiley).
Dr. Simões received the NSF—Faculty Early Career Development (CAREER) Award in 2002. He is an Associate Editor for the IEEE TRANSACTIONS
ON POWER ELECTRONICS. He served as the Program Chair for the Power
Electronics Specialists Conference in 2005, as well as the General Chair of the
Power Electronics Education Workshop in 2005. He is the Chair of the IEEE
Power Electronics Chapter of the Denver Section and Chairman of the IEEE
Power Electronics Society Intersociety.
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