Ripple Current Reduction of a Fuel Cell for aSingle-Phase Isolated Converter
using a DC Active Filter with a Center Tap
Jun-ichi Itoh*, Fumihiro Hayashi**Nagaoka University of Technology
1603-1 Kamitomioka-cho Nagaoka City Niigata, Japan
itoh@vos.nagaokaut.ac.jp
Abstract- This paper proposes a ripple reduction method incurrent without using any additional switching devices. The
current ripple that has double frequency component of the power
supply is generated in the DC part when a single-phase PWM
inverter is used for a grid connection. The current ripple causes
short lifetime for electrolytic capacitors, batteries and fuel cells.
The proposed circuit realizes a DC active filter function without
increasing the number of the switching device because the energy
buffer capacitor is connected to the center tap of the isolationtransformer. In addition, the buffer capacitor voltage iscontrolled by the common mode voltage of the inverter. This
paper describes the features of the proposed circuit, control
strategy and experimental results. As a result, about 1/5 times of
the ripple can be reduced.

I.

INTRODUCTION

Recently, green energy sources such as wind power systems,
photovoltaic cells and fuel cells, have been extensively studied
in response to the global warming and environmental issues.
The fuel cell is an important technology for mobile
applications and power grid distribution system since it does
not draining CO2.
A fuel cell system requires a grid interconnection converterto supply power to the power grid. A grid interconnection
converter using an isolation transformer is preferred for power
grid distribution system in terms of surge protection and noise
reduction. Moreover, size reduction and high efficiency arealso required essentially [1-6].
However, one problem with the fuel cell system is the short
lifetime, which is dependent on the ripple current. Therefore,
in order to extend the lifetime, the fuel cell ripple current must
be reduced in the grid interconnection converter [7]. However,when single-phase PWM inverter is used for grid connection
system, the grid frequency happens to have double frequencyof the power ripple. This current ripple prevents long lifetimeof the fuel cell. Therefore, in conventional grid connection
inverters, large electrolytic capacitors are connected in parallelto the fuel cell in order to reduce the ripple. However, thelarge-sized electrolysis capacitor causes big volume andexpensive device cost .
To reduce the current ripple, some methods without using alarge-sized electrolytic capacitor have been proposed. For
example, an active filter is applied in DC link part [8-10]. The
DC active filter consists of a small capacitor as energy buffer,a reactor to reduce the switching ripple and DC chopper. The
DC chopper injects the ripple current to avoid power ripple.
Capacitance can be lower since the terminal voltage of thecapacitor can be widely changed. However, the numbers of
the switching devices increased results the DC chopper is highcost and large volume. Likewise, other configurations of theDC active filter have the similar problems.
This paper proposes a new circuit topology including a DC
active filter function without extra switching devices. The
proposed circuit consists of the isolated DC/DC converter andinterconnection inverter, and achieved the DC active filter
function by using the center tap of the isolation transformer.
Besides, one feature of the proposed converter is that the
primary side inverter in the DC/DC converter is controlled by
the common mode voltage and differential voltage,
individually. The ripple current is suppressed by the common
mode voltage control of the DC/DC converter and the main
power flow is controlled by the differential mode voltage.
At first, this paper introduces the conventional and proposed
circuit topologies with the principle of the current ripple
suppression. Second, the control method of the proposed
circuit is described. In addition, this paper indicates the designof the energy buffer capacitor and transformer which the
maximum power ripple can be accepted. Furthermore,experimental results are shown in order to confirm the validityof the proposed circuit.
II. PROPOSED CIRCUIT CONFIGURATIONS
Figure 1 shows a conventional circuit that consists of thefirst stage inverter for the medium frequency link, antransformer, a diode rectifier, and a grid interconnection
inverter. When the interconnection current and power gridvoltage are sinusoidal waveforms, the instantiations power pof the grid interconnection is obtained by (1) at unity power
factorp = 2 I sin(ωt ) ⋅ 2V sin(ωt )(1)= VI {1 − cos(2ωt )}where I and V are the RMS value of the interconnection
current and the grid voltage, and ω is the grid angularfrequency.

Thus, the instantaneous power has a ripple that is double
frequency of the power grid frequency. To reduce the ripple
power of the DC power source, such as a fuel cell, battery or
photovoltaic cell, large electric capacitors, CDC1 and CDC2, areused in the converter, as shown in Figure 1. The use of large
electrolytic capacitors precludes reduction in size and cost.
Figure 2 shows the other conventional circuit using a DC
active filter, which is constructed with a DC chopper and an
energy buffer capacitor Cf. The capacitor Cf is used as anenergy buffer to absorb the ripple power. The inductor Lf can
suppress the switching current. The voltage of the capacitor Cf
is controlled at the double frequency of the power gridfrequency. As a result, the ripple power does not appear in Vdc,
despite the use of small capacitors CDC1 and CDC2. However,the problem of this method is that the number of the switchingelements had to increase.
Figure 3 shows the proposed circuit that combines the firststage inverter and DC active filter function. The energy buffer
capacitor Cf is connected to the center tap of the medium
frequency transformer. The zero vector of the full bridge firststage inverter is used to control the center tap potential voltage.
In addition, the leakage inductance of the transformer is used
to suppress the switching current instead of Lf.
Table 1 shows comparison of the number of the switching
device and capacitor capacity among the conventional circuit,the conventional circuit with the DC active filter and the
proposed circuit. The proposed circuit does not require an
additional switching device and an inductor, in comparison
with the conventional circuit with the DC active filter. Notethat the current rating of the power device in the first stage
inverter and transformer are slightly larger than that of theconventional circuit because the DC active filter current flows
in the first stage inverter and the transformer.
III. CONTROL METHOD
The first stage inverter in this proposed circuit has two roles
which are performed as a DC/DC converter and a DC active
filter. These roles are achieved by controlling the common anddifferential mode voltage in the first stage inverter. This
chapter explains the principle of the proposed control method
and the design method for the buffer capacitor and thetransformer.
A. Switching pattern generation method
Figure 4 illustrates the two switching modes of the firststage inverter in the proposed circuit. In the differential mode,
the terminal voltage of the transformer is controlled as shownin Figs. 4(a) and (b), and in the common mode, the center tap
voltage is controlled as shown in Figs. 4(c) and (d). In otherwords, the inverter outputs the zero voltage vectors (00 and 11
are two) in common mode operation. When the zero voltage
vectors are selected, the line to line voltage of transformer iszero. However, the center tap voltage is either Vdc or zero,depending on the zero vector of Figs. 4(c) or (d), respectively.
Thus, by controlling the ratio of the zero vectors, the centertap voltage can be controlled. It should be noted that theFig. 1. Conventional circuit.
Power Grid
200V
50Hz

iin
AC-link
10 kHz

Ls

icom
Vdc C

DC1

CDC2

is

Lf
Cf

Fig. 2. Conventional circuit with DC active filter.

Fig. 3. Proposed circuit.
Table1. Comparison device number
and capacitance of capacitor.
DeviceDC Link
NumberCapacitor

Conventional Circuit

8

large

Conventional circuit

10

small

8

small

With DC active filter
Proposed Circuit

output switching pattern must included the zero vector period.
Therefore, the voltage transfer ratio of the first stage inverter
is limited by the DC active filter control. As a result, theterminal voltage of the transformer is decreased.
Figure 5 shows the control block diagram of the proposed
circuit. To suppress the ripple current of the fuel cell, all theripples current are provided by the energy buffer capacitor.
Therefore, the capacitor current command icom* is obtained by
calculating the power ripple. The grid interconnection control
can be applied to the conventional control method, which uses
an automatic current regulator (ACR) for the interconnection
current command.

(a) Differential mode 1(b) Differential mode 2(c) Common mode 1
Fig. 4. Operation modes of the proposed circuit.

(d) Common mode 2

Fig. 5. Control block diagrams.

The DC active filter voltage command vcom* is obtained by
the PI regulator in the current regulator. The differential mode
voltage command vdif* is set to 1 as the maximum value, inorder to obtain the maximum terminal voltage of thetransformer.
The feature of the proposed circuit control is that the DC
active filter voltage command vcom* is added to the differential
voltage command vdif* as the common mode voltage. The
output voltage commands v1* and v2* for each leg in the firststage inverter are obtained by (2).
⎧ * 1
*
*
⎪⎪v1 = 2 (vcom + vdif )⎨⎪v * = 1 (v * − v * )
com
dif
⎪⎩ 2
2

(2)

B.. Design of the buffer capacitor for the DC active filter operation
The buffer capacitor is used as an energy storage element of
the active filter. The capacitor Cf has to absorb the power
ripple for half cycle of the power grid. Thus, the required
storage energy WC is given by (3) from (1) and the capacitor
energy Wf is obtained by (4)
T/4
P
(3)
WC = ∫ Pin cos 2ωtdt = in
0

(

ω

)

1
2
2
(4)
W f = C f V f max − V f min2
where Pin is the input power, ω is angle frequency of thepower grid, Vfmax and Vfmin are the maximum and minimum
voltage of Cf, respectively.

Therefore, the required capacitance Cf is given by (5) from
(3) and (4).
2 Pin
Pin
(5)
Cf =
=
ω ((Vcf 0 + ΔVcf / 2) 2 − (Vcf 0 − ΔVcf / 2) 2 ) ωVcf 0 ΔVcf

where Vcf0 and ΔVcf are the average voltage and the variation
voltage of Cf, respectively.
Figure 6 shows the relation between the capacitance and thevoltage variation ΔVcf of the capacitor to compensate thepower ripple at 1 kW and Vcf0 of 150 V, according to (5). The
capacitance can be reduced greatly by the capacitor voltage
variation.
It is noted that reactor Lf is set to decrease the switchingripple. That is, the reactor Lf depends on the switchingfrequency of the first stage inverter. The leakage inductance ofthe transformer is used as Lf.
C. Design of the transformer.
The major focus in the design for the transformer is thecurrent capacity. This is because the transformer has twofunctions; first is to be a DC active filter and second is to be
an isolation transformer. The transformer current, Itans ,isequaled to the sum of the active filter current Icom and thecurrent Idif which is according to the output power is shown in
(6).
I
(6)
I trans = I dif + com
2
Note that in (6) the active filter current is divided by 2
because the transformer winding is connected in parallel to thewhere Vdc is the DC voltage of the first stage inverter.
Also, the differential mode current Idif is obtained by (10)using (7) and (8).
Pin
Pin
(9)
I dif ==
Vdc Ddif _ max Vdc − ΔVcf
On the other hands, the common mode current Icom is shownin (10) because the maximum value of the power ripple is the
double of the input power.
2P
I com = in(10)
Vdc
Finally, the transformer current Itrans is obtained by (11)from (6), (9), and (10).
I
Pin
P
(11)
I trans = I dif + com =+ in2
Vdc − ΔVcf Vdc

It is noted that Icom in (10) is the peak value of the common
mode current. Therefore, RMS value Itrans(rms) of thetransformer current, which is used to decide the thickness oftransformer winding , is obtained by (12).
I
Pin
P
(12)
+ in
I trans ( RMS ) = I dif + com =2
Vdc − ΔVcf
2Vdc
Figure 8 presents the relation between the transformer
current and the capacitor voltage variation at Vcf0 =150 V, Pin= 1 kW and Vdc = 300 V. As the capacitor voltage variation
becomes large, a smaller active filter capacitor Cf can beachieved. However, as the larger the capacitor voltage
variation becomes, it results the system required a large
current capacity transformer.

Capacitance [μF]

1500
1000
500
0

0

100
200
Capacitor Voltage ΔVcf[V]300Fig. 6. Required capacitance for power ripple compensation.
(Pin:1 kW,Vdc:300 V,Vcf0:150 V)

V

V
I

I
t

*

=0

t

*

= 0.3 p.u.

Fig. 7. The current and voltage wave pattern by the difference of thecommon mode duty ratio at the constant power.
100
Trans Current Itrans[A]where Dcom is the duty ratio for the common mode voltage.
The duty ratio for the common mode voltage is obtained by(8) because the average voltage Vcf0 of Cf is half of the DC
voltage.
ΔV cf
ΔV cf
D com ==
(8)
2V cf 0
V dc

2000

80
60
40
20
0

0

100
200
Capacitor Voltage ΔVcf[V]300Fig. 8. The current capacity of the transformer which is necessary forcompensation (Pin:1 kW, Vdc:300 V ,Vcf0:150 V)
2000
Capacitance [μF]center tap in the common mode circuit as shown in Figs.4 (c)and (d).
The common mode voltage controls the capacitor voltage
variation and for the differential mode voltage, it controls thetransmission power to the power grid. The common mode
voltage should be changed widely from the viewpoint ofcapacitance suppression as shown in (5). However, the periodof the differential mode becomes short when the common
mode voltage is widely changed.
Figure 7 shows the differential mode current waveforms andthe terminal voltage waveforms of the transformer in the caseof common mode command is 0 and 0.3 p.u respectively,
given at the same output power. The differential mode current
increases when the common mode voltage increases, because
the power transmission period is shaved off by the common
mode voltage. Therefore, the duty ratio Ddiff for thedifferential mode can be constrained by (7).
D dif + D com = 1
(7)

1500
1000
500
0

0

20
40
60
Trans Current Itrans[A]

80

100

Fig. 9. The transformer current capacity and capacitance which isnecessary for compensation.(Pin:1 kW, Vdc:300 V,Vcf0:150 V)

In addition, the relation between the transformer current andthe capacitance is represented by (13) from (5), and (11).
Pin ( Pin − Vdc I trans )
(13)
Cf =
ωVcf 0Vdc (2 Pin − Vdc I trans )Figure 9 will further explain the relation between thetransformer current and the capacitance Vcf0 = 150 V, Pin = 1
kW and Vdc = 300 V. As can see from Fig. 9, a choice ofpicking up a smaller capacitance results a larger current
capacity transformer is chosen. For example, for a 1 kW
system, if a 150 μF capacitor is chosen, then the transformer
which has current capacity of more than 10A needs to beconsidered. In other words, the proposed method requires a fat
winding transformer in comparison with that of theconventional circuit.
Note that the number of turns in a winding for thetransformer is calculated from (14) as same as theconversional transformer.
V(14)
N = dc4 fSB
where, N is the number of turns in a winding, f is theswitching frequency of the first stage inverter, B is the flux
density of the core, S is the section area of the core.

Table 2. Experimental parameters.
Output power
1 kW
Grid frequency
50 Hz
Grid voltage
200 V
AC Link frequency
10 kHzActive filter inductor
5 mH
Energy buffer capacitor
440 μFDC link capacitor
110 μF(Proposed circuit)
DC link capacitor
2200 μF(Conventional circuit)∑I=

0 DC Input Current

5 A/div

Grid Voltage

500 V/div

Grid Current

10 A/div00
10 ms/div

Fig.10. Operation waveforms of the conventional circuit
without a DC active filter.

0

DC Input Voltage

250 V/div

DC Input Current

5 A/div

0
Grid Voltage

500 V/div

Grid Current

10 A/div00
10 ms/div
Fig.11. Operation waveforms of the proposed circuit.

2
n

(15)
I DC
where In is harmonic components and Idc is DC component.
The major harmonic component in the input current is 100
Hz. In a conventional circuit, the DC input current THD
increases accordingly to the increment of the output power. In
contrast, the DC input current THD decreases despite theincrement of the output power in the proposed circuit. That is,
I DC _ THD

250 V/div0

IV. EXPERIMENTAL RESULTS
The proposed converter was tested under experimental
conditions which are shown in Table 2 to confirm the validityof the proposed circuit operation. The auxiliary inductor isconnected to the center tap of the transformer, because theleakage inductance of the transformer is not sufficient to
reduce the switching ripple current.
Figure 10 shows the operation waveforms of theconventional circuit without the DC active filter. The
sinusoidal grid current waveform and unity power factor are
obtained; however, the DC input current has a large ripple
current component of 100 Hz.
Figure 11 shows the operation waveforms of the proposed
converter. The ripple of the DC input current is suppressed to
20% of that with the conventional circuit, indicating that theDC active filter function is effective.
Figure 12 shows the operation waveforms of theconventional circuit using a large electrolytic capacitor of
2200 μF. This circuit was tested in order to determine thereduction in the DC ripple current by using a large electrolytic
capacitor. Although a large electrolytic capacitor, which is 20
times that shown in Figure 11, is used, the DC input current
did not reduce significantly.
Figure 13 shows the DC input current total harmonic
distortion (THD) of the conventional and proposed circuit,which is defined by (14).

DC Input Voltage

Fig.12. Operation waveforms of the conventional circuit with alarge electrolytic capacitor CDC2=2200 μF.

Input Current THD [%]Therefore, these experimental results confirmed that the
proposed converter is valid for the reduction of the DC input
ripple current in the DC power supply, without the need oflarge electrolytic capacitors.
V. CONCLUSIONS

Output Current THD [%]Fig.13. Input DC Current distortion factor

A novel single-phase isolated converter was proposed forgrid interconnection application. The ripple current in a DC
power supply, such as a fuel cell, battery or photovoltaic cell,
can be reduced by a proper operation of the DC active filter.
The main feature of the proposed circuit is that it does not
require additional switching devices, because the zero vectorof the first stage inverter is controlled as the DC active filter.,
A 1kW prototype was constructed based on the proposed
circuit and the experimental results were obtained as follow,
1) The ripple current can be decreased to 20% lower than aconventional circuit.
2) The proposed circuit shows a degree of effectiveness in ahigh power application.
3) The requirement of the total electrolytic capacitor in thesystem decreases to 1/4 times.

4) The DC active filter operation in the proposed method circuit
does not interference the grid interconnection current control.
Fig.14. THD of Grid connection Current.

In future, the optimization of the transformer andconstructing a high power prototype will be carried out.

0

REFERENCES[1]

0

Fig.15. Efficiency and Grid connection power factor.

the proposed circuit is suitable for the high power application
due to effective in high output power region.
Figure 14 shows the THD of the grid interconnection
current of the conventional and proposed circuit. Almost thesame current THD values were obtained. The proposed circuit
can achieve the same performance level as the conventional
does.
Figure 15 shows the efficiency and the grid interconnection
power factor of the conventional and proposed circuit. The
efficiency of the conventional circuit is higher than the
proposed circuit. One of the reasons power loss had increased
is because of the increasing current in the transformer.
Therefore, the efficiency of the proposed circuit can be
improved if the design of the transformer has been optimized.
Note that the proposed converter has good performance as thegrid interconnection converter because both power factors of
the proposed circuit and the conventional circuit were 99%.

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