Battery Energy Storage System with Isolated
Single-phase Matrix Converter using Center-tapped
Transformer for Power Decoupling Capability
Nagisa Takaoka

Jun-ichi Itoh

Dept. of Electrical Engineering
Nagaoka University of Technology
Nagaoka, Niigata, Japanntakaoka@stn.nagaokaut.ac.jp

Dept. of Electrical Engineering
Nagaoka University of Technology
Nagaoka, Niigata, Japan
itoh@vos.nagaokaut.ac.jp

Abstract—This paper discusses a bi-directional isolated singlephase matrix converter using center-tapped transformer, which
has a power decoupling capability for battery energy storage
systems for renewable energy sources such as PV and wind turbine
systems. The proposed converter is comprised of a full bridge
inverter, a high frequency transformer and a matrix converter inorder to eliminate large DC-link smoothing capacitor. The
power decoupling method employs a center-tapped transformer
and a small capacitor without any additional switches, which aims
to achieve high efficiency. In this paper, by harmonic analysis, theeffectiveness of power decoupling capability is verified in twocases: discharging/charging modes of a battery, which need tokeep output power constant from battery for a single-phase grid
system. As a simulation and experimental result, the validity of thepower
decoupling
capability
isconfirmed
inacharging/discharging mode. In addition, the proposed power
decoupling method reduces the DC bus current ripple generated
by a single-phase load to 85.1% in the discharging mode ofexperiments.

converter is expected to achieve higher efficiency, smaller sizeand longer life-time compared to the conventional rectifierinverter system. However, a matrix converter cannot absorb
power ripple which is generated by a single-phase grid becauseof no energy buffer in its topology. As a result, a power
decoupling capability using small passive components isrequired to the isolated DC to single-phase AC converter inorder to remove the power ripple component.

Keywords-component; matrix converter; power decoupling; bidirection converter;I.

INTRODUCTION

Recently, photovoltaic (PV) and wind turbine systems haveactively researched in terms of furtherance for renewable energy[1-4]. Energy storage systems such as using batteries are
required for power leveling because renewable energy sources
generate power fluctuation depending on natural conditions. The
DC to AC converter needs isolation between the battery and theAC equipment for safety. Therefore, a bi-directional isolated DC
to single-phase AC converter is required. Downsizing, long lifetime and high efficiency are required for the isolated DC tosingle-phase AC converter. A conventional converter
constructed by a full bridge inverter as a primary converter, ahigh frequency transformer, and a secondary converter
composed of a diode rectifier and an inverter, has been proposedin [5]. However, a bulky electrolytic capacitor is required in thesecondary converter to smooth DC-link voltage. Therefore, thiscircuit topology restricts downsizing and high efficiency.
On the other hand, a matrix converter has attracted a lot of
attentions because of no energy buffer [6]. Thus, the matrix

In last few years, some circuit topologies with the power
decoupling capability have been proposed [7-9]. However, these
papers employ not only passive components but also additionalswitching devices for the power decoupling. On the other hand,in order to achieve high efficiency, a power decoupling method
using a center-tapped transformer and a small capacitor has beenpresented [10]. This method does not use additional switches
and suppresses the ripple component in the input current
significantly. However, this circuit still restricts downsizing,
long life-time and high efficiency because [10] deals with theisolated DC to single-phase AC converter by using a diodebridge rectifier and a PWM inverter as described previously.
The authors have already proposed the power decoupling
method for the isolated single-phase matrix converter with acenter-tapped transformer in [11]. The proposed system consistsof a full bridge inverter, a center-tapped transformer, a small
capacitor a matrix converter. The proposed converter does not
require extra switching devices for the power decoupling, owing
to the center-tapped transformer. In contrast, a Pulse Density
Modulation (PDM) which is appropriated to convert a highfrequency voltage to a sinusoidal voltage at 50 Hz is applied tothe matrix converter. In order to suppress fluctuation of an
output power, this proposed circuit needs a bi-direction
operation to charge an energy flow between a single-phase grid
system and a battery for PV system. However, the bi-directional
operation of the proposed circuit is not discuss in past works.
This paper demonstrates the bi-directional operation of the
proposed power decoupling method for the isolated single-phasematrix converter with a center-tapped transformer. In addition,
this paper clarifies the validity of the proposed power decoupling
method at mode transition of an energy flow between a singlephase grid system by simulations and experiments.

This paper is organized as follows; first, the circuit
configuration of the proposed DC to single-phase AC converter
and the modulation method of the matrix converter at secondaryside are described. Second, the principle of the power
decoupling is described. Finally, the fundamental operation
waveforms of the proposed system with discharging/charging
operation modes are considered by simulations and experiments.
II.

CIRCUIT TOPOLOGY AND MODULATION METHOD

A. Circuit Toporogy
1) Conventional circuit
Fig. 1 shows the conventional isolated DC to single-phase
AC converter. The conventional circuit comprises of a fullbridge inverter, a high frequency transformer and a rectifierinverter system. The full bridge inverter outputs a square voltage
at high frequency in order to reduce the volume of thetransformer. The secondary rectifier converts the high frequency
voltage to a DC voltage and the PWM inverter control the load
current with a feedback control. When the load current issinusoidal waveform and achieves the unity power factor, an
instantaneous output power pout is expressed by (1).

p out  2V grid sin o t   2 I grid sin o t  V grid I grid 1  cos 2 o t   Pout 1  cos 2 o t 





where, Vload is the load voltage (RMS), Iload is the load current(RMS), Pout is the output average power and ωo is the output
angular frequency. A ripple component shown in the second
term of (1) should be bypassed in order to obtain a constant DC
current in the DC bus. Hence, this system has to adopt a balky
electrolytic capacitor Cdc to absorb the power ripple.
2) Proposed circuit
Fig. 2 shows the isolated single-phase matrix converter usinga small LC buffer. The DC bus voltage Vbus is expressed thebattery. The matrix converter is employed as a secondary
converter in order to eliminate the DC-link capacitor which isused in conventional circuit. In addition, a center-tapped
transformer, whose role is to link the full bridge inverter not only
to the matrix converter for isolation, but also to the power
decoupling which is used to reduce the DC bus current ripple, isemployed. A buffer circuit including a buffer capacitor Cbuf anda buffer inductor Lbuf is used to absorb the power ripple. It should
be noted that the charge and discharge to compensate the power
ripple is implemented at the buffer capacitor Cbuf. In order toyield a DC bus current without the ripple, the relationshipbetween the instantaneous output power pout and the charged
power pbuf is defined as (1), assuming that the load current issinusoidal waveform and achieves the unity power factor.
B. Modulation Method of Matrix Converter at Secondary Side
Fig. 3 shows the control block diagram of the proposed
converter. These current and voltage controls in the proposed
method are implemented by PI controllers. The full bridge
inverter independently provides a differential mode voltage toexcite the transformer and a common mode voltage tocompensate the power ripple [7]. The buffer current control isfor the power decoupling. As shown in Fig. 3, the buffer current
control is applied to vary the voltage of the buffer capacitor vCbuf

which is used to absorb the power ripple caused by the singlephase grid. Moreover, the buffer current reference ibuf* and theoutput current reference igrid* are decided by the direction of thepower flow in order to achieve the bidirectional power flow
operation. Besides, the modulation of the matrix converter isdiscussed in the next chapter.
The matrix converter used as a secondary converter in the
proposed system applies the PDM (Pulse Density Modulation)of [11-13]. The PDM is suitable to the direct AC to AC converter
which provides a sinusoidal voltage at a commercial frequency
from a high frequency voltage, such as the proposed system. The
PDM treats the input voltage waveform at high frequency as apulse and synthesize a grid voltage with a density of the input
voltage pulses. A half cycle of the input voltage pulses are usedas the minimum unit of the grid voltage waveform. Then, thematrix converter switches in zero voltage period of the input
voltage waveform. In consequence, the matrix converter
achieves Zero Voltage Switching (ZVS) which results indecreasing a switching loss. Thus, the PDM is applied to thematrix converter in this paper.
III.

POWER DECOUPLING METHOD

When the DC bus as battery energy storage systems isconnected to power grid distribution systems, DC bus current
includes a ripple component at twice of the grid frequency. In
this chapter, the principle of the power decoupling for power
Ibus
LLPF

Ldc

igrid

N1 N2
Lout

Vbus

Cdc

CLPF

vgrid

vout

Grid
Full bridge Inverter

Diode rectifier

PWM inverter

Figure 1. Conventional isolated DC to single-phase AC converter.
Conventional converter uses bulky electrolytic capacitor Cdc to absorb
power ripple caused by single-phase load.
Energy Flow ofDischarging Mode
Ibus

Energy Flow ofCharging Mode

LLPF
S1ap

S1bp

N1 N2

S2ap

S2bp

Lout

ibuf

Vbus

igrid
vgrid

vout

CLPF
S1an

S1bn

Full bridge Inverter

Lbuf
Cbuf V
Cbuf
Buffer circuit

Grid
S2an

S2bn
Matrix converter

Figure 2. Proposed isolated DC to single-phase AC converter. Secondary
converter consists of matrix converter in order to eliminate DC-linksmoothing capacitor.

flow where the battery is charged/discharged mode is explained.

ibuf*
vCbuf_ave*vdif*Normalization of buffer voltage referencePI

Fbuf (s)Kp_bufvbuf*

1+sTi_buf

1
Vbus

sTi_buf
PI controller

FiltervCbuf

ibuf

Average voltage control for
buffer capacitor

Buffer current control

vcom*
*

2 |vcom |
1

igrid
vgrid

Power flow
direction2

Kp_out*1+sTi_out vLout

sTi_out
PI controller

ibuf*

Detection of output power

Compensation of
secondary voltage
fluctuation of transformer

N2
Vbus
N1

Current control of output filter inductor
igrid*

S1ap
S1an
S1bp
S1bn

Modulation offull bridge
inverter

1/x

vout*

S2ap
S2an
S2bp
S2bn

Modulationof matrix
converter

vout**

vgrid

igrid

Figure.3. Control block diagram of proposed converter. Buffer current control is for power decoupling. Output filter current control is applied as minor loopof output filter capacitor voltage control that is originally required. By PLL, direction of power flow decides amplitude and phase which is assigned to output
current reference and buffer current reference.

A. Discharging Mode of Battery
In this section, the principle of the power decoupling when
the battery is discharged is explained as shown in Fig. 2.

t

2

Vbus
Pout
vCbuf * sin 2o t  4
oCbuf

where, Vbus is an average DC bus voltage, Vbus/2 is an initial
voltage of Cbuf. Finally, ibuf* is derived by using an output powerreference Pout*.
*

ibuf  Cbuf



dvCbufdt*

Pout cos 20 t 
*



2

pout

chargedischarge＝

where, t0 is a start time of operation. The buffer capacitor voltage
vCbuf which needs to absorb the power ripple is derived from (2).


＋

＝

＝

pbuf

＝


t0

t

0

discharget

iload  2 I load sin o t 

t

0

t

0

iload

charget  Pout cos 2o dvload  2Vload sin o t 

0

ibuf
t

0

vload

＋

Ibus

pbus

dvCbuf d
WCbuf   vCbuf ibuf d   vCbuf  Cbuf
t0
t0
d 

t

t

0

＋

Fig. 4 shows the relationship among the DC bus power, an
instantaneous output power pout and the power decoupling with
Cbuf. The feature of Fig. 2 is that the buffer current control tofluctuate the voltage of buffer capacitor vCbuf absorbs the power
ripple which is caused by the single-phase load. First, the buffer
capacitor energy WCbuf is presented by using (1) and a voltage–current equation of a capacitor.

vCbuf
VCmax
VCmin

Vbus

*

Vbus
P
 out sin2o t 4o Cbuf





t

0

Input powerpbus [W]t0

Buffer powerpbuf [W]0tOutput power
pout [W]Figure.4. Principle of power decoupling with buffer capacitor indischarging mode. Buffer power to compensate power ripple is charged ordischarged at Cbuf. As a result, DC bus current without ripple component isobtained.

The power ripple in discharging mode is compensated byusing the buffer current control with the current reference ibuf*
as shown (4).
B. Charging Mode of Battery
In this section, the principle of the power decoupling whenbattery is charged in order to build up the power from a singlephase grid system is explained. The power flow in this mode isreverse from that in the above session as shown in Fig. 2.
Fig. 5 shows the relationship among the DC bus power, an
instantaneous output power pout and power decoupling in a

pbuf

pbus
t

0

DC bus voltage

t

0

0

＋ Charge
Input power
Pbus [W]TABLE Simulation Condition

pout

Discharge＝

Buffer powerpbuf [W]Output power
pout [W]Figure 5. Principle of power decoupling with buffer capacitor in charging
mode to battery at DC bus side.

charging mode. A buffer current control to fluctuate the voltageof buffer capacitor vCbuf absorbs the power ripple caused by thesingle-phase load which is reverse compared to the discharging
mode as described previously. Therefore, the buffer power pbuf
which needs to absorb the power ripple of the charging mode isexpressed by (5) using the power flow direction as shown in Fig.
2.

pbuf  Pbus  pout  Pout cos2o t 





2

vCbuf * 

Vbus
Pout

sin 2o t  4o CbufIn this way, the buffer capacitor voltage vCbuf which absorbs
the power ripple of the charging mode is decided by the power
ripple and the power flow. The buffer capacitor voltage can bechanged to each power flow in detecting the polar of output
power as shown in Fig. 4 and Fig. 5.
IV.

SIMULATION AND EXPERIMENTAL RESULTS

Table 1 shows the simulation condition to verify thefundamental operation of the proposed converter with thedecoupling method as drawn in (3) and (6). In should be noted
that, in simulation was done an ideal condition without an input
filter and a dead-time.
A. Simulation Results in Discharging Mode
Fig. 6 shows the input and the output waveforms in the
discharging mode of a steady state. Fig. 6 (a) and (b) show
results without/with the power decoupling method. The DC bus
current is filtered by a LPF with a cut-off frequency of 1 kHz inorder to remove a switching ripple. As shown in Fig. 6 (a), theDC bus current has a ripple component at 100 Hz caused by thesingle-phase load. The current ripple component at 100 Hz is101% with reference to the average current. In contrast, thecurrent ripple component is reduced in (b), owing to the power
decoupling method. As a result, the ripple of the DC bus current
is suppressed by 4.37%.
B. Simulation Results in Charging Mode
1) Steady stateFig. 7 shows the input and the output waveforms in the
charging mode at steady state. Fig. 7 (a) and (b) show results

Grid voltage

100 Vrms

Rated power

1kW

Grid frequency

50 Hz

Buffer L (Lbuf)

2.0 mH

Buffer C (Cbuf)

200 mF

Grid connectedinductor (Lout)

2.0 mH(6.3%)

Turn ratio oftransformer N2/N1

0.5

Carrier frequency ofCarrier frequency of
100 kHzfull bridge inverter
matrix converter
Natural angular
Natural angularfrequency of buffer 4000 rad/s frequency of gridcurrent control
current control

400
200
0
6
4
2
0

DC bus voltage Vbus [v]

4000 rad/s

400
200
0
6
4
2
0

100
0
-100
20
0
-20

10 kHzDC bus current filtered with LPF Ibus [A]Grid voltage vgrid [V]

where the buffer capacitor voltage vCbuf is derived from (6).


380 Vdc

Grid current igrid [A]

100
0
-100
20
0
-20
10ms/div.

(a)Without power
decoupling method.

(b) With power
decoupling method.

Figure 6. Input and output waveforms in discharging mode at steady state.
Proposed power decoupling method reduces DC bus current ripple by
95.7% in discharging mode.

400
200
0
0
-2
-4
-6

DC bus voltage Vbus [v]DC bus current filtered with LPF Ibus [A]Grid voltage vgrid [V]

0
-2
-4
-6

100
0
-100

100
0
-100
20
0
-20

400
200
0

Grid current igrid [A]

20
0
-20
10ms/div.

(a)Without power
decoupling method.

(b) With power
decoupling method.

Figure 7. Input and output waveforms in charging mode at steady state.
Proposed power decoupling method reduces DC bus current ripple by
82.3% in charging mode.

without/with the proposed power decoupling method. As shownin Fig. 7 (a), in the charging mode, the DC bus current has aripple component at 100 Hz caused by the single-phase load in
an opposite direction compared with the discharging mode asshown in Fig. 6(a). As a result, without the power decoupling
method in the charging mode, current ripple component at 100

Hz is 102% in reference to the average current. In contrast, theproposed power decoupling method reduces the DC bus current
ripple by 13.0%. The reason, which the effectiveness of power
decoupling is difference between the discharging/charging
modes is an effect with the phase of the grid current by PDM.
2) Mode transition
Fig. 8 shows the input and the output waveforms in the modetransition which is changed in charging/discharging mode withthe proposed power decoupling method. As a simulation result,the proposed power decoupling method can achieve tocompensate the DC current ripple at constant, when the
proposed system is changed in discharging/charging mode.
Besides, surge current which is generated by point of changing
power flow is compensated by an input LC filter because thesurge current is enough low. Moreover, between a battery andproposed circuit is isolated. Hence, the effect on surge current toa battery is almost nothing.
C. Experimental Results in Discharging Mode
An experimental setup is demonstrated in order to validate
the proposed system. It should be noted that a LC filter isinserted in the DC bus side.
Fig. 9 shows the experimental waveforms of the proposed
converter with Vbus of 350 V, vgrid of 100 Vrms and an R-L loadof 500 W. Fig. 9 (a) shows a result without the power decoupling
method and (b) shows a result with the method. As shown in Fig.
9(a), the buffer capacitor voltage does not vary because the fullbridge inverter does not output the common mode AC voltage.
As a result, the DC bus current has ripple component at 100 Hz.
In contrast, the proposed power decoupling method provides thecommon mode AC voltage to fluctuate the buffer capacitor
voltage. In consequence, the power ripple component in the DC
bus current is decreased. Besides, the grid voltage THD in (b) is
2.17%.
Fig. 10 shows the harmonic analysis of the DC bus current.
It should be noted that the harmonic number is based on the
output frequency of 50 Hz. From the result, it is understood thatwithout the power decoupling method, the ripple component at
100 Hz is 62.6% with reference to an average current, whereas
the proposed power decoupling reduces the DC bus current
ripple by 85.1%. As a result, these experimental results verified
the effectiveness of the proposed power decoupling in the
discharging mode.
V.

400
200
0
5
0
-5
100
0
-100
20
0
-20

DC bus voltage Vbus [V]DC bus current filtered with LPF Ibus [A]Grid voltage vgrid [V]Grid current igrid [A]

20ms/div.

Power flow is changed fromdischarging mode to charging mode

Power flow is changed fromcharging mode to discharging mode

Figure 8. Input and output waveforms in discharging mode during mode

DC bus current Ibus 5 A/div

00

Buffer current ibuf 5 A/div
Buffer capacitor voltage VCbuf 100 V/div0

Output voltage (LPF) vout 100 V/div(a)4 ms/div

Without power decoupling method.

DC bus current Ibus 5 A/div
Buffer current ibuf 5 A/div
00

Buffer capacitor voltage VCbuf 100 V/div

CONCLUSION

This paper discusses a bi-directional isolated single-phasematrix converter using center-tapped transformer, which has thepower decoupling capability for battery energy storage systems.
The proposed converter eliminates a bulky electrolytic capacitor
because a matrix converter is employed as a secondary converter.
The power decoupling method employs a center-tapped
transformer and a small capacitor without any additionalswitches. In this paper, the effectiveness of the power
decoupling capability is also confirmed in discharging/charging
modes of a battery by simulations as first step. In simulation
results, the proposed power decoupling method reduces the DC
bus current ripple by 82.3% in the charging mode. In addition,
in experiments, the power decoupling method reduces the DC

0

Output voltage (LPF) vout 100 V/div(b)

4 ms/div

Without power decoupling method.

Figure 9. Experimental waveforms in steady state. DC bus current withproposed power decoupling method do not have little harmoniccomponent.

bus current ripple generated by a single-phase load to 85.1%
during the discharging mode.

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Without the power decoupling method

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