Basic Investigation of Three-Phase to Single–PhasePower Converter using Modular Multilevel Converter Topology
Toshiki Nakanishi＊

Jun-ichi Itoh＊*Nagaoka University of Technology
Abstract:- In this paper, an AC-AC direct type power converter that uses a Modular Multilevel Converter (MMC) topology isproposed for medium voltage application. The proposed system can convert a three-phase AC power supply into a high frequency
AC voltage without DC link stage. Moreover, the proposed system can control the input current and capacitor voltage in each cellwhich is consisting of a DC capacitor and a H-bridge converter. The simulation results show that the proposed system can directly
convert a three-phase AC power supply of 200 V, 50 Hz into a high frequency AC voltage of 360 V, 1-kHz. In addition, theproposed system can obtain low harmonic components of sinusoidal input current and maintain the cell capacitor voltage atconstant closed to the capacitor voltage command 200 V.
Keywords : Direct Power Conversion , Modular Multilevel Converter , Capacitor voltage control , High frequency output

1.

vcAr1

vAr1

Introduction

Recently, the modular multilevel converters (MMCs) areactively being researched in order to apply a power converter tomedium voltage such as 3.3 kV or 6.6 kV without hugetransformers. Besides, the MMCs can reduce the harmonic
distortions in the output voltage due to use of many voltage level.
Therefore, the MMCs topology can be applied in the high power
converters; such as STATCOM and medium adjustable speed
drive systems [1]. However, three-phase to single-phase AC-AC
direct converters with the MMC topology has not so many
reported. This converter can be used as front stage of an isolated
AC-DC converter.
This paper proposes a novel three-phase to single-phase AC-AC
direct power conversion system using MMC which can convert
commercial frequency AC voltage into high frequency voltage.
This paper will show the simulation results of the proposed systemwith the output voltage of 360 V, 1-kHz AC. Moreover, theproposed system evaluates the harmonics of the input current
waveform and the fluctuation of the cell capacitor voltages.

A vArn

vcArn

iAr

vr
vsvt

vrs
vss
vts

vcAsn

vcAtn

iAs

LbiAt

LbLbirisitiBr
BvoSa+LoadSb+

vhkm
h=A,B

iBs Lb iBt Lb
vcBr1
vcBs1
vcBt1

LbvBr1

CELLvBrn

CELLvcCSb－

Sa－

k=r,s,tm=1,2,… nvcBrn

vcBsn

vcBtn

(a) Main circuit(b) H-bridge cellFig. 1. Circuit configuration of three-phase to single-phasedirect power converter using MMC.
Voltage Average Controlvc*
Eq.(2)+n2

PI－3Kc－PI+－Group A : +Group B : －-vd*－PI ++-vq*

dq
rst 3+－+vkm*

k=r,s,tm=1,2… n/22nωLbωLb+－+rst
dq

k=r,s,tvckm+

iq* = 0Eq.(1)+

id*－vc_ave

Circuit configuration

E+－1nvO*
Input Current Control

2.1 Main Circuit
Fig. 1(a) shows the main circuit
configuration of the proposed three-phase to single-phase direct
power converter using MMC. Each leg consists of two
interconnection reactors Lb and H-bridge cells. Then, each of theleg is connected to a load in parallel. Due to cascade connection ofcells, the proposed converter can achieve a multi-level voltage
waveform and also reduce the rated voltage of each cell. Thus,
many cascaded cells are used in practical because it can reduce
harmonic distortion and utilize with a low voltage rating devices.
2.2 H-bridge Cell
Fig. 1(b) shows a configuration of
H-bridge cell. Each cell consists of four IGBTs and a DC capacitor.
Furthermore, the output voltage of the cell and capacitor voltage
are controlled by a pulse width modulation (PWM).

3.

vcAt1

CELL

ik

2.

vcAs1

CELL

Balance Control

k=r,s,tm=1,2… n/2

Fig. 2. Control block diagram of three-phase to single-phasedirect power converter using MMC applied to each arm groupof the features in the proposed system is to control each arm groupof A and B as shown in Fig. 1(a). The control block diagram isseparated to the capacitor voltage control block and the input
current control block. Moreover, the capacitor voltage control
block uses two controls, namely an average control and a balancecontrol. [2]
3.1 Average Control
The average control corrects theerror between the average value of the capacitors voltage on eacharm group and the command voltage with a PI control. The
average value of the capacitors voltage vc_ave is given by (1)Control Strategyv c _ ave Fig. 2 shows the control block of the proposed converter. One1

1 n/2

 v

3 m 1crm

 v csm  v ctm  ................................. (1)Table.1. Simulation parameters

The output of the PI controller is connected to the active current
controller in the input current control block because the capacitor
voltages are adjusted by active current id.
3.2 Balance control
The balance control is used to keep
the voltage in each capacitor voltage at constant and same value.
The average value of all capacitor voltage is controlled to beconstant by the average control. However, unbalance voltage
which occurs among the capacitors cannot be controlled by theaverage control only because the average control corrects only theerror between the average value of the capacitors voltage on eacharm group and the command voltage. Therefore, the balance
control is used to correct the error between each capacitors voltage
and the command voltage.
3.3
Input Current Control
The input current control
is implemented by PI controllers to correct the error between input
current command and detected value, and the compensators forcross terms of id and iq which are provided by the transformation
to the rotating frame. The input current control is separated into
the active current control given by the average control and thebalance control, and the reactive current control. Then, thereactive current iq* is set to zero because the reactive current does
not contribute to the control over capacitor voltage.
3.4
Capacitor Voltage
The output voltage commandvo* that uses to obtain the high frequency output voltage is added
to the output of the input current control block. The change ofvoltage value in each cell depends on the number of cells at eachleg since the cells are connected to the load in series. In addition,
each capacitor voltage also depends on the input and output
voltage. The capacitor voltage command vc*is given by (2)1*vc   2n 

Input voltage (line to line)
Input voltage frequency
Output maximum voltage
Output voltage frequency
Carrier frequency

Cell (per one leg)
DC Capacitor
R loadInterconnection reactor

Input phase voltage vr, vs, vt [V]200vrvsvt

42 mF
100 Ω2 mH

10[msec]

1000-100-200
Input current ir, is, it [A]6ir
is
420-2-4-6it

Fig. 3. Waveforms of input voltage and input current
202.0
201.5
201.0

Capacitor voltage vcAr ,vcBr [V]vcAr1, vcBr1

200.5
200.0
199.5
199.0
198.5
198.0

vcAr2, vcBr2

20[msec]

Fig. 4. Waveform of capacitor voltageE  V O  ................................................. (2)32400

Output voltage vo [V]200

where E is an effective value of the input line to line voltage, Vo isa maximum value of output voltage command and n is the numberof cells at each leg. From (2), the output voltage of each cellbecomes smaller as the numbers of cells. Therefore, the capacitor
voltage command can be set lower.

4.

200Vrms
50 Hz
360 V
1 kHz
9 kHz

vo*

1[msec]vo0-200-400

Simulation Results

Fig. 5. Waveform of output voltage
output voltage is caused by switching. The ripple can be reduced
by applying larger interconnection reactor Lb and increasing thecarrier frequency.

Table. 1 shows the simulation parameters. Switching devices
are defined as ideal elements. The output voltage command issquare wave.
Fig. 3 shows the simulation results that demonstrates the input
voltage and input current. From Fig. 3, it is confirmed that theunity power factor can obtain in the input stage. Moreover, thetotal harmonic distortion (THD) of input current is about 1%.
Fig. 4 shows the waveform of the capacitor voltage. From Fig.
4, the error of capacitors voltage within a same arm occurs.
However, a ratio of maximum error against command voltage isabout 0.31%. Thus, the error does not affect the circuit operation.
Moreover, the maximum ripple factor of each capacitor voltage is
obtained 0.47 %.
Fig. 5 shows the waveform of the output voltage. From theresult, it is confirmed that the high frequency AC voltage can beobtained. The time constant of the rising and falling of output
voltage depends on the inductance of the interconnection reactors
and load resistance. In particular, the equivalent inductance is 2/3times as large as the interconnection reactor Lb because each leg isconnected to the load in parallel. In addition, the ripple of the5. Conclusion
The proposed direct power converter using a modular
multilevel converter topology can directly convert a three-phase
AC power supply into 360 V, 1-kHz AC voltage. Moreover, theproposed system can achieve that the THD of input current isabout 1% and the maximum ripple factor of each capacitor voltage
is 0.47 %. In the future work, the control method to correct theerror of capacitors voltage within a same arm with a larger
numbers of cells will be discussed.
References[1] M. Hagiwara, K. Nishimura, H. Akagi : “A Medium-Voltage Motor Drive
With a Modular Multilevel PWM Inverter”, IEEE Transactions on PowerElectronics, Vol.25, No.7, pp.1786-1799 (2010)[2] Y. Hayashi, T. Takeshita, M. Muneshima, Y. Tadano, and K. Ogura : “Control
System of Modular Matrix Converter”, JIASC, pp.I-237–I-240 (2012)2