Evaluation of Step-down Three-Phase AC-DC Converter
using Modular Multilevel Converter Topology
Toshiki Nakanishi＊

Jun-ichi Itoh＊*Nagaoka University of Technology
Abstract:- In this paper, a step-down AC-DC converter that uses a Modular Multilevel Converter (MMC) topology is proposed
for medium voltage application. The conventional MMC cannot convert medium voltage such as 3.3 kV or 6.6 kV to the low
voltage of 1 kV or less. The proposed system can achieve the step-down operation by applying of MMC using H-bridge cells.
Moreover, the proposed system can control the input current and keep the capacitor voltage of H-bridge cell constantly. Thesimulation results demonstrate that the proposed system can convert a three-phase AC voltage of 6.6 kV into a DC voltage of 1 kV.
In addition, the proposed system can obtain the low total harmonic distortion (THD) in the input current which is approximately
0.2% and maintain the cell capacitor voltage at constant close to the capacitor voltage command 2 kV.
Keywords : Modular Multilevel Converter , Capacitor Voltage Control , High Frequency Transformer , H-bridge Cell

1.

Introduction

vcAr1

vAr1vcAs1

vcAt1

cellA

Recently, modular multilevel converters (MMC) are actively
being researched in order to apply a power converter to medium
voltage such as 3.3 kV or 6.6 kV without commercial gridfrequency transformers[1]. The MMC can reduce the harmonic
distortions in the output voltage due to use of many voltage level.
Besides, the MMC is being researched as the three-phase AC-DC
converters in order to supply the DC voltage of 1 kV or less to theelectrical load from medium voltage in the DC distribution
network [2]. However, in the general AC-DC converter using theMMC which is consisted of chopper cells, it is difficult to achieve
the step-down conversion.
This paper proposes a step-down novel AC-DC converter using
MMC which is consisted of H-bridge cells in order to convert
commercial-frequency AC voltage of 6.6 kV into the DC voltageof 1 kV. This paper shows the simulation results of the proposedsystem with the output voltage of 1 kV and the output power 2MW. Moreover, the proposed system evaluates the harmonics of
the input current waveform and the fluctuation of the cell
capacitor voltages.

2.

vAr(n/2)

vcAr(n/2) vcAr(n/2) vcAt(n/2)
H-bridge cellcell

6.6 kV,50 HzvrsiAr
LinLinB

LbiAt

Lbir
isitLin

vss
vts

iAs

LbiBr
vBr1

covo

Sa++SbvcC

vhkm
iBs Lb iBt Lb
vcBr1
vcBs1
vcBt1

Lb

h=A,B
k=r,s,t－S
m=1,2… n/2 a

Sb－cellvcBr(n/2) vcBr(n/2) vcBt(n/2)
vBr(n/2)cell(a) Main circuit(b) H-bridge cellFig. 1. Circuit configuration of a step-down AC-DC converterusing the MMC which is consisted of H-bridge cells.
Input Current
vo * 1nControl
Average Control**-v
idAd+dqvc*
PI3 2 3*iqA
ACR－rst vAk* n=0-vq*
vcA_ave
idA
k=r,s,trst
iAk
dq iqA

Sum.

vAkm*
k=r,s,tm=1,2… n/2

Output Voltage
Control

k=r,s,tEq.(1)

vcAkm

n/2
k=r,s,tm=1,2… n/2

Circuit configuration

2.1 Main Circuit
Fig. 1(a) shows the main circuit
configuration of the proposed system using the MMC. Each leg
consists of two buffer reactors Lb and H-bridge cells. Due tocascade connection of cells, the proposed converter can achieve amulti-level voltage waveform and also reduce the rated voltage ofeach cell. Thus, many cascaded cells are used in practical because
it reduces harmonic distortion and utilize with a low voltage rating
devices. On the other hand, in the MMC which is consisted ofchopper cells, the DC output voltage is depended on thesummation of the average value of cell output voltage.
2.2 H-bridge Cell
Fig. 1(b) shows a configuration ofH-bridge cell. Generally, the MMC as AC-DC converter isconsisted of chopper cells. In the chopper cell, the peak value ofoutput voltage depends on a number of cells and the input phasevoltage in order to control input current. Thus, the average value

vrs
vts

qin

Balance
Control

n/2

Fig. 2. Control block diagram of the proposed systemusing the MMC applied to each arm groupcannot be decreased because the average value should be set halfvalue of the peak value. Therefore, the MMC using chopper cells
cannot convert the AC voltage into the DC voltage which is lessthan the peak value of the line-to-line voltage of the AC input
voltage. On the other hand, MMC using H-bridge cells canachieve the step-down conversion because the DC output voltage
is controlled that the capacitor voltage in the H-bridge cell isconnected to the AC input voltage as the positive or negative
direction. As a result, the average value of cell’s output voltage
can be varied flexibly.

3. Control Strategy
Fig. 2 shows the control block diagram of the proposed1

Table.1. Simulation parametersconverter. One of the features in the proposed system is to control
each arm group of A and B as shown in Fig. 1(a). The control
block diagram is separated to the capacitor voltage control blockand the input current control block. Moreover, the capacitor
voltage control block is consisted of an average control and abalance control [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 outputof the PI controller is connected to the active current controller inthe input current control block because the capacitor voltages areadjusted by active current id.
3.2 Balance Control
The average voltage control isused to keep the voltage of all capacitor voltage. However, an
unbalance voltage which occurs among the capacitors cannot be
suppressed by the average voltage control only, because theaverage control corrects only the error between the average valueof the capacitors voltage in each group [2]. Therefore, the voltage
balance control is used to correct the error between each of thecapacitors voltage and the voltage command. The voltage
command of the balance control is given by (1)Output power
Input voltage (line to line)
Input voltage frequency
Output voltage
Carrier frequency

Input phase voltage vr, vs, vt [V]10[msec]vt

itCapacitor voltage vcAr1, vcBr1 [V]

2100

vcAr1

2200

vc*

vcBr1

1900
1800
1700

10[msec]

Fig. 4. Waveform of capacitor voltage
1020

Output voltage vo [V]

1000
980
960
950

100[msec]

Fig. 5. Waveform of output voltage
4, the capacitor voltage keeps constant. Moreover, a maximum
ripple factor 23.3 % in each capacitor voltage is obtained.
Fig. 5 shows the waveform of the output voltage. From theresult, it is confirmed that DC voltage can be obtained. Amaximum ripple factor of 3.33% in the DC output voltage isobtained. Thus, the proposed system achieves to convert
commercial frequency AC voltage to DC voltage. Moreover, theproposed system also achieves the step-down operation.

 .......................................................
1  2(2)2 E  VO n  3

5. Conclusion
The proposed AC-DC converter using a modular multilevel
converter topology can achieve a three-phase AC voltage of 6.6
kV into DC voltage of 1 kV by step-down operation. Moreover,the proposed system can also achieve that the THD of input
current is about 0.2% and the maximum ripple factor of eachcapacitor voltage is 23.3%. In the future work, the optimum
design method of MMC control will be discussed.

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.

4.

86000 F
6 mH
3 mH
3000 FFig. 3. Waveforms of input voltage and input current
2200

where vcrm, vcsm and vcsm are the capacitor voltage in each arm, KC
is the gain of the balance voltage control, k is the index of eachphase and m is the index of the cell step.
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 command 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 is used to obtain the high frequency output voltage isadded to the output of the input current control block. The changeof voltage value in each cell depends on the number of cells at
each leg since the cells are connected to the load in series. In
addition, each capacitor voltage also depends on the input andoutput voltage. The capacitor voltage command vc*is given by (2).
*Cell (per one leg)
DC Capacitor
Interconnection reactor
Buffer reactor
Output Capacitor

6000vr
vs40002000
0-2000-4000-6000
600 Input current ir, is, it [A]
ir
is
400
200
0-200
-400
-600

*
vce _ km  KC vc * vc _ km  ik k  r ,s ,t m  1,2,...n / 2 .... (1)vc 2 MW6.6 kVrms
50 Hz
1000 V
7 kHz

Simulation Results

Table 1 shows the simulation conditions. The proposed system
converts commercial-frequency AC voltage of 6.6 kV into the DC
voltage of 1 VFig. 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 the input current isapproximately 0.2%.
Fig. 4 shows the waveform of the capacitor voltage. From Fig.

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] T. Nakanishi, J. Itoh : “Evaluation of Control Methods for Isolated
Three-phase AC-DC converter using Modular Multilevel Converter”,
ECCE ,Asia Downunder pp.52–58 (2013)2