S2PC 2014, Yilan, Taiwan

Step-down Rectifier using Modular Multilevel Converter for Wind Power
Generation System connected to DC Micro-gridToshiki Nakanishi＊a), Jun-ichi Itoh＊
Abstract: This paper discusses a step-down rectifier which is constructed by a Modular Multilevel Converter (MMC) for awind power generation system connected to a DC micro-grid. The proposed system which is constructed by the MMC with an
H-bridge cell achieves to convert from a generator voltage of 3.3 kV into DC bus voltage of 340 V. Moreover, as a fundamental
evaluation, the experimental results by miniature model of 700 W confirm that the proposed system achieves the step-down
operation from the input voltage of 200 V into the DC voltage of 65 V. Finally, the proposed system maintains the capacitor
voltage of each cell to the voltage command. Furthermore, the maximum voltage error between the voltage command of the cell
capacitor and the measured voltage is 10% or less.
Keywords : Modular Multilevel Converter, H-bridge cell, DC micro-grid, Wind power generation system, High power rectifier

1.

Introduction

Recently, a micro-grid and a DC power grid are actively
researched as a next generation power supply(1)-(2). Moreover, theDC micro-grid is suitable to connect a battery energy storage
system and a renewable energy source such as a photovoltaic
generation system and a fuel cell because the output voltage ofmany renewable energy sources is DC voltage. Thus, themicro-grid has been applied to the power grid in isolated islands
as a stand-alone power system(3). Additionally, a wind power
generation is also applied to the power grid in isolated islands as
one of the power source(3). However, a size of the transformer inthe wind power generation system is bulky because thetransformer operates in low operation frequency of the generator.
In addition, the reasons which the transformer is bulky include
that a high transformer ratio is required to convert from generator
voltage into DC bus voltage. On the other hand, the wind power
generation system without the transformer by using a Modular
Multilevel Converter (MMC) is an effective solution. However, inthe general AC-DC converter using the MMC which consists ofchopper cells(4), it is difficult to achieve the step-downrectification because chopper cells cannot output negative voltage.
This paper discusses a step-down rectifier using the MMC andthe control system in order to convert from high voltage AC into
low voltage DC. Moreover, from an experimental result by
miniature model of 700 W, the proposed system achieves toconvert from three phase voltage of 200 V into the DC voltage of65 V.

2.

Main Circuit Configuration

Fig. 1 shows the main circuit configuration of the proposed
wind power generation system using the MMC. Each leg consistsof two buffer reactors Lb and H-bridge cells. Due to cascade
connection of cells, the converter achieves a multi-level voltagea) Correspondence to: Toshiki Nakanishi.
E-mail: nakanishi@stn.nagaokaut.ac.jp＊Nagaoka University of Technology
1603-1, Kamitomioka-machi, Nagaoka city, Niigata, Japan 940-2188

vAr1

cell

vcAr1

vAr(n/2)
celliAr
Wind power
generator

Gcell

Sa+ Sb+vcAr(n/2)

ir
isit

LbiBr

LbvBr1

Sa－ Sb－ Cv
H-bridge cell Br(n/2)

DC bus ofMicro-grid

Vmmc

Isolated
DC-DCconv.

vcBr1
cellvcBr(n/2)
cellFig. 1. Circuit configuration of the MMC for the wind power generation
system connected to the micro-grid. The MMC with H-bridge cells is able
to operate the step-down rectifier.

waveform and also reduces the rated voltage of each cell. Thus,
many cascaded cells are used in practical because it reduces theharmonic distortion and utilizes low voltage rating devices. On theother hand, in the MMC, the output DC voltage depends on thesummation of the average value of cell output voltage.

3. Control StrategyFig. 2 shows the control block diagram of the proposed
step-down rectifier. The greatest feature of the proposed control
system is to control each arm as shown in Fig. 1. Additionally, thecontrol block diagram is separated to the capacitor voltage control
block and the input current control block. Moreover, the capacitor
voltage control block consists of a voltage averaging control
system and a voltage balancing control system.
3.1
Voltage Averaging Control System
The voltage
averaging control system is applied in order to control the averagevalue of all capacitor voltage in the arm. The error between thecapacitor voltage command and the average value of capacitor
voltage in the arm is corrected by a PI controller. Moreover, theS2PC 2014, Yilan, Taiwan

output value of the PI controller is given as the command of apositive phase current. Thus, the command of the positive phasecurrent is generated depending on the fluctuation of the averagevalue of all capacitor voltage in the arm.
3.2
Voltage Balancing Control System
The voltage
averaging control system is used to keep the voltage of all
capacitor voltage. However, an unbalance voltage which occurs
among capacitors in same arm cannot be suppressed by voltage
averaging control system only, because the voltage averaging
control system corrects only the error between the voltage
command and the average value of the capacitors voltage in thearm. In addition, the positive phase current in order to keep thecapacitor voltage flows to all cells which exist in 1 arm. Moreover,the common value of the positive phase current which flows eacharm. Thus, it is difficult to adjust the arm current in order tobalance on each cell. Against the problem, the output DC voltageof the cell is varied depending on the capacitor voltage. Thus, theoutput power of the cell is varied depending on the capacitor
voltage. This control principle is given by equation (1).

Vmmc _ mki *

1 vcmki*Vmmc ......................................... (1)2 n/ 2 vcmkx
x 1

where Vmmc_mki* is the output DC voltage command of each cell.
i is the index of the cell number. Moreover, m, k and i are matchedin both side of equation (1).

4.

Conclusion

This paper discusses the step-down rectifier using a modular
multilevel converter (MMC) for the wind power generation
system without the bulky transformer connected to the DC
micro-grid. Moreover, the proposed control system is very simple
because the control system is applied to the each arm of the MMC.
From the experiment result, the proposed control maintains thecapacitor voltage at constant. In the future work, the method toAVR
PI－-1

ACR
Kacr +－+
iAr_p

iAr_p* +2

vcAr_ave
1/E

Elimination ofzero phase current

2/n +vAri*－A :―B :+(+)
Vmmc_Ari*
Eq.(1)

Vmmc*iArvr

Fig. 2. Control block diagram for step-down rectification of MMC. The
control system is applied to each arm of the MMC.
Table 1.

Experimental conditions.

Input voltage rms E

200 V

Output power PO700 W

Input voltage frequency f

50 Hz

Buffer reactor Lb

4 mH (&Z=1.09%)

Number of cell per leg n

4DC capacitor C

1,300 F

Carrier frequency fS

8 kHz

Load R

5.8 

Input Phase Voltage (R-phase) 10 msec

250 V

0Input Phase Current (R-phase)

05A
MMC Output Voltage

Experimental Results

Table I shows the experiment condition of the prototype. In theprototype, the leg is constructed by four cells.
Fig. 3 shows waveforms of the input phase voltage, the input
current and the output DC voltage. Firstly, from the waveforms of
the input phase voltage and the input current, it is confirmed thatthe unity power factor is obtained in the input stage. Moreover, thetotal harmonic distortion (THD) of the input current is 6.5%. Thus,
as a future work, it is necessary to reduce the total harmonic
distortion of the input current.
Second, the waveform of the output DC voltage in lower sideof Fig. 3 shows that the prototype converts from the input voltage
of 200 V into the output DC voltage of 65 V. From this waveform,
the output DC voltage is kept at constant. Therefore, the prototypeof the MMC achieves the step-down rectification.
Fig. 4 shows the waveforms of the cell capacitor voltage which
are connected to the r-phase leg. The cell capacitor voltage iscontrolled according to the capacitor voltage command vc*. As aresult, the proposed system maintains the capacitor voltage of eachH-bridge cell to the voltage command of 120 V. Furthermore, themaximum voltage error between the voltage command of the cell
capacitor and the measured voltage is 10% or less. As a futurework, it is necessary to consider the cause of the error between thevoltage command and the measured voltage.

5.

vc* +

25 V

0Fig. 3. Waveforms of input voltage, input current and output voltage.
The unity power factor is be obtained in the input stage. On the other hand,
the THD of the input current is 6.5%.

Capacitor voltage (R-phase)vcAr1, vcAr2, vcBr1, vcBr2,
120 V

50 V025 msec
Fig. 4. Waveforms of the capacitor voltage in r-phase leg. The proposedsystem maintains the capacitor voltage of each H-bridge cell to the voltage
command of 120 V. The maximum voltage error between the voltage
command of the cell capacitor and the measured voltage is 10% or less.

reduce of the THD of the input current and will be discussed.
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