Evaluation of Isolated
Three-phase AC-DC Converter
using Modular Multilevel Converter Topology
Toshiki Nakanishi

Jun-ichi Itoh

Dept. of Electrical, Electronics and Information Engineering
Nagaoka University of Technology
Nagaoka, Niigata, Japannakanishi@stn.nagoakaut.ac.jp

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

Abstract— This paper discusses an isolated three-phase
AC-DC converter using Modular Multilevel Converter(MMC) topology for a medium voltage application. The
MMC at a primary stage of an isolated AC-DC converter
directly obtains the high frequency AC voltage from thethree-phase AC voltage of the commercial frequency. In
addition, the proposed system obtains the low total
harmonic distortion (THD) in the input current andmaintains the cell capacitor voltage at constant. The
comparative evaluations of the system design based on the
rated voltage of switching devices are considered. In
particular, the numbers of cells and switching devices, theelectrostatic energy of the DC capacitors, the THD of the
input current and device losses are evaluated when highrated voltage devices: 2.5-kV IGBTs, 3.3-kV IGBTs and 3.3kV SiC-MOSFETs are applied to the proposed system.
Keywords— Modular Multilevel Converter; Capacitor
Voltage Control; High Frequency Transformer; H-bridge
Cell; Isolated Three-phase AC-DC converter;I.

INTRODUCTION

Recently, a DC distribution network system in
datacenters has been actively researched in order toachieve size reduction of the network [1-3]. Moreover, inorder to obtain the high efficiency operation, a medium
voltage such as 6.6 kV is employed as the power source.
In addition, DC voltage of 1 kV or less is supplied to aload such as a server supply by an isolation transformer
and a rectifier. However, the conventional system
becomes large due to passive components. A diodebridge or a PWM rectifier which is constructed from highrated voltage devices is applied in the conventional
system. In general, it is difficult to operate in highswitching frequency for a high rated voltage device. Thus,
large passive components are required due to low
switching frequency in order to suppress harmonic
distortion of input current and voltage ripple of output
DC voltage.
On the other hand, the multilevel converter is one ofeffective solution for the problem of above. Recently, themultilevel converter topologies have been actively
discussed for the medium voltage application such as at

3.3 kV or 6.6 kV [4-10]. Especially, Modular Multilevel
Converter (MMC) has been actively studied as one of themultilevel converter topologies [5]. Advantages of the
MMC are as follows; (i) the circuit configuration is
simpler than that of other multilevel converters due tocascade connection of cells. Each of the cells isconstructed from switching devices and a DC capacitor,(ii) the MMC can reduce the harmonic distortions
because the MMC output the multilevel waveform aswith other multilevel converters. (iii) low voltage devices
can be applied to each cells because the output voltage
becomes low by increasing a number of cells. In addition,
the MMC achieves the size reduction of passive
components due to high frequency operation by low
voltage devices. From the advantages of the above, theMMC topology can be applied in high capacity power
applications such as Static Synchronous Compensator(STATCOM) and medium range of adjustable speed
drive systems [11-15].
However, the high power converters which use MMC
result that the volume of the converters becomes large
due to the isolation transformer which is operated withline-frequency. The isolation transformer is applied inorder to separate the grid side and the load side in orderto protect the server supply. One of the effective ways toreduce the transformer size is applying high frequency
operation. The size reduction of the transformer by using
the multi-module converter has been reported [16-17]. In
back-to-back (BTB) systems, multiple bidirectional
isolated DC-DC converters are considered andconsequently the size reduction of the transformer isachieved due to the usage of the high frequency
transformer to each isolated DC-DC converter [9].
However, in principle, it is difficult to achieve highefficiency because the number of switching devices
drastically increases with increasing the number of stages.
Moreover, a DC-DC converter which has high frequency
AC link using MMC has been proposed [18]. In the BTB
system, the size reduction of the transformer can beachieved because the transformer is used in highfrequency AC link [10]. However, the number of cells islarge because MMC has to be applied to both the input
and the output sides.
This paper discusses an isolated three-phase AC-DC
converter using MMC and a control strategy. Three-phaseto single-phase MMC topology is used at a primary stage
in the isolated AC-DC converter. The proposed system
directly converts three-phase voltage into high frequency
single-phase voltage. Furthermore, the volume of the
proposed system becomes small and simple due to a highfrequency single-phase transformer.. It is confirmed by
the simulation that the proposed system achieves low
THD and the cell capacitor voltage remains at constant. In
addition, the comparative evaluations of the system design
based on the rated voltage of switching devices are
considered. In particular, the numbers of cells andswitching devices, the electrostatic energy of the DC
capacitors, the THD of the input current and the device
losses are evaluated when high rated voltage devices: 2.5kV IGBTs, 3.3-kV IGBTs and 3.3-kV SiC-MOSFETs areapplied into the proposed system.
II.

MAIN CIRCUIT CONFIGURATION

Fig. 1(a) shows the circuit configuration of the isolated
three-phase AC-DC converter using MMC topology. Dueto the cascade connection of cells, the proposed converter
can achieve a multilevel voltage waveform and the rated
voltage of each cell is reduced as well. Thus, increasing
the number of cascaded cells are effective because theharmonic distortions of an input current can be reduced,
and low voltage rating switching devices can be used.
Furthermore, the proposed system can directly convert
an input three-phase voltage into a high frequency singlephase voltage. Therefore, the volume of the proposedsystem becomes small due to the use of the highfrequency transformer. In addition, the topology issymmetry from input and output side. Thus, the proposedsystem can control each of group independently. In thispaper, the group in the upper side is defined as an armgroup A, and the group in the lower side is defined as anarm group B.
Fig. 1(b) shows the configuration of an H-bridge cell.

(a) Main circuit configuration.

The output voltage of each cell is controlled by pulse
width modulation (PWM). Moreover, it is necessary toensure the capacitor voltage at constant for a stable
operation of the main circuit.
III.

CONTROL STRATEGY

Fig. 2 shows the control block diagram of the
proposed converter. The proposed control is applied toeach group A and B, respectively as shown in Fig. 1. The
proposed control block diagram is separated into two
blocks, the capacitor voltage control block and the input
current control block. Moreover, two types of control areused in the capacitor voltage control block, an averagecontrol and a balance control [19].
A. Average Voltage Control
The average voltage control corrects the error between
the average value of the capacitors voltage in each armgroup and the voltage command is generated from a PI
regulator. The average value of the capacitor voltage
vc_ave is given by (1)v c _ ave =2 n/2∑ (v crm + v csm + v ctm )3n m =1(1)

where n is the number of cells in each leg and vcrm, vcsm
and vctm are the capacitor voltage in the each arm. The
number of cells is set n/2 in order to calculate the averagevalue of the capacitors voltage in each arm group because
each arm group is independently controlled.
B. Balance Voltage Control
The average voltage control is used to keep the voltageof all capacitors at constant level. However, an unbalance
voltage which occurs among the capacitors cannot be
suppressed by the average voltage control because theaverage control only corrects the error between the
average value of the capacitors voltage in each group andsubjects to the voltage command. Therefore, the balance
voltage control is used in order to correct the error
between each of the capacitors voltage and the voltage(b) H-bridge cell.

Finally, the output voltage of the cell is given by (3).
Note that the sign of the MMC output voltage command
vmmc* has to be changed because the direction of cell
output voltage is different in the group A and the group B.

command. The command of the balance control is givenby (2)

vce _ Akm* = K C (vc * −vc _ km ) ik

(2)

k = r , s ,t m = 1,2 ,...n / 2
where vcrm, vcsm and vctm are the capacitor voltage in eacharm respectively, KC is the gain of the balance voltage
control, k is the index of each phase and m is the index of
number of cells.
Moreover, it is not necessary to eliminate the error
completely because the balance control is an auxiliary
control for the average voltage control. Therefore, thebalance control can be constructed only by using
proportional control.

1(2vAk * * + vmmc *) − vce _ Akm *n1vBkm* = (2vBk * * − vmmc * ) − vce _ Bkm *nvAkm* =

k = r , s ,t⎞1⎛ 2
vc* ≥ ⎜⎜ 2 E + vmmc * ⎟⎟
n⎝ 3
⎠

vc *

+

IV.

iqA *

vcA_ave=0

A. Evaluation Condition
Table 1 shows the parameters of the switching devices
with the high rated voltage. In the proposed system, from(4), the capacitor voltage depends on the input current
control and the MMC output voltage control. Therefore,
the rated voltage of the switching devices depends on thecapacitor voltage of the cell. In the case where the highrated voltage devices are applied, it is possible to reduce
the number of the circuit components in the proposed

m=1,2… n/2

ACR
-vq*

rst
dq iqA

k=r,s,tBalance
Control
k=r,s,t

-vd *

idA

iAk

vcAkm

EVALUATION OF THE PROPOSED SYSTEM

vAk**

Input Current Control

PI

Eq.(2)

n/2
vr
vt

(4)

where E is the effective value of the input line to the line
voltage, vmmc* is the maximum value of the MMC output
voltage command and n is the number of cells at eachleg. From (4), the capacitor voltage of each cell becomes
smaller by increasing the number of cells. Therefore, thecapacitor voltage command becomes lower.

D. Output Voltage Control
The same command of the MMC output voltage
control is given to each cell in order to convert a threephase input voltage to a high frequency voltage. The
output voltage of a cell is vmmc*/n when the command of
the MMC output voltage control is vmmc* and the numberof cells in a leg is n.

idA*

m = 1,2,...n / 2

E. Capacitor Voltage Command
The MMC output voltage command vmmc* is added inorder to obtain the high frequency output voltage, whichis added to the output block of the input current control.
The change of voltage values in each cell depends on thenumber of cells at each leg since the cells are connectedto the load in parallel. In addition, each capacitor voltage
also depends on the input and output voltage. The
capacitor voltages command vc*is given by (4).

C. Input Current Control
The input current flows separately to the upper sidearm and the lower side arm in each phase. Therefore, theinput current can be determined by controlling the current
which flow into the buffer reactors. In addition, in the
input current control, the linkage operation is not needed
between the arm group A and B. Furthermore, theproposed system is able to control the arm groups A and
B respectively.
The input current control is implemented by PI
regulators and the compensators for the cross terms of id
and iq which are provided from the transformation ofrotating frame. The input current control is separated into
the active current control (id) and the reactive current
control (iq). The reactive current command iq* is set tozero in order to control the input power factor to 1 and to
reduce the absolute value of the input current. Therefore,
the same input current command is given to each cellbecause the cascaded cells control the common arm
current.

Average Control

(3)

dq

3

rst

2
n

3

Eq.(3)

vAkm*
k=r,s,tvmmc*
n/2

Output Voltage Control

k=r,s,t1nvce_Akm*θFig. 2. Control block diagrams of the proposed circuit for the arm group A.

m=1,2… n/2

system. On the other hand, in the case where the low
rated voltage devices are applied and the number of cells
is increased, it is possible to suppress the harmonic
distortion of the input current due to the high speed
switching and multiple voltage levels. Thus, the design
factors : the number of cells, the capacitor voltage and the
THD of the input current are different. Therefore,
comparing of the circuit performance based on the rated
voltage of switching devices is required. In order tocompare the circuit performance with different devices,
the number of high rated voltage IGBTs and SiC
MOSFETs which the rated voltage is 3.3-kV are
employed.
The conditions to compare the circuit performance areas follow.
a) The rated voltages of IGBTs are 2.5-kV, 3.3-kV
and 6.5-kV, respectively.
b) The capacitor voltage is set 20% more than thevalue which is calculated by (4). Moreover, the rated
voltage is set 30% more than the capacitor voltage.
c) The output power is set at 400 kW and theoutput voltage is set at 450 V.
d) MMC output voltage command vmmc* is set 2.6
kV, 1 kHz.
e) Buffer reactor Lb is 0.5 mH (the percentage ofimpeadance %Z = 0.31%). Moreover, the interconnection
reactor is 4 mH (the percentage of impeadance %Z =0.125). Both reactors are common value in all system.
f) Switching frequency is chosen so that the
proposed system can achieve the efficiency of 95% withconsidering of the devices loss: the conduction loss, theswitching loss and the conduction loss of the freewheeling diode (FWD).
B. Comparative Evaluation of the proposed system
The number of cells, switching devices and DC
capacitors are estimated by referring (4) and condition (b)is listed in previous part.
Table 2 shows the comparison result of the cellnumbers and switching devices that depends on the rated
voltage of the switching device.
In the case where IGBTs with the rated voltage of 2.5kV are applied, 10 cells are required per leg. Moreover,
120 units of IGBT are required. On the other hand, 2.5kV IGBT can be set as a high response angular frequency
due to high speed switching. Therefore, it is possible toreduce the values of buffer reactors and DC capacitors,
respectively.
On the other hand, in the case where IGBTs with the
rated voltage of 3.3-kV are applied, 8 cells per leg andtotal of 96 units of IGBT are required. The number ofcells and switching devices can be reduced comparedwith the case of 2.5-kV IGBTs. However, the switchingfrequency is limited because the switching speed is lower
than the switching frequency of 2.5-kV IGBTs.
Therefore, it is necessary to increase the value of DC
capacitors since the response angular frequency of thecontrol system is low.
In the case where IGBTs with the rated voltage of 6.5kV are applied, 4 cells per leg and 48 units of IGBT are

Table 1. Parameters of switching devices and switching frequency.
Device Parameters2.5-kV IGBTIXBK64N250

2.5-kV_FWD
SD203N/R25S20

3.3-kV_IGBT
FF200R33KF2C

3.3-kV_FWD
FF200R33KF2C

6.5-kV IGBTFZ400R65KE3

VCE=2.5 kV, Ic = 156 A
Eon = 30.2 mJ@ Ic = 140 A, VCE = 1.25kV,
RGon = 1 Ω, VGE = ±15 V
Eoff = 15.31 mJ@ Ic = 140 A, VCE = 1.25kV,
RGoff = 1 Ω, VGE = ±15 V
VCEsat = 2.5 V@ Ic = 64 A, VGE = 15 V
VRRM= 2.5 kV, Ic= 200 A
VFM = 1.65 V @ Ipk = 628 A
VCE=3.3kV, Ic= 200A
Eon = 235 mJ@ Ic = 200 A, VCE = 1.8kV,
RGon = 7.5 Ω, VGE = ±15 V
Eoff = 215 mJ@ Ic = 200 A, VCE = 1.8kV,
RGoff = 5.6 Ω, VGE = ±15 V
VCEsat = 3.4 V@ Ic = 200 A, VGE = 15 V
VRRM= 3.3 kV, Ic= 200 A
VF = 2.8 V @ IF = 200 A
VCE=6.5 kV, Ic=400 A
Eon = 3450 mJ@ Ic = 400 A, VCE = 3.6kV,
RGon = 1.9 Ω, VGE = ±15 V
Eoff = 2250 mJ@ Ic = 400 A, VCE = 3.6kV,
RGoff = 13 Ω, VGE = ±15 V

fs

5 kHz

1.38 KHz

Under
1 kHz

Table 2. Comparison for numbers of cells and switching devices
depending on the rated voltage of the switching device.
Rated
Numbers of Cells
Numbers of
Voltage@leg
Switching devices
2.5-kV
10120
3.3-kV896
6.5-kV
448

required. The number of cells and switching devices canbe significantly reduced compared to 2.5-kV IGBTs and3.3-kV IGBTs. The switching speed of 6.5-kV IGBT islower than that of 2.5-kV IGBT and 3.3-kV IGBT.
Furthermore, the large values of DC capacitor and buffer
reactors are needed because angular frequency responsesof the average control and the input current control
become low, respectively. In addition, the large
inductance of buffer reactors are also applied since theresponse angular frequency of the input current control
becomes low. In the proposed system, MMC output is ahigh frequency voltage. However, the voltage drop
becomes large since a voltage-dividing condition occurs
between the high frequency transformer and buffer
reactors. In the worst case, it is difficult to transmit a highfrequency power to secondary side of the transformer
when the buffer reactor is large. Therefore, it is necessary
to reduce the inductance of buffer reactors to transmit thehigh frequency power.
C. Evaluation of proposed system by appling SiC
MOSFETsSiC-MOSFET with the rated voltage of 10 kV has
been developed [20]. SiC devices have following
advantages: high rated voltage, low loss, high switching
speed and high operating temperature. Therefore,
Employing SiC devices to high power applications areconsidered nowadays, especially, in the high voltage andhigh power conversion system. However, the dv/dt at
winding wires of a high frequency transformer becomes
greatly large. Thus, the insulation degradation among
winding wires may occur when the proposed system isoperated by using high rated voltage devices. From theprevious topic, it is needed to set the switching frequency
in order to modulate the operation frequency of the highfrequency transformer at 1 kHz. Therefore, it is necessary
to consider the operation of the proposed system when
3.3-kV SiC-MOSFET is applied into the proposed system.
Note that the device data of 3.3-kV SiC-MOSFET was
reported [21].
Table 3 shows the simulation parameters. The
switching frequency of the SiC-MOSFET is set to 15 kHz.
Note that the ideal elements are used in the switching
device. In addition, the output voltage command is asquare-wave of 2.3 kV, 1 kHz.
Fig. 3 shows the input phase voltage and current
waveforms. From Fig. 3, it is confirmed that the unity
power factor can be obtained in the input stage.
Moreover, the total harmonic distortion (THD) of the
input current is approximately 1.74%. The input current
waveform includes few pulse beats. However, the pulse
beats does not affect the THD of the input current since
the frequency of the pulse beats is very high compared to
the fundamental frequency.
Fig. 4 shows the output voltage waveform. From theresult, the output voltage is constantly controlled
approximately at 450 V. Moreover, the ripple voltage is
obtained 0.4%. Thus, the power conversion from threephase AC voltage into a DC voltage is confirmed.
Fig. 5 shows the cell capacitor voltage waveforms at
the r-phase. The cell capacitor voltage is controlledaccording to the capacitor voltage command vc* by theaverage control and the balance control. In the capacitor
voltage, the ripples include two frequency components:
100 Hz and 1 kHz. Then, the ripple of 1 kHz is less thanthat of 100 Hz. Furthermore, the ripple of 100 Hz
comprises a majority of the capacitor voltage ripple.
However, the maximum ripple of each capacitor voltage
is obtained by 2.0%. Thus, it is confirmed that the voltage
ripple does not affect the circuit operation in the proposedsystem.
Fig. 6 shows the output voltage waveforms of the
MMC. It is confirmed that the high frequency AC voltageof 1 kHz is obtained. However, an error occurs between
the output voltage of the MMC and the voltage
command. The error depends on the voltage drop in eachbuffer reactor. Moreover, the output voltage waveform is
dropped when the voltage polarity changes because thehigh frequency current flows in all diode in the rectifier
circuit due to the leakage inductance condition of thehigh frequency transformer. The output voltage depends
on the voltage drop of the leakage inductance.
D. Evaluation of total electrostatic energy
In general electrical products, the size of anelectrolytic capacitor affects the system size. On the other
hand, the size of a DC capacitor depends on theelectrostatic energy because the size of the electrolytic

Table 3 Simulation condition
Output power PO
400 kW Input voltage rms

6.6 kV

Input voltage frequency

50 Hz

Output voltage

450 V

Number of cell per leg n

8Load R

0.5 Ω

Turn ration pri.: sec.

1 : 1.5

Carrier frequency

15 kHzInterconnection reactor Lin

4 mH (%Z = 1.15%)

Buffer reactor Lb

0.5 mH (%Z = 0.31%)

DC capacitor C

1,600 μF

Output capacitor Cr

3,000 μF

Switching device

3.3-kV SiC-MOSFET

Fig. 3. Waveforms of the input phase voltage and current.

Fig. 4. Waveform of the output voltage at the diode bridge rectifier

2030

Capacitor voltage vcAr1, vcBr1 [V]

2020

vcBr1

2010

vcAr1

10[msec]

vc*

2000
1990
1980
1970
Fig. 5. Waveform of the capacitor voltage.

Fig. 6. Waveform of the MMC output voltage.

capacitor depends on the charge voltage of the DC
capacitor and its capacitance. Thus, the system size can12
CVc × N2

(5)where C is a capacitance of each capacitor, N is thenumber of capacitors in MMC and VC is the averagevalue of the capacitor voltage. VC is equal to the capacitor
voltage command vc* when all capacitor voltage is kept
constant by the average control and the balance controls.
On the other hand, the capacitance of the DC capacitor
is designed in order to suppress the voltage ripple. At theDC capacitor, the ripple frequency of the voltage ripple
becomes 100 Hz occurs because each cell rectifies the
input current of the commercial frequency. In this paper,
the capacitance of the DC capacitor is designed in orderto suppress the voltage ripple of 100 Hz. Moreover, theripple is suppressed within 1% against the average valueof the capacitor voltage. Furthermore, it is necessary to
set the capacitance of the DC capacitor to satisfy therequirement. In order to control the input current, thebuffer reactor current ia and the cell output voltage vsc is
given by (6).

ia =2I sin ωt2

v sc = 22
3(6)

E sin ωtwhere I is the effective value of the input current, E is theeffective value of the input line voltage and ω is anangular frequency of the input voltage. Moreover, thecurrent which flows to the DC capacitor is idc. The DC
capacitor voltage vc is kept at constant when vcidc = vsia
holds because input power and the output power are
equal. From (6), the DC capacitor current idc is given by
(7).

idc =

EI
3v c n( 1 − cos 2ωt )(7)

The second term of (7) is the current ripple Δidc. In
addition, the voltage ripple Δvc can be calculated by the
impedance of DC capacitor and the current ripple Δidc.
From the relationship between the voltage ripple Δvc andthe capacitance of DC capacitor for the voltage ripple
suppression is given by (8). Note that vc is equal to thevoltage command vc* in each system. From (8), thecapacitance of each system can be calculated.

C=

EI
*

3Δvc nvc ω(8)Fig. 7 shows the comparison result of the total energy
which all capacitor charges. From the comparison result,
the total electrostatic energy is the largest when 3.3-kV
IGBTs are applied. In the case of applying 3.3-kV IGBTs,
the capacitance should be larger than that is calculated by(8) since the response angular frequency of the average

Total Capacitance Engegy [kJ]Wc =Fig. 7. Comparison of total electrostatic energy in DC capacitors

Input current THD [%]be evaluated by total electrostatic energy of the DC
capacitor. The total electrostatic energy WC is given by(5).

Fig. 8. Comparison of the input current THD
according to the switching devices.

control becomes low.
In the case of applying 3.3-kV SiC-MOSFETs into the
proposed system, the total electrostatic energy is thelowest since the number of cells and the ripple voltage
against the average value of the capacitor voltage are
reduced. Furthermore, by applying 3.3-kV SiCMOSFETs to the system, the size reduction of the
proposed system can be achieved since the number ofcircuit components can be reduced and the power loss inswitching devices can be suppressed.
In the case of applying 2.5-kV IGBTs into the
proposed system, the total electrostatic energy is nearly
identical to that of 3.3-kV SiC MOSFETs because thecapacitance C and the capacitor voltage VC become low.
In conclusion, the size of DC capacitors can be reduced
when 2.5-kV IGBTs are applied into the proposed system
as well as 3.3-kV SiC MOSFETs.
E. Comparison with respect to THD of Input Current
Fig. 8 shows the comparison result of the input current
THD. From Fig. 8, the THD is the lowest in the casewhere IGBTs with the rated voltage of 2.5 kV. When 2.5kV IGBTs are applied into the proposed system, thenumber of cells is the highest. Furthermore, the harmonic
distortion is suppressed because the input voltage
becomes multilevel waveform due to multi-cells. On theother hand, when 3.3-kV SiC-MOSFETs are applied into
the proposes system, the low THD of the input current isachieved as well as 2.5-kV IGBTs because it is possible
to set a higher response angular frequency of the input
current control due to the high switching speed. Finally,
the THD is the highest in cases where IGBTs with the
rated voltage of 3.3-kV. When 3.3-kV IGBTs are appliedF. Comparison Evaluation of the device loss
Fig. 9 shows the comparison result of the device
losses: the conduction loss, the switching loss and the
conduction loss of the FWD against the output power.
Note that a SiC Junction Barrier Schottky Diode isapplied as the FWD of 3.3-kV SiC-MOSFETs. Note thatthe device data of a SiC Junction Barrier Schottky Diodewas reported [22].
In the case of applying 3.3-kV IGBTs to the proposedsystem, the switching loss is larger than the other systembecause turn-on loss and turn-off loss are large comparedwith the system which 2.5-kV IGBTs and 3.3-kV SiCMOSFETs from Table 1. Moreover, the system using
3.3-kV IGBTs cannot achieve the efficiency of 95%. It is
possible to achieve the efficiency of 95% when the low
switching speed is set. However, the system cannot
produce the high frequency output voltage when the low
switching speed is set. In addition, in the system that is
applied with low switching frequency devices, the valueof the DC capacitor is needed to be increase in order tomaintain the voltage of DC capacitors constantly.
In the case of applying 2.5-kV IGBTs, the efficiencyof 95% can be achieved when the switching frequency is
5 kHz. Furthermore, it is possible to obtain the highfrequency output voltage and keep the voltage of DC
capacitors at constant. Moreover, when 2.5-kV IGBTs
are applied into the proposed system, more cells are
required rather than other systems. However, the system
is able to achieve the efficiency of 95%. Thus, in thesystem which is applied 2.5-kV IGBTs, the number ofcells is not significantly affected the efficiency.
In the case of applying 3.3-kV SiC-MOSFETs to the
proposed system, the loss at switching devices is thelowest because both the turn-on loss and turn-off loss arelower than that of both IGBTs. Furthermore, theefficiency is the highest. Moreover, when the losses ofthe devices become half comparing to the system which
is applied to 2.5-kV IGBTs at switching frequency of 5
kHz, the volume of the heat-sink can be dramaticallyreduced.
In conclusion, by applying 2.5-kV IGBTs or 3.3-kV
SiC-MOSFETs, the high efficiency can be achieved in
the proposed system. Moreover, SiC-MOSFET is themost suitable for the proposed system because SiCMOSFET has the high performance features. Thus, theproposed system using 3.3-kV SiC-MOSFET reduces the
harmonic distortion of the input current, can achieve thehigh efficiency and the size reduction of the system. On
the other hand, 2.5-kV IGBTs also has good performancein terms of the efficiency and the suppression of theharmonic distortion.

Device loss [p.u.]

into the proposed system, it is not possible to set a highresponse angular frequency because the switching speedof the IGBT is very low. Thus, it is difficult to improve
the harmonic distortion for the system which is applied
2.5-kV IGBTs when the same values of theinterconnection reactor and the buffer reactor are applied
as well as the system which 2.5-kV IGBTs or 3.3-kV
SiC-MOSFETs are applied.

Fig. 9. Comparison of the switching loss and the conduction loss.

V.

CONCLUSION

In this paper, an isolated three-phase AC-DC converter
using MMC and the control strategy were proposed for a
medium voltage application. The proposed system could
directly convert an input three-phase voltage into a highfrequency single-phase voltage. Therefore, the proposedsystem could reduce the volume of the isolatedtransformer since the single-phase high frequency
transformer could be applied to the high frequency link.
The proposed system is evaluated with numbers ofdifferent rated voltage devices: a 2.5-kV IGBT, a 3.3-kV
IGBT and a 3.3-kV SiC-MOSFETs. As a result, when
3.3-kV SiC-MOSFETs is applied into the proposedsystem, the total electrostatic energy in DC capacitors is
the lowest compared to the other devices. Furthermore,
3.3-kV SiC-MOSFETs could achieve the size reductionof the proposed system when the common value ofenergy density in all capacitor is considered. Moreover,the system using 3.3-kV SiC-MOSFETs could achieve
low THD input current waveform which is approximately
2.36% and the maximum ripple factor of each capacitor
voltage is approximately 2.0%. Finally, from comparison
evaluation of the efficiency when device losses are
considered in the proposed system, it is confirmed thatthe efficiency of 95% can be achieved when 2.5-kV
IGBTs and 3.3-kV SiC-MOSFET are applied into the
proposed system. In conclusion, in the cases of 2.5-kV
IGBTs and 3.3-kV SiC-MOSFETs, the proposed system
can achieve the size reduction, the low THD of input
current and high efficiency. Moreover, the system which
is applied SiC-MOSFETs can achieve better efficiency
compared with 2.5-kV IGBT even the same switchingfrequency is applied.
In future works, the optimized design method of thehigh frequency transformer for the high power
application will be considered. In addition, the volume
evaluation results of the proposed system compared tothe conventional system will be shown.
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