Energy and Power Engineering, 2013, 5, 1172-1175
doi:10.4236/epe.2013.54B222 Published Online July 2013 (http://www.scirp.org/journal/epe)
A Novel Voltage Balancing Method of Modular
Multilevel Converters
Zunfang Chu, Zixin Li, Ping Wang, Yaohua Li
Key Laboratory of Power Electronics and Electric Drive, Institute of Electrical Engineering,
Chinese Academy of Sciences, Beijing, China
Email: chuzf@mail.iee.ac.cn, lzx@mail.iee.ac.cn, wangping@mail.iee.ac.cn, yhli@m ail.iee.ac.cn
Received September, 2012
ABSTRACT
In this paper, a novel voltage balancing method of modular multilevel converters (MMCs) is proposed. This method
divides the voltages of sub-module capacitors in each arm into several groups and the voltage balancing is based on
these groups. The proposed method can save sorting time greatly compared with the conventional method. Simulation
results on a MMC based three-phase inverter show validity of the proposed method.
Keywords: Modular Multilevel Converter; Voltage Balancing
1. Introduction
Modular multilevel converter (MMC) is getting more and
more attention because of its characteristic of highly
suitable for medium to high voltage applications. The
principle of the MMC and the basic method of the volt-
age balancing were introduced in [1]. There are many
published academic papers on MMC [2-7] recent years.
There exist generally two types of PWM methods for
MMC with half bridge as the sub-modules, i.e. the car-
rier-phase-shifted PWM (CPSPWM) method in [2,3,6]
and the sub-modules unified pulse width modulated
(SUPWM) method in [4,5,7]. The SUPWM method can
balance the sub-module cap acitor voltages by sorting an d
selecting the different sub-modules without close-loop
voltage balancing controllers. With the rising of the net-
work voltage level, the number of sub-modules in each
arm is increased. So the sorting time is increased and
influences the control program [6] developed a reduced
switching-frequency (RSF) voltage balancing algorithm.
This method significantly reduces the average device
switching frequency and total switching losses of the
converter. But if this method is used in SUPWM method,
the voltages of the sub-modul e capacit ors fl uct uate la rgel y .
The reason is that some capacitors of sub-modules are
charged (or discharged) in a half fundamental period. [8 ]
reported a voltage-balancing technique using in multi-
modal converter-based high-voltage dc power transmis-
sion which employed selective harmonic elimination
pulse-width-modulation (SHE-PWM). But SHW-PWM
method is difficultly used in condition that the level of
output voltage is more than ten.
To improve the performance of SUPWM method, this
paper presents a novel method to reduce sorting time in
order to balance voltage of sub-module capacitor.
2. The Conventional Voltage Balancing Me-
thod for SUPWM Method for MMC
Figure 1 shows three phase of MMC as an inverter. It
contains three phase and each phase has two arms (upper
arm and lower arm). Each arm has N sub-modules and
one inductor L0. A single sub-module is shown in Figure
1; it consists of one capacitor and two IGBTs. The outpu t
of sub-module uSM has two levels, i.e. 0 (switched off)
and USM (switched on). So each arm will have N + 1 vol-
tage levels.
Figure 1. Three phase of MMC as an inverter.
Copyright © 2013 SciRes. EPE
Z. F. CHU ET AL. 1173
The basic principles of the conventional voltage bal-
ancing method are introduced in [1] as follows.
All of voltages of sub-module capacitors in the
same arm are sorted according to the order from
lower to higher or from higher to lower.
When the arm current charges the capacitors,
sub-module whose capacitor voltage is lower is
switched on and sub-module whose capacitor volt-
age is higher is switched off.
When the arm current discharges the capacitors,
sub-module whose capacitor voltage is higher is
switched on and sub-module whose capacitor volt-
age is lower is switched off.
The conventional voltage balancing method needs to
sort all capacitor voltages in the same arm each control
period. If the number of sub-modules in each arm is very
large, the sorting time can occupy a large pro portion.
3. The Novel Voltage Balancing Method for
SUPWM Method for MMC
The novel voltage balancing method is derived from the
conventional voltage balancing method. The basic prin-
ciples of the novel voltage balancing method are pre-
sented as follows.
The sub-modules in each arm are divided into
several equal groups. For example, if the number
of sub-modules in each arm is 20 and the number
of groups can be selected as 5, each group can
have four sub-modules. Sub-modules 1, 2, 3 and
4 can be assigned to group1, sub-modules 5, 6, 7
and 8 can be assigned to group2, sub-modules 9,
10, 11 and 12 can be assigned to group3, sub-
modules 13, 14, 15 and 16 can be assigned to
group4 and sub-modules 17, 18, 19 and 20 can be
assigned to group 5.
The voltages of sub-module comparators in each
group are sorted according to the order from low-
er to higher or from higher to l ower.
The average voltage of each group is computed
and the average voltages in each arm are sorted
according to the order from lower to higher or
from higher to lower.
According to the arm current and the number of
sub-modules which are needed to switch on, the
groups are labeled by 1 (all of sub-modules in this
group are switched on), 2 (some of sub-modules
in this group are switched on) and 3 (all of
sub-modules in this group are switched off).
When the arm current is charging capacitors, the
label of group whose average voltage is lower is 1
or 2 and the label of group whose average voltage
is higher is 2 or 3. When the arm current is dis-
charging capacitors, the label of group whose av-
erage voltage is higher is 1 or 2 and the label of
group whose average voltage is lower is 2 or 3.
For example, if the sorting result of average volt-
ages is group 4>group 3>group 5>group 1>group
2, the arm current is charging the capacitors and
the number of sub-modules which are needed to
switch on is 10, the labels of group 2 and group1
will be 1, the lab el of group5 will be 2 and the la-
bels of group 3 and group 4 will be 3.
The sub-modules in each group will be switched on or
off based on the sorting result of the capacitor voltages in
group, the label of group, the arm current and the number
of sub-modules which are needed to switch on. If the
label of group is 1 (or 3), all of sub-modules in this group
are switched on (or switched off). When the arm current
is charging capacitors and the label of group is 2,
sub-module whose capacitor voltage is lower is switched
on and sub-module whose capacitor voltage is higher is
switched off. When the arm current is discharging ca-
pacitors and the label of group is 2, sub-module whose
capacitor voltage is higher is switched on and the
sub-module whose capacitor voltage is lower is switched
off. For example, if the arm current is charging capaci-
tors, the number of sub-modules which are needed to
switch on is 10, the labels of group 2 and group 1 are 1,
the label of group 5 is 2, the labels of group3 and group 4
are 3 and the sorting resu lt of capacitor voltage s of group
5 is SM20>SM18>SM17>SM19, all of sub-modules in
group 2 and group 1 are switched on, all of sub-modules
in group 3 and group 4 are switched off, SM17 and SM19
are switched on and SM20 and SM18 are switched off.
4. Simulation Results
In order to verify the validity of the proposed voltage
balancing method in this paper, a MMC based three-
phase inverter is taken as the test example. The setup of
the inverter for simulation is shown in Figure 1. Com-
puter simulation is carried out first in PSIM software and
the parameters are listed in Table 1.
The number of group is set to four, so each group has
three sub-modules. Simulated waveforms of conven-
tional voltage balancing method are depicted in Figure 2
Table 1.
AC system voltage (line-to-line) US = 10kV
Active Power P = 12MW
Reactive Power Q = 6MVA
DC-link voltage UDC = 20kV
Sub-module cap a ci t o r CSM = 8.5mF
Buffer inductors L0 = 8mH
No. of sub-modules in each arm N = 12
Voltage ref. of su b -module capacitor USM_ref = 1667V
Carrier frequency fcarrier = 2kHz
Copyright © 2013 SciRes. EPE
Z. F. CHU ET AL.
1174
0.90.92 0.94 0.96 0.981
Ti me (s)
1.55K
1.6K
1.65K
1.7K
1.75K
1.8K
Voltage[V]
0.9 0.920.940.960.98
Time (s)1
1.55K
1.6K
1.65K
1.7K
1.75K
1.8K
Voltage[V]
0.90.92 0.94 0.96 0.981
Ti me (s)
0K
-0.5K
-1K
-1.5K
0.5K
1K
1.5K
Cu
r
r
ent[A]
0.90.92 0.94 0.96 0.981
Ti me (s)
0K
-5K
-10K
-15K
5K
10K
15K
Figure 2. Simulated waveforms of conventional voltage ba-
lancing method.
0.90.92 0.94 0.96 0.981
Time (s)
1.55K
1.6K
1.65K
1.7K
1.75K
1.8K
Voltage[V]
0.90.92 0.94 0.96 0.981
Time (s)
1.55K
1.6K
1.65K
1.7K
1.75K
1.8K
(b) Voltages of su b-module capacitors in lower arm of phase A
0.90.92 0.94 0.960.981
Time (s )
0K
-0.5K
-1K
-1.5K
0.5K
1K
1.5K
iBiCiA
(c) Three phase currents (iA,iB,iC)
0.90.92 0.94 0.96 0.981
Time (s)
0K
-5K
-10K
-15K
5K
10K
15K
Figure 3. Simulated waveforms of proposed voltage bal-
ancing method.
and Figure 3 shows the simulation results of proposed
voltage balancing method.
Compared Figure 2 with Figure 3, the voltage fluc-
tuation range of sub-modules capacitors using proposed
method is 200 larger than conventional voltage balancing
method 140. From Figures 3(a) and (b), the voltages of
sub-module capacitors are balanced. Although the bal-
ancing of voltage of proposed method is worse than con-
ventional method, the sorting time of proposed method is
shorter than conventional. Because the number of volt-
ages which are needed to be sorted using conventional
method is twelve and it is only four when using proposed
method. So the biggest advantage of proposed method is
that it greatly saves sortin g time.
5. Summary
This paper proposed a novel voltage balancing method
for MMCs. The proposed method is suitable for more
sub-modules in arms. The more sub-modules each arm
has, the more sorting time is saved compared with con-
Copyright © 2013 SciRes. EPE
Z. F. CHU ET AL.
Copyright © 2013 SciRes. EPE
1175
ventional method. Simulation results proved the validity
of this method.
REFERENCES
[1] A. Lesnicar and R. Marquardt, “An Innovative Modular
Multilevel Converter Topology Suitable for a Wide
Power Range,” in Power Tech Conference Proceedings,
2003 IEEE Bologna, Vol. 3, 2003, p. 6.
[2] M. Hagiwara and H. Akagi, “PWM Control and Experi-
ment of Modular Multilevel Converters,” in Power Elec-
tronics Specialists Conference, PESC 2008, IEEE, 2008,
pp. 154-161.
[3] M. Hagiwara and H. Akagi, “Control and Experiment of
Pulsewidth-Modulated Modular Multilevel Converters,”
Power Electronics, IEEE Transactions on, Vol. 24, 2009,
pp. 1737-1746. doi:10.1109/TPEL.2009.2014236
[4] S. Rohner, et al., “Pulse Width Modulation Scheme for
the Modular Multilevel Converter,” in Power Electronics
and Applications, EPE '09. 13th European Conference on,
2009, pp. 1-10.
[5] S. Rohner, et al., “Modulation, Losses, and Semiconduc-
tor Requirements of Modular Multilevel Converters,” In-
dustrial Electronics, IEEE Transactions on, Vol. 57, 2010,
pp. 2633-2642.
[6] T. Qingrui, et al., “Reduced Switching-Frequency Modu-
lation and Circulating Current Suppression for Modular
Multilevel Converters,” Power Delivery, IEEE Transac-
tions on, Vol. 26, 2011, pp. 2009-2017.
doi:10.1109/TPWRD.2011.2115258
[7] Z. Li, et al., “An Improved Pulse Width Modulation
Method for Chopper-Cell Based Modular Multilevel Con-
verters,” Power Electronics, IEEE Transactions on, 2012,
p. 1.
[8] N. Flourentzou and V. G. Agelidis, “Multimodule
HVDC System Using SHE-PWM With DC Ca-
pacitor Voltage Equalization,” Power Delivery,
IEEE Transactions on, Vol. 27, pp. 79-86,
2012.doi:10.1109/TPWRD.2011.2167989