Energy and Power Engineering, 2013, 5, 1512-1516
doi:10.4236/epe.2013.54B286 Published Online July 2013 (http://www.scirp.org/journal/epe)
Research on the Topology and Control Scheme of an
Innovative Modular Multilevel Converter*
Daixiang Zhu, Ming Ding
Department of Electrical Engineering, Hefei University of Technology, Hefei 230009, China
Received 2013
ABSTRACT
This paper presents a new modular multilevel converter (MMC) topology. Compared to conventional multilevel con-
verters, MMC has much lower switching frequency (50 Hz) resulting in lower switching losses, and consequently,
lower total losses of the transmission system. The fundamental concept and the applied control scheme are introduced
in detail. A modified multilevel fundamental switching modulation scheme adopting the multicarrier pulse width
modulation concept is presented. A capacitor voltage balancing technique is proposed. With the established simulation
model of the 11-level MMC, the modulation and balancing strategy presented are confirmed by MATLAB/SIMULINK
simulations. The multicarrier pulse width modulation converter strategy enhances the fundamental output voltage and
reduces total harmonic distortion. This new type of converter is suitable for high-voltage drive systems and power sys-
tem applications such as high voltage dc (HVDC) transmission, reactive power compensation equipment and so on.
Keywords: Sub-Modular; Modular Multilevel Converter; Fundamental Switching Modulation; Topology;
CAPACITOR Voltage Balancing
1. Introduction
Nowadays, multilevel converter is an emerging and
highly attractive topology for medium-voltage (MV) and
high-voltage applications [1,2]. There are three major
multilevel converter topologies [3]: 1) the neutral-point-
clamped converter (NPC), 2) flying capacitor converter
(FC), and 3) cascaded H-bridge converter (CHB). The
three-level NPC is probably the most widely used topol-
ogy for medium-voltage AC motor drives and PWM ac-
tive rectifiers. The NPC suffers from the problem of
voltage imbalance of the dc link capacitors; this problem
increases complexity with an increasing number of levels.
The FC also consists of a series connection of identical
commutation cells-the number of voltage levels in the
line-to-line voltage depends on the number of commuta-
tion cells. One particular challenge is balancing the ca-
pacitor voltages, which is accomplished by the choice of
the converter switching states. The main drawback is the
fact that CHB needs an isolated dc-source, usually pro-
vided by a three-phase rectifier fed by a transformer.
Hence, for a three-phase five-level inverter, a three-phase
transformer with six secondary three-phase windings and
the corresponding diode bridges are necessary, increasing
the volume and cost of the converter.
To solve the aforementioned problems, a modular
multilevel converter (MMC) topology has been proposed
[4,5]. The MMC features the following advantages: 1)
distributed location of capacitive energy storages; 2)
modular construction; 3) simple voltage scaling by a se-
ries connection of cells;4) Lower switching frequencies
(50 Hz) make the overall converter losses closer to the
thyristor technology. Due to its interesting characteristics,
the MMC topology is very attractive for high-voltage dc
transmission and MV converters.
This paper presents a MMC topology. The fundamen-
tal concept and the applied control scheme are introduced.
A modified multilevel fundamental switching modulation
scheme adopting the multicarrier pulse width modulation
concept is presented. A capacitor voltage balancing tech-
nique is proposed. Finally, simulation model of the
11-level MMC is established. The modulation and bal-
ancing strategy presented are confirmed by MATLAB/
SIMULINK simulations. The multicarrier pulse width
modulation converter strategy enhances the fundamental
output voltage and reduces total harmonic distortion.
This new type of converter is suitable for high-voltage
drive systems and power system applications such as
high voltage dc (HVDC) transmission, reactive power
compensation equipment and so on.
*The paper was supported by the National High Technology Research
and Development of China(863 Programme) ( No.2007AA05Z240)and
the 111 Project（B07032）and The Funds for Scientific Research's
Development of Hefei University of Technology(2010HGXJ0063).

D. X. ZHU, M. DING 1513
2. The Modular Multilevel Converter
2.1. Topology of MMC
A basic structure of a MMC is shown in Figure 1. MMC
is composed of many series-connected sub-modules
(SMs) and the primary function of SMs provides many
separate DC sources for synthesizing the desired voltage.
The number of output voltage levels in MMC is N+1,
where N is the number of SMs in per upper or lower arm
of a single phase. Seen from the AC side, all SMs of a
single phase are connected in series. So its AC output
voltage is the sum of all SMs’ output. By switching SMs
in the upper and lower arm, the voltage Udc is adjusted.
In a similar manner, the voltage Ujo (j=a, b, c) can be
adjusted to a desired value if Udc is fix. No additional
external connection or energy transmission to the SMs is
needed, for full 4-quadrant operation of the converter
system.
The arm inductor is in series with the distributed en-
ergy storage capacitors, so the effects of faults arising
inside or outside the converter can be reduced substan-
tially by the arm inductor such as a short circuit between
the DC terminals of the converter. These faults are
swiftly detected, and due to the low current rise rates, the
switching devices can be turned off at absolutely uncriti-
cal current levels. Consequently, this feature provides
very effective and reliable protection of the system. In
addition, the three-phase modules of the converter are
connected in parallel at the DC side. Since the three gen-
erated DC voltages of the phase modules cannot be ex-
actly equal, balancing currents occur between the indi-
vidual phase units. The converter reactors damp these
balancing currents to a very low level and make them
controllable.
2.2. Basic Operating Principle of SM
A suitable and simple realization of the SM is given in
Figure 2. The interface of the SM is composed solely of
two electrical terminals and one bi-directional fiber-optic
Figure 1. Three-phase topology diagram of a MMC.
interface. Table 1 summarises the switch states of a cell
and their resultant influence on associated capacitor
voltages. When the switching device Sm is on and Sc is off,
output voltage VO=0; when the switching device Sm is off
and Sc is on, voltage VO=Vdc.
No VO=-Vdc is possible. Output voltage of SM is uni-
polar. By switching a number of the N SMs in the upper
and lower arm, the voltages in each arm can be synthe-
sized. So the voltage Udc and voltages Ujo (j=a, b, c) can
be adjusted independently.
Figure 3 illustrates single phase topology diagram of a
MMC. Assuming the capacitor voltage is V
dc, output
voltage of each SM can take on one of two different
voltage levels. With S
c on state, it is equal to V dc, and
when Sm is on, is zero. Therefore, it is possible to selec-
tively and separately control each of the individual SMs
to provide a voltage which is either Vdc or zero. The
switches of the SMs are operated so that the individual
voltages from each SM add up to form a multilevel,
near-sinusoidal stepped waveform at the converter ter-
minals. The number of output voltage levels in MMC is
N+1, where N is the number of SMs in per upper or
lower arm of a single phase. The number of capacitors in
charging and discharging state is N. The number of Sc in
the on state in the upper and lower arm is M and L. Then
the following equations can be obtained.
M+L=N (1)
In the upper arm, the number is M of the switching de-
vice Sc and Sm in on and off state; in the lower arm, the
number is M of the switching device Sm and Sc in off and
on state. Then, control state of the SMs in the upper and
lower arm is fully symmetrical, i.e., control scheme of
the SMs in the upper and lower arm is the same. The
relationship among the dc-link voltage Udc and the ca-
pacitor voltage Vdc is given by
Figure 2. Structure of a SM.
Table 1. Commonly control states of a sub-modular.
Mode Sm S
C i O VO Power
path C
1 on off>00 Sm unchanged
2 on off<00 Dm unchanged
3 offon >0Vdc D
c charging
4 offon <0Vdc S
c discharging

D. X. ZHU, M. DING
1514
dc dc
dc
UU
V
M
LN

 (2)
Synthesis of phase output voltage of an 11-level MMC
is shown in Table 2. Here, ua1 and ua2 are output voltage
of the upper and lower arm of phase a; uao is output volt-
age of phase a.
3. Control Scheme
3.1. Multicarrier Fundamental Switching
Modulation Strategy
Fundamental switching modulation is that the switches in
the converters only need to be switched on and off once
during one fundamental cycle; thus, the switching loss of
the devices is reduced to minimum. The typical technol-
ogy is staircase synthesis principle, i.e., it uses a staircase
waveform to approximate the desired sinusoidal wave-
form. Staircase synthesis principle of multicarrier modu-
lation is given by Figure 5 with triangular carrier and
sinusoidal reference waveform. It shows an 11-level con-
verter’s output phase voltage waveform. The goal is to
choose the switching angles 0≤θ1<θ2<…<θ5≤90° so as
to make the staircase waveform approximately equal to
the given desired fundamental voltage uao. For ‘N’ level
converter ‘N-1’ carriers are used. Interaction of particular
carrier and reference is used to generate gating signal for
particular complementary pair of switches in MMC.
Figure 3. Single phase topology diagram of a MMC.
Table 2. Synthesis of phase output voltage.
M L ua1 ua2 uao
5 5 5 Vdc 5Vdc 0
4 6 4Vdc 6Vdc Vdc
3 7 3Vdc 7Vdc 2Vdc
2 8 2Vdc 8Vdc 3Vdc
1 9 Vdc 9Vdc 4Vdc
0 10 0 10Vdc 5Vdc
6 4 6Vdc 4 Vdc -Vdc
7 3 7Vdc 3Vdc -2Vdc
8 2 8Vdc 2Vdc -3Vdc
9 1 9Vdc Vdc -4Vdc
10 0 10Vdc 0 -5Vdc
By Figure 4 we can get the desired sinusoidal wave-
form which is odd quarter-wave symmetric. In quarter of
period, the upper triangular carrier waveform can be ex-
pressed as,
2 0, 1,2,
2
k
ytktk



 5 (3)
Sinusoidal reference waveform is described by
5sin 02
ref
ytt



 (4)
Based on (3) and (4), we can get the switching angles
for an 11-level converter as follows. 110.21


,
220.74


, 331.96


, 444.54

, 560.09


.
For the modular 11-level converter, If we get output
phase voltage waveform at AC side, we must make sub-
modular switches Sc of the upper and lower leg (phase a)
to be triggered with switching angles by Figure 5.
Among the SMs, gate signal generation of Sm is opposite
to Sc of corresponding SM. The advantage of this modu-
lation method is that with the increase of voltage steps,
staircase voltage gradation is finer and finer. Thus, the
approximation becomes more and more accurate.
3.2. Selection of SMs and Capacitor Voltage
Balancing
The capacitor voltages of the individual SMs must be
monitored and kept equal. The voltages of the capacitors
are periodically measured with a typical sampling-rate in
the millisecond-range. According to their voltage, the
capacitors are sorted in descending order by software.
The modulation strategy determines the number of SMs
that should be on in the upper and lower arms of the
MMC (i.e., S
c). Capacitor voltage values and also the
direction of the arm currents are used to select Sc of the
SMs in the upper (lower) arm and to determine which
Figure 4. Staircase synthesis principle of multicarrier
modulation.

D. X. ZHU, M. DING 1515
(a) Gate signals generation of Sc in the upper leg (phase a)
(b) Gate signals generation of Sc in the lower leg (phase a)
Figure 5. Three-phase to
Ms should be switched on. When the current in the up-
feasible of the staircase
pology diagram of a MMC.
S
per (lower) arm is positive, in order to impress the de-
sired arm voltage, the required SMs with the lowest
voltages are switched on. When the current in the upper
(lower) arm is negative, in order to impress the desired
arm voltage, the demanded number of SMs with the
highest voltages is selected. Regardless of the direction
of the upper (lower) arm current, if Sc of the SM is off,
the corresponding capacitor will be bypassed and its
voltage remains unchanged. By this method, continuous
balancing of the capacitor voltages is achieved. Addi-
tionally, the power losses can be kept low by switching
SMs solely, when a change of the output state is re-
quested.
4. Simulation Validation
To validate the correctness and
waveform modulation technology, an 11-level modular
converter is constructed on MATLAB/SIMULINK. The
upper (lower) arm of every phase is composed of ten
identical SM. There is no additional central capacitive
energy storage connected to the DC-bus bar. DC-link
voltage Udc is 20 kV and the capacitance C installed in
each SM is 3000uF. The values of load resistance and
inductance are 100 Ω and 500 mL. The relevant voltage
waveforms and phase current waveforms of this 11-level
topology are shown as Figures 6-8. Figure 9 shows the
capacitor voltages of the ten SMs of the upper arm of
phase a. Simulation waveforms have demonstrated a
good performance of the MMC concept and verified the
correctness and feasible of the model.
Figure 6. Upper and lower phase leg voltages ua1 and ua2 of
phase a, phase voltage uao.
Figure 7. Phase currents.
Figure 8. Line-to-line voltages.

D. X. ZHU, M. DING
1516
Figure 9. Upper phase leg capacitor voltages of phase a.
5. Conclusions
ed the topology of the new mo
r high-voltage dc transmission and MV con-
verters.
ectro Technical Society, Vol. 19, No. 8, 2004,
l Elec-
2008, pp. 28-39.
This paper introducdular
multilevel converter and its modulation control scheme.
The staircase waveform modulation technology is dis-
cussed in detail, very convenient and suitable for multi-
level converters. It turns out to be more and more out-
standing with the increase of voltage levels. The output
AC voltages can be adjusted in very fine increments. The
proposed modulation technology makes the individual
switching devices have much lower switching frequency
(50 Hz) resulting in lower switching losses compared to a
conventional VSC. Therefore, total system losses are
relatively small and efficiency is consequently increased.
Due to its interesting characteristics, the topology is very
attractive fo
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