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A 43-Level 33 kV 3-Phase Modular Multilevel
Cascaded Converter for Direct Grid Integration of
Renewable Generation Systems
Md. Rabiul Islam, Youguang Guo, Senior Member, IEEE, Mohammad Jafari, Zahra Malekjamshidi, and Jianguo Zhu,
Senior Member, IEEE
School of Electrical, Mechanical and Mechatronic Systems
University of Technology Sydney
PO Box 123, Broadway, Sydney, Australia
Md.Islam@uts.edu.au and Rabiulbd@hotmail.com
Abstract—This paper proposed a 43-level 3-phase 33 kV
modular multilevel cascaded (MMC) converter for direct grid
integration of renewable generation systems. A high-frequency
magnetic-link is considered to generate isolated and balanced
multiple dc sources for all of the H-bridge inverters of the MMC
converter. The proposed converter is designed and analyzed
taking into account the specified system performance, control
complexity, cost and market availability of the semiconductors.
The simulation results demonstrate the excellent feature of the
proposed medium-voltage converter. It is expected that the
proposed new technology will have great potential for future
renewable power plants and smart grid applications.
Index Terms—Modular multilevel cascaded converter, medium-
voltage, photovoltaic power plants, grid integration.
I. INTRODUCTION
Different power electronic converters have been developed
using conventional topologies to fulfill the requirements of
renewable generation systems [1], [2]. However, it is hard to
connect the traditional converters to the grids directly, as the
distortion in generated output voltages is high and a single
switch cannot stand the grid voltage level. Many power
semiconductor vendors such as Semikron, ASEA Brown
Boveri (ABB), IXYS, Siemens and Mitsubishi Electric
produce devices specially designed for the diode rectifier
based converter and back-to-back converter for wind turbine
generator systems. All of the devices are in a pack, which
reduces the cost and complexity of the power conditioning
system. Semikron developed compact modules
SKS660FB6U+E1C+B6CI250V06 and IGDD6-4-426-D3816-
E1F2-BL-FA for the diode rectifier based power conditioning
systems. According to the internal circuit configuration, the
module SKS660FB6U+E1C+B6CI250V06 is suitable for
permanent magnet synchronous generator (PMSG)-based
wind turbine generator systems and IGDD6-4-426-D386-
E1F2-BL-FA is suitable for wound rotor synchronous
generator (WRSG)-based wind turbine generator systems.
Mitsubishi Electric developed the module CM00MXA-24S
with this converter topology which can be used for both the
WRSG and PMSG-based wind turbine generator systems.
Semikron also developed the module SKSC120GDD69/11-
A3AWAB1B for power conditioning of synchronous and
doubly-fed generator-based wind power systems.
ABB central inverters are especially designed for medium
scale photovoltaic (PV) power plants. The PVS800 version is
a 3-phase inverter with a power capacity in the range of 100–
500 kW. The transformer steps-up the inverter output voltage
from 300 V ac to grid voltage level (e.g., 6–36 kV). Siemens
developed the SINVERT PVS inverter for medium scale PV
power plants. The ac output voltage and power capacity of the
PVS version inverters are in the range of 288–370 V and 500–
630 kW, respectively.
With these converters, the conventional renewable
generation systems possessing the power-frequency (i.e., 50
or 60 Hz) step-up transformer and the line filter and booster
not only increase the size, weight and loss but also increase
the cost and complexity of the system operation [1]. Today,
the industrial trend is to move away from these heavy and
large size passive components to power electronic systems
that use more and more semiconductor elements controlled
by a digital circuit. In such a way, smart operation is ensured.
In comparison with conventional two level converters,
multilevel converters present lower switching losses, lower
voltage stress on switching devices, and better total harmonic
distortion (THD) [3]–[7]. These remarkable features enable
the connection of renewable energy systems directly to the
grid without using large, heavy and costly power transformers
and also minimize the input and output filter requirements
[8]–[14]. Although several multilevel converter topologies
have been used in low voltage applications, most of the
topologies are not suitable in medium voltage applications.
Because of some special features (e.g., the number of
components scale linearly with the number of levels and
individual modules are identical and completely modular in
constriction hence enabling high level attainability), the
modular multilevel cascaded (MMC) converter topology can
be considered as the best possible candidate for medium-
voltage applications [15], [16]. The high number of levels
means that medium-voltage attainability is possible to
connect the renewable generation units to the medium-
voltage grid directly and it is also possible to improve the
output power quality. The component number and control
complexity increase linearly with the increase in the number
of levels [1], [17]. Fig. 1 plots the component number and
complexity of different level converters.
5 10 15 20
20
40
60
80
100
120
Number of levels
Number of IGBTs/Complexity
Number of IGBTs
Control complexity
Fig. 1. Number of IGBTs/Control complexity versus converter level numbers
of 11 kV system.
8 10 12 14 16 18 20
4
6
8
10
12
Number of levels
THD (%)/Cost (×10000)
Semiconductor cost
(AUD)
THD (%)
Fig. 2. THD (%)/Semiconductor cost versus converter level numbers of 11
kV system.
On the other hand, the distortion in generated output
voltage and semiconductor cost of the converter decrease
dramatically with the increase of the converter number of
levels. Fig. 2 plots THD (%) and semiconductor cost of
different number of MMC converter levels for an 11 kV
system. Due to the unavailability of rated insulated gate
bipolar transistors (IGBTs), the 13 and 17 level converters
used the IGBTs that are used in the 11 and 15 level
converters, respectively. Hence, the semiconductor cost curve
is up-and-down in nature. Moreover, lower switching
frequency, even fundamental switching frequency, can be
used with the high level number converter, which
significantly reduces the switching losses of the converter.
Therefore, the optimal selection in the number of converter
levels is important for the best performance/cost ratio of the
medium-voltage converter systems and this is one of the
central contentions of this paper [17]. In this paper, a 33 kV
system is designed and analyzed taking into account the
specified system performance, control complexity, and cost
and market availability of the power semiconductors. It is
found that the 43-level converters are the optimal choice for
the 33 kV systems. The design and analysis of a 33 kV MMC
converter system is presented in detail in the following
sections.
II. D
ESIGN OF 43-LEVEL 33 KV MODULAR MULTILEVEL
CASCADED CONVERTER
A. Selection of Number of Level of 33 kV converter
Each H-bridge inverter cell commutation voltage of a 13-
level converter is about 4044 V. The highest voltage rating of
a commercially available IGBT is 6.5 kV, which is
recommended for a maximum voltage of 3600 V. Therefore,
a 13-level or lower level converter cannot be used to design
the 33 kV converter. Each H-bridge inverter cell
commutation voltage of a 15-level topology based 33 kV
converter is 3467 V which may be supported by the 6.5 kV
IGBT. Owing to this, at least 15-level topology is required to
design a 33 kV converter. The output power quality of a 55-
level inverter is good enough to directly feed into the 33 kV
ac grid. The cheap 1.7 kV IGBT can be used to design the 55-
level inverter. There are no significant performance
improvements or cost reductions with converters of more
than 55-levels. Moreover, the control complexity increases
with the number of converter levels. Due to these, 15-level to
55-level MMC converter topologies are considered for a 33
kV inverter system.
The arithmetic and logic operations (ALOs) for switching
section, THD of output power, and cost of semiconductors
are calculated. The number of ALOs is used to compare the
complexity of the converters. The THD are calculated
through the MATLAB/Simulink environment. Fig. 3 plots the
component number and complexity of different level
converters for 33 kV system. Fig. 4 plots THD (%) and
semiconductor cost of different number of MMC converter
levels for a 33 kV system. Normalized index values are
calculated. Fig. 5 plots the normalized total index values of
different converters. For the 33 kV converter, the total index
value is the lowest at the 43-level, because there is no
significant output power quality improvement and
semiconductor cost reduction for converters with more than
43 levels. In addition, the component number and control
complexity increase linearly with the increase in the number
of levels. Therefore, 43-level topology is considered as
optimal for 33 kV converter systems.
15 20 25 30 35 40 45 50 55
0
100
200
300
400
Number of levels
Number of IGBTs/complexity
Number of IGBTs
Control complexity
Fig. 3. Number of IGBTs/control complexity versus converter level numbers
of 33 kV system.
15 20 25 30 35 40 45 50 55
2
4
6
8
10
Number of levels
THD (%)/Cost (x30000)
Semiconductor cost
(AUD)
THD (%)
Fig. 4. THD (%)/Semiconductor cost versus converter level numbers of 33
kV system.
15 20 25 30 35 40 45 50 55
1
1.5
2
Number of levels
Normalized total index
Normalized total index
(performance, complexity, and cost)
Fig. 5. Normalized total index versus converter level numbers of 33 kV
system.
B. Power Circuit of 43-Level 33 kV MMC Converter
Each H-bridge inverter dc-link voltage rating of a 33 kV
43-level converter is 1156 V. The available cheap and mature
2.5 kV IGBT can be used to design the 33 kV 43-level
converter, because this IGBT is recommended for 1200 V
maximum applications. About 96% device voltage utilization
factor (DVUF) can be obtained with the 2.5 kV IGBTs. In
total, 21 H-bridge inverter cells are on each phase-leg and 252
active switching devices are required for the 3-phase 43-level
converter. Fig. 6 shows the circuit diagram of 43-level
converter.
Fig. 6. Circuit diagram of a 43-level MMC converter.
However, the MMC converter requires multiple isolated
and balanced dc sources. A high-frequency magnetic-link is
considered to generate multiple isolated and balanced dc
supplies for all of the H-bridge inverter cells of the MMC
converter from a single or multiple renewable source. The grid
electrical isolation and voltage imbalance problems are solved
through the common high-frequency magnetic-link [18].
C. Switching Circuit of 43-Level MMC Converter
The phase-shifted carriers are specially conceived for FC
[19] and MMC [20] converters. Since each FC cell is a two-
level converter, and each H-bridge cell is a 3-level inverter,
the traditional bipolar (using one carrier signal that is
compared to the reference to decide between two different
voltage levels, typically the positive and negative busbars of a
voltage source converter) and unipolar pulse width
modulation (PWM) techniques can be used, respectively. Due
to the modularity of these topologies, each cell can be
modulated independently using the same reference signal in a
phase. If the peak to peak amplitude of the carriers is A
c
, the
amplitude modulation index can be calculated from
c
m
ap
A
A
m
. (1)
Fig. 7 shows the basic block diagram of the phase-shifted
switching scheme for a 3-phase 43-level converter. If
B
n
is
the number of the H-bridge inverter cell or pair on a
particular phase leg and
m the number of converter levels, the
carrier phase-shifting for that particular cell or pair can be
calculated from
)1(
)1(360
m
B
n
o
ps
. (2)
Each compare unit generates one switching signal for the
top switching device of a half-bridge cell or a pair. The
inverted form of this switching signal drives the bottom
switching device. For the left half-bridge cell, one is asserted
when the reference signal value is greater than or equal to the
carrier signal value, and the other is asserted when the
reference signal value is less than the carrier signal value. For
the right half-bridge cell, one is asserted when the inverted
carrier signal value is greater than or equal to the reference
signal value, and the other is asserted when the inverted carrier
signal value is less than the reference signal value. Fig. 8
shows the gate pulse generation technique for the top
switching device and the technique to generate gate pulse for
the bottom switching device is illustrated in Fig. 9.
Fig. 7. Switching control scheme of 43-level MMC converter
For the flying capacitor (FC) multilevel converter, the
advantage of the even power distribution is that once the
flying capacitors are properly charged (initialized to their
corresponding values) no imbalance will be produced due to
the self balancing property of this topology [21], [22] and as a
result there is no need to control the dc-link voltages. Another
interesting feature is that the total output voltage has a
switching pattern with k (number of the power cells) times the
frequency of
the switching pattern of each cell. This
multiplicative effect is produced by the phase shifts of the
carriers. Hence, better THD is obtained at the output, using the
k time’s lower frequency carriers. With the phase-shifted
carrier based modulation scheme, the control signal
assignment to the appropriate semiconductor of the MMC
converter is easy and this remains simple even when the level
number increases to higher values.
Fig. 8. Generation of gate pulse PS
1
with phase shifted carrier.
Signals
Fig. 9. Generation of gate pulse PS
2
with phase shifted carrier.
III. SIMULATION RESULTS
A total of 231 ALOs are involved with the switching
scheme. Although three times more switching devices are
used in the 43-level converter, the semiconductor cost is about
41% lower than that of the 15-level converter, due to the
cheap cost of low voltage rated devices. The line peak to peak
voltage consists of 84 voltage levels and each level contributes
1156 V to the peak to peak line voltage. Due to small step
size, the line voltage waveforms are found to be very
consistent with the reference sine waveforms. Fig. 10 plots the
line voltage waveforms. The output power quality of a 43-
level converter is good enough to feed into the grid. The line
voltage THD is about 3.61%, which satisfies well the 5% limit
by IEEE1547 and IEC61727 standards. The frequency
spectrum of line voltage is shown in Fig. 11. Compared with
the 15-level converter, the 43-level converter provides 44%
better quality output power. As the number of active switching
devices has increased three times, the complexity of the
