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Proceedings Article•DOI•

A 43-level 33 kV 3-phase modular multilevel cascaded converter for direct grid integration of renewable generation systems

Rabiul Islam1, Youguang Guo1, Mohammad Jafari1, Zahra Malekjamshidi1, Jianguo Zhu1 •
University of Technology, Sydney1
20 May 2014-pp 594-599
TL;DR: In this paper, the authors proposed a 43-level 3-phase 33 kV modular multilevel cascaded (MMC) converter for direct grid integration of renewable generation systems, where 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.
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.

Summary (2 min read)

Jump to: [A. Selection of Number of Level of 33 kV converter] – [B. Power Circuit of 43-Level 33 kV MMC Converter] – [C. Switching Circuit of 43-Level MMC Converter] – [III. SIMULATION RESULTS] and [CONCLUSION]

A. Selection of Number of Level of 33 kV converter

  • Each H-bridge inverter cell commutation voltage of a 13level converter is about 4044 V. Moreover, the control complexity increases with the number of converter levels.
  • The arithmetic and logic operations (ALOs) for switching section, THD of output power, and cost of semiconductors are calculated.
  • 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.

B. Power Circuit of 43-Level 33 kV MMC Converter

  • 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.
  • The MMC converter requires multiple isolated and balanced dc sources.
  • 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 twolevel 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.
  • 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.

III. SIMULATION RESULTS

  • A total of 231 ALOs are involved with the switching scheme.
  • The semiconductor cost is about 41% lower than that of the 15-level converter, due to the cheap cost of low voltage rated devices.
  • Due to small step size, the line voltage waveforms are found to be very consistent with the reference sine waveforms.
  • The output power quality of a 43level converter is good enough to feed into the grid.

CONCLUSION

  • The designs of 33 kV converter systems have been analyzed, which helps in selecting the optimal number of levels for a medium-voltage converter.
  • All possible MMC converter topologies have been considered for the 33 kV converter systems.
  • In order to ensure a cost effective design, the DVUFs were calculated and only the selected converters with high DVUF were considered.
  • During the design process, the availability of the semiconductor devices was considered in the first instance.
  • After checking the availability of devices, the converter systems were designed taking into account three main factors: the specified converter output power quality, complexity of the switching controller, and cost of the semiconductors.

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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
Citations
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Journal Article•DOI•
Future Power Distribution Grids: Integration of Renewable Energy, Energy Storage, Electric Vehicles, Superconductor, and Magnetic Bus

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Kashem M. Muttaqi1, Md. Rabiul Islam1, Danny Sutanto1•
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30 Jan 2019-IEEE Transactions on Applied Superconductivity
TL;DR: This paper focuses on a review of the state of the art of future power grids, where new and modern technologies will be integrated into the power distribution grid, and will become the future key players for electricity generation, transmission, and distribution.
Abstract: This paper focuses on a review of the state of the art of future power grids, where new and modern technologies will be integrated into the power distribution grid, and will become the future key players for electricity generation, transmission, and distribution. The current power grids are undergoing an unprecedented transformation from the original design, changing the way how energy has been produced, delivered, and consumed over the past century. This new energy era includes the integration of renewable sources such as wind and solar, supported by the distributed or community energy storage, to power distribution grids through innovative high-frequency magnetic links and power-electronic converters. The use of emission free transportation, such as electric vehicles, and energy efficient technologies, such as superconducting generators and storage systems, are also rapidly emerging and will be integrated into the power grids in the foreseeable future. However, it is necessary to reconsider the current paradigms of system analysis and plan with a focus on how to achieve the most flexible, efficient, and reliable power grid for the future - the one that enables operation in a domain which is very different than the current one to deliver the services to consumers at an affordable cost.

80 citations

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  • ...In order to reduce the size and weight of the energy conversion system, a step-up transformer-less direct grid integration technology has been proposed in recent years [7]....

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Proceedings Article•DOI•
Modeling of magnetic flux in multi-winding toroidal core high frequency transformers using 3D reluctance network model

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Mohammad Jafari1, Zahra Malekjamshidi1, Md. Rabiul Islam1, Jianguo Zhu1•
University of Technology, Sydney1
09 Jun 2015
TL;DR: In this paper, a 3D reluctance network is introduced for modelling of magnetic flux distribution and estimation of self and leakage inductances of multi-winding toroidal core high frequency transformer, and the effective factors on accuracy of model and computation time are studied and the results are compared with experimentally measured parameters to verify the accuracy.
Abstract: Multi-winding transformers as main part of multiport phase shift converters play an important role in integration of renewable energy sources, storage device and loads. The range of power transfer in multi-port phase shift converters particularly depends on the leakage inductance of multi-winding transformer. Numerical analysis such as 2D and 3D finite element methods have been widely used in analysis of magnetic structures and estimation of leakage inductance although their computation time is the main issue especially in online and iterative design processes. In this paper, a 3D reluctance network is introduced for modelling of magnetic flux distribution and estimation of self and leakage inductances of multi-winding toroidal core high frequency transformer. The proposed network is formed in cylindrical coordination, according to the geometrical shape of toroidal core. The effective factors on accuracy of model and computation time are studied and the results are compared with experimentally measured parameters to verify the accuracy of model.

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  • ...The toroidal transformer was already designed as a part of project on integration of renewable energy sources into the grid using multi-level inverters [2], [2]....

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Structure and the space vector modulation for a medium-voltage power-electronic-transformer based on two seven-level cascade H-bridge inverters

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01 Oct 2019-Iet Electric Power Applications
TL;DR: This study presents the structure and the space vector pulse-width modulation (SVPWM) for power electronic transformer (PET) based on two seven-level cascade H-bridge (CHB) inverters and proposes the optimal management of the DC-link voltage distribution.
Abstract: This study presents the structure and the space vector pulse-width modulation (SVPWM) for power electronic transformer (PET) based on two seven-level cascade H-bridge (CHB) inverters. The DC links of CHB inverters are coupled with nine dual-active bridge (DAB) converters with medium-frequency transformers. The DC-link voltages are equalised with two methods - through the control of DAB voltages and through the modulation strategy applied to both CHB inverters. In the proposed SVPWM, the influence of vector sequences on predicted DC-link voltages is analysed, and the optimum vector sequence is selected to equalise them. Regardless of this, the proposed SVPWM strategy enables the proper generation of output voltage vector also in the case of DC-link voltage imbalance - the calculation of the space-vector area takes into consideration the inequality of the DC-link voltages and its influence on the lengths and positions of active vectors. To simplify the modulation algorithm, the multilevel CHB inverter is considered as a set of three-level inverters connected in series. Each of them is controlled using the same SVPWM algorithm. The proposed modulation method reuses the H-bridges with zero duty cycles determined in the initial stages of the output voltage generation process. This enables the optimal management of the DC-link voltage distribution. The experimental research was carried out on a 600 kW/3.3 kV PET. The results are presented in this study.

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  • ...They are commonly used in Static Synchronous Compensator (STATCOM) [3], [4], Energy Storage Systems [5], [6], [28], for the integration of Renewable Generation Systems [7], [8], [29] as well as in electric drive applications [30], [31]....

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Book Chapter•DOI•
Power Converter Topologies for Grid-Integrated Medium-Voltage Applications

[...]

Md. Rabiul Islam1, Youguang Guo1, Jianguo Zhu1•
University of Technology, Sydney1
01 Jan 2014
TL;DR: Different multilevel converter topologies, such as neutral point clamped (NPC), flying capacitor (FC), and modular multilesvel cascaded (MMC), have been considered and compared for the design of an 11 kV converter system and the performance is analyzed and compared in the MATLAB/Simulink environment.
Abstract: Although several converter topologies have been used in low-voltage applications, most of the topologies are not suitable in medium-voltage applications. The selection of converter topology in medium-voltage applications is really a critical problem and highly affects the converter performance and cost. The main aim of this chapter was to find out a suitable converter topology, which can interconnect the renewable generation units directly to the medium-voltage grid with mature semiconductor devices. Different multilevel converter topologies, such as neutral point clamped (NPC), flying capacitor (FC), and modular multilevel cascaded (MMC), have been considered and compared for the design of an 11 kV converter system. The comparison is made in terms of the number of semiconductors, semiconductor cost and availability, total harmonic distortions (THDs), filter size, and control complexity of the converters. The performance is analyzed and compared in the MATLAB/Simulink environment. To ensure quality performance, a level-shifted carrier-based switching scheme is used for the NPC topologies and a phase-shifted carrier-based switching scheme is used for the FC and MMC converter topologies with a carrier frequency of 1–2 kHz and modulation index of 0.8–0.9.

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Journal Article•DOI•
Power electronic transformer based on cascaded H-bridge converter

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Abstract: In this paper the control strategy of power electronic transformer (PET) is proposed. The analyzed structure of PET uses two seven-level cascaded H-bridge (CHB) rectifiers. The electrical power of PET is transferred between DC-links of CHB converters using dual-active-bridges (DABs) and low voltage high frequency transformers. The roposed solution allows for controlling the active and reactive power with a low level of harmonic distortions. The DC-link voltages and the load of the utilized H-bridges are controlled using appropriate modulation strategy. The theoretical issues are confirmed by simulation and experimental results.

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Cites background from "A 43-level 33 kV 3-phase modular mu..."

  • ...They can be used in active filter applications for total harmonic distortion (THD) improvement and reactive power compensation, in energy storage systems, as well as for the integration of renewable generation systems [1, 2]....

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References
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Abstract: The wind turbine generator system requires a power conditioning circuit called power converter that is capable of adjusting the generator frequency and voltage to the grid. Several types of converter topologies have been developed in the last decades; each of them has some advantages and disadvantages. Mainly two converter topologies are currently used in the commercial wind turbine generator systems. Most of the proposed converters require line filters and transformers to improve the power quality and step-up the voltage level, respectively. These heavy and bulky components significantly increase the tower construction, and turbine installation and maintenance costs. Recent advances in power semiconductors and magnetic materials have led to the development of new structure of converters, which would be a possible solution to reducing the size, weight, and cost of power converters. In this chapter, a comprehensive study of power converter technologies, current research and development is conducted.

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25 citations

"A 43-level 33 kV 3-phase modular mu..." refers background in this paper

  • ..., 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 mediumvoltage applications [15], [16]....

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Proceedings Article•
A modular direct-drive permanent magnet wind generator system eliminating the grid-side transformer

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Xibo Yuan1, Yongdong Li1, Jianyun Chai1, Minzheng Ma1•
Tsinghua University1
06 Oct 2009
TL;DR: In this paper, a modular converter for multi-coil direct-drive permanent magnet wind generator system is presented, where the converter modules are connected in series to achieve medium voltage output, thus eliminating the grid-side step-up transformer.
Abstract: This paper has presented a modular converter for multi-coil direct-drive permanent magnet wind generator system. The converter modules are connected in series to achieve medium voltage output, thus eliminating the grid-side step-up transformer. The generator coils with 90 degree phase shift are rectified through Power Factor Correction (PFC) circuit and connected in parallel to get unit power factor and stable dc-link power. The generator armature inductance is used as the AC-side PFC boost inductor, reducing the system size and cost. The required value of the inductor is derived based on the current ripple and zero-crossing distortion. The H-bridge inverters are cascaded to achieve multilevel voltage output and the vector control scheme is adopted to regulate the converter active and reactive power transferred to the grid. Simulation results with a 1.5MW wind system and experimental results with a scaled 3kW system validate the proposed topology and control method.

21 citations

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Frequently Asked Questions (1)
Q1. What have the authors contributed in "A 43-level 33 kv 3-phase modular multilevel cascaded converter for direct grid integration of renewable generation systems" ?

This paper proposed a 43-level 3-phase 33 kV modular multilevel cascaded ( MMC ) converter for direct grid integration of renewable generation systems. It is expected that the proposed new technology will have great potential for future renewable power plants and smart grid applications.