A 43-level 33 kV 3-phase modular multilevel cascaded converter for direct grid integration of renewable generation systems
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)
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  .
C. Switching Circuit of 43-Level MMC Converter
- The phase-shifted carriers are specially conceived for FC  and MMC  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.
- 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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Cites background from "A 43-level 33 kV 3-phase modular mu..."
...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 ....
...The toroidal transformer was already designed as a part of project on integration of renewable energy sources into the grid using multi-level inverters , ....
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]....
Cites methods from "A 43-level 33 kV 3-phase modular mu..."
...They are commonly used in Static Synchronous Compensator (STATCOM) , , Energy Storage Systems , , , for the integration of Renewable Generation Systems , ,  as well as in electric drive applications , ....
...The SPWM strategies for an n-level ML inverter utilize the sinusoidal reference waveform and ‘n-1’ carriers shifted vertically (Level-Shifted PWM (LS-PWM)) ,,  or horizontally (Phase-Shifted PWM (PS-PWM)) , , , ....
"A 43-level 33 kV 3-phase modular mu..." refers background in this paper
...The design and analysis of a 33 kV MMC converter system is presented in detail in the following sections....
...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 mediumvoltage applications , ....
...Due to these, 15-level to 55-level MMC converter topologies are considered for a 33 kV inverter system....
...%) and semiconductor cost of different number of MMC converter levels for a 33 kV system....
...However, the MMC converter requires multiple isolated and balanced dc sources....
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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.