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The Modular Multilevel Converters (MMC) have been a spotlight for the high voltage and high power transmission systems. In the VSC-HVDC (High Voltage Direct Current based on Voltage Source Converter) transmission system, the energy of DC link is stored in the distributed capacitors, and the difference of capacitors in parameters and charge rates causes capacitor voltage balance which affects the safety and stability of HVDC system. A method of MMC based on the expert system for reducing the frequency of the submodules (SMs) of the IGBT switching frequency is proposed. Firstly, MMC with 51 levels for HVDC is designed. Secondly, the nearest level control (NLC) for 51-level MMC is introduced. Thirdly, a modified capacitor voltage balancing method based on expert system for MMC-based HVDC transmission system is proposed. Finally, a simulation platform for 51-level Modular Multilevel Converter is constructed by using MATLAB/SIMULINK. The results indicate that the strategy proposed reduces the switching frequency on the premise of keeping submodule voltage basically identical, which greatly reduces the power losses for MMC-HVDC system.

In High Voltage Direct Current applications, high voltage long distance transmission is often constrained by the fact that the components of power electronics are limited in terms of rated voltage. The two-level VSC has been widely accepted in practical engineering applications. However, its use in high voltage and high power occasions is limited for the reason of the high switching losses and poor voltage quality. Compared with two-level VSC, MMC has much lower switching frequency and switching losses, which makes the MMC suitable for applications in HVDC. The MMC is the most promising topology for its modularity and discretion in HVDC systems. In recent years, there are many topologies of MMC, modulation, capacitor voltage balancing, circulation suppression, the research results of the fault tolerant technique, and so forth [

In practice, the MMC-HVDC contains hundreds of submodules for reaching high voltage; therefore, compared with the 21-level MMC in [

This paper is organized as follows: In Section

A typical structure of MMC-HVDC is DC to three-phase AC converter, as shown in Figure

Topology of MMC-HVDC converter.

The topology shown in Figure

Structure of SM.

Figure

Equivalent circuit of the MMC for phase-

The mathematical equations that govern the dynamic behavior of the MMC phase-

There are lots of modulation methods for multilevel converters; if the number of SMs is small, high frequency PWM multilevel modulation methods such as space vector modulation (SVM), phase disposition PWM (PD-PWM), and carrier phase shift PWM (CPS-PWM) are commonly used in order to meet the requirement of harmonic. However, as the SM number is increasing, the PWM modulation becomes more and more complex. Huge number of SMs of HVDC makes PWM with high frequency and increases switching losses. Compared with PWM modulation, the level modulation has lower switching frequency, so it has lower switching loss, and it is easier to realize without complex pulse width control. For 51-level MMC in HVDC system, the harmonic content of output is very low. The basic principle of NLC modulation mode has been discussed in detail [

Control diagram of NLC with modified voltage balancing method.

The principle traditional voltage balancing method is to keep the minimum voltage deviation of the submodules at any time. According to the different direction of the arm current

If

Without considering lower device switching frequency, conventional voltage balancing control only lays emphasis on the results of the capacitor voltage sequence, which ignores the initial state of submodule. The target of conventional method is to strictly control the difference between capacitor voltage values of the submodules. In practice, the goal of balance control is not completely pursuit of the consistency of each capacitor voltage; relatively small fluctuations of capacitor voltage can be allowed.

Define

IF

In interface engine: make comparison of

IF

IF

The voltage balancing method is modified as follows.

It takes the desired nearest output voltage level

IF

IF

IF

IF

IF

To validate the effectiveness of the voltage balancing method proposed in this paper, the 51-level MMC-HVDC transmission system simulation platform in MATLAB/SIMULINK is taken, and specific parameters are as follows: the number of submodule

The simulation framework of MMC-HVDC is shown in Figure

The framework of MMC-HVDC in MATLAB/SIMULINK.

The SM structure in MATLAB/SIMULINK.

Figures

Line-to-line voltages.

Phase currents.

Figures

Capacitor voltage with different voltage balancing method. (a) Conventional voltage balancing method. (b) Modified voltage balancing method based on expert system. (c) Modified voltage balancing method based on expert system (local zoom).

Figures

Gate signals of phase-B with different modulation schemes. (a) Conventional voltage balancing method. (b) Modified voltage balancing based on expert system.

This paper develops an expert system for RSF voltage balancing method for MMC-HVDC. In this paper, a 51-level MMC for HVDC is designed, whose voltage level is close to engineering practice. The output of converter is with low total harmonic distortion, where there is no need for power filters and the device cost of HVDC is reduced. A NLC method with conventional voltage balancing method is introduced. To reduce the switching frequency of converter, a modified voltage balancing method on expert system is proposed. The simulation results validate the fact that the modified voltage balancing method is effective, which reduces the switching frequency and balances the SM voltage. The simulation results show that the 51-level converter works effectively in HVDC systems.

The authors declare that they have no competing interests.

This work is partially supported by the Fundamental Research Funds for the Central Universities (ZYGX2011J021) and the Scientific and Technical Supporting Programs of Sichuan Province (2013GZ0054).