Research On Modular Multilevel Converter And Real-Time Simulation Technology

In recent years, as a new type of converter, Modular Multilevel Converter (MMC) has drawn more and more attention. It has advantages in the aspects of equivalent switching frequency and expansibility. Its application in flexible dc transmission system also becomes a research hotspot. For the complicated structure of MMC, Traditional off-line simulation software can not meet the high requirements of operation. Real time simulation technology can solve the problem of complicated model. This thesis mainly studies the control strategies of MMC and MMC dc transmission system. Then use the real time simulation platform to verify the effectiveness. The main contents include:(1)The structure of MMC and its operational principle are detailedly analyzed. Then the equivalent mathematical model of MMC is built. Two kinds of carrier phase-shifted modulation method are comparative analyzed with MATLAB/Simulink offline simulation software. The advantages and disadvantages of these two modulation method are concluded.(2)The control strategies of MMC are researched. Firstly, two kinds of voltage balancing control strategies are adopted to accomplish the goal of voltage balancing control. Secondly, a circulating current suppressing strategy based on PR conditioner is proposed to reduce the time delay compensation and improve the circulating current suppressing effects. Thirdly, an online precharging method is adopted to establish the rated voltage of sub-module with ac source from power grid.(3)The application of MMC in the field of flexible dc transmission application is researched. The principle and mathematical model of MMC dc transmission system are introduced. On the basis of this, three-level control method is introduced as follow. The control strategies of two kinds of dc transmission system:double ports active and supplying power to passive network MMC dc transmission system, are researched.(4)Based on the consideration of system scale and simulation efficiency, the real time simulation models of MMC and MMC dc transmission system are built in RT-LAB real time simulation platform. The correctness and effectiveness of control strategy are verified with real time simulation.