Research And Design Of Control System For Modular Multilevel Converter

As the rise of the Voltage Source Converter based High Voltage Direct Current Transmission (VSC-HVDC), the converter with high voltage and high power rate becomes the research focus. Modular Multilevel Converter (MMC) is always adopted to realize VSC-HVDC because of its outstanding characters of high-modularity, low THD and high-reliability. Recently, research of MMC still remain on the control area, such as converter modeling, sub-module capacitor voltage balancing algorithm, internal circulating current concealing, etc., whereas the research of MMC control system architecture design and implementation are rare. This paper takes control system constructing as the research object, and the result are listed below.(1) The structure of MMC and its operational principle are analyzed detailedly. Then the equivalent mathematical model of MMC is built. The control strategies, such as voltage balancing control strategies, circulating current suppressing strategy, precharging method, and so on, are comparative analyzed with MATLAB/Simulink simulation software. The functional requirements of the control system is determined, and select the control strategies that are suitable for the hardware control system.(2) According to the functional requirements of the MMC control system, the existing architecture of MMC control system is analysed. For the MMC traditional three-level control system, a single DSP controller don’t bear the operation burden, and there is a synchronization problem between the sub-module controllers. In parallel operation system of multiple MMC converters, lack of specialized higher-level control module for high-speed communication between each converter. An improved hierasrchical control structure consisted of three-level controller is proposed.(3) In the improved three-level control system architecture implementing, system functions are fully analyzed and classified into modules, and the communication protocol between controllers is specified. The realizeing methods of main function within DSP and auxiliary FPGA are designed, simulated and tested.(4) Based on the research above, a MMC prototype of three-phase 5 level is build. The feasibility of the proposed control system architecture and control strategy is verified through experiments on this platform. Experimental result of various working conditions with main waveform is present.