Research On Modular Multilevel Converter And Its Control Strategy

Along with the rapid development of technology of power electronics, multi-level converters and related technologies have gradually become research focus in the field of the high-voltage and high-power applications. The modular multilevel converter(MMC) as a new multi-level converter topology, with its unique structural advantages to overcome the shortcomings of traditional multi-level converter, its modular structure has good scalability, easy to expand to a higher voltage levels and power levels. It has good potential applications. in HVDC, reactive voltage compensation and power level requirements of the occasion. In this paper, based on MMC’s control strategy and control strategies MMC HVDC system conducted in-depth research and analysis, the main contents include:(1) This paper analyzes the topology of MMC, working principle, the establishment of MMC mathematical model to analyze the four-quadrant operation principles and circulation characteristics of MMC.(2) This paper proposed two MMC precharge control method to solve the DC capacitor voltage initialization problem, analyzed the working of carrier phase shift control strategy and capacitor voltage sequencing control strategy, and compared the two control strategies from the output phase voltage THD, capacitor voltage fluctuations and other indicators, the simulation results show that the two control strategies have good stability state characteristics, to achieve a stable balance control DC capacitor voltage.(3) This paper analyzed the structure of the fuzzy PI controller, working principles, designed based on fuzzy PI controller double-loop vector control MMC-HVDC system, the simulation analysis shows that a variety of conditions, using fuzzy PI controller time adjust the speed and steady state and dynamic performance of the system has been greatly improved.(4) This paper analyzed the control structure of MMC-HVDC system, established mathematical model of MMC-HVDC system in the synchronous rotating coordinate system,proposed suitable MMC-HVDC system control strategy and designed based on double-loop vector control inner current controller and outer voltage controller. Finally, based on Matlab/Simulink simulation software, built to connect the two ends of MMC-HVDC active network simulation model, by analyzing the various conditions of the simulation results proved that the system has built a good steady state performance and dynamic response characteristics.