Control Method Of Modular Multilevel Converter And Application In STATCOM

With the large-scale access of UHVDC transmission and the increasing proportion of wind power and photovoltaic power generation at the end of the grid,the reactive power of the grid is insufficient,and the voltage stability is severely challenged,seriously affecting safety and stablity of the power system operation.Static synchronous compensator(STATCOM)features fast compensation for reactive power regulation and wide adjustment range.STATCOM based on Modular Multilevel Converter(MMC)has high voltage level,large dynamic compensation capacity and high output waveform quality,which can solve the problem of insufficient power supply caused by large-scale feeding of UHVDC transmission,large-scale grid connection of wind power and photovoltaic power generation at the end of power grid.Therefore,deepening the research on MMC control method and its application in STATCOM is of important theoretical significance and engineering practical value.This paper mainly completed the following research work:(1)The topology and working principle of MMC is discussed.Different working conditions of sub-modules(SM)and the working conditions of MMC are analyzed in detail.Based on the mathematical model of MMC,the dynamic mathematical model of MMC AC side and DC side is constructed.The commonly used MMC modulation methods are compared,and the recent level approximation modulation principle is discussed in detail.(2)The capacitor voltage balancing method of MMC is discussed,and the cause of the capacitor voltage imbalance is analyzed.Based on the comparison of the voltage balancing method of hierarchical control and sorting method,in order to solve the problem that the switching loss of the power switching device is large in sorting method and the fluctuation of the capacitor voltage is intense in dual retention factor method,a capacitor voltage balancing method combining dual retention factor and threshold is proposed.By introducing dual retention factor method and setting a threshold,the voltage fluctuation is limited,the switching frequency is greatly reduced,and voltage equalization effect is ensured,thereby reducing the loss of the power switching device and improving the output voltage quality of the AC side.Simulation results verify the effectiveness of the method.(3)Based on the principle of STATCOM reactive power compensation,the mathematical model of STATCOM based on MMC is established.In order to realize the fast tracking and power control of MMC-STATCOM output current,and to solve the problem of circulation caused by the uneven energy distribution between three phases in the MMC topology,a double-closed-loop decoupling control link and circulation suppression method are constructed.The MMC-STATCOM control method based on combination of dual retention factor and threshold is adopted,so that the converter absorbs and emits reactive power in accordance with the demand,so that MMC-STATCOM plays good compensation effect.Simulation results verify the effectiveness of the method.(4)The hardware structure of STATCOM based on MMC is constructed.The core processor and controller are selected.The main parameters of the main circuit part of MMC are determined.The synchronization circuit,sampling and conditioning circuit and drive circuit required for the experiment are constructed.The synchronization circuit,sampling and conditioning circuit,and driving circuit were tested and debugged to achieve the expected results.