Research On The Control Strategy Of Modular Multilevel Converter

With the development of power electronics technology, the device of high powerand high voltage used in the field of power driving and power system was paid moreand more attention by scholars at home and abroad. Modular Multilevel Converter(MMC) is an effective way to realize high power because of its unique properties.This pepar choose the control strategy of MMC as the research object. It researchs onthe modulation method, DC capacitor voltage control strategy, circulating currentsuppressing strategy and experiment machine.The topology and working principle was analyzed, and mathematical model wasdeveloped. The modulation method of the N1levels and2N1levels of outputvoltage was studied, and the modulation method was analyzed by carrier dispositionmodulation and carrier phase-shifted modulation.According to the control target of different stages in the DC capacitor voltage,the control strategy was divided into precharging control and balancing control.According to the characteristics of different operating condition, the thesis putsforward the precharging strategys of the rectifier and inverter operating conditionwithout additional power, and the simulation models were established to verify them.In the balancing control strategy, the way to balance capacitor voltage based onsorting method and closed loop method was analyzed, and combined with theconcrete modulation method, and compared based on the simulation result.The reason of circulating current was analyzed. Its mathematical expression wasderived. Combined with its mathematical model, the circulating current suppressingstrategy was analyzed and verified by simulation. The simulation results show that thefrequency-doubled circulating current was suppressed and the distortion of armcurrent was eliminated.According to the theoretical analysis and simulation model, a three-phaseexperimental machine was designed, and some key problems in the machine wereanalyzed. A control system which was suitable for MMC was designed. Themodulation methods, capacitor voltage balancing strategys and circulating currentsuppressing strategy were verified in the machine, the result of experiment was thesame as the simulation, so the control strategy of the thesis was verified.