Research On Control Strategy Of Modular Multilevel Converter

Power electronic technology plays an irreplaceable role in the HVDC transmission,power transmission,power quality control and new energy power generation and other fields.With the development of the global energy Internet and the improvement of the voltage level in the industrial application,the topology and control methods of the high voltage and power converters are widely concerned by researchers at home and abroad.The modular multilevel converter becomes the best choice to solve the problem of high voltage and power conversion with its advantages such as high modularization,multilevel output and easy to expand.And it's considered to be one of the most promising applications of power electronic converter topology.So,this thesis mainly studies the control strategy of MMC.At firstly the topology and working principle of MMC are introduced in the thesis,and the mathematical model is built to analyze the working characteristics of MMC.It is pointed out that MMC has two working modes,which can output N+1 levels and 2N+1 levels and there will be frequency-doubled circulating current when MMC runs.Thus,the control strategy of MMC is to solve the three problems: MMC modulation,submodule capacitor voltage balance and internal circulating current suppression.The modulation strategy of MMC is to determine the number of submodules cut into the upper and lower arms.In this paper,three kinds of modulation methods are studied and the ladder wave modulation method is considered the most suitable modulation method for MMC.An improved ladder wave modulation method is proposed to make the submodule capacitor voltage not deviate from the reference value when MMC works in the 2N+1 level mode,which improve the stability of the MMC operation.The equalization of the submodule capacitor voltage is the premise for the normal operation of the MMC.In this paper,two kinds of capacitor voltage equalization control methods are analyzed.An improved method is proposed aimed at the shortcomings of the unified equalization control method that based on the capacitor voltage ranking.The method is named variable frequency sorting balancing method,which can effectively reduce the device switching frequency and the operating loss of MMC.The circulating current is caused by the special structure of MMC,which will endanger the operation of MMC.Therefore,it is necessary to suppress the value of the circulating current.On the basis of the analysis of the three kinds of circulation suppression methods and the principle of circulation suppression,this thesis proposes an extremely simple method based on the hysteresis loop control.And the method is simple and effective to suppress the circulating current.Finally,these control methods proposed in this thesis are verified by simulation and compared with other method.The results verify the correctness of the theoretical analysis and the effectiveness of the proposed method.