Research On Control Strategies Of MMC Converter

In recent years,with the rapid development of power electronic technology,the research and development of high voltage and high power electronic equipment has been widely concerned by the academic circles and industrial circles both at home and abroad.Among them,Modular Multilevel Converter(MMC)has rapidly become a research hotspot in the field of high voltage and high power for its unique topological structure and prominent application advantages.In order to make MMC get better application,this paper mainly focuses on its control strategy,including the modulation technology,sub module DC capacitor voltage-balancing strategy and the circulating current restraint strategy,to do related research work.The topological structure and working principle of MMC are analyzed in this paper,and the mathematical model of MMC is established.Furthermore,the research about the two kinds of PWM modulation has been done,which are the implementation methods of carrier disposition modulation and carrier phase-shifting modulation for MMC,and the characteristics of these two kinds of modulation and their effects on MMC's features are analyzed.According to the characteristics of the carrier disposition modulation,this paper studies two kinds of sub module voltage-balancing strategy based on capacitor voltage sequence,which are the sorting method based on distribution switching pulse and the sorting method based on calculating the input number of sub modules.The effectiveness of these two strategies is proved by simulation,and it is concluded that the sorting method based on calculating the input number of sub modules is a more suitable voltage-balancing strategy for the engineering practice.But the problems which make the switching element of MMC system act frequantly and has great loss also exist in the sorting method based on calculating the input number of sub modules,this paper optimizes this strategy depending on these problems,and make the upper and lower limit of sub module's capacitor voltage value as the judgment basis,which effectively reduces the switching loss of the system.According to the characteristics of the carrier phase-shifting modulation,this pa per studies the two kinds of sub module voltage-balancing strategy based on the capacitor voltage closed loop control,which are the voltage-balancing strategy based o n double closed loop control and the voltage-balancing strategy based on the single loop control.The effectiveness of these two strategies is proved by simulation,and it is concluded that the voltage-balancing strategy based on the single loop control i s a more suitable voltage-balancing strategy for the engineering practice.Finally,this paper deduces the expression and nature of the MMC circulation,and according to the nature of the circulation,studies a circulating current restraining strategy based on second harmonic generation negative sequence transformation,and the effectiveness of this strategy is proved by simulation.