The Research Of Fault-tolerant Technology In Modular Multilevel Converter(MMC)

Due to the advantages of modular structure,easy cascade and low switching frequency,the modular multilevel converter has become a hot research in recent years.However,every bridge of MMC is composed of multiple parallel sub-modules(SMs)and each switching device in the SMs is a vulnerable part.The more parallel SMs,the greater the probability of failure.Therefore,how to realize the faulty SMs rapid isolation and the research of fault-tolerant technology in MMC are very important to improve the reliability and stability of MMC operation.This paper first introduces the topological structure,working principle and control strategy of MMC.And the SM structure with fault isolating function,bypassing measure and the design principle of redundant SM number are presented.Then the spinning reserve mathematical model of MMC is derived;the changes and flowing direction of the circulating current using symmetric control strategy and asymmetric control strategy are analyzed respectively and the specific measure to reduce the circulating current is put forward according to the mathematical model;system performance after taking fault-tolerant control strategy is analyzed from the perspective of the fundamental frequency component amplitude of modulation wave;the maximum number of faulty SMs that the system could endure is figured out according to the capacitor voltage fluctuation.Meanwhile,in terms of passive reserve MMC system,this paper puts forward a SM structure that can be put into any phase or three phases simultaneously and proposes an optimized passive reserve control scheme.Simulation of 31-levels MMC system which includes 6 reserve SMs is set up in PSCAD/EMTDC to verify the validation of the fault-tolerant technology of spinning reserve and passive reserve respectively.In the end,in terms of the MMC system without redundant SMs,CPS-SPWM fault-tolerant control strategy and redundancy control strategy of SVPWM are put forward to realize fault-tolerant operation of three-wire MMC system;Simulation of 5-levels MMC system without reserve SMs is set up in PSCAD/EMTDC to verify the validation of the fault-tolerant technology.