Fault-tolerant Method Research Of Modular Multilevel Converter Based On Harmonic Injection

Along with the rapid development of electronic converter technology in recent years,the demand for high voltage power conversion system is fast increasing.And the capacity of the converter becomes the main restriction,the traditional two level converter device is difficult to achieve the demand.Multilevel converter is improved on topology structure to overcome the capacity problems of power semiconductor devices which are widely used in high voltage high power electric energy conversion applications.This paper makes a research on modular multilevel converter(MMC),which includes operation mechanism analysis,sub module capacitor voltage balance control,MMC operation mechanism under sub module fault condition,fault tolerant control strategy.And the simulation and experiment are carried out based on the theoretical research.Firstly,this paper introduces the MMC topology and the operation of sub module and establish its mathematical model.What is more,a simple introduction of the modulation method,voltage balance control method,circulation suppression means of MMC is carried on.Secondly,the redundant and non redundant fault-tolerant system are introduced separately.And the hot and cold operation modes of the redundant fault-tolerant system is introduced,while the fault-tolerant based on zero sequence injection is analyzed emphatically.At the same time,the shortcoming of this method is pointed out,which increases the non fault phase voltage reference.In view of the shortcomings,this paper proposes a novel fault-tolerant method based on the third harmonic injection,which can maintain the normal operation of the system without any increase of additional redundant modules.The simulation model of a seven-level MMC is built on the MATLAB/Simulink.The simulink result verifies the feasibility of the strategy.Lastly,the experimental platform is designed.The system block diagram,the design method of the main circuit parameters(such as the capacitance of the sub module,the inductance of each bridge),and the software flow chart are given.The experiment analog sub module fault by taking off bypass switch.Finally,the feasibility of the strategy is verified by the result of experimental waveform.