Research And Optimization Of Modular Multilevel Inverter Modulation Strategy

With the popularity of flexible DC transmission technology,Modular multilevel converter(MMC)has been rapidly developed as its key technology.Compared to other traditional multilevel inverter topologies,MMC is used in high voltage and high power fields because it can cascade more sub-modules,and can output more levels,and the output waveform quality is extremely high.Modulation strategy and related performance optimization control strategy are the key technologies of MMC-HVDC,so this paper studies and analyzes the modulation strategy and related control strategy of MMC system.The main contents are as follows:(1)The carrier phase shift modulation strategy(CPS-PWM)and the nearest level approximation modulation strategy(NLM)are studied.By comparing the total harmonic distortion rate of the output voltage of the two strategies and the complexity of the modulation algorithm to determine the application of different modulation algorithms,it provides a method for the study of the related control strategies.(2)Three sub-module capacitor voltage equalization strategies are studied,which are capacitor voltage balance strategy based on sorting algorithm,capacitor voltage balance strategy based on common mode voltage injection,and improved capacitor voltage balance strategy based on common mode voltage injection.Taking the capacitor voltage fluctuation rate and the average switching frequency of the switching device as the evaluation index of the capacitor voltage equalization algorithm,the equalization algorithm is determined for the following fault diagnosis method.(3)A fault detect method for MMC sub-modules is studied.The method is based on the difference between the actual value of the sub-module capacitor voltage and the theoretical value.In order to prevent the sampling error of the sensor,the method adopts a method of judging five consecutive cycles to reduce the possibility of misjudgment.After fault detection,the fault-tolerant control with redundant sub-modules and the fault-tolerant control method with no redundant sub-modules are analyzed to ensure the normal operation of the system.Based on the above three research contents,this paper builds the algorithm model of MMC in the platform of Matlab.We can get the conclusion that the equivalent switching frequency of the carrier phase shift modulation strategy is higher,and the recent level approximation algorithm has less complexity.The voltage equalization control strategy based on hierarchical control is better than the other two equalization strategies and has certain circulation suppression function.The proposed fault diagnosis method and fault-tolerant control method can quickly detect faults after faults occur and After fault-tolerant control,the system can operate normally,ensuring stability during system operation.