Research On Control And Protection Strategy Of HVDC System Based On MMC

Modular multilevel converter(MMC)is a new type of topology which has great potential in high voltage direct current(HVDC)and been greatly improved in recent years.In this paper,the development history and research status of MMC-HVDC are first introduced,and then the topology,working principle and control modulation method of half-bridge module MMC are analyzed in detail,at the same time the nearest level modulation strategy is applied in the research of this paper.The main factors that influence the fluctuation of the capacitor voltage are analyzed under the consideration of the charge and discharge mechanism of SM capacitor,then the shortcomings of the existing capacitor voltage control method are pointed out that a large number of voltage signals caused a huge burden on the controller.The paper proposes an improved capacitor voltage control method,which can effectively reduce the amount of data transmission.Aiming at the temporary short circuit fault of DC side,which is the most serious fault in MMC-HVDC system,the dynamic process of short circuit fault is analyzed in detail.Simultaneously,the half-full-bridge sub-module(HFBSM)topology is proposed based on the analysis and comparison of the existing improved sub-module topology,which could block fault current with fewer power electronic devices.When the problem of permanent short circuit fault occurs in MMC-HVDC system,according to the constant load characteristic of power modules,the phenomenon of capacitor divergence is explored.A permanent fault crossing strategy is proposed with MMC remaining controllable during the fault clearance.Finally,the simulation verifies the validity of scheme.