The Control Strategy of Establishing the Voltage of DC Side in MMC

Abstract

This paper discusses the establishing method of DC voltage of modular multilevel converter (MMC). Firstly the structure of MMC and the charging principle of sub-modules (SMs) are introduced in order to control the DC voltage of MMC and reduce the switching loss. It is pointed out that the DC voltage control in MMC includes two parts, the DC voltage establishment of system and the voltages control of SMs. For the DC voltage control in system, the control theory is analyzed, and the corresponding control strategy is proposed. Meanwhile an improved voltage balance control strategy for SM is proposed by the improvements of the traditional voltage sorting method to reduce the high switching frequency caused by the blind action of switching devices in the SM control. Finally, the simulation results based on PSCAD/EMTDC show that DC voltage in MMC can run in the rated value steadily, and the switching frequency is reduced significantly.