Grouping Precharging Method for Modular Multilevel Converters

Before the modular multilevel converter (MMC) enters the normal operation, the capacitor of each sub-module needs to be precharged to its rated operation voltage. This paper presents a grouping precharging method for two-terminal or multi-terminal MMC-HVDC. In the proposed method, the sub-modules on the active side of MMC-HVDC are divided into two groups and the sub-modules on the passive side of MMC-HVDC are divided into four groups. The basic principles and implementation steps of the precharging method are given in details. Compared with the existing one-by-one sub-module precharging method, the proposed precharging method has short charging time and lower power consumption of current-limiting resistors. For the system with 200 sub-modules in each arm as an example, the charging time of the proposed precharging method can be shortened by about 60 times and the limiting resistors power consumption can be reduced about 151 times. Simulation and experimental results verify the feasibility and effectiveness of the proposed precharging method.