Precharging strategy for soft startup process of modular multilevel converters based on various SM circuits

The modular multilevel converter (MMC) has become one of the most promising converter technologies for medium/high-power applications, specifically for high-voltage direct current (HVDC) transmission systems. One of the technical challenges associated with the operation and control of the MMC-based system is to precharge the submodule (SM) capacitors to their nominal voltages during the startup process. In this paper, considering various SM circuits, a general precharging strategy is proposed for the MMC-based systems under ac-and dc-side startup conditions. The proposed startup method does not require any additional feedback control loop, extra measurements, and/or auxiliary power supplies. Based on the developed startup method, the charging current is controllable by adjusting the changing rate of the number of blocked and bypassed SM capacitors. Performance of the proposed strategy for various MMCs is evaluated based on time-domain simulation studies in the PSCAD/EMTDC software environment.