Startup of the Switched-Capacitor Modular Multilevel Converter with Middle Submodule

Among several multilevel converter topologies, the modular multilevel converter (MMC) has received increasing interest due to its modularity, efficiency, and voltage and power scalability features. However, before the MMC can run in normal steady-state conditions, its submodule (SM) capacitors have to be pre-charged to their nominal voltage value. In this paper, we describe an ac-side and a de-side pre-charging strategy, both utilizing Y-matrix modulation (YMM), to startup the switched-capacitor modular multilevel converter (SCMMC) with middle SM. This topology employs a SM between the upper and lower arms, and has no discrete arm inductors. The proposed startup methods do not require any extra hardware besides the current-limiting (charging) resistors, which are bypassed after pre-charging is completed. It is shown that achieving a soft-startup requires balancing the SM voltages at their nominal values during the active charging stage, which necessitates actively controlling the middle SM voltage using the ac line current. Theoretical analysis is presented to quantify the capacitor voltages during the different pre-charging stages, and a time-domain simulation of a 6-level SCMMC with middle SM under YMM is provided to validate the results.