An Improved AC-Precharging Strategy for the Startup Process of Modular Multilevel Converters Based on Time-Domain Analysis

The modular multilevel converter (MMC) has emerged as the most attractive conversion technology for high-voltage dc (HVDC) transmission systems. The startup process of MMC is essential for normal operation. The startup process of HBSM-MMC and FBSM-MMC is analyzed in this paper. Due to the different conduction paths, the capacitor voltage of HBSM and FBSM are different in the uncontrolled precharging stage. Then, it is explained that the reference voltage of FBSM cannot be tracked during controlled AC stratup process. Furthermore, an improved AC-precharging strategy for FBSM-MMC is proposed, which DC-component and AC-component are controlled separately. Finally, the simulation and experiment are performed to verify the effectiveness of the proposed pre-charging strategy.