Closed-Loop Precharge Control of Modular Multilevel Converters During Start-Up Processes

This letter respectively presents two closed-loop precharge control methods to fully charge the submodule (SM) capacitors of the modular multilevel converter (MMC) during dc-side and ac-side start-up processes. The proposed methods are aimed at regulating a constant charging current thus reducing the start-up time and eliminating the inrush current. Besides, capacitor voltage balancing control is also embedded into the controller to equally charge the SM capacitors. Due to the flexibility of the closed-loop control, these methods are applicable to any MMC applications regardless of the number of SMs. Finally, effectiveness of the proposed methods is verified experimentally on a three-phase prototype of MMC.