(1. State Grid Zhejiang Electric Power Research Institute, Hangzhou 310014, China;2. State Grid Zhejiang Electric Power Company, Hangzhou 310007, China)

The charging procedure of a modular multilevel converter (MMC) based HVDC (MMC-HVDC) system with valve-side grounded and ungrounded is analyzed. According to their different charging circuits, the steady DC voltage and the total voltage of the sub-module belonging to one arm is calculated. A strategy for modulated wave pre-tracking is proposed, i.e., realizing tracking of the modulated wave and valve-side voltage prior to deblocking as a preparation for minimizing the inrush current. The mechanism of the inrush current is analyzed, and the corresponding deblocking strategy is designed for the DC voltage control station and the constant power control station. By the active charging mode, the voltage of sub-module is increased, which can remarkably reduce the inrush current brought about during deblocking. A digital model for the MMC-HVDC system with two terminals is finally realized by the PSCAD software, the simulation results showing that the proposed control strategies have satisfactory performance.