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.