In the voltage source converter based high voltage direct current (VSC-HVDC) transmission project

the converter valve of the passive terminal must be pre-charged via the active terminal

while the sub-modules can only be charged to half the rated voltage at most. A very large DC current will be generated if the converter is deblocked in normal logic. So the sub-modules must be charged to the rated voltage by an active charging strategy. According to the characteristics of DC-side uncontrolled charge and normal operations of the modular multilevel converter (MMC)

a DC-side active charging strategy for progressively decreasing the operated sub-modules is designed. The strategy makes possible the smooth transition of the converter valve of the passive terminal from the uncontrolled charging state to the normal deblocked operation state. Besides

the characteristics of the bridge arm current in the DC-side active charging process are analyzed. Finally

simulation results and the project application have verified the strategy designed.