The unified power quality conditioner based on modular multilevel converter (MMC-UPQC) is a comprehensive compensator for power quality in medium and high voltage distribution systems. The precondition of normal work of devices is completing the pre-charging process on the premise of ensuring safety. According to the mathematical modeling and analysis of the three stages in the pre-charging process (the uncontrollable rectification stage

the submodular voltage promotion stage in series converter and the controllable rectification stage)

it is believed that the largest impact current will flow through the anti-paralleled diode paralleled with DC capacitor in submodules in the uncontrollable rectification stage. In the voltage promotion stage of submodules in series converter

current-limiting resistance still plays the key role in limiting the impact current which is less violent than the current in the uncontrollable rectification stage. Then

a pre-charging control strategy for MMC-UPQC is proposed. The digital simulation and low-voltage experimental prototype have verified the validity and feasibility of the control strategy proposed. This work is supported by National Key Technology R&D Program of China (No. 2011BAA01B03).