A common problem in the static DC charging process of modular multilevel converter is voltage polarization. Some sub-modules' voltage sags even cause startup and shutdown repeatedly. In order to analyze the voltage-sharing characteristics of MMC

firstly

equivalent impedance of energy-absorbing source is introduced into the simulation model of sub-module; secondly

the mechanism of voltage polarization is studied based on the improved model; finally

a setting process of static DC charging tolerance time protection is proposed which can reduce the degree of voltage polarization. The static DC charging experiment and single tower test are conducted at Xiamen HVDC flexible transmission project and the results show consistence between simulation and experiment

which prove the feasibility of the improved simulation model of the sub-module

the mechanism analysis result of voltage polarization

the correction of static DC charging tolerance time protection value setting method. This work is supported by State Grid Corporation of China(No. 52130415000Z).