Aiming at the problems of high-voltage and high-capacity modular multilevel converter modulation and sub-modules switching

we propose a method of sorting by group to reduce the calculating capacity for ordering the sub-module capacitor voltage. Moreover

we determine an optimal number of groups of the converter arm sub-module so as to optimize the ordering calculating capacitance of the capacitor voltage. After the establishment of the sub-module capacitor voltage matrix

vertical sorting in each group and horizontal sorting between groups

a sub-module capacitor voltage ordered sub-module matrix is established. In each control cycle period

according to arm current parameters and sub-module voltage rise or decrease direction

switch sub-modules are set and controlled. Simultaneously

we introduce sub-module capacitor voltage deviatio

n control to prevent the excessive capacitor voltage deviation and to reduce the power device switching frequency. Through digital simulation of a 101 level flexible HVDC transmission system and physical simulation of a 37 level flexible HVDC transmission system

it can be concluded that the voltage-sharing control strategy proposed in this paper can significantly reduce the amount of sequencing operations and the switching frequency of the sub-modules

and can ensure that the voltage fluctuation of the sub-module capacitor voltage is maintained within-5%

5

% of the rated value.