This paper establishes a mathematical model of cascaded H-bridge converters and analyzes the factors which cause capacitor voltage unbalance. Further

the differential equation and transfer function are presented to describe the modulated wave adjustment balancing method. The analytical solution of the capacitor voltage steady value is obtained under balanced voltage control condition. In order to set appropriate controller parameters of many sub-modules

phase and proportion coefficient constraints are studied and calculated. Likewise

when the converter switches between the inductive work mode and capacitive work mode

DC capacitor balanced voltage control must conform to the constraints. A model for a three-phase converter is developed via PSCAD/EMTDC. Simulation results have verified that the phase and proportion coefficient constraints are effective. This work is supported by the Science and Technology Development Plan of China Railway Corporation(No. 2015J008-F).