As for the voltage grading effect deterioration of submodule capacitor during the pre-charging process of the modular multilevel converter

a submodule-level refined valve tower simulation model

including the equivalent impedance of submodule absorption power supply

parasitic capacitance of valve tower and water resistance

is set up based on the grading characteristics of capacitive voltage of the submodule during the charging process of converter uncontrolled rectifier. The unevenness of submodules under different parameters is calculated

and the influence mechanism of energy obsorption power supply deviation

accuracy of grading resistor and water conductivity on the grading effect of sub-modules is analyzed. It is found by the research that the different charging currents due to the differences in submodule parameters

and the current to earth due to the parasitic parameters of the valve tower are the main factors that cause the deterioration of the grading effect of the submodule during pre-charging period. The grading resistor can provide a discharge path for the charge so that the charge is transferred from the sub-module with higher voltage to that with lower voltage

which helps to balance the capacitive voltage of each submodule. The research results can provide a theoretical basis for optimizing the operating condition and performance index of the gradign resistor of the modular multilevel converter.