Capacitor selection for modular multilevel converter

A reliable method to select capacitors for a modular multilevel converter (MMC) is proposed in this paper. Based on the ageing mechanisms of the metallized polypropylene film (MPPF) capacitor and operational characteristics of MMC, four requirements, i.e. the maximum capacitor voltage, voltage ripple, ripple current and submodule voltage capability are considered in a procedure for capacitor selection. The effects of 2^nd order harmonic circulating current suppression control and the arm inductors are taken into account. The ripple current stress is found to be less than 126mA/μF for 2kV submodule rated voltage, which is only half of the allowance of the selected MPPF capacitor. The findings are applied to a design example of a 21.8MW, 32kV_dc grid-connected converter. Its operational region is firstly calculated using the proposed theory and then compared with simulation results.