Exploitation of additional degrees of freedom of multiphase drives in low-frequency operation of modular multilevel converters

Abstract

MMC encounters serious limitations in the zero and low-frequency operation of variable-speed motor drives. In MMC, the voltage ripples of SubModule (SM) capacitor is inversely proportional to the phase current frequency. This paper proposes a new approach to solve the zero-/low- frequency operation technical issue of MMC-based drive systems. The proposed approach utilizes the additional degrees of freedom of multiphase machines to inject secondary-plane current components with certain magnitude and frequency in the load current to enable smooth operation during that interval. A simulation model of a three-level five- phase MMC-based Induction Machine (IM) drive system has been built using MATLAB/Simulink. In this paper, the adopted control strategy is explained and the simulation results are presented.