Feasibility Study of a Modular Multilevel DSBC Conversion System Equipped With Medium-Frequency Isolation Transformers for Driving Multiple Medium-Voltage Motors

Abstract

This paper provides an intensive discussion on a modular multilevel cascade conversion system based on double-star bridge cells (DSBCs), which is referred to simply as a DSBC conversion system. This conversion system is defined as a power conversion system consisting of multiple DSBC converters and medium-frequency isolation transformers. A voltage fluctuation appearing across the floating dc capacitor of each bridge cell is investigated. Theoretical and numerical considerations regarding both circulating currents and the medium frequency lead to a reduction in the peak-to-peak voltage fluctuation. The system is therefore applicable to a wide variety of motor drives with small capacitors. Simulated waveforms verify principles of operation, which are obtained from the 6-kV, 5-MW system consisting of a single DSBC converter for grid connection, two DSBC converters for three-phase star-connected RL loads, and two 560-Hz transformers for galvanic isolation among the grid and the loads.