A modular multilevel converter with isolated energy-balancing modules for MV drives incorporating symmetrical six-phase machines

Abstract

A modular multilevel converter (MMC) configuration is presented as a medium-voltage high-power drive for variable-speed applications incorporating symmetrical six-phase machines. The topology employs six standard MMC phase-legs feeding two isolated groups of three-phase machine windings, each with 60° spatial phase-displacement. A novel concept of ripple-power decoupling between adjacent sub-modules (SMs) of MMC phase-legs, while feeding machine windings in a spatial phase-opposition fashion, is realized through isolated dc-dc converter modules. This concept offers a significant reduction in SM capacitance, while eliminating the problem of SM capacitor wide voltage fluctuations, especially at low operating frequencies. Furthermore, the proposed configuration can efficiently operate at near zero frequency, therefore a machine speed-range from zero speed up to the rated speed is possible under rated torque operating conditions. The proposed MMC configuration is analyzed in detail, and its performance is validated using both simulation and experimentation.