Comparative study of the sinusoidal-wave and square-wave circulating current injection methods for low-frequency operation of the modular multilevel converters

The modular multilevel converter (MMC), eliminating the expensive and bulky multi-winding phase-shifting transformer compared with the cascaded H-bridge converter, is shown to be of great potential in the field of medium-voltage motor drive applications. However, special care needs to be taken for MMC driving motor at low speeds, since the voltage fluctuation of the submodule capacitors increases with sinking the output frequency. And this large voltage fluctuation will cause significant voltage unbalance between the submodules and distort the output waveforms. The objective of this paper is to present a comparative study of two existing voltage fluctuation suppression methods: the sinusoidal-wave injection and the square-wave injection. Specifically, comparisons are made in terms of current stress, control limitations, and dc current harmonics. Finally, a down-scaled three-phase MMC prototype has been built and the two injection methods are contrasted and evaluated by experiments.