Energy Control of Modular Multilevel Converters for Drive Applications at Low Frequency Using General Averaging

This article focuses on the energy control of modular multilevel converters (MMCs) for drive applications at low frequency. Utilizing the general averaging method, this work extends the results for MMC dc-operation in a previous work to low-frequency operation, and inherits all improvements with respect to model based controller design in dc-operation analysis, compared to other state-of-art approaches. The proposed energy control system—with either a constant or a stationary regime as reference for the instantaneous MMC energies—is proven to be globally stable, where the stability of the constant reference in low-frequency operation is confirmed theoretically, which is missing in state-of-art research. Besides, an optimization cooperating with two common-mode voltages for the energy control and compensation of ripples at low frequency is provided. The experiments verify the stability of the energy control system, and provide comparison between the two energy references in low-frequency operation for the first time, confirming the constant energy reference as a noteworthy option. The optimization improves the MMC performance, including the stationary operation generated by the optimization in cooperation with the approximated trapezoidal common-mode voltage as an improvement of a state-of-art solution in the sense of smaller arm currents RMS and cell capacitor voltage fluctuation.