Pole-restraining control for Modular Multilevel Converters in electric-ship applications

Abstract

In all-electric ship applications, energy transfer in the range of many megawatts is required. The optimal grid-voltage level in this range is medium voltage, e.g. 10 kV. For DC as well as for AC ship grids, power-electronic converters connect generators and loads to the grid. In case of a DC grid, direct connection of the converter is mandatory, for an AC grid it eliminates the transformer and increases efficiency. The most promising converter type for such requirements is the Modular Multilevel Converter (MMC). However, the control of such a converter - essential for operability and stability - is a challenge, specially during dynamics. This results from the internal single-phase structure of the converter. In this paper, a highly dynamic MMC control for electric-ship applications is introduced. It relies on the Pole-restraining principle, providing fast, stable and robust control. Time-domain analyses of the control dynamics verify the suitability of the chosen approach.