Modulation and commutation of a single stage isolated asymmetrical multilevel converter for the integration of renewables and battery energy storage system in ships

In upcoming all electric ships, for the integration of low-voltage renewables and battery energy storage system with the high-voltage ac distribution, a single stage isolated asymmetrical multilevel converter is advantageous for the following benefits: 1) uses medium frequency transformer (MFT), thereby eliminating the bulky line frequency transformer; hence, resulting in a smaller footprint; 2) the asymmetry introduced in the MFT turns ratio generates high number of output voltage levels with less number of converter modules; and 3) single stage conversion removes the need for a dc-link capacitor, hence, improving the power density. In this paper, a modulation technique using triangular level shifted carriers based on the medium frequency link is investigated for this topology. Unlike previously used techniques, the proposed modulation shifts the dominant harmonics in the output voltage to the sidebands of multiples of twice the switching frequency, thereby reducing the output filter size. The incorporation of the switch nonidealities requires the implementation of a commutation strategy for the single stage conversion. A detailed circuit analysis showing different modes of operation to generate a specific output voltage level, considering the switch nonidealites is outlined in this paper. The analysis aids in the real-time implementation of the converter, and it also explains the distortion in the ideal output voltage profile. The analysis in general can be used for any isolated single stage converter. Simulation and experimental results are provided on a 15-level three-phase converter prototype with MFT isolation.