A Phase-Shifted Square Wave Modulation (PS-SWM) for Modular Multilevel Converter (MMC) and DC Transformer for Medium Voltage Applications

In this letter, a phase-shifted square wave modulation strategy is proposed for modular multilevel converters (MMCs) as well as dc transformers for medium-voltage applications. The proposed technique can achieve a smaller cell capacitor size without increasing the total device rating (TDR) and degrading the dc current control capability. The operation principle of the proposed method is presented on an isolated modular multilevel dc-dc converter, which consists of an MMC stage and applied as a dc transformer for medium-voltage dc grid. The TDR and passive components sizing are derived and compared with those using other modulation methods. Finally a downscaled hardware prototype has been built and tested to verify the proposed modulation method.