Extended modular multilevel converters suitable for medium-voltage and large-current STATCOM applications

This paper discusses a new transformerless shunt static compensator (STATCOM) based on the extended modular multilevel converter (EMMC), to achieve higher performance in a medium-voltage large-current network. It introduces an appropriate control algorithm based on a phase-shifted carrier modulation strategy, to ensure the source-end three-phase currents are sinusoidal and balanced, balancing the voltages of the dc-link capacitors of the modules as well. Compared to the conventional modular multilevel converters (MMC), the EMMC introduces higher reliability and better performance in the distorted large-current systems. Simulation results from a complete model of the proposed STATCOM are presented, confirming the pre-defined objectives. One interesting application for the EMMC-based STATCOM could be the improvement in power quality and performance of the electrified railway power supply systems.