Thyristor/Diode-Bypassed Sub-Module Power-Groups for Improved Efficiency in Modular Multilevel Converters

The half-bridge modular multilevel converter(MMC) is a voltage source converter (VSC) with high efficiency, controllability, and modularity. The topology is weak to dc side faults unless bipolar submodules are used, but this results in decreased efficiency. Power groups (PGs), a thyristor augmented multilevel structure, have been proposed as a way to reduce the power-loss increase arising from achieving dc-fault-tolerance. This paper investigates whether the PG concept can also achieve significant efficiency improvements in VSCs that are not required to be dc fault tolerant. A single submodule voltage (SSMV) method of controlling the turn-on/turn-off of the thyristor assembly within each PG structure is presented, and the differences with the previously detailed dual submodule voltage (DSMV) technique are described. Two thyristor-based PG structures for use in non-dc-fault-tolerant MMCs are proposed, one using SSMV and the other using DSMV. A comparison is made considering the required semiconductor device count, the impact on thyristor snubber design, and the overall power-losses achieved. A further, simplified, variant using a diode bypassed PG structure is presented, which results in power-loss reductions during rectifier mode only. Results show that power-loss reductions of similar to 20%-25% can be achieved by using the proposed PG structures to augment a half-bridge MMC.