The half-bridge Modular Multilevel Converter
(MMC) is a Voltage Source Converter (VSC) with high effi-
ciency, controllability and modularity. The topology is weak to
DC side faults unless bipolar sub-modules 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 Sub-Module
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 Sub-Module
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
∼
20-25% can be achieved by using the
proposed PG structures to augment a half-bridge MMC.