The Hybrid MMC, comprising a mixture of fullbridge
and half-bridge sub-modules, provides tolerance to DC
faults without compromising the efficiency of the converter to a
large extent. The inclusion of full-bridges creates a new freedom
over the choice of ratio of AC to DC voltage at which the
converter is operated, with resulting impact on the converter’s
internal voltage, current and energy deviation waveforms, all
of which impact the design of the converter. A design method
accounting for this, and allowing the required level of derating
of nominal sub-module voltage and up-rating of stack
voltage capability to ensure correct operation at the extremes of
the operating envelope is presented. A mechanism is identified
for balancing the peak voltage that the full-bridge and halfbridge
sub-modules experience over a cycle. Comparisons are
made between converters designed to block DC side faults
and converters that also add STATCOM capability. Results
indicate that operating at a modulation index of 1.2 gives a
good compromise between reduced power losses and additional
required sub-modules and semiconductor devices in the converter.
The design method is verified against simulation results and the
operation of the converter at the proposed modulation index is
demonstrated at laboratory-scale.