Dimensioning and Modulation Index Selection for the Hybrid Modular Multilevel Converter

The Hybrid MMC, comprising a mixture of full-bridge and half-bridge sub-modules, provides tolerance to DC faults within 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 de-rating 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 half-bridge 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 semi-conductor 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.