Dimensioning and Modulation Index Selection for the Hybrid Modular Multilevel Converter

A hybrid modular multilevel converter, comprising a mixture of full-bridge and half-bridge submodules, 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 submodule voltage and uprating 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 submodules 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 submodules 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 the laboratory scale.