Development of a 13.8 kV, 22 kV DC, 1.1 MVA, 7-Level Multi-Cell Converter based on 10 kV SiC MOSFETs

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Abstract
With the emergence of 10 kV Silicon Carbide (SiC) MOSFETS, achieving higher voltage levels becomes feasible for multilevel converters. The multi-cell (MC) topology, aka ‘flying capacitor,’ is a common multilevel circuit used in medium voltage (MV) applications and is selected in this case due to its potential for high power density when operating under high switching frequency, as well as for its ease of capacitor voltage balancing. Accordingly, and adopting 10 kV SiC MOSFETs, this work proposes a 13.8 kV ac, 22 kV dc, 1.1 MVA, 7-level MC converter for grid-interface applications. The converter power stage, its layout, its auxiliary power system architecture, its protection circuitry, its insulation coordination system, and its digital control infrastructure are presented in detail. For verification purposes, a single ac-dc phase-leg building-block prototype is demonstrated experimentally under 22 kV dc operation.