A modular multilevel cascade converter (MMCC) with a resonant high-frequency link

This paper presents a modular multilevel cascade converter (MMCC) topology based on a resonant high-frequency link, which is applicable for transformerless medium-voltage motor drives requiring a wide range of operation frequency. The today´s most practical topology for medium-voltage motor drives proposed in 1990s, contains a bulky and costly phase-shifted multi-winding transformer. On the other hand, existing MMCC topologies based on double or triple star-configured cascade voltage-source converters can incarnate transformerless motor drives, however being subject to limitation of the operation frequency due to the interference between the inter DC-voltage balancing control and the output-voltage control. This paper proposes an MMCC topology based on a resonant high-frequency link, which is characterized by controlling high-frequency zero-sequence voltage and current via the resonant link for the inner DC-voltage balancing control between the supply- and motor-side converters. Simulation results obtained from a 6.6-kV 800-kVA model confirm the validity and effectiveness of the proposed MMCC topology.