A Novel Hybrid DC Transformer Combining Modular Multilevel Converter Structure and Series-Connected Semiconductor Switches

A hybrid dc transformer (DCT) combining the features of modular multilevel converter (MMC) and series-connected semiconductor switches (SSs) is proposed in this article, which is suitable for interconnection between medium-voltage dc networks and low-voltage dc networks. Compared with the conventional MMC-based DCT, the number of submodules (SMs) of the proposed DCT can be reduced under the same device voltage stress. A quasi-square-wave modulation is employed, which can ensure the high-reliability drive of SSs with the realization of zero voltage switching in a wide power and MV voltage range. The voltage self-balancing characteristic of the SM capacitor can be achieved in the proposed DCT, with no requirement of voltage sensor or specific modulation pattern. All the SSs are turned on/off at zero voltage, the dynamic voltage balancing of the SSs is ensured naturally. Based on the analysis of the operation principle, the control scheme and parameters design are elaborated. The theoretical analysis is verified by the simulation and a downscaled 4-kW prototype experiment.