A Novel Solid-State Transformer with Improved Flyback Converter Equipped with Quasi Z-Source Converter for Medium-Voltage Utility Grid

This paper introduces an enhanced solid-state transformer topology for a medium-voltage (MV) utility grid. The main objective of the current study is to develop an improved flyback converter equipped with a quasi z-source converter (qZ_iFC) having a high-voltage conversion capability for the integration of low-input voltage to the DC link of an MV modular multilevel converter (MMC). The system integrates the quasi z-source and flyback converters by operating their existing switches complementary. Furthermore, the high-gain qZ_iFC allows for a reduction in the rated voltage of the input, as well as the use of a high-frequency transformer (HFT) with a unity turns ratio that provides galvanic isolation between the input and the output ports. Thus, using an HFT just for isolation purposes without voltage gain improves the system efficiency. In addition, a controller for (i) qZ_iFC which is regulating complementary switches to prevent the shoot-through current from reaching the HFT resulting in saturation; and (ii) a controller for MMC to produce MV-level AC voltage for loads are suggested. The performance of the proposed system was evaluated for several operating conditions. Results show that the proposed SST smoothly performs the power flow between the ports during steady-state and transient conditions. The power flow capabilities and efficiency values validate the viability and effectiveness of the proposed system.