Four port solid state transformer based on hybrid MMC with enhanced dual half bridge submodules
This study presents a novel solid state transformer (SST) with four ports that can connect the medium voltage (MV) DC bus, the MV AC bus, the low voltage (LV) DC bus, and the LV AC bus, respectively. This SST is suitable for application in the hybrid AC/DC power grid that functions as an energy router between AC and DC systems. Furthermore, the SST submodule is enhanced with two types of dual half bridge converters, namely DHSM1 or DHSM2, depending on if an additional active switch is used to block the possible short circuit. Thanks to the designed structure of the proposed SST that combines DHSM1 and DHSM2 as the front end stage, it achieves reduced hardware complexity with the remaining capability to handle the DC short circuit fault that may occur at the medium or high level voltage DC grid. In addition, an optimised overall control strategy that can reduce the measurement demand and control complexity is proposed, with the possible power flow path inside the SST fully analysed. Simulation and experimental results validate the feasibility of the proposed SST and the effectiveness of the overall control scheme with the DC fault handle capability.