SiC-based unidirectional solid-state transformer concepts for directly interfacing 400V DC to Medium-Voltage AC distribution systems

400 V DC distribution networks present a promising solution for supplying high-power DC loads such as information processing systems, transportation battery charging facilities and DC micro grids, among others. For these applications, high transmission efficiency, reliability and controllability are mandatory. With the current technology, these loads are fed from PWM rectifiers which are connected to the three-phase Low-Voltage (LV) distribution grid (400 V AC in Europe). The LV grid itself is supplied via Low-Frequency Transformers (LFT) from the Medium-Voltage (MV) grid, providing galvanic isolation and the required voltage step down. This paper presents three unidirectional AC/DC SiC-based Solid-State Transformer (SST) topologies with direct connection to the MV grid, which avoid the utilization of the aforementioned LFT by integrating a Medium-Frequency (MF) conversion stage, thus increasing the efficiency and power density of this supply system. The SST topologies are compared by means of a chip area-based comparative evaluation. Finally, the most suited among the presented topologies is Pareto-optimized, achieving a total MV AC to 400 V DC efficiency of 98.3 %. It is shown that the optimized SST features 40 % less overall losses compared to state-of-the-art solutions.