Fault-Tolerant ISOSP Solid-State Transformer for MVdc Distribution

This article introduces a fault-tolerant solid-state transformer (SST) structure to combine the benefits of higher power density and robustness in medium-voltage dc (MVdc) electric distribution systems. A SiC-MOSFET-based 6-MW, 36/6-kV input series output series-parallel (ISOSP) modular stacked dc/dc SST is proposed using medium-frequency (MF) transformer isolation. This structure renders the system with fault tolerance and the capability to operate normally even in a partial fault condition. Small-signal modeling, simulations, and Typhoon-hardware-in-loop (HiL) real-time system were performed to verify the operation of the converter. Experimental results from a scaled-down laboratory prototype prove the feasibility of the proposed isolated dc/dc structure and control system. Furthermore, outcomes from 3-D finite-element analysis done using COMSOL matched the 20-kHz MF transformer design used in the hardware. Overall, the proposed system can lead to significant improvement in power density in mission-critical MVdc applications such as subsea electrification.