Protection Design Considerations of a 10 kV SiC MOSFET Enabled Mobile Utilities Support Equipment Based Solid State Transformer (MUSE-SST)

Abstract

Solid state transformers (SSTs) are evolved as an emerging technology which offer several key features in integrating different grids, storage devices, and renewable energy sources, etc. In this paper, a 10 kV SiC MOSFET enabled Mobile Utility Support Equipment based SST (MUSE-SST) is presented for integrating a medium voltage (MV) AC grid (4.16 kV L-L, 60 Hz) and a low voltage (LV) AC grid (480 V L-L, 60 Hz). The MUSE-SST consist of three power conversion stages: MV side active front end converter (AFEC: MV), dual active bridge (DAB), and LV side active front end converter (AFEC: LV). In order to protect MUSE-SST from various abnormal operating conditions, different protection schemes are discussed. Dedicated gate drivers incorporating device short-circuit protection are implemented for both the MV and the LV SiC MOSFETs. It is observed that the MV short circuit faults are found to be severe and resulting in very high inrush currents from the MV grid which can lead to the impairment of entire SST system. A bi-directional switch in parallel with a high impedance resistor (BSPHR) circuit based protection scheme is presented to limit the grid currents during MV dc-link short circuit conditions. The BSPHR scheme and efficacy of gate drivers are validated through simulations and experimental results, respectively. The avalanche ruggedness of 10kV, SiC MOSFETs is established experimentally using a single shot unclamped inductive switching (UIS) test and the critical avalanche energy for the MOSFET failure is obtained.