Suppression of Device Voltage Stress from Ground Leakage Current for Soft-Switching Solid-State Transformer

Grounding related issues are critical for safe and reliable operation of solid-state transformer (SST) in medium-voltage (MV) utility applications. This paper presents the issue of additional device voltage stress due to grounding-path current for current-source soft-switching solid-state transformer (S4T). The S4T achieves single-stage AC-AC, AC-DC, or DC-DC conversion with full-range ZVS, derived from the current-source/flyback converters. However, the flyback operation means that the device voltage in the inactive reverse-blocking bridge can be influenced by parasitic current. A comprehensive parasitic model of a modular S4T (M-S4T) prototype is developed from direct measurements and datasheets. Using the developed parasitic model and equivalent circuits, the causes of the voltage stress are analyzed. A voltage-stress suppression scheme of connecting additional grounding capacitors to the transformer terminal and the input terminal is proposed. Robust parameter design guidelines of the proposed scheme are given. The effectiveness of the proposed scheme is verified experimentally with a M-S4T prototype based on SiC MOSFETs and diodes under single-module and stacked-module testing cases up to 1.6 kV peak.