DC SST MF Transformer Partial Discharge Characteristics Study with High dV/dT PWM Switching Transients of SiC Devices

Abstract

Tightly packed medium frequency (MF) transformer windings carrying PWM voltages are prone to partial discharges due to its proximity to adjacent conductor and continuously alternating electric fields between the conductors. In addition, for SiC based DC SST applying phase-shift modulation, a peak-potential state is generated between the adjacent primary and secondary winding conductors for the duration of phase shift. This magnified stress on insulation system gives rise to premature inception of partial discharges leading to sooner than predicted MF transformer failure. This paper presents a non-intrusive PD detection strategy for a PWM voltages using high speed optical sensors for foil-type transformers to detect partial discharges in MF transformer's inter-winding insulation at PWM voltages containing high dv/dt switching transients due to SiC devices. PD inception and extinction characteristics are experimentally obtained in the lab using a hardware consisting of 3.3 kV SiC MOSFETs that generates PWM voltages with dv/dt up to 60 V/ns and switching frequency up to 50 kHz on foil winding specimens.