The Effects of PWM with High dv/dt on Partial Discharge and Lifetime of Medium-Frequency Transformer for Medium-Voltage (MV) Solid State Transformer Applications

Abstract

The medium-frequency (MF) transformer in a phase-shift modulated dc solid state transformer (SST) is prone to partial discharge (PD) that impacts the system reliability and lifetime. Tightly packed MF transformer windings carrying pulsewidth modulation (PWM) voltages create favorable conditions for PD inception due to proximity between adjacent winding conductors and electric field enhancement because of spiral geometry. A peak potential condition is created between the adjacent posterior conductors of the primary and secondary windings as a result of phase-shifted modulation. This along with fast alternating electric fields incepts PD at lower than expected operating voltages. Early inception of PD leads to premature transformer failure leading to unanticipated dc SST down-time. The effects of PWM voltages containing high dv/dt switching transients on PD and lifetime characteristics of MF transformer in a dc SST are not yet understood, which is investigated in this article. A nonintrusive PD detection strategy for MF PWM ac voltage is presented using high-speed optical sensors. PD characterization is performed on Kapton polyimide insulated copper foil windings for a foil-type MF transformer. Winding samples are stress tested with steady state PWM voltages up to 2 kV and frequencies up to 50 kHz with high dv/dt up to 60 V/ns generated by a half-bridge inverter using 3.3 kV SiC devices while maintaining the transformer's geometric profile. Accelerated dielectric lifetime characterization is also performed and novel lifetime prediction models are derived based on the acquired results containing both frequency and dv/dt dependent variables.