Solid insulation electrical endurance assessment under square voltage

Abstract

Medium-frequency transformers (MFTs) are subjected to distinct electrical stresses compared to conventional 50 Hz transformers. It is particularly due to the integration of wide bandgap semiconductor devices such as silicon carbide (SiC), which significantly enhances the efficiency of power electronic converters. However, the MFTs are then submitted to medium frequency square voltage with high dv/dt, which can induce a nonuniform distribution on its turns. In this context, Nomex is considered as a candidate for inter-turn insulation in MFTs. To assess inter-turn stress endurance, virgin Nomex samples were subjected to an electric field of 3.6 kV/mm thanks to a 20 kHz square voltage for 904 hours under ambient temperature. The condition of the insulation was monitored using frequency-domain dielectric spectroscopy and partial discharge (PD) measurements. Experimental results show that, in the absence of PD activity, no significant variation was observed in permittivity, dielectric loss factor, PD inception voltage, or PD extinction voltage, indicating stable dielectric properties and no ageing effects under the applied stress conditions.

Mamadou Djan BALDE, Martin GUILLET, Eric VAGNON, Julien HUILLERY

Presented at CEIDP 2025