Breakdown Characteristics of DMD Paper and Epoxy Resin Under the Square Waveform Voltage of High-Frequency Transformers Using Equal-Diameter Cylindrical Electrodes

The breakdown characteristics of insulation materials under square waveform voltage of high-frequency transformers (HFTs) are fundamental to the insulation design of HFTs. Due to the insufficient study of the breakdown characteristics under the square waveform voltage of HFTs, the insulation design of HFTs relies on the breakdown characteristics under line frequency ac or dc voltages, which may compromise the insulation reliability of HFTs. This study establishes a high-voltage high-frequency experimental platform for breakdown tests under bipolar square waveform (BSW) and combined BSW-DC voltages at various temperatures, insulation thicknesses, and rise times of BSW using equal-diameter cylindrical electrodes. The breakdown voltages of DMD paper and epoxy resin are analyzed with the two-parameter Weibull distribution. The effects of BSW frequency (from 4 to 14 kHz), BSW rise time (from 2.25 to 8.15 μs), insulation thickness (from 0.2 to 0.3 mm, DMD paper), dc component (BSW-DC ratio, from 1:1 to 1:3), and temperature (from 20 to 120 °C) on breakdown are experimentally and quantitatively investigated. The experimental results reveal that characteristic breakdown voltages decrease with increasing frequency and temperature, while they increase with a higher dc component. The influence of BSW frequency on breakdown voltage is reduced as the dc component increases. The effect of temperature on breakdown voltage is more pronounced under BSW voltage compared to combined BSW-DC voltage. Breakdown occurs near the edges of the BSW voltage without dc bias and randomly at the high level of combined BSW-DC voltage. As the thickness increases, the breakdown strength decreases. At higher frequencies, the impact of insulation thickness on breakdown strength is reduced. The breakdown voltage increases with the rise time. The influence of BSW frequency is reduced when the rise time of the square waveform voltage increases. These characteristics are explained by the free volume breakdown theory. Based on these findings, suggestions for the insulation design of HFTs are given.