Mechanisms and lifetime modeling of polyimide insulation under coupled electro-thermal-humidity stresses with high-frequency voltage

Accelerated aging of high-frequency transformer insulation is driven by the combined actions of electrical stress and thermo-hygroscopic environments, where electro–thermal feedback may lead to non-negligible factor interactions. In this work, Kapton polyimide (PI) film used for turn-to-turn insulation was investigated under high-frequency sinusoidal and bipolar square-wave voltage. An L9(3 4 ) orthogonal design was employed to evaluate the coupled effects of voltage amplitude (2 kV-4 kV), frequency (20 kHz-40 kHz), temperature (20 °C-100 °C), and relative humidity (25 %-75 %) on lifetime. Two-parameter Weibull statistics were used to extract the characteristic life β for each condition. Range analysis and ANOVA consistently indicate the factor importance as V > f > T > H , and all four main effects are statistically significant at the 1 % level. To directly assess electro-thermal coupling, an additional 2 × 2 extreme-corner f - T sub-design was conducted, and two-way ANOVA confirms a significant f × T interaction. A multiplicative multi-factor lifetime model was established and further improved by introducing an f - T interaction term exp( G ·ln f / T ), which substantially enhances the fitting accuracy. Electro-thermal simulations provide physical insight by showing that dielectric-loss heating promotes localized hotspot formation under high-frequency electrical stress. These results offer an engineering-applicable basis for high-frequency insulation design and lifetime assessment.

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