Frequency-Dependent Surface-Discharge Lifetime and PRPD Failure Signatures of Polyimide Insulation Under High-Frequency Sinusoidal Voltage

Polyimide (PI) film is a widely used insulation material in high-frequency power equipment, yet its frequency–lifetime relation and terminal discharge features have rarely been resolved together over a complete ageing trajectory. This paper reports three contributions. First, a condition-specific frequency–lifetime relation for PI surface insulation is presented, in which the discharge inception voltage is frequency-invariant while the flashover voltage and the surface lifetime both fall with frequency. Second, a stage-resolved discharge morphology derived from phase-resolved patterns is proposed, in which a late-stage finger-like cluster serves as a candidate flashover precursor, complemented by a count-aware, time-normalised discharge activity proxy. Third, a space-charge interpretation linking the per-cycle charge injection budget growth at high frequency to the suppressed inter-cycle dissipation, jointly accounting for the lifetime relation and the non-dendritic damage morphology, is presented. The fitted lifetime relation provides a quantitative planning basis over the tested 10–40 kHz grid for insulation design of high-frequency power equipment, and the finger signature provides a candidate device-level morphological criterion for online monitoring.