Effect of small metal particles on ageing of epoxy insulation

The electric performance (breakdown strength, ageing) of industrially manufactured insulation materials is usually far worse than that of laboratory made samples. The main reason of this discrepancy is commonly attributed to various kinds of impurities as, e.g., small metal particles, introduced by the large scale manufacturing process. We have intentionally introduced small elongated metal particles with well defined shape (length varying from 0.1 to 1 mm, diameter from 10 to 100 μm) into alumina-filled epoxy insulation, oriented in the electric field direction. We have performed accelerated ageing tests at elevated ac electric fields between 10 and 28 kV/mm. The dependence of the time-to-breakdown as a function of the applied electric field and the particle size can be understood in terms of a field enhancement at the particle ends, leading to charge injection. We find a simple scaling relation between the lifetime and the injected space charge