Ageing of DC conduction in thermally aged oil‐impregnated pressboard

Abstract

Ageing of the conduction process in a dielectric allows a better understanding of the phenomena. Available literature shows that the DC conductivity of oil impregnated pressboard increases with applied electric stress, particularly above 40 °C. However, the reason for this conductivity behaviour is not well understood. This paper attempts to understand the conduction mechanism of oil‐pressboard insulation and the effect of thermal ageing on it. For this purpose, DC conductivity measurements have been performed on oil impregnated pressboard specimens with varied ageing status at the electric field range 1 kV mm−1–15 kV mm−1 at 40 °C temperature. After a critical analysis, a new electric field dependent non‐linear conduction model was developed based on charge injection and thermally assisted “hopping” of charge carriers from one site to another site to explain charge transport in oil‐pressboard. The developed model fitted well with the measured experimental values of conductivity. It was also observed that certain model parameters, that is charge carrier density, charge mobility and potential barrier for charge hopping are dependent on the ageing status of the insulation that clearly affects the conduction process.