Chemical and physical degradation effects on epoxy surfaces exposed to partial discharges

Plane-parallel electrodes coated with epoxy resin have been used to investigate partial discharge (PD)-induced degradation of the insulating material. The mean amplitude of pulse-type discharges has been measured as a function of PD exposure duration, while light emission from the discharge space was simultaneously monitored using a sensitive photomultiplier tube (PMT). This combination of discharge diagnostics has allowed us to observe, for all specimens studied, a transition from an initial regime of large-amplitude (≳100 pC) pulses to smaller pulses (~1 pC) superimposed upon a continuous glow (pseudoglow and true glow) regime. Some specimens eventually sustained a pure (pulseless) glow discharge. The transition, typically after a few hundred hours, is attributed to PD-induced chemical modification of the epoxy surface, which drastically increases its electrical conductivity. Using various complementary physical and chemical techniques, we have studied the evolution of the specimen surface and near-surface regions as a function of PD exposure duration up to several thousand hours. For example, certain reaction products of PD-induced oxidative degradation have been identified; not surprisingly, the transition to a pseudoglow or glow regime does not interrupt the degradative processes in the specimens´ surface and bulk (≲300 μm) regions