The effect of thermal aging on the electrical breakdown strength of crosslinked polyethylene

Over the last several decades, lifetime models for crosslinked polyethylene based insulations have been developed based on the combined effects of aging under thermal and electrical stresses. Several of these models include a threshold electrical stress below which no aging occurs, though other modes of aging such as thermo-oxidative aging are still present. The thermo-oxidative aging of crosslinked polyethylene that affects the mechanical properties of tensile strength and elongation at different temperatures can be modeled using the Arrhenius equation. This oxidative model can be used to predict the thermo-oxidative “lifetime” under service conditions at which time the material´s mechanical properties are severely degraded. Some researchers have drawn `apparent´ correlations between decrease in mechanical strength and decrease in electrical strength after thermal aging as a means to construct a thermal endurance curve. However, a generalization of such an approach is questioned relative to lifetime predictions for failure mechanisms involving electrical breakdown. This study assesses the impact of degree of oxidative degradation on the AC dielectric strength of crosslinked polyethylene. The results clarify the utility of thermal oxidative aging with respect to combined aging models for prediction of the electrical “lifetime” of crosslinked polyethylene.