The importance of mechanical properties for increasing the electrical endurance of polymeric insulation

Electrical endurance and mechanical stress-strain experiments have been performed on six grades of polyethylene; two branched low density grades, one cross-linked low density grade and three linear high density grades. A clear empirical correlation is observed between the mechanical stress at yield and the electrical endurance time. The relationship can be explained using a modified version of the Electrokinetic (EK) endurance model of Lewis et al. In the original model, it was assumed that main chain chemical bonds were ruptured during ageing. In the modified model it is proposed that the main ageing mechanism is one involving polymer yield. Matthews et al. have related yield to the α-transition in polyethylene and it has been shown previously (Griffiths et al.), that the activation energy associated with the α-transition (1.2 eV) produces the closest fit between electrical endurance data and the EK model. Cross-linking polyethylene makes it mechanically tough by inducing work hardening but has no measurable effect on the stress at yield. Furthermore, experimental results show that cross-linking has no effect on the electrical endurance time. These observations are consistent with the above idea that polymer yield and electrical ageing are related.