Model of Inception and Growth of Damage from Microvoids in Polyethylene-based Materials for HVDC Cables. 1. Theoretical Approach

A comprehensive physical aging and life model is presented for damage inception and growth in polymeric insulation, starting at the level of microscopic cavities, that are known to be one of the most severe factors limiting design electric field and life of cable insulation. Degradation within a polyethylene-based material for HVDC cables is considered as a hot-electron induced bond-breaking process that cumulates with time into the polymer matrix at microvoid-polymer interface, fed by subsequent electron avalanches generated in microvoids. By this way, a conductive pit is formed, that may exceed eventually a critical size for the start-up of a rapid failure mechanism, thereby leading ultimately to electrical breakdown. The model, essentially based on the physical and microstructural characteristics of the insulation, is virtually free from disposable parameters. It is shown to reproduce qualitatively the dependence of insulation time to failure on applied field, temperature and cavity size and to fit quantitatively experimental times-to-failure relevant to PE-based materials in typical working conditions for HVDC cables. The first part of this paper presents the theoretical background and the derivation of model equations, as well as their discussion. The second part deals with parametric investigation and data fitting.