Model of ageing inception and growth from microvoids in polyethylene-based materials under AC voltage

Several degradation mechanisms may affect polymeric insulation system reliability. Some authors postulate that, even in a perfect dielectric, nanoscale cavities can enlarge due to various mechanisms (from mechanical fatigue to lowering of the degradation reaction energy barrier) up to a point where highly energetic phenomena, which bring about breakdown, can be incepted. Other authors are more focused on the inherent limits of manufacturing processes, which leave cavities within the insulation system whose size is large enough to cause electron avalanches, thus a measurable partial discharge (PD) activity, from the time the system is put in service or as a function of external factors (e.g. mechanical damage, thermal shrinking, overvoltages). Given the time scale of polymeric system failures, this latter mechanism seems to be more plausible. It is therefore worthwhile to investigate in depth the degradation rates associated with PD in micrometric cavities in polymeric insulation systems subjected to AC voltage. The proposed model is based on damage accumulation on cavity surfaces caused by PD phenomena. The main degradation mechanism associated with PD is considered to be the hot-electron induced bond-breaking process. This process accumulates with time, leading to the creation of a damaged region of critical size and, ultimately, to breakdown. The proposed model describes the defect induction and growth phase until treeing phenomena start, which is, normally, the largely prevailing component of breakdown time.