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 degradation rates associated with PD in micrometric cavities in polymeric insulation systems subjected to AC voltage in depth. 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 damage of critical size and, ultimately, to breakdown. The amount of damage generated by the discharges, defined as the number of broken C-H bonds of the polymer chains, is evaluated on the basis of the electron energy distribution, calculated for various electric field values, as in a previously-presented life model for HVDC insulation systems. Here the possibility of the extension to the case of HVAC systems is studied, considering the effect of charge diffusion dynamics and of field modification inside the cavity due to the charge deployed by previous discharges. These phenomena are investigated by means of numerical simulations of the discharge phenomenon, based on a physical model of self-sustaining discharges and on the physical properties of the insulating material. Finally a method for the evaluation of the damage growth rate under partial discharges is proposed, on the basis of the statistical distributions of the amplitude of the partial discharges and of the repetition rate of the discharge activity averaged over a certain period of time.