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

Abstract

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 hotelectron 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.

Evolution of pd patterns in polyethylene insulation cavities under AC voltage

“…3) Depending on cavity size and electric field, hot-electrons, having enough energy to cause the dissociation of bonds of the polymeric chains (e.g. C-H bonds for PE), can be produced in the electron multiplication process [5,14,15]. These hot electrons cause a damage of the polymer matrix through DEA (Dissociative Electron Attachment) according to [11,13].…”

“…Under dc field, the electrons in the polymer matrix can reach the dielectric/cavity interface due to electric conduction. There, they are trapped because of the negative electronic affinity of PE [14]. They can be emitted into the cavity through the Schottky effect, thanks to the fact that the activation energy for electron detrapping,  , can become lower than the actual charge trap depth (work function), U 0 , due to the electric field and the Coulombic repulsion between stored charges on the surface.…”

“…Under ac sinusoidal voltage, the period of the applied field can be comparable to the decay time of the space charge deposited at the dielectric/cavity interface, so that the contribution of space charge decay must be taken into account [14,15]. Indeed, after the first PD occurs under ac field, the main source of the electrons available for injection is provided by the charges at the cavity surface.…”

“…In particular, physical models of PD behavior and relevant damage growth from micro-cavities in dielectric material under dc and ac voltage were presented, see e.g. [2,[11][12][13][14][15]22].…”

Relation between the trend of partial discharges and aging models under AC voltage

“…Significant research [2][3][4][5][6] has been carried out to study the degradation induced by this type of PD activity. In particular, a physical model of inception and growth of damage from micro-voids in polyethylene-based material under DC was presented in [7][8][9][10]. The results presented in this paper are obtained in the framework of developing an analytical model of the PD-induced degradation processes under AC voltage, occurring within cavities present in insulation systems.…”