On the Triggering of Partial Discharges in Polyethylene: Chemical and Electronic Characterization

We report a characterization of the chemical conditions that might cause an electron emission from a polyethylene surface and trigger a partial discharge in an isolated void. In the framework of the electrical power industry, polyethylene is, commonly, the most used material to form the insulating layer of electrical cables. Unfortunately, under AC, it is known that this polymer suffers deterioration, which is usually associated with the treeing process. The latter phenomenon starts within a gaseous defect encased in the polymeric matrix, inside which the electrical strength undergoes a significant decrease and an electron is emitted from the polymer into the void. This creates the conditions for the triggering of a series of partial discharges that degrade the material from within and creates a tree of cavities in continuous, and self-sustaining, expansion. The mechanism by which the electron emission occurs, causing the discharge to be triggered, is, most likely, the Schottky effect. It is, there- fore, very important to define the chemical conditions that favor the initial surface electron ejection. In the present study, we performed a series of density functional theory calculations for the characterization of the electronic structure of several defected polyethylene systems. Our purpose was to find a combination of chemical defects that could significantly reduce the surface work function, and potentially give a Schottky emission consistent with our experimental reference. The work function of each system has been the key parameter we followed for assessing its Schottky emission properties. According to the several tests we conducted, we stress that is really unlikely to have a Schottky emission from polyethylene without any residual electron charge on the surface, which, in turn, needs to be localized thanks to the additive electronic states given by chemical defects. In particular, we found that an oxidized, and negatively charged, polyethylene surface returned a work function in line with the experiment.