Test procedures for the evaluation of defect tolerance of electrically-stressed polymers

The influence of conductive defects, able to amplify electric field magnitude, on insulation breakdown is investigated in this paper by short-term tests. Appropriate test cells were used, made according to a Rogowski profile, where a set of semiconductive needle-shaped indents starting from the outer surface is realized. Samples of XLPE specimens were tested, differing for the types of additives used as voltage stabilizer and electrical tree retardant. The results of electric strength measurements are dealt with in the paper, with the purpose to show that the proposed test cell provides Weibull-distributed breakdown data, characterized by acceptable confidence intervals and repeatability. This enables an effective comparison between materials, even in the case that the presence of additives does not change remarkably material performances. Moreover, the multiplicity of indents helps in averaging the considerable influence of the curvature radius of needles, which may play a fundamental role in the usual test cells realized by needle-plane electrode configuration.