Development and optimization of breakdown strength measurements on polyethylene insulants

In order to study the ageing phenomena of polyethylene insulants in high voltage cables, we carried out electrical measurements, for example the short time dielectric breakdown field in polyethylene. The decrease of the intrinsic dielectric rigidity of polyethylene may be correlated with ageing. That is why we optimized the short time breakdown field measurements on polyethylene samples between plane surface electrodes immersed in a dielectric oil. First, we proceeded to the measurements of the dielectric breakdown voltages on polyethylene samples while increasing the voltage of 20 kV/mm/s. In a second step, the experimental results obtained from a population of 30 samples were treated by a Weibull statistic method : the two parameters model was used in order to deduce the value of the nominal breakdown field when the breakdown probability equals to 63.2% and the threshold breakdown field equals to 0; when the threshold breakdown field wasn´t nil, we applied the three order model. The study of experimental parameters, for example the nature and the surface of the electrodes, the viscosity of the immersion oil, the laboratory responsible for the experiments are presented, then discussed