Observation of non-monotonic dependence of leakage current with temperature during thermal cycling

Conductivity under high voltage DC, is one of the most important characteristics of insulating materials used as insulation in HVDC applications. The volume conductivity of the insulation can be measured on press molded plaque samples by applying DC voltage and measuring the leakage current passing through the insulation. Such measurements are often performed at constant temperatures and constant voltage levels. In this work, the leakage current is measured on 1 mm thick press molded low density polyethylene (LDPE) and crosslinked polyethylene (XLPE) samples under DC voltage of 30 kV while the different temperature conditions including thermal cycling with temperatures between room temperature and 90 °C is applied. While the obtained results with constant temperature are in good agreement with the previously reported measurements, during thermal transients an interesting phenomenon was observed. Based on the theories and previously reported measurements, the leakage current is expected to have an Arrhenius dependence to temperature. But in the new experiments during thermal transients, the leakage current shows a non-monotonic temperature dependence and during heating and cooling, peaks are observed in the measured leakage current. It was discovered that the behavior of leakage current is influenced by the type of protective film used during sample preparation and using PET film leads to a higher apparent conductivity than using aluminum foil. A non-monotonic temperature dependence is observed in samples pressed using PET film while samples pressed with aluminum foil exhibit an Arrhenius temperature dependence.