An Attempt to Estimate DC Endurance Coefficient from Volume Resistivity

Ageing is a slow process of degradation of dielectric properties of cable insulation operating at high thermal stress. Prolonged thermal stress gradually weakens the insulation and ultimately leads to failure. Therefore, ageing studies are important for the design of insulation for high voltage power apparatus. One of the important properties of insulation is the resistivity of the material, especially for use in DC systems. In this paper, the authors have considered volume resistivity as a parameter to identify the changes with respect to ageing time. The ageing of insulation, change in volume resistivity and V-t characteristics are believed to be related. A batch of fresh LDPE samples was subjected to thermal stress for different time durations and the volume resistivity and permittivity of each of the samples had been measured under different electric fields. The variation in volume resistivity was analyzed experimentally and theoretically. An attempt has been made to relate endurance coefficient to changes in resistivity. Endurance coefficient has been estimated using volume resistivity.