Application of oxidation induction time and compensation effect to the diagnosis of HV polymeric cable insulation

Oxidative stability tests were performed on field and laboratory-aged crosslinked polyethylene (XLPE), ethylene-propylene rubber (EPR) and polypropylene (PP) insulations using differential scanning calorimetry. Flat films and miniature cables aged in the laboratory were subjected to a wide range of aging conditions that included thermal, electrical and a combination of thermal-electrical aging, in dry and wet environments. The results were analyzed using the Eyring rate theory. It is shown that for a given material the oxidative stability data can be described by a single linear relationship between the activation entropy ΔS and the activation enthalpy ΔH of the oxidative process, and thus are governed by the so called compensation effect. It is argued that the position of a data point representing a certain operating condition of an insulation on the ΔS vs. ΔH compensation plot is a measure of the degree of degradation and can be used as a diagnostic indicator of the operating conditions of the insulation