Correlation of the thermal history and level of oxidation with field failure in a primary distribution cable study

A 206 foot section of XLPE insulated, 13.8 kV, tripler distribution cable that had failed in service was removed and different chemical analyses were performed on the XLPE insulation at several distances from the point of failure. X-ray photoelectron spectroscopy (XPS) was used to determine the changes in the chemical composition of the insulation including detection of oxygen, nitrogen and other elements. Other diagnostic tests included Fourier transform infrared spectroscopy (FTIR), oxidation induction time (OIT) and differential scanning calorimetry (DSC). In addition AC breakdown tests were performed on two 30 foot lengths of a single leg of the cable. A direct correlation was observed between the localized level of oxidation and the magnitude of the local peak temperature seen in service. The location of the point of failure correlated with higher levels of oxidation and correspondingly with a higher local peak temperature. These results suggest that measurement of oxidation products is one tool useful in estimating the degree of degradation in XLPE cable.