A Study on Long-Term Characteristics of XLPE Cables

Abstract

To study the long-term characteristics of XLPE cables installed in free air and in water, aging tests were conducted under various testing conditions using XLPE cables with both 3.5mm and 6mm insulation.
From the Weibull plots of lifetime distribution under the voltage stress E_L being the minimum breakdown strength, the minimum value of time to breakdown t_L under the constant voltage was estimated. The results of accelerated aging tests of XLPE cables installed in free air demonstrated that the V-t characteristics of XLPE cables could not be described by the conventional inverse power's law (t∝ V^-n) with a single constant life exponent n.
Based on the microscopic observation of sliced insulation of removed XLPE cables, it was concluded that bow-tie trees with longer tree length, observed in cables tested in water, were caused by the moisture from outside, whereas the trees in cables tested in free air were caused by the residual moisure originally existing in the insulation. The breakdown strength of the aged cables tested in water increases through cable drying. It doesn't however recover to the original values.