A Study on Low Participation of Oxygen in Generation and Progress of Bow-tie Trees

Abstract

Bow-tie trees (BTTs) generated by microscopic contaminants or voids in insulators of XLPE cables still remain a factor contributing to the deterioration. It has been reported that redox reactions of ions diffusing in XLPE lead to molecular chain cleavage. In such an electrochemical mechanism, the participation of oxygen other than ions is suspected. In order to examine the influence of oxygen on BTT generation and progress, we performed various experiments controlling oxygen gas by vacuum degassing, a gas barrier film, etc. The number of BTTs and the maximum BTT length of XLPE samples aged at 60°C increased with the water content. However, several of the data deviated significantly from these relationships. These results represent that BTT generation and progress were enhanced by vacuum degassing and inhibited by wrapping the samples with a gas barrier film or bubbling air through the tap water to immerse the samples. On the whole, the behavior of BTT generation toward supplying or shielding of oxygen was not consistent. The experimental results can be qualitatively explained by diffusion or permeation of ions and water rather than oxygen gas, thus oxygen seems not to be directly associated with BTT generation and progress.