Influence of water and ion diffusion on generation and progress of bow-tie tree

Bow‐tie tree (BTT) generated from contaminant, such as metal, carbon, amber (overcured resin) or void is a deterioration factor of XLPE cable. In particular, BTT in contact with inner or outer semiconductive shield could significantly lower residual AC breakdown voltage of HV power cables. To evaluate the influence of water and ion diffusion on generation and progress of BTT, we investigated the relationship between water content of XLPE and the generation of BTT under various accelerated aging conditions. The number of BTT in XLPE samples with accelerated aging under the open condition, involving evaporation of water in which samples were immersed, was very large compared with the closed condition. Furthermore, when samples were intermittently immersed in water, the number of BTT in samples was large compared with samples immersed continuously. In these experiments the generation of BTT seemed to have nothing to do with changes in water content before and after accelerated aging. Therefore, it was suggested that diffusion of ions rather than water in XLPE played an important role in the generation of BTT. © 2007 Wiley Periodicals, Inc. Electr Eng Jpn, 162(3): 9–16, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20413