Experimental Study on Dependence of Bow‐Tie Trees on Nuclei

Abstract

SUMMARYBow‐tie trees (BTTs) generated by contaminants, for example, metal, amber (overcured resin), carbon, and voids in the insulators of XLPE (cross‐linked polyethylene) cables still remain a factor contributing to deterioration. In order to examine the influence of contaminants on the behavior of BTT generation, we performed several experiments using XLPE samples containing copper powder, finely divided overcured resin, or micro voids as BTT nuclei. As a result of the experiments, in spite of the great difference in the distribution of particle diameter, and the shape and permittivity of the nuclei, all samples containing copper powder or overcured resin showed a common dependence of the number of BTTs on the mean size of the BTT nuclei. Consequently, there is no fundamental difference in these nuclei. We consider that the lower limit of the nucleus size suitable for BTT generation is determined by the local electric field at the tip of the nuclei and its upper limit is determined by the extent of stagnation of ions around the nuclei. Moreover, we consider that the increase in the mean size of BTTs with the initial water content was caused by relaxation of the local electric field due to absorbed water in XLPE samples.