Experimental Study on Dependence of Bow-tie Trees on Nuclei

Abstract

Bow-tie trees(BTTs) generated by contaminants, e.g., metal, amber(over cured resin), carbon, voids in insulators of XLPE cables still remain a factor contributing to the 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 over cured 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, the shape and permittivity of the nuclei, each sample containing the copper powder or the over cured resin showed a common dependence of the number of BTTs on the mean size of BTT nuclei. Consequently, there is no fundamental difference in these nuclei. We consider that the lower limit of the nuclei size suitable for BTT generation is determined by the local electric field at the tip of the nuclei and the upper limit of them is determined by the extent of the 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 a relaxation effect on the local electric field due to absorbed water in XLPE samples.