Development of a Nondestructive Method for the Chemical Composition Analysis of the Nuclei of Bow-tie Water Trees in Field-aged Cross-linked Polyethylene Cable Insulation

Water tree formation is the major degradation mechanism of cross-linked polyethylene (XLPE) cables, which are widely used for supplying electric power to urban areas. Three types of water trees can be formed: (1) vented water trees from an inner semiconductive layer, (2) vented water trees from an outer semiconductive layer, and (3) bow-tie water trees. Bow-tie water trees are generated from the nuclei (contaminant or void) in the XLPE insulation and propagate toward the direction parallel to the applied electric field. To clarify the generation and propagation mechanisms of bow-tie water trees, the chemical composition analysis of the nuclei of bow-tie water trees is important. However, the nuclei are very minute and buried into the insulation of the XLPE cable, which hinders the analysis of their chemical composition. We herein selected confocal Raman microscopy and X-ray analytical microscopy because the former method can be used to clarify the chemical bonding between organic compounds, polymers, and inorganic compounds and the latter for the elemental analysis of a microarea of cable insulation. By combining both techniques, a void was revealed as the nucleus of the bow-tie tree formed in the insulation of a service-aged XLPE cable.