Relation between the halo and water trees in XLPE cables

Abstract

The distribution of the halo or microcavity density across the insulation of steam-cured high-voltage XLPE (cross-linked polyethylene) cables was measured precisely by microphotometry and correlated with water-content measurements. The cable studied was the 138-kV type ( approximately=23-mm insulation wall) and had experienced several failures during its six years in service. The density of water trees over 80 mu m long is strongly related to the presence of the halo, i.e. high water-concentration regions. Shorter trees are located mostly in the middle of the insulation where the water content is very low. The halo is formed to act as a water reservoir, facilitating the growth of bow-tie trees. Heat treatment at a temperature higher than the XLPE melting point causes the halo to disappear totally, but not the water trees. >