Relation between the halo and water trees in XLPE cables

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 (≈23-mm insulation wall) and had experienced several failures during its six years in service. The density of water trees over 80 μm 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