Water tree initiation and growth in XLPE cables under static and dynamic mechanical stress

Water treeing is the major aging mechanism in wet-designed cross-linked polyethylene (XLPE) cables causing a reduced lifetime. Several factors affect the initiation and growth of water trees. Without the presence of an electrical field, contaminants or moisture above a certain level, no water trees are observed. In addition, also e.g. mechanical load can influence water treeing. In this paper the effect of dynamic mechanical load on water treeing is studied. A novel test rig was designed for applying simultaneously dynamic mechanical and electrical stress on medium voltage cables in wet environment. The cables are dynamically bent to a given radius to induce a maximum strain of 1 % in the insulation. After ageing the cable insulation was examined for water trees. The water treeing is compared with a reference subjected to a static mechanical load. It is found that mainly small bow-tie trees grow in the insulation even after 0.8 million mechanical cycles. Significantly higher densities of the trees are found at the location with highest dynamic mechanical strain compared to the static reference cable.