Properties of Water Tree Growing in XLPE and composites

Crosslinked polyethylene (XLPE) is widely used as insulation material in medium-voltage and high-voltage power cables due to its excellent electrical performance and aging resistance. However, in humid environment, XLPE cables are apt to be treed and degrade the dielectric performance. The water tree would easily lead to the growth of electrical tree and then cause permanent damage of insulation. The service life of power cable is threatened by those factors. Therefore, it is especially important to study the growth characteristics and influencing factors of water trees in XLPE. In this paper, the lamellar nano-montmorillonite (MMT), spherical nano-silicon dioxide (SiO₂) and polar ethylene-vinyl acetate copolymer (EVA) were blended with XLPE by melt blending method, and the effects of the three kinds of dopants on mechanical properties, structural morphology and water tree growth characteristics of XLPE were studied. The results show that the modified XLPE sample has a higher elastic modulus and better toughening effect that was attributed to the increased absorption of impact energy during plastic deformation and the reduced damage of polyethylene molecular chain caused by the periodic deformation of water droplets under the action of the electric field. Thus the growth of water trees is inhibited. The dopant with different structures significantly modify the crystal morphology of XLPE, homogenizing the crystal size and optimizing the crystal structure, which makes the diffusion path of water molecules more tortuous, and the diffusion of water was slowed down. The accelerated water tree aging tests were conducted on the c omposite samples with water-needle electrode method, the morphology of water-tree was observed by microscope and the length was statistically analyzed. According to the result, it can be concluded that water tree length of composites with MMT, SiO₂ and EVA are decreased compared with pure XLPE. The XLPE/SiO₂ nanocomposite has the shortest branch, which decreased by 54.1% along the electric field direction and by 48.7% in the vertical direction of the main electric field. For the sample with XLPE/MMT, the length of water trees are being reduced by 57.5% and 18.8% along the direction of and in the vertical direction of the main electric field respectively. In addition, the water trees morphologies were obviously different. The water trees of pure XLPE grows uniformly in divergent form, while the growth of the water tree of XLPE/MMT was nonuniform diverging from the tip defect with more “branches”. As to the sample with XLPE/SiO₂, the water tree was relatively uniform with few branches and smooth edge.