Water-tree Detection Of XLPE Cable Based On Residual Charge Method

XLPE cable has excellent electrical performance, easy-installation and degradation-proof property, thus is widely used in electric transmission lines at all voltage levels. Under the mutual influence of water and electrical field, water tree may be generated in insulation layers of cables. As the water tree grows, electrical field at the tip of water tree will become more intensive, and finally result in the generation of electrical tree at the tip of water tree. Once the electrical tree appears, breakdown will almost occur in the insulation layer of cable in a short period of time. Therefore, it is highly important to investigate the growing process and diagnosis methods which have important theory and application value in maintenance of reliable operation of power cables.From microscopic viewpoint, the formation of water tree in XLPE cable insulations is stated, and the microscopic structure of water-tree and its equivalent ellipsoid model structure are introduced. Based on the nonlinear property of resistance of water tree, a new method, Residual Charge Method (RCM), is proposed to diagnose water tree. After the design and building of experiment circuit, experiments are conducted on 12 10kV XLPE cable samples, and the amounts of residual charges are measured. The results show that when the first ramping alternative current (ac) voltage is applied, there is a good correlation between the leakage rate and magnitude of residual charges and the growing information of water tree in insulation layers of cables.