Mechanisms causing nonlinear dielectric response of water treed XLPE cables

The nonlinear dielectric response of water tree degraded XLPE insulation has been found to be an important diagnostic criterion. It is experimental shown that in case of long water trees the dielectric response increase more than proportional with increasing test voltage above a certain voltage level. To explain this nonlinearity, the water treed XLPE insulation is considered to consist of small water filled voids separated by crazing zones. At relatively low content of water and low applied test voltage several of these crazing zones are likely to be closed and insulating. When increasing the test voltage, Maxwell mechanical stresses will cause water to penetrate into the crazing zones, thus making electric contact between the elongated water droplets. Results from FEM calculations of electric fields and losses show that the effect of this will be to enhance the electric field at the tip of these c onductive channels and to increase the dielectric losses of the water treed insulation. Micro-FTIR examination showed that vented water tree structures can contain up to 2% of water.The water content was, however, lower at the tip of the trees and in the region close to the insulation screen than in the middle of the tree structure. Results from AC breakdown expe riments show that it is these regions with lowest water content that are particularly vulnerable to initiation of electrical trees.