Understanding water treeing mechanisms in the development of diagnostic test methods

Recently, several diagnostic methods for measurement of permittivity and dielectric loss have been developed for non-destructive testing of water-tree degraded XLPE cables. Critical degradation is found to be associated with a more than proportional increase of the dielectric properties when increasing the applied test voltage above a certain level. In this paper an explanation for this nonlinearity will be presented, using a mechanical approach to the water treeing phenomenon. The water treed insulation is considered to consist of small water filled voids separated by crazing zones. At relatively low water content 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 finite element method (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 conductive channels and to increase the dielectric losses of the water treed insulation.