The effect of water treeing on the electric field distribution of XLPE - Consequences for the dielectric strength

This paper describes the influence of water trees on dielectric properties of flat samples cut from a HV crosslinked polyethylene (XLPE) cable insulat ion. In order to obtain experimental results in a short period of time, we used accelerated aging conditions of voltage and frequency as well as diffe rent techniques for producing a multitude of artificial water tree inceptio n points. The average value of the water tree permittivity and a law for th e time-dependence of permittivity inside the treed degraded areas were dedu ced from measurements of capacitance and water tree lengths. On the basis o f the experimental knowledge of permittivity and water tree length, we dete rmined with analytical/numerical methods, the distribution of the electric field for different shapes of treed regions. Because of the increase in len gth and permittivity of the water tree, the electric field in front of the degraded area is amplified, which is an important factor of risk leading to the electric breakdown.