A study on the dependence of water tree permittivity with time

During the growth of water trees in the insulation of a cable the distribution of the electric field is modified because of the local change of the dielectric properties of the material. It results in a local enhancement of the electric field which could increase the risk of breakdown. The key factor is the permittivity of the water tree and the aim of this work is to determine its possible values and, particularly, the law of its increase with time during the growth of the trees. The paper presents permittivity measurements in uniform field laboratory models with water trees grown in accelerated conditions. The resulting variations of the electric field in the material are also given.