Correlation of optical and dielectric properties of water trees in polyethylene

Abstract

Many experimental works report that the presence of water trees lowers the voltage needed for the initiation of dielectric breakdown in polyethylene. The influence of water trees on the dielectric breakdown is generally related to their length. Here, in addition to this spatial property, we consider the extinction of the light by the water trees due mainly to scattering of the incident light by water microcavities. This effect is described by means of the water tree absorbance. The absorbance itself is related to another main variable: the water content. We also take into account the existence of a structure in the water tree, i.e., of a heterogeneous distribution of water microcavities inside the trees. Water content and spatial water distribution allows us to define a local permittivity for each region in the tree. These elements are the basis of a model for the computation of the disturbance induced by the tree in the spatial distribution of the electric field (reduction of the field inside the tree, enhancement outside). The field enhancement effect is computed as a function of the water concentration. Measurements of the light transmitted by given water trees are performed; they allow us to propose an estimate of the water contained by different water trees. From these concentrations, different field enhancement coefficients are computed from the model. They are then compared with measurements of electrical treeing inception voltage performed with and without water trees. Theoretical values obtained from the model are found in good agreement with experimental data.