Water tree induced breakdown in polyethylene

A breakdown mechanism based on a local enhancement of the electric field due to the dielectric nature of the water tree was investigated. Using a laboratory model made of a water needle and a facing metallic needle simulating a defect, it was shown that: (i) the presence of a water tree at the water electrode clearly lowers the dielectric strength; (ii) the dielectric strength decreases when the water tree length increases; and (iii) for a fixed water tree length, the dielectric strength decreases when the absorption of the light in the water tree increases. This last result is explained by increasing field enhancement on the metallic needle due to the change in water tree permittivity induced by the increasing water content