The surge characteristics of XLPE insulation containing water trees

Polymers such as polyethylene (PE) and cross-linked polyethylene (XLPE), used as insulation in high voltage cables, are susceptible to a form of degradation known as water treeing. These tree-like structures grow into the insulation from stress concentrations or cavities in the presence of water and electric stress1. Although some cables which have failed in service contain water trees, the exact mechanism by which the water trees contribute to the failure is not known. It is possible that transient overvoltages due to lightning or switching surges produce failure from a water tree2. This paper reports the effect of water trees on the ac, dc and impulse strengths of miniature cross-linked polyethylene cables. The influence of test method and the drying of cables containing water trees are also included. Tests using miniature cables rather than water-filled needle shaped cavities were preferred as the stress enhancement in the cable insulation is usually much less than that for the needle system and also gives a closer representation of a high voltage cable.