Influence of mechanical stress and frequency on water treeing in XLPE cable insulation

A brief review is given of the mechanical damage theory of the formation of vented water trees. It assumes that vented trees are initiated when microcracks or crazes at the semiconductor/insulation interface become filled with water. Field enhancement leads to mechanical stresses high enough to cause crazing of the polymer and further tree growth. Experiments were performed on sharply coiled 12 kV XLPE (cross-linked polyethylene) cables and on mechanically stressed Rogowski-type test objects. The prediction that introduction of strains may ease the initiation and growth of vented trees was confirmed by these experiments. Water tree analysis and breakdown tests of Rogowski objects aged at 50 and 1000 Hz showed that the acceleration effort due to aging at 1000 Hz was mainly a reduction of the initiation period of water tree formation