Investigations of Electrical Trees in XLPE Cable Insulation Samples Using PC Based On-Line Monitoring

Using PC-based on-line monitoring system, extensive measurements were performed in insulation blocks cut from the XLPE insulation of 36, 69 and 132 kV rated cables produced in Saudi Arabia. A needle-plane electrode system was used with a needle radius of 10 mum. The role of ´applied stress step-time duration´ and associated time resolved initiation and growth of different types of tree structures were evaluated. Results show that a step of 3 kV/10 minute duration provides good comparison to evaluate service aged and new XLPE insulation of different voltage ratings. Velocity of propagation of trees is fast at initiation but declines exponentially to a constant rate within 20 seconds of its initiation. Fractal analysis of some of these trees was also carried out. The paper presents discussion on these results and sheds light on the electrical treeing phenomenon