Investigation of electrical treeing in cable grade crosslinked polyethylene (XLPE) insulations

Using PC-based on-line monitoring system, extensive measurements were performed in insulation blocks cut from the crosslinked polyethylene (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 m. The role of 'applied voltage 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 min duration provides good comparison to evaluate service aged and new XLPE insulation of different voltage ratings. Velocity of propagation of electrical trees is fast at initiation, but declines exponentially to a constant rate within 20 s of its initiation. Fractal analysis of such trees was also carried out using different types of technical grade XLPE insulations. This paper presents discussion on these results and sheds light on the electrical treeing phenomenon.