The Effects of Frequency and Temperature on Water Tree Degradation of Miniature XLPE Cables

This paper presents results of water treeing aging tests on miniature crosslinked polyethylene cables. To produce the water trees, the cables were immersed in water and subjected to HV at frequencies of 600, 400, and 1000 Hz. The times to breakdown and ac ramp breakdown voltages after preset times of aging were measured. The effect of temperature cycling was also examined. The ac ramp voltage was found to be a good indicator of the severity of water tree degradation at room temperature as the time to breakdown distributions correlated well with a reduction in the ac ramp breakdown voltage. Temperature cycling of the cables increased the number and the lengths of bow-tie trees compared to room temperature aged cables but decreased the number of vented trees. Correlation between ramp breakdown voltages and times to breakdown was poor for temperature-cycled cables. Microscopic examinations of some of the breakdown channels and possible causes of the different mechanisms of breakdown as the final stage of aging are discussed.