The Voltage Breakdown Characteristics of Miniature XLPE Cables Containing Water Trees

This paper presents the results of 60 Hz, dc, and impulse voltage breakdown tests on miniature XLPE cables containing water trees. To produce the water trees, the cables were immersed in water and subjected to high voltage at a frequency of 1 kHz for periods up to 1000 h. The dc and impulse breakdown voltages of virgin cable were similar and about twice the peak value of the ac breakdown voltage. The breakdown voltages decreased by up to 90% of their original values with time of ageing and also with ageing voltage. For well-developed water trees in the insulation, the ac and dc breakdown strengths were similar. The number and lengths of the water trees also increased with time of ageing and voltage. The breakdown voltages were almost independent of the tree density but decreased significantly with increasing tree length. The effects of ageing temperature and drying out of the cable prior to breakdown testing are also reported.