Vu Thanh Hai, Nguyen Dinh Thang

This report deals with the mechanism of the final breakdown of polymeric
power cables which have been degraded by water trees. Water treeing is one of the major
degradation mechanisms in cable insulation. It has been believed that the final breakdown
takes place when a water tree penetrates the insulation. In order to reproduce penetrating
water trees in a film specimen, a double layer specimen with low-density polyethylene films
was subjected to an aging test to generate water trees. After confirming that penetrating
water trees had bridged across one layer of the ”double-layer” specimen, the breakdown
test was performed with several kinds of combinations of degraded and non-degraded films
by applying an AC voltage. It was concluded from the result that the penetrated water
tree region would withstand relatively high voltage stresses if the ambient relative humidity
was as low as 59 %, however, the breakdown voltage rapidly drops at higher humidities.
Finally, it was suggested that, under less humid conditions, a water tree would extend until
it bridges across the insulation without causing breakdown, however, the breakdown would
likely take place when the ambient humidity turns to be higher.