Electrical ageing and testing of dielectric self-supporting cables for overhead power lines

Long-span, all-dielectric, self-supporting cables have been developed as a result of demand for optical fibre based communication systems on overhead power lines. After considerable experience and successful operation on towers carrying circuits whose system voltages are less than 150 kV, these cables have been installed on higher voltage lines. In such circumstances, degradation of sheathing materials has occurred and this has been attributed to gradual erosion by dry-band arcing. New results of ageing by dry-band arcing are presented and conclusions are reached which are important in the testing of materials. A new feature is added to the dry-band arc model of failure. This allows the length of an arc to be gradually reduced by the movement of moisture along the cable. It is shown experimentally that if such arc compression does take place, the cable can be severely damaged by only a few seconds of this activity. It is suggested that the lifetime of a cable might be effectively determined by the time to one such period of activity rather than the ability of a cable to withstand long periods of stable dry-band arcing.