Extending Service Life of Installed 15-35 KV Extruded Dielectric Cables

This paper describes methods and apparatus used to extend the useful life of extruded dielectric cables rated 15-35 kV having electrochemical trees in the insulation of sufficient severity to cause service failures. In-service treatment of 15 kV cables, ranging in length from 70 to 270 meters, having stranded conductors flushed with dry nitrogen or filled with special liquid are described. Flushing the conductors with dry nitrogen removes moisture and filling the conductors with special liquids, which subsequently permeates into the insulation, displaces moisture. Both methods restore to a large degree the voltage breakdown strength of the cables, reduced by electro- chemical trees, thereby extending their useful life. Laboratory and field test data taken on upgraded cables are presented. Methods to introduce gases and liquids while the cable is energized and apparatus to by-pass system components are described. An assessment of costs for upgrading indicates a significantly lower expense and less disturbance to cust6mers than the cost of replacing the cable.