Long-term characteristics of XLPE insulated cable installed in water

The long-term voltage characteristics (V-t characteristics) have been evaluated for XLPE cables having 6-mm insulation thickness that were energized while immersed and the causes of cable breakdown have been elucidated by means of residual breakdown tests conducted by the pre-interruption method. The life index n was found to increase with decreases in the intensity of the applied field. Among causes of breakdown in these cables are the influence of water penetration itself and the effects of the onset and growth of bow-tie trees. Under high-intensity fields, water acts on existing defects increasing their harmful effects so that cable performance is adversely affected by water alone. This deterioration in performance due to water alone is limited, however, in terms of field intensity, and at lower intensities the onset and growth of bow-tie trees result in the formation of new defects that adversely affect performance. The working field of cables for 22-66 kV service falls into the latter category, and it is the phenomenon of BTT onset and growth that is thought to be the factor determining the life of cables in commercial service. Pre-interruption tests have made it possible to discover electrical trees induced from BTTs and have clarified their harmful effects, and have made it possible to correlate BTT length to breakdown strength, with BTTs considered as a dielectric contaminant in the polyethylene. Evaluations were also made of the effects of cable sheaths, showing that while PVC sheaths delay the penetration of moisture (and thus the onset of BTTs), there is a limit to this effect. The appropriateness of these evaluations of the PVC sheath effect and of the harmful effects of BTTs was confirmed by a study of the deterioration of 66-kV cables that had been removed from service