Long-term characteristics of XLPE insulated cable installed in water

Abstract

Long-term voltage characteristics (V-t characteristics) have been evaluated for XLPE cables having 6-mm mulation thickness that were energized while m e r s e d , 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 increased 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 mto the latter category, and it is the phenomenon of BTT onset and growth that is thought to be the factor determinmg 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 B I T length to breakdown strength, with BITS considered as a dielectnc contaminant in the polyethylene. Evaluations were also made of the effects of cable sheaths, showmg that whle 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 BITS was confirmed by a study of the deterioration of 66-kV cables that had been removed from service.1 Introduction Because of their excellent dielectric properties and easy handlmg, XLPE cables have been videl) used in transmission and distnbution systems.In Japan these cables have been adopted transmission s~stems for up to 275 kV and nowadabs even 500-kV XLPE cable are in senice') To obtam a cable \\ith sufficient reliabilitr ,the understanding of the long-term characteristics and V-t charactenstics of the cable is indispensableThe authors have conducted some research into the V-t charactenstics of XLPE cabl83) and it has been clanfied that long-term cable performance is affected especially if they are used in the presence of water. In this paper,the results of longterm tests of XLPE cable installed in water are reported and the causes of breakdown and the damaging effects of bow-tie trees are discussed.1n addition, the effects of vinyl sheaths and the msulating properties of cable that has been removed from actual lines are studied.