Investigation of incipient faults in 66 kV oil‐filled cable sealing ends

This paper reports on the investigation of incipient fault mechanisms of a widely used family of Cable Sealing Ends (CSEs) which use an earthed metallic clip on top of the stress cone to fasten the cable termination stress grading. Finite element simulations indicate that the clip is causing localised electric field enhancement which could also cause impurities in the oil to move towards the clip under the influence of the dielectrophoretic force. Accelerated ageing tests of stress cones in the laboratory have corroborated the simulation results, with the high electric field causing degradation of the semi‐conducting tape underneath the clip as well as initiation of tracking on the surface of the stress cone originating from the edges of the clip. Partial discharge (PD) testing using a bespoke test rig in the high voltage laboratory has shown that incipient faults originating from the ageing of the solid CSE materials can only be detected after surface tracking has progressed substantially. Even then, the intermittent nature of PD activity can pose a risk to the accurate condition assessment of CSEs if continuous monitoring is not employed and if the ultra‐high frequency method is used on its own.