Influence of thermal cycling on the cable-joint interfacial pressure

Abstract

Cable joints are generally known to be the weak points of underground distribution lines. One of the most common failure modes of premolded cable joints is linked to the occurrence of dielectric breakdown at the interface. Many parameters may influence the dielectric strength of a cable-joint interface: surface roughness, the presence of dielectric grease, and the length of the interface to name a few. Nevertheless, interfacial pressure is known to be a key factor that sets the cable-joint interfacial breakdown strength. AC dielectric breakdown strength is proportional to interfacial pressure up to a threshold pressure. A high interfacial pressure leads to good dielectric breakdown strength whereas a lower pressure leads to a poor long-term performance. Interfacial pressure decreases with the number of thermal cycles. Moreover, partial discharge (PD) monitoring during thermal cycles with thermal gradients in the insulation, following IEEE 404 standard protocol, did reveal a maximum PD intensity at the end of the temperature decrease for each cycle. This could be linked to a decrease in the interfacial pressure. We study the behavior of interfacial pressure when a cable-joint assembly is thermally cycled and identify the worse case for cycles at 75, 90 and 130/spl deg/C with thermal gradients in the bulk of the insulation. The cycling protocol is based on existing IEEE 404 standard for cable joints for use with extruded dielectric cable rated 5-138 kV.