Daniel Fournier

Effect of pressure and length on interfacial breakdown between two dielectric surfaces

The phenomenon of interfacial AC breakdown is considered. Two pieces of EPDM (ethylene-propylene diene monomer) rubber were pressed together and a special jig was conceived to provoke arcing along the interface. The voltage at which breakdown occurred was measured and its dependence on interfacial length and pressure established. In the absence of silicone grease at the interface, breakdown voltage showed a linear dependence on pressure. With silicone grease, breakdown values were consistently higher and reached a plateau at higher pressures. The dependence on length was similar to what is observed for bulk dielectrics. While the dielectric strength curve for a silicone-greased interface decreased with length, the one with a dry interface, although it followed the same general behavior, exhibited an unexplained dip at around 4 mm.<<ETX>>

Influence of thermal cycling on the cable-joint interfacial pressure

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.

Interfacial breakdown in cable joints

Electrical breakdown at the interface of two dielectric surfaces is a complex phenomenon and is one of the major causes of failure for cable accessories. The dielectric strength of an interface increases with interfacial pressure and is substantially improved by the presence of electrical insulating greases. In order to better understand this phenomenon, interfacial breakdown experiments were carried out on interfaces found in cable joints. Measurements of breakdown voltage, interfacial pressure and grease conductivity were made to provide a comprehensive set of data for the investigation. This paper describes the experimental approach and discusses the preliminary results of the investigation on breakdown voltages found in new and field-aged cable splices.<<ETX>>

Effect of the surface roughness on interfacial breakdown between two dielectric surfaces

Cable splices and accessories are the weak link in an underground power distribution system. Investigation of problems related to cable splices and accessories becomes quite intricate once the simpler causes of failures are dismissed to allow more complex phenomena to be examined. The interfacial breakdown between two internal dielectric surfaces represents one of the major causes of failure for power cable joints. In order to better understand this phenomenon, breakdown experiments were performed at interfaces found in cable splices. An experimental jig was designed to induce breakdown between dielectric surfaces longitudinally along their interface. Effects of surface roughness at EPDM/XLPE and EPDM/EPDM interfaces as well as the presence of silicone grease are taken into account.

Morphological study of aging phenomena in XLPE by TEM technique

The long-term dielectric performance of underground power cable XLPE (cross-linked polyethylene) insulation suffers from poorly understood aging phenomena. A study of the morphological modifications of XLPE due to electrical aging may provide insight for a better understanding of aging mechanisms. The TEM technique has been used to study the XLPE morphology of unaged, laboratory-aged, and field-aged cable samples. A suitable image contrast enhancing and a stabilization of radiation damage were both successfully achieved by staining the sections with ruthenium tetroxide (RuO4), as individual XLPE lamellae were neatly and reproducibly resolved. Image analysis was used to help in the determination of any aging-induced morphological changes. XLPE samples from an unaged cable (D1) has been exposed to a low-energy electron beam, in an attempt to simulate certain conditions thought to occur in electric field-stressed dielectrics.