Particle-initiated breakdown with spacer involvement in compressed gas and its mixtures

Abstract

The use of compressed gas as the insulating medium has made it possible to use compact equipment compared to that with air insulation. However, the compact construction increases the operating field intensity. Sulphur hexafluoride (SF/sub 6/) gas insulation is extremely sensitive to local increases in electric field, which results from protrusion on electrode, triple junction (the region where the electrode, insulator and SF/sub 6/ gas meet) in compressed gas, the presence of conducting particles in gas insulation, and the shape of spacers supporting the conductor inside its grounded casing. The influence of a metallic particle attached to the spacer is particularly significant in the decrease of the dielectric strength of the SF/sub 6/ insulated system. Therefore, for development of highly reliable compact gas-insulated systems, it is vital to reduce the effect of metallic particles. The flashover withstand of a gas/spacer interface is a limiting factor in the design and operation of a SF/sub 6/ gas-insulated system. The surface flashover shows a strong sensitivity to the metallic particle contamination of the spacer surface. The particle may cause a flashover at a small fraction of the clean gas-gap breakdown voltage. The particles initiate spacer flashover at low voltage values, not only for AC and DC voltages, but also for impulse and oscillating impulse voltages. Therefore, it is reasonable when commissioning a SF/sub 6/ gas-insulated system to carry out tests with a voltage wave form for which the particle-contaminated insulation is more sensitive and/or to use diagnostic measurements in order to detect the presence of particles.