Study on Effect of Metallic Particle Contamination on the Break Down Voltage in Air Insulated System

The development of compressed gas insulated switchgear (GIS) and gas insulated transmission line (GITL) have progressed rapidly worldwide because of the excellent insulating and arc quenching properties of Sulphur Hexa Fluoride (SF6) gas. However, free conducting particles lower the corona onset and breakdown voltage of these systems. Under the action of the applied electric field in a coaxial geometry, conducting particles acquire charge and lift off from the outer enclosure when the electrostatic force from the electric field becomes equal or larger than the force due to gravity. Metallic particles move randomly in gas insulated systems under the action of electric field. The particle may remain in mid-gap or stay around the central conductor for several voltage cycles. Particle movement therefore plays a crucial role in determining the voltages withstand capability of GIS/GITL systems.Metallic conductors in gas insulated system are protected with a dielectric covering to mitigate the problem of particle initiated breakdown. Dielectric covering reduces the effect of surface roughness of conductors/duct and also increases the dielectric strength of the insulation system. Several researchers have developed computational models for particle movement in co-axial electrode system. These models however make assumptions about the particle charging process and charge exchange mechanism when a moving particle returns to the dielectric coated enclosure. Many experimental techniques have been proposed by researchers to explain the movement of particles in a co-axial bus-duct system. The experimental observations are also compared with the computational model. However, particle contamination in GIS is not fully understood and explained satisfactorily. Though SF6 is accepted universally as the best gaseous dielectric, it is considered to be a Greenhouse Gas. Hence research worldwide focuses on alternatives to SF6 gas without making much compromise on dielectric and other properties. In this context, use of SF6-N2 gas mixtures, containing less than 10% of SF6 gas is seen as a viable alternative for GITL. Hence the present study was taken up with this motivation