Study of metallic particle induced partial discharge activity in 10∶90 SF6-N2 gas mixtures

The use of gas mixtures as replacement to SF₆ gas is getting wider acceptance because of environmental issues. Hence, serious efforts are being made to reduce the use of SF₆ gas in high voltage application, wherever it is possible. In this direction, use of SF₆-N₂ gas mixtures has been attempted in GITL (gas insulated transmission line) and few other cases. Invariably, in these cases, a small percentage of SF₆ gas is used in combination with nitrogen gas and depending upon the requirements of insulation, either the pressure of the gas mixture has to be increased or the percentage of SF₆ gas has to be enhanced to meet the specified target of electric strength. However, irrespective of this, the gas mixture will be subjected to the problems that are encountered in case of pure SF₆ gas. One such serious problem is related to the presence of conducting particles, which may be either fixed or moving in the gas duct. Since the particle size and location can vary, some of the routine dielectric tests may not be sensitive to detect very small particles. Further, detecting a particle in N₂ environment may be more difficult than in SF₆ gas because of the lower sensitivity of the gas to particles and the higher operating stresses that exist in SF₆ insulated systems. Hence detection of metallic particles and their location is very important from the stand point of safe operation of an insulation system consisting of SF₆-N₂ gas mixture.