Lightning impulse breakdown characteristics of SF6 alternative gases for gas-insulated switchgear

Investigations of the lightning impulse breakdown characteristics of a SF₆/N₂ gas mixture, N₂ gas and CO₂ gas have been carried out. In the case of the SF₆/N₂ gas mixture the 50% breakdown (electric) field strength for the positive impulse voltage was slightly lower than that for the negative impulse voltage, and the pressure of the SF₆/N₂ gas mixture must be increased to approximately twice the pressure of SF₆ gas to obtain the same dielectric strength as that of SF₆ gas. For CO₂ gas, the 50% breakdown field strength for the negative impulse voltage was lower than that for the positive impulse voltage in the case of SF₆ gas. However, the dielectric strength of CO₂ gas, even at twice the pressure of SF₆ gas, was approximately 60% of the strength of SF₆ gas, so that the CO₂ gas-insulated busbar may be approximately 1.6 times as large as the SF₆ gas-insulated busbar. In the case of N₂ gas, the 50% breakdown field strength for the positive impulse voltage was lower than that for the negative impulse voltage, and the dielectric strength of N₂ gas, even at twice the pressure of SF₆ gas, was less than 50% of the strength of SF₆ gas, so that the N₂ gas-insulated busbar may be approximately 2.1 times as large as the SF₆ gas-insulated busbar. A remarkable difference between CO₂ and N₂ gas was found from the observation of discharge forms and partial discharge properties.