Quantitative Evaluation of Lightning Surge Range Voltage-time Characteristics in High Pressure SF₆ Gas

Since gas-insulated switchgear (GIS) is subjected to very fast transient overvoltages such as lightning surges or disconnector switching surges. Therefore the sparkover voltage and time (V-t) characteristics of SF6 in a very short time range of less than 1μs are of great interest from the viewpoint of insulation design and coordination for a GIS. This paper describes the V-t characteristics of SF6 at a gas pressure of 0.5MPa using a steep-front square impulse voltage under a quasi-uniform field gap and presents a quantitative evaluation of the V-t characteristics for a nonstandard lightning impulse voltage. In the case of a square impulse, the V-t characteristics of positive polarity were observed to be almost flat over a long time range from 80ns to 10μs, and rose steeply over a short time range from 80ns down to 20ns. For negative polarity, the V-t characteristics exhibit a gentle rise from 200ns down to 40ns. In the estimation of V-t characteristics, the equal-area criterion parameters were quantitatively estimated using the square impulse. For a nonstandard lightning impulse, we found that the application of equal area criterion with these parameters for the non-oscillating impulse and oscillating impulse of up to 5.3MHz as a model of lightning surge and disconnector switching surge is possible.