Evaluation of insulation strength for lightning surge that actually enters into substations is important to estimate insulation reliability of gas-insulated equipment. The standard lightning impulse voltage (1.2/50μs) is used for factory tests. However, the actual lightning surge waveforms in substations are complex and are usually superimposed with various oscillations. Insulation characteristics of SF₆ gas against such complex voltages have not been sufficiently clarified.
This paper deals with gap breakdown characteristics in SF₆ gas under sub-microsecond pulses. Breakdown voltages are lower under a polarity reversal case than under a mono polarity case. The cause of this difference is discussed with a observing discharge propagation using an image converter camera. Moreover the electrode size effect is also discussed.