The breakdown voltage and discharge extension of long gaps in nitrogen-SF6 and air-SF6 gas mixtures

Abstract

The breakdown voltage and the extension of the discharge has been investigated for impulse voltages with rod-plane gaps of 30 and 50 cm in nitrogen-SF6 and air-SF6 mixtures at atmospheric pressure. The addition of 0.4–2% SF6 by volume to nitrogen reduced the 50% breakdown voltage V50 for both polarity impulse voltages below the value for pure nitrogen or pure SF6, typically 50% below the pure SF6 level. However, in air-SF6 mixtures the opposite effect was found, and a small addition of SF6 (e.g., 1%) increased V50 by typically 80% above that for pure SF6. The extension of the breakdown path has been studied with an image converter and still cameras. In nitrogen-SF6 mixtures there was a rapid extension of the discharge corresponding to the condition of the minimum breakdown voltage with the 2% SF6 content. The typical discharge extension characteristic in both types of mixtures was an intermittent propagation of the leader accompanied by ball-like regions of corona streamers at the leader tip. For a 50% SF6–nitrogen mixture, typical pause times between the steps were 1 μsec for negative polarity and 0.08 μsec for positive polarity, and the discharge extension to breakdown was two to three times faster for positive impulses.