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

摘要

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.