Impulse Breakdown and Corona Characteristics for Rod-Plane Gaps in Mixtures of SF6 and Nitrogen with Less Than 1% of SF6 Content

Measurements of 50% impulse breakdown voltage were made mainly in SF6-N2 mixtures containing 0.1% SF6 content by pressure, over the range of 1-5 bar and gap lengths of 10 to 50mm. The gaps were tested under both negative and positive polarities with impulse voltages of 1.8/50 μsec and 310/3500 μsec. Data on the spatial growth of ionization was obtained in the form of still photographs of spark discharges and prebreakdown coronas. These results show that the positive impulse breakdown of nitrogen in the pressure region of 1-2.5 bar increases considerably with addition of small traces of SF6. The increase in the breakdown strength of mixtures with less than 1% SF6 is shown to be associated with a movement of the initiation point of the discharge, away from the tip of the rod electrode. This movement is referred to here as "creep effect" of the spark trajectories and its variation is examined with pressure, gap length and impulse front duration. On the basis of the results of this work and absorption characteristics of SF6, N2 and SF6-N2 mixtures, the anomalous behavior of SF6-N2 mixtures with SF6 content of the order of 0.1% is interpreted as the result of an anode shielding mechanism produced by a positive space charge, which is affected by factors such as polarity and waveform of the applied voltaae, gas pressure and mixture ratio.