On the discharge phenomena in the 10%SF6-90%N2 gas mixture under fast oscillating impulse conditions

The discharge phenomena were investigated in a background uniform field (plate - needle to plate arrangement) varying the needle protrusion (length=3 to 5 mm; tip radius= 200 μm), the gas pressure (300 and 500 kPa) as well as the gas mixture composition (100%SF₆, 10%SF₆-90%N₂, 100%N₂). A positive polarity, low damped, fast oscillating (1.2 MHz), high voltage impulse (up to 450 kV peak) was applied to stress the test gap. The V-t characteristics in the mixture (breakdown voltage vs. time to breakdown) is reported and then compared to both the same curve of pure gases and similar measures available in literature. The discharge mechanism in the mixture is described and compared to the pure gases through voltage (capacitive sensor), discharge current (Rogowski coil), emitted light (photo-multiplier tube) and pictures from an image converter operating in both streak and frame modes.