Streamer Inception Probability in Air - How the Polarity Dependence is Affected by Changes in the Shape of the Applied Voltage Impulse

In this research, the influence of the rise time of the applied voltage pulse on the polarity dependence for streamer inception probability in air is studied. This was done for a needle-plane electrode configuration at 1 bar absolute pressure, where the streamers emerged from a grounded steel needle placed 48 mm below a planar steel disk. The electrode setup was stressed by two different lightning impulses (LI): A standard LI with a rise time of approximately $1.1 \mu \mathrm{s}$ and a fast rise time LI with a rise time of approximately 160 ns. The streamer inception probabilities showed a significant polarity dependence, with negative streamers requiring more than twice the applied voltage to incept for both LI-variations. This is assumed to be due to either differences in inception region or source to first electron, or a combination of both. The rise time of the applied lightning impulse was found not to affect this polarity dependence of streamer inception probability in air. There was, however, a slight difference in streamer inception probability between the two LIs, with standard LI increasing the streamer inception probability. This was only the case when the amplitude of the fast rise time LI was measured as the peak of the voltage overshoot on the LI. When the overshoot was excluded, both LIs resulted in the same streamer inception probability for positive streamers. There was still a difference for negative streamers, but now the standard LI was found to decrease the streamer inception probability.