| Breakdown with Stable Streamer and Leader Coronas |
- Consider a very long gap distance, above 2 m, between positive rod or sphere and plane electrode configuration. On applying preferably a slow changing voltage, that is, ac or si of sufficient large magnitude, at first a very strong and dense filamentary streamer corona appears at the high voltage electrode. This is known as 'first corona' shown in the schematic of breakdown (Fig. 13.7). Unlike in case of shorter gap lengths, in this case the minimum collective current density in discharge channel required to produce a 'leader' (about 200 rnA) may be achieved even without the cathode effect. The first corona goes through constriction phenomenon and is then followed by stable stem and leader discharges in the gap. The stepped form of leader is accompanied with streamer coronas at their tips. The leader discharge propagates, traversing the path laid down by streamer corona, as it provides the leader the required current density.
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- The stable leader corona prevails in the gap until it extends to the opposite electrode, as illustrated in Fig. 13.7. Breakdown of air between electrodes is accomplished with the 'final jump' of a leader channel bridging the gap, followed by an arc. Photographs of stable leader and breakdown with its final jump on a 7m gap length between rod-plane, applying positive polarity si voltage were taken by Lemke [2.18], shown.in Fig. 13.7.
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Fig. 13.7 Breakdown mechanism schematic with stable leader and streamer coronas |
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Fig 13.8 Stable leader corona and breakdown with final jump for a 7 m gap with positive polarity si voltage, Lemke [ 2.18]. |
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