The impulse breakdown voltage and time-lag characteristics of long gaps in air I. The positive discharge

Abstract

The electrical breakdown of rod/rod, rod/sphere and rod/plane gaps in the atmosphere has been examined oscillographically and photographically. Positive polarity impulse potentials of crest value up to 1 MV, of wave-front variable between 0.06 and 2.0 μs and of wave tail 2 ms were used. It has been found that the lack of a sharply defined breakdown potential was due to the existence of long time lags quite distinct from the shorter times to breakdown observed with the conventional short wave-tail impulse. A ‘ dead-time ’ of low probability of breakdown on the wave tail separated the two classes of breakdown. The breakdown voltage of a rod/rod gap has been found to be dependent upon the wave front of the impulse. An accompanying photographic examination of the initial corona phase of breakdown also revealed a variation with the impulse voltage wave front. It is shown that these results were consistent with the electric field distortion arising from space charge. The corona phase of breakdown was responsible for this space charge. The statistical behaviour of long gap breakdown was due to random variations in the corona phase. The effect of the statistical time lag in the production of initiatory electrons upon the corona phase is discussed. A rotating-mirror camera of f/1.0 aperture and a technique for controlled suppression of the breakdown enabled the growth of the discharge with time to be studied in some detail. It was shown that the positive leader either preceded or was coincident with the negative leader, depending upon the gap arrangement. It is concluded that the establishment of the leader at the high-tension electrode is the criterion for breakdown. The role of the earthed cathode in aiding this leader development was dependent upon its size and geometry. For cathodes of small dimensions the occurrence of a negative corona phase increased the anode electric gradient; for large cathodes the surface charge induced by the anode corona discharge became important. The variation of breakdown strength with gap geometry is accountable in these terms.