Double impulse tests of long airgaps. Part 1: Engineering problems and physical processes: the basis of recent tests

Collaborative tests have been performed using nonstandard impulses, and employing a wide range of diagnostic techniques. Part 1 of the paper describes the need for such tests, the experimental arrangement and the digital data techniques developed for the tests. Part 2 is an account of the decay of the leader channel in long airgaps and the recovery of dielectric strength. It was found that the leader conductivity had a lifetime of the order of 1 ms. In part 3 of the paper, the influence of a nonstandard impulse front on the leader growth is examined in detail. The results show that overstressing of the gap can result in a reduced probability of break-down, and the cause of this effect is studied. In phase-to-phase structures, on the other hand, the case of a negative surge preceding a positive can be particularly dangerous: the effect of pre-existing negative space charges on positive leader growth is therefore described in part 4. Large-scale influences which cause more vigorous leader processes have been observed.