Double impulse tests of long airgaps. Part 2: Leader decay and reactivation

The behaviour of large air-insulated structures subjected to complex multiple-peak surge overvoltages is influenced by the presence of already established leader overvoltages in influenced by the presence of already established leader development in the space. The decay processes of a positive leader channel following current interruption appear not to have been studied in detail, and Part 2 provides electrical and optical measurements of leader decay and reactivation in a 6 m rod-plane gap. It is found that, for a positive leader created by a switching impulse (SI) of critical time to crest, the leader path could be rapidly reactivated over at least part of its length by a second voltage application at times of up to about 1 ms after its original formation. For longer times, new leader growth may still retrace the original basic leader path. The temporal and spatial characteristics of the reactivated leader are substantially different from those of the basic leader. The causes of the large statistical variation in behaviour are discussed. The results presented should provide the basis for improved physical knowledge of the leader and corona processes, and contribute to improved engineering models for insulation co-ordination.