Corona and breakdown-voltage characteristics in sphere¿plane and rod-rod gaps under impulse voltages of various wavefront durations

Studies were made of the electrical characteristics of the corona and the leader stroke preceding the impulse breakdown of sphere¿plane and rod¿rod gaps up to 40cm long, in atmospheric air. The studies were made with both positive- and negative-polarity voltages of front durations between 2 and 300 ¿s. The effect of wavefront duration on the breakdown voltage was also studied. The breakdown voltage of the sphere¿plane gap with the sphere positive, and of the rod¿rod gap under both polarities, increased with increasing wavefront duration from 2 to 17 ¿s, and thereafter it decreased for wavefronts up to 300 ¿s. With the sphere negative, the breakdown voltage of the gaps studied remained approximately constant for all durations of the wavefronts. With the sphere positive, the breakdown of all gap lengths studied was preceded by the propagation of a positive leader. When the sphere was negative, however, a change in the development of the leader from a negative leader to a positive leader was observed at a certain gap length. The leader development remained independent of the wavefront duration of the voltage wave. The breakdown of the rod¿rod gap with the high-voltage rod positive was initiated by a positive leader for all gap lengths and voltage wavefronts. With the high-voltage rod negative, two different mechanisms of the leader development were found under the 7 ¿s and 17 ¿s-wavefront voltages. In gaps between 10 and 20cm long, a positive leader was observed, whereas in the longer gaps the breakdown was initiated by a negative leader. Under the 2¿s-wavefront voltage, the breakdown in all gaps was preceded by a positive leader.