Double impulse tests of long airgaps. Part 4: Effects of pre-existing space charge on positive discharge development

The paper describes one of the three major studies with double impulses performed during the fifth test period of the Les Renardiÿres Group. In the third experiment a negative impulse (Tcr=45 ¿s) was used to create an extensive space charge in a 6 m rod-rod gap. The voltage level selected was ~0.8U50¿. Thereafter the effect of this pre-existing space charge on the development of a subsequent positive discharge was investigated. The positive discharge was initiated by applying a positive impulse (240/2500 ¿s) to the previously unenergised electrode at the associated U50+ level. The spatial and temporal characteristics of the pre-existing space charge were varied over a wide range, first, by utilising wavetail values of either 180 or 9000 ¿s, and, secondly, by selecting the time delay ¿t to the application of the positive impulse such that ¿t lay in the range 0¿¿t¿ 300 ms. The evolution of the pre-existing space charge is seen to be strongly dependent on the time-to-half value T1/2 of the negative impulse, and, for T1/2=180 ¿s, the phenomenon of reverse discharges is observed in the region of the gap adjacent to the negative electrode. Development of the positive discharge is observed to be associated with a leader propagation phase which clearly interacts with the pre-existing space charge, such that the characteristic properties of the leaders (velocity, current) remain essentially constant throughout the test range. The breakdown data (U50+) indicate that large reductions in withstand (~50%) are achieved with the short ¿t/T1/2 values, and that recovery of the gap to an effective space-charge-free condition requires ¿t¿100T1/2.