Double impulse tests of long airgaps. Part 3: Voltage front perturbation effects

Withstand voltage ratings for air insulation are partly based on test data obtained with standard biexponential impulses. The importance of the time parameters of the test impulse, with the distinction between lightning (LI) and switching (SI) standard impulses, is well established. The paper (Part 3) describes tests on a 6 m rod-plane gap with stresses which depart substantially from the biexponential voltage-front. Results are presented of the effect on leader growth, propagating under a switching impulse of critical shape, of a superimposed voltage perturbation of either LI or SI shape. For either type of perturbation, the time at which it is applied is important. The LI perturbation could arrest a young leader or enhance the propagation of a more mature leader. The electrical and spatial response of the discharges to such fast voltage perturbations is described. For SI perturbations, the change in the voltage derivative dU/dt, caused by the perturbation, controls the rate of change of the leader growth, and will provide a useful parameter in the modelling of nonstandard impulse breakdown. For such modelling, the results also justify the useful simplification that the linear charge density of 50 ¿C/m of a developing leader is unchanged by either LI or SI perturbations.