Impulse breakdown of large sphere-plane gaps

Experiments are reported on the breakdown level of sphere-plane and rod-plane gaps (from 0.75 to 3 m) for positive- and negative-impulse waveforms with risetimes of 1¿2 ¿s and 80¿130 ¿S. Sphere radii of 125¿375 mm and a 12.5 mm square-cut rod were used. The low breakdown strength for slow positive impulses is due to the high peak field at the anode and is associated with impulse-corona starting early on the voltage rise. The gap strength can be greatly increased and the `U-curve¿ phenomenon avoided on reducing the peak field to below 4.1 MV/m. Breakdown for spheres with radii greater than 10% of the gap follows a simple scaling law: Emean=3.75¿0.81 EpeakMV/m