Investigations of the occurrence of erratic low breakdown voltages in compressed air

Previous investigations of the influence of electrode surface irregularities on the breakdown voltage in compressed gases were continued in order to get a better understanding of the reasons which cause the erratic low breakdown voltages in pressurized gas insulations. This paper contains the results of breakdown voltage investigations of smooth or roughened electrodes in air with pressures up to 30 bar. Measurements were carried out with + d.c. as well as 50 Hz a.c. using uniform and coaxial fields with gap lengths from 2.0 to 5.0 mm. The a.c. breakdown voltage measurements displayed that the lowest spark voltages of polished or sandblasted electrodes are in the same order of magnitude. With- d.c. an irreversible prebreakdown current behaviour of sand-blasted electrodes was found in air at pressures higher than about 5 bar, respectively at field gradients greater than about 80 kV/cm. Scanning electron microscope pictures of a single electrode surface protrusion showed the removal of the top of the irregularity by irreversible prebreakdown current pulses at -d.c.. It is assumed that field electron emission accompanied by space charges and ionization causes a heating as well as a removal of sharp electrode irregularities and without doubt this heating process facilitates the occurence of erratic low breakdown voltages.