Effect of humidity and gas density on switching impulse breakdown of short air gaps

The dual purpose of the present work was first to examine the effect of humidity and gas density on the breakdown characteristics of short airgaps and secondly to investigate whether charge injection data could provide additional information concerning the probability of breakdown at low probability levels. Impulse tests were, therefore, performed in a sealed test vessel using laboratory air for pressures in the range 0.7-1.4 bar and absolute humidity levels from ~2 to 15 g/ m3. Both pressure and humidity could be closely controlled and measured. The impulse shape was +100/2500 fis and the test gap consisted of a 5 cm-diameter sphere at 20 cm above an earthed plane. Statistical flashover tests and measurements of apparent charge injection at the sphere were made and both sets of data were analysed using the method of maximum likelihood. The breakdown probabilities were found to satisfy a normal distribution and the charge measurements provided significant additional information at low probability levels. The measured values of V50 were found to depend linearly upon absolute humidity with a slope of 1.5 kV/g/m3 at all pressures. The withstand voltages, on the other hand, were only weakly dependent upon absolute humidity. The humidity correction factors found in the present work agree well with the IEC standard at 1.2 and 1.4 bar, but depart appreciably at lower pressures.