On the influence of extreme low humidity in air discharge processes under positive switching impulses

Abstract

In the high voltage equipment design, the external air insulation strength depends of several parameters like type of voltage stress, electrode geometry and surroundings, and atmospheric air conditions like humidity, pressure and temperature. It has been long known that the air dielectric strength decreases by reducing its water content and is reduced at higher temperatures. Most of the high voltage equipment is designed and tested for standard reference atmospheric air, ie. 20 °C temperature and 101.3 kPa pressure and 11 g/m3 absolute humidity, but the final use of it is at locations with different conditions and some correction factors are need. International standard committees suggest procedures to perform these corrections based mainly on empirical approaches. However, the great amount of electrode configurations in air one can find in the high voltage design and the limited amount of information available of their breakdown processes and how they are influenced by the water content, demanded a better understanding. The present work aims to study the dependence of the electrical discharge in air gaps tested with positive switching impulses at its early stages, like corona inception and streamer to leader transition, while taking the air humidity to extreme low values. The effect of low air humidity on the time to breakdown was also analyzed.