Effect of humidity and electrode roughness on the AC and impulse breakdown characteristics of dry-air

Dry-air is recognized as an attractive insulating medium for substituting SF₆ gas in high voltage devices. Recently, Japanese manufacture announced that 72.5 kV has been successfully developed using dry-air as a main insulating medium. However, only a few fundamental research works of dry-air have been reported compared to those of SF₆. In this paper, two kinds of experiments were conducted to clarify the breakdown characteristics of dry-air. First, the effect of humidity on the breakdown characteristics of dry-air was deeply investigated. And secondly, the effect of electrode roughness on the breakdown voltage of dry-air has been investigated. In order to make artificial specific humidity of dry-air, the experiment chamber has been controlled in vacuum condition under 0.2 Torr and some water was injected before filling the dry-air. Dry-air used for the experiments was composed of about 21% Oxygen and 79% nitrogen and its humidity was controlled of -85°C dewpoint temperature. Water could be successfully vaporized in dry-air, and then AC withstand test and lightning impulse breakdown test were performed using sphere to plane electrode airgap configurations. Next, similar breakdown tests were performed considering polished condition, and artificially made surface roughness with 16 μm and 25 μm on the high voltage electrode respectively. From the test results, it was shown that impulse breakdown voltages were slightly decreased with increasing electrode roughness. However, AC withstand voltages did not show any remarkable difference regarding surface roughness, and it was revealed that the breakdown voltage of dry-air was not affected by different humidity condition ranging from 300 PPM to 1,500 PPM humidity in dry-air. AC withstand voltage and negative impulse voltage in this humidity range did not show noticeable difference.