AC and impulse breakdown characteristics of dry-air with regard to different moisture content

Dry-air has been regarded as an attractive insulating medium which could substitute SF₆ gas in high voltage electric equipment. Thus, some manufactures announced that they succeeded in developing medium-voltage switchgear only insulated by dry-air. And their final target is to develop extra high voltage level dry-air insulated switchgear. In order to enhance insulation performance of dry-air, it was generally accepted that the control of moisture content in dry-air is critical factor to determine the performance of dry-air But, there are only a few fundamental research works regarding the insulation characteristics affected by the ratio of moisture content in dry-air. In this paper, we´d like to determine the relation between moisture content in dry-air and breakdown characteristics of dry-air. Humidity of dry-air has been successfully controlled by means of water injection in the test chamber. And then AC withstand test and lightning impulse breakdown test were performed using sphere to plane electrode air-gap configurations. From the test results, it was revealed that the AC and impulse breakdown voltage of dry-air was not seriously affected by different humidity condition ranging from 300 PPM to 1,500 PPM humidity in dry-air.