Humidity effects on dielectric strength of air-gaps for indoor HV installations

For indoor HV installations, the electrode configurations of air-gaps are different from outdoor HV apparatus and the fields are usually optimized. The humidity can be lower and the temperature is often higher than the outdoor atmosphere. In order to study the effect of humidity on the breakdown voltage of the indoor air-gaps, laboratory tests were performed on the typical indoor air gaps-toroid/plane as well as reference gaps-rod/plane and sphere/plane under typical indoor atmosphere conditions. The test results indicate that for the reference air-gaps, the correction procedure recommended by IEC 60060-1 is valid, while there is an over-correction of 6-9% for the toroid/plane air gaps, which is caused by humidity correction.

Introduction

Changes of atomospheric humidity have been shown to affect sparkover voltages in gaps with non-uniform field like in many practical insulation configurations. The switching-impulse breakdown voltage of such an air-gap usually decreases with the decrease of atmospheric humidity. The phenomenon behind is complicated and is strongly related to the discharge development characteristics, which are determined by the gap structures and the voltage type. A large number of contributions on these subjects exist in literature and have been well covered in the Cigr reviews [1]–[2].