Humidity dependent conductivity of air in HVDC applications and the role of surfaces

Abstract: High Voltage Direct Current (HVDC) bushings are insulated devices designed to facilitate the safe passage of electric current across an earthed barrier. Understanding air conductivity is of utmost importance in the design of HVDC components, which is determined by ion density and ion mobility. Previous research has indicated a correlation between air conductivity, air humidity and phenomena on HVDC surfaces. This thesis aims to investigate and elucidate how adsorbed water may contribute to air conductivity. To accomplish this objective, an experimental study is conducted employing ellipsometry to measure the water layer thickness as a function of relative humidity and applied voltage. The results exhibit a thickness consistent with prior studies in the absence of applied voltage, but reveal an altered thickness when voltage is applied. While definitive conclusions cannot be drawn from this thesis alone, the findings provide some support for the hypothesis from earlier research, suggesting the involvement of surface phenomena in air conductivity. The experimental procedure presented in this thesis can be further expanded and refined to gain a more comprehensive understanding of the surface reactions at play. This could further develop the understanding of the factors influencing air conductivity in HVDC systems and contribute to the advancement of design methodologies in this field.