Dark current characteristics and their correlation with surface charge accumulation on insulators in clean-air HVDC GIS
Surface charge accumulation on insulators is a critical concern for high-voltage direct current gas-insulated switchgear (HVDC GIS), as it distorts the electric field at the gas–solid interface and further initiates partial discharge or surface flashover. As an environmentally friendly alternative insulating medium, clean air has drawn extensive interest in engineering applications, yet the characteristics of gas-side dark current and its inherent contribution to surface charge accumulation remain unclear. In this work, dark current in clean air is investigated through combined experimental measurements and numerical simulations. A coaxial measurement system is used to examine the influences of electric field strength, gas pressure, electrode surface roughness and electrode material, whereby the dominant dark-current mechanisms under different operating conditions are identified. By correlating dark-current characteristics with surface charge observations on practical GIS insulators, an inherent correlation is established between dark-current mechanisms and two typical surface charge patterns: uniform background charge and localized charge speckles. The results show that, within the tested pressure ranges of 0.3–0.8 MPa for dark-current measurements and 0.2–0.6 MPa for surface-charge comparison, cosmic radiation-induced ionization dominates dark current generation at low electric fields (< ∼3 kV mm−1), whereas micro-discharges become predominant at higher electric fields (> ∼3 kV mm−1) under the present coaxial electrode configuration; field emission contributes only marginally within the investigated field range. Moreover, reducing electrode surface roughness or applying electrode surface coatings effectively suppresses dark current and mitigates charge speckles, providing a practical strategy for surface charge regulation. Among the compared insulating gases, clean air exhibits dark-current and surface-charge characteristics closest to those of SF6, indicating great application potential for HVDC GIS.