Abstract
Replacing SF6-gas in 550 kV high-voltage direct current gas-insulated switchgear (HVDC GIS) with synthetic air requires changes to the GIS design. Design criteria with high precision are required for adequate dimensioning of the DC insulation system. There is little published knowledge on the high-pressure DC breakdown behaviour of synthetic air under different electrode roughness conditions. This work aims to characterize the breakdown behaviour of a smooth and a rough electrode in a sphere-plane geometry in synthetic air at filling pressures relevant for HVDC GIS. This work reports on experiments and simulations of inception and breakdown voltage in a ±DC-stressed 2 cm long sphere-plane gap with synthetic air under gas pressures of 1 bar to 13 bar (absolute). To calculate the expected inception voltage, the Raether-Meek criterion is used. We show where the modelling and design criteria matches experiments, and where further understanding is necessary. The effect of electrode roughness is almost negligible at low filling pressures but becomes significant at high pressures as the size of critical electron avalanches reduces in line with gas density scaling laws. This work documents DC breakdown behaviour of high-pressure synthetic air with experiments. We propose a methodology for evaluating the DC breakdown strength, including the effect of roughness. The presented results are useful for obtaining adequate design limits for HVDC GIS.