Conductor Surface Roughness-dependent Gas Conduction Process for HVDC GIL-Part II: Experiment

In this paper, an electric field ranging from 0.44-5.24 kV/mm was applied to a gas-insulated line (GIL) model filled with SF6 at different values of gas pressure and relative humidity (RH). Dark current was measured using high voltage conductors with different surface roughness (Sz). The results show that the dark current reflects a multi-factor-dependent ionization process, which has a strong correlation with electric field strength, RH, and electrode surface roughness. At 45% RH, an Sz value of 14.152 mu m, and an electric field less than 2.2 kV/mm, the gas pressure was found to have a significant inhibitory effect on dark current. However, at an electric field greater than 2.5 kV/mm, the increase of RH has a significant positive correlation on dark current. From the results, it is verified that the values of conductor surface roughness requirements for AC GIL are too high to be used for DC GIL. We emphasize that the ionization leading to high dark current is due to local protrusions at different environmental conditions. For the DC GIL, the surface roughness of conductors should be minimized to achieve satisfactory performance.