Surface charging characteristics of GIL model spacers under DC stress in C 4 F 7 N/CO 2 gas mixture

This is the authors' abstract. We don't add key points for this paper.

The C4F7N/CO2gas mixture has attracted much attention due to its excellent insulation property and eco-friendly character. In order to evaluate the feasibility of its application in DC GIL, this paper focuses on the surface charging characteristics of spacers under DC stress in the C4F7N/CO2mixture. First, the DC insulation strength was tested under uniform field. The results show that the critical dielectric strength of pure C4F7N is more than twice that of SF6and the dielectric strength of 15%~20% C4F7N/CO2mixture is equivalent to that of SF6. Next, the surface charge measurement of spacers was performed in C4F7N/CO2mixtures, which reveals that the surface charge density decreases slightly with the increase of C4F7N content. However, the surface charging level in 25% C4F7N/CO2mixture is still much higher than that in SF6. It is found that most of the surface charge stems from partial discharges (PDs) near the flange. The PD measurement shows that in SF6, there are more PDs with low magnitude, but much less PDs with high magnitude compared to that in the 15%~25% C4F7N/CO2mixtures. It is the different PD behaviors between C4F7N/CO2mixtures and SF6that lead to their different surface charging characteristics.