Surface charging characteristics of GIL model spacers under DC stress in C₄F₇N/CO₂ gas mixture

The C₄F₇N/CO₂ gas 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/CO₂ mixture. 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 SF₆ and the dielectric strength of 15%∼20% C₄F₇N/CO₂ mixture is equivalent to that of SF₆. Next, the surface charge measurement of spacers was performed in C₄F₇N/CO₂ mixtures, which reveals that the surface charge density decreases slightly with the increase of C₄F₇N content. However, the surface charging level in 25% C₄F₇N/CO₂ mixture 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/CO₂ mixtures. It is the different PD behaviors between C₄F₇N/CO₂ mixtures and SF₆ that lead to their different surface charging characteristics.