Discharge characteristics of long SF6 gas gap with and without insulator in GIS under VFTO and LI

Gas insulator switchgears (GISs), widely used in electric power systems for decades, have many advantages due to their compactness, minimal environmental impact, and long maintenance cycles. However, very fast transient overvoltage (VFTO) increases caused by a rise in voltage levels can lead to GIS insulation failures. In this paper, a generating system of VFTO and standard lightning impulse (LI) is established. The insulation characteristics of SF₆ gas with and without insulators under VFTO and standard LI are investigated. Experimental results show that the 50% breakdown voltages of the inhomogeneous electric field rod-plane gap under positive VFTO and standard LI are higher than that under negative VFTO and standard LI. The research shows that the 50% breakdown voltage under VFTO could be lower than that under standard LI at 0.5 MPa for the negative polarity. Moreover, the polarity effect of the insulator without defect is different from that with defect. Similarly, the breakdown voltage of the defective insulator under VFTO could be lower than that under standard LI by 8%. The flashover channel under VFTO is seen as more than that under standard LI. Based on the analysis of discharge images and experimental results, it is concluded that the polarity effect is related to the distortion effect of ion clusters formed by SF₆ on the electric field. Additionally, the steepness and front time of impulse plays an important role in the initiation and further development of discharge on insulator surface. Finally, the research shows that different discharge characteristics between VFTO and standard LI may be caused by different wave fronts and oscillation on the tails of the impulses.