Voltage-time Characteristics of Long SF_6 Gap Under VFTO and Lightning Impulse

In order to study the voltage-time(U-t) characteristics of SF6 long gap of ultra-high voltage(UHV) gas insulated switchgear(GIS) under very fast transient overvoltage(VFTO) and lightning impulse(LI),a generating system of VFTO and LI was designed.By changing the gap length,gas pressure,and voltage type,the U-t characteristics of rod-plane and sphere-plane SF6 long gaps under negative VFTO and LI were investigated at different gas pressures.Experimental results show that,although the minimum breakdown voltage of the sphere-plane gap under VFTO is higher than that under LI,the U-t curve of the sphere-plane gap under VFTO is flatter than that under LI,resulting in the intersection of U-t curves.However,for the rod-plane gap,the U-t curve under VFTO is below that under LI at the pressure of 0.5 MPa.Based on the analysis of experimental results and physical background of U-t curves,the semi-empirical formula is derived to predict the U-t curves of the given SF6 gap.This formula can be expressed by a power exponent function with three parameters,which are related with gas pressure,gap length,and field nonuniformity factor.It is demonstrated that the predicted U-t curves can reflect breakdown characteristics of SF6 gaps accurately,which is useful for insulation design of GIS.