Discharge characteristics of SF6 gas with and without insulators under VFTO and lightning impulse voltage

A generating system of very fast transient overvoltage (VFTO) and lightning impulse (LI) was established. The effects of voltage polarity, gas pressure and voltage waveform on discharge characteristics of SF
6
gas with and without insulators under VFTO and LI were investigated. Experimental results show that the 50% breakdown voltage of rod-plane gap under VFTO is higher than that under LI in positive polarity. However, for negative polarity, the breakdown voltage and voltage-time curve under VFTO fall below those under LI at the pressure of 0.5MPa. Moreover, the polarity effect of the rod-plane gap under LI is reversed at a certain pressure. The flashover voltage of defective insulator under VFTO can be lower than that under LI by near 10%. The flashover voltage is the lowest when the particle is attached on some position nearby high voltage electrode of insulator. The decrease of pulse rise time will increase the discharge channel number. It is concluded that the polarity effect is related with the distortion effect of ion clusters formed by SF
6
on electric field. The steepness and rise time of impulse play an important role in the initiation and further development of leader discharge in SF
6
and on insulator surface.