Generally

when the up-and-down method is used to research the discharge characteristics of SF

6

under impulse voltage

dispersion of breakdown voltage is required to be less than 3%

which guarantees the reliability of data. But during the process of experiment

the dispersion of breakdown voltage of SF

6

comparing with other gas

is usually significant and even larger than 5%. Therefor

we discuss the changing law of SF

6

breakdown voltage dispersion with electrode size

gas pressure and gap length under positive and negative standard lighting impulse（LI）. The results show that when gap length d=33 mm and radius of

electrode r=40

15

8 mm

the voltage dispersion is lower as the pressure grows. The change trend of voltage dispersion

as the field nonuniformity factor grows under positive impulse

is alike "U" curve trend

which is growing at first and then declining. And the change trend of voltage dispersion is alike inverse "U" curve trend under negative pulse

which is declining at first and then declining; when the radius of the electrode r=8 mm and the gap length d=7

15

33 mm

the voltage dispersion declines as the pressure grows. The coefficient of dispersion grows linearly with gap length under both positive and negative LI.