Reducing the usage of SF

6

is important to a green power grid. Recent research on SF

6

replacement focuses on gas behavior at room temperature and there has been little work on the use of replacement gas in extremely cold regions. CF

4

which has a low liquefaction temperature

is strongly electronegative. The insulation characteristics of CF

4

and its N

2

mixture in slightly non-uniform electric field has been studied under the stress of positive and negative li

ghtning impulses. Results show that the breakdown voltage of CF

4

/N

2

mixture increases with the mixing ratio but reaches saturation fairly quickly. The mixture with 20% of CF

4

has very good insulation strength

low GWP value and low liquefaction temperature. It is potentially a good candidate for use in extremely cold environment. Moreover

the synergistic effect of binary gas mixture is analyzed and results shown that pressure has positive contribution towards the synergistic effect. The synergistic effect of CF

4

/N

2

mixture with negative lightning impulse stress is stronger than that with positive lightning impulse stress

with the synergistic coefficient being less than 0.4.