In recent years

perfluoroisobutyronitrile (C

4

F

7

N)

one of the most promising alternative gases to SF

6

gas

has been widely focused and studied due to its superior insulating performance and stable physical and chemical properties. However

due to its high liquefaction temperature

C

4

F

7

N must be used in conjunction with a buffer gas with a lower liquefaction temperature. Air is chosen as the buffer gas in this paper. Under the slightly and extremely uneven electric fields

the influence of the gas pressure

mixing ratio and electrode gap on the breakdown voltage of C

4

F

7

N-air gas mixture is studied through t

he power frequency breakdown test. It is found by the result that the power frequency breakdown voltages of the C

4

F

7

N-air gas mixture under two electric fields increases approximately linearly with the increase of the electrode gap. Under the extremely uneven electric field

there is an obvious ‘hump’ effect. Under the same condition

the power frequency breakdown voltage of C

4

F

7

N-air gas mixture with mixing ratio of 8%-16% can reach 67% to 86% that of SF

6

. Under the slightly uneven electric field

the insulation strength of the gas mixture with mixing ratio of 8% C

4

F

7

N and 92% air is approximately 0.8 times that of SF

6

under the same condition. The C

4

F

7

N-air gas mixture with mixing ratio of 12%-16% is comparable to SF

6

gas in power frequency insulation performance.