In recent years

C

3

F

7

CN has been widely concerned by researchers in the field of SF

6

alternative gas as an environmental-friendly insulation medium.However

there are few systematic studies on the power frequency breakdown performance of C

3

F

7

CN/CO

2

gas mixtures at present. In this paper

the power frequency breakdown and self-resto

ring characteristics of C

3

F

7

CN/CO

2

gas mixtures were tested under the ball electrodes which were used to simulate quasi-uniform electric field. The influences of mixing ratios and gas pressures on the breakdown voltage and the synergistic effect between C

3

F

7

CN and CO

2

were systematically analyzed and compared with that of SF

6

under the same conditions. The study shows that the power frequency breakdown voltage of the gas mixtures with C

3

F

7

CN content of 2% to 8% increases linearly with the increase of gas pressure

and presents a saturation growth trend with the increase of the mixing ratio.When the content of C

3

F

7

CN is 2% or 8%

the power frequency breakdown voltage of C

3

F

7

CN/CO

2

gas mixure can reach 50% and 75% of that of pure SF

6

under the same conditions

respectively. C

3

F

7

CN has a good restoring insulation performance and has a certain synergistic effect when mixed with CO

2

. The research results confirm that C

3

F

7

CN/CO

2

gas mixtures have a potential to replace SF

6

.