Influence of a Small Percentage of O2 in SF6 Gas Mixtures on the Insulation Properties

The breakdown characteristics of SF
6
/CO
2
/O
2
and SF
6
/N
2
/O
2
ternary gas mixtures were investigated with AC test voltages under both slightly uniform and non-uniform electric fields. In this study, the slightly uniform field represented by a sphere-plane electrode, while a rod-plane electrode configuration provided the non-uniform field. The gap lengths between the electrode varied from 5 mm to 25 mm, and the gas pressure was fixed at 1.0 bar (abs). The tests were measured at various mixing ratios considering a small percentage of O
2
in each gas mixture. The effects of additional O
2
in SF
6
/CO
2
and SF
6
/N
2
on the breakdown voltages were discussed to obtain a reasonable mixing ratio of the ternary gas while maintaining the insulation properties of the gas. The addition of 10% O
2
in the SF
6
/CO
2
mixture showed a significant increase in the breakdown voltage for all gap distances under both electrode configurations. Meanwhile, only 5% of O
2
is required to increase the breakdown voltage in the SF
6
/N
2
mixture under the rod-plane electrode, and 10% of O
2
is required to be added for the sphere-plane configuration. Apart from the experimental work, this paper also discusses a simulation study on the maximum electric field for each breakdown test. It is found that the rod-plane electrode provides a higher electric field compared to the sphere-plane electrode configuration.