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

The breakdown characteristics of SF<inf>6</inf>/CO<inf>2</inf>/O<inf>2</inf> and SF<inf>6</inf>/N<inf>2</inf>/O<inf>2</inf> 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<inf>2</inf> in each gas mixture. The effects of additional O<inf>2</inf> in SF<inf>6</inf>/CO<inf>2</inf> and SF<inf>6</inf>/N<inf>2</inf> 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<inf>2</inf> in the SF<inf>6</inf>/CO<inf>2</inf> mixture showed a significant increase in the breakdown voltage for all gap distances under both electrode configurations. Meanwhile, only 5% of O<inf>2</inf> is required to increase the breakdown voltage in the SF<inf>6</inf>/N<inf>2</inf> mixture under the rod-plane electrode, and 10% of O<inf>2</inf> 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.