Breakdown characteristics of SF6 /CF4 mixtures in 25.8 kV

SF₆ gas has excellent dielectric strength, but it causes global warming about 23900 times more than CO₂ gas which is the main cause of global warming. This shows necessity of the environment friendly dielectric gas which can replace SF₆ gas. We made 25.8 kV GIS of unity bundle operation tank type. And breakdown characteristics are carried out experiment with using LI (lightning impulse) voltage and AC voltage. This paper shows breakdown characteristics as mixed rates and total pressure of SF₆ /CF₄ mixture under the AC voltage and LI voltage. The total pressure of the SF₆ / CF₄ mixtures was varied within the range of 0.1-0.5 Mpa in the test chamber and 0.10-0.22 Mpa in the 25.8 kV GIS. The percentage of SF₆ and CF₄ gas was mixed 0, 20, 40, 60, 80 and 100%. In the result, it was observed that dielectric strength is increased by increasing content of SF₆ gas and increasing pressure of SF₆ / CF₄ mixtures. In the 25.8 kV GIS, the mixed gas of SF₆ and CF₄ showed better insulation characteristics than pure CF₄ gas in LI voltage and AC voltage test.