SF₆/N₂ gas mixture has the advantages of being used at low temperature and reducing the amount of SF₆ gas, which has been applied in practice. Under some conditions, the breakdown voltage of SF₆/N₂ mixture may be higher than that of pure SF₆, which is called positive synergistic effect. In order to explore the conditions of positive synergistic effect of SF₆/N₂ mixture, power frequency discharge research is carried out, and the mechanism of positive synergistic effect is revealed in combination with shorter time scale impulse discharge.
The needle plate electrode was used in the experiment, the electrode distance was 10 mm, and the pressure of SF₆ gas and SF₆/N₂ mixed gas was in the range of 0.1~0.4 MPa. Under the power frequency voltage, the change of breakdown voltage of SF₆ gas and SF₆/N₂ mixture with pressure is obtained. Then the pressure and proportion of SF₆ was found under which the breakdown voltage of SF₆/N₂ mixture is higher than SF₆ gas. Under this pressure and proportion, lightning impulse experiments were carried out to explore the variation law of positive synergy effect under different voltage time scales. Finally, the positive synergy of SF₆/N₂ mixture is revealed from the perspective of the influence of space charge on electric field.
It was found that the power frequency breakdown voltage of SF₆/N₂ mixture is higher than that of pure SF₆ gas when SF₆ accounts for 30% at 0.2 MPa. While the peak of hump curve of SF₆/N₂ mixture was also 0.2 MPa. It shows that space charge is the key factor to improve insulation performance of SF₆/N₂ mixture at 0.2 MPa.
When using SF₆/N₂ gas mixture in practice, if the absolute pressure is 0.2 MPa, such as in medium and low voltage ring cabinet, its insulation performance will be better than that of pure SF₆ gas when protrusion occurs, because space charge will significantly improve its insulation performance.
 

SF6,SF6/N2,synergistic effect,Gas discharge,Gas insulation