Influence of SF6 admixtures to nitrogen on critical parameters of surface streamer inception

Summary form only given. Sulfur hexafluoride (SF₆) is commonly used as a gaseous dielectric in high-voltage equipment due to its excellent insulating properties and chemical stability. However it significantly contributes to the greenhouse effect, and can under certain circumstances (after a discharge event) be poisonous for persons. Therefore, SF₆ - nitrogen mixtures are considered, in order to decrease the negative effects of the gas, while still keeping the required dielectric properties.The goal of the present work was to investigate prebreakdown parameters of discharges originating on the surface of a dielectric in the mixtures of SF₆ with nitrogen. All experiments were performed in a specially designed vessel that allowed setting up and maintaining the parameters of gaseous environment (SF₆/N₂ ratio, gas pressure)¹. Streamers were propagating along the surface of an epoxyresin dielectric, prepared in a shape, typical for various HV technologies¹. The voltage of discharge inception was determined for different amounts of SF₆ admixture. It was shown, that the inception voltage increases rapidly starting at very small admixtures of SF₆ (~5%), and continues to increase at a lower rate with the further increase of SF₆ percentage. The voltage of streamer inception also increases with the increase of gas pressure (in these experiments the concentration of SF₆ was kept constant). The velocity of discharge propagation was estimated from the analysis of time-resolved discharge current measurements. The rise-time of the current increased significantly with the addition of SF₆, moreover, for high concentrations of the above the majority of the discharges were originating as leaders, skipping the streamer phase. The fall time of the current behaves in a similar way, thus the total duration of the discharge decreases almost by a factor of 10 (- rom tens to several nanoseconds for pure nitrogen and 5% SF₆ admixture respectively). Overall, it was shown, that major inception parameters (critical voltage, rise and fall times, current shapes) of streamer discharges propagating on a gas-dielectric interface change considerably starting already with a small (5%) admixture of sulfur hexafluoride to nitrogen.