Partial Discharge Characteristics of Epoxy Insulation Surface Under AC/LI Superimposed Voltages in SF₆/N₂ Gas Mixtures

In this article, the partial discharge (PD) excitation characteristics of SF6/N2 gas mixtures on the epoxy insulation surface under alternating current and lightning impulse (ac/LI) superimposed voltages are investigated. The phenomenon of PD excitation in SF6/N2 gas with 20% SF6 content is explained in detail. The effects of SF6/N2 gas mixtures ratio and the polarity on the PD characteristics under subsequent ac voltage are analyzed. It found that under the influence of ac/LI superimposed voltages, there are significant differences in the PD initiation characteristics between SF6/N2 gas with SF6 content less than 60% and pure SF6. In SF6/N2 gases, the ac superimposed phase that can initiate subsequent discharge is larger, and when superimposed voltages are applied in the positive (or negative) half cycle of ac to initiate discharge, in pure SF6, the discharge only occurs in the subsequent ac’s negative (or positive) half cycle. However, in SF6/N2 gas mixtures with SF6 content less than 60%, the discharge can occur in both the positive and negative half cycles of ac. In addition, the PD phenomenon after ac/PLI superimposed voltages is more prominent than ac/NLI superimposed voltages. The severity of PD under subsequent ac increases with the decrease of ac voltage duration experienced from the superposition moment to the first zero crossing point of the ac voltage.