Effects of electrode chemical reactions on SF6 discharge characteristics in extremely inhomogeneous electric fields

Abstract

SF6 is widely used in the gas-insulated metal-enclosed switchgear or the corona-stabilized gas spark switch applications as a gas dielectric. It is generally believed that the discharge characteristics are only related to the electric field distribution and gas molecular density; however, the electrode chemical reactions can indeed markedly affect the SF6 discharge characteristics in the extremely inhomogeneous electric fields under the steady-state voltage. In this study, we used a needle-plane electrode system to build an extremely inhomogeneous electric field and examined the discharge characteristics within it including U-p characteristics, corona appearance, and corona current. We also analyzed the micro-region characteristics of the electrode surface, including the surface morphology, elemental composition, and chemical state to fully qualitatively determine the role of the electrode chemical reactions in discharge behavior. We found that the N-shaped U-p curve widens, the filamentous leader channels disappear, and the corona current drops suddenly as the duration of the electrode chemical reactions increases. Varying the surface morphology, elemental composition, and chemical state was observed on the electrodes of different polarities through micro-region analysis. The metal fluoride or metal sulfide film on the electrode surface may serve as a resistive coating due to its low electrical conductivity, which obstructs the leader discharge while enhancing the streamer discharge, suppressing the transition from the streamer to leader discharge and altogether significantly altering the discharge characteristics. The resistive coating produced is the primary cause of the electrode chemical reaction effects on the discharge characteristics, primarily as it depresses the transition from the streamer to leader discharge. The results presented here may provide useful guidelines for further research on SF6 discharge under inhomogeneous electric fields.