The Discharge Mode of SF6N2 Gas Mixture in Extremely Inhomogeneous Electric Field under Power Frequency Voltage

Abstract

SF$_{6}/\text{N}_{2}$ gas mixture has been used as insulated medium in power equipment, but the breakdown characteristics and the discharge mode of SF$_{6}/\text{N}_{2}$ gas mixture in extremely inhomogeneous electric field under power frequency voltage are not clear, which will threaten the stable operation of power equipment. The experiment was based on the fully enclosed power frequency experimental device, which includes inflatable power frequency transformer, experimental chamber, control and measurement system. The synchronous camera was used to capture the pre-discharge images of SF$_{6}/\text{N}_{2}$ gas mixture in needle-plate electrode. The results show that the relationship between the breakdown voltage of the SF$_{6}/\text{N}_{2}$ gas mixture and the pressure also has the ‘hump’ phenomenon in extremely inhomogeneous electric field. In the hump area, the leader stepping was observed obviously in the discharge images, and the leader stepping is closely related to the low probability breakdown of the gap, that is, the streamer channel is transformed into leader channel, then the secondary streamer is generated at the head of the leader channel, and the leader channel is continuously stepping to the plate electrode. With the increase of voltage, there is a uniform discharge area at the head of the needle electrode, which prevents the development of the leader channel and leads the increase of breakdown voltage. The leader channel has a tendency to bypass the uniform discharge area and develop from both sides of the needle electrode to the plate electrode, causing breakdown finally.