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DC wall bushing is one of the key equipment in high-voltage direct current (HVDC) transmission project. At present, post insulators based on epoxy composites are widely used for supporting insulation in SF6-insulated apparatus. However, the flashover faults caused by surface charge accumulation can be a severe threat to the long-term operation of the post insulators under dc field. Therefore, the issue of charge accumulation on epoxy post insulators in HVDC wall bushing and its effect on flashover has been widely concerned. Aiming at this problem, the flashover characteristics of epoxy samples in SF6 gas under polarity reversal conditions with dc voltage prestressed for 10–600 min were measured in this article. The flashover process is analyzed through surface charge distribution measurement and electric field computation. The results show that with the increase of prestressed time, the accumulated negative charge density could reach $-14.8\,\,\mu \text{C}/\text{m}^{{2}}$ , which led to a 12% increase in maximum field strength. As a result, the polarity reversal flashover voltage could decrease by 12% and the flashover time delay could decrease by 84.6%. This article can offer as a reference for the effect of surface charge accumulation on flashover in SF6 gas-insulated equipment. [ABSTRACT FROM AUTHOR]

In this work, we present a detailed analysis on the observed streamer characteristics of PEEK (polyetheretherketone) surface in nitrogen at atmospheric pressure driven by positive repetitive pulse voltage, including the forward discharge streamer and the back discharge streamer. Fast oscilloscopic technique and digital photographic technique were employed to observe the streamers of surface discharges, and current pulse waveform analysis technique and image analysis technique were used to analyze the characteristics of the observed streamers. The experimental results show that the current pulse of forward discharge streamer has a shorter rise time, a higher current amplitude, a faster current rising rate, a shorter fall time, a faster current falling rate and a smaller charge quantity than those of back discharge streamer at the same voltage amplitude. Besides, the forward discharge streamer has a longer stopping length, a faster propagation velocity and a stronger maximum light intensity than back discharge streamer at the same voltage amplitude. Furthermore, the correlation between the electrical characteristics and optical characteristics of the streamer was analytically derivated, and the current rising rate of the streamer is proportionally correlated to the product of the streamer stopping length and the streamer velocity square. [ABSTRACT FROM AUTHOR]