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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]

The Gas Insulated Switchgear (GIS) equipment observes surface charge accumulation under DC or AC voltage in real operation, which could distort the electric field and incur flashover eventually. With a 3D high-resolution surface charge measurement system established on actual 220 kV GIS basin insulator, the present research captured the surface charge distribution characteristics of the insulator under DC and AC voltage through high-precision probe scanning on the insulator surface. The test results show that: 1) Surface charge demonstrates obvious polarity effect under DC voltage. Under positive DC voltage, there accumulated mainly positive surface charges. Under negative DC voltage, the polarity of accumulated surface charges is all negative. Regardless of charge polarities, the surface charge density increases as the voltage amplitudes mounts and the voltage application duration prolongs. 2) Surface charge accumulation also takes place under AC voltage application. All charges are negative and distribute in a uniformed manner but in smaller density comparing to the DC scenario. As the voltage amplitudes increase, the surface charge density first increases, then decreases and finally reaches a near saturation state. As voltage application goes on, the surface charge accumulation becomes more obvious. Above all, the insulation issues in relation to surface charge accumulation require due attention in the application and maintenance of GIS equipment, and the present research attempts to provide useful reference for the optimal design of GIS/GIL insulators. [ABSTRACT FROM PUBLISHER]

Recent studies have found that the surface flashover strength of oil-impregnated insulation paper can be improved by the modification of nanoparticles. To reveal the working mechanism, this paper presents an experimental study on the effects of TiO2 nanoparticles on streamer propagation process at the surface of oil-based nanofluid-impregnated insulation paper under lightning impulse voltage. The results show that the surface flashover voltage of nanofluid-impregnated insulation paper is significantly increased by 34.6%. Streamer characteristics including shape and length at the surface of oil-impregnated insulation paper were investigated using the schlieren technique. It is indicated that streamer characteristics are dramatically changed by the addition of TiO2 nanoparticles. For nanofluid-impregnated insulation paper, streamers with more but much shorter branches tend to develop into adjacent nanofluid rather than clinging to the paper compared to that for pure oil-impregnated paper. The reason responsible for the shape dissimilarity lies in the change of electrical force due to the permittivity difference between oil and paper. Furthermore, more shallow traps introduced by nanoparticles contribute to the faster surface charge dissipation of nanofluid-impregnated paper, reducing the charge accumulation at the streamer head and suppressing the electric field distortion. Thus, it is hard for streamers to develop at the surface of nanofluid-impregnated paper, resulting in an increase of surface flashover strength. [ABSTRACT FROM AUTHOR]

This paper presents experimental research on creeping discharge characteristics of TiO2 nanofluid-impregnated pressboards (NPs) under sustained ac voltage. It was found that NPs possess higher resistance to creeping discharge than that of pure oil-impregnated pressboard. The results of permittivity and surface charge accumulation and decay characteristics reveal that the modification of nanoparticles can not only make the electric field more uniformly distributed at the interface of nanofluid/pressboard due to a better matching in permittivity but also significantly increase the surface charge decay rate via a trapping and detrapping process in shallower traps. Thus, the creeping discharge characteristics of NPs are greatly improved by the modification of TiO2 nanoparticles. [ABSTRACT FROM AUTHOR]