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Epoxy post-insulator is one of the key parts in SF6 gas-insulated DC wall bushing, which is irreplaceable in high voltage direct current transmission projects. Flashovers occur on post-insulators frequently, where a great number of tiny metal particles exist. The micron metal particles attached to the epoxy will change the insulation state of the surface. However, this relation between microstructure of material and macroscopic electrical properties on flashover would still arouse controversy. In order to study the effect of particles on the flashover characteristics, the particles generated from wear of spring in DC wall bushing were selected, the surface potential decay along surface and the DC flashover voltage of epoxy attached with particles were measured. The results show that the discrete particles could increase surface trap level by 0.025 eV under the effect of Van der Waals force. Furthermore, the deeper traps could capture the charge during the streamer development and inhibit the flashover, when the particle amount less than 500 per mm2. If the particles are more enough to form the continuous paths, these conductive paths could promote the streamer to propagate, which shortens the insulation distance, increases the electric field, and decreases the flashover voltage by 50% finally. [ABSTRACT FROM AUTHOR]

Epoxy post-insulator is one of the key parts in SF6 gas-insulated DC wall bushing, which is irreplaceable in high voltage direct current transmission projects. Flashovers occur on post-insulators frequently, where a great number of tiny metal particles exist. The micron metal particles attached to the epoxy will change the insulation state of the surface. However, this relation between microstructure of material and macroscopic electrical properties on flashover would still arouse controversy. In order to study the effect of particles on the flashover characteristics, the particles generated from wear of spring in DC wall bushing were selected, the surface potential decay along surface and the DC flashover voltage of epoxy attached with particles were measured. The results show that the discrete particles could increase surface trap level by 0.025 eV under the effect of Van der Waals force. Furthermore, the deeper traps could capture the charge during the streamer development and inhibit the flashover, when the particle amount less than 500 per mm2. If the particles are more enough to form the continuous paths, these conductive paths could promote the streamer to propagate, which shortens the insulation distance, increases the electric field, and decreases the flashover voltage by 50% finally. [ABSTRACT FROM AUTHOR]

The micron metal particles are inevitably produced in long-term operation condition of high voltage direct current SF6 insulated apparatus, which could frequently cause the surface discharge. Degradation of long-term surface discharge will be a severe threat to the insulation strength. Aiming at this problem, the surface charge accumulation and flashover voltage of epoxy itself before and after discharge degradation by particles were compared in this paper. The composition after degradation was measured by EDS and FTIR. The microstructure was observed by SEM. The roughness was measured as well. The trap distribution was measured by SPD. The mechanism of degradation and its effect on insulation characteristics were analyzed. The results show that the surface charge accumulation increase three times after degradation. Additionally, flashover voltage decreases 19% after degradation. The smoothly surface after degradation decrease the trap energy level for 14%, which might be the change reason of surface charge accumulation and flashover voltage. This paper can offer as a reference for the effect of micron metal particles on insulation degradation in SF6 gas insulated equipment. [ABSTRACT FROM AUTHOR]

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]

Highlights • All the measurement were conducted on an actual well-built GIS/GIL platform. • Quantitative influence of surface charge on the flashover voltage are put forward. • Impacting mechanism of surface charges on flashover voltage is proposed. • The conclusion could provide guidance for the insulation design and fault diagnosis. Abstract Under AC, DC and switching impulse voltages, the surface charges accumulated on the basin insulators of Gas Insulated Substation (GIS) or Gas Insulated Metal Enclosed Transmission Line (GIL) could distort local electric field and even reduce the surface flashover voltage. In order to quantitatively measure the influence of surface charge accumulation on flashover voltage of insulator in actual GIS/GIL apparatus, a high-precision 3D surface charge measurement platform and a reasonable model were established for flashover tests in reference to the insulation design and flashover properties of actual 220 kV GIS basin insulator. The surface charge accumulation dynamics and corresponding flashover voltages of the built models under different conditions were captured and analyzed. The results indicate that, as the density of accumulated surface charges increases under either AC, DC or switching impulse voltage application, the surface flashover voltage would decrease from 62.1 kV to 55.7 kV, namely a margin of 10.3% under AC voltage application. In the context of DC voltage, there appears a 22.8% flashover voltage drop, from 61.0 kV to 47.1 kV. While for the case of switching impulse voltage, the charge accumulation reduced the flashover voltage to 52.9 kV with a 13.6% decline from 61.2 kV. Based on the simulation and theoretical calculations, the 'enhanced electric field zone' was identified to justify the influencing mechanism of surface charges on flashover voltage. The research results could contribute to optimal insulation design and fault diagnosis of basin insulator in GIS/GIL. [ABSTRACT FROM AUTHOR]

• A high precision interface charge measuring platform under AC/DC voltages are set up. • The interface charge accumulating mechanism of oil-pressboard are presented. • The impacting mechanism interface charge on flashover features are proposed. • The conclusions could provide guidance for insulation design of oil-pressboard. The interface charges of oil-pressboard insulation could distort the local electric field and even weaken the insulating strength of converter transformer. This paper employs the electrostatic capacitive probe to capture the accumulation characteristics of interface charges in oil-pressboard insulation with needle-plate electrode under AC, DC and combined AC/DC voltages. The research results indicate that, under AC voltage, the polarity of interface charge is the same to that of the instantaneous value of applied voltage, and density of interface charge remains constant with the AC voltage application prolonging. In the context of DC voltage, the density of negative interface charge is about 1.2–1.5 times of that of the positive one. Under AC/DC combined voltage, the time-dependent dynamic accumulating process of interface charge behaves similarly to the waveform of AC voltage, and the larger the amplitude of DC component in combined voltage, the weaker the fluctuation. The tests for the surface flashover of oil-pressboard insulation were also conducted, aiming at discovering the impacting mechanism of interface charge accumulation on flashover voltage. The surface flashover voltage of test model under negative DC superimposed AC voltage is 1.3 times higher than that of positive DC combined with AC voltage. The built interface polarization model made an explanation for the interface charge accumulation, and the impacting mechanism of interface charge on flashover voltage, namely the positive effect of homopolar interface charges, is also proposed. [ABSTRACT FROM AUTHOR]