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As trapping sites for surface charges, the surface trap is an important factor affecting the insulation performance of the epoxy–SF6 interface, which is closely related to the microstructure and its evolution. However, existing studies often ignore the effect of the degradation of the epoxy surface on the surface traps and charge accumulation during the long-term service of the insulation system. In this study, the surface physicochemical evolution and its influence on surface traps of a typical epoxy–SF6 insulation structure were researched under a DC-temperature gradient condition, and an evolution mechanism of surface traps was proposed. The results show that the free radicals introduced during the material degradation process increase the density and depth of both positive and negative charge traps on the epoxy surface. The effect of free radicals formed by the cleavage of epoxy molecules on the electronic orbital and molecular electrostatic potential of epoxy molecules is the intrinsic reason for the evolution of surface trap evolution. This work may help to understand the long-term evolution mechanisms of the physicochemical characteristics of the epoxy–SF6 interface in DC field and may provide a theoretical basis for the long-term failure mechanism revealing and the insulation performance enhancement. [ABSTRACT FROM AUTHOR]

The charge accumulation phenomenon at the gas–solid interface under the co-action of DC electric field and temperature gradient is of great significance, but its long-time evolution characteristics are still unclear. In this work, the accumulation and dissipation characteristics of the surface charge on the epoxy-SF6 interface within 216 h were obtained, indicating that, (a) the normal surface electric field is increased by the temperature gradient along the gas–solid interface, which results in a prominent increase in the surface charge density; (b) there are different evolution processes of surface charge in high and low temperature regions under the co-action of DC-temperature gradient, and the increase of the charge trap density caused by the degradation of solid material is considered to be an important reason for the charge evolution; (c) the total charge dissipation ratio within 600 s decreases with the increase of charging time in DC electric field, and the surface charge dissipates mainly to the gas side of the interface; (d) the large amount of charge accumulated at the interface distorts the surface electric field at the triple junction points. The total electric field strength at the low-temperature tri-junction point increases by 35.5%, while the direction of the tangential field at the high-temperature tri-junction point reverses during the long-time co-action of the DC-temperature gradient. The results of this work may be helpful to understand the long-time charge evolution characteristics of the gas–solid interface under the DC-temperature gradient and to reveal the failure mechanism of the gas–solid interface. [ABSTRACT FROM AUTHOR]

The charge accumulation phenomenon at the gas–solid interface under the co-action of DC electric field and temperature gradient is of great significance, but its long-time evolution characteristics are still unclear. In this work, the accumulation and dissipation characteristics of the surface charge on the epoxy-SF6 interface within 216 h were obtained, indicating that, (a) the normal surface electric field is increased by the temperature gradient along the gas–solid interface, which results in a prominent increase in the surface charge density; (b) there are different evolution processes of surface charge in high and low temperature regions under the co-action of DC-temperature gradient, and the increase of the charge trap density caused by the degradation of solid material is considered to be an important reason for the charge evolution; (c) the total charge dissipation ratio within 600 s decreases with the increase of charging time in DC electric field, and the surface charge dissipates mainly to the gas side of the interface; (d) the large amount of charge accumulated at the interface distorts the surface electric field at the triple junction points. The total electric field strength at the low-temperature tri-junction point increases by 35.5%, while the direction of the tangential field at the high-temperature tri-junction point reverses during the long-time co-action of the DC-temperature gradient. The results of this work may be helpful to understand the long-time charge evolution characteristics of the gas–solid interface under the DC-temperature gradient and to reveal the failure mechanism of the gas–solid interface. [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]

Surface charges have significant impact on the evolution of surface dielectric barrier discharge (SDBD). In this work, the role of residual surface charges on repetitively nanosecond pulsed SDBD in atmospheric air is investigated using a two-dimensional fluid model, based on the assumption of preserving the distribution of surface charges at the end of the previous high voltage (HV) pulse. In the bipolar mode when the polarity of residual surface charges is opposite to that of the current HV pulse, a lower breakdown voltage and more deposited energy can be observed, showing an obvious enhancement of SDBD. In the unipolar mode, residual surface charges suppress the development of discharges and energy deposition. It is found that more residual surface charges are accumulated during the negative pulsed discharge, which have a more pronounced effect on the subsequent positive pulsed one. This is explained by the fact that the negative surface streamers directly contact the dielectric and charge it, while the positive surface streamers float above the dielectric, forming a ion-rich region near the surface. The results in this work demonstrate the mechanism of how residual surface charges affect discharge dynamics, which can be utilized to regulate energy deposition in SDBDs. [ABSTRACT FROM AUTHOR]