Particle-field-charge coupling mechanism and its impact on surface flashover of tri-post insulators in C4F7N/CO2 DC GIL

To clarify the coupling effect between metal particle contamination and surface charge evolution in the environmentally friendly direct current gas-solid composite insulation system, this paper conducted a systematic study on the dynamic adsorption of metal particles, surface charge accumulation, and surface flashover characteristics of full-scale three-pole insulators in a 0.1 MPa 10%C 4 F 7 N/90%CO 2 mixed gas. The results show that the direct current electric field dominates the regional dynamic adsorption behavior of the particles. As the voltage increases from 30 kV to 80 kV, the particle adsorption center evolves from the top of the pole to the middle, and the formed dense adsorption band becomes the key physical boundary for insulation deterioration. The local discharge excited by the tip effect of the adhering particles disrupted the carrier balance at the interface. Pronounced non-uniform bipolar charge accumulation was observed on the insulator surface, indicating that particle attachment significantly alters the interfacial charge distribution under the investigated C 4 F 7 N/CO 2 gas-mixture conditions. The charge peak increased by more than 50% compared to the clean state, and the accumulation range expanded by approximately 30%. This severe charge distortion induced a strong secondary electric field distortion, resulting in a significant time-dependent attenuation of the surface flashover voltage with respect to the DC pre-applied voltage. Among them, the middle part of the pillar was the region with the most intense charge-field distortion, and its flashover voltage decreased by as much as 21.4%, making it the core weak point of this insulating system. This study experimentally demonstrates a strong correlation among particle adsorption, surface charge evolution, and flashover-performance degradation under the investigated C 4 F 7 N/CO 2 gas-mixture conditions. The research results can provide key theoretical support for the insulation structure optimization design and long-term operation reliability assessment of environmental protection type direct current GIL.