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The flashover characteristics at the gas–solid interface are crucial for secure and reliable application of insulating gases and must be carefully evaluated when selecting environmentally friendly insulating gas. This study investigates the flashover characteristics of the epoxy resin surface in various gas mixtures, including C4F7N/N2, C4F7N/CO2, and C4F7N/air, with a focus on the analysis of the generation and impact of solid products using macroscopic and microscopic methods. The results show that when N2 is used as the buffer gas, the deep decomposition of C4F7N generates carbon-based solid particles on the surface of the epoxy resin, forming conductive channel after multiple flashovers and therefore causing surface insulation failure. In case of C4F7N/CO2 and C4F7N/air, the oxygen element can react with the carbon-containing solid products, inhibiting the formation of solid products. Moreover, under high gas pressure ( $\ge 0.3$ MPa in this article) and high C4F7N content, the surface of the epoxy resin sample in the C4F7N/air mixture gas environment can form erosion points containing fluorine-containing solid particles, leading to a lower flashover voltage for C4F7N/air than C4F7N/CO2. These findings provide a better understanding of the flashover characteristics of epoxy resin surfaces in different gas mixtures and contribute to the development of effective insulation medium for gas-insulated equipment.

Corrosion and electrical erosion of metals within transformers lead to the presence of metal ions in the oil. Unlike metal particles, these soluble ions diffuse readily in the oil and are challenging to filter out, posing a significant threat to the transformer’s reliable operation. This article employs experimental evaluations to elucidate the effects of copper ions on the dielectric properties, breakdown performance, and partial discharge characteristics of the insulating oil, coupled with a theoretical analysis. Dielectric spectroscopy test reveals that soluble copper ions alter the dielectric properties of the oil, specifically causing a reduction in its relative permittivity due to ion-induced endogenous electric fields (about 60% decline in the low frequencies). Additionally, the ions result in increased dielectric losses at low frequencies and notably enhanced conductivity at high frequencies, potentially due to electron tunneling and thermally activated polarization. AC breakdown tests indicate that the breakdown voltage of the oil decreases as copper ion concentration increases. The relatively lower ionization potential and charge saturation effects are identified as contributing factors to the decreased breakdown voltage. Partial discharge experiments confirm that copper ions lead to reduced inception voltages (with a maximum decline of 17.5% in the test), and the frequency of discharges between needle–plate gaps in the insulating oil increases with ion concentration. The findings offer critical insights into the influence of metal ions on the electrical properties and discharge behavior of insulating oils.

Transient arc ablation caused by load current during switching is the primary cause of internal contact damage in the on-load tap changers (OLTCs). However, the mechanism of arc ablation on contact in an oil gap, a basis for effective failure assessment, is rarely involved by far. Herein, the arc ablation mechanism was experimentally investigated based on breakdown performance, transient pulse current, and changes to microscopic surface morphology. Silver cathode caused an apparent reduction in the average breakdown voltage, whose data reliability was verified by the Weibull distribution compared to the copper cathode. With the increased applied voltage, the discharge duration decreases, while the discharge pulse’s steepness increases significantly. The materials’ properties of the cathode, including electrical conductivity, melting point, and stability of metal oxides, have a critical impact on arc ablation. As a result, the comparison of the surface morphology in the scanning electron microscopy (SEM) image of the two cathodes shows that the silver electrode has better surface discharge uniformity and remains relatively smooth after arc ablation, whereas the copper oxides produced by arc ablation are relatively stable and irregularly attached to the electrode surface, forming pits and bumps. The transfer process materials during the arc process are also discussed, leading to variations in the quality of contact materials and metal contamination of insulating oil. The results in this article contribute to a better understanding of the damage mechanism of arc ablation on contact and its influencing factors.

Partial discharges (PDs) can erode insulating paper, altering its surface morphology. This morphological transformation, subsequently, influences the evolution of further PDs. This study constructs a numeric streamer model, integrating the hydrodynamic drift-diffusion model in oil, the bipolar charge transport model in the insulating paper, and the Ohmic model for interface charge transfer. The model serves to explore how changes in surface morphology affect the discharge process. Simulation results demonstrate that morphological alterations on the profiled surface accelerate PD development and augment surface charge accumulation on the insulating paper. The generalized concave crater’s charge accumulation effect results in an intensity increase at the bottom of the electric field and a decrease at the top of the profiled surface. The positioning of concave and convex features on the surface primarily impacts the amplitude of local electric fields, which decreases as the distance from the arc center increases. The distortion rate of the electric field in a convex or concave surface is a time-independent characteristic, solely reliant on surface morphology. Given identical surface morphology, a greater value of surface roughness suggests a higher capability for electric field distortion. The results in this article contribute to a better understanding of surface morphology’s effect on the streamer propagation process.

Eco-friendly HFO-1336mzz(E) is promising as an insulating gas in medium-voltage equipment. However, the surface flashover characteristics of epoxy resin in HFO-1336mzz(E) /buffer-gas mixtures, significantly influencing the insulation performance and operational safety, have gained little academic attention by far. This article has studied the effects of typical buffer gas (N2 and CO2), pressure, and mixing ratio on the surface flashover characteristics under negative dc voltage. The results show that more solid by-products have been generated after several flashovers in the HFO-1336mzz(E) /N2 gas mixtures, leading to a lower tolerance than HFO-1336mzz(E) /CO2 gas mixtures. The surface flashover voltage of HFO-1336mzz(E) /CO2 gas mixtures increases with gas pressure or mixing ratio, and there is a positive synergistic effect between HFO-1336mzz(E) and CO2. The available concentration of HFO-1336mzz(E) gas in the mixtures depends on the limiting liquefaction temperature and gas pressure. When the liquefaction temperature is limited to −15 °C, the HFO-1336mzz(E)/CO2 gas mixtures with 4%–6% HFO-1336mzz(E) at 0.40 MPa and 6%–10% HFO-1336mzz(E) at 0.30 MPa can be used to replace pure SF6 at 0.13 MPa in medium-voltage equipment. This article can provide an essential reference for its engineering application.