Effect of micro-particles on the dielectric properties and breakdown characteristics of insulating oil

The presence of carbon particles poses a significant threat to the insulating performance of transformer insulating oil. Specifically, the roles of the micro-sized carbon particles (below 5 μm) on the dielectric properties and AC breakdown performance of insulating oil are neglected by IEC 60422, which have been seldom studied in previous research. This study investigates carbon particles with diameters of 1 μm, 3–5 μm, 8 μm, and 20 μm dispersed in mineral oil at concentrations up to 0.024 wt.%. Results show that electrical properties are significantly influenced by particle concentration and size. Notably, at 0.024 wt.%, the breakdown voltage drops from 39.4 kV (pure oil) to 26.5 kV for 1 μm particles and 18.6 kV for 20 μm particles. Additionally, the dielectric constant and dissipation factor increase slightly with particle loading. Numerical simulations reveal that strong local electric field distortion occurs at the particle-oil interface due to permittivity mismatch. This distortion rate increases with particle size and concentration, consistent with experimental findings. This work aims to clarify the neglected influence of sub-5 μm carbon particles on the dielectric properties and AC breakdown performance of mineral insulating oil, providing critical guidance for insulation system design, condition monitoring, and contamination standards in high-voltage power equipment.