Effect of metal particles on the electrical properties of mineral and natural ester oils

Metal particles, such as copper and iron, not only reduce the breakdown voltage but also accelerate the oxidation of insulating oils. The effects of metal particles on the dielectric properties of mineral and natural ester insulating oils were investigated in this work. The experimental results indicate that the AC and impulse breakdown voltages of the insulating oil decrease with the increase of the size and the quantity of the metal particles. The positive breakdown voltages V_b+ of insulating oils decrease moderately in the beginning, while the particles quantity increases to a certain extent, the V_b+ decreases significantly. In comparison, the metal particles always reduce the negative impulse breakdown voltage V_b- significantly. The effect of the copper particles on the insulating oils is slightly greater than that of the iron particles due to their higher conductivity. The permittivity of contaminated insulating oils increases quasi-linearly with the concentration of copper particles. Improved models considering the polarization of the surface oxidation layer of copper particles were proposed, and the calculated data were in good agreement with the measured permittivity of the contaminated insulating oil.