Influence of nonmetallic particles on the breakdown strength of vegetable insulating oil

The insulating performance of transformer oil is vulnerable to minute particles. As the vegetable insulating oil is increasingly used in transformers while its physical and chemical properties are different from those of mineral oil, special attention should be paid to the influence of particles on its dielectric strength. This paper investigated the influence of nonmetallic particles, namely carbon particles and cellulose particles on the breakdown strength of vegetable insulating oil. The breakdown voltages of vegetable insulating oil and mineral insulating oil contaminated with carbon particles or cellulose particles of different sizes and particle numbers were measured, respectively. The relationships between the breakdown voltages and particle numbers were obtained. The experiment results showed that the breakdown strengths of the vegetable insulating oil decrease with the increase of the particle number, and the breakdown voltages and the denary logarithm of particle numbers approximately satisfy the linear correlation, which is the same for mineral insulating oil. More importantly, the vegetable insulating oil is less sensitive to the particles compared to mineral insulating oil in terms of breakdown voltage, which might be due to its higher viscosity. Therefore, different standard of particle limit should be established for vegetable insulating oil.