Research On The Influence Of Impurities On Dielectric Properties Of Vegetable Insulating Oil

Vegetable insulating oils are fire-resistant and environmental-friendly liquid dielcetrics, which offer excellent electrical and physiochemical properties and have been successfully applied in distribution transformers. Since there are many sources of impurities in insulating oil, such as the impurities generated inevitably in the process of production, transportation and operation of transformers. These impurities directly affect the dielectric properties of transformer oil, and threaten the safe and reliable operation of transformers. Currently, most research focuses on the influence of impurities on dielectric properties of mineral insulating oils. Vegetable insulating oils differ from mineral insulating oils in their molecular structures, components, physicochemical and electrical properties. These differences lead to that the influence of impurities on dielectric properties of vegetable insulating oils is quite different from that of mineral insulating oils. Therefore, the author investigated the influence of impurities on the dielectric properties of vegetable insulating oils. The main contents are shown as follows:(1)Experimental investigation was executed on the influence of impurities on AC breakdown voltage of vegetable oil. The relationships between AC breakdown voltage and impurity number as well as impurity size were obtained. The results show that the AC breakdown voltage of insulating oils decreases with the increasing impurity number, and it has an approximate linearity relationship with the denary logarithm of impurity number. With the same number of impurity in insulating oils, the AC breakdown voltage of insulating oils reduces with the increase of impurity size and the AC breakdown voltage of vegetable oil is greater than that of mineral oil. Since the higher viscosity of vegetable oil, the influence of impurities on AC breakdown voltage of vegetable oil is smaller than that of mineral oil.(2) Experimental research was conducted on the influence of impurities on frequency domain spectroscopy characteristics of vegetable oil. The variation of relative permittivity, dielectric loss and volume resistivity of vegetable oil contaminated with impurities with frequency were obtained. The results show that the fiber and copper particles increase the relative permittivity and dielectric loss of insulating oils, while reduce the volume resistivity of insulating oils, and it is more obvious at low frequencies.(3)Experimental investigation was executed on the influence of temperature and copper volumetric concentration on dielectric properties of vegetable oil. The results show that the relative permittivity of insulating oils contaminated with copper particles increases linearly with copper particle volumetric concentration and decreases linearly with temperature. Based on the insulating oil molecule polarization theory and take the influence of copper particle polarization into account. The relative permittivity polarization model of insulating oils contaminated with copper particles was proposed. Analysis suggests that the relative permittivity of contaminated insulating oils is dominantly determined by insulating oils, and the higher relative permittivity of contaminated insulating oils compared with that of pure oil samples is largely caused by copper particle inner polarization.(4)The results show that the electrical conductivity and dissipation factor of insulating oils contaminated with copper particles increase linearly with copper particle volumetric concentration and increase exponentially with temperature. A modified electrical conductivity model based on the Maxwell conductivity model and electrophoresis phenomenon of copper particles is introduced. Meanwhile, a modified dissipation factor model is proposed following the modified electrical conductivity model and polarization loss of copper particles. Analysis suggests that since the higher electrical conductivity and viscosity of vegetable oil, the dissipation factor of vegetable oil is influenced by its electrical conductivity changed by copper particles, while the dissipation factor of mineral oil is changed by polarization loss generated from copper particle steering motion.