Unraveling Weak Interaction of Kinematic Viscosity of Fatty Acid Methyl Esters in Natural Ester Insulating Oils

Natural ester insulating oil has become a good substitute for mineral insulating oil due to its characteristics of renewability, easy degradation, low carbon emission, and high fire safety. However, the high kinematic viscosity of natural ester insulating oil not only affects the heat dissipation and heat transfer of the transformers but also seriously restricts its safe promotion and application in transformers in cold regions. In this article, in order to explore the influence mechanism of the kinematic viscosity of natural fatty acid methyl ester (FAME), insulating oil molecules from the microscopic scale of weak intermolecular interaction, the electrostatic potential (ESP), van der Waals (vdW) potential, and average local ionization energy (ALIE) of four FAME molecules are analyzed, respectively. The computed results demonstrate that the C=C double bonds in the four FAME molecules weaken the intermolecular electrostatic interaction and vdW interaction, and the weakening effect increases with the increase of the number of the C=C double bonds. This may be for this reason that the C=C double bond reduces the kinematic viscosity. This study can provide theoretical guidance and a basis for quantitative analysis and comparison for the improvement of the kinematic viscosity of natural ester insulating oil, lubricating oil, and biodiesel.