Influence of Natural Ester and Mineral Oil Blending on the Diffusion Behavior of Water Molecules and the Dielectric Properties

The insulating oil is an essential component of the internal insulation in oil-impregnated transformers, and its performance is crucial for the stable operation of the power grid. The basis for ensuring long-term, reliable operation of transformers is the improvement of dielectric properties and insulation properties of insulating oil. Currently, the insulating oil can be modified in three ways: adding nanoparticles, adding antioxidants, and blending vegetable oil with mineral oil. Some scholars have found that the insulating properties can be effectively improved by adding nanoparticles or antioxidants into the insulating oil, but it will introduce impurity particles into the insulating oil. The problem has been addressed by blending vegetable oil with mineral oil in order to improve the insulating properties of insulating oil. While numerous experimental studies have indicated that the method of blending vegetable oil with mineral oil improves the insulation and dielectric properties of insulating oils, the mechanism of the modification remains unclear. To understand the mechanism, Molecular Dynamics (MD) studies were conducted on the 18 groups of mixed oil models established in accordance with different ratios of vegetable oil to mineral oil (as 0:10, 1:9, 2:8, 3:7, 4:6 and 5:5) and with different water contents (1%, 2% and 3%) under the COMPASS force field. After geometric optimization, annealing optimization and dynamic optimization, the model was subjected to a Molecular Dynamics simulation with a duration of 500 ps and a sampling period of 1fs. The result show that with the increasing of vegetable oil content, the diffusion coefficient of water molecules in mixed oil showed a decreasing trend. In addition, with the increasing of water content, the diffusion coefficient of water molecules in mixed oil also showed a decreasing trend. Further studies were conducted to analyze the interaction energy and the average number of hydrogen bonds between water molecules in mixed oil systems. A decrease in the diffusion coefficient of water molecules can be attributed to the introduction of polar groups in vegetable oil molecules, which lead to an increase in interaction energy and the formation of hydrogen bonds. Furthermore, with the increase of vegetable oil content and water content, the static dielectric constant of different mixed oil systems increases gradually, and it’s very sensitive to the water content. The introduction of a large number of polar groups leads to an increase in the number of particles participating in polarization per unit volume. In addition, the interaction between polar groups and water molecules affects the polarization process of the system. Based on the analysis, the following conclusions can be drawn: (1) As vegetable oil proportions increase in the mixed oil, the diffusion coefficient of water molecules decreases. Since the introduction of vegetable oil into the mixed oil increases the number of polar groups, the number of hydrogen bonds and the interaction energy increases, which reduces the diffusion ability of water molecules in the mixed oil system, thereby improving the insulation performance of the mixed oil. (2) Due to the strong polarity of polar groups and water molecules in vegetable oil, the dielectric constant of mixed oil gradually increases with an increase in the proportion of vegetable oil and water content. With the increasing of its content, the dielectric property of mixed oil would be affected to a certain extent. (3) Thus, when conducting insulation oil modification using the method of blending vegetable oil with mineral oil, it is imperative to select the most appropriate type and proportion of vegetable oil. It is necessary to comprehensively consider the inhibitory effect of different types and amounts of polar groups on the diffusion behavior of water molecules and the increasing effect on the intermediate electric constant of vegetable oil. © 2024 China Machine Press. All rights reserved.