基于分子动力学仿真的混合油中水分子扩散行为及其介电常数研究

doi:10.19595/j.cnki.1000-6753.tces.222163

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摘要

绝缘油是油浸式变压器内绝缘的重要组成部分,其绝缘性能对电力系统的稳定运行至关重要。实验研究表明植物油与矿物油共混可有效提升其绝缘性能,但改性机理尚不明确。该文利用分子动力学研究了植物油、矿物油不同共混比例下油中水分子的扩散行为及介电常数。结果表明：随着植物油占比增加,水分子的扩散系数逐渐降低,抑制了油中“水桥”的形成,提升了混合油的绝缘性能;水分子与混合油组分分子之间相互作用能与氢键计算结果表明,植物油中强极性基团的引入增大了混合油与水分子的相互作用能与氢键数目,抑制了水分子的热运动,导致其扩散系数降低;而随着植物油占比增加,混合油的静态介电常数显著增大,且随着含水量提高显著上升,这是由于水分子及植物油中强极性基团的引入增大了混合油中参与极化的粒子数目,导致体系介电常数显著增高。综上所述,强极性基团是影响混合油绝缘性能的重要因素,共混时需综合考虑其对水分子扩散行为的抑制及对介电常数的增大效应,以达到提升混合油绝缘性能的目的。

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关键词 ：混合油, 含水量, 扩散系数, 介电常数, 分子模拟

Abstract：

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.

Key words： Mixed oil water content diffusion coefficient dielectric constant molecular simulation

收稿日期: 2022-11-17

PACS:	TM215
	TM411

基金资助:

山东省电力公司科技项目（520626200070）和中国博士后科学基金项目（2019M653630）资助

通讯作者: 张博 1991年生,男,博士,讲师,研究方向为聚合物绝缘材料改性及介电机理研究。Email：zhangbo@xpu.edu.cn

作者简介: 赵曼卿 1997年生,男,硕士研究生,研究方向为油纸绝缘介电特性分子模拟研究。Email：zhaomanqing2021@163.com

引用本文:

赵曼卿, 张博, 李健飞, 李华强, 朱庆东. 基于分子动力学仿真的混合油中水分子扩散行为及其介电常数研究[J]. 电工技术学报, 0, (): 222163-222163. Zhao Manqing, Zhang bo, Li Jianfei, Li Huaqiang, Zhu Qingdong. Influence of Natural Ester and Mineral Oil Blending on the Diffusion Behavior of Water Molecules and the Dielectric Properties. Transactions of China Electrotechnical Society, 0, (): 222163-222163.

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