Natural ester, as a new environmentally green insulating oil, has been widely used in transformer. In an oil-immersed transformer, the normal aging, thermal failure, and discharge failure could easily lead to the decomposition of the oil-paper insulation system and produce different kinds of gases. Studying gas dissolution in natural ester and mineral oil could provide assistance in applying criteria to make a diagnosis of different kinds of faults in the transformer. In this paper, the molecular dynamics method was used to investigate the diffusion behavior of seven fault characteristic gases (including H₂, CO, CH₄, C₂H₂, CO₂, C₂H₄, C₂H₆) in natural ester and mineral oil. The simulation parameters of free volume, interaction energy, mean square displacement, and diffusion coefficient were compared between the natural ester and mineral oil. Meanwhile, the influence of temperature on the diffusion of gas molecules in two kinds of oils was also analyzed. Results showed that the free volume, the interaction energy, and the relative molecular mass of gas molecules were the factors influenced by the diffusion of gas molecules in natural ester and mineral oil. The order of the diffusion coefficients of gas molecules in natural ester was as follows: H₂ > CH₄ > CO > C₂H₂ > C₂H₄ > CO₂ > C₂H₆ and that in mineral oil was as follows: H₂ > CH₄ > CO> C₂H₂ > C₂ H₄ > C₂H₆ > CO₂. By comparing the diffusion behavior of gas molecules in natural ester and mineral oil, it was found that the smaller free volume and higher interaction energy of gas molecules in natural ester were the major reasons for the gas molecules to be more difficult to diffuse in natural ester. The rising temperature could enhance the free volume and reduce the interaction energy between gas molecules and oil. The diffusion coefficient of gas molecules increased exponentially with the follow of temperature. However, the temperature didn’t affect the ordering of diffusion coefficient, free volume, and interaction energy of gas molecules in natural ester and mineral oil.

Developing green insulation materials has become an important direction for future research. As a new environmental insulation liquid, natural ester can effectively delay cellulose insulation polymer ageing. Mastering the mechanism of natural ester delaying the cellulose ageing could promote its application in power transformers. In this study, based on the accelerated thermal ageing of pressboard immersed in natural ester and 25# mineral oil at 120°C for 221 days, the thermal ageing rate and the levelling off degree of polymerisation (LODP) for two kinds of oil‐impregnated pressboards were compared. The molecular level mechanism of natural ester delaying cellulose degradation and keeping higher LODP value was proposed using molecular dynamics simulation. At the early stage of thermal ageing, cellulose with a higher degree of polymerisation ( DP ) can form many hydrogen bonds with hydronium ions, and their binding energy is stronger, which can accelerate the acid hydrolysis of cellulose, causing the DP of pressboard immersed in natural ester and mineral oil to decrease rapidly. With the breaking of the cellulose molecular chain, the number of hydrogen bonds and binding energy between cellulose and hydronium ions becomes smaller and the thermal ageing rate decreases gradually. At the end stage of ageing, it is difficult for the hydronium ions to enter the crystalline cellulose, acid hydrolysis of crystalline cellulose is hard to conduct, leading to the occurrence of the LODP phenomenon. The degradation rate of cellulose in natural ester is only 50% of that in mineral oil. The cellulose LODP value in natural ester is over 200 larger than that in mineral oil. Compared with the mineral oil‐cellulose system, natural ester can form hydrogen bonds with hydronium ions and lead to fewer hydronium ions in cellulose, fewer hydrogen bonds and smaller binding energy between cellulose and hydronium ions, which can reduce the acid hydrolysis reaction of cellulose. This study provides a theoretical explanation for natural esters to delay the ageing of insulation paper.

The accelerated thermal ageing test was conducted with samples composed of mineral oil (MO), FR3 natural ester, kraft insulation pressboard (KIP) and thermally upgraded paper (TUP) at 120 °C. In the middle ageing period, aged MO was replaced with new FR3. Molecular dynamics was used to simulate the mechanism of delaying cellulose aging by replacing natural esters. The experiment results show that, compared to the controls, KIP and TUP impregnated in FR3 had higher degrees of polymerization (DP) values and lower ageing rates. After the FR3 replacement, the relative chain scission rate of KIP and TUP was significantly slowed. FR3 has better aging retarding effect on TUP than KIP. Atomic scale mechanism analysis shows that FR3 natural ester could inhibit the diffusion of water and acid in the oil-paper insulation system before and after oil change, which has a significant ability to delay the acid hydrolysis of cellulose. These is the main reason why KIP and TUP exhibited a higher DP in natural ester. This paper provides theoretical support for transformer replacement of natural ester.

The lightning impulse breakdown strength of natural ester is lower than that of mineral oil. This article focuses on a discharge mechanism difference analysis of natural ester and mineral oil under a strong impulse electric field (EF). Based on density functional theory (DFT), the ionization energy, electron affinity (EA), and excitation energy of natural ester and mineral oil were calculated under different EF strengths. The microparameters related to the discharge of natural ester and mineral oil were compared, showing that the former has a lower ionization energy and a larger dipole moment. This quality makes natural ester more likely to ionize and produce electrons, which more easily promotes the formation of electron avalanches and streamer. Meanwhile, natural ester molecules are more prone to transition from the ground state to the excited state than mineral oil molecules, exacerbating the photon release and formation of discharges in natural ester. The greater conductivity of natural ester leads to its dispersion and the rapid development of discharge streamers. In addition, an electrostatic potential (ESP) analysis on molecular surfaces was used to predict the active sites of the oil molecular structure. This article provides help for understanding the relationship between the micromolecular structure and the macroscopic discharge characteristics for different types of oil molecules.

The thermal oxidative aging of cross-linked polyethylene (XLPE) during cable operation is a primary factor leading to changes in its mechanical properties. Notably, these properties exhibit an initial increase followed by a decrease as aging progresses. In this study, molecular models of silane-cross-linked polyethylene (Si-XLPE) and peroxide-cross-linked polyethylene (p-XLPE) were constructed to analyze their thermal oxidative aging processes using ReaxFF simulations. The research focused on understanding how structural changes in XLPE during aging impact its mechanical properties from the perspective of intermolecular forces. The simulation results reveal that Si-XLPE initially degrades from the silane cross-linking bonds, eventually forming small hydrocarbon molecules. In contrast, p-XLPE experiences the formation of C=O and C=C bonds during its structural degradation. As the degree of aging increases, both the density and cohesive energy density of XLPE initially rise and then decline, while the free volume fraction exhibits a decrease followed by an increase. This indicates that the early stages of aging enhance the intermolecular forces within XLPE, which explains the observed pattern in mechanical property changes. Furthermore, the study finds that Si-XLPE consistently exhibits stronger intermolecular forces than p-XLPE across all stages of aging, contributing to its superior mechanical properties. These simulation results effectively explain the mechanical behavior observed in experimental studies of XLPE. This paper is expected to provide some new perspectives and ideas on constructing the structure-activity relationship between the microstructure and macroscopic properties of XLPE.