Study On Migration Mechanism Of Ion And Water In Natural Ester Insulating Oil And Computational Method Of Hydrolysis Kinetics

Natural ester insulating oil can extend the service life of insulating paper by the inhibition of water and ion diffusion to insulating paper since its obvious difference in the molecular structure compared to mineral oil,which is one of the advantages of natural ester insulating oil.However,a challenging problem for the application of natural ester insulating oil in transmission-voltage power equipment is the higher deterioration rate and insufficient long-term stability.Therefore,from a microscopic perspective of interactions between natural ester insulating oil and water molecules or ions,both the mechanisms of water and ion migration,and the influence of hydrolysis are studied to determine a method for improving the stability of nature ester insulating oil.The main contents are as follows:(1)Complexation effects of natural ester insulating oil are studied.The electrostatic interaction sites of insulating oil are acquired by the molecular electrostatic potential analysis using quantum-chemical calculations.The difference in spatial contact probabilities between various particles and specific functional groups are obtained by molecular dynamics simulations.At the same time,the complexation effects of nature ester for water molecules and ions are revealed by the radial distribution function.Moreover,various probability functions,such as the spatial distribution function and the lifetime function are introduced to analyze the complexation structures.The results show that with the help of the ester and β-H function groups carrying high electrostatic potentials,natural ester insulating oil has complexation effects on both water and ions,especially the more stable ion cage formed with ions,which reveals the mechanism of natural ester insulating oil for inhibiting water and ion migration.(2)Ion migration under the ion cage effect is investigated.Ion mobilities in insulating oils are computed by the Nernst-Einstein equation to study the influence of the ion cage.Furthermore,combined with the Arrhenius function,a numerical model for calculating ion mobility is established.The migration behaviors of ions in oils under electric fields are studied,and it is found that there is an electric field threshold for ion mobility in nature ester.Based on the molecular scattering theory,the dissociation of ion cages is investigated to explain the enhancement of ion mobility by electric fields.The results reveal that with the help of the ion-cage effect,natural ester insulating oil can effectively localize ions,weaken their electric field responsiveness,and inhibit their directional diffusion to insulating paper.(3)Water diffusion and equilibrium under the complexation effect are investigated.Water diffusion coefficients in oils are computed by molecular dynamics simulations,and the influence of the complexation effect on the water diffusion coefficient is explored.The Arrhenius function is introduced to establish a numerical model for calculating the water diffusion coefficient.Using the Conductor-like screening model for segment activity coefficient combined with the solubility-activity equation and the thermodynamic phase equilibrium equation,a theoretical algorithm,rarely depending on experimental data,for computing water saturation values of insulating oils and oil-paper moisture equilibriums is established,which achieves the mapping relation between molecular structures and moisture equilibrium curves.The results reveal that the complexation effect of natural ester insulating oil is responsible for low water diffusivity,high water affinity,and low equilibrium moisture content of the paper.(4)A reaction kinetics equation of nature ester hydrolysis is established to propose a method for improving hydrolytic stability.The laplace function,valence electron density,and electron localization density are used to investigate the nucleophilic attack sites of nature ester.It is found that the carbonyl carbon is exposed to nucleophiles due to the electron deficiency effect.The reaction pathways and energy barriers of ester hydrolysis by water or ions are studied based on the transition state theory,and a reaction kinetics equation is established,which distinguishes the key factors affecting the hydrolysis of natural ester insulating oil.Furthermore,a surface modification method for the paper is proposed to improve its performance of thermal aging resistance and hydrolytic stability of natural ester insulating oils.The accelerated thermal aging results show that the modified paper has improved anti-aging performance,and its degree of polymerization is 57% higher than that of the normal paper aged in nature ester and 135% in mineral oil.At the same time,the acid values,dielectric losses,and breakdown voltages of nature ester samples are improved during aging.The work is an active exploration for research on the stability of the oil-paper insulation,which could provide the foundation and technical reference for the design and development of new oil-paper insulations.