Thermal Aging Characteristics And Molecular Dynamics Of Oil-impregnated Insulating Paper For Power Transformers

As the main insulation medium inside the transformer,the oil-impregnated paper insulation directly affects the safe and stable operation of the transformer and the safety of the transmitted electricity.At present,research on thermal aging characteristics of oil-impregnated insulating paper is mostly conducted by macroscopic accelerated thermal aging experiments to extract aging parameters for assessing the aging state of insulating paper.The development of molecular simulation technology has led to the development of oil-impregnated paper insulation towards microscopic aging mechanisms.In this paper,the physicochemical and electrical properties of oil-impregnated insulating paper specimens at different aging stages are measured by macroscopic experiments;molecular simulation software is used to analyze and explain the mechanism at the microscopic level to study the molecular dynamics of cellulose molecules at different temperatures and in different systems.This work is of great significance to investigate the evolution of thermal aging of oil-impregnated insulating paper and reveal the microscopic mechanism of aging,and provides guidance to evaluate the service life of transformers and insulating paper modification.In the macroscopic experiments,firstly,the polymerization degree and surface morphology of the paper specimens aged for 0,96,360,720 and 1200 hours were tested and analyzed;secondly,the thermal aging characteristics of oil-impregnated insulating paper were indirectly analyzed by measuring the acid and moisture content in insulating oil at different aging levels;finally,the electrical properties of oil-impregnated insulating paper at different aging levels were tested and analyzed,including: volume Finally,the electrical properties of oil-impregnated insulating paper at different aging levels were tested and analyzed,including volume resistivity,dielectric properties,breakdown field strength and space charge distribution,etc.The experimental results show that as the aging degree of insulating paper deepens,the degree of polymerization decreases,and the macroscopic manifestation is that the degradation color of insulating oil deepens and the insulating paper becomes fragile.The acid value in the oil increases,and the moisture content in the oil increases first and then decreases.Breakdown field strength first increases and then decreases,resistivity plummets,traps increase,and dielectric loss increases.All these performance changes are related to the complex chemical changes of the thermal aging process,which can reflect the aging degree of the insulating paper and assess the aging life of the transformer,which is very intuitive and accurate.For molecular simulation,three hybrid models were built using simulation software,namely,pure cellulose model,cellulose and water hybrid model,and cellulose and oil hybrid model.From the energy analysis,the essence of the thermal cleavage of cellulose molecules is that the rate of increase of the non-bonding forces is not higher than the average potential energy thus leading to the breakage of chemical bonds.And the glass transition temperature of the amorphous region of cellulose is determined by the specific volume temperature profile method,which is important for determining the morphological properties of cellulose.For the kinetics of chain motion of cellulose,the molecular chain motion,the degree of end distance and bending,small molecule diffusion and molecular free volume were investigated.It is shown that the degree of molecular chain motion: cellulose and water mixed system > pure cellulose system > cellulose and oil system,and there are two jump changes with the increase of temperature,and the same change pattern for the bending degree.The small molecule diffusion coefficient and free volume both become larger and the molecular activity increases with the increase of simulated temperature,and the study shows that the working temperature should not be too high to control the entry and generation of water during the aging process of insulating paper.Further,the thermal stability of cellulose molecules and the simulation model of defects are studied at the level of cellulose molecules,and their orbital energy levels and forces are analyzed.It is shown that the process of molecular thermal cleavage is the process of chemical bond breaking and formation of complex products,and the position where the molecule is subjected to large forces,the easier it is for the chemical bond breaking to occur and the larger the trap energy level corresponding to the model of the established defects.The size of the trap energy level at the early stage of aging is 0.54 e V,which is mainly caused by the absence of H atoms or hydroxyl groups,changing into ions in the free state;the size of the trap energy level at the middle stage of aging is 0.73 e V,where the absence of some C atoms occurs;the trap energy level at the end of aging is 0.92 e V,where the breakage of chemical bonds occurs and the molecular destruction is serious.For the stability of the transformer operating condition and to delay the aging of the insulating paper,the protection of the chemical bonds in the places where the molecules are subjected to large forces should be strengthened.