Impact Of Moisture And Acid On The Microscopic Properties Of Oil-immersed Insulating Paper By Molecular Simulation

During the long run process of power transformer, the oil-paper insulationdegrades gradually under the impact of multiple factors, such as electricity, heat,moisture, and organic acids. The ageing process of insulating paper is irreversible,which determines the lifetime of transformer directly. Moisture and acids in insulatingpaper accelerate aging of paper, and it is of great importance to study the influence ofmoisture and acids on the microscopic characteristics of insulating paper, thus explorethe aging mechanism. Currently, the main means to study the insulating paper ismacroscopic experiment, which is hard to obtain the microscopic mechanism. In thisarticle, firstly, based on the molecular dynamics (MD) and the density functional theory(DFT) method, the impact of moisture on the microscope properties of insulating paperhas been studied. Mechanical constant, hydrogen bond and Mulliken population ofcellulose models with different moisture content were calculated. The diffusionbehavior of organic acid molecules with different initial positions has been studied bymolecular simulation. The impact of organic acid on the microscopic characteristics ofcellulose has been studied. The main conclusions are as follows:Water molecules can destroy hydrogen bond network between the neighboringcellulose molecules, which leads to the significant decrease of mechanical strength. Themoisture directly affects the strength of chemical bonds on the pyran ring, while thepyrolytic initiator of ring opening is always the C-O bond on the pyran ring. Thechemical strength of cellulose glycosidic bonds is reduced due to the moisture action,which promotes the fracture of glucosidic bonds and accelerates cellulose aging processfurther. Through forming hydrogen bonds with hydroxyl groups on the cellulose chain,water molecules have a certain impact on the shedding of corresponding chemicalgroups, with hydroxyl and hydrogen groups included. And the tensile test resultscompared well with the simulation results.For the organic acid molecules with the initial position in cellulose, the interactionbetween the acid molecules and cellulose is much larger than that between acid and oil,and acid molecules are limited inside the cellulose tightly. For the organic acidmolecules with the initial position in oil, three small molecules, including formic acid,acetic acid and levulinic acid, can diffuse from oil into cellulose. While macromoleculesincluding naphthenic acid and stearic acid, stay in oil and do not spread to cellulose after the simulation. In addition, the moisture makes a contribution to the increase of thefree volume in fiber system, providing more space of the diffusion of particle, thusincreasing the diffusion coefficient of organic molecules.The influence rules of acid on cellulose is similar to that of moisture, a largeamount of acid molecules lead to a significant increase in MSD, indicating that themovement of cellulose chain is enhanced and structural stability is decreasing. Themechanical constants changes by the influence of acid. Acid molecules involves in theformation of hydrogen bonds, while destroys the original hydrogen bond network.However, the acid content can only reach0.02~0.04gKOH/g at the later stage of aging,which means under practical operating conditions, small organic acid has a limitedimpact on the movement and structural stability of cellulose.