Molecular Dynamics Study On The Impact Of Temperature And Electric Field On The Microscopic Properties Of Oil-immersed Insulation Paper

Oil-immersed paper, which has been universally utilized as insulation in powertransformers, plays an important role in determining the lifespan of transformers.However, in the long-term operation of power transformer, oil-immersed paper issusceptible to temperature and electrical field, which would result in irreversibledegradations of its insulation performance. And moisture in oil-immersed paper canalso accelerate its aging. However, it is very difficult for traditional researches, whichare based on macroscopic experiments, to explain the aging process of oil-paper fromthe micro-mechanism. The rapidly developed molecular simulation technique, whichcould investigate the structure of materials at the molecular level and reproduce themicroscopic physical and chemical process of the material under applied stress, hasbeen considered one of the most promising solutions to the problem above. Therefore,the microscopic properties of oil-paper under different temperature and electric fieldwere studied by molecular dynamics (MD) combined with some research results in thispaper. The main research contents and conclusions are as follows:Firstly, three amorphous models of oil-immersed insulation paper wereconstructed to study the impact of temperature on various microscopic propertiesespecially the glass transition temperature (Tg), aiming to explore the correlativemechanism between them. Results indicate that potential energy of the amorphousmodels will increase when the temperature rises. Meanwhile, the cellulose chainmotion and diffusion of small molecules will be enhanced, accompanied with thereduction of hydrogen bonds and static mechanical properties. During the glasstransition process, the cellulose chain motion including mean square displacement,end-to-end distance and mean square bend have a corresponding significant transition,with a sudden change of the free volume, which indicates the nature of glass transition.Compared with the oil molecules, the addition of water seriously reduces the Tg ofamorphous cellulose in insulation paper, considerably reducing the thermal stability ofthe insulating paper. The formation of hydrogen bonds between the water and chaincontributes to the increase of chain flexibility, finally reducing the Tg of amorphouscellulose, which should be considered in the design of anti-aging insulating paper.Secondly, single chain and multi-chain cellulose models were built to study the yield behavior of cellulose under strong electric field. Results indicate that, whenapplied strong electric field, yield behavior occurs on both of the single and multicellulose chain, whose direction is consistent with the electric field. The stress causedby electric field will result in the yield behavior of cellulose, and eventually lead to thebreakage of the cellulose chains. The yield behavior of cellulose may introduce localspace charge trap, and accelerate aging of paper.Then, moisture diffusion models on surface of crystal cellulose and in amorphouscellulose were constructed respectively to study the influence of the electric fieldpolarization on moisture diffusion. The polarization effect of strong electric fieldrestrains movement of water molecules on the surface of the crystal cellulose, whichleads to the reduction of total diffusion coefficient D and the difference among Dx, Dyand Dz. The interaction energy between crystal cellulose surface and water moleculeswould be increased by electric field, which enhances the hydrophilicity of crystalcellulose surface and affects the directionality of water diffusion coefficient to someextent. Relatively, movement of water in amorphous cellulose with electric field iscomplicated and lack of regularity.Finally, taking into account the effect of temperature and electric field onglucosidic bond of cellulose chain, the fracture models of crystal cellulose wereconstructed to investigate connections between degree of polymerization, fracturedegree and mechanical properties of crystal cellulose. During the fracture process ofcellulose, there is a high degree of positive correlation among elasticity modulus in thedirection of chain (C33), Young’s modulus (E), bulk modulus (K) and shear modulus(G), which has a ladder-like drop with the decrease of degree of polymerization (DP)and increase of fracture degree (FD). The result of nonlinear fitting shows that whenthe fracture degree of model is50%or100%, the relationship between DP and E obeysthe same exponential distribution. Accompanied with a decrease of intermolecularhydrogen bonds, the decrease of DP and increase of DP will lead to the reduction ofC33, which ultimately lead to a reduction of the mechanical strength.The research of this paper would exhibit some important engineering value andacademic significance to develop new aging research method and to explore new agingtheory.