Molecular Dynamics Study Of Thermal Aging Of Oil-impregnated Insulation Paper

In the 12th five-year-plan scheme of China, it is firstly proposed to develop an advanced power network featured by high capacity, high efficiency and long-distance trans-regional transmission. It manifests that UHV power transmission is taken as a strategical cause for the modernization of China. Accompanying the setups of transmission voltage level, electrical materials are expected to have a superior insulation performance to meet such increasing challenges. Oil-paper composite material has been universally utilized as insulation in power transformers, rendering it play an important role in determining the lifespan of transformers, and furthermore, influencing the security of whole power network. However, Oil-paper composite material is susceptible to temperature, electrical field and mechanical stress in the long-term operation of power transformer, which would result in irreversible degradations of its insulation performance and subsequently cause damages to the safe operation of power network. The investigation on ageing mechanism of oil-paper composite materials is considered as the basis and prerequisite of some other study subjects, such as condition assessment, lifetime prediction and anti-ageing treatments. Ageing of insulating material is not only dependent on material's intrinsic properties, such as molecular and supermolecular structure, but also closely correlated with the material's physicochemical behaviors under thermal stress. Therefore, the work, which aims for elucidating the microcosmic mechanism of ageing and exploring the thermal behaviors of insulating materials, exhibits some important academic significance as well as a contribution to practical application.Thermal degradation is a major ageing process of oil-paper insulation. It generates some hazardous substances which in reverse accelerate the ageing rate, such as moisture and acids. A large amount of researches were conducted to study the macroscopic characteristics of oil-paper insulation thermal degradation, with various extents of achievements. Nevertheless, due to the complicated multidisciplinary problems and limitations of conventional experimental resorts, the corresponding microscopic mechanism underlying macroscopic phenomenon is still far from being acknowledged. Computer simulation seems to be an effective technique to solve above questions. Thus, molecular dynamics simulation is adopted in this work. By utilizing a combination of microscopic, mesoscopic and macroscopic interpreting scales, the thermal stability of insulating paper when exposed to thermal stress and water is investigated. Moreover, the diffusion and distribution laws of organic acids and water molecules are discussed as well. All simulation parameters are rigorously according to practical experiences. The innovative results obtained by this paper are as follows:?Molecular dynamics simulation were performed on amorphous and crystalline cellulose, respectively. Results show that the tension modulus of amorphous cellulose is comparatively small. The mechanical modulus and hydrogen bonds structure of amorphous cellulose are much more vulnerable to thermal stress than those of crystalline cellulose. As temperature rises, amount of hydrogen bonds inside amorphous cellulose declines sharply and mobility of cellulose chain is strengthened greatly. By contrast, the amount of hydrogen bonds and mobility of cellulose chain inside crystalline cellulose seem not obviously varied. Simulation results imply that thermal degradation of insulating paper occurs firstly at amorphous region and the ageing extent of amorphous cellulose is increasingly greater than crystalline cellulose as temperature goes higher, which is highly coincident with experimental results.?The bonding effects of cellulose and oil towards organic acids were studied by molecular dynamics simulation, it is found that there are two predominant factors that influence the bonding behaviors. The first factor is polarity effect and free volume, which makes the bonding energy of cellulose is much greater than that of oil towards acids. The other factor is solubility parameter. The deformation energy of low weight acid is smaller than the absorption energy between cellulose and low weight acid; and solubility parameter of low weight acid is approximate to that of cellulose. Contrarily, the deformation energy of high weight acid is greater than the absorption energy between cellulose and low weight acid; and solubility parameter of high weight acid is approximate to that of oil. Due to above impacting factors, it is concluded that low weight acid is more readily absorbed to cellulose, either staying at the surface or penetrating into the inside of cellulose; while for high weight acid, it would absorbed to oil and has a weak influence on paper's aging.?The bonding effects of cellulose and oil towards water molecules, as well as diffusion behaviors of water molecules in oil-cellulose composite system were studied by molecular dynamics simulation. Both oil and cellulose could absorb water molecules. However, bonding effect of oil is notably lower than that of cellulose towards water molecules. Such a difference is largely attributed to polarity effect. The observation can be taken advantage of to explain the experimental phenomenon that majority of water would reside in cellulosic paper while oil's water content is much lower.?The influence of moisture on thermal stability of cellulose was studied. Result shows water exerts a detrimental influence on stability of cellulose. Moisture not only weakens cellulose's mechanical strength but also destroy cellulose's structural stability. The larger the water content is, the more serious the hydrogen bonds of cellulose are destroyed. Simulation results explain why the degree of polymerization of cellulosic paper will undergo an accelerated declination process when water content keeps increasing.