【作者】 杨涛；

【导师】 王有元；

【作者基本信息】 重庆大学 ， 电气工程， 2013， 硕士

【摘要】 油纸绝缘作为大型电力变压器绝缘的主要形式，其绝缘性能直接决定变压器的使用寿命。油浸绝缘纸在变压器长期运行过程中受温度、电场等因素的影响绝缘性能逐渐下降，并且在水分等的作用下加速老化，而传统的油浸绝缘纸老化研究大多基于宏观的实验，很难从微观机理上对绝缘纸的老化过程进行深入分析。近年来迅速发展的分子模拟技术则可以从分子水平上认识材料的结构，重现材料在外界应力作用下的物理和化学变化过程，已成为解决上述问题的关键技术之一。本文以分子动力学模拟为主要手段，结合相关研究结果对比分析，研究了温度及电场对油浸绝缘纸微观特性的影响。主要研究内容及结论如下：首先建立了绝缘纸纤维素无定形模型，研究了温度对油浸绝缘纸的微观特性的影响，探索了以玻璃转化温度(Tg)为核心的各种微观特性的关联机制。研究结果表明，温度的升高会使绝缘纸无定形区势能增加，纤维素链运动和小分子扩散运动增强，氢键数目减少，静态力学特性降低。玻璃转化过程中均方位移、末端距及均方弯曲等链运动特性均一致地出现了明显的跃变，并导致分子运动的自由体积突变。相对于油分子，水分子的加入会严重地降低绝缘纸无定形区的玻璃转化温度，进而降低绝缘纸稳定工作的温度。水分和纤维素链之间的氢键是使纤维素链柔顺性增加乃至玻璃转化温度减小的重要原因，根据这个机理在绝缘纸防老化设计中应注意消除水分的这种作用，提升绝缘纸的热稳定性。其次，建立了纤维素单链和多链模型，研究了强电场下纤维素的屈服行为。研究结果表明，强电场作用下，纤维素单链和多链均会发生和电场方向一致的屈服行为，这种电场应力导致的屈服是纤维素链断裂的根本原因，纤维素链的屈服可能引入局部空间电荷陷阱，加速纤维素的老化。再者，建立了绝缘纸纤维素晶面和无定形区水分的扩散模型，研究了电场极化作用对水分扩散的影响。强电场的极化作用会束缚水分子在绝缘纸晶区表面的扩散，使其扩散系数减小，并表现出在X、Y和Z方向的差异性；电场还会使纤维素晶面和水分子的相互作用能增加，纤维素亲水性增强，这在一定程度上影响水分扩散系数的方向性。相比较而言，水分在纤维素无定形区的运动受电场影响规律性较差。最后，考虑到温度及电场对绝缘纸纤维素链糖苷键的的断裂作用，建立了纤维素晶区链断裂模型，研究了绝缘纸纤维素晶区断裂度、聚合度与机械强度的关系。纤维素链发生断裂时，其链方向上的弹性系数C33和杨氏模量E、体积模量K以及剪切模量G具有高度的正相关性，并随聚合度的减小和断裂度的增加呈阶梯型降低。当模型断裂度分别为50%及100%时，其聚合度和杨氏模量的关系服从相同的指数分布。聚合度的减少和断裂度的增加，伴随体系的弹性系数C33的减少以及分子间氢键数量减少，最终会导致机械强度的减少。本论文的研究对于拓展和加深对油浸绝缘材料的认识，发展新的老化研究手段，探索新的老化理论具有重要的工程价值和学术意义。更多还原

【Abstract】 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.更多还原