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基于分子模拟的油纸绝缘老化机理及气体扩散行为研究

Study on Diffusion Behavior of Gas and Mechanics of Oil-Paper Aging Using Molecular Simulation

【作者】 陆云才

【导师】 廖瑞金;

【作者基本信息】 重庆大学 , 电气工程, 2007, 硕士

【摘要】 变压器油纸绝缘的老化是一个非常复杂的物理和化学变化过程,它不仅涉及电场应力的作用,而且还涉及热应力、机械应力等其它环境应力的作用。近年来,国内外学者开始选择老化过程中的物理化学变化作为重点研究对象,并试图从微观角度揭示油纸绝缘材料的结构与性质的关系,解释油纸绝缘微观结构和宏观性质的联系。九十年代初,随着高科技学科的飞速进步,分子模拟作为一种完全独立而新颖的研究手段,在材料科学领域的应用进入一个新的阶段。分子模拟技术是用计算机基于原子水平的分子模型来模拟分子的结构与行为,它除了能获得准确的物性数据外,还可以对各种复杂现象的机理有更深刻的了解,分子模拟被认为是实现这一目标的关键技术之一。本文以分子动力学为主,结合计算量子化学,对分子模拟在变压器油纸绝缘老化机理和气体扩散行为方面的应用进行了相关研究。首先,建立了绝缘纸纤维素分子模型,模拟计算了绝缘纸纤维素分子LUMO-HOMO能隙随直流电场强度的变化,得出了纤维素分子可能发生绝缘击穿的场强;利用分子动力学方法,分析了在一定外界环境下(温度、电场)纤维素分子断链过程,发现纤维素分子最先在薄弱的苷键位置上进行断链。将动力学模拟结果与绝缘纸老化前后原子力显微镜观测结果比较,证明了实际中纤维素的电热老化会破坏其葡萄糖单体的环状结构,并产生一些活性基团;对比分析绝缘纸纤维素红外光谱的分子模拟结果与实测值,确定了纤维素分子振动特征基团部位,提出了可以利用特征基团来分析纤维素的降解机理。然后,通过分子模拟石蜡基油裂解可能性分析,确定了石蜡基油裂解温度,并分析了温度对石蜡基油热裂解反应速率的影响,拟合计算了裂解反应的活化能。研究表明,石蜡基油热裂解反应速率随温度的升高而加快,具有一级反应的明显特征。此外,从分子动力学出发分析了油中溶解气体产生机理及过程。结果显示,高温加速分子运动是造成分子裂解的主要原因,而电场力会沿电场作用方向对分子链起到巨大的拉伸作用,促进分子的裂解。在油裂解过程中都有甲基、次甲基、含氧基、烃类等自由基产生,并含有大量的碳正离子和氢负离子。这些极不稳定的中间体经过重排、键的断裂、氢的转移等步骤形成稳定的小分子烃类气体。最后,利用分子动力学法,采用COMPASS力场,计算了7种小分子在石蜡基油、纸体系中的自扩散系数,并模拟了温度对油、纸绝缘中二氧化碳分子自扩散性的影响。提出了利用自由体积理论来描述油中气体扩散传质现象,自由体积理论对于理解小分子气体在油绝缘中的扩散行为具有重要意义。通过本论文的研究,增进了对油纸绝缘微观老化机理和气体扩散现象本质的了解,同时也进一步丰富了分子动力学的研究手段,拓宽了其应用领域,为将来分子动力学方法在高电压绝缘材料领域的广泛使用提供了有力的支持。

【Abstract】 The transformer oil-paper insulation is a complicated physical and chemical change process. It not only involves the action of electric field stress, but also the action of other environmental stresses such as hot stress and mechanical stress. In recent years, the domestic and international scholars have begun to regard physical and chemical changes in the aging course as the key research object. They attempt to expose the relationship between structure and property of the oil-paper insulating materials from microcosmic perspective to explain the connection between micro-structure and macro-properties of oil-paper. At the beginning of the 1990s, with the quick development of Hi-Tech, as a kind of totally independent and novel research means, the application of molecular simulation in material science enters a new stage. Molecular simulation is a kind of technology using computer to imitate the molecular structure and behavior based on molecular model of atom level, besides being able to get the accurate property data, it is can also used to get a deeper understanding to the mechanism of various complicated phenomena. Molecular simulation is considered one of the most promising solutions to realize this goal.The molecular dynamics and computational quantum chemistry are used to research the aging mechanism of transformer oil-paper insulation and the application of gas diffusion behavior in this paper.Firstly, the model of cellulose molecule is built up, the LUMO-HOMO energy gap changing with the direct electric fields is calculated simultaneity. The possible breakdown field of cellulose molecule is educed. The chain scission process of cellulose molecule under certain external environment such as temperature and electric field is analyzed. And we found that chain fracture of cellulose molecule occurs in the weakest glycosidic linkage. The comparison of the dynamics simulated result and atomic force microscopy observation before and after ageing proves that the electrical-heating aging can destroy the cyclic structure of glucose actually and it can produce some active groups. Through comparing and analyzing the molecular simulation result and surveying value of the infrared spectroscopy of cellulose, the vibration characteristic group’s position can be confirmed. The degradation mechanism which can be analyzed by active groups is put forward.Then, the crack probability analysis of paraffin base oil is given by molecular simulation. The cracked temperature of paraffin base oil is established and the impact of temperature on hot cracked reaction rate of paraffin oil is analyzed. Besides, the activation energy of cracking reaction is calculated. The result indicates that the hot cracked reaction rate of paraffin base oil quickens with the increasing of temperature. In addition, the generation mechanism of gas dissolve in oil and its course are analyzed from molecular dynamics. The result reveals that high temperature quickening molecular motion speed is the reason of molecular splitting. And electric field stress plays a enormous tension effect on molecular chain along electric field function, which promote the cleavage of the molecule. There are a lot of free radicals such as carbonyl, methane and Hydrocarbons engendered in the progress of oil cracking. These unstable intermediates form the steady light molecule hydrocarbon gases through a sequence of steps such as gas reset, cleavage, as well as hydrogen transformation.Finally, the self-diffusion coefficients of paraffin base oil and paper system are calculated by molecular dynamics under COMPASS force field. The effect of temperature on carbon dioxide in oil insulation and paper insulation is simulated. Using free volume theory to describe the diffuse phenomenon of gas in the oil is proposed. The free volume theory has a significant sense to understand the diffusion behavior of small molecule gas in the oil.Our understanding of micro aging mechanism of oil-paper insulation and the essential of gas diffusion phenomenon are enhanced through the research of this thesis, and the research means of molecular dynamics and its application are widened at the same time. And it has offered a strong support for mass use of molecular dynamics method in the field of high voltage insulating material.

  • 【网络出版投稿人】 重庆大学
  • 【网络出版年期】2007年 05期
  • 【分类号】TM85
  • 【被引频次】16
  • 【下载频次】465
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