【作者】 朱孟兆；

【导师】 廖瑞金；

【作者基本信息】 重庆大学 ， 电气工程， 2011， 博士

【摘要】 2011年,发展特高压等大容量、高效率、远距离先进输电技术已经明确写入十二五规划,特高压输电已经上升为国家战略。电力电压等级的提高,对电力绝缘材料的绝缘性能提出了更为苛刻的要求。油纸复合绝缘介质作为电力变压器的重要绝缘材料,其性能的优劣直接决定着变压器的使用寿命,影响整个电网的运行安全。油纸绝缘在变压器的长期运行过程中,受热、电、机械等多种因素的影响逐渐劣化,导致其绝缘性能逐渐下降,给电网的安全运行造成重大隐患。变压器油纸绝缘老化机理研究是绝缘老化状态评估、寿命预测和制定抗老化措施的前提和基础,故油纸绝缘老化的微观机理及热特性研究具有重要工程价值和学术意义。热老化是变压器油纸绝缘的主要老化形式之一。在油油绝缘的热老化过程中,将会产生水分、酸等一系列有害物质,而它们又会在热场的作用下更一步加速油纸绝缘的老化。长期以来,国内外学者对油纸绝缘的热老化宏观特性进行了广泛的研究,而宏观现象背后的微观机制的基础理论研究,由于涉及到电气、物理、化学等多学科交叉,同时也受传统实验手段的制约,主要实验数据的总结和归纳为主。近年来飞速发展起来的模拟计算技术,为研究油纸绝缘老化的微观机理,提供了可行、有效的手段。本文充分利用分子动力学的优势,在考虑油纸复合绝缘介质的实际运行环境基础上,采用微观、介观、宏观多尺度联合的方法,研究热场对绝缘纸热稳定性的影响,以及由热老化产生的有机酸和水分在油纸绝缘中的扩散规律及分布特点,水分对绝缘纸热稳定性影响等基础问题。论文取得主要创新性成果有:①利用分子动力学对纤维素非晶区和晶区的热稳定性能模拟计算表明,热场作用下纤维素非晶区的拉伸模量小,且随温度的升高,非晶区的力学模量和氢键结构被破坏程度均大于晶区。纤维素非晶区内氢键数随温度升高,下降明显,链运动增加明显,而晶区内氢键数和链运动变化不大。模拟与实验结果对比分析表明,变压器绝缘纸的热老化首先从非晶区开始,并随温度增高,非晶区的老化程度大于晶区。②首次利用分子动力学对油和纤维素对其热老化生成的有机酸的束缚行为及有机酸与纤维素的相互作用进行了研究。油和纤维素对五种有机酸的束缚行为由以下两方面决定:一方面,纤维素的极性和介质中的相对自由体积共同作用下,导致油对有机酸的束缚远小于纤维素对有机酸的束缚;另一方面,小分子有机酸的在纤维素表面的形变能小于吸附能,其溶解度参数与纤维素溶解度参数相近,而大分子有机酸的形变能大于吸附能,其溶解度参数与油的溶解度参数相近。两方面综合,小分子酸将附着于纤维素表面或进入内部,进而对纤维素老化产生影响,大分子酸将吸附于油中,对绝缘纸的老化未形成影响。③利用分子动力学研究了油和纤维素对其热老化生成的水分子的束缚行为以及水分子在油-纤维素复合介质中的扩散行为。油与纤维素均对水分有吸附作用,但由于介质极性的不同,油对水分的束缚作用远小于纤维素对水分的束缚作用,使水分子向纤维素绝缘纸中扩散并停留于纤维素绝缘纸中。模拟结果从微观上阐明了实验研究中绝大部分水份存在于绝缘纸中,油中水份含量很小的原因。④首次利用分子动力学研究了水分对纤维素热稳定性的影响。水分含量对纤维素的稳定性有重要的影响:水分不但降低了纤维素的机械性能,而且影响了纤维素的结构稳定性;水分含量越大,纤维素的氢键被破坏程度越大,分子动力学模拟结果阐明了实验中随着水份含量增加,纤维素绝缘纸的聚合度加速下降的原因。更多还原

【Abstract】 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:①Molecular 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.②The 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.③The 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.④The 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.更多还原