【作者】 施健；

【导师】 司马文霞；

【作者基本信息】 重庆大学 ， 电气工程， 2014， 博士

【摘要】 电力变压器作为电力系统中能量转换和传输的核心设备，其安全稳定运行关系着人们正常的生活和国家的经济命脉，由操作冲击导致的内绝缘放电将带来严重的后果。变压器油是一种绝缘性能优良的液体电介质，并已在电力变压器中广泛应用，随着我国特高压电网的快速发展，电力系统电压等级不断增加，为了节省生产成本的和空间资源，变压器本身逐渐紧凑化，这进一步增加了变压器绝缘的设计与制造难度；同时，变压器油中不可避免地存在空间电荷，空间电荷的出现不一定引起击穿，但能够通过畸变场强，影响变压器油的击穿强度。因此，需要从变压器油本身出发，提高其绝缘性能，这对提升变压器的绝缘水平和保障电力系统的安全运行具有重要的工程和理论研究价值。本文开展了变压器和纳米改性变压器油中电场和空间电荷分布的测量工作，对纳米改性变压器油的操作冲击击穿特性进行了试验研究、仿真模拟与理论分析。主要研究内容和相应的结论有：①以高Kerr常数碳酸丙烯酯液体为载体，分析了其中空间电荷分布的Kerr电光测量系统、测量原理，探索了液体中电场-光强图的成像规律，研制的Kerr电光测量系统为变压器油中电场和空间电荷分布的测量奠定了基础。为了提高电光测量的精度，对光路进行了优化以消除电场等倾线的干扰，得到了一种规范的空间电荷分布的测量系统及其反算方法。并最终利用该系统测量并得到了操作冲击电压下碳酸丙烯酯液体中空间电荷的注入水平及其分布的动态过程，其中CCD（Charge Coupled Device，电荷耦合元件）图像的灰度化处理和频域滤波等技术提高了液体中空间电荷的计算精度。对碳酸丙烯酯液体中空间电荷注入与输运过程进行了仿真，与试验测量结果对比后发现双电层理论和电化学反应过程分别能够很好地解释液体中空间电荷的注入与产生机理。②基于高Kerr常数碳酸丙烯酯液体中空间电荷的测量方法和数据处理方法，利用阵列型光探测量了低Kerr常数纯变压器油和纳米改性变压器油在操作冲击电压作用下的电场和空间电荷分布。试验发现平板不锈钢电极系统下的纳米改性变压器油的耐受电压比纯变压器油的高8.3%，利用阵列型光探测量结果解释了纳米变压器油的耐受电压提高的原因，即不锈钢平板电极间纳米改性变压器油中的双电极同性电荷注入现象更加明显，电极附近的电荷注入水平更高（纯变压器油和纳米改性变压器油空间电荷水平分别为0.010C/m3和0.036C/m3），空间电荷的屏蔽作用更好，从而使纳米改性变压器油具有更高的耐压特性。③研究了纳米改性变压器油中纳米粒子的自然稳定机制和被充电后的纳米粒子在电场作用下的迁移过程，对纳米粒子进行了表面改性试验、油中纳米粒子的分散性测试，结果表明，自然状态下经表面改性后的纳米粒子能够稳定的分散在变压器油中，直流电场下纳米粒子容易吸附到正电极，交流和操作冲击电压下的纳米改性变压器油中的纳米粒子也是稳定的。对介电型（Al2O3）、半导体型（TiO2）和导电型纳米粒子（Fe3O4）改性的变压器油在操作冲击电压作用下的击穿特性进行了测试，并与传统变压器油的击穿特性进行对比，结果表明，Al2O3、TiO2、Fe3O4纳米改性变压器油的正极性操作冲击击穿电压较纯油分别提高了35.9%、33.3%、44.3%，而负极性操作冲击下变压器的击穿电压分别只提高了11.5%、10.2%、8.12%，提高幅度较小。④从导电型纳米粒子与介电型纳米粒子在外加电场下的界面电荷特性入手，分析了它们对电子载流子的捕捉作用，得到了不同介电特性纳米粒子对流注发展过程影响的普适性规律，即不论是导电型纳米粒子还是介电型的纳米粒子，在电场作用下，其界面分别形成的感应电荷和极化电荷，都会产生电荷势垒，吸附变压器油击穿过程中流注中的电子，从而降低流注发展速率，提高了变压器油的击穿电压水平。纳米粒子对电子吸附的饱和电量与其电导率和介电常数有关，每个Al2O3、TiO2、Fe3O4纳米粒子分别能吸附的电子数为7.9e、11e、11.4e。⑤基于对变压器油中电荷载体的注入、产生、消失、复合、迁移等过程的理论分析和参数的确定，建立了变压器油中流注放电发展的场致分子电离模型，基于该模型得到的流注尺寸和流注发展速率与试验结果吻合，同时得到了流注通道内的电场、空间电荷、电位、温度分布等参数。纳米改性变压器油中纳米粒子对流注中电子的捕捉作用降低了流注发展速率和限制了流注尺寸，且流注头部的正离子、负离子、电子及空间电荷密度都有所提高，而流注通道中只有电子密度相对纯油中流注通道是降低的，原因是纳米粒子对电子的捕获作用降低了流注头部的电子数密度，为了维持流注的进一步发展，流注头部更多的中性分子会被电离。上述研究工作，为新型变压器油的研制提供了试验支持和理论依据，有利于大型电力变压器的内绝缘优化设计。更多还原

【Abstract】 Transformer is the core apparatus of power conversion and transmission inelectrical power system, the operation stability of which concerns the common people’slife and the lifeline of national economic. As the impulse overvoltage brings baddamages to internal insulation, transformer oil is widely used in electric powertransformer as liquid dielectric for its excellent insulating properties. With the fastdevelopment of EHV (extra high voltage) power grid, the voltage rate of electrc powersystem is increasing and the miniaturization of transformer which is to save the cost andspace resource intensifies the difficulty of its designing and manufacturing. Meanwhile,even though may not lead to breakdown, the inevitably existing space chargedistribution distorts the local electric field, therefore, decreases the breakdown strength.Therefore, to enhance the insulation strength of transformer oil, measures should betaken from the perspective of transformer oil itself. It has significance of engineeringand theory to enhance the insulating property and guarantee the operation safty ofpower system.In this thesis, the electric field and space charge distributions are measured intransformer oil and oil based nanofluids, and the experiments, simulation and theoreticalanalysis of impulse withstand characteristics of transformer oil based nanofluid isundertaken. The main contents and corresponding conclusions are as follows.①By using the base-liquid (propylene carbonate) with high Kerr constant, theKerr measurements system and its principle are analyzed and the electro-optic mappingrule is obtained. The Kerr system provides a solid basis on electric field and spacecharge measurements in transformer oil. The optical path is optimized to eliminate theisoclinic lines and to improve the measurement precision. The standard anticalculationmethod of space charge distribution is obatained, and by using which the space chargedynamics in propylene carbonate is calculated. The grayscale processing and thespectral filtering techniques for CCD images highly improve the precision. The chargeinjection and transport process in propylene carbonate is simulated. The doubleelectrical layer and the electro-chemical reaction can explain the space charge injectionand the charge production in bulk liquid, respectively, by comparing the the simulatingand experimental results.②Based on the space charge measurement method and the data processing in propylene carbonate, the electric field distribution and space charge injection level intransformer oil and oil based nanofluid with low Kerr constant are obtained underimpulse voltage by using the array photo-electrical detector. It was found that thewithstand voltage of NFs is8.3%higher than that of pure oil under parallel electrodesystem, which was proved by space charge measurements using array photo-electricaldetector. The bipolar charge injection in NF is much higher than that in oil, i.e., themaximum space charge densities in transformer oil and oil based nanofluid are0.010C/m3and0.036C/m3, respectively. Therefore, the conclusion that the bipolar chargeinjection with higher level can increases the withstand voltage of transformer oil isconfirmed.③The natural stability mechanisms of nanoparticle in transformer oil basednanofluid and the mobility of nanoparticle after being charged under electric field areinvestigated. The surface modification of nanoparticle and the dispersion test are carriedout. It was found that the nanoparticles are evenly distributed in the carrier oil undernatural circumtance. The nanoparticles under DC field are absorbed by positiveelectrode, while they are very stable under AC voltage and impulse voltage. Thewithstand voltages of transformer oil-based nanofluids modified by dielectric Al2O3,semiconductive TiO2and conductive Fe3O4under impulse voltage are investigated andcompared with that of pure transformer oil. The results show that withstand voltages oftransformer oil-based nanofluids modified by dielectric Al2O3, semiconductive TiO2andconductive Fe3O4under impulse voltage are35.9%,33.3%and44.3%higher than thatof pure transformer oil, while the enhancements of negative withstand voltage are notobvious and are only11.5%,10.2%and8.12%higher than that of transformer oil.④From the perspective of interface charge characteristics of conductive anddielectric nanoparticles under applied electric field, their electron-capturing process areanalyzed and the universal rule of nanoparticle’s effect on streamer discharge isobtained, i.e., surface inductive and polar charges on nanoparticles with largeconductivity and dielectricity will emerge under external electric field and generateelectron potential traps which capture electrons passing by in the streamer channel. Theprocess restrains the development of streamer discharge and enhances the insulationproperties of transformer oil. The effect of nanoparticle on trapping electrons is relatedto its conductivity and dielectricity. Each Al2O3naoparticle, TiO2naoparticle, and Fe3O4naoparticle can trap7.9e,11e and11.4e, respectively.⑤Based on the analysis and definination of space charge injection, production, vanishment, recombination and mobility, the electric field dependent molecularionization model of streamer discharge in transformer oil is constructed. The simulatedstreamer radius and streamer velocity are in accordance with experimental counterparts.The electric field, space charge, potential and temperature distribution along thestreamer channel are obtained from the model. The electron capturing nanoparticlescontributed to the decreasing streamer velocity and the shrunk dimension in nanofluids.The charge densities of positive ion, negative ion and electrons are higher at streamerhead compared to those in pure oil, while only the electron density in the streamerchannel is lower than that in pure oil. The reason is that the electrons are badly reducedby nanoparticles and more neutral molecules in transformer oil are ionized in order tokeep the streamer developing.The research work carried out in this paper provides the experimental andtheoretical supports for the research and development of new transformer oil, which isalso beneficial for the internal insulation design for large power transformers.更多还原