| 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. |
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