| Vegetable insulating oils are environment-friendly and fire-resistant liquiddielectrics with good insulation properties， and have been used in distributiontransformers for many years. Because the physical and chemical properties of vegetableinsulating oil are different from those of mineral oil, the gases dissolved in vegetable oilunder typical thermal and electrical faults differ from those in mineral oil，as well as themoisture-absorption characteristics and the dispersion of nanoparticles. With the widespreading application of vegetable insulating oil, the problems above are vital to thedesign, manufacture, operation and maintenance of vegetable oil filled transformers.Therefore, dissolved gas analysis in vegetable insulating oil under typical faults,the moisture diffusion and equilibrium characteristics of vegetable oil-paper insulationand vegetable oil based nanofluid are studied. The main contents are as follows:①The dissolved gases in vegetable insulating oil under the typical faults areobtained，and the key gas is proposed. In the simulated typical faults experiment, testingequipment and testing method for thermal faults and electrical faults are designed. IECthree ratios and Duval triangle diagnostic method are utilized to analyze the dissolvedgases in vegetable insulating oil under the typical faults. The results show that the keygas under typical thermal faults in mineral oil is different from that in vegetableinsulating oil is C2H6, IEC three ratio method and Duval triangle diagnostic method arenot suitable for the fault of vegetable insulating oil.②The moisture diffusion characteristics between vegetable insulating oil andinsulating paper are studied. According to Fick’s second diffusion law, the model formoisture diffusion in vegetable oil-paper insulation is proposed and the moisturediffusion coefficient between vegetable oil and paper is obtained from experiment. Thedynamic moisture distribution in the vegetable oil-paper system is simulated.Temperature, thickness of paper and initial moisture content are considered to be thefactors influencing the equilibrium time constant, which are also compared with theminimal oil-paper system. In addition, a2D infrared spectrum method is used to studythe moisture diffusion in vegetable oil-paper system. The experiment and analysisresults show that the diffusion rate of moisture in vegetable oil-paper insulation is abouthalf of that in mineral oil-paper insulation. ③The moisture distribution equilibrium in vegetable oil-paper insulation is studiedand a new mathematical model for the moisture distribution equilibrium is proposed.The moisture equilibrium curves of vegetable oil-paper insulation are obtained by bothdirect and indirect methods. Meanwhile, the results of vegetable oil-paper insulation arecompared with that of the minimal oil-paper. According to the moisture equilibriumcurve, the mathematical model for the moisture distribution equilibrium is set up bydata fitting and approximate methods, and the corresponding parameters are given. Themodel can accurately describe the oil-paper moisture equilibrium curve after verified.④The influence of moisture on the breakdown and dielectric properties ofvegetable insulating oil are studied. Aging experiment of in vegetable oil-paperinsulation with different initial moisture content are carried out, aging parameters suchas breakdown voltage of oil, acid value, furfural concentration and DP of paper aretested. Experiment results indicate that the impact of moisture content on the breakdown,dielectric and aging properties of vegetable insulating oil differs greatly from that ofmineral oil. Vegetable insulating oil has higher moisture absorption ability than mineraloil and makes the insulating paper drier, thus delaying the aging rate of insulating paper.⑤.The vegetable oil based Fe3O4nanofluid are prepared by the co-precipitationmethod and modified by the oleic acid and sodium oleate. The dispersion stability of themodified nanoparticles in vegetable insulating oil is studied. The impulse breakdowncharacteristic of vegetable oil based nanofluid are investigated, the polarization andcharging dynamics model of nanoparticles in vegetable oil are proposed, space chargesaffecting the breakdown performance of nanofluid are subsequently analyzed. Theexperiment results show that feeding mode, stirring time and surfactant coating timehave a great impact on the dispersion stability of the modified nanoparticles invegetable insulating oil, and nanoparticles can significantly improve both the positiveand negative lightning impulse breakdown voltage of vegetable insulating oil.The above work offers the theory and experiment basis for the design,manafacturing, operation and maintenance of vegetable insulating oil filled powertransformer, which has important reference value for the application of the vegetableinsulating oil and vegetable insulating oil based nanofluid in the power transformer. |
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