| Oil-immersed transformer is one of the most important equipment in the electric power system. Transformer failure will bring huge economic losses to the enterprises and national economy. The insulation failure is the main cause of transformer accidents, and oil paper insulation thermal aging is the leading cause of transformer insulation fault. In order to improve the reliability and economy of power equipment operation, to accurately assess the aging state of oil-paper insulation and to predict the transformer remaining life is of great significance. In this paper, research on new characteristic parameters of oil-paper thermal aging is based on the determination of acid product from insulation degradation. Four parts are included in this article.The first part reviews the research status on the thermal aging properties of oil-paper insulation in transformer and detection method of characteristic produces.In the second part, our work is focused on the correlation between the generation of acidic products and the thermal aging of oil-paper insulation of the power transformer was studied. The accelerated thermal aging tests on transformer's oil-paper insulation were conducted using 25# mineral oil and cellulose paper under 130 o C. The reaction rules of the acidic products were studied during the thermal aging of transformer oil-paper insulation, and the relationship between the acid value and the amount of furfural as the aging characteristic parameters and dielectric loss factor were compared and analyzed.Aging reaction of generating acidic products was divided into induction phase and developed phase. Aging reaction is induced by a small amount of free radicals produced from the pyrolysis reactor in the induction phase, and the reaction rate of producing acidic substances is slow. In the developed phase, the acid value of transformer oil significantly increased, and aging reaction was regarded as free radicals chain reaction, the reaction rate increased rapidly. Compared with the rate constant for the induction period, the rate constants of different aged samples increased by 5-11 times. Furfural content of transformer oil has declined in the late stages of induced reaction, and then the furfural content could not accurately reflect the aging situation of transformer oil-paper insulation. The change trend of oil dielectric loss factor and acid value was the same in the aging reaction, and the sharp increase moment of oil acid value and dielectric loss occurred at the same aging time. Under the conditions depicted in this paper, the transformer oil-paper insulation had started to accelerate the deterioration when the rates of generation acidic products increased rapidly. Through theoretic deduction according to the transformer structure and properties of acids produced, there was a positive correlation between the change of acid value and the change of dielectric loss in the transformer aging process. Results showed that, the aging state of transformer oil-paper insulation can be assessed more accurately by simultaneously determining the changes of generated acidic products and the traditional characteristic parameters furfural in oil.In the third part, in view of the phenomenon of oil acid value increase rapidly and furfural content decline in the developed phase of aging reaction, the determination methods of furfural and furoic acid content were studied and the correlation between the furfural and furoic acid and acid value were analyzed. The simulated sample preparation method and extraction factors influencing were studied. Analysis method was developed for the determination of furfural and furoic acid content in transformer oil by centrifugal extraction with high performance liquid chromatography(HPLC). Under the optimized conditions, the calibration curves of furfural was linear in the range of 0.1mg·L-1~40.0mg·L-1 with detection limits 19.1?g/L. The calibration curves of furoic acid was linear in the range of 0.05mg·L-1~5.0mg·L-1 with detection limits 9.6?g/L. The developed method was validated by analyzing the furfural and furoic acid content in Thermal aging samples. The spiked recoveries ranged from 82.9% to 109.5%.Comparison of furfural, furfural acid and acid value changes in oil-paper insulation aging samples of different aging time, furoic acid content in oil of the sample was showing a monotone increasing tendency with the increase of aging time, the fast increasing trend of furoic acid and acid value was the same. Furoic acid content was rapidly increasing when furfural content began to reduce at the developed phase of aging reaction. Furoic acid content change also reflects the change of other substances generated carboxylic acid during the thermal aging process of oil-paper insulation.The last part are the summary of this main results, and some suggestions were gived for the future work. |
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