| Oil-immersed power transformer is the core equipment of transmission and transformation in the power system, the oil-paper compound insulation system internal decides the safe operation of transformer and reliable supply of power. Organic acid and water are two important products of aging oil-paper insulation, but properties of different kinds of organic acids, the influence of organic acids on aging of oil-paper insulation, the relationship between organic acids and the characteristic parameters of different detection methods and the water distribution of aging oil-paper insulation need to be further research and analysis.According to properties of five different organic acids, formic acid and stearic acid have been chosen as objects of this study. Dissolution and distribution characters of formic acid and stearic acid have been contrastively analysis. Then the influence of insulation paper degradation rate by formic acid and stearic acid have been researched based on the change rule of degradation degree with thermal aging. The relationship between acid value and average degradation rate has been established based on zero order dynamical-model. Research results shows that formic could be easily absorbed by paper while stearic could easily dissolve in oil and formic could let water diffuse to oil from paper. The average degradation rate of insulation paper increases linearly with the increasing of acid value. The average degradation of insulation paper by formic is about eight times of stearic, The acid catalysis on further thermal aging of transformer oil-paper insulation will be obvious when the formic content reach 0.1mgKOH/g and stearic content reach 0.3mgKOH/g. The influence of the acid with low molecular and strongly polar on the thermal aging of oil-paper insulation should be particularly concerned in actual analysis of factors which influence the thermal aging of transformer oil-paper insulation.The expression of polarization and depolarization current has been inferred based on dielectric response theory. And the oil-paper insulation characters of polarization and depolarization current, insulation resistance and absorption ratio measured at different stage of thermal aging by acid has been researched. The result shows that polarization and depolarization current curves especially the ending part of polarization current and start part of depolarization current could reflect the thermal aging of oil-paper insulation. The catalysis of formic acid on thermal aging is much more obvious than stearic acid. The decay rate of depolarization current accelerates with the increasing of acid value, especially the samples with formic acid. Compared with insulation resistance and absorption ratio, polarization and depolarization current curves could reflect the aging condition of oil-paper insulation more comprehensively.According to the water distribution of oil-paper insulation after thermal aging, the acid value in oil is suggested as the characteristic parameter of aging to describe the influence of aging on water distribution quantitatively in this paper. The results of accelerated thermal aging test on four different experimental samples show that the aging of oil-paper insulation leads to the increase of water saturation in oil and prompts the water transformation from paper to oil. According to the experimental measurement results, we introduced the shifting factor m related to acid value in oil, established the mathematical expression between acid value in oil, temperature and water saturation in paper based on Arrhenius equation, got the mathematical expression between water content in paper, acid value in oil, temperature and water content in paper based on Freundlich adsorption equation, and obtained the fitting relationship between adsorption equilibrium constant c, coefficient n and temperature. The water content of different aging degree papers could be evaluated accurately by using the above research results and it has important engineering application value on actual motion transformers. |
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