| The internal insulation of a transformer is mainly oil-paper insulation of normal insulating paper and mineral oil. Under combined stress of temperature, moisture and oxygen, the oil-paper insulation may be subject to aging, which is one of the leading reasons of transformer faults. Of all the factors, temperature plays a dominant role; however, the moisture also has significant influence on the lifetime and operation conditions of the oil-paper insulation. With increasing moisture content, the electric performance of oil-paper insulation declines rapidly and the aging rate is also accelerated, hence the lifetime reduces. Therefore, moisture is considered as the‘number one enemy'except temperature to oil-paper insulation. But, the combined effect of temperature and moisture to oil-paper insulation degradation is lack of systematically study at home and abroad, and the model of moisture's accelerating effect on oil-paper insulation aging has never been reported. It is very important to make aging condition assessments, fault diagnoses, and life predictions for transformer insulation by studying the variation tendency and the impact on the aging rate and characteristics of moisture during thermal aging process, and then building a model of the function of water in accelerating aging.This paper designed an accelerated aging test, in which the insulating papers with the initial moisture contents of 1%, 3% and 5% were aged in mineral oil at 90?, 110?and 130?respectively. Firstly, this paper studied the variation trends of moisture content in insulating paper and insulating oil during the aging process, and analyzed how moisture influences the insulating paper's degradation rate, furfural content in oil and oil acidity. Secondly, by utilizing the molecular dynamics simulation technique, the interactions between water and cellulose and oil have been studied. Then, the equilibrium distribution of wter in oil-paper system and how water accelerates the degradation of paper were also involved. In addition, on the basis of cumulative dynamic model of DP loss, we explored the variation law of insulating paper's DP under the combined effects of temperature and moisture; combined with the time-temperature superposition theory and degree of polymerization damage accumulation model, an oil-paper insulation lifetime prediction model with the synergistic effect of temperature and water content was established. Finally, the effects of water and aging on the dielectric characteristic in frequency domain were explored, and the diagnosis water content and aging condition based on frequency dielectric features were aslo studied. The main obtained innovations of the paper are as follows:(1) The influence law of various initial moisture content on characteristic parameters of thermal aging of cellulose insulation paper and mineral oil. With the increasing of water content, the degradation of insulation paper were accelerated remarkably under low temperature (90?). While the temperature is comparatively higher (110?and 130?), the accelartion to the degradation of insulation paper with the increase of water content will become saturation.(2) By utilizing molecular dynamics simulation technology, the interaction between water and oil-paper insulation were analyzed. Various results have been obtained. Firstly, the interaction energies between water and cellulose are far greater than which between water and oil, and stable hydrogen bond were established between water cellulose. It causes that a large portion of water are bounded in cellulose. In addition, with the increasing of temperature, the interaction energies between water and cellulose gradually decreased which lead to decrease of the bounded effect of cellulose to water. As a result, the acceleration effect of water to the degradation of cellulose gradually decreases while the temperature is higher. The simulation results explained the experiment phenomena that the accelartion to the degradation of insulation paper with the increase of water content will become saturation while the temperature is comparatively higher (110?and 130?).(3) Introduced with time-temperature shift factor, the oil-paper apparent activation energy by the synergistic effect of temperature and moisture is calculated, which value is (101±15) kJ/mol. Oil-paper insulation life model equation in transformer is improved based on the time-temperature superposition theory and the degree of polymerization damage accumulation, and this model can predict the oil-paper insulation life under the two factors of water content and temperature.(4) The frequency domain dielectric spectroscopy features (relative permittivity?r and dielectric loss tan?) for the diagnosis of oil-paper insulation water content and aging condition is presented. It has been discovered, between 10-3 Hz to 10-1Hz,?r of insulating paper and tan?are linearly and exponential with water content, respectively, i.e.?r=a×m+b and tan?=a×exp(m/c)+b; from 10-3Hz to 100Hz range, relative permittivity?r and the polymerization degree difference (DPO-DPt) is relatively good in accordance with quadratic function:?r=a×e-b×(DPO–DPt)2+c×(DPO-DPt)+d. |
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