| Transformers are the vital equipment of power system, which are mainly composed of oil-paper insulation and moisture and degree of polymerization (DP) will seriously affect the life of the transformer. In this paper, the characteristics of moisture diffusion, dielectric response and its influence factors in oil-paper insulation are studied based on physical modeling, theoretical analysis and experimental validation. The research is beneficial to understanding the moisture diffusion processes and applying the dielectric response method to evaluate moisture content in oil-paper insulation.In order to understand the physical nature of oil-paper insulation dielectric response characteristics, a physical model of oil-paper insulation and a stochastic statistical model were established to explain the dielectric relaxation process. The equivalent relaxation function was introduced relating to random relaxation waiting time and dielectric relaxation rate of oil-paper insulation. On the basis of limit theorems of probability theory and conditional expectation, that physical model of oil-paper insulation obeys joint probability distribution of the levy distribution and generalized sinusoidal was derived. According to probability distributions of random relaxation waiting time and dielectric relaxation rate of oil-paper insulation, the analytical probability density function of the oil-paper insulation relaxation rate can be obtained from HN empirical equation.To quantitatively describe the variation of frequency domain spectroscopy (FDS) of oil-paper insulation according to changes of moisture content, temperature and DP, HN dielectric relaxation model was introduced to research the characteristics of FDS. Analysis formulas of real and imaginary part of HN dielectric relaxation model were solved based on geometry method and complex analysis. FDS of five different moisture content, temperature and DP of oil-paper insulation samples were carried out. HN dielectric relaxation model were applied to fitting curves of FDS. Variations of parameters of HN dielectric relaxation model changing with moisture content, temperature and DP were analyzed. The relationships between relaxation time constant and moisture content, temperature, DP respectively satisfied the log-linear relationship, Arrhenius equation and linear relationship. High-frequency dielectric constant and its difference with static dielectric constant increase with the rising of moisture content, temperature, DP. Shape parameter β increased with rising of moisture content and temperature, while decreased with fall of DP. The range of shape parameter a was0.7-0.9.DP of insulation paper will affect the moisture equilibrium in oil-paper insulation. Moisture equilibrium of insulation paper in air and oil-paper system were tested. Experimental results show that the lower degree of polymerization of insulation paper is, the higher moisture content is adsorbed at same air of relative humidity and the faster migration rate of moisture into insulation paper, but the lower steady-state moisture content in insulation paper in oil-paper insulation. Microscopic characteristics of aging cellulose: extended amorphous area, reduction of degree of polymerization (DP) and increased hydrophilic impurities attached to insulation paper lead to arise of moisture absorption of insulation paper in the air. In oil-paper insulation, pores in amorphous paper and cellulose filled with oil molecules causing reduction of indirect adsorption moisture, hydrophilic impurities dissolved in oil and the lower hydrophilic group caused by decreasing specific surface area reduce the moisture content in insulation paper. Finally, comparison experimental curves with the Oommen-class curves indicates that influence of cellulose aging on moisture equilibrium in oil-paper insulation could be simplified as Oommen-class curves shift in the abscissa direction.The FDS of equilibrium moisture in non-uniform aging oil-paper insulation were researched. On basis of distribution impedance method, a new oil-paper insulation dielectric response model was established. The FDS of some different oil-paper insulation containing the same moisture content and different equilibrium moisture content were tested. The test results shown that the dielectric loss, real part and imaginary part of complex permittivity increased with rising of moisture content in paper or decreasing of DP of paper. Through comparative analysis of results, the impact of moisture content on FDS is much larger than DP. Five different DP of insulation paper with equilibrium moisture content were overlaid and their FDS were tested. According to the distribution impedance model, their FDS were calculated. Except for some small error in the low frequency band, test results and calculations are basically consistent which indicated that the distribution impedance model is valid. |
PDF Full Text Request