Research On Aging Characteristics Of Power Transformer Oil-Paper Insulation Based On Frequency-Domain Dielectric Spectroscopy

Oil-paper insulation is the main part of the internal insulation of oil immersible power transformer. Its insulation performance determines the operating life of the transformer. The oil paper insulation is easy to degrade under the thermal, electrical, mechanical stresses during transformer operation process, which leads to the degree of polymerization (DP) of insulating paper drop. Meanwhile, the producing water and acid can lead to the copper components corroding and producing copper-based products. Both the declining DP and the existence of copper-based products make a difference to the frequency domain dielectric spectrum of oil-paper insulation. The research on the correlated effects has an important reference value for using frequency domain dielectric spectroscopy to assess the aging statement of oil-paper insulation accurately.The platform for accelerating thermal aging and frequency domain dielectric spectroscopy testing of oil-paper insulation is set up. The producing process and diffusion rule of copper-based products and its impact on the breakdown voltage of the power frequency of oil-paper insulation are studied. The results show that the content of the copper-based conducts in oil-paper insulation will increase with the increasing of oxygen content and aging temperature during the testing process. The producing copper-based conducts diffuses from the side near to the side away of copper. The presence of copper-based product will cause the breakdown voltage of power frequency of oil-paper insulation reduces. The closer of the oil-paper insulation to copper bar the lower of the breakdown voltage.Experiments are conducted for analyzing the accelerating aging function of moisture and acid to oil-paper insulation and the diffusion mechanism of them during the aging process of oil-paper insulation. The experimental results show that when the moisture and acids coexist in oil-paper insulation, the aging acceleration is much stronger than they alone exist. Different molecular weight acids have different equilibrium distribution between oil and paper.At the same time, the influence of copper-based products and DP of insulation paper to the frequency domain dielectric spectrum of oil-paper insulation is tested and analyzed. The test results show that the presence of copper-based products has little effect on the real part of the complex permittivity of oil-paper insulation, but the greater impact on the imaginary part and the dielectric loss, especially in the middle frequency band. The frequency domain dielectric spectrum of oil-paper insulation can be better fitted by double relaxation Cole-Cole model without the presence of copper-based products. When a certain amount of copper-based products exist, the relaxation mechanisms in the oil-paper insulation may change, so that the original relaxation center separates. Thus leads to the frequency domain dielectric spectrum needing to be better fitted by triple relaxation Cole-Cole model. The declining DP has an impact on the complex permittivity of oil-paper insulation according to impacting the distribution of aging products (moisture and small molecular weight acids, etc) between oil and paper. When the frequency domain dielectric spectrum method is used to diagnose the content of moisture or acids, the above impact should be considered.