Repeated Polarization Current Characteristics And Evaluation Method For Aging State Of Oil-paper Composite Insulation

Oil-immersed transformers undertake an important task of transmitting and converting electrical energy in the power system.Insulation deterioration is the main cause of their failures.Therefore,a reliable assessment of the aging state of the oilpaper composite insulation in the oil-immersed transformer is of great significance to the safe operation of the power system.The aging evaluation method of oil-paper composite insulation based on the dielectric response test technology has received a lot of attention due to its non-destructiveness,and has made great progress in quantitative evaluation.However,the aging characteristics obtained by the current research have not been widely promoted and applied,so it is necessary to persisting in finding more reliable aging assessment methods and characteristic parameters of oil-paper composite insulation.In this paper,a test platform was built,and paperboard samples with different degrees of aging were prepared.The polarization depolarization test was carried out on the aged paperboard oil-paper composite insulation,and the influence of the paperboard aging degree and the composite insulation structure on the polarization characteristics was explored.Based on the research on the polarization characteristics of oil-paper insulation aging,a repeated polarization test method was proposed,and the test was carried out on the above samples.The relationship between the repeated polarization currents of oil-paper insulation and the paperboard aging state was studied,and the repeated polarization currents under different structures and different applied electric field strengths are analyzed.In addition,the aging characteristics of oil-paper insulation are extracted with the help of the extended Debye model.The research results show that during the polarization depolarization current test,the polarization current and the depolarization current of the aged paperboard oilpaper composite insulation are asymmetrical,and the depolarization process is slower than the polarization process.As the degree of aging of the paperboard continues to increase,the asymmetry gradually weakens.And the asymmetry is affected by the position of the paperboard,which is more obvious when the paperboard is on the side of the high-voltage electrode.During the repeated polarization current test,the attenuation rate of the middle and front section of the second polarization current of oil-paper insulation is faster than that of the first one.This attenuation difference shows a trend of first increasing and then decreasing with the increase of test time,and there is a maximum value.And the maximum difference decreases with the increase of the paperboard aging degree.The difference of the two polarization currents under different paperboard positions and different applied electric field strengths has a similar changing law.The three-branch Debye model can describe the polarization current of the repeated polarization test well,and the difference in the maximum time constant of the two polarization currents corresponding to the Debye model can quantify the attenuation difference.The peak value of the maximum time constant difference is used as a characteristic quantity to characterize the aging degree of the paperboard,which changes exponentially with the degree of paperboard polymerization,and when the peak value is less than 2.5,the aging of the paperboard reaches the end.The repeated polarization test method proposed in this paper can distinguish the aging state of the paperboard in the oilpaper composite insulation through the difference in the middle and front section of the two polarization currents,and provides a new research method for the aging evaluation of the oil-paper composite insulation.