Research On Aging Degree And Moisture Content Assessment Methods Of Oil-Paper In Transformer Based On Dielectric Response

Under the stress of heating power, moisture, electric field, oxygen, acid and mechanical force, the insulating performance and mechanical performance of oil paper system will get worse and this depravation is not reversible. So the evaluation of oil-paper insulation’s aging degree become research hotspot in this world. As the product of oil-paper insulation aging, moisture will become participant of aging process, promoting degradation of oil and paper and accelerating aging of insulating material irreversibly. Moisture is the second largest killer which led oil-paper insulation become aged, and its content evaluation is taken seriously by many scholars. The aging oil of transformer can be filtered and replaced to improve insulating property. The paper represents the insulating property of transformer because it can’t be replaced, so to achieve accurate and effective diagnostic of paper aging state is more meaningful.In order to simulate oil-paper insulation system whose aging oil is filtered or replaced, the paper made oiled-paper samples with different DP(degree of polymerization) and moisture content by the means of equivalent aging and natural moisture absorption, the mixed them with new insulating oil. Set oil-paper insulation test platform in time and frequency domain with experimental facilities in laboratory, then take PDC and FDS experiment to the samples. Testing results indicate that:in the test time of PDC, DP exert an effect on the front and middle testing period of polarization current and the whole period of depolarization current; content of moisture exert an effect on the whole testing period of polarization current and the middle and last period of depolarization current. In the test frequency range of FDS, real permittivity and imaginary permittivity increase with the decrease of DP in low frequency stage while they stay unchanged in high frequency stage; the increase of moisture content in oiled paper would increase the real permittivity before 102Hz and imaginary permittivity during the whole testing frequency stage; The imaginary permittivity curve tend to pan camera right with increasing of moisture content. This paper analyze the cause of those results based on the theory of electrolyte polarized microscopic mechanism and dielectric response.Base on the test results of PDC, This paper proposes to use initial depolarization current idp1 and steady polarization current id (conduction current) as mapping feature quantity of DP(degree of polymerization) and moisture content respectively, then fit the relationship formula of them, then present the aging degree and moisture content evaluation method of transformer oiled paper based on PDC. Based on the test results of FDS, This paper proposes to use the "Equivalent decline extent" of imaginary permittivity as mapping feature quantity of DP to evaluate the situation of oiled paper, then fit the relationship formula between moisture content and the real permittivity at frequency of 10-4Hz. At last present the aging degree and moisture content evaluation method of transformer oiled paper based on FDS.This paper prepares oiled paper with different aging degree and moisture content, and verifies the validity of the two kind methods proposed above. In experiment process we find that:the worse the aging condition of oiled paper, the smaller its moisture content. Verification results indicate that:the two evaluation methods in this paper are effective in the range of errors permitted and they will be referable in practice.