The Dielectric Loss Characteristics And The Deterioration Mechanism Of Oil-paper Insulation For UHV Transformer

In recent years,because UHV transmission can achieve long-distance,large-scale,efficient power transmission,and increase the security of power grid operation and other advantages,UHV transmission has developed rapidly.UHV transformer is one of the key equipments in UHV transmission,which plays an important role in transforming voltage.For UHV transformers,the m ost important factor determining their reliability and life is the reliability and life of insulation materials.Oil-paper insulation of UHV transformer bears high voltage and high temperature,and its operating conditions are complex and harsh.Long-term operation requires high electrical and thermal performance of oil-paper insulation of UHV transformer.In this paper,oil-paper insulation of UHV transformer is taken as the research object.Oil-immersed insulation pressboard with different aging degree is prepared by accelerating thermal aging.Dielectric frequency spectrum and partial discharge characteristics of samples with different aging degree are tested and analyzed.The degradation mechanism of insulated pressboard is analyzed by surface micro-observation and Fourier infrared spectroscopy.The development process of partial discharge under test electrodes is simulated and analyzed by over-electric field simulation.The specific research results are as follows:By measuring the dielectric spectra in frequency domain of oil-immersed insulated pressboard with different aging degree,it is found that the real and imaginary parts of complex dielectric constant and dielectric loss factor increase with the aging degree of insulated pressboard in low frequency band.The cubic spline interpolation is used to calculate the dielectric loss factor curve.The area under the curve is taken as the characteristic quantity,and the relationship between the characteristic quantity and aging time is studied.The results show that the area under the curve of dielectric loss factor varying with frequency in characteristic frequency band is linearly related to aging time.Through the partial discharge test of oil-immersed insulated pressboard with different aging degree,the partial discharge characteristics of oil-immersed insulated pressboard with different aging degree under different voltage were obtained.The results show that for oil-immersed insulated pressboard with the same aging degree,the partial discharge increases with the increase of voltage.Under the same voltage,the higher the aging degree,the larger the average discharge and maximum discharge,the more the discharge times,and the wider the discharge phase in the voltage cyc le.And the higher the voltage,the greater the difference of discharge characteristics between different aging degree pressboard.Scanning electron microscopy(SEM)was used to observe the surface micro-morphology of pressboard with different aging degree.It was found that the aging process made the fibers on the surface of insulated pressboard shrink and break,increased the gap between fibers on the surface of pressboard,and reduced the insulation performance of insulated pressboard.By using Fourier transform infrared spectroscopy(FTIR)to test the pressboard,it was found that the aging process reduced the content of glycoside bonds and hydroxyl groups in the pressboard cellulose,and ultimately led to the decrease of insulation performance.Through the electric field simulation of the experimental model,it is found that the electric field distribution of the experimental model will be seriously affected by the bubbles generated during the aging process,the development of partial discharge and the cuprous sulfide produced during the aging process,which will easily lead to the further deterioration of partial discharge and pressboard.Through the above analysis of the characteristics of oil-immersed insulated pressboard with different aging degrees,the process and mechanism of degradation of oil-immersed insulated pressboard were understood.Through the analysis of dielectric spectrum of different aging degrees,the method of characterizing the aging degree of pressboard by dielectric frequency domain spectrum was proposed.