Study On Space Charge Characteristics Of Aged Oil-paper Based On Pulse Electro-acoustic Method

Converter transformer is a absolutely substantial position in HVDC system.Oil-paper insulation is the most common insulation medium in converter transformer.At present,domestic and foreign studies agree that the internal space charge of oil-paper insulation strongly distorts the internal field strength and promotes partial discharge in the material.Moreover,under the influence of external factors such as thermal aging and lightning shock,the phenomenon of internal space charge of oil-paper insulation is more obvious,which further reduces the insulation level of oil-paper insulation and give rise to to the protected process of HVDC.Consequently,it is material to ucubrate the space charge distribution of oil-paper insulation in aging period for electric system assurance.In this paper,the principle of measuring space charge by electro-acoustic pulse method is analyzed,and a set of space charge measurement system is designed and made.The system connects Lab VIEW with the upper computer to realize data collection function.In view of the inherent signal overshoot problem of PEA technology,based on the analysis of the cause of signal overshoot,the network function of the system is calculated by deconvolution,and then the measured signal is entered.Row correction.Demonstration indicate that this set of determination system is coherent to manage,preventing puzzling algebraic computation,and can correctly measure the space charge distribution in samples.On the basis of this set of space charge measurement system,this paper studies oil-paper insulation under different thermal aging degree and accumulation of lightning.The parameters of paper aggregation degree,micro-water content in oil and dielectric properties under different treatment conditions are measured.The space charge distribution of the sample is measured under the average field strength of 20 k V/mm,and the corresponding total space charge variation is analyzed.The decay characteristics and decay time constants of the total space charge are calculated,and the trap energy levels of each sample are calculated.The space charge characteristics and trap energy levels of oil-paper insulation under different treatment conditions were analyzed.The test results show that the injection and attenuation of oil-paper insulation are fast and stable,and the attenuation of space charge can be approximated as exponential attenuation.At the beginning of thermal aging,the degree of aggregation of oil-paper insulation does not change much,and the space charge is easily injected due to the increase of internal moisture content.With the enhancement of thermal aging duration,the entirety interior space charge advances swiftly at first and then grows into unchangeable moderately.The falsification of the inner electric field elevates with the development of thermal aging.However,under the cumulative action of lightning shocks,the oil-paper insulation still has a high insulation level in the first 200 lightning shocks,and has a strong inhibition on space charge.After 300 lightning shocks,the inhibition level of oil-paper insulation on space charge decreases rapidly.On the one hand,under the cumulative action of lightning strikes,the insulation paper is damaged,and the degree of aggregation and roughness are changed.On the other hand,the effect of lightning voltage accumulation on polar substances is also discussed.With the increasing number of lightning shocks,the number of shallow traps in oil-paper insulation increases continuously,and then the trap energy level increases continuously,which generates deep traps,which poses a huge threat to the insulation level of oil-paper insulation.Generally speaking,for the running converter transformer,both temperature and lightning shock can promote the interjection of homopolar space charge,while lightning has an extremer impact on the distribution of space charge.The electric damage generated by lightning impulse is accumulating,which encourage the procedure of charge insertion,and distorts field strength.It also further magnifies the deterioration of the insulation level,thus causing a vicious circle.