| The valve-side insulation of the converter transformer is subject to various voltages such as AC,DC,and polarity reversal,with complex and concentrated electric field stress and great difficulty in development.If insulation failure occurs during operation,it may cause the winding to suffer from short-circuit impact and even trigger combustion and explosion accidents.Insulation damage frequently occurs in the factory DC tests of the ultra-high voltage valve-side outgoing device,indicating insufficient research on the oil-paper insulation space charge problem,unclear charge distribution laws in the actual valve-side outgoing structure,and existing studies cannot deeply and effectively explain the mechanistic reasons.Currently,designers generally use the RC(Resistivity-Capacitance)model,which provides an ideal symmetrical distribution of oil-paper interface charges,unable to reflect the actual non-symmetric charge distribution and non-linear phenomena obtained by measurements.Currently,related scholars have carried out a large number of fundamental studies using the Kerr method,but the research objects are all simplified flat plate models or small-sized coaxial models,and the research results only remain at the level of charge characteristics and mechanistic explanation,unable to be used for the analysis and calculation of the electric field of the ultra-high voltage converter valve-side outgoing device.In response to the above problems,this paper conducted the following research.In order to better consider the scale effect and be closer to the actual structure,a large-size coaxial structure outgoing device model(1.6:1)and a spatial electric field measurement system were established according to the equivalent design of the±800kV valve-side outgoing de vice.The multi-gap structure of the outgoing device model consists of 5 coaxial circular pressboard cylinders and 5 oil gaps from inside to outside,with a diameter of 0.8m and an inner height of 1.2m.The oil gap electric field distribution is basically consistent with the ±800kV actual valve-side outgoing device.The measurement accuracy of the model was verified by using laser telemetry,and the deviation between the measured and simulated electric field values of the 5 oil gaps was within 3%,meeting the experimental requirements.Through laser telemetry of the two-gap flat pressboard-oil insulation model under high electric field,the "oil gap effect"phenomenon of charge distribution was discovered。In the two-gap model,when the voltage was ± 4kV,the charge distribution satisfied the polarity effect,that is,the electric field in the positive polarity side oil was always higher than that in the negative polarity side.The positive charge density of the oil-pressboard interface near the high-voltage electrode side is less than the negative charge density of the oil-pressboard interface at the grounding side.However,as the applied voltage increased,the electric field distribution in different oil gaps no longer satisfied the polarity effect.That is,as the upper oil gap spacing increased and the lower oil gap spacing decreased,the charge density near the oilpressboard interface on the high-voltage electrode side gradually increased,surpassing the effect of the "polarity effect" and equivalent charge density changed from65.81μC/m2 to 37.86μC/m2 at+25kV,while the total oil gap spacing remained unchanged at 7mm,and the upper oil gap spacing increased from 1mm to 5mm.The study believes that the high electric field in the oil intensifies the field-induced ionization phenomenon.The greater the distance between the pressboard and the electrode,the more the charge of the field-induced ionization on the electrode side pressboard,which leads to an accumulation of oil-pressboard interface charge on the wide oil gap side that is enough to exceed the effect of the "polarity effect".The distribution law of the spatial electric field intensity in the multi-gap model oil was revealed,and a quantitative analysis method for oil-pressboard interface charge of the flat electrode multi-gap pressboard-oil insulation structure was proposed based on the field-induced ionization effect under high electric field.By carrying out actual measurements on a 1.6:1 model electric field of a realtype valve side lead-out device,the distribution law of charges of the coaxial multi-oil gap oil-pressboard insulation structure and its effect on the electric field in oil were revealed.The model consists of#1,#2,#3,#4,and#5 pressboards and#1,#2,#3,#4,and#5 oil gaps,with oil gap sizes of 10,10,15,20,and 25mm,respectively,distributed coaxially and concentrically from the high-voltage electrode to ground.Through the actual measurement of the electric field in oil,it was found that regardless of polarity,as the pressurization time increases,the spatial electric field intensity of the#1,#4,and#5 oil gaps gradually decrease,but that of the#2 and#3 oil gaps gradually increases.When reaching steady state,compared with the RC model calculation value,the#2 oil gap has the highest distortion rate of 322.4%,while the distortion rate of the#3,#4,and#5 oil gaps gradually decreases,with the#5 oil gap having the lowest distortion rate of 9.2%.By defining the sum of opposite polarity charges between the pressboard and the adjacent two oil gaps as the equivalent charge density,the radial distribution characteristics of interface charges at the coaxial multi channels was discovered.The interface equivalent charge density of the#1 and#4 pressboards is positive,which weakens the electric field intensity in oil,while that of the#2 and#3 pressboards is negative,which enhances the spatial electric field intensity in oil.The#5 pressboard only has one interface that accumulates positive charges,which slightly weakens the electric field in oil.The study shows that this phenomenon is consistent with the phenomenon of increased equivalent charge imbalance between adjacent oil gaps and increased distortion of the electric field in the two-gap model under the "oil gap spacing effect".At higher external applied voltages,the electric field in the oil exceeded the electric field threshold of the "oil gap spacing effect",and the ratio of the#2 and#3 oil gap spacing was 1:1.5.At this time,due to the larger spacing of the#3 oil gap,the field-induced ionization effect resulted in the generation of more charges,which caused severe distortion of the electric field in the#2 and#3 oil gaps.Based on the field-induced ionization effect under high electric fields,a quantified analysis method for the interface charges along the axial direction of the coaxial oil-pressboard insulation structure was proposed,and the accumulation and distribution characteristics of oil-pressboard interface charges under different influencing factors were obtained.Based on the experimental results of multi-oil gap structure,a DC electric field calculation method for the valve-side outlet device of converter transformer considering interface charge is proposed.The actual charge distribution is applied to the oil-pressboard interface in a segmented linear way,which transforms the nonlinear continuous problem into a segmented linear problem and solves the problem that the existing RC model cannot accurately reflect the value of interface charge.The pressboard overlap structure model is used for verification,and the maximum error between the calculated value and the measured value is 4.3%.For the complex structure of the actual valve-side outlet device of converter,the one-dimensional ion flow model is first used to calculate the charge distribution along the radial oil gap and assign values.Secondly,based on the Lorentz function fitting of the nonlinear distribution of interface charge edge and axial assignment,the overall electric field distribution is synthesized by calculating the interface charge electric field and superimposing with the externally applied Laplace field.This method is compared with the measured results of the coaxial multi-oil gap structure model,and the maximum deviation is less than 3%,reflecting the problem that the uneven distribution of charge in the multi-oil gap structure leads to serious distortion of local electric field.Using the method proposed in this pressboard,it is found that the electric field near the equalizing ring of the valve-side outlet device of the ±800kV converter transformer produces significant fluctuations,and the field strength of the second oil gap increases significantly,reaching 6.1 kV/mm,which is 23.1%of the electric field in the pressboard.Based on this,this pressboard proposes suggestions for the insulation design process of ultra-high voltage converter valve-side,which should not only focus on the allowable electric field strength in the pressboard,but also pay attention to whether the electric field of the second oil gap is within the allowable range. |
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