Research On The Generation Mechanism Of Interface Charge And The Impacting Factors In Oil-pressboard Insulation Structure Under DC Voltage

The converter transformer plays an important role in the High Voltage DC transmission system,and the safety and reliability of its insulation performance are crucial to the stable operation of the DC transmission system.At present,the insulation design and verification of converter transformers mainly use the RC(Resistivity-capacitance)model based on the equivalent migration process of positive and negative charges,and the calculation process of which only considers the interface polarization charge of the oil-pressboard insulation structure under DC voltage.As for the electric field and space/interface charge characteristics in oil-pressboard insulation under DC voltage,relevant academic research results have clarified the failure of the application of RC model,namely except the interface polarization charges,there are other forms of electric charge,and which have the features of non-equivalent migration and accumulation,obvious polarity effect.Based on which,some kinds of analysis models were put forward such as the Ion Drift model and the Biploar Carrier Injection model.However,these two electric field models have very different mechanisms for space/interface charge generation and lack of effective experimental verification.Moreover,in the analysis of these models,the influence of the nonlinearity and anisotropy of insulation medium and the effect of structure scale in the actual large-size insulation structure of converter transformer are ignored,which makes the accuracy of the analysis model be very limited.In order to accurately measure the space-time electric field in the oil-pressboard insulation structure,based on the Kerr electro-optical effect,this paper developed a high-precision 32-channel photodetector array for space electric field measurement of the oil-pressboard insulation structure in the world for the first time.The two-dimensional synchronous measurement of the spatial and temporal distribution characteristics of the space electric field,and the performance of the platform was tested with a plate electrode in pure transformer oil.The results show that the spatial resolution of the measurement is 1.4mm2,the sensitivity of the electric field measurement is 0.145kV/mm,and the measurement accuracy is greater than 96.50%.Through the actual measurement and theoretical calculation of the electric field/interface charge in oil of different oil-pressboard/paper structures under DC voltage with plate electrodes,the generation mechanism of oil-pressboard interface charge was explored and confirmed.Based on the resistivity difference between transformer pressboard and electrical insulating paper,this paper constructed the test models,which are in the combination of single-layer transformer oil gap and different types of pressboard/paper composite insulation structures,to obtain the wide value interval(0.03 kV/mm?5.01kV/mm)of electric field strength in oil under the action of applied voltage gradient(5kV?25kV).The test results show that for the composite electric field excited by positive polarity interface charge,when the field strength of oil is low(defined as "interface charge deviation field strength",it varies within different models but has the minimum of about 1.74 kV/mm),the measured calculated value of interface charge density is approximately consistent with the interface polarized charge density calculated by RC model.While for the composite electric field excited by negative interface charge,the interface charge deviation field strength has the minimum of about 1.40 kV/mm.Under the condition of external voltage applied corresponding to the "interface charge deviation field strength",in the compound structure of electrode coated with symmetrical insulating paper,when the oil field strength was higher(>1.71 kV/mm),and the deviation between the measured value of total interface charge density and the calculated value of polarized charge density was up to 1.43 times;However,in the composite structure of insulating paper and insulating paperboard covering electrodes,with the introduction of high-density electrical paperboard(high-resistivity medium),the oil field is strongly decreased,and the measured value of the interface total charge density is approximately consistent with the calculated value of polarized charge.Based on the theoretical calculation results of different types of typical interfacial charge generation mechanisms(interface polarization,field-induced separation and bipolar carrier injection)and the actual measurements,it is proved that in the actual oil-paper composite insulation structure of converter transformer,the interface charge is mainly comprised of interface polarization charges and field-induced separating charges.The simulation results indicated that,with the increase of applied voltage,the proportion of polarization charge density in total interface charges decreased from 66.30%to 26.05%,while for the field-induced separation charges,it rises from 25.95%to 69.34%.As for the bipolar carrier injection,the proportion keeps relatively constant.from 4.61%to 7.75%.The above research results proved that,under the actual operation conditions,in the case of lower electric field strength in oil,the interface charge are main interface polarization charges,while under the condition of relative higher electric field in oil,the field-induced separation charges will dominate.By changing the key dielectric parameters of the transformer pressboard with temperature and moisture,the mathematical expressions of the nonlinear characteristic of the pressboard were obtained.Through the measurement for the space-time electric field distribution in oil of oil-pressboard/paper insulation structure,the characteristics of uneven distribution of space electric field reveals that the impacting mechanism of the anisotropy of insulating materials on the electric field characteristics.In the plate electrode,insulation paper-insulating pressboard composite insulation structure,the intensity of space electric field in different position in transformer oil has a maximum deviation of 23.4%,and the non-uniform coefficient could be up to 1.19;With the increase of applied voltage,the non-uniformity coefficient of oil field strength increases gradually,and the maximum increase can reach 6.25%.Based on volume resistivity dispersion test for samples in the different location of the large pieces of pressboard,this paper reveals that the anisotropy of medium parameters is one of the root causes of the nonuniformity of electric field distribution characteristics.Based on the nonlinear characteristic and anisotropy characteristic of the resistivity of pressboard,this paper revises and optimizes the key parameters of the space electric field calculation model.Through the experimental study of the spatial electric field/interfacial charge characteristics of oil paper insulation structure at three scales of 50/100/150mm,the influence rule of the scale effect on the spatial electric field characteristics was found.The research results show that with the increase of the model scale,the deviation between the measured value of the space electric field in oil and the calculated value of the RC model gradually increases,and the maximum increase can reach 30.8%.The root cause lies in that,with the increase of model size,the total interface charge increases gradually,which has a stronger influence on the spatial electric field in oil.Besides,the large-scale insulation structure magnifies the anisotropy of the pressboard,thereby aggravating the distortion of the interface charge on the space electric field effectBased on the generation mechanism of interfacial charge,combined with the polarity effect,nonlinear and anisotropy of pressboard and the scale effect,the generation and accumulation model of interfacial polarization charge and field-induced separation charge and the space electric field calculation model are optimized and modified.In this paper,the space electric field calculation model of oil-paper insulation structure based on interface charge behavior characteristics under DC electric field is revised.For the equivalent model of the valve side outlet device of the converter transformer with mesoscale structure size,the measured and calculated comparison of the space electric field strength in oil is obtained,the maximum deviation between the measured result and the theoretical calculation result is 4.41%.