| Converter transformer is one of the irreplaceable core equipment in HVDC transmission system,and the reliability of its insulation performance is crucial to the safe operation of the HVDC.At present,the resistance-capacitance model is mainly used for the insulation design and verification of converter transformers,which only considers the polarization charge.Existing studies have shown that in addition to the polarization charges,there are Schottky injected charges at the oil-pressboard interface under high temperature.The existing electric field analysis models for high temperature are mostly based on the results of μm-thick paper.The thickness of the pressboard actually used in converter transformers is mm.The materials,manufacturing process,dielectric constant,volume resistivity and positive/negative charge mobility of paper and pressboard are all different,resulting in different changes of the field strength and charge characteristics of paper and pressboard with temperature.Therefore,it is difficult to use the previous results to calculate the electric field of pressboard directly.In order to solver this problem,this paper the actual measurement of the electric field distribution characteristics in the oil-pressboard insulation at different temperatures and temperature gradients under DC voltage,proposed a calculation model of Schottky injected charge for oil-pressboard insulation,and clarified the effect and reason of size on the electric field.In order to accurately measure the electric field in the oil-pressboard at different temperatures under DC voltage,build a temperature-controlled measurement platform is build for the spatial electric field distribution of the oil-pressboard insulation structure under DC voltage based on the Kerr effect.The controllable temperature range is 20～100℃,the temperature control accuracy is ± 1℃,and the maximum difference of the transformer oil temperature between before and after the 2h test is 0.7℃.The electric field measurement accuracy is 0.1 V/mm,the measurement error is 5%,the influence of temperature on the electric field measurement is 0.1%,and the maximum measurable pressboard area is 324cm2.Three typical oil-pressboard insulation models are extracted according to the valve side outlet structure of the converter transformer.They are the the valve side outlet single oil channel model:the insulating pressboard is attached to the upper/lower electrode,and the valve side outlet double oil channel model:the insulating pressboard evenly divides the oil,the valve side outlet three oil channels model:two insulating pressboards evenly divide three oil channels.Through the actual measurement on the space electric field/interface charge characteristics of the valve side outlet single oil channel model at different temperatures,it is found that with the increase of temperature,the electric field strength in the oil increases,and the interface charge density of the oil-pressboard decreases,the difference in the polarized charge and the the measured interface charge density increases,the polarity of the measured interface charge is opposite to that of the polarized charge at 80℃.In order to analyze the influence of temperature on the interface charge,an equivalent charge analysis model based on the Schottky injection mechanism for pressboard is proposed,the contribution of the Schottky effect to the interface charge is characterized by the equivalently charge injected into the pressboard.The law of the temperature influence on the interface charge is revealed by the analysis model,the interface charge is mainly polarization charge,the Schottky injection charge accounts for 0.95%at 20℃;the interface charge is mainly Schottky injection charge,the Schottky injection charge accounts for 60.11%at 80℃,the equivalent charge density of Schottky injection is increased by a factor of 1713.4.In order to study the effect of materials on the interface charge,the change law of the interface charge of the valve side outlet single oil channel PTFE model with temperature is measured,and it is found that the temperature change had little effect on the interface charge.At 80℃,the Schottky injection accounts for 1.16%,and the interface charge of PTFE is still dominated by polarized charge.The main reason is that the volume resistivity of PTFE changes less with temperature,the injection barrier of PTFE is 1.17 times that of pressboard,and the Schottky injection charge is 2%of the oil-pressboard insulation model.Through the actual measurement on the space electric field/interface charge characteristics of the the valve side outlet two oil passages model at different temperatures,it is found that the field strength of the upper oil passage is stronger than that of the lower oil passage at+25kV 20℃,and the field strength of the lower oil passage is greater than that of the upper oil passage at 60 and 80℃.With the increase of temperature,the interface charge density of oil-pressboard decreases,and the difference between the interface polarized charge density and the measured charge density increases.The polarity of the measured interface equivalent charge at 60 and 80℃ is opposite to that of the equivalent charge at 20℃.The law of the temperature influence on the interface charge is revealed by the analysis model,the interface charge is mainly polarization charge,the Schottky injection charge accounts for 0.10%at 20℃;the interface charge is mainly Schottky injection charge,the Schottky injection charge accounts for 60.00%at 80℃,the equivalent charge density of Schottky injection is increased by a factor of 100.7.Through the actual measurement on the space electric field/interface charge characteristics of the the valve side outlet three oil passages model at different temperatures,it is found that the field strength of the upper oil passage is stronger than that of the middle and lower oil passage at+25kV 20℃,and the field strength of the middle oil passage is greater than that of the upper and lower oil passage at 80℃.With the increase of temperature,the interface charge density of oil-pressboard decreases,and the difference between the interface polarized charge density and the measured charge density increases.the measured interface equivalent charge polarity of the upper pressboard is negative,while the lower pressboard shows no polarity at 20℃.the measured interface equivalent charge of the upper pressboard is positive,and the lower pressboard is negative at 80℃.In order to analyze the influence of temperature on the interface charge accurately,an equivalent charge of pressboard analysis model based on Schottky injected charge trapping and de-trapping is proposed,the contribution of the Schottky effect to the interface charge is characterized by the charge trapped and de-trapped in the different position pressboard.The law of the temperature influence on the interface charge is revealed by the analysis model,the interface charge is mainly polarization charge,the Schottky injection charge accounts for 0.02%at 20℃;the Schottky injection charge accounts for 48.10%at 80℃,the equivalent charge density of Schottky injection is increased by a factor of 9.6.The difference in the number of Schottky injected holes and electrons trapped and de-trapped of the upper and lower pressboards leads to the equivalent charge opposite polarities of upper and lower pressboards.Through the actual measurement on the space electric field/interface charge characteristics of the the valve side outlet two oil passages model at temperature gradient,it is found that the field strength of the upper oil channel is stronger than that of the lower oil channel under the temperature gardient,which is opposite to the field strength distribution when the temperature is uniform,the polarity of the measured interface charge is opposite to that of the polarized charge.The equivalent charge of Schottky injection accounts for 76.5%and 85.4%at △T=20℃(40～60℃)and △T=20℃(60～80℃).T he current density of electron injection is higher than that of holes,the amount of electrons injected into the pressboard is more than that of holes,and the mobility of electrons is slower relative to the uniform temperature under temperature gradient,which causes the pressboard equivalent charge polarity to be opposite to the uniform temperature.Through the actual measurement on the space electric field/interface charge characteristics of the the valve side outlet two oil passages model under 1 0/100/180mm,it is found that with the increase of scale,the electric field strength in the oil increases.The calculated density of the polarized charge and the Schottky injected charge are the same at different scales,but when different scales are considered,the amount of Schottky injected into the pressboard is different.The larger the size of the pressboard,the greater the amount of Schottky injects charge.This is the main reason that the larger the scale of the valveside outlet model,the greater the electric field strength influenced by the Schottky injects charge in the oil is. |
PDF Full Text Request