Research On Spatiotemporal Distribution Characteristics Of Electric Field In Oil-Pressboard Insulation Structure Under Temperature Gradient

The reliable insulation design of converter transformers is of great significance to ensure the safe operation of HVDC transmission systems.There is a temperature gradient inside the converter transformer when it actually operates.At present,the research on the electric field and interfacial charge characteristics in oil of oilpressboard insulating structures under temperature gradients is limited to the small temperature difference model or separately for oil-impregnated pressboard or transformer oil,and lacks the study of electric field characteristics and interfacial charge characteristics in oil under large temperature difference conditions.Based on the Kerr effect,a temperature gradient controllable oil-pressboard insulating structure oil space electric field measurement platform is constructed,and a hollow electrode and external temperature gradient oil circulation system suitable for temperature gradient conditions are designed,with a temperature control range of 10℃～90℃,a temperature control accuracy of ±1℃,and an electric field measurement sensitivity of 2.91%.With the increase of the applied voltage,the amplitude of electric field distortion in the oil,covering the upper electrode of the pressboard,decreased from 326.70%at+5kV to 227.81%at+25kV in the temperature gradient range of 20℃～80℃.The distortion amplitude in the electric field oil under the pressboard covered electrode model decreased from 276.00%at+5kV to 168.46%at+25kV.The amplitude of electric field distortion in the oil of the two oil channel models increased from 95.00%at+5kV to 388.59%at+25kV.At+25kV,when the temperature range changes from 20℃～40℃ to 20℃～80℃ and then to 60℃～80℃,the amplitude of electric field distortion in oil first increases and then decreases.At 20℃～80℃ the temperature difference between the upper and lower electrodes is the largest,the volume resistivity of the pressboard is most affected by temperature,the voltage division ratio between the high temperature area and the low temperature area reaches the largest,and the electric field distortion amplitude is the highest.The temperature of the lower electrode does not change,and as the temperature of the upper electrode increases,the amount of positive charge injected by Schottky from the upper electrode gradually increases,and the polarity of the equivalent charge injected by Schottky gradually becomes positive polarity.The net charge density of the interface of the pressboard covered electrode model is less than the RC polarized charge density,and the measured value of the electric field in oil is gradually greater than the calculated value of RC.The net charge density of the electrode model under the pressboard overlay is greater than the RC polarized charge density,and the measured value of the electric field in oil is always less than the calculated value of RC.The positive charge density of Schottky injection equivalent charge in the two-channel model of the valve side outlet line increases,the equivalent polarity of the pressboard is positive polarity,and the electric field strength in the lower oil channel oil gradually exceeds that of the upper oil passage.The temperature of the upper electrode does not change,and as the temperature of the lower electrode increases,the amount of negative charge injected by Schottky from the lower electrode gradually increases,and the polarity of the equivalent charge injected by Schottky gradually becomes negative.The relationship between the measured charge density and the interface polarized charge density of different models is opposite to that when the upper limit temperature increases.