Study On Dielectric And Breakdown Characteristics Of Oil-paper Insulation Under AC-DC Combined Voltage At Low Temperature

The converter transformer is one of the most important equipment in high-voltage direct current(HVDC)power transmission projects,the insulation near the valve side winding works under AC-DC combined voltage during operation.The electric field in the oil-paper insulation is complex under AC-DC combined voltage,and the space charge accumulation inside the oil-impregnated pressboard is serious.A lot of researches have been carried out on the insulation of converter transformers,which mainly focus on the room temperature and high temperature.As HVDC transmission projects are being built in the cold regions of northern China,converter transformers will face low temperature challenges.However,the dielectric and breakdown characteristics of the oil-paper insulation in the converter transformer at low temperature is not clear,and it is impossible to provide a theoretical basis for the insulation design and safe operation of the converter transformer.Therefore,it is necessary to study the dielectric and breakdown characteristics of the oil-paper insulation at low temperature.To study the effect of moisture on the dielectric characteristic of oil-paper insulation at low temperature,the conductivity,relative permittivity and dielectric loss factor of transformer oil and oil-impregnated paper at different temperatures and moisture were tested in the laboratory.The test results show that the conductivities of transformer oil and oil-impregnated paper decrease with the decrease in the temperature.Moisture has little effect on the relative permittivity and dielectric loss factor of transformer oil.However,the moisture has a significant effect on the relative permittivity and dielectric loss factor of oil-impregnated paper,and the decrease in temperature will reduce the effect of moisture on both.To study the effect of temperature and moisture on the space charge characteristics of oil-impregnated paper,a high-low temperature space charge measurement system was built in the laboratory based on pulsed electro-acoustic(PEA)method.The test results show that the temperature and moisture greatly affect the migration,accumulation and decay of the space charge in oil-impregnated paper.The space charge increases first and then decreases with the increase in the temperature.To study the effect of moisture on the trap distribution of oil-impregnated paper,an isothermal surface potential decay test platform was built.The test results show that the increase in moisture content make the trap level shallower,which leads to faster attenuation of space charge during short circuit test.To simulate the space charge accumulation characteristics of oil-impregnated paper at different temperatures,the impurity ion transport process was introduced to the bipolar charge transport model.The simulation results show that the model proposed in this paper can well simulate the space charge distribution of oil-impregnated paper,and the mean volume charge density of the test and simulation results have the same trend.To study the low-temperature breakdown characteristics of oil-paper insulation under AC-DC combined voltage,a low-temperature breakdown platform was built in the laboratory to study the breakdown characteristics of transformer oil,oil-impregnated paper and oil-paper composite insulation.The test results show that the breakdown field strength of transformer oil with the low moisture content increases with the decrease in the temperature,while that of the transformer oil with high moisture content decrease firstly and then increase.The breakdown field strength of transformer oil decreases with the increase in the DC content of applied voltage,and the effect of moisture on the breakdown field strength is greater near room temperature.The breakdown field strength of oil-impregnated paper increases with the increase in the DC content of applied voltage,and the effect of moisture on the breakdown field strength under high DC content voltage is greater.With the decrease in the temperature,the breakdown field strength of oil-impregnated paper with low moisture content increases under low DC content of applied voltage,and it decreases under high DC content of applied voltage,while the breakdown field strength of oil-impregnated paper with high moisture content increases under different voltage waveform.The breakdown voltage of oil-paper composite insulation increases first and then decreases with the increase in the DC content of applied voltage.According to the above test research results,a mathematical model was proposed to describe the breakdown characteristics of the oil-paper insulation under AC-DC combined voltage,and its calculation results are consistent with the trend of the test results,which can provide reference for the insulation design of the converter transformer.