Study On Insulation Characteristics Of Converter Transformer Bushing Under The Temperature Gradient

Converter transformer bushing is a key equipment for HVDC to realize the converter transformer connecting between the transmission line and semiconductor valve,which is a complex structure of the insulation system.In the operation,the bushing is not only to withstand the effect of the working voltage of the AC and DC and polarity reversal voltage,which leads to improvement of the insulation property.The local field of converter transformer bushing easily exceeds the margin value under strong electric field,while the dielectric resistivity changes with heat generated in the operation,that the distortion of electric field of bushing will more easily result in dielectric breakdown and insulation failure.Therefore,to consider the influence of temperature on the electric field of converter transformer bushing will improve the accurate analysis.A numerical model is built to calculate the temperature distribution of ±600kV converter transformer bushing,and temperature distribution of bushing is obtained with the heat source of Joule heating and dielectric loss,and natural convection of SF6 gas is considered.Then,the numerical model is built to calculate the electric field distribution of converter transformer bushing under the DC voltage and polarity reversal voltage with finite element method.Finally,temperature distribution is loaded to the DC electric field and polarity reversal electric field of converter transformer bushing,and the DC electric field and polarity reversal electric field distribution is analyzed under the condition of constant temperature and temperature gradient.The time of before polarity reversal,during polarity reversal and after the polarity reversal is selected,and different points of insulating medium are selected to study the effect of temperature on the polarity reversal process,and the polarity reversal electric field distribution under different conditions is compared and analyzed.Conclusion: the DC electric field distribution is concentrated in the epoxy resin insulation of converter transformer bushing,when the temperature is considered,the high intensity field of the bushing is concentrated on the outer side of the epoxy resin,that is the low-temperature side,and the low intensity field of the bushing is concentrated on the inner side of the epoxy resin,that is the high-temperature side,but under the condition of constant temperature,the high field intensity of the bushing is concentrated in the inner side of theepoxy resin,and the low field intensity is concentrated on the outer side of the epoxy resin,that is,the concentration of DC electric field is reversed by temperature.The polarity reversal electric field distribution is also concentrated in the epoxy resin insulation of converter transformer bushing.When the temperature is considered,the concentration of electric field is reversed by temperature before,during and after the polarity reversal,and the polarity reversal electric field distribution of the bushing is obviously distorted by the temperature.