Electric Field Adjustment Method Of ±800kV Dry-type High Voltage DC Bushing Under Temperature Gradient

The main insulation material of the high-voltage dry-type DC bushing is epoxy resin/crepe paper composite material.Compared with oil-paper insulation,it has the advantages of less maintenance,fire and explosion prevention,and high mechanical strength.Since the Joule heating of the current carried by the central guide rod of the bushing is much higher than the loss generated when the leakage current flows through the insulating medium,there is a large temperature gradient in the high voltage bushing capacitor core.This is more obvious in dry insulation with poor heat dissipation performance.The existence of a temperature gradient will cause a gradient distribution of the electrical conductivity of the insulating material in the radial dimension,and further affect the electric field distribution under direct current,and the phenomenon of electric field flip occurs.This thesis studies the influence mechanism of temperature gradient on electric field distribution through theoretical analysis and simulation verification,and proposes corresponding electric field optimization methods.The specific content includes:Taking the dry-type bushing of ±800 k V converter transformer as the research object,the cone model commonly used in capacitor core is adopted.The corresponding electro-thermal coupling simulation model is established,and the electric field of the temperature field of the dry-type DC bushing under the temperature gradient is simulated and calculated.Studies have shown that: the temperature gradient will lead to an increase in the electric field strength at the outermost plate.When 5k A is carrying current,the electric field strength at the outermost plate is increased by 44.5% compared to when there is no load.Taking advantage of the good thermal conductivity of boron nitride（BN）,BN nanoparticles are introduced into epoxy impregnated paper for modification.Improve the electric field distribution by reducing the temperature gradient.Studies have shown that the introduction of BN nanoparticles improves the thermal conductivity of epoxy impregnated paper and reduces the temperature gradient inside the casing;thermal conductivity and electrical conductivity temperature coefficient are the main factors that affect the degree of electric field flipping;by selecting a proper BN doping ratio,the degree of electric field flipping under temperature gradients can be reduced while ensuring that the strength of the insulating medium is not significantly affected,thereby reducing the field strength at the outermost plate;choosing 5wt%BN doping can reduce the field strength at the outermost electrode plate of the capacitor core by 3% when carrying current at 5 k A compared to the unmodified one.The insulation structure of the dry-type DC bushing under the temperature gradient is optimized.A design scheme of insulation structure that takes into account both the electric field homogenization under no-load and the electric field flip under load is proposed to reduce the maximum field strength of the insulating outer layer under the temperature gradient.Two optimization schemes for capacitor cores are given.The simulation results show that the maximum field strength of the two schemes is reduced by 16.5% and 19.4%,respectively,compared with the unoptimized ones.