Electrothermal Field Distribution And Optimization For A UHV Converter Transformer Valve Side Bushing

The valve-side bushings of UHV converter transformer is one of the key equipment of UHVDC transmission project,and its performance is closely associated with the stability of the HVDC system.With the increase of the transmission capacity and voltage level,the problems of voltage-uniformity and thermal-uniformity of the bushing are becoming more and more prominent.The valve-side bushings have a complex structure and are prone to overheating faults due to their poor heat dissipation under large load.On the other hand,the temperature gradient between inside and outside of the bushing is large.The insulation conductivity is nonlinearly dependent on the temperature leading to the distortion of the internal electric field,which affects the life and reliable operation of the bushing.This thesis aims to study the temperature distribution characteristics of the bushing and the effect of temperature on the electric field distribution by taking the valve-side bushing of UHV converter as the research object,and improve the electrothermal field distribution by the combination of structural optimization and material modification.The main work and conclusions of this thesis are as follows.A ±800 kV valve-side bushing model of UHV converter transformer was established,and the electrothermal field of the bushing was simulated using the finite element simulation software COMSOL to analyze the electrothermal field distribution characteristics of the bushing.The effects of the diameter of the bushing,the width of the air gap between the inner and outer conductors and the structural parameters of the bushing core on the temperature distribution of the bushing were studied.It is found that the air gap between the inner and outer conductors has the most obvious effect on the temperature field.After optimizing the casing with the preferred structural parameters,it is found that the current-carrying capacity of the bushing can reach4500 A and the maximum temperature dropped from 149.206 ℃ to 109.439 ℃,which meets the 120 ℃ temperature limit specified in IEC60137:2008;After analyzing and comparing the electric field distribution patterns before and after the optimized temperature field,it is concluded that the electric field distribution is improved after optimizing the temperature field.Considering the difficulty of optimizing structural parameters at higher currents,the temperature field optimization based on material modification was explored.Nano-filler and physical blending methods were used to prepare composite materials of epoxy resin and boron nitride(BN).The influence of different BN mass fractions on the thermal conductivity of the material was studied.The heat radiating device was designed to study the heat dissipation ability of the composite sample.The modified sample was applied to the bushing model to simulate the temperature field and the results show that the modified materials could improve the thermal conductivity of insulation,thus improve the temperature distribution of the bushing.Finally,the improved temperature field distribution was conducive to optimizing the electric field distribution of the bushing under DC.In this thesis,the finite element simulation software was used to study and calculate the distribution characteristics of the electrothermal field and the influence of the temperature field on the electric field distribution of the valve-side bushing of a converter transformer.The optimization method of the combination of structural optimization and material modification was proposed and verified.The improvement of both the temeperature and electric field distribution was obsreved,which could provide theoretical basis and experimental support for the optimization and design of valve side bushings of converter transformers in practical projects.