The Finite Element Analysis Of HVDC Earth Electrode Electric Field Coupling With Thermal Field Dynamically Based On Shell Theory

The extremely uneven distribution of energy demand centers and energy resource centers in China,led to the rapid development of HVDC transmission projects;Besides,due to the shortage of land resources and the higher requirements of the electrode site selection,the common grounding electrode technology is widely used.So the working environment of DC grounding electrode is becoming more and more complex and severe.Under monopole ground operation mode,the temperature rise phenomenon will lead to serious ground burning,soil local hardening,or lead to serious corrosion of metal facilities,and the transformer DC bias.The accurate simulation and analysis of DC grounding electrode's bulk flow mechanism and the temperature rise process,considering a low resistivity reservoirs,underground river and a high resistivity layer of rock and other geological circumstances,accurately research on the effect of abnormal resistance region(ARR)on DC grounding pole,and study on the protective distance of buried metal pipeline in actual layered soil structure,are the theoretical basis of UHVDC grounding electrode design and solving the DC corrosion and DC bias problem.Due to the huge gap,sometimes up to 106,between large disperse current domain and very small section size of grounding conductor,the computation will be a sharp rise in traditional three-dimensional finite element method,and sometimes there will be singular point on interface of grounding conductor and soil in the process of FEM geometry subdivision.To solve the above problem,this dissertation introduces the "shell" theory into FEM method,to realize the coupling of 3D finite element and 2-D finite element.In addition,this dissertation introduced the soil electrical and thermal parameters into the calculation process,and the variation of soil parameters caused by soil temperature rise is simulated accurately Then,based on the COMSOL Multiphysics simulation software,the dynamic coupling finite element model considering the variation of soil parameters of DC grounding electrode is established;This dissertation analyzes the influence of temperature characteristics of soil parameters on the DC grounding electrode temperature rise process under different surface soil resistivity.The results show that,considering the temperature characteristics of soil parameters,the grounding resistance of DC grounding system is no longer a fixed value,and changes with working time;In this dissertation,the influence of high resistivity and low resistivity abnormal resistance region(ARR)on the electric field distribution and the DC grounding resistance is analyzed.The results show that the horizontal and vertical depth of ARR have a significant influence on the DC grounding parameters.Therefore,In the actual design,a detailed survey on geological conditions of the polar site near the DC earth electrode should be done.Considering massive ARR,analyzes the influence of ARR near DC earth electrode on electric field distribution.Hope that conclusion of this dissertation can provide reference for actual DC earth electrode design;Finally,based on the extensive applicability of the numerical algorithm to any soil structure,the protection distance of buried metal pipeline under the simple two layered soil is analyzed systematically.The results show that influence of soil resistivity on minimum protection distance of buried metal pipeline is closely related to the soil thickness of that.The greater the thickness of soil is,the greater the influence of the minimum protection distance is,and show a direct proportional phenomenon.The influence of the upper soil thickness d1 on the minimum protection distance depends mainly on the gap between ?1 and ?2;Under the same ratio of ?1/?2,the change trend of minimum protection distance with d1,are assumed to be identical within a range of allowable errors.