| The ±800kV UHVDC transmission project has the advantages of long distance,large capacity and low loss,however,when a DC transmission project operates in the unipolar ground-return mode,the electrode current will form earth surface potential on the ground,and part of the DC will rush in the AC grid and cause the transformer DC bias,thereby generating transformer temperature rise,vibration,harmonics and reactive disturbance,adversely affect the normal operation of the power system.Based on the finite element simulation software,the calculation model of surface potential distribution caused by HVDC transmission ground current is established.The influence factors on surface potential such as the shallow and deep soil resistivity,the shape of the electrode,the lake and mountain,the grounding electrode and the ground current are proposed.Combined with the domestic HVDC transmission project,this paper completes the calculation of the surface potential distribution near DC grounding pole.The main research work is as follows:Firstly,this paper established a horizontal layered earth resistivity model in finite element simulation software,Ansoft Maxwell,applied the simplify method on equivalent grounding resistance of toroidal grounding pole.Besides,this paper introduces the simulation step of Ansoft Maxwell for solving the steady electric field problem.Simulation results of Ansoft Maxwell and the results obtained in the GODES and ANSYS calculation software are compared under the same conditions to verify the feasibility of the calculation method used in this paper.Secondly,the control variable method is used to calculate the earth surface potential(ESP)caused by the change of a certain influencing factor based on the horizontal layered soil model,and the influence degree and characteristic law of various factors on the distribution of ESP is analyzed.The main factors affecting the potential distribution includes the resistivity of deep and shallow soil,the shape of the lake and the mountain,the shape of the grounding electrode and the current into ground.Then,taking the deep ground exploration data near the grounding pole of Zhushuang Village in Shandong Temple to Shandong UHVDC transmission project as an example,a hierarchically-blocking complex soil model of the earth is established,and the potential distribution within 200 km of the grounding pole is calculated,subsequently.Finally,the grounding electrode of the ±800kV Jiuquan-Hunan UHVDC transmission project whose rated ground current is 5000 A and operated in a single-maximum operation mode is taken as the study case,in which the horizontal layered and stratified block complex geodetic model is established and the surface potential distribution of some substations near the grounding pole under two different geodetic models is calculated respectively,according to the geological data of Guazhou County in Jiuquan City and the deep resistivity inversion data surveyed by the geoelectric survey method.The results of this study show that different earth resistivity models cause large differences in ESP of each substation,and the surface potential distribution is greatly affected by factors such as shallow resistivity and high resistivity of deep resistivity and lake(ocean),while the shape of the grounding electrode has little influence on the ESP.The grounding current has a certain influence on ESP,the distance farther away from the grounding pole,the smaller the influence of ground current on ESP.The hierarchical-blocking earth resistivity model and calculation method adopted by this paper in case study can accurately and quickly calculate the surface potential distribution near the grounding pole,whose results can also provide support for DC bias calculation of nearby transformers and provide reference for power grid planning and construction and substation location near DC grounding pole. |
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