Research On Key Issues Of Transient Temperature Rise Of DC Deep-Well Earthing Electrode

HVDC transmission is very suitable for long-distance and large-capacity power transmission.Earthing electrode is an important component of HVDC transmission system.Deep well type high voltage direct current earthing electrode has a wide application prospect because of its small number of sub-electrodes and small occupied area,which effectively alleviates the problem of difficult location of earthing electrode.However,due to its "end effect",the current density at the electrode terminal may be very high,and temperature rise has become an important factor limiting the development of deep-well earthing technology,which is related to its normal and stable operation.According to the structural and environmental characteristics of deep-well earthing electrodes,this paper studies the transient temperature rise characteristics of deep-well earthing electrodes around key issues such as earth resistivity survey,modeling and temperature characteristics of saturated water underground medium.The main research contents and conclusions are as follows:(1)In order to accurately establish the earth resistivity model in the deep layer,a combined measurement and modeling method using quadrupole method,controllable source audio magnetotelluric method and magnetotelluric method is proposed,and a wide-area earth resistivity model from the surface to tens of kilometers underground is obtained.Compared with the measured value of grounding resistance in actual engineering site,the error of the obtained earth resistivity model in calculating the grounding resistance of deep-well earthing electrode is 0.036Ω,which proves the effectiveness and accuracy of the proposed exploration and modeling method.(2)Considering the characteristics of deep-well earthing electrodes that are buried deeply and are in saturated groundwater environment,the resistivity and thermal parameters of saturated water soil and rock from the surface to 1,000 m underground are measured.The change mechanism of resistivity and thermal parameters with temperature is analyzed.Through numerical fitting,the expressions of resistivity and thermal parameters of different types of saturated water underground media varying with temperature are obtained,which establishes a foundation for further analysis of the temperature rise process of earthing electrodes.(3)Using the obtained functions of resistivity,thermal parameters and temperature of saturated water underground medium,the temperature rise model of DC earthing electrode considering the temperature variation characteristics of saturated water underground medium is established by finite element method.Using conventional vertical and circular earthing electrodes,the temperature rise simulation calculation is carried out,and the temperature rise process in saturated water homogeneous soil and layered soil is analyzed.It is concluded that the electrode temperature rise rate in saturated water soil model is obviously lower than that in conventional soil model.The temperature rise test of vertical electrode in saturated water soil is carried out.The comparison between the test and simulation results shows that the maximum temperature error is within 4.93%.(4)The field temperature rise test of deep-well earthingelectrode was carried out,and the error analysis of temperature rise results calculated by wide-area earth resistivity model showed that the local earth resistivity near the deep-well earthing electrode determined the temperature rise of its electrode.On the basis of arithmetic average of electric logging results,a local earth resistivity modeling method suitable for deep-well earthing electrode temperature rise calculation is proposed,and a deep-well earthing electrode temperature rise model considering saturated water underground medium temperature characteristics and local earth model is established.The model is used to compare and calculate the measured data of the field temperature rise in actual projects,and the maximum temperature error is found to be 5.5℃.The trend of electrode temperature rise is in good agreement with the test results.