Research On Technology Of Adding Current Sharing Resistance To Limit Step Voltage Of HVDC Grounding Electrode

With the continuous operation of UHVDC projects in China,in recent years,problems such as DC bias of DC grounding electrode to surrounding transformers and corrosion to pipelines have emerged,which make it more and more difficult for the location and land acquisition of grounding electrode.Sometimes,in order to fully avoid the impact on these facilities,the low deep soil resistivity,surrounding substations and pipelines are often given priority in project location In some cases,the earth electrode has to be built in a narrow and long zone with relatively high shallow resistivity and limited site area.In these areas,it is often difficult to meet the requirements of step voltage through the optimization design of grounding electrode body.At present,most of the researches on limiting the step voltage above the pole site by domestic scholars are to reconstruct the body of the pole and the surrounding soil environment,but the researches on reducing the step voltage above the pole site through the optimization of the internal shunt ratio of the earth pole are relatively lacking.After analyzing and summarizing the distribution law of step voltage of DC grounding electrode,this paper puts forward a method to limit the step voltage of high voltage DC grounding electrode by adding current sharing resistance in the drainage system,calculates the current sharing resistance configuration scheme of different types of DC grounding electrode and the influence of adding current sharing resistance on the operation performance of DC grounding electrode.The main research results are as follows:1)Based on CDEGS,a horizontal multi ring DC grounding electrode model is established.The effects of soil characteristics,anisotropic conductive media and polar ring parameters on the step voltage distribution above the electrode site are studied.The results show that the surface soil resistivity has a great influence on the step voltage distribution above the horizontal multi ring DC grounding electrode,while the surface soil thickness and the bottom soil resistivity have a little influence on the step voltage distribution above the ground electrode;when there is a low resistivity conductive medium near the outside of the pole site,the step voltage level near the low resistance area will be raised,and the distance conductive will be increased The closer the medium is,the higher the maximum step voltage is.When there is anisotropic conductive medium in the surface soil of the internal area of the pole site,its resistivity will affect the uniformity of the overall step voltage distribution of the pole site,and greatly affect the maximum step voltage of the pole site;the effect of reducing the step voltage above the earth electrode by increasing the number of pole rings and optimizing the radius size of each ring is not significant.2)Using the method of cyclic iteration,the current sharing resistance configuration schemes of horizontal multi ring grounding electrode,irregular shape grounding electrode and split grounding electrode are calculated respectively.The results show that the maximum value of step voltage above the pole site is effectively reduced and the step voltage level of each ring section is basically balanced,so as to make full use of the pole site.3)The influence of current sharing resistance on the operation performance of DC grounding electrode is calculated.The results show that the current density of outer ring decreases and the current density of inner ring increases,but the maximum value of current density does not change obviously.The current sharing resistance also causes the grounding of DC grounding electrode The resistance increases,but the degree of increase is very small;the change of the current sharing resistance has little effect on the grounding resistance of the DC grounding electrode,but has great influence on the current density and step voltage.The increase of the current sharing resistance will increase the current density of the inner ring,resulting in the increase of the maximum step voltage above the electrode site and the imbalance degree of step voltage.When the current sharing resistance breaks or breaks down,it has little effect on the grounding resistance of DC grounding electrode,but has great influence on the current density and step voltage.The open circuit fault of current sharing resistance will lead to the increase of current density and step voltage of the pole ring in normal operation,and the short circuit fault of current sharing resistance will lead to the increase of current density and step voltage of the pole ring where the fault current sharing resistance is located.