Differential Design Of Vertical Dc Grounding Electrode And Protection Of Transfer Potential

HVDC is very suitable for power transmission of long-distance and large-capacity.Grounding pole is an important part of HVDC system.The main body of a vertical DC grounding electrode is composed of electrode wells arranged according to certain rules and diversion system.Compared with a horizontal grounding electrode,the area occupied by a vertical DC grounding electrode is greatly reduced,the application prospect of which is broad.At present,schemes of vertical grounding electrode have been adopted at the sending end of +800 k V Puqiao DC project which was put into operation in 2015 and the receiving end of +800 k V Northwest Yunnan DC project which was put into operation in 2018,but the experience of optimal design and operation of grounding electrode is insufficient.This paper focuses on the differential design of electrode parameters of a vertical DC grounding electrode and contact potential of long-span conductors near a grounding electrode.The area of vertical DC grounding electrode of concentric double-ring arrangement is smaller than that of conventional vertical grounding pole,so it is suitable for areas where the area of electrode address is severely limited.The simulation results show that the average current flowing through electrodes at the inner ring is lower than that of electrodes at the outer ring.The reason is that electrodes at the inner ring are greatly affected by the shielding effect,and the obvious difference of the shunting will also cause great difference in the transient temperature rise between electrodes at the inner and outer ring.In order to solve the problem of large difference of shunt and transient temperature rise between electrodes at inner and outer ring electrode.The method by adjusting the length of electrodes at the inner and the outer ring to reduce the difference of shunt,and then adjusting the diameter of coke layer cross section to reduce the difference of transient temperature rise is proposed.By fitting the calculation results of the series of simulation models,a quantitative optimization method for the length of electrodes and the diameter of coke layer cross section at the inner and the outer ring was obtained.The calculation results of typical examples show that this optimization algorithm can effectively reduce the difference of shunt and temperature rise between electrodes at the inner and the outer ring without increasing the grounding electrode engineering quantity,and the maximum transient temperature rise energy of grounding electrode is greatly reduced.The ideal arrangement of vertical DC grounding electrodes is that electrodes are distributed on a circle with equal spacing,but electrodes are usually arranged in an irregular elliptical shape due to the restriction of conditions of pole addresses.Simulation models of typical elliptically arranged vertical DC grounding electrodes are established,results of which show that the stepping potential near the end of the long axis is obviously higher than that of other parts.The reason is that the smaller curvature radius near the end of the long axis results in larger current flowing through the electrode and larger axial component of step potential generated by the electrode.Based on the reason of high step potential near the end of long axis,methods of reducing step potential by increasing electrode density,curvature radius and electrode burial depth at the end of the long axis are proposed.By results of simulation calculation,it is found that the most effective method is to increase the buried depth of several electrodes at the end of the long axis and nearby.In view of the problem that the excessive buried depth of electrodes will increase the risk of collapse in engineering,an equivalent method of increasing the buried depth of electrodes by adding an insulating steel pipe to the interface between coke and soil at the top of electrode is proposed.A long-span conductor near the grounding pole may form a long-distance multi-point grounding conductor system.The large potential difference between the grounding points may lead to a high contact potential on the conductor,which is similar to the principle of transfer potential in the traditional sense.Firstly,a simplified three-point grounding model is taken as an example,and the simplified circuit model and the calculation formula of contact potential of conductors at each grounding point are obtained.It’s found that contact potential of conductors depends on the differences of potential and the grounding resistances between grounding points.Then variation of conductor contact potential is studied from aspects of soil resistivity,relative position relationship between line corridor and grounding electrode,number and distribution of grounding points,grounding resistance of grounding points,etc,according to conclusions of which,methods of restraining the contact potential of conductors are put forward,such as avoiding the grounding point of conductors being too close to the edge of grounding pole,keeping the shape of conductor corridor as close as possible to the contour of grounding pole,avoiding the grounding point being too concentrated at both ends of conductors,reducing the grounding resistance of grounding points at both ends of conductors and grounding points close to the.Finally,combined with typical cases,risks of contact potential of suspended overhead optical cable and metal fence near a grounding electrods are analyzed respectively,according to respective structural characteristics of which,the solutions to eliminate the risk of contact potential are proposed from two aspects of cutoff current path and reduction of conductor length,and the application in engineering proves the effectiveness of the schemes.