Research On The Grounding Resistance Measurement Of Large Substation Grounding Grid With Short Electrode Wire

With the development of electric power industry,the application of EHV(extra-high voltage)transmission technology become more and more widely.The number of UHV(ultra-high voltage)substation is increasing,which has a larger grounding net than before.The grounding net can provide protection for the safe and stable operation of power systems.Grounding resistance is one of the most important indicators to measure the electrical properties of grounding system.So the measurement of grounding resistance is carried out before the substation is put into operation or the substation is launched or annually during their operation.At present,we mainly use the traditional long distance method and short distance method based on the theory of the potential compensation to measurement of ground resistance.However,the long distance method need a wire length of about 4 ~ 5 times the diagonal length,which lead a larger workload.And short distance method based on the theory of the potential compensation need to calculate the potential electrode position based on the grounding network structure.So both the two methods are not applicable apply to the large grounding net if its structure is unknown.Therefore,how to simply and precisely measure the grounding resistance of large grounding grid is an important problem in power systems.In this paper,based on the in-depth analysis the measuring principle of traditional methods,a generally applicable short distance measurement method is proposed.At first,a finite element model of large grounding based on the shell theory is proposed.To deal with the problem of difficult subdivision and large computation induced by traditional finite element method in dealing with large grounding net,making a long sheet metal instead of the grounding conductor.The open boundaries of earth are processed by introducing a spatial transformation which translates the semi-infinite space into the finite space.Compared with the results of experimental results,the presented method is proved to be correct and effective.By calculating and analyzing the ground-potential of different grounding grids in different soil structure based on the model,we get the scope of application ofhemispherical grounding model and tablet grounding model.Then a short distance measurement method is proposed by establishing the equivalent model to calculate the potential of voltage electrode,and derived the grounding resistance through it and the voltage measurements between grounding and voltage electrode.Finally,we measured the grounding resistance of real grounding grids.Compared with the 0.618 method,the presented method is proved to be correct and effective.The results show that,this method can apply to the complex soil structure,and it is still applicable even if the design of substation ground net is missing.The length of the lead wire is proposed to be 2D if we use the hemispherical grounding model,and it is proposed to be 1D if we use the tablet grounding model.The voltage and current lead can lead along the opposite direction,which can effectively eliminate the mutual inductance between the lead.