Study On The Measurement Of Soil Structure And Inversion Of Soil Model In Grounding System

In modern society,the demand for electricity increases constantly with the continuous development of industry.The security and reliability of the operation of the power system maintain the safety of the people’s life and property,and need to be continuously improved.Grounding system is an important part of ensuring the safe and normal operation of power system,and the technology of grounding must be developed rapidly.At the same time,with the gradual expansion of the scale of the power grid in China and the continuous improvement of the system voltage level and capacity,the grounding current is also increasing.Nowadays,large grounding grid has become a trend of development.It is very important to build a safe and reliable grounding system.In grounding systems,we integrate the resistance between electrode and soil and grounding-line resistance as grounding resistance.The value of grounding resistance is not only affected by soil thickness,soil resistivity,but the shape,size and burying depth of grounding electrode,the connection between grounding wire and grounding electrode.Therefore,the rational estimation of soil structure and resistivity is the basis for the design of grounding system.First,in order to explore the relationship between surface potential and soil structure and improve the accuracy of grounding measurement,the surface potential of homogeneous soil and horizontal two-layer soil under different conditions was obtained by simulation.The computational formula of the surface potential of the two types is theoretically analyzed and deduced.It is found that the influence of the measuring current and the distance between electrodes can be excluded by processing the surface potential.The conception of surface apparent resistivity distribution coefficient is put forward,and the distribution coefficient of both homogeneous soil and horizontal twolayer soil was compared with the simulation data.It is found that the distribution coefficient curve of homogeneous soil is independent of the measuring current and electrode spacing.The resistivity distribution coefficient of horizontal two-layer soil is related to the soil interface reflection coefficient,the thickness of the upper soil and the spacing between electrodes.The more non-uniform the soil is,the greater the difference of the distribution curve between the homogeneous soil and the horizontal two-layer soil.Using the distribution coefficient,the parameters of the soil interface reflection coefficient and the upper thickness,which provides a basis for the selection of electrode spacing in the grounding measurement,were calculated effectively.Then,by changing the buried depth of one side electrode,the change regulation of the surface potential with the buried depth of the electrode was studied.The potential of four points was measured at the interval where potential changes obviously.A measuring point was selected as a reference point,and the potential difference between other measuring points and the reference point was calculated.The variance rate of potential difference when the buried depth had been changed was calculated by the method of successive difference.In this way,the data can be fully utilized,the random error can be reduced and the change regulation of the surface potential can be accurately reflected.In actual measurement,there is a contact resistance between the electrode and the soil because of the gap among the soil particles.In the simulation calculation,a coating is needed to be added to the surface of the electrode.It is found that the variance rate of the surface potential is extreme value when the buried depth reaches the thickness of the upper layer soil.Finally,compared the soil models that is inversed by both the Wenner method and the arbitrary voltage-electrode arrangement method,this paper studies how to solve the influence of unfavorable terrain and obstacles in the process of soil resistivity measurement,and improve the response to deep soil resistivity.It is found that for the Wenner method,the inversion results are more and more reliable with the increase of the electrode spacing.The measurement results of the arbitrary voltage-electrode arrangement method reflect the apparent resistivity of the soil at a specific depth.Combined the measurement results of both the Wenner method and the arbitrary voltage-electrode arrangement method under large electrode spacing,the soil model can be inversed to meet the engineering requirements.