Research On The Corrosion Diagnosis Method Of Grounding Grids By Transient Electromagnetic Imaging

Transient electromagnetic method(TEM),as a non-destructive testing method for grounding grids,has the advantages of trenchless and non-stop operation.At present,the breakpoints and structure of grounding gird can be identified successfully.Based on this,this thesis studies a transient electromagnetic diagnosis method which can identify the different corrosions of grounding grid.This thesis starts with the basic principle and configuration of TEM,and select the configuration for grounding girds.Based on the "smoke ring" effect the apparent resistivity formula is derived.Finally,the corrosion degree of grounding grid is defined by a graded semi-quantitative method,and the corrosion diagnosis method of grounding grids is proposed.ANSYS Maxwell was used for the forward modeling of grounding grid corrosion,and an intact single grid was used to analyze the typical characteristics of transient electromagnetic signals.Analyze the effect of different grounding grid states(including different sizes,different burial depths,different materials,different soil environments,etc.)on diagnosis.These will give rise to errors in the calculation of deviations in the definition of corrosion,which will result in a semi-quantitative hierarchical of corrosion rather than a quantitative definition.TEM corrosion diagnosis of grounding grids with different structures was studied.The apparent resistivity of three typical measuring points(the center of the grid,above the flat steel and above the flat steel intersection)has the following characteristics: the maximum value of the apparent resistivity appears above the flat steel intersection,the minimum value of apparent resistivity appears above the center of grid,the apparent resistivity above the flat steel is between them.Based on this feature,the structure of grounding grids can be deduced.In grounding grids,the corrosion of flat steel mainly affects the apparent resistivity of the center of the smallest element gird composed of this flat steel.So we can define the corrosion degree of a grid semi-quantitatively by the change of apparent resistivity of this grid center measuring point.A construction method of transient electromagnetic diagnosis for grounding grid corrosion is proposed instead of the traditional regular straight line.During construction,it is not necessary to arrange the measuring points in a straight line of rules,just to ensure full coverage of the effective area.That is,the measurement point of each single grid center is measured when the structure of grounding grid is known;when the structure is unknown,when structure is unknown,the measurement points are arranged as much as possible,so that the measurement points are arranged near the three feature positions(the center of the grid,above the flat steel and above the flat steel intersection),thereby improving the structure detection accuracy.Through the test of grounding grids in Chongqing University and the corrosion detection of grounding grid of 110 kV substation,the effectiveness of the corrosion diagnosis method for structure detection and corrosion semi-quantitative grading was verified.