Research On The Estimation Of Horizontally Stratified Soil Structure Under The Effect Of Grounding Grid

Grounding system is one of the basic safety precautions. In the regular work, all kinds of protection devices must cooperate with a good grounding device, which can effectively lead the lightning current and fault current into earth. With the rapid development of national economy, demand for electricity takes off and short-circuit capacitance of substation becomes higher, which makes the design and retrofit of grounding system more difficult. Whether the grounding system that has been built or prepare to built, we must know the exactly the soil structure in the site of the substation. Then we can envaluate its ground performance by numerical computation method, accordingly a appropriate design or retrofit scheme can be performed. While, several studies have indicated that the presence of buried metallic structures lowers the measured apparent resistivity values when the measurement is performed near such structures, the affected measure data will lead to a wrong soil structure which makes the ground resistance of a build ground grid bigger than its design value. So far, existing soil structure inverse method can not handle the measured apparent resistivity which is effected by buried metal, it is necessary to come up with a method to handle this problem.On the basis of analyzing the domestic and foreign research literature,this paper presents a method for the estimation of soil structure based on genetic algorithm(GA), which can reveal actual soil structures through the experimental curve of apparent resistivity that was measured near the ground grid. In this paper, the method of optimization is GA, and new soil structures were generated it. Finite element method(FEM) was used to compute the theoretical curve of apparent resistivity under these soil structures. The relative position of ground grid and the measurement profiles were considered in FEM, which can reveal the true measurement site. Last, we have done a error analysis when this method is used in practical engineering application; the soil structure calculation result of a practical converter station proves the valid of this method. The main conclusions obtained in this paper are:① For soil structure inverse problem, we analysis the reasonable precision setting of soil parameters according to the use of soil structure in engineering application. Through the reasonable precision setting of soil parameters, the solution space of GA is compressed, which make the result much easier to converge to global optimum or local oprimum with a smaller fitness value.② On the basis of the compressed solution space, we proposed a GA of variable precision method. This method can control the solution space in a proporate scale by changing precision of soil parameters in the iterations, which makes the inverse problem more easier escape from the local optimum and approach the global optimum. The calculation result of a three-layer soil structure proves that this method is more efficient than basic GA.③ When the proposed method which combined FEM with GA was applied in the soil structure solving of a built substation, we analysis the influence caused by distance deviation from the measuring profile to the ground mesh; it shows that the existing positioning method can ensure the actual application of this method, the distance deviation is controllable.④ we derived the feasibility of simplifing 3D ground grid to 1D from the principle, meanwhile the proposed method was used in soil structure calculation of a large-scale converter station. Compared with the survey data when the ground grid was not built, the calculation result and survey data are in good agreement.