Determination of the Soil Sounding Depth for the Earthing Resistance Calculation of Substations 35 kV

Purpose. Determination of the minimum required sounding depth for calculation of the earthing resistance for substations with a voltage class of 35 kV. Methodology. For each ratio of electrical resistivity values of soil layers, earthing resistance was calculated with changing of the layers separation depth from 0.4 m to h max , where h max is the layers separation depth in a two-layer soil at which the earthing resistance value becomes the same as in a uniform soil. Results. In the experiments carried out, a family of curves was obtained that describes the effect of separation depth of soil layers for various combinations of soil electrical resistivities and geometric dimensions of the earthing arrangement. The accumulated statistical data for substations with a voltage class of 35 kV made it possible to determine the required sounding depth depending on the maximum size of the earthing arrangement and the probability of the relative resistivity falling into the corresponding range of values. An algorithm is proposed for determining the required investigation depth by Wenner method as part of the electromagnetic diagnostics of the earthing arrangement of existing substations with a voltage class of 35 kV. Originality. For the first time, a probabilistic relationship was established between the ratio of the electrical resistivity of soil layers, the size of the earthing arrangement, and the necessary depth investigation of the geological medium. As a result it has been proven that there are substations for which the required sounding depth does not exceed the maximum size of the earthing arrangement. Practical value. The use of the algorithm developed in this work allows increasing the accuracy of the earthing resistance calculation of electrical installations with voltages above 1 kV operating in a network with isolated neutral.