Substation Grounding Grid Analysis with the Variation of Soil layer depth Method

This paper presents a substation grounding grid analyzed with the variation of soil layer depth. The grounding grid system of a practical 22 kV substation, in which the ground grid is made from copper, is calculated by virtue of the CDEGS software. The study, supported by Provincial Electricity Authority (PEA), Thailand, is based the on grounding system of Royal Flora RATCHAPHRUEK substation. The supporting soil resistivity data has been obtained from actual field tested at the substation. The result has shown that soil resistivity at the substation was interpreted to a two-layer earth structure. Moreover, the burial depth of the grid affects the Ground Potential Rise (GPR) to some extent as well as the ground rod, connected with the grid, and also plays an important role in GPR reduction. In case that the top soil-layer resistivity is more than the bottom-layer, the ground grid with and without ground rod will greatly reduce the value of GPR. On the contrary, in case that the top soil-layer resistivity is less than the bottom-layer resistivity, the ground grid with and without ground rod will slightly reduce the value of GPR. This indicates that the current density over the grid affects directly to the current distribution to the soil layer. Therefore, the design and construction of grounding grid in the area which the top soil-layer resistivity is less than the bottom-layer resistivity, can lessen the number of ground rod used in the grid because the value of GPR is insignificantly different. Finally, the deeper the grid buries in the layer, the less of the GPR value is.