# Effect of Non-Homogeneous Soil Characteristics on Substation Grounding-Grid Performances: A Review
## 5 Conclusions  
The existing standards available to design a grounding system are typically relevant only for homogeneous soil conditions, as opposed to the fact that most of the soil structures on this earth are non-homogenous. The layered structure of a non-homogeneous soil can affect the ground resistance and ground potentials on soil profile particularly when the soil resistivity between layers differs enormously. However, owing to the complexities of computations needed to resolve the problem, often the soils are assumed to be homogenous, which resulted in significant inaccuracies. In varying soil conditions, the performance of different grounding grid parameters may dramatically change. Although an exact comparison is impossible because grounding grid designs vary depending on the location to be installed, a broad overview of grounding performance as design changes and their effects as soil layers and their corresponding characteristics change will be useful in providing more information on safe grounding designs in non-homogeneous soil. The design parameters for grounding behavior analysis in a horizontal two-layer soil structure were varied in terms of grid depth, mesh number, and length of horizontal conductors and vertical rods, while the soil characteristics were altered in terms of depth and resistivity of the first soil layer. It can be observed that the resistivity of each soil layer has an impact on grounding behavior and safety. For example, the GPR reduces when the depth of the grid increases until it reaches the border of the top layer and increases after the grid enters the bottom layer with high resistivity when the soil resistivity in the bottom layer \(\rho_{2}\) is higher than the top soil layer \(\rho_{1}\). Longer vertical rods in a horizontal two-layer soil structure are only effective when the bottom layer's soil resistivity, \(\rho_{2}\) is lower than the top soil layer's resistivity, \(\rho_{1}\). When the soil resistivity in the bottom layer \(\rho_{2}\) is higher than the top soil layer \(\rho_{1}\) increasing the top soil layer's depth and resistivity for simulation purposes increases the GPR and grid resistance, while vice versa shows \begin{table}
\begin{tabular}{p{130.1 pt} p{130.1 pt} p{130.1 pt}} \hline \hline  & **Case 1** & **Case 2** \\ \hline  & **(\(\rho_{1}\) =100 \(\Omega\)m), (\(\rho_{2}\)=1000 \(\Omega\)m)** & **(\(\rho_{1}\) =1000 \(\Omega\)m), (\(\rho_{2}\)=100 \(\Omega\)m)** \\ \hline
**Distance, ‘a’ between electrodes increases** & Apparent resistivity started to increase significantly until a certain distance, and then increases with a small rate & Apparent resistivity started to decrease significantly until a certain distance, and then decreases with a small rate \\ \hline
**The distance, ‘d’ increases** & Due to the impact of the second layer with high resistivity, the apparent resistivity reduces. & Due to the impact of the second layer with low resistivity, the apparent resistivity increases. \\ \hline
**The angle, \(\beta\) increases.** & The apparent resistivity increases & The apparent resistivity decreases \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Summary of performance of apparent resistivity with varying parameters ‘a’, ‘d’ and \(\beta\). a substantial reduction. This demonstrates the impact of the first soil layer's depth and corresponding resistivity on grounding behavior. The design parameters for grounding behavior analysis in a horizontal three-layer soil structure were changed in terms of grounding electrode types and vertical rod lengths. The analysis utilized ring-shaped and linear grounding electrodes in a three-layer horizontal soil structure and found that the ring-shaped grounding electrode has the best current and potential dispersion properties, while the linear grounding electrode has the poorest. The findings for the length of vertical rods are similar to the horizontal two-layer soil structure when it reaches the low resistive soil layer.