# Effect of Non-Homogeneous Soil Characteristics on Substation Grounding-Grid Performances: A Review
## 2 Effect of Horizontal Two-Layer Soil Characteristics on Grounding System Performances
### Length of Horizontal Conductors  
The position for the grounding system installation is determined by the soil layers involved; either uniform soil; horizontal soil, or vertical soil layers. The performance of a grounding system would be influenced by the electrical characteristics of each soil layer present. To demonstrate this, a study was conducted on single short and long horizontal conductors of 10 m and 50 m, respectively, with a 0.005 m radius buried within the top or bottom layer of soil at a depth of 0.75 m [52]. The conductor is energized with a current amplitude of 1 A. The horizontal electrodes are buried at 100 \(\Omega\)m soil layer where it is kept constant as shown in Table 3 and Figure 4 and the relative permittivity of both top and bottom soil layers are also kept constant at 10. The (a) in Table 3 represents the soil layer with high resistivity of 1000 \(\Omega\)m while the (b) represents the soil layer with low resistivity of 10 \(\Omega\)m.  
The resistivity of homogeneous soil is assumed as 100 \(\Omega\)m to make a comparison between homogeneous and two-layer soil structures. Table 4 gives the impedance of short and long horizontal grounding conductors, respectively. Although the results presented in [52] consist of various frequencies, only 10 kHz is considered in this paper as it is commonly tested for both lengths of conductors. Both short and long conductors display the same impedance pattern. The impedance of the grounding conductor in Case 1 and Case 2 is higher when the neighbouring soil layer of which the grounding conductor is buried is high (bottom layer-1000 \(\Omega\)m). In contrast, the impedance is lower when the neighbouring soil layer of which the grounding conductor is buried is low (top layer-100 \(\Omega\)m) compared to impedance in homogeneous soil [52]. A summary of the grounding impedance in Case 1 and Case 2 is presented in Table 5.  
Figure 3: Ground resistance, Rg with a different number of meshes in two-layer soil structure [39].  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Case**} & **Depth of Upper** & **Top Layer Resistivity,** & **Bottom Layer** \\  & **Layer, d\({}_{1}\) (m)** & \(\rho_{1}\) (\(\Omega\)m)** & **Resistivity, \(\rho_{2}\) (\(\Omega\)m)** \\ \hline \multirow{2}{*}{**1** (Conductor in a top  soil layer)} & \multirow{2}{*}{1.0} & \multirow{2}{*}{100} & \multirow{2}{*}{(a) 1000} \\ \cline{3-3} \cline{5-5}  & & & (b) 10 \\ \hline \multirow{2}{*}{**2** (Conductor in a  bottom soil layer)} & \multirow{2}{*}{0.5} & \multirow{2}{*}{(a) 1000} & \multirow{2}{*}{100} \\ \cline{3-3} \cline{5-5}  & & & (b) 10 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Parameters of soil layers adapted from [52].  
Even if the grounding conductor is located in a low resistive soil layer, the impedance is affected by the resistivity of the neighbouring soil layer, according to the findings. Compared to the homogeneous soil layer, the impedance in high resistive neighbouring soil (case 1 and case 2) is higher, while the impedance is lower in low resistive neighbouring soil for both cases. In case 2, the grounding conductor's impedance is lower than in case 1 when the neighboring soil is highly resistive. This might be because the neighboring soil's high resistivity (top layer) allows more current to pass to the lower resistivity of the bottom layer, decreasing the grounding impedance. On the other hand, low resistive neighbouring soil (top layer) allows more current flow to the lower resistivity in the top layer, raising grounding impedance. Case 1 exhibits a grounding behavior that is the contrary of case 2.