# Effect of Non-Homogeneous Soil Characteristics on Substation Grounding-Grid Performances: A Review
## 3 Effect of Horizontal Three-Layer Soil Characteristics on Grounding System Performances
### Types of Grounding Electrodes  
The analysis in [60] was conducted for three types of grounding electrodes namely; liner, ring-shaped and cross-shaped. The placement of ring-shaped grounding electrodes and directions for X and Y coordinates of the three-layer soil are shown in Figure 9. The same placement is applicable for the linear grounding electrode. The electrodes are buried  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Structure** & **Soil Layer** & **Resistivity (f\(\mathbf{m}\))** & **Soil Thickness (m)** \\ \hline \multirow{4}{*}{**A**} & 1 & 200 & 2 \\ \cline{2-4}  & 2 & 20–2000 & 2 \\ \cline{2-4}  & 3 & 300 & \(\infty\) \\ \hline \multirow{4}{*}{**B**} & 1 & 200 & 2 \\ \cline{2-4}  & 2 & 1000 & 2 \\ \cline{2-4}  & 3 & 20–2000 & \(\infty\) \\ \hline \hline \end{tabular}
\end{table}
Table 6: Soil data for two different structures adapted from [50].  
Figure 8: The relationship between the soil’s apparent resistivity and the reflection factor (K) (**a**) soil structure (A) (**b**) soil structure (B) [49].  
0.05 m depth into the soil. Only the linear and ring-shaped grounding electrodes will be discussed in this section to summarize the information on the effect of different types of grounding electrodes on grounding performance in horizontal three-layer soil, as the ring-shaped grounding electrode has the best dispersion properties and the linear grounding electrode has the poorest. Table 7 shows the maximum current and potential distribution for linear and ring-shaped grounding electrodes.  
For linear grounding electrodes in Figure 10a,b, the peak current density is likely to be right above the linear grounding electrode's edges. The current peak value of 65.9 A/m\({}^{2}\) occurs at 0.25 m and 0.35 m of Y coordinates. The current density decreases to 25 A/m\({}^{2}\) at the soil structure boundary. The optimum potential can be seen right above the middle of the linear grounding electrode [60].  
The peak current density can be seen at about 0.15 m from the center point in a circular field for the ring-shaped grounding electrode as in Figure 11a,b. The surface current density is near to 0 in a circular field about 0.05 m from the core and two smaller current  
\begin{table}
\begin{tabular}{l c c} \hline \hline  & **Linear** & **Ring-Shaped** \\ \hline
**Peak Current density (A/m\({}^{2}\))** & 67.4 & 43 \\ \hline
**Optimum Potential (V)** & 278 & 173 \\ \hline \hline \end{tabular}
\end{table}
Table 7: Current and potential distribution for linear and ring-shaped grounding electrodes.  
Figure 10: Current density and potential distribution curves for X and Y directions of linear grounding electrode (**a**) potential distribution (**b**) current density distribution [60].  
Figure 9: Placement of ring-shaped grounding electrodes in three-layer soil structure with X and Y coordinates.  
peaks of 1 A/m\({}^{2}\). The current density dropped to 24 A/m\({}^{2}\) at the soil's border. On the other hand, the optimum potential can be seen right above the ring-shaped grounding electrode structure.  
In both X and Y directions, it can be deduced that the current density and potential distribution curves for ring-shaped grounding electrodes overlap. Since both types of grounding electrodes are fully symmetrical, this overlap makes sense. Furthermore, under the same conditions, the current and voltage in the soil surrounding the ring-shaped grounding electrode are 35-40% [60] lower than those around the linear grounding electrode.