# 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 Vertical Rods  
The vertical rod length is a key design element in affecting the grounding behavior and protection in a two-layer soil structure. When it comes to assessing the safety of a grounding system, the placement of vertical rods is crucial [16; 53; 54; 55]. A study on the effect of the length of vertical rods on a grounding system is conducted in [16]. Two horizontal grounding grids with 150 m \(\times\) 150 m and 100 m \(\times\) 100 m were analyzed, and four vertical rods were attached to the horizontal grounding grids as in Figure 5. The grids are placed 0.6 m within the soil and the mesh dimension for both grids is 10 m. Figure 5 shows the \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Soil Structure** & **10 m** & **50 m** \\ \hline  & **Homogeneous** & 14.3 \(\Omega\) & 4.4 \(\Omega\) \\ \hline  & **(a)** & 35.1 \(\Omega\) & 14.1 \(\Omega\) \\ \cline{2-4}
**Case 1** & **(b)** & 8.3 \(\Omega\) & 2.6 \(\Omega\) \\ \hline  & **(a)** & 15.6 \(\Omega\) & 4.6 \(\Omega\) \\ \cline{2-4}
**Case 2** & **(b)** & 10.8 \(\Omega\) & 3.6 \(\Omega\) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The impedance of 10 m and 50 m grounding conductor adapted from [52]. Figure 4: Visual representation of soil layer parameters. \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Case 1 (Grounding** & **Case 2 (Grounding** \\  & **Conductor in the Top** & **Conductor in the Bottom** \\  & **Soil Layer)** & **Soil Layer)** \\ \hline  & **High resistive neighbouring** & & \\  & **soil (a)** & High impedance & Low impedance \\ \hline  & **Low resistive neighbouring** & & \\  & **soil (b)** & Low impedance & High impedance \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Summary of grounding impedance in Case 1 and Case 2.  
comparison between two different grid sizes on grounding resistance in two-layer soil. For two-layer soil, the top layer, \(\rho_{1}\), has a resistivity of 200 m while the bottom layer, \(\rho_{2}\), has a resistivity of 600 m. The top soil layer is 20-m deep. It shows that the two curves in Figure 6 are almost superposed, whereas the grounding resistance reduction rate, \(\zeta\) (at the \(y\)-axis) for different grid sizes are almost similar due to the ratio of the corresponding radius and the vertical rod-length (L/R\({}_{\rm eq}\)) (at the \(x\)-axis) is same. Grounding resistance reduction rate, \(\zeta\) measures the grounding resistance reduction when the length of vertical rod varies, which is calculated using Equation (3), while R\({}_{\rm eq}\) is the equivalent radius of the horizontal grounding grid calculated using Equation (4) [16]. S represents the horizontal grounding grid area. R\({}_{\rm o}\) denotes the horizontal grounding grid's resistance. R\({}_{1}\) represents the grounding resistance after vertical rods are added [16]. The reduction rate for a larger grounding grid (150 m \(\times\) 150 m) is lower than the smaller grid (100 m \(\times\) 100 m) in two-layer soil [16]. \[\zeta=\frac{\rm R_{0}-R_{1}}{\rm R_{0}} \tag{3}\]  
\[\rm R_{eq}=\sqrt{\frac{S}{\pi}}. \tag{4}\]  
The findings show that the curves overlap each other when they are far from the two-layer boundary; whereas they diverge significantly when they are near the soil boundary. The resistance reduction rate differs significantly between two different sizes of grounding grids for the same length of vertical rods. The impact of soil resistivity of the top and bottom layers of a two-layer soil structure might explain this phenomenon. According to research findings in [11; 55; 56; 57], when the length of the rod is long enough to penetrate the high resistive bottom soil layer, its resistivity impacts the behavior of the grounding system, increasing grid resistance. The length of rods to be installed at the grounding grid  
Figure 5: A grounding grid positioning in two-layer soil structure. Figure 6: Effect of vertical rod length on decreasing grounding resistance rate in two-layer soil [16]. is dependent on the resistivity of the soil layer.