# Design, Modeling, and Analysis of IEEE Std 80 Earth Grid Design Refinement Methods Using ETAP
## 3 Earth Grid Design Criteria  
The substation earth grid design IEEE Std 80-2013 specification gives a comprehensive design procedure that should be followed for the design of a compliant and efficient earth grid through its design flowchart shown in Figure 1 adapted from IEEE Std 80-2013 [1]. The flowchart outlines the key parameters that should be considered when designing an earth grid, and it sets out the design criterion that must be met for a substation earth grid to be considered compliant and safe for application. From the IEEE Std 80-2013 earth grid design flowchart, the key design parameters that were assessed for this study that play an essential role in the refinements discussed in Section 2: Literature Review are the touch voltage, step voltage, ground potential rise, and fault current, which are critically associated with steps 3, 6, 7, 9, and 10 of the flowchart. Figure 1: Substation earth grid design flowchart (adapted from IEEE Std 80-2013 [1]). The main purpose of the refinement methods is to improve the design parameters of the safety limits of touch voltage, step voltage, and ground potential rise as described by Equations (1), (2), and (4). The touch voltage is defined by IEEE in [1] as the difference between the GPR and the potential of the surface that they are standing on when they are in contact with any conductive objects connected to the earth grid, and step voltage is the surface potential difference felt by a person with a 1 m distance between their feet when they are not in contact with any conductive objects connected to the earth grid [1]. Lastly, the ground potential rise is defined as the product of the maximum grid current and overall grid resistance [24], and is the highest electrical potential a substation's earth grid can achieve concerning a distant earthing point believed to be at remote earth's potential [1]. Concerning the GPR, another critical parameter is the maximum grid current, which is described by Equation (3), and it is directly proportional to the magnitude of the GPR along with the earth grid resistance of the substation. \[E_{step70} =(1000+6C_{s}\cdot\rho_{s})\ \frac{0.157}{\sqrt{t_{s}}} \tag{1}\] \[E_{touch70} =(1000+1.5C_{s}\cdot\rho_{s})\ \frac{0.157}{\sqrt{t_{s}}}, \tag{2}\]  
where:  
\(C_{s}\) is the surface layer derating factor;  
\(\rho_{s}\) is the resistivity of the surface material;  
\(t_{s}\) is the duration of the shock current. \[I_{G}=3I_{0}D_{f}S_{f} \tag{3}\]  
where:  
\(3I_{0}\) is the system fault current;  
\(D_{f}\) is the decrement factor;  
\(S_{f}\) is the current division factor. \[GPR=I_{G}R_{g} \tag{4}\]  
where:  
\(I_{G}\) is the maximum grid current;  
\(R_{g}\) is the earth grid resistance. The power system parameters consist of two 20 MVA, 11/6.6 kV transformers, and each transformer feeds a switchboard consisting of a 370 kW motor and two lumped loads with a total loading of 400 kVA. Under the N-1 operating condition, the energized transformer will feed twice the normal load. For simulation purposes, the supply transformers were initially solidly earthed. The substation earth grid was superimposed onto the power system to import the actual power system short-circuit study results directly onto the earth grid. These results were then used for the design of the earth grid. Figure 2 shows the earth grid superimposed onto the power system consisting of solidly earthed supply transformers. The ETAP short-circuit study (based on IEC-60909) was conducted for the power system to assess the fault level generated by the supply transformers and back-fed to the supply by the 370 kW motors. The simulation shows that a steady state fault level of 21.877 kA is generated by the transformers and motors, contributing an additional fault level of 0.28 kA, resulting in a total peak fault current of 22.759 kA, which will be seen by the busbars and, subsequently, the substation earth grid through the neutral of the transformers.