Design of Substation Grounding

For a given area to be grounded:
Ground resistance, Rg:
1. Decreases when the number of meshes increases (decrease becomes negligible for a large number of meshes).
2. Gradual decrease with burial depth.
3. Increasing the number or depth of ground rods effectively reduces the  resistance until saturation occurs.

    The worst touch and step voltages occur in the outer meshes. Reducing mesh spacing toward the perimeter reduces these voltages.
    Increasing the concentration of deep ground rods around the perimeter will also effectively reduce the excessive step and touch voltages in the outer area.
    For a given length of grid conductor or ground rod, the ground rod discharges much more current into the earth than does the grid conductor.
    Because the current in the ground rod is discharged mainly in the lower portion, the touch and step voltages are reduced significantly compared with that of the grid alone.

Evaluation of Ground Resistance
where
Rg = station ground resistance, ohm
ρ = average soil resistivity, ohm-meter
A = area occupied by the ground grid, m2
h = depth of the grid, m
For better estimates of the ground resistance of the grids with ground rods, equations
such as the Schwarz formula, given below, should be used.
R1 = resistance of grid conductors
R2 = resistance of all ground rods
R12 = mutual resistance between the grid conductors and group of ground
rods
Schwarz developed a set of convenient formulas, defining R1, R2, and R12 in terms of basic design parameters, assuming uniform soil. However, in practice, it is often desirable to drive ground rods deep into the ground to reach more conductive soil. The following site-dependent critical parameters have been found to have substantial impact on the grid design:

    Maximum grid current (IG)
    Fault duration (tf) and shock duration (ts)
    Soil resistivity (ρ)
    Resistivity of surface layer (ρs)
    Grid geometry

 Design Procedure
Refer to fig. 13.13 (design-procedure block diagram) and the index of design parameters
in tables 13.3 and 13.4. These include

    Obtaining utility (power company) standards and guidelines
    Determining substation area (A) and computing soil resistivity from field data
    Determining uniform soil or two-layer model
    Determining maximum expected future fault current (3I0) and maximum possible clearing times (including backup)
    Determining the conductor size for different applications, such as grid,down leads, and equipment-grounding conductors
    Determining tolerable touch and step voltages
    Carrying out initial design (the initial estimates of conductor spacing and ground rod locations should be based on the current IG and the area being grounded)
    Estimating the grid resistance or resistance of the grounding system (for the final design, more accurate estimates of the resistance may be desired, especially when ground rods are used)
    Determining maximum grid current (IG) that flows between the ground grid and surrounding earth and calculating the ground-potential rise (GPR)
    Computing the mesh (Em) and step (Estep) voltages for the grid (revise the preliminary design if the computed mesh voltage is greater than the tolerable touch voltage)
    ,Revising the design if both the computed touch and step voltages exceed the tolerable voltages (these revisions may include smaller conductor spacing and additional ground rods)
    Adding ground rods that may be required at the base of surge arrestors,transformer neutrals, etc.

Steps in Substation Grounding Design