GROUNDING GRID DESIGN CRITICAL PARAMETERS BASIC INFORMATION

The following site-dependent parameters have been found to have substantial impact on the grid design: maximum grid current IG, fault duration tf, shock duration ts, soil resistivity ρ, surface material resistivity (ρs), and grid geometry.

Several parameters define the geometry of the grid, but the area of the grounding system, the conductor spacing, and the depth of the ground grid have the most impact on the mesh voltage, while parameters such as the conductor diameter and the thickness of the surfacing material have less impact.

Fault duration (tf) and shock duration (ts)

The fault duration and shock duration are normally assumed equal, unless the fault duration is the sum of successive shocks, such as from reclosures. The selection of tf should reflect fast clearing time for transmission substations and slow clearing times for distribution and industrial substations.

The choices tf and ts should result in the most pessimistic combination of fault current decrement factor and allowable body current. Typical values for tf and ts range from 0.25 s to 1.0 s.

Soil resistivity (ρ)

The grid resistance and the voltage gradients within a substation are directly dependent on the soil resistivity. Because in reality soil resistivity will vary horizontally as well as vertically, sufficient data must be gathered for a substation yard.

Because the equations for Em and Es given assume uniform soil resistivity, the equations can employ only a single value for the resistivity.

Resistivity of surface layer (ρs)

A layer of surface material helps in limiting the body current by adding resistance to the equivalent body resistance.

Grid geometry

In general, the limitation on the physical parameters of a ground grid are based on economics and the physical limitations of the installation of the grid. The economic limitation is obvious. It is impractical to install a copper plate grounding system.

Clause 18 describes some of the limitations encountered in the installation of a grid. For example, the digging of the trenches into which the conductor material is laid limits the conductor spacing to approximately 2 m or more.

Typical conductor spacings range from 3 m to 15 m, while typical grid depths range from 0.5 m to 1.5 m. For the typical conductors ranging from 2/0 AWG (67 mm2) to 500 kcmil (253 mm2), the conductor diameter has negligible effect on the mesh voltage.

The area of the grounding system is the single most important geometrical factor in determining the resistance of the grid. The larger the area grounded, the lower the grid resistance and, thus, the lower the GPR.