Earthing Software Benchmark Study

Compares SafeGrid and CDEGS along with others

Table of Contents

1. Overview of the benchmarks

The Annex H in IEEE Standard 80-2013 contains benchmark case results for comparing and evaluating software tools and methodologies used for the analysis of substation earthing.

The results compare the simple equations from IEEE Std 80 with the results given by some of the commercially available software such as CDEGS, ETAP, SGW, SDWorkstation and WinIGS.

The benchmarks are divided into two categories: soil analysis and grid analysis (resistance, touch and step voltages, transfer voltages).

SafeGrid Earthing Software results were computed and compared with the benchmark results.

2. Soil analysis

The following Table 2.1 shows the Wenner (Four-pin) measurements for two-layer soil models:

Table 2.1 - Wenner field measurements for two-layer soil models

2.1 Two-layer soil models

Two-layer soil models derived from four-pin field measurements of Table 2.2 are shown as followed:

Table 2.2 - Two-layer soil models derived from four-pin field measurements of Table 2.1

3. Grounding system analysis

The grid current for all cases is 744.8A. The soil structure has been limited to uniform and two-layer soils.  For consistency in comparing results between the programs, the touch and step voltages were evaluated at very specific points and with specific guidelines on the points evaluated to determine the worst-case voltages. For example, to determine the step voltage at the corner of the grid, the earth surface potentials were determined at points over the corner of the grid and 1m outside the grid along the diagonal. The step voltage was computed as the difference between the potentials at these two points. For cases where several points (i.e. a grid of points) were used to determine the worst-case touch voltage, the evaluated points were spaced 0.5m apart.

3.1 Grid 1 - symmetrically spaced and shaped grid, uniform soil, no ground rods

The ground grid for this comparison is shown in Figure 3.1.  The equations in IEEE Std 80 compute the touch voltage at the centre of the corner mesh (T1), so this point was chosen for comparison.  The actual maximum touch voltage for this grid shape might be on the diagonal near the centre of the corner mesh, but located slightly nearer the perimeter of the grid (T3).  For some cases, it might also be directly over the extreme corner (perimeter) of the grid (T2).  Thus, these two points were also analysed for comparison.  The equations in IEEE Std 80 compute the step voltage as the difference between the earth surface potential 1 m apart, with one point directly over the corner of the grid and the other on a diagonal and 1 m beyond the first point.  Though the actual worst case step voltage might be at a different location, comparisons were limited to this one location (S1) for this case.  The comparisons are shown in Table 3.1.  For SafeGrid, grid resistance, GPR, touch voltages and step voltages are calculated over the whole area with 0.5 m separation.

The conductor area of 2/0 CU is 67.4 mm2.

Figure 3.1 – Grounding system parameters for Grid 1
Table 3.1 – Comparison of results for grid analysis of Grid 1

3.2 Grid 2 - symmetrically spaced and shaped grid uniform soil, with ground rods

This case is the same as Grid 1, with the addition of twenty 7.5 m rods located at each intersection around the perimeter of the grid.  The touch voltage and step voltage were computed at the same locations as for Grid 1.  For SafeGrid, grid resistance, GPR, touch voltages and step voltages are calculated over the whole area with 0.5 m separation.

Figure 3.2 – Grounding system parameters for Grid 2
Table 3.2 – Comparison of results for grid analysis of Grid 2

3.3 Grid 3 - symmetrically spaced and shaped grid, two-layer soil, with ground rods

The ground grid for this comparison is the same as for Grid 2, except the soil model is changed to a two-layer soil with , , .  The touch and step voltages were computed at the same location as for Grid 1.  For SafeGrid, grid resistance, GPR, touch voltages and step voltages are calculated over the whole area with 0.5 m separation.

Figure 3.3 – Grounding system parameters for Grid 3
Table 3.3 - Comparison of results for grid analysis for Grid 3

3.4 Grid 5 - symmetrically spaced non-symmetrically shaped grid, fence grounded to main grid, two-layer soil, with ground rods

The ground grid for this comparison is shown in Figure 3.4. This grid is non-symmetrical in shape (L-shaped), though it still has symmetrically spaced grid conductors. It also has ground rods of uniform length at every other intersection around the perimeter and has a grounded fence within the confines of the main grid and bonded to the grid. The worst-case touch voltage was computed at all points 0.5 m apart within the fence, plus all points within reach (1 m) outside the fence. The worst-case step voltage (S1) was computed at all points 0.5 m apart within an area defined inward from 1 m outside the perimeter of the grid. For direct comparison, the step voltage (S2) was also compared by determining the difference between earth surface potentials 1 m apart along the diagonal at the upper left corner of the grid. The comparisons are shown in Table 3.4.

Figure 3.4 – Grounding system parameters for Grid 5
Table 3.4 – Comparison of results for grid analysis for Grid 5

3.5 Grid 6 - non-symmetrically spaced and shaped grid, non-orthogonal conductors, two-layer soil, with ground rods at random locations and unequal lengths

The final ground grid for comparison is similar to Grid 3, but with conductors on the diagonal and with corner grounds 7.5 m long and all other ground rods 2.5 m long. The soil model is changed to a two-layer soil with ρ1 = 100 Ω-m, ρ2 = 300 Ω-m, and h = 6.091 m (20 ft). See Figure 3.5. For the computer programs, the touch and step voltages were computed at numerous points to determine the worst case for each. The worst case touch voltage was computed at all points 0.5 m separation within the perimeter conductor. The worst case step voltage (S1) was computed at all points 0.5 m separation within an area defined inward from 1 m outside the perimeter of the grid. For direct comparison, the step voltage (S2) was also compared by determining the difference between earth surface potentials 1 m apart along the diagonal at the upper left corner of the grid.  The comparisons are shown in Table 3.5.

Figure 3.5 - Grounding system parameters for Grid 6
Table 3.5 – Comparison of results for grid analysis for Grid 6

References

[1]        IEEE Guide for Safety in AC Substation Grounding, IEEE Std. 80-2013

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Easily design safe earthing systems in compliance with Standards.

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Easily design safe earthing systems in compliance with Standards.

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