The concept behind these new equations is to use, as a model, known basic equations and to multiply them by correction factors to obtain values consistent with the EPRI EL-3099 test results
For the corner mesh voltage E m, the basic equation for the surface voltage at a distance from a conductor has been utilised:
The surface voltage above the grid conductors is a special case of the basic formula
The voltage at a distance X outside a corner is another special case:
where S K is a 2-layer model correction factor.
The geometric factors are functions of n, D and grid plate radius and include a shape factor S s. Their complexity, to be consistent with the test results, is less that of the equations in the new edition of IEEE Std 80. S K is a 2-layer factor which, when combined with the unique method of providing an apparent resistivity value from Wenner test results, provides consistency with the EPRI EL-3099 test results.
The consistency of the above formulae with the test results can be judged from the following table, which indicates the percentage difference to the test results.
That such a close consistency occurs is in itself an indicator that the EPRI EL-3099 test results and the test techniques are representative of actual conditions. The tests reported in EPRI EL3099 span grids with areas differing by 9:1, conductor spacing from 6m to 90m, burial depth from 0 .5m to 2m and includes square, rectangular and L-shaped grids, square and rectangular meshes and 2-layer ground. Yet, as the above table shows, the test results can be so well predicted by equations representing the basic parameters that, in our understanding, effect the changes in surface voltage
graph comparisons between new equation values and test results
In the graphs presented below, the results from model grid tests reported in EPRI EL3099 are plotted with dots. The continuous curve derives from the new E m equation and the dotted curve by the E m equation in IEEE STD 80 1986.
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fig 1 |
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fig 3 |
fig 3 |
The graphs demonstrate the consistency of the new E m equation with the EPRI EL- 3099 test results. Of particular interest may be Figure 2, which shows an irregular curve pattern of the test results when a mixture of square and rectangular meshes are included. The 2nd, 4th , 6th and 7th plotted values along the X-axis are rectangular meshes, the others being square meshes. Due to the inclusion of the shape factor in the new E m equation, the irregularity is depicted.