Ground Rods
In a high soil resistivity area, it is very difficult to meet the requirement of grounding resistance. Especially in gas-insulated substations or small-sized substations, these can be used to decrease the grounding resistance. A vertical ground rod, of large size is also useful in domestic earthing.
Table of Contents
The principle is to make use of an effective low-resistivity layer under the ground and to maintain the stability of the grounding resistance. The soil resistivity is usually non-uniform along both the vertical and horizontal distributions. For the vertical distribution, a layer of soil at different depths has different resistivities. Generally, the soil within a few meters of the ground surface has a relatively higher resistivity, but this resistivity is unstable and changes with season and climate.
The deeper the soil is, the more stable its resistivity is. Especially in a region where soil resistivity is high and the common methods for reducing the grounding resistance cannot be used, the use of these rods which are connected to the main grounding grid is an effective method to decrease the grounding resistance. The ground rods are likely to penetrate the water layer, and then the effect of decreasing grounding resistance is better.
Usage Coefficient of Long Vertical Ground Rods
Vertical ground rods are arranged along the peripheral grounding conductor of the grounding grid. The arrangement of these rods has five different placement styles and the number of rods N in each style is:
- Four (arranged one in each corner),
- Eight (one in each corner and one in each side),
- 12 (one in each corner, two along each side),
- 16 (one in each corner, three along each side),
- 20 (one in each corner, four along each side).
Each rod is arranged with equal spacing and the length L of a vertical ground rod can be 10, 30, 50, 70, 90, 110, 130, or 150 m. The equivalent radius r of the grounding grid is:
, where A is the grounding grid area.
The definition of the decreased ratio of grounding resistance after using the rod is
Where R0 is the grounding resistance of the grounding grid and R is the grounding resistance after adding the rods.
The definition of the usage coefficient η of N rods is
where RC is the grounding resistance of RP and R0 in parallel, and RP is the grounding resistance of N number of rods in parallel.
Figures below show the curves of the decreased ratio of grounding resistance and the usage coefficient η of the rods with L/r and N after the addition of rods. The following conclusions can be obtained
The decreased ratio ζ increases with the increase in the proportion of L/r for the rod’s length L and equivalent radius r of the grounding grid.
As L/r increases, the usage coefficient η decreases when L/r>1, and the usage coefficient η tends towards saturation.
When L is fixed, the usage coefficient η decreases with an increase in the number of vertical electrodes N, equivalent to decreasing the spacing between the rods. The reason for this decrease in η is that the shielding effect between vertical rods increases with an increase in the spacing between their placement.
When L is fixed, the decreased ratio z of grounding resistance increases as the vertical rod’s number N increases. When N increases to a critical value (from Figure 6.8 we can see the value of N is 8), the decreased ratio increases slowly.
Design Considerations
For a long vertical ground rod, the following rules are useful in the design:
In order to decrease the shielding effect between a horizontal grounding grid and vertical ground rods and to increase the usage coefficient of each vertical rod, it is more beneficial to arrange the rods along the peripheral conductors. If the conditions permit, we should arrange the rods as far outside the substation as possible, to make the spacing of ground rods at least equal to their length.
The number of vertical ground rods and their actual length can be determined according to the requirement of the grounding resistance and the geological structure of the substation site. The basic principle is, that when there is no layer of layer low resistivity under the ground, the length of the vertical ground rod is generally not less than the equivalent radius of the horizontal grounding grid.
The number of ground rods should be generally more than four. However, attention should be paid to the fact that:
(i) The decreased ratio begins to become saturated when the number of vertical ground rods increases to a critical value and
(ii) the construction cost of vertical ground rods is relatively high.
A full investigation of the substation site and the soil characteristics nearby should be made to determine the soil structure. If there is a deep low resistivity layer, the vertical ground rod for grounding is suitable.
However, if the resistivity of the deep layer is higher than that of the surface layer, using a vertical ground rod for grounding makes little sense.
