Methods for improving the safety of grounding system
If calculations based on the initial design of the grounding system indicate that dangerous potential differences can exist within a substation, the following remedies should be studied and applied where appropriate for improving the safety of grounding system.
Table of Contents
Decrease the Substation Grounding Resistance
Decreasing the resistance of grounding in a substation will decrease the maximum GPR/EPR, and hence, the maximum transfer potential. The way that is most effective to decrease the grounding resistance is by increasing the area occupied by the earth grid for improving the safety of a grounding system.
Another method is to effectively decrease the resistivity of the soil region neighboring the grounding grid, because the soil resistance of this region provides a large part of the grounding resistance of the substation. In case a deep low-resistivity soil layer exists, the grounding resistance can be effectively decreased by arranging long vertical ground rods or ground wells to penetrate into it. Thus, improving the safety of grounding system
However, decreasing the grounding resistance of a substation does not mean an appreciable decrease in the local potential gradient, because this depends on the arrangement of the grounding conductors.
Closer Grid Spacing
By making the spacing of grid conductors small, the grounding grid behaves similar to a metal plate. Thus, dangerous potentials within the substation can be eliminated, but at a cost. It is much difficult to decrease the dangerous potential differences outside the peripheral conductors, especially for a small substation in a high-resistivity region.
However, by burying the grounding conductors outside the fence of the substation, it is usually possible to ensure steeper gradient is immediately outside the grid perimeter. Thus, the most dangerous touch voltage levels can be controlled. Thus improving the safety of grounding system.
Another economical and effective way to control gradients, improving the safety of grounding system is to increase the density of ground rods around the perimeter, while the density in the center of the grid may be decreased.
Other approach to control the peripheral gradient and step voltage outside the substation fence is to bury two or more parallel conductors around the perimeter at increasingly greater depths as the distance from the fence of the substation is increased.
To attain a more uniform potential distribution on the ground surface above the grounding grid is to arrange the grounding conductors with unequal spacings. This means the spacing near the perimeter of the grid is small, but in the center of the grid the spacing is large. This approach can effectively equalize the potential gradient and limit the number of conductors used in the grounding system. Which will help in improving the safety of grounding system
Increasing the Serial Resistance with the Human Body for improving the safety of grounding system
Paving with a surface-layer material of high resistivity is another important method, because it offers higher resistance in series with a human body, and it decreases the current through the body, consequently allowing for higher touch and step potential. Thus, improving the safety of grounding system.
A layer of high resistivity material, such as gravel and cobblestone, is mostly spread on the soil surface above the grounding grid. The wet cobblestone used in the substation has a resistivity of 5000 Ω m. If the thickness of the layer is 10–15 cm, the possibility of danger is decreased by one-tenth.
Many experiments have been carried out in Germany and other parts of the world and results suggest that, when people stand on the wet cobblestone layer and touch a water tap, the current is about one-20th of the current compared to when people stand on grass without a cobblestone layer.
It should be pointed out that the conclusion above is made under the assumption of a clean layer of cobblestone. However, in reality, some stones are pushed into the earth and the gap is filled with dust and soil, which decreases the insulation of the high resistivity layer.
When a high resistivity material layer has already been spread on the surface, but the safety requirement is not satisfied, the thickness of the layer should be increased. The upper limit of the thickness is 15 cm.
Diverting the Fault Current to Other Paths for improving the safety of grounding system
By connecting overhead ground wires of transmission lines or by decreasing the tower footing resistances in the vicinity of the substation, the part of the fault current will be diverted from the earth grid. In connection with the latter, however, the effect on fault gradients near tower footings should be thoroughly checked.
Meanwhile, the fault current might flow through the ground wire of a transmission tower, therefore selecting a ground wire with a big cross-section and high conductivity for a few spans is a must. For a small substation located at a high resistivity area, this might be the only way for improving the safety of grounding system.
Limiting the Total Fault Current
If it is feasible, limiting the total fault current will decrease the GPR and all gradients in proportion. Other factors of protection system, however, will usually make this impractical. Moreover, it should not be accomplished at the expense of a greater fault clearing time, which can increase the danger rather than decreasing it.
Setting the Groundings of Structures
A ban on approaching any area with a big potential difference would appear when a fault happens, so a pedestrian ban should be set in this area to reduce the possibility of electrical contact. After the initial design, if there is still a dangerous potential difference, methods should be taken to ease the situation. The designers must take care of this problem in the design stage itself to make the construction easier.
Preventing the Use of Isolated Grounding
Generally, an isolated grounding within the substation’s region is a hazard and therefore it is adopted rarely. Because the grounding wire and the protective wire for electric equipment are separated, it is usually assumed that a short-circuit current would not flow through the protective wire and thus the high potential of the protective wire would be avoided.
However, this conclusion is problematic:
The resistance of the protective wire or ground wire is smaller than that of either one when they are connected together.
When the insulation is broken somewhere in the substation, there still is current flowing through the protective wire.
It is inevitable that grounding electrodes are connected to each other in the same area, because absolute separation is impossible.
Even if absolute separation is realized, there still might be a dangerous high potential at the connecting point.
Safety Check of Existing Substation Grounding System
For an existing substation, to check whether the substation grounding is safe or not, the following formula can be adopted. If the following requirement is met, the substation is considered as safe
Where, Km is the Spacing factor for mesh voltage, Ki is the correction factor for grid geometry, ρ is the soil resistivity, Ω·m, ρs is the surface layer resistivity, Ω·m, LT is the total effective length of grounding system conductor, including grid and ground rods in m and IG is the maximum grid current that flows between ground grid and surrounding earth including dc offset in A.
