Methods for decreasing grounding resistance
Decreasing grounding resistance of a system can efficiently bring down the ground potential rise which is fault current multiplied by the grounding resistance. Thus it makes the site safer during faults. Decreasing grounding resistance also makes the transfer potential lower, which is the potential between substation ground and other adjacent ground, can be fatal if too large. The cause of transfer potential can be communication circit, metal pipes, conduicts, fence etc.
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During a earth fault, assuming the ground resistance to be 10 ohms and fault current flowing to the ground is 1500 Amps, then the GPR will be 15KV.
To decrease grounding resistance of a system various methods are practised around the world. These can be expansion of the grounding grid, adding an alternate connection to a different grounding grid, increase the depth of burrial of the grid, adding long electrodes, decreasing the spacing between electrodes, changing the soil, chemical charging of the soil etc. These can bring definite change in the soil resistance.
Expanding the Grounding Grid Area of a Substation
The grounding resistance of a substation is calculated by 
. Where R is the grounding resistance in ohm, ρ is the soil resistivity in ohm-meter, A is the area of the grounding grid in m2, LT is total length of grounding conductors in meters. From this equation, it can be inferred that the grounding resistance is inversly proportional to the area of the grounding grid. Therefore, more area of grounding grid will result in smaller grounding resistance. Therefore, sufficient expansion of the grounding grid of a subsation can bring down the grounding resistance effectively.
This method to increase the grounding grid area is specific to subsations having the area for expansion works. Those substation located in hills and urban areas, where finding the land for expansion is not possible, are restricted from the use of this method.
Connecting with an External Grounding
Connecting with an external grounding grid requires a auxiliary grid nearby the substation with low soil resistivity for decreasing grounding resistance, which can be connected by parallel grounding connections to the main grounding grid.
During fault conditions, there will exist high potential difference between the main and auxiliary grid. In case of lightning impulse the potential difference will be even greater. Therefore, the connection between main and auxiliary grid has to be made with increased numbers of parallel connecting grounding conductors with little resistance.
Increasing the Burial Depth of the Grounding Grid
From, ![R = \rho \left[ \frac{1}{L_T} + \frac{1}{\sqrt{20A}} \left( 1 + \frac{1}{1 + h \sqrt{\frac{20}{A}}} \right) \right]](https://megawatts.in/wp-content/ql-cache/quicklatex.com-61257c6cd2be9390ad24c5d52ff1437a_l3.png "Rendered by QuickLaTeX.com")
, Where R is the grounding resistance in ohm, ρ is the soil resistivity in ohm-meter, A is the area of the grounding grid in m2, LT is total length of grounding conductors in meters, h is the burial depth in meters. From the expression, it can be seen that when the h increases the entire term decreases. It means keeping other things constant, when the burrial depth increases the grounding resistance decreases.
However, this method does not practically provide good result in lowering grounding resistance specially in high soil resistive areas, making this hardly accepted in practice. It may be noted that 0.8m is the normal burial depth of grounding grid in a substation.
Using Natural Groundings
Natural grounding elemnets contains mettallic structures connected to ground which can effectively result in decreasing grounding resistance as they provide parallel path for grounding fault currents. The natural grounding elements include steel reinforced concrete foundations, steel structures, hydro intakes, steel penstocks, burried water pipes and etc. Since, there addition to the grounding grid, increases effective grounding area, also they have low grounding resistance, already a grounded infrastructure, incurrs no high expenditure, therefore use of the natural grounding devices will result in decreasing grounding resistance specially for hydro power stations.
Partially Changing the Soil
In the equation of grounding resistance we have seen that the soil resistivity directly impacts the grounding resistance of a system. In areas with high soil resistivity, the grounding resistance is high and cannot be efficiently lowered by other methods of decreasing grounding resistance. Hence partially charging the soil by replacing existing high resitive soil with low resistive soil around the grounding electrode. Also chemical treatment can be done to lower the resistivity of the soil.
This article is a part of the Safety and Earthing page, where other articles related to topic are discussed in details.
