Methods for decreasing grounding resistance
Decreasing grounding resistance of a grounding system decreases the maximum ground potential rise, it also can decrease the maximum transfer potential. During a earth fault, a serious hazard may take place from transfer of high potential from the substation’s grounding grid to outside locations. This may be transferred by those objects connected with the grid, such as communication circuits, conduits, pipes, metallic fences, low-voltage neutral wires and so on. In a region of high soil resistivity, it is very difficult to reduce the grounding resistance.
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When a ground fault occurs, the GPR will be very high. For example, if the grounding resistance is 5Ω, the GPR/EPR will rise to 10 kV with a 2000A short-circuit current will be injected into the grounding grid.
Several methods have been tried to decrease the grounding resistance of a grounding system. Regular methods includes increasing the grounding grid, connecting the main grounding grid to an additional auxiliary external grounding grid, increasing the burial depth of the grounding grid, utilizing natural grounding objects such as the steel foundations of structures, adding vertically long grounding electrodes and exchanging the soil surrounding the grounding grid for low resistivity materials. These methods are suitable for different geological situations but that does not mean they should be taken up independently.
Expanding the Grounding Grid Area of a Substation
Substation’s grounding resistance can be calculated by 
. From this equation we can see that the grounding resistance is inversely proportional to the square root of the area. The greater the area is, the lower is the grounding resistance. Therefore, expanding the area of the substation grounding grid is an effective method for decreasing grounding resistance effecticely.
The method of expanding the grid area for decreasing grounding resistance can be used in specific regions. For substations in mountain regions, it is not possible to expand the grid area. What is more, for substations in urban districts, it is impossible to find suitable land to expand the grounding grid area.
Connecting with an External Grounding
A connection with an external grounding is a method which involves connecting the main grounding grid of the substation with an auxiliary grounding grid which is laid in a low soil resistivity region outside the substation’s area for decreasing grounding resistance of the whole grounding grid system.
It should be therefore noted that there is a significant potential difference between the substation grid and the auxiliary grounding grid in a fault condition. When a high-frequency impulse, such as lightning, stresses the system, the potential difference will be especially great. So, it must therefore be ensured that there are several grounding conductors in parallel connecting the main grounding grid with the auxiliary grounding grid.
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")
, we can see that if we increase the burial depth of a grounding grid, it will decrease the grounding resistance if other parameters remain unchanged. But the effect of this method is not obvious, especially in a high soil resistivity region. Therefore an engineering project generally does not adopt this method. The burial depth of a substation grounding grid is generally about 0.8 m.
Using Natural Groundings
Natural grounding element contains the steel reinforced concrete frames of buildings, hydropower intake trash racks, gates, water pipes and so on. These natural groundings, themselves have a low grounding resistance, therefore should be connected with the main grounding grid in order to achieve the purpose of decreasing grounding resistance. Especially in a hydropower substation, using natural grounding can potentially be used for decreasing grounding resistance effectively without increasing the investment.
Partially Changing the Soil
Soil resistivity directly affects grounding resistance. For some grounding devices which are located in a high soil resistivity region, if it is difficult to use other methods for decreasing the grounding resistance, the method of partially changing the soil can be used. We can make use of a low resistivity soil or low resistivity material to replace the original high resistivity soil around the grounding device or electrode. Apart from chaning the soil, soil treatment can also be done for the said purpose.
