Design Principles of Substation Grounding System

The most important factor in the substation is the grounding. The system neutral is connected to the ground through grounding. The discharge path for surge arrestors and safety of operating personnels can also be achieved through grounding

       The grounding gives low resistance path to earth and reduces rise in ground potential which is dependent on magnitude of fault current and resistance of grounding system. The low resistance of substation ground can not be obtained in deserts and rocky areas.

       The convenient way for getting proper ground connection is through grids. If the ground rods are used in addition then it reduces the ground resistance. Depending on the size of the substation, nature of soil and the grounding resistance required, the size of grid and number along with length of driven rods can be determined.

       Under the fault condition, the potential of earth and its gradient over the surface out from the electrode is dependent on the ground resistance. The systems with higher values for maximum ground fault current, it is not possible to have lower values for ground resistance so as to have rise in grounding system potential to safe value.  

       The practical solution for grounding at substation yards is through grid or mat. The grid consists of a number of meshes and connected to several earth electrodes driven at intervals.

       The total number of electrodes required is determined by using the expression.

                           Number of electrodes = Maximum Fault Current/ 500

       The grounding resistance for a grounding grid or mat is calculated from the following expression,

       Here ρ_s is resistivity of soil in Ω-m, L is total length of buried conductor in meters and r is radius of circular plate in meters.

       The size of grounding conductor should be appropriate so as to have thermal stability for ground fault current and it should be mechanically strong. The minimum cross section for the grounding conductor having required thermal stability is determined from the following expression,

       Where I_F is fault current in amperes while t is time in seconds for the operation of protective relays including circuit breaker tripping time. C is a constant. Its value is 70 for steel having temperature rise of 400^oC while for copper with temperature rise of 300^oC, its value is 165.

       The minimum size required for proper mechanical strength in case of steel is 61 mm². for copper it is 107.2 mm² while in case of aluminium it is 195 mm².

       In case of grounding conductors made up of steel, its size should be checked for corrosion. If the soil is moderately or severely corrosive then the steel strip of minimum thickness 6 mm and minimum cross section area should be 200 mm².

       The earthing grid system is normally extended over the total substation yard and in few cases, several meters beyond it. The grounding conductors should have low impedance. They should be able to carry prospetcive fault current without getting fused or damaged. They must take account the future expansion of connected power system.