Wednesday, October 12, 2011

Grounding Grids

       The low ground resistance in case of high voltage substations can be obtained with the use of interconnected ground grids. In a typical grounding grid system, a number of interconnected bare soild copper conductors are buried at a depth of 0.3 to 0.6 m and spaced in a grid pattern. It provides common earth for all devices and metallic structures in the substation.
       At each of the junction point, the conductors are bonded together. This system is usually suported by a number of vertical rods about 3 m long at some joints.
       If a is cross-sectional area of copper, in circular miles, t is the fault duration in seconds, Tm is the maximum allowable temperature and Ta is the ambient temperature then the size of grid conductors required which prevents fusing under the fault current is given as,
       If the grid depth is less than 0.25 m then the earthing resistance of the grid is given by,
       Here  R = Grid resistance in ohms
                 a = Ground area occupied by grid in m2
                 L = Total length of buried conductors in m
       But when the grid depth is greater than 0.25 m then earthing resistance is given by,
       The effective grounding of the equipment is possible through the grid. Also the voltage gradient at the surface of the earth can be controlled at safe value for human contacts with the addition of ground rods, the ground resistance further reduces when soil resistivity in the upper layer is more than the soil underneath.

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