MINI-SUBSTATION EARTHING FOR MV SYSTEMS WITHOUT CONTINUOUS EARTHING CONDUCTOR TO SOURCE SUBSTATION



MINI-SUBSTATION EARTHING FOR MV SYSTEMS WITH CONTINUOUS EARTHING CONDUCTOR TO SOURCE SUBSTATION

MINI-SUBSTATION EARTHING FOR MINI-SUBSTATION CONNECTED TO OVERHEAD LINE

GUIDE LINE FOR SUBSTATION EARTHING

Horizontal Grid
Bury a horizontal grid of copper conductors(10 mm Ø round rod) about 1 m below ground level, the grid extending over the whole area occupied by the substation. All metalwork in the substation (e.g. steel structures, fences, etc.) is then bonded to this earth electrode, so that a direct low resistance path to ground is provided for short circuit currents. A continuous conductor loop should surround the perimeter to enclose as much area as practical. Within the loop, conductors should be laid in parallel lines to form a grid and where practical, along the structures or rows of equipment, to provide for short earth connections. The ratio of the mesh sides is usually between 1:1 and 1:3. Frequent cross-connections have a relatively small effect on lowering the resistance of a grid. Their primary role is to assure adequate control of the surface potentials.

Vertical Rods

In substations no ordinary single electrode is adequate in providing a safe earthing system. In turn, when several electrodes, such as earth rods are connected to each other and to all equipment neutrals, frames, and structures that are to be earthed, the result is, essentially, a grid arrangement of earth electrodes. If the connecting links happen to be buried in a soil of good conductivity, this network alone may represent an excellent earthing system. If the magnitude of current dissipated into ground is high, it is seldom possible to install a grid with resistance so low as to ensure that the rise in ground potential will not generate unsafe surface potential. The hazard can then only be eliminated by control of local potentials through the entire area. While horizontal (grid) conductors are most effective in reducing the danger of high step and touch potentials on the earth’s surface, sufficiently long earth rods will stabilise the performance of such a combined system. If the upper layer has a higher soil resistivity than the lower layers, vertical rods penetrating the lower layers are far more effective in dissipating fault currents, whenever a multi-layer soil is encountered. Rods installed predominantly along the grid perimeter will considerably moderate the steep increase of the surface gradient near the peripheral meshes.


 

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