Substation Earthing System ( Grounding System ) – Earthing Mat
The substation earthing system comprises of a grid (earth mat) formed by a horizontal buried conductors.
The grounding system in substation is very important. The functions of grounding systems or earth mat in include:
- Ensure safety to personnel in substations against electrical shocks.
- Provide the ground connection for connecting the neutrals of stat connected transformer winding to earth ( neutral earthing ).
- Discharge the overvoltages from overhead ground wires or the lightning masts to earth. To provide ground path for surge arresters.
- Provide a path for discharging the charge between phase and ground by means of earthing switches.
- To provide earth connections to structures and other non-current carrying metallic objects in the sub-station (equipment earthing).
In addition to such a grid below ground level, earthing spikes (electrodes) are driven into the ground. They are connected electrically to the earth grid, equipment bodies, structures, neutrals, etc. All these are connected to the station earthing system by earthing strips.
If the switchyards have a soil of low resistivity, earth resistance of the earthing system would be low. If the soil resistivity is high, the mesh rods are laid at closer spacing. More electrodes are inserted in the ground.
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| Earth Mat in 110kV side of a 220/110 kV Substation |
The fence, equipment body, tanks, support, structures, towers, structural steelworks, water pipes, etc. should be earthed.
Earth Resistance Value
The value of earth resistance of the ground system determines the voltage rise of the various earthed points during the earth fault.
If earth fault current is I, earth resistance is R, the voltage rise under short circuit condition would be V= IR.
The permissible potential rise and the maximum possible earth fault current set a limit on the maximum value of earth resistance.
To achieve earth resistance within specified limits, enough number of earth spikes and sufficient surface area of the earth grid and closer ground mesh rods are necessary.
The touch potential and earth potential in the switchyard under any earth fault condition should be within safe limits.
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