Abstract:
The earthing of electric installations plays an important part as regards the
behaviour of the network and personal safety when there are disturbances.
Presently all earthing designs are performed by contractors and submitted to
Ceylon Electricity Board for approval. But due to lack of information at the tender
stage majority of the contractors are unable to bid the earthing design part
accurately resulting deviations to the contract during the implementation stage.
To minimize this problem Transmission Design Branch and the Transmission
Maintenance Branch of the Ceylon Electricity Board jointly decided to do the
Earthing Design and submitted with the tender for bidding.
After the initial soil resistivity measurements, it was found that the resistivity
values were very high compared to other Grid Substation locations. Therefore it
was decided to do an earthing design for Horana Grid Substation to identify the
suitabil ity of proposed land and identify the land to be developed to construct an
additional earthing mat to transfer the earth fault current. Without a properly
designed grounding system large potential differences can exist between
different points within the substation itself. Under normal circumstances, the
current flow through the grounding grid from line - to - ground faults that
constitutes the main threat to personnel.
The earthing design for Horana Grid Substation has the following objectives.
1. Ensure such a degree of human safety that a person working or walking in
the vicinity of grounded facilities, is not exposed to the danger of a critical
electric shock. The touch and step voltages produced in a fault condition
have to be at safe levels. A safe value is one that will not produce enough
current within a body to case ventricular fibrillation.
2. Provide means to carry and dissipate electric currents into earth under
normal and faulty conditions without exceeding any operating and
equipment limits or adversely affecting continuity of service.
3. Provide grounding for lightning impulses and the surges occurring from
the switching of substation equipment, which reduces damage to
equipment.
4. Provide a low resistance to protective relays to see and clear ground
faults, which improves protective equipment performance, particularly at
minimum faults.
IV
