Power System Earthing
It is necessary to earth a power system at a suitable point by a suitable method as it offers many advantages as
- It provide Safety to the electrical equipments against over-current
- It provides better safety
- It reduces the maintenance expenditure
- It improves the service reliabilty
- It provides improved lightning protection
When the neutral of the power transformer and generator is directly connected to the earth, then the system is said to be solidly earthed. The solidly earthing does not make a zero impedance circuit as generator or transformer would have its own reactance in series with the neutral circuit. The direct earthing of a generator without external impedance causes earth fault current from the generator to exceed the maximum 3-phase fault current if the impedance of the generator is too low. This results in Stator winding damage as the short circuit current during fault will exceed the short circuit rating of the winding for which it was designed. For this system of earthing, it is necessary that the earth fault current shall be in the range of 25% to 100% of the 3-phase fault current to prevent the development of high transit over voltages.
In resistance earthing the neutral of the generator or transformer is connected to the earth trough a resistance in series.
Advantage Of Resistance Earthing are:
- It reduces the line voltage drop caused when earth fault occurs
- It reduces electric shock hazards to the persons, caused by stray earth fault currents in the return path
- It reduces the mechanical stresses in the circuit carrying fault current
- It reduces the effect of burning of faulted electrical equipment
The magnitude of the resistance to be used should be such that it should limit the earth fault current to a value which will reduce minimum damage at the point of the fault.
In reactance earthing a reactor is connected in between the neutral of the machine and earth. A low reactance is connected in series with the neutral of the machine to limit the earth fault current through the generator. This current should not be greater than the 3-phase fault current of the generator. The earth fault current of the earthed system should not be less than 25% of the 3-phase fault current in order to minimize the transient voltages