Working:
In all the methods of earthing discussed above, the other healthy phases cannot supply power when the ground fault occurs on one line until the fault is cleared.
In Peterson coil type earthing system the other healthy phases can supply power when a ground fault occurs on one phase. Other advantages also result in this method. Peterson coil is an iron cored reactor.
Reactance of the coil is so adjusted that it produces resonance with the capacitance of the healthy phase. Arcing is prevented due to this.
The other two healthy phases continue to supply power when an earth fault occurs on one phase. Peterson coil is also called the arc suppression coil or ground fault neutralizer. The grounding obtained by this method is called resonant grounding.
Phaser diagram is shown in figure
Current vector IBR is ahead of the voltage vector VBR by 90°. Similarly current vector IBY is ahead of voltage vector VBY by 90°. Vector sum of currents IBR and IBY is ICF which is out of phase by 180° with current IF
= 3 x line to neutral charging current
During the condition of fault, the voltage of the faulty phase comes across the arc suppression coil and current IF lags about 90° and is in phase opposition with the current ICF.
So the arc does not persist during the fault and fault current or capacitive current do not flow even then inductance of the coil and system capacitance do not balance exactly so some resonant current flows.
Calculation of the inductance can be made as follows.
Advantages :
1. Capacitive charging currents flowing at the time of fault are neutralized by the inductive reactance of the Peterson coil.
2. Value of the fault current can be kept practically equal to zero so the phenomenon of arcing ground does not occur. So over voltages are not produced.
3. There are tapings in the Peterson coil so if there is change in the length of line or in the capacitance of the line, then the value of inductance can be adjusted with the help of taping.
4. The other two healthy phases continue to supply power when the fault occurs on one phase.
5. The earth fault relay can be employed.
6.The lightning stroke is suppressed.
Disadvantages:
1. The insulation of the equipment has to be kept more as the voltage of the healthy phase becomes times the normal voltage.
2. Due to the above reason the rating of the lightning arrester has to be kept equal to the full voltage.
3. This method is useful for the line fed by transformer but it is not used for the line fed by the alternator as it becomes difficult to provide increased insulation for the alternator winding.