This system is also referred as arc suppression coil grounding. In the previous earthing methods that we have discussed the earth fault on any one of the phases caused total shut down of the system. So continuity of supply can be maintained. This is not he case with ungrounded system whee fault on one phase will not cause other phases to supply power. This method of grounding has this advantage of isolated neutral system along with reduced possibility of arcing grounds and numerous other advantages.
It consists of a coil called Peterson coil or Ground fault neutralizer or arc supression coil whose function is to make arcing earth faults self extinguishing and in the case of sustained faults to reduce the earth current to low value so that system can supply power with one line earthed.
This system works on the principle that when inductance and capacitance are connected in parallel, resonance takes place between them and because of the characteristics of resonance, the fault current is reduced or can be neutralized.
The system with fault on phase B is show in the Fig. 1(a). The corresponding phasor diagram is shown in the Fig. 1(b).
|Fig. 1 Resonant grounding|
An arc supression coil is an iron-cored reactor similar to oil immersed transformer connected between neutral of system and earth. This coil is provided with number of tappings so that it can be tuned with the capacitance which may vary due to varying operational conditions.
As the system operation is similar to isolated neutral system, the phase to earth voltage of healthy phase is times the normal phase voltage and the resultant capacitive current is √3 times the normal charging current of one phase. The resultant capacitive current will lead by with faulty phase voltage while the fault current lags by with faulty phase voltage.Now we have, IF = IC at resonance
IF = Vph/XL ; IC = 3Vph/XC
The rating of the coil is continuous and equal to the maximum earth fault current. If a double phase to ground fault or another ground fault occurs, the current flowing through the coil is more. This can be prohibited with closing of a circuit breaker after certain time lag. The earth fault current flows through the parallel circuit by passing the arc supression coil. Here the circuit breaker is normally open and closes after the closure of relay tripping circuit by passing are supression coil.
This method of neutral grounding is used in medium voltage overhead transmission line which are connected to system generators through intermediate power transformers. This is because the higher insulation requirement on the apparatus associated with arc supression coil grounding system is easily incorporated in power transformers than in generators. Also the overhead lines are usually subjected to earth faults due to lightning. Hence protection is required.
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