A restricted earth fault is a normal earth fault that is restricted to a particular zone. Hence, the restricted earth fault protection takes care of the faults that occurred only within that particular zone neglecting the faults outside the restricted zone.
Protection Scheme for Restricted Earth Fault of Generator :
If the neutral point of an alternator is not accessible for connecting CT in order to provide percentage differential protection and it is grounded through some resistance then a protection scheme known as restricted earth-fault protection is provided. The connection diagram of such a protection scheme is shown in the figure below.
It consists of three current transformers connected in each phase at the outgoing side of the winding. The secondaries of the CTs are connected in parallel. A single CT is connected in the line connecting the alternator to the earth. The relay operating coil is connected across the secondaries of the CT. The neutral grounding is achieved by grounding resistance.
Under normal conditions, the sum of the line currents of the alternator will be zero, thereby inducing zero currents in the secondaries of the CTs. Hence no current flows through the operating coil of the relay since neutral also carries zero current, therefore relay will remain inoperative.
Suppose if the fault appears at F2 i.e., outside the protected zone, all the line side CTs, and neutral side CT will be energized in such a way that the current is balanced. Hence, the relay will not operate.
For an internal line to ground fault i.e., fault at F1 which is in the protected zone, the neutral side CT is more energized and hence the relay operates if the residual current is greater than the relay pickup value.
For an internal line to line faults, no CT is energized and hence the relay does not operate. However, in most cases, the internal line-to-line faults develop into earth faults. Under these circumstances, the relay will operate as the neutral side CT will be more energized.
As this protection scheme provides protection only against internal ground faults and does not respond to internal line faults and through faults, it is named restricted earth-fault protection. Moreover, due to the presence of grounding resistance at the neutral side, this protection scheme does not provide protection to the stator winding completely against ground faults.
It is a general practice, to earth the neutral point of the alternator through a resistance. This grounding resistance will limit the earth-fault current. The earth-fault current will be more for faults away from the neutral point and it will be less for faults nearer to the neutral point.
Thus, for less severe faults (i.e., faults near the neutral point) the fault current may be restricted so that, the secondary current of the CT falls below the pick-up current of the relay. Hence, a certain percentage of the winding nearer the neutral point remains unprotected by the restricted earth-fault protection.
Usually, the emf induced in the stator winding up to 10 to 15% of total winding from the neutral point will not be sufficient enough to produce heavy electrostatic stress which results in insulation failure. Hence, the probability of insulation failure in this part of the winding is almost negligible. Thus, there is no need of providing protection for this part of winding against ground faults.
But, for the major part of winding which is away from the neutral point, there exist the chances of insulation failure. Thus, it is necessary to provide protection for this part of winding against ground faults. Hence, the restricted earth-fault protection provided to alternators does not provide protection against earth-fault to the complete winding.
