Restricted Earth Fault Protection :

➤Usually it is a practice to connect the neutral of star connected generator to the ground. (Various advantages of providing neutral grounding are explained in previous article) When neutral is solidly grounded, it is possible to protect complete generator or transformer winding against phase to ground fault.

➤But usually neutral of generator or transformer winding is earthed through resistance/inductance to limit earth fault currents.

➤winding from earth fault and the % of winding protected depends on the value of neutral earthing resistor and the relay setting.

➤Generally 80 to 85% of generator winding is protected against earth faults using differential protection. The remaining 20 to 15% winding from neutral side left up protected by the differential protection.

➤To achieve 100% protection against earth faults, a separate earth fault protection is provided in addition to differential protection.

➤On the occurrence of earth fault, fault current Ir flows thoght a part of the dc ganartor winding and neutral to ground circuit.

➤The corresponding secondary current of current transformer I. flows through the operating coil and restricted earth fault coil of the differential protection.

➤ If the earth fault occur at point f of the generator winding, voltage Vat is available to drive earth fault current Ir through the neutral to ground connection

➤If fault occurs near to neutral point i.e. terminal 'a', the driving voltage Var will be very less. Hence earth fault current It is having considerably low value.

Percentage Differential Protection with Restricted Earth Fault Relay

➤Relay setting can not be kept to a very low value (i.e. setting the relay too sensitive to sense the earth fault currents of small magnitudes). Because if too sensitive the relay may respond during through faults or other faults due to inaccuracies of CTs or saturation of CTs etc.