Answer / anaz.a

restricted earth fault protection is a protection scheme given in assistance to the differential protection.it is given to protect an earth fault which occurs very close to the neutral point in star connected windings of the transformer.this kind of fault a very low fault current only and this current may not be sufficient to activate the differential protection relays.
The sensitivity of the differential relays cannot be increased beyond a limit since this may lead to unnecessary tripping(the transformer may trip even for an earth fault occurring on the outgoing feeder Side)

Hence a special scheme of protection called REF(Restricted earth fault)is provided in order to protect a restricted area of the winding from internal earth fault.

Answer / muhammad shomail

Yes ; it is true.

Answer / bandarawy

Due to the fact that current transformers always have a
certain error, the differential current always differs from
zero as soon as there is a load current flowing. Especially
during a through-fault condition with high short circuit
current , the differential current may be very high due to
the error of the current transformers. Depending on the
sensitivity of the setting value of the overcurrent
protection relay, when it is used as a simple differential
protection relay, unwanted tripping may occur.
For this reason, it is necessary to stabilize the
differential protection by means of a so called bias
current. For the restricted earth-fault protection using
the biased differential protection principle

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