What is the use of REF (Restricted earth fault protection) on transformer?

Hi,
Restricted earth protection which also known as Differential Earth protection by which a
particular defined zone is protected (restricted).

For REF, the neutral point of the Transformer (star point) is required to be earthed . The REF composed from 4CTs, one to be installed on the Earthed-Neutral while the remaining 3no. to be for phases(R,Y,B).

The REF arranged as follows:
One side of the 4 CTs to be connected together and the other terminals of the phases Cts to be connected to the first terminal of their operated relays. The other terminals of the relays to be connected together to the earth relay & the other Neutral CT lead.

Operation:-
The current passing through the earth relay equal to the difference between the current of earthed-neutral & the sum (resultant) of the phases current (IR+IY+IB-IN)=0.
Therefore, there will be no current through the earth relay unless there a fault occurs within the protected zone (prior to Phases CTs).

thanks sir

The term "restricted" comes from the fact that you are only looking at a restricted zone as aptly described above and not the complete or total zone differential coverage.

Agree above, its purpose is to provide senstive but fast operation for faults within the neutral grounded transformer winding, yet allow "through" neutral currents to flow for long enough for faults in downstream equipment to be detected and cleared by the downstream plants protection equipment.

it can be done by O/L AND E/L relay na?

na.... those are not zonal.

@Omer sir,i have a doubt whether REF protection can be used for delta connected transformer as well?If not,why?Please explain the maximum percentage to which the winding is unprotected?I had read that it is 15%.Is it correct?
With regards

I presume that the Restricted Earth Fault Protection corresponds to the Impedance Grounded Fault Protection in terms of US Standards. In this case the neutral of the trnasformer is grounded (earthed) thru an Impedance. The impedance may be a high resistance, low resistance, or a combination of resistance and inductance (Z). The purpose of this Imdedance grounding (earthing through an impdedance) is to limit or restrain the line to ground fault current to a certaim threshold, to a calculated value desired in the relaying system for managing the fault current. Without the restraining impedance, the ground fault current on a bolted line to ground fault will be maximum

Unlike the wye or the star transformer connection, the delta connection does not have a neutral point to ground. A delta connection will not generate a line to ground fault.

The protection of the winding of the star transformer on ground fault will be the tripping of the line circuit breakers through the sensing / actuation of the sensing CT or relay on the neutral grounding. In some cases, the design do not intend to trip the breaker but instead set an alarm indication that there is a ground fault. This is done on ground fault currents that are manageable / tolerable to operate if the magnitude is within a level that will not harm the transformer windings. Setting of the impedance or resistance value is accurately designed and coordinated with the the grounding (earth) resistance of the soil.

Mr. Wei, you mentioned a neutral point while the connection is delta, how?
Nonetheless, may I confirm that the REF protection used for internal protection of the Transformer and based to its nature the REF will not detect the external earth fault, this is an important note.

But the other lines relays (3nos) assccoiated there on is used for OC as stated on your comments Mr.Wei