REF Protection Offer Any Benefits Over 87T? 1

[111R]

Is there any benefit to implementing REF protection on a transformer that is already protected by 87T protection using CTs on the high and low side bushings? I can see the ability to isolate an internal fault to the wye winding, but I don't know if there's any other benefit or if it's just mainly used on those transformers that don't have full differential protection.

 

[RRaghunath]

REF protection supplements the 87T. The earth faults yield low fault currents, generally speaking and hence the protection needs to be more sensitive. This is the purpose of REF protection.
The 87T cannot be sensitive for two reasons - one it needs to be stable at any tap of the transformer and the other, zero sequence currents are filtered from star windings in a Delta-Star connected transformer zero sequence.