Protections for 4 MVA 3 Winding Transformer

http://www.acronymfinder.com/REF.html

You don't state the secondary voltage.

REF for LV to protect the transformer where there is liable to be load on the neutral.

I take it you do mean neutral - earth over current for HV where the neutral isn't utilised, it protects the load. If possible REF as well to protect the transformer.

I've installed a few transformers where there's no switchgear or protection between the transformer and load on the secondary side other than neutral over current.

Even though IE Rule (Rule 64A, Sub-rule 2 (d)) mandates Differential Protection only for transformers of capacity above 10MVA, it is prudent to go for Differential Protection for your 4MVA Transformer too, considering the repair/replacement costs & production downtime costs.

As for your second question, your 51N will also respond for external earthfaults whereas REF (64R) will respond only to internal earthfaults. Also, 51 N will have to have a time delay setting to co-ordinate with downstream relays and the 64R should have no time delay; it must be instantaneous. Definitely REF is faster, but only for internal earthfaults.

Thanks Mr. Expert (GURU), the reply forwarded by you is very much appreciated. I would like to clear one point from you is that how would you rate the protection of a transformer of rating 4MVA without differential and REF. As you mentioned that 51N is with time delay so the damage incase of fault within the transformer could lead to severe damage which can be minimized by using REF (64R) protection. Is 64R (REF) is mandatory for 4MVA transformer.

I only quoted from the Standards. Required protection is individual's choice. Generally a compromise between equipment availability and economics. If you have enough money, you may go for Differential even for a 1 MVA Transformer.

But, REF protection is mandated for transformers of rating 1 MVA and above.