Differential protection Vs Restricated earth fault protection of Transformer

but differential relay will also get operated on earth fault i nside the its zone.........I think we are keeping time gradient for that.

Please note that the differential relay will operate for earth faults inside the zone only if the earth fault current is more than the bias setting in the relay. The normal bias setting in a differential relay is 20%. So, complete earth fault protection is not possible with differential relay. That is why you need a seperate earth fault relay with sensitive settings like 5%.

Also please note that all Unit Protection Relays like Differential & REF cannot have an intentional time delay. They must operate instantaneously in the event of an internal fault. There is no time grading possible with any unit protection relays.

Can you pls tell me what's unit protection relay meant by?

Unit Protection means any protective relay or protectuve scheme that is intended to protect only a particular unit - like a generator, a transformer or a motor from faults. It will not react if the fault is outside its pre-defined zone.

Iam not able to understand how the transformer is protected if the earth fault occurs on the delta primary side of the transformer if the fault current less than the differential bias setting & the primary source is also have earthed neutral?Is it not harmful?Does the transformer gets protection from the primary side earth fault relays? Would be grateful to you......

If the Delta Primary of a Transformer is connected to an earthed source, it is prudent to have an REF Relay connected in the Residual of all the three phase CTs. In such a connection, during an external (secondary side) earth fault, two lines would sense the current on the Delta side and balance out; hence, the Delta side REF would not act for a Star side winding earth fault.

If the earth fault is in one of the delta windings then only one line CT would sense and the Delta side REF would act.

(Residual Connection means, connect the 'S1' of all three phase CTs together and connect all 'S2' together; now , connect your REF Relay across this 'S1' & 'S2'). But for this scheme to work, the Delta side must be connected to an earthed source.

Can you pls provide the drawing for the same?Means the REF scheme for the delta connected winding?Thank you......

Dear Mr. Sivakumar Sir,

I found your discussion very interesting and useful. I had one question regarding REF protection for Dg sets in Synchronism. We were designing the protection schemes for 4 Nos DG sets of 2000 KVA which were to be synchronised. In this case, Only 1 neutral contactor is closed to ground, while other 3 neutrals are open. In REF protection, we need to connect 1 Class PX CT in neutral. My doubt was, if neutral is not grounded and open, will REF relay work. whether is it necessary to provide directional Earth fault protection in case neutral grounding is open.

Please give your good advice.

Dear Sir,

If your neutral will get open from ground / earth, then both i.e. REF 64R as well as 51G will not work.

Regards

Mahamuni

Dear Sir,

Thanks for you reply.

Can you please provide circuit diagram of both?

Also as per my knowledge REF provide the protection from Secondary winding of transformer to immediet breaker while Diff. Protection provide the protection both for secodary & Primary winding of the transformer.

Is it correct.?

Differential will not detect earthfaults on the primary winding (DELTA) if the fault current magnitude is less than the bias setting in the relay. It is also manadatory to provide REF Relay for DELTA side of a (DELTA/STAR) transformer, provided the supply for the DELTA side is derived from an earthed source.

It is easier to implement REF in a STAR winding as you redaily have an earthed neutral.

As regards diagrams for both the schemes, I do not know how to attach drawing files in this forum. Can you mail me at: SIVAKUMARK@LNTEBG.COM

Thanks for the explaination. it's very clear for me to understand the basic principle before going further study