REF/NEF Protection

[a430]

Hi everyone,

I will please like to know if i can use nuetral earth fault protection in place REF in a star Delta transformer.Wnat is the implication of not providing REF protection on Y/D transformer.

 

[ScottyUK]

By NEF, do you mean a relay connected to a plain CT on the star point earthing conductor or 'neutral'?

If you have any chance of a ground fault on the star side of the transformer and you expect to ride through it without tripping, then your EF relay must be sufficiently slow to allow other protection to operate and clear the fault. The REF system is designed to provide very fast, sensitive protection for faults within the protected zone, and to be insensitive to faults outside of the protected zone. The plain EF relay will respond to all faults, so by making it sufficiently insensitive or slow to allow it to discriminate with other protection in the event of a fault you inherently have less protection for your transformer. It could prove to be a very expensive decision to omit the fast protection, making the difference between a repair to the transformer and counting the fragments of the transformer.

If you don't care about tripping the transformer for faults out on the system, then by all means use a plain E/F relay but expect nusciance trips to occur. I personally think it would be a very bad idea from both the point of view of system security and of protection of plant. Plain E/F on the neutral is typically used as backup protection for a relay scheme in conjunction with an IDMT relay.


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I don't suffer from insanity. I enjoy it...

 

[veritas]

Agree with ScottyUK. Would like to add that if an EF relay must be used, rather residually connect it. This results in the relay tripping much faster for inzone faults than if it were neutral earth connected. Of course this only helps if the source contribution for an inzone fault is far greater than the zero sequence contribution from the trfr itself.

If possible, rather stick to REF.

Regards.

 

[RalphChristie]

Nice summary by Scotty. The type of protection used will be dependant on the size and importance of the transformer.

One thing extra, although not compulsory at this moment: Arc flash protection. Many countries do not even think of it now, but in my opinion it will eventually be implemented through-out the world. A fast protective scheme, like REF-protection, will certainly reduce arc-flash hazards. One thing to consider for the future.


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[a430]

Hi everyone.

please, if i must protect my transformer for REF,then can anyone recommend Alstom VCSU earth fault relay for this purpose.

Thanks.

 

[DiscoP]

The relay designed for this purpose from Alstom is the MCAG14

 

[a430]

Hi everyone.

Iam sorry, i mean MCSU Alstom type.

 

[ScottyUK]

Other possibility is the MFAC14 relay from the same manufacturer. The MCAG14 is a current operated relay and requires an external stabilising resistor if it is used in a Hi-Z scheme. The MFAC14 is voltage operated and is effectively an MCAG with plug-selectable resistors fitted internally. Both would require an external Metrosil.


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I don't suffer from insanity. I enjoy it...