Sir We have a REF relay connected to 10MVA(Y-Y) Transformer.on External earth

Sir We have a REF relay connected to 10MVA(Y-Y) Transformer.on
External earth fault REF relay trips along with earth fault
relay.The REF relay has no complant as we tested relay.Why is
it happening?

Question Posted / manu

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Sir We have a REF relay connected to 10MVA(Y-Y) Transformer.on External earth fault REF relay trip..

Answer / shariff

REF relay shold not operate otherthan its zone of operation
i.e 11kV REFR zone is from 11kV trf busings to 11kV CT
(incomer) and some times PT also if PT is before CT. It is
a mal function only and it can be any of the following.
1) Check the 11kV NCT wiring it may be loose/ got cut in
the NCT wiring box in the yard.
2) The 11kV incomer CT wiring may be loose/ got cut etc.
3) Check the 11kV PT primary resistance it may notbe equal.

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Sir We have a REF relay connected to 10MVA(Y-Y) Transformer.on External earth fault REF relay trip..

Answer / k.prakashchandra

The NCT is responsible..The ungrounded lead from The NCT may
be grounded.

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Sir We have a REF relay connected to 10MVA(Y-Y) Transformer.on External earth fault REF relay trip..

Answer / k.prakashchandra

Also may be due to 1 .reverse connection of NCT leads ( Unbalanced current of lines and NCT current add to each other ). 2. Low saturation point of NCT as Kv of bushing NCT is usually less than that of line CTs

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