Sir we have 12.5 MVA(110/11KV) transformer (Y-Y) connected,which tripped showi

Sir we have 12.5 MVA(110/11KV) transformer (Y-Y)
connected,which tripped showing earth fault,when we charged
transformer it stood OK ,but on connecting 11 KV feeders it
tripped showing earth fault.our service engineers found that
the B phase and R phase phase connecting to neutral broken
from HV OLTC(on load tap changer),and R phase and Neutral is
ok.Dissolved gas analysis shows that accetline content is
high,tan delta value is 12.My dout is it is an internal
fault of transformer,why REF or diffrential relay is not
acted?

Question Posted / manu

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Sir we have 12.5 MVA(110/11KV) transformer (Y-Y) connected,which tripped showing earth fault,when ..

Answer / k.prakashchandra

REF relay does not act as the unbalanced current is equal to
NCT current in all respects. And for diff relay, it also
does not act as the corresponding ct currents in HT and
sides for each phase are equal.If there is current in only
one phase ,EFR will trip as current's return path is through
the EFR.But why there is current in the single good phase as
all dist Trs are delta/ star connected ? May be due to star
connected line capacitors or dist trs/loads are also star
connected with neutrals earthed.

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