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What are the possible causes of REF operation on a power
transformer
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Answer Posted / k.prakashchandra
All cts(Phases and neutral) are connected in parallel.If
there are unbalanced currents in phases, the resultant
current in REFR is zero as the difference is also measured
by NCT .If there is an earth fault inside the transformer
or between cts and bushings ,the fault current is sensed by
NCT only. Hence current balnce is lost in REFR resulting in
trip command by the relay
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