Answer Posted / alpesh

Differential protection operate on difference of primary
and secondary current. Practically load is not balanced and
supply system too and so there will be always current
difference in normal running condition. To avoid
malfunction (during charging also) always differential
protection setting set to protect 70 to 80% zone. 20 to 30%
zone left unprotected with only differential protection.

By using REF, it is possible to protect 20 to 30%
unprotected zone without any maloperation.

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