REF Protection for Transformer

A Restricted Earth Fault (REF) protection is required to protect the transformer from earth fault of a restricted/localized zone of a circuit and are more sensitive.

An REF relay works by measuring the actual current flowing to earth from the frame of the unit. If that current exceeds a certain preset maximum value of milliamps (mA) then the relay will trip to cut off the power supply to the unit.

Differential protection relay operate by sensing the in balance of the windings of a transformer . REF protection is added to transformers in order to detect ground faults on a given winding more sensitively than differential protection relays.

The following link will give detailed information about transformer protection.

http://www.basler.com/downloads/transfguide.pdf

Good Question! But, because of the high bias setting in the differential relay, say, of the order of 10 to 20%, you need 10 to 20% of earth fault current for the diffrential relay to respond to an internal earthfault in the trf. This is particularly problematic, if the trf. neutral is impeadnace earthed and the eath fault in the winding is close to the neutral. In such situations REF relays defilnitely help, as they are more sensitive.