Often both REF and differential protection are employed. A properly designed REF scheme is stable for all faults outside the zone bounded by the CT's which allows the use of an instantaneous relay with a low setting.For example with a star connected winding the fault current decreases as the fault position moves closer to the star point. With a low effective setting you can achieve almost full coverage of the winding. Overall differential schemes are desensitised to make them stable for magnetic inrush, ratio correction (eg tapchanger) and possible overfluxing by applying bias settings. The effective coverage of the winding is reduced so if you had a relay setting of 20% of rated current and a resistance earthed system with a 1pu earthing resistor the differential protection would only cover about 42% of the winding from the line end.
Regards
Marmite
