Conventional earth fault protection using over current elements fails to provide adequate protection for transformer windings. This is particularly the case for a
star-connected winding with an impedance-earthed neutral.
The degree of protection is very much improved by the application of restricted earth fault protection (or REF protection). This is a unit protection scheme for one winding of the transformer. It can be of the high impedance type as shown in below figure, or of the biased low impedance type.
For the high-impedance type, the residual current of three line current transformers is balanced against the output of a current transformer in the neutral conductor. In the biased low-impedance version, the three phase currents and the neutral current become the bias inputs to a differential element.
The system is operative for faults within the region between current transformers, that is, for faults on the star winding in question. The system will remain stable for all
faults outside this zone. The gain in protection performance comes not only from
using an instantaneous relay with a low setting, but also because the whole fault current is measured, not merely the transformed component in the HV primary winding (if the star winding is a secondary winding). Hence, although the prospective current level decreases as fault positions progressively nearer the neutral end of the winding are considered, the square law which controls the primary line current is not applicable, and with a low effective setting, a large percentage of the winding can be covered.
For solidly earthed systems a restricted earth fault protection is often provided as a complement to the normal transformer differential relay. The advantage with the restricted earth fault relays is their high sensitivity. Sensitivities of 2-8% can be achieved. The level is dependent of the current transformers magnetizing currents
whereas the normal differential relay will have sensitivities of 20 -40%. Restricted earth fault relays are also very quick due to the simple measuring principle and the measurement of one winding only. The differential relay requires percentage through fault and second harmonic inrush stabilization which always will limit the minimum operating time.