In power transformers the fault current is controlled mainly by the leakage reactance of the winding, which varies in a complex manner with the position of the fault. The variable fault point voltage is also an important factor, as in the case of impedance earthing. For faults close to the neutral end of the winding, the reactance is very low, and results in the highest fault currents. The variation of current with Earth fault current in solidly earthed star winding is shown in below Figure.
For secondary winding faults, the primary winding fault current is determined by the variable transformation ratio; as the secondary fault current magnitude stays high throughout the winding, the primary fault current is large for most points along the winding. But as you see for low percent turn involved to earth fault current we have not enough primary current to differential relay effective operation whereas we front the high fault current in end turn of windings. In this case REF can detect whole of fault current via neutral CT.
Also 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.
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. 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 advantages to be obtained by the use of restricted earth fault protection lead to the system being frequently used in conjunction with an overall differential system. The importance of this is shown in below Figure from which it will be seen that if the neutral of a star-connected winding is earthed through a resistance of one per unit, an overall differential system having an effective setting of 20% will detect faults in only 42% of the winding from the line end.