Restricted earth faults-a complementary protection to the differential protection

Most vital and important Protection to protect the Transformer is the Biased Differential Protection which provides excellent protection against Ph to Ph faults and phase to earth faults for the faults within the Differential zone protecting the winding covering 80% from the bushing, however it is not capable of providing protection against Single Phase to Earth faults near the neutral in a star connected Transformer, with solidly earthed (grounded) Neutral because of its feature of biasing. To address this constraint REF protection is the only answer. Thus REF Protection may be called as a Complementary Protection to the Biased Differential protection as this protection too is based on Merz-price methodology. [1]