A New Restricted Earth Fault Protection

A new ground differential or restricted earth fault (REF) protection algorithm is proposed. The paper analyses the sensitivity of the overall phase differential protection on impedance grounded delta-star power transformers, and the need for more sensitive ground fault protection. The paper examines the performance of the widely used Dual Slope Percentage Restraint REF principle, as well as the new REF algorithm, their performance under inrush, and/or external faults conditions in conjunction with CT saturation. Playback tests on both REF elements have indicated the proposed new REF is of superior stability during external fault conditions, and is inherently not affected by inrush conditions.