Modified algorithm for directional earth-fault protection without voltage inputs

This paper presents improved algorithm for directional earth-fault relay without voltage inputs. This relay has only currents inputs and it is intended for use in isolated neutral networks. Instead of zero sequence voltage or three phase currents as reference quantities, only one phase current is used as a reference quantity. To obtain reference signal for two other phases, measured phase current is phase shifted for ±2π/3 rad. Thereby voltage signal is completely avoided, and the number of current measurements is reduced from four to two measurements per outgoing feeder. The algorithm detects not only the line containing a phase-to-earth fault, but also the faulted phase. Reduced number of current inputs and no voltage inputs results in low-cost design and fast response of the improved directional relay.