Abstract: Protective relay apparatus for detecting the direction or distance to a fault from the measuring point on an ac electrical power transmission line using current and voltage deviation signals. The deviation signals represent the changes induced by the fault from the steady-state pre-fault magnitudes. Trajectories are plotted on an X-Y plane using the voltage deviation signal as the X-axis coordinate and the differentiated current deviation signal multiplied by the line reactance to the balance point as the Y-axis coordinate. Due to the use of the differentiated current deviation signal, the trajectories are straight lines. The threshold boundaries for determining the distance to or direction of the fault are time-varying lines of unit slope representing the pre-fault voltage at the fault point.

Abstract: Protective relay apparatus and method for determining the faulted phase conductor of a three-phase ac electrical transmission line. The voltage and current on each conductor are measured and the steady-state ac electric power signal removed. The resulting filtered-voltage signals are multiplied by the characteristic admittance of the transmission line and then combined with the filtered-current signals to produce three current signals representative of the current on each conductor caused by an incident traveling wave and three current signals representative of the current on each conductor caused by a forward traveling wave. A comparator compares the six current signals to provide an indication of the direction to the fault and to provide an enable signal when the fault is on the protected line segment. The three incident-wave current signals are filtered to provide a modal signal representing the total current produced on the transmission line by a first propagational mode of the incident traveling wave.