Patent ID: 8159229

Claim:
A method for determining a distance to fault in distance protection of a three-phase power transmission line, where in case of a fault on the line: it is assumed that a fault current is fed from two ends of the transmission line, measurements of phase currents are received, a first impedance of the line representing a distance to fault is estimated using a first fault model, the method comprising: basing a first fault model on an assumption that the fault current corresponds to at least one of the phase currents, estimating a second impedance of the line representing the distance to fault using a second fault model, wherein the second fault model is based on an assumption that the fault current corresponds to a negative-sequence component of the phase currents, estimating a third impedance of the line representing the distance to fault using a third fault model, wherein the third fault model is based on an assumption that the fault current corresponds to a zero-sequence current component of the phase currents, determining a load condition on the line, assigning the load condition to one of classifications normal, over-reach or under-reach, and determining a resulting impedance based on a combination of the first impedance, the second impedance and the third impedance depending on the assigned classification.