Patent ID: 6989977

Claim:
A digital directional relay which converts a voltage value and a current value of a power system into digital voltage data and digital current data by periodic sampling and determines a direction of a fault using the digital voltage data and the digital current data, the digital directional relay comprising: a voltage variation computing unit which computes a variation ΔV u in unbalanced component voltage using both voltage data at a reference point in time and voltage data at a point in time elapsed from the reference point in time by a given sampling interval; a current variation computing unit which computes a variation ΔI u in unbalanced component current using both current data at reference time and current data at time elapsed from the reference time by a given sampling interval; a forward fault determination unit which determines that a forward fault occurs when a following expression is satisfied: K 1 ΔI u −ΔV u cos(φ−Ψ)≧K 2 where K 1 and K 2 are constants and Ψ is a phase difference between current and voltage; and a reverse fault determination unit which determines that a reverse fault occurs when a following expression is satisfied: K 1 ΔI u −V u cos(φ−Ψ)<K 2 where K 1 and K 2 are constants and Ψ is a phase difference between current and voltage.