PROBLEM TO BE SOLVED: To provide a distance relay for an AC feeder circuit capable of restraining unwanted operation caused by the current from an electric rolling stock which regenerates AC power and controls a high power rate, and detecting malfunctions in the feeder circuit speedily and perfectly.
SOLUTION: The voltages and currents of an AC feeder circuit are taken as samples at prescribed intervals. An input impedance vector Zd is computed from the current data id,..., id-n and voltage data vd,..., vd-n, and an accident determining output Zd=1 is performed. From the current data, a fundamental wave 1f component and an arbitrary low harmonic component are determined by filtering, and a harmonic component percentage content Σf% to the fundamental wave 1f component is determined. An electric rolling stock current determining output Hf=1 is performed. A current vector variation ΔIV is determined from the current data, and a current vector rapid change determining output ΔIV=1 is performed. When respective determining outputs Zd=[0, 1], Hf=[0, 1], ΔIV=[0, 1] meet predetermined logical conditions, an operation output OUT=1 for opening a feeder breaker is performed.