PROBLEM TO BE SOLVED: To realize a vector relay for a feeder capable of extracting fundamental wave current, second and third higher harmonics and the like by a combination of digital filters without using Fourier transformation, and attaining reduction in computing load and error.
SOLUTION: The feeder currents in a railway AC feeder circuit are taken as samples at shorter intervals than those of rated frequency thereof and the sample data are stored. The data are computed and stored so that they permit the detection of a change in the magnitude and phase of the feeder current in the data obtained by subtracting the feeder current data of n-cycle before that of system frequency from the present feeder current data stored in a sample holding means. A fundamental wave component is taken out from differential current data by combining digital filters, thereby conducting computation for eliminating other low harmonic components. The amplitude is determined by amplitude computation based on the differential fundamental wave data, and evaluated in comparison with a predetermined value as protective operation.