Use of wavelets for out of step blocking function of distance relays

Out of step blocking function in distance relays is required to distinguish between a power swing and a fault. Detection of symmetrical faults during power swings can present a challenge. In cases when the values of the apparent impedances seen by a distance relay before and after a three-phase fault during a power swing are very close, most of the proposed schemes can run into problems. If such values fall into zone-1 of the relay, not only the detection, but the speed of detection is also crucial. This paper introduces wavelet analysis to detect power swings as well as reliably and quickly detect a symmetrical fault during a power swing. Total number of dyadic wavelet levels of voltage/current waveforms and the choice of particular levels for such detection are carefully studied. Different power swing conditions and fault instants are simulated with PSCAD/EMTDCreg software to test the proposed methodology