Out-of-step blocking function in distance relay using mathematical morphology

Out-of-step condition arising from an unstable power swing requires quick detection of the power swing followed by a controlled separation of different areas of power system at predetermined locations. However, in order to perform such separation, local operation of distance relays needs to be blocked using out-of-step blocking (OSB) function. Conventional blinder-based method to implement the OSB function cannot detect a symmetrical fault once the relay is blocked during a power swing. Different schemes are proposed to overcome this drawback. This study presents detection methods for power swing and symmetrical faults, based on mathematical morphology. These methods offer distinct advantages over other schemes proposed in the literature. The methods are combined and rigorously tested on waveforms generated using PSCAD/EMTDC^®. These methods are then used to propose a reliable and fast OSB tool that imposes a very low computation burden at low sampling rate.