Real-Time Discrimination of Symmetrical Faults from Power Swings

For reliable operation, distance relays have to be blocked in case of stable power swings (SPSs). Because these relays are prone to detect an SPS as a symmetrical 3-phase fault according to their symmetric nature. It should be noted that there are zero and negative sequence components during asymmetrical faults. However, these components do not exist during stable fluctuations or symmetrical faults. Consequently, according to the symmetric nature of the stable fluctuation, the distance relay may experience maloperation. This article proposes a new technique to discriminate a symmetrical 3-phase fault from an SPS. The proposed technique is based on the extraction of the exponentially decaying DC component in the 3-phase current by using the MIMIC impedance. This technique can detect the symmetrical fault in less than a quarter of one power cycle. The suitability of the technique is shown by simulating various symmetrical faults during fast and slow SPS conditions.