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the impedance measured during the power swing varies and give tripping to distance protection relay, in distance relays to block the tripping due to power swing a delay timer is used.
Obviously the power swings affect the distance relays, the impedance measured during the power swings changes and may enter the tripping characteristics of zone 3 , zone 2 or even zone 1 (in case of severe power swing), a feature is added in distance relays to block the tripping when due to power swings, impedance locus passes through zone 3 or zone 2 so that the system may adjust to new rotor angle following the faults in the system.