Operation condition for zone 3 impedance relays during low-frequency power swings

Low-frequency power swings during contingencies and loads surges may undesirably activate impedance relays on transmission lines, and enlarge the blackouts. In this paper, the operation condition of zone 3 impedance relays (mho relays) during unstable low-frequency power swings is newly described by a polynominal function to quantify the critical swing angles and frequency with different relay setting and system parameters, and find the contributing factors leading to relay operations. The numerical results shows: (1) the swing loci are more densely populated inside than outside of the protection region, which corresponds to long residence time, and possibly causes relay operation; (2) with different relay settings and system parameters, the relays may be sensitive to load encroachments and power swings; (3) relays more probably operate with shorter time setting and lower swing frequency.