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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...
Zone 3 impedance relays are sensitive to power-flow distribution due to their overreaching property, and may undesirably operate in stressed system conditions with reactive power shortages or neighboring faults. In this paper, the distance of the apparent impedance to the protection boundary on the R-X plane is defined by the operation margin, which is expressed as the function of bus voltages. The...
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