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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 sensitivities of the operation margin to bus voltages, bus powers, and shunt susceptances quantify the influence of various disturbances on the relay performance. The numerical results show: (1) the operation margin is quantified by the sensitivities with satisfying accuracy and less computation effort compared with power-flow schemes; (2) the variation of bus powers and shunt susceptances at different locations may improve or decrease the operation margin; and (3) the variation of active and reactive powers at the same bus may yield opposite effects on the operation margin.