Operation margin analysis of zone 3 impedance relay based on sensitivities to power injection

Zone 3 impedance relays (mho relays) on transmission lines have an overreaching protection region. The relays are sensitive to power flow redistribution and to power swing caused by outage or switch operation outside the protected region. Traditionally, the operation margin of mho relays is expressed with impedance. Various load flow schemes that require extensive calculation are necessary to decide the optimal location to control the operation margin. The equivalent expression of the operation margin expressed by a function of bus voltage has been established, and a preventive control model based on the sensitivities of the operation margin to power injection has been proposed. On the basis of the simulation results on the IEEE 14-bus test system, it has been found that: (1) the operation margin can be effectively controlled using the sensitivity method; (2) active and reactive power controls at the same bus may yield opposite control effects; (3) the proposed method is especially effective for stressed power systems; (4) relative control error with is very small as compared to the load flow method and (5) the calculation efficiency is much higher than that of the load flow method