A monitoring and modifying scheme for Zone 3 of critical distance relays in view of voltage stability
Unwarranted operation of Zone 3 of the distance relays during voltage-degraded conditions may cause cascade outages and voltage instability in the power system. The present paper proposes a novel scheme to identify the critical distance relays considering voltage stability and online calculating of Zone 3 setting of the relays subjected to maloperation. The proposed scheme is based on time-based and line loading-based indices defined by the combination of a new concept of relay margin (RM) and the rate of change of the apparent impedance calculated by local voltage and current measurements. The indices appropriately indicate the power system situation taking into account the voltage stability. Therefore, the scheme uses the mentioned indices in online Zone 3 setting of the critical relays for keeping the maximum backup protection. Developing analytical formulation for indices and Zone 3 settings, using only local measurements, simplicity, and the applicability for the modern numerical relays are the main advantages of the proposed scheme. Concepts, how to apply, and the simulation results are demonstrated with the help of the New England 39-bus system.