Out-of-step Protection combining Three-impedance Element Model with the Equal Area Criterion

This paper proposes a new procedure for out-of-step protection through the combination of a three-impedance element model with equal area criterion. The P   curve of the system is transferred into Pt  curve and then Pt  curve is applied to forecast whether the oscillation between the generator and the power system is out-of-step or stable. The forecasting results determine the operation of the out-of-step protection: If the oscillation is unstable, the signals will be sent during the first period of unstable oscillation, and then the number of sliding pole will be counted to achieve a delay action. Otherwise, no response will be made. A three-machine infinite bus system has been used to test the proposed method by simulation with PSCAD software. The simulations test the protection in a variety of faults within the system, including unstable and stable oscillation, loss of excitation and short-circuit faults. Simulation results confirm that this method can work effectively as a backup protection when faults such as loss of excitation and short-circuit fault are not promptly removed. Moreover, the simulations provide a reliable basis for the scheme and design of out-of-step protection in the power plant.