This paper proposes a novel self-tuning and response-based method for estimation of a single generator instability after large disturbances in multi-machine power systems. The proposed method takes advantage of the Equal-Area Criterion (EAC) and least-squares method. The instability is determined by monitoring the cumulative acceleration and deceleration areas in the power-angle plane. These areas are continuously computed in order to determine whether or not the rotor angle reaches the Unstable Equilibrium Point (UEP). A realtime curve-fitting algorithm based on the least-squares method is suggested to estimate the UEP. The generator rotor angle is calculated readily using three-phase voltages and currents taken at the generator terminal, and no further information such as fault clearing instant is required. The algorithm is independent upon changes in network parameters and only needs locally measured quantities. Self-tuning and response-based operation are the main advantages of the proposed algorithm over the other existing algorithms. The performance of the proposed algorithm and its independence from the network configuration are successfully validated and compared with that of impedance-based out-of-step protection relays.

This paper proposes a novel-predictive and setting-free out-of-step protection technique based on synchronized phasor measurements and equal area criterion. Generator instability is predicted by comparing the acceleration and deceleration areas corresponding to the fault and postfault conditions, respectively. For each generator, the acceleration area during fault condition is calculated directly using the generator output power and rotor angle in real time. As soon as the fault is cleared, the deceleration area is predicted based on estimating the postfault Thévenin equivalent of the power network. The proposed technique is verified on the New England 39-bus test network. The effect of the proposed technique on improvement of the power network stability is compared with that of the well-known impedancebased out-of-step relay. KEYWORDS equal area criterion, impedance-based OOS relay, out-of-step, phasor measurement unit, Thévenin equivalent

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