In the field of power system stability there is provided a method of predicting the presence of an out-of-step condition in a power system that includes a plurality of generators, the method including the steps of:
(a) obtaining a differential value ({tilde over (δ)}COIk) between a rotor angle (δk) of an individual one of the plurality of generators and an equivalent rotor angle (δCOIk) of the centre of inertia of the remainder of the plurality of generators;
(b) processing the differential value ({tilde over (δ)}COIk) to determine whether the differential value ({tilde over (δ)}COIk) is predicted to reach a predefined reference threshold (δthreshold); and
(c) predicting the presence of the out-of-step condition in the power system if the differential value ({tilde over (δ)}COIk) is predicted to reach the predefined reference threshold (δthreshold).