Today, the presence of renewable energy sources (RES) in the power system has increased, and this has created challenges for the inertia of the power system. System inertia is due to rotor rotation of large synchronous generators. At the initial moments after occurrence of a disturbance in the power system, inertia reacts to the causing further outages in the system and prevents the rapid reduction of the system frequency, and thereby improving power system stability. If the power system operators are aware of the value of system inertia, they can obtain the dynamic performance of the power system against the disturbance. Therefore, it is possible to estimate the power system's ability in the initial moments after disturbance and implement appropriate control measures. In this paper, an online method to estimate system inertia considering the rotor speed variations is proposed. Based on the estimated inertia, the concept of the oscillation radius is defined and used for identification of the coherent generators. The case study system is the two-area four-machine power system. The simulation results show the validity and efficacy of the proposed method for estimating the system inertia and identifying the coherent groups of generators.
