-

A hybrid method based on time domain simulation technique and the Boundary of stability region based Controlling Unstable equilibrium point method (BCU method) for direct analysis of power system transient stability using the Center of Inertia (COI) formulation is presented. The BCU method is based on the relationship between the boundary of stability region of a power system and that of a reduced system. This method finds the Controlling Unstable Equilibrium Point (CUEP) of the original system via a reduced system whose CUEP is easier to compute. Features distinguishing the BCU method from the existing direct methods are that it finds the exact controlling UEP relative to a fault-on trajectory. Having obtained the closest point to the controlling UEP using the BCU method, we calculate the exact controlling UEP by employing a modified Newton-Raphson technique. The proposed method has been successfully applied on two test systems and the results are given.