In this paper, a preventive control method against transient instabilities due to a set of critical contingencies that could occur in a large power system is proposed. The generation rescheduling, adopted as the preventive control, is based on the coherency between the generators. The rescheduling is done by attempting to bring the generators' rotor speeds equal after a three phase fault that might cause instability. The proposed methodology involves off-line simulations to determine the system response and to check the severity of each contingency. For each critical contingency, a scaling factor is assigned to scale the speed trajectory of the contingency. Active-set sequential quadratic programming (SQP) is used to optimize the scaling factors in such a way that the rescheduling method based on the scaling factors improves the dynamic security and restores the system's stability for all contingencies that are taken into account.