Copyright © 2003 IFAC.Dynamics and stability of an interconnected power system can be analyzed through an approach involving its clusters, which are characterized as coherently oscillating segments of the system. Oscillatory stability or instability between these clusters is strongly related to the electromechanical modes associated with inter-cluster dynamics. In this paper, we determine the feasibility regions for the system and their boundaries corresponding to Hopf bifurcations of inter-cluster oscillatory modes. Feasibility regions are defined as the set of operating points at which the system can be operated without losing local stability. Thus, we present a method of investigating the presence and the conditions for oscillatory instability between die clusters formed in an interconnected power system. Feasibility boundaries given in different parameter sub-spaces of control parameters and power exchanges play an important role in determining the conditions for a safe operation.