In this paper, we analyze Hopf bifurcation related oscillatory behavior occurring between clusters of an interconnected power system. This phenomenon is discussed through feasibility boundary calculations associated with utter-cluster oscillatory modes of the system's real-power-angle dynamics. We demonstrate examples of Hopf bifurcation related instabilities occurring between power Bow distribution based clusters (weakly coupled power systems) and/or between any coherency based clusters which may also be geographically scattered. This work gives an explanation for the nature of sustained oscillations between coherent groups of generators formed in a large interconnected power system. © 2003 IEEE.