Clarifications of the BCU method for transient stability analysis

Energy function methods have been studied for many years, and have been applied to practical power system stability analysis problems of multi-machine power systems. Developments in real-time power system monitoring suggest that dynamic events can be monitored at the power system control centers, and naturally the energy function methods were tried as real-time stability prediction tools. However, a number of instances were uncovered, where the energy function methods which use the potential energy boundary surface as an approximation of the stability boundary produce unreliable results. In particular, during several transient stability studies, the boundary controlling unstable (BCU) equilibrium point method seemed to predict stable swings, whereas in reality the swings turned out to be unstable. This paper presents these counter-examples, and suggests an explanation as to why these methods produce a wrong result