A thorough understanding of the phase portrait, or more precisely the stability region and the stability boundary of a nonlinear dynamic system is the foundation of all methods in stability studies for testing the stability of an initial state, that is whether it is inside the stability region. This paper presents quite complete results on the phase portrait of a class of large nonlinear dynamic systems which includes the power system. Also the connection between the constant energy surface and the stability boundary of the power system is explored. This gives valuable insight into classical stability tests which are based on the constant energy surface. The approach to stability monitoring of the power system deriving from these results is outlined.
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