I.Latroduncion There is extensive literature on voltage collapse and most of the papers use directly or indirectly Venikov's criterion for static voltage instability by moniitoring the singularity of the load flow Jacobian (1-5]. Equally popular is the multiple load flow solution approach (6]. In this paper we formulate voltage stability a a dynamic problem and show that the excitation system plays a key role in determining voltage stability through the relevant eigenvalues of the linearized system A matrix. In most studies using a linearized dynamic approach, the electromechlanical modes are of concern for stabilizing the system but in this paper it will be shown that the electrical variables associated with the excitation system play a dominant role. In the limiting case when there is no representation of the excitation system the determinant of the load fow Jacobian becomes the key determining factor. Under these conditions,Venikov's criterion is valid. We consider both the limited Q and the unlimited Q case, i.e the Q limits on the reactive power generation.In [7), the voltage collapse was related to misoordination of the ULTC's. The same formulation with slight modification allows static var compensators and tap changing under load transformers to be included in this model.
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