Electric power systems are frequently nonlinear and, when faced with increasing power demands, may behave in unpredictable and rather irregular ways. We investigated the nonlinear dynamics of a single machine infinite bus power system model in order to study the appearance of coexistent periodic and chaotic attractors, characterizing multi-stable behavior. The corresponding basins of attraction present fractal boundaries, for which we have determined the uncertain fraction scaling in phase space. The bifurcation diagrams are studied with respect to variations of the mechanical power input and may lead to voltage collapse under certain circumstances, which we relate to a boundary crisis suffered by a chaotic attractor. Ó 2002 Published by Elsevier Science Ltd.Voltage collapse in electric power systems have caused blackouts around the world, with losses of billions of dollars and many side effects as damages in household appliances, increase of crime rates, and car crashes, just to name a few [1]. Although the exact mechanism of a voltage collapse is still a matter of investigation, it is known that in such events the voltage magnitudes at electric power systems decrease rapidly under a heavy load [2]. From a more fundamental point of view, the ubiquitous presence of bifurcations in the dynamic behavior of nonlinear electric power systems may be related to the occurrence of voltage collapses [3]. In this picture, a normal operation of the power system would correspond to a stable equilibrium state. When the production or transmission of electric energy is insufficient to supply an increasing power demand, an electric power system may lose its operational stability through a bifurcation, where the actual state becomes unstable and new stable states are created [4].The current literature on this subject assigns as possible causes of voltage collapse: (i) a saddle node bifurcation [5], where stable and unstable equilibria coalesce at the bifurcation point and disappear; (ii) a chaotic blue-sky bifurcation [6], where a chaotic attractor collides with an unstable equilibrium, also known as an interior crisis [7,8]. This chaotic attractor is achieved through a flip bifurcation cascade that begins with a limit cycle produced through a supercritical Hopf bifurcation, after which the former stable equilibrium becomes unstable. The connection between these description lies in the fact that the unstable equilibrium that collides with the chaotic attractor coalesces with the stable equilibrium that undergoes a Hopf bifurcation, through a saddle-node bifurcation [9].Besides voltage collapse, the existence of long-lived chaotic transients is also an issue of major safety concern in the operation of an electric power system. The latter commonly has protective relay devices, designed not to interfere with the transient voltage oscillations. Hence they do not play a stabilizing role, which could be actually necessary to avoid undesirable voltage transients, which may cause severe damage to expensive equipment such as rotor...