Effects of generator and static-load nonlinearities on electromechanical oscillations

Linear analysis methods, such as eigenanalysis and Prony analysis, are becoming standard tools in studying power system electromechanical oscillations. Because the actual system is nonlinear, large-amplitude oscillatory swings will not necessarily have frequency and damping characteristics indicated by the eigensolution. Understanding the differences can be critical in comparing eigenanalysis and simulated results, and in applying Prony analysis to the nonlinear swings. This paper investigates the effects of excitation limiting, magnetic saturation, the swing equation, and static-load model properties on the linearity of a power swing. Simulation, eigenanalysis, and Prony analysis results are compared on a four-machine system operating under several conditions. It is found that significant differences occur between the eigenanalysis and simulation results when heavy excitation limiting occurs during the swing. Generator magnetic saturation tends to cause the simulated results to have more effective damping than the eigensolution. Other nonlinearities cause little or no differences. Prony analysis tends to match the simulated swing, except if excitation limiting occurs.