Incorporation of hard excitation limits into power system normal form analysis

In this paper a theoretical framework for the incorporation of the effect of excitation limits on normal form analysis is presented. The limiter action is approximated by a smooth nonlinear type atan function which is incorporated in the power system dynamic model using singular perturbation theory. This generalized limiter model is employed to characterize the limits of a simple DC excitation system. Based upon this representation, a second order normal form model is developed and applied to evaluate the impact of excitation limiter on the nonlinear inter-area phenomena. Such a modeling capability offers improved understanding of nonlinear behavior, and should allow quantitative analysis of limit-induced oscillatory behavior. The proposed technique is tested on a two-area, four-machine system in which the effect of excitation limits is considered. Detailed numerical results are presented to verify the results.