For power systems with detailed excitation and power system stabiliser (PSS) controller, the authors set up a singular perturbation dynamical model with saturation non-linearities for their small-signal stability analysis. To estimate the stability region of this kind of dynamical systems, they establish a set of conditions under which the stability region can be decomposed into the Cartesian product of a contractive high-dimensional ellipse and a sufficiently large set. A method is provided for estimating the stability region with least conservativeness by introducing an linear matrix inequality-based optimisation model. A simulation on a simple power system is described as well.