Effects of stator transients on steady-state stability regions of electric power systems

Using linear integral manifolds, the assumption of neglecting stator transients is reexamined in the context of steady-state stability of electric power systems under varying system parameters and operating conditions. This widely-accepted assumption is shown to give erroneous results in two examples consisting of a synchronous machine or an induction machine connected to an infinite bus by a transmission line with a varying stator resistance. In the first example, the classical quasi-steady-state order reduction yields the same stability regions for two synchronous machines with different inertias. The stability regions, while being adequate for the larger machine, are erroneous for the small machine, for relatively large values of stator resistance. These results are also confirmed by the Routh-Hurwitz criterion of stability. Using a first-order integral manifold correction, almost exact stability regions are reconstructed for both machines. In the second example, the stability regions obtained from the classical quasi-steady-state order reduction are inadequate in generating mode for both small and large induction machines and require a first-order correction. These higher-order corrections using the method of integral manifolds can be applied in conventional small-signal stability programs to obtain better eigenvalue results near stability boundaries without the need for reintroducing the neglected stator transients.