Frequency stability boundary of small isolated power systems

This paper explores new frequency protection schemes based on the analysis of the ω-dω/dt (frequency - rate of change of frequency) phase plane and on the definition of frequency stability boundary in particular. Due to their nature, these frequency protection schemes belong to advanced UFLS schemes and they could be applied in small isolated power systems. The definition of the frequency stability boundary is inspired by the definition of the region of angular stability. The derivation of the frequency stability boundary is based upon a simplified power-system model which allows summing power outputs of n generators without creating an (n+1)th-order system. It can be shown that the frequency stability boundary is a separatrix, describing a region of frequency stability in the sense of Lyapunov stability theory. This region helps determining if and how much load must be shed to guarantee frequency stability for a particular disturbance. The region of frequency stability is described by an ellipsis in the phase plane.