Assessment of transient stability in power systems with FACTS

The deregulation of the energy-supply industry has increased the importance of security in an electric power system (EPS). The analysis of transient stability in electric power systems plays an important role in this issue. Since an analytical solution to a set of EPS differential equations does not exist, many approaches for finding a solution were developed in the past. Existing methods can be divided into three types. First are the so-called time-domain (T-D) digital simulations that numerically integrate differential equations and obtain a solution in the form of a system trajectory. The second approach uses direct methods that employ Lyapunov energy functions. In addition to information on systems stability, these methods can also provide stability margin estimation and various stability indices that help to identify a critical operation point. They are less accurate but substantially faster than T-D simulations. Complementary characteristics, speed and accuracy, are combined in hybrid methods, which emerged by combining digital simulation and direct methods. This paper gives an insight into all three methods for cases when transient stability assessment of power systems with FACTS (flexible AC transmission systems) is under discussion. The discussion focuses on direct methods and the derivation of appropriate energy functions that embed the action of FACTS devices. In this sense we provide methods for the implementation of the FACTS effect in energy functions.