Discrete control of unified power flow controller for stability improvement

The emerging flexible ac transmission system (FACTS) controllers are considered to be suitable for transient stability control due to their speed and flexibility. In this paper, a control strategy is developed to achieve maximal improvement in transient stability and damp the rotor oscillations using unified power flow controller (UPFC). The control strategy involves maximization and minimization of power flowin a line. In general, this involves the solution of a constraine doptimization problem at each step to determine the voltage and current injected by the UPFC. For the special case of a lossless symmetrical line, when the UPFC is located at the midpoint of the line, analytical solution can be derived. The control strategy can be extended to multimachine system and maximizing and minimizing power flow on a critical line can improve transient stability. The control strategy is evaluated by simulation studies