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This paper deals with optimal power flow control in electric power systems by use of unified power flow controller (UPFC). Models suitable for incorporation in power flow programs are developed and analysed. The application of UPFC for optimal power flow control is demonstrated through numerical examples. It is shown that a UPFC has the capability of regulating the power flow and minimising the power losses simultaneously. An algoriithm is proposed for determining the optimum size of UPFC for power flow applications. The performance of UPFC is compared with that of a phase shifting transformer (PST).

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It has been veri"ed that a controllable series capacitor with a suitable control scheme can improve transient stability and help to damp electromechanical oscillations. A question of great importance is the selection of the input signals and a control strategy for this device in order to damp power oscillations in an e!ective and robust manner. Based on Lyapunov theory a control strategy for damping of electromechanical power oscillations in a multi-machine power system is derived. Lyapunov theory deals with dynamical systems without inputs. For this reason, it has traditionally been applied only to closed-loop control systems, that is, systems for which the input has been eliminated through the substitution of a predetermined feedback control. However, in this paper, we use Lyapunov function candidates in feedback design itself by making the Lyapunov derivative negative when choosing the control. This control strategy is called control Lyapunov function for systems with control inputs. Also, two input signals for this control strategy are used. The "rst one is based on local information and the second one on remote information derived by the single machine equivalent method.

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