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In order to be able to successfully apply interline power flow controllers (IPFCs) for power-system transient-stability improvement and for the damping of oscillations, proper control strategies should be applied to these devices. The basis for the implementation of such a strategy that is proposed in this paper is to know the energy function of a power system that includes IPFCs. Already-known energy functions that proved to be suitable for an electric-power system do not include such a device. Therefore, in this paper an energy function that considers the IPFC's action in the form of a supplement to the already-known structure-preserving energy functions was constructed. Furthermore, the strategy for implementing the newly developed IPFC energy functions in order to ensure a globally optimum control strategy in the Lyapunov sense is presented. The results of the numerical examples show that the magnitudes of the IPFC's series-injected voltages remain set to their maximum values and that only the angles of these voltages change, similar to the 'bang-bang' control strategies proposed for some other FACTS devices. Index Terms--FACTS devices, interline power flow controller (IPFC), Lyapunov energy function, power system control, power system dynamic stability.

In order to be able to successfully apply direct methods or other energy-function-based calculations in power systems, which include flexible AC transmission systems (FACTS) devices, the influence of those devices should be properly considered. This is currently not always possible in power systems incorporating unified power flow controllers (UPFCs) because the present energy functions for electric power systems do not involve proper UPFC actions. This paper presents a way of incorporating the transient-stability augmentation action of the most versatile FACTS device, i.e., the UPFC, into an energy function for multimachine systems. After making some assumptions, a new term of the structure-preserving energy function (SPEF) that represents a UPFC's energy function was constructed. This term can simply be added to any existing SPEF. The extended SPEF was tested for one UPFC in a longitudinal test system and for one and two UPFCs in an IEEE nine-bus machine test system. A comparison between the critical clearing times (CCTs) acquired directly with the use of the newly constructed SPEF and those obtained with time-simulation results shows that the proposed UPFC's energy function is suitable because the correct CCTs were obtained. Index Terms--FACTS devices, Lyapunov methods, power system control, power system transient stability.

It is currently not possible to effectively apply direct methods for transient stability assessment in general multimachine systems incorporating Static Series Synchronous Compensators (SSSCs) because present energy functions do not involve SSSC actions. This paper presents a way to incorporate the transient stability augmentation action of an SSSC into an energy function for multimachine systems. We first assessed damping control strategies for an SSSC using the control Lyapunov function theory. After making some assumptions a new term of a structure-preserving energy function (SPEF) that represents an SSSCs energy function was constructed. This term can simply be added to any existing SPEF. The extended SPEF was tested in longitudinal and IEEE nine-bus test systems. A comparison between critical clearing times (CCTs) acquired directly and those obtained by time simulation results show that the proposed SSSCs energy function is suitable and a proper CCT can be obtained. Index Terms--Power system control, Lyapunov methods, power system transient stability.

In this paper, new energy functions that incorporate the action of controllable series devices (CSDs) are presented. They make it possible for the direct method to acquire the critical clearing time (CCT) of a single machine-infinite bus (SMIB) power system and the certain series flexible ac transmission systems (FACTS) devices directly. Dealing with this matter, optimal control strategies for the controllable series compensation (CSC) and the static synchronous series compensator (SSSC) are first derived. These and later tasks make use of the advantages of mathematical models which represent the controllable parameter of a FACTS device in relation to the transmitted real power through the given line. Using the derived control laws and some assumptions, Lyapunov-like energy functions were carried out. The results of a numerical example attained by a direct method and a digital simulation are shown to be promising. The final consideration suggests where future efforts should be directed. Index Terms--Control strategy, controllable series devices, critical clearing time, CSC, FACTS, Lyapunov energy function, SSSC, transient stability.