# Asea Brown Boveri Ltd. (ABB) Award 2005 - Deborah Koch

31.12.05 - Damping power system oscillations with UPFC devices. Dir.: Prof. A. Germond.

Pour la qualité de son travail dans le domaine de l'électronique de puissance appliquée à la sécurité des lignes de transmission ; ce travail de master a été réalisé à la University Pontificia Comillas de Madrid.

**Damping power system oscillations with UPFC devices**

This diploma thesis presents a theoretical study of a third generation FACTS controller, the UPFC (United Power Flow Controller). Such a device is of largest interest due to its potential applications in a power system, such as power flow controlling and suppression of power system oscillations. In particular this work is focused on the application of eigenvalue sensitivities to design controllers for an UPFC device, aimed at damping electromechanical oscillations.

Besides a brief survey of FACTS devices in general and the UPFC device in particular, the first chapters (chapter 1-3) are devoted to modeling of various approaches to achieve power system stabilization in the small signal stability regime. The main mathematical tools are presented in this context, but they will be re-used in the following chapters. As a surprising result the stabilization effect in the small signal stability regime shows to be the same in all approaches, this is an external control of the excitation voltage, series voltage ratio and phase and shunt reactance.

In the next chapters (chapter 4-7) a detailed derivation of a realistic steady-state model of an UPFC device is presented. Two different approaches are employed, which are referred to as Noroozian model and Hingorani model. They differ in the expression of the series voltage, i.e. the Hingorani approach assumes that the voltage over the DC link is constant, whereas in the Noroozian model the series voltage depends on the bus voltage. The corresponding controllability and observability factors, which are essential to determine the eigenvalue sensitivities, are developed in both approaches. All the results discussed in the following chapters are obtained in the Hingorani representation, since it is assumed to be more realistic. In chapter 8 the design of a damping controller for an UPFC device is discussed, including two parts: design of the phase compensation network and the calculation of the controller gain. All the derived mathematical tools are included into Matlab SSST (Small Signal Stability Toolbox).

The application of eigenvalue sensitivities to design the damping controllers of UPFC devices is illustrated in chapter 9. For this purpose a test system is chosen. The controller modulates either the UPFC ratio or the UPFC phase and uses the current as input, which showed to be the most appropriate signal. The damping by phase and ratio modulating is evaluated and it is shown that it is impossible to achieve simultaneous damping for a given initial phase. Also the computed results are dependent on the UPFC reactance. For a higher value the UPFC damping efficiencies are more homogeneous over the initial phase, i.e. they approach the same value for a different initial phase. However, the highest damping efficiencies at a certain initial phase are found for a smaller value of the UPFC reactance.