Interconnection of remotely power systems with large generation capacity and system load is progressively widespread due to the increase of the power exchanges between countries as well as regions within countries in many parts of the world. In the cases of long distance AC transmission, as in interconnected power systems, care has to be taken for maintaining synchronism as well as stable system voltages, particularly in conjunction with system faults and line switching. With series compensation, bulk AC power transmission over very long distances (1000 km and more) is in existence today. These long distance power transfers cause, however, the system low-frequency oscillations, typically within the range of 0.4 to 2 Hz, to become more lightly damped. For this reason, many power network operators and utilities are taking steps to add supplementary controls in their systems to provide extra system damping aiming to improve the system security by damping these undesirable oscillations. This thesis reports the results of time-domain simulation studies that are carried out to investigate the effectiveness of supplemental controls of a phase imbalance hybrid single-phase-Thyristor Controlled Series Capacitor (TCSC) compensation scheme and a static synchronous compensator in damping power system oscillations. In this context, studies are conducted on a typical large power system incorporating several series capacitor compensated transmission lines and large load centers with their reactive power support provided by static synchronous compensators (STATCOM). Several case studies investigating the effects of the location of the hybrid single-phase-TCSC compensation scheme, the degree of compensation provided by the scheme, the stabilizing signals of the supplemental controls, the fault clearing time, as well as the fault location on the damping of power system oscillations are documented. The results of the investigations conducted in this thesis demonstrate that the supplemental controls are very effective in damping power system oscillations resulting from clearing system faults. The time-domain simulation studies are conducted using the ElectroMagnetic Transients program Restructured Version (EMTP-RV).

Η εργασία πραγματεύεται την απόσβεση των ταλαντώσεων ισχύος με χρήση του εξελιγμένου ελεγχόμενου αντισταθμιστή σειράς. / SSSC is used in order to improve power system oscillation stability and some useful analytical conclusions are presented.

L'intégration des liaisons à courant continu dans les systèmes électriques permet d’accroitre les possibilités de pilotage des réseaux, ce qui permet d’en améliorer la sûreté et de raccorder de nouveaux moyens de production. Pour cela la technologie VSC-HVDC est de plus en plus plébiscitée pour interconnecter des réseaux non synchrones, raccorder des parcs éoliens offshore, ou contrôler le flux d’énergie notamment sur des longues distances au travers de liaisons sous-marines (liaison NorNed). Les travaux de cette thèse portent sur la modélisation, la commande non-linéaire et la stabilisation des systèmes VSC–HVDC, avec deux axes de travail. Le premier se focalise sur la conception et la synthèse des lois de commandes non-linéaires avancées basées sur des systèmes de structures variables (VSS). Ainsi, les commandes par modes glissants (SMC) et le suivi asymptotique de trajectoire des sorties (AOT) ont été proposées afin d’assurer un degré désiré de stabilité en utilisant des fonctions de Lyapunov convenables. Ensuite, la robustesse de ces commandes face à des perturbations et/ou incertitudes paramétriques a été étudiée. Le compromis nécessaire entre la robustesse et le comportement dynamique requis dépend du choix approprié des gains. Ces approches robustes, qui sont facile à mettre en œuvre, ont été appliquées avec succès afin d’atteindre des performances dynamiques élevées et un niveau raisonnable de stabilité vis-à-vis des diverses conditions anormales de fonctionnement, pour des longueurs différentes de liaison DC. Le deuxième vise à étudier l’influence de la commande du convertisseur VSC-HVDC sur l'amélioration de la performance dynamique du réseau de courant alternatif en cas d’oscillations. Après une modélisation analytique d’un système de référence constitué d’un groupe connecté à un convertisseur VSC-HVDC via un transformateur et une ligne, un contrôleur conventionnel simple PI est appliqué au niveau du convertisseur du système pour agir sur les oscillations rotoriques de la machine synchrone. Cette commande classique garantie une amélioration acceptable des performances dynamiques du système; surtout pour l'amortissement des oscillations de l'angle de puissance de la machine synchrone lors de défauts. / The integration of nonlinear VSC-HVDC transmission systems in power grids becomes very important for environmental, technical, and economic reasons. These systems have enabled the interconnection of asynchronous networks, the connection of offshore wind farms, and the control of power flow especially for long distances. This thesis aims the non-linear control and stabilization of VSC-HVDC systems, with two main themes. The first theme focuses on the design and synthesis of nonlinear control laws based on Variable Structure Systems (VSS) for VSC-HVDC systems. Thus, the Sliding Mode Control (SMC) and the Asymptotic Output Tracking (AOT) have been proposed to provide an adequate degree of stability via suitable Lyapunov functions. Then, the robustness of these commands has been studied in presence of parameter uncertainties and/or disturbances. The compromise between controller’s robustness and the system’s dynamic behavior depends on the gain settings. These control approaches, which are robust and can be easily implemented, have been applied to enhance the system dynamic performance and stability level in presence of different abnormal conditions for different DC link lengths. The second theme concerns the influence of VSC-HVDC control on improving the AC network dynamic performance during transients. After modeling the Single Machine via VSC-HVDC system in which the detailed synchronous generator model is considered, the conventional PI controller is applied to the converter side to act on damping the synchronous machine power angle oscillations. This simple control guarantees the reinforcement of the system dynamic performance and the power angle oscillations damping of the synchronous machine in presence of faults.