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Purpose Self-commuted voltage source converter (VSC) can significantly extend the flexibility and operability of an HVDC system and be used to implement the concept of multi-terminal HVDC (MTDC) grid. To take full advantage of MTDC systems, its overall behaviour must be characterized in quasi static and dynamic states. Based on the numerous literatures, a dedicated two-level VSC model and its local controllers and DC grid voltage regulators are developed for this purpose. Furthermore, the requirement of the system to guarantee all the physical constrains must be well assessed and concrete demonstrations must be provided by numerical simulations. Design/methodology/approach First, a two-level VSC model and its local controllers and DC grid voltage regulators are developed. Then, DC cable models are investigated and their characteristics are assessed in the frequency domain. Those developed models are combined to form a three-terminal HVDC grid system on Matlab/Simulink platform. To analyze the stability of this electrical system, the dynamics of the system against variations of power dispatch are observed. Findings To analyze the stability of this electrical system, the dynamics of the system against variations of power dispatch are observed. The differences in the DC grid voltage dynamics and the power flow of the converter stations coming from the embedded primary controls are analysed, and the technical requirements for both cases are assessed. Originality/value In this paper, the dynamic stability of an MTDC system has been analysed and assessed through an adequate simulation model, including its control scheme and the cable models. The interest of the improved PI model for cables is highlighted.

The role of Modular Multilevel Converters (MMCs) in HVDC grid greatly differs depending on whether it is an offshore or an onshore station. From the common point in their control schemes, an unexploited ability of the MMC—the controllability of the internally stored energy—is identified in both offshore and onshore applications. The virtual capacitor control, previously proposed by the authors, makes use of this degree of freedom to provide energy contribution to the DC grid. The impact of this control is demonstrated by time-domain simulations of a five-terminal HVDC grid.

The Modular Multilevel Converter (MMC) is a most promising converter technology for the High Voltage DC application. The complex topology of the MMC requires several additional controllers to balance the energy in the capacitors which are distributed all over the converter. Typically, there is a requirement of two controls; one is the regulation of the total energy in each leg, and the other is the distribution of the energy between the upper and the lower arms. This paper presents control strategies for the latter one being capable of distributing the energy only by internal power flow, so that undesired interference with the associated grids can be completely avoided. The proposed controls are achieved by forcing the common mode currents to be balanced while keeping the classic cascaded control structure as much as possible. The effectiveness and advantage of the proposed solutions are demonstrated by simulations.

The complex topology of the Modular Multilevel Converter (MMC) requires some additional controllers to keep its functionalities. One of the important requirements on the MMC control is to balance the energy stored in the distributed capacitors in the arms on the three legs. However, due to the superimposed internal DC and AC power flows in the converter, the energy stored in the arms contains intrinsic oscillations. This paper provides a thorough analysis on those intrinsic oscillations on the internal energy of the MMC. Based on the analysis, a novel analytic filter is proposed, which enables to extract average value of the energy while keeping other internal dynamics stable. The proposed filter is implemented on an EMTP-RV platform. The simulation demonstrates its improved dynamic response and reduction of the internal losses compared to the existing solutions.

Cette thèse porte sur la commande de réseaux multi-terminaux à courant continu (MTDC) basés sur des convertisseurs multiniveaux modulaires (MMCs).Tout d’abord, notre attention se focalise sur l'énergie stockée en interne dans le MMC qui constitue un degré de liberté additionnel apporté par sa topologie complexe. Afin d’en tirer le meilleur parti, les limites de l’énergie interne sont formulées mathématiquement.Afin de maîtriser la dynamique de la tension DC, l’utilisation de ce nouveau degré de liberté s’avère d’une grande importance. Par conséquent, une nouvelle de stratégie de commande, nommée «Virtual Capacitor Control», est proposée. Cette nouvelle méthode de contrôle permet au MMC de se comporter comme s’il possédait un condensateur de taille réglable aux bornes, contribuant ainsi à l’atténuation des fluctuations de la tension DC.Enfin, la portée de l’étude est étendue au réseau MTDC. L'un des défis majeurs pour un tel système est de faire face à une perte soudaine d'une station de convertisseur qui peut entraîner une grande variation de la tension du système. A cet effet, la méthode de statisme de tension est la plus couramment utilisée. Cependant, l'analyse montre que l'action de contrôle souhaitée risque de ne pas être réalisée lorsque la marge disponible de réserve de puissance du convertisseur est insuffisante. Nous proposons donc une nouvelle structure de contrôle de la tension qui permet de fournir différentes actions en fonction du signe de l'écart de la tension suite à une perturbation, associée à un algorithme qui détermine les paramètres de statisme en tenant compte du point de fonctionnement et de la réserve disponible à chaque station. Convertisseurs modulaires multiniveaux (MMCs), Transmission à haute tension courant continu (HVDC), Réseaux multi- terminaux DC (MTDC), Commande de convertisseur, Gestion de l'énergie, Réglage primaire de tension, Statisme de tension, Modélisation, Convertisseurs multiniveaux, Réseaux électriques (énergie), Modèles mathématiques, Réseaux électriques (circuits), Énergie gestion, Électronique de puissance Control and energy management of MMC-based multi-terminal HVDC grids The scope of this thesis includes control and management of the Modular Multilevel Converter (MMC)-based Multi-Terminal Direct Current (MTDC).At first, our focus is paid on the internally stored energy, which is the important additional degree of freedom brought by the complex topology of MMC. In order to draw out the utmost of this additional degree of freedom, an in-depth analysis of the limits of this internally stored energy is carried out, and they are mathematically formulated.Then, this degree of freedom of the MMC is used to provide a completely new solution to improve the DC voltage dynamics. A novel control strategy, named Virtual Capacitor Control, is proposed. Under this control, the MMC behaves as if there were a physical capacitor whose size is adjustable. Thus, it is possible to virtually increase the equivalent capacitance of the DC grid to mitigate the DC voltage fluctuations in MTDC systems.Finally, the scope is extended to MMC-based MTDC grid. One of the crucial challenges for such system is to cope with a sudden loss of a converter station which may lead to a great variation of the system voltage. The voltage droop method is commonly used for this purpose. The analysis shows that the desired control action may not be exerted when the available headroom of the converter stations are insufficient. We thus propose a novel voltage droop control structure which permits to provide different actions depending on the sign of DC voltage deviation caused by the disturbance of system voltage as well as an algorithm that determines the droop parameters taking into account the operating point and the available headroom of each station. Modular multilevel converters (MMCs), High-voltage direct current (HVDC) transmission, Multi-terminal DC (MTDC) grid, Converter control, Energy management, Primary voltage control, Voltage droop control, Modeling