Voltage source converter high voltage direct current (VSC-HVDC) technology has become increasingly cost-effective and technically feasible in recent years. It is likely to play a vital role in integrating remotely-located renewable generation and reinforcing existing power systems. Multi-terminal VSC-HVDC (MTDC) systems, with superior reliability, redundancy and flexibility over the conventional point-to-point HVDC, have attracted a great deal of attention globally. MTDC however remains an area where little standardisation has taken place, and a series of challenges need to be fully understood and tackled before moving towards more complex DC grids. This thesis investigates modelling, control and stability of MTDC systems. DC voltage, which indicates power balance and stability of DC systems, is of paramount importance in MTDC control. Further investigation is required to understand the dynamic and steady-state behaviours of various DC voltage and active power control schemes in previous literature. This work provides a detailed comparative study of modelling and control methodologies of MTDC systems, with a key focus on the control of grid side converters and DC voltage coordination. A generalised algorithm is proposed to enable MTDC power flow calculations when complex DC voltage control characteristics are employed. Analysis based upon linearised power flow equations and equivalent circuit of droop control is performed to provide further intuitive understanding of the steady-state behaviours of MTDC systems. Information of key constraints on the stability and robustness of MTDC control systems has been limited. A main focus of this thesis is to examine these potential stability limitations and to increase the understanding of MTDC dynamics. In order to perform comprehensive open-loop and closed-loop stability studies, a systematic procedure is developed for mathematical modelling of MTDC systems. The resulting analytical models and frequency domain tools are employed in this thesis to assess the stability, dynamic performance and robustness of active power and DC voltage control of VSC-HVDC. Limitations imposed by weak AC systems, DC system parameters, converter operating point, controller structure, and controller bandwidth on the closed-loop MTDC stability are identified and investigated in detail. Large DC reactors, which are required by DC breaker systems, are identified in this research to have detrimental effects on the controllability, stability and robustness of MTDC voltage control. This could impose a serious challenge for existing control designs. A DC voltage damping controller is proposed to cope with the transient performance issues caused by the DC reactors. Furthermore, two active stabilising controllers are developed to enhance the controllability and robust stability of DC voltage control in a DC grid.

This thesis treats the subject of a complete steady state and dynamic model of the VSC-HVDC covering both the AC and DC system-side of the converter. The topology of the model is recreated after the scheduled transmission line in the south of Sweden, called SydVästlänken. The topology covers both a simple two terminal connection as well as a multi-terminal one. This model is to be implemented in the power system simulation program ARISTO. The main directive of the model is operation planning and education during real-time scenarios. The model is deliberately designed as a complete and complex model but with methods of reducing it's complexity to suit the users needs at the time for implementation. Further more the author have made sure that it is a complete generic model to suit the application of the program as the technology to the day of this study are unknown. Lastly, a method of controlling the converters are presented where Droop-control take a prominent roll as the AC and DC voltage regulator.

Les réseaux multi-terminaux à courant continu sont une solution efficace pour intégrer l’énergie électrique produite en grande quantité par de grands parcs éoliens offshore. Bien que le recours à la technologie HVDC soit maitrisé pour des applications point-à-point, des verrous technologiques sont encore à lever pour permettre une exploitation sûre d’un réseau à courant continu. La protection est le principal domaine technique pour lequel des progrès sont encore attendu. Des stratégies de protection fiables et assurant le meilleur ratio technico-économique sont à l’étude. Ces travaux de thèse ont pour objectif la mise en œuvre d’une philosophie de protection à sélectivité totale, identique à celle utilisée dans les réseaux de transport traditionnels. Cette étude considère l’utilisation de liaison par câbles uniquement, de convertisseurs VSC-MMC composées de sous-modules en demi-pont et de disjoncteurs hybrides à courant continu. Une association de deux algorithmes de détection de défaut a été proposée. Une étude du temps disponible pour l’élimination du défaut a été menée. Enfin, des simulations numériques avec le logiciel EMTP ont permis d’évaluer la fiabilité de la stratégie de protection. / In a near future, multi-terminal High Voltage Direct Current grids (MT-HVDC grids) appear to be a suitable solution for the integration of power electricity produced by remote offshore windfarms into the AC transmission system. Though the recourse to HVDC point-to-point links is well-known, challenges still remain for a safe operation of HVDC grids. Protection is the main technical field still under study and reliable protection strategies ensuring the best technological and economic ratio are investigated. This thesis focused on a full selective protection philosophy similar to the one applied to AC transmission systems. The consideration of cable links, Half-Bridge VSC-MMC converters and hybrid DC circuit breakers defines the frame of the study. An association of two algorithms for the identification of faults is suggested. The time available for the fault clearing process has been investigated. Simulations performed with EMTP software have been used to evaluate the reliability of the suggested strategy.

In order to meet the increasing demand ofenergy in today’s society while at the same time minimizing theenvironmental impact, renewable energy sources will be requiredto be integrated into the existing energy mix. Technologicaladvances in high voltage direct current (HVDC) grids playa crucial role in making this possible. Therefore the purposeof this project has been to validate the properties of basiccontrol strategies in terms of how they respond to four differentsimulation cases. All simulations have been conducted on asimplified version of the CIGR ́E B4 test grid, consisting offour monopolar HVDC converters. After analyzing the resultsobtained from each control strategy it became evident thatprovided if the benefits of the redundancy introduced by amulti-terminal grid are to be fully utilized, a distributed voltagecontrol should be used. Moreover, after substituting one ofthe four internal controllers with an external one, it becameclear that simply deciding the droop constants based on resultsfrom the simulation model wouldn’t be sufficient for real worldapplications. / För att möta det ökande energibehovet i dagens samhälle, samtidigt som energiproduktionens miljöpåverkan ska minimeras, krävs det att förnyelsebara energikällor integreras i den existerande energimixen. Tekniska framsteg inom högspända likströmsnät (HVDC) spelar en avgörande roll i att göra detta möjligt. Därför har syftet med detta projekt varit att validera egenskaperna hos grundläggande kontrollstrategier efter hur dem reagerar på fyra olika simuleringsfall. Alla simuleringar har genomförts på en förenklad version av CIGRE´ B4 testsystem, bestående av fyra monopolära HVDC omriktare. Efter att analyserat de erhållna resultaten från varje kontrollstrategi blev det uppenbart att om fördelarna med multiterminala elnät skulle uppnås, bör en distribuerad spänningskontroll användas. Dessutom, efter att ha bytt ut en av dem fyra interna kontrollerna med en extern, visade det sig att endast bestämma droppkonstanterna baserat på resultat från simuleringsmodellen inte är tillräckligt för verkliga applikationer. / Kandidatexjobb i elektroteknik 2020, KTH, Stockholm

La multiplication des projets HVDC de par le monde démontre l'engouement toujours croissant pour cette technologie de transport de l'électricité. La grande majorité de ces transmissions HVDC correspondent à des liaisons point-à-point et se basent sur des convertisseurs AC/DC de type LCC ou VSC à 2 ou 3 niveaux. Les travaux de cette thèse se focalisent sur l'étude, le contrôle et la commande de systèmes HVDC de type multi-terminal (MTDC), avec des convertisseurs de type VSC classique ou modulaire multi-niveaux. La première étape consiste à obtenir les modèles moyens du VSC classique et du MMC. La différence fondamentale entre ces deux convertisseurs, à savoir la possibilité pour le MMC de stocker et de contrôler l'énergie des condensateurs des sous-modules, est détaillée et expliquée. Ces modèles et leurs commandes sont ensuite linéarisés et mis sous forme de représentations d'état, puis validés en comparant leur comportement à ceux de modèles de convertisseurs plus détaillés à l'aide de logiciels de type EMT. Une fois validés, les modèles d'état peuvent être utilisés afin de générer le modèle d'état de tout système de transmissions HVDC, qu'il soit point-à-point ou MTDC. La comparaison d'une liaison HVDC à base de VSCs classiques puis de MMCs est alors réalisée. Leurs valeurs propres sont étudiées et comparées, et les modes ayant un impact sur la tension DC sont identifiés et analysés. Cette étude est ensuite étendue à un système MTDC à 5 terminaux, et son analyse modale permet à la fois d'étudier la stabilité du système, mais aussi de comprendre l'origine de ses valeurs propres ainsi que leur impact sur la dynamique du système. La méthode de décomposition en valeurs singulières permet ensuite d'obtenir un intervalle de valeurs possibles pour le paramètre de"voltage droop", permettant ainsi le contrôle du système MTDC tout en s'assurant qu'il soit conforme à des contraintes bien définies, comme l'écart maximal admissible en tension DC. Enfin, une proposition de "frequency droop" (ou "statisme"), permettant aux convertisseurs de participer au réglage de la fréquence des réseaux AC auxquels ils sont connectés, est étudiée. Le frequency droop est utilisé conjointement avec le voltage droop afn de garantir le bon fonctionnement de la partie AC et de la partie DC. Cependant, l'utilisation des deux droop génère un couplage indésirable entre les deux commandes. Ces interactions sont mathématiquement quantifiées et une correction à apporter au paramètre de frequency droop est proposée. Ces résultats sont ensuite validés par des simulations EMT et par des essais sur la plate-forme MTDC du laboratoire L2EP. / HVDC transmission systems are largely used worldwide, mostly in the form of back-to-back and point-to-point HVDC, using either thyristor-based LCC or IGBT-based VSC. With the recent deployment of the INELFE HVDC link between France and Spain, and the commissioning in China of a three-terminal HVDC transmission system using Modular Multilevel Converters (MMCs), a modular design of voltage source converters, the focus of the scientific community has shifted onto the analysis and control of MMC-based HVDC transmission systems. In this thesis, the average value models of both a standard 2-level VSC and an MMC are proposed and the most interesting difference between the two converter technologies -the control of the stored energy in the MMC- is emphasised and explained. These models are then linearised, expressed in state-space form and validated by comparing their behaviour to more detailed models under EMT programs. Afterwards, these state-space representations are used in the modelling of HVDC transmission systems, either point-to-point or Multi-Terminal HVDC (MTDC). A modal analysis is performed on an HVDC link, for both 2-level VSCs and MMCs. The modes of these two systems are specifed and compared and the independent control of the DC voltage and the DC current in the case of an MMC is illustrated. This analysis is extended to the scope of a 5-terminal HVDC system in order to perform a stability analysis, understand the origin of the system dynamics and identify the dominant DC voltage mode that dictates the DC voltage response time. Using the Singular Value Decomposition method on the MTDC system, the proper design of the voltage-droop gains of the controllers is then achieved so that the system operation is ensured within physical constraints, such as the maximum DC voltage deviation and the maximum admissible current in the power electronics. Finally, a supplementary droop "the frequency-droop control" is proposed so that MTDC systems also participate to the onshore grids frequency regulation. However, this controller interacts with the voltage-droop controller. This interaction is mathematically quantified and a corrected frequency-droop gain is proposed. This control is then illustrated with an application to the physical converters of the Twenties project mock-up.