Bibliographies: 'Multi-terminal network' – Grafiati (2024)

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Many networks, particularly infrastructure networks, have multiple source nodes and multiple terminal nodes. And many such networks exhibit community structures, wherein the network is partitioned into groups of densely connected nodes with sparse connections between groups, based on topology or spatial characteristics, among others. This article proposes an approach for evaluating the effects of disruptive events, or the disconnection of network components due to failures or attacks, to the community structures and to the total network. The approach enables the assessment of resilience, evaluating both the vulnerability of the network and the recoverability enabled by different network restoration sequences. Different predefined restoration sequences are compared from different perspectives, including cost and strategy characteristics as well as resilience objectives (partial or complete restoration). The approach is illustrated with the topology of an electric power network.

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This paper presents a simple and general theory on power absorbed by multi-terminal networks and the way to measure it, which has direct application to unification of active and reactive power measurement procedures of polyphase loads. This approach to the subject is highly efficient, not only for its simplicity, but also because it eliminates all particular demonstrations or proofs of different measurement methods, with consequent time and effort savings for students. It also offers absolute certainty about the scope of every power measurement procedure and their possible variations.

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Network operation may require that a specified number k of nodes be able to communicate via paths consisting of operating edges and nodes. In an environment of node and edge failure, this leads to associated reliability measures. When the k nodes are known in advance, this has been widely studied as k-terminal reliability; when the k nodes are chosen uniformly at random, this has been studied as k-resilience. A third notion, when it suffices to have anyk nodes communicate, is related to the expected size of the largest component in the network. We generalize these three measures to the probability that given h nodes chosen in advance and i nodes chosen at random, they appear in a component of size at least k = h + i + j. As expected, for general networks, for most choices of (h, i, j) the computation is #P-complete and hence unlikely to admit a polynomial time algorithm. We develop polynomial time algorithms in the special case that the network is series-parallel, which subsume and generalize earlier methods for k-terminal reliability and k-resilience.

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Contraflow technique is the widely accepted model on network optimization. It allows arc reversal that increases the arc capacities. The earliest arrival transshipment contraflow is an important model that transship the given flow value by sending the maximum amount at each time point from the beginning within given time period by reversing the direction arcs from the sources to the sinks at time zero. This problem has not been solved polynomially on complex networks, i.e., multi-terminal networks yet. However, its 2-value-approximation solution has been found by Pyakurel and Dhamala [13] in pseudopolynomial time complexity. Moreover, they have claimed that for the special case of zero transit time on each arc, the 2-value-approximation solution can be computed in polynomial time complexity. In this paper, we solve their claim presenting an efficient algorithm.

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With the existing reliability evaluation methods, it is difficult to realize the overall evaluation of multi-voltage levels distribution systems, and sense-control terminal faults are not considered in the process of reliability evaluation. Therefore, a reliability evaluation method for multi-voltage levels distribution networks considering the influence of the fault of the sensing and control terminal is proposed. Firstly, based on the component reliability model, system state selection method, and reliability index, a reliability evaluation method for distribution networks considering sense-control terminal faults has been proposed. Secondly, the features of a multi-voltage levels distribution network are analyzed. The sequential Monte Carlo simulation method and the failure mode effects analysis method are combined, and a collaborative evaluation method for power supply reliability of multi-voltage levels distribution systems is proposed. Eventually, the effectiveness of the proposed method is proved by an example comparison, and the impact of the terminal fault on the reliability of distribution network is analyzed. The reliability evaluation results can provide technical support for the planning of the distribution network.

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Heterogeneous wireless networks (HWNs) are capable of integrating the different radio access technologies that make it possible to connect mobile users based on the performance parameters. Further quality of service (QoS) is one of the major topics for HWNs, moreover existing radio access technology (RAT) methodology are designed to provide network QoS criteria. However, limited work has been carried out for the RAT selection mechanism considering user QoS preference and existing models are developed based on the multi-mode terminal under a given minimal density network. For overcoming research issues this paper present quality of experience (QoE) RAT (QOE-RAT) selection methodology, incorporating both network performance criteria and user preference considering multiple call and multi-mode HWNs environment. First, this paper presents fuzzy preference aware weight (FPAW) and multi-mode terminal preference aware TOPSIS (MMTPA-TOPSIS) for choosing the best RAT for gaining multi-services. Experiment outcomes show the QOE-RAT selection method achieves much superior packet transmission outcomes when compared with state-of-art Rat selection methodologies.

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Introduction. In recent years, transmission systems comprise more direct current structures; their effects on alternating current power system may become significant and important. Also, multi-terminal direct current is favorable to the integration of large wind and solar power plants with a very beneficial ecological effect. The novelty of the proposed work consists in the effects of the aforementioned modern devices on transient stability, thus turn out to be an interesting research issue. In our view, they constitute a new challenge and an additional complexity for studying the dynamic behavior of modern electrical systems. Purpose. We sought a resolution to the problem of the transient stability constrained optimal power flow in the alternating current / direct current meshed networks. Convergence to security optimal power flow has been globally achieved. Methods. The solution of the problem was carried out in MATLAB environment, by an iterative combinatorial approach between optimized power flow computation and dynamic simulation. Results. A new transient stability constrained optimal power flow approach considering multi-terminal direct current systems can improve the transient stability after a contingency occurrence and operate the system economically within the system physical bounds. Practical value. The effectiveness and robustness of the proposed method is tested on the modified IEEE 14-bus test system with multi-objective optimization problem that reflect active power generation cost minimization and stability of the networks. It should be mentioned that active power losses are small in meshed networks relative to the standard network. The meshed networks led to a gain up to 46,214 % from the base case.

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Smart phones and tablet computers have greatly boosted thedemand for services via wireless access points, keeping constantpressure on the network providers to deliver vast amounts of dataover the wireless infrastructure. To enlarge coverage and enhancethroughput, relaying has been adopted in the new generation ofwireless communication systems, such as in the Long-Term EvolutionAdvanced standard, and will continue to play an important role inthe next generation wireless infrastructure. Depending onfunctionality, relaying can be characterizing into three maincategories: amplify-and-forward (AF), compression-and-forward(CF), and decode-and-forward (DF). In this thesis, we investigatedifferent cooperative strategies in wireless networks whenrelaying is in use. We first investigate the capacity outer and inner bounds for awireless multicast relay network where two sources, connected byerror-free backhaul, multicast to two destinations with the helpof a full-duplex relay node. For high-rate backhaul scenarios,we find the exact cut-set bound of the capacity regionby extending the proof of theconverse for the Gaussian relay channel. For low-rate backhaulscenarios, we present two genie-aided outer bounds by extendingthe previous proof and introducing two lemmas on conditional(co-)variance. Our inner bounds are derived from variouscooperative strategies by combining DF/CF/AF relaying with networkcoding schemes. We also extend the noisy network coding scheme andtheshort-message noisy network coding approach tocorrelated sources. For low-rate backhaul, we propose a new codingscheme, partial-decode-and-forward based linear network coding. Wederive the achievable rate regions for these schemes and measurethe performance in term of achievable rates over Gaussianchannels. By numerical investigation we observe significantgains over benchmark schemes and demonstrate that the gap between upper and lower bounds is ingeneral not large. We also show that for high-rate backhaul, thecut-set bound can be achieved when the signal-to-noise ratios liein the sphere definedby the source-relay and relay-destination channel gains. For wireless networks with independent noise, we propose a simpleframework to get capacity outer and inner bounds based on the``one-shot'' bounding models. We first extend the models fortwo-user broadcast channels to many-user scenarios and thenestablish the gap between upper and lower bounding models. Fornetworks with coupled links, we propose a channel decouplingmethod which can decompose the network into overlappingmultiple-access channels and broadcast channels. We thenapply the one-shot models and create an upper bounding network withonly bit-pipe connections. When developing the lower bounding network,we propose a two-step update of these models for each coupledbroadcast and multiple-access channels. We demonstrate by someexamples that the resulting upper bound is in general very goodand the gap between the upper and lower bounds is usually notlarge. Forrelay-aided downlink scenarios, we propose a cooperation scheme bycancelling interference at the transmitter. It is indeed asymbol-by-symbol approach to one-dimension dirty paper coding(DPC). For finite-alphabet signaling and interference, we derivethe optimal (in terms of maximum mutual information) modulatorunder a given power constraint. A sub-optimal modulator is alsoproposed by formulating an optimization problem that maximizes theminimum distance of the signal constellation, and this non-convexoptimization problem is approximately solved by semi-definiterelaxation. Bit-level simulation shows that the optimal andsub-optimal modulators can achieve significant gains over theTomlinson-Harashima precoder (THP) benchmark and over non-DPCreference schemes, especially when the power of the interferenceis larger than the power of the noise.

QC 20121015

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Les réseaux sans fil de la prochaine génération devraient prendre en charge les techniques de détection. Des exemples importants sont les systèmes de transport intelligents, où les véhicules détectent en permanence les changements environnementaux et échangent des informations avec les véhicules ou les serveurs centraux. Il existe des solutions naïves pour réaliser ces deux tâches, qui proposent de partager les ressources entre les deux. Cependant, les coûts élevés de spectre et de matériel de ces approches encouragent l’intégration des tâches de détection et de communication (ISAC) via une seule forme d’onde et une seule plateforme matérielle. Cette thèse se concentre sur l’ISAC théorique de l’information. Nous examinons le premier modèle informationnel théorique pour ISAC dans [1] où un canal sans mémoire dépendant de l’état (SDMC) avec des signaux de rétroaction généralisés est observé au niveau de l’émetteur (Tx). Notre première contribution est de caractériser le compromis fondamental entre les taux de communication et la distorsion de détection des canaux de diffusion (BC) dépendants de l’état, mono-Tx et bi-Rx, qui sont physiquement dégradés. Nous fournissons également des limites intérieures et extérieures sur les compromis taux-distorsion réalisables pour les canaux de diffusion généraux. La stratégie optimale de détection des Tx uniques est un simple estimateur symbole par symbole et l’optimalité de cet estimateur découle du fait que les canaux de rétroaction généralisés et la séquence d’état sont tous deux sans mémoire. Ce n’est pas nécessairement le cas dans les configurations avec plus d’une Tx. Plus précisément, pour le MAC, nous proposons une détection collaborative où chaque Tx compresse d’abord les sorties et les entrées obtenues pour extraire les informations d’état, puis transmet l’indice de compression à l’aide d’un code de canal pur aux autres Tx. Nous décrivons également deux schémas ISAC collaboratifs pour D2D, basés sur la séparation source-canal/le schéma de canal bidirectionnel de Han et basés sur le codage conjoint source-canal (JSCC). Dans le scénario MAC et D2D, nos schémas ISAC sont strictement concaves dans les paires taux-distorsion et améliorent donc également les stratégies classiques de partage du temps ou des ressources
Next-generation wireless networks are expected to support sensing techniques. Important examples are intelligent transport systems, where vehicles continuously sense environmental changes and exchange information with vehicles or central servers. There are some naive solutions to do both tasks which propose to share the resources between the two. But, the high spectrum and hardware costs of these approaches encourage to integrate the sensing and communication (ISAC) tasks via a single waveform and a single hardware platform. This thesis focuses on information-theoretic ISAC. We review the first information-theoretic model for ISAC in [1] where a statedependent memoryless channel (SDMC) with generalized feedback signals observed at the transmitter (Tx). Our first contribution is to characterize the fundamental tradeoff between communication rates and sensing distortion of statedependent single-Tx two-Rx broadcast channels (BC) that are physically degraded. We also provide inner and outer bounds on the achievable rate-distortion tradeoffs for general BCs. The single-Txs’ optimal sensing strategy is a simple symbol-by-symbol estimator and the optimality of this estimator stems from the fact that the generalized feedback channels and the state-sequence both are memoryless. This is not necessarily the case in setups with more than one Tx. Specifically, for the MAC, we propose collaborative sensing where each Tx first compresses the obtained outputs and inputs to extract state information, then transmits the compression index using a pure channel code to the other Tx. Also, we describe two collaborative ISAC schemes for D2D, based on source-channel separation/Han’s two-way channel scheme and based on joint source-channel coding (JSCC). In both the MAC and the D2D scenario, our ISAC schemes are strictly concave in the rate-distortion pairs and thus also improve over classical time- or resource-sharing strategies

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La diversité des technologies d'accès radio (e.g., GPRS, UMTS, HSDPA, Wi-Fi, WiMAX, LTE ...), leur complémentarité en termes de couverture, des caractéristiques (e.g., la bande passante, QoS) et des possibilités commerciales pour les opérateurs conduisent au développement des terminaux mobiles intégrant simultanément plusieurs interfaces radio. La capacité des terminaux mobiles utilisant simultanément différentes interfaces offre de nombreux avantages intéressants, tels que l'accès permanent et omniprésent, la fiabilité, le partage de charge, l'agrégation de bande passante disponible et la sélection d'interface basée sur plusieurs critères, etc. Les terminaux mobiles avec plusieurs interfaces radio ont la possibilité de choisir la "meilleure" interface en fonction de plusieurs paramètres tels que les caractéristiques des applications, les préférences des utilisateurs, les caractéristiques du réseau, les politiques d'opérateur et les contraintes tarifaires, etc. Il devient également possible d'associer les applications aux différentes interfaces de réseau basant sur les exigences d'application. Dans cette thèse, nous abordons le problème de sélection d'interface où un terminal mobile équipé de plusieurs interfaces peut sélectionner à tout moment la meilleure interface ou la meilleure technologie d'accès selon plusieurs critères. Nous considérons le problème de décision pour la sélection d'interface. Le problème de décision est un problème très complexe. On peut avoir les différent approches pour la section d'interface (e.g., fonction de coût, fonction d'utilité, ou la politique). Chaque approche est considérée comme un angle d'attaque. Nous nous intéressons l'approche MADM qui est une approche prometteuse pour la décision avec plusieurs attributs. Nous investiguons ces méthodes dans le contexte de la sélection d'interface. L'objectif fondamental des méthodes MADM est de déterminer la solution optimale parmi plusieurs solutions. MADM comprend de nombreuses méthodes, comme SAW (Simple Additive Weighting), WP (Weighting Product) et TOPSIS (Technique for Order Preference by Similarity to Ideal Solution). Le premier objectif de ma thèse est d'étudier et d'analyser les méthodes de MADM pour le problème de sélection d'interface. La première contribution est de proposer une étude de simulation qui met en évidence des limites des méthodes de MADM dans le contexte de la sélection d'interface. Par exemple, TOPSIS a le problème "d'anomalie de classem*nt". Ce problème se produit lorsqu'une interface à faible classem*nt est retirée de la liste du candidat (e.g., un réseau est déconnecté), l'ordre de classem*nt des interfaces changera anormalement. La deuxième contribution propose l'algorithme de DiA (Distance to the ideal Alternative) qui permet au terminal mobile de sélectionner dynamiquement la meilleure interface. Nous montrons que DiA n'a pas le problème "d'anomalie de classem*nt" qui est le défaut de la méthode TOPSIS. Les résultats de simulation valident l'algorithme de DiA. La troisième contribution s'attaque au problème d'association flux/interface où un terminal mobile équipé de plusieurs interfaces doit associer une application à l'interface spécifique appropriée. Nous proposons tout d'abord une fonction d'utilité interface. Cette fonction d'utilité permet d'identifier l'interface qui satisfait des besoins d'application et économise la consommation d'énergie du terminal mobile. Nous proposons ensuite un premier modèle d'association flux/interface qui permet d'associer séquentiellement des applications aux interfaces. Les attributs de réseau tels que le délai d'accès et le coût monétaire sont également pris en compte dans le régime. L'algorithme de DiA est utilisé pour classer les interfaces basées sur les valeurs d'utilité interface et les attributs de réseau. Les résultats de simulation sont présentés pour valider le schéma proposé. De plus, nous proposons un deuxième modèle d'association flux/interface. Dans ce modèle, un terminal peut associer simultanément plusieurs applications aux interfaces de réseau. Le modèle vise à maximise l'utilité globale du terminal. Ce problème est un problème d'optimisation. En particulaire, il est lié aux problèmes d'optimisation heuristique stochastique (i.e., méta-heuristique) qui sont principalement basées sur les techniques de recherche dont les solutions et l'ordre de recherche basent sur les procédures aléatoires. En première étape, nous étudions et réalisons une étude de simulation des méthodes d'optimisation heuristique stochastique, e.g., la recherche locale, la recherche de Tabou, la méthode de recuit simulée. Nous proposons ensuite une technique de diversification orientée pour la recherche Tabou comme une amélioration. Cela permet à la recherche Tabou d'éviter de se retrouve piégée plusieurs fois dans l'optimum local et d'augmenter les performances de la recherche Tabou dans notre contexte. Les résultats de simulation montrent que la méthode modifiée a meilleure performance comparée avec les autres algorithmes méta-heuristique dans notre contexte. Nous nous dirigeons ensuite vers une approche au niveau de réseau pour le problème d'association flux/interface. Nous considérons un système des terminaux mobiles multi-interface. Chaque terminal peut associer des applications aux interfaces. Comme plusieurs terminaux en concurrence pour les ressources de réseau commun, le système est modélisé comme un jeu stratégique. Notre objectif est de trouver des stratégies d'équilibre de Nash pour le jeu. Nous avons laissé le jeu évoluer en fonction de la dynamique de Replicateur et observons si le système converge et si les points stationnaires sont des équilibres de Nash. Nous montrons que la dynamique de Replicateur est positivement corrélée et le système est un jeu potentiel. Notre système converge vers des points stationnaires qui comprennent tous les équilibres de Nash. En outre, les points stationnaires sont efficaces car ils optimisent l'utilité du système. Un point intéressant est que nos résultats sont validés pour une fonction d'utilité générale qui dépend de l'état du système. Pour valider notre modèle et démontrer que le système converge vers des équilibres de Nash, nous mettons en œuvre les scénarios de simulation en utilisant un algorithme d'apprentissage Nash avec un schéma d'allocation de bande passante spécifique ainsi que d'une fonction d'utilité qui prend en compte le niveau de satisfaction d'application et la consommation d'énergie.

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La diversité des technologies d'accès radio (e. G. , GPRS, UMTS, HSDPA, Wi-Fi, WiMAX, LTE. . . ), leur complémentarité en termes de couverture, des caractéristiques (e. G. , la bande passante, QoS) et des possibilités commerciales pour les opérateurs conduisent au développement des terminaux mobiles intégrant simultanément plusieurs interfaces radio. La capacité des terminaux mobiles utilisant simultanément différentes interfaces offre de nombreux avantages intéressants, tels que l'accès permanent et omniprésent, la fiabilité, le partage de charge, l’agrégation de bande passante disponible et la sélection d’interface basée sur plusieurs critères, etc. Les terminaux mobiles avec plusieurs interfaces radio ont la possibilité de choisir la «meilleure» interface en fonction de plusieurs paramètres tels que les caractéristiques des applications, les préférences des utilisateurs, les caractéristiques du réseau, les politiques d'opérateur et les contraintes tarifaires, etc. Il devient également possible d'associer les applications aux différentes interfaces de réseau basant sur les exigences d'application. Dans cette thèse, nous abordons le problème de sélection d’interface où un terminal mobile équipé de plusieurs interfaces peut sélectionner à tout moment la meilleure interface ou la meilleure technologie d'accès selon plusieurs critères. Nous considérons le problème de décision pour la sélection d’interface. Le problème de décision est un problème très complexe. On peut avoir les différent approches pour la section d’interface (e. G. , fonction de coût, fonction d’utilité, ou la politique). Chaque approche est considérée comme un angle d’attaque. Nous nous intéressons à l’approche MADM qui est une approche prometteuse pour la décision avec plusieurs attributs. Nous investiguons ces méthodes dans le contexte de la sélection d’interface
The diversity of radio access technologies (e. G. , GPRS, UMTS, HSDPA,Wi-Fi, WiMAX, LTE…), their complementary in terms of coverage area, technical characteristics (e. G. , bandwidth, QoS) and commercial opportunities for the operators lead to the development of mobile terminals integrating multiple radio interfaces. The ability of mobile terminals to support various interfaces provides many interesting benefits, such as permanent and ubiquitous access, reliability, load sharing/load balancing, bandwidth aggregation, and muti-criteria interface selection. Mobile terminals with several radio interfaces have the possibility to choose the ―best‖ interface according to several parameters such as application characteristics, user preferences, network characteristics, operator policies, tariff constraints, etc. It becomes also possible to associate the applications to the available network interfaces basing mainly on application requirements. In the thesis, we tackle the interface selection issue where a mobile terminal equipped with several interfaces has to select at any time the best interface or the best access technology according to multiple criteria. We particularly focus on the decision schemes and investigate the MADM methods. The fundamental objective of the MADM methods is to determine among a finite set of alternatives the optimal one. MADM includes many methods such as Simple Additive Weighting (SAW), Weighting Product (WP), and Technique for Order Preference by Similarity to Ideal Solution (TOPSIS)

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There is a trend for offshore wind farms to move further from the point of common coupling to access higher and more consistent wind speeds to reduce the levelised cost of energy. To accommodate these rising transmission distances, High Voltage Direct Current (HVDC) transmission has become increasingly popular. However, existing HVDC wind farm topologies and converter systems are ill suited to the demands of offshore operation. The HVDC and AC substations have been shown to contribute to more than 20% of the capital cost of the wind farm and provide a single point of failure. Therefore, many wind farms have experienced significant delays in construction and commissioning, or been brought off line until faults could be repaired. What is more, around 75% of the cost of the HVDC and AC substations can be attributed to structural and installation costs. Learning from earlier experiences, industry is now beginning to investigate the potential of a modular approach. In place of a single large converter, several converters are connected in series, reducing substation individual size and complexity. While such options somewhat reduce the capital costs, further reductions are possible through elimination of the offshore substations altogether. This thesis concerns the design and evaluation the Hybrid HVDC Transformer, a high power, high voltage, DC transformer. This forms part of the platform-less (i.e. without substations) offshore DC power collection and distribution concept first introduced by the Offshore Renewable Energy Catapult. By operating in the medium frequency range the proposed Hybrid HVDC Transformer can be located within each turbine’s nacelle or tower and remove the need for expensive offshore AC and DC substations. While solid state, non-isolating DC-DC transformers have been proposed in the literature, they are incapable of achieving the step up ratios required for the Hybrid HVDC transformer [1]– [3]. A magnetic transformer is therefore required, although medium frequency and non-sinusoidal operation does complicate the design somewhat. For example, inter-winding capacitances are more significant and core losses are increased due to the added harmonics injected by the primary and secondary converters [1], [2]. To mitigate the impact of these complications, an investigation into the optimal design was conducted, including all power converter topologies, core shapes and winding configurations. The modular multilevel converter in this case proved to be the most efficient and practical topology however, the number of voltage levels that could be generated on the primary converter was limited by the DC bus voltage. To avoid the use of pulse width modulation and hence large switching losses, a novel MMC control algorithm is proposed to reduce the magnitude of the converter generated harmonics while maintaining a high efficiency. The development and analysis of this High Definition Modular Multilevel Control algorithm forms the bulk of this thesis’ contribution. While the High Definition Modular Multilevel Control algorithm was developed initially for the Hybrid HVDC Transformer, analysis shows it has several other potential applications particularly in medium and low voltage ranges.

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This thesis explains the working principles of an AC/DC converter topology intended for HVDC applications, called the Alternate Arm Converter (AAC). It consists of a hybrid between the modular multi-level converter (MMC) through the presence of H-bridge cells and the 2-level converter because of the director switches in each arm. Thanks to its cells, the AAC is able to generate a multi-level staircase AC voltage waveform which results a low distortion AC current while the director switches control which arms are conducting at any given time. By synchronising the conduction period of an arm with the zero-crossing points of the AC voltage waveform, the voltage rating of the stacks can be reduced, hence minimizing the number of cells. In case of a DC-side fault, an AAC with enough cells is able to keep control of the current in the phase reactor and even be operated to support the AC grid by providing reactive power similarly to a STATCOM. Since the AAC relies on pre-charged H-bridge cells, an effective energy management is required to control their level of charge. An ideal working point has been identified, called 'Sweet-Spot'. This operating point describes a set of conditions where the incoming and outgoing energy flows equate, in effect nullifying the average energy drift of the cells. Additional energy techniques based on current modulation have been developed in order to redistribute the energy inside the converter. The study of the AAC has provided an understanding of its working and device requirement and a hierarchical structure for its control system has been developed. Simulation results confirm these findings both on the operation of the AAC under normal and abnormal situations and on the effectiveness of the developed energy management system. Post-processing of the simulation data has also shown that the AAC is on par with the half-bridge MMC on power efficiency.

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For the past century, ac networks have been established as the standard technology for electrical power transmission system s. However, the de technology has not disappeared completely from this picture. The capability of de systems to transmit higher power over longer distances, the possibility of interconnecting asynchronous networks, and their high efficiency has maintained the interest of both industry and academia. Historically, systems based on dc-generators and mercury valves were used for de power transmission applications, but, by the 90's, all installations were thyrsi tor-based line commutated converters (LCC). In 1999, the first system based on voltage source converters (VSC) was installed in Gotland, Sweden, marking the beginning of a new era for de transmission. Over the past 15 years, the power rating of VSC-based de transmission systems has increased from 50 to 700 MVV, the operating voltage from 120 to 500 kV, meanwhile , the covered distances have become as long as 950 km (ABB's HVDC-light installation in Namibia in 2010). The work presented in this thesis is oriented towards the control and operation of multi-terminal VSC de (MTDC) networks. The proposed approach is a hierarchical control architecture, inspired by the well-established automatic generation control strategy applied to ac networks. In the proposed architecture, the primary control of the MTDC system is decentralized and implemented using a generalized droop strategy More than analyzing the behavior of the primary control, this thesis provides a methodology for designing the various parameters that influence this behavior. The importance of correctly dimensioning the VSC's output capacitor is underlined as this element, when set in the context of a MTDC network, becomes the inertial element of the grid and it has a direct impact on the voltage overs hoots that appear during transients. Further on, an improved droop control strategy that attenuates the voltage oscillations during transients is proposed. Also part of the proposed hierarchical control, the secondary control is centralized and it regulates the operating point of the network so that optimal power flow (OPF) is achieved . Compared to other works, this thesis elaborates, both analytically and through simulations, on the coordination between the primary and secondary control layers.
Durante el siglo pasado, las redes de corriente alterna se han consolidado como la tecnología estándar para los sistemas de transmisión de energía eléctrica. Sin embargo, los sistemas de transmisión en continua se han seguido utilizando en algunas aplicaciones. La capacidad de estos para transmitir mayores potencias a distancias más largas, la posibilidad de interconectar redes asincrónicas, y su alta eficiencia han propiciado que se mantuviera el interés académico, de investigación e industrial en esta tecnología . Aunque históricamente se utilizaron sistemas basados en generadores de continua y válvulas de mercurio para las redes de transmisión, en la década de los 90 todas las instalaciones ya contaban con convertidores conmutados basados en tiristores (LCC). En 1999, se instaló el primer sistema basado en convertidores en fuente de tensión (VSC) en Gotland, Suecia, marcando el comienzo de una nueva era para la transmisión en corriente continua. En los últimos 15 años, la potencia de los sistemas de transmisión en continua basados en VSC ha aumentado desde los 50 hasta los 700 MN, la tensión de servicio de 120 a 500 kV y las distancias recorridas han llegado a ser, en algunos casos, de hasta 950 kilómetros (HVDC-light de ABB en Namibia en 201 O). El trabajo presentado en esta tesis se centra en el control y operación de redes de corriente continua VSC multi-terminal (MTDC). El enfoque propuesto se basa en una arquitectura de control jerárquico, inspirada en la estrategia de control de generación automática aplicada a redes de corriente alterna. En la arquitectura propuesta, el control primario del sistema MTDC está descentralizado e implementado mediante una estrategia de 'droop' generalizada. Más allá del análisis del comportamiento del control primario, esta tesis presenta una metodología para el diseño de los diferentes parámetros que influyen en el mismo. Se destaca la importancia de dimensionar correctamente condensador de salida del VSC, ya que este elemento, cuando se encuentra en el contexto de una red MTDC, se convierte en el elemento inercial de la red y tiene un impacto directo en el comportamiento transitorio de las tensiones. Asimismo, se propone una estrategia de control de 'droop' mejorada que atenúa las oscilaciones de tensión durante los transitorios. En el marco del control jerárquico propuesto, el control secundario está centralizado y regula el punto de funcionamiento de la red de manera que se consigue un flujo de potencia óptimo (OPF). En comparación con otros trabajos, esta tesis lleva a cabo, tanto de forma analítica como a través de simulaciones, un estudio detallado sobre la coordinación entre las capas de control primario y secundario en redes MTDC.

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Abstract:

Cette thèse traite de la commande hiérarchique de réseaux à courant continu multi-terminaux à haute tension (MT-HVDC) intégrant des sources d'énergie renouvelables à grande échelle. Le schéma de contrôle proposé est composé de quatre ‘couches’ : le contrôle local où se trouvent les convertisseurs de puissance, avec une échelle de temps de l’ordre de la milliseconde ; le contrôle primaire qui est décentralisé et appliqué à plusieurs terminaux avec une échelle du temps de l’ordre de la seconde ; un niveau de commande où la communication est prise en compte et où l’approche de Modèle du Commande Prédictive (MPC) assure la planification de la tension et de la puissance à leur état d'équilibre, pour l'ensemble du système; enfin, le contrôleur de niveau supérieur, qui est principalement basé sur les techniques d'optimisation, où les aspects économiques sont pris en compte (il s’agit du réglage dit tertiaire).Au niveau des convertisseurs, un accent particulier est mis sur les convertisseurs bidirectionnels DC/DC. Dans cette thèse, trois topologies différentes sont étudiées en profondeur: deux phases Dual Active Bridge (DAB), trois phases DAB, et l’utilisation de la technologie Modular Multilevel converter (MMC) comme convertisseur DC/DC. Pour chaque topologie, une commande non-linéaire spécifique est discutée. D’autre part une nouvelle fonction pour le convertisseur DC/DC est étudiée. Il s’agit de son utilisation comme disjoncteur à courant continu (DC-CB). En ce qui concerne le contrôle primaire, qui permet de maintenir le niveau de tension continue dans le réseau, nous avons étudié trois philosophies de contrôle: celle de maître/esclave, celui du contrôle « voltage margin control » et celle de la commande du statisme (droop control). Enfin, nous avons choisi d'utiliser le droop control, entre autres, parce que la communication entre les nœuds n’est pas nécessaire. Concernant la commande secondaire, son principal objectif est de planifier le transfert de puissance entre les nœuds du réseau, qui fournissent la tension et la puissance de référence aux contrôleurs locaux et primaires, même lorsque des perturbations apparaissent. Dans cette partie, nous avons proposé une nouvelle approche pour résoudre les problèmes de flux de puissance (équations non-linéaires) basée sur le théorème du point fixe de l’application contractive. Ceci permet d'utiliser plus d'un slack bus, contrairement à l’approche classique basée sur la méthode de Newton-Raphson. Par ailleurs, le réglage secondaire joue un rôle très important dans les applications pratiques, en particulier lorsque les sources d'énergie renouvelables (variables dans le temps). Dans de tels cas, il est intéressant de considérer des dispositifs de stockage afin d'améliorer la stabilité de tout le système. Il est également possible d'envisager différents types de prévisions (météo, charge, ..) basées sur la gestion des réserves de stockage. Toutes ces caractéristiques ont suggéré l'utilisation d'une approche MPC. Dans ce contexte, plusieurs critères d'optimisation ont été considérés, en particulier la minimisation des pertes de transmission ou des congestions dans le réseau.La tâche principale de réglage tertiaire est de d'atteindre l'optimisation économique de l'ensemble du réseau. Dans cette thèse, nous avons pu maximiser le profit économique du système en agissant sur le marché réel, et en optimisant l'utilisation des périphériques de stockage. Dans le but de mettre en œuvre la philosophie de contrôle hiérarchique présentée dans cette thèse, nous avons construit un banc d'essai expérimental. Cette plate-forme dispose de quatre terminaux reliés entre eux par l'intermédiaire d'un réseau à courant continu, et connectés au réseau principal de courant alternatif. Ce réseau DC peut fonctionner à un maximum de 400 V, et avec une courant maximal de 15 A
This thesis focuses on the hierarchical control for a multi-terminal high voltage direct current (MT-HVDC) grid suitable for the integration of large scale renewable energy sources. The proposed control scheme is composed of 4 layers, from the low local control at the power converters in the time scale of units of ms; through distributed droop control (primary control) applied in several terminals in the scale of unit of seconds; and then to communication based Model Predictive Control (MPC) that assures the load flow and the steady state voltage/power plan for the whole system, manage large scale storage and include weather forecast (secondary control); finally reaching the higher level controller that is mostly based on optimization techniques, where economic aspects are considered in the same time as longer timespan weather forecast (tertiary control).Concerning the converters' level, special emphasis is placed on DC/DC bidirectional converters. In this thesis, three different topologies are studied in depth: two phases dual active bridge (DAB), the three phases DAB, and the use of the Modular Multilevel Converter (MMC) technology as DC/DC converter. For each topology a specific non-linear control is presented and discussed. In addition, the DC/DC converter can provide other important services as its use as a direct current circuit breaker (DC-CB). Several operation strategies are studied for these topologies used as DC-CB.With respect to primary control, which is the responsible to maintain the DC voltage control of the grid, we have studied several control philosophies: master/slave, voltage margin control and droop control. Finally we have chosen to use droop control, among other reasons, because the communication between nodes is not required. Relative to the secondary control, its main goal is to schedule power transfer between the network nodes providing voltage and power references to local and primary controllers, providing steady state response to disturbances and managing power reserves. In this part we have proposed a new approach to solve the power flow problem (non-linear equations) based on the contraction mapping theorem, which gives the possibility to use more than one bus for the power balance (slack bus) instead of the classic approach based on the Newton-Raphson method. Secondary control plays a very important role in practical applications, in particular when including time varying power sources, as renewable ones. In such cases, it is interesting to consider storage devices in order to improve the stability and the efficiency of the whole system. Due to the sample time of secondary control is on the order of minutes, it is also possible to consider different kinds of forecast (weather, load,..) and to achieve additional control objectives, based on managing storage reserves. All these characteristics encourage the use of a model predictive control (MPC) approach to design this task. In this context, several possibilities of optimization objective were considered, like to minimize transmission losses or to avoid power network congestions.The main task of tertiary control is to manage the load flow of the whole HVDC grid in order to achieve economical optimization. This control level provides power references to the secondary controller. In this thesis we were able to maximize the economic profit of the system by acting on the spot market, and by optimizing the use of storage devices. In this level it is again used the MPC approach.With the aim of implementing the hierarchical control philosophy explained in this thesis, we have built an experimental test bench. This platform has 4 terminals interconnected via a DC grid, and connected to the main AC grid through VSC power converters. This DC grid can work at a maximum of 400 V, and with a maximum allowed current of 15 A

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Cette thèse a été consacrée à l'étude des réseaux multi-terminaux haute tension à courant continu (MTDC). Les principales contributions étaient dans le domaine du contrôle automatique non linéaire, appliquées aux systèmes électriques, électronique de puissance et les sources d'énergie renouvelables. Le travail de recherche a été lancé avec l'intention de combler certaines lacunes entre la théorie et la pratique, en particulier: 1) d'enquêter sur diverses approches de contrôle pour le but d'améliorer la performance des systèmes MTDC; 2) d'établir des connexions entre la conception du contrôle empiriques existantes et analyse théorique; 3) d'améliorer la compréhension du comportement multi-échelle de temps des systèmes MTDC caractérisés par la présence de transitoires lents et rapides en réponse aux perturbations externes. En conséquence, ce travail de thèse peut être mis en trois domaines, à savoir la conception non linéaire de commande de systèmes MTDC, analyse des comportements dynamiques de système MTDC et l'application de systèmes MTDC pour le contrôle de fréquence des systèmes de climatisation
This dissertation was devoted to the study of multi-terminal high voltage direct current (MTDC) networks. The main contributions were in the field of nonlinear automatic control, applied to power systems, power electronics and renewable energy sources. The research work was started with the intention of filling some gaps between the theory and the practice, in particular: 1) to investigate various control approaches for the purpose of improving the performance of MTDC systems; 2) to establish connections between existing empirical control design and theoretical analysis; 3) to improve the understanding of the multi-time-scale behavior of MTDC systems characterized by the presence of slow and fast transients in response to external disturbances. As a consequence, this thesis work can be put into three areas, namely nonlinear control design of MTDC systems, analysis of MTDC system's dynamic behaviors and application of MTDC systems for frequency control of AC systems

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This thesis investigated the capability of multi-terminal high voltage direct current (HVDC) networks based on voltage source converter (VSC) technology to transfer power generated from offshore wind farms to onshore grids and interconnect the grids of different countries. Variable speed wind turbines and other low-carbon generators or loads that are connected through inverters do not inherently contribute to the inertia of AC grids. A coordinated control scheme for frequency support from multi-terminal VSC-HVDC (MTDC) scheme was designed to transfer additional power to AC grids from the kinetic energy stored in the wind turbine rotating mass and the active power transferred from other AC systems. The wind turbine inertia response limited the rate of change of AC grid frequency and the active power transferred from the other AC system reduced the frequency deviation. The wind turbines recovered back to their original speed after their inertia response and transferred a recovery power to the AC grid. An alternative coordinated control scheme with a frequency versus active power droop controller was designed for frequency support from MTDC schemes, in order to transfer the recovery power of wind turbines to other AC systems. This prevented a further drop of frequency on the AC grid. The effectiveness of the alternative coordinated control scheme was verified using the PSCAD simulation tool and demonstrated using an experimental test rig. A scaling method was demonstrated for a multi-terminal DC test rig to represent the equivalent steady state operation of different VSC-HVDC systems. The method uses a virtual resistance to extend the equivalent DC cable resistance of the test rig through the action of an additional DC voltage versus DC current droop controller. Three different VSC-HDC systems were modelled using the PSCAD simulation tool and demonstrated on the DC test rig with virtual resistance, showing good agreement.

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The problem of identifying and removing bottlenecks in a multi-terminal oil & gas pipeline network while achieving quality and delivery targets is a very real and complex problem. The most effective way to meet the above business objective is to develop a terminal network simulation model. This paper is a case study describing the approach in designing a complex multi-nodal pipeline network simulation model with objective to resolve a critical inter-company storage problem for a major refiner. Various complex system modeling techniques and approaches are elaborated with a focus on practical application. A case study is also presented to demonstrate the practical application of the modeling techniques for terminal network simulation model development.

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