Management and Protection of High-Voltage Direct Current Systems Based on Modular Multilevel Converters

The electrical grid is undergoing large changes due to the massive integration of renewable energy systems and the electrification of transport and heating sectors. These new resources are typically non-dispatchable and dependent on external factors (e.g., weather, user patterns). These two aspects make the generation and demand less predictable, facilitating a larger power variability. As a consequence, rejecting disturbances and respecting power quality constraints gets more challenging, as small power imbalances can create large frequency deviations with faster transients. In order to deal with these challenges, the energy system needs an upgraded infrastructure and improved control system. In this regard, high-voltage direct current (HVdc) systems can increase the controllability of the power system, facilitating the integration of large renewable energy systems. This thesis contributes to the advancement of the state of the art in HVdc systems, addressing the modeling, control and protection of HVdc systems, adopting modular multilevel converter (MMC) technology, with focus in providing services to ac systems. HVdc system control and protection studies need for an accurate HVdc terminal modeling in largely different time frames. Thus, as a first step, this thesis presents a guideline for the necessary level of deepness of the power electronics modeling with respect to the power system problem under study. Starting from a proper modeling for power system studies, this thesis proposes an HVdc frequency regulation approach, which adapts the power consumption of voltage-dependent loads by means of controlled reactive power injections, that control the voltage in the grid. This solution enables a fast and accurate load power control, able to minimize the frequency swing in asynchronous or embedded HVdc applications. One key challenge of HVdc systems is a proper protection system and particularly dc circuit breaker (CB) design, which necessitates fault current analysis for a large number of grid scenarios and parameters. This thesis applies the knowledge developed in the modeling and control of HVdc systems, to develop a fast and accurate fault current estimation method for MMC-based HVdc system. This method, including the HVdc control, achieved to accurately estimate the fault current peak value and slope with very small computational effort compared to the conventional approach using EMT-simulations. This work is concluded introducing a new protection methodology, that involves the fault blocking capability of MMCs with mixed submodule (SM) structure, without the need for an additional CB. The main focus is the adaption of the MMC topology with reduced number of bipolar SM to achieve similar fault clearing performance as with dc CB and tolerable SM over-voltage

Dynamic Averaged Models of VSC-Based HVDC Systems for Electromagnetic Transient Programs

RÉSUMÉ Les systèmes d’haute tension à courant continu (HTCC) basés sur technologies de convertisseur de source de tension (CST) offrent des prometteur opportunités dans une variété de domaines au sein de l'industrie des systèmes de puissance en raison de leurs avantages reconnus par rapport aux systèmes HTCC classiques basés à convertisseurs de commutation de ligne (CCL). La technologie CST-HTCC combine des convertisseurs de puissance, basé sur des IGBT (Insulated Gate Bipolar Transistor), avec des liens au courant continus pour transmettre la puissance dans l'ordre de milliers de mégawatts. En plus de contrôler le flux d'énergie entre deux réseaux à courant alternatif, les systèmes CST-HTCC peuvent fournir de réseaux faibles et même des réseaux passifs. Les systèmes CST-HTCC présentent une réponse dynamique plus rapide grâce à la méthode de modulation de largeur d'impulsions (MLI) en comparaison avec l'opération de commutation de fréquence fondamentale des systèmes HTCC traditionnels. Représentation détaillée des systèmes CST-HTCC dans les programmes d’Électromagnétique Transitoire (EMT) comprend la modélisation des valves IGBT et doit normalement utiliser de pas d'intégration petit pour représenter avec précision les événements de commutation rapides. Les simulations et les calculs informatiques introduits par les modèles détaillés compliquent l'étude des événements en régime permanent et transitoire mettant en évidence la nécessité de développer des modèles plus efficaces qui assurent un comportement similaire de la réponse dynamique. L'objectif de cette thèse est de développer des modèles moyennés qui reproduit avec précision le comportement statique et dynamique, en plus les transitoires des systèmes CST-HTCC dans des programmes de type EMT. Ces modèles simplifiés représentent la valeur moyenne des réponses des dispositifs de commutation, convertisseurs, et des contrôles à l'aide de techniques de valeur moyenne, de sources contrôlées et des fonctions de commutation. Cette thèse contribue également à l'élaboration de modèles CST détaillés utilisés pour valider les modèles moyenne proposés. Les modèles détaillés développés comprennent convertisseur avec topologies à deux et à trois niveaux et la plus récente topologie du convertisseur modulaire multiniveaux (CMM). Comparaison des différentes topologies de convertisseur approprié pour VSC-HVDC transmission, y compris leurs avantages et leurs limitations, sont également discutés.----------ABSTRACT High Voltage Direct Current (HVDC) systems based on Voltage-sourced Converter (VSC) technologies present a bright opportunity in a variety of fields within the power system industry due to their recognized advantages in comparison to conventional line-commutated converter (LCC) based HVDC systems. VSC-HVDC technology combines power converters, based on IGBTs (Insulated Gate Bipolar Transistors), with dc links to transmit power in the order of thousands of megawatts. In addition to controlling power flow between two ac networks, VSC-HVDC systems can supply weak and even passive networks. VSC-HVDC systems present a faster dynamic response thanks to its Pulse-width Modulation (PWM) control in comparison with the fundamental switching frequency operation of traditional HVDC systems. Detailed representation of VSC-HVDC systems in Electro Magnetic Transient (EMT) programs includes the modeling of IGBT valves and must normally use small integration time-steps to accurately represent fast switching events. Computational burden introduced by such a detailed models complicates the study of steady-state and transient events highlighting the need to develop more efficient models that provide similar behavior and dynamic response. The objective of this thesis is to develop, test and validate averaged models to accurately replicate the steady-state, dynamic and transient behavior of VSC-based HVDC systems in EMT-type programs. These simplified models represent the average response of switching devices and converters by using averaging techniques involving controlled sources and switching functions. The work also contributes to the development of detailed VSC models used to validate the proposed average models. The detailed models developed include two- and three-level converter topologies and the most recent Modular Multilevel Converter (MMC) topology. Comparison of different converter topologies suitable to VSC-HVDC transmission, including their advantages and limitations, are also discussed. A control system is implemented based on vector control which permits independent control both active and reactive power (and/or voltage) at each VSC terminal. Available modulation techniques are presented and compared in terms of performance and power quality

Modular Multilevel Converters in Hybrid Multi-Terminal HVDC Systems

High-voltage direct current (HVDC) systems are becoming commonplace in modern power systems. Line commutated converters (LCCs) are suitable for bulk power and ultra-HVDC (UHVDC) transmission, while with inflexible power reversal capability and possible commutation failures. However, voltage source converters (VSCs) possess flexible power reversal capability and provide immunity to commutation failures. Modular VSC topologies offer improved performance compared to conventional 2 level/3 level VSC-based HVDC. The family of modular VSCs includes the well-established modular multilevel converter (MMC) and other emerging modular VSC topologies such as the DC-fault tolerant alternate arm converter (AAC) that share topological and operational similarities with the MMC. It is noteworthy that the integration of LCC and modular VSCs leads to unique benefits despite the challenges of different HVDC configurations. Hence, it is necessary to explore the system performance of different HVDC converter topologies, especially more complex hybrid multiterminal HVDC (MTDC) systems and DC-grids combining different converters. This thesis focuses on the combination of the LCC, MMC and AAC to constitute different hybrid HVDC transmission systems. It is of significance to provide a common platform where the proper comparison and evaluation of different HVDC systems and control methods can be completed and independently validated. Therefore, this thesis also provides an overview of current HVDC benchmark models available in the existing literature. In addition, the detailed modeling methods of HVDC systems are discussed in this thesis. For ensuring the static security of HVDC systems especially the future DC-grids, this thesis proposes a generalized expression of DC power flow under mixed power/voltage (P/V) and current/voltage (I/V) droop control, considering the DC power flow for normal operation and after converter outage. Detailed simulation models are established in PLECS-Blockset and Simulink to study the hybrid HVDC/MTDC systems and DC grid combining the LCC with the MMC and (or) AAC. The detailed sets of results demonstrate the functionalities of developed hybrid HVDC systems and validate the performance of systems complying with widely accepted HVDC operating standards. The developed LCC/AAC-based HVDC/MTDC systems and LCC/MMC/AAC-based DC grid in this thesis are prime steps towards the study of more complex MTDC systems and a key element in the development of future DC super grids