8 research outputs found
Accelerated switching function model of hybrid MMCs for HVDC system simulation
An accelerated switching function model (SFM) of the hybrid modular multilevel converter comprising both full-bridge (FB) and half-bridge (HB) submodules (SMs) in each arm is presented for HVDC system simulation, where auxiliary circuits are adopted to represent all possible current paths during normal and fault conditions. The proposed SFM can represent the negative voltage generating capability of the FB SMs and the equivalent switching functions in the blocking states of the FB and HB SMs are also introduced in the proposed model to accurately replicate the potential charging of the SM capacitors, yielding improved simulation accuracy compared to other alternatives. In addition to the faster simulation speed, the proposed model accurately reproduces the converter behaviour during various operating conditions, including normal operation, AC fault, and DC fault, etc. The proposed SFMs are assessed in MATLAB/Simulink environment using both down- and full-scale HVDC links and the simulation results confirm the validity of the proposed model in terms of model accuracy and improved simulation speed
Modeling and enhanced control of hybrid full bridge–half bridge MMCs for HVDC grid studies
Modular multilevel converters (MMCs) are expected to play an important role in future
high voltage direct current (HVDC) grids. Moreover, advanced MMC topologies may include various
submodule (SM) types. In this sense, the modeling of MMCs is paramount for HVDC grid studies.
Detailed models of MMCs are cumbersome for electromagnetic transient (EMT) programs due to
the high number of components and large simulation times. For this reason, simplified models that
reduce the computation times while reproducing the dynamics of the MMCs are needed. However, up
to now, the models already developed do not consider hybrid MMCs, which consist of different types
of SMs. In this paper, a procedure to simulate MMCs having different SM topologies is proposed.
First, the structure of hybrid MMCs and the modeling method is presented. Next, an enhanced
procedure to compute the number of SMs to be inserted that takes into account the different behavior
of full-bridge SMs (FB-SMs) and half-bridge submodules (HB-SMs) is proposed in order to improve
the steady-state and dynamic response of hybrid MMCs. Finally, the MMC model and its control are
validated by means of detailed PSCAD simulations for both steady-state and transients conditions
(AC and DC faults)
Modeling of Direct Current Grid Equipment for the Simulation and Analysis of Electromagnetic Transients
RÉSUMÉ Les transmissions à base de courant continu sont capables de répondre mieux que les transmissions traditionnelles à base de courant alternatif aux enjeux de nos jours tels que l’intégration des énergies renouvelables, les difficultés avec l’installation des nouvelles lignes aériennes pour les raisons socio-environnementaux, la gestion des flux de puissance sur le réseau électrique. Ceci est grâce aux systèmes de contrôle performants et rapides, à un niveau de fiabilité accrue des composants utilisés, à l’efficacité énergétique des technologies de pointe, telles que les convertisseurs modulaires multiniveaux (Modular Multilevel Converter ou MMC en anglais). Ces avantages ont contribué à une croissance rapide du nombre de transmissions à courant continu à travers le monde dans les dernières années, avec les plans d’établir des réseaux multi-terminaux d’un niveau supérieur aux réseaux électriques traditionnels dans le but de les renforcer. Les outils de simulation numériques sont nécessaires pour faciliter et accélérer la mise en œuvre de ce type de projets d’envergure. Ils permettent d’analyser et d’étudier les systèmes électriques de plus en plus complexes et par conséquent d’éviter les problèmes opérationnels, d’augmenter la fiabilité et l’efficacité des réseaux électriques. La complexité accrue des réseaux électriques modernes qui contiennent les composants à base de l’électronique de puissance tels que les liaisons à courant continu exige une recherche sur les outils de simulation et les modèles avancés. Ainsi, cette thèse se focalise sur le développement d’un cadre pour les simulations précises et rapides des liaisons à courant continu. À la suite d’une revue de la littérature il est démontré que la modélisation des MMCs a un impact particulièrement important sur la précision et l’accélération des simulations et par conséquent une grande partie de cette thèse est dédiée aux différentes méthodes pour réduire le temps de simulation et améliorer la précision des résultats dans les études avec les MMCs.
Le cœur du sujet commence par la présentation de la modélisation des MMC hybrides et leurs systèmes de contrôle. Les modèles sont classés en quatre catégories selon le niveau de précision : le modèle détaillé permet de représenter les non-linéarités au niveau des composants semiconducteurs.----------ABSTRACT
Compared to the traditional alternating current technology-based electrical grids, High-Voltage Direct Current (HVDC) transmission systems can more effectively respond to the challenges of the modern power grid related to the integration of renewable energy sources, difficulty to install new overhead lines due to socio-environmental reasons, and power flow management. This is mainly due to high performance of control systems, fast response times, reliable components and energy efficiency of the state-of-the-art HVDC technologies of today, such as the Modular Multilevel Converter (MMC). These advantages have contributed to the rapid growth in the number of HVDC projects in recent years with plans of having overlay HVDC grids that can reinforce the existing electrical grids.
To facilitate and accelerate the implementation of large-scale HVDC projects, it is required to use numerical simulation tools. Such tools allow to perform advanced analysis of involved electrical systems for preventing operating problems, increasing robustness and efficiency in power grids. The increased level of complexity of modern power grids with power electronics-based components, such as HVDC, requires research on advanced simulation tools and models.
Therefore, this thesis aims to develop a framework allowing for accurate modeling and fast simulations of HVDC projects. After analysis of existing literature, the areas with high potential impact on accuracy and acceleration of electromagnetic transient simulations are found, and it is the modeling of MMCs that is considered in this thesis. Thus, a significant part of this thesis is dedicated to research on efficient modeling techniques that allow to reduce simulation time and improve accuracy for MMC-based HVDC systems.
The modeling aspects and control systems of hybrid MMCs are presented first. The MMC models used in electromagnetic transient simulations are grouped into four categories. The detailed model represents the nonlinear current-voltage characteristics of semiconductor switches. The detailed equivalent model represents the switches as two-value resistances: a small value for the closed state and a large value for the open state. The arm equivalent model assumes all capacitors in each arm have identical voltages, so a single equivalent capacitor is used to represent the whole arm, thus greatly reducing the computational burden of the model
Potential of Bipolar Full-Bridge MMC-HVdc Transmission for Link and Overlay Grid Applications
Bipolare HGÜ Systeme in Multi-Level-Ausführung stellen ein attraktives Lösungskonzept zur Bewältigung einer Vielzahl von Herausforderungen im Kontext heutiger Energiesysteme dar. Da dies jedoch auf Kosten einer deutlich erhöhten Systemkomplexität geschieht, ist ein tiefgreifendes Verständnis des transienten Verhaltens sowie der dynamischen Charakteristik von enormer Wichtigkeit.
Diese Doktorarbeit beinhaltet eine detaillierte Analyse von grundlegenden Zusammenhängen bezogen auf bipolare HGÜ Systeme in Multi-Level-Ausführung und stellt ein generisches Regelungs-, Bilanzierungs- und Schutzkonzept vor. Die generelle Leistungsfähigkeit des Konzepts wird durch elektro-magnetische Transientensimulationen nachgewiesen
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
Protection and fault location schemes suited to large-scale multi-vendor high voltage direct current grids
Recent developments in voltage source converter (VSC) technology have led to an increased interest in high voltage direct current (HVDC) transmission to support the integration of massive amounts of renewable energy sources (RES) and especially, offshore wind energy. VSC-based HVDC grids are considered to be the natural evolution of existing point-to-point links and are expected to be one of the key enabling technologies towards expediting the integration and better utilisation of offshore energy, dealing with the variable nature of RES, and driving efficient energy balance over wide areas and across countries. Despite the technological advancements and the valuable knowledge gained from the operation of the already built multi-terminal systems, there are several outstanding issues that need to be resolved in order to facilitate the deployment of large-scale meshed HVDC grids. HVDC protection is of utmost importance to ensure the necessary reliability and security of HVDC grids, yet very challenging due to the fast nature of development of DC faults and the abrupt changes they cause in currents and voltages that may damage the system components. This situation is further exacerbated in highly meshed networks, where the effects of a DC fault on a single component (e.g. DC cable) can quickly propagate across the entire HVDC grid. To mitigate the effect of DC faults in large-scale meshed HVDC grids, fast and fully selective approaches using dedicated DC circuit breaker and protection relays are required. As the speed of DC fault isolation is one order of magnitude faster than typical AC protection (i.e. less than 10 ms), there is a need for the development of innovative approaches to system protection, including the design and implementation of more advanced protection algorithms. Moreover, in a multi-vendor environment (in which different or the same type of equipment is supplied by various manufacturers), the impact of the grid elements on the DC fault signature may differ considerably from case to case, thus increasing the complexity of designing reliable protection algorithms for HVDC grids. Consequently, there is a need for a more fundamental approach to the design and development of protection algorithms that will enable their general applicability. Furthermore, following successful fault clearance, the next step is to pinpoint promptly the exact location of the fault along the transmission medium in an effort to expedite inspection and repair time, reduce power outage time and elevate the total availability of the HVDC grid. Successful fault location becomes increasingly challenging in HVDC grids due to the short time windows between fault inception and fault clearance that limit the available fault data records that may be utilised for the execution of fault location methods. This thesis works towards the development of protection and fault location solutions, designed specifically for application in large-scale multi-vendor HVDC grids. First, a methodology is developed for the design of travelling wave based non-unit protection algorithms that can be easily configured for any grid topology and parameters. Second, using this methodology, a non-unit protection algorithm based on wavelet transform is developed that ensures fast, discriminative and enhanced protection performance. Besides offline simulations, the efficacy of the wavelet transform based algorithm is also demonstrated by means of real-time simulation, thereby removing key technical barriers that have impeded the use of wavelet transform in practical protection applications. Third, in an effort to reinforce the technical and economic feasibility of future HVDC grids, a thorough fault management strategy is presented for systems that employ efficient modular multilevel converters with partial fault tolerant capability. Finally, a fault location scheme is developed for accurately estimating the fault location in HVDC grids that are characterised by short post-fault data windows due to the utilisation of fast acting protection systems.Recent developments in voltage source converter (VSC) technology have led to an increased interest in high voltage direct current (HVDC) transmission to support the integration of massive amounts of renewable energy sources (RES) and especially, offshore wind energy. VSC-based HVDC grids are considered to be the natural evolution of existing point-to-point links and are expected to be one of the key enabling technologies towards expediting the integration and better utilisation of offshore energy, dealing with the variable nature of RES, and driving efficient energy balance over wide areas and across countries. Despite the technological advancements and the valuable knowledge gained from the operation of the already built multi-terminal systems, there are several outstanding issues that need to be resolved in order to facilitate the deployment of large-scale meshed HVDC grids. HVDC protection is of utmost importance to ensure the necessary reliability and security of HVDC grids, yet very challenging due to the fast nature of development of DC faults and the abrupt changes they cause in currents and voltages that may damage the system components. This situation is further exacerbated in highly meshed networks, where the effects of a DC fault on a single component (e.g. DC cable) can quickly propagate across the entire HVDC grid. To mitigate the effect of DC faults in large-scale meshed HVDC grids, fast and fully selective approaches using dedicated DC circuit breaker and protection relays are required. As the speed of DC fault isolation is one order of magnitude faster than typical AC protection (i.e. less than 10 ms), there is a need for the development of innovative approaches to system protection, including the design and implementation of more advanced protection algorithms. Moreover, in a multi-vendor environment (in which different or the same type of equipment is supplied by various manufacturers), the impact of the grid elements on the DC fault signature may differ considerably from case to case, thus increasing the complexity of designing reliable protection algorithms for HVDC grids. Consequently, there is a need for a more fundamental approach to the design and development of protection algorithms that will enable their general applicability. Furthermore, following successful fault clearance, the next step is to pinpoint promptly the exact location of the fault along the transmission medium in an effort to expedite inspection and repair time, reduce power outage time and elevate the total availability of the HVDC grid. Successful fault location becomes increasingly challenging in HVDC grids due to the short time windows between fault inception and fault clearance that limit the available fault data records that may be utilised for the execution of fault location methods. This thesis works towards the development of protection and fault location solutions, designed specifically for application in large-scale multi-vendor HVDC grids. First, a methodology is developed for the design of travelling wave based non-unit protection algorithms that can be easily configured for any grid topology and parameters. Second, using this methodology, a non-unit protection algorithm based on wavelet transform is developed that ensures fast, discriminative and enhanced protection performance. Besides offline simulations, the efficacy of the wavelet transform based algorithm is also demonstrated by means of real-time simulation, thereby removing key technical barriers that have impeded the use of wavelet transform in practical protection applications. Third, in an effort to reinforce the technical and economic feasibility of future HVDC grids, a thorough fault management strategy is presented for systems that employ efficient modular multilevel converters with partial fault tolerant capability. Finally, a fault location scheme is developed for accurately estimating the fault location in HVDC grids that are characterised by short post-fault data windows due to the utilisation of fast acting protection systems
Méthodes scalables de commande par allocation pour le convertisseur modulaire multiniveaux : de la modélisation à l'implémentation temps réel
Dans le cadre de la montée en puissance des convertisseurs statiques, les différents avantages qu’il y a à utiliser les Convertisseurs Modulaires Multiniveaux (MMC) ont mené à leur popularisation. Cependant, à mesure que le nombre de niveaux de tension et le nombre de phase augmentent, ces convertisseurs présentent un nombre de plus en plus important de degrés de liberté pour en effectuer la commande. Ainsi les MMC représentent un défi pour la commande car le nombre de variables de commande est alors supérieur aux contraintes à satisfaire, faisant d’eux des systèmes redondants ou encore sous-déterminés ce qui ouvre la voie de l’optimisation. D’abord apparues dans les années 1980 dans l’aéronautique pour tirer profit de la multiplicité des surfaces aérodynamiques et des redondances associées que présente un avion afin d’en contrôler sa trajectoire (volets, ailerons, gouvernes…), les méthodes de commande par allocation ont fait leurs preuves en étant progressivement appliquées dans différents domaines technologiques. En parallèle ces algorithmes ont fait l’objet de travaux pour améliorer les performances obtenues et notamment s’adapter aux systèmes commandés.
Le sujet de la thèse concerne donc le développement et l’implémentation en temps réel de méthodes de commande par allocation, avec un souci d’optimisation en ligne, pour un système de conversion d’énergie à base de MMC.
La première partie de la thèse portent sur la modélisation du convertisseur MMC en vue de sa commande à partir de méthodes d’allocation. Ce qui implique le développement de différents modèles de commande avec différents niveaux de détails et de complexité. Un résultat fort issu de cette première partie est un modèle de commande dont la complexité n’est plus influencée par le nombre de phases du système électrique considéré.
La deuxième étape des travaux concerne le développement d’une nouvelle méthode d’allocation qui met à profit les avantages des méthodes présentes dans l’état de l’art pour en concevoir une nouvelle plus adaptée. Ainsi cette démarche a conduit à la programmation d’un nouvel algorithme d’allocation présentant des caractéristiques dynamiques et statiques réglables et adaptables simplement, son intégration aux méthodes déjà existantes est aisée et presque immédiat.
La troisième étape des travaux combine les travaux précédents. Tout d’abord en simulation, la méthode de commande par allocation du convertisseur est programmée puis testée pour finalement être validée. Pour la commande différentes architectures sont conçues permettant de réaliser des comparatifs afin d’évaluer leur capacité à atteindre les performances requises pour le bon fonctionnement du système. Il en découle une analyse des différents algorithmes de commande proposés. Le résultat principal de cette partie est la conception d’un nouvel algorithme d’allocation permettant de contrôler les tensions aux bornes des condensateurs ainsi que les tous les courants du convertisseur dans chacune des branches et ce indépendamment du nombre de phases.
La quatrième étape porte sur la validation expérimentale des méthodes développées. Pour se faire, le convertisseur MMC disponible au laboratoire LAPLACE est utilisé ainsi qu’un ensemble d’outils de prototypage rapide (OPAL-RT) permettant de tester et mettre au point les algorithmes de façon sûre et efficace.
La cinquième partie des travaux concerne l’extension, hors de la zone de fonctionnement nominale du convertisseur, des algorithmes de commande développés. En effet une ouverture est proposée mettant en exergue les capacités des méthodes d’allocation à reconfigurer le fonctionnement du MMC lorsqu’un défaut apparait dans l’un des sous-modules. Les résultats obtenus en simulation montrent une amélioration de la disponibilité du convertisseur, c’est-à -dire une continuité de fonctionnement en présence de défauts ce qui justifie l’intérêt de poursuivre les travaux dans cette direction
Providing Virtual Inertia Through Power Electronics
VSC-HVDC (voltage source converter based HVDC) system with its inherent merits for renewable energy integration has captured increasing research attentions. However, compared with AC systems dominated by synchronous generators (SGs), VSC-HVDC systems with general vector control cannot provide inertia for the grid due to lack of kinetic energy. This tends to degrade the safety and stability of the grid with the increasing penetration of renewable energy sources. To cope with this issue, virtual synchronous generator (VSG) has been proposed. In this thesis, firstly, a comprehensive introduction of various typologies of VSG schemes is made to illustrate their deficiencies and merits. The simulation results established in Simulink/Plecs show that VSG can not only participate into the regulation of frequency and voltage in case of power disturbances but guarantee the inertia provision for the grid.
Although the integration of VSG control enhances the inertia and damping response of inverts, researches show that plenty of issues relative with VSG should be ameliorated. The fluctuation performances of SGs are introduced into the output active power and current of inverters when incorporates VSG control. This threatens the stability and safety of VSG operation, for power electronic based inverters are more vulnerable during the oscillations of current and frequency. Hence, to solve these issues, various enhanced VSG strategies have been constructed to improve its robustness and output performance. In this thesis, the structures and properties of enhanced VSG schemes are fully discussed. The results show that the dynamic properties of VSG during transient periods are enhanced in comparison of that of normal VSG.
Modular multilevel converters (MMC) and alternate arm converters (AAC), as the representatives for enhanced topologies of VSC-HVDC system, have more complicated inner structures in comparison with 2/3 level converters. In this thesis, VSG control is applied into MMC/AAC models to strengthen their power and frequency regulation ability. In addition, a four-terminal multi terminal direct current (MTDC) system is incorporated with VSG control to provide primary frequency and voltage response for the grid. The results show that the integration of VSG improves the stability operation and inertia response of MMC/AAC/MTDC systems