Дисертації з теми "Modular multilevel"

Recent research established the effectiveness of applying a pulsed electric field to deactivate harmful microorganisms (such as bacteria and E. coli). Successful deactivation is achieved by lethal electroporation; a process that produces electric pores in the biological cell membrane of the harmful microorganisms when subjected to high-voltage (HV) pulses. The HV pulses are designed to create pores beyond a critical size at which the biological cell can reseal. In contrast when applying non-lethal electroporation, the cell-membrane survives after the electroporation process. This is required, for example, when inserting protein cells in the cell-membrane. In both lethal and non-lethal electroporation, HV pulses in the kilo-Volt range (1-100 kV) with durations ranging between nanoseconds and milliseconds are required. This thesis proposes nine pulse generator (PG) topologies based on power electronic devices and modular multilevel converter sub-modules. The proposed topologies are divided into two main groups namely: PGs fed from a HV DC supply and PGs fed from an LV DC supply. The first group presents a new family of HV DC fed topologies that improve the performance of existing HV DC fed PGs, such as flexible pulse-waveform generation and full utilisation of the DC link voltage. The second group is dedicated to a new family of LV DC fed PG topologies which have flexible pulse-waveform generation, controlled operation efficiency, and high voltage gain. All the proposed PG topologies share the important aspect in the newly developed HV PGs, that is modularity, which offers redundancy and robust pulse generation operation. The presented PG topologies are supported by theoretical analysis, simulations, and experimentation.

L’électronique de puissance a pénétré depuis quelques décennies les applications à forte puissance dans de nombreux domaines de l’industrie électrique. Au-delà de l’apparition des technologies d’interrupteur à forte puissance commutable en moyenne tension, ces applications imposaient également des avancées dans le domaine des topologies de convertisseurs statiques : les principaux défis à affronter concernaient l’atteinte de niveaux de tension compatibles avec le domaine de puissance des applications, l’augmentation de la fréquence de commutation apparente en sortie afin d’augmenter la bande passante de la commande, de réduire la taille des éléments de filtrage et de limiter les harmoniques de courant injectés dans le réseau d’alimentation. Les topologies de convertisseurs modulaires multiniveaux (MMC) sont issues de cette problématique de recherche : elles permettent grâce à l’association de cellules de commutation d’atteindre des niveaux de tension exploitables en grande puissance avec les technologies d’interrupteurs existantes, de limiter les fréquences et les pertes de commutation des interrupteurs élémentaires tout en maîtrisant la distorsion harmonique totale (THD). La modularité, la redondance, les degrés de liberté et les fonctionnalités des MMC leur permettent aussi d’augmenter la tolérance aux défauts. Ils pénètrent à présent une large gamme d'applications comme le transport à courant continu en haute tension (HVDC), les systèmes d'énergie renouvelable, les entraînements à vitesse variables de grande puissance, la traction ferroviaire et maritime ainsi que des applications spécifiques très contraignantes en matière de performance dynamique comme les systèmes d’alimentation des électro-aimants dans les accélérateurs de particules. Les topologies MMC sont composées de cellules de commutation élémentaires utilisant des interrupteurs électroniques tels que le Thyristor à Commande Intégrée (IGCT) standard ou les dernières génération d’IGBT. Les convertisseurs MMC ont fait l’objet de nombreux travaux de recherche et de développement en ce qui concerne les topologies, la modélisation et le calcul du fonctionnement en régime permanent et transitoire, le calcul des pertes, le contenu harmonique des grandeurs électriques et les systèmes de commande et de régulation. Par contre le dimensionnement de ces structures est rarement abordé dans les travaux publiés. Comme la grande majorité des topologies de convertisseurs statiques, les convertisseurs MMC sont composés non seulement d’interrupteurs mais aussi d’organes de stockage d’énergie de type composants diélectriques (condensateurs) et magnétiques (inductances, coupleurs) qui sont essentiels pour assurer la conversion des grandeurs électriques en entrée et en sortie. Ces composants ont une forte influence sur la taille, le volume et le rendement des convertisseurs et le dimensionnement optimal de ces derniers résulte souvent de compromis entre la taille des composants passifs, la fréquence et la puissance commutable par les interrupteurs élémentaires. Le travail de recherche présenté dans ce mémoire concerne le développement d’une méthodologie de dimensionnement optimal et global des MMC intégrant les composants actifs et passifs, respectant les contraintes des spécifications de l’application et maximisant certains objectifs de performance. Cette méthodologie est utilisée pour analyser divers compromis entre le rendement global du convertisseur et sa masse, voire son volume. Ces divers scénarios peuvent être également traduits en termes de coût si l’utilisateur dispose du prix des composants disponibles. Diverses solutions concurrentes mettant en œuvre un nombre de cellules spécifique adaptées à des interrupteurs de caractéristiques différentes en termes de calibre de tension, de courant et de pertes associés peuvent ainsi être comparées sur la base de spécifications d’entrée-sortie identiques. La méthodologie est appliquée au dimensionnement d’un convertisseur MMC utilisé comme étage d’entrée (« Active Front-end » : AFE) d’une alimentation d’électro-aimant pulsée de grande puissance. Dans une première partie, une méthode de calcul rapide, précise et générique du régime permanent du convertisseur MMC est développée. Elle présente la particularité de prendre en compte la fréquence de commutation contrairement aux approches conventionnelles utilisant la modélisation en valeurs moyennes. Cet outil se révèle très utile dans l’évaluation du contenu harmonique qui est contraint par les spécifications, il constitue le cœur de l’environnement de conception du convertisseur. Contrairement aux convertisseurs conventionnels, il existe des courants de circulation dans les convertisseurs MMC qui les rendent complexe à analyser. Les inductances de limitation incorporées dans les bras de la topologie sont généralement volumineux et pénalisants en termes de volume et de masse. Il est courant d’utiliser des inductances couplées afin de réduire l'ondulation , la THD et la masse. Dans le travail présenté, un circuit équivalent des inductances couplée tenant compte de l'effet de saturation est développé et intégré à l’environnement. L’utilisation d’inductances couplée augmente la complexité de l'analyse du fonctionnement et la précision de leur méthode de dimensionnement est critique pour l’optimisation globale du convertisseur. Un modèle analytique de dimensionnement de ces composants a été développé et intégré dans l’environnement ainsi qu’un modèle de complexité supérieure qui utilise le calcul des champs par éléments finis. La méthodologie de conception optimale et globale proposée utilise une procédure d’optimisation non linéaire avec contraintes qui pilote l’outil de calcul de régime permanent, le modèles de dimensionnements à plusieurs niveaux de complexité des composants passifs ainsi que d’autres modules permettant de quantifier les régimes de défaut. Pour pallier à la précision réduite des modèles analytiques, une approche d'optimisation hybride est également implantée dans l’environnement. Dans la boucle d'optimisation hybride, le modèle de dimensionnement des inductances peut être corrigé par le modèle de complexité supérieure qui utilise le calcul des champs. On obtient ainsi un meilleure compromis entre la précision de la solution optimale et le temps de convergence de la méthode itérative d’optimisation globale.
In the last decades, power electronics has penetrated high power applications in many areas of the electrical industry. After the emergence of high-voltage semiconductor switch technologies these applications also required advances in the field of static converter topologies: The main challenges were to achieve voltage levels compatible with the application power domain, to increase the apparent switching frequency at the output, to increase the control bandwidth, to reduce the size of the elements of filtering and of limiting the current harmonics injected into the supply network. The topologies of multi-level modular converters (MMC) are based on this research problem: they enable the use of switching cells to achieve high power levels that can be used with existing switch technologies, frequencies and switching losses of the elementary switches while controlling the total harmonic distortion (THD). Modularity, redundancy, degrees of freedom and MMC functionality also allow them to increase fault tolerance. They now penetrated a wide range of applications, such as high-voltage DC (HVDC), renewable energy systems, high-speed variable speed drives, rail and marine traction, and very specific applications in terms of dynamic performance such as electromagnet power systems in particle accelerators. MMC topologies are composed of elementary switching cells using electronic switches such as the standard Integrated Control Thyristor (IGCT) or the latest generation of IGBTs. MMC converters have been the subject of extensive research and development work on topologies, modeling, and calculation of steady-state and transient operation, loss calculation, the harmonic content of electrical quantities and systems control and regulation functions. On the other hand, the dimensioning methodology of these structures is rarely addressed in the published works. Like most static converter topologies, MMC converters are composed not only of switches but also passive components of energy storage devices (capacitors) and magnetic (inductors, couplers) that are essential to ensure the conversion of the input and output electrical quantities. These components have a strong influence on the size, the volume and the efficiency of the converters and the optimal dimensioning of the latter often result from a compromise between the size of the passive components, the frequency and the power switchable by the elementary switches. The research presented in this thesis concerns the development of an optimal and comprehensive design methodology for MMCs integrating active and passive components, respecting the constraints of the application specifications and maximizing certain performance objectives. This methodology is used to analyze the various trade-off between the overall efficiency of the converter and its mass, or even its volume. These various scenarios can also be translated into cost if the user has the price of the available components. Various competing solutions using a specific number of cells adapted to switches with different characteristics in terms of voltage, current, and associated losses can thus be compared on the basis of identical input-output specifications. The methodology is applied to the dimensioning of an MMC converter used as an active front-end (AFE) input of a high-power pulsed solenoid power supply. In the first part, a fast, precise and generic method for calculating the steady-state model of MMC converter is developed. It has the particularity of taking into account the switching frequency as opposed to conventional approaches using modeling in mean values. This tool is very useful in evaluating the harmonic content that is constrained by the specifications, it is the heart of the design environment of the converter. Unlike conventional converters, there are circulation currents in MMC converter structure that make it complex to analyze. The inductors which are used in the arms of the topology are generally bulky and expensive in terms of volume and mass. It is common to use coupled inductors to reduce ripple, THD, and mass. In the presented work, an equivalent circuit of coupled inductances considering the saturation effect is developed and integrated. The use of coupled inductors increases the complexity of the analysis and the precision of its sizing method is critical for the overall optimization of the converter. An analytical model for the dimensioning of these components has been developed and integrated as well as a higher complexity model which uses the finite element method calculation. The proposed optimal and global design methodology uses a nonlinear optimization procedure with constraints that drive the steady-state computing tool, multi-level design models of passive component complexity, and other modules to quantify the fault state. To compensate the low precision of the analytical models, a hybrid optimization approach is also implemented. In the hybrid optimization loop, the inductance-sizing model can be corrected by the higher complexity model that uses finite element computation. A better compromise is thus obtained between the precision of the optimal results and convergence time of the iterative global optimization method.

This thesis examines alternatives for power supply for a heavy truck application based on five different modular multilevel converter configurations that ultimately feed a 3-phase motor. Advantages and disadvantages of the different configurations are being discussed as well as other important factors that play a role in what configuration that is beneficial for the intended application. How half- or full-bridge submodules and battery cells relate to each other to achieve a desired voltage are being explained and calculated. Power losses of the converter submodules are being calculated as well as how a specific battery capacity, with increasing average power consumption, performs uphill according to set requirements. It turns out to be the double-armed modular multilevel converter configurations that has the best performance when it comes to utility, energy storage and the lowest power losses.

The integration of renewable energy sources in the electrical grid is reducing our dependence on fossil fuels. However, to ensure feasibility and reliability of distributed energy generation, more efficient and higher power converters are required. The modular multilevel converter (MMC) is a modern topology of multilevel converter that is very attractive for medium- and high-voltage/power applications, including high-voltage direct current transmission systems and high-power motor drives. The main features of the MMC are modularity, scalability to different power and voltage levels, redundancy and high quality output voltages and currents. However, the operation of the MMC is complex, and there are some issues that still have to be further investigated. One of these issues is the voltage ripples of the submodule (SM) capacitors. The voltage ripples define the minimum value of the capacitances needed for the converter, and therefore its overall size and cost. The use of a proper circulating current controller can reduce the voltage ripples. In this thesis, three techniques for calculating the circulating current reference are presented: two techniques based on optimization functions for minimizing the capacitor voltage ripples; and a fast-processing technique that provides results close to optimal. The capacitor voltage ripples can also be reduced by adding a zero-sequence signal to the modulation signals. In this thesis, the application of discontinuous modulation to the MMC is proposed for the first time. This technique is based on the injection of a discontinuous zero-sequence signal and highly reduces the switching power losses and capacitor voltage ripples. Real applications of the MMC are composed of a high number of SMs. This implies a challenge in the control system, including the data acquisition system. A new technique for measuring the capacitor voltages with only a few sensors has been presented in this thesis. From the output voltage provided by a group of SMs, the individual voltage of each one of them can be acquired. Since acquisition cannot be performed at each sampling time, the capacitor voltages are calculated between samples using an estimation algorithm. Reliability is a feature required in industrial applications. The structure of the MMC facilitates the existence of redundant SMs, but faults need to be detected and localized for deactivating the faulty component. This thesis presents a robust and fast strategy for detecting, localizing and correcting faults in SMs and voltage sensors. The technique is based on three additional sensors per arm, which measure the output voltage of a group of SMs and compare it with the expected voltage. Capacitance differences between the SMs can appear due to component tolerance or ageing of the capacitors. Capacitance mismatches cause uneven distribution of the power losses, thus increasing the thermal stress of some semiconductors, and therefore, their probability of failure. A power loss balancing technique has been proposed, equalising the losses in all the SMs and therefore avoiding the concentration of power losses in some SMs. Application of the MMC to motor drive applications has also been studied in this thesis. The operation of the MMC at low motor speeds/frequencies is still a challenge, since the capacitor voltage ripples are inversely proportional to the current frequency. In this thesis, it has been demonstrated that discontinuous modulation can help to reduce capacitor voltage ripples in motor drive applications, achieving very low speed operation. The technique is compared with other state-of-the-art methods, and it achieves similar capacitor voltage ripples and a significant reduction in power losses. All the control and modulation techniques proposed in this thesis have been studied by simulation in the MATLAB/Simulink environment and corroborated experimentally on low-power laboratory prototypes.
La integració de fonts d’energia renovables a la xarxa elèctrica està reduint la nostra dependència dels recursos fòssils. Però per tal d’assegurar la viabilitat i fiabilitat de la generació d’energia distribuïda, fan falta convertidors estàtics més eficients i de més potència. El convertidor multinivell modular (MMC) és una topologia de convertidor multinivell recent, molt prometedora per aplicacions de mitja i alta potència, com són els sistemes de transmissió d’energia en corrent continua o els accionaments de motors d’alta potència. Els principals avantatges del MMC són modularitat, escalabilitat en tensió i potència, redundància i gran qualitat de la tensió i corrent de sortida. El funcionament del MMC, però, és complex i encara hi ha alguns problemes que s’han d’investigar amb més profunditat. Un dels problemes és l’arrissat de tensió del condensador de sub-mòdul (SM). L’arrissat de tensió defineix el valor mínim d’aquests condensadors i per tant, el seu cost. L’ús d’un corrent circulant adequat pot reduir l’arrissat de tensió. En aquesta tesi es presenten tres tècniques per calcular la consigna del corrent circulant: dues tècniques basades en funcions d’optimització que minimitzen l’arrissat de tensió i una tècnica d’aplicació més simple, la qual proporciona resultats pròxims als òptims però que es pot calcular més ràpidament. L’arrissat de tensió també es pot reduir afegint un component homopolar en els senyals de modulació. En aquesta tesi es proposa per primera vegada l’ús de la modulació discontinua per al MMC. Aquesta tècnica, basada en la injecció d’un component homopolar, permet una gran reducció de l’arris s at de tens ió i de les pèrdues de commutació. Les aplicacions reals del convertidor MMC es componen per un gran nombre de SMs. Això implica un repte en el disseny del sistema de control, particularment en l’etapa d’adquisició de dades. Aquesta tesi presenta un nou sistema de mesura per a les tensions dels condensadors de SM, en el qual es necessiten pocs sensors. A partir de la tensió de sortida d’un grup de sensors, el sistema pot adquirir la tensió de cada un d’ells. Com que l’adquisició no es pot fer a cada període de mostreig, entre adquisicions la tensió es calcula mitjançant un algoritme d’estimació. Un dels requisits de les aplicacions industrials és la fiabilitat. L’estructura del MMC permet l’ús de SMs redundants, però les fallades s’han de detectar i localitzar per tal de desactivar el component erroni. En aquesta tesi es presenta un sistema ràpid i robust de detecció, localització i correcció de fallades en SMs i sensors de tensió. El sistema es basa en l’ús de tres sensors addicionals per semi-branca, els quals mesuren la tensió de sortida d’un grup de SMs i la comparen amb la tensió esperada. A causa de la tolerància o l’envelliment dels condensadors , poden aparèixer diferències en la capacitat dels SMs. Aquestes diferències causen una mala distribució de les pèrdues dels semiconductors, incrementant l’estrès tèrmic d’alguns dels components i la probabilitat de fallada. Per això, es proposa un algoritme d’equilibrat de pèrdues, el qual iguala les pèrdues dels SMs i n’evita la concentració en algun SM. En aquesta tesi també s’ha estudiat l’aplicació del MMC en accionaments de motors. El funcionament del MMC a baixa velocitat/freqüència del motor és un repte encara no resolt, ja que l’arrissat de tensió dels condensadors és inversament proporcional a la freqüència del corrent. Aquesta tesi demostra que la modulació discontinua es pot utilitzar per reduir l’arrissat de tensió en aquesta situació, aconseguint un bon funcionament a molt baixa velocitat. En comparació amb altres tècniques actuals de baixa velocitat, la modulació discontinua aconsegueix un arrissat de tensió similar i una reducció de les pèrdues. Totes les tècniques proposades en aquesta tesi s’han estudiat mitjançant simulació en l’entorn MATLAB/Simulink i s’han corroborat experimentalment en prototips de laboratori.

This thesis discusses the operation of the grid-tied modular multilevel converters (MMC) applied on the dc power transmission, particularly on the medium and high-voltage applications. First, it is presented the evolution of the power converters used on the high-voltage dc transmission field (HVdc) with special focus on the modular multilevel-based power converters. Then, due to the intrinsic nature of the converter, besides the control requirements for its dc and ac buses interactions, its energy storage should be carefully managed in order to achieve a safe and knowledgeable operation of this power converter. Hence, its control requirements are presented and mathematically supported. Moreover, the progressive design and validation of its control loops is addressed in this thesis by means of the converter simulation over a broad range of operating conditions. One key-point factor of the MMC performance is the strategy followed to modulate the voltages generated on its arms. In this vision, different modulation techniques were combined with peculiar zero sequence signals in order to analyze their impact on the voltages across the converter arms and its intrinsic performance. This study was also complemented by different procedures followed to balance the energy storage of its capacitors. A transversal research question of this voltage source converter topology is its efficiency. Then, besides the analysis of the ac power flow impact on the power losses produced by its semiconductors, it is deduced and proposed a mathematical expression that that can describe the power losses produced semiconductors, over a broad range of operating conditions of the MMC. Finally, it is explored the possible degrees of freedom of an half-bridge-based MMC whenever it is operating in the static synchronous compensation (STATCOM) mode. Depending on the converter operation aspect that is required to be optimized, the voltage across its dc poles can be adjusted to achieve an improved performance of the MMC
La presente tesis trata sobre el funcionamiento de los convertidores modulares de multinivel (MMC) utilizados en la transmisión de energía eléctrica en corriente continua, en particular para aplicaciones de media y alta tensión. En primer lugar, se presenta la evolución de los convertidores utilizados en el campo de la transmisión de energía eléctrica mediante enlaces en corriente continua de alta tensión(HVdc), haciendo especial énfasis en los convertidores de topología multinivel. Debido a la naturaleza intrínseca del convertidor MMC, se debe regular el intercambio de potencia entre las redes de corriente alterna y continua a las que se conecta, junto con la energía interna almacenada, para asegurar un buen funcionamiento del mismo. Por ello, se presenta una descripción del control del convertidor soportada por un riguroso análisis matemático. El diseño de los diferentes lazos de control se valida mediante simulaciones representando diferentes condiciones de funcionamiento posibles. Un factor clave del rendimiento del MMC es la estrategia de modulación utilizada para aplicar voltajes en cada una de sus ramas. Para evaluar sus diferencias a nivel de pérdidas, se presenta una comparativa entre diferentes técnicas de modulación incorporando secuencia homopolar. Este estudio se complementa con el estudio de diferentes procedimientos seguidos para equilibrar el almacenamiento de energía en los condensadores de una rama. Una cuestión de investigación transversal de esta topología de convertidor de tensión es su eficiencia. Posteriormente, se obtiene una expresión matemática que permite describir las pérdidas de los semiconductores del convertidor en funcionamiento, para diferentes niveles de transferencia de potencia. Finalmente, se analizan los posibles grados de libertad de un MMC operando en modo de compensación de potencia reactiva (STATCOM). En base a la operación de dicho convertidor y de la variable que se requiera optimizar, resulta posible variar la tensión entre sus polos DC para lograr un mejor funcionamiento del convertidor

Les travaux présentés dans ce mémoire ont été réalisés dans le cadre d'une collaboration entre le LAboratoire PLAsma et Conversion d'Énergie (LAPLACE), Université de Toulouse, et la Seconde Université de Naples (SUN). Ce travail a reçu le soutien de la société Rongxin Power Electronics (Chine) et traite de l'utilisation des convertisseurs multi-niveaux pour le transport d'énergie électrique en courant continu Haute Tension (HVDC). Depuis plus d'un siècle, la génération, la transmission, la distribution et l'utilisation de l'énergie électrique sont principalement basées sur des systèmes alternatifs. Les systèmes HVDC ont été envisagés pour des raisons techniques et économiques dès les années 60. Aujourd'hui il est unanimement reconnu que ces systèmes de transport d'électricité sont plus appropriés pour les lignes aériennes au-delà de 800 km de long. Cette distance limite de rentabilité diminue à 50 km pour les liaisons enterrées ou sous-marines. Les liaisons HVDC constituent un élément clé du développement de l'énergie électrique verte pour le XXIème siècle. En raison des limitations en courant des semi-conducteurs et des câbles électriques, les applications à forte puissance nécessitent l'utilisation de convertisseurs haute tension (jusqu'à 500 kV). Grâce au développement de composants semi-conducteurs haute tension et aux architectures multicellulaires, il est désormais possible de réaliser des convertisseurs AC/DC d'une puissance allant jusqu'au GW. Les convertisseurs multi-niveaux permettent de travailler en haute tension tout en délivrant une tension quasi-sinusoïdale. Les topologies multi-niveaux classiques de type NPC ou " Flying Capacitor " ont été introduites dans les années 1990 et sont aujourd'hui couramment utilisées dans les applications de moyenne puissance comme les systèmes de traction. Dans le domaine des convertisseurs AC/DC haute tension, la topologie MMC (Modular Multilevel Converter), proposée par le professeur R. Marquardt (Université de Munich, Allemagne) il y a dix ans, semble particulièrement intéressante pour les liaisons HVDC. Sur le principe d'une architecture de type MMC, le travail de cette thèse propose différentes topologies de blocs élémentaires permettant de rendre le convertisseur AC/DC haute tension plus flexible du point de vue des réversibilités en courant et en tension. Ce document est organisé de la manière suivante. Les systèmes HVDC actuellement utilisés sont tout d'abord présentés. Les configurations conventionnelles des convertisseurs de type onduleur de tension (VSCs) ou de type onduleur de courant (CSCs) sont introduites et les topologies pour les systèmes VSC sont ensuite plus particulièrement analysées. Le principe de fonctionnement de la topologie MMC est ensuite présenté et le dimensionnement des éléments réactifs est développé en considérant une commande en boucle ouverte puis une commande en boucle fermée. Plusieurs topologies de cellules élémentaires sont proposées afin d'offrir différentes possibilités de réversibilité du courant ou de la tension du côté continu. Afin de comparer ces structures, une approche analytique de l'estimation des pertes est développée. Elle permet de réaliser un calcul rapide et direct du rendement du système. Une étude de cas est réalisée en considérant la connexion HVDC d'une plateforme éolienne off-shore. La puissance nominale du système étudié est de 100 MW avec une tension de bus continu égale à 160 kV. Les différentes topologies MMC sont évaluées en utilisant des IGBT ou des IGCT en boitier pressé. Les simulations réalisées valident l'approche analytique faite précédemment et permettent également d'analyser les modes de défaillance. L'étude est menée dans le cas d'une commande MLI classique avec entrelacement des porteuses. Enfin, un prototype triphasé de 10kW est mis en place afin de valider les résultats obtenus par simulation. Le système expérimental comporte 18 cellules de commutations et utilise une plate-forme DSP-FPGA pour l'implantation des algorithmes de commande.

Denna avhandling behandlar analys och styrning av den modulära multinivå omvandlaren (M2C). M2C är en lovande omvandlarteknologi för högspända högeffekttillämpningar. Anledningen till detta är låg distorsion i utstorheterna kan uppnås med låg  medelswitchfrekvens per switch och utan utgångsfilter. Med M2C har utspänningen så lågt övertonsinnehåll att drift av högeffektmotorer är möjlig utan reduktion av märkeffekten. Emellertid innebär det stora antalet styrda switchar att styrningen blir mer komplex än för motsvarande tvånivåomvandlare. Styrningen av M2C måste måste konstrueras så att submodulernas kondensatorspänningar balanseras och är stabila oberoende av driftfall. En aktiv mekanism för val av submoduler, som är integrerad i modulatorn, har visat sig vara effektiv för att ombesörja den interna balanseringen av omvandlararmarna. Utöver balanseringen av de individuella kondensatorerna krävs en strategi för styrning av den totalt upplagrade energin i omvandlaren. Med utgångspunkt i en analytisk beskrivning av omvandlaren föreslås styrlagar för både öppen styrning och sluten reglering, vilka genom både simuleringar och med hjälp av experiment har visat sig vara stabila i hela arbetsområdet. Den potentiella växelverkan mellan den inre omvandlarstyrningen och en yttre strömreglering undersöks också. Både simuleringar och experiment bekräftar att eventuell interaktion inte innebär några avsevärda problem vare sig för omvandlaren eller motorn. En  hårdvaruimplementering av en nedskalad trefasig 10kVA-omvandlare har genomförts för att verifiera modellering och styrning. Implementeringen av styrningen beskrivs i detalj. Styrningen är anmärkningsvärt snabb och kan utökas till godtyckligt antal nivåer. Den kan därför användas för en fullskaleimplementering i MW-klassen.
This thesis deals with the analysis and control of the modular multilevel converter (M2C). The M2C is a promising converter technology for various high-voltage high-power applications. The reason to this is that low-distortion output quantities can be achieved with low average switching frequencies per switch and without output filters. With the M2C the output voltage has such a low harmonic content that high-power motors can be operated without any derating. However, the apparent large number of devices, requires more complex converter control techniques than a two-level counterpart. The internal control of an M2C must be designed so that the submodule capacitor voltages are equalized and stable independent of the loading conditions. An active submodule selection mechanism, included in the modulator, has been shown able to provide voltage sharing inside the converter arm. Apart from the individual capacitor voltage sharing, a strategy has to be designed to ensure that the total amount of energy stored inside the converter will always be controlled. Based on an analytical description of the converter, both feedback and open-loop control methods are suggested, simulated and experimentally evaluated, which will ensure stable operation in the whole operation range. The potential interaction of the internal controllers with an external motor current controller is also investigated. Both simulation and experimental results show that any interaction will not result in any problems neither for the converter nor for the motor control itself. A hardware implementation of a down-scaled 10 kVA three-phase laboratory prototype converter is performed, in order to evaluate the modeling and the controllers developed. The controller implementation is described in detail, as it exhibits remarkably fast response, and can be expanded up to an arbitrary number of levels. Therefore it can be used even by a full-scale converter implementation in the MW range.
QC 20110628

This paper studies the possibility of building a modular multilevel converter (M2C) using silicon carbide (SiC) switches. The main focus is on a theoretical investigation of the conduction losses of such a converter and a comparison to a corresponding converter with silicon-insulated gate bipolar transistors. Both SiC BJTs and JFETs are considered and compared in order to choose the most suitable technology. One of the submodules of a down-scaled 3 kVA prototype M2C is replaced with a submodule with SiC JFETs without antiparallel diodes. It is shown that the diode-less operation is possible with the JFETs conducting in the negative direction, leaving the possibility to use the body diode during the switching transients. Experimental waveforms for the SiC submodule verify the feasibility during normal steady-state operation. The loss estimation shows that a 300 MW M2C for high-voltage direct current transmission would potentially have an efficiency of approximately 99.8% if equipped with future 3.3 kV 1.2 kA SiC JFETs.
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Modular multilevel converters (MMCs) are envisaged as the key power electronic converter topology to enable a multi-terminal pan-European high voltage direct current (HVDC) Supergrid for the interconnection of o�shore wind farms and exchange of energy between di�erent countries. A key feature of MMCs in the large number of semiconductor devices employed in each converter station, distributed over a stack of series-connected sub-modules (SMs). These semiconductors possess strict thermal limits, which can constrain the operating range on the converter by limiting its capability of providing enhanced functionalities to the AC grid such as short-term power overloads. Furthermore, due to di�erent loading conditions and ageing, signi�cant temperature di�erences can exist between SMs which can lead to a very di�erent lifetime expectancies for the semiconductor modules. This thesis proposes active thermal control methodologies to act of two distinct converter levels. Firstly, two novel dynamic rating strategies are proposed to de�ne the converter current injection limit as a response to the maximum semiconductor temperature feedback. This enables the exploitation of the semiconductors thermal headroom to provide short-term power overloads, which can be used for the improvement of the frequency support of a power-distressed AC grid. Secondly, a SM-level temperature regulation and balancing algorithm is proposed, aiming at the equalisation of the maximum semiconductor die temperature in all the SMs of an MMC arm. The proposed methods are validated in a detailed and combined electro-thermal simulation model with 3 and 10 SMs per arm developed in MATLAB®/Simulink® using PLECS® Blockset. An experimental platform has been designed and utilised to verify the e�ectiveness of the dynamic rating strategies and the SM temperature regulation and balancing strategy.

Modular multilevel converter (MMC) has a prominent potential to take over the high-power converter market thanks to its exceptional characteristics, including modularity, flexibility to adapt to any voltage level, significant reduction in average switching frequency without compromising remarkable harmonic performance and many other. However, due to structural constraints of the existing submodule arrangements in classical MMCs the power scalability at submodule levels is limited. To address this issue, this PhD thesis reports a novel Modular Multilevel Converter with Interleaved half-bridge Sub-Modules (ISM-MMC). The ISM-MMC exhibits a higher modularity and scalability in terms of current ratings with respect to conventional MMCs, while preserves the typical voltage level adaptiveness. The ISM-MMC brings the known advantages of classical MMC to low/medium-voltage, high-current applications, where classical MMCs are rarely used. A detailed description of operating principle along with the converter’s average model, outer and internal control methods, a hybrid modulation scheme that helps to exploit advantages of the interleaving scheme and converter efficiency analysis are given in this thesis. This dissertation also concerns a current balancing problem that is typical in interleaved converters, while it is very new issue in MMC-based structures. The problem has been rigorously studied and a new control strategy, which relies on interleaved currents estimation, has been proposed in this work. This technique minimizes the number of required current sensors in ISM-MMC, thereby reducing the converter’s cost, weight, and volume. To make operation of such current regulators possible, a new capacitor voltage balancing strategy suitable for both ISM-MMCs and conventional MMCs is developed. Extensive numerical simulations, hardware-in-the-loop, and experimental tests on a scaled-down, single-phase ISM-MMC laboratory prototype are carried out to demonstrate the feasibility of the proposed topology, implemented modulation and control schemes.

This thesis provides insight into state-of-the-art Modular Multilevel Converters (MMC) for medium and high voltage applications. Modular Multilevel Converters have increased in interest in many industrial applications, as they offer the following advantages: modularity, scalability, reliability, distributed location of capacitors, etc. In this study, the modeling, control and design considerations of modular based multilevel converters, with an emphasis on the reliability of the converter, is carried out. Both modular multilevel converters with half-bridge and full-bridge sub-modules are evaluated in order to provide a complete analysis of the converter. From among the family of modular based hybrid multilevel converters, the newly released Alternate Arm Converter (AAC) is considered for further assessment in this study. Thus, the modular multilevel converter with half-bridge and full-bridge power cells and the Alternate Arm Converter as a commercialized hybrid structure of this family are the main areas of study in this thesis. Finally, the DC fault analysis as one of the main issues related to conventional VSC converters is assessed for Modular Multilevel Converters (MMC) and the DC fault ride-through capability and DC fault current blocking ability is illustrated in both the Modular Multilevel Converter with Full-Bridge (FB) power cells and in the Alternate iii Arm Converter (AAC). Accordingly, the DC fault control scheme employed in the converter and the operation of the converter under the fault control scheme are explained. The main contributions of this study are as follows: The new D-Q model for the MMC is proposed for use in the design of the inner and outer loop control. The extended control scheme from the modular multilevel converter is employed to control the Alternate Arm Converters. A practical reliability-oriented sub-module capacitor bank design is described based on different reliability modeling tools. A Zero Current Switching (ZCS) scheme of the Alternate Arm Converter is presented in order to reduce the switching losses of the Director Switches (DS) and, accordingly, to implement the ZCS, a design procedure for the Arm inductor in the AAC is proposed. The capacitor voltage waveform is extracted analytically in different load power factors and the waveforms are verified by simulation results. A reliability-oriented switching frequency analysis for the modular multilevel converters is carried out to evaluate the effect of the switching frequency on the MMC's operation. For the latter, a DC fault analysis for the MMC with Full-Bridge (FB) power cells and the AAC is performed and a DC fault control scheme is employed to provide the capacitor voltage control and DC fault current limit, and is illustrated herein.
Master of Science

Le convertisseur multiniveau modulaire (MMC) est une solution appropriée pour les réseaux HVDC grâce à sa modularité, sa faible fréquence de commutation et sa tension alternative quasi-sinusoïdale. En raison de sa topologie, son modèle mathématique est assez complexe et est donc souvent simplifié au stade de la conception. En particulier, la résistance équivalente au bras R, l'inductance du bras L et le courant circulant sont souvent négligés. Toutefois, les résultats expérimentaux obtenus avec notre prototype monophasé de MMC à pont complet à six niveaux ont montré que ces hypothèses ne sont pas toujours acceptables. Dans ce contexte, l'objectif de cette thèse est d'étudier l'impact de R, L et du courant de circulation sur la tension du condensateur du module et sur la zone de fonctionnement du MMC. Premièrement, nous avons étendu le modèle basé sur les intégrales communément utilisé et nous avons clarifié les hypothèses sur lesquelles il repose. Entre autres, des expressions pour les courants de circulation et courant DC ont été développées et comparées à celles que l’on trouve dans la littérature. Cela nous a permis d'analyser l'ondulation de la tension du condensateur du module en fonction de R et L, sans courant de circulation. Deuxièmement, pour surmonter les limites du modèle basé sur l'intégrale, nous avons proposé d'utiliser un modèle MMC invariant dans le temps en régime permanent dans le système dq0. Quelques hypothèses seulement sont nécessaires pour obtenir ce modèle, mais une évaluation numérique est requise. Cela nous a permis d'analyser la tension moyenne du condensateur du module et l'ondulation de tension du condensateur du module en fonction de R et L, avec et sans courant de circulation. Troisièmement, en utilisant le modèle invariant dans le temps en régime permanent, nous avons développé un diagramme PQ détaillé du MMC. Outre la limite de courant AC, la limite de courant DC et la limite d'indice de modulation classiques, nous avons ajouté plusieurs limites internes: courant de l'IGBT, courant efficace des bras et ondulation du courant et de la tension du condensateur du module. Les résultats ont été confirmés par simulation numérique à l'aide d'un modèle détaillé Matlab Simulink SimPowerSystems. Les résultats présentés dans cette thèse pourraient être utilisés pour optimiser le dimensionnement des composants de la MMC en fonction de sa zone d’exploitation et pour évaluer l’impact de différents paramètres sur les performances du MMC
The modular multilevel converter is a suitable solution for HVDC grids thanks to its modularity, low switching frequency and quasi-sinusoidal AC voltage. However, due to its topology, its mathematical model is quite complex and is therefore often simplified at the design stage. In particular, the arm equivalent resistance R, the arm inductance L and the circulating current are often neglected. But experimental results obtained with our 1-ph 6-level full-bridge MMC prototype showed that these hypotheses are not always acceptable. In this context, the goal of this thesis is to study the impact of accounting for R, L and the circulating current on the module capacitor voltage and on the operating area of the converter. First, we extended the commonly used integral based model and we clarified the hypotheses behind it. Among others, expressions for the circulating and dc currents have been developed and compared with the one that can be found in the literature. It allowed us to analyze the module capacitor voltage ripple as a function of R and L, without circulating current only. Second, to overcome the limitations of the integral based model, we proposed to use a steady state time invariant (DeltaSiga) MMC model in dq0 frame. Only few hypotheses are required to obtain this model, but a numerical evaluation is required. It allowed us to analyze the module capacitor average voltage and the module capacitor voltage ripple as a function of R and L, with and without circulating current. Third, using the steady state time invariant model, we developed a detailed PQ diagram of the MMC. In addition to the conventional AC current limit, DC current limit and modulation index limit, we added several internal limits: IGBT current, arm rms current and module capacitor voltage and current ripple. The results have been confirmed by numerical simulation using a detailed Matlab Simulink SimPowerSystems model. The results presented in this thesis could be used to optimize the sizing of the components of the MMC considering its operating area, and to assess the impact of different parameters on the MMC performance

This thesis proposes a new modular multilevel converter with embedded cell balancing for battery electric vehicles. In this topology, the battery cells are directly connected to the half-bridge choppers of the sub-modules, allowing the highest flexibility for the discharge and recharge of each individual cell. Tht: traditional battery management system is replaced by the control of the converter, which individually balances all the cells. A new balancing algorithm is presented and discussed in. the thesis, showing that the converter generates symmetric three-phase voltages with low harmonic distortion even for significantly unbalanced cells. The thesis also analyses stationary recharge of the battery cells from both three-phase and single-phase ac sources. The performance of the converter as a traction drive is assessed in terms of torque-speed characteristic and power losses for the full frequency range, including field weakening. A simplified model for estimating conduction and switching losses for the proposed modular multilevel converter is presented and the results for a typical driving cycle are compared with a traditional two-level converter. Simulation and experimental results on a kW-size prototype have confirmed the feasibility of the proposed traction modular converter in terms of effectiveness of the cell balancing control, validity of the proposed loss model, suitability of use for traction and effectiveness of recharging operations.

This thesis aims to bring clarity to the dimensioning aspects and limiting factors of the modular multilevel converter (MMC). Special consideration is given to the dc capacitors in the submodules as they are a driving factor for the size and weight of the converter. It is found that if the capacitor voltages are allowed to increase by 10% the stored energy must be 21 kJ/MW in order to compensate the capacitor voltage ripple. The maximum possible output power can, however, be increased by injecting a second-order harmonic in the circulating current. A great advantage of cascaded converters is the possibility to achieve excellent harmonic performance at low switching frequencies. Therefore, this thesis also considers the relation between switching harmonics, capacitor voltage ripple, and arm quantities. It is shown that despite subharmonics in the capacitor voltages, it is still possible to achieve periodic arm quantities. The balancing of the capacitor voltages is also considered in further detail. It is found that it is possible to balance the capacitor voltages even at fundamental switching frequency although this will lead to a comparably large capacitor voltage ripple. Therefore, in order to limit the peak-to-peak voltage ripple, it is shown that a predictive algorithm can be used in which the resulting switching frequency is approximately 2–3 times the fundamental frequency. This thesis also presents two new submodule concepts. The first submodule simply improves the trade-off between the switching frequency and capacitor voltage balancing. The second submodule includes the possibility to insert negative voltages which allows higher modulation indices compared to half-bridge submodules. A brief comparison of cascaded converters for ac-ac applications is also presented. It is concluded that the MMC appears to be well suited for ac-ac applications where input and output frequencies are close or equal, such as in interconnection of ac grids. In low-frequency applications such as low-speed drives, however, the difficulties with handling the energy variations in the converter arms are much more severe in the MMC compared to the other considered topologies.

QC 20141010

Recent research into industrial applications of electric power conversion shows an increase in the use of renewable energy sources and an increase in the need for electric power by the loads. The Medium-Voltage DC (MVDC) concept can be an optimal solution. On the other hand, the Modular Multilevel Converter (MMC) is an attractive converter topology choice, as it has advantages such as excellent harmonic performance, distributed energy storage, and near ideal current and voltage scalability. The fault response, on the other hand, is a big challenge for the MVDC distribution systems and the traditional MMCs with the Half-Bridge submodule configuration, especially when a DC short circuit fault happens. In this study, the fault current behavior is analyzed. An alternative submodule topology and a fault operation control are explored to achieve the fault current limiting capability of the converter. A three-phase SiC-based MMC prototype with the Full-Bridge configuration is designed and built. The SiC devices can be readily adopted to take advantage of the wide-bandgap devices in MVDC applications. The Full-Bridge configuration provides additional control and energy storage capabilities. The full in-depth design, controls, and testing of the MMC prototype are presented, including among others: component selection, control algorithms, control hardware implementation, pre-charge and discharge circuits, and protection scheme. Systematical tests are conducted to verify the function of the converter. The fault current behavior and the performance of the proposed control are verified by both simulation and experiment. Fast fault current clearing and fault ride-through capability are achieved.
Master of Science

The modular multilevel converter (MMC) is being developed as a core technology for the next generation of high-power, voltage source converters (VSCs). The focus of this thesis lies in the SM dimensioning, testing method and topology innovation for the MMC. First, the thesis presents a new submodule (SM) capacitor selection method, considering the three main voltage requirements: the maximum capacitor voltage, the voltage ripple and the SM voltage capability. The effect of the arm inductor is included. A quick way to estimate the capacitor ripple current stress is also provided to check the selection. Second, the thesis proposes two model assisted SM testing schemes for the MMC. The prototype SM can be thoroughly tested according to the targeted operating modes without having to build a complete MMC. During the test, the converter arm current can be faithfully achieved, which contains not only the fundamental frequency component, but also dc offset and harmonic circulating current components. One scheme is the uncompensated testing scheme, which uses fewer devices, and has simpler control and faster transient dynamics. The other is the compensated testing scheme, which requires much lower dc supply voltage, smaller coupling inductance, and provides higher current tracking accuracy in steady state. Both testing schemes have been verified through simulation and experiments. Third, the thesis proposes a compact SM topology for the MMC based on stacked switched capacitor (SSC) architecture. Feasibility study shows that the total physical volume of all capacitors in each SM can be reduced by more than 40% without significantly increasing the power loss. Design concept and control principles are presented. Practical considerations for a high-voltage, high-power system are also provided, which are demonstrated through experiments on a scaled down laboratory prototype SM. Finally, this thesis evaluates the offshore 50/3 Hz ac power transmission and the use of back-to-back (B2B) MMC for frequency conversion. The high-level design of a B2B MMC is presented. System performance is briefly evaluated using computer simulation.

High Voltage Direct Current (HVDC) is a power electronics -based technology that enables the transmission of large amounts of power over long distances, the integration of remote offshore wind power to the main land, and the interconnection of asynchronous AC systems. The Modular Multilevel Converter (MMC) is the state-of-the-art technology for Voltage Source Converter (VSC) based HVDC applications. As compared to the two-level converter, the MMC presents a more complex control scheme. However, it brings additional flexibility into the system. The present work focuses on the (energy-based) control of the MMC for HVDC applications, aiming to understand the additional degrees of freedom related to the internal energy of the MMC. First, DC voltage regulation in HVDC point-to-point links. Moreover, an experimental validation using a scaled MMC-based point-to-point link is carried out, particularly focusing on a novel experimental design of an HVDC cable emulator. With such a laboratory setup, the simulated system dynamics are contrasted with experiments . Furthermore, a generic controller for the same application is presented, and different optimal tuning techniques are addressed. Thus, the most suitable control gains are obtained automatically based on system constraints. In HVDC applications such as remote offshore wind farm clusters or isolated systems with low or non-existing synchronous generation, the MMC needs to operate as grid-forming. The present work explores the role of the internal energy of the MMC through different control structures . Finally, a multi-terminal HVDC grid where some terminals share the regulation of the DC voltage and others operate in grid-forming mode is considered, addressing the distributed DC voltage droop control design.
L'alta tensió en corrent continu (HVDC) és una tecnologia basada en electrònica de potència que permet la transmissió de gran potència en distàncies llargues, la integració de parcs eòlics marins remots a la xarxa terrestre, i la interconnexió de sistemes asíncrons de corrent altern. El convertidor modular multinivell (MMC) és la tecnologia més recent per aplicacions HVDC basada en convertidors en font de tensió (VSC). Comparat amb el convertidor de dos nivells, l'MMC presenta un esquema de control més complex, però aporta una major flexibilitat al sistema. Aquest treball es centra en el control dels MMC per aplicacions HVDC, amb l'objectiu d'entendre els graus de llibertat addicionals relacionats amb la seva energia interna. En primer lloc, s 'estudia el control de l'MMC per aplicacions de control de tensió contínua en enllaços punt a punt. Després, es realitza una validació experimental mitjançant un enllaç punt a punt a escala, posant l'èmfasi en el disseny d'un emulador de cable HVDC. D'aquesta manera, els resultats de les simulacions es poden contrastar amb els experiments de laboratori. A continuació, es presenta una estratègia de control genèrica i el càlcul òptim dels seus paràmetres amb diferents mètodes. Així doncs, els guanys més adequats pels controladors s'obtenen automàticament, basats en un conjunt de restriccions sobre el sistema. En aplicacions HVDC com els grans parcs eòlics marins o els sistemes aïllats amb poca o cap generació síncrona, un o diversos convertidors han de generar la xarxa d'alterna. En aquest treball s'investiga el rol de l'energia interna de l'MMC, implementant diferents estructures de control. Finalment, es considera una xarxa HVDC multiterminal, en la qual un conjunt de convertidors controla la tensió contínua i d'altres formen la xarxa d'alterna. En aquest escenari, es planteja el disseny del control distribuït de tensió contínua

Reliability and efficiency of power transmission has been at the forefront of research for some time and is currently being given critical consideration due to the increased dependence on electrical energy. With the increased demand for electricity, engineers are considering different methods of supply arrangement to improve the security of electricity supply. High Voltage Direct Current (HVDC) transmission is a technology that avails itself for distance power transmission, interconnection of asynchronous networks and cross sea or offshore power transmission. The main element of an HVDC system is the AC/DC or DC/AC power converter. Recently, a new breed of power converters suitable for HVDC transmission has been the subject of considerable research work. These converters are modular in structure with high efficiency and their operation results in higher power quality, with reduced filtering components when compared to the use of Line Commutated and two-level or three-level Voltage Source Converter (VSC) based transmission systems. One such modular circuit is the Parallel Hybrid Modular Multilevel Voltage Source Converter (PH-M2L-VSC). This research investigates the operation and control of the PH-M2L-VSC for HVDC applications. Control schemes supporting the operation of the converter as would be expected of an HVDC VSC are proposed, including operation with an unbalanced AC network. Simulation results from a medium voltage demonstrator and experimental results from a laboratory scale prototype are presented to validate the methods proposed and enable a performance comparison to be made with other topologies.

For the future of sustainable energy, renewable energy will need to significantly penetrate existing utility grids. While various renewable energy sources are networked with high-voltage DC grids, integration between these high-voltage DC grids and the existing AC grids is a significant technical challenge. Among the limited choices available, the modular multi-level converter (MMC) is the most prominent interface converter used between the DC and AC grids. This subject has been widely pursued in recent years. One of the important design challenges when using an MMC is to reduce the capacitor size associated with each module. Currently, a rather large capacitor bank is required to store a certain amount of line-frequency related circulating energy. Several control strategies have been introduced to reduce the capacitor voltage ripples by injecting certain harmonic current. Most of these strategies were developed using trial and error and there is a lack of a systematic means to address this issue. Most recently, Yadong Lyu has proposed to control the modulation index in order to reduce capacitor ripples. The total elimination of the unwanted circulating power associated with both the fundamental line frequency and the second-order harmonic was demonstrated, and this resulted in a dramatic reduction in capacitor size. To gain a better understanding of the intricate operation of the MMC, this thesis proposes a state-space analysis technique in the present paper. Combining the power flow analysis with the state trajectory portrayed on a set of two-dimensional state plans, it clearly delineates the desired power transfer from the unwanted circulating energy, thus leading to an ultimate reduction in the circulation energy and therefore the required capacitor volume.
Master of Science

The modular multi-level converter (MMC) is the most prominent interface converter used between the HVDC grid and the HVAC grid. One of the important design challenges in MMC is to reduce the capacitor size. In the current practice, a rather large capacitor bank is required to store line-frequency related circulating energy, even though a number of control strategies have been introduced to reduce the capacitor voltage ripples. In the present paper, a novel control strategy is proposed by means of harmonic injections in conjunction with gain control to completely eliminate both the line frequency and the second-order harmonic of the capacitor voltage ripple. Ideally, the proposed method works with the full bridge topology. However, the concept also works with half bridge topology with a significant reduction of line frequency related ripple. To gain a better understanding of the nature of circulating energy and the means of reducing it, the method of state plane analysis is employed to offer visual support. In addition, the design trade-off between full bridge MMC and half bridge MMC is presented and a novel control strategy for a hybrid MMC is proposed. Finally, the work is supported with a scaled down hardware demonstration.
Master of Science

During the last four decades there has been considerable development in voltage source converters (VSCs), which are widely contributed in multilevel converter topologies. Since then, multilevel VSC topologies have been used for applications with different power rating owing to the improvement of the output waveforms quality and minimising filtering requirements. In comparison with the conventional multilevel converters, modular multilevel converter (MMC) is considered as the most attractive topology for high and medium-power applications mainly due to the series connection of a high number of submodules (SMs). The challenges associated with the implementation of a high number of SMs includes: voltage-balancing of the distributed SM, cost, reliability and the increased complexity in the circuit configuration. Furthermore, achieving efficient and fast closed-loop control of the MMC requires the accurate knowledge of the voltage and current measurements, which means a considerable number of sensors are usually required to operate the MMCs. The main objective of this research is to propose several novel strategies for the converter to achieve voltage-balancing with fewer number of sensors to produce comparable performance to the sensor-based method. Four different sensorless schemes have been investigated, where two are current sensorless-based techniques and two are voltage sensorless-based techniques. The proposed current sensorless schemes are based on developed sorting algorithm, and the proposed voltage sensorless schemes employ two novel different recursive algorithms with the standard sorting algorithm. In regards to the voltage sensorless schemes, the first proposed method uses an exponentially weighted recursive least square (ERLS) algorithm, while the second proposed method employs a Kalman filter (KF) to estimate the SM capacitor voltages. Capacitance uncertainty has been investigated for the proposed voltage sensorless schemes. The proposed methods have been implemented via simulation but also on a scaled-down laboratory prototype. II The thesis also deals with capacitor diagnosis where a new scheme has been proposed which may be used for health monitoring technique, a comparison with an existing technique has been evaluated. Detailed simulations and experimental tests are carried out to investigate the performance of the proposed sensorless schemes, and results are compared with the sensor-based approach. These various schemes have been implemented and tested in real-time using a commercial floating point microcontroller where a 4-level single-phase MMC was employed. The results achieved for these novel schemes show an important improvement in the performance of the MMC under different operation conditions while fewer sensors were used.

Voltage and current unbalance are major problems in distribution networks, particularly with the integration of distributed generation systems. One way of mitigating these issues is by injecting negative sequence current into the distribution network using a Static Synchronous Compensator (STATCOM) which normally also regulates the voltage and power factor. The benefits of modularity and scalability offered by Modular Multilevel Cascaded Converters (MMCC) make them suitable for STATCOM application. A number of different types of MMCC may be used, classified according to the sub-module circuit topology used. Their performance features and operational ranges for unbalanced load compensation are evaluated and quantified in this research. This thesis investigates the use of both single star and single delta configured five-level Flying Capacitor (FC) converter MMCC based STATCOMs for unbalanced load compensation. A detailed study is carried out to compare this type of sub-module with several other types namely: half bridge, 3-L H-bridge and 3-L FC half bridge, and reveals the one best suited to STATCOM operation. With the choice of 5-L FC H-bridge as the sub-module for STATCOM operation, a detailed investigation is also performed to decide which pulse width modulation technique is the best. This was based on the assessment of total harmonic distortion, power loss, sub-module switch utilization and natural balancing of inner flying capacitors. Two new modulation techniques of swapped-carrier PWM (SC-PWM) along with phase disposed and phase shifted PWM (PS-PWM) are analyzed under these four performance metrics. A novel contribution of this research is the development of a new space vector modulation technique using an overlapping hexagon technique. This space vector strategy offers benefits of eliminating control complexity and improving waveform quality, unlike the case of multilevel space vector technique. The simulation and experimental results show that this method provides superior performance and is applicable for other MMCC sub-modules. Another contribution is the analysis and quantification of operating ranges of both single star and delta MMCCs in rating the cluster dc-link voltage (star) and current (delta) for unbalanced load compensation. A novel method of extending the operating capabilities of both configurations uses a third harmonic injection method. An experimental investigation validates the operating range extension compared to the pure sinusoidal zero sequence voltage and current injection. Also, the superiority of the single delta configured MMCC for unbalanced loading compensation is validated.

The nominal power of single Wind Energy Conversion Systems has been steadily growing, reaching power ratings close to 10 MW. In the power conversion stage, medium-voltage power converters are replacing the conventional low-voltage back-to-back topology. Modular Multilevel Converters have appeared as a promising solution for Multi-MW WECSs due to their characteristics such as modularity, reliability and the capability to reach high nominal voltages. Thereby, this thesis discusses the application of the Modular Multilevel Matrix Converter to drive Multi-MW Wind Energy Conversion Systems (WECSs). The modelling and control systems required for this application are extensively analysed and discussed in this document. The proposed control strategies enable decoupled operation of the converter, providing maximum power point tracking capability at the generator-side, grid-code compliance and Low Voltage Ride Through Control at the grid-side and good steady-state and dynamic performance for balancing the capacitor voltages of the converter. The effectiveness of the proposed control strategies is validated through simulations and experimental results. Simulation results are obtained with a 10MW, 6.6 kV Modular Multilevel Matrix Converter based WECS model developed in PLECS software. Additionally, a 5 kVA downscale prototype has been designed and constructed during this Ph.D. The downscale prototype is composed of 27 H-Bridges power cells. The system is controlled using a Digital Signal Processor connected to three Field Programmable Gate Array which are equipped with 50 analogue-digital channels and 108 gate drive signals. Two programmable AMETEK power supplies emulate the electrical grid and the generator. The wind turbine dynamics is programmed in the generator-side power supply to emulate a generator operating in variable speed/voltage mode. The output port of the Modular Multilevel Matrix Converter is connected to another power source which can generate programmable grid sag-swell conditions. Simulation and experimental results for variable-speed operation, grid-code compliance, and capacitor voltage regulation have confirmed the successful operation of the Modular Multilevel Matrix Converter based WECSs. In all the experiments, the proposed control systems ensure proper capacitor voltage balancing, keeping the flying capacitor voltages bounded and with low ripple. Additionally, the performance of the generator-side and grid-side control system have been validated for Maximum Power Point Tracking and Low-Voltage Ride Through, respectively.

Doctor en Ingeniería Eléctrica. Doctor of Philosophy in Electrical and Electronic Engineering
La potencia nominal de los Sistemas de Conversión de Energía Eólica se ha incrementado constantemente alcanzando niveles de potencia cercanos a los 10 MW. Por tanto, convertidores de potencia de media tensión están reemplazando a los convertidores Back-to-Back de baja tensión habitualmente empleados en la etapa de conversión de energía. Convertidores Modulares Multinivel se han posicionado como una solución atractiva para Sistemas de Conversión de Energía Eólica de alta potencia debido a sus buenas prestaciones. Algunas de estas prestaciones son la capacidad de alcanzar altos voltajes, modularidad y confiabilidad. En este contexto, esta tesis discute la aplicación del Convertidor Modular Matricial Multinivel para conectar Sistemas de Conversión de Energía Eólica de alta potencia. Los modelos matemáticos y estrategias de control requeridas para esta aplicación son descritos y discutidos en este documento. Las estrategias de control propuestas habilitan una operación desacoplada del convertidor, proporcionando seguimiento del máximo punto de potencia en el lado del generador eléctrico del sistema eólico, cumplimiento de normas de conexión en el lado de la red eléctrica y regulación de los condensadores flotantes del convertidor. La efectividad de las estrategias de control propuestas es validada a través de simulaciones y experimentos realizados con un prototipo de laboratorio. Las simulaciones se realizan con un Sistemas de Conversión de Energía Eólica de 10 MW operando a 6.6 kV. Dicho sistema se implementa en el software PLECS. Por otro, se ha desarrollado un prototipo de laboratorio de 6kVA durante el desarrollo de este proyecto. El prototipo de laboratorio considera un Convertidor Modular Matricial Multinivel de 27 módulos Puente-H . El sistema es controlado empleando una plataforma de control basada en una Digital Signal Processor conectada a tres tarjetas del tipo Field Programmable Gate Array que proveen de 50 mediciones análogo-digital y 108 señales de disparo. La entrada del convertidor es conectada a una fuente programable marca Ametek que emula el comportamiento de la turbina eólica. A su vez, la salida del convertidor es conectada a otra fuente programable con capacidad de producir fallas en la tensión. Los resultados obtenidos, tanto en el prototipo experimental como en simulación, confirman la operación exitosa del Convertidor Modular Matricial Multinivel en aplicaciones eólicas de alta potencia. En todos los casos, las estrategias de control propuestas aseguran regulación de la tensión en los condensadores flotantes, seguimiento del máximo punto de potencia en el lado del generador eléctrico del sistema eólico y cumplimiento de normas de conexión en el lado de la red eléctrica.
The nominal power of single Wind Energy Conversion Systems has been steadily growing, reaching power ratings close to 10MW. In the power conversion stage, medium-voltage power converters are replacing the conventional low-voltage back-to-back topology. Modular Multilevel Converters have appeared as a promising solution for Multi-MW WECSs due to their characteristics such as modularity, reliability and the capability to reach high nominal voltages. Thereby, this thesis discusses the application of the Modular Multilevel Matrix Converter (\mc) to drive Multi-MW Wind Energy Conversion Systems (WECSs). The modelling and control systems required for this application are extensively analysed and discussed in this document. The proposed control strategies enable decoupled operation of the converter, providing maximum power point tracking capability at the generator-side, grid-code compliance and Low Voltage Ride Through Control at the grid-side and good steady state and dynamic performance for balancing the capacitor voltages of the converter.\\ The effectiveness of the proposed control strategies is validated through simulations and experimental results. Simulation results are obtained with a 10MW, 6.6 kVM3C based WECS model developed in PLECS software. Additionally, a 5 kVA downscale prototype has been designed and constructed during this Ph.D. The downscale prototype is composed of 27 H-Bridges power cells. The system is controlled using a Digital Signal Processor connected to three Field Programmable Gate Array which are equipped with 50 analogue-digital channels and 108 gate drive signals. Two programmable AMETEK power supplies emulate the electrical grid and the generator. The wind turbine dynamics is programmed in the generator-side power supply to emulate a generator operating in variable speed/voltage mode. The output port of the M3C is connected to another power source which can generate programmable grid sag-swell conditions. Simulation and experimental results for variable-speed operation, grid-code compliance, and capacitor voltage regulation have confirmed the successful operation of the \mc{} based WECSs. In all the experiments, the proposed control systems ensure proper capacitor voltage balancing, keeping the flying capacitor voltages bounded and with low ripple. Additionally, the performance of the generator-side and grid-side control system have been validated for Maximum Power Point Tracking and Low-Voltage Ride Through, respectively.

Voltage fluctuation and power losses in the distribution line are problems in distribution networks. One method to mitigate these problems is by injecting reactive power into the network using a Static Synchronous Compensator (STATCOM). This can be used both for regulating the voltage and reducing the losses. A STATCOM is critically dependent on a grid synchronisation scheme that can accurately track the changes occurring in the grid phase and frequency. The Modular Multilevel Converter (MMC) is a promising topology for STATCOM applications because of its simple modular circuit structure that allows for higher voltage ratings, and conventionally uses a stack of sub-modules which are either two-level half or H-bridge converters. As a novel alternative, the thesis investigates the practicality of a STATCOM based on a three-level flying capacitor (FC) converter. Two variants of this topology are presented; the FC Half-bridge and FC H-bridge. A comprehensive study is undertaken to compare these with the Half and H-bridge sub-module under STATCOM operation. Most importantly, an FC H-bridge-based STATCOM is investigated for reactive power compensation. The challenges of multilevel, multi-module PWM control schemes achieving good waveforms at low switching frequency, whilst maintaining module capacitor voltage balance, are thoroughly addressed. Simulation results validate the operation for both line voltage regulation and power factor correction. An experimental power system with an FC-based STATCOM rig is designed and built, and validates the simulation results for power factor correction. It demonstrates correct operation of a control scheme that includes a system for maintaining capacitor voltage balance. Another new contribution is the investigation of a phase locking technique based on the Energy Operator (EO). The method, combining two different EO computations, is shown to achieve fast and accurate detection of frequency and phase angle when combined with an appropriate filter, and crucially operates well under unbalanced voltage conditions. The technique is compared with two other well-known phase locked loop (PLL) schemes, showing that it outperforms the others in terms of speed and accuracy. A hardware implementation of the EO-PLL validates the principle, showing the simplicity of the method.

This thesis investigates the design, analysis, and operation of modular multilevel converters (MMC) for HVDC applications. Based on the operation principles of the MMC, the operation of MMC under asymmetrical arm impedance conditions is analysed using three equivalent sub-circuits at different freqeuncy. Detail analysis of the impact of asymmetrical conditions on the differential-mode current, the common-mode current and sub-module (SM) capacitor voltages, is performed. Based on the analysis, the corresponding control targets and an improved control strategy are designed to improve the operation performance. Considering the advantages of half-bridge based SM (HBSM) and full-bridge based SM (FBSM), a hybrid MMC (H-MMC) configuration consisting of FBSMs and HBSMs is proposed. By adopting the negative voltage state for some of the FBSMs, the output voltage range is extended to increase converter power transmission capability. By considering the relationships between the AC and DC voltages, AC, DC and arm currents, the ratio of the numbers of the FBSM to HBSM is analysed in order to maintain capacitor voltage balance and retain DC fault blocking capability. An equivalent circuit for the H-MMC is proposed, which considers each arm to be consisted of two individual voltage sources. This model is used to analyse SM capacitor voltage balancing and ripple. A two-stage selection and sorting algorithm is developed to ensure capacitor voltage balancing among the SMs. The proposed H-MMC is compared to other topologies in terms of power device utilization and power losses, and it shows that the H-MMC has higher device utilization and lower power loss than the conventional FBSM based MMC; Furthermore, The DC fault ride-through capability of the H-MMC are discussed. It is found that the H-MMC can not only isolate the DC fault, but also coniture operating at a wide DC voltage range from zero to rated value. Such two features of the H-MMC show the advantages in the hybrid configurations over the conventional FBSM and HBSM systems. Finally, two applications based on the proposed H-MMC are presented; one is a high power DC/DC converter with fault blocking capability for interconnecting large HVDC systems, and the other is a hybrid HVDC transmission system comprising a wind farm side VSC based on the H-MMC and a grid side LCC for transmitting wind power to AC grid.

Doctor en Ingeniería Eléctrica
Hoy en día, la construcción de maquinaria y plantas industriales exigen soluciones de accionamiento con un diseño flexible y escalable. En sectores industriales como la petroquímica, la minería, la generación de energía, etc., existe una demanda de soluciones con alta eficiencia, seguridad integrada y respaldo de las compañías proveedoras. En este contexto, el Convertidor Multinivel Modular (MMC) ha aparecido como una opción prometedora para accionamientos de media tensión de alta potencia debido a sus características, como modularidad total, flexibilidad de control, niveles de media tensión, calidad de potencia y posible operación sin transformador. Por lo tanto, esta tesis discute la aplicación del MMC para accionamientos de máquinas. El modelado del MMC y sus sistemas de control asociados para esta aplicación son analizados ampliamente en este documento. Específicamente, se ha propuesto un nuevo modelado basado en coordenadas dq y su sistema correspondiente sistema de control para regular el valor instantáneo de las tensiones de los condensadores del MMC. Además, se ha propuesto la integración de los sistemas de control de la máquina y del MMC para mejorar el rendimiento general del sistema. Por ejemplo, se demostró que las corrientes circulantes requeridas durante bajas frecuencias ac se reducen al considerar la interacción de ambos sistemas de control. La efectividad de las estrategias de control propuestas se validó a través de extensos resultados experimentales, que se han publicado en dos artículos (IEEE Transactions on Industrial Electronics) y seis artículos de conferencia (indexados en la base de datos Scopus), así como contribuciones importantes en otros proyectos relacionados con el control de convertidores multinivel modulares. El prototipo utilizado se compone de 18 celdas de potencia. El sistema se controla utilizando un procesador de señales digitales y dos FPGAs. Un segundo MMC con 12 celdas de potencia también se usó para algunas pruebas, conformando una unidad Back-to-Back MMC. Se probó el rendimiento dinámico y en estado estable de las metodologías de control propuestas, considerando el arranque del MMC, cambios escalón tanto en el par y las corrientes de magnetización, rampas de velocidad, pruebas de cruce por velocidad cero, operación de rotor bloqueado, operación con flujo debilitado, diferentes condiciones de carga, manipulación de la tensión dc del MMC, etc. En todos los casos, el rendimiento alcanzado es consistente con los resultados esperados. Nowadays, machinery and plant construction are demanding drive solutions with flexible and scalable design. In industrial sectors such as petrochemical, mining, power generation, etc., there is a demand for solutions with high efficiency, integrated safety and support from the supplier companies. In this context, the Modular Multilevel Converter (MMC) has appeared as a promising option for high-power medium-voltage drives due to their characteristics, such as full modularity, control flexibility, medium-voltage levels, power quality and possible transformer-less operation. Thereby, this thesis discusses the application of the MMC as a machine drive. The modelling and control systems required for this application are extensively analysed and discussed in this document. Specifically, a novel dq-based modelling of the MMC and its associated control system has been proposed to regulate the instantaneous value of the MMC capacitor voltages. Additionally, the integration of the machine and MMC control systems has been proposed to enhance the performance of the overall system. For example, it was demonstrated that the required circulating currents during low-ac frequencies are reduced by considering the interaction of both control systems. The effectiveness of the proposed control strategies is validated through extensive experimental results, which have been published in two journal papers (IEEE Transaction on Industrial Electronics) and six conference papers (indexed in the Scopus database), as well as important contributions in other projects related to the control of modular multilevel converters. The downscaled prototype utilised is composed of 18 power cells. The system is controlled using a Digital Signal Processor and two Field Programmable Gate Arrays (FPGAs). A second MMC with 12 power cells was also used for some tests, conforming a Back-to-Back MMC-based drive. The dynamic and steady-state performance of the proposed control methodologies were tested, considering the MMC starting-up, step changes in both the torque and magnetising currents, speed-ramps, zero-speed crossing test, rotor-locked operation, flux-weakening operation, different loading conditions, manipulation of the input voltage of the MMC, etc. In all cases, the achieved performance is consistent with the expected results.

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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior
The modular multilevel converter emerged as a new topology of multilevel converters, being introduced in 2002. The advantages of this topology are related to its modularity and scalability. This work presents the study and implementation of this converter, which includes the presentation of the main methods of modulation and voltage balancing of the foating capacitors and startup. The used modulation in modeled using switching functions, its allow one minimize the current ripple at system inductor due the correct selection of carriers shift angles. Moreover a current control and voltages equalization methodology are proposed. It is performed dynamic modeling and quantitative analysis of the converter and it is derived a design methodology. This methodology is used to design and build a 3 kVA prototype with bus voltage of 800 V. The results include transient analyses, efficiency, voltage charging and steady state.
O conversor modular multinível emergiu como uma nova topologia de conversores mutiníveis, sendo introduzido a partir de 2002. As vantagens desta topologia estão relacionadas a sua modularidade e escalabilidade. Este trabalho apresenta o estudo e implementação deste conversor, o que inclui a apresentação das principais metodologias de modulação e equilíbrio da tensão e pre-carga dos capacitores flutuantes. Apresenta-se um estudo da modulação por meio de funções de chaveamento que permite a minimização da ondulação de corrente nos indutores por meio da escolha adequada dos ângulos de defasagem das portadoras empregadas. Para que o projeto da estrutura seja possível, e realizada a modelagem dinâmica e a analise quantitativa do conversor em diferentes condições de operação, sendo derivada uma metodologia de projeto. Esta metodologia de posta a prova com a construção de um protótipo de 3 kVA com tensão de barramento de 800 V. Os resultados obtidos do protótipo incluem avaliações transitórias, verificação do rendimento, pre-carga e operação em regime.

This thesis is an effort to investigate the operation and the performanceof modular multilevel converters (M2Cs). Proven to be the most promisingtopology in high-voltage high-power applications, it is necessary to put aneffort in understanding the physical laws that govern the internal dynamicsof such converters, in order to design appropriate control methods. AlthoughM2Cs belong to the well-studied family of voltage-source converters (VSCs),and claim a modular structure, their control is significantly more complicatedcompared to two- or three-level VSCs, due to the fact that a much highernumber of switches and capacitors are needed in such a topology. This thesishighlights the important parameters that should be considered when designingthe control for an M2C, through analyzing its internal dynamics, and alsosuggests ways to control such converters ensuring stable operation withoutcompromising the performance of the converter.Special focus is given on ac motor-drive applications as they are very demandingand challenging for the converter performance. Interactions betweenthe internal dynamics and the dynamics of the driven motor are experimentallyinvestigated. The problem of operating the converter when connectedto a motor standing still is visited, even under the condition that a greatamount of torque and current are requested, in order to provide an idea forthe converter requirements under such conditions. Finally, an optimization ofthe converter operation is suggested in order to avoid overrating the convertercomponents in certain operation areas that this is possible.All analytical investigations presented in this thesis are confirmed by experimentalresults on a laboratory prototype converter, which was developedfor the purposes of this project. Experimental verification proves the validityof the theoretical investigations, as well as the correct performance of thecontrol methods developed during this project on a real, physical converter,hoping that the results of this thesis will be useful for large-scale implementations,in the mega- or even giga-watt power range.
Denna avhandling är ett försök att undersöka drift och egenskaper avmodulära multinivåomvandlare (M2C:er). Eftersom denna topologi anses varaden mest lovande inom högspänings-högeffekt-tillämpningar är, och somett underlag för att kunna formulera lämpliga styrmetoder, är det nödvändigtatt lägga kraft i att försöka förståde fysikaliska lagar som styr den inredynamiken i sådana omvandlare. Även om M2C:erna tillhör den välstuderadefamiljen av spänningsstyva omvandlare (VSC:er), och har en modulärstruktur, är deras reglering avsevärt mer komplicerad jämfört med två- ellertre-nivåomvandlare, eftersom ett mycket större antal switchar och kondensatorerär nödvändiga i en sådan topologi. Denna avhandling sätter fingretpå de parametrar som måste beaktas när man konstruerar regleringen för enM2C, genom att analysera den interna dynamiken, samt att föreslå sätt attstyra sådana omvandlare såatt stabil drift kan säkerställas utan att negativtpåverka prestanda.Ett speciellt fokus läggs på växelströmsmotordrifter eftersom de är särskiltutmanande vad gäller prestanda. Växelverkan mellan den interna dynamikenoch motorns dynamik undersöks experimentellt. Problemet att driva motornvid stillestånd behandlas även i fallet med hög ström och högt moment för atterhålla kunskap om kraven påomvandlaren i sådana fall. Slutligen föreslås enoptimering av omvandlarens drifttillstånd för att undvika överdimensioneringav omvandlarens komponenter i de fall detta är möjligt.Alla analytiska undersökningar som läggs fram i denna avhandling är bekräftadegenom experimentella resultat från en laboratorieomvandlare, somutvecklats inom ramen för detta arbete. Den experimentella verifieringen bevisargiltigheten av alla teoretiska undersökningar. Den visar också på demycket goda prestanda som de utvecklade styrmetoderna har vid drift aven verklig fysisk omvandlare. Förhoppningen är att resultaten från detta arbetekan komma till använding i storskaliga implementerinar i mega- ellergiga-wattklassen.

QC 20141201

Le convertisseur modulaire à plusieurs niveaux est un sujet d'intérêt important et actuel dans le contexte des applications de systèmes de transmission haute tension à courant continu. Cette topologie convient à plusieurs applications, en raison de pertes de commutation plus faibles dues à une fréquence de commutation plus petite, à une faible distorsion harmonique de courant alternatif, à une structure modulaire permettant une construction évolutive, et une maintenance plus simple. Cependant, une stratégie de contrôle plus complexe est nécessaire pour contrôler le courant circulant, pour compenser le déséquilibre de tension entre les circuits et l'équilibrage de tension de SM, de manière à maintenir constantes les tensions des condensateurs de SM. Cette thèse présente deux contrôleurs non linéaires pour un MMC, capables de contrôler les courants circulants et l'énergie dans le convertisseur. Le premier est conçu selon la théorie bilinéaire basée sur le contrôle de rétroaction quadratique. Le deuxième contrôleur proposé est développé en utilisant la théorie de Lyapunov, fortement basé sur des techniques de perturbation singulière et de linéarisation par bouclage. Pour les deux, une étude mathématique est réalisée sur la stabilité, basée sur la théorie de Lyapunov. Ce résultat assure une stabilisation asymptotique pour les trois phases MMC. L'utilisation d'une fonction de Lyapunov implique une vérification formelle de la stabilité et une région explicite d'attraction pour le modèle considéré. Les deux techniques de contrôle sont développées à partir d'un modèle bilinéaire moyen, et la robustesse et les performances sont vérifiées au moyen d'un modèle de commutation MMC provenant des simulations électriques Matlab Simscape. L'évaluation comprend des variations de référence de puissance active et réactive, des conditions de déséquilibre du réseau, des incertitudes de paramètres et même une comparaison avec un contrôleur PI standard. Aussi, pour les contrôleurs non linéaires sont étudiés: l'effet des gains du contrôle sur la dynamique du système et les performances du contrôleur en cas de changement du point de fonctionnement. Les contributions principales de la thèse sont les deux algorithmes de contrôle non linéaires distincts, basés sur un modèle mathématique bilinéaire, conçus pour les convertisseurs MMC; Les deux algorithmes sont capables de contrôler l’équilibrage du courant et énergie du convertisseur au niveau du modèle détaillé du MMC par commutation; Il existe une analyse formelle de la stabilité par la théorie de Lyapunov pour ces systèmes; et une fois que le contrôle proposé n'est pas basé sur un modèle linéarisé, une vaste région d'opération est garantie
MMC is a very important topic in the context of high voltage direct-current transmission systems applications. This topology is suitable for several applications, as a result of smaller switching losses due to lower switching frequency, low alternating-current harmonic distortion, modular structure enabling scalability construction and practical maintenance. However, a more complex control strategy is required to control circulating current, to compensate the voltage imbalance between legs and voltage balancing of SM, such as to maintain SM's capacitors voltages constant. This thesis presents two nonlinear controllers for an MMC, able to control circulating currents, and the energy in the converter. First proposed controller is developed using Lyapunov theory, strongly based on singular perturbation and feedback linearization techniques. Second one is designed following bilinear theory based on quadratic feedback control. For both, a mathematical proof is given for its stability, which is based on Lyapunov's theory. This result provides asymptotic stabilization for the three-phases MMC. The use of a Lyapunov function implies a formal verification of stability and a broad region of attraction for the considered model. Both control techniques are developed by means of an average bilinear model and performances are verified by means of a detailed MMC switching model at Matlab Simscape Electrical environment. The evaluation includes active and reactive power reference variations, grid imbalance conditions, parameters uncertainties and even a comparison with a standard PI controller. Also, for the nonlinear controllers, it is studied the effect of control gains on the system's dynamics. The main thesis' contributions can then be stated as the two distinct nonlinear control algorithms, based on a bilinear mathematical model, designed for MMC converters; Both algorithms are able to control circulating currents and converter's energy at the switching MMC model; There are formal stability analysis by Lyapunov theory for these controllers; and once these proposed controllers are not based on a linearized model, a broad operation region is obtained
Conversor multinível modular é o tópico de interesse amplo e atual no contexto de aplicações de sistemas de transmissão de corrente contínua de alta tensão. Essa topologia é adequada para várias aplicações, como resultado de menores perdas de chaveamento, devido à menor frequência de comutação dos IGBTs, baixa distorção harmônica na corrente alternada, estrutura modular que permite escalabilidade na construção e manutenção prática. No entanto, é necessária uma estratégia de contrôle mais complexa para controlar a corrente circulante, para compensar o desequilíbrio de tensão entre as pernas e o equilíbrio de tensão dos sub-módulos, de forma a manter constantes as tensões dos capacitores dos sub-módulos. Esta tese apresenta dois controles não-lineares para conversores MMC, capazes de controlar correntes circulantes e a energia no conversor. O primeiro é projetado seguindo a teoria bilinear baseada no controle de feedback quadrático. O segundo controlador proposto é desenvolvido usando a teoria de Lyapunov, fortemente baseada em técnicas singular perturbation e feedback linearization. Para ambos, é definida uma prova matemática de sua estabilidade, baseada na teoria de Lyapunov. Este resultado fornece estabilização assintotica para as três fases MMC. O uso de uma função de Lyapunov implica uma verificação formal da estabilidade e uma região explícita de atração para o modelo considerado. Ambas as técnicas de controle são desenvolvidas por meio de um modelo médio bilinear e a robustez e o desempenho são verificados por meio de um modelo chaveado de conversores MMC nas simulações do Matlab Simscape Electrical. A avaliação inclui variações de referência de potência ativa e reativa, condições de desequilíbrio da rede, incertezas de parâmetros e até uma comparação com um controlador PI. Além disso, para os controladores não lineares, são estudados: o efeito do controle de ganho na dinâmica do sistema e no desempenho do controlador em caso de alteração no ponto de operação. As principais contribuições da tese são os dois algoritmos distintos de controle não-linear, baseados em um modelo matemático bilinear, projetados para conversores MMC; Ambos os algoritmos são capazes de controlar o equilíbrio de corrente circulante e a energia do conversor; Há uma análise formal de estabilidade pela teoria de Lyapunov para esse sistema; e uma vez que os controles propostos não se baseiam em um modelo linearizado, uma vasta região de operação é alcançável

This thesis uses computer simulation techniques to develop and test a distributed control structure for a single-sided bridge-cell, multilevel modular cascade converter STATCOM. The control technique was designed to track a reference current and balance the voltage in the modules. A master controller was designed to track the reference current while each module had a controller which oversaw balancing the voltage among the modules. The master controller was also able to estimate the total dc voltage in the system, reducing the number of sensors required and making the control structure truly distributed. The research findings will contribute towards making multilevel modular cascade converters more modular in nature.

Modular Multilevel Converters (MMCs) are distinguished by their modular nature that makes them suitable for wide range of high power and high voltage applications. However, they are vulnerable to internal faults because of the large number of series connected Sub-Modules. Additionally, it is highly recommended not to block the converter even if it is subjected to internal faults to secure the supply, to increase the reliability of the system and prevent unscheduled maintenance. This thesis introduces a fault tolerant control system for controlling the MMC in normal as well as abnormal operating conditions. This is done through developing a new adaptive voltage balancing strategy based on capacitor voltage estimation utilizing ADAptive LInear NEuron (ADALINE) and Recursive Least Squares (RLS) algorithms. The capacitor voltage balancing techniques that have been proposed in literature are based on measuring the capacitor voltage of each sub-module. On contrary, the proposed strategy eliminates the need of these measurements and associated communication links with the central controller. Furthermore, the thesis presents a novel fault diagnosis algorithm using the estimated capacitor voltages which are utilized to detect and localize different types of sub-module faults. The proposed fault diagnosis algorithm surpasses the methods presented in literature by its fast fault detection capability without the need of any extra sensing elements or special power circuit. Finally, a new Fault Tolerant Control Unit (FTCU) is proposed to tolerate the faults located inside the MMC submodules. The proposed FTCU is based on a sorting algorithm which modifies the parameters of the voltage balancing technique in an adaptive manner to overcome the reduction of the active submodules and secure the MMC operation without the need of full shut-down. Most of fault tolerant strategies that have been proposed by other researchers are based on using redundant components, while the proposed FTCU does not need any extra components. The dynamic performance of the proposed strategy is investigated, using PSCAD/EMTDC simulations and hardware in the loop (HIL) real-time simulations, under different normal and faulty operating conditions. The accuracy and the time response of the proposed fault detection and tolerant control units result in stabilizing the operation of the MMC under different types of faults. Consequently, the proposed integrated control strategy improves the reliability of the MMC.

The increased demand for electrical power and the concern of environmental pollution drive the development of bulk-power transmission over long distance and renewable energy. The use of multi-terminal (MT) modular multilevel converter (MMC) high-voltage direct current (HVDC) technology to integrate power from the offshore wind farm (OWF) is becoming increasingly popular. However, some technological barriers and potential risks may exist in the new technology, which requires comprehensive research and innovative developments. This thesis investigates several important aspects of an offshore integrated MMC multi-terminal HVDC (MTDC) system, including start-up control, control and protection under AC and DC fault conditions. For the start-up control, a hierarchical start-up scheme is proposed for the terminals with active AC networks and a reduced DC voltage start-up scheme is proposed for the terminal with the OWF. Synthesising both schemes forms a comprehensive start-up control scheme for the start-up control of the MTDC system, which can effectively mitigate the voltage spikes and current surges during the start-up process. For control and protection against AC fault conditions, associated control and protection strategies and detailed control and protection sequences are proposed for the faults occurring at the converter AC-side. In addition, a special control and protection strategy is proposed when the faulted-side MMC experiences blocking failure following the fault. For the DC fault management, a fault isolation strategy is proposed and the system recovery scheme is comprehensively investigated after the fault isolation, with delayed-auto-re-configuration (DARC) schemes being proposed. Combining the DARC scheme and the fault isolation strategy, a complete control and protection sequence is proposed. Effectiveness of the proposed schemes is evaluated on the RTDS simulation platform.

Global power consumption has increased by approximately 3% each year over the past 15 years. The growing demand for energy has stimulated the spread of clean and reliable renewable energy networks and power grid interconnections throughout the world. For example, in Europe, there are 23 High Voltage Direct Current (HVDC) Transmission lines under construction which are scheduled for completion before 2024. The Modular Multilevel Converter (MMC) is one of the most attractive candidates for the HVDC transmission system converter technology. Its high flexibility and controllability make it an attractive option for HVDC transmission. However, the higher initial investment and the unfavourable conditions for using associated DC circuit breakers have always been a barrier to further installations. Since ABB successfully developed the HVDC DC circuit breakers in 2012, there is increasing interest in DC grids using the MMC HVDC transmission system. However, one of the common problems existing in the HVDC transmission system is the control of the capacitor volt-age in each submodule of the MMC. However, in the transmission systems, especially in the renewable energy systems, there are disturbances existing. The conventional voltage balancing control is weak to the disturbances, such as power and sampling frequency changes. Therefore, the proposed voltage balancing control in this thesis has improved the responding time and precision of the control. It determines the charging state of each submodule by deriving the capacitor voltage variations, thereby ensuring the voltage of each capacitor is within pre-defined range regardless the disturbance. In later study, both simulation and experimental results have shown the proposed control approach has strong immunity to the sampling frequency noise compared to the conventional control. However, even with the proposed voltage balancing control, the capacitor voltage difference cannot be eliminated entirely. They will cause circulating current flowing among the phases of the circuit. Therefore, causing unnecessary pressures to the affected components. The circulating current suppression control pro-posed in this thesis can eliminate the AC component of the circulating current, by regulating it according to the power going through the converter. It gets rid of the two PID controllers and abc-dq transformation which are commonly used in conventional circulating current control approach. The simulation and experiment results have shown the suppression of the proposed control approach regarding the AC components in the circulating current, and the fast response time taking effect within one control cycle. In this thesis, both proposed control approaches are presented with simulation results and validated with the scaled down experiment model.

Energy security concerns and the impact of traditional sources of power generation on the climate have prompted a rise in renewable energy expansion around the world. Power transmission from remote generation sites to consumers over long distance is most efficient using High-Voltage Direct Current (HVDC) transmission lines. Consequently, HVDC and the integration of renewable resources are considered as key perspectives in the improvement of sustainable energy systems capable of secure and stable electric power supply. With the intention of huge energy demand in the future, the multi-terminal DC grid concept is proposed based on various converter topologies like Line Commutated Converter (LCC), Voltage Sourced Converter (VSC), and Modular Multilevel Converter (MMC) HVDC technologies. These converters play a vital role in integrating remotely-located renewable generation and reinforcing existing power systems. The MMC has become increasingly popular in HVDC transmission compared to conventional line commutated converters, two-level and multilevel voltage source converters. Low generation of harmonics, a low switching frequency of semiconductors, sine formed AC voltages and currents, black start capability and higher overall efficiency are a few of the unique features of MMC. The MMC is characterised by a modular arm structure, formed by a cascade connection of a vast number of simple cells with floating DC capacitors. These cells are called Sub-Modules (SMs) and can be easily assembled into a converter for high voltage power conversion systems. Compared with traditional VSCs, the analytical modelling of MMC is more challenging. This is because of technical issues such as higher order system, the discontinuous and non-linear nature of signal transfer through converters, the complexity of the interaction equations between the AC and DC variables, and harmonic frequency conversion through AC side and DC side of the converter. This work intends to resolve these challenges by developing a detailed non-linear model using fundamental switching Selective Harmonic Elimination (SHE) modulation technique, an average MMC model in DQ0 frame and an analytical dynamic MMC model, which can be suitable for small-signal stability studies, and control design. Firstly, the detailed model of MMC using fundamental switching SHE modulation scheme has been developed using PSCAD/EMTDC (Power systems computer aided design Electromagnetic transients for DC) software. The basic terms and equations of the MMC have been presented along control loops. The significance of the switching frequency on the performance of the MMC has been studied as well as the relation between the switching frequency, the Total Harmonic Distortion (THD) and the number of output voltage levels. Detailed representation of MMC systems in PSCAD/EMTDC programs incorporates the modelling of Insulated-Gate Bipolar Transistor (IGBT) valves and should typically utilise small integration time-steps to represent fast switching events precisely. Computational burden introduced by such detailed models make the study of steady-state and transient events more complex, highlighting the need to implement more efficient models that provide comparative behaviour and dynamic response. Secondly, average DQ0 models has been implemented to accurately replicate the steady-state, dynamic and transient behaviour of MMC in PSCAD/EMTDC programs. These simplified models represent the average response of switching devices and converters by using averaging techniques involving controlled sources and switching functions. Developing the MMC average model in DQ0 frame was a challenging task because of the multiplication terms in the MMC average model in ABC frame. The proposed approach to overcome this challenge is considering generic form for the product variables and multiplying them in ABC frame and then transferring only the DC and fundamental frequency components of the results to DQ0 frame. The comparisons between detailed model and the average model validated the effectiveness of the average model in representing the dynamics of MMC. It is at least one hundred times faster than the detailed model for the same simulation time step. Finally, a dynamic analytical MMC model and associated controls have been proposed. To enable the model application to a broad range of system configurations and various dynamic studies, the model is built on a modular modelling approach using four sub-systems; an AC system, Phase Locked Loop (PLL) system, MMC system and a DC arrangement. The developed MMC system model has been linearized and implemented in state-space form. To select the best open-loop controller gains, eigenvalue analysis is performed for each particular test system. The rationality and correctness of the proposed model are verified against non-linear PSCAD/EMTDC simulations, and good accuracy is obtained in the time domain analysis. Further, the model is also verified in the frequency domain, and it is concluded that the developed model can be employed for dynamic analysis below 300 Hz.

This thesis presents improved modulation and control schemes for an H-bridge modular multilevel converter (MMC) that can be used to enhance the transient response of DC transmission systems. The schemes enable H-bridge MMC cell capacitor voltages to be regulated independent of the DC link voltage in a DC transmission system, and also permit operation with variable DC link voltage, down to zero voltage, with full control over active and reactive power exchange. The proposed schemes also offer protection functionality during a pole-to-pole DC fault by restraining the DC fault current magnitude in the converter arms to a level compatible with the current rating of the converter switching devices. The modulation and control schemes use the perturbations in the cell capacitor voltages and common mode currents of an individual phase to eliminate the second-order harmonics from each converter arm. This is achieved without a dedicated controller for suppression of the second-order harmonics . The validity of the proposed modulation and control schemes is confirmed using simulations and experimentation in open and close loop using a scaled down H-bridge MMC. Their viability in DC transmission systems is assessed using simulation of point-to-point and multi-terminal DC networks; this includes power transmission with reduced DC link voltage and survival from permanent and temporary DC faults with DC link controlled recharging following fault clearance. The major practical implication of the proposed modulation and control schemes is that they offer the possibility for voltage source converter based DC transmission systems to ride-through DC faults without the need for expensive and fast DC circuit breakers, as is being pursued by HVDC manufacturers. This thesis demonstrates the possibility of operation without converter blocking, without risk of converter damage from excessive current stresses. In this manner, a converter station of the DC transmission system can be used during a DC fault to provide voltage support to an AC grid.

Dans le cadre du programme de croissance Européen 2020, la commission européenne a mis en place officiellement un chemin à long terme pour une économie à faible émission de carbone, en aspirant une réduction d’au moins 80% des émissions de gaz à effet de serre, d’ici 2050. Répondre à ces exigences ambitieuses, impliquera un changement majeur de paradigme, et notamment en ce qui concerne les infrastructures du réseau électrique. Les percées dans la technologie des semi-conducteurs et les avancées avec les nouvelles topologies d’électronique de puissance et leurs contrôle-commandes, ont contribué à l’impulsion donnée au processus en cours de réaliser un tel SuperGrid. Une percée technologique majeure a eu lieu en 2003, avec le convertisseur modulaire multi-niveaux (MMC ou M2C), présenté par le professeur Marquardt, et qui est actuellement la topologie d’électronique de puissance la plus adaptée pour les stations HVDC. Cependant, cette structure de conversion introduit également un certain nombre de défis relativement complexes tels que les courants “additionnels” qui circulent au sein du convertisseur, entrainant des pertes supplémentaires et un fonctionnement potentiellement instable. Ce projet de thèse vise à concevoir des stratégies de commande “de haut niveau” pour contrôler le MMC adaptées pour les applications à courant continue-haute tension (HVDC), dans des conditions de réseau AC équilibrés et déséquilibrés. La stratégie de commande optimale identifiée est déterminée via une approche pour la conception du type “de haut en bas”, inhérente aux stratégies d’optimisation, où la performance souhaitée du convertisseur MMC donne la stratégie de commande qui lui sera appliquée. Plus précisément, la méthodologie d’optimisation des multiplicateurs de Lagrange est utilisée pour calculer le signal minimal de référence du courant de circulation du MMC dans son repère naturel
Following Europe’s 2020 growth program, the Energy Roadmap 2050 launched by the European Commission (EC) has officially set a long term path for a low-carbon economy, assuming a reduction of at least 80% of greenhouse gas emissions by the year 2050. Meeting such ambitious requirements will imply a major change in paradigm, including the electricity grid infrastructure as we know it.The breakthroughs in semi-conductor technology and the advances in power electronics topologies and control have added momentum to the on-going process of turning the SuperGrid into a reality. Perhaps the most recent breakthrough occurred in 2003, when Prof. Marquardt introduced the Modular Multilevel Converter (MMC or M2C) which is now the preferred power electronic topology that is starting to be used in VSC-HVDC stations. It does however, introduce a number of rather complex challenges such as “additional” circulating currents within the converter itself, causing extra losses and potentially unstable operation. In addition, the MMC will be required to properly balance the capacitive energy stored within its different arms, while transferring power between the AC and DC grids that it interfaces.The present Thesis project aimed to design adequate “high-level” MMC control strategies suited for HVDC applications, under balanced and unbalanced AC grid conditions. The resulting control strategy is derived with a “top-to-bottom” design approach, inherent to optimization strategies, where the desired performance of the MMC results in the control scheme that will be applied. More precisely, the Lagrange multipliers optimization methodology is used to calculate the minimal MMC circulating current reference signals in phase coordinates, capable of successfully regulating the capacitive arm energies of the converter, while reducing losses and voltage fluctuations, and effectively decoupling any power oscillations that would take place in the AC grid and preventing them from propagating into the DC grid

With the deployment of sensors in hardware equipment and advanced metering infrastructure, system operators have access to unprecedented amounts of data. Simultaneously, grid-connected power electronics technology has had a large impact on the way electrical energy is generated, transmitted, and delivered to consumers. Artificial intelligence and machine learning can help address the new power grid challenges with enhanced computational abilities and access to large amounts of data. This thesis discusses the fundamentals of neural networks and their applications in power systems such as load forecasting, power system stability analysis, and fault diagnosis. It extends application of neural networks to inverter-based resources by studying the implementation and performance of a neural network controller emulator for voltage-sourced converters. It delves into how neural networks could enhance cybersecurity of a component through multiple hardware and software implementations of the same component. This ensures that vulnerabilities inherent in one form of implementation do not affect the system as a whole. The thesis also proposes a comprehensive support vector classifier (SVC)--based submodule open-circuit fault detection and localization method for modular multilevel converters. This method eliminates the need for extra hardware. Its efficacy is discussed through simulation studies in PSCAD/EMTDC software. To ensure efficient usage of neural networks in power system simulation softwares, this thesis entails the step by step implementation of a neural network custom component in PSCAD/EMTDC. The custom component simplifies the process of recreating a neural network in PSCAD/EMTDC by eliminating the manual assembly of predefined library components such as summers, multipliers, comparators, and other miscellaneous blocks.
Master of Science
Data analytics and machine learning play an important role in the power grids of today, which are continuously evolving with the integration of renewable energy resources. It is expected that by 2030 most of the electric power generated will be processed by some form of power electronics, e.g., inverters, from the point of its generation. Machine learning has been applied to various fields of power systems such as load forecasting, stability analysis, and fault diagnosis. This work extends machine learning applications to inverter-based resources by using artificial neural networks to perform controller emulation for an inverter, provide cybersecurity through heterogeneity, and perform submodule fault detection in modular multilevel converters. The thesis also discusses the step by step implementation of a neural network custom component in PSCAD/EMTDC software. This custom component simplifies the process of creating a neural network in PSCAD/EMTDC by eliminating the manual assembly of predefined library components.

The Modular Multilevel Converter (M2C) is proven to be a key converter technology which is suitable for various high-voltage high-power applications. It offers several advantages over the conventional Voltage Source Converters(VSC) and multilevel converters.This Thesis deals with the measurement system for an M2C. Different methods to measure submodule capacitor voltages are analyzed, implemented on circuit boards and verified experimentally. The aim is to define the best approach to measure submodule capacitor voltages from reliability, speed,accuracy and simplicity point of view. Initially, a detailed study on the operation of a M2C is given in order to define the importance of having a fast and accurate measurement system of the submodule capacitor voltages. Secondly, a study of different methods to measure capacitor voltages is carried out. First the configuration of an ADC (Analog-to-Digital Converter) is presented and afterwards an alternative method based on voltage-to-frequency conversion is presented. Then, a research of the electronic components which are suitable to fulfill the demands of such measurement systems and which are available on the market has been carried out. In particular, two different families of components are examined; VCOs (Voltage Controlled Oscillators) and VFCs(Voltage-to-Frequency Converters). As next step, the description of the digital interface between the submodule and the Field Programmable Gate Array (FPGA) is given. The FPGA receives information about the submodule capacitor voltages. This process is programmed using VHDL (VHSIC-very-high-speed-integrated-circuit Hardware Description Language). Finally, a hardware implementation of the measurement systems is performed, in order to verify the effectiveness of the proposed methods.

With the rise in demand for electric vehicles increasing, the need for high efficiency electrification systems is in high demand. One challenge is keeping full output power to the electric drives as the vehicle battery drops. This thesis presents a GaN based three-phase semi-quasi-z-source boost inverter that can produce twice the output voltage of a traditional inverter without the need for a boost converter stage. This single stage approach is great when the AC output voltage is relatively low. A second approach presented in this paper is a novel GaN based composite boost converter topology which is made up of a very efficient unregulated converter topology with an integrated partial power voltage regulation stage. This approach offers the benefits of very high efficiency from the unregulated converter stage and the regulated output voltage with the voltage regulation stage. This design can offer an estimated efficiency up to 98.6%.

Nowadays, it is problematic to connect only one power semiconductor switch directly to the grid due to the high voltage range. In order to solve this difficulty, a new type of power converter has been introduced as a solution in high power applications. Multilevel Converters use high speed switching components, avoiding the problem of linking them directly to the grid by connecting single devices among multiple DC levels. Differents Multilevel topologies have been developed in the last few years. Multilevel Converters are more complex to modulate than the two level traditional converters because of the number of switching alternatives that are available. The latest and most promising such topology for high power applications is the Modular Multilevel Converter (M2C). Several control and modulation methods have been suggested for this topology. The aim of this master thesis project is to deeply investigate and evaluate one of them, based on a carrier phase-shifted Pulse Width Modulation (PWM) techniques. Four different control topologies using phase shift PWM techniques on M2C are studied and explored in this work. These topologies include the following loops of control: Averaging Control based on the currents inside the converter, Individual Balancing Control based on the output current and capacitors voltages, and Arm Balancing Control based on the voltage difference between the arms of the converter. The operation principle of an M2C is presented. This project proposes a switching frequency that meets the two required criteria: low enough to maintain cost feasibility, and high enough to reach a harmonic performance target. Additionally, this work proposes an analytic expression for the output voltage spectrum of the converter, which enables prediction of harmonic performance. Three distinct simulations were performed each one using different control topologies and switching frequencies. The first controller simulated took into account the Averaging Control topology, based on the circulating current. Within this topology both individual and arm balancing techniques are also explored. A second controller is also simulated using Averaging Control, based on the arms currents, as well as the other control loops. For the last case an Averaging Control, based on the arm currents, without the Arm Balancing is simulated. The results of each simulation are discussed and compared. Finally, these topologies are implemented and verified experimentally on a 10-KVA M2C prototype. The experiments are performed using only one phase and 11-level modulation methods. The controller efficiency is studied and verified through step response analysis.

HVDC transmission lines are a competitive and in some cases are proven to bea superior choice compared to AC transmission applications. Suitable convertershave been developed for that matter where Modulär Multilevel Converters (MMC)are highly preferred due to their low losses, no filtering requirementsand direct andfast control of AC and DC side. However, the overall eciency of the converteris higher than of a six pulse voltage source converter, it is still lower than the linecommutated converter type.In this master thesis an attempt to decrease the conduction losses of the MMC isinvestigated. A new cell structure design used in MMC is proposed along with itsassociated control strategy. The main idea is to divert the current at steady stateoperation through thyristors, which have lower conduction resistance than IGBTsthat are used in MMC topologies, at time intervals where the capacitor is bypassedfrom the cell. This new cell commutation is tested initially in the lab and thenthe whole structure operation is validated on a 3 phase MMC PSCAD model. Theresults from the lab confirmed the commutation of the new cell and the results fromthe 3 phase model showed that the new cell structure does not disturb the normaloperation of the MMC. A rough loss comparison that have been conducted betweenthe new cell structure and a half bridge that is used in a typical MMC, showed thatthe first one was less efficient. For that reason a generalized concept is introducedwhich promise higher efficiency than of the proposed concept.
HVDC transmission är ett fördelaktigt sätt att överföra eekt i jämförelse med ACtransmission. Omriktare har utvecklats för att passa applikationen, där ModularMultilevel Converters (MMC) har visat sig passa bra för HVDC på grund av de lågaförlusterna och dess obentliga krav på lter. Dessutom har de en direkt och snabbkontrollteori på både AC och DC sidan. Även om dess totala verkningsgrad är högreän hos six-pulse voltage source converter (VSC) men lägre än Line CommutatedConverter (LCC).Detta exjobb innefattar att minska ledningsförlusterna i MMCn. En ny designav cell strukturen föreslås, tillsammans med en passande kontrollteori. Idén äratt, på grund av dess lägre ledningsresistans använda tyristorer snarare än IGBTervilka annars är vanliga i MMCer, detta då kondensatorn är förbikopplad.. Dennya cellstrukturen testas initialt experimentellt i laboratorium och hela systemetvalideras genom simulering av en 3-fas MMC modell i PSCAD. De experimentellaresultaten bekräftade att den nya modellen fungerar och de simulerade resultatenvisar att den föreslagna topologin inte stör funktionen hos MMCn. En jämförelsemellan den nya topologin och den konventionella halvbridge strukturen har gjorts,där den föreslagna topologin hade lägre verkningsgrad. Istället har en generelltkoncept introducerats för att utlova en högre verkningsgrad än den först föreslagnatopologin.

Calculate solutions of a dynamic MMC energy-based model, when the system variables, i.e. the voltages and currents, are given as periodic signals. The signals are represented by a finite number distinct frequency components. As a result, the arm energies and cell voltages are given in this signal domain and can easily be translated to time domain as well.:cplx_series.m cplx_series_demo.m energy_series.m denergy_series.m check_symmetry.m transf2arm.m LICENSE.GNU_AGPLv3 sconv2.m

Nowadays high power density has become an emerging need for the medium-voltage (MV) high-power converters in applications of power distribution systems in urban areas and transportation carriers like ship, airplane, and so forth. The limited footprint or space resource cost such immensely high price that introducing expensive advanced equipment to save space becomes a cost-effective option. To this end, replacing conventional Si IGBT with the superior SiC MOSFET to elevate the power density of MV modular converters has been defined as the concentration of this research work. As the modular multilevel converter (MMC) is the most typical modular converter for high power applications, the research topic is narrowed down to study the SiC MOSFET-based MMC. Fundamentals of the MMC is firstly investigated by introducing a proposed state-space switching model, followed by unveiling all possible operation scenarios of the MMC. The lower-frequency energy fluctuation on passive components of the MMC is interpreted and prior-art approaches to overcome it are presented. By scrutinizing the converter's switching states, a new switching-cycle control (SCC) approach is proposed to balance the capacitor energy within one switching cycle is explored. An open-loop model-predictive method is leveraged to study the behavior of the SCC, and then a hybrid-current-mode (HCM) approach to realize the closed-loop SCC on hardware is proposed and verified in simulation. In order to achieve the hybrid-current-mode SCC (HCM-SCC), a high-performance Rogowski switch-current sensor (RSCS) is proposed and developed. As sensing the switching current is a critical necessity for HCM-SCC, the RSCS is designed to meet all the requirement for the control purposes. A PCB-embedded shielding design is proposed to improve the sensor accuracy under high dv/dt noises caused by the rapid switching transients of SiC MOSFET. The overall system and control validations have been conducted on a high-power MMC prototype. The basic unit of the MMC prototype is a SiC Power Electronics Building Block (PEBB) rated at 1 kV DC bus voltage. Owing to the proposed SCC, the PEBB development has achieved high power density with considerable reduction of passive component size. Finally, experimental results exhibit the excellent performance of the RSCS and the HCM-SCC.
Ph. D.

This thesis presents a longitudinal study of undergraduate achievement within a modular first degree course, analysing the academic records of a cohort of students who graduated from the Modular Degree Programme at Oxford Brookes University. Multilevel models are fitted to the marks achieved by members of this cohort in each module taken. Level 1 units are individual module entries, nested within occasions within individual student's programmes. These models were fitted by maximum likelihood and used to study the effects of both student and module characteristics on performance. The effects of these factors on mean marks, on the consistency of students performance and on the variation between students were studied by including complex variation at level 1 and random effects at student level in the models. In addition, individual progress charts were fitted, showing how patterns of progress vary from one student to another. Reviewing the hierarchical structure, it was found that a more complex, crossclassified structure is needed to represent the data accurately. This recognises that individual module entries are clustered within modules, as well as within students. Fitting large multilevel cross-classified models is computationally difficult, however newly developed MCMC estimation techniques allowed a model based on the more complex structure and including random effects and complex variation to be fitted. This analysis shows how MCMC estimation techniques can be used to fit a large cross-classified multilevel model, incorporating random effects and complex variation. The results obtained describe students' progress over the period of their degree course and measure the effects, other things being equal, of factors such as assessment methods, age and subject on mean levels of achievement, consistency of performance and the variation between students, providing a model for future studies of achievement within a modular framework.