Search

Medium frequency transformers in the power electronic system suffer from unusual electrical stress. With short rise time and higher frequency of the PWM voltage, interturn/layer overvoltage and overheating can be induced in the transformer's winding. Partial discharge may be triggered with higher probability and may lead to premature failure of the insulation. Therefore, it is necessary to improve the insulation capability of medium frequency transformers with respect to resisting partial discharge generation and deterioration under PWM-like voltage and high temperature. To achieve this goal, selection of better insulation material is critical. This paper presents comparative partial discharge inception voltage and endurance lifetime studies on three different materials including polyimide, polyester and Nomex paper, which are usually used for the insulation of medium frequency transformers. Results show that polyimide and polyester behave better than Nomex paper in resisting partial discharge inception. While polyimide shows longest endurance against continuous partial discharge.

Working under PWM voltage with fast voltage rise time and high frequency, medium frequency transformers may suffer from overvoltage and temperature rise in the winding. As a result, partial discharge may be triggered with high probability, threating the insulation reliability. Therefore, it is necessary to evaluate the insulation capability of medium frequency transformers with respect to partial discharge and deterioration. This paper introduces a test system based on power electronic modules that can generate repetitive pulse voltage. Then, by using this system, partial discharge inception voltage test under different temperatures and endurance lifetime test are conducted on Nomex paper, which is an insulation material usually used in medium frequency transformers. The results show the change trending of the partial discharge inception voltage of Nomex paper with temperature and indicate an accelerated aging of this material with the presence of continuous discharge.

Transformer design procedure may vary essentially in respect of the transformer type and its operating frequency (ranging between 50/60 Hz and a few megahertz). This paper presents a simple and straightforward method based on the optimal choice of core geometry of a high frequency transformer (HFT) used in Solid State Transformer (SST) applications. The core of SST is the HFT which largely influences its size and overall performance. The proposed design procedure for HFT focuses on optimizing the core geometry coefficient (in cm5) with a constraint inflicted on loss density. The core geometry coefficient has direct impact on the regulation and copper loss and the procedure results in a robust overall design with minimal footprint. Also, the procedure intends to bring all the operating parameters like regulation, losses and temperature rise within permissible limits while retaining desired efficiency. Thus an energy-efficient design is achieved with minimal footprint. The optimization procedure is implemented using recently developed Moth-flame Optimization (MFO) algorithm. The results of the MFO algorithm are compared with the wellestablished PSO technique. An experimental prototype is built to validate the findings.

The multi-winding medium frequency transformer (MMFT) has attracted widespread attention, since its application in power electronic transformers can simplify the system structure and reduce the volume and weight. However, the special working conditions of MMFT due to high voltage and high power density increase the difficulty of insulation design for MMFT. For this issue, this paper presents a comprehensive analysis of electric filed characteristics for MMFT. First, the electric field model of MMFT is established using the 2-D finite element method. Based on it, the influences of connection mode, core structure, and hollow winding on the electric field characteristics of MMFT are studied, including the overall electric field distribution, maximum electric field intensity, and the electric field intensity along a fixed path. The results show that there are differences in the maximum electric field intensity for different connection modes and different core structures, which provides references for MMFT insulation design. The proposed modeling method and analysis results in this paper are important for insulation improvement of MMFT.

The advancement and breakthrough in technology are associated with the characteristics of materials and their improvement in performance. In the context of this research, we focus on medium frequency applications up to a few tens of kHz. The objective is to evaluate the contribution of unconventional magnetic materials in the design and realisation of high-speed rotating machines or medium frequency transformer. Local and global systematic approaches will be carried out based on experimentation and modelling. Furthermore, original structures of machine will be studied for the best integration of specialties of materials and their implementation., Les avancées et les ruptures technologiques passent par la maîtrise des propriétés des matériaux et leurs améliorations. Nous envisageons dans le cadre de cette étude de se focaliser sur des applications moyenne fréquence allant jusqu'à quelques dizaines de kHz. L'objectif sera d'évaluer l'apport des matériaux magnétiques non conventionnels dans la conception et la réalisation de machines tournantes rapides ou encore de transformateur moyenne fréquence. Des approches locales et globales du matériau au système seront menées en s'appuyant sur l'expérimentation et la modélisation. De même des structures originales de machines seront recherchées pour intégrer au mieux les spécificités des matériaux et leur mise en œuvre.

The development of Medium Frequency Transformers (MFTs) from a novel perspective is essential for the advancement of today&acute, s various relevant applications such as the emerging solid-state transformers, along with interfaces for the interconnection of photovoltaic parks and electric vehicles. The analysis, design and implementation of MFTs pursuing the achievement of characteristics such as high power density, high efficiency, and a specific dispersion inductance is a key goal for designers. There are several parameters and design methods that influence the final performance of an MFT, such as the geometry and material of the core. The advantages/disadvantages of each material/geometry combination, about the dispersion inductance for instance, are not well known, even considering a single material but various geometries. This paper presents the analysis, design and experimental development of three nanocrystalline-core MFTs, each one with a different core geometry (toroidal, type CC and shell-type). The purpose of this work is to evaluate and compare the most favourable characteristics and performance of each type of geometry, tested at 5 kHz and 1.75 kVA. The cases studied, in simulation and experimentation with scaled prototypes, focus on evaluating the power density, the core losses, the winding losses, the geometric dimensions, and the dispersion inductance obtained in each MFT, as well as its performances operating with sinusoidal and square waveforms. The results show that: 1) the toroid core has higher efficiency, 2) the shell core has the lowest dispersion inductance and is easier to build, and 3) the CC type has the highest dispersion inductance. This new information is a step to further understand how to get more controllable, more efficient MFTS, with a higher power density and lower cost, depending on the intended application of cutting-edge DC-DC DAB-type converters.

The magnetizing inductance of the medium frequency transformer (MFT) impacts the performance of the isolated dc-dc power converters. The ferrite material is considered for high power transformers but it requires an assembly of type &ldquo, I&rdquo, cores resulting in a multi air gap structure of the magnetic core. The authors claim that the multiple air gaps are randomly distributed and that the average air gap length is unpredictable at the industrial design stage. As a consequence, the required effective magnetic permeability and the magnetizing inductance are difficult to achieve within reasonable error margins. This article presents the measurements of the equivalent B(H) and the equivalent magnetic permeability of two three-phase MFT prototypes. The measured equivalent B(H) is used in an FEM simulation and compared against a no load test of a 100 kW isolated dc-dc converter showing a good fit within a 10% error. Further analysis leads to the demonstration that the equivalent magnetic permeability and the average air gap length are nonlinear functions of the number of air gaps. The proposed exponential scaling function enables rapid estimation of the magnetizing inductance based on the ferrite material datasheet only.

The evolutions in power systems and electric vehicles, related to the economic opportunities and the environmental issues, bring the need of high power galvanically isolated DC-DC converter. The medium frequency transformer (MFT) is one of its key components, enabled by the increasing switching frequency of modern power semiconductors like silicon carbide transistors or diodes. The increased operating frequency offers small converter size, leading to the decrease in raw material usage. Most likely this will result in the converter cost reduction what will further increase the demand for solid state transformer solutions. The modelling and analysis are essential in the development of the MFT technology which is attracting lots of research and industrial interest.In this dissertation the isolated DC-DC converter topologies are introduced with the particular focus on the dual active bridge (DAB). The key components of the isolated DC-DC converters, power semiconductors and medium frequency transformer are reviewed.A mathematical model of a 3-phase MFT in the isolated DC-DC power converter, suitable in electromagnetic transient and steady state simulation is developed. The transformer modelling methods are reviewed and the Lagrange energy method is used to derive a physically motivated model for circuit analysis. The model involves a matrix of nonlinear magnetizing inductances and a matrix of linear leakage inductances, both including self and mutual values. The macroscopic models of magnetic hysteresis are reviewed and the feedback Preisach model is developed.The design of a 3-phase 20 kHz transformer for a 100 kW 1.2 kV isolated DC-DC power converter is presented. The particular focus is put on the winding and core design, and power loss and thermal calculations which are the most critical aspects of the high-power density transformer. The design results in two 3-phase MFT prototypes, first of its kind worldwide.A finite element model of the transformer is developed allowing to determine the magnetic flux characteristic Φ(Θ) and the related inductances required in the circuit model. The finite element model is based on the measured equivalent B(H) and homogenized material properties. Other model parameters are calculated analytically and compared against the measurement on the prototype MFT.The dissertation is concluded showing the technical feasibility and benefits of the 3-phase MFT. The developed MFT prototype operating at 20 kHz is more than 10 times lighter than the equivalent 50 Hz transformer. The 3-phase 100 kW DC-DC converter efficiency is measured 99.2% what is an impressive result. The efficiency of the 3-phase DC-DC is higher than its equivalent single-phase variant.A challenge of high power MFT design related to the parasitic air gaps in the core is highlighted. The influence of the air gaps on core power loss is confirmed showing that the increase in the air gap size in a certain range causes a decrease in the core power loss. In the 3-phase MFT prototype the parasitic air gaps do not cause any measurable effect on winding power loss and temperature. It is shown that the relative magnetic permeability is nonlinearly decreasing with the increase of the number of parasitic air gaps. An exponential interpolation function is proposed allowing to estimate the equivalent magnetic permeability, average air gap length and magnetizing inductance for any high-power ferrite core MFT with a similar core assembly.The proposed MFT equivalent circuit model is proven accurate in steady state and transient analyses. The no-load inrush test confirms the importance of the magnetic cross saturation involved in the magnetizing inductance model. The influence of the mutual leakage inductance on the operation of the DAB converter is shown. The feedback Preisach model of hysteresis is proven accurate in the modelling of hysteresis loops in the multi air gap ferrite core MFT., L'évolution des systèmes d'alimentation et des véhicules électriques, liée aux opportunités économiques et aux problèmes environnementaux, fait naître le besoin d'un convertisseur continu-continu à isolation galvanique de forte puissance. Le transformateur à moyenne fréquence (MFT) est l'un de ses composants clés, rendu possible par l'augmentation de la fréquence de commutation des semi-conducteurs de puissance modernes tels que les transistors au carbure de silicium ou les diodes. L'augmentation de la fréquence de fonctionnement permet de réduire la taille du convertisseur, ce qui entraîne une diminution de l'utilisation des matières premières. Il en résultera très probablement une réduction du coût des convertisseurs, ce qui augmentera encore la demande de solutions de transformation à l'état solide. La modélisation et l'analyse sont essentielles dans le développement de la technologie MFT qui suscite beaucoup d'intérêt dans la recherche et l'industrie.Dans cette thèse, les topologies de convertisseurs DC-DC isolés sont présentées avec un accent particulier sur le double pont actif (DAB). Les composants clés des convertisseurs DC-DC isolés, les semi-conducteurs de puissance et le transformateur moyenne fréquence sont passés en revue.Un modèle mathématique d'un MFT triphasé dans le convertisseur de puissance DC-DC isolé, adapté à la simulation électromagnétique transitoire et en régime permanent, est développé. Les méthodes de modélisation du transformateur sont passées en revue et la méthode de l'énergie de Lagrange est utilisée pour dériver un modèle physiquement motivé pour l'analyse du circuit. Le modèle implique une matrice d'inductances magnétisantes non linéaires et une matrice d'inductances de fuite linéaires, les deux incluant des valeurs propres et mutuelles. Les modèles macroscopiques d'hystérésis magnétique sont passés en revue et le modèle de Preisach à rétroaction est développé.La conception d'un transformateur triphasé de 20 kHz pour un convertisseur de puissance DC-DC isolé de 100 kW 1,2 kV est présentée. L'accent est mis sur la conception du bobinage et du noyau, ainsi que sur les calculs de perte de puissance et les calculs thermiques, qui sont les aspects les plus critiques du transformateur à haute densité de puissance. La conception aboutit à deux prototypes de MFT triphasés, les premiers de ce type dans le monde.Un modèle par éléments finis du transformateur est développé, permettant de déterminer la caractéristique du flux magnétique Φ(Θ) et les inductances associées requises dans le modèle de circuit. Le modèle par éléments finis est basé sur l'équivalent B(H) mesuré et les propriétés homogénéisées du matériau. Les autres paramètres du modèle sont calculés analytiquement et comparés à la mesure sur le prototype MFT.La conclusion de la dissertation montre la faisabilité technique et les avantages du MFT triphasé. Le prototype MFT développé fonctionnant à 20 kHz est plus de 10 fois plus léger que le transformateur 50 Hz équivalent. L'efficacité du convertisseur CC-CC triphasé de 100 kW est mesurée à 99,2%, ce qui est un résultat impressionnant. L'efficacité du convertisseur continu-continu triphasé est supérieure à celle de sa variante monophasée équivalente.Un défi de la conception de MFT de haute puissance lié aux entrefers parasites dans le noyau est mis en évidence. L'influence des entrefers sur la perte de puissance du noyau est confirmée, montrant que l'augmentation de la taille de l'entrefer dans une certaine gamme entraîne une diminution de la perte de puissance du noyau. Dans le prototype MFT triphasé, les entrefers parasites n'ont pas d'effet mesurable sur la perte de puissance et la température du bobinage. Il est démontré que la perméabilité magnétique relative diminue de façon non linéaire avec l'augmentation du nombre d'entrefers parasites. Une fonction d'interpolation exponentielle est proposée, permettant d'estimer la perméabilité magnétique équivalente, la longueur moyenne de l'entrefer et l'inductance magnétisante pour n'importe quel MFT à noyau de ferrite de forte puissance avec un assemblage de noyau similaire.Le modèle de circuit équivalent MFT proposé s'avère précis dans les analyses en régime permanent et transitoire. Le test d'appel sans charge confirme l'importance de la saturation croisée magnétique impliquée dans le modèle d'inductance magnétisante. L'influence de l'inductance de fuite mutuelle sur le fonctionnement du convertisseur DAB est démontrée. Le modèle d'hystérésis de Preisach à rétroaction s'est avéré précis dans la modélisation des boucles d'hystérésis dans le noyau de ferrite à entrefer multiple MFT., Postępujące zmiany w systemach elektroenergetycznych i pojazdach elektrycznych, związane z możliwościami ekonomicznymi i kwestiami środowiskowymi, wytworzyły zapotrzebowanie na izolowane galwanicznie przekształtniki DC-DC dużej mocy. Transformator średniej częstotliwości (MFT) jest jednym z ich kluczowych elementów. Technologia MFT rozwijana jest dzięki dostępności nowoczesnych półprzewodników mocy o coraz większej częstotliwości przełączeń, takich jak np. tranzystory i diody wykonane z węglika krzemu. Zwiększona częstotliwość pracy skutkuje małymi rozmiarami przekształtnika, co z kolei prowadzi do zmniejszenia zużycia materiałów konstrukcyjnych. Opracowanie technologii umożliwiających wieloseryjną produkcję przemysłową spowoduje dalszą redukcję kosztów przekształtnika, co dodatkowo zwiększy zapotrzebowanie na rozwiązania oparte na transformatorach z energoelektronicznymi przekształtnikami. Projektowanie oparte na modelowaniu, symulacji i analizie jest podstawą rozwoju technologii MFT i przyciąga coraz większe zainteresowanie w świecie nauki i przemysłu.W niniejszej rozprawie przedstawiono izolowane topologie przekształtników DC-DC, skupiając się na topologii DAB (ang. dual active bridge). Przeanalizowano kluczowe elementy przekształtników DC-DC takich jak półprzewodniki mocy i transformator średniej częstotliwości.Opracowano model matematyczny trójfazowego MFT zastosowany w izolowanym przekształtniku DC-DC. Model ten nadaje się do symulacji elektromagnetycznych stanów ustalonych i nieustalonych. Opisano różne metody modelowania transformatorów, a metoda Lagrange'a została wybrana do opracowania szczegółowego modelu obwodowego. Model transformatora uwzględnia macierz nieliniowych indukcyjności magnesujących oraz macierz liniowych indukcyjności rozproszenia. Obie macierze zawierają zarówno wartości własne i wzajemne. Opisano makroskopowe modele histerezy magnetycznej. Model Preisach'a ze sprzężeniem zwrotnym został opracowany dla analizowanego MFT.Przedstawiono projekt trójfazowego transformatora 20 kHz zastosowany w prototypowym przekształtniku DC-DC o mocy 100 kW i napięciu 1,2 kV. Szczególny nacisk położono na aspekty istotne przy projektowaniu transformatora o dużej gęstości mocy. Opisano proces projektowania konstrukcji uzwojenia i rdzenia transformatora, uwzględniając zagadnienia symulacji i obliczeń strat mocy oraz temperatur. Wynikiem projektu są dwa, unikatowe w skali światowej, w pełni funkcjonalne trójfazowe transformatory średniej częstotliwości.Opracowano model polowy transformatora, który umożliwia wyznaczenie charakterystyki strumienia magnetycznego w funkcji przepływu Φ(Θ). W zaproponowanym modelu obwodowym użyto indukcyjności wynikających z charakterystyki Φ(Θ). Opracowano metodę wyznaczania zastępczej charakterystyki magnesowania, w oparciu o pomiar na prototypie MFT. Zastępcza charakterystyka magnesowania została wykorzystana w uproszczonym modelu polowym MFT. Pozostałe parametry modelu obwodowego wyznaczono na podstawie zależności analitycznych. Poprawność metod i modeli została zweryfikowana w oparciu o porównanie wyników symulacji i pomiarów.Rozprawa wykazuje techniczną wykonalność i zalety trójfazowego MFT. Opracowany prototyp MFT pracujący przy 20 kHz jest ponad 10 razy lżejszy niż porównywalny transformator 50 Hz. Sprawność trójfazowego przekształtnika DC-DC 100kW wynosi 99,2%, co jest bardzo dobrym wynikiem. W oparciu o wyniki pomiarów stwierdzono, że sprawność trójfazowego przekształtnika DC-DC jest wyższa niż wariant jednofazowy.W rozprawie szczególny nacisk położono na analize konsekwencji konstrukcji wielosegmentowego rdzenia ferrytowego MFT dużej mocy. Konstrukcja związana jest z licznymi niepożądanymi szczelinami powietrznymi. Potwierdzono, że wzrost długości szczeliny powietrznej w pewnym zakresie, powoduje zmniejszenie strat mocy w rdzeniu. Na podstawie badań symulacyjnych i wyników pomiarów stwierdzono, że w analizowanym MFT, szczeliny powietrzne nie powodują żadnego mierzalnego wpływu na straty mocy i temperaturę uzwojenia. Wykazano, że przenikalność magnetyczna rdzenia zmniejsza się nieliniowo wraz ze wzrostem liczby szczelin powietrznych. Zaproponowano wykładniczą funkcję pozwalającą oszacować zastępczą przenikalność magnetyczną, średnią długość szczeliny powietrznej i indukcyjność magnesującą dla dowolnego MFT o podobnej konstrukcji rdzenia z materiału ferrytowego.Wykazano, że zaproponowany model obwodowy MFT pozwala na dokładną analizę stanów ustalonych i nieustalonych. Próba włączenia transformatora potwierdza istotny wpływ nasycenia magnetycznego pomiędzy kolumnami, które modelowane jest w macierzy indukcyjności magnesującej, na wartość prądu udarowego. Wykazano wpływ wzajemnej indukcyjności rozproszenia na pracę przekształtnika DAB. Wykazano, że model histerezy Preisach'a ze sprzężeniem zwrotnym charakteryzuje się dużą dokładnością w modelowaniu pętli histerezy rdzenia ferrytowego MFT z wieloma szczelinami powietrznymi.

Medium Frequency Transformers (MFT) are an innovative technology compared to low frequency transformers, with the promise of reduced volume and increased efficiency. This PhD thesis focuses in particular on their design for high voltage, high power applications, such as high voltage and medium voltage DC networks, as well as railway networks. In these applications, MFTs are used in converters that can generate specific constraints to be taken into account during their design: non-sinusoidal signals, polarization voltage, target inductance values. Moreover, the technological choices currently available for the realization of MFTs are numerous, and there is currently no consensus on any technology for any given application. Trends could be identified using a tool to classify MFT designs from the literature. Thus, the most promising technologies were selected and retained for the future. Based on these technologies, a design methodology was developed to quickly and semi-automatically design and compare MFTs with different technological choices. It consists of three steps: pre-design, analytical design, and validation. The complete analytical design of the MFT with different technological choices is carried out using an automated design tool developed during this thesis, named SUITED (SUpergrid Institute TransformEr Design). This methodology requires models and data for each of the components and phenomena of the MFT. Concerning the magnetic core, a review and selection of models from the literature were carried out for the evaluation of the magnetizing inductance and magnetic losses. In addition, magnetic characterizations have made it possible to highlight the impact of certain technological processes on the levels of loss of magnetic cores made of nanocrystalline material, which is an excellent candidate for MFTs. Concerning the windings, analytical models to calculate the magnetic field, leakage inductance and skin and proximity effects were developed and compared with those in the literature and simulations. These models are proving to be more accurate on the MFT geometries considered. On top of that, a new method for evaluating the parasitic capacitances of windings with rectangular turns has been successfully implemented and validated. Thermal networks have been identified for the different MFT geometries. The thermal resistances of conduction, convection and radiation are calculated from detailed models. In particular, the anisotropy of materials is taken into account for thermal conduction, and the convection coefficients are evaluated via different correlations for each face of the MFT. The thermal networks are then solved iteratively and analytically to take into account the non-linearity of the thermal resistances while optimizing the required computation time. Finally, this entire design methodology was applied to three case studies corresponding to the target applications: high voltage, medium voltage and rail. The results obtained do show the performance and necessity of this approach., Les Transformateurs Moyenne Fréquence (TMF) sont une technologie innovante par rapport aux transformateurs basse fréquence, avec la promesse d’une réduction de volume et d’une augmentation du rendement. Cette thèse s’intéresse en particulier à leur conception pour des applications haute tension forte puissance, telles que les réseaux haute tension et moyenne tension à courant continu, ainsi que les réseaux ferroviaires. Dans ces applications, les TMF sont utilisés au sein de convertisseurs pouvant générer des contraintes spécifiques à prendre en compte durant leur conception : signaux non-sinusoïdaux, tension de polarisation, valeurs d’inductances cibles. De plus, les choix technologiques actuellement disponibles pour la réalisation des TMF sont nombreux, et aucun de ces choix ne fait actuellement consensus quelle que soit l’application visée. Des tendances ont pu être identifiées à l’aide d’un outil de classification des designs de TMF issus de la littérature. Ainsi, les technologies les plus prometteuses ont été sélectionnées et retenues pour la suite. A partir de ces technologies, une méthodologie de conception permettant de concevoir et comparer rapidement et ce de façon semi-automatique des TMF avec différents choix technologiques a été mise en place. Elle est constituée de trois étapes : pré-design, design analytique et validation. Le design analytique complet du TMF avec différents choix technologiques est réalisé à l’aide d’un outil de conception automatisée développé durant cette thèse, que nous avons nommé SUITED (SUpergrid Institute TransformEr Design). Cette méthodologie requiert des modèles et données pour chacun des composants et phénomènes du TMF. Concernant le noyau magnétique, une revue et une sélection de modèles issus de la littérature ont été effectuées pour l’évaluation de l’inductance magnétisante et des pertes magnétiques. De plus, des caractérisations magnétiques ont permis de mettre en évidence l’impact de certains procédés technologiques sur les niveaux de pertes de noyaux magnétiques en matériau nanocristallin, qui est un excellent candidat pour les TMF. Au niveau des bobinages, des modèles analytiques pour calculer le champ magnétique, l’inductance de fuite et les effets de peau et de proximité ont été développés et comparés avec ceux de la littérature ainsi que des simulations. Ces nouveaux modèles s'avèrent obtenir une meilleure précision sur les géométries de TMF considérées que ceux de la littérature. De plus, une nouvelle méthode d’évaluation des capacités parasites de bobinages à spires rectangulaires a été mise en place et validée avec succès. Des schémas thermiques équivalents ont été identifiés pour différentes géométries de TMF. Les résistances thermiques de conduction, convection et radiation sont calculées à partir de modèles détaillés. En particulier, l’anisotropie des matériaux est prise en compte pour la conduction thermique, et les coefficients de convection sont évalués via des corrélations différentes pour chaque face du TMF. Les schémas thermiques sont ensuite résolus de façon itérative et analytique pour prendre en compte les non-linéarités des résistances thermiques tout en optimisant le temps de calcul nécessaire. Enfin, l’ensemble de la méthodologie de conception mise en place a été appliqué sur trois cas d’études correspondant à des applications cibles : haute tension, moyenne tension et ferroviaire. Les résultats obtenus montrent effectivement la performance et la nécessité de cette approche

International audience; The article presents a three-phase Medium Frequency Transformer being a part of a 100kW 1.2kV 20kHz Dual Active Bridge DC-DC converter. The transformer design is detailed focusing on winding and core power loss calculation. The high power three-phase MFT prototype is presented. The experimental results include the transformer impedance characteristics, no load test and three-phase DAB full load test waveforms.

High performance, highly efficient DC-DC converters play a key role in improving the penetration of renewable energy sources in the context of smart grids in applications such as solid-state transformers, built-in power drives in electric vehicles and interfacing photovoltaic and wind-power systems. Advanced medium-frequency transformers (MFTs) are fundamental to enhance DC-DC converters and determining its behavior, therefore MFT design procedures have become increasingly important in this context. This paper investigates which type of core material, between nanocrystalline and silicon steel, has the best properties for designing MFTs for distinct applications. Unlike to other proposals, in this work, two 1 kVA-120 V/240 V-1 kHz lab MFT prototypes, with a different type of core material, are developed for the purpose of comparing its physical characteristics, behavior, and performance under real-life conditions. A final section, the experimental results show that the nanocrystalline MFT has greater power density and efficiency. The results of this work introduce nanocrystalline MFTs as an option in a wider range of applications in niches in which other materials are currently used.

Medium-frequency (MF) transformer has gained much popularity in power conversion systems. Temperature control is a paramount concern, as the unexpected high temperature declines the safety and life expectancy of transformer. The scrutiny of losses and thermal-fluid behavior are thereby critical for the design of MF transformers. This paper proposes a coupled, semi-numerical model for electromagnetic and thermal-fluid analysis of MF oil natural air natural (ONAN) transformer. An analytical model that is based on spatial distribution of flux density and AC factor is exploited to calculate the system losses, while the thermal-hydraulic behavior is modelled numerically leveraging the computational fluid dynamics (CFD) method. A close-loop iterative framework is formulated by coupling the analytical model-based electromagnetic analysis and CFD-based thermal-fluid analysis to address the temperature dependence. Experiments are performed on two transformer prototypes with different conductor types and physical geometries for validation purpose. Results suggest that the proposed model can accurately model the AC effects, losses, and the temperature rises at different system components. The proposed model is computationally more efficient than the full numerical method but it reserves accurate thermal-hydraulic characterization, thus it is promising for engineering utilization. Published version

Novel high-power medium-voltage converter technologies offering galvanic insulation are needed to support the development of the emerging medium-voltage direct-current grids and further improve the performance of various other applications such as traction, renewable energy and e-mobility. With the recent advancements in the power semiconductor industry, resulting in faster and more efficient switches with extended voltage and current capabilities, these converters, often referred to as solid state transformers (SSTs) or power electronic transformers (PETs), have become increasingly attractive. Besides the power semiconductor modules, the central component of any such converter, having a significant impact on its efficiency and power density, is the medium frequency transformer (MFT) that provides the necessary galvanic insulation and input-output voltage matching. However, the progress of the magnetic components has not been following the same pace as the semiconductor industry. Unlike the traditional line frequency transformers (LFTs), MFTs have not yet reached the technological maturity, thus leaving many areas open for research. To that end, this thesis focuses on the technical challenges tied to modeling and design optimization of the MFTs for the emerging SSTs. The available technologies and materials, suitable for medium frequency operation are identified and classified in respect to different application requirements. A detailed analysis and modeling of all the relevant phenomena governing the MFT electrical behavior as well as limiting the operation and design range is performed. A synthesis of all these models is done in form of a design optimization algorithm capable of generating the set of all feasible transformer designs. Moreover, design filters are developed allowing to interactively search for the most preferable design alternatives in terms of hot-spot temperatures, weight, volume and efficiency. As a proof of concept, a 100kW, 10kHz MFT prototype has been realized according to the optimal specifications resulting from the proposed design optimization tool. The accuracy of the utilized models was confirmed via thorough testing, including: electric parameter identification, partial-discharge test, full-power loading test and an extensive thermal run within the realized back-to-back resonant test setup. With the established design methodology and reliable models, a technology coordination and an MFT design sensitivity study has been performed on the SST level. A 0.5MW, 10kV input-series output-parallel series resonant converter based topology has been selected for the case study, taking into account different available semiconductor ratings and the resulting converter modularity. This study has shown that, considering the available technologies and materials, the expected MFT power density reaches its apex at around 10 −20kHz. While modern wide-band-gap semiconductors will for sure increase the efficiency and power density of the converter stage, this result indicates that further size reduction of the magnetic components above these frequencies will only be possible through improvements in the materials - providing core and winding materials with better high frequency loss characteristic.

The interest in the hybrid electric aircraft propulsion is increasing. Different goals must be guaranteed for a correct performance of an electric propulsion system for aircraft: reliability, low weights, performance in critical conditions. The paper is focused on the thermal analysis of the medium/high frequency core transformers used for the coupling between the battery storage system and the electric motor in electric aircrafts. A multi-physics simulation is carried out to solve the transient thermal problem for the transformer during electrical assisted phase of the aircraft.

One of the most important innovation expectation in railway electrical equipment is the replacement of the on-board transformer with a high power converter. Since the transformer operates at line-frequency (i.e., 50 Hz or 16 2/3 Hz), it represents a critical component from weight point of view and, moreover, it is characterized by quite poor efficiency. High power converters for this application are characterized by a medium frequency inductive coupling and are commonly referred as Power Electronic Transformers (PET), Medium Frequency Topologies or Solid-State Transformers (SST). Many studies were carried out and various prototypes were realized until now, however, the realization of such a system has some difficulties, mainly related to the high input voltage (i.e., 25 kV for 50 Hz lines and 15 kV for 16 2/3 Hz lines) and the limited performance of available power electronic switches. The aim of this study is to present a survey on the main solutions proposed in the technical literature and, analyzing pros and cons of these studies, to introduce new possible circuit topologies for this application.

At the interface of Power Electronics and High Voltage Engineering, this work combines theoretical considerations and experimental investigations to assess the endurance of polymeric insulation exposed to inverter-type medium-voltage stress, which is characterized by amplitudes in the kilovolt range, switching frequencies of several (tens of) kilohertz and slew rates of several kilovolts per microsecond.

This paper is introducing a new method of operation for a series resonant converter, with intended application in megawatt high-voltage dc wind turbines. Compared to a frequency controlled series resonant converter operated in subresonant mode, the method (entitled pulse removal technique) allows the design of the medium frequency transformer for highest switching frequency, while being operated at lower frequency without saturation. The main focus of this paper is to identify and analyze the operating modes of the converter with pulse removal technique. With the use of variable frequency and variable phase displacement in subresonant mode, the new method of operation promises transformer size reduction and facilitates soft-switching transition of the insulated gate bipolar transistors (IGBTs) and line frequency diodes on rectifier side. Four modes of operation are identified, while equations for output power, voltage, and current stress are identified. Experimental results are concluded on a 1 kW, 250 V/500 V prototype.

A new modulation scheme is introduced for a single-phase series-resonant converter, which permits continuous regulation of power from nominal level to zero, in presence of variable input and output dc voltage levels. Rearranging the circuit to locate the resonant LC tank on the rectifier side of the high turns-ratio transformer combined with frequency control and phase-shifted inverter modulation keep transformer flux constant from nominal frequency down to dc, always in subresonant continuous or discontinuous conduction mode. This overcomes the principal deficit of series-resonant converters, and the resulting compact and efficient transformer, and soft-commutated inverter, present particular advantages in high-power, high-voltage applications, such as dc offshore wind turbines. With transformer excitation frequency in hundreds of Hz range, line-frequency diodes can be employed in the high-voltage rectifier valve. Circuit operation and conduction modes, governing equations, and sample waveforms are presented, together with experiments from a scaled demonstrator.

Medium frequency transformers (MFTs) are a key component of DC&ndash, DC dual active bridge (DAB)-type converters. These technologies are becoming a quintessential part of renewable energy solutions, such as photovoltaic systems and wind energy power plants, as well as in modern power grid interfaces functioning as solid-state transformers in smart-grid environments. The weight and physical dimensions of an MFT are key data for the design of these devices. The size of an MFT is reduced by increasing its operating frequency. This reduction implicates higher power density through the transformer windings, as well as other design requirements distinct to those used for conventional 60/50 Hz transformers, therefore, new MFT design procedures are needed. This paper introduces a novel methodology for designing MFTs, using nanocrystalline cores, and tests it using an MFT&ndash, DAB lab prototype. Different to other MFT design procedures, this new design approach uses a modified version of the area-product technique, which consists of smartly modifying the core losses computation, and includes nanocrystalline cores. The core losses computation is supported by a full analysis of the dispersion inductance. For purposes of validation, a model MFT connected to a DAB converter is simulated in Matlab-Simulink (The MathWorks, v2014a, Mexico City, Mexico). In addition, a MFT&ndash, DAB lab prototype (1 kVA at 5 kHz) is implemented to experimentally probe further the validity of the methodology just proposed. These results demonstrate that the analytic calculations results match those obtained from simulations and lab experiments. In all cases, the efficiency of the MFT is greater than 99%.