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In this work, a novel superconducting (SC) inductor topology for an axial flux synchronous machine is presented and tested. The proposed device combines HTS YBaCuO bulks and coils supplied with DC current to create a variable air gap flux density distribution. In fact, the two SC bulks modulate and redirect the flux lines produced by the coil thanks to their magnetic field shielding property. This results in a periodic space variation of the axial component of the flux density. A 3D electromagnetic modeling based on a finite element solution is developed to demonstrate the relevance of using the magnetic shielding properties of SC bulks. In order to verify the screening properties of the SC bulk, a prototype of the proposed inductor was constructed and tested in the laboratory.

A new device, referred to as Superconducting Power Filter (ScPF), is introduced to improve the stability of aircraft electric DC grids. This device is made of a non-inductive superconducting coil connected in series with a conventional RLC filter. The superconducting coil behaves as a nonlinear current-dependent resistance. By design, the superconductor does not undergo a full transition to the normal-resistive state and remains in the current-sharing regime. Thus, a quick response and recovery of the superconductor to transients can be achieved. In the present work, the thermo-electric model of the device incorporates a refined description of the transition from the superconducting state to the normal-resistive state of the superconductor. To check the feasibility of passive stabilization of embedded DC grids using ScPF, different widths and lengths of commercially available 2 $^\text{nd}$ generation High Temperature Superconducting (2G HTS) tapes were considered. We chose a case study simulating an equivalent circuit of a More Electric Aircraft (MEA) with an EPS-A3 architecture. For this particular case study, it is shown that the stability limit of the DC grid can be significantly increased, while preserving the reliability (no quench) and low cost (only a few meters of superconducting tape) of the ScPF.

International audience; This article presents a semianalytical modeling approach for ac losses computation in high-temperature superconducting (HTS) stacks and coils near ferromagnetic parts. The Brandt's model is combined with the equivalent current model to take into account of the proximity of ferromagnetic materials with finite relative permeability. Large stacks of HTS tapes are considered. A homogenization is applied to reduce the computing time. The obtained results are compared with finite element analysis. Very good agreements are obtained on both local and global quantities.

International audience; In this article, we developed a 3-D integro-differential model in the time domain for the electromagnetic field computation in high-temperature superconductive (HTS) composite materials which are characterized by a nonlinear anisotropic electrical conductivity and multiscale dimensions. The model is formulated in terms of the electric vector potential and discretized by using the finite-difference method coupled to a collocation method for the integral part. Only the active parts of the system are discretized. A relaxation method is applied to improve the convergence of the solution and to reduce the computation time. In the aim validation, a numerical example is considered where the numerical evaluation of ac losses in a Bi-Sr-Ca-Cu-O type HTS tape is compared to an analytical calculation using measured parameters.

Superconducting YBa2Cu3Oy (YBCO) foams were prepared using commercial open-cell, polyurethane foams as starting material to form ceramic Y2BaCuO5 foams which are then converted into superconducting YBCO by using the infiltration growth process. For modelling the superconducting and mechanical properties of the foam samples, a Kelvin-type cell may be employed as a first approach as reported in the literature for pure polyurethane foams. The results of a first modelling attempt in this direction are presented concerning an estimation of the possible trapped fields (TFs) and are compared to experimental results at 77 K. This simple modelling revealed already useful information concerning the best suited foam structure to realize large TF values, but it also became obvious that for various other parameters like magnetostriction, mechanical strength, percolative current flow and the details of the TF distribution, a refined model of a superconducting foam sample incorporating the real sample structure must be considered. Thus, a proper description of the specific microstructure of the superconducting YBCO foams is required. To obtain a set of reliable data, YBCO foam samples were investigated using optical microscopy, scanning electron microscopy and electron backscatter diffraction (EBSD). A variety of parameters including the size and shape of the cells and windows, the length and shape of the foam struts or ligaments and the respective intersection angles were determined to better describe the real foam structure. The investigation of the foam microstructures revealed not only the differences to the original polymer foams used as base material, but also provided further insights to the infiltration growth process via the large amount of internal surface in a foam sample.

This paper presents a fast-integral modeling approach for the design and characterization of large-scale non-inductive high temperature superconducting (HTS) coils made of first-generation HTS tape. The integral modeling is combined with a strategy of far tape approximation allowing to model a reduced set of consecutive tapes instead of the entire coil. It makes it possible to estimate the losses and the current density distribution in the coil, under different current supply. This leads to a considerable reduction in the computation time. The calculated AC losses show a good agreement with the Norris ellipse formula and measurements, highlighting the importance of considering the elliptical section of the superconductor in the tape, in the modeling. The developed modeling approach is in particular useful in the AC + DC mode, where Norris’s formulas cannot be used to determine the losses. Besides, some interesting phenomena have been observed, related to the current distribution in the tapes at different AC and DC current values.

We compare the temperature and field dependence of the critical current densities of high- $T_c$ superconductor materials intended for various bulk applications such as trapped-field magnets. This comprises bulk samples of YBa $_2$ Cu $_3$ O $_x$ (YBCO), MgB $_2$ , and iron-based materials, including also various versions of the YBCO compound such as melt-textured ones, infiltration-growth processed ones, and YBCO foams. Critical current densities and flux pinning forces were obtained from magnetization loops measured using Quantum Design SQUID and physical property measurement system (PPMS) systems with applied magnetic fields of up to $\pm$ 9 T. The obtained data are compared to each other with respect of the optimal cooling temperature possible using modern cryocoolers. Furthermore, we plot the temperature dependencies of the critical current densities versus the normalized temperature $t=T/T_c$ . This enables a direct judgement of the performance of the material in the trapped-field applications.

High-Temperature Superconductors (HTS) considerably accelerate the development of superconducting machines for electrical engineering applications such as fully electrical aircraft. This present contribution is an overview of different superconducting materials that can be used as magnetic screens for the inductor of high specific power electrical machines. The impact of the material properties, such as the critical temperature (Tc) and the critical current density (Jc), on the machine performances is evaluated. In addition, the relevance to flux modulation machines of different HTS bulk synthesis methods are addressed.

International audience; Electrical machines are important parts of different power systems. The application of high temperature superconductors (HTS) in electrical machines is very promising due to high transport currents. This paper reviews various topologies of superconducting motors and generators using HTS published in the literature in recent time. It begins with a brief presentation of the HTS material used in electrical machines. The description of AC losses and cryogenic systems is done afterwards. Then we offer a striking description of the various realizations of HTS electrical machines such as half HTS synchronous machines, fully HTS synchronous machine, machines with HTS bulks and stacks. Some of these machines are totally innovative compared to conventional ones and their operating principle is strictly related to the presence of HTS materials.

National audience

International audience; For possible applications as trapped field (TF) magnets, it is essential to fabricate large, polycrystalline bulk samples from the FeSe compound, the simplest high-Tc superconductor (HTSc) possible. FeSe is relatively cheap to prepare, and does not contain any rare-earth material. The grain boundaries in this compound are not acting as weak links as it is the case for the YBCO compound. Although the transition temperature, Tc, is just below 10 K, the upper critical fields are comparable with other HTSc. Preparing the FeSe samples using solid-state sintering yields samples exhibiting strong magnetic hysteresis loops (MHLs), and the superconducting contribution is only visible after subtracting MHLs from above Tc. Due to the complicated phase diagram [1], the samples are a mixture of several phases, α-FeSe, β-FeSe, δ-FeSe (Fe7Se8) and metallic α-Fe [2]. The amount of the latter two phases depends directly on the Se loss during the sintering process. The δ-FeSe is antiferromagnetic, and α-Fe is ferromagnetic [3]. In the present contribution, we show MHLs of a variety of samples measured up to ±7 T and determine the magnetic characteristics, together with the amount of superconductivity determined from M(T) measurements. We performed a thorough analysis of the microstructures using polarization microscopy, Kerr effect, MFM, SEM, EBSD and TEM in order to establish a relation between microstructure and sample properties. To prepare good superconducting samples, the presence of the (anti)ferromagnetic phases must be reduced by carefully adjusting the Se content using Ti foils as getter materials. Measuring magnetoresistance of these samples [4] implies that the samples are always cooled in the own local field, and thus, the analysis of the resistance data calculating the fluctuation-induced conductivity above Tc [5] is strongly affected by this local magnetic field. We demonstrate the importance of preparing phase-pure FeSe samples, which are essential for the various applications envisaged. This work is part of the SUPERFOAM international project funded by ANR and DFG under the references ANR-17-CE05-0030 and DFG-ANR Ko2323-10, respectively. References [1] H. Okamoto, J. Phase Equilibria 12, 383 (1991). [2] P. Diko et al., Physica C 476, 29 (2012). [3] A. J. Williams, T. M. McQueen, R. J. Cava, Solid State Commun.149, 1507 (2009). [4] T- Karwoth et al., J. Phys. Conf. Ser. 1054, 012018 (2018). [5] A. Almessiere et al., Mat. Chem. Phys. 243, 122665 (2020).

International audience; The development of the sintering process for the synthetization of superconducting Iron-selenide (FeSe) polycrystal enables a quick and cheap fabrication of superconducting bulks for trapped field applications without the use of Rare Earths or toxic elements. A relevant number of papers were published in the literature in order to determine the best way to produce a pure β-phase, responsible of the superconductivity without any other non-desirable phases (α-FeSe, unreacted Fe, FeSe – also called δ-FeSe). Using the collected information, we produced a pure β-phase by classic sintering process at temperatures above 685°Cfollowed by annealing below 450°C for optimizing the sample and improvement of the critical currents. Using the Spark Plasma Sintering method enables an improved homogeneity and a better grain coupling, which leads to a higher quality for FeSe superconducting bulks and an increased sample density. The consequence is another increase of the critical currents which makes the samples competitive for trapped bulks applications.

A new application of high temperature superconducting technology for HVDC grids is introduced. The device referred to as “superconducting power filter” (ScPF) aims at increasing the stability of HVDC grids by adding a current-dependent resistance to the grid. In comparison with other stabilization techniques, a ScPF achieves a fully passive stabilization with virtually no losses in nominal operation. To clarify its feasibility for HVDC grids, the CIGRE B4 DSC1 benchmark is considered. Using an electrical-thermal model of the device, the stability of the DC grid is numerically assessed. The first steps towards an experimental proof of concept are presented.

In this paper, a three-dimensional semi-analytical approach, based on the volume integral method, is developed to compute the forces and torque in fully high temperature superconducting (HTS) ironless axial flux machine (IAFM). The use of particular volumetric basis elements helps to speed up the computation of the magnetic field. The Maxwell stress tensor method is used for the force and torque calculations, where the electromagnetic state of the superconductor is taken into account. The developed model is then introduced into a multi objective optimization procedure, based on the genetic algorithms, to maximize the magnetic torque and minimize the superconducting wire length in the considered structure, while respecting the constraints linked to the physical and mechanical properties of the considered HTS tape. It is shown that the considered fully HTS machine has a higher torque density compared to conventional one.

International audience; PEM Fuel Cells (PEMFCs), fueled by hydrogen, are electrochemical devices that convert hydrogen to useful power and two by-products: heat and water. They cover an important part of power applications namely in the transportation area, and in other practical applications that are either stationary or portable. In particular, the domestic refrigerator is one of the daily and indispensable applications but with a high-energy demand due to the high running time cycles. This work is a technical assessment of the feasibility of building a coupled "PEM Fuel Cell - Refrigerator" system. Real technical data for the refrigerator are collected, processed and evaluated. The obtained results show reasonable flows consumption rates. In fact, the refrigerator requires a flow rate of 1.607 slpm of hydrogen and 8 slpm of air at a pressure of respectively 3 atm and 1 atm. The water is produced at a rate of 1.285 10(-3) slpm. The annual amount of hydrogen consumed by the refrigerator is estimated to 28, 47 kg. The energy provided to the refrigerator is about 130 W and the energy needed by the air compressor is 28, 24 W. A technical solution is suggested at the end of this work to reduce the start and stop cycles of the fuel cell.

International audience

In this paper, a fast and realistic approach is proposed for the DC modeling of superconducting coils made of first-generation (1G) high temperature superconducting (HTS) tapes, taking into consideration the non-uniformity of the electric current density distribution in the HTS tapes cross-section, by using a power minimization criterion with the dependence of the critical current density on the magnetic flux density (MFD). Integral equations are used to evaluate the MFD, enabling to discretize only the active parts of the system. The model is then combined with an inverse procedure for the characterization of non-inductive HTS coils using measurements.

International audience; With the objective to concurrence the well-known melt texturing YBCO samples in term of irreversible field and critical current, infiltration growth YBCO bulks (IG-processed) show a recent increase of interest due to their advantage of better controlling the shape of the final product and better mechanical stability. Two types of IG-processed YBCO samples were developed: standard bulk samples and superconducting foams which show the advantage of a huge cooling capacity and a low weight due to his low density, particularly appreciated for space applications. Intended for use as permanent superconducting magnets (trapped field (TF) magnets), precedents works were done to evaluate the pinning properties of such materials by pinning force scaling (Dew-Hughes and Kramer) but only under relatively low magnetic field (up to 9 T). Considering a very high irreversible field (Hirr), even at 77 K, a lot of assumptions and extrapolations were needed. Small pieces of both designs (standard bulk and foam) were cut and mechanically polished (1 x 1 x 0.1 mm3) according to the crystallographic orientation (H // c). To experimentally determine the pinning properties for magnetic field above 9 T, we were able to make measurements at high magnetic field (above 30 T) for different temperatures between 85 K and 40 K with the help of EMFL facilities using Cantilever Torque Magnetometry at the High Magnetic Field Laboratory of Nijmegen.

Poster; International audience; Pieces of bulk, superconducting FeSe samples prepared by solid-state sintering were irradiated with 2.5 MeV electrons (T = 23.5 K) at SIRIUS facility using two different fluences, 2×10^19 electrons/cm2 and 4×10^19 electrons/cm2. In contrast to previous work performed on FeSe single crystals, the bulk FeSe samples are polycrystalline without specific texture and exhibit a density of ~4 g/cm3 (= 78% of theoret. density) due to pores. Furthermore, the samples contain magnetic impurities which lead to a ferromagnetic signal of the sample. Such bulk FeSe samples are intended as base material for superconducting trapped field magnets operating at 5 K, with the benefit of a higher upper critical field, Hc2, and reduced flux jump probability as compared to MgB2. The electron irradiation introduced point defects to the FeSe grains. As result, the superconducting transition temperature, Tc, is slightly reduced, depending on the fluence, but the critical currents are increased by about 20-30%. We discuss the changes of Tc, of the magnetic background and the superconducting properties of the samples before and after the electron irradiation.

Poster; International audience; We suggest the use of superconducting trapped field magnets (TF) for Flux-Pinning Docking Interfaces (FPDI) in satellite systems. Previous work introducing the FPDI concept to cube-sat satellites was using bulk high-Tc superconductors and permanent magnets of Nd-Fe-B. This arrangement was proven to work both experimentally and theoretically. However, such FPDIs have limited magnetic field strength, consist of heavy-weight material, and can only be used with a single purpose. Replacing the magnetic material in the FPDI by a TF-magnet would enable the interface to operate for both purposes, i.e., generating a (stronger) magnetic field and trapping it. We show the requirements for such a system and discuss the possible gains when using a TF-FPDI in satellites. A further possibility to reduce the system weight would be the use of superconducting foams as superconducting material.

Purpose The purpose of this paper is to develop a rapid and realistic modelling approach for the design and characterization of high temperature superconducting (HTS) coils and windings carrying DC currents. Indeed, the strong dependence of the electromagnetic properties of such materials on the magnetic field makes the design and characterization of HTS systems a delicate operation where local quantities have to be evaluated. Design/methodology/approach A volume integral modelling approach has been developed taking into account the electric nonlinearity of the HTS material which is represented by power law. The variations of the characteristic quantities of the HTS (critical current density and power law exponent) with the magnetic flux density are also taken into account by using Kim’s law. The volume integral modelling allows to model only the active parts of the system and thus to overcome the difficulties linked to the multiscale dimensions. Findings The model has been tested in a case study in which simulation results were compared to measurements and to finite element analysis. A good agreement was found which validates the model as a rapid and efficient tool for HTS coils and windings design and modelling. Practical implications HTS coils are important elements of emerging superconducting devices which require a high level of reliability, such as generators or motors. The proposed approach is interesting to speed up the design and optimization procedures of such systems. Originality/value Advanced structures of the basic elements have been used in the volume integral modelling, which results in a considerable gain in computation time and in memory-space saving while keeping a high level of precision and realism of the modelling, which has been verified experimentally.

Pour faire face au caractère intermittent des énergies renouvelables (EnR), la production et le stockage de l'hydrogène sont considérés comme une solution intéressante. De nos jours, les EnR (éoliennes, photovoltaïques, etc.) et les combustibles fossiles (charbon, gaz naturel, etc.) sont les diverses ressources disponibles sur Terre pour produire de l'hydrogène. L'électrolyse de l'eau est l'un des moyens les plus attractifs pour produire de l'hydrogène à partir d'EnR; cette réaction électrochimique est réalisée par un électrolyseur. Généralement, les convertisseurs DC-DC de type abaisseur sont utilisés dans les systèmes de production d'hydrogène car les électrolyseurs doivent être alimentés avec une tension continue très basse pour produire de l'hydrogène à partir d'eau dé-ionisée. Bien que ces convertisseurs soient largement utilisés, ils présentent plusieurs inconvénients du point de vue des ondulations du courant de sortie et de l'efficacité énergétique. Afin de répondre aux exigences des applications électrolyseur, un convertisseur DC-DC entrelacé amélioré a été utilisé pour effectuer ce travail de recherche. L'objectif de cet article est de développer une loi de commande du convertisseur étudié sur la base du courant traversant l'une des phases du convertisseur. Cette commande permet de minimiser le dépassement de courant de phase et les oscillations, et par conséquent les pertes de conduction. Enfin, une étude expérimentale a permis d'évaluer la performance de la commande en courant du point de vue des performances de l'électrolyseur.

International audience; The FeSe compound is the simplest high-temperature superconductor (HTSc) possible, and relatively cheap, not containing any rare-earth material. Although the transition temperature, Tc, is just below 10 K, the upper critical fields are comparable with other HTSc. Preparing FeSe using solid-state sintering yields samples exhibiting strong ferromagnetic hysteresis loops (MHLs), and the superconducting contribution is only visible after subtracting MHLs from above Tc. Due to the complicated phase diagram, the samples are a mixture of several phases, the superconducting beta-FeSe, and the non-superconducting delta-FeSe and gamma-FeSe. Furthermore, antiferromagnetic Fe7Se8 and ferromagnetic alpha-Fe may be contained, depending directly on the Se loss during the sintering process. Here, we show MHLs measured up to ±7 T and determine the magnetic characteristics, together with the amount of superconductivity determined from M(T ) measurements. We also performed a thorough analysis of the microstructures in order to establish a relation between microstructure and the resulting sample properties.

International audience; —An electromagnetic simulation of YBCO bulks was performed and the resulting trapped magnetic flux density was compared to Field Cooling experimental measurements for an applied magnetic flux density up to 3 T. The simulation relied on an axisymmetric problem implementing an A-formulation of the Maxwell’s equations solved by means of the Finite Element Method so that the time evolution of the magnetic flux density was computed over the cross section of the bulk. To derive its electrical conductivity, a classic power law was adopted that includes the dependence of the critical current density upon temperature and external magnetic field modelled on the basis of a Modified Kim-Anderson relation. It was found that this model tends asymptotically towards the critical state for which the n-value becomes a free parameter that should be then estimated experimentally. The experimental results could be fairly reproduced at different operating temperatures with the best trapping at temperatures below 77 K benefiting from an increasing critical current density.

The complete penetration magnetic field $B_{P}$ is the main feature of a superconducting pellet submitted to an axial applied magnetic field. The electric $E{-}J$ characteristics of HTS bulk is generally described by a power law $E(J) = E_{C}(J/J_{C})^{n}$ . The influence of the $n$ -value and applied magnetic field rise rate $V_{b}$ on the $B_{P}$ of a HTS cylindrical pellet has been presented in a previous paper. The numerical results presented come from a numerical resolution of a nonlinear diffusion problem. With the help of these simulations, a linear relationship between $B_{P}$ , ln $V_{b}$ , and $n$ -value has been deduced. This comparison allows determining the critical current density $J_{C}$ and the $n$ -value of the power law based on direct measurement of $B_{P}$ in the gap between two bulk HTS pellets. In this paper, an improvement of this method is presented. The influence of geometric parameters $R$ and $L$ is studied to give generality to the relationship between $B_{P}$ , $V_{b}$ , and $n$ -value. A previous $B_{P}$ formula is confirmed by these new simulations. To correctly connect simulation and experimental results, the influence of spacing $e$ between bulks is studied and presented. A relationship between $B_{P}$ and measured complete penetration magnetic field $B_{PM}$ is determined.

The low-temperature superconducting (LTS) coils are designed to produce strong magnetic fields at the working temperature of liquid helium. Some kinds of them have been created to produce strong and homogeneous magnetic fields, as in magnetic resonance imaging systems. However, the critical and mechanical parameters of superconducting material limit the performances of the coils. Most of the time, LTS coils are built to produce a homogeneous magnetic flux density in one single direction. Consequently, it is necessary to rotate the subject or the sample, when a study is needed depending on the magnetic field orientation. This is not suitable for special kinds of experiments involving high-temperature superconducting systems or biomedical applications, in which the subject must not be rotated. In order to satisfy these requirements, we have designed a new coil setup. The design was made using a 3-D electromagnetic software in order to calculate the performances of the coils system, to optimize the magnetic field homogeneity and to find a cost effective solution. The first results reported in this paper concern the design of a magnet of three Helmholtz coils, each of which generates a unidirectional magnetic field ( $x$ , $y$ , and $z$ ). Each Helmholtz coil can generate 1.73 T; thus, the flux density magnitude in the center of system could reach 3 T. The price of the all system, including the cryostat, is around 45 k $\geneuro$ .

International audience; Various MgB2 thin films and single crystals were found in the literature to exhibit a sharp, narrow peak at low fields in the volume pinning force, Fp(H)-diagrams. The origin of this peak is associated with a steep drop of the current density when applying external magnetic fields and is ascribed to sample purity. We show here that bulk MgB2 prepared by spark-plasma sintering also shows the sharp, narrow peak in Fp. The peak is also seen in the volume pinning force scaling, Fp/Fp,max vs h = H/H irr. Furthermore, polycrystalline bulk MgB2 samples prepared close to the optimum reaction temperature reveal this peak effect as well, but other samples of the series show a regular scaling behavior. The combination of magnetization data with data from electric transport measurements on the same samples demonstrates the origin of this peak effect. On increasing preparation temperature, the pinning force scaling changes from grain boundary pinning to point pinning and the grain connectivity gets worse. Hence, the sharp, low-field peak in Fp vanishes. Therefore, the occurrence of the peak effect in Fp gives important information on the grain coupling in the MgB2 samples.

Oral; International audience; Toutes les méthodes d’aimantation des supraconducteurs sont basées sur la variation du champ magnétique appliqué qui résulte des caractéristiques de l’inducteur utilisé. Dans ce papier nous avons développé un modèle analytique pour montrer l’effet de la présence d’une pastille supraconductrice sur la valeur de l’inductance d’une bobine circulaire de section transversale rectangulaire placée dans un circuit ferromagnétique avec entrefer. Ce modèle est basé sur la détermination du potentiel vecteur magnétique à partir de la résolution les équations de Laplace et de poisson dans les différentes régions d’intérêt en utilisant la méthode de séparation des variables. Cette résolution analytique s’effectue à l’aide d’un code de calcul développé sous MATLAB. Les résultats obtenus sont comparés avec ceux obtenus par une simulation numérique basée sur la méthode des éléments finis implémentée sous COMSOL. Une concordance remarquable est observée entre les deux résultats.

National audience; L’histoire des machines supraconductrices a débuté dans les années 1960. La première machine fabriquée en 1967 était un alternateur entièrement supraconducteur utilisant des fils NbTi. Ensuite, les supraconducteurs à haute température (HTS) ont été découverts et ce fut le point de départ de nouvelles études sur les machines supraconductrices. Cet article passe en revue diverses topologies de moteurs supraconducteurs et de générateurs utilisant les supraconducteurs à haute température critique et qui ont été étudiées et réalisées au GREEN.

International audience; Recently, the use of electrolyzers for hydrogen production through water electrolysis is of great interest in the industrial field to replace current hydrogen production pathways based on fossil fuels (e.g. oil, coal). The electrolyzers must be supplied with a very low DC voltage in order to produce hydrogen from the deionized water. For this reason, DC-DC step-down converters are generally used. However, these topologies present several drawbacks from output current ripple and voltage gain point of view. In order to meet these expectations, interleaved DC-DC step-down converters are considered as promising and interesting candidates to supply proton exchange membrane (PEM) electrolyzers. Indeed, these converters offer some advantages including output current ripple reduction and reliability in case of power switch failures. In addition, over the last decade, many improvements have been brought to these topologies with the aim to enhance their conversion gain. Hence, the main goal of this paper is to carry out a thorough state-of-the-art of different interleaved step-down DC-DC topologies featuring a high voltage gain, needed for PEM electrolyzer applications.

International audience; Over the last decade, hydrogen production through water electrolysis has benefited from a growing interest from industrials and from the international scientific community. In order to minimize the environmental impact in the hydrogen production pathways, renewable energy sources (wind, solar, hydraulic) are attractive solutions. Like fuel cells, electrolysers need a DC/DC converter since the required DC voltage is quite low. Therefore, DC-DC buck converters are generally used. On the one hand, these converters present several drawbacks for electrolyser applications, especially from conversion ratio and output current ripple point of view. On the other hand, interleaved buck topologies offer more perspective from availability and output current ripple reduction point of view. Hence, the main purpose of this paper is to present and compare different candidate interleaved buck DC-DC topologies featuring a high voltage gain, needed for electrolysers.

The authors would like to thank Thierry Lubin and Alexandre Colle for their useful discussions on this topic.; International audience; The development of bulk magnets based on REBCO type superconductors that can generate up to 17 T requires the implementation of a magnetization system, sometimes pulsed. Therefore, at least one coil around the High Temperature Superconducting (HTS) bulk is required. In this paper, we suggest an analytical approach in order to assess the magnetic vector potential and to compute the inductance of a coil having a rectangular cross-section and concentric with a magnetic circuit including an air-gap. On a proposed geometry, analytical results are compared with those obtained by using Finite Element Analysis (FEA). The gained analytical expressions give accurate results with an error less than 1% on the inductance value of the coil. The induced current density and trapped magnetic field in a HTS bulk during a Pulsed Field Magnetization (PFM) are also calculated using the proposed analytical expressions of the magnetic vector potential combined with an integral method. The achieved results show a remarkable match with those obtained with COMSOL.

Electromagnetic characterization and modelling of superconducting material are very important for end users. Firstly main features of superconducting material are presented. Secondly two methods of characterization to obtain experimentally the influence of magnetic field on critical current density JC are presented. These two methods are commonly used for applications in electrical engineering. The electrical method is based on measuring the voltage and current of a superconducting sample. It has the advantage of simplicity in implementation, but the main drawback as the presence of the self magnetic field. This prevents the determination of the parameters of JC(B) for weak magnetic fields. The magnetic method using the cycle of magnetization has the advantage of making measurements without contact with the sample but has two drawbacks: it is based on a theoretical model in which JC is constant and the assumption of infinite length sample.

Complete penetration magnetic field BP is a feature of a superconducting sample submitted to an applied magnetic field. It is very important to know this for applications such as an electrical motor or levitation. The electric E-J characteristics of a high-temperature superconductor (HTS) bulk is generally described by a power law. The main purpose of this paper is to investigate the influence of the n-value and the applied magnetic field rise rate Vb on the BP of a cylindrical HTS pellet. The numerical results presented come from the resolution of a nonlinear diffusion problem with commercial software. In this paper, cylindrical HTS pellets are submitted to an axial applied magnetic field. With the help of these simulations, a linear relationship between BP , Vb, and the n-value has been found. A comparison between measurements and simulations is done for the magnetization of cylindrical bulk superconducting samples. This comparison allows to determine the critical current density J C and n-value of the power law E(J) = EC(J/J C )n. The experiment is based on the direct measurement of the local magnetic field in the gap between two bulk HTS pellets. The field penetration measurements have been carried out on HTS pellets at 77 K by applying increasing magnetic fields with a quasi-constant sweep rate for the axial direction of the applied magnetic field. Two values of complete penetration magnetic field BP have been measured at two different rise rates Vb. The n-value of the real HTS pellet has been deduced.

The optimization of a HTS cable design with respect to AC-losses is of crucial importance for the economic viability of the respective concept. However the experimental determination of AC-losses is not straightforward since for short cable samples the distribution of current among the super-conducting tapes is mainly determined by the contact resistances of the individual tapes. The resulting inhomogeneous current distribution definitely falsifies the results. To solve this experimental problem we present a new experimental technique. The setup is a 2m-long three phase concentric cable model for which, within each phase, the superconducting tapes (up to 30) are connected in series. The Cu-braid backwards conductors were assembled in a rotational symmetric cage type arrangement, such that their self fields at the cable cancel. If experimental peculiarities of this setup, as the strong inductive coupling between the phases and the suitable positioning of the voltage contact leads, are correctly taken into account, the currents can be controlled independently and the electrical properties of the cable can be measured unambiguously. In this paper preliminary results are presented. The work is part of the German government funded cable project AMPACITY (1 km / 20 kV/ 2 kA)

Oral; International audience; In this work, we propose a numerical model for a rapid computation of the electromagnetic field in multifilamentary composite HTS tapes carrying sinusoidal AC currents or submitted to external sinusoidal time varying magnetic fields. The proposed model is based on an integrodifferential formulation in terms of the electric vector potential and allows taking into account the nonlinearity of the constitutive law, relating the electrical current density to the electrical field, in the frequency domain rather than in the time domain as it is commonly done. Only the active parts are discretized; the multiscale dimensions are thus easily treated.

Poster; International audience; With the rapid advance of power electronics technologies, DC electrical grids are expanding at every scale and various high temperature superconducting devices are being developed to operate in such grids. Up to now applications of superconductivity with a clear impact in the electrical sector have been HTS power cables and fault current limiters. Various prototypes have been built and tested around the world either in the industry or research institutions. These technologies have demonstrated the potential to address increasing power transmission demand and protection against faults in electrical grids. In the present work, a new application of HTS technology for DC grids is introduced. The device referred to as “superconducting power filter” aims at increasing the stability of DC grids by adding a current-dependent resistance to a classical RLC filter. The device is closely related to a resistive fault current limiter (r-ScFCL), but here the superconductor is expected to be operated around its critical current thereby remaining in the superconducting state during load fluctuations, with the option of providing timely over-current protection if required. In comparison with other stabilization techniques, a superconducting power filter achieves a fully passive stabilization without limiting significantly the energy transferred to the load in nominal operation. To clarify its operation, a simple circuit constituted of an ideal constant DC voltage source and a controlled power load connected through a RLC filter in series with a superconducting power filter is considered. Using an electrical-thermal model of the device, the stability of the DC grid is assessed depending on the design parameters of the device. The corresponding experimental setup has been assembled to back up the numerical results. We built a superconducting power filter with a non-inductive coil wound with BSCCO and cooled down at 77 K in liquid nitrogen. The hereafter reported results confirmed that a superconducting power filter can indeed provide a stabilizing functionality in DC grids.

Poster; International audience; Our basic method to characterize HTS pellets, which has been presented in earlier publications, is based on the measurement of their complete penetration magnetic flux density BP. The method allows assessing the critical current density JC and the n-value through the classical power law model modeling the magnetic behavior of pellets and the direct measurement of BP. A first method used two slightly axially separated HTS disks to determine BP in the gap between the disks. However, this first method exhibits two main disadvantages. The first disadvantage is that the magnitude of the derived values depends on the spacing e between the two pellets. The second one is that the two pellets ought to be strictly identical for the method to lead to representative values of a given pellet. To overcome those constrains and limitations, we have been looking for a new improved method that relies only on the measurement involving a single pellet. The following work introduces the principle of this new method. The main idea is thus to deduce BP from the magnetic flux density measured at the center of the top surface of the pellet referred to as BT when the applied magnetic flux density Ba equals BP. The influence of the applied magnetic flux density ramping rate and the geometric dimensions of the pellet on the ratio BP /BT is studied and used to assess BP from the measured BT. It is shown that the proposed method allows achieving a substantial improvement on the accuracy of the Jc and n-value measurements of a given superconducting disk over the past method based on two HTS pellets.

International audience; In this work, we present a numerical model for a rapid computation of eddy currents in composite materials, adapted for both carbon fiber reinforced polymers (CFRPs) for NDT applications and multifilamentary high temperature superconductive (HTS) tapes for coupling loss evaluation. The proposed model is based on an integro-differential formulation in terms of the electric vector potential in the frequency domain. It allows taking into account both the high anisotropy and the nonlinearity of the considered materials.

The authors would like to thank Isabelle Schwenker for its help during the setup of the experimental parts. We are also grateful to Mariusz Stepien and Boguslaw Grzesik for organizing in 2014 the visit of the Silesian University of Technology where we have seen such a solar motor for the first time.; International audience; The concept of the motor, described here, has been presented for the first time in 1959. It allows to convert light energy into mechanical energy, without the need of any brushes or other power electronics. An original improvement that we have made to this structure concerns the superconducting suspension which tends to limit frictions and ensures natural stability of the rotor. The second major novelty is that we have designed the entire motor for operating in liquid nitrogen. The motor structure is detailed in this article, including the electromagnetic design, the electrical and mechanical equations. Two different configurations of superconducting bearings are studied in order to make levitate the rotors. Finally, for the designed motors, only 10% of the installed onboard power is converted into mechanical power. All the experimental results, especially the speed of the motor, are consistent with the estimated values by the model.