arXiv subjects

Enric Pardo

Publications and source records attributed to Enric Pardo.

At least 37 records · Page 2Linked to original sources

arXiv2019

Modeling of Airgap Influence on DC Voltage Generation in a Dynamo-Type Flux Pump

High-temperature superconducting (HTS) Flux pumps are promising devices to maintain steady current mode in HTS magnets or to energize rotor windings in motors and generators in a contactless way. Among different types of flux pumps, the dynamo-type flux pump has been very common due to its simple structure and ease of maintenance. However, understanding the principle of dynamo-type flux pump has been challenging despite the recent progress. For addressing this challenge, the numerical modeling has proved to be an appropriate tool. Modeling is usually fast, precise, cost-efficient and enables to examine some details that is hardly possible via measurements. For this purpose, an efficient numerical model based on Minimum Electromagnetic Entropy Production (MEMEP) method has been used to study the performance of the flux pump in open-circuit mode. This model is the fastest and most efficient model of a flux pump with good agreement with experiments. In addition to the main behavior of the flux pump, the influence of airgap on open-circuit DC voltage of the flux pump has been investigated. This study revealed an important property in HTS flux pumps that with increasing airgap, pumping voltage in superconducting tape does not cease, but only reaches to insignificantly low values, which is not measurable via experiments. Furthermore, the modeling results of open-circuit DC voltage were compared to experimental ones obtained from the article published in 2016 by Bumby \textit{et al.} \cite{bumby2016anomalous} and the results showed good agreement.

physics.app-ph↗

arXiv2019

T-A Formulation to Model Electrical Machines with HTS Coated Conductor Coils

Modelling high temperature superconductor (HTS) motors remains challenging mainly due to the high aspect ratio of these conductors but also because of the properties of the magnetic materials. This paper presents a 2D time dependent model to assess the AC losses of superconducting motors based on the new T-A formulation, which by using Finite Element Methods (FEM), allows its implementation in commercial software. The T-A formulation computes the magnetic flux density with different Maxwell's equations depending on the areas of the motor and makes it possible to use the thin strip approximation i.e. the HTS tapes are modelled as infinitely thin lines. The model is then expected to tackle the high aspect ratio of the HTS as well as decreasing both the mesh complexity and the computing time. The first objective of the paper is to validate the method in 2D by evaluating the AC losses of a specific synchronous motor called SUTOR; the computed results are compared with good agreements to those assessed with the MEMEP method, already validated. In a second part, the same losses are computed, taking into account the anisotropy of Jc with the implementation of a data set based on experimentally measured Ic at 65 K and 77 K.

cond-mat.supr-con↗

arXiv2019

AC loss modelling in superconducting coils and motors with parallel tapes as conductor

Many superconducting power applications contain REBCO coils. Superconducting motors are very interesting for mobile applications like aviation, thanks to their high power per unit weight. In order to reduce the inductance, multi-tape conductors are often used in windings but parallel tapes present coupling AC loss. In this article, we analyze the AC loss of single coils and a full stator of a motor made of windings of parallel tapes, taking several coupling situations into account. Our computer modeling method combines self-programmed and commercial finite-element methods to calculate the current density and AC loss. We found that isolating the tapes along the length provides almost the same AC loss reduction as that obtained by full transposition. This reveals that there are still many unexploited strategies to reduce the AC loss in multi-tape conductors.

cond-mat.supr-con↗

arXiv2018

3D modelling of macroscopic force-free effects in superconducting thin films and rectangular prisms

When the magnetic field has a parallel component to the current density ${\bf J}$ there appear force-free effects due to flux cutting and crossing. This results in an anisotropic ${\bf E}({\bf J})$ relation, being ${\bf E}$ the electric field. Understanding force-free effects is interesting not only for the design of superconducting power and magnet applications but also for material characterization. This work develops and applies a fast and accurate computer modeling method based on a variational approach that can handle force-free anisotropic ${\bf E}({\bf J})$ relations and perform fully three dimensional (3D) calculations. We present a systematic study of force-free effects in rectangular thin films and prisms with several finite thicknesses under applied magnetic fields with arbitrary angle $θ$ with the surface. The results are compared with the same situation with isotropic ${\bf E}({\bf J})$ relation. The thin film situation shows gradual critical current density penetration and a general increase of the magnitude of the magnetization with the angle $θ$ but a minimum at the remnant state of the magnetization loop. The prism model presents current paths with 3D bending for all angles $θ$. The average current density over the thickness agrees very well with the thin film model except for the highest angles. The prism hysteresis loops reveal a peak after the remnant state, which is due to the parallel component of the self-magnetic-field and is implicitly neglected for thin films. The presented numerical method shows the capability to take force-free situations into account for general 3D situations with a high number of degrees of freedom. The results reveal new features of force-free effects in thin films and prisms.

cond-mat.supr-con↗

arXiv2017

3D magnetization currents, magnetization loop, and saturation field in superconducting rectangular prisms

Bulk superconductors are used in both many applications and material characterization experiments, being the bulk shape of rectangular prism very frequent. However the magnetization currents are still mostly unknown for this kind of three dimensional (3D) shape, specially below the saturation magnetic field. Knowledge of the magnetization currents in this kind of samples is needed to interpret the measurements and the development of bulk materials for applications. This article presents a systematic analysis of the magnetization currents in prisms of square base and several thicknesses. We make this study by numerical modeling using a variational principle that enables high number of degrees of freedom. We also compute the magnetization loops and the saturation magnetic field, using a definition that is more relevant for thin prisms than previous ones. The article presents a practical analytical fit for any aspect ratio. For applied fields below the saturation field, the current paths are not rectangular, presenting 3D bending. The thickness-average results are consistent with previous modeling and measurements for thin films. The 3D bending of the current lines indicates that there could be flux cutting effects in rectangular prisms. The component of the critical current density in the applied field direction may play a role, being the magnetization currents in a bulk and a stack of tapes not identical.

cond-mat.supr-con↗

arXiv2017

3D computation of non-linear eddy currents: variational method and superconducting cubic bulk

Computing the electric eddy currents in non-linear materials, such as superconductors, is \E{not straightforward}. The design of superconducting magnets and power applications needs electromagnetic computer modeling, being in many cases a three-dimensional (3D) problem. Since 3D problems require high computing times, novel time-efficient modeling tools are highly desirable. This article presents a novel computing modeling method based on a variational principle. The self-programmed implementation uses an original minimization method, which divides the sample into sectors. This speeds-up the computations with no loss of accuracy, while enabling efficient parallelization. This method could also be applied to model transients in linear materials or networks of non-linear electrical elements. As example, we analyze the magnetization currents of a cubic superconductor. This 3D situation remains unknown, in spite of the fact that it is often met in material characterization and bulk applications. We found that below the penetration field and in part of the sample, current flux lines are not rectangular and significantly bend in the direction parallel to the applied field. In conclusion, the presented numerical method is able to time-efficiently solve fully 3D situations without loss of accuracy.

cond-mat.supr-con↗

arXiv2017

Roebel cables from REBCO coated conductors: a one-century-old concept for the superconductivity of the future

Energy applications employing high-temperature superconductors (HTS), such as motors/generators, transformers, transmission lines and fault current limiters, are usually operated in the alternate current (AC) regime. In order to be efficient, the HTS devices need to have a sufficiently low value of AC loss, in addition to the necessary current-carrying capacity. Most applications are operated with currents beyond the current capacity of single conductors and consequently require cabled conductor solutions with much higher current carrying capacity, from a few kA to up to 20-30 kA for large hydro-generators. A century ago, in 1914, Ludwig Roebel invented a low-loss cable design for copper cables, which was successively named after him. The main idea behind Roebel cables is to separate the current in different strands and to provide a full transposition of the strands along the cable direction. Nowadays, these cables are commonly used in the stator of large generators. Based on the same design concept of their conventional material counterparts, HTS Roebel cables from REBCO coated conductors were first manufactured at the Karlsruhe Institute of Technology (KIT) and have been successively developed in a number of varieties that provide all the required technical features such as fully transposed strands, high transport currents and low AC losses, yet retaining enough flexibility for a specific cable design. In the past few years a large number of scientific papers have been published on the concept, manufacturing and characterization of such cables. Times are therefore mature for a review of those results. The goal is to provide an overview and a succinct and easy-to-consult guide for users, developers, and manufacturers of this kind of HTS cables.

cond-mat.supr-con↗

arXiv2017

AC loss calculation in coated conductor coils with a large number of turns

For superconducting coils with many turns, it is widely believed that the magnetic field from magnetization currents is negligible compared to the one generated by the whole coil. This article shows that this is not true for coils with a considerably large number of turns (up to thousands). We introduce a method to accurately calculate the AC loss in coils with virtually any number of turns that takes the magnetization currents into account. The current density and AC loss is analyzed for two windings made of coated conductor consisting of 32 pancake coils totaling 768 and 6400 turns, respectively.

cond-mat.supr-con↗

arXiv2016

Modeling of screening currents in coated conductor magnets containing up to 40000 turns

Screening currents caused by varying magnetic fields degrade the homogeneity and stability of the magnetic fields created by $RE$BCO coated conductor coils. They are responsible for the AC loss, which is also important for other power applications containing windings. Since real magnets contain coils exceeding 10000 turns, accurate modeling tools for this number of turns are necessary for magnet design. This article presents a fast numerical method to model coils with no loss of accuracy. We model a 10400-turn coil for its real geometry and coils of up to 40000 turns with the continuous approximation, which we checked that introduces negligible errors. The screening currents, the Screening Current Induced Field (SCIF) and the AC loss is analyzed in detail. The SCIF is maximum at the remnant state with a considerably large value and decreases substantially after one day of relaxation. The instantaneous AC loss for an anisotropic magnetic-field dependent $J_c$ is qualitatively different than for a constant $J_c$, although the loss per cycle is similar. Saturation of the magnetization currents at the end pancakes causes that the maximum AC loss at the first ramp increases with $J_c$. The presented modeling tool can accurately calculate the SCIF and AC loss in practical computing times for coils with any number of turns used in real windings, enabling parameter optimization.

cond-mat.supr-con↗

arXiv2016

Flexible Dual-Connectivity Spectrum Aggregation for Decoupled Uplink and Downlink Access in 5G Heterogeneous Systems

Maintaining multiple wireless connections is a promising solution to boost capacity in fifth-generation (5G) networks, where user equipment is able to consume radio resources of several serving cells simultaneously and potentially aggregate bandwidth across all of them. The emerging dual connectivity paradigm can be regarded as an attractive access mechanism in dense heterogeneous 5G networks, where bandwidth sharing and cooperative techniques are evolving to meet the increased capacity requirements. Dual connectivity in the uplink remained highly controversial, since the user device has a limited power budget to share between two different access points, especially when located close to the cell edge. On the other hand, in an attempt to enhance the uplink communications performance, the concept of uplink and downlink decoupling has recently been introduced. Leveraging these latest developments, our work significantly advances prior art by proposing and investigating the concept of flexible cell association in dual connectivity scenarios, where users are able to aggregate resources from more than one serving cell. In this setup, the preferred association policies for the uplink may differ from those for the downlink, thereby allowing for a truly decoupled access. With the use of stochastic geometry, the dual connectivity association regions for decoupled access are derived and the resultant performance is evaluated in terms of capacity gains over the conventional downlink received power access policies.

cs.NI↗

arXiv2015

Electromagnetic modelling of superconductors with a smooth current-voltage relation: variational principle and coils from a few turns to large magnets

Many large-scale applications require electromagnetic modelling with extensive numerical computations, such as magnets or 3-dimensional (3D) objects like transposed conductors or motors and generators. Therefore, it is necessary to develop computationally time-efficient but still accurate numerical methods. This article develops a general variational formalism for any ${\bf E}({\bf J})$ relation and applies it to model coated-conductor coils containing up to thousands of turns, taking magnetization currents fully into account. The variational principle, valid for any 3D situation, restricts the computations to the sample volume, reducing the computation time. However, no additional magnetic materials interacting with the superconductor are taken directly into account. Regarding the coil modelling, we use a power law $E(J)$ relation with magnetic field-dependent critical current density, $J_c$, and power law exponent, $n$. We test the numerical model by comparing the results to analytical formulas for thin strips and experiments for stacks of pancake coils, finding a very good agreement. Afterwards, we model a magnet-size coil of 4000 turns (stack of 20 pancake coils of 200 turns each). We found that the AC loss is mainly due to magnetization currents. We also found that for an $n$ exponent of 20, the magnetization currents are greatly suppressed after 1 hour relaxation. In addition, in coated conductor coils magnetization currents have an important impact on the generated magnetic field; which should be taken into account for magnet design. In conclusion, the presented numerical method fulfills the requirements for electromagnetic design of coated conductor windings.

cond-mat.supr-con↗

arXiv2014

Self-field Effects and AC Losses in Pancake Coils Assembled from Coated Conductor Roebel Cables

In this contribution we develop a refined numerical model of pancake coils assembled from a coated conductor Roebel cable, which includes the angular dependence of the critical current density $J_c$ on the magnetic field and the actual (three-dimensional) shape of the current lead used to inject the current. Previous works of ours indicate that this latter has an important influence on the measured value of the AC losses. For the simulation of the superconductor, we used two alternative models based on different descriptions of the superconductor's properties and implemented in different mathematical schemes. For the simulation of the current lead we use a full three-dimensional finite-element model. The results of the simulation are compared with measurements and the main issues related to the modeling and the measurement of Roebel coils are discussed in detail.

cond-mat.supr-con↗

arXiv2014

Modelling of AC loss in coils made of thin tapes under DC bias current

Many applications, such as magnets and SMES, are usually charged and discharged under a bias DC current, which may increase the AC loss. For their design, it is necessary to understand and predict the AC loss. This article analises the AC loss in magnet-like coils under DC bias contribution super-imposed to the AC current. The analysis is based on a numerical model that takes the interaction between magnetization currents in all turns into account. The studied example is a stack of 32 pancake coils with 200 turns each made of thin tape, such as $Re$BCO coated conductor. We present the current density, the instantaneous power loss, and loss per cycle. We have found that the loss increases with the DC bias current. The instantaneous power loss is the largest in the initial rise of the the AC current. In following cycles, the power loss is higher in the current increase than in the decrease. The loss per cycle is the largest at the end pancakes. In conclusion, the highest cooling power should be supplied to the top and bottom pancakes and during current rise, specially the initial one. The presented model has a high potential to predict the AC loss in magnet-size coils, useful for their design.

cond-mat.supr-con↗

arXiv2014

Determination of Intrinsic Magnetic Response from Local Measurements of Fringing Fields

Micron-sized Hall bars and micro-SQUIDs are now used routinely to measure the local static and dynamic magnetic response with micron-scale spatial resolution. While this provides a powerful new tool, determining the intrinsic magnetization presents new challenges, as it requires correcting for demagnetization fields that vary widely with position on a sample. In this paper we develop a method to correct for the demagnetization effect at local points of a rectangular prism shaped sample using a finite element analysis of Maxwell's equation applied to local Hall sensor measurements calibrated by bulk measurements of the magnetization. This method can be generalized to other geometric shapes to analyze data obtained with local magnetic probes.

cond-mat.mes-hall↗

arXiv2013

Computation of Losses in HTS Under the Action of Varying Magnetic Fields and Currents

Numerical modeling of superconductors is widely recognized as a powerful tool for interpreting experimental results, understanding physical mechanisms and predicting the performance of high-temperature superconductor (HTS) tapes, wires and devices. This is especially true for ac loss calculation, since a sufficiently low ac loss value is imperative to make these materials attractive for commercialization. In recent years, a large variety of numerical models, based on different techniques and implementations, have been proposed by researchers around the world, with the purpose of being able to estimate ac losses in HTSs quickly and accurately. This article presents a literature review of the methods for computing ac losses in HTS tapes, wires and devices. Technical superconductors have a relatively complex geometry (filaments, which might be twisted or transposed, or layers) and consist of different materials. As a result, different loss contributions exist. In this paper, we describe the ways of computing such loss contributions, which include hysteresis losses, eddy current losses, coupling losses, and losses in ferromagnetic materials. We also provide an estimation of the losses occurring in a variety of power applications.

cond-mat.supr-con↗

arXiv2011

Angular dependenceof the magnetisation AC loss: coated conductors, Roebel cables and double pancake coils

The AC loss in ReBCO coated conductors is large in situations when the conductors are subjected to a considerable magnetic field, like in rotating machines, transformers and high-field magnets. Roebel cables can reduce the AC loss in these cases. However, computer simulations are needed to interpret the experiments, understand the loss mechanisms, reduce the AC loss by optimising the Roebel cable and design the cryogenic system. In this article, we simulate and discuss the AC loss due to an applied magnetic field making an arbitrary angle with the cable and taking into account a realistic anisotropic field dependence of the critical current density. We study the AC loss in the superconductor parts for the limits of very high coupling currents and completely uncoupled strands. The simulations for the uncoupled case also describe a double pancake coil with no transport current. For the simulations, we use two different numerical methods with complementary strengths. This serves as a mutual check of the correctness of the simulation results, which agree to each other. Opposite than expected, we found that the AC loss does not only depend on the perpendicular component of the applied magnetic field. We also found that the AC loss for applied fields with an orientation below 7 degrees with the strands surface is reduced more than one order of magnitude comparing to an untransposed cable. Therefore, we recommend to use Roebel cables for windings with important parallel components of the magnetic field, such as transformers and high-field magnets.

cond-mat.supr-con↗

arXiv2004

Simple Riesz groups having wild intervals

We prove that every partially ordered simple group of rank one which is not Riesz embeds into a simple Riesz group of rank one if and only if it is not isomorphic to the additive group of the rationals. Using this result, we construct examples of simple Riesz groups of rank one $G$, containing unbounded intervals $(D_n)_{n\geq 1}$ and $D$, that satisfy: (a) For each $n\geq 1$, $tD_n\ne G^+$ for every $t< q_n$, but $q_nD_n=G^+$ (where $(q_n)$ is a sequence of relatively prime integers); (b) For every $n\geq 1$, $nD\ne G^+$. We sketch some potential applications of these results in the context of K-Theory.

math.OA↗

arXiv2002

Magnetic properties of superconducting multifilamentary tapes in perpendicular field. I: Model and vertical stacks

Current and field profiles, and magnetization and AC losses are calculated for an array of infinitely long superconducting tapes arranged vertically in a perpendicularly applied magnetic field. Calculations are based on the critical state model. The finite thickness of the tapes and the effects of demagnetizing fields are considered. The influence of the magnetic coupling of the filaments in the magnetic properties of the arrays are studied. The general model can be applied to an arbitrary arrangement of tapes as long as there is reflection symmetry with respect to the vertical central plane.

cond-mat.supr-con↗