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Superconducting stacks can be used as strong permanent magnets in several applications. One of their uses is to build light and compact superconducting motors for aviation, where these magnets can be used in the rotor, but they can demagnetize quickly in the presence of cross fields. In this article, we propose a new configuration of soldered stacks face-to-face, which can be constructed by relatively simple joining techniques. Based on numerical modeling of the cross-field demagnetization of stacks of two and 16 tapes, we show that such a sample can withstand around twice as high ripple field amplitudes than isolated stacks. This is due to the increase in the parallel penetration field by around a factor 2. For cross-field amplitudes below this value, a soldered stack can retain higher permanent magnetization than isolated stacks. This method of reducing cross-field demagnetization does not decrease the power or torque rating of a motor, compared to other strategies like the increase in the gap between rotor and stator. [ABSTRACT FROM AUTHOR]

High temperature superconducting (HTS) dynamos are promising devices that can inject large DC currents into the winding of superconducting machines or magnets in a contactless way. Thanks to this, troublesome brushes in HTS machines or bulky currents leads with high thermal losses will be no longer required. The working mechanism of HTS dynamo has been controversial during the recent years and several explanations and models have been proposed to elucidate its performance. In this paper, we present the first three-dimensional (3D) model of an HTS flux pump, which has good agreement with experiments. This model can be beneficial to clarify the mechanism of the dynamo and pinpoint its unnoticed characteristics. Employing this model, we delved into the screening current and electric field distribution across the tape surface in several crucial time steps. This is important, since the overcritical screening current has been shown to be the reason for flux pumping. In addition, we analyzed the impact of both components of electric field and screening current on voltage generation, which was not possible in previous 2D models. We also explored the necessary distance of voltage taps at different airgaps for precise measurement of the voltage across the tape in the dynamo. [ABSTRACT FROM AUTHOR]

Modeling 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 article presents a 2-D time dependent model to assess the ac losses of superconducting motors based on the new T–A-formulation, which by using the finite-element method, 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 modeled 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 article is to validate the method in 2-D by evaluating the ac losses of a specific synchronous motor called superconducting torque motor; the computed results are compared with good agreements to those assessed with the minimum electromagnetic entropy production method, already validated. In a second part, the same losses are computed, taking into account the field anisotropy of Jc with the implementation of a dataset based on experimentally measured Ic at 65 and 77 K. With the model proposed in this article, it is possible to calculate the magnetic field of a whole electrical machine cross section and compute the ac losses in the individual tapes of its windings within the same numerical model. [ABSTRACT FROM AUTHOR]

We present results from commissioning testing of a 1 MVA three-phase transformer demonstration project. The three-phase transformer utilizes a 15 strand Roebel cable for the low voltage windings and 4 mm wide conductor for the high voltage windings. Both windings use ReBCO conductor and the iron core is external to the cryogenic region. The target operating temperature is 70 K. Key design features and operational performance measures of the advanced sub-cooled liquid nitrogen circulation system are described. The overall thermal budget is presented along with ac loss measurements for the windings and electrical test results of the complete transformer. There remain some unresolved issues with the stability of the cryogenic cooling system when operated sub-cooled, and with the high voltage withstand performance of the HV bushings. These issues are described and discussed. The utility of HTS Roebel cable as a convenient means of producing a compact, high current winding with low ac loss characteristics has been successfully demonstrated. [ABSTRACT FROM PUBLISHER]

Superconducting stacks and bulks can act as very strong magnets (more than 17 T), but they lose their magnetization in the presence of alternating (or ripple) transverse magnetic fields, due to the dynamic magneto-resistance. This demagnetization is a major concern for applications requiring high run times, such as motors and generators, where ripple fields are of high amplitude and frequency. We have developed a numerical model based on dynamic magneto-resistance that is much faster than the conventional Power-Law-resistivity model, enabling us to simulate high number of cycles with the same accuracy. We simulate demagnetization behavior of superconducting stacks made of 10–100 tapes for up to 2 million cycles of applied ripple field. We found that for high number of cycles, the trapped field reaches non-zero stationary values for both superconducting bulks and stacks; as long as the ripple field amplitudes are below the parallel penetration field, being determined by the penetration field for a single tape in stacks. Bulks keep substantial stationary values for much higher ripple field amplitudes than the stacks, being relevant for high number of cycles. However, for low number of cycles, stacks lose much less magnetization as compared to bulks. [ABSTRACT FROM AUTHOR]

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 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 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. Therefore it is timely 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 cable. [ABSTRACT FROM AUTHOR]