Analysis of AC losses in superconducting electrical components for application in the design of electrical systems
- Autores: José María Ceballos Martínez
- Directores de la Tesis: Alfredo Álvarez García (dir. tes.), María Pilar Suárez Marcelo (dir. tes.)
- Lectura: En la Universidad de Extremadura ( España ) en 2010
- Idioma: inglés
- ISBN: 978-84-694-8518-7
- Número de páginas: 170
- Tribunal Calificador de la Tesis: Amadeu Leâo Rodrigues (presid.), Belén María Pérez Caballero (secret.), Ricard Bosch Tous (voc.), Xavier Granados García (voc.), Fedor Gömöry (voc.)
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- Resumen
The work presented here was conducted within the framework of one of the research lines of the "Benito Mahedero" Group of Electrical Applications of Superconductors, at the Industrial Engineering School of the University of Extremadura (Badajoz, Spain). The work was mainly carried out at the Group's Laboratory in Badajoz, but a part was carried out at the Institute of Electrical Engineering of the Slovak Academy of Science of Bratislava (Slovakia).
Partial financial support for the work was given by the Extremadura Government through a research project (ref. 1PR98A045).
The purpose of the work was to study, characterize, and measure the different components of AC losses in superconductors that are part of such electrical systems as transformers, electrical motors, etc. The reason for such a study is because, if the study of losses is an important part of the design of any electrical application, in superconducting electrical systems losses determine not only their efficiency but also the capacity of the corresponding cooling system.
A difference from most of the previous AC loss studies published by other workers is that the focus of interest is not a single tape carrying current in a possible external magnetic field. Rather, our interest is in the tape as part of a multilayer coil, because this is the most usual way that the tape is used in electrical systems. The behaviour of each section of tape is different from that of an isolated piece because of the influence of the superconducting layer wound just next to it.
In order to analyze the different components of the AC loss including the influence one one section of tape of another wound together with it, we made a comparative study of an isolated tape and of the same tape in the same conditions except for the proximity of another tape, with and without current, located just over the first one.
This study was done with different tapes during the last year: first we used multifilament BSCO tape, then YBCO tape with ferromagnetic substrate, and finally YBCO tape with non-ferromagnetic substrate.
In a first stage of the work (with BSCO tape) we studied the coil forming part of a multilayer magnetic coupling, investigating the dependence of the losses on the coil's geometric parameters. A practical formulation for the calculation of the parameters was proposed.
Experimentally, the parallel field in the coil was observed to have a greater effect on the losses than the perpendicular field [1, 2].
But this effect is also observed to be different in the different layers of the coil. In the second stage (with BSCO tapes) we therefore studied the behaviour of isolated tapes under different conditions of current and magnetic field with the aim of determining, in the third stage, the variation of this behaviour when another tape is located nearby.
The results of the second stage [3-5] showed the losses to have a strong dependence on the phase difference between the transport current and the magnetic field.
The third step was the design of a procedure to evaluate the influence of nearby tapes on the losses in a section of tape, comparing the results with the known behaviour of isolated tape. Two pieces of BSCO tape close together were used, carrying the same current (amplitude and phase) as in a multilayer coil. One of the pieces was cut longer than the other in order to take some measurements in the part of the longer tape not in the immediate proximity of the shorter one. This thus provided new data with which to add further precision to our conclusions.
One of the most interesting results of this stage was the revelation of how the proximity of tapes carrying the same current causes a reduction of the practical critical current in them [6].
During the time in which the foregoing work was being carried out, new tapes based on YBCO were replacing BSCO tape in superconducting electrical system designs. To complete the study of AC losses, we began the study of this type of tape in the fourth and last stage of the thesis work. Samples of 2G (second generation) YBCO tape were tested in the same way as the BSCO tape in stages 1-3 of this work. The first results and conclusions of this study were presented in [7].
The thesis document presented here was closed after this publication, but the study of YBCO tape is now the focus of one of our Group's research lines.
Further work with this tape includes:
1 Study of the differences between losses in tapes with and without magnetic substrate.
2 Study of the influence of the magnetic substrate on nearby tapes and coil AC losses.
3 Study of the anisotropy of YBCO tape with and without magnetic substrate.
4 Study of the influence of the tape's anisotropy on the losses and practical critical current of a coil, depending on the bending curvature of the tape.
The results of the work described have led to our participation in 5 international conferences in applied superconductivity, and to the publication of the articles [1-7] referenced in this summary and attached to the document.
