STUDY OF AC LOSSES IN SUPERCONDUCTING ELECTRICAL COMPONENTS FORELECTRICAL SYSTEM DESIGNJosé María CeballosSUMMARYThe work presented here was conducted within the framework of one of the research linesof the Benito Mahedero Group of Electrical Applications of Superconductors, at theIndustrial Engineering School of the University of Extremadura (Badajoz, Spain). Thework was mainly carried out at the Group's Laboratory in Badajoz, but a part was carriedout at the Institute of Electrical Engineering of the Slovak Academy of Science ofBratislava (Slovakia).Partial financial support for the work was given by the Extremadura Government through aresearch project (ref. 1PR98A045).The purpose of the work was to study, characterize, and measure the differentcomponents of AC losses in superconductors that are part of such electrical systems astransformers, electrical motors, etc. The reason for such a study is because, if the study oflosses is an important part of the design of any electrical application, in superconductingelectrical systems losses determine not only their efficiency but also the capacity of thecorresponding cooling system.A difference from most of the previous AC loss studies published by other workers is thatthe focus of interest is not a single tape carrying current in a possible external magneticfield. Rather, our interest is in the tape as part of a multilayer coil, because this is the mostusual way that the tape is used in electrical systems. The behaviour of each section oftape is different from that of an isolated piece because of the influence of thesuperconducting layer wound just next to it.In order to analyze the different components of the AC loss including the influence one onesection of tape of another wound together with it, we made a comparative study of anisolated tape and of the same tape in the same conditions except for the proximity ofanother 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 multifilamentBSCO tape, then YBCO tape with ferromagnetic substrate, and finally YBCO tape withnon-ferromagnetic substrate.In a first stage of the work (with BSCO tape) we studied the coil forming part of a multilayermagnetic coupling, investigating the dependence of the losses on the coil's geometricparameters. 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 thelosses than the perpendicular field [1, 2].But this effect is also observed to be different in the different layers of the coil. In thesecond stage (with BSCO tapes) we therefore studied the behaviour of isolated tapesunder different conditions of current and magnetic field with the aim of determining, in thethird 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 onthe 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 onthe losses in a section of tape, comparing the results with the known behaviour of isolatedtape. 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 theother in order to take some measurements in the part of the longer tape not in theimmediate proximity of the shorter one. This thus provided new data with which to addfurther precision to our conclusions.One of the most interesting results of this stage was the revelation of how the proximity oftapes 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 onYBCO were replacing BSCO tape in superconducting electrical system designs. Tocomplete the study of AC losses, we began the study of this type of tape in the fourth andlast stage of the thesis work. Samples of 2G (second generation) YBCO tape were testedin the same way as the BSCO tape in stages 1-3 of this work. The first results andconclusions of this study were presented in [7].The thesis document presented here was closed after this publication, but the study ofYBCO 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 magneticsubstrate.2 Study of the influence of the magnetic substrate on nearby tapes and coil AClosses.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 criticalcurrent of a coil, depending on the bending curvature of the tape.The results of the work described have led to our participation in 5 internationalconferences in applied superconductivity, and to the publication of the articles [1-7]referenced in this summary and attached to the document.REFERENCES[1] B. Pérez, A. Álvarez, P. Suárez, J.M. Ceballos, X. Obradors, X. Granados, R. Bosch.Ac losses in a toroidal superconductor transformer IEEE Transactions on AppliedSuperconductivity, 13, pp. 2341-2343 (2003).[2] P. Suárez, A. Álvarez, B. Pérez, D. Cáceres, E. Cordero, J.M. Ceballos. Influence ofthe shape in the losses of solenoidal air-core transformersIEEE Transactions onApplied Superconductivity,15, pp. 1855-1858 (2005).[3] F Gomöry, J. Souc M. Vojenciak, E. Seiler, B. Klincok, J.M. Ceballos, E. Pardo, A.Sánchez, C. Navau, S. Farinon, P. Fabbricatore. Predicting ac loss in practicalsuperconductors. Supercond. Sci. Technol., 19, pp. 60-66 (2006).[4] M. Vojenciak, J. Souc J.M. Ceballos, F Gomöry, B. Klincok, F. Grilli. Study of acloss in Bi-2223/Ag tape under the simultaneous action of ac transport current and acmagnetic field shifted in phase Supercond. Sci. Technol., 19, pp. 397-404 (2006).[5] E. Pardo, F Gomöry, J. Souc J.M. Ceballos. Current distribution and ac loss for asuperconducting rectangular strip with in-phase alternating current and applied field.Supercond. Sci. Technol., 20, pp. 351-364 (2007).[6] P. Suárez , A. Álvarez, B. Pérez And J.M. Ceballos. Influence of the current throughone turn of a multilayer coil on the nearest turn in a consecutive layer. Journal OfPhysics: Conference Series, 97 (2008).[7] P. Suárez, A. Álvarez, J.M. Ceballos and B. Pérez Losses in 2G tapes wound closetogether: Comparison with similar 1G tape configurationsIEEE Transactions on Applied Superconductivity Vol. 19, pp. 2395-2398, 2009
