Research On AC Losses Of HTS

Superconducting technology is high-tech in the21st century with a strategicimportance. The implementation of superconductive device into power system is wellconcerned which can overcome several important technical problems of modern powersystem in a new technical way. Most of the superconductive device will produce AC lossesin operation which will not only aggravate the burden of the refrigeration system but alsowill lead to instability of superconducting devices. So, the study on AC loss of thesuperconductor was one of the major tasks before the superconducting power devices canbe put into practical use. This thesis is focused on the AC loss of high-temperaturesuperconductors (HTS).The main work is concentrated on the measuring and modeling AC loss on YBCOconductors at77K. The analysis of AC losses in YBCO conductors is the leading thread inthe structure of the thesis. An introduction to high-temperature superconductivity is madewith special emphasis on the mechanisms of AC losses in HTS. AC transport current lossesof YBCO coated conductor is measured in liquid nitrogen (77K) by a sensitive electricalmethod. The AC losses of YBCO tape which was produced by Superpower Inc. is measuredunder different currents and different frequencies respectively. The measured AC transportcurrent losses results are compared with the Norris formula calculative results when thepower frequency equals to50Hz. The comparison shows that the measured results wereslightly lower than calculative values. Besides, the measured results in different frequencieswhich varied from15Hz to300Hz are also provided. A number of these results show thatAC losses become larger with the increasing of frequency.For precise calculation of the AC losses and studying the electromagnetic properties ofHTS with smooth current-voltage characteristics, the finite element method (FEM) hasbeen used. The implementation of the power-law model of the E-J characteristic of HTS into the FEM method is presented. The implementation of FEM method has been validatedby means of comparison with results from electrical measurements. The AC losses inYBCO tape of square geometry in various operating conditions have been calculated andcompared. The effect of parallel field on AC loss is small relatively compared to verticalfield under the same amplitude of magnetic field.