AC losses in conductors based on high {Tc} superconductors

SUENAGA,M.

Brookhaven National Laboratory

关键词: Magnetic Fields;    75 Condensed Matter Physics, Superconductivity And Superfluidity;    Ac Losses;    Strontium Oxides;    Calcium Oxides;   

DOI  :  10.2172/759046
RP-ID  :  BNL--67513
RP-ID  :  AC02-98CH10886
RP-ID  :  759046

美国|英语

来源: UNT Digital Library

【 摘 要 】

In electrical power devices, ac losses from a superconductor is a primary factor which determines their usefulness as commercial power equipment. For this reason, extensive studies have been carried out on the losses of Bi{sub 2}Sr{sub 2}Ca{sub 2}Cu{sub 3}O{sub 10}/Ag, [Bi(2223)/Ag], tapes. These studies were mostly limited to a single isolated tape. However, a conductor in a power device is surrounded by other conductors and the precise magnetic field distribution around it is very different from that for a single conductor carrying currents or in ac fields. Since the precise field distribution in and around a superconductor is critical in determining the losses, it is very important to measure and to understand the losses in Bi(2223)/Ag tapes which are surrounded by other tapes as in a power device. Taking this fact into consideration, recently the authors have studied ac losses in stacks of Bi(2223)/Ag tapes in parallel and perpendicular applied fields and shown that they can calculate the losses in these cases utilizing the critical state model if a number of appropriate factors about properties of the tape are taken into a consideration. However, in a power device such as a transformer, magnetic fields near the ends of a solenoid vary from parallel to perpendicular with the tape face. Thus, it is important to learn the behavior of the losses in the stacks of Bi(2223)/Ag tapes with respect to the variations in the angle between the applied field direction and the tape face. In order to accomplish this, they measured the angular dependence of the losses in the stacks which were made from two different Bi(2223)/Ag tapes. Here they report this result and discuss under what conditions they can calculate the losses with a reasonable accuracy. The angular dependence of the losses in ac applied fields were measured using a series of stacked Bi(2223)/Ag tapes having the angles with the direction of applied fields of 0, 7.5, 15, 30, 45, 60, and 90 degrees. The measured values of the losses were compared with the calculated values. The calculations were performed using the measured losses for the 0 and 90 degrees orientations and assuming the losses can be separated for the currents circulating in the plane of and across the tapes. It was shown that at very high fields, i.e., well above the full penetration fields, this assumption was justified by the observed good agreement between the measured and the calculated losses for both tapes. However, at the low fields, significant deviations between the calculated and the measured were seen for both of the tapes. One possible contributing factor to this discrepancy is the non-ideal shape of the tape cross sections. This can causes the field distribution near the edges of the slabs to deviate from that for an assumed uniform infinite slab. This makes the actual fields at the edge regions to be greater than the applied fields. Thus, the losses will be higher than those calculated. Although not having a full understanding of the losses at low fields is of concern, it will not be of a great importance in the designs of power devices in practice. This is due to the fact that the values of the losses at low fields are orders of magnitudes lower than those at high fields and thus the primary concern for the power device development is the reduction of the losses at high fields.