Transport AC losses analysis of twisted quasi-isotropic conductor
Introduction
wning to the characteristics of high current capacity with low AC loss, mechanical flexibility and anti-electromagnetic interference, the second-generation (2 G) high temperature superconducting (HTS) conductor has become a promising choice for commercial application [1,2]. In order to improve the current carrying capacity and weaken the magnetic anisotropy of HTS conductor, a quasi-isotropic HTS conductor structure was proposed by North China Electric Power University (NCEPU) [3], [4], [5]. It is well known that when 2 G HTS conductor transmitting alternating current (AC), AC loss will be generated in the superconducting layer and the metal layer, which includes the hysteresis loss (pH), coupling loss (Pc) and eddy current loss (Pe). The heat caused by AC loss will increase the difficulty for cooling, reduce the current carrying capacity and affect the stability of superconducting devices. AC loss of the quasi-isotropic conductor stacked by 2 G HTS tapes with large current capacity was well studied by using finite element method (FEM) based on 2D H formulation for its centrosymmetric structure and some related reports have already been available [6,7].
With the continuous research on twisted stacked-tape cable conductor (TSTC), it has been found that the twisting operation of HTS conductors can effectively improve their overall characteristics, especially in terms of reducing AC losses [8], [9], [10]. Therefore, to carry out some studies on the twisted quasi-isotropic superconducting conductor is essential nowadays. However, the twisted quasi-isotropic structure stacked by high aspect ratio of HTS tapes and copper tapes cannot be used for dimensional reduction, which resulting in huge number of mesh elements and intensive heavy computation burden by using H formulation.
In this paper, an efficient 3D simulation based on the T-A formulation will be adopted to estimate the current distribution over the twisted HTS tapes and AC loss over the entire conductor. At the same time, the transport AC loss of the quasi-isotropic conductor with different twisted angles will be measured by four-probe method under various frequencies in liquid nitrogen (LN2) bath. In order to fix the conductor and change the twist angel, a twisting apparatus will be designed and fabricated. The critical current of the conductor with different twist angles will be measured as well.
Section snippets
Configuration of the quasi-isotropic superconducting conductor
The three-dimensional and cross-section view of the quasi-isotropic conductor is presented in Fig. 1(a), which has a square shape consists of 4 units and is banded with sticky aluminum foil. Copper and HTS tapes with the same width of 2 mm are stacked alternatively. The HTS tapes used in this paper are fabricated by Shanghai Superconductor Technology Co. Ltd., with 1 µm superconducting layer and the thickness of copper tape is about 200 µm. A 300 mm long quasi-isotropic conductor was prepared
Results and discussion
While the quasi-isotropic conductor transports alternative current, coupling loss tends to generate when the magnetic field parallel to the tape. It is known that the induced current and the time constant of the coupling current are both proportional to the square of the twist pitch length of a twisted stacked-tape cable [18]. Therefore, in terms of the twisted quasi-isotropic conductor, it is desirable to minimize the coupling losses by reducing the twist pitch length. However, based on the
Conclusion
In this paper, transport AC losses of twisted quasi-isotropic conductor were analyzed by 3D T-A formulation and experiment. At 5% degradation of the critical current, the minimum twisted pitch length was measured. Transport AC losses at several twist angles were measured and calculated under different frequencies and the results are in good agreement. Transport AC loss can be reduced to be 79% by decreasing twist pitch length. Compared with non-twisted quasi-isotropic conductor, the twisted one
CRediT authorship contribution statement
Wei Pi: Writing - review & editing, Supervision, Funding acquisition. Yiran Meng: Investigation, Writing - original draft. Tie Guo: Formal analysis. Defu Wei: Resources. Yueyin Wang: Software, Visualization. Ziqiu Liu: Validation. Guoqing Li: Data curation. Yinshun Wang: Methodology, Project administration.
Declaration of Competing Interest
None.
Acknowledgements
This work was supported in part by State Grid under Grant DG71–19–004 and the National Natural Science Foundation of China under Grant 51877083.
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