Quan Li

AC loss characteristics of superconducting power transmission cables: gap effect and Jc distribution effect

Four groups of superconducting power transmission cables composed of coated conductors with 4.0xa0mm width and 2xa0µm superconductor thickness have been designed, including mono-layer, two-layer, four-layer and six-layer cables. In each group, three types of cables have been constructed with gaps of different size between adjacent coated conductors, which are classified into small gap, medium gap and large gap. Moreover, different lateral critical current density (Jc) distributions, specifically a uniform distribution and a trapezoidal distribution with a sloping shoulder, have been assumed while calculating the AC losses of these cables numerically by using a one-dimensional FEM model. Numerical results show that AC losses in mono-layer cables are significantly influenced by gaps between coated conductors as well as lateral Jc distribution, while cables with many layers are hardly affected by them. This proves that small gaps between coated conductors are not absolutely essential to reduce AC losses in multi-layer cables, and a sloping shoulder of the Jc distribution is more allowable in multi-layer cables than in mono-layer cables. The AC loss distributions among layers in six-layer cables are presented and the reasons for different influences of the gap as well as the lateral Jc distribution in mono-layer and multi-layer cables are discussed.

Ac loss reduction of multilayer superconducting power transmission cables by using narrow coated conductors

The ac loss characteristics of coated conductors are dominated by the magnetic field component normal to their superconductor layer. Multilayer cables as well as monolayer cables consisting of 4?mm-wide coated conductors (named 4?mm cables) and those consisting of 2?mm-wide coated conductors (named 2?mm cables) were designed, and numerical electromagnetic field analyses were performed in their cross sections to calculate their ac losses. Trapezoidal lateral critical current density Jc distributions with shoulders as well as uniform ones were assumed in coated conductors for the analyses. The former models the degraded Jc near the edges of coated conductors. In the case of the monolayer, the calculated ac losses of the 2?mm cables were comparable to those of the 4?mm cables. In the cases of the multilayers, the calculated ac losses of the 2?mm cables were obviously less than those of the 4?mm cables. The degraded Jc near the edges of coated conductors more seriously affects the ac loss characteristics of the 2?mm cables than those of the 4?mm cables. However, even if we consider the influence of the degraded Jc near the edges of coated conductors, 2?mm-wide coated conductors are more profitable than 4?mm-wide coated conductors in multilayer cables from the viewpoint of ac loss reduction.

Thermally actuated magnetization flux pump in single-grain YBCO bulk

Recent progress in material processing has proved that high temperature superconductors (HTS) have a great potential to trap large magnetic fields at cryogenic temperatures. For example, HTS are widely used in MRI scanners and in magnetic bearings. However, using traditional ways to magnetize, the YBCO will always need the applied field to be as high as the expected field on the superconductor or much higher than it, leading to a much higher cost than that of using permanent magnets. In this paper, we find a method of YBCO magnetization in liquid nitrogen that only requires the applied field to be at the level of a permanent magnet. Moreover, rather than applying a pulsed high current field on the YBCO, we use a thermally actuated material (gadolinium) as an intermedia and create a travelling magnetic field through it by changing the partial temperature so that the partial permeability is changed to build up the magnetization of the YBCO gradually after multiple pumps. The gadolinium bulk is located between the YBCO and the permanent magnet and is heated and cooled repeatedly from the outer surface to generate a travelling thermal wave inwards. In the subsequent experiment, an obvious accumulation of the flux density is detected on the surface of the YBCO bulk.

Effects of Lateral-Tailoring of Coated Conductor for Ac Loss Reduction of Superconducting Power Transmission Cables

The effects of removing low-Jc edges of coated conductors by a laser for ac loss reduction were studied in two-layer superconducting power transmission cables. By removing the low-Jc edges, narrower coated conductor with more uniform Jc distribution can be obtained. The original 5 mm-wide coated conductors as well as the edge-removed 4 mm-wide and 3 mm-wide coated conductors were assembled spirally around cyrindrical formers in two layers to form a cable. The measured ac losses were compared with the ac losses calculated using a numerical model where the spiral structure is neglected. For ac loss calculations, we used the lateral Jc distribution of the coated conductor measured by the magnetic knife method or those which are assumed based on the losses of the critical current by the edge removal.

Transport AC Loss Measurements in Single- and Two-Layer Parallel Coated Conductor Arrays With Low Turn Numbers

We measure transport ac losses in planar one-layer four-turn parallel coated conductor arrays (1 × 4) and in two-layer (2L) four-turn parallel coated conductor arrays (2 × 4) with a frequency up to 1 kHz. The horizontal separation between the conductors, i.e., gh, and the vertical separation between the neighboring superconducting layers, i.e., gv, were varied to investigate the transport ac loss dependence on gh and gv. In 2L arrays, the tapes in the top layer sit either aligned with the tapes in the bottom layer or aligned with the gaps between the tapes in the bottom layer. We show that the losses differently scale in arrays with low turn numbers to the scaling expected with an infinite array of tapes.

Effects of Unevenly Distributed Critical Currents and Damaged Coated Conductors to AC Losses of Superconducting Power Transmission Cables

Two groups of superconducting power transmission cables composed of two layers of coated conductors with 4 mm width and 2 μm superconductor-thickness have been designed. In one group, four styles of unevenly distributed critical currents (<i>I</i>_c) are applied, while in the other group there is one damaged coated conductor in each cable. Trapezoidal critical current density (<i>J</i>_c) distribution with a sloping shoulder of 0.3 mm is assumed while calculating the AC losses of these cables numerically by using a one dimensional FEM model. Numerical results show that unevenly distributed <i>I</i>_c increase AC losses along with the increasing variance ratio which defines the difference between individual <i>I</i>_c and the average <i>I</i>_c. Even only one damaged coated conductor can dramatically increase AC losses no matter which layer it locates at. The detailed AC loss distributions among coated conductors are presented and the reasons for different influences of unevenly distributed <i>I</i>_c as well as damaged coated conductor are discussed.

Thermally Actuated Magnetization Method in High Temperature Superconductor Bulks

It is particularly attractive for High Temperature Superconductors (HTS), such as the melt-processed YBCO single-grain bulks, to trap strong magnetic fields at cryogenic temperatures. With the flux density orders of magnitude much greater than the rare earth magnets, the YBCO bulks can be formed as small and as compact as them. As a result, YBCO bulks are used as magnets in magnetic bearings, MRI scanners and motors, etc. The trapped field by the YBCO magnet is decided by the magnetization, which normally includes three different ways. However, the traditional ways to magnetize the YBCO will always need the applied field to be as high as the expected field on the superconductor or much higher than it. In this paper, we will describe a technique which facilitates the creation of the high magnetic field and utilize a normal permanent magnet in stead. By using this rare earth permanent magnet which is not as strong, this technique involves the superconducting flux pump concept, which means that a much larger field being trapped by the superconductor bulk is caused by a small field repeatedly applied to it. To achieve such pumping effect, an intermedia material is necessary and in the related experiments, Gadolinium is used as such an important thermally actuated material. Traveling magnetic field through it will magnetize the YBCO in such a way that the flux density will be accumulated step by step on the surface on the bulk.

Potential for Torque Density Maximization of HTS Induction/Synchronous Motor by Use of Superconducting Reluctance Torque

We try to realize the ultimate maximization of torque density in a high-temperature superconducting induction/synchronous motor (HTS-ISM) for transportation equipment. Although the basic structure of the HTS-ISM is almost the same as that of the conventional squirrel-cage induction motor, it can rotate at high-efficiency synchronous speed by the use of superconducting (zero resistance) rotor windings. It has been also shown that torque density can be enormously enhanced at such synchronous rotation mode. In this study, in order to further enhance such torque density, a rectangular superconducting bulk shield is installed in the rotor core. First, we clarified the effectiveness of the HTS bulk based on the analysis and the experiment. Moreover, we succeeded in obtaining no-load and load curves for the speed range from 300 to 1800 r/min.

Numerical analysis of thermally actuated magnets for magnetization of superconductors

Superconductors, such as YBCO bulks, have extremely high potential magnetic flux densities, comparing to rare earth magnets. Therefore, the magnetization of superconductors has attracted broad attention and contribution from both academic research and industry. In this paper, a novel technique is proposed to magnetize superconductors. Unusually, instead of using high magnetic fields and pulses, repeatedly magnetic waves with strength of as low as rare earth magnets are applied. These magnetic waves, generated by thermally controlling a Gadolinium (Gd) bulk with a rare earth magnet underneath, travel over the flat surface of a YBCO bulk and get trapped little by little. Thus, a very small magnetic field can be used to build up a very large magnetic field. In this paper, the modelling results of thermally actuated magnetic waves are presented showing how to transfer sequentially applied thermal pulses into magnetic waves. The experiment results of the magnetization of YBCO bulk are also presented to demonstrate how superconductors are progressively magnetized by small magnetic field

Magnetization of Bulk Superconductors Using Thermally Actuated Magnetic Waves

A novel technique is proposed to magnetize bulk superconductors, which has the potential to build up strong superconducting magnets. Instead of conventionally using strong magnetic pulses, periodical magnetic waves with strength as low as that of rare-earth magnets are applied. These magnetic waves travel from the periphery to the center of a bulk superconductor and become trapped little by little. In this way, bulk superconductors can gradually be magnetized. To generate these magnetic waves, a thermally actuated magnet was developed, which is constructed by a heating/cooling switch system, a rare-earth bulk magnet, and a Gadolinium (Gd) bulk. The heating/cooling switch system controls the temperature of the Gd bulk, which, along with the rare-earth magnet underneath, can transform thermal signals into magnetic waves. The modeling results of the thermally actuated magnet show that periodical magnetic waves can effectively be generated by applying heating and cooling pulses in turn. A YBCO bulk was tested in liquid nitrogen under the magnetic waves, and a notable accumulation of magnetic flux density was observed.