Indirect measurement of AC current distributions in HTS tapes by a pickup coil method

It is necessary to develop HTS conductors with a large current capacity and low AC loss characteristics for practical use of HTS power devices. For large capacity power applications, HTS tapes such as Bi-2223 tapes and YBCO coated conductors are assembled. So, it is important to evaluate the characteristics of current distributions in such assembled HTS tapes. However, an adequate measurement method has not been established. In this study, we have tried the development of an indirect measurement method to evaluate the current distributions in stacked HTS tapes. In this method, the current distributions are indirectly found from the results of the field distributions measured by a pickup coil located around the surface of the sample tapes. To confirm the accuracy and the sensitivity of this method, current distributions in a test conductor made of copper tapes are measured. Numerical and measured current distributions are compared. Using this method, the current distributions in stacked Bi-2223 tapes are measured at room temperature and 77 K. On the basis of the obtained results, the characteristics of the AC current distributions in the HTS tapes are discussed.     

准TEM高温超导微带线电容等效电路模型及其分电容模型

本文根据高温超导厚膜和薄膜场和电流的分布特点,给出厚膜和薄膜准TEM微带线电容等效电路及分电容表达式.这些表达式的正确性通过三个例子证实.     

Calculating transport AC losses in stacks of high temperature superconductor coated conductors with magnetic substrates using FEM

In this paper, the authors investigate the electromagnetic properties of stacks of high temperature superconductor (HTS) coated conductors with a particular focus on calculating the total transport AC loss. The cross-section of superconducting cables and coils is often modeled as a two-dimensional stack of coated conductors, and these stacks can be used to estimate the AC loss of a practical device. This paper uses a symmetric two dimensional (2D) finite element model based on the H formulation, and a detailed investigation into the effects of a magnetic substrate on the transport AC loss of a stack is presented. The number of coated conductors in each stack is varied from 1 to 150, and three types of substrate are compared: non-magnetic weakly magnetic and strongly magnetic. The non-magnetic substrate model is comparable with results from existing models for the limiting cases of a single tape (Norris) and an infinite stack (Clem). The presence of a magnetic substrate increases the total AC loss of the stack, due to an increased localized magnetic flux density, and the stronger the magnetic material, the further the flux penetrates into the stack overall. The AC loss is calculated for certain tapes within the stack, and the differences and similarities between the losses throughout the stack are explained using the magnetic flux penetration and current density distributions in those tapes. The ferromagnetic loss of the substrate itself is found to be negligible in most cases, except for small magnitudes of current. Applying these findings to practical applications, where AC transport current is involved, superconducting coils should be wound where possible using coated conductors with a non-magnetic substrate to reduce the total AC loss in the coil.     

Single-phase AC losses in prototype HTS conductors for superconducting power transmission lines

We report single-phase AC loss measurements on 8-, 4-, and 3-layer, multi-strand, HTS prototype conductors for power transmission lines. We use both calorimetric and electrical techniques. The agreement between the two techniques suggests that the interlayer current distribution in 1-m long conductors are representative of those in long conductors. The losses for the 8- and 4-layer conductors are in rough agreement, with the 8-layer losses being somewhat lower. The 3-layer conductor losses are substantially higher — probably due to unbalanced azimuthal currents for this configuration.     

新型高温超导扭绞缆线的临界电流评估方法及其特性研究

高温超导缆线是研究各种大电流高温超导应用的基础.将超导材料通过不同方式组合成复合导体,不仅可以让缆线的载流量得到提升,更可以在机械强度和电磁特性等方面得到改善.在国内外众多研究缆线的团队中,上海交通大学超导团队提出了一种基于YBCO高温超导带材的新型高温超导扭绞缆线结构.这种结构在理论上不仅实现了载流量和机械强度的平衡,还通过扭绞改善了电磁特性[4-5].但是目前对于这种缆线的研究还处于起步阶段,特别是对其临界电流特性的研究.因此为了研究新型高温超导扭绞缆线的临界电流特性,本文从新型高温超导扭绞缆线的结构出发,通过仿真和实验两个角度对新型高温超导扭绞缆线的临界电流特性进行研究.在仿真方面,用H方程法计算缆线的临界电流,并探究扭绞节距是否会对临界电流的衰减产生影响.在实验方面,研究出了一种最适合于该缆线结构的临界电流评估方法“SVWE-copperwire-TU”.并用该方法测量了新型高温超导扭绞缆线的临界电流特性.本研究的结论对于后续制作更大电流的新型高温超导扭绞缆线具有重要意义.     

EAST 装置新超导馈线系统冷却回路设计

EAST 装置电流馈线系统由13 对NbTi 材料CICC 超导母线组成,连接超导磁体系统和高温超导电流引线,为磁体系统的运行提供馈电通道和失超状态下能量的释放通道。馈线系统中超导母线由液氦制冷机通过低温分配阀箱提供的4.5K 超临界氦冷却,冷却通道进口压力为0.45MPa,出口压力不小于0.25MPa。通过对超导母线和接头内流道中的迫流氦的压降计算确定了冷却流程方案为1 对TF 超导母线单独串联组成1 个冷却通道, 12 对PF 超导母线分为6 个冷却通道,每个通道由2 对PF 超导母线串联组成。     

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EAST 装置电流馈线系统由13 对NbTi 材料CICC 超导母线组成,连接超导磁体系统和高温超导电流引线,为磁体系统的运行提供馈电通道和失超状态下能量的释放通道。馈线系统中超导母线由液氦制冷机通过低温分配阀箱提供的4.5K 超临界氦冷却,冷却通道进口压力为0.45MPa,出口压力不小于0.25MPa。通过对超导母线和接头内流道中的迫流氦的压降计算确定了冷却流程方案为1 对TF 超导母线单独串联组成1 个冷却通道, 12 对PF 超导母线分为6 个冷却通道,每个通道由2 对PF 超导母线串联组成。     

Japanese efforts on coated conductor processing and its power applications: New 5 year project for materials and power applications of coated conductors (M-PACC)

Five years of Japanese national project (FY2003–FY2007) was ended last spring with remarkable success. The national project was originally aimed for development of coated conductors which have high superconductivity performance and long length enough to fabricate high temperature superconducting (HTS) electric power devices. Preliminary research and development of HTS electric power devices were carried out as well. A series of R&D results will be summarized and reviewed in this paper.The new 5 years Japanese national project has started last June (FY2008–FY2012) to develop HTS electric power applications including SMES, cables and transformers and to develop/produce coated conductors satisfying the requirements from the respective power devices. Collaborative R&D efforts by national laboratories, universities and private companies have been started with the supports of NEDO and METI.The accomplishment of the previous project will be summarized and the plans and goals of the new project will be presented in this paper.     

YBCO超导体的电工学应用研究进展

超导体具有一些完全区别于传统电工导体和电工磁性材料的电磁特性,因而在电工学领域具有广泛的应用价值。自从1987年发现高温超导体以来,高温超导体的电工学应用研究得到了快速的发展。YBCO超导材料具有比BSCCO超导材料更加优越的电工学应用性能,因而随着YBCO超导材料逐渐实现小批量供货,近年来有关YBCO超导体的电工学应用研究得以广泛的开展。文章重点介绍国内外在YBCO材料(包括块材和带材)及其在电力技术和磁体技术等方面的应用研究进展,以此纪念YBCO超导体发现30周年。     

5 kVA高温超导变压器双饼线圈不同有限元模型对比分析

本文基于5 kVA 高温超导变压器的高压绕组双饼线圈, 运用有限元软件, 完成了均质化建模. 相比于全模型, 采用均质化模型不仅不影响计算精度, 而且大大减少了计算时间, 同时能够快速有效地计算出双饼线圈的磁场分布、 电流密度分布以及线圈交流损耗. 为了验证双饼线圈有限元计算模型的正确性, 本文进行了 H 方程全模型、H 方程均质化模型、T-A 方程全模型及T-A 方程均质化模型下的对比仿真分析. 仿真结果的一致性表明了双饼线圈有限元计算模型的正确性. 本文为后续基于T-A 方程下高温超导变压器均质化模型的研究提供了参考.     

高温超导变压器中带材临界电流随磁场变化的研究

通过自行设计的小型磁体产生稳态磁场,测量了高温超导带材在不同磁场下的临界电流,得到带材临界电流随外加磁场变化的曲线．并提出了高温超导变压器漏磁场优化设计的目标．     

35kJ高温超导储能磁体的优化设计

介绍了35kJ高温超导储能磁体的设计步骤。为了减少常规寻优方法的计算量,将加权粒子群优化算法(WPSO)引入到高温超导储能磁体的优化设计中,给出了用B i-2223超导线材进行35kJ储能磁体设计的优化结果,分析了高温超导线材的各向异性对磁体临界电流的影响,并比较了两种不同的单螺线管磁体结构在不同优化目标的情况下,超导线材用量和杂散磁场的分布的变化。     

Induced current of high temperature superconducting loops by combination of exciting coil and thermal switch

With its commercialization, the second-generation (2G) high temperature superconducting (HTS) RE—Ba—Cu—O (REBCO, RE is rare earth) tape is extensively applied to the superconducting magnets in the high magnetic fields. However, unlike low temperature superconducting (LTS) magnets, the HTS magnet cannot operate in the persistent current mode (PCM) due to the immature superconducting soldering technique. In this paper, an exciting method for two HTS sub-loops, so-called charging and load loops, is proposed by flux pump consisting of exciting coil and controllable thermal switch. Two HTS sub-loops are made of an REBCO tape with two slits. An exciting coil with iron core is located in one sub-loop and is supplied with a triangular waveform current so that magnetic field is generated in another sub-loop. The influence of magnetic flux on induced current in load loop is presented and verified in experiment at 77 K. The relationship between the induced magnetic flux density and the current on the sub-loops having been calibrated, magnetic flux density, and induced current are obtained. The results show that the HTS sub-loops can be excited by a coil with thermal switch and the induced current increases with magnetic flux of exciting coil increasing, which is promising for persistent current operation mode of HTS magnets.     

Towards beyond-1 GHz solution NMR: internal 2H lock operation in an external current mode

We have commenced a project to develop a beyond-1 GHz solution NMR spectrometer using a HTS coil. Due to a small residual resistance present in the HTS conductor and joint resistance between conductors, a stable persistent current sufficient for NMR measurements is unlikely. Therefore, a current has to be supplied to the HTS coil from an external power supply. The ripple of an external power supply causes a field fluctuation which must be stabilized. In this study we show results of NMR measurements using a 500-600 MHz NMR in such an external current mode: the field fluctuations are stabilized by an internal 2H lock. The field fluctuation from the external power supply comprises a major field fluctuation component at low frequencies, 0.003-0.005 Hz, and superimposed minor field ripples at 2 Hz and 50 Hz. The former limits the time interval of the internal 2H lock, while the latter generates sidebands in the NMR spectrum. Sideband and baseline noise are controlled by appropriate selection of the feedback loop parameters of the lock. The quality of the 1D-solution NMR spectra observed in external current mode is equivalent to that obtained in persistent current mode. However, if the feedback loop time is as short as the gradient pulse width, refocusing of the NMR signal is lost and NMR peaks disappear. The 2D-NOESY and the 2D-HSQC spectra of ubiquitin in an external current mode have been acquired. The quality of the 2D spectra is equivalent to those obtained in persistent current mode; i.e. the internal 2H lock operates stably over an experimental time interval of 40-50 min. To realize a beyond-1 GHz NMR spectrometer, further investigations must be made of (i) the long term stability of a DC power supply, (ii) the enhancement of the compensation field limit for the internal 2H lock, (iii) the extension of the helium refill time interval, and (iv) a method to correct the field homogeneity in the external current mode.     

YBCO线圈在低温下的温度仿真分析

近几年,高温超导线材的制造技术已日趋成熟。作为第二代高温超导带材的代表,YBCO在低温下具有的优点使其在超导磁体领域的应用越来越突出。但是在高温超导磁体的运行中会因为受到扰动而失超。为了探讨这个问题,模拟了一个磁体在低温下(20K左右)有加热点扰动时的稳定性情况,即对YBCO线圈在低温下的工作情况进行了模拟仿真。文中根据实验设计的YBCO单饼线圈参数,利用热传导方程和基本电路方程,采用有限元计算方法,模拟出线圈内部的温度分布。分析仿真结果可以为后续实验设计提供参考。     

Investigation of Bi-2223 high temperature superconducting tape as the material for gradient coil in MRI

The use of Bi-2223 high temperature superconducting (HTS) tape as a material for gradient coils in MRI is evaluated in this paper. Bi-2223 tapes have a very high critical current and a very low power loss. A HTS tape gradient coil is expected to provide much higher gradient strength and generate much lower heating than a copper coil. Measurements of the AC power loss of Bi-2223 tapes at typical operating frequencies for gradient coils are presented. The degradation of the critical current and its effect on the increase of AC power loss is analyzed. Practical technical issues such as resistance, gradient strength and mechanical performance are also discussed. A prototype Bi-2223 HTS tape gradient coil is evaluated to verify the concept.     

The transport current redistribution between the core layers on the models of HTS cables

The multilayer conductors with insulated and non-insulated layers are analyzed. The space–time current redistributions between the layers of the conductor are investigated for the next parameters: values of electric resistance at the current leads, transverse contact electric resistance between the layers, the model length, time, the conductor dimension, the current—its peak value, the change law and the ramp of the total current input. Most of the published test results for short (from 1 to 10 m) conductor models do not reflect the true current distribution of the real long length cable conductors. The criterion of experimental model constructions and, in particular, the calculation of its minimum length are presented. The calculations demonstrate the electromagnetic processes in real cable conductors.     

30 mm wide process of GdBCO PLD/clad-type textured metal substrates

Gd₁Ba₂Cu₃O_x (GdBCO) coated conductors using bi-axially textured clad-type substrates have been developed for high temperatures superconducting (HTS) power cables. In this project, large amount of coated conductors (CCs) are necessary for the several tests of basic properties of HTS power cables and the fabrication of a 15 m long 3-in-one HTS model cable. In order to ensure an adequate amount and properties of coated conductors, the stable manufacturing technology and the high production rate at each process are required.In this report, we have newly installed a high power laser of 300 W for the PLD system, resulting in improvement of the manufacturing stability and the deposition rate for GdBCO film. In addition, we have optimized target–substrate distance in PLD process, and the uniformity of I_c distribution across 30 mm wide tapes were remarkably improved. Furthermore, the surface morphology of buffer layers were enhanced by optimizing the deposition condition using electron beam (EB) evaporation method, and the maximum I_c value for a short sample has achieved 497 A/cm at 77 K in the self-field. Based on these progresses, we are currently developing the stable manufacturing process for long CCs with over 400 A/cm.     

ITER高温超导电流引线的高安全分流器研究

电流引线是室温电源电缆与低温磁体之间的电连接部件.高温超导材料在液氮温度下具有零电阻率和低热导率的特性,用它做成的电流引线可以大大减小低温系统的热负荷,从而减少制冷设备投资及系统运行费.高温超导电流引线可以分为阻性换热器段和高温超导段两部分(其中还包括各部件间的连接部分).高温超导段的分流器设计关系到冷端热负荷大小以及超导段失超后的安全问题.为了研究国际热核聚变试验堆(ITER)电流引线高安全性能,专门设计、试验了68kA引线的1/90实验样品.本文通过对比全CuBe(cu-2%Be)分流器、全不锈钢分流器和二元分流器的失冷故障(LOFA)实验结果,证明二元分流器能够克服安全性和冷端漏热矛盾,可以满足ITER高安全性的要求.     

不同载流运行下冷绝缘高温超导电缆的温度场数值分析

针对冷绝缘高温超导电缆(Cold Dielectric High Temperature Superconducting cable,CD HTS cable)在不同载流下的运行稳定性,采用有限元数值分析方法对超导电缆内部温度场进行准确计算和分析。根据超导电缆内部各层传热特征,建立基于ANSYS的CD HTS电缆有限元模型,给出正常载流及故障载流下超导电缆温度场分析传热边界条件,计算得到不同载流情况下超导电缆本体温度分布变化规律,从而为其通流能力影响下的运行稳定性分析提供参考依据,对超导电缆的故障保护具有一定的指导意义。