Current distribution in parallel-connected YBCO high-temperature superconducting elements with different critical currents

A high-temperature superconductor is investigated for application to a fault-current limiter. However, the current-carrying capacity of a single element is too low for practical use. This parallel connection of superconducting elements is required to increase the current capacity. This paper discusses an ac current distribution between two YBa₂Cu₃O_7-x superconducting elements connected in parallel that differ in electrical aspects such as a critical current level and voltage-current characteristic. As the instantaneous value of the current rises from zero to peak, the following processes are observed: (1) the transition from superconducting to normal conducting state is initiated in the element with the smaller critical current; (2) the current value in the element is kept constant until the other element with the larger critical current level becomes normal conducting state; and (3) only when the instantaneous value of the ac current exceeds the summation of the individual critical current values of each element does the voltage across parallel-connected elements appear. These results are caused by the quick recovery characteristic of YBCO superconductors from the normal conducting state to the superconducting state.     

Observation of the behavior of a laser-ablated plume by laser imaging spectroscopic techniques

In order to investigate the behavior of species in the laser-ablation plume during the pulsed-laser deposition (PLD) process, the time-resolved two-dimensional laser-induced fluorescence spectroscopy (2D-LIF) and Rayleigh-Mie scattering (2D-RMS) systems have been developed. Results on PLD of high-temperature superconducting thin films of YBa₂Cu₃O_7-x (YBCO) are presented. First, the effects of Doppler and collisional quenching on the sensitivity of 2D-LIF are examined. Then the spatial distributions of the ground state Cu atoms observed by these techniques, as well as excited particles in the laser ablation process of YBCO, are presented. The effect of substrate heating on plume propagation has been observed for the first time. 2D-RMS was applied to visualize the behavior of particles directly ejected from the target and of particles condensed in the gas phase.     

Proposal of flux‐lock‐type fault current limiter with high‐TC superconducting element

This paper proposes a new type of fault current limiter (FCL), which consists of a high‐T_C superconducting (HTS) element and two coils wound on the same core without any leakage magnetic flux. In this FCL, either the limiting impedance or the initial limiting current level can be controlled by adjusting the inductances and the winding direction of the coils. Therefore, this FCL could relax the material restrictions on high‐T_C superconducting FCL. A current‐limiting experiment by a model FCL was carried out, and the limiting performance was observed. The initial limiting current level of the model FCL was 1.7 times higher than the critical current of the HTS element, and the fault current is suppressed to 52% immediately after the short‐circuit in the test. Considering voltage–current characteristics of a high‐T_C superconductor in a computer simulation, the calculated results almost agreed with the experimental results. © 1999 Scripta Technica, Electr Eng Jpn, 127(1): 31–38, 1999     

A shell type superconducting transformer for experiments using Nb3Sn superconducting cables

A shell-type superconducting transformer was developed for experiments using Nb₃Sn superconducting cables. The designed capacity is 667 kVA (single phase), the voltage is 440/220 V, the current is 1515/3030 A and the percent impedance is 16 percent. Main features of the transformer are as follows: (1) Magnetic field in superconducting coils is decreased by increasing the number of high and low coil groups. (2) Large-scale superconducting cables are not needed when the number of high and low coil groups is increased. (3) Epoxy impregnated coils are used to withstand an electromagnetic force at 120 Hz. The Nb₃Sn basic strand was manufactured by the internal tin diffusion process. The cable consists of seven insulated subcables, and the subcable consists of seven strands. The primary (HV) coil of the transformer was excited, in which the secondary (LV) coil was shortened. The primary current reached 1618 A_rms without quenching, and the reached capacity corresponds to 712 KA. © 1997 Scripta Technica, Inc. Electr Eng Jpn, 118 (3): 13–21, 1997     

高温超导线圈在工频交流下的失超判断

运行于电力系统中的超导电力装置可能会遭受交流过电流故障,从而导致超导态转变到常态(失超),因此检测失超是超导电力装置实际应用的重要基础。然而在交流工作条件下,超导线圈产生很大的电感电压,其幅值远大于超导线圈失超后的常态电阻电压,给失超判断带来困难。笔者应用超导线圈在交流下电感电压和电阻电压的相位变化关系原理,经过实验研究,提出了基于相位变化的超导线圈在交流下的失超判断方法,为超导线圈在交流下的失超检测提供了新的技术途径。     

Possible interrelations between ferroelectrics and superconductors

Abstract

Ferroelectrics and high-temperature superconductors have a variety of features in common which suggest similar mechanisms to be responsible for their occurrence. Ferroelectricity has been shown to be due to an onsite anharmonic electron-phonon interaction. In conventional superconductors, an attractive electron-phonon interaction compensates the Coulomb repulsion, thus leading to the paired electron state. For high-temperature superconductors it will be shown that again a mechanism based on electron-phonon interactions can lead to strong increases in the superconducting transition temperature T_c, depress the isotope effect and vary the gap to T_c ratio. In spite of the close relationship between ferroelectrics and oxide superconductors, it is concluded that both phenomena are mutually exclusive.     

Development of NbTi superconducting strands and cables for the field winding of a 200‐MW‐class high‐energy‐density superconducting generator

The NbTi superconducting strands and cables for the field winding of the 200‐MW‐class high‐energy‐density‐type superconducting generator are developed. They are composed of Cu/Cu‐10wt%Ni/Nb‐46.5wt%Ti superconducting strands and the 10‐kA (at 5 T)‐class 9‐strand compacted cables. The diameter of strands is 1.33 mm, and the 9‐strand compacted cables are 2.4 mm thick and 6.0 mm wide. In order to produce high‐current‐density NbTi strands, we made strands under controlled aging heat treatments, the total and final strains, and the strains between heat treatments, by using large‐scale extruder. Moreover, in order to produce high‐stability and low‐AC‐loss NbTi strands and cables, the matrix ratio of strands and the cross sections of strands are optimized. The current density of NbTi filaments for the four‐time‐aging manufactured 1.33‐mm‐diameter strands was J_C=3150 A/mm² at 5 T, 1150 A/mm² at 8 T. The critical current of the 9‐strand compacted cable is 10.7 kA at 5 T. The AC losses of the final compacted cables are less than 100 kW/m³ at 5 T, 5 T/s, that is, decreased to less than half of the target of the AC loss value (< at 5 T, 5 T/s). Compared with the strand (Cu ratio 1.77), the minimum quench energy (MQE) of the strand (Cu ratio>2) increased about 40% at the operation mode current of the superconducting generator. © 2006 Wiley Periodicals, Inc. Electr Eng Jpn, 156(3): 24– 31, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20266     

Development of a 400 kJ Nb3Sn superconducting magnet for an SMES system

An Nb₃Sn superconducting magnet to store 400 kJ was developed as a unit magnet for a 2.4-MJ SMES system used for stabilization studies of electrical power systems. The superconducting magnet consists of a cryostat and an Nb₃Sn coil. The dimensions of the coil are: 340 mm inner diameter, 700 mm outer diameter and 177 mm axial length. The pool-cooled coil is a stack of 20 Nb₃Sn double pancakes, and the cooling channels are aligned between pancake coils. To reduce Joule loss in electrical power converters, the maximum operating current of the coil is designed to be 350 A, which is one order of magnitude less than the operating currents of similar scale coils for pulse use. The conductor is an Nb₃Sn monolithic conductor with cross section 1.50 × 2.38 mm. For good superconducting stability and high dielectric strength of the coil, the Nb₃Sn double pancakes were wound by the react-and-wind technique. Operation of dc current to 105% (367.5 A) of the design operating current was achieved without quench. After the whole of the coil was exposed out of liquid helium, the coil did not quench under 120 A current operation for more than 2 hours. It was verified that the coil was stable for the SMES system. © 1998 Scripta Technica, Inc. Electr Eng Jpn, 121(3): 44–52, 1997     

Magnetic-shield type superconducting fault current limiter with high Tc superconductors

The continuous development of electric power utilities has led to the increase in problems concerning fault currents. The use of fault current limiters, which suppress fault currents below a certain level, has been examined widely. The authors have studied a superconducting fault current limiter that is based on the magnetic shielding characteristics of superconductors. In this paper, three kinds of fault current limiters with high T_c superconductors and their results tested at liquid nitrogen temperature are presented. The experimental results show that the impedances of limiters in the fault condition became higher than those in the normal condition but were lower than the anticipated ones. Next, a numerical calculation model to analyze the experimental results is presented. The calculation results with this model agreed with the aforementioned experimental ones very well. From these studies it is concluded that it is necessary for the sudden transition from the superconducting state to the fully resistive state to be caused by suitable means.     

Design guidelines of a flux‐lock superconducting fault current limiter with ac magnetic field coil for a 6.6‐kV distribution system

We have proposed a new type of fault current limiter, which consists of a flux‐lock reactor with high‐Tc superconducting (HTS) elements and an ac magnetic field coil (Flux‐Lock‐Type Fault Current Limiter: FLT‐FCL). The FLT‐FCL can increase both the current capacity and the limiting impedance by means of a transformer action and an ac magnetic field application mechanism. This paper reports the conceptual design of an FLT‐FCL for application to a 6.6‐kV/200‐A distribution system. Theoretical expressions for the current limiting behavior are derived and the new concept of “quench power” is proposed in order to estimate the required number of HTS elements for two types of FLT‐FCL and for a basic FCL type consisting only of HTS elements. Design guidelines for the FLT‐FCL are derived from the calculation results. © 2001 Scripta Technica, Electr Eng Jpn, 135(4): 17–25, 2001     

SPECTRAL-DOMAIN MODELLING OF SUPERCONDUCTING MICROSTRIP STRUCTURES

We present an analysis of thin microstrip superconducting structures in which the effects of the superconducting material, of finite complex conductivity and finite thickness, are taken into account through the concept of sheet impedance coupled with an extension of the Spectral Domain Immittance Approach. The effect of the magnetic field on the penetration depth in the superconductor is included by solving the Ginzburg–Landau equations within the thin-film approximation (t/λ₀ ≪ 1). The state of the material is determined self-consistently in such a way that when the critical field is exceeded, the material systematically shifts from the superconducting to the normal state. Special attention is paid to the behaviour of the system in the neighbourhood of the critical temperature, T/T_c ≃ 1. We report that, in some cases, the propagation constant of a superconducting microstrip line attains a maximum when the real and imaginary parts of the conductivity are approximately equal. Numerical results for a microstrip line and a patch resonator are presented and compared with available data to document the validity of the approach. © 1997 John Wiley & Sons, Ltd.     

Nb3Sn single-phase targets for fiber-reinforced-superconductors and technique of preparing superconducting films without post annealing

A new type of superconductor is being developed for the application of high-field pulsed superconducting magnets, which are called fiber-reinforced-superconductors (FRS). A filament of FRS consists of a reinforcement fiber with high elastic modulus and a superconducting layer around it. The strain can be reduced against electromagnetic force because of highelastic modulus fibers. If Nb₃Sn target is utilized, the preparation process of FRSs is shortened and there is the possibility to produce Nb₃Sn layer which leads to high current density because of its uniform stoichiometric Nb₃Sn layer. This paper presents first a technique of preparing a single-phase target of Nb₃Sn to produce superconducting layers on reinforcement fibers of FRSs. Mixture of niobium and tin powder was reacted in a furnace filled with argon gas to be Nb₃Sn, and then it was broken and cast into a stoichiometric target with a diameter of 77 mm. The critical temperature of the target itself was 17.5 K. The thin film deposited with the target showed no superconductivity without post annealing, but it had critical temperature from 14.5 K to 12.5 K after heat treatment. Second, a new target containing rich tin was prepared and the film deposited with it had superconductivity without post annealing.     

超导磁体失超检测电路的设计

超导磁体的失超检测与保护是超导电力技术实用化的一个重要课题。为了实现对超导储能混合磁体的失超检测与保护,本论文根据混合磁体的特点及结构,在现有有源功率检测的基础上,提出了一种针对超导混合磁体的电压校正电路,通过对混合超导磁体上产生的失超信号进行隔离、放大、滤波以及比较等,实现失超信号的检测。设计并完成了失超检测系统硬件电路的制作与调试。通过搭建高温超导线圈的实验装置,在高温超导储能磁体上进行失超检测的实验研究,得出了电压矫正前后的线圈电压波形。给线圈分别以不同速度充电,研究充电速度对阀值电压、失超的起始点及临界电流的影响。实验线圈电压波形表明,该失超检测系统能及时、有效地检测超导混合磁体失超的发生。     

Current distribution in superconducting multistrands for ac use before and after quenching

A superconducting multistranded cable is used to realize high current capacity for ac use. The critical current value of the cable is reported to be less than the simple summation of the individual critical current value of each strand. The causes for such a degradation of the critical current value have not been revealed. This paper investigates the current distribution in multistrands before and after their quenching by using seven-strand superconducting cable and 7x7 cable. The following experimental results are derived: (1) the quenching is initiated at one strand in the cable; (2) the current in the quenched strand is transferred into the other strands; (3) an avalanche of quenching is induced among the strands; and (4) the central strand is quenched finally among the strands. The critical current values of the 7- and 7 × 7-stranded cables also are measured. These values are in good agreement with the predicted values based on the mutual inductance among the strands. It is concluded that the unbalance of the current distribution in the superconducting multistrands can be one of the promising causes for the degradation of the critical current value.     

New type of Nb3Sn fiber-reinforced superconductors for high-field pulsed magnet and effect of thermal stress

A new type of superconductor with high elastic modulus fibers is being developed for the application of high-field pulsed superconducting magnets called Fiber-Reinforced Superconductors (FRS). FRSs have great potential for the construction of a 15-T class pulses magnet with its size kept equal to ITER because stainless steels of cable-in-conduit-conductors could be reduced considerably. This paper presents a technique of preparing FRS and measuring its superconducting characteristics including strain-Ic relation. FRS has a critical current density of 600 (A/mm²/initial niobium) at 15 T, which is almost equal to one of the commercial bronze-processed wires. The intrinsic strain vs. I_c characteristics are similar to that of bronze-processed wire. Thermal strain on FRS also is discussed because materials with high elastic modulus tend to have low thermal contraction which leads to degradation of the superconducting characteristics of the Nb₃Sn layer. Possibilities of overcoming the degradation while maintaining very high elastic modulus of tungsten fiber for reinforcement are shown.     

Fundamental performance of superconducting fault current limiter with two air core coils

In this paper, we theoretically compared the fundamental characteristics of fault current limiter (FCL) with high‐Tc superconductor (HTS) and two coaxial air‐core coils based on steady‐state analysis. Two types of FCL are possible. One is parallel type and the other is transformer type. The parallel type can be divided into two types according to the combination of the winding direction of coils. That is, there are two cases that the coils are wound so that the magnetic fluxes induced by coils reduce and increase each other. In this paper, we called them parallel type 1 and 2, respectively. There is no significant difference in the HTS volume required to satisfy both the specified limiting impedance Z_FCL and initial current I_ini in limiting operation among those three FCLs although the HTS in each type of FCL has different length and cross‐sectional area. In the cases of those FCLs, we can improve the current limiting performance by arranging the HTS in the coils and applying the magnetic flux to the HTS in the limiting operation. The magnitudes of the magnetic flux density are almost the same. From the viewpoint of the FCL impedance in normal operation, parallel type 1 has the most desirable structure. On the other hand, transformer type is the best to eliminate the magnetic flux applied to the HTS in the normal operation. © 2008 Wiley Periodicals, Inc. Electr Eng Jpn, 165(3): 29–36, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20477     

Electrical insulation technology for superconducting magnets inJapan

Electrical insulation of superconducting magnets needs to endure a variety of voltages with a large range of shapes, magnitudes and duration under normal operation, during coil quenching, and at withstand voltage tests. The author reviews the current state of electrical insulation technology for superconducting magnets in Japan. He maintains that Japan will continue to aggressively pursue the research, development and application of superconducting magnets. In accordance with this, the main technical subjects to be solved in the insulation of the superconducting magnets in the near future will be: (1) the development of insulation that has high electrical, mechanical and radiation-resistant properties at the cryogenic condition; and (2) investigation into the long-range effects of insulation on the stability of superconducting magnets     

Impedance spectroscopy study and electrical behavior of PMN-PZT actuators joined by ceramic slurry

The solid solution of Pb(Mg,Nb)O₃-Pb(Zr,TiO) O₃ materials have high piezoelectric constant of 600 ∼650 pC/N and electromechanical coupling coefficient k _p of 0.65. Due to such high piezoelectric constant and electromechanical coupling coefficient, Pb(Mg,Nb)O₃-Pb(Zr,TiO)O₃ materials have been attracted attentions for the applications of multilayer ceramic actuators. Actuators can be produced by staking piezoelectric ceramic materials and inner electrodes, alternatively. However, it is difficult to fabricate huge ceramic actuators without any serious problems during the process conditions. Because ceramic are easily cracked during the sintering process, due to strikingly different shrinkage rate between the ceramic materials and metal electrodes. In this research, new jointing methods will be proposed for huge ceramic actuators, and then their electrical properties were investigated. Time dependent leakage current and impedance spectroscopy were employed to expect device performances.     

Increase of inductance ratio of flat type fault current limiter with high‐Tc superconducting disk

Fault current limiters (FCLs) are expected to reduce fault current and improve stability of power systems. A flat type fault current limiter consisting of the pancake primary winding and a high‐T_c superconducting (HTS) disk was proposed as a modified version of a conventional magnetic shield type of the FCL with the superconducting cylinder. The flat type FCL has the advantage of being manufactured smaller than the cylinder type FCL. It is pointed out, from a magnetic field analysis, that the volume of the flat type FCL is smaller than that of the cylinder type FCL with same magnitude of the limiting inductance. Further, the magnetic field analysis suggests that a high inductance ratio can be realized by radially enlarging both the primary winding and the HTS disk, stacking the FCL modules in layers and sandwiching the primary winding with superconducting disks. © 2006 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

Magnetic instability of ac multifilamentary superconductors in spatially distributed magnetic field

Alternating current (ac) superconducting machines such as superconducting generators, transformers, or resistive current limiters need large‐current‐capacity conductors. These conductors are generally fabricated as multistrand cables stacked with multifilamentary NbTi superconductors whose current capacity is a few tens of amperes. ac quench current degradation has often been observed in ac use of such multistrand cables. Several reasons for this degradation have been pointed out: nonuniformity of each strand current, mechanical disturbances, thermomagnetic instability, and ac losses. However, it has not as yet been overcome. Since the angle between the strand axis and the cable axis changes along the cable axis in multistrand cables, the strands are exposed to a spatially (axially) distributed magnetic field, which has longitudinal and transverse components changing periodically due to multiple cabling. This paper mainly discusses the thermomagnetic instability due to the distributed transverse magnetic field, which was compared with the self‐field instability and the longitudinal field instability experimentally and theoretically. It was confirmed that the ac quench current degradation due to the distributed transverse field could be induced by the following: nonlinear E–j characteristic, strong dependence of E–j characteristic on magnetic field in low field region, uniformity of the axial current profile inside the superconductor with the high resistive matrix, and the poor thermal diffusion of CuNi/NbTi composites. © 2000 Scripta Technica, Electr Eng Jpn, 131(1): 45–55, 2000