Stabilization for thermomagnetic instability of CuNi/NbTi superconducting wire in spatially distributed magnetic field

Thermomagnetic instability in superconducting wires composing multistrand cables is a problem in the development of cables with large current capacity. This paper elucidates the quenching properties of ac superconducting wires in a distributed magnetic field applied to the strands in the cable, and the stabilization of the ac superconducting wires considering the effect of the longitudinal magnetic field or the fraction of copper embedded in each strand. First, the degradation of the quench current of CuNi/NbTi superconducting wires in a distributed magnetic field is exhibited with simple test samples. Second, the quench properties of the strand in a (6 + 1)³ cable and the optimal twist pitch of the cable for high stabilization are discussed. Last, the effect of copper on the quench properties of the strand and the appropriate fraction of copper for suppression of quench current degradation in a distributed magnetic field are discussed. © 2001 Scripta Technica, Electr Eng Jpn, 135(4): 26–34, 2001     

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     

Cooling characteristics of a large-current capacity AC superconducting cable

Diameter of the superconducting ac composite strand is small, typically 0.1 mm, because the strand must be twisted in a very short pitch to reduce coupling losses. Therefore, current capacity of the single strand is small and in the range of ～10 A. In large-scale electric power apparatus, the conductors must be able to carry large currents and hundreds of these composite strands should be bundled. The composite strand is highly unstable and susceptible to a very small disturbance due to a frictional wire motion because the main matrix of the wire is highly resistive CuNi. For stable operation of an ac superconducting winding, every strand in the bundle cable should be fixed firmly. An effective technique to fix strands is to impregnate the winding by epoxy. In this case, the ac losses in the winding are to be cooled by heat conduction through the epoxy. Therefore, it is important to estimate the temperature rise of the winding to discuss the fact that the epoxy impregnation technique is applicable to the ac superconducting electric power apparatus. In this paper, the mechanism of thermal conduction of the epoxy-impregnated winding at the 4.2 K region is discussed based on experimental data and the temperature rise of a large-scale cable bundles by 7 × 7 × 7 strands calculated considering thermal resistivity at the interface between the epoxy and the strand. The calculated value agrees well with the measured value.     

管内电缆导体传输交流损耗的理论计算和实验

交流损耗是大型超导核聚变实验装置如国际热核聚变实验堆和中国聚变工程实验堆稳定性的重要指标。然而应用于核聚变装置的管内电缆导体(CICC)在无外场下传输损耗数值计算很少。首先分析基于分布参数等效电路方法对CICC的N根股线的电流进行描述和分析;之后算出整个导体的电流分布,从而求出导体的传输电流损耗;同时,将数值计算结果与临界态模型结果作对比,以验证该计算模型的可行性;提出改进的CICC交流损耗测量方法,为国内下一步测试CICC及其线圈奠定了实验基础;最后,提出适用于单根圆股线的定标法则,为计算CICC的传输电流损耗奠定理论基础。     

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.     

HT-7U管内电缆导体稳定性的仿真与实验研究

管内电缆导体(cable-in-conduit conductor, CICC)是目前大型超导磁体的首选导体,它在大型超导核聚变实验装置及超导储能磁体中的应用具有不可比拟的优越性。为了减小导体的成本,提出超导股线和铜股线混合在一起的结构,因为增加铜比对导体稳定有利,该文制作了4个带有纯铜股线的管内电缆导体,并应用于HT-7U超导托克马克中。利用一维数学模型(Gandalf)对托克马克实际运行模式下CICC的稳定性进行仿真,研究了CICC的稳定性裕度与质量流速率、磁场、运行电流和铜比之间的关系。同时,将理论结果和实验结果进行比较,得到了样品中分离铜的有效比率。     

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     

Development of kA-class alternating current superconducting conductors and fabrication of a prototype coreless superconducting autotransformer

Recently, multifilamentary superconducting wires with very low ac losses have been produced and practical applications will now be considered. To realize actualsize power machines and apparatuses, it is necessary to develop 1 - 10 kA ac conductors. However, the critical currents of multifilamentary wires at 1 T are several tens of A, and therefore it is necessary to use multistrand conductors consisting of several tens or several hundreds of strands. Such conductors sometimes show ac current degradation because of such factors as (1) nonuniform current distribution, (2) wire motion, (3) temperature increase, (4) longitudinal magnetic field effect, etc. Formerly, a coreless transformer was considered unpractical because of its large exciting current. However, Yamamoto and others proposed that a coreless superconducting transformer could be used as a stepdown autotransformer at the receiving side, utilizing its large exciting current as the reactive power source to cancel the charging current of an underground transmission line or UHV line, and therefore the shunt reactors could be eliminated. In this paper, development of ac-superconducting conductors aimed at prevention of current degradation are discussed, as well as quench test results of two small coils made with these conductors. In these conductors, low ac low strands with ultrafine NbTi filaments are twisted around a central bundle of stainless steel wires. One of the coils has been designed as a model coreless autotransformer, and its test result is also described.     

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     

Development of Nb-Ti cable with ultrafine filament and high-resistivity matrix for ac use

Superconducting ac machines such as transformers and reactors are expected to have an important role in future electric power transport lines. In these machines, superconducting coils are wound with superconducting cables that have low ac loss, stable ac quenching current, and high normal resistivity. We have developed Nb-Ti superconducting cables with ultrafine filaments and high-resistivity matrix for these coils. One such cable is a double-stranded round structure using 0.2-mm strands with 0.14-μm filaments and Cu-30wt%Ni as a matrix material. The 50-Hz quenching current without external magnetic field exceeds 1400 Arms. The ac loss is 15 kW/m³ at a transverse external magnetic field of 0.5 T, 50 Hz, and the normal resistivity is 0.21 Ω/m at 0 T, 10 K. © 1997 Scripta Technica, Inc. Electr Eng Jpn, 120(2): 8–18, 1997     

Quenching current level coordination in superconducting power transmission systems

In the present power transmission system, electric power devices are equipped under the concept of insulation coordination and are protected from lightning overvoltage and switching surge by the arrester. However, the future superconducting power transmission system is characterized by the heavy current transmission capability, the lower transmission loss and lightning-surge-free circumstances. Thus, in the superconducting power transmission system, superconducting devices should be protected rather from quenching by the overcurrent of a short-circuit fault than from the breakdown by over-voltage. In this paper, a new concept of the coordination for the superconducting transmission system is proposed as a “quenching current level coordination” instead of the insulation coordination. Furthermore, it is pointed out that the quenching current level coordination has the duality relationship with the present insulation coordination. A superconducting fault current limiter should play an important part of the quenching current level coordination to prevent the other superconducting devices from quenching.     

改进漏感电动势法计算换位线棒股线电流及环流损耗

针对大型水轮发电机定子绕组换位线棒股线电流及环流损耗的计算问题,提出了一种解析计算方法——改进漏感电动势法,解决了现有计算定子换位线棒股线电流分布及环流损耗的漏感电动势法(又称漏感电势法)中存在的漏磁场假设不合理、电路约束不完整以及忽略了股线的实际换位路径等问题。将定子线棒中的每根股线看成由股线电阻和漏感电动势组成,分别给出了槽部漏感电动势和端部漏感电动势的计算方法。以一台140 MW大型水轮发电机为例,采用改进漏感电动势法计算360°全换位和不足换位时的每根股线的电流分布,进而求解定子线棒的环流损耗。通过解析计算结果与实验测量结果对比表明,改进漏感电动势法计算的电流分布及环流损耗值与实验测量值基本一致,验证了解析法的正确性。     

对企业标准OPGW光缆热稳定参数的分析

在OPGW光缆工程设计中,短路电流热容量是一个重要参数,相同组成结构的OPGW光缆,企业标准和国网物料库中的推荐值差异较大。依据规程规定,对短路电流热容量的计算方法和参数取值进行了全面的比较、分析、研究。针对铝包钢组成结构的光缆,合理选择钢、铝导电率计算参数,利用铝包钢导线综合比热法,对光缆短路电流热容量的计算方法进行简化。结果表明,企业标准推荐的短路电流热容量值明显偏少,对企业标准和国网物料库中的推荐值,可用简化计算法验算、修正。     

应用第2代高温超导体的冷绝缘超导电缆输电导体层间均流技术

冷绝缘高温超导电缆的导电层一般设计为多层结构以满足大电流载流特性,但伴随层数的增加,超导体上的集肤效应会引起电缆输电导体各层电流分布不均匀的问题,从而造成电缆损耗增加和传输性能下降。采用基于动态惯性权重因子的粒子群优化算法,提出了电缆导体层电流层间均流优化的设计方法。应用第2代高温超导材料钇钡铜氧涂层导体,通过建立超导电缆的等效电路模型,考虑电场、磁场等约束因素,对一根1km长,110kV/3kA等级的冷绝缘高温超导电缆进行优化设计,获得了电缆本体结构参数及输电导体层和屏蔽层的电流分布。比较优化前后层电流的结果可知,优化后超导电缆各导体层电流与平均电流相比最大不平衡率小于3.5%,各屏蔽层电流达到均布,较好地实现了电缆各导体层电流均匀分布的优化目标。最后,超导模型样缆载流特性实验也验证了优化设计方法的有效性。     

Long‐time constants of irregular AC losses in a large superconducting coil

A large superconducting coil wound with a Cable‐in‐Conduit (CIC) conductor caused an additional AC loss, which cannot be estimated from the short conductor sample test results. It was confirmed that the additional AC loss was generated by long current loops in the CIC conductor. Magnetic field decays of the loops with various long‐time constants were observed through Hall probes. We propose a mechanism for the formation of the long loops. The CIC conductor is composed of several staged subcables. If one strand on the surface of a subcable contacts the other strand on the surface of the adjacent subcable, the two strands must encounter each other again at the LCM (Least Common Multiplier) distance of all staged cable pitches and thereby form a pair of long loops. We numerically traced each strand in the CIC according to a method in which the subcables at all substages rotate around the center of inertia. The calculated long‐time constants of the long loops were slightly shorter than the observed ones. We labeled all strands by order in a real CIC conductor, disassembling the cable carefully after peeling the conduit. It was found that the strands in a triplex were widely displaced from their original positions, so that their contacting lengths became longer than the calculated ones. This fact makes the time constant of the loop longer and hence can explain the observed long‐time constants. The proposed mechanism is effective for estimating the long loops causing additional AC losses in the coil. © 2003 Wiley Periodicals, Inc. Electr Eng Jpn, 143(1): 50–57, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.10064     

汽轮发电机换位线棒环流损耗对定子温度场的影响

为了研究定子换位线棒环流损耗分布对汽轮发电机定子温度场的影响,依据流体力学以及传热学理论,以一台水-氢-氢冷汽轮发电机为例,建立定子三维流体—传热耦合场物理模型和数学模型,采用等效电路网络法计算当定子线棒采用0°/540°/0°换位方式时定子线棒每根股线的环流损耗,在此基础上采用有限体积法求解三维流体—传热耦合方程,得到定子换位股线的环流损耗按平均分布以及按实际分布两种情况下的定子温度。结果表明:定子温度的分布规律与定子换位线棒环流损耗的分布规律有关,当考虑定子换位股线的实际环流损耗分布时,定子铁心和定子线棒最高温度的位置和大小都发生变化。     

高温超导电缆交直流伏安特性测试与分析

为了研究高温超导电缆在直流和交流载流情况下伏安特性的变化规律,提出基于第二代YBCO高温超导体的冷绝缘超导电缆交直流伏安特性测试方法,搭建了伏安特性测试实验系统,通过对一根0.2m长,110k V/1.5k A高温超导电缆样缆的交直流伏安特性进行测试,获得了超导电缆在直流、30Hz、100Hz和工频载流下伏安特性变化规律。结果表明,超导电缆的直流伏安特性曲线呈现E-J指数关系,且失超变化清楚,而交流下当超导电缆通流值低于直流临界值时,交流伏安特性曲线变化平缓,之后随通流能力增加后该曲线呈逐渐上升趋势,超导电缆没有出现明确的失超变化点。研究结果对于开展超导电缆运行稳定性研究提供了较大的参考价值。     

Numerical simulation of critical current performance in Nb3Sn strand subjected to periodic bending deformation

In the ITER Engineering Design Activity (EDA), four NB₃Sn model coils were developed and successfully tested. However, it was revealed that the critical current of the conductor degraded with the increase of electromagnetic force. One of the explanations of this phenomenon is a strand bending caused by enormous electromagnetic force. The authors therefore developed a simulation code using the distributed circuit model to investigate dependency of the critical current performance on the periodic bending deformation. The simulation results were in good agreement with the experiments. The dependence of the critical current on the periodic transverse load, temperature, periodic load pitch, thickness of Ta barrier which prevents Cu stabilizer from being contaminated by Sn, twist pitch of the strand, and RRR of the bronze matrix was investigated using the developed code. The results showed that the critical current degraded less with decreasing the pitch of the transverse load and increasing the Ta barrier thickness. It suggests that the shorter cabling pitch and the larger bending stiffness prevent the critical current degradation. © 2010 Wiley Periodicals, Inc. Electr Eng Jpn, 171(3): 7–15, 2010; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20923     

Stability of the multistrand superconducting cables under partial current distribution

The stability of the multistrand cables under partial current distribution among strands is discussed. The stability is influenced by a current sharing process among the strands. Stability tests under controlled current distribution are carried out for some test samples that had various electric resistance among strands. Then numerical analysis of the stability is carried out with a simplified electric circuit model that simulates current sharing process among strands. The results of the numerical calculations had good agreement with the experimental data. For stability of the multistrand cables, the electrical resistance among strands, which is the key parameter, is required to be lower than the normal resistance. This corresponds to the length of the heated region.     

Operation of superconducting fault current limiter using vacuum interrupter driven by electromagnetic repulsion force for commutating switch

Using conventional high‐temperature superconducting wire, a model superconducting fault current limiter (SFCL) is made and tested. Solenoid coil using Bi2223 silver sheath wire is so made that inductance is as small as possible and a vacuum interrupter is connected in series to it. A conventional reactor coil is connected in parallel. When the fault current flows in this equipment, superconducting wire is quenched and current is transferred into the parallel coil because of voltage drop of superconducting wire. This large current in parallel coil actuates magnetic repulsion mechanism of vacuum interrupter. Due to opening of vacuum interrupter, the current in superconducting wire is broken. By using this equipment, current flow time in superconducting wire can be easily minimized. On the other hand, the fault current is also easily limited by large reactance of parallel coil. © 2008 Wiley Periodicals, Inc. Electr Eng Jpn, 164(1): 52–61, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20315