Ac losses in superconducting coils

AC superconducting wire is being developed for such electrical equipment as superconducting transformers and superconducting generators. AC loss reduction is of primary concern in the development of such high-efficiency equipment. In reducing ac losses, it is necessary to develop assessment methods for ac loss in test samples which have shapes similar to the end-product equipment. This paper described a least squares calculation of ac losses of superconducting wire as a function of frequency and magnetic field strength measured in test coils. Two sample solenoid coils were made to test the influence of different capacities and winding methods on ac losses in ac superconducting coils with rated capacities of 500 kVA and 20 kVA, and impregnated (epoxy resin) and nonimpregnated windings. The ac losses in the superconducting coils were measured by a calorimetric method using the evaporating rates of liquid helium. Estimated ac losses in the superconducting wire of the two coils were compared with Joule losses of copper conductors at ambient temperature. As a result of this comparison, a low-loss ac superconducting wire winding can be made for electrical equipment rather than employing conventional copper winding when used under low magnetic fields less than 0.5 T.     

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.     

定子无铁心轴向磁场永磁轮毂电机损耗分析及效率优化

减速装置的引入使得轮毂电机分布式驱动系统对电机本体的效率提出了更高的要求。为了实现定子无铁心轴向磁场永磁轮毂电机在其常用工况下的高效率,该文分析了利兹线和扁线对定子槽满率的影响规律,建立了不同线型绕组的交流损耗计算模型,对不同线型绕组电机的交直流铜损进行了计算与对比分析,并基于损耗产生机理提出扁线绕组分股和换位的交流损耗抑制方法。在此基础上,针对轮毂电机驱动系统的常用工况,对不同线型绕组电机的损耗和效率进行全面对比。结果表明,采用该文提出的效率优化方法可以实现扁线绕组的低损耗及电机的高效率,使用改进型扁线绕组的定子无铁心轴向磁场永磁电机适合作为减速驱动轮毂电机应用,尤其是对转矩需求较高的轮毂电机。试验结果验证了理论和仿真分析方法的正确性。     

Development of new conductors of the cable‐in‐conduit type for HTS coils with high performance

A new type of cable‐in‐conduit conductor composed of HTS tapes was proposed as a winding for high‐performance HTS coils with high current capacities and low AC losses. In the fabrication of the conductor, the twist of stacked tapes around their axes was made before inserting them inside conduits. The twist angle should be changed continuously along the axis to reduce face‐on oriented magnetic fields applied to the tape in the winding conductor during coil operation. In order to confirm the high current capacity of this type conductor, two single‐layered solenoidal coils wound with sample conductors composed of five stacked Bi‐2223 tapes with bias angles of 20 and 0° were fabricated and tested in liquid nitrogen. A copper magnet system was used in this experiment to generate the spread magnetic field with a spread angle of 20° from the coil axis to the radial direction, which simulates the profile of magnetic fields near the edge windings of practical coils. A large improvement on critical‐current degradation affected by spread magnetic fields was successfully observed for the test coil wound with the 20° bias conductor. © 2005 Wiley Periodicals, Inc. Electr Eng Jpn, 153(4): 12–19, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20218     

Fabrication and excitation testing of a fully superconducting brushless generator with magnetic flux pump

Development of a highly efficient excitation system is important in enhancing the potential of superconducting ac generators. A new conceptual design in which electricity is generated with a brushless exciter system using a superconducting dynamo (the magnetic flux pump) has been proposed. Research on a fully superconducting generator using both a superconducting armature and a superconducting field winding has been under way for the past 7 years. In this paper an experimental model of the machine is described and test results on brushless excitation with the magnetic flux pump, with the machine operating as a fully superconducting brushless generator with an estimated capacity of 20 kVA, are presented. © 1997 Scripta Technica, Inc. Electr Eng Jpn. 118 (4): 35–45, 1997     

超导无线电能传输技术

近年来,无线电能传输技术已成为了热点技术。与传统电能传输方式相比,无线电能传输更为方便、安全,并已被应用于多个领域。由于无线电能传输效率取决于发射与接收线圈本身的电阻,线圈电阻越小,传输效率越高。超导体所具有的直流零电阻、交流低损耗的特性,使得超导材料用于无线电能传输具有显著的效率优势。介绍了超导无线电能传输技术的研究现状,中国科学院应用超导重点实验室关于超导无线电能传输方面已经开展和正在进行的研究工作,指出了超导无线电能传输技术潜在的应用前景。     

Development of high-Jc NbTi multifilamentary superconducting wire with Nb artificial pins for AC applications

High critical current densities of 1.61 × 10¹⁰ A/m² at 1 T, 6.1 × 10⁹ A/m² at 3 T and 2.9 × 10⁹ A/m² at 5 T were achieved by controlling the distribution of Nb artificial pins in NbTi multifilamentary superconducting wires for ac applications. The critical current densities attained are over two times higher than those of conventional ac superconducting wires. This increase can be attributed to the shift of the peak of pinning force density to higher magnetic field by optimizing the pinning parameters. The benefits of increasing critical current densities for ac applications are demonstrated. A 2.5 T/100 kVA ac superconducting magnet was designed and made by using high J_c wire with controlled distribution of Nb artificial pins. Compared with conventional ac superconducting magnets, the new magnet exhibits a drastic decrease in size as well as in ac losses.     

反激式平面变压器绕组交流损耗的分析

基于平面磁性元件绕组的一维模型,分析利用初、次级交叉换位技术减少反激式平面变压器绕组交流损耗的原理,进一步分析了绕组电流的上升(或下降)斜率与绕组交流损耗的关系.比较了CCM与DCM模式下绕组的交流损耗,得出在相同输出功率下,DCM比CCM的交流损耗更大.DCM和CCM两种工作模式下应用交叉换位技术减少绕组交流损耗的效果均很明显,且减少的比例相近.通过有限元分析软件和实验证实了分析结果的正确性和有效性.     

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     

First trial of the electric power transmission of 3.8 kV–460 kVA through the prospective power transmission model system integrated under superconducting environment (PROMISE)

Superconducting technology is regarded as a breakthrough to future electric power transmission because of its highly densified and large transmission capability. This paper proposes a concept of the future power system composed of various superconducting apparatuses. A prototype model system called “PROMISE (PROspective power transmission Model system Integrated under Superconducting Environment)” is constructed to prove the realization of the above concept. PROMISE is composed of a superconducting transformer (60 Hz, 6/3 kV, 1000 kVA class), superconducting fault current limiter (6 kV, 200 A class), and superconducting power cable (5 m, 6 kV, 650 A class). This paper also shows that PROMISE realized the transmission of the electric power of 3.8 kV–460 kVA (50 Hz). This is the first achievement in the world. The voltage-current synthetic test verified that PROMISE can withstand ac voltage of 6 kV while carrying ac current of 170 A (60 Hz). The ac loss of superconducting cables, the heat leak of cryostat and the core loss of the superconducting transformer are measured to estimate the transmission loss of PROMISE. These fundamental performances of PROMISE may indicate the feasibility of the future introduction of superconducting technology for electric power systems.     

The high-frequency phenomenon of copper clad aluminum wires which are lower the alternating current resistance than copper solid wires,and the application of copper clad aluminum wires to the electrical equipment

The paper is proposed approximate formulas of the ac resistance of lead wires and coils which made by using copper clad aluminum (CCA) wires. There are the frequency range that the ac resistance of CCA wires is lower than it of cupper solid wires for lead wires and coils of an electrical equipment. It is shown that the application of CCA wires is economical as comparing with copper solid wires, if the design is due to successful calculation in the electrical equipment. The measured resistance is compared with the formula. CCA wires is an example of multiple conductors which shall be expected to apply in future.     

开关功率变换器中高频电感器的分析与设计

对高频率和大电流的开关功率变换器,电感器损耗对温升及变换器效率指标具有重要影响.通过高频电感器线圈损耗机制的深入分析,对铜箔线圈以及里兹线线圈进行了比较,提出了线圈避开气隙扩散磁通的设计思想和原则.研究了基于功率铁氧体材料性能因素(PF)指标的铁芯材料选用依据.改进、设计了一种高频串联谐振型逆变器的谐振电感器,在1 kVA容量时,使逆变器整机效率提高了4%.     

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.     

Performance of a 3000/6000 V, 1000 kVA class superconducting transformer developed for a prospective power transmission model system integrated under superconducting environment (PROMISE)

A 3000/6000V, 1000 kVA class superconducting transformer (SC-Tr) was developed for a Prospective Power Transmission Model System Integrated under Superconducting Environment (PROMISE). In this transformer, the core and superconducting windings are immersed in liquid helium and the major insulation is provided by the liquid helium. This paper describes both the design features and measured characteristics of the SC-Tr. Fundamental characteristics of the SC-Tr are obtained through no-load, short-circuit tests and quench experiments. The results of the no-load test have verified that the SC-Tr has the capability to withstand ac voltage of 3000/6000 V of 60 Hz without any partial discharge. The short-circuit tests have proved that the SC-Tr is capable of carrying ac current of 170 Arms without quench in the superconducting windings. Furthermore, in a real-load experiment with the PROMISE, electric power of 3800 V-460 kVA of 50 Hz in high-voltage side is transmitted through this SC-Tr.     

Three‐Phase AC Filter Inductor Design for Three‐Phase PWM Inverter for Conversion Efficiency Improvement at Low Load

The inductor losses in a three‐phase ac filter inductor used in a three‐phase pulse‐width modulation (PWM) inverter are evaluated. First, a three‐phase inductor is designed to obtain the same value of inductance for each phase. Then, based on the design, a three‐phase inductor that uses two magnetic materials is proposed. The conversion efficiency of a 1 kVA three‐phase PWM inverter that uses the conventional and proposed ac filter inductors is simulated. Simulation results show that conversion efficiency improves. Finally, the conversion efficiency of an actual three‐phase 1 kVA PWM inverter that uses the conventional and proposed ac filter inductors is measured. In the experiment, the conversion efficiency obtained for the case of the proposed inductor improves by approximately 1% at low power load as compared to the conventional inductor. Furthermore, the calculated inductor losses are in good agreement with measured losses. Improvement in efficiency is verified trough simulations and experiments.     

海底超导电缆交流损耗的抗损伤特性研究

海上新能源发电的快速发展急需大容量高效率的海上电力传输网络与之匹配。为促进海底超导电缆的发展,需要研究局部损伤对超导电缆的影响,尤其是对其损耗特性的影响。这关系到损伤是否会导致损耗骤升而冷却系统无法及时排出多余热量,进而导致低温平衡被破坏,超导电缆无法继续工作,甚至发生进一步大范围损伤。基于陆上超导电缆建立了包含单层到多层电缆的物理模型,并使用有限元方法分析电缆的电流分布、内部磁场分布和输电损耗。通过比较损伤前后电缆损耗特性的变化,分析并总结得出了最适宜用于海底输电的超导电缆结构。结果表明,在单根带材损伤的情况下,一层和二层电缆的损耗大幅度提升,而六层电缆的损耗不到5%。因此六层电缆较为适应海底环境。     

Characteristics of superconducting generator at high response excitation

Superconducting generators have many advantages, one of which is their ability to improve the stability of power systems because of their low synchronous impedances. The advantage may be increased by adoption of high-response excitation. The first 100 kVA high-response excitation superconducting generator has now been built. One of the problems in the design of such generators involves the characteristics of the field circuit, i.e., the field armature winding, losses in the component materials, and loss in the superconducting field winding. In this paper, a 2-dimensional electromagnetic field analysis of field circuit characteristics is presented.     

A new separate‐identification method of two stator equivalent core‐loss resistances corresponding to hysteresis and eddy‐current losses for parallel‐type mathematical models of AC motors

This paper proposes a new unified method for identifying equivalent stator core‐loss resistance of AC motors, which can be applied to both induction and synchronous motors. In order to realize AC motors that exhibit high performance such as precise torque generation and/or efficient energy transmission, stator core loss cannot be neglected in designing vector control systems. It is common to model stator core loss in magnetic circuits as loss caused by equivalent resistance in electrical circuit. One of the best mathematical models for controlling AC motors with core loss is a kind of parallel‐type model that succeeds in modeling both eddy‐current and hysteresis losses. The newly proposed method succeeds in identifying separately and simultaneously two kinds of equivalent core‐loss resistances on the model corresponding to eddy‐current and hysteresis losses. The practical usefulness of the method is evaluated and confirmed through experiments using two induction motors of 5.5 and 2.0 kW having relatively high core losses and a permanent magnet synchronous motor of 750 W having relatively low core losses. © 2003 Wiley Periodicals, Inc. Electr Eng Jpn, 143(4): 50–63, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.10152     

一种确定交直流混合激励电磁构件杂散损耗的有效方法

交直流混合激励条件下,大型电磁装置构件中杂散损耗的分析与验证,以及如何有效降低杂散损耗在产品分析与设计中备受关注。平波电抗器是高压直流输电系统(HVDC)中重要的电气装备之一,其工作时绕组中通过较大的直流电流以及高次谐波电流(交直流混合激励)。本文从实验与仿真分析两方面系统地研究交直流混合激励时,导磁钢板中杂散损耗有效、实用的确定方法,并通过已建立的国际基准模型进行相关验证。提出采用间接处理方法准确确定平波电抗器类电磁装备结构件中杂散损耗的工程实用措施,所得结果可为产品电磁设计提供参考。     

高阻值合金基体材料对Bi-2223高温超导带材交流损耗影响的实验研究

飞速发展的高温超导材料的研究和制各，加速了高温超导材料在变压器、限流器、电缆、同步电机和储能磁体等方面的应用。影响高温超导材料应用的关键因素之一是它的交流损耗。在多芯高温超导带材中，交流损耗依赖于各种参数。其中，基体的有效电阻率对带材的交流损耗有重要影响。为了研究高阻值基体材料对带材交流损耗的影响，该文制各了6种具有不同阻值的基体材料的带材。在77K下，研究了高阻值基体材料对Bi-2223带的交流损耗的影响.结果发现，在较低频率下，通过引进高阻值基体材料，可有效地减少带材的交流外场损耗。同时也研究了高阻值基体材料电阻率对耦合损耗时间常数的影响。