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     

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     

Study of the magnetic field in superconducting alternators

Contents In this paper, the radial and tangential components of the magnetic field inside a three-phase turbogenerator with superconducting field winding and with a rotating screen of any number of elements are calculated with reference to a twodimensional study and to steady state and transient running.The expressions obtained are valid for any order of the field harmonic and any number of pole-pairs of the generator, and refer to a finite thickness of the rotor and stator windings and take account of the real distribution of the eddy currents in the screen elements.As an example of the applications, with reference to a preliminary design for a two-pole turboalternator of around 1300 MVA with double rotor screen, the distribution of the flux density with no load and its content of harmonics are found and the steady state and transient behaviour of the rotor screen is analyzed.

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Das magnetische Feld eines supraleitenden Synchrongenerators

Übersicht Im Rahmen der vorliegenden Abhandlung werden die radialen und tangentialen Komponenten des magnetischen Feldes innerhalb eines dreiphasigen, mit supraleitender Feldwicklung ausgeführten Turbogenerators mit rotierender Abschirmung an Hand einer ebenen Betrachtung stationär und transient berechnet.Die darin angegebenen Beziehungen gelten für beliebige Ordnung der Feldharmonischen und für beliebige Polpaarzahlen des Generators, wobei die Dicke der Rotor- und Statorwicklung endlich ist und die Wirbelstromverteilung den tatsächlichen Verhältnissen entspricht.Am Beispiel eines zweipoligen leerlaufenden Turbogenerators von ca. 1300 MVA mit zweifacher Rotorabschirmung wurde die Verteilung der magnetischen Flußdichte, deren Oberwellengehalt sowie das stationäre und transiente Verhalten der rotierenden Abschirmung untersucht.

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List of Symbols A potential vector - B magnetic flux density vector - C magnetic field attenuation factor - G winding current density vector - G ^* eddy current density vector - H magnetic field vector - J _np ,Y _np Bessel functions of the first and second kind andnp order - n stator or rotor harmonic number - p number of pole-pairs - P power losses in the screen per unit length along the axis - r radial coordinate - s complex variable - t time variable - z axial coordinate - angular coordinate - 0 _a , 0 _f respectively, armature and field winding angle (mechanical) - absolute magnetic permeability - ₀ magnetic permeability of free space - _r relative magnetic permeability - electrical conductivity - angular frequency of armature current - _m rotational speed of the rotor - ² Laplacian operator     

A high-temperature superconducting transformer with localized magnetic field

This paper describes a high-temperature superconducting transformer with a bar-type magnetic core and concentric windings with alternating layers, with single-channel and multi-channel arrangements. There is given the design concept of high-temperature superconducting windings of the transformer, made in the form of newly developed first-generation high-temperature superconducting ribbon wires, with localized magnetic field intended for producing maximum transport currents in the windings, as well as for reducing the consumption of a high-temperature superconducting material, cooling agent, and energy losses in these windings.     

Transient magnetic fields in the superconducting alternator

Contents The Fourier Transform method is used to compute transient magnetic fields in a 1.3 GVA superconducting alternator. The necessary frequency-response functions are measured in a scale-model and are used to show the spatial variation of the transient field. The study brings out the points on the field winding at which magnetic transients will be most severe.

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Transiente Magnetfelder beim Generator mit supraleitender Erregerwicklung

Übersicht Zur Berechnung transienter Magnetfelder bei einem 1,3 GVA Generator mit supraleitender Erregerwicklung wird die Fourier-Transformation verwendet. Die erforderlichen Frequenzgänge werden an einem maßstabgerechten Modell gemessen und daran die räumliche Änderung des transienten Feldes dargestellt. Die Untersuchung zeigt die Punkte stärkster Flußänderung auf der Erregerwicklung.

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List of Symbols B Flux density,T - f Frequency, Hz - f Frequency sampling interval, Hz - F/2 Folding frequency, Hz - i _o (t), I _o (f) Driving-current and its Fourier Transform - i _d (t), I _d (f),i _q (t), I _q (f) d- andq-axis transient stator currents and their Fourier Transforms - N Number of samples in Fourier Transform - S Screening-ratio frequency-response function - T Duration of transient calculated by Fourier Transform, sec - t Time sampling interval, sec - Permeability, H/m - Resistivity, Ohm-m     

Magnetic field gradient system for nuclear magnetic resonance microimaging

In this study we present an orthogonal magnetic field gradient system for nuclear magnetic resonance (NMR) microimaging applications. The construction details are given for a prototype assembly for proton microscopy inside a 50-mm vertical bore magnet, which is designed to fit into a commercial 300-MHz NMR probe. This system has been used to acquire images of the human spinal cordin vitro. Its performance has been evaluated and compared to that predicted by computer simulation.     

圆筒式永磁联轴器磁场仿真分析及结构优化设计

为确保乏燃料后处理厂混合澄清槽搅拌装置的永磁联轴器长期安全可靠运行,对永磁联轴器进行磁场仿真分析及结构优化设计。采用有限元方法建立三维磁场仿真模型,建模过程充分考虑联轴器的实际运行工况。首先,考察磁极对数、磁极厚度、磁场气隙、磁转角对磁转矩的影响。其次,通过温度场仿真分析考察转速对联轴器温升的影响。最后,以降低联轴器的隔离套温升为目标,对其进行结构优化设计并进行仿真分析。结果表明:优化结构的隔离套温升得到明显减小。控制联轴器温升可有效降低永磁联轴器退磁失效风险。研究成果可为同类型联轴器设计提供参考。     

空间电磁场对被复线有线通信干扰问题分析

野外有线通信受干扰时,被复线因耦合空间电磁能量而产生的干扰常常是被忽视的因素。基于Agrawl场线耦合传输线理论,利用时域有限差分技术建立了双线传输线电磁场耦合计算模型,对传输线对空间电磁波的时域耦合响应进行了仿真计算,总结了场线耦合的一般性规律,分析了对差分信号接收电路产生干扰的机理,验证了被复线的空间电磁能量耦合是野外有线通信受扰分析不可忽视的因素,提出了利用被复线进行有线通信时减少空间电磁耦合的建议。     

超导磁储能装置的功率控制

以电流源型和电压源型变流器作为研究对象,探讨了可对电流源型变流器和电压源型变流器交流侧电流的幅值和相位进行有效控制的SPWM开关策略。在此基础上,研究了能够按照系统要求对2种超导磁储能装置进行有功和无功功率调节的功率控制方法。仿真结果表明所研究的功率调节方法能够在四象限内进行超导磁储能装置输入输出有功和无功功率的快速解耦控制,仿真同时验证了所研究的功率控制策略的正确性和可行性。     

非线性PID控制器在超导磁储能装置中的应用

基于非线性比例积分微分PID(Proportional Integration Differential)控制器在设计上具有不依赖于被控系统数学模型的特点,设计了用于电力系统的超导磁储能装置SMES(Superconducting Magnetic Energy Storage)的非线性PID控制器。概述了非线性PID控制器利用“跟踪-微分器”非线性结构产生控制所需的比例、积分、微分信号的原理。介绍了含SMES的电力系统模型及非线性PID控制器的设计。数字仿真结果验证了所设计的控制器是可行的,同时表明该控制器结构简单、易实现。     

超导同步电动机温度场计算与分析

为判断高温超导电机超导体及永磁体运行的稳定性,准确计算电机内的温度场十分必要.以全超导样机热性能为研究对象,首先利用时步有限元方法建立二维正弦时变电磁场模型,得到了电机定转子损耗分布.根据传热学原理,通过数值分析给出定子表面散热系数的计算方法.然后计算分析高温超导电机的温度场.通过计算结果与实测数据的对比分析,验证所给...     

Magnetic field analysis of a 70‐MW‐class superconducting generator by the 3D finite element method

The three‐dimensional magnetic field and eddy current distribution of the 70‐MW‐class superconducting generator was calculated by a finite element method. The condition for the exciting armature coil was calculated by the jω‐method. In the magnetic flux distribution of the armature winding bore, the value obtained by analysis had good agreement with the measurement. An analysis that let the rotor move by the synchronizing speed was also done. The armature voltage for the nonload condition and the armature current for the three‐phase short‐circuit condition obtained by calculations had good agreement with the measurements. The validity of the analytical model for three‐dimensional magnetic field analysis of superconducting generators was confirmed from these results. In addition, the synchronous reactances were calculated using these results with eddy current in the facing. It was found that the facing had the effect of decreasing synchronous reactance by about 5%. © 2000 Scripta Technica, Electr Eng Jpn, 134(2): 53–60, 2001     

Magnetic damping of the electrodynamic suspension‐type superconducting levitation system

The damping characteristic of the bogie motion is one of the most important issues to be investigated in the superconducting magnetically levitated system based on electrodynamic suspension (EDS). We have therefore studied the damping characteristics and the mechanism of magnetic damping. Electric circuits consisting of coil resistances and inductances can describe the electromagnetic behavior of the levitation system. By numerical calculation based on the electric‐circuit equations combined with the equations of motion, damping characteristics of the bogie are examined for the system consisting only of levitation coils and superconducting coils, and also for the configuration with magnetic shielding structures incorporated. Furthermore, paying attention to the energy balance in the levitation and propulsion system when the bogie is forced to oscillate sinusoidally, we have investigated the relation between the input energy and the running speed, and compared the waveforms of levitation force and bogie oscillation to consider the characteristics and the mechanism of the damping. © 1999 Scripta Technica, Electr Eng Jpn, 127(2): 49–60, 1999