The reduced impedance function for cup-core eddy-current probes

Eddy-current nondestructive evaluation makes use of ferrite-core probes for exciting alternating currents in the test piece. The effectiveness of the probe for detecting flaws in the material through changes in its impedance may be assessed from its interaction with an unflawed conductor. For an important class of eddy-current probes, those with ferrite cup-cores, the measured impedance characteristics are governed by a rule-based behavior found from experiments. These findings are in conflict with the elementary circuit theory model of probe-workpiece interaction but are confirmed by a general field-theory probe model     

Induced circumferential currents and losses in flexible superconducting cables

In most designs for flexible ac superconducting cables each phase is carried by a co-axial pair of conductor tubes formed from a single layer of helically laid conductor strands. It is shown that the cable current would generate a net axial magnetic flux and hence an alternating circumferential electric field outside each co-axial pair. If, as in some cable designs, each conductor pair is to be contained in its own helium pipe, circumferential currents will be induced in the pipe wall. The losses depend on the pipe material but are typically three orders of magnitude too large. One solution is to line the pipe with superconductor, such as lead, but this could require more niobium in the conductor itself. Alternatively the co-axial pair could be redesigned so that there is no net axial flux. One possibility is to form conductor tubes from two layers of conductor strands laid in helices of opposite sense.The induced current problem is avoided if all three phase conductors are contained in a common helium pipe, provided that there are no zero sequence components to the phase currents. Losses from any zero sequence component could be readily reduced to an acceptable level, for example by laying a ferromagnetic strip alongside the conductors inside the helium pipe.Since circulating currents will also be induced in the electrostatic screens adjacent to the conductor strands the screens must be of semiconducting rather than metallic, tape.     

高阶扭转模态耦合下覆冰导线的稳定性和影响因素分析

利用Galerkin法建立面内前四阶和扭转前四阶模态耦合的覆冰导线动力学模型。借助分岔理论分析各阶模态的失稳临界条件,研究导线系统在不同风速、扭转阻尼比、档距及初始拉力下各阶模态的失稳规律,并利用数值模拟对理论分析结果进行验证。研究结果表明:考虑了扭转前四阶模态的导线模型,其面内前四阶模态特征值实部随风速变化的响应曲线先后经历2次Hopf分岔,呈限幅振动;扭转阻尼比的增大扩大了面内模态的失稳风速区域;随着档距增大,面内模态的2个Hopf分岔点和扭转模态的一个Hopf分岔点分别左移,表明大档距时,扭转模态逐渐代替面内模态的舞动;初始拉力对面内模态的失稳区域影响显著,而对扭转模态的影响很小。以上结论可为工程中导线的优化设计提供理论依据。     

Dynamic Behavior of Torsional Eddy-Current Dampers: Sensitivity of the Design Parameters

Eddy-current devices are promising for the passive and semi-active control of vibrations. From the electromechanical point of view, their behavior is characterized by the torque-to-speed curve and the mechanical impedance. In this paper, we report an investigation of the electromechanical behavior of eddy-current dampers/couplers consisting of permanent magnets and a massive conductor. Our goal was to determine the influence of the main design parameters on the torque-to-speed characteristic and on the mechanical impedance. To this end, we studied the dependence of the electrical pole and static damping coefficient on the design parameters under a given current density distribution within the conductor. This approach is equivalent to the procedure used to characterize the inductance and the torque constant of a standard electrical machine and it allows us to show that the electrical pole is determined by the inductive and resistive nature of the conductive part. Even if the conductor is realized by a solid conductor, the approach allows us to determine its equivalent resistance and inductance as a function of the design parameters, such as the number of pole pairs and the thicknesses of the permanent magnets and the conductor. We validated the analytical expressions of the equivalent lumped parameters, of the electrical pole, and of the static damping coefficient by finite-element analysis. This analysis provides the guidelines to identify the project parameters that can be varied to optimize the performances of the device as a coupler, damper, or brake.     

Modeling the dynamic effects of ice shedding on spacer dampers

Effects of ice-shedding-induced cable vibration on spacer dampers are studied by applying a dynamic model. The model is based on the authors' recent development, which is improved here by applying spacer properties obtained experimentally. The model simulates cable vibration and bundle rotation at mid-span, where the spacer is attached to the cables, as well as the spacer deformation and forces acting on the spacer during the vibration of twin, triple and quad conductor bundles. Ice accumulation on a spacer may change the latter's elastic and damping properties, whose influence on the vibration is considered in the model together with the effect of varying cable properties. Simulation results show how the risk of bundle collapse and the load on the spacer is reduced when the number of subconductors in the bundle is increased. The model also predicts that the most important property which can reduce forces acting on the spacer during vibration is spacer damping, whereas the bundle rotation is affected to the greatest extent by cable damping.     

Assessment of the fatigue failure of an All Aluminium Alloy Cable (AAAC) for a 230 kV transmission line in the Center-West of Brazil

The aim of this work was to conduct an evaluation on the causes for the premature fatigue failure of an AAAC 900 MCM conductor used in a 230 kV transmission line located at the Center-West of Brazil. Each phase of the line had a double cable configuration separated by vertical rigid spacer clamps. To carry out the analysis, vibration recorders were installed on the spacer clamps at two different positions in the span and fatigue tests on the cable were conducted. The Miner's rule was applied and the remaining life of the cable was estimated to be greater than 33 years. The fact that the actual life of the cable is much smaller than the estimated one was associated to the conjoint action of three factors: (i) the use of rigid spacer clamps between the two conductors; (ii) the choice of an AAAC conductor (whose fatigue resistance proved to be significantly less than that of an ACSR) and (iii) the existence of a high frequency laminar wind regime. To avoid new failures the rigid spacer clamps were uninstalled. This action drastically reduced vibration in the line so that life estimates increased for more than 100 years.     

Superconductor Stability Against Quench and Its Correlation with Current Propagation and Limiting

This paper presents a numerical (finite element) analysis of superconductor stability and current propagation under random variations of critical superconductor parameters. Instead of using singular (homogeneous) values, random variations potentially are appropriate to take into account any conductor inhomogeneity that can be considered as an obstacle to current propagation. Traditional assumptions like homogeneous current distribution, critical temperature, critical current density and critical magnetic fields are not justified in general; a local disturbance (for example, release of mechanical stress energy), if not immediately distributed by solid conduction, would generate a transient increase of local conductor temperature. Local critical current density and magnetic field then will be reduced, and current distribution will change. Disturbances may arise also from transport currents that locally exceed the critical current of the superconductor. Disturbances of all kinds may increase the conductor temperature above its critical value. A local analysis of all superconductor states thus is mandatory to safely avoid a quench. As an extension of standard stability models, also flux flow resistive states are taken into account. We will try to find a possibly existing correlation between current propagation and superconductor stability. Fault current limiting is discussed as a special case of current propagation. The analysis is applied to a bundle of high-temperature superconductor (HTSC) filaments. As will be shown, temperature profiles in a superconductor do not allow a clear distinction between Ohmic resistive or flux flow resistive fault current limiting. Though frequently made in the literature, this separation is highly questionable, because Ohmic resistive and flux flow resistive states may locally coexist, side by side, but are not very stable in the superconductor volume.     

Analysis of the voltage harmonic distortion at buses feeding office loads

The harmonic distortion of the phase-to-neutral voltage at low-voltage buses feeding office load is investigated for various cable configurations, lengths and cross-sections. The investigation is carried out with a software model that was validated with analytical expressions and measurements taken at a real installation. It is shown that the selection of a cable should be based not only on its ampacity and allowable voltage drop but also on its configuration, in order to avoid excessive voltage harmonic distortion. Configurations where the neutral conductor is part of a four-core cable lead to relatively small harmonic distortion, whereas cable configurations where the neutral conductor runs separately cause high-voltage harmonic distortion. In the latter case, the neutral conductor develops a significant voltage with respect to the ground. Increasing the cross-section of the neutral conductor, as the existing standards dictate, may prevent its overloading from zero-sequence harmonic currents but hardly improves the voltage harmonic distortion at the load bus.     

Probing heterogeneous electron transfer at an unbiased conductor by scanning electrochemical microscopy in the feedback mode

The theory of the feedback mode of scanning electrochemical microscopy is extended for probing heterogeneous electron transfer at an unbiased conductor. A steady-state SECM diffusion problem with a pair of disk ultramicroelectrodes as a tip and a substrate is solved numerically. The potential of the unbiased substrate is such that the net current flow across the substrate/solution interface is zero. For a reversible substrate reaction, the potential and the corresponding tip current depend on SECM geometries with respective to the tip radius including not only the tip-substrate distance and the substrate radius but also the thickness of the insulating sheath surrounding the tip. A larger feedback current is obtained using a probe with a thinner insulating sheath, enabling identification of a smaller unbiased substrate with a radius that is approximately as small as the tip radius. An intrinsically slow reaction at an unbiased substrate as driven by a SECM probe can be quasi-reversible. The standard rate constant of the substrate reaction can be determined from the feedback tip current when the SECM geometries are known. The numerical simulations are extended to an SECM line scan above an unbiased substrate to demonstrate a "dip" in the steady-state tip current above the substrate center. The theoretical predictions are confirmed experimentally for reversible and quasi-reversible reactions at an unbiased disk substrate using disk probes with different tip radii and outer radii.     

考虑垂度影响的拉索-双粘滞阻尼器系统振动分析

随着斜拉索长度的不断增加,在索近梁端单点安装阻尼器已经难以满足拉索减振的需要。同时,已有研究表明索垂度会减弱索端阻尼器的减振效果。因此,针对超长斜拉索,考虑垂度的影响,分析了斜拉索上两处安装粘滞阻尼器(拉索-双粘滞阻尼器)系统的复模态特性。考虑实际中阻尼器一般安装于近索锚固端位置,推导出了该情况下系统模态阻尼比的近似解析表达式,进行了阻尼器参数对索阻尼的影响分析和优化。结果表明:对于双阻尼器系统,垂度仍对拉索对称振动模态的阻尼比有减弱效果,对反对称振动模态无影响;阻尼器对称安装于索两端时,即使考虑垂度索所获得的最优阻尼相比于安装单个阻尼器时可以提高至2倍,能有效解决单阻尼器提供阻尼不足的问题。双阻尼器同端布置相比于仅安离索端较远的阻尼器时,索单阶最优模态阻尼无提高,但对于同时抑制拉索高、低阶振动具有优势。     

高压输电线风雨激振特性研究及数值分析

高压输电线风雨激振严重威胁着输电线路的寿命和安全运行,其激振机理及防治措施成为电力系统急待研究的难题之一。在降雨、风载和电场作用下,在输电导线表面形成极化水线, 水线的运动诱发气动力变化导致输电线线路振动。本文建立了水线运动与输电导线振动耦合的风雨激振理论模型。采用龙格-库塔法对输电导线和水线运动方程进行数值求解,得到了输电导线和水线的响应,讨论了风速、导线结构阻尼和电场力等参数对输电导线振幅的影响     

The effect of interturn heat flow on the recovery currents of a superconducting magnet: A comparison of calculated and measured values

Cryostatic stabilization is appropriate for superconducting magnets producing steady fields over large volumes. The measured recovery currents for a 10 cm bore 6.4 T experimental solenoid are compared with values derived from a stability analysis which includes heat flow from the resistive region to adjacent conductors as well as to the helium coolant. Agreement is excellent despite some uncertainty in the helium heat transfer characteristic and in the thermal conductivity of the interturn insulation. Given good data for these it should be possible, using the method presented, to design cryostatically stabilized windings with precision just from basic data for the conductor and its intended environment.Details of the behaviour of large and small resistive regions in this solenoid are given. The critical currents are also accurately predicted from the performance of short samples of the conductor after taking account of anisotropy with respect to the direction of the ambient magnetic field.     

输电线路防舞阻尼器系统参数分析及设计研究EI北大核心CSCD

针对目前覆冰导线舞动频发现状,提出了一种通过在导线靠近输电塔位置处设置阻尼器来减振耗能从而实现输电线路舞动抑制的方法。基于Hamilton原理运用多阶伽辽金函数推导得到了导线-阻尼器系统的广义运动方程,并以某750 kV单档八分裂输电线路为例进行运动方程特征值分析,得到了导线-阻尼器系统的动力特性,探索了导线的垂度参数、阻尼器安装位置、阻尼系数及刚度系数等对系统等效阻尼比的影响,阻尼器安装位置越靠近跨中,系统的最大阻尼比提升效果越明显,两侧对称安装阻尼器可以有效地减小最优阻尼系数。采用数值算例和有限元数值仿真技术比较了粘滞阻尼器与负刚度阻尼器(NSD)对导线系统的减振效果,并针对NSD提出了参数的优化设计方法。研究表明:相比传统阻尼器,NSD可以在较低阻尼系数下有效地提高系统各阶的最大阻尼比,且能够明显降低导线系统的自振频率;系统所能达到的一阶最大阻尼比对NSD安装位置的变化不敏感;通过有限元仿真证实了,基于NSD的输电导线阻尼器设置方案相较于传统阻尼器方案具有更好的防舞性能。     

拉索-阻尼器-弹簧系统的阻尼特性分析

该文研究得到了拉索-阻尼器-弹簧系统的复特征频率方程。在阻尼器和弹簧安装点距拉索锚固点长度与拉索长度之比远远小于1的假设条件下,得到了拉索-阻尼器-弹簧系统模态阻尼比的近似解析解,该近似解析解与数值计算得到的精确解对比吻合良好。当弹簧和阻尼器处于同一侧时将会减小拉索所能获得的最大模态阻尼值,而当阻尼器与弹簧在拉索两端时弹簧对阻尼的影响几乎可以忽略。当阻尼器仍处于拉索锚固点附近而弹簧位置向中间移动时由阻尼器引起的频率变化量仍是小量的条件下,得到了结合数值频率解的拉索-阻尼器-弹簧系统模态阻尼比近似解析式。此时拉索所能获得最大模态阻尼比、对应的最优阻尼系数、无量纲频率与弹簧位置、刚度之间存在明确的变化关系。该文研究成果对于拉索端部同时附加橡胶减振器和阻尼器、附加阻尼器的索网结构减振设计具有重要的参考价值。     

A study of breakdown strength of liquid nitrogen cooled power cables

A 66 KV liquid nitrogen cooled cryogenic cable was manufactured and tested using ac and impulse voltages.Liquid nitrogen cooled cryogenic cables will be in practical use over the 275–500 kV range. Study of the 66 kV class cable is the first step towards the development. The insulation system of this cable is polyethylene paper tape wrapped insulation impregnated with liquid nitrogen and its insulation thickness is about 7.75 mm. The influence of bending, conductor surface effect, and thickness dependence on the breakdown strength are discussed.     

Superconductive dc transmission lines: design study study and cost estimates

A conservative design study of a dc superconductive power line based on present day technology is presented. Power is transmitted by opposing currents in coaxial conductors, formed from helically wound tapes of composite, with a potential difference between them. Conductor temperature stability and fault current conditions are shown to be the factors which dominate the conductor cost; both are analysed. The capital cost of a cable is expressed as a function of a cable dimension, the basic cable parameters, and the material properties. This cost may be minimized with respect to cable radius. The resultant optimum magnetic fields vary little with power rating and are quite low at about 1–2 × 10⁵ A m^?1. The capital cost of such cables, exclusive of terminal costs, is less than the cost of conventional dc cables at power levels above about 1 GVA. Although extensive development would be necessary, the basic principles needed to build a dc superconductive cable are known.     

用于微纳米几何量尺寸测量的三维微接触式测头

为实现微纳尺度器件三维几何形貌测量及表征,基于电容和压阻原理,开发了两种三维微接触式测头。其中电容测头测量范围4.5μm,轴向分辨力和横向分辨力分别为10 nm和25 nm;压阻测头测量范围4.6μm,轴向分辨力和横向分辨力分别为5 nm和10 nm。两种测头均可集成到纳米测量机,实现微结构几何参数的测量。     

Field test of liquid nitrogen cooled cryogenic power cable

A liquid nitrogen cooled cryogenic cable, 66 kV, 100 MVA, 30 m was constructed and tested with loads simulating actual operating loads from October 1970 to December 1971. The successful operation of the cable makes us hopeful it can be used for large capacity transmission. Our test line is composed of aluminium stranded hollow conductor, liquid-nitrogen impregnated polyethylene paper electrical insulation, and polyurethane foam thermal insulation.     

Piezo-resistive semi-conductor gauges

Though the piezo–resistive semi–conductor strain gauge has a very high gauge factor compared with wire or foil resistance strain gauges, some precautions ate necessary in its use if a reasonable degree of stability and accuracy are to be attained. The introduction of thermistors in the bridge circuit can compensate for errors caused by temperature changes.     

Reduction of static strain for increased life of transmission line conductors

Fatigue failures of transmission line conductors are known to occur close to the suspension clamps. The causes of such failures are known to be due to high static stresses combined with dynamic stresses resulting from vibration. The overall stress in the aluminium strands of a conductor could be limited by reducing the static bending stress near the clamps. Passive protective devices such as neoprene lining, damping shims introduced in the suspension clamps would bring down the stress level, thereby increasing the service life of conductors. Results of tests conducted on a laboratory span are brought out in this paper. The experimental results reveal that neoprene lining and damping shims introduced in the suspension clamp would be very effective in bringing down the static bending stress and thus the overall stress level.