Conductor Motion Research Articles

Optimum conditions of stabilizer ratio and critical current margin for maximizing the current density of a tightly-wound superconducting magnet

A new stability criterion for a tightly wound superconducting magnet is proposed. In a tightly wound superconducting magnet such as a resin-impregnated or a nonimpregnated magnet, conductor motion within the winding or cracking of epoxy resin often causes a quench of the magnet. Based on an experimental result and theoretical analysis of conductor motion, their time constants were 200 to 500 mu s. The time constant of temperature rise due to heating in a conductor was calculated to be about 20 mu s so that conductor motion can be regarded thermally as a steady-state phenomenon. In a model where the heat due to local steady disturbance is cooled by conduction to both ends of the conductor with a heated length l, the optimum stabilizer ratio and critical current margin for maximizing the current density of the tightly wound superconducting magnet are obtained theoretically. In this theoretical analysis, the stability parameter B, the limited disturbance l/sup 2/ p, and evaluation factor EF are proposed. Stability evaluations on some magnets that were constructed and tested are presented. >

Stabilization of dry-wound high-field NbTi solenoids

Dry-wound, Formvar-insulated NbTi superconducting solenoids are frequently used for small-bore and low-field applications because of their low manufacturing costs. The more widespread use of dry-wound coils for large-bore and high-field applications has been limited by their tendencies toward conductor-motion-induced degradation. Simple models for estimating the occurrence of conductor-motion disturbances in dry-wound solenoids are described. These models were used to design and analyze the training behaviors of three small, high-field NbTi test coils. The experimental results suggest that stabilization is best achieved by forcing all potential conductor motions to occur during low-field portions of the magnet's energization sequence, where its stability margin is greatest. >

Study on fluctuations in supporting force of conductors caused by fluctuations in conductor dimensions

The authors are investigating fluctuations of contact force between a conductor and a spacer, and the possibility of local conductor motion in the helical coils of the Large Helical Device (LHD) which will be installed in National Institute for Fusion Science (NIFS). They performed an experiment to study the rigidity of the helical coils and fluctuations in the contact force caused by the irregularity of conductor bends and spacer dimensions by making a model coil pack. A theory is proposed to statistically estimate the fluctuations in the contact force. The theory is explained and the experimental results are shown and compared with the theory. The theory agrees well with the experimental results.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Disturbance due to conductor motion and stability of large-scale conductors

It is generally understood that cryostatic conductors are too conservative in design and have too low current density to be applied to the magnets for next generation fusion experiment devices and realistic magnets. To increase the current density beyond the cryostatic stability criteria, it is necessary to estimate the energy of disturbance causing quenches and to design the conductor stable against the estimated disturbance. The most difficult disturbances to cope with are conductor motions. In this paper, the size of the disturbances due to conductor motions are related to mechanical properties, bending irregularity of the magnet conductors and winding and conductor structures. Based on those relations, the expected number of quenches of a magnet is also estimated.

Acoustic emission monitoring on a model field winding for the 70 MW class superconducting generator

Acoustic emission monitoring on a model field winding for the 70 MW class superconducting generator was performed during cooldown, excitation and quenches to diagnose the integrity of the model field winding and improve the reliability of the field windings for the generator. The results confirm that the acoustic emission signals are mainly due to conductor motions which cause premature quenching of the winding. It is therefore important to support the field windings as rigidly as possible to prevent conductor motions and to increase the stability of the field windings.

Influence of copper-to-superconductor ratio on stability

Copper-to-superconductor ratio (Cu/Sc ratio) is an important design parameter for a conductor for a high-current-density magnet. In this paper its influence on stability is studied, focusing on the disturbance due to a conductor motion. It is pointed out that minimum quench energy (MQE) is not significantly dependent on the Cu/Sc ratio and that is it difficult to explain the experimental results on training characteristics of SSC conductors with CU/Sc ratio of 1.2 and 1.5 in terms of the dependence of MQE on the Cu/Sc ratio. In this paper, experimental results for SSC conductors are investigated by estimating the movable length of the conductor.

Probability of premature quenches due to conductor motion in superconducting windings

The probability of quench caused by abrupt motion of a conductor is discussed in this paper. There are fluctuations in the contact forces in the interfaces between the conductors and the spacers which are caused by local bending of the conductor and fluctuations in the dimensions of the spacer. Conductor motion occurs where the contact force is not enough to support the conductor. In this analysis, it is assumed that any irregularity of the conductor and spacer follows Gaussian stochastic processes, and the probability of conductor motion and the number of premature quenches expected are quantitatively calculated.

Two-degree-of-freedom analysis of power line galloping by describing function methods

Estimates of maximum amplitudes of conductor galloping are needed in order to determine appropriate phase-to-phase clearances of overhead power lines. A new method for computing these estimates is obtained by the use of mathematical models of conductor galloping that include both vertical and torsional motion. The method gives estimates of the maximum gallop amplitude and frequency for various wind speeds. The stability of the predicted gallop limit cycle is determined by perturbation of the limit cycle parameters. Computer simulations show good agreement with the maximum amplitude and frequency estimates predicted by the model. The maximum gallop amplitudes predicted by the model also conform well with those observed on actual galloping lines. In addition, this analysis verifies what a number of galloping conductor researchers had long expected, namely, that torsional conductor motion limits maximum gallop amplitudes.

Stability against the frictional motion of conductor in superconducting windings

An experiment was performed to investigate quenches of superconducting wires caused by conductor motions. In the experiment, the disturbance energies of conductor motions causing quenches were measured. The conductor moved in a series of step motions, which was demonstrated by the acoustic emission (AE) measurement. A disturbance where the conductor moved at three spacers caused a quench, whereas a conductor motion at one spacer did not trigger a quench. Experimental data are compared to the values derived from the theory, which quantitatively estimate the size of the conductor motion. The theory estimates the disturbance energy quite well.

A ground-based sensor for the detection of transmission line conductor motion

A novel lower-cost method of detecting transmission line conductor motion has been developed. The detector is ground based and can be installed by two people with no line outage or physical connection to the transmission line structure. Detection sensitivity is approximately 0.3 m (1 ft) p-p (peak-to-peak). Field testing has shown that the detector can detect several modes of conductor motion, including both single and double-loop motion, as well conductor blowout conditions. It is concluded that this detector can greatly aid in conductor motion data gathering with reduced cost and enhanced personnel safety over alternate approaches.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

On the efficacy of mechanical damping and frequency detuning in alleviating wake-induced flutter of overhead power conductors

A nonlinear, quasi-steady analysis of the wake-induced instability of a leeward conductor in the wake of a second identical windward conductor has been developed. In addition to the nonlinear analysis, the aerodynamic forces acting on the conductors are linearized, and a linear form of the equations of motion formulated. Both the linear and nonlinear analyses account for motion of the windward conductor. Although the quasi-steady assumption is employed, the time delay between flow leaving the windward conductor and influencing the aerodynamic forces on the leeward conductor is accounted for, as is the retardation of wake flow approaching the leeward conductor. Accounting for the above “unsteady effects” results in considerable differences in the equations of motion vis-a-vis the fixed windward conductor analysis; in particular, they change significantly the stability of the two conductors and induce a new form of instability—a negative damping instability. This new instability mechanism results in some configurations of the two conductors, which the fixed windward conductor analysis predicts to be stable, being unstable. Using both the linear and nonlinear analyses, the individual effects of mechanical damping and frequency detuning are investigated, the particular areas of interest being the threshold wind speed at which instability will occur and the limit cycle amplitudes once instability has occurred. In general, increasing mechanical damping, in both the in-plane and out-of-plane modes, is the most effective way of enhancing the stability of the system; it can, in some circumstances, raise the threshold velocity necessary to cause instability, and reduce the limit cycle amplitude of an unstable system. For the frequency coalescence instability mechanism, detuning the modes can be beneficial, but there are specific combinations of modes for which frequency detuning has little effect on either the minimum wind speed for instability to occur, or on the limit cycle amplitudes. Thus, for transmission line bundles, in which a multitude of modes may coalesce, it is unlikely that frequency detuning will be particularly beneficial. Also, not surprisingly, frequency detuning has almost no effect on the negative damping instabilities.

Stability analysis of superconducting magnet: an approach to quantification of energy of disturbance caused by conductor motion

One of the most intricate disturbances causing quenches in high current-density superconducting magnets is an abrupt conductor motion. There have been few papers investigating the disturbance energy of the conductor motion and it is generally understood that its quantification is very difficult. In this paper a model to quantify the disturbance energy due to conductor motion is proposed. The model relates the disturbance energy to mechanical properties of a conductor, electromagnetic force and winding structure. The stability of a superconducting composite conductor subject to an abrupt conductor motion is analysed based on the proposed model. It is pointed out that the stability of the conductor is improved by decreasing distance between spacers placed between the conductors. However, it is also pointed out that there is a limit to the improvement of the stability by decreasing the spacer distance, because of limited accuracies of the conductor and spacer dimensions.

Magnetic conditioning in superfluid

Improvements in superconducting magnet technology have reduced to a handful the number of training quenches typical of dipole magnets. The number of training quenches in long (17 m) and short (1–2 m) SSC magnets are now about the same (operating at 6.6 T and 4.4 K), yet the steps necessary to totally eliminate training are in future research and development plans for magnet construction and conductor motion prevention. The accepted hypothesis is that Lorentz forces and poor mechanical properties of superconducting cables are the cause of conductor motion. Conductor motion reduces the stored energy in the cable by converting it into heat. The small amount of heat generated (millijoules) during motion is usually enough to quench the magnet when it is close to short sample. During training, the magnet performance normally improves with the number of quenches. It is not the quench itself that improves magnet performance but rather the fact that once conductor motion has occurred it will probably not repeat itself unless subjected to higher forces. Conditioning is a process that enables the magnet to reduce its stored energy without causing a premature quench 1,2. During the conditioning process, the magnet is further cooled from its operating temperature of 4.4 to 1.8 K by converting He I into He II. As a result, the magnet is placed in a state where it has excess stability as well as excellent heat transfer capabilities 3. Although this does not eliminate motion, if the magnet is now cycled to ≈ 10% above its operating field at 4.4 K (which is above short sample) the excess stability should be enough to prevent quenching and reduce the probability of conductor motion and training once the magnet has been warmed back up to its operating temperature of 4.4 K.

Quantification of disturbance energy due to conductor motion and stability analysis of superconducting composite wire

The authors propose a model to quantify the disturbance energy due to conductor motion. The model relates the disturbance energy to mechanical properties of the conductor, electromagnetic force, and winding structure. The stability of a superconducting composite conductor subject to an abrupt conductor motion is analyzed on the basis of the proposed model. It is pointed out that the stability of the conductor is improved by decreasing the distance between spacers placed between the conductors. However, it is pointed out that there is a limit to the improvement, because of limited accuracies of the conductor and spacer dimensions.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Non-linear motion of a rigid conductor in a magnetic multipole field

The non-linear motion of an ideally conducting current-carrying rigid cylinder in a magnetic multipole field is analyzed. It is found that non-linear stability is achieved provided the current in the external conductor rods is sufficiently large. Since the results agree qualitatively with experimental observations of the Extrap z-pinch, it is suggested that the so far unexplained stability of this configuration is due to non-linear effects.

Estimating the size of disturbances due to conductor motion in superconducting windings

An abrupt conductor motion is one of the dominant origins of a premature quench in a high-performance, high-current-density, magnet. A model based on microscopic slip phenomena is proposed to estimate the disturbance energy due to conductor motion from the mechanical properties of the conductors and the winding configurations, and the stability of the conductor against the disturbance. Stability of a rectangular-cross-section conductor is better than that of a round-cross-section conductor. To improve the stability by reducing the disturbance energy, the aspect ratio (for the rectangular conductor) and the diameter (for the round conductor) must be increased. >

1.8 K conditioning (non-quench training) of model SSC dipoles

The accepted hypothesis is that training quenches are caused by heat generation when conductors move under Lorentz force. Afterwards no conductor motion will occur until a higher field and greater Lorentz force acts. If superior heat transfer and/or greater temperature margin is provided by operating at lower bath temperature, one might expect that the heat generated by conductor motion will not cause a runaway temperature increase, or quench. To test this hypothesis, the central dipole field in SSC model magnets was ramped at 1.8 K to 7.1 tesla without the magnets' quenching. The bath was then raised to 4.4 K and the magnets quenched at their short sample limits of 6.6 tesla or higher. Comparison with similar magnets trained in He I at 4.4 K is made and the significance of the nonquench training on system operation is discussed.

Relation between radial stress and quench current for tightly wound dry solenoids

Premature quenches in superconducting solenoids, wound by Formvar coated NbTi conductors, has been studied. Radial compression between winding layers is supposed to be essential to suppress conductor motions; the relation between radial stress distribution in the winding and quench current is investigated by experiment for a model magnet. The result is discussed based on the stress calculation and frictional characteristics of the Formvar coated conductor.

Frictional motion of conductors of a pancake‐type superconducting magnet

Frictional motion of conductors of a pancake‐type superconducting magnet

Dynamics of a Guyed Transmission Tower System

A free vibration analysis is presented for a transmission line. The guyed supporting towers are somewhat novel, consisting of a Y-shaped frame made of hollow steel sections. Transverse vibrations in both the conductors and the guy-wires were modelled by using special parabolic cable elements. The lowest frequency modes primarily involve conductor motions but are affected by insulator swing and by coupling with the towers. It was determined from such mode shapes that loads transferred to the towers during galloping are not catastrophic, at least when the lines are iced lightly.