Value Of Transport Current Research Articles

Numerical study on self-field losses of 30 m BSCCO HTS transmission cable

The self-field losses of the one phase of high- T C superconducting (HTS) transmission cable are calculated by the electric circuit (EC) model. The one phase of HTS cable is constructed by the former of fine-strands copper rod, HTS conductor with four superconducting layers, the insulation made by polypropylene laminated paper, and HTS shielding with two superconducting layers, which was fabricated by Sumitomo Electric Industries (SEI). The length of the cable is 30 m. Each HTS layer comprises BSCCO tapes. The current-dependent resistance of HTS layers in EC model is estimated on the base of Norris expressions for ellipse. The calculated losses are compared with the experimental results measured by 4-terminal method by SEI. The calculation of alternating current (AC) losses, a summation of the self-field losses in HTS layers and the eddy-current losses in the former, is almost equal to the measurement at wide transport-current range below the lowest value of the layer critical current. This result indicates that the numerical calculation by EC model is quite reliable. The minimum AC loss is also calculated by obtaining the optimum helical-pitch lengths of HTS layers at transporting 1 kA rms. The minimum loss is 36% lower than the loss of HTS cable designed by SEI at the transport current value. In HTS cable with the optimum helical-pitch lengths, the calculation of the layer currents are not uniform in HTS conductor but are almost uniform in HTS shielding, which is contradict to SEI’s one. It is considered that the numerical calculation by EC model is useful to obtain the optimum helical-pitch lengths in HTS cable with the minimum AC loss.

Slow voltage oscillations in Ag-doped superconducting Y–Ba–Cu–O

The time effects in Ag-doped YBa 2Cu 3O 7− x sample (YBCO/Ag) were examined by means of transport relaxation measurements ( V- t curves). At well-defined values of transport current ( I), temperature ( T) and external magnetic field ( H), an abrupt rise in sample voltage was observed at the early stage of the relaxation process. After reducing the initial current to a finite value, the sample voltage levels off within a very short time. The rapid voltage drops seen in V- t curves were attributed to the rapid dynamic reorganization of flux lines traversing the sample edges. These observations were also interpreted as an indication of doping of YBCO with Ag and easy suppression of superconducting order parameter due to the presence of Ag. In addition, we investigated the influence of bidirectional square wave (BSW) current on the evolution of V- t curves at different temperatures and external magnetic fields. It was observed that a nonlinear response seen in V- t curves to BSW current with sufficiently short periods or sufficiently low amplitude reflects itself as regular sinusoidal- type voltage oscillations, which were discussed mainly in terms of the dynamic competition between pinning and depinning.

Self-Field Loss in AC Transport Current of<tex>$rm Ni$</tex>-Sheathed<tex>$rm MgB_2$</tex>Superconducting Tapes

AC transport current loss (self field loss) of ex-situ-processed <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$ MgB_2$</tex> superconducting tapes with <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$ Ni$</tex> sheath was measured under different conditions of transport current value, transport current frequency, and temperature. Experimental results were compared with Norris' theoretical curves and previous measured losses of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$ MgB_2$</tex> tapes with <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$ CuNi$</tex> and stainless-steel sheaths, which were larger than the theoretical ones and previous data. The loss vs. normalized transport current (transport current normalized by critical current) curves varied with temperature and the loss at 9 K was larger than that at 14 K. The measured loss approached Norris' curves in the high normalized transport current region. In order to gain insight into these origins, FEM analysis of magnetic flux concentration by <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$ Ni$</tex> and estimations of hysteresis loss of ferromagnetic nickel-sheath and of homogeneity of the core were carried out. As a result, the hysteresis loss of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$ Ni$</tex> was very small based on the estimation and the loss measurement of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$ Ni$</tex> . Main contributions to large loss are considered to be self-field enhancement due to magnetic flux concentration by the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$ Ni$</tex> -sheath and critical current density distribution of the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$ MgB_2$</tex> core caused by the inhomogeneity of critical current density and critical temperature.

The field-free region in a coated superconductor according to the Bean model for the case of transport current

This paper is concerned with the question of whether the basic Bean model which applies for the description of transport current and its loss in ordinary superconductors can also be used for describing the ac transport current in a high-temperature coated superconductor. A corollary which goes with the Bean model is the existence of a field-free region in the interior of the conductor, and a direct computation of the loss depends strongly on an accurate knowledge of this region. A calculation of the field-free region is performed here, with the loss calculation reserved for a companion paper. The shape of the field-free region is computed for various values of transport current in zero applied magnetic field. It is found that for a ratio of peak transport current to critical current greater than about 0.46 the field-free region is described by an ellipse-like curve in a reduced plot, while in a similar plot for a ratio less than about 0.46 it has a shape that more closely resembles a rectangle. The former is similar to the field-free region assumed by Norris and this case is computed in the present analysis quite accurately for ratios of peak transport current to critical current near unity. For the case of small peak current the calculation is less accurate because of difficulties of computation near the edges. Nevertheless the result can serve as an aid to making a loss estimate.

AC loss measurements of a high critical temperature superconductor transporting sinusoidal or non-sinusoidal current

In electrical engineering, static converters generate harmonics; consequently the losses in the conductors increase. To use High-Tc superconductors for current transportation, it is important to evaluate and to measure their losses. Under the assumption that the superconductor is only exposed to its self-field, Bean's model is used to compute these losses. This work deals with the losses in a cylindrical High-Tc superconductor current-lead. It is made of compacted composite of Bi-Pb-Sr-Ca-Cu-O-2223, the value of its critical current is about 100 A (measured with I /spl mu/V/cm criteria). Two types of measuring methods are used. The first of them uses a differential amplifier associated with a computer, which calculate the instantaneous power by numerical integration. The second series of experiments, valid only with sinusoidal current, are made using a lock-in amplifier measuring the R.M.S. voltage. The value of transport current is high enough to be compatible with applications. Both methods give nearly the same results for sinusoidal signal. Measurements are made with non-sinusoidal current, to do that the superconductor sample is fed by a dimmer switch. The experiments show that the losses are essentially a function of the maximum value of the current, not of the firing angle. We conclude that the shape of the current is not important compared to its peak value.

超電導撚線における高速常電導転移現象 Ｉ 発生機構

The mechanism of the so-called fast quench, i.e., an abnormal quench process with a high propagation velocity of normal zone in the multi-strand cables is discussed. Firstly, a set of basic equations were proposed so as to describe precisely the electromagnetic coupling among strands, propagation of normal zone and thermal interaction among strands. A good agreement of the theoretical value of transport current, at which the fast quench began to appear, with our observed data for a 2-strand cable supported strongly the availability of the proposed equations for a description of the fast quench. Then numerical calculations were also carried out for the commutation of transport current and temperature distribution inside a 2-strand cable when one strand was locally quenched. With the aid of the obtained results, a detailed mechanism for the occurrence of the fast quench was proposed.

Superconducting current stability in composite superconductors

The maximum transport current in multifilament composite superconductors has been studied. The experimentally obtained values of transport current exceed the normal phase minimum propagation current substantially. The current-voltage characteristics of the investigated samples have been studied. It is shown that the relatively high value of the maximum transport current is attributed, under the given experimental conditions, to the behaviour of the current-voltage characteristic in the region of low values of the electric field. The theory and experiment are in good agreement.