AC Losses In Coils Research Articles

AC loss and critical current characterization of a noninductive coil of two-in-hand RABiTSYBCO tape for fault current limiter applications

The combination of noninductive winding and the two-in-hand tape configuration hasbeen found to be very effective in reducing the AC loss of coils of RABiTS™ YBCOcoated conductor for fault current limiting (FCL) applications. In this study, anoninductive coil composed of 11 turns of two-in-hand RABiTS™ YBCO tape wasdesigned and constructed for characterizing and investigating the AC loss, criticalcurrent and inductance. The AC losses in this coil were also calculated by finiteelement modeling (FEM) using the COMSOL Multiphysics package. With themagnetic field dependent permeability and ferromagnetic loss of the substratematerial taken into account, the simulation results are in good agreement withthe experimental data. The simulations therefore were employed to study theeffect on the AC loss behavior of the coil of the width of YBCO tape and of thespatial separation between the coil turns to suggest a more effective coil design.

Numerical analysis and finite element modelling of an HTS synchronous motor

This paper investigates the electromagnetic properties of high-temperature superconductors with a particular focus on the AC loss in coils made from YBCO superconductors. The numerical analysis and finite element modelling of the YBCO superconductors used in Cambridge’s superconducting permanent magnet synchronous motor currently in development is described. The stack of tapes in the superconducting coils is modelled using the direct H formulation, a B-dependent critical current density and a bulk approximation. Magnetic boundary conditions for this model are derived from a 2D finite element method (FEM) motor model. The combination of these models allows the total AC loss (combined transport and magnetisation losses) in the HTS coils used in an all-superconducting machine design to be estimated. The raw AC loss figures are compared to the output power of the motor for two test cases, and it is found that the AC loss contributes significantly to the total loss and therefore efficiency. An experimental rig is also described, which has been built in order to test the electromagnetic properties and performance of the motor. It is explained how this rig will be used to investigate the magnetisation of the rotor and carry out AC loss measurements on the stator coils.

Edge and top/bottom losses in non-inductive coated conductor coils with small separationbetween tapes

In this paper we consider two different finite-element models for computing ac losses incoils composed of coated conductors: a 2D model based on solving Maxwell’s equations bymeans of edge elements and a 1D model based on solving the integral equations for thecurrent density in the tapes. The models are tested for a configuration of practical interest,a non-inductive solenoidal coil for fault current limiter applications. We focused ourattention on the conditions when differences between the two models are expected toemerge, for example when the tapes are closely packed or when the dependence of thecritical current density on the local magnetic field is taken into account. We present anddiscuss several cases, offering possible explanations for the observed differences of ac lossvalues.

Development of 1 kA class HTS coil for superconducting power transformers

The winding technology with high current capacity is one of the important factors in power application of 20 MVA/66 kV-class superconducting power transformers. High-current power transformer is achieved by transposed conductors made of Rare Earth-based superconducting coated conductors. By transpositions, the improvement of uniformity of strand current distribution makes it possible to reduce the total number of strands and AC loss of coils. In this study, the high- T c superconducting coil was made by transposed 24 strands of 5 mm-wide Rare Earth-based superconducting coated conductors and operated at a current of 1 kA. The test results of AC and DC current characteristics and AC loss are reported.

AC Losses in a Conduction-Cooled LTS Pulse Coil With Stored Energy of 1 MJ for UPS-SMES as Protection From Momentary Voltage Drops

AC losses in the conduction-cooled low temperature superconducting (LTS) pulse coil with stored energy of 1 MJ are estimated. The 1 MJ coil is a superconducting pulse coil for 1 MW, 1 sec UPS-SMES. UPS-SMES is an uninterruptible power supply (UPS) with superconducting magnetic energy storage (SMES) for protection of production lines of an industrial plant or large-scale experimental devices such as a fusion device, from a momentary voltage drop and an instant power failure. The winding conductor for the 1 MJ coil is a NbTi/Cu Rutherford cable, which is extruded with aluminum. The 1 MJ coil was wound by a new twist winding method. A 1 MJ coil was fabricated and cooling and excitation tests were carried out. In this paper, two methods to estimate ac losses in conduction-cooled LTS coils are proposed. One method estimates ac losses in the coil under steady-state conditions. The other method estimates ac losses in the coil under transient-state conditions. For estimation of ac losses in the 1 MJ coil, measurement of temperature in the coil during tests and thermal analysis using the two-dimensional finite element method are compared. These procedures clarified that the 1 MJ coil has low losses.

Effect of Thermal Cycles on Critical Current and AC Loss for Superconducting Coils Having Positive or Negative Thermal Expansion Bobbin

We investigated the effect of thermal cycles between room and liquid-nitrogen temperatures on the critical current (Ic) and AC loss of two superconducting coils: one with a bobbin that expands during cooling from room temperature to cryogenic temperature and one with a bobbin that contracts during the cooling. After 100 cycles, neither bobbin suffered degradation. The Ic of the contraction-bobbin coil did not decrease, and that of the expansion-bobbin coil decreased due to the repeated thermal strain. The expansion coil's AC loss was significantly smaller than the contraction coil's loss at the first cycle; however, it increased with the thermal cycles and eventually surpassed the contraction coil's loss because the Ic of the expansion coil decreased due to the thermal fatigue. These results indicate that a moderate expansion in the size of the bobbin effectively decreases AC loss and that excessive expansion reduces the Ic and increases the AC loss.

Analysis of AC Loss Characteristics of High Temperature Superconducting Coil

This paper reports an analytical study on AC loss characteristics of a high temperature superconducting coil wound with a Bi2223/Ag tape. The electromagnetic AC loss in the high temperature superconducting coil made by stacking pancake coils wound with the Bi2223/Ag tape was numerically calculated by using the measured DC data of the HTS tape. The calculated AC loss was compared with measured AC losses in Bi2223 HTS coils obtained in a previous study. It had been found that the measured AC losses in the coils contained large mechanical loss. For the comparison of the measured and calculated losses, the mechanical loss was subtracted from the measured total loss data to estimate the electromagnetic loss. The calculated AC losses were compared with the measured electromagnetic losses in the coils, producing rather good agreement. A possible way to reduce the AC loss in the HTS coil was also presented and discussed.

AC coil, reactor and cable demonstrations of low AC loss elementary and assembledBSCCO conductors

A European consortium has progressed towards AC coil, transformer and cable applicationsmade of low AC loss Bi-2223 conductors. Internally assembled ring-bundle-barrierconductors were used in AC coils and an externally assembled conductor was realized fortransformer and reactor windings. For the design and optimization of HTS AC coils, anumerical tool for the prediction of AC losses in coils and coil sets had been developed. Thedemonstrations with air-core coils and iron-core reactors showed that the numerical toolcan already be confidently used in the design and optimization of HTS AC coils at nominaloperation conditions. A series of three power cable prototypes was manufactured toestimate the influence of optimized low AC loss tapes on the total losses of a cable. In orderto estimate the impact of the twist pitch on the losses we also manufactured cableswith short and long twist pitches. The advantages and drawbacks of low ACloss tapes in power applications are discussed on the grounds of the test results.

AC loss measurement of HTS coils with ferromagnetic disks

Manufacturing techniques for low AC loss high-T/sub c/ superconducting coils are necessary to realize high efficiency superconducting power equipments. High-T/sub c/ superconducting tape conductors have anisotropic magnetic field dependence on critical current and AC magnetization loss. A magnetic field component perpendicular to the tape surface degrades HTS coil performance. We numerically investigated a magnetic field adjusting method and AC loss reduction effects by attaching ferromagnetic materials to both ends of HTS coils. We also measured total AC loss of an HTS coil comprising 10 HTS double pancake coils by the boil-off method. Total AC loss of the coil was dominated by mechanical loss due to vibration and movement of conductors and pancake coils. In this paper, methods to reduce total AC loss of HTS coils are discussed.

Determination of the AC losses of Bi-2223 HTS coils at 77 K at power frequencies using a mass boil-off calorimetric technique

A mass boil-off measurement system has been used to accurately measure and characterize the ac loss of high-temperature superconductor (HTS) coils at frequencies between 50 and 200 Hz, and in applied ac fields of up to 0.04 T. The mass boil-off calorimeter incorporated a glass cryostat, a copper field coil, and two mass flow meters. The response of the gas flow to a step change in the applied magnetic field was found to have a time constant of about 600 s. Under suitable experimental conditions, it was possible to measure the ac losses of coils with an accuracy of /spl plusmn/0.3 W. The ac loss characteristics of an HTS pancake and two HTS solenoid coils are presented and the accuracy of mass flow calorimetry in liquid nitrogen is reported on.

AC losses of NbTi superconducting wires with fine filament

An investigation was conducted of the AC losses in coils with NbTi superconducting wires. The authors used the multiple-stacking procedure to prepare the wires, which varied in filament size from 10 mu m to 0.07 mu m. Three kinds of wires with 0.07- mu m filament were specially fabricated to estimate the proximity effect on AC losses. The distances between the filaments of these wires were 21 nm, 42 nm, and 70 nm, respectively. AC losses were measured for small coils at 60 Hz. The proximity effect was observed for the wires with 0.07- mu m filament when the distance was 21 nm. Conversely, when the distance was 42 nm, the loss decreased to the small value of 11 kW/m/sup 3/ at 0.5 T and 4.2 K. >

Ac losses of superconducting solenoidal coils

ac losses of superconducting solenoidal coils were investigated with a calorimetric method while applying a current or uniform magnetic field of 50 Hz with a sinusoidal wave form. The peak value of the ac current was varied from 0 to 15 rms A and that of the ac magnetic field from 0 to 0.02 T. These experimental conditions are for a practical regime where loss theories are difficult to apply and empirical data of the type generated are directly useful. Nine coil samples of various sizes were constructed with different fine multifilamentary superconducting wires. The ac losses observed are proportional to the square of the amplitude of an applied ac magnetic field or an applied ac current and appear to be mainly eddy current losses. The ac losses of coils of the same size caused by an applied ac current were the same regardless of the kind of wire used. All losses in the superconducting coils appeared to be equivalent to those expected from eddy currents in solid copper wires 0.72 times the diameter of the wires used, independent of the wire size and internal configuration.