Stephen P. Ashworth

A new finite-element method simulation model for computing AC loss in roll assisted biaxially textured substrate YBCO tapes

This paper presents a new finite-element simulation model for computing the electromagnetic properties and AC losses in systems of YBCO (yttrium barium copper oxide) conductors on roll assisted biaxially textured substrates (RABiTS). In this model, the magnetic field dependent permeability and ferromagnetic loss of the substrates in RABiTS YBCO tapes are taken into account. The simulations were employed to simulate the AC loss in stacks of two parallel connected YBCO tapes. The simulation results are compared with the experimental data to check the validity of the simulation model. The result reveals an effective way of significantly reducing AC loss in YBCO tapes by stacking two RABiTS YBCO coated conductors with the appropriate relative tape orientation.

Measuring transport AC losses in YBCO-coated conductor coils

Several AC power applications of YBCO-coated conductors (CCs) involve superconducting tapes wound in coils. In such a configuration the superconducting tape is arranged as closely packed turns, which strongly interact due to the generated magnetic field. This has a strong influence on the AC losses, which are different from those of an isolated tape, and need to be precisely quantified in order to predict and reduce the refrigeration requirements of applications. In this paper we experimentally evaluate the transport AC losses in a pancake coil composed of 25 turns of superconducting tape. We describe in detail the measuring technique utilized, pointing out the issues in this kind of measurement. We also present preliminary results of AC loss computation by finite-element modelling.

Comparison of the AC losses of BSCCO and YBCO conductors by means of numerical analysis

This paper presents a quantitative comparison of the AC loss performance of two BSCCO and two YBCO conductors, characterized by the same self-field critical current of 150 A. In particular, compared are a 37-filamentary BSCCO tape, a 16-filamentary BSCCO square wire, a standard YBCO tape, and a stack of four narrower YBCO tapes. The comparison is made using a numerical technique, based on the finite-element method, which employs a non-linear E-J relation with the dependence of the critical current density Jc on the local magnetic field. For the simulations of YBCO, a new shell-region model is utilised, which allows overcoming the geometry and mesh problems, typical for superconductors with very high aspect ratio. Different AC working conditions are simulated: self-field, applied external field, and combined transport current and external field of varying orientation. Outlined are the advantages of using BSCCO or YBCO conductors for the different applications. Various magnetic field and current density profiles are investigated in order to illustrate the reasons for the loss difference in the four conductors. Particular attention is drawn to the YBCO tape and the YBCO stack, whose AC loss characteristics are less studied than those of BSCCO conductors.

AC loss study of antiparallel connected YBCO coated conductors

Some applications of high temperature superconducting conductors require a non-inductive winding, which may be constructed from antiparallel connected YBCO (yttrium barium copper oxide) tapes. In the case of AC applications, this antiparallel winding changes the AC losses from that of an isolated conductor. This study focuses on the effect of the spatial separation and misalignment between conductors on their AC loss behavior for YBCO conductors on both rolling assisted biaxially textured substrate (RABiTS) and ion beam assisted deposition templates in an effort to fully understand the behavior of these conductors in real world applications. For RABiTS samples, the study was carried out for all three possible configurations (the so-called back-to-back, front-to-front and same-way configurations) to clarify the effect of the ferromagnetic substrate on the AC loss behavior in these conductor configurations. Numerical simulations were also employed in some cases to compare with and elucidate experimental observations.

Interaction of magnetic field and magnetic history in high-temperature superconductors

Yttrium barium copper oxide (YBCO) coated conductors are now the most promising high-temperature superconducting tapes in terms of current capacity and price. One form of these conductors utilizes YBCO films on Ni–W metallic tapes and is being considered for a number of power engineering applications. In these applications, the conductor will carry an ac current, leading to energy losses, which are the focus of significant technical and experimental efforts. Our measurements of the ac losses of YBCO/Ni–W conductors carrying ac currents in applied dc magnetic fields have revealed a complex interaction between the magnetic materials present, the geometry of the conductor, the ac and dc magnetic fields, and the electromagnetic “history” of the sample. The investigation of this interaction is the main subject of this paper.

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

The combination of noninductive winding and the two-in-hand tape configuration has been found to be very effective in reducing the AC loss of coils of RABiTS™ YBCO coated conductor for fault current limiting (FCL) applications. In this study, a noninductive coil composed of 11 turns of two-in-hand RABiTS™ YBCO tape was designed and constructed for characterizing and investigating the AC loss, critical current and inductance. The AC losses in this coil were also calculated by finite element modeling (FEM) using the COMSOL Multiphysics package. With the magnetic field dependent permeability and ferromagnetic loss of the substrate material taken into account, the simulation results are in good agreement with the experimental data. The simulations therefore were employed to study the effect on the AC loss behavior of the coil of the width of YBCO tape and of the spatial separation between the coil turns to suggest a more effective coil design.

Quantifying AC Losses in YBCO Coated Conductor Coils

In superconducting coils tapes are often closely packed in a configuration similar to a ldquoz-stackrdquo, which strongly influences the AC losses with respect to when tapes can be considered as separate objects: losses are reduced in the presence of externally applied AC magnetic field, but are increased in the presence of AC transport current. In this paper, we quantify the magnitude of the tape interaction and the AC losses of these two cases, both experimentally and by means of finite-element method simulations. We also discuss the experimental issues arisen with the utilized AC loss measurement technique.

Current Density Distribution in Multiple YBCO Coated Conductors by Coupled Integral Equations

In this paper we present a new model to compute the current density distribution and the AC losses in multiple YBCO coated conductors by means of coupled integral equations implemented in a finite-element (FE) environment. The model considers the superconducting tapes as 1D objects and allows overcoming the problem of the mesh of conductors of large aspect ratio, typical of 2D implementations; as a result, it is also much faster. The interaction between the different conductors is taken into account by means of a coupling of the electromagnetic variables. The magnetic field distribution in the space between the conductors is computed by coupling the 1D model with a standard 2D magnetic model, which uses the computed current density distributions as sources for the magnetic field. Results are compared with those obtained with standard 2D implementations and with experimental results.

Measurements of AC Losses and Current Distribution in Superconducting Cables

This paper presents our new experimental facility and techniques to measure ac loss and current distribution between the layers for High Temperature Superconducting (HTS) cables. The facility is powered with a 45 kVA three-phase power supply which can provide three-phase currents up to 5 kA per phase via high current transformers. The system is suitable for measurements at any frequency between 20 and 500 Hz to better understand the ac loss mechanisms in HTS cables. In this paper, we will report techniques and results for ac loss measurements carried out on several HTS cables with and without an HTS shielding layer. For cables without a shielding layer, care must be taken to control the effect of the magnetic fields from return currents on loss measurements. The waveform of the axial magnetic field was also measured by a small pick-up coil placed inside a two-layer cable. The temporal current distribution between the layers can be calculated from the waveform of the axial field.

Fast Numerical Computation of Current Distribution and AC Losses in Helically Wound Thin Tape Conductors: Single-Layer Coaxial Arrangement

In this paper, we introduce a very fast method to compute the current distribution in helically wound thin conductors when one or many of them are arranged in a symmetrical manner to form a single-layer power cable. The method relies on two different approaches to find the magnetic vector potential due to helically wound current sheets. By invoking relevant symmetry arguments associated with the geometry of the problem and neglecting the thickness of the tape conductors, we show that this 3-D problem can be reduced to a computationally small 1-D problem whose domain lies along the half-width of any of the constituting conductor. As a consequence, the proposed method is very efficient in terms of computational time, and it is more accurate than many previous 2-D methods that cannot take into account the twist pitch. Since the nonlinear resistivity of the superconducting material can easily be treated with this method, it can be used to find current and field distributions, as well as ac losses in high-temperature superconductor coils and cables made of coated tapes. To verify the validity of the proposed method, we performed experimental measurements of ac losses in two configurations of solenoid-type cables made of a sample of YBCO-coated conductor tape. Excellent agreement was observed between the experimental data and the simulation results.