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HTS wire cost is a critical factor for successful commercialization of HTS traction transformer technology. Wire cost might be minimized without significantly increasing AC loss by introducing a hybrid winding structure: the end-part of the windings is wound with high-cost high- I c wires; the central-part of the windings is wound with low-cost low -I c wires. We report AC loss simulation results on HTS windings with both HV and LV windings wound with REBCO wires. The 2D axisymmetric FEM simulation was carried out using H -formulation. The HV windings are wound with 4 mm-wide wires and LV windings are wound with 8/5 (eight 5 mm – wide strands) Roebel cables. Both HV and LV windings have a hybrid structure in order to reduce the wire cost. Flux diverters are placed at the end of the windings to reduce AC loss. Significant HTS wire cost reduction could be achieved without compromising AC loss by using hybrid windings. This may help commercialize HTS traction transformer technology.

The objective of the paper is to describe the application of artificial intelligence software to predict formation evaluation logs (compressional sonic, shear sonic and density) using only gamma ray, and resistivity log data and drilling dynamics data as received by the electronic drilling recorder (EDR). The software was applied real-time as a well was being drilled in deepwater Gulf of Mexico. Thorough examination and conditioning of EDR and wireline data give way to a training model construction for the artificial neural network (ANN) using full suites of log-data in offset wells. Next, a neural network architecture and associated hyperparameters are chosen and tested. The fully trained and validated model is applied to the gamma ray, resistivity and EDR of the target well while drilling. Real-time EDR and wireline data flow via WITSML from rig to cloud and data is delivered to the client. The results of the study indicate the simulated log data were comparable to those measured from conventional logging tools over the study area. In both blind well tests the density agreed with the conventional log results within 1.1 % and the compressional within 2.51 % (Figure 1). Each of these is well within the range of variance expected of repeat runs of a conventional logging tool. A primary driver for near real-time logs was to confirm structural depth of the target sands along the well bore. There was a depleted sand below the expected TD of the well that, if encountered, could have led to total losses and possible loss of the wellbore. It was critical to have real-time logs to characterize the sands above the depleted sand, using every possible petrophysical and geologic character to refine the log correlation. This integration of all the logs provided the best interpretation of the sand quality and led toward the completion decision. AI-based logs are a highly cost-effective alternative to LWD logging. It presents an environmentally friendly approach as there is no logging personnel on-site and no expensive and potentially dangerous nuclear sources in the hole The deployment of this patented, machine learning-driven, real-time simulation of formation evaluation logs is unique in using only gamma ray, resistivity and drilling data. It is particularly useful in the overburden section where formation evaluation tools are often not run for cost reasons, in side-tracks, in HP/HT settings and operational risk mitigation. It provides additive data for other petrophysical/QI/rock property analyses including seismic inversion, shale content, porosity, log QC/editing, real-time LWD, drilling optimization, etc.

Traction transformer is one of the critical electrical devices in high-speed trains. To increase transformer efficiency up to 99% and reduce weight and fire risks, we are currently building a 6.5 MVA superconducting traction transformer. Assuming 15 as a realistic cooling penalty, thermal loss from the transformer windings cannot exceed 4 kW in order to meet 99% of efficiency. There are non-ignorable high frequency harmonics in the traction side of the transformer, so it is important to investigate the effect of harmonic current on ac loss in transformer windings. We explored the dependency of ac loss in transformer on the amount of the 13th harmonic in traction side, by means of numerical calculation in a 2-D axisymmetric model using the H formulation. Simulations were also carried out to study the influence of the phase state of the 13th harmonic, either in phase or out of phase compared to the fundamental current, and multiple harmonics together in traction winding. Results show that the 13th harmonic must be limited under 1.5% of the rated current to fulfill the aim of 99% efficiency. AC loss in transformer increases when maximum amplitude of distorted currents carrying the same order in-phase harmonic but different amount increases. With various harmonic components in the LV winding, it is important to consider both the maximum current and waveforms to have a clear idea of loss behavior in the transformer.

In this paper, seven groups of sheet insulation for use in liquid nitrogen immersed power transformers have been tested under three different standard electrodes from ASTM D149-97a. The seven groups consist of three different types of paper/film, nonadhesive Kapton 100 HN (Kapton), THERNOMID 14.20 paper (NMN), and Nomex 410 paper (Nomex). The porous paper, Nomex 410, is found to have ac dielectric strength of 62-68 kV/mm, which is likely to be independent from its thickness. The nonporous film, Kapton, shows the dielectric strength of 288 kV/mm, whereas NMN paper has a dielectric strength of 74-78 kV/mm, larger than Nomex with similar thickness. Liquid nitrogen gaps near the triple junction of electrodes as well as liquid nitrogen pockets between the fibers are the main sources of partial discharge (PD) for NMN and Nomex. For Kapton, PDs are more likely to occur in the cavities filled with liquid nitrogen, either on the surface of the electrode or around the impurities on the surface of the film. Electrodes with a small stressed volume are more appropriate for dielectric tests of thin paper/film insulation, such as Kapton, due to the lower influence of cavities on the electrode surface. In contrast, electrodes with a large stressed volume and uniform electric field distribution are suitable for the papers such as Nomex and NMN.

Power transformers using a high temperature superconductor (HTS) ReBCO coated conductor and liquid nitrogen dielectric have many potential advantages over conventional transformers. The ac loss in the windings complicates the cryogenics and reduces the efficiency, and hence it needs to be predicted in its design, usually by numerical calculations. This article presents detailed modelling of superconducting transformers with Roebel cable in the low-voltage (LV) winding and a high-voltage (HV) winding with more than 1000 turns. First, we model a 1 MVA 11 kV/415 V 3-phase transformer. The Roebel cable solenoid forming the LV winding is also analyzed as a stand-alone coil. Agreement between calculations and experiments of the 1 MVA transformer supports the model validity for a larger tentative 40 MVA 110 kV/11 kV 3-phase transformer design. We found that the ac loss in each winding is much lower when it is inserted in the transformer than as a stand-alone coil. The ac loss in the 1 and 40 MVA transformers is dominated by the LV and HV windings, respectively. Finally, the ratio of total loss over rated power of the 40 MVA transformer is reduced below 40% of that of the 1 MVA transformer. In conclusion, the modelling tool in this work can reliably predict the ac loss in real power applications. This is the Accepted Manuscript version of an article accepted for publication in 'Superconductor Science and Technology'. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record is available online at https://doi.org/10.1088/0953-2048/28/11/114008.

One of critical issues for HTS transformers is achieving sufficiently low AC loss in the windings. Therefore, accurate prediction of AC loss is critical for the HTS transformer applications. In this work, we present AC loss simulation results employing the H-formulation for a 1 MVA 3-Phase HTS transformer. The high voltage (HV) windings are composed of 24 double pancakes per phase wound with 4 mm – wide YBCO wire. Each double pancake coil has 38 ¼ turns. The low voltage (LV) windings are 20 turn single-layer solenoid windings wound with 15/5 (15 strands of 5 mm width) Roebel cable per phase. The numerical method was first verified by comparing the numerical and experimental AC loss results for two coil assemblies composed of two and six double pancake coils (DPCs). The numerical AC loss calculated for the transformer was compared with the measured AC loss as well as the numerical result obtained using the minimum magnetic energy variation (MMEV) method. The numerical AC loss result in this work and experimental result as well as the numerical result using MMEV at the rated current agree to within 20%. Further simulations were carried out to explore the dependence of the AC loss on the gap between the turns of the LV winding. The minimum AC loss at rated current in the 1 MVA HTS transformer appears when the gap between turns is approximately 2.1 mm turn gap in the LV winding. This is due to the change of relative heights between the HV and LV windings which results in optimal radial magnetic field cancellation. The same numerical method can be applied to calculate AC loss in larger rating HTS transformers.

According to our previous research on the dielectric strength of Nomex T410 paper under a horizontal turn-to-turn electrode system, a significant decrease by approximately 38% of the dielectric strength was found after the introduction of film boiling. In this paper, a wide range of sheet insulation samples has been tested under a turn-to-turn electrode system in vertical arrangement, more practical to the real design of high-temperature superconducting power transformers. Samples from three different types of paper (NMN, Nomex, and Kapton) have been investigated under a static situation with no boiling introduced. In addition, their dynamic dielectric characteristics under different levels of electric stress have also been tested by introducing a transient thermal stress. The experiment has been conducted in open bath liquid nitrogen (77 K) as well as in subcooled liquid nitrogen (67 K). Both of the tests are conducted under one atmospheric pressure. The result shows that all samples have enhanced dielectric performance when immersed in the subcooled liquid nitrogen. Moreover, the Kapton film is found to always have the best dielectric performance. However, its poor impregnation of liquid nitrogen largely affects its dielectric strength, especially at 77 K. When compared to Nomex paper with a similar thickness, the NMN paper is better due to its nonporous structure.

Increasing the current-carrying capacity of bifilar coils is essential for the high-temperature superconducting (HTS) fault current limiter (FCL) application. We built three bifilar stacks with two, four, and six conductors using 4 mm wide stabilized SuperPower YBCO wires to investigate ac loss dependence of the bifilar stacks on the number of conductors. In the three basic configurations, the upper half conductors carry current in one direction, and the lower half conductors carry current in the opposite direction. We report transport ac loss results on these stacks measured at three different frequencies. The measured loss values were compared with those in stacks with the same conductor number and geometry but producing different degrees of field cancellation. We also explored the dependence of ac loss on a central gap in bifilar stacks composed of four and six conductors and the results were compared with the numerical results obtained from a two-dimensional FEM model. Results show that the bifilar stacks where each turn is stacked with multiple wires is a promising option which compromise both high current-carrying capacity and lower ac loss for HTS FCL application.

We have been seeking effective ways to reduce ac losses in REBCO coil assemblies. The ac loss characteristics of assemblies of REBCO double-pancake coils (DPCs) are influenced by the magnetic interactions between coils as well as the electric field (E)−current density (J) characteristics of REBCO coated conductors. We compared the ac loss characteristics of two assemblies of DPCs obtained by electromagnetic field analysis and experiment: one consisting of two DPCs and another consisting of six DPCs. Prior to the electromagnetic field analyses, we measured the current transport properties of the coated conductors and formulated their E−J characteristics. In the assembly of six DPCs, ac loss concentrates in the end PCs. In order to reduce the ac loss there, we replaced the end DPCs with ones wound with coated conductors with higher critical currents. However, the conductor replacing is not effective so much to reduce the ac loss because of magnetic interactions between DPCs.

Assembled coated conductors are essential in many high-current high-temperature superconducting (HTS) applications. A promising option is to use Roebel cable, as it offers both low ac loss and mechanical flexibility. In this work, we measured magnetization loss at 77 K in REBCO Roebel cables with varying strand numbers from 6 to 14 to investigate the strand number dependence of ac loss characteristics of the Roebel cables. The 4-mm-wide Roebel strands were punched from nonstabilized 10-mm-wide Fujikura REBCO tapes. The applied field amplitude, frequency, and the field angle (the angle between the magnetic field and the tape wide surface) were varied. Three 10-mm-wide vertical stacks with conductor number of three, five, and seven were prepared as reference stacks using the same source Fujikura tapes. The measured magnetization loss values of the Roebel cables were compared with those of the equivalent stacks as well as with the results of COMSOL modeling of the stacks using the H -formulation. At magnetic-field amplitudes greater than the effective penetration field of the Roebel cables the cables have lower ac loss than equivalent stacks.

It is crucial for the successful application of high-temperature superconducting (HTS) power transformers that they are able to survive under abnormal operations, such as a short-circuit fault. Modeling, based on the design of a four-winding three-phase HTS power transformer, suggests severe boiling will occur on the surface of the winding during a short-circuit fault and will jeopardize the turn-to-turn insulation of the HTS power transformer. In this paper, a turn-to-turn insulation breakdown experiment platform involving the introduction of boiling on the surface of electrode has been built. Nomex 410 paper, one of the candidates for turn-to-turn insulation in the design of the HTS power transformer, was tested. The characteristics of dielectric strength of Nomex 410 in three different thicknesses (0.13, 0.18, and 0.25 mm) have been investigated, without boiling, as well as under different boiling situations, in an open bath of liquid nitrogen. The voltage application methods applied during both experiments test the electrical breakdown of Nomex 410 under a dynamic situation. The experiment results show that when under film boiling, compared to the test results for nucleate boiling, the dielectric strength of Nomex 410 paper decreases by approximately 38% of the value when no boiling is involved. Moreover, it is verified that the presence of an electric field accelerates the heat transfer between the liquid nitrogen and the heated electrode.

Wire cost and ac loss are paramount issues for the future uptake of high-temperature superconductor (HTS) ac machines. In particular, new approaches which reduce the total manufacturing cost of HTS coils are critical to the future commercial uptake of HTS technology. Here, we present the results of ac loss measurements of a hybrid HTS coil assembly comprising end windings (EWs) wound with BSCCO wire and a central winding (CW) wound with REBCO wire. As a cost mitigation measure, we have chosen to use REBCO wire which possesses a relatively low self-field $I_{{\rm{c}}}$ at 77 K, equating to approximately half that of the BSCCO wire. Transport ac loss measurements were made on the total coil assembly at both 77 K and 65 K, using a recently developed lock-in amplifier approach. We show the ac losses in the assembly at these temperatures scale with the overall transport critical current threshold of the hybrid coil assembly. The results are then compared with a reference coil assembly having similar geometrical dimensions, and in which both the EWs and CW were wound with identical BSCCO wire. Surprisingly, we observe that the ac loss of the hybrid coil is lower than that of the BSCCO coil, despite the CW of the hybrid coil employing wire a lower overall average $I_{{\rm{c}}}$ . We attribute this effect to the lower ac loss of coated conductor REBCO wire under parallel field, which is the dominant field orientation in the CW region. Based on this observation, we conclude that the hybrid assembly design is a good design strategy for HTS coil assemblies, enabling both wire cost and ac loss to be balanced.

We present results from commissioning testing of a 1 MVA three-phase transformer demonstration project. The three-phase transformer utilizes a 15 strand Roebel cable for the low voltage windings and 4 mm wide conductor for the high voltage windings. Both windings use ReBCO conductor and the iron core is external to the cryogenic region. The target operating temperature is 70 K. Key design features and operational performance measures of the advanced sub-cooled liquid nitrogen circulation system are described. The overall thermal budget is presented along with ac loss measurements for the windings and electrical test results of the complete transformer. There remain some unresolved issues with the stability of the cryogenic cooling system when operated sub-cooled, and with the high voltage withstand performance of the HV bushings. These issues are described and discussed. The utility of HTS Roebel cable as a convenient means of producing a compact, high current winding with low ac loss characteristics has been successfully demonstrated.

This work describes the fabrication process to realise a highly columnar porous microstructured piezoceramic, and provides 2D array element pulse response plots for both porous and nonporous material.

AC loss in high temperature superconducting (HTS) coils affects the performance of HTS devices. Using magnetic flux diverters (MFDs) is an effective way to reduce AC loss in HTS coils. In this paper, measurement and finite element method simulation of AC loss results in a REBCO coil assembly comprising four double pancake coils with two molypermalloy-powder MFDs are presented. Both experimental and numerical results show that MFDs can significantly reduce the AC loss in the REBCO coil assembly while generating negligible loss in themselves. Further, the influence of the distance between the coil assembly and the diverters on AC loss reduction is explored. Compared with the AC loss data in the coil assembly without MFDs, over 80% AC loss reduction is achieved when the distance between the coil assembly and the diverters is at its minimum value, 2 mm. The simulation results reveal that the AC loss reduction in the coil assembly is mainly due to the reduction of the radial (perpendicular) magnetic field component to the surface of REBCO wires in the end windings of the coil assembly.

High-temperature superconducting (HTS) transformer windings operate subcooled at atmospheric pressure to temperatures as low as 65 K, with minimal power dissipation from AC loss in normal operation. During short circuits, HTS transformers are subjected to transient heating by currents as large as 10 times the rated current for durations of up to 2 s. After isolating the fault, HTS transformers are required to cool back down to their base temperature while carrying the operating current. HTS transformer conductors are insulated, typically with wrapped self-adhesive polyimide tape, and windings consist of close-packed conductors wound on composite formers. In this paper, the heat transfer, transient thermal response, and recovery performance of brass-laminated coated conductor HTS wire as well as metallic conductors in liquid nitrogen (LN2) were measured at a range of temperatures from 77 K to 64 K and pressures from atmospheric to 0.14 bar in heating and cooling situations. Measurements were made on bare tapes, and on tapes wrapped with polyimide insulation tape and aramid paper as well as polymer-coated tapes with a range of coating thicknesses. Heat transfer from tapes mounted with one face in contact with a glass-epoxy composite cylindrical former was measured for comparison with free-standing tapes. Using conductors with solid polymer coatings of optimized thickness immersed in subcooled LN2 results in the highest heat transfer and fast recovery following heating to 300 K by a high current pulse. Compared to a bare tape in LN2 at ambient pressure at 77 K, the heat transfer in a coated tape in 65 K subcooled LN2 can be very significantly enhanced by up to a factor of 15, and recovery can be seven times faster.

Traction transformers are critical components of Chinese high-speed-trains. We are currently building a single–phase 6.5 MVA superconducting traction transformer which can achieve targets of less than 3 tons of transformer system weight, better than 99% efficiency, and 43% short-circuit impedance. The proposed transformer consists of four single-phase 25 kV/1.9 kV HTS windings, operating at 65 K, each of which drives a motor. The design incorporates Roebel cable in the LV windings to cope with large current and minimize AC loss. We present 2D FEM AC loss modelling results that identify the critical parameters that contribute to AC loss. We show that the combination of winding length ≥1 m, high performance Fujikura wires, and flux diverters arranged at the end of HV and LV windings, can restrain AC loss in the HTS windings to under 2 kW. We introduce an open-loop cooling system concept with sub-cooler integrated inside the transformer cryostat that can achieve total system weight under 3 tons assuming 2.5 kW total heat load and 8 h of continuous running time. A nominal efficiency of 99.5% can be achieved for this total heat load. The entire superconducting transformer system can be readily fit in the space allocated for conventional transformers in the Chinese Fuxing trains.

We measured ac loss in Roebel cables consisting of six REBCO strands and ten REBCO strands. The amplitude of the ac transport current and the applied ac magnetic field, and the angle of the applied ac magnetic field to the wide face of the Roebel cable were varied. Measured ac losses of the Roebel cables were compared with those of reference stacks of coated conductors to discuss on the influence of the strand number and the three-dimensional structure of the Roebel cables on their ac loss characteristics.

Superconducting transformers using high current density High Temperature Superconductor (HTS) wire cooled with liquid nitrogen can be lighter and more efficient than conventional power transformers. This paper describes the 1 MVA 11/0.415 kV HTS transformer developed by a New Zealand - Australian team, featuring HTS Roebel cable in the 1.4 kA-rated low voltage winding. Comparison of HTS and conventional transformer designs at 40 MVA rating shows lower lifetime cost of losses makes HTS base-load transformers cost-competitive in higher energy cost markets. Power density - more MVA in a restricted footprint - could be a decisive advantage in mobile applications.

Progress is reported on the development of a 3-phase 1 MVA 11 kV/415 V HTS transformer using Roebel cable for the secondary windings. We describe efforts to address risks associated with short circuits, insulation, and heat transfer. We present modelling results for the response of the windings to a short circuit. Sample strands have been tested to demonstrate that cables immersed in liquid nitrogen can survive short circuits. The primary windings use insulated conductor to withstand computed impulse voltages. Breakdown and partial discharge testing confirms that the production insulation scheme is adequate for our design. Heat transfer results are presented for a sample Roebel cable winding

The transport AC loss in a short length of 8/2 YBCO Roebel cable, (i.e., with 8 × 2 mm width strands) is measured. The results are compared with those in a stack composed of four 4 mm-wide YBCO tapes carrying the same current in each tape to verify the advantage of Roebel cables over vertical stacks in terms of transport AC loss. The vertical separation between the superconductor layers in the Roebel cable and the stack is made the same to facilitate comparison. The normalized transport AC losses in the cable are smaller than those in the stack. This is a clear demonstration that Roebel cable is advantageous over stacks with and without transposition of the tapes composing the stacks. Results from a numerical calculation are in accord with the experimental results.

We present design details for the planned construction of a 3-phase 1 MVA 11 kV/415 V transformer using YBCO Roebel cable (YBCO-Yttrium Barium Copper Oxide second generation high temperature superconducting coated conductor). The YBCO Roebel cable is a promising technology for carrying high currents. It simplifies manufacture of the winding while managing AC loss. We present the transformer winding design which follows a simple layout with the low-voltage windings utilizing a 15 strand × 5 mm (15/5) Roebel cable in a single-layer 20 turn solenoid. The target rated current capacity of the cable is 1500 A rms at 77 K. The high-voltage winding will be in the form of a stack of double pan cake coils arranged on a composite former outside the low-voltage windings. The coils will be immersed in sub-cooled liquid nitrogen with a target maximum operating temperature of 70 K. The development methodology is described along with the results of experiments and modeling to validate the performance characteristics of the windings.

We present results for magnetic losses in a five strand HTS Roebel cable with 2 mm wide insulated strands. The loss as a function of field is compared to the isolated strands and an un-insulated cable. The loss as a function of the angle of the applied field to the cable normal is found to scale simply with the normal component of field. The dependence of the loss on the frequency of the applied field is presented and a small intrinsic frequency dependence of the superconductor is observed. The frequency dependence of the loss in a cable coupled by copper bridges is presented. The coupling loss is found to follow the Debye form at low fields. Transport AC losses in HTS Roebel cable with different strand insulation thicknesses are found to vary with the vertical separation of strands.

Transport AC loss in a short length of 5/2 YBCO Roebel cable (five 2 mm wide strands) with weakly magnetic substrate is measured. The AC loss data are compared with those for a single strand. All the strands composing the cable and the single strand are cut from the same stock material. Transport AC loss in the 5/2 Roebel cable is larger than that predicted by both the Norris strip and ellipse models over a large range of the cable current. The difference between normalized AC losses in the Roebel cable and the single strand is larger at low It/Ic than at high It/Ic, where It is the amplitude of transport current. These AC loss characteristics were not observed in transport AC loss results for YBCO Roebel cables composed of coated conductor with non-magnetic substrate. Results from an electromagnetic analysis of two parallel stacks of wire with magnetic substrate are presented to elucidate the results.

We have developed a numerical technique to estimate the current carrying capability of HTS Roebel cable composed from coated conductor strands. The influence of self-field on the critical current density is studied computationally for a Roebel cable using a realistic field and angle dependence behaviour of critical current. The computations are carried out for N /2 (number of strands/strand width in mm), and N /5 Roebel cable for N = 2–15. The local current distribution in each strand satisfies the self-consistent criteria J = J c ( B ( J )) except for a small region where the current density is set to zero to maintain the condition of equal currents in all strands. The variation of critical current with vertical and horizontal separation between the strands is also investigated. Results are compared with the measured values of critical current for some of our cables. The comparison shows an error of up to 10% which we attribute mostly to the model not accounting for the spread in I c values of the constituent strands.

REBCO Roebel cables are promising candidates for bifilar coils (or non-inductive coils) in large scale high temperature superconducting resistive type fault current limiters (SFCLs), as they enable large current-carrying capability in conjunction with low AC loss. Here we report transport AC loss measurement results in two Roebel cable bifilar stacks (RBS) with different vertical spacing between the turns assembled from a straight 4/2 (four 2 mm width strands) Roebel cable. We also report two-dimensional finite element method AC loss simulation results using the H-formulation in the RBSs, in two parallel bifilar stacks (TPBS) which have the same geometrical dimensions as the RBS with 0.284 mm spacing (g) but have unequal current distribution between the conductors composing the stacks, and in an equivalent bifilar stack (EBS) comprising four 4 mm wide conductors while keeping identical vertical geometrical dimensions with the RBS with g = 0.284 mm but has unequal current distribution between the conductors. We show that the measured transport AC loss values in the RBS with g = 0.284 mm are approximately one fifth of those in the parent straight 4/2 Roebel cable due to cancellation of the perpendicular magnetic field components in the bifilar stack, and also show the AC loss in RBSs increases with increasing g. We then show the calculated transport AC loss values in the RBS with g = 0.284 mm are approximately one third of the values in the TPBS and EBS when I t,turn/I c0,cable > 0.6, where I t,turn is the current amplitude for upper or lower turn of the bifilar stacks and I c0,cable is nominal I c value for the 4/2 Roebel cable. We numerically demonstrate that the result is caused by flux flow loss in the inner conductors in the TPBS and EBS due to unequal current distributions in conductors. Roebel cables show clear advantage over simple vertical stacks for SFCL applications in terms of AC loss.

We present the results of finite-element modeling of a YBCO thin-film superconductor deposited on a nonmagnetic substrate and on a weakly ferromagnetic substrate. The model was implemented using commercial software to calculate the ac loss in the presence of a sinusoidal external magnetic field applied perpendicular to the surface of the superconducting tape. A strategy is demonstrated to overcome the difficulties in the finite-element method due to the high aspect ratio of the YBCO film by using the computed values of ac loss for thicker samples to extrapolate the results to the actual physical thickness of the superconductor ( ~ 1 mum). The effect of the width of the weekly ferromagnetic substrate upon the ac loss of the superconductor film has also been studied.

Roebel cable has been successfully manufactured using second generation (2G) YBCO wire. Roebel shaped strands of width 2 mm and a transposition length of 90 mm were cut from 40 mm wide tape supplied by American Superconductor Corporation. The tape has a weakly ferromagnetic Ni-5 at%W alloy substrate. Eleven insulated strands were assembled to form the cable. This paper reports results for the magnetic AC loss as a function of field amplitude for an insulated 2G Roebel strand and insulated multi-strand Roebel cable in perpendicular and parallel field orientation. In parallel field, the loss at the highest field amplitude measured is reduced by a factor of approximately 30 below the losses in the perpendicular field. In perpendicular field, the losses at the low field amplitude of the Roebel cable sample is influenced by the magnetic substrate. At high field amplitudes ( >0.1 T), the cable and total strand loss converge as expected for an insulated cable, where coupling currents are absent.

This chapter discusses high-temperature superconducting (HTS) transformers with an emphasis on the technical and cost barriers they must surmount to supplant conventional transformers in the power grid. Requirements for conventional transformers with respect to short circuit current limitation, overload capacity, electrical insulation, voltage regulation, efficiency, and mitigation of fire hazards are reviewed since these constrain the operation of HTS transformers. Salient features of HTS conductors and cryogenic operations and the status and achievements of HTS transformer development worldwide are reviewed. AC loss in HTS transformer windings is considered, especially the use of multiple conductors to attain high currents using transposition at the coil level or as continuously transposed cable. Cryostat and cryocooler technology and progress on achieving transformer performance goals, especially relating to fault current performance and conductor loss, are reviewed. The economic viability of HTS transformers is reviewed from the viewpoint of total cost of ownership.

We report the development of a laboratory electromagnet utilizing high temperature superconducting (HTS) coils which is designed to generate fields in excess of 3 Tesla, but with power consumption and size significantly below that possible with copper-wound coils for an equivalent useful field volume. The components are complete and the system is currently being assembled. The electromagnet is based around a compact iron yoke with four HTS double pancake coils, employing a total of 1.6 km of first generation HTS (Bi-2223/Ag composite) wire, to generate the field across an air-gap of 50 mm. The electromagnetic design was tailored to take into account the anisotropic in-field performance of the HTS conductor, requiring a minimization of the radial field component at the coils while retaining sufficiently open access for installation of the components of a vibrating sample magnetometer with which this magnet will be used. The HTS coils are conduction cooled with a single-stage cryocooler that delivers approximately 27 W of cooling power at the design operating temperature of 35 K; HTS current leads are employed to reduce the thermal load at the coils. In the present configuration the coils are housed in a single cryostat, but the design is easily adaptable to provide for a number of specific application requirements. Contrary to the generally held view that HTS magnets are still too expensive, this project demonstrates that utilization of HTS conductor is now an attractive option in a number of application areas.

Accurate measurements of the magnetic AC loss in short samples of conductor are essential for the design of superconducting power devices and guide the development of superconducting wire with low AC losses. The authors report quantitative AC loss measurements at power frequencies at 77 K obtained with a Hall sensor magnetometer (HSM) using a calibration method which involves assumptions about the current distribution within the sample. The samples included strips of pure silver and multi-filamentary Bi-2223 tapes. A comparison using AC susceptometry and their Hall sensor magnetometer technique shows good agreement between the two methods for both the HTS and silver tape samples. In the case of the silver tapes the results also agree with the predicted power loss. AC susceptometer measurements were made using a pick-up coil enclosing the entire sample so that the global magnetic moment was measured, including any effects of the sample ends. The HSM results are unaffected by sample end-currents. The comparison confirms the accuracy of the calibrated HSM technique.

A new magnetic grain alignment technique has been applied to produce biaxially aligned YBCO coated tapes. A biaxially aligned dispersion of orthorhombic Y2Ba4Cu7O15 (Y-247) powder was settled on untextured silver substrates. The Y-247 tapes were then melt processed to achieve high critical current YBa2Cu3O7 (Y-123) tapes with CuO as a secondary phase. The biaxial alignment is preserved after the densification process and a clear enhancement of Jc relative to identically prepared untextured or uniaxially textured samples is obtained. Critical current densities of up to 5000 A cm-2 at 77 K in self-field and 1500 A cm-2 in 0.5 T magnetic field at 65 K were obtained in films from 20 to 40 µm thick. Problems were experienced in achieving fully densified thick films while retaining biaxial texture. The initial grain size distribution was found to have a major influence on the final microstructure. Provided significant improvements in Jc can be obtained this method offers an alternative to coated tape processes based on epitaxial growth which has the advantage that it does not require textured substrates. The biaxial alignment technique described here intrinsically acts on the bulk material rather than at surfaces. This offers the possibility of texturing without thickness limitations.

Evidence is presented from the scaling of the Knight shift, entropy and transport properties together with the sharp peaking of condensation energy, critical currents, superfluid density and a variety of other physical properties for the occurrence of a common critical doping point in lightly overdoped high-Tc superconductors (HTS). This critical doping lies at the point where the doping-dependent normal-state pseudogap energy, Eg, falls to zero and bears a strong, though incomplete, resemblance to a quantum critical point (QCP). A QCP scenario could lead directly to an explanation of the non-Fermi liquid behaviour of the normal-state metallic phase and the overall generic behaviour of the HTSC.

We report a new approach to the preparation of biaxially textured YBCO coated tape conductors. Using a dynamic magnetic grain alignment technique, a biaxially aligned dispersion of orthorhombic Y-247 powder is settled on untextured silver substrates. In subsequent heat treatment the Y-247 is transformed to Y-123 and CuO involving a partial melt. Critical current densities at 77 K in self-field of up to 5000 A/cm/sup 2/ have been obtained in films from 10 to 30 /spl mu/m thick. There is a clear enhancement of J/sub c/ relative to identically prepared untextured or uniaxially textured samples. J/sub c/ appears to be limited at present by the difficulty of achieving fully densified films while retaining biaxial texture. Provided significant improvements in J/sub c/ can be obtained this method offers an alternative to coated tape processes based on epitaxial growth which does not demand textured substrates and can give much higher fill factors. The grain alignment technique is quite general for materials with anisotropic magnetic or electric susceptibility and can therefore be utilised in a wide range of applications.

Oxygen pressure-temperature phase diagrams for Ln/sub 2/Ba/sub 4/Cu/sub 7/O/sub 15-/spl delta// (Ln-247) compounds (Ln=Er,Eu,Gd,Y and Yb) in the pressure range 2-32 bars of oxygen and temperature range 900-990/spl deg/C were studied. The required pressures for Ln-247 single phase synthesis are much lower than was previously found. In addition, phase-pure Ln-247 (Ln=Dy,Nd,Pr and Sm) samples were prepared at high oxygen pressure. The temperatures of the superconducting transition (T/sub c/) for fully oxygen loaded Ln-247 samples are in the range 63-94 K, except for Pr-247 which is not superconducting at temperatures above 4.2 K. These transition temperatures are, for the wider range of rare earth, considerably higher than previously reported. Evidence is presented for progressive underdoping of carriers with increasing rare earth ionic radius, suggesting that T/sub c/ has not been maximized.

We report measurements of the anisotropic thermopower in the normal state of YBa 2 Cu 3 O 7 − δ thin films with three different orientations: a -axis perpendicular to the plane of the substrate; c -axis perpendicular; and coherently tilted on a (305) SrTiO 3 substrate respectively. We have also measured the thermopower of magnetically aligned YBa 2 Cu 3 O 7 − δ ceramic samples. The thermopower in the direction of the c -axis is found to be positive and increases as the temperature increases. The thermopower along the b -axis direction has only a small temperature dependence, while the averaged thermopower in the ab -plane in c -axis films decreases as temperature increases. These results are consistent with those made on single crystals.

Y 2 Ba 4 Cu 7 O 15−δ superconductors with similar oxygen contents but different T c values have been produced by varying the reaction conditions. Resistivity, 89 Y NMR and thermopower measurements were used to ascertain the origin of the different T c 's. We find that the sample with the lowest T c has a lower hole concentration on the CuO 2 planes and the normal-state energy gaps are similar to those found in oxygen-depleted YBa 2 Cu 3 O 7−δ superconductors. Thus the electronic behaviour of Y 2 Ba 4 Cu 7 O 15−δ is similar to that of YBa 2 Cu 3 O 7−δ and different T c values can be attributed to different hole concentrations on the CuO 2 planes and not to different pairing potentials or pair-breaking. The difference in the temperature dependence of the thermopower between these samples can be understood in terms of the different relative CuO chain and CuO 2 plane contributions.

Both AC loss and wire cost in coil windings are critical factors for high temperature superconductor (HTS) AC machinery applications. We present AC loss measurement results in three HTS coil assemblies at 77 K and 65 K which have a hybrid coil structure comprising one central winding (CW) and two end windings (EWs) wound with ReBCO and BSCCO wires with different self-field I c values at 77 K. All AC loss results in the coil assemblies are hysteretic and the normalized AC losses in the coil assemblies at different temperatures can be scaled with the I c value of the coil assemblies. The normalised results show that AC loss in a coil assembly with BSCCO CW can be reduced by using EWs wound with high I c ReBCO wires, whilst further AC loss reduction can be achieved by replacing the BSCCO CW with ReBCO CW. The results imply that a flexible hybrid coil structure is possible which considers both AC loss and wire cost in coil assemblies.

The irreversibility lines Hirr(T) for the series of superconducting compounds TlmA2Can−1CunO2+m+2n, where A is Ba or Sr, are measured between 0 and 1 T for m = 1 and 2 and n = 1, 2, 3 and 4 and show a marked steepening with decreasing anisotropy, i.e. as m is lowered from 2 to 1. In addition, for m = 2 material Hirr(T) is observed to steepen with decreasing number of CuO2 layers. The hole dependence of the irreversibility line is also investigated in Tl2Ba2CuO6+δ by changing the oxygen stoichiometry δ, Hirr(T) progressively steepens with increasing hole concentration on the overdoped side of Tc(max). These results are consistent with a model of flux motion arising from a cross-over from 3D vortex lines to 2D pancake vortices which are decoupled in the c-direction, and provide guidance for materials selection and optimum doping. The manufacture of superconducting, silver-clad tapes using Tl0.5Pb0.5Sr2Ca0.8Y0.2Cu2O7 is described. Zero field transport critical currents in excess of 104 A cm−2 are obtained but the material is untextured and weak-linked, as shown by the strong field dependence of Jc.

The room temperature infrared and Raman spectra of ceramic YBa 2 Cu 4 O 8 and La substituted YBa 2 Cu 4 O 8 are presented and discussed. All the predicted B lu (infrared) and A g (Raman) modes are observed and assigned. When the spectra are compared with YBa 2 Cu 3 O 7−δ it is found that modes involving the CuO chain atoms are significantly altered, as many be expected given that the structure of these two compounds differs by the insertion of an extra CuO chain parallel to the b -axis. On the other hand, modes involving largely motion of the oxygens on the CuO 2 planes and of Y are observed at similar frequencies in both compounds. With La 3+ substitution for Ba 2+ , most of the B lu and A g modes harden with contraction of the c -axis. Modes that show softening trends in contrast to the generally observed hardening are the IR and Raman modes that involve largely motion of the CuO 2 plane oxygens, which can be understood in terms of a decrease in the hole density on the planes with La 3+ substitution.

Far-infrared reflectivity measurements on ceramic samples of YBa 2 Cu 4 O 8 , a high- T c superconductor with T c = 80 K, were made over the temperature range of 20–300 K and frequency range of 20–1000 cm −1 . Six phonons were observed at 130 cm −1 , 192 cm −1 , 300 cm −1 (doublet), 500 cm −1 , and 603 cm −1 and their temperature dependences were studied. Through comparisons with 1–2–3 compounds, these phonons were identified and assigned to the respective lattice vibrations of B 1u symmetry (i.e. along the c -axis). The observed temperature dependences of phonon frequencies, lineshapes and reflectivity ratios all point to the existence of two superconducting energy gaps, one in the c direction at about 150 cm −1 and another (somewhat more speculatively) in the ab plane at about 450 cm −1 . While the c -axis gap occurs at a nearly BCS-like value of ∼ 2.7 kT c , the a−b plane gap corresponds to ∼ 8 kT c , in agreement with the work of others on single crystals of YBa 2 Cu 3 O 7 .

We report the preparation and characterization of the homologous series of high temperature superconductors, Y2Ba4Cu6+nO14+n−ξ with n=0 (123), n=1 (247) and n=2 (124) as a function of Ca and La substitution through measurement of Tc, lattice parameters, selected Rietveld refinements and thermogravimetric analysis. Solubility limits of Ca and La are investigated and found to be widely different for each member. Data is presented showing that Ca substitutes on both the Y- and Ba- sites and as the Ca concentration increases the relative fraction on the Y site decreases. Systematic trends indicate that for n=0 123 is slightly overdoped, 247 is slightly underdoped and 124 is well underdoped relative to Tc(max) such that 0.06 La substitution, 0.04 Ca substitution and 0.15 Ca substitution is required to maximise Tc in 123, 247 and 124, respectively. Differences in the orthorhombicity between the members of the series as a function of substitution are interpreted in terms of differences in hole doping and in oxygen stoichoimetry for each member. For instance, the observed rapid decrease in the orthorhombicity in 124 at a high La content is interpreted as evidence that oxygen loads into the interchain site. This destabilises the ribbons leading to the formation of tetragonal 123.

The internal friction of YBa2Cu3O7−δ is measured at 7.8, 40, and 144 kHz as a function of temperature (from 300 to 1100 K) and oxygen partial pressure. At 40 kHz a marked damping peak occurs at ∼800 K which is attributable to oxygen hopping in the basal plane. The oxygen diffusion coefficient calculated from the hopping relaxation time is in good agreement with other estimates yielding an activation enthalpy, hm=1.07±0.05 eV and D0=1.8×10−4 cm2/s. At a critical value of δ∼0.25 the diffusion coefficient begins to rise rapidly similar to the occurrence of a fast‐ion transition.