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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.

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.

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.

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.

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 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.

We present results from testing to verify the performance of major subsystem components for a 1-MVA three-phase transformer demonstration project. The transformer utilizes a 15-strand Roebel cable for the low-voltage windings and a 4-mm-wide superconductor for the high-voltage windings. Both windings use a YBCO conductor. The winding assemblies will be housed in three individual vacuum insulated glass-fiber composite cryostats and will be cooled by circulated liquid nitrogen and subcooled to a target operating temperature of 70 K. Results from thermal insulation performance tests on a sample composite cryostat are presented. AC loss test results of a low-voltage winding assembly are also presented. The Roebel cable is proven to exhibit low AC loss operation at high current. We conclude that AC loss is not a fundamental obstacle to HTS transformer commercialization.

We measure transport ac losses in an 8/2 (eight 2-mm-wide strands) YBCO Roebel cable at 77 and 65 K without dc parallel magnetic field, and at 77 K under dc parallel magnetic fields of 0-1.5 T. The transport ac loss in the Roebel cable scales with temperature and dc parallel magnetic field as the square of the cable's critical current.

We measure transport ac losses in planar one-layer four-turn parallel coated conductor arrays (1 × 4) and in two-layer (2L) four-turn parallel coated conductor arrays (2 × 4) with a frequency up to 1 kHz. The horizontal separation between the conductors, i.e., gh, and the vertical separation between the neighboring superconducting layers, i.e., gv, were varied to investigate the transport ac loss dependence on gh and gv. In 2L arrays, the tapes in the top layer sit either aligned with the tapes in the bottom layer or aligned with the gaps between the tapes in the bottom layer. We show that the losses differently scale in arrays with low turn numbers to the scaling expected with an infinite array of tapes.