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A 6.5 MVA/25 kV high temperature superconducting (HTS) transformer for the Chinese Fuxing high-speed train has been proposed to replace the oil-based transformers while achieving higher efficiency, lighter weight, and minimized volume. The high targeted efficiency (> 99%) makes AC loss reduction a vital issue. HTS coated conductors generally exhibit asymmetric critical current characteristics as a function of magnetic field angle Ic(B, θ), leading to a non-trivial influence on the AC loss of coil windings. The fast-computing T-A homogenization method is proposed to calculate the AC loss of the 6.5 MVA/25 kV traction transformer with large turn numbers. The variables, T and A, are the current and magnetic vector potentials, respectively. The AC loss of the transformer windings is analyzed for various coil configurations with and without flux diverters considering Ic(B, θ). At the rated current and 65 K, employing the flux diverters with a square-shape cross-section, the total AC loss is decreased by 73.7% and an extra 150 W loss reduction was also obtained. Moreover, an additional reduction of 37 W is realized upon utilizing the asymmetric Ic(B, θ) characteristic. The reduced 187 W in AC loss at 65 K corresponds to a reduction in ambient power requirement of over 5.6 kW. Therefore, considering asymmetric Ic(B, θ), can lead to a non-trivial reduction in AC loss, even incorporating flux diverters.

AC homopolar motors, with high-temperature superconductors (HTS) are considered to be a promising technology for high-power applications. With superconducting components being placed exclusively in non-rotating parts, a motor can reach high rotational speeds. However, the dynamic behaviour of the system creates a non-negligible magnetic ripple field which generates heat and makes thermal management of the cryogenic system more challenging. In this paper we present a case study for the thermal behaviour of a conduction-cooled, high-temperature superconducting field coil assembly in a 500 kW asynchronous homopolar motor. The analysis focusses on the investigation of the magnetic ripple field and its impact on heat generation due to AC loss in the HTS field coil and eddy current loss in copper parts present in the assembly. Using electromagnetic and thermal finite-element models, we construct a detailed description of the thermal bus design which considers different cooling scenarios and sizing of the conductive elements. Depending on the geometry of the thermal bus, we observe heat loads from AC losses of up to 0.4 W, and eddy current losses in the copper elements of up to 1 W. These heat loads translate to temperature increases as high as 10 K. Techniques to reduce the temperature rise based on thermal bus redesign are presented, along with recommendations for the most promising cooling scenarios and optimal thermal bus geometry.

AC loss generated in Superferric magnets composed of high temperature superconducting (HTS) coil windings and iron cores is one of critical issues for rapid-cycling synchrotrons (RCSs) application. In this work, we carried out 3D FEM AC loss simulations in HTS coil assemblies coupled with an iron core using the T-A homogenization method. The HTS coil assemblies consist of a stack of double pancake coils (DPCs) wound with 4 mm - wide Shanghai Superconductor wires with an average self-field critical current of 192.6 A. The numerical and experimental AC loss results of a DPC (1 DPC) without the iron core were compared to validate the 3D T-A homogenization method. Then, 3D AC loss simulations were carried out on the DPC assemblies with different number of DPCs with/without the iron core to investigate the influence of the iron core and the coil number on AC loss. The iron core causes increase in AC loss values of the DPC assemblies. AC loss increases by a factor of five in the 4 DPC assembly with the iron core compared with that without the iron core. The simulation result in the 4 DPC assembly with the iron core reveals that strong perpendicular magnetic field penetration occurs even in the center DPCs and this explains the substantial AC loss increase in the 4 DPC assembly.

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Funding Information: Financial support was provided by the Swiss National Science Foundation (Grant No. 200021_184940). Funding Information: Networking support provided by the European Cooperation in Science and Technology, COST Action CA19108 (Hi-SCALE) is acknowledged. Funding Information: Supported by Fundação para a Ciência e a Tecnologia, Portugal, with reference UIDB/00066/2020. Funding Information: A part of this work was supported by the Russian National Technology Initiative Foundation (Grant ID 0000000007418QR20002). Funding Information: The research was also supported by the European Synchrotron Radiation Facility (Grant No. MA-2767). Funding Information: This work was supported in part by the New Zealand Ministry of Business, Innovation and Employment (MBIE) by the Strategic Science Investment Fund ‘‘Advanced Energy Technology Platforms’’ under Contract RTVU2004. Funding Information: Y Wang acknowledges support from the National Science Foundation (NSF) Award DMR-2132338. R P Camata and C-C Chen are supported by the FTPP Program funded by NSF EPSCoR RII Track-1 Cooperative Agreement OIA-2148653. C-C Chen also acknowledges support from the NSF Award DMR-2142801. Publisher Copyright: © 2023 The Author(s). Published by IOP Publishing Ltd. This paper presents a roadmap to the application of AI techniques and big data (BD) for different modelling, design, monitoring, manufacturing and operation purposes of different superconducting applications. To help superconductivity researchers, engineers, and manufacturers understand the viability of using AI and BD techniques as future solutions for challenges in superconductivity, a series of short articles are presented to outline some of the potential applications and solutions. These potential futuristic routes and their materials/technologies are considered for a 10-20 yr time-frame. publishersversion published

Iron-core electromagnets are widely used as applied-field switches in high-temperature superconducting (HTS) flux pumps. It was recently reported that the critical current ( I c ) of the HTS wire is suppressed by up to 45% in the air gap of the iron core. Here, it is reported that the self-field I c of a single tape (unifilar) is suppressed from 357 A to 195 A while in the air gap of an iron core, agreeing closely with the previous study. This is extended by showing that the I c response to a perpendicular magnetic field is asymmetric, with a peak I c of 249 A found to be at 0.16 T of applied magnetic field. This effect can be eliminated using an anti-parallel current configuration (bifilar), which is shown experimentally to recover close to the no-core self-field I c of 357 A with no observed asymmetry. The results of both types of switch configurations are closely replicated in finite-element modelling using COMSOL Multiphysics®. This is caused by the asymmetric interaction between the self-induced magnetic field of the HTS switch element and the magnetic circuit of the iron-core electromagnet.

Future all-electric aircraft will employ highly efficient compact and lightweight machines for power generation and propulsion applications. Previous studies have indicated that the most attractive options include synchronous machines employing superconducting excitation and stator windings for achieving high power density and efficiency. The dc excitation winding could be constructed using a variety of superconductors, but the choice for ac stator windings is limited. High conductivity aluminum conductor at cryogenic temperature is a possible option but it is expected to be heavier than a superconducting winding and will require more extensive cooling as the losses within the winding pack will be much larger. A conductor-on-round-core (CORC) cable using REBCO tapes looks like an attractive option as it enables compound bending for constructing saddle coils. Here, we investigate using this cable for the stator winding of a 3 MW, 4500 RPM, 4-pole propulsion motor and compare with an identical motor employing an MgB2 stator winding. The motor configuration assumes saddle-shaped coils for the stator winding. Each saddle coil constitutes a double pancake. A round conductor capable of bending in three-dimensional is preferred for such coils, making the round CORC cable ideal for this application. The cable employs striated REBCO with striation width as small as 1/3 mm. The ac field in the stator winding region is about 0.7 T at 150 Hz and the cable is operated at 50% of its critical current for limiting ac losses and for providing a safety margin. Individual tapes in the CORC cable experience field perpendicular to its broad face resulting in high ac losses. Comparing the performance of motors employing CORC and MgB2 cables for the stator, we find that the ac losses in the CORC stator are much larger than the MgB2 stator. High losses in the CORC stator winding make this option less attractive unless coolant power is freely available, such as in LH2-fueled planes. However, this machine with CORC coils is the most compact and lightweight option. Such compact light mass motors could be attractive for LH2 fueled applications, such as interstate trucks, heavy machinery, locomotive hauling goods trains, ships (especially for podded propellers), etc.

Flux diverters (FDs) are used in High Temperature Superconducting (HTS) transformers for AC loss reduction and flux optimization. In this paper, a 2D axial symmetric superconducting winding model is proposed and two designs of flux diverters are applied to the windings. A homogenization approach is used to analyze the windings with large turn numbers. The key parameters including the number, width, height and the spatial positions of the FDs are adjusted for AC loss and magnetic flux analysis. The means of obtaining optimum designs of the FDs is provided and can be used to develop new winding designs with FDs, which contributes to better electromagnetic performance and higher efficiency of HTS transformers.

In many applications of high-temperature superconductivity, such as superconducting magnetic bearings, precise knowledge of the non-linear voltage-current ( E-J ) relation is necessary for making accurate quantitative predictions of performance. Magnetic microscopy allows the spatial mapping of coated conductor superconducting properties. There is no equivalent spatial mapping technique for bulk superconductors. A convenient method of mapping the E-J relation for superconductor bulks to be used in superconducting magnetic bearings is desired. The maglev force in a superconducting bearing decays logarithmically over time due to flux creep. The non-linear E-J behaviour describes this flux-creep behaviour and maximum force. It should, therefore, be possible to map the E-J power law of a bulk superconductor using levitation force decay characteristics. Using the H -formulation finite-element method we have simulated the levitation force decay under zero field cooled conditions and identified a relation between the logarithmic force decay parameters and the n -index of the E-J power law. This potentially offers a non-destructive method of determining the E-J relation in large superconducting bulks.

High-temperature superconducting (HTS) magnets have been widely used in various applications due to their excellent performance. One long-lasting problem, however, is that they have to be powered by electronic power supplies via a pair of thick current leads, which go through room temperature environment into a cryogenic environment. The considerable heat load generated by these resistive current leads at a cryogenic temperature substantially limits the operating current and the energy density of the magnet. In this article, we report a novel mechanism of inductively energizing closed-loop HTS dc magnets. This exploits a newly discovered effect, which appears within a superconducting loop when a global screening current interacts with local screening current. This results in a dc voltage across the superconductor, which enables an alternating superconducting current to be rectified in order to energize a superconducting magnet. Based on this principle, a superconducting transformer–rectifier prototype is realized and demonstrated. Test results show that the prototype can output a dc voltage of up to 25 mV and a maximum direct current over 500 A. It is envisaged that this work will enable future HTS dc magnets to be operated in a closed cryogenic environment, eliminating the need for electronic power supplies and bulky current leads. This would greatly reduce the footprint and power demand of HTS magnet systems and unlock many new opportunities for the applications of this technology.

(RE)Ba2Cu3O7-d (REBCO) coated conductors are becoming a preferred wire choice for many potential high temperature superconducting (HTS) applications, where additional metal layers are usually attached to the REBCO conductors to improve reliability and stability. In such cases, the eddy current loss in the metal layers is affected by the magnetic field shielding by adjacent REBCO coated conductors. Here we present the ac loss measurement and simulation results in three copper-superconductor stacks comprising a 10-mm-wide Fujikura-coated conductor with one, two, and four 0.1-mm-thick, 10-mm-wide copper strips, under various applied magnetic field angles (defined by the angle between the applied magnetic field and the wide face of the superconductor) from 15° to 90° at 77 K. The amplitude of the magnetic field is up to 100 mT. The shielding effect, which reduces the eddy current loss in the metal layers stacked with a superconductor layer, increases with decreasing field angles, due to the change of the effective penetration field at the different field angles. The eddy current loss in the copper strips in the copper-superconductor stacks under the magnetic fields with various orientations is dominated by the perpendicular magnetic field component.

This paper presented the design and performance analysis of a dynamo-type HTS flux pump for a 10 kW superconducting generator. The HTS field coil of a 10 kW superconducting generator was designed and fabricated. The induced DC current, output voltage, and dynamic resistance of the flux pump connected to the HTS field coil were estimated using electric circuit equations. They were tested at various rotational speeds and air gaps, and the test results were compared with the estimated results. The performance was confirmed by applying a flux pump to the 10 kW superconducting generator. As a result, as the rotational speed of the flux pump increased, the induced DC current of the flux pump connected to the HTS field coil increased proportionally. As the air gap of the flux pump increased, the induced DC current decreased. A sufficient current was induced in the field coil to produce a 10 kW output.

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.

An asymmetric dependence of the critical current on the direction of an applied magnetic field in HTS coated conductors has a non-trivial influence on the AC loss of coil windings. We report the modelled influence of real conductor critical current asymmetry on the AC loss characteristics of a 1 MVA HTS transformer design previously demonstrated by the Robinson Research Institute as well as a stand-alone coil having the same geometrical and electrical parameters as the low voltage (high current) winding of the transformer. We compare two commercial HTS conductors with distinctive differences in their critical current asymmetry and show a maximum variation of 15% and 29% in the calculated AC loss of the transformer and the stand-alone coil winding, respectively, when the conductor orientation is varied in the top and bottom halves of the windings. AC loss simulation giving consideration to asymmetric conductor critical current before winding the transformer could lead to substantial AC loss reduction even using the same amount of conductor and the same transformer design.

This paper reports on a method for strain calibration of fiber optic sensors which have been adhesively bonded to a structural member. Generally, the coefficient of strain transfer ( $k_{\mathrm {ST}}$ ) can be used to calibrate the strain sensitivity of the surface bonded FBG. Here we propose an alternative method to determine $k_{\mathrm {ST}}$ which is based solely on observing the temperature-dependent behaviour of the adhesively mounted FBG sensor. In this paper we prove the validity of our method by comparing $k_{\mathrm {ST}}$ determined via a conventional strain (mechanical) calibration and via our alternative temperature (thermal) calibration. A set of 18 surface bonded FBGs with coefficients ranging from 0.5 to 1 are investigated. The average deviation of $k_{\mathrm {ST}}$ determined via a temperature calibration versus a mechanical strain calibration was ~5%, with a maximum deviation of 13% obtained from the set of bonded sensors measured in this work. This led to an average relative accuracy of < 6%, when the strain sensors were calibrated using our temperature-based calibration method.

Optical fiber Bragg gratings (FBGs) are well suited for applications as temperature or/and strain sensors in harsh environments, e.g., detecting thermal hot spots in high-temperature superconductor (HTS) fusion energy magnets at cryogenic temperatures and high radiation environments. To maximize the signal-to-noise ratio (SNR) of the FBGs to a hot spot, we propose to have them mounted in V-shaped grooves of HTS’ copper former. To investigate the differences between different adhesives on transferring strain and heat in this configuration, five arrays of FBGs are mounted in the V-shaped grooves of a copper dog-bone using Scotch-Weld epoxy, Stycast 2850 FT, Apiezon N, and Loctite 5145 silicone. The copper is cycled through tensile forces in a modified universal tensile tester, subjected to a thermal cycle between 293 K and 77 K, and exposed to heat pulse propagations at 293 K and 80 K. The FBGs that are bonded using Stycast show the highest temperature and strain sensitivities at room and cryogenic temperatures. No major differences in the temperature and strain sensitivities have been found between Ormocer and polyimide coated FBGs. Apiezon N is found to transfer strain consistently well below 245 K, which is comparable with other bonding materials in the temperature range between 77 K and 110 K. The FBGs bonded with the four adhesives in the V-groove configurations are shown to have comparable SNRs to a temperature rise of 20 K at 80 K. This paper emphasizes the importance of maximizing the thermal strain transferred from the host material through the bonding agents to achieve high temperature sensitivity of FBGs.

For HTS dc magnet applications, a large proportion of the cooling load on the cryocooler is imposed by the external current leads. Removing the need for these current leads would allow these systems to be built using smaller, cheaper cryocoolers and reduce the ongoing operating costs. This can be achieved by using an HTS superconducting dynamo as a fully superconducting dc current source. Superconducting dynamos remove the need for external current leads by providing a wireless method of injecting dc current into a closed superconducting circuit. Most previous work on HTS dynamos has considered devices employing just one coated conductor stator tape, but dynamos employing multiple parallel stator tapes can achieve substantially higher output currents. In this study, we present an HTS dynamo design in which eight parallel HTS stator tapes are arranged in a cylindrical geometry, excited by an axially concentric rotor carrying multiple equally spaced Nd–Fe–B magnets. Here, we focus on the differing behavior observed between the synchronous case when the number of magnet nm is a factor of the stator number ns = 8 (i.e., nm = 1, 2, 4, 8) and the asynchronous case when it is not (i.e., nm = 3, 5). Our results indicate that both synchronous and asynchronous configurations can exhibit output short-circuit currents of up to ∼1.3 kA. In both cases, a relative drop in performance is observed for higher values of nm . We find that the asynchronous magnet arrangement does not greatly improve the dynamo performance compared to the synchronous arrangement. At present, the thermal load associated with conduction-cooled current leads has prevented the practical realization of cryocooled HTS magnets in the kilo-amp (kA) range. However, the dynamo device reported here now offers a practically feasible route to novel kA-class portable cryocooled HTS magnet systems.

The energy stored in a 2 MW motor coil, or a generator field winding, may be as high as 25 kJ (for a 5 H field windings with 100 A current) and this energy release would be catastrophic if rapidly released. Therefore quench detection and protection in large machine coils is a significant concern. In this work, the feasibility of uncoated Fiber Bragg Grating (FBG) sensors for high temperature superconductor (HTS) magnet quench detection has been studied by studying the thermal distribution of a vertical HTS tape stack without applying current. The ceramic heater and PT100 platinum resistance sensors were employed to trigger the hotspot and provide a temperature reference. A proposed thermal-mechanical model was used to predict the response of the attached FBG sensors. The measured results agreed well with the prediction results, and the temperature sensitivity of uncoated FBG can be improved significantly by the thermal expansion of HTS tape and epoxy at the operating temperature of HTS. The thermal distribution results were studied by the attached FBG sensors. Finally, a mechanical stress test was carried out to simulate the effect of magnetic force on the FBG temperature sensor. The result shows that the slow change of stress can be ignored in comparison with the rapid temperature change in the ramping process, the FBG sensor can work properly for quench detection on HTS coil operation state. This paper provides the feasibility of the application of bare FBG sensors for the quench detection of HTS magnets. Our purpose is to monitor the temperature in real time, and give a quick feedback for any possible heat disturbance.

High-temperature superconductor (HTS) dynamos provide a means of wirelessly exciting dc currents in HTS coils. By eliminating the need to connect a coil to a room-temperature power supply, an HTS dynamo of comparable performance would present significant improvements to the cost, complexity, and efficiency of HTS-based electromagnetic machines. But due to the lack of a predictive model, development of HTS dynamos must be carried out through empirical investigation. In this paper, we examine the effects of varying either the rotor speed or the number of magnets arranged symmetrically upon the rotor. Both approaches affect the frequency at which magnets cross the HTS stator wire. Here we present experimental results from an axial-flux HTS dynamo, employing an externally mounted rotor carrying m permanent magnets, where 2 ≤ m ≤ 9. We find that the open-circuit dc voltage, short-circuit dc current, and internal dc resistance are all determined solely by the magnet-crossing frequency, regardless of the number of rotor magnets and speed employed to achieve this frequency. For a fixed speed, all three parameters are found to be proportional to m across the full range of conditions studied in this work. However, for a fixed number of rotor magnets, we observe that the open-circuit voltage and short-circuit current both exhibit a similar sublinear dependence on rotor speed. Examination of the transient voltage waveform reveals that this nonlinearity is likely due to a periodic transient heat pulse absorbed by the stator wire each time the magnet traverses it at high speed.

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.

This paper reports on the characterization of a palladium (Pd)-based fiber-optic hydrogen (H2) sensor for health monitoring of distribution and power transformers in the electrical grid. The sensor consists of a Pd foil, which expands due to H2 absorption, and a fiber Bragg grating that measures this expansion. Fifteen sensors were manufactured and characterized in gas and oil environments at various H2 concentrations and temperatures. In gas, the sensors were evaluated at 60 °C, 75 °C, 90 °C, 105 °C, and 120 °C and H2 concentrations from 0.01% to 5%. In oil, the same sensors were evaluated at 90 °C and dissolved H2 concentrations from 5 to 2700 ppm. Furthermore, the influence of carbon monoxide (CO), which is often present in transformers and can impact H2 response of the sensor, was investigated. At 90 °C in gas, the response to 0.01% and 5% H2 was on average 28 and 719 pm, respectively. At 90 °C in oil, the response to 5 and 2700 ppm dissolved H2 was on average 11 and 763 pm, respectively. The average, relative accuracy is better than 20% over the whole measurement range in gas and for measurements above 100 ppm in oil. These specifications make the sensor a promising candidate for online monitoring of H2 in transformers.

The recent adoption of high-T c superconductor (HTS) wires for ultra-high field magnet windings provide great promise for future applications, such as high-power generators and Tokamak fusion reactors. However, an open issue with the use of HTS is the challenge of rapidly detecting a hot spot which could lead to a quench. Optical fiber sensors have been shown to be promising alternatives to the voltage-based quench detection method. In this paper, we report on ultra-long fiber Bragg gratings (ULFBG) for hotspot detection at cryogenic temperatures using a new detection algorithm. This novel sensing system is suitable for applications in which solely the occurrence of a hotspot but not its precise location is of importance. This is the case e.g. for quench detection in HTS. The developed system provides the advantages of cost reduction and faster response time compared to conventional FBGs with wavelength-division multiplexing and continuous FBGs with time-division multiplexing, respectively. We demonstrate the functionality of the system for the ULFBG with a grating length of 100 mm and 500 mm at 77 K and 90 K respectively. The ULFBG is shown to respond as fast as conventional FBG, to a hot spot as small as 1 K temperature rise. Furthermore, using the proposed signal processing algorithm, ULFBG exhibits much higher signal to noise ratio than that from the conventional FBG. It is believed that artificial intelligence based technique can assist the signal processing algorithm in detecting a small hot spot more rapidly from the big spectral data in real-time.

Electric motors based on conventional conductive materials such as copper and aluminium are limited in efficiency and power density by the resistive losses of these materials. Lighter and more efficient motors are possible when High Temperature Superconductor (HTS) materials are used as there are no resistive losses. Additionally, the high current density that may be supported by these materials enables alternative motor designs which leverage high magnetic fields to remove the requirement for iron components and reduce the overall motor volume. Despite HTS materials exhibiting superconductivity at much higher temperatures than other superconductive materials, cryogenic cooling to below 90 K is still required. Suitable cryocoolers typically have efficiencies below 20%. This unfavorably alters both the specific power and efficiency of HTS based motors when considering the motor as including all the associated hardware required for operation. Previously it has been shown that energizing the superconducting components of a motor wirelessly using a HTS dynamo can reduce the required mass of both the cryogenic refrigeration equipment, and the motor drive electronics. The work presented here investigates the reduction in total fuel burn of a 737 sized turboelectric aircraft that may be achieved when replacing a conventional DC power supply for the HTS motor field coils with a HTS Dynamo DC power supply.