Superconducting Field Winding Research Articles

AC loss analysis on an HTS field winding of the generator with T-A formulation

In this study, we proposed a coupling model utilizing finite element software to estimate AC loss on the high-temperature superconducting (HTS) field winding of a 1 MW generator. The model comprises three distinct physical modules: rotating machinery and magnetic (RMM), electrical circuit (CE) and partial differential equation (PDE). These modules collaborate to resolve the generator's magnetic field distribution, rated output parameters and degree of current magnetization on the HTS field winding, employing Maxwell equations and T-A formulation. Given the placement of the HTS field winding on the rotor core, accurately calculating its loss in an electromagnetic field that varies both temporally and spatially presents a considerable challenge. To address this issue, we adjust the coordinate system within the PDE module to vary with the material rather than space. Our analysis reveals that the instantaneous AC loss of an HTS field winding comprises two primary components: the attenuation envelope and the oscillatory variation, resulting from the charging process and external harmonic magnetic fields. Additionally, we comprehensively discuss and analyze the influence of current excitation speed on instantaneous AC loss. Accurately estimating AC loss in HTS field windings is a critical and meaningful aspect of preliminary machine design. This study provides an essential reference for addressing this issue.

Airplane motors employing superconducting DC field windings and conventional conductor AC windings

Many organizations are developing compact lightweight highly efficient rotating machines for airplane applications. These machines include permanent magnets for excitation and an iron-core with and without superconducting windings. Air-core (no magnetic iron) machines have the potential to be the most lightweight and efficient. Such machines can use superconductors for both DC excitation field coils and AC armature coils, which need conductors under development, like MgB2 and Bi2212. If liquid-hydrogen (LH2) is available on a plane and can be used as a coolant, it becomes feasible to develop machines with AC armature coils made from conventional conductors like copper, aluminium, and high-conductivity aluminium.This paper describes conceptual designs for a 3 MW, 4,500 RPM motor employing REBCO CORC conductor for the DC field coils and conventional conductor Litz cable for the AC armature coils cooled by LH2 available on the plane. Both rotor and stator coils are contained in separate cryostats. The DC excitation coils on the rotor are operated at 40 K to work successfully with a brushless flux pump exciter. Likewise, stator AC coils are cooled with available LH2 to take advantage of the lower resistivity of conventional conductors at cryogenic temperatures. Motor size, mass and losses are compared for stator windings employing copper, aluminium, and high-conductivity aluminium (Hyper-Al). Compared with copper and aluminium machines, the machine employing Hyper-Al has smaller size, mass and total losses.

Design and Test of Quench Protection for Superconducting Generator Field Coils

The quench protection for an LTS superconducting field winding within a 15+ MW direct-drive superconducting wind generator is presented. The quench protection circuit comprises multiple superconducting field coils, one quench protection switch (QPS), a set of trigger heaters thermally attached to the QPS, and a set of quench protection heaters in thermal contact with each field coil. Trigger heaters are electrically connected across each field coil. All quench protection heaters are electrically connected across the QPS. The quench of any field coil will power at least one trigger heater and heat the QPS to quench. Consequently, the QPS voltage will power all the quench protection heaters and trigger all field coils to quench nearly simultaneously. This QPS-based quench protection circuit has been successfully validated by experiments. Alternatively, one of the field coils, designated as trigger-coil, can also function as the QPS. Simulation results of the two quench circuits are compared.

Thermal Characteristic Analysis of Fully Superconducting Motors Employing Dilute Gas Rotor Cooling Structure

We have proposed a rotor cooling structure using dilute gas for a fully superconducting motor for aircraft application. Numerical analysis was performed to investigate the feasibility of the proposed cooling method and structure, which can make a fully superconducting motor lighter and more compact than a partially superconducting motor. The helium gas pressure in the gap, the rotor rotational speed, and the rotor loss are key parameters of the cooling, and thermal analysis was performed to clarify the dependence of the field winding temperature on these parameters. In the analysis, the motor model has the maximum output of 5.5 MW and the maximum speed of 5000 rpm. The superconducting armature windings are cooled down to 20 K by conduction cooling with liquid hydrogen, and the superconducting field winding is cooled down to about 50 K indirectly using helium gas in the gap. Numerical analysis results indicate that the helium gas pressure should be increased when the rotor loss is larger, but if the pressure is increased too much, the temperature of the field winding becomes higher due to wind loss. When the rotor loss is several hundred watts, the helium gas pressure should be in the range of 104 to 105 Pa.

AC Loss Analysis and Modular Cryostat Design of a 10-MW High-Temperature Superconducting Double Stator Flux Modulation Machine

Superconducting (SC) machines are one of the best candidates for large-scale direct-drive wind power generation because of its high torque density. In a traditional SC synchronous machine, either armature windings or field windings need to rotate, which makes the structure of the machine complicated. To solve this problem, a high temperature superconducting (HTS) double stator flux modulation (DSFM) machine was proposed. In this machine, the armature windings and SC field windings are placed in outer stator and inner stator, respectively, and the machine works through flux modulation effect by reluctance rotor. Due to modulation effect, there is a high content of harmonic in this machine. Thus, it is important to analyze harmonics influence on SC coils, especially the SC coils ac loss. In this article, the losses of a 10MW HTS-DSFM machine are calculated and analyzed. It turns out that the ac losses of SC coils in this machine is higher compared with the traditional SC synchronous machine. From analytical calculation, the ac magnetic field perpendicular to the tape surface have strong impacts on the SC coils. In order to reduce the ac losses, some suppression methods are adopted on machine structures and SC thermostat. Then, cryostats are designed and the cooling budgets are evaluated. At last, the comprehensive performances of the machine are calculated and its economy in the long term is verified.

Characteristics of the Superconducting Field Winding of an HTS Wind Turbine Generator During a Short Circuit Fault

High temperature superconducting generators (HTSGs) have great potential of high power density for future large direct-drive wind turbines. As synchronous machines, HTSGs have rapid rise of field currents during a sudden short circuit fault which cause AC losses in the HTS tapes. This paper applies <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H-A</i> formulation of Maxwell equations for finite element simulation of electromagnetic behavior of a 10 MW, 9.6 rpm HTSGs. The model is integrated to a short circuit model to calculate the characteristics of the HTS field winding, such as field currents and AC losses during a three-phase no-load short circuit fault at the armature winding terminal. Two HTSG designs are compared. The <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$n$</tex-math></inline-formula> value characterizing the sharpness of the superconducting-resistive state transition of the HTS tape and the number of armature winding segments are analyzed as key variables. The result indicates high AC losses are produced but more armature winding segments can effectively reduce the AC loss and the field current. A lower <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$n$</tex-math></inline-formula> value can also lower the AC loss level but only for the non-magnetic rotor design. The field current is not affected by the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$n$</tex-math></inline-formula> value for both HTSG designs.

High Temperature Superconducting Flux Pumps for Contactless Energization

The development of superconducting technology has seen continuously increasing interest, especially in the area of clean power systems and electrification of transport with low CO2 emission. Electric machines, as the major producer and consumer of the global electrical energy, have played a critical role in achieving zero carbon emission. The superior current carrying capacity of superconductors with zero DC loss opens the way to the next-generation electric machines characterized by much higher efficiency and power density compared to conventional machines. The persistent current mode is the optimal working condition for a superconducting magnet, and thus the energization of superconducting field windings has become a crucial challenge to be tackled, to which high temperature superconducting (HTS) flux pumps have been proposed as a promising solution. An HTS flux pump enables current injection into a closed superconducting coil wirelessly and provides continuous compensation to offset current decay, avoiding excessive cryogenic losses and sophisticated power electronics facilities. Despite many publications regarding the design and analyses of various types of HTS flux pumps, the practical application of HTS flux pumps in a high-performance superconducting machine has been rarely reported. Therefore, it is of significance to specify the main challenges for building and implementing a reliable HTS flux pump. In addition, the physical mechanisms of distinct HTS flux pumps have caused some confusion, which should be clarified. Above all, a systematic review of the recent development and progress of HTS flux pumps remains lacking. Given the above-mentioned issues, this paper summarized the most up-to-date advances of this emerging technology, clarified the working mechanisms and commonly adopted modeling approaches, presented objective analyses of the applicability of various HTS flux pumps, specified the primary challenges for implementing HTS flux pumps, and proposed useful suggestions to improve this wireless excitation technology. The overall aim of this work is to bring a deep insight into the understanding of HTS flux pumps and provide comprehensive guidance for their future research and applications.

Electromagnetic characteristic analysis of a REBCO magnet with a current bypass/distribution winding technique under an asynchronous rotating magnetic field

Generally, high-temperature superconducting rotating machines (HTSRMs) are considered synchronous machines. If the output of the HTSRM fluctuates based on frequent changes in the electrical or mechanical loads, there is the concern that an asynchronous rotating magnetic field (RMF) is applied from the stationary copper armature winding to the high-temperature superconducting (HTS) field winding in the rotary. This may act as a magnetic disturbance to the HTS field magnet, resulting in permanent damage. To enhance the reliability of HTS magnets in wind power and electric propulsion applications, winding methods with current bypass/distribution characteristics, such as no-insulation (NI) and metal-insulation (MI), have attracted scholarly attention because of their high thermal and electrical stabilities, resulting in their self-stabilizing and protective performances. To verify the feasibility of the NI and MI winding techniques for wind power generators, the basic characteristics under a time-varying magnetic field must be studied, contrary to HTS magnet applications under a time-static magnetic field. Therefore, the electromagnetic characteristics of rare-earth barium copper oxide (REBCO) magnets applied with NI and MI winding technologies were compared and analyzed in this study, considering the magnetically transient situations in which an asynchronous RMF is applied to REBCO magnets. In addition, we developed a characteristic evaluation device similar to a synchronous rotating machine to generate the unsynchronized RMF. Moreover, various basic tests were performed to target the small racetrack-type field windings. The critical current, n-value, terminal voltage, and center magnetic field are investigated under various values of the frequency and current amplitude of the three-phase armature winding, and their behaviors are discussed in detail based on the characteristic resistances of the two test magnets.

Study on Electrical Design of Large-Capacity Fully Superconducting Offshore Wind Turbine Generators

A topological structure of 10 MW fully superconducting offshore wind turbine synchronous generator (FSWTG) is proposed, which stator and rotor windings are both composed with high temperature superconductors (HTS) in this paper. And an electrical design method is presented, which integrates the generator structure design, calculation analysis, and parameter optimization. It is all completed based on the electrical design method that the basic parameter design and the optimization of the number of poles, the pole pitch and the electrical load of the 10 MW FSWTG. Finally, the different design parameters are compared and analyzed for FSWTG, partial superconducting wind turbine synchronous generators (PSWTG: traditional armature winding + high temperature superconducting field winding) and the traditional wind turbine synchronous generators (WTG). The results show that the FSWTG has higher efficiency, lower weight and smaller size than those.

Analysis of Superconducting Synchronous Motors With Halbach Array Field Excitation

This article evaluates the novel design approach of a 3 MW partially superconducting synchronous machine for electric aircraft propulsion. Its special feature is the application of an electromagnetic Halbach array for the field excitation to reduce the back yoke weight and thereby increase the overall output power density. The electromagnetic Halbach array is obtained by reproducing a permanent magnet Halbach array with high-temperature superconducting coils. The arrangement is further optimized, and its field characteristics are compared to a conventional superconducting field winding design. Even though the Halbach array has a higher space and superconductor consumption, it shows better characteristics concerning the intensity of inside and outside magnetic field. Applied in a superconducting motor, the Halbach array field winding achieves a back yoke thickness reduction of up to 85% and a back yoke weight reduction of up to 78% compared to reference models. The gravimetric power density is increased by up to 30% at the expense of volumetric power density. In exchange for a back yoke weight reduction, other components, such as field winding and cryostat, become heavier.

Stator Optimization of Wind Power Generators With High-Temperature Superconducting Armature Windings and Permanent Magnet Rotor

Electrical machines with high-temperature superconducting (HTS) armature windings can benefit from high power density and efficiency. Furthermore, by using permanent magnets (PM) on the rotor, the cryogenic system can be significantly simplified compared with the fully superconducting (SC) machines or the machines with SC field windings. Compared with the conventional electrical machines, the performance of machines with HTS armature windings can be more sensitive to the design parameters due to the interaction between the operating current and the flux density. In this article, the influence of stator geometry on the machine with HTS armature windings and PM rotor is investigated. By considering the interaction between the operating current and the flux density, an iteration procedure for calculating the operating current of the HTS armature windings is proposed. A 10-MW surface-mounted PM wind power generator with HTS armature windings is studied by the finite element analysis. It is found that machines with different coil angle have different behaviors when changing the tooth width and the dimensions of tooth tips, which is different from conventional electrical machines. A new design and optimization method is then developed. Finally, the optimal design is identified with global optimization.

T - formulation based numerical modelling of dynamic loss with a DC background field

High-temperature superconducting (HTS) generators offer the advantages of high efficiencies and increased power densities. Most superconducting generator designs feature DC field windings to provide the required magnetomotive force. The superconducting field windings in HTS machines are subject to complex magnetic fields, which lead to dynamic losses occurring in the winding. This magnetic field environment has a large DC background component due to the self-field of the superconducting field windings. This paper investigates the dynamic loss in combination with a DC background field using a T – formulation based numerical model, where the dynamic region is used to identify the dynamic loss. Our double claw pole generator design, which offers a high power density at low superconducting tape requirements, is used as a case study for dynamic loss analysis with a DC background field. Results show that DC background field has a strong effect on the dynamic loss due to the reduced critical current. In addition it was shown that the T – formulation based numerical model in conjunction with the dynamic region requires further research to accurately predict the dynamic loss due to the changing DC current transport region.

Comparison of Electromagnetic Performance of 10-MW HTS Double-Stator Flux Modulation Wind Generators With Different Topologies

For conventional HTS synchronous generator, the superconducting (SC) field windings or armature windings are rotating, which could complicate their industrial feasibility for the offshore wind generation. In order to overcome this drawback, a kind of HTS double-stator flux modulation generator (HTS-DSFMG) without rotating windings is proposed. This paper compares the electromagnetic performance of 10-MW HTS double-stator flux modulation generators with two different topologies, i.e., iron-cored stators with both field and armature coils, air-cored stator with field windings and iron-cored stator with armature windings. In addition, the system comparison of HTS-DSFMG with conventional HTS synchronous machine is conducted under the no-load, rated load and short-circuit fault conditions. It is found that the proposed machine can dramatically reduce the peak current and peak torque during a symmetrical three-phase short-circuit fault, and its active material mass and cost are comparable with the synchronous one under rated load condition. Finally, the ac losses analysis of the SC coils for HTS-DSFMGs is conducted and three effective methods are proposed to reduce the ac losses.

Overhang Effect Analysis of a Homopolar HTS Synchronous Generator Using 3D Finite Element Method

In this paper, the overhang effect of a homopolar synchronous generator with high-temperature superconducting (HTS) field winding is analyzed. Homopolar topology can be used to implement stationary field winding, which is advantageous for superconducting electrical rotating machine applications that rotate at a relatively high speed. The homopolar machine has an axial-winding radial-flux structure, unlike general heteropolar machines that have a radial-winding radial-flux structure. Therefore, in order to analyze the characteristics of the homopolar machine more accurately, use of 3D finite element method (FEM) should be considered. The overhang of rotor poles in electrical rotating machines can be used to enhance the flux linkage in armature windings and to improve the output power. Therefore, applying the overhang structure in the development of an electrical rotating machine is a method that can further improve the performance of the machines. In this paper, we use 3D FEM to analyze the overhang effect of a homopolar HTS synchronous generator and present the overhang length required to improve the performance of the machine.

Electromagnetic Performance of 10-MW HTS Double-Stator Flux Modulation Generators Considering Distributed/Concentrated Armature Winding

For HTS conventional synchronous generator, the superconducting (SC) field windings or armature windings are rotating, which could complicate their industrial feasibility for the off-shore wind generation. In order to overcome this drawback, a kind of HTS double-stator flux modulation generator (HTS-DSFMG) with stationary seal is proposed. Meanwhile, the electromagnetic performances of 10-MW HTS double-stator flux modulation generators with two different armature winding structure, i.e., distributed and concentrated armature windings are compared in this paper. In detail, the magnetomotive force (MMF) harmonic and the saturation effect of each armature winding structure have been studied. The objective is to intensive study the advantages and disadvantages of HTS-DSFMG with different armature windings, and to find the suitable configuration of the armature winding for large scale HTS double-stator flux modulation generators.

Loss characteristics of HTS coated conductors in field windings of electric aircraft propulsion motors

High-temperature superconducting (HTS) coated conductors (CCs) are widely regarded as a promising candidate to enable very high power density motors. These machines operate at high rotational speeds, with some designs going up to 12 000 rpm. HTS CCs are applied to the field windings of these motors to increase the magnetic loading and hence the power density. Even though the superconducting field windings operate with a DC current, due to the magnetic field environment, losses are present. This paper examines the dynamic and total loss characteristics of YBCO-coated conductors in the frequency range relevant to high-speed motors for electric aircraft propulsion. A multi-layer model was created using the H-formulation and the losses for each layer were highlighted. For the first time, it was shown that the DC transport current region in the HTS layer shrinks as the frequency of the applied field increases due to the increased magnetisation current around the edges of the CC, which reduces the dynamic loss per cycle as the frequency increases. To fully understand the loss distribution in the HTS CC, the total loss in the conductor was investigated. For an applied magnetic field of 100 mT and 800 Hz, more than 30% of the total loss occurs in the copper layer due to the decreased penetration depth of the magnetic field and the skin effect. Results show that to accurately model and understand the losses in superconducting field windings, a multi-layer model should be used, since a significant proportion of loss shifts towards the copper stabilizers. Over all, it was shown that both the dynamic loss as well as magnetisation loss play a crucial role in the estimation of the loss in superconducting field windings.

Modular and stackable power generators for efficient renewable power generation

Stackable power generators enable very compact electric machines with high power density. How to identify an effective way of stacking is necessary and requires new knowledge for future guidance. In this study, both possibilities of stacking modules concentrically and axially are explored and compared. An axial flux generator based on the existing double claw pole design is used as a case study. The claw pole generator, which is modular and stackable, significantly benefits on-site installation due to its flexible small modules that can be easily transported. This is particularly important to the installation of offshore wind turbines. Superconducting field windings are applied instead of copper in such power generators, which greatly increases their power density. In order to further increase their power density and reduce mass, the authors improved the modularity of 5 MW generators and stacked them in both ways. Results show that the power density versus mass as well as volume can be improved through stacking the modules both concentrically and axially, while the former approach is much more effective. Stacking two modules of 5 MW concentrically results in a 12.1 MW power generator with a more compact structure and higher power density than the original 10 MW generator.

Design and Analysis of a Novel Dual-Stator Cylindrical Linear Vernier Machine With LTS Field Windings

The cylindrical linear vernier machine (CLVM), in which the magnetic gearing effect is adopted, is a specific kind of the linear vernier machines (LVMs) and is suitable for the direct-drive applications that both low speed and high thrust are demanded at the same time. In order to further improve the thrust density of the LVMs, a novel dual-stator CLVM with low-temperature superconducting field windings (DCLVM-LTS) is proposed and analyzed in this paper. The operation principles of the DCLVM-LTS will be introduced in the paper by the analysis of the air-gap flux density. The superiorities of the proposed DCLVM-LTS are calculated by the finite element method. Compared to a DCLVM with PM arrays, the thrust density of the proposed DCLVM-LTS exhibits a considerable improvement in terms of thrust density, power factor, and economy efficiency.

Mass reduction of superconducting power generators for large wind turbines

High temperature superconductors (HTS) enable very compact electric machines with high power density. Here, an HTS power generator based on authors’ previous double claw pole design is studied and improved. The original design of the double claw pole machine features a stationary field core to provide support and access to the superconducting field winding and its cryostat. It does not serve any other purpose. However, a homopolar field passes through it. The improvement was made to replace the field core by iron-cored copper coils, creating an inner stator within the generator to increase the electric loading. Through this step, the power output of the generator was increased from 10 to 11.6 MW, increasing the power density from 54.28 to 63.54 W/kg while maintaining the same outer diameter.

Measurement of outgassing rates of Kevlar and S-Glass materials used in torque tubes of High Tc Superconducting (HTS) Motors

Torque tubes in High Temperature Superconducting (HTS) motor transfer torque from superconducting field winding rotor to the room temperature shaft. It should have minimum heat conduction property for minimizing the load on cryo-refrigerator. Generally, these torque tubes are made with stainless steel material because of high strength, very low outgassing and low thermal contraction properties at cryogenic temperatures and vacuum conditions. With recent developments in composite materials, these torque tubes could be made of composites such as Kevlar and S-Glass, which have the required properties like high strength and low thermal conductivity at cryogenic temperatures, but with a reduced weight. Development and testing of torque tubes made of these composites for HTS motor are taken up at Bharat Heavy Electricals Limited (BHEL), Hyderabad in collaboration with Central Institute of Plastics and Engineering Technology (CIPET), Chennai and Indian Institute of Technology (IIT), Kharagpur. As these materials are subjected to vacuum, it is important to measure their outgassing rates under vacuum conditions before manufacturing prototype torque tubes. The present study focusses on the outgassing characteristics of Kevlar and S-Glass, using an Outgassing Measurement System (OMS), developed at IIT Kharagpur. The OMS facility works under vacuum environment, in which the test samples are exposed to vacuum conditions over a sufficient period of time. The outgassing measurements for the composite samples were obtained using pressure-rise technique. These studies are useful to quantify the outgassing rate of composite materials under vacuum conditions and to suggest them for manufacturing composite torque tubes used in HTS motors.