HTS Field Winding Research Articles

Investigating the impact of magnetic air gap variations on short circuit characteristics of partially high temperature superconducting generator

Partially high-temperature superconducting generators (PHTSGs) feature large magnetic air gaps imposed by using cryostats for HTS field windings. This increased air gap significantly affects end winding inductance and can lead to elevated fault torque levels. This study investigates the influence of magnetic air gap variation on generator design and end winding inductance and its implications for short-circuit faults in PHTSGs. Through a combination of numerical simulations and analytical analysis, the study explores how changes in the magnetic air gap affect end winding inductance and subsequently influence short-circuit fault behavior. The results reveal a direct correlation between magnetic air gap length, end winding inductance, and key short-circuit parameters such as stator current, field currents, and electromagnetic torque. Notably, an increase in the magnetic air gap is observed to elevate stator and field currents and electromagnetic torque during short-circuit events. These insights underscore the importance of considering magnetic air gap variation and its impact on end winding inductance and its relationship with short circuit characteristics in the design and operation of PHTSGs, providing valuable insights for enhancing the resilience and performance of high-temperature superconductor-based generator systems.

Electrical machines with HTS windings – prospective schemes, design algorithm, and specific power dependences

The parameters calculation results for electric machines with HTS armature windings have been provided in this Article. Various machine schemes using HTS field winding, permanent magnets, tooth and annular armature windings have been considered. A design algorithm that takes into account system level requirements, analytical calculations, finite element modeling, as well as heat and mechanical calculation have been proposed. Specific power dependences for 77K and 20K machines have been found from the analytical method and optimization calculation based on it. It has been proved that the method developed allows finding specific power for various machines. Specific power dependences presented in this Article can be used for system level design.

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.

Electrical design and structure optimization of 10 MW fully superconducting wind turbine generators

In this paper, two types of conceptual structures of 10 MW fully superconducting offshore wind turbine generators (FSWTG) with an efficient electrical design method are presented. Four types of generator models, Gen. A - D, are compared according to different armature winding structures, and it's found that Gen. A is the preferable one. Optimization of the main generator parameters reveals that 36 poles with a pole pitch of 474 mm and electrical load of 6.0 kA/cm yields the best performance. The different design parameters of FSWTGs are compared to partially superconducting wind turbine generators (PWTG) with HTS field windings and copper armature windings and traditional wind turbine generators (WTG). Results show that FSWTGs are smaller in size, weigh 0.7 and 0.4 times less and are more efficient by 1.9–2.1% and 2.9–3.1% than PWTGs and WTGs, respectively. Lastly, the AC losses generated in the superconducting armature windings of the four types of FSWTGs are calculated and discussed.

Fabrication and Charging Test of HTS Field Windings Using HTS Contactless Rotary Excitation Device

This paper presents the results of the fabrication and preliminary charging test of second-generation high-temperature superconducting (2G HTS) coils for the rotor field winding of a 1-kW-class HTS rotating machine (HTSRM). This machine technically employs an HTS contactless rotary excitation device (CRED), which is the so-called rotary flux pump, to charge the HTS field windings using a noncontact excitation method. In this study, the major components of the 1-kW-class HTSRM, such as the 2G HTS coils for the rotor field pole of the rotating machine, HTS strands with a toroidal head for the rotary part of CRED, and stationary part of CRED, were fabricated and assembled. Then, the charging performance of the field operating current was tested in preliminary experiments. In particular, to confirm the technical feasibility of CRED on application to the 1-kW-class HTSRM, the HTS field coils were charged under various operating conditions of the HTS CRED in a stationary flux-pump mode.

A Five-Phase Doubly Fed Doubly Salient HTS Linear Motor for Vertical Transportation

In this paper, a doubly fed doubly salient high temperature superconducting linear motor (DFDS-HTSLM) is proposed for vertical transportation. The proposed DFDS-HTSLM adopts a five-phase doubly salient configuration. The stator consists of iron lamination only with no windings or permanent magnets. The mover houses both the copper armature winding and the HTS field winding in its outer layer and inner layer, respectively. By using the HTS field winding, the air-gap flux can be flexibly adjusted by controlling the DC current flowing in it. In addition, by using five-phase configuration, the proposed motor can realize the fault-tolerance operation easily, hence improving the reliability.

Comparison Study on Harmonic Loss of MW-Class Wind Generators With HTS Field Winding

This paper investigates the effect of slot-pole combination on harmonic loss such as iron loss and eddy-current loss in wind generators with high-temperature superconductor field winding. In order to study the effect of stator configuration, three wind generators having the same rotor structure but different stators were purposely designed to produce 11 MW. In the analysis, 2-D time-stepped finite element analysis was used to estimate flux density waveforms, and then harmonic iron loss in the stator yoke and rotor eddy-current loss were obtained. From the simulation results, we showed that the harmonic effect on stator iron loss is not significant because magnetic loading by field winding dominates. Meanwhile, we found that the eddy-current loss in the rotor by harmonics of magneto motive force strongly depends on the slot-pole combination.

Armature reaction effects on a high temperature superconducting field winding of an synchronous machine: experimental results

This paper presents experimental results from the Superwind laboratory setup. Particular focus in the paper has been placed on describing and quantifying the influence of armature reaction on performance of the HTS filed winding. Presented experimental results have confirmed the HTS field winding sensitivity to both armature reaction intensity and angular position with respect to the HTS coils. Furthermore, the characterization of the HTS field winding has been correlated to the electromagnetic torque of the machine where the maximal Ic reduction of 21% has been observed for the maximum torque.

A Cryogenic Dc-Dc Power Converter for a 100kW Synchronous HTS Generator at Liquid Nitrogen Temperatures

Abstract A dc-dc converter has been developed for retrofitting inside the vacuum space of the HTS rotor of a synchronous generator. The heavy copper sections of the current leads used for energising the HTS field winding were replaced by cryogenic power electronics; consisting of the converter and a rotor control unit. The converter board was designed using an H-bridge configuration with two 5A rated wires connecting the cryogenic boards to the stator control board located on the outside of the generator and drawing power from a (5A, 50 V) dc power source. The robustness of converter board was well demonstrated when it was powered up from a cold start at 82K. When charging the field winding with moderate currents (30A), the heat in-leak to the ‘cold’ rotor core was only 2W. It continued to function down to 74K, surviving several quenches. However, the quench protection function failed when injecting 75A into the field winding, resulting in the burn out of one of the DC-link capacitors. The magnitudes of the critical currents measured with the original current leads were compared to the quench currents, which was defined as the current which triggered quench protection protocol. The difference between the two currents was rather large, (∼20A). However, additional measurements using a single HTS coil in liquid nitrogen found that this reduction should not be so dramatic and in the region of 4A. Our conclusions identified the converter's switching voltage and its operating frequency as two parameters, which could have contributed to lowering the quench current. Magnetic fields and eddy currents are expected to be more prominent the field winding and its impact on the converter also need further investigation.

An HTS Machine Laboratory Prototype

Abstract This paper describes Superwind HTS machine laboratory setup which is a small scale HTS machine designed and build as a part of the efforts to identify and tackle some of the challenges the HTS machine design may face. One of the challenges of HTS machines is a Torque Transfer Element (TTE) which is in this design integral part of the cryostat. The discussion of the requirements for the TTE supported with a simple case study comparing a shaft and a cylinder as candidates for TTE are presented. The discussion resulted with a cylinder as a TTE design rated for a 250Nm and with more then 10 times lower heat conduction compared to a shaft. The HTS machine was successfully cooled to 77K and tests have been performed. The IV curves of the HTS field winding employing 6 HTS coils indicate that two of the coils had been damaged. The maximal value of the torque during experiments of 78Nm was recorded. Loaded with 33%, the TTE performed well and showed suffcient margin for future experiments.

Further Testing of an “Iron-Cored” HTS Synchronous Generator Cooled by Liquid Air

The systematic performance test program of the 100 kW iron-core HTS synchronous generator has been underway since 2007 at the University of Southampton. The early measurements were conducted at liquid nitrogen temperatures (77 K) and sub-cooled liquid nitrogen close to its triple point (63 K). Further measurements at these operating temperatures for both open circuit and loaded configurations have enabled us to confirm the machine's synchronous reactance. Circulating liquid air through the closed loop cooling system enabled us to lower the rotor temperature down to 61.7 K and boost the critical current I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> of the HTS winding up to 144 A. A number of transient tests were carried out to assess the impact on the machine, in particular the current in the HTS field winding when the stator circuit was suddenly shorted. The results show that the peak current can be almost 3 times the initial operating field current in the first cycle of the response. No `thermal quench' of HTS winding was witnessed, despite the occurrence of large over currents. The recovery of the HTS winding back to the initial operating current occurred in several seconds and depended on the initial operating current and the reaction of the power supply.

Study of 10 MW-Class Wind Turbine Synchronous Generators With HTS Field Windings

We study the electro-magnetic design of 10 MW-class wind turbine generator with high temperature superconducting field winding by using the FEM analysis. The design examples are presented and the generator characteristics are investigated. The 10 MW-class HTS wind turbine generator is considered to be feasible from the stand point of the electro-magnetic design. In this study, 8-pole and 12-pole generators are investigated. From the results, it is considered that the 8-pole design is preferable compared with the 12-pole design. The analysis also reveals the small synchronous reactance of the HTS wind turbine generator. Therefore, proper protection is necessary for the large short circuit current.

Development of an HTS hydroelectric power generator for the hirschaid power station

This paper describes the development and manufacture of a 1.7MW, 5.25kV, 28pole, 214rpm hydroelectric power generator consisting of superconducting HTS field coils and a conventional stator. The generator is to be installed at a hydro power station in Hirschaid, Germany and is intended to be a technology demonstrator for the practical application of superconducting technology for sustainable and renewable power generation. The generator is intended to replace and uprate an existing conventional generator and will be connected directly to the German grid. The HTS field winding uses Bi-2223 tape conductor cooled to about 30K using high pressure helium gas which is transferred from static cryocoolers to the rotor via a bespoke rotating coupling. The coils are insulated with multi-layer insulation and positioned over laminated iron rotor poles which are at room temperature. The rotor is enclosed within a vacuum chamber and the complete assembly rotates at 214rpm. The challenges have been significant but have allowed Converteam to develop key technology building blocks which can be applied to future HTS related projects. The design challenges, electromagnetic, mechanical and thermal tests and results are presented and discussed together with applied solutions.

Design study of high-temperature superconducting generators for wind power systems

Design study on high-temperature superconducting machines (HTSM) for wind power systems was carried out using specially developed design program. Outline of the design program was shown and the influence of machine parameters such as pole number, rotor outer diameter and synchronous reactance on the machine performance was clarified. Three kinds of generator structure are considered for wind power systems and the HTSM operated under highly magnetic saturated conditions with conventional rotor and stator has better performance than the other types of HTSM. Furthermore, conceptual structure of 8 MW, 20 pole HTSM adopting salient-pole rotor as in the case of water turbine generators and race-truck shaped HTS field windings like Japanese Maglev was shown.

Development of a 100 hp Synchronous Motor With HTS Field Coils

A 3-phase, 100 hp, 4 pole, 1800 rpm superconducting synchronous motor built and tested in Korea Electrotechnology Research Institute(KERI). This machine consists of HTS rotor and air-core stator. The HTS field windings are composed of the double-pancake coils wound with AMSC's stainless steel-reinforced Bi-2223 tape conductor. These were assembled on the support structure and fixed by a bandage of glass-fiber composite. The cooling system is based on the heat transfer mechanism of the thermosyphon by using GM cryocooler as cooling source. The cold head is in contact with the condenser of a Ne-filled thermosyphon. The rotor assembly was tested independently at the stationary state and combined with stator. The open circuit, no-load, and short circuit characteristic were obtained. Also, load tests in motor mode driven by inverter and generator mode connected to resister load bank were conducted. This paper will present design, construction, and experimental test results of the 100 hp HTS machine.

Development status of superconducting motors

The article gives a brief history of the development of superconducting motors with emphasis on the use of high temperature superconductors. At present, effort is directed toward the demonstration of a 1000 hp HTS motor, the test of which will mark another significant milestone in HTS motor development. For the first time, a closed-loop refrigeration system will be used to maintain an operating temperature near 30 K for the HTS field winding. Following the conclusion of the 1000 hp HTS motor project, a 5000 hp HTS motor will be constructed and tested to demonstrate its economical viability. The progress in developing HTS motors to date is shown. The rotor for the 1000 hp motor was designed to accommodate other coils and can provide a test bed for wire developed in the future.

Status of the 1000 HP HTS motor development

Progress has been made in the development of high efficiency HTS motors with the aid of Department of Energy funding under the Superconductivity Partnership Initiative. This effort includes the fabrication and testing of synchronous motors with HTS field windings. The objectives of this development effort include saving half the losses of conventional motors in a package with half the volume. In Phase I of the present program, a 125 HP synchronous motor with an HTS field winding was designed and tested to levels in excess of 200 HP. This paper summarizes the status of a 1000 HP motor development which is part of a Phase II effort. The program elements to be reviewed include the overall design characteristics of the motor, the status of the field coils and refrigeration system, and a description of other components of the 1000 HP motor system. The 1000 HP motor fully represents the design issues to be addressed in the 5000 HP motor also to be developed in Phase II.

Development of synchronous motors and generators with HTS field windings

Synchronous machines with superconducting field windings offer reduced losses, size and weight. Continuous advances in the HTS conductor development at American Superconductor Corporation enable the demonstration of machines with increasing power. The latest achievements include a > 200 h.p. (150 kW) motor operated on an inverter, built and tested at Reliance Electric Corporation, and coils for a 1 MW generator for airborne applications. The HTS coils provide MMFs of the order of 70 000 A turns per pole and operate between 20 and 27 K. At this temperature, gaseous helium or hydrogen is used as the coolant. The cooling is more efficient by a factor of 10 than for cooling systems required for LTS materials operating at liquid helium temperatures. The elevated operating temperature offers significant benefits for the overall efficiency as the coefficient of performance for removing the parasitic heat load (cryostat and current lead loss) is improved. On this path to commercially viable machine sizes, American Superconductor currently is developing and manufacturing HTS coils for a 1000 h.p. (750 kW) synchronous motor which will be delivered to Reliance Electric for demonstration in 1998.

High-temperature superconducting magnet motor demonstration

A superconducting field winding, assembled with coils constructed with Bismuth 2223 HTS wire, has been successfully operated in an electric homopolar motor. At a field winding temperature of 4.2 K, the motor produced 125 kW of output power. At a temperature of 28 K, the motor power developed was 91 kW. These measured power levels are the highest that have been produced by an electric motor containing a HTS field winding.