High Temperature Superconducting Machines Research Articles

Electromagnetic-mechanical coupling analysis of high-temperature superconducting racetrack coil

The second-generation high-temperature superconducting (HTS) coated conductor has been recognized as one of the most promising materialsfor high field magnets due to its superior electromagnetic and mechanical performances. HTS racetrack coils wound with the coated conductors are an extensively used configuration in engineering applications, such as HTS machines and high-speed maglevs. In this paper, in order to analyze the electromagnetic and mechanical behaviors of HTS racetrack coils, a 3D coupled electromagnetic-mechanical model is used to consider the effect of coil deformation and the strain dependence of critical current. The effectiveness of the coupled model is validated by comparing the numerical results with experimental data in the literature. A numerical simulation of a 3D HTS racetrack coil subjected to an external electromagnetic field is carried out using coupled and uncoupled models. The results indicate that the structure deformation can reduce the penetration depth of the screening current, and the hoop stress and strain are mainly concentrated on the circular part of the racetrack coil. Afterwards, the influences of various parameters on the electromagnetic and mechanical responses of the HTS racetrack coil are also investigated.

Time-variant magnetic field, voltage, and loss of no-insulation (NI) HTS magnet induced by dynamic resistance generation from external AC fields

High-temperature superconducting (HTS) coils serving as DC magnets can be operated under non-negligible AC fields, like in synchronous machines of maglev trains and wind turbines. In these conditions, dynamic resistance is generated in HTS tapes, causing redistribution/bypassing of the transport current inside the no-insulation (NI) coil and its unique operational features. This issue was studied by experiments on an NI coil with DC current supply put into external AC fields. Due to the current redistribution induced by dynamic resistance, the central magnetic field and voltage of the NI magnet initially undergo various transient processes, and eventually exhibit a stable central magnetic field reduction and a DC voltage. These time evolutions have implications for the time-varying torque and loss of an HTS machine. These time evolutions are strongly affected by the contact resistivity distribution, and whether it is the first time that the NI magnet has been exposed to the AC field, showing several qualitatively different waveforms (e.g. some are even non-monotonic with time). The magnitudes of the stable central field reductions, and their observed linear correlation with the DC voltages are found to be decided by the local contact resistivity of the innermost and outermost several turns. It is also noted that the non-insulated turn-to-turn contact help lessening the loss induced by the dynamic resistance. A numerical model is established to analyze/explain these experimental results by observing the microscopic current distribution. Two risks of quench are noticed: (i) the azimuthal current of the middle part turns increases as the AC field is applied; (ii) a concentration of radial current is observed near the terminals of the NI coil.

Performance Analysis of rotating pole shoe superconducting machine with dual coils

With the improvement of high temperature superconducting (HTS) materials, HTS machines have shown better competitive advantages in wind power generation and other fields. However, the conventional structure of superconducting machines has led to low reliability of the machines due to the complex structures such as brush and rotating dewar. Based on the analysis of the properties of second-generation high-temperature superconducting strips — YBCO strips, a kind of rotating pole shoe superconducting machine (RPHTSM) with dual HTS coils is proposed in this paper, which avoids the use of brush and rotating dewar used in the conventional HTS machine. The three-dimensional model for magnetic field analysis of the RPHTSM is established and the structure of the RPHTSM is optimized according to the calculation. A two-dimensional simplified model was developed and based on which the output performance of the machine was calculated. By comparing with the conventional HTS machine, it is concluded that the dual-coil RPHTSM has great advantage in reducing the consumption of superconducting strips and the inertia of the rotor.

An Improved Critical Current Calculation Method of HTS Field-Excitation Coil for Double-Stator HTS Generator With Stationary Seal

In this article, a calculation method for critical current of high-temperature superconducting (HTS) excitation coils made by Bi-2223 tapes is proposed, which can provide a reference for the design and manufacture of HTS excitation coils. Based on the test data provided by the manufacturer, the suppression effect of the applied parallel and perpendicular magnetic field on the critical current density is separately described by the proposed Jc-B model of the Bi-2223 tapes. Then, taking a 10 kW double-stator HTS machine (DS-HTSM) as an example, the target excitation magnetomotive force (MMF) required to meet its designed electromagnetic properties is obtained. After then, the turns number and the critical current of the HTS excitation coils required to meet the target MMF are calculated by the finite element analysis (FEA). Further, the FEA model of the DS-HTSM will be built by using the homogenization modeling method of HTS coils, and the influence of the armature reaction magnetic field on the critical current of the HTS excitation coils is analyzed. Finally, the correctness and effectiveness of the proposed improved critical current calculation method is verified by fabricating the 10 kW DS-HTSM prototype and conducting experimental test.

Influence of Copper Rings and Plates on Eddy Losses of No-Insulation High Temperature Superconductor Coil in Synchronous Machines

No-insulation (NI) high temperature superconducting (HTS) machine is a semi-superconducting machine with improved thermal-stability and power density, in which the rotor windings are replaced with NI HTS coils. However, the AC background field from stator windings leads to significant eddy current in NI HTS windings. The induced eddy current and loss affect the operating efficiency and safety of HTS machine. Traditionally, copper discs and rings are used for conduction cooling and mechanical support of NI HTS coil. In this paper, the influence of copper discs and rings on reducing the eddy current and loss of double pancake NI HTS coil is studied. Three structures of copper discs, copper ring, and both copper discs and ring are analyzed, by both simulation and experiment. The results show that all these three structures of copper shielding can effectively reduce the eddy loss on NI HTS coil within the HTS machine operating temperature of 20–30 K. The combination of copper discs and ring has the best shielding performance.

Iron Loss Analysis and Efficiency Calculation of Double Stator HTS Machine With Stationary Seal

This paper mainly analyzes the iron loss of a double stator high-temperature superconducting (HTS) machine with stationary seal and calculates its efficiency. Firstly, according to the field-modulation theory, the air-gap flux density harmonic components of the excitation magnetic field and armature-reaction magnetic field are derived. Secondly, the air-gap flux density harmonic components corresponding to the flux density harmonic components of iron core are derived, and the main iron core flux density harmonic components are confirmed and verified by finite element analysis (FEA). When using the semi-FEA method to calculate the iron loss, according to the analysis results of main core flux density harmonic components, the appropriate step-interval of the FEA is selected. In addition, according to the characteristics of the iron core flux density, this paper proposes an alternative method of iron core flux density, which greatly shortens the calculation time of semi-FEA method of iron loss calculation. Then, iron loss model with variable coefficients is used to calculate the iron loss of the machine, and the calculation results are compared with those of the FEA iron loss calculation method. Finally, other losses and efficiency of the machine are calculated.

Numerical Study on Transient Behaviours of a 30-kW Generator Using a Coupling Method

As research and production technologies for high-temperature superconducting (HTS) materials have become increasingly mature, many high-efficiency and low-weight electrical machines made of HTS conductors have been successfully manufactured. In this paper, a transient numerical model for processing a three-phase symmetrical short circuit in a 30-kW HTS generator is proposed. It is based on an electromagnetic and thermal model, coupling the multiphysics of the rotating machinery and magnetic, circuit, and heat transfer modules. Some macroscopic characteristics of the generator, such as the output voltage and current waveforms, can be obtained. It also simulates the microscopic current shunting phenomenon between layers and temperature variation across HTS armature coils during a three-phase symmetrical short circuit fault. Then, some parameters of HTS conductors that are relevant to the temperature rise in coils are considered in this model. This simulation is an effective tool for analysing the transient performance of HTS generators in various fault operating conditions and could correspondingly provide an important guidance for the HTS machines’ preliminary design stage.

3D modeling of a superconducting dynamo-type flux pump

High temperature superconducting (HTS) dynamos are promising devices that can inject large DC currents into the winding of superconducting machines or magnets in a contactless way. Thanks to this, troublesome brushes in HTS machines or bulky currents leads with high thermal losses will be no longer required. The working mechanism of HTS dynamo has been controversial during the recent years and several explanations and models have been proposed to elucidate its performance. In this paper, we present the first three-dimensional (3D) model of an HTS flux pump, which has good agreement with experiments. This model can be beneficial to clarify the mechanism of the dynamo and pinpoint its unnoticed characteristics. Employing this model, we delved into the screening current and electric field distribution across the tape surface in several crucial time steps. This is important, since the overcritical screening current has been shown to be the reason for flux pumping. In addition, we analyzed the impact of both components of electric field and screening current on voltage generation, which was not possible in previous 2D models. We also explored the necessary distance of voltage taps at different airgaps for precise measurement of the voltage across the tape in the dynamo.

Design of a 10 MVA Fully HTS Synchronous Generator With Dual Field Windings and 4 Poles Spanned Armature Windings

We have proposed a fully high temperature superconducting (HTS) synchronous generator with dual field windings to reduce the circumferential component of the magnetic field on the HTS tape in the armature. In this paper, we suggested a design of a 10 MVA fully HTS synchronous generator with dual field windings. We varied the distance between inner and outer field windings, the shape of the cross-sections of the fields, and the height of the armature windings to find the structure with minimum losses. The structure of the HTS armature windings should be different from the one of the copper armature windings because the overhang of the HTS armature windings in cryostats cannot be bent. So, we also proposed 4 poles spanned 3 phase armature windings. Normally one set of 3 phase copper armature windings is installed in 2 poles span with the structure of double layer winding, but the proposed 3 phase HTS armature windings are installed in 4 poles span in single layer. We analyzed AC losses of various designs of the fully HTS machines and suggested the de-sign parameters with minimum AC loss.

Fully superconducting machine for electric aircraft propulsion: study of AC loss for HTS stator

Fully superconducting machines provide the high power density required for future electric aircraft propulsion. However, superconducting windings generate AC losses in AC electrical machine environments. These AC losses are difficult to eliminate at low temperatures, and they add an extra burden to the aircraft cooling system. Due to the heavy cooling penalty, AC loss in the HTS stator is one of the key topics in HTS machine design. In order to evaluate the AC loss of superconducting stator windings in a rotational machine environment, we designed and built a novel axial-flux high temperature superconducting (HTS) machine platform. The AC loss measurement is based on the calorimetric boiling-off of liquid nitrogen. Both total AC loss and magnetisation loss in the HTS stator are measured under the condition of a rotational magnetic field. This platform represents a key element in studying ways to minimise AC losses in an HTS stator, in order to maximise the efficiency of fully HTS machines.

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.

3D Semi-Analytical Modeling and Optimization of Fully HTS Ironless Axial Flux Electrical Machines

In this paper, a three-dimensional semi-analytical approach, based on the volume integral method, is developed to compute the forces and torque in fully high temperature superconducting (HTS) ironless axial flux machine (IAFM). The use of particular volumetric basis elements helps to speed up the computation of the magnetic field. The Maxwell stress tensor method is used for the force and torque calculations, where the electromagnetic state of the superconductor is taken into account. The developed model is then introduced into a multi objective optimization procedure, based on the genetic algorithms, to maximize the magnetic torque and minimize the superconducting wire length in the considered structure, while respecting the constraints linked to the physical and mechanical properties of the considered HTS tape. It is shown that the considered fully HTS machine has a higher torque density compared to conventional one.

Design, Fabrication, and Testing of a YBCO Racetrack Coil for an HTS Synchronous Motor With HTS Flux Pump

An experimental platform of high temperature superconducting (HTS) synchronous motor with HTS flux pump has been designed and under construction. The HTS flux pump as an exciter eliminates thermal load from the metal current leads, which will reduce the cryogenic volume and improve the efficiency of the HTS machine. A finite element model (FEM) using T-A formulation was developed to analyze the electromagnetic characteristics of a double-pancake (DP) racetrack coil. The no-insulation (NI) racetrack coil wound by YBCO tapes was fabricated with a winding machine in our lab. We proposed a novel mechanical structure to avoid the impregnation of epoxy resin and make the coil more reliable. The turn-to-turn pressure of the racetrack coil can be changed by driving the screws on the straight section. Besides, we manufactured a rotating HTS flux pump as a DC power supply to charge the racetrack coil without contact. The self-field critical current test with a direct current (DC) power supply and the charging experiment with the HTS flux pump of the constructed YBCO racetrack coil were performed at 77 K.

A Novel HTS Wind Generator Having Permanent Magnets Between the Rotor Pole-Tips

Superconducting direct-drive wind generators have been proposed as a possible approach for offshore wind energy application due to their inherent high torque density and small mass compared with conventional electrical machine. In this paper, a novel 10 MW high temperature superconducting (HTS) salient pole synchronous wind generator with additional permanent magnets inserted between the rotor silent pole-tips is proposed. First, the electromagnetic structure of this new topology is designed in detail and the performance improvements of this machine are investigated by finite-element analysis. These two kinds of assist effect are also clarified. Then, the proposed structure is compared with conventional HTS wind generator without assistant magnets under the same scale constraints. The comparison results show that with the accessorial magnets, the torque density can be increased by 22%. Finally, the symmetrical three-phase short-circuit fault performances of the proposed HTS machine are conducted to verify the generator reliability.

A Novel HTS Flux-Switching Transverse-Flux Machine With Partitioned Stator

This paper proposes and analyzes a novel high-temperature superconducting (HTS) flux-switching transverse-flux (HTS-FSTF) machine with partitioned stator, which keeps the merits of stator-excitation HTS machine with static seals. The key is to adopt the axial partitioned stator structure to realize the independent partition arrangement of armature windings and HTS-excitation windings, as well as artfully place the flux barrier between them to minimize the impact of armature field on the critical field state of the HTS excitation. Moreover, by using the split-pole design, only a single ring-shaped HTS-excitation coil is employed, thus, easily implementing static seals of simple cooling Dewar pattern to improve the refrigeration system reliability and reduce costs. The operation principle based on the transverse-flux path analysis and the cooling Dewar design are described in details. Also, by using the three-dimensional finite-element analysis, the electromagnetic performances of the proposed machine are investigated, and its validity is verified.

Design and Experimental Study of the Wireless Online Monitoring System of a High-Temperature Superconducting Machine

As an important research topic of ship electric propulsion and new power generation, high-temperature superconducting (HTS) machines have become a research hotspot in the field of HTS applications in recent years. With high power density, high efficiency, and outstanding dynamic characteristics, HTS motors could bring much more great social and economic benefits. Meanwhile, the application conditions of HTS materials are harsh, and any parameters of the HTS materials beyond the safe range of operation are probable to lead the HTS materials to quench. The quenching of HTS materials changes the state parameters of power devices; what's more, it will bring a huge impact on the safety and stability of HTS power system. To ensure the safe operation of the HTS machine, by detecting the voltage, environment temperature, and liquid nitrogen level, this paper designed a wireless online monitoring system to determine whether the HTS material has failed. The system is based on the brief principle of superconductor quench. In addition, the system was applied to a 500-kW HTS power generator. The experiment shows the sufficient accuracy and reliability of the wireless online monitoring system.

Calculation Method Study of HTS Coils’ Critical Current

A method to calculate second-generation (2G) high temperature superconducting (HTS) coils’ critical current is presented. Critical current and n value of HTS tapes measured under different magnet field are used for the calculation of the coil. The distribution of HTS coils’ magnetic field intensity, current density, and electric field intensity is calculated with COMSOL 2-D based on H formulation. A partial differential equation (PDE) model and the magnetic field formulation (mfh) model of HTS tape are constructed. The modeling method and key points are also introduced and discussed. AC loss of the HTS tape is calculated with the above-mentioned two models, respectively. The calculation results of the two modeling approaches are coincident. The cross sections of the straight segment and bending segment of a HTS racetrack double-pancake coil are modeled, respectively, with a 2-D and 2-D-axisymmetric model in COMSOL, and the critical current is calculated. E – I properties of each turn of the coil and the critical current are calculated. HTS coils with seven pairs of voltage lead are wound. Voltages at different parts of the coils and the critical current are measured. The measurement results of the magnetic field, electric field, and critical current agree with the calculation results. The method presented in this paper can also be referred to when designing an HTS machine with HTS tapes or coils.

An effective way to reduce AC loss of second-generation high temperature superconductors

Second-generation (2G) high temperature superconducting (HTS) devices have high power density and efficiency for energy applications. However, in alternative current conditions, the vortex movement in 2G HTS leads to energy dissipation at low temperatures, which significantly affects efficiency and adds an extra burden to cooling systems. This energy dissipation is identified as AC loss. Reducing AC loss to improve efficiencies of 2G HTS devices, e.g. HTS machines, cables and fault current limiters, has become a key research focus. We report here on an effective way to reduce AC losses by using a new 2G HTS wire. The principle is to stack narrow 2G HTS tapes into a wire structure, which is called a soldered-stacked-square (3S) wire. Our experiments on solenoid coils have proved that the 3S wire helps to reduce the AC loss by 80%. Moreover, the critical current of the 3S wire can be easily adjusted for various applications. It is envisioned that the 3S wire will become a powerful technology enabling the development of 2G HTS power devices with high efficiency.

Electromagnetic Design of a Large-Scale Double-Stator Direct Driving HTS Wind Generator

High-temperature superconducting (HTS) direct driving wind generators have been extensively researched due to their inherent high torque density and small mass. In this paper, a double-stator HTS wind generator with rotating HTS field windings is proposed to further improve the torque density. The electromagnetic design of this generator is made and electromagnetic performances of different topologies with air-gap windings or iron teeth are compared and analyzed. In addition, two regular HTS wind generators and one double-stator permanent magnet wind generator with the same scale are put forward to make comparison with proposed double-stator HTS machines, and the feasibility and practical value of these five topologies are evaluated in terms of the weight, volume and cost of active materials. The results reveal double-stator HTS wind generator has great advantages in volume and mass reduction.

Design and Analysis of a Stator HTS Field-Modulated Machine for Direct-Drive Applications

This paper proposes a new stator high-temperature superconducting (HTS) field-modulated machine with the merit of high torque density for direct-drive applications, which incorporates the attractive features of both stator excitation HTS machine and field-modulated machine. The key is to artfully place the HTS windings on the stator to implement static seals as well as utilizing the “magnetic gearing effect” to achieve high torque capability. Based on the geometry of the fabricated double-layer Dewar assembly for cooling the HTS excitation coil, the proposed machine is initially designed. By using the finite element analysis (FEA), the machine performances are analyzed, and its validity is verified