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An investigation into the corona onset characteristics of small curvature electrode with rough surface under combined ac–dc voltages was presented. The experimental platform under combined voltages with different electrode fillet radius and electrode spacing was built. The corona inception voltage of the small curvature electrode was obtained by collecting the corona pulse signal and changes in the number of ultraviolet photons. Based on the finite element algorithm, the corona onset electric field strength under combined ac–dc voltages was calculated. Results showed that the corona inception voltage increased with the increase of electrode fillet radius and electrode spacing. The corona inception voltage of positive dc–ac was higher than that of negative dc–ac. The linear superposition of ac and dc voltages when corona occurred was regarded as combined corona inception voltage. It increased first and then decreased slightly with the increase of ac voltage. The combined corona electric field strength on the electrode surface generally decreased with the increase of ac voltage. The research of this article can provide useful references for the maintenance of electrode surface and insulation optimization of converter valve. [ABSTRACT FROM AUTHOR]

The development of high-temperature superconducting (HTS) electrical machines results in applications of various fields, as they have higher efficiency and power density. Numerical modeling of superconducting electrical machines is usually performed by finite element method based their electrical behavior, and H-Formulation of the Maxwell's equations is one of the models that are used tremendously due to its high flexibility and simplicity. Axial field machine (AFM) is a machine topology with higher efficiency and power density, but with more complexity when modeling. This paper proposed a new 2D model for the superconducting AFM, where the superconducting coil is simulated with a 2D infinitely long model, this model also divides the electrical machine into two parts: the superconducting parts (HTS coils, bulks, etc.) and non-superconducting parts (e.g., conventional conductors, machine structure). This paper compares the results of H-A Formulation and T-A Formulation with this approach, which requires appropriate boundary conditions and continuity between the two regions, along with correct modeling of fixed and moving parts of the electrical machine, where the geometry of this model is based on a linear synchronous machine, which means the rotor is moving linearly instead of rotation. [ABSTRACT FROM AUTHOR]

This article analyzed and compared the complex permeance model (CPM) and nonlinear analytical model (NAM) for surface-mounted permanent magnet (SPM) motor using a voltage source inverter (VSI). For linear CPM, the slotting effect is represented using the improved complex permeance function that describes the real slot shape rather than infinitely deep slot. NAM is extended from CPM and it replaces the magnetic potential drop of stator iron by the equivalent virtual current in the slot-opening and tooth-tip to account for the nonlinearity effect. The nonlinear inductance of SPM motor including the main phase inductance, slot leakage inductance, tooth-tip leakage inductance, and end winding leakage inductance is calculated using the frozen permeability method, which is the key to solve the electric circuit with VSI. The instantaneous back electromotive force (EMF) is calculated from the nonlinear back EMF coefficient and the back EMF neglecting iron nonlinearity. The electromagnetic torque obtained from motor model is used in the mechanical model and then the rotational speed and rotor position are obtained for the electric circuit model. Hence, the iterative solving process for the motor system is established to calculate the transient performance of SPM motor using VSI. Compared with CPM, NAM can greatly improve the calculation accuracy, which is also validated by both finite-element model and experiment. Besides, the proposed NAM is computationally efficient, which is useful for both motor design and motor control. [ABSTRACT FROM AUTHOR]

With regard to the state-of-the-art technologies in the fields of applied superconductivity and electromagnetic devices, research and development highlights are presented. The recent progress and achievement described with principle and technical details include mainly i) applied superconducting materials; ii) superconducting magnets and their applications such as in ITER and Tokamaks; iii) high Tc superconducting (HTS) magnetic levitation and applications; iv) HTS smart grids; v) superconducting and electromagnetic material modelling and characterization; and vi) advanced electromagnetic devices. The applied superconductivity technology and availability are especially focused and verified with the trend of development prospection. [ABSTRACT FROM AUTHOR]

As the output power of wind farm is fluctuating, it is one of the important ways to improve the schedule ability of wind power generation to predict the output power of wind farm. The operation mode of tracking planned output takes the planned value issued by the grid dispatching as the control basis of wind power generation. This operation mode is easy to control, which not only meets the needs of power grid dispatching, but also improves the reliability and controllability of new energy generation. This paper introduces the traditional PI control strategy of SMES and combines the advantages of SMES and the characteristics of application scenarios, and introduces sliding mode variable structure control into SMES. Through simulation and comparison of two different control modes, it is verified that SMES adopting sliding mode variable structure control have the advantages of fast response speed and strong robustness, which can make the wind farm output stable power according to the planned value. [ABSTRACT FROM AUTHOR]

When a superconductor is exposed to an AC magnetic field whilst carrying a constant DC transport current, a DC electrical resistance can be observed, commonly referred to as “dynamic resistance”. This dissipative effect can play a critical role in many potential high-temperature superconducting (HTS) applications, especially for the AC-field-actuated HTS switch, which is the key component in the HTS transformer-rectifier flux pump (TRFP). The characteristics of the HTS switch determine the performance and reliability of TRFP. In this paper, we report a numerical study of a new HTS switch design made of series-connected YBCO stacks. This study aims to obtain insights into the characteristics of dynamic resistance and power loss, including their relationships with the amplitude and the frequency of the AC magnetic field. The energy conversion efficiency of the HTS switch is also investigated. This work will help design a high-performance AC-field-actuated HTS switch. [ABSTRACT FROM AUTHOR]

With excellent mechanical properties and electromagnetic properties, compact superconducting cables are considered as potential candidates for large capacity power applications and fusion magnets. In this paper, the concept of a novel compact cable, conductors in tube (CIT) cable is introduced from the aspect of design, fabrication, and performance evaluation, respectively. The principal advantages of CIT cables are that the cable can release the residual stress created from the fabrication process, and the outer former of the cable can be used as both the supporter as well as the container of refrigerant liquid. Firstly, the topological structure and the detailed fabrication process of a CIT cable are presented. Then, a typical CIT cable is manufactured. The feasibility of this new structure is verified by its critical current test. Moreover, bending characteristics of CIT cables are evaluated and compared with Conductors on Round Core (CORC) cables from the aspect of both simulation and experiments. The superior bending performance enables it to be potential in the future compact magnet application. [ABSTRACT FROM AUTHOR]

The Bi-2212 round wire has a multifilamentary structure, and the use of numerical method to study ac loss of such multifilamentary superconducting wire requires a long calculation time and large memory. Adopting a homogenization method at low frequency to study the ac loss of multifilamentary structural wire/tape has been proven to have high enough accuracy and can save a lot of calculation time. With the expansion of the application range of superconducting materials, however, superconducting power devices exceeding the frequency of tens of kHz have huge potential applications in the future such as wireless power transfer. Therefore, it is necessary to study the applicability of the homogenization model at high frequencies. This article mainly presents an ac loss numerical study of Bi-2212 round wire at wider frequency (50 Hz–500 kHz) by homogenization method. The homogenization is achieved by two different methods. The first method is to treat the superconducting bundle and the Ag matrix between the bundles as a superconducting domain (bundle-matrix homogenized model, BHM). The second method is to regard the Ag matrix between the superconducting filaments and the filaments as a superconducting domain (filament-matrix homogenized model, FHM). In order to evaluate the feasibility of the two different homogenization methods, we compare the calculation results of the two different homogenization models with the original model, OM. The results show that the FHM can effectively improve the calculation speed while satisfying the high calculation accuracy. This conclusion can be used to quickly evaluate the ac energy loss of large-scale superconducting devices at wider frequencies. [ABSTRACT FROM AUTHOR]

Taking the advantage of quench behavior, high-temperature superconductors (HTS), such as YBCO tapes, are gradually used in the resistive superconducting fault current limiters (RSFCLs) for the direct current power system. However, due to the high cost, as well as the mechanical vulnerability, the complexity of HTS still remains. In this article, first YBCO tapes were sustained under different pressures, and then the transient characteristics of the tapes were tested under a huge pulsed current. Results show that while applying pressures on YBCO tapes, the resistance would increase significantly. Thus, a new approach to increase the quench resistance of RSFCLs was proposed, which could potentially reduce the consumption of YBCO tapes and cut down the cost. [ABSTRACT FROM AUTHOR]

Experimental and advanced superconducting Tokamak (EAST) was designed by the Institute of Plasma Physics Chinese Academy of Sciences. The magnet system of EAST includes poloidal field (PF) coils, central solenoid (CS) coils, and toroidal field coils. The heat generated by the ac losses of PF and CS coils increases the burden of the cooling system, which reduces the critical current of the superconducting cable. Once the superconducting magnet quenches, it will pose a serious threat to the stability of the whole device. In this paper, the piecewise linear method and the finite-element analysis were used to analyze the ac loss of the EAST PF and CS coils under the specific operating current. The whole calculation process is described in detail. The final results meet the stability margin of the design of the EAST device. Meanwhile, the ac losses of PF and CS coils for EAST are theoretically evaluated in terms of the structure of magnet, the simplified current, and the distribution of the coil. The results of this paper not only provide data support for the next design and experiment of the EAST device, but provide some references for similar calculation of other large nuclear fusion magnets. [ABSTRACT FROM AUTHOR]

The superconducting fault-current limiter (SFCL) has been effective in coping with large fault currents and is expected to be largely applicated in the power system to solve the problem of increasing fault-current level in power grid nowadays. However, the coordination between SFCLs and traditional power system devices such as protective relaying has not been researched enough. This paper studies the influences of the resistive-type SFCL on the incremental power frequency relay of the transmission lines. The resistive SFCL is simulated by using thermal-electric analog method. A model of a 220-kV transmission system with the incremental power frequency relay element was built and simulated with the SIMULINK MATLAB software. Three compensation methods to eliminate the negative effects of the integration of SFCL are proposed and compared. Finally, numerical simulation tests have demonstrated the correctness and validity of theoretical analyses. [ABSTRACT FROM AUTHOR]

A key factor concerning the superconducting application of HTS tapes is the volume of current they can carry. The resistivity of the HTS tape is theoretically zero if lower than the critical temperature, but it cannot carry an infinitely large current. Therefore, the critical current of different HTS tapes must be evaluated to determine their maximum current carrying capacity. First, this study proposed an experimental structure based on controllable dc current ramp rate (R-value) and the critical current of three kinds of tapes were measured, respectively. Finally, this research provides new insight into the understanding of HTS tapes with the magnetic substrates and the nonmagnetic substrates. [ABSTRACT FROM AUTHOR]

High temperature superconductor (HTS) is becoming more and more attractive for the application in high field magnets, which can be utilized in the magnetic resonance imaging (MRI) and rotating machines. To charge the HTS magnets and operate them in persistent current mode, a persistent current switch (PCS) is very necessary. This paper will focus on how to build a fast persistent current switch controlled by ac magnetic field. The PCS can present a large resistance rapidly compared with conventional PCS and recover to zero-resistance in a short recovery time. Impacting factors and strategy to control the proposed PCS are discussed. An experimental system is built and tests are carried out to verify the practicality of the proposed PCS. [ABSTRACT FROM AUTHOR]

For some high-temperature superconducting (HTS) devices, such as fusion magnet, fault current limiters (FCLs), and superconducting energy magnetic system, the ac loss of the HTS current leads is one of the most important heat sources in the cryogenic system. When HTS current lead is carrying alternating current, ac loss occurs. However, the HTS current leads of the above HTS devices usually carry the nonsinusoidal current, and the ac loss is difficult to be analyzed when the applied transport current is nonsinusoidal waveform. In this paper, we firstly proposed two kinds of structure of HTS current leads, and the ac losses of these two current leads with the specific current waveform were calculated subsequently by using finite element method software COMSOL. Furthermore, the ac losses of two structure current leads were measured by using the integral method, respectively. Finally, the ac loss results of these two structures of current leads were compared and analyzed theoretically and experimentally, which aims to select the preferred structure of current lead for HTS applications in terms of ac loss. [ABSTRACT FROM AUTHOR]

Flux pumps are able to compensate slow current decay in high-TC superconducting (HTS) magnets through noncontact approaches. It is a promising alternative for power sources and thick current leads in operating HTS coils. Following the previous work of a rectifier flux pump, we developed a feedback control system to achieve flexible control of load current. Experimental results show that the flux pump can stabilize load current at a preset level with the help of the control system. Power loss of using the flux pump is also compared with that of using current leads. [ABSTRACT FROM AUTHOR]

This paper presents the design and modeling of a second-generation (2G) high-temperature superconducting (HTS) magnet, which is suitable for Lorentz force electrical impedance tomography (LFEIT). The concept of constructing a superconducting magnet using the configuration of a Halbach array is proposed, which can achieve a favorable thin geometry for the LFEIT system, and is able to build a homogeneous magnetic field with high intensity. The modeling of this superconducting Halbach array magnet has been executed by COMSOL Multiphysics. \boldsymbolH-formulation together with \boldsymbolB-dependent critical current density and bulk approximation are used as the basic tools for modeling. Results reveal that this magnet can achieve over 1-T magnetic field in the cross section required. The optimization of HTS coils' location and different numbers of coils are carried out to balance the strength and homogeneity of magnetic field. [ABSTRACT FROM PUBLISHER]