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In a cryogenic liquid medium, shock waves will be formed by the expansion of compressed gas that has been vaporized due to the injection of a large amount of energy. This may severely threaten the reliability of high- T c superconducting (HTS) apparatus. Several studies have documented the destructive forces of evaporating nitrogen produced by arc energy. However, the properties of and propagation mechanisms relating to pressure waves in liquid nitrogen have yet to be understood. The aim of this study was to clarify the evolution of pressure waves within an enclosed pipeline and reveal the effects of several factors such as the pipe size and the injected energy on the shock-wave impact using explosion dynamics simulations. The results provide evidence for the strengthening of shock waves due to multiple reflection and superposition. In addition, analysis of the pressure impulse and effective strain reveals that, in the case of moderate injected energy, the overall shape of the inside wall of the pipe will remain unchanged except at the points closest to the explosion center; in contrast, the ends of the pipeline may suffer from more severe deformation. Finally, the calculations suggest that the shock-wave impact increases almost linearly with the injected energy, and in logarithmic coordinates, the pressure is inversely proportional to the explosion distance. These findings provide a better understanding of the characteristics and propagation patterns of shock waves in liquid nitrogen, and they lay a foundation for evaluating the safety of HTS cables and energy pipelines. [ABSTRACT FROM AUTHOR]

The influence of the operating parameters such as magnetic field, cathode voltage, and gas pressure on the target erosion of the rectangular planar dc magnetron is discussed in this paper. Through particle simulation for a prototype rectangular magnetron, with the same shape, stronger magnetic field could make the target erosion much sharper; with the same ratio of the field intensity in the end region to that in the straight region, stronger magnetic field could make the anomalous erosion in the curve-out region severer. Furthermore, higher cathode voltage could not only make the target erosion wider but also make the anomalous erosion weaker. Gas pressure has less influence on the target erosion in the straight region, but higher pressure could make the anomalous erosion severer. Finally, how to improve the magnetic field is discussed. Through shimming ferromagnetic and permanent magnet bars in the race track, not only the target erosion becomes wider but also the anomalous erosion could be reduced. Index Terms--Erosion, magnetron sputtering, particle simulation, plasma.

High-temperature superconducting (HTS) power equipment and tests at present require operating temperatures higher than the liquid nitrogen (LN2). LN2 is an important cryogenic refrigerant and liquid insulation material, which is widely used in HTS equipment. Although pressurized LN2 can achieve temperatures higher than 77 K, it requires the use of sealed pressure vessels. Thus, a liquid cryogen that can offer temperatures higher than LN2 at atmospheric pressure, excellent electrical insulation performance, and high thermal conductivity has to be explored. Tetrafluoromethane (CF4) is another cryogenic refrigerant and electrical insulation material. The mixture of LN2 and liquid CF4 has excellent thermal properties. When mixed in appropriate proportions, it has a melting point of 50 K and a boiling point of 145 K. The LN2/CF4 mixture is used in the insulating layer of the superconducting energy pipeline to provide a low-temperature environment (85–100 K) for the HTS dc cable. However, the insulation characteristics of the LN2/CF4 mixture at cryogenic temperatures have not been studied, quantitatively. In this article, a test platform is developed to measure the breakdown voltage of the liquid mixture. The dc breakdown voltages of LN2/CF4 liquid mixture under varying mixing proportions are obtained at cryogenic temperature. The insulation failure probability is estimated using a two-parameter Weibull statistical method. The results reveal that the breakdown strength of the LN2/CF4 mixture is lower than that of pure LN2. As the proportion of CF4 increases, the breakdown field strength of liquid mixture first decreases and then increases. When the mole fraction of CF4 is 60%, the average breakdown strength decreases to the lowest value (14.07 kV/mm), about half of that of pure LN2 (28.26 kV/mm). The results of this article can provide a reference for the applications of LN2/CF4 liquid mixture in HTS power equipment and a scheme to expand the working temperature range of superconducting applications. [ABSTRACT FROM AUTHOR]

High Tc superconducting (HTS) DC cable is a promising solution for large-scale power transmission over long distance. However, the refrigeration system for HTS cable is indispensable but undesirable. Considering that LNG (liquid natural gas) is now one kind of clean energy that is used more and more widely, it is possible to integrate HTS cable and LNG transportation into a single pipeline, resulting in a new energy transportation system- a HTS energy pipeline. In this paper, the principle and structure of the energy pipeline are first discussed and then the basic physical properties of the HTS tapes and liquid insulation medium in the LNG temperature range are analyzed. On this basis, a 10 kV/1 kA energy pipeline is designed, fabricated and tested. [ABSTRACT FROM AUTHOR]

In this article, a general scheme of a 40-kV/2-kA resistive type dc superconducting fault current limiter (SFCL) is suggested, which could be utilized to suppress the surge current caused by short-circuit faults in voltage-source-converter HVdc (VSC-HVdc) power grids. This dc SFCL is designed and fabricated with 6 coil modules in parallel and 4 in series. In order to investigate the current-limiting performance of the dc SFCL under the rapidly rising fault current, the SFCL is tested on the platform with a pulsed dc impact generator. Besides the property of current limitation, the current capacity, insulation and quench recovery performance of such SFCL are also tested. Results indicate that the critical current of SFCL is larger than 2050 A at 69 K, and the dc withstand voltage is larger than 74 kV in 2 h. During the 9-kV dc impact test, the short-circuit current could be limited to 8842 A, and the quench recovery time is less than 200 ms. [ABSTRACT FROM AUTHOR]

As one of the most effective ways for the suppression and interruption of short-circuit current in voltage source converter based high-voltage direct current (VSC-HVdc) power systems, superconducting fault current limiter (SFCL) has received more and more attention, especially the resistive SFCL. However, when the resistive SFCL is in the current limiting state or overload operation, a large amount of bubbles will be generated on the surface of superconducting tape, thereby affecting the insulation performance of the SFCL. Experiments are performed by using a heater-built-in electrode system where bubbles are generated from the surface of the ground electrode. The test can simulate the actual working condition. The effect of bubbles on breakdown voltage of liquid nitrogen (LN2) is investigated under different types of voltage (negative lightning impulse, positive dc voltage, and composite voltage of negative lightning impulse and positive dc voltage) and different pressures. The results show that the breakdown voltage of LN2 decreases with the increase of the number of bubbles regardless the type of voltage, and elevating pressure is an effective way to improve the insulation strength of LN2. All the test data can be used as the theoretical basis for insulation design of resistive SFCL. [ABSTRACT FROM AUTHOR]

It is crucial to calculate the levitation force and model the levitation behaviour in the designation of superconducting magnetic bearing (SMB). At present, the related research focuses on the single degree of freedom (DoF) levitation behaviour of the SMB independently, for instance the axial or radial levitation force. Practically, the superconducting magnetic levitation is passive levitation at directions of multi‐DoF. Additionally, the non‐ideal conditions in fabrication, such as the joint effect of superconducting bulks, lead to complex 3‐D‐induced current distributions which generate the levitation force. There are coupling effects which affect the levitation forces at the directions of different DoFs. This paper investigates the multi‐DoF modelling and levitation behaviour of the radial‐type SMB. By adopting the Power‐law E‐J relation, the dependence of conductivity on electric field is derived to define the superconductive material. Then, the software MagNet is employed to establish the 3‐D multi‐DoF finite element model. Under the external disturbance of the PM rotor with axial constant speed and constant load, the multi‐DoF levitation behaviours of radial‐type SMB are discussed considering the joint effect of superconducting bulks. It is also obtained that the influence of the revolving speed, the static, and dynamic eccentricity on the axial levitation force. [ABSTRACT FROM AUTHOR]

It is usually difficult to obtain the radial levitation force of the radial‐type superconducting magnetic bearing (SMB) due to the massive numerical calculations and the sophisticated experimental measurements. This study presents a method for fast calculating the radial levitation force of radial‐type SMB, which is based on the infinitesimal method and the 2D finite‐element method. During the modelling, the radial‐type SMB is assumed to be composed circumferentially of infinite superconductor‐permanent magnet (SC‐PM) infinitesimals with shape of rectangular thin slice. The dependence of levitation force for the SC‐PM infinitesimal on the air‐gap length between the SC stator and the PM rotor can be obtained using the 2D finite‐element model established in Cartesian coordinate system. Moreover, a simplified analytical model of calculating radial levitation force is developed based on the infinitesimal method and the circumferential distribution formula of non‐uniform air‐gap when the PM rotor exhibits radial eccentricity. The results of theoretical calculation and experimental measurement of radial levitation force show a good agreement, which verifies the feasibility of the proposed method. It has the advantages of fastness and easiness, and can be used as a theoretical tool for the design, optimisation and performance prediction of the radial‐type SMB. [ABSTRACT FROM AUTHOR]

A three-phase 1250-kVA/10.5-kV/0.4-kV high-temperature superconducting (HTS) transformer has been designed and fabricated. The windings of the HTS transformer are wound by copper-alloy-reinforced multifilamentary Bi2223/Ag tapes, and the structures of the primary and secondary windings are solenoid and double pancake, respectively. The three-phase three-limb core is made of domain-refined high-induction-type grain-oriented silicon sheet steel, and the cryostat is made of glass-fiber-reinforced plastics. According to the tests, the no-load loss is 2319.2 W at an exciting current of 0.254% of the rated level, the short-circuit impedance of the transformer is 5.6% of the rated value, and the average load loss is 249.6 W. In addition, the leakage current between the primary and secondary windings is about 12.6 mA under the condition of 35 kV/1 min/50 Hz, and the negative full-wave lighting impulse test with a peak voltage of 75\ \mboxkV/1.0\ \mu\mboxs/50\ \mu\mboxs from the primary winding is carried out without breakdown. The experimental results show that the developed HTS transformer meets the requirements of the Chinese standards, and the demonstration of the HTS transformer at the superconducting power substation since September 9, 2014 has been reliable, showing its potential for long-term operation in the grid. [ABSTRACT FROM AUTHOR]

Because high-temperature superconducting (HTS) coils possess low-resistivity, large-current-carrying, and state transition characteristics, the design of HTS transformers is much different from that of conventional ones. The general design of 10.5-kV/1250-kVA HTS transformer is given first. In the electromagnetic design of the 1250-kVA HTS transformer windings via field-circuit coupling simulation, the influence of leakage magnetic field on the critical current of tapes and the circulating current of parallel branches was considered. Moreover, the transient stability of transformer windings combined with superconducting fault current limiter in the HTS power substation is discussed. Based on electromagnetic analysis, the primary and the secondary windings are constructed, and a special transposed method for double pancakes wound with four parallel tapes is proposed. Finally, the current-carrying ability of the primary and the secondary windings is tested in 77 K. Their critical currents are higher than 200 and 3200 A, respectively, which is enough in rated operating condition. [ABSTRACT FROM PUBLISHER]

Flywheel energy system (FES) is a non-pollution energy storage system which uses high-speed rotation flywheel to store energy. As high temperature superconducting (HTS) bearings have very low friction and self-stability characteristic, they are applied to FES increasingly. In this paper, for the optimal structure of permanent magnet rotor, 3 topologies were presented and their magnetic field characteristics were analyzed. For effective cooling of YBCO bulks, a thermal model of the superconductor magnet stator using alumina shims was set up and its temperature distribution was analyzed. The result shows that the structure of the stator in which alumina shims were put between the neighboring YBCO bulks is better than that without alumina shims, and the temperature of YBCO bulks in the structure is lower. Then a radial-type HTS bearing prototype with outer diameter of 200 mm, inner diameter of 40 mm and height of 120 mm was built. The axial force, the axial stiffness and the levitation characteristics of the device were measured and analyzed in field cooling and zero-field cooling respectively. [ABSTRACT FROM PUBLISHER]

A superconducting magnetic energy storage system (SMES), with stored energy of 1 MJ and compensation power of 0.5 MVA, has been developed successfully, and now is operating at the world's first superconducting power substation at Baiyin National High-Tech Industrial Development Zone, Gansu Province, China. The SMES employs a high Tc superconducting magnet, which consists of 44 pancakes, operates at 4.2 K in liquid helium, and is cooled down by 4 G-M cryo-coolers. The SMES connects to a 10.5 kV power grid by the use of a power conversion system. Since 16 February, 2011, the SMES has been operating reliably, and providing good-quality power for three companies. [ABSTRACT FROM PUBLISHER]

A 0.5 MVA/1 MJ superconducting magnetic energy storage system (SMES) has been installed in a superconducting power substation. The SMES is designed to compensate active power fluctuations, current harmonics and reactive power of the downstream load. The system configurations and control strategies are described. Experimental and field test results show that the proposed SMES can effectively improve the power quality of the power system. [ABSTRACT FROM PUBLISHER]

With the increasing depletion of fossil fuels and growing environmental pressure, the mankind has got known the need to vigorously develop the renewable energy and the energy-saving technology. The high Tc superconducting (HTS) power technology will be very helpful to enhance the stability, reliability, and efficiency and transmission capacity of the power grid which would be dominated by the renewable energy. In this paper, we will report the installation and operation of a 10 kV HTS power substation which includes a 75 m/1.5 kA HTS power cable, a 10 kV/1.5 kA HTS fault current limiter, a 1 MJ/0.5 MVA high Tc SMES and a 630 kVA/10 kV/0.4 kV HTS power transformer. [ABSTRACT FROM PUBLISHER]

High temperature superconducting (HTS) generator was promising in the wind power applications for its advantages in the weight, size and thermal stability against load fluctuations, especially in the “direct-driven” design. A 100 kW generator was proposed for the demonstration of the feasibility of using HTS in 10 MW wind turbine generators. The electromagnetic design and optimization of the rotor were done using finite element method (FEM). The excitation coil of the rotor was designed considering variable working conditions. According to the numerical results, a number of suggestions in the design and winding techniques of the superconducting rotor were proposed. [ABSTRACT FROM AUTHOR]

• N2/CF4 mixture has a wide liquid temperature range from 50 K to 100 K (@ 0.5 MPa with a 60 ~ 70 mol% of N2); • N2/CF4 has a relatively broad HOMO-LUMO gap with the energy of 14.96 eV; • The binding energy of N2/CF4 is −1.758 kJ/mol; • C...N contact in N2/CF4 can be recognized as a tetrel bond; • N2/CF4 mixture has an electric strength superior to N2 but inferior to CF4. Nitrogen / carbon tetrafluoride (N 2 /CF 4) may have a feasibility for use as the coolant or insulator of power apparatus but few studies have been carried out to figure out the characteristics of it. In order to have a better understanding of such mixture, some molecular features and properties concerning N 2 , CF 4 and their compound are calculated through computational quantum chemistry methods and discussed in this paper with the emphasis focusing on molecular surfaces and intermolecular interactions. Concretely, the phase equilibrium curves are predicted according to several parameters obtained through the analysis of molecular surfaces. Furthermore, surface electrostatic potentials (ESPs) are studied and the mutual interpenetration distance was obtained. In addition, in order to have a further understanding of intermolecular interactions between N 2 and CF 4 , the density of states (DOS) and charge decomposition analysis (CDA) are discussed as well as an analysis of electron density difference (EDD) and quantum theory of atoms in molecules (QTAIM). Finally, a prediction of electric strength is performed and it could be found from the prediction that N 2 /CF 4 has an electric strength superior to N 2 but inferior to CF 4. [ABSTRACT FROM AUTHOR]

Highlights • We have studied the characteristics of the YBCO tapes under ms and kA level. • The heat transfer characteristics during the impulse was analyzed with experiments and calculations. • The temperature gradient was studied though simulations. Abstract Compared to the AC application, different research should be emphasized when the superconducting fault current limiter is used in VSC-HVDC. In the AC application, over-current flowing in the superconducting will last for 45 ms–60 ms before the circuit breaker is shut down. Two or three times of the critical current of the superconducting tapes will be enough. And the heat generation will be absorbed a lot by the liquid N 2 during 45 ms–60 ms. However, in HVDC situation, the time will decrease to 4 ms–10 ms and the over current could be ten times of the critical current of the tapes. This paper would explore the threshold value of the YBCO tapes in DC over-current situation and the heat transfer between the tapes and the liquid nitrogen. We obtained the relationships between time and the threshold value of over-current according to the DC over-current impulse experiments under different time conditions. The heat flux of liquid nitrogen with temperature difference was obtained by calculations. Combinations with the experiment results and the temperature-resistance curve, we got the heat transfer law. Finite element simulation model is also established in Comsol to analyze the heat transfer law between the layers during the impulse. The results will be a reference for design of the superconducting fault current limiter (SFCL). [ABSTRACT FROM AUTHOR]

With the fast development of voltage source converter based high voltage direct current (VSC HVDC) power system, an effective fault current method is in demand. In HVDC systems, the short-circuit current generally exceeds the capacity of the fast circuit breaker. Meanwhile, a superconducting fault current limiter (SFCL) is able to limit the short circuit current level. If installed in an overhead line system, SFCL has to meet the requirement of the reclosing time. The fault current is much larger than the critical current of superconducting tapes, which generates huge heat and the temperature of the tapes in the resistive type SFCL will increase sharply. After removing the fault, the tapes need certain time to recover to the superconducting state. Thus, the recovery characteristics in DC faults are an important feature of the resistive type SFCL, and they are mainly determined by the corresponding features of the superconducting tapes. In this paper, a method is presented to measure the recovery time under transient DC over current impulse. The recovery characteristics of high temperature superconducting (HTS) tapes under transient DC over current impulse are studied. The recovery time under different impulse duration and different impulse amplitude are measured by experiments. The influence factors of the recovery characteristics are discussed, which will be useful for the design of SFCL. [ABSTRACT FROM AUTHOR]

• Under a constant axial load, the axial levitation force and displacement vary in a similar form of dynamic variance. Both of them fluctuate sinusoidally: the period keeps constant, while the amplitude decays gradually to be stable. • Even though the steady-state average of axial levitation force equals the constant load, both the load size and the initial velocity drift the work point of the PM rotor. • Subjected to the sinusoidal load with different frequencies, the axial levitation force and the axial displacement present a complex variation rule and no longer sinusoidal law, because of the superposition effect of the natural frequency of SMB and the sinusoidal load frequency. Due to the external disturbances or the time-variable load, it is necessary to study the dynamic levitation behavior in order to guarantee the stable levitation of superconducting magnetic bearing (SMB). This paper investigates the dynamic levitation behaviors of a radial-type SMB under axial load condition. By adopting the Power-law E - J relation, the simplified H -formulation finite element model of a radial-type SMB was established to calculate the axial dynamic levitation behavior with the aid of the PDE module and ODE module in software COMSOL. The experimental measurements of the axial levitation force were performed in the field cooling condition so as to validate the model. Then, the dynamic levitation behaviors of the radial-type SMB were discussed when the PM rotor suffers from the external disturbances including the axial constant load with different initial velocities and the sinusoidal time-variable loads. The results show that both the load size and the initial velocity drift the work point of PM rotor. And the sinusoidal load leads to a complex time-evolution of the levitation behavior. The study is useful to predict the effects of external disturbances on the dynamic levitation behaviors in the practical applications of the SMB. [ABSTRACT FROM AUTHOR]