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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]

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]

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]