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The development of microwave heating technology promotes the improvement of industry quality and efficiency, but the drawbacks of microwave heating, such as uneven heating, prevent its wide application in industry. In this paper, a double‐layer ring tube is proposed for improving the heating uniformity and achieving the target temperature. The gap between the inner and outer tubes is evenly divided into four regions, and the cooling liquid flows through the gap to control the temperature of the heating target placed in the inner tube. Maxwell's equation, heat conduction equation, Navier‐Stokes equation and the mass conservation equation are coupled to simulate the heating process, and the experimental data and simulation data are compared to verify the correctness of the model. Meanwhile, the heating uniformity of structure with or without heating exchanger is compared, and the effect of different fluid velocities, different sizes of the reactor and different dielectric constant of heating target on the heating performance is discussed. [ABSTRACT FROM AUTHOR]

Partial discharge (PD) is the main factor that can cause failure of the cold dielectric insulation system of superconducting cables. Nine insulation model cables with different insulating materials and wrapping styles are prepared. PD experiments under liquid nitrogen and room temperature are carried out on these model cables. Assuming that the lifetime of superconducting cable is 35 years, 40 thermal cycles from 77 –323 K are carried out on three model cables. Experimental results show that the PD is 100 times lower in liquid nitrogen than at room temperature for both polypropylene-laminated paper (PPLP) and polyimide (PI). Compared with model cables that used PI, the ones that used PPLP show much higher inception electric field. And no decay in partial discharge inception electric filed (PDIE) is found during the thermal cycles. The study will be helpful to the insulation system design for superconducting cable. [ABSTRACT FROM AUTHOR]

The resistive-type superconducting fault current limiter (R-SFCL) is simple in configuration, near zero impedance at normal state, and can be quickly triggered by the fault current. So, it is a possible solution to suppress the fault current in the power grid. In this paper, the design and development of a 220-kV R-SFCL were reported. The core component of it is a module constructed by 128 bifilar coils. The bifilar coils are connected in series and parallel to form the module according to the application requirements. The bifilar coils are made of 12-mm-wide steel-stabilized 2G tapes, while 16 bifilar coils are connected in parallel to form a sub-module and 8 sub-modules are connected in series to form the module. For this fault current limiter, 10.75-km 2G tapes are used in total, and the resistance of the R-SFCL module is 4.5 Ω at 300 K. Besides, the half-size sub-module and the cryogenic insulation support (CIS) are developed and tested. [ABSTRACT FROM AUTHOR]

The resistivetype superconducting fault current limiter (R-SFCL) is simple in structure, near zero impedance at normal state and can be quickly triggered by the fault current. Thus, it is a possible solution to limit the fault current in the power grid. In this paper, we report the fabrication and test for an R-SFCL module The R-SFCL module consists of seven bifilar coils connected in parallel, which were wound with two parallel 2G tapes. A 5mm spacer was co-wound between adjacent turns to act as the highvoltage insulation. In the tests, we test the current distribution among the bifilar coils, and then simulated short-circuit tests were conducted on the module to obtain its performance under fault current surge. [ABSTRACT FROM AUTHOR]

Resistive type superconducting fault current limiter (R-SFCL) is one of the most promising candidates to suppress fault currents in power grids. However, the violently boiled cryogen liquids, commonly liquid nitrogen, occurred due to fault currents, poses a crucial cryogenic insulation challenge for R-SFCL. Here we report the design and development of a Cryogenic Insulation Supporter Module (CISM) for a 220 kV R-SFCL. First, we present the design and test of the prototype Cryogenic Insulations Supporter (CIS). The developed CIS passed the high voltage tests under operating conditions. Following on this, we then present the design of CISM, which consists of 14 CISs, with the length and outer diameter of 12 m and 2.5 m, respectively. [ABSTRACT FROM AUTHOR]

A 10 kV High Temperature Superconducting power substation (10 kV HTS substation), supported by Chinese State 863 projects, was developed and has been running to supply power for several factories for more than two years at an industrial park of Baiyin, Gansu province in Northwest China. The system of the 10 kV HTS substation compositions, including a HTS cable, a HTS transformer, a SFCL, and a SMES, are introduced. The SMES works at liquid helium temperature and the other three apparatus operates under liquid nitrogen condition. There are mainly four types of heat losses existing in each HTS apparatus of the 10 kV HTS substation, including AC loss, Joule heat loss, conductive heat, and leak-in heat from cryostat. A small quantity of AC loss still exists due to the harmonic component of the current when it carries DC for HTS apparatus. The principle and basis for analysis of the heat losses are introduced and the total heat loss of each apparatus are calculated or estimated, which agree well with the test result. The analysis and result presented are of importance for the design of the refrigeration system. [ABSTRACT FROM AUTHOR]

Abstract: Poloidal Field (PF) coils, made of CICC (Cable-In-Conduit Conductors), are the most important part of ITER (International Thermonuclear Experimental Reactor) superconducting magnet system. The structure of superconducting magnet system in ITER and the CICC are introduced. The main factors influenced the cabling of the superconducting cable for CICC is discussed. Analyzed the outer diameter controlling technique and the relationship between ac loss and structure parameters of the superconducting cable. The technique route of cabling has been established. Especially, the technique of dies setting for shaping+multi-rolling compacting dies+half-dies for diameter keeping is explained in detail, which can efficiently control the diameter and the cabling quality of the superconducting cable, without injury to strands line. The technical parameters for cabling is fixed and one 765m long dummy cable and one 115m long superconducting cable of CICC for PF coil is manufactured successfully for ITER the last. [Copyright &y& Elsevier]

Renewable energy sources (RESs) are generally connected to the grid through power electronic interfaces, which generate electrical power instantaneously with little inertia. With the increasing penetration of RESs, the grid will gradually develop into a low inertia and underdamped power system, which results in serious grid frequency stabilization problems. The virtual synchronous generator (VSG) is an emerging technology that mimics the operation characteristics of traditional synchronous generators (SGs). Virtual inertia and damping are therefore introduced, which help to stabilize grid frequency. This paper gives a comprehensive overview of the VSG. The basic operation principle of VSG is introduced and analyzed in depth. The key issues related to VSG are summarized and discussed, including hardware configuration, software control strategies, energy supporting methods, and typical applications. [ABSTRACT FROM AUTHOR]

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]

A 10 kA/360 m warm-dielectric high-temperature superconducting direct current (DC) power cable system (10 kA cable), supported jointly the Chinese government and industrial enterprise, was developed and has been operating as a branch circuit to transmit power for a 320 kA aluminum electrolyzing production line for more than 10 months at an industrial plant in central China. Both the 10 kA cable and its supporting system of the cable system are introduced. The cryogenic system for the 10 kA cable adopts closed loop and the sub-cooled liquid nitrogen is forced to flow inside by a pump. The design of corrugated cryogenic envelope pipe is modularized and every independent module has two standardized joints, which makes it easy to integrate with the other pipes and the terminations. The heat loss sources and the structure including both the termination and the cryogenic envelope pipe of the 10 kA cable are discussed. The total heat loss of the 10 kA cable excluding the loss of cryogenic pipe for liquid nitrogen backward flowing is designed to be less than 1698 W at 10 kA, and the heat loss was compared and discussed with that of the aluminum bar. The field test and commissioning of the cable show that the 10 kA cable performs steadily and its heat loss is less than the expected value. [ABSTRACT FROM AUTHOR]

Nitrogen/tetrafluoromethane (N 2 /CF 4) has emerged as an effective refrigerant mixture, which plays a significant role in various high- T c superconducting (HTS) power devices and energy systems. Several studies have documented the superior performance of such zeotropic binary mixtures in heat transfer and energy conservation compared to pure liquids. However, boiling heat transfer characteristics of liquid mixtures on the molecular scale are not fully understood, especially the mechanisms associated with the role of the additive in regulating thermal properties. Here, we performed the molecular dynamics simulations for the pure N 2 and the mixtures containing 20 and 40 mol% of CF 4 to probe into the boiling heat transfer process on an ideal copper substrate. Analyses suggest that the boiling process of the N 2 /CF 4 mixture shares similar features with the pure N 2 , but the binary refrigerant manifests advantages at higher substrate temperatures. Specifically, the additive CF 4 delays the onset of film boiling, and the operational temperature range could be enlarged by almost 40% compared to pure N 2 in terms of the 40 mol% mixture. Moreover, the mixture with CF 4 additive maintains a small interfacial resistance even if the substrate temperature exceeds the critical value of film boiling for pure N 2 , highlighting the potential to use such mixtures for devices that may suffer from high heat flux levels. Finally, analyses regarding the mixture composition and the solid-liquid interactions confirm the essential role of the additive CF 4 in mitigating the mismatch of the vibrational density of states at a high substrate temperature, which may reveal the mechanisms concerning the CF 4 improving the heat transfer performance of the original N 2. These findings provide a better understanding of the advantage of N 2 /CF 4 at high substrate temperatures, and they lay a foundation for designing the cooling systems of HTS apparatuses. • The boiling heat transfer of N 2 /CF 4 liquid mixture was investigated via molecular dynamics simulations. • The CF 4 delays the onset of film boiling and enlarges the operational temperature range. • The mixtures containing CF 4 maintain efficient heat transfer at high substrate temperatures. • The additive CF 4 mitigates the VDOS mismatch, playing a vital role in promoting heat transfer performance. [ABSTRACT FROM AUTHOR]

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]

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]

• The flashover voltage and current of G10 in LN 2 are obtained through experiments and the flashover data is analyzed by Weibull model. • The flashover process is observed by OES, and the surface evolution after G10 flashover is analyzed by optical microscope and FTIR. • The relationship between polymer flashover and trap in LN 2 is established through test of surface trap distribution and calculation of charge transport equation. The insulation failure is a bottleneck to restrict the safe operation and development of direct current (DC) superconducting power devices. The solid–liquid flashover test is an important evaluation index, but the previous researches mostly focus on engineering application, and less involve in the mechanism analysis. As a characterization method for defects existing in the dielectric, the trap has been applied to the study of flashover to occur polymer surface in liquid nitrogen (LN 2) in recent years. However, the mechanism of trap action in solid–liquid flashover is still confusing. Therefore, this paper carries out the following research. Firstly, the flashover voltage and current of glass epoxy laminate sheet (G10) in LN 2 were obtained through experiments and analysis using the Weibull model in this paper. Secondly, the flashover process of G10 was monitored. The changes of G10 surface morphology and chemical bond before and after flashover were also observed. Finally, the relationship between polymer flashover and trap in LN 2 was analyzed based on the surface trap distribution test and theoretical calculation of the charge transport equation. The results are beneficial to guide the selection and performance improvement of dielectric materials and the design of insulating structures in superconducting devices. [ABSTRACT FROM AUTHOR]

Abstract: High temperature superconducting (HTS) DC power cable shows a wide application prospect in the field of power transmission for its nearly lossless and rather high capacity. A 360m/10kA HTS DC power cable system, which connects the rectifier output of a substation with the bus bar of an electrolytic aluminium cell, will be put into operation at Henan Zhongfu Industrial Co., Ltd. As one of the items in this project, a 5m/10kA HTS DC power cable was developed, which is used to investigate the conductor design, fabrication, current-carrying capacity and stability of the 360m/10kA HTS power cable. The HTS DC power cable core consists of five conductor layers wound with spliced Bi-2223 wires with the length of 600m. The cable core has five layers and 23 conductors in each layer with the outer diameter of 45.42mm. The superconducting power cable is fabricated and tested. The critical current is about 14.3kA at 77K. The superconducting power cable is charged to 10kA with rate of 10A/s and operates at steady-state for 30min. In this paper, the 10kA HTS DC power cable design, fabrication and test are presented. The experimental research of the performance of spliced superconducting wire and charging, steady-state operating performance of the cable was carried out. [Copyright &y& Elsevier]

The new energy revolution which will be dominated by renewable energy will need to develop a corresponding new power grid. Based on the characteristics of renewable energy resources, and the stability problem of AC network, it was proposed that the DC-based transmission grid should be developed in the future. The high Tc Superconducting (HTS) power cable will be a competitive candidate for large-capacity power transmission of renewable energy. In this paper, the progress of the development of a 360 m/10 kA HTS DC power cable is presented, and the prospects of HTS cable is discussed. [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]

The effects of local characteristics on the performance of full length Bi2223 multifilamentary tapes are investigated computationally and experimentally at 77 K and self-field. Generally the current–voltage characteristics of superconductors are described by the standard power law model with parameters such as critical current Ic and index n. By measuring the critical current {Ici} and index {ni} values of local tapes, we can get the critical current Ic and n value of full length tapes by means of statistical method. The results show that the distribution of local critical currents are non-uniform, and local critical currents have important effect on the performance of the entire tapes. The critical current of the entire tape is different from the mean value of local critical current based on Gaussian statistical distribution along the long tape. [Copyright &y& Elsevier]

• The temperature distribution of an energy pipeline is simulated and tested. • A simulation model of the energy pipeline principle prototype is proposed. • The factors and laws that affected the temperature distribution are obtained. • An empirical formula to calculate the outlet temperature is achieved. Superconducting DC energy pipeline is an energy channel that simultaneously transmits electrical energy as well as the liquid fuel. It improves the efficiency of energy transmission and reduces the overall cost by cooling high-temperature superconducting DC cables with the low-temperature liquid fuel, such as liquid hydrogen, liquefied shale gas, and liquified natural gas (LNG). In order to properly design this, it is very important to know the temperature distribution along the pipeline, which is yet to be carried out by the researchers. In this paper, a 10 m energy pipeline with LNG cooling and having a concentric nested structure is simulated and tested experimentally. The simulation results of the temperature rise along the pipeline were in good agreement with the experimental results. The effect of various parameters on the pipeline temperature were also obtained. Finally, based on the theoretical equations and simulation results, an empirical formula is proposed to calculate the temperature at the outlet of the energy pipeline. [ABSTRACT FROM AUTHOR]

• Based on the requirement of integrated transportation of electricity and LNG, an effective structure with high safety and efficient heat exchange is proposed. • The HTS tape and insulation medium used in LNG temperature range are discussed. • Based on the comparison of different types of bipolar cable and pipeline, the overall design scheme of ±100 kV/1 kA superconducting DC energy pipeline with optimized comprehensive performance is given. High Tc Superconducting (HTS) DC cable is a promising solution for large-scale power transmission over long distance. However, the indispensable refrigeration system for HTS cable might be undesirable. Considering that LNG (liquid natural gas) now is one kind of clean energy that is used more widely and widely, it is possible to integrate the HTS cable and LNG transportation into an energy pipeline, resulting an acceptable new energy transportation system. In this paper, the principle and structure of the energy pipeline are discussed firstly, and then the basic physical properties of HTS tapes and liquid insulation media in LNG temperature range are analyzed. On the base as above, the general design of ±100 kV/1kA energy pipeline is made. [ABSTRACT FROM AUTHOR]