Two categories of high-temperature superconducting (HTS) tapes, the silver-sheathed BSCCO tape (1G HTS tape) and REBCO coated conductor (2G HTS tape), have been commercialized worldwide and widely used in applications. In this study, the 1G HTS tape produced by Sumitomo and 2G HTS tape produced by Superpower were selected for a systematic comparative study in critical current (I c) performances under various temperature and magnetic field conditions. The anisotropy and I c variation patterns of these two types of samples in the temperature range of 25-77 K, magnetic field from 0 to 7 T and field angle from 0 to 90 degrees were compared and analyzed. The results of this study reveal certain comprehensive characteristic features of these two categories of HTS tapes, supplementing some new systematic information to the literature.

Nitrogen/tetrafluoromethane (N-2/CF4) 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 CF4 to probe into the boiling heat transfer process on an ideal copper substrate. Analyses suggest that the boiling process of the N-2/CF4 mixture shares similar features with the pure N-2, but the binary refrigerant manifests advantages at higher substrate temperatures. Specifically, the additive CF4 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 CF4 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 CF4 in mitigating the mismatch of the vibrational density of states at a high substrate temperature, which may reveal the mechanisms concerning the CF4 improving the heat transfer performance of the original N-2. These findings provide a better understanding of the advantage of N-2/CF4 at high substrate temperatures, and they lay a foundation for designing the cooling systems of HTS apparatuses.

The high-temperature superconducting (HTS) ac and dc cables are gradually finding their way toward commercialization due to their ability to transfer bulk electric power with negligible dielectric loss. In this context, this article critically reviews the different aspects of HTS cables used in various applications and related accessories. The operational, laboratory-scale, and prototype HTS cable projects are discussed in detail. Different dielectrics, electrical insulations, superconductors, cooling systems, cryogens, and upcoming projects of HTS cables are reviewed. The underlying challenges and future directions in ac and dc cables, electric aircraft and ships, associated costs, cryogenic insulation materials, cooling technologies, superconductors, and electrical insulations of HTS cables are discussed in detail. The space charge performance of presently used polypropylene laminated paper insulation and future insulation, i.e., Kapton material, is experimentally analyzed, to show the future challenges in the insulation design. The technology readiness level, market trends, and relevant test standards are explored. The research and market trends suggest that HTS cables can replace conventional transmission methods. However, their consistent growth and adaptation of HTS cables will depend on the design of cryocoolers, raw material cost of superconductors, and large-scale production.

The DC superconducting energy pipeline (DC SEP) is a promising technology, which has the ability to transmit electricity and fossil energy such as liquefied natural gas (LNG) at the same pipeline so that LNG could serve as the refrigerant for the high-temperature superconducting (HTS) cables. The collaborative transportation of electricity and LNG increases the efficiency while lowering the cost. However, the operation performance of the SEP, which is crucial for HTS cables and LNG, is of greater complexity on account of multi-physics interactions. Herein, a +/- 100 kV/1 kA SEP model with electric, magnetic, fluid and ther-mal fields is established in COMSOL Multiphysics to analyze the temperature distribution of SEP via parametric scanning on SEP heat leakage and LNG flow rate. Finally, the relationship between temperature rise and LNG flow rate of a SEP has been estimated based on the interactions of the multi-physics fields. The results indicate that the temperature rises by 11.6 K for every kilometer of SEP. Moreover, the influences of heat leakage and LNG flow on temperature rise are revealed. Temperature rise increases pro-portionally with heat leakage and it decreases not monotonously with LNG flow rate. This study validates the feasibility of SEP and provides the theoretical references for the demonstration of SEP.

采用实体电缆制作了典型气隙缺陷模型,利用脉冲电流法研究不同电压下气隙缺陷的直流局部放电特性。根据不同外施电压下气隙缺陷放电量Q和放电重复率N随时间t变化的图谱,提取出两类典型统计特征H（q）和H（Δt）。结果表明,气隙缺陷的局部放电起始电压为-14.5 kV;气隙缺陷的Q-t图谱呈"山丘"状,放电量Q和放电重复率N随外施电压升高而升高;随着外施电压升高, H（Δt）的偏斜度和峰度值逐渐增大,表明放电密度随外施电压升高而增加。研究结果为直流XLPE输电电缆局部放电研究提供了数据基础和理论依据。

随着高温超导高压直流输电技术的高速发展,电压等级和输电容量进一步提升,输电系统在低温下的绝缘问题已成为研究的热点。环氧树脂作为高温超导直流输电系统终端的关键绝缘材料,在高场强下产生空间电荷以及绝缘的老化击穿问题,严重制约着超导输电系统的发展,而提高环氧树脂在低温下的直流电气性能已成为高温超导直流输电系统安全稳定运行的关键因素。为此,本文以纯环氧树脂为基体,通过掺杂表面修饰过的纳米Al2O3颗粒,制备不同Al2O3含量的纳米复合电介质试样,对试样进行理化性能的表征分析,并且对纳米复合电介质低温和常温下的直流电气性能进行了试验研究以及理论分析。理化分析测试表明,使用KH560处理过的纳米Al2O3失重过程分为物理失重和化学失重两个阶段,体现出良好的表面修饰作用。从试样的断面结构表征发现Al2O3含量为1wt%和3wt%的试样内部纳米颗粒分散比较均匀,而5wt%的试样会出现微米级的团聚。热稳定性分析发现添加Al2O3颗粒在5wt%以内时,对环氧树脂热稳定性能影响不大。电气性能测试表明,随着Al2O3含量的增多,直流击穿强度先增大后减小,且试样在低温下的直流击穿强度比常温高,低温下击穿强度试验结果的分散性更小。表面电位的初始值随着纳米Al2O3颗粒含量的增加先增大后减小,且纳米颗粒的加入能够使表面电位衰减变慢;此外,低温下的表面电位初始值高于常温;添加Al2O3后陷阱能级范围增大,陷阱中心能级升高,陷阱密度随着纳米Al2O3颗粒的含量先增大后减小;低温下的陷阱中心能级和陷阱密度与常温环境下相比均出现小幅增大。电压上升时间越短,越容易引发直流电树枝;直流电压下环氧树脂及其纳米复合电介质的电树形态呈现出多枝状和单枝状,低温下为单枝状的概率更大,Al2O3颗粒的含量对电树枝形态无明显影响;纳米Al2O3能够降低电树枝的引发概率,减小电树枝的生长速度,抑制作用随着Al2O3颗粒含量的增加而增强;低温下直流电树枝的引发概率和长度均低于常温;在起树阶段以及电压上升和下降阶段均出现了较高幅值的局部放电脉冲簇,Al2O3颗粒的添加会抑制材料的局部放电活动,低温下可测量到更高的局部放电幅值,但局部放电的重复率明显降低。

This article presents the effect of dielectric barrier discharge (DBD) plasma treated nano-Al2O3 particles on the dc insulation properties of epoxy resin (EP) nanocomposites at cryogenic temperature.The surface of the nano-Al2O3 particles was treated by a designed DBD plasma setup. The pure EP, 3 wt% untreated EP/nano-Al2O3 composite, and plasma-treated EP/Al2O3 nanocomposites with 3, 5, and 10 wt% were prepared by a physical mixture and casting method. The scanning electron microscopy and energy dispersive X-ray spectroscopy were used to investigate the dispersion of the nano-Al2O3 and elemental composition, respectively. The insulation properties such as DC breakdown strength, surface potential decay (SPD), dc conductivity and thermally stimulated depolarized current were measured under a cryogenic environment using cryogenic thermostat. The obtained results showed that under the cryogenic environment, the dc breakdown strength of the EP was evidently enhanced by the addition of the plasma treated nano-Al2O3 up to 5 wt%, but that was decreased for 10 wt%. The dc conductivity and SPD rate were reduced with the addition of the plasma treated nano-Al2O3 up to 5 wt%. It was found that the plasma-treated EP/Al2O3 nanocomposites had a deeper energy level and higher trap density compared to the pure EP. It was elucidated that the enhanced dc insulation properties of the plasma-treated EP/Al2O3 nanocomposites was attributed to the reduced charge carrier mobility and stronger chemical bonds in the materials.

With the development of superconducting energy pipelines as well as the increase in voltage levels and transmission energy, the insulation performance of the cable terminal greatly affects the safety and reliability of the system. Therefore, it is crucial to further study the characteristics and mechanism of surface flashover for insulating materials in liquid nitrogen. In this paper, a non-uniform electric field was constructed by applying a negative polarity DC voltage, and the evolution of the surface flashover characteristics under consecutive discharges was obtained using photoelectric integration diagnostics, considering the influence of various parameters such as hydrostatic pressures, electrode materials, and electrode gaps. The results indicate that the flashover voltage rises significantly by increasing the gap length or the pressure. Furthermore, compared with aluminum and lead, copper electrodes can cause surface flashover with much higher inception voltage. More importantly, the flashover voltage decreases continuously during the consecutive discharges and eventually reaches saturation, but the insulation strength can fully recover after removing the surface charges. Therefore, the periodic removal of the accumulated charge from the superconducting terminals is considered to contribute to the achievement of stable insulation properties. ©2022 Chin.Soc.for Elec.Eng.

For some energy storage devices, an efficient connection structure is important for practical applications. Recently, we proposed a new kind of energy storage composed of a superconductor coil and permanent magnets. Our previous studies demonstrated that energy storage could achieve mechanical -> electromagnetic -> mechanical energy conversion with high efficiency and low loss. Additionally, we have found an optimized configuration of magnets to increase the capacity of the energy storage/conversion device. This article discusses a series connection structure to further enhance the capacity of the energy storage device. Two sets of experiments were carried out to investigate the effectiveness of the connection structure. The experimental results indicate that the energy capacity of a series connection of two such storage units will be the sum of the two units. The energy conversion efficiency is not compromised in the connection structure. Based on the results, it was concluded that an energy storage array with a much larger capacity may be achievable by a proper serial and parallel arrangement of multiple such energy storage/conversion units.

Up to today, a coil made of high temperature superconductor (HTS) usually needs a soldered joint to form a closed circuit. It is very difficult to have the joint fully superconducting. The Ohmic joint will cause Joule loss when the coil carries a quasi-persistent current. As a result, the electromagnetic energy stored in an HTS coil declines with time. We propose an approach to reduce the Joule loss of an HTS coil during the energy storing stage. The principle of the approach is to tune the current of the HTS coil smaller by introducing an iron core into the coil to increase the inductance of the coil. With this approach, the Joule loss on the HTS coil will be significantly reduced and the initially stored energy in the coil will be kept with little attenuation in a relatively long energy storing period. When a larger current is required for some functions, the current can be almost returned to the initial charged value by removing the iron core from the HTS coil. In this paper, we report our results in analytical deduction and experimental verification of this principle. Besides, the current value can be tuned to any value in a certain range by controlling the position of the iron core inside the HTS coil. This feature may be useful for some applications in which the current or magnetic field needs to be adjusted. © 2022 Author(s).