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 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。

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).

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).

Our previous studies had proved that a permanent magnet and a closed superconductor coil can construct an energy storage/convertor. This kind of device is able to convert mechanical energy to electromagnetic energy or to make an energy conversion cycle of mechanical -> electromagnetic -> mechanical. In this study, we focus on the investigations into the application potential of this kind of device. First, we confirmed our proposed optimized configuration with theoretical analysis and experiments on a scaled-up testing platform. Furthermore, a new prototype with a large permanent magnet and a grouped coil composed of three separated closed superconducting coils was built and tested. It was proved that both practical meaningful mechanical force and energy storing capacity could be achieved with this kind of device by using enlarged pair of magnet and superconductor coil. Finally, we investigated the attenuation characteristic of the current in the superconducting coil at a stable energy storing state for a duration of about two hours, which shown the attenuation being practically tolerable for an energy storage with a typical charging-discharging cycle less than a few hours. It is concluded that this kind of device is of some advantages and promising application potentials as a short-term energy storage, particularly to replace fly-wheels in the case of mechanical -> electromagnetic -> mechanical conversion.