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For extra-high-voltage high-temperature superconducting (HTS) cables, not only high electrical insulation performance but also low dielectric loss is required. Therefore, we focus on a porous synthetic paper with lower dielectric loss, e.g., Tyvek polyethylene nonwoven fabric, than that of polypropylene laminated paper (PPLP), which has been used for a conventional HTS cable insulation system. In this paper, we compare partial discharge inception characteristics for different liquid nitrogen/synthetic paper composite insulation systems. The partial discharge inception electric field strength in the composite insulation system depends on the electric field strength in the porous area of synthetic paper.

Breakdown (BD) and partial discharge (PD) characteristics of insulating paper and transformer board materials are investigated as cost-effective alternatives to high-price materials like polyimide (PI) or polytetrafluoroethylene and fiber reinforced plastic materials typically used in superconducting devices based on high temperature superconductors cooled with liquid nitrogen (LN 2 ). Three different cotton and cellulose based insulating papers are tested in LN 2 at atmospheric pressure and compared to synthetic sheet materials and polypropylene laminated paper (PPLP). Additionally, the BD performance and PD characteristics of three different transformer board materials are investigated in LN 2 . The results reveal that the paper and board materials have nearly the same or even slightly higher BD voltages in LN 2 in comparison to the values in insulating oil. The pure paper materials with thicknesses of 130 μm have a dielectric strength of 70-90 kV/mm, whereas values of 130-140 kV/mm are measured for PPLP and PI with nearly the same thickness. The dielectric strength of the board materials has a maximum value of 76 kV/mm. In the frame of the investigations, the BD performance of the pure paper materials in LN 2 is not influenced by the water content of the materials, dried and undried specimens have nearly the same BD voltage. Only the PPLP material has a 15% higher BD voltage in the dried state in comparison to the undried state.

For extra-high-voltage high-temperature superconducting (HTS) cables, not only high electrical insulation performance but also low dielectric loss is required. Therefore, we focus on a porous synthetic paper with lower dielectric loss, e.g., Tyvek polyethylene nonwoven fabric, than that of polypropylene laminated paper (PPLP), which has been used for a conventional HTS cable insulation system. In this paper, we compare partial discharge inception characteristics for different liquid nitrogen/synthetic paper composite insulation systems. The partial discharge inception electric field strength in the composite insulation system depends on the electric field strength in the porous area of synthetic paper.

In this paper, seven groups of sheet insulation for use in liquid nitrogen immersed power transformers have been tested under three different standard electrodes from ASTM D149-97a. The seven groups consist of three different types of paper/film, nonadhesive Kapton 100 HN (Kapton), THERNOMID 14.20 paper (NMN), and Nomex 410 paper (Nomex). The porous paper, Nomex 410, is found to have ac dielectric strength of 62-68 kV/mm, which is likely to be independent from its thickness. The nonporous film, Kapton, shows the dielectric strength of 288 kV/mm, whereas NMN paper has a dielectric strength of 74-78 kV/mm, larger than Nomex with similar thickness. Liquid nitrogen gaps near the triple junction of electrodes as well as liquid nitrogen pockets between the fibers are the main sources of partial discharge (PD) for NMN and Nomex. For Kapton, PDs are more likely to occur in the cavities filled with liquid nitrogen, either on the surface of the electrode or around the impurities on the surface of the film. Electrodes with a small stressed volume are more appropriate for dielectric tests of thin paper/film insulation, such as Kapton, due to the lower influence of cavities on the electrode surface. In contrast, electrodes with a large stressed volume and uniform electric field distribution are suitable for the papers such as Nomex and NMN.

One of the key components for dc HTS cable for long-distance line is a stop joint box (SJB). Up to now, the SJB has not been commercialized yet due to some difficulties of dc electrical insulation matters in LN 2 . Basically, the structure of SJB would be similar to the SJB of oil-filled cable because they adopt liquid insulation and sectionalized cooling separations. The insulation structure of conventional SJB based on ac electric field was no more suitable for dc; hence, there is a need to design suitable insulation structure, which can be applied for dc. In this work, in order to find a solution to reduce dc electric field concentration on the polypropylene laminated paper (PPLP) above epoxy spacer, double layer structure composed of PPLP and Kraft were suggested. To verify the effectiveness of this structure for relieving dc electric field, simulation and experiments were performed. From analytical works, the Kraft layer between PPLP and epoxy has shown noticeable electric field mitigation. Furthermore, surface breakdown tests considering PPLP/Kraft combined layers were conducted to verify the insulation properties of double layer structure. Finally, it was deduced that Kraft layer was effective to enhance surface breakdown voltage. Surface breakdown characteristics of PPLP and Kraft were relevant to the surface roughness of insulating materials.

It is crucial for the successful application of high-temperature superconducting (HTS) power transformers that they are able to survive under abnormal operations, such as a short-circuit fault. Modeling, based on the design of a four-winding three-phase HTS power transformer, suggests severe boiling will occur on the surface of the winding during a short-circuit fault and will jeopardize the turn-to-turn insulation of the HTS power transformer. In this paper, a turn-to-turn insulation breakdown experiment platform involving the introduction of boiling on the surface of electrode has been built. Nomex 410 paper, one of the candidates for turn-to-turn insulation in the design of the HTS power transformer, was tested. The characteristics of dielectric strength of Nomex 410 in three different thicknesses (0.13, 0.18, and 0.25 mm) have been investigated, without boiling, as well as under different boiling situations, in an open bath of liquid nitrogen. The voltage application methods applied during both experiments test the electrical breakdown of Nomex 410 under a dynamic situation. The experiment results show that when under film boiling, compared to the test results for nucleate boiling, the dielectric strength of Nomex 410 paper decreases by approximately 38% of the value when no boiling is involved. Moreover, it is verified that the presence of an electric field accelerates the heat transfer between the liquid nitrogen and the heated electrode.

Studies on developing an HTS (High-Temperature Superconducting) power cable with high transmission capacity and less loss are actively being performed worldwide. In Korea, an HTS power cable is being developed which applies cold dielectric type that a cable core wrapped by insulating papers is impregnated in liquid nitrogen. The insulating paper used in cold dielectric type is LPP (Laminated Polypropylene Paper) which has less dielectric loss and a superior dielectric property. In this paper, AC and lightning impulse breakdown strength and V-t characteristics with mini-model cables which were manufactured as the same structure with an HTS power cable were researched. Also, a thickness of insulation layer for a 154 kV class HTS power cable was designed as a result.

High gradient magnetic separation (HGMS) for purification of wastewater from paper factory has been developed and withstand for 1 year operation. The system developed is a high gradient magnetic separation (HGMS) system with superconducting magnet which purifies wastewater continuously from a paper mill. The magnetic field generated by the superconducting magnet for HGMS is 2 T at the center of the bore, operating current 100 A, 680 mm long and 400 mm bore NbTi solenoid. The developed system which treats wastewater of 2000 t/day has been confirmed to show the expected performance. The purpose of the developed system is to separate COD constituents from the wastewater efficiently and economically in a small space. The required performances are follows, i) the set-up area is less than 6 m times 6 m, ii) COD of the treated water is less than 100 ppm and water can be recycled, and iii) a system can treat wastewater more than 2000 t/day. The design and the performance of the system are reported.

Paper is widely used in various engineering applications due to its physical properties and ease of manufacture. As a result paper has been selected or designed as an electrical insulation material for parts and components in high voltage technology. In the current study we select a paper employed in conventional transformers as the electrical insulation material. The potential of this paper is investigated at cryogenic temperatures to determine its physical properties for high temperature superconducting power applications. Dielectric measurements were performed using impedance spectroscopy at a constant frequency. Dielectric breakdown tests were performed on samples at 77 K using a liquid nitrogen bath.

As polypropylene laminated paper (PPLP) is usually wound around the high-voltage conductor of a dc high-temperature superconducting power cable several times, it is needed to determine whether electrical conductivity of PPLP could be increased according to the number of layers. In thin films without layers, electrical conductivity increases according to the thickness of materials. For this reason, the effect of the number of PPLP layers on electrical conductivity needs to be verified because the dc electric field distribution is highly influenced by electrical conductivity. In this paper, to determine the effect of PPLP layers on electrical conductivity, we measured the conductivity of specimens with single, double, and quadruple layers of PPLP in liquid nitrogen (LN2), and compared the results according to layer number. In addition, we measured the electrical conductivity of specimens with single, double, and quadruple layers of Kraft to verify if there was a layer effect or not. The electrical conductivity of the PPLP and Kraft specimens did not change with an increase in the number of layers. Therefore, we concluded that the electrical conductivity of single-layered PPLP and Kraft specimens can be used in dc electric field simulations.

110 kV cold dielectric high temperature superconducting (HTS) cable has been developing in China. Generally, the lapped dielectric structure is adapted for cable body, so dielectric characteristics of dielectric sheets are very important. Polyimide, Polypropylene laminated paper (PPLP) and polyimide and poly tetra fluoro ethylene (PTFE) are widespread dielectric sheet materials for HTS equipments with lapped structure. In this paper, the lightning impulse dielectric strengths of three materials under tensile statuses are tested by a special electrode system. The experimental results are processed by method of Weibull plot. As a result, the impulse dielectric strengths of all three materials have good adaptive capacity for tensile stresses in liquid nitrogen. The impulse dielectric strengths of polyimide are highest of three materials under normal using stresses. The AC dielectric strengths of three materials are also concluded from previous papers. Combining experimental results, data in previous papers and design methods related, the multi-layered dielectric thickness and layers of PPLP used for 110 kV cold dielectric HTS cable body are designed.

The determination of the neutrino mass is still an open issue in particle physics. The calorimetric measurement of the energy released in a nuclear beta decay allows measuring the whole energy, except the fraction carried away by the neutrino: due to the energy conservation, a finite neutrino mass m ν causes the energy spectrum to be truncated at Q-m ν , where Q is the transition energy of the decay. The electron capture of 163 Ho (Q ~ 2.5 keV) is an ideal decay, due to the high fraction of events close to the endpoint (i.e., the maximum energy of the relaxation energy spectrum). In order to achieve enough statistics, a large number of detectors (~10 4 ) are required. Superconducting microwave microresonators are detectors suitable for large-scale multiplexed frequency-domain readout, with theoretical energy and time resolution on the order of electronvolts and microseconds, respectively. Our aim is to develop arrays of microresonator detectors applicable to the calorimetric measurement of the energy spectrum of 163 Ho. Currently, a study aimed at the selection of the best design and material for the detectors is in progress. In order to obtain low-Tc detectors, with Tc ranging between ~0.5 and 2 K, different Ti/TiN (titanium nitride) multilayer films were produced. The reduced Tc was obtained by superposing thin layers of stoichiometric TiN to pure Ti layers, and the Tc was tuned by varying the ratio between the thickness of the layers. In this contribution, a comparison between the measurements (critical temperature, gap parameter, and X-ray energy spectra) made with stoichiometric and substoichiometric TiN and Ti/TiN multilayer film microresonators is presented.

It is environmentally important to recycle the wastewater, since paper factories use a large amount of water and equivalent amount of wastewater is generated. Conventional water treatment facilities like precipitation process need large-scale equipment and wide space to purify the wastewater of paper factory. In case of massive waste water, high gradient magnetic separation (HGMS) parts are more effective to purify it rapidly and to occupy relatively small space, since large voids at filter with HGMS are adopted. In this respect, we prepared two types of superconducting magnets in order to apply HGMS parts for wastewater purification process. We made cryo-cooled Bi-2223 (HTS) superconducting magnet parts with room temperature bore in diameter of 70 mm and 200 mm in height. Also, cryo-cooled Nb-Ti superconducting magnet with room temperature bore in diameter of 100 mm and 600 mm in height was used for magnetic separator. Magnetic filters were designed by the analysis of magnetic field distribution at superconducting magnets. The faster wastewater's flow rate was, the worse the degree of clarity in treated water was. The ability of wastewater treatment in the case of magnetic separation used with 10 of magnetite particles was better than that in the case of 5 of magnetite particles.

High gradient magnetic separation (HGMS) can be one of the promising ways for a new environmental purification technique because of producing no contamination such as flocculants and having possibility of treating large amount of wasted water within a short time. The magnetic separation system for purification of wastewater from the paper factory has been developed. The test plant of 2000 ton/day was set up in the actual paper manufacturing process and a purification test of the wastewater was performed. The magnetic seeding of the organic pulp and dyes were successfully performed and the Chemical Oxygen Demand (COD) value for recycling was reached to less than 40 ppm after magnetic separation. The superconducting magnet for the system is 3 T, 680 mm long and 400 mm bore NbTi solenoid. The system mainly consists of a mixing tank (magnetic seeding tank), a settling tank, and a superconducting magnet. Floating magnetic flocks composed of magnetite particles and organic polymers such as pulp and dye are captured by magnetic force in the superconducting magnet chamber. Some magnetic flocks are precipitated at the settling tank by the gravity, which helps to reduce the amount of magnetic flocks going through the magnet chamber. The system has been successfully operated for several months

Among various factors influencing the service life of electrical equipment, the performance of dielectric insulation materials has an important role. Degradation of insulating materials caused by aging is fatal to insulation breakdown. The dielectric sheets of most conventional and high-temperature superconducting electrical equipment are working at tensile status when lapped on the conductor. Therefore, the study of the dielectric characteristic for insulation materials at aging and tensile status is very important to safely operate electrical equipment. In this paper, a special electrode system was designed to measure the breakdown voltages of polypropylene laminated paper, polyimide, and polytetrafluoroethylene sheets at tensile status both in normal environment and liquid nitrogen. Before the breakdown experiment, the specimens of the three materials all endured the aging process of abrupt temperature change. According to the test results and the Weibull plot analysis, the polyimide presents good dielectric strength at aging and tensile status both in normal environment and in liquid nitrogen.

The electrical insulation performance of dielectric materials is an important parameter in the insulation design of a cold dielectric (CD) high-temperature superconducting (HTS) cable. Polypropylene laminated paper (PPLP) and Kraft paper are suitable for the insulation design of the CD HTS cable. The experimental study on the cryogenic insulation properties of PPLP and Kraft paper at 0.1 MPa liquid nitrogen were carried out, and the dielectric strength of a mini-model cable, which were assembled as the same structure with CD HTS cable in dc breakdown, lightning impulse breakdown, and ac breakdown, were obtained according to the three-parameter Weibull distribution based on grey model GM(1,1). The results have an important practical value for the insulation design of the CD HTS cable.