• 한국초전도ㆍ저온공학회논문지
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• 제25권1호
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• pp.16-20
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• 2023

The International Electrotechnical Commission (IEC) 61788-26:2020 provides guidelines for measuring the critical current of Rare-earth barium copper oxide (REBCO) tapes using two methods: linear ramp and step-hold methods. The critical current measurement criterion, 1 or 0.1 μV/cm of electric field from IEC 61788-26 has been normally applied to REBCO coils or magnets. No-insulation (NI) winding technique has many advantages in aspects of electrical and thermal stability and mechanical integrity. However, the leak current from the NI REBCO coil can cause distortion in critical current measurement due to the characteristic resistance which causes the radial current flow paths. In this paper, we simulated the NI REBCO coil by applying both linear ramp and step-hold methods based on a simplified equivalent circuit model. Using the circuit analysis, we analyzed and evaluated both methods. By using the equivalent circuit model, we can evaluate the critical current of the NI REBCO coil, resulting in an estimation error within 0.1%. We also evaluate the accuracy of critical current measurement using both the linear ramp and step-hold methods. The accuracy of the linear ramp method is influenced by the inductive voltage, whereas the accuracy of the step-hold method depends on the duration of the hold-time. An adequate hold time, typically 5 to 10 times the time constant (τ), makes the step-hold method more accurate than the linear ramp method.

• 한국초전도ㆍ저온공학회논문지
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• 제12권3호
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• pp.16-20
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• 2010

For large scale power applications of HTS conductor, it is getting more important to have a stacked HTS coated conductor with low loss and large current capacity. But it was not easy to measure some electric properties. Stabilizer free YBCO CC for striated/ stacked conductors is easily burned out during the measurement of the critical current density because it has no stabilizer and it is difficult to set-up the current lead and voltage taps because it has many pieces of YBCO CC in a conductor. Instead of direct measuring the critical current of a stacked HTS coated conductor, indirect estimation from measuring a magnetization loss of HTS coated conductor could be useful for practical estimation of the critical current. The magnetization loss of a superconductor is supposed to be affected by a full penetrating magnetic field, and it tends to show an inflection point at the full penetrating magnetic field when we generate the graph of magnetization loss vs. external magnetic field. The full penetrating magnetic field depends on the shape of the conductor and its critical current density, so we can estimate the effective critical current density from measuring the magnetization loss. In this paper, to prove the effectiveness of this indirect estimation of the critical current, we prepared several different kinds of YBCO CC(coated conductor) including a stacked conductor short samples and measured the magnetization losses and the critical currents of each sample by using linked pick up coils and direct voltage measurement with transport current respectively.

• 한국초전도ㆍ저온공학회논문지
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• 제22권4호
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• pp.62-66
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• 2020

We fabricated superconducting quantum interference devices (SQUIDs) based on Nb Josephson junctions, and characterized the key parameters of the SQUIDs. The SQUIDs are double relaxation oscillation SQUIDs (DROSs) having larger flux-to-voltage transfer coefficient than the standard DC-SQUIDs. SQUID sensors were fabricated by using Nb junction technology consisted of a DC magnetron sputtering and a conventional photolithography process. In multichannel SQUID systems for whole-head magnetoencephalography measurement with a helmet-type SQUID array, we need about 336 SQUID sensors for each system. In this paper, we fabricated a few hundred SQUID sensors, measured the critical current, flux modulation voltage and decided if each tested SQUID can be used for the multichannel systems. As the criterion for the acceptance of the sensors, we chose the critical current and amplitude of the modulation voltage to be 8 ㎂ and 80 ㎶, respectively. The average critical current of the SQUIDs was 10.58 ㎂. The typical flux noise of the SQUIDs with input coil shorted was 2 μΦ0/√Hz at white region.

• 한국초전도ㆍ저온공학회논문지
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• 제19권2호
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• pp.48-52
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• 2017

Discrepancy between a power supply current and an actual "spiral" coil current makes the conventional 4-probe measurement of a critical current ($I_c$) of a no-insulation (NI) high temperature superconductor (HTS) coil inaccurate and time-consuming. This paper presents a fast and accurate approach for $I_c$ measurement of NI HTS coils. With an NI HTS coil energized at a constant ramping rate, a complete analytic expression for the spiral coil current was obtained from a first-order partial differential equation that derived from an equivalent circuit model of the NI coil. From the analytic solution, both spiral coil current and radial leak current can be obtained simultaneously, which enables fast and accurate measurement of the NI coil $I_c$. To verify the proposed approach, an NI double-pancake (DP) coil, wound with GdBCO tapes of $6mm{\times}0.1mm$, was constructed and its $I_c$ was repeatedly measured with various ramping rates in a bath of liquid nitrogen at 77 K. The measured results agreed well with the calculated ones, which validates the proposed approach to measure $I_c$ of an NI HTS coil.

• Progress in Superconductivity
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• 제3권1호
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• pp.99-103
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• 2001

We have conducted two runs of interlaboratory comparison on Ag-sheathed Bi-2223 tapes to evaluate the level of measurement techniques for the critical current measurement. Two classes of specimens were prepared for parallel and serial routings and sent to four participating laboratories. The critical currents of specimens were measured at 77 K in zero magnetic field. In the first comparison, we used twenty different Bi-2223 tapes as specimens for comparison and participating laboratories measured the specimens using their own instruments and procedures. As a result, the scattering of data on the first comparison showed -3.0% to +l2.2% for the parallel routing and -0.7% to +l5.1% for the serial routing. Major sources of these variations were attributed to different measurement techniques. Thus, the second comparison of measurement was done on the same specimens under specified measurement conditions, particularly in terms of cooling procedure and sweep rate of the test current. The variations for the second comparison were decreased -3.1% to +3.2% far the parallel routing and -1.8% to +7.7% fur the serial routing.

• 한국전기전자재료학회:학술대회논문집
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• 한국전기전자재료학회 2010년도 춘계학술대회 논문집
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• pp.12-12
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• 2010

The hall probe measurement system was used to measure the critical current distribution of superconducting coated conductor. The system consists of reel to reel moving apparatus, 7 array hall probe, a rotary encoder and permanent magnet. The magnetic field profile across the width of superconducting coated conductor using Bean's critical state model was calculated. The effect of various parameters of the formulas on the magnetic field distribution and the effect of shape and size of artificial defects, which were formed on the surface of SmBa2Cu3O7-d(SmBCO) coated conductor using laser marking system, on the hall probe magnetic field signal of the hall probe measurement system was investigated.

• 한국전기전자재료학회:학술대회논문집
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• 한국전기전자재료학회 2009년도 춘계학술대회 논문집
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• pp.7-8
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• 2009

Non-contact critical current measurement apparatus was developed using hall probe which measures the magnetic field distribution across the width of superconducting tape. The hall probe consists of 7 independent hall sensors which lie in a line 600 ${\mu}m$. The difference between maximum and minimum magnetic field in the magnetic filed distribution is a main parameter to determine the critical current. As preliminary research, we calculated the magnetic field intensity at the middle sensor, which is a minimum magnetic field and generated by the circular shielding current modeled by Bean model. We confirmed that there are some parameters that affect on the minimum magnetic field; the distance between superconducting layer and hall sensor, the width of superconducting tape, and the critical current distribution across the width of superconducting tape. Among these parameters, the distance between superconducting layer and hall sensor highly influences on the minimum magnetic field.

• 한국전기전자재료학회:학술대회논문집
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• 한국전기전자재료학회 2009년도 하계학술대회 논문집
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• pp.363-364
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• 2009

본 연구는 IEC/TC 90 규격을 참조하여 Cu stabilizer/Ag/$LaMnO_3$/IBAD-MgO/$Y_2O_3$/$Al_2O_3$/Hastelloy C276/Cu stabilizer의 구조를 가지는 coated conductor의 DC 임계 전류의 결정에 대한 사전 연구를 위해 (1) 시편의 냉각시간, (2) 전류 선의 용량을 변화하여 임계 전류의 변화를 관찰하여 다음과 같은 결과를 얻었다. (1) 5분 이상의 시편 냉각시간은 COV가 약 0.4% 정도로 거의 임계 전류 결정에 영향을 주지 않았으며, 냉각 시간이 길면 길수록 임께 전류 이하에서 전압의 기저면이 낮아지는 경향을 보였다. (2) 전류 선의 용량이 클수록 임계 전류 이하에 전압의 기저면이 낮아지는 경향을 보였다.

• 한국초전도학회:학술대회논문집
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• 한국초전도학회 2001년도 High Temperature Superconductivity Vol.XI
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• pp.57-57
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• 2001

• 대한전기학회논문지:전기기기및에너지변환시스템부문B
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• 제53권7호
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• pp.424-429
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• 2004

The 4-probe method with a voltage tap on terminals has been used for the measurement of the critical current of multi-strand high-T$_{c}$ superconducting(HTS) cables. And the critical current of cables is obtained as the measured total current divided by the number of conductor when the terminal voltage exceeds the predetermined criterion of critical current. However, because of the non-uniform current distribution due to the different critical current, shapes, and other characteristics of each conductor, this is not applicable method to the multi-strand HTS cable. To determine the critical current of multi-strand HTS cable, the critical current of each conductor must be measured with different method. h this paper, the current distribution and the critical current of each conductors in multi-strand cable were measured with specially made Pick-up coils and voltage taps. It is presented that the real critical current of multi-strand is smaller than sum of each conductors. The main cause of non-uniform current distribution is the difference between the resistances appeared in each HTS wires.s.