Over-current Experiments Research Articles

Analysis of Overcurrent Performance of YBCO Tape Considering Different Heat Exchange Conductions

The performance degradation of High Temperature Superconducting (HTS) tape, which may be influenced by various heat exchange conditions due to different cooling methods, is of significant for HTS devices under repeated overcurrent. In this article, considering different heat exchange conditions, a series of repeated overcurrent experiments were carried on YBCO tapes under 50Hz sinusoidal alternating current. The degradation of critical current of YBCO tapes after repeated overcurrent was recorded. the resistance characteristics of YBCO tapes under different cooling conditions were analyzed. Through finite element simulation, the temperature and current distribution of YBCO tape under different heat exchange conditions were simulated. The experimental results show that the overcurrent capability of the conductor under conduction cooling is far inferior to the liquid nitrogen immersion method.

Critical Current Degradation Behavior of Coated Conductor Subjected to Repeat Overcurrent

The performance degradation of superconducting equipment under repeat fault is a key issue for the operation of superconducting equipment in grid operation. The mechanism of the High Temperature Superconducting (HTS) tape performance degradation behavior is an important issue that need to be figured out. Therefore, it's necessary to carry out the performance degradation study of HTS tape under repeat overcurrent. In this paper, considering overcurrent duration, a series of repeated overcurrent experiments were carried on YBCO tapes under 50 Hz sinusoidal alternating current. The degradation of critical current of YBCO tapes after repeat overcurrent was recorded. The critical current density of the YBCO tape was scanned to track sample's structural change during experiment. The mechanism of the degradation development of YBCO tapes after their first degradation was described. The research could provide a reference for the design of fault protection and life assessment of HTS devices.

Quench and Recovery Characteristics of SFCL Based on Double-Sided YBCO Thin Films

In previous studies, we have preliminarily verified the feasibility of resistive superconducting fault current limiters (SFCLs) by using double-sided thin films. However, there are still many details that need to further research on double-sided thin films. In this present study, we fabricated two SFCL components juxtaposed on two sides of 2-in diameter sapphire substrates, coated with a gold layer and patterned into meander lines by photolithography. The double-sided SFCL components in serial connection were subjected to the ac fault currents, and their quench and recovery characteristics were investigated at various source voltages. From the experiments results, it was revealed that the double-sided configuration induced simultaneous quench and recovery between the components, and the temperature distribution in double-sided SFCL components was nearly close during quench and recovery. These results can be interpreted with the thermal interaction between top and bottom SFCL components by the sapphire substrates. The thermal effect of electrodes significantly affected the temperature distribution during quench and recovery. In addition, in overcurrent experiments, we defined the concept of perpendicular quench propagation to explain the rapid limitation of fault current at the beginning of short circuit and to optimize the structure of double-sided SFCL module.

Fluorescent thermal imaging of a non-insulated pancake coil wound from high temperature superconductor tape

We have wound a 157-turn, non-insulated pancake coil with an outer diameter of 85 mm and we cooled it down to 77 K with a combination of conduction and gas cooling. Using high-speed fluorescent thermal imaging in combination with electrical measurements we have investigated the coil under load, including various ramping tests and over-current experiments. We have found that the coil does not heat up measurably when being ramped to below its critical current. Two over-current experiments are presented, where in one case the coil recovered by itself and in another case a thermal runaway occurred. We have recorded heating in the bulk of the windings due to local defects, however the coil remained cryostable even during some over-critical conditions and heated only to about 82–85 K at certain positions. A thermal runaway was observed at the center, where the highest magnetic field and a resistive joint create a natural defect. The maximum temperature, ∼100 K, was reached only in the few innermost windings around the coil former.

Study on Quenching Characteristics and Resistance Equivalent Estimation Method of Second-Generation High Temperature Superconducting Tape under Different Overcurrent.

In this paper, through AC and DC overcurrent tests on second generation high temperature superconducting tape (2G HTS tape), we respectively summarize the typical types of quenching resistance and corresponding quenching degree, in which there are three types under AC overcurrent and two types under DC overcurrent. According to experimental results, a rule was found that, when 2G HTS tape quenches to normal state, the relationship between quenching resistance and joule heat generated from 2G HTS tape presents a fixed trend line, and the influence of liquid nitrogen can be ignored. Then, the characteristics and rules of quenching resistance found in experiments are well explained and confirmed by a detailed 3D finite element model of 2G HTS tape including electromagnetic field and thermal field. Finally, based on above works, our group proposes a new equivalent method to estimate the quenching resistance, where the results of AC and DC overcurrent experiments can be equivalent to each other within a certain range. Compared with FEM, the method has the following advantages: (i) The method is simple and easy to implement. (ii) This method combines precision and computational efficiency. (iii) With superconducting tape quenching to normal state, this method presents a good consistency with experimental results.

Quench Behavior of 2G HTS Coils With Polyimide Film and MIT Material Under Over Pulse-Current

This paper presents the quench behavior of second-generation (2G) high-temperature superconducting (HTS) coil insulated with a Kapton polyimide film or metal-insulator transition (MIT) materials, which could enhance the thermal stability and magnetic field-control performance of a 2G HTS magnet. For the quench test, overcurrent experiments using pulse-current shooting were conducted to investigate the thermal quench behavior and thermal stabilities of two HTS coils co-wound by polyimide or MIT materials against a large overcurrent pulse. The experimental results of the two insulation materials were compared to verify the superior thermal performance of the MIT material as the turn-to-turn insulation. Further, the applicability of the MIT material on a 2G HTS magnet system was discussed according to the experimental results.

Experiments and Numerical Simulations on Local Degradation Characteristics of Coated Conductor Due to Overcurrent

Coated conductors are subjected to short-circuit currents 10 to 30 times greater than the operating current in the use of superconducting power cables. The <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> and thickness of the stabilizer are considered to be nonuniformly distributed in the real manufacturing process. Thus, the coated conductor may be damaged locally by the hot spot due to the overcurrent. Therefore, it is important to clarify the local degradation characteristics of the coated conductor and determine its tolerance against the fault current in the actual operation. In a previous study, overcurrent experiments and numerical simulations on thermal stress-strain were carried out on a coated conductor with a nonuniformly distributed <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> . In this study, two samples with nonuniformly distributed copper plating thickness were tested. Numerical simulations based on the finite element method were performed to simulate the temperature and thermal stress-strain distribution caused by the overcurrent. The nonuniform thickness of the copper plating and the longitudinal length of the nonuniform area were considered as the simulation parameters.

Overcurrent Tests and Numerical Simulations on a 66-kV-Class RE123 High-Temperature Superconducting Model Cable

In Japan, the development of a 66-kV-class superconducting power cable was started in 2008 as a national project. A high-temperature superconducting (HTS) power cable typically consists of a copper former, HTS conductor layers, electrical insulation layers, HTS shield layers, and copper shield layers. A 66-kV-class superconducting power cable may be subjected to a fault current of 31.5 kA <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rms</sub> for 2 s. Therefore, in order to ensure the stability and feasibility of the cable, the thermal characteristics and current distributions of the cable need to be investigated under fault conditions. In this study, overcurrent experiments were performed on a 2-m-long HTS model cable. Numerical simulations were also performed on the model cable by using a computer program on the basis of a 3D finite element method and an electrical circuit model.

Current Margin of 66 kV Class HTS Power Cable Against Fault Current

In practical applications, high temperature superconducting (HTS) power cables can be subjected to short-circuit fault currents. Further, these cables are assumed to operate over a period of 30 years. Therefore, it is important to investigate the current margin and aging degradation against the fault current. In order to ensure the over-current characteristics of 66 kV class HTS power cables against the fault current, preliminary experiments were carried out on some HTS model cables. Concurrently, numerical simulations were performed to clarify the electromagnetic and thermal behaviors of the HTS model cables under fault conditions. Moreover, the validity of our computer simulation was confirmed by comparing the experimental results with the simulation results. In this study, the maximum fault current in one GdBCO coated conductor assembled in the HTS power cable was numerically simulated. AC over-current experiments were carried out on an HTS model cable by using the maximum fault current of the simulation result to evaluate the current margin against the fault current under conduction cooling and liquid nitrogen bath cooling condition.

The electric characteristics of HTS double-pancake coil with AC pulse over currents

A double pancake coil was designed and manufactured with a 36-m long Bi-2223/Ag tape. The tape was insulated by 25 μm thick Kapton tapes, which can stand a voltage of 400 V rms in liquid nitrogen. The whole double pancake was impregnated with epoxy resin. AC over-current experiments of the coil were performed by applying constant AC voltages to the two terminals of the coil and lasted for 3 s. The experiment began first at a lower voltage of 33.6 V rms, and then the voltage stepped up till the coil was burned out at the pulse voltage of 202.7 V rms. All of the experiments were carried out with the coil dipped in liquid nitrogen. The current waveforms were measured. The impedance and resistance characters of the HTS coil with its over pulse currents were analyzed from the experiment results. At the end of this paper, some conclusions derived from the experiment results and their analyses are given, which are helpful for the safety operating of the HTS coils in power applications.

Over-Current Characteristics of 66-kV RE123 HTS Power Cable

In Japan, the development of the 66-kV-class superconducting power cable was begun in 2008 as a national project. A high-temperature superconducting (HTS) power cable typically consists of a copper former, HTS conductor layers, electrical insulation layers, HTS shield layers, and copper shield layers. 66-kV-class superconducting power cables may be subjected to a fault current of 31.5 kA <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rms</sub> for 2 s. Therefore, in order to ensure the stability and feasibility of superconducting power cables, we need to investigate these cables with respect to their thermal characteristics and current distribution under fault conditions. In this study, we carried out over-current experiments on a 2-m-long HTS model cable. We also performed numerical simulations on the model cable by using a computer program developed by us on the basis of a 3D finite element method (FEM) and an electrical circuit model.

Over-Current Characteristics of 275-kV Class YBCO Power Cable

In Japan, a project for the development of a 275-kV class YBCO power cable started in 2008. High-temperature superconducting (HTS) power cables typically consist of a copper former, an HTS conductor layer, an electrical insulation layer, an HTS shield layer, and a copper shield layer. In practical applications, the 275-kV class transmission line may be subjected to short-circuit fault currents such as 63 kA <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rms</sub> for a duration of 0.6 s. Therefore, in order to ensure the stability and demonstrate the feasibility of the cable, it is important to estimate the current distribution and temperature increase under the fault condition. We designed the copper former, copper shield layer, and copper plating of the YBCO coated conductor carefully so as to fulfill the requirements of practical applications. In this study, we carried out over-current experiments on a 2-m long YBCO model cable and performed numerical simulations by a computer program developed using the finite element method (FEM) and an electric circuit model. We investigated the electromagnetic and thermal behaviors of the cable under fault conditions from the experimental and simulation results.

Repetitive Over-Current Characteristics of the Joints Between the YBCO Coated Conductor

In recent years, the YBCO coated conductor (CC) has been developed as a suitable material for power apparatuses. To fabricate a superconducting apparatus of a large capacity, the joint between the superconducting tapes are inevitable because of the need to have a long length conductor. Consequently, the joint characteristic is very important for the efficiency of the superconducting part in the apparatuses. Besides, maintaining the characteristic of the joint between the superconducting tapes even after repetitive over-current is also important for stability of power apparatuses. Therefore the joints between CC tapes were developed in differing joint length and the characteristics of the joints were compared. In this paper, we present the experimental results of several over-current experiments with the jointed CC tapes. The results of the over-current experiments and the trend of the joint characteristics are compared in point of stability of the joints. Resistance and critical current of the joints were measured from the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> - <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> curve before and after several over-current experiments for observation of the trend of the joint characteristics. This study of repetitive over-current characteristics of the joints may be useful to the development of superconducting apparatuses in forms of magnets.

Alternating Over-Current Characteristics of HTSC Tape Using DFT

The quench characteristics of high-TC superconducting (HTSC) tape for alternating over-current applications have been investigated at various amplitudes of the over-current and its quench developments have been analysed by applying discrete Fourier transform (DFT) for the voltage and the current waveforms of HTSC tape. Generally, the HTSC tape shows a broad superconducting and normal transition region above its critical current due to the metal sheath unlike HTSC thin film or HTSC bulk. Moreover, the voltage-current characteristics of HTSC tape are complicated because the quench and the recovery between the superconducting state and the normal state are repeated. In this paper, the alternating over-current experiments for the HTSC tape were performed to analyse its quench developments. The fundamental frequency components for the current and the voltage waveforms of the HTSC tape were abstracted by applying DFT. From those fundamental frequency components, the numerical formula for HTSC tape's resistance has been derived. By applying its numerical formula into circuit equation, the quench developments of HTSC tape dependent on alternating over-current have been estimated and the agreement with the experimental results has been found

Overcurrent experiments on HTS tape and cable conductor

Overcurrents in the power grid can have a magnitude of up to 20 times or higher than the rated current. This may cause problems and permanent damage to electrical equipment in the grid. High temperature superconducting (HTS) tapes are known to be sensitive to currents much larger than their critical current. In this light, it is important to investigate the response of HTS tapes and cable conductors to overcurrents several times the critical current. A number of experiments have been performed on HTS tapes and cable conductors, with currents up to 20 times the critical current. During overcurrent experiments, the voltage, and the temperature were measured as functions of time in order to investigate the dynamic behavior of the HTS tape and cable conductor. After each experiment, damage to the superconductors was assessed by measuring the critical current. Preliminary results show that within seconds an HTS tape (critical current=17 A) heats above room temperature with an overcurrent larger than 140 A. Similar overcurrent experiments showed that a HTS cable conductor could sustain damage with overcurrents exceeding 10 times the critical current of the cable conductor.