Normal Zone Propagation Velocity Research Articles

Enhanced Normal Zone Propagation Velocity in REBCO Coated Conductors Using an Intermetallic Stabilizer Coating

Enhanced Normal Zone Propagation Velocity in REBCO Coated Conductors Using an Intermetallic Stabilizer Coating

The Effect of the Interfacial Resistance of the Superconducting-stabilizer Film on the Typical Sector Diffusion Pace for 2G HTS Tapes

High temperature superconducting (HTS) tapes of the second generation have been widely used as energy storage materials, such as in superconducting magnetic energy storage (SMES) devices. In order to enhance the current-carrying characteristics, these systems are typically run close to the critical currents of the coated conductors; as a result, hot spots may develop, which could cause the superconductor to become quenched. In order to prevent the start of hot spots and to reduce the amount of faulting, efforts have been made to raise the normal zone propagation velocity (NZPV) in this manuscript. The interfacial resistance between the superconductor and stabilizer layer, which can act as a current flow diverter during fault circumstances, has been shown to be the key to producing massive NZPV. The tape's architecture has been slightly modified by the addition of a high resistive layer between the superconducting and stabiliser layers, where various interfacial resistances have been utilised to forecast the temperature distribution between 10 cm lengths of HTS tape. A 2D numerical model was created using COMSOL to assess the NZPV and temperature distribution of the 2G superconducting tape. It has been concluded that larger NZPVs can be achieved by using substantial interfacial resistances to prevent superconducting tape quenching.

Fault current limiter based on high temperature superconductors

High Temperature Superconductors (HTS) made the Superconducting Fault Current Limiter (SFCL) a reality in power electric grids. Resistive SFCL, which is the subject of this article, is based on a unique property of superconductors to quench from a non-dissipative to a high dissipative state above a given current. SFCL is an attractive response to the poorly solved problem of fault currents in power electric grids and makes it possible to enhance the security and quality of electricity, something that matters more than ever. Liquid nitrogen cryogenics – which is possible thanks to HTS- makes the cryogenics user friendly. In-field SFCL operations confirm this. Second generation (2 G) HTS materials or Coated Conductors suit well to SFCL. Due to its high interest, SFCL has been the subject of a great deal of research and development. In terms of Technology Readiness Level (TRL) it reaches 8 with several in grid test experiences.Here a short history of SFCL is presented with a highlight about Curl 10, Ampacity and SuperOx devices. The two major locations in the grid are reported: grid coupling and feeder protection. The quench phenomena is described in a bit more detail before the basic design (length and cross section) is presented. The design is primarily thermal, considering two extreme operations: fault with very high prospective current (clear short-circuit) and fault with prospective current close to the minimum runaway current along the length. This operation is called the hot spot one. It is due to the intrinsic variation of the critical current along the length and to the low Normal Zone Propagation Velocity (NZPV). The cost of the superconducting conductor is given, showing no bottleneck for an economic development. All the more it is still possible to lower the cost with some developments. This article ends with two real advances presently underway: Current Flow Diverter (CFD) and Sapphire substrate superconducting 2 G tape. CFD aims to increase the NZPV, to still further secure the operation and to reduce the conductor cross section. The use of sapphire substrates allows to reduce significantly the length of the design.

A sulfurization method for creating the buffer-layers current flow diverter architecture in REBa2Cu3O7 coated conductors

The current flow diverter (CFD) is a known concept that has proven to effectively reduce the probability of destructive hot spots in REBa2Cu3O7 (REBCO; RE = rare earth) coated conductors (CCs) by boosting the normal zone propagation velocity. However, the implementation of the CFD concept requires additional steps in a fabrication process that is already complex and has struggled to find a simple reel-to-reel fabrication method. This work reports on the details of a fabrication route for the buffer-layers-CFD (bCFD) architecture using a solid-vapor silver sulfurization technique to tune the geometry of the metal stabilizer in the high-temperature superconductor tape. The analysis of the microstructure and superconducting properties of the Ag2S/Ag/GdBCO trilayer processed under different conditions shows how we achieved a new customized functional CC with the bCFD architecture. In DC limitation experiments, this bCFD-sulfide architecture allowed to generate an electric potential much faster than the conventional architecture (60 V s−1 vs. 1.2 V s−1) thanks to the strong enhancement of the NPZV.

Quench initiation and normal zone propagation characteristics of smart insulated high-temperature superconducting coil

A smart insulation (SI) winding technique utilizing vanadium trioxide (V2O3) as a turn-to-turn insulation material is an effective means of achieving a suitable time constant and thermal stability in high-temperature superconducting (HTS) coils. However, the low electrical conductivity of the V2O3 makes the SI coil vulnerable to damage during quenching. To address this problem, this study explored a mixture of V2O3 and molybdenum (Mo) particles to improve the thermal stability of the SI coil. Two GdBCO coils, insulated with V2O3 and V2O3:Mo, respectively, were fabricated, and quench tests were performed at various operating currents in liquid nitrogen at 77 K. The findings revealed that the V2O3:Mo-insulated coil demonstrated improvements of 82%, 25%, and 160% in the minimum quench energy, and the normal-zone propagation velocities in the longitudinal and transverse directions, respectively, at 60% of the current-carrying capacity of the coil, which are essential performance parameters in the operation of a superconducting magnet.

Influence of Interturn Contact Stress on the Quench Behavior of Insulated YBCO Pancake Magnets: A Study Based on Numerical Simulations

Insulated high temperature superconducting (HTS) magnets are still widely used in low field applications, such as HTS motors and generators. In this paper, a two-dimensional/three-dimensional (2D/3D) mixed electrothermal model and a homogeneous 3D electrothermal model are employed to study the influence of inter-turn contact stress on the minimum quench energy (MQE), longitudinal and transverse normal zone propagation velocity (NZPV) of insulated YBCO magnets for the temperature range 30 K – 60 K. This temperature range is of interest usually in applications, such as HTS motors, generators and transformers, because it allows the use of low cost cooling system. It is found that MQE increases significantly with increasing inter-turn contact stress. The inter-turn contact stress has small influence on the longitudinal NZPV of insulated YBCO pancake magnet. However, the transverse NZPV rises significantly with the increase of inter-turn contact stress. The transverse NZPV is nearly proportional to the inter-turn contact stress. The 3D homogeneous model gives results that are in agreement with those of 2D/3D mixed model, especially for the temperature range 45 K – 60 K; however, the MQE and transverse NZPV given by the two models are different for the temperature range 30 K – 45 K. That means for insulated HTS YBCO pancake magnets operating at the temperature range 45 K – 60 K, the 3D homogeneous electro-thermal model can be a good replacement of the 2D/3D mixed model to study the quench behavior, which can achieve better computational efficiency without significant loss of accuracy.

Quench experiments on sub-size HTS Cable-In-Conduit Conductors for fusion applications: Data analysis and model validation

The recent experimental campaign, carried out in the upgraded SULTAN facility and supported by EUROfusion, investigated quench propagation in high field, sub-size HTS Cable-In-Conduit Conductors. The available experimental data consist of voltage, jacket and helium temperatures measured along the length of the conductors. In this work, the analysis of experimental data is carried out, focusing on the normal zone propagation velocity and the hotspot temperature reached during the quench. The validation of one-dimensional (1D) thermal-hydraulic and electric numerical models implemented in the H4C code is presented, showing very good agreement between experimental and computed results, both in terms of global and local quantities. The validated tool is then used to assess the impact on the hotspot measured in SULTAN and that reached in a uniform magnetic field configuration, showing that SULTAN measurements provide conservative assessments. The tool is then used to simulate a quench in a real-case scenario, i.e., the EU DEMO Central Solenoid operation, showing that a conventional quench detection strategy based on voltage measurements should suffice, leading to hotspot temperature around 250 K.

MEMS microphone arrays for quench detection in superconducting cables

An acoustic array is proposed as a quench detection method in superconducting magnets. A quench occurs when the current density in the superconductor exceeds a critical value, resulting in a loss of superconductivity and rapid local heating. This event is destructive and must be rapidly detected. It is thought that the quench may act as an acoustic source (Takayasu, 2019), which could be detected and localized by a microphone array inserted into the cryogenic coolant. A main advantage of this method is that acoustics propagate 1000 times faster than the normal zone propagation velocity in HTS conductors, providing for fast detection times. To demonstrate this concept, we first characterized the performance of a piezoelectric MEMS microphone and several potential preamplifiers under cryogenic conditions. An acoustic sense node was then constructed that operates down to 10 K. A cryogenic probe incorporating the MEMS array was used to study a quench event in a segment of a 2 mm wide REBCO tape. Quench experiments were carried out in a 7.6 cm diameter, 111 cm tall cryostat in Helium gas at 20 to 50 K. The MEMS array clearly detects a quench induced failure. Other observed acoustic features of unknown origin will be described.

Quench detection and early warning based on thermoelastic strain rate for HTS tapes thermally triggered by heat spots

The rapid detection and comprehensive monitoring of quench onset and evolution in high-temperature superconducting (HTS) materials remains immensely challenging because the normal zone propagation velocity in HTS materials is two or three orders of magnitude less than that in low-temperature superconductors. In this study, we experimentally and numerically explore quench events triggered by heat spots in yttrium barium copper oxide HTS tapes to characterize the quench onset and propagation. A multiplexed fiber Bragg grating sensor with multiple gratings was used to perform highly accurate strain measurements. Conventional voltage and temperature measurements were performed synchronously on the HTS tapes. A systematic comparison of these multifield signals during quench onset and development illustrated that the evolution of thermoelastic strain and strain-rate in HTS tapes captures the quench onset and propagation. A distinct feature was exhibited during the quenching of a pre-tensioned HTS tape: the thermoelastic strain initially relaxed but subsequently increased until the strain rate exhibited a significant slope change, which corresponded to the quench onset time. The thermoelastic strain in a nearly unconstrained HTS tape gradually increased until quenching occurred. A prominent characteristic for detecting quench onset in HTS materials have been revealed based on the change in the slope of the thermoelastic strain-rate or the second derivative of the strain remaining nearly constant. For a pre-tensioned HTS tape, the minimization of the thermoelastic strain or the strain-rate becoming zero may be a predictor that preceded the quench by ∼1–2 s, which can be, to a certain extent, regarded as an early warning. Another important and novel result was the experimental demonstration of global strain responses distant from the quench location in the pre-tensioned HTS tape, while the temperature and voltage detection are commonly localized methods. The mechanism behind these thermoelastic strain characteristics was further discussed and simulated from the induced Joule heating throughout the quench event. The measurements and numerical predictions suggested a new paradigm of quench detection based on the thermoelastic strain-rate in HTS materials.

Detecting Quench in HTS Magnets With LTS Wires—A Theoretical and Numerical Analysis

Protecting a high temperature superconducting (HTS) magnet from a quench event is a challenging task. Because of the slow normal zone propagation velocity, the long reliable quench detection method by directly monitoring coil voltage may not be timely for HTS anymore, leaving HTS magnets under danger of overheating. Using a NbTi low temperature superconducting (LTS) wire to detect quench in coils wound with ReBCO HTS tapes have recently been experimentally proved, yet a theoretical study is still needed to further develop this technique and make it prepared to be applied more generally in high field magnets. In this manuscript, we have demonstrated that it is the significant difference in the temperature dependence of critical current between LTS and HTS but not the normal zone propagation velocity (NZPV), that makes LTSs good quench detectors. Simulations show that LTS quench detectors should have low matrix fraction or high matrix resistivity. At last, at field up to 15 T or 20 T, Nb3Sn is proven to be a good quench detector.

Quench Protection Study of a Large Scale REBCO Magnet With Additional Copper Tapes

REBCO coated conductors (CCs) are expected to be applied to high field magnets due to their high longitudinal tensile strength and excellent critical current characteristics in high fields. For large scale magnets with large stored energy, quench protection is a key issue. Although various quench protection methods have been proposed for REBCO magnets, the most traditional and straight forward approach is to increase the copper stabilizer and reduce the current density. This approach leads to a significant increase in conductor length to generate a required magnetic field in case a small bore diameter magnet, while the conductor increase would not be serious in case a large bore diameter one. Based on this approach, we have successfully fabricated a practical cryogen-free superconducting magnet (CSM) with a large bore of 200 mm-diameter and a central field of 5 T using REBCO CCs laminated with 0.3 mm-thick copper tapes. For further study, we designed a MJ-class 10 T-CSM with the same inner-diameter as the 5 T-CSM and investigated the feasibility of the magnet through quench tests in this study. We conducted quench tests on short samples of copper-plated REBCO CCs with copper tape soldered on different surfaces, and quench tests on a small coil assuming an energy recovery method with external resistance. In the short sample tests, it was found that the temperature rise was suppressed regardless of the position of the copper tape lamination, and the normal zone propagation velocity was improved when the copper tape was laminated to the substrate side. In the small coil tests, the decay time constant after shutdown was varied to find the conditions that can protect the magnet with high stored energy. We confirmed that quench protection of 10 T-CSM was feasible under practical conditions.

Numerical and Experimental Study on Thermal Stability of BSCCO/REBCO Hybrid Tapes

This paper studies the thermal stability of BSCCO (Bi-2223) /REBCO hybrid tapes with different stacking methods under various operating currents in liquid nitrogen (LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) bath. The thermal stability of single REBCO tape and BSCCO tape are also studied as the comparison. A 3D numerical model considering the anisotropic thermal conductivity is established to calculate the normal zone propagation velocity (NZPV) and minimum quench energy (MQE) of different hybrid tapes, and the temperature distributions of the tapes are obtained as well. The experiments of thermal stability of single tape and RB hybrid tape are also carried out. Results show the hybrid tape has better thermal stability than the REBCO tape and faster NZPV than the BSCCO tape.

Some Results of the EU Project FASTGRID

The EU FASTGRID project aimed at improving the REBCO conductor in order to enhance its economical attractiveness for the Superconducting Fault Current Limiter (SFCL). Two approaches were simultaneously investigated: i) the reduction of the tape length through the increase of the electric field during limitation, ii) the reduction of the tape cost through improved yield and higher critical current, including the lowering of the liquid nitrogen bath temperature to 65 K. We carried out a base line using an upgraded THEVA tape. Different approaches have been tried to improve the shunt layer, both with metallic and non-conductive materials. As a first solution, we developed and successfully tested a conductor with a Hastelloy shunt able to withstand 130 Vrms/m during 50 ms at 65 K and low prospective current faults. This conductor was used in two pancake coils. These have been successfully tested at IPH in Berlin. The second shunt solution developed consisted of a ceramic in epoxy matrix coating. Another significant improvement of the tape is the successful implementation of the Current Flow Diverter (CFD) concept with a simple sulfidation process or a deposition of an insulating yttria nanolayer by Inkjet Printing. An increase by one order of magnitude of the Normal Zone Propagation Velocity with respect to tapes without CFD has been measured. A low cost and fast hot spot detection system has also been developed. To reach much higher electric fields under limitation, the FASTGRID team also developed advanced tapes based on a sapphire substrate, which can tolerate ultra-high electric fields up to about 1 kV/m. Validated at laboratory scale, this game-changing technology needs to be implemented in long lengths with an industrial process. One pancake was integrated and tested with a low-cost optical fiber and fast hot spot detection system, developed and patented within the FASTGRID project. This manuscript will showcase most of these FASTGRID results.

Feasibility study of capacitance based quench detection technique for HTS power transmission cables

2nd Generation High Temperature Superconducting (2G HTS) tapes are replacing the conventional Low Temperature Superconducting (LTS) wires in various felds of applied superconductivity, especially in the power transmission applications. Transients in the electrical grid can result in the loss of superconductivity leading to thermal runaway. This phenomenon is known as quench. To avoid catastrophic consequences, timely detection of quench is imperative. However, the HTS materials possess a low Normal Zone Propagation Velocity (NZPV) compared to LTS materials during quench. This property combined with a low electromagnetic shielding makes the conventional voltage tap detection technique unreliable for HTS power transmission cables. A capacitance based quench detection technique is proposed and its feasibility is studied. A variation in the permittivity of the cryogenic fuid and the dielectric can be observed in association with the heat generation during faults. This can be refected as the variation in the measured capacitance between cryostat and cable former. In this study, this capacitance is computed using the FEM model of HTS cable operating at various temperatures along with diferent vapour liquid ratios of cryogen. The results of this work can be extended into experiments for quench detection in HTS power transmission cables.

Normal zone propagation in various REBCO tape architectures

The normal zone propagation velocity (NZPV) of three families of REBCO tape architectures designed for superconducting fault current limiters and to be used in high voltage direct current transmission systems has been measured experimentally in liquid nitrogen at atmospheric pressure. The measured NZPVs span more than three orders of magnitude depending on the tape architectures. Numerical simulations based on finite elements allow us to reproduce the experiments well. The dynamic current transfer length (CTL) extracted from the numerical simulations was found to be the dominating characteristic length determining the NZPV instead of the thermal diffusion length. We therefore propose a simple analytical model, whose key parameters are the dynamic CTL, the heat capacity and the resistive losses in the metallic layers, to calculate the NZPV.

Thermal stability of quasi-isotropic strands with different cross-sections and with/without metal sheaths

This paper mainly focuses on the thermal stability performance, such as minimum quench energy (MQE) and normal zone propagation velocity (NZPV), of a quasi-isotropic strand (Q-IS) with three different configurations made from 2G high-temperature superconducting (HTS) tapes in the adiabatic approximation. Due to the anisotropic performance of thermal conductivity in 2G HTS tapes, the tensor form of the heat conduction equation was used instead of conventional isotropic forms. We first measured the MQE and NZPV of three strands in the adiabatic approximation. Subsequently, the experimental results were verified by finite element analysis and compared with the values of MQE and NZPV in a liquid nitrogen (LN2) bath. Finally, the effect of a metal sheath on the current distribution and generation of Joule heat during the quench process were analyzed through simulation. The results show that the arc-shaped aluminum filler brings slightly higher thermal stability of Q-IS while the copper sheath largely improves stability. Moreover, compared with the thermal stability of the strand fully exposed to the LN2 bath, for the strand in the adiabatic approximation, the MQE decreases a lot while the maximum of NZPV is increased by 2–3 times.

Thermally conductive composites with hydroxylated boron nitrides for the efficient thermal management of superconducting coils

The thermal conductivity (κ) of polymer nanocomposites with boron nitride (BN) nanoplates is critically dependent on the distribution of BN in the polymer matrix. Here, we heavily functionalized BN nanoplates with hydroxyl groups by ultrasonication in solution with boric acid. Then, the hydroxylated BN nanoplates could be homogeneously distributed in an epoxy matrix, which improved the through-plane κ of 2.09 and 1.96 W/m·K at 25 and -90 °C, respectively. Processable composite resin with hydroxylated BN was utilized as an adhesive for a low-temperature superconducting (LTS) coil to enhance its normal zone propagation velocity (NZPV) and cooling rate performance. Consequently, fast heat dissipation through the adhesive between superconductors contributed to a 65.8% of increase in NZPV and a 13.1% improvement in the cooling rate of the device compared with a commercial adhesive.

Development of a Normal Zone Propagation Insert Set-up for YBCO Superconducting Tape

For practical application, stability investigation of high temperature superconductor (HTS) is of great importance. An insert set-up has been developed at the Institute of Plasma Physics, Chinese Academy of Sciences (ASIPP). Measurements are performed as a function of transport current and magnetic field for several operating temperatures between 30 and 50 K. This work focuses on the design of insert set-up and the estimation of normal zone propagation velocity (Vnzp) and minimum quench energy (MQE). The experimentally obtained temperature dependence of Vnzp is compared with a simplified adiabatic model.

Cryogenic testing of microphones and preamplifiers for MEMS quench detection in superconducting magnets

High Temperature Superconducting (HTS) tapes are needed for high field magnets in high-energy physics and nuclear fusion. However, quench events, in which the superconductor loses its superconductivity, can cause significant and costly damage to the device if allowed to propagate. Consequently, rapid quench detection methods are crucial. Existing methods such as voltage taps can be less effective in HTS applications, as the normal zone propagation velocities of HTS devices are slow, drastically increasing quench detection time (Iwasa, Cryogenics, 2003). We propose an alternative method that utilizes a linear array of MEMS microphones embedded in the cryogenic coolant channel (Takayasu, IEEE Transactions on Applied Superconductivity, 2019). The system requires amplifiers and acoustic sensors capable of operation at cryogenic temperatures as low as 4.2 K, although operation at 20 K may be sufficient. We report on cryogenic testing of the Vesper microphone preamplifier ASIC, as well as opamp based preamplifiers using the OPA838, LT1464, AD8591, and AD8057 ICs. The amplifiers are being used with the Vesper VM4 MEMS microphone to achieve acoustic measurements in gaseous helium at temperatures as low as 8 K. A charge amp scheme using the AD8591 achieved the best results to date. [Support from DOE STTR DE-SC0019905.]

Design and Characteristic Analysis of a Homopolar Synchronous Machine Using a NI HTS Field Coil

This paper deals with a homopolar synchronous machine (HSM) applying high-temperature superconducting (HTS) field coils. Superconductors, especially high-temperature superconductors, have high potential as advanced materials for next-generation electrical machines due to their high critical current density and excellent mechanical strength. However, coils made with high-temperature superconductors have a high risk of being damaged in the event of a quench due to the intrinsic low normal zone propagation velocity (NZPV). Therefore, the coil protection issue has been regarded as one of the most important research fields in HTS coil applications. Currently, the most actively studied method for quench protection of the HTS coils is the no-insulation (NI) winding technique. The NI winding technique is a method of winding an HTS coil without inserting an insulating material between turns. This method can automatically bypass the current to the adjacent turn when a local quench occurs inside the HTS coil, greatly improving the operating stability of the HTS coils. Accordingly, many institutions are conducting research to develop advanced electrical machines using NI HTS coils. However, the NI HTS coil has its intrinsic charge/discharge delay problem, which makes it difficult to successfully develop electrical machines using the NI HTS coil. In this study, we investigated how this charging/discharging problem appeared when the NI HTS coil was used in an HTS homopolar synchronous machine (HSM) which is one of the electrical machines with a high possibility of applying the HTS coil in the future because it has a stationary field coil structure. For this, the characteristic resistances of HTS coils were experimentally obtained and applied to the simulation model.