Normal Zone Propagation Velocity Research Articles

Stability and quench behaviors of conduction-cooled 2G HTS coil co-wound with SS tape

We fabricated a second-generation (2G) high-temperature superconducting (HTS) model coil cowound by a 2G HTS tape and a stainless steel (SS) tape. The inner diameter of the model coil was 120 mm, and the number of turns was 100. A 2G HTS tape 12 mm in width manufactured by SuNAM Co., Ltd. in South Korea was used. The total length of the coil was 44 m, and the critical current $(I_{{c}})$ of the model coil was about 174 A in liquid nitrogen. The minimum quench energy (MQE) was 19.1–22.6 J, and the normal zone propagation velocity (NZPV) was 2.4–2.7 mm/s at 0.977 $I_{{c}}$ in liquid nitrogen. MQE was 2.85 J at 0.86 $I_{{c}}$ , and NZPV was 2.7 mm/s at 77 K in conduction cooling. A 2G HTS coil cowound with an SS tape was robust in the event of the quench induced by heater and overcurrent flow above critical current.

Thermal conductivity and stability of commercial MgB2 conductors

This paper presents a study of the thermal transport properties of MgB2 tapes differing in architecture, stabilization and constituent materials. The temperature and field dependence of thermal conductivity, was investigated both along the conductor and in the direction perpendicular to the tape. These data provide fundamental input parameters to describe the 3D heat diffusion process in a winding. Thermal transport properties—even in field—are typically deduced using semi-empirical formulas based on the residual resistivity ratio of the stabilizer measured in absence of magnetic field. The accuracy of these procedures was evaluated comparing the calculated κ values with the measured ones. Based on the experimental thermal conduction properties and critical current surface we determined the dependence of minimum quench energy and normal zone propagation velocity on the operating parameters of the conductor. The correlation between thermal properties and tape layout allowed us to provide information on how to optimize the thermal stability of MgB2 conductors.

Enhanced Quench Protection in REBa2Cu 3Oδ-7-Based Coils by Enhancing Three-Dimensional Quench Propagation via Thermally Conducting Electrical Insulation

This simulation explores the effects of insulation properties on quench propagation in ReBa 2Cu 3O δ-7-based coils. At present, superconducting magnets primarily use insulators that are electrically and thermally insulating, for example, Kapton. Here, the impact of varying the thermal conductivity of the electrical insulation on quench behavior is reported. In particular, the behavior of a Kapton-insulated coil is compared with one insulated with doped TiO 2, one insulated with “ideal Al 2O 3”, and one noninsulated coil. The effects on minimum quench energy and normal zone propagation behavior are investigated. In addition, a new concept, the current sharing volume (CSV), which accounts for two- or three-dimensional normal zone propagation, is introduced. The CSV is defined as the volume of coil for which the temperature is above the current sharing temperature. The simulation results show that the transverse thermal conductivity and insulation thickness strongly influence the normal zone propagation velocity, thus impacting the quench detection time and hotspot temperature. As expected, the coils insulated with the higher thermal conductivity alternatives exhibited faster normal zone growth and lower hotspot temperatures relative to CSV growth. The impact of improved thermal conductivity of turn-to-turn insulation becomes even greater when distributed sensing replaces voltage-based sensing.

Experimental Study of the Normal Zone Propagation Velocity in Double-Layer 2G-HTS Wires by Thermal and Electrical Methods

The Normal zone propagation velocity (NZPV) of a double-layer second generation high temperature superconducting wire manufactured by American Superconductor has been measured by electrical and thermal methods, and the results have been compared and discussed. The NZPV values determined by the voltage traces are ranging from 3.8 mm/s at 0.4 I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> to 19.2 mm/s at 0.9 I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ; while from 5.9 to 18.3 mm/s by the temperature traces. NZPV determined by these two approaches agrees well with each other. Also, NZPV of double-layer YBCO tape is close to that of conventional single-layer superconducting tape.

Quench Analysis of a Superconducting Magnet for RISP 28 GHz ECR Ion Source

This paper presents quench analysis of a superconducting magnet system for 28 GHz electron cyclotron resonance (ECR) ion source. The magnet system consists of a hexapole coil and four solenoid coils located outside of the hexapole one. All coils were wound with NbTi wire and impregnated by epoxy. To analyze the characteristic of superconducting coil when the quench occurs, a numerical code was developed. The analysis procedures are as follows. First, normal zone propagation (NZP) velocity which is as a function of magnetic field was calculated. Second, a fraction of the winding volume was obtained by transient analysis, considering longitudinal and transverse NZP velocities. Third, a generated resistance and temperature rising over time were simulated. Lastly, current trace of the coil was calculated. The current trace calculated by simulation well agrees with the test result. Also the result of hot-spot temperature is reasonable. Since simulated hot-spot temperature and experimental result are 60.32 K and 63 K when the operating current is 169 A. The normal zone resistances are also identical for 1.13 s which is the convergence time of simulation. The final resistances are about 10.2 Ω and 11.23 Ω. Therefore it is expected that the analysis code can be used to estimate the characteristic of superconducting magnet when the quench occurs.

A Study on Normal Zone Propagation Behavior of Partially Insulated GdBCO Coil

This paper reports the quench initiation and propagation characteristics of two GdBCO single-pancake coils, one wound with Kapton tape every two turns and the other completely wound with Kapton tape, to investigate the effect of a partial insulation (PI) winding technique. The thermal/electrical stabilities of the PI coil were improved by turn-to-turn contacts between the partially uninsulated turns, leading to an increased minimum quench energy. Furthermore, transverse normal zone propagation velocities of the PI coil increased because local heat was well transferred through uninsulated turns during quench events.

Impact of the Normal Zone Propagation Velocity of High-Temperature Superconducting Coated Conductors on Resistive Fault Current Limiters

The engineering critical current, i.e., Ic, of high-temperature superconducting coated conductors (HTS-CCs), today available on the market, is not a uniform parameter and varies significantly along the length of the conductors. In addition, commercial HTS-CCs have a low normal zone propagation velocity (NZPV). This property, together with the Ic inhomogeneity, exposes the HTS-CCs to local thermal instabilities. A crucial challenge for the design of resistive fault current limiters (RFCLs) based on HTS-CCs is to avoid the thermal runaway of the conductors; and in this respect, the enhancement of the NZPV is a promising solution. In recent years, several methods have been proposed, and many various techniques are now available. In this paper, we are interested to quantify the impact the enhancement of NZPV will have on the design of RFCLs based on HTS-CCs whichever is the adopted technical solution. For this reason, we use numerical models to analyze the effects of the NZPV enhancement on the limitation performance of an RFCL integrated in a medium-voltage power grid.

Hot Spot Temperature in an HTS Coil: Simulations With MIITs and Finite Element Method

MIITs, a zero-dimensional concept to study hot spot temperature, has been previously used to estimate hot spot temperatures and quench heater delays in NbTi and Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn magnets. However, quench behavior is completely different in high-temperature superconducting (HTS) magnets due to the slow normal zone propagation velocity and the high temperature margin. Because the MIITs concept does not take into account thermal diffusion in the magnet, opposite to the finite-element method (FEM) analysis, the difference of these concepts is studied in this paper. Here, we have taken the approach to compute the hot spot temperatures for a future HTS magnet, designed to be built from REBCO Roebel cable, with MIITs and FEM simulations. The magnet protection is accomplished with a dump resistor, and the effect of quench detection threshold voltage on the hot spot temperature has been studied. Furthermore, the inductance of the magnet increases with the magnet length. Thus, there exists a maximum inductance of the magnet, which should not be exceeded to be able to protect the magnet only with a dump resistor. The hot spot temperatures with different values of inductance are also studied in this paper. Our simulations show that the hot spot temperatures computed with MIITs are from 60 to 150 K higher than those of FEM analysis. Thus, the MIITs concept seems unreliable when considering hot spot temperatures in HTS magnets protected with only dump resistors. However, the MIITs concept might be a usable tool when comparing different magnet designs. If 400 K is the upper limit for the hot spot temperature and the protection scheme includes only a dump resistor, the length of the investigated magnet can be increased to only such value that the magnet inductance is at most 50 mH.

Magnetic field dependent stability and quench behavior and degradation limits in conduction-cooled MgB2 wires and coils

Long lengths of metal/MgB2 composite conductors with high critical current density (Jc), fabricated by the powder-in-tube process, have recently become commercially available. Owing to its electromagnetic performance in the 20−30 K range and relatively low cost, MgB2 may be attractive for a variety of applications. One of the key issues for magnet design is stability and quench protection, so the behavior of MgB2 wires and magnets must be understood before large systems can emerge. In this work, the stability and quench behavior of several conduction-cooled MgB2 wires are studied. Measurements of the minimum quench energy and normal zone propagation velocity are performed on short samples in a background magnetic field up to 3 T and on coils in self-field and the results are explained in terms of variations in the conductor architecture, electrical transport behavior, operating conditions (transport current and background magnetic field) and experimental setup (short sample versus small coil). Furthermore, one coil is quenched repeatedly with increasing hotspot temperature until Jc is decreased. It is found that degradation during quenching correlates directly with temperature and not with peak voltage; a safe operating temperature limit of 260 K at the surface is identified.

Variation of Quench Propagation Velocities in YBCO Cables

We show by modelling that the quench propagation velocity is not constant in HTS coils but it changes during the quench. Due to the large temperature margin between the operation and the current sharing temperatures, the normal zone does not propagate with the temperature front. This means that the temperature will rise in a considerably larger volume when compared to the quenched volume. Thus, the evolution of the temperature distribution below current sharing temperature T c s after the quench onset affects the normal zone propagation velocity in HTS more than in LTS coils. This can be seen as an acceleration of the quench propagation velocities while the quench evolves when margin to T c s is high. In this paper, we scrutinize quench propagation in a stack of YBCO cables with an in-house finite element method software which solves the heat diffusion equation. We compute the longitudinal and transverse normal zone propagation velocities at various distances from the hot spot to demonstrate the distance-variation of these velocities. According to the results in our particular simulation case, the longitudinal normal zone propagation velocity is 30 % higher far away from the quench origin compared to its immediate vicinity when T op=4.2 K and T c s =15 K.

Two-Dimensional Anisotropic Model of YBCO Coated Conductors

The ability of second-generation YBCO coated conductor (CC) tapes to transport high current densities at high temperature, i.e., up to 77 K, and at very high magnetic fields, i.e., above 20 T, are pushing the use of these tapes in various applications, from magnet to power system technologies. An accurate study of their quench behavior is mandatory for the design and safe use of cables and magnets manufactured with this ceramic superconducting material. A new 2-D finite-element method (FEM) numerical quench model, which is called anisotropic model of YBCO CCs, was built for this purpose in the COMSOL Multiphysics environment. One of the most difficult issues in the modeling of the YBCO tapes with the FEM is their high aspect ratio due to the very small thickness of the YBCO layer, about 1 μm. In the model developed, the problem of the high aspect ratio of the tape is tackled by multiplying the tape thickness by a constant factor and then compensating the heat and electrical balance equations through the introduction of material anisotropic properties. The FEM model is validated by comparison with literature experimental data on minimum quench energy and normal zone propagation velocity.

The Influence of the Al Stabilizer Layer Thickness on the Normal Zone Propagation Velocity in High Current Superconductors

We present a two dimensional numerical model for predicting the normal zone propagation velocity in Aluminum stabilized Rutherford NbTi cables. By solving two coupled differential equations under adiabatic conditions, the model takes into account the thermal diffusion and the current redistribution process following a quench. Both the temperature and magnetic field dependencies of the materials properties are included. We study the influence of the thickness of the cladding on the propagation velocity for varying operating current and magnetic field. To assist in the comprehension of the numerical results, we introduce an analytical formula for the longitudinal normal zone propagation.

Study of Quench Behavior of ReBCO Impregnated Pancake Coil with a 75-μm-thick Copper Stabilizer Under Conduction-cooled Conditions

Recently, high performance ReBa2Cu3Ox (ReBCO) coated conductor, such as those with over 500 A per 1cm in width at 77K, have become commercially available. The quench detection of ReBCO pancake coils under conduction-cooled conditions is an important issue for the safe operation of superconducting applications. Although experimental data on the quench characteristics of ReBCO coated conductors have been reported, there is no sufficient database available regarding ReBCO impregnated pancake coils.In this work, a ReBCO impregnated pancake coil was fabricated using ReBCO coated conductors laminated with a 75-μm-thick copper stabilizer. The inner diameter of the ReBCO coil was 50mm, and the outer diameter was approximately 73mm. The measured longitudinal normal-zone propagation velocities (NZPV) in the ReBCO coil were 3–27mm/s at 50–20K. It was also confirmed that the ReBCO coil was shut down from 100 A to 240 A by a balance voltage detection at 40–100mV under conduction-cooled conditions without degradation.

Measurement and Analysis of Normal Zone Propagation in a ReBCO Coated Conductor at Temperatures Below 50 K

Measurementsof the quasi-adiabatic normal zone propagation velocity and quench energies of a Superpower SCS4050 copper stabilised ReBCO superconducting tape are presented over a temperature range of 23 − 47K; in parallel applied magnetic fields of 6, 10 and 14 T; and over a current range from 50% to 100% of Ic. The data are compared to results of analytic predictions and to one-dimensional numerical simulations. The availability of long lengths of ReBCO coated conductor makes the material interesting for many HTS applications operating well below the boiling point of liquid nitrogen, such as magnets and motors. One of the main issues in the design of such devices is quench detection and protection. At higher temperatures, the quench velocities in these materials are known to be about two orders of magnitude lower compared to low temperature superconductors, resulting in significantly smaller normal zones and the risk of higher peak temperatures. To investigate whether the same also holds for lower temperatures more extended data sets are needed, both as input and as validation for numerical design tools.

Analysis of the influence of the normal zone propagation velocity on the design of resistive fault current limiters

Commercial high-temperature superconducting coated conductors (HTS-CCs) have low thermal diffusivity and nonuniform critical current density. These two factors lead commercial HTS-CCs to a partial quench when they are subjected to a transport current around their average critical current (). The consequence is the appearance of localized resistive zones, and a high risk of thermal runaway can arise when HTS-CCs are used for resistive fault current limiter (RFCL) purposes. The enhancement of the normal zone propagation velocity (NZPV) of HTS-CCs is a desirable solution for achieving sufficient thermal stability while keeping the cost of RFCLs under an acceptable threshold. Even though in recent years, several valid methods to increase the NZPV have been proposed, their impact on the design of RFCLs is not clear. For this reason, we developed a one-dimensional numerical model that enables us to simulate HTS-CCs with enhanced NZPV and to study the limitation performance of a HTS-CC-based RFCL in real operating conditions. Our preliminary re sults demonstrate that the NZPV enhancement can effectively limit the needed amount of HTS-CCs with important economic benefits for the design of RFCLs.

Thermal conductivity and dielectric properties of a TiO2-based electrical insulator for use with high temperature superconductor-based magnets

Quench protection is a remaining challenge impeding the implementation of high temperature superconductor (HTS)-based magnet applications. This is due primarily to the slow normal zone propagation velocity (NZPV) observed in Bi2Sr2CaCu2OX (Bi2212) and (RE)Ba2Cu3O7 − x (REBCO) systems. Recent computational and experimental findings reveal significant improvements in turn-to-turn NZPV, resulting in a magnet that is more stable and easier to protect through three-dimensional normal zone growth (Phillips M 2009; Ishmael S et al 2013 IEEE Trans. Appl. Supercond. 23 7201311). These improvements are achieved by replacing conventional insulation materials, such as Kapton and mullite braid, with a thin, thermally conducting, electrically-insulating ceramic oxide coating. This paper reports on the temperature-dependent thermal properties, electrical breakdown limits and microstructural characteristics of a titanium oxide (TiO2) insulation and a doped-TiO2-based proprietary insulation (doped-TiO2) shown previously to enhance quench behavior (Ishmael S et al 2013 IEEE Trans. Appl. Supercond. 23 7201311). Breakdown voltages at 77 K ranging from ∼1.5 kV to over 5 kV are reported. At 4.2 K, the TiO2 increases the thermal conductivity of polyimide by about a factor of 10. With the addition of a dopant, thermal conductivity is increased by an additional 13%, and a high temperature heat treatment increases it by nearly an additional 100%. Similar increases are observed at 77 K and room temperature. These results are understood in the context of the various microstructures observed.

Experiment and Numerical Simulation on Quench Detection in Cryocooler-Cooled YBCO Coil for SMES Application

In the real application of high-temperature superconducting (HTS) coils to a superconducting magnetic energy storage (SMES) system, coated conductors are cyclically subjected to tensile strain due to electrical charging and discharging. A quench is not only caused by failure in the power supply and cooling system but also could be induced by local deterioration of the superconducting characteristics because of the cyclic strain during operation. In such a local deterioration case, the conventional detection method using voltage signal for low-temperature superconducting (LTS) coils is not applicable for the HTS coils because of the significantly slow velocity of normal-zone propagation. Furthermore, the voltage detection method is considered to be extremely difficult because the noise of the converters and other equipment is much larger than the local normal-zone voltage of the HTS coils. Therefore, a new quench-detection method for HTS coils is required. In our previous studies, a current detection method was developed for a cryocooler-cooled SMES coil wound with a kA-class laminated bundle conductor composed of four electrically insulated coated conductors. In the present study, experiments and numerical simulations were carried out on a double pancake model coil that assumes real SMES operation to verify the validity of the current detection method.

Thermal Quench Behaviors of No-Insulation Coils Wound Using GdBCO Coated Conductor Tapes With Various Lamination Materials

This paper presents the quench initiation and propagation characteristics of no-insulation racetrack coils wound using three types of Cu-stabilized GdBCO coated conductors (CCs), with SUS and brass laminations, and without lamination. The test results show that the minimum quench energy values of the brass-laminated GdBCO CC coil were larger than those of the SUS-laminated one. Moreover, the normal zone propagation velocities of the brass-laminated GdBCO CC coil were greater than those of the SUS-laminated one due to the higher thermal conductivity of brass. Moreover, the nonlaminated GdBCO CC coil exhibited greatly improved thermal stability compared to the brass/SUS-laminated GdBCO CC coils, because the local heat in the event of quenching was easily dissipated due to the absence of lamination.

New Fast Response Thin Film-Based Superconducting Quench Detectors

Quench detection on superconducting bus bars and other devices with a low normal zone propagation velocity and low voltage build-up is quite difficult with conventional quench detection techniques. Currently, on ATLAS superconducting bus bar sections, superconducting quench detectors (SQD) are mounted to detect quench events. A first version of the SQD essentially consists of an insulated superconducting wire glued to a superconducting bus line or windings, which in the case of a quench rapidly builds up a relatively high resistance that can be easily and quietly detected. We now introduce a new generation of drastically improved SQDs. The new version makes the detection of quenches simpler, more reliable, and much faster. Instead of a superconducting wire, now a superconducting thin film is used. The layout of the sensor shows a meander like pattern that is etched out of a copper coated 25 μm thick film of Nb-Ti glued in between layers of Kapton. Since the sensor is now much smaller and thinner, it is easier to install and build up a high resistance with a much shorter response time. The design of the sensors is explained. The test results of the new sensors in a few variants in a calibration setup as well as when mounted on the windings surface of a magnet are reported.

High normal zone propagation velocity in second generation high-temperature superconductor coated conductors with a current flow diverter architecture

A high normal zone propagation velocity (NZPV) is a desirable feature in second generation (2G) high-temperature superconductor (HTS) coated conductors (CCs) in order to reduce the probability of developing destructive hot spots. In this work we investigated experimentally the impact of inserting a highly resistive layer that partially covers the HTS–stabilizer interface of 2G HTS CCs. This new layer was called a ‘current flow diverter’ (CFD). The purpose of the CFD is to concentrate the current at the edges of the tape when the current transfers from the HTS to the stabilizer upon a quench event. A series of commercial 2G HTS tapes were modified in order to integrate a CFD into them. Measurements realized on these modified tapes showed that the CFD architecture allowed the NZPV to be enhanced by at least two orders of magnitude in comparison with unmodified commercial tapes. Furthermore, it was shown that the NZPV can be significantly enhanced with a very small increase in the HTS–stabilizer interfacial resistance, which is of prime importance for the reliability of current contacts in real applications.