Normal Zone Propagation Velocity Research Articles

Influence of the Normal Zone Propagation Velocity on the Thermal Performance of HTS Coated Conductors Under DC Over-Current Impact

The voltage source converter (VSC)-based DC distribution system that uses self-turnoff power electronic devices is getting attractive, and the cooperation of resistive superconducting fault current limiters (R-SFCLs) and DC circuit breakers has become a practical strategy to isolate the DC fault. However, high temperature coated conductors (HTS-CCs) widely used in R-SFCLs are prone to generate localized resistive zones and have a very low normal zone propagation velocity (NZPV). Therefore, it is meaningful to investigate the effect of different NZPV values on the thermal performance of HTS-CCs used in the R-SFCLs of VSC-based DC distribution systems. This paper developed a prototype of the VSC-based DC distribution system and a model of the HTS-CC in software, PSCAD and COMSOL respectively. The results showed that the increase of the NZPV could improve the superconductor's uniformity of temperature distribution after the DC over-current impact.

Measurement of splice resistance and normal zone propagation velocity in REBCO tapes for the Superconducting Link of HL-LHC

The Superconducting Link (SC-Link) being developed at CERN in the context of the LHC High-Luminosity upgrade (HL-LHC) will supply the current to superconducting magnets of the HL-LHC Inner Triplets and Matching Sections. The SC-Link consists of MgB2 highcurrent cables, and it includes high temperature superconducting REBCO cables making the electrical transition between the MgB2 and the current leads. Electrical protection of superconducting devices made of REBCO against unexpected quenches is challenging especially when operation is at high current density. Adequate electrical protection of REBCO cables is crucial to insure good performance of the circuit over the lifetime of the machine; protection relies on the detailed knowledge of the quench propagation velocity. In this paper we reviewed the electromagnetic properties of REBCO tapes procured for prototype work. We measured the splice resistance (Rs ) of the REBCO conductors and found a significant variation among the different batches of conductor. The normal zone propagation velocity (NZPV) was reported to be affected by the interfacial resistance which is governing the splice resistance: increasing the interfacial resistance enhances the NZPV. In order to determinate the influence of the internal resistance on the quench behaviour of the tapes, we investigated the normal zone propagation velocity of the epoxy-impregnated tapes at 77 K in self-field and 4.2 K in field up to 7 T in the FRESCA test station. We found that despite the Rs variation observed (up to factor 4), the tapes show similar NZPV at both temperatures. In addition we observed a significant field dependence of the NZPV of the tapes at 4.2 K. The results are compared to numerical simulation and discussed.

Critical aspects in the measure of the quench propagation velocities on HTS tapes

This paper concerns the investigation of the quench propagation phenomena in High Temperature Superconducting HTS tapes. In particular the determination of the Normal Zone Propagation Velocity NZPV, is based on the measure of the time propagation of the normal zone by reading the rising voltages detected by voltage taps placed at known distances. Several experimental aspects related to the measured times and voltages with their dependencies on temperature have to be considered. In particular, the rise up of the temperature determines a time limit that has to be considered to avoid the damage of the tape. Based on our measures of NZPV and in agreement with the results of a numerical model developed for our experimental environment, it is shown that the time limit of the measures is set by the value of the bias current. The tape length traveled by the normal zone front wave is related to this measure time and depends on the value of the NZPV that have to be measured. Adopting a simplified approach, it is possible to forecast the values of the tape length that may be used for the NZPV measurements and how its value may change as a function of the bias current.

A Novel Fault Diagnosis Method for High-Temperature Superconducting Field Coil of Superconducting Rotating Machine

In this paper, a new method is presented for sensitive quench detection in high-temperature superconductor (HTS) rotating machinery. The normal zone propagation velocity of an HTS is about 1000 times slower than that of a low-temperature superconductor. Therefore, the propagation of normal zone resistance, which occurs when the HTS transits from the superconducting state to the normal state, is also slower. Thus, it is difficult to detect the abnormal signals by voltage measurement using voltage taps. Moreover, the monitoring signal includes noise generated by interaction between the HTS rotating machinery and the industrial environment. Therefore, when quenching occurs in the HTS rotating machinery, a thermal runaway occurs in the hot spot. Furthermore, the magnetic energy stored in the HTS coil can damage the machinery. For these reasons, a new method is proposed for sensitive quench detection that reduces the noise generated from the power supply and from the HTS rotating machinery, using both an RLC resonance circuit and fast Fourier transform analysis.

Quench Experiments of Conduction-Cooled Coated Conductors With Various Copper-Stabilizer Thicknesses

We experimentally studied the quench properties of REBa2Cu3Oy (RE-123) coated conductors with various plated-copper thicknesses (20 and 40 μm). A short sample of coated conductors was conduction-cooled to 45 K, a magnetic field (μ0H, up to 2 T) was applied perpendicular to its wide face to control the critical current, an operating current was supplied, and subsequently quench was initiated using a small heater. Normal zone propagation velocities (NZPVs) were measured at various operating currents, and the NZPVs of coated conductors with various copper thicknesses were compared with each other. To understand the impact of the copper stabilizer on quench protection, hot-spot temperatures were measured during the processes that simulate quench detection using voltage taps and protection using dump resistor. The maximum hot-spot temperatures were plotted against the operating current as well as the overall current density, and the impact of the thickness of the copper stabilizer on hot-spot temperature was examined. The impact of the initial temperature on hot-spot temperature was also studied.

Quench, normal zone propagation velocity, and the development of an active protection scheme for a conduction cooled, react-and-wind, MgB2 MRI coil segment

The development of coils that can survive a quench is crucial for demonstrating the viability of MgB2-based main magnet coils used in MRI systems. Here we have studied the performance and quench properties of a large (outer diameter: 901 mm; winding pack: 44 mm thick × 50.6 mm high) conduction-cooled, react-and-wind, MgB2 superconducting coil. Minimum quench energy (MQE) values were measured at several coil operating currents (Iop), and distinguished from the minimum energy needed to generate a normal zone (MGE). During these measurements, normal zone propagation velocities (NZPV) were also determined using multiple voltage taps placed around the heater zone. The conduction cooled coil obtained a critical current (Ic) of 186 A at 15 K. As the operating currents (Iop) varied from 80 to 175 A, MQE ranged from 152 to 10 J, and NZPV increased from 1.3 to 5.5 cm s−1. Two kinds of heater were involved in this study: (1) a localized heater (‘test heater’) used to initiate the quench, and (2) a larger ‘protection heater’ used to protect the coil by distributing the normal zone after a quench was detected. The protection heater was placed on the outside surface of the coil winding. The test heater was also placed on the outside surface of the coil at a small opening made in the protection heater. As part of this work, we also developed and tested an active protection scheme for the coil. Such active protection schemes are of great interest for MgB2-based MRIs because they permit exploitation of the relatively large MQE values of MgB2 to enable the use of higher Je values which in turn lead to competitive MgB2 MRI designs. Finally, the ability to use a quench detection voltage to fire a protection heater as part of an active protection scheme was also demonstrated.

Normal Zone Propagation Velocity and Minimum Quench Energy of Stainless Steel Double-Layered Superconducting Wires Under External Magnetic Fields

A comprehensive study of the quench properties of single laminated wires having a double-layered superconducting structure is presented. In particular, we have focused on the influence of the angle and intensity of an external magnetic field perpendicular to direction of the applied current on the minimum quench energy (MQE) and normal zone propagation velocity (NZPV) values. We conclude that strong changes on the NZPV are mainly determined by the intensity of the applied current, while the orientation and magnitude of the applied field have a less but not negligible impact on the resulting NZPV. The quench parameters are also reported for the case when the applied current is considered as a constant parameter in which case the MQE has been found to reach a maximum value when the in-field critical current is the largest.

Experimental Analysis of Quench Characteristic in HTS Tapes and Coils

Normal zone propagation velocity (NZPV) and minimum quench energy (MQE) are the important parameters for quench protection of superconducting magnets. In this research, a series of experiments were conducted on single-length conductor, small coils, and parallel double conductors made of YBCO and BSCCO high-temperature superconducting (HTS) tapes at the liquid nitrogen temperature, and their normal zone propagation velocities and minimum quench energies were tested and analyzed. The experimental results show that the NZPVs of HTS single-length tape, parallel double tapes, and coils are closely related to the critical current ratio i of HTS tapes. When i is in a certain range, the NZPV is approximately proportional to i, and similarly, MQE is inversely proportional to i.

Experiment and Simulation for Normal Zone Propagation of Multifilament MgB2 Superconducting Wire Cooled by Liquid Hydrogen

Developing applications of liquid-hydrogen (LH 2 )-cooled superconducting devices is a challenging issue. Since the boiling point of LH 2 is 20.4 K, MgB 2 , whose critical temperature is 39 K, can be cooled with a sufficient temperature margin. Furthermore, MgB 2 wire is expected to be used for superconducting equipment due to the low production and material cost. Therefore, in order to design MgB 2 superconducting equipment, the knowledge of normal zone propagation phenomena is important for the thermal stability and the quench protection. In this study, normal zone propagation and minimum quench energy (MQE) with a multi-filamentary MgB 2 superconducting wire produced by Hitachi, Ltd. were observed under immersed in LH 2 . In the experiment, heat pulse, which initiates a normal zone, was injected to the center area of 200 cm long MgB 2 wire. Then, the MQE and the normal zone propagation velocity (NZPV) were measured under specific conditions. NZPV was in the order of several cm/s and MQE was in the order of a few J at 30 K under LH 2 cooling. In order to clarify temperature distribution along the wire during the normal zone propagating phenomena, the simulation model of MgB 2 wire cooled by LH 2 was created and analyzed using a finite element method simulation software.

Impact of Current Flow Diverter on Innovative HTS Tape Architectures for DC Fault Current Limitation at Electric Fields up to 150 V/m

In this work, we use numerical simulations to compare the performance of different promising architectures of HTS tape for DC fault current limitation. A 1-D finite-element model which solves the heat equation through the thickness of a high-temperature superconducting (HTS) tape at the location of a hot spot was coupled with a simple electrical circuit model to perform the simulations. Using the normal zone propagation velocity (NZPV) obtained from a distinct 3-D finite-element electrothermal model, the quench dynamics in HTS tapes can be predicted, which allows estimating quite accurately the fault current level vs time. The calculations indicate that the insertion of a current flow diverter in the tape architecture allows decreasing the fault current level more rapidly and reducing the temperature elevation, thanks to the increased NZPV.

Development and Study of Nb3Sn Wires With High Specific Heat

Recently, a new type of Nb3Sn conductor was developed, which, by incorporating substances with high specific heat at 2–8 K in a proper design, has demonstrated significant improvement of minimum quench energy (MQE). This approach to improving conductor stability is promising to reduce Nb3Sn magnet training. This paper continues studying this type of conductor. Voltage–current ( V-I ) tests from 15 to 0 T and voltage–field ( V-B ) sweeps at various currents were conducted to investigate the influence of increased specific heat on the intrinsic stability. MQE and normal zone propagation velocity (NZPV) were also measured. Finally, a scheme was put forward to introduce new types of substances with not only high specific heat but also high thermal diffusivity to Nb3Sn conductors, which is expected to work more effectively.

Systematic Study of the Contact Resistance Between REBCO Tapes: Pressure Dependence in the Case of No-Insulation, Metal Co-Winding and Metal-Insulation

No-insulation and partial-insulation are novel approaches for compact, high field rare-earth-Ba2Cu3O7-x-coated conductor magnets. They maximize the current density and overcome some of the coated conductors’ intrinsic challenges such as the low normal zone propagation velocity and the resulting difficulties to quench REBCO magnets safely. However, this comes at a reduction of the field quality and an increase of the charging time. The performance of this type of coils is mainly determined by the contact resistance between the different layers of the winding. In the case of partially-insulated coils, the contact resistance can be adjusted by a tailored choice of the material used for the metal co-winding or for the metallic coating of the REBCO tape. We present here a systematic study of the impact of pressure on the contact resistance in the cases of direct contact (no-insulation) as well as metal co-winding and metal coating (partial-insulation). Different “insulation” metals as well as various “insulation” to REBCO tape ratios are tested at 77 K in self-field varying the contact pressure in the range of 10–200 MPa. The goal is to provide coil manufacturers with the means of achieving a controlled contact resistance to optimize the coil performance.

Status of the European Union Project FASTGRID

High voltage direct current super-grids are one attractive solution for the transmission of bulk power of renewable electricity over long distances. Their protection is still an issue and superconducting fault current limiters (SCFCL) offer attractive perspectives. However, the actual superconducting tapes are not yet properly designed for operation at high voltages (>100 kV): The electric field developed during the current limitation is still too low (approximately 50 V/m for 50 ms) and the limiter requires too long lengths of tape. The European project FASTGRID aims at improving the properties of the REBCO tapes to enhance significantly (by 2-3 times) the electric field limit and so the economical SCFCL attractiveness. We use advanced THEVA tapes. Substantial improvements are also planned on the stabilizer (shunt) layer. Several shunt ways are under investigations. Other improvement on the tape properties will be carried out, namely the increase of the normal zone propagation velocity by at least one order of magnitude, with the help of the innovative current flow diverter architecture. The optimized conductor will be used in a SCFCL module (≈1.5 kA-50 kV) tested at 65 K. We will monitor the temperature along the conductor using an attached optical fibre. FASTGRID will also develop innovative tapes based on sapphire substrate, which can tolerate very high electric fields, in the range of kilovolts per meter. Validated on the laboratory scale, this game-changing technology needs to be implemented at long lengths with an industrial process. We will provide an overview of the project and its first results.

Numerical simulation of quench initiation and propagation in multi-filamentary Bi2Sr2CaCu2Ox round wires at high magnetic fields

In order to protect high-field superconducting magnets during a quench with rapidly rising hot spot temperatures, it is important to have a quantitative understanding of quench behaviors of superconducting wires at high magnetic fields. In this article, we developed a two-dimensional numerical model of quench initiation and propagation in Bi2Sr2CaCu2Ox/Ag/Ag alloy (Bi2212) multifilamentary composite round wires with the H-formulation and the heat conduction equation. The results show that the minimum quench energy (MQE) of Bi2212 round wires decreases with increasing magnetic field and closely relates to the current sharing temperature (Tcs). For a constant ratio of the transport current to the critical current (It/Ic), the normal zone propagation velocity (NZPV) decreases with increasing magnetic field. However, the declining trend of NZPVs slows down beyond 10 T. Moreover, for a constant transport current It, the Ic and Tcs decrease with the increasing magnetic field, which results in a simultaneous increase of NZPV with a decrease of MQE.

Quench dynamics study in 2nd generation HTS tapes

The faster quench detection is required for high temperature superconductor (HTS) to avoid the local damage of conductor due to overheating. This problem is particularly challenging for high temperature superconducting magnets due to the slower normal zone propagation velocity (NZPV) in comparison to low temperature superconductor (LTS). As a result, quench detection and protection for HTS coils is matter of concerned for fusion community working for the 2nd generation high field tokomak. The Di-BSCCO HTS tape samples of critical current around 160 A are selected to study the quench dynamic experiment. The Normal zone voltage rise and the temperature evolution after thermal quench were experimentally measured in Di-BSCCO tapes. The measured voltage and temperature signals have been analysed and elaborated in this paper.

A Numerical Adiabatic Model for the Quench Behavior Analysis of the Ag-Matrix Bi-2212 Round Wire

To study the quench behavior of the Bi-2212 round wire as a promising superconductor for future fusion reactor coils and high-field magnets, a numerical adiabatic model has been developed at the Institute of Plasma Physics, Chinese Academy of Sciences. The model is in 1-D and the partial differential equation is solved with the implicit method. Quench energy is simulated by introducing heat in a section of the wire, and the temperature and voltage are calculated as the function of time and space. The minimum quench energy and normal zone propagation velocity were calculated at different conditions. The results from the model were compared with analytical solution and experiment data. The analysis and discussion are presented in this paper.

Quench Behavior Comparison Between Solid Nitrogen and Conduction Cooled REBCO Coated Conductor

The quench properties for REBCO conductor in SN2 and conduction cooled system are studied. The normal zone propagation velocity determined by voltage, temperature, and strain methods is compared to demonstrate the feasibility of strain method to detect the quench of the REBCO conductor.

Use of the buffer layers as a current flow diverter in 2G HTS coated conductors

This paper presents a simple and effective approach to increase the normal zone propagation velocity (NZPV) in (RE)BaCuO thin films grown on a flexible metallic substrate. The concept is an extension of the current-flow-diverter concept already known in the literature. The key idea behind the novel approach is to use a specific geometry of the silver thermal stabilizer that surrounds the superconducting tape. More specifically, a very thin layer of silver stabilizer is deposited on top of the superconductor layer, typically less than 100 nm, while the remaining stabilizer (still silver) is deposited on the substrate side. Normal zone propagation velocities up to 170 cm s−1 at 77 K have been measured experimentally for the first time with this novel architecture, corresponding to a stabilizer thickness of 20 nm on top of the superconductor layer. This is at least one order of magnitude faster than the NZPV speeds measured on commercial 2G HTS tapes. Our results clearly demonstrate that a very thin stabilizer on top of the superconductor layer leads to high normal zone propagation velocities. The experimental values are in good agreement with predictions realized by finite element simulations. Furthermore, the propagation of the normal zone during the quench was recorded in situ and in real time using a high-speed camera. Due to high Joule losses generated on both edges of the tape sample, a ‘U-shaped’ profile could be observed at the boundaries between the superconducting and the normal zones, which matches very closely the profiles predicted by the simulations. The most direct application of this new HTS tape architecture is in high field magnets, since faster quench propagation allows easier detection and protection, which is likely to accelerate substantially the development of HTS magnets used as high-field MRI/NMR systems, etc.

Development of design for large scale conductors and coils using MgB2 for superconducting magnetic energy storage device

MgB2 wires are commercially available, and their superconducting characteristics have been continuously developed in the last decade. The relatively high critical temperature of these wires has attracted the attention of researchers, especially in the field of superconducting magnetic energy storage (SMES) coil applications in terms of its relatively high critical temperature, as it enables the use of liquid hydrogen for cooling the coils. The sensitivity of multi-filament MgB2 wires to bending strain makes the design of large-scale conductors and coils for an SMES system technologically difficult, and the careful investigation of the applied strains during manufacturing is required. Two-conventional methods have been introduced for the fabrication of the coils: wind-and-react (W&R) and react-and-wind (R&W). These methods have been demonstrated to be suitable for the production of large-scale MgB2 magnets to maximize the coil performance. The W&R and R&W methods have been successfully applied to the designs of conductors and coils, and small W&R test coil fabrication, as well as stability demonstrations are performed in this study. Our study is the first to demonstrate the feeding of hundreds of amperes of transport current using multifilamentary MgB2 wires at around liquid hydrogen temperature in the practical background magnetic field of 2 T. The minimum quench energy and normal zone propagation velocity are also experimentally investigated for the protection of the actual coils for SMES application.

Experimental Study of Normal Zone Propagation in the Short Sample of Bi-2223/Ag/AgMg Tape in Two Different Cooling Modes

The normal zone propagation evolution of a short sample of Bi-2223/Ag/AgMg high-temperature superconducting tape has been investigated. We considered two different cooling modes; conduction–convection and only convection. For different currents, the minimum quench energy and the normal zone propagation velocity (NZPV) were measured and compared. The results show that the NZPV changes are more dominated by cooling system than the transport current.