YBCO Coil Research Articles

The mechanism of thermal runaway due to continuous local disturbances in the YBCO-coated conductor coil winding

Though YBCO coils are stable against transient disturbances such as conductor motion, they suffer from thermal runaway at a current below the coil critical current due to continuous local disturbances attributed to partial degradation of the conductor in the coil winding. Continuous heat generation in the degraded layer induces thermal runaway in adjacent layers; thermal runaway does not occur in the degraded layer spontaneously due to the small n index of the degraded YBCO-coated conductor. The thermal runaway current depends on the cooling conditions of the winding. For a paraffin-impregnated YBCO coil under quasi-adiabatic conditions, the thermal runaway current is far below the coil critical current, while it is close to the coil critical current in the case of a dry-wound coil. The permissible temperature rise following a thermal runaway for YBCO conductors in the degraded layer is demonstrated to be 340 K. If the YBCO coils are operated at a temperature below 20 K, the current density, typically 600–800 A mm−2, is much higher than that at 77 K. Therefore, the time interval between thermal runaway initiation and the melting temperature becomes less than 0.5 s, posing a difficult problem for protection; i.e., thermal runaway due to continuous local disturbances is hazardous to the safe operation of high current density YBCO coils.

Dynamic Simulation Test Research on Power Fluctuation Compensation Using Hybrid SMES of YBCO and BSCCO Tapes

A kJ class high temperature superconducting magnetic energy storage system (HTS-SMES) composed of YBCO and BSCCO coils with a cryogenic system using sub-cooled LN2 is optimal designed. This SMES is applied for the dynamic power fluctuation compensation considering the quench protection in power system. A dual processor combined with the digital signal processor (DSP) and the micro-programmed control unit (MCU) is proposed to construct a LCL filtered voltage source converter system for the SMES. Based on the SVPWM pulse modulation, the power fluctuation compensation strategy of the SMES and its DSP programs are proposed. Finally, a dynamic simulation test system with this SMES for compensation of power fluctuation is build up and the electric power experimental results show that the SMES can output the required compensation power correctly and restrain the power fluctuation in milliseconds.

The experimental research and analysis on the quench propagation of YBCO coated conductor and coil

Quench propagation characteristics of YBCO tapes and coils were studied in this paper. Quench propagation in each test was initiated by a NiCr wire heater mounted on the sample tape and coil. Quench propagation velocity was measured through voltage taps and temperature sensors across different zones of the tapes and coil. Voltage taps and temperature sensors are arranged in longitudinal and radial direction of the coil. The thermal stability margin, the longitudinal quench propagation velocity for the tape and coil, radial quench propagation velocity for coil and the minimum propagation current were measured. In this study, we investigated the relationship between heat inputs and normal zone propagation velocities and the impact of operating currents on normal zone propagation velocities. We also compared the longitudinal quench propagation velocity and the radial quench propagation velocity. Furthermore, we performed numerical simulations by using a computer program based on a one-dimensional heat flow equation model for YBCO tape. The temperature longitudinal distributions of YBCO tape were simulated. The simulation results also prove the relation between heat input and normal zone propagation velocities and agree with the experimental results.

Development of a 5.1 T conduction-cooled YBCO coil composed of a stack of 12 single pancakes

We fabricated and tested a 5T-class conduction-cooled high-temperature superconducting (HTS) coil composed of a stack of 12 single pancake coils wound with YBCO-coated conductors. The length of each single pancake coil was 25m, and the inner diameter of the coil was 50mm. The voltage–current characteristics were measured in liquid nitrogen and under conduction-cooled conditions at 10–60K. At 10K, the central magnetic field of the 12 stacked pancake coils was as high as 5.1T at 305A. We also tested the coil operation in a 4T background magnetic field.

Removal of degradation of the performance of an epoxy impregnated YBCO-coated conductor double pancake coil by using a polyimide-electrodeposited YBCO-coated conductor

Degradation of the epoxy impregnated YBCO coil performance is due to tensile radial stress concentration on the outer edge of the conductor during cool down. This stress acts as a cleavage stress and opens the conductor edge, fracturing the YBCO layer. The fracture propagates to another edge of the conductor, resulting in degradation of the coil performance. Degradation of the epoxy impregnated YBCO coil is eliminated, if we use a polyimide-electrodeposited YBCO-coated conductor: tensile radial stress concentration on the outer edge of the conductor is reduced due to plastic deformation of the ductile polyimide. Polyimide electrodeposition onto the YBCO-coated conductor is reliable, uniform, easy to apply, and can be extended to larger YBCO magnets, removing the risk of coil degradation.

Critical Currents and AC Losses in YBCO Coils

Measurements of V-I characteristics and AC losses of YBCO single pancakes and multipancake coils have been performed. YBCO pancakes and coils were wet-wound using non-insulated YBCO tapes. Layer-to-layer insulation was achieved by using epoxy filled with alumina particles that ensures higher effective current density of the coil in comparison with coils of standard insulated tapes. This procedure also improves the coil's thermal conductivity in the direction perpendicular to the tape plane. Static and alternating magnetic fields have been applied to the AC current carrying coils during measurements. DC magnetic fields up to 50 mT affect the V-I curves only slightly. On the other hand, AC magnetic fields with amplitudes up to 50 mT and frequencies 4-100 Hz change the form of the V-I curves and strongly reduce the critical current. AC losses and their distribution along the coil were measured by electrical and calorimetric methods. It was found that AC losses peak at the central region of the coil and depend strongly on distance between adjacent pancakes. Surprisingly, adjacent pancakes, even if kept in an open circuit state, increase the losses of a current carrying pancake. It is suggested that introduction of gaps between pancakes/double pancakes can be used as means for decreasing AC losses in multi-pancakes YBCO coils.

Magnitude of the Screening Field for YBCO Coils

Screening current induced in a YBCO-coated conductor coil causes two major problems; (i) reduction in the central magnetic field and (ii) temporal magnetic field drift due to flux creep. They constitute disadvantages for YBCO coil applications such as NMR, MRI, accelerator and high field magnets. The second problem is effectively suppressed by current sweep reversal, while the first remains unsolved. The present paper demonstrates that the screening current-induced magnetic field (screening field) is dominated by (a) the YBCO coil shape, (b) the YBCO-coated conductor width, (c) the coil inner diameter and (d) the ratio of operating current to the coil critical current. The dependence on these quantities is systematically investigated by numerical simulations. We conclude that coils with a smaller width of YBCO-coated conductor, a larger inner diameter and a higher ratio of operating current to the coil critical current generate a smaller central screening field ratio.

Quench Detection Method of HTS Model Coil Using a Series-Type Thermocouple

This paper proposes a quench detection method applied to a HTS coil and also discusses the results of tests on an HTS BSCCO wire and an HTS YBCO coil. A series-type thermocouple was fabricated to detect quench, where several thermocouples were welded in series. As the series-type thermocouple has many points of detection, variation in temperature was detected at each point by using thermo-electromotive force. The feasibility of the proposed method was investigated through the quench detection tests on the YBCO coil. This method has advantages such as a simple structure and cost effectiveness and also protects the HTS coil by detecting partial quench and its propagation.

Thermal Stability of Conduction-Cooled YBCO Pancake Coil

When a high-temperature superconducting (HTS) coil is operated using a conduction-cooling technique, the coil should be impregnated with epoxy resin because the coil is not sufficiently cooled without it. On the other hand, it is important that the coil should have no damaged area in the winding because the damaged area would generate heat locally and the coil would eventually quench. However, whenever we fabricated an impregnated HTS coil wound with YBCO tapes and evaluated its V-I characteristics, the n-value of the coil was much lower than we expected, indicating that the critical currents of some areas in the winding drastically decreased. Therefore, we have started to improve the performance of an impregnated HTS coil wound with YBCO tapes. In this paper, we investigated the cause of the degradation and found that the degradation did not occur when we decreased the radial tensile stress in the windings. Then we fabricated four single-pancake coils, stacked them, and tested them in a conduction-cooled condition. The measured V-I curves were in good agreement with the calculated ones, suggesting that we successfully developed a technique of fabricating an impregnated HTS coil wound with YBCO tapes with no degradation. We also measured thermal runaway currents of a conduction-cooled HTS coil composed of two single-pancake coils wound with YBCO tapes and numerically simulated the thermal properties by using a three-dimensional heat conduction equation in order to study the thermal stability of the YBCO coil. The measured thermal runaway currents were in good agreement with the calculated ones.

Quench Behavior and Protection in Cryocooler-Cooled YBCO Pancake Coil for SMES

The thermal behavior of a high-temperature superconducting (HTS) coil is significantly different from that of low-temperature superconducting (LTS) coil because it has a greater volumetric heat capacity at the temperature required for practical use. Therefore, the possibility of quench in HTS coils is much lower than that in LTS coils. In the application of the YBCO coil to Superconducting Magnetic Energy Storage (SMES) system, electrical charging and discharging are repeated; therefore, the superconducting characteristics of the YBCO coated conductor may deteriorate as a result of cyclic subjection to tensile strain. To enhance the reliability and safety of HTS coil, protection scheme assuming a quench is also required for HTS coil. In this study, we focus on the coil wound with YBCO laminated bundle (parallel) conductor supposing SMES application and investigate the characteristics of normal-zone propagation and the thermal behavior within the coil during a quench by using a newly developed computer code based on the finite element method and an equivalent circuit. And we also propose a quench detection method using observation of nonuniform current in YBCO laminated bundle conductor and discuss the validity of the detection method comparing with conventional quench-voltage detection.

Remarkable weakness against cleavage stress for YBCO-coated conductors and its effect on the YBCO coil performance

Cleavage strength for an YBCO-coated conductor at 77K was investigated with a model experiment. The nominal cleavage strength for an YBCO-coated conductor is extremely low, typically 0.5MPa. This low nominal cleavage strength is due to stress concentration on a small part of the YBCO-coated conductor in cleavage fracture. Debonding by the cleavage stress occurs at the interface between the buffer layer and the Hastelloy substrate. The nominal cleavage strength for a slit edge of the conductor is 2.5-times lower than that for the original edge of the conductor; cracks and micro-peel existing over the slit edge reduce the cleavage strength for the slit edge. Cleavage stress and peel stress should be avoided in coil winding, as they easily delaminate the YBCO-coated conductor, resulting in substantial degradation of coil performance. These problems are especially important for epoxy impregnated YBCO-coated conductor coils. It appears that effect of cleavage stress and peel stress are mostly negligible for paraffin impregnated YBCO-coated conductor coils or dry wound YBCO-coated conductor coils.

Technology Development Status of Yttrium Based Coil for SMES

A new Japanese national project has started in 2008 to develop high temperature superconducting electric power devices. In this project, we have developed a superconducting magnetic energy storage (SMES) system, which are highly expected as a stable power supply. Main issues are a large current capacity conductor made of YBCO-coated tapes and a compact coil wound the conductors. Aiming at the manufacturing of compact coil with higher energy density through the creation of higher magnetic field, we produced YBCO coil, and by carrying out hoop stress tests. Also, aiming at the development of 20 MJ class SMES component coils necessary for picturing the technological outlook of 2 GJ class SMES coil for power system control, we manufactured component coils (outer diameter 650 mm), adopting the bundled conductor that would enable the realization of high strength and low loss, proving that its current capacity was large enough to exceed 2.6 kA, through basic verification test for energization. In addition, we have challenged the development of high efficiency coil conduction cooling technology, stable manufacturing technology of coated conductors for SMES and the highly reliable/highly durable SMES coil component technology.

Degradation of the performance of a YBCO-coated conductor double pancake coil due to epoxy impregnation

Now that YBCO-coated conductors have been commercialized, a number of YBCO coils have been developed. However, their basic performances have not been systematically investigated so far. Here, we demonstrate that of a YBCO double pancake coil. The critical current of an epoxy impregnated YBCO double pancake coil was substantially degraded, i.e. the normal voltage appears above 8A, only 18% of that for the dry coil. It was inferred that degradation occurs if the cumulative radial stress developed during cool down exceeds the critical transverse stress for the YBCO-coated conductor (typically 10MPa). Under these conditions, the conductor was debonded at the interface between the buffer layer and YBCO layers, or fractured in the YBCO layer itself, causing cracks on the YBCO layer, resulting in a significant decline of the critical current. These negative effects are suppressed if the coils are dry wound or impregnated with paraffin, as the bonding strengths between turns are negligible and therefore turns are separated if the cumulative radial stress tends to be tensile. For non-circular coils in which epoxy impregnation is inevitable, degradation due to cumulative tensile transverse stress is still the major problem.

Effect of YBCO-Coil Shape on the Screening Current-Induced Magnetic Field Intensity

A numerical simulation method which deals with the screening current-induced magnetic field for YBCO coils, including the self field effect induced by the transport current, has been developed. The simulation agrees well with the experimental results for an YBCO solenoid. Based on the numerical simulation, the effect of coil shape on the screening current-induced magnetic field intensity for the YBCO coils has been investigated. The field was demonstrated to reach a maximum if the solenoid corresponds to the minimum-volume design; it amounts to as large as -18% of the central magnetic field. Two major problems must be considered for YBCO coils regarding the screening current: (a) a reduction in the central magnetic field by the screening current and (b) a temporal drift of the apparent magnetic field due to relaxation of the screening current by flux creep. It is suggested that the latter can be suppressed by a current sweep reversal technique.

High Field Magnets With HTS Conductors

Development of high-field magnets using high temperature superconductors (HTS) is a core activity at the NHMFL. Magnet technology based on both YBCO-coated tape conductors and Bi-2212 round wires is being pursued. Two specific projects are underway. The first is a user magnet with a 17 T YBCO coil set which, inside an LTS outsert, will generate a combined field of 32 T. The second is a 7 T Bi2212 demonstration coil set to be operated in a large bore resistive magnet to generate a combined magnetic field of 25 T. Owing to the substantial technological differences of the two conductor types, each project faces different conductor and magnet technology challenges. Two small coils have been tested in a 38-mm cold bore cryostat inserted in a 31 T resistive magnet: a Bi2212 round-wire layer-wound insert coil that generated 1.1 T for a total of 32.1 T and a YBCO double-pancake insert that generated 2.8 T for a total central field of 33.8 T. Four larger layer-wound coils have been manufactured and tested in a 20 T, 186-mm cold bore resistive magnet: a sizeable Bi-2212 coil and three thin large-diameter YBCO coils. The test results are discussed. The current densities and stress levels that these coils tolerate underpin our conviction that >30 T all-superconducting magnets are viable.

The Characteristics of the Normal Transition in the Longitudinal and Transverse Directions in Cryocooled YBCO Coils

Recently, the superconducting devices using cryogenic and superconducting technologies were widespread in the field of electric power. The 2G wires with YBCO superconducting layer, metal substrate and stabilizer layer are expected to be used in future superconducting applications, because it has high Jc properties in high temperature and high magnetic fields. In order to operate safely at high transport currents, the thermal stability of the wounding wire should be addressed in terms of detection and protection against the quench. On the other hand, the understanding of the 3-dimensional normal zone expansion during a quench is very important to design the superconducting devices such as magnets and cables. This paper presents the results of an experimental study for normal zone propagation properties in the longitudinal and transverse directions using the pancake coil wound with 2G wires which have Cu and SUS stabilizers under the cryocooler cooling. And the minimum quench energy (MQE) of the pancake coils wound with two different kinds of stabilizers as functions of operating temperature and transport current will be described.

Determination of stabilizer thickness for YBCO coated conductors based on coil protection

The possibility of the occurrence of quench caused by mechanical disturbance in high-temperature superconductors (HTS’s) is much lower than that in low-temperature superconductors. However, to enhance the reliability and safety of HTS coils, it is necessary to establish a stability criterion and a protection scheme on the basis of the assumption that the quench is caused by the failure in power supply, cooling systems, etc. In this study, we assume that the stored magnetic energy in the HTS coil is dissipated by an external resistance that acts as a quench protection. Under this assumption, we derive a function that examines the heat balance of quenching YBCO coil under an adiabatic condition and propose a criterion using the derived function to determine an appropriate stabilizer thickness of YBCO conductors.

System Coordination of 2 GJ Class YBCO SMES for Power System Control

YBCO superconducting wire has a relatively low decrease in power distribution at high temperatures and under a high magnetic field. A high-intensity substrate is used for the wire, so the wire has high machine characteristics. Therefore, it is expected that this wire can be used for large-scale high magnetic field coils. Here, coordination between the SMES system for 100 MVA/2 GJ class load fluctuation compensating was conducted using IBAD/CVD-YBCO wire. The SMES system includes a toroidal type YBCO coil consisting of 180 compact, high magnetic field multi-unit coils, a large coil cooling system that uses the conduction cooling method, which does not use a refrigerant medium, and a multi-cell power converter that achieves multi-unit coil connection with relatively low current and low voltage. Studies were conducted for each individual device and for the whole system. Based on the study plan in this paper, it has become possible to develop and coordinate each device of the 100 MVA/2 GJ class power system load fluctuation compensation SMES system using YBCO wire, which up until now had seem impossible as an actual system.

Detection of temperature rise in YBCO coil by time–frequency visualization of AE signals

The normal zone propagation velocities in the high T C superconductors are slow at the high operation temperature and therefore the local and excessive temperature rise generates when the superconducting-to-normal transition occurs, and then the superconductors are degraded or burned. Therefore it is essential to detect the temperature rise in the high T C superconducting coils as soon as possible to protect the coils. In general, the temperature rise can be detected by many voltage taps and thermocouples, however, the taps and thermocouples make the electric insulation of the windings degraded. The author has presented the time–frequency visualization of AE signals as a method to detect the temperature rise with non electric contact and the usefulness for Bi 2Sr 2Ca 2Cu 3O x (Bi2223) superconductors has been presented. In this paper, the usefulness of the proposed method for YBa 2Cu 3O x (YBCO) superconductors is verified and the experimental results show that the proposed method can detect the temperature rise in a YBCO coil.

Coil Development using YBCO Tape Coated Conductor for Power Applications

Each superconducting coil manufacturing process using YBCO tape coated conductor was evaluated, and double pancake type copper stabilized coils were verified, in order to develop base technologies of a YBCO coil for power applications. Cost of YBCO tape including a stabilization process needs to be low in order for it to be a practical use wire for power application coils. Conduction cooling is also needed below the 50K temperature range, which is optimal for high magnetic field coils using YBCO tape. Therefore, we developed technology for each process for manufacturing of a conduction cooling type coil. Techniques for winding, stabilization, electrode formation, and impregnation were developed, considering YBCO tape configuration and a conduction cooling method. Several wire coils were then manufactured and verified using a conduction cooling system, and the high performances of thermal stability, low electrical joint resistance, etc. were achieved without degradation.