HTS Pancake Coils Without Turn-to-Turn Insulation (original) (raw)
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Superconductor Science and Technology, 2013
This paper presents experimental and analytical studies on the characteristic resistance of NI (noinsulation) ReBCO pancake coils, which are used in an equivalent circuit model to characterize 'radial as well as spiral' current paths within the NI coils. We identified turn-to-turn contact resistance as a major source of the characteristic resistance of an NI coil. In order to verify this, three single pancake NI HTS coils-60, 40, 20 turns-were fabricated with their winding tension carefully maintained constant. A sudden discharge test was performed on each coil to obtain its characteristic resistance, and the relation between the turn-to-turn contact and the characteristic resistance was investigated. Based on the characteristic resistance and the n-value model, an equivalent circuit model was proposed to characterize the time-varying response of the NI coils. Charging tests were performed on the three test coils and the experimental results were compared with the simulated ones to validate the proposed approach with the equivalent circuit model.
Effect of Winding Tension on Electrical Behaviors of a No-Insulation ReBCO Pancake Coil
IEEE Transactions on Applied Superconductivity, 2014
This paper presents a study on the effects of winding tension on the characteristic resistance of a no-insulation (NI) coil. Two ReBCO NI test pancake coils, having the same winding i.d. (60 mm), o.d. (67.6 mm) and number of turns (60), were sequentially prepared in a way that the first test coil was wound with a winding tension of 12-N, tested, and then rewound with a new winding tension of 20-N for the same tests. In each test, the test coil was energized at a target current, the power supply was "suddenly" disconnected, and then the temporal decay of the coil center field was measured, from which the time constant of the test coil and the consequent characteristic resistance were obtained. To check the reproducibility of experimental data, each test was repeated 4 times and each time the test coil was unwound and rewound with a given winding tension. The experimental results were analyzed with equivalent circuit analyses. Correlation between the winding tension and the characteristic resistance was discussed in detail.
Partial-Insulation Winding Technique for NbTi Coils
IEEE Transactions on Applied Superconductivity, 2014
In 2010, we reported, for the first time, the noinsulation (NI) winding technique for high-temperature superconducting (HTS) pancake coils. Based on our test results of small NbTi NI coils, reported in 2011, the key benefits, i.e., enhanced mechanical integrity, compactness, and thermal stability, of the NI winding technique, appear intact for lowtemperature superconducting (LTS) windings. However, the intrinsic charging delay observed in NI coils, caused by the lack of insulation, is more pronounced in LTS coils of a bare round wire than in HTS pancakes of thin, wide tape. Thus, to significantly reduce the charging delay in LTS coils of a bare round wire, we proposed a partial-insulation (PI) winding technique, a variation of the NI technique. In the PI winding of a bare round wire, a thin insulation sheet is introduced every few layers-note that in the PI winding, there are no turn-to-turn insulations. This paper reports results, experimental and analytical, of the PI winding technique in which bare-round-wire NI and PI coils were prepared to quantify the effects of PI winding technique. Three LTS coils of the identical dimension and magnet constant were wound with 0.4 mm diameter NbTi mono-filament wire and tested in a bath of liquid helium at 4.2 K, respectively, with three winding techniques: insulated (INS); NI; and PI. We analyzed the experimental results by applying an equivalent circuit model that had earlier been successfully applied to another set of experimental results. A graph model of resistance matrix was applied to estimate characteristic resistance of both NI and PI coils.
Theoretical and experimental study of AC loss in HTS single pancake coils
2008
The electromagnetic properties of a pancake coil in AC regime as a function of the number of turns is studied theoretically and experimentally. Specifically, the AC loss, the coil critical current and the voltage signal are discussed. The coils are made of Bi2Sr2Ca2Cu3O10/Ag (BiSCCO) tape, although the main qualitative results are also applicable to other kinds of superconducting tapes, such as coated conductors. The AC loss and the voltage signal are electrically measured using different pick up coils with the help of a transformer. One of them avoids dealing with the huge coil inductance. Besides, the critical current of the coils is experimentally determined by conventional DC measurements. Furthermore, the critical current, the AC loss and the voltage signal are simulated, showing a good agreement with the experiments. For all simulations, the field dependent critical current density inferred from DC measurements on a short tape sample is taken into account.
Effects of Thermal Cycling and Thermal Stability on 2G HTS Pancake Coils
IEEE Transactions on Applied Superconductivity, 2015
Degradation of superconducting critical currents of 2G-HTS coils may appear upon thermal cycling due to the tensile stress differences of its elements that induce increasing tape damages. We report on the thermal cycling and thermal stability effects of round, epoxy impregnated pancake coils under two cooling conditions (liquid nitrogen and conduction cooling). Thermal stability has been analyzed by measuring the quench development and propagation. Minimum quench energy values and longitudinal and radial propagation velocities are reported. The influence of different epoxy impregnations and number of turns in the thermal cycling effects and quench behavior has been studied. Index Terms-2G-HTS conductors, Quench , Superconducting magnets, Thermal cycling.
Characteristic Comparison for the Various Winding Methods of HTS Magnets
IEEE Transactions on Applied Superconductivity, 2012
Recently various high temperature superconducting (HTS) conductors have been commercialized in the power applications. While a BSCCO becomes tape or wire, others coated conductors (CC) are tapes. An HTS magnet utilized with these conductors can be fabricated as a form of pancake winding or layer winding. In the case of HTS magnet using a thin and rectangular tape, the pancake winding method is satisfied in the practical cases because of its suitable features. However, one of the disadvantages of this winding method is increasing brands and splices. From this point of view, we propose a layer winding method using an HTS tape and fabricated HTS layer coils. To verify the feasibility of the proposed winding method, critical current and over-current tests are investigated in liquid nitrogen and the results are compared with those of pancake coils. In addition, operating characteristics on the condition of various winding tensions are analyzed. Index Terms-Critical current test, high temperature superconducting (HTS) magnet, layer wound coil, over-current test, pancake wound coil, winding tension.
IEEE Transactions on Applied Superconductivity, 2014
This paper proposes a numerical approach to calculate the characteristic resistance (R c) of partial-insulation (PI) and no-insulation (NI) high-temperature superconductor pancake coils with the non-uniform current path in such coils taken into consideration. Recently, an analytic approach has been proposed to estimate R c of an NI coil, where the coil current is assumed to be "uniform" over the entire coil. This model, however, is not effective to explain the increase of R c when a coil is modified from NI to PI. In this paper, we firstly introduce our numerical approach based on a finite element analysis. Then, the charging characteristics of selected PI and NI coils that we had previously reported are analyzed by the proposed approach. Reasonable agreement between the measured and calculated data validates the proposed approach to estimate R c of a PI as well as an NI coil.
Calculated E-I characteristics of HTS pancakes and coils exposed to inhomogeneous magnetic fields
Journal of Physics: Conference Series, 2006
The upper limit of the operating current of LTS solenoids can be estimated as the coordinate of the crossing point of its load line with I C (B) line of the superconductor. For HTS coils this approach seems to underestimate the allowable operating current of the coil. A better approach is to obtain a full electric field distribution over the coil and to use it as the base for a more sophisticated coil design criteria. We developed an algorithm and a Matlab program for calculating distributions of the current density, magnetic field and electric field in HTS solenoids made of pancakes, considering the inhomogeneous current density distribution inside the anisotropic tape. I-V curves of several Bi-2223 coils are calculated and good agreeement of the calculated and mesured critical currents, I C , and indexes, n, are attained. One can utilize the program in the coil design choosing his own criteria of coil's critical current, e.g., 1) The average electric field 10 -4 V/m over the coil, 2) The electric field 10 -4 V/m at the weak point of the coil, 3) The energy dissipation in the entire coil, 4) Distribution of local energy dissipation.
Exploiting asymmetric wire critical current for the reduction of AC loss in HTS coil windings
Journal of Physics Communications
Critical current and AC loss in coil windings are two important factors for various HTS applications. Many coated conductors exhibit asymmetry in the variation of the critical current with magnetic field angle. This asymmetry results in different coil critical current values depending on the orientation of the conductors in the coil windings. We report critical current and AC loss results at 77 K and 65 K for three hybrid coil assemblies which have a common central winding and different arrangements of the end windings. We found a difference greater than 13% in both the critical current and the AC loss results for the different arrangements. The results imply that if we wind coil assemblies smartly even using the same materials and the same design, we can not only improve critical current but also reduce AC loss significantly.
Recent Developments in 2G HTS Coil Technology
IEEE Transactions on Applied Superconductivity, 2009
Recent developments in 2G HTS coil technology are presented highlighting the ability of 2G HTS wire to function under difficult operating conditions without degradation. The challenges of use in various coil constructions and applications are discussed. Several applications where the conductor is subjected to high stress levels include high field insert coils and rotating machinery. While these applications present different challenges, the ability of the conductor to operate under high stress levels has been demonstrated in both direct sample measurement and test coils. The high winding current density that is available with SuperPower's thin 2G HTS wire was utilized in a high field insert coil demonstration generating central fields in excess of 26.8 T [1]. The ability of the wire to be tailored (stabilization, insulation, ac losses) to fit various operating parameters will also be discussed.