SMES Coil Research Articles

A mechanism causing an additional AC loss in a large CICC coil

A large superconducting coil wound with cable-in-conduit (CIC) conductor caused an additional AC loss which cannot be estimated from short conductor sample test results. It was confirmed that the additional AC loss was generated by long current loops in the CIC conductor. Magnetic field decays of the loops with various long time constants were observed through Hall probes. We propose a mechanism forming the long loops. The CIC conductor is composed of several staged sub-cables. If one strand on the surface of a sub-cable contacts with the other strand on the surface of the adjacent sub-cable, the two strands must encounter each other again at LCM (least common multiplier) distance of all staged cable pitches and thereby result in forming a pair of a long loop. We traced each strand in the CIC according to a method that the sub-cables at all sub-stages rotate around a center of inertia. The long time constants were calculated and their results can explain the data measured in a large SMES coil. The proposed mechanism is effective for estimating the additional AC loss in the coil.

Experimental results of the model coil for cooling design of a 1 T cryocooler-cooled pulse coil for SMES

The authors have been developing high-Tc superconducting coils for SMES applications. Their primary goal is to make a HTS coil which is cooled to 40 K by a single-stage cryocooler and continuously operated at 1 Hz with a field amplitude of 1 T. The coil has heat drains of AlN plates to remove heat because of AC losses. They made a cooling model coil system to study the effective arrangement of the heat drains. The system consisted of a model coil using Cu conductors, current leads and a cryocooled system. The test coil was divided into three sections in different arrangement of heat drains. The model coil was daubed with a high thermal conductivity epoxy resin to improve thermal contact resistance between the conductors and AlN plates. They tested the coil by Joule heating which was equal to AC losses. They measured the temperature distribution in the coil and the temperature difference between Cu conductors and AlN plates. The temperature difference was measured between 0.2 K and 0.7 K. The results will be applied to the 1 T HTS coil design.

Transient stability of large aluminum stabilized superconductors

Very large current composite superconductors have been considered and adopted to use in SMES coils and fusion applications, such as the Large Helical Device (LHD). These superconductors have large cross-sectional area of high purity aluminum stabilizer to improve their stability and to enhance the overall current density. Once a normal-zone is initiated in such a composite superconductor, the current transfers to the aluminum stabilizer according to the temperature distribution. The time constant of current diffusion in the stabilizes however is very long due to the low electrical resistivity of aluminum and the large conductor size. Therefore, an excess Joule heat is generated in a small area near superconducting filaments and the temperature increases locally. In this paper, to evaluate this peculiar property, we carry out some simulations with regard to quench process in the superconductor applied to the helical coil of LHD in the National Institute for Fusion Science. The simulations, by using a newly developed computer code, are compared with the experimental results of the stability tests on the short samples of LHD conductor. Furthermore, we focus on the influence of the CuNi alloy clad adopted to the LHD conductor on the normal transition and normal-zone propagation properties.

Force-balanced coil for large scale SMES

In large scale SMES, huge electromagnetic force caused by high magnetic fields and coil currents is a serious problem. In order to solve this problem, we propose a concept of force-balanced coil (FBC) applied to the SMES. The FBC balances the centering force with the hoop force, both of which are exerted in the major radius direction. Moreover, for the optimization of large aspect ratio superconducting coils, we propose a stress-balanced coil (SEC) concept, improving the concept of FBC, which balances magnetic pressures at the coil nose part where the produced magnetic field reaches its maximum value. Comparing the toroidal magnetic field coil (TFC) and FBC with SEC, the last can reduce coil stresses and obtain large stored energy with shorter length conductors and/or lower magnetic fields.

SMES coil configurations with reduced stray field

The stray field of SMES restricts its site location, although SMES has an attractive potential for power management and quality control. The stray field outside a solenoid is analyzed by a series of Legendre polynomials and the result is applied to the stray fields of various SMES coil configurations. As long as the summation of magnetic moments from all coils is zero, the term of a stray field decreasing as r/sub p//sup -3/ can be cancelled out. The higher order of the stray field can vanish if the coil arrangement is optimized. In this paper, the authors consider a single solenoid as a reference, active shield coils, axially displaced coils and multipole coils to reduce the stray field. The multipole coil configuration has high potential to drop the stray field, since the stray field behaves like r/sub p//sup (3+n/2)/, where n is the coil number.

Technologies for high field HTS magnets

Six partners from five European countries are cooperating within a R&D BRITE EURAM project named SHIFT (superconducting high-T/sub c/ coils for high field technologies). The objective of this 3-year project is to develop basic technologies for manufacturing magnets producing high fields operating in the temperature range 20-30 K: HTS wires, coils based on bulk parts, coil structures and cooling system. The potential applications are pulsed magnets for power quality SMES and DC magnets for MRI, instrumentation and laboratory use. Specifications and designs of SMES for the voltage sags smoothing application have been performed and will be presented together with SMES designs for flicker mitigation. Main deliverables are the realization of a 6 Tesla demonstration coil cooled by closed cycle refrigeration and the design of a SMES coil for power quality applications. Target performances, conductors, test coil developments and designs are discussed in this paper.

ＳＭＥＳコイル配置の漏洩磁界

ＳＭＥＳコイル配置の漏洩磁界

Protection study of the Babcock and Wilcox SMES coil

Babcock and Wilcox (B&W) is working with Anchorage Municipal Light and Power (ML&P) to design, manufacture and install a 30 MW, 1800 MJ SMES system in Anchorage, Alaska. This will be the first mid-sized SMES system that will demonstrate full four-quadrant utility application. One design option considered for the SMES magnet is an enthalpy margin stabilized coil. A protection code has been established to study the quench performance of the coil. This paper presents the results of this study and compares different protection methods. The paper also discusses the impact of some key design parameters on coil quench performance.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

大容量超電導コイルのクエンチ時温度上昇の解析

Considering both self-heating of the normal zones propagating in coils wound with large-capacity superconductors and power consumption by external resistors, we have analyzed the temperature-rises in the coils and deduced new analytical formulas by introducing effective decay time constants and by considering energy-balancing in the system. Calculations by using this formula and computer simulations were conducted for small-scaled SMES coils wound with 20kA superconducting cable conductor and comparison was made between these results. It was found that our formula gave a temperature-rise higher by about twenty percents than the computer simulation result, while Wilson's formula gave a temperature-rise much higher than ours and of too safety side. It has been demonstrated that the formula is physically more reasonable and practical than Wilson's. However, the formula is more limited than computer simulations in taking-in of rigorous temperature-dependence of various properties and in dealing with complicated propagations of normal zones, so computer simulations are desirable for us to get more real temperature-rises to be expected in the coils.

AC losses in the SMES conductor and coil structure

The authors present a methodology for calculating AC losses in the SMES/ETM (Superconducting Magnetic Energy Storage/Engineering Test Model) conductor and coil structure. Coupling and eddy current losses are discussed for a 100-s discharge of a 143-m-diameter ETM with a total stored energy of 21 MWh. The chosen pitch length, filament size, and winding configuration are shown to result in coupling losses that are small during this 400-MW. The total coupling losses are about 0.0048 J/cm/sup 3/. The structural current losses result in energy deposition of about 0.05 J/cm/sup 3/ during the same discharge. The energy is being deposited in the structure (away from the conductor) and over a very long time as far as stability is concerned (100/s). In addition, the highest power deposition occurs toward the end of the discharge, when the conductor current is lowest. The AC losses during slow utility discharge, calculated using the proposed methodology, represent part of the steady-state heat load to be removed by the refrigeration system. >

Conductor qualification tests for the 30-MJ Bonneville Power Administration SMES coil

The 30-MJ energy storage coil for the Bonneville Power Administration requires a low-loss, cryostable conductor that is able to carry 4.9 kA in a field of 2.8 T and will maintain its properties over 108partial discharge cycles. The multi-level cable which satisfies these requirements has been extensively tested at various stages in its development and in its final form. Tests have been performed to determine the effect of manufacturing options on ac losses, low temperature electrical resistivity, stability, and fatigue resistance of the insulated conductor. This paper will concentrate on the stability and fatigue tests which have not previously been reported.