Since high-temperature superconducting (HTS) conductors have a greater volumetric heat capacity at operating temperatures envisaged for practical applications, the possibility of quench caused by a mechanical disturbance such as friction due to wire movement is much lower than that for low-temperature superconducting (LTS) coils. In real applications such as superconducting magnetic energy storage (SMES) systems, however, electrical charging and discharging are repeated and the HTS conductor may deteriorate locally due to cyclic mechanical strain. We also have to consider the possibility of quench caused by a failure in the power supply, cooling system or similar problems. Therefore, a protection scheme assuming quench is also required for HTS coils. In this report, we describe a method of determining the appropriate stabilizer thickness of the YBCOcoated conductors from the perspective of the quench protection of the YBCO coil, which dumps the stored magnetic energy in the coils via external dump resistance. We have been developing a quench detection method for a cryocooler-cooled SMES coil wound with a kA-class laminated bundle conductor composed of an electrically insulated YBCO-coated conductor. Since the normal-zone propagation velocity is quite slow in HTS coils, the detection of a non-recovering normal zone using a voltage signal is quite difficult. Therefore, quench must be detected using some other method. We numerically show that quench in the coil can be detected by observing the transposition of current in the bundle conductor caused by local normal transition. Furthermore, we show the results of experiments on a small model coil wound with a four-YBCO-strand bundle conductor to examine the possibility of a quench detection method by observing the transposition of current in the bundle conductor. This work was mainly supported by the New Energy and Industrial Technology Development Organization (NEDO) as part of the technological development of yttrium-based superconducting power equipment.
REBCO conductors were commercialized only recently in 2009 and therefore the magnet technology for REBCO conductors remains undeveloped. The major technological problems for REBCO coils encountered thus far are (a) degradation in the superconducting performance for mechanical reasons, (b) difficulty in protecting the magnet in the case of a super-normal transition of the REBCO coil and (c) the enormous effect of a screening-current-induced magnetic field. In this paper, the basic mechanism of degradation in the REBCO superconducting coil performance due to mechanical reasons is investigated through experiments and numerical simulations. Effective remedies for the degradation are also discussed.