The strain effect on critical current (I c (ε)) in YBa 2 Cu 3 O 7−δ (YBCO) coated conductors was evaluated at temperatures in the range 20-83 K under magnetic fields parallel to the c-axis up to 10 T. The peaked reversible variation of I c with applied uniaxial strain was confirmed in the self-field at all tested temperatures. The strain sensitivity increases with increasing temperature, resulting in a more pronounced reversible suppression with strain at higher temperature. Interestingly, it was found that the peak strain corresponding to the maximum of I c shifts to the compressive side with decreasing temperature. Such a peak shift cannot be explained by a change in the thermal residual strain of the YBCO film, suggesting that the peak strain of the I c (ε) in YBCO coated conductors is not determined only by relaxation of the residual strain. The strain sensitivity of I c (ε) at 60 K becomes greater with increasing magnetic field, while the influence of the magnetic field is much less pronounced at 20 K. The in-field I c (ε), including the compressive strain region as well as the tensile region, shows a double peak behavior at low magnetic field at 77 and 83 K. The temperature and magnetic field effect on I c (ε) in YBCO coated conductors is discussed considering flux pinning within the grains and on grain boundaries.

A SMES system of MVA class for bridging instantaneous voltage dips has been developed using Bi-2212 wire. The Bi-2212 wire has high-performance conductive characteristics that do not deteriorate at a low temperature in high magnetic fields beyond 10 T. These characteristics enable a compact design of a SMES system of the Bi-2212 wire. In addition, coils of the Bi-2212 wire can be adequately insulated due to a high temperature margin. Therefore, the SMES system designed by using the coils has advantages to enhance dielectric strength and output power of the system. In our previous study, a SMES system consisting of 4 unit coils was constructed and the various properties were examined. Up to the present, the total 18 unit coils were stacked to make a coil system (outer diameter: 700 mm, height: 554 mm, stored energy: 984 kJ) and installed into a SMES system of 1 MVA for bridging instantaneous voltage dips. Also, the cooling system of the HTS SMES has been improved. The characteristics of the conduction cooled HTS coils of 1 MJ class were investigated in the operations of 1 MVA SMES system for bridging instantaneous voltage dips. Thermal reliability was verified during each operations of exciting, standby, bridging and current damping. Moreover, the repetitive bridging operations even worked out every 5 minutes. Advantages of the conduction cooled HTS coils for SMES were verified.

The thermal behavior of a high-temperature superconducting (HTS) coil is significantly different from that of lowtemperature 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.

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