Thermal-quench behavior of non-insulated high-temperature superconducting (HTS) racetrack pancake coil with cooling channels through the epoxy surface

Abstract

The high-temperature superconducting (HTS) racetrack pancake (RP) coil is recommended to be used as a field coil in rotating machines such as generators and motors. Epoxy impregnation is normally required in the field coil to protect it against mechanical stresses due to the variation in the magnetic field and to enhance its mechanical stability to withstand rotor vibrations. However, the thermal conductivity of epoxy is very low compared with that of HTS. Consequently, when quenching is induced in an epoxy-impregnated coil, the Joule heating is poorly dissipated outward because of the absence of thermal drain to dissipate the heat in liquid nitrogen (LN2), which damages the coil through heat accumulation. We designed cooling channels (pores) through the surface of an epoxy-impregnated coil to boost the circulation of LN2 in the coil during the cooling process and to improve the operation of the coil during quenching. In this study, cooling-down and quench tests were performed on the epoxy-impregnated RP coils to investigate the effect of cooling channel on the thermal-quench characteristics of the RP coils. Moreover, the minimum quench energy and normal-zone propagation velocity of the hotspot induced in the coils were discussed on the basis of the quench-test results.