Charge–Discharge and Thermal–Electrical Characteristics of GdBCO Coils Wound With Various Types of Grease as an Insulation Material

This paper reports the charge-discharge and thermal-electrical characteristics of GdBCO single-pancake coils employing various types of grease, such as N-grease (N coil), Si-based grease (Si coil), and vacuum grease (V coil), as an insulation material to ameliorate the charging-discharging delay observed in no-insulation (NI) coils. The charge-discharge test results confirmed that the use of grease as an insulation material can effectively reduce leakage current because the turn-to-turn grease layers serve as an effective insulation material. In sudden discharge tests, the time constant values of coils wound with N-grease, Si-based grease, and vacuum grease were drastically lower than those of the NI coil. In the over-current test at 1.5 Ic, the maximum voltages of N and Si coils were 3.8 times lower than that of the V coil, indicating that the thermal and electrical stabilities of N and Si coils were considerably enhanced. Moreover, the heat dissipating test results indicated that the peak temperature of the Si coil was lower than those of other coils because the Si-based grease acted as the most effective heat dissipater in the event of local hot spot generation in the coil. In conclusion, among the various types of grease tested in this study, Si-based grease may be the most effective turn-to-turn insulator for the development of highly stable second-generation (2G) HTS magnets with ameliorated charging-discharging delay of NI coil.