Turn-to-Turn Contact Resistance Measurement of No-Insulation REBCO Pancake Coil at DC Current Operation

Abstract

No-insulation (NI) (RE)Ba2Cu3Oy (REBCO) magnets are very promising for practical applications to generate high magnetic field. The NI winding technique provides high thermal stability so that quench protection is unnecessary. However, the turn-to-turn contact resistance (sometimes called characteristic resistance) is an important factor to characterize the thermal stability and charging delay of NI REBCO pancake magnets. Although the conventional sudden-discharging method is widely used to measure the turn-to-turn contact resistance, it is not applicable to various conditions; e.g., the change of DC operating current and temperature.Therefore, we have previously proposed a turn-to-turn contact resistance measurement method using a low-frequency AC (LFAC) current. In this paper, we measured the contact resistance for different DC operating currents or change of DC current to confirm the validity of LFAC method. In addition, by heating a test coil, the contact resistances were also measured during the change of DC operating current. Since the electromagnetic stress is small enough not to change the contact condition, the change in contact resistance is negligible. The measured contact resistance was unchanged, so we could conclude that the LFAC method is effective in measurement of the contact resistance when DC operating current was applied.

Turn-to-Turn Contact Resistance Measurement of No-Insulation REBCO Pancake Coil: External Field Dependence

“…We have previously proposed a low-frequency-AC current method to measure the turn-to-turn contact resistance of NI REBCO pancake coil [10], [11]. In the proposed LFAC method, by applying a low-frequency AC current to an NI REBCO pancake coil or superimposing on a DC transport current, the coil impedance is measured.…”

“…where j, ω, and L are the imaginary unit, the AC current angular frequency, and the coil inductance, respectively. The AC current frequency of 1 Hz to 50 Hz is preferred to a small single pancake coil with small inductance [10], [11].…”

Investigation into accuracy of turn-to-turn contact resistance measurement in low-frequency AC method by 3D numerical simulation

“…where V p and I p are the peak of voltage and current, respectively. Through measurements, the validity of the LFAC method was confirmed under several conditions [28][29][30]. The measurement accuracy was discussed against various frequencies in [31].…”

“…It is noted that the contact resistivity is assumed to be uniform in the whole REBCO pancake coil. Table 1 lists the specifications of the REBCO coil simulated in this paper, identical to the NI REBCO coil with 10 turns shown in [29]. In the experiment, the 10-turn NI REBCO pancake coil was wound at a tension of 1 kg tension, and it was cooled down to 77 K by immersion in liquid nitrogen.…”

“…As an alternative method, we have previously proposed a low-frequency AC current (LFAC) method [28]. The LFAC method can measure the contact resistances under different conditions, such as operating current or field dependencies [29,30]. In the LFAC method, an AC current whose frequency satisfies the following condition is applied to an NI REBCO pancake coil:…”