Alternating current losses of a 10 metre long low loss superconducting cable conductor determined from phase sensitive measurements

Abstract

The ac loss of a superconducting cable conductor carrying an ac current is small. Therefore the ratio between the inductive (out-of-phase) and the resistive (in-phase) voltages over the conductor is correspondingly high. In vectorial representations this results in phase angles between the current and the voltage over the cable close to 90 degrees. This has the effect that the loss cannot be derived directly using most commercial lock-in amplifiers due to their limited absolute accuracy. However, by using two lock-in amplifiers and an appropriate correction scheme the high relative accuracy of such lock-in amplifiers can be exploited. In this paper we present the results from ac-loss measurements on a low loss 10 metre long high temperature superconducting cable conductor using such a correction scheme. Measurements were carried out with and without a compensation circuit that could reduce the inductive voltage. The 1 µV cm-1 critical current of the conductor was 3240 A at 77 K. At an rms current of 2 kA (50 Hz) the ac loss was derived to be 0.6±0.15 W m-1. This is, to the best of our knowledge, the lowest value of ac loss of a high temperature superconducting cable conductor reported so far at these high currents.

Non-uniform current distribution as the cause of false voltage signals in the ac loss measurement on a superconducting cable

“…There is widespread experience with signals containing a false component that, in spite of being in phase with the cable current, does not have anything to do with the true loss. Sometimes it is preferred to utilize the voltage drop as measured between the current terminations, containing the loss signal from superconductor on a large resistive background [5][6][7][8]. In this case, the results are obtained after subtracting the loss in current termination from the total measured loss.…”

Estimation of Apparent AC Transport Losses for Cylindrical Conductors Composed of Superconductor Tapes

“…2 shows the calculation results of transport losses (used as "true values") for all conductors, plotted against the current amplitude . The dashed lines represent the analytical loss values in the monoblock model for a continuous superconductor cylinder with the perimeter of 64 mm (equal to the products of tape width and number of tape strands in both conductors) and the superconductor thickness equal to the thickness of a tape strand in each conductor [8]. As can be seen, the calculated loss values of the conductor C1 are larger than the analytical values of the monoblock model in higher current range near the critical current , while they are well agree with the analytical value .…”

Contactless electrical measurements of transport ac losses in a 3 m long superconducting cable

“…The voltage taps for loops 1 and 2 were soldered onto each end of the HTS conductor in the region where the HTS tapes were soldered to the Cu terminations. In this way the need for post experimental subtraction of the resistive signal [4] from the total measured signal was avoided. The voltage lead distance to the cylindrical cable conductor was 13 mm, thus picking up a significant inductive signal, however, presumably also avoiding complicated local distribution of the magnetic fields due to the discrete nature of the cable conductor.…”

“…The loss components of the different loops, i.e. the voltage in phase with the conductor current, were measured by the use of two lock-in amplifiers (Stanford Research SR830) [4].…”

“…An important parameter to study is the ac transport losses generated in an HTS cable conductor. The ac losses of single HTS conductors of length 1, 3 and 10 m have been measured electrically and calorimetrically [4,5]. Until now, only the total transport loss has been assessed.…”