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

Abstract

Electrical measurement of ac loss in a short model of superconducting cable plays an important role in the development of low-loss cables. The main problem in this type of measurement is the appearance of false loss voltage signals. For the experiments when the total current splits into parallel paths, inevitably with different impedances, a voltage component in phase with the cable current is induced in signal wires. The source of this voltage originates in the phase differences between the total current in the cable and the currents flowing through individual tapes. A simple equivalent circuit model has been developed that allows us to estimate these phase shifts. Analytical formulae have also been derived for the false voltage signals. According to these formulae reliable data can be achieved by taking the sum of voltage signals read by signal wires symmetrically surrounding the cable. While not contributing to the ac loss measurement, the false voltage signals can be used to detect the phase shifts in tape currents referred to the phase of the total cable current. Theoretical predictions have been experimentally verified with the help of a 1 m cable model manufactured to allow for the theoretical model to be evaluated. Experiments demonstrated that the variation in contact resistances is the most critical factor in the testing of short cable models.