Partner und Internationale Organisationen
(Englisch)
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BICC Superconductors Ltd (UK), Institute of Cryogenics, University of Southampton (UK), ENEL SRI, Milano (I), Universitad de Zaragoza (E), Red Electrica de Espana, Madrid (E)
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Abstract
(Englisch)
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The objective of this programme was to advance the implementation of high temperature superconducting electrical power engineering devices through increased knowledge of the fundamental mechanisms of ac losses in Bi(2223)/Ag tapes.
EPFL was srongly involved in four tasks in this project:
Task 2a: Electrical measurements of ac loss at 77K in self field at power frequencies.
As the purpose of the a.c. loss measurement in the SACPA programme is to give recommendations to the tape manufacturers, it is important that all members of the consortium have confidence in the measurements. To provide a coherent framework for the measurement of the a.c. losses it is essential that the measurement systems of the various partners be standardised. For this reason, a round robin study between the four partners responsible for measurements (IOC, EPFL, BICC and CISE) was initiated as a major part of sub-task 2a.
Task 2b: Electrical Measurements of ac losses in self field at high frequencies (> 150Hz).
Electrical measurements of self-field a.c. losses in 14 BICC tapes with different number of filaments and sheath composition have been performed in the range of 59-2500 Hz by means of a large-bandwidth dual lock-in amplifier. Three contributions to the total self-field a.c. loss in the tapes - hysteresis loss, eddy current loss, and resistive loss (for high transport currents) - have been quantitatively dissociated and described for various current ratios I/Ic = 0.05¸0.99. The effect of the sheath composition and tape geometry on the eddy current loss has been studied by computer simulations and compared to the results from the measured tapes.
Task 2e: To determine the loss behavior and stability of Bi-2223 tapes subject to perturbations in the transport current with values above Ic.
A digital lock-in amplifier has been used to perform V-I time series acquisition when currents with amplitude above Ic are driven through Bi-2223 tapes. The current waveform is triangular, sinusoidal or with arbitrary shape, and is applied for several consecutive cycles. The generator frequencies are selected to be 0.05, 0.1, 0.5, 1, 10 and 59 Hz. The amplitude of the applied transport current is varied from 2 to 15 times the value of Ic. The invariance of the V-I curves during several consecutive cycles and the resistive behavior of the tapes in the over-Ic regime show that Bi-2223 tapes have very good stability to such type of transport current perturbations.
The four Bi-2223 tapes have been subjected to over-critical-current excursions of different waveform, frequency and amplitude, reaching 20 times the value of Ic. Even after 10 consecutive cycles of over-critical-current perturbation with amplitude up to 15?Ic, the samples have not deteriorated their superconducting properties (when I goes repeatedly below Ic). A sample has been destroyed only when 10 cycles at 20?Ic were applied at low frequency (10 Hz). The performed experiments lead to the conclusion that Bi-2223/Ag or Ag-alloyed tapes show very good stability (no deterioration of their superconducting properties) when over-critical-current perturbations occur for up to 10 consecutive cycles at frequencies up to 59 Hz, and current amplitudes up to 15?Ic. The estimated temperature rise in over-Ic regime - from 77 to 112 K for Bicc-46 and from 77 to 93 K for Bicc-21 - does not change significantly the tape's resistivity. The overall stability of the tapes is also demonstrated by the identical response to over-Ic perturbations at different frequencies, repeated for up to 10 cycles.
Task 3b: To investigate how a dc field, applied at different angles to the tape surface, affects the ac losses, measured by electrical characterisation method.
A digital lock-in amplifier has been used to perform electrical measurements of AC losses with transport current at 59 Hz. A DC magnetic field between 2 and 400 mT is applied with a varying angle from parallel to perpendicular to the tape's wide side, thus achieving a complete view of the tape's behavior under DC applied field. After a simultaneous V-I time series acquisition, the dependencies of Ic and n on the magnetic field and angle have been quantified. For device modeling purposes, a phenomenological law expressing Ic and n as a function of the applied magnetic field's intensity and direction has been proposed. The effect of the DC background field on the AC losses has been qualified. The measured AC loss is compared to Norris elliptical prediction for a broad Ip/Ic range.
The exponent n and Ic exhibit a very similar behaviour when the applied DC field amplitude and angle are increased. The performance of the tapes under DC applied field is not only characterised by a drastic diminution of the critical current, but also by a corresponding decrease of the exponent value n. Even for excellent samples, a value of n = 20 at zero field vanishes down to n = 2 or 3 at perpendicular fields with amplitude ~ 400 mT. This has to be taken into consideration for AC loss and electromagnetic calculations in new HTS devices. The perpendicular field component has considerably greater influence on the diminution of Ic and n: a decrease by a factor of 10, while the corresponding decrease is a factor of 2.5 when the field is parallel to the width of the tape,. A function, which gives the Ic and n dependence on the applied field magnitude and angle, has been proposed. It may be used, for example, for modelling purposes in device simulations. The measured transport AC losses in applied DC field are comparable to Norris elliptical prediction, for 0.1< i < 1. With the increase of the field amplitude, the AC loss is augmented as well, and for field amplitudes B = 100 mT, it is higher by a factor of 10 than at zero field in the greater part of the sub-critical range.
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