FellowIng., PhD. Michal Vojenčiak
Project NameThermo-electrical Stability of Superconductors in Unconventional Cooling Conditions
Host organisationInstitute of Electrical Engineering
Duration of the project02.04.2015 - 01.04.2018

Abstract
Superconductors are materials with strongly non-linear current-voltage characteristic. When the transported electrical current is below the critical value they exhibit zero resistance, while at higher currents their resistance is high. Latter regime is associated with energy dissipation and temperature increase; it can lead to the temperature excursion causing a damage of superconducting material. Stability of the superconducting wire describes how big is the margin against transition to non-superconducting state and whether it will be followed by a material degradation. In this project we investigate the stability in conditions representing a situation typical for two configurations: the fault current limiter and the cable for a coil winding. The project proposes two novel ideas: 1.Non-uniformities in material properties and distributions of currents will be considered as an intrinsic factor that must be taken into account in the stability study. 2.Exploration of stability at cooling conditions different from the conventional regimes of liquid helium or nitrogen, will aim at discovery of “stability islands” with better overall performance. Fault current limiter suffers from inhomogeneities of the superconducting material itself as well as from a non-uniform distribution of temperature. Both effects cause local transition to non-superconducting state and thus local overheating. This effect will be studied by means of experiments as well as numerical modeling. In superconducting cable the current distribution among parallel wires is often non-uniform. The common reason is a variation of contact resistances at current terminations. We will investigate the way to realize more homogeneous current distribution and increase the cable stability. It is questionable nowadays if the conditions of high stability can be achieved without liquid coolant. With this project we want to demonstrate that a model of fault current limiter can safely operate in these conditions.

Project Summary with Interim Results

Superconductors are materials with strongly non-linear current-voltage characteristic. At current lower than the critical one they exhibit zero resistance, while at high current their resistance is high.  Latter regime is associated with dissipative processes and thus temperature increase. In extreme cases it can lead to damage of the superconducting material.

            Stability of the superconducting wire defines how much of theoretical current carrying capacity is safe to use in practice and whether the wire can withstand transition to non-superconducting state without degradation. In this project we investigate stability of superconductors for two applications: fault current limiter and cable for a coil winding.

First task in the project was building new experimental apparatus for pulse measurement of superconductor electrical current limitation characteristics. Thanks to new setup we have found strong positive influence of heat conducting ceramic blocks attached to superconducting tape. Maximum reached temperature of superconducting tape during current limitation can be substantially reduced using ceramic blocks acting as thermal ballast.

Next part of the project aimed in manufacturing and characterization of a superconducting cable. We focused on Conductor On Round Core (CORC) type of cable which consists of superconducting tapes helically wound on a former. About 1 m long cable sample with new type of current leads was manufactured, using optimized length of soldered contacts. Finally we characterized the cable using DC and AC transport current. This cable is the first CORC cable characterized in details and results will be used as a reference for future cables.




The CORC cable during manufacturing – winding of last superconducting tape (figure a) and layout of optimized CORC cable termination (figure b)