Marie-Françoise Devismes

Vacuum Interrupters: Design optimisation requires reliable experimental data

Vacuum interrupter technology is mature with respect to manufacturing quality, reliability and performance. One would expect that this leads to a convergence in terms of conception resulting in a dominating design principle. Yet still a large number of designs and dimensions coexist in the market. Numerical calculation techniques are used to optimise the geometry in order to reduce the overall dimension of the VI. This approach requires criteria for maximum allowable electric field or current density values. Yet a look at the state of the art shows that the maximum allowable electric field may vary at least by a factor of 2 and current density on interruption by 20% depending in a subtle way on process parameters. Mastering these details is the drive to more compact designs and new applications.

Thermal effects in YBCO/metal bilayers at high current densities

Abstract We report on the thermal aspects associated with the switching properties at high current densities of YBCO thin films shunted by a metallic layer. The role of the shunt is first detailed in a current ramp mode experiment under various conditions of temperature and magnetic field. We discuss specifically a regime associated with an almost total current diversion by the metallic layer, observed when the critical current density is reduced at temperatures close to the critical temperature T c or under magnetic field. This regime is associated to a thermal anomaly, as seen in the temperature signal measured on the rear side of the substrate, which can explain the partial recovery of the superconductive state under magnetic field. Under current limitation conditions at 50 Hz, 77 K and self-field, the temperature reaches 260 K at the rear side of the sapphire substrate after 0.4 s without any damage to the superconducting device.

41 kVa Bi/Y transformer design

Abstract YBCO coated conductors have very large critical current density and the field distribution in a transformer is favourable to obtain tapes with low ac losses. The objective of the READY European project is the construction of a transformer that uses first lengths of YBCO tapes produced by some of the partners. The secondary winding will use a YBCO conductor (limited length), the primary will use a PIT Bi 2223 tape. We chose a cold magnetic circuit with low iron loss sheets. The cryostat is then very simple with metallic vessels and only one interface with the pulsed tube is required. Helium is the coolant fluid since a wide range of operating temperatures (40–80 K) is foreseen. The transformer design and optimisation will be presented as well as preliminary experiments (He cooling, ac loss measurements, inrush current PIT behaviour,…) to validate some chosen solutions and theoretical calculations.

Switching of YBCO thin films into the dissipative state at high current densities

We report on the switching properties at high current densities of YBCO thin films shunted by an Ag layer. These experiments are performed either in liquid nitrogen or liquid argon (87.3K). Different transition regimes associated to specific electric field thresholds are observed. The nature of the thermal exchange with the bath and the role of the silver shunt and substrate are discussed. While only two regimes in the current-voltage characteristics are observed in liquid nitrogen, successively reversible and hysteretic, an intermediate behaviour appears at 87.3K associated with a total current diversion by the silver layer.