Patrick Bonnet

Experiments with a high voltage (40 kV) superconducting fault current limiter

Abstract Fault current limiters are devices that restrict fault currents without appreciable delay, so that they can protect electrical grids and devices more efficiently than classical circuit-breakers. Extremely low response times (a few microseconds) are provided by low- T c superconducting conductors, which combine a high critical current density and a high normal-state resistivity, with a vanishing heat capacity. Due to the risks of dielectric breakdown and overheating, the rated voltages have been limited to a few kV up to now. A significant step forward was recently attained with the successful tests of a 40 kV rms prototype; the device accepts rated currents of 315 A rms , and protects the line from any current over 1100 A peak (less than one tenth of the ordinary fault current). Its design and test results are presented here, with a tentative extrapolation to higher-rated currents and voltages.

A new step in AC superconducting conductors

High current AC superconducting cables are made of numerous co-assembled wires. The assembly mode may be a source of performance degradation, due to different causes, such as wire deformation, mechanical instabilities, heating, anomalous current distribution among the different wires, longitudinal self magnetic field, inadequate junctions. These possible causes have been systematically investigated. Significant results are presented for possible applications of high-performance multi-kA conductors. >

Performances of Industrial Superconducting Wires for 50–60 Hz Applications

Multifilamentary superconducting wires with a greatly reduced level of losses have been produced in lengdis of several tens of kilometers. In spite of the reduction of the filament diameter, proximity effects are avoided and we make die best possible use of die reversible motion of the flux lines, so that die hysteretic losses are lower. The reduction of losses due to induced currents can be obtained by choosing a twist pitch as short as possible. These concepts lead us to realize, first at a small scale, then at an industrial scale, conductors comprising filaments of Nb-Ti with a diameter of 0.1 to 0.2 μm and a highly resistive CuNi matrix. The twist pitch of these filaments is four times the diameter of die conductor. Some coils have been tested at the frequency of 50 Hz and show very reduced losses.

Completion of the Polo model coil

A superconducting poloidal field model coil as needed for tokamaks has been constructed by GEC ALSTHOM within the Polo project in effective collaboration with Forschungszentrum Karlsruhe, Institut fur Technische Physik, Germany. The manufacturing procedures for the coil and its terminals are described.