Y. Laumond

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.

Preliminary tests on a superconducting power transformer

Test results for a 50-Hz single-phase power superconducting transformer are presented and discussed. The near-ambient-temperature iron core, of conventional shape, is situated in a fiber-glass reinforced-composite inner vessel which is vacuum insulated from the He bath. The iron core is cooled by an inner channel. Inter-leaved low-voltage and high-voltage windings are wound around the inner vessel by halved formers and cooled by natural convection in a 4.21 K He bath. The tests demonstrated construction feasibility (showing considerable weight reduction compared with nonsuperconducting designs); increased efficiency for power levels of about 1 MVA; and fault current strongly limited by the high value of conductor matrix resistivity. >

Reduction of AC losses in ultra-fine multifilamentary NbTi wires

The authors present AC loss measurements for a series of wires in which the proximity effect has been greatly reduced by increasing the distance between the filaments while retaining high values of the overall critical current density, typically 3000 A/mm/sup 2/ at 0.5 T. For the best performing wires, 50-Hz AC losses in W/(Am) (=(losses in W/m/sup 3/)/(overall J/sub c/ in A/m/sup 2/)) for small test coils are a factor of ten lower for 0.5-T peak field and a factor of three lower at 1-T peak field. For wires with negligible proximity coupling, losses at low fields are reduced compared to that calculated by the Bean model, due to the small number of flux lines that thread across the width of each filament. >

Investigations of HV and EHV superconducting fault current limiters

In close cooperation, GEC Alsthom, Alcatel Alsthom Recherche and Electricite De France are investigating the technical feasibility and the economical impact of low T/sub c/ superconducting fault current limiters, intended for the French HV and EHV electrical grids. The program includes the complete design of a 63 kV/1.25 kA/5 kA/spl circ/ version, experienced on current and voltage models. Extensions to the HV prototype demonstration, and to the EHV fault current limiters, are also discussed in this paper.<<ETX>>

Thermo-electromagnetic stability of ultrafine multifilamentary superconducting wires for 50-60 Hertz use

The authors present calculations of the theoretical thermoelectromagnetic stability of ultrafine multifilamentary superconducting wires. These calculations take into account the self-field effect of the transport current. The authors then compare theoretical and experimental results for conductors comprising typically several hundred thousand 0.17- mu m diameter filaments. These first measurements of the electromagnetic stability of multifilamentary wires under AC conditions have given encouraging results and have shown good agreement with theory. The results are pertinent to the development of Nb-Ti ultrafine filamentary wires for 50-60-Hz applications. >

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. >

Progress on superconducting current limitation project for the French electrical grid

Electricite de France, GEC ALSTHOM and Alcatel Alsthom Recherche, initiated in project for the definition, experimental validation and economic evaluation of superconducting fault current limiters adapted to the French grid system. As a first step, a device of 63 kV-1.25 kA is investigated, which has the function to limit the 25 kA fault current at 5300 A/sub peak/. A very compact and low-inductance design has been selected, based on resistive limitation. Considerations concerning the general design, conductor protection, high voltage insulation, cryogenic system and interaction with the grid, are developed in this paper. >

25 kV Superconducting Fault Current Limiter

Our technological progress in die field of superconductivity over the last ten years made possible the manufacture of industrial lengths of conductors, consisting of NbTi ultra-fine filaments, embedded in a Cu-30 wt % Ni matrix; 50 Hz losses are greatly reduced, and the electrical resistance beyond the critical current is very large. Such conductors offer numerous new perspectives, through which the design of electrotechnical machines could be reconsidered. This paper describes the main features of a 50 Hz single-phase fault current limiter, constructed in our laboratories, which present some electrical and cryogenic properties: S-N transition limiting the fault current to a value a few percent above the threshold current within a few µs, high voltage insulation capabilities, moderate cryogenic losses during steady state and in transient conditions.

Thermo-Electromagnetic Stability of Ultrafine Multifilamentary Superconducting Cables for Industrial Frequency Use

The development from 1983 onwards of Nb-Ti ultrafine multifilamentary wires for 50—60 Hz applications has opened up many interesting perspectives in electrotechnology. The AC losses have been greatly reduced due to decreasing the filament diameter to values of 0.1 to 0.2 µm and by using a highly resistive CuNi matrix with a 30% nickel content. The low thermal conductivity and specific heat of this CuNi matrix and the very high critical current densities induce a very acute problem of thermoelectromagnetic stability. As the electromagnetic diffusivity is much higher than the thermal diffusivity, the stability is governed by an adiabatic criterion. The last experiments about various coils and different assembly configurations of industrial cables give us a large panel of results which permit us to check the validity of the theoretical calculations.

Application of 50 Hz superconductors close to self field conditions

Applications of 50 Hz superconductors like the transformer and the fault current limiter correspond to relatively low magnetic fields, so that AC losses and stability are mainly governed by the conductor self field. AC loss calculations as they are performed in most cases for superconductors, are based on the Bean critical state model which states that everywhere in a superconductor, the current density has a modulus equal to the critical current density J/sub c/. This model is applicable when the superconducting transition E(J) is very sharp, but sizeable discrepancies appear for 50 Hz superconductors, as they present a relatively smooth superconducting transition. AC loss calculations have been developed using the Maxwell equations combined with the actual E(J) relationship. The heat generation in the conductor is then used as an input for a numerical calculation of the temperature distribution through the superconductor. The stability limits are directly derived from the thermal model.<<ETX>>