Julian Cave

Short circuit test performance of inductive high T/sub c/ superconducting fault current limiters

The current limiting performance of inductive fault current limiters based on Bi-2212 high temperature superconducting tubes has been evaluated. Fault current limitation is due to the nonlinear impedance of this device when the induced current in the superconducting Bi-2212 tubes exceeds the critical current. A power load in a test circuit is first operated in nominal power conditions (<10 kVA). A fault condition is then provoked by short circuiting the load. The limiters current and voltage characteristics are recorded prior to and during the fault. Fault currents are limited to about 5-8 times the nominal current and limitation always occurs in the first cycle. Using an approximate transformer analysis with a shorted single turn secondary, the current in the superconductor and its effective resistance are obtained. The superconductors effective resistivity is of the order of 10 /spl mu//spl Omega/cm at the onset of a fault and increases gradually as the fault progresses. Results for the evolution of the limiting impedance with time as the nominal voltage of the circuit is increased are presented. The materials requirements for this type of fault current limiter are discussed.<<ETX>>

Testing and modelling of inductive superconducting fault current limiters

Fault current limiters are expected to be amongst the first applications of high-temperature superconductors in power engineering. The use of the intrinsic property of a superconductor to transit from a zero (or near zero in the case of AC currents) resistance state to a highly resistive state when the critical current is exceeded is the basis for a fast acting fault current limiter. In this article, the authors report test results (43 kVA nominal power: 450 V RMS and 95 A RMS) and comparisons to theoretical simulations. Circuit analysis is used to study design issues and to extract the superconducting materials (BSCCO 2212) properties during the application of the short-circuit. EMTP simulation is employed to predict fault current limiter behaviour in the utility network in order to coordinate with other equipment.

Development of inductive fault current limiters up to 100 kVA class using bulk HTS materials

Since 1992, Hydro-Quebec has been developing prototypes of screened iron core fault current limiters for medium voltage distribution networks. Fault current limiters (FCLs) are amongst the most promising applications of high temperature superconductors. The unique properties of HTS such as high T/sub c/ and high resistivity in the flux flow state make them suitable for a design based on bulk materials. A 100 kVA class inductive study prototype of SFCL built around five commercial Bi-2212 hollow tubes (Hoechst) has been repeatedly tested at /spl sim/600 V RMS in nominal and fault conditions. In particular, the thermal stability has been investigated as a function of the temperature of the nitrogen bath (67-77 K). Simulation of the behaviour of the inductive limiter using differential equations are presented where both electromagnetic and thermal behaviour are considered. Part of this work is done within the joint collaboration between Hydro-Quebec and Siemens.

Calorimetric ac loss measurements of silver sheathed Bi-2223 superconducting tapes

A new calorimetric method has been developed to measure ac losses in superconductors. Calibration of the losses requires a reference heater of the same size and shape as the superconductor mounted in a symmetric environment. Its main advantage is the direct determination of the total losses. Variations of the method include a null technique and averaging of the main Fourier component to reduce noise effects. The method has been tested on Bi-2223 tapes and gives the expected hysteresis losses as well as a frequency independent loss which can be attributed to flux creep. The measured losses are similar to those obtained by other groups with other methods.

Improvements and validation of the null calorimetric method for a.c. loss measurements in superconductors

Abstract A new calorimetric method for a.c. loss measurements in superconductors has been developed over the last few years, based on a null detection heating method. The measurements have shown general agreement with theoretical predictions and with values obtained by other methods. More recently, a second version of the set-up has been built to solve some initial problems: mainly, the insufficient thermalization of the tape extremities and thermocouple reference junctions. These have been corrected by the use of copper centred feed-throughs. It was then found that the new set-up yields correct results but with the use of an exchange gas. The method has been validated through a comparison of the a.c. losses measured by the null calorimetric method for a copper tape with those deduced from standard electrical measurements. The new version of the null calorimetric method has been used to measure a.c. losses of silver and silver alloy sheathed Bi-2223 tapes. Dependencies on current and on frequency of the a.c. losses are correlated with the V–I curves, for a “poor condition” tape sample and a “good condition” tape sample, characterized by different n-values according to a fit V=k·In.

Detailed analysis of magnetization data and transport critical current measurements for Ag(Bi,Pb)SrCaCuO composite tapes☆

Abstract Unlike traditional superconductors, bismuth-based high temperature superconductor wires are known for their complex electromagnetic behaviour arising from their crystal structure and anisotropic nature. To gain insight into their flux line pinning behaviour, it is necessary to understand the current flow and its distribution in these materials. Silver-sheathed (Bi,Pb)SrCaCuO tape samples (with nominal 2223 phase composition and T c values of 100–110 K) have been characterized by d.c. and a.c. susceptibility, magnetization, transport critical current and microstructural analysis. The transport critical current densities for these samples approach 10 4 A cm −2 at 77 K and in self-field. Detailed analysis of the d.c. magnetization as the applied field is cycled, for both transverse and parallel applied fields, has been used to investigate the critical currents and the scaling lengths at several temperatures. The correlations and discrepancies from the application of the critical state analysis to both magnetization data and transport critical current data are discussed. When applying simple versions of this analysis to high temperature superconductors (such as using the width of a magnetization loop to obtain the critical current density) serious errors may arise.

Nonlinear Magnetic Diffusion in a Bi2212 Hollow Cylinder: Measurements and Numerical Simulations

This paper compares numerical simulations and measurements of the nonlinear magnetic diffusion phenomenon in a Bi2212 high temperature superconducting (HTS) hollow cylinder. Although the measurements were done many years ago, it is only recently that computer power has become sufficient to reproduce the experiment by simulation on a personal computer in a reasonable time. Observations of flux density (magnitude and phase) both inside and outside the cylinder were carried out at different frequencies and magnitudes of a sinusoidal applied field. In addition, the computed current and flux density profiles inside the superconductor were obtained. The superconductor was modeled by E - J characteristics that depend on Jc and a power law exponent n. Both parameters are isotropic functions of B. The paper shows that numerical methods are useful to extract model parameters and to predict the behavior of HTS elements entering in the conception of real applications, such as the inductive fault current limiters.

Measurement of AC losses in superconducting tapes by reproduction of thermometric dynamic response

Abstract We have developed a dynamic response thermometric method for the measurement of AC losses in high T c superconductors. This method is based on the comparison of a temperature response caused by a known dissipation in the sample with that produced by the AC losses. By passing a DC current and measuring the DC voltage and corresponding temperature response the sample can be used as its own power dissipation reference. The advantages of this method are the short measurement duration time and the possibility to vary many experimental conditions: for example, AC and DC transport currents and AC, DC and rotating applied magnetic fields. In this article we present the basic method using variable short pulses of constant DC current for calibration and similarly of constant amplitude AC current to create the losses. The losses are obtained by numerical modelling and comparison of the thermometric dynamic response in the two above conditions. Finally, we present some experimental results for a Bi2223 superconducting tape at 50 Hz and 77 K.

Calorimetric study of the thermal conductivity of superconducting tapes: experimental method and simulations

The method consists in the monitoring of the temperature variation of a superconductor tape subjected to a trapezoidal current pulse. Simulations of the experiment were performed using the lumped capacitance model and also the two dimensional finite element method. Both were successful in reproducing the samples reactions to the heat pulse. This increases our understanding of the propagation of internally generated heat in superconductors and will facilitate, for example, the study of AC losses in these materials. The thermal conductivity deduced from the measurements are consistent with the literature.

Static electric potential measurements on the surface of Ag/Bi-2223 high temperature superconducting tapes

By using a fine sliding contact it is possible to measure the surface potential on a superconductor in the flux flow state as a continuous function of position. This method has been applied to the investigation of the current flow distribution in silver-clad Bi-2223 tapes at 77 K. A four point sliding contact has been constructed (gauge lengths 1-2 mm) which can slide over several cms length of a sample. The probes are arranged in a diamond or square configuration so that the local potential difference can be sampled both along and across the tape sample. Several modes of operation are possible: V-I curves can be measured at several locations along the tape and continuous traces of the transverse and longitudinal surface potential difference can be obtained for fixed values of current and applied field. Deliberately induced defects such as cracks produce characteristic correlating transverse and longitudinal potential signatures from which the local current flow direction can be deduced. Correlations between different local J/sub c/ values and microstructures are discussed.<<ETX>>