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Design of a 30 m long 1 kA 10 kV YBCO cable

In this paper a 30 m long one-phase coaxial YBCO cable with 1 kA transport current and 10 kV operating voltage was designed for the Super3C project to check the feasibility of YBCO tapes for low-loss cables. The final design incorporates cryogenic, mechanical and electromagnetic aspects. The electromagnetic losses during normal operation must be minimized. The cryogenic design must also take into account the generation of heat during short circuit conditions. Mechanical restrictions set the minimum gaps between the coated conductor tapes and the minimum lay angles in order to make the cabling feasible and to enable handling of the cable. The design of the electric insulation should be according to the international standard as far as applicable. The final design has to take into account all of the above restrictions.

AC Losses and Current Sharing in an YBCO Cable

Power cables constitute one of the most promising industrial applications for HTS materials. The Super3C project aims at establishing the feasibility of a low loss 30 m long 1 kA 10 kV YBCO cable. A major goal of the design is to minimize AC losses. Therefore, a circuit analysis model was tailored for predicting the losses in YBCO cables accurately. During this project, also a 0.5 m long one layer cable was constructed to test the behavior of a real YBCO cable. The AC losses measured from this cable were in good agreement with the computed results and thereby the feasibility of the developed design tool was verified. However, the measurements revealed that differences in contact resistances caused uneven current sharing between the tapes but computational analysis predicted the current sharing to be nearly even in the final 30 m cable.

Accuracy of numerical analysis for Hall sensor magnetometer measurements

The Hall sensor magnetometer is a widely used tool to characterize the homogeneity of high temperature superconducting tapes. However, it is well known that the current density distribution inside the sample, J, cannot be uniquely solved from the measured magnetic flux density, B. By discretising the Biot-Savart law the problem can be reduced to a solvable set of linear equations. Inaccurate assumptions about the sample geometry, noise in the measured data, and numerical errors can cause considerable error in the computed J. In this paper, a statistical approach to compare different inversion methods is presented. Example runs are carried out in order to find optimal locations for the measurement points. The results show that the relative error can be reduced by several orders of magnitude with the proper choice of measurement points, but the problem always remains ill-conditioned.

Magnetic flux density maps caused by DC, AC and remanence currents around artificial defects in Bi-2223/Ag tapes

Abstract The homogeneity of HTS tapes can be studied by measuring magnetic flux density maps with the Hall sensor magnetometer. The measurements are usually performed above samples carrying either the DC transport current or the remanence currents after switching off the transport current. In this paper time dependent maps above samples carrying AC transport current at 50 Hz are presented together with the DC and remanence maps. Several current amplitudes are used and measurements are repeated after drilling a hole into each sample. Finally, the applicability of the different types of maps to the homogeneity studies is discussed.

Hall sensor magnetometer for ac characterization of high temperature superconducting tapes

During the last five years, the ac performance of high temperature superconductor (HTS) tapes has been under intense development due to the need of practical applications. In order to measure and understand the current density distributions inside these tapes, several methods have already been developed. However, no system has been able to give the time dependent, spatial distribution of the magnetic flux density at a selected distance above the HTS tape carrying any cyclic current signal. In this paper, such a system operating at a frequency range of 0?400 Hz is presented, and possible error sources are extensively investigated. Post-processing of the measured data is explained in detail and error analysis of several example measurements is given. Measurements with HTS and copper samples are compared in order to demonstrate the usefulness of the system.

Fault current model for YBCO cables

Superconducting cables can be used to transport large amounts of energy with small losses in considerably smaller volume compared to conventional cables. At present, the first YBCO cable demonstrations are under development and they are expected to outperform BSCCO cables. Due to extremely high current densities in very thin superconducting films these cables are sensitive to overcurrents and therefore cable designers should be able to predict their behaviour during faults. In this paper, the temperature distribution in various 1 kA YBCO cable geometries was computed with several fault current waveforms in order to study the cable stability. FEM models were used to determine simultaneously both the current density and the temperature distributions of the cable as functions of time. Real temperature dependent properties of the cable materials and the strong magnetic field and current density dependence of the superconductor resistivity were taken into account.

Hall sensor magnetometer measurements of Bi-HTS tapes for low AC loss applications

In order to reduce ac losses in Bi-2223 tapes twisted filaments and different kinds of oxide barriers around filaments have been developed. In this paper, the homogeneity and electromagnetic behavior of these tapes are studied. The magnetic flux density maps over current carrying samples are measured with Hall sensor magnetometer. The homogeneity of tapes is most easily visible when dc transport currents at different amplitudes and remanence currents after switching off the dc transport current are examined. Ac transport currents at 50 Hz are used to determine the current of full penetration. The results of different samples are compared and discussed in relation to the design-parameters of the tapes.

Fault current analysis for a superconducting 1 kA YBCO cable

Superconducting cables can be used to transport large amounts of energy with small losses in considerably smaller volume compared to the conventional ones. The first YBCO cable demonstrations are under development and they are expected to outperform BSCCO cables. In the design work, the temperatures in the YBCO cables need to be simulated under any fault current conditions. In this paper, the temperature distribution in a 1 kA YBCO-cable was computed with various fault currents in order to study the thermal stability of the cable. The fault current is shared between the layers of superconducting YBCO tape and the copper shunt. FEM models were used to determine simultaneously both the current density and the temperature distributions of the cable as a function of time. Adiabatic conditions were assumed in order to obtain an upper limit for the temperature. According to the model, the copper core was able to absorb most of the heat. The cable maintained the thermal stability during 1 s with 10 and 20 kA (rms) fault currents. With 30 and 40 kA fault currents the cable was predicted to quench.

AC coil, reactor and cable demonstrations of low AC loss elementary and assembled BSCCO conductors

A European consortium has progressed towards AC coil, transformer and cable applications made of low AC loss Bi-2223 conductors. Internally assembled ring-bundle-barrier conductors were used in AC coils and an externally assembled conductor was realized for transformer and reactor windings. For the design and optimization of HTS AC coils, a numerical tool for the prediction of AC losses in coils and coil sets had been developed. The demonstrations with air-core coils and iron-core reactors showed that the numerical tool can already be confidently used in the design and optimization of HTS AC coils at nominal operation conditions. A series of three power cable prototypes was manufactured to estimate the influence of optimized low AC loss tapes on the total losses of a cable. In order to estimate the impact of the twist pitch on the losses we also manufactured cables with short and long twist pitches. The advantages and drawbacks of low AC loss tapes in power applications are discussed on the grounds of the test results.

Discrete Fourier transformation analysis of AC Hall sensor magnetometer measurements of high temperature superconductor tapes

During the last five years, the AC performance of high temperature superconductor (HTS) tapes has been under intense development due to the needs of practical applications. Previously, a system capable of measuring time dependent, spatial distribution of the magnetic flux density at a selected distance above the HTS tape has been presented. The tape can carry any cyclic current signal in the frequency range of 0–400 Hz. In this paper, a calculation model to extract information about the current densities, which create the measured magnetic flux density, is developed and used to interpret these measurements. Discrete Fourier transformation is found to provide a numerically stable method that is well suited to the interpretation of results and also for the signal noise exclusion. The third harmonic of the measured Hall voltage is seen to play an integral role in the comparison of the penetration models.