L Rostila

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.

Magnesium Diboride Wires With Nonmagnetic Matrices—AC Loss Measurements and Numerical Calculations

In the superconducting applications, the wires are exposed to time-varying magnetic field when the current changes. This generates losses which can be minimized by reducing filament size, twisting the wire, and increasing the transverse resistivity. However, the high losses of magnesium diboride wires often arise from magnetic sheath materials, and therefore, this work presents new type of wires with nonmagnetic matrix and multi-filamentary structure. The results of AC loss measurements, in external sinusoidal magnetic field, are presented. Two MgB2 samples were measured both in two temperature ranges, as two different set-ups were used, one with fixed LHe bath temperature 4.2 K. Second one enabled operation temperatures from 23 K up to the critical temperature of 39 K. Amplitude of magnetic field of the former set-up was up to 0.8 T and frequency range was from 0.1 to 1.4 Hz. In the latter one, the maximum amplitude was 28 mT, and the frequencies were 72 and 144 Hz. The results evidenced that the superconducting filaments were uncoupled and the measurements agreed with theoretical models based on this assumption. In practice, the uncoupling was modeled so that the net current in each filament was set to zero.

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.

Low field critical current density of titanium sheathed magnesium diboride wires

Magnesium diboride (MgB2) is replacing some of the conventional superconductors due to its low cost and availability in kilometer lengths. MgB2 has also been considered for AC applications. In order to model the AC losses and the critical currents of the applications, intrinsic Jc(B)-dependence is an important factor also at low fields. In this work Jc(B)-dependence of an MgB2 sample is extracted from the standard in field voltage-current measurements. The proposed method is applied to a non magnetic titanium sheathed sample at 16 and 20 K and a simple formula for Jc(B) aligns with the measurements. In the fitting process, the critical current distribution inside the wire is numerically simulated in order to take the self field of the sample into account. Moreover, the same formula aligns with measurements of a different sample. These critical current measurements, performed at 4.2 K, were based on magnetization. In the self field computations, the superconducting cross section must be determined accurately. Therefore, we tailored an image processing tool for MgB2 wires to obtain the geometry from a photograph.

A Time-Harmonic Approach to Numerically Model Losses in the Metal Matrix in Twisted Superconductors in External Magnetic Field

The first NbTi superconductor was developed in 1962 at Westinghouse. During 50 years, the manufacturing process of NbTi wires became highly optimized, and complex wire structures, which are needed in reducing ac losses, can be produced these days. Twisted multifilamentary structures generate many challenges from the modelers point of view. Considering numerical modeling, NbTi wires are too complicated to model with filament-level details. This is due to the high number and very nonlinear resistivity of filaments. Consequently, simplified approaches are needed for wire modeling. In this paper, a time-harmonic approach to model losses in the metal matrix is introduced. It is based on the linear approximation of filaments. The method to determine the linear resistivity of filaments is based on the definition of the skin depth and the radius of filaments. This can be done in advance without solving a nonlinear problem in 3-D. The suitability of the time-harmonic approach is benchmarked against the H-formulated 3-D eddy current model (ECM) with power-law resistivity. According to simulations, the losses in the metal matrix predicted by the time-harmonic approach agreed well with the nonlinear ECM when the filaments were uncoupled. The results achieved with the proposed approach similarly followed the effects of geometrical changes in the wire structure on losses as the nonlinear ECM and predicted the circumstances where the filaments became partially coupled. The simulation times were considerably lower with the new approach. From the manufacturers perspective, it is important to design conductors where the filaments stay uncoupled and have low ac losses in such a situation. Thus, the new approach can provide an effective design tool in developing new superconductors.

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.

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.

Critical current in nonhomogeneous YBCO coated conductors

The critical current of an YBCO tape is determined by the magnetic field inside the YBCO layer and the quality of YBCO material. In thick YBCO layers the average critical current density is reduced by the self-field and decreased material quality. In this paper the combined influence of the material nonhomogeneities and self-field on the critical current of YBCO tapes is scrutinised. First, the zero field critical current density was assumed to decrease along the YBCO thickness. Secondly, the possible defects created in the cutting of YBCO tapes were modelled as a function of lowered critical current density near the tape edges. In both cases the critical current was computed numerically with integral element method. The results suggest that the variation of zero field critical current density, Jc0, along the tape thickness does not effect on the critical current if the mean value of Jc0 is kept constant. However, if Jc0 is varied along the tape width the critical current can change due to the variated self-field. The computations can be used to determine when it is possible to evaluate the average zero field critical current density from a voltage-current measurement with an appropriate accuracy.