Marijn Pieter Oomen

Large area YBCO-coated stainless steel tapes with high critical currents

Recent progress in developing large area HTS-coated stainless steel tapes is reported. YBCO films have been grown on IBAD-YSZ buffered 0.1 mm-thick steel tapes using a high-rate pulsed-laser-deposition technique which allows a deposition rate up to 70 nm /spl middot/ m/sup 2//h. The coated conductors (CC) are provided with an optimized shunt layer and current contacts which reduce the generation of Joules heat. An advanced technique for critical current I/sub c/ measurements was developed. The effects of self-field and time relaxation of current in helically wound tapes are analyzed. In long tapes of 10 m /spl times/ 4 mm, the highest I/sub c/ of 78 A at 77 K was observed. Critical currents of 67-75 A were reproducibly achieved in (8-10) m /spl times/ 4 mm coated tapes. For shorter tapes with a length of 0.2-1 m, critical currents of 317-391 A/cm width of the tape were observed. Fifteen wider CC tapes of 0.6 m /spl times/ 43 mm with I/sub c/ = 700 A have been manufactured and employed in newly developed fault current limiters with a nominal current of 3 kA. Critical current densities J/sub c/ = (1.3-2.5) MA/cm/sup 2/ at 77 K were observed in YBCO films. The CC tapes exhibit favorable behavior under axial stress and sufficiently small ac losses.

Modelling and measurement of ac loss in BSCCO/Ag-tape windings

High-temperature superconducting (HTS) transformers promise decreased weight and volume and higher efficiency. A 1 MVA HTS railway transformer was built and tested at Siemens AG. This paper deals with the prediction of ac loss in the BSCCO/Ag-tape windings. In a railway transformer the tape carries ac current in alternating field, the temperature differs from 77 K, tapes are stacked or cabled and overcurrents and higher harmonics occur. In ac-loss literature these issues are treated separately, if at all. We have developed a model that predicts the ac loss in sets of BSCCO/Ag-tape coils, and deals with the above-mentioned issues. The effect of higher harmonics on the loss in HTS tapes is considered for the first time. The paper gives a complete overview of the model equations and required input parameters. The model is validated over a wide range of the input parameters, using the measured critical current and ac loss of single tapes, single coils and sets of coils in the 1 MVA transformer. An accuracy of around 25% is achieved in all relevant cases. Presently the model is developed further, in order to describe other HTS materials and other types of applications.

Field-angle dependence of alternating current loss in multifilamentary high-Tc superconducting tapes

The energy loss in multifilamentary superconducting tapes has been measured for various orientations of the external ac magnetic field. When the ac loss in parallel and perpendicular fields is known, the loss at intermediate field angles can be predicted over a wide range of field amplitudes. The prediction is based upon the following assumptions: For low-field amplitudes, the current patterns that would be induced in the tape by the parallel and perpendicular field components can be summed. For high-field amplitudes, the tape only carries the current pattern induced by the perpendicular field component. These assumptions are supported by theory.

Dynamic resistance in a slab-like superconductor with Jc(B) dependence

The dynamic resistance in a slab-like superconductor is calculated, taking into account a field-dependent critical current density. In superconductors carrying DC transport current in an AC external magnetic field, the dynamic resistance causes a transport loss which depends on the amplitude and frequency of this field as well as on the transport current. This resistance is calculated analytically in a critical-state model applied to a superconducting slab in a parallel field. The field has a general periodic time dependence and for the superconductor a relation between critical current and momentary magnetic field as in the Kim model is assumed. The dynamic resistance appears only at field amplitudes larger than the so-called threshold field, which depends on the transport current. The model predictions are compared with experimental results obtained with a Bi-2223/Ag tape at liquid-nitrogen temperature. At small field amplitude and at low transport current, the derived model predicts the observed dependence of dynamic resistance on field amplitude, field frequency and transport current. For a larger field amplitude and simultaneous high transport current, the resistance is found to be larger than the model predicts. This is probably due to the not completely slab-like geometry and/or to a different field dependence of the critical current density in a high AC field.

Development and application of superconducting transformers

Superconducting transformers are an important innovation for future power transmission and transportation systems. Powerful, lightweight, energy-saving and environmentally friendly they offer enormous benefits compared to their conventional counterparts. Siemens is developing a 1-MVA demonstrator transformer for laboratory testing, exhibiting innovative features like horizontal design, cabled-conductor windings and a closed cooling cycle with sub-cooled nitrogen. Being one of the most promising applications Siemens has started a programme towards the development of on-board transformers for electrical rail vehicles. This paper summarises world-wide efforts in the development of superconducting transformers and reports on the progress achieved at Siemens.

Magnetic AC loss in multi-filamentary Bi-2223/Ag tapes

The ac loss in high-Tc superconducting tapes with twisted and non-twisted filaments has been studied by a magnetic method. A brief overview of the theoretical background and the experimental set-up is presented. Measurements were made at 77 K in a magnetic field of 50 Hz frequency and 0.001-0.7 T amplitude. Application of dc transport current made it possible to distinguish between the loss components, arising from intra-grain and from filament currents. The magnitude of the filament loss component indicates that the filaments are fully coupled, which agrees with theory. In other measurements, the orientation of the external field with respect to the tape was varied. Knowledge of the ac loss in parallel and in perpendicular field is sufficient to predict the ac loss for any intermediate orientations of the field.

AC losses in multifilamentary Bi(2223) tapes with an interfilamentary resistive carbonate barrier

Abstract For the most common AC application frequencies, the main component of the AC losses in multifilamentary Bi(2223) tapes are caused by hysteresis- and coupling losses. These losses can be reduced enhancing the matrix resistivity and applying a twist to the filaments. We report on the AC loss properties of 37-filament tapes with AgAu (8 wt.%) matrix, and novel 19-filament tapes with SrCO 3 barriers between the filaments. We performed transport AC loss and magnetic AC loss measurements in parallel and perpendicular magnetic fields. Both kinds of tapes were also prepared with filament twists below a twist pitch of 20 mm. The influence of the different tape modifications on the AC loss behaviour is presented and compared with theoretical models to understand the effect of the resistive matrix. In the case of magnetic AC loss measurements, reduced AC losses due to decoupled filaments were observed for the twisted tapes with a resistive matrix in low parallel fields.

HTS flux pump for cryogen-free HTS magnets

A reliable method for persistent-current mode operation of HTS magnets is still not available. As an alternative solution to external high-current power supplies we have developed a cryogen-free operating transformer-rectifier type HTS flux pump. Using this device only small ac currents have to be supplied into the cryogenic system. Both thermally triggered HTS thin film switches as well as cryogenic MOSFET switches have been used to operate a conduction-cooled 0.5 T HTS magnet coil. This paper presents results of experimental test and modeling of the flux pump operation.

Decrease of magnetic AC loss in twisted-filament Bi-2223 tapes

In AC power-engineering applications, the energy dissipation in the superconductor is dominated by the magnetization due to alternating fields. To reduce this type of loss, conductors are being developed with twisted filaments and an increased matrix resistivity. The magnetic AC loss has been well described for low-T/sub c/ (wire) conductors. In Bi-2223 tapes the picture is different due to strong anisotropy, granularity, flux creep and large aspect ratio of the tape. The magnetic AC loss is investigated at power frequencies in various Bi-2223 tapes (twisted and nontwisted) and with different materials for the matrix (Ag, Ag alloys and ceramic barriers). When the field is parallel to the tape plane, the filaments in twisted tapes can be decoupled and the AC loss is decreased even when the matrix is silver. In tapes with ceramic barriers between the filaments, first indications of filament decoupling are observed also in perpendicular field. Compared to a round wire, there are essential differences between the AC loss mechanisms occurring in a long twisted tape and those in a short piece of nontwisted tape.

AC loss in high-temperature superconducting conductors, cables and windings for power devices

High-temperature superconducting (HTS) transformers and reactor coils promise decreased weight and volume and higher efficiency. A critical design parameter for such devices is the AC loss in the conductor. The state of the art for AC-loss reduction in HTS power devices is described, starting from the loss in the single HTS tape. Improved tape manufacturing techniques have led to a significant decrease in the magnetization loss. Transport-current loss is decreased by choosing the right operating current and temperature. The role of tape dimensions, filament twist and resistive matrix is discussed and a comparison is made between state-of-the-art BSCCO and YBCO tapes. In transformer and reactor coils the AC loss in the tape is influenced by adjacent tapes in the coil, fields from other coils, overcurrents and higher harmonics. These factors are accounted for by a new AC-loss prediction model. Field components perpendicular to the tape are minimized by optimizing the coil design and by flux guidance pieces. High-current windings are made of Roebel conductors with transposed tapes. The model iteratively finds the temperature distribution in the winding and predicts the onset of thermal instability. We have fabricated and tested several AC windings and used them to validate the model. Now we can confidently use the model as an engineering tool for designing HTS windings and for determining the necessary tape properties.