Volker Hussennether

kA-Class High-Current HTS Conductors and Windings for Large Scale Applications

Multistrand conductors with kA-class current capacity are a key requirement for many large scale applications of HTS in power engineering and magnet technology. Siemens has developed a cabling technology for continuously transposed conductors (CTC) for transformer and generator applications with critical current up to 1.2 kA at liquid nitrogen temperature. For technical issues of coil winding several demonstrator coils were fabricated in solenoid and racetrack geometry. The current capacity of the CTCs and of the coils are in agreement with model calculations considering magnetic field distributions within the conductor and coils, respectively. To account for mechanical loads during operations such as Lorentz forces and centrifugal forces robust conductors are required. We constructed dedicated devices in order to study the performance of continuously transposed conductors in coil-like geometry.

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.

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.

Current Capacity of Ag/Bi-2223 Wires for Rotating Electric Machinery

With focus on the application in rotating electric machines we measured the dependence of current capacity of Ag/Bi-2223 wires on temperature and magnetic field. Even for wires stemming from a single manufacturer we observe a significant spread of wire properties. We study different temperature and magnetic field dependence by a parallel path model which allows for a quantitative analysis. The implications of experiments and modelling are discussed with regard to the further wire development and for application within windings.