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.

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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.

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