End-winding region configuration of an HTS transformer

Single-phase 14 kVA transformer model with Bi-2223 superconducting windings in a nonmetallic cryostat has been designed and fabricated. Rated voltage of the primary and the secondary winding is 400 V and 200 V, respectively. Considerable attention was paid to the minimization of the radial component of the magnetic field at the end of the winding, where this field component, perpendicular to the tape, reduces critical current and increases AC losses. Additional laminated ferromagnetic rings shaping the magnetic field around the winding edges are used to reduce the radial field component. The additional rings also eliminate stray field differences along winding circumference. A finite element method (software ANSYS) was used for optimization of the ring cross-section and their position with respect to the winding. The calculations show that the rings reduce the radial component of the magnetic field by about 50% and the critical current of the winding can be increased from 53 A to 65 A.