Analysis of magnetic field and circulating current for HTS transformer windings

In a high-temperature superconducting (HTS) transformer, the leakage magnetic field decreases the critical current and increases the ac loss in the tapes. Moreover, because of nearly zero resistance of HTS tapes, a slight unbalance of the branch inductances of the windings might result in heavy circulating current. So, the numerical analysis of the leakage magnetic field and circulating current is especially necessary for an HTS transformer design. In this paper, the influence of the winding configurations on the stray field and circulating current is studied. That is, the magnetic field distribution is analyzed by finite-element method and then, based on the inductance matrix obtained after a magnetic field analysis, the circulating current is calculated by circuit analysis. Some measures for improving the leakage field and circulating current distribution are also proposed to make HTS transformers more efficient.