Research On Electromagnetic Design And Characteristics Of High Temperature Superconducting Transformer With 300KVA/25000V/860V Single-Phase

The characteristics of HTS (High Temperature Superconducting) transformers are first presented in this paper, and the development of HTS transformers now at home and broad is described in detail, then, the electromagnetic characteristics are analyzed which are critical current, magnetic field distribution, circulating current, leakage impedance, AC loss, and short circuit current and close circuit current at no load, etc.. 1) Critical current of a superconductor is dependent on the magnetic field and temperature. The critical current in a superconducting device decrease by 40~60% compared with that under only self-field. The critical current of the stacked samples are smaller than those of the stacked number times the critical current of single tape. Moreover, because of anisotropy of HTS tape, the influence of radial field component on the critical current is much greater than that of axial one on it. 2) Leakage magnetic field and circulating current in the windings must be paid much more attention to in a HTS transformer than in a conventional one. The leakage field, especially its radial component decreases critical current and increases AC loss in a HTS tape. The characteristics of no resistance in a tape will cause heavy circulating current among the paralled brabches of windings. A. In this paper we study comparatively the effect of 2 types of cores and 7 different winding configurations on the magnetic field distribution by a method of coupling magnetic field with electric circuit. The conclusions are that it is not practical that joining together the the paralleled tapes at each end of the layers in the secondary and connecting two layers in series with a copper wires are used as the substitution for the transposition needed for parallel tapes, and the "E-core"type of core has smaller radial components than the "C-core"type of core. We also investigate some measures on reduction of radial field component, such as placing flux diverters on the ends of windings. A methed for decrease the radial magnetic field is presented that putting unequal lengths of gapes among the pancakes or turns of a coil from the middle to the ends of the secondary windings, etc.. B. The circulating current distributions of 8 types of winding configurations are thoroughly analyzed, and this is a pioneer work in the study on circulating current of HTS transformer. We conclude from it that if a winding is wound in parallel with more than one tape, a cylindrical winding would be the proper choice, for the circulating current can be reduced to zero, at least theoretically, by fully transposing the parallel tapes, and it is, however, impossible in a pancake winding because no two or more magnetically equivalent coil pancakes exist. 3) Leakage impedance is an important parameter of a transformer. The windings which are made of different types of coils for the primary winding and the secondary winding have uneven distribution of magnetic-force, and produce the radial component of magnetic field which should be considered in the calculation of leakage reactance. In this paper, a calculation method of leakage reactance is proposed for the winding configurations which is concentrically arranged "HV-LV-HV"on a core limb. An equivalent transformation about the uneven distributed magnetic force is proposed for calculating leakage reactance. The investigation shows that the small the number of the coil pancakes is, the greater the leakage reactance changes with the gap lengths among coil pancakes. Finally the method is verified valid by the result based on the calculation of leakage magnetic field distribution by finite element method (FEM). 4) Because of zero resistance of HTS tape, the characteristics of a HTS transformer in an electromagnetic temporary process is some different from that of a conventional one. Moreover, because the non-periodic component of current declines slowly, the electromagnetic force in a steady short state has a component of 50 Hz frequency, which doesn't exist in a conventional transformer. In the paper, the electromagnetic force and mechanical stress in each coil in short steady state are calculated, and the close circuit current at no load is also investigated under consideration of core saturation. 5) AC loss is an important parameter of a HTS transformer, and it is one of difficulties to study it. The AC loss of the HTS transformer is calculated in terms of the theoretical formula presented by some related references. The result shows that although the axial field component is higher than the radial component, the AC loss produced by the radial field component is greater than that by the axial one.