Study On Electromagnetics Of High-Temperature Superconducting Transformers

The high-temperature superconducting(HTS) transformer is one of the most promising superconducting applications in power systems. In this paper, according to the electromagnetic characteristics of HTS tapes, we adopted the finite element model to conduct the systemic study on electromagnetics of high-temperature superconducting transformers, especially on the electromagnetic analyses different from the conventional ones, for instance, the transformer performance influenced by the thickness of the low temperature Dewar, the high current density of HTS tapes, the AC loss analysis of HTS windings, and so on. The research may provide some useful theory basis for the development of HTS transformers.Firstly, the preliminary design of HTS transformers was completed based on the material experiments in this thesis. According to the preliminary design results, electromagnetic properties of HTS transformer were studied in detail. The analytic solution, two-dimensional finite element method(FEM), and three-dimensional FEM were applied to calculate the stray field distributions in HTS transformer windings considering the electromagnetic properties of the high-temperature superconductor, respectively. According to the analyzed results, the two-dimensional plane suitable to the stray field analysis was determined. The effects of the ferrite core material, the operating temperature, the core structure, the coil type, as well as the material, size, and position of the flux diverters at the end edge of the coils, on the transformer performance were studied, in order to choose the most suitable structure pattern to HTS transformer. The current distribution principle among parallel superconducting tapes or coils was determined, and the influence of the standard transposition on the circulation among stacked tapes was studied. Due to the low impedance characteristics of the tapes, incomplete transposition can not avoid the circumfluence effectively, the complete transposition is necessary to stack tapes.Then the core losses, the AC losses of HTS windings, especially of the parallel superconducting tapes or coils, the electromotive forces carried by HTS windings, and the stresses recieved by HTS tapes were analyzed when a HTS transformer was operating with the rated load and when it was short-circuited, respectively. Considering the loading capability, the short-circuited losses, and the short-circuited stresses of the superconducting tapes, the double pancake coils placed concentrically were adopted. The global optimization to constructural variables of the HTS transformer was done according to the different optimal goals in this thesis. The constructure variables of theflux diverters were optimized to improve the performance of the end edges of HTS coils.At last, this thesis confirmed that the analysis of the current distribution and the leakage magnetic field distribution of a 3-phase HTS transformer can be simlified to that of a single-phase transformer. A further research on the design of large-capacity 3-phase/single-phase HTS transformer was carried through.