Traction transformers are key components for the Chinese high speed train system, and it is hoped that a superconducting version will replace oil-based conventional transformers in this application. Since 2018 Beijing Jiaotong University has been leading a six partner project, funded by the Chinese Ministry of Science and Technology (MOST), to develop a 6.5 MVA HTS traction transformer. The transformer consists of four single-phase 25 kV/1.9 kV HTS windings, operating at 65 K, each of which drive a motor. The rated currents for each of the HV (high voltage) and LV (low voltage) windings are 63 A and 846 A, respectively. The HTS transformer should demonstrate improved performance, achieving 99% efficiency and 3 tonne total system weight. Minimization of AC loss in the HTS windings is critical to achieve both efficiency and weight targets; the weight of the cooling system scales directly with AC loss.
Despite this improved performance, the cost of the HTS windings is critical for commercialization of the HTS traction transformer technology. Wire costs can be minimized without significantly increasing AC loss by using hybrid windings: the end-part of the windings is wound with high-cost and high-performance wire/Roebel cable; the central part of the windings is wound with low-cost and low-performance wire/Roebel cable.
We report H-formulation 2D axisymmetric FEM AC loss simulation results on hybrid structure HTS windings with both HV and LV windings wound with REBCO coated conductors. The simulation uses measured Jc(B) curves at 65 K for each conductor. The HV windings utilize 4 mm coated conductors and LV windings utilize 8/5 Roebel cables assembled using Roebel strands from 12 mm conductors. Both windings have a hybrid structure in order to reduce the wire cost. Flux diverters are placed at the end of the windings to reduce AC loss. AC loss values in the HTS transformers with the hybrid structure are compared with HTS transformers with non-hybrid structures and the feasibility of the hybrid winding structure is discussed.