Superconducting traction transformer

An ongoing project to develop HTS traction transformers for the Chinese Fuxing high-speed train is demonstrating that the high-power density can be reached using high-temperature superconductors (HTS). The findings are spectacular: the existing 6.5 MVA traction transformers can be replaced with drop-in superconducting transformers, achieving targets of less than 3 tons transformer system weight and 99.5 % efficiency compared to 6 tons and 95 % in the existing devices. The key to achieving these impressive figures is minimizing the AC loss of the HTS windings. New high-performance wire, high current HTS Roebel conductor, high aspect-ratio windings, and flux diverters placed at the winding ends all contribute to reducing the electrical loss to less than 2 kW

AC Loss Calculation on a 6.5 MVA/25 kV HTS Traction Transformer with Hybrid Winding Structure

HTS wire cost is a critical factor for successful commercialization of HTS traction transformer technology. Wire cost might be minimized without significantly increasing AC loss by introducing a hybrid winding structure: the end-part of the windings is wound with high-cost high-I c wires; the central-part of the windings is wound with low-cost low-I c wires. We report AC loss simulation results on HTS windings with both HV and LV windings wound with REBCO wires. The 2D axisymmetric FEM simulation was carried out using H-formulation. The HV windings are wound with 4 mm-wide wires and LV windings are wound with 8/5 (eight 5 mm - wide strands) Roebel cables. Both HV and LV 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. Significant HTS wire cost reduction could be achieved without compromising AC loss by using hybrid windings. This may help commercialize HTS traction transformer technology

Depairing critical current achieved in superconducting thin films with through-thickness arrays of artificial pinning centers

Large area arrays of through-thickness nanoscale pores have been milled into superconducting Nb thin films via a process utilizing anodized aluminum oxide thin film templates. These pores act as artificial flux pinning centers, increasing the superconducting critical current, Jc, of the Nb films. By optimizing the process conditions including anodization time, pore size and milling time, Jc values approaching and in some cases matching the Ginzburg-Landau depairing current of 30 MA/cm^2 at 5 K have been achieved - a Jc enhancement over as-deposited films of more than 50 times. In the field dependence of Jc, a matching field corresponding to the areal pore density has also been clearly observed. The effect of back-filling the pores with magnetic material has then been investigated. While back-filling with Co has been successfully achieved, the effect of the magnetic material on Jc has been found to be largely detrimental compared to voids, although a distinct influence of the magnetic material in producing a hysteretic Jc versus applied field behavior has been observed. This behavior has been tested for compatibility with currently proposed models of magnetic pinning and found to be most closely explained by a model describing the magnetic attraction between the flux vortices and the magnetic inclusions.Comment: 9 pages, 10 figure

Design of a single-phase 6.5 MVA/25 kV superconducting traction transformer for the Chinese Fuxing high-speed train

Traction transformers are critical components of Chinese high-speed-trains. We are currently building a single–phase 6.5 MVA superconducting traction transformer which can achieve targets of less than 3 tons of transformer system weight, better than 99% efficiency, and 43% short-circuit impedance. The proposed transformer consists of four single-phase 25 kV/1.9 kV HTS windings, operating at 65 K, each of which drives a motor. The design incorporates Roebel cable in the LV windings to cope with large current and minimize AC loss. We present 2D FEM AC loss modelling results that identify the critical parameters that contribute to AC loss. We show that the combination of winding length ≥1 m, high performance Fujikura wires, and flux diverters arranged at the end of HV and LV windings, can restrain AC loss in the HTS windings to under 2 kW. We introduce an open-loop cooling system concept with sub-cooler integrated inside the transformer cryostat that can achieve total system weight under 3 tons assuming 2.5 kW total heat load and 8 h of continuous running time. A nominal efficiency of 99.5% can be achieved for this total heat load. The entire superconducting transformer system can be readily fit in the space allocated for conventional transformers in the Chinese Fuxing trains

Superconducting traction transformer:Traction - the HTS Transformer Killer Application?

An ongoing project to develop HTS traction transformers for the Chinese Fuxing high-speed train is demonstrating that the high power density accessible using high temperature superconductors (HTS) can produce spectacular results: the existing 6.5 MVA traction transformers can be replaced with drop-in superconducting transformers which can achieve targets of less than 3 tons transformer system weight and 99.5% efficiency compared to 6 tons and 95% in the existing devices. The key to achieving these impressive figures is minimising the AC loss of the HTS windings. New high-performance wire, high current HTS Roebel conductor, high aspect-ratio windings, and flux diverters placed at the winding ends all contribute to reducing the electrical loss to less than 2 kW

15% reduction in AC loss of a 3-phase 1 MVA HTS transformer by exploiting asymmetric conductor critical current

An asymmetric dependence of the critical current on the direction of an applied magnetic field in HTS coated conductors has a non-trivial influence on the AC loss of coil windings. We report the modelled influence of real conductor critical current asymmetry on the AC loss characteristics of a 1 MVA HTS transformer design previously demonstrated by the Robinson Research Institute as well as a stand-alone coil having the same geometrical and electrical parameters as the low voltage (high current) winding of the transformer. We compare two commercial HTS conductors with distinctive differences in their critical current asymmetry and show a maximum variation of 15% and 29% in the calculated AC loss of the transformer and the stand-alone coil winding, respectively, when the conductor orientation is varied in the top and bottom halves of the windings. AC loss simulation giving consideration to asymmetric conductor critical current before winding the transformer could lead to substantial AC loss reduction even using the same amount of conductor and the same transformer design

The interpretation of the field angle dependence of the critical current in defect-engineered superconductors

We apply the vortex path model of critical currents to a comprehensive analysis of contemporary data on defect-engineered superconductors, showing that it provides a consistent and detailed interpretation of the experimental data for a diverse range of materials. We address the question of whether electron mass anisotropy plays a role of any consequence in determining the form of this data and conclude that it does not. By abandoning this false interpretation of the data, we are able to make significant progress in understanding the real origin of the observed behavior. In particular, we are able to explain a number of common features in the data including shoulders at intermediate angles, a uniform response over a wide angular range and the greater discrimination between individual defect populations at higher fields. We also correct several misconceptions including the idea that a peak in the angular dependence of the critical current is a necessary signature of strong correlated pinning, and conversely that the existence of such a peak implies the existence of correlated pinning aligned to the particular direction. The consistency of the vortex path model with the principle of maximum entropy is introduced.Comment: 14 pages, 7 figure

Utilising full angle-dependent critical current data in the electromagnetic modelling of HTS coils

International audienceA methodology is presented for modelling HTS coils using full angle-dependent critical current data. The results are contrasted with those obtained using more common techniques such as a minimum critical current method or a field magnitude-dependent approximation. Several interesting design consequences that emerge only when the full anisotropy of the wire is taken into account are outlined and discussed

Utilising full angle-dependent critical current data in the electromagnetic modelling of HTS coils

International audienceA methodology is presented for modelling HTS coils using full angle-dependent critical current data. The results are contrasted with those obtained using more common techniques such as a minimum critical current method or a field magnitude-dependent approximation. Several interesting design consequences that emerge only when the full anisotropy of the wire is taken into account are outlined and discussed