Superconducting transformers – Part I
Superconducting transformers using high current density High Temperature Superconductor (HTS) wire cooled with liquid nitrogen can be lighter and more efficient than conventional power transformers. This paper describes the 1 MVA 11/0.415 kV HTS transformer developed by a New Zealand – Australian team, featuring HTS Roebel cable in the 1.4 kA-rated low voltage winding. Comparison of HTS and conventional transformer designs at 40 MVA rating shows lower lifetime cost of losses makes HTS base-load transformers cost-competitive in higher energy cost markets. Power density – more MVA in a restricted footprint – could be a decisive advantage in mobile applications.
Keywords: superconductor, Roebel cable, cryogenic
Transformers using High Temperature Superconductor (HTS) wire instead of copper conductor, and liquid nitrogen instead of dielectric oil, have been in development for almost two decades now. What are the prospects that HTS transformers will find a place in the electricity grid of the future? After giving a brief background to high temperature superconducting wire and the basic technology of HTS transformers, we describe the design and construction of our recently completed demonstration transformer featuring superconducting Roebel cable low voltage windings. The agreement of measurement and modelling of the load loss of this transformer allow us now to predict the load loss of larger HTS transformers with confidence. A clearer picture emerges of the outlook for this technology.
1.1 High temperature superconductors
The discovery of a new class of High Temperature Superconductor (HTS) materials in 1987 inspired the vision of a power grid populated with superconducting devices: motors and generators, transformers, cables, fault current limiters, and magnetic energy storage. Although the future has seemed to be rather a long time coming, HTS versions of all these devices have since been demonstrated. A roadmap by Wolsky  reviews progress to 2013 and includes extensive references.
The HTS materials of most interest for grid applications, named BSCCO (Bismuth Strontium Calcium Copper Oxide) and REBCO (Rare Earth Barium Copper Oxide) for the initial letters of their constituent elements, make the transition to the superconducting state with its vanishing DC electrical resistivity at critical temperatures T_c above the normal boiling point of liquid nitrogen, 77 kelvin (-196 °C). Fig. 1 contrasts the temperature variation of the resistivity of copper and REBCO HTS conductor, plotted on a log scale. At room temperature, the HTS material behaves like a highly resistive metal, with resistivity almost 1,000 times that of copper. While the resistivity of copper drops more than a factor of seven in cooling to liquid nitrogen temperature, at about 90 K the apparent resistivity of the superconductor falls abruptly to immeasurably low levels.
Figure 1. Variation of resistivity of HTS conductor with temperature compared to copper