Transformers-a successful application of high temperature superconductors

With the exception of core steel and improvements in insulation, power transformers have changed little in the last fifty years. In general, modern power transformers larger than 10 MVA have full load efficiencies exceeding 99% and are routinely subjected to a form of life-cycle cost evaluation as a part of the purchase process. However, resistive losses in the windings constitute a significant proportion of losses at full load and the desire to further economically reduce these losses using superconducting windings has been an elusive goal for greater than a quarter century. Using high temperature superconducting (HTS) windings this goal is now within reach both technically and economically. Major HTS transformer projects are underway in Europe, Japan and the United States with all projects reporting success in their initial phases. With the desire for improved efficiency and lower losses within technical reach, additional benefits such as reduced size and weight, improved fire safety, overload without loss of transformer life and improved system dynamics through lower impedance and controlled fault current levels have also been identified. These additional benefits may well prove to be significantly more important to the applications of HTS transformers than improved efficiency through loss reduction in the windings. This paper summarizes the status of current HTS transformer projects and the remaining economic and technical challenges that must be met.