We describe the design, construction, and testing of a proof-of-principle 45 kVA, 450/450 V, single phase superconducting transformer with fault tolerant capability. This small-scale transformer demonstrates extended short circuit withstand time as a result of the high thermal mass of the windings as well as improved recovery under load due to the high heat transfer achieved in subcooled liquid nitrogen with wire coated to optimise boiling heat transfer. Primary and secondary windings consist of 38-turn single layer solenoids wound in helical trenches milled in glass-reinforced epoxy formers. The thermal mass of the 0.4 mm thick brass-laminated conductor provides for adiabatic fault withstand times over 1 s, allowing greater flexibility in the design of protection systems required to isolate faults. Enhanced heat transfer from the windings to the liquid nitrogen coolant allows the transformer to recover from the short circuit event, cooling from around 300 K to return to the superconducting state even while current close to the rated current continues to flow. Two factors contribute to the high heat transfer. Firstly the HTS transformer operates in subcooled liquid nitrogen, at 65 K–66 K at atmospheric pressure, significantly increasing the heat transfer compared to operation at saturated vapour pressure. Secondly, the HTS conductor is coated with solid polymer insulation with optimised thickness, which allows efficient cooling by nucleate boiling to take place over an extended range of conductor temperature.

This paper describes recent experimental results in a continuing program to develop a 1 MVA demonstration transformer that employs HTS Roebel cable for the high current windings. The electrical design parameters of the transformer are presented, including the configuration of the specially developed Roebel cable. This paper discusses estimation of the cable I c from measured I c (B) data for samples of the strands, AC loss minimization using flux diverters, proving the suitability of the polyimide insulation scheme to withstand the effects of a high voltage impulse and modelling and experimental verification of the fault current limiting behaviour expected from the HTS conductor.

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