An Experimental High Temperature Superconducting Transformer: Design, Construction and Testing
In 2005, the development of a 50Hz, 15kVA, 30V:115V partial core high temperature superconductor (HTS) transformer was completed. The transformer was designed purely as a proof of concept, rather than as a unit that would be put into service. The transformer windings were layer wound using Bi2223 HTS tape from American Superconductors. The transformer failed while endurance testing under full load with the internal primary winding failing to open circuit. This prompted an investigation into the cause of failure. Results from the investigation suggested the cause of failure to be insufficient cooling of the windings. This paper presents a new experimental full core HTS transformer with an alternative winding design to enable greater cooling for the HTS wire. The design involves the use of cooling channels allowing direct contact of the HTS wire to the liquid nitrogen coolant. A mock up transformer was constructed first using copper wire of similar dimensions to the HTS wire. The idea was to construct two similar windings, while using the same core, but using two different winding material types so that a direct performance comparison could be made between the two. Open circuit, short circuit and loaded tests were performed on the copper mock up transformer submerged in liquid nitrogen. Test results gave an efficiency of 88% in liquid nitrogen, and a 17% voltage regulation at 11kVA load. In terms of mechanical integrity, the copper mock-up transformer withstood all stresses subjected to it when submerged in liquid nitrogen. Following successful testing of the copper mock up transformer, a set of HTS windings were constructed using Bi2223 superconductors. The transformer was a full core, 50Hz, 15kVA, 230V:230V two winding transformer. Open circuit, short circuit and loaded tests were performed on the HTS transformer while submerged in liquid nitrogen. Test results gave an efficiency of 97%, and a 13% voltage regulation at 14kVA load.