This thesis first describes the reverse method of transformer design. An existing magnetic model for full-core shell-type transformers, based on circuit theory, is summarised. A magneto-static finite element model is introduced and two sample transformers are analysed. The magnetic model based on finite element analysis is shown to be more accurate than the model based on circuit theory. Partial-core resonant transformers are then introduced and their characteristics are explained using an equivalent circuit model. A method of measuring the winding inductances under resonant operation is developed and used to investigate the characteristics of two different tuning methods. A finite element model of the partial-core resonant transformer is developed by adopting the model for full-core shell-type transformers. The model results accurately match the measured inductance variation characteristics of three sample transformers and predict the onset of core saturation in both axial-offset and centre-gap arrangements. A new design of partial-core resonant transformer is arrived at, having an alternative core and winding layout, as well as multiple winding taps. The finite element model is extended to accommodate the new design and a framework of analysis tools is developed. A general design methodology for partial-core resonant transformers with fixed inductance is developed. A multiple design method is applied to obtain an optimal design for a given set of specifications and restrictions. The design methodology is then extended to devices with variable inductance. Three design examples of partial-core resonant transformers with variable inductance are presented. In the first two design examples, existing devices are replaced. The new transformer designs are significantly lighter and the saturation effects are removed. The third design example is a kitset for high-voltage testing, with the capability to test any hydro-generator stator in New Zealand. The kitset is built and tested in the laboratory, demonstrating design capability. Other significant test results, for which no models have yet been developed, are also presented. Heating effects in the core are reduced by adopting an alternative core construction method, where the laminations are stacked radially, rather than in the usual parallel direction. The new kitset is yet to be used in the field.

The thesis begins with an introduction to transformer theory. The partial core transformer is then introduced and compared with a full core design. A brief introduction to superconductors and high temperature superconductors is then presented. High temperature superconducting fault current limiters are then examined and the advantage of a high temperature superconducting partial core transformer and fault current limiter as a single unit is highlighted. The reverse design model is discussed followed by the model parameters that are used in designing the high temperature superconducting partial core transformer. Partial core transformers with copper windings and high temperature superconductor windings at the University of Canterbury were then tested and the measured results compared with the results calculated from the reverse design model, to validate the model. The high temperature superconducting partial core transformer failed during an endurance run and the investigation of the failure is then presented. The results of the failure investigation prompted an alternative winding insulation design. A model to calculate the time at which the high temperature superconducting winding of the partial core transformer would melt at different currents was then built. The time was calculated to be used in the operation of the quench detection mechanism and it could also be used in choosing a circuit breaker with a known operating time. The design of the high temperature superconducting partial core transformer and fault current limiter is then presented. Design configurations with different core length and winding length are examined. The idea behind choosing the final design for the high temperature superconducting partial core transformer and fault current limiter is then discussed. The final design of the high temperature superconducting partial core transformer and fault current limiter is then presented. A new 7.5 kVA, 230-248 V high temperature superconducting partial core transformer and fault current limiter was designed, built and tested. The windings are layer wound with first generation Bi2223 high temperature superconductor. A series of electrical tests were performed on the new device including open circuit, short circuit, resistive load, overload and fault ride through. These tests were performed to determine the operational characteristics of the new high temperature superconducting partial core transformer and fault current limiter. The measured results from the tests were compared with the calculated results. The fault ride through test results were then compared to a 15 kVA high temperature superconducting partial core transformer that was designed and built at the University of Canterbury. Since the resistive component of the silver matrix in Bi2223 high temperature superconductor plays a very little role in controlling the fault current, the current limited by the leakage reactance is compared between the two devices. The high temperature superconducting partial core transformer and fault current limiter was found to be 99.1% efficient at rated power with 5.7% regulation and fault current limiting ability of 500 % over the 15 kVA high temperature superconductor partial core transformer from University of Canterbury.