High-voltage partial-core resonant transformers

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. LIST OF PUBLICATIONS The following papers have either been published, accepted or submitted during the course of the research described in this thesis: JOURNAL PAPERS 1. Bell, S.C. , and Bodger, P.S. , ‘Equivalent circuit for high-voltage partial-core resonant transformers’, IET Electr. Power Appl. , vol. 2, no. 3, pp. 155-163, May 2008. 2. Bell, S.C. , and Bodger, P.S. , ‘Inductive reactance component model for highvoltage partial-core resonant transformers’ IET Electr. Power Appl. , vol. 2, no. 5, pp. 288-297, September 2008. 3. Bell, S.C. , and Bodger, P.S. , ‘Power transformer design using magnetic circuit theory and finite element analysis’, submitted to IJEEE on 15/8/07. 4. Bell, S.C. , and Bodger, P.S. , ‘New design of high-voltage partial-core resonant transformer’, submitted to Electric Power Systems Research, 30/07/08. CONFERENCE PAPERS 1. Bodger, P.S. , and Bell, S.C. , ‘Power transformer analytical design approaches’, paper presented at the Power Transformer Convention & Workshop, Christchurch, New Zealand, 2-3 July 2007. 2. Bendre, V.D. , Bell, S.C. , Enright, W.G. , and Bodger, P.S. , ‘AC high potential testing of large hydro-generator stators using open core transformers’, paper presented at the 15th International Symposium on High Voltage Engineering, Ljubljana, Slovenia, 27-31 August 2007, paper T3-303. 3. Lynch, K. , Bodger, P.S. , Enright, W.G. , and Bell, S.C. , ‘Partial core transformer for energisation of high voltage arc-signs’, paper presented at the 15th International Symposium on High Voltage Engineering, Ljubljana, Slovenia, 27-31 August 2007, paper T3-304. vi 4. Bell, S.C. , and Bodger, P.S. , ‘Power transformer design using magnetic circuit theory and finite element analysis a comparison of techniques’, paper presented at Australasian Universities Power Engineering Conference (AUPEC), Perth, Western Australia, 9-12 December 2007, pp. 607-612. 5. Bodger, P.S. , Enright, W.G. , Bell, S.C. and Bendre, V.D, ‘Partial core transformers for HV testing and power supplies’, Invited paper presented at Techcon Asia Pacific, Sydney, Australia, 31 March-2 April, 2008, pp. 67-79. 6. Enright, W.G. , Bendre, V.D. , Bell, S.C. and Bodger, P.S. , ‘Field experiences using a prototype open core resonating transformer for A.C. high potential testing of hydro-generator stators’, Invited paper presented at Techcon Asia Pacific, Sydney, Australia, 31 March-2 April, 2008, pp. 265-277. Additionally, a 30-minute oral presentation titled ‘High-voltage testing of hydro-generator stators using partial-core transformers’ was presented at the Seminar for the next generation of researchers in power systems, University of Manchester, United Kingdom, 16-19 September 2007. ACKNOWLEDGEMENTS The final reading of this thesis has prompted me to review my initial goals and ambitions as a post-graduate student. My original thesis topic was superconducting transformers, which, for reasons beyond my control, I never quite managed to study. My original time-line of 3 years was exceeded by more than 7 months. My anticipated savings over the study period were not quite achieved. Nevertheless, I am very happy with the outcomes of this thesis. The years spent as a post-graduate student have been some of the best of my life so far (but I hope for even better times in the years to come). None of this would have been possible without the technical and social support of the other students and staff. I would like to give particular thanks to my supervisor Pat Bodger and co-supervisor Wade Enright. Thanks also to the technicians, especially Jac Woudberg and David Healy for building the transformers, and Ken Smart for general advise and constant loaning of equipment. The postgraduate students in the department have also made my life more interesting. Some of the best conversations I had were at the small hours of the morning. I recall, on more than one occasion, having the cleaning ladies tell me to go home when they arrived at 6am. Special thanks to my friends in the industry who kept telling me to get a real job, and my family, who always managed to appear interested when listening to me talk about my research. Thanks to Geoff Cardwell for informing us about the conference in Slovenia, which I subsequently attended, and for arranging the site visit to an ABB transformer factory in Germany. Thanks also to Mirko, Anna and Rayk for showing me around Germany during my holiday in Europe. I would also like to acknowledge phdcomics.com and dc++ for the much needed distractions from research. Finally, I would like to acknowledge the financial support I received through Scholarships from the University of Canterbury and the Electric Power Engineering Centre. CONTENTS ABSTRACT iii LIST OF PUBLICATIONS v ACKNOWLEDGEMENTS vii LIST OF FIGURES xv LIST OF TABLES xxi GLOSSARY xxiii CHAPTER