Design features of a 240 MVA 275/132 kV site-assembled autotransformer

After a brief review of the conflict between increasing unit sizes and transport-weight limitations and the various methods of overcoming this, the paper outlines the design philosophy underlying the principle of subdivision and reassembly on site and discusses the reasons for adopting the particular method described in the paper and applied to a pilot exercise. Vertical subdivision of the complete unit between phases was adopted, and the various problems encountered in the design, construction and assembly are discussed and illustrated with particular reference to the means adopted for ensuring mechanical stability and accurate realignment of the sections. As adequate dryness of the windings during transport and reassembly is essential, the paper describes the methods used for sealing the windings during transport and for creating suitable conditions of atmospheric dryness and cleanliness during reassembly on an open plinth on site. The steps taken were deemed sufficiently safe to obviate the need for special high-voltage tests on the insulation after reassembly, and this judgment was proved to be correct by the successful commissioning of the unit. The principle so successfully demonstrated on a pilot exercise can be applied to much larger units, and it can also be of great value in those countries where the present transport-weight limitations impose severe restrictions on the maximum size of a 3-phase unit that can be installed on certain sites.