Concentrations of population and business activities result in high electricity demand in urban areas. This requires the construction of large-capacity underground substations. Oilless, nonflammable and nonexplosive equipment is recommended for underground substations. Therefore, several types of large-capacity gas-insulated transformer have been developed. Because the gas forced cooling type was considered to be available up to approximately 60 MVA, all of these gas-insulated transformers are liquid cooled. But the liquid cooling type has the disadvantage of a complex structure for liquid cooling. For this reason, the authors have been studying the development of a simple design for a gas forced cooling, large-capacity gas-insulated transformer. This paper discusses research and development of cooling and insulation technology for a large-capacity gas-insulated transformer and the development of a 275 kV, 300 MVA gas-insulated transformer.
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