This transformer is the first time that the company has developed a high-voltage and large capacity 220kV transformer. The successful development of this product has improved the production and manufacturing level of high-voltage and large capacity products for SEC , and is of great significance for the company's entry into the power grid market. This product has fully explored and demonstrated the traditional design theory of strengthening the strength and bearing capacity of pancake electrical appliances and the insulation structure of transformer coils. Through independent research, development, and trial production, it proposes the use of small oil gap volume theory and the design of insulation structure based on composite field strength, achieving the increase of electrical strength, reduction of materials, reduction of losses, and safety and energy saving of transformers.