The fully sealed power transformer eliminates the oil reservoir and uses the fins of the corrugated oil tank body as the heat dissipation and cooling elements. At the same time, it expands and contracts with the increase and decrease of the transformer oil, isolating the interior of the transformer from the atmosphere, preventing the deterioration of the oil, insulation moisture absorption and aging, and enhancing the reliability of operation. The core is made of high-quality cold-rolled silicon steel sheets, effectively improving the distribution of magnetic flux density within the core, reducing noise and loss. The coils are wound with high-strength enameled wire (or paper-covered wire), in a cylindrical (or toroidal) structure, with uniform ampere-turn distribution and a reasonable insulation structure, having strong anti-short-circuit capability. The fasteners used in the oil tank are all stop-back nuts, which can ensure that they do not loosen during long-distance transportation and during operation. After phosphating treatment of the oil tank surface, it is coated with three-proof paint (anti-humidity, anti-mildew, anti-salt fog), and is widely used in harsh operating environments such as metallurgy, mining, and petrochemical industries.