Oil-immersed transformer is a new type of high performance transformer with more reasonable structure and better performance. Because its three core columns are three-dimensional arrangement of equilateral triangle, its magnetic circuit has no air gap and is more tightly wound. The three magnetic circuits have the same length and are the shortest, and the cross-sectional area of the core column is closer to the circle. Three balance, reduce the three harmonic components, the product is more suitable for urban and rural, industrial and mining enterprises grid transformation, more suitable for combined transformer and pre-installed transformer substation.