An oil-immersed transformer is a new, high-performance transformer with a more rational structure and superior performance. Its three-dimensional rolled iron core, with its three core columns arranged in an equilateral triangular立体 pattern, has no air gap in its magnetic circuit, allowing for tighter winding and consistent magnetic circuit lengths. The cross-sectional area of the iron core columns is closer to circular, thereby further enhancing performance, reducing losses, noise, and achieving three-phase balance. This product is more suitable for urban and rural, industrial and mining enterprise grid transformations, and is especially suitable for transformers used in modular transformers and pre-assembled substation transformers.