According to the distribution of iron core winding, the transformer divide into core transformer and shell transformer. The three phase core transformer has two structures of three phase three core and three phase and five core. The single phase of core transformer is equipped with center core limb and two branch core limb. The width of center core limb is sum of two iron core width.
All the windings put into center core limb. The branch core limb surround with windings like shell. So we called it shell transformer. iron-shell three-phase transformer whose core can be considered to be composed of three independent single-phase shell transformers and discharged together.
The structure of the core transformer is relatively simple, the distance between the high voltage winding and the iron core is far, and the insulation is easy to handle. The structure of the shell-type transformer is relatively strong, the manufacturing process is relatively complicated, the distance between the high-voltage winding and the iron core column is relatively close, and the insulation treatment is difficult. The shell structure is easy to strengthen the mechanical support of the winding, so that it is subjected to a large electromagnetic force, and is particularly suitable for a transformer that passes a large current. The shell structure is also used for large capacity power transformers.