The active part of a transformer consists of the core, coils, upper cover, tap changer, bushings, and protection and connection members on the upper cover. The coils are mounted by squeezing against the core and each other by means of laths in order to carry the radial forces, and the upper yoke is arranged. Connections of the active part can be made on the upper cover or directly on side walls of the tank as well.

Assembly is process of assembling the core and the winding up to the end of production. When assembling the active part, the transformer must have a structure to provide the best strength to resist the predicted short circuit mechanical forces. Therefore, strength stability is increased by additional measures during the construction of the active part