A transformer includes a transformer core, a first wire, which forms a first winding, and a second wire, which forms a second winding. The first and second windings are wound around the transformer core. A preformed insulation structure is arranged between the first and second winding and designed to space apart the second winding from the first winding and the transformer core. The preformed insulation structure further includes a first shell which at least partially encloses the transformer core with the first winding, and a second shell which at least partially encloses the transformer core with the first winding. The first and second shells are identical. One or more holes are defined in the first shell and the second shell. The one or more holes cover more than 10% of a surface of the preformed insulation structure.