A transformer containing an electrical active part arranged in an insulating space and including primary and secondary winding wound around a magnetic core. and radially spaced apart from each other by a winding space having an insulation fluid. A block defines a compartment closed and separated from a remaining insulating space, the compartment being at least partially arranged within the winding space. The compartment contains an insulation fluid different from insulation fluid in the remaining insulating space, and having a higher dielectric breakdown field strength than the insulation fluid in the remaining insulating space.