Abstract: An electrical transformer is disclosed which includes an enclosure; a magnetic core assembly arranged within the enclosure, the magnetic core assembly having a first core limb, a second core limb and a third core limb; and three coil assemblies having a first coil assembly, a second coil assembly, and a third coil assembly. At least two diaphragms are arranged within the enclosure, the diaphragms being essentially sealed to a first outermost coil of the first coil assembly, and arranged for guiding a cooling fluid in series through a first inner fluid duct, a second inner fluid duct, a third inner fluid duct and an extra-coil volume along outsides of third, second and first outermost coil coils of the third, second and first coil assemblies.