An insulation system for a winding structure. The insulation system includes an innermost barrier pair arranged to cover a majority of the winding structure in the axial direction of the winding structure inside and outside the barrier structure relative the curvature of winding turns of windings of the winding structure, wherein at least one barrier of the innermost barrier pair defines a first flow path allowing flow of a dielectric fluid mainly in a first axial direction between the winding structure and the at least one barrier when the insulation system is in a assembled state; and a first outer barrier arranged radially inwards or radially outwards relative each barrier of the innermost barrier pair, wherein the first outer barrier defines a second flow path, parallel to the first flow path, allowing flow of a dielectric fluid mainly in a second axial direction opposite the first axial direction.