A transformer comprising a core and winding wound around a winding axis extending along a limb of the core, said winding terminating in an axial end surface extending in a direction perpendicular to said winding axis, said transformer comprising a ring comprising magnetic material, said ring being located outside said winding and adjacent to said axial end surface. The ring comprises a set of magnetic metal components, such as magnetic metal sheets, said magnetic metal components being distributed about the winding axis and electrically insulated from each other. The core comprises a yoke, said yoke extending radially across the ring, at one or more crossing locations, from a radial inside of the ring to a radial outside of the ring. The cross-sectional height of the ring varies about the winding axis such that magnetic metal components at the crossing locations have lower height along the winding axis than magnetic metal components further away from the crossing locations.