In transformers and reactors with sheet-wound or foil-wound windings, problems may arise due to different electric powers towards the edges of the sheet. One such problem is that a heavy displacement of current may arise, which results in heavy additional losses as well as in powerful localized heating of the sheet edges. The current displacement is caused by the fact that the substantially axial magnetic leakage flux extending between the windings is deflected in a more or less radial direction at the ends of the windings, instead of continuing axially and passing into the yokes. This causes the ends of the windings to be traversed by a magnetic flux with a radial component which generates eddy currents in the winding conductor and causes losses in addition to the unavoidable ohmic losses caused by the conductor current. To reduce these problems, several solutions have been proposed, one of which is described in SE 418 234. Here a method is described which is characterized in that the conductor sheet at its end portions, at least in a region at the periphery of the winding, follows a funnel-shaped double-curved surface. Another method is described in SE 428 979, which is characterized in that the cooling channels include spacers which exhibit an increasing thickness towards the end surfaces of the winding.
The double curvature which is needed to reduce the effect of the current displacement is necessary above all in those parts of the sheet winding which, viewed in a radial direction, are furthest away from the axial centre line of the winding. For practical reasons, sheet windings are therefore sometimes formed with an end surface in a plane perpendicular to the centre line for that part which, viewed in a radial direction, lies nearest the axial centre line of the winding. These parts will be referred to below as the central and peripheral parts of the sheet winding, respectively.
Another known electrical phenomenon is that high electric field intensities arise at sharp edges or pointed projections, and these field intensities may cause corona and electric flashover. Reducing these field intensity concentrations by increasing the radius of curvature of the edges or projections in different ways belongs to the state of the art.
In existing sheet winding designs, a potentially dangerous region, with respect to corona and possibly electric flashover, exists in those parts of a sheet winding which adjoin both sides of a cooling channel. This is true both for those parts of the sheet winding which comprise the central and the peripheral parts. The reason is that for the central parts of the sheet winding, the end surface of the sheet winding forms a right angle with the internal and external walls of the cooling channels, that is, the sheet winding forms a right-angled edge. For the peripheral parts of the sheet winding the risk of corona and discharge will be even greater since the end surface of the sheet winding forms an angle with the cylindrical surfaces of the cooling channels, especially with the internal cylindrical surfaces, which is smaller than a right angle, that is, the sheet winding forms an even more acute angle with the cooling channels. This is shown very clearly in the accompanying drawings.