Transformer

Abstract

Transformer (100) comprising a core (10) extending along a winding axis (A). The transformer (100) further com prises a winding (20) wound around the core (10) and extendi ng along the winding axis (A), wherein the winding (20) term inates in a first axial end surface (21) and in a second axia l end surface (22). The first axial end surface (21) is a rranged opposite the second axial end surface (22) along th e winding axis (A). The first axial end surface (21) and the second axial end surface (22) extend along a radial direct ion (R), which is perpendicular to the winding axis (A). The transformer (100) further comprises a ring element (30) arranged at the first axial end surface (21). The r ing element (30) comprises ferromagnetic sheets (31), a ligned along the winding axis (A). The ring element (30) f urther comprises an insulating shell (32) covering the fer romagnetic sheets (31).

Description

[0001] P2025, 0432 WO N / P250092WO01 October 15, 2025

[0002] - 1 -

[0003] Description

[0004] TRANSFORMER

[0005] The present disclosure relates to a transformer, in particular to a transformer for application in power grid systems, for example it relates to high voltage transformers.

[0006] Transformers are used in power systems for voltage level control. In particular, a transformer is used to step up and step down voltage in electric power systems in order to transmit and utilize electrical power. Document US 2011 / 316662 Al discloses a winding arrangement for a transformer or for a reactor. The winding arrangement includes an annular winding cover part disposed on the front face of a winding, wherein a side surface of the winding cover part overlaps a front surface of the winding, wherein the annular winding cover part is designed to be magnetically conductive at least in a partial area and comprises a convex side surface facing away from the winding.

[0007] Document US 3 983 523 A discloses a clamping ring and static plate used for axially compressing the ends of a cylindrically shaped power transformer winding. The electrically conductive static plate portion of the combination device is formed around and is insulated from a foundation providing, high- strength material, clamping ring.

[0008] It is desirable to provide a transformer that can be operated reliably.

[0009] Embodiments of the disclosure relate to a transformer. The transformer comprises a core. The core extends along a P2025, 0432 WO N / P250092WO01 October 15, 2025

[0010] winding axis. The transformer further comprises a winding. The winding is wound around the core. The winding extends along the winding axis. The winding terminates in a first axial end surface and in a second axial end surface. The first axial end surface is arranged opposite the second axial end surface along the winding axis. The first axial end surface and the second axial end surface extend along a radial direction. The radial direction is perpendicular to the winding axis. The transformer further comprises a ring element. The ring element is arranged at the first axial end surface. The ring element comprises ferromagnetic sheets. The ferromagnetic sheets are aligned along the winding axis. The ring element further comprises an insulating shell. The insulating shell covers the ferromagnetic sheets. The ring element and the associated insulating shell are positioned at a first distance from the first axial end surface along the winding axis.

[0011] The distance between the ring element and the axial end surface allows the transformer to be cooled.

[0012] The magnetic flux generated by the windings generates eddy current losses in the winding's conductors. These eddy current losses contribute to overall load losses and local losses, which create hotspots. During operation of the transformer, radial flux also generates axial forces. For example, these forces cause vibrations in the windings and resulting noises. The ring element is arranged to reduce losses, hotspots and also forces. Due to the ring element, the magnetic flux generated by the winding generates less eddy current losses in conductors of the winding and less axial forces as well. The ring element is arranged at the axial end surface, where radial flux is higher and tends to go out from the winding and reach the core or the tank. This P2025,0432 WO N / P250092WO01 October 15, 2025

[0013] - 3 -

[0014] radial flux, going through the core surfaces generates also eddy losses in the core sheets. This may also be reduced by the ring element. Thus, losses and hotspots generated by the radial flux at the end of the winding are reduced.

[0015] The ring element is used to reduce the radial eddy current losses in the winding and / or the exposed area of the core. In particular, the ring element can be an integral part of an equipotential ring which is arranged at the axial end of the winding. The equipotential ring is extended by a magnetic flux collector functionality.

[0016] Therefore, the insulating shell is provided, which surrounds the ferromagnetic sheets. The insulating shell acts as an equipotential ring or a part of an equipotential ring, and the ferromagnetic sheets act as a magnetic flux collector.

[0017] Equipotential rings are used to reduce the electrical stress at the axial ends of the winding. For example, they are made of laminated wood or transformer board and are covered with aluminum crepe paper to keep their voltage at a def ined voltage. The aluminum is extremely thin so that the losses in the equipotential ring, which is used as a magnetic shield ring, are neglectable. They are not magnetic, so they do not influence the magnetic field generated by the windings.

[0018] In order to avoid additional components and an increase in complexity and cost of the transformer design, the ring element is used as an equipotential ring and as a magnetic flux collector. For this purpose, the ferromagnetic sheets are arranged inside the insulating shell. The ferromagnetic sheets are arranged around the winding axis. For example, the ferromagnetic sheets are spirally wound around the winding P2025,0432 WO N / P250092WO01 October 15, 2025

[0019] - 4 -

[0020] axis. In this case, one or more ferromagnetic sheets are wound around the winding axis. For example, a single ferromagnetic sheet is spirally wound around the winding axis. Alternatively, the ferromagnetic sheets are coaxially arranged around the winding axis. The ferromagnetic sheets extend mainly along the winding axis. The extension of the ferromagnetic sheets along the winding axis and around the winding axis is larger than the extension of the ferromagnetic sheets along the radial direction. The ferromagnetic sheets are concentrically arranged and adjacent to each other when viewed along the winding axis. The ferromagnetic sheets are in direct or indirect contact to each other. For example, the ferromagnetic sheets are glued together. For example, the ferromagnetic sheets are glued together with resin. Alternatively, or additionally layers of electrically insulating material are placed between the ferromagnetic sheets, when viewed along the winding axis.

[0021] The ring element extends in a closed ring-like shape around the core. For example, the ring element comprises a radius around the center of the core with a value between 30 cm and 3 meters, depending on the transformer type of the transformer. The closed ring-like shaped ring element may be circular or elliptical.

[0022] Alternatively, the ring element for example is not formed as a closed ring but with a ring segment shape. The ring element comprises an opening. At the opening two ends of the ring element are arranged facing each other and opposite each other and are arranged at a distance from each other. The ring element is interrupted at the opening. For example, ring flows along the ring element are avoided. The ring segment shape may be circular or elliptical. P2025,0432 WO N / P250092WO01 October 15, 2025

[0023] According to an embodiment, the ferromagnetic sheets comprise a ferromagnetic metal or a ferromagnetic material.

[0024] For example, the ferromagnetic sheets comprise iron or another ferromagnetic metal. For example, the ferromagnetic sheets are made of a ferromagnetic material. For example, the ferromagnetic sheets comprise grain-oriented steel or amorphous steel. For example, the ferromagnetic sheets comprise electrical steel like the one used to produce transformer cores. Electrical steel is a special kind of steel used in the cores of electromagnetic devices such as motors, generators, and transformers because it reduces power loss. It is an iron alloy with silicon as the main additive element.

[0025] According to embodiments, the insulating shell comprises crepe paper, cellulose based insulation materials or resin. Cellulose based insulation materials are for example textiles or paper.

[0026] The insulating shell completely surrounds the ferromagnetic sheets. The insulating shell protects the ferromagnetic sheets and / or forms an electrical insulation. Alternatively, or additionally, the insulating shell is used to smooth the surface.

[0027] According to embodiments, the insulating shell is covered by an inner layer. The inner layer faces the ferromagnetic sheets. The inner layer is arranged between the insulating shell and the ferromagnetic sheets. The inner layer is a conductive or semi-conductive layer. The inner layer may be in electrical contact with the ferromagnetic sheets. This can P2025,0432 WO N / P250092WO01 October 15, 2025

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[0029] mean that the inner layer is directly or indirectly connected to the ferromagnetic sheets.

[0030] According to embodiments, the inner layer comprises a metallic layer or carbon layer.

[0031] The inner layer comprises a carbon layer and / or a metallic layer. For example, the carbon layer and / or the metallic layer is formed as a coating of the inner layer. For example, the carbon layer comprises carbonized paper. For example, the metallic layer comprises aluminum. For example, the metallic layer comprises aluminum crepe paper.

[0032] For example, the inner layer comprises several layers. For example, the inner layer comprises a paper layer, a carbon and / or metallic layer, and a further paper layer. For example, the carbon layer and / or the metallic layer is used to keep the voltage of the ring element at a defined voltage during operation. By electrically connecting the inner layer to the ferromagnetic sheets, the ferromagnetic sheets can be kept at said voltage as well.

[0033] According to embodiments, the ring element comprises an insulation element. The insulation element is arranged along the radial direction between the ferromagnetic sheets and the insulating shell.

[0034] The rounding radius of the insulating shell is shaped with the insulation element for proper electric stress shielding.

[0035] According to embodiments, the insulation element is an in-shape insulation element. The insulation element is form- P2025,0432 WO N / P250092WO01 October 15, 2025

[0036] - 7 -

[0037] fittingly filling the space between the ferromagnetic sheets and the insulating shell.

[0038] For example, the cross-section of the insulation element along the radial direction is of a rounded shape that can be easily produced. For example, the cross-section of the insulation element is semi-circular, circular, elliptical, semi-elliptical, semi-circular or has different radii on its top and bottom surface. The top surface of the insulation element is facing away from the respective axial end surface. The bottom surface of the insulation element is facing to the respective axial end surface.

[0039] According to embodiments, the insulation element comprises at least one of plastic, laminated plastic, wood, transformer board, laminated paper, rubber, resin, and ceramics.

[0040] According to embodiments, the insulation element encloses the ferromagnetic sheets at least partially. For example, the insulation element completely encloses the ferromagnetic sheets. That the insulation element completely encloses the ferromagnetic sheets can mean that the ferromagnetic sheets are surrounded by the insulation element in all directions. However, the ferromagnetic sheets may still be accessible through a recess, channel or slot, for example for an electrical connection to the inner layer of the insulating shell. It is also possible that the insulation element only covers portions of the ferromagnetic sheets.

[0041] According to embodiments, the insulation element comprises at least two pieces joined together. The two or more pieces can be assembled and / or glued together, wherein the ferromagnetic sheets are inserted into these assembled pieces. The pieces P2025,0432 WO N / P250092WO01 October 15, 2025

[0042] - 8 -

[0043] can be fabricated with numerical control machines to obtain the above-mentioned rounded shape. Thus, fabrication of the insulation element can be carried out efficiently. In addition, assembly of the insulation element and the ferromagnetic sheets can be carried out easily. However, it is also possible that the insulation element is integrally formed, i.e. formed by a single piece.

[0044] According to embodiments, the insulation element is molded or comprises one or more molded pieces. The insulation element can be molded around the ferromagnetic sheets. Alternatively, one or more pieces are molded and then assembled, wherein the ferromagnetic sheets are inserted into this assembled structure. In addition, after molding, the insulation element or the pieces, respectively, may be machined with numerical control machines to obtain the needed rounded shape mentioned above. Molding may be a cost-efficient way to fabricate the insulation element. In case of a molded insulation element, a material of the same may preferably be plastic or laminated plastic, for example.

[0045] According to embodiments, the insulation element comprises a recess or slot or channel, wherein an interconnection electrically connecting the ferromagnetic sheets to the inner layer of the insulating shell is provided through the recess, slot, or channel. The recess, slot, or channel may accommodate the electrical interconnection. For example, the electrical interconnection comprises a wire or electrically conductive sheet. By means of the recess, slot or channel the ferromagnetic sheets are accessible for electrical connection. P2025,0432 WO N / P250092WO01 October 15, 2025

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[0047] According to embodiments, the insulation element comprises a first rounding radius at a side facing the winding, and a second rounding radius at a side facing away from the winding. The side facing away from the winding may refer to as the top surface of the insulation element mentioned above. The top surface is facing away from the respective axial end surface of the winding. The side facing the winding may refer to as the bottom surface mentioned above. The bottom surface is opposite the top surface along the winding axis.

[0048] According to embodiments, the first rounding radius is different from, in particular smaller than, the second rounding radius. In an example, the first rounding radius is at least 0.5 mm and at most 10 mm. The second rounding radius may be at least 3 mm and at most 30 mm. An appropriate rounding of the insulation element aims for electric stress shielding. In particular, the rounded shape of the insulation element helps to smooth a voltage distribution outside the winding. Since the first rounding radius can be small, an edge of the winding can be substantially covered by the ring element, so that bending of the field lines of the electric field in this region can be reduced. Since the second rounding radius can be large, in particular larger than the first rounding radius, the field lines of a strong electric field can be deflected smoothly at the top surface of the ring element. In other words, the field lines of the electric field follow the shape of the ring element with the insulation element.

[0049] The ring element can also be positioned at the second axial end surface. The ring element is either positioned directly at the second axial end surface, or the ring element is P2025,0432 WO N / P250092WO01 October 15, 2025

[0050] positioned at a second distance from the second axial end surface.

[0051] According to embodiments, the transformer comprises a further ring element. The further ring element is arranged at the second axial end surface. The further ring element is positioned directly or at a second distance from the second axial end surface along the winding axis.

[0052] The first distance and the second distance have the same value. Alternatively, the first distance and the second distance have different values.

[0053] The further ring element is designed correspondingly to the ring element, for example. For example, the ring element is arranged at the first axial end surface of the winding and the further ring element is arranged at the second axial end surface of the winding or vice versa. At both axial end surfaces of the winding, a respective ring element is arranged to reduce eddy losses and / or collect the magnetic flux at both axial end surfaces.

[0054] According to embodiments, the transformer comprises a plurality of windings. The plurality of windings extends along the winding axis. Each winding is terminating in a respective first axial end surface from a plurality of first axial end surfaces and in a respective second axial end surface from a plurality of second axial end surfaces.

[0055] According to embodiments, the windings of the plurality of windings are coaxially wound around the core. P2025, 0432 WO N / P250092W001 October 15, 2025

[0056] The plurality of windings is arranged coaxially to the core. For example, the windings of the plurality of windings comprises a common center in a ready-to-operate condition. For example, the center corresponds to the winding axis.

[0057] The winding closest to the core is arranged directly adjacent to and at a distance to the core. Thus, the ring element is arranged at the winding which is arranged nearest to the core. For example, the ring element is arranged at the winding with the smallest diameter. Each subsequent winding is arranged at a further distance to the core than the winding closest to the core. Alternatively, or additionally two or more windings are arranged along the winding axis with the same distance to the core.

[0058] According to embodiments, the core comprises a plurality of legs extending along the winding axis. The windings of the plurality of windings are wound around the plurality of legs.

[0059] In particular, the legs of the core extend parallel to each other along the winding axis. The core has two or more legs. For each leg, one or more windings are wound around the leg. For example, the ring element and / or the further ring element is arranged at the respective winding wound around each leg. For example, each winding closest to its respective leg has a ring element and / or a further ring element on its first axial end surface and / or on its second axial end surface.

[0060] According to embodiments, the transformer comprises a plurality of ring elements. The plurality of ring elements is arranged at the first axial end surface. Additionally, or alternatively, the plurality of ring elements is arranged at the second axial end surface. The projection of each ring P2025, 0432 WO N / P250092W001 October 15, 2025

[0061] 12

[0062] element of the plurality of ring elements onto one respective winding of the plurality of windings, covers at least partially one respective first axial end surface from the plurality of first axial end surfaces. Additionally, or alternatively, the projection of each ring element of the plurality of ring elements onto one respective winding of the plurality of windings covers at least partially one respective second axial end surface from the plurality of second axial end surfaces.

[0063] The ring elements are arranged coaxially to the core. For example, the ring elements of the plurality of ring elements comprises a common center in a ready-to-operate condition. For example, the center corresponds to the winding axis.

[0064] For example, respective ring elements are arranged on each of the windings. Alternatively, respective ring elements are arranged only on a part of the windings. In particular, the respective ring element is arranged on the windings, which are arranged facing the core or its legs. For example, windings that are arranged facing away from the core or its legs do not have a ring element. For example, the ring elements are arranged on the windings with the smallest radii and not on windings with a larger radius which are arranged further away from the core or its legs.

[0065] It is possible that respective ring elements are arranged on both axial end surfaces of the windings around the core, or at just one single axial end surface. Alternatively, or additionally, it is possible that respective ring elements are arranged at both axial end surfaces of the windings around one or more legs of the core, or at just one single P2025, 0432 WO N / P250092W001 October 15, 2025

[0066] 13

[0067] axial end surface of the windings around one or more legs of the core.

[0068] Each ring element covers the first axial end surface or the second axial end surface of its respective winding. The respective ring element can cover the axial end surface completely or partially. Completely covering means in this context, that the ring element does not protrude beyond the boundaries of the axial end surface when viewed along the winding axis.

[0069] According to embodiments, the projection of at least one ring element of the plurality of ring elements onto the plurality of windings covers at least partially at least two first axial end surfaces of the plurality of first axial end surfaces. Additionally, or alternatively, the projection of at least one ring element of the plurality of ring elements onto the plurality of windings covers at least partially at least two second axial end surfaces of the plurality of second axial end surfaces.

[0070] At least one ring element of the plurality of ring elements covers two or more first axial end surfaces of adjacent windings of the plurality of windings. For example, the one ring element covers the axial end surface of the winding closest to the core and the axial end surface of the winding adjacent to the winding closest to the core.

[0071] The respective ring element can cover each of the two or more adjacent axial end surfaces completely or partially.

[0072] Completely covering means in this context, that the ring element covers the whole axial end surface of both windings P2025, 0432 WO N / P250092W001 October 15, 2025

[0073] - 14 -

[0074] and the space between both adjacent windings when viewed along the winding axis.

[0075] All of the features and advantages as described herein relating to the ring element are applicable to further ring elements and to a plurality of ring elements.

[0076] Hereinafter, the transformer will be explained in more detail with reference to the drawings on the basis of exemplary embodiments. The accompanying figures are included to provide a further understanding. In the figures, elements of the same structure and / or functionality may be referred by the same reference signs. It is to be understood that the embodiments shown in the figures are illustrative representations and are not necessarily drawn to scale. Insofar as elements or components correspond to one another in terms of their function in different figures, the description thereof is not necessarily repeated for each of the following figures.

[0077] Figure 1 to 5 show schematic cross-sections of a transformer according to embodiments.

[0078] Figure 6 shows a schematic cross-section of a ring element according to an embodiment.

[0079] Figure 7 shows a schematic cross-section of a ring element according to another embodiment.

[0080] Figure 8 shows a schematic cross-section of a ring element according to another embodiment. P2025, 0432 WO N / P250092W001 October 15, 2025

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[0082] Figure 1 schematically shows parts of a transformer 100. The transformer 100 is enclosed in a tank (not explicitly shown) which, for example, is filled with a dielectric fluid. The transformer 100 comprises a core 10. A winding 20 is wound around the core 10. In particular, the core 10 and the winding 20 are symmetrically wound around a winding axis A.

[0083] The winding 20 extends along the winding axis A and terminates in a first axial end surface 21 and in a second axial end surface 22. The first axial end surface 21 is arranged opposite the second axial end surface 22 along the winding axis A. The first axial end surface 21 and the second axial end surface 22 extend along a radial direction R. The radial direction is perpendicular to the winding axis A. The winding 20 is arranged at a distance from the core 10 along the radial direction R.

[0084] The transformer 100 comprises a ring element 30. The ring element 30 is arranged at the first axial end surface 21 at a first distance d1. Alternatively, the ring element 30 is in direct contact to the first axial end surface 21. The ring element 30 comprises ferromagnetic sheets 31. The ferromagnetic sheets 31 are aligned along the winding axis A. For example, the ferromagnetic sheets 31 are oriented parallel to the winding axis A. The ferromagnetic sheets 31 comprise, for example, iron, grain-oriented steel or amorphous steel.

[0085] The ring element 30 further comprises an insulating shell 32. The insulating shell 32 covers the ferromagnetic sheets 31. The insulating shell 32 comprises, for example, crepe paper. The insulating shell 32 is covered by a conductive or semi - P2025, 0432 WO N / P250092W001 October 15, 2025

[0086] 16

[0087] conductive layer 321 facing the ferromagnetic sheets 31. The conductive or semi conductive layer 321 comprises, for example, aluminum crepe paper or carbonized paper.

[0088] The ring element 30 further comprises an insulation element 33. The insulation element 33 is arranged along the radial direction R between the ferromagnetic sheets 31 and the insulating shell 32. The insulation element 33 is, for example, an in-shape insulation element 33, which form-fittingly fills the space between the ferromagnetic sheets 31 and the insulating shell 32. For example, the cross-section of the insulation element 33 along the radial direction R is of a rounded shape that can be easily produced. For example, the cross-section of the insulation element 33 is semi-circular. Alternatively, the cross-section of the insulation element 33 is circular, elliptical, or semi-elliptical. Such an in-shape insulation element 33 is easy to produce for mass production and saves further costs. The insulation element 33 comprises at least one of plastic, wood, rubber, resin, or ceramics.

[0089] The ring element 30 has a ring-like shape or a ring segment shape. The ring element 30 for example is not formed as a closed ring but with a ring segment shape.

[0090] The ring element 30 extends in a closed ring-like shape around the core 10. For example, the ring element 30 comprises a radius around the center of the core with a value between 30 cm and 3 meters, depending on the transformer type of the transformer 100. The closed ring-like shaped ring element 30 may be circular or elliptical. P2025, 0432 WO N / P250092W001 October 15, 2025

[0091] Alternatively, the ring element 30 for example is not formed as a closed ring but with a ring segment shape. For example, ring flows along the ring element 30 are avoided. The ring segment shape may be circular or elliptical.

[0092] Figure 2 to 5 show different configurations of the transformer 100.

[0093] Figure 2 shows the transformer 100, which is basically the same as in figure 1 with the difference, that the ring element 30 is arranged on the first axial end surface 21 and a further ring element 300 is arranged on the second axial end surface 22. The further ring element 300 is arranged at the second axial end surface 22 at a second distance d2. Alternatively, the further ring element 300 is in direct contact to the second axial end surface 22. The first distance d1 and the second distance d2 may be the same or different in their values. At both axial end surfaces 21, 22 of the winding 20, a respective ring element 30, 300 is arranged to reduce eddy losses and / or collect the magnetic flux at both axial end surfaces 21, 22.

[0094] Figure 3 shows the transformer 100, which is basically the same as in figure 2 with the difference, that a plurality of windings 20n is wound around the core 10. The plurality of windings 20 extends along the winding axis A. The plurality of windings 20n is arranged coaxially to the core 10. The windings 20 of the plurality of windings 20n comprise a common center in a ready- to - operate condition. For example, the center corresponds to the winding axis A. The winding 20 closest to the core 10 is arranged directly adjacent to and at a distance to the core 10. Each winding 20 is terminating in a respective first axial end surface 21 from a plurality P2025, 0432 WO N / P250092W001 October 15, 2025

[0095] 18

[0096] of first axial end surfaces 21n and in a respective second axial end surface 22 from a plurality of second axial end surfaces 22n. In figure 3 two windings 20 are shown. The transformer 100 is not limited to two windings 20 and may have more than two windings 20 wound around the core 10.

[0097] Furthermore, the transformer 100 comprises a plurality of ring elements 30n. The ring elements 30 are arranged coaxially to the core 10. For example, the ring elements 30 of the plurality of ring elements 30n comprises a common center in a ready- to - operate condition. For example, the center corresponds to the winding axis A.

[0098] In figure 3 four ring elements 30 are shown. Each winding 20 has one ring element 30 on its respective first axial end surface 21 and one ring element 30 on its respective second axial end surface 22. The transformer 100 is not limited to four ring elements 30 and may have more than four or less than four ring elements 30.

[0099] Each of the plurality of ring elements 30n may be arranged at the respective winding 20 of the plurality of windings 20n. The plurality of ring elements 30n may be arranged at the respective first axial end surface 21 of the plurality of first axial end surfaces 21n or at the respective second axial end surface 22 of the plurality of first axial end surfaces 22n.

[0100] In figure 3 respective ring elements 30 are arranged on each of the windings 20. Alternatively, respective ring elements 30 are arranged only on a part of the windings 20. For example, the respective ring element 30 is arranged on the windings 20, which are arranged facing the core 10. For P2025, 0432 WO N / P250092W001 October 15, 2025

[0101] 19

[0102] example, windings 20 that are arranged facing away from the core 10 do not have a ring element 30. For example, the ring elements 30 are arranged on the windings 20 with the smallest radii and not on windings 20 with a larger radius which are arranged further away from the core 10.

[0103] It is possible that respective ring elements 30 are arranged on both axial end surfaces 21, 22 of the windings 20 around the core 10, or at just one single axial end surface.

[0104] Each ring element 30 covers the first axial end surface 21 or the second axial end surface 22 of its respective winding 20. The respective ring element 30 covers the respective axial end surface 21, 22 completely. Alternatively, the respective ring element 30 covers the respective axial end surface 21, 22 partially. Completely covering means in this context, that the ring element 30 does not protrude beyond the boundaries of the axial end surface 21, 22 when viewed along the winding axis A.

[0105] Figure 4 shows the transformer 100 in a different configuration. The transformer 100 is basically the same as shown in figure 3 with the difference, that one ring element 30 covers the first axial end surfaces 21 of two windings 20. The further ring element 300 covers the second axial end surfaces 22 of the two windings 20. The transformer 100 is not limited to two windings 20 and may have more than two windings 20 wound around the core 10. In particular one ring element 30 is not limited to cover the axial end surfaces 21, 22 of two windings 20 and may cover the axial end surfaces 21, 22 of three or more windings 20 from the plurality of windings 20n. In particular one further ring element 300 is also not limited to cover the axial end surfaces 21, 22 of P2025, 0432 WO N / P250092W001 October 15, 2025

[0106] 20

[0107] two windings 20 and may cover three or more windings 20 from the plurality of windings 20n. Furthermore, a combination of ring elements 30 which cover exactly one axial end surface 21, 22 of the respective winding 20 and ring elements 30 which cover simultaneously the axial end surfaces 21, 22 of two or more windings 20 is possible.

[0108] Figure 5 shows the transformer 100 with the core 10 comprising a plurality of legs 10n extending along the winding axis A. The legs 10n of the core 10 extend parallel to each other along the winding axis A.

[0109] In figure 5 the core 10 comprises two legs 10n. The core 10 is not limited to two legs 10n and may have more than two legs 10n. One winding 20 of the plurality of windings 20n is wound around one respective leg 10n. The transformer 100 is not limited to one winding 20 per leg 10n and may have more than one winding 20 wound around the respective leg 10n.

[0110] Each ring element 30 of the plurality of ring elements 30n is arranged at the respective winding 20 wound around each leg 10n. Alternatively only some windings 20 are covered by the ring elements 30. For example, each winding 20 closest to its respective leg 10n has a ring element 30 on its first axial end surface 21 and / or on its second axial end surface 22.

[0111] For example, windings 20 that are arranged facing away from the core 10 or its legs 10n do not have a ring element 30. For example, the ring elements 30 are arranged on the windings 20 with the smallest radii and are not arranged on windings 20 with a larger radius which are arranged further away from the core 10 or its legs 10n. P2025, 0432 WO N / P250092W001 October 15, 2025

[0112] 21

[0113] The number of ring elements 30 is purely exemplary and may deviate from the quantity shown in figures 1 to 5. Respective ring elements 30 can be arranged at each opposite axial end surface 21, 22 or only at one axial end surface and the other axial end surface is free of the ring element 30.

[0114] Furthermore, the number of windings 20 around the core 10 or around each of the plurality of legs lOn is not limited to one or two windings 20 and may deviate from the quantity shown in figure 1 to 5.

[0115] Figure 6 shows the ring element 30 in a schematic cross-section. The depicted ring element 30 may be used in the transformer 100 as discussed in figures 1 to 5. The ring element 30 comprises ferromagnetic sheets 31. The ferromagnetic sheets 31 are aligned along the winding axis A. The ferromagnetic sheets 31 comprise ferromagnetic metal or material. For example, the ferromagnetic sheets 31 comprise iron or another ferromagnetic metal. For example, the ferromagnetic sheets 31 are made of a ferromagnetic material. For example, the ferromagnetic sheets 31 comprise grain-oriented steel or amorphous steel. For example, the ferromagnetic sheets 31 comprise electrical steel like the one used to produce transformer cores.

[0116] The ring element 30 further comprises the insulating shell 32. The insulating shell 32 covers the ferromagnetic sheets 31. For example, the insulating shell 32 comprises crepe paper, cellulose based insulation materials or resin.

[0117] Cellulose based insulation materials are for example textiles or paper. The insulating shell 32 completely surrounds the ferromagnetic sheets 31. The insulating shell 32 protects the ferromagnetic sheets 31 and / or forms an electrical P2025, 0432 WO N / P250092W001 October 15, 2025

[0118] - 22 -

[0119] insulation. Alternatively, or additionally, the insulating shell 32 is used to smooth the surface.

[0120] The insulating shell 32 is covered by an inner layer 321. The inner layer 321 faces the ferromagnetic sheets 31. The inner layer 321 is arranged between the insulating shell 32 and the ferromagnetic sheets 31. The inner layer 321 is a conductive or semi-conductive layer. For example, the inner layer 321 comprises an aluminum foil or carbonized paper. For example, the inner layer 321 comprises several layers. For example, the inner layer 321 comprises a paper layer, a carbon and / or metallic layer, and a further paper layer. For example, the carbon layer and / or the metallic layer is used to keep the voltage of the ring element at a defined voltage during operation.

[0121] The ring element 30 further comprises an insulation element 33. The insulation element 33 is arranged along the radial direction R between the ferromagnetic sheets 31 and the insulating shell 32. The rounding radius of the insulating shell 32 is shaped with the insulation element 33 for proper electric stress shielding. For example, the insulation element 33 is an in-shape insulation element 33. The insulation element 33 is form-fittingly filling the space between the ferromagnetic sheets 31 and the insulating shell 32.

[0122] For example, the cross-section of the insulation element 33 when cut along the radial direction R is of a rounded shape that can be easily produced. For example, the cross-section of the insulation element 33 is semi - circular. Alternatively, the cross-section of the insulation element 33 is circular, elliptical, semi - elliptical or has different rounding radii P2025, 0432 WO N / P250092W001 October 15, 2025

[0123] at a top surface 331 and a bottom surface 332. The top surface 331 of the insulation element 33 is facing away from the respective axial end surface 20 of the winding 20. The bottom surface 332 is opposite the top surface 331 along the winding axis A. The bottom surface 332 of the insulation element 33 is facing to the respective axial end surface.

[0124] Such an in-shape insulation element 33 is easy to produce for mass production and saves further costs. For example, the insulation element 33 comprises at least one of plastic, laminated plastic, wood, transformer board, laminated paper, rubber, resin, and ceramics.

[0125] With Figure 7 a ring element 30 according to another embodiment is shown in a schematic cross-section. In this embodiment, the insulation element 33 encloses the ferromagnetic sheets 31. The insulation element 33 comprises two pieces 33a, 33b joined together. The ferromagnetic sheets 31 are inserted into the two pieces. The two pieces may be molded pieces. It is however also possible that the pieces are fabricated by a numerical control machine. At least the rounded shape of the insulation element 33 or its pieces, respectively, may be realized by a numerical control machine.

[0126] The insulation element 33 of Figure 7 further comprises a slot or channel 333 through which the ferromagnetic sheets can be electrically connected to the inner layer 321. For example, an electrical connection can be established by a wire or electrically conductive sheet arranged in said slot or channel. The insulation element 33 may also comprise a recess, such that the insulation element 33 does only partially enclose the ferromagnetic sheets, which are exposed through the recess. P2025, 0432 WO N / P250092W001 October 15, 2025

[0127] - 24 -

[0128] The edges of the insulation element 33, and therefore also of the ring element 30, are rounded, wherein in the example of Figure 7 a rounding radius at the top surface 331 is equal as a rounding radius at the bottom surface 332.

[0129] With Figure 8 a ring element 30 according to another embodiment is shown in a schematic cross-section. The ring element 30 according to Figure 8 is different from the ring element 30 according to Figure 7 in that the insulation element 33 comprises a first rounding radius rl at a side facing the winding 20, and a second rounding radius r2 at a side facing away from the winding 20, wherein the first rounding radius is different from, in particular smaller than, the second rounding radius. As described above, the side facing the winding 20 may be referred to as bottom surface 332, while the side facing away from the winding 20 may referred to as top surface 331. In other words, the bottom surface 332 is flatter than the top surface 331. In even other words, a portion of the bottom surface 332 that extends parallel to the radial direction R is larger than a corresponding portion of the top surface 331. In that way, the edges of the winding 20 are substantially covered by the bottom surface 332. The differently rounded edges prevent the electric field lines from becoming strongly curved at the bottom surface 332 and at the top surface 331, which in turn reduces the electrical stress at the axial ends of the winding 20.

[0130] The ring element 30 is simple to implement and achieves lower losses and / or lower forces in the transformer 100. The material requirements for the ring element 30 are reduced compared to other solutions. Efficiency and reliability of P2025, 0432 WO N / P250092W001 October 15, 2025

[0131] 25

[0132] the transformer 100 can be increased without any need for a substantive design change of the transformer 100.

[0133] This patent application claims the priority of European patent application 24215773.3, the disclosure content of which is hereby incorporated by reference. P2025, 0432 WO N / P250092W001 October 15, 2025

[0134] 26

[0135] Reference Signs

[0136] 100 transformer

[0137] 10 core

[0138] 20 winding

[0139] 20n plurality of windings

[0140] 21 first axial end surface

[0141] 22 second axial end surface

[0142] 21n plurality of first axial end surfaces 22n plurality of second axial end surfaces 30 ring element

[0143] 300 further ring element

[0144] 30n plurality of ring elements

[0145] 31 ferromagnetic sheets

[0146] 32 insulating shell

[0147] 321 inner layer

[0148] 33 insulation element

[0149] 331 top surface

[0150] 332 bottom surface

[0151] 333 slot / channel / recess

[0152] dl first distance

[0153] d2 second distance

[0154] A winding axis

[0155] R radial direction

[0156] rl first rounding radius

[0157] r2 second rounding radius

Claims

P2025, 0432 WO N / P250092W001 October 15, 2025Claims1. Transformer (100) comprisinga core (10) extending along a winding axis (A),a winding (20) wound around the core (10) and extending along the winding axis (A), wherein- the winding (20) terminates in a first axial end surface (21) and in a second axial end surface (22), wherein - the first axial end surface (21) is arranged opposite the second axial end surface (22) along the winding axis (A), and wherein- the first axial end surface (21) and the second axial end surface (22) extend along a radial direction (R), which is perpendicular to the winding axis (A),a ring element (30) arranged at the first axial end surface (21), comprising:- ferromagnetic sheets (31), aligned along the winding axis (A) and- an insulating shell (32) covering the ferromagnetic sheets (31)characterized in that the ring element (30) and the associated insulating shell (32) are positioned at a first distance (dl) from the first axial end surface (21) along the winding axis (A).

2. Transformer (100) according to claim 1, wherein the ferromagnetic sheets (31) comprise a ferromagnetic metal or a ferromagnetic material.

3. Transformer (100) according to claim 1 or 2, wherein the insulating shell (32) comprises crepe paper, cellulose based insulation materials or resin.P2025, 0432 WO N / P250092W001 October 15, 2025284. Transformer (100) according to any of the preceding claims, wherein the insulating shell (32) is covered by an inner layer (321) facing the ferromagnetic sheets ( 31), wherein the inner layer (321) is a conductive or semi-conductive layer.

5. Transformer (100) according to claim 4, wherein the inner layer (321) comprises a metallic layer or a carbon layer.

6. Transformer (100) according to any of the preceding claims, wherein the ring element (30) comprises an insulation element (33 ) arranged along the radial direction (R) between the ferromagnetic sheets (31) and the insulating shell (32).

7. Transformer (100) according to claim 6, wherein the insulation element (33 ) is an in-shape insulation element (33) form-fittingly filling the space between the ferromagnetic sheets (31) and the insulating shell (32).

8. Transformer (100) according to claim 6 or 7, wherein the insulation element (33 ) comprises at least one of plastic, laminated plastic, wood, transformer board, laminated paper, rubber, resin and ceramics.

9. Transformer (100) according to one of claims 6 to 8, wherein the insulation element (33 ) encloses the ferromagnetic sheets (31) at least partially.

10. Transformer (100) according to one of claims 6 to 9, wherein the insulation element (33 ) comprises at least two pieces (33a, 33b) joined together.P2025, 0432 WO N / P250092W001 October 15, 2025- 29 -11. Transformer (100) according to one of claims 6 to 10, wherein the insulation element (33 ) is molded or comprises one or more molded pieces.

12. Transformer (100) according to one of claims 6 to 11, wherein the insulation element (33 ) comprises a recess or slot or channel (333 ), wherein an interconnection electrically connecting the ferromagnetic sheets (31) to the inner layer (321) of the insulating shell (32) is provided through the recess, slot, or channel (333 ).

13. Transformer (100) according to one of claims 6 to 12, wherein the insulation element (33 ) comprises a first rounding radius (rl) at a side facing the winding (20), and a second rounding radius (r2) at a side facing away from the winding (20).

14. Transformer (100) according to claim 13, wherein the first rounding radius (rl) is different from, in particular smaller than, the second rounding radius (r2).

15. Transformer (100) according to any of the preceding claims, comprising a further ring element (300), arranged at the second axial end surface (22), wherein the further ring element (300) is positioned directly or at a second distance (d2) from the second axial end surface (22) along the winding axis (A).

16. Transformer (100) according to any of the preceding claims, comprising a plurality of windings (20n) extending along the winding axis (A), wherein each winding (20) is terminating in a respective first axial end surface (21) from a plurality of first axial end surfaces (21n) and in aP2025, 0432 WO N / P250092W001 October 15, 2025respective second axial end surface (22) from a plurality of second axial end surfaces (22n).

17. Transformer (100) according to claim 16, wherein the windings (20) of the plurality of windings (20n) are coaxially wound around the core (10).

18. Transformer (100) according to claim 16 or 17, wherein the core (10) comprises a plurality of legs (lOn) extending along the winding axis (A), wherein the windings (20) of the plurality of windings (20n) are wound around the plurality of legs (lOn).

19. Transformer (100) according to one of claims 16 to 18, comprising a plurality of ring elements (30n), arranged at the first axial end surface (21) and / or at the second axial end surface (22), wherein the projection of each ring element (30) of the plurality of ring elements (30n), onto one respective winding (20) of the plurality of windings (20n) covers at least partially one respective first axial end surface (21) from the plurality of first axial end surfaces (21n) or covers at least partially one respective second axial end surface (22) from the plurality of second axial end surfaces (22n).

20. Transformer (100) according to claim 19, wherein the projection of each ring element (30) of the plurality of ring elements (30n), onto the plurality of windings (20n) covers at least partially at least two first axial end surfaces (21) of the plurality of first axial end surfaces (21n) or covers at least partially at least two second axial end surfaces (22) of the plurality of second axial end surfaces (22n).

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