The present invention generally relates to transformer cores. The present invention further relates to a transformer comprising a transformer core and a method of manufacturing a transformer core.

Background

Electrical transformers are used in many applications, e.g. at various stages of power distribution systems for distributing electrical power. A transformer generally comprises one or more coils (also referred to as winding) of electrically conducting wire. The transformer further comprises a circuit of material, generally referred to as transformer core. The transformer core comprises a number of limbs or legs that are surrounded by one or more of the coils. The parts of the transformer core connecting respective legs are also referred to as yokes. Multi-phase transformers and, in particular, three-phase transformers are frequently used, and a variety of different transformer types exist.

WO 2000/14753 discloses a transformer core which comprises at least one leg and at least one yoke part, wherein the cross section of the leg or the legs is regularly multi-edged with more than four edges. The core is made up of rings rolled from strips of constant width, whereby good electrical properties are achieved. The transformer is also easy to manufacture and avoids waste of material.

WO 2010/027290 discloses another example of a wound delta magnetic core for a three phase transformer. US 2,458,052 discloses a magnetic core for a three-phase transformer with a triangular geometry that is made of steel ribbon. The core further includes magnetic filler or bridge filler means for magnetically interconnecting the yoke portions of the several cores at each end of the three-phase transformer.

However, it remains desirable to provide a more efficient transformer core. In particular, it is generally desirable to provide a transformer core that is relatively easy to manufacture, involves relatively low material costs and involves small magnetic losses.

Summary

According to a first aspect, disclosed herein are embodiments of a

transformer core. The transformer core comprises at least a first leg, a second leg and a third leg, each leg being configured to be surrounded by one or more transformer coils of electrically conducting wire. Each leg has two ends opposite each other. The transformer core further comprises a first yoke extending between a first end of the first leg and a first end of the second leg, and a second yoke extending between the first end of the first leg and a first end of the third leg.

Embodiments of the transformer core comprise a first flux bypass component extending outside of said one or more coils and configured to provide a magnetic flux path through the first flux bypass component between the first yoke and the second yoke, and wherein the first flux bypass component has a first end extending into the first yoke and a second end extending into the second yoke.

Accordingly, the transformer core comprises at least three legs, each leg configured to be surrounded by one or more transformer coils, each leg comprising a first end and a second end, opposite the first end. Embodiments of the transformer core comprise a plurality of yokes each yoke extending between end portions of respective ones of the legs. The transformer core may further comprise a plurality of flux bypass components, wherein each flux bypass component extends outside of said one or more coils between two yokes which extend from a same end of one of the legs and provides a magnetic flux path through said flux bypass component between said two yokes, and wherein said flux bypass component has a first end extending into one of said two yokes and a second end extending into another of said two yokes. In particular, the yokes extending from the same end of one of the legs may provide separate magnetic flux paths from said one leg to respective ones of the other legs where the separate flux paths do not share any part of the yokes. In particular the first and second yokes may be separate from each other along their entire length. The flux bypass components allow a portion of the magnetic flux to flow directly from one yoke to another yoke without passing through any of the legs, i.e. to bypass the legs of the transformer, which allows for greater freedom for the free interchange or transfer of flux between the various legs of the core which reduces the building factor and increases the efficiency of the transformer, e.g. by reducing the core material while maintaining the no load loss level or by reducing the no load losses for the same amount of core material. Moreover, as the flux bypass component extends into the yoke, a more efficient flux exchange between the yokes and the bypass components is achieved, thus reducing energy losses of the resulting transformer. Accordingly, the flux bypass components, and the two yokes which the flux bypass component connects, together form an additional flux path between two legs, in addition to, and separate from, a flux path through the third yoke that connects said two legs with each other. In particular, embodiments of the transformer core may comprise a third yoke extending between the first end of the second leg and a first end of the third leg and providing a flux path between the second leg and the third leg; and wherein a portion of the first yoke, the first flux bypass component and a portion of the second yoke together provide a further flux path between the second leg and the third leg which flux path does not extend through the first leg and is separate from the flux path provided by the third yoke. Each flux bypass component may extend between two or more yokes, i.e. may provide a flux path between two or more yokes.

Embodiments of the transformer core may be configured for a multi-phase transformer, such as a three-phase transformer. The legs of the transformer core may be straights, elongated members. They may be arranged with their longitudinal axes parallel with each other thus defining a first end of the transformer core (corresponding to the respective first ends of the legs) and a second end of the transformer

core(corresponding to the respective second ends of the legs). The

transformer core has a set of yokes at its first end and another set of yokes at its second end. The yokes of the first set connect the first ends of respective pairs of legs and the yokes of the second set connect second ends, opposite the first ends, of respective legs with each other. In some embodiment, each pair of yokes of the first set is interconnected by a respective flux bypass component and each pair of yokes of the second set are interconnected by a respective flux bypass component.

Some embodiments of the transformer core may be made as layered structures where the core comprises thin layers or laminations of a suitable material such as electrical steel. In particular, some embodiments of transformer cores are made from transformer plates that are stacked so as to provide the transformer core; these cores are also referred to as stacked cores. In other embodiments of a transformer core, the layers form ring- shaped components where each layer is bent and extends in a ring-shaped form. These transformer cores are also referred to as wound cores, as they may be made from strips or ribbon of sheet material that are wound into generally ring-shaped components, e.g. generally rectangular rings, so as to form the components of the transformer core. In some embodiments, the rings may be cut during manufacture of the transformer. In other

embodiments, the complete rings may be assembled to form the transformer core. It will be appreciated, however, that wound cores may also be manufactured in other ways, e.g. by forming segments of stacked strips that are bent into shape and then assembled to form ring shaped components of a transformer core, e.g. as disclosed in US 6,668,444. Transformer cores and, in particular, wound cores, may be manufactured in various geometric arrangements, such as delta cores and planar cores. In both types of transformer cores, the legs extend substantially parallel to each other but laterally displaced from each other. The ends of the legs that extend out of the coils are connected with each other by respective yokes. In a planar core the legs are substantially arranged along a straight line while the legs of a delta core are arranged in a triangular arrangement, normally forming an equilateral triangle. Generally, transformers with delta cores have the advantage that the coils are all positioned at equal distances from each other which results in improved electrical properties. Moreover, a delta core may be manufactured using less steel than a comparable planar transformer core.

A wound transformer core may comprise multiple core rings where each core ring connects two of the legs of the transformer core. Each core ring may comprise a layered structure where the layers extend along the

circumferential direction of the core ring. In particular, in some embodiments, the transformer core comprises a plurality of core rings each core ring defining two leg portions and two yoke portions. The core rings may have a generally rectangular shape, e.g. with rounded corners and resemble a frame; the leg portions of the ring may be substantially straight while the yoke portions of the ring may be straight or curved. The leg portions may correspond to the long sides of the rectangle while the yoke portions correspond to the short sides of the rectangle. Each leg portion forms part of a respective one of said two legs connected by said core ring; and each yoke portion forms at least a part of a yoke extending between the legs connected by said core ring. A first leg portion of a first core ring of the plurality of core rings thus forms part of the first leg, a second leg portion of the first core ring forms part of the second leg, and one of the yoke portions of the first core ring forms at least a part of the first yoke; a first leg portion of a second core ring of the plurality of core rings forms part of the first leg, a second leg portion of the second core ring forms part of the third leg and at one of the yoke portions of the second core ring forms at least a part of the second yoke. Accordingly, each leg of the transformer core may comprise leg portions of different core rings. In these embodiments, the provision of flux bypass components may be particular useful, as they allow the provision of flux paths between respective parts of one leg and another leg.

The core rings may be formed from strips of sheet material or otherwise as a layered structure. In particular, in some embodiments, each of the plurality of core rings comprises a stack of a plurality of core ring layers, each core ring layer extending along the circumferential direction of the core ring. Suitable material for the core rings include electrical steel such as grain-oriented steel, or amorphous metal.

Similarly, the flux bypass components may be formed from plates of sheet material or otherwise as a layered structure, e.g. of electrical steel such as grain-oriented steel, non-grain-oriented steel, or amorphous metal. In particular, in some embodiments, the first flux bypass component comprises a plurality of bypass layers. The flux bypass components may be made from the same material as the legs and the yokes or from a different material than the legs and the yokes. Flux bypass components made from non-grain oriented steel may allow a larger degree of freedom in respect of the geometry of the flux bypass components while maintaining an efficient flux path. On the other hand, flux bypass components made from grain-oriented steel may reduce core losses even more.

At the ends of the flux bypass component that extend into the yokes, the bypass layers and the core ring layers may form an interleaved stacked structure where end portions of the bypass layers are sandwiched between respective ones of the core ring layers. In some embodiments, each layer of the bypass structure is sandwiched between two layers of the core ring such that the both surfaces of the layer of the flux bypass component are in contact with surfaces of the layers of the yoke. In other embodiments, other interleaving patterns may be used, e.g. a single layer of the flux bypass component or two or more adjacent layers of the flux bypass component may be sandwiched between one, two or more layers of the core ring. In both cases each of the end portions of the flux bypassed layers are sandwiched between two core ring layers, either alone or with other layers. In such an interleaved structure a particularly efficient flux transfer between the bypass layers and the core ring layers is achieved. It will be appreciated that, in some embodiments, the end portions may extend all the way through the yoke, such that a part of the end portion protrudes out of the yoke or at least extends all the way to an opposite side of the yoke.

The interleaved structure may be formed such that one or more first core ring layers of a yoke portion of the first core ring that forms part of the first yoke is sandwiched between an end portion of a bypass layer of the first flux bypass component and an end portion of a bypass layer of a second flux bypass component, the second flux bypass component extending between the first yoke and the third yoke. Hence, between some pairs or each pair of core ring layers, only flux bypass layers towards a single yoke is inserted at any position along the yoke; the flux bypass layers between respective pairs of core layers may thus alternatingly belong to flux bypass components that extend to respective other yokes. Such an alternating structure allows efficient flux transfer between different pairs of core rings through respective layers of the layered core ring structures. In particular, an incoming flux through a layer of one flux bypass component is fed into the yoke portion at a different level of the layered yoke structure than the incoming flux that enters the yoke into a different direction from another flux bypass component, i.e. the incoming flux from different flux bypass components that are oppositely directed inside the yoke, enter the yoke at different levels of the yoke. In other words, the crossing flux paths enter at different levels of the layered yoke structure. Moreover, such an alternating structure ensures that the flux bypass layers have a large contact area with the core ring layers, and it even allows for relatively wide flux bypass layers that have contact with layers of the core ring across the entire width of the flux bypass layers.

In some embodiments, e.g. when the first yoke is made from grain-oriented steel, the first yoke defines a principle flux direction of the first yoke. The size of the angle at which the first flux bypass component extends into the first yoke relative to the principle flux direction has less influence on the efficient flux exchange between the first yoke and the first flux bypass component when the first flux bypass component is made from non-grain-oriented steel as compared to a first flux bypass components made from grain-oriented steel. However, in some embodiments, e.g. when the first yoke is made from grain-oriented steel, the first end of the first flux bypass component defines a principle flux extraction direction, wherein the principle flux extraction direction defines an angle relative to the principle flux direction of the yoke.

When the flux bypass components are made from non-grain-oriented steel, relative sharp turns of the flux direction at the transition between the yoke and the flux bypass components are often acceptable, even turns larger than 90°. While angles up to 90° between the principle flux direction and the principle flux extraction direction may be acceptable even for flux bypass components made from grain-oriented steel, smaller angles, such as angles smaller than 50°, such as smaller than 45°, such as smaller than 40°, such as 30° or smaller, may be preferred, in particular when the flux bypass components are made from grain-oriented steel. Angles smaller than 45° and, preferably smaller than 40°, such as 30° or smaller, facilitate the flux transfer between the yoke and the flux bypass component, e.g. when the flux bypass component is made from grain-oriented steel. In particular, the angle may be between 5° and 45°, such as between 10° and 40°, such as between 15° and 35°.

The principle flux direction and the principle flux extraction direction may be defined by the material properties of the material of the yoke and of the flux bypass component, respectively. In particular, the principle flux direction and the principle flux extraction direction may be defined as the directions of maximum magnetic flux density of the respective materials. The principle flux extraction direction may be defined as the principle flux direction of the material of the flux bypass component at the first end of the first flux bypass component.

The transformer core, or at least parts of the transformer core, may be made from any material suitable for transformer cores such as iron alloys, e.g. steel, or other suitable metals. In some embodiments the transformer core is made from electrical steel or other iron alloys having desirable magnetic properties rendering the material suitable as a material for transformer cores. Suitable examples for use at least for parts of the transformer core include grain-oriented steel, non-grain-oriented steel, and amorphous metal.

Generally, the flux bypass components may be made from the same material as other parts (e.g. the legs and/or the yokes) of the transformer core or, partly or in whole, from a different material.

In particular, in some embodiments, the transformer core, including the flux bypass components, is made from grain-oriented steel. In other

embodiments, the parts of the transformer core other than the flux bypass components (i.e. the legs and yokes) are made from grain-oriented steel while the flux bypass components are made from non-grain-oriented steel. In embodiments where the yoke and/or flux bypass component is/are made from grain-oriented metal such as grain-oriented steel, the principle flux direction and/or principle flux extraction direction is/are determined by the dominant crystal orientation and the orientation of magnetic domain walls of the grain-oriented metal such as steel. Grain-oriented metal such as grain- oriented steel is normally available as rolled, in particular cold-rolled, strips or sheets. Typical thicknesses of the rolled strips or sheets may be less than 2 mm. These strips may be cut to shape to make laminations which are stacked together or they may be wound into rings to form the

laminated/wound transformer core or they may be assembled to a wound transformer core in another suitable manner. In such embodiments, the principle flux direction and/or principle flux extraction direction normally coincides with the rolling direction. When the flux bypass component extends into the yoke, the yoke material is in contact with an overlap portion of a surface of the flux bypass component, e.g. the upper surface and/or the lower surface of the end portions of a layer of the flux bypass component. In particular, the interleaving pattern may comprise an alternating sequence of one or more layers of a flux bypass component followed by one or more layers of the yoke. In some

embodiments, the flux bypass component is shaped and sized such that the extent of the overlap portion along the principal flux direction of the yoke, at least at one portion of the overlap region, is long enough to allow an efficient flux exchange through the interface surfaces of the flux bypass component and the yoke, respectively, such as at least 0.5 cm, such as at least 1 cm, such as at least 2 cm, such as at least 3 cm, such as at least 4 cm, thereby facilitating the magnetic flux to travel across the surface interface between the yoke material and the flux bypass component. In particular, when the flux bypass component comprises elongated plates of sheet metal being inserted along their longitudinal direction, the metal plates may have a width (at least at the end that is inserted into the yoke) large enough to cause the extent of the overlap portion along the principal flux direction of the yoke to be at least 0.5 cm, such as at least 1 cm, such as at least 2 cm, such as at least 3 cm, such as at least 4 cm. The yoke has a width, measured in the direction across the principle flux direction of the yoke. When the yoke is a layered structure, the width is defined as a width of one of the layers and, in particular, of the layer that is in contact with the flux bypass component. In some embodiments, the flux bypass component is shaped and sized, and is inserted far enough into the yoke, such that the extent of the overlap portion along the principal flux direction of the yoke and across the entire width of the first yoke is at least 0.5 cm, such as at least 1 cm, such as at least 2 cm, such as at least 3 cm, such as at least 4 cm, thereby facilitating the magnetic flux to travel across the surface interface between the yoke material and the flux bypass component over a large flux transfer area.

As mentioned above, the flux bypass components may be formed from plates, such as strips, of sheet material or otherwise as a layered structure. Accordingly, in some embodiments, the first flux bypass component comprises a plurality of plates arranged as a single stack of plates extending into the first yoke and into the second yoke. In particular, a first end of each plate of the stack extends into the first yoke and a second, opposite, end of said plate extends into the second yoke. Accordingly, in some embodiments, each flux bypass component is formed as a single stack of plates of electrical steel, where opposite ends of each plate extend into the respective yokes such that the opposite end portions of each plate of the flux bypass component are sandwiched between adjacent layers of the respective yokes. It has been found that, though simple in construction and including relatively few components, this arrangement provides a surprisingly efficient

transformer core. The flux bypass components of this embodiment may be made of grain-oriented steel or of non-grain-oriented steel. For example, the flux bypass components may be made of non-grain-oriented steel while the remaining parts of the transformer core may be made from grain-oriented steel. In alternative embodiments, the first flux bypass component comprises a plurality of plates including a first stack of plates extending into the first yoke and a second stack of plates extending into the second yoke. The provision of the flux bypass component as multiple, separate stacks of plates that are interleaved with each other at their respective adjacent ends facilitates manufacturing of the core. Respective stacks of plates may be inserted into respective yokes prior to, or during, assembling the stacks of plates into a combined flux bypass component. Adjacent stacks are placed next to each other but partially overlapping. Their layers are generally parallel with each other and arranged in extension of each other within the plane defined by them, except for the overlap region where they are stacked into a combined stack.

In some embodiments, the first and second stacks of plates are connected with each other so as to form an overlap region where the plates of the first and second stacks form a combined stack where the plates of the first and second stacks are interleaved with each other so as to form an alternating sequence of plates of the first and second stacks in the overlap region.

Accordingly the stacks of plates may first be inserted into respective yokes and then interleaved with each other when the transformer parts including the respective yokes are, or have been, assembled with each other. In

alternative embodiments, the first flux bypass component comprises one or more intermediate stacks. At least a first intermediate stack may be arranged to be connected to the first stack of plates so as to form a first overlap region. In the overlap region, the plates of the first stack and of the first intermediate stack form a combined stack where the plates of the first stack and of the first intermediate stack are interleaved with each other. A second end of a second intermediate stack of plates is connected to the second stack of plates so as to form a second overlap region where the plates of the second stack and of the second intermediate stack form a combined stack. In the overlap region, the plates of the second stack and of the second intermediate stack are interleaved with each other. The first and second stacks may be the same stack or different stacks. When they are different, they may be directly interleaved with each other or via a further intermediate stack. In

embodiments where the first flux bypass component comprises a single intermediate stack of plates, the intermediate stack of plates has a first end and a second end; wherein the first end of the intermediate stack of plates is connected to the first stack of plates so as to form a first overlap region where the plates of the first stack and of the intermediate stack form a combined stack where the plates of the first stack and of the intermediate stack are interleaved with each other; and wherein the second end of the intermediate stack of plates is connected to the second stack of plates so as to form a second overlap region where the plates of the second stack and of the intermediate stack form a combined stack where the plates of the second stack and of the intermediate stack are interleaved with each other. Accordingly, in embodiments where the flux bypass component comprises one or more intermediate stacks of plates, the first and second stacks of plates may first be inserted into respective yokes and then interleaved with the intermediate stack(s) when the transformer parts including the respective yokes are, or have been, assembled with each other. When the flux bypass components are made from non-grain-oriented steel, relative sharp turns of the flux direction at the transition between the interleaved stacks are often acceptable, even turns larger than 90°. In some embodiments the layers that form the respective stacks each define respective principle flux directions, e.g. when the respective stacks are made from grain-oriented steel. While angles up to 90° between the principle flux directions of the interleaved stacks may be acceptable, even for flux bypass components made from grain-oriented steel, smaller angles may be preferred. The layers of the first stack and the layers of the intermediate stack may be arranged such that the principle flux direction of the layers of the first stack form an angle of less than 50°, such as less than 45°, such as less than 40°, e.g. 30° or smaller, with the principle flux direction of the layers of the intermediate stack. In particular, the angle may be between 5° and 45°, such as between 10° and 40°, such as between 15° and 35°. Similarly, the layers of the second stack and the layers of the intermediate stack may be arranged such that the principle flux direction of the layers of the second stack forms an angle of less than 50°, such as less than 45°, such as less than 40°, e.g. 30° or smaller, with the principle flux direction of the

intermediate layer. In particular, the angle may be between 5° and 45°, such as between 10° and 40°, such as between 15° and 35°. Smaller angles between the principle flux directions of the stacks provide a more efficient flux transfer between the respective stacks, in particular when grain-oriented steel is used as material for the respective stacks. When the flux bypass components are made from non-grain-oriented steel, the orientation of the stacks relative to each other has less influence on the flux transfer between stacks. Generally, it will be appreciated that the intermediate stack itself may be made up of multiple interleaved stacks that form an angle relative to each other so as to reduce the transition angles at each transition between interleaved stacks.

In some embodiments, the intermediate stack may be divided into two or more stacks arranged next to each other where the layers of adjacent intermediate stacks are interleaved with each other in an overlap area.

In some embodiments, the transformer core may be a planar core. In particular, in such embodiments, the yokes may all be parallel to each other. The flux bypass components may have end portions extending into

respective ones of the yokes and center portions extending parallel to the yokes. The parallel center portions and the end portions may be formed as respective stacks of plates that are interleaved with each other to form the transition between the end portions and the center portion. In some embodiments, the transformer core is a delta core where the legs define corners of a triangle and the yokes define sides of the triangle. In particular, the triangle may be an equilateral triangle. From each end of each of the legs, a pair of two yokes protrude towards respective ends of the corresponding other two legs. The two yokes of each pair thus define an angle of about 60° between them. Each pair of yokes that extend from the same end of one of the legs may be connected by a flux bypass component as described herein. The flux bypass components extend inside the triangle defined by the legs and yokes, thereby allowing the flux bypass components to be kept relatively short, thus reducing the amount of additional material used for the flux bypass components.

The present disclosure relates to different aspects including the transformer core described above and in the following, corresponding apparatus, systems, methods, and/or products, each yielding one or more of the benefits and advantages described in connection with the first mentioned aspect, and each having one or more embodiments corresponding to the embodiments described in connection with the first mentioned aspect and/or disclosed in the appended claims.

In particular, according to one aspect, the present disclosure relates to transformer comprising a transformer core as disclosed herein, and a plurality of coils wound from electrically conducting wire; wherein the coils extend around respective ones of the legs of the transformer core. The transformer may be a multi-phase, such as a three-phase transformer.

In some embodiments, the transformer comprises additional windings extending around one or more, such as around all, flux bypass components. The additional windings may be connected to a control circuit configured to feed electrical currents through the additional windings so as to influence the magnetic flux through the flux bypass components. According to another aspect, the present disclosure relates to a method for manufacturing a transformer core as disclosed herein. Embodiments of the method comprise:

- providing a layered core ring; wherein one or more plates are inserted at respective positions between adjacent layers of the first layered ring;

- optionally heat treating the layered core ring;

- assembling the layered core ring with one or more other core rings so as to form a transformer core having flux bypass components as described herein;

- optionally heat treating the assembled transformer core.

In some embodiments the inserted plates are layers of a bypass component. The other core rings may be manufactured in a similar manner, i.e. with a stack of layers of one or more bypass components interleaved between layers of the wound core. When the layered core ring and the one or more other core rings are assembled, the stacks of layers of the respective bypass components may protrude towards each other. The process may then comprise inserting layers of an intermediate stack of a flux bypass

component between the layers of the protruding stacks such that the layers of the intermediate stack are interleaved with end portions of two of the protruding stacks. Hence, this embodiment is particularly suitable for manufacturing wound transformer cores where the flux bypass components comprise three of more stacks of layers, e.g. as described in connection with FIGs 4, 5, 7A, 7C, 8A-C, 9A-B, 12 and 12. In particular, FIGs. 7A and 7C illustrate the assembled rings with protruding layers 206a-b prior to insertion of the layers 206c of the intermediate stack. In this embodiments, it may be beneficial to heat treat the assembled transformer core after insertion of the layers of the intermediate stack. In other embodiments, the plates may be placeholder plates. They may be removed prior to assembly of the core rings with each other, optionally after heat treatment of the individual core rings, so as to expose gaps between respective layers of the yoke into which layers of a flux bypass component may subsequently be inserted. Accordingly, assembling the first core ring with one or more other core rings may then comprise inserting layers of a flux bypass components into the exposed gaps, Inserting the layers of the flux bypass component may further comprise interleaving the inserted layers with corresponding layers that are inserted into another one of the core rings. After insertion of the layers of the flux bypass components, the transformer core may again be heat treated. This embodiment of the manufacturing process may be particularly suitable for manufacturing wound transformer cores where the flux bypass components comprise only one or two stacks of layers, e.g. as described in connection with FIGs 1A-B, 2A-C, 3A-B, and 7B. The layered core rings may be wound from strips of electrical steel or from other types of sheet material with suitable magnetic properties, such as magnetic ribbon or strips. The plates may be inserted at predetermined positions along the ring between consecutive layers of the core ring during the winding process. One or more plates may be inserted between each pair of adjacent layers or only between some pairs of adjacent layers. In some embodiments, for each yoke portion, plates may be inserted at first and second positions along the length of the yoke. In some embodiments one or more plates are inserted between respective pairs of layers, alternatingly at the first and second position. When manufacturing a thee-legged delta core, the process may comprise manufacturing three core rings, each having plates inserted. The three core rings may then, e.g. after removal of the placeholder plates when the plates are placeholder pates, be assembled so as to form a delta core, e.g. such that each leg of the delta core is formed from leg portions of two of the core rings, the two leg portions may thus be joined such the their yoke portions form an angle of 60° relative to each other. In some embodiments, the transformer core may be manufactured from more than three core rings, e.g. such that each side of the transformer core (corresponding to a side of the triangle defined by the transformer core) is formed by more than one ring. In such embodiments bypass components may be inserted into each ring or only into a subset of rings, e.g. the thickest ring and/or a ring facing inwards with respect to the triangle.

The heat treatment of the individual core rings and/or of the assembled transformer core may be performed in a conventional manner. Brief description of the drawings

FIGs. 1 A-B illustrate an example of a three-phase transformer having a delta core for a three-phase transformer.

FIGs. 2A-C illustrate another example of a transformer core with flux bypass components for a three-phase transformer. FIGs. 3A-B illustrate another example of a transformer core with flux bypass components for a three-phase transformer.

FIG. 4 shows a cross sectional view of a transformer core with flux bypass components for a three-phase transformer.

FIG. 5 schematically illustrates an embodiment of a transformer core with flux bypass components for a three-phase transformer.

FIG. 6 schematically illustrates another example of a transformer core with flux bypass components for a three-phase transformer.

FIGs. 7A-C illustrate yet further examples of transformer cores for a three- phase transformer with flux bypass components. FIG.s 8A-C schematically show top views of examples of transformer cores for a 5-legged planar transformer with flux bypass components.

FIGs. 9A-B illustrate embodiments of a 3-legged planar transformer with flux bypass components. FIG. 10 shows an example of a plate configured to form at least a part of a layer of a flux bypass component.

FIGs. 1 1 - 12 illustrate yet further examples of transformer cores for a three- phase transformer with flux bypass components.

Detailed description Various aspects and embodiments of a transformer core disclosed herein will now be described with reference to the drawings.

FIG. 1 A shows an example of a three-phase transformer having a delta core. FIG. 1 B shows the transformer core of the transformer of FIG. 1 A. The transformer, generally designated 100, comprises three coils 101 and a transformer core 102. The coils are made from electrically conducting wire. The transformer core comprises three legs 103 that protrude through the respective coils 101 . The legs are positioned such that they are parallel to each other and that their centers define an equilateral triangle. From each end of a leg, two yokes extend to corresponding ends of the respective other two legs. Accordingly, the transformer core comprises six yokes, a set of three yokes forming an upper triangle when the legs extend in an upright orientation and interconnect the upper ends of the respective legs. The set of the remaining three yokes forms a lower triangle when the legs extend in an upright orientation and interconnect the lower ends of the respective legs. From each end of a leg, a pair of yokes extends. The pair of yokes that extend from the same end of a leg is connected by a flux bypass component 106 extending between the yokes of a pair such that the opposite ends of the flux bypass components extend into the respective yokes. Hence, each pair of the yokes that form the upper triangle is interconnected by a respective flux bypass component and each pair of the yokes that form the lower triangle is interconnected by a respective flux bypass component. In particular, the transformer core is made from wound strips of electrical steel. The strips are wound in a ring shape, each ring having a shape of a rounded rectangle where to opposite sides of the rectangle are leg portions. The remaining two sides of the ring form yoke portions that extend outside of the coils between respective legs of the transformer core. Each leg portion forms, together with one or more leg portions of one of the other rings, a leg of the transformer core.

Each side of the transformer core is thus formed like a generally rectangular frame 22, 23 and 24, respectively.

It will be appreciated that each frame of the transformer core may be formed by a single wound ring. In such an embodiment, each leg is made up of two leg portions, one from two different rings. Alternatively, each frame may be formed by multiple rings. Accordingly, e.g. leg may thus be formed by leg portions of two or more rings of one frame and by leg portions of two or more rings of another frame. Such a multi-ring arrangement may allow the cross sectional shape of the legs to more closely approximate a circular cross section.

In the particular example shown in FIGs. 1A-B, frame 22 comprises rings 22a-b, frame 23 comprises rings 23a-c, while frame 24 comprises rings 24a- b. The rings are wound from strips of different width and the rings have different number of layers, i.e. the have different thickness.

The flux bypass components are made from stacks of electrical steel or another suitable material. Examples of suitable materials for the flux bypass components include grain-oriented steel, non-grain-oriented steel, amorphous metal. The material of the flux bypass components may be the same material as the wound core rings or it may, partly or in whole, be a different material.

The ends of the individual plates of the bypass components extend into the wound rings and the ends of one or more plates may be sandwiched between respective layers of the wound rings. In the example of FIG. 1 B, only the rings having an inward facing side not obstructed by another ring are interconnected by respective flux bypass components. However, in other embodiments, all rings - or another subset of rings - may be interconnected by flux bypass components. Generally, the connected rings may be the rings having largest thickness (i.e. the largest number of layers) and/or the largest width, so as to allow for the most efficient flux transfer between different yokes. In particular, in the present example, the flux bypass components extend between the inner rings 22a, 23a and 24a. FIGs. 2A-C illustrate another example of a transformer core. In particular, FIG. 2A shows a 3D view of the transformer core while Fig. 2B shows a top view of the transformer core. The transformer core of FIGs. 2A-B is similar to the transformer core of FIG. 1 B in that the transformer core is a delta core having three legs 103A-C and yokes 205 where the legs are arranged in a triangular configuration. The transformer core is made from wound strips of electrical steel and, in particular, from three frames 22, 23, and 24 of wound strips. In the example of FIGs. 2A-B, each frame is made from a single core ring made from wound strip material. Hence, each leg is formed by leg portions of two rings: Leg 103A is formed from leg portions of rings 22 and 23, leg 103B is formed from leg portions of rings 23 and 24 while leg 103C is formed from leg portions of rings 22 and 24. Each ring comprises leg portions of two legs and yoke portions connecting these leg portions. Each yoke portion 205 comprises multiple layers of electrical steel. The transformer core further comprises flux bypass components 206A-C extending between pairs of yokes as described in connection with FIG. 1 B. In particular, the flux bypass components are stacks of plates of electrical steel whose ends extend into the respective yokes such that the end portions of each plate of the flux bypass component are sandwiched between adjacent layers of the respective yokes. As can most easily be seen from FIG. 2B, the transformer core provides flux paths between different leg portions of each pair of legs. The leg portions of legs 103A and 103C are connected by flux paths 265 and 266. Flux path 265 is a path formed by the yoke portion of the ring that connects the two legs 103A-B, i.e. the yoke portion that is part of the ring 22. The other flux path 266 is formed by parts of the other yokes (i.e. yoke portions of rings 23 and 24) and by the flux bypass component 206B that connects yoke portions of rings 23 and 24. The flux paths 265 and 266 bypass the third leg 103B. The flux bypass components of this embodiment may be made of non-grain-oriented steel while the remaining parts of the transformer core may be made from grain-oriented steel. The use of non- grain-oriented steel for the flux bypass components of the present

embodiment is beneficial, as the angles at which the flux bypass components are inserted into the yokes is relatively large.

FIG. 2C illustrates the shape of one of the plates 216 that are stacked so as to form the flux bypass components 206. The ends of the plate 216 that are inserted between adjacent layers of the yoke 205 are indicated by dotted lines in FIG. 2C. Moreover, FIG. 2C shows how the plate 216 may be cut or punched out from sheet steel.

FIG. 3A illustrates another example of a transformer core. The transformer core of FIG. 3A is similar to the example of FIGs. 2A-C but where the plates 216 forming the flux bypass components have different shapes in the upper and lower portions of the transformer core, respectively. In particular, FIG. 3B illustrates the shape of one of the plates 216 used at the upper portion as it may be cut or punched out of sheet material. The plates of the flux bypass components at the lower portion are similar as the plates of the embodiments of FIGs. 2A-C. In any event, the plates 216 are stacked so as to form the flux bypass components 206 of the transformer core of FIG. 3A. The ends of the plate 216 that are inserted between adjacent layers of the yoke 205 are indicated by dotted lines in FIG. 3B.lt will be appreciated that, in an alternative embodiment, the plates at both the upper and the lower portion may have a shape as shown in FIG. 3B.

Embodiments of the transformer cores described herein may be

manufactured by winding a band of electrical steel to form the frames, e.g. frames 22, 23, and 24 in Figs. 1A, 2A-B and 3A. During the winding process placeholder plates may be inserted between consecutive layers of the band at suitable positions of the yoke portions of the frames, so as to establish space for subsequent insertion of the plates of the flux bypass components. The frames may be heat treated and then assembled to form the transformer core. Before or after assembly of the frames with each other, the placeholder plates may be pulled out of the yoke portions. The plates of the flux bypass may then be consecutively inserted into the slots that had previously been occupied by the placeholder plates and stacked in an overlapping fashion, optionally with an intermediate plate so as to build up the flux bypass components. Finally, the assembled transformer core may be heat treated again. As in the previous example, the flux bypass components of this embodiment may be made of non-grain-oriented steel while the remaining parts of the transformer core may be made from grain-oriented steel.

FIG. 4 shows a schematic cross-sectional view of a transformer core. The transformer core of FIG. 4 is similar to the transformer cores of the previous examples in that it is a delta core comprising legs 103, yokes 205 and flux bypass components 206. The yokes are made of laminated steel plates or from wound steel strips as described in connection with the previous examples. In particular, the transformer core may be made from three frames, each frame being formed by multiple wound rings that may have different sizes, e.g. as illustrated in the example of FIG. 1A-B, so as to provide the legs with a cross section that approximates a circular cross section. In FIG. 4 this is schematically illustrated by a hexagonal cross section of the legs. For ease of illustration, in FIG. 4 the yoke portions 205 of only one ring for each frame are shown. The yoke portions of other rings may optionally also be provided with flux bypass components as described herein. The flux bypass components are made from thin steel plates, e.g. of non- grain-oriented steel, whose ends are inserted between layers of the yoke portions of the core rings. In the example of FIG. 4, each layer of the flux bypass components is not formed by a single plate but by three separate plates 206a-c, respectively. In particular each layer of the flux bypass component comprises a first plate 206a whose one end is inserted into one of the yokes, a second plate 206b whose one end is inserted into another one of the yokes, and an intermediate plate 206c which overlaps both the first and the second plates so as to provide an overlap area where magnetic flux can flow from the first plate 206a into the intermediate plate 206c and from the intermediate plate into the second plate 206b. In this example, the principle flux direction of one of the yoke portions 205 is indicated by arrows. In this example, layers of flux bypass components towards different yokes are inserted at the same level of the layered yoke; in fact, corresponding layers of flux bypass components towards different yokes abut each other at the inserted edge 467. When the bypass components are made from non- grain-oriented steel, such an arrangement still allows for an efficient flux exchange to/from the yoke.

FIG. 5 schematically illustrates an embodiment of a transformer core with a layered flux bypass component connecting two yokes 205 where each layer of the flux bypass component is made up from three plates 206a, 206b and 206c as in the example of FIG. 4, i.e. where each flux bypass component comprises a sequence of three interleaved stacks. The flux bypass

components and the remaining parts of the transformer core of this

embodiment may be made from grain-oriented steel. The principle flux directions of the various elements are indicated by arrows. As can be seen from FIG. 5, the plates of the flux bypass component may be arranged such that, at each transition between the yoke and a plate of the flux bypass component and at each transition between plates of the different stacks of the flux bypass component, the change in principle flux direction may be chosen to be by the same angle a. In a delta core transformer where the legs and yokes form an equilateral triangle and the angle between adjacent yokes is 60°, the change of principle flux direction at the transitions between the plates of the various stacks may thus be chosen to be a = 30°. It will be appreciated, however, that in other embodiments the angles between principle flux directions of the respective layers may different, e.g. have one value at the transition between the yoke and flux bypass component and a different value at the transition between the various stacks of the flux bypass component, e.g. as illustrated in FIG. 7C below.

As is further illustrated in FIG. 5, the width W of the plates of the flux bypass component may be chosen such that the minimum length L, measured along the direction of the principle flux direction of the yoke 205, of the overlap area between the plate 206a and the yoke 205 has a desired length, e.g. no smaller than 0.5 cm, such as no smaller than 1 cm, such as no smaller than 2 cm, such as no smaller than 3 cm, so as to allow for an efficient flux transition between the yoke and the flux bypass component. In particular, the plate 206a extends into the yoke 205 across the entire width of the yoke. In the example of FIG. 5, the extent of the overlap is even larger than L at most positions across the width of the yoke. Moreover, in the example of FIG. 5, the bypass plate 206a is shaped such that its one edge is aligned with a side of the yoke, i.e. the plate only protrudes out of the yoke on one side of the yoke. In other examples the bypass plates may extend all the way through the yoke and protrude out of the yoke on both sides of the yoke.

FIG. 6 schematically illustrates another example of a transformer core having legs 103 and yokes 205. As in FIG. 4, the legs are illustrated with a hexagonal cross section which may be formed by respective leg portions of multiple wound core rings that form each side of the transformer core.

Nevertheless, for ease of illustration, only the yoke portions 205 of a single core ring of each side are shown. The transformer core of the example of FIG. 6 is similar to the transformer cores of the examples of FIGs. 4 and 5 in that the transformer core is a delta core and that the yokes 205 and legs 103 are made of wound strips of electrical steel and that the flux bypass components are made from stacked plates 206a-b where each layer of the bypass components is made up of separate plates that overlap with each other. In the example of FIG. 6, each layer of each bypass component is made up of two plates 206a and 206b, respectively. As is further illustrated in FIG. 6, the plates of the flux bypass layer are inserted between the layers of the yokes 205 in an alternating pattern such that one, two, or even more yoke layers is/are sandwiched between two plates 206a and 206b of respective flux bypass components. In particular, one plate 206a is part of a flux bypass component that connects the yoke with one of the remaining yokes while the other plate 206b is part of a flux bypass component that connects the yoke with the other of the remaining yokes. In this fashion, the plates 206a-b of the flux bypass components can be made relatively wide without unduly increasing the thickness of the yoke, as there is only one plate of one flux bypass component inserted between any two layers of the yoke and at the same position along the length of the yoke. When two yoke layers are sandwiched between two plates of 206a and 206b of respective flux bypass components, one yoke layer is in direct contact with one of the two plates and the other yoke layer is in direct contact with the other of the two plates. This allows for an efficient flux exchange and avoids crossing flux paths within the same layer of the layered structure.

FIGs. 7A-C illustrate yet further examples of transformer cores with flux bypass components. All examples of FIGs. 7A-C are delta cores having legs 103 and yokes 205. The legs and yokes may be made from wound rings 22- 24 of steel strips as described in connection with FIG. 2A. Hence, each of the legs 103 is formed from respective leg portions of two of the wound rings. The flux bypass components are formed as stacks of steel plates. The number of plates used in each layer of the flux bypass components and their geometrical shape differ in the various examples of FIGs. 7A-C. It is noted that, for ease of illustration, in each of the FIGs. 7A-C, only the plates of a single layer of a single flux bypass component are shown. It will be

appreciated, even though not explicitly shown in FIGs. 7A-C, that the transformer cores also include flux bypass components connecting the other pairs of yokes; however, at least in the examples of FIGs 7B and C and when the flux bypass components are made from grain-oriented steel, the layers of the other flux bypass components may be inserted between different layers of the yoke, e.g. as described in connection with FIG. 6. Hence, generally, layers of different flux bypass components that extend into the same stacked yoke portion may be inserted at respective levels of the stacked yoke portion, e.g. at alternating levels.

In the example of FIG. 7A, the flux bypass components comprise elongated plates 206a, b that are inserted between the layers of the yokes and that extend along a major part of the length of the yokes. The plates 206A protrude out of the side of the yoke toward the interior of the transformer core. They extend all the way into the yoke, i.e. across the entire width of the yoke. The plates inserted into two yokes are then connected by inserting an intermediate plate 206C which, in FIG. 7A, is illustrated prior to insertion. The intermediate plate may be slid in the plane of the drawing upwards until it overlaps the elongated plates 206a, b. The arrangement of the example of FIG. 7A is particularly suited for flux bypass components made from non- grain-oriented steel where the direction of the magnetic flux can easily turn. The example of FIG. 7A has the advantage that it is easy to manufacture as the elongated layers 206a, b of the bypass components can easily be inserted into the yoke portions of the core rings during manufacturing of the core rings. The example of FIG. 7B is similar to the example of FIG. 6 in that each layer of the flux bypass component is formed by two plates 206a and 206b. Each of the plates is inserted between layers of a respective one of the yokes, and the ends of the plates that protrude out of the yokes overlap with each other so as to provide a flux path across the overlap zone. This embodiment is suitable for transformer cores where also the flux bypass components are made from grain-oriented steel. The principle flux direction of the plates 206a-b is indicated by arrows in the figure. Alternatively, the flux bypass components of this embodiment may be made from non-grain-oriented steel. As can most easily be seen from FIG. 7B, the transformer core provides flux paths between different leg portions of each pair of legs. Flux paths 265 and 266 between respective leg portions of two of the legs are schematically illustrated by arrows in FIG. 7. Flux path 265 is a path between the leg portions of ring 24; the path is formed by the yoke portion of ring 24. The other flux path 266 is formed by parts of the other yokes (i.e. yoke portions of rings 22 and 23) and by the flux bypass component that connects yoke portions of rings 22 and 23, i.e. by the flux bypass component made up of stacks 206a and 206b.The flux path 266 connects leg portions of rings 22 and 23. The example of FIG. 7C is similar to the examples of FIGs. 4 and 5 in that each layer of the flux bypass component is formed by three plates 206a-c. Respective ends of two of the plates (plates 206a and 206b) are inserted between layers of a respective one of the yokes, and an intermediate plate 206c is inserted so as to overlap with the ends of the plates 206a-b that protrude out of the yokes so as to provide a flux path across the overlap zone. As in FIG. 7A, the intermediate plate 206c is shown prior to assembly with plates 206a-b. This embodiment is suitable for transformer cores where the flux bypass components are made from grain-oriented steel. The principle flux direction of the plates 206a-c is indicated by arrows in the figure. Alternatively, the flux bypass components of this embodiment may be made from non-grain-oriented steel.

FIGs. 8A-C schematically show top views of yet further examples of transformer cores with flux bypass components. In particular, FIGs. 8A-C show examples of planar transformers where the legs 103 and yokes 205 are arranged along a straight line. The transformer cores of FIGs. 8A-C are 5- legged transformer cores where only three of the legs (the center ones) are surrounded by coils (not explicitly shown). The yokes and legs of the transformer core are formed by core rings. The core rings of the transformer cores of FIGs 8A-C may be made from wound strips of steel or otherwise as wound core rings. For example, the legs and yokes of the transformer core may be made from four rings of wound strips of steel such that each of the center legs that are surrounded by a coil is formed by respective leg portions of two adjacent core rings. The flux bypass components may be cut or punched from stacked plates of steel sheet material. Each flux bypass component is made from three or more stacks of strips including respective stacks that are interleaved with the yokes and intermediate stacks, some of which extend parallel to the plane defined by the legs and yokes.

FIG. 8A schematically illustrates an embodiment of a transformer core with flux bypass components 206a-e suitable for a transformer core where the flux bypass components are made of grain-oriented steel. As in the previous examples, the bypass components are made from stacks of elongated plates. In particular the flux bypass components connecting the yoke portions of adjacent rings comprise stacks 206a extending into the respective yoke portions; the layers of these stacks are interleaved with the layers of the core ring as described herein. The flux bypass components further comprise intermediate stacks 206b connecting the respective stacks 206a which extend into adjacent yokes with each other. The outer rings are connected by a flux bypass component that comprises two stacks 206c extending into their respective yoke portions, intermediate stacks 206d arranged to form arcuate portions extending around the respective outer legs, and a long intermediate stack 206e extending parallel to the plane defined by the planar core and connecting the two arcuate portions with each other. At the transitions between stacks, the plates of the stacks are interleaved so as to create an overlap area for flux transfer. The number of intermediate stacks forming the arcuate portions may be selected such that the angles between adjacent stacks are kept sufficiently small so as to allow for an efficient flux exchange.

FIGs. 8B-C illustrate examples of transformer cores with flux bypass configurations suitable for transformer cores where the flux bypass

components are made from non-grain-oriented steel. As can be seen from FIGs. 8A-C, the embodiment for flux bypass components made from grain- oriented steel includes a longer flux bypass component with multiple stacks 206c-e (see FIG. 8A) and transitions, so as to keep the bend angle at each transition small. In the examples of FIG. 8B-C, the number of different stacks of the flux bypass component that connects the outer rings is smaller, as illustrated by stacks 206c and 206d in FIG. 8B.

In the transformer core of the example shown in FIG. 8C, the flux bypass components share an elongated intermediate stack 206b which is connected to respective stacks 206a that extend into the respective yoke portions. In the example of FIG. 8C, the transformer core comprises flux bypass components on one side of the planar core. It will be appreciated, however, that other embodiments may comprise corresponding flux bypass components on both sides of the planar core.

FIGs. 9A-B illustrate embodiments of a 3-legged planar transformer core having legs 103a-c and yokes 205a-b. FIG.s 9A shows a 3D view of an upper portion of one embodiment of a planar, three-legged transformer core while FIG. 9B shows a top view of another embodiment of a three-legged transformer core. As can best be seen from FIG. 9A, both embodiments of the transformer core are made from multiple rings 22, 23 and 24, each ring made from wound steel strips. In particular, the transformer core comprises two inner rings 22 and 23 comprising yoke portions 205a, b, respectively, and an outer ring 24 comprising yoke portion 205c so as to provide flux paths connecting all pairs of legs with each other. Leg 103a is thus made up of respective leg portions from rings 22 and 24, leg 103b is made up of leg portions from rings 23 and 24 while leg 103c is made up of ring portions from rings 22 and 23.

FIG. 9A schematically illustrates an embodiment of a three-legged planar transformer core with a flux bypass configuration suitable for non-grain- oriented steel. The flux bypass components comprise stacks 206a-e of metal plates that are shaped and, optionally, bent so as to provide paths between respective pairs of the yokes 205a-c. In particular the core comprises stacks 206a, c,d of plates whose ends are interleaved with the layers of the yoke as described herein. Stacks 206a and 206c are interleaved with yoke portions 205a-b of respective inner rings 22 and 23 while stacks 206d are interleaved with yoke portion 205c of the outer ring 24. Stacks 206a are connected by an intermediate stack 206b so as to provide a flux bypass component between the yoke portions 205a and 205b of the inner rings 22 and 23, respectively. Each of the stacks 206c is connected with a corresponding one of stacks 206d by a respective intermediate stack 206e so as to connect the yoke portion 205c of the outer ring 24 with the yoke portions 205a-b, respectively. To this end the plates forming the intermediate stack 206e are bent as illustrated in FIG. 9C. The flux bypass component made from stacks 206a-b provides a flux connection between the inner yokes 205a-b while the flux bypass components made from stacks 206c-e provide flux connections between the outer yoke 205c with respective ones of the inner yokes 205a-b. It will be appreciated that the yoke portions at the lower end of the

transformer core may be connected by corresponding flux bypass

components. FIG. 9B schematically illustrates a flux bypass configuration particularly suitable for grain-oriented steel. The flux bypass components comprise stacks of metal plates. At the transitions between the stacks, the respective stacks are interleaved so as to allow magnetic flux to transfer from the plates of one stack to the adjacent stack. In particular, a flux bypass component connecting corresponding yoke portions of the inner ring is made up from stacks 206a that extend into, and are interleaved with, the respective yoke portions, and an intermediate stack 206b that connects the stacks 206a with each other. This flux bypass component thus provides a flux connection between the inner yokes. A further pair of flux bypass components provides flux connections between the outer yoke 205c and respective ones of the inner yokes. Each of these flux bypass components comprise stacks 206c of plates that extend into one of the yoke rings (one stack extends into a yoke portion of the outer ring and the other stack extends into a corresponding yoke portion of one of the inner rings). The stacks 206c are interconnected by a sequence of intermediate stacks 206d-e so as to form an arcuate flux bypass component extending around a respective one of the outer legs 103a,b of the transformer core. Hence, one or more of the intermediate stacks of the flux bypass components 206b may be made from bent plates, e.g. as illustrated in FIG. 9C. In the example of FIG. 9B, the central intermediate plate 206e is made from bent plates. The number and orientation of the stacks 206a-e may be chosen such that the angle between adjacent stacks and between stacks 206a, c and the corresponding yoke are kept sufficiently small so as to facilitate efficient flux exchange at the transitions between stacks and between stacks and yokes. Accordingly, this embodiment may be particularly suitable for transformer cores where the flux bypass components are made from grain-oriented steel.

FIG. 9C illustrates a plate 206e of one of the stacks 206e of the

embodiments of FIGs 9A-B. FIG. 10 shows an example of a plate 216 configured to form at least a part of a layer of a flux bypass component. The plate has a first end 1261 that is shaped and sized to be inserted between respective layers of a yoke 205 and a part 1262 that protrudes out of the yoke at an angle relative to the yoke. The distal end 1263 of the protruding part 1262 may be inserted between layers of another stack of plates 206c that form a part of a flux bypass component. The principle flux direction of the parts are illustrated by arrows.

FIGs. 1 1 -12 illustrate further embodiments of transformer cores comprising flux bypass components. As in the previous examples, the transformer cores are wound cores comprising multiple rings of stacked metal strips, e.g. made from grain-oriented steel, and the bypass components are made from stacks of metal plates, preferably made from non-grain-oriented steel. In the examples of FIGs. 1 1 -12 different bypass components share at least one intermediate stack of plates, i.e. the shared intermediate stack of plates is common to two or more flux bypass components.

The transformer core of FIG. 1 1 is a delta core comprising legs 103 and yokes 205. In this example, the bypass components comprise stacks 206a of plates that extend from respective yoke portions inwardly towards the centre of the triangle defined by the delta core. The core further comprises a shared intermediate stack 206b that is located about the centre of the triangle defined by the delta core and that is interleaved with each of the stacks 206a.

FIG. 12 illustrates an embodiment of a 3-legged planar transformer core having legs 103a-c and yokes 205a-b, similar to the embodiments of FIG.s 9A-B but with a different configuration of flux bypass components. In particular, the yoke portions 205a-b of the inner rings 22 and 23 of the transformer core are connected by a flux bypass component that comprises stacks 206a of plates that extend into the respective yoke portions between respective layers of the yoke portions as described herein. The stacks are interconnected by an intermediate stack 206b. To this end the ends of the plates of the intermediate stack 206b are interleaved with the ends of respective ones of the stacks 206a as described herein. The intermediate stack 206b is a shared intermediate stack as it is also a part of flux bypass components connecting the yoke portion 205c of the outer ring 24 with the yoke portions 205a, b of the inner rings 22 and 23, respectively. In particular, the transformer core comprises a stack 206c of plates that extend into a yoke portion of the outer ring. The transformer core further comprises an intermediate stack 206d formed by plates that are bent into a C-shaped form. One end of the C-shaped stack 206d is interleaved with the ends of stack 206c that protrudes out of yoke 205c. The other end of the C-shaped stack 206d is interleaved with the shared intermediate stack 206b. In particular the C-shaped stack is interleaved at a center portion of the shared intermediate stack 206b. Accordingly the flux bypass components form flux paths between the yoke portion 205c of the outer ring via stacks 206c, 206d, 206b and 206a to respective ones of the yoke portions 205a-b of the inner core rings. As can be seen from a comparison from FIG. 12 with FIGs. 9A-B, the embodiment of FIG. 12 requires less material to form the flux bypass components. In the example of FIG. 12, the transformer core comprises flux bypass components on one side of the planar core. It will be appreciated, however, that other embodiments may comprise corresponding flux bypass components on both sides of the planar core.

In the claims enumerating several means, several of these means can be embodied by one and the same element, component or item of hardware. The mere fact that certain measures are recited in mutually different dependent claims or described in different embodiments does not indicate that a combination of these measures cannot be used to advantage.

It should be emphasized that the term "comprises/comprising" when used in this specification is taken to specify the presence of stated features, elements, steps or components but does not preclude the presence or addition of one or more other features, elements, steps, components or groups thereof.