WO2014086948A2

WO2014086948A2 - Transformer assembly

Info

Publication number: WO2014086948A2
Application number: PCT/EP2013/075723
Authority: WO
Inventors: Joseph DURON; Luc GROSJEAN; Philippe Haeberlin; Philippe Stefanutti; Toufann Chaudhuri
Original assignee: Abb Technology Ag
Priority date: 2012-12-05
Filing date: 2013-12-05
Publication date: 2014-06-12
Also published as: WO2014086948A3; EP2929551A2; EP2929551B1; CN104995699A; CN104995699B

Abstract

The present invention relates to a transformer assembly (20), comprising: a cylindrical inner housing (21) and a cylindrical outer housing (26) partially surrounding the cylindrical inner housing (21), the cylindrical inner housing (21) and the cylindrical outer housing (26) forming an enclosure (30), wherein the enclosed volume of the enclosure (30) between the cylindrical inner housing (21) and the cylindrical outer housing (26) is filled with an insulating liquid; - at least one winding (22, 23) in the enclosed volume (30); and - a first cooling means arranged in or at an inner surface of the outer housing (26) to provide cooling for the at least one winding (22, 23).

Description

Transformer assembly

Technical field

The present invention relates to transformer assemblies, in particular transformer assemblies for high-power applications, such as for use in traction applications and the like.

Related art

In traction applications, transformers are conventionally used for galvanic decoupling and transformation of electrical power. To provide high-power conversion, transformers need to be designed with a substantial size and weight. Due to the high power involved, cooling and insulation constraints are to be considered in the transformer design.

Transformers used in the distribution of electrical power are usually of the vacuum- cast or resin block type and have air insulation. In particular for traction applications, size and weight reduction is important. However, power density and heat dissipation are high, so that vacuum casting or resin block solutions cannot be applied in compact transformer designs since they do not provide sufficient heat dissipation to keep the temperature of the transformer within an allowable range.

In order to meet the requirements of traction applications, traction transformers are usually encased in oil-filled tanks having forced oil circulation and forced air cooling. Due to the restricted heat dissipation through oil, the size and weight of the above kind of transformers cannot be further reduced. In document CN 2 891 235, a cavity-type water-cooled transformer is disclosed having a core water tank which is surrounded by a primary side winding and a ring- shaped water chamber.

Document CN 103035370 discloses an oil-immersed transformer device including a transformer disposed in a transformer tank. The transformer is mounted in the transformer tank. The transformer tank is filled with oil. A cooling duct for cooling the oil is provided in the transformer tank, wherein water is fed through the cooling duct.

Document CN 202917292 discloses a transformer with a cavity between a primary coil and a secondary coil for feeding cooling water there through.

It is an object of the present invention to provide a compact transformer design which allows for high heat dissipation and good electrical insulation.

Summary of the invention

This object is achieved by the transformer assembly according to claim 1 .

Further embodiments of the present invention are indicated in the dependent subclaims.

According to a first aspect, a transformer assembly is provided, comprising:

a cylindrical inner housing and a cylindrical outer housing partially surrounding the cylindrical inner housing , the cylindrical inner housing and the cylindrical outer housing forming an enclosure , wherein the enclosed volume of the enclosure between the cylindrical inner housing and the cylindrical outer housing is filled with an insulating liquid;

- at least one winding in the enclosure; and

- a first cooling means arranged in or at an inner surface of the outer housing to provide cooling for the at least one winding. Furthermore, the inner housing and the outer housing may be made of an electrically insulating material.

According to an embodiment, a portion of a transformer core may extend through the inner housing, wherein a second cooling means is at least partly located between the portion of the transformer core and the inner housing.

It may be provided that the transformer core forms a closed loop, so that an outer portion of the transformer core partly extends along an outer surface of the outer housing, wherein one or more support plates are securely attached to the enclosure, wherein each of the one or more support plates comprises a support plate fin which abuts at a side surface of the outer portion of the transformer core.

At least one of the support plates may be provided with an earthing connector.

Furthermore, a first and a second cover are arranged at the axial ends of the enclosure. The first cover, the inner housing and the outer housing may be formed integrally.

It may be provided that an inner surface of the first cover is provided with a structuring to form channels for the insulating liquid to flow within the enclosure in a radial and/or tangential direction due to convection.

Moreover, a pressure membrane for flexibly adapting the volume of the insulating liquid within the enclosure may be arranged at one axial end of the enclosure.

The first cooling means may comprise a first cooling pipe which extends along the inner surface of the outer housing or within the wall of the outer housing at least partly along the axial and circumferential direction thereof, so that the first cooling pipe forms at least one loop. According to a further aspect, a transformer assembly is provided, comprising:

- a cylindrical support element;

- at least one winding wound around the support element;

- a core portion extending through the support element; and

- a cooling means included in the support element to provide cooling for the core portion and the at least one winding.

One idea of the above transformer assembly is to provide an inner support element which includes a core portion and a cooling means. One or more windings are concentrically arranged around the support element. This assembly allows effectively dissipating heat produced both by the core and by the one or more windings through the cooling means. Hence, the above assembly allows for efficient cooling of the transformer assembly, in particular for the dissipation of heat generated in an inner region of the transformer assembly.

Moreover, the support element may be made of an electrically insulating material or an electrically conductive material, in particular a metal, with at least one electrically insulating portion which fully extends over the axial length of the support element to prevent a circular current flowing therethrough.

According to an embodiment, at least two windings may be provided with an inner winding and an outer winding around the support element wherein the outer winding at least partly encompass the inner winding to serve as primary and secondary coils of the transformer assembly.

A housing element may be provided around the at least one winding to form a winding enclosure.

The housing element may be made of an electrically insulating material or an electrically conductive material, in particular a metal, with at least one electrically insulating portion which fully extends over the axial length of the support element to prevent a circular current flowing therethrough. Furthermore, the housing element may be provided with further cooling means.

Moreover, the housing element may be provided with protrusions which are configured to couple with an outer core portion to provide cooling of the core.

It may be provided that the protrusions axially extend over the full length of the housing element and/or abut at the outer core portion in an electrically insulating manner.

A plurality of cylindrical support elements may be provided around respective core portions of a loop-shaped core, wherein a winding is wound around each of the support elements, wherein cooling means are included in the support elements to provide cooling for the respective core portion and the respective winding.

A single housing element may be arranged to enclose at least two of the plurality of support elements and the respective windings.

Brief description of the drawings

Embodiments of the present invention are described in more detail in the following description in conjunction with the accompanying drawings, in which:

Figure 1 shows a perspective view of an inner portion of a transformer assembly according to an embodiment;

Figure 2 shows a cross-section across an axial direction through the transformer assembly of Figure 1 ;

Figure 3 shows a perspective view of the complete transformer assembly of

Figure 1 ; Figure 4 shows a perspective view of a transformer assembly according to a further embodiment;

Figure 5 shows a perspective view of a transformer assembly according to a further embodiment;

Figure 6 shows a sectional view of a transformer assembly according to a further embodiment;

Figure 7 shows a perspective view of a transformer assembly according to a further embodiment;

Figure 8 shows a cross-sectional view of the transformer assembly of Figure

7 in the axial direction;

Figure 9 shows a perspective cut-off view through the housing of the transformer assembly of Figure 7;

Figure 1 0 shows a cross-sectional view of a transformer assembly of Figure 7 in the axial direction;

Figure 1 1 shows another perspective view of a transformer assembly of Figure

7; and

Figure 1 2 shows another perspective view of a transformer assembly of Figure

7.

Description of embodiments

In the following, a first embodiment is described in conjunction with the views according to Figures 1 to 3. The transformer assembly 1 has an inner support element 2. In the present embodiment, the inner support element 2 is cylindrical and has an annular cross- section. However, the cylindrical inner support element 2 can also be shaped with different cross-sections.

Around the cylindrical inner support element 2, a inner winding 3 is wound. The conductors of the inner winding 3 can be wire-like, such as a coil of metal wire, e. g. copper wire, or plate-like, coated with an electrical insulation layer, and are spirally wound around the cylindrical inner support element 2. The inner winding 3 may act as a primary or secondary winding of the transformer assembly 1 .

Around the inner winding 3, a outer winding 4 is arranged. The outer winding 4 concentrically encompasses the inner winding 3. The outer winding 4 can be directly wound onto the inner winding 3. The conductors of the outer winding 4 can be wirelike, such as a coil of metal wire, e. g. copper wire, or plate-like, coated with an electrical insulation layer, and are spirally wound around the inner winding 3. The outer winding 4 may act as a primary or secondary winding of the transformer assembly 1 .

In order to ensure electrical insulation between the inner and outer windings 3, 4, it could also be provided that an insulation layer is arranged between the inner winding 3 and the outer winding 4 in case the electrical insulation coating of the conductors is damaged.

The material of the inner support element 2 provides good thermal conductivity and is preferably made of metal or the like. The inner support element 2 is provided with cooling means. For example, the cylindrical inner support element 2 can include one or more cooling pipes 7 or cooling channels which extend through the inner support element 2 in an axial direction. The cooling pipes 7 are configured to allow a cooling medium to flow therethrough. The cooling medium can be air, water, oil, SF6 or the like. The cylindrical inner support element 2 firstly serves as a cold plate for the inner surface of the inner winding 3. Secondly, the inner support element 2 is configured to also provide good thermal conductivity to a portion of a transformer core 5.

The portion of the transformer core 5 axially extends through the interior of the inner support element 2 to provide cooling of the transformer core 5. The transformer core 5 is made of a ferromagnetic material which allows to direct the magnetic flux within the core 5. The transformer core 5 forms a closed loop, wherein one core portion extends in an axial direction through the interior of the inner support element 2. The other portion of the transformer core 5 further extends around the exterior of the outer winding 4.

Substantially, the transformer core 5 can be made of a ferromagnetic material, e. g. ferrite, amorphous materials, nano-crystalline materials and the like. The transformer core 5 can have a shape to be partly encompassed by the cylindrical inner support element 2 and be shaped like an E-core, C-core or the like. The portion of the transformer core 5 that passes through the cylindrical inner support element 2 can be cooled by the cooling means and is mechanically supported.

To avoid a short circuit, the cylindrical inner support element 2 must not act as a turn of a parasitic secondary coil. Hence, the inner support element 2 must be electrically open. Therefore, the cylindrical inner support element 2 must include at least one insulating gap 8 along its circumferential direction. The insulating gap 8 prevents the flow of a circular current. For instance, the cylindrical inner support element 2 can be made of two half cylinders which are arranged in an insulated manner to form the inner support element 2, so that no circular current can flow therethrough.

Alternatively, the cylindrical inner support element 2 may be fully made of an electrically insulating material, like epoxy or the like.

Encompassing the outer surface of the outer winding 4, a cylindrical outer housing element 6 is provided. The cylindrical outer housing element 6 is arranged in good thermal contact with the outer winding 4 and provides good thermal conductivity, so that heat generated in the outer winding 4 can be dissipated via the cylindrical outer housing element 6. Along its tangential or axial direction, the outer housing element 6 can be formed integrally or of several parts.

The cylindrical outer housing element 6 may be made of thermally conductive material, such as metal. The outer housing element 6 may be provided with second cooling means. For example, the outer housing element 6 can include one or more further cooling pipes 9 or cooling channels which extend through the outer housing element 6 in an axial direction. The further cooling pipes 9 are configured to allow a cooling medium to flow therethrough. The cooling medium can be air, water, oil, SF6 or the like. Additionally or alternatively, cooling fins can be provided on the outer surface of the outer housing element 6.

The cylindrical inner support element 2 and the outer housing element 6 substantially have the same axial length and are arranged concentrically to enclose the inner and outer windings 3, 4. At their axial ends, covers 1 1 (one on each side) are provided to close the space formed between the cylindrical inner support element 2 and the cylindrical outer housing element 6 in which the windings 3, 4 are housed. The cylindrical outer housing element 6 can be used as a mechanical winding enclosure and an insulating material can be filled in the space formed by the cylindrical inner support element 2 and the cylindrical outer housing element 6.

As can be seen in Figure 3, the cylindrical outer housing element 6 has a protrusion 10 to be coupled to a yoke portion of the transformer core 5 so as to collect heat generated within the yoke portion of the transformer core 5 and/or heat generated in that portion of the transformer core 5 which extends into the interior of the inner support element 2. The protrusion 10 protrudes radially and substantially extends axially along the full axial length of the support and housing elements 2, 6.

The second cooling means of the cylindrical outer housing element 6 can be configured such that heat generated in the outer winding 4 and in the outer core portion is consumed and transported away from the transformer assembly 1 . The protrusion 10 of the cylindrical outer housing element 6 may form a gap 8 to avoid a short circuit of a parasitic turn formed by the cylindrical outer housing element 6. To avoid an electrical shortcut, the protrusions 10 of the cylindrical outer housing element 6 are coupled to the outer core portion in an electrically insulating manner but with good thermal conductivity on both sides of the outer core portion.

The covers 1 1 at both ends of the transformer assembly 1 may include bushings 12 to electrically connect the inner and outer windings 3, 4 of the transformer assembly 1 .

It may be provided that the outer portion of the core 5 is not thermally connected to the cylindrical outer housing element 6, but is provided with a further cooling means to dissipate heat generated within the core 5.

Figure 4 shows a further embodiment of a transformer assembly 1 , wherein the core 5 forms a closed loop and has two core portions arranged in parallel, each of which is provided with transformer assembly parts. The core portions are connected with yoke portions of the core 5, so that yoke portions and core portions for the closed loop core 5.

Accordingly, the core portions are each encompassed with cylindrical inner support elements 2, as described above. The cylindrical inner support elements 2 are each provided with one single coil, so that each of the cylindrical inner support elements 2 is surrounded by the windings of the single coil. Substantially, each of the transformer assembly parts has a design which corresponds to that of the previously described embodiments. Thus, each of the windings is surrounded by a cylindrical outer housing element 6. In other words, in contrast to the embodiments described in conjunction with Figures 1 to 3, the embodiment of Figure 4 shows a single winding between the cylindrical inner support element 2 and the cylindrical outer housing element 6, which are coupled by a closed-loop core 5 to act as a transformer. In contrast to the previous embodiments, the embodiment of Figure 4 has cylindrical outer housing element 6 which are not provided with a protrusion. However, to avoid a short-circuit current tangentially flowing in the cylindrical outer housing element 6, an insulating means has to be provided which extends over the full axial length of the cylindrical outer housing element 6.

Both axial ends of the cylindrical outer housing elements 6 are closed with covers 1 1 , wherein a single cover 1 1 for each two axial ends of the two cylindrical outer housing elements 6 is provided.

The embodiment shown in Figure 5 substantially corresponds to an assembly as shown in Figure 4, with the difference that the two cylindrical outer housing elements 6 are replaced by a single outer casing element 13 which is formed to enclose both windings 3, 4 arranged on the inner support elements 2 for the two parallel core portions.

The cooling means for the cylindrical outer housing elements 6 can be formed as tubes integrated in the cylindrical outer housing element 6 or, as shown in Figure 6, in the form of separate cooling pipes 14 arranged between the outer surface of the windings 3, 4 arranged between the cylindrical outer housing element 6 and the cylindrical inner support element 2. The separate cooling pipes 14 may be attached to an inner wall of the cylindrical outer housing element 6.

Instead of having an insulating material in the winding enclosure formed between the cylindrical inner support element 2 and the outer housing element 6, 13, a fluid can be provided with a forced circulation in the interior of the winding enclosure.

Figures 7 to 12 show different views of a further embodiment of a transformer assembly 20. Figure 7 shows a perspective view onto the transformer assembly 20. The transformer assembly 20 has a cylindrical outer housing 26 which is closed on both ends thereof by a first cover 28 and a second cover 31 to form a tight enclosure 30 with an enclosed volume in the interior thereof. As can be further seen in the cross-sectional view of Figure 8, the transformer assembly 20 has an inner housing 21 which my serve as an inner support element for windings. In the present embodiment, the inner housing 21 is cylindrical and has a cylinder axis which is substantially parallel to the cylinder axis of the outer housing 26. Both the outer housing 26 and the inner housing 21 may have an annular cross- section. However, the outer housing 26 and the inner housing 21 may also be shaped with different cross-sections (across the axial direction thereof).

The cylindrical inner housing 21 and the cylindrical outer housing 26 may have a ring shape with an axial length which may be shorter or larger than a respective span across the cross-section of the inner housing 21 and the cylindrical outer housing 26.

The first and second covers 28, 31 , the inner housing 21 and the outer housing 26 may be made of an electrically insulating material, such as a non-metal material, epoxy or the like. Furthermore, the inner housing 21 , the outer housing 26 and the first cover 28 may be integrally formed of said insulating material.

Around the inner housing 21 , an inner winding 22 is wound. The inner winding 22 may contact an outer wall surface of the inner housing 21 or may be spaced therefrom. The conductors of the inner winding 22 can be wire-like, such as a coil of metal wire, e. g. copper wire, or plate-like, e.g. coated with an electrical insulation layer, and are spirally wound around the cylindrical inner housing 21 . The inner winding 22 may act as a primary or secondary winding of the transformer assembly 20.

Encompassing the inner winding 22, an outer winding 23 may be arranged. The outer winding 23 can be directly wound onto the inner winding 22. The conductors of the outer winding 23 can be wire-like, such as a coil of metal wire, e. g. copper wire, or plate-like, e.g. coated with an electrical insulation layer, and are spirally wound around the inner winding 22. The outer winding 23 may act as a primary or secondary winding of the transformer assembly 20. In order to ensure electrical insulation between the inner and outer windings 22, 23, it could also be provided that an insulation layer (not shown) is arranged between the inner winding 22 and the outer winding 23 to prevent a shortcut in case the electrical insulation coating of the winding conductors is damaged.

A transformer core 25 is provided which forms a closed loop. An inner portion of the transformer core 25 axially extends through the interior of the transformer assembly 20, i.e. through the interior of the cylindrical inner housing 21 , in the axial direction thereof. Another portion, i. e. an outer portion, of the transformer core 25 further extends around the exterior of the outer housing 26.

The transformer core 25 allows to direct the magnetic flux within the core 25. Substantially, the transformer core 25 can be made of a ferromagnetic material, e. g. ferrite, amorphous materials, nano-crystalline materials and the like. The transformer core 25 can have a shape to be partly encompassed by the cylindrical inner housingt 21 .

The inner and outer windings 22, 23 are accommodated in the enclosure 30 between the inner housing 21 and the outer housing 26. The enclosure 30 is filled with an electrically insulating fluid such as oil. The interior of the inner housing 21 in which the first cooling means and the magnetic core 25 are arranged may be void of the electrically insulating fluid such as the oil.

As can be seen in the cross-sectional view of Figure 8 and the perspective view of the cut-through housing of the transfer assembly 20 in Figure 9, a first cooling means and a second cooling means are provided. The first cooling means is arranged on an inner surface or in the wall of the outer housing 26 and the second cooling means is provided at least partly extending within the interior of the cylindrical inner housing 21 for cooling the portion of the transformer core 25 that passes through the cylindrical inner housing 21 . For example, on (or close to) an inner surface or in the wall of the outer housing 26, one or more first cooling pipes 29 or cooling channels as first cooling means may be arranged which at least partly extend through the outer housing 26 along an axial direction. In one embodiment, the first cooling pipes 29 are arranged meandering or in loops to provide a large surface for a thermal contact with the electrically insulating fluid within the enclosure 30. The first cooling pipes 29 are configured to allow a cooling medium to flow therethrough, so that heat transferred into the insulating fluid from the inner and outer windings 22, 23 may be consumed and dissipated via the first cooling pipes 29. The cooling medium can be air, water, oil, SF6 or the like.

The second cooling means is configured as one or more second cooling pipes 27 or cooling channels which are fed through the inner housing 21 in an axial direction. The second cooling pipes 27 are configured to allow a cooling medium to flow therethrough. The cooling medium can be air, water, oil, SF6 or the like. The inner volume with the inner portion of the transformer core 25 and the second cooling pipes 27 can be casted from a thermally conducting material to firstly fixate a relative position between the transformer core 25 and the inner housing 21 and to further thermally couple the second cooling pipes 27 to the transformer core 25 in order to dissipate heat therefrom.

As can be seen in Figure 9, for further support of the convection flow of the insulating liquid in the enclosure 30, an inner surface of the first cover 28 is provided with structuring forming channels 32 through which the insulating liquid is able to flow and to also reach parts of the inner winding 22. The liquid flow and to reach all parts of the windings with liquid is therefore supported by the formation of the liquid flow channels. A first set of the channels 32 may be directed in a radial direction with respect to the cylinder axis of the outer housing 26. A second set of the channels 32 may be directed at least partially in a tangential direction. For example as shown in Figure 9, the second set of the channels 32 form a square to allow a convection flow of the insulating fluid along a tangential direction. The second set of the channels 32 may also comprise a circular channel around the cylinder axis of the outer housing 26. Furthermore, the second cover 31 is arranged so that the enclosure 30 formed by the inner housing 21 , the first cover 28 and the outer housing 26 is tightly closed. As specifically shown in Figure 10, in order to allow a thermal expansion of the insulating liquid in the enclosure 30, a pressure membrane 38 is attached between the outer housing 26 and the second cover 31 . The pressure membrane 38 firstly tightens the enclosure 30 and, secondly, is configured to deform in case thermal pressure occurs in the insulating liquid in the enclosure 30. To allow a deformation of the pressure membrane 38, the second cover 31 may be provided with a cavity to accommodate the deformed pressure membrane 38. The second cover 31 can be attached to the outer housing 26 by means of screws 39 thereby fixing the pressure membrane 38 with its edges between the second cover 31 and the outer housing 26 to provide a tight sealing of the enclosure 30.

The outer housing 26 may include a first bushing arrangement 34 which may substantially be arranged opposite to the outer portion of the transformer core 25 extending along an outer surface of the outer housing 26. The first bushing arrangement 34 may be arranged at the outer housing 26 substantially in a central portion along the axial direction of the enclosure 30. The first bushing arrangement 34 is configured to electrically connect the inner or outer windings 22, 23 of the transformer assembly 20. As the outer housing 26 is made of an electrically insulating material, a strong electrical insulation between one or more terminals 35 of the first bushing arrangement 34 can be achieved.

Furthermore, a second bushing arrangement 40 is provided at the second cover 31 . The second bushing arrangement 40 has terminals 41 to electrically connect the inner or the outer windings 22, 23 of the transformer assembly 20. It is preferred that the terminals 41 of the second bushing 40 are tangentially displaced with respect to the terminals 35 of the first bushing arrangement 34 to provide a larger distance between the terminals 35, 41 . As can be seen from Figures 1 1 and 12, support plates 42 are provided to fixate the outer portion of the transformer core 25 at the first and second covers 28, 31 . The support plates 42 can be attached to the first and second covers 28, 31 , e. g. by means of screws 43, so that a support plate fin 44 of the respective support plate 42 is arranged radially extending along a lateral surface of the outer portion of the transformer core 25, respectively. The support plate fins 44 abut the outer portion of the transformer core 25 thereby fixating the relative position of the transformer core 25 in the transformer assembly 20. Moreover, the support plate fins 44 are made of an electrically conducting material to electrically contact the transformer core 25. The support plate fins 44 are further provided with an earthing boss 45 to connect to ground so that the transformer core 25 can be electrically grounded.

As can be seen in Figures 1 1 and 12, the first cooling means and second cooling means are connectable with cooling liquid terminals 36 to supply cooling liquid to the transformer assembly 20 and to circulate the cooling liquid through the first and second cooling pipes 29, 27. In the present embodiment, the first and second cooling pipes 29, 27 are connected in parallel via branching elements 37. The branching elements 37 are placed at the first cover 28 on both sides of a portion of the transformer core 25 which extends along an outer surface of first cover 28.

In an alternative embodiment, the first and second cooling pipes 29, 27 may be serially connected, so that one end/terminal of the first cooling pipe 29 is connected to an end/terminal of the second cooling pipe 27. One other end/terminal of the first cooling pipe 29 and one other end/terminal of the second cooling pipe 27 correspond to cooling liquid terminals 36.

Reference list transformer assembly

support element

inner winding

outer winding

core

housing element

cooling pipes

gap

further cooling pipes

protrusions

covers

bushings

housing element

separate cooling pipes

transformer assembly

inner housing

inner winding

outer winding

transformer core

outer housing

second cooling pipes

first cover

first cooling pipes

enclosure

second cover

first bushing arrangement

terminals cooling liquid terminal pressure membrane screws

second bushing arrangement terminals

support plate

screws

support plate fin

earthing boss

Claims

1 . Transformer assembly (20), comprising:

- a cylindrical inner housing (21 ) and a cylindrical outer housing (26) partially surrounding the cylindrical inner housing (21 ), the cylindrical inner housing (21 ) and the cylindrical outer housing (26) forming an enclosure (30), wherein the enclosed volume of the enclosure (30) between the cylindrical inner housing (21 ) and the cylindrical outer housing (26) is filled with an insulating liquid;

- at least one winding (22, 23) in the enclosed volume and

- a first cooling means arranged in or at an inner surface of the outer housing (26) to provide cooling for the at least one winding (22, 23).

2. Transformer assembly (20) according to claim 1 , wherein the inner housing (21 ) and the outer housing (26) are made of an electrically insulating material.

3. Transformer assembly (20) according to claim 1 or 2, wherein a portion of a transformer core (25) extends through the inner housing (21 ), wherein a second cooling means is at least partly located between the portion of the transformer core (25) and the inner housing (21 ).

4. Transformer assembly (20) according to any one of claims 1 to 3, wherein the transformer core (25) forms a closed loop, so that an outer portion of the transformer core (25) partly extends along an outer surface of the outer housing (26), wherein one or more support plates (42) are securely attached to the enclosure (30), wherein each of the one or more support plates (42) comprises a support plate fin (44) which abuts a lateral surface of the outer portion of the transformer core (25).

5. Transformer assembly (20) according to claim 4, wherein at least one of the support plates (42) is provided with an earthing connector.

6. Transformer assembly (20) according to any one of claims 1 to 5, wherein first and second covers (28, 31 ) are arranged at the axial ends of the enclosure (30).

7. Transformer assembly (20) according to claim 6, wherein the first cover (28), the inner housing (21 ) and the outer housing (26) are formed integrally.

8. Transformer assembly (20) according to claim 6 or 7, wherein an inner surface of the first cover (28) is provided with a structuring to form channels (32) for the insulating liquid to flow within the enclosure (30) in a radial and/or tangential direction due to convection.

9. Transformer assembly (1 ) according to any one of claims 6 to 8, wherein at one axial end of the enclosure (30) a pressure membrane (38) is arranged for flexibly adapting the volume of the insulating liquid within the enclosure (30).

10. Transformer assembly (1 ) according to any one of claims 1 to 9, wherein the first cooling means comprises a first cooling pipe (29) which extends along the inner surface of the outer housing (26) or within the wall of the outer housing (26) at least partly along the axial direction and circumference direction thereof, so that the first cooling pipe (29) forms at least one loop.