Subsea transformer Field of the invention

The invention relates to a subsea transformer comprising a transformer arranged inside a transformer tank. Background

Oil platforms are often used in offshore oil and gas production. More recently, processing facilities are being relocated to the ocean floor. Such subsea installations often re- quire electric power to operate. Electric power can be produced subsea or can be transported to the subsea installation from a topside installation, e.g. via an umbilical from an oil platform or ship, or via a subsea cable from an onshore site. Higher voltages are often used for transporting elec- trie energy from the topside installation to the subsea installation, in particular to limit losses. Accordingly, a transformation to a lower voltage at which subsea equipment is to be operated is required. On the other hand, a transformer may step up a voltage supplied by offshore power gen- erating devices, such as wind turbines, for the transmission to an onshore site. For this purpose, a subsea transformer can be provided at a subsea installation.

The subsea installation may be located at a great water depth, for example more that 1000, more than 2000 or even more than 3000 meters deep. The high pressures that prevail at such depths of water can be dealt with by providing a pressure resistant enclosure for the transformer, the inside of which is kept at a close to atmospheric pressure. The problem with such enclosures is that they are very bulky and heavy, as they need to withstand pressure differences up to 300 bar. To overcome such problems, the transformer may be provided in an enclosure that is pressure compensated, in particular in which the inside pressure is equalized to the outside pressure. The pressure difference can thus be kept small, ena- 5 bling the use of a lighter enclosure having thinner walls.

Although this greatly facilitates the design of the enclosure, a number of problems remain.

To ensure a safe operation of the transformer, the transit) former enclosure needs to be tightly sealed against the ambient sea water. In particular, a proper sealing of the enclosure means that providing pressure compensation may be difficult. Furthermore, the transformer needs to be contacted electrically. The electric connections through the enclosure 15 to the transformer are again difficult to seal.

Enclosed volumes that are additionally provided for sealing either need to be provided with thick walls that are capable of withstanding the high outside pressures, or also need to

20 be pressure compensated. Accordingly, pressure compensation of the transformer enclosure is difficult to implement, it can be complex in configuration and can require a considerable amount of space, making the transformer enclosure bulkier. Further, due to the technical complexity, the pressure

25 compensation can be cost intensive to implement. Each pressure compensation furthermore requires sealing which is a potential weak point regarding the water-tightness of the subsea transformer enclosure.

30 Assembly of the subsea transformer is thus complex and time intensive. It is desirable to facilitate the assembly and to reduce the number of weak points in the sealing of the subsea transformer to prevent the ingress of surrounding sea water. A compact and cost efficient design of the subsea transformer

35 is further desirable. Summary

Accordingly, there is a need for an improved subsea transformer, which in particular provides good sealing while fa- cilitating assembly thereof.

This need is met by features of the independent claims. The dependent claims describe embodiments of the invention. An embodiment provides a subsea transformer comprising a transformer and a transformer tank adapted to accommodate the transformer. The transformer tank has an opening which is sized so as to enable the insertion of the transformer into the transformer tank through the opening. The transformer is arranged inside the transformer tank. The subsea transformer further comprises a closing plate adapted to close the opening of the transformer tank, and at least one component mounted to the closing plate and having a double barrier against the ingress of an ambient medium surrounding the sub- sea transformer when installed subsea, such as sea water. A fluid tight seal between the transformer tank and the closing plate is further provided, the fluid tight seal being formed by welding the closing plate to the transformer tank so as to close the opening in the transformer tank.

By welding the closing plate to the transformer tank, a good seal can be provided which makes it unnecessary to implement a double barrier at this joint. Furthermore, since the closing plate comprises the component having a double barrier, assembly of the subsea transformer is facilitated, since the transformer may be sealed inside the transformer tank by a single mounting step, in particular by welding the closing plate to the transformer tank. In an embodiment, the transformer tank may itself be a welded steal tank, and may as such only provide a single barrier against the ambient medium. The closing plate may be a top plate, it may for example be positioned onto an opening ar- ranged on an upper side of the transformer tank, i.e. it may be placed on top of the transformer tank.

In an embodiment, the fluid tight seal comprises a circumfer- entially continuous weld around the opening, the weld being formed with a flange of the transformer tank surrounding the opening and a peripheral edge of the closing plate. The peripheral edge may be any portion of the closing plate that is adjacent to the contact area between the closing plate and the transformer tank. The fluid tight seal between the transformer tank and the closing plate can thus be provided in an efficient matter.

The fluid tight seal of the opening may further comprise an 0-ring seal located inwards as seen from the welding. Inwards in this respect means towards the inside of the transformer tank. The additional 0-ring seal may be useful during the assembly of the subsea transformer or may provide an additional barrier against the ingress of the ambient medium.

In an embodiment, all components of the subsea transformer having a double barrier sealing against the ambient medium are mounted to the closing plate. Accordingly, the transformer tank may not require any further openings or penetra- tions other than the opening that is closed by the closing plate. The number of weak points in the sealing of the subsea transformer may thus be reduced. Furthermore, assembly of the subsea transformer can be facilitated, as the top plate with all double barrier sealings may be preassembled and may then be mounted to the transformer tank and sealed by the welding. In an embodiment, the transformer tank may not comprise any further openings other than the opening that is being closed by the closing plate. In an embodiment, the at least one component mounted to the closing plate comprises an electric through connection enabling an electric contacting of the transformer from outside the subsea transformer and/or a pressure compensation device _.

adapted to balance the pressure inside the transformer tank with the pressure of the ambient medium surrounding the sub- sea transformer when installed subsea. The transformer enclosure, formed by the transformer tank and the closing plate, may for example be a pressure compensated enclosure which is filled with a dielectric medium. The pressure of the dielectric medium may be adjusted to the pressure prevailing outside the subsea transformer by means of the pressure compensation device. The transformer tank can thus be relatively thin walled, resulting in a compact subsea transformer. By providing the electric through connections to the transformer through the closing plate, no penetrations through the transformer tank are necessary for the electric connections.

The at least one component mounted to the closing plate may comprise a pressure compensation device having a first flexible element providing a fluid tight seal between the ambient medium and an intermediate volume and a second flexible element providing a fluid tight seal between the intermediate volume and the inside of the transformer tank. By means of the two flexible elements, a pressure compensation with a double barrier can be provided. If the outer barrier, i.e. the first flexible element fails and passes sea water, the sea water will only reach the intermediate volume and will not get into the transformer tank and thus into contact with the transformer.

Both the first and the second flexible element may be mounted to an outer side of the closing plate, i.e. a side of the closing plate facing away from the inside of the transformer tank. The second flexible element may be mounted inside a volume enclosed by the first flexible element. The intermediate volume may thus be confined between the first and the second flexible elements and the outer side of the closing plate. A simple but effective configuration for providing a double barrier protection against the ingress of the ambient medium, in particular sea water, can thus be achieved for the pressure compensation device. The first and the second flexible elements may for example be provided by a first and a second bellow, respectively. The second bellow may be arranged within the first bellow, thus providing the above described configuration.

The pressure compensation device may comprise a first and a second pressure compensator each comprising the respective first or second flexible element, the second pressure compen- sator being mounted to an outer side of the closing plate.

The second pressure compensator may enclose a volume with the closing plate, and the closing plate may have a fluid connection between the volume enclosed by the second pressure compensator and the interior of the transformer tank. In par- ticular, the closing plate may have a flow channel or a flow tube which provides the fluid connection. If the volume of a medium comprised in the transformer tank becomes larger or smaller, e.g. due to temperature changes, the medium can flow through the fluid connection into or out of the volume en- closed by the second pressure compensator, thereby keeping the pressure inside the transformer tank constant. The pressure compensator may thus also be termed volume compensator.

In an embodiment, the subsea transformer may comprise two of the above mentioned pressure compensation devices mounted distant to each other to the closing plate. It may for example comprise two double bellows, with one of the bellows enclosing a volume which is in fluid communication with the interior of the transformer tank and the intermediate volume being enclosed between the two bellows (and possibly the closing plate) . Volume changes of the medium in the transformer tank can thus be compensated, while the overall design of the subsea transformer can be kept compact with such arrangement .

The transformer tank and the pressure compensator may be filled with a medium, in particular with dielectric liquid. In a further embodiment, the at least one component mounted to the closing plate comprises at least one electric through connection enabling an electric contacting of the transformer from outside the subsea transformer. The electric through 5 connection can comprise a bushing chamber mounted to the

closing plate, the bushing chamber having a liquid tight seal to the inside of the transformer tank and to the outside of the subsea transformer. The bushing chamber may comprise at least one electric connection to the inside of the transit) former tank and at least one electric connection to the outside of the subsea transformer. A conductor inside the bushing chamber may electrically couple the two electric connections. The bushing chamber may be mounted to an outer side of the closing plate. By means of the bushing chamber, a double 15 barrier for the electric through connection to the transformer can be provided at the closing plate. In particular, since both the bushing chamber and a pressure compensation device can be provided on the closing plate, assembly of the subsea transformer is facilitated.

20

The bushing chamber may be pressure compensated against the intermediate volume of the pressure compensation device. The bushing chamber can enclose a fist volume and the pressure compensation device can enclose a second volume (different to 25 the first volume) , and pressure compensation can be provided between these two different volumes.

As an example, the closing plate may comprise a fluid connection between the volume enclosed by the bushing chamber and

30 the intermediate volume. In particular, a flow channel or

flow tube may be provided in the closing plate. As mentioned above, the intermediate volume can be the volume enclosed between a first and a second pressure compensator and the closing plate, for example the volume enclosed between two bel-

35 lows that are placed one within the other and both of which are mounted to the closing plate. The fluid connection can then reach from the intermediate volume to the volume enclosed by the bushing chamber to provide pressure balancing. Other possibilities of providing the pressure balancing are also conceivable, such as providing a movable element or a flexible element in such fluid connection. The electric connection to the inside of the transformer tank may be provided by a transformer bushing. The electric connection to the outside of the subsea transformer may be provided by a penetrator. In an embodiment, the subsea transformer comprises two bushing chambers, one of which provides electric through connections to a primary side of the transformer, and the other of which provides electric through connections to a secondary side of the transformer. Depending on whether the transformer is a step-up or a step-down transformer, the voltage on the secondary side of the transformer may be higher or lower than the voltage supplied to the primary side of the transformer. By providing two bushing chambers, the higher voltage electric through connections can be kept separate from the lower voltage through connections. The configuration of the subsea transformer may thus be simplified, as the bushing chamber comprising low voltage electric connections does not need to be configured for high voltages. Further, the risk of applying a high voltage to a low voltage connection in case of a failure can be reduced.

The transformer tank may be shaped so as to accommodate the transformer and to minimize the overall volume of the transformer tank, i.e. it can be adapted to the size of the par- ticular transformer. It may have a bottom plate, and side walls extending orthogonally to the bottom plate, so that the transformer tank is open at its upper side. At the opening of the transformer tank, a flange may be provided which is continuous in circumferential direction of the opening. The closing plate may be a top plate which is placed on top of the transformer tank for closing the opening. Both in the flange of the transformer tank and the top plate, through holes may be provided for bolting the top plate to the flange of the transformer tank. The fluid tight seal may be provided by welding the edge of the top plate abutting the flange of the transformer tank to the flange of the transformer tank. The transformer tank may be provided with cooling ribs. In particular, it may have elongated projections on inner walls of the transformer tank, and/or it may have elongate projections on the outer walls of the transformer tank. When the transformer tank is filled with a dielectric liquid, heat produced by the transformer can be transported efficiently through the wall of the transformer tank and from there into the ambient medium, in particular sea water. As an example, the elongate projections may be cooling ribs extending from a bottom plate of the transformer tank to the top plate mounted to the transformer tank.

The transformer tank, the pressure compensation device and/or the bushing chamber can be filled with dielectric liquid. The features of the embodiments of the invention mentioned above end those yet to be explained below can be combined with each other unless noted to the contrary.

Brief description of the drawings

The foregoing and other features and advantages of the invention will become further apparent from the following detailed description read in conjunction with the accompanying drawings. In the drawings, like reference numerals refer to like elements.

Figure 1 is a schematic diagram illustrating a subsea transformer in accordance with an embodiment of the invention. Figure 2 is a sectional side view of a subsea transformer according to a further embodiment of the invention.

Figure 3 is a top view of the subsea transformer of figure 2. Figure 4 is a perspective view of the subsea transformer of figures 2 and 3. Figure 5 is a sectional side view illustrating the sealing of a penetrator to a wall of the bushing chamber in accordance with an embodiment.

Figure 6 is a sectional side view of the sealing of a pene- trator to a wall of the bushing chamber in accordance with another embodiment .

Detailed description In the following, the embodiments illustrated in the accompanying drawings are described in more detail. It should be clear that the following description is only illustrative and non restrictive. The drawings are only schematic representations, and elements in the drawings are not necessarily to scale with each other.

Figure 1 illustrates a subsea transformer 10 including a subsea transformer enclosure 11 and a transformer 12 arranged in a transformer chamber 14 of the subsea transformer enclosure 11. The subsea transformer enclosure 11 comprises a transformer tank 15 which defines the transformer chamber 14 and which has an opening at its upper side. The opening is closed by the closing plate 16. A first component 20 and a second component 30 are mounted to closing plate 16,, each having a double barrier against the ingress of sea water surrounding the subsea transformer 10 when it is installed subsea.

Transformer 12 may for example be a step down transformer, which transforms a higher voltage supplied from a topside in- stallation, e.g. a marine vessel or an onshore location, by an umbilical or a subsea cable into a lower voltage, which is then supplied to components of the subsea installation of which subsea transformer 10 forms part. In other configura- tions, the subsea transformer may be used to combine electric energy collected from different offshore energy generating devices, such as wind turbines, and may transform it to a voltage suitable for transmission over a subsea cable to an onshore site or via an umbilical to a topside installation.

As an example, the transformer 12 which is housed in the subsea transformer enclosure 11 can be adapted to convert the subsea cable step out transmission voltage to the subsea switch gear distribution voltage.

Subsea transformer 10 and thus subsea transformer enclosure 11 are adapted to be deployed and operated at depth of water of more than 1000, more than 2000 or even more than 3000 me- ters . In particular, the subsea transformer enclosure 11, i.e. the transformer tank 15 and the closing plate 16 can be adapted to be operable in a depth range of 1000 to 3000 meters . For this purpose, the transformer chamber 14 is filled with dielectric liquid. The dielectric liquid provides electric isolation and furthermore transports heat from transformer 12 to the walls of the subsea transformer enclosure 11. The component 20 mounted to transformer chamber 14 is a pressure compensation device. The pressure compensation device 20 comprises a first flexible element 21 realizing a first pressure compensator and a second flexible element 22 realizing a second pressure compensator, with an intermediate volume 23 being bordered or confined by the first and second pressure compensators. The intermediate volume 23 is also filled with dielectric liquid.

The flexible elements 21, 22 are in figure 1 schematically indicated by the dashed lines. The flexible element of each pressure compensator provides a pressure balancing, in particular a pressure equalization between the media which it borders. The flexible element may for example be a membrane, a bladder, a bellow or the like. The first pressure compensator provides pressure balancing between the ambient medium (e.g. sea water) surrounding the subsea transformer 10 and the intermediate volume 23, while the second pressure compen- sator provides pressure balancing between the intermediate volume 23 and the inside of transformer chamber 14, which is in fluid communication with the second pressure compensator

22 via the fluid connection 25, here an opening in the closing plate 16. Furthermore, the first flexible element 21 pro- vides a fluid tight seal between the ambient medium and the intermediate volume 23, and the second flexible element 22 provides a fluid tight seal between the intermediate volume

23 and the inside of the transformer chamber 14. Filling the intermediate volume 23 and the transformer chamber 14 with dielectric liquid has the advantage that if the pressure outside the subsea transformer enclosure 11 is increased, only small volume changes of the dielectric liquid confined in the transformer chamber 14 or in the intermediate volume 23 result. The flexible elements of the pressure compensators transmit the increased outside pressure to the intermediate volume 23 and to the transformer chamber 14, respectively, as the flexible elements 21, 22 can be deformed and can thus compensate for volume changes. The pressure com- pensators may thus also be termed volume compensators. Note that if the transformer chamber 14 was filled with a gas, an increase of the ambient pressure by a factor of 2 would lead to a volume reduction of factor 2, which is difficult to compensate for by a pressure compensator.

Furthermore, the pressure compensators 21, 22 also compensate for changes of a volume of the dielectric liquid which may be caused by temperature changes. This can be of particular importance during transport of the subsea transformer 10. With- out the flexible element of the respective pressure compensator, a change in temperature would lead to a change of volume, which may result in a drastic increase in pressure inside the transformer chamber 14, for example if the dielec- trie liquid expands. By means of the flexible element of the pressure compensator, such volume expansion or reduction can be allowed, while keeping the pressure inside the transformer chamber 14 balanced to the outside pressure.

It should be noted that "pressure balance" does not necessarily mean that the pressure inside the transformer chamber 14 and the pressure outside the subsea transformer enclosure 11 are exactly the same. A small pressure difference may be maintained, for example by biasing preloading the flexible element 21 and/or 22 of the pressure compensation device. This is advantageous as for example if a small overpressure is maintained in transformer chamber 14, the ingress of e.g. sea water from the surroundings of the subsea transformer en- closure 11 can be prevented, as dielectric liquid will flow out through the leak due to the overpressure. A biasing can for example be achieved by making use of a weight placed on top of a bellow, by biasing a bellow by means of a spring, by biasing a membrane or the like.

Pressure compensators 21, 22 thus compensate for volume changes which would otherwise result in pressure imbalances between the inside of transformer chamber 14 and the outside of subsea transformer 10.

Providing the pressure compensation device 20 enables the use of a transformer tank 15 having relatively thin walls.

Although due to the pressure balancing, the pressure differ- ence between the medium inside transformer chamber 14 and outside of the subsea transformer 10 is relatively small, a reliable sealing of the transformer chamber 14 still has to be ensured, in particular in view of the long service life that is required for a subsea transformer. In the embodiment of figure 1, a fluid tight seal 18 is thus provided between the transformer tank 15 and the closing plate 16. The fluid tight seal 18 is formed by welding the closing plate 16 to the transformer tank 15, so that the opening on the upper side of transformer tank 15 is closed.

The sectional side view of figure 1 only schematically illus- trates that a peripheral edge of the closing plate 16 is welded to an upper part of the transformer tank 15 which surrounds the opening. The implementation of the fluid tight seal 18, in particular the position at which the closing plate 16 is welded to transformer tank 15 can be different from the schematic illustration of figure 1 and will be chosen in accordance with the particular layout of subsea transformer 10. As an example, the closing plate 16 may have a flange or a protrusion extending outward or downward which may be welded to an upper rim of the transformer tank 15, or the transformer tank 15 may be provided with a flange surrounding the opening, towards which closing plate 16 is welded, as for example illustrated in figure 2.

By providing the fluid tight seal 18 in form of a circumfer- entially continuous weld around the opening in the transformer tank, a very good sealing can be achieved so that the fluid tight seal 18 can be the only barrier against the ingress of sea water, i.e. so that only a single barrier is required at the joint between the closing plate 16 and the transformer tank 15. Even so, an additional seal may be provided, which may for example be beneficial during the assembly of the subsea transformer enclosure 11, e.g. for providing a preliminary sealing when dielectric liquid is filled into the transformer chamber 14.

For electrically contacting transformer 12, electric through connections are provided which reach from outside the subsea transformer 10 into the transformer chamber 14. The electric through connections are implemented by a bushing chamber 30 mounted to the closing plate (or top plate) 16, electric connections 31 from the bushing chamber 30 to the inside of transformer chamber 14 and electric connections 32 from the bushing chamber 30 to the outside of the subsea transformer 10. Electric connections 32 penetrate an outer wall of the bushing chamber 30 while electric connections 31 penetrate the closing plate 16. Bushing chamber 30 can be welded to the closing plate 16, and can be filled with a dielectric liquid. Reference numeral 34 indicates a sealing element which may be provided by a flange comprising a gasket, such as an 0-ring or the like, and/or by welding. By such implementation of the electric through connections for contacting the transformer 12, a reliable double barrier sealing can be achieved. Water entering the bushing chamber 30 in case of a failure will not reach the transformer chamber 14.

Similarly, the connections between a wall of one of the pressure compensators and the transformer chamber 14 can be welded. Sealings are provided at the mounting positions of the flexible elements 21, 22. The first flexible element 21 provides a first seal against the ambient medium, while the second flexible element 22 provides a second seal between the intermediate volume and the transformer chamber 14, so that transformer chamber 14 is protected by a double barrier against ambient sea water. Similarly, the sealings of the electric connections 32 provide a first barrier, and the sealings of the electric connections 31 provide a second barrier for the transformer chamber 14. The configuration illustrated in figure 1 thus achieves a double barrier sealing for the transformer chamber 14, while only requiring a simple pressure compensator arrangement.

As can be seen, all components 20, 30 that require some type of access into transformer chamber 14 (for the electrical connections or for pressure balancing) are provided with double barriers and are mounted to the closing plate 16. Accordingly, no further openings or feed-through connections are provided in the transformer tank 15 (other than the opening which is closed by closing plate 16) . Transformer tank 15 may thus have a relatively simple configuration, it may be a welded steel tank comprising a bottom plate and side walls. Since all joints of transformer tank 15 are welded, the sin- gle barrier formed by transformer tank 15 provides sufficient protection against the ingress of sea water, even when subsea transformer 10 is deployed at great depths. Since the components having double barriers are mounted to the closing plate 16, and may thus be preassembled, the assembly of subsea transformer 10 is facilitated. Assembly may be performed by placing transformer 12 into the transformer tank 15, electrically contacting the transformer 12 (by means of electric connections 31) , placing the closing plate 16 on the transformer tank 15 and providing the fluid tight seal 18 by welding the closing plate 16 to transformer tank 15. Dielectric liquid can be filled into the transformer chamber 14 before closing transformer tank 15.

When installed subsea, a sea cable (or umbilical) connection may be terminated at the electrical connections 32. In particular, a jumper cable may be connected to electric connections 32 and may on its other side be connected to an umbili- cal termination assembly (UTA) . The umbilical termination assembly may terminate an umbilical which connects to a topside installation located on a vessel or at an onshore site. For a long lifetime, the jumper cable may be connected via dry mate connectors to the electric connections 32. Inside bushing chamber 30, an electric conductor connects the electric connections 31 and 32, which is explained further below in more detail with respect to figure 2.

The volume enclosed by bushing chamber 30 is filled with di- electric liquid. The inner volume of bushing chamber 30 is pressure balanced against the intermediate volume 23. In the example of figure 1, a fluid connection 40 is provided between the bushing chamber 30 and the intermediate volume 23. Fluid connection 40 enables the dielectric liquid to flow be- tween the bushing chamber 30 and the intermediate volume 23. Thus, by means of the first pressure compensator 21, the interior of the bushing chamber 30 is pressure balanced against the ambient medium surrounding the subsea transformer 10. Note that in other embodiments, other means for pressure balancing between the bushing chamber 30 and the intermediate volume 23 may be provided. As an example, pressure balancing may occur by means of a movable piston, a flexible element such as a bladder, a membrane, a bellow or the like.

By making use of only two pressure compensators, a double barrier pressure compensation for the inside of transformer chamber 14 is achieved, and a pressure compensation for the inside of bushing chamber 30 is achieved.

Note that the arrangement of the pressure compensators 21, 22 is only schematically illustrated in figure 1, so that the general function and purpose of the pressure compensation device 20 becomes clear. As mentioned above, the pressure compensators 21, 22 can be configured in a variety of different ways, they may be arranged distant to one another, or one within the other, and may comprise different means for pres- sure compensation.

Figure 2 shows a particular implementation of the subsea transformer 10 described above with respect to figure 1. Accordingly, the explanations given above are similarly appli- cable to the subsea transformer 10 of figure 2 and described hereinafter .

In the example of figure 2, the transformer tank 15 comprises a flange portion at its upper rim which extends radially out- wards. The flange portion abuts the closing plate 16, and the fluid tight seal 18 is provided by a weld between the flange portion and the outer peripheral edge of the closing plate 16. Further, the joint between closing plate 16 and transformer tank 15 comprises an additional seal 17 located in- wards with respect to the fluid tight seal 18. The additional seal 17 is implemented as an 0-ring seal, in which a resilient sealing ring is disposed in a recess in the closing plate 16 and is compressed between the closing plate 16 and the flange portion. The additional seal 17 can be beneficial when mounting the closing plate 16 to the transformer tank 15, e.g. for providing a preliminary sealing of the transformer chamber 14.

The transformer tank 15 is again provided by a single completely welded wall which is adapted to effectively transport the heat losses of the transformer to the ambient sea water. The closing plate 16 can be a steel plate. Note that for the purpose of a comprehensive presentation, only the subsea transformer enclosure 11 is shown in figures 2-4, although it should be clear that in an assembled state, a transformer 12 will be located in the transformer chamber 14 and be contacted via the electric connections 31.

In the example of figure 2, the pressure compensation device 20 comprises flexible elements 21, 22 in form of two bellows, wherein the second bellow (second flexible element 22) is placed within a first bellow (first flexible element 21) . The intermediate volume 23 is thus confined between the two bellows 21, 22. The fluid connection 40 between the interior of the bushing chamber 30 and the intermediate volume 23 is provided by a flow channel. These so called "compensation tubes" allow the dielectric liquid to flow between the intermediate volume 23 and the inside of the bushing chamber 30. Fluid connection 25 between the interior of the second bellow 22 and the inside of transformer chamber 14 is similarly provided by a compensation tube. For the purpose of redundancy and for achieving a compact configuration, a first and a second pressure compensation device 20 are mounted to the closing plate 16, each having two bellows 21, 22, one placed within the other. If one pressure compensation device 20 fails, the other can still provide pressure compensation. Further, each pressure compensation device can be smaller than one large volume pressure compensation device, making each pressure compensation device less prone to damage . The external electric power connections from and to the transformer (in particular to the primary and secondary side of the transformer) are grouped into two separate dielectric liquid filled bushing chambers 30. In the side view of figure 2, only one bushing chamber 30 is visible. The second bushing chamber can be seen in figure 3, showing a top view of the subsea transformer enclosure 11 of figure 2. The bushing chamber 30 is provided in form of a bushing box. The bushing box acts as a double barrier to the ambient sea water as seen from the inside of transformer chamber 14. The electric connections 31 to the inside of the transformer chamber 14 are provided in form of transformer bushings. They penetrate the lower wall of the bushing box 30 (i.e. the closing plate 16) and provide an electric through connection for an electric conductor (not shown in figure 2; see figures 5 and 6) . The electric connections 32 to the outside of the subsea transformer enclosure 11 are provided by penetrators, which may be high voltage penetrators, and which can attach to a jumper cable by means of dry mate connectors. In other configurations, the penetrators may be medium voltage penetrators. A high voltage may be a voltage in a range between about 50 kV and about 200 kV. A medium voltage may be a voltage in a range between about 1.000 V and about 50 kV. On the primary side of the transformer, high voltage penetra- tors may be used while on the secondary side, medium voltage penetrators may be used.

An electric conductor 33 is provided between the transformer bushings 31 and the penetrator 32. The conductor 33 and/or the transformer bushing 31 and/or the penetrator 32 may be provided with boot seals. In case of leakage of sea water into the bushing box 30, the boot seals can provide protection from water. This can prevent that a flashover occurs in the case of water leakage into the bushing box. The boot seals prevent the water from coming into contact with the conductors . Another possibility to improve the protection of the electric connections from sea water is the use of double barriers in each penetration of a wall of bushing chamber 30. This is illustrated in figures 5 and 6. In the example of figure 5, the penetrator 62 is provided with an axial sealing 68 and a radial sealing 67. The wall 61 of the bushing chamber comprises a penetrator adaptation 63, against which the penetrator 62 is sealed. The seals 67, 68 may for example be provided in form of 0-rings. Reference nu- meral 65 designates the electric connection towards the sea cable, or towards the conductor 33 in the bushing chamber, while reference numeral 66 designates the electric connection to the inside of the bushing chamber, or to the inside of the transformer housing, respectively. Each of the electric con- nections 31, 32 can be configured similarly to the electric connection 60 shown in figure 5. Penetrator 62 may be made of a resin or a plastic material and provides electric isolation between the wall 61 of the bushing chamber and the electric conductor (indicated by reference numerals 69 and 66) in the penetrator.

Figure 6 illustrates another possibility of implementing the electric connections 31, 32 with a double barrier. In the configuration of figure 6, the electric connection 60 com- prises a radial sealing 67 in form of an 0-ring. Furthermore, a radial edge 64 of the penetrator 62 is metalized. The metalized penetrator edge 64 is welded to the penetrator adaptation 63 (indicated by reference numeral 69) . This configuration provides a liquid tight seal and a double barrier. Again, the configuration of figure 6 may be used in any of the electric connections 31, 32. Figure 3 shows a top view of the subsea transformer enclosure 11 of figure 2. The two pressure compensation devices 20 are mounted to the closing plate 16. Furthermore, the two bushing boxes 30 can be seen, with the electric connections 32 to- wards which a sea cable or umbilical can be connected. From each bushing chamber 30, a fluid connection can be provided to the intermediate volume of either one of the pressure compensation devices 20, or to the intermediate volume of both pressure compensation devices 20. It is also possible to pro- vide from one bushing chamber 30 a fluid connection to one pressure compensation device 20, and to provide a fluid connection from the other bushing chamber 30 to the other pressure compensation device 20. If water enters one of the bushing chambers or pressure compensation devices, the others will remain unaffected. On the other hand, if connections to both pressure compensation devices 20 are provided, pressure compensation continues even if one of the pressure compensation devices fails. Figure 4 shows a perspective view of the subsea transformer enclosure 11 of figures 2 and 3. The transformer tank 15 is provided with cooling ribs inside and outside to ensure that heat losses of the transformer are effectively transported to the sea water. The transformer tank 15 can be relatively thin walled, as the differential pressure between inside the transformer chamber 14 and the surrounding medium is kept minimal due to the pressure compensation. As mentioned above, an internal overpressure may result from biasing preloading the pressure compensators, and the transformer tank 15 only needs to be constructed to withstand the maximum internal overpressure. The overpressure ensures that an accidental leakage in a system will not result in any water inside the transformer chamber. The dielectric liquid will, to a limit, leak out into the sea water. This way, it is also possible to detect a leakage before a large failure occurs, like a short circuit inside the transformer. As can be seen, the closing plate 16 comprises the fluid connections 25 between the transformer chamber and the pressure compensation device 20 (in particular the inside of the second bellow 22) . Reference numeral 26 indicates the mounting position of the double bellow pressure compensation device. Bushing boxes 30 have a flange by means of which they are mounted to the closing plate 16. The bushing box may furthermore be welded to the closing plate 16. The transformer tank 15 can be shaped so that the volume of dielectric liquid can be kept to a minimum, i.e. the shape can be adapted to the shape of the transformer which it houses. The transformer tank 15 and the top plate 16 can be made of carbon steel. The pressure compensation devices may be made of stainless steel, or a comparable material, such as Inconel® 625.

In the example of figure 4, the flange at the upper end of the transformer tank 15 and the peripheral edge of closing plate 16 are provided with mating through holes. These may be used to initially bolt the closing plate 16 to transformer tank 15, before the fluid tight seal 18 is provided by welding . In summary, the above outlined configurations provide a sub- sea transformer that is easy to assemble and in which the sealing against ambient sea water is improved. A closing plate is provided on which components requiring a penetration into the transformer chamber are mounted, resulting in a sim- plified assembly. As the closing plate is welded to the transformer tank, there is no need for a double barrier at this joint. A secure sealing having a long operating life can thus be achieved for the subsea transformer.