Power transformer

Abstract

Power transformer A power transformer (1) comprising a plurality of active components (11) immersed in a fluid (3), a tank (51 ), and an expansion vessel (52) connected to the tank (51) is specified, wherein a volume (2) in the expansion vessel (52) comprises a porous filler material (4) configured to dampen a sloshing motion of the fluid (3) within the volume (2).

Description

[0001] P2025,0053 WO N / P240264WO001 March 24, 2026

[0002] 1

[0003] Description

[0004] Power Transformer

[0005] The present disclosure relates to a power transformer.

[0006] Active components of a power transformer are often immersed in a fluid. Under operating conditions where the power transformer is subjected to dynamic motions, however, sloshing of the fluid may lead to mechanical damage.

[0007] Embodiments of the disclosure relate to a way to improve the reliability of power transformers.

[0008] According to an embodiment, a power transformer comprises a plurality of active components immersed in a fluid, a tank and an expansion vessel connected to the tank wherein a volume of the expansion vessel comprises a porous filler material configured to dampen a sloshing motion of the fluid within the volume.

[0009] In particular, the expansion vessel comprises a gas-filled portion and a fluid-filled portion separated from the gas-filled portion, and the porous filler material occupies at least 5% or at least 10% or at least 20% or at least 40% of the fluid- filled portion of the expansion vessel. This volume occupation helps to efficiently suppress sloshing motions of the fluid. In particular, the porous filler material may be configured to impede bulk fluid displacement while permitting fluid flow through the material, such that sloshing - induced fluid motion within the fluid- filled portion is reduced during operational movement of the expansion vessel.P2025,0053 WO N / P240264WO001 March 24, 2026

[0010] 2

[0011] The expansion vessel may be configured to compensate for a temperature - induced volume change of the fluid.

[0012] The porous filler material may dampen the kinetic energy of the fluid, while allowing its flow to provide a temperature-induced volume compensation of the fluid.

[0013] This may help to reduce the mechanical stress on the components of the power transformer. Further, the porous filler material may act as a barrier to fire propagation or help to suppress explosions.

[0014] The porous filler material may be used in connection with any type of expansion vessels. For example, the expansion vessel may be arranged on top of the tank.

[0015] For example, the expansion vessel may have a cylindrical shape mounted to the tank. Alternatively, the expansion vessel may have a cuboid shape. For example, the expansion vessel may cover at least 20% or at least 30% or at least 50% of the tank in a top view onto the tank. This approach may help to reduce the height of the expansion vessel and of the power transformer.

[0016] The porous filler material may be any solid material that comprises voids, cells, or other open spaces that are filled or fillable with gaseous or liquid matter. During operation of the power transformer, an exchange between the voids of the porous filler material and the adjoining gaseous or liquid matter may occur. Alternatively, there is no or at least no significant exchange between the voids of the porous filler material and the adjoining gaseous or liquid matter.P2025,0053 WO N / P240264WO001 March 24, 2026

[0017] 3

[0018] For example, the porous filler material 4 has a porosity in a range from 20% to 95% of its volume. The porosity is the fraction of the volume of the voids over the total volume of the porous material. If the porous filler material is compressible, the volume of the porous material refers to a state where the porous filler material is not compressed.

[0019] As the porous filler material located in the expansion vessel is spaced apart from the active components of the power transformer, the material requirements with respect to electrical isolation properties and / or high temperature stability are reduced. Thus, the material of the porous filler material may be selected based on other criteria such as mechanical properties like elasticity, compressibility, or density and / or chemical stability against the liquid.

[0020] For example, the fluid may comprise a liquid such as an oil like a mineral oil, a synthetic oil, an ester or any other electrically insulating liquid capable of withstanding the conditions during operation of the power transformer.

[0021] According to a further embodiment, the expansion vessel comprises a gas - filled portion and a fluid- filled portion separated from the gas - filled portion. For example, the expansion vessel may comprise a separator separating the gas-filled portion from the fluid- filled portion.

[0022] For example, the separator extends horizontally in space so that the air- filled portion of the expansion vessel is located above the fluid- filled portion during operation of the power transformer.P2025,0053 WO N / P240264WO001 March 24, 2026

[0023] 4

[0024] For example, the separator may be a membrane that may be connected to sidewalls of the expansion vessel or a shell of a gas bag configured to receive the gas of the gas - filled portion.

[0025] Alternatively, the fluid- filled portion and the gas - filled portion may be arranged in the expansion vessel laterally side by side.

[0026] According to a further embodiment, at least a portion of the porous filler material is located in the fluid- filled portion of the expansion vessel. Thus, the fluid may be in direct contact with the porous filler material.

[0027] According to a further embodiment, at least a portion of the porous filler material is in contact with the separator. For example, the porous filler material is attached to the separator. Alternatively, the porous filler material may be configured to float freely underneath and in contact with the separator.

[0028] According to a further embodiment, the porous filler material completely occupies the fluid- filled portion of the expansion vessel. Thus, the entire fluid located within the expansion vessel is arranged within pores or cells of the porous filler material. Sloshing motions of the fluid may be efficiently suppressed in this way.

[0029] According to a further embodiment, the porous filler material is configured to exhibit an anisotropic permeability, so that the permeability parallel to a dominant fluid motion within the expansion vessel is larger than in a direction perpendicular to the dominant fluid motion. Thus, the porousP2025,0053 WO N / P240264WO001 March 24, 2026

[0030] 5

[0031] filling material may be provided with increased permeability in the primary sloshing direction (i.e., substantially parallel to the dominant fluid motion within the expansion vessel) and reduced permeability in the transverse and / or vertical directions. This directional permeability promotes controlled fluid penetration and energy dissipation during sloshing events, while limiting bulk fluid displacement that would otherwise amplify slosh dynamics.

[0032] According to a further embodiment, the porous filler material comprises a gradient density structure. For example, the pore size and / or material density varies across the thickness or length of the porous element. For example, a lower - density, higher-porosity region may be positioned facing the incoming sloshing fluid to allow partial penetration and initial energy absorption, while a higher - density, lower-porosity region downstream increases flow resistance and dissipates kinetic energy through viscous losses. Such a gradient may be formed continuously or discretely (e.g. layered) and may be tailored to the expected sloshing frequency and / or amplitude.

[0033] According to a further embodiment, the porous filler material comprises a plurality of stacked layers. One of the stacked layers may directly adjoin at least one further layer of the plurality of stacked layers. In particular, two or more stacked layers of the porous filler material may be located within the fluid- filled portion. The layers may differ from one another with respect to at least one property such as permeability, density or elasticity.

[0034] According to a further embodiment, the porous filler material is configured to thermally insulate the liquid within theP2025,0053 WO N / P240264WO001 March 24, 2026

[0035] 6

[0036] expansion vessel from walls of the expansion vessel at least in regions.

[0037] The inclusion of a thermally insulating porous material may provide additional benefits in applications where the stored fluid is temperature - sensitive or where temperature gradients may influence fluid properties such as viscosity, density, or vapor pressure.

[0038] In particular, the inventors have found that a reduction of the heat transfer between the fluid and the expansion vessel walls by means of the porous filler material helps maintain more stable fluid properties, which in turn contributes to more predictable sloshing behaviour and improved system performance.

[0039] The porous filler material further provides significant vibration damping. While porous structures inherently provide some level of vibration damping, the vibration attenuation described herein is enhanced through the specific selection and configuration of the porous filler material. In particular, viscoelastic or elastomeric porous materials may convert mechanical vibration energy into heat through internal friction within the solid matrix and through viscous losses of fluid moving within the pores.

[0040] It should be understood that the described structural configurations and material functionalities may be implemented individually or in combination. The porous material may simultaneously provide anti - sloshing, vibration damping, thermal insulation, and / or filtration effects, depending on the application. The invention is therefore defined by the functional interaction between the porousP2025,0053 WO N / P240264WO001 March 24, 2026

[0041] 7

[0042] structure and the fluid dynamics, rather than by any single material property or geometric parameter.

[0043] According to a further embodiment, at least a portion of the porous filler material is located in the gas - filled portion of the expansion vessel. In this case, the porous filler material may be arranged on both sides of the separator of the expansion vessel. This may help to further reduce the kinetic energy of the fluid if the power transformer is moved. Alternatively, the porous filler material may be located in the gas - filled portion only.

[0044] According to a further embodiment, the porous filler material is subdivided into a plurality of separate blocks. In this context, the term "block" does not imply any limitation on the geometric shape of the individual blocks. For example, the blocks are arranged laterally side by side in contact with the separator. The blocks may be attached to one another and / or attached to the separator. Alternatively, the blocks may be configured to float freely underneath the separator.

[0045] According to a further embodiment, the fluid penetrates into at least a portion of the porous filler material. For example, the porous filler material comprises open pores or cells that are accessible to the fluid. In other words, the porous filler material may have a sponge- like structure.

[0046] According to a further embodiment, at least a portion of the porous filler material is sealed against the fluid. For example, the porous filler material has a closed cell structure so that the fluid does not penetrate into cells or pores of the porous filler material that are spaced apart from the surface of the porous filler material.P2025,0053 WO N / P240264WO001 March 24, 2026

[0047] Alternatively, a coating or shell surrounding the porous filler material may be used to seal a porous filler material like an open cell porous filler material against the fluid.

[0048] For example, the porous filler material may float underneath and in contact with the separator as a single block or as separate blocks.

[0049] According to a further embodiment, the porous filler material comprises at least one of: a crosslinked polymer material such as a polyurethane or a polysiloxane or a silicone, a meshed metal, a glass fiber, a ceramic material such as a ceramic foam, a composite combining different material types in its structure.

[0050] Foams made from these materials may be formed as open cell foam or as closed cell foam. These foams may be produced with varying density.

[0051] According to a further embodiment, the porous filler material is elastically compressible. In this context, "compressible" means that the porous filler material compresses in response to pressure acting on the porous filler material during operation of the power transformer. For example, the porous filler material may be compressed by the gas - filled portion of the expansion vessel if the volume of the fluid in the expansion vessel decreases. For example, the porous filler material may comprise a metal foam such as an aluminum metal foam.

[0052] According to a further embodiment, the porous filler material is rigid. For example, rigid foams may be made of a polymerP2025,0053 WO N / P240264WO001 March 24, 2026

[0053] 9

[0054] material, a ceramic material, a glass fiber or a perforated rigid structure. In this context, "rigid" means that the porous filler material does not, or at least not significantly, change its volume in response to pressure acting on the porous filler material during operation of the power transformer.

[0055] For example, the porous filler material may comprise a Silicon Carbide (SiC) foam which is a porous, open- celled structure made from an interconnected lattice of ceramic ligaments. The resulting foam structure has a high void volume, large surface area, and low flow resistance. It is also lightweight, strong, fracture and thermal shock resistant.

[0056] According to a further embodiment, the power transformer comprises a further porous filler material arranged in the tank. The further porous filler material may help to reduce the kinetic energy of the fluid within the tank.

[0057] Features described above in connection with the porous filler material may also apply for the further porous filler material. For example, the further porous filler material may be attached to the inner wall of the tank. Different materials may be used for the porous filler material and the further porous filler material depending on the location within the power transformer.

[0058] According to a further embodiment of the power transformer, at least a portion of the further porous filler material is arranged between two of the active components of the power transformer. For example, the further porous filler materialP2025,0053 WO N / P240264WO001 March 24, 2026

[0059] 10

[0060] may be arranged between two coil windings or between a coil winding and a tap changer of the power transformer.

[0061] At these locations the requirements on the porous filler material with respect electrical isolation properties and / or high temperature stability may be increased compared to porous filler material located in the expansion vessel. For example, a ceramic foam may be used as further porous filler material near or between active components of the power transformer.

[0062] According to a further embodiment, the power transformer is configured to be operable while it is in motion. For example, the power transformer is part of an offshore installation.

[0063] For example, the power transformer may be operated on a fixed bottom or floating platform. As the porous filler material efficiently reduces sloshing motions of the fluid, the reliability of the power transformer can be increased under these operating conditions.

[0064] According to a further embodiment, the porous filler material is configured to increase a natural frequency of the fluid at lowest occupational rate within the expansion vessel to a frequency that is at least by a factor of 2 (~ 1.41) greater than a floating frequency of the power transformer. In particular, the natural frequency of the fluid may be by at least a factor of 2 and at most a factor of 1.8 greater than the floating frequency of the power transformer. Thus, the natural frequency of the fluid is intentionally detuned with respect to the floating frequency to provide an efficient vibration isolation and to reduce the kinetic energy of the fluid in the expansion vessel.P2025,0053 WO N / P240264WO001 March 24, 2026

[0065] 11

[0066] In offshore applications, typical floating frequencies caused by waves are in a range from 0.1 Hz to 1 Hz. A typical natural frequency of oil within a conventional expansion vessel without porous filling material is about 0.3 Hz. Using the porous filling material the natural frequency of the fluid within the expansion vessel may be increased to values above 1 Hz, so that the transfer of kinetic energy from the waves to the fluid within the expansion vessel can be significantly reduced.

[0067] However, the described setup is also suitable for stationary power transformers. In this case the porous filler material in particular reduces the risk of sloshing of the f luid during the transport of the power transformer damaging components of the power transformer, such as the active components immersed in the fluid or components of the tank or the expansion vessel.

[0068] In particular, the tank may already be filled with the fluid during the transport of the power transformer to its final destination.

[0069] Features described above in connection with one embodiment of the power transformer may be combined with other features described in connection with other embodiments of the power transformer unless the features are contradictory.

[0070] The accompanying figures are included to provide a further understanding. In the figures, elements of the same structure and / or functionality may be referenced by the same reference signs. It is to be understood that the embodiments shown inP2025,0053 WO N / P240264WO001 March 24, 2026

[0071] 12

[0072] the figures are illustrative representations and are not necessarily drawn to scale.

[0073] In the Figures:

[0074] Figures 1A and 1B show an exemplary embodiment of a power transformer wherein Figure 1B shows a sideview of the power transformer and Figure 1A shows a sectional view of the expansion vessel of the power transformer;

[0075] Figure 2 shows an exemplary embodiment of an expansion vessel of a power transformer in a cross - sectional view;

[0076] Figure 3 shows an exemplary embodiment of a power transformer in a sectional view;

[0077] Figure 4 shows an exemplary embodiment of a power transformer in a sectional view; and

[0078] Figure 5 shows an exemplary embodiment of a power transformer in a sectional view.

[0079] While the disclosure is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the figures and will be described in detail. It should be understood, however, that the intention is not to limit the disclosure to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the scope of the disclosure defined by the appended claims.

[0080] The power transformer 1 comprises a tank 51 wherein active components 11 are immersed in a fluid 3. The activeP2025,0053 WO N / P240264WO001 March 24, 2026

[0081] 13

[0082] components such as coil windings are not explicitly shown in Fig. 1B for easer representation.

[0083] The tank 51 receiving the fluid 3 is connected to an expansion vessel 52 via one or more pipes 53.

[0084] As illustrated in the sectional view of Figure 1A extending perpendicular to the drawing plane of Figure 1B, a volume 2 of the expansion vessel 52 comprises a porous filler material 4.

[0085] The porous filler material 4 is located in a fluid- filled portion 522 of the expansion vessel 52. The fluid- f illed portion 522 is separated from a gas - filled portion 521 of the expansion vessel 52 by means of a separator 520.

[0086] In the exemplary embodiment of Figure 1A, the gas - f illed portion 521 is formed by a gas bag 525. The gas bag 525 is configured to provide a counterpressure against the fluid 3 so that the gas bag 525 expands in case the volume of the fluid 3 decreases with temperature.

[0087] The porous filler material 4 occupies the entire fluid- filled portion 522 of the expansion vessel 52 so that the fluid 3 located within the expansion vessel 52 is arranged within cells or pores of the porous filler material 4. Thus, the porous filler material 4 efficiently suppresses sloshing motions of the fluid 3 within the expansion vessel 52.

[0088] For example, the porous filler material 4 has a porosity in a range from 50% to 95% of its volume in a decompressed state so that at least 50% of the fluid- filled portion 522 can be occupied by the fluid 3 within the porous filler material 4.P2025,0053 WO N / P240264WO001 March 24, 2026

[0089] 14

[0090] The porous filler material 4 is formed from an elastically compressible material so that the porous filler material 4 compresses as the gas - filled portion 521 expands and vice versa.

[0091] However, a complete occupation of the fluid- filled portion is not required to suppress sloshing motions. The required occupation rate depends on the specific configuration of the expansion vessel. For example, the porous filler material occupies at least 5% or at least 10% or at least 20% or at 40% of the fluid- filled portion of the expansion vessel.

[0092] The porous filler material 4 may be configured to exhibit an anisotropic permeability, so that the permeability parallel to a dominant fluid motion within the expansion vessel is larger than in a direction perpendicular to the dominant fluid motion.

[0093] The porous filler material comprises a gradient density structure. For example, the pore size and / or material density varies across the thickness or length of the porous element. For example, a lower - density, higher-porosity region may be positioned facing the incoming sloshing fluid to allow partial penetration and initial energy absorption, while a higher - density, lower-porosity region downstream increases flow resistance and dissipates kinetic energy through viscous losses. Such a gradient may be formed continuously within the porous filler material or discretely by means of a plurality of sublayers that differ from one another with respect to the pore size and / or material density.P2025,0053 WO N / P240264WO001 March 24, 2026

[0094] 15

[0095] Alternatively, stacked layers of the porous filler material may differ from one another with respect to other properties such as permeability or elasticity.

[0096] The porous filler material may also thermally insulate the liquid within the expansion vessel from walls of the expansion vessel at least in regions. This may contribute to a more predictable sloshing behaviour and improved system performance.

[0097] The specific configuration of the porous filler material with respect to its location and material properties may be appropriately adapted to the specific configuration and may also depend on the operation conditions of the transformer. In particular, the porous material may simultaneously provide anti - sloshing, vibration damping, thermal insulation, and / or filtration effects.

[0098] In the exemplary embodiment of Figures 1A and 1B, the expansion vessel 52 is configured as a separate compensator mounted to the tank 51. For example, the expansion vessel 52 has a cylindrical base shape. However, other shapes or other types of expansion vessels 52 may also be used.

[0099] During operation of the power transformer 1, the fluid 3 does not come into contact with air or other gases. The gas - filled portion 521 is externally accessible via a connection 61. The fluid- filled portion 522 is connected to the tank 51 via a connection 62.

[0100] Compressible open cell foams may be formed from crosslinked polymers such as polyurethane or polysiloxane or silicon rubber from meshed metal or from glass fiber, for example. InP2025,0053 WO N / P240264WO001 March 24, 2026

[0101] 16

[0102] particular, the material for the porous filler material may be chosen depending on the fluid 3 used in the power transformer 1.

[0103] The fluid 3 may comprise a mineral oil, a synthetic oil, or an ester, for example.

[0104] The exemplary embodiment of Figure 2 substantially corresponds to the exemplary embodiment described in connection with Figures 1A and 1B. In contrast to the previous exemplary embodiment, the porous filler material 4 does not occupy the complete volume of the fluid- filled portion 522 of the expansion vessel 52.

[0105] In the exemplary embodiment of Figure 2, the porous filler material 4 is subdivided into a plurality of individual blocks 41. The blocks 41 may be attached to the separator 520 of the expansion vessel 52. Alternatively, the blocks 41 may freely float in the fluid 3. In this case, the porous filler material 4 follows the movement of the separator 520 provided by the gas bag 525, thereby dampening the kinetic energy of the fluid 3.

[0106] The porous filler material 4 may be formed from a compressible open cell material as described in connection with Figures 1A and 1B.

[0107] Alternatively, the porous filler material 4 may also be formed from a closed cell filler material or from an encapsulated filler material so that the fluid 3 does not penetrate into the porous filler material 4 of the blocks 41.P2025,0053 WO N / P240264WO001 March 24, 2026

[0108] 17

[0109] Further it is not necessary for the porous filler material 4 to be compressible in the exemplary embodiment of Figure 2. Rather, a rigid material may be used as porous filler material 4 as well. For example, rigid foams can be made of a polymer material, a ceramic, a glass fiber or a perforated rigid structure.

[0110] For example, ceramic open cell foam such as zirconia ceramic foam represents a rigid foam with superior mechanical and thermal properties. Thus, these materials are also suitable at locations within the power transformer where high temperatures occur, for example in close proximity to a transformer winding or a tap changer of the power transformer 1. This will be described in more detail in connection with Figure 3.

[0111] The exemplary embodiment of Figure 3 substantially corresponds to the exemplary embodiment described in connection with Figures 1A and 1B.

[0112] In contrast to the exemplary embodiment of Figures 1A and 1B, the expansion vessel 52 is configured as a compact expansion vessel located above the tank 51. In a top view onto the power transformer the expansion vessel 52 may cover at least 20% or at least 30% or at least 50% of the tank 51. This helps to reduce the height of the expansion vessel and of the entire power transformer 1.

[0113] The separator 520 is configured as a separator membrane 526 that extends between sidewalls of the expansion vessel 52 to separate the gas - filled portion 521 from the fluid- filled portion 522.P2025,0053 WO N / P240264WO001 March 24, 2026

[0114] 18

[0115] The expansion vessel 52 is connected to the tank 51 via one or more ports 54.

[0116] In the exemplary embodiment of Figure 3, both the gas - filled portion 521 and the fluid- filled portion 522 are filled with elastic porous filler material 4. The materials in the two volumes can be identical or different in composition, structure and / or material properties according to the specific of the system design. Thus, the kinetic energy of the fluid 3 within the expansion vessel 52 is reduced by porous filler material 4 arranged on opposite sides of the separator 520.

[0117] Optionally, the tank 51 may be provided with a further porous filler material 45. The further porous filler material 45 may help to further reduce fluid sloshing motion within the tank 51. The materials described above in connection with the porous filler material 4 may also be used for the further porous filler material 45. Materials capable of withstanding high temperatures are particularly suited.

[0118] As illustrated in Figure 3, the further porous filler material 45 may be attached to the tank 51 and / or be arranged between active components 11 of the power transformer 1. For example, the further porous filler material 45 may be arranged between adj acent coil windings 111 or between a coil winding 111 and a tap changer 112.

[0119] Of course, the further porous filler 45 may also be used in the tank 51 of the exemplary embodiments described in connection with Figures 1A and IB as well as Figure 2.P2025,0053 WO N / P240264WO001 March 24, 2026

[0120] 19

[0121] Likewise, the active components 11 of the power transformer 1 illustrated in Figure 3 may be present in previous exemplary embodiments as well.

[0122] In principle, the porous filler material 4 and / or the further filler material 45 may be provided at any location where a high fluid motion amplitude is expected and / or at any location where sloshing motions might cause damage.

[0123] The exemplary embodiment of Figure 4 substantially corresponds to the exemplary embodiment of Figure 3.

[0124] In contrast to the exemplary embodiment of Figure 3, the porous filler material 4 is subdivided into a plurality of blocks 41.

[0125] As in the exemplary embodiment of Figure 2, the porous filler material 4 does not completely fill the fluid- filled portion 522 of the expansion vessel 52. Thus, the blocks 41 may also be formed from a rigid material and / or from a closed cell or encapsulated filler material.

[0126] As described in connection with Figure 2, the gas - f illed portion 521 is arranged above the fluid- filled portion 522.

[0127] The blocks 41 may be attached to the separator membrane 526. Alternatively, the blocks 41 may freely float underneath and in contact with the separator membrane 526.

[0128] The exemplary embodiment of Figure 5 substantially corresponds to the exemplary embodiment described in connection with Figure 3.P2025,0053 WO N / P240264WO001 March 24, 2026

[0129] 20

[0130] In contrast to the exemplary embodiment of Figure 3, the gas-filled portion 521 is configured as a bellow 527. In this case the bellow 527 and the fluid- filled portion 522 are arranged laterally side by side in the expansion vessel 52. The bellow 527 is externally accessible via a connection 61.

[0131] As in the previous exemplary embodiments, the bellow 527 is configured for a temperature - induced volume compensation of the fluid 3.

[0132] The exemplary embodiments of the power transformer 1 described above provide a passive system for the mitigation of power transformer fluid sloshing motions induced by external mechanical excitations such as wave motion in floating offshore installations. In particular, sloshing motions of the fluid can be reduced without hindering the flow of the fluid 3 required for volume compensation.

[0133] For example, the porous filler material may be conf igured to increase a natural frequency of the fluid at lowest occupational rate within the expansion vessel to a frequency that is at least by a factor of √2 (~ 1.41) greater than a floating frequency of the power transformer. In particular, the natural frequency of the fluid may be by at least a factor of √2 and at most a factor of 1.8 greater than the floating frequency of the power transformer. Thus, the porous filler material intentionally detunes the natural frequency of the fluid with respect to the floating frequency such that transfer of kinetic energy of waves surrounding the transformer to the fluid in the expansion vessel is reduced.

[0134] The material for the porous filler material 4 may be selected in view of specific application conditions regardingP2025,0053 WO N / P240264WO001 March 24, 2026

[0135] 21

[0136] temperatures occurring during operation regarding the chemical stability against the fluid used in the power transformer, dielectric properties and / or mechanical properties such as compressibility.

[0137] The embodiments shown in the Figures 1A to 5 as stated represent exemplary embodiments of the improved power transformer; therefore, they do not constitute a complete list of all embodiments according to the improved power transformer. Actual power transformers may vary from the embodiments shown in terms of arrangements, components, and materials for example.P2025,0053 WO N / P240264WO001 March 24, 2026

[0138] - 22 -

[0139] Reference Signs

[0140] 1 power transformer

[0141] 11 active component

[0142] 111 coil winding

[0143] 112 tap changer

[0144] 2 volume

[0145] 3 fluid

[0146] 4 porous filler material

[0147] 41 block

[0148] 45 further porous filler material 51 tank

[0149] 52 expansion vessel

[0150] 520 separator

[0151] 521 gas - filled portion

[0152] 522 fluid- filled portion

[0153] 525 gas bag

[0154] 526 separator membrane

[0155] 527 bellow

[0156] 53 pipe

[0157] 54 port

[0158] 61 connection to gas

[0159] 62 connection to tank

Claims

P2025,0053 WO N / P240264WO001 March 24, 202623Claims1. A power transformer (1) comprising a plurality of active components (11) immersed in a fluid (3 ) comprising a liquid, a tank (51), and an expansion vessel (52) connected to the tank (51), wherein- a volume (2) in the expansion vessel (52) comprises a porous filler material (4) configured to dampen a sloshing motion of the fluid (3 ) within the volume (2),- the expansion vessel (52) comprises a gas - filled portion (521) and a fluid- filled portion (522) separated from the gas - filled portion (521), and- the porous filler material (4) occupies at least 5% of the fluid- filled portion (522) of the expansion vessel (52).

2. The power transformer according to claim 1,wherein the porous filler material (4) occupies at least 20% of the fluid- filled portion (522) of the expansion vessel (52).

3. The power transformer according to claim 1,wherein the porous filler material (4) completely occupies the fluid- filled portion (522) of the expansion vessel (52).

4. The power transformer according to any one of the preceding claims,wherein at least a portion of the porous filler material (4) is in contact with a separator (520) separating the gas-filled portion (521) from the fluid- filled portion (522) of the expansion vessel (52).

5. The power transformer according to any one of the preceding claims,P2025,0053 WO N / P240264WO001 March 24, 202624wherein the porous filler material (4) is configured to exhibit an anisotropic permeability, so that the permeability parallel to a dominant fluid motion within the expansion vessel (52) is larger than in a direction perpendicular to the dominant fluid motion.

6. The power transformer according to any one of the preceding claims,wherein the porous filler material comprises a gradient density structure.

7. The power transformer according to any one of the preceding claims,wherein the porous filler material comprises a plurality of stacked layers.

8. The power transformer according to any one of the preceding claims,wherein the porous filler material is configured to thermally insulate the liquid within the expansion vessel (52 ) from walls of the expansion vessel at least in regions.

9. The power transformer according to any one of the preceding claims,wherein a portion of the porous filler material (4) is located in the gas - filled portion (521) of the expansion vessel ( 52 ).

10. The power transformer according to any one of the preceding claims,wherein the porous filler material (4) is subdivided into a plurality of separate blocks (41).P2025,0053 WO N / P240264WO001 March 24, 20262511. The power transformer according to any one of the preceding claims,wherein the fluid (3 ) penetrates into at least a portion of the porous filler material (4).

12. The power transformer according to any one of the preceding claims,wherein at least a portion of the porous filler material (4) is sealed against the fluid (3 ).

13. The power transformer according to any one of the preceding claims,wherein the porous filler material (4) comprises at least one of: a crosslinked polymer material, a meshed metal, a glass fiber, a ceramic foam.

14. The power transformer according to any one of the preceding claims,wherein the porous filler material (4) is elastically compressible.

15. The power transformer according to any one of claims 1 to 13,wherein the porous filler material (4) is rigid.

16. The power transformer according to any one of the preceding claims,wherein the power transformer (1) comprises a further porous filler material (45) arranged in the tank (51).

17. The power transformer according to claim 16,wherein at least a portion of the further porous fillerP2025,0053 WO N / P240264WO001 March 24, 202626material (45) is arranged between the active components (11) of the power transformer (1).

18. The power transformer according to any one of the preceding claims,wherein the power transformer (1) is configured to be operable while it is in motion.

19. The power transformer according to claim 18,wherein the porous filler material is configured to increase a natural frequency of the fluid at lowest occupational rate within the expansion vessel (52) to a frequency that is at least by a factor of √2 greater than a floating frequency of the power transformer.

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