EP4679463A1

EP4679463A1 - Preservation system for a liquid-immersed transformer and transformer system

Info

Publication number: EP4679463A1
Application number: EP24188313.1A
Authority: EP
Inventors: Alan SBRAVATI; Julian CARDONA
Original assignee: Hitachi Energy Ltd
Current assignee: Hitachi Energy Ltd
Priority date: 2024-07-12
Filing date: 2024-07-12
Publication date: 2026-01-14
Also published as: WO2026013173A1

Abstract

The present disclosure relates to a preservation system (10) for a liquid-immersed transformer (30), comprising an expansion tank (11) configured for holding variable volumes of a first liquid (4) and a gas (18), respectively, at least one chamber (14) formed separate from the expansion tank (11) and configured for holding a further volume of the gas (18), the chamber (14) being fluidically connected to an upper part of the expansion tank (11), such that the gas (18) above the variable volume of the first liquid (4) can flow to and from the at least one chamber (14), and a u-shaped, tube-like vessel (16) with a first upper part (16a) connected to the at least one chamber (14), a second upper part open to an environment, and a base part (16b) fluidically connecting the first upper part (16a) and the second upper part and being configured for holding a second liquid (20) separating the gas (18) from ambient air.

The present disclosure further relates to a transformer system (1).

Description

The present disclosure relates to a preservation system for a liquid-immersed transformer, in particular an insulating liquid preservation system for an oil-immersed transformer, comprising an expansion tank configured for holding variable volumes of a first liquid and a gas, respectively. The present disclosure further relates to a transformer system comprising a corresponding preservation system.
Transformer windings of high power or high voltage transformers are usually immersed in a liquid, such as mineral oil, for insulation and/or cooling. The liquid is held by a transformer tank surrounding the transformer windings. Typically, the transformer tank is filled entirely with the liquid and sealed against the environment. During operation, the liquid in the tank will expand due to the heat generated by a current flowing through the transformer windings. To stop the transformer tank from bursting, this additional pressure must be relieved, while keeping the insulating liquid isolated from the ambient air.
In conventional designs, both functions are implemented by means of an expansion tank, which is partially filled with the liquid and partly with a gas, such as air. The gas acts as a pressure cushion for the variable volume of the liquid contained in the expansion tank. To avoid unwanted reactions between the gas and liquid, a flexible barrier is provided between the liquid and the gas. For example, the gas can be contained in a sealed rubber bag, which is provided within the expansion tank. Alternatively, the expansion tank may comprise a flexible diaphragm, which allows an expansion and reduction of the expansion tank's effective volume by sucking in or pushing out the flexible diaphragm.
Such rubber bags or flexible diaphragms used in expansion tank of electrical transformers are made from a polymeric material and are subject to repeated expansion and contraction. Moreover, the material is directly exposed to the liquid on one side and to the gas, such as air, on the other side. Suitable flexible polymeric materials age and have, at best, a maximum lifespan of approximately 10 years. In contrast, a liquid-immersed high voltage transformer typically has a lifespan of several decades, e.g., 40 years. As a consequence, regular maintenance of the expansion tank is necessary to replace the rubber bag or flexible diaphragm used for pressure compensation. Such maintenance is both costly and dangerous, as high voltage transformers are often installed in remote locations and/or at elevated heights. Moreover, it requires de-energizing of the transformer, which may lead to temporary interruptions in an electrical supply network. Moreover, when such a rubber bag develops a crack or another leak, the insulating liquid is exposed to the air within. The air inside the rubber bag is expected to have a low moisture content, due to the use a silica-gel desiccant. However, practical experience suggest that the desiccant is very poorly effective on reducing the water content inside the rubber bag, and thus requires replacing the silica-gel.
Accordingly, it is an object to provide an improved transformer system in general and an improved preservation system for a liquid-immersed transformer in particular. In particular, it is desirable to eliminate elements requiring maintenance in a transformer system, in particular for transformers located at unmanned substations.
According to a first aspect, a preservation system for a liquid-immersed transformer is provided. The preservation system comprises:

an expansion tank configured for holding variable volumes of a first liquid and a gas, respectively;
at least one chamber formed separate from the expansion tank and configured for holding a further volume of the gas, the chamber being fluidically connected to an upper part of the expansion tank, such that the gas above the variable volume of the first liquid can flow to and from the at least one chamber; and
a u-shaped, tube-like vessel with a first upper part connected to the at least one chamber, a second upper part open to an environment, and a base part fluidically connecting the first upper part and the second upper part and being configured for holding a second liquid separating the gas from ambient air.

According to the proposed system, a volume of the gas in the preservation system is increased compared to conventional designs by providing at least one separate chamber in addition to an expansion tank. A larger amount of gas can be compressed more easily, thus resulting in a cushioning effect. In addition, the gas contained partially in the expansion tank and partially in the at least one chamber is sealed from the environment using a second liquid held in the bottom part of a u-shaped, tube-like vessel. In this way, the gas can be separated from ambient air without the need for any flexible parts, in particular made from rubber or other elastic polymeric materials. Note that the proposed preservation system therefore serves multiple purposes, including providing a pressure control for the transformer tank, minimizing any internal overpressure, and providing a seal for the gas. It may therefore also be referred to as pressure relief system and/or sealing system.
The tube-like vessel is u-shaped, such that the first upper part corresponds to a first leg of the u-shaped vessel and the second upper part corresponds to a second leg of the u-shaped vessel. Such a vessel is easy to manufacture, enables relatively large height differences in a column of the second liquid, and it also serves as an oil level indicator and/or pressure gauge.
Optionally, the preservation system further comprises a conduit interconnecting an upper port of the expansion tank with a first port of the at least one chamber. By providing a conduit for interconnecting the two separately formed parts, it becomes possible to arrange the at least one chamber at a different location, for example at a sidewall of a transformer tank below the typically elevated expansion tank.
Optionally, a height of the first upper part and/or the second upper part of the tube-like vessel exceeds 1 meter. Moving the column of the second liquid in the tube-like vessel by a height of about 1 meter corresponds to a pressure change of about 0.1 bar (10,0000 Pascal). This corresponds to a change in pressure in the gas for typical dimensions and operating temperatures of high voltage transformer systems.
Optionally, the expansion tank is configured as a solid cylindrical tank made from a metal material without any internal or external flexible parts.
Optionally, the at least one chamber is configured as a solid rectangular cuboid made from a metal material, with the tube-like vessel being mounted at a sidewall of the rectangular cuboid.
The use of the above materials and geometries enables an easy mechanical integration and attachment of the respective parts of the preservation system to the liquid-immersed transformer.
Optionally, the preservations system is configured such that the further volume of the gas held in the at least one chamber limits the variation of an internal pressure of the expansion tank to plus or minus 25% of an external pressure of the ambient air or less. For example, a volume of the at least one chamber may equal or exceed a total volume of the expansion tank. Due to the different compressibility of the first liquid and the gas, the provision of at least one chamber with the similar size or larger volume than the total volume of the expansion tank leads to a reduction of the operational pressure differences to an acceptable level, such as within a range of 25% above and below the ambient air pressure.
According to another aspect, a transformer system is provided. The transformer system comprises a transformer tank configured for holding at least one transformer winding and a preservation system as described above. The transformer tank is fluidically connected with the expansion tank and filled with the first liquid, such that the at least one transformer winding is completely immersed in the first liquid. The expansion tank is partially filled with the first liquid and partially filled with the gas. The at least one chamber is filled with the gas, and at least the base part of the tube-like vessel is filled with the second liquid.
Such a transformer system ensures that the transformer windings are always completely immersed in the first liquid, and that the first liquid and the second liquid never come in direct contact with each other. Moreover, the gas held partly in the expansion tank and partly in the at least one chamber acts as a pressure cushion, whereas the second liquid serves as a hydraulic barrier between the gas in the expansion tank and the ambient air as described above. Together, the second liquid in the tube-like vessel and the gas in the expansion tank will create a barrier for preventing the contact of the first liquid with the ambient without the use of any flexible bag or diaphragm. Moreover, the system ensures the pressure on the expansion tank remains in equilibrium with the atmospheric pressure, not generating any relevant overpressure or partial vacuum condition.
Optionally, the expansion tank is arranged above the transformer tank and is fluidically connected to the transformer tank with a first conduit. Optionally, the at least one chamber is arranged at a sidewall of the transformer tank and is fluidically connected to the expansion tank with a second conduit, in particular the conduit as described above with regard to the preservation system. The above arrangement of components enables a simple construction and integration of the described parts of the transformer system.
Optionally, a density of the second liquid exceeds the density of the first liquid. For example, the first liquid may be an insulating mineral oil and the second liquid may be silicon or an ester, in particular a synthetic ester, such as Envirotemp^™ 360 (E360) or Midel^™ 7131, or another high-density liquid. By choosing liquids with different densities, a mixing of the two liquids is prevented even in the unlikely case that the two liquids should enter the same part, such as the at least one chamber, due to overpressure or underpressure situations experienced by the transformer system.
Optionally, the gas comprises at least one of confined ambient air, dry air or dry nitrogen. Provision of a volume of confined ambient and/or dehumidified air or dry nitrogen inhibits any unwanted chemical interactions between the gas and the first and second liquid, respectively.
Optionally, the second upper part of the tube-like vessel comprises a desiccant, in particular a silica gel. Provision of the desiccant removes water from the second liquid and/or the ambient air surrounding the open end, thereby inhibiting unwanted chemical reactions between them.
Optionally, at least one of the first and the second upper part of the tube-like vessel is transparent and configured as a level indicator for the first liquid in the expansion tank. By monitoring the level of the second liquid in the first and/or second upper part of the tube-like vessel, one can indirectly measure the level of the first liquid in the expansion tank, thus obliviating the need for a separate oil level indicator there.
The present disclosure comprises several aspects of a transformer system. Every feature described with respect to one of the aspects is also disclosed herein with respect to the other aspects, even if the respective feature is not explicitly mentioned in the context of the specific aspect.
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 in the figures are illustrative representations and are not necessarily drawn to scale.

Figures 1A to 1D show different views of an improved transformer system.
Figure 2 shows, in a schematic manner, the operating principle of an improved preservation system.
Figures 3A and 3B show two views of a u-shaped, tube-like vessel holding a liquid acting as a hydraulic barrier.
Figures 4 and 5 show the volumetric expansion and density of various liquids suitable for the disclosed systems.
Figure 6 shows a perspective view of a high voltage transformer comprising an improved preservation system.

Figures 1A to 1D show, in a schematic manner, different views of an improved transformer system according to a first embodiment of the present disclosure. In particular, Figure 1A shows a first side view, wherein parts of a support structure have been removed for better understanding. Figures 1B to 1D show, respectively, a back view, a normal side view and a top view of the transformer system of Figure 1A.
Figures 1A to 1D only show the main components of the transformer system, which are relevant with regard to insulating liquid preservation of a liquid used for insulating and/or cooling one or more transformer coils and sealing a gas of the transformer system from the environment. Other components of a complete transformer system are shown, for example, in the perspective view of Figure 6.
As can be seen in Figures 1A to 1D, the transformer system 1 comprises a relatively large transformer tank 2 capable of holding one or more transformer coils (not shown). The transformer tank is filled completely with a first liquid, or at least to a degree sufficient to completely immerse the transformer coils in a specified temperature range such as -30 °C to +160 °C. The first liquid typically is an insulating mineral oil. Since liquids in general and mineral oil in particular have a very low compressibility and a positive volumetric expansion coefficient, the first liquid held in the transformer tank 2 will increase as the transformer windings warm up during operation. To prevent the transformer tank 2 from bursting and avoid the contact of the insulating liquid with ambient air, a preservation system 10 is provided, e.g., system for preserving the insulating liquid of the transformer system.
The preservation system 10 comprises an expansion tank 11, which is partially filled with the first liquid and partially filled with a gas, such as a non-renewed volume of normal air, dehumidified air or dry nitrogen. The expansion tank 11 is physically arranged above a lid 6 of the transformer tank 2 and attached thereto by means of several support struts 12. It is fluidically connected to the inside of the transformer tank 2 by means of a first conduit 13. In the embodiment shown in Figures 1A to 1D, a first end of the conduit 13 is connected to an upper port at the lid 6 of the transformer tank 2 and a second end of the conduit 13 is connected to a lower port of the expansion tank 11.
During operation of the transformer system 1, the level of the first liquid within the expansion tank 11 will rise due to a heating and thermal expansion of the first liquid. Accordingly, the volume available for the gas filling the remaining part of the expansion tank 11 is reduced. To avoid an overpressure within the expansion tank 11, the expansion tank 11 is connected to a further chamber 14. The chamber 14 and the expansion tank 11 are fluidically connected by means of the second conduit 15 which connects an upper port of the expansion tank 11 with a first port of the chamber 14.
In the described embodiment, the chamber 14 is attached directly to a sidewall of the transformer tank 2 and is physically separated from the expansion tank 11. As can be best seen in Figure 1B, the chamber may be arranged between two struts 12 for holding the expansion tank 11. Note that the chamber 14 does not hold any liquids under normal operating conditions of the transformer system 1. Instead, it simply enlarges the volume of gas provided in the preservation system 10. Since gas, unlike liquids, can be relatively easily compressed, it effectively serves as a pressure cushion or buffer in case the level of the first liquid in the expansion tank 11 rises.
To compensate for the remaining increases in pressure in the preservation system 10, a second port of the chamber 14 is connected to a tube-like vessel 16. In the depicted embodiment shown in Figures 1A to 1D, the tube-like vessel 16 is directly attached to the outside wall of the chamber 14. As described in more detail below with respect to Figures 2, 3A and 3B, the tube-like vessel 16 comprises a first upper part, which is fluidically connected to the second port of the chamber 14 and a second upper part which is open to the environment, i.e., to ambient air surrounding the transformer system 1. To avoid any direct contact between the ambient air and the gas held in the expansion tank 11 and the chamber 14, a second liquid (not shown in Figures 1A to 1D) is provided in a base part of the tube-like vessel 16, which acts as a flexible seal of the preservation system 10.
Figure 2 shows, in a schematic manner, a cross-section through a transformer system 1, such as the transformer system 1 of Figures 1A to 1D. Note that in Figure 2 some of the components of the preservation system 10 are indicated as separate parts, such as the chamber 14 and the tube-like vessel 16. This is done for representational simplicity and does not imply that these parts are separate from each other in an assembled state of the transformer system 1 as shown, for example, for the high voltage transformer of Figure 6.
As can be seen in the cross-section of Figure 2, the transformer tank comprises three transformer windings 3, which are immersed in an insulating first liquid 4, such as mineral oil. Optionally, the transformer tank 2 may be internally divided into different chambers (not shown). For example, each transformer winding 3 may be placed in a separate chamber of the transformer tank 2. In this case, each chamber of the transformer tank 2 can be separately filled with an insulating first liquid 4.
The main chamber of the transformer tank 2 and/or turrets 5 formed at a lid 6 of the transformer tank 2 are connected by one or several conduits 13a and 13b to a protection device 7 through which the first liquid 4 can flow. Note that the turrets 5 are also filled with the first liquid 4. The protection device 7 may be a so-called Buchholz relay, which is used for identifying an excessive presence of gases in the first liquid 4 and/or for identifying an excessive flow of the first liquid 4 from the transformer tank 2 (including the turrets 5) to the expansion tank 11 (and vice-versa).
Parts of the first liquid 4 displaced from the transformer tank 2 flow through the conduits 13a, 13b and/or 13c to a lower part 11a of the expansion tank 11. The remainder of the volume of the expansion tank 11 is filled with a gas 18, such as dry air or dry nitrogen. Note that there is no physical barrier between the first liquid 4 in the lower part 11a and the gas 18 in the upper part 11b. Accordingly, the gas 18 should be chosen such that it does not cause any unwanted chemical reactions with the first fluid 4. This is in contrast to conventional solutions, in which a rubber bag or diaphragm is often used as a physical barrier between the liquid and the air, leading to the maintenance issues described earlier.
The upper part 11b comprises an upper port 19, which is connected by means of the second conduit 15 to a corresponding port of the chamber 14. The chamber 14 is further connected by means of a third conduit 17, with a first upper part 16a of the tube-like vessel 16. Note that at least a part of the first upper part 16a is also filled with the gas 18. In contrast, a base part 16b of the vessel 16 is filled with a second liquid 20. The second liquid may be an ester, such as FR3 or E360. Such materials do not evaporate under normal atmospheric conditions and also do not react with the gas 18. If the pressure in the chamber 14 is increased, a height of a column of the second liquid 20 in the first upper part 16a is reduced. Inversely, if a pressure in the chamber 14 is reduced, the height of the column of the second liquid 20 in the first upper part 16a rises.
Figures 3A and 3B show an exemplary configuration of the tube-like vessel 16 at two different temperatures. For example, the situation shown in Figure 3A may correspond to a minimum temperature of the transformer system 1, when is has been switched off. Accordingly, the temperature of the first liquid 4 corresponds, more or less, to the temperature of the ambient air. In contrast, the situation depicted in Figure 3B may correspond to a maximum operating temperature of the transformer system 1, when the transformer windings 3 are fully loaded.
As can be seen in Figures 3A and 3B, the vessel 16 is formed by a u-shaped tube. It comprises a first leg 21, a second leg 22 and a connecting part 23 interconnecting the first leg 21 and the second leg 22. The first leg 21 comprises a first port 24 and is otherwise closed with respect to the environment. The first port 24 serves, as shown in Figure 2, to connect the vessel 16 to the chamber 14. The second leg 22 comprises a further port 25, which may simply be an open end of the tube, or a hole or grate in the wall of the tube. Accordingly, the second leg 22 is essentially open to the environment of the transformer system 1. As the temperature of the transformer windings 3 and hence the first liquid 4 rises, the level of the first liquid 4 in the expansion tank 11 also rises. Accordingly, a pressure of the gas 18 in the chamber 14 is increased, and the second liquid 20 arranged in the connecting part 23 of the vessel 16 is pushed partially from the first leg 21 to the second leg 22.
Based on typical configurations of the transformer tank 2, the maximum pressure of the gas 18 inside the chamber 14, acting as a pressure cushion chamber, does not change by more than ± 0.1 bar. Note that this corresponds to roughly to a height difference corresponding to ± 1 meter of the column of the second liquid 20 in the first leg 21 and/or second leg 22 of the vessel 16. In a transformer system having a height of several meters, a correspondingly large u-shaped tube may be provided simply at the outside wall of the chamber 14 as shown, for example, in Figure 1B. Thus, neither any further measures or components nor any parts extending beyond the dimensions of the transformer tank 2 are needed to compensate for the pressure changes within the transformer system 1. Moreover, in case all or relevant parts of the u-shaped tube are transparent, the absolute height of the second liquid in the first leg 21 or the second leg 22, or the difference in height indicates the level of the first liquid 4 in the expansion tank, alleviating the need for a separate oil-level indicator there.
To further improve the sealing function of the preservation system 10 and avoid any unintentional reactions between the ambient air and the second liquid 20, a desiccant 26 that is lighter than the second liquid 20 is placed in the upper part of the second leg 22. For example, a film of silica gel may be placed as a protective layer over the second liquid 20 as shown in Figures 3A and 3B. Silica gel only evaporates at temperatures above 250 °C, which lies well above the normal operating temperature of the transformer system 1. Thus, under normal conditions, neither the second liquid 20 nor the desiccant 26 needs to be replaced.
Figures 4 and 5 show the volumetric expansion of mineral oil, FR3, a natural ester, and Envirotemp^™ 360 (E360), a synthetic ester. Other esters, such as Midel^™ 7131 synthetic ester, or other high-density liquids may also be used. As can be derived from Figure 5 in particular, both esters have a higher density than mineral oil. Moreover, their volumetric expansion coefficient is lower at the relevant temperature range of -20 to +160°C as shown in Figure 4.
Based on the coefficients derived from Figures 4 and 5, and the dimensions of the various parts, one can derive a minimum height displacement Dh_min of the second liquid 20 in the first leg 21 at a minimum temperature, and a maximum level height displacement Dh_max of the second liquid 20 in the first leg 21 at a maximum temperature. In particular, for a transformer tank of approximately 5m x 2.4m x 4m, an oil volume of 35,9m³ at T = 25°C, and an expansion coefficient of 0.00075 1/K, the oil volume varies by 3.1m³ over an assumed temperature difference of 115 °C. This corresponds to a pressure of roughly 3020 N/m² or 0.03 bar at the level of the cover. Further assuming an additional gas volume of 1.4m³ provided by the chamber 14, and a height Hturret of 0,35m, one obtains Dh = (Hexp-Hexpmin)*(ρoil/ρse) and P_min = Dh*g*ρ. For T_min = - 0°C and E360, P_min is roughly 3120 N/m², corresponding to 03.1 bar, resulting in Dh_min = 323 mm as shown in Figure 3A. For T_max = -20°C and E360, P_min is roughly 3120 N/m², corresponding to 0.31 bar, resulting in Dh_min = 323 mm as shown in Figure 3A. Correspondingly, for the maximum operating temperature, one obtains Pmax=10030 N/m² corresponding to 0.99 bar, resulting in Dh_max = 1065 mm as shown in Figure 3B.
The maximum pressure of the gas 18 inside the chamber 14 and in contact with the first liquid 2, will be exposed to +/- 1 meter of column of the first liquid 4, which converts for insulating mineral oil to less than +/- 0.1 bar. Accordingly, there is no risk of excessive dissolution of the gas 18 in the first liquid 4. Any vapors or water contained in the gas 18 will condensate in the chamber 14, ensuring that only the confined gas, in particular dry air, flows back and forward between the expansion tank 11 and the chamber 14.
Figure 6 shows a practical implementation of a preservation system 10 of the described type in a complete high voltage transformer 30, such as transformer used in electrical substations.
As can be seen in Figure 6, the transformer tank 2 is reinforced by a number of support ribs 8 attached to the outside of a transformer tank 2. To limit the overall size of the high voltage transformer 30, instead of one large chamber 14, three separate chambers 14a to 14c in the form of rectangular cuboid are arranged between and next to two of the reinforcement bars 8 provided at a front wall of the transformer tank. Note that the three chambers 14a to 14c are mounted below an expansion tank 11, e.g., at or near a ground-level of the high voltage transformer 30. The three chambers 14a to 14c are interconnected by a series of conduits 15 which equalize the gas pressure in the chambers 14a to 14c.
Note that the other parts of the high voltage transformer 30 remain essentially unchanged with respect to previous designs. In particular, the location and general shape of the expansion tank 11 may remain the same. However, due to the fact that the expansion tank 11 does not need to comprise an internal rubber bag and/or elastic diaphragm, it may be constructed in a rigid manner. Moreover, the overall volume and, hence, size of the expansion tank 11 can be reduced since it is not necessary to accommodate a rubber bag or an oil level indicator at the expansion tank 11. Instead, as shown in Figure 6, the tube-like vessel 16 serves directly as an oil-level indicator as detailed before with respect to Figures 3A and 3B.
The described preservation system 10 and overall construction of the transformer system 1 and high voltage transformer 30 alleviates the need for regular maintenance with respect to the preservation system 10. In the unlikely situation of an overflow of the first liquid 4 from the expansion tank 11, the excess liquid will accumulate in the chamber 14 and can be easily drained from there. In the opposite case, if the expansion tank 11 is completely emptied, for example due to a leak in the transformer tank 2, a volume of the second liquid 20 from the tube-like vessel 16 may accumulate in the chamber 14. Even in this case, there is no risk of the second liquid 20 entering the expansion tank 11 or of the two liquids 4 and 20 mixing. However, in case the entire volume of the second liquid 20 is emptied into the chamber 14, a sealing function between the ambient air and the gas 18 and the first liquid 4 in the expansion tank 11 is no longer provided.
The embodiments shown in Figures 1 to 6 as stated represent exemplary embodiments of the improved transformer system and combine a preservation and sealing system. Therefore, they do not constitute a complete list of all embodiments according to the improved systems and devices. Actual transformer systems and preservation systems may vary from the embodiments shown in terms of the arrangements of their components, the specific configuration of the components, as well as the materials used, for example.

Reference Signs

1: transformer system
2: transformer tank
3: transformer coil
4: first liquid
5: turret
6: lid (of the transformer tank)
7: protection device
8: support rib

10: preservation system
11: expansion tank
11a: lower part
11b: upper part
12: support strut
13: first conduit
14: chamber
15: second conduit
16: (tube-like) vessel
16a: first upper part
16b: base part
17: third conduit
18: gas
19: upper port
20: second liquid
21: first leg
22: second leg
23: connecting part
24: first port
25: second port
26: desiccant

30: high voltage transformer

Claims

A preservation system (10) for a liquid-immersed transformer (30), comprising:
- an expansion tank (11) configured for holding variable volumes of a first liquid (4) and a gas (18), respectively;

- at least one chamber (14) formed separate from the expansion tank (11) and configured for holding a further volume of the gas (18), the chamber (14) being fluidically connected to an upper part (11b) of the expansion tank (11), such that the gas (18) above the variable volume of the first liquid (4) can flow to and from the at least one chamber (14); and

- a u-shaped, tube-like vessel (16) with a first upper part (16b) connected to the at least one chamber (14), a second upper part open to an environment, and a base part (16b) fluidically connecting the first upper part (16a) and the second upper part and being configured for holding a second liquid (20) separating the gas (18) from ambient air.
The preservation system (10) of claim 1, further comprising a conduit (15) interconnecting an upper port (19) of the expansion tank (11) with a first port of the at least one chamber (14).
The preservation system (10) of claim 1 or 2, wherein a height of the first upper part (16a) and/or the second upper part of the tube-like vessel (16) exceeds 1 meter.
The preservation system (10) of any one of claims 1 to 3, wherein the expansion tank (11) is configured as a solid cylindrical tank made from a metal material without any internal or external flexible parts.
The preservation system (10) of any one of claims 1 to 4, wherein the at least one chamber (16) is configured as a solid rectangular cuboid made from a metal material, the tube-like vessel (16) being mounted at a sidewall of the rectangular cuboid.
The preservation system (10) of any one of claims 1 to 5, wherein the preservations system (10) is configured such that the further volume of the gas held in the at least one chamber (14) limits the variation of an internal pressure of the expansion tank (11) to plus or minus 25% of an external pressure of the ambient air or less.
A transformer system (1), comprising:
- a transformer tank (2), configured for holding at least one transformer winding (3); and

- a preservation system (10) according to any one of claim 1 to 6, wherein

- the transformer tank (2) is fluidically connected with the expansion tank (11) and filled with the first liquid (4), such that the at least one transformer winding (3) is completely immersed in the first liquid (4);

- the expansion tank (11) is partially filled with the first liquid (4) and partially filled with the gas (18);

- the at least one chamber (14) is filled with the gas (18); and

- at least the base part (16b) of the tube-like vessel (16) is filled with the second liquid (20).
The transformer system (1) of claim 7, wherein the expansion tank (11) is arranged above the transformer tank (2) and is fluidically connected to the transformer tank (2) with a first conduit (13).
The transformer system (1) of claim 7 or 8, wherein the at least one chamber (14) is arranged at a sidewall of the transformer tank (2) and is fluidically connected to the expansion tank (11) with a second conduit (15), in particular the conduit (15) according to claim 2.
The transformer system (1) of any one of claims 7 to 9, wherein a density of the second liquid (20) exceeds a density of the first liquid (4).
The transformer system (1) of any one of claims 7 to 10, wherein the first liquid (4) is an insulating mineral oil.
The transformer system (1) of any one of claims 7 to 11, wherein the second liquid (20) is silicon or an ester or another high-density liquid.
The transformer system (1) of any one of claims 7 to 12, wherein the gas (18) comprises confined ambient air or at least one of dehumidified air or dry nitrogen.
The transformer system of any one of claims 7 to 13, wherein the second upper part of the tube-like vessel (16) comprises a desiccant (26), in particular a silica gel.
The transformer system of any one of claims 7 to 14, wherein at least one of the first upper part (16a) and the second upper part of the tube-like vessel (16) is transparent and configured as a level indicator for the first liquid (4) in the expansion tank (11), in particular as an oil-level indicator.