Preservation system for a liquid-immersed transformer and transformer system

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 an d 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

[0001] P2024,0538 WO N / P240009WO01 July10,2025 -1 - DescriptionPreservation system for a liquid-immersed transform er andtransformersystemThe present disclosure relates to a preservation sy stem for aliquid-immersed transformer, in particular an insul atingliquid preservation system for an oil-immersed tran sformer,comprising an expansion tank configured for holding variablevolumesofa firstliquid and a gas,respectively. Thepresent disclosure further relates to a transformer systemcomprising a corresponding preservation system. Transformerwindingsofhigh powerorhigh voltagetransformers are usually immersed in a liquid, such asmineral oil, for insulation and / or cooling. The liq uid isheld by a transformer tank surrounding the transfor merwindings. Typically, the transformer tank is filled entirelywith the liquid and sealed against the environment. Duringoperation, the liquid in the tank will expand due t o the heatgenerated by a current flowing through the transfor merwindings. To stop the transformer tank from burstin g, thisadditional pressure must be relieved, while keeping theinsulating liquid isolated from the ambientair.In conventional designs, both functions are impleme nted bymeans of an expansion tank, which is partially fill ed withthe liquid and partlywith a gas,such asair.The gasactsas a pressure cushion for the variable volume of th e liquidcontained in the expansion tank.To avoid unwanted reactionsbetween the gas and liquid, a flexible barrier is p rovidedbetween the liquid and the gas. For example, the ga s can becontained in a sealed rubber bag, which is provided within P2024,0538 WO N / P240009WO01 July10,2025 -2 -the expansion tank. Alternatively, the expansion ta nk maycomprise a flexible diaphragm, which allows an expa nsion andreduction ofthe expansion tank'seffective volume bysucking in orpushing outthe flexible diaphragm.Such rubber bags or flexible diaphragms used in exp ansiontank of electrical transformers are made from a pol ymericmaterialand are subjectto repeated expansion andcontraction. Moreover, the material is directly exp osed tothe liquid on one side and to the gas, such as air, on theother side. Suitable flexible polymeric materials a ge andhave,atbest,a maximum lifespan ofapproximately 10 years.In contrast, a liquid-immersed high voltage transfo rmertypicallyhasa lifespan ofseveraldecades,e.g., 40 years.As a consequence, regular maintenance of the expans ion tankisnecessaryto replace the rubberbag orflexible diaphragmused for pressure compensation. Such maintenance is bothcostlyand dangerous,ashigh voltage transformers are ofteninstalled in remote locations and / or at elevated he ights.Moreover, it requires de-energizing of the transfor mer, whichmay lead to temporary interruptions in an electrica l supplynetwork.Moreover,when such a rubberbag develops a crackoranother leak, the insulating liquid is exposed to t he airwithin. The air inside the rubber bag is expected t o have alow moisture content, due to the use a silica-gel d esiccant.However, practical experience suggest that the desi ccant isvery poorly effective on reducing the water content insidethe rubber bag, and thus requires replacing the sil ica-gel.Accordingly, it is an object to provide an improvedtransformer system in general and an improved prese rvationsystem for a liquid-immersed transformer in particu lar. Inparticular, it is desirable to eliminate elements r equiring P2024,0538 WO N / P240009WO01 July10,2025 -3 - maintenance in a transformersystem,in particular for transformerslocated atunmanned substations. According to a firstaspect,a preservation system foraliquid-immersed transformer is provided. The preser vationsystem comprises:- an expansion tank configured for holding variable v olumesofa firstliquid and a gas,respectively;- at least one chamber formed separate from the expan siontank and configured for holding a further volume of thegas,the chamberbeing fluidicallyconnected to an upper part of the expansion tank, such that the gas above thevariable volume of the first liquid can flow to and fromthe atleastone chamber;and- a u-shaped, tube-like vessel with a first upper par tconnected to the at least one chamber, a second upp er partopen to an environment, and a base part fluidicallyconnecting the first upper part and the second uppe r partand being configured forholding a second liquid separating the gasfrom ambientair.According to the proposed system, a volume of the g as in thepreservation system is increased compared to conven tionaldesignsbyproviding atleastone separate chamber inaddition to an expansion tank. A larger amount of g as can becompressed more easily, thus resulting in a cushion ingeffect. In addition, the gas contained partially in theexpansion tank and partially in the at least one ch amber issealed from the environment using a second liquid h eld in thebottom part of a u-shaped, tube-like vessel. In thi s way, the P2024,0538 WO N / P240009WO01 July10,2025 -4 -gas can be separated from ambient air without the n eed foranyflexible parts,in particularmade from rubber orotherelastic polymeric materials. Note that the proposedpreservation system therefore serves multiple purpo ses,including providing a pressure control for the tran sformertank, minimizing any internal overpressure, and pro viding aseal for the gas. It may therefore also be referred to aspressure reliefsystem and / orsealing system.The tube-like vessel is u-shaped, such that the fir st upperpart corresponds to a first leg of the u-shaped ves sel andthe second upper part corresponds to a second leg o f the u-shaped vessel. Such a vessel is easy to manufacture , enablesrelativelylarge heightdifferencesin a column of the secondliquid, and it also serves as an oil level indicato r and / orpressure gauge.Optionally, the preservation system further compris es aconduit interconnecting an upper port of the expans ion tankwith a first port of the at least one chamber. By p roviding aconduit for interconnecting the two separately form ed parts,it becomes possible to arrange the at least one cha mber at adifferentlocation,forexample ata sidewallofatransformer tank below the typically elevated expan sion tank.Optionally, a height of the first upper part and / or thesecond upper part of the tube-like vessel exceeds 1 meter.Moving the column of the second liquid in the tube- likevesselbya heightofabout1 metercorrespondsto a pressurechange of about 0.1 bar (10,0000 Pascal). This corr esponds toa change in pressure in the gas for typical dimensi ons andoperating temperaturesofhigh voltage transformer systems. P2024,0538 WO N / P240009WO01 July10,2025 -5 -Optionally, the expansion tank is configured as a s olidcylindrical tank made from a metal material without anyinternalorexternalflexible parts. Optionally,the atleastone chamberisconfigured asa solidrectangular cuboid made from a metal material, with the tube-like vessel being mounted at a sidewall of the rect angularcuboid.The use of the above materials and geometries enabl es an easymechanical integration and attachment of the respec tive partsofthe preservation system to the liquid-immersed transformer. Optionally,the preservationssystem isconfigured such that the furthervolume ofthe gasheld in the atleast onechamber limits the variation of an internal pressur e of theexpansion tankto plusorminus25% ofan external pressureof the ambient air or less. For example, a volume o f the atleast one chamber may equal or exceed a total volum e of theexpansion tank. Due to the different compressibilit y of thefirst liquid and the gas, the provision of at least onechamber with the similar size or larger volume than the totalvolume of the expansion tank leads to a reduction o f theoperational pressure differences to an acceptable l evel, suchas within a range of 25% above and below the ambien t airpressure.According to another aspect, a transformer system i sprovided. The transformer system comprises a transf ormer tankconfigured for holding at least one transformer win ding and apreservation system as described above. The transfo rmer tankis fluidically connected with the expansion tank an d filled P2024,0538 WO N / P240009WO01 July10,2025 -6 -with the first liquid, such that the at least one t ransformerwinding is completely immersed in the first liquid. Theexpansion tank is partially filled with the first l iquid andpartially filled with the gas. The at least one cha mber isfilled with the gas,and atleastthe base partof the tube- like vesselisfilled with the second liquid.Such a transformer system ensures that the transfor merwindings are always completely immersed in the firs t liquid,and that the first liquid and the second liquid nev er come indirect contact with each other. Moreover, the gas h eld partlyin the expansion tank and partly in the at least on e chamberacts as a pressure cushion, whereas the second liqu id servesas a hydraulic barrier between the gas in the expan sion tankand the ambient air as described above. Together, t he secondliquid in the tube-like vessel and the gas in the e xpansiontank will create a barrier for preventing the conta ct of thefirst liquid with the ambient without the use of an y flexiblebag ordiaphragm.Moreover,the system ensuresthe pressureon the expansion tank remains in equilibrium with t heatmosphericpressure,notgenerating anyrelevant overpressure orpartialvacuum condition.Optionally, the expansion tank is arranged above th etransformer tank and is fluidically connected to th etransformertankwith a firstconduit.Optionally, the at leastone chamberisarranged ata sidewallofthetransformer tank and is fluidically connected to th eexpansion tank with a second conduit, in particular theconduit as described above with regard to the prese rvationsystem. The above arrangement of components enables a simpleconstruction and integration of the described parts of thetransformersystem. P2024,0538 WO N / P240009WO01 July10,2025 -7 - Optionally,a densityofthe second liquid exceeds thedensity of the first liquid. For example, the first liquidmay be an insulating mineral oil and the second liq uid may besilicon or an ester, in particular a synthetic este r, such asEnvirotemp™ 360 (E360) or Midel™ 7131, or another h igh-densityliquid.Bychoosing liquidswith different densities,a mixing of the two liquids is prevented even in th e unlikelycase that the two liquids should enter the same par t, such asthe at least one chamber, due to overpressure or un der-pressure situationsexperienced bythe transformer system.Optionally, the gas comprises at least one of confi nedambientair,dryairordrynitrogen.Provision of a volume ofconfined ambientand / ordehumidified airordry nitrogeninhibits any unwanted chemical interactions between the gasand the firstand second liquid,respectively. Optionally,the second upperpartofthe tube-like vessel comprisesa desiccant,in particulara silica gel. Provisionof the desiccant removes water from the second liqu id and / orthe ambient air surrounding the open end, thereby i nhibitingunwanted chemicalreactionsbetween them.Optionally, at least one of the first and the secon d upperpart of the tube-like vessel is transparent and con figured asa level indicator for the first liquid in the expan sion tank.By monitoring the level of the second liquid in the firstand / or second upper part of the tube-like vessel, o ne canindirectly measure the level of the first liquid in theexpansion tank, thus obliviating the need for a sep arate oillevelindicatorthere. P2024,0538 WO N / P240009WO01 July10,2025 -8 -The present disclosure comprises several aspects of atransformer system. Every feature described with re spect toone of the aspects is also disclosed herein with re spect tothe other aspects, even if the respective feature i s notexplicitly mentioned in the context of the specific aspect.The accompanying figuresare included to provide a furtherunderstanding. In the figures, elements of the same structureand / orfunctionalitymaybe referenced bythe same reference signs.Itisto be understood thatthe embodiments shown inthe figures are illustrative representations and ar e notnecessarilydrawn to scale.Figures 1A to 1D show different views of an improve dtransformersystem.Figure 2 shows, in a schematic manner, the operatin gprinciple ofan improved preservation system.Figures 3A and 3B show two views of a u-shaped, tub e-likevessel holding a liquid acting as a hydraulic barri er.Figures 4 and 5 show the volumetric expansion and d ensity ofvarious liquids suitable for the disclosed systems.Figure 6 shows a perspective view of a high voltagetransformer comprising an improved preservation sys tem.Figures 1A to 1D show, in a schematic manner, diffe rent viewsof an improved transformer system according to a fi rstembodiment of the present disclosure. In particular , Figure1A shows a first side view, wherein parts of a supp ortstructure have been removed for better understandin g. Figures P2024,0538 WO N / P240009WO01 July10,2025 -9 - 1B to 1D show,respectively,a backview,a normal side view and a top view ofthe transformersystem ofFigure 1A.Figures 1A to 1D only show the main components of t hetransformersystem,which are relevantwith regard toinsulating liquid preservation of a liquid used forinsulating and / or cooling one or more transformer c oils andsealing a gas of the transformer system from the en vironment.Other components of a complete transformer system a re shown,forexample,in the perspective view ofFigure 6.As can be seen in Figures 1A to 1D, the transformer system 1comprises a relatively large transformer tank 2 cap able ofholding one ormore transformercoils(notshown). The transformertankisfilled completelywith a first liquid,orat least to a degree sufficient to completely immer se thetransformercoilsin a specified temperature range suchas -30 °C to +160 °C. The first liquid typically is aninsulating mineral oil. Since liquids in general an d mineraloil in particular have a very low compressibility a nd apositive volumetric expansion coefficient, the firs t liquidheld in the transformer tank 2 will increase as thetransformer windings warm up during operation. To p revent thetransformer tank 2 from bursting and avoid the cont act of theinsulating liquid with ambientair,a preservation system 10is provided, e.g., system for preserving the insula tingliquid ofthe transformersystem.The preservation system 10 comprises an expansion t ank 11,which is partially filled with the first liquid and partiallyfilled with a gas,such asa non-renewed volume of normalair, dehumidified air or dry nitrogen. The expansio n tank 11is physically arranged above a lid 6 of the transfo rmer tank P2024,0538 WO N / P240009WO01 July10,2025 -10 - 2 and attached thereto bymeansofseveralsupport struts12.It is fluidically connected to the inside of the tr ansformertank 2 by means of a first conduit 13. In the embod imentshown in Figures 1A to 1D, a first end of the condu it 13 isconnected to an upper port at the lid 6 of the tran sformertank 2 and a second end of the conduit 13 is connec ted to alowerportofthe expansion tank11.During operation of the transformer system 1, the l evel ofthe firstliquid within the expansion tank11 will rise dueto a heating and thermal expansion of the first liq uid.Accordingly, the volume available for the gas filli ng theremaining part of the expansion tank 11 is reduced. To avoidan overpressure within the expansion tank 11, the e xpansiontank 11 is connected to a further chamber 14. The c hamber 14and the expansion tank 11 are fluidically connected by meansof the second conduit 15 which connects an upper po rt of theexpansion tank11 with a firstportofthe chamber 14.In the described embodiment, the chamber 14 is atta cheddirectly to a sidewall of the transformer tank 2 an d isphysically separated from the expansion tank 11. As can bebest seen in Figure 1B, the chamber may be arranged betweentwo struts 12 for holding the expansion tank 11. No te thatthe chamber 14 does not hold any liquids under norm aloperating conditions of the transformer system 1. I nstead, itsimplyenlargesthe volume ofgasprovided in the preservation system 10.Since gas,unlike liquids, can berelatively easily compressed, it effectively serves as apressure cushion or buffer in case the level of the firstliquid in the expansion tank11 rises. P2024,0538 WO N / P240009WO01 July10,2025 -11 -To compensate for the remaining increases in pressu re in thepreservation system 10, a second port of the chambe r 14 isconnected to a tube-like vessel 16. In the depictedembodiment shown in Figures 1A to 1D, the tube-like vessel 16is directly attached to the outside wall of the cha mber 14.As described in more detail below with respect to F igures 2,3A and 3B, the tube-like vessel 16 comprises a firs t upperpart,which isfluidicallyconnected to the second portofthe chamber 14 and a second upper part which is ope n to theenvironment, i.e., to ambient air surrounding the t ransformersystem 1. To avoid any direct contact between the a mbient airand the gas held in the expansion tank 11 and the c hamber 14,a second liquid (notshown in Figures1A to 1D)is providedin a base part of the tube-like vessel 16, which ac ts as aflexible sealofthe preservation system 10.Figure 2 shows, in a schematic manner, a cross-sect ionthrough a transformer system 1, such as the transfo rmersystem 1 of Figures 1A to 1D. Note that in Figure 2 some ofthe components of the preservation system 10 are in dicated asseparate parts, such as the chamber 14 and the tube -likevessel 16. This is done for representational simpli city anddoes not imply that these parts are separate from e ach otherin an assembled state of the transformer system 1 a s shown,for example, for the high voltage transformer of Fi gure 6.As can be seen in the cross-section of Figure 2, th etransformer tank comprises three transformer windin gs 3,which are immersed in an insulating first liquid 4, such asmineral oil. Optionally, the transformer tank 2 may beinternally divided into different chambers (not sho wn). Forexample, each transformer winding 3 may be placed i n aseparate chamber of the transformer tank 2. In this case, P2024,0538 WO N / P240009WO01 July10,2025 -12 -each chamber of the transformer tank 2 can be separ atelyfilled with an insulating firstliquid 4.The main chamber of the transformer tank 2 and / or t urrets 5formed at a lid 6 of the transformer tank 2 are con nected byone or several conduits 13a and 13b to a protection device 7through which the first liquid 4 can flow. Note tha t theturrets5 are also filled with the firstliquid 4. Theprotection device 7 may be a so-called Buchholz rel ay, whichis used for identifying an excessive presence of ga ses in thefirstliquid 4 and / orforidentifying an excessive flow ofthe first liquid 4 from the transformer tank 2 (inc luding theturrets 5) to the expansion tank 11 (and vice-versa ).Parts of the first liquid 4 displaced from the tran sformertank 2 flow through the conduits 13a, 13b and / or 13 c to alower part 11a of the expansion tank 11. The remain der of thevolume of the expansion tank 11 is filled with a ga s 18, suchas dry air or dry nitrogen. Note that there is no p hysicalbarrier between the first liquid 4 in the lower par t 11a andthe gas18 in the upperpart11b.Accordingly,the gas18should be chosen such that it does not cause any un wantedchemicalreactionswith the firstfluid 4.Thisis incontrast to conventional solutions, in which a rubb er bag ordiaphragm is often used as a physical barrier betwe en theliquid and the air, leading to the maintenance issu esdescribed earlier.The upper part 11b comprises an upper port 19, whic h isconnected bymeansofthe second conduit15 to acorresponding port of the chamber 14. The chamber 1 4 isfurther connected by means of a third conduit 17, w ith afirst upper part 16a of the tube-like vessel 16. No te that at P2024,0538 WO N / P240009WO01 July10,2025 -13 -least a part of the first upper part 16a is also fi lled withthe gas 18. In contrast, a base part 16b of the ves sel 16 isfilled with a second liquid 20. The second liquid m ay be anester, such as FR3 or E360. Such materials do not e vaporateunder normal atmospheric conditions and also do not reactwith the gas18.Ifthe pressure in the chamber14 isincreased, a height of a column of the second liqui d 20 inthe firstupperpart16a isreduced.Inversely,if a pressurein the chamber 14 is reduced, the height of the col umn of thesecond liquid 20 in the first upper part 16a rises.Figures 3A and 3B show an exemplary configuration o f thetube-like vessel16 attwo differenttemperatures. Forexample, the situation shown in Figure 3A may corre spond to aminimum temperature of the transformer system 1, wh en is hasbeen switched off.Accordingly,the temperature of the firstliquid 4 corresponds, more or less, to the temperat ure of theambient air. In contrast, the situation depicted in Figure 3Bmay correspond to a maximum operating temperature o f thetransformer system 1, when the transformer windings 3 arefullyloaded. Ascan be seen in Figures3A and 3B,the vessel16 isformed bya u-shaped tube.Itcomprisesa firstleg 21,a second leg22 and a connecting part 23 interconnecting the fir st leg 21and the second leg 22. The first leg 21 comprises a firstport 24 and is otherwise closed with respect to theenvironment.The firstport24 serves,asshown in Figure 2,to connect the vessel 16 to the chamber 14. The sec ond leg 22comprises a further port 25, which may simply be an open endofthe tube,ora hole orgrate in the wallofthe tube. Accordingly,the second leg 22 isessentiallyopen to theenvironment of the transformer system 1. As the tem perature P2024,0538 WO N / P240009WO01 July10,2025 -14 -of the transformer windings 3 and hence the first l iquid 4rises, the level of the first liquid 4 in the expan sion tank11 also rises. Accordingly, a pressure of the gas 1 8 in thechamber 14 is increased, and the second liquid 20 a rranged inthe connecting part 23 of the vessel 16 is pushed p artiallyfrom the firstleg 21 to the second leg 22. Based on typicalconfigurationsofthe transformer tank2,the maximum pressure of the gas 18 inside the chamb er 14,acting as a pressure cushion chamber, does not chan ge by morethan ± 0.1 bar. Note that this corresponds to rough ly to aheight difference corresponding to ± 1 meter of the column ofthe second liquid 20 in the first leg 21 and / or sec ond leg 22ofthe vessel16.In a transformersystem having a heightofseveral meters, a correspondingly large u-shaped tu be may beprovided simplyatthe outside wallofthe chamber 14 asshown, for example, in Figure 1B. Thus, neither any furthermeasures or components nor any parts extending beyo nd thedimensionsofthe transformertank2 are needed to compensatefor the pressure changes within the transformer sys tem 1.Moreover, in case all or relevant parts of the u-sh aped tubeare transparent,the absolute heightofthe second liquid inthe first leg 21 or the second leg 22, or the diffe rence inheight indicates the level of the first liquid 4 in theexpansion tank, alleviating the need for a separate oil-levelindicatorthere.To further improve the sealing function of the pres ervationsystem 10 and avoid any unintentional reactions bet ween theambient air and the second liquid 20, a desiccant 2 6 that islighterthan the second liquid 20 isplaced in the upperpartof the second leg 22. For example, a film of silica gel maybe placed as a protective layer over the second liq uid 20 as P2024,0538 WO N / P240009WO01 July10,2025 -15 -shown in Figures 3A and 3B. Silica gel only evapora tes attemperatures above 250 °C, which lies well above th e normaloperating temperature ofthe transformersystem 1. Thus, undernormalconditions,neitherthe second liquid 20 northe desiccant26 needsto be replaced.Figures 4 and 5 show the volumetric expansion of mi neral oil,FR3, a natural ester, and Envirotemp™ 360 (E360), a syntheticester.Otheresters,such asMidel™ 7131 synthetic ester,orother high-density liquids may also be used. As can bederived from Figure 5 in particular, both esters ha ve ahigher density than mineral oil. Moreover, their vo lumetricexpansion coefficient is lower at the relevant temp eraturerange of-20 to +160°C asshown in Figure 4.Based on the coefficients derived from Figures 4 an d 5, andthe dimensions of the various parts, one can derive a minimumheight displacement Dh min of the second liquid 20 in the firstleg 21 at a minimum temperature, and a maximum leve l heightdisplacement Dh max of the second liquid 20 in the first leg 21at a maximum temperature. In particular, for a tran sformertank of approximately 5m x 2.4m x 4m, an oil volume of 35,9m³at T = 25°C, and an expansion coefficient of 0.0007 51 / K, theoil volume varies by 3.1m³ over an assumed temperat uredifference of 115 °C. This corresponds to a pressur e ofroughly 3020 N / m² or 0.03 bar at the level of the c over.Furtherassuming an additionalgasvolume of1.4m³ providedby the chamber 14, and a height Hturret of 0,35m, o ne obtainsDh = (Hexp-Hexpmin)*( ρoil / ρse) and P min = Dh*g* ρ. ForTmin = - 0°C and E360, P min is roughly 3120 N / m², correspondingto 03.1 bar, resulting in Dh min = 323 mm as shown in Figure3A. 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 P2024,0538 WO N / P240009WO01 July10,2025 -16 -shown in Figure 3A. Correspondingly, for the maximu moperating temperature,one obtainsPmax=10030 N / m²corresponding to 0.99 bar, resulting in Dh max = 1065 mm asshown in Figure 3B.The maximum pressure of the gas 18 inside the chamb er 14 andin contact with the first liquid 2, will be exposed to + / - 1meter of column of the first liquid 4, which conver ts forinsulating mineral oil to less than + / - 0.1 bar. Ac cordingly,there is no risk of excessive dissolution of the ga s 18 inthe first liquid 4. Any vapors or water contained i n the gas18 will condensate in the chamber 14, ensuring that only theconfined gas, in particular dry air, flows back and forwardbetween the expansion tank11 and the chamber14.Figure 6 shows a practical implementation of a pres ervationsystem 10 ofthe described type in a complete high voltagetransformer 30, such as transformer used in electri calsubstations. Ascan be seen in Figure 6,the transformertank2 isreinforced by a number of support ribs 8 attached t o theoutside of a transformer tank 2. To limit the overa ll size ofthe high voltage transformer 30, instead of one lar ge chamber14,three separate chambers14a to 14cin the form ofrectangular cuboid are arranged between and next to two ofthe reinforcement bars 8 provided at a front wall o f thetransformertank.Note thatthe three chambers14a to 14caremounted below an expansion tank 11, e.g., at or nea r aground-level of the high voltage transformer 30. Th e threechambers14a to 14care interconnected bya series of conduits15 which equalize the gaspressure in the chambers 14a to 14c. P2024,0538 WO N / P240009WO01 July10,2025 -17 -Note that the other parts of the high voltage trans former 30remain essentially unchanged with respect to previo usdesigns. In particular, the location and general sh ape of theexpansion tank 11 may remain the same. However, due to thefact that the expansion tank 11 does not need to co mprise aninternal rubber bag and / or elastic diaphragm, it ma y beconstructed in a rigid manner. Moreover, the overal l volumeand, hence, size of the expansion tank 11 can be re ducedsince it is not necessary to accommodate a rubber b ag or anoil level indicator at the expansion tank 11. Inste ad, asshown in Figure 6, the tube-like vessel 16 serves d irectly asan oil-level indicator as detailed before with resp ect toFigures3A and 3B.The described preservation system 10 and overall co nstructionof the transformer system 1 and high voltage transf ormer 30alleviates the need for regular maintenance with re spect tothe preservation system 10. In the unlikely situati on of anoverflow of the first liquid 4 from the expansion t ank 11,the excess liquid will accumulate in the chamber 14 and canbe easily drained from there. In the opposite case, if theexpansion tank 11 is completely emptied, for exampl e due to aleak in the transformer tank 2, a volume of the sec ond liquid20 from the tube-like vessel 16 may accumulate in t he chamber14. Even in this case, there is no risk of the seco nd liquid20 entering the expansion tank 11 or of the two liq uids 4 and20 mixing. However, in case the entire volume of th e secondliquid 20 is emptied into the chamber 14, a sealing functionbetween the ambient air and the gas 18 and the firs t liquid 4in the expansion tank11 isno longerprovided. P2024,0538 WO N / P240009WO01 July10,2025 -18 -The embodiments shown in Figures 1 to 6 as stated r epresentexemplary embodiments of the improved transformer s ystem andcombine a preservation and sealing system. Therefor e, they donot constitute a complete list of all embodiments a ccordingto the improved systems and devices. Actual transfo rmersystemsand preservation systemsmayvaryfrom theembodiments shown in terms of the arrangements of t heircomponents, the specific configuration of the compo nents, aswellasthe materialsused,forexample.

[0002] P2024,0538 WO N / P240009WO01 July10,2025 -19 - Reference Signs 1 transformersystem 2 transformertank 3 transformercoil 4 firstliquid 5 turret 6 lid (ofthe transformertank) 7 protection device 8 supportrib 10 preservation system 11 expansion tank 11a lowerpart 11b upperpart 12 supportstrut 13 firstconduit 14 chamber 15 second conduit 16 (tube-like)vessel 16a firstupperpart 16b base part 17 third conduit 18 gas 19 upperport 20 second liquid 21 firstleg 22 second leg 23 connecting part 24 firstport 25 second port 26 desiccant 30 high voltage transformer

Claims

P2024,0538 WO N / P240009WO01 July10,2025 -20 - Claims1. A preservation system (10) for a liquid-immersedtransformer(30),comprising:- an expansion tank (11) configured for holding varia blevolumesofa firstliquid (4)and a gas(18), respectively;- at least one chamber (14) formed separate from theexpansion tank (11) and configured for holding a fu rthervolume of the gas (18), the chamber (14) being flui dicallyconnected to an upper part (11b) of the expansion t ank(11), such that the gas (18) above the variable vol ume ofthe first liquid (4) can flow to and from the at le ast onechamber(14);and- a u-shaped, tube-like vessel (16) with a first uppe r part(16b) connected to the at least one chamber (14), a secondupperpartopen to an environment,and a base part (16b) fluidically connecting the first upper part (16a) a nd thesecond upperpartand being configured forholding a second liquid (20) separating the gas (18) from amb ientair.

2. The preservation system (10) of claim 1, furthercomprising a conduit(15)interconnecting an upper port(19)of the expansion tank (11) with a first port of the at leastone chamber(14).

3. The preservation system (10) of claim 1 or 2, wh erein aheight of the first upper part (16a) and / or the sec ond upperpartofthe tube-like vessel(16)exceeds1 meter.

4. The preservation system (10) of any one of claim s 1 to 3,wherein the expansion tank(11)isconfigured asa solidP2024,0538 WO N / P240009WO01 July10,2025 -21 -cylindrical tank made from a metal material without anyinternalorexternalflexible parts.

5. The preservation system (10) of any one of claim s 1 to 4,wherein the at least one chamber (16) is configured as asolid rectangular cuboid made from a metal material , thetube-like vessel (16) being mounted at a sidewall o f therectangularcuboid.

6. The preservation system (10) of any one of claim s 1 to 5,wherein the preservations system (10) is configured such thatthe furthervolume ofthe gasheld in the atleast onechamber (14) limits the variation of an internal pr essure ofthe expansion tank(11)to plusorminus25% ofan external pressure ofthe ambientairorless. 7.A transformersystem (1),comprising:- a transformer tank (2), configured for holding at l eastone transformerwinding (3);and- a preservation system (10) according to any one of claim 1to 6,wherein- the transformer tank (2) is fluidically connected w ith theexpansion tank (11) and filled with the first liqui d (4),such thatthe atleastone transformerwinding (3) is completelyimmersed in the firstliquid (4);- the expansion tank (11) is partially filled with th e firstliquid (4)and partiallyfilled with the gas(18);- the at least one chamber (14) is filled with the ga s (18);and- at least the base part (16b) of the tube-like vesse l (16)isfilled with the second liquid (20).P2024,0538 WO N / P240009WO01 July10,2025 -22 -8. The transformer system (1) of claim 7, wherein t heexpansion tank (11) is arranged above the transform er tank(2) and is fluidically connected to the transformer tank (2)with a firstconduit(13).

9. The transformer system (1) of claim 7 or 8, wher ein the atleast one chamber (14) is arranged at a sidewall of thetransformer tank (2) and is fluidically connected t o theexpansion tank(11)with a second conduit(15),in particular the conduit(15)according to claim 2.

10. The transformer system (1) of any one of claims 7 to 9,wherein a density of the second liquid (20) exceeds a densityofthe firstliquid (4).

11. The transformer system (1) of any one of claims 7 to 10,wherein the first liquid (4) is an insulating miner al oil.

12. The transformer system (1) of any one of claims 7 to 11,wherein the second liquid (20) is silicon or an est er oranotherhigh-densityliquid.

13. The transformer system (1) of any one of claims 7 to 12,wherein the gas (18) comprises confined ambient air or atleastone ofdehumidified airordrynitrogen.

14. The transformer system of any one of claims 7 t o 13,wherein the second upper part of the tube-like vess el (16)comprisesa desiccant(26),in particulara silica gel.

15. The transformer system of any one of claims 7 t o 14,wherein atleastone ofthe firstupperpart(16a) and thesecond upper part of the tube-like vessel (16) is t ransparentP2024,0538 WO N / P240009WO01 July10,2025 -23 -and configured as a level indicator for the first l iquid (4)in the expansion tank (11), in particular as an oil -levelindicator.

Images