Method for Operating a Cooling System of a Transformer

Abstract

A method for operating a cooling system of a transformer, wherein the transformer is cooled via a cooling liquid that circulates in the cooling system that includes a heat-exchanger, devices for increasing heat exchange performance of the at heat-exchanger and a controller, where in a normal operating state, the controller adjusts power of the devices for increasing the heat exchange performance as a function of a measured upper temperature and where, irrespective of the measured upper temperature, the controller refrains from activating the devices and/or operates the devices at a reduced power relative to the normal operating state if the lower temperature of the cooling liquid lies below a lower threshold value during operation of the transformer to achieve improved characteristics of the cooling system during operation under low environmental temperatures of the transformer, particularly in the case of a turn-on operation following lengthy storage in a cold state.

Claims

1. A method for operating a cooling system of a transformer having at least one transformer winding, a cooling liquid of the cooling system surrounding the at least one transformer winding and the cooling liquid circulating in the cooling system in a normal operating state of the transformer, at least heat produced in the at least one transformer winding being released into a surrounding atmosphere via the circulating cooling liquid, said cooling system comprising at least one heat-exchanger unit through which the cooling liquid can flow, for release of heat from the cooling liquid into the surrounding atmosphere, devices for increasing heat exchange performance of the at least one heat-exchanger unit, said devices interacting with the at least one heat-exchanger unit, a control unit for adjusting the heat exchange performance of the at least one heat-exchanger unit, an upper temperature being measured in at least one of (i) the cooling system and (ii) at the transformer and, in a normal operating state, the control unit adjusting a power of the devices for increasing heat exchange performance of the at least one heat-exchanger unit as a function of the measured upper temperature, the method comprising: measuring a lower temperature of the cooling liquid in the cooling system; and irrespective of the measured upper temperature, at least one of: i) refraining from activating, by the control unit, the devices for increasing heat exchange performance of the at least one heat-exchanger unit if the lower temperature of the cooling liquid lies below a lower threshold value during operation of the transformer and ii) operating, by the control unit, the device for increasing heat exchange performance of the at least one heat-exchanger unit at a reduced power relative to the normal operating state if the lower temperature of the cooling liquid lies below a lower threshold value S.sub.u during operation of the transformer.

2. The method as claimed in claim 1, wherein the devices for increasing heat exchange performance of the at least one heat-exchanger unit comprise at least one of (i) at least one ventilator unit and (ii) at least one pump unit which transports the cooling liquid through the at least one heat-exchanger unit in the normal operating state.

3. The method as claimed in claim 1, wherein the transformer is arranged inside a housing which is filled with the cooling liquid, and the heat-exchanger unit is arranged in a cooling circuit which is fluidically connected to the housing; and wherein the cooling liquid circulates through the cooling circuit in the normal operating state.

4. The method as claimed in claim 2, wherein the transformer is arranged inside a housing which is filled with the cooling liquid, and the heat-exchanger unit is arranged in a cooling circuit which is fluidically connected to the housing; and wherein the cooling liquid circulates through the cooling circuit in the normal operating state.

5. The method as claimed in one of claim 1, wherein the lower temperature of the cooling liquid is measured in that region of the cooling system in which the lowest temperature of the cooling liquid is to be expected.

6. The method as claimed in claim 1, wherein the lower temperature of the cooling liquid is measured in that region of the housing of the cooling system.

7. The method as claimed in claim 3, wherein the lower temperature is measured via a first temperature sensor; and wherein the first temperature sensor is arranged in at least one of (i) a floor region of the housing and (ii) a region of a junction of an outlet of the cooling circuit into the housing.

8. The method as claimed in claim 6, wherein the lower temperature is measured via a first temperature sensor; and wherein the first temperature sensor is arranged in at least one of (i) a floor region of the housing and (ii) a region of a junction of an outlet of the cooling circuit into the housing.

9. The method as claimed in claim 1, wherein the devices for increasing heat exchange performance of the at least one heat-exchanger unit comprises at least one pump unit which transports the cooling liquid through the at least one heat-exchanger unit in the normal operating state; and wherein the at least one pump unit is activated by the control unit if the lower temperature of the cooling liquid during the operation of the transformer lies below the lower threshold value S.sub.u and above a lower limit temperature.

10. The method as claimed in claim 1, wherein the devices for increasing heat exchange performance of the at least one heat-exchanger unit comprise at least one pump unit which transports the cooling liquid through the at least one heat-exchange unit in the normal operating state; and wherein the at least one pump unit is periodically activated and deactivated by the control unit if the lower temperature of the cooling liquid during the operation of the transformer lies below a lower limit temperature, which lower limit temperature is lower than the lower threshold value.

11. The method as claimed in claim 1, wherein the devices for increasing heat exchange performance of the at least one heat-exchanger unit comprise at least one ventilator unit and at least one pump unit which transports the cooling liquid through the at least one heat-exchanger unit in the normal operating state; and wherein the control unit: deactivates the at least one ventilator unit and one of (i) deactivates and (ii) periodically activates and deactivates the at least one pump unit if the lower temperature of the cooling liquid lies below a lower limit temperature, said lower limit temperature being lower than the lower threshold value; deactivates the at least one ventilator unit and activates the at least one pump unit if the lower temperature of the cooling liquid lies between the lower limit temperature and the lower threshold value; and operates the at least one ventilator unit and the at least one pump unit based on the normal operating state if the lower temperature lies above the lower threshold value.

12. The method as claimed in claim 1, wherein the lower threshold value lies in a range between 10 C. and 40 C. above a congealing temperature of the cooling liquid.

13. The method as claimed in claim 12, wherein the lower threshold value lies between 20 C. and 30 C.

14. The method as claimed in claim 1, wherein the lower threshold value lies in a range between 10 C. and 40 C. above that temperature at which the kinematic viscidity of the cooling liquid is greater than or equal to 1800 mm.sup.2/s.

15. The method as claimed in claim 1, wherein the lower threshold value lies in a range between 20 C. and 30 C.

16. The method as claimed in claim 1, wherein the cooling liquid is selected from the group comprising mineral-oil-based liquids, synthetically produced oils comprising silicone oil, synthetically produced esters, and biologically produced liquids.

17. The method as claimed in claim 1, further comprising: measuring a current value, which specifies or is proportional to the current flowing through the transformer on a primary side or secondary side of the transformer; and increasing the power of the devices for increasing heat exchange performance by the control device in comparison with the normal operating state, irrespective of the measured at least one upper temperature, if the measured current value exceeds an upper threshold value and if the lower temperature of the cooling liquid lies above the lower threshold value.

18. The method as claimed in claim 1, wherein the transformer comprises one of a power transformer and a reactor.

19. A control device for a cooling system of a transformer having at least one transformer winding, the cooling system containing a cooling liquid \ which circulates within the cooling system during operation of the at least one transformer; wherein the control device adjusts at least a power of devices for increasing heat exchange performance of at least one heat-exchanger unit, said devices interacting with the at least one heat-exchanger unit; wherein the at least one heat-exchanger unit is configured to release heat from the cooling liquid into the surrounding atmosphere, wherein the control device is connected to a first temperature sensor (12) for measuring a lower temperature of the cooling liquid; and wherein the control device is configured to: measure a lower temperature of the cooling liquid in the cooling system; and irrespective of the measured upper temperature, at least one of: (i) refrain from activating the devices for increasing heat exchange performance of the at least one heat-exchanger unit if the lower temperature of the cooling liquid lies below a lower threshold value during operation of the transformer and (ii) operate the devices for increasing heat exchange performance of the at least one heat-exchanger unit at a reduced power relative to the normal operating state if the lower temperature of the cooling liquid lies below a lower threshold value S.sub.u during operation of the transformer.

20. The control device as claimed in claim 19, wherein the transformer comprises one of a power transformer and a reactor.

21. A transformer comprising: at least one transformer winding; and a first temperature sensor for determining a lower temperature of a coolant; a cooling system containing the cooling liquid for cooling the at least one transformer winding; wherein the cooling system includes: at least one heat-exchanger unit for release of heat from the cooling liquid into a surrounding atmosphere; devices for increasing heat exchange performance of the at least one heat-exchanger unit, said devices interacting with the at least one heat-exchanger unit; and a control unit for adjusting at least the power of the devices for increasing heat exchange performance of the at least one heat-exchanger unit; wherein the control device is configured as claimed in claim 13.

22. The transformer as claimed in claim 21, wherein the transformer comprises one of (i) a power transformer and (ii) reactor.

23. A non-transitory computer-readable medium encoded with instructions which, when executed by a processor, cause the control device to compare a measured lower temperature of the cooling liquid with a lower threshold value and to send at least one of (i) control signals and (ii) input/output signals to devices for increasing heat exchange performance of at least one heat-exchanger unit, the instructions comprising: program code for, measuring a lower temperature of the cooling liquid in the cooling system; and program code for, irrespective of the measured upper temperature, at least one of: i) refraining from activating, by the control unit, the devices for increasing heat exchange performance of the at least one heat-exchanger unit if the lower temperature of the cooling liquid lies below a lower threshold value during operation of the transformer and ii) operating, by the control unit, the devices for increasing heat exchange performance of the at least one heat-exchanger unit at a reduced power relative to the normal operating state if the lower temperature of the cooling liquid lies below a lower threshold value during operation of the transformer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to explain the invention further, reference is made in the following part of the description to the figures, from which further advantageous details and possible application fields of the invention can be derived. The figures are understood to be exemplary and, while exposing the character of the invention, do not restrict or even conclusively depict it in any way, in which:

[0032] FIG. 1 shows a schematic illustration of a transformer according to the invention;

[0033] FIG. 2 shows a schematic illustration of the lower temperature; and

[0034] FIG. 3 is a flowchart of the method in accordance with the invention.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

[0035] FIG. 1 shows a schematic illustration of a transformer 1 in accordance with the invention, with a transformer winding 3 that is wound around a transformer core 10. The transformer winding 3 comprises at least one lower-voltage winding and at least one higher-voltage winding, which are not illustrated further in the figure. The transformer 1 can also be formed as a reactor. Moreover, the transformer winding 3, more precisely the electrical conductors thereof, can be enwound with cellulose paper for the purpose of electrical insulation.

[0036] The transformer winding 3 and the transformer core 10 are arranged in a housing 2 that is filled with a cooling liquid 7. In addition to the cooling of the transformer 1, described in greater detail below, i.e., the cooling of at least the transformer winding 3 and the transformer core 10, the cooling liquid 7 also serves to electrically insulate the transformer 1. It is therefore particularly advantageous for the cooling liquid 7 to be a cooling liquid that is suitable for transformers, e.g., a transformer oil. Use is typically made of mineral-oil-based cooling liquids in this case, though synthetic liquids based on, e.g., silicone oil or esters, or ultimately even biological liquids that nonetheless have raised congealing temperatures, are also viable for use in transformers.

[0037] In order to dissipate the heat that is generated in the transformer winding 3 during the operation of the transformer 1 and that increases with the electrical load of the transformer 1, a cooling system is provided in which the cooling liquid 7 circulates. In order to release the heat energy in the cooling system efficiently from the cooling liquid 7 into the environment or the surrounding atmosphere, in particular the surrounding air, the cooling system comprises at least one heat-exchanger unit 5 and devices 15 that interact with the heat-exchanger unit 5 for increasing the heat exchange performance of the at least one heat-exchanger unit 5. In the present exemplary embodiment, the heat-exchanger unit 5 is linked via a cooling circuit 4 that is connected to the housing 2. The circulation of the cooling liquid 7 through the cooling system, i.e., in particular in the interior of the housing 2 and within the cooling circuit 4, can in principle occur as a result of natural convection. It should not be forgotten that the housing 2 of the transformer 1 also functions as a heat exchanger, though no separate devices 15 for increasing the heat exchange performance are usually provided for the housing 2.

[0038] In the exemplary illustrated embodiment, the devices 15 for increasing the heat exchange performance comprise both a ventilator unit 6 via which the heat-exchanger unit 5 can be cooled, and a pump unit 11 via which the natural convection of the cooling liquid 7 in the cooling system can be assisted or a forced convection can be effected, at least through the cooling circuit 4. It should be understood the number of heat-exchanger units 5, ventilator units 6, pump units 11 and cooling circuits 4 is unlimited and is freely selectable in accordance with the specific application, where systems comprising only one or a plurality of ventilator units 6 or only one or a plurality of pump units 11 are also conceivable.

[0039] For the purpose of the heat exchange, the heat-exchanger unit 5 in the present exemplary embodiment is cooled by the surrounding air, where the surrounding air absorbs heat from the heat-exchanger unit 5 or from the cooling liquid 7 that flows through the heat-exchanger unit 5. Cooler surrounding air can be supplied to the heat-exchanger unit 5 by natural convection and/or via at least the ventilator unit 6. If the cooling of the heat-exchanger unit 5 is produced via the ventilator unit 6 (as in the presently illustrated exemplary embodiment), surrounding air is sucked in by the ventilator unit 6 and blown onto the heat-exchanger unit 5 on an outlet side 9 of the ventilator unit 6, where the outlet side 9 faces towards the radiator 5.

[0040] In order to adjust the cooling of the transformer 1 when the transformer 1 is under load and generates heat, provision is made for a control device 8 which is connected in the present exemplary embodiment to the pump unit 11, the ventilator unit 6 and to a first temperature sensor 12 and a second temperature sensor 13, at least for the purpose of information transfer, this being indicated by the broken and dash-dot lines. In conventional control devices 8, an upper temperature T.sub.o of the cooling system is captured as an input variable and the required cooling power is adjusted as a function of the upper temperature T.sub.o. The upper temperature T.sub.o can be captured by the second temperature sensor 13, for example. In the present exemplary embodiment, the second temperature sensor 13 is configured as a sensor for hot cooling liquids, in particular a hot-oil sensor, which captures an upper temperature T.sub.o of the cooling liquid 7. It is, however, also conceivable for the upper temperature T.sub.o to be measured as a temperature in the transformer winding 3 or in the transformer core 10 or in the interior of a winding assembly, where combinations of different temperature measurements are also conceivable.

[0041] The devices 15 for increasing the heat exchange performance is usually driven by the control device 8 such that they are activated or their cooling power is increased when the measured upper temperature T.sub.o exceeds a predefined upper temperature threshold. It should be understood it is also possible to define a plurality of upper threshold values, where each threshold value is assigned a different cooling power.

[0042] In the case of operation involving problematic cold temperatures, i.e., if the temperature of the coolant 7 is low due to low environmental temperatures, or in the case of a cold start of the transformer 1, i.e., when the transformer 1 is started up under low environmental temperatures, then the cooling liquid 7 can exhibit a particularly high viscosity due to the low environmental temperature. As a result, the ability of the cooling liquid 7 to flow through the cooling circuit 4, in particular through the heat-exchanger unit 5, is reduced in an unacceptable manner, such that the cooling power of the heat-exchanger unit 5 is unavailable and the remaining cooling power of the housing 2 of the transformer 3 is insufficient. This behavior of the cooling liquid 7 under low environmental temperatures can, as described below, result in unacceptable operating states such as high temperatures in the transformer 1.

[0043] While the cooling liquid 7 in the region of the transformer winding 3 is heated by the heat that is produced during operation, and the viscosity reduces such that part of the cooling liquid 7 circulates in the cooling system, other parts of the cooling liquid 7, which are situated in the cooling circuit 4 and in the heat-exchanger unit 5, remain undercooled and therefore prevent a flow of the cooling liquid 7 through the cooling circuit 4 and heat-exchanger unit 5. This can be explained, e.g., by the fact that these elements of the cooling system are arranged further away from the transformer winding 3 and by the fact that these elements usually have a large surface area in order to achieve a heat exchange and/or occupy an exposed position and are significantly affected by the environmental temperatures. It can therefore occur that the temperature of the cooling liquid 7 in the cooling circuit 4 and in the heat-exchanger unit 5 remains at a very low temperature level over an extended time period. If a conventional control device 8 is then used, a high upper temperature T.sub.o would be measured (in the already heated cooling liquid 7, in particular in the upper region of the housing 2, or in the transformer winding 3 itself), which would trigger an additional cooling of the heat-exchanger unit 5 by the ventilator unit 6 and the pump unit 11.

[0044] However, this must be considered as disadvantageous, because the cooling liquid 7 that flows back from the heat-exchanger unit 5 and that should heat the remaining parts of the cooling liquid 7 stays at a lower temperature due to the additional cooling effected via the ventilator unit 6, such that a flow of the cooling liquid 7 through the cooling circuit 4 is not achieved as a result of the continued high viscosity of the cooling liquid 7 in the region of the heat-exchanger unit 5. In other words, the additional cooling of the cooling liquid 7 via the ventilator unit 6 can result in only part of the coolant 7 contained in the cooling system circulating in the cooling system, such that the heat cannot be dissipated efficiently and overheating of the transformer 1 can occur. It is particularly critical in this case that, due to the additional cooling, the heat-exchanger unit 5 cannot then contribute to the cooling of the circulating cooling liquid 7 and therefore the cooling circuit 4 cannot fulfill its function. The reason for this (as mentioned above) is the high viscosity of the cooling liquid 7, which has been cooled as a result of the low environmental temperature and additional cooling, in the cooling circuit 4 and in the heat-exchanger unit.

[0045] In order to overcome these disadvantages, in accordance with the present invention, a lower temperature T.sub.u of the cooling liquid 7 is measured in the cooling system and, irrespective of the measured at least one upper temperature T.sub.o, the control device 8 does not activate the devices 15 for increasing the heat exchange performance or operates the devices 15 for increasing the heat exchange performance at a reduced power relative to the normal operating state if the lower temperature T.sub.u of the cooling liquid 7 lies below a lower threshold value S.sub.u during the operation of the transformer 1. For the purpose of capturing the lower temperature T.sub.u of the cooling liquid 7, the first temperature sensor 12 in the present exemplary embodiment is arranged in a floor region 14 of the housing 2. The cooling liquid 7 having the lowest temperature sinks towards the floor of the housing 2 due to the high viscosity or the high density, while the cooling liquid 7 that is heated in the region of the transformer winding 3 rises. As a result, a first temperature sensor 12 arranged in the floor region 14 is particularly suitable for capturing the lower temperature T.sub.u of the cooling liquid 7, because the lowest temperature of the cooling liquid 7 can be expected in the floor region 14.

[0046] It is advantageous in this case for the lower threshold value S.sub.u, which is dependent on the composition of the cooling liquid 7 used, to be defined in relation to the congealing point or the congealing value of the cooling liquid 7. For example, it is found to be particularly advantageous if the lower threshold value S.sub.u lies e.g. 40 C. above the congealing point. Therefore, if the measured lower temperature T.sub.u of the cooling liquid 7 lies below the lower threshold value S.sub.u, e.g., possibly more than 20 C. below the lower threshold value S.sub.u, depending on the viscosity and the hydraulic relationships (e.g. the hydraulic resistance and the buoyancy force acting against this), adequate circulation of the cooling liquid 7 in the cooling circuit 4 or through the heat-exchanger unit 5 is not possible. In this case, the control device 8 will inventively prevent any additional cooling of the cooling liquid 7 in the heat-exchanger unit 5.

[0047] In according with the disclosed method of the invention, the additional cooling of the heat-exchanger unit 5 by the ventilator unit 6 in a cold start situation, which is detected if the measured lower temperature T.sub.u lies below the lower threshold value S.sub.u, is therefore either completely stopped or at least reduced, such that the cold cooling liquid 7 in the lower region of the housing 2 and the cooling liquid 7 in the cooling circuit 4 is more quickly brought into line with the temperature level of the already circulating cooling liquid 7, such that a viscosity of the cooling liquid 7 is obtained which allows the circulation of as far as possible all the cooling liquid 7 in the cooling system, in particular in the cooling circuit 4 and through the heat-exchanger unit 5, and functional cooling of the transformer 1 is thus achieved even when problems arise in relation to cold. As soon as the lower temperature T.sub.u reaches the lower threshold value S.sub.u or exceeds the lower threshold value S.sub.u, e.g., in a predefined transition temperature range T that lies above the lower threshold value S.sub.u (see FIG. 2), the normal operating state can be reactivated by the control device 8, such that the additional cooling of the heat-exchanger unit 5 is reactivated or the power of the ventilator unit 6 is increased to a normal value and is adjusted or controlled as a function of the measured upper temperature T.sub.o.

[0048] In a further embodiment of the method of the invention, the pump unit 11 is used as part of the devices 15 for increasing the heat exchange performance. The pump unit 11 contributes significantly to the transfer of heat away from the heated regions of the transformer 1 to the heat-exchanger units 5. By virtue of the heat transport and the heat that is released due to operation, the pump unit 11 can also be used to heat the cold (or even thickening) cooling liquid 7 in the region of the heat-exchanger unit 5. The pump unit 11 is also used to improve the flow situation by increasing the delivery pressure in the cooling circuit 4 in addition to the natural buoyancy factors. If a lower temperature T.sub.u of the cooling liquid 7 which lies below the lower threshold value S.sub.u is detected, then the control device 8 can control the pump unit 11 such that, by virtue of the pump unit 11, a greater volumetric flow of the insufficient throughput is effected in the cooling circuit 4 and/or in the heat-exchanger unit 5 and the cooling liquid 7 that has been heated by the transformer 1 is thus supplied to the heat-exchanger unit 5. Here, the ventilator unit 6 therefore remains deactivated because it has been detected that the lower threshold value S.sub.u has not been reached, while the pump unit 11 is activated.

[0049] In this context, reference is made briefly to FIG. 2, which depicts a schematic illustration of the relevant values of the lower temperature T.sub.u. The lower threshold value S.sub.u, at which the power of the devices 15 for increasing the heat exchange performance is inventively adapted, has already been discussed in detail. This lower threshold value S.sub.u can lie between 20 C. and 30 C. above the congealing point, for example.

[0050] The transition temperature range T, whose lower limit forms the lower threshold value S.sub.u, was also cited above. In this range of generally between 20 C. and 25 C. above the lower threshold value S.sub.u, it is possible to maintain an inventive reduction of the cooling, e.g., in order to ensure a corresponding heating of the cooling liquid 7 in the cooling circuit 4 and/or in the heat-exchanger unit 5, which allows operation of the devices 15 for increasing the heat exchange performance in normal operation. It is therefore possible to make allowance for, e.g., a time delay between the increase in the measured lower temperature T.sub.u in the housing 2 and the actual heating in the complete cooling circuit 4 and/or in the heat-exchanger unit 5.

[0051] As discussed above, the operation of the pump unit 11 below the lower threshold value S.sub.u can be advantageous. However, if the temperature T.sub.u lies below a lower limit temperature G.sub.u that lies in the region of the congealing point of the cooling liquid, e.g., only up to 5 C. above the congealing point, the viscosity of the cooling liquid 7 would be so high that continuous activation of the pump unit 11, due to the insufficient cooling resulting from the reduced throughput of cooling liquid 7 through the pump unit 11, would trigger the motor protection of the pump unit 11, and therefore the pump unit would remain deactivated for an extended period of time. The control unit 8 can therefore be configured such that the pump unit 11 either remains completely deactivated or is periodically activated and deactivated, while the ventilator unit 6 remains constantly deactivated, if the lower temperature T.sub.u lies below the lower limit temperature G.sub.u. Periodical operation can be achieved, for example, by alternately activating the pump unit 11 for a first time period, such as 10 min, and then deactivating the pump unit 11 for a second time period, e.g., of equal length to the first time period. In the temperature range between the lower limit temperature G.sub.u and the lower threshold value S.sub.u, and possibly within the transition temperature range T, the pump unit 11 remains activated, while the ventilator unit 6 remains deactivated or is operated at reduced power. The same naturally applies analogously for the pump unit 11 alone, if no ventilator unit 6 is present. If the lower temperature T.sub.u exceeds the lower threshold value S.sub.u or the transition temperature range T, the cooling can occurs in the normal operating state again based on the upper temperature T.sub.o in the context of the normal operating state.

[0052] If the cooling liquid 7 is, e.g., a mineral-oil-based cooling liquid, then the congealing temperature or the temperature at which the cooling liquid 7 exceeds a dynamic viscosity of 1800 mm.sup.2/s is approximately 45 C. The lower limit temperature G.sub.u, that is relevant for the operation of the pump unit 11 in particular, can be approximately 40 C. for exemplary purposes, while the lower threshold value S.sub.u, which is relevant for the operation of the ventilator unit 6 in particular, can be approximately 15 C. The associated transition temperature range T therefore terminates at a lower temperature of approximately 10 C. Beyond this, the control occurs in the normal operating state again based on the upper temperature T.sub.o.

[0053] Finally, it is also conceivable for a current value to be measured by a converter at the transformer 1, where the current value specifies or is at least proportional to the current flowing on a primary side or secondary side of the transformer 1. Various scenarios are suitable for a computer-based solution, which can also obtain the information from other regions and if necessary from the network control system. Taking the measured lower temperature T.sub.u of the cooling liquid 7 into account, it is therefore possible by including the load of the transformer 1 to achieve an advance cooling of the transformer 1 via the control device 8, if the transformer 1 is not in a situation with a critically low lower temperature T.sub.u of the cooling liquid 7 but an upper threshold value of the upper temperature T.sub.o of the cooling system has not yet been reached. In other words, an additional cooling can already be effected before the actual upper threshold value of the upper temperature T.sub.o is reached.

[0054] FIG. 3 is a flowchart of a method for operating a cooling system of a transformer 1 having at least one transformer winding 3, where the cooling liquid 7 of the cooling system surrounds the at least one transformer winding 3 and the cooling liquid 7 circulates in the cooling system in a normal operating state of the transformer 1, at least heat produced in the at least one transformer winding 3 is released into a surrounding atmosphere via the circulating cooling liquid 7, and where the cooling system comprises at least one heat-exchanger unit 5 through which the cooling liquid 7 can flow, for a release of heat from the cooling liquid 7 into the surrounding atmosphere, means 15 for increasing a heat exchange performance of the at least one heat-exchanger unit 5, where the means 15 interacts with the at least one heat-exchanger unit 5, a control unit 8 for adjusting the heat exchange performance of the at least one heat-exchanger unit 5, where an upper temperature T.sub.o is measured in either the cooling system or at the transformer 1 and where, in a normal operating state, the control unit 8 adjusts the power of the means 15 to increase heat exchange performance of the at least one heat-exchanger unit 5 as a function of the measured upper temperature T.sub.o.

[0055] The method comprises measuring a lower temperature T.sub.u of the cooling liquid 7 in the cooling system, as indicated in step 310.

[0056] Next, irrespective of the measured upper temperature T.sub.o, the control unit either (i) refrains from activating the means 15 for increasing the heat exchange performance of the at least one heat-exchanger unit 5 if the lower temperature T.sub.u of the cooling liquid 7 lies below a lower threshold value S.sub.u during operation of the transformer 1 or (ii) operates the means 15 for increasing the heat exchange performance of the at least one heat-exchanger unit 5 at a reduced power relative to the normal operating state if the lower temperature T.sub.u of the cooling liquid 7 lies below a lower threshold value S.sub.u during operation of the transformer 1, as indicated in step 320.

[0057] Thus, while there have been shown, described and pointed out fundamental novel features of the invention as applied to a preferred embodiment thereof, it will be understood that various omissions and substitutions and changes in the form and details of the methods described and the devices illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit of the invention. For example, it is expressly intended that all combinations of those elements and/or method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Moreover, it should be recognized that structures and/or elements and/or method steps shown and/or described in connection with any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.