A method for leak decreasing or repairing a cooling circuit in a power plant generator stator

Abstract

A method for preventing, repairing or decreasing a leak in a cooling circuit connection ring (22) of an electric generator, e.g. stator (20), of a power plant. This is especially to allow to extend the operation to said connection ring during a (planned) outage or to prolong an end-of-life electric generator. Said method comprises a step of disconnecting cooling fluid inlet of and cooling fluid outlet said connection ring from a cooling fluid circuit of said electric generator, afterwards a step of applying a coating to the cooling fluid inlet with first compressed gas and finally a step of curing the applied coating with second compressed gas.

Description

A METHOD FOR LEAK DECREASING OR REPAIRING A COOLING CIRCUIT IN A POWER PLANT GENERATOR STATORTECHNICAL FIELD

[0001] The present disclosure relates to methods for preventing, decreasing or repairing a leak in an electrical conduction element of an electric generator of a power plant. Specifically, for preventing, decreasing or repairing a leak in a connection ring of an electric generator connected to a steam turbine or a gas turbine. This can serve especially for extending connection ring operation to a (planned) outage or to prolong operation of an end-of-life electric generator. This in particular relates to electric generators connected steam turbines with an output of ca. 250 MW or higher.BACKGROUND

[0002] Electrical machines, such as motors and generators, generally comprise a rotor structure and a stator structure. Large electric generators may be e.g. electrically excited generators or permanent magnet excited generators (PMG). The rotor of an electrical machine rotates with respect to the stator. The rotor may be the inner structure and the stator the outer structure. The stator in this case thus surrounds, e.g. radially, the rotor. Alternatively, the configuration may be the opposite, i.e. the rotor surrounds, e.g. radially, the stator.

[0003] Electric generators for turbines can include an internal rotor and an external stator. In this configuration, the rotor is connected with a steam turbine or a gas turbine. Such turbines are used in power plants.

[0004] In electric generators, cooling is important. Generators generally include electrical conduction elements which heat up in use. An increase in temperature of at least some electrical conduction elements may lead to decrease of performance or even to failure of the electrical conduction elements. Decrease of performance leads to efficiency losses of the generator. To reduce the temperature of the electrical conduction elements, a cooling fluid may be used. This cooling fluid is passed along an inside of the electric conduction elements. Specifically, the cooling fluid is going through an inside, e.g. through an internal cooling fluidpassage, of the electrical conduction element to remove heat from the electrical conduction elements and reduce their temperature.

[0005] An electrical conduction element of a generator may for example be a stator bar. A stator bar may include a plurality of solid and hollow electrical conductors or strands for carrying electrical current induced in the electrical conductors. A cooling fluid may be circulated through the hollow electrical conductors for cooling the stator bar. Likewise, a rotor bar could be cooled by circulating a cooling fluid through it.

[0006] Connection rings, also known as phase rings, are another example of electrical conduction elements of an electric generator. Connection rings generally conduct the stator current (considering that the stator is the armature of the generator) out of the stator winding, e.g. the stator bars.

[0007] A cooling system may be provided for reducing the temperature of the electrical conduction elements. A cooling system may comprise a pump which causes the cooling fluid to move from e.g. a tank, to and through the electrical conduction element to be cooled, and then out of the electrical conduction element and to a heat exchanger. When going through the electrical conduction element, the cooling fluid is heated, and when going through the heat exchanger afterwards, the cooling fluid is cooled back down. The cooled cooling fluid can be directed again to the electrical conduction element. A cooling fluid circuit for cooling can thereby be provided.

[0008] A cooling fluid may be a liquid, e.g. water, distilled water, water glycol, mineral oil, or any other liquid suitable for cooling. A cooling fluid may be a gas in other examples. For example, a cooling gas may be sulfur hexafluoride. Other suitable gases for cooling may also be used.

[0009] If the electrical conduction elements for a generator deteriorate, they may leak the cooling fluid or allow cooling liquid to be contaminated with gas, e.g., hydrogen used for further cooling. For example, leaking is caused by hydrogen that surrounds a liquid cooling within a generator such that hydrogen is penetrating the liquid cooling due to a pressure difference. For instance, a crevice may appear in a wall of an electrical conduction element, allowing the leakage of the cooling fluid. The passage of time may increase the risk of having leakages of the cooling fluid. If an electrical conduction element comprises several portions that are attached, e.g. brazed to each other, the attachment region between the portions may have an increased risk of deterioration, and therefore of leaking the cooling fluid.

[0010] A leak can be detected during operation or during an outage. When a leakage is detected during an outage of the generator due to maintenance operations, the electricalconduction element should be replaced. If not replaced, the damage to the electrical conduction element, and possibly to adjacent elements, may be greater later on. However, replacing a leaking electrical conduction element may be very difficult and may require disassembly of the entire generator or significant parts thereof. Especially problematic and time consuming is removing the rotor from an electric generator if that is needed for the repair. This greatly increases the time of the outage and thereby extends the duration in which there is no supply of electrical power. It should be noted that both removing and installing the rotor is a task that includes considerable safety (e.g., EHS) risks.

[0011] In some cases, a leakage may force an unplanned outage. An unplanned outage is an outage that is not included in a schedule for maintenance of an electrical generator. This kind of outage creates an interruption in energy production. It may create accumulation of e.g., unused coal or other fuel for a power plant which requires space, and can be an environmental hazard and a major financial burden.

[0012] In some cases, the location of the leakage may be determined, but accessing that location may be complicated or impossible without overhaul or disassembly. In other cases, a leakage may be detected, but the exact location may be unknown. In both cases, the electrical conduction element may have to be at least partially removed from the generator, which may require removing other adjacent components and may be time consuming and economically costly. Several days may be needed to unmount, repair and mount again the affected electrical conduction element, and possibly the other removed components. In some cases, for example if a stator bar is leaking, the stator may need to be rewound. If the location of the leakage cannot be determined, the repair process may be even more complicated and time consuming. The known methods of repairing a leakage require determining a place of leakage in order to patch it. This is specifically challenging for leaking connection rings. For connection rings, an external application of e.g., anaerobic cement is a known method of repairing. This method requires vacuum and so is difficult in practice. Another method is tig brazing which requires access to a connection ring.SUMMARY

[0013] The present disclosure seeks to overcome problems known in the art. Specifically, the present disclosure provides safer and faster methods for preventing, decreasing or repairing a leak in electrical conduction elements of an electric generator and which do not require determining the place of leakage. In particular, said leaking electrical conducting elements are connection rings. Additionally or alternatively, the present disclosure seeks to limit or eliminate unplanned outages of electrical generators including connection rings thatare due for maintenance. The purpose of the invention is not to substitute replacement of a used connecting ring with a new one, but to allow to extend connection ring operation to a (planned) outage. It can also be used for end-of-life operation.

[0014] The principles of the invention will be disclosed below and explained. It should be noted that any explanations are provided for a better understanding of the invention and should not be understood as exhaustive or limiting to the invention. Expressions like comprising and including are not exhaustive and other elements can be present in addition to the ones that are mentioned as included or comprised.

[0015] In an aspect, this disclosure provides a method for preventing, decreasing or repairing a leak in a connection ring of an electric generator of a power plant.

[0016] Preventing can include prevention of occurrence of leaking when a connection ring (electrical conduction element) can be expected to start leaking. This is meant to include a situation when a used connection ring is expected to fail due to occurrence of leaking and the intention is to allow this used connection ring to operate until a (planned) outage during which it will be replaced with a permanent solution, i.e., a new connection ring. Decreasing can include decreasing of an intensity, for example magnitude or flow rate, of a leak, limiting a number of leaks or both. This is meant to include a situation when a connection ring is failing due to leaks and a generator including said connection ring cannot operate anymore. Said decreasing of a leak is to allow the electric generator to operate despite leaking connection ring which will allow, for example, to produce energy during the time when a new connection ring is transported to the electric generator. Repairing can include stopping leaks, at least for a period of time. This is to include a situation in which a leak or leaks are stopped at least to allow an electric generator to operate until a (e.g., planned) outage during which a repaired connection ring is replaced with a new connection ring. In other words, the invention addresses prolonging of operation of an electric generator including a leaking connection ring until a (planned) outage. The invention also can prolong operation of end-of-life electric generators. It follows that the invention should not be confused or be treated as a substitute for providing a new connection ring.

[0017] The electric generator comprises a generator rotor installed in the electric generator and a connection ring cooling fluid circuit. In embodiments, the cooling fluid circuit is for cooling down both the electric generator and the connection ring or is a connection ring electric generator cooling fluid circuit. The connection ring comprises a cooling fluid inlet, a cooling fluid outlet, and an internal cooling fluid passage extending from the cooling fluid inlet to the cooling fluid outlet. The cooling fluid inlet and also the cooling fluid outlet can include a portion of a connection ring. The internal cooling fluid passage can be described as an internal coolingfluid passage for cooling the connection ring. The method comprises: disconnecting the cooling fluid inlet and the cooling fluid outlet from the cooling fluid circuit; applying a coating to the cooling fluid inlet with first compressed gas (for example, air) such that an inside dimeter of the internal cooling fluid passage is coated from the cooling fluid inlet to the cooling fluid outlet; curing the applied coating with second compressed gas which is, optionally, at a higher temperature than the first compressed gas. Using higher temperature shortens the curing time which can be beneficial. The method steps are conducted with the generator rotor installed in the electric generator. The coating has to be thermally conductive to allow cooling of the connection ring. Thermally conductive materials are known in the art and selecting those is within the knowledge and skills of a skilled person. Thermally conducting may herein be regarded as having a sufficient thermal conductivity to allow heat exchange between a connection ring (electrical conduction element) and a cooling liquid sufficient for cooling of the connection ring (electrical conduction element). Cooling can be understood as at least one of preventing temperature from rising, slowing pace of temperature increase and decreasing temperature. In normal operation, for example of a connection ring, cooling can mean preventing temperature from rising, for example, above operating temperatures and decreasing temperature, for example, to operating temperatures. It should be noted that a generator rotor is installed in an electric generator even if some elements of the generator rotor are removed, e.g., shrouds.

[0018] The method provides coating from the cooling fluid inlet to the cooling fluid outlet and so across the entirety of the internal cooling fluid passage of the connection ring. This omits the need to know where exactly a leak is present. This is relevant as for connection rings it is often impossible to locate the place where there is a leak as some places are not accessible when the connection ring is installed in an electric generator and when a rotor is installed in the electric generator. Since there is no need to know where a leak is or is expected to be, any steps relating to finding the location (if that is possible and accessible) can be omitted and so this allows to perform the repair, limitation or prevention operation faster. The method allows servicing of a connection ring without the need to remove the rotor which additionally allows for much faster repair of the connection ring. This additionally allows the repair of leaks that are in inaccessible places. It should be noted that in addition to saving time, omitting of the need to remove a generator rotor increases safety of operation as the procedure for removing such a heavy equipment is risky. Same or similar benefits are provided when the method is used for decreasing of leaking and when the method is used to prevent leaking for connection rings expected to start leaking. Those will not be listed for the reasons of brevity.

[0019] A coating used for preventing, decreasing or repairing a leak in a connection ring of an electric generator of a power plant is any coating suitable for its purpose. A coating for connection rings according to the disclosure offers thermal stability within the ranges of operation of a given connection ring. Depending on a type of cooling fluid, a coating for connection rings can be stable up to at least 65°C, 70°C or higher, for example, 75°C, 80°C, 85°C, 90°C, 95°C, 100°C or more. In other words, within the limits in which a given connection ring is used. Additionally, this coating for connection rings is not chemically reactive with a given cooling fluid used within the range of operating temperatures of a connection ring. As the result, a use of chemically reactive or not thermally stable coatings will not lead to prevention, decreasing or repairing of a connection ring. A coating for connection rings according to the disclosure is not a thermal insulator, but operates as a thermally conducting coating. Providing a coating that is a thermal insulator to a connection ring would prevent the connection ring from being cooled down and, hence, would be responsible for overheating of the connection ring. A coating for connection rings according to the disclosure can be applied using gas, for example compressed air. It follows that various types of polymers can be used as the coating for connection rings. In embodiments, a coating includes an epoxy coating. An epoxy coating proved to be suitable for most applications. This coating is a good example of not chemically reactive, thermally stable and thermally conductive coating.

[0020] Optionally, the method further includes reconnecting the cooling fluid inlet and the cooling fluid outlet to the cooling fluid circuit such that the cooling fluid can be circulated through the internal cooling fluid passage. This step is to enable a restart of the electric generator and so it is not essential for the servicing of the connection ring. Servicing can mean preventing, decreasing or repairing.

[0021] In embodiments, the cooling fluid is selected from the group including, but not limited to, deionized water, oil and Sulphur hexafluoride. The first and second compressed gas can be air. Selecting air as the gas is beneficial due to its availability.

[0022] In embodiments, the applied coating is an epoxy coating. An epoxy coating tends to adhere well and dry quickly, and it is a durable coating once cured. In some of these examples, the epoxy coating has viscosity in the range of 440-700 mPa*s. This epoxy coating proved to provide a required stability of coating used in water-cooled connection rings thereby enabling to arrive at a connection ring that will work with the coating for a required period of time, e.g., until a planned outage or until a new connection ring is delivered. The viscosity in the range of 440-700 mPa*s for an epoxy coating provided the highest performance in terms of how fast the coating can be applied and also in terms of ease of application. It remains unclear why the combination of epoxy coating with viscosity within the range of 440-700 mPa*sprovides the best performance. Without wishing to be bound by any theory, it can be explained that the coating thickness is the most uniform for this combination of features. Uniform thickness of the coating ensures that all leakage is minimized or stopped and that curing time is decreased. Additionally, a possibility for formation of blockages within the connection rings during application of the coating is prevented or minimized.

[0023] In embodiments, the method steps of disconnecting the connection ring from the cooling fluid circuit is, comprises or is limited to removing hydraulic and electrical connections between the connection ring and the electric generator.

[0024] In embodiments, the method further comprises attaching a connector to the cooling fluid inlet, before applying the coating, for securing a device for applying the coating. This step facilitates the application of the coating. The connector can include braze threaded fittings or clamps.

[0025] In embodiments, the method further comprises checking whether there is a blockage in the internal cooling fluid passage before applying the coating. Said checking is selected from the group including, but not limited to, using a borescope and using thermal analysis. Using the borescope is done after disconnecting the cooling fluid inlet and the cooling fluid outlet from the cooling fluid circuit. Thermal analysis can be used before disconnecting the cooling fluid inlet and the cooling fluid outlet from the cooling fluid circuit or after, e.g., with the use of an auxiliary circuit that includes a heated medium. This step is entirely optional and can be omitted. Specifically because any blockage would create overheating of the connection ring which would be visible for example due to damaged winding or discoloration.

[0026] In embodiments, the method further comprises removing a passivation layer from at least one element selected from the group including the cooling fluid inlet, the cooling fluid outlet, and the internal cooling fluid passage, wherein said step is completed before applying the coating. This step improves the stability of the coating as it prevents or limits liberation of elements of the coating together with elements of the passivation layer. The most beneficial is removal the passivation layer from the cooling fluid inlet, the cooling fluid outlet, and the internal cooling fluid passage. This step is not essential and is optional. For example, it is not needed in situations in which there is no or insignificant amount of said passivation layer. Some liberation may also be permitted in other situations, depending on a particular connection ring. In embodiments, removing a passivation layer can include at least one element selected from the group including, but not limited to, blasting cleaning and acid etching. Optionality, following the removing of the passivation layer, the method can include further checking for blockages with a borescope. At this point, the usage of thermal analysis is not preferable as there is no temperature difference to observe. In any case, attaching an auxiliary circuit for providing aheating medium needed in that situation by thermal analysis takes more time and so negatively affects the total time of repair, prevention or decreasing of a leak.

[0027] In embodiments, the method further comprises testing the connection ring for leaks before applying the coating. The testing can comprise at least one from performing a pressure test and performing a vacuum decay test. This test prevents from the need for servicing of a connection ring that ultimately does not possess any leaks. A pressure test can include, for example, Helium testing. Electric generators include typically more than a single connection ring and it may be difficult to observe which connection ring is leaking. Confirming leaking or lack thereof is relevant for determination whether the method is for repairing, decreasing or for preventing of a leak.

[0028] In embodiments, the internal cooling fluid passage has substantially the same (i.e. substantially constant) internal diameter from the cooling fluid inlet to the cooling fluid outlet. Said substantially same (constant) internal diameter offers simplification in application of the coating. This has an impact on how fast the coating can be applied. Additionally, this configuration of the internal cooling fluid passage limits or prevents formation of any blockages with the coating within the internal cooling fluid passage.

[0029] Another aspect relates to a connection ring of an electric generator of a power plant, the connection ring comprising a cooling fluid inlet, a cooling fluid outlet, and an internal cooling fluid passage extending from the cooling fluid inlet to the cooling fluid outlet. The inside diameter of the internal cooling fluid passage is coated from the cooling fluid inlet to the cooling fluid outlet with a coating. The coating is thermally conductive. In an embodiment, the coating is an epoxy coating.

[0030] A further aspect of this disclosure relates to an electric generator of a power plant. The electric generator comprises a connection ring, a generator rotor installed in the electric generator, and a connection ring cooling fluid circuit. The connection ring comprises a cooling fluid inlet, a cooling fluid outlet, and an internal cooling fluid passage extending from the cooling fluid inlet to the cooling fluid outlet. The connection ring comprises a coating applied on the internal cooling fluid passage such that it is coated from the cooling fluid inlet to the cooling fluid outlet. Optionally, the coating is an epoxy coating, and / or the cooling fluid is deionized water, and / or the coating is stable up to at least 100°C.

[0031] The above-mentioned aspects and embodiments can be altered such that a connection ring is replaced by a conducting element. In line with this, in another aspect of the present disclosure, the method as described in any of the above embodiments with the exception that that the connection ring is replaced with an electrical conduction element of anelectric generator of a power plant is provided. The electrical conduction element comprises a cooling fluid inlet, a cooling fluid outlet, and an internal cooling fluid passage extending from the cooling fluid inlet to the cooling fluid outlet. The internal cooling fluid passage cools down the electrical conduction element.

[0032] This method has the same benefits as those listed above, i.e., walls of the internal cooling fluid circuit can be repaired, improved or protected even if they are far from the inlet, and / or if they are at locations which cannot be easily accessed or accessible at all. Also, knowing the precise location of a crack through which leaking occurs is not needed, as the internal walls are coated in their entirety.

[0033] Therefore, the electrical conduction element may be repaired or protected in an easy and fast way. Also, this method may be particularly useful for deferring or avoiding significant repair expenses as the electrical conduction element approaches the end of its serviceable life. Instead of performing replacements which may require a long period of time that includes outage of the electric generator, this method may be applied. The use of the electrical conduction element may be extended until a planned outage.

[0034] Throughout this disclosure, an electrical conduction element may be understood as an element which is able to conduct electricity. For example, an electrical conduction element may comprise one or more components such as electric wires, cables or busbars which are configured to this end. As indicated before, an electrical conduction element comprises an internal cooling passage extending between an inlet and an outlet. The internal cooling passage is configured to receive a cooling fluid, e.g. water, water glycol, oil, or in general any suitable cooling fluid. The cooling fluid may be liquid. In other examples, the cooling fluid may be a gas.

[0035] Throughout this disclosure, an inlet may refer to an opening through which the coating will be applied to the internal walls of the electrical conduction element. It may, or may not, correspond to the opening of the electrical conduction element through which the cooling fluid enters the electrical conduction element for cooling it when the cooling fluid is circulated through the cooling fluid circuit. Similarly, an outlet of the electrical conduction element may be an outlet through which the cooling fluid exits the electrical conduction element when being part of the cooling fluid circuit, but this is not necessarily the case. At the same time, a cooling liquid inlet can be an inlet for a cooling liquid to enter a device, e.g., a connection ring, and a cooling liquid outlet can be an outlet for a cooling liquid to leave a device, e.g., a connection ring.

[0036] Herein, a leaking electrical conduction element (e.g., a connection ring) may be understood as an electrical conduction element which is damaged such that, if a fluid is circulated through at least one of the internal cooling fluid passages, from an inlet to an outlet, the fluid leaks. For example, the electrical conduction element may comprise one or more cracks through which the fluid escapes. The leaking electrical conduction element can be damaged from the inside, from the outside, or from both inside and outside. The damage can originate from corrosion which can be mechanical (e.g., pressure, hit), chemical (e.g., dissolution, chemical reaction) or which can be both mechanical and chemical.

[0037] The coating applied, for example to the internal walls, may be an epoxy coating. An epoxy coating tends to adhere well and dry quickly, and it is a durable coating once cured.

[0038] Disconnecting the electrical conduction element may comprise not completely removing the electrical conduction element from the electric generator. As with the above method it is not necessary to have access to the region of the electrical conduction element which is damaged, the electrical conduction element can be kept close to the original position of the electrical conduction element with respect to the generator. Less components adjacent to the electrical conduction element may need to be removed. The procedure may therefore be easier and faster.

[0039] The electrical conduction element may be a stator bar, a bus bar, may comprise a diode bridge, or, as mentioned above, may be a connection ring.

[0040] Particular aspects, examples, embodiments and elements of aspects, embodiments, and examples disclosed herein can be combined together in any number, repetition, and order to form new aspects, embodiments, and examples that form part of this disclosure. All those combinations are not described herein solely for the reason of conciseness of the disclosure.BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 shows a flowchart of a method for preventing, decreasing or repairing a leak in a connection ring of an electric generator of a power plant.

[0042] Figure 2 schematically illustrates a perspective view of an example of a stator of an electric generator with a generator rotor removed for the sake of clarity.

[0043] Figure 3 schematically illustrates an example of a plurality of connection rings.

[0044] Figure 4 schematically illustrates an example of a single connection ring.

[0045] Figure 5 schematically illustrates a cut opened portion of a connection ring whose internal walls have been coated by applying and curing a coating as described herein.

[0046] Figure 6 schematically illustrates a coated inner wall of a connection ring.DETAILED DESCRIPTION OF EXAMPLES

[0047] Reference now will be made in detail to embodiments of the present disclosure, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation only, not as a limitation. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present disclosure. For instance, features illustrated or described as part of one embodiment can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present disclosure covers such modifications and variations as come within the scope of the appended claims and their equivalents.

[0048] Figure 1 shows a flowchart of a method 10 for repairing a leaking, or for decreasing or preventing a leak in a connection ring of an electric generator of a power plant. In examples, this includes an electric generator that is due for a planned outage, for example in 1 -2 weeks, 3-4 weeks, 1-2 months or in a different period. In examples, this may be an electric generator that is an end-of-life electric generator with a number of parts requiring servicing. The electric generator comprises a generator rotor installed in the electric generator and a connection ring cooling fluid circuit. The connection ring 22 comprises a cooling fluid inlet 27, a cooling fluid outlet 26, and an internal cooling fluid passage extending from the cooling fluid inlet 27 to the cooling fluid outlet 26 for cooling the connection ring 22. In other embodiments, an electrical conduction element is used instead of the connection ring. In some or all embodiments, the power plant can be a fossil power plant or any other power plant including a liquid cooled electric generator. In examples, the electric generator is a 250 MW unit. In other examples, the electric generator is in range of 250 MW - 1700 MW or above. In same or other embodiments, each electric generator includes from 3 to 20 connection rings. For example, 12, 15, 17, 18, or 19. Figure 2 schematically illustrates a perspective view of an example of a stator of an electric generator. The stator 20 comprises several connection rings 22 in which the method 10 may be performed.

[0049] The method steps are conducted with the generator rotor installed in the electric generator. Installed can be understood as a generator rotor being supported by bearings within the electric generator. Thus, installed does not require an electrical connection. On the one hand, known methods of repairing connection rings require removing the generator rotor to beable to approach the connection ring. It follows that the herein proposed methods are not limited as the known methods are. On the other hand, removing of the generator rotor is time consuming and involves risks (e.g., damaging, EHS) and so keeping the generator rotor offers benefits in terms of pace of servicing and decreasing risks.

[0050] The internal cooling fluid passage may have a substantially the same (constant) internal diameter from the cooling fluid inlet 27 to the cooling fluid outlet 26.

[0051] The method comprises, at block 11 , disconnecting the cooling fluid inlet 27 and the cooling fluid outlet 26 from the cooling fluid circuit. The cooling fluid circuit can be for cooling down the electric generator and at least one connection ring. The cooling fluid circuit can include one or more elements selected from the group including a tube, a pipe, a valve, a heat exchanger, and a pump. In other examples, the method comprises disconnecting the connection ring 22 from a remainder of a cooling fluid circuit. A cooling fluid circuit may be formed by the connection ring and one or more elements which are connected to the connection ring. Such additional elements may include further connection rings, tubes or pipes, a heat exchanger, a pump or compressor and others.

[0052] A cooling fluid may be circulated through the cooling fluid circuit for decreasing a temperature at a desired region or component. A fluid may for example be a liquid. In other examples, a fluid may be a gas. The cooling fluid may be selected from the group including, but not limited to, deionized water, oil and sulphur hexafluoride. A compressed gas may be air.

[0053] When the connection ring 22 is disconnected from the cooling fluid circuit, the cooling fluid circuit is open and the path for the fluid is broken. In specific examples, disconnecting includes disconnecting 11 the cooling fluid inlet 27 and the cooling fluid outlet 26 from the cooling fluid circuit. Only a single connection ring can be disconnected or two or more connection rings may be disconnected. Disconnecting may comprise removing hydraulic and electrical connections between the connection ring 22 and the electric generator.

[0054] The method further comprises, at block 12, applying a coating 28 to the cooling fluid inlet 27 with a first compressed gas such that an inside diameter of the internal cooling fluid passage is coated from the cooling fluid inlet 27 to the cooling fluid outlet (26). In a more specific example, applying 12 a coating 28 to the cooling fluid inlet 27 may comprise applying first compressed air such that an inside diameter of the internal cooling fluid passage is coated from the cooling fluid inlet 27 to the cooling fluid outlet 26. At block 13, the method further comprises curing the applied coating with second compressed gas which can be at a higher temperature than the first compressed gas. The first gas and the second gas may be the same gas. The coating 28 is thermally conductive.

[0055] Curing can involve an initial curing which can be made in 4-5 hours at 50°C. Very dry air can allow for a faster initial curing. Elevating temperature for faster curing may not always be desired at least for the reasons of safety. Curing can further involve a final curing that is done over a longer period, for example 8-12 hours. The gas flow during application of the coating and curing is adjusted as needed. Application of a coating requires a certain magnitude of gas flow, and a skilled person can derive that magnitude by observing how the coating is spreading in the internal cooling fluid passage. The exact value depends, among others, on the coating physicochemical parameters, diameter and shape of the internal cooling fluid passage. Gas flow can be kept at a stable rate or varied during each step. In specific examples, higher values of gas flow rate are used in the application of the coating and during the initial stage of curing, and lower values of airflow rate are used in the last stage of curing.

[0056] At block 14, the method may optionally include reconnecting the cooling fluid inlet 27 and the cooling fluid outlet 26 to the (remainder of the) cooling fluid circuit such that a cooling fluid can be circulated through the cooling fluid circuit, in particular through the internal cooling fluid passage.

[0057] The coating cannot be a thermal insulator and so is thermally conductive. The coating has to be able to maintain its stability at a working temperature of the cooling fluid circuit.

[0058] Regarding the step of block 11 , if the connection ring 22 is for example mechanically attached to the stator 20, the connection ring may be disconnected from the stator.

[0059] For example, the connection ring 22, see figures 3 and 4, may be separated from other connection rings 22 and / or separated from a corresponding stator bar 25 (see figure 1 ). As the connection ring has been disconnected from other components of the cooling fluid circuit, a path for a cooling fluid is broken. A closed loop for liquid is no longer present.

[0060] In some examples, disconnecting 11 may comprise not completely removing the connection ring 22 from the electric generator. That is to say, the connection ring 22 may be detached from other elements of the cooling fluid circuit, optionally from the generator, but it may not have to be completely removed from the generator. The connection ring may be partially removed from the generator in some of these examples. In other examples, a partial removal may not even be necessary, and the connection ring may be kept in its original position in the generator.

[0061] After having detached the connection ring 22 and before applying the coating, a connector may be attached to the cooling fluid inlet 27 (the cooling fluid inlet may for example be a lead plug of a connection ring 22, see figure 4) for securing a device for applying thecoating to the connection ring 22. As compressed gas is used, the connector may ensure that the connection ring 22 is able to withstand the pressure during the application of the coating. One or more clamps, or a threaded fitting, may be used in some examples as connectors. The device for applying the coating may therefore be secured to the connection ring 22 later on, in particular to the inlet, through the connector. Optionally, the threaded fitting may be brazed to the inlet of the connection ring 22.

[0062] The method may further comprise checking whether there is a blockage in the internal cooling fluid passage before applying the coating. This step may be performed before attaching a connector to an inlet or may be performed afterwards. Any suitable method for checking may be used. For example, a borescope may be used to check whether there is a blockage in the internal cooling fluid passage for cooling the connection ring 22. Another option is to use thermal analysis. An infrared red camera may for instance be used to check the temperature of the connection ring when a liquid is circulated through it. If there is a blockage, it will show as a region having a higher temperature in the image taken. If a blockage is found, it can be removed prior to applying a coating.

[0063] Also before applying the coating, the method may further comprise removing a passivation layer from at least one element selected from the group including the cooling fluid inlet 27, the cooling fluid outlet 26, and the internal cooling fluid passage. This step may for example be performed after checking for blockages in the internal cooling fluid passage. Removing the passivation layer may for example comprise blasting cleaning or acid etching.

[0064] After removing the passivation layer, the method may further comprise checking whether there is a blockage in the internal cooling fluid passage of the connection ring caused by the removal of the passivation layer. This may be performed similarly as already explained before.

[0065] The method may further comprise, also before applying the coating, testing the connection ring 22 in search of leaks. This may serve as a confirmation that the connection ring 22 is indeed damaged, e.g. it comprises one or more cracks, and needs to be repaired. Testing may be performed after removing the passivation layer in some examples, but it could also be performed before this step. Testing may comprise performing a pressure test or a vacuum decay test in some examples. Other suitable ways of testing may be used.

[0066] The device for applying the coating at block 12 may be secured to the inlet of the connection ring 22. A nozzle of the device may for example be attached to a connector previously attached to the inlet of the connection ring 22, as explained before. A compressor may be comprised in the coating applicator device or may be separate from the coatedapplicator device. The compressor may be connected to the coating applicator device before starting to apply the coating.

[0067] At block 12, the coating may be applied in a continuous manner. This means that the flow of the coating product is maintained during a certain period of time such that the coating product spreads along the (walls of the) internal cooling passage of the connection ring 22 and for example reaches a desired thickness.

[0068] The coating applied at block 12 may be an epoxy coating. The epoxy coating may form a durable coating once cured. The epoxy coating may also adhere easily to the internal walls of the internal cooling fluid passage of the connection ring 22. In some examples, the epoxy coating may be a two component epoxy including a resin and a curing agent. In some examples, the epoxy coating has a viscosity in the range of 440-700 mPa*s.

[0069] After the coating has been applied and cured, the method may comprise checking whether there is a blockage in the internal cooling fluid passage due to the application and curing of the coating. If a blockage is found, it will be removed.

[0070] The method may further comprise testing the connection ring in search of leaks. In this manner, it may be checked that the coating has been applied correctly and that the connection ring is therefore not damaged and will not leak when a cooling fluid is circulated through it.

[0071] After the connection ring has been repaired or protected, and after it has been connected to the remainder of the cooling fluid circuit, one or more further actions may be performed. For example, if insulation had to be removed for disconnecting the connection ring from the cooling fluid circuit, the insulation may be added. The connections between the connection ring 22 and the generator may be checked prior to insulating. Paint may also be added if deemed necessary.

[0072] Method 10 may be performed on a connection ring 22. The stator 20 of figure 1 schematically shows a plurality of connection rings 22.

[0073] In a further aspect of the disclosure, a connection ring of an electric generator of a power plant is provided. The connection ring 22 comprises a cooling fluid inlet 27, a cooling fluid outlet 26, and an internal cooling fluid passage extending from the cooling fluid inlet 27 to the cooling fluid outlet 27. An inside of the internal cooling fluid passage is coated from the cooling fluid inlet 27 to the cooling fluid outlet 26 with a coating 28. The coating 28 is thermally conductive.

[0074] Explanations, features and details of the previous aspect (method 10) may be applied and incorporated to this aspect as well as to the following aspects and vice versa.

[0075] Figure 3 schematically illustrates a plurality of connection rings 22. A single connection ring is schematically illustrated in figure 4. As can be seen in figure 3 and in figure 4, a connection ring 22 may comprise more than one outlet 26. For example, if the coating is to be applied to the connection ring 22 of figure 4 through the lead plug 23, the lead plug 23 may be deemed to comprise the inlet 27. The connection ring 22 of figure 4 comprises three backsets 24, and therefore the backsets 24 may be deemed to comprise outlets 26 which may be sealed before the coating is applied through the inlet 27.

[0076] Figure 5 illustrates a portion of a connection ring 22 which has been coated by applying and curing a coating to the internal walls of the connection ring 22 as previously explained. The walls covered by the coating 28 can be seen in this figure. In this example, the connection ring comprises two internal cooling fluid passages. In other examples, a connection ring may comprise a single passage or more than two passages. A closer look at a coated inner wall of an internal cooling fluid passage of a connection ring 22 can be seen in figure 6.

[0077] In a further aspect of the disclosure, an electric generator of a power plant is provided. The electric generator comprises a connection ring 22, a generator rotor installed in the electric generator, and a connection ring cooling fluid circuit. The connection ring 22 comprises a cooling fluid inlet 27, a cooling fluid outlet 26, and an internal cooling fluid passage extending from the cooling fluid inlet 27 to the cooling fluid outlet 27. The connection ring 22 comprises a coating 28 applied on the internal cooling fluid passage from the cooling fluid inlet 27 to the cooling fluid outlet 26. The coating 28 is thermally conductive.

[0078] The generator may be connected to a steam turbine or to a gas turbine in examples.

[0079] The coating may be an epoxy coating. The cooling fluid may be deionized water.The coating may be stable up to at least 1002C.

[0080] This written description uses examples to disclose a teaching, including the preferred embodiments, and also to enable any person skilled in the art to put the teaching into practice, including making and using any devices or systems and performing any incorporated methods. The patentable scope is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims. Aspects from the various embodiments described, as well as other known equivalents for each such aspects, can be mixed andmatched by one of ordinary skill in the art to construct additional embodiments and techniques within the scope of this disclosure. If reference signs related to drawings are placed in parentheses in a claim, they are solely for attempting to increase the intelligibility of the claim, and shall not be construed as limiting the scope of the claim.

Claims

CLAIMS1 . A method (10) for preventing, decreasing or repairing a leak in a connection ring (22) of an electric generator of a power plant, wherein the electric generator comprises a generator rotor installed in the electric generator and a connection ring cooling fluid circuit, the connection ring (22) comprising a cooling fluid inlet (27), a cooling fluid outlet (26), and an internal cooling fluid passage extending from the cooling fluid inlet (27) to the cooling fluid outlet (27), the method comprising: disconnecting (11 ) the cooling fluid inlet (27) and the cooling fluid outlet (26) from the cooling fluid circuit, applying (12) a coating (28) to the cooling fluid inlet (27) with first compressed gas such that an inside diameter of the internal cooling fluid passage is coated from the cooling fluid inlet (27) to the cooling fluid outlet (26); curing (13) the applied coating (28) with second compressed gas; and wherein the method steps are conducted with the generator rotor installed in the electric generator, and wherein the coating (28) is thermally conductive.

2. The method of claim 1 , wherein the method further includes reconnecting (14) the cooling fluid inlet (27) and the cooling fluid outlet (26) to the cooling fluid circuit such that the cooling fluid can be circulated through the internal cooling fluid passage.

3. The method of any of claims 1 - 2, wherein the cooling fluid is selected from the group including deionized water, oil, and Sulphur hexafluoride.

4. The method of any of claims 1 - 3, wherein disconnecting (11 ) comprises removing hydraulic and electrical connections between the connection ring (22) and the electric generator.

5. The method of any of claims 1 - 4, further comprising attaching a connector to the cooling fluid inlet before applying the coating (28), for securing a device for applying (12) the coating (28).

6. The method of any of claims 1 - 5, further comprising removing a passivation layer from at least one element selected from the group including the cooling fluid inlet (27), the cooling fluid outlet (26), and the internal cooling fluid passage, before applying (12) the coating (28).

7. The method of claim 6, wherein removing the passivation layer includes at least one element selected from the group including blasting cleaning and acid etching.

8. The method of any of claims 1 - 7, wherein the internal cooling fluid passage has substantially a constant internal diameter from the cooling fluid inlet (27) to the cooling fluid outlet (26).

9. The method of any of claims 1 - 8, wherein the coating (28) is an epoxy coating.

10. The method of claim 9, wherein the epoxy coating has a viscosity in the range of 440-700 mPa*s.

11. The method of any of claims 1 - 10, wherein the first compressed gas and the second compressed gas is compressed air and wherein the second compressed gas has a higher temperature than a temperature the first compressed gas.

12. A connection ring (22) of an electric generator of a power plant, the connection ring (22) comprising a cooling fluid inlet (27), a cooling fluid outlet (26), and an internal cooling fluid passage extending from the cooling fluid inlet (27) to the cooling fluid outlet (27), an inside of the internal cooling fluid passage is coated from the cooling fluid inlet (27) to the cooling fluid outlet (26) with a coating (28), wherein the coating (28) is thermally conductive.

13. The connection ring (22) of claim 12 wherein the coating (28) is an epoxy coating.

14. The connection ring of claim 13, wherein the epoxy coating is stable until at least 1002C.

15. An electric generator of a power plant, wherein the electric generator comprises a generator rotor and a connection ring according to any of claims 12 - 14.

16. The electric generator of claim 15, wherein the cooling fluid is deionized water.

17. The electric generator of claim 15 or 16, wherein the electric generator is connected to a steam turbine or a gas turbine.

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