Abstract:
The invention relates to a thermal power plant including a turbine ( 1, 2, 3 ) operated using at least one water-steam-loop ( 4 ), and a generator ( 5 ) driven by said turbine ( 1, 2, 3 ). According to the present invention, means ( 6, 7, 8, 9, 10 ) for cooling said generator ( 5 ) are provided, which means ( 6, 7, 8, 9, 10 ) are de signed for extracting condensate from said water-steam-loop ( 4 ), transporting it to said generator ( 5 ) and returning it into said water-steam-loop ( 4 ).

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates in general to a thermal power plant including a turbine operated using at least one water-steam-loop, and a generator driven by said turbine. The thermal power plant referred to can e.g. be a conventional steam power plant, a combined gas and steam power plant or a nuclear power plant.  
       BACKGROUND OF THE INVENTION  
       [0002]     Thermal power plants known in the prior art comprise a turbine, which is supplied with pressurized steam from a steam raising unit. The pressurized steam expands in the turbine and thereby causes the turbine to rotate. This rotational motion is transferred via a drive shaft to an electric generator, which generator converts the mechanical energy into electrical energy. The inevitable energy losses during the energy conversion in the generator lead to an undesirable heating of the generator. In order to avoid excessive generator temperatures, it is necessary to cool the generator during operation. In the prior art this task is achi eved using air, hydrogen or pure water as cooling media. The heated up cooling media is thereafter released directly into the environment or is re-cooled by a cooling device if a closed cycle cooling system is employed, in which case the thermal energy is released into the environment indirectly. However, the thermal efficiencies displayed by the above described thermal power plants of the prior art are considered unsatisfactory.  
         [0003]     Due to the above drawbacks in the prior art it is an object of the present in vention to provide a thermal power plant with an improved thermal efficiency.  
       SUMMARY OF THE INVENTION  
       [0004]     In order to solve the above object, according to the present invention, a power plant of the aforementioned type is provided, which is characterized in that means for cooling the generator are provided, which means are designed for extracting condensate from the water-steam-loop, transporting it to the generator and returning it into the water-steam-loop.  
         [0005]     The present invention is based on the insight, that by using condensate from the water-steam-loop for cooling the generator and returning the heated up condensate into the loop, the excess thermal energy produced by the generator can be transferred into the water-steam-loop with minimal energy loss. This measure improves the thermal efficiency of the overall thermal power plant significantly.  
         [0006]     It is advantageous, if the means for cooling the generator comprise an extraction device for extracting the condensate from the water-steam-loop, wherein the extraction device is arranged close to a location of the lowest condensate temperature within the water-steam-loop. Therewith the condensate transported to the generator is essentially of the lowest temperature available within the water-steam-loop. This allows for an optimum cooling effect of the generator. The generator can therefore be cooled to a relatively low temperature, which increases its efficiency. Further, no or very little additional cooling, like air cooling etc., needs to be provided to the gen erator, which results in the thermal energy produced by the generator being transferred to the water-steam-loop to a very high degree. This again leads to a high thermal efficiency of the overall power plant.  
         [0007]     Further, it is practical if the extraction device is arranged in close proximity to a condenser converting the steam exiting the turbine into condensate. After the expanded steam exits the turbine it needs to be cooled down further in order to cause a phase transition into water. This task can be achi eved using a condenser, which may be connected via a cooling circuit to a cooling facility, like a cooling tower etc. The water or condensate exiting the condenser is of a temperature considerably below the vaporization temperature and is therefore well suited for cooling the generator. In order to utilize the condensate of relatively low temperature for generator cooling, it is therefore practical to position the extraction device in close proximity after the condenser.  
         [0008]     Additionally, it is advantageous if the extraction device is arranged close to a condensate pump. A condensate pump is usually positioned immediately after the condenser in order to move the condensate on in the loop. If the extraction device is arranged as above, it is due to the proximity to the condenser ensured, that the temperature of the condensate is very low, and further the momentum provided to the condensate by the condensate pump may be of assistance for moving the condensate to the generator.  
         [0009]     Further, it is expedient in case the extraction device is arranged between the condensate pump und a low-pressure pre-heating device designed for pre-heating the condensate and fed with excess steam from the turbine. The low -pressure pre-heating device represents the first stage for raising the temperature of the condensate before it is again converted into high-pressure steam for driving the turbine. For this purpose thermal energy provided by excess steam supplied from the turbine is used. As the condensate used for cooling the generator needs to be of a comparatively low temperature, it is expedient to extract the respective condensate for this purpose before it enters the low -pressure pre-heating device.  
         [0010]     In addition, it is advantageous if the means for cooling the generator further comprise a return device for returning the condensate into the water-steam-loop after the condensate has been heated up during cooling the generator. The thermal energy extracted from the generator in the form of an elevated temperature of the condensate is mo st effectively fed back into the water-steam-loop by returning it into the loop before pre-heating takes place. As the condensate in the loop is at this point still of a relatively low temperature, the injection of higher temperature condensate has a signi ficant effect on the resulting temperature of the combined condensate. In this case the generator takes over part of the function of the low pressure pre -heating device, as the condensate entering the device already has an elevated temperature. Thereby, a higher temperature can be achieved for the condensate after passing the low-pressure pre-heating device.  
         [0011]     It is further advantageous, if the turbine comprises a low pressure turbine, which feeds the low-pressure pre-heating device with excess steam. The turbine expediently comprises a high-pressure turbine, a medium-pressure turbine and a low-pressure turbine, through which turbines the high pressure steam produced in a steam raising unit is routed consecutively, while successively loosing pressure and temp erature. As the low pressure pre-heating device is the first stage of heating the condensate and therefore operates at non-maximum temperatures, it is expedient to heat the device with excess steam from the turbine operating at the lowest temperature of all turbines, which is the low-pressure turbine.  
         [0012]     It is further expedient, if the means for cooling the generator comprise a circulation pump for transporting the extracted condensate to the generator. This measure speeds up the circulation of the extracted condensate and therefore ensures a sufficient stream of condensate to circle to the generator in order to provide for the required cooling effect. By adjusting the pumping speed of the circulation pump and therewith its suction effect, it is further possible to control the amount of condensate extracted from the water-steam-loop. It is therefore not required to provide a special mechanical diverting device within the extraction device in order to divert enough condensate towards the generator. The amount of condensate required for cooling the generator can rather be easily controlled by adjusting the operating speed of the circulation pump.  
         [0013]     Additionally, it is advantageous if the means for cooling the generator comprise a generator cooling device, through which the condensate extracted from the water-steam-loop is routed. The generator cooling device is expediently in contact with the generator, advantageously attached to it, and operates as a heat exchanger between the generator and the condensate flowing through the generator cooling device.  
         [0014]     All features included in the dependent claims can be essential to the invention individually and/or in any combination with other features independent of the formal references of the claim(s) containing the respective features. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0015]     A detailed description of the present invention is provided herein below with reference to the following drawing, in which:  
         [0016]      FIG. 1  is a schematic representation of a thermal power plant according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]     A preferred embodiment of the thermal power plant according to the present invention is shown in  FIG. 1 . The power plant comprises three turbines, namely a high pressure turbine  1 , a medium pressure turbine  2  and a low pressure turbine  3 , which are all mounted on a single drive shaft  21  connected to an electric generator  5 . The turbines are operated by a water-steam-loop  4 . The high pressure turbine  1  is supplied with highly pressurized hot steam from a steam raising unit  19 . The steam expands to a certain extent in the high pressure turbine  1  and thereby causes the turbine and therewith the drive shaft  21  to rotate. The expanded steam exiting the high pressure turbine  1  is both of reduced pressure and of reduced temperature. Some of this steam is then diverted to a high-pressure pre-heater  18  arranged prior to the steam raising unit  19  in the water-steam-loop  4 . The remaining portion of the steam is routed to a re-heater  20  from where it is forwarded to the medium-pressure turbine  2 . In the medium-pressure turbine  2  the steam expands further, thereby applying torque to the medium-pressure turbine  2  and accordingly to the drive shaft  21 . A portion of the steam exiting the medium-pressure turbine  2  is guided to a feed water container arranged prior to the high-pressure pre-heater  18  in the water-steam-loop  4 . The remaining steam is routed to the low-pressure turbine  3 , where the steam expands even further causing torque to be applied to the low pressure turbine  3  and accordingly to the drive shaft  21 . The combined torque of the three turbines drives the electric generator  5  via the drive shaft  21 . The generator  5  converts this mechanical energy into electrical energy.  
         [0018]     A portion of the steam exiting the low-pressure turbine  3  is fed to a low-pressure pre-heating device  13  arranged prior to the feed water container  16  in the water-steam-loop  4 . The remaining steam, which is of very low pressure and of reduced temperature is routed to a condenser  11 , in which the steam is cooled down in order to transfer into the liquid phase. The cooling can be achieved by a cooling tower  14 , which is thermally connected to the condenser  11  via a cooling circuit  15 . The condensate in the form of water exiting the condenser  11  is then moved on in a loop-pipeline  22  to the low pressure pre-heating device  13 . For this purpose a condensate pump  12  is arranged immediately after the condenser  11 . Closely after the condensate pump  12  some of the condensate is diverted from the loop -pipeline  22  using an extraction device  6 . This extraction device  6  consists of branching means, which allow some of the condensate to branch off into a cooling pipeline  8 . The amount of condensate to be diverted into the cooling pipeline  8  can be controlled by adjusting the operating speed of a circulation pump  7  arranged shortly after the extraction device  6  in the path of the cooling pipeline  8 . When running at a high operating speed the circulation pump  7  generates a significant suction activity causing more condensate to be diverted into the cooling pipeline  8 .  
         [0019]     The condensate in the cooling pipeline  8  is then moved through a generator cooling device  9 , which is attached to the generator  5 . The generator  5  is thereby cooled, i.e. the thermal energy is transferred from the generator  5  to the condensate. The heated up condensate is then returned into the loop-pipeline  22  prior to entering the low pressure pre-heating device  13 . This is achieved using a return device  10 , which consists of branching means similar to the extraction device  6 .  
         [0020]     Extracting the condensate from the loop-pipeline  22 , as described, at a location shortly after the condensate pump  12  takes advantage of the fact that the condensate at this location has essentially the lowest temperature within the overall water-steam-loop  4  due to the close proximity to the condenser  11 . This leads to a high cooling efficiency of the generator  9 . Further, returning the condensate heated up by the generator  5  back into the loop-pipeline  22  before the pipeline enters the low-pressure pre-heating device  13  increases the temperature of the overall condensate stream entering the low-pressure pre-heating device  13 . The low-pressure pre-heating device  13  can therefore achieve a higher target temperature for the condensate. This improves the thermal efficiency of the overall thermal power plant.  
         [0021]     The low-pressure pre-heating device  13  uses, as described above, excess steam from the low-pressure turbine  3  in order to pre -heat the condensate under low pressure conditions. Thereafter, the condensate enters the feed water container  16 , which is supplied with excess steam from the medium-pressure turbine  2 . A feed pump  17  forwards the condensate to the high-pressure pre-heater  18 , which uses excess steam from the high pressure turbine  1  for further pre-heating the condensate under high pressure conditions. Finally, the condensate reaches the steam raising unit  19 , which uses the thermal energy produced by the power plant in order to transfer the condensate into high pressure steam.