Abstract:
The cooling system cools the stator and/or the rotor of a generator. Oxidation-resistant cooling channels are provided in the stator and/or in the rotor. Low conductivity for the coolant flowing through the cooling circuit is ensured by feeding fresh coolant into the cooling circuit, whereby the fresh coolant has a lower electric conductivity than the coolant in the cooling circuit.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a continuation of copending international application PCT/DE99/00044, filed Jan. 13, 1999, which designated the United States. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a cooling system for cooling the stator and/or the rotor of a generator, and to a method for cooling the stator and/or the rotor of a generator. 
     A hydrogen-cooled synchronous generator with a water-cooled stator winding is described in the book “Synchronmaschinen” [Synchronous Machines], AEG Telefunken Handbücher, Volume 12, Berlin 1970, on page 53. Generators with a water-cooled stator winding require a water loop. It comprises pumps for circulating the primary cooling water, return coolers and filters which ensure that the stator winding is not soiled, and an expansion vessel which is fitted on top on the machine. Fine filters and an ion exchanger for preparing the water are connected in the shunt circuit to this main circuit. Since the cooling water must be fed to the stator winding via insulating hoses, a small fraction of hydrogen can diffuse via them from the machine interior into the water circuit. This hydrogen fraction is given the opportunity of degassing from the water in the not entirely filled expansion vessel. It is led to the outside via a pressure control valve and a gas meter. 
     German patent application DE 22 22 487 describes a device for removing non-absorbed gases in liquids in the case of liquid-filled electric machines. In accordance with FIGS. 1 and 2 of that document, two concepts are applied for a cooling circuit. On the one hand, a coolant compensating container via which coolant is supplemented is connected to the cooling circuit via a spur line. In that compensating container which is disposed outside of the cooling circuit, a degassing container through which the entire coolant flow passes is connected into the cooling circuit. Outgassing of the coolant occurs in the degassing container. The gases are discharged to the outside. In accordance with the other concept, the compensating container for the coolant is integrated into a shunt circuit connected in parallel with the main cooling circuit. In this case, a smaller coolant flow of the shunt circuit passes continuously through the compensating container which serves simultaneously as a degassing container. 
     A cooling water circuit for a water-cooled electric machine is described with the aid of FIG. 5 in Siemens-Zeitschrift [Siemens Journal], Vol. 41, 1967, issue 10, pages 838-39. The cooling water must have a low electric conductivity for reasons of insulation. For this reason, chemical filters or ion exchangers, which continuously reduce the ion concentration in the cooling water, are connected into the cooling circuit. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to provide a cooling system and a method of cooling a generator, i.e., the stator and/or the rotor of the generator, which overcomes the above-noted deficiencies and disadvantages of the prior art devices and methods of this kind, and which is simple and cost-effective. 
     With the above and other objects in view there is provided, in accordance with the invention, a cooling system for cooling a generator, comprising: 
     a cooling circuit having oxidation-resistant cooling channels in a stator and/or rotor of a generator conducting coolant for cooling the generator; 
     a compensating container communicating with the cooling circuit for the coolant in the cooling circuit; and 
     a source of fresh coolant communicating with the cooling circuit for feeding fresh coolant and maintaining an electric conductivity of the coolant in the cooling circuit below a predetermined threshold. 
     In other words, the cooling system for cooling the stator and/or the rotor of a generator has a cooling circuit or coolant loop and a compensating container for a coolant flowing through the cooling circuit. Oxidation-resistant cooling channels are provided in the stator and/or in the rotor, and the electric conductivity of the coolant is bounded above essentially by virtue of the fact that fresh coolant is fed to the cooling circuit. 
     The following advantages can be achieved by means of oxidation-resistant cooling channels, for example cooling channels made from high-grade steel: 
     The limits of the oxygen content of the coolant can be generously dimensioned. 
     It is possible to dispense with nitrogen purging for the purpose of minimizing the oxygen content in the coolant. 
     The level of the pH value is of subordinate importance. 
     An ion exchanger can be eliminated. 
     The substantial outlay on time for the purpose of conditioning the water during commissioning of the generator is eliminated. 
     The invention is based on the finding that it is possible to dispense with chemical filters or ion exchangers in the case of a cooling system having oxidation-resistant cooling channels. A low electric conductivity of the coolant is achieved in a simple and cost-effective way by feeding deionized fresh coolant. By contrast with cooling channels made from, for example, copper, in the case of oxidation-resistant cooling channels there is no need to observe a rigorously closed coolant loop. 
     In accordance with an added feature of the invention, the compensating container is integrated into a parallel section connected in parallel with the cooling circuit. 
     The compensating container is preferably integrated into a parallel section connected in parallel with the cooling circuit. As set forth above, in the case of water-cooled generators a compensating container has frequently been arranged such that the entire coolant flows through it. This requires a compensating container of very large dimensions. Such a compensating container is a substantial cost factor. Alternatively, a compensating container has been connected to the cooling circuit via a spur line. In the case of such a design, the stagnates in the compensating container and becomes enriched with ions. The integration of the compensating container into a shunt section parallel to the cooling circuit renders it possible, on the one hand, to design the compensating container to be small. On the other hand, coolant flows continuously through, with the result the coolant the latter does not stagnate. Consequently, there is no substantial increase in conductivity, as a result of which it is possible, in turn, to set a limitation on conductivity particularly effectively by feeding fresh coolant. 
     The compensating container preferably holds between 50 and 800 l, in particular between 100 and 300 l. It is also preferably possible to guide a primary coolant flow through the cooling circuit and to guide a secondary coolant flow through the parallel section, the primary coolant flow being larger by a factor of 10 to 1000, in particular by a factor of 50 to 200, than the secondary coolant flow. The primary coolant flow is preferably between 10 and 100 m 3 /h, in particular between 20 and 40 m 3 /h. The secondary coolant flow is preferably between 10 and 500 l/h, in particular between 100 and 250 l/h. 
     The compensating container is preferably connected to a discharge line which serves to discharge surplus coolant from the cooling circuit. It is thereby possible for surplus coolant to be discharged simply via a discharge line, whereas previously there was a need to provide an overpressure valve. 
     It is further preferred for the discharge line to have a U-shaped bend in the region of which a gas outlet opening is arranged such that upon overshooting of a limiting gas pressure in the compensating container gas can be discharged from the compensating container via the gas outlet opening. An overpressure valve can thereby be eliminated. The coolant level can preferably be monitored via a sight glass. 
     In accordance with an additional feature of the invention, the compensating container is positioned separately from the generator. The compensating container therefore does not form a structural unit with the generator. Such an embodiment is possible owing to the compensating container of smaller dimensions which is arranged outside the cooling circuit. In particular, this results in the advantage that the compensating container does not have to be provided with any expensive vibration damping. Such vibration damping is required when, as previously, the compensating container forms a structural unit with the generator, that is to say, for example, is arranged on the generator. The compensating container is thereby exposed to the vibrations which are caused by the generator during operation. 
     Preferably provided in the cooling circuit is a calming section which serves for degassing and calming the coolant. Such a calming section can, for example, simply be a line connected in parallel with the cooling circuit. In such a calming section, the coolant flows more slowly and can be degassed. The parallel section in which the compensating container is arranged preferably branches off from this calming section. 
     In accordance with again an additional feature of the invention, there is integrated a cooler with a coolant inlet in the cooling circuit, the calming section being connected upstream of the coolant inlet. It is preferred to provide two coolers whose coolant inlets are connected by means of the calming section. This configuration exhibits a particularly suitable arrangement of the calming section. 
     The cooling system is preferably used for cooling the stator of a water-cooled turbo-driven generator, in particular a turbo-driven generator with a power of between 500 and 1300 MVA. 
     With the above and other objects in view there is also provided, in accordance with the invention, a method of cooling a stator of a generator, which comprises: conducting coolant through oxidation-resistant coolant channels in a generator; and 
     supplementing the coolant with fresh coolant for maintaining an electric conductivity of the coolant at below a predefined threshold, wherein the fresh coolant has a lower electric conductivity than the coolant. 
     In other words, the objects with regard to the method are satisfied with the method for cooling the stator and/or the rotor of a generator. The coolant is guided through oxidation-resistant coolant channels of the stator and/or the rotor. The electric conductivity of the coolant is essentially bounded above by supplementing the coolant with fresh coolant, which has a lower electric conductivity than the coolant in the coolant loop. 
     In accordance with a concomitant feature of the invention, the coolant is degassed and calmed in a calming section, in particular in a calming section connected upstream of the cooler. 
     Other features which are considered as characteristic for the invention are set forth in the appended claims. 
     Although the invention is illustrated and described herein as embodied in a cooling system and a method for cooling a generator, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
     The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The sole FIGURE is a diagrammatic illustration of a cooling system for the stator of a generator. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the FIGURE in detail, there is illustrated a cooling system  1  for a turbo-driven generator  4 . It is understood that the FIGURE is not drawn to scale. The turbo-driven generator  4  comprises a rotor  3  and a stator  2  surrounding the rotor  3 . The stator  2  has an electric winding with a multiplicity of electric conductors. Cooling channels  19  made from high-grade steel and of which one cooling channel  19  is indicated diagrammatically lead through the electric winding. The cooling channels  19  are integrated into a cooling circuit  5 . A coolant  7 , here water, flows in the cooling circuit  5 . The cooling channels  19  are connected to a hot water manifold  41 . A line  50  leads from the hot water manifold  41  to two parallel-connected coolers  17  ( 17   a,    17   b ). A volumetric flow measuring instrument  39  is integrated into the line  50 . The temperature in the hot water manifold  41  is determined via a temperature sensor or temperature measuring instrument  49 . The first cooler  17   a  has a coolant inlet  18   a.  The second cooler  17   b  has a coolant inlet  18   b.  The coolant inlets  18   a  and  18   b  are connected in parallel to a calming section  16 . A line  52  leads from the coolers  17   a  and  17   b  to a pump unit  53 . 
     A refreshing unit  20  which can be switched in and out of the coolant loop by a valve  43  opens into the line  52 . The refreshing unit  20  serves to supply deionized fresh coolant  7 A. The fresh coolant  7 A has a lower electric conductivity than the coolant  7 . The mixing in of the fresh coolant  7 A bounds electric conductivity of the coolant  7  above and satisfies the insulation requirements at any instant. By contrast with deionizing with the aid of ion exchangers, this control of the electric conductivity has the advantage of freedom from maintenance and simplicity, and is cost-effective. 
     A line  54  leads to a filter  30  from the pump unit  53 . The filter  30  serves to filter out dirt particles. Furthermore, a conductivity measuring instrument  32  and a temperature measuring instrument  33  are built into the line  54 . A line  55  leads from the filter  30  to a cold water manifold  42 . The cold water manifold  42  is connected, in turn, to the cooling channels  19 . 
     A line  12   a  which leads to a compensating container  6  branches off from the calming section  16 . A line  12   b  leads from the compensating container  6  to the line  52  hydrodynamically downstream of the coolers  17  (i.e., in the flow direction). A discharge line  12  leads away from the compensating container  6 . The discharge line has a U-shaped bend  13 . Gas can be discharged in the region of the U-shaped bend  13  via a gas outlet opening  14  when its gas pressure is so high in the compensating container  6  that coolant  7  is pressed below the gas outlet opening  14 . Furthermore, a filling level display  44  is provided on the compensating container  6 . 
     The cooling circuit  5  for cooling the stator  2  is formed by 
     a) the cooling channels  19  in the stator  2 , 
     b) the hot water manifold  41 , 
     c) the line  50 , 
     d) the calming section  16  and the coolers  17 , 
     e) the line  52 , 
     f) the line  54 , 
     g) the line  55 , and 
     h) the cold water manifold  42 . 
     The cooling circuit  5  also comprises the measuring and operating units integrated into it, for example the pump unit  53  or the filter  30 . 
     There is further provided a bypass line  34  in parallel with the generator  4 . It can be switched in or out via a valve  35 . A valve  36  closes or opens the line  55  to the generator  4 . The bypass line serves, inter alia, to protect the pump unit  53  when the cooling system  1  is being started up. 
     The cooling circuit  5  is connected in parallel with a parallel section  9  which comprises 
     a) the line  12   a,    
     b) the compensating container  6 , and 
     c) the line  12   b.    
     An important advantage of the cooling system  1  is that the compensating container  6  is arranged outside the cooling circuit  5 . Only a relatively small secondary coolant flow  11  is led through the compensating container  6  via the parallel section  9 . The compensating container  6  can thereby be of relatively small design. The compensating container  6  preferably holds between 50 and 800 l, in particular between 100 and 300 l. The secondary coolant flow  11  is smaller in this case by a factor of 50 to 200, for example, than a primary coolant flow  10  which is led in the cooling circuit  5 . The small configuration of the compensating container  6  is particularly cost-effective. In addition, the compensating container  6  does not form a structural unit with the generator  4 . Consequently, firstly it can be designed without vibration damping, and secondly it can be set up at any desired, particularly suitable site. 
     A further advantage of the cooling system  1  resides in the fact that a calming section  16  is connected upstream of the coolers  17 . The water flows more slowly and can be degassed in this calming section  16 . The gas is fed via the line  12   a  to the compensating container  6 . It can be let out of the latter in a simple fashion via the cover, for example. An overpressure valve for the compensating container  6  is further eliminated. The overpressure equalization is performed via the discharge line  12 . 
     The cooling system  1  shown can be used, in particular, for generators with oxidation-resistant cooling channels  19 . 
     The individual components are described in more detail in the following. 
     Piping of the Generator 
     Cold water is fed to the generator via an ultrafine filter at the generator inlet, and is fed into the cold water manifold  42  at the bottom in the vertical middle. The hot water heated by the generator  4  is extracted from the hot water manifold  41  at the top in the vertical middle and led downwards outside the generator  4  to the coolers  17 . This incorporation ensures the automatic ventilation of manifolds and starts during operation. Shut-off valves  36 ,  37  are disposed inline in the supply line immediately upstream of the generator and in the return line. The valves permit a flushing operation via the bypass line  34  without the cooling channels  19  running full of water in the process or being undesirably wetted. The volumetric flow in the bypass line  34  is restricted to the nominal volumetric flow of the pump unit  53  by suitable measures. 
     Pumps 
     Two centrifugal pumps  53   a,    53   b  of identical power are available for circulating the water circuit. Each pump  53   a,    53   b  can be selected as an operating pump or standby pump. The standby pump is automatically switched in as soon as the operating pump fails. Three-phase AC motors which are fed from different networks are provided for driving the pumps  53   a,    53   b.    
     Filters 
     The water filled in and circulated must be largely free from suspended matter which can be deposited and build up flow impediments. The water of the primary coolant flow  10  must therefore be led to the filter  30  with a suitable filter grade. There is no need for bypassing. The degree of soiling can be detected via a pressure difference measurement. A pressure difference meter with a binary limit monitor is provided as standard. 
     Coolers 
     The coolers  17  serve for return cooling of the primary water flow  10 . Provided as standard are two coolers  17   a,    17   b.  Each cooler  17   a,    17   b  assumes 50% of the cooling power. Soldered plate coolers are used as the coolers  17 . All wetted surfaces consist of stainless steel. No bypass is provided on the primary water side for the coolers  17 . Consequently, a cooler  17  takes over the full primary water volumetric flow in the case of malfunction. The fall in the volumetric flow effected by the rise in the pressure difference does not initiate protection in this case. Located on the primary-side coolant inlets  18   a,    18   b  of the coolers  17  is a parallel switching pipe, the calming section  16 , which serves as a bubble separator. It ensures that the flow is calmed and that gas bubbles can be eliminated. The cross section of the calming section  16  is dimensioned for this additional task. A small cooler bypass quantity flows steadily to the compensating container  6  via the line  12   a.  This secondary coolant flow is used to carry eliminated bubbles out of the calming section  16  into the compensating container  6 . 
     Compensating Container 
     The compensating container  6  is connected to the cooling circuit  5  via spur lines  12   a,    12   b.  It absorbs the thermally conditioned volumetric change in the water, discharges surplus water and serves as ventilating and degassing tank. When the cooling channels  19  are being filled up during commissioning, it temporarily covers the additionally required water demand. There is a low forced volumetric flow through the compensating container  6 . This volumetric flow carries gas bubbles of the cooling circuit  5  into the compensating container  6 . The filling level of the compensating container  6  can be detected from outside, and the undershooting of the minimum filling level is notified by a warning. The dewatering and degassing are combined via a water supply such that water can flow without pressure, whereas in the case of overpressure in the compensating container  6  gas is discharged into an exhaust line  65 . The water supply is continuously renewed via the water of the refreshing unit  20 . There is no need for maintenance and monitoring of the water filling. Hydrogen penetrates into the water by diffusion and mini-leakages in the case of a hydrogen-cooled generator. There builds up in the compensating container  6  an overpressure which presses the water column in the pressure limb of the U-shaped bend as far as the level of the gas outlet opening  14  for the exhaust line  65 . Hydrogen which has further penetrated is led via the discharge line  12  into the exhaust line  65 , and does not cause any additional rise in pressure. A nitrogen rise at the bubble separator, the calming section, renders it possible to flush the compensating container  6  with inert gas. 
     Coolant Feed 
     The water fed in for the purpose of refreshment is extracted from a deionized water network of low conductivity. The water is fed in upstream of the pumps  53   a,    53   b  and led via a fine filter before being fed in. The volumetric flow is set by hand using a control valve and displayed locally. A return-flow lock or check valve prevents loss of primary water when the deionizing water network is unpressurized.