Abstract:
In a generator with a cooling system which draws in, from the generator, cooling medium heated by the heat-generating elements of the generator and which guides the hot cooling medium to at least two cooling units ( 23 ), which cooling units ( 23 ) operate in parallel and cool the cooling medium before it is led back to the heat-generating elements of the generator, operation substantially uninfluenced by failures of the cooling units ( 23 ) is made possible by means being provided which mix together the cooling medium flows ( 31 ) flowing from the different cooling units ( 23 ) after they emerge from the cooling units ( 23 ) and before they are supplied to the heat-generating elements of the generator.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of electrical generators. It relates to a generator with a cooling system which draws in, from the generator, cooling medium heated by the heat-generating elements of the generator and which guides the hot cooling medium to at least two cooling units, which cooling units operate in parallel, and in which cooling system the cooling units cool the cooling medium before it is led back to the heat-generating elements of the generator. 
     BACKGROUND OF THE INVENTION 
     In generators, the problem generally arises that the ventilation work carried out on the cooling medium, the rotor surface friction and, in particular, the electrical losses in the conductor windings and the stator laminae lead to a large development of heat. This demands efficient cooling of the central components of the generator. 
     In the current increasingly widespread high performance generators, the ventilation losses in particular increase because of the increasing peripheral velocities and this makes careful design of the cooling systems essential. In such cooling systems, a cooling medium—usually air or another gas, and also liquid media in special cases—is normally driven through the generator in a cooling circuit. The heat occurring at the hot components of the generator is transported away by the cooling medium and is extracted again from the cooling medium by means of cooling units at another location in the cooling circuit. 
     In generators which are operated on the suction cooling principle, the cooling medium heated by the heat-generating elements of the generator is drawn in from the generator by main fans fastened to the rotor shaft. These main fans are normally arranged at the front and rear ends of the generator and a ducting system guides the cooling medium expelled by the main fans through cooling ducts to a cooling arrangement, which is usually located under the generator in a foundation pit. Heat is then extracted from the cooling medium as it flows through the cooling arrangement extending essentially over the complete length of the generator and composed of a plurality of cooling units operating in parallel. The cold medium is then ducted back, over the complete length, to the heat-generating elements within the generator, thus forming a closed cooling circuit. 
     In order to meet the current demands for cooling in machines operating at their performance limits, very efficient cooling systems with small flow losses and high efficiency are necessary. Particularly with respect to operational reliability and avoiding damage to the components, it is then necessary to ensure that the cooling system is as insensitive as possible to faults. This is, for example, achieved by installing a plurality of units, which operate in parallel and adjacent to one another in the cooling medium flow, instead of a single large cooling unit. Compensation for the failure of a cooling unit can, by this means, be at least partially provided by other units and destruction of the components due to overheating can be substantially avoided. 
     In modern generators, even the use of a plurality of cooling units operating in parallel cannot prevent components suffering damage in the case of a failure of even one unit. If, namely, a cooling unit fails, hot gas streaks immediately form behind the failed unit (behind in the flow direction) and these lead, within an extremely short time, to the critical material temperatures being exceeded in the components over which flow occurs. The situation is particularly critical in the case of the failure of boundary coolers because, in this case, strong and substantially isolated hot gas streaks occur immediately. 
     SUMMARY OF THE INVENTION 
     The invention is therefore based on the object of making means available in which 
     the formation of hot gas streaks is prevented in the case of the failure of individual coolers, 
     major modification to the conventionally used cooling systems does not become necessary 
     no significant pressure losses are generated. 
     This object is achieved, in a generator of the type mentioned at the beginning, by providing means which mix together the cooling medium flows flowing from different cooling units after they emerge from the cooling units and before they are supplied to the heat-generating elements of the generator. 
     A first preferred embodiment of the invention is characterized in that the mixing is effected by rigid guide plates being mounted in an equalizing space arranged downstream of the cooling units, which guide plates deflect the various cooling medium flows. 
     A further embodiment is characterized in that the guide plates mix together the cooling medium flows from respectively adjacent cooling units. This is, in particular, advantageous because antarbitrary number of cooling units, which are connected in parallel adjacent to one another, together with the guide plates can be provided in a modular manner and because it is possible to dispense with a bundling arrangement for the flows. 
     Another embodiment is based on the fact that the guide plates are arranged in the equalizing space in such a way that the cooling medium flows are provided with a vortex when they are mixed together. The result of this is that the mixing takes place much more efficiently. 
     In a particularly preferred embodiment, at least two guide plates are present which—starting from the sides adjoining one another of the adjacent cooling units—are extended to a ceiling level above the outlet surface of the cooling units in such a way that they engage in-one another in saw-tooth fashion in an alternating manner inclined towards the side of the one and of the other adjacent cooling unit. By this means, the equalizing space is subdivided into a distribution space and a mixing space, the distribution space being located directly behind the cooling units in the flow direction of the cooling medium and the mixing space being arranged immediately behind the distribution space in the flow direction of the cooling medium. The cooling medium flows of adjacent cooling units are mixed, in the mixing space, by the deflection at the guide plates in such a way that they are subjected to vortices when flowing from the distribution space to the heat-generating elements of the generator. In addition, a further advantage follows from the fact that the partial flows have counterflow relative to one another due to this guidance system and this, in the case of a failure of one unit, has the result of optimum heat exchange, due to the counterflow cooling, between the uncooled hot and cooled cooling medium flows. 
     Further embodiments are given by the subclaims. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     The invention is to be explained in more detail below using embodiment examples in association with the drawings. In these 
     FIG. 1 shows a diagrammatic representation of a generator with suction cooling principle and mixing of the cooling medium in the equalizing space; 
     FIG. 2 shows a perspective view of an excerpt of an equalizing space with guide plates; 
     FIG. 3 shows a plan view of an equalizing space with lateral contiguous arrangement of guide plates; 
     FIG. 4 shows a plan view of several contiguously arranged cooling units with guide plates. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a diagrammatic longitudinal section through a generator operated on the suction cooling principle. The generator is bounded at the front and back by machine casing covers  13  and is longitudinally enclosed by an essentially cylindrical machine casing  14 . The casing encloses a stator lamination body formed from partial lamination bodies  20 , radial ventilation slots  26  being present in the stator lamination body between the various partial lamination bodies  20 . A rotor  22  is located in the centre of the stator lamination body and the associated rotor shaft  12  is supported in bearing pedestals  32 , which are located on brackets  25  erected on a foundation  24 . 
     The foundation  24  has a foundation pit  10 , which extends axially over the complete length of the machine casing  24  and essentially includes the total width of the machine casing  24 . A cooling arrangement consisting of a plurality of cooling, units  23  is arranged in this foundation pit  10 . In this arrangement, the inlet openings of the cooling unit  23  are connected to outlet spaces of main fans  11  arranged at both ends of the rotor  22  and the outlet openings of the cooling units  23  open into an equalizing space  16 . The main fan  11  is solidly connected to the rotor shaft  12  and rotates with the same speed as the rotor  22 . 
     The flow paths of cooling gas flowing through a generator are indicated by arrows- in the right-hand part of FIG.  1 . This cooling principle involves so-called reverse or suction cooling in which hot gas  30 ,  32  is supplied to the cooling units  23  by means of fans  11 . Behind the cooling units  23 , in the flow direction, the cold cooling gas flow  31  is subdivided after the equalizing space  16  into the cold gas chambers  18  so that partial flows are formed. A first partial flow flows between guide plates  28  and an inner shell  17  directly to the rotor  22 , a second partial flow flows through the winding head  21  into the machine air gap  29  between rotor and stator and a third cooling gas flow passes through the cold gas chambers  18  and ventilation slots  26  into the machine air gap  29 . The cooling gas flow is drawn in from the machine air gap  29  by the main fans  11  through ventilation slots  26  and the hot gas chambers  19  between an inner shell  17  and an outer shell  15 . The air  32  driven by the main fan  11  is then deflected and led through the cooling duct casing  33  into the foundation pit  10  and to the cooling units  23 . It can now be easily realized that in the case, for example, of failure of the boundary-end cooling unit, the winding head  21  will heat up rapidly because a hot gas streak forms precisely at this location. 
     The left-hand half of FIG. 1 indicates how guide plates  34  can be arranged in the equalizing space  16  over the various cooling units  23  in the equalizing space  16 . The latter is subdivided by the guide plates  34  into distribution spaces  36  and mixing spaces  35  and, in this arrangement, the guide plates  34  form—in side view—an essentially zigzag-shaped wall. The guide plates  34  do not, however, separate the distribution space  36  and the mixing space  35  from one another but, rather, engage with one another in saw-toothed manner segmentally at right angles to the section plane of FIG.  1  and permit the medium to flow through. 
     FIG. 2 shows a perspective view of the arrangement of the guide plates  34  in the equalizing space for two adjacently arranged cooling units. The engagement of the guide plates  34  within one another in inclined saw-tooth manner may be seen. If it is assumed that one unit  40  of the cooling units  23  is no longer functional whereas the adjacent unit  41  still operates, hot air flows out of the unit  40  and cold air flows out of the unit  41 . Because of the guide plates  34 , cold and hot cooling medium flows are now mixed in counterflow and subjected to vortices when passing through the slots between the guide plates  34 . In the situation of two adjacently arranged cooling units  23  shown in FIG. 2, it is found to be advantageous for the outlet flow behaviour of the cooling medium to be particularly influenced at the sides by the use of cover plates  37 . Such cover plates  37  extend, adjoining the side walls  47  and parallel to the outlet surfaces of the cooling unit  23 , at the ceiling level  44  at which the sides of the guide plates  34  facing away from the cooling units  23  meet. By this means, the cover plates  37  abut the upper edges of the guide plates  34  inclined towards them and ensure that the whole of the cooling medium flowing out of the cooling unit  23  located underneath is mixed with the medium flowing out of the adjacent unit and that, in the case of the failure of one unit, no boundary-end streaks of hot cooling medium can form. 
     FIG. 3 shows the situation of FIG. 2 in a plan view. In this case also, it is assumed that the black cooling unit  40  is not functional whereas the other cooling unit  41  functions satisfactorily. The hot air  42  flowing out of the faulty unit  40  is indicated by black arrows and the cooled air flowing out of the functional unit  41  is shown by white arrows  43 . The counterflow mixing of the flows  42  and  43  may be clearly recognized in the plan view. The counterflow leads to optimum heat exchange between and mixing of the two flows  42  and  43 . The guide plates  34  can be alternately arranged adjacent to one another to correspond with the width  46  of the units  23 . Depending on the requirements, mixing cells  38  consisting of two oppositely extending guide plates  34  can, for example, be arranged adjacently in modular fashion. The relationship between the width  46  of the cooling units  23  and the mixing cells  38  can, in principle, be freely selected but it is found that a ratio of the guide plate width  45  to the unit width  46  of between {fraction (1/10)} and {fraction (1/15)} is particularly advantageous. If, in addition, the slope of the guide plates  34  relative to the outlet surface of the cooling units  23  is set to between 25° and 30° and a total height of the equalizing space  16  (between the outlet plane of the cooling units and the top edge of the foundation pit) of between 1 and 1.5 m is permitted, the result is optimum flow and mixing of the cooling air. 
     FIG. 4 shows, in a plan view, how the above cooling medium mixing concept can also be applied to a plurality of cooling units  23  arranged adjacent to one another. In this arrangement, the guide plates  34  are arranged so that they simply engage relative to one another with respect to adjacently located units  23 . In this way, the air which flows out of one unit, which is adjacent to two further units, is mixed with the air from both adjacent units. In order to avoid the particularly critical boundary-end hot air streaks, it is again necessary to extend cover plates  37  at the boundary end coolers. 
     Arrangements of guide plates other than those described in the above embodiment example are, of course, possible and the inventive idea can be realized in a similar manner. It is, for example, conceivable to additionally tilt the guide plates sideways or to structure them so that they are not flat but are in an aerodynamically favourable shape.