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.