Abstract:
A method for acceleration of a shaft run that a turbine, in particular a gas turbine, a generator and an excitation machine connected to one another by a common shaft, includes accelerating the turbine is accelerated using acceleration apparatus during a starting process, wherein the excitation machine is used as the acceleration apparatus. In addition, an apparatus for performing the method.

Description:
This patent application is a continuation for International Patent Application No. PCT/EP2003/050713, filed on Oct. 13, 2003, which claims priority to German Patent Application No. DE 102 47 905.4, filed on Oct. 14, 2002. The entire disclosure of both applications is incorporated by reference herein. 

   The present invention relates to the field of electrical power generation. It relates in particular to a method for acceleration of a shaft run and to an apparatus for carrying out the method. 
   BACKGROUND 
   A method such as this and an apparatus such as this are known, for example, from the document U.S. Pat. No. 5,097,195, which is incorporated by reference herein. 
   Gas turbines are being used to an ever increasing extent for the generation of electrical power. In this case, the oil or gas which is burnt in the gas turbine is converted to movement energy and then, via a generator, to electrical energy. The gas turbine and the generator are in this case coupled to one another and form a shaft run. The gas turbine cannot be started until it reaches a specific rotation speed. Below this rotation speed, the shaft run must be accelerated by some other component. This component then still has to assist the gas turbine over a wide rotation speed range. 
   Nowadays, the generator is itself used for this starting process. In this case, the generator is used as a motor rather than as a generator. However, owing to the high regulation quality required for the starting process and owing to the load on the mains and the generator, the generator must not be connected directly to the mains. It is therefore now normal for the generator to be fed via a static frequency converter (SFC) during this process. In this case, the size of the SFC is governed in particular by the required torque at medium to high rotation speeds. In addition to this starting and acceleration apparatus, a second device is required in order to control the excitation of the generator. The generator field winding is fed either via sliprings or via a brushless excitor. In the case of brushless excitation, a second apparatus may be required for frequency conversion (in this context, see the document U.S. Pat. No. 5,097,195, which was cited above). 
   In some cases, however, the gas turbine is also started by means of an independent drive acting on the shaft. This can be achieved in various ways. In this case, by way of example, asynchronous motors are used with appropriate frequency converters. 
   The known solutions for this starting process have the disadvantage of the comparatively high degree of complexity. During acceleration via the generator, the frequency converter has to be matched to the voltage level of the main stator circuit, and switching means must be provided for disconnection in the generator mode. In the case of an independent drive, the mechanics of the shaft run are more complex. 
   SUMMARY OF THE INVENTION 
   An object of the invention is to provide a method and an apparatus for acceleration of a shaft run, which avoid the disadvantages of the prior art. 
   The present invention provides a method for acceleration of a shaft run, which shaft run comprises a turbine, in particular a gas turbine, a generator and an excitation machine, which are connected to one another by a common shaft, in which method the turbine is accelerated during the starting process by means of an acceleration apparatus, wherein the excitation machine is used as an acceleration apparatus. The invention involves use of the excitation machine, which is seated on the shaft run, as the acceleration apparatus. In this case, there is no need for the SFC installation which is otherwise normally used and feeds the generator directly during acceleration. 
   According to one refinement of the invention, the excitation machine has an excitation rotor with a rotor winding, and an excitation stator with a stator winding. During acceleration, the rotor winding is fed with a variable-frequency alternating current, and the stator winding is fed with a constant-frequency alternating current, in particular the mains frequency. A frequency converter connected to the mains is used, in particular, for feeding the rotor winding of the excitation machine. However, conversely, it is also possible to feed the stator winding with a variable-frequency alternating current, and the rotor winding with a constant-frequency alternating current. 
   Further simplification is obtained if, according to another preferred refinement of the invention, in which the generator has a field winding, the rotor winding of the excitation machine feeds the field winding of the generator after the end of the starting process, and the frequency converter is connected or remains connected to the stator winding of the excitation machine in order to regulate the excitation voltage. 
   The use of the field winding of the generator by means of the rotor winding of the excitation machine preferably makes use of electronic elements, in particular in the form of a rotating rectifier, with the rotor winding of the excitation machine being permanently connected to the field winding of the generator via the electronic elements, and the electronic elements being switched on or off by a signal, depending on whether the shaft run is being accelerated or whether the generator is being excited. 
   The present invention also provides an apparatus for carrying out the method that includes a shaft run with a turbine, in particular a gas turbine, a generator and an excitation machine, which are connected to one another by means of a common shaft, and has means for acceleration of the shaft run, wherein the acceleration means comprise a frequency converter which is connected to the mains and can optionally be connected to a rotor winding of the excitation machine. One preferred refinement of the apparatus according to the invention is characterized in that the rotor winding of the excitation machine is connected to a field winding of the generator via controllable electronic elements which can be switched on and off via a control signal line, and in which the electronic elements form a rotating rectifier. 
   Further embodiments can be found in the claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be explained in more detail in the following text with reference to exemplary embodiments and in conjunction with the drawings, in which: 
       FIG. 1  shows a simplified circuit diagram of a first preferred exemplary embodiment of the invention, in which, during acceleration, the rotor winding is fed with a variable-frequency alternating current and the stator winding is fed with a constant-frequency alternating current; 
       FIG. 2  uses a circuit diagram comparable to that in  FIG. 1  to show a second preferred exemplary embodiment of the invention, in which, during acceleration, the stator winding is fed with a variable-frequency alternating current, and the rotor winding is fed with a constant-frequency alternating current; and 
       FIG. 3  shows examples of curves of the rotor voltage UR (curves A and B) and of the stator voltage U St  (curves C and D) during acceleration according to the invention with (curves B and D) and without (curves A and C) a variable-voltage transformer. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows one preferred exemplary embodiment of the invention, in the form of a simplified circuit diagram. A gas turbine  10 , a generator  22 , of which only the generator rotor  11  is illustrated, and an excitation machine  14  are connected by means of a common shaft  21  and form a shaft run. The excitation machine  14  has an excitation stator  16  with a stator winding  25 , and an excitation rotor  15  with a rotor winding  23 . The stator winding  25  can optionally be connected to the mains  24  or to the output of a frequency converter  20 , which is connected to the mains  24 , via two switches S 1  and S 3 . The rotor winding  23  is connected to a field winding  12  in the generator rotor  11  via electronic elements in the form of a rotating rectifier  13 . The rotating rectifier  13  contains electronic elements (for example thyristors), which can be switched on and off via a control signal line  18 . When the electronic elements are switched on, the rotor winding  23  is connected to the field winding  12 . In contrast, when the electronic elements are switched off, this connection is interrupted. The rotor winding  23  is connected to the output of the frequency converter  20  via a further switch S 2 . A variable-voltage transformer  19  can be inserted into the feed circuit. 
   During normal or continuous operation, the switches S 2  and S 3  are open, while the switch S 1  is closed. The stator winding  25  of the excitation machine  14  is fed by the frequency converter  20 . The voltage which is induced in the rotor winding  23  is rectified in the rotating rectifier  13 , and feeds the field winding  12  of the generator  22 . The electrical power that is generated is available on the stator winding (which is not illustrated) of the generator  22 . The frequency converter  20  can in this case be used to regulate the excitation voltage. 
   During the starting process, the switches S 2  and S 3  are closed, and the switch S 1  is open, in order to accelerate the shaft run. The stator winding  25  of the excitation machine  14  is thus fed from the mains  24 , and the rotor winding  23  is fed via sliprings  26  from the frequency converter  20 . Appropriate control signals are tapped off via current transformers  17  in the supply lines between the variable-voltage transformer  19  and the rotor winding  23 , and are passed via a control signal line  18  to the rotating rectifier  13 , whose electronic elements are switched off, thus interrupting the connection between the rotor winding  23  and the field winding  12 . The excitation machine  14  thus operates as a drive motor for the shaft run. The starting process can in this case be controlled via the frequency converter  20 . Once the rotation speed required for operation of the gas turbine  10  has been reached, it is possible to switch over to the continuous mode, as described above. The variable-voltage transformer  19  can be switched on in order to load the frequency converter in a protective manner, with the switching between the various taps preferably being carried out electronically. During this starting process, it may be advantageous for the stator winding  25  of the excitation machine  14  to be fed initially, at low rotation speeds, with a variable-frequency alternating current, in particular from the frequency converter  20  which is connected to the mains  24 , and when the excitation resistance is used as a load while the stator winding  25  of the excitation machine  14  is being fed with a variable-frequency alternating current. 
     FIG. 2  shows the opposite feed to that shown in  FIG. 1 . In this case, the rotor winding  23  is connected to the mains via sliprings  26 , the switch S 2  and, if required, the variable-voltage transformer  19 , and is fed with a constant-frequency alternating current, in particular the mains frequency. The stator winding  25 , in contrast, is fed with a variable-frequency alternating current from the frequency converter  20 . 
     FIG. 3  shows the voltage profiles U(r) for various situations as a function of the rotation speed r during acceleration. The curve A shows the rotor voltage UR without the use of the variable-voltage transformer  19 , the curve B shows the rotor voltage UR with the variable-voltage transformer  19  being used, the curve C shows the stator voltage U St  without the use of the variable-voltage transformer  19 , and the curve D shows the stator voltage U St  with the variable-voltage transformer  19  being used. 
   Overall, the solution according to the invention results in the following characteristic properties, features and advantages:
         the variable rotation speed form of the excitation machine of a brushless generator allows the excitation machine to be used as an acceleration apparatus. There is no need for the SFC installation, which is otherwise normally required and feeds the generator directly.   An accelerator excitor such as this allows starting of generator turbines in which the generator has a high-voltage winding (for example in the case of the so-called “Powerformer”; in this context, see M. Leijon—Powerformer—a radically new rotating machine, ABB Review 2 (1998) pp. 21–26) without the variable-frequency feed being transformed up via an intermediate transformer to the voltage level of the generator, and without any complex additional winding being required.   The use of a variable rotation speed generator is assisted by an acceleration excitor since, in consequence, the generator and the converter can be designed solely for the generator mode, but not for the motor mode.   The stator winding of the generator is no longer subjected to the high-frequency voltage peaks of the starting apparatus during starting. This reduces the risk of damage to the generator insulation. Shaft voltages caused by voltage peaks are likewise reduced.   The solution is also suitable for deceleration of the shaft run.   The brushes are suitable for excitor boosting, that is to say for briefly increasing the excitation current, during operation.   The brushes can be raised during operation.   The method and the installation can be used for the turning mode, in which the shaft run is rotated slightly.