Abstract:
A method for testing an overspeed protection mechanism of a single-shaft combined-cycle plant that includes, during testing, an electrical load is first connected to the generator and the load is disconnected and a trigger threshold value can be achieved at a test moment in time, thereby triggering the overspeed protection mechanism.

Description:
[0001]    EP 2 372 482 A2 discloses a method and a system for testing an overspeed protection system of a turbomachine. 
         [0002]    DE 299 08 581 U1 discusses a device for monitoring the operational safety of a turbine in the event of load shedding. 
         [0003]    FR 2 947 300 A1 discloses a method for testing a turbomachine. 
         [0004]    Hitherto, the test mechanisms for overspeed protection of steam turbine and gas turbine plants could be monitored by setting the triggering limit value for the overspeed protection mechanism to a lower speed than the operating speed. In test operation, it is possible to exceed this lower test speed and to check whether the overspeed protection mechanism works. 
         [0005]    It would however be desirable to also carry out this test operation with original triggering limit values. That means that the overspeed protection mechanism should be checked proceeding from the operational speeds. This is also desirable given that in certain countries such an overspeed protection, which is to take place with original triggering limit values, is prescribed by law. In the case of single-shaft plants, such a check can be performed only for the gas and steam turbines together, taking into account the permissible configuration parameters. 
       SUMMARY OF INVENTION 
       [0006]    The invention has an object of indicating a method for testing a single-shaft combined-cycle plant, wherein the overspeed protection mechanism test can be performed proceeding from operational speeds. 
         [0007]    This object is achieved with a method for testing an overspeed protection mechanism of a single-shaft combined-cycle plant, wherein, during test operation, the gas turbine and the steam turbine are operated at a test speed, wherein the generator is operated with a connected load, wherein, during the test operation, load is shed, wherein the speed of the steam turbine increases and, upon reaching a ST triggering limit value, a ST overspeed protection mechanism is triggered. 
         [0008]    An essential consideration of the invention is thus that the steam and gas turbine drives an electric generator at a test speed which corresponds to the operating speed of 50 Hz or 60 Hz, wherein an electric load is arranged on the electric generator. This electric load leads to increased torque on the rotors of the gas and steam turbine. Shedding load, i.e. switching off the electric load abruptly, changes the counteraction of the torque on the gas and steam turbine rotors, the consequence of which is that the rotational speed increases more or less abruptly since the inertia of the system means that the control of the steam supply and fuel supply to the gas turbine is not sufficient. 
         [0009]    The load shedding thus causes an increase in the rotational speed of the steam turbine and necessarily of the gas turbine, and a ST overspeed protection mechanism should be triggered upon reaching a ST triggering limit value. 
         [0010]    Advantageous developments are specified in the dependent claims. 
         [0011]    In a first advantageous development, the steam turbine reaches the ST triggering limit value first, triggering the ST overspeed protection mechanism, and then the gas turbine reaches a GT limit value, the GT overspeed protection mechanism being triggered once the GT limit value has been reached. Thus, in this advantageous development, it is necessary for two triggering conditions to be reached in succession in order to first trigger the overspeed protection mechanism of the steam turbine and subsequently that of the gas turbine. The ST triggering limit value must be reached first, and then the GT limit value. 
         [0012]    In a further advantageous development, in test operation, the steam turbine is in a fully warmed state. That means that, in test operation, the operating parameters of the steam turbine are ideally reached and no temporary effects in live operation need be taken into account. 
         [0013]    In an advantageous development, in test operation, the gas turbine is operated at low power. 
         [0014]    In a further advantageous development, in test operation, the gas turbine is operated with a constant exhaust gas temperature. 
         [0015]    In a further advantageous development, a time t Triggering  elapses between the triggering of the ST overspeed protection mechanism and the load shedding, and t Triggering &lt;t max , wherein the steam turbine is tripped when t Triggering &gt;t max  and the ST overspeed protection mechanism has not yet been triggered. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    An exemplary embodiment of the invention is described in more detail below. Schematically, in the figures: 
           [0017]      FIG. 1  shows a schematic overview of a single-shaft combined-cycle plant, 
           [0018]      FIG. 2  shows a rotational speed profile after load shedding. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0019]      FIG. 1  shows a single-shaft combined-cycle plant  1 . This single-shaft combined-cycle plant  1  comprises a steam turbine  2 , a gas turbine  3  and an electric generator  4 , which are connected to one another in a torque-transmitting manner via a common shaft  5 . Between the gas turbine  3  and the electric generator  4 , there is arranged a coupling  6  by means of which the transmission of torque can be interrupted. 
         [0020]    An electric consumer  9  or an electric load  9  is connected to an output  7  of the electric generator  4  via a switch  8 .  FIG. 1  shows the switch  8  in the closed state. 
         [0021]      FIG. 2  shows a rotational speed profile of the gas turbine (n GT ) and of the steam turbine (n ST ). The rotational speed profiles illustrated in  FIG. 2  show the rotational speed profile of the gas turbine  3  and of the steam turbine  2  when the coupling  6  is closed. First, the gas turbine  3  and steam turbine  2  are operated at a constant speed of 3000 revolutions per minute. At time t=t Load   _   shedding , the electric consumer  9  is separated from the generator  4  by means of the switch  8 . The consequence of this is that the speed of the gas turbine (n GT ) and of the steam turbine (n ST ) increases temporarily and once a triggering limit value is reached, the steam turbine  2   3  is tripped, which leads to a sharp drop in the rotational speed, as shown in  FIG. 2 . 
         [0022]    The live overspeed protection must take place with a triggering limit value of the overspeed protection mechanism which is unchanged with respect to normal operation. During the test, the gas turbine  3  and steam turbine  2  are accelerated in a defined manner to the triggering limit value of the associated overspeed protection mechanism. When the triggering limit values are exceeded, the overspeed protection mechanism must shut down the corresponding actuating elements of the gas turbine  3  and the steam turbine  2 , and thus prevent critical overspeeds. In the context of functional safety, the live overspeed protection mechanism test is not a real demand on the protection mechanism since the controllers approach the corresponding triggering limit values with defined dynamics and a critical overspeed does not materialize. 
         [0023]    In the case of single-shaft combined-cycle plants  1 , both the gas turbine  3  and the steam turbine  2  are each equipped with a separate overspeed protection mechanism. Due to the mechanical coupling  6  between the gas turbine  3  and the steam turbine  2 , it is impossible in single-shaft plants for the speed of the steam turbine  2  to be higher than the speed of the gas turbine  3 . Furthermore, the gas turbine  3  must provide sufficient boiler power for the live overspeed test of the steam turbine  2 . Thus, the overspeed test of the steam turbine  2  cannot take place independently of the gas turbine  3 . The method for testing the overspeed protection mechanism of the single-shaft combined-cycle plant  1  is as follows: during test operation, the gas turbine  3  and the steam turbine  2  are operated at a test speed equal to the operational speed of 3000 revolutions per minute. The generator  4  is operated with a connected load  9 , wherein, during the test operation, load is shed at time t Load   _   shedding , raising the speed of the steam turbine  2  and of the gas turbine  3 , a ST overspeed protection mechanism being triggered when a ST triggering limit value is reached and a GT overspeed protection mechanism being triggered when the GT triggering limit value is reached. The consequence of this is that the speed of the steam turbine  2  and of the gas turbine  3  is reduced. 
         [0024]    The mass moment of inertia and/or the response time constant influences the dynamic behavior of the gas turbine  3  and the steam turbine  2  after the load shedding. The ratio of the response time constants has an influence on the choice of triggering limit value. 
         [0025]    In the case of the single-shaft combined-cycle plant  1 , load shedding leads automatically to quick shutdown of the steam turbine  2 . For that reason, the triggering limit value of the steam turbine overspeed protection mechanism can also be set to a value marginally above (e.g. 104% of) the maximum grid frequency (for example 61.5 Hz) without reducing the plant availability. The difference between the maximum permissible grid frequency and the triggering limit value of the steam turbine overspeed protection mechanism is chosen such that no undesired trip takes place during operation. 
         [0026]    A further limiting condition is that the triggering limit value of the steam turbine overspeed protection mechanism is not greater than the triggering limit value of the gas turbine overspeed protection mechanism. 
         [0027]    The triggering limit value of the gas turbine overspeed protection mechanism must be set to a value above the maximum speed after load shedding and below the critical overspeed. The triggering limit value must be chosen such that the steam turbine  2  reaches the triggering limit value before the gas turbine  3 . 
         [0028]    The control system technology of the gas turbine  3  and steam turbines  2  of a single-shaft combined-cycle plant  1  is augmented such that a switch is installed by means of which the live overspeed test is activated. This function is deactivated automatically after a maximum time period which can be set, in order to protect the steam turbine  2  from contact with too-cold steam. The maximum time period must be chosen so as to correspond to the duration of the live overspeed test with the limiting condition that the steam temperatures remain within the permitted range during the test. The method is characterized by the following: in dependence on the switch, in the case of the gas turbine  3  first the load shedding detection is blocked and automated, for example time-delayed, opening of the switch  8  of the generator  4  is effected. In the case of the steam turbine  2 , in dependence on the switch, first the rotational speed controller setpoint value is set to a value marginally above the triggering limit value of the steam turbine overspeed protection mechanism, which can for example be 105%, wherein the controller should approach the limit value with a defined acceleration. The magnitude of the acceleration influences the test duration. Subsequently, the limit frequency influence is deactivated and then the influence of the gas turbine fire power on the steam turbine controller is suppressed. In this context, it is then necessary for possibly the overall rapid shutdown mechanism in the event of coupling disturbances to be controlled for the duration of the overspeed test, which is necessary only if the gas turbine  3  is tripped by the overall rapid shutdown mechanism before reaching the triggering limit value. 
         [0029]    The method for testing the overspeed protection mechanism of the single-shaft combined-cycle plant  1  is carried out as long as the steam turbine  2  is fully hot and has been in operation for longer than 5 hours. The gas turbine  3  is operated with lowest possible power and constant exhaust gas temperature, which is reflected by the IGV point. The overspeed test is activated by actuating the switch in the control system technology. When the overspeed test is activated, the necessary switchovers will take place in automated fashion, and the automated opening of the switch  8  takes place with a certain delay. The gas turbine  3  and steam turbine  2  then approach the triggering limit value in a defined fashion. The steam turbine  2  reaches its triggering limit value first, the gas turbine  3  reaching its triggering limit value shortly after. This overall procedure must not exceed a certain time, which is to be set, since the steam then becomes impermissibly cold. For that reason, the operation of live overspeed testers is actively automatically deactivated after a few seconds, which can be set, which causes the steam turbine to be tripped.