Abstract:
A method for operating a combined cycle power plant, according to which shortly before the planned start-up of a parked load, the steam turbine is lowered to a very low output, the gas turbine is then operated in parked load, and next the steam turbine is powered up to a parked output. The GT operating power can be the rated power of the gas turbine. The ST operating power can be the rated power of the steam turbine.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the US National Stage of International Application No. PCT/EP2014/064182 filed Jul. 3, 2014, and claims the benefit thereof. The International Application claims the benefit of European Application No. EP13177932 filed Jul. 25, 2013. All of the applications are incorporated by reference herein in their entirety. 
     
    
     FIELD OF INVENTION 
       [0002]    The invention relates to a method for operating a combined cycle power plant, wherein the gas turbine is operated at a GT operating power and the steam turbine is operated at an ST operating power, wherein the power of the steam turbine is reduced to an ST part power, wherein the ST part power is lower than the ST operating power. 
       BACKGROUND OF INVENTION 
       [0003]    Combined cycle power plants are used to generate electrical energy for communal energy supply. In general, a combined cycle power plant supplies electrical energy to a supply grid, the energy requirement being dependent on the time profile. This means that the energy requirement is not constant over the course of a day. The electrical supply grid is supplied with electrical energy by multiple power plants. Thus, use is made for example of conventional power plants and power plants which convert renewable energies into electrical energy. Feeding-in of the renewable energies is subject to fluctuations, which leads to increasing demands on the conventional power plants. This means that conventional power plants must be operated longer and lower in what are termed part loads or parked loads. In combined cycle power plants, such low part loads are associated—depending on the configuration of the gas turbine—with reduced gas turbine outlet temperatures. 
         [0004]    As a result, the steam turbine inlet temperature also drops. Thus, as soon as the plant is operated in part load, the steam inlet temperature is reduced. However, this means that the hot components of the steam turbine are exposed to cold steam, which leads to thermal stresses. 
         [0005]    If the parked load is then abandoned again, the steam temperatures rise again, which once again leads to thermal stresses. In order to prevent these thermal stresses, it is possible not to run the gas turbine so far in part load in order that the steam temperature does not drop so much. It is also possible to slowly reduce the steam temperature by spraying prior to the actual load reduction. Then, the load change takes place at a lower—but therefore more constant—temperature. After increasing the load, the steam temperature is once again raised slowly to the rated temperature. 
         [0006]    Another possibility for preventing thermal stresses consists in running the steam turbine down prior to reducing the gas turbine power. Then, the components of the steam turbine will cool down with very low thermal stresses. Once the components have cooled sufficiently, the steam turbine could be started up again at a reduced gas turbine power and thus at a low steam inlet temperature. This would lead to very low service life consumption. 
       SUMMARY OF INVENTION 
       [0007]    The invention is based on an object of indicating another possibility for reducing thermal stresses. 
         [0008]    This object is achieved by a method for operating a combined cycle power plant, wherein the gas turbine is operated at a GT operating power and the steam turbine is operated at an ST operating power, wherein the power of the steam turbine is reduced to an ST part power, wherein the ST part power is lower than the ST operating power, wherein the power of the gas turbine is then reduced to a GT parked power, wherein the GT parked power is lower than the GT operating power. 
         [0009]    Once the GT parked power of the gas turbine has been reached, the power of the steam turbine is increased to an ST parked power, wherein the ST parked power is 20% to 60% of the ST operating power. 
         [0010]    The invention thus proposes indicating an operating method wherein the steam turbine is involved in the parked load. Thus, for the purposes of grid stability, the steam turbine rotor keeps as much rotating mass as possible on the grid. 
         [0011]    The ST power of the steam turbine is reduced to a very low power shortly before the planned commencement of the parked load. The gas turbine is then operated in parked load. On account of the markedly lower heat transfers between steam and steam turbine components in part load, service life consumption is significantly lower as a consequence of lowering the steam temperature. In that context, the steam turbine cools down slowly. 
         [0012]    Advantageous developments are specified in the dependent claims. Thus, in a first advantageous development, the ST part power is set at 5% to 40%, 5% to 30%, 5% to 20%, or 5% to 10% of the ST operating power. 
         [0013]    In another advantageous refinement, the GT parked power is 20% to 60% of the gas turbine operating power. 
         [0014]    It is thus proposed to once again take up load after the steam turbine has cooled down slowly at a reduced gas turbine power and thus a low steam inlet temperature. 
         [0015]    In one alternative embodiment, the steam turbine could be held in this low part load until the end of the parked load. 
         [0016]    The invention thus proposes reducing the power of the steam turbine to an ST part power. The ST part power is lower than the ST operating power. Reducing to the ST part power is effected by closing a steam inlet valve. In that context, the steam inlet valve is controlled such that almost no fresh steam flows through the steam turbine. In that context, a bypass station is formed such that there results a fluidic connection between the steam inlet and the condenser. Thus, downstream of the steam generator, steam is not fed to the steam turbine but directly to the condenser, which has a disadvantageous effect on the efficiency. The steam turbine then cools down. After this, the power of the gas turbine is reduced to a GT parked power. This affects the steam inlet temperature. That means that the steam inlet temperature drops. After a certain time, the steam inlet valve is then once again opened and the fluidic connection between the steam inlet and the condenser is interrupted. Thus, all of the steam generated in the steam generator is then fed through the steam turbine. 
         [0017]    In one advantageous development, the steam turbine comprises a high-pressure, intermediate-pressure and low-pressure turbine section, wherein—the high-pressure turbine section,—the high-pressure turbine section and the intermediate-pressure turbine section,—the intermediate-pressure turbine section,—the intermediate-pressure turbine section and the low-pressure turbine section or—the low-pressure turbine section is/are not charged with steam. 
         [0018]    Thus, ideally, one pressure stage is completely closed. In the switched-off turbine section, the service life consumption is even lower since here the components cool down naturally. Advantageously, the pressure in the steam turbine or in the remaining operational turbine sections is reduced as much as possible, which is made possible by drains, evacuation lines, start-up lines or also process steam lines. 
         [0019]    Thus, the significant reduction in the pressure in the steam turbine reduces the heat transfer and significantly reduces the service life consumption during part load. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    The invention will now be explained in more detail with reference to an exemplary embodiment. In the drawing: 
           [0021]      FIG. 1  shows a schematic representation of a combined cycle power plant. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0022]      FIG. 1  shows a schematic representation of a combined cycle power plant. In essence, a combined cycle power plant  1  comprises a gas turbine  2  that can be operated with fossil fuels. This gas turbine  2  comprises a compressor part  3  in which air is heated and compressed, a combustion chamber  4  in which the air from the compressor part  3  is mixed with fuel and ignited, and a turbine part  5  in which—in various stages consisting of guide vanes and rotor blades that are not shown—the hot exhaust gases turn a rotor. A shaft  6  transfers this rotation to a generator  7 . The generator  7  then supplies a supply grid with electrical energy (not shown). 
         [0023]    The hot exhaust gases from the gas turbine  2  are fed into a steam generator  8 . In this steam generator  8 , fresh steam is generated by means of a line  9  and is then fed via a steam turbine fresh steam line  10  into a high-pressure turbine section  11 . An HP valve  12  is arranged in the steam turbine fresh steam line  10 . The steam leaving the HP turbine section  11  is conveyed to an intermediate superheater  13 . This takes place via the cold intermediate superheater line  14 . Once the steam has been heated in the intermediate superheater  13 , the hot intermediate superheater line  15  supplies steam to an intermediate-pressure turbine section  16 . From the intermediate-pressure turbine section  16 , the steam flows via an overflow line  17  into two low-pressure turbine sections  18 . After the low-pressure turbine section  18 , the cold, expanded steam flows into a condenser  19  where it condenses to water which is conveyed, via a pump  20 , back into the fresh steam generator  8  via the fresh steam line  9 . 
         [0024]    The steam turbine fresh steam line  10  is fluidically directly connected to the condenser  19  via a redirection station  21 . An overflow valve  22  is arranged in the overflow line  21 . An electric generator  23  is connected, in a torque-transmitting manner via a common shaft  24 , to the high-pressure turbine section  11 , the intermediate-pressure turbine section  16  and the low-pressure turbine section  18 . The HP turbine section  11 , the IP turbine section  16  and the LP turbine sections  18  form the steam turbine  25 . 
         [0025]    In one alternative embodiment, the combined cycle power plant comprises a redirection system. This redirection system comprises a high-pressure redirection station  22  and a high-pressure redirection valve  21  arranged in the high-pressure redirection station  22 , wherein the high-pressure redirection station  22  establishes a fluidic connection between the steam turbine fresh steam line  10  and the cold intermediate superheater line  14 . Furthermore, the redirection system comprises an intermediate-pressure redirection station  22   a  and an intermediate-pressure redirection valve  21   a  arranged in the intermediate-pressure redirection station  22   a,  wherein the intermediate-pressure redirection station  22   a  establishes a fluidic connection between the hot intermediate superheater line  15  and the condenser  19 . 
         [0026]    Thus, steam can flow from the steam turbine fresh steam line  10  to the condenser  19 , via the redirection system comprising the high-pressure redirection station  22  and the intermediate-pressure redirection station  22   a.    
         [0027]    Furthermore, the combined cycle power plant comprises an intermediate-pressure valve  12   a  arranged in the hot intermediate superheater line  15 . 
         [0028]    Now, according to the invention, the combined cycle power plant is operated as follows. First, the gas turbine  2  is operated at a gas turbine operating power. Equally, the steam turbine  25  is operated at an ST operating power. The power of the steam turbine  25  is reduced to an ST part power, wherein the ST part power is lower than the ST operating power. Then, in this context, the ST part power is 5% to 40%, 5% to 30%, 5% to 20%, or 5% to 10% of the ST operating power. 
         [0029]    This is achieved by nearly closing the HP valve  12  and the intermediate-pressure valve  12   a,  such that almost no steam flows through the steam turbine  25 . Thus, the components in the steam turbine  25  cool down. After a certain time, the power of the gas turbine  2  is then reduced to a GT parked power, wherein the GT parked power is lower than the GT operating power. In this case, the GT parked power is 20% to 60% of the gas turbine operating power. As a result, the temperature of the hot exhaust gas from the gas turbine  2  is lower, leading to a reduction in the temperature of the fresh steam which is generated in the steam generator  8  and passes through the steam turbine fresh steam line  10  and the hot intermediate superheater line  15 . 
         [0030]    Once the HP valve  12  is almost closed, the overflow valve  22  or the redirection system  22 ,  21 ;  22   a,    21   a  is opened such that the majority of the steam generated in the steam generator  8  is fed directly into the condenser  19 . However, this is disadvantageous for the overall efficiency of the combined cycle power plant. 
         [0031]    Once the GT parked power of the gas turbine  2  has been reached, the power of the steam turbine  25  is increased to an ST parked power. This ST parked power is 20% to 60% of the ST operating power. This is achieved by opening the HP valve  12  and the intermediate-pressure valve  12   a.  The overflow valve  22  in the overflow line  21  is closed again. Thus, the steam—now conveyed in the steam turbine fresh steam line  10  and in the hot intermediate superheater line  15  as a consequence of the reduced steam inlet temperature of the steam—can be fed into the HP turbine section  11 . As a consequence of the lower fresh steam temperature, the volumetric flow of fresh steam is also lower. 
         [0032]    The power of the steam turbine  25  is reduced by reducing the pressure of the steam. It is now possible, once the ST part power and the GT parked power have been reached, to operate the steam turbine  25  as follows. The steam turbine  25  comprises a high-pressure turbine section  11 , an intermediate-pressure turbine section  16  and a low-pressure turbine section  18 , wherein the high-pressure turbine section  11 , the high-pressure turbine section  11  and the intermediate-pressure turbine section  16 , the intermediate-pressure turbine section  16 , the intermediate-pressure turbine section  16  and the low-pressure turbine section  18  or the low-pressure turbine section  18  is/are not charged with steam. The remaining turbine sections remain closed and can cool down naturally. 
         [0033]    The pressure of the steam in the turbine sections not charged with steam is then reduced as far as possible. To that end, drains, evacuation lines, start-up lines or process steam lines are opened.