Abstract:
A combined-cycle system includes a compressor, a gas turbine, a steam turbine, and an electric generator, which are coupled to the same shaft. A method of controlling the system envisages detecting a current compression ratio of the compressor, calculating a normalized compression ratio on the basis of the current compression ratio, and determining a load condition of the gas turbine on the basis of the normalized compression ratio. Moreover, a setpoint is selected, for at least one operating quantity of the gas turbine, and regulating signals are applied to actuators of the gas turbine so that the operating quantity of the gas turbine tends to reach the setpoint.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    As is known, for a gas turbine to operate efficiently it is necessary for different parameters to be set in an optimal way as a function of the load, i.e., of the power that the gas turbine is effectively supplying, and of the ambient conditions. A wrong setting of the parameters leads in fact to a deterioration in the conditions of combustion, and this causes, on the one hand, a reduced efficiency and, on the other hand, an increase in pollutant emissions. 
         [0002]    It has been noted that in modern gas turbines, which use burners with low NOx emissions, the flowrate of fuel fed to the pilot burners and the temperature of the exhaust gas are particularly critical, and setting of reference values that are not adequate as a function of the operating conditions has severe consequences on the efficiency of the machine. 
         [0003]    The identification of the operating conditions does not in general pose problems in autonomous gas-turbine systems and in combined-cycle systems of a “multishaft” type (i.e., in which each gas turbine is mounted on a respective shaft that is independent with respect to the shaft of the steam turbine and is coupled to a respective electric generator). In this case, in fact, the power supplied by the gas turbine can be easily estimated from the electric power supplied by the generator coupled to the gas turbine itself. The measurement of the electric power is practically always available. 
         [0004]    Difficulties arise, instead, in the case of combined-cycle systems of a “single-shaft” type, where a gas turbine, a steam turbine, and an electric generator are coupled to the same shaft. The power supplied by the gas turbine can hence not be estimated from the electric power supplied by the generator, which also contains a contribution of the steam turbine. In addition, it should be considered that, in many operating conditions, where correct setting of the parameters is particularly important, the load associated, respectively, to the gas turbine and to the steam turbine can depart sensibly from the power references provided by the system controller. In some transients, for instance, the system may be required to deliver a supplementary power rapidly, and the system controller consequently modifies the power reference for the gas turbine and for the steam turbine. The response of the steam turbine is, however, very slow with respect to that of the gas turbine, which in the initial steps of the transient supplies practically all the supplementary power required. The power references are hence unreliable at least during the transients. 
         [0005]    The operating conditions of the steam turbine may vary in an unforeseeable way also for other reasons. For instance, a fraction of the steam can be drawn off to be used for district-heating systems. 
       SUMMARY OF THE INVENTION 
       [0006]    The aim of the present invention is hence to provide a method of controlling a combined-cycle system and a combined-cycle system that is free from the limitations described and, in particular, enables the operating parameters of the gas turbine to be set correctly as a function of the load. 
         [0007]    According to the present invention a method of controlling a combined-cycle system and a combined-cycle system are provided as defined, respectively, in Claims  1  and  10 . 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0008]    The present invention will now be described with reference to the annexed drawings, which illustrate a non-limiting example of embodiment thereof and in which: 
           [0009]      FIG. 1  is a simplified block diagram of a combined-cycle system for the production of electric energy in accordance with one embodiment of the present invention; 
           [0010]      FIG. 2  is a more detailed block diagram of a part of the system of  FIG. 1 ; and 
           [0011]      FIG. 3  is a flowchart regarding a method for controlling a combined-cycle system in accordance with, one embodiment of the present invention, 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0012]    With reference to  FIG. 1 , a combined-cycle system for the production of electric power is designated by the number  1 . The system  1  comprises a gas-turbine assembly  2 , a steam turbine  3 , and an electric generator  4  in single-shaft configuration, i.e., all coupled on one and the same shaft  5 . 
         [0013]    A control device  7  controls the gas-turbine assembly  2 , the steam turbine  3 , and the electric generator  4  on the basis of measurement signals supplied by measuring devices  6  in such a way that the operating conditions of the system  1  are optimised for supplying an electric power PE required by the loads. 
         [0014]    The gas-turbine assembly  2  comprises a compressor  8 , a combustion chamber  9 , and a gas turbine  10 . The compressor  8  and the gas turbine  10  are mounted, on the shaft  5 . 
         [0015]    The compressor  10  is provided with an anti-icing device  11  that comprises a recirculation line  12  and a regulating valve  13 . The recirculation line  12  connects the outlet and the inlet of the compressor  8 , and the regulating valve  13  enables an anti-icing air flowrate Q AI  (at approximately 400° C.) to be taken from the outlet of the compressor  8  and to be fed back at inlet to prevent the formation of ice. 
         [0016]    An input stage of the compressor  8  is provided with a stage  8   a  of inlet guide vanes (IGVs), which are controlled by the control device  7  through an IGV actuator  15  for regulating an air flowrate Q A  taken in by the compressor  8 . The air flowrate Q A  taken in by the compressor  8  in turn enables regulation of the exhaust gas temperature T E . 
         [0017]    The combustion chamber  9  is provided with pilot burners  9   a  and premixing burners  9   b  (see  FIG. 2 .). A pilot fuel flowrate Q FP  fed to the pilot burners  9   a  is regulated by the control device  7 , which acts on a fuel valve  16 . 
         [0018]    The system  1  further comprises a recovery boiler  17 , which uses hot exhaust gas from the gas turbine  10  to generate steam for the steam turbine  3 , and a condenser  18 , which receives the steam processed by the steam turbine  3 . 
         [0019]    The measuring devices  6  supply measurement signals indicative of operating quantities of the system  1 . In particular, the measurement signals comprise: 
         [0000]    a signal S β  indicative of a current compression ratio β C  of the compressor  8 ;
 
a signal S T  indicative of the ambient temperature;
 
a signal S TE  indicative of the exhaust gas temperature T E ;
 
a signal S N  indicative of the angular speed of the shaft  5 ; and
 
a signal S B  indicative of the position of the regulating valve  13 .
 
         [0020]    The control device  7  shares the load requested to the system between the gas turbine  10  and the steam turbine  3 , detects the current operating conditions at least of the gas turbine  10  and selects optimal reference values (setpoints) of operating quantities of the system  1  as a function of the detected operating conditions. Moreover, the control device  7  applies regulating signals to the actuators of the system  1 , in particular to the IGV actuator  15  and to the fuel valve  16  in such a way that the operating quantities tend to reach the respective setpoints. The control device  8  also performs a function of supervision of the accessory apparatuses, such as, for example, the anti-icing device  11 . In particular, the control device  7  acts on the regulating valve  13  so as to activate the anti-icing device  11  and regulate the anti-icing air flowrate Q AI  when the ambient temperature drops below a threshold. 
         [0021]    In order to determine the operating conditions of the gas turbine  10 , the control device  7  uses the current compression ratio β C  of the compressor  8 , which is obtained from the signal S β  supplied by the measuring devices  6 . 
         [0022]    The current compression ratio pc is normalized with respect to reference operating conditions and corrected to take into account the effect of factors such as the ambient conditions, the rotation speed of the shaft  5 , and the action of the anti-icing device  11 . 
         [0023]    The reference conditions may be ISO conditions (temperature T=15° C.; pressure P=1.013 bar). 
         [0024]    The normalized compression ratio β N  is given by 
         [0000]    
       
         
           
             
               
                 
                   
                     β 
                     N 
                   
                   = 
                   
                     
                       
                         β 
                         C 
                       
                       
                         β 
                         R 
                       
                     
                      
                     
                       C 
                       1 
                     
                      
                     
                       C 
                       2 
                     
                      
                     
                       C 
                       3 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where β RIF  is the compression ratio in the reference conditions, and C T , C N , C Q  are, respectively, a corrective temperature coefficient, a corrective speed coefficient, and a corrective flowrate coefficient. 
         [0025]    The corrective temperature coefficient C T  takes into account the effect of the ambient temperature, which is detected by the measuring devices  6  (signal S T ). 
         [0026]    The corrective speed coefficient C 3  depends upon the rotation speed, of the gas turbine  10 , which can also be detected by the measuring devices  6  (signal S N ). 
         [0027]    The corrective flowrate coefficient C 3  depends upon an estimate of the anti-icing air flowrate Q AI  that is taken at output from the compressor  8  and fed back at input to the compressor  8  itself. In practice, the third corrective coefficient C 3  takes into account that not the entire air flowrate Q A  taken in by the compressor  8  is introduced into the combustion chamber  9  and, moreover, the recirculation of air from the outlet to the inlet of the compressor  8  modifies the conditions of temperature. In one embodiment, estimation of the anti-icing air flowrate Q AI  is determined on the basis of the position of the regulating valve  13 , which is set by the control device  7 . In a different embodiment, the anti-icing air flowrate Q AI  is measured, for instance with a flowmeter. 
         [0028]    It has been found that the normalized compression ratio β N  defined above represents the power supplied by the gas turbine  10 , normalized with respect to the same conditions (for example ISO or standard conditions). Hence, in practice, it is possible to determine the load conditions of the gas turbine  10  starting from the calculation of the normalized compression ratio β N . 
         [0029]    The control device  7  operates as described hereinafter, with reference to  FIG. 3 , to optimize operation of the gas turbine  10  as a function of the load conditions. 
         [0030]    The control device  7  first of all acquires the measurement signals supplied by the measuring devices  6 , amongst which, in particular (block  100 ): 
         [0000]    the signal S β  indicative of the current compression ratio β C  of the compressor  8 ;
 
the signal S T  indicative of the ambient temperature;
 
the signal S TE  indicative of the exhaust gas temperature T E ;
 
the signal S N  indicative of the angular speed of the shaft  5 ; and
 
the signal S R  indicative of the position of the regulating valve  13 .
 
         [0031]    Once the measurement signals have been acquired, the control device calculates the current compression ratio β C  from the signal S β  (block  110 ) and determines the values of the corrective temperature coefficient C T , of the corrective speed coefficient C N , and of the corrective flowrate coefficient C Q  (block  120 ) using functions determined experimentally and stored, for example, in the form of tables. 
         [0032]    Once the current compression ratio β C  and the current values of the corrective temperature coefficient C T , of the corrective speed coefficient C N , and of the corrective flowrate coefficient C Q  are available, the control device  7  calculates the normalized compression ratio β N  applying Eq. (1) (block  130 ). 
         [0033]    Next (block  140 ), the control device  7  determines the load conditions of the gas turbine  10  on the basis of the value of the normalized compression ratio β N . For this purpose, a function is used, stored for instance in the form, of a table in the control device  7 . The function may be defined experimentally, starting from historic series, or else using a model of the system  1 , which can be described with sufficient precision to yield reliable results. 
         [0034]    In an alternative embodiment, instead of determining the load conditions of the gas turbine  10  directly from the normalized compression ratio β N , the control device  7  calculates an estimate of the power supplied by the gas turbine  10  on the basis of the normalized, compression ratio β N . The load, conditions of the gas turbine  10  are then determined as a function of the estimate of the power delivered. 
         [0035]    After determining the current load conditions of the gas turbine  10 , the control device  7  selects respective setpoints for the critical quantities that significantly affect, the efficiency of the gas turbine  10  (block  150 ). In particular, the control device  7  defines a first setpoint SP TE , indicative of a target temperature of the exhaust gas, and a second setpoint SP P , indicative of a target pilot fuel flowrate to be fed to the pilot burners of the combustion chamber  9 . 
         [0036]    Finally (block  160 ), the control device  7  applies a first regulating signal S IGV  to the IGV actuator  15  and a second regulating signal S FV  to the fuel valve  16  in such a way that the temperature exhaust gas T E  and the pilot fuel flowrate Q FP  supplied to the pilot burners  9   a  tend to reach the first setpoint SP TE  and the second setpoint SP P , respectively. 
         [0037]    Thanks to the method described, the power supplied by the gas turbine of a single-shaft combined-cycle system can be easily estimated with good precision and in a reliable way, also considering the fact that the measurement of the current compression ratio β C  is normally available in the systems. The parameters of the gas turbine can thus be correctly set as a function of the load and of the ambient conditions, and it is possible to maintain optimal conditions of combustion with high efficiency and low emissions of pollutant substances, in particular NOx. 
         [0038]    Finally, it is evident that modifications and variations may be made to the method and to the system described herein, without departing from the scope of the present invention, as defined in the annexed claims.