Abstract:
A method is provided for operating a gas turbine, which especially feeds power to a local isolated power supply network, and which comprises a compressor for compressing combustion air which is drawn in from the environment, a combustion chamber for combusting supplied fuel by the compressed combustion air, a turbine which is driven by the hot gas from the combustion chamber, and a generator, which is driven by the turbine, for generating electric power. With such a method, an improvement of controlling is achieved by one or more parameters of the gas turbine being measured or determined, by the effective thermal output power of the gas turbine being calculated from the measured or determined parameters, and by the calculated effective thermal output power being used for controlling the gas turbine.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of International Application No. PCT/EP2007/055813 filed Jun. 13, 2007, which claims priority to Swiss Patent Application No. 01087/06, filed Jul. 6, 2006 the contents of both of which are incorporated by reference as if fully set forth. 
     
    
     FIELD OF INVENTION 
       [0002]    The present invention refers to the field of technology of gas turbine installations. It relates to a method for operating a gas turbine and to a gas turbine for carrying out the method. 
       BACKGROUND 
       [0003]    A gas turbine installation for generating electric power customarily comprises a compressor which draws in air from the environment via a compressor inlet  18 , compresses this, and delivers the compressed air via a compressor outlet to a subsequent combustion chamber where it is used for combusting a liquid or gaseous fuel which is introduced through a fuel feed line. The hot gas which results during the combustion is transmitted via a turbine inlet to a turbine which is connected downstream, where it is expanded, performing work. The expanded gas is discharged as exhaust gas at a turbine exhaust. Via a common shaft, the turbine drives both the compressor and a generator, at the terminals of which electric power can be tapped and transmitted via a transformer to a local or national power supply network. 
         [0004]    The output power of the gas turbine installation at the generator terminals is one of the main control parameters of the power plant. If the gas turbine is part of a so-called “power island” (note: an isolated local network, which is separated from the national grid, with limited electrical capacity is to be understood by “power island” in this connection; typical examples of power islands are metallurgical plants, paper mills or rolling plants), the controlling of the gas turbine and its precise and reliable operation takes place in an environment which is characterized by a continuous power demand of the individual, often fluctuating consumers on the power island. In such a complex environment, the controlling of the gas turbine requires particular attention. 
         [0005]    While the efficient and accurate controlling of a gas turbine installation in a generally “rigid” national grid already represents a challenge, the requirements increase if a comparatively smaller isolated local network with individual consumers and associated critical processes is to be operated and kept alive. 
         [0006]    The controlling of the gas turbine installation especially requires improved and further developed control strategies for modern gas turbine installations during faster transition phases with potential network frequency fluctuations. 
         [0007]    A special demand upon gas turbine controlling in isolated power islands with potential network frequency fluctuations results from the fact that the active power at the generator terminals (P GENO ) comprises a kinetic power (P KINETIC ) in addition to the thermal power (P GT ) of the gas turbine, which kinetic power is proportional both to the time derivative of the network frequency (dn/dt) and to the total inertia moment (J ISLAND ) of the consumers which are connected up to the island during such an event. 
         [0008]    A device for calculating the mechanical output power of a gas turbine is known from publication US-A1-2005/0131616, in which by the use of a wattmeter and a tachometer the electric power at the generator terminals and the speed of the turbine are measured, and from the two values the mechanical output power of the turbine is calculated by an equation. This solution is dependent upon measuring at the generator terminals and therefore cannot be applied in cases in which this measuring cannot be carried out, is not quick enough, or is falsified by interruptions. This is especially the case with the aforementioned power islands. 
         [0009]    A method for controlling the power of a turbogroup is known from EP-A1-0 903 469, in which the power which is delivered by the generator is determined and the thermal power of the turbine is controlled in dependence upon the measured electric power of the generator, wherein the kinetic power which is absorbed or delivered by the shaft is additionally determined and the thermal power is controlled in accordance with the sum of the electric power and kinetic power. The electric power in this case can especially be calculated from the rotational frequency of the shaft and the torque acting on the shaft. This solution is not suitable for power islands either. 
       SUMMARY 
       [0010]    The present disclosure is directed to a method for operating a gas turbine that feeds power to a local isolated power supply network. The turbine includes a compressor, for compressing combustion air which is drawn in from the environment, and a combustion chamber for combusting supplied fuel using the compressed combustion air. The gas turbine also includes a turbine which is driven by hot gas from the combustion chamber, and a generator, which is driven by the turbine, for generating electric power. The method includes measuring or determining at least one of the following parameters of the gas turbine: an ambient temperature, a compressor inlet temperature, a compressor outlet temperature, a turbine exhaust temperature, an ambient air pressure, a total or static absolute or gauge pressure at a compressor outlet, a total or static absolute or gauge pressure at a turbine inlet, a pressure loss between compressor outlet and turbine inlet, a speed of the gas turbine or a frequency of a power supply network, a measured or predetermined mass flow of the fuel which is fed to the combustion chamber, and a lower heating value of the fuel. The method also includes calculating an effective thermal output power of the gas turbine from the measured or determined parameters; and controlling the gas turbine based on the calculated effective thermal output power. 
         [0011]    The present disclosure is also directed to a gas turbine including a compressor for compressing combustion air which is drawn in from the environment, a combustion chamber for combusting supplied fuel by the compressed combustion air, a turbine which is driven by the hot gas from the combustion chamber and a generator, which is driven by the turbine, for generating electric power which is fed to a local isolated power supply network. A control unit is provided for controlling the operation of the gas turbine, and at associated points of the gas turbine transducers are provided for measuring at least one of the following parameters: an ambient temperature, a compressor inlet temperature, a compressor outlet temperature, a turbine exhaust temperature, an ambient air pressure, a total or static absolute or gauge pressure at a compressor outlet, a total or static absolute or gauge pressure at a turbine inlet, a pressure loss between compressor outlet and turbine inlet, a speed of the gas turbine or a frequency of the power supply network, a measured or predetermined mass flow of the fuel which is fed to the combustion chamber, and a lower heating value of the fuel. The transducers are connected to the control unit, and the control unit is configured to calculate the effective thermal output power of the gas turbine from the data which is generated by the transducers. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0012]    The invention is to be subsequently explained in more detail based on exemplary embodiments in conjunction with the drawing. The single FIGURE shows a connection diagram of a gas turbine with a control unit according to a preferred exemplary embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Introduction to the Embodiments 
       [0013]    It is the object of the invention to disclose a method for operating a gas turbine, and also a gas turbine for carrying out the method, which avoid the disadvantages of known solutions and which are especially suitable for installations which are used in power islands. 
         [0014]    The object is achieved as set forth below. It is essential for the invention that one or more of the following parameters of the gas turbine, specifically
       the ambient temperature,   the compressor inlet temperature,   the compressor outlet temperature,   the turbine exhaust temperature,   the ambient air pressure,   the total or static absolute or gauge pressure at the compressor outlet,   the total or static absolute or gauge pressure at the turbine inlet   the pressure loss between compressor outlet and turbine inlet,   the speed of the gas turbine or the frequency of the power supply network,   the measured or predetermined mass flow of the fuel which is fed to the combustion chamber, and   the lower heating value of the fuel,
 
are measured or determined, that from the measured or determined parameters the effective thermal output power of the gas turbine is calculated, and that the calculated effective thermal output power is used for controlling the gas turbine.
       
 
         [0026]    According to one embodiment of the invention, the compressor comprises variable inlet guide vanes and the position of the variable inlet guide vanes is taken into consideration as a further parameter when calculating the effective thermal output power. 
         [0027]    In another embodiment of the invention a cooling medium, especially water or steam, is fed to the gas turbine at a chosen point, and in that the mass flow of the cooling medium is taken into consideration as a further parameter when calculating the effective thermal output power. 
         [0028]    In a preferred embodiment of the invention, the effective thermal output power of the gas turbine is calculated in accordance with the equation 
         [0000]      P GT αƒ1(p 3 )·ƒ2(m fuel )·ƒ3(LHV)·ƒ4(n GT )·ƒ5(T 2 ) 
         [0029]    wherein f 1 , . . . , f 5  are functions which are to be determined for the respective case, p 3  is the compressor outlet pressure, m fuel  is the fuel mass flow, LHV is the lower heating value of the fuel, n GT  is the speed of the gas turbine, and T 2  is the compressor inlet temperature. 
         [0030]    In this case, the ambient temperature can especially be used as a variable in the equation instead of the compressor inlet temperature as long as available devices for cooling or for preheating the ambient air which is drawn in by the compressor are not activated. 
         [0031]    In a preferred embodiment, in order to take into consideration ageing effects and other changes in the gas turbine, the effective thermal output power of the gas turbine is measured at determined time points. The coefficients which occur in the equation are adjusted by comparison with the calculated effective thermal output power. Furthermore, the lower heating value of the fuel in the equation can also be adjusted from time to time. 
         [0032]    In a further embodiment of the invention, the effective thermal output power of the gas turbine is continuously measured, in that for the controlling of the gas turbine the measured or calculated value of the effective thermal output power is selectively used, and in that the controlling of the gas turbine is automatically changed over from one to the other value depending upon the state of the gas turbine and of the power supply network. 
         [0033]    The measured value of the effective thermal output power can especially be used in this case for controlling the gas turbine if the gas turbine is in a steady operating state, and the calculated value of the effective thermal output power is used if the gas turbine is in a fast changing transition state and/or the power supply network is unstable. 
         [0034]    In another embodiment of the gas turbine according to the invention, the compressor comprises variable inlet guide vanes, and transducer, which is connected to the control unit, is arranged on the compressor for sensing the position of the variable inlet guide vanes. 
         [0035]    In another embodiment, devices for cooling the gas turbine by a cooling medium are provided on the gas turbine, and in that a transducer, which is connected to the control unit, is arranged on the gas turbine for measuring the mass flow of the cooling medium. 
         [0036]    According to a further embodiment of the invention, devices, which are connected to the control unit, are attached on the terminals of the generator for measuring the power which is delivered at the generator terminals. 
         [0037]    In this case the invention is not limited to a gas turbine according to the description and FIGURE which follow here, but it also covers gas turbines with a sequential firing, as originates from EP-0620362-B1. This whole publication in this case forms an integral part of this application. 
       DETAILED DESCRIPTION 
       [0038]    The invention is based on the fact that, as already mentioned above, the active power at the generator terminals (P GENO ) comprises a kinetic power (P KINETIC ) in addition to the thermal power (P GT ) of the gas turbine, which kinetic power is proportional both to the time derivative of the network frequency (dn/dt) and to the total inertia moment (J ISLAND ) of the consumers which are connected up to the island during such an event. 
         [0039]    Since the inertia moment is to be predicted only with difficulty due to the practically unlimited combination possibilities of the individual consumers, it is simpler to determine the effective thermal power (P GT ) by suitable physical/mathematical methods as input values for the control system of the gas turbine. 
         [0040]    The physical formulation for the description of the aforementioned transient effects uses the following equation: 
         [0000]    
       
         
           
             
               
                 
                   
                     P 
                     GENO 
                   
                   = 
                   
                     
                       
                         P 
                         GT 
                       
                       + 
                       
                         P 
                         KINETIC 
                       
                     
                     = 
                     
                       
                         P 
                         GT 
                       
                       + 
                       
                         4 
                          
                         
                             
                         
                          
                         
                           
                             π 
                             2 
                           
                           · 
                           
                             J 
                             ISLAND 
                           
                           · 
                           n 
                           · 
                           
                             ( 
                             
                               
                                  
                                 n 
                               
                               
                                  
                                 t 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0041]    wherein the equation for the steady state can be simplified to 
         [0000]      P GENO =P GT   (2) 
         [0042]    The proposed method is based on the determining of the effective thermal output power of the gas turbine by various measured characteristic parameters. Taking into consideration the thermodynamics of an open gas turbine cyclic process (Joule-Brayton cyclic process), the thermal output power of a gas turbine in simple values follows the relationship 
         [0000]      P GT αm exh ·(h 6 −h 7 ),  (3) 
         [0000]    wherein the indices “6” and “7” refer to the conditions at the turbine inlet ( 21 ) and turbine exhaust ( 22 ). 
         [0043]    In the case of an idealized uncooled turbine, the value m exh  represents the total exhaust gas flow which passes through the turbine ( 12 ), while the inlet enthalpy h 6  is linked with the turbine inlet temperature (TIT) according to international standard ISO 2314:1989. 
         [0044]    If a constructed gas turbine with a prespecified swallowing capacity and a defined operating concept is taken as a starting point, the operating range of the gas turbine is consequently established, and the thermal output power of the gas turbine can be approximated by a mathematical function of the following type: 
         [0000]      P GT αƒ1(p 3 )·ƒ2(m fuel ).  (4) 
         [0045]    The compressor outlet pressure p 3  in the equation (4) is directly linked with the exhaust gas mass flow m exh , while the fuel mass flow m fuel  represents a suitable measurement for the firing degree of the gas turbine, i.e. for the inlet enthalpy h 6  from equation (3). 
         [0046]    In order to compensate for possible fluctuations in the lower heating value (LHV) of the fuel, the equation (4) can be expanded as follows: 
         [0000]      P GT αƒ1(p 3 )·ƒ2(m fuel )·ƒ3(LHV).  (5) 
         [0047]    If, furthermore, fluctuations are to be compensated for in the network frequency (which are equivalent to the relative change in the rotor speed of the gas turbine), and changes in the ambient temperature are to be taken into consideration, the following equation finally results from the equation (5): 
         [0000]      P GT αƒ1(p 3 )·ƒ2(m fuel )·ƒ3(LHV)·ƒ4(n GT )·ƒ5(T 1 ).  (6) 
         [0048]    If the ambient temperature T 1  is substituted by the compressor inlet temperature T 2 , there finally follows: 
         [0000]      P GT αƒ1(p 3 )·ƒ2(m fuel )·ƒ3(LHV)·ƒ4(n GT )·ƒ5(T 2 ).  (7) 
         [0049]    The last-mentioned equation (7) can be referred to as a so-called “power formula”. 
         [0050]    The most suitable structure of the functions f 1 , . . . , f 5  in equation (7) follows from the specific operating characteristic of the respective gas turbine and must be determined individually on the basis either of suitable calculations of the cyclic process or from direct measurements on the gas turbine. 
         [0051]    If the functions f 1 , . . . , f 5  of the equation (7) are defined in a suitable manner, the correlation for the effective thermal power according to equation (6) or (7) can be integrated in a simple manner into the control system of the gas turbine, in which the following parameters are measured individually or in combination and processed in the control unit of the gas turbine:
       the ambient temperature T 1 ,   the compressor inlet temperature T 2 ,   the compressor outlet temperature   the turbine exhaust temperature   the ambient air pressure   the total or static absolute or gauge pressure p 3  at the compressor outlet  19 ,   the total or static absolute or gauge pressure at the turbine inlet  21 ,   the pressure loss between compressor outlet  19  and turbine inlet  21 ,   the speed n GT  of the gas turbine  10  or the frequency of the power supply network,   the position of the variable inlet guide vanes VIGV of the compressor  11 ,   the measured or predetermined mass flow m fuel  of the fuel which is fed to the combustion chamber  15 ,   the measured or predetermined mass flow of the water or steam if either of these is additionally injected into a component of the gas turbine; and   the lower heating value (LHV) of the fuel on the basis of for example an online gas chromatograph (GC).       
 
         [0065]    In order to provide corresponding parameter values for the control unit  17 , according to the FIGURE various transducers  23 , . . . ,  32  are provided in a distributed manner in the gas turbine  10  and are connected to the control unit  17 :
       the transducer  23  is provided for the position of the variable inlet guide vanes VIGV of the compressor  11 ;   the transducers  24  at the compressor inlet  18  are provided for the pressure and/or for the temperature at the compressor inlet  18 ;   the transducer  25  is provided for the ambient temperature T 1  and/or for the ambient air pressure;   the transducer  26  on the fuel feed line  20  measures the fuel mass flow m fuel ;   the transducer  27  on the fuel feed line  20  for example is formed as a gas chromatograph and measures the lower heating value LHV of the fuel;   the transducer  28  which is arranged at the turbine inlet  21  measures the turbine inlet pressure;   the transducer  29  which is arranged at the turbine exhaust  22  is provided for measuring the turbine exhaust temperature;   the transducer  30  which is arranged on the shaft  14  senses the speed of the shaft  14 ;   the transducer  31  which is arranged on the cooling medium feed line  34  measures the mass flow of the cooling medium;   the transducer  32  which is provided at the compressor outlet  19  measures the compressor outlet temperature and the compressor outlet pressure; and   the transducer which is arranged on the generator terminals measures the electric power at the generator terminals.       
 
         [0077]    From the incoming measured values the control unit calculates the effective thermal power of the gas turbine  10  in accordance with the equations (6) or (7), and from it derives control signals which are delivered at the output  35  of the control unit for controlling the gas turbine  10  in a manner known per se. 
         [0078]    It can be advantageous within the scope of the invention if the “power formula” according to equation (6) or (7) is adapted continuously (online adaptation) to permanent or temporary changes in the gas turbine  10 . Thus, it is difficult to incorporate ageing effects and the thermal state of the gas turbine into the “power formula” from the outset. The calculated power, therefore, will not be exactly the same as the power which is measured by the transducer  33 . In this case, assistance can be created by the coefficients of the “power formula” being automatically altered online in order to correlate the measured power with the calculated power. 
         [0079]    In case the lower heating value LHV of the fuel is not measured online by a gas chromatograph, or the delay time of the gas chromatograph is too long, it is practical to correspondingly adjust the lower heating value LHV in the “power formula”. 
         [0080]    If the gas turbine  10  is in a steady state or stable state, the power which is measured at the generator terminals is of higher accuracy. If, on the other hand, the gas turbine  10  is in a fast changing transient operating state, or if the network is unstable, the inaccuracy of the measured power is great. This inaccuracy can have the following causes:
       the lacking dynamics of the power measuring (i.e. the measuring is not quick enough);   and/or the kinetic power of the power train.
 
In these cases, the calculated power can be used for controlling the gas turbine  10  instead of the measured power. An automatic switching can be provided in the control unit  17  so as to use the suitable power value (measured or calculated) in accordance with the operating state of the gas turbine  10  and of the network.
       
 
         [0083]    The described method can not only be advantageously used in power islands but can generally be used in the following situations:
       1. The active power of the gas turbine cannot be directly measured at the generator terminals (P GENO ). This is especially the case with
           a. a gas turbine operation for supporting a power island, i.e. especially during fast transient operating states and/or network fluctuations;   b. gas turbine operation in a combined cycle power plant with a single-shaft configuration.   
           2. The measuring of the active power at the generator terminals (P GENO ) is not quick enough.   3. The measuring of the active power at the generator terminals (P GENO ) is interrupted.       
 
         [0089]    In all, a gas turbine operation is created with the invention in which the controlling both in the steady state and in transient operating states is improved and the availability and reliability especially of the critical local isolated networks (“power islands”) is increased. 
       LIST OF DESIGNATIONS 
       [0000]    
       
           10  Gas turbine 
           11  Compressor 
           12  Turbine 
           13  Generator 
           14  Shaft 
           15  Combustion chamber 
           16  Transformer 
           17  Control unit 
           18  Compressor inlet 
           19  Compressor outlet 
           20  Fuel feed line 
           21  Turbine inlet 
           22  Turbine exhaust 
           23 , . . . ,  33  Transducer 
           34  Cooling medium feed line 
           35  Control output