Abstract:
A system and method control a gas turbine subject to fuel composition variation. The method includes operating a first effector to control the gas turbine based on fuel composition. The method also includes operating a second effector to maintain operation of the first effector within a first boundary limit, the second effector operation being initiated when the operating the first effector reaches a second boundary limit within the first boundary limit.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The subject matter disclosed herein relates to gas turbine control and, more specifically, to control that accommodates changes in fuel composition. 
         [0002]    The stability of power grid frequency requires that the power supplied to the grid equals the power demand from the grid. One of the factors that can cause variation in the power supplied to the grid by gas turbine engines is a change in the composition of the fuel supplied to the gas turbine. Composition variation of the natural gas supply is a common issue. Variation in fuel composition poses concerns for combustion dynamics, combustor blowout, emissions compliance (e.g., NOx, CO), auto-ignition, and flashback. Most premixed combustion systems are designed with sufficient auto-ignition and flashback margin to accommodate expected pipeline compositional variation. Thus, combustion dynamics, emissions compliance, and blowout remain the primary gas turbine operability concerns associated with fuel quality variation. 
         [0003]    Most prior approaches to maintain acceptable combustor operability in the face of compositional variation in the fuel have proven costly and slow. For example, because combustor operability is acceptable within a defined range of the Modified Wobbe Index (MWI) [an extension of Wobbe Index (WI), which captures normalized energy output of a given gas, that includes fuel temperature], one approach involves compensating for changes in gas composition with changes in fuel temperature to maintain a constant MWI. This approach involves closed-loop control of MWI by varying the temperature set-point of a gas fuel heater. However, the approach is costly, because it requires a Wobbe meter and/or gas chromatograph, and slow because of the time constant typically associated with heating and cooling large amounts of natural gas. 
         [0004]    One form of gas turbine control that is fast enough to accommodate rapid variations in fuel composition involves distribution of the fuel supply among the multiple nozzles of a combustor in a method referred to as combustor fuel staging or fuel split scheduling. When controlled with a model-based algorithm, fuel splits have proven to be a fast effector that accommodates rapid changes in fuel composition. While extremely effective when authority is available, the fuel split control is subject to limits beyond which it is ineffective in maintaining stability. When fuel split is thus limited, it will be unable to accommodate additional rapid changes in fuel composition. Thus, the ability to accommodate rapid fuel variation across the widest possible range of fuels would be appreciated in the power industry. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0005]    According to one aspect of the invention, a method to control a gas turbine subject to fuel composition variation includes operating a first effector to control the gas turbine based on fuel composition; and operating a second effector to maintain operation of the first effector within a first boundary limit, the operating the second effector being initiated when the operating the first effector reaches a second boundary limit within the first boundary limit. 
         [0006]    According to another aspect of the invention, a system to control a gas turbine subject to fuel composition variation includes a first effector configured to control the gas turbine based on the fuel composition variation, the first effector maintaining authority to control the gas turbine up to a first boundary limit; and a second effector configured to maintain the first effector within the first boundary limit, the second effector being operational when the first effector has reached a second boundary limit within the first boundary limit. 
         [0007]    These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0008]    The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0009]      FIG. 1  is a block diagram of the gas turbine control system according to an embodiment of the invention; 
           [0010]      FIG. 2  is an explanatory illustration of fuel splits according to the fuel split module of the gas turbine control system shown in FIG.  1 ;... 
           [0011]      FIG. 3  is an explanatory illustration of a determination of the fuel temperature set-point signal used by the fuel heater module of the gas turbine control system shown in  FIG. 1 ; 
           [0012]      FIG. 4  illustrates two exemplary characteristics of combustion dynamics tones (frequencies) of the gas turbine combustion system; and 
           [0013]      FIG. 5  depicts processes involved in gas turbine control according to embodiments of the invention. 
       
    
    
       [0014]    The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    As noted above, model-based fuel staging or fuel split control may accommodate rapid variations in fuel composition. However, fuel splits have boundaries that limit their authority. These boundaries (upper and lower limits beyond which the fuel split controller does not have authority to control) are caused in part by the compromises needed to control various parameters such as, for example, NOx emissions, PK 1  and PK 2  combustion dynamics, and lean blowout (LBO). The split schedule is defined off-line and tuned in-line. To say that the fuel split controller has reached its limits of authority is to say that a physical or parameter boundary limit has been reached at which the fuel split controller can no longer adjust fuel distribution to maintain all the required parameters within acceptable values. An effector at the limit of its authority is said to be saturated. An example of the fuel split effector saturation involves the physical limits of the gas control valves used to adjust the fuel splits to each injector. As another example, when saturated, the fuel split effector cannot simultaneously maintain both NOx emissions and dynamics at acceptable levels. 
         [0016]    With saturated split effectors, further fuel variation could be accommodated by a fuel temperature controller, which adjusts the fuel temperature set-point to perform close-loop control on combustion dynamics and/or emissions. This approach would certainly increase the allowable variation in fuel quality. In addition, because both controls (fuel split and fuel temperature) are based on emissions and dynamics and not on the Wobbe Index or Modified Wobbe Index (MWI), a costly Wobbe meter or gas chromatograph is not needed. However, because the thermal inertia of the fuel heater limits the speed with which the fuel temperature control system could react to variations in fuel composition, the fuel temperature control represents a slow effector and does not evidence a similar ability to accommodate rapid changes in fuel composition as the fuel split controller. 
         [0017]    In light of the above, embodiments of a gas turbine controller detailed herein employ the fast effector (fuel split control), not in parallel with fuel temperature control, but, instead, with the lower-bandwidth fuel temperature effector maintaining close-loop control of the fuel split margin to a limiting boundary. That is, the relatively slower effector (fuel temperature control) is used to maintain enough margin (with amount of margin determined as detailed below) to allow the fast effector (fuel split control) to maintain control authority and to accommodate rapid changes in fuel composition over an increased range of fuel quality variation. 
         [0018]      FIG. 1  is a block diagram of the gas turbine control system  100  according to an embodiment of the invention. The gas turbine control system  100  includes a controller  140  which may comprise one or more processors  142  and memory devices  144 . The controller  140  may also include a user interface  146  to facilitate user interaction with the control of the gas turbine  110  and an output device  148  that may include a display device. While a single controller  140  is shown interacting with the various modules of the gas turbine control system  100 , in alternate embodiments, each of the modules (e.g., fuel temperature module  130 ) may be coupled to a different controller  140  which may be in communication with other controllers  140 . The gas fuel  105  supplies the gas turbine  110  through the fuel input  127 . As  FIG. 1  shows, two modules—the fuel temperature module  130  and the fuel split module  120 —are in the path of the fuel  105  that is supplied to the gas turbine  110 . The fuel split module  120  may be regarded as the primary controller accounting for fuel  105  composition variation, because the fuel split module  120  is able to react quickly to rapid changes in fuel  105  composition. The fuel split module  120 , which is detailed further below, uses a fuel split effector signal  122  from the controller  140  and supplies fuel system sensor data  125  to the controller  140 . The fuel split module  120  includes various valves  121  to control the fuel input  127 . The fuel temperature module  130 , which is also detailed further below, keeps the fuel split module  120  operating within an established boundary limit. That is, rather than control fuel  105  temperature to achieve a particular MWI, the temperature module  130 , which includes a heater, controls fuel temperature, as needed, to facilitate the fuel split module  120  maintaining its authority. The fuel temperature module  130  uses a fuel temperature set-point signal  132  from the controller  140  and supplies a fuel temperature measurement  135  to the controller  140 . The gas turbine  110  has additional controls in the form of gas turbine effectors  112  and supplies gas turbine sensor data  115  to the controller  140 . 
         [0019]      FIG. 2  is an explanatory illustration of fuel splits according to the fuel split module  120  of the gas turbine control system  100  shown in  FIG. 1 . The fuel split module  120  operates according to a model-based control processed by the processor  142 . As previously noted, the gas turbine  110  must match the load in order to maintain frequency stability. The fuel split module  120  maintains combustor operability for varying loads on the gas turbine  110  by changing the distribution of incoming fuel  105  mixed with air or pre-mix (PM) directed to each fuel injector  210  of each of the multiple cans  220  of the gas turbine  110 . The algorithm that controls this fuel  105  distribution or fuel split is model-based. The model processed by the controller  140  uses information from the gas turbine sensor data  115  and the fuel system sensor data  125  and generates a fuel split effector signal  122  that the fuel split module  120  uses to control various valves to affect the fuel split at the fuel input  127 . The model-based algorithm may lose authority to continue control of the gas turbine  110  with the fuel split module  120  when a boundary limit is reached. For example, if fuel  105  composition changes rapidly when the valves  121  of the fuel split module  120  are in a given position, that physical state of the valves  121  may prevent further control of the fuel splits to handle the change in fuel  105  composition. As another example, the fuel split module  120  may approach a boundary limit because, beyond a certain set of conditions for the gas turbine  110 , the fuel split module  120  cannot respond to a fuel  105  composition variation to maintain both dynamics and NOx emissions at acceptable levels. The model processed by the controller  140  can determine the boundary limits of authority of the fuel split module  120 . 
         [0020]      FIG. 3  is an explanatory illustration of a determination of the fuel temperature set-point signal  132  used by the fuel temperature module  130  of the gas turbine control system  100  shown in  FIG. 1 . Only the PM 3  split (shown in Mode  4  and Mode  6  of  FIG. 2 ) is considered for explanatory purposes, but it should be understood that the discussion below applies to each split affected by the fuel split module  120 . The determination of the fuel temperature set-point signal  132 , discussed below, is part of the processing done by the controller  140 . As noted above, fuel  105  temperature control is used by the fuel temperature module  130  to keep the fuel split module  120  within its boundary limits of authority. That is, the fuel split module  120  provides high-bandwidth control to rapidly modify fuel splits, as needed, based on fuel  105  composition variations, and the fuel temperature module  130  provides low-bandwidth control in affecting gas turbine  110  conditions such that the fuel split module  120  authority stays within an acceptable margin of its authority boundary limit. The fuel temperature module  130  provides closed-loop control on the fuel split module  120  authority as detailed below. 
         [0021]    The premise used to implement the logic used to determine the fuel temperature set-point signal  132  is shown at the upper right of  FIG. 3 . Specifically, two exemplary frequencies of combustion dynamics (PK 1  and PK 2 ) are considered and are shown to have the opposite PM 3 /dynamics relationships. That is, to reduce dynamics for the PK 1  case, PM 3  must be increased, and to reduce dynamics for the PK 2  case, PM 3  must be decreased. Keeping that in mind, the logic used to determine the fuel temperature set-point signal  132  in the example is detailed. In path  301 , gas turbine sensor data  115  and fuel system sensor data  125  are input to the PK 1  Model Perturbation Algorithm, which outputs the boundary limit of authority (PM 3 _PK 1 max). Because in the PK 1  case, PM 3  must be increased to reduce dynamics, the desired buffer  310  of authority is added to output the desired PM 3   320   a.  For example, if the PK 1  Model Perturbation Algorithm outputs 63.2% PM 3  and the buffer  310  is 1% PM 3 , then the desired PM 3   320   a  is the aggregate of those two values or 64.2 PM 3 . The actual PM 3  is subtracted from that desired PM 3   320   a  value to give the error  330   a.  The proportional and integral (P+I) computer control is used to determine the Tfuel_PK 1 max or fuel temperature needed to make the error  330   a  equal to 0 or, in other words, to have the desired PM 3   320   a  match the actual PM 3 . The recursive nature of the control system implemented by the processor  140  is used to integrate the control over time. In path  302 , the logic is similar to that discussed for path  301 . However, because PM 3  must be decreased to reduce dynamics for the PK 2  case, the buffer  310  is subtracted from the PM 3 _PK 2 max (limit of authority in the PK 2  case) to provide the desired PM 3   320   b.  It is important to note that, in a similar way to paths  301  and  302 , other logic paths  303  are used to determine the fuel temperature needed to protect other factors (e.g., NOx). A priority is used by the controller  140  to determine the fuel temperature set-point signal  132  based on the different outcomes of the different factors (paths  301 ,  302 ,  303 ). It also bears noting again that the exemplary processor logic shown in  FIG. 3  only addresses one of the splits (PM 3  in this case), but a similar procedure is followed for the other splits, as well. 
         [0022]    The amount of buffer  310  provided by the fuel temperature module  130  (i.e., how close the fuel split module  120  is permitted to get to its boundary limit) may be user determined. The determination of the buffer  310  may be based on, for example, a desire to accommodate a certain level of fuel  105  composition variation or a certain percentage of MWI. Once the level of fluctuation to be tolerated is determined, the controller  140  may run scenarios in the model to determine when the fuel split module  120  will run out of authority according to the scenarios. 
         [0023]      FIG. 4  illustrates two exemplary frequencies of combustion dynamics, PK 1  and PK 2 , of the gas turbine  110 . This illustration, like the one included in  FIG. 3 , clarifies the direction of control of fuel temperature needed to keep the fuel split module  120  in authority, as detailed below. The relationship between MWI (fuel composition) and dynamics is shown for each of PK 1  and PK 2 . Specifically, in the case of PK 1 , cold fuel  105  is assumed and the dashed line indicates the relationship between MWI (which is affected by fuel composition and fuel temperature) and dynamics. In the PK 1  case, when the determined buffer  310  of the fuel split module  120  is reached, the fuel temperature module  130  must heat the fuel  105  to increase the fuel temperature (Tfuel) in order to reduce dynamics and keep the fuel split module  120  in authority. In the case of PK 2 , heated fuel  105  is assumed and the solid line indicates the relationship between MWI and dynamics. In the PK 2  case, when the determined buffer  310  of the fuel split module  120  is reached, the fuel temperature module  130  must reduce Tfuel (allow the fuel  105  to cool) in order to reduce dynamics and keep the fuel split module  120  in authority.  FIG. 3  focuses on PM 3  (fuel injector  210  associated with pre-mix  3 ) shown in modes  4  and  6  at  FIG. 2 . 
         [0024]      FIG. 5  depicts processes  500  involved in gas turbine control according to embodiments of the invention. At block  510 , determining a buffer  310  for the fuel split module  120  authority may include user input. As noted with reference to  FIG. 3 , a system operator may determine the amount of fluctuation that the fuel split module  120  must accommodate and use the models processed by the controller  140  to determine a buffer  310  based on that desired level of fluctuation capability. The processes  500  may include determining or modifying fuel splits (block  520 ) at the controller  140  to generate a fuel split effector signal  122  to the fuel split module  120 . The fuel splits may be modified based on a change in composition of the fuel  105  supplied to the gas turbine  110 . Operating the fuel split module  120 , at block  530 , may be thought of as operating a high-bandwidth effector to maintain gas turbine  110  stability in the face of changes (even rapid changes) in fuel  105  composition. However, when the fuel split module  120  reaches the buffer  310  of its boundary limit or authority limit, the processes  500  may include (block  540 ) determining a fuel temperature set-point and outputting the fuel temperature set-point signal  132  at the controller  140 . At block  550 , operating the fuel temperature module  130  to control the fuel  105  to the fuel temperature set-point affects (increases) gas turbine  110  stability such that the fuel split module  120  is maintained within the desired buffer  310  of its authority limit. 
         [0025]    While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.