BACKGROUND
TECHNICAL FIELD
Embodiments of the subject matter disclosed herein generally relate to methods and systems and  more particularly  to mechanisms and techniques for controlling a gas turbine.

DISCUSSION OF THE BACKGROUND
Turbomachinery used  for example  in power plants or jet engines  are continuously evolving based on new discoveries and better materials. In addition  the manufacturers of these machines are under increased pressure to produce or improve the machines to be “greener ” i.e.  to reduce the amount of pollution produced while operating.
Thus  there is ongoing research for lowering the exhaust emissions of turbo machineries especially considering the desire to use a wide range of gaseous fuels. Meeting these requirements becomes more and more difficult  particularly when considering the wide range of operation of these devices. An accurate turbomachine exhaust temperature control becomes  under these conditions  a relevant factor in order to develop successful applications.
One approach for lowering the pollution produced by a turbomachine is based on a paradigm of exhaust temperature versus compressor pressure ratio. In this regard  U.S. Patent Application Publication 2008/0243352  the entire content of which is included here by reference  describes that current control systems may execute scheduling algorithms that adjust the fuel flow  inlet guide vanes (IGV)  and other control inputs to provide safe and efficient operation of a gas turbine. Gas turbine control systems may receive as inputs operating parameters and settings that  in conjunction with scheduling algorithms  determine turbine control settings to achieve the desired operation. Measured operating parameters may include compressor inlet pressure and temperature  compressor exit pressure and temperature  turbine exhaust temperature  and generator power output. Desired operating settings may include generator power output and exhaust energy. The schedules (e.g.  exhaust temperature vs. compressor pressure ratio  fuel splits vs. combustion reference temperature  inlet bleed heat (IBH) vs. IGV  compressor operating limit line vs. corrected speed and inlet guide vane  etc.) are defined to protect the turbine against known operational boundaries (e.g.  emissions  dynamics  lean-blow-out  compressor surge  compressor icing  compressor clearances  aero-mechanical  etc.) based on off-line field tests or laboratory data. The output of the schedules then determines the appropriate adjustment of the control system inputs. Typical control inputs managed by a control system may include fuel flow  combustor fuel distribution (which may be referred to as "fuel splits")  compressor inlet guide vane position  and inlet bleed heat flow.
Figure 1  which is similar to Figure 1 of U.S. Patent Application Publication 2008/0243352  illustrates an example of a gas turbine 10 having a compressor 12  a combustor 14  a turbine 16 coupled to the compressor 12  and a computer control system (controller) 18. An inlet duct 20 to the compressor 12 may feed ambient air to compressor 12. The inlet duct 20 may have ducts  filters  screens and noise abatement devices that contribute to a pressure loss of ambient air flowing through the inlet 20 and into inlet guide vanes 21 of the compressor 12. An exhaust duct 22 for the turbine directs combustion gases from the outlet of the turbine 10 through  for example  emission control and noise abatement devices. The amount of inlet pressure loss and back pressure may vary over time due to the addition of components and due to dust and dirt clogging the inlet 20 and exhaust ducts 22. The turbine 10 may drive a generator 24 that produces electrical power.
As described in U.S. Patent Application Publication 2008/0243352  the operation of the gas turbine 10 may be monitored by several sensors 26 designed to measure different performance-related variables of the turbine 10  the generator and the ambient environment. For example  groups of redundant temperature sensors 26 may monitor ambient temperature surrounding the gas turbine 10  compressor discharge temperature  turbine exhaust gas temperature  and other temperature measurements of the gas stream through the gas turbine 10. Similarly  groups of redundant pressure sensors 26 may monitor ambient pressure  and static and dynamic pressure levels at the compressor inlet and outlet turbine exhaust  at other locations in the gas stream through the gas turbine 10. Groups of redundant humidity sensors 26  for example  wet and dry bulb thermometers  may measure ambient humidity in the inlet duct of the compressor 12. Groups of redundant sensors 26 may also include flow sensors  speed sensors  flame detector sensors  valve position sensors  guide vane angle sensors  or the like  that sense various parameters pertinent to the operation of gas turbine 10. As used herein  "parameters" refer to items that can be used to define the operating conditions of the turbine  such as but not limited to temperatures  pressures  and gas flows at defined locations in the turbine.
Also described in U.S. Patent Application Publication 2008/0243352A  the fuel control system 28 regulates the fuel flowing from a fuel supply to the combustor 14  one or more splits between the fuel flowing into primary and secondary fuel nozzles  and the amount of fuel mixed with secondary air flowing into a combustion chamber. The fuel control system 28 may also select the type of fuel for the combustor. The fuel control system 28 may be a separate unit or may be a component of the main controller 18. The controller 18 may be a computer system having at least one processor that executes programs and operations to control the operation of the gas turbine using sensor inputs and instructions from human operators. The programs and operations executed by the controller 18 may include  among others  sensing or modeling operating parameters  modeling operational boundaries  applying operational boundary models  applying scheduling algorithms  and applying boundary control logic to close loop on boundaries. The commands generated by the controller 18 may cause actuators on the gas turbine to  for example  adjust valves (actuator 27) between the fuel supply and combustors that regulate the flow  fuel splits and type of fuel flowing to the combustors; adjust inlet guide vanes 21 (actuator 29) on the compressor; adjust inlet bleed heat; as well as activate other control settings on the gas turbine.
U.S. Patent Application Nos. 2002/0106001 and 2004/0076218  the entire contents of which are incorporated here by reference  describe a method and system for adjusting turbine control algorithms to provide accurate calculation of a firing temperature and combustion reference temperature of a gas turbine as the water vapor content in a working fluid varies substantially from a design value. These references disclose using turbine temperature exhaust and turbine pressure ratio for controlling the firing temperature.
However  the traditional methods and systems are limited in their capability of controlling the gas turbine and accordingly  it would be desirable to provide systems and methods that obtain a more accurate firing temperature control  and/or a more accurate combustion parameters control  and/or a more accurate exhaust emissions control.

SUMMARY
According to one exemplary embodiment  there is a method for controlling an operating point of a gas turbine that includes a compressor  a combustor and at least a turbine. The method includes determining an exhaust pressure drop at an exhaust of the turbine; measuring a compressor pressure discharge at the compressor; determining a turbine pressure ratio based on the exhaust pressure drop and the compressor pressure discharge; calculating an exhaust temperature reference curve of the turbine as a function of the turbine pressure ratio; determining whether both conditions (1) and (2) are true  wherein condition (1) is IGVmin + DIGV1 £ IGVset point £ IGVmax + DIGV2  and condition (2) is ttx ³ ttxh + Dttx3  with IGVset point being a target angle of inlet guide vanes (IGV) to be provided at an inlet of the compressor  IGVmin is a minimum value for the IGV  IGVmax is a maximum value for the IGV  DIGV1 is a first predetermined positive IGV angle increase for lean-lean to premixed transfer  DIGV2 is a second predetermined positive IGV angle increase for lean-lean to premixed transfer  ttx is a current exhaust temperature  ttxh is the exhaust temperature reference curve  and Dttx3 is a predetermined negative temperature characterizing an exhaust temperature dead band for lean-lean to premixed transfer; and changing  if both conditions (1) and (2) are true  a split fuel quantity from a first value to a second value or otherwise maintaining the first value  the first value characterizing a lean-lean steady state mode and the second value characterizing a premixed secondary mode of a premixed mode.
According to anohter exemplary embodiment  there is a controller for controlling an operating point of a gas turbine that includes a compressor  a combustor and at least a turbine. The controller includes a pressure sensor configured to measure a compressor pressure discharge at the compressor and a processor connected to the pressure sensor. The processor is configured to determine an exhaust pressure drop at an exhaust of the turbine  determine a turbine pressure ratio based on the exhaust pressure drop and the compressor pressure discharge  calculate an exhaust temperature reference curve of the turbine as a function of the turbine pressure ratio  determine whether both conditions (1) and (2) are true  wherein condition (1) is IGVmin + DIGV1 £ IGVset point £ IGVmax + DIGV2  and condition (2) is ttx ³ ttxh + Dttx3  with IGVset point being a target angle of inlet guide vanes (IGV) to be provided at an inlet of the compressor  IGVmin is a minimum value for the IGV  IGVmax is a maximum value for the IGV  DIGV1 is a first predetermined IGV angle increase for lean-lean to premixed transfer  DIGV2 is a second predetermined negative IGV angle increase for lean-lean to premixed transfer  ttx is a current exhaust temperature  ttxh is the exhaust temperature reference curve  and Dttx3 is a predetermined negative temperature characterizing an exhaust temperature band for lean-lean to premixed transfer during which no action is taken  and change  if both conditions (1) and (2) are true  a split fuel quantity from a first value to a second value or otherwise maintaining the first value  the first value characterizing a lean-lean steady state mode and the second value characterizing a premixed secondary mode of a premixed mode.
According to still another exemplary embodiment  there is a computer readable medium including computer executable instructions  wherein the instructions  when executed  implement a method for controlling an operating point of a gas turbine that includes a compressor  a combustor and at least a turbine. The method includes determining an exhaust pressure drop at an exhaust of the turbine; measuring a compressor pressure discharge at the compressor; determining a turbine pressure ratio based on the exhaust pressure drop and the compressor pressure discharge; calculating an exhaust temperature reference curve of the turbine as a function of the turbine pressure ratio; determining whether both conditions (1) and (2) are true  wherein condition (1) is IGVmin + DIGV1 £ IGVset point £ IGVmax + DIGV2  and condition (2) is ttx ³ ttxh + Dttx3  with IGVset point being a target angle of inlet guide vanes (IGV) to be provided at an inlet of the compressor  IGVmin is a minimum value for the IGV  IGVmax is a maximum value for the IGV  DIGV1 is a first predetermined positive IGV angle increase for lean-lean to premixed transfer  DIGV2 is a second predetermined negative IGV angle increase for lean-lean to premixed transfer  ttx is a current exhaust temperature  ttxh is the exhaust temperature reference curve  and Dttx3 is a predetermined negative temperature characterizing an exhaust temperature band for lean-lean to premixed transfer during which no action is taken; and changing  if both conditions (1) and (2) are true  a split fuel quantity from a first value to a second value or otherwise maintaining the first value  the first value characterizing a lean-lean steady state mode and the second value characterizing a premixed secondary mode of a premixed mode.

BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings  which are incorporated in and constitute a part of the specification  illustrate one or more embodiments and  together with the description  explain these embodiments. In the drawings:
Figure 1 is a schematic diagram of a conventional gas turbine;
Figure 2 is a schematic diagram of a gas turbine considered in an embodiment of the subject matter disclosed;
Figure 3 is a graph illustrating the variation of an exhaust temperature versus a pressure ratio of the turbine according to an exemplary embodiment;
Figure 4 is a schematic illustration of a relationship between operating points and optimal operating points of the gas turbine according to an exemplary embodiment;
Figure 5 is a schematic diagram of an exhaust temperature versus turbine pressure ratio plane according to an exemplary embodiment;
Figure 6 is a schematic diagram of a reference exhaust temperature curve in the plane of Figure 5 according to an exemplary embodiment;
Figure 7 is a flow chart illustrating steps for calculating an exhaust temperature set point for the turbine according to an exemplary embodiment;
Figures 8 – 10 are schematic diagrams showing various operating modes of the gas turbine according to exemplary embodiments;
Figure 11 is a flow chart illustrating steps for calculating a primary to lean-lean mode transfer threshold curve according to an exemplary embodiment;
Figure 12 is a graph showing a split fuel versus time for primary to lean-lean mode transfer according to an exemplary embodiment;
Figure 13 is a graph showing a trajectory of an operating point of the turbine in a plane defined by exhaust temperature versus turbine pressure ratio according to an exemplary embodiment;
Figure 14 is a flow chart illustrating steps of a method for calculating a transfer from primary to lean-lean modes according to an exemplary embodiment;
Figure 15 is a graph showing an exhaust temperature band for a lean-lean to premixed mode transfer according to an exemplary embodiment;
Figure 16 is a graph showing a split fuel versus time for lean-lean to premixed mode transfer according to an exemplary embodiment;
Figure 17 is a graph showing a change of an operating point of a turbine when changing from lean-lean to premixed operation according to an exemplary embodiment;
Figure 18 is a graph showing a change of an operating point of a turbine when changing from premixed to lean-lean operation according to an exemplary embodiment;
Figure 19 is a graph showing a split fuel versus time for premixed to lean-lean mode transfer according to an exemplary embodiment;
Figure 20 is a flow chart illustrating steps of a method for calculating a transfer from lean-lean to premixed modes according to an exemplary embodiment; and
Figure 21 is a schematic diagram of a controller used to control the turbine.

DETAILED DESCRIPTION
The following description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. Instead  the scope of the invention is defined by the appended claims. The following embodiments are discussed  for simplicity  with regard to the terminology and structure of a single shaft gas turbine system. However  the embodiments to be discussed next are not limited to these systems  but may be applied to other systems  for example multiple shaft gas turbines.
Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature  structure  or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus  the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification is not necessarily referring to the same embodiment. Further  the particular features  structures or characteristics may be combined in any suitable manner in one or more embodiments.
As discussed above with regard to Figure 1  various parameters of the turbine 10 may be measured and/or calculated for determining a desired quantity to be monitored. Such a quantity is the firing temperature of the turbine. By maintaining the firing temperature of the turbine within an optimum range  the operation of the turbine is considered to be smooth and under control. When the firing temperature of the turbine exits the optimum range  the controller 18 is configured to change  for example  the compressor’s air flow rate and thus  a compressor pressure ratio  for adjusting the firing temperature. Events that may determine the firing temperature to exit the optimum range is  for example  a change in a load of the gas turbine or a gas fuel composition change.
However  the novel embodiments to be discussed next do not rely on the traditional paradigms for controlling the gas turbine but rather rely on a novel paradigm  e.g.  controlling a turbine temperature exhaust based on a turbine pressure ratio. This novel paradigm offers a more accurate estimation of the state of the gas turbine and is also more sensitive to changes occurring in the functioning of the gas turbine  e.g.  load change.

Determining an exhaust temperature as a function of turbine pressure ratio
According to an exemplary embodiment  an exhaust temperature is determined as a function of the turbine pressure ratio and the exhaust temperature is monitored and maintained within certain boundaries for ensuring an efficient operation of the gas turbine  e.g.  accommodating base load  low load  high load  etc. More details about determining the exhaust temperature and the turbine pressure ratio are discussed next with regard to Figure 2. Figure 2 shows a gas turbine 30 having a compressor 32 configured to receive a fluid (e.g.  air) through an inlet duct 36. Sensors 34 may be disposed at inlet duct 36 for measuring at least one of a pressure  temperature  humidity  etc.
The fluid is compressed by the compressor 32 and the compressed fluid is sent to a combustor 40 via a path 42 to be mixed with fuel (e.g.  natural gas) supplied by a feeding duct 44. More sensors 34 may be disposed in or around combustor 40 for measuring characteristics of the compressed fluid and/or the fuel. A combustion in combustor 40 takes place and this combustion raises the temperature of the mixture of compressed fluid and fuel to a firing temperature. Fuel is provided via feeding duct 44 to primary and secondary burners  as disclosed later. Valves 45a and 45b are used to provide fuel to the primary and secondary burners. A control unit 70 is also configured to adjust valves 45a and 45b to provide a desired percentage of the fuel to the primary and secondary valves. The flow of combusted gas  having a high energy  is supplied via ducts 52 to a turbine 50  which may be mechanically connected with a shaft 56 to a generator 54. The generator 54 may produce electric power. Turbine 50 is also mechanically connected via a shaft 58 to the compressor 30  thus supplying the required driving power to the compressor 30. Discharge gases are expelled from the turbine 50 through an outlet duct 60. Both the inlet duct 52 and the outlet duct 60 may be monitored by sensors 34.
Data from sensors 34 is provided to the control unit 70. The control unit 70 may receive additional data via an input port 72. Based on processes computed by the control unit 70  various commands are provided via an output port 74 to different parts of the gas turbine 30  e.g.  commands to rotate the vanes  to modify a rotational speed of the shaft  etc. A detailed structure of the control unit 70 is discussed later.
According to an exemplary embodiment  the proposed new control of the gas turbine is based on a turbine temperature exhaust (ttx)  which is measured/determined at outlet 60  versus the turbine pressure ratio (tpr)  which is measured/determined as a ratio between a discharge pressure of the compressor 32 and an exhaust pressure of the turbine 50. With reference to Figure 2  the discharge pressure of the compressor 32 is measured at point 80 and the exhaust pressure of the turbine 50 is measured at point 60. However  according to an exemplary embodiment  the exhaust pressure may be measured/estimated inside combustor 40  at an inlet of turbine 50 or inside turbine 50. These pressures are discussed later in more details. It is noted that the particulars discussed next for determining ttx are for illustrative purposes and not to limit the subject matter disclosed.
Figure 3 shows a (ttx  tpr) plane. Each point in this plane may be considered as belonging to a set A as shown in Figure 4. Set A is defined as including operating points for the gas turbine 30 based on a combustion model. Set A includes a subset B of points. These points are determined as discussed next and are defined as optimum operating points for the gas turbine 30.
Those points of the plane (ttx  tpr)  i.e.  points from set A  that correspond to constant firing temperature  constant speed  constant IGV angle  constant air""s specific humidity and constant bleed conditions may be represented by a curve 90  which may have an upwards concavity. The turbine pressure ratio tpr may vary with the compressor inlet temperature. An error introduced when approximating curve 90  which may be a parabola with its osculatory straight line 92 at tpr = tpr0 is small and may be neglected for values of tpr near to tpr0. One skilled in the art would recognize that other approximating functions may be used.
Varying gradually the compressor inlet temperature  the compressor speed and the IGV angle  the curve 90 changes gradually  for example  without any discontinuity in its prime derivative. Therefore  the constant firing temperature locus  which may be calculated based on ttx  can be approximated by the linear interpolation of the osculatory straight line 92.
Based on the points in set B discussed above  a function f to be discussed later  is applied to determine points belonging to a set C. The points of set C are set points for the gas turbine operation as per control logic. In other words  points belonging to set C are calculated  as discussed next  and the operator of the gas turbine 30 controls some parameters for maintaining the gas turbine within set C. Figure 4 illustrates this concept.
According to an exemplary embodiment  function f may be defined as f = g∙h∙l  where g  h  and l are mathematical functions or operators. For example  g may be a linear interpolation with an opportune fuel characteristic  h may be a bilinear interpolation of the angles of IGV and gas turbine speed  and l may be a polytropic correction given by p∙T((1-γ)/ γ)=constant. Setting the domain B  the codomain C is entirely defined through function f. Local perturbations in B produce local perturbations in C. Depending on the application  more or less functions or different functions may be used for defining function f. In other words  instead of the g  h  and l functions discussed above  other functions may be used or a different number of functions.
The determination of a set ttx temperatures  which is desired to be maintained for an efficient operation of the gas turbine 30 is now discussed. Assume that the gas turbine may operate in the following ranges: for an ambient temperature tamb  consider a range tambi-1 ≤ tamb ≤ tambi  for an IGV angle igv  consider a range igvj-1 ≤ igv ≤ igvj  and for a gas turbine speed tnh  consider a range tnhk-1 ≤ tnh ≤ tnhk. Also suppose that the gas turbine is controlled at optimum firing temperature. Based on the above ranges  operational points of the gas turbine may be represented in the (ttx  tpr) space shown in Figure 5 by curves defined by the following points. There are four points A1 to A4 for a lean fuel and for the lowest ambient temperature; there are four points B1 to B4 for the lean fuel and for the highest ambient temperature; there are four points C1 to C4 for a rich fuel and the lowest ambient temperature; and there are four points D1 to D4 for the rich fuel and the highest ambient temperature. The number of points may vary according to the nature of the interpolating function.
The lean fuel and the rich fuel are defined as follow. Gas turbines for industrial applications are using natural gas that includes CH4 more than 90%. Natural gas is considered to be a rich gas fuel. Blending the natural gas with inert gases  for example  nitrogen  carbon dioxide  and argon  produces leaner gas fuels  i.e.  lower LHV value (LHV is the lower heating value of the gas and describes the amount of energy that can be obtained from a unit of mass of the gas by burning the gas). The rich fuel may be obtained by blending the natural gas with heavier hydrocarbons  like ethane  propane  and/or butane.
For each of the above discussed set of points  a central point (A5  B5  C5 and D5) is calculated using two bilinear interpolations (function g discussed above). A bilinear interpolation is an extension of linear interpolation for interpolating functions of two variables on a regular grid. The bilinear interpolation performs linear interpolation first in one direction  and then again in the other direction. Points A5 and B5 define a temperature control curve 100 for the lean gas and points C5 and D5 define a temperature control curve 102 for the rich gas. As discussed above  another function than a bilinear interpolation may be used.
A ttxset point is determined by using a linear interpolation (function h discussed above or  in another application  other functions) of the two ordinates corresponding to the actual pressure ratios on the two control curves 100 and 102  based on the LHVactual gas  the LHVrich gas and the LHVlean gas·
If more points are calculated for other conditions and/or values of the considered parameters  more ttxset point may be determined. Plotting these points versus a corresponding tpr ratio results in a reference exhaust temperature curve 104  which is shown in Figure 6. It is noted that the reference exhaust temperature curve 104 lies between the two control curves 100 and 102. According to an exemplary embodiment (not illustrated)  curve 104 is parallel to curves 100 and 102.
Steps for calculating the ttxset point may be represented in the block diagram shown in Figure 7. According to this figure  data selector unit 110 receives as input the ambient temperature tamb  the rotation angle of the vanes IGV  the rotational speed tnh of the shaft and the rich gas matrix data. An example of the rich gas matrix data is:
ttxr
ttxri j k tambi
igv1 igv2 … igv5 igv6
tnh1 ttxri 1 1 ttxri 2 1 … ttxri 5 1 ttxri 6 1
tnh2 ttxri 1 2 … … … …
tnh3 ttxri 1 3 … … … …
tnh4 ttxri 1 4 … … … ttxri 6 4
and the turbine pressure ratio matrix for rich gas is given by:
tprr
tprri j k tambi
igv1 igv2 igv3 igv5 igv6
tnh1 tprri 1 1 tprri 2 1 tprri 3 1 tprri 5 1 tprri 6 1
tnh2 tprri 1 2 … … … …
tnh3 tprri 1 3 … … … …
tnh4 tprri 1 4 … … … tprri 6 4
Eight points C1 to C4 and D1 to D4 (shown in Figure 5) are output by the data selector unit 110. This output is provided as input to interpolator unit 112. The same process is repeated by data selector unit 114 for the same parameters except that a lean gas matrix data is used instead of the rich gas matrix data. Output from interpolators 112 and 116  i.e.  rich gas ttx versus tpr actual control curve and lean gas ttx versus tpr actual control gas  are provided as input to calculation unit 118 for calculating two ttx set points. Linear interpolator 120 receives the two ttx set points and interpolates them to produce a final point  the ttxset point. Based on the output of the linear interpolator 120  a firing unit 122 may calculate variations of the ttxset point of the gas turbine. It is noted that the linear interpolator 120 and the firing unit 122 may receive directly information about the fuel gas LHV.
Having the ttxset point  controller 70 may be programmed to monitor this value and to adjust various parameters of the gas turbine 30 (e.g.  angle of IGV  fuel amount  etc.) to maintain the ttxset point in a predetermined range for an efficient operation of the gas turbine. In one exemplary embodiment in which a single shaft gas turbine is used  the ttxset point may be adjusted by controlling the IGV angle. The reference exhaust temperature curve ttxh 104  which the gas turbine is desired to follow  is now calculated.
Consider three vectors that identify the gas turbine operating parameters. These vectors are tamb  igv  and tnh  and they correspond to the ambient temperature  angle of IGV vanes  and shaft’s rotational speed. The mathematical expressions for these three vectors are:
tamb= [tambi] = [tamb1  tamb2  ...   tamb7]
with index i being:
2 if tamb < tamb2
3 if tamb2 ≤ tamb < tamb3
4 if tamb3 ≤ tamb < tamb4
5 if tamb4 ≤ tamb < tamb5
6 if tamb5 ≤ tamb < tamb6
7 if tamb6 ≤ tamb 
where tamb is the actual ambient temperature.
[0001] The igv angle vector is defined as:
igv = [igvj] = [igv1  igv2  ...   igv6] with index j being:
2 if igv < igv2
3 if igv2 ≤ igv < igv3
4 if igv3 ≤ igv < igv4
5 if igv4 ≤ igv < igv5
6 if igv5 ≤ igv 
where igv is the actual igv angle.
[0002] The tnh shaft speed vector is defined as:
tnh = [tnhk] = [tnh1  tnh2  tnh3  tnh4] with index k being:
2 if tnh < tnh2
3 if tnh2 ≤ tnh < tnh3
4 if tnh3 ≤ tnh 
where tnh is the actual shaft speed percentage. The values for i  j  and k differ from application to application and may include a large number of possibilities.
Four 3D matrices are introduced for calculating the reference exhaust temperature curve ttxh  i.e.  a reference curve used by the operator for controlling the gas turbine. According to an exemplary embodiment  ttxh can be seen as a locus of points where the gas turbine operates at optimal ttx and tpr values. The four matrices are the exhaust temperature lean fuel matrix ttxl  the pressure ratio lean fuel matrix tprl  the exhaust temperature rich fuel matrix ttxr  and the pressure ratio rich fuel tprr. Elements of these matrices are listed below:
ttxl = [ttxli j k] for lean fuel 
tprl = [tprli j k] for lean fuel 
ttxr = [ttxri j k] for rich fuel  and
tprr = [tprri j k] for rich fuel.
Assuming that the actual operating conditions tamb  igv and tnh are within ranges tambi-1 ≤ tamb < tambi; igvj-1 ≤ igv < igvj; and tnhk-1 ≤ tnh < tnhk  the actual reference curve ttxh is given by
ttxh = ttxha + Δttxh 
where ttxha defines a reference curve for the operation of the gas turbine at optimal ttx and tpr points  but also taking into account compressor inlet pressure and gas turbine exhaust pressure drop  and Δttxh is a correction of ttxha that is used to maintain the turbine firing temperature at optimum values while the inlet and exhaust pressure drops of the turbine vary.
Reference curve ttxha is defined as
ttxha = ttxhr ∙ (LHV - LHVl) / (LHVr – LHVl) + ttxhl ∙ (LHVr - LHV) / (LHVr – LHVl) 
where the parameters defining ttxha are defined as follows:
ttxhr = ttxri-1 + (ttxri - ttxri-1) / (tprri - tprri-1) ∙ (tpr-tprri-1) 
ttxhl = ttxli-1 + (ttxli -ttxli-1) / (tprli - tprli-1) ∙ (tpr-tprli-1) 
LHV is the lower heating value of the actual fuel 
LHVl is the lower heating value of the lean fuel 
LHVr is the lower heating value of the rich fuel.

The following bilinear interpolations are applied:
ttxli-1 = BilinearInterpolation(ttxli-1 j-1 k-1  ttxli-1 j k-1  ttxli-1 j k  ttxli-1 j-1 k  igv  tnh) =
ttxli-1 j-1 k-1 (igvj - igv) / (igvj - igvj-1) ∙ (tnhk - tnh) / (tnhk - tnhk-1) +
ttxli-1 j  k-1 ∙ (igv - igvj-1) / (igvj - igvj-1) ∙ (tnhk - tnh) / (tnhk - tnhk-1) +
ttxli-1 j k ∙ (igv - igvj-1) / (igvj - igvj-1) ∙ (tnh - tnhk-1) / (tnhk - tnhk-1) +
ttxli-1 j-1 k ∙ (igvj - igv) / (igvj - igvj-1) ∙ (tnh - tnhk-1) / (tnhk - tnhk-1) 

ttxli = BilinearInterpolation(ttxli j-1 k-1  ttxli j k-1  ttxli j k  ttxli j-1 k  igv  tnh) =
ttxli j-1 k-1 ∙ (igvj - igv) / (igvj - igvj-1) ∙ (tnhk - tnh) / (tnhk - tnhk-1) +
ttxli j  k-1 ∙ (igv - igvj-1) / (igvj - igvj-1) ∙ (tnhk - tnh) / (tnhk - tnhk-1) +
ttxli j k ∙ (igv - igvj-1) / (igvj - igvj-1) ∙ (tnh - tnhk-1) / (tnhk - tnhk-1) +
ttxli j-1 k ∙ (igvj - igv) / (igvj - igvj-1) ∙ (tnh - tnhk-1) / (tnhk - tnhk-1) 

tprli-1 = BilinearInterpolation(tprli-1 j-1 k-1  tprli-1 j k-1  tprli-1 j k  tprli-1 j-1 k  igv  tnh) =
tprli-1 j-1 k-1 ∙ (igvj - igv) / (igvj- igvj-1) ∙ (tnhk - tnh) / (tnhk - tnhk-1) +
tprli-1 j k-1 ∙ (igv - igvj-1) / (igvj - igvj-1) ∙ (tnhk - tnh) / (tnhk - tnhk-1) +
tprli-1 j k ∙ (igv – igvj-1) / (igvj – igvj-1) ∙ (tnh -tnhk-1) / (tnhk - tnhk-1) +
tprli-1 j-1 k ∙ (igvj - igv) / (igvj – igvj-1) ∙ (tnh - tnhk-1) / (tnhk – tnhk-1) 

tprli = BilinearInterpolation(tprli j-1 k-1  tprli j k-1  tprli j k  tprli j-1 k  igv  tnh)=
tprli j-1 k-1 ∙ (igvj - igv) / (igvj – igvj-1) ∙ (tnhk - tnh) / (tnhk - tnhk-1) +
tprli j  k-1 ∙ (igv - igvj-1) / (igvj - igvj-1) ∙ (tnhk - tnh) / (tnhk - tnhk-1) +
tprli j k ∙ (igv – igvj-1) / (igvj – igvj-1) ∙ (tnh - tnhk-1) / (tnhk - tnhk-1) +
tprli j-1 k ∙ (igvj - igv) / (igvj – igvj-1) ∙ (tnh - tnhk-1) / (tnhk - tnhk-1) 

ttxri-1 = BilinearInterpolation (ttxri-1 j-1 k-1  ttxri-1 j k-1  ttxri-1 j k  ttxri-1 j-1 k  igv  tnh)=
=ttxri-1 j-1 k-1 ∙ (igvj - igv) / (igvj – igvj-1) ∙ (tnhk - tnh) / (tnhk - tnhk-1) +
ttxri-1 j k-1 ∙ (igv - igvj-1) / (igvj - igvj-1) ∙ (tnhk - tnh) / (tnhk - tnhk-1) +
ttxri-1 j k ∙ (igv – igvj-1)/ (igvj – igvj-1) ∙ (tnh - tnhk-1) / (tnhk - tnhk-1) +
ttxri-1 j-1 k ∙ (igvj - igv) / (igvj – igvj-1) ∙ (tnh - tnhk-1) / (tnhk - tnhk-1) 

ttxri = BilinearInterpolation(ttxri j-1 k-1  ttxri j k-1  ttxri j k  ttxri j-1 k  igv  tnh)=
ttxri j-1 k-1 ∙ (igvj - igv) / (igvj – igvj-1) ∙ (tnhk - tnh) / (tnhk - tnhk-1) +
ttxri j k-1 ∙ (igv – igvj-1) / (igvj – igvj-1) ∙ (tnhk - tnh) / (tnhk - tnhk-1) +
ttxri j k ∙ (igv – igvj-1) / (igvj – igvj-1) ∙ (tnh - tnhk-1) / (tnhk - tnhk-1) +
ttxri j-1 k ∙ (igvj - igv) / (igvj – igvj-1) ∙ (tnh - tnhk-1) / (tnhk - tnhk-1) 

tprri-1 = BilinearInterpolation(tprri-1 j-1 k-1  tprri-1 j k-1  tprri-1 j k  tprri-1 j-1 k  igv  tnh)=
tprri-1 j-1 k-1 ∙ (igvj - igv) / (igvj – igvj-1) ∙ (tnhk - tnh) / (tnhk - tnhk-1) +
tprri-1 j k-1 ∙ (igv - igvj-1) / (igvj - igvj-1) ∙ (tnhk - tnh) / (tnhk - tnhk-1) +
tprri-1 j k ∙ (igv – igvj-1) / (igvj – igvj-1) ∙ (tnh - tnhk-1) / (tnhk - tnhk-1) +
tprri-1 j-1 k ∙ (igvj - igv) / (igvj – igvj-1) ∙ (tnh - tnhk-1) / (tnhk - tnhk-1)  and

tprri = BilinearInterpolation(tprri j-1 k-1  tprri j k-1  tprri j k  tprri j-1 k  igv  tnh)=
tprri j-1 k-1 ∙ (igvj - igv) / (igvj – igvj-1) ∙ (tnhk - tnh) / (tnhk - tnhk-1) +
tprri j k-1 ∙ (igv - igvj-1) / (igvj - igvj-1) ∙ (tnhk - tnh) / (tnhk - tnhk-1) +
tprri j k ∙ (igv – igvj-1)/ (igvj – igvj-1) ∙ (tnh - tnhk-1) / (tnhk - tnhk-1) +
tprri j-1 k ∙ (igvj - igv) / (igvj – igvj-1) ∙ (tnh - tnhk-1) / (tnhk - tnhk-1)
The correction Δttxh is given by:
Δttxh = ttxh ∙ ((pambactual + Δpexhaust ref) / (pambactual + Δpexhaust))( γ / (1- γ) ) – 1) +
((pambactual - Δpinlet ref) / (pambactual - Δpinlet))(γ / (1- γ) ) - 1))  where
γ = a ∙ tpr + b with a and b being constants and γ is made to fit the gas turbine polytropic expansion (p ∙ t((1 - γ)/ γ) = constant).
The correction Δttxh takes into account  among other things  the actual gas turbine exhaust and inlet pressure drops. As the gas turbine temperature control curves (ttxh for example) depend on a reference exhaust pressure drop Δpexhaust ref and reference inlet pressure drop Δpinlet ref  it is possible to correct these curves for different exhaust and inlet pressure drops by using  for example  function Δttxh.
The actual inlet pressure drop value Δpinlet act may be measured instead of being estimated due to the amount of dirt at the input of the compressor. In other words  the compressor inlet system pressure drop depends on the flow conditions and on the dirt in the inlet filter  the periodic dirt deposition and removal may cause an unpredictable variability of the inlet pressure drop over the time. In an application  if the LHV signal is not available  for example due to calorimeter fault or calibration problems  the controller 70 may be configured to use a LHVdefault to override the actual LHV.
The above bilinear interpolations  linear interpolation and polytropic expansion  when applied as indicated above to the parameters of the gas turbine  for example  IGV angles and shaft rotational speed at various points i  j  and k of the allowed ranges  generate the ttxset point on the reference curve ttxh. In one exemplary embodiment  multiple ttxset point are calculated for the gas turbine for various conditions and all these points ttxset point are part of the ttxh curve. Other reference curves may be determined from ttxh  as discussed next. These additional reference curves may also be used to control the operation of the gas turbine.
According to an exemplary embodiment  a reference exhaust temperature versus compressor pressure ratio curve TTRX may be used to control the gas turbine. The TTRX curve may be defined as TTRX = Min(IsothermNO  ttxh)  where IsothermNO is defined as an isotherm of the gas turbine at normal operating conditions. In one application  the IsothermNO represents the maximum temperature to which the rotor of the turbine may be exposed. A control curve for the exhaust temperature versus IGV may be defined as TTRXGV = TTRX. A control curve for the exhaust temperature versus fuel may be defined as TTRXB = TTRXBNO if a peak load mode is off and TTRXB = TTRXBPK if the peak load mode is on. The peak load mode is defined as a gas turbine that runs at constant operating conditions (ambient temperature  pressure  shaft speed  IGV position  and fuel gas composition) and delivers a power higher than the nominal one. This condition occurs when the gas turbine’s operating firing temperature is higher than the nominal temperature. TTRXBNO is given by TTRX + Min((IGVmax - IGVset point) ∙ Δ1  Δ2)  where Δ2 is a value that limits the value of the Min function  and TTRXBPK is given by Min(IsothermPK  ttxh + ΔPK).
ΔPK is given by
ΔPK = Δttxr ∙ (LHV -LHVl) / (LHVr - LHVl) + Δttxl ∙ (LHVr - LHV) / (LHVr - LHVl)  with LHV being the lowest heating value of the actual fuel 
LHVI being the lowest heating value of the lean fuel 
LHVr being the lowers heating value of the rich fuel 
Δttxl = Δttxli-1 + (Δttxli - Δttxli-1) ∙ (tamb – tambi-1) / (tambi - tambi-1)  and
Δttxr = Δttxri-1 + (Δttxri-1 - Δttxri-1) ∙ (tamb – tambi-1) / (tambi - tambi-1).
The above exhaust temperature control via IGV and exhaust temperature control via fuel curves may be used in the control of the gas turbine as follows. A gas turbine may be controlled by varying  for example  the speed of the shaft of the turbine  the angle of IGV (that directly controls the amount of air provided to the compressor)  the amount of fuel provided to the combustor  the ratio of fuel/air provided to the combustor  etc. According to an exemplary embodiment  for a single shaft gas turbine  the angle of the IGV is used first to control the operation of the gas turbine  i.e.  to maintain the ttxact point on the ttxh curve calculated above (in the ttx versus tpr plane). In other words  when the actual ttxact point deviates from the ttxh curve due to various conditions of the gas turbine (e.g.  change in load)  a first control adjusts the angle of IGV for bringing the ttxact point of the gas turbine to the ttxset point. However  this control may reach a saturation point  i.e.  a point at which the angle of IGV may not be modified further or it is not desired to be modified further. At this point  an amount of fuel to be provided to the gas turbine may be varied until the ttxact point is made to coincide with the ttxset point. If this control becomes saturated  it is possible to change a ratio between the fluid provided by the compressor and the fuel injected into the combustor  thus limiting the fuel flow rate and further regulating ttxact point.
To fully determine the ttxh curve in the ttx versus tpr plane  it is next discussed the determination of the turbine pressure ratio tpr. The gas turbine exhaust pressure is less difficult to be estimated than to be measured. Although the pressures involved in the turbine pressure ratio tpr may be measured  it is preferred to calculate tpr as discussed next because it is more accurate than the measured tpr. In this regard  it is noted that vortices may appear at locations 80 and 60 in the gas turbine  which make the measured pressures less accurate as they may vary across a small distance. The estimation may be performed based on characteristics of a flue pressure drop  exhaust gas data and ambient pressure. According to an exemplary embodiment  the turbine pressure ratio tpr is determined based on the estimated exhaust pressure drop and the absolute compressor discharge pressure. In one embodiment  the exhaust pressure drop is determined at point 60 (see Figure 2) while the absolute compressor discharge pressure is determined at point 80 (see Figure 2). In another embodiment  for a compressor having multiple stages  the absolute compressor discharge pressure is determined after the discharge diffuser  which is downstream of the last stage. According to this exemplary embodiment  the absolute compressor discharge pressure is measured.
According to an exemplary embodiment  the exhaust pressure drop is made up by two terms  the pressure drop due to a mass flowing in the flue of the turbine 50 and a pressure recovery due to a chimney effect. The chimney effect may appear if there is a height difference between the gas turbine exhaust and the flue discharge to the atmosphere. The first term is given by aa ∙ ρexhaust ∙ v2 and the second term is given by (ρair - ρexhaust) ∙ Δh. The meaning of each constant  parameter and variable used in the present calculations is provided later. Thus  the total exhaust pressure drop due to the flowing mass in the flue can be expressed as:
Δpexhaust = aa ∙ pexhaust ∙ v2 - (ρair - ρexhaust) ∙ Δh  which may be rewritten as
aa ∙ ρexhaust ∙ v2 = aa ∙ ρexhaust ∙ (Wexhaust / (ρexhaust ∙ ab))2 =
aa ∙ (Wexhaust / ab)2 / ρexhaust = a / ρexhaust ∙ Wexhaust2.
[0003] To simplify this expression  assume that the density ρ of the gas in the flue is independent of the actual exhaust pressure drop and depends only on the
discharge pressure  which here is the ambient pressure  as the exhaust pressure
drop is assumed to be only a small fraction of the ambient pressure. Thus  the error introduced by this simplification can be neglected. The exhaust gas density ρexhaust can be expressed as:
ρexhaust = ρexhaust ref ∙ ttxref / ttxact ∙ pambact / pambref.
The ambient air density can be expressed as:
ρair = ρair ref ∙ tambref / tambact ∙ pambact / pambref 
where:
ρexhaust is the density of the exhaust gas at the ttxact temperature and pambact ambient pressure 
ρexhaust ref is the density of the exhaust gas at the ttxref temperature and pambref
ambient pressure 
ρair is the density of the ambient air at the actual pressure and temperature 
ρair ref is the density of the ambient air at the reference pressure and temperature 
Δh is the elevation difference between the gas turbine exhaust and the flue discharge to the atmosphere 
v is the exhaust speed inside the flue 
ttxref is the reference exhaust temperature 
ttxact is the actual exhaust temperature 
pambref is the reference ambient pressure 
pambact is the actual ambient pressure 
Wexhaustact is the actual exhaust gas mass flow rate  and
a is a constant typical for the specific exhaust duct.
It is assumed in this exemplary embodiment that the exhaust gas composition is substantially constant over a premixed mode operation  and thus  its density is substantially constant at a given temperature.
The exhaust gas mass flow rate may be estimated as follows. Assume that the compressor air mass flow rate is independent of the compressor pressure ratio as the error introduced by this assumption is negligible for the purpose of the exhaust pressure drop estimation. The gas turbine’s axial compressor air mass flow rate can be estimated by the following transfer function:
Wairact = SGha ∙ pinletact / pinletref ∙ (f3 ∙ x3 + f2 ∙ x2 + f1 ∙ x + f0) ∙ f4 ∙ Wairref ∙ k  where
f0 = a0 ∙ y3 + b0 ∙y2 + c0 ∙ y 
f1 = a1 ∙ y3 + b1 ∙y2 + c1 ∙ y 
f2 = a2 ∙ y3 + b1 ∙y2 + c2 ∙ y 
f3 = a3 ∙ y3 + b1 ∙y2 + c3 ∙ y 
f4 = a41 ∙z3 + b41 ∙ z2 + c41 ∙ z + d41 if tnhact / tnhref < tnhthreshold 
a42 ∙ z3 + b42 ∙ z2 + c42 ∙ z + d42 if tnhact / tnhref ≥ tnhthreshold 
x = igvact / igvref 
y = tnhact / tnhref ∙ (tinletref / tinletact)0.5 
z = tnhact / tnhref ∙ (tinletref / tinletact)  and
ai and aij are application-specific constants.
Because the gas turbine is equipped with an IBH system  at some partial load operating conditions  a fraction of the compressor’s air mass flow rate is recirculated and does not enter the exhaust duct. Moreover  the fuel gas mass flow rate enters entirely through the exhaust duct. Therefore  Wexhaustact = Wairact ∙ (1- IBHfraction) + Wfuelact. In this exemplary embodiment  it has been assumed that the air to the bearings compensates the air from the cooling blowers.
[0004] Considering that while the gas turbine is on exhaust temperature control and the fuel/air mass ratio is substantially constant for a specific fuel gas composition  the fuel/air mass flow ratio may be evaluated as follows:
faratio = Wfuelact / Wairact = Wfuelref / Wairref ∙ LHVref / LHVact = faratio ref ∙ LHVref / LHVact.
The IBHfraction is a set point generated by the control panel and controlled while the system is not at fault. Then  the exhaust mass flow rate may be evaluated as:
Wexhaustact = Wairact ∙ (1 - IBHfraction) ∙ (1 + faratio ref ∙ LHVref / LHVact).
The specific gravity of humid air SGha can be evaluated based on the specific humidity as follows:
SGha = ρha / ρda 
mha = mda + mwv 
mda = mha ∙ (1-sh) 
mwv = mha ∙ sh  and
vha = mha / ρha = mda / ρda + mwv / ρwv.
Multiplying this last expression by ρha  the following equation is obtained:
mha = mda ∙ ρha / ρda + mwv ∙ ρha / ρwv  where
ρha / ρda = SGha and ρha / ρwv= ρha ∙ ρda / ρda ∙ ρwv = SGha / SGwv.
[0005] Thus 
mha = mda ∙ ρha / ρda + mwv ∙ ρha / pwv = mda ∙ SGha + mwv ∙ SGha / SGwv  or
mha = (1-sh) ∙ mha ∙ SGha + sh ∙ mha ∙ SGha / SGwv.
Dividing this last expression by mha
1= (1-sh) ∙ SGha + sh ∙ SGha / SGwv  or
SGwv = SGha ∙ ((1-sh) ∙ SGwv + sh).
Finally 
SGha = SGwv / ((1-sh) ∙ SGwv + sh).
If the specific humidity signal is not available or the transmitter is in a fault mode  the specific humidity signal can be substituted by a curve of specific humidity versus ambient temperature generated by the interpolation of the data shown in Table 1:
Table 1
shdefault Average air specific humidity vs. ambient temperature
tamb tamb1 tamb2 … … … tamb6 tamb7
shi sh1 sh2 … … … sh6 sh7
The following notations have been used in the above calculations:
pinletact is the actual air pressure at the compressor inlet 
pinletref is the reference air pressure at the compressor inlet 
tamb is the ambient temperature 
tinletact is the actual air temperature at the compressor inlet  may be measured with at least two thermocouples such that the maximum reading of the thermocouples is considered to be tinletact or in case that one thermocouple is faulty and/or the difference in readings is too large (for example 10 F)  tamb is considered to be tinletact 
tinletref is the reference air temperature at the compressor inlet 
tnhact is the compressor actual speed 
tnhref is the compressor reference speed 
igvact is the actual igv angle 
igvref is the reference igv angle 
Wairact is the actual air mass flow rate at the compressor inlet 
Wairref is the reference air mass flow rate at the compressor inlet 
Wexhaustact is the actual exhaust gas mass flow rate 
Wfuelact is the fuel mass flow rate 
IBHfraction is the fraction of air bled from the compressor discharge 
faratio ref is the reference fuel air mass ratio 
LHVref is the reference gas fuel""s LHV 
LHVact is the actual gas fuel""s LHV 
sh is the air specific humidity 
SGxx is the specific gravity of xx (see subscript list below) 
ρxx is the density of xx (see subscript list below) 
mxx is the mass of xx (see subscript list below) 
Vxx volume of xx (see subscript list below) 
ha is humid air 
wv is water vapor  and
da is dry air.
Having calculated the specific gravity  the mass flow rate through the compressor and other parameters as discussed above  it is now possible to calculate the turbine pressure ratio tpr. The algorithm for calculating tpr may be summarized as follows:
- calculate SGha to be SGwv / ((1-sh) ∙ SGwv + sh) if sh signal is valid and available and shdefault if sh transmitter signal fault;
- assume x = igvact / igvref  y = tnhact / tnhref ∙ (tinletref / tinletact)0.5  and z = tnhact / tnhref ∙ (tinletref / tinletact);
- f0 = a0 ∙ y3 + b0 ∙y2 + c0 ∙ y 
- f1 = a1 ∙ y3 + b1 ∙y2 + c1 ∙ y 
- f2 = a2 ∙ y3 + b1 ∙y2 + c2 ∙ y 
- f3 = a3 ∙ y3 + b1 ∙y2 + c3 ∙ y 
- f4 = a41 ∙z3 + b41 ∙ z2 + c41 ∙ z + d41 if tnhact / tnhref < tnhthreshold  and
- a42 ∙ z3 + b42 ∙ z2 + c42 ∙ z + d42 if tnhact / tnhref ≥ tnhthreshold;
- define Wairact = SGha ∙ pinletact / pinletref ∙ (f3 ∙ x3 + f2 ∙ x2 + f1 ∙ x + f0) ∙ f4 ∙ Wairref ∙ k 
- evaluate Wexhaustact = Wairact ∙ (1-IBHfraction) ∙ (1 + faratio ref ∙ LHVref / LHVact) 
- calculate ρair = ρair ref ∙ tambref / tambact ∙ pambact / pambref 
- calculate ρexhaust = ρexhaust ref ∙ ttxref / ttxact ∙ pambact / pambref.
- calculate Δpexhaust = aa ∙ ρexhaust ∙ v2 - (ρair - ρexhaust) ∙ Δh  and
- evaluate tpr = cpd / (pambact + Δpexhaust)  where cpd is the absolute compressor discharge pressure  which is measured in this application.
Thus  the ttxh curve 104 (see Figure 6) is fully determined at this stage. If the temperature control curves for the gas turbine have been set up for a reference exhaust pressure drop Δpexhaust ref and reference inlet pressure drop Δpinlet ref  it is possible to correct the temperature control curves for a different exhaust and inlet pressure drops  for example  the actual one  by using the correction Δttxh  as already discussed above.
One or more advantages of the temperature control logic described above are discussed now. Because the entire procedure developed above for controlling the gas turbine is matrix based  the procedure is flexible and allows for easy site tuning. The procedure may bias the controlled exhaust temperature  during normal and peak load operation  based on the actual fuel""s LHV (or other fuel characteristics if differently specified). Based on this bias  it is possible to better control pollutant emission  combustion dynamics and combustor""s turn down margins.
When the peak mode is enabled  the gas turbine may stay at normal firing temperature if the base load power is enough to cover the driven machine""s power demand and the gas turbine may stay in over-firing if the base load power does not cover the driven machine""s power demand. The peak firing value may be biased by the fuel characteristics. Based on this "smart" behavior  maintaining the peak mode always enabled  it is possible to configure the gas turbine to be more reactive in case of a variation of a modified Wobbe index (MWI) base load  and/or to undertake a larger load step up starting from any operating point (largest spinning reserve).
The MWI is given by LHVgas/(SGgas · Tgas)0.5  with LHVgas being the lower heating value of the gas  SGgas the specific gravity of gas  and Tgas the temperature of the fuel gas.

Calculating a threshold for determining an operation mode change
The above exemplary embodiments have described controlling a gas turbine based on a exhaust temperature reference curve. However  for an improved control of the gas turbine  other parameters and curves may be calculated. One such example  which is discussed next  is the primary to lean-lean mode transfer threshold curve ttxth.
Prior to calculating the ttxth curve  the modes of the gas turbine are discussed. However  for a better understanding of the modes  the following discussion about the gas turbine is believed to be appropriate. According to an exemplary embodiment  the combustor 40 shown in Figure 2 may have the structure shown in Figure 8. More specifically  combustor 40 may have a wall 110 that encloses primary burners 112 and at least a secondary burner 114. One or more secondary burners 114 may be used. Both primary and secondary burners 112 and 114 may be connected via corresponding fuel supply lines 116 and 118 to one or more fuel sources (not shown). The primary burner 112 injects the fuel to a primary region 120  in which the fuel in contact with a fluid (e.g.  air  oxygen  etc.) supplied by compressor 32  are ignited  resulting in flames 124 produced in the primary region 120. The secondary burner 114 injects the fuel to a secondary region 126  where additional flames may be produced by the ignition of the additional fuel from the secondary burner 114 in the presence of the fluid from the compressor.
The modes of operation of the gas turbine may be grouped in main modes and sub modes. The main modes are characterized by the amount of fuel being supplied to the primary and/or secondary burners and the regions where the ignition takes place. The main modes are the primary mode  the lean-lean mode  and the premixed mode. Other modes may be defined and used depending on the application  the type of turbine  etc. The primary mode is illustrated in Figure 8 and is characterized by more than half of the fuel being supplied to the primary burners 112 and most of the flames being present in the primary region 120. A small amount of fuel or no fuel is supplied to the secondary burner 114. In one application  the entire fuel is supplied to the primary burners and no fuel to the secondary burner. The primary mode is used while the gas turbine is started or loaded up to a first predetermined percentage of the base load. The first predetermined percentage depends with the application. In one exemplary embodiment  the first predetermined percentage is approximately 20% of the base load. The primary mode is a diffusion flame mode  i.e.  the fuel is not premixed with an oxidant (e.g.  air) prior to being ignited. This is in contrast with the premixed mode in which the fuel is premixed with the oxidant prior to being ignited. The lean-lean mode may include burners operating in the diffusion flame mode and burners operating in the premixed mode.
The gas turbine’s operating mode changes to the lean-lean mode when the load increases over the first predetermined percentage and the load is between the first predetermined percentage and a second predetermined percentage. In one exemplary embodiment  the second predetermined percentage is 75% but can have different values depending on the application. Also  for the lean-lean mode  the secondary burner 114 is activated as shown in Figure 9 and around 60 % of the fuel is supplied to the primary burners and around 40 % of the fuel is supplied to the secondary burner. However  the percentages are for illustrative purposes and they may change from application to application. For this mode  there are flames both in the primary region 120 and the secondary region 126.
The gas turbine’s mode further changes to the premixed mode when the load increases to a third predetermined percentage  which may be  for example  about 80% to 100%. At this stage  most of the fuel is provided to the primary burners 112 while the remaining of the fuel is provided to the secondary burner 114. However  it is noted that the flames have moved from the primary region 120 to the secondary region 126 as shown in Figure 10. At this mode  the gas turbine operates with the lowest emissions  i.e.  low NOx/CO pollutants.
The relevant sub modes of the main modes discussed above are as follows: (1) lean-lean pre-fill  (2) lean-lean transient  and (3) lean-lean steady state for the lean-lean mode and (1) premixed secondary  (2) premixed transient  and (3) premixed steady state for the premixed mode. Each mode and sub mode has specific conditions that trigger their activation. Only one mode of operation at a time can be active.
Next  it is discussed how to calculate the threshold curve ttxth for transitioning from primary mode to the lean-lean mode. The lean-lean to premixed mode transfer threshold are based on the ttxh curves calculated above with regard to Figures 5 and 6. The threshold curve ttxth is calculated similar to the reference ttxh curves  i.e.  tamb  igv and tnh vectors are defined based on various ranges of these parameters  3D matrices ttxtl  tprtl  ttxtr and tprtr  which identify the turbine exhaust temperatures and pressure ratios are generated  and the actual threshold curve ttxth is calculated based on equation ttxth = ttxtha + Dttxth. It is noted that in terms of mathematical procedure  the difference between calculating ttxh and ttxth is the extra symbol “t”. Thus  for this reason  the entire algorithm for calculating ttxth is not repeated again but it is assumed to be the one used for calculating ttxh. Although the algorithm used for calculating the ttxh and the ttxth curves is the same  the difference in values of these two curves is determined by the specific values of the 3D matrices ttxtl  tprtl  ttxtr and tprtr  i.e. 
ttxtl=[ttxtli j k] for lean fuel 
tprtl=[tprtli j k] for lean fuel 
ttxtr=[ttxtli j k] for rich fuel  and
tprtr=[tprtli j k] for rich fuel.
In addition  as discussed above with ttxh  other functions f may be used to calculate ttxth curves.
Similar to the Dttxh correction  the Dttxth correction is used to take into account the actual gas turbine’s exhaust and inlet pressure drops. The gas turbine’s temperature control curves refer to a reference exhaust pressure drop Dpexhaust ref and reference inlet pressure drop Dpinlet ref. Similar to the reference curve ttxh  it is possible to correct the threshold curve ttxth for different exhaust and inlet pressure drops by using correction Dttxth.
According to an exemplary embodiment  a flow chart summarizing the calculations performed for determining Dttxth is shown in Figure 11. According to this figure  data selector unit 140 receives as input the ambient temperature tamb  the angle of rotation of the vanes IGV  the rotational speed tnh of the shaft and the rich gas matrix data (defined above with regard to ttxh). Eight points similar to C1 to C4 and D1 to D4 (shown in Figure 5) are output by the data selector unit 140. Depending on the function selected  more or less points may be used. This output is provided as input to bi-linear interpolator unit 142. The same process is repeated by data selector unit 144 for the same parameters except that a lean gas matrix data is used instead of the rich gas matrix data. An output from data selector unit 144 is provided as input to interpolator unit 146. Output from interpolators 142 and 146  i.e.  rich gas ttxth versus tprt actual control curve and lean gas ttxth versus tprt actual control gas are provided as input to calculation unit 148 for calculating two ttxth set points. Linear interpolator 150 receives the two ttxt set points and calculates the ttxth threshold of the gas turbine. It is noted that the linear interpolator 150 may receive directly information about the fuel gas LHV while the interpolator units 142 and 146 may directly receive igv and tnh data.
Having now the reference exhaust temperature ttxh curve determined as a function of the turbine pressure ratio tpr and the primary to lean-lean transfer threshold reference curve ttxth as a function of the turbine pressure ratio tpr  a mode transfer (change) of the gas turbine may be determined based on these curves and other information as discussed next. It is noted that the (ttx  tpr) plane has been used for determining both the ttxh curve and the ttxth curve  and thus  the entire discussion regarding the control of the gas turbine may be based on this plane.

Primary to lean-lean and lean-lean to primary mode transfers
The primary to lean-lean and the lean-lean to primary mode transfers are triggered by the operating point of the gas turbine passing the ttxth curve in the (ttx  tpr) plane  which was discussed above with regard to Figures 5 and 6. According to an exemplary embodiment  the primary to lean-lean and lean-lean to primary transfer sequences are described with reference to Figure 12. Figure 12 illustrates a split S on the Y axis versus time t on the X axis. The split S is indicative of a percentage of the total fuel supplied to the combustor 40 (see Figure 2) that is provided to the primary burners (see 112 in Figure 8) and a percentage of the total fuel that is provided to the secondary burner (see 114 in Figure 8). This percentage ratio is called split S. For example  a split S may be 40/60  i.e.  40 % of the total fuel being provided to the primary burners and 60 % of the total fuel being provided to the secondary burner.
With regard to Figure 12  it is assumed that split S1 is provided by the controller to the gas turbine at a time t1. Suppose that at time t1  the ttx temperature (of the actual operating point) reaches the primary to lean-lean transfer threshold curve ttxth 220 shown in Figure 13 and remains on or above it. The actual operating point is assumed to move on curve 222 prior to reaching the threshold curve ttxth 220. After a predetermined time interval of Dt1 seconds from the beginning (t1) of this condition  if the condition persists  i.e.  the actual operating time remains above or on the primary to lean-lean transfer threshold curve ttxth  the controller is configured to change the fuel split from S1 to S2. This change is happening gradually as shown in Figure 12. According to an exemplary embodiment  the change from S1 to S2 may have a constant rate of change. The fuel split remains at such value S2 for a time interval until a second time t2. According to an exemplary embodiment  the time difference t2 – t1 is precalculated.
When the fuel split is modified from S1 to S2  the gas turbine moves from the primary mode 200 to the lean-lean mode (202  204  and 206). In other words  the primary mode 200 is characterized by S1 while the lean-lean mode is characterized  among other values  by S2. However  it is noted that the lean-lean mode has plural sub modes  each having its own fuel split Si. The fuel split S2 characterizes the lean-lean pre-fill sub mode 202. For simplicity  the sub modes are referred to as modes. A purpose of this mode is to purge away the air inside the manifold (combustor) and the flexible hoses by using the fuel gas  in order to make the transfer sequence between modes more stable.
At the time t2  the pre-fill mode is complete and the controller is configured to change the fuel split from S2 to S3. S3 characterizes the lean-lean transient mode 204. The lean-lean transient mode 204 is maintained for the time interval t3-t4 in order to stabilize the flame in the secondary region shown in Figure 8. Time interval t3-t4 is also precalculated. At the time t4 the controller changes the fuel split to S4  a steady state split value  which characterizes the lean-lean steady state mode 206. In one application  the split S3 may be defined as S3 = S4 + DS1  where D is a small variation  i.e.  in the range of about 1 to 10 %.
At time t5  when the ttx temperature of operating point 226 decreases below a ttxth + Dttx1 threshold curve 224 as shown in Figure 13  the lean-lean to primary transfer is triggered and the split is changed by the controller from S4 to S1. Dttx1 is the exhaust temperature dead band for primary to lean-lean transfer threshold. The band defined by curves ttxth to ttxth + Dttx1 is used to prevent the quick change of the gas turbine back and forth between two modes as the exhaust temperature of the operating point may slightly vary in time above and/or below curve 220. The lean-lean transient 204 and the lean-lean steady state 206 modes (not the lean-lean pre-fill 202) may be aborted anytime the ttx temperature decreases below the ttxth + Dttx1 threshold curve 224.
As shown in Figure 12  when the split is changed from Si to Sj  with Si > Sj  an average split ramp rate is R1  and when the split is changed from Sk to Sh  with Sk < Sh  an average split ramp rate is R2  different than R1. These ramp rates may be constant and/or may depend with the final and initial values Si and Sj. According to an exemplary embodiment  when the primary to lean-lean transfer starts  an allowable exhaust temperature spread is increased by DSP1 (exhaust temperature spread increase over primary to lean-lean transfer) for Dtpr->ll seconds  where “pr” represents primary  “ll” represents lean-lean  and Dtpr->ll represents time duration of the spread relaxation during primary to lean-lean tranfer. The values of ramp rates R1 and R2  the split values S2 and S4 and the shift DS1 may be tuned during the shop/site tuning of the gas turbine.
According to an exemplary embodiment  which is not intended to limit the other exemplary embodiments  the following values may be used for the parameters discussed above. Split S1 in the primary mode may have a value of substantially 100%  split S2 in the lean-lean pre-fill mode may have a value of substantially 90%  split S4 in the lean-lean steady state may have a value of substantially 65%  split variation DS1 may have a value of substantially -3%  DSP1 may have a value of substantially 200 F  Dt1 may be in the order of 3 s  Dtpr->ll may be in the order of 60 s  and Dttx1 may be in the order of -25 F. The term substantially is used here to indicate that the actual values may be off the stated values by an amount that depends on the application without departing from the intended scope.
Thus  based on the above discussed exemplary embodiments that relate to the primary to lean-lean and lean-lean to primary mode transfers  it is possible to control the operating point of the gas turbine such that both the mode and a split fuel are controlled. More specifically  according to an exemplary embodiment illustrated in Figure 14  there is a method for controlling an operating point of a gas turbine that includes a compressor  a combustor and at least a turbine. The method includes a step 1400 of determining an exhaust pressure drop at an exhaust of the turbine  a step 1402 of measuring a compressor pressure discharge at the compressor  a step 1404 of determining a turbine pressure ratio based on the exhaust pressure drop and the compressor pressure discharge  a step 1406 of calculating a primary to lean-lean mode transfer threshold reference curve as a function of the turbine pressure ratio  where the primary to lean-lean mode transfer threshold curve includes points at which an operation of the gas turbine is changed between a primary mode and a lean-lean mode  a step 1408 of determining at a first time when an exhaust temperature associated with the operating point is higher than an exhaust temperature of the primary to lean-lean mode transfer threshold reference curve for the same turbine pressure ratio  and a step 1410 of changing  after a predetermined time after the first time  a split fuel quantity from a first value to a second value if the exhaust temperature associated with the operating point remains higher than the exhaust temperature of the primary to lean-lean mode transfer threshold reference curve.
The primary mode is defined in this embodiment as providing most of the fuel to primary burners and providing a remaining or no fuel to a secondary burner of the combustor and also igniting the provided fuel in a primary region of the combustor  the primary region being adjacent to the secondary region  and the lean-lean mode is defined as providing fuel to both the primary burners and the secondary burner and burning the provided fuel both in the primary region and the secondary region. The split fuel quantity describes in percentages a first amount of the total fuel that is received by the primary burners and a second amount of the total fuel that is received by the secondary burner.

Lean-lean to premixed and premixed to lean-lean mode transfers
While the gas turbine is operated on exhaust temperature control via IGV’s  the locus of the exhaust temperature set points is represented by the TTRXGV temperature control curve discussed above. In the plane (ttx  tpr) illustrated in Figures 5 and 6  and as shown in Figure 15  lowering the curve ttxh 240 by Dttx to curve 242 is equivalent to lowering the firing temperature by DTTRF. In other words  curves 240 and 242 in Figure 15 shows a correspondence between ttxh and the firing temperature TTRF.
Therefore  a dead band between curves 240 and 242 in the (ttx  tpr) plane can replace a traditional firing temperature dead band. A dead band is used in this disclosure to define a part of the plane in which no action is taken even if a given parameter passes the band. Based on this fact  the lean-lean to premixed transfer sequences are described now with regard to Figure 16.
Consider that the gas turbine operates in the lean-lean steady state mode 206 (see Figure 16) at a time prior to time t7. The split fuel S for this mode is S4  similar to the one discussed above with regard to Figure 12. The lean-lean steady state mode 206 to premixed secondary 250 transfer sequence is initiated at time t7 and the fuel split is changed from S4 to S5. The transfer starts when the logic “AND” of the following conditions is TRUE for at least Dt2 seconds:
IGVmin + DIGV1 £ IGVset point £ IGVmax + DIGV2 
ttx ³ ttxh + Dttx3 
“premixed_mode_set” = TRUE  and
“premixed_mode_enable” = TRUE 
where DIGV1 >0  DIGV2 <0  Dttx3 <0  and IGVset point is a point that may be calculated continuosly or periodically by a controller and is a target for the IGV vanes.
The meaning of the above parameters and equations are discussed now with regard to Figure 17. Suppose that an operating point of the gas turbine is 260  as shown in Figure 17. Suppose that there is a cold day  i.e.  an ambient temperature less than 10 0C. By adjusting the IGV angle of the compressor  the operating point 260 (which represents the actual exhaust temperature) may cross curve 264 to arrive on ttxh curve 266  which is desired for the operation of the gas turbine. Curve 264 is offset from ttxh curve 266 by a value Dttx3  which is an exhaust temperature dead band for the lean-lean to premixed transfer threshold and may have a value around -12.5 F. Curve 262  which is discussed later  is offset from ttxh curve 266 by Dttx3 and Dttx4  which is an exhaust temperature dead band from lean-lean to premixed transfer threshold.
When the operating point reaches position 260a on ttxh 266  the IGVs are fully closed. By opening the IGVs to a value IGVmin + DIGV1  the operating point of the gas turbine moves to position 260b. DIGV1 range is used to reduce the firing temperature swing due to the instability introduced by the lean-lean to premixed transfer sequence  because the IGV maintains the firing temperature while controlling the gas turbine exhaust temperature. DIGV2 prevents the lean-lean to premixed transfer at a high load. A similar behaviour of the operating point 260 takes place for a hot day with the exception that prior to passing curve 264  the operating point 260c reaches a point from which the ttx temperature cannot increase and the operating point evolves on an isothermal curve 270 prior to reaching ttxh curve 266.
The premixed secondary mode 250 in Figure 16 is changed either when it is detected that the primary flame is not present (see for example Figure 10) and this determination starts the premixed secondary 250 to premixed transient 280 sub-sequence at time t8  or when a predetermined maximum time duration for the premixed secondary mode 250 has been reached.
When the premixed secondary mode 250 terminates  the fuel split is changed from S5 to S6 as shown in Figure 16. The fuel split S6 characterizes the premixed transient mode 280. The premixed transient mode 280 is maintained for a predetermined time Dt3  for example  about 5 s  and then the premixed transient mode 280 is terminated at time t10 in Figure 16. The premixed transient mode 280 is used to stabilize the flame in the secondary zone 126 in Figure 10. A premixed steady state 290 is initiated at time t11 and this mode has a fuel split value S7. The fuel split value S6 of the premixed transient mode 280 is calculated based on the fuel split value S7 of the premixed steady state 290 and may have the value S6 = S7 + DS2  with DS2 being in one embodiment about -2 %.
When the premixed transient mode 280 terminates at time t10  the controller controls the gas turbine to enter the premixed steady state mode 290  thus changing the fuel split value from S6 to S7. The fuel split value S7 is expected to depend on the actual reaction conditions  which are related to the fuel’s characteristics  the reaction temperature and the induction time. The reaction temperature is a temperature at which combustion takes place close to the completion phase and the induction time is a time spent to produces intermediate chemical substances (radicals) which are the promoter of the fast reaction (fast reaction is usually called “flame”)  starting from the reactants mixing.
According to an exemplary embodiment  the fuel split S7 may be set to achieve a desirable compromise between emissions  dynamics and stability. This compromise value may be given by the following formula  which is not intended to limit the exemplary embodiments:
S7 = split = splithr · (LHV - LHVl) / (LHVr - LHVl) + splithl · (LHVr - LHV) / (LHVr -LHVl) + Dsplit  where
LHV is the lower heating value of the actual fuel 
LHVl is the lower heating value of the lean fuel 
LHVr is the lower heating value of the rich fuel 
splithr = LinearInterpolation(splitr  igv) 
splithl = LinearInterpolation(splitl  igv)  and
igv is the actual IGV angle.
According to an exemplary embodiment  splitr and splitl matrices may be defined as
splitr
igv1 igv2 igv3 igv4 igv5 igv6
splitr1 … splitr3 … … splitr6

splitl
igv1 igv2 igv3 igv4 igv5 igv6
splitl1 … splitl3 … … splitl6

Dsplit = LinearInterpolation(Dsplit  Dttx5  igv) 

Dsplit
igv1 igv2 igv3 igv4 igv5 igv6
Dttx5 1 Dsplit1 1 … … … … …
Dttx5 2 Dsplit2 1 … … … … …
Dttx5 3 Dsplit3 1 … … … … …
Dttx5 4 Dsplit4 1 … Dsplit3 4 … … splitl6 4
with Dttx5 = ttxh – ttx.
In case the LHV signal is not available  for example  calorimeter fault or calibration fault  a value (LHVr + LHVl)/2 may be used instead of the actual value. Other values may be used depending on the application. According to an exemplary embodiment  the premixed mode (which includes the premixed secondary  premixed transient and premixed steady state) may be aborted anytime the conditions for the premixed to lean-lean or extended lean-lean are met. In one application  when the fuel split quantity is changed from Si to Sj  with Si > Sj  a split ramp rate may be R3. In another application  when the fuel split quantity is changed from Sk to Sh  with Sk < Sh  a split ramp rate may be R4. Still in another application  the split ramp rates may depend on the final and initial values of the fuel split quantity.
According to an exemplary embodiment  when the lean-lean to premixed transfer starts  the allowable exhaust temperature spread is increased by DSP2 for Dtll->pm seconds. A value of DSP2 may be around 200 F and a value of Dtll->pm may be around 60 s. The ramp rates R3 and R4  the split values S7 and the shift DS2 may be adjusted as desired by the operator of the gas turbine during turbine tuning.
Next  the opposite process is discussed  i.e.  the premixed to lean-lean mode transfer. As a lean-lean extended mode may be defined as being equivalent to the lean-lean mode but operated in a gas turbine operating range where the premixed mode works properly  a transition from the premixed to the lean-lean extended mode is similar to the transition from the premixed to the lean-lean mode. For this reason  only the transition from premixed to lean-lean is discussed next. The premixed to lean-lean transfer is initiated when the logic OR of the following conditions is TRUE for at least Dt4 seconds:
IGVset point £ IGVmin
ttx £ ttxh + Dttx3 + Dttx4
“primary reignition” = TRUE
“premixed_mode_enable” = FALSE
with Dttx4 < 0. The premixed mode can be enabled/disabled by the operator. Thus  the premix_mode_enable is a logical variable that may be used to control this mode of the turbine.
With regard to Figure 18  an operating point 300 of the gas turbine may be on the ttxh curve 266. When the OR logic above happens  the operating point 300 moves to a new position 300a. This transition may happen when a load requested from the gas turbine is decreasing. When the operating point passes curve 262  which was discussed above  the transfer from the premixed mode to the lean-lean mode is triggered. This transfer may happen for a cold day or a hot day  with a difference between the cold and hot days being that the operating point for the hot day may move along an isothermal curve 270  similar to that described in Figure 17. It is noted that the threshold curve 262 for changing the premixed mode to the lean-lean mode may be different from the threshold curve 264 for changing from the lean-lean mode to the premixed mode.
Assuming that the mode transfer between the premixed mode 290 and the lean-lean mode 206 (or other lean-lean sub modes) takes place at time t12 as shown in Figure 19  spark plugs (not shown) of the combustor are activated to reignite the fuel. If a primary re-ignition is detected at time t13  the fuel split is changed from S7 to S4 with an average ramp rate R5.
If the primary re-ignition is not detected within Dt5 seconds  the gas turbine is configured to trip. The premixed mode 290 transfer-out (both to lean-lean and to extended lean-lean) resets the “premixed_mode_set” condition to FALSE in the controller. The “premixed_mode_set” is set again to TRUE when the logic AND of the following conditions is TRUE for at least Dt6 seconds:
IGVset point £ IGVmin  and
ttx £ ttxh + Dttx3 + Dttx4.
This mechanism is configured to prevent the continuous premixed transfer-in and transfer-out in case of large gas turbine power fluctuations.
After a time interval of Dt7 seconds after the “premixed_mode_set” has been reset to FALSE  the gas turbine operator is enabled to set it to TRUE again. This configuration allows the operator to prepare the gas turbine for entering again the premixed mode  thus avoiding reducing the load of the gas turbine required by the existing algorithms (IGV’s full closure).
According to an exemplary embodiment  the following numerical values may be used for the various time intervals discussed above. Dt4 may have a value of about 0.5 s  Dt5 may have a value of about 8 s  Dt6 may have a value of about 5 s  and Dt7 may have a value of about 10 s.
According to an exemplary embodiment illustrated in Figure 20  there is a method for controlling an operating point of a gas turbine that includes a compressor  a combustor and at least a turbine. The method includes a step 2000 of determining an exhaust pressure drop at an exhaust of the turbine  a step 2002 of measuring a compressor pressure discharge at the compressor  a step 2004 of determining a turbine pressure ratio based on the exhaust pressure drop and the compressor pressure discharge  a step 2006 of calculating an exhaust temperature at the exhaust of the turbine as a function of the turbine pressure ratio  a step 2008 of determining whether both conditions (1) and (2) are true  wherein condition (1) is IGVmin + DIGV1 £ IGVset point £ IGVmax + DIGV2  and condition (2) is ttx ³ ttxh + Dttx3  with IGVset point being a predetermined angle of inlet guide vanes (IGV) provided at an inlet of the compressor  IGVmin is a minimum value for the IGV  IGVmax is a maximum value for the IGV  DIGV1 is a first predetermined positive IGV angle increase for lean-lean to premixed transfer  DIGV2 is a second predetermined negative IGV angle increase for lean-lean to premixed transfer  ttx is a current exhaust temperature  ttxh is an exhaust temperature reference curve  and Dttx3 is a predetermined negative temperature characterizing an exhaust temperature dead band for lean-lean to premixed transfer  and a step 2010 of changing  if both conditions (1) and (2) are true  a split fuel quantity from a first value to a second value or otherwise maintaining the first value  the first value characterizing a lean-lean steady state mode and the second value characterizing a premixed secondary mode of a premixed mode. The premixed mode is defined as providing fuel to primary burners and to a secondary burner of the combustor and igniting the provided fuel in a secondary region of the combustor while primary regions are free of flames  the primary regions being adjacent to the secondary region  the lean-lean mode is defined as providing fuel to both the primary burners and the secondary burner and igniting the provided fuel both in the primary regions and the secondary region  and the split fuel quantity describes in percentages a first amount of the total fuel that is received by the primary burners and a second amount of the total fuel that is received by the secondary burner.
According to an exemplary embodiment  the exhaust temperature reference curve ttxh  the exhaust temperature threshold curve ttxth and other curves represented in plane (ttx  tpr) may be calculated based on other parameters that characterize a fuel instead of the lower heating value (LHV). Such parameters may be  for example  a NOx (oxides of Nitrogen) factor  an upper to lower flammability ratio (a lower flammability limit is the smallest percentage of combustible in a given volume of a mixture of fuel and air (or other oxidant) that supports a self propagating flame and an upper flammability limit is the highest percentage of the combustible in the given volume that supports a self propagating flame)  etc. In other words  ttxh curve has been calculated in an exemplary embodiment discussed above as being ttxh = ttxha + Δttxh  where ttxha = ttxhr ∙ (LHV - LHVl) / (LHVr – LHVl) + ttxhl ∙ (LHVr - LHV) / (LHVr – LHVl). However  the ttxha depends on the lower heating value LHV of the fuel and not from  for example  the NOx factor  the upper to lower flammability ratio  etc.
Thus  if a gas turbine is fed sequentially with first and second fuels  which have the same MWI index but different NOx factors  the algorithm discussed above for calculating the ttxh is not sensitive to the NOx factor as this factor is not part of the ttxha function. As the MWI factor depends from the LHV  which is reflected in the formula for the ttxha  ttxha and implicitly the ttxh curve are influenced by a change in the MWI index of the fuel. However  as the first and second fuels have similar MWI indexes  the ttxh curve and other curves based on the LHV variable will not be able to “see” that different fuels are provided to the gas turbine.
For this reason  according to an exemplary embodiment  the ttxh  ttxth  and other curves may be calculated as a function of the NOx factor  the upper to lower flammability ratio  or other parameters characteristics for a fuel. In one application  the same mathematical functions and algorithm may be used to calculate the new ttxh  ttxth curves but with the LHV parameter replaced by the new parameter. However  other functions and/or algorithms may be used to calculate the ttxh  ttxth and other curves based on the NOx factor  the upper to lower flammability ratio  etc. In other words  the controller 70 may be configured to calculate the desired curves in multiple (ttx  tpr) planes  each corresponding to a given fuel parameter.
According to an exemplary embodiment  the controller may be configured to use a parameter indicative of a characteristic of the fuel to determine the exhaust temperature reference curve. The parameter  as discussed above  may be one of a lower heating value of the fuel  a NOx factor of the fuel  an upper to lower flammability ratio of the fuel  or a combination thereof. Further  the controller may be configured to calculate exhaust temperature reference curves based on a corresponding parameter  for example  three reference curves for the three noted parameters  and to select one of the calculated exhaust temperature reference curves and control the gas turbine based on the selected exhaust temperature reference curve (the NOx factor based reference curve for the example discussed above).
For purposes of illustration and not of limitation  an example of a representative controller 2100 capable of carrying out operations in accordance with the exemplary embodiments is illustrated in Figure 21. The controller 70 discussed above with regard to Figure 2 may have the structure of controller 2100. It should be recognized  however  that the principles of the present exemplary embodiments are equally applicable to a processor  computer system  etc.
The exemplary controller 2100 may include a processing/control unit 2102  such as a microprocessor  reduced instruction set computer (RISC)  or other central processing module. The processing unit 2102 need not be a single device  and may include one or more processors. For example  the processing unit 2102 may include a master processor and associated slave processors coupled to communicate with the master processor.
The processing unit 2102 may control the basic functions of the system as dictated by programs available in the storage/memory 2104. Thus  the processing unit 2102 may execute the functions described in Figures 14 and 20. More particularly  the storage/memory 2104 may include an operating system and program modules for carrying out functions and applications on the controller. For example  the program storage may include one or more of read-only memory (ROM)  flash ROM  programmable and/or erasable ROM  random access memory (RAM)  subscriber interface module (SIM)  wireless interface module (WIM)  smart card  or other removable memory device  etc. The program modules and associated features may also be transmitted to the controller 2100 via data signals  such as being downloaded electronically via a network  such as the Internet.
One of the programs that may be stored in the storage/memory 2104 is a specific program 2106. As previously described  the specific program 2106 may store relevant parameters of the gas turbine and also may include instructions for calculating the primary to lean-lean mode transfer threshold curve and sending instructions to close or open IGV  etc. The program 2106 and associated features may be implemented in software and/or firmware operable by way of the processor 2102. The program storage/memory 2104 may also be used to store data 2108  such as the relevant parameters of the gas turbine  or other data associated with the present exemplary embodiments. In one exemplary embodiment  the programs 2106 and data 2108 are stored in non-volatile electrically-erasable  programmable ROM (EEPROM)  flash ROM  etc. so that the information is not lost upon power down of the controller 2100.
The processor 2102 may also be coupled to user interface 2110 elements associated with a control station in a power plant. The user interface 2110 of the power plant may include  for example  a display 2112 such as a liquid crystal display  a keypad 2114  speaker 2116  and a microphone 2118. These and other user interface components are coupled to the processor 2102 as is known in the art. The keypad 2114 may include alpha-numeric keys for performing a variety of functions  including dialing numbers and executing operations assigned to one or more keys. Alternatively  other user interface mechanisms may be employed  such as voice commands  switches  touch pad/screen  graphical user interface using a pointing device  trackball  joystick  or any other user interface mechanism.
The controller 2100 may also include a digital signal processor (DSP) 2120. The DSP 2120 may perform a variety of functions  including analog-to-digital (A/D) conversion  digital-to-analog (D/A) conversion  speech coding/decoding  encryption/decryption  error detection and correction  bit stream translation  filtering  etc. The transceiver 2122  generally coupled to an antenna 2124  may transmit and receive the radio signals associated with a wireless device.
The controller 2100 of Figure 21 is provided as a representative example of a computing environment in which the principles of the present exemplary embodiments may be applied. From the description provided herein  those skilled in the art will appreciate that the present invention is equally applicable in a variety of other currently known and future mobile and fixed computing environments. For example  the specific application 2106 and associated features  and data 2108  may be stored in a variety of manners  may be operable on a variety of processing devices  and may be operable in mobile devices having additional  fewer  or different supporting circuitry and user interface mechanisms. It is noted that the principles of the present exemplary embodiments are equally applicable to non-mobile terminals  i.e.  landline computing systems.
The disclosed exemplary embodiments provide a gas turbine  computer system and a method for controlling the gas turbine based on a novel paradigm and threshold. It should be understood that this description is not intended to limit the invention. On the contrary  the exemplary embodiments are intended to cover alternatives  modifications and equivalents  which are included in the spirit and scope of the invention as defined by the appended claims. Further  in the detailed description of the exemplary embodiments  numerous specific details are set forth in order to provide a comprehensive understanding of the claimed invention. However  one skilled in the art would understand that various embodiments may be practiced without such specific details.
Although the features and elements of the present exemplary embodiments are described in the embodiments in particular combinations  each feature or element can be used alone without the other features and elements of the embodiments or in various combinations with or without other features and elements disclosed herein.
This written description uses examples to disclose the invention  including the best mode  and also to enable any person skilled in the art to practice the invention  including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims  and may include other examples that occur to those skilled in the art. Such other example are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims  or if they include equivalent structural elements within the literal languages of the claims.

We Claim:
1. A method for controlling an operating point of a gas turbine that includes a compressor  a combustor and at least a turbine  the method comprising:
determining (2000) an exhaust pressure drop at an exhaust of the turbine;
measuring (2002) a compressor pressure discharge at the compressor;
determining (2004) a turbine pressure ratio based on the exhaust pressure drop and the compressor pressure discharge;
calculating (2006) an exhaust temperature reference curve of the turbine as a function of the turbine pressure ratio;
determining (2008) whether both conditions (1) and (2) are true  wherein condition (1) is IGVmin + DIGV1 £ IGVset point £ IGVmax + DIGV2  and condition (2) is ttx ³ ttxh + Dttx3  with IGVset point being a target angle of inlet guide vanes (IGV) to be provided at an inlet of the compressor  IGVmin is a minimum value for the IGV  IGVmax is a maximum value for the IGV  DIGV1 is a first predetermined positive IGV angle increase for lean-lean to premixed transfer  DIGV2 is a second predetermined negative IGV angle increase for lean-lean to premixed transfer  ttx is a current exhaust temperature  ttxh is the exhaust temperature reference curve  and Dttx3 is a predetermined negative temperature characterizing an exhaust temperature dead band for lean-lean to premixed transfer; and
changing (2010)  if both conditions (1) and (2) are true  a split fuel quantity from a first value to a second value or otherwise maintaining the first value  the first value characterizing a lean-lean steady state mode and the second value characterizing a premixed secondary mode of a premixed mode 
wherein the premixed mode is defined as providing fuel to primary burners and to a second burner of the combustor and igniting the provided fuel in a secondary region of the combustor while primary regions are free of flames  the primary regions being adjacent to the secondary region 
the lean-lean mode is defined as providing fuel to both the primary burners and the secondary burner and igniting the provided fuel both in the primary regions and the secondary region  and
the split fuel quantity describes in percentages a first amount of the total fuel that is received by the primary burners and a second amount of the total fuel that is received by the secondary burner.

2. The method of Claim 1  further comprising:
determining whether at least one of conditions (3)  (4)  (5) and (6) is true for at least a predetermined amount of time  wherein condition (3) is IGVset point £ IGVmin  condition (4) is ttx £ ttxh + Dttx3 + Dttx4  condition (5) is “primary reignition” = TRUE  and condition (6) is “premixed_mode_enable” = FALSE  Dttx4 is a predetermined negative temperature characterizing an exhaust temperature dead band for premixed to lean-lean transfer  primary_reignition is a logical variable indicating whether the fuel in the primary region in the turbine is reignited  and premixed_mode_enable is a logical variable input by an operator; and
changing  if one of conditions (3)  (4)  (5) and (6) is true  the split fuel quantity from a third value to the first value or otherwise maintaining the value  wherein the value characterizes a premixed steady state mode of the premixed mode.

3. The method of Claim 1  further comprising:
changing the second value of the fuel split quantity to a fourth value either when no flame is detected in the primary regions or after a first predetermined amount of time  wherein the fourth value characterizes a premixed transient mode of the premixed mode.

4. The method of Claim 3  further comprising:
changing the fourth value of the fuel split quantity to the third value after a second predetermined amount of time.

5. The method of Claim 4  wherein the second value is the lowest  the fourth value is the highest  the third value is between the second value and the fourth value  and the first value is between the second value and the third value.

6. The method of Claim 4  further comprising:
changing the split fuel quantity between modes with a predetermined slope.

7. The method of Claim 1  wherein the step of calculating the exhaust temperature reference curve comprises:
using a parameter indicative of a characteristic of the fuel to determine the exhaust temperature reference curve  wherein the parameter is one of a lower heating value of the fuel  NOx factor of the fuel  upper to lower flammability ratio of the fuel  or a combination thereof.

8. A controller (70  2100) for controlling an operating point of a gas turbine (30) that includes a compressor (32)  a combustor (40) and at least a turbine (50)  the controller (70  2100) comprising:
a pressure sensor (34) configured to measure a compressor pressure discharge at the compressor; and
a processor (2102) connected to the pressure sensor (34) and configured to 
determine an exhaust pressure drop at an exhaust of the turbine 
determine a turbine pressure ratio based on the exhaust pressure drop and the compressor pressure discharge 
calculate an exhaust temperature reference curve of the turbine as a function of the turbine pressure ratio 
determine whether both conditions (1) and (2) are true  wherein condition (1) is IGVmin + DIGV1 £ IGVset point £ IGVmax + DIGV2  and condition (2) is ttx ³ ttxh + Dttx3  with IGVset point being a target angle of inlet guide vanes (IGV) to be provided at an inlet of the compressor  IGVmin is a minimum value for the IGV  IGVmax is a maximum value for the IGV  DIGV1 is a first predetermined IGV angle increase for lean-lean to premixed transfer  DIGV2 is a second predetermined positive IGV angle increase for lean-lean to premixed transfer  ttx is a current exhaust temperature  ttxh is the exhaust temperature reference curve  and Dttx3 is a predetermined negative temperature characterizing an exhaust temperature dead band for lean-lean to premixed transfer  and
change  if both conditions (1) and (2) are true  a split fuel quantity from a first value to a second value or otherwise maintaining the first value  the first value characterizing a lean-lean steady state mode and the second value characterizing a premixed secondary mode of a premixed mode 
wherein the premixed mode is defined as providing fuel to primary burners and to a second burner of the combustor and igniting the provided fuel in a secondary region of the combustor while primary regions are free of flames  the primary regions being adjacent to the secondary region 
the lean-lean mode is defined as providing fuel to both the primary burners and the secondary burner and igniting the provided fuel both in the primary regions and the secondary region  and
the split fuel quantity describes in percentages a first amount of the total fuel that is received by the primary burners and a second amount of the total fuel that is received by the secondary burner.

9. The controller of Claim 8  wherein the processor is further configured to:
determine whether at least one of conditions (3)  (4)  (5) and (6) is true for at least a predetermined amount of time  wherein condition (3) is IGVset point £ IGVmin  condition (4) is ttx £ ttxh + Dttx3 + Dttx4  condition (5) is “primary reignition” = TRUE  and condition (6) is “premixed_mode_enable” = FALSE  Dttx4 is a predetermined negative temperature characterizing an exhaust temperature dead band for premixed to lean-lean transfer  primary_reignition is a logical variable indicating whether the fuel in the primary region is reignited  and premix_mode_enable is a logical variable input by an operator; and
changing  if one of conditions (3)  (4)  (5) and (6) is true  the split fuel quantity from a third value to the first value or otherwise maintaining the third value  wherein the third value characterizes a premixed steady state mode.

10. The controller of Claim 8  wherein the processor is further configured to:
change the second value of the fuel split quantity to a fourth value when no flame is detected in the primary regions for a certain time or change the fuel split to the first value if the flame is detected in the primary regions after a first predetermined amount of time  wherein the fourth value characterizes a premixed transient mode.

11. The controller of Claim 10  wherein the processor is further configured to:
change the fourth value of the fuel split quantity to the third value after a second predetermined amount of time.

13. The controller of Claim 7  wherein the processor is further configured to:
use a parameter indicative of a characteristic of the fuel to determine the exhaust temperature reference curve  wherein the parameter is one of a lower heating value of the fuel  a NOx factor of the fuel  an upper to lower flammability ratio of the fuel  or a combination thereof.

14. The controller of Claim 13  wherein the processor is configured to:
calculate exhaust temperature reference curves based on a corresponding parameter; and
select one of the calculated exhaust temperature reference curves and control the gas turbine based on the selected exhaust temperature reference curve.
Dated this the 26th Day of May  2012

MANISHA SINGH NAIR
Agent for the Applicant [IMN/PA-740]
LEX ORBIS IP PRACTICE

EXHAUST TEMPERATURE BASED MODE CONTROL METHOD FOR GAS TURBINE AND GAS TURBINE
Abstract of the Invention
Gas turbine and method for controlling an operating point of the gas turbine that includes a compressor  a combustor and at least a turbine. The method includes calculating an exhaust temperature reference curve of the turbine as a function of a turbine pressure ratio; determining whether condition IGVmin + DIGV1 £ IGVset point £ IGVmax + DIGV2  and condition ttx ³ ttxh + Dttx3  are true; and changing  if both conditions are true  a split fuel quantity from a first value to a second value or otherwise maintaining the first value  the first value characterizing a lean-lean steady state mode and the second value characterizing a premixed secondary mode of a premixed mode.