Field The present invention relates generally to controllers for gas turbines, to gas turbines comprising such controllers and to methods of controlling such gas turbines.

Background to the invention An example of a typical gas turbine arrangement is shown in Figure 1. The gas turbine comprises an air inlet 10 at one end followed by a compressor stage 1 1 in which incoming air is compressed for application to one or more combustors 12, which are distributed circumferentially around the turbine axis 13. Fuel is introduced into the combustors at 14 and is there mixed with a part of the compressed air leaving the compressor stage 1 1. Hot gases created by combustion in the combustors are directed to a set of turbine blades 15, being guided in the process by a set of guide vanes 16, and the turbine blades 15 and the shaft forming the axis 13 are turned as a result. The turbine blades 15 in turn rotate the blades of the compressor stage 1 1 , so that the compressed air is supplied by the gas turbine itself once this is in operation.

Part of a typical combustor is shown in Figure 2A. Figure 2B shows a section along a line Ill-Ill shown in Figure 2A. The combustor is in four parts: a front-end part 20, a swirler part 21 , a burner pre-chamber part 22 and a combustion volume 23. Main fuel is introduced into the swirler 21 by way of the front-end part 20 through a conduit 24, while pilot fuel enters the burner space through a conduit 25 having at its end a pilot- fuel nozzle 29. The main and pilot fuel-flows are derived from a fuel-split valve 26, which is fed with a fuel supply 27 representing the total fuel supply to the combustor. The main fuel flow enters the swirler through a set of main-fuel nozzles (or injector) 28, from where it is guided along swirler vanes 30, being mixed with incoming compressed air in the process. The fuel may be gaseous fuel or liquid fuel. The resulting air/fuel mixture maintains a burner flame 30. The hot air from this flame enters the combustion volume 23. A gas turbine will often comprise a number of such combustors, in which case the main and pilot fuel-flow distribution will usually be as shown in Figure 3. Due to environmental concerns, there is a continued drive to reduce pollutant emissions from gas turbines. Potential pollutant emissions include oxides of nitrogen (NO and N0 ₂ , generally referred to as NOx), carbon monoxide (CO), unburned hydrocarbons (UHCs, typically expressed as equivalent methane), oxides of sulphur (S0 ₂ and S0 ₃ ) and particulate matter (PM). UHCs typically include volatile organic compounds (VOCs), which contribute to the formation of ground level atmospheric ozone, in addition to compounds such as methane and ethane, that do not contribute to ozone formation. The amounts of S0 ₂ , UHC and PM are usually considered negligible when burning natural gas. However, NOx and potentially CO emissions may be of significance when combusting natural gas and/or fuel oil in gas turbines.

An amount of NOx produced depends on combustion temperature and/or fuel to air ratio. When combustion takes place at lower temperatures and/or at lower fuel to air ratios, the NOx emissions are reduced. Conventional methods of reducing NOx emissions include Wet Low Emission (WLE), in which water or steam injection reduces the fuel to air ratios, and Dry Low Emission (DLE) and Dry Low NOx (DLN), which use principles of lean premixed combustion. DLE may reduce NOx and CO emissions to less than 25 ppmv or even to less than 10 ppmv while DLN may reduce NOx emissions to less than 25 ppmv.

Figure 4A shows a graph of a Turbine Entry Temperature (TET) as a function of load L for a typical gas turbine. For such a typical gas turbine without part load emissions requirements (particularly CO), a term natural turndown may be used whereby low emissions are achieved for a restricted load but without employing any control techniques to maintain a high turndown control temperature (TCT) such as Variable Guide Vane (VGV) or bleed to exhaust (B2E). The dashed line in Figure 4A depicts a relationship between the TCT and the load L when the part load emissions techniques are not employed. In order to achieve lower emissions at lower loads, Variable Guide Vane (VGV) modulation or bleed to exhaust (B2E) may be used in order to maintain a constant Turbine Entry Temperature (TET). This constant temperature line is known as the turndown control temperature (TCT) line and is shown as a solid line in Figure 4A. Typically, pilot fuel split settings are defined based on gas turbine TET. This means that as the load L is decreased along the TCT, a pilot split is held constant throughout the load range. As a result, there is limited control over combustion performance in the way of combustion dynamics, for example flame stability (also known as combustion stability), and/or emissions, for example, at different loads L on the TCT line. Figure 4B shows a graph of a Turbine Entry Temperature (TET) and pilot split as a function of load L for a typical gas turbine.

Particularly, Figure 4B shows pilot split behaviour along the TCT line. Known pilot fuel control algorithms typically maintain constant, predefined pilot split values along the constant TCT line as the load L is decreased. This may limit control over combustion parameters such as flame stability and/or NOx emissions at each load condition. The known pilot fuel control algorithms rely on inputs from low frequency dynamics and on burner tip measurements and may be mapped against TET, such that when the TET is constant, control of the known pilot fuel control algorithms is somewhat limited.

However, the conventional methods of reducing NOx emissions at the lower temperatures, for example TETs, and/or at the lower fuel to air ratios result in reduction of thermodynamic efficiency of the gas turbines. This is contrary to a typical goal of increasing thermodynamic efficiency. Further, such conventional methods of reducing NOx emissions are typically concerned with operating the gas turbines at full loads. In addition, known pilot fuel control algorithms for lower loads provide limited control of combustion dynamics, for example flame stability, and/or emissions. Furthermore, flame stability and/or emissions may be influenced by complex interplay between environmental and operating conditions, together with gas turbine specific factors such as age and contamination.

Hence, there is a need to improve control of gas turbines to improve flame stability and/or emissions. Summary of the Invention

It is one aim of the present invention, amongst others, to provide a controller for a gas turbine, a gas turbine comprising such a controller and a method of controlling such a gas turbine that improves flame stability and/or emissions, for example at lower temperatures and/or lower loads. According to a first aspect, there is provided a controller for a gas turbine arranged to supply a load L, the gas turbine comprising a total fuel supply means arranged to supply fuel at a fuel flow rate FF to a combustor, wherein the fuel supply means comprises a first fuel supply means and a second fuel supply means, wherein the controller is arranged to:

determine one or more ratios R of one or more combustor operating parameters COP respectively at the load L to respective reference combustor operating parameters COPR at a reference load LR; and

control a proportion P of the fuel flow rate FF supplied via the first fuel supply means based, at least in part, on the determined one or more ratios R.

According to a second aspect, there is provided a gas turbine arranged to supply a load L, the gas turbine comprising a fuel supply means arranged to supply fuel at a fuel flow rate FF to a combustor, wherein the fuel supply means comprises a first fuel supply means and a second fuel supply means, wherein the gas turbine comprises a controller arranged to:

determine one or more ratios R of one or more combustor operating parameters COP respectively at the load L to respective reference combustor operating parameters COPR at a reference load LR; and

control a proportion P of the fuel flow rate FF supplied via the first fuel supply means based, at least in part, on the determined one or more ratios R.

According to a third aspect, there is provided a method of controlling a gas turbine arranged to supply a load L, the gas turbine comprising a total fuel supply means arranged to supply fuel at a fuel flow rate FF to a combustor, wherein the fuel supply means comprises a first fuel supply means and a second fuel supply means, the method comprising:

determining one or more ratios R of one or more combustor operating parameters COP respectively at the load L to respective reference combustor operating parameters COPR at a reference load LR; and

controlling a proportion P of the fuel flow rate FF supplied via the first fuel supply means based, at least in part, on the determined one or more ratios R;

whereby a flame stability and/or an emission is improved.

Preferably, control or controlling a proportion P of the fuel flow rate FF supplied via the first fuel supply means based, at least in part, on the determined one or more ratios R is without any other condition-based (internal or external) correction factors. This means the present invention is advantageous because it is relatively simple using predefined values of the pilot fuel split which is based on the COP calculated on condition rather than time-traces and post-processing of other input parameters to calculate the fuel split. The present invention is simple and therefore computationally less demanding so it is quicker to respond.

According to a fourth aspect, there is provided a tangible non-transient computer- readable storage medium having recorded thereon instructions which when implemented by a controller for a gas turbine arranged to supply a load L, the gas turbine comprising a fuel supply means arranged to supply fuel at a fuel flow rate FF to a combustor, wherein the fuel supply means comprises a first fuel supply means and a second fuel supply means, cause the controller to perform a method of controlling the gas turbine, the method according to the third aspect. According to a fifth aspect, there is provided a controller for a gas turbine arranged to supply a load L, the gas turbine comprising a fuel supply means arranged to supply fuel at a fuel flow rate FF to a combustor, wherein the fuel supply means comprises a first fuel supply means and a second fuel supply means, wherein the controller is arranged to control a proportion P of the fuel flow rate FF supplied via the first fuel supply means based, at least in part, on the fuel flow rate FF.

According to a sixth aspect, there is provided a gas turbine arranged to supply a load L, the gas turbine comprising a fuel supply means arranged to supply fuel at a fuel flow rate FF to a combustor, wherein the fuel supply means comprises a first fuel supply means and a second fuel supply means, wherein the gas turbine comprises a controller arranged to control a proportion P of the fuel flow rate FF supplied via the first fuel supply means based, at least in part, on the fuel flow rate FF.

According to a seventh aspect, there is provided a method of controlling a gas turbine arranged to supply a load L, the gas turbine comprising a fuel supply means arranged to supply fuel at a fuel flow rate FF to a combustor, wherein the fuel supply means comprises a first fuel supply means and a second fuel supply means, the method comprising:

controlling a proportion P of the fuel flow rate FF supplied via the first fuel supply means based, at least in part, on the fuel flow rate FF;

whereby a metal temperature and/or an emission is improved. According to a eighth aspect, there is provided a tangible non-transient computer- readable storage medium having recorded thereon instructions which when implemented by a controller for a gas turbine arranged to supply a load, the gas turbine comprising a fuel supply means arranged to supply fuel at a fuel flow rate FF to a combustor, wherein the fuel supply means comprises a first fuel supply means and a second fuel supply means, cause the controller to perform a method of controlling the gas turbine, the method according to the seventh aspect and/or the third aspect.

Detailed Description of the Invention

According to the present invention there is provided a controller for a gas turbine, as set forth in the appended claims. Also provided is a gas turbine, a method of controlling a gas turbine and a tangible non-transient computer-readable storage medium. Other features of the invention will be apparent from the dependent claims, and the description that follows.

Throughout this specification, the term "comprising" or "comprises" means including the component(s) specified but not to the exclusion of the presence of other components. The term "consisting essentially of" or "consists essentially of" means including the components specified but excluding other components except for materials present as impurities, unavoidable materials present as a result of processes used to provide the components, and components added for a purpose other than achieving the technical effect of the invention, such as colourants, and the like.

The term "consisting of" or "consists of" means including the components specified but excluding other components.

Whenever appropriate, depending upon the context, the use of the term "comprises" or "comprising" may also be taken to include the meaning "consists essentially of" or "consisting essentially of, and also may also be taken to include the meaning "consists of" or "consisting of". The optional features set out herein may be used either individually or in combination with each other where appropriate and particularly in the combinations as set out in the accompanying claims. The optional features for each aspect or exemplary embodiment of the invention, as set out herein are also applicable to all other aspects or exemplary embodiments of the invention, where appropriate. In other words, the skilled person reading this specification should consider the optional features for each aspect or exemplary embodiment of the invention as interchangeable and combinable between different aspects and exemplary embodiments. According to the first aspect, there is provided a controller for a gas turbine arranged to supply a load L, the gas turbine comprising a total fuel supply means arranged to supply fuel at a fuel flow rate FF to a combustor, wherein the total fuel supply means comprises a first fuel supply means and a second fuel supply means, wherein the controller is arranged to:

determine one or more ratios R of one or more combustor operating parameters COP respectively at the load L to respective reference combustor operating parameters COPR at a reference load LR; and

control a proportion P of the fuel flow rate FF supplied via the first fuel supply means based, at least in part, on the determined one or more ratios R.

By controlling the proportion P (i.e. a split) of the fuel flow rate FF supplied via the first fuel supply means, for example a pilot fuel supply means, based, at least in part, on the determined one or more ratios R, the proportion P may be varied along a Turndown Control Temperature (TCT) line, for example a substantially constant TCT line, as the load L is varied, for example reduced from a full load L supplyable by the gas turbine. In this way, control of the gas turbine may be improved so as to improve flame stability and/or emissions, as the load L is varied for example.

Controlling the proportion P in this way may be applied, for example, as modifications to conventional pilot fuel control algorithms, to calculate and apply pilot split offsets for conventional pilot split maps for each load L and/or to split map envelopes, based, at least in part, on the determined one or more ratios R.

In this way, improved, for example more accurate, initial conditions for control of pilot splits (i.e. the proportion P) at each load L along the substantially constant TCT line may be provided, so as to achieve improved combustion performance in terms of flame stability and/or emissions. By improving flame stability, reliability of the gas turbine may be improved. By improving emissions, for example reducing NOx emissions, environmental impact may be improved. For example, a conventional pilot split may be a predefined constant pilot split value along the constant TCT line as the load L is decreased.

In contrast, the proportion P (i.e. a pilot split according to the invention) is non constant along a constant TCT line as the load L is decreased, being controlled based, at least in part, on the determined one or more ratios R, as described above.

Particularly, the proportion P may increase linearly as the load L is increased along the TCT line, for example. At the low end of the constant TCT line, the proportion P may be less than the conventional pilot split and at the high end of the constant TCT line, the proportion P may tend towards and/or equal the conventional pilot split. At loads L below the low end of the constant TCT line, the proportion P may increase as the TET is reduced, being less than but tending towards the conventional pilot split as the load L is reduced. At loads L above the high end of the constant TCT line, the proportion P may decrease as the TET is increased to the full load L, substantially equal to the conventional pilot split value. That is, the proportion P may be at most the conventional pilot split value for a given load L, and may be less than the conventional pilot split value for intermediate loads L, for example in a range from about 10% to 60% of the full load L. The gas turbine may be as described with respect to Figures 1 to 3.

In one example, the combustor comprises and/or is a can, annular or cannular combustor. In one example, the gas turbine includes a plurality of combustors, for example such combustors. In one example, the first fuel supply means comprises and/or is a pilot fuel supply means. In one example, the first fuel supply means is a single pilot fuel supply means. In one example, the second fuel supply means comprises and/or is a main fuel supply means. In one example, the gas turbine comprises a plurality of combustors, the first fuel supply means comprises a plurality of pilot fuel supply means, for example corresponding and/or respective pilot fuel supply means, and the second fuel supply means comprises a plurality of main fuel supply means, for example corresponding and/or respective main fuel supply means. In other words, each combustor may include a pilot fuel nozzle and a main fuel nozzle associated with the first fuel supply means and the second fuel supply means respectively. In one example, the fuel is a gas fuel, for example natural gas. In one example, the fuel is a liquid fuel, for example fuel oil.

In one example, the reference load LR is a full load (i.e. 100% load L) supplyable by the gas turbine.

In one example, the combustor operating parameter COP is one selected from a group comprising a combustion intensity CI of the combustor, an equivalence ratio ER of the combustor and a combustor inlet function CIF of the combustor.

The inventors have determined that one or more of these combustor operating parameters COP may be particularly beneficial in improving a flame stability and/or an emission, for example a NOx emission.

In one example, the combustion intensity CI is determined based, at least in part, on a heat input HI to the gas turbine, a compressor exit pressure CEP of the gas turbine and/or a combustor volume CV of the combustor and the controller is arranged to: determine the combustion intensity CI;

determine a first ratio R1 of the combustion intensity CI at the load L to a reference combustion intensity CIR at the reference load LR; and

control the proportion P of the fuel flow rate FF supplied via the first fuel supply means based, at least in part, on the determined first ratio R1 .

In one example, the reference load LR is a full load supplyable by the gas turbine and the reference combustion intensity CIR at the reference load LR is the combustion intensity CIR at full load supplyable by the gas turbine. In one example, the combustion intensity CI is determined by CI = HI / (CEP x CV) and the controller is arranged to determine the heat input HI, the compressor exit pressure CEP and/or the combustor volume CV.

The heat input HI may be derived based on the fuel flow rate FF, calorific value of the fuel and specific gravity of the fuel. The compressor exit pressure CEP may be derived based on a gas turbine (engine) pressure ratio and/or may be measured.

The combustor volume CV may be measured/determined based on geometric measurements of the combustion system, for example, (prechamber + can + tduct).

In more detail, the combustion intensity CI may be determined by Equation 1 below as:

Combustion Intensity CI (MW /bar ^■ m ³ )

Heat Input to Gas Turbine HI (MW)

Compressor Exit Pressure CEP (bar ^■ a) x Combustor Volume CV (m ³ )

In one example, the gas turbine includes a plurality of combustors and the combustor volume CV is the total combustor volume CV of the plurality of combustors. In one example, the combustor volume is predetermined, for example by measurement, and the controller is arranged to store the combustor volume CV.

In one example, the gas turbine comprises one or more sensors arranged to sense the heat input HI and/or the compressor exit pressure CEP and the controller is arranged to obtain the sensed the heat input HI and/or the compressor exit pressure CEP therefrom.

In one example, the equivalence ratio ER (also known as Φ) is determined based, at least in part, on the fuel flow rate FF to the combustor, an air flow rate FA to the combustor and/or a stoichiometric fuel to air ratio SFAR and wherein the controller is arranged to:

determine the equivalence ratio ER;

determine a second ratio R2 of the equivalence ratio ER at the load L to a reference equivalence ratio ERR at the reference load LR; and

control the proportion P of the fuel flow rate FF supplied via the first fuel supply means based, at least in part, on the determined second ratio R2.

In one example, the equivalence ratio ER is determined by ER = ( FF / FA ) / SFAR and wherein the controller is arranged to determine the fuel flow rate FF, the air flow rate FA and/or the stoichiometric fuel to air ratio SFAR. The fuel flow rate FF may be determined, for example measured, using a flow meter, for example. The air flow rate FA may be determined, for example computed, based on a compressor speed and an engine pressure ratio.

The stoichiometric fuel to air ratio SFAR may be determined, for example calculated, as a value specific to a certain fuel composition and may be a theoretical value that implies that a reaction between carbon and oxygen constituents is completed without any excess of the former or latter, to achieve a complete consumption of both constituents. If more oxygen exists within the mixture, then the mixture is referred to as lean mixture. This is typical for gas turbines, for which reference to lean premixed systems is typically made. However, when an excess of fuel exist, then reference is made instead to rich systems, as also referred to for gas turbines, but not as often.

In more detail, the equivalence ratio ER may be determined by Equation 2 below as:

Actual Fuel FF ^■ ■ Air FA Ratio

Equivalence Ratio ER

Stoichiometric Fuel ·■ Air Ratio SFAR where

Mass Fuel Flow Rate (kg/s)

Actual Fuel ·■ Air Ratio = —— r-^—

Mass Air Flow Rate kg/s)

In one example, the gas turbine comprises one or more sensors arranged to sense the fuel flow rate FF and/or the air flow rate FA and the controller is arranged to obtain the sensed fuel flow rate FF and/or the air flow rate FA therefrom.

In one example, the combustor inlet function CIF is determined based, at least in part, on an air flow rate FA to the combustor, a compressor exit temperature CET of the gas turbine and/or a compressor exit pressure CEP of the gas turbine and the controller is arranged to:

determine the combustor inlet function CIF; determine a third ratio R3 of the combustor inlet function CIF at the load L to a reference combustor inlet function CIFR at the reference load LR; and

control the proportion P of the fuel flow rate FF supplied via the first fuel supply means based, at least in part, on the determined third ratio R3.

In one example, the combustor inlet function CIF is determined by CIF = ( FA x VCET ) / CEP and the controller is arranged to determine the air flow rate FA, the compressor exit temperature CET and/or the compressor exit pressure CEP. The air flow rate FA may be determined, for example computed, based on a compressor speed and an engine (i.e. gas turbine) pressure ratio.

The compressor exit temperature CET may be determined, for example measured, by thermocouples and/or may be derived based on engine (i.e. gas turbine) pressure ratio.

The compressor exit pressure CEP may be determined, for example derived, based on engine pressure ratio and/or may be measured. In more detail, the combustor inlet function CIF may be determined by Equation 3 as:

Combustor Inlet Function CIF

Mass Air Flow Rate (kg/s) x J Compressor Exit Temperature CET (K)

Compressor Exit Pressure CEP (bar ^■ a)

In one example, the gas turbine comprises one or more sensors arranged to sense the air flow rate FA, the compressor exit temperature CET and/or the compressor exit pressure CEP and the controller is arranged to obtain the sensed air flow rate FA, the compressor exit temperature CET and/or the compressor exit pressure CEP therefrom.

In one example, controller is arranged to:

determine a bleed flow rate FB; and

control the proportion P of the fuel flow rate FF supplied via the first fuel supply means based, at least in part, on the determined bleed flow rate FB. In one example, the gas turbine comprises one or more sensors arranged to sense the bleed flow rate FB and the controller is arranged to obtain the sensed bleed flow rate FB therefrom. In one example, the bleed flow rate FB is a mass air flow rate to an exhaust and/or an inlet of the gas turbine.

In one example, the gas turbine comprises one or more sensors arranged to sense the mass air flow rate to an exhaust and/or an inlet of the gas turbine and the controller is arranged to obtain the sensed mass air flow rate to an exhaust and/or an inlet of the gas turbine therefrom.

In one example, the load L and/or the fuel flow rate FF and/or a Turbine Entry Temperature TET is substantially constant.

In one example, the gas turbine comprises one or more sensors arranged to sense the load L and/or the fuel flow rate FF and/or the Turbine Entry Temperature TET and the controller is arranged to obtain the sensed the load L and/or the fuel flow rate FF and/or the Turbine Entry Temperature TET therefrom.

In one example, the controller comprises a memory and a processor, wherein the memory includes instructions which when executed by the processor, cause the controller to perform a method of controlling the gas turbine as described herein, for example as described above and/or according to the third aspect. In other words, the controller may be arranged, for example, to determine the one or more ratios R of the one or more combustor operating parameters COP respectively at the load L to the respective reference combustor operating parameters COPR at the reference load LR, determine the combustor operating parameters COP as described above and/or control the proportion P of the fuel flow rate FF supplied via the first fuel supply means based, at least in part, on the determined one or more ratios R, according to the instructions executed by the processor. In one example, the controller comprises a communication unit, arranged to communicate with one or more sensors, as described above. In one example, the controller comprises a storage, arranged to store one or more than one or more ratios R of the one or more combustor operating parameters COP respectively at the load L, the respective reference combustor operating parameters COPR at the reference load LR, and/or the determined combustor operating parameters COP, as described above.

According to the second aspect, there is provided a gas turbine arranged to supply a load L, the gas turbine comprising a total fuel supply means arranged to supply fuel at a fuel flow rate FF to a combustor, wherein the total fuel supply means comprises a first fuel supply means and a second fuel supply means, wherein the gas turbine comprises a controller arranged to:

determine one or more ratios R of one or more combustor operating parameters COP respectively at the load L to respective reference combustor operating parameters COPR at a reference load LR; and

control a proportion P of the fuel flow rate FF supplied via the first fuel supply means based, at least in part, on the determined one or more ratios R. The gas turbine, the load L, the fuel supply means, the fuel flow rate FF, the combustor, the first fuel supply means, the second fuel supply means, the controller, the ratios R, the combustor operating parameters COP, the reference combustor operating parameters COPR, the reference load LR and/or the proportion P may be as described with respect to the first aspect.

According to the third aspect, there is provided a method of controlling a gas turbine arranged to supply a load L, the gas turbine comprising a total fuel supply means arranged to supply fuel at a fuel flow rate FF to a combustor, wherein the total fuel supply means comprises a first fuel supply means and a second fuel supply means, the method comprising:

determining one or more ratios R of one or more combustor operating parameters COP respectively at the load L to respective reference combustor operating parameters COPR at a reference load LR; and

controlling a proportion P of the fuel flow rate FF supplied via the first fuel supply means based, at least in part, on the determined one or more ratios R;

whereby a flame stability and/or an emission is improved.

The gas turbine, the load L, the fuel supply means, the fuel flow rate FF, the combustor, the first fuel supply means, the second fuel supply means, the controller, the ratios R, the combustor operating parameters COP, the reference combustor operating parameters COPR, the reference load LR and/or the proportion P may be as described with respect to the first aspect and/or the second aspect. The emission may be, for example, a NOx emission.

In one example, the combustor operating parameter COP is one selected from a group comprising a combustion intensity CI of the combustor, an equivalence ratio ER of the combustor and a combustor inlet function CIF of the combustor.

In one example, the combustion intensity CI is determined based, at least in part, on a heat input HI to the gas turbine, a compressor exit pressure CEP of the gas turbine and/or a combustor volume CV of the combustor and wherein the method comprises: determining the combustion intensity CI;

determining a first ratio R1 of the combustion intensity CI at the load L to a reference combustion intensity CIR at the reference load LR; and

controlling the proportion P of the fuel flow rate FF supplied via the first fuel supply means based, at least in part, on the determined first ratio R1.

In one example, the combustion intensity CI is determined by CI = HI / (CEP x CV) and wherein the method comprises determining the heat input HI, the compressor exit pressure CEP and/or the combustor volume CV.

In one example, the equivalence ratio ER is determined based, at least in part, on the fuel flow rate FF to the combustor, an air flow rate FA to the combustor and/or a stoichiometric fuel to air ratio SFAR and wherein the method comprises:

determining the equivalence ratio ER;

determining a second ratio R2 of the equivalence ratio ER at the load L to a reference equivalence ratio ERR at the reference load LR; and

controlling the proportion P of the fuel flow rate FF supplied via the first fuel supply means based, at least in part, on the determined second ratio R2. In one example, the equivalence ratio ER is determined by ER = ( FF / FA ) / SFAR and wherein the method comprises determining the fuel flow rate FF, the air flow rate FA and/or the stoichiometric fuel to air ratio SFAR.

In one example, the combustor inlet function CIF is determined based, at least in part, on an air flow rate FA to the combustor, a compressor exit temperature CET of the gas turbine and/or a compressor exit pressure CEP of the gas turbine and wherein the method comprises:

determining the combustor inlet function CIF;

determining a third ratio R3 of the combustor inlet function CIF at the load L to a reference combustor inlet function CIFR at the reference load LR; and

controlling the proportion P of the fuel flow rate FF supplied via the first fuel supply means based, at least in part, on the determined third ratio R3.

In one example, the combustor inlet function CIF is determined by CIF = ( FA x VCET ) / CEP and wherein the method comprises determining the air flow rate FA, the compressor exit temperature CET and/or the compressor exit pressure CEP.

In one example, method comprises:

determining a bleed flow rate FB; and

controlling the proportion P of the fuel flow rate FF supplied via the first fuel supply means based, at least in part, on the determined bleed flow rate FB.

In one example, the load L and/or the fuel flow rate FF and/or a Turbine Entry Temperature TET is substantially constant.

In one example, the reference load LR is a full load supplyable by the gas turbine. In one example, the first fuel supply means is a pilot fuel supply means. According to the fourth aspect, there is provided a tangible non-transient computer- readable storage medium having recorded thereon instructions which when implemented by a controller for a gas turbine arranged to supply a load L, the gas turbine comprising a total fuel supply means arranged to supply fuel at a fuel flow rate FF to a combustor, wherein the total fuel supply means comprises a first fuel supply means and a second fuel supply means, cause the controller to perform a method of controlling the gas turbine, the method according to the third aspect.

According to the fifth aspect, there is provided a controller for a gas turbine arranged to supply a load L, the gas turbine comprising a total fuel supply means arranged to supply fuel at a fuel flow rate FF to a combustor, wherein the total fuel supply means comprises a first fuel supply means and a second fuel supply means, wherein the controller is arranged to control a proportion P of the fuel flow rate FF supplied via the first fuel supply means based, at least in part, on the fuel flow rate FF.

By controlling the proportion P (i.e. a split) of the fuel flow rate FF supplied via the first fuel supply means based, at least in part, on the fuel flow rate FF, control of the gas turbine may be improved so as to improve a metal temperature and/or an emission, as the load L is varied for example. This improvement is particularly observed at high loads, for example >90%, and/or at higher temperatures, as described below. Controlling the proportion P in this way may be applied, for example, as modifications to conventional pilot fuel control algorithms, to calculate and apply pilot split offsets for conventional pilot split maps for each load L and/or to split map envelopes, based, at least in part, on the fuel flow rate FF. By improving metal temperature, reliability of the gas turbine may be improved. By improving emissions, for example reducing NOx emissions, environmental impact may be improved.

The inventors have determined that ambient variations may have a significant effect on NOx emissions and/or on metal temperatures in the pilot region where fuel is burnt rich for flame stability. That is, NOx emissions may be a function of TFIRE which depends on the air and fuel properties e.g. temperature, pressure, massflow etc. NOx emissions also depend on the pilot fuel percentage. Fine balancing of pilot fuel percent is required as lower pilot fuel results in poor flame stability and higher pilot fuel results in higher NOx emissions.

Conventional control methods of gas turbines do not consider all these factors, being typically based on pilot percent (percentage of the total fuel) and, as described previously, used to keep the flame stable.

The inventors have determined that NOx emissions may be a strong function of pilot percent using gas and/or liquid fuel. When the fuel flow rate increases at relatively colder temperatures, the pilot fuel flow rate into the primary combustion zone also increases conventionally, as described above. Further, NOx emissions may change due to volumetric heat release and/or generation. For example, at relatively colder temperatures, heat release may be relatively higher if higher amounts of pilot fuel flow (at a same percent pilot split) are present in a stability region, which is often a vortex core region, of the combustor. Hence, a percentage pilot split as a function of a combustor exit temperature TX or a turbine inlet temperature Tl may not account for NOx emissions and/or metal temperatures.

The controller may be further arranged as described with respect to the first aspect. Advantageously, the fifth aspect may improve a metal temperature and/or an emission at higher loads L and/or temperatures, as described previously, while the first aspect may be improve a flame stability and/or an emission at intermediate loads L, thereby providing synergistic advantages.

In one example, there is provided a controller for a gas turbine arranged to supply a load L, the gas turbine comprising a total fuel supply means arranged to supply fuel at a fuel flow rate FF to a combustor, wherein the total fuel supply means comprises a first fuel supply means and a second fuel supply means, wherein the controller is arranged to:

control a proportion P of the fuel flow rate FF supplied via the first fuel supply means based, at least in part, on the fuel flow rate FF;

determine one or more ratios R of one or more combustor operating parameters COP respectively at the load L to respective reference combustor operating parameters COPR at a reference load LR; and

control the proportion P of the fuel flow rate FF supplied via the first fuel supply means based, at least in part, on the determined one or more ratios R. The gas turbine may be as described with respect to Figures 1 to 3.

In one example, the combustor comprises and/or is a can, annular or cannular combustor. In one example, the gas turbine includes a plurality of combustors, for example such combustors. In one example, the first fuel supply means comprises and/or is a pilot fuel supply means. In one example, the first fuel supply means is a single pilot fuel supply means. In one example, the second fuel supply means comprises and/or is a main fuel supply means. In one example, the gas turbine comprises a plurality of combustors, the first fuel supply means comprises a plurality of pilot fuel supply means, for example corresponding and/or respective pilot fuel supply means, and the second fuel supply means comprises a plurality of main fuel supply means, for example corresponding and/or respective main fuel supply means. In other words, each combustor may include a pilot fuel nozzle and a main fuel nozzle associated with the first fuel supply means and the second fuel supply means respectively. In one example, the fuel is a gas fuel, for example natural gas. In one example, the fuel is a liquid fuel, for example fuel oil. For gas fuel, instead of a pilot pressure drop as described herein, a pressure ratio across the pilot fuel nozzle may be more important. Generally, a pressure ratio should be such that there is no reverse flow and/or high combustor to combustor variation.

It should be understood that the fuel flow rate FF is the total fuel flow rate FF. That is, the fuel flow rate FF includes that fuel supplied to the combustor via the first fuel supply means and a second fuel supply means. For example, if the gas turbine includes a plurality of combustors, the fuel flow rate FF includes that fuel supplied to the plurality of combustors via the first fuel supply means and a second fuel supply means. In one example, the fuel flow rate FF is determined by the load L. That is, if the load L changes, the fuel flow rate FF may change accordingly.

In one example, the gas turbine comprises one or more sensors arranged to sense, for example measure, the fuel flow rate FF and/or the load L and the controller is arranged to obtain the measured the fuel flow rate FF and/or the load L therefrom.

In one example, the controller is arranged to control the proportion P of the fuel flow rate FF supplied via the first fuel supply means based, at least in part, on a reference fuel flow rate FFR. In one example, the reference fuel flow rate FFR is at a first predetermined temperature T1 , preferably 323 K of inlet air temperature to the gas turbine, and/or a first predetermined load L1 , preferably 100%.

In one example, the controller is arranged to control the proportion P of the fuel flow rate FF supplied via the first fuel supply means based, at least in part, on a reference proportion PR of the fuel flow rate FF supplied via the first fuel supply means.

In one example, the controller is arranged to control the proportion P of the fuel flow rate FF supplied via the first fuel supply means based, at least in part, on a reference proportion PR of the fuel flow rate FF supplied via the first fuel supply means. In one example, the controller is arranged to control the proportion P of the fuel flow rate FF supplied via the first fuel supply means based, at least in part, on a reference proportion split PRS of the fuel flow rate FF supplied via the first fuel supply means. Additionally and/or alternatively, the proportion P of the fuel flow rate FF supplied via the first fuel supply means may be described by equation 1 below as:

Scaling Constant SC

Proportion P = —

Fuel Flow Rate FF where

Scaling Constant SC

= Reference Fuel Flow Rate FFR x Reference Proportion Split PRS

In one example, the reference proportion split PRS is at a first predetermined temperature T1 , preferably 323 K, and/or a first predetermined load L1 , preferably 100%.

In one example, the reference proportion split PRS is a pilot percentage as a function of a turbine inlet temperature Tl.

In one example, the reference proportion split PRS is based, at least in part, on minimum fuel flow rate below which liquid spray cone from a total fuel supply means nozzle, for example the first fuel supply means nozzle, collapses.

In one example, the reference proportion split PRS is based, at least in part, on a minimum fuel flow rate below which there is no positive and/or net flow into the combustor via a total fuel supply means nozzle, for example the first fuel supply means nozzle.

In this way, the proportion P of the fuel flow rate FF supplied via the first fuel supply means based, at least in part, on the fuel flow rate FF considers, for example, physics of a liquid fuel spray pressure drop and/or a gas fuel pressure ratio. For example, if the pressure drop is too low, variation between combustors may be significant, with different combustion zones having different fuel flow rates, making it difficult to control the gas turbine in such weak fuel flows. For example, for a liquid spray pressure drop from pressure swirl injectors or nozzles, a minimum required pressure drop may be 0.5 bar. For a gas fuel, a minimum required pressure ratio may be 1.005 or a 0.5% pressure drop.

In one example, the reference proportion PR of the fuel flow rate FF supplied via the first fuel supply means is at a second predetermined temperature T2, preferably 323 K, and/or a second predetermined load L2, preferably 100%. In one example, the proportion P of the fuel flow rate FF supplied via the first fuel supply means is determined by P = ( FFR ^* PR ) / FF.

In one example, the reference fuel flow rate FFR is at the first predetermined temperature T1 and/or the first predetermined load L1 and/or the reference proportion PR is at the second predetermined temperature T2 and/or the second predetermined load L2.

In one example, the first predetermined temperature T1 and the second predetermined temperature T2 are the same (i.e. equal). In one example, the first predetermined load L1 and the second predetermined load L2 are the same (i.e. equal).

In one example, the controller is arranged to control the fuel flow rate FF based, at least in part, on the ambient temperature TA.

In one example, the gas turbine comprises one or more sensors arranged to sense, for example measure, an ambient temperature TA and the controller is arranged to obtain the measured ambient temperature TA therefrom. In one example, the controller is arranged to control the proportion P of the fuel flow rate F supplied via the first fuel supply means based, at least in part, on the fuel flow rate FF if a combustor exit temperature TX is greater than a third predetermined temperature T3 and/or if a turbine inlet temperature Tl is greater than a fourth predetermined temperature T4. In one example, the gas turbine comprises one or more sensors arranged to sense, for example measure, a combustor exit temperature TX and/or a turbine inlet temperature Tl and the controller is arranged to obtain the measured combustor exit temperature TX and/or the measured turbine inlet temperature Tl therefrom.

In one example, the third predetermined temperature T3 is in a range from about 1400 K to 1900 K, preferably in a range of from about 1500 K to 1700 K, more preferably in a range of from about 1550 K to 1650 K. In one example, the fourth predetermined temperature T4 is in a range from about 1400 K to 1900 K, preferably in a range of from about 1500 K to 1700 K, more preferably in a range of from about 1550 K to 1650 K.

The fourth predetermined temperature T4 may depend on a particular design of the combustor.

In this way, the control of the proportion P of the fuel flow rate FF supplied via the first fuel supply means based, at least in part, on the fuel flow rate FF is at temperatures where thermal NOx emissions are dominant and/or when metal tip temperatures are a concern. At temperatures higher than T3 and/or T4, the fuel flow rate FF supplied via the first fuel supply means is at least a minimum value such that NOx emissions may be reduced across the ambient temperature range and/or metal temperatures in the vicinity of a pilot zone may be reduced, improving reliability and/or leading to higher component life.

A minimum required pressure drop for a reasonable spray is assumed to be at an ambient temperature of 50 °C and at a load L of 100%. Below this fuel flow rate FF, there may be a relatively higher combustor to combustor variation. According to the control provided by the fifth aspect, a pilot split map may be changed while maintaining a reasonable pressure drop.

According to the control provided by the fifth aspect, a pilot split map may be changed based on the turbine inlet temperature Tl while ensuring that a 50 °C ambient temperature and a load L of 100% is the worst condition or limiting condition at which a scaling factor is calculated. In this way, control of the gas turbine may be readily implemented. According to the control provided by the fifth aspect, differences with respect to at least NOx emissions between site testing and actual operating conditions upon installation on site, for example at relatively colder ambient temperatures and/or 100% load on site, may be reduced.

In one example, the controller is arranged to control the proportion P of the fuel flow rate FF supplied via the first fuel supply means to be a constant, if the combustor exit temperature TX is at most the third predetermined temperature T3.

In one example, the controller comprises a memory and a processor, wherein the memory includes instructions which when executed by the processor, cause the controller to perform a method of controlling the gas turbine as described herein, for example as described above and/or according to the third aspect. In one example, the controller comprises a communication unit, arranged to communicate with one or more sensors, as described above. In one example, the controller comprises a storage, arranged to store one or more predetermined loads, for example the first predetermined load L1 , and/or predetermined temperatures, for example the first predetermined temperature T1 , the second predetermined temperature T2, the third predetermined temperature T3 and/or the fourth predetermined temperature T4, as described above.

According to the sixth aspect, there is provided a gas turbine arranged to supply a load L, the gas turbine comprising a total fuel supply means arranged to supply fuel at a fuel flow rate FF to a combustor, wherein the total fuel supply means comprises a first fuel supply means and a second fuel supply means, wherein the gas turbine comprises a controller arranged to control a proportion P of the fuel flow rate FF supplied via the first fuel supply means based, at least in part, on the fuel flow rate FF. The gas turbine, the load L, the fuel supply means, the fuel flow rate FF, the combustor, the first fuel supply means, the second fuel supply means, the controller, and/or the proportion P may be as described with respect to the fifth aspect and/or the first aspect. According to the seventh aspect, there is provided a method of controlling a gas turbine arranged to supply a load L, the gas turbine comprising a total fuel supply means arranged to supply fuel at a fuel flow rate FF to a combustor, wherein the total fuel supply means comprises a first fuel supply means and a second fuel supply means, the method comprising:

controlling a proportion P of the fuel flow rate FF supplied via the first fuel supply means based, at least in part, on the fuel flow rate FF;

whereby a metal temperature and/or an emission is improved.

The gas turbine, the load L, the fuel supply means, the fuel flow rate FF, the combustor, the first fuel supply means, the second fuel supply means, the controller, and/or the proportion P may be as described with respect to the fifth aspect, the sixth aspect, the first aspect and/or the second aspect. The emission may be, for example, a NOx emission.

In one example, there is provided a method of controlling a gas turbine arranged to supply a load L, the gas turbine comprising a total fuel supply means arranged to supply fuel at a fuel flow rate FF to a combustor, wherein the total fuel supply means comprises a first fuel supply means and a second fuel supply means, the method comprising:

controlling a proportion P of the fuel flow rate FF supplied via the first fuel supply means based, at least in part, on the fuel flow rate FF;

determining one or more ratios R of one or more combustor operating parameters COP respectively at the load L to respective reference combustor operating parameters COPR at a reference load LR; and

controlling the proportion P of the fuel flow rate FF supplied via the first fuel supply means based, at least in part, on the determined one or more ratios R;

whereby a metal temperature and/or a flame stability and/or an emission is improved.

In one example, the controlling the proportion P of the fuel flow rate FF supplied via the first fuel supply means is based, at least in part, on a reference fuel flow rate FFR.

In one example, the reference fuel flow rate FFR is at a first predetermined temperature T1 , preferably 323 K, and/or a first predetermined load L1 , preferably 100%. In one example, the controlling the proportion P of the fuel flow rate FF supplied via the first fuel supply means is based, at least in part, on a reference proportion PR of the fuel flow rate FF supplied via the first fuel supply means. In one example, the reference proportion PR of the fuel flow rate FF supplied via the first fuel supply means is at a second predetermined temperature T2, preferably 323 K, and/or a second predetermined load L2, preferably 100%.

In one example, method comprises controlling the proportion P of the fuel flow rate FF supplied via the first fuel supply means based, at least in part, on a reference proportion split PRS of the fuel flow rate FF supplied via the first fuel supply means.

In one example, the reference proportion split PRS is at a first predetermined temperature T1 , preferably 323 K, and/or a first predetermined load L1 , preferably 100%.

In one example, the reference proportion split PRS is a pilot percentage as a function of a turbine inlet temperature Tl. In one example, the reference proportion split PRS is based, at least in part, on minimum fuel flow rate below which liquid spray cone from a total fuel supply means nozzle, for example the first fuel supply means nozzle, collapses.

In one example, the reference proportion split PRS is based, at least in part, on a minimum fuel flow rate below which there is no positive and/or net flow into the combustor via a fuel supply means nozzle, for example the first fuel supply means nozzle.

In one example, the proportion P of the fuel flow rate FF supplied via the first fuel supply means is determined by P = ( FFR ^* PR ) / FF.

In one example, the reference fuel flow rate FFR is at the first predetermined temperature T1 and/or the first predetermined load L1 and/or the reference proportion PR is at the second predetermined temperature T2 and/or the second predetermined load L2. In one example, the method comprises measuring an ambient temperature TA and wherein the fuel flow rate FF is based, at least in part, on the ambient temperature TA.

In one example, the method comprises measuring a combustor exit temperature TX and/or a turbine inlet temperature Tl and wherein the controlling the proportion P of the fuel flow rate F supplied via the first fuel supply means based, at least in part, on the fuel flow rate FF if the combustor exit temperature TX is greater than a third predetermined temperature T3 and/or if the turbine inlet temperature Tl is greater than a fourth predetermined temperature T4.

In one example, the third predetermined temperature T3 is in a range from about 1400 K to 1900 K, preferably in a range of from about 1500 K to 1700 K, more preferably in a range of from about 1550 K to 1650 K. In one example, if the combustor exit temperature TX is at most the third predetermined temperature T3, the proportion P of the fuel flow rate FF supplied via the first fuel supply means is constant.

According to the eighth aspect, there is provided a tangible non-transient computer- readable storage medium having recorded thereon instructions which when implemented by a controller for a gas turbine arranged to supply a load L, the gas turbine comprising a total fuel supply means arranged to supply fuel at a fuel flow rate FF to a combustor, wherein the total fuel supply means comprises a first fuel supply means and a second fuel supply means, cause the controller to perform a method of controlling the gas turbine, the method according to the seventh aspect and/or the third aspect.

Brief description of the drawings For a better understanding of the invention, and to show how exemplary embodiments of the same may be brought into effect, reference will be made, by way of example only, to the accompanying diagrammatic Figures, in which:

Figure 1 schematically depicts a longitudinal section of a typical gas turbine; Figure 2A schematically depicts a longitudinal section of a typical combustor and Figure 2B schematically depicts a section along line Ill-Ill in Figure 2A;

Figure 3 schematically depicts a block diagram illustrating derivation of main and pilot fuel supplies in a typical gas turbine with multiple combustors;

Figure 4A shows a graph of a Turbine Entry Temperature as a function of load for a typical gas turbine; Figure 4B shows a graph of a Turbine Entry Temperature and pilot split as a function of load for a typical gas turbine;

Figure 5 schematically depicts a controller for a gas turbine according to an exemplary embodiment;

Figure 6 schematically depicts a gas turbine according to an exemplary embodiment;

Figure 7 schematically depicts a method of controlling a gas turbine according to an exemplary embodiment;

Figure 8A shows a graph of a Turbine Entry Temperature and combustor operating parameters as a function of load for a gas turbine according to an exemplary embodiment; and Figure 8B shows a graph of a Turbine Entry Temperature and pilot split as a function of load for a gas turbine according to an exemplary embodiment.

Detailed Description of the Drawings Figure 5 schematically depicts a controller 50 for a gas turbine (not shown) according to an exemplary embodiment.

In more detail, the controller 50 is for a gas turbine arranged to supply a load L, the gas turbine comprising a total fuel supply means arranged to supply fuel at a fuel flow rate FF to a combustor, wherein the total fuel supply means comprises a first fuel supply means and a second fuel supply means. The controller 50 is arranged to determine one or more ratios R of one or more combustor operating parameters COP respectively at the load L to respective reference combustor operating parameters COPR at a reference load LR. The controller 50 is further arranged to control a proportion P of the fuel flow rate FF supplied via the first fuel supply means based, at least in part, on the determined one or more ratios R.

By controlling the proportion P (i.e. a split) of the fuel flow rate FF supplied via the first fuel supply means, for example a pilot fuel supply means, based, at least in part, on the determined one or more ratios R, the proportion P may be varied along a Turndown Control Temperature (TCT) line, for example a substantially constant TCT line, as the load L is varied, for example reduced from a full load L supplyable by the gas turbine. In this way, control of the gas turbine may be improved so as to improve flame stability and/or emissions, as the load L is varied for example. The controller may be arranged as described previously.

Figure 6 schematically depicts a gas turbine 600 according to an exemplary embodiment. In more detail, the gas turbine 600 is arranged to supply a load L. The gas turbine 600 comprises a total fuel supply means 60 arranged to supply fuel at a fuel flow rate FF to a combustor 70, wherein the total fuel supply means 60 comprises a first fuel supply means 61 and a second fuel supply means 62. The gas turbine 600 comprises the controller 50, as described above with reference to Figure 5. Particularly, the controller 50 is arranged to determine one or more ratios R of one or more combustor operating parameters COP respectively at the load L to respective reference combustor operating parameters COPR at a reference load LR. The controller 50 is arranged to control a proportion P of the fuel flow rate FF supplied via the first fuel supply means 61 based, at least in part, on the determined one or more ratios R.

Figure 7 schematically depicts a method of controlling a gas turbine according to an exemplary embodiment.

In more detail, the method is of controlling a gas turbine arranged to supply a load L, the gas turbine comprising a total fuel supply means arranged to supply fuel at a fuel flow rate FF to a combustor, wherein the total fuel supply means comprises a first fuel supply means and a second fuel supply means.

At S701 , one or more ratios R of one or more combustor operating parameters COP respectively at the load L to respective reference combustor operating parameters COPR at a reference load LR are determined.

At S702, a proportion P of the fuel flow rate FF supplied via the first fuel supply means based, at least in part, on the determined one or more ratios R is controlled, whereby a flame stability and/or an emission is improved.

The method may include any of the steps described previously.

Figure 8A shows a graph of a Turbine Entry Temperature (TET) and combustor operating parameters as a function of load L for a gas turbine according to an exemplary embodiment.

Particularly, Figure 8A shows a graph of a combustion intensity CI of the combustor, an equivalence ratio ER of the combustor and a combustor inlet function CIF of the combustor as a load L is varied, showing the behaviours of these combustor operating parameters along the Turndown Control Temperature (TCT) line. The combustion intensity CI of the combustor, the equivalence ratio ER of the combustor and the combustor inlet function CIF are determined according to the equations 1 to 3, as described above. The combustion intensity CI of the combustor, the equivalence ratio ER of the combustor and the combustor inlet function CIF are normalised to full load L values (i.e. 100% load L). That is, the graph shows three ratios R of the three combustor operating parameters COP respectively at the load L to respective reference combustor operating parameters COPR at a reference load LR (i.e. 100% load L). While the TET remains constant along the constant TCT line, these combustor operating parameters are not constant.

In detail, the normalised combustion intensity CI of the combustor decreases linearly as the load L is increased along the TCT line, having a maximum greater than unity (i.e. greater than at the full load) at the low end of the constant TCT line and a minimum at the high end of the constant TCT line. At loads L below the low end of the constant TCT line, the normalised combustion intensity CI of the combustor decreases as the TET is reduced. At loads L above the high end of the constant TCT line, the normalised combustion intensity CI of the combustor increases as the TET is increased to the full load L. In detail, the normalised equivalence ratio ER decreases linearly as the load L is increased along the TCT line, having a maximum at the low end of the constant TCT line and a minimum at the high end of the constant TCT line. At loads L below the low end of the constant TCT line, the normalised equivalence ratio ER decreases as the TET is reduced. At loads L above the high end of the constant TCT line, the normalised equivalence ratio ER increases as the TET is increased to the full load L.

In detail, the normalised combustor inlet function CIF increases linearly as the load L is increased along the TCT line, approaching a minimum at the low end of the constant TCT line and a maximum at the high end of the constant TCT line. At loads L below the low end of the constant TCT line, the normalised combustor inlet function CIF decreases marginally as the TET is reduced. At loads L above the high end of the constant TCT line, the normalised equivalence ratio ER of the combustor decreases slightly as the TET is increased to the full load L. Figure 8B shows a graph of a Turbine Entry Temperature (TET) and pilot split as a function of load for a gas turbine according to an exemplary embodiment.

Particularly, Figure 8B shows a graph of a conventional pilot split (i.e. a proportion P of the fuel flow rate FF supplied via the first fuel supply means) according to a conventional pilot split map (solid line) compared with an exemplary pilot split (i.e. a proportion P of the fuel flow rate FF supplied via the first fuel supply means) according to an exemplary embodiment (dashed line).

In detail, the conventional pilot split is a predefined constant pilot split value along the constant TCT line as the load L is decreased, as described above with reference to Figure 4B.

In contrast, the exemplary pilot split value is non constant along the constant TCT line as the load L is decreased, being controlled based, at least in part, on the determined one or more ratios R, as described above. In detail, the exemplary pilot split value increases linearly as the load L is increased along the TCT line. At the low end of the constant TCT line, the exemplary pilot split value is less than the conventional pilot split and at the high end of the constant TCT line, the exemplary pilot split value tends towards and/or equals the conventional pilot split. At loads L below the low end of the constant TCT line, the exemplary pilot split value increases as the TET is reduced, being less than but tending towards the conventional pilot split as the load L is reduced. At loads L above the high end of the constant TCT line, the exemplary pilot split value decreases as the TET is increased to the full load L, substantially equal to the conventional pilot split value. That is, the exemplary pilot split value is at most the conventional pilot split value for a given load L, and is less than the conventional pilot split value for intermediate loads L, for example in a range from about 10% to 60% of the full load.

As described above, controlling the proportion P in this way may be applied, for example, as modifications to conventional pilot fuel control algorithms, to calculate and apply pilot split offsets for conventional pilot split maps for each load L and/or to split map envelopes, based, at least in part, on the determined one or more ratios R.

Although a preferred embodiment has been shown and described, it will be appreciated by those skilled in the art that various changes and modifications might be made without departing from the scope of the invention, as defined in the appended claims and as described above.

Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

All of the features disclosed in this specification (including any accompanying claims and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.