Patent Description:
Gas turbine systems are used in a wide variety of applications to generate power. In operation of a gas turbine system ("GT system"), air flows through a compressor and the compressed air is supplied to a combustion section. Specifically, the compressed air is supplied to a number of combustors each having a number of fuel nozzles, i.e., burners, which use the air in a combustion process with a fuel. The compressor includes a number of inlet guide vanes (IGVs), the angle of which can be controlled to control an air flow to the combustion section, and thus a combustion temperature. The combustion section is in flow communication with a turbine section in which the combustion gas stream's kinetic and thermal energy is converted to mechanical rotational energy. The turbine section includes a turbine that rotatably couples to and drives a rotor. The compressor may also rotatably couple to the rotor. The rotor may drive a load, like an electric generator.

The combustion section includes a number of combustors that can be used to control the load of the GT system, e.g., a plurality of circumferentially spaced combustor 'cans. ' Advancements have led to the use of combustors having two combustion stages. A header (or head end) combustion stage may be positioned at an upstream end of the combustion region of each combustor. The header combustion stage includes a number of fuel nozzles that act to introduce fuel for combustion. Advanced gas turbine systems also include a second combustion stage, referred to as an axial fuel staging (AFS) or late lean injection (LLI) combustion stage, downstream from the header combustion stage in the combustion region of each combustor. The AFS combustion stage includes a number of fuel nozzles or injectors that introduce fuel diverted (split) from the header combustion stage for combustion in the AFS combustion stage. The AFS combustion stage provides increased efficiency and assists in emissions compliance for the GT system by ensuring a higher efficacy of combustion that reduces harmful emissions in an exhaust of the GT system. Each fuel nozzle in the header combustion stage can be controlled to be on or off to control flow of fuel for combustion. Conventionally, a combustion section reference temperature is used to control the combustion section. The combustion section reference temperature is an estimation of the temperature of the combustion flow at the exit of the combustion region prior to entering the turbine section.

Loading or unloading a GT system presents a number of challenges relative to controlling emissions as the GT system gradually increases or decreases its power output. For example, a start up may begin with the rotor being rotated by a motor until a speed is reached allowing the compressor to begin flowing air to the combustion section (i.e., purge speed). The speed may then be reduced at which point fuel flow is initiated to the combustion section, and fuel combustion begins. At this point, the GT system goes through a number of 'combustion modes' in which a number of fuel nozzles of the header combustion stage become operative, and then eventually all fuel nozzles in the header combustion stage and the AFS combustion stage become operative. During this process, air flow intake is set by controlling an angle of a stage(s) of IGVs on the compressor that control air flow volume.

During the progression through the combustion modes, it is very difficult to control emissions at certain times. To illustrate, <FIG> shows an illustrative, conventional start up load path graph plotting exhaust temperature (Tx) versus load between full speed no load (FSNL) (<NUM>% load) to <NUM>% of rated operation, with a schematic rendition of a carbon monoxide (CO) amount in the GT exhaust in a quasi-steady state with the load path. Changes in combustion modes are shown with vertical marks on the load path line. Initially, IGVs are set to a desired angle and an initial exhaust temperature is achieved at FSNL (<NUM>% load). As startup progresses from <NUM>% to <NUM>% load through three illustrative combustion modes (vertical marks on load path), exhaust temperature rises as more fuel nozzles are activated and net fuel flow increases while the air flow remains constant. Exhaust temperature rises until it reaches a plateau at an isotherm exhaust temperature limit for the GT system. As shown by the CO emissions schematic rendition, as loading progresses through the combustion modes, CO amounts rise and fall through the different modes until the final combustion mode in the sequence is employed, i.e., with all fuel nozzles in header and AFS combustion stages activated. Each combustion mode typically includes at least one period during which the emissions are higher than desired at low loads. Once the final combustion mode in the sequence has been engaged, the CO emissions will eventually decrease to compliant levels as the unit loads. The lowest load at which emissions compliance is satisfied is referred to as the minimum emissions compliance load (MECL) and reflects the turndown capability of the GT system. Conventional load path control typically utilizes various control strategies for stage(s) of IGVs, inlet bleed heating, or compressor extraction flow modulation to extend a GT system's low load capability, e.g., during turndown. Using such a strategy may result in low load capability of <NUM>%-<NUM>% of rated power depending on ambient conditions and the technology employed in the GT system.

<CIT> shows a gas turbine system including a compressor feeding air to a combustion section that is coupled to a turbine. The combustion system includes a plurality of combustors. The method and or the gas turbine system progresses through each of a plurality of progressive combustion modes during loading of the gas turbine system.

<CIT> discloses a dual fuel concentric nozzle for a gas turbine, whereby a plurality of fuel nozzles is provided in a secondary combustion stage of a sequential combustor.

A first aspect of the invention provides a loading/unloading method for a gas turbine system according to claim <NUM>.

A second aspect of the invention provides a gas turbine (GT) system according to claim <NUM>.

The illustrative aspects of the present invention are designed to solve the problems herein described and/or other problems not discussed.

It is noted that the drawings of the disclosure are not to scale.

As an initial matter, in order to clearly describe the current invention it will become necessary to select certain terminology when referring to and describing relevant machine components within a gas turbine (GT) system. When doing this, if possible, common industry terminology will be used and employed in a manner consistent with its accepted meaning. Unless otherwise stated, such terminology should be given a broad interpretation consistent with the context of the present application and the scope of the appended claims. Those of ordinary skill in the art will appreciate that often a particular component may be referred to using several different or overlapping terms. What may be described herein as being a single part may include and be referenced in another context as consisting of multiple components. Alternatively, what may be described herein as including multiple components may be referred to elsewhere as a single part.

In addition, several descriptive terms may be used regularly herein, and it should prove helpful to define these terms at the onset of this section. These terms and their definitions, unless stated otherwise, are as follows. As used herein, "downstream" and "upstream" are terms that indicate a direction relative to the flow of a fluid, such as the combustion gas stream in a combustion section or, for example, the flow of air through the compressor. The term "downstream" corresponds to the direction of flow of the fluid, and the term "upstream" refers to the direction opposite to the flow. The terms "forward" and "aft," without any further specificity, refer to directions, with "forward" referring to the front or compressor end of the engine, and "aft" referring to the rearward or turbine end of the engine. It is often required to describe parts that are at differing radial positions with regard to a center axis. The term "radial" refers to movement or position perpendicular to an axis. In cases such as this, if a first component resides closer to the axis than a second component, it will be stated herein that the first component is "radially inward" or "inboard" of the second component. If, on the other hand, the first component resides further from the axis than the second component, it may be stated herein that the first component is "radially outward" or "outboard" of the second component. The term "axial" refers to movement or position parallel to an axis. Finally, the term "circumferential" refers to movement or position around an axis. It will be appreciated that such terms may be applied in relation to the center axis of the turbine.

Where an element or layer is referred to as being "on," "engaged to," "disengaged from," "connected to" or "coupled to" another element or layer, it may be directly on, engaged, connected or coupled to the other element or layer, or intervening elements or layers may be present. In contrast, when an element is referred to as being "directly on," "directly engaged to," "directly connected to" or "directly coupled to" another element or layer, there may be no intervening elements or layers present.

As indicated above, the invention provides a load/unload method for a gas turbine (GT) system that may allow for emissions compliance during periods when it normally is not provided. The invention also includes a GT system including a compressor feeding air to a combustion section that is coupled to a turbine section. The combustion section includes a plurality of combustors with each combustor including a primary combustion stage including a first plurality of fuel nozzles and a secondary combustion stage downstream from the primary combustion stage. The secondary combustion stage includes a second plurality of fuel nozzles. Hence, the combustion section is a two stage combustion section. In accordance with embodiments of the invention, during loading or unloading, the method progresses through each of a plurality of progressive combustion modes that sequentially activate a different number of at least one of the first and second plurality of fuel nozzles. That is, each progressive combustion mode turns on more or less fuel nozzles to, respectively, increase or decrease the combustion temperature and combustion flow. During loading, the method progresses through each of a plurality of progressive combustion modes that sequentially activate a higher number of at least one of the first and second plurality of fuel nozzles. Similarly, during unloading, the method progresses through each of a plurality of progressive combustion modes that sequentially activate a lower number of at least one of the first and second plurality of fuel nozzles. In contrast to current load/unload methods that control a temperature at an exit of the combustor, during each combustion mode, a primary combustion stage exit temperature of a combustion gas flow (i.e., between primary and secondary combustion stages, referred to herein as primary combustion stage exit temperature (PCSET) or mid-combustor temperature) is controlled to be within a predefined target range corresponding to the respective combustion mode. As a result, emissions are better controlled to remain emissions compliant.

<FIG> shows a cross-sectional view of an illustrative GT system <NUM> in which teachings of the invention may be employed. In <FIG>, GT system <NUM> includes an intake section <NUM>, and a compressor <NUM> downstream from intake section <NUM>. Compressor <NUM> feeds air to a combustion section <NUM> that is coupled to a turbine section <NUM>. Compressor <NUM> may include one or more stages of inlet guide vanes (IGVs) <NUM>. As understood in the art, the angle of stages of IGVs <NUM> can be controlled to control an air flow volume to combustion section <NUM>, and thus, among other things, the combustion temperature of section <NUM>. Combustion section <NUM> includes a plurality of combustors <NUM>. Each combustor <NUM> includes a primary combustion stage <NUM> including a first plurality of fuel nozzles, and a secondary combustion stage <NUM> downstream from primary combustion stage <NUM>. Secondary combustion stage <NUM> includes a second plurality of fuel nozzles, different than the first plurality of fuel nozzles. Exhaust from turbine section <NUM> exits via an exhaust section <NUM>. Turbine section <NUM> through a common shaft or rotor connection drives compressor <NUM> and a load <NUM>. Load <NUM> may be any one of an electrical generator and a mechanical drive application and may be located forward of intake section <NUM> (as shown) or aft of exhaust section <NUM>. Examples of such mechanical drive applications include a compressor for use in oil fields and/or a compressor for use in refrigeration. When used in oil fields, the application may be a gas reinjection service. When used in refrigeration, the application may be in liquid natural gas (LNG) plants. Yet another load <NUM> may be a propeller as may be found in turbojet engines, turbofan engines and turboprop engines.

Referring to <FIG> and <FIG>, combustion section <NUM> may include a circular array of a plurality of circumferentially spaced combustors <NUM>. <FIG> shows a cross-sectional side view of combustor <NUM>. A fuel/air mixture is burned in each combustor <NUM> to produce the hot energetic combustion gas flow, which flows through a transition piece <NUM> (<FIG>) to turbine nozzles <NUM> (<FIG>) of turbine section <NUM>. For purposes of the present description, only one combustor <NUM> is illustrated, it being appreciated that all of the other combustors <NUM> arranged about combustion section <NUM> are substantially identical to the illustrated combustor <NUM>. Although <FIG> shows a plurality of circumferentially spaced combustors <NUM> and <FIG> shows a cross sectional side view of a combustor <NUM> that have come to be known in the art as can combustor systems, it is contemplated that the present invention may be used in conjunction with other combustor systems including and not limited to annular combustor systems.

Referring now to <FIG>, there is shown generally a combustor <NUM> for GT system <NUM> (<FIG>) including primary combustion stage <NUM> and secondary combustion stage <NUM>. A transition piece <NUM> flows hot combustion gas flow to turbine nozzles <NUM> and the turbine blades (not shown). Primary combustion stage <NUM> may include a casing <NUM>, an end cover <NUM>, a first plurality of premixing fuel nozzles <NUM> (hereinafter simply "fuel nozzles <NUM>"), a cap assembly <NUM>, a flow sleeve <NUM>, and a combustion liner <NUM> within flow sleeve <NUM>. An ignition device (not shown) is provided and preferably comprises an electrically energized spark plug. Combustion in primary combustion section <NUM> occurs within combustion liner <NUM>. Combustion air is directed within combustion liner <NUM> via flow sleeve <NUM> and may enter combustion liner <NUM> through a plurality of openings formed in cap assembly <NUM>. The air enters combustion liner <NUM> under a pressure differential and mixes with fuel from start-up fuel nozzles (not shown) and/or first plurality of fuel nozzles <NUM> within combustion liner <NUM>. Consequently, a combustion reaction occurs within combustion liner <NUM> releasing heat for the purpose of driving turbine section <NUM> (<FIG>). Highpressure air for primary combustion stage <NUM> may enter flow sleeve <NUM> and a transition piece impingement sleeve <NUM>, from an annular plenum <NUM>. Compressor <NUM> (<FIG>), which is represented by a series of vanes and blades at <NUM> and a diffuser <NUM> in <FIG>, supplies this highpressure air.

Each of first plurality of fuel nozzles <NUM> in primary combustion stage <NUM> can take a variety of forms. In the example of <FIG>, each fuel nozzle <NUM> may include a swirler <NUM>, consisting of a plurality of swirl vanes that impart rotation to the entering air and a plurality of fuel spokes <NUM> that distribute fuel in the rotating air stream. The fuel and air then mix in an annular passage within fuel nozzle <NUM> before reacting within primary reaction zone <NUM>. However, other forms of (premixing) fuel nozzles <NUM> may be employed.

As shown in <FIG>, secondary combustion stage <NUM> includes a second plurality of fuel nozzles <NUM> for transversely injecting a secondary fuel mixture into a combustion gas flow product of primary combustion stage <NUM>. Fuel nozzles <NUM> may include any variety and number of injection elements for injecting the second fuel mixture. Fuel nozzles <NUM> may extend radially into the combustion gas flow path. In one example, four circumferentially spaced fuel nozzles <NUM> are employed. However, any number may be possible.

With further regard to first plurality of fuel nozzles <NUM> in <FIG>, fuel nozzles <NUM> may also have a variety of layouts, e.g., relative to cap assembly <NUM>. <FIG> and <FIG> are plan views of alternate embodiments of a combustor cap assembly <NUM>, as viewed from an aft end of combustion section <NUM> looking in an upstream direction. Cap assembly <NUM> illustrated in <FIG> corresponds to that shown in more detail in <FIG>, although it should be understood that cap assembly <NUM> illustrated in <FIG> is equally well-suited for combustion section <NUM> shown in <FIG>.

In <FIG>, a center fuel nozzle <NUM>, which is disposed about a centerline <NUM> of combustion section <NUM>, is secured within a respective opening (not separately labeled) in a cap plate <NUM>. A plurality (in this example, five) outer fuel nozzles <NUM> are disposed about center fuel nozzle <NUM> and likewise are secured within respective openings in cap plate <NUM>. Each outer fuel nozzle <NUM> has a centerline <NUM>. Each fuel nozzle <NUM>, <NUM> is a bundled tube fuel nozzle having a plurality of parallel, non-concentric mixing tubes <NUM> that extend through a common fuel plenum. Cap plate <NUM> may include a plurality of cooling holes to facilitate cooling of the cap face, and/or cap assembly <NUM> may include a second plate upstream of cap plate <NUM> to direct cooling flow against the upstream surface of cap plate <NUM>.

In <FIG>, a center fuel nozzle <NUM> is surrounded by a plurality (in this case, five) outer fuel nozzles <NUM>. Each outer fuel nozzle <NUM> has a truncated wedge shape, such that outer fuel nozzles <NUM> may be positioned in close proximity to center fuel nozzle <NUM> and cover a majority of the head end area. The truncated wedge shape may be defined as having a pair of radial sides <NUM> that extend in opposite directions and that are joined by a first (radially inner) arcuate side <NUM> and a second (radially outer) arcuate side <NUM>. Radially outer sides <NUM> define a radially outer perimeter of fuel nozzles <NUM> and, collectively, of cap assembly <NUM>. Each fuel nozzle <NUM> has a respective centerline <NUM> radially outward of centerline <NUM> of center fuel nozzle <NUM> and combustion section <NUM>. In this illustrative configuration, each fuel nozzle <NUM>, <NUM> may have its own respective nozzle face <NUM> in a shape corresponding to the shape of outer fuel nozzle <NUM> (wedge) or <NUM> (round). Alternatively, tubes <NUM> that are part of each respective fuel nozzle <NUM>, <NUM> may extend through a common cap plate (not shown). In this configuration, outer fuel nozzles <NUM> have respective fuel plenums defining a wedge shape, and center fuel nozzle <NUM> has a fuel plenum defining a round shape. The upstream ends of mixing tubes <NUM> of each fuel nozzle <NUM>, <NUM> extend through a respective fuel plenum for each fuel nozzle <NUM>, <NUM>. It should be noted that the specific size, spacing, and number of mixing tubes <NUM> shown in the Figures (including <FIG> and <FIG>) is intended to be representative of the present bundled tube fuel nozzles <NUM>, <NUM>, <NUM>, <NUM> and should not be construed as limiting the present bundled tube fuel nozzles as having tubes of any particular size, spacing, or number. Moreover, it should be not construed as limiting the present bundled tube fuel nozzles as having tubes with a single tube diameter.

In <FIG> and <FIG>, fuel nozzles <NUM>, <NUM>, <NUM>, <NUM> are each denoted with an alphanumerical "PM" indicator, e.g., PM1, PM2 or PM3. As will be described elsewhere herein, these indicators are used to indicate which fuel nozzles are activated or operational, i.e., burning fuel, during a particular 'combustion mode. ' While primary combustion stage <NUM> is shown as including six fuel nozzles <NUM>, <NUM>, <NUM>, <NUM> (<FIG>), and secondary combustion stage <NUM> is shown as including four fuel nozzles <NUM> (<FIG>), it is emphasized that the teachings of the invention are not limited to stages with any particular number of fuel nozzles. Further, while certain types of fuel nozzles <NUM>, <NUM> have been described herein, it is emphasized that a wide variety of fuel delivery elements can be employed.

As understood in the art, a plurality of sensors <NUM> detect various operating conditions of GT system <NUM>, and/or the ambient environment during operation of the system. In many instances, multiple redundant control sensors may measure the same operating condition. For example, groups of redundant temperature control sensors <NUM> may monitor ambient temperature, compressor discharge temperature, turbomachine exhaust gas temperature, and/or other operating temperatures of combustion gas flow (not shown) through GT system <NUM>. Similarly, groups of other redundant pressure control sensors <NUM> may monitor ambient pressure, static and dynamic pressure levels at compressor <NUM>, GT system <NUM> exhaust, and/or other parameters in GT system <NUM>. Control sensors <NUM> may include, without limitation, flow sensors, pressure sensors, speed sensors, flame detector sensors, valve position sensors, guide vane angle sensors, and/or any other device that may be used to sense various operating parameters during operation of GT system <NUM>.

It is further recognized that while some parameters are measured, i.e., are sensed and are directly known, other parameters are calculated by a model and are thus estimated and indirectly known. Some parameters may also be initially input by a user <NUM> (<FIG>) to GT control system <NUM>. In terms of the modeled parameters and applicable to the present invention, a primary combustion stage exit temperature (PCSET) of a combustion gas flow <NUM> (i.e., located conceptually between primary combustion stage <NUM> and secondary combustion stage <NUM> at location <NUM> in <FIG>) may be calculated using a model of GT system <NUM>. Similarly, a combustor exit temperature, i.e., conceptually at the end of transition piece <NUM> at location <NUM> in <FIG>, may be calculated using a model of GT system <NUM>. Historically, measured cycle parameters including but not limited to compressor pressure ratio (CPR), compressor discharge temperature (TCD), exhaust temperature (Tx), and GT output power have been used as modeling inputs to predict combustion exit temperatures from combustion section <NUM>. In accordance with embodiments of the invention, PCSET of a combustion gas flow will be controlled within a combustion mode(s) during loading and/or unloading of GT system <NUM> to control emissions.

A load/unload control system <NUM> regulates an amount of fuel flow from a fuel supply(ies) (not shown) to combustion section <NUM>, and in particular, to each of fuel nozzles <NUM> (<FIG>)(<NUM>, <NUM>, <NUM>, <NUM> (<FIG>), hereinafter simply "fuel nozzles <NUM>" for brevity) and/or fuel nozzles <NUM> (<FIG>) by controlling fuel valves <NUM>. Each fuel valve <NUM> is not limited to a single type of valve and may include a variety of types including but not limited to fuel control valves and fuel stop valves. Load/unload control system <NUM> can also control an amount of fuel split between primary combustion stage fuel nozzles <NUM> and secondary combustion stage fuel nozzles <NUM>, and an amount mixed with air flowing into combustion section <NUM>. Load/unload control system <NUM> may also control a split of fuel between combustion stages <NUM> and <NUM>. Although not always applicable, load/unload control system <NUM> may also select a type of fuel for use in combustion section <NUM>. Load/unload control system <NUM> may be a separate unit or may be a component of an overall GT control system <NUM> (<FIG>), e.g., as part of GT controller <NUM> (<FIG>). Load/unload control system <NUM> can implement a method according to embodiments of the invention, as will be described further herein.

<FIG> shows an illustrative environment demonstrating load/unload control system <NUM> coupled with GT system <NUM> (<FIG>) via at least one computing device <NUM>. Load/unload control system <NUM> may be a computer system (computing device <NUM>, <FIG>) that includes at least one processor (processing component <NUM>, <FIG>) and at least one memory device (storage component <NUM>, <FIG>) that executes operations to control the operation of GT system <NUM> based at least partially on models, sensor inputs, calculations and/or on instructions from human operators. Load/unload control system <NUM> may include, for example, a model of GT system <NUM>. Operations executed by load/unload control system <NUM> may include sensing or modeling operating parameters, modeling operational boundaries, applying operational boundary models, or applying scheduling algorithms that control operation of GT system <NUM>, such as by regulating a fuel flow to combustion section <NUM>. Load/unload control system <NUM> compares operating parameters of GT system <NUM> to operational boundary models, or scheduling algorithms used by load/unload control system <NUM> to generate control outputs, such as, without limitation, control instructions for valves <NUM> controlling activation of fuel nozzles <NUM> (<FIG>) and/or fuel nozzles <NUM> (<FIG>) based on PCSET of a combustion gas flow of primary combustion stage <NUM>. For example, the model may accept measured parameters as inputs such as compressor air discharge temperature (TCD), fuel flow(s), exhaust temperature (Tx), GT system output power, etc., and calculate PCSET of the combustion gas flow after primary combustion stage <NUM> along with other parameters used in the control such as but not limited to: firing temperature (Tfire) and combustor exit temperature (at location <NUM> (<FIG>)). Commands generated by load/unload control system <NUM> may cause valve(s) <NUM> on GT system <NUM> to selectively regulate fuel flow, fuel splits, and/or a type of fuel channeled between the fuel supply(ies) and combustors <NUM>. Other commands may be generated to cause actuators to adjust a relative position of stages of IGVs <NUM> (<FIG>) or activate other control settings on GT system <NUM>.

<FIG> shows a flow diagram illustrating a load/unload method for GT system <NUM>, which can be performed by a computing device such as load/unload control system <NUM>. More specifically, the <FIG> example shows a loading setting such as a start up of GT system <NUM>. <FIG> shows an emission compliant start up (loading) load path graph plotting exhaust temperature (Tx) versus load from FSNL (<NUM>% load) up to <NUM>%, according to embodiments of the invention. <FIG> also includes a schematic rendition of a carbon monoxide (CO) amount in the GT exhaust in a quasi-steady state with the load path.

With reference to <FIG> and <FIG>, a load/unload method can be performed (e.g., executed) using at least one computing device <NUM>, implemented as a computer program product (e.g., a non-transitory computer program product) for load/unload control system <NUM>. Generally, the process includes progressing through each of a plurality of progressive combustion modes that sequentially activate a different number of at least one of the first or second plurality of fuel nozzles, while controlling PCSET of a combustion gas flow to be constant or within a predefined target range corresponding to the respective combustion mode. For example, for loading, the process includes progressing through each of a plurality of progressive combustion modes that sequentially activate a higher number of at least one of the first or second plurality of fuel nozzles, while controlling a PCSET of a combustion gas flow of the primary combustion stage <NUM> to be within a predefined target range corresponding to the respective combustion mode. As used herein, a "combustion mode" is a state in which a certain number of fuel nozzles in primary combustion stage <NUM>, or primary combustion stage <NUM> and secondary combustion stage <NUM> are activated or operational, i.e., they are burning fuel. Each combustion mode may also include a number of other operational parameter settings for GT system <NUM> such as but not limited to: IGV stage <NUM> (<FIG>) positioning to control air flow to combustion system <NUM>.

As shown in <FIG>, in process P1, an initial start up process may be performed. The initial start up may include processing according to embodiments of the invention in which PCSET of a combustion gas flow is maintained within a predefined target range corresponding to the respective combustion mode, or process P1 can be conventional, i.e., with no PCSET control. For example, a start up may begin with the rotor being rotated by a motor at a speed to prevent bowing of the rotor (i.e., turning gear speed) and then increasing to a speed that allows compressor <NUM> (<FIG>) to begin flowing air to combustion section <NUM> (i.e., purge speed). The speed may then be reduced at which point fuel flow is initiated to combustion section <NUM>, i.e., load/unload control system <NUM> initiates fuel flow via valve(s) <NUM>, and fuel combustion begins. At this point, GT system <NUM> may go through a number of `combustion modes' in which a typically increasing number of fuel nozzles <NUM>, <NUM>, <NUM>, <NUM> (<FIG>) of primary combustion stage <NUM> are activated. In the example shown, combustion modes <NUM>, <NUM> and <NUM> are implemented apart from the teachings of the invention. For example, with reference to <FIG> or <FIG>: a combustion mode <NUM> may activate fuel nozzle denoted PM1; a combustion mode <NUM> may activate fuel nozzles denoted PM2; and a combustion mode <NUM> may activate fuel nozzles denoted PM1 and PM2 (center and <NUM> outer) in primary combustion stage <NUM>. No secondary combustion stage fuel nozzles <NUM> are activated in secondary combustion stage <NUM> at this time. It is emphasized that process P1, as described, is only illustrative and any now known or later developed initial start up process can be employed prior to implementing the teachings of the invention.

In process P2, each of a plurality of progressive combustion modes that sequentially activate a different number of at least one of the first or second plurality of fuel nozzles are progressed through, while also controlling PCSET to be within a predefined target range corresponding to the respective combustion mode. For loading, as shown in <FIG>, each of the plurality of progressive combustion modes sequentially activate a higher number of at least one of the first or second plurality of fuel nozzles. Throughout process P2, fuel flow and load output of GT system <NUM> are increasing.

<FIG> shows a load path <NUM> (darker, solid line) according to embodiments of the invention that illustrates the progressive combustion modes and controlling PCSET of a combustion gas flow. As will be described, some of the progressive combustion modes may have a different predefined target range for PCSET, e.g., a predefined target range for PCSET for combustion mode M6 is higher than combustion mode <NUM>, and predefined target range for PCSET for combustion mode <NUM> is higher than combustion mode <NUM>. However, a predefined target range for other combustion modes, e.g., modes <NUM>, 6A2 and 6A, as will be described, may be the same. Certain combustion modes may have a larger or smaller predefined target range than other combustion modes. It will be appreciated that the predefined target range for PCSET for each combustion mode is influenced by a number of factors such as but not limited to: GT system size, location/environment; government regulations; fuel used; or other settings. Consequently, exact statements of value for each predefined target range may vary widely. "Predefined" as applied to the predefined target range simply indicates that load/unload control system <NUM> calculates the acceptable range prior to implementation thereof, e.g., based on the above noted factors and the particular combustion mode. <FIG> also shows conventional load path <NUM> (shown in dashed line) for comparison purposes. In <FIG>, an illustrative next combustion mode upon which teachings of the invention may be applied, i.e., after process P1, may be combustion mode <NUM>. It is noted that another combustion mode could also be the point in which the teachings of the invention are applied, e.g., combustion mode <NUM> or <NUM>.

In process P2A, load/unload control system <NUM> can determine whether to progress to a next combustion mode in a number of ways. In one embodiment, progressing to a next successive combustion mode of the plurality of combustion modes occurs in response to one of the following: in process P2A1, a compressor pressure ratio of compressor <NUM> exceeds a respective threshold for a current combustion mode; or, in process P2A2, PCSET exceeds a respective maximum threshold for the current combustion mode. Here, each combustion mode may have a pre-assigned compressor pressure ratio (CPR) threshold, and a maximum PCSET threshold. When either threshold is exceeded, i.e., directly exceeded or within an unacceptable range, load/unload control system <NUM> activates more fuel nozzles to move GT system <NUM> along load path <NUM> to the next combustion mode, if more combustion modes exist. At process P2B, load/unload control system <NUM> determines whether any additional combustion modes exist. If there are additional combustion modes, i.e., Yes at process P2B, processing proceeds to process P2C. If not, i.e., No at process P2B, processing proceeds to process P3, described elsewhere herein. In the current pass, additional combustion modes exist, and processing proceeds to process P2C.

At process P2C in <FIG>, load/unload control system <NUM> may implement a next combustion mode (e.g., combustion mode <NUM>) by activating fuel nozzles denoted PM1 and PM3 (center and <NUM> outer)(<FIG>) in primary combustion stage <NUM>. Line <NUM> in <FIG> reflects a constant PCSET value as well as the combustion mode <NUM> PCSET control target. The shaded band shows a predefined target range of PCSET that is emissions compliant for combustion mode <NUM>, and in which PCSET may vary. As noted, an amount PCSET may vary will depend on a number of factors such as but not limited to: GT system size, location/environment; government regulations; fuel used; or other settings. PCSET may be maintained during combustion mode <NUM>, at process P2C in <FIG>, by load/unload control system <NUM> in any now known or later developed fashion. For example, load/unload control system <NUM> may control at least one of: a fuel flow rate (e.g., via valves <NUM>) of each fuel nozzle activated during a respective combustion mode; or a position of at least one stage of IGVs <NUM> (<FIG>) that control an air flow volume to combustion section <NUM> from compressor <NUM>. In the example shown in <FIG>, the position of stages of IGVs <NUM> is used to regulate PCSET for operation in combustion mode <NUM>. Maintaining PCSET within a predefined target range suitable for combustion mode <NUM> results in better emissions control during the loading process shown, see fairly consistent, quasi-steady state CO emissions in <FIG>.

Subsequently, processing returns to process P2A (after process P2C), where load/unload control system <NUM> again determines whether to progress to a next combustion mode. For example, in response to one of the following: in process P2A1, a compressor pressure ratio of compressor <NUM> exceeding a respective threshold for a current combustion mode; or, in process P2A2, a PCSET may exceed a respective maximum threshold for the current combustion mode. If one of the thresholds is exceeded, processing progresses to process P2B. At process P2B, load/unload control system <NUM> determines whether any additional combustion modes exist. If there are additional combustion modes, i.e., Yes at process P2B, processing proceeds to process P2C. If not, i.e., No at process P2B, processing proceeds to process P3, described elsewhere herein.

Returning to <FIG>, point <NUM> indicates when combustion mode <NUM> is implemented at process P2C of <FIG> by load/unload control system <NUM>. Load/unload control system <NUM> may activate more fuel nozzles in primary combustion stage <NUM> such as fuel nozzles denoted PM2 and PM3 (<NUM> outer) (<FIG> and <FIG>) to implement combustion mode <NUM>. Additionally, PCSET target used by load/unload control system <NUM> is updated to an appropriate level for combustion mode <NUM>. In the example shown in <FIG>, the PCSET predefined target range, and thus exhaust temperature Tx, increases to a higher level, shown by line <NUM>. The shaded band shows a range of PCSET that is emissions compliant for combustion mode <NUM>, and in which PCSET may vary based on the aforedescribed factors. PCSET may be maintained during combustion mode <NUM>, at process P2C in <FIG>, by load/unload control system <NUM> in any now known or later developed fashion, e.g., by controlling at least one of: a fuel flow rate of each fuel nozzle activated during a respective combustion mode, or a position of at least one stage of IGVs <NUM> (<FIG>) that control an air flow volume to combustion section <NUM> from compressor <NUM>. In the example shown in <FIG>, the position of the IGVs <NUM> may be used to regulate PCSET for operation in combustion mode <NUM>. Maintaining PCSET within a predefined target range suitable for combustion mode <NUM> results in better emissions control during the loading process shown, as illustrated by the quasi-steady state CO emissions line.

Returning to <FIG>, in process P2A (after process P2C), load/unload control system <NUM> can determine whether to progress to a next combustion mode, assuming one exists (process P2B).

Returning to <FIG>, point <NUM> indicates when combustion mode <NUM> is implemented at process P2C of <FIG> by load/unload control system <NUM>. Load/unload control system <NUM> may again activate more fuel nozzles in primary combustion stage <NUM> such as fuel nozzles denoted PM1, PM2 and PM3 (<FIG>)(all primary stage fuel nozzles) to implement combustion mode <NUM>. Additionally, PCSET target used by load/unload control system <NUM> is updated to an appropriate level for combustion mode <NUM>. In the example shown in <FIG>, PCSET target, and thus exhaust temperature Tx, increase to a higher level. Line <NUM> reflects a constant PCSET value as well as the combustion mode <NUM> PCSET control target. The shaded band shows a predefined target range for combustion mode <NUM> in which PCSET may vary. Again, PCSET may be maintained during combustion mode <NUM>, at process P2C in <FIG>, by load/unload control system <NUM>, e.g., by controlling at least one of: a fuel flow rate of each fuel nozzle activated during a respective combustion mode; and a position of at least one stage of IGVs <NUM> (<FIG>) that control an air flow volume to combustion section <NUM> from compressor <NUM>. In the example shown in <FIG>, the position of stage(s) of IGVs <NUM> is used to regulate PCSET for operation in mode <NUM>. Maintaining PCSET within a predefined target range suitable for combustion mode <NUM> results in better emissions control during the loading process shown.

The above-identified process can repeat as before with the notable exception now that once combustion mode <NUM> (all fuel nozzles in primary combustion stage <NUM> are active), load/unload control system <NUM> may start to activate fuel nozzles <NUM> in secondary combustion stage <NUM>. For example, a seventh combustion mode, referred to as 6A2 in <FIG>, may have load/unload control system <NUM> activate fuel nozzles denoted PM1, PM2 and PM3 (<FIG>) in primary combustion stage <NUM> (all primary stage fuel nozzles) and also at least one of the second plurality of fuel nozzles <NUM> (<FIG>) of secondary combustion stage <NUM>. In one example, half of fuel nozzles <NUM> may be activated, e.g., <NUM> of <NUM>. The transition may occur, after process P2A (<FIG>), i.e., in response to load/unload control system <NUM> determining whether to progress to a next combustion mode, assuming one exists. The transition point is indicated with numeral <NUM> in <FIG>. Again, PCSET may be maintained during combustion mode 6A2, at process P2C in <FIG>, by load/unload control system <NUM> - see line <NUM>. During any combustion mode in which all of the first plurality of fuel nozzles <NUM> (<FIG>) of primary combustion stage <NUM> and the at least one of the second plurality of fuel nozzles <NUM> (<FIG>) of secondary combustion stage <NUM> are active, load/unload control system <NUM> may also modify a split of fuel flow between the primary and secondary combustion stages <NUM>, <NUM> to decrease a fuel flow to fuel nozzles <NUM> (<FIG>) of primary combustion stage <NUM> and increase the fuel flow to fuel nozzle(s) <NUM> (<FIG>) of secondary combustion stage <NUM>. This modification on the split of fuel flow may be used to maintain PCSET of the combustion gas flow of primary combustion stage <NUM> substantially constant, i.e., within a predefined target range. That is, the fuel split, rather than IGV position as in previous combustion modes, may be used to regulate PCSET. This allows the position of the IGVs <NUM> to be used for other control applications, in this example maintaining a constant, sub-isotherm, exhaust temperature. While no shading is shown, it is understood, some form of a predefined target range is acceptable for combustion modes 6A2 (and subsequent mode 6A). Again, maintaining PCSET within a predefined target range suitable for combustion mode 6A2 results in better emissions control during the loading process shown.

An eighth and final combustion mode, referred to as 6A in <FIG>, may have load/unload control system <NUM> activate fuel nozzles denoted PM1, PM2 and PM3 (<FIG>) in primary combustion stage <NUM> (all primary combustion stage fuel nozzles) and also all of fuel nozzles <NUM> of secondary combustion stage <NUM>. The transition may occur, after process P2A (<FIG>), i.e., in response to load/unload control system <NUM> determining whether to progress to a next combustion mode. The transition point is indicated with numeral <NUM> in <FIG>. Again, PCSET may be maintained during combustion mode 6A, at process P2C in <FIG>, by load/unload control system <NUM> - see line <NUM>. In the example shown in <FIG>, fuel split between primary and secondary combustion stages <NUM>, <NUM> may be used to regulate PCSET. Simultaneously, the position of stage(s) of IGVs <NUM> are used to keep the exhaust temperature at or below the maximum limit.

As illustrated by <FIG> and <FIG>, for loading by load/unload control system <NUM>, each successive combustion mode of the plurality of progressive combustion modes initially activates a higher number of just first plurality of fuel nozzles <NUM>, <NUM>, <NUM>, <NUM> (<FIG>) of the primary combustion stage <NUM> than a preceding combustion mode. This is the case for combustion modes <NUM>-<NUM>, as described, which provide a first set of progressive combustion modes. Subsequently, each successive combustion mode of the plurality of progressive combustion modes may then activate all of the first plurality of fuel nozzles <NUM> (<FIG>) of primary combustion stage <NUM> and more of the second plurality of fuel nozzles <NUM> (<FIG>) of secondary combustion stage <NUM> than a preceding combustion mode. This is the case for combustion modes 6A2 and 6A, as described, which create a second set of progressive combustion modes that follow the first set of progressive combustion modes.

It is emphasized that the combustion modes described herein are only illustrative and that other sequences of combustion modes than described may be employed. For example, successive combustion modes can activate more than one additional fuel nozzle. Furthermore, the first set of progressive combustion modes may use any combination of at least two successive combustion modes selected from: a first mode in which a first number of fuel nozzles of the first plurality of fuel nozzles of the primary combustion stage is activated; a second mode in which a second number of the first plurality of fuel nozzles of the primary combustion stage are activated (second number higher than the first number); a third mode in which a third number of the first plurality of fuel nozzles of the primary combustion stage are activated (third number higher than the first and second numbers); a fourth mode in which a fourth number of the first plurality of fuel nozzles of the primary combustion stage are activated (fourth number higher than the first, second and third numbers); a fifth mode in which a fifth number of the first plurality of fuel nozzles of the primary combustion stage are activated (fifth number higher than the first, second, third and fourth numbers); and a sixth mode in which a full number of the first plurality of fuel nozzles of the primary combustion stage are activated (full number is higher than the first, second, third, fourth and fifth numbers). As noted, during the first set of progressive combustion modes, the second plurality of fuel nozzles <NUM> of the secondary combustion stage <NUM> are inactive. As noted, the second set of progressive combustion modes may include a seventh mode in which all of the first plurality of fuel nozzles of the primary combustion stage are activated and a partial number of the second plurality of fuel nozzles of the secondary combustion stage are activated (partial number is less than all of the second plurality of fuel nozzles of the secondary combustion stage); and an eighth mode in which all of the first plurality of fuel nozzles of the primary combustion stage are activated and all of the second plurality fuel nozzles of the secondary combustion stage are activated. In an alternative embodiment, at least one secondary combustion stage <NUM> fuel nozzle <NUM> may be activated prior to all of primary combustion zone <NUM> fuel nozzles <NUM> being fully activated.

Returning to <FIG> and <FIG>, in process P3, load/unload control system <NUM> (or GT controller <NUM>) (<FIG>) may proceed with applying further load to GT system <NUM> in a conventional fashion. At process P3, as load increases and stages of IGVs <NUM> open, exhaust temperature Tx will naturally begin to decrease from the isotherm limit. Throughout this remainder of unit loading, i.e., until base load operation is attained, limits that govern the trajectory of the load path may include but are not limited to combustor exit temperature limits (at location <NUM>, <FIG>), exhaust temperature (Tx) targets, firing temperature targets, limits on temperature rise across combustion section <NUM>, and/or baseload control settings.

The above-described process is also applicable in an unloading process for GT system <NUM>. In this case, as load/unload control system <NUM> progresses through each of a plurality of progressive combustion modes, it sequentially activates a lower number of at least one of the first or second plurality of fuel nozzles. <FIG> shows an emission compliant unload path graph plotting exhaust temperature (Tx) versus load down from <NUM>%, and with a schematic rendition of a carbon monoxide (CO) amount in the GT exhaust in a quasi-steady state with the load path, according to embodiments of the invention. <FIG> is generally the inverse of <FIG>. During unloading, load/unload control system <NUM> progresses through the plurality of progressive combustion modes including: a first set of progressive combustion modes in which all of the first plurality of fuel nozzles <NUM> (<FIG>) of primary combustion stage <NUM> are activated and in which each successive combustion mode activates a lower number of second plurality of fuel nozzles <NUM> (<FIG>) of secondary combustion stage <NUM> than a preceding combustion mode of the first set of progressive combustion modes. Further, load/unload control system <NUM> implements a second set of progressive combustion modes, following the first set of progressive combustion modes, during which all of second plurality of fuel nozzles <NUM> (<FIG>) of the secondary combustion stage <NUM> are de-activated and each successive combustion mode activates a lower number of first plurality of fuel nozzles <NUM> (<FIG>) of primary combustion stage <NUM> than a preceding combustion mode of the second set of progressive combustion modes. During each combustion mode, as shown in <FIG>, PCSET may be maintained by load/unload control system <NUM> in any now known or later developed fashion. For example, load/unload control system <NUM> may control at least one of: a fuel flow rate of each fuel nozzle activated during a respective combustion mode; a fuel split between stages <NUM>, <NUM>; or a position of at least one stage of IGVs <NUM> (<FIG>) that control an air flow volume to combustion section <NUM> from compressor <NUM>. Similar to <FIG>, each combustion mode has a PCSET that is maintained by load/unload control system <NUM>. In one embodiment, progressing to a next successive combustion mode of the plurality of combustion modes occurs in response to one of the following: a compressor pressure ratio of compressor <NUM> receding below a respective threshold for a current combustion mode. Here, each combustion mode may have a pre-assigned compressor pressure ratio (Pcd) threshold. When the compressor pressure ratio recedes below the threshold, load/unload control system <NUM> activates less fuel nozzles to move GT system <NUM> along load path <NUM> shown in <FIG> to the next combustion mode, if more combustion modes exist. In an alternative embodiment, certain combustion mode(s) may temporarily activate more fuel nozzles during the unloading.

As described herein and shown in <FIG>, GT control system <NUM> (including load/unload control system <NUM>) can include any conventional control system components used in controlling a GT system <NUM>. For example, GT control system <NUM> can include electrical and/or electromechanical components for actuating one or more components in the GT system <NUM>. Control system <NUM> can include conventional computerized sub-components such as a processor, memory, input/output, bus, etc. GT control system <NUM> can be configured (e.g., programmed) to perform functions based upon operating conditions from an external source (e.g., at least one computing device <NUM>), and/or may include pre-programmed (encoded) instructions based upon parameters of GT system <NUM>.

As noted herein, GT control system <NUM> can also include at least one computing device <NUM> connected (e.g., hard-wired and/or wirelessly) with GT controller <NUM>, load/unload control system <NUM>, and other parts of GT system <NUM> such as valves <NUM>. In various embodiments, computing device <NUM> is operably connected with valves <NUM> and other parts of GT system <NUM>, e.g., via a plurality of conventional sensors such as flow meters, temperature sensors, etc., as described herein. Computing device <NUM> can be communicatively connected with GT controller <NUM>, e.g., via conventional hard-wired and/or wireless means. GT control system <NUM> is configured to monitor GT system <NUM> during operation according to various embodiments.

Further, computing device <NUM> is shown in communication with a user <NUM>. A user <NUM> may be, for example, a programmer or operator. Interactions between these components and computing device <NUM> are discussed elsewhere in this application.

As noted herein, one or more of the processes described herein can be performed, e.g., by at least one computing device, such as computing device <NUM>, as described herein. In other cases, one or more of these processes can be performed according to a computer-implemented method. In still other embodiments, one or more of these processes can be performed by executing computer program code (e.g., load/unload control system <NUM>) on at least one computing device (e.g., computing device <NUM>), causing the at least one computing device to perform a process, e.g., progressing through combustion modes according to approaches described herein.

In further detail, computing device <NUM> is shown including a processing component <NUM> (e.g., one or more processors), a storage component <NUM> (e.g., a storage hierarchy), an input/output (I/O) component <NUM> (e.g., one or more I/O interfaces and/or devices), and a communications pathway <NUM>. In one embodiment, processing component <NUM> executes program code, such as load/unload control system <NUM>, which is at least partially embodied in storage component <NUM>. While executing program code, processing component <NUM> can process data, which can result in reading and/or writing the data to/from storage component <NUM> and/or I/O component <NUM> for further processing. Pathway <NUM> provides a communications link between each of the components in computing device <NUM>. I/O component <NUM> can comprise one or more human I/O devices or storage devices, which enable user <NUM> to interact with computing device <NUM> and/or one or more communications devices to enable user <NUM> and/or other GT component(s) <NUM> to communicate with computing device <NUM> using any type of communications link. To this extent, GT control system <NUM> can manage a set of interfaces (e.g., graphical user interface(s), application program interface, and/or the like) that enable human and/or system interaction with control system <NUM>.

In any event, computing device <NUM> can comprise one or more general purpose computing articles of manufacture (e.g., computing devices) capable of executing program code installed thereon. As used herein, it is understood that "program code" means any collection of instructions, in any language, code or notation, that cause a computing device having an information processing capability to perform a particular function either directly or after any combination of the following: (a) conversion to another language, code or notation; (b) reproduction in a different material form; and/or (c) decompression. To this extent, GT control system <NUM> (and load/unload control system <NUM>) can be embodied as any combination of system software and/or application software. In any event, the technical effect of computing device <NUM> is to progress through combustion modes during a load/unload of GT system <NUM> according to various embodiments herein.

Further, GT control system <NUM> (and load/unload control system <NUM>) can be implemented using a set of modules <NUM>. In this case, a module <NUM> can enable computing device <NUM> to perform a set of tasks used by GT control system <NUM>, and can be separately developed and/or implemented apart from other portions of GT control system <NUM>. GT control system <NUM> may include modules <NUM> which comprise a specific use for machine/hardware and/or software. Regardless, it is understood that two or more modules, and/or systems may share some/all of their respective hardware and/or software. Further, it is understood that some of the functionality discussed herein may not be implemented or additional functionality may be included as part of computing device <NUM>.

When computing device <NUM> comprises multiple computing devices, each computing device may have only a portion of GT control system <NUM> (and/or load/unload control system <NUM>) embodied thereon (e.g., one or more modules <NUM>). However, it is understood that computing device <NUM> and GT control system <NUM> are only representative of various possible equivalent computer systems that may perform a process described herein. To this extent, in other embodiments, the functionality provided by computing device <NUM> and GT control system <NUM> can be at least partially implemented by one or more computing devices that include any combination of general and/or specific purpose hardware with or without program code. In each embodiment, the hardware and program code, if included, can be created using standard engineering and programming techniques, respectively.

Regardless, when computing device <NUM> includes multiple computing devices, the computing devices can communicate over any type of communications link. Further, while performing a process described herein, computing device <NUM> can communicate with one or more other computer systems using any type of communications link. In either case, the communications link can comprise any combination of various types of wired and/or wireless links; comprise any combination of one or more types of networks; and/or utilize any combination of various types of transmission techniques and protocols.

As discussed herein, GT control system <NUM> (and load/unload control system <NUM>) enables computing device <NUM> to control and/or monitor combustion section <NUM>. GT control system <NUM> may include logic for performing one or more actions described herein. In one embodiment, GT control system <NUM> may include logic to perform the above-stated functions. Structurally, the logic may take any of a variety of forms such as a field programmable gate array (FPGA), a microprocessor, a digital signal processor, an application specific integrated circuit (ASIC) or any other specific use machine structure capable of carrying out the functions described herein. Logic may take any of a variety of forms, such as software and/or hardware. However, for illustrative purposes, GT control system <NUM> (and load/unload control system <NUM>) and logic included therein will be described herein as a specific use machine. As will be understood from the description, while logic is illustrated as including each of the above-stated functions, not all of the functions are necessary according to the teachings of the invention as recited in the appended claims.

In various embodiments, GT control system <NUM> may be configured to monitor operating parameters of combustion section <NUM>, i.e., each combustor <NUM> therein, as described herein. Additionally, GT control system <NUM> is configured to control combustion section <NUM>, according to various functions described herein.

It is understood that in the flow diagram shown and described herein, other processes may be performed while not being shown, and the order of processes can be rearranged. Additionally, intermediate processes may be performed between one or more described processes. The flow of processes shown and described herein is not to be construed as limiting of the various embodiments defined by the claims.

The technical effect of the various embodiments of the invention, including, e.g., the GT control system <NUM> and load/unload control system <NUM>, is to run a load/unload method for GT system <NUM>, as described herein. The teachings of the invention can be applied to any GT system <NUM> with two combustion stages to significantly drop emissions at low load. During loading, for example, teachings of the invention minimize CO emissions during startup to achieve emissions compliance down to FSNL.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention as defined by the claims.

Accordingly, a value modified by a term or terms, such as "about," "approximately" and "substantially," are not to be limited to the precise value specified. "Approximately" as applied to a particular value of a range applies to both values, and unless otherwise dependent on the precision of the instrument measuring the value, may indicate +/- <NUM>% of the stated value(s).

Claim 1:
A loading/unloading method for a gas turbine (GT) system (<NUM>), the gas turbine system (<NUM>) including a compressor (<NUM>, <NUM>) feeding air to a combustion section (<NUM>) that is coupled to a turbine (<NUM>), the combustion system (<NUM>) including a plurality of combustors (<NUM>, <NUM>), each combustor (<NUM>, <NUM>) including a primary combustion stage (<NUM>) including a first plurality of fuel nozzles (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) and a secondary combustion stage (<NUM>) downstream from the primary combustion stage (<NUM>), the secondary combustion stage (<NUM>) including a second plurality of fuel nozzles (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>), the method comprising during unloading or loading of the GT system (<NUM>):
progressing through each of a plurality of progressive combustion modes that sequentially activate a higher number of the second plurality of fuel nozzles (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) than a preceding combustion mode during loading and that sequentially activate a lower number of the second plurality of fuel nozzles (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) than a preceding combustion mode during unloading, and
during each combustion mode regardless of whether loading or unloading, controlling a primary combustion stage (<NUM>) exit temperature (PCSET) of a combustion gas flow to be within a predefined target range corresponding to the respective combustion mode.