Abstract:
In a gas turbine electric power generator where rotational speed of the gas turbine is synchronized to the electrical frequency of a power distribution grid and the gas turbine includes a compressor component, an air extraction path, and a control system for controlling an amount of compressor air extraction, a method is provided for controlling output power produced by a gas turbine. The method includes initiating compressor air extraction and controlling the amount of compressor air extraction.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates generally to a method for operating a gas turbine during select operating conditions such as under-frequency operation through extraction of air from the compressor. 
     Large increases in the electrical power consumptive demand placed upon an electrical power distribution grid will tend to reduce the electrical operational frequency of the grid, causing an “under-frequency” event. For example, a heavy or sudden electrical demand may cause a particular power distribution grid having a nominal operational frequency of 50 Hz to momentarily operate at 49 Hz. In conventional electrical power generation systems that utilize one or more heavy-duty industrial gas turbine for supplying electrical power to the grid, the physical speed of each turbine supplying power to the grid is synchronized to the electrical frequency of the grid. Unfortunately, as the physical speed of a gas turbine decreases with other things being equal, its power output correspondingly decreases. Consequently, during an under-frequency event, a gas turbine will tend to output a lower power. In the past, a common practice in response to a power grid under-frequency event (occurrence) is to increase the firing temperature of the gas turbine to produce more power in an effort to maintain a predetermined level of output power. Unfortunately, such over-firing of the gas turbine may reduce the operational life expectancy of various hot gas path components within the turbine. 
     Grid code regulations typically require that power production equipment have the capability to maintain load during under-frequency excursions. Various regions around the world have different requirements that must be satisfied in order for power equipment to be considered compliant. Typically, gas turbine generators meet these requirements by increasing firing temperature to maintain generator output within requirements. Increases in firing temperature increase power output at a given pressure ratio, which works adequately when the gas turbine does not approach any operating limits such as maximum pressure ratio capability or maximum inlet guide vane (IGV) position. A firing temperature increase is typically achieved by an increase the fuel flow supplied to the combustor. All things otherwise equal, the increase in fuel flow results in a higher pressure at the turbine inlet, which in turn applies backpressure on the compressor. Eventually, adding more flow results in a compressor pressure limit, which typically is observed by limiting the flow through the turbine through the diversion of compressor discharge air to inlet (inlet bleed heating) and/or reduction of fuel flow (and consequently firing temperature). However, this method has limited capability to meet grid code requirements for cool ambient conditions and/or low Btu fuels (e.g. syngas) applications, due to operability limits encountered by the gas turbine compressor. 
     Some conventional gas turbines, used for power generation, incorporate variable inlet guide vanes (IGV). Such variable stator vanes provide the ability to adjust compressor airflow by changing incidence angle (i.e., the difference between the air angle and the mean line angle at the compressor blade leading edge) in the front stages of the compressor. These variable IGVs permit an acceptable compressor surge-free operation margin to be maintained. Typically, maintaining surge-free operation is a vital operational criterion of the compressor component for gas turbines. 
     Wickert et al. (U.S. Pat. No. 6,794,766) provides a method for over-firing of gas turbines equipped with variable stator vanes (blades) to compensate for power output during under-frequency events. Wickert utilizes the variable stator vanes to increase the amount of airflow consumed by the compressor component in a predefined manner so to preclude and/or minimize a decrease in the level of output power generated during a grid under-frequency event and maintaining a safe margin during such an event. However, not all gas turbines are equipped with variable stator vanes to permit employing such a technique. Further, this action alone may not be sufficient if the maximum vane position is reached and a pressure ratio limit is encountered simultaneously while attempting to increase output. In this situation, other action must be taken to alleviate the pressure limit. 
     It would therefore be desirable to utilize an operational method, which would improve the power output during select operations and result in improved grid code compliance during under-frequency operation. 
     BRIEF DESCRIPTION OF THE INVENTION 
     Briefly, in accordance with one aspect of the present invention, in a gas turbine electric power generator where rotational speed of the gas turbine is synchronized to the electrical frequency of a power grid and the gas turbine includes a compressor component, an air extraction path, and means for controlling an amount of compressor air extraction, a method is provided for controlling output power produced by a gas turbine. The method includes initiating the compressor air extraction and controlling the amount of compressor air extraction. 
     In accordance with another aspect of the present invention, in a gas turbine electric power generator where rotational speed of the gas turbine is synchronized to the electrical frequency of a power grid and the gas turbine includes a compressor component, an air extraction path, and means for controlling an amount of compressor air extraction, a method is provided for controlling output power produced by a gas turbine. The method includes initiating compressor air extraction and controlling the amount of compressor air extraction during a power grid under-frequency condition through at least one of a discharge path to atmosphere, a discharge path to energy recovery equipment; reducing diluent flow to the combustor and raising the firing temperature. 
     In accordance with a further aspect of the present invention, the gas turbine electric power generator wherein a rotational speed of a gas turbine is synchronized to the electrical frequency of a power grid, a control system is provided that controls initiating compressor air extraction and controlling extracting compressor air to increase margin to compressor pressure ratio limits. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
         FIG. 1  illustrates a typical gas turbine generator set incorporating standard air, fuel, and combustion product flow. 
         FIG. 2  illustrates a gas turbine generator set with a plurality of elements that permit gas turbine operation during under-frequency operation through use of air extraction. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The previously described aspects of the present invention have many advantages, including using compressor air extraction to provide a simple and effective method of operating the gas turbine during under-frequency events. 
       FIG. 1  illustrates combined cycle gas turbine equipment  5 , including a compressor  50 , a combustor  52 , a gas turbine  54 , a heat recovery steam generator (HRSG)  56  and it associated steam turbine  58 . Air, under ambient conditions, enters the axial flow compressor  50  at air intake  10 . The compressed air  12  enters the combustor  52  where fuel is injected at  28  and combustion occurs. The combustion mixture  14  leaves the combustor and enters the gas turbine  54 . In the turbine section, energy of the hot gases is converted into work. This conversion takes place in two steps. The hot gases are expanded and the portion of the thermo-energy is converted into kinetic energy in the nozzle section of the gas turbine  54 . Then a portion of the kinetic energy is transferred to the rotating bucket of the bucket section of the gas turbine  54  and converted to work. A portion of the work developed by the gas turbine  54  is used to drive the compressor  50  whereas the remainder is available for generating electric power. The exhaust gas  16  leaves the gas turbine and flows to the HRSG  56 , providing energy to produce steam for driving steam turbine  58 . Electric power is generated from the gas turbine driven generator  60  and the steam turbine driven generator  62  and supplied to an electric power grid  64 . 
     The Brayton cycle is the thermodynamic cycle upon which gas turbines operate. Every Brayton cycle can be characterized by pressure ratio and firing temperature. The pressure ratio of the cycle is the compressor discharge pressure at  12  divided by the compressor inlet pressure at  10 . The firing temperature is defined as the mass flow mean total temperature at the stage 1 nozzle trailing edge plane. It is well known that an elevated firing temperature in the gas turbine is a key element in providing a higher output per unit mass flow and therefore a higher output power. The maximum pressure ratio that the compressor can deliver in continuous operation is commonly defined in terms of a margin from a surge pressure ratio line. Compressor surge is defined as a low frequency oscillation of flow where the flow separates from the blades and reverses flow direction 
       FIG. 2  shows different extraction points and discharge paths for air extraction on the combined cycle gas turbine equipment  5 , which may be used alone or in combination. In one aspect of the invention, extraction air would be taken from the compressor  50  outlet and/or combustor  52  at  20  and vented to atmosphere at  22  via discharge to atmosphere control valve  40 . Compressor air may be further extracted at  34  from the compressor upstream of the compressor outlet. Specific location points for extraction of air from the gas turbine depend on the particular device. For example, air extraction from the General Electric “E” Series gas turbines is typically from the outlet of the compressor while the air extraction point from the General Electric “F” Series gas turbines is typically from the combustor. In another aspect of the invention, extracted air may be discharged to air extraction energy recovery equipment  66  through discharge to energy recovery equipment control valve  42 . The air extraction energy recovery equipment  66  may include an air separation unit (ASU)  68  and other recovery equipment  76 . The ASU  68  separates N 2  and O 2  in the air. The O 2  may then be used in the production of syngas fuel for a gas turbine in a gasification process while N 2  may be used as a diluent or vented. Still another aspect of the invention provides extraction of compressor  50  outlet air through inlet bleed control valve  44  to the inlet side of the compressor  50  at  26 . 
     Air extraction alone will typically result in a decrease in power output, all other factors being equal, due to decreased mass flow rate input. However, simultaneously with the air extraction, additional fuel is supplied to the combustor  52 . at  28 . The reduction in compressor airflow through air extraction provides relief of the compressor pressure ratio limits typically encountered. Because compressor airflow extraction provides relief of the compressor pressure ratio limits, increased fuel flow can be accommodated within the compressor pressure ratio limits. The resulting gas turbine output power is increased while maintaining margin to the compressor pressure ratio. During under-frequency conditions, employing air extraction with increased firing will increase gas turbine output power to assist in meeting grid code requirements. 
     Yet another aspect of the present invention reduces diluents inflow  30  to the combustor  52 . Lower diluent flow to the combustor reduces the overall fuel/air flow rate. With a lower diluent flow rate, the margin to the compressor-pressure ratio limit is increased and more fuel may be added in its place to increase power. 
     In still a further aspect of the present invention, the combustor  52  may be co-fired with a richer alternative fuel at  32 , such as natural gas or distillate or blends with the richer alternative fuels, if a primary fuel is leaner as is typical of syngas and process fuels. Because the co-firing with the richer alternative fuel permits a higher power output with the same fuel flow rate, higher output power can be achieved with a lower overall fuel/air flow rate, thereby maintaining a margin to the compressor pressure ratio limit. 
     Individual elements described above for permitting a higher power output from the gas turbine may be used alone or in combination. 
     Efficient operation of the gas turbine requires that a number of critical turbine operating parameters be processed to determine optimal settings for controllable parameters such as fuel flow and intake air flow. Such operating parameters include compressor inlet and outlet temperatures and pressures, exhaust temperature and pressure and the like. One example of a control system or means for controlling a gas turbine is the General Electric Co.&#39;s Speedtronic™ Mark V Control System, which is designed to fulfill all gas turbine control, including speed and load control functions. Such a control system is described in Andrew et al. (U.S. Pat. No. 6,226,974). Andrew describes a controller that is coupled to receive input from a plurality of sources such as operations controls and a plurality of sensors coupled to the turbine and power output means. The controller is coupled to a system of turbine actuators that are used to maintain or establish a particular turbine operating regime. The actuators include, but are not limited to, an air flow control actuator and a fuel flow control actuator. 
     In an aspect of the present invention, a similar control system to Andrew et al. may be employed, with or without IGV control. The control system may also employ controls over one or a combination of control valves. Referring to  FIG. 2 , the control system  80  may control additional actuating controls, such as discharge to atmosphere control valve  40 , discharge to energy recovery equipment control valve  42  and inlet bleed control valve  44  that extract part of the air flowing from the discharge of the compressor for improving margin to compressor pressure ratio limits, thereby allowing increased firing for power control. The control system  80  initiates the compressor air extraction and controls the amount of compressor air extraction from discharge to atmosphere control valve  40 , discharge to energy recovery equipment control valve  42 , and inlet bleed valve  44 . Further, the control system  80  will further control fuel input to the combustor  70 , diluent control  72 , and alternate fuel control  74 . Because such sensing and actuating controls are well known in the art, they need not be described herein with respect to actuator controls for air extraction operation. 
     While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.