Abstract:
A method for determining an operating set point for a combined cycle power plant, the method includes: simulating the operation of the power plant; correcting the simulation of the operation of the power plant; optimizing the simulation of the operation by simulating the operation at different operating settings and selecting at least one of the operating settings as being optimal, and generating the operating set point based on the optimized simulation of the power plant.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to control systems for a combined cycle power plant and systems which optimize the performance of such power plants. 
         [0002]    Combined cycle power plants are typically used to generate electrical power delivered to a power grid. The power is distributed by the grid to consumers, such as residences, businesses and other consumers of electrical power. Combined cycle power plants are paid for the power delivered to the grid. 
         [0003]    The price paid for power delivered to the grid varies based on the demand for power. The price may be higher for power delivered during normal business hours than for power delivered in the middle of the night. Similarly, the price may be higher during summer and winter than during fall and spring. Typically the prices are set periodically, such as each day, week or month. The prices are given to the power plants. 
         [0004]    The operators of the plants use the prices to determine how much power to generate and deliver to the power grid. The operators also consider the cost to generate power by their power plants. The difference between the price for power and the cost to generate the power is a measure of the profitability of the power plant. The profitability of the power plant is an important indicator to the economic viability of the plant. 
         [0005]    Determining the cost to generate power involves several factors, such as the cost of fuel and ambient conditions, such as outside temperature and humidity. The operating state of the gas and steam turbines in the power plant are also factors influencing the cost to generate power. Other factors affecting the cost to generate power and the power output capacity of the plant include the costs introduced by power augmentation devices, such as a chiller to cool the air entering a gas turbine and a duct firing system to supplement the generation of steam for the steam turbines. The relationship between these factors is complex. It has long been difficult to operate a combined cycle power plant with power augmentation devices in a manner that minimizes the cost of generating power and maximizes the profitability of the plant. 
         [0006]    Similarly, the long term and short term variations in the demand for power are factors influencing the setting of the loads on the steam turbines and gas turbines in a combined cycle power plant. Steam turbines typically operate at a steady state to generate a constant amount of power. Gas turbines are well suited to produce varying amounts of power, such as for peak power during certain hours of a day. 
         [0007]    Due to the complexities in determining the cost to generate power, variations in the price of power and variations in fuel cost, there is a long felt need for systems and methods to assist operators in controlling combined cycle power plants and in determining optimal operational settings for the power plant. The optimal operational settings would be useful to increase the profitability of the power plant and increase the effectiveness of the power plant in meeting a need for power by a power grid. The complexities in determining the optimal operational settings are greatly expanded when power augmentation equipment, such as air chillers and duct firing devices, are added to the power plant. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    A method has been conceived for determining an operating set point for a combined cycle power plant, the method includes: simulating the operation of the power plant; correcting the simulation of the operation of the power plant; optimizing the simulation of the operation by simulating the operation at different operating settings and selecting at least one of the operating settings as being optimal, and generating the operating set point based on the optimized simulation of the power plant. 
         [0009]    In the method, the combined cycle power plant includes one or more gas turbines, one or more heat recovery steam generators (HRSG), one or more steam turbines, one or more chiller systems and one or more duct firing systems for the HRSG, wherein the simulation of the operation of the power plant includes digitally modeling each of the gas turbine, the steam turbine, the chiller system and the duct firing system and correlating the digital models using an artificial neural network. 
         [0010]    The method may further comprise adjusting the simulation of the operation of the power plant using a Kalman filter which receives and compares data regarding the actual operation of the power plant to data regarding the simulation, wherein the Kalman filter generates adjustments applied to the simulation. The optimization of the simulated operation may include determining a cost of operating the plant at each of the different operating settings. The correction of the simulation may occur during a first period, and the optimization occurs during a second period that does not overlap with the first period. 
         [0011]    A method has been conceived to determine an optimal operating setting for a combined cycle power plant including at least one each of: a gas turbine, a chiller system, a HRSG, a steam turbine and a duct firing device for the HRSG, the method comprising: modeling the thermodynamic performance of each of the gas turbine, the chiller system, the steam turbine and the duct firing system; simulating the thermodynamic performance of the power plant by correlating the modeling of the gas turbine, the chiller system, the steam turbine and the duct firing system; comparing the simulated thermodynamic performance to the actual thermodynamic performance of the power plant and determining a difference between the simulated thermodynamic performance and the actual thermodynamic performance; adjusting the modeling based on the comparison and reducing the difference; optimizing the simulation of the thermodynamic performance by simulating the thermodynamic performance of the power plant at various operating settings and selecting at least one of the operating settings as being optimal, and displaying the selected operating setting. The operator of the plant may adjust the plant settings based on the selected operational setting shown on the display. 
         [0012]    The method may include comparing the simulated thermodynamic performance to the actual thermodynamic performance of the power plant during a first period, and the optimization occurs during a second period that does not overlap with the first period. The modeling of the thermodynamic performance of the chiller system may include an algorithm correlating the power needed to drive the chiller system to a desired temperature of inlet air entering the gas turbine, wherein the algorithm accounts for desired temperatures above and below the dew point of the inlet air. The comparison and the adjustment may be performed by a Kalman filter. The optimization may include determining a cost of operating the power plant at each of the various operating settings. 
         [0013]    A thermo-economic controller has been conceived for a combined cycle power plant including at least one each of: a gas turbine, a chiller system, a HRSG, steam turbine and a duct firing device for the HRSG, the thermo-economic controller comprising a processor accessing a non-transitory memory storing digital instructions which when executed by the processor cause the controller to: model the thermodynamic performance of each of the gas turbine, the chiller system, the steam turbine and the duct firing system; simulate the thermodynamic performance of the power plant by correlating the modeling of the gas turbine, the chiller system, the steam turbine and the duct firing system; compare the simulated thermodynamic performance to the actual thermodynamic performance of the power plant and determining a difference between the simulated thermodynamic performance and the actual thermodynamic performance; adjust the modeling based on the comparison and to reduce the difference; optimize the simulation of the thermodynamic performance by simulating the thermodynamic performance of the power plant at various operating settings and selecting at least one of the operating settings as being optimal, and display the selected operating setting on a user interface device. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The accompanying drawings in conjunction with the text of this specification describe an embodiment of the invention. 
           [0015]      FIG. 1  is a diagram of a combined cycle power plant. 
           [0016]      FIG. 2  is a diagram of a computer model of the combined cycle power plant, an artificial neural network, an optimization system and a Kalman Filter. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    Methods and systems have been conceived to assist operators of combined cycle power plants to determine optimal operational settings of a plant, increase the profitability of the plant and meet the needs of a power grid for power generation. The conceived methods and system include computer models of major components in a combined cycle power plant, an artificial neural network (ANN) interacting with the models to simulate the entire power plant, an optimizer to optimize the operation of the power plant based on the plant simulation, the price of power and the cost to generate power, and automatic calibration of the plant simulation using a Kalman Filter. The Kalman filter provides a closed control loop that allows the system to automatically calibrate the computer models of the major components and the simulation of the entire plant based on data of physical and thermodynamic operating conditions of the power plant. The methods and systems for optimizing the settings of a combined cycle power plant are applicable to other types of plants, such as a cogeneration power plant. 
         [0018]      FIG. 1  is a schematic diagram of a combined cycle power plant  10  having a thermo-economic controller  12 . The primary power generators in the power plant  10  are a gas turbine system  14  and a steam turbine system  16 . Their power generation may be augmented by a chiller system  18  and a duct firing system  19 . The chiller system cools the air entering the gas turbine to enhance the power generation capacity of the gas turbine. The duct firing system burns fuel to generate heat used to supplement the production and heating of steam flowing into the steam turbine. The duct firing system augments the hot exhaust gases from the gas turbine that are also used to produce and heat the steam. 
         [0019]    The thermo-economic controller  12  simulates the operation of the entire combined cycle power plant  10 . The thermo-economic controller includes or communicates with digital models, such as a gas turbine model  20 , a chiller model  22  and a steam turbine model  24 . Using these digital models, the thermo-economic controller  12  simulates the actual operation, e.g., thermodynamic performance, of the entire combined cycle power plant  10 . 
         [0020]    The thermo-economic controller  12  seeks optimal settings for operating the combined cycle power plant. An optimizer system, e.g., a digital software optimization program, may run the digital models at various operational settings and identify an optimal operational setting(s). The variations in the operational settings may be generated by perturbations applied to the value of one or more of the operational settings. The optimal setting(s) be determined based on the lowest cost to produce power by the plant or the maximum profitability of the plant. 
         [0021]    To determine costs and profitability, the thermo-economic controller  12  includes or communicates with an economic model  26  that tracks the price of power and certain costs, such as the costs of fuels used in the gas turbine and duct firing system. The economic models provide data used by the thermo-economic controller to judge which of the operational setting(s) generated by the controller is best in terms of lowest cost or maximum profitability. 
         [0022]    The thermo-economic controller  12  includes a Kalman filter  28  to assist in adjusting and calibrating the digital models so that they accurately simulate the thermodynamic operation of the power plant. The adjustment and calibration of the digital models are performed while the operating settings of the digital models are the same as the operational settings of the actual power plant. The Kalman filter may be used to adjust or calibrate the models and the artificial neural network (ANN) in real time or in near real time, such as every hour, day or week. 
         [0023]    The thermo-economic controller  12  generates suggested operating settings for the power plant, such as power output (Megawatts—MW) for the gas turbine, the temperature and water mass flow for the chiller and the amount of duct firing. These suggested operating settings are provided to a human operator via a user interface device  30  such as a computer display screen, printer or sound speaker. Knowing the suggested operating settings, the operator inputs operating settings into control systems for the plant or for plant components, such as the gas turbine, chiller, HRSG, steam turbine and duct firing system. The control information causes the power plant to operate in the manner specified by the operator and as indicated by the control information. Alternatively, the thermo-economic controller  12  may directly input the suggested operating setting to the combined cycle power plant at the suggested operating settings. 
         [0024]    The gas turbine system includes a gas turbine  32  and a control system  34  for the gas turbine. Fuel  36  flows to and powers the gas turbine. The gas turbine drives an electrical generator (not shown) to generate electrical energy  38  to be delivered to a power grid  40 . The control system  34  for the gas turbine may be dedicated to the gas turbine. The control system  34  may set commands for the gas turbine regarding the fuel flow rate and receive data from the gas turbine such as the air inlet temperature and humidity, power output, shaft speed and temperatures of the exhaust gas. The control system  34  may also collect actual operating data from pressure and temperature sensors, flow control devices and other devices monitoring the gas turbine. Some of this operating data  29  may be transmitted to the Kalman filter  28 . 
         [0025]    The air entering the gas turbine may be cooled by cold water supplied to an inlet air duct  42  from the chiller system  18 . Cooling the air entering a gas turbine augments the capacity of the gas turbine to generate power. 
         [0026]    The chiller system  18  includes a refrigeration system  44  for cooling water and a control system  46 . The control system  46  receives information regarding the temperature of the cooling water  48  and commands regarding the desired cooling of the intake air. These commands may come from the gas turbine controller  34 . The control system  46  for the chiller system may also issue commands to cause the refrigeration system  44  to produce the cooling water  48  having a certain temperature and flow rate. The control system  46  sends data regarding the operation of the chiller system  18  to the thermo-economic controller  12 . 
         [0027]    The steam turbine system  16 , e.g., an HRSG, includes a steam turbine  50  and a duct firing device  19 , such as a furnace. Hot exhaust gas  54  from the gas turbine  32  are further heated or reheated by the duct firing device  19  to provide additional heat energy to produce steam  58 . The steam powers the steam turbine  50  and drives a generator (not shown) to produce electrical energy  38  delivered to the grid  40 . In some cases, the gas turbine and steam turbine are coupled to the same shaft line powering the same generator. 
         [0028]    The control system  60  for the steam turbine sets the flow rate of fuel  56  burned by the duct firing device  19 . Heat generated by the duct firing device increase the generation of steam  58  beyond the amount of steam produced by the exhaust gases from the gas turbine. The control system  60  sends data to the thermo-economic controller  12  regarding the operation of the steam system. 
         [0029]    The controller  12  communicates with the operator  30  and external sources  62  to, for example, receive data on the prices and demand for power delivered to the grid  40 . The thermo-economic controller  12  issues recommendations to the operator regarding desired operational settings to the gas turbine system  14 , chiller system  18  and the steam system  16 . The controller also receives and stores data on the operation of these systems  14 ,  18  and  16 . 
         [0030]    The thermo-economic controller  12  is a computer system having a processor and non-transitory memory storing data, the digital models  20 ,  22 ,  24  and  26 , the optimization program  64 , the Kalman Filter  28  and other computer programs. The computer system may be embodied in a single physical or virtual computing device or distributed over local or remote computing devices. 
         [0031]    The digital models  20 ,  22 ,  24  may be embodied as a set of algorithms, e.g. transfer functions, that relate operating parameters of each of the systems. Each model  20 ,  22 ,  24  may be a physics-based aero-thermodynamic computer model, a regression-fit model, neural-net model, or other suitable computer model of the system represented by the model. 
         [0032]    The models  20 ,  22 ,  24  and  26  may be regularly, automatically and in real-time or near real-time adjusted or calibrated using the Kalman filter  28 . The Kalman filter  28  receives data inputs regarding actual physical and thermodynamic operating conditions of the combined cycle power plant  10 . The data inputs may be supplied to the Kalman filter in real-time or near real-time, such as periodically every 15 minutes, every hour, every day, or weekly, during the operation of the power plant. 
         [0033]    The Kalman filter compares the data inputs regarding actual operating conditions of the power plant to the same operating conditions predicted by the models and artificial neural network (ANN)  62  that simulate the operation of the power plant. Differences between the actual condition data and the predicted data are used by the Kalman filter to adjust the ANN and digital models. For example, the Kalman filter may generate performance multipliers applied to inputs or outputs of the digital models and ANN or modify the weights applied to the logic units and algorithms used in the ANN and digital models. These actions by the Kalman filter reduce the differences between the actual condition data and the predicted data. The Kalman filter  28  continues to operate to reduce the differences between the data of actual conditions and the data of the predicted conditions. 
         [0034]    By way of example, the Kalman filter may generate performance multipliers for the predicted data regarding the compressor discharge pressure and temperature in the gas turbine, the efficiency of the gas and steam turbines, and the flow of fuel to the gas turbine and duct firing system. These types of data, e.g., representing compressor discharge pressure and temperature, efficiencies and fuel flow, reflect operating conditions that are subject to degradation of the performance of the power plant over time. By providing performance multipliers for these types of data, the Kalman filter may be particularly useful in adjusting the models and ANN to account for degradation in the performance of the power plaint. 
         [0035]      FIG. 2  is a schematic diagram of the thermo-economic controller  12 . Each of the digital models  20 ,  22 ,  24  and  26  of components of the combined cycle power plant are represented by graphs illustrating algorithms used to model the corresponding system. The models interact and communicate with the ANN  62 . The ANN connects the digital models together to form a model of the entire combined cycle power plant that simulates the thermodynamic operation of the plant. The ANN collects data outputted by the digital models and provides data to be used as inputs by the digital models. 
         [0036]    The thermo-economic controller  12  also includes an optimization system  64 , such as an optimization computer program, that interacts with the ANN to search for optimal settings of the gas turbine, chiller, steam turbine and duct firing device to achieve a defined goal. The goal may be to maximize the profitability of the combined cycle power plant. The optimization system may cause the ANN to run the digital models  20 ,  22  and  24  at various operational settings. The optimization system may have perturbation algorithms that assist in varying the operational settings of the models. The perturbation algorithms cause the simulation of the combine cycle power plant provided by the digital models and ANA to operate at settings different that the current operational setting for the plant. By simulating the operation of the plant at different settings, the optimization system searches for operational settings that would cause the plant to operate more economically. 
         [0037]    The economic digital model  26  provides data (information) used by the optimization system  64  to determine which settings of the power plant are most profitable. The economic model  26  may, for example, receive and store fuel cost data formatted such as a chart  260  that correlates the cost of fuel over time, such as during the seasons of a year. Another chart  262  correlates the price received for electrical power (kilowatts—kW) at different times of a day, week or month. The economic model provides data regarding the price to be received for power and the cost of fuel (gas turbine fuel and duct firing fuel). This data from the economic model is used by the optimizer to evaluate each of the operational states of the combined cycle power plant. The optimizer may identify which of the operational states for the power plant that best meets the desired goal. 
         [0038]    The digital models each simulate the operation of a component of the power plant, such as modeling the thermodynamic operation of the gas turbine, the chiller system or the steam turbine. The models may include algorithms such as mathematical equations and look-up tables that simulate the response of component to specific input conditions. 
         [0039]    The digital model  20  of the gas turbine may include an algorithm  200  that correlates the effect of the temperature of the inlet air entering the gas turbine to the power output (MW) of the gas turbine. The algorithm may show that power output decreases from a maximum value  201 , e.g., base load, as the inlet air temperature increases beyond a threshold  202  temperature. 
         [0040]    The model  20  of the gas turbine may receive data from the controller  34  for the gas turbine. This controller  34  may include a digital model of the gas turbine. 
         [0041]    The gas turbine digital model may also include an algorithm  204  that correlates the heat rate (HR) of the gas turbine at different power output levels (MW) of the gas turbine. Heat rate represents the efficiency of a gas turbine or other power generation system. Heat rate is inversely related to efficiency. A lower heat rate indicates a higher thermodynamic performance efficiency of the gas turbine. 
         [0042]    The digital model  22  of the chiller system simulates the thermodynamic operation of this system in providing cool air to the gas turbine. The model  22  may include an algorithm  220  that correlates the amount of chilling capacity produced based on the energy (Aux. Pwr) applied to run the refrigeration system  44 . The chilling capacity indicates the amount of cooling applied to the air entering the gas turbine. There may be a maximum chilling capacity value  221  that can be achieved by the chiller. A related algorithm  222  correlates the energy (Chiller Aux. Pwr) applied to run the refrigeration system  44  to the temperature of the chilled air entering a gas turbine. The power required to run the chiller increases dramatically to reducing the temperature of the air entering the gas turbine below the dew point  224  of the ambient air. 
         [0043]    The digital model  24  of duct firing system  19  for the HRSG may include an algorithm  240  to correlate the power output of the steam turbine (ST Output) to the energy added by duct firing, such as the amount of fuel consumed by the duct firing. There may be an upper threshold level  242  to the increase in steam turbine output that can be achieved by the duct firing system. 
         [0044]    The ANN  62  interacts with and provides communications between each of the digital models  20 ,  22  and  24  of the physical components of the power plant. The interaction includes collecting output data from the models and generating input data used by the models to generate the output data. The ANN may be a digital network of connected logic elements. The logic elements may each embody an algorithm that accepts data inputs to generate one or more data outputs. A simple logic element may sum the values of the inputs to produce output data. Other logic elements may multiple the values of the inputs or apply other mathematical relationships to the input date. 
         [0045]    The data inputs to each of the logic elements of the ANN may be assigned a weight, such as multiplier between one and zero. The weights may be modified during a learning mode which adjusts the ANN to better model the performance of the combined cycle power plant. The weights may also be adjusted based on commands provided by the Kalman filter. Adjusting the weights of the data inputs to the logic units in the ANN is one example of means by which the ANN may be dynamically modified during operation of the combined cycle power plant. Other examples include modifying weights of data inputs to algorithms (which are an example of a logic unit) in each of the thermodynamic digital models for the steam turbine, chiller and gas turbine. The thermo-economic controller  12  may be modified in other ways, such as be adjustments to the logic units and algorithms, based on the data provided by the optimizer and Kalman filter. 
         [0046]    The thermo-economic controller  12  may generate and output to the user interface  30  of recommended set points  66  for the combined cycle power plant. These recommended set points  66  may include: a temperature and mass flow rate (T&amp;M) for the cooling water generated by the chiller and used to cool the air entering the gas turbine; a power output (MW) for the gas turbine of the gas turbine if not set to its base load; and the duct firing rate for the HRSG. 
         [0047]    By way of example, the thermo-economic controller  12  may be used to properly optimize the setting of the chiller system  18 . The power needed to drive the refrigeration system  44  dramatically increases if the cooling duct  42  is set to cool the gas turbine inlet air below the dew point  224 . This power increase is accounted for in the relationship  222  between gas turbine inlet air temperature and power required for the chiller system. Because this relationship  222  is incorporated into the thermo-economic controller  12 , the optimization of the simulation of the plant by the controller takes into account that cooling gas turbine inlet air below the dew point dramatically increases the power required by the chiller system. 
         [0048]    While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.