Abstract:
Methods and apparatus for controlling load on a machine are provided. The method includes determining a maximum value of a process variable using a predictive model of the machine while holding a control output associated with the process variable substantially constant over a prediction period, incrementing the control output if the determined maximum value of the process variable is within an allowable limit range, and setting the control output to the last value of the process variable that did not cause the process value to exceed the allowable range.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to systems and methods for controlling a process system and more particularly, to implementing model predictive control (MPC) in a real time controller. 
     Model predictive control typically is used to determine an optimum control output profile based on the results of a model of the process involved. The model predicts the future outcome of control output changes. This technique is particularly useful when the process involved is complex or has a long time constant. One such application is steam turbine rotor stress and axial clearance control. The effects of some control outputs, such as, turbine load on steam turbine rotor stress are not realized for 30 minutes or more. A standard approach to model predictive control involves extensive linear algebraic manipulation. The size of the computing problem is driven by several factors: prediction horizon (time), control step size, and number of model states. The resulting control profile can be complex, changing in value over the prediction horizon as needed to produce an optimal solution. Accordingly, implementing MPC using traditional techniques in existing control systems is not possible due to the large computational effort required. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In one embodiment, a method of controlling load on a machine includes determining a maximum value of a process variable using a predictive model of the machine while holding a control output associated with the process variable substantially constant over a prediction period, incrementing the control output if the determined maximum value of the process variable is within an allowable limit range, and setting the control output to the last value of the process variable that did not cause the process value to exceed the allowable range. 
     In another embodiment, a turbine engine control system includes a plurality of sensors configured to determine a state of the turbine engine, and a processor programmed to determine a maximum value of a process variable using a predictive model of the machine while holding a control output associated with the process variable substantially constant over a prediction period, increment the control output if the determined maximum value of the process variable is within an allowable limit range, and set the control output to the last value of the process variable that did not cause the process value to exceed the allowable range. 
     In yet another embodiment, a computer program embodied on a computer readable medium for controlling load on a machine is provided. The program includes a code segment that determines a maximum value of a process variable using a predictive model of the machine while holding a control output associated with the process variable substantially constant over a prediction period, increments the control output if the determined maximum value of the process variable is within an allowable limit range, and sets the control output to the last value of the process variable that did not cause the process value to exceed the allowable range. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of an exemplary steam turbine-generator system in accordance with an embodiment of the present invention; 
         FIG. 2  is a simplified schematic diagram of the steam turbine-generator shown in  FIG. 1 ; and 
         FIG. 3  is a flow chart of an exemplary method of controlling an output of a turbine controller that may be used with the turbine shown in  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The principal controls available to a shift operator of a steam turbine-generator system include boiler controls which determine the temperature and pressure of the main steam and reheat steam supplies and a main steam admission control valve or valves, which determine the amount of steam admitted to the first or high pressure turbine stage. Practical guidance to an operator of such a steam turbine-generator system includes evaluations of the substantially instantaneous operating parameters in a manner that can be interpreted easily, quickly and without detailed technical analysis to facilitate the manipulation of these principal controls. A technical effect of the present invention is the providing of permissible load guidance during turbine transient operation. 
       FIG. 1  is a schematic view of an exemplary steam turbine-generator system  10  in accordance with an embodiment of the present invention. Steam turbine-generator system  10  includes a machine, such as a steam turbine-generator  12  receiving a thermal input from a steam boiler  14 . Boiler  14  may be of any convenient type, such as a coal-fired, oil-fired, or heat recovery steam generator. Steam turbine-generator  12  is controlled by a turbine controller  17  and boiler  14  is controlled by a plant and real-time controller  15 , with operator inputs represented by a line  16  from an operator  18 . Turbine controller  17  is configured to control outputs that are transmitted to various control elements associated with steam turbine-generator  12  to control the load of steam turbine-generator  12 . Electric power output is produced and represented by a line  20 . A set of measured parameters from steam turbine-generator  12  are applied on a line  22  to a data processing subsystem  24 . The outputs of a data processing subsystem  24  are transmitted to an operator interface subsystem  26  which may be of a conventional type such as, for example, a cathode ray tube display, a printer or other types of analog or digital display devices. The output from data processing subsystem  24  may also be applied to a data storage subsystem  28  wherein the data may be stored for short-term or long-term purposes. Data storage subsystem  28  may be of any convenient type including a printer. However, in an embodiment used as an example herein, data processing subsystem  24  includes a digital processor and data storage subsystem  28  preferably includes a digital storage device. 
     Coupled in parallel with operator interface subsystem  26  is a performance engineer interface subsystem  27 . Interface  27  allows a performance engineer  29  to study the outputs of data processing subsystem  24  on a more leisurely basis as compared with operator  18 . Performance engineer  29  communicates with operator  18  to improve the long-term performance of turbine-generator system  10  due in part to the higher level, sophisticated analysis with which the performance engineer views the data. The performance engineer also determines the maintenance procedures for the system and subsystem  27  assists in the promulgation of those procedures. 
       FIG. 2  is a simplified schematic diagram of steam turbine-generator  12  (shown in  FIG. 1 ). Various embodiments of the present invention uses temperature and pressure measurements at various locations throughout steam turbine-generator system  10 , including a measurement of the generated electrical power output, and compares their relationship to corresponding design values to determine the power losses, efficiencies and heat rates throughout the system. 
     Steam turbine-generator  12  includes a steam turbine  30  coupled through a mechanical connection  32 , to an electric generator  34  which generates an electric power output. A transducer (not shown) in electric generator  34  produces an electric power output signal W 1  which is applied to line  22  for transmission to data processing subsystem  24 . The operator input on line  16  is applied by hydraulic, electrohydraulic, digital or other well known means, to a main control valve actuator  36  which affects a main control steam admission valve  38  as illustrated by line  40 . A valve position signal V 1 , is generated by appropriate means and represents the amount by which main control valve  38  is opened, and the signal is applied to line  22  for transmission to data processing subsystem  24 . It is to be understood that valve  38  is representative of a number of steam admission control valves commonly associated with a steam turbine. 
     A steam generator  42 , which is part of boiler  14 , produces a supply of hot pressurized steam that is applied to main control valve  38  on a line  44 . The steam passing through main control valve  38  is applied on a main steam line  46  to an input of a high pressure turbine  48 . As utilized herein, the term “HP” refers to high pressure turbine  48 . Steam exiting from HP turbine  48 , now partially expanded and cooled, but still containing substantial energy, is applied on a cold reheat line  50  to a reheater  52  which is also part of boiler  14 . The pressure and temperature of the steam in line  44 , upstream of main control valve  38  and generally at its inlet are measured by sensors (not shown) to produce a representative first pressure signal P 1  and a first temperature signal T 1  which are transmitted to data processing subsystem  24 . The pressure and temperature of the steam in cold reheat line  50 , downstream of high pressure turbine  48  at substantially its exit, are measured by sensors (not shown) to produce a representative third pressure signal P 3  and a third temperature signal T 3  which are also transmitted to data processing subsystem  24 . 
     A pressure sensor (not shown) produces a pressure signal P 2 , representing the pressure sensed proximate the first stage of HP turbine  48 , and the signal is transmitted to data processing subsystem  24 . 
     An intermediate pressure turbine  54  (hereinafter “IP” turbine) receives reheated steam from reheater  52  on a hot reheat line  56 , expands the steam to extract energy from it and exhausts the steam through an exhaust line  58  to a low pressure turbine  60 . Mechanical outputs of HP turbine  48 , IP turbine  54  and low pressure turbine  60  (hereinafter “LP” turbine) are interconnected mechanically as shown by coupling means  62  and  64  which are, in turn, mechanically coupled to connection  32  and to the generator  34 . A fourth temperature T 4  and pressure P 4  in hot reheat line  56 , upstream of IP turbine  54  are measured by sensors (not shown) and representative signals are transmitted to data processing subsystem  24 . In addition, a fifth temperature T 5  and pressure P 5  of the steam in line  58 , downstream of IP turbine  54 , is measured by sensors (not shown) and signals representing those quantities are also transmitted to data processing subsystem  24 . In another embodiment, T 5  and P 5  are measured at the low pressure bowl of LP turbine  60 . 
     Exhaust steam from LP turbine  60  is applied on a line  66  to a condenser  68  wherein the steam is condensed to water and thereafter conveyed on a line  70  to steam generator  42  for reuse. One of the factors that can degrade system efficiency is deficient operation of condenser  68  which can result in higher than normal back pressure at the exhaust of low pressure turbine  60 . Such back pressure is an indication that the operation of condenser  68  requires adjustment for improved efficiency. A pressure sensor (not shown) in line  66  produces an exhaust pressure signal P 6  which is transmitted to data processing subsystem  24  for further processing and display. 
     It should be noted that the temperature sensors used may be of any convenient type, however, in an embodiment described herein, each temperature sensor includes a plurality of high accuracy chromel constantan (Type E) thermocouples disposed in a well and positioned to give access to the steam whose temperature is to be measured. By using a plurality of thermocouples for each sensor, the results from the plurality of thermocouples may be averaged to substantially reduce individual thermocouple errors or minor differences in system temperatures. In addition, the availability of more than one thermocouple offers a measure of redundancy in case of failure of one or more of the thermocouples at a sensor location. Transmission of the temperature signals may be accomplished using analog voltages or the temperature signals may be digitized before transmission to make the measurements less susceptible to the lengths of cable runs and to noise. Similarly, the pressure sensors may be of any convenient type. 
       FIG. 3  is a flow chart of an exemplary method  300  of controlling an output of a turbine controller that may be used with turbine  30  (shown in  FIG. 2 ). In the exemplary embodiment, method  300  is programmed to execute on a processor associated with turbine  30 , such as controller  17 . Also programmed into controller  17  is an algorithmic model of system  10  including turbine  30 . Method  300  includes initializing  302  the states of the model the current states of turbine  30 . For example, the measured parameters input into data processing subsystem  24  through line  22  are used to replace corresponding values in the model such that a time (t 0 ) of the model is the current time with respect to turbine  30 . Method  300  iteratively calculates  304  the value of a process variable of interest as a function of a control output that is held constant and time. The current process variable value is compared  306  to the maximum (or minimum) process variable value thus far and if the current process variable value is larger (or smaller) than the maximum (or minimum) process variable value, the maximum (or minimum) process variable value is replaced  308  with the current process variable value. If not, then processing skips step  308  and continues at a step  310 , where method  300  checks  310  if the current time increment is greater than the current prediction horizon. If the current time increment is not greater than the prediction horizon, the time is incremented  312  one unit and processing continues at step  304 . If the current time exceeds the prediction horizon, method  300  compares  314  the maximum value of the process variable achieved during the prediction horizon timeframe to an allowed maximum limit. In cases where the minimum value of the process variable is of interest, step  314  compares the minimum process variable value found during the prediction horizon to an allowed minimum limit. 
     If the value of the process variable is determined by the comparison to be outside the limits, the previous value of the control output is set to the output of controller  17  to limit the change of the process variable value to the allowed limit. The maximum value of the process variable is set  318  to zero and the iterative process of method  300  begins again at step  302 . If the value of the process variable is determined by the comparison to not be outside the limits at the end of the prediction horizon, the time increment counter is reset  320  to zero time. The control output, which was held constant during the current iteration is incremented  322  and method  300  continues at step  304 . Accordingly, method  300  seeks a higher control output that will permit the process variable value to quickly reach predetermined limit for that process variable. 
     In operation, turbine rotors are typically limited to a predetermined heat-up rate based at least in part on engineering studies of the material of manufacture of the rotor, the configuration of component parts and the response of those parts to changes in temperature and stress. To limit the possibility of exceeding the allowable heat-up rate, operators are typically required to control heat-up rate manually to a rate that is within the predetermined heat-up limit or to heat-up the rotor in steps wherein the rotor can “soak” at the new temperature while temperatures across the rotor equalize. Method  300  permits establishing a control signal that will load the turbine at a relatively faster rate by comparing a response of a model of the turbine to an actual response of the turbine using measured parameters. 
     As used herein, with reference to a real-time controller, real-time refers to outcomes occurring at a substantially short period after a change in the inputs affecting the outcome. The period is the amount of time between each iteration of a regularly repeated task. Such repeated tasks are called periodic tasks. The time period is a design parameter of the real-time system that may be selected based on the importance of the outcome and/or the capability of the system implementing processing of the inputs to generate the outcome. 
     As used herein, the term controller may include any processor-based or microprocessor-based system, such as a computer system, that includes microcontrollers, reduced instruction set circuits (RISC), application-specific integrated circuits (ASICs), logic circuits, and any other circuit or processor that is capable of executing the functions described herein. The examples given above are exemplary only, and are not intended to limit in any way the definition and/or meaning of the term controller. 
     The various embodiments, or the components thereof, may be implemented as a part of the computer system. The computer system may include a computer, an input device, a display unit, and an interface, for example, to access the Internet. It may also include a microprocessor, which may be connected to a communication bus. The computer may include a memory, which may include a Random Access Memory (RAM) and a Read Only Memory (ROM), as well as a storage device, which may be a hard disk drive or a removable storage drive such as a floppy disk drive, an optical disk drive, and so forth. The storage device can also be other similar means of loading computer programs or other instructions into the computer system. 
     The computer system executes a set of instructions that are stored in one or more storage elements, in order to process input data. The storage elements may also hold data or other information, as desired or required, and may be in the form of an information source or a physical memory element in the processing machine. The set of instructions may include various commands that instruct the computer system to perform specific operations, such as the processes of the various embodiments of the invention. The set of instructions may be in the form of a software program. The software may be in various forms, such as system software or application software. Further, the software may be in the form of a collection of separate programs, a program module within a larger program, or a portion of a program module. The software may also include modular programming in the form of object-oriented programming. The processing of input data by the processing machine may be in response to user commands, to results of previous processing, or to a request made by another processing machine. 
     As used herein, the terms ‘software’ and ‘firmware’ are interchangeable and include any computer program that is stored in the memory, to be executed by a computer, which includes RAM memory, ROM memory, EPROM memory, EEPROM memory, and non-volatile RAM (NVRAM) memory. The memory types mentioned above are only exemplary and do not limit the types of memory used to store computer programs. 
     The above-described model predictive control method is cost-effective and highly reliable. The method permits outputting a machine loading profile that quickly reaches a hold point that is determined to be the highest possible, followed by a controlled ramp and an early release to full load. Accordingly, the model predictive control method facilitates operation of machines in a cost-effective and reliable manner. 
     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.