VARIABLE RATE FEEDFORWARD CONTROL BASED ON SET POINT RATE OF CHANGE

• US 20090118873A1
• Filed: 11/02/2007
• Published: 05/07/2009
• Est. Priority Date: 11/02/2007
• Status: Active Grant

• Current Assignee: Emerson Process Management Power & Water Solutions Incorporated (Emerson Electric Company)
• Original Assignee: Emerson Process Management Power & Water Solutions Incorporated (Emerson Electric Company)
• Inventors: kephart, Richard W., Cheng, Xu, Menten, Charles H.
• Granted Patent: US 7,826,908 B2
• Time in Patent Office: Days
• US Class Current: 700/296
• CPC Class Codes: G05B 13/021 in which a variable is automatically adjusted to optimise the performance
• CPC Class Codes: G05B 2219/41221 Fuzzy shaping
• CPC Class Codes: G05B 2219/41435 Adapt coefficients, parameters of feedforward

Abstract

A method of controlling a power generating unit or other process equipment with a slow reaction time includes creating a feedforward control signal to selectively include a fast response rate component or a slow response rate component based on the average rate at which a load demand set point signal has changed during a particular previous period of time. The method then uses the developed feedforward control signal to control the power generating equipment or other slowly reacting process equipment. In particular, a control method switches between introducing a fast or a slow response component within a feedforward control signal based on whether the change in the load demand set point over a particular period of time in the past (e.g., an average rate of change of the load demand set point signal) is greater than or less than a predetermined threshold. This method is capable of providing a relatively fast control action even if the expected load demand set point change is in a small range. In addition, this method does not require knowledge of the final or target load demand set point during the time in which the load demand set point is ramping up to a final target value and is not dependent on the ramp size, i.e., the ultimate difference between the load demand set point at the beginning of the load demand set point change and the final or target value of the load demand set point, making it more versatile than prior art systems.

Claims

1. A method of controlling a power generating unit using a feedforward control signal developed from a load demand set point signal indicating a desired output of the power generating unit, comprising:

determining a magnitude of a rate of change of the load demand set point signal during a particular period of time prior to a current time;

comparing the determined magnitude of the rate of change of the load demand set point signal during the particular period of time to a threshold;

creating the feedfoward control signal with a fast response rate characteristic when the determined magnitude of the rate of change of the load demand set point signal during the particular period of time is below the threshold and creating the feedforward control signal with a slow response rate characteristic when the determined magnitude of the rate of change of the load demand set point signal during the particular period of time is greater than the threshold; and

using the feedforward control signal to control the power generating unit.

2. The method of controlling a power generating unit of claim 1, wherein the load demand set point signal ramps between two values at a fixed rate and wherein the fast response rate characteristic results in a feedforward control signal that changes at a rate greater than the fixed rate and wherein the slow response rate characteristic results in a feedforward control signal that changes at a rate less than the fixed rate.

3. The method of controlling a power generating unit of claim 1, wherein determining the magnitude of the rate of change of the load demand set point signal during the particular period of time prior to the current time includes determining a difference between a current value of the load demand set point signal and a value of the load demand set point signal at the beginning of the particular period of time.

4. The method of controlling a power generating unit of claim 1, wherein determining the magnitude of the rate of change of the load demand set point signal during the particular period of time prior to the current time includes determining a moving average of the rate of change of the load demand set point signal during the particular period of time.

5. The method of controlling a power generating unit of claim 1, wherein determining the magnitude of the rate of change of the load demand set point signal during the particular period of time prior to the current time includes determining a weighted average of the rate of change of the load demand set point signal during the particular period of time with different weights being associated with different times.

6. The method of controlling a power generating unit of claim 1, wherein creating the feedforward control signal includes creating the feedforward control signal with the fast response rate characteristic when the determined magnitude of the rate of change of the load demand set point signal during the particular period of time is well below the threshold, and creating the feedforward control signal with the slow response rate when the determined magnitude of the rate of change of the load demand set point signal during the particular period of time is well above the threshold and creating the feedforward control signal as a weighted combination of the fast response rate and the slow response rate when the determined magnitude of the rate of change of the load demand set point signal during the particular period of time is near or at the threshold.

7. The method of controlling a power generating unit of claim 6, wherein creating the feedforward control signal includes using a fuzzy logic technique to create the feedforward control signal as a weighted combination of the fast response rate and the slow response rate.

8. The method of controlling a power generating unit of claim 7, wherein using the fuzzy logic technique includes combining a first control response rate and a second control response rate based on the value of the magnitude of the rate of change of the load demand set point signal during the particular period of time.

9. The method of controlling a power generating unit of claim 7, wherein using the fuzzy logic technique includes combining a first feedforward control signal calculated using a first response rate and a second feedforward control signal calculated using a second response rate based on the value of the magnitude of the rate of change of the load demand set point signal during the particular period of time.

10. The method of controlling a power generating unit of claim 1, wherein creating the feedforward control signal further includes using a feedforward control rate calculated for a previous period of time when the load demand set point signal is equal to a target value for the load demand set point signal.

11. A power plant controller, comprising:

an input to receive a load demand set point signal specifying a load demand set point for each of a series of times;

a calculation unit that determines a magnitude of a rate of change of the load demand set point associated with a particular period of time prior to a current time;

a comparison unit that compares the determined magnitude of the rate of change of the load demand set point during the particular period of time to a threshold; and

a control signal generator that generates a control signal with a fast response rate when the determined magnitude of the rate of change of the load demand set point during the particular period of time is below the threshold and that creates a control signal with a slow response rate when the determined magnitude of the rate of change of the load demand set point during the particular period of time is greater than the threshold.

12. The controller of claim 11, wherein the input receives a load demand set point signal that specifies a target load demand at a particular time in the future and a ramp rate to be used to determine the load demand set point at each of the times between the current time and the particular time in the future.

13. The controller of claim 11, wherein the load demand set point signal ramps between two values at a fixed rate and wherein the control signal generator generates a control signal with the fast response rate by creating a feedforward control signal that changes at a rate greater than the fixed rate and wherein the control signal generator generates a control signal with the slow response by creating a feedforward control signal that changes at a rate less than the fixed rate.

14. The controller of claim 11, wherein the calculation unit determines the magnitude of the rate of change of the load demand set point associated with a particular period of time prior to the current time by determining a difference between a load demand set point value at the current time and a load demand set point value at a time in the past offset from the current time by the particular period of time.

15. The controller of claim 11, wherein the calculation unit determines the magnitude of the rate of change of the load demand set point associated with a particular period of time prior to the current time by determining an average rate of change of the load demand set point during the particular period of time.

16. The controller of claim 11, wherein the control signal generator generates the control signal with the fast response rate when the determined magnitude of the rate of change of the load demand set point during the particular period of time is well below the threshold, generates the control signal with the slow response rate when the determined magnitude of the rate of change of the load demand set point during the particular period of time is well above the threshold and generates the control signal as a weighted combination of the fast response rate and the slow response rate when the determined magnitude of the rate of change of the load demand set point during the particular period of time is near or at the threshold.

17. The controller of claim 11, wherein the comparison unit comprises a fuzzy logic block.

18. The controller of claim 17, wherein the fuzzy logic block switches between or combines a first feedforward control signal calculated using the fast response rate and a second feedforward control signal calculated using the second response rate, based on the difference between the value of the magnitude of the rate of change of the load demand set point during the particular period of time and the threshold.

19. A method of producing a control signal for use in controlling equipment in a plant, comprising:

obtaining a set of set point signal values for a set point signal specifying the desired operation of the equipment over a particular period of time;

determining a magnitude of a rate of change of the set point signal during the particular period of time based on the set of set point signal values;

comparing the determined magnitude of the rate of change of the set point signal during the particular period of time to a threshold;

creating a control signal for use in controlling the equipment based on the comparison between the determined magnitude of the rate of change of the set point signal during the particular period of time and the threshold, wherein the control signal is created with a first response characteristic when the determined magnitude of the rate of change of the set point signal during the particular period of time is below the threshold and the control signal is created with a second response characteristic when the determined magnitude of the rate of change of the set point signal during the particular period of time is greater than the threshold.

20. The method of producing a control signal of claim 19, wherein determining the magnitude of the rate of change of the set point signal during the particular period of time includes determining a difference between a current value of the set point signal and a value of the set point signal at the beginning of the particular period of time.

21. The method of producing a control signal of claim 19, wherein determining the magnitude of the rate of change of the set point signal during the particular period of time includes determining an average of the rate of change of the set point signal over the particular period of time.

22. A boiler operated power plant, comprising:

a turbine;

a boiler coupled to the turbine that operates to create steam to drive the turbine;

a control unit communicatively connected to the boiler to control the operation of the boiler, the control unit including;

a feedback controller that produces a feedback control signal;

a feedforward controller that produces a feedforward control signal, the feedforward controller including;

an input that receives a load demand set point signal specifying a load demand set point for each of a series of times of operation of the power plant;

a calculation unit that determines a magnitude of a rate of change of the load demand set point over a particular period of time prior to a current time;

a comparison unit that compares the determined magnitude of the rate of change of the load demand set point over the particular period of time to a threshold; and

a feedforward control signal generator that generates the feedfoward control signal to include a first response characteristic when the determined magnitude of the rate of change of the load demand set point over the particular period of time is below the threshold and that creates the feedfoward control signal with a second and different response characteristic when the determined magnitude of the rate of change of the load demand set point over the particular period of time is greater than the threshold; and

a control signal combiner that combines the feedforward control signal and the feedback control signal to create a master control signal for controlling the boiler.

23. The boiler operated power plant of claim 22, wherein the comparison unit or the feedforward control signal generator comprises a fuzzy logic block that switches between first and second feedforward control signals having the first and second response characteristics, respectively, or that switches between first and second response rates wherein the first and second response rates are used to produce the first and second feedforward control signals having the first and second response characteristics, respectively.

24. The boiler operated power plant of claim 22, wherein the calculation unit determines the magnitude of the rate of change of the load demand set point over the particular period of time prior to the current time by determining a difference between a current load demand set point and a load demand set point at the particular time in the past offset from the current time by the particular period of time.