Variable Rate Feedforward Control for Power Plant Load Response
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Solution Overview
Problem
Power generating equipment, such as steam turbines in thermal power plants, face significant lag in response time due to their physical characteristics, leading to overshoot and swing issues when adjusting power output to meet changing load demand set points, especially when the changes are small or incremental, and when the final target load demand set point is unknown.
Innovation Solution
A variable rate feedforward control circuit that dynamically switches between fast and slow response characteristics based on the average rate of change of the load demand set point over a previous period, eliminating the need for knowledge of the final target load demand set point and reducing overshoot by applying a high response rate for initial changes and a low response rate for sustained changes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a fixed fast response rate is used in feedforward control, then the initial response to load demand changes is improved, but overshoot and swing occur when the change is sustained
Solution Approach 1:
The patent applies dynamics by making the feedforward control signal's response rate variable rather than fixed. The system dynamically adjusts between fast and slow response rates based on the current operating phase (initial change vs. sustained change), allowing the control characteristics to adapt to the changing conditions throughout the load demand transition.
Solution Approach 2:
The patent changes the parameter of response rate from a constant value to a variable value that depends on the phase of load demand change. By modifying the response rate parameter based on whether the system is in an initial or sustained change phase, the control system achieves both fast initial response and stable sustained operation.
2Stability of the object's composition
If a fixed slow response rate is used in feedforward control, then system stability is improved, but the response time to load demand changes increases
Solution Approach 1:
The system dynamically selects between slow and fast response rates based on the phase of change. During initial load demand changes, the fast response rate is applied to reduce response time. During sustained changes, the slow response rate is applied to maintain stability, thus optimizing both response time and stability at different phases.
Solution Approach 2:
The response rate parameter is changed from a fixed slow value to a variable value that increases during initial changes and decreases during sustained changes. This parameter adaptation allows the system to achieve fast response when needed while maintaining stability during prolonged transitions.
3Speed
If derivative action is applied to increase response rate, then the response speed is improved, but large overshoot and swing occur when load demand changes are large or sustained
Solution Approach 1:
The patent applies partial derivative action only during the initial phase of load demand changes when it is most beneficial, rather than applying it continuously. By limiting the derivative action to the initial response phase and removing it during sustained changes, the system achieves fast initial response without the harmful overshoot and swing that would result from continuous derivative action.
Solution Approach 2:
The control action is segmented into two distinct phases: initial change phase and sustained change phase. Derivative action (fast response) is applied only to the initial phase, while feedback control (slower response) handles the sustained phase. This segmentation allows the system to benefit from fast response when needed while avoiding the instability caused by continuous fast action.
4Adaptability or versatility
If the final target load demand set point is unknown, then adaptability to incremental changes is improved, but control precision deteriorates
Solution Approach 1:
The system serves itself by using only the current load demand set point information that is already available, without requiring knowledge of the final target value. The feedforward control signal is generated based on the current set point and the selected response rate, allowing the system to adapt to incremental changes while maintaining control precision through the variable response rate mechanism.
Data Source
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AI summary
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.