Model-Based Control for Combined Cycle Power Plant Load Regulation
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Solution Overview
Problem
Combined cycle power plants face challenges in rapidly adjusting power output due to the significant lag in response time of steam turbine systems, leading to periods of unnecessary gas turbine over or under demand, as they operate with steam turbines in a 'valves wide open' mode, which limits load regulation and results in inefficient power control.
Innovation Solution
A model-based control scheme is implemented using a steam turbine to gas turbine transfer function within a closed-loop feedback control system, optimizing gas turbine operation and incorporating a predictive model to account for steam turbine output, thereby improving unit MW set-point tracking and disturbance rejection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If steam turbines operate with valves wide open to minimize throttling losses, then energy efficiency is improved, but load regulation capability deteriorates
Solution Approach 1:
The control system performs preliminary action by predicting future steam turbine output based on current gas turbine demand and system dynamics. This prediction allows the gas turbine controller to proactively adjust gas turbine output to compensate for anticipated steam turbine lag, rather than reactively responding after the lag occurs. The predictive model calculates the expected steam turbine contribution and uses this information to pre-adjust gas turbine loading, maintaining both energy efficiency and load regulation capability.
2Speed
If steam turbine response time is reduced through faster control, then load regulation improves, but system stability deteriorates due to oscillations
Solution Approach 1:
The control system implements feedback by continuously monitoring actual steam turbine output and comparing it with predicted output. The difference between actual and predicted values is fed back to the controller, which adjusts future predictions and gas turbine control signals accordingly. This feedback mechanism allows the system to adapt to changing conditions and correct for model inaccuracies, maintaining stability while achieving faster effective response through predictive compensation.
3Productivity
If gas turbine output is increased to compensate for steam turbine lag, then power demand met improves, but operational efficiency deteriorates due to over/under demand periods
Solution Approach 1:
The control system changes parameters by dynamically adjusting gas turbine output based on predicted steam turbine contribution. Rather than maintaining fixed gas turbine loading or using simple proportional control, the system calculates optimal gas turbine output by considering the predicted steam turbine response to current and future demand changes. This parameter adjustment ensures gas turbine operates at optimal loading levels, meeting power demand without excessive over/under generation, thereby improving operational efficiency while maintaining productivity.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This control scheme enhances the robustness of power plant control, allowing for better load regulation and cost savings by optimizing gas turbine operation and reducing the lag in steam turbine response, leading to more efficient power generation and reduced operational costs.
Implementation Method 1
a steam turbine power generation unit having a steam inlet system, a steam turbine coupled to the steam inlet system and powered by steam from the steam inlet system
Implementation Method 2
interconnected such that the steam inlet system is coupled to the combusted gas exhaust to absorb heat from combusted gas in the combusted gas exhaust to produce heated steam within the steam inlet system
Data Source
AI summary
A control system uses a modeled steam turbine megawatt (power) change attributed to a gas turbine demand change (i.e., a steam turbine to gas turbine transfer function) within a conventional closed loop feedback control scheme to perform control of a combined cycle power plant. This control system implements a form of internal model control and provides better unit megawatt (power) set-point tracking and disturbance variable rejection for overall more robust control, and thus operates to optimize the gas turbine operation of the combined cycle power plant in a manner that provides cost savings over time.