Combined-Cycle Plant Load Distribution via Sliding-Pressure Curves
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Solution Overview
Problem
Combined-cycle plants face challenges in flexibility and load management due to insufficient autonomous response capacity of steam turbines, particularly during rapid load variations and network frequency control, which affects efficiency and compliance with energy market and environmental standards.
Innovation Solution
A method for controlling combined-cycle plants that includes a load set-point generation unit distributing load between gas and steam turbines, using a processing stage and correction stage to determine and adjust set-points, incorporating a pressure regulator and modified sliding-pressure curve to ensure steam turbine participation in primary grid frequency control and maintain plant stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If the plant controller subtracts steam turbine power from overall load set-point to determine gas turbine load, then the control system maintains simple structure, but the steam turbine lacks autonomous response capacity for rapid load variations
Solution Approach 1:
The control system is segmented into multiple independent controllers: a gas turbine load controller that receives the overall load set-point and a steam turbine pressure regulator that operates autonomously based on sliding-pressure curves. This segmentation allows the steam turbine to independently respond to load variations without requiring complex centralized control logic, thus improving adaptability while managing complexity.
Solution Approach 2:
The steam turbine control system uses pre-defined sliding-pressure curves that are established beforehand. These curves provide predetermined pressure set-points as a function of load, enabling the steam turbine to autonomously respond to rapid load variations without requiring real-time complex calculations or centralized control decisions, thereby enhancing responsiveness while keeping the control structure relatively simple.
2Speed
If the pressure regulator controls the steam turbine according to sliding-pressure set-point curve, then thermodynamic efficiency is maximized, but the system cannot adequately respond to fast load variations and network frequency control requirements
Solution Approach 1:
The control system transitions from static thermodynamic optimization to dynamic control by implementing a sliding-pressure curve that adapts pressure set-points based on load conditions. This dynamic approach allows the steam turbine to respond faster to load variations while maintaining thermodynamic efficiency through curve-based optimization rather than fixed set-points.
Solution Approach 2:
The steam turbine control system incorporates feedback mechanisms where the actual steam turbine power and pressure measurements are continuously monitored and compared against the sliding-pressure curve targets. This feedback enables real-time adjustments that maintain thermodynamic efficiency while achieving faster response to load changes through autonomous corrective actions.
3Reliability
If the gas turbine controls the entire load distribution, then the control structure remains simple, but the plant cannot maintain stability during start-up and shut-down phases
Solution Approach 1:
The steam turbine control system operates autonomously using self-service principles, where the sliding-pressure curve and pressure regulator work together without requiring continuous intervention from the central plant controller. This autonomy enables the steam turbine to maintain stability during start-up and shut-down phases independently, improving reliability while adding minimal complexity to the overall control structure.
Data Source
AI summary
A method of controlling a combined-cycle plant for production of electric energy, comprising at least one gas turbine assembly (2) and a steam turbine (5) with respective generators (3) and (6). A provisional load set- point (SPTV*) for the steam turbine (5) is determined on the basis of the plant load set-point (SP0) and an actual steam pressure (PA) supplied to the steam. turbine (5). A gas turbine load set-point (SPTG) is thus determined on the basis 'of the plant load set-point (SP0) and the provisional load set-point (SPTV*) and is used to control the gas turbine assembly (2). A steam turbine load set- point (SPTV), indicative of a second power (WTV) to be supplied through the steam turbine (5), is finally determined on the basis of the provisional load set- point (SPTV*) and of a load error (ETG) of the gas turbine assembly (2) and is used to control the steam turbine (5).