Combined-Cycle Plant Control for Subfrequency Transients
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Solution Overview
Problem
Combined-cycle plants face limitations in responding to subfrequency transients due to the slow increase in steam flowrate to the intermediate-pressure section of the steam turbine, resulting in a limited power delivery during frequency primary regulation, as the pressure increase from the high-pressure section reduces the superheated steam flowrate from the intermediate-pressure drum.
Innovation Solution
A control device with regulation modules and valves that dynamically adjust the steam flowrates and superheated steam mixing to maintain steam flow consistency and enhance power delivery, by increasing the high-pressure steam flowrate and adjusting the regulation valve to minimize hydraulic resistance and maintain superheated steam flow, thereby optimizing the steam turbine's contribution during subfrequency transients.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If the steam flowrate to the high-pressure section is increased suddenly to increase power delivery, then the power output increases, but the superheated steam flowrate from the intermediate-pressure drum decreases due to reduced pressure drop
Solution Approach 1:
The control device predicts the future steam flowrate to the intermediate-pressure section based on current operating conditions and the planned increase in high-pressure steam flowrate. This prediction allows the system to prepare compensatory actions in advance, specifically by pre-opening the regulation valve to maintain superheated steam flowrate despite the upcoming pressure drop reduction.
Solution Approach 2:
The control device continuously monitors the actual steam flowrate to the intermediate-pressure section and compares it with the predicted flowrate. Based on this feedback, the control device dynamically adjusts the regulation valve opening degree to compensate for deviations, ensuring that the superheated steam flowrate is maintained at the desired level throughout the transient process.
2Power
If the steam flowrate to the high-pressure section is increased suddenly, then the power delivery increases, but the steam turbine response time is limited due to slow intermediate-pressure section flowrate increase
Solution Approach 1:
The control device predicts the future steam flowrate to the intermediate-pressure section based on current operating conditions and the planned increase in high-pressure steam flowrate. This prediction allows the system to prepare compensatory actions in advance, specifically by pre-opening the regulation valve to maintain superheated steam flowrate despite the upcoming pressure drop reduction.
Solution Approach 2:
The control device continuously monitors the actual steam flowrate to the intermediate-pressure section and compares it with the predicted flowrate. Based on this feedback, the control device dynamically adjusts the regulation valve opening degree to compensate for deviations, ensuring that the superheated steam flowrate is maintained at the desired level throughout the transient process.
3Stability of the object's composition
If the regulation valve opening degree is increased to maintain superheated steam flowrate, then the steam flow consistency is improved, but the device complexity increases due to coordinated control of multiple valves
Solution Approach 1:
The control device is designed to perform multiple functions: it predicts future steam flowrates, determines optimal regulation valve opening degrees, controls the regulation valve, and monitors actual flowrates. This multi-functional approach consolidates what would otherwise require separate dedicated systems into a single integrated control device, managing complexity while achieving steam flow consistency.
Solution Approach 2:
The control device continuously monitors the actual steam flowrate to the intermediate-pressure section and compares it with the predicted flowrate. Based on this feedback, the control device dynamically adjusts the regulation valve opening degree to compensate for deviations, ensuring that the superheated steam flowrate is maintained at the desired level throughout the transient process.
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
The solution allows for a more rapid and effective increase in power delivery during subfrequency transients, ensuring the steam turbine can contribute optimally to frequency primary regulation without degrading steam conditions, thus improving the overall response of the combined-cycle plant.
Implementation Method 1
The steam generator generally comprises cylindrical bodies or drums for separating the phases, one for the high-pressure section of the steam turbine, one for the intermediate-pressure section, and one for the low-pressure section.
Implementation Method 2
In addition, in order to increase the available energy jump and hence the efficiency of the plant, the steam coming from each of the drums is superheated before being fed to the respective section of the steam turbine.
Implementation Method 3
a steam turbine 3, which comprises a high-pressure section 12, an intermediate-pressure section 13, and a low-pressure section 14
Data Source
Figure 1
Figure 2
AI summary
A method for controlling a combined-cycle plant envisages regulating a high-pressure steam flowrate (QHP) fed to a high-pressure section (12) of a steam turbine (3). An output flowrate (QO) from the high-pressure section (12) and a superheated steam flowrate (QS), supplied by a steam generator (7) through a regulation valve (30), are mixed to obtain an intermediate-pressure steam flowrate (QIP) to be supplied to an intermediate-pressure section (13) of the steam turbine (3). The high-pressure steam flowrate (QHP) is increased in response to detection of a subfrequency transient of an electrical network (36) connected to the plant (1). The regulation valve (30) is normally maintained at a partial opening degree. The opening degree of the regulation valve (30) is increased in response to a subfrequency transient so as to compensate for an increase in pressure at output from the high-pressure section (12) with a decrease of a head loss associated to the regulation valve (30) and prevent a reduction in the superheated steam flowrate (QS).