Feedforward Steam Drum Water Level Control for FCB Tests
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Solution Overview
Problem
During a fast cut back (FCB) test or load rejection test, conventional steam drum water level control systems in steam turbines experience rapid water level fluctuations, leading to false readings and potential main fuel trip (MFT) protection actions due to sharp changes in steam pressure and flow, which existing three-impulse control modes fail to manage effectively.
Innovation Solution
A feedforward control structure that measures steam drum water level and steam flow, using a feedforward signal to adjust feedwater flow based on steam production per ton of coal, stabilizing water levels by switching to a converted steam flow during FCB or load rejection tests, thereby preventing false water level triggers and maintaining system stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If conventional three-impulse control mode is used for steam drum water level control, then the control system structure is simple, but the water level control accuracy deteriorates during FCB test causing false water level and triggering MFT protection
Solution Approach 1:
The feedforward control calculates the required feedwater adjustment in advance based on the detected steam flow change, and applies this adjustment before the water level actually changes. This preliminary action prevents the false water level phenomenon from occurring in the first place, rather than reacting after the problem arises.
Solution Approach 2:
The control system combines feedforward control (based on steam flow changes) with feedback control (based on actual water level measurements). The feedback mechanism continuously monitors the water level and adjusts the feedforward calculation parameters to compensate for disturbances, ensuring reliable water level control during FCB tests.
2Speed
If feedwater flow is quickly reduced to respond to false water level rise, then the water level control responds rapidly, but the water level drops sharply below safe levels triggering MFT protection
Solution Approach 1:
The feedforward control applies a counteracting feedwater adjustment in advance based on the detected steam flow reduction. When the steam turbine reduces steam flow, the system proactively increases feedwater flow to compensate, preventing the water level from rising falsely in the first place, rather than reacting to the false rise and potentially overcorrecting.
3Stress or pressure
If steam inlet control valve is quickly closed during FCB test, then the steam pressure control is effective, but the steam flow drop causes steam drum water level to rise rapidly creating false water level
Solution Approach 1:
The feedforward control acts as an intermediary mechanism between the steam flow change and the water level. It translates the steam flow change into a compensatory feedwater adjustment, mediating the relationship between steam pressure control actions and water level to prevent false measurements.
Data Source
AI summary
The present disclosure relates to a feedforward structure for controlling water level, in particular to a feedforward structure for controlling a steam drum water level in a steam turbine FCB test and a control method therefor. The feedforward structure includes a steam drum provided with a steam drum water level measurement sensor therein, a feed water pipe provided with a feed water flow measurement sensor therein, and a steam inlet pipe provided with a steam flow measurement sensor therein. A steam drum water level measurement value I in the steam drum water level measurement sensor is compared with a set value of the steam drum water level.