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During a 0.1Hz frequency drop test during online operation, the test results revealed that our units fell significantly short of the required response speed and compliance rate.
The key findings from our internal inspection and analysis are as follows:
* **Low Compliance Rate:** The frequency response compliance rates for the tested units were 32.3% for GT#1, 35.1% for GT#2, and 36.42%–39.69% for GT#3 and #4, which are significantly below the target value of 100%.
* **Limitations of Control Tuning:** To improve the speed response, we modified the Speed Droop Function Value (f(x)), the PID values of the MW Reference logic, and the PID values of the Fuel Valve TRIM logic for fuel signal correction. We also attempted to expand the response range of the STG A/B valves. However, the impact on the actual output response was minimal (only about a 10-15% increase). Furthermore, excessively increasing these tuning parameters raises concerns about combustion instability and control divergence due to valve over-response, making it impossible to apply safely in practice.
* **Root Cause (STG Valve Flow Response Delay):** Although the flow changes in STG A and STG B are major factors driving the output response, we observed that the change in the fuel valve opening command is not proportional to the actual flow change. Notably, there is a dead time of over 10 seconds before the flow actually changes after the valve position adjusts. During the output increase phase, the time delay between the valve opening and the resulting flow change extends to over 40 seconds.
For these reasons, we have concluded that the revised GF (Governor Free) standard—which mandates output recovery within 10 seconds after an event—cannot be met solely through DCS control logic tuning. We believe physical hardware improvements to the equipment are necessary at this stage.
In this regard, I would greatly appreciate your insights on the following three points:
1. Have there been any reported cases of similar, significant delays between STG valve opening changes and actual flow changes in identical W501F units at other power plants?
2. Regarding the root cause of this delay, do you think it is more likely attributable to a mechanical issue (such as mechanical linkage slip or actuator hysteresis) or to dynamics within the combustion environment (such as backpressure variations)?
3. If physical interventions (such as hardware improvements or replacements) beyond control logic tuning are required as we suspect, what are your recommended solutions or technical approaches?
The key findings from our internal inspection and analysis are as follows:
* **Low Compliance Rate:** The frequency response compliance rates for the tested units were 32.3% for GT#1, 35.1% for GT#2, and 36.42%–39.69% for GT#3 and #4, which are significantly below the target value of 100%.
* **Limitations of Control Tuning:** To improve the speed response, we modified the Speed Droop Function Value (f(x)), the PID values of the MW Reference logic, and the PID values of the Fuel Valve TRIM logic for fuel signal correction. We also attempted to expand the response range of the STG A/B valves. However, the impact on the actual output response was minimal (only about a 10-15% increase). Furthermore, excessively increasing these tuning parameters raises concerns about combustion instability and control divergence due to valve over-response, making it impossible to apply safely in practice.
* **Root Cause (STG Valve Flow Response Delay):** Although the flow changes in STG A and STG B are major factors driving the output response, we observed that the change in the fuel valve opening command is not proportional to the actual flow change. Notably, there is a dead time of over 10 seconds before the flow actually changes after the valve position adjusts. During the output increase phase, the time delay between the valve opening and the resulting flow change extends to over 40 seconds.
For these reasons, we have concluded that the revised GF (Governor Free) standard—which mandates output recovery within 10 seconds after an event—cannot be met solely through DCS control logic tuning. We believe physical hardware improvements to the equipment are necessary at this stage.
In this regard, I would greatly appreciate your insights on the following three points:
1. Have there been any reported cases of similar, significant delays between STG valve opening changes and actual flow changes in identical W501F units at other power plants?
2. Regarding the root cause of this delay, do you think it is more likely attributable to a mechanical issue (such as mechanical linkage slip or actuator hysteresis) or to dynamics within the combustion environment (such as backpressure variations)?
3. If physical interventions (such as hardware improvements or replacements) beyond control logic tuning are required as we suspect, what are your recommended solutions or technical approaches?
