Testing and Adjustment of the Turbine Control System 1

7. What is the purpose of the closing time test? How is it conducted? The closing time test is used to measure the closing time of the automatic main steam valve control devices, as well as those of the oil-driven mechanisms that are used to close the valves, in order to determine whether their response speed meets the required standards. The tests were conducted in both stationary and no-load conditions (with steam parameters at rated values). The testing method is: (1) Use a chronometer for measurement; the requirement for connecting the chronometer is that it should be activated when the automatic main steam valve and the control steam valve start to close ； The electric stopwatch stops when the steam valve is fully closed. Before the test, both the automatic main steam valve and the control steam valve are in the fully open position (when testing under no-load conditions, the control steam valve can be made fully open by reducing the opening of the bypass for the electric main steam valve). During the test, manually operate the emergency cut-off valve and record the closing times of the automatic main steam valve and the control steam valve. (2) Measurement using a data recorder: Recording the closing process with a data recorder allows for more accurate determination of the closing time and facilitates analysis. Stroke signal transmitters are installed on the automatic main steam valve and the hydraulic actuator to convert displacement signals into electrical signals, which are sent to the recorder; at the same time, these signals also serve as the tripping signals for the emergency shutdown device (indicating the time when tripping begins). During the test, the manual emergency cut-off switch is activated, and the recorder automatically records the closing process of the main steam valve and the control steam valve. 8. What tests should be carried out on the overspeed protection device? How are they conducted? To ensure the reliability of the overspeed protection device during the operation of the turbine, it is necessary to conduct tests on this device in accordance with relevant regulations after the turbine is installed, after each major overhaul, or after 2000 hours of operation. These tests include: (1) Manual test: The purpose of this test is to check whether the emergency shutdown valve, the automatic main steam valve, and the control valves function properly. The tests are conducted with the turbine at rest and under no-load conditions, and can be carried out simultaneously with the tightness tests and closing time tests of the automatic main steam valve and the control steam valve. (2) Oil injection test: Large steam turbine units are equipped with oil injection test devices for emergency safety valves, which allow tests to be conducted either under no-load conditions or under load. The no-load oil injection test can determine the oil injection strike speed of the emergency safety device. The load-bearing oil injection test is a test to verify the operation of the emergency safety device while the system is operating under normal load conditions. During the test, the trip device of the safety device under test is disconnected from the normal operating system (at this time, the other safety device and its trip device remain in normal operation and can still provide protection), and it is connected to the testing system. After the test is completed, it is returned to normal operating status, after which the test is conducted on the other safety device. (3) Over-speed test: The purpose is to determine the actual operating speed of the emergency safety device. The test was conducted under no-load conditions. During the test, the speed is raised to its maximum using a synchronizer, and then the overspeed testing device is used to increase the speed gradually, causing the emergency shutdown device to activate; the speed at which this occurs is then recorded. 9. What issues should be considered when conducting an overspeed test? The following points should be taken into account when performing an overspeed test: (1) The test should be carried out twice under identical conditions, and the difference between the rotational speeds during these two tests should not exceed 0.6% of the rated speed. For newly installed or overhauled units, a second test should be conducted; the difference between the speed at which it operates during this second test and the average of the speeds at which it operated in the previous two tests should not exceed 1% of the rated speed. (2) If the unit has two emergency safety devices, an no-load test is conducted first to determine which one operates at a lower speed. Using the oil injection test device, switch the test handwheel to the emergency shutdown devices with a lower operating speed, so that the operating speeds of both emergency shutdown devices can be measured simultaneously in a single test. (3) For high-power units, sufficient warm-up time should be allowed before the overspeed test to prevent the rotor metal temperature from dropping below the brittle transition temperature, which could affect the safety of the unit; this is particularly important for units that start under sliding-parameter conditions. 10. When the operating speed of the emergency safety device does not meet the requirements, how should the adjustment be made? If the manufacturer provides values indicating how many turns the adjustment nut needs to be turned in order to change the operating speed, then the number of turns required can be calculated using the following formula: L0 = (n – n0) / nm. Here, L0 represents the number of turns the adjustment nut needs to be turned ； nm: The value specified by the manufacturer for the change in speed per rotation of the adjustment nut, in rpm ； n: Operating speed of the emergency safety device before order adjustment, rpm ； n0: The specified operating speed at which the emergency safety device activates, in rpm. If the nm values provided by the manufacturer are not available, the following method can be used to determine the number of turns required for adjustment: If the speed at which the emergency safety device activates is measured as n1 for the first time, turn the adjustment nut L1 by L1 turns (usually L10); the adjustment should be made in the direction that increases the speed ； If L0

13. What is the reason why the control system cannot maintain operation at no load? When the automatic main steam valve is fully open while the control steam valve has not yet been opened, the speed of the turbine continues to increase, even reaching the speed at which the emergency safety device activates; as a result, the unit cannot maintain stable operation at no load. The reasons for this are: (1) The control steam valve is not sealed properly, allowing steam leakage； (2) If the joint between the valve seat and the steam vessel is not tight, steam will leak into the turbine ； (3) Adjust the valve disc to eliminate sticking, so that the steam valve does not close properly ； (4) The cold-state clearance between the cam and the roller in the cam-type valve distribution mechanism is too small; under hot conditions this clearance disappears, or improper installation of the cam results in the control valve not closing properly and thus gas leakage occurs ； (5) The operating stroke of the synchronizer is not adjusted properly; the minimum allowable stroke is too small. When the governor and speed exceed the rated values, the synchronizer’s adjustment capacity is insufficient or it loses its ability to adjust. This often happens in spring-type synchronizers. 14. How is the operating range test for the synchronizer conducted? During the test, the synchronizer is first placed in its lowest position, and the first measurement is taken. Then the synchronizer is operated to increase the rotational speed; a measurement is taken every 25–30 r/min until it reaches its highest position. The recorded parameters are synchronizer stroke and turbine speed. For systems with non-assisted spring synchronizers, the travel of the governor sleeve must also be recorded; whereas for hydraulic regulation systems, the pulse oil pressure value is recorded. Through this test, it is primarily checked whether the turbine can maintain idling, as well as whether it can be connected to and disconnected from the power grid at lower grid frequencies, in order to determine the operating range of the synchronizer. 15. What is the purpose of the no-load speed-up and speed-down tests? How are they carried out? What issues need to be considered? The purpose of the no-load speed-up and speed-down tests is to determine the static characteristic curves of the induction mechanism and the transmission amplification mechanism. During the test, the synchronizer was placed at the middle and upper positions respectively, and the main steam valve was slowly closed to reduce the unit’s speed; relevant data were recorded once the speed stabilized. The interval between measurements of the speed at various points should be such that there are no fewer than 8 measurements within the full open range of the hydraulic actuator. Generally, a measurement is taken every time the speed drops by 25–30 r/min, until the speed reduction test with the hydraulic actuator fully open is completed. After that, the main steam valve is slowly opened to carry out a speed increase test, until the original speed is reached again. The following points should be taken into consideration during testing: (1) The speed of increasing or decreasing the rotation rate should not be too fast; generally, the rate at which the rotation rate is changed should be 100 r/min ； (2) The test is allowed to proceed in only one direction, that is, deceleration tests and acceleration tests are conducted separately; reciprocating speed changes are not permitted ； (3) During testing, keep the steam parameters at their rated values as much as possible. 16. How to conduct a load test? What issues should be noted? A load test is carried out when the unit is operating connected to the grid. Record the no-load data before connecting to the grid. After connection to the grid, there should be no fewer than 12 measurement points between no-load and rated load conditions; the measurement points near no-load and rated load should be more frequent. Load changes are adjusted by operating the synchronizer, and recordings are taken after the load has stabilized for a certain period of time. If the steam parameters and exhaust vacuum remain relatively stable during the test, it may not be necessary to conduct a load-reduction test; otherwise, the load-reduction test can be carried out after completing the load-increase test. The following points should be noted during the load test: (1) During the test, try to keep the parameters of the fresh steam within the design range ； (2) Before the start of the test, the exhaust vacuum is reduced to its maximum value; no adjustments are made during the test, allowing it to change according to load variations ； (3) The regenerative system shall be fully activated, but it shall not be operated during the testing process. 17. What is a test of the dynamic characteristics of a control system? The dynamic characteristics of a control system refer to the non-steady-state behavior of the parameter being controlled, such as speed, as it changes over time. That is, the characteristics of the transition process from one stable state to another. The purpose of testing the dynamic characteristics of the control system is mainly to determine the speed rise curve during load rejection, so as to accurately assess the quality of the transition process; meanwhile, the data obtained from these tests can provide a basis for improving the dynamic performance of the system. 18. What conditions are required for conducting dynamic tests on control systems? During dynamic tests, also known as load-shedding tests, the speed of the turbine must increase above its rated speed. This is a complex and important test, and it is necessary to carry out all related tasks with great care to ensure the safety of the turbine during the load-shedding process. Before the test, the unit must meet the following conditions: (1) The speed variation rate, lag rate of the control system, as well as the stroke range of all components must be within specified limits ； (2) The tightness tests of the automatic main steam valve and control steam valve passed, and the performance of the extraction check valve was good with its tightness also meeting the requirements. (3) The manual emergency shutdown device operates normally and reliably. The operating speed during the overspeed test meets the requirements. (4) Automatic main steam valve, which adjusts the closing time of the steam valve; this time should generally be less than 0.4–1 s. (5) The generator cut-off button operates reliably.

19. What issues should be considered during the load shedding test? The following points should be taken into account during the load shedding test: (1) The frequency should be kept close to the rated value, avoiding values that are too high. (2) The unit should be operating at normal load, with the deviations of the initial steam temperature and pressure from their rated values being less than 5%. (3) Dedicated personnel should be assigned to monitor the manual emergency safety devices, automatic main steam valves, extraction check valves, and vacuum break valves, so that rapid action can be taken in the event of automatic failure. (4) When loading is reduced, a dedicated person must monitor the speed. The tachometer is equipped with a maximum speed mark; if the speed exceeds this mark and the emergency shutdown device does not activate, the machine should be stopped immediately by shutting off the switch. (5) Operational data should be recorded comprehensively and accurately before and after the test. 20. What are the methods for adjusting the rate of speed variation in a control system? By changing the slope of one of the characteristic curves – those of the governor, the transmission amplification mechanism, or the valve timing mechanism – it is possible to alter the slope of the control system’s characteristic curve, thereby changing the rate of speed variation in that system. (1) Changing the governor characteristic curve: For mechanical speed sensing mechanisms, the spring stiffness of the governor can be altered in order to change the rate of speed variation. Spring stiffness is proportional to the rate of speed variation; an increase in stiffness leads to an increase in this rate, while a decrease in stiffness results in a decrease in it. Changing the spring stiffness generally involves altering the number of effective windings of the spring, and the number of windings is inversely proportional to the stiffness. To increase the rate of speed variation, it is necessary to reduce the number of windings in the spring ； Conversely, increasing the number of working turns of the spring reduces the rate of speed variation. For hydraulic speed sensing elements, such as rotary damping governors, the rate of speed variation can be adjusted by changing the diameter of the disc valve, the diameter of the bellows, the inner and outer diameters of the rotary damper, or the diameter of the throttle hole through which pressure oil flows into the secondary pumping chamber. (2) Changing the characteristic curve of the transmission amplification mechanism: A convenient way to change the slope of the characteristic curve of the transmission amplification mechanism is to adjust the feedback rate; by increasing the amount of feedback generated by the same piston displacement in the hydraulic actuator, the rate of speed variation can be increased. For example, increasing the slope of the feedback cone or feedback ramp that controls the opening degree of the feedback oil port, or increasing the transmission ratio of the feedback lever and the lift of the feedback cam at the same angle, can all increase the rate of speed variation. (3) Changing the characteristic curve of the steam distribution mechanism: In this case, it is required to change the displacement of the hydraulic motor piston while keeping the flow area constant, so as to generate the same power. For control valves with a throttle cone profile, the profile of the throttle cone can be altered through calculation ； For adjustment systems using cam mechanisms, it is possible to change the cam rotation angle at which the control steam valve reaches its maximum lift under full load; moreover, it is more convenient to alter the displacement of the oil motor piston by changing the overlap degree of the control steam valve. Changing the valve overlap alters the displacement of the hydraulic motor piston; however, it should be noted that the local velocity variation rate also changes at this time. 21. How to adjust the operating range of the synchronizer? When the upper limit position of the synchronizer meets the requirements for the operation of the unit, it is advantageous to expand its operating range; at this point, lowering the lower limit position of the synchronizer is beneficial for the unit’s operation. Adjustment is only necessary when the synchronizer’s operating range is insufficient. If the speed variation rate of the control system is too high and the operating range of the synchronizer is insufficient, the speed variation rate should be changed ； If the position of the synchronizer limit point is incorrect, the position of the limit point should be adjusted ； More commonly, the initial tension of the synchronizer spring is not adjusted properly; in such cases, the thickness of the spring shims should be changed in order to adjust the initial tension of the synchronizer spring. 22. How to adjust the positions of the upper and lower limits on the static characteristic curve? If the operating range of the synchronizer meets the requirements, but the positions of the upper and lower limits are too high or too low, it is possible to change the position of the starting point of the static characteristic curve by adjusting factors such as the initial tension of the spring, the initial height of the oil port, the position of the feedback cone on the piston rod of the hydraulic actuator, and the initial operating position of the synchronizer. This allows the positions of the characteristic curves for the various intermediate stages to change, thereby altering the positions of the upper and lower limits on the static characteristic curve. 23. How to conduct tightness tests on the control steam valve and the automatic main steam valve? A tightness test is a test to determine the degree of tightness with which these valves close. (1) Sealing test of the automatic main steam valve: The test is carried out at the rated steam parameters, vacuum level, and rated speed. With the steam valve adjusted to be open, close the automatic main steam valve, and record the stable speed and the time taken to reach it; generally, the stable speed is required to be maintained for 15 minutes, with the speed dropping below 1500 r/min. (2) Tightness test of the control steam valve: At the rated steam parameters, vacuum level, and rated speed, the main steam valve is opened automatically; the synchronizer is used to close the control steam valve. The stable speed and the rate at which the turbine’s speed drops after the control steam valve is closed are recorded. It is generally required that this rate be essentially consistent with the coasting curve of the turbine after shutdown under the same steam parameters and vacuum level. 24. How to conduct tests on the closing time of the regulating steam valve and the automatic main steam valve? In addition to requiring tight sealing, the regulating steam valve and the automatic main steam valve also need to operate quickly and smoothly. Generally, it is required that the closing time of the automatic main steam valve be no more than 0.5–0.8 seconds, and the closing time of the hydraulic actuator be no more than 1 second. Therefore, a shutdown time test is required. The tests were conducted in a stationary state with no steam pressure and under no-load (rated parameter) conditions. The method is as follows ； (1) Measure the closing time with an electric stopwatch. Connect an electric stopwatch to the end points of the stroke at both ends of the automatic main steam valve and the hydraulic actuator, as well as to the stroke indicator. It is required that as soon as the automatic main steam valve and the hydraulic actuator start to close, the electric stopwatch shall immediately start counting; it shall then stop automatically once the closing process is complete. When both the automatic main steam valve and the hydraulic actuator are in the fully open position, the manual emergency safety device is activated, and the times at which the automatic main steam valve and the hydraulic actuator close are recorded. (2) Measure the entire process of shutdown using a data recorder. Install travel sensors on the automatic main steam valve and the hydraulic actuator to convert displacement signals into electrical signals that are sent to the data recorder. A tripping signal at the emergency safety device indicates the start time; by manually activating the emergency safety device, recording can be done, thereby capturing the entire process of shutdown.