What are the operations and precautions for a steam turbine to shed load?

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I. Operations after load shedding 1. After load shedding occurs, the turbine operator must first check whether the turbine speed is too high, whether the main valve, control valves, and heating check valves are closed, and whether the main oil pressure and lubricating oil pressure are within normal ranges. They also need to check the safety oil pressure, the pressures of the first and second stage oils, as well as the axial displacement oil pressure. If necessary, the turbine oil pump should be started, and all relevant data should be recorded. Conduct a comprehensive inspection of the steam turbine; if the problem is not related to the turbine, try to maintain the rated speed while waiting for reconnection to the grid. 2. In the event of a load rejection related to electrical issues, adjust the synchronizer promptly to restore normal rotation speed, then measure vibrations and check all operating parameters of the turbine. Once it is confirmed that there are no abnormalities with the turbine, reconnect it to the grid as requested by the dispatch team, shift leader, or on-duty supervisor. 3. In the event of load shedding due to issues with the turbine, ensure the lubricating oil pressure is maintained; if necessary, start the turbine oil pump. 4. If the control valve is closed, check whether the main steam valve is also closed. If it isn’t closed, turn the synchronizer to its lower limit [i.e., the topmost position] to see if the rotational speed decreases. If there is no response or if the rotational speed increases, immediately engage the emergency brake to shut down the machine. Then check whether the main steam valve is closed and turn the synchronizer to the topmost position. Attempt to open the start valve again and check whether the pulse oil pressure at position 1 is normal, to ensure that the control valve can operate. After the start valve is fully open, slowly open the automatic main valve to reach the rated speed. After verification shows no abnormalities, connect to the grid. 5. If the main valve trips and shuts down, activate the emergency safety device, open the start valve, adjust the synchronizer to its lowest position, and maintain the rotational speed above 2200 r/Min. (It is strictly prohibited to open the main valve when the speed is between 1200 r/Min and 2200 r/Min.) After restarting the machine, check that it is connected to the grid properly. The so-called load rejection accident refers to an accident phenomenon in which a steam turbine generator set suddenly loses all or part of its load. There are mainly the following types of load shedding: 1) The unit’s load cannot be delivered normally due to a sudden trip of the power supply transmission lines ； 2) Generator protection trips, opening the generator outlet switch ； 3) Turbine protection activates; the automatic main steam valve closes suddenly ； 4) During operation, one automatic main steam valve, governing valve, or one oil actuator suddenly closes. II. Determining load shedding When a unit experiences load shedding, the operating staff must quickly identify the cause of it before taking appropriate actions to address it. The main methods for making such determinations are as follows: (1) When load shedding is caused by electrical issues (the first two types mentioned above), the generator loses all or most of its load. In this case, the most noticeable characteristic is an increase in the rotor speed; if the dynamic characteristics of the turbine’s speed control system are not optimal, it may lead to the activation of the turbine’s over-speed protection mechanism, resulting in shutdown of the turbine. (2) When the unit loses load due to a turbine protection action (the third type mentioned above), the generator set will shed all of its load, at which point the unit’s speed remains essentially unchanged compared to before the load loss. Due to the closure of the automatic main steam valve, all steam flowing into the turbine was cut off. At this point, the unit was able to maintain a stable rotational speed solely thanks to the power supplied back from the power grid; in other words, the generator set operated in motor mode, a condition known as reverse power operation. (3) When a load rejection of the unit occurs due to a sudden closure of the governing valve (the aforementioned Type 4), only a portion of the load is shed from the generator set, while the unit’s rotational speed remains unchanged. The amount of load shedding depends on the flow rate through the suddenly closed valve, as a proportion of the steam intake of the unit at that time, and it is also related to the type of valve. III. Hazards of load rejection accidents
When a turbine-generator set experiences load rejection during operation, it not only has certain negative effects on the stable operation of the power grid, but also poses a direct threat to the safe operation of the unit. The hazards are mainly manifested in the following aspects:
(1) Load rejection is a primary cause of overspeed in the unit. Overspeed often triggers the overspeed protection system, resulting in the shutdown of the unit; in severe cases, it may even lead to the destruction of the turbine-generator set due to runaway rotation. (2) The load rejection caused a significant thermal shock to the unit. After a load rejection, the unit’s load undergoes a significant change, resulting in a decrease in the amount of steam entering the turbine. Due to the throttling effect of the governing valves, the temperature of the steam flowing through the turbine’s flow passages drops considerably. This causes the surfaces of the cylinder and rotor to be rapidly cooled, thereby generating substantial thermal stresses. Data show that when a running unit suddenly loses 50% of its load, the thermal stresses generated in the cylinder and the metal components of the rotor are the most severe. If all the load is suddenly removed due to the activation of the turbine protection system and the main steam valve closes automatically, although no steam flows through the turbine for a temporary period, subsequent accident handling requires the turbine to be fed with steam again, and starting under extremely high temperature conditions often results in greater thermal shock. (3) The load rejection process is accompanied by a relatively large mechanical shock. After a load rejection, the sudden change in the unit’s load causes the flow rate and state of the steam passing through the turbine’s flow path to change accordingly. Consequently, the axial thrust acting on the rotor also changes, resulting in a sudden shift in the indicated axial displacement value. This, in turn, subjects the thrust bearing and coupling bolts to a significant mechanical shock. (4) Load rejection causes a significant disturbance to the rotor of the steam turbine generator. When a unit in operation suddenly loses load, the rotor, which was previously operating relatively stably, is subjected to an uneven steam flow impact, causing a sudden change in the unit’s vibration. (5) After load shedding, the boiler pressure vessel may still operate under overpressure, which can easily cause the safety valve to activate.