Automatic control method for shaft seal system of steam turbine under full-automatic load rejection working condition

Abstract

A kind of automatic control method of steam turbine shaft seal system under full-automatic load rejection working condition, its control strategy includes: A, shaft seal steam supply regulating valve control strategy: after load rejection or FCB triggers, shaft seal steam supply regulating valve first opens to a certain degree, then automatically invests shaft seal pressure;B, shaft seal overflow valve control strategy: after load rejection or FCB triggers, shaft seal overflow valve cuts manual, automatically gives instruction to 0%;C, shaft seal steam supply desuperheating water regulating valve control strategy: after load rejection or FCB triggers, shaft seal steam supply desuperheating water regulating valve cuts manual, automatically gives instruction to 0%;D, shaft seal mother tube drain valve control strategy: after load rejection or FCB triggers, shaft seal mother tube drain valve automatically opens.This application is successfully applied to a million unit, and the unit 75% FCB and 100% full-automatic load rejection test are all successful, and the unit has the condition of fast grid connection with load after test completion, and fully meet the requirements of safe and stable operation of unit.

Description

Technical Field

[0001] This invention relates to the field of thermal power unit control, specifically an automatic control method for a turbine shaft sealing system under fully automatic load shedding conditions. Background Technology

[0002] Fast Cut Back (FCB) is a solution proposed to enable generating units to quickly restart and reconnect to the grid after a severe grid failure. In the event of an emergency grid failure, the unit rapidly disconnects from the grid and enters island operation mode. The boiler maintains stable combustion, the turbine maintains a speed of 3000 rpm, and the generator operates to supply power to the plant's auxiliary load. When the grid returns to normal, subject to grid dispatch permission, it can quickly reconnect to the grid and supply power to important users, playing a crucial role in ensuring grid security and stability.

[0003] The FCB (Load Shedding) test is the riskiest and most challenging test for thermal power units. While there are successful FCB cases for some thermal power units, successful high-load FCB tests are few and far between, especially for 1,000 MW units. There are no known cases of 100% fully automated load shedding tests. The main reason for this is that when a unit transitions from normal operation to FCB conditions, various operating parameters change drastically, placing higher demands on the operation of main and auxiliary equipment. Even slight errors can easily lead to test failure, or even equipment damage accidents such as turbine water ingress, abnormal unit vibration, boiler overpressure, power outages, oil shortages, and bearing failure. The special requirements from some owners for fully automated control of the entire FCB and load shedding test process further increase the test difficulty and place higher demands on the automatic control strategies of the main and auxiliary equipment.

[0004] The shaft sealing system is a crucial component of a steam turbine, its primary function being to provide sealing steam to the turbine's shaft seals. The normality of its operating parameters directly impacts the safe and stable operation of the main unit. During normal unit operation, the pressure and temperature of the shaft sealing system remain relatively stable, having little impact on the main unit's safety and economic efficiency. However, once the unit enters the Full Load Sheet (FCB) condition, the shaft seal steam source switches from self-sealing to external steam supply. At this point, the shaft seal pressure fluctuates significantly, and the shaft seal temperature drops drastically. Conventional control methods struggle to quickly bring the shaft seal parameters back to normal within a short timeframe, easily leading to abnormal phenomena such as condenser vacuum drop, dynamic and static friction, abnormal unit vibration, and water carryover in the lubrication system. This can cause the unit to trip due to low condenser vacuum and excessive vibration, triggering protective measures, and even resulting in serious accidents such as seal wear, rotor / journal / blade damage, and bearing failure. Therefore, the automatic control strategy of the turbine shaft sealing system is critical to the success of fully automatic load shedding or FCB tests of thermal power units. Summary of the Invention

[0005] This invention primarily addresses the shortcomings of existing technologies by providing an automatic control method for a turbine shaft sealing system under fully automatic load shedding conditions. This method can achieve the goal of automatically controlling the turbine shaft sealing system pressure to quickly stabilize and meet the normal operation of the unit, while also avoiding the risk of accidents such as abnormal turbine vibration and dynamic-static friction caused by excessively low shaft sealing temperatures. This improves the success rate of fully automatic load shedding and FCB (Full Load Break) in thermal power units, and provides favorable conditions for rapid grid connection of the unit.

[0006] An automatic control method for a turbine shaft sealing system under fully automatic load shedding conditions, the control strategy of which includes:

[0007] A. Control strategy for shaft seal steam supply regulating valve

[0008] After load shedding or FCB triggering, the shaft seal steam supply regulating valve first opens to a certain degree, and then the shaft seal pressure is automatically activated.

[0009] B. Shaft seal overflow valve control strategy

[0010] After load shedding or FCB triggering, the shaft seal relief valve switches to manual mode and automatically sends the command to 0%.

[0011] C. Control strategy for shaft seal steam supply desuperheating water regulating valve

[0012] After load shedding or FCB triggering, the shaft seal steam desuperheating water regulating valve switches to manual mode, and the automatic command is sent to 0%.

[0013] D. Shaft seal header drain valve control strategy

[0014] After load shedding or FCB triggering, the shaft seal header drain valve automatically opens.

[0015] Furthermore, in the control strategy for the shaft seal steam supply regulating valve, the current shaft seal steam supply pressure function is used as the setpoint for the joint opening of the shaft seal steam supply regulating valve.

[0016] Furthermore, in the control strategy for the shaft seal steam supply regulating valve, the shaft seal pressure function is obtained as follows: when the unit is carrying 3-5% of the rated load and the shaft seal main pipe drain valve is fully open, under the premise that the shaft seal system main pipe pressure is normal, the shaft seal steam supply regulating valve opening value corresponding to different shaft seal steam supply pressures is obtained.

[0017] Furthermore, in the control strategy of the shaft seal steam supply regulating valve, the shaft seal steam supply regulating valve is always in automatic control mode, and the shaft seal pressure setpoint is the recommended value of the main engine manufacturer or the empirical value under long-term operating conditions of the unit; when the FCB is reset, the shaft seal steam supply regulating valve is still in automatic control mode, and the pressure setpoint remains unchanged. At this time, the pressure setpoint can be manually modified or switched to manual control mode.

[0018] This invention offers the following advantages: The automatic control method for the turbine shaft sealing system under fully automatic load shedding conditions proposed in this invention can quickly stabilize shaft sealing parameters and meet the goal of normal unit operation without adding additional equipment or changing the original logic function of the unit. It achieves fully automatic control throughout the process, offering high safety, reliability, and ease of implementation without manual intervention. It has been successfully applied to a million-kilowatt unit. Test results show that the unit successfully completed 75% FCB and 100% fully automatic load shedding tests on the first attempt. During the tests, the shaft sealing parameters quickly stabilized and met operational requirements, with minimal vibration changes and condenser vacuum within the normal range. The unit was ready to connect to the grid and carry load at any time, demonstrating that this automatic control strategy for the turbine shaft sealing system has strong engineering application value and can provide a reference for the design and optimization of automatic control of turbine shaft sealing systems during FCB and fully automatic load shedding tests for similar million-kilowatt units and other types of units. Attached Figure Description

[0019] Figure 1 This describes the pressure and temperature changes of the turbine shaft sealing system under the 75% FCB test condition of this invention.

[0020] Figure 2 These are the curves showing the changes in key parameters such as condenser vacuum under the 75% FCB test conditions of this invention.

[0021] Figure 3 The curves showing the changes in key parameters such as condenser vacuum under the 100% fully automatic load shedding test conditions of this invention are shown in the embodiments of the present invention. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0023] This invention provides an automatic control method for a turbine shaft sealing system under fully automatic load shedding conditions, the control strategy of which includes:

[0024] (1) Shaft seal pressure control strategy

[0025] During the initial startup phase of the turbine unit, the shaft seal inlet valve controls the pressure of the shaft seal steam header, supplying shaft seal steam to various sections of the high, medium, and low pressure cylinders. At this time, the shaft seal relief valve is basically closed. As the unit load increases, steam in the high, medium, and low pressure cylinders overflows into the shaft seal header, causing the shaft seal header pressure to exceed the set value. The shaft seal inlet valve gradually closes, and the shaft seal relief valve gradually opens to control the shaft seal header pressure, allowing excess shaft seal steam to enter the condenser. This stage is also called the self-sealing stage.

[0026] Since the shaft seal inlet valve and shaft seal relief valve operate based on the shaft seal header pressure, regardless of whether the turbine is in a self-sealing or non-self-sealing state, the shaft seal pressure can be controlled by adjusting the opening degree of the shaft seal inlet valve and shaft seal relief valve.

[0027] (1.1) Control strategy for shaft seal steam supply regulating valve

[0028] After load shedding or FCB triggering, the shaft seal steam source will switch from self-sealing to external steam supply, causing significant fluctuations in shaft seal pressure. To prevent excessively low shaft seal system pressure, the shaft seal steam supply regulating valve is first opened to a certain degree, then the shaft seal pressure is switched to automatic control and remains in automatic control mode. The shaft seal pressure setpoint can be the OEM's recommended value or an empirical value under long-term unit operating conditions. After load shedding or FCB reset, the shaft seal steam supply regulating valve remains in automatic control mode, and the pressure setpoint remains unchanged. At this time, the pressure setpoint can be manually modified, or the system can be switched to manual control mode.

[0029] Regarding the opening degree of the shaft seal steam supply regulating valve, it is recommended to use the current shaft seal steam supply pressure function as the set value for the opening degree when the shaft seal steam supply regulating valve is in operation. The pressure function can be obtained as follows: under the condition that the unit is carrying 3-5% of the rated load and the shaft seal main pipe drain valve is fully open, and assuming that the shaft seal system main pipe pressure is normal, the opening degree value of the shaft seal steam supply regulating valve corresponding to different shaft seal steam supply pressures.

[0030] (1.2) Shaft seal relief valve control strategy

[0031] After load shedding or FCB triggering, the steam source for the shaft seal switches from self-sealing to an external steam source, at which point the shaft seal system has no excess steam. If the shaft seal relief valve remains open, steam from the shaft seal system will enter the condenser through the relief valve, causing a further decrease in the shaft seal system pressure. Therefore, after load shedding or FCB triggering, the shaft seal relief valve needs to be closed quickly. The specific control strategy is as follows: after load shedding or FCB triggering, the shaft seal relief valve immediately switches to manual mode and automatically sends a command to 0%.

[0032] (2) Shaft seal temperature control strategy

[0033] The steam supply to the shaft seal is in direct contact with the turbine's main shaft, and its temperature directly affects the shaft's expansion and contraction. During normal operation, both the rotor and shaft seal temperatures are high. If the steam supply temperature to the shaft seal suddenly drops, not only will the steam seal be cooled, but the main shaft section in the shaft seal section may also experience rapid cooling and contraction. This alters the clearance between the rotor and cylinder. If the contraction is excessive, dynamic and static friction can occur, leading to abnormal vibration of the unit and affecting the safe and stable operation of the main engine. When the shaft seal temperature is too low, water may be carried into the steam supply to the shaft seal, causing water hammer in the steam supply pipeline. In severe cases, water may enter the turbine, leading to serious equipment damage accidents such as cylinder deformation, rotor and steam seal damage. Therefore, changes in shaft seal temperature have a significant impact on turbine safety.

[0034] (2.1) Control strategy for the steam supply desuperheating water regulating valve for shaft seal

[0035] Under high-load or full-load conditions, the high-pressure shaft seal temperature of the steam turbine can reach over 500℃. After load shedding or FCB triggering, the shaft seal steam source will switch from self-sealing to external steam source. To avoid severe unit vibration, the inlet steam temperature of the shaft seal system should be increased as much as possible to match the temperature of the high-pressure shaft seal and cylinder. Considering that the desuperheating water regulating valve for the shaft seal supply may be open during unit operation, it is necessary to close the desuperheating water regulating valve to increase the shaft seal inlet steam temperature. The specific control strategy is as follows: after load shedding or FCB triggering, the desuperheating water regulating valve for the shaft seal supply is switched to manual mode, and the automatic mode sends the command to 0%.

[0036] (2.2) Control strategy for the drain valve of the shaft seal header

[0037] After load shedding or FCB triggering, the steam source for the shaft seal is switched from self-sealing to an external steam source. The steam inlet temperature of the shaft seal is approximately 300 degrees Celsius, while the maximum temperature of the turbine shaft seal can reach over 500 degrees Celsius. At this time, the drain valve needs to be opened in time to remove any cold steam or accumulated water that may be generated in the shaft seal system during the steam source switching process. The specific control strategy is as follows: after load shedding or FCB triggering, the drain valve of the shaft seal main pipe is automatically opened.

[0038] Based on the FCB test project of the 1×1000MW ultra-supercritical unit in Cilacap Phase III, Indonesia, the inventors of this application summarized the control difficulties of the turbine shaft sealing system under FCB and load shedding conditions, and proposed corresponding countermeasures, forming a control strategy for the turbine shaft sealing system under FCB and load shedding conditions. After applying the research results to the Cilacap Phase III 1000MW unit project in Indonesia, the unit's 75% FCB and 100% fully automatic load shedding tests were both successful on the first attempt, proving that the control strategy is feasible in practice. It can provide a reference for the automatic control design and optimization of the turbine shaft sealing system for FCB and fully automatic load shedding tests of similar 1000MW units and other types of units.

[0039] 1. Project Overview and Introduction of Main Equipment

[0040] The Cilazza Phase III 1×1000MW ultra-supercritical coal-fired power plant in Cilazza, Central Java Province, Indonesia, is the largest single-unit capacity power plant in Indonesia and also the first domestically produced 1,000MW unit exported. Due to Indonesia's weak power infrastructure, large grid frequency fluctuations, small grid capacity, and unstable grid load, newly built units are required to have FCB (Fuel-Cooled Grid) functionality, which is one of the necessary conditions for new units to enter commercial operation.

[0041] (1) The boiler is a DG2953 / 28.25-Ⅱ3 type ultra-supercritical parameter variable pressure operation spiral tube once-through boiler manufactured by Dongfang Boiler Factory. It is a single furnace, with one intermediate reheat, adopts front and rear wall opposed combustion mode, balanced ventilation, solid slag discharge, all-steel suspended structure Π type, and open-air coal-fired boiler.

[0042] (2) The steam turbine is the N1000-27 / 600 / 600 type ultra-supercritical, single-stage intermediate reheat, four-cylinder four-exhaust, single-shaft, eight-stage regenerative, double back pressure, condensing steam turbine manufactured by Shanghai Steam Turbine Works.

[0043] (3) The generator is a QFSN-1000-2 type water-hydrogen-hydrogen static excitation three-phase synchronous steam turbine generator manufactured by Shanghai Electric Machinery Factory.

[0044] 2. Requirements for turbine shaft sealing system under FCB or fully automatic load shedding conditions

[0045] The main function of the turbine shaft sealing system is to provide sealing steam to the shaft seals of the turbine, feedwater pump, and small turbine, while also properly guiding or extracting leaking steam from each seal. During normal operation, the shaft seal pressure and temperature do not change significantly, having little impact on the safety and economy of the main unit. However, when the unit enters FCB or load shedding mode, the shaft seal steam source switches from self-sealing to external steam supply, causing drastic changes in shaft seal parameters. This can easily lead to abnormal phenomena such as condenser vacuum drop, dynamic and static friction, abnormal unit vibration, and water carryover in the lubrication system. These abnormalities can cause the unit to trip due to protective actions such as low condenser vacuum and excessive vibration, or even serious accidents such as seal wear, rotor / blade damage, and bearing failure.

[0046] Therefore, after the unit's FCB or fully automatic load shedding is triggered, whether the shaft sealing system can achieve automatic control of shaft sealing pressure and temperature, and quickly control each shaft sealing parameter within the normal range, is the key to the success of the unit's FCB or fully automatic load shedding. This puts forward the following requirements for the shaft sealing system control strategy.

[0047] (1) Prevent the shaft seal pressure from being too high or too low.

[0048] During unit operation, high shaft seal pressure can lead to steam leakage from the shaft seal, causing water ingress into the lubricating oil, resulting in deterioration of the turbine oil quality, causing bearing vibration and abnormal temperature increases, and even serious accidents such as bearing damage. Low shaft seal pressure has a significant impact on the low-pressure cylinder. At this time, the unit vacuum decreases, and cold air may enter the shaft seal or even the cylinder, causing the shaft seal section or the cylinder to be cooled, resulting in changes in dynamic and static clearances, and even dynamic and static friction, seriously threatening equipment safety. At the same time, a decrease in condenser vacuum will lead to an increase in exhaust steam temperature. The exhaust cylinder and bearing housing will expand due to heat, which may cause changes in the shaft system and cylinder center, causing turbine vibration. Therefore, it is evident that changes in shaft seal pressure have a significant impact on the safety and economy of the turbine.

[0049] (2) Prevent the shaft seal temperature from being too low

[0050] The steam supply to the shaft seal is in direct contact with the turbine's main shaft, and its temperature directly affects the shaft's expansion and contraction. During normal unit operation, both the rotor and shaft seal temperatures are high. If the steam supply temperature to the shaft seal suddenly drops, not only will the shaft end seals and rotor seals be cooled, but the main shaft section in the shaft seal section is also prone to rapid cooling and contraction. This can alter the clearance between the rotor and cylinder. If the contraction is excessive, dynamic and static friction may occur, leading to abnormal unit vibration and affecting the safe and stable operation of the main unit. When the shaft seal temperature is too low, water may be carried into the steam supply to the shaft seal, causing water hammer in the steam supply pipeline. In severe cases, water may enter the turbine, leading to serious equipment damage accidents such as cylinder deformation, rotor and seal damage. Therefore, changes in shaft seal temperature have a significant impact on turbine safety.

[0051] 3. Application of the Automatic Control Strategy of the Invention in the Cilacap Phase III Shaft Sealing System in Indonesia

[0052] 3.1 Automatic control logic of the shaft sealing system during the test

[0053] Based on the above shaft seal control strategy, and considering the design parameters and actual conditions of each piece of equipment on site, the following logic was adopted during the test:

[0054] (1) After FCB is triggered, the shaft seal relief valve switches to manual mode and automatically sends the command to 0%.

[0055] (2) After FCB is triggered, the shaft seal steam supply regulating valve is opened to 45%, and then the shaft seal pressure is automatically activated with a pressure set value of 4 kPa.

[0056] (3) After FCB is triggered, the shaft seal steam desuperheating water regulating valve is switched to manual and the command is automatically sent to 0%.

[0057] (4) After FCB is triggered, the drain valve of the shaft seal mother pipe is opened.

[0058] 3.2 Introduction to the 75% FCB test (relevant curves are shown in the figure) Figure 1 , Figure 2 (As shown)

[0059] At 19:12:02 on November 1, 2019, the FCB was triggered by simulating an external grid fault, which tripped the 7A3 and 7AB3 switches on the high-voltage side of the main transformer, disconnected the unit from the grid, and put the unit into FCB mode.

[0060] (1) After the FCB is triggered, the turbine switches from "load control" to "speed control", with a maximum speed of 3129.5 r / min, a minimum speed of 2973.2 r / min, and finally stabilizes at 3000 r / min, and runs stably with 33.95 MW of plant power.

[0061] (2) After FCB is triggered, the shaft seal overflow valve is closed from 42.2% to 0%; the shaft seal steam supply regulating valve is opened from 20.7% to 44%, and then the shaft seal pressure is automatically activated with a pressure setting of 4 kPa; the shaft seal main pipe drain valve is automatically opened.

[0062] (3) Under FCB conditions, the shaft seal pressure fluctuated significantly in the initial stage of the test, then gradually approached the set value of 4 kPa; the shaft seal temperature gradually decreased from 307 degrees to 294 degrees, with little temperature change; the condenser vacuum gradually increased from the initial -93.847 kPa to the maximum value of -89.62 kPa, and then gradually stabilized at -91.2 kPa. Throughout the entire test, the condenser vacuum remained within the normal range, and the vibration of each bearing of the unit did not change significantly, fully meeting the requirements for safe and stable operation of the unit.

[0063] At 19:14:40, as required by the dispatch, the unit was reconnected to the grid, and the FCB was automatically reset. At this time, the shaft seal pressure was 3.941 kPa, the shaft seal temperature was 294.1 degrees, and the condenser vacuum was -91.28 kPa.

[0064] This invention summarizes the control challenges of turbine shaft sealing systems under FCB and load shedding conditions, proposes corresponding countermeasures, and formulates an automatic control strategy for turbine shaft sealing systems under FCB and load shedding conditions. After applying the research results to the Cilacap Phase III 1MW unit project in Indonesia, the unit successfully completed both 75% FCB and 100% fully automatic load shedding tests on the first attempt. After the tests, the unit could quickly connect to the grid and operate under load, proving that the control strategy is feasible in practice and can provide a reference for the design and optimization of automatic control of turbine shaft sealing systems during FCB and fully automatic load shedding tests of similar units.

[0065] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. An automatic control method for a turbine shaft sealing system under fully automatic load shedding conditions, characterized in that: Its control strategies include: A. Control strategy for shaft seal steam supply regulating valve After load shedding or FCB triggering, the shaft seal steam supply regulating valve first opens to a certain degree, and then the shaft seal pressure is automatically activated. B. Shaft seal overflow valve control strategy After load shedding or FCB triggering, the shaft seal relief valve switches to manual mode and automatically sends the command to 0%; C. Control strategy for shaft seal steam supply desuperheating water regulating valve After load shedding or FCB triggering, the shaft seal steam desuperheating water regulating valve switches to manual mode, and the automatic command is sent to 0%. D. Shaft seal header drain valve control strategy After load shedding or FCB triggering, the shaft seal header drain valve will automatically open. In the control strategy of the shaft seal steam supply regulating valve, the shaft seal steam supply pressure function is used as the setpoint for the joint opening of the shaft seal steam supply regulating valve; In the control strategy of the shaft seal steam supply regulating valve, the shaft seal steam supply pressure function is obtained as follows: when the unit is carrying 3~5% of the rated load and the shaft seal main pipe drain valve is fully open, under the premise that the shaft seal system main pipe pressure is normal, the shaft seal steam supply regulating valve opening value corresponding to different shaft seal steam supply pressures.

2. The automatic control method for a turbine shaft sealing system under fully automatic load shedding conditions as described in claim 1, characterized in that: In the control strategy of the shaft seal steam supply regulating valve, the shaft seal steam supply regulating valve is always in automatic control mode, and the shaft seal pressure setpoint is the recommended value of the main engine manufacturer or the empirical value under long-term operating conditions of the unit. When the FCB is reset, the shaft seal steam supply regulating valve is still in automatic control mode, and the pressure setpoint remains unchanged. At this time, the pressure setpoint can be manually modified or switched to manual control mode.

Images