Mechanical overspeed device and protection control method for a hydropower plant

Abstract

This invention provides a mechanical overspeed device and protection control method for a hydropower station. By adding two pairs of pressure switches and a set of pressure sensors to the mechanical overspeed hydraulic circuit, the oil pressure in the mechanical overspeed pipeline can be more accurately determined when the mechanical overspeed device operates. The unit's LCU protection control uses an OR relationship between the pressure switch's digital signal and the pressure sensor's analog signal, connecting them in series with the mechanical overspeed signal node. This improves the redundancy and reliability of signal judgment, reduces the probability of false operation, and lowers the risk of failure to operate. The turbine backup protection control uses the pressure switch and the mechanical overspeed node in series, further accurately determining the oil pressure in the mechanical overspeed pipeline when the mechanical overspeed device operates, thus accurately determining the system's normal operation. Simultaneously, the turbine backup protection control introduces a GCB trip signal, further avoiding the risk of false operation of the mechanical overspeed device during unit operation, thereby solving the problems mentioned in the background art.

Description

Technical Field

[0001] This invention belongs to the field of electrical automatic control of hydropower stations, and in particular relates to a mechanical overspeed device and protection and control method for hydropower stations. Background Technology

[0002] Mechanical overspeed protection devices are the most important protection in the event of an accident in a hydropower station unit. The mechanical overspeed protection device is fixed to the main shaft of the turbine-generator unit and rotates with the unit. It is usually set as the next level of protection after the electrical speed signal device. When the speed reaches or exceeds the factory-set operating value, it directly controls the shutdown via hydraulic signals. Simultaneously, when the unit runs away from the grid, the mechanical overspeed protection device sends a protection signal, which is transmitted to the unit's LCU and the turbine backup protection control via a limit switch. The limit switch is installed on the mechanical overspeed protection device itself, and its contacts are normally closed, with the limit switch contacts connected to the trip unit. Currently, the following problems exist:

[0003] (1) The signal sent from the limit switch node of the mechanical overspeed device to the unit LCU and the water turbine backup protection control is a single element. The control logic is imperfect and the reliability is low. In particular, there is a risk of malfunction during equipment maintenance or operation, which will directly lead to unplanned shutdown of the unit.

[0004] (2) The limit switch is installed on the mechanical overspeed device body. Its contacts are normally closed. The cam of the limit switch contact is directly connected to the trip unit. It is a point-to-point contact with a small contact area. When the limit switch fixing bolt is loose or the limit switch cam contact is loose, the limit switch will be activated. At this time, its signal is sent directly to the unit LCU protection control and water turbine backup protection, which is prone to false activation.

[0005] (3) When the mechanical overspeed device is activated, its signal is directly sent to the unit LCU protection control and water turbine backup protection. The oil pressure on the mechanical overspeed pipeline is not judged, and the judgment basis is incomplete.

[0006] (4) If the unit is in operation, the unit will also be shut down unplanned if the mechanical overspeed device is activated. This method is simple but the logic is incomplete because the unit will not experience mechanical overspeed when it is in operation. Therefore, this logic has a loophole. Summary of the Invention

[0007] The technical problem this invention aims to solve is to provide a mechanical overspeed device and its protection and control method for a hydropower station. By adding two pairs of pressure switches and a set of pressure sensors to the mechanical overspeed hydraulic circuit, the oil pressure in the mechanical overspeed pipeline can be more accurately determined when the device operates. The unit's LCU protection control uses an OR relationship between the pressure switch signals and the pressure sensor analog signals, connecting them in series with the mechanical overspeed signal node. This improves the redundancy and reliability of signal judgment, reduces the probability of false tripping, and lowers the risk of failure to operate. The turbine backup protection control connects the pressure switches and the mechanical overspeed node in series, further accurately determining the oil pressure in the mechanical overspeed pipeline when the device operates, thus accurately determining the system's normal operation. Simultaneously, the turbine backup protection control introduces a GCB trip signal, further avoiding the risk of false tripping of the mechanical overspeed device during unit operation, thereby solving the problems mentioned in the background art.

[0008] To achieve the above-mentioned technical features, the present invention aims to provide a mechanical overspeed protection device for a hydropower station, comprising a mechanical overspeed device installed on the main shaft of a turbine, the mechanical overspeed device cooperating with a limit switch assembly, the signal output terminal of the limit switch assembly being connected in parallel to a first switch node GS1 and a second switch node GS2 of the mechanical overspeed device; the signal node of the first switch node GS1 of the mechanical overspeed device is output to the turbine backup protection control system, and the signal node of the second switch node GS2 of the mechanical overspeed device is output to the unit LCU protection control system;

[0009] It also includes a GCB circuit breaker, which is installed on the generator outlet side. The GCB circuit breaker leads out two GCB circuit breaker trip nodes, and the signals from the two GCB circuit breaker trip nodes are respectively output to the water turbine backup protection control system and the unit LCU protection control system.

[0010] It also includes pressure switches YLD1 and YLD2 installed on the mechanical overspeed hydraulic circuit and pressure sensor SP. The signal node of pressure switch YLD1, which is the low pressure 1 switch action node of the mechanical overspeed hydraulic circuit, is output to the water turbine backup protection control system. The signal node of pressure switch YLD2, which is the low pressure 2 switch action node of the mechanical overspeed hydraulic circuit, is output to the unit LCU protection control system. The signal node of pressure sensor SP, which is the pressure sensor less than the set value node of the mechanical overspeed hydraulic circuit, is output to the unit LCU protection control system.

[0011] The mechanical overspeed device consists of only one set, including a pendulum mounted on the turbine main shaft, which works in conjunction with a limit switch assembly;

[0012] The limit switch assembly includes a protective cover, inside which a limit switch is installed. The limit switch engages with a trip unit via a limit switch cam, and the trip unit engages with a pendulum.

[0013] The protective cover is a grid-shaped cover.

[0014] The trip unit is cut into a cylindrical shape and is designed in a planar manner. The trip unit and the limit switch cam are made to make face-to-face contact to increase the contact area and ensure reliable contact.

[0015] The pressure switches YLD1 and YLD2 and the pressure sensor SP are respectively equipped with quick-connect pressure test connectors and pre-valve valves to facilitate the calibration and replacement of the pressure switches and pressure sensors.

[0016] The water turbine backup protection control system adopts a hard-wired control loop, which is independent of the unit's LCU protection control system and has an independent control panel.

[0017] The unit's LCU protection and control system adopts an automatic control mode using a PLC controller and is equipped with an independent control panel.

[0018] A control method for overspeed protection of hydropower station machinery using an overspeed protection device:

[0019] For the water turbine backup protection control system, which uses a hard-wired control loop, there are two types of water turbine backup protection control logic, specifically:

[0020] The first option is to determine whether the GCB circuit breaker tripping node is in place when the first switch node GS1 of the mechanical overspeed device is activated, ensuring that the GCB circuit breaker is in the tripping position. Here, the GCB circuit breaker tripping node is a switching signal. This method can prevent the mechanical overspeed device from malfunctioning and causing the unit to stop running and the high-speed door to close. It can also prevent the mechanical overspeed device from shutting down due to malfunction of the limit switch node or loose bolts, which could cause the normally closed limit switch node of the mechanical overspeed device to operate and thus shut down the unit.

[0021] The second solution: When the first switch node GS1 of the mechanical overspeed device is activated, it is determined whether the low pressure switch node YLD1 of the mechanical overspeed hydraulic circuit is activated. This ensures that the low pressure switch node YLD1 is activated to its set value. Here, the low pressure switch node YLD1 is a switching signal. This method avoids the possibility of malfunction of the first switch node GS1 of the mechanical overspeed device. If the limit switch node of the mechanical overspeed device is actually activated, the low pressure switch node YLD1 must be used as the interlock. This is because when the mechanical overspeed device is activated, the oil pressure in the mechanical overspeed hydraulic circuit drops rapidly to 0MPa, ensuring that the first switch node GS1 of the mechanical overspeed device has actually been activated.

[0022] There are three types of control logic for the unit's LCU protection control system, specifically:

[0023] The first solution: When the second switch node GS2 of the mechanical overspeed device is activated, it is determined whether the switch YLD2 of the mechanical overspeed hydraulic circuit low pressure 2 switch is activated. This ensures that the mechanical overspeed hydraulic circuit low pressure 2 switch YLD2 is activated to the set value. Here, the mechanical overspeed hydraulic circuit low pressure 2 switch YLD2 is a switch signal. This method is used to avoid false activation of the second switch node GS2 of the mechanical overspeed device. If the limit switch node of the mechanical overspeed device is activated, the mechanical overspeed hydraulic circuit low pressure 2 switch YLD2 must be used as the interlock. This is because when the mechanical overspeed device is activated, the oil pressure in the mechanical overspeed hydraulic circuit drops rapidly to 0MPa, ensuring that the second switch node GS2 of the mechanical overspeed device has actually been activated.

[0024] The second solution: When the second switch node GS2 of the mechanical overspeed device is activated, the analog signal method is used to determine whether the pressure acquisition value of the pressure sensor SP on the mechanical overspeed hydraulic circuit is lower than the low pressure setting value SPo. The analog signal method is used to more accurately determine the pressure value on the mechanical overspeed hydraulic circuit, so as to make up for the dead zone of the pressure switch node itself when it is activated. The combination of digital and analog signal methods makes the judgment logic more complete and reliable.

[0025] The third option is to determine whether the falling edge of the mechanical overspeed device's second switch node GS2 is in the grid-connected state when the unit is not in the grid-connected state. In other words, the protection control logic of the unit's LCU protection control system can only be triggered when the mechanical overspeed device's second switch node GS2 is in the non-grid-connected state. The grid-connected state is the calculation and integration point of the unit's LCU. When the unit is in the grid-connected state, its rising edge is activated, and its signal node is 1. When the unit's grid-connected state disappears, i.e., the no-load state, its falling edge is activated, and its signal node is 0.

[0026] The beneficial effects of this invention are as follows:

[0027] 1. This invention employs a point-to-surface contact method between the travel switch contact cam and the trip unit of the mechanical overspeed device, increasing the contact area and improving the contact reliability between the cam and the trip unit. The design is reasonable and highly practical. Simultaneously, the mechanical overspeed device body uses a grid-like protective cover, which is highly adaptable and avoids the risk of accidental human contact, improving the reliability and safety of the device's operation. This device effectively solves the problems existing in traditional methods, greatly reducing the risk of accidental human contact and the recurrence of existing defects in the device body.

[0028] 2. The technical problem to be solved by this invention is to provide a method for protecting and controlling the mechanical overspeed of a hydropower station. Two pairs of pressure switches and a set of pressure sensors are added to the hydraulic pipeline for mechanical overspeed to more accurately determine the oil pressure in the pipeline when the mechanical overspeed device is activated. The unit's LCU protection control system uses an OR relationship between the pressure switch signals and the analog signals from the pressure sensors, connecting them in series with the mechanical overspeed signal node. This improves the redundancy and reliability of signal judgment, reduces the probability of false activation, and lowers the risk of failure to activate. The turbine backup protection control uses the pressure switches and the mechanical overspeed node in series to more accurately determine the oil pressure in the mechanical overspeed pipeline when the mechanical overspeed device is activated, thus accurately determining the normal operation of the system. Simultaneously, the turbine backup protection control introduces a GCB trip signal, further avoiding the risk of false activation of the mechanical overspeed device during unit operation. Attached Figure Description

[0029] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0030] Figure 1 This is a system and control logic diagram of the present invention.

[0031] Figure 2 This is a structural diagram of the limit switch assembly of the present invention.

[0032] Figure 3 This is the electrical control diagram of the first switch node GS1, the GCB circuit breaker tripping node, and the mechanical overspeed hydraulic circuit pressure low 1 switch action node YLD1 of the present invention. Detailed Implementation

[0033] The embodiments of the present invention will be further described below with reference to the accompanying drawings.

[0034] Example 1:

[0035] See Figure 1-3A mechanical overspeed protection device for a hydropower station includes a mechanical overspeed device 3, which is installed on the turbine main shaft 14. The mechanical overspeed device 3 cooperates with a limit switch assembly. The signal output terminal of the limit switch assembly is connected in parallel to a first switch node GS18 and a second switch node GS211 of the mechanical overspeed device. The signal node of the first switch node GS18 is output to the turbine backup protection control system 12, and the signal node of the second switch node GS211 is output to the unit LCU protection control system 13. It also includes a GCB circuit breaker 1, which is installed on the generator outlet side. The GCB circuit breaker 1 leads out two GCB circuit breaker trip nodes 6. The signal from circuit breaker tripping node 6 is output to the water turbine backup protection control system 12 and the unit LCU protection control system 13, respectively. It also includes pressure switches YLD12 and YLD24, and a pressure sensor SP5 installed on the mechanical overspeed hydraulic circuit. The signal node YLD17 of pressure switch YLD12, which is the mechanical overspeed hydraulic circuit low pressure 1 switch action node, is output to the water turbine backup protection control system 12. The signal node YLD29 of pressure switch YLD24, which is the mechanical overspeed hydraulic circuit low pressure 2 switch action node, is output to the unit LCU protection control system 13. The signal node SP5 of pressure sensor SP5, which is the mechanical overspeed hydraulic circuit pressure sensor less than set value node 10, is output to the unit LCU protection control system 13. By adding two pairs of pressure switches and one set of pressure sensors to the mechanical overspeed hydraulic circuit, the oil pressure in the mechanical overspeed pipeline can be more accurately determined when the mechanical overspeed device is activated. The unit LCU protection control uses an OR relationship between the pressure switch switching signals and the pressure sensor analog signals, connecting them in series with the mechanical overspeed signal node. This improves the redundancy and reliability of signal judgment, reduces the probability of false activation, and reduces the risk of failure to activate. The water turbine backup protection control uses pressure switches and mechanical overspeed terminals in series to more accurately determine the oil pressure on the mechanical overspeed pipeline when the mechanical overspeed device is activated, so as to accurately determine the normality of the system. At the same time, the water turbine backup protection control introduces GCB trip signals to further avoid the risk of mechanical overspeed device malfunction during unit operation, so as to solve the problems mentioned in the background technology.

[0036] Furthermore, the mechanical overspeed device 3 consists of only one set, including a pendulum 15 mounted on the turbine main shaft 14. The pendulum 15 cooperates with a limit switch assembly. The limit switch assembly includes a protective cover 19, inside which a limit switch 18 is installed. The limit switch 18 cooperates with a trip unit 16 via a limit switch cam 17, and the trip unit 16 cooperates with the pendulum 15. The aforementioned mechanical overspeed device 3 can be used to determine the overspeed signal of the turbine main shaft 14.

[0037] Furthermore, the protective cover 19 is a grid-shaped protective cover. The protective cover is designed to prevent maintenance personnel from accidentally touching the mechanical overspeed device trip unit during equipment maintenance, cleaning, and sweeping, and to prevent the mechanical overspeed device from sending false signals.

[0038] Furthermore, the trip unit 16 is cut into a cylindrical shape and is in a planar manner. The trip unit 16 and the limit switch cam 17 are in a face-to-face contact manner to increase the contact area and ensure reliable contact.

[0039] Furthermore, quick-connect pressure testing connectors are installed on the pipelines containing pressure switches YLD12 and YLD24, and pressure sensor SP5, and pre-valve valves are provided to facilitate the calibration and replacement of the pressure switches and pressure sensors, thereby enhancing adaptability.

[0040] Furthermore, the water turbine backup protection control system 12 adopts a hard-wired control loop, which is independent of the unit LCU protection control system 13 and has an independent control panel.

[0041] Furthermore, the unit's LCU protection and control system 13 adopts an automatic control mode using a PLC controller and is equipped with an independent control panel.

[0042] Example 2:

[0043] A control method for overspeed protection of hydropower station machinery using an overspeed protection device:

[0044] The water turbine backup protection control system 12 adopts a hard-wired control loop. There are two types of water turbine backup protection control logic, specifically:

[0045] The first option is to determine whether the GCB circuit breaker 1 tripping node is in place when the mechanical overspeed device first switch node GS18 is activated. This ensures that the GCB circuit breaker 1 is in the tripping position. Here, the GCB circuit breaker tripping node 6 is a switching signal. This method prevents the mechanical overspeed device 3 from malfunctioning and causing the unit to stop running and the high-speed door to close. It also avoids the unit from being shut down due to the failure or loosening of the limit switch node of the limit switch assembly or the activation of the normally closed limit switch node of the mechanical overspeed device.

[0046] The second solution: When the first switch node GS18 of the mechanical overspeed device is activated, check whether the low pressure switch node YLD17 of the mechanical overspeed hydraulic circuit is activated. Ensure that the low pressure switch node YLD17 is activated to its set value. Here, the low pressure switch node YLD17 is a switching signal. This method avoids the possibility of malfunction of the first switch node GS18 of the mechanical overspeed device. If the limit switch node of the mechanical overspeed device is actually activated, the low pressure switch node YLD17 must be used as the interlock. This is because when the mechanical overspeed device is activated, the oil pressure in the mechanical overspeed hydraulic circuit drops rapidly to 0MPa, ensuring that the first switch node GS18 of the mechanical overspeed device has actually been activated.

[0047] Example 3:

[0048] There are three types of control logic for the unit's LCU protection control system 13, specifically:

[0049] The first solution: When the second switch node GS211 of the mechanical overspeed device is activated, it is determined whether the switch YLD29 of the mechanical overspeed hydraulic circuit pressure low 2 switch is activated. This ensures that the mechanical overspeed hydraulic circuit pressure low 2 switch YLD29 is activated to the set value. Here, the mechanical overspeed hydraulic circuit pressure low 2 switch YLD29 is a switch signal. This method is used to avoid the mechanical overspeed device's second switch node GS211 from malfunctioning. If the mechanical overspeed device's limit switch node is activated, the mechanical overspeed hydraulic circuit pressure low 2 switch YLD29 must be used as a lockout. This is because when the mechanical overspeed device is activated, the oil pressure in the mechanical overspeed hydraulic circuit drops rapidly to 0MPa, ensuring that the mechanical overspeed device's second switch node GS211 has actually been activated.

[0050] The second solution: When the second switch node GS211 of the mechanical overspeed device is activated, the analog signal method is used to determine whether the pressure acquisition value of the pressure sensor SP5 on the mechanical overspeed hydraulic circuit is lower than the low pressure setting value SPo. The analog signal method is used to more accurately determine the pressure value on the mechanical overspeed hydraulic circuit, so as to make up for the dead zone of the pressure switch node itself when it is activated. The combination of digital and analog signal methods makes the judgment logic more complete and reliable.

[0051] The third option is to determine whether the falling edge of the mechanical overspeed device's second switch node GS211 is in the grid-connected state when the unit is not in the grid-connected state. In other words, the protection control logic of the unit's LCU protection control system 13 can only be triggered when the mechanical overspeed device's second switch node GS211 is in the non-grid-connected state. The grid-connected state is the calculation and integration point of the unit's LCU. When the unit is in the grid-connected state, its rising edge is activated, and its signal node is 1. When the unit's grid-connected state disappears, i.e., the no-load state, its falling edge is activated, and its signal node is 0.

Claims

1. A mechanical overspeed protection device for a hydropower station, characterized in that, The system includes a mechanical overspeed device (3), which is installed on the turbine main shaft (14). The mechanical overspeed device (3) is used in conjunction with a limit switch assembly. The signal output terminal of the limit switch assembly is connected in parallel with the first switch node GS1 (8) and the second switch node GS2 (11) of the mechanical overspeed device. The signal node of the first switch node GS1 (8) of the mechanical overspeed device is output to the turbine backup protection control system (12), and the signal node of the second switch node GS2 (11) of the mechanical overspeed device is output to the unit LCU protection control system (13). It also includes a GCB circuit breaker (1), which is installed on the generator outlet side. The GCB circuit breaker (1) leads out two GCB circuit breaker trip nodes (6), and the signals of the two GCB circuit breaker trip nodes (6) are respectively output to the water turbine backup protection control system (12) and the unit LCU protection control system (13). It also includes pressure switch YLD1 (2), pressure switch YLD2 (4) and pressure sensor SP (5) installed on the mechanical overspeed hydraulic circuit. The signal node of pressure switch YLD1 (2) at the mechanical overspeed hydraulic circuit low pressure 1 switch action node YLD1 (7) is output to the water turbine backup protection control system (12). The signal node of pressure switch YLD2 (4) at the mechanical overspeed hydraulic circuit low pressure 2 switch action node YLD2 (9) is output to the unit LCU protection control system (13). The signal node of pressure sensor SP (5) at the mechanical overspeed hydraulic circuit pressure sensor less than set value node (10) is output to the unit LCU protection control system (13). The mechanical overspeed device (3) has only one set, including a pendulum (15) installed on the turbine main shaft (14), which cooperates with the limit switch assembly; The limit switch assembly includes a protective cover (19), inside which a limit switch (18) is installed. The limit switch (18) is engaged with a trip unit (16) via a limit switch cam (17), and the trip unit (16) is engaged with a pendulum (15). The trip unit (16) is cut into a cylindrical shape and is in a planar manner. The trip unit (16) and the limit switch cam (17) are in a face-to-face contact manner to increase the contact area and ensure reliable contact.

2. The hydropower station mechanical overspeed protection device according to claim 1, characterized in that, The protective cover (19) is a grid-shaped protective cover.

3. The hydropower station mechanical overspeed protection device according to claim 1, characterized in that, The pressure switches YLD1 (2), YLD2 (4) and SP (5) are respectively equipped with quick pressure testing connectors and pre-valve valves to facilitate the calibration and replacement of pressure switches and pressure sensors.

4. The hydropower station mechanical overspeed protection device according to claim 1, characterized in that, The water turbine backup protection control system (12) adopts a hard-wired control loop, which is independent of the unit LCU protection control system (13) and has an independent control panel.

5. The hydropower station mechanical overspeed protection device according to claim 1, characterized in that, The unit's LCU protection and control system (13) adopts PLC controller automatic control mode and is equipped with an independent control panel.

6. A control method for overspeed protection of hydropower station machinery using the overspeed protection device of any one of claims 1-5, characterized in that: For the water turbine backup protection control system (12), which adopts a hard-wired control loop, there are two types of water turbine backup protection control logic, specifically: The first option is to determine whether the GCB circuit breaker (1) is in place when the first switch node GS1 (8) of the mechanical overspeed device is activated. This ensures that the GCB circuit breaker (1) is in the open position. Here, the GCB circuit breaker open node (6) is a switching signal. This method is used to prevent the mechanical overspeed device (3) from malfunctioning during the operation of the unit, which could lead to the unit being stopped and the fast door being closed. It can also prevent the unit from being stopped due to the failure or damage of the limit switch node of the limit switch assembly or the loosening of the bolts. The second option is to determine whether the mechanical overspeed device's first switch node GS1 (8) is activated when the mechanical overspeed hydraulic circuit pressure low 1 switch node YLD1 (7) is activated. This ensures that the mechanical overspeed hydraulic circuit pressure low 1 switch node YLD1 (7) is activated to the set value. Here, the mechanical overspeed hydraulic circuit pressure low 1 switch node YLD1 (7) is a switch signal. This method avoids the situation where the mechanical overspeed device's first switch node GS1 (8) is malfunctioning. If the mechanical overspeed device's limit switch node is actually activated, the mechanical overspeed hydraulic circuit pressure low 1 switch node YLD1 (7) must be used as a lockout. This is because when the mechanical overspeed device is activated, the oil pressure on the mechanical overspeed hydraulic circuit drops rapidly to 0MPa, ensuring that the mechanical overspeed device's first switch node GS1 (8) has actually been activated. There are three types of control logic for the unit's LCU protection control system (13), specifically: The first option is to determine whether the switch node YLD2 (9) of the mechanical overspeed device is activated when the second switch node GS2 (11) of the mechanical overspeed hydraulic circuit is activated. This ensures that the set value of the switch node YLD2 (9) of the mechanical overspeed hydraulic circuit is activated. Here, the switch node YLD2 (9) of the mechanical overspeed hydraulic circuit is a switch signal. This method is to avoid the mechanical overspeed device's second switch node GS2 (11) from malfunctioning. If the mechanical overspeed device's limit switch node is activated, the switch node YLD2 (9) of the mechanical overspeed hydraulic circuit must be used as a lockout. This is because when the mechanical overspeed device is activated, the oil pressure on the mechanical overspeed hydraulic circuit drops rapidly to 0 MPa, ensuring that the mechanical overspeed device's second switch node GS2 (11) has actually been activated. The second option is to use analog signals to determine whether the pressure value collected by the pressure sensor SP (5) on the mechanical overspeed hydraulic circuit is lower than the low pressure setting value SPo. Using analog signals will more accurately determine the pressure value on the mechanical overspeed hydraulic circuit to compensate for the dead zone of the pressure switch node itself. Combining digital and analog signals will make the judgment logic more complete and reliable. The third option: When the second switch node GS2 (11) of the mechanical overspeed device is activated, it is determined whether the falling edge of the unit in grid-connected state is in place. That is to say, the protection control logic of the unit LCU protection control system (13) can only be triggered when the second switch node GS2 (11) of the mechanical overspeed device is activated in the non-grid-connected state. The grid-connected state of the unit is the calculation and integration point of the unit LCU. When the unit is in grid-connected state, its rising edge is activated and its signal node is 1. When the unit is in grid-connected state, i.e., the no-load state, its falling edge is activated and its signal node is 0.

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