Shutdown energy recycling and using system and method for large mixed-flow water turbine

Abstract

The shutdown energy recovery and use system comprises a transformer, the input end of the transformer is connected with the output end of a three-phase power supply of a generator, and the transformer is sequentially connected with an energy dissipation resistor R, a bidirectional AC / DC rectifier, a capacitor bank and a bidirectional DC / DC rectifier to form a series shutdown energy recovery device; the control module is used for controlling semiconductor elements in the bidirectional AC / DC rectifier and the bidirectional DC / DC rectifier, carrying out smooth conversion of stored energy and output energy, improving the transient stability of the generator and carrying out power compensation; the shutdown energy recovery device uses the recovered energy as the excitation current of the generator excitation system when the generator rotor is shut down. The current generated in the stator in the shutdown process of the water turbine is rectified and stored and then conveyed to the generator rotor for recycling and using the shutdown amount. The energy utilization rate is improved, the shutdown process is accelerated, and the transient stability of the system is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of Francis turbine shutdown energy recovery, and in particular relates to a large-scale Francis turbine shutdown energy recovery and use system and method. Background Art

[0002] Hydroelectric generators have the advantage of short start-up and shutdown time. They are used for peak load and frequency regulation in power systems and are frequently started and shut down. However, large turbines have large rotating parts and large rotational inertia. If there is no external force intervention, the unit will run in the low-speed zone for a long time. Therefore, when the unit speed drops to a certain value, external force intervention is required to stop the unit quickly. At present, there are two main types of domestic hydroelectric generator shutdown braking systems: mechanical braking and electrical braking.

[0003] Electrical braking mainly adopts the principle of synchronous motor. When the turbine unit is disconnected from the system, the generator end is first short-circuited, and then the excitation current is passed through the rotor of the generator to make the motor generate braking electromagnetic braking torque, weakening the rotational inertia of the unit, thereby reducing the speed and shortening the downtime.

[0004] like Figure 6 As shown, when the hydro-generator set is decoupled and demagnetized, disconnect the generator outlet switch X1, wait until the speed drops to 60% of the rated speed, use the brake switch X0 to short-circuit the three-phase generator stator at the machine end outlet, and close the switch X2 and the brake energy switch X3. The brake rectifier cabinet of the generator excitation system rectifies the factory power and sends it into the rotor circuit for electrical braking. Flexible electrical braking is used to make the braking current climb smooth and adjustable.

[0005] The disadvantage of the prior art is that, in the electrical braking process, a resistor is used as a braking element, which is an energy-consuming braking method and fails to effectively utilize the energy of the generator rotor and the heat generated on the stator. Summary of the invention

[0006] The technical problem to be solved by the present invention is to provide a system and method for recovering and using the shutdown energy of a large-scale mixed-flow turbine. The present invention can accelerate the shutdown process without affecting the normal shutdown process of the unit, and also realize the recovery and reuse of the shutdown energy of the large-scale mixed-flow turbine generator set, thereby improving the energy utilization efficiency.

[0007] In order to solve the above technical problems, the technical solution adopted by the present invention is: A large-scale Francis turbine shutdown energy recovery and utilization system includes a transformer, an energy dissipation resistor R, a bidirectional AC / DC rectifier, a capacitor bank, a bidirectional DC / DC rectifier and a control module; the transformer input end is connected to the generator three-phase power supply output end, and the transformer is sequentially connected with the energy dissipation resistor R, the bidirectional AC / DC rectifier, the capacitor bank and the bidirectional DC / DC rectifier to form a shutdown energy recovery device in series; The control module is used to control the semiconductor components in the bidirectional AC / DC rectifier and the bidirectional DC / DC rectifier, to smoothly convert the stored energy and the output energy, to improve the transient stability of the generator and to perform power compensation; When the generator rotor is shut down, the shutdown energy recovery device uses the recovered energy as the excitation current of the generator excitation system.

[0008] Preferably, the shutdown energy recovery device is connected in parallel with the brake rectifier cabinet, the shutdown energy recovery device controls whether to put the generator excitation system into operation through switch X5, and the brake rectifier cabinet controls whether to put the generator excitation system into operation through switch X3.

[0009] Preferably, it also includes a management module, which is used to detect whether the capacitor group is faulty, monitor the charging status of the capacitor group, and send the status of the capacitor group to the control module. If the equipment fails or the chargeable amount is insufficient, the electric braking recovery process will not be executed, and it will be determined whether to recover energy for utilization. Preferably, the bidirectional AC / DC rectifier maintains the voltage across the capacitor bank at a constant value during energy storage, and transmits the energy stored in the capacitor bank to the rotor for enhanced braking with high quality during auxiliary braking.

[0010] Preferably, the capacitor bank stores energy generated in the stator.

[0011] Preferably, the bidirectional DC / DC rectifier maintains the voltage across the capacitor bank at a constant value during energy storage, and transmits the energy stored in the capacitor bank to the rotor for enhanced braking with high quality during auxiliary braking.

[0012] Preferably, the shutdown energy recovery device is connected to the three-phase power output terminal of the generator through a switch X4.

[0013] An operating method of a large Francis turbine shutdown energy recovery and utilization system includes the following working conditions: If the unit can be shut down normally, the generator outlet switch X1 is opened after load reduction and decoupling; the rotor inverter is demagnetized, and the internal switch of the generator excitation system is disconnected; it is judged that the shutdown energy recovery and use system is abnormal or the chargeable capacity of the capacitor bank is not satisfied, and the brake switch X0 is turned on when the subsequent conditions are met, and the non-energy recovery electric braking process is entered; If there is no abnormality in the shutdown energy recovery and use system and the chargeable capacity of the capacitor bank is sufficient, keep the brake switch X0 disconnected; the turbine guide vanes are fully closed, the unit speed naturally drops to the set speed N1, and switch X4 is turned on; the control module controls the bidirectional AC / DC rectifier to stabilize the output voltage, and outputs direct current to charge the capacitor bank; when the energy in the supercapacitor is full, the brake switch X0 is closed, and the non-energy recovery electric braking process is entered; When the unit speed drops to the set speed N2, disconnect switches X3 and X4, stop the electrical braking, fully engage the mechanical braking, and gradually reduce the unit speed to 0.

[0014] Preferably, it is determined that the shutdown energy recovery and use system is abnormal or the capacitor bank has not reached the dischargeable capacity, the switch X5 is kept disconnected, the switch X3 is closed, and power is supplied through the brake rectifier cabinet; If the shutdown energy utilization system is normal and the capacitor bank reaches the dischargeable capacity and the management module determines that the unit is in the electrical braking process, close switch X5 and disconnect switch X3; always detect the energy supply system and capacity status of the supercapacitor, maintain it if there is no abnormality, and switch to the brake rectifier cabinet for power supply when there is a fault or insufficient capacity, close switch X3, and disconnect switch X5; determine that the unit is in the mechanical braking state and disconnect switch X5.

[0015] Preferably, the stator potential of the generator set is defined as E, the direct-axis synchronous reactance of the generator is defined as Xd, and E and Xd satisfy the following formula: ; Stator current during braking I k It can be expressed as: ; because

[0016] Stator current during braking I k It can be expressed as: ; Where: f is the frequency; ω is the angular frequency; L is the inductor; Φ is the magnetic flux; n is the unit speed; P is the pole pair number; W is the number of coil turns; K f is the pitch coefficient; K d , K E is a constant coefficient; R is the effective stator resistance.

[0017] After the unit is disconnected, the three phases of the stator are short-circuited, and a constant DC current is input into the rotor winding, which generates an induced current in the stator. This current generates a copper loss braking torque in the rotor winding, causing the unit to slow down and brake to a stop. The short-circuit current in the stator I k It is a constant value and does not change with the decrease of the motor speed. According to the synchronous motor theory, during the electrical braking process, the excitation current passing through the rotor coil is relatively constant. Therefore, it can be considered that the fundamental wave magnetic flux per pole Φ It is also constant, proving that the shutdown energy recovery and utilization system can provide a stable excitation current.

[0018] The present invention can achieve the following beneficial effects: During the process of reducing the speed of existing large-scale hydro-turbine generators during shutdown, it is necessary to go through three stages: closing the guide vanes, electrical braking, and mechanical braking, so that the rotor changes from a 100% speed to a stationary state. During electrical braking, the three-phase stator of the generator is short-circuited, and a copper loss braking torque is generated in the rotor. The unit is quickly stopped by converting the rotor kinetic energy into the heat energy of the stator short-circuited winding. The current generated by the stator short circuit during the electrical braking process is only the heat dissipation of the stator bars, resulting in energy waste. Under the premise of not affecting the normal shutdown process of the unit, the present invention can not only accelerate the shutdown process, but also realize the recovery and reuse of the shutdown energy of large-scale mixed-flow hydro-turbine generator sets, thereby improving the energy utilization efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The present invention will be further described below in conjunction with the accompanying drawings and embodiments: Figure 1 This is an internal structure diagram of the shutdown energy recovery device of the present invention; Figure 2 This is a system relationship diagram of the shutdown energy recovery device of the present invention; Figure 3 This is a relationship diagram of the stator current, rotor current and trigger angle of the electric brake of the present invention; Figure 4 Flow chart of the present invention (energy recovery process); Figure 5 This is the flow chart of the present invention (shutdown energy utilization process); Figure 6 This is a diagram of the existing shutdown system. DETAILED DESCRIPTION

[0020] The preferred solution is Figures 1 to 5 As shown, a large-scale Francis turbine shutdown energy recovery and utilization system is composed of a transformer, an energy dissipation resistor R, a bidirectional AC / DC rectifier, a capacitor bank, a bidirectional DC / DC rectifier and a control module.

[0021] Transformer: converts the AC voltage at the generator outlet into a voltage acceptable to the switching elements; Energy dissipation resistor: weakens the current fluctuation in the circuit and protects the circuit; Bidirectional AC / DC rectifier: During the energy storage process, the voltage across the capacitor bank is maintained at a constant value. During the auxiliary braking process, the energy stored in the capacitor bank is transmitted to the rotor with high quality to enhance braking; Capacitor bank: stores the energy generated in the stator; Bidirectional DC / DC rectifier: During the energy storage process, the voltage across the capacitor bank is maintained at a constant value. During the auxiliary braking process, the energy stored in the capacitor bank is transmitted to the rotor with high quality to enhance braking; Management module: detects whether the capacitor bank is faulty and the charging status of the capacitor bank, and sends the status of the capacitor bank to the control module. If the equipment fails or the chargeable capacity is insufficient, the electric braking recovery process will not be executed, and it is determined whether to recover energy for utilization.

[0022] Control module: controls the semiconductor components in the bidirectional AC / DC rectifier and bidirectional DC / DC rectifier, performs smooth conversion of stored energy and output energy, improves the transient stability of the generator and performs power compensation.

[0023] Shutdown energy recovery device Figure 1 As shown, when the generator enters the shutdown process and reaches the speed of N1, switch X4 is put into operation. The generator excitation system is connected to the factory power using a regulator and a thyristor rectifier, and the factory power is rectified and sent to the rotor circuit. The electromagnetic induction generator stator generates an induced current. Since the current amplitude is approximately constant, the voltage is reduced by the transformer, and the energy dissipation resistor is filtered and stabilized. The braking energy is stored in the capacitor bank to achieve the purpose of turbine shutdown energy recovery. When the generator speed continues to drop to N2, the electrical brake is exited and the mechanical brake is put into operation, and the switch X4 is opened to stop charging the supercapacitor. If the supercapacitor cannot be put into operation due to equipment failure, the brake switch X0 can be directly put into operation to restore it to the conventional electric brake shutdown process.

[0024] The excitation current passed into the generator rotor during shutdown and normal processes is DC current. Therefore, the energy recovered during turbine shutdown can be directly used to provide auxiliary power for the generator rotor when the generator is shut down, realizing braking energy recovery and reuse. When it is detected that the energy in the energy recovery device meets the output requirements and the unit is in the electrical braking process, switch X5 is closed and switch X3 is disconnected, and the energy in the device is used to build the rotor magnetic field during the shutdown process. If the equipment fails or the battery energy is insufficient, switch X3 is closed and the energy output switch X5 is disconnected, switching to the factory power to provide energy for the rotor to ensure the normal operation of the unit.

[0025] When the shutdown energy recovery device is put into use in the generator excitation system, it constitutes a shutdown energy recovery and use system. The system framework is as follows: Figure 2 shown.

[0026] A method for recovering and using a large-scale Francis turbine shutdown energy system comprises the following steps: If the unit is shut down without electric braking due to an electrical accident, the process ends. If the unit can be shut down normally, the generator outlet switch X1 is opened after the load is reduced and the unit is disconnected. The rotor inverter is demagnetized and the internal switch of the generator excitation system is disconnected. It is judged that the shutdown energy recovery system is abnormal or the chargeable capacity of the capacitor bank is not satisfied. When the subsequent conditions are met, the brake switch X0 is put into operation to enter the non-energy recovery electric braking process. If the shutdown energy recovery system is normal and the chargeable capacity of the capacitor bank is satisfied, the brake switch X0 is kept disconnected. When the turbine guide vanes are fully closed and the unit speed naturally drops to 50% of the rated speed, switch X4 is put into operation. The bidirectional AC / DC rectifier is controlled by the control module to stabilize the output voltage and output DC to charge the capacitor bank. When the energy in the supercapacitor is full, the brake switch X0 is closed to enter the non-energy recovery electric braking process. When the unit speed drops to 15% of the rated speed, switches X3 and X4 are disconnected to stop the electric braking, so that the mechanical brake is fully put into operation and the unit speed gradually drops to 0.

[0027] The shutdown energy utilization system and method include the following steps: judging that the shutdown energy utilization system is abnormal or the capacitor bank has not reached the dischargeable capacity, keeping switch X5 disconnected, closing switch X3, and supplying power through the brake rectifier cabinet; if the shutdown energy utilization system is normal and the capacitor bank has reached the dischargeable capacity and the management module judges that the unit is in the electrical braking process, closing switch X5 and disconnecting switch X3; constantly detecting the energy supply system and capacity status of the supercapacitor, maintaining the status if there is no abnormality, and switching to the brake rectifier cabinet for power supply when there is a fault or insufficient capacity, closing switch X3, and disconnecting switch X5; judging that the unit is in a mechanical braking state, disconnecting switch X5.

[0028] The feasibility analysis process of the present invention is as follows: After the unit is disconnected, the three phases of the stator are short-circuited, and a constant DC current is input into the rotor winding, which generates an induced current in the stator. This current generates a copper loss braking torque in the rotor winding, causing the unit to slow down and brake to a stop. The short-circuit current in the stator I k It is a constant value and does not change with the decrease of the motor speed. According to the synchronous motor theory, during the electrical braking process, the excitation current passing through the rotor coil is relatively constant. Therefore, it can be considered that the fundamental wave magnetic flux per pole Φ It is also constant.

[0029] Generator stator potential E , Generator direct-axis synchronous reactanceX d It can be expressed as:

[0030] Stator current during braking I k It can be expressed as:

[0031] because

[0032] Stator current during braking I k It can be expressed as:

[0033] When the electrical braking is in steady state (the trigger angle of the rectifier cabinet is basically a constant), the relationship between the stator current, rotor current and trigger angle is as follows: Figure 3 By utilizing this feature, the shutdown energy recovery and utilization system can be used for energy recovery.

[0034] The above embodiments are only preferred technical solutions of the present invention and should not be regarded as limiting the present invention. The protection scope of the present invention shall be the technical solutions recorded in the claims, including equivalent replacement solutions of the technical features in the technical solutions recorded in the claims. That is, equivalent replacement improvements within this scope are also within the protection scope of the present invention.

Claims

1. A large-scale Francis turbine shutdown energy recovery and utilization system, characterized by: It includes a transformer, an energy dissipation resistor R, a bidirectional AC / DC rectifier, a capacitor bank, a bidirectional DC / DC rectifier and a control module; the transformer input end is connected to the three-phase power output end of the generator, and the transformer is sequentially connected with the energy dissipation resistor R, the bidirectional AC / DC rectifier, the capacitor bank and the bidirectional DC / DC rectifier to form a shutdown energy recovery device in series; The control module is used to control the semiconductor components in the bidirectional AC / DC rectifier and the bidirectional DC / DC rectifier, to smoothly convert the stored energy and the output energy, to improve the transient stability of the generator and to perform power compensation; When the generator rotor is shut down, the shutdown energy recovery device uses the recovered energy as the excitation current of the generator excitation system.

2. A large-scale Francis turbine shutdown energy recovery and utilization system according to claim 1, characterized in that: The shutdown energy recovery device is connected in parallel with the brake rectifier cabinet. The shutdown energy recovery device controls whether to put the generator excitation system into operation through switch X5, and the brake rectifier cabinet controls whether to put the generator excitation system into operation through switch X3.

3. A large-scale Francis turbine shutdown energy recovery and utilization system according to claim 1, characterized in that: It also includes a management module, which is used to detect whether the capacitor group is faulty, monitor the charging status of the capacitor group, and send the status of the capacitor group to the control module. If the equipment fails or the chargeable amount is insufficient, the electric braking recovery process will not be executed, and it will be determined whether to recover energy for utilization.

4. A large-scale Francis turbine shutdown energy recovery and utilization system according to claim 1, characterized in that: The bidirectional AC / DC rectifier maintains the voltage across the capacitor bank at a constant value during energy storage, and transmits the energy stored in the capacitor bank to the rotor for enhanced braking with high quality during auxiliary braking.

5. A large-scale Francis turbine shutdown energy recovery and utilization system according to claim 1, characterized in that: The capacitor bank stores the energy generated in the stator.

6. A large-scale Francis turbine shutdown energy recovery and utilization system according to claim 1, characterized in that: The bidirectional DC / DC rectifier maintains the voltage across the capacitor bank at a constant value during energy storage, and transmits the energy stored in the capacitor bank to the rotor for enhanced braking with high quality during auxiliary braking.

7. A large-scale Francis turbine shutdown energy recovery and utilization system according to claim 1, characterized in that: The shutdown energy recovery device is connected to the three-phase power output terminal of the generator through the switch X4.

8. The method for operating a large-scale Francis turbine shutdown energy recovery and utilization system according to any one of claims 1 to 7, characterized in that The following conditions are included: If the unit can be shut down normally, the generator outlet switch X1 is opened after load reduction and disconnection; The rotor inverter is demagnetized and the internal switch of the generator excitation system is disconnected; it is judged that the shutdown energy recovery and use system is abnormal or the chargeable capacity of the capacitor bank is not satisfied, and when the subsequent conditions are met, the brake switch X0 is turned on to enter the non-energy recovery electric braking process; If there is no abnormality in the shutdown energy recovery and use system and the chargeable capacity of the capacitor bank is sufficient, keep the brake switch X0 disconnected; the turbine guide vanes are fully closed, the unit speed naturally drops to the set speed N1, and switch X4 is turned on; the control module controls the bidirectional AC / DC rectifier to stabilize the output voltage, and outputs direct current to charge the capacitor bank; when the energy in the supercapacitor is full, the brake switch X0 is closed, and the non-energy recovery electric braking process is entered; When the unit speed drops to the set speed N2, disconnect switches X3 and X4, stop the electrical braking, fully engage the mechanical braking, and gradually reduce the unit speed to 0.

9. The operating method of a large Francis turbine shutdown energy recovery and utilization system according to claim 8, characterized in that: If it is determined that the shutdown energy recovery and use system is abnormal or the capacitor bank has not reached the discharge capacity, keep switch X5 open, switch X3 closed, and supply power through the brake rectifier cabinet; If the shutdown energy utilization system is normal and the capacitor bank reaches the dischargeable capacity and the management module determines that the unit is in the electrical braking process, close switch X5 and disconnect switch X3; always detect the energy supply system and capacity status of the supercapacitor, maintain it if there is no abnormality, and switch to the brake rectifier cabinet for power supply when there is a fault or insufficient capacity, close switch X3, and disconnect switch X5; determine that the unit is in the mechanical braking state and disconnect switch X5.

10. The operating method of a large-scale Francis turbine shutdown energy recovery and utilization system according to claim 1, characterized in that: The stator potential of the generator set is defined as E , the direct-axis synchronous reactance of the generator is X d , E and X d Satisfies the following formula: ； Stator current during braking I k It can be expressed as: ； because Stator current during braking I k It can be expressed as: ； Where: f is the frequency; ω is the angular frequency; L is the inductor; Φ is the magnetic flux; n is the unit speed; P is the pole pair number; W is the number of coil turns; K f is the pitch coefficient; K d , K E is a constant coefficient; R is the stator effective resistance; After the unit is disconnected, the three phases of the stator are short-circuited, and a constant DC current is input into the rotor winding, which generates an induced current in the stator. This current generates a copper loss braking torque in the rotor winding, causing the unit to slow down and brake to a stop. The short-circuit current in the stator I k It is a constant value and does not change with the decrease of the motor speed. According to the synchronous motor theory, during the electrical braking process, the excitation current passing through the rotor coil is relatively constant. Therefore, it can be considered that the fundamental wave magnetic flux per pole Φ It is also constant, proving that the shutdown energy recovery and utilization system can provide a stable excitation current.

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