Supercritical carbon dioxide power generation system based on four-quadrant frequency converter

Abstract

The invention provides a supercritical carbon dioxide power generation system based on a four-quadrant frequency converter, belongs to the technical field of supercritical carbon dioxide cycle power generation, and can at least partially solve the problems that an existing constant-speed compressor turbine or a one-way frequency converter cannot adjust the rotating speed in the starting, stopping and low-load stages; surging or liquid phase risks are easily caused; the load increasing and decreasing speed is limited by a power grid or a load box, and the peak regulation capacity is weak; in order to solve the problems that braking energy can only be dissipated through a resistance box, the system efficiency is low, adjustment is complex, and load shedding accidents are likely to happen, the four-quadrant frequency converter is arranged, electric speed increasing is provided in the starting stage, mechanical impact is reduced, and the starting and stopping success rate is increased; in the rated stage, mechanical energy output by the turbine is efficiently converted into electric energy, the electric energy is sent to a power grid or a plant alternating current bus through a grid-connected converter, and the overall energy efficiency is improved.

Description

Technical Field

[0001] This invention belongs to the field of supercritical carbon dioxide cycle power generation technology, specifically relating to a supercritical carbon dioxide power generation system based on a four-quadrant frequency converter. Background Technology

[0002] With the development of power generation technology, supercritical carbon dioxide, as an excellent working fluid to replace water vapor, has attracted the attention of many researchers due to its higher cycle efficiency, more compact equipment layout, and more economical initial investment. Existing supercritical carbon dioxide cycles generally use constant-speed compressor turbines or unidirectional frequency converters.

[0003] In existing technologies, the use of fixed-speed compressor turbines or unidirectional frequency converters makes the speed unadjustable during startup, shutdown, and low-load phases; this can easily lead to surge or liquid phase risks; the rate of load increase and decrease is limited by the power grid or load cell, resulting in weak peak-shaving capability; braking energy can only be dissipated through the resistor box, resulting in low system efficiency and complex adjustment, and a high risk of load shedding accidents. Therefore, we propose a supercritical carbon dioxide power generation system based on a four-quadrant frequency converter. Summary of the Invention

[0004] The present invention aims to solve at least one of the technical problems existing in the prior art, and to provide a supercritical carbon dioxide power generation system based on a four-quadrant frequency converter.

[0005] This invention provides a supercritical carbon dioxide power generation system based on a four-quadrant frequency converter, comprising: A power generation module, comprising a compressor, a regenerator, a heat exchanger, a turbine, and a generator connected in sequence; A four-quadrant frequency converter, wherein the AC side of the four-quadrant frequency converter is connected to the stator winding of the generator; A logic control unit, electrically connected to the four-quadrant frequency converter, is used to enable bidirectional communication with the four-quadrant frequency converter during operation; and The DC bus is electrically connected to the four-quadrant frequency converter, and the DC bus is connected to the DC side of the grid-connected converter; The logic control unit includes a shaft system coordination control module, used to calculate the weighted target speed ω_s of the compressor demand speed and the turbine demand speed in real time during operation, and a liquid phase boundary protection module, used to calculate the liquid phase approximation index LI in real time based on the compressor inlet pressure P_in and temperature T_in during operation, and to limit the rate of decrease of the weighted target speed ω_s when the liquid phase approximation index LI>0.9; The four-quadrant frequency converter includes the following operating modes: an electric mode that increases the speed of the compressor rotor and the turbine rotor during the startup phase of the power generation module; a power generation mode that converts mechanical energy into electrical energy and feeds it back to the grid through a grid-connected converter during the rated phase of the power generation module; and a braking mode that absorbs the coasting energy of the compressor rotor and the turbine rotor and feeds it back to the DC bus during the load reduction or shutdown phase of the power generation module.

[0006] Furthermore, the power generation module also includes a cooler connected to the regenerator pipeline, the cooler being connected to the gas storage tank pipeline, and the gas storage tank being connected to the compressor pipeline.

[0007] Specifically, the liquid phase boundary protection module is electrically connected to the four-quadrant frequency converter to continuously monitor P_in and T_in during operation, and to update the liquid phase approximation index LI in real time by looking up a table or fitting a formula. Among them, the liquid phase approximation index T_quasi-critical is the quasi-critical temperature under the inlet pressure P_in.

[0008] Specifically, the logic control unit also includes a surge warning module, which detects the flow-pressure signal of the compressor during operation and compares the signal with a preset surge trajectory curve. Based on the comparison result, it triggers the acceleration or deceleration command of the four-quadrant frequency converter.

[0009] Preferably, the surge warning module stores an anti-surge curve, a surge approach curve, and the surge trajectory curve.

[0010] Specifically, the horizontal axis of the anti-surge curve, the surge approach curve, and the surge trajectory curve represents the flow rate, and the vertical axis of the anti-surge curve, the surge approach curve, and the surge trajectory curve represents the pressure ratio, which is the ratio of the compressor's outlet pressure to its inlet pressure.

[0011] Furthermore, the shaft system coordinated control module uses the following logic to generate the target rotational speed ω_s: ω_s = k_c·ω_c + k_t·ω_t + k_sync·Δθ Where k_c and k_t are load distribution coefficients, k_sync is the coaxial torsional vibration damping coefficient, and Δθ is the real-time shaft system torsional angle deviation.

[0012] Furthermore, the logic control unit also includes a power management module electrically connected to the four-quadrant frequency converter to switch between the power generation mode and the braking mode during operation and thereby suppress DC bus voltage fluctuations.

[0013] Specifically, the power management module is electrically connected to the four-quadrant frequency converter to achieve the following during operation: After the power grid dispatch load reduction command arrives, the four-quadrant frequency converter is switched from the power generation mode to the braking mode; In the braking mode, the four-quadrant frequency converter absorbs the kinetic energy of the compressor rotor and the turbine rotor, and converts the kinetic energy into electrical energy, which is then fed back to the plant AC bus via the grid-connected converter. When the voltage of the DC bus exceeds the threshold, the power management module activates the chopper resistor on the DC bus to dissipate the voltage of the DC bus and prevent the DC bus from overvoltage.

[0014] Furthermore, the logic control unit is used to control the four-quadrant frequency converter and the power generation module according to the liquid phase approximation index LI during operation, wherein: When the liquid phase approximation index LI > 0.95, the speed reduction operation of the four-quadrant frequency converter is stopped and the speed of the compressor rotor and the turbine rotor is increased; When the liquid phase approximation index LI > 0.98, the heat exchanger's thermal power is increased and the cooler's heat exchange capacity is reduced to increase the temperature of the power generation module.

[0015] The beneficial effects of this invention are as follows: Equipped with a four-quadrant frequency converter, it provides electric acceleration during startup, reducing mechanical shock and improving start-stop success rate; during rated operation, it efficiently converts the mechanical energy output from the turbine into electrical energy, which is then sent to the power grid or plant AC bus via a grid-connected converter, improving overall energy efficiency; the shaft system coordinated control module weightedly synthesizes the speed requirements of the compressor side and the turbine side, and introduces damping adjustment for shaft torsional deviation, so that the target speed satisfies both the load distribution at the hot and cold ends and suppresses coaxial torsional vibration; the liquid phase boundary protection module evaluates the liquid phase approach index in real time based on the compressor inlet pressure and temperature, and limits the deceleration slope, stops deceleration, or moderately accelerates according to a graded strategy. Attached Figure Description

[0016] Figure 1 This is a connection diagram of a supercritical carbon dioxide power generation system based on a four-quadrant frequency converter, according to a specific embodiment of the present invention. Figure 2 The operating logic diagram of a supercritical carbon dioxide power generation system based on a four-quadrant frequency converter, according to a specific embodiment of the present invention. The components include: 1. compressor, 2. regenerator, 3. heat exchanger, 4. turbine, 5. generator, 6. cooler, 7. air tank, 8. four-quadrant frequency converter, and 9. DC bus. Detailed Implementation

[0017] To enable those skilled in the art to better understand the technical solution of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0018] like Figure 1 As shown in the figure, a supercritical carbon dioxide power generation system based on a four-quadrant frequency converter provided by a specific embodiment of the present invention includes: The power generation module includes a compressor 1, a regenerator 2, a heat exchanger 3, a turbine 4, and a generator 5 connected in sequence; a four-quadrant frequency converter 8, the AC side of which is connected to the stator winding of the generator 5; a logic control unit electrically connected to the four-quadrant frequency converter 8 for bidirectional communication with the four-quadrant frequency converter 8 during operation; and a DC bus 9 electrically connected to the four-quadrant frequency converter 8, the DC bus 9 being connected to the DC side of the grid-connected converter, the AC side of which is connected to the power grid or the plant's AC bus. The logic control unit includes a shaft coordination control module for real-time calculation of the weighted target speed ω_s of the compressor 1 and turbine 4 required speeds during operation, and a liquid phase boundary protection module for real-time calculation of the liquid phase approximation index LI based on the compressor inlet pressure P_in and temperature T_in during operation, and limiting the rate of decrease of the weighted target speed ω_s when the liquid phase approximation index LI>0.9. The four-quadrant frequency converter 8 includes the following operating modes: an electric mode that increases the rotor speeds of compressor 1 and turbine 4 during the generator module startup phase; a generator mode that converts mechanical energy into electrical energy and feeds it back to the grid through a grid-connected converter during the generator module rated phase; and a braking mode that absorbs the coasting energy of compressor 1 and turbine 4 during the generator module load reduction or shutdown phase and feeds it back to the DC bus.

[0019] Specifically, the four-quadrant frequency converter 8 can increase the shaft speed, convert mechanical energy or inertial kinetic energy into electrical energy and feed it back to the DC bus 9 or the power grid, and can also perform regenerative braking to absorb coasting energy; the AC side of the four-quadrant frequency converter 8 is directly connected to the stator of the generator 5, and performs closed-loop regulation according to the target speed / torque given by the upper logic control unit; in conjunction with power management, it suppresses bus voltage fluctuations, and when necessary, it uses chopper resistors to dissipate energy.

[0020] Furthermore, compressor 1 and turbine 4 are coaxially arranged, generator 5 is a coaxial high-speed generator, and the shaft system coordination control module is a coaxial shaft system coordination control module. It integrates the required speed of compressor 1, the required speed of turbine 4 and the torsional state of the shaft system into a target speed ω_s, which takes into account both the load distribution at the hot and cold ends and suppresses coaxial torsional vibration. The shaft system coordination control module acts as the "dispatch center" of the speed master, and issues a smooth and limited speed reference to the four-quadrant frequency converter.

[0021] Furthermore, the liquid phase boundary protection module prevents the compressor 1 inlet from approaching / crossing the liquid phase boundary of supercritical carbon dioxide, i.e., condensation / wet compression, thus protecting the compressor 1 and the stability of the cycle. When approaching the liquid phase boundary, the liquid phase boundary protection module limits the deceleration or even reverses the deceleration, and coordinates with the hot end to perform actions such as increasing the heat work of the heat exchanger or reducing the cooler load, bringing the operating condition back to the safe zone.

[0022] Based on the above basic implementation method, the power generation module also includes a cooler 6 connected to the regenerator 2 pipeline, the cooler 6 being connected to the gas storage tank 7 pipeline, the gas storage tank 7 being connected to the compressor 1 pipeline, and the heat exchanger 3 connected to the regenerator pipeline being the main heat exchanger.

[0023] Specifically, the liquid phase boundary protection module is electrically connected to the four-quadrant frequency converter 8 to continuously monitor P_in and T_in during operation, and update the liquid phase approximation index LI in real time by looking up a table or fitting a formula. Among them, the liquid phase approximation index T_quasi-critical is the quasi-critical temperature under the inlet pressure P_in condition.

[0024] In one specific embodiment, the logic control unit further includes a surge warning module, which detects the flow-pressure signal of the compressor 1 during operation and compares the signal with a preset surge trajectory curve. Based on the comparison result, it triggers an acceleration or deceleration command for the four-quadrant frequency converter 8.

[0025] In this embodiment, the surge warning module stores anti-surge curves, surge approach curves, and surge trajectory curves.

[0026] Furthermore, the horizontal axis of the anti-surge curve, surge approach curve, and surge trajectory curve represents the flow rate, while the vertical axis represents the pressure ratio, which is the ratio of the outlet pressure to the inlet pressure of compressor 1. The approach curve and anti-surge curve are used to trigger the four-quadrant inverter 8 to perform a "rapid pass-through" action: increasing ω_s by 5% of the rated speed within 2000ms to escape the surge boundary. The selection and calibration of the approach surge curve and anti-surge curve should be determined comprehensively based on the rated operating pressure and flow rate of the supercritical carbon dioxide cycle. The parameter selection and control method in this embodiment are only an example of a certain cycle operating condition.

[0027] In another specific embodiment, the shaft system coordinated control module uses the following logic to generate the target rotational speed ω_s: ω_s = k_c·ω_c + k_t·ω_t + k_sync·Δθ Where k_c and k_t are load distribution coefficients, k_sync is the coaxial torsional vibration damping coefficient, and Δθ is the real-time shaft system torsional angle deviation.

[0028] Specifically, coefficients such as k_c, k_t, and k_sync are correction parameters for the shaft speed of the integrated compressor 1-turbine 4 unit (especially the speed when the load changes). There are no special mandatory requirements. They should be comprehensively calibrated and corrected in combination with the characteristics of the overall supercritical carbon dioxide power generation cycle system and the mechanical characteristics of the integrated unit itself.

[0029] In one specific implementation, the logic control unit further includes a power management module electrically connected to the four-quadrant inverter for switching between generation and braking modes during operation and thereby suppressing DC bus voltage fluctuations. The switching of the four-quadrant inverter 8 between motoring, generation, and braking modes is completed by the logic control unit 10 through a single control word, with a switching time of <10 ms, and the electromagnetic torque of the coaxial rotor is maintained continuously during the switching process to avoid mechanical shock.

[0030] In this embodiment, the power management module is electrically connected to the four-quadrant inverter 8 to achieve the following during operation: after the grid dispatch load reduction command arrives, the four-quadrant inverter 8 is switched from the generation mode to the braking mode; in the braking mode, the four-quadrant inverter 8 absorbs the kinetic energy of the rotor of compressor 1 and the rotor of turbine 4, and converts the kinetic energy into electrical energy and feeds it back to the plant AC bus via the grid-connected converter; when the voltage of DC bus 9 exceeds the threshold, the power management module activates the chopper resistor on DC bus 9 to dissipate the voltage of DC bus 9 and prevent DC bus 9 from overvoltage. The voltage threshold of DC bus 9 can be set with reference to 105%~110% of the voltage of DC bus 9.

[0031] Specifically, when the liquid phase approximation index LI > 0.95, the four-quadrant frequency converter 8 is forced to stop the speed reduction operation, stopping any possible speed reduction operation, and slowly increasing the speed. The speed change rate can be referenced as |dω_s / dt|≤0.05ω_n / s, where ω_n is the rated speed. When the liquid phase approximation index LI > 0.98, the speed reduction command is immediately blocked and the "temperature recovery" subroutine is triggered. By increasing the heat power of heat exchanger 3 or increasing the opening of the bypass valve of cooler 6, the heat exchange of cooler 6 is reduced, and T_in is increased. Regarding the pseudo-critical temperature and the approximation coefficient, it is recommended to force real-time display for easy comparison by operators. If necessary, it should also be connected to the unit's DCS or PLC alarm logic. Regarding the speed change rate, the value in this embodiment is only a suggestion. The change rate should be comprehensively considered based on the total power of the power generation system, the main parameters of the compressor and turbine, and the current operating conditions.

[0032] In another specific embodiment, the method of operating this system includes the following steps: Includes the following steps: Start-up phase: The logic control unit sets the initial speed ω_s=10%ω_n, where ω_n is the rated speed of the power generation module; the four-quadrant frequency converter 8 accelerates to the warm-up speed ω_warm with constant torque, and continuously monitors LI<0.85 during this period; During the grid connection phase: when ω_s=ω_n and the phase synchronization error is <1°, the four-quadrant frequency converter 8 switches from motor mode to generator mode, and the power closed-loop time constant is ≤100 ms; During peak shaving: Based on the grid AGC or the unit peak shaving command ΔP_demand, the power management module controls the four-quadrant frequency converter 8 to complete the power target value adjustment within 1 second; the adjustment rate can be achieved by the dispatcher through AGC, or manually input by the operator; Shutdown phase: After the surge warning module detects no abnormalities, the system first decelerates to 50%ω_n in braking mode, then switches to electric mode to maintain coasting cooling, and finally disconnects the four-quadrant frequency converter 8.

[0033] Furthermore, after the shutdown phase ends, the logic control unit outputs a "shaft station confirmation" signal, triggering the pressure holding valve of the air tank 7 to close. At the same time, the four-quadrant frequency converter 8 enters standby mode, and the DC bus 9 voltage is maintained at ±2% of the rated voltage parameter, waiting for the next start command.

[0034] Furthermore, during the working fluid filling stage, the system pressure is increased to the minimum allowable starting pressure of compressor 1 via the gas storage tank 7, generally not lower than 2MPa. It is important to note that compressor 1, generator 5, and turbine 4 are arranged coaxially. Before starting compressor 1, the pre-start preparations for generator 5 and turbine 4, as well as the corresponding auxiliary machines or subsystems (e.g., lubrication system, sealing gas system), should also be completed and checked. After compressor 1 starts, the generator 5 should be accelerated via a four-quadrant frequency converter 8 to the target rated speed, i.e., electric mode. The acceleration rate and acceleration curve should conform to the design requirements of the unit and avoid the critical zone of the unit itself. During the acceleration process, the inlet parameters (temperature, pressure) of compressor 1 should be closely monitored, and the inlet carbon dioxide should be kept as far away as possible from its quasi-critical value. At the same time, the operating boundary requirements of regenerator 2, heat exchanger 3, turbine 4, and cooler 6 should be fully considered. After compressor 1 reaches its rated speed, heat exchanger 3 should gradually increase its heating power. Depending on the type of heat exchanger 3, the main system and the bypass valves of compressor 1, turbine 4, and cooler 6 (cooling medium side) should be comprehensively adjusted to ensure that the start-up conditions of heat exchanger 3 are met as soon as possible and to shorten the preparation time. When the inlet parameters of turbine 4 rise, the four-quadrant inverter 8 should switch to power generation mode. When switching modes, the inlet parameters of compressor 1, the parameters of the four-quadrant inverter 8 (such as current), and the voltage of DC bus 9 should be the focus. Regarding load changes, similar to the mode switching principle, it is important to note that changes in turbine 4 exhaust pressure can lead to an increase in system back pressure, affecting the approximation index LI's shift towards the liquid phase. The power management module should be fully tested and then put into operation. Under high load conditions, the inlet parameters of compressor 1 cause the approximation index LI to approach 1. In this case, attention should be paid to adjusting the load rate and adjusting the cooling medium flow rate of cooler 6 through the liquid phase boundary protection module. Regarding shutdown, as heat exchanger 3 exits operation, four-quadrant frequency converter 8 should switch to electric mode and reduce speed. Considering that auxiliary systems such as sealing gas can work normally during and after shutdown, high pressure and temperature parameters and low load conditions should not be sustained for too long. The compressor can be tripped and shut down after its speed drops to 60. Regarding fault conditions, the surge warning module is used to prevent compressor 1 from entering the surge zone. When the compressor operating point is approaching the surge curve, the "shutdown command" of the compressor 1 bypass should be locked, and there should be a clear alarm prompt. At this time, the system flow should be increased appropriately, and the compressor 1 bypass can be opened if necessary. Under high load conditions, the shutdown of the fault condition requires emergency response measures for the load shedding condition. To ensure that the voltage of DC bus 9 is within a safe and controllable range when the four-quadrant inverter 8 is forcibly braked, the system should immediately start the emergency venting to minimize the enthalpy of the working fluid.

[0035] To aid in a better understanding of the present invention, a more comprehensive and specific embodiment is described, in which the present invention provides, on one hand, a supercritical carbon dioxide power generation system based on a four-quadrant frequency converter, comprising: The power generation module includes a compressor 1, a regenerator 2, a heat exchanger 3, a turbine 4, and a generator 5 connected in sequence; a four-quadrant frequency converter 8, the AC side of which is connected to the stator winding of the generator 5; a logic control unit electrically connected to the four-quadrant frequency converter 8 for bidirectional communication with the four-quadrant frequency converter 8 during operation; and a DC bus 9 electrically connected to the four-quadrant frequency converter 8, the DC bus 9 being connected to the DC side of the grid-connected converter, the AC side of which is connected to the power grid or the plant's AC bus. The logic control unit includes a shaft coordination control module for real-time calculation of the compressor's required speed and the weighted target speed ω_s of the turbine's required speed during operation, and a liquid phase boundary protection module for real-time calculation of the liquid phase approximation index LI based on the compressor inlet pressure P_in and temperature T_in during operation, and limiting the rate of decrease of the weighted target speed ω_s when the liquid phase approximation index LI>0.9. The four-quadrant frequency converter 8 includes the following operating modes: an electric mode that increases the speed of the compressor 1 rotor and turbine 4 rotor during the power generation module startup phase; a power generation mode that converts mechanical energy into electrical energy and feeds it back to the grid through the grid-connected converter during the power generation module rated phase; and a braking mode that absorbs the coasting energy of the compressor 1 rotor and turbine 4 rotor and feeds it back to the DC bus during the power generation module load reduction or shutdown phase.

[0036] In this embodiment, the power generation module also includes a cooler 6 connected to the regenerator 2 via a pipeline. The cooler 6 is connected to the gas storage tank 7 via a pipeline, and the gas storage tank 7 is connected to the compressor 1 via a pipeline.

[0037] Specifically, the liquid phase boundary protection module is electrically connected to the four-quadrant frequency converter 8 to continuously monitor P_in and T_in during operation, and update the liquid phase approximation index LI in real time by looking up a table or fitting a formula. Among them, the liquid phase approximation index T_quasi-critical is the quasi-critical temperature under the inlet pressure P_in condition; the logic control unit also includes a surge warning module, which detects and analyzes the flow-pressure signal of the compressor during operation and compares the signal with the preset surge trajectory curve. Based on the comparison result, it triggers the acceleration or deceleration command of the four-quadrant frequency converter 8; the surge warning module has pre-stored anti-surge curve, surge approach curve and surge trajectory curve; the horizontal axis of the anti-surge curve, surge approach curve and surge trajectory curve is the flow rate, and the vertical axis of the anti-surge curve, surge approach curve and surge trajectory curve is the pressure ratio, which is the ratio of the outlet pressure of compressor 1 to the inlet pressure; The shaft system coordinated control module uses the following logic to generate the target rotational speed ω_s: ω_s = k_c·ω_c + k_t·ω_t + k_sync·Δθ Where k_c and k_t are load distribution coefficients, k_sync is the coaxial torsional vibration damping coefficient, and Δθ is the real-time shaft system torsional angle deviation; The logic control unit also includes a power management module, which is electrically connected to the four-quadrant inverter 8 to switch between generation and braking modes during operation and thereby suppress voltage fluctuations on the DC bus 9. The power management module, electrically connected to the four-quadrant inverter 8, is used to: switch the four-quadrant inverter 8 from generation mode to braking mode after a load reduction command from the grid dispatch arrives; in braking mode, the four-quadrant inverter 8 absorbs the kinetic energy of the compressor 1 rotor and turbine 4 rotor, converting the kinetic energy into electrical energy and feeding it back to the plant's AC auxiliary bus via the grid-connected converter; when the voltage on the DC bus 9 exceeds a threshold, the power management module activates the chopper resistor on the DC bus 9 to dissipate the voltage on the DC bus 9, preventing overvoltage on the DC bus 9. The logic control unit is used to control the four-quadrant frequency converter 8 and the power generation module according to the liquid phase approximation index LI during operation. Specifically: when the liquid phase approximation index LI > 0.95, the speed reduction operation of the four-quadrant frequency converter 8 is stopped and the rotor speed of compressor 1 and turbine 4 is increased; when the liquid phase approximation index LI > 0.98, the heat power of heat exchanger 3 is increased and the heat exchange of cooler 6 is reduced to increase the temperature of the power generation module.

[0038] In summary, the embodiments disclosed herein have at least the following technical effects: The system achieves continuous and controllable energy management and speed control under all operating conditions, including startup, rated operation, load reduction, and shutdown. The four-quadrant frequency converter 8 provides electric acceleration during startup, reducing mechanical shock and improving the success rate of start-up and shutdown. During rated operation, it efficiently converts the mechanical energy output by the turbine into electrical energy, which is then sent to the power grid or the plant's AC bus via the grid-connected converter, improving overall energy efficiency. During load reduction and shutdown, it enters a braking state, absorbing the inertial kinetic energy of the shaft system and feeding it back to the DC bus. When the voltage of the DC bus 9 increases, it dissipates the energy by triggering the chopper resistor through power management, thereby suppressing overvoltage and related tripping risks and ensuring the safety and predictability of the shutdown process. The shaft system coordinated control module weighted and synthesized the speed requirements of compressor 1 and turbine 4, and introduced damping adjustment for shaft torsional deviation, ensuring that the target speed satisfies both the load distribution at the hot and cold ends and suppresses coaxial torsional vibration. Through limiting and ramping the target speed, the system achieves smooth transitions under load disturbances and scheduling commands, reducing linkage fluctuations and structural fatigue, and extending the service life of key components such as bearings, couplings, and seals. Simultaneously, the parametric design facilitates tuning for different unit sizes and seasonal operating conditions, improving adaptability and maintainability. The liquid phase boundary protection module assesses the liquid phase approach index in real time based on the compressor inlet pressure and temperature, and limits the deceleration slope, stops deceleration, or moderately increases the speed according to a graded strategy. When the index further increases, the module coordinates with the hot end adjustment to increase the heat power of heat exchanger 3 or reduce the heat exchange of cooler 6, so that the inlet temperature quickly moves away from the liquid phase boundary, avoiding wet compression and the resulting vibration, wear, and efficiency reduction. This process relies on continuous monitoring and table lookup or fitting calculation to achieve adaptive protection against the strong nonlinearity in the supercritical region, significantly reducing the probability of unplanned downtime. The surge warning module performs online trajectory discrimination on the flow rate and pressure ratio signals of compressor 1 and compares them with the anti-surge curve and the approach curve. When the compressor approaches the unstable region, it triggers acceleration or deceleration commands in advance. This pre-control measure suppresses backflow and pressure pulsation, protects the regenerator 2, heat exchanger 3 and pipelines, reduces circulation shock and noise, and creates more stable boundary conditions for shaft coordination and liquid phase protection. The power management module is responsible for quickly switching between generation and braking modes upon arrival of grid-side dispatch and for stabilizing the voltage of DC bus 9. It prioritizes energy feedback to absorb shaft kinetic energy and promptly activates the chopper resistor to safely release abnormal energy when the voltage exceeds the threshold. This closed-loop energy path is clearly defined, and the three domains of the electric motor and heater coordinate smoothly, significantly improving the system's grid-friendliness and peak-shaving capability.

[0039] It is understood that the above embodiments are merely exemplary implementations used to illustrate the principles of the present invention, and the present invention is not limited thereto. For those skilled in the art, various modifications and improvements can be made without departing from the spirit and essence of the present invention, and these modifications and improvements are also considered to be within the scope of protection of the present invention.

Claims

1. A supercritical carbon dioxide power generation system based on a four-quadrant frequency converter, characterized in that, include: A power generation module, comprising a compressor, a regenerator, a heat exchanger, a turbine, and a generator connected in sequence; A four-quadrant frequency converter, wherein the AC side of the four-quadrant frequency converter is connected to the stator winding of the generator; A logic control unit is electrically connected to the four-quadrant frequency converter to enable bidirectional communication with the four-quadrant frequency converter during operation. as well as The DC bus is electrically connected to the four-quadrant frequency converter, and the DC bus is connected to the DC side of the grid-connected converter; The logic control unit includes: a shaft system coordination control module, used to calculate the weighted target speed ω_s of the compressor's required speed and the turbine's required speed in real time during operation; and a liquid phase boundary protection module, used to calculate the liquid phase approximation index LI in real time based on the compressor inlet pressure P_in and temperature T_in during operation, and to limit the rate of decrease of the weighted target speed ω_s when the liquid phase approximation index LI>0.9; The four-quadrant frequency converter includes the following operating modes: an electric mode that increases the speed of the compressor rotor and the turbine rotor during the startup phase of the power generation module; a power generation mode that converts mechanical energy into electrical energy and feeds it back to the grid through a grid-connected converter during the rated phase of the power generation module; and a braking mode that absorbs the coasting energy of the compressor rotor and the turbine rotor and feeds it back to the DC bus during the load reduction or shutdown phase of the power generation module.

2. The supercritical carbon dioxide power generation system based on a four-quadrant frequency converter according to claim 1, characterized in that, The power generation module also includes a cooler connected to the regenerator pipeline, which is connected to the gas storage tank pipeline, and the gas storage tank is connected to the compressor pipeline.

3. The supercritical carbon dioxide power generation system based on a four-quadrant frequency converter according to claim 1, characterized in that, The liquid phase boundary protection module is electrically connected to the four-quadrant frequency converter to continuously monitor P_in and T_in during operation, and to update the liquid phase approximation index LI in real time by looking up a table or fitting a formula. Among them, the liquid phase approximation index T_quasi-critical is the quasi-critical temperature under the inlet pressure P_in.

4. The supercritical carbon dioxide power generation system based on a four-quadrant frequency converter according to claim 1, characterized in that, The logic control unit also includes a surge warning module, which is used to detect the flow-pressure signal of the compressor during operation, compare the signal with a preset surge trajectory curve, and trigger the acceleration or deceleration command of the four-quadrant frequency converter according to the comparison result.

5. The supercritical carbon dioxide power generation system based on a four-quadrant frequency converter according to claim 4, characterized in that, The surge warning module pre-stores anti-surge curves, surge approach curves, and surge trajectory curves.

6. The supercritical carbon dioxide power generation system based on a four-quadrant frequency converter according to claim 5, characterized in that, The horizontal axis of the anti-surge curve, the surge approach curve, and the surge trajectory curve represents the flow rate, and the vertical axis of the anti-surge curve, the surge approach curve, and the surge trajectory curve represents the pressure ratio, which is the ratio of the compressor's outlet pressure to its inlet pressure.

7. The supercritical carbon dioxide power generation system based on a four-quadrant frequency converter according to claim 1, characterized in that, The shaft system coordinated control module uses the following logic to generate the target rotational speed ω_s: ω_s = k_c·ω_c + k_t·ω_t + k_sync·Δθ Where k_c and k_t are load distribution coefficients, k_sync is the coaxial torsional vibration damping coefficient, and Δθ is the real-time shaft system torsional angle deviation.

8. The supercritical carbon dioxide power generation system based on a four-quadrant frequency converter according to claim 1, characterized in that, The logic control unit also includes a power management module, which is electrically connected to the four-quadrant frequency converter to switch between the power generation mode and the braking mode during operation and thereby suppress DC bus voltage fluctuations.

9. The supercritical carbon dioxide power generation system based on a four-quadrant frequency converter according to claim 8, characterized in that, The power management module is electrically connected to the four-quadrant frequency converter to achieve the following during operation: After the power grid dispatch load reduction command arrives, the four-quadrant frequency converter is switched from the power generation mode to the braking mode; In the braking mode, the four-quadrant frequency converter absorbs the kinetic energy of the compressor rotor and the turbine rotor, and converts the kinetic energy into electrical energy, which is then fed back to the plant AC bus via the grid-connected converter. When the voltage of the DC bus exceeds the threshold, the power management module activates the chopper resistor on the DC bus to dissipate the voltage of the DC bus and prevent the DC bus from overvoltage.

10. The supercritical carbon dioxide power generation system based on a four-quadrant frequency converter according to any one of claims 1 to 9, characterized in that, The logic control unit is used to control the four-quadrant frequency converter and the power generation module according to the liquid phase approximation index LI during operation, wherein: When the liquid phase approximation index LI > 0.95, the speed reduction operation of the four-quadrant frequency converter is stopped and the speed of the compressor rotor and the turbine rotor is increased; When the liquid phase approximation index LI > 0.98, the heat exchanger's thermal power is increased and the cooler's heat exchange capacity is reduced to increase the temperature of the power generation module.

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