A method for rapid grid connection control after power frequency disconnection from low-frequency MG transmission systems

Abstract

This invention provides a rapid grid connection control method for low-frequency M-G transmission systems after power frequency grid disconnection, relating to the field of new energy low-frequency power transmission technology. It solves the problems of existing low-frequency transmission systems, such as difficulty in quickly restoring grid connection after power frequency side disconnection, and the high complexity and cost of the systems. The method includes: when the power frequency grid disconnects due to a fault, controlling the power frequency grid-side switch to trip while keeping the wind power low-frequency side switch closed; controlling the speed of the motor-generator frequency converter to increase, absorbing excess active power on the wind power low-frequency side, and maintaining the unit speed within the safe speed limit; responding to the power frequency grid fault being cleared, limiting the wind farm's output power, and controlling the unit speed to decrease to synchronize with the power frequency grid; controlling the unit to reconnect to the restored power frequency grid in a preset grid connection mode. This invention enables low-frequency offshore wind turbines to operate without disconnecting from the grid and to quickly reconnect after fault recovery, without the need for re-establishing voltage and grid connections.

Description

Technical Field

[0001] This invention relates to the field of new energy low-frequency power transmission technology, specifically to a method for rapid grid connection control after power frequency disconnection from a low-frequency MG power transmission system. Background Technology

[0002] As offshore wind power expands into mid- and far-sea areas, low-frequency AC transmission technology has become an important technological direction due to its comprehensive advantages in transmission capacity, transmission distance, and cost. Currently, low-frequency transmission systems for mid- and far-sea wind power primarily employ solutions based on power electronic frequency converters. For example... Figure 1 As shown, this system typically includes key components such as an offshore low-frequency wind farm, an offshore booster station, submarine AC cables, and a modular multilevel matrix converter (M3C) located on land. The electricity generated by the offshore wind turbines is transformed and transmitted at low frequency to the shore station via submarine cables, where it is then converted to power frequency by the M3C and finally connected to the onshore power grid.

[0003] In systems based on M3C (Multi-Chip Power Electronics) technology, when a fault occurs in the receiving-end power frequency grid, the conventional control strategy to protect the frequency converter is to quickly disconnect the switch connecting the power frequency grid and the frequency converter, and simultaneously disconnect the switch connecting the wind farm and the frequency converter. This aims to prevent excess power from surging in due to the inertia of wind turbine power regulation, thereby avoiding the risk of overvoltage on the DC side of the converter. During the fault, the wind farm needs to dissipate energy using its own protection circuits and strive to maintain the turbine speed within a safe range. However, when the fault on the power frequency side is cleared and reconnection to the grid is prepared, because the entire low-frequency transmission network has been disconnected, the system needs to rebuild the voltage and network from scratch. This process is time-consuming and severely affects the rapid recovery capability after a grid fault.

[0004] To overcome the aforementioned drawback of long recovery times, existing technologies propose an improved solution: configuring a dedicated energy-dissipating resistor device at the frequency converter station. Under this solution, when the power frequency side disconnects from the grid, the connection between the wind farm and the frequency converter remains unbroken. At this time, the excess power output by the wind turbine is consumed by the connected energy-dissipating resistor, effectively maintaining the stability of the converter's DC voltage and providing a buffer time for the wind turbine to adjust its output power, while simultaneously maintaining the continuous operation of the low-frequency network. Once the power frequency side fault is cleared, reconnection to the grid can be achieved simply by disconnecting the energy-dissipating resistor, without needing to re-voltage the transmission cable, thus achieving rapid reclosing. However, this solution introduces additional energy-dissipating resistor equipment, increasing the complexity of the system structure and its footprint, while also increasing equipment investment and maintenance costs.

[0005] On the other hand, low-frequency MG transmission systems composed of motor-generator rotating frequency converters exhibit unique advantages in grid affinity. These units possess the inherent characteristics of synchronous generators, providing natural rotational inertia and short-circuit current support, and demonstrating excellent performance in reactive power regulation and power quality assurance. However, research on rapid reclosing and grid connection for such transmission systems based on rotating equipment remains lacking when facing similar power frequency side disconnection scenarios. If their control logic follows the approach of traditional power electronics solutions, they will also face problems such as cumbersome recovery processes and time delays.

[0006] Therefore, how to solve the problem of rapid reclosing and grid connection of offshore wind power low-frequency transmission systems based on motor-generator rotating frequency switching after disconnection from the power frequency side without significantly increasing system complexity and cost has become a key technical focus in this field. Summary of the Invention

[0007] The purpose of this invention is to address the difficulty in quickly restoring grid connection to low-frequency transmission systems employing motor-generator rotating frequency converters after grid disconnection on the power frequency side, and the high system complexity and cost of existing solutions. Therefore, this invention proposes a rapid grid connection control method for low-frequency (MG) transmission systems after power frequency grid disconnection. This invention can maintain the operation of the low-frequency grid in the event of a power frequency side fault disconnection, enabling low-frequency offshore wind turbines to operate without disconnection. After the power frequency side fault is restored, rapid grid reconnection can be achieved without the need for rebuilding the submarine cable network.

[0008] The present invention employs the following technical solutions to achieve its objective: A method for rapid grid connection control after power frequency disconnection from a low-frequency MG transmission system, wherein the low-frequency MG transmission system includes a wind power low-frequency side and a power frequency grid side connected by a motor-generator frequency converter; the method includes the following steps: S1. When the power frequency grid side disconnects from the grid due to a fault, the switch on the power frequency grid side is controlled to trip, while the switch on the wind power low frequency side remains closed. S2. Control the speed of the motor-generator inverter unit to increase, so as to absorb the excess active power generated by the wind power low-frequency side due to the grid disconnection event, and maintain the speed of the motor-generator inverter unit within the preset safe speed limit. S3. In response to the power grid fault being cleared, limit the output power of the wind farm and control the speed of the motor-generator inverter unit to decrease to the speed synchronized with the power grid. S4. Control the motor-generator frequency converter unit to reconnect to the restored power frequency grid in a preset grid connection mode.

[0009] Preferably, in step S2, the preset safe speed limit is 1.1 times the rated speed of the motor-generator frequency converter.

[0010] Preferably, step S2 further includes: when the speed of the motor-generator inverter unit reaches the upper limit of the safe speed, engaging at least one auxiliary active power consumption device to consume the excess active power on the low-frequency side of the wind power. The auxiliary active power consumption device includes at least one of the following: an energy bypass circuit installed on the wind turbine side, an energy-consuming resistor installed on the power frequency grid side or the motor-generator frequency converter side, and an energy storage element connected to the low-frequency MG transmission system.

[0011] Furthermore, from the moment the power frequency grid is disconnected until the speed of the motor-generator inverter unit drops from the rated speed... The speed increases to the upper limit of the safe speed range. up to that point, the time elapsed Determined according to the following formula:

[0012] In the formula, For the motor-generator frequency converter unit at a speed from Rise to The increase in kinetic energy absorbed during the process; The active power output by the low-frequency side of the wind power system within a preset time period after grid disconnection; the activation time of the at least one auxiliary active power consumption device is within the specified time. Completed within the specified time.

[0013] Preferably, step S2 further includes controlling the wind farm to reduce its output active power; If the wind farm requires adjustment time to reduce its output active power Less than the time Then, by controlling the speed of the motor-generator inverter unit to increase, activating the at least one auxiliary active power consumption device, and coordinating the wind farm to reduce the output active power, the speed of the motor-generator inverter unit is maintained within the safe speed limit. If the adjustment time Greater than or equal to the time Then the configured capacity of the low-frequency MG transmission system is not less than Energy storage devices or energy consumption devices; among which, Determined according to the following formula:

[0014] In the formula, the integration interval is from the moment of disconnection from the power frequency grid to the adjustment time. The moment of ending.

[0015] Specifically, kinetic energy increment Determined based on the following formula:

[0016] In the formula, Let be the moment of inertia of the rotor of the motor-generator frequency converter unit.

[0017] Preferably, in step S2, the type and capacity of the at least one auxiliary active power consumption device are adjusted according to the adjustment time required for the wind farm to reduce its output active power. The moment of inertia of the rotor of the motor-generator frequency converter unit and the time mentioned Associate and match selections.

[0018] Specifically, in step S3, limiting the output power of the wind farm involves issuing a command to reduce the active power output through the central power controller of the wind farm or the local controller of each wind turbine, so that the total active power output value of the wind farm drops below the active power level that the motor-generator frequency converter can transmit at synchronous speed.

[0019] Optionally, in step S4, the preset grid connection mode is a coasting grid connection mode; in the coasting grid connection mode, the excitation of the motor in the motor-generator inverter unit is disconnected, so that the motor-generator inverter unit decelerates inertially under the action of wind resistance and mechanical loss, and the voltage frequency and phase on the generator side are monitored; when the generator speed drops to the synchronous speed with the power grid on the power frequency side, and the voltage frequency and phase meet the synchronous closing conditions, the switch on the power frequency side is closed to complete the grid connection.

[0020] Optionally, in step S4, the preset grid connection mode is a frequency conversion feedback grid connection mode; in the frequency conversion feedback grid connection mode, a starting frequency converter is connected to the power frequency winding side of the motor-generator frequency converter; the starting frequency converter is controlled to work in energy feedback state, converting the rotational mechanical energy of the motor-generator frequency converter into electrical energy and feeding it back to the power frequency grid or energy-consuming device, thereby causing the speed of the motor-generator frequency converter to drop rapidly; when the generator speed in the motor-generator frequency converter drops to a speed synchronized with the power frequency grid, and the voltage frequency and phase meet the synchronous closing conditions, the power frequency winding side is switched from the starting frequency converter to the power frequency grid connection, and the switch on the power frequency grid side is closed to complete the grid connection.

[0021] In summary, due to the adoption of this technical solution, the beneficial effects of this invention are as follows: This invention maintains the operation of the low-frequency grid during power frequency side disconnection, preventing large-scale grid disconnection of offshore wind turbines due to power inability. The core of the method lies in utilizing the inherent rotating mechanical inertia of the motor-generator frequency converter unit. By actively allowing and controlling its rotational speed to increase within a safe range, the excess active power continuously transmitted from the wind power side is temporarily converted into rotor kinetic energy and stored. This process provides the wind farm with crucial buffer time to adjust its power output, enabling the entire transmission system to maintain transient stability during grid-side faults and creating the preconditions for rapid subsequent grid restoration.

[0022] This invention significantly reduces reliance on traditional passive energy absorption devices such as energy-consuming resistors by utilizing the rotational inertia of the wind turbine to store energy. Especially in scenarios where wind turbine power regulation is rapid and output can be reduced promptly, the system can primarily or entirely rely on the turbine's own kinetic energy storage to absorb excess power, thereby reducing or even eliminating the need for large-capacity energy-consuming resistors. This not only simplifies the equipment configuration of onshore frequency converter stations and reduces the complexity of system construction and equipment investment costs, but also improves the overall operational economy of the system.

[0023] After a fault is cleared on the power frequency side, this invention enables rapid and smooth reconnection to the grid. Because the low-frequency grid remains operational, the submarine cable voltage is maintained, eliminating the time-consuming process of rebuilding the voltage and grid from scratch. By pre-limiting wind power output and combining two selectable modes—coasting or inverter energy feedback—the speed of the motor-generator unit can be actively and controllably reduced to synchronize with the power frequency grid, thus achieving synchronous grid connection. This significantly shortens the time required from fault recovery to full system energization, improving power supply reliability and wind power utilization efficiency.

[0024] This invention also enhances the system's reliability and adaptability through a multi-layered active power balancing mechanism. In addition to absorbing energy through variations in turbine speed, it also coordinates the bypass circuits on the wind turbine side, potential energy storage components, and grid-side energy dissipation devices when necessary. These measures are matched and coordinated based on parameters such as the actual time required for wind turbine power adjustment and the turbine's moment of inertia, forming a complementary power absorption system. This comprehensive control strategy ensures that the turbine speed is strictly limited within a safe range under any operating conditions, effectively preventing the risk of equipment overspeed damage and guaranteeing the safety of critical system equipment. Attached Figure Description

[0025] The present invention is described in detail with reference to the following figures, which include three figures as follows: Figure 1This is a schematic diagram of a COSCO offshore wind power low-frequency transmission system implemented using M3C technology in an existing scheme. Figure 2 This is a schematic diagram illustrating the overall process of the rapid grid connection control method after power frequency disconnection according to the present invention. Figure 3 This is a schematic diagram of the architecture of the low-frequency MG power transmission system used in the method of the present invention. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The parts of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0027] Therefore, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0028] A method for rapid grid connection control after power frequency disconnection from a low-frequency MG transmission system. Figure 2 This document provides a brief overview of the overall process of this method, which can be viewed concurrently. For details on low-frequency MG transmission systems within this method, please refer to [link / reference needed]. Figure 3 The diagram illustrates the connection between the low-frequency side of the wind power system and the power grid side via a motor-generator frequency converter. The key steps of the method in this embodiment can be summarized as follows: S1. When the power frequency grid side is disconnected due to a fault, the switch on the power frequency grid side is controlled to trip, while the switch on the low frequency side of the wind power is kept closed. S2. Control the speed of the motor-generator frequency converter to increase in order to absorb the excess active power generated by the low-frequency side of the wind power due to grid disconnection, and maintain the speed of the motor-generator frequency converter within the preset safe speed limit. S3. In response to the clearing of the fault on the power frequency grid side, limit the output power of the wind farm and control the speed of the motor-generator frequency converter to decrease to the speed synchronized with the power frequency grid side; S4. Control the motor-generator frequency converter unit to reconnect to the power frequency grid side of the grid in the preset grid connection mode to restore normal operation.

[0029] In this embodiment, such as Figure 3As shown, a low-frequency MG (magnetic generator) transmission system typically includes low-frequency wind turbines, a data collection system, and an offshore substation located on the offshore wind power side, connected to the onshore side via submarine AC cables. The onshore portion includes an onshore step-down substation, low-frequency side switches, a motor-generator frequency converter (MG) unit, an onshore substation, and a power frequency side switch. The motor-generator frequency converter, or MG unit, is the core equipment for frequency conversion. Its motor and generator are mechanically connected coaxially, and by setting different numbers of pole pairs, it converts the low-frequency electrical energy from offshore wind power into power frequency electrical energy, which is then fed into the power grid.

[0030] When a fault occurs on the power frequency grid side, causing the power frequency switch to trip and the system to disconnect from the power frequency grid, the method proceeds to step S1. The control decision at this point is to keep the switch connecting the low-frequency side of the wind power plant closed, without disconnecting the electrical connection between the offshore wind farm and the onshore frequency converter. Due to the inertia of the wind turbine's own power regulation, its output cannot instantly follow changes in the grid state. Therefore, for a period of time, the offshore wind farm will continuously transmit active power to the land via submarine cables. This power becomes excess power because it cannot be fed into the power frequency grid. If left unaddressed, it will lead to system voltage and frequency runaway, ultimately forcing the entire low-frequency transmission system to shut down.

[0031] In step S2 of this embodiment, when faced with an excess of active power, the core strategy is to actively utilize the rotating mechanical inertia of the motor-generator inverter unit. The rotational speed of the motor-generator inverter unit is allowed and controlled to increase, temporarily converting the electrical energy that cannot be delivered in time into the kinetic energy of the unit's rotor for storage.

[0032] In this embodiment, the classical expression for the rotor's rotational kinetic energy is: , The moment of inertia representing the rotor of the electric motor-generator frequency converter unit. This represents the mechanical angular velocity. An increase in rotational speed means an increase in rotor kinetic energy, which comes from the absorbed excess active power. In this way, a crucial time window is gained for offshore wind farms to adjust their power output, avoiding immediate system collapse caused by power blockage.

[0033] As a preferred embodiment of this invention, to ensure equipment safety, the speed increase of the motor-generator frequency converter unit is strictly limited to a preset safe speed limit. This safe speed limit is set at 1.1 times the rated speed of the unit; the control logic monitors the unit speed in real time and ensures that its increase does not exceed this 1.1 times rated speed threshold. As long as the speed is maintained within this safe range, the stress on the unit's mechanical structure is within the allowable value and will not cause equipment damage.

[0034] As the speed of the motor-generator inverter unit increases and it absorbs excess power, the control system sends instructions to the offshore wind farm, requiring it to reduce its active power output. In this embodiment, this is coordinated by the wind farm's central power controller, or it can be executed autonomously by each wind turbine unit based on received instructions and local frequency voltage signals. The wind turbine units gradually reduce the power injected into the collector bus by adjusting their pitch angle or generator torque, with the goal of eventually reducing their total output power to a lower level suitable for the post-fault system state.

[0035] However, the process of reducing the power output of a wind turbine requires a certain adjustment time. If, during this adjustment time, the energy absorbed by the increased turbine speed is insufficient to balance all the excess power, the turbine speed may exceed the safe range. Therefore, as a preferred embodiment, step S2 also introduces at least one auxiliary active power consumption device as a backup measure. The auxiliary active power consumption device includes, but is not limited to: an energy bypass circuit installed on the wind turbine side, commonly referred to as a crowbar circuit; an energy-consuming resistor installed on the power frequency side of the motor-generator inverter unit or at a nearby grid point; and energy storage elements connected to the system, such as capacitors or battery energy storage systems.

[0036] In this embodiment, the timing and capacity configuration of the auxiliary active power consumption device need to be calculated. Starting from the initial moment of disconnection from the power grid, assuming the wind farm's output power remains at a certain value, the motor-generator inverter unit starts from its rated speed... Rising to the speed corresponding to the safe speed limit Time elapsed This can be estimated based on the principle of energy conservation. That is, the increase in kinetic energy absorbed by the unit as it rises from its initial speed to its safe upper limit speed is equal to the excess productive energy output by the wind farm during that time period. From this, the time... The expression is as follows:

[0037] In the formula, For the motor-generator frequency converter unit at a speed from Rise to The increase in kinetic energy absorbed during the process; This refers to the active power output of the low-frequency side of the wind power system within a preset time period after grid disconnection. Therefore, the auxiliary active power consumption device needs to be installed within this estimated time. The system is put into operation within the specified window to ensure that additional power consumption pathways are activated when the unit speed reaches the safe upper limit, thereby curbing further increases in speed.

[0038] In this embodiment, the increase in kinetic energy It can be calculated using the rotor's moment of inertia and rotational speed, and can be determined according to the following formula:

[0039] In the formula, This refers to the rotational inertia of the rotor in a motor-generator frequency converter unit.

[0040] In this embodiment, the actual adjustment time required for the wind turbine to reduce power output This is also a key variable. If the actual adjustment time... Shorter than the time window estimated above By combining the energy absorption from increased rotational speed, the energy consumption from auxiliary devices, and the rapid power reduction of the wind farm itself, the unit's rotational speed can be reliably stabilized within safe limits.

[0041] But if the actual adjustment time Longer than or equal to the aforementioned time window This means that the turbine speed may have already reached its safe limit before the wind farm's power output has completely decreased, and short-term auxiliary equipment may not be sufficient to support the wind farm until it has been fully adjusted. In this case, it is necessary to configure the system with a larger capacity energy storage device or energy dissipation device.

[0042] In this embodiment, the minimum capacity required here The excess energy output of the wind farm during the complete adjustment period can be determined by integrating the wind farm's output power from the moment of grid disconnection to the end of the adjustment period, as shown in the following formula:

[0043] In the formula, the integration interval is the time from the moment of disconnection from the power frequency grid to the adjustment time. The moment of ending.

[0044] In step S3 of this embodiment, after the fault on the power frequency grid side is cleared and the grid returns to normal, the control system enters the grid connection recovery phase. First, it is ensured that the output power of the offshore wind farm is limited to a low level, lower than the power capacity that the motor-generator inverter unit can stably transmit at power frequency synchronous speed. This can be achieved by issuing a command to reduce active power output through the wind farm's central power controller or the local controller of each wind turbine unit. This is to prevent power surges from occurring again during subsequent grid connection. Subsequently, in step S4, this embodiment provides two selectable modes to reconnect the motor-generator inverter unit to the power frequency grid.

[0045] The first mode is the coasting grid-connected mode. In this mode, the control system disconnects the excitation power supply to the motor, causing the motor-generator inverter unit to lose driving torque and naturally decelerate due to losses such as mechanical friction and wind resistance, i.e., coasting. Simultaneously, the control system continuously monitors the voltage frequency and phase of the generator output and compares them with the voltage frequency and phase on the power grid side. As the unit speed decreases, its generating frequency also gradually decreases. When the generator speed drops to the synchronous speed of the power grid, and the difference between its voltage phase and the grid voltage phase meets the synchronization closing conditions, the control system immediately issues a command to close the power grid side switch, thereby smoothly reconnecting the unit to the power grid.

[0046] The second mode is the frequency conversion feedback grid connection mode, which is suitable for applications requiring faster grid connection. In this mode, the control system connects a starting frequency converter to the power frequency side winding of the motor-generator frequency converter unit. The starting frequency converter is controlled to operate in energy feedback mode, where the mechanical energy of the high-speed rotating unit is converted into electrical energy through the generator-starting frequency converter circuit. This electrical energy can be fed back to the restored power frequency grid or sent to a dedicated energy-consuming device. Through this electrical braking method, the unit speed can be reduced quickly and controllably. Similarly, the control system continuously monitors synchronization. When the unit speed is pulled back to the power frequency synchronous speed and the synchronization conditions are met, the control system performs a switching operation, disconnecting the power frequency side winding from the starting frequency converter and reconnecting it to the power frequency grid. Subsequently, the power frequency side switch is closed to complete the grid connection. Compared with the coasting mode, this mode significantly shortens the time from fault recovery to reconnection.

[0047] Furthermore, throughout the entire control process of this embodiment, the selection and capacity configuration of auxiliary active power consumption devices require comprehensive technical and economic considerations. The selection criteria include, but are not limited to: the statistical time of the wind turbine power adjustment characteristics, the specific rotational inertia parameters of the motor-generator frequency converter rotor, and the system's allowable speed rise time constant.

[0048] Different auxiliary devices have their own characteristics in terms of response speed, cost, and reusability. Crowbar circuits have extremely fast response but can usually only operate for short periods; energy-dissipating resistors have a simple structure but the energy dissipated is converted into heat; energy storage devices can absorb and temporarily store energy, which may be utilized later, but they are more expensive. The control system can select one or more devices to combine based on specific system parameters and operating requirements to achieve a reliable balance of active power during faults with the best cost-effectiveness.

[0049] The method described in this implementation begins with a power frequency side fault disconnection. By maintaining low-frequency side connectivity, utilizing the unit's inertial energy storage, coordinating wind farm power reduction, and activating auxiliary devices, the stability of the low-frequency side grid during the fault is successfully maintained. After the fault is cleared, wind power is pre-limited, and the unit is resynchronized using coasting or frequency conversion feedback, ultimately achieving rapid and smooth reconnection to the grid. The entire process fully utilizes the physical characteristics of the motor-generator frequency converter unit itself, combined with multi-level active power balance control, forming a highly efficient, reliable, and adaptable system protection and recovery scheme.

Claims

1. A method for rapid grid connection control after power frequency disconnection from a low-frequency MG transmission system, wherein the low-frequency MG transmission system comprises a wind power low-frequency side and a power frequency grid side connected by a motor-generator frequency converter, characterized in that, The method includes the following steps: S1. When the power frequency grid side disconnects from the grid due to a fault, the switch on the power frequency grid side is controlled to trip, while the switch on the wind power low frequency side remains closed. S2. Control the speed of the motor-generator inverter unit to increase, so as to absorb the excess active power generated by the wind power low-frequency side due to the grid disconnection event, and maintain the speed of the motor-generator inverter unit within the preset safe speed limit. S3. In response to the power grid fault being cleared, limit the output power of the wind farm and control the speed of the motor-generator inverter unit to decrease to the speed synchronized with the power grid. S4. Control the motor-generator frequency converter unit to reconnect to the restored power frequency grid in a preset grid connection mode.

2. The method for rapid grid connection control after power frequency disconnection according to claim 1, characterized in that: In step S2, the preset safe speed limit is 1.1 times the rated speed of the motor-generator frequency converter.

3. The method for rapid grid connection control after power frequency disconnection according to claim 1 or 2, characterized in that, Step S2 further includes: when the speed of the motor-generator frequency converter reaches the upper limit of the safe speed, at least one auxiliary active power consumption device is put into operation to consume the excess active power on the low-frequency side of the wind power. The auxiliary active power consumption device includes at least one of the following: an energy bypass circuit installed on the wind turbine side, an energy-consuming resistor installed on the power frequency grid side or the motor-generator frequency converter side, and an energy storage element connected to the low-frequency MG transmission system.

4. The method for rapid grid connection control after power frequency disconnection according to claim 3, characterized in that: From the moment the power frequency grid is disconnected until the speed of the motor-generator inverter unit drops from the rated speed... The speed increases to the upper limit of the safe speed range. up to that point, the time elapsed Determined according to the following formula: In the formula, For the motor-generator frequency converter unit at a speed from Rise to The increase in kinetic energy absorbed during the process; The active power output by the low-frequency side of the wind power system within a preset time period after grid disconnection; the activation time of the at least one auxiliary active power consumption device is within the specified time. Completed within the specified time.

5. The method for rapid grid connection control after power frequency disconnection according to claim 4, characterized in that: Step S2 also includes controlling the wind farm to reduce its output active power; If the wind farm requires adjustment time to reduce its output active power Less than the time Then, by controlling the speed of the motor-generator inverter unit to increase, activating the at least one auxiliary active power consumption device, and coordinating the wind farm to reduce the output active power, the speed of the motor-generator inverter unit is maintained within the safe speed limit. If the adjustment time Greater than or equal to the time Then the configured capacity of the low-frequency MG transmission system is not less than Energy storage devices or energy consumption devices; among which, Determined according to the following formula: In the formula, the integration interval is from the moment of disconnection from the power frequency grid to the adjustment time. The moment of ending.

6. The method for rapid grid connection control after power frequency disconnection according to claim 5, characterized in that, Kinetic energy increment Determined based on the following formula: In the formula, Let be the moment of inertia of the rotor of the motor-generator frequency converter unit.

7. The method for rapid grid connection control after power frequency disconnection according to any one of claims 4-6, characterized in that: In step S2, the type and capacity of the at least one auxiliary active power consumption device put into operation are related to the adjustment time required for the wind farm to reduce its output active power. The moment of inertia of the rotor of the motor-generator frequency converter unit and the time mentioned Associate and match selections.

8. The method for rapid grid connection control after power frequency disconnection according to claim 1, characterized in that, In step S3, limiting the output power of the wind farm specifically involves issuing a command to reduce the active power output through the central power controller of the wind farm or the local controller of each wind turbine, so that the total active power output value of the wind farm drops below the active power level that the motor-generator frequency converter can transmit at synchronous speed.

9. The method for rapid grid connection control after power frequency disconnection according to claim 1, characterized in that: In step S4, the preset grid connection mode is the coasting grid connection mode. In the coasting grid connection mode, the excitation of the motor in the motor-generator inverter unit is disconnected, so that the motor-generator inverter unit decelerates inertially under the action of wind resistance and mechanical loss, and the voltage frequency and phase on the generator side are monitored. When the generator speed drops to the synchronous speed with the power grid on the power frequency side, and the voltage frequency and phase meet the synchronous closing conditions, the switch on the power frequency side is closed to complete the grid connection.

10. The method for rapid grid connection control after power frequency disconnection according to claim 1, characterized in that: In step S4, the preset grid connection mode is a variable frequency feedback grid connection mode. In the variable frequency feedback grid connection mode, a starting frequency converter is connected to the power frequency winding side of the motor-generator frequency converter. The starting frequency converter is controlled to work in energy feedback mode, converting the rotational mechanical energy of the motor-generator frequency converter into electrical energy and feeding it back to the power frequency grid or energy-consuming device, thereby causing the speed of the motor-generator frequency converter to drop rapidly. When the generator speed in the motor-generator frequency converter drops to a speed synchronized with the power frequency grid, and the voltage frequency and phase meet the synchronous closing conditions, the power frequency winding side is switched from the starting frequency converter to the power frequency grid connection, and the switch on the power frequency grid side is closed to complete the grid connection.

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