Start control method and device for offshore wind power low-frequency power transmission M-G grid-connected system

Abstract

The invention provides an offshore wind power low-frequency power transmission M-G grid-connected system starting control method and device, and the method comprises the steps: controlling an M-G starting device to provide rotor excitation for an M-G system when a shore power grid is powered off; working modes of a synchronous motor and a synchronous generator in the M-G system are controlled, charging and voltage building are carried out on the offshore power grid, and the voltage of the offshore power grid rises to a rated value; after the offshore wind speed is greater than the cut-in wind speed, starting the offshore wind plant to enter a pre-synchronization stage operation mode; then switching the working modes of a synchronous motor and a synchronous generator in the M-G system and the state of an M-G starting device, and supplying power to the M-G starting device and the shore thermal power generating unit auxiliary equipment through the offshore wind plant; and starting the ashore power grid, connecting the offshore wind plant into the ashore power grid through the M-G system, and controlling the offshore wind plant to transmit electric energy to the ashore power grid through the M-G system. According to the method, stable starting of the grid-connected system can be realized, and the production cost is effectively reduced.

Description

Technical Field

[0001] This invention relates to the field of new energy technology, and in particular to a startup control method and device for an offshore wind power low-frequency transmission MG grid-connected system. Background Technology

[0002] In recent years, the installed capacity of offshore wind power has been continuously expanding, and wind farm construction has gradually developed towards offshore and large-capacity applications. In medium- and long-distance power transmission scenarios, low-frequency AC transmission (LFAC) technology has broken through the transmission distance bottleneck of traditional power frequency AC transmission systems, enabling efficient transmission of offshore wind energy. In LFAC technology, the motor-generator (MG) system, as a new type of frequency converter, can replace power electronic converters to achieve frequency conversion between offshore low-frequency and onshore power frequencies.

[0003] In existing startup strategies for offshore wind power low-frequency transmission (MG) grid-connected systems, the startup of MG systems largely relies on onshore power grid supply. When the onshore power grid experiences a power outage, the system cannot start. Furthermore, there is limited research on methods for using offshore wind power low-frequency transmission (MG) grid-connected systems as black-start power sources and addressing the switching of control strategies before and after grid connection. Patent CN114784859B discloses a black-start method for offshore wind farms based on a diesel-storage integrated system, comprising: Step S1, the energy storage device adopts a zero-voltage startup control strategy to reduce inrush current; Step S2, starting the diesel generator set and using auxiliary loads to assist in starting the diesel generator set; Step S3, engaging a balancing load and supplying power to the auxiliary units of the wind turbines; Step S4, starting each wind turbine one by one and capturing wind energy to generate electricity; Step S5, after the appropriate active power units have started, engaging wind turbines operating in phase-shifting mode; Step S6, reducing the adjustable load according to the rated power of the motor load to be engaged, and engaging the motor load after the system enters a steady state to achieve black start. However, this method also requires the introduction of a diesel generator, which makes the system architecture complex and the production cost high. Summary of the Invention

[0004] This invention provides a startup control method and device for a low-frequency transmission (MG) grid-connected system for offshore wind power, which can achieve a smooth startup of the grid-connected system and effectively reduce production costs.

[0005] A startup control method for an offshore wind power low-frequency transmission (MG) grid-connected system, comprising an offshore wind farm, an MG system, and an MG startup device; the offshore wind farm is connected to an offshore power grid, the offshore power grid is connected to the MG system via a low-frequency AC submarine cable, and the MG system is connected to an onshore power grid; the onshore power grid is connected to a thermal power unit, and the thermal power unit is connected to auxiliary equipment of the thermal power unit; The method includes: When the onshore power grid fails, the system controls the disconnection of the offshore wind farm from the offshore power grid, the disconnection of the MG system from the onshore power grid, and the disconnection of the MG starting device from the onshore power grid. The system also controls the start of the MG starting device to provide rotor excitation for the MG system. The operating modes of the synchronous motor and synchronous generator in the MG system are controlled to charge and build up the voltage of the offshore power grid, so that the voltage of the offshore power grid rises to the rated value; Once the offshore wind speed exceeds the cut-in wind speed, the offshore wind farm will enter the pre-synchronization phase operation mode. After the pre-synchronization phase is completed, the offshore wind farm is connected to the offshore power grid, the working modes of the synchronous motors and synchronous generators in the MG system and the status of the MG starting device are switched, and the offshore wind farm supplies power to the MG starting device and the auxiliary equipment of the thermal power unit. The onshore power grid is activated, and the offshore wind farm is connected to the onshore power grid via the MG system. The offshore wind farm is controlled to transmit power to the onshore power grid through the MG system.

[0006] Furthermore, the MG starting device includes an energy storage device, an energy storage-side converter, and a filter inductor; controlling the starting of the MG starting device to provide rotor excitation for the MG system includes: The energy storage-side converter is controlled to operate in inverter mode, so that the energy storage device is in a discharge state, providing rotor excitation for the MG system.

[0007] Furthermore, the MG system also includes a first excitation device and a second excitation device. The synchronous motor is connected to the first excitation device, and the synchronous generator is connected to the second excitation device. The rotor of the synchronous motor and the rotor of the synchronous generator are coaxially connected. The ratio of the number of pole pairs of the synchronous motor to the number of pole pairs of the synchronous generator is set to be equal to the ratio of the frequency required by the offshore power grid to the frequency of the onshore power grid. The synchronous motor is connected to the low-frequency AC submarine cable, and the synchronous generator switches its connection with the onshore power grid via a connection switch. Controlling the operating modes of the synchronous motors and synchronous generators in the MG system to charge and build up voltage for the offshore power grid includes: The second excitation device controls the electromagnetic torque of the synchronous generator based on the rotor excitation, so that the synchronous generator operates in motor mode; The synchronous motor is controlled to operate in generator mode by the first excitation device to charge and build up voltage for the offshore power grid, so that the voltage of the offshore power grid rises to the rated value.

[0008] Furthermore, the offshore wind farm includes grid-connected wind turbines, grid-connected wind turbine generator-side converters, grid-connected wind turbine grid-side converters, grid-following wind turbines, grid-following wind turbine generator-side converters, and grid-following wind turbine grid-side converters. The offshore wind farm is initiated into a pre-synchronization phase operation mode, including: Control the start-up of the generator-side converter of the grid-type wind turbine and maintain the stability of the DC bus capacitor voltage of the generator-side converter of the grid-type wind turbine; The grid-side converter of the control grid-type wind turbine is started, so that the grid-side converter of the control grid-type wind turbine operates in the pre-synchronization stage mode.

[0009] Furthermore, after the pre-synchronization phase is completed, the offshore wind farm is connected to the offshore power grid, and the operating modes of the synchronous motors and synchronous generators in the MG system and the status of the MG starting device are switched, including: Establish a connection between the grid-side converter of the grid-connected wind turbine and the offshore power grid, so that the offshore wind farm can be integrated into the offshore power grid; The grid-side converter of the control grid-type wind turbine operates in independent mode; The synchronous motor in the MG system is controlled to operate in motor mode, and the synchronous generator is controlled to operate in generator mode. The state of the energy storage side converter in the MG starting device is switched to rectification mode. The energy storage device in the MG starting device is charged through the offshore wind farm. The connection between the MG system and the auxiliary equipment of the thermal power unit is controlled so that the offshore wind farm supplies power to the auxiliary equipment of the thermal power unit through the MG system.

[0010] Furthermore, the onshore power grid is activated, and the offshore wind farm is connected to the onshore power grid via the MG system. Controlling the offshore wind farm to transmit power to the onshore power grid through the MG system includes: Control the connection between the grid-side converter of the grid-connected wind turbine and the offshore power grid; Start the grid-side converter of the grid-connected wind turbine and maintain the DC bus capacitor voltage of the generator-side converter of the grid-connected wind turbine stable. Once the offshore wind speed exceeds the cut-in wind speed, the inverter on the machine side of the grid-type wind turbine will be started. The onshore power grid is activated, and the offshore wind farm is connected to the onshore power grid via the MG system, enabling the offshore wind farm to transmit power to the onshore power grid through the MG system.

[0011] Furthermore, after activating the onshore power grid, the following also includes: Disconnect the onshore power grid from the MG starting device, control the synchronous generator in the step-by-step operation mode, and when the grid connection conditions are met, connect the offshore wind power low-frequency transmission MG grid connection system to the onshore power grid and enter the grid connection operation mode.

[0012] Furthermore, the offshore wind power low-frequency transmission MG grid-connected system also includes a quasi-synchronous paralleling device, which is used to detect the voltage amplitude difference, voltage phase difference, and voltage frequency difference between the synchronous generator and the onshore power grid connection switch, and control the closing of the connection switch when the grid connection conditions are met, so as to realize the grid connection of the offshore wind power low-frequency transmission MG grid-connected system with the onshore power grid and the system enters the grid-connected operation mode.

[0013] A start-up control device for an offshore wind power low-frequency transmission MG grid-connected system applied to the above method, comprising: The connection control module is used to control the disconnection of the offshore wind farm from the offshore power grid, the disconnection of the MG system from the onshore power grid, and the disconnection of the MG starting device from the onshore power grid when the onshore power grid is interrupted, and to control the start of the MG starting device to provide rotor excitation for the MG system. The charging and voltage-building module is used to control the operating mode of the synchronous motor and synchronous generator in the MG system, and to charge and build up the voltage of the offshore power grid so that the voltage of the offshore power grid rises to the rated value. The pre-synchronization module is used to initiate the pre-synchronization phase operation mode of the offshore wind farm after the offshore wind speed exceeds the cut-in wind speed. The state switching module is used to control the offshore wind farm to connect to the offshore power grid after the pre-synchronization phase is completed, switch the working mode of the synchronous motor and synchronous generator in the MG system and the state of the MG starting device, and supply power to the MG starting device and auxiliary equipment of the thermal power unit through the offshore wind farm. The onshore power grid startup module is used to start the onshore power grid. The offshore wind farm is connected to the onshore power grid via the MG system, and the module controls the offshore wind farm to transmit power to the onshore power grid through the MG system.

[0014] Furthermore, the MG starting device includes an energy storage device, an energy storage-side converter, and a filter inductor; the connection control module controls the starting of the MG starting device to provide rotor excitation for the MG system, including: The energy storage-side converter is controlled to operate in inverter mode, so that the energy storage device is in a discharge state, providing rotor excitation for the MG system.

[0015] Furthermore, the MG system also includes a first excitation device and a second excitation device. The synchronous motor is connected to the first excitation device, and the synchronous generator is connected to the second excitation device. The rotor of the synchronous motor and the rotor of the synchronous generator are coaxially connected. The ratio of the number of pole pairs of the synchronous motor to the number of pole pairs of the synchronous generator is set to be equal to the ratio of the frequency required by the offshore power grid to the frequency of the onshore power grid. The synchronous motor is connected to the low-frequency AC submarine cable, and the synchronous generator switches its connection with the onshore power grid via a connection switch. The charging and voltage-building module controls the operating modes of the synchronous motors and synchronous generators in the MG system to charge and build up voltage for the offshore power grid, including: The second excitation device controls the electromagnetic torque of the synchronous generator based on the rotor excitation, so that the synchronous generator operates in motor mode; The synchronous motor is controlled to operate in generator mode by the first excitation device to charge and build up voltage for the offshore power grid, so that the voltage of the offshore power grid rises to the rated value.

[0016] Furthermore, the offshore wind farm includes grid-connected wind turbines, grid-connected wind turbine generator-side converters, grid-connected wind turbine grid-side converters, grid-following wind turbines, grid-following wind turbine generator-side converters, and grid-following wind turbine grid-side converters. The pre-synchronization module initiates the offshore wind farm into a pre-synchronization phase operation mode, including: Control the start-up of the generator-side converter of the grid-type wind turbine and maintain the DC bus capacitor voltage of the generator-side converter of the grid-type wind turbine stable; The grid-side converter of the control grid-type wind turbine is started, so that the grid-side converter of the control grid-type wind turbine operates in the pre-synchronization stage mode.

[0017] Furthermore, after the pre-synchronization phase is completed, the state switching module controls the offshore wind farm to connect to the offshore power grid, switching the operating modes of the synchronous motors and synchronous generators in the MG system as well as the state of the MG starting device, including: Establish a connection between the grid-side converter of the grid-connected wind turbine and the offshore power grid, so that the offshore wind farm can be integrated into the offshore power grid; The grid-side converter of the control grid-type wind turbine operates in independent mode; The synchronous motor in the MG system is controlled to operate in motor mode, and the synchronous generator is controlled to operate in generator mode. The state of the energy storage side converter in the MG starting device is switched to rectification mode. The energy storage device in the MG starting device is charged through the offshore wind farm. The system connects the MG system to the auxiliary equipment of the thermal power unit, so that the offshore wind farm supplies power to the auxiliary equipment of the thermal power unit through the MG system.

[0018] Furthermore, the onshore power grid startup module starts the onshore power grid, and the offshore wind farm is connected to the onshore power grid via the MG system. The module controls the offshore wind farm to transmit power to the onshore power grid through the MG system, including: Control the connection between the grid-side converter of the grid-connected wind turbine and the offshore power grid; Start the grid-side converter of the grid-connected wind turbine and maintain the DC bus capacitor voltage of the generator-side converter of the grid-connected wind turbine stable. Once the offshore wind speed exceeds the cut-in wind speed, the inverter on the machine side of the grid-type wind turbine will be started. The onshore power grid is activated, and the offshore wind farm is connected to the onshore power grid via the MG system, enabling the offshore wind farm to transmit power to the onshore power grid through the MG system.

[0019] Furthermore, after the onshore power grid startup module starts the onshore power grid, it also includes: Disconnect the onshore power grid from the MG starting device, control the synchronous generator in the step-by-step operation mode, and when the grid connection conditions are met, connect the offshore wind power low-frequency transmission MG grid connection system to the onshore power grid and enter the grid connection operation mode.

[0020] Furthermore, the offshore wind power low-frequency transmission MG grid-connected system also includes a quasi-synchronous paralleling device, which is used to detect the voltage amplitude difference, voltage phase difference, and voltage frequency difference between the synchronous generator and the onshore power grid connection switch, and control the closing of the connection switch when the grid connection conditions are met, so as to realize the grid connection of the offshore wind power low-frequency transmission MG grid-connected system with the onshore power grid.

[0021] An electronic device includes a processor and a memory, the memory storing a plurality of instructions, the processor being configured to read the instructions and execute the method described above.

[0022] The startup control method and apparatus for offshore wind power low-frequency transmission (MG) grid-connected system provided by this invention have at least the following beneficial effects: (1) The MG can be started by energy storage system without taking power from the onshore power grid or through diesel engine, so as to achieve the smooth start of the grid-connected system, supply power to key equipment, so that offshore wind power can be used as black start power source to assist the onshore power grid to restore power supply and reduce production costs. (2) The proposed start-up control strategy for the offshore wind power low-frequency transmission MG grid-connected system achieves accurate synchronization between the output voltage of the offshore wind farm grid-type wind turbine and the voltage at the collection bus before grid connection through pre-synchronization technology. This can reduce the fluctuation of voltage amplitude and frequency of the offshore power grid after grid connection and realize seamless switching of the grid-type wind turbine from off-grid to grid connection. (3) It can achieve smooth start-up of MG, improve the success rate of offshore wind turbines connecting to the offshore power grid and the system connecting to the onshore power grid, and has good dynamic performance. Attached Figure Description

[0023] Figure 1 This is a flowchart of one embodiment of the start-up control method for the offshore wind power low-frequency transmission MG grid-connected system provided by the present invention.

[0024] Figure 2 This is a schematic diagram of the structure of an embodiment of the offshore wind power low-frequency transmission MG grid-connected system in the start-up control method of the offshore wind power low-frequency transmission MG grid-connected system provided by the present invention.

[0025] Figure 3 This is a schematic diagram illustrating the working principle of the energy storage-side converter in the startup control method of the offshore wind power low-frequency transmission MG grid-connected system provided by the present invention.

[0026] Figure 4 This invention provides a schematic diagram illustrating the working principle of the grid-side converter of a grid-type wind turbine under different modes in the start-up control method for the offshore wind power low-frequency transmission MG grid-connected system.

[0027] Figure 5 A flowchart of one embodiment of the start-up control device for the offshore wind power low-frequency transmission MG grid-connected system provided by the present invention. Detailed Implementation

[0028] To better understand the above technical solutions, the following will provide a detailed explanation of the technical solutions in conjunction with the accompanying drawings and specific implementation methods.

[0029] refer to Figure 1 and Figure 2 In some embodiments, a startup control method for an offshore wind power low-frequency transmission (MG) grid-connected system is provided. The offshore wind power low-frequency transmission (MG) grid-connected system includes an offshore wind farm 1, an MG system 2, and an MG startup device 3. The offshore wind farm 1 is connected to an offshore power grid, which is connected to the MG system 2 via a low-frequency AC submarine cable 4. The MG system 2 is connected to an onshore power grid. The onshore power grid is connected to a thermal power unit 7, and the thermal power unit is connected to auxiliary equipment 6. The method includes: S1. When the onshore power grid fails, control the disconnection of the offshore wind farm from the offshore power grid, disconnect the MG system from the onshore power grid, disconnect the MG starting device from the onshore power grid, and control the start of the MG starting device to provide rotor excitation for the MG system. S2. Control the working mode of the synchronous motor and synchronous generator in the MG system to charge and build up the voltage of the offshore power grid, so that the voltage of the offshore power grid rises to the rated value; S3. After the offshore wind speed exceeds the cut-in wind speed, the offshore wind farm will be activated to enter the pre-synchronization stage operation mode. S4. After the pre-synchronization phase is completed, the offshore wind farm is connected to the offshore power grid, the working mode of the synchronous motor and synchronous generator in the MG system and the status of the MG starting device are switched, and the offshore wind farm supplies power to the MG starting device and the auxiliary equipment of the thermal power unit. S5. Start the onshore power grid. The offshore wind farm is connected to the onshore power grid via the MG system. Control the offshore wind farm to transmit power to the onshore power grid through the MG system.

[0030] Specifically, refer to Figure 1 The offshore wind power low-frequency transmission MG grid connection system in this embodiment includes an offshore wind farm 1, an MG system 2, an MG starting device 3, a low-frequency AC submarine cable 4, and a quasi-synchronous paralleling device 5. The offshore wind farm 1 is connected to the offshore power grid, the offshore power grid is connected to the MG system 2 through the low-frequency AC submarine cable 4, the MG starting device 3 is connected to the MG system 2, the MG starting device 3 is also connected to the auxiliary equipment 6 of the thermal power unit of the onshore power grid, the auxiliary equipment 6 of the thermal power unit is connected to the thermal power unit 7, the thermal power unit 7 is connected to the onshore power grid, and the MG system 2 is connected to the onshore power grid.

[0031] Furthermore, the offshore wind farm 1 includes a grid-connected wind turbine 11, a grid-connected wind turbine generator-side converter 12, a grid-connected wind turbine grid-side converter 13, a grid-following wind turbine 14, a grid-following wind turbine generator-side converter 15, and a grid-following wind turbine grid-side converter 16.

[0032] The MG system 2 includes a synchronous motor 21 and a synchronous generator 22 that are connected to each other and are controlled by a first excitation device 23 and a second excitation device 24, respectively.

[0033] The MG starting device 3 includes an energy storage device 31, an energy storage-side converter 32, and a filter inductor L.

[0034] Energy storage device 31, energy storage-side converter 32 and filter inductor L Connect and via the first switch Br 1. To enable the connection and disconnection of the MG starting device 3 and the MG system 2.

[0035] The grid-type wind turbine 11, the wind turbine machine-side converter 12, and the grid-type wind turbine grid-side converter 13 are connected in sequence. The grid-type wind turbine grid-side converter 13 is connected via a second switch. Br 2. Enable connection and disconnection with the offshore power grid.

[0036] The grid-connected fan 14, the grid-connected fan machine-side converter 15, and the grid-connected fan grid-side converter 16 are connected in sequence. The grid-connected fan grid-side converter 16 is connected via a third switch. Br 3. Enable connection and disconnection with the offshore power grid.

[0037] MG starting device 3 also uses a fourth switch Br 4. To achieve connection and disconnection with the auxiliary equipment of thermal power units 6 of the onshore power grid.

[0038] MG System 2 via the fifth switch Br 5. Enables connection and disconnection with the onshore power grid; quasi-synchronous paralleling device 5 and the fifth switch. Br 5. Connecting to detect the fifth switch. Br 5. The state of both ends and control of the fifth switch Br 5. On / off state.

[0039] The grid-connected wind turbine's machine-side converter 12 employs a DC voltage control strategy oriented towards the wind turbine rotor flux linkage, while the grid-connected wind turbine's grid-side converter 13 employs a virtual synchronous generator (VSG) control strategy. The grid-following wind turbine's machine-side converter 15 employs a maximum power point tracking (MPPT) control strategy oriented towards the wind turbine rotor flux linkage, and the grid-following wind turbine's grid-side converter 16 employs a DC voltage control strategy oriented towards the offshore grid voltage synthesis vector.

[0040] The MG system 2 includes a low-frequency synchronous motor 21 and a power-frequency synchronous generator 22, with their rotors coaxially connected. Frequency conversion from low-frequency AC to power-frequency AC is achieved by setting the ratio of the number of pole pairs. The ratio of the number of pole pairs of the synchronous motor to the number of pole pairs of the synchronous generator is equal to the ratio of the frequency required by the offshore power grid to the frequency of the onshore power grid, and is controlled by a first excitation device 23 and a second excitation device 24, respectively. The synchronous motor 21 is connected to the low-frequency AC submarine cable 4, and the synchronous generator 22 switches its connection to the onshore power grid via a connection switch (fifth switch Br5).

[0041] The low-frequency AC submarine cable 4 enables the transmission of low-frequency AC power from the offshore power grid to the synchronous motor terminals of the MG system.

[0042] The energy storage-side converter 32 in the MG starting device 3 has two control modes: rectification and inversion. The energy storage-side converter 32 is filtered by an inductor. L Then through the first switch Br 1 is connected to the generator terminal of synchronous generator 21 in MG system 2.

[0043] Furthermore, in step S1, when the onshore power grid is de-energized, the connection between the offshore wind farm and the offshore power grid is disconnected, the connection between the MG system and the onshore power grid is disconnected, and the connection between the MG starting device and the onshore power grid is disconnected, i.e., the second switch is disconnected. Br 2. Third switch Br 3. Fourth switch Br 4 and the fifth switch Br 5.

[0044] Controlling the start of the MG starting device to provide rotor excitation for the MG system includes: The energy storage-side converter is controlled to operate in inverter mode, so that the energy storage device is in a discharge state, providing rotor excitation for the MG system.

[0045] Specifically, the energy storage-side converter is controlled to operate in inverter mode, that is, the first switch is controlled to be closed. Br 1. Start the energy storage device 31 and the energy storage side converter 32. The second excitation device 24 draws power from the energy storage device 31 to achieve rotor excitation.

[0046] Further, in step S2, controlling the operating mode of the synchronous motor and synchronous generator in the MG system to charge and build up voltage for the offshore power grid includes: S21. The second excitation device controls the electromagnetic torque of the synchronous generator based on the rotor excitation, so that the synchronous generator operates in motor mode. S22. The synchronous motor is controlled to operate in generator mode by the first excitation device to charge and build up voltage for the offshore power grid, so that the voltage of the offshore power grid rises to the rated value.

[0047] Specifically, in step S21, the energy storage side converter operates in inverter mode, and the second excitation device 24 controls the electromagnetic torque of the synchronous generator, so that the synchronous generator operates in motor mode. When the speed is stable, the excitation current of the second excitation device 24 is adjusted so that the terminal voltage of the synchronous motor is equal to the rated value.

[0048] refer to Figure 3 The specific calculation formulas for the reference values ​​of the d-axis and q-axis currents of the energy storage-side converter in inverter mode are as follows: (1) In the formula, and These are the reference values ​​for the d-axis and q-axis currents of the energy storage-side converter in inverter mode. and f G These are the frequency reference value and actual frequency value of the energy storage side converter in inverter mode. k fp and kfi These are the proportional and integral coefficients for synchronous generator speed control, and the frequency reference value in inverter mode. , s For the Laplace operator.

[0049] The specific calculation formulas for the reference values ​​of the d-axis and q-axis voltages of the energy storage-side converter in inverter mode are as follows: (2) In the formula, and These are the reference values ​​for the d-axis and q-axis voltages at the converter ports on the energy storage side in inverter mode. u MGd and u MGq These are the d-axis and q-axis components of the voltage output after the L-type filter, respectively. ω G It is the angular frequency on the generator side of the MG. L It is the inductance value of the filter inductor. k ip and k ii These are the proportional and integral coefficients of the inner current loop; i MGd and i MGq These are the d-axis and q-axis current components of the energy storage-side converter, respectively.

[0050] In step S22, the first excitation device 23 is started, so that the synchronous motor works in generator mode to charge and build up voltage to the low-frequency AC submarine cable, and the voltage of the offshore power grid rises to the rated value to supply power to the auxiliary equipment of each wind turbine in the wind farm.

[0051] Furthermore, in step S3, after detecting that the sea wind speed is greater than the cut-in wind speed, the generator-side converter 12 of the grid-type wind turbine is started and the DC bus capacitor voltage of the generator-side converter of the grid-type wind turbine is kept stable; then the grid-side converter 13 of the grid-type wind turbine is started, so that the grid-side converter of the grid-type wind turbine operates in the pre-synchronization stage operation mode.

[0052] Specifically, refer to Figure 4 The voltage amplitude of the virtual synchronous machine in the grid-side converter 13 of the pre-synchronization stage of the grid-type wind turbine E VSG1 for: (3) In the formula, E VSG1 This represents the voltage amplitude of the virtual synchronous machine in the grid-side converter of a grid-type wind turbine during the pre-synchronization phase. k qi It is the integral coefficient for reactive power control of the virtual synchronous machine. and U o These are the reference and actual output voltage values ​​of the grid-side converter for grid-type wind turbines. Q ov This is the virtual reactive power output value of a grid-connected wind turbine. D q It is the voltage droop factor of the virtual synchronous machine during the pre-synchronization phase. D q =0.

[0053] Pre-synchronization stage virtual synchronizer voltage phase angle θ VSG1 The specific calculation formula is as follows: (4) In the formula, This is the reference value for the virtual synchronizer speed during the pre-synchronization phase. , p Δ is the ratio of the number of pole pairs of the synchronous generator to the number of pole pairs of the synchronous motor in MG. ω 1 represents the speed change of the virtual synchronizer during the pre-synchronization phase. J It is the moment of inertia of the virtual synchronizer. D It is the damping coefficient of the virtual synchronizer. k ωp and k ωi These are the proportional and integral coefficients for speed control. P ov It is the virtual active power output of the grid-side converter of the grid-type wind turbine; During the pre-synchronization phase, the grid-side converter of the grid-connected wind turbine outputs virtual active power and virtual reactive power. P ov and Q ov The specific calculation formula is as follows: (5) In the formula, U g It is the voltage of the offshore power grid. R v and X v It is virtual impedance. θ g It is the voltage phase angle of the offshore power grid.

[0054] The specific calculation formula for the reference value of the output voltage of the grid-side converter of the pre-synchronization stage wind turbine after passing through the LC filter is as follows: (6) In the formula, X virFor virtual sensing in VSG control, i od and i oq The d-axis and q-axis components of the output current of the grid-side converter of a grid-type wind turbine after passing through an LC filter. and The reference values ​​for the d-axis and q-axis output voltages of the grid-side converter of the pre-synchronization stage wind turbine are after passing through an LC filter.

[0055] Further, in step S4, after the pre-synchronization phase is completed, the offshore wind farm is connected to the offshore power grid, and the operating modes of the synchronous motors and synchronous generators in the MG system and the status of the MG starting device are switched, including: S41. Establish the connection between the grid-side converter of the grid-type wind turbine and the offshore power grid, so that the offshore wind farm can be integrated into the offshore power grid; S42. Control the grid-side converter of the grid-type wind turbine to operate in independent operation mode; S43. Control the synchronous motor in the MG system to work in motor mode and control the synchronous generator to work in generator mode. Switch the state of the energy storage side converter in the MG starting device to rectification mode. Charge the energy storage device in the MG starting device through the offshore wind farm. Control the connection between the MG system and the auxiliary equipment of the thermal power unit so that the offshore wind farm supplies power to the auxiliary equipment of the thermal power unit through the MG system.

[0056] Specifically, after the pre-synchronization is completed, control the second switch. Br 2. Closing the circuit establishes the connection between the grid-side converter of the grid-connected wind turbine and the offshore power grid, enabling the offshore wind farm to be integrated into the offshore power grid. The grid-side converter 13 operates in independent operation mode. The offshore wind farm transmits electricity, the synchronous motor switches to motor mode, the synchronous generator switches to generator mode, the energy storage-side converter switches from inverter mode to rectifier mode, and the energy storage device switches from discharging state to charging state. The virtual synchronous machine voltage amplitude of the grid-side converter 13 in independent operation mode... E VSG1 The formula for calculating ′ is: (7) In the formula, and Q o These are the reference and actual output power values ​​for grid-type wind turbines, and the voltage droop coefficient. D q ≠0.

[0057] Virtual synchronous machine voltage phase angle in independent operation mode θ VSG1 The specific calculation formula for ′ is: (8) In the formula, Δ ω 1′ represents the speed change of the virtual synchronous machine in independent operation mode. and P o These are the reference value and the actual value of the output active power of the grid-side converter of the grid-type wind turbine.

[0058] When a grid-connected wind turbine operates in independent mode, the frequency reference value for the energy storage-side converter switching from a discharging state to a charging state remains the same. .

[0059] Further, in step S5, the onshore power grid is activated, and the offshore wind farm is connected to the onshore power grid via the MG system. Controlling the offshore wind farm to transmit power to the onshore power grid through the MG system includes: S51. Control the connection between the grid-side converter of the grid-connected wind turbine and the offshore power grid; S52. Start the grid-side converter of the grid-connected wind turbine and maintain the DC bus capacitor voltage of the generator-side converter of the grid-connected wind turbine stable. S53. After the offshore wind speed exceeds the cut-in wind speed, control the start-up of the inverter on the machine side of the grid-type wind turbine. S54. Start the onshore power grid. The offshore wind farm is connected to the onshore power grid through the MG system, so that the offshore wind farm can transmit power to the onshore power grid through the MG system.

[0060] Specifically, close the fourth switch. Br 4. Connect the MG starting device to the onshore power grid. The offshore wind farm transmits electricity to power the energy storage device and auxiliary equipment of the thermal power unit. Control the establishment of the connection between the grid-side converter of the grid-connected wind turbine and the offshore power grid, specifically by controlling the closing of the third switch. Br 3. Start the grid-side converter 16 of the grid-connected wind turbine and maintain the DC bus capacitor voltage of the generator-side converter 15 of the grid-connected wind turbine stable; after detecting that the offshore wind speed is greater than the cut-in wind speed, control the generator-side converter 15 of the grid-connected wind turbine to start. The synchronous generator in the MG system operates in generator mode, and the synchronous motor in the MG system operates in motor mode. The actual power supply for the energy storage system of the MG starting device and the auxiliary equipment of the thermal power unit is the offshore wind farm.

[0061] Furthermore, after activating the onshore power grid, the following also includes: Disconnect the onshore power grid from the MG starting device, control the synchronous generator in the step-by-step operation mode, and when the grid connection conditions are met, connect the offshore wind power low-frequency transmission MG grid connection system to the onshore power grid and enter the grid connection operation mode.

[0062] Specifically, after the onshore thermal power units started up, the onshore power grid returned to normal, and the fourth switch... Br 4. When disconnected, the thermal power unit supplies power to the onshore load and its own auxiliary equipment. The offshore wind power low-frequency transmission MG grid connection system enters the synchronization phase operation mode. The voltage amplitude calculation formula of the virtual synchronous machine control in the synchronization phase is the same as that in the independent operation mode, but the voltage phase angle of the virtual synchronous machine in the synchronization phase is different. θ VSG2 The specific calculation formula is as follows: (9) In the formula, Reference speed value of the virtual synchronizer during the full-step phase. Δ ω 2 represents the speed change of the virtual synchronizer during the full-step phase, Δ. f max It is the maximum frequency difference between the voltages on both sides of the switch that is allowed in a quasi-synchronous parallel device.

[0063] When the MG grid-connected system operates in the synchronization phase mode, the energy storage-side converter, which is in a charging state, has a corresponding frequency reference value. The calculation formula is: (10) Furthermore, the quasi-synchronous paralleling device is used to detect the voltage amplitude difference, voltage phase difference, and voltage frequency difference between the synchronous generator and the onshore power grid connection switch, and controls the closing of the connection switch when the grid connection conditions are met, so as to realize the grid connection of the offshore wind power low frequency transmission MG grid connection system with the onshore power grid.

[0064] Furthermore, after the energy storage device has finished charging, the first switch... Br 1. After disconnection and grid connection, the grid-side converter of the grid-connected wind turbine enters the grid-connected operation mode. In the grid-connected operation mode, the voltage amplitude calculation formula of the virtual synchronous machine control remains unchanged, but the voltage phase angle remains unchanged. θ VSG3 The specific calculation formula is as follows: (11) In the formula, This is the reference value for the virtual synchronous machine speed in grid-connected operation mode. Δ ω 3 represents the change in the speed of the virtual synchronous machine under grid-connected operation mode.

[0065] Table 1 shows the corresponding control modes. Figure 4 The switch corresponds to the variable.

[0066] Table 1

[0067] refer to Figure 5 In some embodiments, a start-up control device for an offshore wind power low-frequency transmission MG grid-connected system applied to the above method is provided, comprising: The connection control module 201 is used to control the disconnection of the offshore wind farm from the offshore power grid, the disconnection of the MG system from the onshore power grid, and the disconnection of the MG starting device from the onshore power grid when the onshore power grid is cut off, and to control the start of the MG starting device to provide rotor excitation for the MG system. The charging and voltage-building module 202 is used to control the working mode of the synchronous motor and synchronous generator in the MG system, and to charge and build up the voltage of the offshore power grid so that the voltage of the offshore power grid rises to the rated value. The pre-synchronization module 203 is used to start the offshore wind farm into the pre-synchronization stage operation mode after the offshore wind speed is greater than the cut-in wind speed. The state switching module 204 is used to control the offshore wind farm to connect to the offshore power grid after the pre-synchronization phase is completed, switch the working mode of the synchronous motor and synchronous generator in the MG system and the state of the MG starting device, and supply power to the MG starting device and auxiliary equipment of the thermal power unit through the offshore wind farm. The onshore power grid startup module 205 is used to start the onshore power grid. The offshore wind farm is connected to the onshore power grid through the MG system, and the offshore wind farm is controlled to transmit power to the onshore power grid through the MG system.

[0068] Furthermore, the MG starting device includes an energy storage device, an energy storage-side converter, and a filter inductor; the connection control module 201 controls the starting of the MG starting device to provide rotor excitation for the MG system, including: The energy storage-side converter is controlled to operate in inverter mode, so that the energy storage device is in a discharge state, providing rotor excitation for the MG system.

[0069] Furthermore, the MG system also includes a first excitation device and a second excitation device. The synchronous motor is connected to the first excitation device, and the synchronous generator is connected to the second excitation device. The rotor of the synchronous motor and the rotor of the synchronous generator are coaxially connected. The ratio of the number of pole pairs of the synchronous motor to the number of pole pairs of the synchronous generator is set to be equal to the ratio of the frequency required by the offshore power grid to the frequency of the onshore power grid. The synchronous motor is connected to the low-frequency AC submarine cable, and the synchronous generator switches its connection with the onshore power grid via a connection switch. The charging and voltage-building module 202 controls the operating mode of the synchronous motor and synchronous generator in the MG system to charge and build up voltage for the offshore power grid, including: The second excitation device controls the electromagnetic torque of the synchronous generator based on the rotor excitation, so that the synchronous generator operates in motor mode; The synchronous motor is controlled to operate in generator mode by the first excitation device to charge and build up voltage for the offshore power grid, so that the voltage of the offshore power grid rises to the rated value.

[0070] Furthermore, the offshore wind farm includes grid-connected wind turbines, grid-connected wind turbine generator-side converters, grid-connected wind turbine grid-side converters, grid-following wind turbines, grid-following wind turbine generator-side converters, and grid-following wind turbine grid-side converters. The pre-synchronization module 203 initiates the offshore wind farm into the pre-synchronization phase operation mode, including: Control the start-up of the generator-side converter of the grid-type wind turbine and maintain the DC bus capacitor voltage of the generator-side converter of the grid-type wind turbine stable; The grid-side converter of the control grid-type wind turbine is started, so that the grid-side converter of the control grid-type wind turbine operates in the pre-synchronization stage mode.

[0071] Furthermore, the state switching module 204 controls the offshore wind farm to connect to the offshore power grid, switching the operating modes of the synchronous motors and synchronous generators in the MG system as well as the state of the MG starting device, including: Establish a connection between the grid-side converter of the grid-connected wind turbine and the offshore power grid, so that the offshore wind farm can be integrated into the offshore power grid; The grid-side converter of the control grid-type wind turbine operates in independent mode; The synchronous motor in the MG system is controlled to operate in motor mode, and the synchronous generator is controlled to operate in generator mode. The state of the energy storage side converter in the MG starting device is switched to rectification mode, so that the energy storage device is in charging state. The MG starting device is charged through the offshore wind farm, and the connection between the MG system and the auxiliary equipment of the thermal power unit is controlled, so that the offshore wind farm supplies power to the auxiliary equipment of the thermal power unit through the MG system.

[0072] Furthermore, the onshore power grid startup module 205 starts the onshore power grid, and the offshore wind farm is connected to the onshore power grid via the MG system. It controls the offshore wind farm to transmit power to the onshore power grid through the MG system, including: Control the connection between the grid-side converter of the grid-connected wind turbine and the offshore power grid; Start the grid-side converter of the grid-connected wind turbine and maintain the DC bus capacitor voltage of the generator-side converter of the grid-connected wind turbine stable. Once the offshore wind speed exceeds the cut-in wind speed, the inverter on the machine side of the grid-type wind turbine will be started. The onshore power grid is activated, and the offshore wind farm is connected to the onshore power grid via the MG system, enabling the offshore wind farm to transmit power to the onshore power grid through the MG system.

[0073] Furthermore, after the onshore power grid startup module 205 starts the onshore power grid, it also includes: Disconnect the onshore power grid from the MG starting device, control the synchronous generator in the step-by-step operation mode, and when the grid connection conditions are met, connect the offshore wind power low-frequency transmission MG grid connection system to the onshore power grid and enter the grid connection operation mode.

[0074] Furthermore, the offshore wind power low-frequency transmission MG grid-connected system also includes a quasi-synchronous paralleling device, which is used to detect the voltage amplitude difference, voltage phase difference, and voltage frequency difference between the synchronous generator and the onshore power grid connection switch, and control the closing of the connection switch when the grid connection conditions are met, so as to realize the grid connection of the offshore wind power low-frequency transmission MG grid-connected system with the onshore power grid.

[0075] In some embodiments, an electronic device is also provided, including a processor and a memory, the memory storing a plurality of instructions, the processor being configured to read the instructions and execute the methods described above.

[0076] The offshore wind power low-frequency transmission MG grid-connected system start-up control method and apparatus provided in the above embodiments have at least the following beneficial effects: (1) The MG can be started by energy storage system without taking power from the onshore power grid or through diesel engine, so as to achieve the smooth start of the grid-connected system, supply power to key equipment, so that offshore wind power can be used as black start power source to assist the onshore power grid to restore power supply and reduce production costs. (2) The proposed start-up control strategy for the offshore wind power low-frequency transmission MG grid-connected system achieves accurate synchronization between the output voltage of the offshore wind farm grid-type wind turbine and the voltage at the collection bus before grid connection through pre-synchronization technology. This can reduce the fluctuation of voltage amplitude and frequency of the offshore power grid after grid connection and realize seamless switching of the grid-type wind turbine from off-grid to grid connection. (3) It can achieve smooth start-up of MG, improve the success rate of offshore wind turbines connecting to the offshore power grid and the system connecting to the onshore power grid, and has good dynamic performance.

[0077] Although preferred embodiments of the invention have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including both the preferred embodiments and all changes and modifications falling within the scope of the invention. Clearly, those skilled in the art can make various alterations and modifications to the invention without departing from its spirit and scope. Thus, if these modifications and modifications of the invention fall within the scope of the claims and their equivalents, the invention is also intended to include these modifications and modifications.

Claims

1. A startup control method for an offshore wind power low-frequency transmission (MG) grid-connected system, characterized in that, The offshore wind power low-frequency transmission MG grid connection system includes an offshore wind farm, an MG system, and an MG start-up device; the offshore wind farm is connected to the offshore power grid, the offshore power grid is connected to the MG system through a low-frequency AC submarine cable, and the MG system is connected to the onshore power grid; the onshore power grid is connected to the thermal power unit, and the thermal power unit is connected to the auxiliary equipment of the thermal power unit; The method includes: When the onshore power grid fails, the system controls the disconnection of the offshore wind farm from the offshore power grid, the disconnection of the MG system from the onshore power grid, and the disconnection of the MG starting device from the onshore power grid. The system also controls the start of the MG starting device to provide rotor excitation for the MG system. The operating modes of the synchronous motor and synchronous generator in the MG system are controlled to charge and build up the voltage of the offshore power grid, so that the voltage of the offshore power grid rises to the rated value; Once the offshore wind speed exceeds the cut-in wind speed, the offshore wind farm will enter the pre-synchronization phase operation mode. After the pre-synchronization phase is completed, the offshore wind farm is connected to the offshore power grid, the working modes of the synchronous motors and synchronous generators in the MG system and the status of the MG starting device are switched, and the offshore wind farm supplies power to the MG starting device and the auxiliary equipment of the thermal power unit. The onshore power grid is activated, and the offshore wind farm is connected to the onshore power grid via the MG system. The offshore wind farm is controlled to transmit power to the onshore power grid through the MG system.

2. The method according to claim 1, characterized in that, The MG starting device includes an energy storage device, an energy storage-side converter, and a filter inductor; Controlling the start of the MG starting device to provide rotor excitation for the MG system includes: The energy storage-side converter is controlled to operate in inverter mode, so that the energy storage device is in a discharge state, providing rotor excitation for the MG system.

3. The method according to claim 2, characterized in that, The MG system also includes a first excitation device and a second excitation device. The synchronous motor is connected to the first excitation device, and the synchronous generator is connected to the second excitation device. The rotor of the synchronous motor and the rotor of the synchronous generator are coaxially connected. The ratio of the number of pole pairs of the synchronous motor to the number of pole pairs of the synchronous generator is set to be equal to the ratio of the frequency required by the offshore power grid to the frequency of the onshore power grid. The synchronous motor is connected to the low-frequency AC submarine cable, and the synchronous generator switches its connection with the onshore power grid via a connection switch. Controlling the operating modes of the synchronous motors and synchronous generators in the MG system to charge and build up voltage for the offshore power grid includes: The second excitation device controls the electromagnetic torque of the synchronous generator based on the rotor excitation, so that the synchronous generator operates in motor mode; The synchronous motor is controlled to operate in generator mode by the first excitation device to charge and build up voltage for the offshore power grid, so that the voltage of the offshore power grid rises to the rated value.

4. The method according to claim 2, characterized in that, The offshore wind farm includes grid-type wind turbines, grid-type wind turbine generator-side converters, grid-type wind turbine grid-side converters, grid-connected wind turbines, grid-connected wind turbine generator-side converters, and grid-connected wind turbine grid-side converters. The offshore wind farm is initiated into a pre-synchronization phase operation mode, including: Control the start-up of the generator-side converter of the grid-type wind turbine and maintain the DC bus capacitor voltage of the generator-side converter of the grid-type wind turbine stable; The grid-side converter of the control grid-type wind turbine is started, so that the grid-side converter of the control grid-type wind turbine operates in the pre-synchronization stage mode.

5. The method according to claim 4, characterized in that, After the pre-synchronization phase is completed, the offshore wind farm is connected to the offshore power grid, and the operating modes of the synchronous motors and synchronous generators in the MG system and the status of the MG starting device are switched, including: Establish a connection between the grid-side converter of the grid-connected wind turbine and the offshore power grid, so that the offshore wind farm can be integrated into the offshore power grid; The grid-side converter of the control grid-type wind turbine operates in independent mode; The synchronous motor in the MG system is controlled to operate in motor mode, and the synchronous generator is controlled to operate in generator mode. The state of the energy storage side converter in the MG starting device is switched to rectification mode. The energy storage device in the MG starting device is charged through the offshore wind farm. The connection between the MG system and the auxiliary equipment of the thermal power unit is controlled so that the offshore wind farm supplies power to the auxiliary equipment of the thermal power unit through the MG system.

6. The method according to claim 5, characterized in that, Activating the onshore power grid, connecting the offshore wind farm to the onshore power grid via the MG system, and controlling the offshore wind farm to transmit power to the onshore power grid through the MG system include: Control the connection between the grid-side converter of the grid-connected wind turbine and the offshore power grid; Start the grid-side converter of the grid-connected wind turbine and maintain the DC bus capacitor voltage of the generator-side converter of the grid-connected wind turbine stable. Once the offshore wind speed exceeds the cut-in wind speed, the inverter on the machine side of the grid-type wind turbine will be started. The onshore power grid is activated, and the offshore wind farm is connected to the onshore power grid via the MG system, enabling the offshore wind farm to transmit power to the onshore power grid through the MG system.

7. The method according to claim 6, characterized in that, After activating the onshore power grid, the following is also included: Disconnect the onshore power grid from the MG starting device, control the synchronous generator in the step-by-step operation mode, and when the grid connection conditions are met, connect the offshore wind power low-frequency transmission MG grid connection system to the onshore power grid and enter the grid connection operation mode.

8. The method according to claim 7, characterized in that, The offshore wind power low-frequency transmission MG grid-connected system also includes a quasi-synchronous paralleling device, which is used to detect the voltage amplitude difference, voltage phase difference, and voltage frequency difference between the synchronous generator and the onshore power grid connection switch, and control the closing of the connection switch when the grid connection conditions are met, so as to realize the grid connection of the offshore wind power low-frequency transmission MG grid-connected system with the onshore power grid.

9. A start-up control device for an offshore wind power low-frequency transmission MG grid-connected system applied to the method described in any one of claims 1-8, characterized in that, include: The connection control module is used to control the disconnection of the offshore wind farm from the offshore power grid, the disconnection of the MG system from the onshore power grid, and the disconnection of the MG starting device from the onshore power grid when the onshore power grid is interrupted, and to control the start of the MG starting device to provide rotor excitation for the MG system. The charging and voltage-building module is used to control the operating mode of the synchronous motor and synchronous generator in the MG system, and to charge and build up the voltage of the offshore power grid so that the voltage of the offshore power grid rises to the rated value. The pre-synchronization module is used to initiate the pre-synchronization phase operation mode of the offshore wind farm after the offshore wind speed exceeds the cut-in wind speed. The state switching module is used to control the offshore wind farm to connect to the offshore power grid after the pre-synchronization phase is completed, switch the working mode of the synchronous motor and synchronous generator in the MG system and the state of the MG starting device, and supply power to the MG starting device and auxiliary equipment of the thermal power unit through the offshore wind farm. The onshore power grid startup module is used to start the onshore power grid. The offshore wind farm is connected to the onshore power grid via the MG system, and the module controls the offshore wind farm to transmit power to the onshore power grid through the MG system.

10. An electronic device, characterized in that, It includes a processor and a memory, the memory storing multiple instructions, and the processor being used to read the instructions and execute the method as described in any one of claims 1-8.

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