Hybrid offshore wind plant black-start method based on DRU-HVDC sending-out system

Abstract

The invention discloses a hybrid offshore wind plant black-start method based on a DRU-HVDC sending-out system, and belongs to the technology of a power transmission system due to the economical efficiency and the reliability of the power transmission system. In order to solve the problem that a control strategy of a DRU-HVDC power transmission system based on a diode rectifier unit is still challenged in the black-start process of an offshore wind plant, the invention aims to solve the black-start problem of an offshore wind plant low-frequency alternating current collection and direct current sending-out system adopting the DRU-HVDC and realize stable grid connection and mixed operation of wind turbine generators after starting. The control method comprises the steps that a switchable control strategy is provided, undisturbed switching between a GFM operation mode and a GFL operation mode is supported, after starting, the system can stably operate in the mixed mode of the GFM operation mode and the GFL operation mode, and a reliable black-start and mixed operation solution is provided for an open-sea wind power plant without synchronous power supply support.

Description

Technical Field

[0001] This invention belongs to the field of power transmission control technology, specifically relating to the control strategy of offshore wind power transmission systems, and in particular a black start method for hybrid offshore wind farms based on a DRU-HVDC transmission system. Background Technology

[0002] With the rise of clean energy and the transformation of the energy structure, offshore wind power, as a key form of renewable energy, has received widespread attention and developed rapidly. As offshore wind power projects expand to large scale and long distances, ensuring system reliability and stability during the construction and operation of wind farms has become an urgent challenge.

[0003] The power transmission architecture based on diode rectifier units (DRUs) is gradually becoming a research hotspot in the industry due to its significant potential in improving the economics and operational reliability of offshore wind power systems. As a type of active commutator converter, the normal operation of a DRU relies on an AC voltage source to provide the necessary commutation voltage support. Compared to traditional power frequency AC transmission technology, low-frequency transmission schemes effectively extend the feasible transmission distance of offshore wind power by reducing the inductive reactance of transmission lines and the capacitive charging current of submarine cables. Therefore, combining the low-frequency AC collection method within the wind farm with DRU-based high-voltage DC rectification and transmission technology can further optimize the overall system cost and enhance its economic competitiveness.

[0004] There are two technical approaches to controlling offshore wind farms based on DRU (Dual Voltage Source Unit) transmission systems: First, additional equipment is required as the offshore AC voltage support source. Existing technologies use parallel AC submarine cables to connect to the onshore power grid, including methods involving parallel connection of small-capacity voltage source converters to the AC bus of the rectifier station. In this approach, the wind turbines operate in grid-following (GFL) mode. Commissioned units generally employ GFL control strategies, synchronizing the unit's output voltage phase via a phase-locked loop (PLL). These current-source characteristic units have a significant risk of instability in weak grids and must rely on an active grid for operation. Although onshore grids or U / f-controlled parallel voltage source converters can start the wind farm, allowing the wind turbines to operate in GFL mode, relying on external energy sources weakens the economic advantages of DRUs. Second, grid-forming (GFM) operation can be achieved by changing the unit control strategy, eliminating the need for an additional voltage support source. GFM units exhibit external voltage source characteristics, showing excellent stability in weak grids but limited performance in strong grids. Mainstream control methods include: grid-based control based on a globally unified reference coordinate system, phase-locked loop control, and grid-based control schemes based on reactive power-frequency droop.

[0005] Given that the operation of the diode rectifier unit (DRU) relies on externally provided commutation voltage and its energy transfer path is unidirectional (only transmitting power from offshore to onshore), neither the DRU rectifier station nor the onshore AC grid can provide initial startup power to the offshore wind farm. Therefore, a dedicated black-start power source must be independently configured for the offshore wind farm to meet its startup energy requirements. This step is crucial for system safety and stability. Existing technologies use diesel generators as black-start sources, but do not consider DRU integration scenarios. For DRU DC transmission systems, black-start solutions such as additional converters, energy storage devices, and photovoltaics have been compared. Other research has designed an adaptive switching mechanism based on the complementary characteristics of grid-connected / grid-linked control in strong and weak grids to balance stability and economy. However, existing solutions still struggle to simultaneously meet the low-cost black-start requirements of DRU-HVDC systems and the economical and reliable operation requirements of wind farms with multiple turbine types.

[0006] Therefore, this invention employs a marine low-frequency AC aggregation system based on DRU-HVDC to reduce operating costs and improve economic efficiency and stability. Summary of the Invention

[0007] Purpose of the invention: This invention aims to solve the black start problem of low-frequency AC collection DC transmission system for offshore wind farms using DRU-HVDC, and to achieve stable grid connection and hybrid operation of wind turbine units after startup. It proposes a black start method for hybrid offshore wind farms based on DRU-HVDC transmission system, which is used to realize autonomous black start of wind farms and smooth grid connection of GFM units after startup.

[0008] Technical solution: A black start method for a hybrid offshore wind farm based on a DRU-HVDC transmission system, wherein the system includes wind turbine generators, a DRU rectifier station, and a full-power converter; The full-power converter includes a machine-side converter and a grid-side converter. The method includes setting a phase-locked loop circuit in the grid-side converter to monitor the phase of the port voltage of the grid-side converter. During black start, the frequency deviation output by the PLL is used as the frequency input of the reactive power-frequency controller. The frequency controlled by the grid-side converter is obtained by superimposing the PLL frequency deviation, the frequency deviation generated by the reactive power-frequency controller, and the wind farm frequency setpoint, thereby obtaining the true frequency and angle of the point of common coupling (PCC) and avoiding startup power fluctuations.

[0009] The method is based on the fact that when the PLL starts up, the reactive power reference value is zero, the reactive power output is zero, and the frequency deviation generated by the reactive power-frequency controller is zero. After the PLL starts up and exits, the deviation generated by the reactive power-frequency controller will compensate for the PLL deviation and be superimposed with the wind farm setpoint to output the frequency, so that the new unit can participate in reactive power distribution and frequency construction.

[0010] Furthermore, the method is to start the wind turbine in a grid-connected operation model, and after startup, it can adapt to mixed grid-connected and grid-following operation modes. The startup control includes the following steps: S1. Auxiliary equipment startup: Start the diesel generator equipped with the first wind turbine to supply power to the auxiliary equipment of the wind turbine (this is disconnected after the wind farm starts up, and the auxiliary equipment is then powered by the offshore AC system).

[0011] S2. Mechanical Start: Once the wind speed reaches the target, the rotor brake is released, and the wind turbine drives the permanent magnet synchronous generator (PMSG) to rotate. The turbine-side and grid-side converters remain locked.

[0012] S3. Converter Start-up: The wind turbine drives the PMSG rotor to rotate, generating AC voltage. This voltage is used to uncontrolled charge the DC bus capacitor through the anti-parallel diodes of the IGBTs on the turbine side. Once the DC-side capacitor voltage rises to a certain level, the converter unlocks and stabilizes the DC voltage to the rated value according to the control strategy. The PMSG speed is controlled at the rated speed by pitch angle control. The wind turbine outputs very little mechanical energy, which is only lost within the unit itself before the unit outputs active power.

[0013] S4. Grid-side converter startup and voltage establishment: After the generator side stabilizes, the grid-side converter is started and the AC circuit breaker is closed to charge the transformer and the collector cable. The grid side operates in constant U / f mode, setting a voltage amplitude of 0.8 pu lower than the DRU conduction value (DRU AC side voltage amplitude is greater than DC side voltage) as the d-axis reference. Through voltage inner loop, current inner loop, and PWM control, the voltage amplitude and frequency at the turbine port and collection point are established. The active power-voltage amplitude controller does not operate, and there is no active power output.

[0014] S5. Self-synchronization between units: Start the grid-type wind turbine units one by one according to the above stages, and carry out self-synchronization and reactive power balance between units until the number meets the charging power requirements of the wind farm.

[0015] S6. Collection Point Closure and DRU Charging: After the unit starts up and voltage is established, the collection point circuit breaker is closed to charge the DRU converter transformer. Under the action of the reactive power-frequency controller, the unit dynamically maintains reactive power balance to avoid converter overload.

[0016] S7. Active power output: After charging is completed, the active power-voltage amplitude controller is unlocked. The AC voltage of the wind farm gradually rises to the DRU conduction value. The DRU conducts and the wind turbine outputs active power to the outside world. The active power is sent out through the DRU.

[0017] Furthermore, the wind turbine is equipped with a full-power converter for operation control. The full-power converter includes a turbine-side converter and a grid-side converter, and its control method is as follows: The generator-side converter rectifies the AC power generated by the permanent magnet synchronous generator into DC power. Its control method adopts zero d-axis current control, while maintaining the constant DC bus voltage. The generator-side converter control system is a voltage and current dual closed-loop control. By controlling the rectification of the generator-side converter, the constant DC bus voltage is achieved. The grid-side converter adopts a three-layer control architecture consisting of a power controller, a voltage controller, and a current controller, and uses a voltage-current dual closed-loop system to control the output voltage of the grid-side converter.

[0018] The three-layer control architecture of the grid-side converter performs active power-voltage amplitude control and reactive power-frequency control, as detailed below: Active power-voltage amplitude control satisfies the following calculation expression: ; In the formula, , These are the reference values ​​for the d-axis and q-axis components of the AC voltage at the fan outlet, respectively. This is the initial value of the d-axis voltage. , These are the proportional and integral coefficients of the active power control loop. It is the active power output of the wind turbine. This is a reference value for the active power of the wind turbine generated by the maximum power point tracking (MPPT) control.

[0019] The reactive power-frequency control of the grid-side converter satisfies the following calculation expression: , In the formula, It is the frequency of the low-frequency AC system (reference value for the frequency of the fan converter). It is the proportional coefficient of the reactive power control loop. It is the reactive power output of the wind turbine. This is the reference value for the reactive power of the wind turbine. This is the initial value of the frequency for a low-frequency AC system. rad / s.

[0020] After connecting to the reactive power-frequency controller, the frequency of the AC voltage at the wind turbine outlet is obtained based on the calculation expression of reactive power-frequency, thereby establishing a frequency reference for the wind farm and realizing synchronous operation of the units under conditions without communication; the frequency value output by the reactive power-frequency controller generates the phase angle required for coordinate transformation through the integration stage, and then controls the frequency of the offshore AC system by adjusting the reactive power of the wind turbine.

[0021] Furthermore, the DRU-HVDC power transmission system includes a full-power converter direct-drive permanent magnet synchronous wind turbine generator, a high-voltage low-frequency AC current collection submarine cable, a diode rectifier station, a converter transformer, reactive power compensation and filtering devices, a high-voltage DC submarine cable, and an onshore modular multilevel converter station. The direct-drive permanent magnet synchronous wind turbine generator transmits the generated electricity to the offshore common junction point via a high-voltage low-frequency AC submarine cable, and then connects to a diode rectifier station to convert the low-frequency AC power into DC power. Subsequently, it is transmitted to the shore via a long-distance DC submarine cable, and finally the DC power is inverted into AC power by the onshore modular multilevel converter station and connected to the power grid. A filter device is installed on the AC bus to eliminate the characteristic harmonics generated by the diode rectifier station; The system is equipped with a reactive power compensation device to suppress the capacitive effect caused by the AC submarine cable charging power; The system's direct-drive permanent magnet synchronous wind turbines can operate in either grid-connected or grid-connected modes.

[0022] Furthermore, the direct-drive permanent magnet synchronous wind turbine in the DRU-HVDC power transmission system includes a hybrid mode of grid-connected and grid-following operation. The turbine-side converter corresponding to the direct-drive permanent magnet synchronous wind turbine operates in a grid-connected model, and the grid-side converter operates in a grid-connected model during startup. This method is designed for offshore wind farms transmitting power via DRU-HVDC. Setting the direct-drive permanent magnet synchronous wind turbine to operate in a grid-connected model enables black start and system power transmission.

[0023] This method controls the generator frequency of the direct-drive permanent magnet synchronous wind turbine unit to be generated by a reactive power-frequency controller or PLL, and reduces switching disturbances by externally resetting the following operating point through an integrator.

[0024] Beneficial effects: The improved GFM unit converter control strategy and wind farm black start strategy proposed in this invention enable wind turbine units to establish stable AC voltage under conditions without external synchronous power supply, providing reliable commutation support for DRU; subsequently, a seamlessly switchable grid-connected / grid-following wind turbine unit is designed to overcome the adaptability limitations of a single control mode in strong and weak grid scenarios, and achieve a seamless transition of the GFM unit to GFL mode after black start. Attached Figure Description

[0025] Figure 1 It is a topology for offshore wind power to be transmitted via DRU-HVDC; Figure 2 This is the control logic diagram of the machine-side converter; Figure 3 This is the control logic diagram of the grid-side converter; Figure 4 This is the control logic diagram for a grid-side converter that can switch between grid-connected and grid-connected modes. Figure 5 This is a diagram showing the voltage vector between the grid-side converter coordinate system and the wind turbine connection point. Figure 5 (a) is the use of the pre-synchronous grid-side converter coordinate system and the voltage vector at the wind turbine connection point without employing the pre-synchronous grid-side converter coordinate system. Figure 5 (b) is the voltage vector at the connection point of the wind turbine using the coordinate system of the pre-synchronous grid-side converter; Figure 6 This is the control logic diagram of the improved grid-side converter described in this invention; Figure 7 This is a flowchart of the black start strategy for offshore wind farms; Figure 8 This is a flowchart of the startup strategy for grid-connected wind turbines in offshore wind farms; Figure 9(a) shows the DC bus voltage variation of wind turbine 1; Figure 9(b) shows the rotor speed variation of the generator of wind turbine 1; Figure 9(c) shows the pitch angle variation of wind turbine 1; Figure 9(d) shows the voltage amplitude variation of wind turbine 1; Figure 9(e) shows the active and reactive power variation of wind turbine 1; Figure 9(f) shows the active and reactive power variation of wind turbine 2; Figure 9(g) shows the active and reactive power variation of wind turbine 3; Figure 9(h) shows the frequency variation of wind turbine units 1-3; Figure 9(i) shows the active and reactive power variation of the AC side of the DRU; Figure 9(j) shows the active power variation of the DC side of the DRU; Figure 9(k) shows the voltage and current waveform variation of wind turbine unit 1. Figure 10(a) shows the changes in active and reactive power of wind turbine 1; Figure 10(b) shows the changes in active and reactive power of wind turbine 2; Figure 10(c) shows the changes in active and reactive power of wind turbine 3; Figure 10(d) shows the changes in active and reactive power on the AC side of the DRU; Figure 10(e) shows the changes in active power on the DC side of the DRU; Figure 10(f) shows the frequency change curve of wind turbine 3; and Figure 10(g) shows the voltage and current waveforms of wind turbine 1. Detailed Implementation

[0026] Combination Figure 1As shown, the DRU-HVDC power transmission system of this invention comprises: a direct-drive permanent magnet synchronous wind turbine equipped with a full-power converter, a 66 kV high-voltage low-frequency AC collector submarine cable, a marine diode rectifier unit (DRU), a converter transformer, reactive power compensation and filtering devices, a high-voltage DC submarine cable, and an onshore modular multilevel converter (MMC) station. Within the wind farm, a fixed number of wind turbines form a series via a medium-voltage low-frequency AC submarine cable. Multiple such series of turbines transmit their generated electricity to the offshore point of common coupling (PCC) via the AC submarine cable, and then connect to the DRU rectifier station. To eliminate the characteristic harmonics generated by the DRU, an AC filter is installed on the AC bus; simultaneously, a reactive power compensation device is configured to suppress the capacitive effect caused by the charging power of the AC submarine cable. The marine low-frequency AC power is converted to DC power by the diode rectifier, and then transmitted to the shore via a long-distance DC submarine cable. Finally, the onshore MMC converter station inverts the DC power back into AC power and integrates it into the power grid. Given that diode rectifiers lack controllability and are difficult to generate low-frequency AC voltage, offshore wind turbines need to adopt grid-type control to autonomously establish and maintain voltage.

[0027] For direct-drive permanent magnet synchronous wind turbine generators, their operating mode is controlled by a full-power converter, which includes a machine-side converter and a grid-side converter. The control strategy for the machine-side converter is as follows: Figure 2 As shown, its function is to rectify the AC power generated by the permanent magnet synchronous generator into DC power while maintaining a constant DC bus voltage. Its control method employs zero-d-axis current control. The generator-side converter control system is a voltage-current dual closed-loop control. Through rectification control of the generator-side converter, the DC bus voltage is ultimately kept constant. The grid-side converter control system adopts a three-layer architecture: the first layer is the power controller, the second layer is the voltage controller, and the third layer is the current controller. A voltage-current dual closed-loop system is used to achieve rapid and accurate control of the grid-side converter output voltage. The control strategy is as follows: Figure 3 As shown.

[0028] This invention addresses the scenario of offshore wind farms transmitting power via DRU-HVDC. By configuring GFM units, this invention enables black start and power transmission while maintaining compatibility with existing GFL units to reduce costs. The generator-side control strategy is the same as that of GFM units; grid-side control is based on the GFM strategy, with the addition of a GFL control strategy. Synchronization is achieved during startup using the GFM control strategy. The unit frequency is generated by a reactive power-frequency controller (GFM mode) or a PLL (GFL mode), and the operating point is externally reset via an integrator to reduce switching disturbances. The control strategy for grid-connected / grid-following switchable grid-side converters is as follows... Figure 4 As shown.

[0029] The synchronization mechanism for starting grid-connected wind turbines is: when the first... When the typhoon starts ( (where the integer is greater than 1), assuming the previous -1 unit has been put into operation. For example... Figure 5 As shown, the wind turbine grid-side converter uses an abc stationary coordinate system and a dq rotating coordinate system. The dq coordinate system is based on angular velocity... Rotate, and the angle between its d-axis and the stationary a-axis is . Assume that at startup, the angle between the unit's grid connection point voltage vector Us and the a-axis is... Its rotational angular velocity is equal to the angular frequency of the power grid. According to instantaneous power theory, the reactive power output of a wind turbine is... ,Depend on Figure 5 (a) Under the action of voltage and current dual-loop control, the reactive power output of the wind turbine is... This will be increased accordingly. Under the condition of increased reactive power, the rotational speed of the d-axis in the dq rotating coordinate system will be increased. This will increase accordingly. The synchronization process will continue until the d-axis is perfectly aligned with the voltage vector Us. At this point, the voltage phase generated inside the converter... Phase with external system voltage Us Achieving a precise match signifies that the system has successfully entered a synchronized operating state.

[0030] When a wind farm has started up and is outputting active power, if newly started grid-connected wind turbines are directly synchronized using a reactive power-frequency controller, their initial frequency and angle values ​​will not match the actual angle of the grid-side converter voltage. Under the dual closed-loop action of voltage and current, this will lead to significant power fluctuations, potentially causing system instability. Figure 5 (b) shows that using PLL pre-synchronization can make the voltage vector Us coincide with the d-axis, thus avoiding power fluctuations.

[0031] The aforementioned grid-side converter control strategy and Figure 5 The synchronization method shown is only applicable to the black start process of wind farms (when there is no voltage or power output). During black start, the first unit establishes a PCC voltage to provide a frequency reference, and subsequent units synchronize with it. In this scenario, the PCC current is small, the synchronization impact is small, and the use of a reactive power-frequency controller for synchronization is permissible. However, after the wind farm successfully starts up and outputs active power, the PCC current is large. If the controller is still used directly for synchronization at this time, the initial frequency and angle of the new unit will not match the actual values, resulting in large power fluctuations due to the double closed-loop effect, threatening stability. Therefore, an improved control strategy for grid-side converters in grid-connected systems is proposed.

[0032] The improved strategy involves activating a PLL to monitor the port voltage phase before converter startup. During startup, the frequency deviation output by the PLL is used as the frequency input to the reactive power-frequency controller. The frequency controlled by the grid-side converter is determined by the PLL deviation. Frequency deviation generated by the controller Wind farm setpoint The three factors are superimposed to obtain the true frequency and angle of the PCC, avoiding startup power fluctuations. Since the reactive power reference value is zero, the reactive power output is zero during startup, and the frequency deviation generated by the controller is zero. The PLL exits after startup, without affecting system stability. After exiting, the deviation generated by the controller will compensate for the PLL deviation, and the output frequency will be superimposed with the wind farm setpoint, allowing the new unit to participate in reactive power distribution and frequency construction. Improvement strategies include... Figure 6 As shown.

[0033] This invention is based on the black start strategy process of grid-connected wind turbines and offshore wind farms as follows: Figure 7 As shown.

[0034] Start-up procedures for other wind turbines in offshore wind farms: Grid-type wind turbines should be started according to... Figure 8 The flowchart shown indicates that starting the system enables plug-and-play operation of grid-connected wind turbines. Grid-connected / grid-synchronized wind turbines operate in the same manner during grid-connected mode, performing grid connection and self-synchronization.

[0035] To verify the effectiveness of the proposed black-start strategy, this invention uses Matlab / Simulink software for simulation experiments. Three wind turbines are used to represent a large actual wind farm to verify the effectiveness of the control strategy and the black-start strategy. The simulation parameters are shown in Table 1: Table 1. Simulation Experiment Parameters

[0036] The black start experiment setup for a wind farm is as follows: The No. 1 and No. 2 grid-type wind turbine units start the wind farm. After the No. 3 grid-type wind turbine unit starts up and delivers active power in the wind farm, it performs a black start and realizes the synchronous operation of the wind turbine unit with respect to the wind farm and the wind turbine unit.

[0037] The generator starts at 0 seconds. Before the generator-side converter unlocks, the generator charges the DC bus capacitor through the anti-parallel diodes of the IGBTs in the generator-side converter. At 0.5 seconds, the generator-side converter starts, controlling the DC bus voltage to the rated value of 1500 V. At 1 second, the pitch angle control is activated to control the PSMG rotor speed at the rated speed.

[0038] At 2 seconds, the No. 1 grid-connected wind turbine is started, establishing the PCC voltage and frequency. At 3 seconds, the No. 2 grid-connected wind turbine is started, and the two turbines synchronize under the action of the reactive power-frequency controller. At 4 seconds, the turbine series circuit breaker is closed, and the two grid-connected wind turbines begin charging the wind farm. At 7 seconds, the active power-voltage amplitude controller is unlocked, outputting the active power at the rated value. Due to the limitation of the turbine-side output power, the reference value cannot be directly at full power; it needs to slowly rise from 0 to 4.5 MW full power under the action of the first-order inertial element. During the gradual increase of active power, the AC side voltage of the DRU also reaches the conduction value, the DRU starts successfully, and the wind farm can output power externally.

[0039] At 13 seconds, the circuit breaker at the output port of the No. 3 grid-connected wind turbine is closed, and voltage is established on the low-voltage side of the No. 3 wind turbine step-up transformer. At 13.2 seconds, the No. 3 wind turbine PLL is started to lock the voltage on the low-voltage side of the wind turbine step-up transformer. At 14 seconds, the grid-side control of the No. 3 grid-connected wind turbine is unlocked, and the PLL is disconnected. Under the action of the reactive power-frequency controller, it synchronizes with the wind turbines that have already started and participates in the average sharing of reactive load on the offshore AC system.

[0040] The simulation results are presented in Figure 9. The simulation results show that, after adopting the proposed grid-type and grid-type black-start strategies, the voltage and power fluctuations during system startup are relatively small, the DC bus voltage and speed can remain stable at their rated values, and there will be no significant impact on the system. All electrical quantities can smoothly reach their pre-normal operating state, and the entire startup process is relatively smooth.

[0041] This invention addresses the technical challenges of stable operation of offshore wind farms, overcoming the black start problem of low-frequency AC-DC transmission systems in offshore wind farms. It proposes an improved grid-connected wind turbine control strategy to achieve autonomous black start of the wind farm and smooth grid connection of GFM turbines after start-up. Simulation examples verify the feasibility of the proposed strategy, leading to the following conclusions: Based on the improved GFM control black start strategy, a stable offshore AC voltage platform was successfully established without external auxiliary power supply, providing reliable commutation support for DRU and enabling the wind farm to start autonomously from zero power state. The design incorporates a seamless switching mechanism between GFM and GFL modes, overcoming the limitations of a single control mode in adaptability to varying system intensity scenarios. This ensures that wind turbines can flexibly switch between grid-connected (providing system inertia) and grid-following (optimizing power transmission) modes after black start, supporting the dynamic expansion and stable operation of wind farms.

Claims

1. A hybrid offshore wind farm black start method based on DRU-HVDC sending-out system, characterized in that, The system comprises a wind turbine, a DRU rectifier station and a full-power converter; The full-power converter comprises a machine-side converter and a grid-side converter, and the method comprises providing a phase-locked loop circuit in the grid-side converter to monitor the phase of the port voltage of the grid-side converter, using the frequency deviation of the PLL output as the frequency input of the reactive power-frequency controller during black start, and superimposing the frequency deviation of the PLL, the frequency deviation of the reactive power-frequency controller and the frequency set value of the wind farm to obtain the frequency and angle of the point of common coupling (PCC), thereby avoiding the fluctuation of the starting power.

2. The hybrid offshore wind farm black start method of claim 1, wherein, The method is based on the fact that the reactive reference value is zero, the reactive output is zero and the frequency deviation of the reactive power-frequency controller is zero when the PLL is started; the PLL is exited after starting, and the deviation of the reactive power-frequency controller compensates the deviation of the PLL, and the superimposed output frequency of the set value of the wind farm enables the new unit to participate in the reactive power distribution and frequency construction.

3. The hybrid offshore wind farm black start method according to claim 1 or 2, characterized in that, The method is based on the fact that the reactive reference value is zero, the reactive output is zero and the frequency deviation of the reactive power-frequency controller is zero when the PLL is started; the PLL is exited after starting, and the deviation of the reactive power-frequency controller compensates the deviation of the PLL, and the superimposed output frequency of the set value of the wind farm enables the new unit to participate in the reactive power distribution and frequency construction. The method is based on the fact that the reactive reference value is zero, the reactive output is zero and the frequency deviation of the reactive power-frequency controller is zero when the PLL is started; the PLL is exited after starting, and the deviation of the reactive power-frequency controller compensates the deviation of the PLL, and the superimposed output frequency of the set value of the wind farm enables the new unit to participate in the reactive power distribution and frequency construction. The method is based on the fact that the reactive reference value is zero, the reactive output is zero and the frequency deviation of the reactive power-frequency controller is zero when the PLL is started; the PLL is exited after starting, and the deviation of the reactive power-frequency controller compensates the deviation of the PLL, and the superimposed output frequency of the set value of the wind farm enables the new unit to participate in the reactive power distribution and frequency construction. The method is based on the fact that the reactive reference value is zero, the reactive output is zero and the frequency deviation of the reactive power-frequency controller is zero when the PLL is started; the PLL is exited after starting, and the deviation of the reactive power-frequency controller compensates the deviation of the PLL, and the superimposed output frequency of the set value of the wind farm enables the new unit to participate in the reactive power distribution and frequency construction. The method is based on the fact that the reactive reference value is zero, the reactive output is zero and the frequency deviation of the reactive power-frequency controller is zero when the PLL is started; the PLL is exited after starting, and the deviation of the reactive power-frequency controller compensates the deviation of the PLL, and the superimposed output frequency of the set value of the wind farm enables the new unit to participate in the reactive power distribution and frequency construction. The method is based on the fact that the reactive reference value is zero, the reactive output is zero and the frequency deviation of the reactive power-frequency controller is zero when the PLL is started; the PLL is exited after starting, and the deviation of the reactive power-frequency controller compensates the deviation of the PLL, and the superimposed output frequency of the set value of the wind farm enables the new unit to participate in the reactive power distribution and frequency construction. The method is based on the fact that the reactive reference value is zero, the reactive output is zero and the frequency deviation of the reactive power-frequency controller is zero when the PLL is started; the PLL is exited after starting, and the deviation of the reactive power-frequency controller compensates the deviation of the PLL, and the superimposed output frequency of the set value of the wind farm enables the new unit to participate in the reactive power distribution and frequency construction. The method is based on the fact that the reactive reference value is zero, the reactive output is zero and the frequency deviation of the reactive power-frequency controller is zero when the PLL is started; the PLL is exited after starting, and the deviation of the reactive power-frequency controller compensates the deviation of the PLL, and the superimposed output frequency of the set value of the wind farm enables the new unit to participate in the reactive power distribution and frequency construction. ​ ​ ​ ​ 4. The hybrid offshore wind farm black start method of claim 1, wherein, The wind turbine is configured with a full-power converter to realize operation control, and the full-power converter comprises a machine-side converter and a grid-side converter, and the control mode is as follows: The machine-side converter rectifies alternating current generated by the permanent magnet synchronous generator into direct current, and the control method adopts zero d-axis current control while maintaining the constant of the direct current bus voltage. The machine-side converter control system is a voltage and current double closed loop control. The constant of the direct current bus voltage is realized through the rectification control of the machine-side converter. The grid-side converter adopts a three-layer control architecture constructed by a power controller, a voltage controller and a current controller, and realizes the control of the outlet voltage of the grid-side converter by adopting a voltage-current double closed loop system.

5. The hybrid offshore wind farm black start method of claim 4, wherein, The three-layer control architecture of the grid-side converter realizes active power-voltage amplitude control and reactive power-frequency control, and the specific control is as follows: The active power-voltage amplitude control satisfies the following calculation expression: ； wherein, , are the d-axis and q-axis components of the reference value of the AC voltage at the fan outlet, respectively, is the initial value of the d-axis voltage. , are the proportional and integral coefficients of the active control loop, is the active power output of the wind turbine, is the reference value of the active power of the wind turbine generated by the maximum power tracking control of the fan. The reactive power-frequency control of the grid-side converter satisfies the following calculation expression: ， wherein is the low frequency AC system frequency, is the proportional gain of the reactive power control loop, is the reactive power output of the wind turbine, is the reactive power reference value of the wind turbine, is the initial value of the low frequency AC system frequency; After the reactive power-frequency controller is connected, the outlet alternating current voltage frequency of the wind turbine is obtained according to the calculation expression of the reactive power-frequency, so that the frequency reference of the wind farm is established and the synchronous operation of the unit is realized under the condition of no communication. The frequency value output by the reactive power-frequency controller generates the phase angle required for coordinate transformation through an integrator, and then the frequency of the offshore alternating current system is controlled by regulating the reactive power of the wind turbine.

6. The hybrid offshore wind farm black start method of claim 1, wherein, The DRU-HVDC transmission system based on the direct-driven permanent magnet synchronous wind turbine with a full-power converter, a high-voltage low-frequency alternating current collection sea cable, a diode rectifier station, a converter transformer, a reactive power compensation and filtering device, a high-voltage direct current sea cable and a land-based modular multilevel converter station; The direct-driven permanent magnet synchronous wind turbine generates electric energy, which is transmitted to the offshore point of common coupling through the high-voltage low-frequency alternating current sea cable, then connected to the diode rectifier station to convert low-frequency alternating current into direct current, then transmitted to the shore by the long-distance direct current sea cable, and finally converted into alternating current by the land-based modular multilevel converter station and connected to the power grid. The filtering device is arranged on the alternating current bus to eliminate the characteristic harmonics generated by the diode rectifier station. The system is provided with a reactive power compensation device to suppress the capacitive effect caused by the charging power of the alternating current sea cable. The direct-driven permanent magnet synchronous wind turbine in the system includes a grid-forming mode or a grid-following mode.

7. The hybrid offshore wind farm black start method of claim 1, wherein, The direct-driven permanent magnet synchronous wind turbine in the system includes a grid-forming mode and a grid-following mode, the machine-side converter corresponding to the direct-driven permanent magnet synchronous wind turbine operates in the grid-forming mode, and the grid-side converter operates in the grid-forming mode when starting. For the DRU-HVDC transmission scene of the offshore wind farm, the direct-driven permanent magnet synchronous wind turbine is set to operate in the grid-forming mode to realize black start and system power transmission.

8. The hybrid offshore wind farm black start method of claim 7, wherein, The method controls the unit frequency of the direct-driven permanent magnet synchronous wind turbine generated by the reactive power-frequency controller or the PLL, and resets the working point outside the integrator to reduce switching disturbance.

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