Starting and power control method of hybrid converter wind power direct current sending-out grid-connected system

Abstract

The invention provides a starting and power control method for a hybrid converter wind power direct current output grid-connected system, and the method comprises the steps: firstly bypassing a DRU converter station, building a direct current voltage through the voltage reduction operation of a shore MMC converter station to charge an offshore MMC converter station, and then charging an offshore wind power plant through the passive starting capability of the offshore MMC converter station. According to the method, grid connection of partial units of the offshore wind plant is achieved, then a bypass DRU converter station is connected and started, grid connection of the remaining units is achieved, sending-end alternating-current voltage is established, power of the offshore wind plant at the sending end is output, the method is easy to implement, and distribution automatic control of active power of DRU and MMC converters can be achieved.

Description

Technical Field

[0001] This invention relates to the field of offshore wind power grid connection technology, and in particular to the start-up and power control method of a hybrid converter wind power DC transmission grid connection system. Background Technology

[0002] During the large-scale and continuous development of offshore wind power, diode-based DC transmission systems (DRU-HVDC) offer advantages such as low loss, small size, and low cost. However, diode converters lack active commutation capability and rely entirely on external AC voltage, while modular multilevel converters (MMCs) possess grid-connection and flexible control capabilities. Adopting a hybrid DRU-MMC scheme to construct a novel hybrid converter topology, complementing the technological advantages of both, and enhancing the adaptability of HVDC technology in large-scale offshore wind power grid-connection scenarios, represents one of the important development directions for future offshore wind power output technology.

[0003] Based on the configuration of the DRU and MMC on the DC side, hybrid HVDC topologies are mainly divided into parallel and series types. Among them, the parallel type involves the DRU operating in parallel with a low-power MMC on both the AC and DC sides. The MMC operating in parallel with the DRU still requires a high voltage rating, which increases the cost and the complexity of converter coordination control.

[0004] Existing research mainly focuses on steady-state control and stability analysis. However, since diode uncontrolled rectifiers only have unidirectional conduction capability, onshore power grids cannot directly start DRUs and offshore wind farms, making it difficult to achieve system black start.

[0005] The solution proposed in this invention to address the aforementioned problems includes a DRU-MMC hybrid converter with a series connection on the DC side and a parallel connection on the AC side. This allows the low-cost DRU to handle most of the power transmission, while the MMC provides a stable AC synchronous power supply for offshore wind farms, maintaining stable amplitude and frequency. This solution fully utilizes the advantages of diode devices—high current and low loss—while requiring a smaller MMC capacity, resulting in high overall system economics. Summary of the Invention

[0006] This invention proposes a startup and power control method for a hybrid converter wind power DC transmission grid-connected system, which is easy to implement and can realize automatic control of the distribution of active power between DRU and MMC converters.

[0007] The present invention adopts the following technical solution.

[0008] A startup method for a hybrid converter-based offshore wind power DC transmission grid-connected system is provided for step-down startup and active power control of the hybrid converter-based offshore wind power DC transmission grid-connected system. The method first bypasses the DRU converter station, establishes DC voltage through the step-down operation of the onshore MMC converter station to charge the offshore MMC converter station, and then utilizes the passive startup capability of the offshore MMC converter station to charge the offshore wind farm, enabling some units of the offshore wind farm to be connected to the grid. Then, the bypassed DRU converter station is connected, started, and the remaining units are connected to the grid to establish the sending-end AC voltage, and the power from the offshore wind farm is output at the sending end.

[0009] Assume the onshore MMC converter station is MMC1 and the offshore MMC converter station is MMC2; the startup method of the grid-connected system includes the following steps;

[0010] Step 1: Start up the onshore MMC converter station and supply the required rated DC voltage U to the offshore MMC converter station. DC_MMC1 ;

[0011] Step 2: The offshore MMC converter station is started up to charge the offshore wind farm;

[0012] Step 3: Start some wind turbine units for grid connection and when the DC voltage on the offshore wind farm side is greater than the DC voltage at the receiving end, turn on the DRU converter station;

[0013] Step 4: Connect the DRU converter station to the grid with all wind turbines of the offshore wind farm.

[0014] Step one is as follows: First, bypass the DRU converter station and charge the onshore converter station MMC2 using the onshore AC system. After unlocking, operate at a reduced voltage to establish a DC-side voltage equal to the rated DC voltage U of the offshore converter station MMC1. DC_MMC1 .

[0015] Step two is as follows: After the DC side voltage of the offshore converter station MMC1 is stabilized, the sub-modules of the offshore converter station MMC1 are charged; after charging is completed, the offshore converter station MMC1 is unlocked and operates in passive operation mode to establish the offshore AC system voltage for charging the offshore wind farm.

[0016] Step 3 specifically involves the following: If the offshore MMC1 converter station can only start some wind turbine units, the startup logic is as follows: The grid-side converter GSC of the wind turbine units establishes the DC-side voltage of the wind turbine units, and the stator-side switch BRK detects the no-load voltage U of the turbine-side converter. MSC With the stator voltage U of the motor stator After the amplitude and phase meet the closing criteria, the circuit is closed, the stator-side switch BRK is closed, and the wind turbine is connected to the grid for power generation.

[0017] The closing criterion is expressed by the following formula:

[0018]

[0019] In the formula U on δ on These are the machine-side converter MSC voltages U MSC Stator side voltage U stator The start-up thresholds for amplitude difference and phase angle difference.

[0020] When the AC voltage on the offshore wind farm side exceeds the threshold range, and the DC voltage on the sending end of the offshore wind farm side is greater than the DC voltage on the receiving end, the DRU converter station is turned on.

[0021] Step four specifically involves: After the DRU converter station is connected to the grid, the voltage of the onshore converter station MMC2 is boosted to its required rated DC voltage U. DC_MMC2 After the DRU starts transmitting power, all remaining wind turbines are connected to the grid, realizing the power transmission of the entire offshore wind farm.

[0022] When the aforementioned startup method uses the sending-end DRU, sending-end MMC, and receiving-end MMC control to achieve large-scale offshore wind power transmission, there is no need to configure black-start energy storage devices or backup AC return lines, and the control and grid connection strategies of the wind turbine units in the offshore wind farm do not need to be modified.

[0023] The grid-connected system includes wind turbines for offshore wind farms, offshore DRU-MMC hybrid converters, DC transmission cables, and onshore MMC2 converter stations.

[0024] The DRU-MMC hybrid converter consists of a DRU connected in series on the DC side and an MMC1 connected in parallel on the AC side. The DRU is a low-cost DRU with a twelve-pulse structure that reduces harmonics.

[0025] When the grid-connected system is working, the low-cost DRU is used to handle a large proportion of power transmission, while the MMC1 is responsible for providing the offshore wind farm with stable AC synchronous power supply in terms of amplitude and frequency and a small proportion of power transmission.

[0026] The grid-connected system also includes circuit breakers QF1, QF2, QF3, QF4, and QF5, as well as filters and disconnect switches QS1, QS2, and QS3;

[0027] The MMC1 is equipped with disconnector QS3 and circuit breaker QF1; the DCU is equipped with disconnector QS1, QS2 and circuit breaker QF2; the wind turbine units of the wind farm are equipped with circuit breakers QF3 and QF4; and the filter is equipped with circuit breaker QF5.

[0028] MMC2 is the output terminal of the grid-connected system, which is connected to the onshore high-voltage AC power grid.

[0029] A power control method for a hybrid converter wind power DC transmission grid-connected system is used for active power control after startup of the hybrid converter offshore wind power DC transmission grid-connected system. The controlled active power includes the active power of the onshore converter station MMC2, the active power of the offshore converter station MMC1, and the active power of the DRU converter station. In the grid-connected system, the active power control of the grid-connected system depends on the active power control of the onshore MMC converter station and the offshore MMC converter station.

[0030] The diodes in the DRU converter station do not have active commutation capability and lack active control. When the grid-connected system is in stable operating condition, the DRU converter station and the offshore MMC1 converter station in the grid-connected system operate in series. The relationship between voltage, current, and active power is expressed by the following formula:

[0031]

[0032] The onshore converter station MMC2 uses constant DC voltage control to maintain the DC submarine cable voltage U. DC_MMC2 Maintaining a constant value, the MMC1 of the offshore converter station controls the amplitude U of the offshore AC voltage. AC_PCC To maintain a constant active power, when the active power of the offshore wind turbine generator changes, the active power of the DRU-MMC hybrid converter is adjusted according to the DC voltage U of the DRU converter station. DC_DRU DC voltage U of the offshore MMC1 converter station DC_MMC1 The ratio k automatically allocates changes in active power.

[0033] This invention provides a step-down start-up and active power control method for a hybrid converter-based offshore wind power DC transmission and grid connection system. First, a DC voltage is established through step-down operation of the onshore MMC (Multi-Mechanical Control Unit). Then, the black-start capability of the offshore MMC is utilized to establish the AC voltage at the sending end. Compared to a pure DRU (Dual-Run Unit) sending end, this step-down start-up method eliminates the need for black-start energy storage devices or backup AC return lines. Simultaneously, the control and grid connection strategies for the wind turbines do not require modification. Large-scale offshore wind power transmission is achieved through coordinated control of the sending-end DRU, MMC, and receiving-end MMC. Regarding active power control of the hybrid converter, automatic control of the active power distribution between the DRU and MMC converters is achieved without requiring an active power coordination control strategy.

[0034] The present invention proposes a DRU-MMC hybrid converter with a series connection on the DC side and a parallel connection on the AC side. This allows the low-cost DRU to handle most of the power transmission, while the MMC provides a stable AC synchronous power supply for offshore wind farms. This design fully utilizes the advantages of diode devices—high current and low loss—while requiring a smaller MMC capacity, resulting in high overall system economics.

[0035] Compared with existing technologies, this application presents a method for step-down startup and active power control of a hybrid converter-based offshore wind power DC transmission grid-connected system. First, a DC voltage is established through step-down operation of the onshore MMC (Multi-Mechanical Control Unit). Then, the black-start capability of the offshore MMC is utilized to establish the AC voltage at the sending end. Compared to a pure DRU (Dual-Run Unit) sending end, this startup method does not require a black-start energy storage device or a backup AC return line. Simultaneously, the control and grid connection strategies of the wind turbines do not need modification. Large-scale offshore wind power transmission is achieved through the coordinated control of the sending-end DRU, MMC, and receiving-end MMC. This application enables black-start and stable power control of a hybrid converter-based offshore wind power DC transmission grid-connected system without requiring an active power coordination control strategy. It is highly efficient, simple, and easily implementable, improving the stability and operational reliability of the entire hybrid converter, and possesses high practical value and economic benefits. Attached Figure Description

[0036] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments:

[0037] Appendix Figure 1 This is a schematic diagram of the grid-connected system in the present invention.

[0038] Appendix Figure 2 This is a schematic diagram of the simulation results of DC voltage in an embodiment of the present invention;

[0039] Appendix Figure 3 This is a schematic diagram of the simulation results of active power in an embodiment of the present invention. Detailed Implementation

[0040] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the scope of protection of this application.

[0041] As shown in the figure, the startup method of the hybrid converter wind power DC transmission grid-connected system is used for the step-down startup and active power control of the hybrid converter offshore wind power DC transmission grid-connected system. The method first bypasses the DRU converter station, establishes DC voltage through the step-down operation of the onshore MMC converter station to charge the offshore MMC converter station, and then uses the passive startup capability of the offshore MMC converter station to charge the offshore wind farm, realizing the grid connection of some units of the offshore wind farm. Then, the bypassed DRU converter station is connected, started and realizes the grid connection of the remaining units to establish the sending-end AC voltage, and outputs the power of the offshore wind farm at the sending end.

[0042] Assume the onshore MMC converter station is MMC1 and the offshore MMC converter station is MMC2; the startup method of the grid-connected system includes the following steps;

[0043] Step 1: Start up the onshore MMC converter station and supply the required rated DC voltage U to the offshore MMC converter station. DC_MMC1 ;

[0044] Step 2: The offshore MMC converter station is started up to charge the offshore wind farm;

[0045] Step 3: Start some wind turbine units for grid connection and when the DC voltage on the offshore wind farm side is greater than the DC voltage at the receiving end, turn on the DRU converter station;

[0046] Step 4: Connect the DRU converter station to the grid with all wind turbines of the offshore wind farm.

[0047] Step one is as follows: First, bypass the DRU converter station and charge the onshore converter station MMC2 using the onshore AC system. After unlocking, operate at a reduced voltage to establish a DC-side voltage equal to the rated DC voltage U of the offshore converter station MMC1. DC_MMC1 .

[0048] Step two is as follows: After the DC side voltage of the offshore converter station MMC1 is stabilized, the sub-modules of the offshore converter station MMC1 are charged; after charging is completed, the offshore converter station MMC1 is unlocked and operates in passive operation mode to establish the offshore AC system voltage for charging the offshore wind farm.

[0049] Step 3 specifically involves the following: If the offshore MMC1 converter station can only start some wind turbine units, the startup logic is as follows: The grid-side converter GSC of the wind turbine units establishes the DC-side voltage of the wind turbine units, and the stator-side switch BRK detects the no-load voltage U of the turbine-side converter. MSC With the stator voltage U of the motor stator After the amplitude and phase meet the closing criteria, the circuit is closed, the stator-side switch BRK is closed, and the wind turbine is connected to the grid for power generation.

[0050] The closing criterion is expressed by the following formula:

[0051]

[0052] In the formula U on δ on These are the machine-side converter MSC voltages U MSC Stator side voltage U stator The start-up thresholds for amplitude difference and phase angle difference.

[0053] When the AC voltage on the offshore wind farm side exceeds the threshold range, and the DC voltage on the sending end of the offshore wind farm side is greater than the DC voltage on the receiving end, the DRU converter station is turned on.

[0054] Step four specifically involves: After the DRU converter station is connected to the grid, the voltage of the onshore converter station MMC2 is boosted to its required rated DC voltage U. DC_MMC2 After the DRU starts transmitting power, all remaining wind turbines are connected to the grid, realizing the power transmission of the entire offshore wind farm.

[0055] When the aforementioned startup method uses the sending-end DRU, sending-end MMC, and receiving-end MMC control to achieve large-scale offshore wind power transmission, there is no need to configure black-start energy storage devices or backup AC return lines, and the control and grid connection strategies of the wind turbine units in the offshore wind farm do not need to be modified.

[0056] The grid-connected system includes wind turbines for offshore wind farms, offshore DRU-MMC hybrid converters, DC transmission cables, and onshore MMC2 converter stations.

[0057] The DRU-MMC hybrid converter consists of a DRU connected in series on the DC side and an MMC1 connected in parallel on the AC side. The DRU is a low-cost DRU with a twelve-pulse structure that reduces harmonics.

[0058] When the grid-connected system is working, the low-cost DRU is used to handle a large proportion of power transmission, while the MMC1 is responsible for providing the offshore wind farm with stable AC synchronous power supply in terms of amplitude and frequency and a small proportion of power transmission.

[0059] The grid-connected system also includes circuit breakers QF1, QF2, QF3, QF4, and QF5, as well as filters and disconnect switches QS1, QS2, and QS3;

[0060] The MMC1 is equipped with disconnector QS3 and circuit breaker QF1; the DCU is equipped with disconnector QS1, QS2 and circuit breaker QF2; the wind turbine units of the wind farm are equipped with circuit breakers QF3 and QF4; and the filter is equipped with circuit breaker QF5.

[0061] MMC2 is the output terminal of the grid-connected system, which is connected to the onshore high-voltage AC power grid.

[0062] A power control method for a hybrid converter wind power DC transmission grid-connected system is used for active power control after startup of the hybrid converter offshore wind power DC transmission grid-connected system. The controlled active power includes the active power of the onshore converter station MMC2, the active power of the offshore converter station MMC1, and the active power of the DRU converter station. In the grid-connected system, the active power control of the grid-connected system depends on the active power control of the onshore MMC converter station and the offshore MMC converter station.

[0063] The diodes in the DRU converter station do not have active commutation capability and lack active control. When the grid-connected system is in stable operating condition, the DRU converter station and the offshore MMC1 converter station in the grid-connected system operate in series. The relationship between voltage, current, and active power is expressed by the following formula:

[0064]

[0065] The onshore converter station MMC2 uses constant DC voltage control to maintain the DC submarine cable voltage U. DC_MMC2 Maintaining a constant value, the MMC1 of the offshore converter station controls the amplitude U of the offshore AC voltage. AC_PCC To maintain a constant active power, when the active power of the offshore wind turbine generator changes, the active power of the DRU-MMC hybrid converter is adjusted according to the DC voltage U of the DRU converter station. DC_DRU DC voltage U of the offshore MMC1 converter station DC_MMC1 The ratio k automatically allocates changes in active power.

[0066] Example:

[0067] The embodiments of this application are based on a hybrid converter offshore wind power DC transmission and grid connection system, which is based on DRU-MMC hybrid converter offshore wind power DC grid connection technology.

[0068] like Figure 1 As shown, it consists of a wind farm, wind turbine generators, offshore DRU-MMC hybrid converters, DC transmission cables, onshore MMC2 converter station, circuit breakers QF1-QF5, disconnecting switches QS1-QS3, filters, etc.

[0069] The DRU-MMC hybrid converter consists of a DRU connected in series on the DC side and an MMC1 connected in parallel on the AC side. The low-cost DRU is used to handle most of the power transmission, while the MMC1 is responsible for providing stable AC synchronous power supply with stable amplitude and frequency for offshore wind farms and a small portion of the power transmission.

[0070] The buck operation startup method provided in this application embodiment includes the following steps:

[0071] S1: Bypass the DRU converter station; the onshore MMC2 converter station operates at reduced voltage to establish DC voltage. Close QS1 and QS3, open QS2 to bypass the DRU converter station. The onshore MMC2 converter station starts operating at reduced voltage, establishing a DC side voltage equal to the rated DC voltage U of the offshore MMC1 converter station. DC_MMC1 ;

[0072] S2: Establish the offshore AC voltage. After the DC side voltage stabilizes, the offshore MMC1 converter station submodule charges. After charging is complete, it unlocks and operates in passive mode. QF1 is closed to establish the offshore AC system voltage U. AC_PCC ;

[0073] S3: DRU converter station connected to the system and charging of the wind farm. Disconnect QS1 and close QS2 to connect the DRU converter station to the system. Close QF3, QF4, and QF5 to charge the offshore wind farm using the offshore AC system voltage established by the offshore MMC1 converter station;

[0074] S4: Partial grid-connected operation of wind turbines. The grid-side converter (GSC) of the wind turbine establishes the DC-side voltage of the wind turbine, and detects the no-load voltage U of the turbine-side converter (MSC). MSC With the stator voltage U of the motor stator After the amplitude and phase meet the synchronization closing criteria, the stator-side switch BRK is closed, and the wind turbine is connected to the grid. The closing criteria are as follows:

[0075]

[0076] In the formula U on δ on These are the machine-side converter MSC voltages U MSC Stator side voltage U stator The start-up thresholds for amplitude difference and phase angle difference.

[0077] S5: DRU converter station startup. When the AC voltage on the offshore wind farm side exceeds the threshold range, and the DC voltage on the offshore wind farm side is greater than the receiving-end DC voltage, the diode conducts in the forward direction.

[0078] S6: Wind turbine grid connection complete. Close QF2, DRU converter station connected to grid, and step up the voltage of onshore MMC2 converter station to rated DC voltage U. DC_MMC2 After the DRU begins transmitting power, all remaining wind turbines are connected to the grid, enabling the entire offshore wind farm to transmit electricity.

[0079] like Figure 2 , Figure 3 As shown in Table 1, the simulation verification results of this embodiment are presented. The simulation parameters are also shown in Table 1. The simulation results show that initially, the onshore MMC2 converter station steps down from its rated voltage of 1000kV to 250kV to establish the rated DC voltage for the offshore MMC1. After the offshore MMC1 is unlocked, it operates in passive mode, establishing the AC voltage for the wind farm. At 1.0s, some wind turbines begin to connect to the grid, and the system operates in MMC-HVDC mode to transmit power. At 5s, the DRU converter station connects to the offshore AC grid, subsequently connecting all offshore wind farm turbines to the grid, realizing the power transmission of the entire offshore wind farm.

[0080] The DRU converter station transmits 1500MW of active power, and the offshore MMC1 converter station transmits 500MW of active power. The DC voltage of the DRU converter station is U. DC_DRU DC voltage U of the offshore MMC1 converter station DC_MMC1The ratio k is 3:1. Simulation results verify that the active power of the DRU-MMC hybrid converter is based on the DC voltage U of the DRU converter station. DC_DRU DC voltage U of the offshore MMC1 converter station DC_MMC1 The ratio k was used for allocation. This verified the correctness and effectiveness of the method proposed in this patent application.

[0081] Table 1

[0082] Parameter name DRU MMC1 MMC2 Rated active power / MW 1500 500 2000 Rated DC voltage / kV 750 250 1000 AC voltage / kV 66 132 525 Converter transformer turns ratio 66 / 400 66 / 132 525 / 521

[0083] The above provides a detailed description of a method for step-down startup and active power control of a hybrid converter offshore wind power DC transmission grid-connected system provided in the embodiments of this application. The principles and implementation methods of this application have been explained. The description of the above embodiments is only for the purpose of helping to understand the technical solutions and core ideas of this application. Those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. These modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application.

Claims

1. A startup method for a hybrid converter-based wind power DC transmission grid-connected system, used for step-down startup and active power control of a hybrid converter-based offshore wind power DC transmission grid-connected system, characterized in that: The method first bypasses the DRU converter station, establishes DC voltage through the onshore MMC converter station's step-down operation to charge the offshore MMC converter station, and then utilizes the passive start-up capability of the offshore MMC converter station to charge the offshore wind farm, enabling some units of the offshore wind farm to be connected to the grid. Then, the bypassed DRU converter station is connected, started, and the remaining units are connected to the grid to establish the sending-end AC voltage, and the power from the offshore wind farm at the sending end is output.

2. The startup method of the hybrid converter wind power DC transmission grid-connected system according to claim 1, characterized in that: The onshore MMC converter station is designated as MMC1, and the offshore MMC converter station is designated as MMC2. The startup method of the grid-connected system includes the following steps. Step 1: Start up the onshore MMC converter station and supply the required rated DC voltage U to the offshore MMC converter station. DC_MMC1 ; Step 2: The offshore MMC converter station is started up to charge the offshore wind farm; Step 3: Start some wind turbine units for grid connection and when the DC voltage on the offshore wind farm side is greater than the DC voltage at the receiving end, turn on the DRU converter station; Step 4: Connect the DRU converter station to the grid with all wind turbines of the offshore wind farm.

3. The startup method of the hybrid converter wind power DC transmission grid-connected system according to claim 2, characterized in that: Step one is as follows: First, bypass the DRU converter station and charge the onshore converter station MMC2 using the onshore AC system. After unlocking, operate at a reduced voltage to establish a DC-side voltage equal to the rated DC voltage U of the offshore converter station MMC1. DC_MMC1 .

4. The startup method of the hybrid converter wind power DC transmission grid-connected system according to claim 2, characterized in that: Step two is as follows: After the DC side voltage of the offshore converter station MMC1 is stabilized, the sub-modules of the offshore converter station MMC1 are charged; after charging is completed, the offshore converter station MMC1 is unlocked and operates in passive operation mode to establish the offshore AC system voltage for charging the offshore wind farm.

5. The startup method of the hybrid converter wind power DC transmission grid-connected system according to claim 2, characterized in that: Step 3 specifically involves the following: If the offshore MMC1 converter station can only start some wind turbine units, the startup logic is as follows: The grid-side converter GSC of the wind turbine units establishes the DC-side voltage of the wind turbine units, and the stator-side switch BRK detects the no-load voltage U of the turbine-side converter. MSC With the stator voltage U of the motor stator After the amplitude and phase meet the closing criteria, the circuit is closed, the stator-side switch BRK is closed, and the wind turbine is connected to the grid for power generation. The closing criterion is expressed by the following formula: In the formula U on δ on These are the machine-side converter MSC voltages U MSC Stator side voltage U stator The start-up thresholds for amplitude difference and phase angle difference. When the AC voltage on the offshore wind farm side exceeds the threshold range, and the DC voltage on the sending end of the offshore wind farm side is greater than the DC voltage on the receiving end, the DRU converter station is turned on.

6. The startup method of the hybrid converter wind power DC transmission grid-connected system according to claim 2, characterized in that: Step four specifically involves: After the DRU converter station is connected to the grid, the voltage of the onshore converter station MMC2 is boosted to its required rated DC voltage U. DC_MMC2 After the DRU starts transmitting power, all remaining wind turbines are connected to the grid, realizing the power transmission of the entire offshore wind farm.

7. The startup method of the hybrid converter wind power DC transmission grid-connected system according to claim 2, characterized in that: When the aforementioned startup method uses the sending-end DRU, sending-end MMC, and receiving-end MMC control to achieve large-scale offshore wind power transmission, there is no need to configure black-start energy storage devices or backup AC return lines, and the control and grid connection strategies of the wind turbine units in the offshore wind farm do not need to be modified.

8. The startup method of the hybrid converter wind power DC transmission grid-connected system according to claim 2, characterized in that: The grid-connected system includes wind turbines for offshore wind farms, offshore DRU-MMC hybrid converters, DC transmission cables, and onshore MMC2 converter stations; The DRU-MMC hybrid converter consists of a DRU connected in series on the DC side and an MMC1 connected in parallel on the AC side. The DRU is a low-cost DRU with a twelve-pulse structure that reduces harmonics. When the grid-connected system is working, the low-cost DRU is used to handle a large proportion of power transmission, while the MMC1 is responsible for providing the offshore wind farm with stable AC synchronous power supply in terms of amplitude and frequency and a small proportion of power transmission. The grid-connected system also includes circuit breakers QF1, QF2, QF3, QF4, and QF5, as well as filters and disconnecting switches QS1, QS2, and QS3; The MMC1 is equipped with disconnector QS3 and circuit breaker QF1; the DCU is equipped with disconnector QS1, QS2 and circuit breaker QF2; the wind turbine units of the wind farm are equipped with circuit breakers QF3 and QF4; the filter is equipped with circuit breaker QF5. MMC2 is the output terminal of the grid-connected system, which is connected to the onshore high-voltage AC power grid.

9. A power control method for a hybrid converter wind power DC transmission grid-connected system, used for active power control after startup of the hybrid converter offshore wind power DC transmission grid-connected system as described in claim 1, characterized in that: The active power it controls includes the active power of the onshore converter station MMC2, the active power of the offshore converter station MMC1, and the active power of the DRU converter station; in the grid-connected system, the active power control of the grid-connected system depends on the active power control of the onshore MMC converter station and the offshore MMC converter station.

10. The power control method for a hybrid converter wind power DC transmission grid-connected system according to claim 9, characterized in that: The diodes in the DRU converter station do not have active commutation capability and lack active control. When the grid-connected system is in stable operating condition, the DRU converter station and the offshore MMC1 converter station in the grid-connected system operate in series. The relationship between voltage, current, and active power is expressed by the following formula: The onshore converter station MMC2 uses constant DC voltage control to maintain the DC submarine cable voltage U. DC_MMC2 Maintaining a constant value, the MMC1 of the offshore converter station controls the amplitude U of the offshore AC voltage. AC_PCC To maintain a constant active power, when the active power of the offshore wind turbine generator changes, the active power of the DRU-MMC hybrid converter is adjusted according to the DC voltage U of the DRU converter station. DC_DRU DC voltage U of the offshore MMC1 converter station DC_MMC1 The ratio k automatically allocates changes in active power.

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