Offshore wind power bidirectional asymmetric feeding type dc transmission system and control method thereof

Abstract

The application relates to the technical field of offshore wind power DC transmission, and discloses an offshore wind power bidirectional asymmetric feeding type DC transmission system and a control method thereof, which comprises a first conversion module, a second conversion module, a third conversion module, a switch module and an onshore conversion station, wherein the internal current flow path of the switch module is controlled, the third conversion module is used to establish an offshore collection voltage during black start, and after the black start is completed, the first conversion module and the second conversion module are connected to the system, so that the problem that an offshore wind farm cannot be black started is solved.

Description

Technical Field

[0001] This invention relates to the field of offshore wind power DC transmission technology, specifically to an offshore wind power bidirectional asymmetrical feeding DC transmission system and its control method. Background Technology

[0002] Long-distance offshore wind power transmission typically employs high-voltage direct current (HVDC) transmission technology. Offshore wind turbines generate electricity, which is then collected via AC cables. Offshore converter stations convert this AC power into DC power, which is then transmitted to onshore converter stations via DC submarine cables. Currently, the converter valves in offshore wind power HVDC transmission projects all utilize the MMC (Multi-Mechanical Controlled) structure. However, MMC converter valves are expensive and bulky, thus requiring further optimization for large-scale offshore wind power transmission schemes. To adapt to various application scenarios and reduce transmission project costs, various collection and networking transmission schemes with distinct characteristics have been proposed both domestically and internationally. These include various collection and transmission topologies and networking schemes, such as those using diode-based unidirectional current-type uncontrolled rectifier converters at the sending end. While using diode-based unidirectional current-type uncontrolled rectifier converters at the sending end helps achieve lightweight offshore platforms, existing schemes have the significant drawback of being unable to black-start offshore wind farms. Summary of the Invention

[0003] In view of this, the present invention provides a bidirectional asymmetrical feeder DC transmission system for offshore wind power and its control method to solve the problem of offshore wind farms being unable to start in black.

[0004] In a first aspect, the present invention provides a bidirectional asymmetrical feeder DC transmission system for offshore wind power, comprising: a first converter module, a second converter module, a third converter module, a switch module, and an onshore converter station, wherein the wind turbines of the offshore wind farm are fed to an offshore busbar via medium-voltage AC cables; the AC side of the first converter module is connected to the offshore busbar, the upper end of the DC side of the first converter module is connected to an upper DC submarine cable, and the lower end of the DC side of the first converter module is connected to the upper end of the DC side of the second converter module; the AC side of the second converter module... The second converter module is connected to the offshore busbar. The lower DC side of the second converter module is connected to the first terminal of the switch module. The AC side of the third converter module is connected to the offshore busbar. The upper DC side of the third converter module is connected to the second terminal of the switch module. The lower DC side of the third converter module is connected to the third terminal of the switch module. The fourth terminal of the switch module is connected to the lower DC submarine cable. The upper DC side of the onshore converter station is connected to the upper DC submarine cable. The lower DC side of the onshore converter station is connected to the lower DC submarine cable. The AC side of the onshore converter station is connected to the power grid.

[0005] In this invention, by controlling the current flow path inside the switching module, the third converter module is used to establish the offshore collection voltage during black start. After the black start is completed, the first and second converter modules are then connected to the system, thereby solving the problem of offshore wind farms being unable to black start.

[0006] In one optional implementation, the first converter module includes: a first transformer, a first controllable switch, and a first converter, wherein the primary side of the first transformer is connected to the marine busbar, and the secondary side of the first transformer is connected to the AC side of the first converter through the first controllable switch; the upper end of the DC side of the first converter is connected to the upper DC submarine cable, and the lower end of the DC side of the first converter is connected to the upper end of the DC side of the second converter module.

[0007] In one optional implementation, the second converter module includes: a second transformer, a second controllable switch, and a second converter, wherein the primary side of the second transformer is connected to the offshore busbar, and the secondary side of the second transformer is connected to the AC side of the second converter through the second controllable switch; the upper end of the DC side of the second converter is connected to the lower end of the DC side of the first converter, and the lower end of the DC side of the second converter is connected to the first end of the switch module.

[0008] In one optional implementation, the third converter module includes: a third transformer and a third converter, wherein the primary side of the third transformer is connected to the offshore busbar, and the secondary side of the third transformer is connected to the AC side of the third converter; the upper end of the DC side of the third converter is connected to the second end of the switch module, and the lower end of the DC side of the third converter is connected to the third end of the switch module.

[0009] In one alternative implementation, the first and second converters are both 12-pulse rectifier diode valves; the third converter is a half-bridge MMC converter valve.

[0010] This invention employs a diode valve connected in series with a MMC on the DC side, which enables negative pressure start-up of the MMC in the offshore converter station, thereby achieving black start-up of the offshore wind farm.

[0011] The MMC used in this invention is a half-bridge structure, which has a greater cost advantage compared to the existing full-bridge MMC solution.

[0012] In one optional embodiment, the switch module includes: a third controllable switch, a fourth controllable switch, and a fifth controllable switch, wherein the first end of the third controllable switch is connected to the lower end of the DC side of the second converter, and the second end of the third controllable switch is connected to the lower DC submarine cable; the first end of the fourth controllable switch is connected to the upper end of the DC side of the third converter, and the second end of the fourth controllable switch is connected to the first end of the third controllable switch; the first end of the fifth controllable switch is connected to the lower end of the DC side of the third converter, and the second end of the fifth controllable switch is connected to the second end of the third controllable switch.

[0013] In one optional embodiment, the onshore converter station includes: a full-bridge and half-bridge hybrid MMC converter valve and a fourth transformer, wherein the upper end of the DC side of the full-bridge and half-bridge hybrid MMC converter valve is connected to an upper DC submarine cable, the lower end of the DC side of the full-bridge and half-bridge hybrid MMC converter valve is connected to a lower DC submarine cable, the AC side of the full-bridge and half-bridge hybrid MMC converter valve is connected to the primary side of the fourth transformer; and the secondary side of the fourth transformer is connected to the power grid.

[0014] In one alternative implementation, the offshore wind power bidirectional asymmetric feeder DC transmission system further includes: installing a filter on the offshore combiner bus.

[0015] Secondly, the present invention provides a control method for an offshore wind power bidirectional asymmetrical feeder DC transmission system, based on an optional embodiment of the first aspect. The method includes: unlocking and starting the onshore converter station; closing the fourth and fifth controllable switches, unlocking the third converter, and using open-loop control to gradually increase the AC side voltage of the third converter from zero to the rated value; gradually starting the offshore wind turbine under no-load after the offshore collected voltage is established; gradually increasing the active power output of the offshore wind turbine after the no-load start-up is completed; disconnecting the fourth and fifth controllable switches when the current flowing through the fourth and fifth controllable switches is zero, and simultaneously closing the third controllable switch; closing the first and second controllable switches as the onshore converter station gradually increases its DC side voltage to the rated value; and gradually increasing the output power of the offshore wind farm until the offshore wind farm can output the maximum value.

[0016] Thirdly, the present invention provides a control device for a bidirectional asymmetrical feeder DC transmission system for offshore wind power, comprising: a first control module for unlocking and starting the onshore converter station; a second control module for closing the fourth and fifth controllable switches, unlocking the third converter, and using open-loop control to gradually increase the AC side voltage of the third converter from zero to the rated value; a third control module for gradually starting the offshore wind turbine under no-load after the offshore collected voltage is established; a fourth control module for gradually increasing the active power output of the offshore wind turbine after the offshore wind turbine has completed no-load start-up; a fifth control module for disconnecting the fourth and fifth controllable switches and simultaneously closing the third controllable switch when the current flowing through the fourth and fifth controllable switches is zero; a sixth control module for closing the first and second controllable switches as the onshore converter station gradually increases its DC side voltage to the rated value; and a seventh control module for gradually increasing the output power of the offshore wind farm until the offshore wind farm can output the maximum value. Attached Figure Description

[0017] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0018] Figure 1 This is a composition diagram of an offshore wind power bidirectional asymmetric feeding DC transmission system according to an embodiment of the present invention;

[0019] Figure 2 This is a composition diagram of another offshore wind power bidirectional asymmetric feeding DC transmission system according to an embodiment of the present invention;

[0020] Figure 3 This is a flowchart illustrating the control method of a bidirectional asymmetric feeder DC transmission system for offshore wind power according to an embodiment of the present invention. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0022] In the description of this invention, it should be noted that the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0023] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can also refer to the internal connection of two components; and they can refer to a wireless connection or a wired connection. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0024] Furthermore, the technical features involved in the different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0025] In related technical solutions, although using a diode-based unidirectional current-mode uncontrolled converter at the sending end helps to achieve lightweight offshore platforms, it has the following disadvantages:

[0026] (1) Pure diode check valves cannot transfer power from land to sea, making black start of offshore wind farms difficult;

[0027] (2) In the scheme of connecting the diode valve in series with the MMC on the DC side, when transmitting offshore wind power, the DC side current needs to flow through the diode valve and the MMC at the same time, which means that the power device of the MMC needs to be selected with a larger overcurrent capacity and capacitance value.

[0028] (3) For the DC side MMC of the diode valve in series, in order to have the ability to start under negative pressure, the marine MMC needs to use a full bridge structure, which is more expensive than the half bridge structure.

[0029] (4) The scheme of connecting the DC side of the diode valve in series with the MMC generally requires closed-loop control of the AC side voltage or the DC side voltage of the MMC, and the control algorithm is relatively complex.

[0030] Based on the above problems, this embodiment provides a bidirectional asymmetric feeder DC transmission system for offshore wind power, such as... Figure 1 As shown, it includes: a first converter module 1, a second converter module 2, a third converter module 3, a switch module 4, and an onshore converter station 5.

[0031] like Figure 1 As shown, the wind turbines of the offshore wind farm are connected to the offshore busbar via medium-voltage AC cables; the AC side of the first converter module 1 is connected to the offshore busbar, the upper end of the DC side of the first converter module 1 is connected to the upper DC submarine cable, and the lower end of the DC side of the first converter module 1 is connected to the upper end of the DC side of the second converter module 2; the AC side of the second converter module 2 is connected to the offshore busbar, and the lower end of the DC side of the second converter module 2 is connected to the first end of the switch module 4; the AC side of the third converter module 3 is connected to the offshore busbar, the upper end of the DC side of the third converter module 3 is connected to the second end of the switch module 4, and the lower end of the DC side of the third converter module 3 is connected to the third end of the switch module 4; the fourth end of the switch module 4 is connected to the lower DC submarine cable; the upper end of the DC side of the onshore converter station 5 is connected to the upper DC submarine cable, the lower end of the DC side of the onshore converter station 5 is connected to the lower DC submarine cable, and the AC side of the onshore converter station 5 is connected to the power grid.

[0032] Specifically, during normal operation of the wind farm, the first converter module 1 and the second converter module 2 are put into the system by controlling the internal current path of the switch module 4, while the third converter module 3 is switched out. In order to achieve black start, the third converter module 3 is put into the system first by controlling the internal current path of the switch module 4. When the start-up conditions are met, the third converter module 3 is switched out, so that the first converter module 1 and the second converter module 2 are put into the system.

[0033] In some alternative implementations, such as Figure 2 As shown, the first converter module 1 includes: a first transformer T1, a first controllable switch K1, and a first converter 11. The primary side of the first transformer T1 is connected to the marine busbar, and the secondary side of the first transformer T1 is connected to the AC side of the first converter through the first controllable switch K1. The upper end of the DC side of the first converter is connected to the upper DC submarine cable, and the lower end of the DC side of the first converter is connected to the upper end of the DC side of the second converter module 2 (i.e., the second converter 21).

[0034] In some alternative implementations, such as Figure 2 As shown, the second converter module 2 includes: a second transformer T2, a second controllable switch K2, and a second converter 21. The primary side of the second transformer T2 is connected to the marine busbar, and the secondary side of the second transformer T2 is connected to the AC side of the second converter 21 through the second controllable switch K2. The upper end of the DC side of the second converter 21 is connected to the lower end of the DC side of the first converter, and the lower end of the DC side of the second converter 21 is connected to the first end of the switch module 4 (i.e., the first end of the third controllable switch K3).

[0035] In some alternative implementations, such as Figure 2 As shown, the third converter module 3 includes: a third transformer T3 and a third converter 31. The primary side of the third transformer T3 is connected to the marine busbar, and the secondary side of the third transformer T3 is connected to the AC side of the third converter 31. The upper end of the DC side of the third converter 31 is connected to the second end of the switch module 4 (i.e., the first end of the fourth controllable switch K4), and the lower end of the DC side of the third converter 31 is connected to the third end of the switch module 4 (i.e., the first end of the fifth controllable switch K5).

[0036] In some alternative implementations, the first converter and the second converter 21 are both 12-pulse rectifier diode valves; the third converter 31 is a half-bridge MMC converter valve, but this is only an example and is not a limitation.

[0037] In some alternative implementations, such as Figure 2As shown, the switch module 4 includes: a third controllable switch K3, a fourth controllable switch K4, and a fifth controllable switch K5. The first end of the third controllable switch K3 is connected to the lower DC side of the second converter 21, and the second end of the third controllable switch K3 is connected to the lower DC submarine cable. The first end of the fourth controllable switch K4 is connected to the upper DC side of the third converter 31, and the second end of the fourth controllable switch K4 is connected to the first end of the third controllable switch K3. The first end of the fifth controllable switch is connected to the lower DC side of the third converter 31, and the second end of the fifth controllable switch is connected to the second end of the third controllable switch K3.

[0038] Specifically, when the third controllable switch K3 is closed and the fourth and fifth controllable switches are open, the first and second converters 21 are connected to the system and the third converter 31 is disconnected. When the third, fourth, and fifth controllable switches K3, the third converter 31 is connected to the system and the first and second converters 21 are disconnected.

[0039] In some alternative implementations, such as Figure 2 As shown, the onshore converter station 5 includes: a full-bridge and half-bridge hybrid MMC converter valve 41 and a fourth transformer T4. The upper end of the DC side of the full-bridge and half-bridge hybrid MMC converter valve is connected to the upper DC submarine cable, the lower end of the DC side of the full-bridge and half-bridge hybrid MMC converter valve is connected to the lower DC submarine cable, and the AC side of the full-bridge and half-bridge hybrid MMC converter valve is connected to the primary side of the fourth transformer. The secondary side of the fourth transformer is connected to the power grid.

[0040] In some alternative implementations, such as Figure 2 As shown, the offshore wind power bidirectional asymmetric feeding DC transmission system also includes: installing a filter on the offshore combiner bus.

[0041] This embodiment provides a control method for an offshore wind power bidirectional asymmetrical feeder DC transmission system, based on the optional implementation methods of the above embodiments, such as... Figure 3 As shown, the method includes:

[0042] Step S1: Unlock and start the onshore converter station 5.

[0043] Specifically, the full-bridge and half-bridge hybrid MMC converter valve is unlocked and started, and its operation can be controlled according to a preset control method. At this time, the full-bridge and half-bridge hybrid MMC converter valve rectifies the voltage of the power grid into DC power to the DC submarine cable.

[0044] Step S2: Close the fourth controllable switch K4 and the fifth controllable switch to unlock the third converter 31, and use open-loop control to gradually increase the AC side voltage of the third converter 31 from zero to the rated value.

[0045] Specifically, when the fourth controllable switch K4 and the fifth controllable switch are closed, the third converter 31 is unlocked and started. At this time, the third converter 31 inverts the DC power of the DC submarine cable into AC power to the marine collection bus.

[0046] Step S3: Once the offshore collected voltage is established, gradually start the offshore wind turbine under no-load conditions.

[0047] Specifically, after the third converter 31 is started, the voltage of the offshore collection bus gradually rises. When the voltage of the offshore collection bus rises to the preset voltage, it indicates that the offshore collection voltage has been established. At this time, the wind turbines of the offshore wind farm are gradually started under no-load conditions.

[0048] Step S4: After the offshore wind turbine completes its no-load start-up, gradually increase the active power output of the offshore wind turbine.

[0049] Step S5: When the current flowing through the fourth controllable switch K4 and the fifth controllable switch is zero, disconnect the fourth controllable switch K4 and the fifth controllable switch, and at the same time close the third controllable switch K3.

[0050] Specifically, after the offshore wind turbines are gradually started under no-load conditions, the offshore wind turbines output active power, and the current flowing through the fourth controllable switch K4 and the fifth controllable switch is detected. When the current flowing through the fourth controllable switch K4 and the fifth controllable switch is zero, the third controllable switch K3 is activated.

[0051] Step S6: When the onshore converter station 5 gradually increases its DC side voltage to the rated value, close the first controllable switch K1 and the second controllable switch K2.

[0052] When the onshore converter station 5 gradually increases its DC side voltage to the rated value, it indicates that the black start is complete. At this time, the third converter 31 needs to be disconnected and the first converter and the second converter 21 need to be connected.

[0053] Step S7: Gradually increase the output power of the offshore wind farm until it reaches its maximum output value.

[0054] Specifically, refer to Figure 2 The process from black start to normal operation of an offshore wind farm is as follows:

[0055] Step 1: Onshore converter station 5 is unlocked and started, with negative voltage output on the DC side;

[0056] Step 2: Close K4 and K5, unlock and start the offshore half-bridge MMC, and gradually increase its AC side voltage from zero to the rated value through open-loop control;

[0057] Step 3: After the offshore collection voltage is established, the offshore wind turbines are gradually started under no-load;

[0058] Step 4: After the offshore wind turbine completes the no-load start-up, gradually increase the output active power. When the current flowing through K4 and K5 approaches zero, K4 and K5 are disconnected, and K3 is closed at the same time.

[0059] Step 5: The onshore converter station 5 gradually increases its DC side voltage to the rated value, and then closes K1 and K2;

[0060] Step 6: Gradually increase the output power of the offshore wind farm until it reaches its maximum output value. At this point, the entire offshore wind power system has completed black start and normal power transmission operation.

[0061] This embodiment of the offshore converter station adopts a diode valve DC-side series half-bridge MMC, which can realize negative voltage start-up of the offshore converter station MMC; and adopts the operation mode of diode valve DC-side series MMC during black start-up, and after black start-up is completed, the MMC DC side is disconnected from the diode valve DC side, so the power devices and capacitors selected for the MMC can be smaller; the MMC used in this embodiment is a half-bridge structure, which has a cost advantage compared to a full-bridge MMC; finally, the MMC in this embodiment adopts an open-loop control method for AC side voltage, making the control method simpler.

[0062] This embodiment also provides a control device for an offshore wind power bidirectional asymmetrical feeder DC transmission system. This device is used to implement the above embodiments and preferred embodiments, and details already described will not be repeated. As used below, the term "module" can refer to a combination of software and / or hardware that performs a predetermined function. Although the device described in the following embodiments is preferably implemented in software, hardware implementation, or a combination of software and hardware, is also possible and contemplated.

[0063] This embodiment provides a control device for an offshore wind power bidirectional asymmetrical feeder DC transmission system, including:

[0064] The first control module is used to unlock and start the onshore converter station.

[0065] The second control module is used to close the fourth controllable switch K4 and the fifth controllable switch, unlock the third converter 31, and use open-loop control to gradually increase the AC side voltage of the third converter 31 from zero to the rated value.

[0066] The third control module is used to gradually start the offshore wind turbine under no-load conditions after the offshore collected voltage is established.

[0067] The fourth control module is used to gradually increase the active power output of the offshore wind turbine after it has completed its no-load start-up.

[0068] The fifth control module is used to disconnect the fourth controllable switch K4 and the fifth controllable switch when the current flowing through the fourth controllable switch K4 and the fifth controllable switch is zero, and at the same time close the third controllable switch K3.

[0069] The sixth control module is used to close the first controllable switch K1 and the second controllable switch K2 when the onshore converter station gradually increases its DC side voltage to the rated value.

[0070] The seventh control module is used to gradually increase the output power of the offshore wind farm until the offshore wind farm can output the maximum value.

[0071] Further functional descriptions of the above modules and units are the same as those in the corresponding embodiments described above, and will not be repeated here.

[0072] In this embodiment, the control device of the offshore wind power bidirectional asymmetric feed DC transmission system is presented in the form of a functional unit. Here, a unit refers to an ASIC (Application Specific Integrated Circuit) circuit, a processor and memory that execute one or more software or fixed programs, and / or other devices that can provide the above functions.

[0073] Although embodiments of the invention have been described in conjunction with the accompanying drawings, those skilled in the art can make various modifications and variations without departing from the spirit and scope of the invention, and such modifications and variations all fall within the scope defined by the appended claims.

Claims

1. A DC transmission system for bidirectional asymmetrical feeding of offshore wind power, characterized in that, include: The system comprises a first converter module, a second converter module, a third converter module, a switch module, and an onshore converter station. The wind turbines of the offshore wind farm are connected to the offshore busbar via medium-voltage AC cables; The AC side of the first converter module is connected to the marine busbar, the upper end of the DC side of the first converter module is connected to the upper DC submarine cable, and the lower end of the DC side of the first converter module is connected to the upper end of the DC side of the second converter module. The AC side of the second converter module is connected to the marine busbar, and the lower end of the DC side of the second converter module is connected to the first end of the switch module. The AC side of the third converter module is connected to the marine busbar, the upper end of the DC side of the third converter module is connected to the second end of the switch module, and the lower end of the DC side of the third converter module is connected to the third end of the switch module. The fourth terminal of the switch module is connected to the lower DC submarine cable; The upper end of the DC side of the onshore converter station is connected to the upper DC submarine cable, the lower end of the DC side of the onshore converter station is connected to the lower DC submarine cable, and the AC side of the onshore converter station is connected to the power grid. The first converter module includes: a first transformer, a first controllable switch, and a first converter connected in series; The second converter module includes: a second transformer, a second controllable switch, and a second converter connected in series. The third converter module includes: a third transformer and a third converter connected in series. The first and second converters are both 12-pulse rectifier diode valves; the third converter is a half-bridge MMC converter valve. The switching module includes a third controllable switch, a fourth controllable switch, and a fifth controllable switch. The first terminal of the third controllable switch is connected to the lower DC side of the second converter, and the second terminal of the third controllable switch is connected to the lower DC submarine cable. The first terminal of the fourth controllable switch is connected to the upper DC side of the third converter, and the second terminal of the fourth controllable switch is connected to the first terminal of the third controllable switch. The first terminal of the fifth controllable switch is connected to the lower DC side of the third converter, and the second terminal of the fifth controllable switch is connected to the second terminal of the third controllable switch. Unlock and start the onshore converter station; close the fourth and fifth controllable switches to unlock the third converter, and use open-loop control to gradually increase the AC side voltage of the third converter from zero to the rated value; after the offshore collected voltage is established, gradually start the offshore wind turbine under no-load; after the offshore wind turbine completes the no-load start-up, gradually increase the output active power of the offshore wind turbine; when the current flowing through the fourth and fifth controllable switches is zero, disconnect the fourth and fifth controllable switches, and simultaneously close the third controllable switch; as the onshore converter station gradually increases its DC side voltage to the rated value, close the first and second controllable switches; gradually increase the output power of the offshore wind farm until the offshore wind farm can output the maximum value.

2. The offshore wind power bidirectional asymmetrical feeder DC transmission system according to claim 1, characterized in that, The primary side of the first transformer is connected to the marine busbar, and the secondary side of the first transformer is connected to the AC side of the first converter through the first controllable switch. The upper end of the DC side of the first converter is connected to the upper DC submarine cable, and the lower end of the DC side of the first converter is connected to the upper end of the DC side of the second converter module.

3. The offshore wind power bidirectional asymmetrical feeder DC transmission system according to claim 2, characterized in that, The primary side of the second transformer is connected to the marine busbar, and the secondary side of the second transformer is connected to the AC side of the second converter via the second controllable switch. The upper end of the DC side of the second converter is connected to the lower end of the DC side of the first converter, and the lower end of the DC side of the second converter is connected to the first end of the switching module.

4. The offshore wind power bidirectional asymmetrical feeder DC transmission system according to claim 3, characterized in that, The primary side of the third transformer is connected to the marine busbar, and the secondary side of the third transformer is connected to the AC side of the third converter. The upper DC side of the third converter is connected to the second terminal of the switching module, and the lower DC side of the third converter is connected to the third terminal of the switching module.

5. The offshore wind power bidirectional asymmetrical feeder DC transmission system according to claim 1, characterized in that, The onshore converter station includes: a full-bridge and half-bridge hybrid MMC converter valve and a fourth transformer, wherein... The upper DC side of the full-bridge and half-bridge hybrid MMC converter valve is connected to the upper DC submarine cable, the lower DC side of the full-bridge and half-bridge hybrid MMC converter valve is connected to the lower DC submarine cable, and the AC side of the full-bridge and half-bridge hybrid MMC converter valve is connected to the primary side of the fourth transformer. The secondary side of the fourth transformer is connected to the power grid.

6. The offshore wind power bidirectional asymmetrical feeder DC transmission system according to claim 1, characterized in that, Also includes: A filter is installed on the marine confluence bus.

7. A control method for an offshore wind power bidirectional asymmetrical feeder DC transmission system, characterized in that, Based on the offshore wind power bidirectional asymmetric feeder DC transmission system according to claim 1, the method includes: Unlock and start up the onshore converter station; Close the fourth and fifth controllable switches to unlock the third converter, and use open-loop control to gradually increase the AC side voltage of the third converter from zero to the rated value. Once the offshore voltage is established, the offshore wind turbines will be gradually started under no-load conditions. After the offshore wind turbine completes its no-load start-up, gradually increase the active power output of the offshore wind turbine. When the current flowing through the fourth and fifth controllable switches is zero, disconnect the fourth and fifth controllable switches and simultaneously close the third controllable switch; When the onshore converter station gradually increases its DC side voltage to the rated value, the first controllable switch and the second controllable switch are closed. Gradually increase the output power of offshore wind farms until they reach their maximum output capacity.

8. A control device for a bidirectional asymmetrical feeder DC transmission system for offshore wind power, characterized in that, The offshore wind power bidirectional asymmetric feeder DC transmission system according to claim 1 includes: The first control module is used to unlock and start the onshore converter station; The second control module is used to close the fourth and fifth controllable switches, unlock the third converter, and use open-loop control to gradually increase the AC side voltage of the third converter from zero to the rated value. The third control module is used to gradually start the offshore wind turbine under no-load conditions after the offshore collected voltage is established. The fourth control module is used to gradually increase the active power output of the offshore wind turbine after it has completed its no-load start-up. The fifth control module is used to disconnect the fourth and fifth controllable switches and simultaneously close the third controllable switch when the current flowing through the fourth and fifth controllable switches is zero. The sixth control module is used to close the first and second controllable switches when the onshore converter station gradually increases its DC side voltage to the rated value. The seventh control module is used to gradually increase the output power of the offshore wind farm until the offshore wind farm can output the maximum value.

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