Offshore wind power bidirectional asymmetric feed type direct current sending-out system and control method thereof

Abstract

The invention relates to the technical field of offshore wind power direct current transmission, and discloses an offshore wind power bidirectional asymmetric feed type direct current sending-out system and a control method thereof.The offshore wind power bidirectional asymmetric feed type direct current sending-out system comprises a first current conversion module, a second current conversion module, a third current conversion module, a switch module and an onshore converter station, the third conversion module is used for establishing an offshore collection voltage, and after black start is completed, the first conversion module and the second conversion module are connected to the system, so that the problem that the offshore wind power plant cannot be subjected to black start is solved.

Description

Technical Field

[0001] The present invention relates to the technical field of offshore wind power direct current transmission, and in particular to an offshore wind power bidirectional asymmetric feeding type direct current transmission system and a control method thereof. Background Art

[0002] Long-distance offshore wind power transmission generally uses high-voltage direct current transmission technology. After the offshore wind turbines generate electricity through AC cables, the offshore converter station converts the AC into DC, and then transmits the electricity to the onshore converter station through the DC submarine cable. At present, the converter valves of the offshore converter station in the offshore wind power DC transmission project all adopt the MMC structure, but the MMC converter valve is expensive and has a large volume and weight, so the large-scale offshore wind power transmission scheme needs to be further optimized. In order to adapt to various application scenarios and reduce the cost of transmission projects, a variety of aggregation and networking transmission schemes with their own characteristics have been proposed at home and abroad, including the use of unidirectional current-type uncontrolled rectifier converters based on diode elements at the sending end and other aggregation and transmission topologies and networking schemes. The use of unidirectional current-type uncontrolled rectifier converters based on diode elements at the sending end helps to achieve lightweight offshore platforms, but the existing schemes have the significant disadvantage that offshore wind farms cannot be black-started. Summary of the invention

[0003] In view of this, the present invention provides an offshore wind power bidirectional asymmetric feeding type DC transmission system and a control method thereof to solve the problem that an offshore wind farm cannot be black-started.

[0004] In a first aspect, the present invention provides an offshore wind power bidirectional asymmetric feeding type DC transmission system, 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 connected to an offshore busbar through a medium-voltage AC cable; 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 the upper end 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 lower end of the DC side of the second converter module is connected to the offshore 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 offshore 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 end 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.

[0005] In the present invention, by controlling the current flow path inside the switch module, during a black start, the third converter module is used to establish the offshore collection voltage. After the black start is completed, the first converter module and the second converter module are connected to the system, thereby solving the problem that the offshore wind farm cannot be black started.

[0006] In an optional embodiment, 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 offshore 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 an optional embodiment, 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 an optional embodiment, 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 an optional implementation, the first converter and the second converter are both 12-pulse rectifier diode valves; and the third converter is a half-bridge MMC converter valve.

[0010] The present invention adopts a method of connecting the MMC in series on the DC side of the diode valve, which can realize the negative pressure start of the MMC of the offshore converter station, thereby realizing the black start of the offshore wind farm.

[0011] The MMC adopted in the present invention is a half-bridge structure, which has greater cost advantages than the full-bridge MMC of the existing solution.

[0012] In an 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 an optional embodiment, the onshore converter station includes: a full-bridge half-bridge hybrid MMC converter valve and a fourth transformer, wherein the upper end of the DC side of the full-bridge 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 half-bridge hybrid MMC converter valve is connected to the lower DC submarine cable, and the AC side of the full-bridge 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.

[0014] In an optional implementation, the offshore wind power bidirectional asymmetric feeding type DC transmission system further includes: a filter is arranged on the offshore busbar.

[0015] In a second aspect, the present invention provides a control method for an offshore wind power bidirectional asymmetric feeding type DC transmission system. The offshore wind power bidirectional asymmetric feeding type DC transmission system is an optional implementation mode of the first aspect, and the method includes: unlocking and starting an onshore converter station; closing a fourth controllable switch and a fifth controllable switch, unlocking a third converter, and using open-loop control to gradually increase the AC side voltage of the third converter from zero to a rated value; when the offshore collection voltage is established, gradually starting the offshore wind turbine at no-load; when the offshore wind turbine completes the no-load start, gradually increasing the output active power of the offshore wind turbine; when the current flowing through the fourth controllable switch and the fifth controllable switch is zero, disconnecting the fourth controllable switch and the fifth controllable switch, and closing the third controllable switch at the same time; when the onshore converter station gradually increases its DC side voltage to the rated value, closing the first controllable switch and the second controllable switch; gradually increasing the output power of the offshore wind farm until the maximum value that the offshore wind farm can output.

[0016] In a third aspect, the present invention provides a control device for an offshore wind power bidirectional asymmetric feeding type DC transmission system, comprising: a first control module, used to unlock and start an onshore converter station; a second control module, used to close a fourth controllable switch and a fifth controllable switch, unlock a third converter, and adopt open-loop control to gradually increase the AC side voltage of the third converter from zero to a rated value; a third control module, used to gradually start the offshore wind turbine at no-load after the offshore collection voltage is established; a fourth control module, used to gradually increase the output active power of the offshore wind turbine after the offshore wind turbine completes the no-load start; a fifth control module, used to disconnect the fourth controllable switch and the fifth controllable switch when the current flowing through the fourth controllable switch and the fifth controllable switch is zero, and close the third controllable switch at the same time; a sixth control module, used to close the first controllable switch and the second controllable switch when the onshore converter station gradually increases its DC side voltage to the rated value; a seventh control module, used to gradually increase the output power of the offshore wind farm until the maximum value that the offshore wind farm can output. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

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

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

[0020] Figure 3 It is a flow chart of a control method of an offshore wind power bidirectional asymmetric feeding type DC transmission system according to an embodiment of the present invention. DETAILED DESCRIPTION

[0021] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative work are within the scope of protection of the present invention.

[0022] In the description of the present invention, it should be noted that the terms “first”, “second” and “third” are only used for descriptive purposes and cannot be understood as indicating or implying relative importance.

[0023] In the description of the present invention, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, it can also be the internal connection of two components, it can be a wireless connection, or it can be a wired connection. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0024] In addition, 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 the related technical solutions, although the use of a unidirectional current-type uncontrolled converter based on diode elements at the sending end helps to achieve lightweight offshore platforms, it has the following disadvantages:

[0026] (1) Pure diode check valves cannot transmit power from onshore to offshore, making it difficult to black-start offshore wind farms;

[0027] (2) The solution of connecting the MMC in series on the DC side of the diode valve. When transmitting offshore wind power, the DC side current needs to flow through the diode valve and the MMC at the same time, resulting in the MMC power device overcurrent capacity and capacitance value to be larger;

[0028] (3) The solution of connecting the MMC in series on the DC side of the diode valve. In order to have the capability of negative pressure starting, the offshore MMC needs to adopt a full-bridge structure, which is more expensive than the half-bridge structure.

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

[0030] Based on the above problems, in this embodiment, a bidirectional asymmetric feeding type DC transmission system for offshore wind power is provided, 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 through a medium-voltage AC cable; 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 the normal operation of the wind farm, the internal current path of the switch module 4 is controlled so that the first converter module 1 and the second converter module 2 are put into the entire system, and the third converter module 3 is cut out; in order to achieve a black start, it is necessary to control the internal current path of the switch module 4 so that the third converter module 3 is first put into the system, and when the start conditions are met, the third converter module 3 is cut out, so that the first converter module 1 and the second converter module 2 are put into the entire system.

[0033] In some optional embodiments, 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, wherein the primary side of the first transformer T1 is connected to the offshore 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 end 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 optional embodiments, 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, wherein the primary side of the second transformer T2 is connected to the offshore 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 optional embodiments, such as Figure 2 As shown, the third converter module 3 includes: a third transformer T3 and a third converter 31, wherein the primary side of the third transformer T3 is connected to the offshore 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 optional 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 intended to be limiting.

[0037] In some optional embodiments, 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, wherein the first end of the third controllable switch K3 is connected to the lower end of the DC side of the second converter 21, and the second end of the third controllable switch K3 is connected to the lower end DC submarine cable; the first end of the fourth controllable switch K4 is connected to the upper end of the 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 end of the 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 controllable switch K4 and the fifth controllable switch are opened, the first converter and the second converter 21 are connected to the system, and the third converter 31 is cut out. When the third controllable switch K3, the fourth controllable switch K4 and the fifth controllable switch are closed, the third converter 31 is connected to the system, and the first converter and the second converter 21 are cut out.

[0039] In some optional embodiments, such as Figure 2 As shown, the onshore converter station 5 includes: a full-bridge half-bridge hybrid MMC converter valve 41 and a fourth transformer T4, wherein the upper end of the DC side of the full-bridge 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 half-bridge hybrid MMC converter valve is connected to the lower DC submarine cable, and the AC side of the full-bridge 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 optional embodiments, such as Figure 2 As shown, the offshore wind power bidirectional asymmetric feeding type DC transmission system also includes: a filter is set on the offshore busbar.

[0041] In this embodiment, a control method for an offshore wind power bidirectional asymmetric feeding type DC transmission system is provided. Based on the offshore wind power bidirectional asymmetric feeding type DC transmission system of the optional implementation mode of the above embodiment, 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 valves are unlocked and started, and the operation of the full-bridge and half-bridge hybrid MMC converter valves can be controlled according to a preset control method. At this time, the full-bridge and half-bridge hybrid MMC converter valves rectify the voltage of the power grid into direct current to the DC submarine cable.

[0044] Step S2: 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 a rated value.

[0045] Specifically, the fourth controllable switch K4 and the fifth controllable switch are closed, unlocking and starting the third converter 31. At this time, the third converter 31 inverts the DC power of the DC submarine cable into AC power and transmits it to the offshore collection bus.

[0046] Step S3: After the offshore aggregate voltage is established, the offshore wind turbine is gradually started at no-load.

[0047] Specifically, after the third converter 31 is started, the voltage of the offshore busbar begins to gradually rise. When the voltage of the offshore busbar rises to the preset voltage, it means that the offshore busbar voltage is established. At this time, the wind turbines of the offshore wind farm are gradually started at no load.

[0048] Step S4: After the offshore wind turbine completes no-load startup, gradually increase the output active power 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, the fourth controllable switch K4 and the fifth controllable switch are opened, and the third controllable switch K3 is closed at the same time.

[0050] Specifically, after gradually starting the wind turbines of the offshore wind farm at no-load, the offshore wind turbines output active power, and detect the current flowing through 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, the third controllable switch K3.

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

[0052] When the onshore converter station 5 gradually increases its DC side voltage to the rated value, it indicates that the black start is completed. 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 the maximum value that the offshore wind farm can output is reached.

[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: The onshore converter station 5 is unlocked and started, and the DC side outputs a negative voltage;

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

[0057] Step 3: After the offshore voltage is established, the offshore wind turbines are gradually started without load;

[0058] Step 4: After the offshore wind turbine completes no-load startup, the output active power is gradually increased. When the current flowing through K4 and K5 is close to 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 completes black start and normal power transmission operation.

[0061] The offshore converter station in this embodiment adopts a method of connecting a half-bridge MMC in series on the DC side of a diode valve, which can realize negative pressure starting of the MMC of the offshore converter station; and adopts an operation mode of connecting the MMC in series on the DC side of the diode valve during the black start phase. After the black start is completed, the MMC DC side is disconnected from the DC side of the diode valve, so that the power devices and capacitors selected by the MMC can be smaller; the MMC used in this embodiment is a half-bridge structure, which has more cost advantages than the full-bridge MMC; finally, the MMC in this embodiment adopts an open-loop control method of the AC side voltage, and the control method is simpler.

[0062] In this embodiment, a control device for an offshore wind power bidirectional asymmetric feeding type DC transmission system is also provided, which is used to implement the above-mentioned embodiments and preferred implementation modes, and the descriptions that have been made will not be repeated. As used below, the term "module" can implement a combination of software and / or hardware for a predetermined function. Although the devices described in the following embodiments are preferably implemented in software, the implementation of hardware, or a combination of software and hardware, is also possible and conceivable.

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

[0064] A first control module, used for unlocking and starting 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 a rated value;

[0066] The third control module is used to gradually start the offshore wind turbine at no-load after the offshore voltage is established;

[0067] The fourth control module is used to gradually increase the output active power of the offshore wind turbine after the offshore wind turbine completes no-load startup;

[0068] a fifth control module, configured to disconnect the fourth controllable switch K4 and the fifth controllable switch and simultaneously close the third controllable switch K3 when the current flowing through the fourth controllable switch K4 and the fifth controllable switch is zero;

[0069] A sixth control module, configured 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 a rated value;

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

[0071] The further functional description of each of the above modules and units is the same as that of the above corresponding embodiments and will not be repeated here.

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

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

Claims

1. An offshore wind power bidirectional asymmetric feeding type DC transmission system, characterized in that: include: 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 connected to the 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 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 offshore 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 offshore 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 end 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.

2. The offshore wind power bidirectional asymmetric feeding type DC transmission system according to claim 1 is characterized in that: 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 offshore 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 asymmetric feeding type DC transmission system according to claim 2 is characterized in that: 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.

4. The offshore wind power bidirectional asymmetric feeding type DC transmission system according to claim 3 is characterized in that: 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.

5. The offshore wind power bidirectional asymmetric feeding type DC transmission system according to claim 4 is characterized in that: The first converter and the second converter are both 12-pulse rectifier diode valves; The third converter is a half-bridge MMC converter valve.

6. The offshore wind power bidirectional asymmetric feeding type DC transmission system according to claim 4 is characterized in that: 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 end 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; A first end of the fifth controllable switch is connected to a lower end of the DC side of the third converter, and a second end of the fifth controllable switch is connected to a second end of the third controllable switch.

7. The offshore wind power bidirectional asymmetric feeding type DC transmission system according to claim 1, characterized in that: The onshore converter station includes: a full-bridge 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 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; A secondary side of the fourth transformer is connected to the power grid.

8. The offshore wind power bidirectional asymmetric feeding type DC transmission system according to claim 1, characterized in that: Also includes: A filter is arranged on the offshore busbar.

9. A control method for an offshore wind power bidirectional asymmetric feeding type DC transmission system, characterized in that: Based on the offshore wind power bidirectional asymmetric feeding type DC transmission system according to claim 6, the method comprises: Unlock and start up the onshore converter station; Closing the fourth controllable switch and the fifth controllable switch, unlocking the third converter, and using open-loop control to gradually increase the AC side voltage of the third converter from zero to a rated value; When the offshore voltage is established, the offshore wind turbines are gradually started at no load; After the offshore wind turbine completes no-load startup, gradually increase the active power output of the offshore wind turbine; When the current flowing through the fourth controllable switch and the fifth controllable switch is zero, the fourth controllable switch and the fifth controllable switch are opened, and the third controllable switch is closed at the same time; 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 the maximum value that the offshore wind farm can output is reached.

10. A control device for an offshore wind power bidirectional asymmetric feeding type DC transmission system, characterized in that: include: A first control module, used for unlocking and starting the onshore converter station; a second control module, configured to close the fourth controllable switch and the fifth controllable switch, unlock the third converter, and use open-loop control to gradually increase the AC side voltage of the third converter from zero to a rated value; The third control module is used to gradually start the offshore wind turbine at no-load after the offshore voltage is established; The fourth control module is used to gradually increase the output active power of the offshore wind turbine after the offshore wind turbine completes no-load startup; a fifth control module, configured to disconnect the fourth controllable switch and the fifth controllable switch and simultaneously close the third controllable switch when the current flowing through the fourth controllable switch and the fifth controllable switch is zero; A sixth control module, configured to close the first controllable switch and the second controllable switch when the onshore converter station gradually increases its DC side voltage to a rated value; The seventh control module is used to gradually increase the output power of the offshore wind farm until it reaches the maximum value that the offshore wind farm can output.

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