Offshore wind power multi-terminal multi-element heterogeneous direct current sending-out system and operation control method thereof

Abstract

The invention provides an offshore wind power multi-terminal multi-element heterogeneous direct current sending-out system and an operation control method. The offshore wind power multi-terminal multi-element heterogeneous direct current sending-out system comprises an offshore part and a shore part. According to the offshore part, a main wind power plant is any one of offshore wind power clusters; the offshore MMC is connected with the main wind power plant, so that flexible direct-current transmission of the main wind power plant is transmitted to the shore; the wind power plants are remaining wind power plants in the offshore wind power cluster; the offshore DRUs are connected with the secondary wind farms in a one-to-one correspondence manner, so that the secondary wind farms can be sent out ashore through uncontrolled rectification; the offshore contact module is connected with the main wind power plant and the auxiliary wind power plant, and the main wind power plant and the offshore MMC establish the alternating current bus voltage and frequency of the auxiliary wind power plant through the offshore contact module to provide black-start power for the auxiliary wind power plant; the ashore part comprises a plurality of ashore MMC (Modular Multilevel Converters) which are connected with the offshore MMC; and the load centers are in one-to-one correspondence connection with the ashore MMC. Based on a maritime contact module, a main wind power plant and a maritime MMC establish alternating current bus voltage and frequency of an auxiliary wind power plant, and black start power is provided for the auxiliary wind power plant to complete start.

Description

Technical Field

[0001] The present invention relates to the field of power grid technology, and in particular to an offshore wind power multi-terminal multi-heterogeneous direct current transmission system and an operation control method thereof. Background Art

[0002] The development scale, development area and development depth of renewable energy have now developed to a relatively high level. In the east and south, from nearshore to deep sea, the offshore wind power development area is vast. Large-scale wind power resources are developed in clusters and multiple clusters are widely distributed, and the power grid is relatively weak. The use of AC systems to gather fluctuating power in a wide area faces technical bottlenecks in voltage stability and grid connection stability. Therefore, the scheme of multi-terminal DC transmission aggregation and grid connection has received widespread attention. Since 2014, there have been many applications such as the Nan'ao three-terminal flexible DC project, the Zhoushan five-terminal flexible DC project and the Zhangbei new energy base coupled pumped storage four-terminal flexible DC grid connection project.

[0003] For clustered offshore wind power, the use of multi-terminal DC transmission convergence and grid connection can effectively achieve the complementarity of wide-area wind power and smooth the fluctuation of grid-connected power. However, the current offshore flexible DC converter station based on modular multilevel (MMC) topology is large in size and weight, and has high construction and operation and maintenance costs, which has become a key bottleneck affecting the cost of offshore wind power transmission. Compared with the modular multilevel converter currently used in multi-terminal DC transmission systems, diode rectifiers can greatly reduce the size and cost of offshore platforms. However, due to the problems that diode rectifiers cannot provide black start power for wind turbines, cannot actively establish the voltage of the wind farm intranet, and generate voltage harmonics and current harmonics, they cannot be directly used for offshore wind power transmission.

[0004] After searching, the Chinese patent with application number CN201910563358.8 provides a hybrid DC converter topology structure and its corresponding fault crossing method. The topology structure includes an uncontrolled rectifier, an auxiliary converter, and a reactor, which realizes low-cost transmission based on diodes and ensures technical economy. However, its auxiliary converter still adopts a modular multi-level topology, which requires a certain platform area, and the auxiliary converter needs to withstand high voltage. Considering the insulation clearance, the reduction in the volume of the converter valve is limited. Since both the auxiliary converter and the diode rectifier need to be placed on the offshore platform, this solution has limited reduction in the overall construction cost of the offshore platform.

[0005] After searching, the Chinese patent with application number CN202210531669.8 provides a grid-type wind turbine control method for an offshore wind power diode rectifier unit delivery system. By transforming the control strategy of the wind turbine from a grid-following type to a grid-forming type, the voltage and frequency construction of the offshore AC system and the compensation of the reactive power consumed by the diode are realized, so that the offshore modular multi-level converter valve can be completely replaced by a diode uncontrolled rectifier, realizing the lightweight and low-cost offshore delivery platform. This solution requires the transformation of wind turbines and the coordinated networking between wind turbines. At present, the networking technology of wind turbines is not mature and has not been applied in engineering. Especially in the case of large-scale offshore wind power DC grid connection, wind turbines may be provided by multiple manufacturers. How wind turbines from different manufacturers can be coordinated to form a network is a problem that cannot be solved at present. Summary of the invention

[0006] In view of the defects in the prior art, the object of the present invention is to provide an offshore wind power multi-terminal multi-heterogeneous DC transmission system and an operation control method thereof.

[0007] According to one aspect of the present invention, there is provided an offshore wind power multi-terminal multi-heterogeneous direct current transmission system, comprising an offshore part and an onshore part, wherein:

[0008] The offshore part includes:

[0009] A main wind farm, wherein the main wind farm is any one of the offshore wind farm clusters;

[0010] An offshore MMC, wherein the offshore MMC is connected to the main wind farm to realize flexible direct current transmission of power from the main wind farm to the shore;

[0011] Auxiliary wind farms, which are the remaining wind farms in the offshore wind power cluster;

[0012] An offshore DRU, wherein the offshore DRU is connected to the auxiliary wind farm in a one-to-one correspondence, so as to realize uncontrolled rectification and transmission of the auxiliary wind farm to the shore;

[0013] An offshore contact module, wherein the offshore contact module connects the main wind farm and the auxiliary wind farm, and through the offshore contact module, the main wind farm and the offshore MMC establish the AC bus voltage and frequency of the auxiliary wind farm to provide black start power for the auxiliary wind farm;

[0014] The onshore part includes:

[0015] An onshore MMC, wherein a plurality of the onshore MMCs receive the DC power transmitted by the offshore portion;

[0016] a load center connected to the onshore MMC;

[0017] A receiving-end power grid, wherein the receiving-end power grid is connected to the onshore MMC.

[0018] Preferably, the main wind farm is connected to the offshore MMC via an AC submarine cable; the offshore MMC is connected to the shore via a DC submarine cable;

[0019] The offshore DRUs in the auxiliary wind farm are connected via AC submarine cables, and the offshore DRUs are connected to the shore via DC submarine cables;

[0020] The onshore MMC is connected to the load center or the receiving power grid through an AC overhead line.

[0021] Preferably, the power transmission path at the main wind farm is:

[0022] After the electric energy is collected from the main wind farm through the AC submarine cable, it is transmitted to the offshore MMC station; at the offshore MMC station, it is stepped up and converted into high-voltage direct current through a transformer, and then transmitted to the shore through the DC submarine cable, and then converted into high-voltage alternating current by the shore MMC, and transmitted to the load center or the receiving-end power grid through the AC overhead line;

[0023] The power transmission path at the auxiliary wind farm is:

[0024] After the electric energy is collected from the auxiliary wind farm through the AC submarine cable, it is transmitted to the offshore DRU station, where it is stepped up and converted into high-voltage DC by a transformer, transmitted to the shore through the DC submarine cable, and then converted into high-voltage AC by the onshore MMC, and transmitted to the load center or the receiving power grid through the AC overhead line.

[0025] Preferably, the offshore connection module is arranged between a low-voltage side common connection point of a coupling transformer of an offshore MMC of the main wind farm and a low-voltage side common connection point of a coupling transformer of an offshore DRU of the auxiliary wind farm.

[0026] Preferably, the working modes of the offshore communication module include industrial frequency alternating current, low frequency alternating current and direct current;

[0027] When the industrial frequency AC form is adopted, the offshore communication module includes an industrial frequency AC submarine cable, and the industrial frequency AC submarine cable connects the two common connection points;

[0028] When the low-frequency AC form is adopted, the offshore communication module includes a low-frequency AC submarine cable and two AC-AC converters, the two AC-AC converters are respectively connected at the two common connection points to realize the conversion between industrial frequency AC and low-frequency AC, and the low-frequency AC submarine cable connects the two AC-AC converters;

[0029] When the DC form is adopted, the offshore communication module includes a low-frequency AC submarine cable and two AC-DC converters, the two AC-DC converters are respectively connected at the two common connection points to realize the conversion between industrial frequency AC and DC, and the low-frequency AC submarine cable connects the two AC-DC converters.

[0030] Preferably, through the offshore communication module, the offshore MMC establishes a connection with the offshore DRU, establishes the AC voltage and frequency of the common connection point on the low-voltage side of the connecting transformer of the auxiliary wind farm offshore DRU, and the auxiliary wind farm draws power from the common connection point on the low-voltage side of the connecting transformer of the auxiliary wind farm offshore DRU and completes the black start.

[0031] According to a second aspect of the present invention, there is provided an operation control method of an offshore wind power multi-terminal multi-heterogeneous direct current transmission system, comprising:

[0032] Simultaneously start the offshore MMC and a connected onshore MMC to establish the AC voltage and frequency of the main wind farm;

[0033] Start the offshore communication module, establish the AC voltage and frequency of the auxiliary wind farm, and provide black start power for the auxiliary wind farm. The auxiliary wind farm draws power and completes the startup.

[0034] Preferably, the simultaneously starting the offshore MMC and an onshore MMC connected thereto to establish the AC voltage and frequency of the main wind farm includes:

[0035] Randomly select an onshore MMC and start it together with the offshore MMC connected to the main wind farm;

[0036] The onshore MMC and the offshore MMC are pre-charged. After the pre-charging is completed, the onshore MMC adopts the DC voltage control mode; the offshore MMC adopts the constant AC voltage and frequency control mode.

[0037] Preferably, the offshore communication module is started to establish the AC voltage and frequency of the auxiliary wind farm and provide black start power for the auxiliary wind farm, so that the auxiliary wind farm takes power and completes the startup, including:

[0038] Select a secondary wind farm, start its offshore communication module, and establish a connection between the secondary wind farm and the offshore MMC;

[0039] The AC voltage and frequency of the common connection point on the low-voltage side of the connection transformer on the auxiliary wind farm side are established through the offshore MMC, and the auxiliary wind farm draws power from the common connection point on the low-voltage side of the connection transformer and completes a black start;

[0040] Other auxiliary wind farms were started one after another.

[0041] Preferably, after the auxiliary wind farm is started, the offshore MMC and the offshore communication module will enter a steady-state operation mode, and the power transmitted by the offshore communication module is 0; at this time, the output power of the auxiliary wind farm is only sent out by the offshore DRU to which it is connected, and the offshore communication module is disconnected or continues to maintain the connection.

[0042] Compared with the prior art, the embodiments of the present invention have at least one of the following beneficial effects:

[0043] The novel offshore wind power multi-terminal multi-heterogeneous DC transmission system and its operation control method in the embodiment of the present invention complement the advantages of modular multilevel converter (MMC) and diode uncontrolled rectifier (DRU). In a large-scale offshore wind power cluster, one of the wind farms is selected as the main wind farm, and the main wind farm adopts the flexible DC transmission and transmission technology based on offshore MMC. The other wind farms are used as auxiliary wind farms, and the auxiliary wind farms adopt the uncontrolled rectification and transmission technology based on DRU, thereby reducing the transmission cost of the offshore wind power cluster. At the same time, the main wind farm and the auxiliary wind farm are connected through an offshore communication module, and the main wind farm and the offshore MMC establish the AC bus voltage and frequency of the auxiliary wind farm, and provide black start power for the auxiliary wind farm, solving the problems that pure diode rectification cannot provide black start power for the wind turbine, cannot actively establish the voltage of the wind farm intranet, and generates voltage harmonics and current harmonics, and cannot be directly used for the transmission of offshore wind power. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Other features, objects and advantages of the present invention will become more apparent from the detailed description of non-limiting embodiments made with reference to the following drawings:

[0045] Figure 1 A schematic diagram of the topological structure of an offshore wind power multi-terminal heterogeneous direct current transmission system in a preferred embodiment of the present invention;

[0046] Figure 2 This is a connection diagram of the offshore communication module in a preferred embodiment of the present invention in the form of industrial frequency AC;

[0047] Figure 3 This is a connection diagram of a marine communication module in a preferred embodiment of the present invention in the form of low-frequency AC;

[0048] Figure 4 This is a schematic diagram of a connection of an offshore communication module in a preferred embodiment of the present invention in a direct current form;

[0049] Figure 5 It is a simulation wave diagram of an MMC station in a specific embodiment of the present invention;

[0050] Figure 6 A simulation waveform diagram of a diode station in a specific embodiment of the present invention;

[0051] Figure 7 This is a waveform diagram of an AC voltage injected into a diode in a specific embodiment of the present invention. DETAILED DESCRIPTION

[0052] The present invention is described in detail below in conjunction with specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form. It should be noted that, for those of ordinary skill in the art, several variations and improvements may be made without departing from the concept of the present invention. These all belong to the protection scope of the present invention.

[0053] It should be noted that the user information (including but not limited to user device information, user personal information, etc.) and data (including but not limited to data used for analysis, stored data, displayed data, etc.) involved in this application are all information and data authorized by the user or fully authorized by all parties, and the collection, use and processing of relevant data must comply with relevant regulations.

[0054] In one embodiment of the present invention, a multi-terminal multi-heterogeneous direct current transmission system for offshore wind power is provided, such as Figure 1 As shown, it includes the offshore part and the onshore part.

[0055] The offshore part includes: the main wind farm, offshore modular multilevel converter (offshore MMC), auxiliary wind farm, offshore pure diode uncontrolled rectifier (offshore DRU), and offshore communication module;

[0056] The main wind farm is any one of the offshore wind farm clusters; the offshore MMC is connected to the main wind farm to realize flexible DC transmission of the main wind farm to the shore; the auxiliary wind farm is the remaining wind farm in the offshore wind farm cluster; the offshore DRU is connected to the auxiliary wind farm one-to-one to realize uncontrolled rectification and transmission of the auxiliary wind farm to the shore; the offshore communication system connects the main wind farm and the auxiliary wind farm, and through the offshore communication module, the main wind farm and the offshore MMC establish the AC bus voltage and frequency of the auxiliary wind farm to provide black start power for the auxiliary wind farm.

[0057] The onshore part includes: onshore modular multilevel converter (onshore MMC for short), load center and receiving power grid.

[0058] Multiple onshore MMCs receive direct current from the offshore part; the load center and the receiving-end power grid are connected to the onshore MMCs to obtain power supply.

[0059] In order to achieve smooth and rapid energy transmission between the offshore part and the onshore part, in a preferred embodiment of the present invention,

[0060] The main wind farm is connected to the offshore MMC via an AC submarine cable; the offshore MMC is transmitted to the shore via a DC submarine cable; the auxiliary wind farm is connected to the offshore DRU via an AC submarine cable, and the offshore DRU is transmitted to the shore via a DC submarine cable; the onshore MMC is transmitted to the load center or the receiving power grid via an AC overhead line.

[0061] Based on the topological results of the offshore wind power multi-terminal multi-heterogeneous transmission system in the above embodiment, in a preferred embodiment of the present invention, a preferred transmission path for wind farm electric energy is provided. Specifically:

[0062] The power transmission path at the main wind farm is:

[0063] Electric energy is collected from the main wind farm through the AC submarine cable and then transmitted to the offshore MMC station. At the offshore MMC station, the voltage is stepped up and converted into high-voltage DC by the transformer, and then transmitted to the shore through the DC submarine cable. Then, it is converted into high-voltage AC by the onshore MMC and transmitted to the load center or the receiving grid through the AC overhead line.

[0064] The power transmission path at the auxiliary wind farm is:

[0065] After the electric energy is collected from the auxiliary wind farm through the AC submarine cable, it is transmitted to the offshore DRU station, where it is stepped up and converted into high-voltage DC by a transformer, transmitted to the shore via the DC submarine cable, and then converted into high-voltage AC by the onshore MMC, and transmitted to the load center or the receiving power grid via AC overhead lines.

[0066] The diode rectifier cannot provide black start power for the wind turbine, cannot actively establish the voltage of the wind farm network, and generates voltage harmonics and current harmonics, and cannot be directly used for the delivery of offshore wind power. Therefore, in a preferred embodiment of the present invention, the offshore connection module is installed between the low-voltage side common connection point of the coupling transformer of the offshore MMC of the main wind farm (referred to as the main PCC) and the low-voltage side common connection point of the coupling transformer of the offshore DRU of the auxiliary wind farm (referred to as the auxiliary PCC).

[0067] Furthermore, in a preferred embodiment of the present invention, the offshore communication module can adopt various forms, including industrial frequency alternating current, low frequency alternating current and direct current.

[0068] When using industrial frequency AC, the connection method is as follows: Figure 2 As shown, the offshore communication module includes an industrial frequency AC submarine cable;

[0069] When low frequency AC is used, the connection method is as follows: Figure 3 As shown, the offshore communication module includes a low-frequency AC submarine cable and two AC-AC converters. The two AC-AC converters are respectively connected to two common connection points to realize the conversion between industrial frequency AC and low-frequency AC. The low-frequency AC submarine cable connects the two AC-AC converters.

[0070] When using DC, the connection method is as follows: Figure 4 As shown, the offshore communication module includes a low-frequency AC submarine cable and two AC-DC converters. The two AC-DC converters are respectively connected to two common connection points to realize the conversion between industrial frequency AC and DC. The low-frequency AC submarine cable connects the two AC-DC converters.

[0071] In the above embodiment, by establishing a communication module between the main wind farm and the auxiliary wind farm, the auxiliary wind farm can establish an AC voltage domain frequency through the offshore MMC and obtain black start power from the offshore MMC, so that the auxiliary wind farm can complete power acquisition and start-up. The above problem of the diode rectifier is solved.

[0072] Based on the same inventive concept, in other embodiments of the present invention, an operation control method of an offshore wind power multi-terminal multi-heterogeneous DC transmission system is also provided, and its main steps are as follows:

[0073] Step 1: start the offshore MMC and a connected onshore MMC at the same time to establish the AC voltage and frequency of the main wind farm;

[0074] Step 2: Start the offshore communication module, establish the AC voltage and frequency of the auxiliary wind farm, and provide black start power to the auxiliary wind farm. The auxiliary wind farm takes power and completes the startup. Thus, the system enters steady-state operation.

[0075] In order to smoothly start the main wind farm, in a preferred embodiment, a preferred process for starting the main wind farm is provided, specifically:

[0076] S1.1, select an onshore MMC and start it together with the offshore MMC connected to the main wind farm;

[0077] S1.2, control the onshore MMC and offshore MMC to pre-charge;

[0078] S1.3, after the pre-charging is completed, the onshore MMC adopts the DC voltage control mode; the offshore MMC adopts the constant AC voltage or frequency control mode. At this point, the AC voltage and frequency of the main wind farm are established.

[0079] Similarly, in order to realize the startup of the auxiliary wind farm using the diode rectifier, in a preferred embodiment, a preferred process for starting the auxiliary wind farm is provided, specifically:

[0080] S2.1, select a secondary wind farm, start its offshore communication module, and establish a connection between the secondary wind farm and the offshore MMC;

[0081] S2.2, through the offshore MMC, establish the AC voltage and frequency of the secondary PCC (the common connection point on the low-voltage side of the connecting transformer of the offshore DRU of the secondary wind farm), and the secondary wind farm draws power from the secondary PCC and completes the black start;

[0082] S2.3, other auxiliary wind farms are started in sequence according to steps S2.1-S2.2.

[0083] When the auxiliary wind farm is started, the offshore MMC and offshore communication system will enter the steady-state operation mode, and the power transmitted by the offshore communication module will be controlled to 0. At this time, the output power of the auxiliary wind farm is only sent by the offshore DRU connected to it; so the offshore communication module can be disconnected or kept connected.

[0084] In the above embodiment, all offshore wind farms are successfully started up through the offshore communication module, thereby significantly reducing the size and cost of the offshore platform.

[0085] In order to verify the feasibility and effectiveness of the offshore wind power multi-terminal multi-heterogeneous DC transmission system and its operation control method in the above embodiment, in a specific embodiment of the present invention, the following is established in PSCAD / EMTDC: Figure 1 The simulation model of the system shown in the figure has the following system parameters:

[0086] Table 1MMC converter valve parameters

[0087]

[0088]

[0089] Table 2 Fan parameters

[0090]

[0091] During the simulation, at t = 2s, the system starts to start, and the simulation results of the MMC station and the DRU station are as follows: Figure 5 , Figure 6 and Figure 7 As shown, it can be found that the multi-heterogeneous DC transmission system and operation control method proposed in the embodiment of the present invention are feasible.

[0092] The above describes the specific embodiments of the present invention. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art can make various modifications or variations within the scope of the claims, which does not affect the essence of the present invention. The above preferred features can be used in any combination without conflicting with each other.

Claims

1. An offshore wind power multi-terminal multi-heterogeneous direct current transmission system, characterized in that: It includes the offshore part and the onshore part, including: The offshore part includes: A main wind farm, wherein the main wind farm is any one of the offshore wind farm clusters; An offshore MMC, wherein the offshore MMC is connected to the main wind farm to realize flexible direct current transmission of power from the main wind farm to the shore; Auxiliary wind farms, which are the remaining wind farms in the offshore wind power cluster; An offshore DRU, wherein the offshore DRU is connected to the auxiliary wind farm in a one-to-one correspondence, so as to realize uncontrolled rectification and transmission of the auxiliary wind farm to the shore; An offshore contact module, wherein the offshore contact module connects the main wind farm and the auxiliary wind farm, and through the offshore contact module, the main wind farm and the offshore MMC establish the AC bus voltage and frequency of the auxiliary wind farm to provide black start power for the auxiliary wind farm; The onshore part includes: An onshore MMC, wherein a plurality of the onshore MMCs receive the DC power transmitted by the offshore portion; a load center connected to the onshore MMC; A receiving-end power grid, wherein the receiving-end power grid is connected to the onshore MMC.

2. The offshore wind power multi-terminal multi-heterogeneous direct current transmission system according to claim 1 is characterized in that: The main wind farm is connected to the offshore MMC via an AC submarine cable; the offshore MMC is connected to the shore via a DC submarine cable; The offshore DRUs in the auxiliary wind farm are connected via AC submarine cables, and the offshore DRUs are connected to the shore via DC submarine cables; The onshore MMC is connected to the load center or the receiving power grid through an AC overhead line.

3. The offshore wind power multi-terminal multi-heterogeneous direct current transmission system according to claim 2 is characterized in that: The power transmission path at the main wind farm is: Electric energy is collected from the main wind farm through the AC submarine cable and then transmitted to the offshore MMC station; The voltage is stepped up and converted into high-voltage direct current by a transformer at the offshore MMC station, transmitted to the shore through the direct current submarine cable, and then converted into high-voltage alternating current by the shore MMC, and transmitted to the load center or the receiving end power grid through the alternating current overhead line; The power transmission path at the auxiliary wind farm is: After the electric energy is collected from the auxiliary wind farm through the AC submarine cable, it is transmitted to the offshore DRU station, where it is stepped up and converted into high-voltage DC by a transformer, transmitted to the shore through the DC submarine cable, and then converted into high-voltage AC by the onshore MMC, and transmitted to the load center or the receiving power grid through the AC overhead line.

4. The offshore wind power multi-terminal multi-heterogeneous direct current transmission system according to claim 1 is characterized in that: The offshore contact module is arranged between the low-voltage side common connection point of the coupling transformer of the offshore MMC of the main wind farm and the low-voltage side common connection point of the coupling transformer of the offshore DRU of the auxiliary wind farm.

5. The offshore wind power multi-terminal multi-heterogeneous direct current transmission system according to claim 4 is characterized in that: The working modes of the offshore communication module include industrial frequency AC, low frequency AC and DC; When the industrial frequency AC form is adopted, the offshore communication module includes an industrial frequency AC submarine cable, and the industrial frequency AC submarine cable connects the two common connection points; When the low-frequency AC form is adopted, the offshore communication module includes a low-frequency AC submarine cable and two AC-AC converters, the two AC-AC converters are respectively connected at the two common connection points to realize the conversion between industrial frequency AC and low-frequency AC, and the low-frequency AC submarine cable connects the two AC-AC converters; When the DC form is adopted, the offshore communication module includes a low-frequency AC submarine cable and two AC-DC converters, the two AC-DC converters are respectively connected at the two common connection points to realize the conversion between industrial frequency AC and DC, and the low-frequency AC submarine cable connects the two AC-DC converters.

6. The offshore wind power multi-terminal multi-heterogeneous direct current transmission system according to claim 4 is characterized in that: Through the offshore communication module, the offshore MMC establishes a connection with the offshore DRU, establishes the AC voltage and frequency of the common connection point on the low-voltage side of the connecting transformer of the auxiliary wind farm offshore DRU, and the auxiliary wind farm draws power from the common connection point on the low-voltage side of the connecting transformer of the auxiliary wind farm offshore DRU and completes the black start.

7. An operation control method for an offshore wind power multi-terminal heterogeneous direct current transmission system, characterized in that: include: Simultaneously start the offshore MMC and a connected onshore MMC to establish the AC voltage and frequency of the main wind farm; Start the offshore communication module, establish the AC voltage and frequency of the auxiliary wind farm, and provide black start power for the auxiliary wind farm. The auxiliary wind farm draws power and completes the startup.

8. The operation control method of an offshore wind power multi-terminal multi-heterogeneous direct current transmission system according to claim 7 is characterized in that: The simultaneous starting of the offshore MMC and a connected onshore MMC to establish the AC voltage and frequency of the main wind farm includes: Randomly select an onshore MMC and start it together with the offshore MMC connected to the main wind farm; The onshore MMC and the offshore MMC are pre-charged. After the pre-charging is completed, the onshore MMC adopts the DC voltage control mode; the offshore MMC adopts the constant AC voltage and frequency control mode.

9. The operation control method of an offshore wind power multi-terminal multi-heterogeneous direct current transmission system according to claim 7, characterized in that: The offshore contact module is started to establish the AC voltage and frequency of the auxiliary wind farm and provide black start power for the auxiliary wind farm. The auxiliary wind farm takes power and completes the startup, including: Select a secondary wind farm, start its offshore communication module, and establish a connection between the secondary wind farm and the offshore MMC; The AC voltage and frequency of the common connection point on the low-voltage side of the connection transformer on the auxiliary wind farm side are established through the offshore MMC, and the auxiliary wind farm draws power from the common connection point on the low-voltage side of the connection transformer and completes a black start; Other auxiliary wind farms were started one after another.

10. The operation control method of an offshore wind power multi-terminal multi-heterogeneous direct current transmission system according to claim 7, characterized in that: After the auxiliary wind farm is started, the offshore MMC and the offshore communication module will enter the steady-state operation mode, and the power transmitted by the offshore communication module is 0; at this time, the output power of the auxiliary wind farm is only sent out by the offshore DRU to which it is connected, and the offshore communication module is disconnected or continues to maintain the connection.

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