Offshore alternating-current wind power multi-cluster direct-current collecting and direct-current sending-out system

Abstract

The invention relates to the technical field of offshore alternating-current wind power direct-current sending-out, and discloses an offshore alternating-current wind power multi-cluster direct-current collecting and direct-current sending-out system which comprises an offshore alternating-current wind power cluster, a multi-level direct-current voltage collecting device and an onshore flexible direct-current converter station which are connected in sequence. Comprising a lowest voltage grade direct current pooling station layer, at least one intermediate voltage grade direct current pooling station layer and a highest voltage grade direct current pooling station which are connected from bottom to top, and is used for performing layered pooling on alternating current converged by a plurality of offshore alternating current wind power clusters through alternating current submarine cables based on a preset voltage level; and then the high-voltage direct-current cable is sent to an onshore flexible direct-current converter station. Based on a layered and modularized multi-layer voltage level direct current collection mechanism, the advantages of new energy direct current collection and transmission are fully exerted, the networking flexibility and the operation toughness of the direct current transmission system can be improved, and the transmission cost and loss are reduced.

Description

Technical Field

[0001] This invention relates to the field of offshore wind power DC transmission technology, and in particular to an offshore AC wind power multi-cluster DC collection and DC transmission system. Background Technology

[0002] Offshore wind power technology has been vigorously developed due to its advantages such as high stability, high wind speed, high power generation efficiency, no land occupation, and proximity to load centers. The all-DC collection and transmission scheme, which uses DC boosting to collect and transmit electricity, is gradually becoming an option for offshore wind power transmission because it ensures higher utilization of transmission lines and can use lower voltages than AC schemes while transmitting the same power.

[0003] However, existing all-DC collection and transmission systems in the context of large-scale isolated offshore wind power transmission rely on a massive, centralized offshore converter platform. This not only incurs extremely high manufacturing, installation, and maintenance costs, but also results in high costs and losses from long-distance AC submarine cables, making fault isolation difficult and impacting the overall system resilience. Furthermore, these all-DC collection and transmission systems are still at a single DC voltage level, and adding new wind turbine clusters typically requires redesigning or expanding the entire collection network or converter platform, making them highly inflexible and significantly limiting the application of wind farms in terms of scale, location, and phased construction. Therefore, providing a DC collection and transmission system capable of meeting the needs of offshore multi-cluster collection and transmission scenarios with wide collection ranges and long transmission distances is of great significance. Summary of the Invention

[0004] To address the aforementioned technical issues, this invention provides a multi-cluster DC collection and transmission system for offshore AC wind power. Based on a hierarchical and modular multi-level voltage DC collection mechanism, it fully leverages the advantages of new energy DC collection and transmission while enhancing the networking flexibility and operational resilience of the DC transmission system, reducing transmission costs and losses, and effectively meeting the needs of multi-cluster DC collection and transmission scenarios with wide collection ranges and long transmission distances.

[0005] This invention provides a multi-cluster DC collection and DC transmission system for offshore AC wind power, comprising an offshore AC wind power cluster, a multi-level DC voltage collection device, and an onshore flexible DC converter station connected in sequence: the multi-level DC voltage collection device includes a lowest voltage level DC collection station layer, at least one intermediate voltage level DC collection station layer, and a highest voltage level DC collection station connected from bottom to top. The multi-level DC voltage collection device is used to collect AC power from multiple offshore AC wind power clusters via AC submarine cables, and then send it to the onshore flexible DC converter station via high-voltage DC cables after the AC power is collected in layers based on a preset voltage level. Furthermore, the offshore AC wind power cluster is determined based on the division of offshore wind power areas and wind resource conditions.

[0006] Furthermore, the DC voltage level of the multi-level DC voltage collection device is determined based on the analysis of the new energy collection capacity and transmission distance.

[0007] Furthermore, the multi-level DC voltage collection device adopts a radial topology layout.

[0008] Furthermore, the lowest voltage level DC aggregation station in the lowest voltage level DC aggregation station layer is connected to different intermediate voltage level DC aggregation stations in the next intermediate voltage level DC aggregation station layer via DC submarine cables; each intermediate voltage level DC aggregation station in the intermediate voltage level DC aggregation station layer is connected to the next level via DC submarine cables, until it is connected to the highest voltage level DC aggregation station.

[0009] Furthermore, bipolar DC transmission lines are used to connect the lowest voltage level DC collection station layer with the intermediate voltage level DC collection station layer, adjacent intermediate voltage level DC collection station layers, and the intermediate voltage level DC collection station layer with the highest voltage level DC collection station.

[0010] Furthermore, the highest voltage level DC aggregation station includes a DC / DC transformer, and each of the intermediate voltage level DC aggregation stations in the intermediate voltage level DC aggregation station layer includes a multi-port DC / DC converter; each of the lowest voltage level DC aggregation stations in the lowest voltage level DC aggregation station layer includes an AC / DC converter.

[0011] Furthermore, when there is only one intermediate voltage level DC aggregation station layer, the AC / DC converter is used to control the speed of the offshore AC wind turbine; the multi-port DC / DC converter in the intermediate voltage level DC aggregation station layer is used to control the DC voltage stability of each corresponding lowest voltage level DC aggregation station in the lowest voltage level DC aggregation station layer; the DC / DC transformer in the highest voltage level DC aggregation station is used to control the discrete port voltage balance of each intermediate voltage level DC aggregation station in the intermediate voltage level DC aggregation station layer.

[0012] Furthermore, the multi-level DC voltage collection device controls the bus voltage and system reactive power based on a preset three-level control strategy; the preset three-level control strategy includes system-level control, converter-level control and valve-level control.

[0013] Furthermore, based on a preset three-level control strategy, the bus voltage and system reactive power are controlled, including: The system-level control is executed based on a preset system-level control strategy to generate top-level control commands; the top-level control commands include the operating power command and reactive power command of the offshore AC wind turbine, the DC-side voltage command of the DC / DC converter, the discrete voltage command of the DC / DC transformer, and the bus voltage command and reactive power command of the onshore flexible DC converter station. In response to the top-level control command, the converter-level control is executed based on a preset converter-level control strategy to generate converter control commands. The preset converter-level control strategy includes: generating a wind turbine voltage modulation signal based on vector control according to the operating power command and reactive power command of the offshore AC wind turbine; generating a phase shift signal based on dual closed-loop control according to the DC-side voltage command of the DC / DC converter; generating a duty cycle signal based on the discrete voltage command of the DC / DC transformer; and generating a receiving-end converter voltage modulation signal based on vector control according to the bus voltage command and reactive power command of the onshore flexible DC converter station. In response to the converter control command, the valve-level control is executed based on a preset valve-level control strategy. The preset valve-level control strategy includes: generating a converter valve switching pulse signal of the AC / DC converter based on sinusoidal pulse width modulation according to the wind turbine voltage modulation signal; generating a converter valve switching pulse signal of the DC / DC converter based on phase shift modulation according to the phase shift signal; generating a converter valve switching pulse signal of the DC / DC transformer based on DC chopper modulation according to the duty cycle signal; and generating a converter valve switching pulse signal of the onshore flexible DC converter station based on sinusoidal pulse width modulation according to the receiving-end converter voltage modulation signal.

[0014] This invention provides a multi-cluster DC collection and transmission system for offshore AC wind power, comprising, in sequence, offshore AC wind power clusters, a multi-level DC voltage collection device, and an onshore flexible DC converter station. The multi-level DC voltage collection device includes, from bottom to top, a lowest voltage level DC collection station layer, at least one intermediate voltage level DC collection station layer, and a highest voltage level DC collection station. It is used to collect AC power from multiple offshore AC wind power clusters via AC submarine cables, perform layered collection based on preset voltage levels, and then transmit the power to the onshore flexible DC converter station via high-voltage DC cables. Compared with existing technologies, this invention, based on a layered and modular multi-level voltage level DC collection mechanism, fully leverages the advantages of new energy DC collection and transmission while improving the networking flexibility and operational resilience of the DC transmission system, reducing transmission costs and losses. It effectively meets the needs of multi-cluster DC collection and transmission scenarios with wide collection ranges and long transmission distances, providing strong technical support for the future construction of offshore wind power transmission systems. Attached Figure Description

[0015] Figure 1This is a schematic diagram of the offshore AC wind power multi-cluster DC collection and DC transmission system in an embodiment of the present invention; Figure 2 This is a schematic diagram of the structure of the offshore multi-layer DC collection system in an embodiment of the present invention; Figure 3 This is a schematic diagram of the DC aggregation scenario at the lowest voltage level DC aggregation station layer in this embodiment of the invention; Figure 4 This is a schematic diagram of a DC collection scenario at the intermediate voltage level DC collection station layer in an embodiment of the present invention; Figure 5 This is a schematic diagram of the DC aggregation scenario at the highest voltage level DC aggregation station layer in this embodiment of the invention; Figure 6 This is a schematic diagram illustrating the application scenario of the three-level DC voltage collection device in an embodiment of the present invention; Figure 7 This is a schematic diagram of the operation control architecture of the offshore AC wind power multi-cluster DC collection and DC transmission system corresponding to the three-level DC voltage collection device in this embodiment of the invention. Among them, 1. Offshore wind power cluster; 2. Multi-level DC voltage collection device; 3. Onshore flexible DC converter station; 21. Lowest voltage level DC collection station layer; 211. Lowest voltage level DC collection station; 22. Intermediate voltage level DC collection station layer; 221. Intermediate voltage level DC collection station; 23. Highest voltage level DC collection station. Detailed Implementation

[0016] 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.

[0017] like Figure 1 As shown, this embodiment of the invention provides a multi-cluster DC collection and DC transmission system for offshore AC wind power, comprising an offshore AC wind power cluster 1, a multi-level DC voltage collection device 2, and an onshore flexible DC converter station 3 connected in sequence. The multi-level DC voltage collection device 2 is used to collect AC power from multiple offshore AC wind power clusters 1 via AC submarine cables, and then collect it in layers based on preset voltage levels before sending it to the onshore flexible DC converter station 3 via high-voltage DC cables. The multi-level DC voltage collection device 2 can be understood as a full DC collection device designed based on the principle of layering and partitioning, which collects DC power obtained by rectifying offshore AC wind power through multiple voltage levels to the highest voltage level before sending it to the onshore flexible DC converter station 3.

[0018] In one embodiment, such as Figure 2 As shown, the multi-level DC voltage aggregation device 2 includes a lowest voltage level DC aggregation station layer 21, at least one intermediate voltage level DC aggregation station layer 22, and a highest voltage level DC aggregation station 23 connected from bottom to top. It should be noted that, for ease of description, all voltage level DC aggregation station layers between the lowest voltage level DC aggregation station layer 21 and the highest voltage level DC aggregation station 23 are uniformly named intermediate voltage level DC aggregation station layers 22. However, in practical applications, the voltage levels corresponding to each intermediate voltage level DC aggregation station layer 22 increase progressively from bottom to top. In practical applications, the number of intermediate voltage level DC aggregation station layers 22 varies depending on the actual application scenario. To ensure the safe operation of the DC power grid while effectively reducing system construction costs and transmission losses, this embodiment preferably sets the number based on the analysis of the new energy aggregation capacity (scale) and transmission distance. For example, in practical applications, the DC transmission voltage level can be initially estimated using the Swedish Ullman empirical formula, and then appropriately adjusted according to the relationship between the DC voltage level and the distance and power of power transmission obtained from actual engineering statistics.

[0019] like Figure 3 As shown, each of the offshore AC wind power clusters 1 is connected to one of the lowest voltage level DC collection stations 211 in the lowest voltage level DC collection station layer 21 via AC submarine cables. Considering the fault isolation requirements of offshore wind farms to improve the power supply reliability of the system, in this embodiment, the offshore wind farms are preferably divided into multiple offshore AC wind power clusters 1 based on the offshore wind power area division and wind resource conditions. The specific number of offshore AC wind power clusters and the number of wind farms in each offshore AC wind power cluster vary depending on the actual application scenario and are not limited here.

[0020] To further enhance fault isolation while effectively improving the flexibility, stability, operational efficiency, and economy of the system construction, this embodiment preferably employs a radial topology for the multi-level DC voltage collection device 2. For example... Figure 4As shown, the lowest voltage level DC aggregation station 211 in the lowest voltage level DC aggregation station layer 21 is connected to different intermediate voltage level DC aggregation stations 221 in the previous intermediate voltage level DC aggregation station layer 22 via DC submarine cables; as Figure 5 As shown, the intermediate voltage level DC collection stations 221 in each of the intermediate voltage level DC collection station layers 22 are connected to the next higher level via DC submarine cables, until they are connected to the highest voltage level DC collection station 23. The highest voltage level DC collection station 23 is connected to the onshore flexible DC converter station 3 via a high-voltage DC cable.

[0021] In practical applications, each offshore wind farm within an offshore AC wind power cluster 1 is connected to the same lowest voltage level DC collection station 211 in the lowest voltage level DC collection station layer 21 via an AC submarine cable (AC transmission line). This rectifies the AC voltage input to the offshore AC wind power cluster 1 into the lowest voltage level DC voltage. Based on the voltage level collection requirements corresponding to the lowest voltage level DC collection station 211, multiple offshore AC wind power clusters 1 will converge to the same lowest voltage level DC collection station 211. Then, based on the intermediate voltage level collection requirements corresponding to the upper-level intermediate voltage level DC collection station layer 22, such as... Figure 4 As shown, multiple lowest voltage level DC collection stations 211 in the lowest voltage level DC collection station layer 21 will be connected to the same intermediate voltage level DC collection station 221 in the next intermediate voltage level DC collection station layer 22 via DC submarine cables for DC voltage boosting; when there are two or more intermediate voltage level DC collection station layers 22 at the same time, after each intermediate voltage level DC collection station layer 22 boosts the DC voltage of the next level, it will simultaneously connect multiple intermediate voltage level DC collection stations 221 in this layer to the next level according to the voltage level collection requirements of the adjacent upper layer, until it is connected to the highest voltage level DC collection station 23 corresponding to the highest voltage level.

[0022] The AC power from the offshore wind farm is collected in multiple layers by the multi-level DC voltage collection device 2 provided in this embodiment, and then sent to the onshore flexible DC converter station 3 through the high-voltage DC cable connected to the multi-level DC voltage collection device 2.

[0023] In practical applications, to improve the operational reliability, fault ride-through capability, and voltage level matching flexibility of the multi-level DC voltage aggregation device 2, this embodiment preferably uses bipolar DC transmission lines to connect the lowest voltage level DC aggregation station layer 21 with the intermediate voltage level DC aggregation station layer 22, adjacent intermediate voltage level DC aggregation station layers 22, and the intermediate voltage level DC aggregation station layer 22 with the highest voltage level DC aggregation station 23. Specific bipolar DC transmission line connection methods can be directly referenced from existing technologies and will not be elaborated here.

[0024] To further enhance the high integration of the multi-level DC voltage aggregation device 2, reduce the number of converters used, lower device cost and size, improve system efficiency and performance, and enhance the flexibility and intelligence of multi-voltage level DC aggregation control, this embodiment preferably sets the highest voltage level DC aggregation station 23 to include a DC / DC transformer, each intermediate voltage level DC aggregation station 221 in the intermediate voltage level DC aggregation station layer 22 to include a multi-port DC / DC converter, and each lowest voltage level DC aggregation station 211 in the lowest voltage level DC aggregation station layer 21 to include an AC / DC converter. To facilitate a clear understanding of the application scenarios of the multi-level DC voltage aggregation device 2 described in this embodiment, the following will use... Figure 6 The following is an example of a three-level DC voltage collection device (containing only one intermediate voltage level DC collection station layer): like Figure 6 As shown, the DC voltage levels of the three-tiered DC voltage collection device, from high to low, are ±VDC1H, ±VDC2H, and ±VDC3H, exhibiting a radial topology. In practical applications, the offshore wind power is first grouped according to the offshore wind power area division and wind resource conditions, forming multiple offshore wind power clusters 1. Then, it is connected via AC submarine cable (AC transmission line) to the lowest voltage level DC collection station 211, which contains an AC / DC converter, to convert the AC voltage V... AC3 After rectification to a DC voltage of ±VDC3H, multiple lowest voltage level DC collection stations 211 are connected to the intermediate voltage level DC collection station 221, which includes a multi-port DC / DC converter, via a DC submarine cable (bipolar DC transmission line). The DC voltage is then boosted to ±VDC2H and connected to the highest voltage level DC collection station 23 via a DC submarine cable (bipolar DC transmission line). The DC voltage is then boosted to ±VDC1H and sent out.

[0025] In this embodiment, the multi-level DC voltage aggregation device mainly ensures that all the collected energy from the wind farm side can be effectively delivered by controlling the bus voltage and the reactive power of the system. Specifically, when there is only one intermediate voltage level DC aggregation station layer 22, the AC / DC converter is used to control the speed of the offshore AC wind turbine to achieve efficient wind energy capture; the multi-port DC / DC converter in the intermediate voltage level DC aggregation station layer 22 is used to control the DC voltage stability of each corresponding lowest voltage level DC aggregation station 211 in the lowest voltage level DC aggregation station layer 21, that is, the multi-port DC / DC converter provides a guarantee for the maximum power tracking operation of the wind turbine by controlling the DC voltage stability on the turbine side; the DC DC / DC transformer in the highest voltage level DC aggregation station 23 is used to control the discrete port voltage balance of each intermediate voltage level DC aggregation station 221 in the intermediate voltage level DC aggregation station layer 22, that is, the DC DC / DC transformer adjusts the output power of each wind turbine by controlling the discrete port voltage of each multi-port DC / DC converter to achieve discrete port voltage balance.

[0026] To ensure the reliable and stable operation of the offshore AC wind power multi-cluster DC aggregation and DC transmission system by effectively regulating the AC / DC converter, multi-port DC / DC converter, and DC / DC transformer in the multi-level DC voltage aggregation device 2, in this embodiment, the multi-level DC voltage aggregation device 2 preferably controls the bus voltage and system reactive power based on a preset three-level control strategy; the preset three-level control strategy includes system-level control, converter-level control, and valve-level control.

[0027] System-level control can be understood as the global coordinated control of the offshore AC wind power multi-cluster DC collection and DC transmission system. It is mainly responsible for generating commands to control the operating power and reactive power of offshore AC wind turbines, commands to control the discrete port voltages of the multi-port DC / DC converters in the intermediate voltage level DC collection station 22 by the DC DC / DC transformer in the highest voltage level DC collection station 23, commands to control the DC side voltage in the lowest voltage level DC collection station 21 by each multi-port DC / DC converter in the intermediate voltage level DC collection station 22, and commands to control the DC bus voltage and reactive power of the onshore flexible DC converter station 3. The converter level is mainly responsible for calculating and generating control signals based on the commands generated by system-level control to control the output voltage of the AC / DC converter, the phase shift of the multi-port DC / DC converter, the duty cycle of the DC DC / DC transformer, and the output voltage of the onshore flexible DC converter station 3. The valve level control is mainly responsible for converting the upper-level control signals received by each converter into corresponding switching pulse signals through appropriate modulation methods, and acting on the corresponding converter valves to achieve stable operation of the system.

[0028] Specifically, such as Figure 7 As shown, the control of bus voltage and system reactive power based on a preset three-level control strategy includes: The system-level control is executed based on a preset system-level control strategy to generate top-level control commands; the top-level control commands include the operating power commands for the offshore AC wind turbines. and reactive power command DC / DC converter DC side voltage command DC / DC transformer discrete voltage command The HVDC bus voltage command of the onshore flexible DC converter station 3 and reactive power command ; In response to the top-level control command, the converter-level control is executed based on a preset converter-level control strategy, generating converter control commands; the preset converter-level control strategy includes: based on the operating power command of the offshore AC wind turbine. and reactive power command The wind turbine voltage modulation signal (modulation wave) is generated based on vector control. According to the DC-side voltage command of the DC / DC converter Phase shift signal generated based on dual closed-loop control According to the discrete voltage command of the DC / DC transformer Generate duty cycle signal According to the bus voltage command of the onshore flexible DC converter station 3 and reactive power command The receiving-end commutator voltage modulation signal (modulation wave) is generated based on vector control. ; In response to the converter control command, valve-level control is executed based on a preset valve-level control strategy; the preset valve-level control strategy includes: based on the fan voltage modulation signal The switching pulse signal of the commutator valve of the AC / DC converter is generated based on sinusoidal pulse width modulation; according to the phase shift signal... The switching pulse signal of the commutator valve of the DC / DC converter is generated based on phase shift modulation; according to the duty cycle signal... The switching pulse signal of the converter valve of the DC / DC transformer is generated based on DC chopper modulation; the switching pulse signal is generated based on the receiving-end commutation voltage modulation signal. The converter valve switching pulse signal of the onshore flexible DC converter station is generated based on the sinusoidal pulse width modulation. It should be noted that the sinusoidal pulse width modulation, phase shift modulation and DC chopper modulation in this embodiment can all be implemented with reference to existing technologies, and will not be described in detail here.

[0029] This invention provides a multi-cluster DC collection and transmission system for offshore AC wind power, comprising a series of interconnected offshore AC wind power clusters, a multi-level DC voltage collection device, and an onshore flexible DC converter station. The multi-level DC voltage collection device includes, from bottom to top, a lowest voltage level DC collection station layer, at least one intermediate voltage level DC collection station layer, and a highest voltage level DC collection station. It is used to collect AC power from multiple offshore AC wind power clusters via AC submarine cables, perform layered collection based on preset voltage levels, and then transmit the power to the onshore flexible DC converter station via high-voltage DC cables. This system, based on a layered, modular multi-level voltage DC collection mechanism and corresponding operational control logic, fully leverages the advantages of new energy DC collection and transmission while improving the grid flexibility and operational resilience of the DC transmission system, reducing transmission costs and losses. It effectively meets the needs of multi-cluster DC collection and transmission scenarios with wide collection ranges and long transmission distances, providing strong technical support for the future construction of offshore wind power transmission systems.

[0030] The various embodiments in this specification are described in a progressive manner. For directly identical or similar parts of the embodiments, refer to each other. Each embodiment focuses on describing the differences from other embodiments. In particular, the system embodiments are basically similar to the method embodiments, so the description is relatively simple; relevant parts can be referred to the descriptions in the method embodiments. It should be noted that the technical features of the above embodiments can be combined arbitrarily. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as the combination of these technical features does not contradict each other, it should be considered within the scope of this specification.

[0031] The above-described embodiments are merely preferred embodiments of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the invention. It should be noted that those skilled in the art can make various improvements and substitutions without departing from the principles of the present invention, and these improvements and substitutions should also be considered within the scope of protection of the present invention. Therefore, the scope of protection of this invention should be determined by the scope of the claims.

Claims

1. A marine AC wind power multi-cluster DC collection and DC transmission system, characterized in that, The system comprises a marine alternating current wind power cluster, a multi-level direct current voltage collection device and a land flexible direct current converter station connected in sequence. The multi-level direct current voltage collection device is used for collecting alternating current from a plurality of marine alternating current wind power clusters through an alternating current submarine cable, collecting the alternating current based on preset voltage levels, and sending the alternating current to the land flexible direct current converter station through a high-voltage direct current cable.

2. The offshore AC wind power multi-cluster DC collection and DC transmission system according to claim 1, characterized in that, The marine alternating current wind power cluster is divided based on a marine wind power region division and wind resource conditions.

3. The offshore AC wind power multi-cluster DC collection and DC transmission system according to claim 1, characterized in that, The direct current voltage levels of the multi-level direct current voltage collection device are determined based on new energy collection capacity and transmission distance analysis. 4.The offshore AC wind power multi-cluster DC collection and DC transmission system of claim 1, wherein, The multi-level direct current voltage collection device adopts a radial topology structure. 5.The offshore AC wind power multi-cluster DC collection and DC transmission system of claim 4, wherein, The lowest voltage level direct current collection station in the lowest voltage level direct current collection station layer is connected to different intermediate voltage level direct current collection stations in the previous intermediate voltage level direct current collection station layer through a direct current submarine cable; the intermediate voltage level direct current collection stations in each intermediate voltage level direct current collection station layer are connected to the next level through a direct current submarine cable, until the highest voltage level direct current collection station is connected. 6.The offshore AC wind power multi-cluster DC collection and DC transmission system of claim 4, wherein, The lowest voltage level direct current collection station layer and the intermediate voltage level direct current collection station layer, adjacent intermediate voltage level direct current collection station layers, and the intermediate voltage level direct current collection station layer and the highest voltage level direct current collection station are connected by bipolar direct current transmission lines.

7. The offshore AC wind power multi-cluster DC collection and DC transmission system of claim 1, wherein, The highest voltage level direct current collection station comprises a direct current DC / DC transformer, each intermediate voltage level direct current collection station in the intermediate voltage level direct current collection station layer comprises a multi-port DC / DC converter, and each lowest voltage level direct current collection station in the lowest voltage level direct current collection station layer comprises an AC / DC converter. 8.The offshore AC wind power multi-cluster DC collection and DC transmission system of claim 7, wherein, When there is only one intermediate voltage level direct current collection station layer, the AC / DC converter is used to control the speed of the marine alternating current wind turbine; the multi-port DC / DC converter in the intermediate voltage level direct current collection station layer is used to control the direct current voltage stability of each lowest voltage level direct current collection station in the lowest voltage level direct current collection station layer; The direct current DC / DC transformer in the highest voltage level direct current collection station is used to control the discrete port voltage balance of each intermediate voltage level direct current collection station in the intermediate voltage level direct current collection station layer. 9.The offshore AC wind power multi-cluster DC collection and DC transmission system of claim 8, wherein, The multi-level direct current voltage collection device controls the bus voltage and system reactive power based on a preset three-level control strategy; the preset three-level control strategy comprises system-level control, converter-level control and valve-level control. 10.The offshore AC wind power multi-cluster DC collection and DC transmission system of claim 9, wherein, The control of the bus voltage and system reactive power based on the preset three-level control strategy comprises: The system-level control is performed based on a preset system-level control strategy to generate a top-level control instruction; the top-level control instruction includes an operating power instruction and a reactive power instruction of an offshore AC fan, a DC / DC converter DC side voltage instruction, a DC / DC transformer discrete voltage instruction, a bus voltage instruction and a reactive power instruction of the onshore flexible DC converter station; The converter-level control is performed based on a preset converter-level control strategy in response to the top-level control instruction to generate a converter control instruction; the preset converter-level control strategy includes: generating a fan voltage modulation signal based on vector control according to the operating power instruction and the reactive power instruction of the offshore AC fan; generating a phase shift amount signal based on double closed loop control according to the DC / DC converter DC side voltage instruction; generating a duty cycle signal according to the DC / DC transformer discrete voltage instruction; and generating a receiving end converter voltage modulation signal based on vector control according to the bus voltage instruction and the reactive power instruction of the onshore flexible DC converter station; The valve-level control is performed based on a preset valve-level control strategy in response to the converter control instruction; the preset valve-level control strategy includes: generating a converter valve switching pulse signal of the AC / DC converter based on sinusoidal pulse width modulation according to the fan voltage modulation signal; generating a converter valve switching pulse signal of the DC / DC converter based on phase shift modulation according to the phase shift amount signal; generating a converter valve switching pulse signal of the DC / DC transformer based on DC chopper modulation according to the duty cycle signal; and generating a converter valve switching pulse signal of the onshore flexible DC converter station based on the sinusoidal pulse width modulation according to the receiving end converter voltage modulation signal.

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