Offshore wind power DC transmission system

Abstract

This application provides an offshore wind power DC transmission system. The system includes: an onshore DC switch station, multiple offshore converter stations, and multiple onshore converter units; the AC side of the offshore converter stations is connected to the wind turbine generators, and the DC side of the offshore converter stations is connected to the input side of the onshore DC switch station's busbar via DC cables; the input side of the busbar is equipped with a DC switch for connection to the DC cables, and the output side of the busbar is connected to an overhead DC line via a DC circuit breaker. The overhead DC line is connected to the DC side of the onshore converter units via a DC circuit breaker, and each overhead DC line is connected to one or more onshore converter units; the DC side of each onshore converter unit is equipped with a high-speed parallel DC switch, which connects to the overhead DC line; the AC side of each onshore converter unit is connected to a load center; both the offshore converter stations and the onshore converter units adopt a half-bridge MMC topology. This application enables reliable and low-cost ultra-high capacity DC transmission.

Description

Technical Field

[0001] This application relates to the field of DC power transmission technology, and in particular to a DC power transmission system for offshore wind power. Background Technology

[0002] In recent years, to promote energy transition, my country's policies have strongly supported the development and utilization of offshore wind power. Offshore wind power development is showing a trend from near-shore, shallow-water, small-scale demonstrations to large-scale, centralized development in open ocean and deep water. The transmission methods for offshore wind power to connect to the onshore power grid include high-voltage AC transmission and high-voltage DC transmission. High-voltage AC transmission is suitable for near-shore wind power. However, when offshore wind power is located beyond a certain distance from the shore, long-distance AC cable transmission suffers from problems such as excessive charging power leading to voltage rise requiring compensation and overvoltage. High-voltage AC transmission cannot meet the demands of large-capacity, long-distance offshore wind power transmission and is also costly.

[0003] High-voltage direct current (HVDC) transmission offers advantages such as not requiring synchronization with the onshore power grid, long transmission distances, and flexible operation and control, making it more suitable for large-capacity, long-distance power transmission scenarios. Flexible HVDC, in particular, avoids commutation failure issues, allows independent adjustment of active and reactive power, and exhibits low harmonic levels, making it the mainstream method for large-scale offshore wind power transmission to the onshore power grid. Achieving ultra-high capacity transmission requires a multi-terminal flexible HVDC transmission scheme; however, current HVDC transmission system architectures struggle to achieve stable transmission of ultra-large capacities. Summary of the Invention

[0004] This application provides an offshore wind power DC transmission system that enables reliable and low-cost ultra-large capacity DC transmission and improves system stability.

[0005] This application provides an offshore wind power DC transmission system.

[0006] Includes: onshore DC switch stations, multiple offshore converter stations, and multiple onshore converter units;

[0007] The AC side of the offshore converter station is used to connect to the wind turbine generator, and the DC side of the offshore converter station is connected to the input side of the busbar of the onshore DC switch station via a DC cable.

[0008] The input side of the busbar is equipped with a DC switch for connecting to a DC cable. The output side of the busbar is connected to a DC overhead line via a DC circuit breaker. The DC overhead line is connected to the DC side of the onshore converter unit via a DC circuit breaker, and each DC overhead line is connected to one or more onshore converter units.

[0009] The DC side of the onshore converter unit is equipped with a high-speed DC parallel switch, which is connected to the DC overhead line. The AC side of each onshore converter unit is used to connect to a load center.

[0010] Both the offshore converter station and the onshore converter unit adopt a half-bridge MMC topology.

[0011] In one embodiment, the DC switch configured on the input side of the busbar includes a high-speed DC parallel switch or a DC circuit breaker.

[0012] In one embodiment, a DC power dissipation device is disposed between the positive and negative terminals of the busbar.

[0013] In one embodiment, both the offshore converter station and the onshore converter unit adopt a symmetrical unipolar topology.

[0014] The positive terminal of the busbar of the onshore DC switch station is connected to the positive terminal of two or more onshore converter units via the first DC overhead line of each circuit, and the negative terminal of the busbar is connected to the negative terminal of the onshore converter unit connected to the first DC overhead line in the same circuit via the second DC overhead line of each circuit.

[0015] The positive terminals of each onshore converter unit in the same circuit are interconnected through the third overhead DC line, and the negative terminals of each onshore converter unit in the same circuit are interconnected through the fourth overhead DC line.

[0016] The positive terminal of the busbar is connected to the positive terminal of an offshore converter station via the first DC cable of each circuit, and the negative terminal of the busbar is connected to the negative terminal of the offshore converter station connected to the first DC cable of the same circuit via the second DC cable of each circuit.

[0017] In one embodiment,

[0018] Both the offshore converter station and the onshore converter unit adopt a symmetrical single-pole topology.

[0019] The positive terminal of the onshore DC switch station's busbar is connected to the positive terminal of at least one onshore converter unit via the first DC overhead line of each circuit, and the negative terminal of the busbar is connected to the negative terminal of the onshore converter unit connected to the first DC overhead line of the same circuit via the second DC overhead line of each circuit.

[0020] The DC side of each onshore converter unit in each loop is interconnected with the DC side of at least one onshore converter unit in a different loop, wherein the positive terminal of each onshore converter unit is interconnected via a third overhead DC line and the negative terminal is interconnected via a fourth overhead DC line.

[0021] The positive terminal of the busbar is connected to the positive terminal of an offshore converter station via the first DC cable of each circuit, and the negative terminal of the busbar is connected to the negative terminal of the offshore converter station connected to the first DC cable of the same circuit via the second DC cable of each circuit.

[0022] In one embodiment, a high-speed DC parallel switch is configured at the outlet where the onshore converter unit connects to the third DC overhead line, and a high-speed DC parallel switch is configured at the outlet where the onshore converter unit connects to the fourth DC overhead line.

[0023] In one embodiment, both the offshore converter station and the onshore converter unit adopt a bipolar topology.

[0024] The positive terminal of the busbar of the onshore DC switch station is connected to the positive terminal of two or more onshore converter units via the first DC overhead line of each circuit. The negative terminal of the busbar is connected to the negative terminal of the onshore converter unit connected to the first DC overhead line in the same circuit via the second DC overhead line of each circuit. The neutral line of the busbar is connected to the neutral line of the onshore converter unit via the fifth DC overhead line of each circuit.

[0025] The neutral line of the busbar is connected to the fifth overhead DC line via an AC circuit breaker, and the neutral line of the onshore converter unit is connected to the fifth overhead DC line via an AC circuit breaker.

[0026] The positive terminals of each onshore converter unit in the same circuit are interconnected through the third DC overhead line, the negative terminals of each onshore converter unit in the same circuit are interconnected through the fourth DC overhead line, and the neutral lines of each onshore converter unit in the same circuit are interconnected through the sixth DC overhead line.

[0027] The positive terminal of the busbar is connected to the positive terminal of an offshore converter station via the first DC cable of each circuit. The negative terminal of the busbar is connected to the negative terminal of the offshore converter station connected to the first DC cable of the same circuit via the second DC cable of each circuit. The neutral line of the busbar is connected to the neutral line of the offshore converter station connected to the first DC cable of the same circuit via the third DC cable of each circuit. The neutral line of the busbar is connected to the third DC cable via an AC circuit breaker.

[0028] In one embodiment, both the offshore converter station and the onshore converter unit adopt a bipolar topology.

[0029] The positive terminal of the busbar of the onshore DC switch station is connected to the positive terminal of at least one onshore converter unit via the first DC overhead line of each circuit. The negative terminal of the busbar is connected to the negative terminal of the onshore converter unit connected to the first DC overhead line in the same circuit via the second DC overhead line of each circuit. The neutral line of the busbar is connected to the neutral line of the onshore converter unit via the fifth DC overhead line of each circuit.

[0030] The neutral line of the busbar is connected to the fifth overhead DC line via an AC circuit breaker, and the neutral line of the onshore converter unit is connected to the fifth overhead DC line via an AC circuit breaker.

[0031] The DC side of each onshore converter unit in each loop is interconnected with the DC side of at least one onshore converter unit in a different loop, wherein the positive terminal of each onshore converter unit is interconnected via a third overhead DC line, the negative terminal is interconnected via a fourth overhead DC line, and the neutral line is interconnected via a sixth overhead DC line.

[0032] The positive terminal of the busbar is connected to the positive terminal of an offshore converter station via the first DC cable of each circuit. The negative terminal of the busbar is connected to the negative terminal of the offshore converter station connected to the first DC cable of the same circuit via the second DC cable of each circuit. The neutral line of the busbar is connected to the neutral line of the offshore converter station connected to the first DC cable of the same circuit via the third DC cable of each circuit. The neutral line of the busbar is connected to the third DC cable via an AC circuit breaker.

[0033] In one embodiment, a high-speed DC parallel switch is configured at the outlet where the onshore converter unit is connected to the third DC overhead line, a high-speed DC parallel switch is configured at the outlet where the onshore converter unit is connected to the fourth DC overhead line, and an AC circuit breaker is configured at the outlet where the onshore converter unit is connected to the sixth DC overhead line.

[0034] In one embodiment, the AC sides of two or more of the aforementioned onshore converter units are interconnected.

[0035] As can be seen from the above technical solutions, the embodiments of this application have the following advantages:

[0036] The offshore wind power DC transmission system provided in this application includes an onshore DC switch station and multiple offshore converter stations and multiple onshore converter units, all adopting a half-bridge MMC topology. Power is transmitted offshore via DC cables and onshore via overhead DC lines. The onshore and offshore sides are connected by a busbar at the onshore DC switch station. This reduces the transmission cost of large-capacity offshore wind power, saves transmission corridor space, and can support transmission demands at the multi-megawatt level. DC circuit breakers are installed at both ends of the overhead DC lines to achieve fault isolation. When both the offshore converter stations and onshore converter units adopt a half-bridge MMC topology without DC fault clearing capability, this ensures reliable fault isolation in the event of a fault, improving system stability. Attached Figure Description

[0037] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0038] Figure 1 This is a schematic diagram of the structure of an offshore wind power DC transmission system in one embodiment;

[0039] Figure 2 This is a schematic diagram of the converter station topology using a symmetrical unipolar topology in one embodiment;

[0040] Figure 3 This is a schematic diagram of the structure of an offshore wind power DC transmission system with a symmetrical monopole topology and a single overhead line, as shown in one embodiment.

[0041] Figure 4 This is a schematic diagram of the structure of an offshore wind power DC transmission system with a symmetrical unipolar topology and multiple overhead lines, as shown in one embodiment.

[0042] Figure 5 This is a schematic diagram of the structure of an offshore wind power DC transmission system using a bipolar topology and a single-circuit overhead line, as shown in one embodiment.

[0043] Figure 6 This is a schematic diagram of the structure of an offshore wind power DC transmission system employing a bipolar topology and consisting of multiple overhead lines, as shown in one embodiment.

[0044] Figure 7 This is a schematic diagram of a marine wind power DC transmission system using a bipolar topology and multiple overhead lines, as described in another embodiment. Detailed Implementation

[0045] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0046] like Figure 1 As shown in the figure, this application provides an offshore wind power DC transmission system, including: an onshore DC switch station 110, multiple offshore converter stations 120 and multiple onshore converter units 130, wherein the offshore converter stations 120 and the onshore converter units 130 both adopt a half-bridge MMC topology. The offshore converter station 120 has an AC side for connecting to the wind turbine WT, and a DC side for connecting to the input side of the busbar of the onshore DC switch station 110 via a DC cable. The input side of the busbar is equipped with a DC switch 111 for connecting to the DC cable. The output side of the busbar is connected to the DC overhead line via a DC circuit breaker (DCCB). The DC overhead line is connected to the DC side of the onshore converter unit 130 via the DC circuit breaker (DCCB), and each loop of the DC overhead line is connected to one or more onshore converter units 130. The DC side of the onshore converter unit 130 is equipped with a DC high-speed parallel switch (HSS) and is connected to the DC overhead line via the DC high-speed parallel switch (HSS). The AC side of each onshore converter unit 130 is used to connect to a load center G.

[0047] The core component of a Modular Multilevel Converter (MMC) is the sub-module connected in series within the bridge arm. A half-bridge MMC topology is one where the sub-modules are half-bridge modules. Due to its lower cost and wider applicability, the half-bridge MMC topology is widely used in offshore wind power DC transmission systems. However, half-bridge sub-modules lack DC fault clearing capabilities. In a flexible DC transmission system, if a fault occurs in the DC line, the converter and DC-side energy storage components will discharge rapidly, causing the fault current to reach the withstand limit of the power electronic devices within milliseconds, threatening the safe operation of the equipment and system. (Reference) Figure 2 As shown (taking a symmetrical unipolar topology as an example), in this embodiment, both the offshore converter and the onshore converter adopt a half-bridge MMC topology, which does not have the ability to clear DC faults. DC circuit breakers (DCCBs) are configured on both sides of the DC overhead line. The DC circuit breakers (DCCBs) can realize the rapid clearing and isolation of DC faults. When a DC fault occurs, the fault isolation can be achieved through the DC circuit breakers (DCCBs) on both sides of the DC overhead line, thereby improving the stability of system operation.

[0048] Furthermore, this embodiment proposes a transmission method using DC cables on the offshore side and DC overhead lines on the onshore side. Compared to the European method of using DC land cables for onshore power transmission, this significantly reduces costs. Compared to the current method in my country where AC overhead lines are used for onshore power transmission, this method reduces the number of transmission corridors in large-scale offshore wind power transmission. Within the capacity limits, each DC overhead line can connect to multiple onshore converter units 130, further saving transmission corridors and reducing costs. In this embodiment, an onshore DC switch station 110 is set up to connect DC cables and DC overhead lines, realizing the conversion between offshore cables and overhead lines. In long-distance power transmission, the DC switch 111 of the onshore DC switch station 110 can be used to isolate faulty sections of the line, avoiding the impact on the entire transmission line when the fault is isolated, and improving the flexibility of the system power supply.

[0049] The DC cables include DC submarine cables and / or DC land cables. Since the busbar may be located in a land area at a certain distance from the coastline, DC submarine cables are used for the part transmitted at sea, and DC land cables are used for transmission between the coastline and the busbar.

[0050] In some embodiments, a land-based converter unit refers to a land-based converter station. In other embodiments, a land-based converter unit refers to a single converter unit within a single land-based converter station.

[0051] In some embodiments, the load centers connected to different onshore converter units may be the same load center or may be connected to different load centers.

[0052] The offshore wind power DC transmission system provided in this application includes an onshore DC switch station 110 and multiple offshore converter stations 120 and multiple onshore converter units 130, all of which adopt a half-bridge MMC topology. Power is transmitted offshore via DC cables and onshore via overhead DC lines. The onshore and onshore sides are connected by the busbar of the onshore DC switch station 110. This reduces the transmission cost of large-capacity offshore wind power, saves transmission corridor space, and can support transmission demands at the multi-megawatt level. DC circuit breakers (DCCBs) are configured at both ends of the overhead DC lines to achieve fault isolation. When both the offshore converter station 120 and the onshore converter unit 130 adopt a half-bridge MMC topology without DC fault clearing capability, this ensures reliable fault isolation in the event of a fault, improving system stability.

[0053] In one embodiment, the DC switch 111 configured on the input side of the busbar is a DC circuit breaker (DCCB).

[0054] A DC circuit breaker (DCCB) is configured on the input side of the busbar to connect to the DC cable. In the event of a DC fault, the DC circuit breaker (DCCB) can cut off the short-circuit current in the DC cable, thereby achieving fault isolation.

[0055] In one embodiment, the DC switch 111 configured on the input side of the busbar is a DC high-speed parallel switch (HSS).

[0056] Configuring a high-speed DC parallel switch (HSS) connected to the DC cable on the input side of the busbar enables fault isolation, improving the reliability and availability of the entire DC system. Using an HSS prevents power interruption during converter station commissioning or decommissioning. Although the HSS cannot interrupt large DC currents, it is less expensive, smaller, and simpler in structure than a DC circuit breaker (DCCB). Combined with DC circuit breakers (DCCBs) on overhead DC lines, it can reliably isolate system faults.

[0057] like Figure 3 As shown, in one embodiment, a DC power dissipation device 140 is disposed between the positive and negative terminals of the busbar; wherein, the first end of the DC power dissipation device 140 is connected to the positive terminal DC+ of the busbar, and the second end of the DC power dissipation device 140 is connected to the negative terminal DC- of the busbar.

[0058] To further ensure the system's fault clearing capability in the event of a DC fault, a DC energy dissipation device 140 can be installed between the positive and negative poles of the busbar. After the DC circuit breaker DCCB clears the DC fault, the DC energy dissipation device 140 can operate at full voltage to achieve surplus power balance during the DC fault.

[0059] In one embodiment, if the offshore converter station 120 or the wind turbine is equipped with an energy-consuming device, the DC energy-consuming device 140 may not be configured. The surplus power can be balanced through the energy-consuming device of the offshore converter station 120 or the energy-consuming device of the wind turbine. However, this method has higher requirements for the configuration of the offshore converter station 120 or the wind turbine and requires a larger cost.

[0060] refer to Figure 3As shown, in one embodiment, both the offshore converter station 120 and the onshore converter unit 130 have a symmetrical unipolar topology. The positive terminal DC+ of the onshore DC switch station 110 is connected to the positive terminals of two or more onshore converter units 130 via the first DC overhead line of each circuit, and the negative terminal DC- of the busbar is connected to the negative terminal of the onshore converter unit 130 connected to the first DC overhead line in the same circuit via the second DC overhead line of each circuit. The positive terminals of each onshore converter unit 130 in the same circuit are interconnected via a third DC overhead line, and the negative terminals of each onshore converter unit 130 in the same circuit are interconnected via a fourth DC overhead line. The positive terminal DC+ of the busbar is connected to the positive terminal of an offshore converter station 120 via the first DC cable of each circuit, and the negative terminal DC- of the busbar is connected to the negative terminal of the offshore converter station 120 connected to the first DC cable in the same circuit via the second DC cable of each circuit.

[0061] In this embodiment, a converter station with a symmetrical monopole topology is adopted. For converter stations in large-scale offshore wind power transmission systems, the symmetrical monopole topology allows for the use of flexible DC transformers with an odd or even total number, providing greater flexibility in system equipment configuration. The capacity of a single onshore converter unit 130 is greater than that of a single offshore converter station 120. Within the allowable capacity range, a single overhead line can simultaneously connect to multiple converter stations. In one embodiment, a high-speed parallel DC switch (HSS) is configured on both the third and fourth DC overhead lines. When fault isolation is required on either the third or fourth DC overhead line, especially when the electrical distance between the third and fourth DC overhead lines reaches a certain level, isolation can be achieved through the high-speed parallel DC switch (HSS), improving system stability.

[0062] like Figure 4 As shown, in one embodiment, both the offshore converter station 120 and the onshore converter unit 130 adopt a symmetrical unipolar topology, and the DC overhead line uses a multi-circuit connection; the positive terminal DC+ of the onshore DC switch station 110 is connected to the positive terminal of at least one onshore converter unit 130 via the first DC overhead line of each single circuit, and the negative terminal DC- of the busbar is connected to the negative terminal of the onshore converter unit 130 connected to the first DC overhead line in the same circuit via the second DC overhead line of each single circuit; the onshore in each circuit... The DC side of converter unit 130 is interconnected with the DC side of at least one onshore converter unit 130 of a different circuit. The positive terminal of each onshore converter unit 130 is interconnected via a third overhead DC line, and the negative terminal is interconnected via a fourth overhead DC line. The positive terminal DC+ of the busbar is connected to the positive terminal of an offshore converter station 120 via the first DC cable of each single circuit, and the negative terminal DC- of the busbar is connected to the negative terminal of the offshore converter station 120 connected to the first DC cable of the same circuit via the second DC cable of each single circuit.

[0063] In this embodiment, "multiple circuits" can refer to dual circuits or more circuits. Using multiple circuit connections ensures that if one circuit fails, the other circuits can maintain the normal operation of the onshore converter unit 130 and the corresponding load center G, guaranteeing power reliability. In one embodiment, a high-speed DC parallel switch (HSS) is configured at the outlet connecting the onshore converter unit to the third DC overhead line, and a high-speed DC parallel switch (HSS) is configured at the outlet connecting the onshore converter unit to the fourth DC overhead line. When fault isolation is required on the third or fourth DC overhead line, especially when the electrical distance between the third and fourth DC overhead lines reaches a certain distance, isolation can be achieved through the high-speed DC parallel switch (HSS), improving system stability.

[0064] like Figure 5 As shown, in one embodiment, both the offshore converter station 120 and the onshore converter unit 130 adopt a bipolar topology. The positive terminal DC+ of the onshore DC switch station 110 is connected to the positive terminals of two or more onshore converter units 130 via the first DC overhead line of each circuit. The negative terminal DC- of the busbar is connected to the negative terminal of the onshore converter unit 130 connected to the first DC overhead line in the same circuit via the second DC overhead line of each circuit. The neutral line N of the busbar is connected to the neutral line of the onshore converter unit 130 via the fifth DC overhead line of each circuit. The neutral line N of the busbar is connected to the fifth DC overhead line via the AC circuit breaker BRK (Breaker), and the neutral line of the onshore converter unit 130 is connected to the fifth DC overhead line via the AC circuit breaker BRK. The positive terminals of each onshore converter unit 130 in the circuit are interconnected via a third DC overhead line, the negative terminals of each onshore converter unit 130 in the same circuit are interconnected via a fourth DC overhead line, and the neutral lines of each onshore converter unit 130 in the same circuit are interconnected via a sixth DC overhead line. The positive terminal DC+ of the busbar is connected to the positive terminal of a marine converter station 120 via the first DC cable of each circuit, the negative terminal DC- of the busbar is connected to the negative terminal of the marine converter station 120 connected to the first DC cable in the same circuit via the second DC cable of each circuit, the neutral line of the busbar is connected to the neutral line of the marine converter station 120 connected to the first DC cable in the same circuit via the third DC cable of each circuit, and the neutral line N of the busbar is connected to the third DC cable via the AC circuit breaker BRK.

[0065] When using a bipolar topology, the fault current in the circuit can be isolated by the operation of the DC circuit breaker DCCB on the pole line. The neutral line N of the busbar is equipped with an AC circuit breaker BRK and connected to the fifth DC overhead line. The neutral line of the onshore converter unit 130 is also equipped with an AC circuit breaker BRK and connected to the fifth DC overhead line. The AC circuit breaker BRK has a lower cost, which can reduce the cost of the system. Similarly, on the offshore side, the neutral line N of the busbar is equipped with an AC circuit breaker BRK and connected to the third DC cable.

[0066] like Figure 6 As shown, in one embodiment, both the offshore converter station 120 and the onshore converter unit 130 adopt a bipolar topology. The positive terminal DC+ of the onshore DC switch station 110 is connected to the positive terminal of at least one onshore converter unit 130 via the first DC overhead line of each circuit. The negative terminal DC- of the busbar is connected to the negative terminal of the onshore converter unit 130 connected to the first DC overhead line of the same circuit via the second DC overhead line of each circuit. The neutral line N of the busbar is connected to the neutral line of the onshore converter unit 130 via the fifth DC overhead line of each circuit. The neutral line N of the busbar is connected to the fifth DC overhead line via the AC circuit breaker BRK. The neutral line of the onshore converter unit 130 is connected to the fifth DC overhead line via the AC circuit breaker BRK. The onshore converter unit 130 in each circuit is connected to the fifth DC overhead line. The DC side of 30 is interconnected with the DC side of at least one onshore converter unit 130 of a different circuit. The positive terminal of each onshore converter unit 130 is interconnected via a third DC overhead line, the negative terminal is interconnected via a fourth DC overhead line, and the neutral line is interconnected via a sixth DC overhead line. The positive terminal DC+ of the busbar is connected to the positive terminal of an offshore converter station 120 via the first DC cable of each circuit. The negative terminal DC- of the busbar is connected to the negative terminal of the offshore converter station 120 connected via the second DC cable of each circuit. The neutral line N of the busbar is connected to the neutral line of the offshore converter station 120 connected via the third DC cable of each circuit. The neutral line N of the busbar is connected to the third DC cable via the AC circuit breaker BRK.

[0067] In this embodiment, a multi-circuit interconnection wiring method is adopted. When one circuit fails, the other circuits can maintain the normal operation of the onshore converter unit 130 and the corresponding load center G, ensuring power reliability. When using a bipolar topology, since there is no fault isolation requirement when the neutral line fails, the neutral line N of the busbar is equipped with an AC circuit breaker BRK connected to the fifth DC overhead line. The neutral line of the onshore converter unit 130 is also equipped with an AC circuit breaker BRK connected to the fifth DC overhead line. The AC circuit breaker BRK has a lower cost, reducing system cost. Similarly, on the offshore side, the neutral line N of the busbar is equipped with an AC circuit breaker BRK connected to the third DC cable. In one embodiment, a DC high-speed parallel switch HSS is configured at the outlet where the onshore converter unit connects to the third DC overhead line, a DC high-speed parallel switch HSS is configured at the outlet where the onshore converter unit connects to the fourth DC overhead line, and an AC circuit breaker BRK is configured at the outlet where the onshore converter unit connects to the sixth DC overhead line.

[0068] refer to Figure 7 As shown, in one embodiment, when the offshore converter station 120 and the onshore converter unit 130 adopt a bipolar topology, the offshore side can share a neutral line to reduce costs; the onshore side can also share a neutral line to reduce costs.

[0069] refer to Figures 4 to 7 As shown, in one embodiment, the AC sides of two or more onshore converter units 130 are interconnected.

[0070] In this embodiment, the AC sides of two or more onshore converter units 130 are interconnected so that the onshore converter units 130 can participate in the construction of the load center G power grid. The onshore converter units 130 do not need to perform load dispatching. The interconnected onshore converter units 130 on the AC side and their respective connected load centers G together form a large power grid, which can automatically coordinate the load and improve the safe and stable operation level of the power grid.

[0071] Finally, it should be noted that in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0072] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this application, "multiple" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0073] It should be noted that when one element is considered to be "connected" to another element, it can be directly connected to the other element or connected to the other element through an intermediary element. Furthermore, in the embodiments of this application, "connection" should be understood as "electrical connection," "communication connection," etc., if the connected circuits, modules, units, etc., transmit electrical signals or data to each other.

[0074] The various embodiments in this specification are described in a progressive manner. Each embodiment focuses on the differences from other embodiments. The various embodiments can be combined as needed, and the same or similar parts can be referred to each other.

[0075] The above description of the disclosed embodiments enables those skilled in the art to make or use this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A DC transmission system for offshore wind power, characterized in that, include: Onshore DC switch station, multiple offshore converter stations and multiple onshore converter units; The AC side of the offshore converter station is used to connect to the wind turbine generator, and the DC side of the offshore converter station is connected to the input side of the busbar of the onshore DC switch station via a DC cable. The input side of the busbar is equipped with a DC switch for connecting to a DC cable. The output side of the busbar is connected to a DC overhead line via a DC circuit breaker. The DC overhead line is connected to the DC side of the onshore converter unit via a DC circuit breaker, and each DC overhead line is connected to one or more onshore converter units. The DC side of the onshore converter unit is equipped with a high-speed DC parallel switch, which is connected to the DC overhead line. The AC side of each onshore converter unit is used to connect to a load center. Both the offshore converter station and the onshore converter unit adopt a half-bridge MMC topology. Both the offshore converter station and the onshore converter unit adopt a symmetrical single-pole topology. The positive terminal of the busbar of the onshore DC switch station is connected to the positive terminal of two or more onshore converter units via the first DC overhead line of each circuit, and the negative terminal of the busbar is connected to the negative terminal of the onshore converter unit connected to the first DC overhead line in the same circuit via the second DC overhead line of each circuit. The positive terminals of each onshore converter unit in the same circuit are interconnected through the third overhead DC line, and the negative terminals of each onshore converter unit in the same circuit are interconnected through the fourth overhead DC line. The positive terminal of the busbar is connected to the positive terminal of an offshore converter station via the first DC cable of each circuit, and the negative terminal of the busbar is connected to the negative terminal of the offshore converter station connected to the first DC cable of the same circuit via the second DC cable of each circuit.

2. The offshore wind power DC transmission system according to claim 1, characterized in that, The DC switch configured on the input side of the busbar includes a high-speed DC parallel switch or a DC circuit breaker.

3. The offshore wind power DC transmission system according to claim 1 or 2, characterized in that, A DC power dissipation device is configured between the positive and negative terminals of the busbar.

4. The offshore wind power DC transmission system according to claim 1, characterized in that, Both the offshore converter station and the onshore converter unit adopt a symmetrical single-pole topology. The positive terminal of the onshore DC switch station's busbar is connected to the positive terminal of at least one onshore converter unit via the first DC overhead line of each circuit, and the negative terminal of the busbar is connected to the negative terminal of the onshore converter unit connected to the first DC overhead line of the same circuit via the second DC overhead line of each circuit. The DC side of each onshore converter unit in each loop is interconnected with the DC side of at least one onshore converter unit in a different loop, wherein the positive terminal of each onshore converter unit is interconnected via a third overhead DC line and the negative terminal is interconnected via a fourth overhead DC line. The positive terminal of the busbar is connected to the positive terminal of an offshore converter station via the first DC cable of each circuit, and the negative terminal of the busbar is connected to the negative terminal of the offshore converter station connected to the first DC cable of the same circuit via the second DC cable of each circuit.

5. The offshore wind power DC transmission system according to claim 1 or 4, characterized in that, A high-speed DC parallel switch is configured at the outlet where the onshore converter unit connects to the third DC overhead line, and a high-speed DC parallel switch is configured at the outlet where the onshore converter unit connects to the fourth DC overhead line.

6. The offshore wind power DC transmission system according to claim 1, characterized in that, Both the offshore converter station and the onshore converter unit adopt a bipolar topology. The positive terminal of the busbar of the onshore DC switch station is connected to the positive terminal of two or more onshore converter units via the first DC overhead line of each circuit. The negative terminal of the busbar is connected to the negative terminal of the onshore converter unit connected to the first DC overhead line in the same circuit via the second DC overhead line of each circuit. The neutral line of the busbar is connected to the neutral line of the onshore converter unit via the fifth DC overhead line of each circuit. The neutral line of the busbar is connected to the fifth overhead DC line via an AC circuit breaker, and the neutral line of the onshore converter unit is connected to the fifth overhead DC line via an AC circuit breaker. The positive terminals of each onshore converter unit in the same circuit are interconnected through the third DC overhead line, the negative terminals of each onshore converter unit in the same circuit are interconnected through the fourth DC overhead line, and the neutral lines of each onshore converter unit in the same circuit are interconnected through the sixth DC overhead line. The positive terminal of the busbar is connected to the positive terminal of an offshore converter station via the first DC cable of each circuit. The negative terminal of the busbar is connected to the negative terminal of the offshore converter station connected to the first DC cable of the same circuit via the second DC cable of each circuit. The neutral line of the busbar is connected to the neutral line of the offshore converter station connected to the first DC cable of the same circuit via the third DC cable of each circuit. The neutral line of the busbar is connected to the third DC cable via an AC circuit breaker.

7. The offshore wind power DC transmission system according to claim 1, characterized in that, Both the offshore converter station and the onshore converter unit adopt a bipolar topology. The positive terminal of the busbar of the onshore DC switch station is connected to the positive terminal of at least one onshore converter unit via the first DC overhead line of each circuit. The negative terminal of the busbar is connected to the negative terminal of the onshore converter unit connected to the first DC overhead line in the same circuit via the second DC overhead line of each circuit. The neutral line of the busbar is connected to the neutral line of the onshore converter unit via the fifth DC overhead line of each circuit. The neutral line of the busbar is connected to the fifth overhead DC line via an AC circuit breaker, and the neutral line of the onshore converter unit is connected to the fifth overhead DC line via an AC circuit breaker. The DC side of each onshore converter unit in each loop is interconnected with the DC side of at least one onshore converter unit in a different loop, wherein the positive terminal of each onshore converter unit is interconnected via a third overhead DC line, the negative terminal is interconnected via a fourth overhead DC line, and the neutral line is interconnected via a sixth overhead DC line. The positive terminal of the busbar is connected to the positive terminal of an offshore converter station via the first DC cable of each circuit. The negative terminal of the busbar is connected to the negative terminal of the offshore converter station connected to the first DC cable of the same circuit via the second DC cable of each circuit. The neutral line of the busbar is connected to the neutral line of the offshore converter station connected to the first DC cable of the same circuit via the third DC cable of each circuit. The neutral line of the busbar is connected to the third DC cable via an AC circuit breaker.

8. The offshore wind power DC transmission system according to claim 6 or 7, characterized in that, A high-speed DC parallel switch is configured at the outlet where the onshore converter unit connects to the third DC overhead line; a high-speed DC parallel switch is configured at the outlet where the onshore converter unit connects to the fourth DC overhead line; and an AC circuit breaker is configured at the outlet where the onshore converter unit connects to the sixth DC overhead line.

9. The offshore wind power DC transmission system according to claim 1, characterized in that, Interconnection of the AC side of two or more of the aforementioned onshore converter units.

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