Offshore wind power direct-current transmission system

Abstract

An offshore wind power direct-current transmission system, which relates to the technical field of direct-current power transmission. The system comprises: an onshore direct-current switching station, a plurality of offshore converter stations and a plurality of onshore converter units, wherein an alternating-current side of each offshore converter station is used for connecting to a wind turbine generator unit, and a direct-current side of each offshore converter station is connected to an input side of a busbar of the onshore direct-current switching station by means of a direct-current cable; an output side of the busbar is connected to a direct-current side of each onshore converter unit by means of a direct-current overhead line, the input side of the busbar is configured with direct-current high-speed switches, each of which is connected to a direct-current cable, and the output side of the busbar is configured with direct-current high-speed switches, each of which is connected to a direct-current overhead line; the direct-current overhead line is connected to the direct-current side of an onshore converter unit by means of a direct-current high-speed switch; an alternating-current side of each onshore converter unit is used for connecting to a load center; and each of the offshore converter stations and the onshore converter units is of a topological structure having a direct-current fault clearing capability. The present application can realize reliable and low-cost ultra-high-capacity direct-current transmission.

Description

Offshore wind power DC transmission system

[0001] This application claims priority to the Chinese patent application filed with the China Patent Office on November 20, 2023, with application number 202311548715.6 and invention name “Offshore Wind Power DC Transmission System”, the entire contents of which are incorporated by reference into this application. Technical Field

[0002] The present application relates to the technical field of direct current (DC) transmission, and in particular to an offshore wind power DC transmission system. Background Art

[0003] In recent years, my country has vigorously supported the development and utilization of offshore wind power to promote energy transition. Offshore wind power development has trended from nearshore, shallow-water, small-scale demonstration projects to offshore, deep-water, large-scale, centralized development. Transmission methods for connecting offshore wind power to onshore power grids include high-voltage AC (HVAC) and high-voltage DC (HVDC). High-voltage AC is suitable for nearshore wind power. However, when offshore wind power exceeds a certain distance from shore, long-distance AC cable transmission presents challenges such as excessive charging power, resulting in voltage increases requiring compensation and overvoltage. High-voltage AC transmission is difficult to meet the needs of large-capacity, long-distance offshore wind power transmission and is also costly.

[0004] High-voltage direct current (HVDC) transmission offers advantages such as the absence of synchronization with the onshore power grid, long transmission distances, and flexible operation and regulation. This makes it particularly suitable for large-capacity, long-distance transmission. Because flexible HVDC does not suffer from commutation failures, can independently regulate active and reactive power, and has low harmonic levels, it is currently the mainstream method for transmitting large-scale offshore wind power from offshore locations to onshore power grids. To achieve ultra-high-capacity transmission, a multi-terminal flexible HVDC solution is required, but current HVDC system architectures make it difficult to achieve stable transmission of ultra-large capacities.

[0005] Summary of the Invention

[0006] The embodiments of the present application provide an offshore wind power DC transmission system that can achieve reliable and low-cost ultra-large capacity DC transmission and improve system stability.

[0007] The present application provides an offshore wind power DC transmission system, comprising: an onshore DC switch station, multiple offshore converter stations, and multiple onshore converter units;

[0008] The AC side of the offshore converter station is used to connect to the wind turbine generator set, and the DC side of the offshore converter station is connected to the input side of the busbar of the onshore DC switch station through a DC cable;

[0009] The output side of the busbar is connected to the DC side of the onshore converter unit via a DC overhead line, the input side of the busbar is equipped with a DC high-speed parallel switch connected to the DC cable, and the output side of the busbar is equipped with a DC high-speed parallel switch connected to the DC overhead line;

[0010] The DC overhead line is connected to the DC side of the onshore converter unit via a DC high-speed parallel switch;

[0011] The AC side of each onshore converter unit is used to connect to a load center;

[0012] Wherein, the offshore converter station and the onshore converter unit both adopt a topological structure with DC fault clearing capability.

[0013] In one embodiment, the offshore converter station and the onshore converter unit both adopt a full-bridge / half-bridge hybrid MMC topology structure.

[0014] In one embodiment, the offshore converter station and the onshore converter unit both adopt a full-bridge / half-bridge hybrid MMC topology structure.

[0015] In one embodiment, the offshore converter station and the onshore converter unit both adopt a symmetrical monopole topology structure;

[0016] The positive pole of the busbar of the onshore DC switch station is connected to the positive terminals of two or more onshore converter units via the first DC overhead line of each loop, and the negative pole of the busbar is connected to the negative terminal of the onshore converter unit connected to the first DC overhead line in the same loop via the second DC overhead line of each loop;

[0017] The positive terminals of the onshore converter units in the same circuit are interconnected via a third DC overhead line, and the negative terminals of the onshore converter units in the same circuit are interconnected via a fourth DC overhead line.

[0018] The positive pole of the busbar is connected to the positive terminal of an offshore converter station via the first DC cable of each loop, and the negative pole of the busbar is connected to the negative terminal of the offshore converter station to which the first DC cable in the same loop is connected via the second DC cable of each loop.

[0019] In one embodiment, the offshore converter station and the onshore converter unit both adopt a symmetrical monopole topology structure;

[0020] The positive pole 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 loop, and the negative pole of the busbar is connected to the negative terminal of the onshore converter unit connected to the first DC overhead line in the same loop via the second DC overhead line of each loop;

[0021] The DC side of the onshore converter unit in each loop is interconnected with the DC side of the onshore converter unit in at least one different loop, wherein the positive terminals of each onshore converter unit are interconnected via a third DC overhead line, and the negative terminals are interconnected via a fourth DC overhead line;

[0022] The positive pole of the busbar is connected to the positive terminal of an offshore converter station via the first DC cable of each loop, and the negative pole of the busbar is connected to the negative terminal of the offshore converter station to which the first DC cable in the same loop is connected via the second DC cable of each loop.

[0023] In one embodiment, a DC high-speed parallel switch is configured at the outlet where the onshore converter unit is connected to the third DC overhead line, and a DC high-speed parallel switch is configured at the outlet where the onshore converter unit is connected to the fourth DC overhead line.

[0024] In one embodiment, the offshore converter station and the onshore converter unit both adopt a bipolar topology structure;

[0025] The positive pole of the busbar of the onshore DC switch station is connected to the positive terminals of two or more onshore converter units via the first DC overhead line of each loop, the negative pole of the busbar is connected to the negative terminal of the onshore converter unit connected to the first DC overhead line in the same loop via the second DC overhead line of each loop, and the neutral pole of the busbar is connected to the neutral line of the onshore converter unit via the fifth DC overhead line of each loop;

[0026] The neutral line of the busbar is connected to the fifth DC overhead line via an AC circuit breaker, and the neutral line of the onshore converter unit is connected to the fifth DC overhead line via an AC circuit breaker;

[0027] The positive terminals of the onshore converter units in the same circuit are interconnected via a third DC overhead line, the negative terminals of the onshore converter units in the same circuit are interconnected via a fourth DC overhead line, and the neutral lines of the onshore converter units in the same circuit are interconnected via a sixth DC overhead line.

[0028] The positive pole of the busbar is connected to the positive terminal of an offshore converter station via the first DC cable of each loop, the negative pole of the busbar is connected to the negative terminal of the offshore converter station to which the first DC cable in the same loop is connected via the second DC cable of each loop, the neutral line of the busbar is connected to the neutral line of the offshore converter station to which the first DC cable in the same loop is connected via the third DC cable of each loop, and the neutral line of the busbar is connected to the third DC cable via an AC circuit breaker.

[0029] In one embodiment, the offshore converter station and the onshore converter unit both adopt a bipolar topology structure;

[0030] The positive pole 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 loop, the negative pole of the busbar is connected to the negative terminal of the onshore converter unit connected to the first DC overhead line in the same loop via the second DC overhead line of each loop, and 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 loop;

[0031] The neutral line of the busbar is connected to the fifth DC overhead line via an AC circuit breaker, and the neutral line of the onshore converter unit is connected to the fifth DC overhead line via an AC circuit breaker;

[0032] The DC side of the onshore converter unit in each circuit is interconnected with the DC side of the onshore converter unit in at least one different circuit, wherein the positive terminals of each onshore converter unit are interconnected via a third DC overhead line, the negative terminals are interconnected via a fourth DC overhead line, and the neutral terminals are interconnected via a sixth DC overhead line;

[0033] The positive pole of the busbar is connected to the positive terminal of an offshore converter station via the first DC cable of each loop, the negative pole of the busbar is connected to the negative terminal of the offshore converter station to which the first DC cable in the same loop is connected via the second DC cable of each loop, the neutral line of the busbar is connected to the neutral line of the offshore converter station to which the first DC cable in the same loop is connected via the third DC cable of each loop, and the neutral line of the busbar is connected to the third DC cable via an AC circuit breaker.

[0034] In one embodiment, a DC high-speed parallel switch is configured at the outlet where the onshore converter unit is connected to the third DC overhead line, a DC high-speed 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.

[0035] In one embodiment, each offshore converter station is connected to the neutral line of the busbar via the same third DC cable;

[0036] Each onshore converter unit is connected to the neutral line of the busbar through the same fifth DC overhead line.

[0037] In one embodiment, the AC sides of two or more of the onshore converter units are interconnected.

[0038] It can be seen from the above technical solutions that the embodiments of the present application have the following advantages:

[0039] The offshore wind power DC transmission system provided in the present application includes an onshore DC switch station and multiple offshore converter stations and multiple onshore converter units, all of which adopt a topological structure with DC fault clearing capability. Power is transmitted via DC cables on the offshore side and via DC overhead lines on the onshore side. The offshore and onshore sides are connected via a busbar of the onshore DC switch station, thereby reducing the transmission cost of large-capacity offshore wind power and saving transmission demand. Moreover, since both the offshore converter station and the onshore converter units have DC fault clearing capability, fault isolation can be achieved by simply configuring DC high-speed parallel switches on the input and output sides of the busbar and the DC side of the onshore converter unit, thereby improving system stability at a low cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0041] FIG1 is a schematic structural diagram of an offshore wind power DC transmission system in one embodiment;

[0042] FIG2 is a schematic diagram of a full-bridge and half-bridge hybrid MMC topology structure in one embodiment;

[0043] FIG3 is a schematic structural diagram of an offshore wind power DC transmission system using a symmetrical monopole topology and multiple overhead lines in one embodiment;

[0044] FIG4 is a schematic structural diagram of an offshore wind power DC transmission system using a bipolar topology and a single-circuit overhead line in one embodiment;

[0045] FIG5 is a schematic structural diagram of an offshore wind power DC transmission system using a bipolar topology and multiple overhead lines in one embodiment;

[0046] FIG6 is a schematic structural diagram of an offshore wind power DC transmission system using a bipolar topology and multiple overhead lines in another embodiment. DETAILED DESCRIPTION

[0047] The following will be combined with the drawings in the embodiments of this application to clearly and completely describe the technical solutions in the embodiments of this application. Obviously, the embodiments described are only part of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0048] As shown in FIG1 , an embodiment of the present application provides an offshore wind power DC transmission system, comprising: an onshore DC switch station 110, multiple offshore converter stations 120, and multiple onshore converter units 130. Both the offshore converter stations 120 and the onshore converter units 130 employ a topology with DC fault clearing capabilities. The AC side of the offshore converter station 120 is used to connect to wind turbines (WTs), and the DC side of the offshore converter station 120 is connected to the input side of the busbar of the onshore DC switch station 110 via a DC cable. The output side of the busbar is connected to the DC side of the onshore converter unit 130 via a DC overhead line. The input side of the busbar is equipped with a DC high-speed parallel switch (HSS) connected to the DC cable, and the output side of the busbar is equipped with a DC high-speed parallel switch (HSS) connected to the DC overhead line. The DC overhead line is connected to the DC side of the onshore converter unit 130 via the DC high-speed parallel switch (HSS). The AC side of each onshore converter unit 130 is connected to a load center (G).

[0049] 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 cables for onshore transmission, this method significantly reduces costs. Compared to the current method of using AC overhead lines for onshore transmission in my country, this method can reduce the number of transmission corridors in large-scale offshore wind power transmission. Within the permitted capacity, a single DC overhead line can be connected to multiple onshore converter units 130, further conserving transmission corridors and reducing costs. Furthermore, by establishing an onshore DC switch station 110 to connect the DC cables and DC overhead lines, conversion between submarine cables and overhead lines is achieved. During long-distance transmission, the DC high-speed parallel switch (HSS) in the onshore DC switch station 110 can be used to clear the faulty line section, preventing the impact of clearing the fault on the entire transmission line and improving the flexibility of the system power supply. Since the offshore converter station 120 and the onshore converter unit 130 themselves have DC fault clearing capabilities, reliable fault isolation can be achieved by simply cooperating with a DC high-speed parallel switch HSS, without the need for a DC circuit breaker. Compared to a DC circuit breaker, a DC high-speed parallel switch HSS has low cost, small size, and simple structure, which can reduce system costs.

[0050] Among them, the DC cable includes a DC submarine cable and / or a DC land cable. Since the busbar may be located in an onshore area at a certain distance from the coastline, the DC submarine cable is used for the offshore transmission part, and the DC land cable is used for transmission between the coastline and the busbar.

[0051] In some embodiments, an onshore converter unit refers to an onshore converter station. In other embodiments, an onshore converter unit refers to a single converter unit in a single onshore converter station.

[0052] In some embodiments, the load center to which different onshore converter units are connected may be the same load center, or may be connected to different load centers.

[0053] The offshore wind power DC transmission system provided in the present 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 topological structure with DC fault clearing capability. Power is transmitted via DC cables on the offshore side and via DC overhead lines on the onshore side. The offshore side and the onshore side are connected by the busbar of the onshore DC switch station 110, thereby reducing the transmission cost of large-capacity offshore wind power and saving transmission demand. In addition, since both the offshore converter station 120 and the onshore converter unit 130 have DC fault clearing capability, fault isolation can be achieved by simply configuring a DC high-speed parallel switch HSS on the input side, output side of the busbar, and DC side of the onshore converter unit 130, thereby improving system stability at a low cost.

[0054] As shown in FIG2 , in one embodiment, the offshore converter station 120 and the onshore converter unit 130 both adopt a full-bridge / half-bridge hybrid MMC topology structure.

[0055] Currently, most modular multilevel converters (MMCs) utilize a half-bridge submodule structure. However, these half-bridge MMCs cannot handle DC transmission line faults through the converter's own operation. This is because even when the insulated gate bipolar transistors (IGBTs) are turned off in the half-bridge submodule topology, the AC system still feeds current to the fault point through the IGBT's anti-parallel diodes. This impacts the AC system equivalent to a three-phase short circuit, which is detrimental to the transient stability of the AC / DC transmission system. To address these shortcomings of the half-bridge MMC, full-bridge MMCs with DC fault clearing capabilities have emerged. After a DC line fault occurs, HVDC transmission systems based on full-bridge MMCs can rapidly block or output negative voltage to cut off the DC fault current without tripping the AC-side circuit breaker. After the fault is cleared, the system can quickly resume operation. Therefore, full-bridge MMCs are more suitable for hybrid DC transmission systems based on long-distance overhead lines. However, full-bridge MMCs use twice as many power switching devices as half-bridge MMCs, significantly increasing cost and losses. In this embodiment, an MMC that is a mixture of half-bridge sub-modules and full-bridge sub-modules is used, so that the MMC can reduce costs and losses during operation while having the ability to clear DC faults.

[0056] 1 , in one embodiment, the offshore converter station 120 and the onshore converter unit 130 both have a symmetrical monopole topology. The positive DC+ busbar 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 DC- 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. The positive terminals of the onshore converter units 130 in the same circuit are interconnected via the third DC overhead line, and the negative terminals of the onshore converter units 130 in the same circuit are interconnected via the fourth DC overhead line. The positive DC+ busbar is connected to the positive terminal of one offshore converter station 120 via the first DC cable of each circuit, and the negative DC- 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.

[0057] In this embodiment, a converter station employs a symmetrical monopole topology. For converter stations in large-scale offshore wind power transmission systems, this symmetrical monopole topology allows for the use of an odd or even number of flexible DC transformers, resulting in greater room for optimization of 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 permitted capacity range, a single overhead line can simultaneously connect to multiple onshore converter units 130, thus conserving transmission corridors. In one embodiment, a high-speed DC paralleling switch (HSS) is configured at the outlet where the onshore converter unit connects to the third DC overhead line, and a high-speed DC paralleling switch (HSS) is configured at the outlet where the onshore converter unit connects to the fourth DC overhead line. When fault isolation is required on the third or fourth DC overhead line, particularly when the electrical distance between the third and fourth DC overhead lines reaches a certain value, the high-speed DC paralleling switch (HSS) can be used to achieve isolation, improving system stability.

[0058] As shown in FIG3 , in one embodiment, the offshore converter station 120 and the onshore converter unit 130 both have a symmetrical monopole topology. The positive pole DC+ of the busbar 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, and the negative pole 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. The DC side of the onshore converter unit 130 in each circuit is interconnected with the DC side of at least one onshore converter unit 130 in a different circuit, wherein the positive terminal of each onshore converter unit 130 is interconnected via the third DC overhead line, and the negative terminal is interconnected via the fourth DC overhead line. The positive pole DC+ of the busbar is connected to the positive terminal of one offshore converter station 120 via the first DC cable of each circuit, and the negative pole 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.

[0059] In this embodiment, "multiple circuits" can be two or more circuits. By interconnecting multiple circuits, if one circuit fails, the other circuits can maintain the normal operation of the onshore converter unit 130 and the corresponding load center G of that circuit, thereby ensuring power reliability. In one embodiment, a DC high-speed parallel switch (HSS) is configured at the outlet where the onshore converter station connects to the third DC overhead line, and a DC high-speed parallel switch (HSS) is configured at the outlet where the onshore converter station connects 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 using the DC high-speed parallel switch (HSS), thereby improving system stability.

[0060] As shown in FIG4 , in one embodiment, the offshore converter station 120 and the onshore converter unit 130 both adopt a bipolar topology structure; the positive pole DC+ of the busbar 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 loop, the negative pole 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 loop via the second DC overhead line of each loop, and 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 loop; the neutral line N of the busbar is connected to the fifth DC overhead line via the AC circuit breaker BRK, and the neutral line of the onshore converter unit 130 is connected to the fifth DC overhead line via the AC circuit breaker BRK; in the same loop, The positive terminals of the onshore converter units 130 are interconnected via a third DC overhead line, the negative terminals of the onshore converter units 130 in the same circuit are interconnected via a fourth DC overhead line, and the neutral lines of the onshore converter units 130 in the same circuit are interconnected via a sixth DC overhead line. The positive pole DC+ of the busbar is connected to the positive terminal of an offshore converter station 120 via a first DC cable in each circuit, the negative pole 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 a second DC cable in each circuit, the neutral line N of the busbar is connected to the neutral line of the offshore converter station 120 connected to the first DC cable in the same circuit via a third DC cable in each circuit, and the neutral line N of the busbar is connected to the third DC cable via an AC circuit breaker BRK.

[0061] When a bipolar topology is used, since there is no need for fault isolation in the event of a neutral line fault, the neutral line N of the busbar is configured 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 configured with an AC circuit breaker BRK and connected to the fifth DC overhead line. The AC circuit breaker BRK is relatively low in cost and can reduce system costs. Similarly, on the offshore side, the neutral line N of the busbar is configured with an AC circuit breaker BRK and connected to the third DC cable.

[0062] As shown in FIG5 , in one embodiment, the offshore converter station 120 and the onshore converter unit 130 both adopt a bipolar topology structure; the positive pole DC+ of the busbar 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 loop, the negative pole 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 loop via the second DC overhead line of each loop, and 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 loop; the neutral line N of the busbar is connected to the fifth DC overhead line via the AC circuit breaker BRK, 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 onshore converter unit 130 in each loop The DC side of the unit 130 is interconnected with the DC side of at least one onshore converter unit 130 of a different circuit, wherein the positive terminal of each onshore converter unit 130 is interconnected through a third DC overhead line, the negative terminal is interconnected through a fourth DC overhead line, and the neutral line is interconnected through a sixth DC overhead line; the positive pole 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 pole 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, the neutral line N of the busbar is connected to the neutral line of the offshore 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.

[0063] In this embodiment, "multiple circuits" can be two or more circuits. By interconnecting multiple circuits, if 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. In a bipolar topology, since there is no need for fault isolation in the event of a neutral line fault, the neutral line N of the busbar is connected to the fifth DC overhead line with an AC circuit breaker BRK. The neutral line of the onshore converter unit 130 is also connected to the fifth DC overhead line with an AC circuit breaker BRK. The AC circuit breaker BRK is relatively low-cost, thus reducing system costs. Similarly, on the offshore side, the neutral line N of the busbar is connected to the third DC cable with an AC circuit breaker BRK. In one embodiment, a DC high-speed paralleling switch HSS is configured at the outlet of the onshore converter unit connecting to the third DC overhead line, a DC high-speed paralleling switch HSS is configured at the outlet of the onshore converter unit connecting to the fourth DC overhead line, and an AC circuit breaker BRK is configured at the outlet of the onshore converter unit connecting to the sixth DC overhead line.

[0064] As shown in FIG6 , in one embodiment, each offshore converter station 120 is connected to the neutral line N of the busbar through the same third DC cable; each onshore converter unit 130 is connected to the neutral line N of the busbar through the same fifth DC overhead line.

[0065] 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 share a neutral line to reduce costs.

[0066] 3 to 6 , in one embodiment, the AC sides of two or more onshore converter units 130 are interconnected.

[0067] In this embodiment, the AC sides of two or more onshore converter units are interconnected to enable the onshore converter units to participate in the construction of the load center power grid. The onshore converter units do not need to perform load allocation. The interconnected onshore converter units on the AC side and the load centers to which they are connected together constitute a large power grid, which can automatically coordinate the load internally and improve the safe and stable operation level of the power grid.

[0068] Finally, it should be noted that, in this document, relational terms such as first and second, etc., are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "comprises," "comprising," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or device comprising a series of elements includes not only those elements, but also other elements not explicitly listed, or elements inherent to such process, method, article, or device. In the absence of further limitations, an element defined by the phrase "comprising a ..." does not exclude the presence of additional identical elements in the process, method, article, or device comprising the element.

[0069] 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 the technical features being referred to. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of such features. Throughout the description of this application, "plurality" means at least two, for example, two, three, etc., unless otherwise specifically defined.

[0070] It should be noted that when an 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 intermediate element. In addition, the "connection" in the embodiments of the present application should be understood as "electrical connection", "communication connection", etc., if the connected circuits, modules, units, etc. have electrical signals or data transmission between each other.

[0071] The various embodiments in this specification are described in a progressive manner, and 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 referenced to each other.

[0072] The above description of the disclosed embodiments is intended to enable one skilled in the art to implement or use the present application. Various modifications to these embodiments will be readily apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application is not limited to the embodiments shown herein, but is intended to conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. An offshore wind power DC transmission system, 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 the wind turbines, and the DC side of the offshore converter station is connected to the input side of the busbar of the onshore DC switch station through a DC cable; The output side of the busbar is connected to the DC side of the onshore converter unit via a DC overhead line, the input side of the busbar is provided with a DC high-speed parallel switch connected to the DC cable, and the output side of the busbar is provided with a DC high-speed parallel switch connected to the DC overhead line; The DC overhead line is connected to the DC side of the onshore converter unit via a DC high-speed parallel switch; The AC side of each onshore converter unit is used to connect to a load center; Wherein, the offshore converter station and the onshore converter unit both adopt a topological structure with DC fault clearing capability.

2. The offshore wind power DC transmission system according to claim 1, characterized in that: The offshore converter station and the onshore converter unit both adopt a full-bridge and half-bridge hybrid MMC topology structure.

3. The offshore wind power DC transmission system according to claim 2, characterized in that: The offshore converter station and the onshore converter unit both adopt a symmetrical monopole topology structure; The positive pole of the busbar of the onshore DC switch station is connected to the positive terminals of two or more onshore converter units via the first DC overhead line of each loop, and the negative pole of the busbar is connected to the negative terminal of the onshore converter unit connected to the first DC overhead line in the same loop via the second DC overhead line of each loop; The positive terminals of the onshore converter units in the same circuit are interconnected through a third DC overhead line, and the negative terminals of the onshore converter units in the same circuit are interconnected through a fourth DC overhead line; The positive pole of the busbar is connected to the positive terminal of an offshore converter station via the first DC cable of each loop, and the negative pole of the busbar is connected to the negative terminal of the offshore converter station connected to the first DC cable in the same loop via the second DC cable of each loop.

4. The offshore wind power DC transmission system according to claim 2, characterized in that: The offshore converter station and the onshore converter unit both adopt a symmetrical monopole topology structure; The positive pole 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 loop, and the negative pole of the busbar is connected to the negative terminal of the onshore converter unit connected to the first DC overhead line in the same loop via the second DC overhead line of each loop; The DC side of the onshore converter unit in each loop is interconnected with the DC side of the onshore converter unit of at least one different loop, wherein the positive terminal of each onshore converter unit is interconnected through a third DC overhead line, and the negative terminal is interconnected through a fourth DC overhead line; The positive pole of the busbar is connected to the positive terminal of an offshore converter station via the first DC cable of each loop, and the negative pole of the busbar is connected to the negative terminal of the offshore converter station connected to the first DC cable in the same loop via the second DC cable of each loop.

5. The offshore wind power DC transmission system according to claim 3 or 4, characterized in that: A DC high-speed parallel switch is configured at an outlet where the onshore converter unit is connected to the third DC overhead line, and a DC high-speed parallel switch is configured at an outlet where the onshore converter unit is connected to the fourth DC overhead line.

6. The offshore wind power DC transmission system according to claim 2, characterized in that: The offshore converter station and the onshore converter unit both adopt a bipolar topology structure; The positive pole of the busbar of the onshore DC switch station is connected to the positive terminals of more than two onshore converter units via the first DC overhead line of each loop, the negative pole of the busbar is connected to the negative terminal of the onshore converter unit connected to the first DC overhead line in the same loop via the second DC overhead line of each loop, and the neutral pole of the busbar is connected to the neutral line of the onshore converter unit via the fifth DC overhead line of each loop; The neutral line of the busbar is connected to the fifth DC overhead line via an AC circuit breaker, and the neutral line of the onshore converter unit is connected to the fifth DC overhead line via an AC circuit breaker; The positive terminals of the onshore converter units in the same circuit are interconnected through the third DC overhead line, the negative terminals of the onshore converter units in the same circuit are interconnected through the fourth DC overhead line, and the neutral lines of the onshore converter units in the same circuit are interconnected through the sixth DC overhead line; The positive pole of the busbar is connected to the positive terminal of an offshore converter station through the first DC cable of each loop, and the negative pole of the busbar is connected to the negative terminal of the offshore converter station connected to the first DC cable in the same loop through the second DC cable of each loop. The third DC cable of a loop is connected to the neutral line of the offshore converter station to which the first DC cable in the same loop is connected, and 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 2, characterized in that: The offshore converter station and the onshore converter unit both adopt a bipolar topology structure; The positive pole 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 loop, the negative pole of the busbar is connected to the negative terminal of the onshore converter unit connected to the first DC overhead line in the same loop via the second DC overhead line of each loop, and 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 loop; The neutral line of the busbar is connected to the fifth DC overhead line via an AC circuit breaker, and the neutral line of the onshore converter unit is connected to the fifth DC overhead line via an AC circuit breaker; The DC side of the onshore converter unit in each loop is interconnected with the DC side of the onshore converter unit of at least one different loop, wherein the positive terminal of each onshore converter unit is interconnected through a third DC overhead line, the negative terminal is interconnected through a fourth DC overhead line, and the neutral line is interconnected through a sixth DC overhead line; The positive pole of the busbar is connected to the positive terminal of an offshore converter station via the first DC cable of each loop, the negative pole of the busbar is connected to the negative terminal of the offshore converter station connected to the first DC cable in the same loop via the second DC cable of each loop, the neutral line of the busbar is connected to the neutral line of the offshore converter station connected to the first DC cable in the same loop via the third DC cable of each loop, and 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 DC high-speed parallel switch is configured at the outlet where the onshore converter unit is connected to the third DC overhead line, a DC high-speed 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.

9. The offshore wind power DC transmission system according to claim 6 or 7, characterized in that: Each offshore converter station is connected to the neutral line of the busbar through the same third DC cable; Each onshore converter unit is connected to the neutral line of the busbar through the same fifth DC overhead line.

10. The offshore wind power DC transmission system according to claim 1, characterized in that: The AC sides of two or more of the above-mentioned onshore converter units are interconnected.

Images