Direct current system for uniformly and directly transmitting offshore wind power to land power grids in multiple regions

Abstract

The invention provides a direct current system for uniformly and directly transmitting offshore wind power to a land power grid in multiple regions. The topology of the direct current system mainly comprises an offshore wind power alternating current collecting-direct current sending-out system, a coastal direct current receiving end system, an inland direct current receiving end system and a coastal direct current bus, the offshore wind power alternating current collecting and direct current sending-out system is used for collecting and sending out grid connection of offshore wind power; the coastal direct-current receiving end system is used for transmitting direct-current electric energy to a coastal power grid and controlling system direct-current voltage; the inland direct current receiving end system is used for transmitting direct current electric energy to an inland power grid; the coastal direct-current bus is used for connection and electric energy exchange between the offshore direct-current line and the land direct-current line. According to the invention, the direct transmission of the electric energy of the offshore wind field to the power grids in multiple regions on land is realized, the access and consumption of the offshore wind power in the coastal and inland power grids are ensured, the offshore wind power consumption capability and power transmission capability of the power grids are improved, and the economic benefits and environmental benefits are improved at the same time.

Description

Technical Field

[0001] The present invention relates to a direct current system for uniformly transmitting offshore wind power directly to land power grids in multiple regions, belonging to the technical field of power transmission and distribution of electric power systems. Background Art

[0002] Large offshore wind farms are an important part of my country's current new energy development and construction plan. With the continuous expansion of offshore wind power planning capacity, the demand for grid connection and consumption of large-scale offshore wind power is also increasing.

[0003] However, when absorbing the output power of the large-scale offshore wind farms connected to them, the onshore power grid using the traditional AC transmission grid is often faced with a series of problems such as long transmission distance, large power loss, uneven distribution of tidal currents, and excessive line loads due to the constraints of the original grid structure, which greatly reduces the onshore power grid's ability to absorb large-scale offshore wind power. In addition, the randomness, volatility, and intermittency of offshore wind farm power generation have brought severe difficulties and challenges to the operation stability of the onshore power grid. At the same time, with the further expansion of the scale of coastal new energy access and the further increase in the scale of power grid load, the need to transmit the power of newly added coastal new energy units to the onshore load center is more prominent. Affected by geography and economic development levels, there are usually load centers distributed in multiple onshore regions, which further increases the difficulty of power transmission. If multiple onshore transmission projects are built independently, the construction investment cost will be relatively high, and the difficulty of controlling the coordinated operation of the offshore wind power transmission DC system and multiple onshore transmission systems will also be further increased. Summary of the invention

[0004] In order to solve the problems existing in the prior art, the present invention provides a direct current system for directly transmitting offshore wind power to land power grids in multiple regions, which is safe, stable and economical.

[0005] In order to achieve the above-mentioned object, the technical solution proposed by the present invention is: a DC system for uniformly directly transmitting offshore wind power to multi-regional land power grids, comprising an offshore wind power AC collection-DC transmission system, a seaside DC receiving end system, a seaside DC busbar and at least one inland DC receiving end system;

[0006] The offshore wind power AC collection-DC transmission system comprises an offshore wind farm group, an AC collection network, an offshore converter station and a DC transmission submarine cable connected in sequence, wherein the DC transmission submarine cable is connected to a DC busbar near the sea; the offshore wind farm group is used to generate AC electric energy, the AC collection network is used to collect the electric energy generated by the offshore wind farm group and transmit it to the offshore converter station, and the offshore converter station is used to convert the electric energy received from the offshore wind farm group into AC to DC, and transmit the converted DC electric energy from the DC transmission submarine cable to the DC busbar near the sea;

[0007] The coastal DC receiving end system comprises an interconnected coastal AC power grid and a coastal converter station, wherein the coastal converter station is connected to the coastal DC bus and is used to convert the DC power of the coastal DC bus into AC power and transmit it to the coastal power grid, or to convert the AC power of the coastal power grid into DC power and transmit it to the coastal DC bus;

[0008] The inland DC receiving end system includes an inland power grid, an inland converter station and an inland DC transmission line connected in sequence, wherein the inland DC transmission line is connected to a coastal DC bus; the inland converter station is used to convert the DC power of the coastal DC bus into AC power and transmit it to the inland power grid, or to convert the AC power of the inland power grid into DC power and transmit it to the coastal DC bus via the inland DC transmission line; the offshore converter station is controlled by a constant AC voltage and frequency control method; the inland converter station is controlled by a constant power control method of the inland AC power grid; and the coastal converter station is controlled by a constant DC output voltage control method.

[0009] A further design of the above technical solution is as follows: the coastal converter station and / or inland converter station includes multiple converters, each converter includes a DC port and an AC port; the DC ports of the multiple converters are connected in series to form the DC end of the converter station; the AC ports of the multiple converters are connected in parallel to form multiple AC ends of the converter station.

[0010] In the coastal converter station and / or inland converter station, if the DC port of a converter constitutes a DC end, the converter adopts a voltage source converter; otherwise, the converter adopts a voltage source converter or a grid-commutated converter.

[0011] The offshore converter station comprises at least one converter. When a plurality of converters are included, the plurality of converters are connected in series, in parallel or in series and in parallel.

[0012] The converter comprises at least one sub-converter, wherein the sub-converter is a voltage source converter, a grid commutation converter, or a converter based on a diode rectifier unit, and the sub-converters are connected in series or in parallel.

[0013] The AC collection network includes an AC collection bus, a first type of AC collection submarine cable, a second type of AC collection submarine cable and a third type of AC collection submarine cable; the first type of AC collection submarine cable connects the offshore AC wind turbines in an offshore wind farm to each other; the second type of AC collection submarine cable connects the offshore wind farms in an offshore wind farm group to each other; one end of the third type of AC collection submarine cable is connected to the offshore wind farm group, and the other end is connected to one end of the AC collection bus, and the other end of the AC collection bus is connected to the offshore converter station.

[0014] An offshore converter station connection transformer is provided between the AC busbar and the offshore converter station; a offshore converter station connection transformer is provided between the offshore converter station and the offshore power grid; and an inland converter station connection transformer is provided between the inland converter station and the inland power grid.

[0015] The DC transmission submarine cable adopts a pseudo bipolar wiring method or a true bipolar wiring method.

[0016] The constant power control method includes constant active power and / or constant reactive power.

[0017] The voltage level of the DC power generated by each offshore wind farm group after the AC power is converted from AC to DC at the offshore converter station is the same; the voltage level of the DC power obtained by the coastal converter station is the same as the voltage level of the DC power after the AC to DC conversion at the offshore converter station; the voltage level of the DC power obtained by the inland converter stations is the same.

[0018] The beneficial effects of the present invention are:

[0019] The present invention solves the problems of long transmission distance, uneven tidal distribution and excessive line load that occur when traditional AC grids in coastal power grids absorb large-scale offshore wind power, and realizes the joint operation of offshore wind power transmission DC and inland DC transmission systems. While ensuring the access and absorption of large-scale offshore wind power, the present invention realizes the long-distance transmission of electric energy to inland power grids, improves the offshore wind power absorption capacity and power transmission capacity of the power grid. Compared with independent offshore wind power transmission grid-connected DC systems and inland transmission DC systems, the present invention improves the compactness of offshore wind power and land DC systems, effectively improves the coordination and safety of joint control operation, improves the utilization rate of transmission channels, and at the same time improves economic and environmental benefits.

[0020] The present invention effectively addresses the problem of insufficient grid absorption and load capacity of the original power grid in coastal areas through the synergistic combination of the offshore wind power transmission DC system and the inland DC transmission system, and improves the stability and strength of the grid. In addition, the present invention directly connects and merges the inland DC transmission system with the offshore wind power transmission DC system, which further improves the overall economy and coordination of control operation compared to the independent sea-land DC system.

[0021] The present invention proposes to adopt a mature AC collection-DC transmission system to undertake the grid-connected transmission of large-scale offshore wind power, which can have the advantages of both AC and DC collection and improve the economy of large-scale offshore wind power transmission. The present invention uses a land DC transmission system to undertake the task of transmitting electric energy from inland power grids, which plays a role in expanding and supplementing the original AC grid, and improving the transmission capacity and overall strength of the original inland AC power grid.

[0022] The present invention provides a direct current system and control method for direct transmission of offshore wind power to multiple regional land power grids, which connects the offshore wind power AC collection-DC transmission system, the coastal DC receiving system and the inland DC receiving system through the coastal DC busbar, so as to realize the joint coordinated control and operation of the sea-land DC system, improve the overall strength of the grid of the coastal power grid, and ensure the access and consumption of large-scale offshore wind power. The direct connection and merging of the sea-land DC system lines are realized through the coastal DC busbar, so as to realize the mutual assistance of DC power between the two systems. The operational flexibility of the offshore wind power grid-connected system and the inland DC system is improved, a new solution is provided for the offshore wind power access method, and a solution is provided for the DC construction of the transmission channel of the coastal power grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a schematic diagram of the overall structure of the first embodiment of the present invention;

[0024] Figure 2 It is a schematic diagram of a control method using constant AC voltage and frequency control;

[0025] Figure 3 It is a schematic diagram of a control method using constant DC voltage and reactive power control;

[0026] Figure 4 It is a schematic diagram of a control method using constant power control;

[0027] Figure 5 It is a schematic diagram of the overall structure of the second embodiment of the present invention.

[0028] 1 is the offshore wind power AC collection-DC transmission system, 11 is the offshore wind farm, 12 is the AC collection submarine cable, 13 is the AC collection bus, 14 is the offshore converter station connecting transformer, 15 is the offshore converter station, 16 is the DC transmission submarine cable, 111 is the wind turbine string, and 112 is the wind turbine; 2 is the coastal DC receiving system, 21 is the coastal converter station, 22 is the coastal converter station connecting transformer, and 23 is the coastal power grid; 3 and 5 are the inland DC receiving system, 31 and 51 are the inland converter station, 32 and 52 are the inland converter station connecting transformer, 33 and 53 are the inland power grid, 34 and 54 are DC transmission lines; 4 is the coastal DC bus. DETAILED DESCRIPTION

[0029] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0030] Embodiment 1

[0031] The present invention provides a system and control method for the combined operation of offshore wind power DC transmission and inland DC transmission, and its topology mainly includes: an offshore wind power AC collection-DC transmission system 1, a coastal DC receiving system 2, inland DC receiving systems 3, 5 and a coastal DC bus 4; the offshore wind power AC collection-DC transmission system 1 is used for collecting and transmitting offshore wind power and connecting to the grid; the coastal DC receiving system 2 is used for connecting the offshore DC line to the coastal power grid 23 and controlling the DC voltage of the system; the inland DC receiving system 3 is used for transmitting DC power to the inland power grid; the coastal DC bus 4 is used for connecting the offshore and onshore DC lines and exchanging power.

[0032] The offshore wind power AC collection-DC transmission system 1 comprises an offshore wind farm group, an AC collection network, an offshore converter station 15 and a DC transmission submarine cable 16 connected in sequence; the offshore wind farm group comprises at least one offshore wind farm 11 for generating AC power. The AC collection network comprises a plurality of AC collection submarine cables 12 and a group of AC collection busbars 13 for collecting the power generated by the offshore wind farm group, wherein the plurality of AC collection submarine cables 12 are used to connect the AC collection busbars 13 and a plurality of offshore wind farms 11. The multiple AC collection submarine cables 12 include AC collection submarine cables inside the offshore wind farm and AC collection submarine cables outside the offshore wind farm. The AC collection submarine cables inside the offshore wind farm are AC collection submarine cables between offshore wind turbines, used to connect at least one offshore wind turbine for power collection; the AC collection submarine cables outside the offshore wind farm are AC collection submarine cables between offshore wind farms, connecting the offshore wind farms in the offshore wind farm group to each other. There is also an AC collection submarine cable used to connect at least one offshore wind farm group with an AC collection bus. The other end of the AC collection bus 13 is connected to an offshore converter station 15. The offshore converter station 15 is used to convert the received electric energy of the offshore wind farm group from AC to DC, and send the converted DC electric energy from the DC submarine cable 16 to the coastal DC bus 4. The DC sending submarine cable adopts a pseudo-bipolar wiring method or a true bipolar wiring method.

[0033] The offshore wind farm group can be one offshore wind farm 11 or multiple offshore wind farms 11. The offshore wind farm 11 collects electric energy through an AC collection network. The AC ports of multiple AC wind turbines 112 can be individually connected in parallel and collected, or the AC ports of an AC wind turbine string 111 formed by connecting multiple AC wind turbines 112 in parallel can be grouped and connected in parallel, or the AC ports of multiple AC wind turbine strings 112 can be connected to an AC transformer for centralized step-up and collection.

[0034] The coastal DC receiving end system 2 is used for landing and grid connection of the DC line for offshore wind power transmission, including a coastal converter station 21 and a coastal AC power grid 23 connected thereto. Among them, the coastal converter station 21 is used to convert electric energy from DC to AC or AC to DC, including a DC port formed by the DC end of the converter and an AC port formed by the AC end of the converter. The AC port is connected to the coastal AC power grid, and the DC port is connected to the coastal DC bus 4. When the coastal converter station 21 performs a DC to AC conversion, the converter station receives the DC power in the system, converts it into AC power, and then transmits it to the substation through the AC line, and then inputs it into the coastal power grid; when the coastal converter station performs an AC to DC conversion, the AC power is transmitted to the coastal converter station through the AC line and the transmission substation, and the DC power obtained after the conversion is input into the system.

[0035] The inland DC receiving system 3 is used for the transmission of DC power on land, and includes an inland DC transmission line 34, an inland converter station 31 and an inland power grid 33 which are connected in sequence. Among them, the inland DC transmission line 34 is used to transmit the DC power of the system from the coast to the inland converter station located inland; the inland converter station 31 is used to convert the power from DC to AC or AC to DC, including a DC port composed of the DC end of the converter and an AC port composed of the AC end of the converter; when the inland converter station 31 performs DC to AC conversion, the inland DC transmission line 34 will transmit the DC power to the inland converter station 31, and the converted power is transmitted to the substation through the AC line, and then input into the inland power grid 33; when the inland converter station 31 performs AC to DC conversion, the AC line is used to transmit the power generated by the substation to the inland converter station 31, and the converted power is sent to the DC system through the inland DC transmission line 34; the AC port is connected to the inland power grid 33, and the DC port is connected to the coastal DC bus 4 via the inland DC transmission line 34. Inland DC transmission lines can be constructed using land DC cables that meet the corresponding voltage level and current carrying capacity requirements, overhead lines that meet the corresponding index requirements, or a hybrid line of the two.

[0036] The coastal DC busbar 4 is the port after the DC port of at least one inland converter station is connected in parallel with the coastal converter station and the offshore wind power transmission DC line. It is used for connecting the offshore wind power AC collection-DC transmission system 1, the coastal DC receiving system 2 and the inland DC receiving system line 3, and is used for the transmission and exchange of DC power between the systems.

[0037] There may be several offshore wind power AC collection-DC transmission systems 1, and the components and parameters of each offshore wind power AC collection-DC transmission system 1 may not be completely the same. The voltage level of the DC power after the AC power generated by each offshore wind farm group is converted from AC to DC via the offshore converter station must be the same, and the remaining system components and parameters may be different.

[0038] The coastal DC receiving system 2 is a system that is connected to at least one offshore wind power AC collection-DC transmission system 1 to achieve its landing and grid connection. The voltage level of the DC electric energy converted by the coastal converter station 21 must be the same as the voltage level of the DC electric energy after the AC-to-DC conversion at the offshore converter station 15.

[0039] The coastal converter station 21 includes a converter and a coastal converter station connection transformer 22 connected to the converter, and the inland converter station 31 includes a converter and an inland converter station connection transformer 32 connected to the converter. The converter contains at least one sub-converter, and the sub-converter can be a voltage source converter, or a grid commutation converter or other converter with a bidirectional current flow capability. The sub-converter can be connected in series or in parallel. When the AC voltage level of the converter station matches the AC voltage level of the substation side, the inland converter station connection transformer 32 can be cancelled.

[0040] Each converter includes a DC port and an AC port; the DC ports of multiple converters are connected in series to form a DC end of the converter station; the AC ports of multiple converters are connected in parallel to form multiple AC ends of the converter station. In the coastal converter station 21 and / or the inland converter station 31, if the DC port of a converter constitutes a DC end, the converter adopts a voltage source converter; otherwise, the converter adopts a voltage source converter or a grid commutation converter.

[0041] The offshore converter station 15 includes a converter and a connecting transformer thereof. The converter includes at least one sub-converter, and the sub-converter can be a voltage source converter, a grid commutation converter, or a converter based on a diode rectifier unit. The sub-converters can be connected in series or in parallel. When the AC side of the converter station matches the AC voltage level of the collection network, the connecting transformer 14 of the offshore converter station can be eliminated.

[0042] In this embodiment, the voltage levels of the coastal power grid and the inland power grid, and the power grid topologies within each AC power grid may be different. The power grid topology includes the connection mode, number of loops, and AC voltage level of the AC line.

[0043] Figure 1 The PCC in the diagram is the Point of Common Coupling (PCC). Figures 2 to 4 Middle U d , U q are the d-axis and q-axis components of the AC voltage of the public AC busbar, respectively; P and Q are the active power and reactive power of the inland DC grid and / or inland AC grid, respectively; P ref , Q ref are the reference values ​​of active power and reactive power respectively; U dc is the DC voltage; Udcref is the reference value of DC voltage; I dref and I qref are the reference values ​​of the d-axis and q-axis components of the AC current respectively; M d and M q are the d-axis and q-axis components of the AC modulation ratio respectively; θ is the reference phase angle of the converter; V PCC is the voltage at the PCC point; f is the frequency.

[0044] The control method of this embodiment is designed as follows:

[0045] (1) Offshore converter station: constant AC constant voltage and frequency control is adopted. The control method diagram is as follows Figure 2 As shown, it is used to maintain the stability of the AC voltage amplitude and frequency of the AC collection bus 13 of offshore wind power, thereby ensuring the safe and stable collection of large-scale offshore wind power.

[0046] (2) Inland converter station: In this embodiment, the DC voltage between the DC submarine cable 16 and the inland DC transmission line 34 in the system is controlled by the coastal converter station 21 in the coastal DC receiving end system to maintain stability. Therefore, the coastal converter station 21 adopts constant DC voltage control, and its control method is shown in the schematic diagram as follows: Figure 3 The inland DC receiving system is responsible for transmitting a certain value of power to the inland power grid through the inland DC transmission line. To achieve this function, the inland converter station 31 adopts constant power control, that is, a constant power control method. The schematic diagram of the control method is shown in Figure 4 As shown, the active power value transmitted by the system to the inland power grid can be controlled to be constant; at the same time, the reactive power on the AC side of the inland converter station 31 is controlled, which can be used to provide a certain voltage support when the system fails.

[0047] The above three types of control are all based on double closed-loop vector control, which includes an inner loop current controller and an outer loop power controller. Among them, the outer loop controller controls the active and reactive power targets respectively, among which the outer loop controller of constant DC voltage control controls the DC voltage and reactive power to be constant respectively; the outer loop controller of constant power control controls the active power and reactive power to be constant respectively; the outer loop controller of constant AC voltage and frequency control controls the AC voltage components of the d-axis and q-axis to be constant respectively, and generates a reference phase angle according to the frequency. The inner loop current controller tracks and controls the output current of the converter. On the one hand, it can realize the decoupling control of the dq-axis current, so that the d-axis component and the q-axis component of the current can quickly track their respective command values, and on the other hand, it can realize the suppression of circulating current. The AC modulation ratios Md and Mq generated by the inner loop current controller are calculated by the modulation strategy to obtain the number of sub-modules N that should be put into each bridge arm, and then the specific sub-modules to be put into use are determined by the capacitor voltage equalization control, and then the modulation signal is generated to generate a trigger pulse through the trigger logic, and finally the corresponding sub-module is triggered.

[0048] When the inland converter station 31 adopts the above control method, since the system DC voltage is constant and the active power received by the inland converter station 31 is constant, the DC current flowing through the inland DC transmission line 34 is constant. Based on the DC current magnitude of the DC submarine cable 16 and the inland DC transmission line 34, the DC current magnitude flowing through the coastal converter station 21 can be obtained. One of the advantages of the grid-connected system in this embodiment is that the DC current flowing through the coastal converter station 21 is less than the DC current input by the DC submarine cable 16. Further, the operating power of the coastal converter station 21 is less than the operating power of the inland converter station in the traditional offshore wind power grid-connected system, which reduces the operating loss and investment cost.

[0049] Embodiment 2

[0050] In this embodiment, Figure 5 As shown, it includes: an offshore wind power AC collection-DC transmission system 1, a coastal DC receiving end system 2, two inland DC receiving end systems 3 and 5, and a coastal DC busbar 4.

[0051] The offshore wind power AC collection-DC transmission system 1 includes an offshore wind farm 11, an AC collection network 12-13, an offshore converter platform 14-15, and a DC transmission submarine cable 16. The coastal DC receiving end system 2 includes a coastal converter station 21, an inland converter station connection transformer 22, and a coastal power grid 23. The inland DC receiving end systems 3 and 5 include inland converter stations 31 and 51, inland converter station connection transformers 32 and 52, inland power grids 33 and 53, and inland DC transmission lines 34 and 54.

[0052] The offshore wind power transmission DC and inland DC combined operation system contains two inland DC receiving systems; in the inland DC receiving systems, the inland converter station can adopt different converter structures and capacity configurations, and the AC voltage levels, short-circuit capacities and specific topologies of the connected inland power grids 33 and 53 may be different, but the DC voltage levels of the inland DC transmission lines 34 and 54 should be the same.

[0053] The control method of this embodiment is the same as that of the first embodiment, and both the inland converter stations 31 and 51 adopt constant power control.

[0054] The technical solutions of the present invention are not limited to the above-mentioned embodiments, and any technical solutions obtained by equivalent replacement methods fall within the scope of protection required by the present invention.

Claims

1. A DC system for direct transmission of offshore wind power to multiple regional land power grids, characterized by: It includes an offshore wind power AC collection-DC transmission system, a coastal DC receiving system, a coastal DC busbar and at least one inland DC receiving system; The offshore wind power AC collection-DC transmission system comprises an offshore wind farm group, an AC collection network, an offshore converter station and a DC transmission submarine cable connected in sequence, wherein the DC transmission submarine cable is connected to a DC busbar near the sea; the offshore wind farm group is used to generate AC electric energy, the AC collection network is used to collect the electric energy generated by the offshore wind farm group and transmit it to the offshore converter station, and the offshore converter station is used to convert the electric energy received from the offshore wind farm group into AC to DC, and transmit the converted DC electric energy from the DC transmission submarine cable to the DC busbar near the sea; The coastal DC receiving end system comprises an interconnected coastal AC power grid and a coastal converter station, wherein the coastal converter station is connected to the coastal DC bus and is used to convert the DC power of the coastal DC bus into AC power and transmit it to the coastal power grid, or to convert the AC power of the coastal power grid into DC power and transmit it to the coastal DC bus; The inland DC receiving end system comprises an inland power grid, an inland converter station and an inland DC transmission line connected in sequence, wherein the inland DC transmission line is connected to the coastal DC busbar; the inland converter station is used to convert the DC power of the coastal DC busbar into AC power and transmit it to the inland power grid, or to convert the AC power of the inland power grid into DC power and transmit it to the coastal DC busbar via the inland DC transmission line; The offshore converter station is controlled by a constant AC voltage and frequency control method; The inland converter station is controlled by using a power constant control method of an inland AC power grid; The coastal converter station is controlled by a constant DC output voltage control method.

2. According to claim 1, the DC system for direct transmission of offshore wind power to multiple regional land power grids is characterized by: The coastal converter station and / or inland converter station comprises a plurality of converters, each of which comprises a DC port and an AC port; The DC ports of multiple converters are connected in series to form the DC end of the converter station; The AC ports of multiple converters are connected in parallel to form multiple AC terminals of the converter station.

3. The DC system for directly transmitting offshore wind power to multiple regional land power grids according to claim 2 is characterized by: In the coastal converter station and / or inland converter station, if the DC port of a converter constitutes a DC end, the converter adopts a voltage source converter; otherwise, the converter adopts a voltage source converter or a grid-commutated converter.

4. The DC system for direct transmission of offshore wind power to multiple regional land power grids according to claim 3 is characterized by: The offshore converter station comprises at least one converter. When a plurality of converters are included, the plurality of converters are connected in series, in parallel or in series and in parallel.

5. The DC system for direct transmission of offshore wind power to multiple regional land power grids according to claim 4 is characterized by: The converter comprises at least one sub-converter, wherein the sub-converter is a voltage source converter, a grid commutation converter, or a converter based on a diode rectifier unit, and the sub-converters are connected in series or in parallel.

6. The DC system for direct transmission of offshore wind power to multiple regional land power grids according to claim 1 is characterized by: The AC collection network includes an AC collection bus, a first type of AC collection submarine cable, a second type of AC collection submarine cable and a third type of AC collection submarine cable; The first type of AC collection submarine cable interconnects the offshore AC wind turbines in the offshore wind farm; The second type of AC aggregation submarine cable interconnects the offshore wind farms in the offshore wind farm group; One end of the third type of AC collection submarine cable is connected to an offshore wind farm group, and the other end is connected to one end of an AC collection busbar, and the other end of the AC collection busbar is connected to an offshore converter station.

7. The DC system for direct transmission of offshore wind power to multiple regional land power grids according to claim 6 is characterized by: An offshore converter station connection transformer is provided between the AC collection bus and the offshore converter station; a offshore converter station connection transformer is provided between the offshore converter station and the offshore power grid; and an inland converter station connection transformer is provided between the inland converter station and the inland power grid.

8. The DC system for direct transmission of offshore wind power to multiple regional land power grids according to claim 1 is characterized by: The DC transmission submarine cable adopts a pseudo bipolar wiring method or a true bipolar wiring method.

9. The DC system for direct transmission of offshore wind power to multiple regional land power grids according to claim 1 is characterized by: The constant power control method includes constant active power and / or constant reactive power.

10. The DC system for direct transmission of offshore wind power to multiple regional land power grids according to claim 1 is characterized by: The voltage level of the DC power generated by each offshore wind farm group after the AC power is converted from AC to DC at the offshore converter station is the same; the voltage level of the DC power obtained by the coastal converter station is the same as the voltage level of the DC power after the AC to DC conversion at the offshore converter station; the voltage level of the DC power obtained by the inland converter stations is the same.

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