Offshore wind power unit multi-region land grid direct current system

Abstract

The application provides a kind of offshore wind power unified direct transmission multi-regional land grid DC system, and its topology mainly includes: offshore wind power AC collection-DC sending system, coastal DC receiving end system, inland DC receiving end system and coastal DC bus; Offshore wind power AC collection-DC sending system is used for the collection and sending of offshore wind power and grid connection; Coastal DC receiving end system is used for the transmission of DC power to coastal power grid and control system DC voltage; Inland DC receiving end system is used for the transmission of DC power to inland power grid; Coastal DC bus is used for the connection and power exchange of offshore and land DC line. The application realizes the direct transmission of offshore wind farm power to multiple regional power grids on land, ensures the access and consumption of offshore wind power in coastal and inland power grids, improves the offshore wind power consumption capacity and power transmission capacity of the power grid, and at the same time improves the economic benefit and environmental benefit.

Description

TECHNICAL FIELD

[0001] The application relates to a direct-current system for transmitting offshore wind power to multiple regional land power grids, and belongs to the technical field of power system transmission and distribution. BACKGROUND

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

[0003] However, the land power grid using the traditional alternating current transmission network is often faced with a series of problems such as too long transmission distance, large power loss, uneven power flow distribution, and line load exceeding the standard, etc. when consuming the output power of the large-scale offshore wind farm connected thereto, which greatly reduces the consumption capacity of the land power grid for large-scale offshore wind power. In addition, offshore wind power generation has the characteristics of randomness, volatility and intermittency, which brings severe difficulties and challenges to the operation stability of the land power grid. At the same time, with the further expansion of the scale of new energy access along the coast and the further increase of the scale of power grid load, the demand for transmitting the power of the newly added new energy units along the coast to the load center on land is more prominent. Due to the influence of geography and economic development level, the load center is usually distributed in multiple regions on land, which further increases the difficulty of power transmission. If multiple land transmission projects are independently constructed, the construction investment cost will be relatively high, and the difficulty of controlling the coordinated operation of the offshore wind power transmission direct-current system and multiple land transmission systems will be further increased. SUMMARY

[0004] The application provides a direct-current system for transmitting offshore wind power to multiple regional land power grids, which has safety and stability and economy.

[0005] In order to achieve the above purpose, the technical scheme provided by the application is as follows: a direct-current system for transmitting offshore wind power to multiple regional land power grids, comprising an offshore wind power alternating current collection-direct current transmission system, a coastal direct-current receiving end system, a coastal direct-current bus and at least one inland direct-current receiving end system.

[0006] The offshore wind power alternating current collection-direct current transmission system comprises an offshore wind farm group, an alternating current collection network, an offshore converter station and a direct-current transmission submarine cable connected in sequence, and the direct-current transmission submarine cable is connected with the coastal direct-current bus; the offshore wind farm group is used to output alternating current power, the alternating current collection network is used to collect the power output by the offshore wind farm group and transmit the power to the offshore converter station, the offshore converter station is used to convert the alternating current power received from the offshore wind farm group into direct current, and the converted direct-current power is transmitted from the direct-current transmission submarine cable to the coastal direct-current bus;

[0007] The coastal AC receiving system comprises a coastal AC power grid and a coastal converter station connected to each other, and the coastal converter station is connected to a coastal DC bus for converting DC power of the coastal DC bus into AC power for transmission to the coastal AC power grid or converting AC power of the coastal AC power grid into DC power for transmission to the coastal DC bus.

[0008] The inland DC receiving system comprises an inland power grid, an inland converter station and an inland DC transmission line connected in sequence, and the inland DC transmission line is connected to the coastal DC bus; the inland converter station is configured to convert DC power of the coastal DC bus into AC power for transmission to the inland power grid or convert AC power of the inland power grid into DC power for transmission 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] Further design of the above technical solution is that the coastal converter station and / or the inland converter station comprises a plurality of converters, each of which comprises a DC port and an AC port; the DC ports of the plurality of converters are connected in series to form a DC end of the converter station; and the AC ports of the plurality of converters are connected in parallel to form a plurality of AC ends of the converter station.

[0010] In the coastal converter station and / or the inland converter station, if the DC port of a certain converter forms the DC end, the converter is a voltage source converter; otherwise, the converter is a voltage source converter or a line-commutated converter.

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

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

[0013] The AC collection network comprises an AC collection bus, a first AC collection submarine cable, a second AC collection submarine cable and a third AC collection submarine cable; the first AC collection submarine cable connects offshore AC wind turbines in a sea wind farm to each other; the second AC collection submarine cable connects sea wind farms in a sea wind farm group to each other; and one end of the third AC collection submarine cable is connected to the sea wind farm group, the other end of the third AC collection submarine cable 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] The AC collection bus is provided with a sea converter station connecting transformer between the sea converter station and the sea grid; the sea converter station is provided with a sea converter station connecting transformer between the sea converter station and the sea grid; and the inland converter station is provided with an inland converter station connecting transformer between the inland converter station and the inland grid.

[0015] The DC transmission cable adopts a pseudo bipolar connection mode or a true bipolar connection mode.

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

[0017] The AC power generated by each offshore wind farm group is converted into DC power with the same voltage level by the sea converter station; the DC power converted by the sea converter station has the same voltage level as the DC power converted by the sea converter station; and the DC power converted by each inland converter station has the same voltage level.

[0018] The present application has the following advantages:

[0019] The present application solves the problems of long transmission distance, uneven current distribution, and overloading of lines when the traditional AC grid in the coastal area absorbs large-scale offshore wind power, realizes the joint operation of the offshore wind power DC transmission system and the inland DC transmission system, ensures the access and absorption of large-scale offshore wind power, realizes the long-distance transmission of electric energy to the inland grid, improves the offshore wind power absorption capacity and power transmission capacity of the grid, and improves the compactness of the offshore wind power and the land DC system, effectively improves the coordination and safety of joint control operation, improves the utilization rate of the power transmission channel, and improves the economic and environmental benefits.

[0020] The present application solves the problems of long transmission distance, uneven current distribution, and overloading of lines when the traditional AC grid in the coastal area absorbs large-scale offshore wind power, realizes the joint operation of the offshore wind power DC transmission system and the inland DC transmission system, ensures the access and absorption of large-scale offshore wind power, realizes the long-distance transmission of electric energy to the inland grid, improves the offshore wind power absorption capacity and power transmission capacity of the grid, and improves the compactness of the offshore wind power and the land DC system, effectively improves the coordination and safety of joint control operation, improves the utilization rate of the power transmission channel, and improves the economic and environmental benefits.

[0021] The present application proposes to use 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, improve the economy of large-scale offshore wind power transmission, and use the land DC transmission system to undertake the transmission task of the inland grid electric energy, which plays a role in expanding and supplementing the original AC grid, and improves the transmission capacity and overall strength of the original inland AC grid.

[0022] The application provides a kind of offshore wind power unified direct transmission multi-regional land grid DC system and control method, will offshore wind power AC collection-DC sending system, coastal DC receiving end system and inland DC receiving end system be connected by coastal DC bus, realize the joint control and operation of sea-land DC system, improve the overall strength of coastal area power grid, guarantee the access and consumption of large-scale offshore wind power.And through coastal DC bus, realize the direct connection of sea-land DC system line merging, realize the DC energy interconnection between two systems.Improve the operation flexibility of offshore wind power grid-connected system and inland DC system, provide a new solution for offshore wind power access mode, provide solution ideas for the direct current of coastal area power grid transmission channel. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 It is the overall structure schematic diagram of the embodiment one of the application;

[0024] Figure 2 It is the control method schematic diagram of fixed AC voltage and frequency control;

[0025] Figure 3 It is the control method schematic diagram of fixed DC voltage and reactive power control;

[0026] Figure 4 It is the control method schematic diagram of fixed power control;

[0027] Figure 5 It is the overall structure schematic diagram of the embodiment two of the application.

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

[0029] The application will be described in detail below in combination with the drawings and specific embodiments.

[0030] Embodiment one

[0031] The application provides a system and a control method for offshore wind power DC transmission and inland DC combined operation, and the topology mainly comprises: an offshore wind power AC collection-DC transmission system 1, a coastal DC receiving end system 2, an inland DC receiving end system 3 and 5, and a coastal DC bus 4; the offshore wind power AC collection-DC transmission system 1 is used for collecting and transmitting the offshore wind power and connecting to the grid; the coastal DC receiving end 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 end system 3 is used for transmitting the DC power to the inland power grid; and the coastal DC bus 4 is used for connecting the offshore and land DC lines and exchanging the 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 and is used for emitting AC power; the AC collection network comprises a plurality of AC collection submarine cables 12 and a group of AC collection buses 13 and is used for collecting the power emitted by the offshore wind farm group, wherein the plurality of AC collection submarine cables 12 are used for connecting the AC collection buses 13 and the plurality of offshore wind farms 11; the plurality of AC collection submarine cables 12 comprise 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 turbine generators and are used for connecting at least one offshore wind turbine generator to collect power; the AC collection submarine cables outside the offshore wind farm are AC collection submarine cables between offshore wind farms and are used for connecting the offshore wind farms in the offshore wind farm group to each other; and another AC collection submarine cable is used for connecting at least one offshore wind farm group to the AC collection bus; the other end of the AC collection bus 13 is connected to the offshore converter station 15; the offshore converter station 15 is used for converting the received power of the offshore wind farm group from AC to DC and transmitting the converted DC power from the DC submarine cable 16 to the coastal DC bus 4; and the DC transmission submarine cable adopts a pseudo-bipolar wiring mode or a true bipolar wiring mode.

[0033] The offshore wind farm group can be one offshore wind farm 11 or a plurality of offshore wind farms 11; the offshore wind farm 11 collects power through the AC collection network, which can be to separately parallelly collect the AC ports of a plurality of AC wind turbine generators 112, or to parallelly collect the AC ports of an AC wind turbine generator string 111 formed by a plurality of AC wind turbine generators 112 in groups, or to connect the AC ports of a plurality of AC wind turbine generator strings 112 to an AC transformer for centralized voltage boosting.

[0034] The coastal DC receiving end system 2 is used for landing and grid connection of the offshore wind power DC transmission line, and includes a coastal converter station 21 and a coastal AC power grid 23 connected thereto. The coastal converter station 21 is used for converting DC to AC or AC to DC, and includes a DC port formed by a DC end of a converter and an AC port formed by an AC end of the converter. The AC port is connected to the coastal AC power grid, and the DC port is connected to a coastal DC bus 4. When the coastal converter station 21 converts DC to AC, the DC power in the receiving system is converted into AC power by the converter station, and then transmitted to a transformer substation through an AC line, and then input to the coastal power grid. When the coastal converter station converts AC to DC, the AC power is transmitted to the coastal converter station through the AC line and the transmission transformer substation, and the DC power obtained after conversion is input to the system.

[0035] The inland DC receiving end system 3 is used for 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 connected in sequence. The inland DC transmission line 34 is used to transmit DC power from the coastal area to the inland converter station. The inland converter station 31 is used for converting DC to AC or AC to DC, and includes a DC port formed by a DC end of a converter and an AC port formed by an AC end of the converter. When the inland converter station 31 converts DC to AC, the inland DC transmission line 34 transmits DC power to the inland converter station 31, and the converted power is transmitted to a transformer substation through an AC line, and then input to the inland power grid 33. When the inland converter station 31 converts AC to DC, the AC line is used to transmit power generated by the transformer substation to the inland converter station 31, and the converted power is transmitted 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 through the inland DC transmission line 34. The inland DC transmission line can be built by using a land DC cable meeting the requirements of corresponding voltage levels and current carrying capacity, an overhead line meeting the requirements of corresponding indicators, or a mixed line of the two.

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

[0037] The offshore wind power AC collection-DC transmission system 1 can be provided with a plurality of systems, and the components and parameters of each offshore wind power AC collection-DC transmission system 1 can be different. The voltage levels of the DC power converted from the AC power generated by each offshore wind farm group through the offshore converter station must be the same, and the remaining components and parameters can be different.

[0038] The coastal DC receiving-end system 2 is a system that connects 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 power obtained by the coastal converter station 21 must be the same as the voltage level of the DC power after 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. The inland converter station 31 includes a converter and an inland converter station connection transformer 32 connected to the converter. Each converter contains at least one sub-converter, which can be a voltage source converter or a grid-commutated converter, or other converters capable of bidirectional current flow. Sub-converters can be connected in series or in parallel. When the AC voltage levels of the converter station and the substation match, the inland converter station connection transformer 32 can be omitted.

[0040] Each converter includes a DC port and an AC port; the DC ports of multiple converters are connected in series to form the DC terminal of the converter station; the AC ports of multiple converters are connected in parallel to form multiple AC terminals of the converter station. In the coastal converter station 21 and / or the inland converter station 31, if the DC port of a certain converter constitutes the DC terminal, then the converter is a voltage source converter; otherwise, the converter is a voltage source converter or a grid-commutated converter.

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

[0042] In this embodiment, the coastal power grid and the inland power grid can have different voltage levels and different internal grid topologies. The grid topology includes the connection method of AC lines, the number of circuits, and the AC voltage level.

[0043] Figure 1 In this context, PCC stands for Point of Common Coupling. Figures 2 to 4 Middle U d U q , , represent the d-axis and q-axis components of the AC voltage of the common AC bus; P and Q represent the active and reactive power of the inland DC grid and / or the inland AC grid, respectively; P ref Q ref These are reference values ​​for active power and reactive power, respectively; U dc DC voltage; Udcref Reference value of direct current voltage; I dref and I qref Reference value of d, q-axis component of alternating current, respectively; M d and M q d, q-axis component of alternating current modulation ratio, respectively; θ is the reference phase angle of the converter; V PCC Voltage of PCC point; f is the frequency.

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

[0045] (1) Offshore converter station: fixed AC voltage and frequency control is adopted, and a control method diagram thereof is shown in FIG. 3, which is used to maintain the stability of the AC voltage amplitude and frequency of the AC collection bus 13 of offshore wind power, and to ensure the safe and stable collection of large-scale offshore wind power. Figure 2

[0046] (2) Inland converter station: in the embodiment, the direct current voltage of the direct current sea cable 16 and the inland direct current transmission line 34 in the system is controlled by the coastal converter station 21 in the coastal direct current receiving end system to maintain stability. Therefore, the coastal converter station 21 adopts fixed direct current voltage control, and a control method diagram thereof is shown in FIG. 4. The inland direct current receiving end system is responsible for transmitting a certain value of power to the inland power grid through the inland direct current transmission line. In order to realize this function, the inland converter station 31 adopts fixed power control, that is, the control mode of constant power, and a control method diagram thereof is shown in FIG. 5. The active power value transmitted to the inland power grid by the system 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. Figure 3 Figure 4

[0047] The above three controls are based on double closed loop vector control, which includes inner loop current controller and outer loop power controller. The outer loop controller controls the active and reactive power target respectively, in which the outer loop controller of fixed direct current voltage control controls the direct current voltage and reactive power constant respectively; the outer loop controller of fixed power control controls the active power and reactive power constant respectively; the outer loop controller of fixed AC voltage and frequency control controls the d-axis and q-axis AC voltage components constant respectively, and generates the reference phase angle according to the frequency. The inner loop current controller controls the output current of the converter, which can realize the decoupling control of dq-axis current on the one hand, so that the d-axis component and the q-axis component of the current can quickly track the respective command values, and can realize the circulation suppression on the other hand. The alternating current modulation ratio Md and Mq generated by the inner loop current controller calculate the number of sub-modules N to be put into each bridge arm through the modulation strategy, and then determine the specific sub-modules to be put into by the capacitor voltage equalization control, and then generate the modulation signal to generate the trigger pulse through the trigger logic, and finally trigger the corresponding sub-modules.

[0048] ​​​When the inland converter station 31 adopts the above control method, the active power accepted by the inland converter station 31 is constant due to the constant system DC voltage, and thus the DC current flowing through the inland DC transmission line 34 is constant. Based on the DC current of the DC submarine cable 16 and the DC current of the inland DC transmission line 34, the DC current flowing through the offshore converter station 21 can be obtained. One of the advantages of the grid-connected system in the embodiment is that the DC current flowing through the offshore converter station 21 is smaller than the DC current input by the DC submarine cable 16, and further, the operating power of the offshore converter station 21 is smaller than the operating power of the inland converter station in the conventional offshore wind power grid-connected system, thereby reducing the operating loss and investment cost.

[0049] Embodiment two

[0050] The embodiment comprises Figure 5 As shown in the figure, the embodiment comprises: an offshore wind power AC collection-DC transmission system 1, an offshore DC receiving system 2, two inland DC receiving systems 3 and 5, and an offshore DC bus 4.

[0051] The offshore wind power AC collection-DC transmission system 1 comprises an offshore wind farm 11, AC collection networks 12-13, offshore converter platforms 14-15, and a DC transmission submarine cable 16. The offshore DC receiving system 2 comprises an offshore converter station 21, an inland converter station connection transformer 22, and an offshore power grid 23. The inland DC receiving systems 3 and 5 comprise 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 comprises two inland DC receiving systems. In the inland DC receiving systems, the inland converter stations can adopt different converter structures and capacity configurations, the connected inland power grids 33 and 53 can have different AC voltage levels, short-circuit capacities, and specific topologies, but the DC voltage levels of the inland DC transmission lines 34 and 54 should be the same.

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

[0054] The technical solutions of the present application are not limited to the above embodiments, and any technical solution obtained by equivalent replacement falls within the scope of the present application.

Claims

1. A direct current system for a multi-zone land grid from an offshore wind power plant, characterized in that: The offshore wind farm AC collection-DC transmission system, the coastal DC receiving system, the coastal DC bus, and at least one inland DC receiving system are connected in series. The offshore wind farm 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 series. The offshore wind farm group is used to generate AC power. The AC collection network is used to collect the power generated by the offshore wind farm group and transmit the power to the offshore converter station. The offshore converter station is used to convert the AC power received from the offshore wind farm group into DC power and transmit the converted DC power to the coastal DC bus through the DC transmission submarine cable. The coastal DC receiving system comprises a coastal AC power grid and a coastal converter station connected in series. The coastal converter station is connected to the coastal DC bus and used to convert the DC power from the coastal DC bus into AC power and transmit the AC power to the coastal AC power grid, or convert the AC power from the coastal AC power grid into DC power and transmit the DC power to the coastal DC bus.

2. The offshore wind power system according to claim 1, wherein: The inland DC receiving system comprises an inland power grid, an inland converter station, and an inland DC transmission line connected in series. The inland converter station is used to convert the DC power from the coastal DC bus into AC power and transmit the AC power to the inland power grid, or convert the AC power from the inland power grid into DC power and transmit the DC power to the coastal DC bus through the inland DC transmission line. The offshore converter station is controlled by a constant AC voltage and frequency control method.

3. The offshore wind power system according to claim 2, wherein: The inland converter station is controlled by a constant power control method of the inland AC power grid.

4. The offshore wind power system according to claim 3, wherein: The coastal converter station is controlled by a constant DC output voltage control method.

5. The offshore wind power system according to claim 4, wherein: The coastal converter station and / or the inland converter station comprises a plurality of converters.

6. The offshore wind power system according to claim 1, wherein: The DC ports of the plurality of converters are connected in series and form a DC port of the converter station. The AC ports of the plurality of converters are connected in parallel and form a plurality of AC ports of the converter station. If the DC port of a converter forms the DC port of the converter station, the converter is a voltage source converter; otherwise, the converter is a voltage source converter or a line-commutated converter. The offshore converter station comprises at least one converter. The converter comprises at least one sub-converter. The sub-converter is a voltage source converter, a line-commutated converter, or a converter based on a diode rectifier unit. The sub-converter is connected in series or in parallel. The AC collection network comprises an AC collection bus, a first AC collection submarine cable, a second AC collection submarine cable, and a third AC collection submarine cable. The first AC collection submarine cable connects the offshore AC wind turbines in the offshore wind farm. The second AC collection submarine cable connects the offshore wind farms in the offshore wind farm group. One end of the third 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. The other end of the AC collection bus is connected to the offshore converter station.

7. The offshore wind power plant HVDC system of claim 6, wherein: The AC collection bus is provided with an offshore converter station connecting transformer between the offshore converter station and the offshore power grid; the offshore converter station is provided with an offshore converter station connecting transformer between the offshore converter station and the offshore power grid; and the inland converter station is provided with an inland converter station connecting transformer between the inland converter station and the inland power grid.

8. The system of claim 1, wherein the system is a multi-zone land grid DC system. The DC transmission cable adopts a pseudo-bipolar connection mode or a true bipolar connection mode.

9. The system of claim 1, wherein the system is a multi-zone land grid DC system. The constant power control mode includes active power constant and / or reactive power constant.

10. The system of claim 1, wherein the system is a multi-zone land grid DC system. The voltage levels of the AC power emitted by each offshore wind farm group are the same after AC-DC conversion by the offshore converter station; the voltage levels of the DC power converted by the offshore converter station are the same as the voltage levels of the AC power emitted by each offshore wind farm group after AC-DC conversion by the offshore converter station; and the voltage levels of the DC power converted by each inland converter station are the same.

Images