Offshore wind power plant direct current sending-out system and method

Abstract

The invention provides an offshore wind power plant direct current sending-out system and method, in a black start stage of the system, a diode auxiliary current converter and an auxiliary current converter of a shore converter station, or a diode in anti-parallel connection with a main current converter and the auxiliary current converter form a low-voltage black start loop which is connected with a shore alternating current power grid; the shore alternating current power grid provides starting energy for the network-forming reactive power compensation device and the offshore wind power plant; in a power path transfer stage of the system, transferring the low-voltage black-start loop to a power transmission path formed by the diode rectifier and a main converter of the shore converter station; and in the steady-state operation stage of the system, an alternating-current voltage of the common connection point is established by the network-forming reactive power compensation device, so that the offshore wind plant transmits rated power through the diode rectifier. Compared with a modular multilevel converter, the diode rectifier is small in size, so that the construction cost economy of the offshore platform is better.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of flexible direct current power transmission, and particularly relates to a direct current transmission system and method for an offshore wind farm. BACKGROUND

[0002] To support the realization of the "double carbon" strategic goal, large-scale deep-sea offshore wind power has been rapidly developed. Modular multilevel converters (MMC) have the characteristics of modularity, high efficiency and low harmonic content.

[0003] Modular multilevel converters are the mainstream converter topologies for large-scale deep-sea wind power direct current transmission. However, due to the large size and heavy weight of modular multilevel converters, the construction cost of offshore platforms is high and the economic efficiency is poor. SUMMARY

[0004] To overcome the shortcomings of the prior art, in a first aspect, the application provides a direct current transmission system for an offshore wind farm, comprising: an offshore wind farm, an offshore alternating current grid, a hybrid converter, an onshore converter station and an onshore alternating current grid connected in sequence; the hybrid converter comprises a diode rectifier, a grid-forming reactive power compensation device and an auxiliary converter; the diode rectifier and the auxiliary converter are connected in parallel to the onshore converter station, and the diode rectifier, the grid-forming reactive power compensation device and the offshore alternating current grid are connected through a common connection point.

[0005] In the black start phase of the system, the diode auxiliary converter of the onshore converter station and the auxiliary converter, or the diode of the onshore converter station and the auxiliary converter, form a low-voltage black start loop, which is connected to the onshore alternating current grid, so that the onshore alternating current grid provides start-up energy for the grid-forming reactive power compensation device and the offshore wind farm.

[0006] In the power path transfer phase of the system, the external output power of the offshore wind farm is limited, and the low-voltage black start loop is transferred to the power transmission path formed by the diode rectifier and the main converter of the onshore converter station.

[0007] In the steady-state operation phase of the system, the grid-forming reactive power compensation device establishes an alternating current voltage at the common connection point, so that the offshore wind farm transmits rated power through the diode rectifier.

[0008] Preferably, in the black start phase of the system, the diode auxiliary converter of the onshore converter station is turned on by the onshore alternating current grid to charge the auxiliary converter, and the auxiliary converter provides start-up energy for the grid-forming reactive power compensation device and the offshore wind farm after being unlocked by the charging.

[0009] Preferably, the main converter and the diode auxiliary converter are connected in parallel in the onshore converter station; the insulated gate bipolar transistor inside the main converter is connected in reverse parallel with the diode;

[0010] In the power path transfer stage of the system, the main converter is provided with starting energy by the onshore AC power grid, so that the main converter is unlocked and the rated DC voltage is established.

[0011] Preferably, in the steady state operation stage of the system, the operating voltage of the point of common coupling is established by the network-forming type reactive power compensation device, and the offshore wind farm is provided with reactive power compensation, so that the energy of the offshore wind farm can be sent to the onshore AC power grid through the diode rectifier and the main converter.

[0012] Preferably, the onshore converter station further comprises an energy consumption branch; the main converter is connected in parallel with the energy consumption branch.

[0013] When the onshore AC power grid fails, the power of the offshore wind farm is absorbed by the energy consumption branch.

[0014] Preferably, the onshore converter station further comprises a first fast switch and a second fast switch; the first fast switch is connected between the DC end of the main converter and the DC end of the diode rectifier; the second fast switch is connected between the DC end of the diode auxiliary converter and the DC end of the main converter.

[0015] In the black start stage of the system, the first fast switch is open, and the second fast switch is closed.

[0016] In the power path transfer stage of the system, the first fast switch is closed, and the second fast switch is open.

[0017] Preferably, the hybrid converter further comprises a third fast switch and a fourth fast switch; the third fast switch is connected between the DC end of the diode rectifier and the DC end of the main converter; the fourth fast switch is connected between the DC end of the diode rectifier and the DC end of the auxiliary converter.

[0018] In the black start stage of the system, the third fast switch is open, and the fourth fast switch is closed.

[0019] In the power path transfer stage of the system, the third fast switch is closed, and the fourth fast switch is open.

[0020] Preferably, the system further comprises a local AC energy consumption device, and the offshore wind farm is connected with the local AC energy consumption device.

[0021] In the power path transfer stage of the system, the offshore wind farm is limited to output power by the local AC power consumption device, and then the main converter establishes the rated DC voltage.

[0022] Preferably, the system further comprises an adjustable transformer, the onshore AC power grid is connected to the adjustable transformer, and the adjustable transformer is connected to the main converter.

[0023] In the black start stage of the system, the adjustable transformer reduces the voltage on the side of the main converter by adjusting the tap position of itself, so that the diode of the main converter is turned on in anti-parallel, and then a low-voltage black start loop is formed with the auxiliary converter.

[0024] In a second aspect, the application further provides a method for transmitting offshore wind power through DC, which is applied to the offshore wind power transmission system described above and comprises the following steps:

[0025] In the black start stage of the system, a low-voltage black start loop is formed by the diode auxiliary converter of the onshore converter station and the auxiliary converter, or the diode of the onshore converter station and the auxiliary converter, and the onshore AC power grid is connected to the low-voltage black start loop, so that the onshore AC power grid provides start-up energy for the grid-forming reactive power compensation device and the offshore wind farm.

[0026] In the power path transfer stage of the system, the offshore wind farm is limited to output power, and the low-voltage black start loop is transferred to the power transmission path formed by the diode rectifier and the main converter of the onshore converter station.

[0027] In the steady-state operation stage of the system, the grid-forming reactive power compensation device is used to establish the AC voltage of the point of common coupling, so that the offshore wind farm transmits rated power through the diode rectifier.

[0028] Preferably, in the black start stage of the system, the diode auxiliary converter of the onshore converter station is turned on by the onshore AC power grid to charge the auxiliary converter, and after the auxiliary converter is unlocked by the charging, the auxiliary converter provides start-up energy for the grid-forming reactive power compensation device and the offshore wind farm.

[0029] Preferably, the main converter and the diode auxiliary converter are connected in parallel in the onshore converter station, and the insulated gate bipolar transistor inside the main converter is connected in anti-parallel to the diode.

[0030] In the power path transfer stage of the system, the onshore AC power grid is used to provide start-up energy for the main converter, so that the main converter is unlocked and the rated DC voltage is established.

[0031] Preferably, during the steady state operation stage of the system, the networked reactive power compensation device is used to establish the operating voltage of the point of common coupling and provide reactive power compensation for the offshore wind farm, so that the offshore wind farm energy can be transmitted to the onshore AC power grid through the diode rectifier and the main converter.

[0032] Preferably, the onshore converter station further comprises an energy dissipation branch, and the main converter is connected in parallel with the energy dissipation branch.

[0033] When the onshore AC power grid fails, the energy dissipation branch is used to absorb the power of the offshore wind farm.

[0034] Preferably, the onshore converter station further comprises a first fast switch and a second fast switch, and the DC end of the main converter is connected with the DC end of the diode rectifier through the first fast switch, and the DC end of the diode auxiliary converter is connected with the DC end of the main converter through the second fast switch.

[0035] During the black start stage of the system, the first fast switch is open, and the second fast switch is closed.

[0036] During the power path transfer stage of the system, the first fast switch is closed, and the second fast switch is open.

[0037] Preferably, the hybrid converter further comprises a third fast switch and a fourth fast switch, and the DC end of the diode rectifier is connected with the DC end of the main converter through the third fast switch, and the DC end of the diode rectifier is connected with the DC end of the auxiliary converter through the fourth fast switch.

[0038] During the black start stage of the system, the third fast switch is open, and the fourth fast switch is closed.

[0039] During the power path transfer stage of the system, the third fast switch is closed, and the fourth fast switch is open.

[0040] Preferably, the system further comprises a local AC energy dissipation device, and the offshore wind farm is connected with the local AC energy dissipation device.

[0041] During the power path transfer stage of the system, the local AC energy dissipation device is used to limit the external output power of the offshore wind farm, and then the main converter establishes the rated DC voltage.

[0042] Preferably, the system further comprises an adjustable ratio transformer, and the onshore AC power grid is connected with the adjustable ratio transformer, and the adjustable ratio transformer is connected with the main converter.

[0043] During the black start phase of the system, the adjustable transformer is used to adjust its own tap position to reduce the voltage on the main converter side, so that the anti-parallel diode of the main converter is turned on, and then forms a low-voltage black start circuit with the auxiliary converter.

[0044] In a third aspect, the present invention also proposes an electronic device, comprising: at least one processor and a memory; the memory and the processor are connected via a bus;

[0045] The memory is used to store one or more programs;

[0046] When the one or more programs are executed by the at least one processor, the method for transmitting direct current from an offshore wind farm is implemented.

[0047] In a fourth aspect, the present application further proposes a readable storage medium having an execution program stored thereon, which, when executed, implements the method for transmitting direct current from an offshore wind farm.

[0048] Compared with the closest prior art, the present invention has the following beneficial effects:

[0049] The present invention provides an offshore wind farm direct current transmission system and method, comprising: an offshore wind farm, an offshore AC power grid, a hybrid converter, an onshore converter station and an onshore AC power grid connected in sequence; the hybrid converter comprises: a diode rectifier, a grid-type reactive power compensation device and an auxiliary converter; the diode rectifier and the auxiliary converter are connected in parallel and then connected to the onshore converter station, the diode rectifier, the grid-type reactive power compensation device and the offshore AC power grid are connected via a common connection point; in the black start phase of the system, the diode auxiliary converter of the onshore converter station and the auxiliary converter, or the onshore converter, are connected in parallel. The diodes at the flow station and the auxiliary converter form a low-voltage black-start circuit that connects to the onshore AC power grid, enabling the onshore AC power grid to provide startup energy for the grid-type reactive power compensation device and the offshore wind farm. During the system's power path transfer phase, the offshore wind farm's external power output is restricted, and the low-voltage black-start circuit is transferred to the diode rectifier and the main converter of the onshore converter station to form a power transmission path. During the system's steady-state operation phase, the grid-type reactive power compensation device establishes the AC voltage at the common connection point, enabling the offshore wind farm to deliver rated power through the diode rectifier. Compared to modular multilevel converters, the use of diode rectifiers is smaller in size. Relying on the onshore AC power grid for auxiliary pre-charging and black-start power for the offshore wind farm, while using the grid-type reactive power compensation device to maintain the operating voltage during the black-start phase, further reduces the construction cost of the offshore platform, making the construction cost of the offshore platform more economical. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 The present invention provides an architecture of a DC transmission system for an offshore wind farm Figure 1 ;

[0051] Figure 2 The present invention provides a topological structure of a diode rectifier for a DC transmission system of an offshore wind farm;

[0052] Figure 3 The topological structure of a grid-type reactive power compensation device for an offshore wind farm via a DC transmission system provided by the present invention;

[0053] Figure 4 The present invention provides a topological structure of an auxiliary converter and a main converter of an offshore wind farm through a DC transmission system;

[0054] Figure 5 The present invention provides an architecture of a DC transmission system for an offshore wind farm Figure 2 ;

[0055] Figure 6 A path diagram of a black start phase of an offshore wind farm through a DC transmission system provided by the present invention;

[0056] Figure 7 A path diagram of the steady-state operation phase of an offshore wind farm through a DC transmission system provided by the present invention;

[0057] Figure 8 A flow chart of a method for transmitting DC power from an offshore wind farm provided by the present invention;

[0058] Figure 9 This is a schematic diagram of the operation of an electronic device provided in the present invention application. DETAILED DESCRIPTION

[0059] The specific implementation methods of the present invention will be further described in detail below with reference to the accompanying drawings.

[0060] Example 1:

[0061] like Figure 1 As shown, the present invention proposes a direct current transmission system for an offshore wind farm, comprising: an offshore wind farm, an offshore AC power grid, a hybrid converter, an onshore converter station, and an onshore AC power grid connected in sequence; the hybrid converter comprises: a diode rectifier, a grid-type reactive power compensation device, and an auxiliary converter; the diode rectifier and the auxiliary converter are connected in parallel and then connected to the onshore converter station; the diode rectifier, the grid-type reactive power compensation device, and the offshore AC power grid are connected via a common connection point;

[0062] In the black start phase of the system, the diode auxiliary converter of the onshore converter station and the auxiliary converter, or the diode of the onshore converter station and the auxiliary converter, form a low-voltage black start loop, connect the onshore AC power grid, and enable the onshore AC power grid to provide start-up energy for the grid-forming reactive power compensation device and the offshore wind farm.

[0063] In the power path transfer phase of the system, the output power of the offshore wind farm is limited, the low-voltage black start loop is transferred to the power transmission path formed by the diode rectifier and the main converter of the onshore converter station, and the power transmission path is formed.

[0064] In the steady-state operation phase of the system, the grid-forming reactive power compensation device establishes an AC voltage at the point of common coupling, and enables the offshore wind farm to transmit rated power through the diode rectifier.

[0065] The above system is mainly designed to further reduce the volume and weight of the offshore station. The hybrid converter (offshore) can be composed of a grid-forming reactive power compensation device, an auxiliary converter, and a diode rectifier (DRU). A high-pass filter (HPF) can be connected to the system to filter out harmonic currents above 20 times. The grid-forming reactive power compensation device can be a grid-forming static var generator (SVG). The diode rectifier is as shown in Figure 2 The grid-forming reactive power compensation device has the structure as shown in Figure 3 The grid-forming reactive power compensation device has the structure as shown in 3...... S6 is an IGBT (Insulate Gate Bipolar Transistor) in the full-bridge sub-module, D 3...... D6 is a diode in the full-bridge sub-module, and C2 is a capacitor in the full-bridge sub-module.

[0066] The system can be divided into a black start phase, a power path transfer phase, and a steady-state operation phase.

[0067] The diode rectifier is responsible for transmitting all active power of the wind farm, and the transmission capacity of the wind farm can reach 100%. The DC voltage level is the rated DC voltage Udc. The auxiliary converter establishes the voltage of the offshore wind farm in the black start phase, realizes the black start function of the offshore wind farm and the grid-forming reactive power compensation device, and provides 5% reactive power compensation of the offshore wind farm capacity in the steady-state power transmission phase. The DC voltage level can be 10% of the rated DC voltage Udc. The high-pass filter can filter out harmonic currents above 20 times of the offshore wind farm.

[0068] Furthermore, during the black start phase of the system, the onshore AC power grid turns on the diode auxiliary converter of the onshore converter station and charges the auxiliary converter. After being charged and unlocked, the auxiliary converter provides starting energy for the grid-type reactive compensation device and the offshore wind farm.

[0069] Furthermore, the main converter and the diode auxiliary converter are connected in parallel and are located in the onshore converter station; the insulated gate bipolar transistor inside the main converter is connected in anti-parallel to the diode;

[0070] During the power path transfer phase of the system, the onshore AC grid provides starting energy to the main converter, so that the main converter is unlocked and a rated DC voltage is established.

[0071] Alternatively, a low-voltage black start circuit may be formed by the anti-parallel diode of the main converter and the auxiliary converter;

[0072] As mentioned above, the onshore converter station consists of a main converter and a diode-assisted converter; the onshore converter station includes a main converter and a diode-assisted converter; among them, the main converter is responsible for establishing the rated DC voltage; the diode-assisted converter is responsible for establishing a low-voltage black start circuit; the diode-assisted converter and the auxiliary converter constitute a low-voltage black start circuit (the voltage of the low-voltage black start circuit can be 10% of the rated DC voltage), which is connected to the onshore AC power grid to provide starting energy for the grid-type reactive compensation device and the offshore wind farm; the configuration of the diode-assisted converter can be selected according to actual needs.

[0073] The offshore diode rectifier and the onshore main converter form a power transmission path, which transmits the offshore wind farm power to the onshore AC grid according to the rated DC voltage;

[0074] To reduce costs, such as Figure 4 As shown, the auxiliary converter and the main converter on the shore can adopt a pure half-bridge structure, where HBSM is a half-bridge submodule, S1 and S2 are IGBTs in the half-bridge submodule, D1 and D2 are diodes in the half-bridge submodule, C is a capacitor in the half-bridge submodule, and U C is the voltage across capacitor C, L arm The bridge arms are A, B, and C, which are three-phase AC power. The main converter is responsible for establishing the rated DC voltage Udc to establish a high-voltage DC transmission path.

[0075] The diode-assisted converter on the shore is responsible for establishing a low-voltage black start circuit, and the DC voltage level is 10% Udc.

[0076] Further, in the steady state operation phase of the system, the network-forming reactive power compensation device establishes the operating voltage of the point of common coupling and provides reactive power compensation for the offshore wind farm, so that the offshore wind farm energy can be sent to the onshore AC power grid through the diode rectifier and the main converter.

[0077] Further, after the black start is completed, the network-forming reactive power compensation device establishes the voltage of the point of common coupling and can provide 25% of the wind farm capacity of reactive power compensation for the offshore wind farm power transmission, and has the function of harmonic control of the point of common coupling;

[0078] Further, the onshore converter station further comprises an energy consumption branch; and the main converter is connected in parallel with the energy consumption branch.

[0079] When the onshore AC power grid fails, the energy consumption branch absorbs the power of the offshore wind farm.

[0080] Further, the onshore converter station further comprises a first fast switch and a second fast switch; the first fast switch is connected between the DC end of the main converter and the DC end of the diode rectifier; and the second fast switch is connected between the DC end of the diode auxiliary converter and the DC end of the main converter.

[0081] In the black start phase of the system, the first fast switch is open, and the second fast switch is closed.

[0082] In the power path transfer phase of the system, the first fast switch is closed, and the second fast switch is open.

[0083] Further, the hybrid converter further comprises a third fast switch and a fourth fast switch; the third fast switch is connected between the DC end of the diode rectifier and the DC end of the main converter; and the fourth fast switch is connected between the DC end of the diode rectifier and the DC end of the auxiliary converter.

[0084] In the black start phase of the system, the third fast switch is open, and the fourth fast switch is closed.

[0085] In the power path transfer phase of the system, the third fast switch is closed, and the fourth fast switch is open.

[0086] In the power path transfer phase, the first fast switch is closed and the second fast switch is open, which can realize the quick switching of the low-voltage black start loop to the high-voltage loop. In this phase, the network-forming static reactive power compensation device establishes the offshore AC voltage and compensates for the part of the reactive power required by the diode rectifier for power transmission, and has the function of offshore harmonic control. The diode rectifier is responsible for sending the wind turbine power of the offshore wind farm to the onshore AC power grid.

[0087] Further, the system further comprises: a local AC power consumption device, and the offshore wind farm is connected to the local AC power consumption device.

[0088] In the power path transfer stage of the system, the offshore wind farm is limited by the local AC power consumption device, and then the main converter establishes a rated DC voltage.

[0089] When the black start path is transferred to the power path transfer stage, first, the offshore wind farm can start and generate power in the grid-connected control strategy, at this time the offshore wind farm transmits power to the onshore AC power grid through the low-voltage black start circuit, and the PCC point voltage is transferred to the grid-connected static reactive power compensation device, at this time the offshore wind farm is connected to the local AC power consumption device, the offshore wind farm is limited to transmit power to the outside, the second fast switch and the fourth fast switch are opened, the first fast switch and the third fast switch are closed, the first fast switch and the third fast switch are soft switches, and the main converter is unlocked to establish a rated DC voltage Udc, and then the AC power consumption device of the wind farm is exited, and the power path is completely transferred from the black start path to the power path transfer stage.

[0090] Further, the system further comprises: an adjustable transformer; the onshore AC power grid is connected to the adjustable transformer, and the adjustable transformer is connected to the main converter.

[0091] In the black start stage of the system, the adjustable transformer is used to reduce the voltage on the main converter side by adjusting the tap position of the adjustable transformer, so that the diode of the main converter is turned on, and then a low-voltage black start circuit is formed with the auxiliary converter.

[0092] The above is mainly achieved by adjusting the tap position of the adjustable transformer to reduce the voltage on the main converter side, so that the low-voltage black start circuit can be applied, and then a low-voltage black start circuit is formed by the diode and the auxiliary converter.

[0093] Further, in the steady-state operation stage of the system, the grid-connected reactive power compensation device provides reactive power compensation for the offshore wind farm. At the same time, the auxiliary converter also provides reactive power compensation for the offshore wind farm.

[0094] The offshore wind farm DC transmission system is not only suitable for offshore grid-connected wind turbine transmission, but also suitable for grid-connected wind turbine system, the difference is that the offshore AC voltage is established by the grid-connected wind farm; the system includes three stages, black start stage, power path transfer stage, and steady-state power transmission stage; as Figure 5 shown, the HVDC+ and HVDC- represent the positive and negative bus voltages;

[0095] As Figure 6 shown, in the black start phase of the system, the low-voltage black start loop composed of the diode auxiliary converter on the shore and the auxiliary converter, the low-voltage black start loop is 10% of the rated DC voltage Udc; the specific black start process is as follows; in the black start phase, the second fast switch and the fourth fast switch are in the closed position, and the first fast switch and the third fast switch are in the open position, at this time, the DC voltage is built by the diode auxiliary converter on the shore, and the energy is transmitted to the auxiliary converter, first, the auxiliary converter on the sea builds the voltage and frequency of the offshore PCC point, and provides a black start power source for the grid-connected static reactive power compensation device and the offshore wind farm;

[0096] As Figure 7 shown, in the black start phase of the system, the low-voltage black start loop composed of the diode auxiliary converter on the shore and the auxiliary converter, the low-voltage black start loop is 10% of the rated DC voltage Udc; the specific black start process is as follows; in the black start phase, the second fast switch and the fourth fast switch are in the closed position, and the first fast switch and the third fast switch are in the open position, at this time, the DC voltage is built by the diode auxiliary converter on the shore, and the energy is transmitted to the auxiliary converter, first, the auxiliary converter on the sea builds the voltage and frequency of the offshore PCC point, and provides a black start power source for the grid-connected static reactive power compensation device and the offshore wind farm;

[0097] The present application proposes a diode rectifier, an auxiliary converter and a grid-connected static reactive power compensation device, which has the following advantages compared with the traditional diode rectifier and auxiliary converter series-parallel sending scheme;

[0098] 1) The diode rectifier sends active power, which can compensate 30% of the reactive power, because the capacitor voltage fluctuation of the grid-connected static reactive power compensation device is 100Hz, while the auxiliary converter capacitor voltage can be 50Hz, and the capacitor in the grid-connected static reactive power compensation device is only 25% of the auxiliary converter; under the same capacity, the number of sub-modules in the grid-connected static reactive power compensation device is half of the auxiliary converter, and the use of the grid-connected static reactive power compensation device instead of the auxiliary converter function can greatly reduce the volume and cost of the offshore platform.

[0099] 2) The series reactance of the grid-connected static reactive power compensation device is very small, about 3% of the system impedance, while the auxiliary converter needs bridge arm reactance and transformer, and the series reactance rate is about 20% of the system impedance, in terms of harmonic control on the sea, the grid-connected static reactive power compensation device needs smaller harmonic voltage to filter out the same harmonic current, and the number of sub-modules needed to absorb harmonics is less than that of the auxiliary converter.

[0100] 3) For the diode rectifier parallel auxiliary converter and diode rectifier series auxiliary converter scheme, the auxiliary converter on the sea needs a 30% capacity transformer, and in the present application, the grid-connected static reactive power compensation device does not need a transformer, and the connection transformer capacity of the auxiliary converter on the sea can only be 5%, so the volume and construction cost of the transformer of the offshore station can be effectively reduced.

[0101] 4) The present invention can achieve a black start by utilizing a low-voltage black start circuit composed of a diode-assisted converter and an offshore auxiliary converter. Compared to a solution without a low-voltage starting circuit, this solution can effectively reduce the DC voltage level, thereby reducing the size and weight of the offshore auxiliary converter. Compared to the existing low-voltage black start circuit composed of a main converter onshore, the diode-assisted converter can reduce construction costs.

[0102] Those skilled in the art should know that: in order to improve the economic efficiency of offshore converter platforms and to reduce the construction cost of offshore wind power transmission schemes based on diode rectifiers and auxiliary converters, the present invention proposes a DC transmission scheme for offshore wind farms. Based on the natural advantages of diode rectifiers in terms of small size and low cost compared to auxiliary converters, they have become a popular choice for the development of offshore wind power technology. Currently, offshore wind power DC transmission schemes based on diode rectifiers mainly include pure diode rectifier transmission, which relies on grid-type wind turbines to establish AC voltage and achieve their own black start, but the stability of grid-type wind farms is still unknown; a hybrid converter transmission scheme consisting of a diode rectifier and an auxiliary converter in parallel, which is suitable for traditional grid-type wind turbines; the present invention uses a small-capacity auxiliary converter to establish offshore AC voltage, and relies on the onshore converter station to provide pre-charging of the auxiliary converter and black start power supply for the wind farm. In order to further reduce the construction cost of offshore platforms, in the scheme of the present invention, offshore wind power is transmitted in parallel via a diode rectifier, an auxiliary converter, and a grid-type static VAR compensation device.

[0103] Example 2:

[0104] like Figure 8 As shown, the present invention further provides a method for transmitting DC power from an offshore wind farm, which is applied to the DC power transmission system for an offshore wind farm in the above-mentioned embodiment 1, comprising:

[0105] Step 1: During a black start phase of the system, a low-voltage black start circuit is formed by using the diode auxiliary converter of the onshore converter station and the auxiliary converter, or by using the diode of the onshore converter station and the auxiliary converter, and connected to the onshore AC power grid, so that the onshore AC power grid provides starting energy for the grid-type reactive power compensation device and the offshore wind farm;

[0106] Step 2: During the power path transfer phase of the system, the power output of the offshore wind farm is restricted, and the low-voltage black start loop is transferred to the diode rectifier and the main converter of the onshore converter station to form a power transmission path;

[0107] Step 3: During the steady-state operation phase of the system, the grid-type reactive compensation device is used to establish an AC voltage at the common connection point, so that the offshore wind farm transmits rated power through the diode rectifier.

[0108] Further, in step 1, during the black start phase of the system, the shore AC grid is used to charge the auxiliary converter of the shore converter station after turning on the diode auxiliary converter, and the auxiliary converter is used to provide start-up energy for the grid-forming reactive power compensation device and the offshore wind farm after being unlocked.

[0109] Further, in step 1, during the power path transfer phase of the system, the shore AC grid is used to provide start-up energy for the main converter, and the main converter is used to establish a rated DC voltage after being unlocked.

[0110] Further, in step 2, during the steady-state operation phase of the system, the grid-forming reactive power compensation device is used to establish an operating voltage for the point of common coupling and provide reactive power compensation for the offshore wind farm, so that the energy of the offshore wind farm can be sent to the shore AC grid through the diode rectifier and the main converter.

[0111] Further, when the shore AC grid fails, the energy consumption branch is used to absorb the power of the offshore wind farm.

[0112] Further, during the black start phase of the system, the first fast switch is opened and the second fast switch is closed.

[0113] During the power path transfer phase of the system, the first fast switch is closed and the second fast switch is opened.

[0114] Further, during the black start phase of the system, the third fast switch is opened and the fourth fast switch is closed.

[0115] During the power path transfer phase of the system, the third fast switch is closed and the fourth fast switch is opened.

[0116] Further, during the power path transfer phase of the system, the local AC energy consumption device is used to limit the external output power of the offshore wind farm, and then the main converter establishes a rated DC voltage.

[0117] Further, during the black start phase of the system, the adjustable transformer is used to reduce the voltage on the main converter side by adjusting the tap position of the transformer, so that the diodes in anti-parallel connection of the main converter are turned on, and then a low-voltage black start loop is formed with the auxiliary converter.

[0118] Further, in step 3, during the steady-state operation phase of the system, the grid-forming reactive power compensation device is used to provide reactive power compensation for the offshore wind farm.

[0119] Embodiment 3:

[0120] As Figure 9 shown, the present application also provides an electronic device, which can be a computer device, a single-chip microcomputer device, a smart mobile device, etc. The electronic device in the embodiment can include a processor, a memory, a transceiver component, etc. The memory, the processor and the transceiver component are connected through a bus; the memory can be used to store an execution program, and the exemplary execution program can include instructions; the processor is used to execute the instructions stored in the memory. The memory can also be used to store data, which can be called and / or modified when the instructions are executed.

[0121] The processor can be a central processing unit (CPU), and can also be other general-purpose processors, digital signal processors (DSP), application specific integrated circuits (ASIC), field-programmable gate arrays (FPGA) or other programmable logic devices, discrete gates or transistor logic devices, discrete hardware components, etc., which are the computing core and control core of the terminal, and are suitable for implementing one or more instructions, and are specifically suitable for loading and executing one or more instructions in the storage medium to implement a corresponding method flow or a corresponding function, so as to implement the steps of the offshore wind farm direct current transmission method in the above embodiment.

[0122] Embodiment 4:

[0123] Based on the same inventive concept, the present application also provides a readable storage medium, specifically an electronic device readable storage medium (Memory), which is a memory device in the electronic device, and is used to store programs and data. It can be understood that the storage medium here can include the built-in storage medium in the electronic device, and of course can also include the expansion storage medium supported by the electronic device. The storage medium provides a storage space, and the storage space stores the operating system of the terminal. Moreover, one or more instructions suitable for being loaded and executed by the processor are also stored in the storage space, and these instructions can be one or more execution programs (including program codes). It should be noted that the storage medium here can be a high-speed RAM memory, or a non-volatile memory, such as at least one disk memory. The processor loads and executes one or more instructions stored in the storage medium, which can implement the steps of the offshore wind farm direct current transmission method in the above embodiment.

[0124] Those skilled in the art will appreciate that embodiments of the present application can be readily used as software, hardware, or a combination of software and hardware. In a software embodiment, the present application can be implemented with computer programs (also referred to as software instructions, software code, computer code, and the like) that execute on programmable hardware including computer processors, digital signal processors, microprocessors, central processing units, microcontrollers, programmable hardware logic devices, and the like. Generally, the present application can be implemented in hardware, software, or any combination of hardware and software.

[0125] The present application is described in reference to the flowchart illustrations and / or block diagrams of methods, apparatus (systems) and computer program products according to embodiments of the present application. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general purpose computer, special purpose computer, embedded processing device or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions specified in the flowchart Figure 1 one or more functions specified in the flowchart block or blocks. Figure 1 one or more functions specified in the flowchart block or blocks.

[0126] These computer program instructions can also be stored in a computer- readable memory that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable memory produce an article of manufacture including instructions which implement the function specified in the flowchart block or blocks. Figure 1 one or more functions specified in the flowchart block or blocks. Figure 1 one or more functions specified in the flowchart block or blocks.

[0127] The computer program instructions can also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer-implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart block or blocks. Figure 1 one or more functions specified in the flowchart block or blocks. Figure 1 one or more functions specified in the flowchart block or blocks.

[0128] Finally, it should be noted that the above-described embodiments are merely intended for describing and illustrating, not limiting, the technical solutions of the present application. Although the present application has been described in detail with reference to the above embodiments, those skilled in the art should understand that, after reading the present application, they can make various changes, modifications or equivalent replacements to the specific embodiments, and these changes, modifications or equivalent replacements are all within the scope of protection of the claims of the present application.

Claims

1. A DC transmission system for an offshore wind farm, characterized in that: include: An offshore wind farm, an offshore AC power grid, a hybrid converter, an onshore converter station, and an onshore AC power grid connected in sequence; the hybrid converter comprises a diode rectifier, a grid-type reactive power compensation device, and an auxiliary converter; the diode rectifier and the auxiliary converter are connected in parallel and then connected to the onshore converter station; the diode rectifier, the grid-type reactive power compensation device, and the offshore AC power grid are connected via a common connection point; During the black start phase of the system, the diode auxiliary converter of the onshore converter station and the auxiliary converter, or the diode of the onshore converter station and the auxiliary converter, form a low-voltage black start circuit, which is connected to the onshore AC power grid, so that the onshore AC power grid provides starting energy for the grid-type reactive power compensation device and the offshore wind farm; During the power path transfer phase of the system, the offshore wind farm is restricted from outputting external power, and the low-voltage black start loop is transferred to the diode rectifier and the main converter of the onshore converter station to form a power transmission path; During the steady-state operation phase of the system, the grid-type reactive power compensation device establishes an AC voltage at the common connection point, so that the offshore wind farm transmits rated power through the diode rectifier.

2. The system according to claim 1, wherein: During the black start phase of the system, the onshore AC power grid turns on the diode auxiliary converter of the onshore converter station and charges the auxiliary converter. After being charged and unlocked, the auxiliary converter provides starting energy for the grid-type reactive power compensation device and the offshore wind farm.

3. The system according to claim 2, characterized in that The main converter and the diode auxiliary converter are connected in parallel and are located in the onshore converter station; the insulated gate bipolar transistor inside the main converter is connected in anti-parallel to the diode; During the power path transfer phase of the system, the onshore AC grid provides starting energy to the main converter, so that the main converter is unlocked and a rated DC voltage is established.

4. The system according to claim 3, characterized in that During the steady-state operation phase of the system, the grid-type reactive compensation device establishes an operating voltage for the common connection point and provides reactive compensation for the offshore wind farm, so that the energy of the offshore wind farm is transmitted to the onshore AC power grid through the diode rectifier and the main converter.

5. The system according to claim 4, characterized in that The onshore converter station further includes: an energy-consuming branch; the main converter is connected in parallel to the energy-consuming branch; When the onshore AC power grid fails, the power of the offshore wind farm is absorbed by the energy consumption branch.

6. The system according to claim 5, characterized in that The onshore converter station further includes: a first fast switch and a second fast switch; the first fast switch is connected between the DC end of the main converter and the DC end of the diode rectifier; the second fast switch is connected between the DC end of the diode auxiliary converter and the DC end of the main converter; During a black start phase of the system, the first fast switch is opened and the second fast switch is closed; During a power path transfer phase of the system, the first fast switch is closed and the second fast switch is open.

7. The system according to claim 1, wherein: The hybrid converter further includes: a third fast switch and a fourth fast switch; the third fast switch is connected between the DC end of the diode rectifier and the DC end of the main converter; the fourth fast switch is connected between the DC end of the diode rectifier and the DC end of the auxiliary converter; During a black start phase of the system, the third fast switch is opened and the fourth fast switch is closed; During the power path transfer phase of the system, the third fast switch is closed and the fourth fast switch is open.

8. The system according to any one of claims 1 to 7, characterized in that: Also includes: A local AC energy consumption device, to which the offshore wind farm is connected; During the power path transfer phase of the system, the local AC energy consumption device limits the external output power of the offshore wind farm, and then the main converter establishes the rated DC voltage.

9. The system according to claim 1, wherein: Also includes: An adjustable transformer; the onshore AC grid is connected to the adjustable transformer, and the adjustable transformer is connected to the main converter; During the black start phase of the system, the adjustable transformer adjusts its own tap position to reduce the voltage on the main converter side, causing the anti-parallel diode of the main converter to conduct, and then forms a low-voltage black start circuit with the auxiliary converter.

10. A method for transmitting DC power from an offshore wind farm, applied to the DC power transmission system for an offshore wind farm as claimed in any one of claims 1 to 9, characterized in that: include: During the black start phase of the system, a low-voltage black start circuit is formed by using the diode auxiliary converter of the onshore converter station and the auxiliary converter, or the diode of the onshore converter station and the auxiliary converter, and connected to the onshore AC power grid, so that the onshore AC power grid provides starting energy for the grid-type reactive power compensation device and the offshore wind farm; During the power path transfer phase of the system, the offshore wind farm is restricted from outputting external power, and the low-voltage black start loop is transferred to the diode rectifier and the main converter of the onshore converter station to form a power transmission path; During the steady-state operation phase of the system, the grid-type reactive compensation device is used to establish an AC voltage at the common connection point, so that the offshore wind farm transmits rated power through the diode rectifier.

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