Parallel-type offshore wind power direct current transmission system, and starting control method and apparatus

Abstract

The present application relates to the technical field of offshore direct current transmission, and provides a parallel-type offshore wind power direct current transmission system, and a starting control method and apparatus. The system comprises: an onshore power grid, an onshore converter station, an offshore converter station, and an offshore wind farm; the onshore power grid is connected to the onshore converter station; the onshore converter station is connected to the offshore converter station, and the offshore converter station comprises an offshore auxiliary flexible direct current converter valve and a diode valve connected in parallel to the offshore auxiliary flexible direct current converter valve; a first bypass switch is connected between one end at the direct current side of the offshore auxiliary flexible direct current converter valve and one end at the direct current side of the diode valve; a second bypass switch is connected between the other end at the direct current side of the offshore auxiliary flexible direct current converter valve and the other end at the direct current side of the diode valve; and the offshore auxiliary flexible direct current converter valve and the diode valve are separately connected to the offshore wind farm. According to the present application, the size of the converter valve in the offshore converter station is reduced, thereby reducing the costs of the converter valve.

Description

A parallel offshore wind power direct current transmission system, startup control method and device

[0001] This application claims priority to the Chinese patent application filed with the Patent Office of China on August 10, 2023, with application number 202311002636.5 and invention name “A parallel offshore wind power DC transmission system, startup control method and device”, the entire contents of which are incorporated by reference into this application. Technical Field

[0002] The present application relates to the technical field of offshore direct current transmission, and in particular to a parallel offshore wind power direct current transmission system, a startup control method and a device. Background Art

[0003] Current DC transmission solutions for offshore wind power typically utilize flexible DC transmission solutions based on modular multilevel converters (MMCs). However, these solutions require bulky offshore converter platforms and high investment costs. To reduce the weight of offshore converters and platforms and lower the cost of offshore wind power transmission systems, diode-based solutions are gaining favor in academia and industry both domestically and internationally. Current diode-based transmission solutions include pure diode solutions and those combining diodes with auxiliary MMCs in parallel.

[0004] However, the pure diode solution has the problem of difficulty in black start. Some literature has proposed a solution to add auxiliary submarine cables for black start, but the laying of additional submarine cables in this solution increases costs. The solution of connecting diodes in parallel with auxiliary MMCs can solve the black start problem, but because the auxiliary MMCs are connected in parallel on the DC side, the DC voltage of the auxiliary MMCs is the same as the system DC voltage, resulting in a large number of auxiliary MMC sub-modules, which cannot effectively reduce the size and cost of the converter valve.

[0005] Summary of the Invention

[0006] In order to reduce the volume of converter valves in offshore converter stations and lower the cost of converter valves, the present application proposes a parallel offshore wind power direct current transmission system, a startup control method and a device.

[0007] In a first aspect, the present application provides a parallel offshore wind power direct current transmission system, the system comprising: an onshore power grid, an onshore converter station, an offshore converter station, and an offshore wind farm;

[0008] The onshore power grid is connected to the onshore converter station;

[0009] The onshore converter station is connected to the offshore converter station. The offshore converter station includes an offshore auxiliary flexible DC converter valve and a diode valve connected in parallel with the offshore auxiliary flexible DC converter valve. A first bypass switch is connected between one end of the DC side of the offshore auxiliary flexible DC converter valve and the diode valve. A second bypass switch is connected between the other end of the DC side of the offshore auxiliary flexible DC converter valve and the diode valve.

[0010] The offshore auxiliary flexible DC converter valve and diode valve are respectively connected to the offshore wind farm.

[0011] Considering that in the scheme where the offshore auxiliary flexible DC converter valve is connected in parallel with the diode valve, the DC voltage value of the offshore auxiliary flexible DC converter valve is the same as the rated DC voltage value of the system, resulting in a large number of offshore auxiliary flexible DC converter valve sub-modules. Through the above system, the offshore auxiliary flexible DC converter valve in the offshore converter station is connected in parallel with the diode valve through the first bypass switch and the second bypass switch respectively. During the startup of the offshore wind farm, the DC voltage value of the offshore auxiliary flexible DC converter valve is lower than the rated DC voltage value of the system; after the offshore wind farm is started, the first bypass switch and the second bypass switch are disconnected, that is, the offshore auxiliary flexible DC converter valve is disconnected from the DC side of the system, and the offshore auxiliary flexible DC converter valve is only used to balance the reactive power of the offshore converter station. At this time, the DC voltage value of the offshore auxiliary flexible DC converter valve is lower than the rated DC voltage value of the system. Therefore, in the system provided by the invention, the number of offshore auxiliary flexible DC converter valve sub-modules is reduced, and the volume and cost of the offshore auxiliary flexible DC converter valve are reduced.

[0012] In combination with the first aspect, in a first embodiment of the first aspect, the onshore converter station includes a starting transformer, an onshore flexible DC converter valve, and an operating transformer;

[0013] The onshore power grid is connected to the onshore flexible DC converter valve via the first circuit breaker, the starting transformer, and the second circuit breaker in sequence;

[0014] The onshore power grid is connected to the onshore flexible DC converter valve via the third circuit breaker, the operating transformer, and the fourth circuit breaker;

[0015] The onshore flexible DC converter valve is connected to the offshore converter station via a DC submarine cable.

[0016] In combination with the first embodiment of the first aspect, in the second embodiment of the first aspect, a fifth circuit breaker is connected between one end of the DC submarine cable and the diode valve, and a sixth circuit breaker is connected between the other end of the DC submarine cable and the diode valve.

[0017] In combination with the second embodiment of the first aspect, in a third embodiment of the first aspect, the offshore converter station further includes a first transformer and a second transformer;

[0018] The diode valve is connected to the grid connection point of the offshore wind farm through the first transformer and the seventh circuit breaker in sequence;

[0019] The offshore auxiliary flexible DC converter valve is connected to the grid connection point through the eighth circuit breaker, the second transformer, and the ninth circuit breaker in sequence.

[0020] In combination with the first aspect or the third embodiment of the first aspect, the system further includes a network power supply unit, the network power supply unit including: a battery, a converter and a third transformer;

[0021] The grid power supply unit is used to provide grid voltage to the grid connection point;

[0022] The battery is connected to the grid connection point through the converter, the tenth circuit breaker, the third transformer, and the eleventh circuit breaker in sequence.

[0023] In a second aspect, the present application further provides a startup control method for a parallel offshore wind power DC transmission system, which is used in the parallel offshore wind power DC transmission system of the first aspect. The method comprises:

[0024] Control the onshore power grid to supply power to the offshore wind farm via the onshore converter station and the offshore auxiliary flexible DC converter valve at the offshore converter station, so as to start the offshore wind farm;

[0025] Controlling the output power from the offshore wind farm to the offshore converter station until the current value passing through the first bypass switch and the second bypass switch decreases to a preset current value, and then disconnecting the first bypass switch and the second bypass switch;

[0026] The diode valve in the offshore converter station is turned on to complete the startup of the parallel offshore wind power DC transmission system.

[0027] In a parallel-type offshore wind power DC transmission system, the offshore auxiliary flexible DC converter valve in the offshore converter station is connected in parallel with a diode valve via a first bypass switch and a second bypass switch, respectively. Through the above method, during the startup process of the offshore wind farm, the DC voltage of the offshore auxiliary flexible DC converter valve is lower than the rated DC voltage of the system. After the offshore wind farm is started, the first and second bypass switches are disconnected, disconnecting the offshore auxiliary flexible DC converter valve from the DC side of the system. The diode valve in the offshore converter station is then connected, completing the startup of the parallel-type offshore wind power DC transmission system. At this time, the offshore auxiliary flexible DC converter valve is only used to balance the reactive power of the offshore converter station, and the DC voltage of the offshore auxiliary flexible DC converter valve is lower than the rated DC voltage of the system. That is, throughout the entire startup control process of the parallel-type offshore wind power DC transmission system, the DC voltage of the offshore auxiliary flexible DC converter valve is lower than the system DC voltage, reducing the number of offshore auxiliary flexible DC converter valve submodules and lowering the size and cost of the offshore auxiliary flexible DC converter valve.

[0028] In conjunction with the second aspect, in a first embodiment of the second aspect, the onshore converter station includes a starting transformer, an onshore flexible DC converter valve, and an operating transformer; the onshore power grid is connected to the onshore flexible DC converter valve via a first circuit breaker, a starting transformer, and a second circuit breaker in sequence; the onshore power grid is connected to the onshore flexible DC converter valve via a third circuit breaker, an operating transformer, and a fourth circuit breaker; the onshore flexible DC converter valve is connected to the offshore converter station via a DC submarine cable; a fifth circuit breaker is connected between one end of the DC submarine cable and the diode valve, and a sixth circuit breaker is connected between the other end of the DC submarine cable and the diode valve; the offshore converter station further includes a first transformer and a second transformer; the diode valve is connected to the grid connection point of the offshore wind farm via the first transformer and a seventh circuit breaker in sequence; the offshore auxiliary flexible DC converter valve is connected to the grid connection point via an eighth circuit breaker, a second transformer, and a ninth circuit breaker in sequence;

[0029] Controlling the onshore power grid to supply power to the offshore wind farm via the onshore converter station and the offshore auxiliary flexible DC converter valves at the offshore converter station to start the offshore wind farm, including:

[0030] Closing the first circuit breaker, the second circuit breaker, the first bypass switch, and the second bypass switch allows the onshore power grid to charge the onshore flexible DC converter valve by starting the transformer, and at the same time, the onshore power grid charges the offshore auxiliary flexible DC converter valve in the offshore converter station through the onshore flexible DC converter valve until the onshore flexible DC converter valve and the offshore auxiliary flexible DC converter valve are fully charged;

[0031] Unlock the onshore flexible DC converter valve, set the DC voltage value of the onshore flexible DC converter valve to a preset DC voltage value, and set the reactive power value of the onshore flexible DC converter valve to a preset reactive power value;

[0032] Close the seventh, eighth and ninth circuit breakers, unlock the offshore auxiliary flexible DC converter valve, and control the offshore auxiliary flexible DC converter valve to increase the voltage from zero to make the grid connection point voltage reach the preset grid connection point voltage value, thereby starting a preset number of wind turbines in the offshore wind farm.

[0033] Through the above embodiment, during the startup of the offshore wind farm, the DC voltage value of the offshore auxiliary flexible DC converter valve is the same as the preset DC voltage value of the onshore flexible DC converter valve, that is, the DC voltage value of the offshore auxiliary flexible DC converter valve is lower than the rated DC voltage value of the system. At this time, compared with the existing technology, the number of modules of the offshore auxiliary flexible DC converter valve is reduced and the cost is reduced.

[0034] In combination with the first embodiment of the second aspect, in the second embodiment of the second aspect, controlling the output power from the offshore wind farm to the offshore converter station until the current value passing through the first bypass switch and the second bypass switch decreases to a preset current value, and then disconnecting the first bypass switch and the second bypass switch, includes:

[0035] The offshore auxiliary flexible DC converter valve is controlled to reduce the voltage value at the grid connection point, so that the offshore wind farm enters a controllable energy consumption mode after detecting a drop in the voltage value at the grid connection point, and controls the output power of the offshore wind farm to the offshore converter station until the current value passing through the first bypass switch and the second bypass switch is reduced to a preset current value, and then the first bypass switch and the second bypass switch are disconnected.

[0036] In combination with the first embodiment of the second aspect, in a third embodiment of the second aspect, controlling the output power from the offshore wind farm to the offshore converter station until the current values ​​passing through the first bypass switch and the second bypass switch decrease to preset current values, and then disconnecting the first bypass switch and the second bypass switch, includes:

[0037] Control the offshore auxiliary flexible DC converter valve to reduce the voltage value at the grid connection point, implement constant power control on a preset number of wind turbines to reduce the output power from the offshore wind farm to the offshore converter station, until the current value passing through the first bypass switch and the second bypass switch is reduced to a preset current value, and then disconnect the first bypass switch and the second bypass switch.

[0038] In combination with the first embodiment of the second aspect, in a fourth embodiment of the second aspect, the system further includes a network power supply unit, the network power supply unit including: a battery, a converter, and a third transformer;

[0039] The grid power supply unit is used to provide grid voltage to the grid connection point;

[0040] The battery is connected to the grid connection point through the converter, the tenth circuit breaker, the third transformer, and the eleventh circuit breaker in sequence;

[0041] Controlling the output power from the offshore wind farm to the offshore converter station until the current value passing through the first bypass switch and the second bypass switch decreases to a preset current value, disconnecting the first bypass switch and the second bypass switch, comprising:

[0042] The grid power supply unit is controlled to reduce the output power from the offshore wind farm to the offshore converter station, and then the offshore auxiliary flexible DC converter valve is controlled to be in a no-load condition, and the first bypass switch and the second bypass switch are disconnected.

[0043] In combination with the second embodiment of the second aspect, in a fifth embodiment of the second aspect, turning on the diode valve in the offshore converter station to complete the startup of the parallel offshore wind power direct current transmission system includes:

[0044] When the DC voltage value of the onshore flexible DC converter valve is increased to the rated DC voltage value, the first circuit breaker and the second circuit breaker are opened, and the third circuit breaker and the fourth circuit breaker are closed;

[0045] Close the fifth and sixth circuit breakers, control the offshore auxiliary flexible DC converter valve to increase the grid connection point voltage to the rated AC voltage, and turn on the diode valve;

[0046] Control the offshore wind farm to exit the controllable energy consumption mode, control the DC voltage value of the offshore auxiliary flexible DC converter valve to be constant and the q-axis voltage value to be zero, and complete the startup of the parallel offshore wind power DC transmission system.

[0047] In combination with the third embodiment of the first aspect, in a sixth embodiment of the second aspect, turning on the diode valve in the offshore converter station to complete the startup of the parallel offshore wind power direct current transmission system includes:

[0048] When the DC voltage value of the onshore flexible DC converter valve is increased to the rated DC voltage value, the first circuit breaker and the second circuit breaker are opened, and the third circuit breaker and the fourth circuit breaker are closed;

[0049] Close the fifth and sixth circuit breakers, control the offshore auxiliary flexible DC converter valve to increase the grid connection point voltage to the rated AC voltage, and turn on the diode valve;

[0050] The DC voltage value of the offshore auxiliary flexible DC converter valve is controlled to be constant and the q-axis voltage value is zero, completing the startup of the parallel offshore wind power DC transmission system.

[0051] In combination with the fourth embodiment of the second aspect, in a seventh embodiment of the second aspect, turning on the diode valve in the offshore converter station to complete the startup of the parallel offshore wind power direct current transmission system includes:

[0052] When the DC voltage value of the onshore flexible DC converter valve is increased to the rated DC voltage value, the first circuit breaker and the second circuit breaker are opened, and the third circuit breaker and the fourth circuit breaker are closed;

[0053] Close the fifth and sixth circuit breakers, control the grid power supply unit to increase the grid connection point voltage to the rated AC voltage, and turn on the diode valve;

[0054] The DC voltage value of the offshore auxiliary flexible DC converter valve is controlled to be constant and the q-axis voltage value is zero, the grid-building power supply unit exits the grid-building operation mode, and the startup of the parallel offshore wind power DC transmission system is completed.

[0055] In a third aspect, the present application further provides a parallel offshore wind power DC transmission system startup control device, used in the parallel offshore wind power DC transmission system of the first aspect or any embodiment of the first aspect, the device comprising:

[0056] The first control module is used to control the onshore power grid to supply power to the offshore wind farm via the onshore converter station and the offshore auxiliary flexible DC converter valve of the offshore converter station, so as to start the offshore wind farm;

[0057] a second control module, configured to control the output power from the offshore wind farm to the offshore converter station until the current value passing through the first bypass switch and the second bypass switch decreases to a preset current value, and then disconnect the first bypass switch and the second bypass switch;

[0058] The conduction module is used to conduct the diode valve in the offshore converter station to complete the startup of the parallel offshore wind power direct current transmission system.

[0059] In a fourth aspect, the present application also provides a computer device comprising a memory and a processor, the memory and the processor being communicatively connected to each other, the memory storing computer instructions, and the processor executing the computer instructions to thereby execute the steps of the parallel offshore wind power direct current transmission system startup control method of the second aspect or any embodiment of the second aspect. BRIEF DESCRIPTION OF THE DRAWINGS

[0060] In order to more clearly illustrate the specific implementation methods of the present application or the technical solutions in the prior art, the following is a brief introduction to the drawings required for use in the specific implementation methods or the description of the prior art. Obviously, the drawings described below are some implementation methods of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0061] FIG1 is a circuit diagram of a parallel offshore wind power direct current transmission system according to an exemplary embodiment;

[0062] FIG2 is a schematic diagram of the structure of a half-bridge submodule in an example;

[0063] FIG3 is a flow chart of a startup control method for a parallel offshore wind power direct current transmission system according to an exemplary embodiment;

[0064] FIG4 is a schematic structural diagram of a parallel offshore wind power direct current transmission system startup control device according to an exemplary embodiment;

[0065] FIG5 is a schematic diagram of a hardware structure of a computer device according to an exemplary embodiment. DETAILED DESCRIPTION

[0066] The following will clearly and completely describe the technical solution of this application in conjunction with the accompanying drawings. Obviously, the embodiments described are only part of the embodiments of this application, not all of them. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of this application.

[0067] In addition, the technical features involved in the different embodiments of the present application described below can be combined with each other as long as they do not conflict with each other.

[0068] In order to reduce the volume of converter valves in offshore converter stations and lower the cost of converter valves, the present application proposes a parallel offshore wind power direct current transmission system, a startup control method and a device.

[0069] FIG1 is a circuit diagram of a parallel offshore wind power direct current transmission system according to an exemplary embodiment. The system includes an onshore power grid 1 , an onshore converter station 2 , an offshore converter station 3 , and an offshore wind farm 4 .

[0070] The onshore power grid 1 is connected to the onshore converter station 2 .

[0071] In an optional embodiment, the onshore power grid 1 may be connected to the onshore converter station 2 via a DC submarine cable 5, or may be connected to the onshore converter station 2 via a DC grid line or the like, without specific limitation herein.

[0072] In an optional embodiment, the onshore converter station 2 can use a conventional half-bridge MMC converter valve. The structural diagram of one half-bridge submodule is shown in Figure 2. In Figure 2, one half-bridge submodule includes a capacitor C, a diode D1, a diode D2, and fully controlled power switches T1 and T2. T1 and T2 can be insulated gate bipolar transistors (IGBTs) or integrated gate-commutated thyristors (IGCTs).

[0073] The onshore converter station 2 is connected to the offshore converter station 3. The offshore converter station 3 includes an offshore auxiliary flexible DC converter valve 31 and a diode valve 32 connected in parallel with the offshore auxiliary flexible DC converter valve 31. A first bypass switch 33 is connected between one end of the DC side of the offshore auxiliary flexible DC converter valve 31 and the diode valve 32. A second bypass switch 34 is connected between the other end of the DC side of the offshore auxiliary flexible DC converter valve 31 and the diode valve 32.

[0074] In an optional embodiment, the offshore auxiliary flexible DC converter valve (offshore auxiliary MMC) 31 may be a conventional half-bridge MMC converter valve.

[0075] In an alternative embodiment, the diode valve 32 may employ a twelve-pulse rectifier.

[0076] In an optional embodiment, when the offshore converter station 3 is connected to the onshore converter station 2 via the DC submarine cable 5 , the DC side end of the diode valve 32 is the end where the DC submarine cable 5 is connected to the offshore converter station 3 .

[0077] In an optional embodiment, bypass switches with a certain current breaking capability may be used as the first bypass switch 33 and the second bypass switch 34 .

[0078] The offshore auxiliary flexible DC converter valve 31 and the diode valve 32 are respectively connected to the offshore wind farm 4 .

[0079] Considering that the DC voltage value of the offshore auxiliary flexible DC converter valve 31 is the same as the system rated DC voltage value in the scheme of connecting the offshore auxiliary flexible DC converter valve 31 in parallel with the diode valve 32, the number of sub-modules of the offshore auxiliary flexible DC converter valve 31 is huge. Through the system provided in the embodiment of the present application, the offshore auxiliary flexible DC converter valve 31 in the offshore converter station 3 is connected in parallel with the diode valve 32 through the first bypass switch 33 and the second bypass switch 34 respectively. During the startup of the offshore wind farm 4, the offshore auxiliary flexible DC converter valve 31 outputs a smaller DC voltage value by controlling the onshore converter station to input a small number of sub-modules, so that the DC voltage value of the offshore auxiliary flexible DC converter valve 31 is lower than the rated DC voltage value of the system; after the offshore wind farm 4 is started, the first bypass switch 33 and the second bypass switch 34 are disconnected, that is, the offshore auxiliary flexible DC converter valve 31 is disconnected from the DC side of the system. The offshore auxiliary flexible DC converter valve 31 is used to balance the reactive power of the offshore converter station 3. At this time, the DC voltage value of the offshore auxiliary flexible DC converter valve 31 is lower than the rated DC voltage value of the system, and the number of sub-modules of the offshore auxiliary flexible DC converter valve 31 is proportional to its DC voltage value. Therefore, in the system provided by the invention, the number of sub-modules of the offshore auxiliary flexible DC converter valve 31 is reduced, and the volume and cost of the offshore auxiliary flexible DC converter valve 31 are reduced.

[0080] In FIG1 , the onshore converter station 2 includes a starting transformer 21 , an onshore flexible DC converter valve 22 and an operating transformer 23 .

[0081] The onshore power grid 1 is connected to the onshore flexible DC converter valve 22 through the first circuit breaker 24, the starting transformer 21, and the second circuit breaker 25 in sequence; the onshore power grid 1 is connected to the onshore flexible DC converter valve 22 through the third circuit breaker 26, the operating transformer 23, and the fourth circuit breaker 27.

[0082] In an optional embodiment, starting transformer 21 is a small-capacity transformer used during the black-start phase (i.e., the startup phase of offshore wind farm 4). Operating transformer 23 is a large-capacity transformer used during the steady-state operation phase after offshore wind farm 4 is started. During the black-start phase, onshore converter station 2 uses starting transformer 21, maintaining the conventional half-bridge MMC design. By utilizing fewer submodules, a smaller DC voltage is output to match the output voltage of starting transformer 21 during startup, preventing overmodulation at the onshore converter station.

[0083] In Figure 1, the onshore flexible DC converter valve 22 is connected to the offshore converter station 3 via a DC submarine cable 5. A fifth circuit breaker 35 is connected between one end of the DC submarine cable 5 and the diode valve 32, and a sixth circuit breaker 36 is connected between the other end of the DC submarine cable 5 and the diode valve 32.

[0084] In an optional embodiment, one end of the DC submarine cable 5 is the DC side end of the diode valve 32 .

[0085] In FIG1 , the offshore converter station 3 further includes a first transformer 37 and a second transformer 38 .

[0086] The diode valve 32 is connected to the grid connection point (PCC) of the offshore wind farm through the first transformer 37 and the seventh circuit breaker 39 in sequence; the offshore auxiliary flexible DC converter valve 31 is connected to the grid connection point through the eighth circuit breaker 40, the second transformer 38, and the ninth circuit breaker 41 in sequence.

[0087] In FIG1 , the system further includes a grid-forming power supply unit 6 , which includes a battery 61 , a converter 62 and a third transformer 63 .

[0088] The grid power supply unit 6 is used to provide grid voltage for the grid connection point.

[0089] The battery 61 is connected to the grid connection point via the converter 62 , the tenth circuit breaker 64 , the third transformer 63 , and the eleventh circuit breaker 65 in sequence.

[0090] In FIG1 , after the system is started up, the offshore auxiliary flexible DC converter valve 31 is used to implement functions such as active filtering and dynamic reactive power compensation, which are not specifically limited here.

[0091] FIG3 is a flow chart of a startup control method for a parallel offshore wind power DC transmission system according to an exemplary embodiment, which is applicable to the above-mentioned parallel offshore wind power DC transmission system. The method includes the following steps:

[0092] Step S101 : controlling the onshore power grid 1 to supply power to the offshore wind farm 4 via the onshore converter station 2 and the offshore auxiliary flexible DC converter valve 31 of the offshore converter station 3 , so as to start the offshore wind farm 4 .

[0093] In an optional embodiment, the onshore power grid 1 is controlled to perform uncontrolled charging on the offshore auxiliary flexible DC converter valves 31 of the onshore converter station 2 and the offshore converter station 3. After the charging of the offshore auxiliary flexible DC converter valves 31 of the onshore converter station 2 and the offshore converter station 3 is completed, the onshore converter station 2 and the offshore auxiliary flexible DC converter valves 31 are unlocked to provide starting power for the offshore wind farm 4 to start a preset number of wind turbines in the offshore wind farm 4.

[0094] Step S102: Control the output power of the offshore wind farm 4 to the offshore converter station 3 until the current value passing through the first bypass switch 33 and the second bypass switch 34 decreases to a preset current value, and then disconnect the first bypass switch 33 and the second bypass switch 34.

[0095] In an optional embodiment, the output power from the offshore wind farm 4 to the offshore converter station 3 is output active power.

[0096] In an optional embodiment, the offshore wind farm 4 can be controlled to enter a controllable energy consumption mode so that the output active power from the offshore wind farm 4 to the offshore converter station 3 is 0, until the current value passing through the first bypass switch 33 and the second bypass switch 34 is reduced to a preset current value, and the first bypass switch 33 and the second bypass switch 34 are disconnected.

[0097] In an optional embodiment, constant power control can be implemented on a preset number of wind turbines to reduce the output power from the offshore wind farm 4 to the offshore converter station 3 until the current value passing through the first bypass switch 33 and the second bypass switch 34 is reduced to a preset current value, and then the first bypass switch 33 and the second bypass switch 34 are disconnected.

[0098] In an optional embodiment, the grid power supply unit 6 can also be controlled to reduce the output power from the offshore wind farm 4 to the offshore converter station 3 until the current value passing through the first bypass switch 33 and the second bypass switch 34 is reduced to a preset current value, and the first bypass switch 33 and the second bypass switch 34 are disconnected.

[0099] Step S103: Turn on the diode valve 32 in the offshore converter station 3 to complete the startup of the parallel offshore wind power direct current transmission system.

[0100] In an optional embodiment, when the DC voltage value of the onshore flexible DC converter valve 22 is controlled to increase to the rated DC voltage value, the first circuit breaker 24 and the second circuit breaker 25 are disconnected, and the third circuit breaker 26 and the fourth circuit breaker 27 are closed; the fifth circuit breaker 35 and the sixth circuit breaker 36 are closed, and the offshore auxiliary flexible DC converter valve 31 or the grid power supply unit 6 is controlled to increase the grid connection point voltage value to the rated AC voltage value, at which time the diode valve 32 is turned on.

[0101] In a parallel-type offshore wind power DC transmission system, the offshore auxiliary flexible DC converter valve 31 in the offshore converter station 3 is connected in parallel with the diode valve 32 via the first bypass switch 33 and the second bypass switch 34. Through the above method, during the startup of the offshore wind farm 4, the DC voltage of the offshore auxiliary flexible DC converter valve 31 is lower than the rated DC voltage of the system. After the offshore wind farm 4 is started, the first bypass switch 33 and the second bypass switch 34 are disconnected, that is, the offshore auxiliary flexible DC converter valve 31 is disconnected from the DC side of the system. Then, the diode valve 32 in the offshore converter station 3 is connected, completing the startup of the parallel-type offshore wind power DC transmission system. At this time, the offshore auxiliary flexible DC converter valve 31 is used to balance the reactive power of the offshore converter station, and the DC voltage of the offshore auxiliary flexible DC converter valve 31 is lower than the rated DC voltage of the system. That is, during the entire startup control process of the parallel offshore wind power DC transmission system, the DC voltage of the offshore auxiliary flexible DC converter valve 31 is lower than the system DC voltage, which reduces the number of sub-modules of the offshore auxiliary flexible DC converter valve 31 and reduces the volume and cost of the offshore auxiliary flexible DC converter valve 31.

[0102] In one example, in step S101 above, the offshore wind farm is started up by:

[0103] First, close the first circuit breaker 24, the second circuit breaker 25, the first bypass switch 33 and the second bypass switch 34, so that the onshore power grid 1 charges the onshore flexible DC converter valve 22 by starting the transformer 21. At the same time, the onshore power grid 1 charges the offshore auxiliary flexible DC converter valve 31 in the offshore converter station 3 through the onshore flexible DC converter valve 22 until the onshore flexible DC converter valve 22 and the offshore auxiliary flexible DC converter valve 31 are fully charged.

[0104] In an optional embodiment, the charging mode of the onshore flexible DC converter valve 22 and the offshore auxiliary flexible DC converter valve 31 is uncontrolled charging.

[0105] Then, the onshore flexible DC converter valve 22 is unlocked, and the DC voltage of the onshore flexible DC converter valve 22 is set to a preset DC voltage value, and the reactive power of the onshore flexible DC converter valve 22 is set to a preset reactive power value. At this point, the preset DC voltage value is less than the rated DC voltage value. For example, the preset DC voltage value can be set to 10% of the rated DC voltage value. The preset reactive power value can be set to 0.

[0106] In an alternative embodiment, a predetermined number of submodules in an onshore flexible DC converter valve are controlled to output a smaller predetermined DC voltage, lower than the rated DC voltage, to provide startup power for wind turbines in an offshore wind farm. Simultaneously, the first transformer 37 corresponding to the diode valve 32 is energized to reduce the current surge when the diode valve is conducting.

[0107] In an optional embodiment, to ensure capacitor voltage balance across all submodules in the onshore flexible DC converter valve during startup, the submodules are alternately activated in a rotating manner. During the rotation of different submodules, a preset number of submodules are always in operation. For example, 10% of the submodules can be set to be in operation.

[0108] Finally, close the seventh circuit breaker 39, the eighth circuit breaker 40, and the ninth circuit breaker 41, unlock the offshore auxiliary flexible DC converter valve 31, and control the offshore auxiliary flexible DC converter valve 31 to increase the voltage from zero so that the grid connection point voltage reaches the preset grid connection point voltage value, thereby starting a preset number of wind turbines in the offshore wind farm 4.

[0109] In an optional embodiment, after unlocking the offshore auxiliary flexible DC converter valve 31 , constant voltage amplitude and frequency control are used to establish the AC side voltage from zero to provide starting power for the wind turbines in the offshore wind farm 4 .

[0110] Through the above embodiment, during the startup of the offshore wind farm 4, the DC voltage of the offshore auxiliary flexible DC converter valve 31 is the same as the preset DC voltage of the onshore flexible DC converter valve 22. That is, the DC voltage of the offshore auxiliary flexible DC converter valve 31 is lower than the rated DC voltage of the system. Compared to the prior art, the number of modules of the offshore auxiliary flexible DC converter valve 31 is reduced, thereby lowering costs. Furthermore, during the startup of the offshore wind farm 4, the output voltage of the starting transformer 21 is used to prevent overmodulation of the onshore converter station 2 during the startup process.

[0111] In one example, the above step S102 disconnects the first bypass switch 33 and the second bypass switch 34 in the following manner:

[0112] The offshore auxiliary flexible direct current converter valve 31 is controlled to reduce the voltage value at the grid connection point, so that the offshore wind farm 4 enters a controllable energy consumption mode after detecting a drop in the voltage value at the grid connection point, and controls the output power of the offshore wind farm 4 to the offshore converter station 3 until the current value passing through the first bypass switch 33 and the second bypass switch 34 is reduced to a preset current value, and then the first bypass switch 33 and the second bypass switch 34 are disconnected.

[0113] In an optional embodiment, the offshore auxiliary flexible DC converter valve 31 is controlled to reduce the grid connection point voltage. For example, the grid connection point voltage can be set to 0.8 times the per-unit value. The offshore wind farm 4 is then controlled to enter a controllable energy consumption mode, so that the active power output from the offshore wind farm 4 to the offshore converter station 3 is zero. Without the active power from the offshore wind farm 4, the DC voltage of the offshore auxiliary flexible DC converter valve 31 decreases. At this point, the DC voltage of the offshore auxiliary flexible DC converter valve 31 is lower than the DC voltage of the onshore flexible DC converter valve 22. The onshore flexible DC converter valve 22 then reverses power to the offshore auxiliary flexible DC converter valve 31, charging the capacitor of the offshore auxiliary flexible DC converter valve 31. When the DC voltage of the offshore auxiliary flexible DC converter valve 31 rises to a value close to that of the onshore flexible DC converter valve 22, and the DC current decreases to a preset current value, the first bypass switch 33 and the second bypass switch 34 are disconnected. For example, the preset current value can be set to 0.

[0114] In one example, the above step S102 disconnects the first bypass switch 33 and the second bypass switch 34 in the following manner:

[0115] The offshore auxiliary flexible DC converter valve 31 is controlled to reduce the voltage value at the grid connection point, and a preset number of wind turbines are subjected to constant power control to reduce the output power from the offshore wind farm 4 to the offshore converter station 3, until the current value passing through the first bypass switch 33 and the second bypass switch 34 is reduced to a preset current value, and then the first bypass switch 33 and the second bypass switch 34 are disconnected.

[0116] In an optional embodiment, the offshore auxiliary flexible DC converter valve 31 is controlled to reduce the grid connection point voltage. Constant power control is then implemented for a preset number of wind turbines to reduce the output power from the offshore wind farm 4 to the offshore converter station 3. This output power satisfies the load requirements of the first and second transformers 37, 38, and the offshore wind farm 4, while also leaving a small surplus. At this point, the DC-side current of the offshore auxiliary flexible DC converter valve 31 is reduced to a preset current value, disconnecting the first and second bypass switches 33, 34. Similarly, the preset current value can be set according to actual needs.

[0117] In one example, the above step S102 disconnects the first bypass switch 33 and the second bypass switch 34 in the following manner:

[0118] The grid power supply unit 6 is controlled to reduce the output power from the offshore wind farm 4 to the offshore converter station 3. For example, the output power can be set to be slightly lower than the rated power of the grid connection point; then the offshore auxiliary flexible DC converter valve 31 is controlled to be in a no-load condition (that is, the offshore auxiliary flexible DC converter valve 31 does not output active power, and the DC current of the offshore auxiliary flexible DC converter valve 31 is 0 at this time), and the first bypass switch 33 and the second bypass switch 34 are disconnected.

[0119] In one example, after the offshore wind farm 4 is controlled to enter the controllable energy consumption mode and the first bypass switch 33 and the second bypass switch 34 are disconnected, the above step S103 completes the startup of the parallel offshore wind power direct current transmission system through the following contents:

[0120] First, when the DC voltage of the onshore flexible DC converter valve 22 increases to the rated DC voltage, the first and second circuit breakers 24 and 25 are opened, and the third and fourth circuit breakers 26 and 27 are closed. At this time, the start transformer 21 is switched to the operating transformer 23.

[0121] Then, the fifth circuit breaker 35 and the sixth circuit breaker 36 are closed, and the offshore auxiliary flexible DC converter valve 31 is controlled to increase the grid connection point voltage value to the rated AC voltage value, and the diode valve 32 is turned on.

[0122] Finally, the offshore wind farm 4 is controlled to exit the controllable energy consumption mode. At this time, the offshore wind farm 4 gradually increases the power transmitted to the offshore converter station 3 under the controllable energy consumption mode, controls the DC voltage value of the offshore auxiliary flexible DC converter valve 31 to be constant and the q-axis voltage value to be zero, completes the startup of the parallel offshore wind power DC transmission system, and the entire system returns to normal operation.

[0123] In one example, after constant power control is implemented on a preset number of wind turbines to disconnect the first bypass switch 33 and the second bypass switch 34, step S103 completes the startup of the parallel offshore wind power direct current transmission system by the following steps:

[0124] First, when the DC voltage of the onshore flexible DC converter valve 22 is increased to the rated DC voltage, the first and second circuit breakers 24 and 25 are opened, and the third and fourth circuit breakers 26 and 27 are closed. After the first and second bypass switches 33 and 34 are opened, the surplus power causes the energy in the auxiliary MMC capacitor to slowly increase. Before the capacitor energy reaches the threshold, the DC voltage of the onshore flexible DC converter valve 22 is increased to the rated DC voltage. The first and second circuit breakers 24 and 25 are opened, and the third and fourth circuit breakers 26 and 27 are closed, switching the startup transformer 21 to the operating transformer 23.

[0125] Then, the fifth circuit breaker 35 and the sixth circuit breaker 36 are closed, and the offshore auxiliary flexible DC converter valve 31 is controlled to increase the grid connection point voltage value to the rated AC voltage value, and the diode valve 32 is turned on.

[0126] Finally, the DC voltage value of the offshore auxiliary flexible DC converter valve 31 is controlled to be constant and the q-axis voltage value is zero, completing the startup of the parallel offshore wind power DC transmission system, and the entire system returns to normal operation.

[0127] In one example, after controlling the grid power supply unit 6 to reduce the output power from the offshore wind farm 4 to the offshore converter station 3 and disconnecting the first bypass switch 33 and the second bypass switch 34, the above-mentioned step S103 completes the startup of the parallel offshore wind power direct current transmission system through the following contents:

[0128] First, when the DC voltage of the onshore flexible DC converter valve 22 increases to the rated DC voltage, the first and second circuit breakers 24 and 25 are opened, and the third and fourth circuit breakers 26 and 27 are closed. At this time, the start transformer 21 is switched to the operating transformer 23.

[0129] Then, the fifth circuit breaker 35 and the sixth circuit breaker 36 are closed, and the grid power supply unit 6 is controlled to increase the grid connection point voltage value to the rated AC voltage value, and the diode valve 32 is turned on.

[0130] Finally, the DC voltage value of the offshore auxiliary flexible DC converter valve 31 is controlled to be constant and the q-axis voltage value is zero, the grid power supply unit 6 exits the grid operation mode, the startup of the parallel offshore wind power DC transmission system is completed, and the entire system returns to normal operation.

[0131] Based on the same inventive concept, an embodiment of the present application further provides a parallel offshore wind power DC transmission system startup control device, as shown in FIG4 , which includes:

[0132] The first control module 401 is used to control the onshore power grid to supply power to the offshore wind farm via the onshore converter station and the offshore auxiliary flexible DC converter valve of the offshore converter station to start the offshore wind farm; for details, please refer to the description of step S101 in the above embodiment, which will not be repeated here.

[0133] The second control module 402 is used to control the output power from the offshore wind farm to the offshore converter station until the current value passing through the first bypass switch and the second bypass switch decreases to a preset current value, and then disconnect the first bypass switch and the second bypass switch; for details, please refer to the description of step S102 in the above embodiment and will not be repeated here.

[0134] The conduction module 403 is used to conduct the diode valve in the offshore converter station to complete the startup of the parallel offshore wind power direct current transmission system. For details, please refer to the description of step S103 in the above embodiment, which will not be repeated here.

[0135] In one example, an onshore converter station includes a starting transformer, an onshore flexible DC converter valve, and an operating transformer; the onshore power grid is connected to the onshore flexible DC converter valve via a first circuit breaker, a starting transformer, and a second circuit breaker in sequence; the onshore power grid is connected to the onshore flexible DC converter valve via a third circuit breaker, an operating transformer, and a fourth circuit breaker; the onshore flexible DC converter valve is connected to the offshore converter station via a DC submarine cable; a fifth circuit breaker is connected between one end of the DC submarine cable and the diode valve, and a sixth circuit breaker is connected between the other end of the DC submarine cable and the diode valve; the offshore converter station also includes a first transformer and a second transformer; the diode valve is connected to the grid connection point of the offshore wind farm via the first transformer and the seventh circuit breaker in sequence; the offshore auxiliary flexible DC converter valve is connected to the grid connection point via an eighth circuit breaker, a second transformer, and a ninth circuit breaker in sequence;

[0136] The first control module 401 includes:

[0137] The first closing submodule is used to close the first circuit breaker, the second circuit breaker, the first bypass switch and the second bypass switch, so that the onshore power grid charges the onshore flexible DC converter valve by starting the transformer, and at the same time, the onshore power grid charges the offshore auxiliary flexible DC converter valve in the offshore converter station through the onshore flexible DC converter valve until the onshore flexible DC converter valve and the offshore auxiliary flexible DC converter valve are fully charged; for details, please refer to the description in the above embodiment and will not be repeated here.

[0138] The unlocking submodule is used to unlock the onshore flexible DC converter valve and set the DC voltage value of the onshore flexible DC converter valve to a preset DC voltage value, and the reactive power value of the onshore flexible DC converter valve to a preset reactive power value; for details, please refer to the description in the above embodiment and will not be repeated here.

[0139] The second closing submodule is configured to close the seventh, eighth, and ninth circuit breakers, unlock the offshore auxiliary flexible DC converter valve, and control the offshore auxiliary flexible DC converter valve to raise the grid connection point voltage to a preset grid connection point voltage by means of a zero-start voltage ramp-up method, thereby starting a preset number of wind turbines in the offshore wind farm. For details, refer to the description in the above embodiment and will not be repeated here.

[0140] In one example, the second control module 402 includes:

[0141] The first control submodule is configured to control the offshore auxiliary flexible DC converter valve to reduce the voltage at the grid connection point. Upon detecting a decrease in the grid connection point voltage, the offshore wind farm enters a controllable energy consumption mode and controls the output power from the offshore wind farm to the offshore converter station until the current flowing through the first and second bypass switches decreases to a preset current value, at which point the first and second bypass switches are disconnected. For details, refer to the description in the above embodiment and are not repeated here.

[0142] In one example, the second control module 402 further includes:

[0143] The second control submodule is configured to control the offshore auxiliary flexible DC converter valve to reduce the voltage at the grid connection point, implement constant power control on a preset number of wind turbines, and reduce the output power from the offshore wind farm to the offshore converter station until the current flowing through the first and second bypass switches decreases to a preset current value, at which point the first and second bypass switches are disconnected. Details are described in the above embodiment and are omitted here.

[0144] In one example, the system further includes a grid-forming power supply unit, which includes: a battery, a converter, and a third transformer;

[0145] The grid power supply unit is used to provide grid voltage to the grid connection point;

[0146] The battery is connected to the grid connection point through the converter, the tenth circuit breaker, the third transformer, and the eleventh circuit breaker in sequence;

[0147] The second control module 402 further includes:

[0148] The third control submodule is configured to control the network power supply unit to reduce the output power from the offshore wind farm to the offshore converter station, and then control the offshore auxiliary flexible direct current converter valve to operate in a no-load state, disconnecting the first bypass switch and the second bypass switch. For details, refer to the description in the above embodiment and will not be repeated here.

[0149] In one example, the conducting module 403 includes:

[0150] The fourth control submodule is used to control the onshore flexible DC converter valve to disconnect the first circuit breaker and the second circuit breaker and close the third circuit breaker and the fourth circuit breaker when the DC voltage value increases to the rated DC voltage value; for details, please refer to the description in the above embodiment and will not be repeated here.

[0151] The fifth control submodule is used to close the fifth circuit breaker and the sixth circuit breaker, control the offshore auxiliary flexible DC converter valve to increase the grid connection point voltage value to the rated AC voltage value, and turn on the diode valve; for details, please refer to the description in the above embodiment and will not be repeated here.

[0152] The sixth control submodule is used to control the offshore wind farm to exit the controlled energy consumption mode, maintain a constant DC voltage across the offshore auxiliary flexible DC converter valves, and maintain a zero q-axis voltage, thereby completing the startup of the parallel offshore wind power DC transmission system. For details, refer to the description in the above embodiment and will not be repeated here.

[0153] In one example, the conducting module 403 further includes:

[0154] The seventh control submodule is used to control the onshore flexible DC converter valve to disconnect the first circuit breaker and the second circuit breaker and close the third circuit breaker and the fourth circuit breaker when the DC voltage value increases to the rated DC voltage value; for details, please refer to the description in the above embodiment and will not be repeated here.

[0155] The eighth control submodule is used to close the fifth circuit breaker and the sixth circuit breaker, control the offshore auxiliary flexible DC converter valve to increase the grid connection point voltage value to the rated AC voltage value, and turn on the diode valve; for details, please refer to the description in the above embodiment and will not be repeated here.

[0156] The ninth control submodule is used to control the DC voltage of the offshore auxiliary flexible DC converter valve to be constant and the q-axis voltage to be zero, thereby completing the startup of the parallel offshore wind power DC transmission system. For details, please refer to the description in the above embodiment and will not be repeated here.

[0157] In one example, the conducting module 403 further includes:

[0158] The tenth control submodule is used to control the onshore flexible DC converter valve to disconnect the first circuit breaker and the second circuit breaker and close the third circuit breaker and the fourth circuit breaker when the DC voltage value increases to the rated DC voltage value; for details, please refer to the description in the above embodiment and will not be repeated here.

[0159] The eleventh control submodule is used to close the fifth circuit breaker and the sixth circuit breaker, control the network power supply unit to increase the grid connection point voltage value to the rated AC voltage value, and turn on the diode valve; for details, please refer to the description in the above embodiment and will not be repeated here.

[0160] The twelfth control submodule is used to control the DC voltage of the offshore auxiliary flexible DC converter valve to be constant and the q-axis voltage to be zero, causing the grid-forming power supply unit to exit the grid-forming operation mode, thereby completing the startup of the parallel offshore wind power DC transmission system. For details, please refer to the description in the above embodiment and will not be repeated here.

[0161] Figure 5 is a schematic diagram of the hardware structure of a computer device according to an exemplary embodiment. As shown in Figure 5 , the device includes one or more processors 510 and memory 520. Memory 520 includes persistent memory, volatile memory, and a hard disk. Figure 5 uses one processor 510 as an example. The device may also include an input device 530 and an output device 540.

[0162] The processor 510 , the memory 520 , the input device 530 and the output device 540 may be connected via a bus or other means. FIG. 5 takes the bus connection as an example.

[0163] The processor 510 may be a central processing unit (CPU). The processor 510 may 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 gate or transistor logic devices, discrete hardware components, or a combination of the above chips. The general-purpose processor may be a microprocessor or any conventional processor.

[0164] Memory 520, as a non-transitory computer-readable storage medium, includes persistent memory, volatile memory, and a hard disk, and can be used to store non-transitory software programs, non-transitory computer executable programs, and modules, such as the program instructions / modules corresponding to the parallel offshore wind power DC transmission system startup control method in the embodiments of the present application. Processor 510 executes the non-transitory software programs, instructions, and modules stored in memory 520 to execute various server functional applications and data processing, thereby implementing any of the aforementioned parallel offshore wind power DC transmission system startup control methods.

[0165] The memory 520 may include a program storage area and a data storage area, wherein the program storage area may store an operating system and application programs required for at least one function; the data storage area may store data required for use, etc. In addition, the memory 520 may include a high-speed random access memory, and may also include a non-volatile memory, such as at least one disk storage device, a flash memory device, or other non-volatile solid-state storage device. In some embodiments, the memory 520 may optionally include a memory remotely located relative to the processor 510, and these remote memories may be connected to the data processing device via a network. Examples of the above-mentioned network include, but are not limited to, the Internet, an intranet, a local area network, a mobile communication network, and combinations thereof.

[0166] The input device 530 can receive input digital or character information and generate signal input related to user settings and function control. The output device 540 can include a display device such as a display screen.

[0167] One or more modules are stored in the memory 520 , and when executed by one or more processors 510 , the method shown in FIG. 3 is performed.

[0168] The above-mentioned product can execute the method provided in the embodiment of this application, and has the functional modules and beneficial effects corresponding to the execution method. For technical details not fully described in this embodiment, please refer to the relevant description in the embodiment shown in Figure 3.

[0169] The present application also provides a non-transitory computer storage medium storing computer-executable instructions that can execute the startup control method in any of the above method embodiments. The storage medium can be a magnetic disk, an optical disk, a read-only memory (ROM), a random access memory (RAM), a flash memory, a hard disk drive (HDD), or a solid-state drive (SSD); the storage medium can also include a combination of the above types of memory.

[0170] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "comprises," "comprising," or any other variations thereof are intended to cover non-exclusive inclusion, so that a process, method, article, or device that includes a series of elements includes not only those elements, but also other elements not explicitly listed, or elements inherent to such process, method, article, or device. In the absence of further limitations, an element defined by the phrase "comprising a ..." does not exclude the presence of other identical elements in the process, method, article, or device that includes the element.

[0171] The above are merely specific embodiments of the present application to enable those skilled in the art to understand or implement the present application. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to the embodiments shown herein, but rather is intended to conform to the widest scope consistent with the principles and novel features of the present application.

Claims

1. A parallel offshore wind power direct current transmission system, characterized in that: The system comprises: an onshore power grid, an onshore converter station, an offshore converter station and an offshore wind farm; The onshore power grid is connected to the onshore converter station; The onshore converter station is connected to the offshore converter station, and the offshore converter station includes an offshore auxiliary flexible direct current converter valve and a diode valve connected in parallel with the offshore auxiliary flexible direct current converter valve; a first bypass switch is connected between the offshore auxiliary flexible direct current converter valve and one end of the DC side of the diode valve; a second bypass switch is connected between the offshore auxiliary flexible direct current converter valve and the other end of the DC side of the diode valve; The offshore auxiliary flexible direct current converter valve and the diode valve are respectively connected to the offshore wind farm.

2. The system according to claim 1, characterized in that The onshore converter station includes a starting transformer, an onshore flexible DC converter valve and an operating transformer; The onshore power grid is connected to the onshore flexible direct current converter valve via the first circuit breaker, the starting transformer and the second circuit breaker in sequence; The onshore power grid is connected to the onshore flexible direct current converter valve via a third circuit breaker, the operating transformer and a fourth circuit breaker; The onshore flexible DC converter valve is connected to the offshore converter station via a DC submarine cable.

3. The system according to claim 2, characterized in that A fifth circuit breaker is connected between one end of the DC submarine cable and the diode valve, and a sixth circuit breaker is connected between the other end of the DC submarine cable and the diode valve.

4. The system according to claim 3, characterized in that The offshore converter station further includes a first transformer and a second transformer; The diode valve is connected to the grid connection point of the offshore wind farm via the first transformer and the seventh circuit breaker in sequence; The offshore auxiliary flexible direct current converter valve is connected to the grid connection point via the eighth circuit breaker, the second transformer and the ninth circuit breaker in sequence.

5. The system according to claim 4, characterized in that The system further comprises a network power supply unit, which comprises: a battery, a converter and a third transformer; The grid-connection power supply unit is used to provide a grid-connection voltage for the grid-connection point; The storage battery is connected to the grid connection point via the converter, the tenth circuit breaker, the third transformer and the eleventh circuit breaker in sequence.

6. A startup control method for a parallel offshore wind power direct current transmission system, characterized in that: For the parallel offshore wind power direct current transmission system according to claim 1, the method comprises: Controlling the onshore power grid to supply power to the offshore wind farm via the onshore converter station and the offshore auxiliary flexible direct current converter valve of the offshore converter station, so as to start the offshore wind farm; Controlling the output power of the offshore wind farm to the offshore converter station until the current value passing through the first bypass switch and the second bypass switch decreases to a preset current value, and disconnecting the first bypass switch and the second bypass switch; The diode valve in the offshore converter station is turned on to complete the startup of the parallel offshore wind power DC transmission system.

7. The method according to claim 6, characterized in that The onshore converter station includes a starting transformer, an onshore flexible DC converter valve and an operating transformer; the onshore power grid is connected to the onshore flexible DC converter valve through a first circuit breaker, the starting transformer and a second circuit breaker in sequence; the onshore power grid is connected to the onshore flexible DC converter valve through a third circuit breaker, the operating transformer and a fourth circuit breaker; the onshore flexible DC converter valve is connected to the offshore converter station via a DC submarine cable; a fifth circuit breaker is connected between one end of the DC submarine cable and the diode valve, and a sixth circuit breaker is connected between the other end of the DC submarine cable and the diode valve; the offshore converter station also includes a first transformer and a second transformer; the diode valve is connected to the grid connection point of the offshore wind farm through a first transformer and a seventh circuit breaker in sequence; the offshore auxiliary flexible DC converter valve is connected to the grid connection point through an eighth circuit breaker, a second transformer and a ninth circuit breaker in sequence; The controlling the onshore power grid to supply power to the offshore wind farm via the onshore converter station and the offshore auxiliary flexible direct current converter valve of the offshore converter station to start the offshore wind farm includes: The first circuit breaker, the second circuit breaker, the first bypass switch and the second bypass switch are closed, so that the onshore power grid charges the onshore flexible direct current converter valve by starting the transformer, and at the same time, the onshore power grid charges the offshore auxiliary flexible direct current converter valve in the offshore converter station through the onshore flexible direct current converter valve, until the onshore flexible direct current converter valve and the offshore auxiliary flexible direct current converter valve are completely charged; Unlocking the onshore flexible direct current converter valve, and setting the DC voltage value of the onshore flexible direct current converter valve to a preset DC voltage value, and setting the reactive power value of the onshore flexible direct current converter valve to a preset reactive power value; Close the seventh circuit breaker, the eighth circuit breaker, and the ninth circuit breaker, unlock the offshore auxiliary flexible DC converter valve, and control the offshore auxiliary flexible DC converter valve to increase the voltage from zero so that the grid connection point voltage reaches the preset grid connection point voltage value, thereby starting a preset number of wind turbines in the offshore wind farm.

8. The method according to claim 7, characterized in that The controlling the output power of the offshore wind farm to the offshore converter station until the current value passing through the first bypass switch and the second bypass switch is reduced to a preset current value, disconnecting the first bypass switch and the second bypass switch, comprises: The offshore auxiliary flexible direct current converter valve is controlled to reduce the voltage value of the grid connection point, so that the offshore wind farm enters a controllable energy consumption mode after detecting a decrease in the voltage value of the grid connection point, and controls the output power of the offshore wind farm to the offshore converter station until the current value passing through the first bypass switch and the second bypass switch is reduced to a preset current value, and then the first bypass switch and the second bypass switch are disconnected.

9. The method according to claim 7, characterized in that: The controlling the output power of the offshore wind farm to the offshore converter station until the current value passing through the first bypass switch and the second bypass switch is reduced to a preset current value, disconnecting the first bypass switch and the second bypass switch, comprises: The offshore auxiliary flexible direct current converter valve is controlled to reduce the voltage value of the grid connection point, and a preset number of wind turbines are subjected to constant power control to reduce the output power of the offshore wind farm to the offshore converter station, until the current value passing through the first bypass switch and the second bypass switch is reduced to a preset current value, and the first bypass switch and the second bypass switch are disconnected.

10. The method according to claim 7, characterized in that The system further comprises a network power supply unit, wherein the network power supply unit comprises: a battery, a converter and a third transformer; The grid-connection power supply unit is used to provide a grid-connection voltage for the grid-connection point; The battery is connected to the grid connection point through the converter, the tenth circuit breaker, the third transformer, and the eleventh circuit breaker in sequence; The controlling the output power of the offshore wind farm to the offshore converter station until the current value passing through the first bypass switch and the second bypass switch is reduced to a preset current value, disconnecting the first bypass switch and the second bypass switch, comprises: The grid power supply unit is controlled to reduce the output power from the offshore wind farm to the offshore converter station, and then the offshore auxiliary flexible direct current converter valve is controlled to be in a no-load condition, and the first bypass switch and the second bypass switch are disconnected.

11. The method according to claim 8, characterized in that The method of turning on the diode valve in the offshore converter station to complete the startup of the parallel offshore wind power direct current transmission system includes: When the DC voltage value of the onshore flexible DC converter valve is increased to the rated DC voltage value, the first circuit breaker and the second circuit breaker are disconnected, and the third circuit breaker and the fourth circuit breaker are closed; The fifth circuit breaker and the sixth circuit breaker are closed, and the offshore auxiliary flexible DC converter valve is controlled to increase the voltage value of the grid connection point to the rated AC voltage value, and the diode valve is turned on; The offshore wind farm is controlled to exit the controllable energy consumption mode, the DC voltage value of the offshore auxiliary flexible DC converter valve is controlled to be constant and the q-axis voltage value is controlled to be zero, and the startup of the parallel offshore wind power DC transmission system is completed.

12. The method according to claim 9, characterized in that The method of turning on the diode valve in the offshore converter station to complete the startup of the parallel offshore wind power direct current transmission system includes: When the DC voltage value of the onshore flexible DC converter valve is increased to the rated DC voltage value, the first circuit breaker and the second circuit breaker are disconnected, and the third circuit breaker and the fourth circuit breaker are closed; The fifth circuit breaker and the sixth circuit breaker are closed, and the offshore auxiliary flexible DC converter valve is controlled to increase the voltage value of the grid connection point to the rated AC voltage value, and the diode valve is turned on; The DC voltage value of the offshore auxiliary flexible DC converter valve is controlled to be constant and the q-axis voltage value is controlled to be zero, thereby completing the startup of the parallel offshore wind power DC transmission system.

13. The method according to claim 10, characterized in that The method of turning on the diode valve in the offshore converter station to complete the startup of the parallel offshore wind power direct current transmission system includes: When the DC voltage value of the onshore flexible DC converter valve is increased to the rated DC voltage value, the first circuit breaker and the second circuit breaker are disconnected, and the third circuit breaker and the fourth circuit breaker are closed; Closing the fifth circuit breaker and the sixth circuit breaker, controlling the grid power supply unit to increase the grid connection point voltage value to the rated AC voltage value, and the diode valve is turned on; The DC voltage value of the offshore auxiliary flexible DC converter valve is controlled to be constant and the q-axis voltage value is controlled to be zero, the grid-building power supply unit exits the grid-building operation mode, and the startup of the parallel offshore wind power DC transmission system is completed.

14. A parallel offshore wind power DC transmission system startup control device, characterized in that: For the parallel offshore wind power direct current transmission system according to claim 1, the device comprises: A first control module is used to control the onshore power grid to supply power to the offshore wind farm via the onshore converter station and the offshore auxiliary flexible direct current converter valve of the offshore converter station, so as to start the offshore wind farm; A second control module is used to control the output power of the offshore wind farm to the offshore converter station until the current value passing through the first bypass switch and the second bypass switch is reduced to a preset current value, and then the first bypass switch and the second bypass switch are disconnected; The conduction module is used to conduct the diode valve in the offshore converter station to complete the startup of the parallel offshore wind power DC transmission system.

15. A computer device, characterized in that: The invention comprises a memory and a processor, wherein the memory and the processor are communicatively connected to each other, the memory stores computer instructions, and the processor executes the steps of the startup control method of the parallel offshore wind power direct current transmission system according to any one of claims 6 to 13 by executing the computer instructions.

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