Offshore wind power dc transmission system and starting method and device thereof

Abstract

The application relates to the field of wind power generation and direct current transmission technology, in particular to an offshore wind power direct current transmission system and a starting method thereof. The starting method comprises the following steps: using a negative voltage output function of an offshore direct current voltage variable type conversion module and a shore direct current voltage variable type converter to complete black start of a first offshore wind turbine set; using an alternating current output voltage controllable function of the offshore direct current voltage variable type conversion module and a low voltage ride through function of an offshore wind turbine to make a diode rectifier module access an offshore alternating current power grid in a mode of starting from an initial voltage and rising at a preset rising rate; and connecting offshore wind turbines in an offshore wind farm except the first offshore wind turbine set to the offshore alternating current power grid by controlling the offshore direct current voltage variable type conversion module and the shore direct current voltage variable type converter. The application can complete starting of the offshore wind power direct current transmission system, and the starting process is stable and free of impact.

Description

Technical Field

[0001] This application relates to the fields of wind power generation and DC power transmission technology, and in particular to an offshore wind power DC transmission system and its starting method and apparatus. Background Technology

[0002] Offshore wind resources are extremely abundant, wind speeds are relatively stable, and the vast space allows for larger wind turbines, making it an inevitable trend for future wind power development. Offshore wind power technology has gradually become a key technology for achieving climate goals. To obtain larger sea areas and more stable wind energy, the development of offshore wind power is increasingly becoming a focus of attention. With offshore wind farms being located further and further from shore and with increasingly larger capacities, how to economically and reliably transmit offshore wind power to shore has become a critical issue. Summary of the Invention

[0003] This application aims to at least partially address one of the technical problems in the related art.

[0004] Therefore, the first objective of this application is to propose an offshore wind power DC transmission system, which provides an offshore wind power DC transmission system with low output cost, high transmission efficiency, and good transmission effect.

[0005] The second objective of this application is to propose a startup method for an offshore wind power DC transmission system.

[0006] The third objective of this application is to provide a start-up device for an offshore wind power DC transmission system.

[0007] To achieve the above objectives, the first aspect of this application proposes an offshore wind power DC transmission system, comprising: a diode rectifier module, an offshore DC voltage variable converter module, and an onshore DC voltage variable converter;

[0008] The AC side of the diode rectifier module and the offshore DC voltage variable converter module is connected to the offshore wind farm. The DC side of the diode rectifier module and the DC side of the offshore DC voltage variable converter module are connected in series and then connected to the DC side of the onshore DC voltage variable converter. The AC side of the onshore DC voltage variable converter is connected to the onshore AC power grid.

[0009] Optionally, in one embodiment of this application, the diode rectifier module includes: a first AC switch, a marine rectifier transformer, and a marine diode rectifier;

[0010] The AC side of the offshore diode rectifier is connected to the offshore wind farm via the offshore rectifier transformer and the first AC switch, and the DC side of the offshore diode rectifier is the DC side of the diode rectifier module.

[0011] Optionally, in one embodiment of this application, the marine DC voltage variable converter module includes: a second AC switch, an auxiliary connection transformer, and a marine DC voltage variable auxiliary modular multilevel converter (MMC).

[0012] The AC side of the offshore DC voltage variable auxiliary MMC is connected to the offshore wind farm through the auxiliary connection transformer and the second AC switch, and the DC side of the offshore DC voltage variable auxiliary MMC is the DC side of the offshore DC voltage variable converter module.

[0013] In summary, the offshore wind power DC transmission system proposed in the first aspect of this application includes: a diode rectifier module, an offshore DC voltage variable converter module, and an onshore DC voltage variable converter. The AC sides of the diode rectifier module and the offshore DC voltage variable converter module are connected to the offshore wind farm. The DC sides of the diode rectifier module and the offshore DC voltage variable converter module are connected in series and then connected to the DC side of the onshore DC voltage variable converter. The AC side of the onshore DC voltage variable converter is connected to the onshore AC power grid. This application, by employing a diode rectifier module and an offshore DC voltage variable converter module connected in series, provides an offshore wind power DC transmission system with low output cost, high transmission efficiency, and good transmission effect.

[0014] To achieve the above objectives, the second aspect of this application provides a startup method for an offshore wind power DC transmission system, comprising:

[0015] The black start of the first offshore wind turbine assembly was completed by utilizing the negative voltage output function of the offshore DC voltage variable converter module and the onshore DC voltage variable converter.

[0016] By utilizing the controllable AC output voltage function of the offshore DC voltage variable converter module and the low-voltage ride-through function of the offshore wind turbine, the diode rectifier module is connected to the offshore AC power grid in a manner that the AC voltage is raised from the initial voltage according to a preset rise rate.

[0017] By controlling the offshore DC voltage variable converter module and the onshore DC voltage variable converter, the offshore wind turbines in the offshore wind farm, except for the first offshore wind turbine set, are connected to the offshore AC power grid to complete the startup of the offshore wind power DC transmission system.

[0018] Optionally, in one embodiment of this application, the offshore DC voltage variable converter module includes an offshore DC voltage variable auxiliary MMC, the offshore DC voltage variable auxiliary MMC includes at least two sub-modules, and the black start of the first offshore wind turbine assembly is completed by utilizing the negative voltage output function of the offshore DC voltage variable converter module and the onshore DC voltage variable converter, including:

[0019] Determine the target DC voltage value, and control the onshore DC voltage variable converter to operate in constant DC voltage mode according to the target DC voltage value. The target DC voltage value is determined based on the preset start-up voltage value of the submodule capacitor voltage of the offshore DC voltage variable auxiliary MMC.

[0020] The sub-modules of the marine DC voltage variable auxiliary MMC are put into operation sequentially, and the capacitors of the sub-modules are charged by the onshore DC voltage variable converter until the capacitor voltages of all sub-modules reach the preset start-up voltage value, so as to complete the start-up of the marine DC voltage variable auxiliary MMC.

[0021] A first DC current target value is determined, and the onshore DC voltage variable converter is controlled to operate in constant DC current mode according to the first DC current target value. The first DC current target value is the rated DC current of the offshore wind power DC transmission system.

[0022] The variable DC voltage auxiliary MMC is used to provide AC voltage to the first offshore wind turbine assembly to complete the black start of the first offshore wind turbine assembly.

[0023] Optionally, in one embodiment of this application, before using the offshore DC voltage variable auxiliary MMC to provide AC voltage to the offshore AC grid to complete the black start of the first offshore wind turbine assembly, the method further includes:

[0024] The AC side of the marine DC voltage variable auxiliary MMC is set to operate in constant AC voltage mode, so that the AC voltage output by the marine DC voltage variable auxiliary MMC is an AC voltage with rated amplitude and rated frequency.

[0025] The DC side of the marine DC voltage variable auxiliary MMC is set to operate in stator module capacitor voltage mode. By controlling the DC output voltage of the marine DC voltage variable auxiliary MMC, the DC side of the marine DC voltage variable auxiliary MMC is controlled to absorb or output active power, so that the sub-module capacitor voltage corresponding to the marine DC voltage variable auxiliary MMC is the rated voltage value.

[0026] Optionally, in one embodiment of this application, before determining the target DC voltage value and controlling the onshore DC voltage variable converter to operate in constant DC voltage mode according to the target DC voltage value, the method further includes:

[0027] The onshore DC voltage variable converter is started up via the onshore AC power grid.

[0028] Optionally, in one embodiment of this application, the diode rectifier module includes a first AC switch, an offshore rectifier transformer, and an offshore diode rectifier; the offshore DC voltage variable converter module includes an offshore DC voltage variable auxiliary MMC; the offshore DC voltage variable auxiliary MMC includes at least two sub-modules; and the step of utilizing the controllable AC output voltage function of the offshore DC voltage variable converter module and the low-voltage ride-through function of the offshore wind turbine to allow the diode rectifier module to connect to the offshore AC grid in a manner that the AC voltage is raised from the initial voltage according to a preset rise rate includes:

[0029] A second DC current target value is determined, and the onshore DC voltage variable converter is controlled to operate in constant DC current mode according to the second DC current target value. The second DC current target value is determined based on the capacity of the offshore DC voltage variable auxiliary MMC and the offshore diode rectifier.

[0030] Control the AC voltage output of the marine DC voltage variable converter module to drop to the initial voltage;

[0031] Close the first AC switch to connect the marine rectifier transformer and the marine diode rectifier to the marine AC power grid;

[0032] The AC voltage output by the marine DC voltage variable converter module is controlled to rise from a preset voltage threshold at a preset rise rate until the rated AC voltage amplitude is reached.

[0033] Optionally, in one embodiment of this application, the offshore DC voltage variable converter module includes an offshore DC voltage variable auxiliary MMC, and the step of controlling the offshore DC voltage variable converter module and the onshore DC voltage variable converter to connect the offshore wind turbines in the offshore wind farm, excluding the first offshore wind turbine set, to the offshore AC power grid includes:

[0034] The initial value, increase rate, and final value of the third DC current target value are determined, and the onshore DC voltage variable converter is controlled to operate in constant DC current mode according to the third DC current target value. The initial value of the third DC current target value is determined based on the capacity of the offshore DC voltage variable auxiliary MMC and the offshore diode rectifier. The increase rate of the third DC current target value is determined based on the connection rate of the offshore wind turbines (excluding the first offshore wind turbine set) to the offshore AC grid. The final value of the third DC current target value is the rated DC current of the offshore wind power DC transmission system.

[0035] In summary, the method proposed in the second aspect of this application, firstly, by utilizing the negative voltage output function of the offshore DC voltage variable converter module and the onshore DC voltage variable converter, allows the offshore wind power DC transmission system to operate in a forward current, reverse voltage state. This enables the transmission of active power from the shore to the sea, providing black-start power for the offshore wind turbines and thus completing the black start of the first offshore wind turbine assembly. Secondly, by utilizing the controllable AC output voltage function of the offshore DC voltage variable converter module and the low-voltage ride-through function of the offshore wind turbines, the diode rectifier module is connected to the offshore AC grid by gradually increasing the AC voltage from the initial voltage at a preset rate. This avoids the impact problem caused by the inrush current during rectifier transformer switching and allows the DC output voltage of the offshore diode rectifier to rise slowly, preventing the impact on the DC system caused by sudden step changes in the output DC voltage of the diode rectifier. Finally, during the startup process, by controlling the offshore DC voltage variable converter module and the onshore DC voltage variable converter, the active power and DC transmission power of the offshore wind farm can be balanced. Then, the offshore wind turbines except for the first offshore wind turbine set can be connected to the offshore AC grid to complete the startup of the offshore wind power DC transmission system. The startup process is smooth and without shock.

[0036] To achieve the above objectives, a third aspect of this application provides a starting device for an offshore wind power DC transmission system, comprising:

[0037] The offshore DC voltage variable converter module and part of the offshore wind turbine assembly start-up unit are used to complete the black start of the first offshore wind turbine assembly by utilizing the negative voltage output function of the offshore DC voltage variable converter module and the onshore DC voltage variable converter.

[0038] The diode rectifier access unit is used to utilize the AC output voltage controllable function of the offshore DC voltage variable converter module and the low voltage ride-through function of the offshore wind turbine to enable the diode rectifier module to connect to the offshore AC power grid in a manner that the AC voltage is raised from the initial voltage according to a preset rise rate.

[0039] The remaining offshore wind turbine assembly start-up unit is used to control the offshore DC voltage variable converter module and the onshore DC voltage variable converter to connect the offshore wind turbines (excluding the first offshore wind turbine assembly) in the offshore wind farm to the offshore AC power grid, thereby completing the start-up of the offshore wind power DC transmission system.

[0040] In summary, the apparatus proposed in the third aspect of this application, firstly, utilizes the negative voltage output function of the offshore DC voltage variable converter module and the onshore DC voltage variable converter to complete the black start of the first offshore wind turbine assembly. This allows the offshore wind power DC transmission system to operate in a forward current, reverse voltage state, thereby enabling the transmission of active power from the shore to the sea and providing black start power for the offshore wind turbines, thus completing the black start of the first offshore wind turbine assembly. Secondly, through the diode rectifier access unit, utilizing the controllable AC output voltage function of the offshore DC voltage variable converter module and the low-voltage ride-through function of the offshore wind turbines, the diode rectifier module is connected to the offshore AC grid by gradually increasing the AC voltage from the initial voltage at a preset rate. This avoids the impact problem caused by the inrush current of the rectifier transformer during closing and allows the DC output voltage of the offshore diode rectifier to also increase slowly, avoiding the impact on the DC system caused by sudden step changes in the DC output voltage of the diode rectifier. Finally, during startup, the remaining offshore wind turbine assembly startup unit controls the offshore DC voltage variable converter module and the onshore DC voltage variable converter to connect the offshore wind turbines (excluding the first offshore wind turbine assembly) to the offshore AC power grid, thus completing the startup of the offshore wind power DC transmission system. This achieves a balance between the active power and DC transmission power of the offshore wind farm, allowing the offshore wind turbines (excluding the first offshore wind turbine assembly) to be connected to the offshore AC power grid, completing the startup of the offshore wind power DC transmission system smoothly and without shocks.

[0041] Additional aspects and advantages of this application will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of this application. Attached Figure Description

[0042] The above and / or additional aspects and advantages of this application will become apparent and readily understood from the following description of the embodiments taken in conjunction with the accompanying drawings, wherein:

[0043] Figure 1 This is a schematic diagram of the structure of an offshore wind power DC transmission system provided in an embodiment of this application;

[0044] Figure 2This is a schematic diagram of the structure of a diode rectifier module provided in an embodiment of this application;

[0045] Figure 3 This is a schematic diagram of the structure of a marine DC voltage variable converter module provided in an embodiment of this application;

[0046] Figure 4 This is a schematic diagram of the structure of a marine DC voltage variable auxiliary MMC provided in an embodiment of this application;

[0047] Figure 5 This is a schematic diagram of the structure of a half-bridge submodule provided in an embodiment of this application;

[0048] Figure 6 This is a schematic diagram of the structure of a bidirectional current-mode full-bridge submodule provided in an embodiment of this application;

[0049] Figure 7 This is a schematic diagram of the structure of a unidirectional current-type full-bridge submodule provided in an embodiment of this application;

[0050] Figure 8 A flowchart of a startup method for an offshore wind power DC transmission system provided in this application embodiment;

[0051] Figure 9 A control block diagram of a marine DC voltage variable auxiliary MMC during startup is provided in an embodiment of this application;

[0052] Figure 10 This is a schematic diagram of the starting device of an offshore wind power DC transmission system provided in an embodiment of this application. Detailed Implementation

[0053] The embodiments of this application are described in detail below, examples of which are illustrated in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain this application, and should not be construed as limiting this application. Rather, the embodiments of this application include all variations, modifications, and equivalents falling within the spirit and scope of the appended claims.

[0054] In transmitting offshore wind power to shore, the charging current of cable capacitors limits the transmission distance of high-voltage AC power. When offshore wind farms are far from shore, DC transmission becomes almost inevitable. Modular multilevel converter (MMC) technology has enabled the rapid development of flexible DC transmission, but MMCs suffer from large size and weight. Furthermore, as the capacity of offshore wind farms continues to increase, the challenges of large offshore converter platforms will become more prominent, even unrealistic, becoming a critical factor restricting the development of offshore wind power.

[0055] In related technologies, in order to fundamentally change the voltage source converter method, the solution of using a diode rectifier (DR) to replace MMC on the offshore wind farm side is receiving increasing attention.

[0056] However, for diode rectifier DC transmission schemes, since diode rectifiers cannot transmit power in reverse, it is impossible to achieve black start of offshore wind farms solely relying on diode rectifier DC transmission lines. Additional black start equipment is required to achieve black start of offshore wind farms.

[0057] One approach is to install an auxiliary diesel generator or energy storage power source on the offshore platform to provide black start power for the offshore wind turbines; however, if the black start power supply capacity requirement is large, the investment and land occupation for installing a diesel generator or energy storage power source on the offshore platform will be relatively large.

[0058] Another approach is to construct an auxiliary power line to provide starting power from the shore. This could involve building an additional low-voltage, small-capacity AC transmission line to provide black start power from the shore, or building an additional flexible DC transmission line with a relatively low voltage and small capacity to provide black start power from the shore. However, since the DC transmission scheme is designed for offshore wind farms, the additional auxiliary power line will still bring additional investment issues and increase the amount of equipment on the offshore platform.

[0059] In addition, the black start power supply equipment added by the above two methods only works during black start and has no effect when the DC transmission system is running normally, so the equipment has low efficiency.

[0060] Secondly, the diode rectifier scheme lacks control over AC / DC side voltage / current / power. It also lacks control over the voltage / frequency and transmitted power of the offshore AC bus, failing to provide synchronous grid-connected AC voltage for offshore wind turbines. Coordinated control of dozens or even hundreds of wind turbines is required to achieve voltage and frequency control of the offshore AC bus and control of the active power transmitted by the diode rectifier. To solve this problem, a converter method combining the diode rectifier with a smaller capacity auxiliary voltage source converter can be adopted.

[0061] One converter method involves connecting a small-capacity auxiliary MMC and a diode rectifier in parallel on the AC and DC sides to form a hybrid converter. During startup, the small-capacity auxiliary MMC provides black-start power to the offshore wind turbine. During normal operation, the small-capacity auxiliary MMC can also participate in active power regulation, playing a role in providing synchronous AC grid-connected voltage for the wind farm. However, when the diode rectifier and the auxiliary MMC are connected in parallel on the DC side, the auxiliary MMC still needs to be designed with a higher rated DC voltage, which is not advantageous in terms of economy and land use.

[0062] Another conversion method is to connect the diode in series with the auxiliary MMC, allowing the auxiliary MMC to be designed with a lower rated voltage. However, if a conventional half-bridge module-based MMC is used, the auxiliary MMC's DC voltage can usually only operate near its rated value. Due to the unidirectional DC current characteristic determined by the diode rectifier, the series connection method makes it difficult to achieve pre-charging of the auxiliary MMC and black start of the offshore wind farm. Alternatively, the auxiliary MMC can be designed with bidirectional voltage adjustment capability, allowing it to output a negative voltage during the startup phase, and the onshore converter can also be designed to output a small negative voltage. In this way, during the startup phase, the DC transmission system operates in a forward current and reverse voltage state, enabling the transfer of active power from the shore to the sea, providing conditions for pre-charging of the offshore auxiliary MMC and black start of the offshore wind farm. However, this scheme still faces the challenges of how to smoothly connect the rectifier transformer and diode rectifier during startup, and how to ensure the balance of active power of the wind farm, active power of the auxiliary MMC, and active power of the onshore converter during black start.

[0063] Finally, diode rectification also brings significant reactive power and harmonic problems, requiring the installation of reactive power compensation and filtering equipment, which brings additional investment and space requirements.

[0064] The present application will now be described in detail with reference to specific embodiments.

[0065] Figure 1 This is a schematic diagram of the structure of an offshore wind power DC transmission system provided in an embodiment of this application.

[0066] like Figure 1As shown in the figure, an embodiment of this application provides an offshore wind power DC transmission system, including: a diode rectifier module, an offshore DC voltage variable converter module, and an onshore DC voltage variable converter;

[0067] The diode rectifier module and the offshore DC voltage variable converter module are connected to the offshore wind farm on their AC sides. The diode rectifier module and the offshore DC voltage variable converter module are connected in series to the DC side of the onshore DC voltage variable converter. The onshore DC voltage variable converter is connected to the onshore AC power grid on its AC side.

[0068] According to some embodiments, when the DC side of the diode rectifier module and the DC side of the offshore DC voltage variable converter module are connected in series and then connected to the DC side of the onshore DC voltage variable converter, the DC side of the diode rectifier module and the DC side of the offshore DC voltage variable converter module can be connected to the DC side of the onshore DC voltage variable converter via a DC cable.

[0069] In some embodiments, when the AC side of the onshore DC voltage variable converter is connected to the onshore AC grid, it can be connected to the onshore AC grid through a third AC switch.

[0070] In the embodiments of this application, Figure 2 This is a schematic diagram of a diode rectifier module provided in an embodiment of this application. Figure 2 As shown, the diode rectifier module includes: a first AC switch K1, a marine rectifier transformer, and a marine diode rectifier;

[0071] The AC side of the offshore diode rectifier is connected to the offshore wind farm via an offshore rectifier transformer and the first AC switch K1, while the DC side of the offshore diode rectifier is the DC side of the diode rectifier module.

[0072] In the embodiments of this application, Figure 3 This is a schematic diagram of the structure of a marine DC voltage variable converter module provided in an embodiment of this application. Figure 3 As shown, the offshore DC voltage variable converter module includes: a second AC switch K2, an auxiliary connection transformer, and an offshore DC voltage variable auxiliary modular multilevel converter (MMC).

[0073] Among them, the AC side of the offshore DC voltage variable auxiliary MMC is connected to the offshore wind farm through an auxiliary connection transformer and the second AC switch K2, and the DC side of the offshore DC voltage variable auxiliary MMC is the DC side of the offshore DC voltage variable converter module.

[0074] According to some embodiments, Figure 4This is a schematic diagram of a marine DC voltage variable auxiliary MMC provided as an embodiment of this application. Figure 4 As shown, the marine DC voltage variable auxiliary MMC includes three phase units: a first phase unit, a second phase unit, and a third phase unit. Each phase unit includes an upper bridge arm, an upper bridge arm inductor, a lower bridge arm inductor, and a lower bridge arm.

[0075] In each phase unit, the lower end of the upper bridge arm and the upper end of the lower bridge arm are connected through the upper bridge arm inductor and the lower bridge arm inductor. The upper end of the upper bridge arm is connected to the DC positive bus, and the lower end of the lower bridge arm is connected to the DC negative bus. The connection point of the upper bridge arm inductor and the lower bridge arm inductor is connected to the AC bus corresponding to the phase unit.

[0076] Both the upper and lower bridge arms consist of at least two sub-modules connected in series.

[0077] The first phase unit is connected to AC bus A. The second phase unit is connected to AC bus B. The third phase unit is connected to AC bus C. The upper ends of the upper bridge arms corresponding to the first, second, and third phase units are all connected to the positive DC bus DC+. The lower ends of the lower bridge arms corresponding to the first, second, and third phase units are all connected to the negative DC bus DC-.

[0078] According to some embodiments, when the sub-modules corresponding to the upper bridge arm and the sub-modules corresponding to the lower bridge arm are connected in series, the series connection method of the sub-modules includes, but is not limited to, interleaved series connection, overall series connection, etc.

[0079] In some embodiments, when the submodules are connected in series as a whole, all half-bridge submodules are connected in series to form a half-bridge submodule group, and all full-bridge submodules are connected in series to form a full-bridge submodule group. Then, the half-bridge submodule groups and the full-bridge submodule groups are connected in series. The series connection order of the half-bridge submodule groups and the full-bridge submodule groups can be either half-bridge submodule groups first, followed by the full-bridge submodule groups, or vice versa.

[0080] In some embodiments, when the submodules are connected in an interleaved manner, the interleaving pattern of the full-bridge submodules and half-bridge submodules during the connection process does not specifically refer to a certain fixed form. For example, two full-bridge submodules can be connected in series through one half-bridge submodule, or two full-bridge submodules can be connected in series through two half-bridge submodules.

[0081] According to some embodiments, when both the upper and lower bridge arms include at least two sub-modules connected in series, the sub-module can be a full-bridge sub-module. The sub-module can also include both half-bridge and full-bridge sub-modules simultaneously.

[0082] In some embodiments, the full-bridge submodule includes a bidirectional current-type full-bridge submodule and a unidirectional current-type full-bridge submodule.

[0083] According to some embodiments, Figure 5 This is a schematic diagram of the structure of a half-bridge submodule provided in an embodiment of this application. Figure 5 As shown, the half-bridge submodule includes: a first switch S1, a second switch S2, a first diode D1, a second diode D2, and a first energy storage capacitor C1;

[0084] Wherein, the collector of the first switch S1 and the emitter of the second switch S2 are the first terminals of the half-bridge module, the collector of the second switch S2 is connected to the positive terminal of the first energy storage capacitor C1, and the emitter of the first switch S1 and the negative terminal of the first energy storage capacitor C1 are the second terminals of the half-bridge module.

[0085] The anode of the first diode D1 is connected to the emitter of the first switch S1, and the cathode of the first diode D1 is connected to the collector of the first switch S1. The anode of the second diode D2 is connected to the emitter of the second switch S2, and the cathode of the second diode D2 is connected to the collector of the second switch S2.

[0086] In some embodiments, Figure 6 This is a schematic diagram of the structure of a bidirectional current-mode full-bridge submodule provided in an embodiment of this application. Figure 6 As shown, the bidirectional current-type full-bridge submodule includes: a third switch S3, a fourth switch S4, a fifth switch S5, a sixth switch S6, a third diode D3, a fourth diode D4, a fifth diode D5, a sixth diode D6, and a second energy storage capacitor C2.

[0087] Among them, the emitter of the third switch S3 and the collector of the fourth switch S4 are the first terminals of the bidirectional current-type full-bridge submodule, the emitter of the fifth switch S5 and the collector of the sixth switch S6 are the second terminals of the asymmetric full-bridge submodule, the collector of the third switch S3 and the collector of the fifth switch S5 are connected to the positive terminal of the second energy storage capacitor C2, and the emitter of the fourth switch S4 and the emitter of the sixth switch S6 are connected to the negative terminal of the second energy storage capacitor C2.

[0088] The anode of the third diode D3 is connected to the emitter of the third switch S3, and the cathode of the third diode D3 is connected to the collector of the third switch S3. The anode of the fourth diode D4 is connected to the emitter of the fourth switch S4, and the cathode of the fourth diode D4 is connected to the collector of the fourth switch S4. The anode of the fifth diode D5 is connected to the emitter of the fifth switch S5, and the cathode of the fifth diode D5 is connected to the collector of the fifth switch S5. The anode of the sixth diode D6 is connected to the emitter of the sixth switch S6, and the cathode of the sixth diode D6 is connected to the collector of the sixth switch S6.

[0089] In some embodiments, when the full-bridge submodule includes four switches, the full-bridge submodule is a bidirectional current-mode full-bridge submodule. This bidirectional current-mode full-bridge submodule can be converted to a special full-bridge submodule topology by removing at least one switch. For example, by simultaneously removing the third switch S3 and the sixth switch S6, or simultaneously removing the fourth switch S4 and the fifth switch S5, the bidirectional current-mode full-bridge submodule can be converted to a unidirectional current-mode full-bridge submodule, such as... Figure 7 As shown.

[0090] According to some embodiments, the switches provided in this application, such as the first switch S1, the second switch S2, the third switch S3, the fourth switch S4, the fifth switch S5, and the sixth switch S6, do not specifically refer to a certain fixed type of switch. The types of switches include, but are not limited to, bipolar junction transistors (BJTs), gate turn-off thyristors (GTOs), insulated gate bipolar transistors (IGBTs), integrated gate commutated thyristors (IGCTs), and metal-oxide-semiconductor field-effect transistors (MOSFETs), etc.

[0091] In summary, the offshore wind power DC transmission system proposed in this application includes: a diode rectifier module, an offshore DC voltage variable converter module, and an onshore DC voltage variable converter. The AC sides of the diode rectifier module and the offshore DC voltage variable converter module are connected to the offshore wind farm. The DC sides of the diode rectifier module and the offshore DC voltage variable converter module are connected in series and then connected to the DC side of the onshore DC voltage variable converter. The AC side of the onshore DC voltage variable converter is connected to the onshore AC power grid. By employing a series connection of the diode rectifier module and the offshore DC voltage variable converter module, this application provides an offshore wind power DC transmission system with low output cost, high transmission efficiency, and good transmission effect.

[0092] To achieve the above embodiments, this application also proposes a startup method for an offshore wind power DC transmission system.

[0093] Figure 8 A flowchart illustrating a startup method for an offshore wind power DC transmission system provided in this application embodiment.

[0094] like Figure 8As shown, a startup method for an offshore wind power DC transmission system includes:

[0095] S110 utilizes the negative voltage output function of the offshore DC voltage variable converter module and the onshore DC voltage variable converter to complete the black start of the first offshore wind turbine assembly.

[0096] S120 utilizes the AC output voltage controllability of the offshore DC voltage variable converter module and the low-voltage ride-through capability of the offshore wind turbine to enable the diode rectifier module to connect to the offshore AC power grid by raising the AC voltage from the initial voltage according to a preset rise rate.

[0097] S130 connects all offshore wind turbines except the first offshore wind turbine assembly to the offshore AC grid by controlling the offshore DC voltage variable converter module and the onshore DC voltage variable converter, thereby completing the startup of the offshore wind power DC transmission system.

[0098] In this embodiment, the offshore DC voltage variable converter module includes an offshore DC voltage variable auxiliary MMC. The offshore DC voltage variable auxiliary MMC includes at least two sub-modules. Utilizing the negative voltage output function of the offshore DC voltage variable converter module and the onshore DC voltage variable converter, the black start of the first offshore wind turbine assembly is completed, including:

[0099] Determine the target value of DC voltage V dc1 And based on the target value V of the DC voltage dc1 The onshore DC voltage variable converter is controlled to operate in constant DC voltage mode. The target DC voltage value is determined based on the preset start-up voltage value of the submodule capacitor voltage of the offshore DC voltage variable auxiliary MMC.

[0100] The sub-modules of the marine DC voltage variable auxiliary MMC are put into operation in sequence, and the capacitors of the sub-modules are charged by the onshore DC voltage variable converter until the capacitor voltages of all sub-modules reach the preset start-up voltage value, so as to complete the start-up of the marine DC voltage variable auxiliary MMC.

[0101] Determine the first DC current target value, and control the onshore DC voltage variable converter to operate in constant DC current mode according to the first DC current target value. The first DC current target value is the rated DC current of the offshore wind power DC transmission system.

[0102] The variable DC voltage auxiliary MMC is used to provide AC voltage to the first offshore wind turbine assembly to complete the black start of the first offshore wind turbine assembly.

[0103] According to some embodiments, the DC side of the diode rectifier module and the DC side of the offshore DC voltage variable converter module are connected in series and can be connected to the DC side of the onshore DC voltage variable converter via a DC cable. When the submodule capacitor in the offshore DC voltage variable auxiliary MMC is charged using the onshore DC voltage variable converter, the submodule capacitor can be charged by the current supplied through the DC cable.

[0104] According to some embodiments, the target value of the DC voltage V dc1 It does not refer to a specific fixed value. For example, the target DC voltage value V dc1 It can be -200kV; the target DC voltage value V dc1 It can also be -300kV.

[0105] In some embodiments, the preset start-up voltage value does not specifically refer to a fixed voltage value. The preset start-up voltage value can change when a modification command for the rated preset start-up voltage value is received. For example, the preset start-up voltage value can be 1500V. The preset start-up voltage value can also be 1600V.

[0106] According to some embodiments, the offshore DC voltage variable converter module also includes a second AC switch K2. When the offshore DC voltage variable auxiliary MMC is used to provide AC voltage to the offshore AC power grid, the offshore DC voltage variable auxiliary MMC provides AC voltage to the offshore AC power grid by closing the second AC switch K2.

[0107] In some embodiments, when performing a black start on the first offshore wind turbine assembly, the offshore wind turbines in the first offshore wind turbine assembly can be connected to the offshore AC power grid in sequence, thereby completing the black start and grid connection of the first offshore wind turbine assembly.

[0108] In this embodiment of the application, before using the offshore DC voltage variable auxiliary MMC to provide AC voltage to the offshore AC grid to complete the black start of the first offshore wind turbine assembly, the method further includes:

[0109] The AC side of the marine DC voltage variable auxiliary MMC is set to operate in constant AC voltage mode, so that the AC voltage output by the marine DC voltage variable auxiliary MMC is AC voltage with rated amplitude and rated frequency.

[0110] The DC side of the marine DC voltage variable auxiliary MMC is set to operate in stator module capacitor voltage mode. By controlling the DC output voltage of the marine DC voltage variable auxiliary MMC, the active power absorbed or output by the DC side of the marine DC voltage variable auxiliary MMC is controlled so that the sub-module capacitor voltage corresponding to the marine DC voltage variable auxiliary MMC is the rated voltage value.

[0111] According to some embodiments, the offshore DC voltage variable converter module further includes an offshore DC voltage variable auxiliary MMC AC side controller and an offshore DC voltage variable auxiliary MMC DC side controller. Operating the offshore DC voltage variable auxiliary MMC AC side controller in a constant AC voltage control mode allows the AC side of the offshore DC voltage variable auxiliary MMC to operate in a constant AC voltage mode. Operating the offshore DC voltage variable auxiliary MMC DC side controller in a stator module capacitor voltage control mode allows the DC side of the offshore DC voltage variable auxiliary MMC to operate in a stator module capacitor voltage mode.

[0112] In this embodiment of the application, before determining the target value of the DC current and controlling the onshore DC voltage variable converter to operate in constant DC current mode according to the target value of the DC current, the method further includes:

[0113] The onshore DC voltage variable converter is started up by using the onshore AC power grid.

[0114] According to some embodiments, when the AC side of the onshore DC voltage variable converter is connected to the onshore AC power grid, it can be connected to the onshore AC power grid via a third AC switch. Therefore, when starting up the onshore DC voltage variable converter via the onshore AC power grid, the startup of the onshore DC voltage variable converter can be completed by closing the third AC switch via the onshore AC power grid.

[0115] In this embodiment, the diode rectifier module includes a first AC switch K1, an offshore rectifier transformer, and an offshore diode rectifier. Utilizing the controllable AC output voltage function of the offshore DC voltage variable converter module and the low-voltage ride-through function of the offshore wind turbine, the offshore DC voltage variable converter module includes an offshore DC voltage variable auxiliary MMC. The offshore DC voltage variable auxiliary MMC includes at least two sub-modules. The diode rectifier module is connected to the offshore AC grid by gradually increasing the AC voltage from an initial voltage at a preset rate. This includes:

[0116] Determine the second DC current target value, and control the onshore DC voltage variable converter to operate in constant DC current mode according to the second DC current target value. The second DC current target value is determined based on the capacity of the offshore DC voltage variable auxiliary MMC and the offshore diode rectifier.

[0117] Control the AC voltage output of the marine DC voltage variable converter module to drop to the initial voltage;

[0118] Close the first AC switch to connect the offshore rectifier transformer and the offshore diode rectifier to the offshore AC power grid;

[0119] The AC voltage output of the marine DC voltage variable converter module is controlled to rise from a preset voltage threshold at a preset rise rate until the rated AC voltage amplitude is reached.

[0120] According to some embodiments, the second DC current target value is less than the target value of the rated DC current. This second DC current target value is not specifically a fixed value. For example, the second DC current target value can be 700A; the second DC current target value can also be 800A.

[0121] According to some embodiments, controlling the onshore DC voltage variable converter to operate in constant DC current mode based on the second DC current target value can ensure the balance between the active power of the offshore wind farm, the active power of the offshore DC voltage variable auxiliary MMC, and the active power of the onshore DC voltage variable converter.

[0122] According to some embodiments, the initial voltage does not specifically refer to a fixed voltage. For example, the voltage range of the initial voltage can be 0.2 pu to 0.5 pu. The initial voltage can be 13 kV; the initial voltage can also be 20 kV.

[0123] In some embodiments, the preset lifting rate does not specifically refer to a fixed rate. For example, the preset lifting rate may be no less than 0.5 pu / s.

[0124] In this embodiment, the offshore DC voltage variable converter module includes an offshore DC voltage variable auxiliary MMC, which controls the offshore DC voltage variable converter module and the onshore DC voltage variable converter, connecting the offshore wind turbines in the offshore wind farm, excluding the first offshore wind turbine assembly, to the offshore AC power grid, including:

[0125] The initial value, increase rate, and final value of the third DC current target value are determined, and the onshore DC voltage variable converter is controlled to operate in constant DC current mode based on the third DC current target value. The initial value of the third DC current target value is determined based on the capacity of the offshore DC voltage variable auxiliary MMC and the offshore diode rectifier. The increase rate of the third DC current target value is determined based on the connection rate of the offshore wind turbines (excluding the first offshore wind turbine set) to the offshore AC grid. The final value of the third DC current target value is the rated DC current of the offshore wind power DC transmission system.

[0126] According to some embodiments, the third DC current target value is less than the target value of the rated DC current. This third DC current target value is not specifically a fixed value. For example, the third DC current target value can be 700A; the third DC current target value can also be 800A.

[0127] According to some embodiments, the rate of increase of the third DC current target value is not specifically a fixed rate. For example, the rate could be 2.5 pu / s.

[0128] According to some embodiments, Figure 9 This application provides a control block diagram for a marine DC voltage variable auxiliary MMC during startup, as illustrated in an embodiment. Figure 9 As shown, when controlling the marine DC voltage variable auxiliary MMC, an AC reference voltage can be generated by an AC reference voltage generation module, a DC reference voltage can be generated by a capacitor voltage balancing module, and a trigger pulse signal can be generated through a modulation stage. Finally, the marine DC voltage variable auxiliary MMC can be controlled according to the trigger pulse signal.

[0129] In summary, the method proposed in this application, firstly, by utilizing the negative voltage output function of the offshore DC voltage variable converter module and the onshore DC voltage variable converter, allows the offshore wind power DC transmission system to operate in a forward current, reverse voltage state. This enables the transmission of active power from the shore to the sea, providing black-start power for the offshore wind turbines and thus completing the black start of the first offshore wind turbine assembly. Secondly, by utilizing the controllable AC output voltage function of the offshore DC voltage variable converter module and the low-voltage ride-through function of the offshore wind turbines, the diode rectifier module is connected to the offshore AC grid by gradually increasing the AC voltage from the initial voltage at a preset rate. This avoids the impact problem caused by the inrush current during rectifier transformer switching and allows the DC output voltage of the offshore diode rectifier to rise slowly, preventing the impact on the DC system caused by sudden step changes in the output DC voltage of the diode rectifier. Finally, during the startup process, by controlling the offshore DC voltage variable converter module and the onshore DC voltage variable converter, the active power and DC transmission power of the offshore wind farm can be balanced. Then, the offshore wind turbines except for the first offshore wind turbine set can be connected to the offshore AC grid to complete the startup of the offshore wind power DC transmission system. The startup process is smooth and without shock.

[0130] To achieve the above embodiments, this application also proposes a starting device for an offshore wind power DC transmission system.

[0131] Figure 10 This is a schematic diagram of the starting device of an offshore wind power DC transmission system provided in an embodiment of this application.

[0132] like Figure 10 As shown, a starting device 100 for an offshore wind power DC transmission system includes:

[0133] The offshore DC voltage variable converter module and part of the offshore wind turbine assembly start-up unit 101 are used to complete the black start of the first offshore wind turbine assembly by utilizing the negative voltage output function of the offshore DC voltage variable converter module and the onshore DC voltage variable converter.

[0134] The diode rectifier access unit 102 is used to utilize the AC output voltage controllable function of the offshore DC voltage variable converter module and the low voltage ride-through function of the offshore wind turbine to enable the diode rectifier module to connect to the offshore AC power grid in a manner that the AC voltage is raised from the initial voltage according to a preset rise rate.

[0135] The remaining offshore wind turbine assembly start-up unit 103 is used to connect the offshore wind turbines in the offshore wind farm, except for the first offshore wind turbine assembly, to the offshore AC power grid by controlling the offshore DC voltage variable converter module and the onshore DC voltage variable converter, so as to complete the start-up of the offshore wind power DC transmission system.

[0136] In summary, the apparatus proposed in this application, firstly, utilizes the negative voltage output function of the offshore DC voltage variable converter module and the onshore DC voltage variable converter to complete the black start of the first offshore wind turbine assembly. This allows the offshore wind power DC transmission system to operate in a forward current, reverse voltage state, thereby enabling the transmission of active power from the shore to the sea and providing black start power for the offshore wind turbines, thus completing the black start of the first offshore wind turbine assembly. Secondly, through the diode rectifier access unit, utilizing the controllable AC output voltage function of the offshore DC voltage variable converter module and the low-voltage ride-through function of the offshore wind turbines, the diode rectifier module is connected to the offshore AC grid by gradually increasing the AC voltage from the initial voltage at a preset rate. This avoids the impact problem caused by the inrush current of the rectifier transformer during closing and allows the DC output voltage of the offshore diode rectifier to also increase slowly, avoiding the impact on the DC system caused by sudden step changes in the DC output voltage of the diode rectifier. Finally, during the startup process, the remaining offshore wind turbine assembly startup unit controls the offshore DC voltage variable converter module and the onshore DC voltage variable converter to connect the offshore wind turbines (excluding the first offshore wind turbine assembly) to the offshore AC grid, thereby completing the startup of the offshore wind power DC transmission system. This achieves a balance between the active power and DC transmission power of the offshore wind farm, and allows the offshore wind turbines (excluding the first offshore wind turbine assembly) to be connected to the offshore AC grid, thus completing the startup of the offshore wind power DC transmission system. The startup process is smooth and shock-free.

[0137] It should be noted that in the description of this application, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance. Furthermore, in the description of this application, unless otherwise stated, "a plurality of" means two or more.

[0138] Any process or method described in the flowchart or otherwise herein can be understood as representing a module, segment, or portion of code comprising one or more executable instructions for implementing a particular logical function or process, and the scope of the preferred embodiments of this application includes additional implementations in which functions may be performed not in the order shown or discussed, including substantially simultaneously or in reverse order depending on the function involved, as will be understood by those skilled in the art to which embodiments of this application pertain.

[0139] It should be understood that various parts of this application can be implemented using hardware, software, firmware, or a combination thereof. In the above embodiments, multiple steps or methods can be implemented using software or firmware stored in memory and executed by a suitable instruction execution system. For example, if implemented in hardware, as in another embodiment, it can be implemented using any one or a combination of the following techniques known in the art: discrete logic circuits having logic gates for implementing logical functions on data signals, application-specific integrated circuits (ASICs) having suitable combinational logic gates, programmable gate arrays (PGAs), field-programmable gate arrays (FPGAs), etc.

[0140] Those skilled in the art will understand that all or part of the steps of the methods in the above embodiments can be implemented by a program instructing related hardware. The program can be stored in a computer-readable storage medium, and when executed, the program includes one or a combination of the steps of the method embodiments.

[0141] Furthermore, the functional units in the various embodiments of this application can be integrated into a processing module, or each unit can exist physically separately, or two or more units can be integrated into a module. The integrated module can be implemented in hardware or as a software functional module. If the integrated module is implemented as a software functional module and sold or used as an independent product, it can also be stored in a computer-readable storage medium.

[0142] The storage media mentioned above can be read-only memory, disk, or optical disk, etc.

[0143] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0144] Although embodiments of this application have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting this application. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of this application.

Claims

1. A startup method for an offshore wind power DC transmission system, characterized in that, include: The black start of the first offshore wind turbine assembly is achieved by utilizing the negative voltage output function of the offshore variable DC voltage converter module and the onshore variable DC voltage converter. The offshore variable DC voltage converter module includes an offshore variable DC voltage auxiliary MMC, which comprises at least two sub-modules. The black start of the first offshore wind turbine assembly by utilizing the negative voltage output function of the offshore and onshore variable DC voltage converters includes determining a target DC voltage value and controlling the onshore variable DC voltage converter to operate in a constant DC voltage mode based on the target DC voltage value. The target DC voltage value is determined by the capacitor values ​​of the sub-modules of the offshore variable DC voltage auxiliary MMC. The preset start-up voltage value is determined; the sub-modules of the offshore DC voltage variable auxiliary MMC are sequentially activated, and the negative voltage output of the onshore DC voltage variable converter is used to charge the capacitors of the sub-modules until the capacitor voltages of all sub-modules reach the preset start-up voltage value, thereby completing the start-up of the offshore DC voltage variable auxiliary MMC; a first DC current target value is determined, and the onshore DC voltage variable converter is controlled to operate in constant DC current mode according to the first DC current target value, wherein the first DC current target value is the rated DC current of the offshore wind power DC transmission system; the offshore DC voltage variable auxiliary MMC is used to provide AC voltage to the first offshore wind turbine assembly, thereby completing the black start of the first offshore wind turbine assembly; By utilizing the controllable AC output voltage function of the offshore DC voltage variable converter module and the low-voltage ride-through function of the offshore wind turbine, the diode rectifier module is connected to the offshore AC power grid in a manner that the AC voltage is raised from the initial voltage according to a preset rise rate. By controlling the offshore DC voltage variable converter module and the onshore DC voltage variable converter, the offshore wind turbines in the offshore wind farm, except for the first offshore wind turbine set, are connected to the offshore AC power grid to complete the startup of the offshore wind power DC transmission system.

2. The startup method as described in claim 1, characterized in that, Before utilizing the offshore DC voltage variable auxiliary MMC to provide AC voltage to the offshore AC grid to complete the black start of the first offshore wind turbine assembly, the method further includes: The AC side of the marine DC voltage variable auxiliary MMC is set to operate in constant AC voltage mode, so that the AC voltage output by the marine DC voltage variable auxiliary MMC is an AC voltage with rated amplitude and rated frequency. The DC side of the marine DC voltage variable auxiliary MMC is set to operate in stator module capacitor voltage mode. By controlling the DC output voltage of the marine DC voltage variable auxiliary MMC, the DC side of the marine DC voltage variable auxiliary MMC is controlled to absorb or output active power, so that the sub-module capacitor voltage corresponding to the marine DC voltage variable auxiliary MMC is the rated voltage value.

3. The startup method as described in claim 1, characterized in that, Before determining the target DC voltage value and controlling the onshore DC voltage variable converter to operate in constant DC voltage mode based on the target DC voltage value, the method further includes: The onshore DC voltage variable converter is started up via the onshore AC power grid.

4. The startup method as described in claim 1, characterized in that, The diode rectifier module includes a first AC switch, an offshore rectifier transformer, and an offshore diode rectifier. The offshore DC voltage variable converter module includes an offshore DC voltage variable auxiliary MMC, which includes at least two sub-modules. The method of utilizing the controllable AC output voltage function of the offshore DC voltage variable converter module and the low-voltage ride-through function of the offshore wind turbine to allow the diode rectifier module to connect to the offshore AC grid by gradually increasing the AC voltage from an initial voltage at a preset rate includes: A second DC current target value is determined, and the onshore DC voltage variable converter is controlled to operate in constant DC current mode according to the second DC current target value. The second DC current target value is determined based on the capacity of the offshore DC voltage variable auxiliary MMC and the offshore diode rectifier. Control the AC voltage output of the marine DC voltage variable converter module to drop to the initial voltage; Close the first AC switch to connect the marine rectifier transformer and the marine diode rectifier to the marine AC power grid; The AC voltage output by the marine DC voltage variable converter module is controlled to rise from a preset voltage threshold at a preset rise rate until the rated AC voltage amplitude is reached.

5. The startup method as described in claim 4, characterized in that, The offshore DC voltage variable converter module includes an offshore DC voltage variable auxiliary MMC. Controlling the offshore DC voltage variable converter module and the onshore DC voltage variable converter to connect the offshore wind turbines (excluding the first offshore wind turbine assembly) in the offshore wind farm to the offshore AC power grid includes: The initial value, increase rate, and final value of the third DC current target value are determined, and the onshore DC voltage variable converter is controlled to operate in constant DC current mode according to the third DC current target value. The initial value of the third DC current target value is determined based on the capacity of the offshore DC voltage variable auxiliary MMC and the offshore diode rectifier. The increase rate of the third DC current target value is determined based on the connection rate of the offshore wind turbines (excluding the first offshore wind turbine set) to the offshore AC grid. The final value of the third DC current target value is the rated DC current of the offshore wind power DC transmission system.

6. A DC transmission device for offshore wind power, characterized in that, include: The offshore DC voltage variable converter module and a partial offshore wind turbine assembly starting unit are used to complete the black start of a first offshore wind turbine assembly by utilizing the negative voltage output function of the offshore DC voltage variable converter module and the onshore DC voltage variable converter. The offshore DC voltage variable converter module includes an offshore DC voltage variable auxiliary MMC, which includes at least two sub-modules. The black start of the first offshore wind turbine assembly by utilizing the negative voltage output function of the offshore DC voltage variable converter module and the onshore DC voltage variable converter includes determining a DC voltage target value and controlling the onshore DC voltage variable converter to operate in a constant DC voltage mode according to the DC voltage target value. The DC voltage target value is determined based on the offshore DC voltage. The preset start-up voltage value of the submodule capacitor voltage of the variable DC voltage auxiliary MMC is determined; the submodules of the offshore DC voltage variable auxiliary MMC are sequentially put into operation, and the negative voltage output of the onshore DC voltage variable converter is used to charge the submodule capacitors until the capacitor voltage of all submodules reaches the preset start-up voltage value, so as to complete the start-up of the offshore DC voltage variable auxiliary MMC; a first DC current target value is determined, and the onshore DC voltage variable converter is controlled to operate in constant DC current mode according to the first DC current target value, wherein the first DC current target value is the rated DC current of the offshore wind power DC transmission system; the offshore DC voltage variable auxiliary MMC is used to provide AC voltage to the first offshore wind turbine assembly to complete the black start of the first offshore wind turbine assembly; The diode rectifier access unit is used to utilize the AC output voltage controllable function of the offshore DC voltage variable converter module and the low voltage ride-through function of the offshore wind turbine to enable the diode rectifier module to connect to the offshore AC power grid in a manner that the AC voltage is raised from the initial voltage according to a preset rise rate. The remaining offshore wind turbine assembly start-up unit is used to control the offshore DC voltage variable converter module and the onshore DC voltage variable converter to connect the offshore wind turbines (excluding the first offshore wind turbine assembly) in the offshore wind farm to the offshore AC power grid, thereby completing the start-up of the offshore wind power DC transmission system.

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