Offshore wind power delivery system and black start method thereof

Abstract

The invention discloses an offshore wind power delivery system and a black start method thereof, and relates to the field of offshore wind power transmission, and the black start method comprises the steps: charging an onshore MMC through an onshore power grid to unlock the control of the onshore MMC; closing the bypass switch, controlling the land MMC to charge the offshore MMC so as to unlock the control on the offshore MMC, and starting the fan; the DC modulation ratio of the offshore MMC is controlled to adjust the DC flowing through the offshore MMC, and when the DC flowing through the offshore MMC is zero, the bypass switch is switched off to enable the offshore MMC to absorb surplus power generated by starting of the fan; the DC modulation ratio of the offshore MMC is controlled to maintain the DC voltage, and the PCC voltage amplitude is reduced to a first reference value, so that inrush current is prevented from being input into a converter valve; and a converter valve is put in, the output power of the offshore wind power plant is increased, and black start of the offshore wind power sending-out system is completed. According to the invention, the black start of the offshore wind power delivery system can be completed on the premise of avoiding extra cost and high control difficulty.

Description

Technical Field

[0001] This application relates to the field of offshore wind power transmission, and in particular to an offshore wind power transmission system and its black start method. Background Technology

[0002] Currently, in offshore wind power transmission schemes, the scheme based on the main valve of a diode rectifier unit (DRU) and the auxiliary valve of a small-capacity modular multilevel converter (MMC) in series is widely used because it can significantly reduce the size, weight, and cost of offshore converter valves and eliminates the need for grid-type wind turbines. However, due to the inherent unidirectional uncontrollability of the DRU main valve, this scheme hinders the return of system energy and cannot meet the black start requirements of offshore wind power systems. Therefore, it is necessary to study how to achieve black start of the system under this scheme to meet actual operational needs.

[0003] Existing solutions primarily achieve black start by adding auxiliary equipment or utilizing the low-voltage ride-through (LVRT) characteristics of wind turbines. However, all have varying degrees of drawbacks: In the solution of adding a bypass circuit breaker and thyristors as the starting circuit on the DC side of the DRU, there is an overvoltage risk in the offshore MMC, and high withstand voltage requirements are placed on the thyristors; in the solution of using full-bridge MMCs on both land and sea, utilizing negative voltage for black start, there is an overvoltage risk in the offshore MMC during wind turbine startup; in the solution of configuring energy storage in the submodule of the DC energy-consuming device to achieve black start, the energy storage device is costly and difficult to maintain; in the solution of using a parallel bypass on the DC side of the DRU to activate the DRU using the wind turbine's LVRT characteristics, the wind turbine's LVRT time is limited, the black start operation sequence corresponding to meeting the fault ride-through time requirements is complex, and the engineering feasibility is low. All of the above existing methods require the addition of corresponding auxiliary equipment, leading to additional costs. Furthermore, the design of the black start method relies on auxiliary equipment, making control complex, and neither the cost nor the control difficulty meets the actual operational requirements. Therefore, how to design a black start method for offshore wind power transmission systems to reduce system costs and control difficulties remains a pressing technical problem to be solved in the existing technology. Summary of the Invention

[0004] This application provides an offshore wind power transmission system and its black start method to solve the technical problems of additional cost and excessive control difficulty in existing offshore wind power systems.

[0005] According to a first aspect of the embodiments of this application, a black start method for an offshore wind power transmission system is provided, which is applied to the offshore wind power transmission system, the offshore wind power transmission system including an onshore transmission end and an offshore power generation end; the onshore transmission end includes an onshore power grid and an onshore MMC; the offshore power generation end includes a first converter valve, a second converter valve, an offshore MMC, a first bypass switch, a second bypass switch and an offshore wind farm; The first end of the first converter valve is connected to the first end of the onshore MMC, and the second end of the first converter valve is connected to the first end of the offshore MMC; the first end of the second converter valve is connected to the second end of the offshore MMC, and the second end of the second converter valve is connected to the second end of the onshore MMC; the third end of the first converter valve is connected to the offshore wind farm; the third end of the second converter valve is connected to the offshore wind farm; and the third end of the offshore MMC is connected to the offshore wind farm. The first terminal of the first bypass switch is connected to the first terminal of the first converter valve, and the second terminal of the first bypass switch is connected to the second terminal of the first converter valve; the first terminal of the second bypass switch is connected to the first terminal of the second converter valve, and the second terminal of the second bypass switch is connected to the second terminal of the second converter valve. The black start method includes: Charge the terrestrial MMC via the terrestrial power grid to unlock control of the terrestrial MMC; Close the first and second bypass switches to control the onshore MMC to charge the offshore MMC, thereby unlocking control of the offshore MMC and starting the wind turbines in the offshore wind farm at a preset ratio. Control the DC modulation ratio of the offshore MMC to regulate the DC current flowing through the offshore MMC, and disconnect the first bypass switch and the second bypass switch when the DC current flowing through the offshore MMC is zero, so that the offshore MMC can continuously absorb the surplus power generated by the wind turbine startup. Control the DC modulation ratio of the marine MMC to maintain the DC voltage of the marine MMC at the rated value and reduce the PCC voltage amplitude to the first reference value to prevent the converter valve from engaging in surge current. By engaging the first and second converter valves, the output power of the offshore wind farm is gradually increased, thus completing the black start of the offshore wind power transmission system.

[0006] This application first charges the onshore MMC via the onshore power grid, unlocking control of the onshore MMC. Then, it closes the bypass switch to control the onshore MMC to charge the offshore MMC, unlocking control of the offshore MMC and starting some wind turbines. Next, by controlling the DC modulation ratio of the offshore MMC, it first disconnects the bypass switch to allow the offshore MMC to absorb the surplus power generated by the wind turbine startup. Then, it maintains the DC voltage of the offshore MMC and reduces the PCC voltage amplitude to prevent the converter valve from engaging inrush current. Finally, it engages the converter valve to increase the output power of the offshore wind farm, completing the system's black start. Compared to existing technologies, this application features a special design for the system's connection and control process. The design enables black start of the system without adding auxiliary equipment, avoiding the additional costs associated with adding extra equipment. Furthermore, the absence of auxiliary equipment avoids the excessive control complexity that would result from adding additional equipment. Simultaneously, through a special design of the system's control flow, the DC modulation ratio of the offshore MMC is controlled twice. First, the offshore MMC absorbs the surplus power from the wind turbine startup. Then, by maintaining the DC voltage of the offshore MMC and reducing the voltage amplitude of the PCC, inrush current is prevented from engaging the converter valve. This avoids potential hazards caused by surplus power or inrush current in the system when the converter valve is engaged, ensuring the stability and feasibility of the system's black start.

[0007] In some embodiments of this application, the step of charging the terrestrial MMC via the terrestrial power grid to unlock control of the terrestrial MMC specifically includes: Uncontrolled charging is performed on each submodule of the onshore MMC via the onshore power grid, and the onshore MMC submodule controller is started after the uncontrolled charging of each submodule of the onshore MMC is completed. By gradually controlling each submodule of the onshore MMC to bypass and disconnect it, the DC voltage of the onshore MMC is made equal to the rated DC voltage of the offshore MMC, thus unlocking control of the onshore MMC.

[0008] This application first performs uncontrolled charging of each submodule of the onshore MMC through the onshore power grid. After the uncontrolled charging is completed, the onshore MMC submodule controller is started, and then the onshore MMC submodules are gradually controlled to bypass. The control of the onshore MMC is unlocked by controlling the DC voltage of the onshore MMC, so as to provide an implementation basis for charging the marine MMC through the onshore MMC in the future, and further provide a basis for the black start of the system.

[0009] In some embodiments of this application, the onshore MMC is a hybrid full-bridge and half-bridge MMC, comprising multiple full-bridge submodules and multiple half-bridge submodules; the step of activating the onshore MMC submodule controller after the uncontrolled charging of each submodule of the onshore MMC is completed specifically includes: After the uncontrolled charging of each submodule of the onshore MMC is completed, the multiple full-bridge submodules are driven to operate in half-bridge mode, thereby raising the capacitor voltage of each submodule of the onshore MMC to start the onshore MMC submodule controller; wherein, at the end of the uncontrolled charging, the capacitor voltage of the full-bridge submodule is twice the capacitor voltage of the half-bridge submodule.

[0010] After the uncontrolled charging of the onshore MMC is completed, this application drives multiple full-bridge submodules of the onshore MMC to operate in half-bridge mode, which can raise the capacitor voltage of each submodule of the onshore MMC, thereby starting the onshore MMC submodule controller, providing a basis for subsequent charging of the marine MMC through the onshore MMC, and further enabling the system's black start.

[0011] In some embodiments of this application, controlling the onshore MMC to charge the offshore MMC in order to unlock control of the offshore MMC and start a preset proportion of wind turbines in the offshore wind farm, specifically including: Control the land-based MMC to supply power to the marine MMC so as to perform uncontrolled charging of each submodule of the marine MMC, and start the marine MMC submodule controller after the uncontrolled charging of each submodule of the marine MMC is completed. By gradually controlling each sub-module of the offshore MMC to bypass and disconnect, the DC voltage of the offshore MMC is controlled to be equal to the rated value of the DC voltage of the offshore MMC, thus unlocking the control of the offshore MMC. Control the onshore power grid, onshore MMC and offshore MMC to supply power to the offshore wind farm and start the wind turbines in the offshore wind farm; when starting the wind turbines, start them at a preset ratio to prevent overcurrent.

[0012] This application first controls the onshore MMC to perform uncontrolled charging of each submodule of the offshore MMC. After the uncontrolled charging is completed, the offshore MMC submodule controller is started, and then the offshore MMC submodules are gradually controlled to bypass. By controlling the DC voltage of the offshore MMC, the control of the offshore MMC is unlocked, thereby controlling the onshore power grid, the onshore MMC and the offshore MMC to supply power to the offshore wind farm to start the wind turbines. This provides a basis for the subsequent absorption of surplus power and the activation of the converter valve, and further provides a basis for the implementation of the system's black start.

[0013] In some embodiments of this application, controlling the DC modulation ratio of the offshore MMC to adjust the DC current flowing through the offshore MMC, and disconnecting the first bypass switch and the second bypass switch when the DC current flowing through the offshore MMC is zero, so that the offshore MMC can continuously absorb the surplus power generated by the wind turbine startup, specifically includes: The DC modulation ratio of the marine MMC is controlled by a preset DC current controller to reduce the DC current flowing through the marine MMC and enable the marine MMC to absorb the first surplus power; wherein, the DC current controller is obtained by dynamic modeling of the DC side of the marine MMC; When the DC current flowing through the offshore MMC is zero, the first bypass switch and the second bypass switch are disconnected, and the DC voltage of the onshore MMC is increased to the rated value so that the offshore MMC can absorb the second surplus power; wherein, the first surplus power and the second surplus power are both generated by the wind turbine startup.

[0014] This application first controls the DC modulation ratio of the offshore MMC through a DC current controller, which reduces the DC current flowing through the offshore MMC, allowing the offshore MMC to absorb the first surplus power. Then, when the current flowing through the offshore MMC is zero, the bypass switch is disconnected and the DC voltage of the onshore MMC is increased, allowing the offshore MMC to absorb the second surplus power. This avoids the potential risks caused by the system's surplus power when the converter valve is put into operation, thereby ensuring the stability and feasibility of the subsequent black start of the system.

[0015] In some embodiments of this application, controlling the DC modulation ratio of the marine MMC, maintaining the DC voltage of the marine MMC at a rated value, and reducing the PCC voltage amplitude to a first reference value to prevent the converter valve from engaging in surge current, specifically includes: The DC modulation ratio of the marine MMC is controlled by a preset DC voltage controller to maintain the DC voltage of the marine MMC at the rated value. Based on the relationship between the DC voltage of the converter valve and the DC voltage of the marine MMC, a first reference value for the PCC voltage amplitude is determined when the converter valve is engaged, and the PCC voltage amplitude is controlled to decrease to the first reference value to prevent the converter valve from engaging in surge current; wherein, the relationship is determined based on the external characteristics of the converter valve.

[0016] This application first controls the DC modulation ratio of the marine MMC through a DC voltage controller, which can maintain the DC voltage of the marine MMC. Then, the relationship between the DC voltage of the converter valve and the DC voltage of the marine MMC determines the first reference value of the PCC voltage amplitude when the converter valve is engaged. By using the first reference value, inrush current is prevented when the converter valve is engaged, thus avoiding the potential hazards caused by inrush current when the converter valve is engaged in subsequent operations, thereby ensuring the stability and feasibility of black start of the subsequent system.

[0017] In some embodiments of this application, the activation of the first and second converter valves to gradually increase the output power of the offshore wind farm and complete the black start of the offshore wind power transmission system specifically includes: The first and second converter valves are engaged to gradually increase the output power of the offshore wind farm and restore the DC modulation ratio of the offshore MMC so that the PCC voltage amplitude gradually increases from the first reference value along with the output power, thus completing the black start of the offshore wind power transmission system.

[0018] This application first activates the converter valve to gradually increase the output power of the offshore wind farm and restores the DC modulation ratio of the offshore MMC. This enables the PCC voltage amplitude to gradually increase with the output power of the offshore wind farm, ensuring normal system operation conditions and thus guaranteeing the smooth completion of the system's black start.

[0019] According to a second aspect of the embodiments of this application, an offshore wind power transmission system is provided, including an onshore transmission end and an offshore power generation end; the onshore transmission end includes an onshore power grid and an onshore MMC; the offshore power generation end includes a first converter valve, a second converter valve, an offshore MMC, a first bypass switch, a second bypass switch, and an offshore wind farm; The first end of the first converter valve is connected to the first end of the onshore MMC, and the second end of the first converter valve is connected to the first end of the offshore MMC; the first end of the second converter valve is connected to the second end of the offshore MMC, and the second end of the second converter valve is connected to the second end of the onshore MMC; the third end of the first converter valve is connected to the offshore wind farm; the third end of the second converter valve is connected to the offshore wind farm; and the third end of the offshore MMC is connected to the offshore wind farm. The first terminal of the first bypass switch is connected to the first terminal of the first converter valve, and the second terminal of the first bypass switch is connected to the second terminal of the first converter valve; the first terminal of the second bypass switch is connected to the first terminal of the second converter valve, and the second terminal of the second bypass switch is connected to the second terminal of the second converter valve. The offshore wind power transmission system is used to execute a black start method, which includes: Charge the terrestrial MMC via the terrestrial power grid to unlock control of the terrestrial MMC; Close the first and second bypass switches to control the onshore MMC to charge the offshore MMC, thereby unlocking control of the offshore MMC and starting the wind turbines in the offshore wind farm at a preset ratio. Control the DC modulation ratio of the offshore MMC to regulate the DC current flowing through the offshore MMC, and disconnect the first bypass switch and the second bypass switch when the DC current flowing through the offshore MMC is zero, so that the offshore MMC can continuously absorb the surplus power generated by the wind turbine startup. Control the DC modulation ratio of the marine MMC to maintain the DC voltage of the marine MMC at the rated value and reduce the PCC voltage amplitude to the first reference value to prevent the converter valve from engaging in surge current. By engaging the first and second converter valves, the output power of the offshore wind farm is gradually increased, thus completing the black start of the offshore wind power transmission system.

[0020] In some embodiments of this application, the step of charging the terrestrial MMC via the terrestrial power grid to unlock control of the terrestrial MMC specifically includes: Uncontrolled charging is performed on each submodule of the onshore MMC via the onshore power grid, and the onshore MMC submodule controller is started after the uncontrolled charging of each submodule of the onshore MMC is completed. By gradually controlling each submodule of the onshore MMC to bypass and disconnect it, the DC voltage of the onshore MMC is made equal to the rated DC voltage of the offshore MMC, thus unlocking control of the onshore MMC.

[0021] In some embodiments of this application, the onshore MMC is a hybrid full-bridge and half-bridge MMC, comprising multiple full-bridge submodules and multiple half-bridge submodules; the step of activating the onshore MMC submodule controller after the uncontrolled charging of each submodule of the onshore MMC is completed specifically includes: After the uncontrolled charging of each submodule of the onshore MMC is completed, the multiple full-bridge submodules are driven to operate in half-bridge mode, thereby raising the capacitor voltage of each submodule of the onshore MMC to start the onshore MMC submodule controller; wherein, at the end of the uncontrolled charging, the capacitor voltage of the full-bridge submodule is twice the capacitor voltage of the half-bridge submodule.

[0022] In some embodiments of this application, controlling the onshore MMC to charge the offshore MMC in order to unlock control of the offshore MMC and start a preset proportion of wind turbines in the offshore wind farm, specifically including: Control the land-based MMC to supply power to the marine MMC so as to perform uncontrolled charging of each submodule of the marine MMC, and start the marine MMC submodule controller after the uncontrolled charging of each submodule of the marine MMC is completed. By gradually controlling each sub-module of the offshore MMC to bypass and disconnect, the DC voltage of the offshore MMC is controlled to be equal to the rated value of the DC voltage of the offshore MMC, thus unlocking the control of the offshore MMC. Control the onshore power grid, onshore MMC and offshore MMC to supply power to the offshore wind farm and start the wind turbines in the offshore wind farm; when starting the wind turbines, start them at a preset ratio to prevent overcurrent.

[0023] In some embodiments of this application, controlling the DC modulation ratio of the offshore MMC to adjust the DC current flowing through the offshore MMC, and disconnecting the first bypass switch and the second bypass switch when the DC current flowing through the offshore MMC is zero, so that the offshore MMC can continuously absorb the surplus power generated by the wind turbine startup, specifically includes: The DC modulation ratio of the marine MMC is controlled by a preset DC current controller to reduce the DC current flowing through the marine MMC and enable the marine MMC to absorb the first surplus power; wherein, the DC current controller is obtained by dynamic modeling of the DC side of the marine MMC; When the DC current flowing through the offshore MMC is zero, the first bypass switch and the second bypass switch are disconnected, and the DC voltage of the onshore MMC is increased to the rated value so that the offshore MMC can absorb the second surplus power; wherein, the first surplus power and the second surplus power are both generated by the wind turbine startup.

[0024] In some embodiments of this application, controlling the DC modulation ratio of the marine MMC, maintaining the DC voltage of the marine MMC at a rated value, and reducing the PCC voltage amplitude to a first reference value to prevent the converter valve from engaging in surge current, specifically includes: The DC modulation ratio of the marine MMC is controlled by a preset DC voltage controller to maintain the DC voltage of the marine MMC at the rated value. Based on the relationship between the DC voltage of the converter valve and the DC voltage of the marine MMC, a first reference value for the PCC voltage amplitude is determined when the converter valve is engaged, and the PCC voltage amplitude is controlled to decrease to the first reference value to prevent the converter valve from engaging in surge current; wherein, the relationship is determined based on the external characteristics of the converter valve.

[0025] In some embodiments of this application, the activation of the first and second converter valves to gradually increase the output power of the offshore wind farm and complete the black start of the offshore wind power transmission system specifically includes: The first and second converter valves are engaged to gradually increase the output power of the offshore wind farm and restore the DC modulation ratio of the offshore MMC so that the PCC voltage amplitude gradually increases from the first reference value along with the output power, thus completing the black start of the offshore wind power transmission system.

[0026] This application first charges the onshore MMC via the onshore power grid, unlocking control of the onshore MMC. Then, it closes the bypass switch to control the onshore MMC to charge the offshore MMC, unlocking control of the offshore MMC and starting some wind turbines. Next, by controlling the DC modulation ratio of the offshore MMC, it first disconnects the bypass switch to allow the offshore MMC to absorb the surplus power generated by the wind turbine startup. Then, it maintains the DC voltage of the offshore MMC and reduces the PCC voltage amplitude to prevent the converter valve from engaging inrush current. Finally, it engages the converter valve to increase the output power of the offshore wind farm, completing the system's black start. Compared to existing technologies, this application features a special design for the system's connection and control process. The design enables black start of the system without adding auxiliary equipment, avoiding the additional costs associated with adding extra equipment. Furthermore, the absence of auxiliary equipment avoids the excessive control complexity that would result from adding additional equipment. Simultaneously, through a special design of the system's control flow, the DC modulation ratio of the offshore MMC is controlled twice. First, the offshore MMC absorbs the surplus power from the wind turbine startup. Then, by maintaining the DC voltage of the offshore MMC and reducing the voltage amplitude of the PCC, inrush current is prevented from engaging the converter valve. This avoids potential hazards caused by surplus power or inrush current in the system when the converter valve is engaged, ensuring the stability and feasibility of the system's black start. Attached Figure Description

[0027] Figure 1 This is a flowchart illustrating a black start method for an offshore wind power transmission system according to certain embodiments of this application. Figure 2 This is a topology diagram of an offshore wind power transmission system shown in certain embodiments of this application. Detailed Implementation

[0028] The embodiments of this application are described in detail below. Examples of the embodiments are shown 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 in conjunction with the accompanying drawings are exemplary and are only used to explain some embodiments of this application, and should not be construed as limiting the embodiments of this application. All other embodiments obtained by those skilled in the art based on the embodiments shown in this application without inventive effort are within the protection scope of this application.

[0029] In the description of this application, it should be understood that the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this application, unless otherwise explicitly specified, "a plurality of" or "several" means two or more.

[0030] Currently, black-start methods for offshore wind power transmission systems mainly rely on adding auxiliary equipment or utilizing the low-voltage ride-through characteristics of the wind turbines. However, the need for these auxiliary devices leads to unnecessary additional costs and complicates the timing control of the black-start process, increasing the overall control difficulty. Therefore, existing solutions fail to meet practical operational requirements in terms of both cost and control complexity. Consequently, designing a black-start method for offshore wind power transmission systems that reduces system cost and control complexity remains a pressing technical challenge.

[0031] Based on the above technical background, please refer to Figure 1 and Figure 2 The embodiments of this application provide, as follows: Figure 1 A black start method for offshore wind power transmission systems, applied to, for example Figure 2 A wind power transmission system is provided, comprising an onshore transmission end and an offshore generation end, wherein the onshore generation end and the offshore generation end are connected via a DC cable; the onshore transmission end includes an onshore power grid and an onshore MMC; the offshore generation end includes an offshore MMC, a first bypass switch BP1, a second bypass switch BP2, and a first converter valve DRU1 (i.e., Figure 1 The diode converter valve connected in parallel with BP1), and the second converter valve DRU2 (i.e. Figure 1 The system consists of a diode converter valve connected in parallel with BP2 and an offshore wind farm; the offshore MMC, the first converter valve DRU1, and the second converter valve DRU2 constitute the offshore converter station; the onshore MMC constitutes the onshore converter station. The output power of the offshore wind farm is represented by: BrkDR, the AC circuit breaker of the first converter valve DRU1, which is initially in the open state; BrkDR2, the AC circuit breaker of the second converter valve DRU2, which is initially in the open state; BrkMMC, the AC circuit breaker of the offshore MMC, which is initially in the open state; BrkGrid, the AC circuit breaker of the onshore power grid, which is initially in the closed state; and PCC, which represents the point of common coupling at sea.

[0032] The first end of the first converter valve DRU1 is connected to the first end of the onshore MMC, and the second end of the first converter valve DRU1 is connected to the first end of the offshore MMC; the first end of the second converter valve DRU2 is connected to the second end of the offshore MMC, and the second end of the second converter valve DRU2 is connected to the second end of the onshore MMC; the third end of the first converter valve DRU1 is connected to the offshore wind farm; the third end of the second converter valve DRU2 is connected to the offshore wind farm; and the third end of the offshore MMC is connected to the offshore wind farm. The first end of the first bypass switch BP1 is connected to the first end of the first converter valve DRU1, and the second end of the first bypass switch BP1 is connected to the second end of the first converter valve DRU1; the first end of the second bypass switch BP2 is connected to the first end of the second converter valve DRU2, and the second end of the second bypass switch BP2 is connected to the second end of the second converter valve DRU2.

[0033] In some embodiments of this application, the first converter valve DRU1 and the second converter valve DRU2 in the offshore converter station are both twelve-pulse DRUs, and the offshore MMC is a half-bridge MMC; the offshore MMC is used to establish the PCC (AC) voltage, and its voltage capacity is relatively small; the converter valves DRU1 and DRU2 are used to transmit most of the active power, and their voltage capacity is relatively larger; the bypass switches BP1 and BP2 connected in parallel with the converter valves DRU1 and DRU2 are used to backfeed power from the onshore power grid to the offshore wind farm during the black start process; the onshore MMC in the onshore converter station is a full-half-bridge hybrid MMC, which is used to control the DC voltage and convert the high-voltage DC to AC and send it to the onshore power grid.

[0034] like Figure 1 A black start method for an offshore wind power transmission system is provided, comprising steps S101 to S105, the specific steps of which are as follows: Step S101: Charge the terrestrial MMC via the terrestrial power grid to unlock control of the terrestrial MMC.

[0035] In some embodiments of this application, the step of charging the terrestrial MMC via the terrestrial power grid to unlock control of the terrestrial MMC specifically includes: Uncontrolled charging is performed on each submodule of the onshore MMC via the onshore power grid, and the onshore MMC submodule controller is started after the uncontrolled charging of each submodule of the onshore MMC is completed. By gradually controlling each submodule of the onshore MMC to bypass and disconnect it, the DC voltage of the onshore MMC is made equal to the rated DC voltage of the offshore MMC, thus unlocking control of the onshore MMC.

[0036] Specifically, during the uncontrolled charging phase, the submodule controller is not yet engaged, and the charging of the onshore MMC is uncontrollable. Since the full-bridge submodule (FBSM) in the onshore MMC can charge regardless of whether the charging current is positive or negative, while the half-bridge submodule (HBSM) can only charge when the charging current is positive, the charging voltage of each full-bridge submodule (FBSM) in the onshore MMC will be... This will reach the charging voltage of the HBSM of each half-bridge submodule. The relationship between the full-bridge submodule FBSM and the half-bridge submodule HBSM is as follows: (This is twice the size of the previous value.) ; Solving and Specifically: ; in, The phase amplitude of the AC side voltage of the system. This represents the number of bridge arm submodules in the land-based MMC. This represents the number of full-bridge submodules in a single bridge arm.

[0037] Depend on and The analytical expression shows that the capacitor voltage of the half-bridge submodule is smaller than that of the full-bridge submodule. Therefore, the capacitor voltage of the half-bridge submodule needs to be increased to enter controlled charging. Considering that at the end of the uncontrolled charging stage, the capacitor voltage of the full-bridge submodule is twice that of the half-bridge submodule, the switching transistors in the full-bridge submodule are first driven to remain constantly on, so that each full-bridge submodule operates in half-bridge mode. Let the capacitor voltage increment at this time be... When the onshore MMC reaches steady state, the steady-state relation is satisfied as follows: ; Solving .

[0038] From the capacitor voltage increment According to the analytical expression, when the full-bridge submodule operates in half-bridge mode, the capacitor voltages of both the full-bridge and half-bridge submodules will increase, and the capacitor voltage of the half-bridge submodule can increase to twice the original capacitor voltage. At this time, it can drive the DC / DC power supply to work and start the submodule controller of the land-based MMC.

[0039] After starting the submodule controller of the onshore MMC, by gradually reducing the number of full-bridge and half-bridge submodules in operation, the capacitor voltage of each submodule can be further increased to the rated value, so that the DC voltage of the onshore MMC is equal to the rated value of the DC voltage of the offshore MMC, ensuring the normal operation of the onshore MMC.

[0040] This application first performs uncontrolled charging of each submodule of the onshore MMC through the onshore power grid. After the uncontrolled charging is completed, the onshore MMC submodule controller is started, and then the onshore MMC submodules are gradually controlled to bypass. The control of the onshore MMC is unlocked by controlling the DC voltage of the onshore MMC, so as to provide an implementation basis for charging the marine MMC through the onshore MMC in the future, and further provide a basis for the black start of the system.

[0041] In some embodiments of this application, the onshore MMC is a hybrid full-bridge and half-bridge MMC, comprising multiple full-bridge submodules and multiple half-bridge submodules; the step of activating the onshore MMC submodule controller after the uncontrolled charging of each submodule of the onshore MMC is completed specifically includes: After the uncontrolled charging of each submodule of the onshore MMC is completed, the multiple full-bridge submodules are driven to operate in half-bridge mode, thereby raising the capacitor voltage of each submodule of the onshore MMC to start the onshore MMC submodule controller; wherein, at the end of the uncontrolled charging, the capacitor voltage of the full-bridge submodule is twice the capacitor voltage of the half-bridge submodule.

[0042] After the uncontrolled charging of the onshore MMC is completed, this application drives multiple full-bridge submodules of the onshore MMC to operate in half-bridge mode, which can raise the capacitor voltage of each submodule of the onshore MMC, thereby starting the onshore MMC submodule controller, providing a basis for subsequent charging of the marine MMC through the onshore MMC, and further enabling the system's black start.

[0043] Step S102: Close the first bypass switch BP1 and the second bypass switch BP2 to control the onshore MMC to charge the offshore MMC, thereby unlocking the control of the offshore MMC and starting the wind turbines in the offshore wind farm at a preset ratio.

[0044] In some embodiments of this application, controlling the onshore MMC to charge the offshore MMC in order to unlock control of the offshore MMC and start a preset proportion of wind turbines in the offshore wind farm, specifically including: Control the land-based MMC to supply power to the marine MMC so as to perform uncontrolled charging of each submodule of the marine MMC, and start the marine MMC submodule controller after the uncontrolled charging of each submodule of the marine MMC is completed. By gradually controlling each sub-module of the offshore MMC to bypass and disconnect, the DC voltage of the offshore MMC is controlled to be equal to the rated value of the DC voltage of the offshore MMC, thus unlocking the control of the offshore MMC. Control the onshore power grid, onshore MMC and offshore MMC to supply power to the offshore wind farm and start the wind turbines in the offshore wind farm; when starting the wind turbines, start them at a preset ratio to prevent overcurrent.

[0045] Specifically, similar to the charging phases of onshore MMCs, offshore MMCs also undergo two phases: uncontrolled charging and controlled charging, with identical control logic in both. When the uncontrolled charging phase of the offshore MMC ends, the capacitor voltages of each half-bridge submodule are... ( The rated voltage of the half-bridge submodule of the offshore MMC is sufficient to meet the operating requirements of the DC / DC power supply. The submodule controller of the offshore MMC can start and send a trigger signal normally to control the offshore MMC to perform controlled charging. During the controlled charging phase of the offshore MMC, the control logic is the same as that of the controlled charging phase of the onshore MMC. After the controlled charging is completed, each submodule of the offshore MMC is unlocked, and the AC circuit breaker BrkMMC of the offshore MMC can be closed to establish the offshore AC voltage.

[0046] After establishing the offshore AC voltage, electrical energy can be transmitted through the onshore power grid via the MMC-HDVC channel formed by the onshore MMC and the offshore MMC to start the wind turbines of the offshore wind farm. However, to prevent overcurrent, not all wind turbines should be started. At this time, it is necessary to ensure that the transmitted power is lower than the rated value of the offshore MMC. The ratio of transmitted power to the number of wind turbines started satisfies the following relationship: ; in, To activate the fan ratio, This refers to the rated total power of an offshore wind farm. This refers to the rated power of the marine MMC.

[0047] This application first controls the onshore MMC to perform uncontrolled charging of each submodule of the offshore MMC. After the uncontrolled charging is completed, the offshore MMC submodule controller is started, and then the offshore MMC submodules are gradually controlled to bypass. By controlling the DC voltage of the offshore MMC, the control of the offshore MMC is unlocked, thereby controlling the onshore power grid, the onshore MMC and the offshore MMC to supply power to the offshore wind farm to start the wind turbines. This provides a basis for the subsequent absorption of surplus power and the activation of the converter valve, and further provides a basis for the implementation of the system's black start.

[0048] Because surplus power is generated after the wind turbine starts, the converter valves DRU1 and DRU2 cannot be safely engaged. Therefore, it is necessary to consume or absorb the surplus power and adjust the system circuit state accordingly, i.e., steps S103 to S104, to safely engage the converter valves. During this process, it is necessary to ensure that the bypass switches BP1 and BP2 are safely disconnected with zero current (step S103), and that the DC voltage of the offshore MMC remains stable after the bypass switches are disconnected (step S104).

[0049] Step S103: Control the DC modulation ratio of the offshore MMC to adjust the DC current flowing through the offshore MMC, and when the DC current flowing through the offshore MMC is zero, disconnect the first bypass switch BP1 and the second bypass switch BP2 so that the offshore MMC can continuously absorb the surplus power generated by the wind turbine startup.

[0050] In some embodiments of this application, controlling the DC modulation ratio of the offshore MMC to adjust the DC current flowing through the offshore MMC, and disconnecting the first bypass switch and the second bypass switch when the DC current flowing through the offshore MMC is zero, so that the offshore MMC can continuously absorb the surplus power generated by the wind turbine startup, specifically includes: The DC modulation ratio of the marine MMC is controlled by a preset DC current controller to reduce the DC current flowing through the marine MMC and enable the marine MMC to absorb the first surplus power; wherein, the DC current controller is obtained by dynamic modeling of the DC side of the marine MMC; When the DC current flowing through the offshore MMC is zero, the first bypass switch and the second bypass switch are disconnected, and the DC voltage of the onshore MMC is increased to the rated value so that the offshore MMC can absorb the second surplus power; wherein, the first surplus power and the second surplus power are both generated by the wind turbine startup.

[0051] Since the internal capacitors of an offshore MMC are energy storage components, they can store corresponding energy. Under normal operating conditions, the energy that an offshore MMC can store can be expressed as: ( This refers to the capacitance value of a submodule in an offshore MMC. The capacitor voltage of the sub-module of the offshore MMC is 1.5 pu (per unit, representing the ratio relative to the reference value). It has a certain overvoltage capability and can absorb energy higher than the rated value. According to the capacitor withstand standard, the overvoltage withstand of the internal capacitor of the MMC can usually reach 1.5 pu (per unit). Therefore, the surplus power can be absorbed through the sub-module capacitor of the offshore MMC to create a current zero crossing point for the offshore wind power transmission system and realize the safe opening of the bypass switch.

[0052] During the process of absorbing surplus power by the offshore MMC, in order to prevent overvoltage of the submodule capacitors of the offshore MMC, the total time required for the converter valve to be engaged must not exceed the maximum absorption time of the offshore MMC for absorbing surplus power. This constraint can be specifically expressed by the energy stored in the offshore MMC under rated operating conditions as follows: ; in, The system surplus power is equal to the difference between the output power of the started fans and the losses of the converter valves; The rated voltage of the capacitors in the sub-modules of the offshore MMC; To start the investment time, This represents the maximum absorption time for marine MMC.

[0053] Meanwhile, the power absorbed by the internal capacitors of the marine MMC satisfies energy balance, specifically as follows: ; in, These are the DC voltage and DC current of the marine MMC, respectively. Power is absorbed by the internal capacitors of the marine MMC.

[0054] The DC modulation ratio of marine MMC Analyzing the voltage of the submodule capacitors reveals that when the offshore MMC reaches steady state, the following relationship holds: ; in, This represents the average value of the capacitor voltage of the submodule in the offshore MMC.

[0055] When considering the internal dynamics of the DC side of the offshore MMC, the dynamic modeling relationship of the offshore MMC can be obtained by summing the three phases of the offshore MMC, specifically: ; in, The inductance of a single bridge arm of an offshore MMC. The resistance of a single bridge arm of an offshore MMC.

[0056] Therefore, a DC current controller can be constructed based on the dynamic modeling relationship, specifically as follows: ; in, These are the PI parameters for the DC current controller; This is a reference value for DC current. Since it is necessary to create a zero-crossing point for the current, its preferred value is 0.

[0057] Based on the analytical expression of the DC current controller, it can be seen that by controlling the DC modulation ratio of the marine MMC... It can guarantee The voltage is close to 0, thus ensuring the safety of disconnecting the bypass switch and raising the DC voltage of the onshore MMC to the rated value; before bypass switches BP1 and BP2 are disconnected, the DC voltage of the offshore MMC is clamped by the onshore MMC. As a constant, it can be seen from the analytical expression of the energy balance above that, with... As the voltage decreases, the surplus power is absorbed by the submodule capacitors of the offshore MMC, at which point the average capacitor voltage of the submodules of the offshore MMC will increase.

[0058] This application first controls the DC modulation ratio of the offshore MMC through a DC current controller, which reduces the DC current flowing through the offshore MMC, allowing the offshore MMC to absorb the first surplus power. Then, when the current flowing through the offshore MMC is zero, the bypass switch is disconnected and the DC voltage of the onshore MMC is increased, allowing the offshore MMC to absorb the second surplus power. This avoids the potential risks caused by the system's surplus power when the converter valve is put into operation, thereby ensuring the stability and feasibility of the subsequent black start of the system.

[0059] Step S104: Control the DC modulation ratio of the marine MMC, maintain the DC voltage of the marine MMC at the rated value, and reduce the PCC voltage amplitude to the first reference value to prevent the converter valve from engaging in surge current.

[0060] In some embodiments of this application, controlling the DC modulation ratio of the marine MMC, maintaining the DC voltage of the marine MMC at a rated value, and reducing the PCC voltage amplitude to a first reference value to prevent the converter valve from engaging in surge current, specifically includes: The DC modulation ratio of the marine MMC is controlled by a preset DC voltage controller to maintain the DC voltage of the marine MMC at the rated value. Based on the relationship between the DC voltage of the converter valve and the DC voltage of the marine MMC, a first reference value for the PCC voltage amplitude is determined when the converter valve is engaged, and the PCC voltage amplitude is controlled to decrease to the first reference value to prevent the converter valve from engaging in surge current; wherein, the relationship is determined based on the external characteristics of the converter valve.

[0061] Specifically, after the bypass switch is turned off, When the power is 0, the submodules of the offshore MMC continue to absorb surplus power. At this point, the analytical expression for the energy balance above can be simplified to: .

[0062] Combining the simplified energy balance equation and the above-mentioned steady-state relationship equation for offshore MMC, it can be deduced that if the DC modulation ratio of the offshore MMC is not considered... During control, the DC voltage of the offshore MMC will continuously rise as the capacitor voltage of the submodules increases, causing a drop in the DC side voltage of the converter valve. This could lead to the converter valve prematurely turning on before the surplus power is fully absorbed, resulting in inrush current and creating a safety hazard. Therefore, to prevent the converter valve from turning on prematurely, after the bypass switch is disconnected, the control target needs to be switched from DC current to DC voltage to ensure... The DC voltage controller is stable and specifically comprises: ; in, These are the PI parameters for the DC voltage controller; This is a DC voltage reference value, as it needs to be guaranteed. Since it is a constant value, its preferred value is the rated DC voltage of the marine MMC.

[0063] The external characteristics of the converter valve can be expressed as: ; in, The rated DC voltage of the converter valve. The turns ratio of the transformer connected to the converter valve. This refers to the phase amplitude of the PCC voltage. The leakage reactance of the transformer connected to the converter valve.

[0064] Due to the external characteristics of the converter valve, it can be known that as the system power decreases, the DC voltage of the converter valve will increase, leading to a decrease in the DC voltage of the offshore MMC, thus posing an overmodulation risk. To ensure the DC voltage stability of the offshore MMC, the reference value of the PCC voltage amplitude needs to be reduced to compensate for the change in the DC voltage of the converter valve. The reference value after the change in the PCC voltage amplitude can be specifically expressed as follows: ; Meanwhile, to prevent inrush current during commissioning, the offshore MMC must be controlled to reduce the PCC voltage amplitude to no-load before the converter valve is engaged. Reference value for PCC voltage.

[0065] This application first controls the DC modulation ratio of the marine MMC through a DC voltage controller, which can maintain the DC voltage of the marine MMC. Then, the relationship between the DC voltage of the converter valve and the DC voltage of the marine MMC determines the first reference value of the PCC voltage amplitude when the converter valve is engaged. By using the first reference value, inrush current is prevented when the converter valve is engaged, thus avoiding the potential hazards caused by inrush current when the converter valve is engaged in subsequent operations, thereby ensuring the stability and feasibility of black start of the subsequent system.

[0066] Step S105: Engage the first converter valve DRU1 and the second converter valve DRU2 to gradually increase the output power of the offshore wind farm and complete the black start of the offshore wind power transmission system.

[0067] In some embodiments of this application, the activation of the first and second converter valves to gradually increase the output power of the offshore wind farm and complete the black start of the offshore wind power transmission system specifically includes: The first and second converter valves are engaged to gradually increase the output power of the offshore wind farm and restore the DC modulation ratio of the offshore MMC so that the PCC voltage amplitude gradually increases from the first reference value along with the output power, thus completing the black start of the offshore wind power transmission system.

[0068] Specifically, engaging the first and second converter valves actually means closing the AC circuit breakers corresponding to the first converter valve DRU1 and the second converter valve DRU2, that is, closing the AC circuit breakers BrkDR1 and BrkDR2, so as to engage the converter valves in the current offshore wind power transmission system.

[0069] Specifically, during the process of activating the converter valve and gradually increasing the output power of the offshore wind farm, the wind turbines that have already started operating in the offshore wind farm will continue to operate, while the wind turbines that have not yet started will start in sequence until all the wind turbines in the offshore wind farm are in operation. At this time, the output power of the offshore wind farm will be increased to the rated value, or to the power obtained by maximum power point tracking. During this period, the PCC voltage amplitude will gradually increase to the rated value as the output power of the offshore wind farm increases, and the DC modulation ratio of the offshore MMC will gradually recover to 1, thus completing the black start of the offshore wind power transmission system.

[0070] This application first activates the converter valve to gradually increase the output power of the offshore wind farm and restores the DC modulation ratio of the offshore MMC. This enables the PCC voltage amplitude to gradually increase with the output power of the offshore wind farm, ensuring normal system operation conditions and thus guaranteeing the smooth completion of the system's black start.

[0071] Compared to existing technologies, this application first charges the onshore MMC via the onshore power grid to unlock control of the onshore MMC. Then, it closes a bypass switch to control the onshore MMC to charge the offshore MMC, unlocking control of the offshore MMC and starting some wind turbines. Furthermore, by controlling the DC modulation ratio of the offshore MMC, it first disconnects the bypass switch to allow the offshore MMC to absorb the surplus power generated by the wind turbine startup. Secondly, it maintains the DC voltage of the offshore MMC and reduces the PCC voltage amplitude to prevent inrush current from entering the converter valve. Finally, it engages the converter valve to increase the output power of the offshore wind farm, completing the system's black start. Compared to existing technologies, this application improves the system's connection and control process. This system features a special design that enables black-start operation without the need for additional auxiliary equipment, avoiding the extra costs associated with adding extra equipment. Furthermore, the absence of additional auxiliary equipment also avoids the excessive control complexity that would result from adding extra equipment. Simultaneously, this application employs a unique control flow design, controlling the DC modulation ratio of the offshore MMC twice. First, the offshore MMC absorbs the surplus power generated during wind turbine startup. Then, by maintaining the DC voltage of the offshore MMC and reducing the voltage amplitude of the PCC, inrush current is prevented from activating the converter valve. This avoids potential hazards caused by surplus power or inrush current in the system when the converter valve is activated, ensuring the stability and feasibility of the system's black-start operation.

[0072] For the methods described above, please refer again. Figure 2This application provides an offshore wind power transmission system, including an onshore transmission end and an offshore power generation end; the onshore transmission end includes an onshore power grid and an onshore MMC; the offshore power generation end includes a first converter valve, a second converter valve, an offshore MMC, a first bypass switch, a second bypass switch and an offshore wind farm; The first end of the first converter valve is connected to the first end of the onshore MMC, and the second end of the first converter valve is connected to the first end of the offshore MMC; the first end of the second converter valve is connected to the second end of the offshore MMC, and the second end of the second converter valve is connected to the second end of the onshore MMC; the third end of the first converter valve is connected to the offshore wind farm; the third end of the second converter valve is connected to the offshore wind farm; and the third end of the offshore MMC is connected to the offshore wind farm. The first terminal of the first bypass switch is connected to the first terminal of the first converter valve, and the second terminal of the first bypass switch is connected to the second terminal of the first converter valve; the first terminal of the second bypass switch is connected to the first terminal of the second converter valve, and the second terminal of the second bypass switch is connected to the second terminal of the second converter valve. The offshore wind power transmission system is used to execute a black start method, which includes: Charge the terrestrial MMC via the terrestrial power grid to unlock control of the terrestrial MMC; Close the first and second bypass switches to control the onshore MMC to charge the offshore MMC, thereby unlocking control of the offshore MMC and starting the wind turbines in the offshore wind farm at a preset ratio. Control the DC modulation ratio of the offshore MMC to regulate the DC current flowing through the offshore MMC, and disconnect the first bypass switch and the second bypass switch when the DC current flowing through the offshore MMC is zero, so that the offshore MMC can continuously absorb the surplus power generated by the wind turbine startup. Control the DC modulation ratio of the marine MMC to maintain the DC voltage of the marine MMC at the rated value and reduce the PCC voltage amplitude to the first reference value to prevent the converter valve from engaging in surge current. By engaging the first and second converter valves, the output power of the offshore wind farm is gradually increased, thus completing the black start of the offshore wind power transmission system.

[0073] In some embodiments of this application, the step of charging the terrestrial MMC via the terrestrial power grid to unlock control of the terrestrial MMC specifically includes: Uncontrolled charging is performed on each submodule of the onshore MMC via the onshore power grid, and the onshore MMC submodule controller is started after the uncontrolled charging of each submodule of the onshore MMC is completed. By gradually controlling each submodule of the onshore MMC to bypass and disconnect it, the DC voltage of the onshore MMC is made equal to the rated DC voltage of the offshore MMC, thus unlocking control of the onshore MMC.

[0074] In some embodiments of this application, the onshore MMC is a hybrid full-bridge and half-bridge MMC, comprising multiple full-bridge submodules and multiple half-bridge submodules; the step of activating the onshore MMC submodule controller after the uncontrolled charging of each submodule of the onshore MMC is completed specifically includes: After the uncontrolled charging of each submodule of the onshore MMC is completed, the multiple full-bridge submodules are driven to operate in half-bridge mode, thereby raising the capacitor voltage of each submodule of the onshore MMC to start the onshore MMC submodule controller; wherein, at the end of the uncontrolled charging, the capacitor voltage of the full-bridge submodule is twice the capacitor voltage of the half-bridge submodule.

[0075] In some embodiments of this application, controlling the onshore MMC to charge the offshore MMC in order to unlock control of the offshore MMC and start a preset proportion of wind turbines in the offshore wind farm, specifically including: Control the land-based MMC to supply power to the marine MMC so as to perform uncontrolled charging of each submodule of the marine MMC, and start the marine MMC submodule controller after the uncontrolled charging of each submodule of the marine MMC is completed. By gradually controlling each sub-module of the offshore MMC to bypass and disconnect, the DC voltage of the offshore MMC is controlled to be equal to the rated value of the DC voltage of the offshore MMC, thus unlocking the control of the offshore MMC. Control the onshore power grid, onshore MMC and offshore MMC to supply power to the offshore wind farm and start the wind turbines in the offshore wind farm; when starting the wind turbines, start them at a preset ratio to prevent overcurrent.

[0076] In some embodiments of this application, controlling the DC modulation ratio of the offshore MMC to adjust the DC current flowing through the offshore MMC, and disconnecting the first bypass switch and the second bypass switch when the DC current flowing through the offshore MMC is zero, so that the offshore MMC can continuously absorb the surplus power generated by the wind turbine startup, specifically includes: The DC modulation ratio of the marine MMC is controlled by a preset DC current controller to reduce the DC current flowing through the marine MMC and enable the marine MMC to absorb the first surplus power; wherein, the DC current controller is obtained by dynamic modeling of the DC side of the marine MMC; When the DC current flowing through the offshore MMC is zero, the first bypass switch and the second bypass switch are disconnected, and the DC voltage of the onshore MMC is increased to the rated value so that the offshore MMC can absorb the second surplus power; wherein, the first surplus power and the second surplus power are both generated by the wind turbine startup.

[0077] In some embodiments of this application, controlling the DC modulation ratio of the marine MMC, maintaining the DC voltage of the marine MMC at a rated value, and reducing the PCC voltage amplitude to a first reference value to prevent the converter valve from engaging in surge current, specifically includes: The DC modulation ratio of the marine MMC is controlled by a preset DC voltage controller to maintain the DC voltage of the marine MMC at the rated value. Based on the relationship between the DC voltage of the converter valve and the DC voltage of the marine MMC, a first reference value for the PCC voltage amplitude is determined when the converter valve is engaged, and the PCC voltage amplitude is controlled to decrease to the first reference value to prevent the converter valve from engaging in surge current; wherein, the relationship is determined based on the external characteristics of the converter valve.

[0078] In some embodiments of this application, the activation of the first and second converter valves to gradually increase the output power of the offshore wind farm and complete the black start of the offshore wind power transmission system specifically includes: The first and second converter valves are engaged to gradually increase the output power of the offshore wind farm and restore the DC modulation ratio of the offshore MMC so that the PCC voltage amplitude gradually increases from the first reference value along with the output power, thus completing the black start of the offshore wind power transmission system.

[0079] This application first charges the onshore MMC via the onshore power grid, unlocking control of the onshore MMC. Then, it closes the bypass switch to control the onshore MMC to charge the offshore MMC, unlocking control of the offshore MMC and starting some wind turbines. Next, by controlling the DC modulation ratio of the offshore MMC, it first disconnects the bypass switch to allow the offshore MMC to absorb the surplus power generated by the wind turbine startup. Then, it maintains the DC voltage of the offshore MMC and reduces the PCC voltage amplitude to prevent the converter valve from engaging inrush current. Finally, it engages the converter valve to increase the output power of the offshore wind farm, completing the system's black start. Compared to existing technologies, this application features a special design for the system's connection and control process. The design enables black start of the system without adding auxiliary equipment, avoiding the additional costs associated with adding extra equipment. Furthermore, the absence of auxiliary equipment avoids the excessive control complexity that would result from adding additional equipment. Simultaneously, through a special design of the system's control flow, the DC modulation ratio of the offshore MMC is controlled twice. First, the offshore MMC absorbs the surplus power from the wind turbine startup. Then, by maintaining the DC voltage of the offshore MMC and reducing the voltage amplitude of the PCC, inrush current is prevented from engaging the converter valve. This avoids potential hazards caused by surplus power or inrush current in the system when the converter valve is engaged, ensuring the stability and feasibility of the system's black start.

[0080] It should be understood that the system provided in this application corresponds to the aforementioned method. The offshore wind power transmission system provided in this application can implement the black start method of an offshore wind power transmission system provided in any of the embodiments of this application.

[0081] Adaptively, embodiments of this application also provide a computer device and a computer-readable storage medium.

[0082] The computer device includes: a processor, a memory, and a computer program stored in the memory and configured to be executed by the processor; The processor executes the computer program to implement a black start method for an offshore wind power transmission system according to this application.

[0083] The computer-readable storage medium stores multiple instructions adapted for loading by a processor to execute a black-start method for an offshore wind power transmission system according to this application.

[0084] The above description represents some embodiments of this application, providing a further detailed explanation of the purpose, technical solution, and beneficial effects of this application. It should be understood that the above-described embodiments of this application should not be construed as limiting this application. In particular, any changes, modifications, equivalent substitutions, and variations made by those skilled in the art within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A black start method for an offshore wind power transmission system, characterized in that, This invention is applied to offshore wind power transmission systems, which include an onshore transmission end and an offshore power generation end; the onshore transmission end includes an onshore power grid and an onshore MMC; the offshore power generation end includes a first converter valve, a second converter valve, an offshore MMC, a first bypass switch, a second bypass switch, and an offshore wind farm. The first end of the first converter valve is connected to the first end of the onshore MMC, and the second end of the first converter valve is connected to the first end of the offshore MMC; the first end of the second converter valve is connected to the second end of the offshore MMC, and the second end of the second converter valve is connected to the second end of the onshore MMC; the third end of the first converter valve is connected to the offshore wind farm; the third end of the second converter valve is connected to the offshore wind farm; and the third end of the offshore MMC is connected to the offshore wind farm. The first terminal of the first bypass switch is connected to the first terminal of the first converter valve, and the second terminal of the first bypass switch is connected to the second terminal of the first converter valve; the first terminal of the second bypass switch is connected to the first terminal of the second converter valve, and the second terminal of the second bypass switch is connected to the second terminal of the second converter valve. The black start method includes: Charge the terrestrial MMC via the terrestrial power grid to unlock control of the terrestrial MMC; Close the first and second bypass switches to control the onshore MMC to charge the offshore MMC, thereby unlocking control of the offshore MMC and starting the wind turbines in the offshore wind farm at a preset ratio. Control the DC modulation ratio of the offshore MMC to regulate the DC current flowing through the offshore MMC, and disconnect the first bypass switch and the second bypass switch when the DC current flowing through the offshore MMC is zero, so that the offshore MMC can continuously absorb the surplus power generated by the wind turbine startup. Control the DC modulation ratio of the marine MMC to maintain the DC voltage of the marine MMC at the rated value and reduce the PCC voltage amplitude to the first reference value to prevent the converter valve from engaging in surge current. By engaging the first and second converter valves, the output power of the offshore wind farm is gradually increased, completing the black start of the offshore wind power transmission system.

2. The black start method for an offshore wind power transmission system according to claim 1, characterized in that, The method of charging the terrestrial MMC via the terrestrial power grid to unlock control of the terrestrial MMC specifically includes: Uncontrolled charging is performed on each submodule of the onshore MMC via the onshore power grid, and the onshore MMC submodule controller is started after the uncontrolled charging of each submodule of the onshore MMC is completed. By gradually controlling each submodule of the onshore MMC to bypass and disconnect it, the DC voltage of the onshore MMC is made equal to the rated DC voltage of the offshore MMC, thus unlocking control of the onshore MMC.

3. The black start method for an offshore wind power transmission system according to claim 2, characterized in that, The onshore MMC is a hybrid full-bridge and half-bridge MMC, comprising multiple full-bridge submodules and multiple half-bridge submodules; after uncontrolled charging of each submodule of the onshore MMC is completed, the onshore MMC submodule controller is activated, specifically including: After the uncontrolled charging of each submodule of the onshore MMC is completed, the multiple full-bridge submodules are driven to operate in half-bridge mode, thereby raising the capacitor voltage of each submodule of the onshore MMC to start the onshore MMC submodule controller; wherein, at the end of the uncontrolled charging, the capacitor voltage of the full-bridge submodule is twice the capacitor voltage of the half-bridge submodule.

4. The black start method for an offshore wind power transmission system according to claim 1, characterized in that, The control of the onshore MMC to charge the offshore MMC unlocks control of the offshore MMC and starts a preset proportion of wind turbines in the offshore wind farm, specifically including: Control the land-based MMC to supply power to the marine MMC so as to perform uncontrolled charging of each submodule of the marine MMC, and start the marine MMC submodule controller after the uncontrolled charging of each submodule of the marine MMC is completed. By gradually controlling each sub-module of the offshore MMC to bypass and disconnect, the DC voltage of the offshore MMC is controlled to be equal to the rated value of the DC voltage of the offshore MMC, thus unlocking the control of the offshore MMC. Control the onshore power grid, onshore MMC and offshore MMC to supply power to the offshore wind farm and start the wind turbines in the offshore wind farm; when starting the wind turbines, start them at a preset ratio to prevent overcurrent.

5. The black start method for an offshore wind power transmission system according to claim 1, characterized in that, The control of the DC modulation ratio of the offshore MMC to adjust the DC current flowing through the offshore MMC, and disconnecting the first and second bypass switches when the DC current flowing through the offshore MMC is zero, so that the offshore MMC can continuously absorb the surplus power generated by the wind turbine startup, specifically includes: The DC modulation ratio of the marine MMC is controlled by a preset DC current controller to reduce the DC current flowing through the marine MMC and enable the marine MMC to absorb the first surplus power; wherein, the DC current controller is obtained by dynamic modeling of the DC side of the marine MMC; When the DC current flowing through the offshore MMC is zero, the first bypass switch and the second bypass switch are disconnected, and the DC voltage of the onshore MMC is increased to the rated value so that the offshore MMC can absorb the second surplus power; wherein, the first surplus power and the second surplus power are both generated by the wind turbine startup.

6. The black start method for an offshore wind power transmission system according to claim 1, characterized in that, The control of the DC modulation ratio of the marine MMC, maintaining the DC voltage of the marine MMC at the rated value and reducing the PCC voltage amplitude to the first reference value to prevent the converter valve from engaging inrush current, specifically includes: The DC modulation ratio of the marine MMC is controlled by a preset DC voltage controller to maintain the DC voltage of the marine MMC at the rated value. Based on the relationship between the DC voltage of the converter valve and the DC voltage of the marine MMC, a first reference value for the PCC voltage amplitude is determined when the converter valve is engaged, and the PCC voltage amplitude is controlled to decrease to the first reference value to prevent the converter valve from engaging in surge current; wherein, the relationship is determined based on the external characteristics of the converter valve.

7. The black start method for an offshore wind power transmission system according to claim 1, characterized in that, The activation of the first and second converter valves gradually increases the output power of the offshore wind farm, completing the black start of the offshore wind power transmission system. Specifically, this includes: The first and second converter valves are engaged to gradually increase the output power of the offshore wind farm and restore the DC modulation ratio of the offshore MMC so that the PCC voltage amplitude gradually increases from the first reference value along with the output power, thus completing the black start of the offshore wind power transmission system.

8. An offshore wind power transmission system, characterized in that, It includes an onshore power transmission terminal and an offshore power generation terminal; the onshore power transmission terminal includes an onshore power grid and an onshore MMC; the offshore power generation terminal includes a first converter valve, a second converter valve, an offshore MMC, a first bypass switch, a second bypass switch, and an offshore wind farm; The first end of the first converter valve is connected to the first end of the onshore MMC, and the second end of the first converter valve is connected to the first end of the offshore MMC; the first end of the second converter valve is connected to the second end of the offshore MMC, and the second end of the second converter valve is connected to the second end of the onshore MMC; the third end of the first converter valve is connected to the offshore wind farm; the third end of the second converter valve is connected to the offshore wind farm; and the third end of the offshore MMC is connected to the offshore wind farm. The first terminal of the first bypass switch is connected to the first terminal of the first converter valve, and the second terminal of the first bypass switch is connected to the second terminal of the first converter valve; the first terminal of the second bypass switch is connected to the first terminal of the second converter valve, and the second terminal of the second bypass switch is connected to the second terminal of the second converter valve. The offshore wind power transmission system is used to execute a black start method, which includes: Charge the terrestrial MMC via the terrestrial power grid to unlock control of the terrestrial MMC; Close the first and second bypass switches to control the onshore MMC to charge the offshore MMC, thereby unlocking control of the offshore MMC and starting the wind turbines in the offshore wind farm at a preset ratio. Control the DC modulation ratio of the offshore MMC to regulate the DC current flowing through the offshore MMC, and disconnect the first bypass switch and the second bypass switch when the DC current flowing through the offshore MMC is zero, so that the offshore MMC can continuously absorb the surplus power generated by the wind turbine startup. Control the DC modulation ratio of the marine MMC to maintain the DC voltage of the marine MMC at the rated value and reduce the PCC voltage amplitude to the first reference value to prevent the converter valve from engaging in surge current. By engaging the first and second converter valves, the output power of the offshore wind farm is gradually increased, completing the black start of the offshore wind power transmission system.

9. A offshore wind power transmission system according to claim 8, characterized in that, The control of the DC modulation ratio of the offshore MMC to adjust the DC current flowing through the offshore MMC, and disconnecting the first and second bypass switches when the DC current flowing through the offshore MMC is zero, so that the offshore MMC can continuously absorb the surplus power generated by the wind turbine startup, specifically includes: The DC modulation ratio of the marine MMC is controlled by a preset DC current controller to reduce the DC current flowing through the marine MMC and enable the marine MMC to absorb the first surplus power; wherein, the DC current controller is obtained by dynamic modeling of the DC side of the marine MMC; When the DC current flowing through the offshore MMC is zero, the first bypass switch and the second bypass switch are disconnected, and the DC voltage of the onshore MMC is increased to the rated value so that the offshore MMC can absorb the second surplus power; wherein, the first surplus power and the second surplus power are both generated by the wind turbine startup.

10. An offshore wind power transmission system according to claim 8, characterized in that, The control of the DC modulation ratio of the marine MMC, maintaining the DC voltage of the marine MMC at the rated value and reducing the PCC voltage amplitude to the first reference value to prevent the converter valve from engaging inrush current, specifically includes: The DC modulation ratio of the marine MMC is controlled by a preset DC voltage controller to maintain the DC voltage of the marine MMC at the rated value. Based on the relationship between the DC voltage of the converter valve and the DC voltage of the marine MMC, a first reference value for the PCC voltage amplitude is determined when the converter valve is engaged, and the PCC voltage amplitude is controlled to decrease to the first reference value to prevent the converter valve from engaging in surge current; wherein, the relationship is determined based on the external characteristics of the converter valve.

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