Offshore wind power transmission system and reactive power network construction control strategy determination method, device and equipment thereof

Abstract

The application relates to the technical field of offshore power generation, and provides an offshore wind power transmission system, a reactive power network construction control strategy determination method, device and equipment thereof. The method comprises the following steps: acquiring an operation condition of the offshore wind power transmission system; determining a reactive power network construction control strategy according to the operation condition; if the operation condition is a wind farm black start condition, the reactive power network construction control strategy is an auxiliary converter reactive power network construction control strategy; and if the operation condition is a wind turbine running-in condition, the reactive power network construction control strategy is the auxiliary converter reactive power network construction control strategy, a centralized wind turbine reactive power distribution link and a network-following wind turbine control strategy. The method determines different reactive power network construction control strategies through the operation condition, realizes wind turbine cooperative auxiliary converter reactive power network construction, makes the auxiliary converter not need to bear a high reactive power output, reduces the capacity requirement of the auxiliary converter, realizes auxiliary converter compactness and low cost, and thus reduces the construction cost of the offshore wind power transmission system.

Description

Technical Field

[0001] This application relates to the field of offshore power generation technology, and in particular to an offshore wind power transmission system and a method, apparatus and equipment for determining the reactive power grid control strategy. Background Technology

[0002] The development of offshore wind power into deep-sea areas has made flexible DC transmission based on modular multilevel converters (MMCs) the main transmission method. However, the large size and weight of MMCs lead to problems such as high cost, construction difficulty, and high operation and maintenance costs for offshore wind power transmission systems.

[0003] Currently, the design of compact and low-cost offshore wind power transmission systems has become a hot topic in engineering. Among them, diode rectifier (DR) DC transmission has advantages such as low cost, small size, and light weight, and has great application potential. However, since diode rectifiers (DR) lack control capabilities, they cannot be used to construct offshore AC power grids in the manner of modular multilevel converters (MMC), which is one of the main factors restricting the application of diode rectifiers (DR).

[0004] Currently, there are no practical engineering solutions for grid construction using diode rectifier (DR) systems in offshore wind farms. In research, the construction of AC grids for DR wind farms using diode rectifiers can be categorized into two types based on whether additional auxiliary converter equipment is required. The first type attempts to complete grid construction control entirely using existing wind farm equipment, primarily leveraging the control margin of the wind turbines and relying on their coordinated control to construct the offshore AC grid. This type of research includes self-synchronous grid construction control using active-voltage / reactive-frequency coupling relationships; achieving system frequency stability control by providing a common phase reference for the wind turbines; and virtual synchronization control using onshore converters and wind turbines. The second type of solution requires adding auxiliary converters with control capabilities to the diode rectifier DR offshore wind power transmission system to solve the grid construction problem. There are various ways for auxiliary converters to participate in grid construction, including Vf grid construction with a given voltage and frequency, and frequency control using reactive power injection. The wind turbines can continue to use mature grid-following control methods.

[0005] As mentioned above, the first type of solution heavily relies on wind turbines with grid-connected control capabilities. However, grid-connected wind turbines currently lack practical engineering applications and present significant technical challenges. Coordinating the operation of hundreds of turbines in a large-scale wind farm using a grid-connected approach may pose a risk of oscillation and instability, raising potential feasibility issues. Furthermore, grid-connected wind turbines require a redesign of their internal control systems, incurring additional design and manufacturing costs.

[0006] The second type of grid configuration places high demands on the capacity of the auxiliary converters. For grid configurations using reactive power injection frequency control, the auxiliary converters, while performing control functions, also bear the reactive power requirements of the entire system. The reactive power demand of diode rectifier DR wind farms is substantial, leading to high reactive power output requirements for the auxiliary converters. For grid control methods using open-loop Vf input, the auxiliary converters will bear a portion of the active power, and when disturbances occur in the wind farm, the auxiliary converters may experience transient power transfer and overcurrent problems, again requiring a large capacity configuration. However, large-capacity auxiliary converters reduce the compactness and low-cost advantages of diode rectifier DR offshore wind power transmission systems. Summary of the Invention

[0007] This application provides an offshore wind power transmission system and a method, apparatus, and equipment for determining the reactive power grid control strategy, which solves the technical problems of transient power transfer, overcurrent, and high cost in existing offshore wind power transmission systems that use auxiliary converters to achieve grid construction.

[0008] To achieve the above objectives, this application provides the following technical solution: On the one hand, a method for determining the reactive power grid control strategy of an offshore wind power transmission system is provided, which is applied to the offshore wind power transmission system. The offshore wind power transmission system includes a wind farm and several wind turbines, main converters, and auxiliary converters connected to the grid connection point of the wind farm. The auxiliary converters are connected to power supply equipment for providing active power. The method for determining the reactive power grid control strategy includes the following steps: Obtain the operating status of the offshore wind power transmission system; Based on the operating conditions, determine the reactive power grid control strategy for the offshore wind power transmission system; If the operating condition is the black start condition of the wind farm, then the reactive power grid control strategy is the auxiliary converter reactive power grid control strategy used to provide dynamic reactive power support capability. If the operating condition is the wind turbine in operation, then the reactive power grid control strategy is the grid-connected wind turbine control strategy, the auxiliary converter reactive power grid control strategy, and the centralized wind turbine reactive power distribution link used to undertake the steady-state reactive power in the wind farm.

[0009] Optionally, the content of the auxiliary converter reactive power grid control strategy includes: The following parameters are obtained for the offshore wind power transmission system: grid connection frequency, minimum current limit, DC voltage reference value, first current on the DC side of the main converter, target value of the q-axis voltage component of the auxiliary converter, second current on the DC side of the auxiliary converter, target value of capacitor voltage of sub-module in the auxiliary converter, average value of capacitor voltage of sub-module in the auxiliary converter, active power injected into the wind farm by the auxiliary converter, reference value of active power injected into the wind farm by the auxiliary converter, and three-phase voltage at the grid connection point of the auxiliary converter in the wind farm. The reference value of the DC voltage for the auxiliary converter DC side control is determined based on the target value of the capacitor voltage, the average value of the capacitor voltage, the reference value of the DC voltage, the second current, and the reference value of the DC voltage. The three-phase voltage is subjected to Parker transformation to obtain the q-axis voltage component; based on the target value of the q-axis voltage component and the q-axis voltage component, the q-axis current control reference value for the AC side control of the auxiliary converter is determined; based on the minimum current limit, the first current, the active power, and the active power reference value, the d-axis current control reference value for the AC side control of the auxiliary converter is determined. The grid formation frequency is integrated to obtain the grid formation reference phase; based on the grid formation reference phase, the q-axis current control reference value, and the d-axis current control reference value, the AC voltage reference value for the AC side control of the auxiliary converter is determined. Based on the DC voltage reference value and the AC voltage reference value, a pulse control signal is determined to control the operation of the sub-module in the auxiliary converter.

[0010] Optionally, the main converter is also connected to several reactive power compensation devices, and the centralized wind turbine reactive power distribution system includes: The system acquires the following information: the number of wind turbines in operation of the offshore wind power transmission system; the switching status of each reactive power compensation device; the rated capacity of each reactive power compensation device; the output active power of each wind turbine; the grid-connected AC voltage of the main converter connected to the grid; the reactive power output power injected into the wind farm by the auxiliary converter; the reference value of the reactive power output power injected into the wind farm by the auxiliary converter; and the main commutation reactance, main DC voltage, and main DC current of the main converter. Based on the output active power of all the wind turbines, the main active power flowing into the main converter is determined; based on the grid-connected AC voltage, the main commutation reactance, the main DC voltage, the main DC current, and the main active power, the main reactive power flowing into the main converter is calculated. The reactive power demand data of the wind farm is determined based on the main reactive power, the switching status of all the reactive power compensation equipment, and the rated capacity. The reactive power output data of the auxiliary converter is determined based on the reactive power output power and the reactive power output power reference value. Based on the reactive power demand data of the wind farm and the reactive power output data, the total reactive power output data of the offshore wind power transmission system is determined; The total reactive power output data is evenly distributed to each of the wind turbines in operation based on the number of wind turbines put into operation; Among them, the reactive power output data allocated to each of the wind turbines put into operation is used as the reference value for the reactive power output of the wind turbines.

[0011] Optionally, the centralized wind turbine reactive power distribution link further includes: calculating the main reactive power flowing into the main converter based on the grid-connected AC voltage, the main commutation reactance, the main DC voltage, the main DC current, and the main active power using the reactive power demand formula of the main converter; the reactive power demand formula is: ; In the formula, u pcc For grid-connected AC voltage, u dc and i dc These are the main DC voltage and the main DC current, respectively. Main commutation reactor, This is the commutation overlap angle. The power factor angle on the AC side of the main converter. P DR and Q DR These are the main active power and main reactive power flowing into the main converter, respectively.

[0012] Optionally, the content of the grid-connected wind turbine control strategy includes: Obtain the reference value of reactive power output, the actual value of reactive power output, the DC bus voltage and the rated value of DC bus voltage of each of the aforementioned wind turbines; Based on the DC bus voltage and rated DC bus voltage of each wind turbine, determine the reference value of the d-axis current of the corresponding wind turbine; based on the reference value of the reactive power output and the actual value of the reactive power output of each wind turbine, determine the reference value of the q-axis current of the corresponding wind turbine. The d-axis current reference value and the q-axis current reference value of each wind turbine are decoupled by dq to obtain the reference voltage of the grid-side converter connected to the corresponding wind turbine.

[0013] On the other hand, an offshore wind power transmission system is provided, including a control module, a wind farm, and several wind turbines, a main converter, and an auxiliary converter connected to the grid connection point of the wind farm. The auxiliary converter is connected to a power supply device for providing active power via a tie line. The DC side of the main converter is connected to an onshore converter. The control module is used to control the reactive power grid connection control strategy of the offshore wind power transmission system according to the reactive power grid connection control strategy determination method of the offshore wind power transmission system described above.

[0014] Optionally, the main converter is a 12-pulse or dual 12-pulse cascaded diode rectifier, and the auxiliary converter is a modular multilevel converter.

[0015] On another front, a reactive power grid control strategy determination device 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 includes a wind farm and several wind turbines, main converters and auxiliary converters connected to the grid connection point of the wind farm. The auxiliary converters are connected to power supply equipment for providing active power. The reactive power grid control strategy determination device includes: a data acquisition module and a control strategy determination module. The data acquisition module is used to acquire the operating conditions of the offshore wind power transmission system; The control strategy determination module is used to determine the reactive power grid control strategy of the offshore wind power transmission system based on the operating conditions. Wherein, if the operating condition is the wind farm black start condition, then the reactive power grid control strategy is the auxiliary converter reactive power grid control strategy for providing dynamic reactive power support capability. If the operating condition is the wind turbine in operation, then the reactive power grid control strategy is the grid-connected wind turbine control strategy, the auxiliary converter reactive power grid control strategy, and the centralized wind turbine reactive power distribution link used to undertake the steady-state reactive power in the wind farm.

[0016] Optionally, the main converter is also connected to several reactive power compensation devices, and the reactive power grid control strategy of the auxiliary converter includes: The following parameters are obtained for the offshore wind power transmission system: grid connection frequency, minimum current limit, DC voltage reference value, first current on the DC side of the main converter, target value of the q-axis voltage component of the auxiliary converter, second current on the DC side of the auxiliary converter, target value of capacitor voltage of sub-module in the auxiliary converter, average value of capacitor voltage of sub-module in the auxiliary converter, active power injected into the wind farm by the auxiliary converter, reference value of active power injected into the wind farm by the auxiliary converter, and three-phase voltage at the grid connection point of the auxiliary converter in the wind farm. The DC voltage reference value for the auxiliary converter DC side control is determined based on the target value of the capacitor voltage, the average value of the capacitor voltage, the second current, and the DC voltage reference value. The three-phase voltage is subjected to Parker transformation to obtain the q-axis voltage component; based on the target value of the q-axis voltage component and the q-axis voltage component, the q-axis current control reference value for the AC side control of the auxiliary converter is determined; based on the minimum current limit, the first current, the active power, and the active power reference value, the d-axis current control reference value for the AC side control of the auxiliary converter is determined. The grid formation frequency is integrated to obtain the grid formation reference phase; based on the grid formation reference phase, the q-axis current control reference value, and the d-axis current control reference value, the AC voltage reference value for the AC side control of the auxiliary converter is determined. Based on the DC voltage reference value and the AC voltage reference value, determine the pulse control signal for controlling the operation of the sub-module in the auxiliary converter; And / or, The contents of the centralized wind turbine reactive power distribution process include: The system acquires the following information: the number of wind turbines in operation of the offshore wind power transmission system; the switching status of each reactive power compensation device; the rated capacity of each reactive power compensation device; the output active power of each wind turbine; the grid-connected AC voltage of the main converter connected to the grid; the reactive power output power injected into the wind farm by the auxiliary converter; the reference value of the reactive power output power injected into the wind farm by the auxiliary converter; and the main commutation reactance, main DC voltage, and main DC current of the main converter. Based on the output active power of all the wind turbines, the main active power flowing into the main converter is determined; based on the grid-connected AC voltage, the main commutation reactance, the main DC voltage, the main DC current, and the main active power, the main reactive power flowing into the main converter is calculated. The reactive power demand data of the wind farm is determined based on the main reactive power, the switching status of all the reactive power compensation equipment, and the rated capacity. The reactive power output data of the auxiliary converter is determined based on the reactive power output power and the reactive power output power reference value. Based on the reactive power demand data of the wind farm and the reactive power output data, the total reactive power output data of the offshore wind power transmission system is determined; The total reactive power output data is evenly distributed to each of the wind turbines in operation based on the number of wind turbines put into operation; Among them, the reactive power output data allocated to each of the wind turbines put into operation is used as the reactive power output reference value of the wind turbines; And / or, The content of the grid-connected wind turbine control strategy includes: Obtain the reference value of reactive power output, the actual value of reactive power output, the DC bus voltage and the rated value of DC bus voltage of each of the aforementioned wind turbines; Based on the DC bus voltage and rated DC bus voltage of each wind turbine, determine the reference value of the d-axis current of the corresponding wind turbine; based on the reference value of the reactive power output and the actual value of the reactive power output of each wind turbine, determine the reference value of the q-axis current of the corresponding wind turbine. The d-axis current reference value and the q-axis current reference value of each wind turbine are decoupled by dq to obtain the reference voltage of the grid-side converter connected to the corresponding wind turbine.

[0017] On the other hand, a terminal device is provided, including a processor and a memory; The memory is used to store program code and transmit the program code to the processor; The processor is used to execute the above-described method for determining the reactive power grid control strategy of the offshore wind power transmission system according to the instructions in the program code.

[0018] This invention relates to an offshore wind power transmission system and its reactive power grid control strategy determination method, apparatus, and equipment. The offshore wind power transmission system includes a wind farm and several wind turbines, a main converter, and auxiliary converters connected to the grid connection point of the wind farm. The auxiliary converters are connected to power supply equipment for providing active power. The reactive power grid control strategy determination method includes acquiring the operating conditions of the offshore wind power transmission system; determining the reactive power grid control strategy of the offshore wind power transmission system based on the operating conditions; if the operating condition is a black start condition of the wind farm, the reactive power grid control strategy is an auxiliary converter reactive power grid control strategy for providing dynamic reactive power support; if the operating condition is a wind turbine commissioning condition, the reactive power grid control strategy is a grid-connected wind turbine control strategy, an auxiliary converter reactive power grid control strategy, and a centralized wind turbine reactive power distribution link for undertaking the steady-state reactive power within the wind farm.

[0019] As can be seen from the above technical solutions, this application has the following advantages: The method for determining the reactive power grid connection control strategy of the offshore wind power transmission system determines different reactive power grid connection control strategies based on the operating conditions of the offshore wind power transmission system, realizing reactive power grid connection of wind turbines and auxiliary converters. This ensures that the auxiliary converter does not need to bear a high reactive power output, avoiding the problem of excessive power transfer during grid connection. The auxiliary converter only needs to have a certain transient reactive power output function, which can greatly reduce the capacity requirement of the auxiliary converter. Further optimization of the capacity of the auxiliary converter can achieve compactness and low cost, thereby reducing the construction cost of the offshore wind power transmission system. This solves the technical problems of transient power transfer, overcurrent, and high cost in the existing offshore wind power transmission system using auxiliary converters for grid connection.

[0020] The reactive power grid connection control strategy determination device for this offshore wind power transmission system obtains the operating conditions of the offshore wind power transmission system through a data acquisition module and a control strategy determination module to determine different reactive power grid connection control strategies. This enables reactive power grid connection of wind turbines and auxiliary converters, allowing the auxiliary converters to avoid bearing high reactive power output and preventing excessive power transfer during grid connection. The auxiliary converters only need to have a certain transient reactive power output function, which can greatly reduce the capacity requirements of the auxiliary converters. Further optimization of the capacity of the auxiliary converters can achieve compactness and low cost, thereby reducing the construction cost of the offshore wind power transmission system. Attached Figure Description

[0021] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 This is a flowchart illustrating the steps of the method for determining the reactive power grid control strategy of an offshore wind power transmission system as described in this application embodiment; Figure 2 This is a schematic diagram of the topology of the offshore wind power transmission system described in the embodiments of this application; Figure 3 This is a schematic diagram of the framework of the reactive power grid control strategy of the auxiliary converter in the method for determining the reactive power grid control strategy of the offshore wind power transmission system described in the embodiments of this application. Figure 4 This is a schematic diagram of the framework of the centralized wind turbine reactive power distribution link in the reactive power grid control strategy determination method of the offshore wind power transmission system described in the embodiments of this application. Figure 5This is a schematic diagram of the framework of the grid-following wind turbine control strategy in the reactive power grid control strategy determination method of the offshore wind power transmission system described in the embodiments of this application; Figure 6 A schematic diagram of the framework of the reactive power grid control strategy determination device for an offshore wind power transmission system according to another embodiment of this application; Figure 7 This is a schematic diagram of the terminal device described in an embodiment of this application. Detailed Implementation

[0023] To make the inventive objectives, features, and advantages of this application more apparent and understandable, the technical solutions in the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the embodiments described below are only some embodiments of this application, and not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0024] In the description of the embodiments of this application, 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 one or more of that feature. In the description of the embodiments of this application, "multiple" means two or more, unless otherwise explicitly specified.

[0025] In the embodiments of this application, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in the embodiments of this application according to the specific circumstances.

[0026] Patent terminology used in this application: The operational status of a wind turbine typically refers to the state in which the turbine has completed the startup process, is normally connected to the grid, and is generating electricity. In other words, all systems of the unit are normal, the generator is connected to the grid, and is outputting electrical energy.

[0027] The Parker transformation is a commonly used coordinate transformation method in motors and power systems. The coordinate system transformation of the Parker transformation is to transform the three-phase stationary coordinate system (a, b, c) to the rectangular coordinate system (d, q axis).

[0028] Reactive power compensation equipment is a collection of devices used in a power system to regulate reactive power. Reactive power compensation equipment can include capacitor reactors, static var compensators (SVCs), static synchronous compensators (STATCOMs), etc.

[0029] A power module is an industrial product that combines and packages power semiconductor devices such as IGBTs and MOSFETs according to specific circuit functions. Its core function is to realize the conversion and control of electrical energy.

[0030] This application provides an offshore wind power transmission system and a method, apparatus, and equipment for determining its reactive power grid control strategy, which solves the technical problems of transient power transfer, overcurrent, and high cost in existing offshore wind power transmission systems that use auxiliary converters to achieve grid construction.

[0031] Example 1: Figure 1 This is a flowchart illustrating the steps of the method for determining the reactive power grid control strategy of an offshore wind power transmission system according to an embodiment of this application. Figure 2 This is a topology diagram of the offshore wind power transmission system described in the embodiments of this application.

[0032] In the embodiments of this application, the offshore wind power transmission system includes a control module, a wind farm 10, and several wind turbines 20, a main converter DR, and an auxiliary converter MMC connected to the grid connection point PCC of the wind farm 10. The auxiliary converter MMC is connected to a power supply device 30 for providing active power via a tie line. The control module is used to control the reactive power grid connection control strategy of the offshore wind power transmission system according to the reactive power grid connection control strategy determination method of the offshore wind power transmission system.

[0033] It should be noted that the power supply equipment 30 can be other wind farms or active power sources, such as generators, energy storage converters, frequency converters, rectifiers, energy storage units (such as batteries, capacitors, etc.). The main converter DR can be selected as a diode rectifier. In this embodiment, as... Figure 2 As shown, all wind turbines in wind farm 10 are connected to the offshore wind power transmission system via an offshore medium-voltage AC system, with the common connection point denoted as the point of common connection (PCC). At the PCC, the main converter DR and auxiliary converter MMC of the offshore wind power transmission system are connected separately. The DC terminal of the main converter DR is connected to the multi-terminal DC grid in the wind farm cluster. The DC terminal of the auxiliary converter MMC is connected to other wind farms or small-capacity power sources within the cluster via small-capacity DC tie lines.

[0034] In the embodiments of this application, the main converter DR is a 12-pulse or dual 12-pulse cascaded diode rectifier, and the auxiliary converter MMC can be selected as a modular multilevel converter (MMC). The modular multilevel converter (MMC) includes several sub-modules connected in series.

[0035] It should be noted that the main converter DR can adopt a 12-pulse topology main converter DR, with the DC end connected to the onshore converter. The auxiliary converter MMC can adopt a voltage source converter structure such as a modular multilevel converter MMC to solve the grid connection problem of wind farms. In addition, the auxiliary converter MMC can also further solve the problem of active power source in wind farms during black start and no-wind operation caused by the main converter DR's inability to feed back power by connecting a small-capacity power source. The submodules can be selected as half-bridge or full-bridge power modules.

[0036] like Figure 2 As shown in the embodiments of this application, in the sub-module topology design of the auxiliary converter MMC, although the sub-module adopts a half-bridge power module, it can meet all the functional requirements of the auxiliary converter MMC, but its size and cost issues make it basically not feasible in engineering. Preferably, the sub-module should preferably use a full-bridge power module.

[0037] It should be noted that, as Figure 2 As shown, the auxiliary converter MMC exhibits an asymmetry between AC and DC power: the DC side of the auxiliary converter MMC only needs to transmit the active power required by the wind farm, resulting in low power demand. However, the AC side of the auxiliary converter MMC, in addition to transmitting the same active power, also has a greater reactive power output requirement due to grid connection functionalities. Conventionally, the DC-side voltages of the auxiliary converter MMC, constructed using half-bridge power modules, have a fixed coupling relationship. If a higher AC voltage is configured based on the high reactive power demand on the AC side, the DC-side voltage will be forced to rise. For the DC side, which only transmits active power, an excessively high DC voltage will cause the tie lines to operate in an inefficient state of high voltage and low current, resulting in idle cable current-carrying capacity and redundant insulation costs. Therefore, hybrid submodules (such as the parallel half-bridge / full-bridge hybrid submodule P-HFBSM disclosed in announcement number CN107800317A) can be used to construct the auxiliary converter MMC to reduce the DC-side voltage and improve the economy of the auxiliary converter MMC and tie lines. Hybrid submodules can reduce the number of submodules without changing the total number of switching devices (such as IBGT transistors). Because grid connection functions place significant reactive power pressure on the auxiliary converter MMC, a reactive power grid connection control strategy for offshore wind power transmission systems is adopted. This strategy utilizes the slower-responding wind turbines to handle the system's steady-state reactive power demand, while the auxiliary converter MMC only provides temporary support after the wind farm is disturbed. The auxiliary converter MMC outputs almost no reactive power in steady state, only rapidly compensating for reactive power imbalances during transient processes and providing some control capability, thus significantly reducing capacity requirements.

[0038] like Figure 1 As shown in the figure, this application provides a method for determining the reactive power grid control strategy of an offshore wind power transmission system, which is applied to an offshore wind power transmission system. The method for determining the reactive power grid control strategy of an offshore wind power transmission system includes the following steps: S1. Obtain the operating conditions of the offshore wind power transmission system.

[0039] It should be noted that in the process of determining the reactive power grid control strategy for offshore wind power transmission systems, such as Figure 2 As shown, the offshore wind power transmission system obtains its operating conditions through step S1, providing fundamental data for subsequent steps to determine the reactive power grid control strategy. In this embodiment, the operating conditions of the offshore wind power transmission system include the wind farm black start condition and the wind turbine start-up condition. The wind turbine start-up condition refers to the operating condition where the wind turbine starts to output power, such as the normal operation condition and the start-up condition. The wind farm black start condition refers to the process where, after the entire power system is shut down due to a fault, the system starts up without external network assistance, through generators with self-starting capabilities (such as wind farms), or through external power supply, driving other units to gradually restore the operation of the offshore wind power transmission system. The start-up condition of the wind turbine start-up condition is the process of the wind turbine transitioning from a static state to a normal operating state. The normal operating condition of a wind turbine can be understood as the most stable and common operating condition of the wind turbine during normal operation. At this time, the wind turbine is in its best working state, in grid-connected power generation state, and can output active power according to maximum power tracking or given power command, providing stable air flow or wind power generation.

[0040] In this embodiment of the application, the method for determining the reactive power grid control strategy of the offshore wind power transmission system can determine the operating conditions of the wind farm based on the operating data of the offshore wind power transmission system.

[0041] It should be noted that the operating condition of the wind farm—whether it is a black start condition or a wind turbine in operation condition—is determined based on whether the active power output of the wind farm is lower than a set threshold (e.g., 0.15pu, where pu is the per-unit value of active power). Specifically: if the active power output of the wind farm is not greater than the set threshold, it needs to enter the black start condition, and the wind farm's operating condition is then the wind farm's black start condition. If the active power output of the wind farm is greater than the set threshold, the wind farm's operating condition is the wind turbine in operation condition.

[0042] S2. Determine the reactive power grid connection control strategy for the offshore wind power transmission system based on the operating conditions. If the operating condition is the black start condition of the wind farm, the reactive power grid connection control strategy is the auxiliary converter reactive power grid connection control strategy to provide dynamic reactive power support capability; if the operating condition is the wind turbine commissioning condition, the reactive power grid connection control strategy is the grid-connected wind turbine control strategy, the auxiliary converter reactive power grid connection control strategy, and the centralized wind turbine reactive power distribution link to undertake the steady-state reactive power within the wind farm.

[0043] It should be noted that step S2 determines the reactive power grid control strategy based on preset rules according to the operating conditions of the offshore wind power transmission system obtained in step S1. In this embodiment, based on... Figure 2 The illustrated offshore wind power transmission system topology shows that the steady-state reactive power demand of wind farm 10 is borne by wind turbine 20. The auxiliary converter MMC only needs to provide rapid grid connection support when disturbances occur, significantly reducing the capacity requirement of the auxiliary converter MMC. Therefore, the method for determining the reactive power grid connection control strategy of this offshore wind power transmission system can formulate the reactive power grid connection control strategy based on the operating conditions of the offshore wind power transmission system. Specifically, during the black start operation of the wind farm, some control loops may not operate. In the initial stage of black start, the wind turbines are not yet operational, so the grid-connected turbine control strategy and the centralized turbine reactive power distribution link are ineffective. Only the reactive power grid connection control strategy of the auxiliary converter MMC is active. The wind turbines do not bear any reactive power; only the auxiliary converter balances all the reactive power of the offshore wind power transmission system when performing its reactive power grid connection function. As the wind turbines gradually start up, the grid-connected turbine control strategy and the centralized turbine reactive power distribution link become effective. The preset rules are as follows: if the operating condition is the black start condition of the wind farm, the reactive power grid control strategy is the auxiliary converter reactive power grid control strategy to provide dynamic reactive power support capability; if the operating condition is the wind turbine in operation, the reactive power grid control strategy is the grid-connected wind turbine control strategy, the auxiliary converter reactive power grid control strategy, and the centralized wind turbine reactive power distribution link to undertake the steady-state reactive power in the wind farm.

[0044] In the embodiments of this application, the auxiliary converter MMC can adopt a voltage source converter topology with highly flexible control capabilities, such as a modular multilevel converter (MMC). In addition to the network construction function, it also has other potential control capabilities, such as filtering and bidirectional active power transmission.

[0045] In this embodiment, the AC grid construction of the DR offshore wind farm based on diode rectifiers relies on the auxiliary converter reactive power grid construction control strategy, the centralized wind turbine reactive power distribution link, and the grid-connected wind turbine control strategy. These three reactive power grid construction control strategies operate continuously throughout the entire operating cycle of the wind farm. The auxiliary converter reactive power grid construction control strategy is used for reactive power grid construction. The centralized wind turbine reactive power distribution link transfers the steady-state reactive power of the offshore wind power transmission system from the auxiliary converter to the wind turbines, thereby reducing the capacity pressure on the auxiliary converter's MMC. The grid-connected wind turbine control strategy controls the grid-connected operation of wind turbine 20.

[0046] It should be noted that the auxiliary converter bears all the steady-state reactive power demand of the offshore wind power transmission system, requiring the auxiliary converter to have extremely high reactive power output capability. The method for determining the reactive power grid construction control strategy of the offshore wind power transmission system is based on the operating condition of the offshore wind power transmission system as the black start condition of the wind farm. The auxiliary converter reactive power grid construction control strategy is used as the AC grid construction of the offshore wind power transmission system. It is clear that the steady-state reactive power undertaken by the auxiliary converter is only an additional effect brought about by this type of auxiliary converter reactive power grid construction control strategy. The steady-state reactive power is used to balance the reactive power demand of the diode rectifier (DR) and transformer within the wind farm and will not affect the grid construction function of the auxiliary converter. The grid construction control of the auxiliary converter mainly relies on the coupling relationship between dynamic reactive power injection and frequency. Therefore, when the offshore wind power transmission system operates under the condition of wind turbine operation, the centralized wind turbine reactive power distribution link and the grid-connected wind turbine control strategy are combined as the reactive power grid connection control strategy. The method for determining the reactive power grid connection control strategy of the offshore wind power transmission system adopts the centralized wind turbine reactive power distribution link to distribute the steady-state reactive power that does not undertake the grid connection function to each wind turbine 20, thereby reducing the capacity requirement of the auxiliary converter MMC. The grid-connected wind turbine control strategy is adopted to control the operation of wind turbine 20, so that wind turbine 20 undertakes the steady-state reactive power of the offshore wind power transmission system. The auxiliary converter MMC dynamically adjusts the reactive current injection to achieve frequency stability control.

[0047] This application provides a method for determining the reactive power grid connection control strategy of an offshore wind power transmission system. The offshore wind power transmission system includes a wind farm and several wind turbines, a main converter, and auxiliary converters connected to the grid connection point of the wind farm. The auxiliary converters are connected to power supply equipment for providing active power. The method for determining the reactive power grid connection control strategy includes acquiring the operating conditions of the offshore wind power transmission system; determining the reactive power grid connection control strategy of the offshore wind power transmission system based on the operating conditions; if the operating condition is a black start condition of the wind farm, the reactive power grid connection control strategy is an auxiliary converter reactive power grid connection control strategy for providing dynamic reactive power support capability; if the operating condition is a wind turbine commissioning condition, the reactive power grid connection control strategy is a grid-connected wind turbine control strategy, an auxiliary converter reactive power grid connection control strategy, and a centralized wind turbine reactive power distribution link for undertaking the steady-state reactive power within the wind farm. This method for determining the reactive power grid connection control strategy of an offshore wind power transmission system determines different reactive power grid connection control strategies based on the operating conditions of the offshore wind power transmission system. It enables reactive power grid connection of wind turbines and auxiliary converters, allowing the auxiliary converters to avoid bearing high reactive power output and preventing excessive power transfer during grid connection. The auxiliary converters only need to have a certain transient reactive power output function, which can greatly reduce the capacity requirements of the auxiliary converters. Further optimization of the capacity of the auxiliary converters can achieve compactness and low cost, thereby reducing the construction cost of the offshore wind power transmission system. This method solves the technical problems of transient power transfer, overcurrent, and high cost in the use of auxiliary converters for grid connection in existing offshore wind power transmission systems.

[0048] In this embodiment, the reactive power grid control strategy determination method for the offshore wind power transmission system is realized by relying on the dynamic change of injected reactive power to control the grid frequency of the reactive power grid control strategy. This proposes a grid control approach in which the wind turbine undertakes steady-state reactive power and the auxiliary converter only performs dynamic reactive power injection adjustment, thereby reducing the capacity requirements of the auxiliary converter.

[0049] It should be noted that in the reactive power grid connection control strategy using the auxiliary converter, grid connection control of the wind farm 10, which includes diode rectifiers, is achieved through reactive current injection from the auxiliary converter. Compared to existing solutions, the auxiliary converter MMC does not have steady-state power output; it only needs to have dynamic reactive current injection capability, which significantly reduces capacity requirements. The auxiliary converter MMC can also realize potential functions such as active power transmission, etc. Figure 3 As shown.

[0050] Figure 3 This is a schematic diagram of the framework of the reactive power grid control strategy of the auxiliary converter in the method for determining the reactive power grid control strategy of the offshore wind power transmission system described in the embodiments of this application.

[0051] like Figure 3 As shown, in one embodiment of this application, the content of the auxiliary converter reactive power grid control strategy includes: The following parameters are obtained for the offshore wind power transmission system: grid connection frequency, minimum current limit, DC voltage reference value, first current on the DC side of the main converter, target value of q-axis voltage component of the auxiliary converter, second current on the DC side of the auxiliary converter, target value of capacitor voltage of sub-module in the auxiliary converter, average value of capacitor voltage of sub-module in the auxiliary converter, active power injected into the wind farm by the auxiliary converter, reference value of active power injected into the wind farm by the auxiliary converter, and three-phase voltage at the grid connection point of the auxiliary converter in the wind farm. Based on the target value of capacitor voltage, the average value of capacitor voltage, the second current, and the DC voltage reference value, determine the DC voltage reference value for the DC side control of the auxiliary converter; The three-phase voltage is subjected to Parker transformation to obtain the q-axis voltage component; based on the target value of the q-axis voltage component and the q-axis voltage component, the q-axis current control reference value for the AC side control of the auxiliary converter is determined; based on the minimum current limit, the first current, the active power, and the active power reference value, the d-axis current control reference value for the AC side control of the auxiliary converter is determined. The grid construction frequency is integrated to obtain the grid construction reference phase; based on the grid construction reference phase, the q-axis current control reference value, and the d-axis current control reference value, the AC voltage reference value for the auxiliary converter AC side control is determined. Based on the DC voltage reference value and the AC voltage reference value, determine the pulse control signal for controlling the operation of the sub-module in the auxiliary converter.

[0052] It should be noted that in the auxiliary converter reactive power grid construction control strategy, the grid construction control of the auxiliary converter reactive power grid construction strategy mainly relies on the q-axis current control of the auxiliary converter AC side and the grid construction reference phase. θ The generation section includes the q-axis current control of the auxiliary converter's AC side, which generates the q-axis current control reference value for the auxiliary converter. I q，ref The auxiliary converter has an open-loop grid frequency. ω 0 generated network reference phase θ When the three-phase voltage of the offshore wind power transmission system V abc AC voltage vector and network reference phase θ When a deviation occurs (i.e., the q-axis voltage component) V q When the value is ≠ 0, the q-axis current control of the auxiliary converter AC side will generate a corresponding q-axis current reference value. I q，ref By injecting reactive current into the wind farm, the three-phase voltage is increased. V abcThe AC voltage vector is again in phase with the grid reference phase. θ Overlapping, thereby achieving the goal of controlling the frequency stability of three-phase voltage, three-phase voltage V abc The AC voltage vector can always be in phase with the grid reference. θ Alignment. In addition to network control, Figure 3 The auxiliary converter reactive power grid control strategy shown can also obtain active power transmission function through external power supply equipment, serving as an emergency power source for wind farm black start and no-wind operation. The active power transmission function of the auxiliary converter is mainly realized through the auxiliary converter MMC AC-side active power control and DC-side control of the auxiliary converter reactive power grid control strategy.

[0053] In this embodiment of the application, the three-phase voltage of the auxiliary converter connected to the grid connection point is... V abc After passing through the network reference phase θ After Parker transformation based on the standard, the three-phase voltage V abc Vector and mesh reference phase θ The d-axis of the constructed dq coordinate system is deviated (i.e., the voltage vector shows a q-axis voltage component in the reference dq coordinate system of the network). V q ).

[0054] like Figure 3 As shown in the embodiments of this application, the q-axis current control of the auxiliary converter AC side can be understood as performing a Parker transformation on the three-phase voltage to obtain the q-axis voltage component. V q Based on the target value of the q-axis voltage component V q，ref and q-axis voltage component V q The reference value for q-axis current control of the auxiliary converter AC side is obtained by using the first formula of the first PI controller.

[0055] It should be noted that the first formula is: ; In the formula, S For the Laplace operator, k p1 The proportional gain of the first PI controller. k i1 The integral coefficient of the first PI controller is... I q，ref This is the reference value for q-axis current control, used to assist in the AC side control of the converter. The target value for the q-axis voltage component is... V q，ref Set to 0.

[0056] like Figure 3 As shown in the embodiments of this application, the active power control on the AC side of the auxiliary converter MMC can be understood as being based on the minimum current limit. DRmin First current I DR Active power P c and active power reference value P c，ref The d-axis current control reference value for the AC side control of the auxiliary converter is determined using a preset formula or preset rule. I d，ref .

[0057] It should be noted that the active power control on the AC side of the auxiliary converter MMC includes: setting the minimum current limit. DRmin With the first current I DR Subtracting the two values ​​yields the current difference; this current difference is then processed by a PI controller and a filter to obtain a reference value for the active power injected into the wind farm by the auxiliary converter. P c，ref According to active power P c and active power reference value P c，ref The reference value for d-axis current control of the auxiliary converter AC side is obtained by calculating using the second formula of the second PI controller. I d，ref The second formula is: ; In the formula, k p2 The proportional gain of the second PI controller. k i2 is the integral coefficient of the second PI controller.

[0058] like Figure 3 As shown in the embodiments of this application, in the auxiliary converter reactive power grid control strategy, the inner current loop control can be based on the input d-axis current control reference value. I d，ref and q-axis current control reference value I q，ref The AC voltage reference value for the AC side control of the auxiliary converter is obtained using the third formula and the Parker inverse transform formula. V acref .

[0059] It should be noted that the third formula is: ; The formula for the inverse Parker transform is: ; In the formula, L To assist the external equivalent reactance of the converter, k p This is the proportional coefficient for the inner current loop control. k i This represents the integral coefficient for the current inner loop control. v acref( d )and v acref ( q The reference AC voltage value for the three-phase AC side control input is obtained after Parker inverse transformation. V acref .in, I d and I q These represent the three-phase currents at the grid connection point of the auxiliary converter in the wind farm. I abc The corresponding d-axis current components and q-axis current components after Parker transformation.

[0060] like Figure 3 As shown in the embodiments of this application, in the DC-side control of the auxiliary converter reactive power grid control strategy, the DC-side control of the auxiliary converter MMC includes: based on the target value of the capacitor voltage. cavg Average capacitor voltage U cavg Second current I dc and DC voltage reference value V dc0 The reference value of the DC voltage for the auxiliary converter DC side control is determined through a two-stage PI controller. V dcref .

[0061] It should be noted that the DC-side control of the auxiliary converter MMC can be understood as being based on the target value of the capacitor voltage. cavg and average capacitor voltage U cavg The DC current control reference value is obtained by calculating using the third formula of the third PI controller. I dcref Then, the reference value is controlled based on the DC current. I dcref Second current I dc and DC voltage reference value V dc0The DC voltage reference value for the auxiliary converter DC side control is calculated using the fourth formula of the fourth PI controller. V dcref The third formula is: ; The fourth formula is: ; In the formula, k p3 The proportional gain of the third PI controller. k i3 The integral coefficient of the third PI controller. k p4 The proportional gain of the fourth PI controller. k i4 This represents the integral coefficient of the fourth PI controller.

[0062] like Figure 3 As shown in the embodiments of this application, in the valve-level control of the reactive power grid control strategy of the auxiliary converter, valve-level control can be understood as converting the voltage reference value generated by the upper-level control into the bridge arm voltage reference value on each bridge arm in the auxiliary converter, and generating control signals for each switching device (such as IGBT) on one bridge arm accordingly. Specifically, in valve-level control, the data input to valve-level control includes DC voltage reference values. V dcref AC voltage reference value V acref The data output by the valve-level control is the control signal for each arm in the auxiliary converter MMC, such as a certain phase. j upper arm bridge arm voltage reference value V jp A certain phase j Lower arm bridge arm voltage reference value V jn It can be obtained through DC voltage reference value V dcref AC voltage reference value V acref The calculation is performed using the fifth formula, which is: ; ; In the formula, V jp for j Mutually( j The reference values ​​for the bridge arm voltages of phases a, b, and c are given. V jn for j Mutually( jThe reference values ​​for the bridge arm voltages of phases a, b, and c are used. Then, based on the rated voltage of each submodule and the bridge arm voltage reference values, the number of auxiliary converter MMC submodules to be engaged in each bridge arm is determined, and finally, a pulse control signal is generated for each bridge arm submodule.

[0063] To achieve reactive power distribution, the reactive power grid control strategy of this offshore wind power transmission system employs a centralized wind turbine reactive power distribution link. The controller of this centralized link is located on the offshore wind power transmission system, issuing reactive power output data allocated to each operating wind turbine as a reference value for the turbine's reactive power output. The centralized wind turbine reactive power distribution link uses a combination of feedforward compensation and closed-loop control, combining a feedforward link with high-speed response but steady-state error with a closed-loop control with very low bandwidth but zero steady-state error. Therefore, it can maintain good control stability and dynamic characteristics even under actual operating conditions with communication delays in the wind turbines, and possesses a high response speed.

[0064] Figure 4 This is a schematic diagram of the framework of the centralized wind turbine reactive power distribution link in the reactive power grid control strategy determination method of the offshore wind power transmission system described in the embodiments of this application.

[0065] like Figure 4 As shown, in one embodiment of this application, the main converter DR is also connected to several reactive power compensation devices, and the centralized wind turbine reactive power distribution process includes: The system obtains the number of wind turbines in operation, the switching status of each reactive power compensation device, the rated capacity of each reactive power compensation device, the output active power of each wind turbine, the grid-connected AC voltage of the main converter at the grid connection point, the reactive power output of the auxiliary converter injected into the wind farm, the reference value of the reactive power output of the auxiliary converter injected into the wind farm, and the main commutation reactance, main DC voltage, and main DC current of the main converter. Based on the output active power of all wind turbines, determine the main active power flowing into the main converter; based on the grid-connected AC voltage, main commutation reactance, main DC voltage, main DC current and main active power, calculate the main reactive power flowing into the main converter. The reactive power demand data of the wind farm is determined based on the main reactive power, the switching status and rated capacity of all reactive power compensation equipment; The reactive power output data of the auxiliary converter is determined based on the reactive power output power and the reactive power output power reference value. Based on the reactive power demand data and reactive power output data of the wind farm, the total reactive power output data of the offshore wind power transmission system is determined; The total reactive power output data is evenly distributed to each operating wind turbine based on the number of wind turbines in operation. Among them, the reactive power output data allocated to each wind turbine put into operation is used as the reference value for the reactive power output of the wind turbine.

[0066] It should be noted that the reactive power output reference value of the auxiliary converter... Q cref It can be set to 0. The centralized wind turbine reactive power distribution stage is used to handle the steady-state reactive power demand of the wind farm's AC system. Wind turbine 20 receives reactive power control commands generated by the upper-level control system and outputs reactive power accordingly. Q WT It can be fully adapted to the currently widely used grid-connected direct-drive wind turbines, thus possessing excellent engineering adaptability. In this embodiment, the centralized wind turbine reactive power distribution process can be understood as: firstly obtaining the output active power from each wind turbine within the wind farm. P WTs , s For the first s The wind turbine transmits data to the centralized controller via the wind farm's communication lines. The centralized controller calculates the main reactive power flowing into the main converter based on the DR mathematical model (e.g., using the reactive power demand formula). Q DR Furthermore, considering the deployment of reactive power compensation devices within the wind farm, through methods such as... Figure 4 The open-loop calculation stage of the centralized wind turbine reactive power distribution system shown in the figure initially obtains the rough steady-state reactive power demand data of the wind farm. Q WF In addition, such as Figure 4 The reactive power distribution stage of the centralized wind turbine shown also monitors the reactive power output of the auxiliary converter through a closed-loop controller with extremely low bandwidth. Q WT This forms a closed-loop control section for reactive power demand, which can eliminate errors present in the open-loop feedforward calculation stage. Simultaneously, such as Figure 4 The low-bandwidth discrete pre-controlled controller in the centralized wind turbine reactive power distribution system shown can avoid closed-loop control oscillation problems caused by wind turbine communication delays. This allows the wind farm's reactive power demand data to be processed. Q WF With reactive power output Q WT The total reactive power output data of the offshore wind power transmission system is superimposed. This total reactive power output data is then evenly distributed to each wind turbine based on the number of turbines in operation. The reactive power output data of each operating turbine is used as a reference value for its reactive power output and is denoted as [value missing]. Q WTrefs In the DR mathematical model, the active power of each wind turbine is first... P WTs Adding them together gives the main active power flowing into the main converter DR. P DRThe main reactive power flowing into the main converter is then calculated using the reactive power demand formula. Q DR .

[0067] In this embodiment, the centralized wind turbine reactive power distribution process further includes: calculating the main reactive power flowing into the main converter based on the grid-connected AC voltage, main commutation reactance, main DC voltage, main DC current, and main active power using the reactive power demand formula of the main converter; the reactive power demand formula is: ; In the formula, u pcc For grid-connected AC voltage, u dc and i dc These are the main DC voltage and the main DC current, respectively. Main commutation reactor, This is the commutation overlap angle. The power factor angle on the AC side of the main converter. P DR and Q DR These are the main active power and main reactive power flowing into the main converter, respectively.

[0068] It should be noted that in the reactive power demand formula, all values ​​are per unit.

[0069] In this embodiment, in the centralized wind turbine reactive power distribution process, the reactive power demand data of the wind farm is calculated using the reactive power demand formula based on the main reactive power, the switching status of all reactive power compensation equipment, and their rated capacity. Q WF The reactive power demand formula is: ; In the formula, S caps This indicates the switching status of each reactive power compensation device. Q caps This refers to the rated capacity of each reactive power compensation device. If the switching status of the reactive power compensation device refers to the device being in operation, then... S caps =1; If the switching status of the reactive power compensation equipment refers to the reactive power compensation equipment being disconnected and not working, then S caps =0.

[0070] Figure 5 This is a schematic diagram illustrating the framework of the grid-connected wind turbine control strategy in the reactive power grid control strategy determination method for offshore wind power transmission systems described in this application embodiment. Figure 5 middle, This refers to the fan speed. This refers to the active power of the wind turbine. U abc,w and I abc,w These represent the three-phase voltage and output current at the AC port of the wind turbine, θ w This is the output phase of the PLL phase-locked loop. U dq,w and I dq,w These are the dq-axis voltage and current components after Park transformation, respectively.

[0071] like Figure 5 As shown, in one embodiment of this application, the content of the grid-type wind turbine control strategy includes: Obtain the reference value of reactive power output, the actual value of reactive power output, the DC bus voltage and the rated value of DC bus voltage for each wind turbine; Based on the DC bus voltage and rated DC bus voltage of each wind turbine, determine the reference value of the d-axis current of the corresponding wind turbine; based on the reference value of the reactive power output and the actual value of the reactive power output of each wind turbine, determine the reference value of the q-axis current of the corresponding wind turbine. The d-axis current reference value and q-axis current reference value of each wind turbine are decoupled by dq to obtain the reference voltage of the grid-side converter connected to the corresponding wind turbine.

[0072] It should be noted that the active power of the wind turbine The wind turbine DC bus voltage, after being controlled by the generator-side converter, is converted into the DC bus voltage fluctuation inside the wind turbine. U dcw The grid-connected wind turbine control strategy can be understood as follows: the reactive power command is issued from the reactive power distribution link of the previous centralized wind turbine and the reactive power output reference value of the wind turbine. Q wref The corresponding command. The fan controls the q-axis current reference value of the fan via a reactive power command. I qw,ref This allows the corresponding reactive power to be output to the wind farm. The steady-state reactive power demand of the wind farm can therefore be transferred from the auxiliary converter MMC to each wind turbine 20.

[0073] In this embodiment of the application, in the grid-following wind turbine control strategy, the d-axis active power of the grid-following wind turbine control strategy can be understood as the reference value of the d-axis current of the corresponding wind turbine calculated using the fifth formula of the fifth PI controller based on the wind turbine DC bus voltage and the rated value of the wind turbine DC bus voltage of each wind turbine. I dw,refThe q-axis active power of the grid-type wind turbine control strategy can be understood as the reference value of the q-axis current of the corresponding wind turbine, calculated using the sixth formula of the sixth PI controller based on the reference value of the reactive power output of each wind turbine and the actual value of the reactive power output. I qw,ref .

[0074] It should be noted that the fifth formula is: ; The sixth formula is: ; In the formula, k p5 The proportional gain of the fifth PI controller. k i5 The integral coefficient of the fifth PI controller. k p6 The proportional gain of the sixth PI controller. k i6 This represents the integral coefficient of the sixth PI controller. In this embodiment, the reference value for the d-axis current of the fan is... I dw,ref Reference value of q-axis current of blower I qw,ref The reference voltage is generated by the current inner loop control after dq decoupling. e abc，k The working principle of the current inner loop control of the grid-connected wind turbine control strategy is the same as that of the current inner loop control of the auxiliary converter reactive power grid control strategy, and the working principle of the current inner loop control of the grid-connected wind turbine control strategy will not be described again.

[0075] In this embodiment, taking a 2GW wind farm as an example, when using existing reactive power grid connection strategies, the auxiliary converter needs to be designed to have a capacity of 40% of the wind farm's rated capacity, i.e., about 800MVA, in order to maintain a certain control margin while balancing the reactive power demand of the entire system. However, by using the reactive power grid connection control strategy determination method for this offshore wind power transmission system, the auxiliary converter only needs to handle reactive power injection during transient disturbances, and its capacity design can be reduced to about 10%~20% of the wind farm's rated capacity, i.e., 200~400MVA. Furthermore, since the auxiliary converter MMC only handles significant reactive power during transient disturbances, and its reactive power output is close to zero most of the time, the topology design of the offshore wind power transmission system can be further optimized by combining the transient overload capacity of the auxiliary converter MMC.

[0076] Example 2: Figure 6This is a schematic diagram of the framework of the reactive power grid control strategy determination device for an offshore wind power transmission system according to an embodiment of this application.

[0077] like Figure 6 As shown, this application embodiment provides a reactive power grid control strategy determination device for an offshore wind power transmission system, which is applied to an offshore wind power transmission system. The offshore wind power transmission system includes a wind farm and several wind turbines, main converters and auxiliary converters connected to the grid connection point of the wind farm. The auxiliary converters are connected to power supply equipment for providing active power. The reactive power grid control strategy determination device includes: a data acquisition module 100 and a control strategy determination module 200. Data acquisition module 100 is used to acquire the operating conditions of offshore wind power transmission systems; The control strategy determination module 200 is used to determine the reactive power grid control strategy of the offshore wind power transmission system based on the operating conditions. If the operating condition is the black start condition of the wind farm, the reactive power grid control strategy is the auxiliary converter reactive power grid control strategy used to provide dynamic reactive power support capability. If the operating condition is that the wind turbines are in operation, the reactive power grid control strategy is a grid-connected wind turbine control strategy, an auxiliary converter reactive power grid control strategy, and a centralized wind turbine reactive power distribution link used to handle the steady-state reactive power within the wind farm.

[0078] It should be noted that the content of each module in the device of Embodiment 2 corresponds to the content of each step in the method of Embodiment 1. Therefore, the content of each module of the device for determining the reactive power grid connection control strategy of the offshore wind power transmission system will not be repeated in this embodiment. This device for determining the reactive power grid connection control strategy of the offshore wind power transmission system obtains the operating conditions of the offshore wind power transmission system through a data acquisition module and a control strategy determination module to determine different reactive power grid connection control strategies. This enables reactive power grid connection of the wind turbine and auxiliary converter, allowing the auxiliary converter to avoid bearing a high reactive power output and preventing excessive power transfer during grid connection. The auxiliary converter only needs to have a certain transient reactive power output function, which can greatly reduce the capacity requirement of the auxiliary converter. Further optimization of the auxiliary converter's capacity can achieve compactness and low cost, thereby reducing the construction cost of the offshore wind power transmission system.

[0079] In this embodiment of the application, the main converter is also connected to several reactive power compensation devices, and the content of the reactive power grid control strategy of the auxiliary converter includes: The following parameters are obtained for the offshore wind power transmission system: grid connection frequency, minimum current limit, DC voltage reference value, first current on the DC side of the main converter, target value of q-axis voltage component of the auxiliary converter, second current on the DC side of the auxiliary converter, target value of capacitor voltage of sub-module in the auxiliary converter, average value of capacitor voltage of sub-module in the auxiliary converter, active power injected into the wind farm by the auxiliary converter, reference value of active power injected into the wind farm by the auxiliary converter, and three-phase voltage at the grid connection point of the auxiliary converter in the wind farm. Based on the target value of capacitor voltage, the average value of capacitor voltage, the second current, and the DC voltage reference value, determine the DC voltage reference value for the DC side control of the auxiliary converter; The three-phase voltage is subjected to Parker transformation to obtain the q-axis voltage component; based on the target value of the q-axis voltage component and the q-axis voltage component, the q-axis current control reference value for the AC side control of the auxiliary converter is determined; based on the minimum current limit, the first current, the active power, and the active power reference value, the d-axis current control reference value for the AC side control of the auxiliary converter is determined. The grid construction frequency is integrated to obtain the grid construction reference phase; based on the grid construction reference phase, the q-axis current control reference value, and the d-axis current control reference value, the AC voltage reference value for the auxiliary converter AC side control is determined. Based on the DC voltage reference value and the AC voltage reference value, determine the pulse control signal for controlling the operation of the sub-module in the auxiliary converter; And / or, The reactive power distribution process for centralized wind turbines includes: The system obtains the number of wind turbines in operation, the switching status of each reactive power compensation device, the rated capacity of each reactive power compensation device, the output active power of each wind turbine, the grid-connected AC voltage of the main converter at the grid connection point, the reactive power output of the auxiliary converter injected into the wind farm, the reference value of the reactive power output of the auxiliary converter injected into the wind farm, and the main commutation reactance, main DC voltage, and main DC current of the main converter. Based on the output active power of all wind turbines, determine the main active power flowing into the main converter; based on the grid-connected AC voltage, main commutation reactance, main DC voltage, main DC current and main active power, calculate the main reactive power flowing into the main converter. The reactive power demand data of the wind farm is determined based on the main reactive power, the switching status and rated capacity of all reactive power compensation equipment; The reactive power output data of the auxiliary converter is determined based on the reactive power output power and the reactive power output power reference value. Based on the reactive power demand data and reactive power output data of the wind farm, the total reactive power output data of the offshore wind power transmission system is determined; The total reactive power output data is evenly distributed to each operating wind turbine based on the number of wind turbines in operation. Among them, the reactive power output data allocated to each wind turbine put into operation is used as the reference value for reactive power output of the wind turbine; And / or, The content of the control strategy for grid-type wind turbines includes: Obtain the reference value of reactive power output, the actual value of reactive power output, the DC bus voltage and the rated value of DC bus voltage for each wind turbine; Based on the DC bus voltage and rated DC bus voltage of each wind turbine, determine the reference value of the d-axis current of the corresponding wind turbine; based on the reference value of the reactive power output and the actual value of the reactive power output of each wind turbine, determine the reference value of the q-axis current of the corresponding wind turbine. The d-axis current reference value and q-axis current reference value of each wind turbine are decoupled by dq to obtain the reference voltage of the grid-side converter connected to the corresponding wind turbine.

[0080] Example 3: Figure 7 This is a schematic diagram of the terminal device described in an embodiment of this application.

[0081] like Figure 7 As shown, this application provides a terminal device, including a processor and a memory; Memory is used to store program code and transfer the program code to the processor; The processor is used to execute the above-mentioned method for determining the reactive power grid control strategy of the offshore wind power transmission system according to the instructions in the program code.

[0082] It should be noted that the processor is used to execute the steps in the above-described embodiment of a method for determining reactive power grid control strategy for an offshore wind power transmission system according to the instructions in the program code. Alternatively, when the processor executes the computer program, it implements the functions of each module / unit in the above-described system / device embodiments.

[0083] For example, a computer program can be divided into one or more modules / units, one or more of which are stored in memory and executed by a processor to complete this application. One or more modules / units can be a series of computer program instruction segments capable of performing a specific function, which describe the execution process of the computer program in a terminal device.

[0084] Terminal devices can be computing devices such as desktop computers, laptops, handheld computers, and cloud servers. Terminal devices may include, but are not limited to, processors and memory. Those skilled in the art will understand that this does not constitute a limitation on the terminal device, which may include more or fewer components than illustrated, or combinations of certain components, or different components. For example, a terminal device may also include input / output devices, network access devices, buses, etc.

[0085] The processor can be a Central Processing Unit (CPU), or other general-purpose processors, digital signal processors (DSPs), application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. A general-purpose processor can be a microprocessor or any conventional processor.

[0086] Memory can be an internal storage unit of a terminal device, such as a hard drive or RAM. Memory can also be an external storage device, such as a plug-in hard drive, SmartMedia Card (SMC), Secure Digital (SD) card, or Flash Card. Furthermore, memory can include both internal and external storage units. Memory is used to store computer programs and other programs and data required by the terminal device. Memory can also be used to temporarily store data that has been output or will be output.

[0087] Those skilled in the art will clearly understand that, for the sake of convenience and brevity, the specific working processes of the systems, devices, and units described above can be referred to the corresponding processes in the foregoing method embodiments, and will not be repeated here.

[0088] In the several embodiments provided in this application, it should be understood that the disclosed systems, apparatuses, and methods can be implemented in other ways. For example, the apparatus embodiments described above are merely illustrative; for instance, the division of units is only a logical functional division, and in actual implementation, there may be other division methods. For example, multiple units or components may be combined or integrated into another system, or some features may be ignored or not executed. Furthermore, the coupling or direct coupling or communication connection shown or discussed may be an indirect coupling or communication connection between apparatuses or units through some interfaces, and may be electrical, mechanical, or other forms.

[0089] The units described as separate components may or may not be physically separate. The components shown as units may or may not be physical units; that is, they may be located in one place or distributed across multiple network units. Some or all of the units can be selected to achieve the purpose of this embodiment according to actual needs.

[0090] Furthermore, the functional units in the various embodiments of the present invention can be integrated into one processing unit, or each unit can exist physically separately, or two or more units can be integrated into one unit. The integrated unit can be implemented in hardware or as a software functional unit.

[0091] If the integrated unit is implemented as a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present invention, in essence, or the part that contributes to the prior art, or all or part of the technical solution, can be embodied in the form of a software product. This computer software product is stored in a storage medium and includes several instructions to cause a computer device (which may be a personal computer, server, or network device, etc.) to execute all or part of the steps of the methods described in the various embodiments of the present invention. The aforementioned storage medium includes various media capable of storing program code, such as USB flash drives, portable hard drives, read-only memory (ROM), random access memory (RAM), magnetic disks, or optical disks.

[0092] The above-described embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application.

Claims

1. A method for determining the reactive power grid control strategy of an offshore wind power transmission system, applied to an offshore wind power transmission system, characterized in that, The offshore wind power transmission system includes a wind farm and several wind turbines, a main converter, and an auxiliary converter connected to the grid connection point of the wind farm. The auxiliary converter is connected to power supply equipment for providing active power. The method for determining the reactive power grid control strategy includes the following steps: Obtain the operating status of the offshore wind power transmission system; Based on the operating conditions, determine the reactive power grid control strategy for the offshore wind power transmission system; If the operating condition is the black start condition of the wind farm, then the reactive power grid control strategy is the auxiliary converter reactive power grid control strategy used to provide dynamic reactive power support capability. If the operating condition is the wind turbine in operation, then the reactive power grid control strategy is the grid-connected wind turbine control strategy, the auxiliary converter reactive power grid control strategy, and the centralized wind turbine reactive power distribution link used to undertake the steady-state reactive power in the wind farm.

2. The method for determining the reactive power grid control strategy of an offshore wind power transmission system according to claim 1, characterized in that, The content of the auxiliary converter reactive power grid control strategy includes: The following parameters are obtained for the offshore wind power transmission system: grid connection frequency, minimum current limit, DC voltage reference value, first current on the DC side of the main converter, target value of the q-axis voltage component of the auxiliary converter, second current on the DC side of the auxiliary converter, target value of capacitor voltage of sub-module in the auxiliary converter, average value of capacitor voltage of sub-module in the auxiliary converter, active power injected into the wind farm by the auxiliary converter, reference value of active power injected into the wind farm by the auxiliary converter, and three-phase voltage at the grid connection point of the auxiliary converter in the wind farm. The reference value of the DC voltage for the auxiliary converter DC side control is determined based on the target value of the capacitor voltage, the average value of the capacitor voltage, the reference value of the DC voltage, the second current, and the reference value of the DC voltage. The three-phase voltage is subjected to Parker transformation to obtain the q-axis voltage component; based on the target value of the q-axis voltage component and the q-axis voltage component, the q-axis current control reference value for the AC side control of the auxiliary converter is determined; based on the minimum current limit, the first current, the active power, and the active power reference value, the d-axis current control reference value for the AC side control of the auxiliary converter is determined. The grid formation frequency is integrated to obtain the grid formation reference phase; based on the grid formation reference phase, the q-axis current control reference value, and the d-axis current control reference value, the AC voltage reference value for the AC side control of the auxiliary converter is determined. Based on the DC voltage reference value and the AC voltage reference value, a pulse control signal is determined to control the operation of the sub-module in the auxiliary converter.

3. The method for determining the reactive power grid control strategy of an offshore wind power transmission system according to claim 1, characterized in that, The main converter is also connected to several reactive power compensation devices, and the centralized wind turbine reactive power distribution process includes: The system acquires the following information: the number of wind turbines in operation of the offshore wind power transmission system; the switching status of each reactive power compensation device; the rated capacity of each reactive power compensation device; the output active power of each wind turbine; the grid-connected AC voltage of the main converter connected to the grid; the reactive power output power injected into the wind farm by the auxiliary converter; the reference value of the reactive power output power injected into the wind farm by the auxiliary converter; and the main commutation reactance, main DC voltage, and main DC current of the main converter. Based on the output active power of all the wind turbines, the main active power flowing into the main converter is determined; based on the grid-connected AC voltage, the main commutation reactance, the main DC voltage, the main DC current, and the main active power, the main reactive power flowing into the main converter is calculated. The reactive power demand data of the wind farm is determined based on the main reactive power, the switching status of all the reactive power compensation equipment, and the rated capacity. The reactive power output data of the auxiliary converter is determined based on the reactive power output power and the reactive power output power reference value. Based on the reactive power demand data of the wind farm and the reactive power output data, the total reactive power output data of the offshore wind power transmission system is determined; The total reactive power output data is evenly distributed to each of the wind turbines in operation based on the number of wind turbines put into operation; Among them, the reactive power output data allocated to each of the wind turbines put into operation is used as the reference value for the reactive power output of the wind turbines.

4. The method for determining the reactive power grid control strategy of an offshore wind power transmission system according to claim 3, characterized in that, The centralized wind turbine reactive power distribution process further includes: calculating the main reactive power flowing into the main converter based on the grid-connected AC voltage, the main commutation reactance, the main DC voltage, the main DC current, and the main active power using the reactive power demand formula of the main converter; the reactive power demand formula is: ； In the formula, u pcc For grid-connected AC voltage, u dc and i dc These are the main DC voltage and the main DC current, respectively. Main commutation reactor, This is the commutation overlap angle. The power factor angle on the AC side of the main converter. P DR and Q DR These are the main active power and main reactive power flowing into the main converter, respectively.

5. The method for determining the reactive power grid control strategy of an offshore wind power transmission system according to claim 1, characterized in that, The content of the grid-connected wind turbine control strategy includes: Obtain the reference value of reactive power output, the actual value of reactive power output, the DC bus voltage and the rated value of DC bus voltage of each of the aforementioned wind turbines; Based on the DC bus voltage and rated DC bus voltage of each wind turbine, determine the reference value of the d-axis current of the corresponding wind turbine; based on the reference value of the reactive power output and the actual value of the reactive power output of each wind turbine, determine the reference value of the q-axis current of the corresponding wind turbine. The d-axis current reference value and the q-axis current reference value of each wind turbine are decoupled by dq to obtain the reference voltage of the grid-side converter connected to the corresponding wind turbine.

6. An offshore wind power transmission system, characterized in that, The system includes a control module, a wind farm, and several wind turbines, a main converter, and an auxiliary converter connected to the grid connection point of the wind farm. The auxiliary converter is connected to a power supply device for providing active power via a tie line. The DC side of the main converter is connected to an onshore converter. The control module is used to control the reactive power grid connection strategy of the offshore wind power transmission system according to the reactive power grid connection strategy determination method of the offshore wind power transmission system as described in any one of claims 1-5.

7. The offshore wind power transmission system according to claim 6, characterized in that, The main converter is a 12-pulse or dual 12-pulse cascaded diode rectifier, and the auxiliary converter is a modular multilevel converter.

8. A device for determining the reactive power grid control strategy of an offshore wind power transmission system, applied to an offshore wind power transmission system, characterized in that, The offshore wind power transmission system includes a wind farm and several wind turbines, a main converter, and an auxiliary converter connected to the grid connection point of the wind farm. The auxiliary converter is connected to power supply equipment for providing active power. The reactive power grid control strategy determination device includes a data acquisition module and a control strategy determination module. The data acquisition module is used to acquire the operating conditions of the offshore wind power transmission system; The control strategy determination module is used to determine the reactive power grid control strategy of the offshore wind power transmission system based on the operating conditions. Wherein, if the operating condition is the wind farm black start condition, then the reactive power grid control strategy is the auxiliary converter reactive power grid control strategy for providing dynamic reactive power support capability. If the operating condition is the wind turbine in operation, then the reactive power grid control strategy is the grid-connected wind turbine control strategy, the auxiliary converter reactive power grid control strategy, and the centralized wind turbine reactive power distribution link used to undertake the steady-state reactive power in the wind farm.

9. The reactive power grid control strategy determination device for offshore wind power transmission systems according to claim 8, characterized in that, The main converter is also connected to several reactive power compensation devices, and the reactive power grid control strategy of the auxiliary converter includes: The following parameters are obtained for the offshore wind power transmission system: grid connection frequency, minimum current limit, DC voltage reference value, first current on the DC side of the main converter, target value of the q-axis voltage component of the auxiliary converter, second current on the DC side of the auxiliary converter, target value of capacitor voltage of sub-module in the auxiliary converter, average value of capacitor voltage of sub-module in the auxiliary converter, active power injected into the wind farm by the auxiliary converter, reference value of active power injected into the wind farm by the auxiliary converter, and three-phase voltage at the grid connection point of the auxiliary converter in the wind farm. The DC voltage reference value for the auxiliary converter DC side control is determined based on the target value of the capacitor voltage, the average value of the capacitor voltage, the second current, and the DC voltage reference value. The three-phase voltage is subjected to Parker transformation to obtain the q-axis voltage component; based on the target value of the q-axis voltage component and the q-axis voltage component, the q-axis current control reference value for the AC side control of the auxiliary converter is determined; based on the minimum current limit, the first current, the active power, and the active power reference value, the d-axis current control reference value for the AC side control of the auxiliary converter is determined. The grid formation frequency is integrated to obtain the grid formation reference phase; based on the grid formation reference phase, the q-axis current control reference value, and the d-axis current control reference value, the AC voltage reference value for the AC side control of the auxiliary converter is determined. Based on the DC voltage reference value and the AC voltage reference value, determine the pulse control signal for controlling the operation of the sub-module in the auxiliary converter; And / or, The contents of the centralized wind turbine reactive power distribution process include: The system acquires the following information: the number of wind turbines in operation of the offshore wind power transmission system; the switching status of each reactive power compensation device; the rated capacity of each reactive power compensation device; the output active power of each wind turbine; the grid-connected AC voltage of the main converter connected to the grid; the reactive power output power injected into the wind farm by the auxiliary converter; the reference value of the reactive power output power injected into the wind farm by the auxiliary converter; and the main commutation reactance, main DC voltage, and main DC current of the main converter. Based on the output active power of all the wind turbines, the main active power flowing into the main converter is determined; based on the grid-connected AC voltage, the main commutation reactance, the main DC voltage, the main DC current, and the main active power, the main reactive power flowing into the main converter is calculated. The reactive power demand data of the wind farm is determined based on the main reactive power, the switching status of all the reactive power compensation equipment, and the rated capacity. The reactive power output data of the auxiliary converter is determined based on the reactive power output power and the reactive power output power reference value. Based on the reactive power demand data of the wind farm and the reactive power output data, the total reactive power output data of the offshore wind power transmission system is determined; The total reactive power output data is evenly distributed to each of the wind turbines in operation based on the number of wind turbines put into operation; Among them, the reactive power output data allocated to each of the wind turbines put into operation is used as the reactive power output reference value of the wind turbines; And / or, The content of the grid-connected wind turbine control strategy includes: Obtain the reference value of reactive power output, the actual value of reactive power output, the DC bus voltage and the rated value of DC bus voltage of each of the aforementioned wind turbines; Based on the DC bus voltage and rated DC bus voltage of each wind turbine, determine the reference value of the d-axis current of the corresponding wind turbine; based on the reference value of the reactive power output and the actual value of the reactive power output of each wind turbine, determine the reference value of the q-axis current of the corresponding wind turbine. The d-axis current reference value and the q-axis current reference value of each wind turbine are decoupled by dq to obtain the reference voltage of the grid-side converter connected to the corresponding wind turbine.

10. A terminal device, characterized in that, Including the processor and memory; The memory is used to store program code and transmit the program code to the processor; The processor is configured to execute the reactive power grid control strategy determination method for offshore wind power transmission systems as described in any one of claims 1-5, according to the instructions in the program code.

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