Offshore wind power direct current parallel sending-out system through DRU-MMC and control method

Abstract

The invention discloses a DRU-MMC direct-current parallel sending-out system for offshore wind power and a control method, and aims at achieving high-economical-efficiency and high-efficiency electric power sending-out of deep and far sea wind power clusters. The system comprises a first offshore wind power plant based on a network construction type fan, a second offshore wind power plant based on a network following type fan, an offshore MMC converter station and an offshore DRU converter station. The offshore MMC converter station adopts a voltage-frequency control strategy added with active power-voltage amplitude negative feedback control and reactive power-frequency positive feedback control, the land MMC converter station adopts constant direct current voltage control, the first offshore wind power plant fan adopts network construction type control, and the second offshore wind power plant fan adopts network following type control. According to the direct current parallel sending scheme of the DRU and the MMC, the technical advantages of the MMC and the economical efficiency of the DRU are considered, the stability and reliability of the system are improved by adopting the specific control strategy, and the economical efficiency of the offshore wind power sending system is improved.

Description

Technical Field

[0001] This invention belongs to the field of power system transmission and distribution technology, specifically relating to a system and control method for the parallel DC transmission of offshore wind power via DRU-MMC. Background Technology

[0002] Currently, offshore wind power projects are gradually expanding to deep-sea areas. Under this trend, flexible DC transmission technology based on modular multilevel converters (MMC-HVDC) has become the preferred solution for grid connection of deep-sea wind power due to its mature control strategy, high operational flexibility, and excellent adaptability to passive systems.

[0003] However, the flexible DC transmission scheme for offshore wind power also has certain limitations. The MMC (Modular Multilevel Converter) has a complex internal structure, integrating a large number of sub-modules, resulting in a large overall size, heavy weight, and high operating losses. These factors not only increase the initial investment cost of the converter station but also the subsequent operation and maintenance costs. At the same time, the offshore platform supporting the converter station must have a large space and high load-bearing capacity, which undoubtedly exacerbates the construction cost and engineering difficulty of the offshore converter platform.

[0004] To improve the economic efficiency of deep-sea wind power transmission systems, the industry has begun exploring alternative solutions, among which the DC transmission solution based on diode rectifier units (DRUs) proposed by Siemens has attracted considerable attention. Compared with traditional flexible DC transmission solutions, the DRU solution can significantly reduce the size and weight of offshore converter platforms, thereby substantially reducing the total engineering cost of deep-sea wind power projects. However, this technology also has two characteristics under current conditions: First, as an uncontrolled rectifier, the DRU cannot actively provide voltage and frequency support for offshore wind farms, thus requiring the wind turbines themselves to have grid-connected operation capabilities; second, the power of the DRU converter can only be transmitted in one direction and does not have the ability to transmit power in the reverse direction, therefore it cannot support the black start of offshore wind farms.

[0005] Currently, both domestic and international efforts are actively seeking DC power transmission system solutions based on a hybrid MMC and DRU configuration on the rectifier side, attempting to address the wind farm grid and black start problems inherent in pure DRU solutions. The literature [G. Zhang, W. Xiang and J. Wen. Impedance Modeling and Stability Analysis of DR-MMC Based Hybrid HVDC for Offshore Wind Farm Integration. in IEEE Transactions on PowerDelivery, vol. 38, no. 6, pp. 4397-4409, Dec. 2023] proposes a topology where the DRU and MMC are connected in parallel on the AC side and in series on the DC side. While the MMC can provide reactive power and harmonic compensation for the AC system, this structure cannot achieve reverse power transmission and therefore cannot support black start for wind farms. Based on this, the literature [Yang Mingrui, Xiang Wang, Wen Jinyu. Start-up strategy of offshore wind power DC transmission system via DR-MMC series [J]. High Voltage Engineering, 2025, 51(06): 2807-2821] proposes an improved DRU-MMC series scheme. By introducing switching devices and combining it with onshore full-half-bridge hybrid MMC, reverse transmission of DC power is achieved. However, this scheme requires additional switching equipment, and the full-half-bridge hybrid MMC is less economical than the half-bridge MMC. The literature [Cheng Fan, Yao Liangzhong, Xie Lijun. Start-up and coordinated control strategy of offshore wind power DC transmission system via DR-MMC parallel [J]. Global Energy Internet, 2020, 3(2): 117-124] proposes a topology in which DRU and MMC are connected in parallel on both AC and DC sides. The MMC is used to provide black start power for the wind farm. However, the DC voltage of the MMC is high in this structure, and the number of sub-modules required is large, which leads to increased costs. The literature [Yuan Qingwei, Xie Yeyuan, Jiang Tiangui. Control strategy for DRU+MMC hybrid converter at the sending end of offshore wind farms compatible with grid connection and grid connection [J]. Power System Technology, 2024, 48(11): 4482-4492] further proposes an improved DRU-MMC parallel scheme, which provides black start energy for offshore wind farms by constructing a loop through low-voltage MMCs at both the sending and receiving ends, thus improving economic efficiency, but there is a problem of complex control mode switching timing.

[0006] Against the backdrop of large-scale and clustered development of offshore wind power, scenarios will emerge where multiple adjacent wind farm clusters need to transmit power simultaneously. To balance the technological advantages of MMC (Multi-Fuel Controlled Wind Power) with the economic efficiency of DRU (Dual-Fuel Utility Unit), and to meet the complex scenarios of transmitting power from multiple wind farm clusters, it is urgent to overcome existing technological bottlenecks and develop a new type of offshore wind power transmission system and its control strategy. This will enable highly economical and efficient power transmission from deep-sea wind farm clusters, thereby promoting the sustainable development of the offshore wind power industry. Summary of the Invention

[0007] In view of the above, the present invention provides a system and control method for the parallel DC transmission of offshore wind power via DRU-MMC, which can take into account the technical advantages of MMC and the economics of DRU, and realize the high-economic and high-efficiency power transmission of deep-sea wind power clusters.

[0008] A parallel DC transmission system for offshore wind power via DRU-MMC includes: a first offshore wind farm based on grid-connected wind turbines, a second offshore wind farm based on grid-connected wind turbines, an offshore MMC converter station, an offshore DRU converter station, a first onshore MMC converter station, and a second onshore MMC converter station, wherein: The first offshore wind farm is connected to the first AC bus via a first AC gathering submarine cable, and the second offshore wind farm is connected to the second AC bus via a second AC gathering submarine cable. The first AC bus and the second AC bus are connected by a tie line submarine cable to realize interconnection and power exchange between the AC buses. The AC side of the offshore DRU converter station is connected to the first AC bus via a converter transformer, and the DC side is connected to the DC side of the first onshore MMC converter station via a first DC submarine cable, which is used to transmit the power of the first offshore wind farm to the onshore power grid. The AC side of the offshore MMC converter station is connected to the second AC bus via a converter transformer, and the DC side is connected to the DC side of the second onshore MMC converter station via a second DC submarine cable, which is used to transmit the power of the second offshore wind farm to the onshore power grid. The AC sides of both the first and second onshore MMC converter stations are connected to their respective receiving-end AC power grids via converter transformers, completing AC-DC conversion and injecting electrical energy into the onshore power grid.

[0009] Furthermore, the offshore DRU converter station consists of an even number of 6-pulse DRU rectifiers, with the AC-side converter rectifiers using a configuration where half are Y / Y wiring and half are Y / Δ wiring.

[0010] Furthermore, the capacity of the first offshore wind farm is equal to the capacity of the offshore DRU converter station and the capacity of the first onshore MMC converter station, and the capacity of the second offshore wind farm is equal to the capacity of the offshore MMC converter station and the capacity of the second onshore MMC converter station.

[0011] Furthermore, both the wind turbines in the first and second offshore wind farms are equipped with DC energy dissipation devices, and both the DC outlets of the first and second onshore MMC converter stations are equipped with DC energy dissipation devices.

[0012] The control method for the aforementioned offshore wind power transmission system via the DRU-MMC DC parallel transmission system is as follows: The offshore MMC converter station employs a voltage-frequency control strategy to maintain the voltage and frequency stability of the offshore AC system (including offshore wind farms, AC collection submarine cables, tie-line submarine cables, offshore DRU converter stations, and AC-side equipment of the offshore MMC converter station). Both the first and second onshore MMC converter stations employ a constant DC voltage control strategy to ensure the stable operation of the DC transmission system (including the DC-side equipment of the offshore DRU converter station and offshore MMC converter station, DC submarine cables, and DC-side equipment of the onshore MMC converter station) and meet energy transmission requirements. The wind turbines in the first offshore wind farm employ a grid-based control strategy, enabling them to actively establish grid voltage and frequency. The wind turbines in the second offshore wind farm employ a grid-following control strategy.

[0013] Furthermore, regarding the control strategy for offshore MMC converter stations, in addition to the conventional dual-loop control, active power-voltage amplitude negative feedback control and reactive power-frequency positive feedback control are added. The active power-voltage amplitude negative feedback control loop uses the active power flowing from the second AC bus to the connecting submarine cable. P tie2 The input signal is used to adjust the output voltage amplitude of the offshore MMC converter station to maintain the balance of active power in the offshore AC system; the reactive power-frequency positive feedback control loop directs the reactive power flowing from the second AC bus to the connecting submarine cable. Q tie2 The input signal is used to adjust the frequency of the marine AC system to maintain the balance of reactive power.

[0014] Furthermore, considering the limited power transmission capacity of AC submarine cables, the tie line submarine cables cannot support high-power transmission, and the increased cable transmission distance on the AC side will lead to additional power loss; in the control strategy of the offshore MMC converter station, the active power command value flowing from the offshore MMC converter station to the tie line submarine cable is... and reactive power command value All are set to 0, and the voltage command and reference frequency are calculated and determined by the following formula, thereby performing voltage and current dual-loop control on the offshore MMC converter station; in: and u sd0These are the commanded and rated AC voltage values ​​for the offshore MMC converter station, respectively. f m and f 0 represents the reference frequency and rated frequency of the AC voltage at the offshore MMC converter station, respectively. k pv1 and k iv1 These are the proportional gain coefficient and integral gain coefficient for the active power-voltage amplitude negative feedback control, respectively. k pf1 and k if1 These are the proportional gain coefficient and integral gain coefficient for reactive power-frequency positive feedback control, respectively.

[0015] Furthermore, the grid-type control strategy for the wind turbines in the first offshore wind farm comprises a three-layer control structure: The first layer involves active and reactive power control: The active power control in this layer uses the active power generated by the wind turbine's MPPT (Maximum Power Point Tracking) control loop as a reference value. The difference between this reference value and the actual active power is then processed by a PI (Proportional-Integral) controller to generate an AC voltage d-axis reference value. The reactive power control in this layer uses the reactive power reference value... With actual reactive power Q wt1 The difference generated after comparison is used by a P (proportional) controller or a PD (proportional-derivative) controller to generate the frequency of the wind turbine grid-side AC system. f w The phase of the grid-side voltage is obtained after integration and conversion. θ g ; The second layer is AC voltage control: This layer uses the AC voltage dq axis reference value as input. After comparing this input with the actual AC voltage amplitude, the PI controller generates the AC current dq axis command value, where the AC voltage q axis reference value is 0. The third layer is current control: This layer takes the command value of the AC current dq axis as input. After comparing the input with the actual AC current amplitude, the PI controller generates the AC output voltage reference value of the wind turbine grid-side converter, and uses it to control the wind turbine grid-side converter.

[0016] Furthermore, the reactive power reference values ​​for all wind turbines. All parameters are set to 0, and the first-level reactive power control uses uniform controller parameters. This design enables wind turbines to operate synchronously without the need for phase-locked loops or high-speed communication, simplifying control complexity. At the same time, this strategy ensures that the reactive power of the offshore AC system is evenly distributed among all grid-connected wind turbines, avoiding reactive power overload of a single wind turbine and improving the overall reliability of the system.

[0017] Furthermore, under steady-state conditions, the actual reactive power of the wind turbines in the first offshore wind farm... Q wt1 Frequency of the communication system with the network side f w The following relationship must be satisfied: in: k pf2 The proportional gain coefficient of the controller used in the first-level reactive power control. f 0 represents the rated frequency of the AC voltage at the offshore MMC converter station.

[0018] Based on the above technical solution, this invention enables the parallel transmission of deep-sea wind power via DRU-MMC DC, significantly improving the economic efficiency of offshore wind power transmission systems while ensuring system stability and reliability. The system supports independent or coordinated operation of the DRU and MMC, possessing high flexibility and reliability, and can achieve clustered transmission from multiple offshore wind farms. The design of the offshore DRU converter station effectively reduces platform investment and operation and maintenance costs, while the offshore MMC provides voltage support for the offshore AC system and solves the black start problem of offshore wind farms. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the offshore wind power transmission system via DRU-MMC parallel DC transmission according to the present invention.

[0020] Figure 2 This is a block diagram of the control system for an offshore MMC converter station.

[0021] Figure 3 This is a block diagram of the control system for the wind turbines in the first offshore wind farm.

[0022] Figure 4 The following is a simulation waveform diagram of a marine AC system failure in an embodiment of the present invention, wherein (a) is the DC voltage of the wind turbine, (b) is the output power of the DRU, (c) is the output power of the grid-connected wind turbine, (d) is the output power of the grid-connected wind turbine, (e) is the power flowing from the MMC to the DRU, and (f) is the AC bus voltage of the MMC. Detailed Implementation

[0023] To describe the present invention in more detail, the technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0024] like Figure 1As shown, this embodiment provides an offshore wind power parallel DC transmission system via DRU-MMC, including a first offshore wind farm based on grid-connected wind turbines, a second offshore wind farm based on grid-connected wind turbines, an offshore MMC converter station, an offshore DRU converter station, a first onshore MMC converter station, and a second onshore MMC converter station. The first offshore wind farm is connected to a first AC bus via a first AC collection submarine cable, and the second offshore wind farm is connected to a second AC bus via a second AC collection submarine cable. The first AC bus is connected to the second AC bus via a connecting submarine cable. The AC side of the offshore DRU converter station is connected to the first AC bus via a converter transformer. The DC side of the offshore DRU converter station is connected to the DC side of the first onshore MMC converter station via a first DC submarine cable. The AC side of the offshore MMC converter station is connected to the second AC bus via a converter transformer. The DC side of the offshore MMC converter station is connected to the DC side of the second onshore MMC converter station via a second DC submarine cable. The AC sides of the first and second onshore MMC converter stations are connected to their respective receiving-end AC power grids via converter transformers.

[0025] The offshore DRU converter station consists of two 6-pulse DRU rectifiers, and its step-up transformers adopt Y / Y connection structure and Y / Δ connection structure respectively.

[0026] The capacity of the first offshore wind farm is equal to that of the offshore DRU converter station and the first onshore MMC converter station, both being 1000MW; the capacity of the second offshore wind farm is equal to that of the offshore MMC converter station and the second onshore MMC converter station, both being 1000MW.

[0027] The wind turbines of the first and second offshore wind farms are equipped with DC energy dissipation devices; at the same time, the DC outlets of the first and second onshore MMC converter stations are also equipped with DC energy dissipation devices.

[0028] The offshore MMC converter station uses voltage-frequency control to maintain the voltage and frequency stability of the offshore AC system; both the first and second onshore MMC converter stations use constant DC voltage control to ensure the stable operation of the DC transmission system and meet energy transmission requirements; the wind turbines of the first offshore wind farm use grid-connected control to actively establish grid voltage and frequency; the wind turbines of the second offshore wind farm use grid-following control.

[0029] The control strategy for offshore MMC converter stations, based on conventional dual-loop control, adds active power-voltage amplitude negative feedback control and reactive power-frequency positive feedback control, such as... Figure 2 As shown, the active power-voltage amplitude negative feedback control loop of the MMC uses the active power flowing from the MMC AC bus to the tie line. P tie2The input signal is used to adjust the output voltage amplitude of the offshore MMC converter station to maintain the active power balance of the offshore AC system; the reactive power-frequency positive feedback control loop controls the reactive power flowing from the MMC AC bus to the tie line. Q tie2 The input signal is used to adjust the frequency of the marine AC system to maintain the balance of reactive power.

[0030] Considering the limited power transmission capacity of AC submarine cables, the tie line submarine cable cannot support high power transmission, and the increased cable transmission distance on the AC side will lead to additional power loss. Therefore, the active power command value for the MMC flowing to the tie line is... and reactive power command value All values ​​are set to 0, and controlled using the following formula: In the formula: and u sd0 For MMC AC voltage amplitude command value and rated value, f m and f 0 represents the MMC AC voltage reference frequency and the system rated frequency. k pv1 and k iv1 These are the proportional gain and integral gain coefficients of the active power-voltage amplitude controller. k pf1 and k if1 These are the proportional gain coefficient and integral gain coefficient of the reactive power-frequency controller.

[0031] The grid-type control strategy for the wind turbines of the first offshore wind farm includes a three-layer control structure, such as... Figure 3 As shown: The first layer is active power / reactive power control: The active power control layer uses the active power generated by the wind turbine's maximum power point tracking control loop as a reference value. This reference value is compared with the actual active power, and then a proportional-integral controller generates a reference value for the AC voltage d-axis amplitude. The reactive power control layer can use proportional or proportional-derivative control to generate the frequency of the wind turbine's grid-side AC system. f w .

[0032] The second layer is AC voltage control: The voltage control layer takes the AC voltage dq axis amplitude reference value as input. The AC voltage d axis amplitude reference value is generated by the first layer controller, and the AC voltage q axis amplitude reference value is 0. After comparing with the actual AC voltage amplitude value, the proportional-integral controller generates the command value of the AC current dq axis component.

[0033] The third layer is current control: The current control layer takes the command value of the AC current dq axis component generated by the second layer as input, compares it with the actual value of the AC current, and then generates a reference signal for the AC side output voltage of the wind turbine grid-side converter through the proportional-integral controller.

[0034] Meanwhile, the reactive power reference values ​​of all wind turbine units All parameters are set to 0, and the reactive power control layer uses uniform controller parameters. This design enables wind turbines to operate synchronously without phase-locked loops or high-speed communication, simplifying control complexity. Simultaneously, this strategy ensures that the reactive power of the offshore AC system is evenly distributed among all grid-connected wind turbines, avoiding reactive power overload of individual turbines and improving the overall system reliability. Under steady-state conditions, the reactive power of the grid-connected wind turbines... Q wt1 and f w The following relationship must be satisfied: In the formula: k pf2 This is the proportional gain coefficient of the reactive power-frequency controller for grid-type wind turbines.

[0035] To verify the effectiveness of the system and control method of this invention, we built a system in PSCAD / EMTDC as follows: Figure 1 The simulation model of the offshore wind power transmission system via DRU-MMC DC parallel transmission is shown, and the simulation analysis of the offshore AC system fault is carried out. The relevant parameters of the system are shown in Table 1. The rated frequency of the offshore AC system is 50Hz.

[0036] Table 1 Offshore AC system fault: Assume the system has reached steady state before t=3.5 seconds. At t=3.5 seconds, a three-phase metallic ground fault occurs on the AC bus of the DRU converter. The fault is cleared after 0.1 seconds. The system response is as follows: Figure 4 As shown. By Figure 4 As can be seen, the offshore wind power transmission system of the present invention via DRU-MMC DC parallel transmission can achieve fault ride-through, and the system can return to a stable state after the fault is cleared.

[0037] The above description of the embodiments is provided to enable those skilled in the art to understand and apply the present invention. Those skilled in the art can readily make various modifications to the above embodiments and apply the general principles described herein to other embodiments without creative effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made to the present invention by those skilled in the art based on the disclosure thereof should be within the scope of protection of the present invention.

Claims

1. A system for parallel DC transmission of offshore wind power via DRU-MMC, characterized in that, include: The project includes a first offshore wind farm based on grid-type wind turbines, a second offshore wind farm based on grid-following wind turbines, an offshore MMC converter station, an offshore DRU converter station, a first onshore MMC converter station, and a second onshore MMC converter station, among which: The first offshore wind farm is connected to the first AC bus via a first AC gathering submarine cable, and the second offshore wind farm is connected to the second AC bus via a second AC gathering submarine cable. The first AC bus and the second AC bus are connected by a tie line submarine cable to realize interconnection and power exchange between the AC buses. The AC side of the offshore DRU converter station is connected to the first AC bus via a converter transformer, and the DC side is connected to the DC side of the first onshore MMC converter station via a first DC submarine cable, which is used to transmit the power of the first offshore wind farm to the onshore power grid. The AC side of the offshore MMC converter station is connected to the second AC bus via a converter transformer, and the DC side is connected to the DC side of the second onshore MMC converter station via a second DC submarine cable, which is used to transmit the power of the second offshore wind farm to the onshore power grid. The AC sides of both the first and second onshore MMC converter stations are connected to their respective receiving-end AC power grids via converter transformers, completing AC-DC conversion and injecting electrical energy into the onshore power grid.

2. The offshore wind power parallel DC transmission system via DRU-MMC according to claim 1, characterized in that: The offshore DRU converter station consists of an even number of 6-pulse DRU rectifiers, with the AC-side converter rectifiers using a configuration where half are Y / Y wiring and half are Y / Δ wiring.

3. The offshore wind power parallel DC transmission system via DRU-MMC according to claim 1, characterized in that: The capacity of the first offshore wind farm is equal to the capacity of the offshore DRU converter station and the capacity of the first onshore MMC converter station, and the capacity of the second offshore wind farm is equal to the capacity of the offshore MMC converter station and the capacity of the second onshore MMC converter station.

4. The offshore wind power parallel DC transmission system via DRU-MMC according to claim 1, characterized in that: Both the first and second offshore wind farms are equipped with DC energy dissipation devices inside their wind turbines, and both the first and second onshore MMC converter stations are equipped with DC energy dissipation devices at their DC outlets.

5. The control method for the offshore wind power parallel DC transmission system via DRU-MMC as described in any one of claims 1 to 4, characterized in that: The offshore MMC converter station adopts a voltage-frequency control strategy to maintain the voltage and frequency stability of the offshore AC system; both the first and second onshore MMC converter stations adopt a constant DC voltage control strategy to ensure the stable operation of the DC transmission system and meet energy transmission requirements; the wind turbines in the first offshore wind farm adopt a grid-based control strategy, which can actively establish grid voltage and frequency; the wind turbines in the second offshore wind farm adopt a grid-following control strategy.

6. The control method according to claim 5, characterized in that: For the control strategy of offshore MMC converter stations, based on the conventional dual-loop control, active power-voltage amplitude negative feedback control and reactive power-frequency positive feedback control are added. The active power-voltage amplitude negative feedback control loop uses the active power flowing from the second AC bus to the connecting submarine cable. P tie2 The input signal is used to adjust the output voltage amplitude of the offshore MMC converter station to maintain the balance of active power in the offshore AC system; the reactive power-frequency positive feedback control loop directs the reactive power flowing from the second AC bus to the connecting submarine cable. Q tie2 The input signal is used to adjust the frequency of the marine AC system to maintain the balance of reactive power.

7. The control method according to claim 5, characterized in that: In the control strategy of offshore MMC converter stations, the active power command value flowing from the offshore MMC converter station to the submarine tie line cable is... and reactive power command value All are set to 0, and the voltage command and reference frequency are calculated and determined by the following formula, thereby performing voltage and current dual-loop control on the offshore MMC converter station; in: and u sd0 These are the commanded and rated AC voltage values ​​for the offshore MMC converter station, respectively. f m and f 0 represents the reference frequency and rated frequency of the AC voltage at the offshore MMC converter station, respectively. k pv1 and k iv1 These are the proportional gain coefficient and integral gain coefficient for the active power-voltage amplitude negative feedback control, respectively. k pf1 and k if1 These are the proportional gain coefficient and integral gain coefficient for reactive power-frequency positive feedback control, respectively.

8. The control method according to claim 5, characterized in that: The grid-based control strategy for the wind turbines in the first offshore wind farm includes a three-layer control structure: The first layer involves active and reactive power control: Active power control at this layer uses the active power generated by the wind turbine's MPPT control loop as a reference value. The difference between this reference value and the actual active power is then processed by a PI controller to generate an AC voltage d-axis reference value. Reactive power control at this layer uses the reactive power reference value... With actual reactive power Q wt1 The difference generated after comparison is used by the P controller or PD controller to generate the frequency of the wind turbine grid-side AC system. f w The phase of the grid-side voltage is obtained after integration and conversion. θ g ; The second layer is AC voltage control: This layer uses the AC voltage dq axis reference value as input. After comparing this input with the actual AC voltage amplitude, the PI controller generates the AC current dq axis command value, where the AC voltage q axis reference value is 0. The third layer is current control: This layer takes the command value of the AC current dq axis as input. After comparing the input with the actual AC current amplitude, the PI controller generates the AC output voltage reference value of the wind turbine grid-side converter, and uses it to control the wind turbine grid-side converter.

9. The control method according to claim 8, characterized in that: Reference values ​​for reactive power of all wind turbines All parameters are set to 0, and the first-level reactive power control uses uniform controller parameters.

10. The control method according to claim 8, characterized in that: Under steady state, the actual reactive power of the wind turbines in the first offshore wind farm Q wt1 Frequency of the communication system with the network side f w The following relationship must be satisfied: in: k pf2 The proportional gain coefficient of the controller used in the first-level reactive power control. f 0 represents the rated frequency of the AC voltage at the offshore MMC converter station.

Images