Direct-drive wind farm flexible HVDC transmission system and control method

Abstract

The application discloses a flexible direct-current power transmission system and a control method for a direct-drive wind turbine wind farm, and belongs to the field of flexible direct-current power transmission. The system solves the problems of large grid-connection impact, complex start control and poor voltage balance. The core scheme is as follows: after system initialization, a phased charging strategy is adopted, the initial charging of the land / sea converter station sub-modules is completed and the initial direct-current voltage is established through uncontrolled charging; then, the offshore converter station is unlocked for controllable charging, the voltage balance of the two sides of the sub-modules is realized, and stable alternating-current support is established at the wind farm collection point. Through precise control of the on-off timing of the circuit breaker, the "first break and then connect" grid-connection logic is executed, and there is no large impact current in the grid-connection process. After grid-connection, the wind farm electric energy is stably transmitted to the land power grid through the links of voltage step-up, alternating-current / direct-current conversion, direct-current transmission and rectification. The application improves the start and operation stability of the system, guarantees the reliable and safe transmission of electric energy, and is suitable for the flexible direct-current grid-connection scene of large-scale direct-drive wind turbine wind farms.

Description

Technical Field

[0001] This invention belongs to the field of power electronics technology, specifically relating to a flexible DC transmission system and control method for direct-drive wind turbine wind farms, applicable to long-distance power transmission startup scenarios in offshore direct-drive wind farms. Background Technology

[0002] Flexible DC transmission systems have become the mainstream solution for long-distance power transmission from offshore wind farms due to their advantages such as flexible control, low harmonic content, and ability to achieve passive network power supply. The stability of the system startup process directly determines the reliability of the overall operation afterwards. However, the charging startup strategy of traditional flexible DC transmission systems mostly adopts a dual-end charging mode, which cannot be directly applied to flexible DC transmission systems connected to offshore wind farms.

[0003] When offshore wind farms are connected to flexible DC transmission systems, a stable AC voltage is required. Therefore, the flexible DC transmission system must first establish a DC voltage and then build a stable AC voltage at the offshore converter station to ensure the connection of wind turbines. Current research largely focuses on the startup and grid connection process of doubly-fed induction generator wind farms with flexible DC transmission, lacking detailed and reliable startup methods for direct-drive wind farms connected to the grid via flexible DC transmission.

[0004] The topology and operating characteristics of direct-drive wind farms differ significantly from those of doubly-fed induction generator wind farms, and traditional starting methods cannot meet their requirements for power supply stability and current surge limiting. Therefore, it is urgent to design a flexible DC power transmission system adapted to direct-drive wind turbine wind farms and a corresponding charging and starting method. By optimizing the charging sequence of sub-modules, control strategies, and unit coordination mechanisms, the system can achieve safe and stable startup. Summary of the Invention

[0005] The purpose of this invention is to provide a flexible DC transmission system and control method for direct-drive wind turbine wind farms, which solves the problem that the traditional double-end charging mode is not suitable for flexible DC systems in direct-drive wind farms, and enables the system to start up by relying solely on the onshore power grid, with small current surges during grid connection, thus improving the safety and stability of the startup process.

[0006] To solve the above problems, the technical solution of the present invention is as follows: The flexible DC transmission system for direct-drive wind turbine wind farms includes an onshore power grid, an onshore converter station GSMMC, an onshore transformer T1, an offshore converter station WFMMC, an offshore transformer T2, an offshore wind farm, an auxiliary power supply for the wind farm, a step-up transformer T3, a circuit breaker KG, a current-limiting resistor R, a circuit breaker KR, a circuit breaker KW, and a circuit breaker KF. The onshore power grid is connected in series with the input terminal of the onshore transformer T1, and the circuit breaker KG and the current-limiting resistor R are connected in series in the line between the onshore power grid and the onshore transformer T1. The circuit breaker KR is connected in parallel with the current-limiting resistor R to bypass the current-limiting resistor R; The output terminal of the onshore transformer T1 is connected to the AC side of the onshore converter station GSMMC, and the DC side of the onshore converter station GSMMC is connected to the DC side of the offshore converter station WFMMC through a DC line. The AC side of the offshore converter station WFMMC is connected to the input terminal of the offshore transformer T2, the output terminal of the offshore transformer T2 is connected to one end of the circuit breaker KW, and the other end of the circuit breaker KW is connected to the output terminal of the offshore wind farm. The step-up transformer T3 is connected in series in the line between the offshore wind farm and the circuit breaker KW to boost the output voltage of the offshore wind farm. The wind farm auxiliary power supply is connected in parallel with the offshore wind farm, and the circuit breaker KF is connected in series in the connection line between the wind farm auxiliary power supply and the offshore wind farm. The AC voltage amplitude and frequency of the wind farm auxiliary power supply are consistent with those of the onshore power grid. The offshore wind farm uses direct-drive wind turbines.

[0007] Furthermore, both the onshore converter station GSMMC and the offshore converter station WFMMC adopt a modular multilevel converter (MMC) topology and contain multiple sub-modules, each with built-in capacitors.

[0008] Furthermore, each arm of the onshore converter station GSMMC is configured with N series-connected sub-modules, and each arm of the offshore converter station WFMMC is configured with N series-connected sub-modules. The upper and lower arms of the offshore converter station WFMMC contain a total of 2N operational sub-modules.

[0009] Furthermore, the resistance value of the current-limiting resistor R is determined based on the system rated current and the allowable charging inrush current, and is used to limit the current peak during the charging process of the submodule.

[0010] Furthermore, the circuit breakers KG, KR, KW, and KF are all high-voltage circuit breakers.

[0011] A charging start-up method for a direct-drive wind turbine wind farm via a flexible DC transmission system includes the following steps: Step 1: System initialization, put circuit breakers KG, KR, KW, and KF in the open state, and put all sub-modules of the onshore converter station GSMMC and the offshore converter station WFMMC in the locked state. Step 2: Uncontrolled charging of submodules in the flexible DC transmission system: Close circuit breaker KG, connect current-limiting resistor R, and the onshore power grid charges the submodule capacitors of GSMMC and WFMMC through diode uncontrolled rectification until the submodule capacitor voltage reaches the operating voltage of the control system, thus completing the uncontrolled charging. Step 3: Controllable charging of WFMMC submodules: Unlock the WFMMC submodules, start passive AC voltage control, and continuously charge the WFMMC submodule capacitors through the DC line until the WFMMC submodule capacitor voltage is equal to the GSMMC submodule capacitor voltage. At the same time, establish a stable AC voltage and frequency at the wind farm collection PCC. Step 4: Controllable charging of GSMMC submodules: Close circuit breaker KR, bypass current limiting resistor R, unlock GSMMC DC voltage control function with slope control, so that the submodule capacitor voltage of GSMMC rises slowly at a preset slope until the submodule capacitor voltage reaches the rated voltage Udc / N and the DC side voltage reaches the rated value. Step 5: Wind farm startup and grid connection: The offshore wind farm is connected to the auxiliary power supply of the wind farm through circuit breaker KF and enters a stable operating state with the support of the auxiliary power supply; after the flexible DC transmission system completes the charging in step 4, it sends control signals to circuit breakers KW and KF to control circuit breaker KF to open and circuit breaker KW to close, and the offshore wind farm is successfully connected to the flexible DC transmission system.

[0012] Furthermore, in step 2, during the uncontrolled charging process, the voltage applied by the onshore power grid to each arm of the GSMMC is the peak value U of the AC system valve side line voltage. Lrms When charging is complete, the capacitor voltage u of the GSMMC submodule is... sm_GSMMC1 Satisfying formula (1), the DC side voltage U dc1 Satisfying formula (2), the submodule voltage u of WFMMC usm_WFMMC1 Satisfying formula (3): Formula (1):

[0013] Formula (2):

[0014] Formula (3):

[0015] Where N represents the number of submodules in a single bridge arm of GSMMC and WFMMC.

[0016] Furthermore, in step 3, when the WFMMC submodule is unlocked, the number of submodules put into operation is halved, causing the DC bus voltage to temporarily drop to half of the voltage at the end of step 2, and then gradually rise to the rated value during the charging process.

[0017] Furthermore, in step 3, after the controllable charging is completed, the submodule capacitor voltages of both GSMMC and WFMMC satisfy formula (4): Formula (4):

[0018] Among them, usm_GSMMC2 The voltage of the GSMMC submodule capacitor, u sm_WFMMC2 This refers to the capacitor voltage of the WFMMC submodule.

[0019] Furthermore, in step 5, when the offshore wind farm is connected to the flexible DC transmission system, the voltage amplitude and frequency of the offshore wind farm are kept consistent with those of the flexible DC transmission system, and the opening and closing sequence of circuit breakers KF and KW is controlled to limit the grid connection inrush current.

[0020] The beneficial effects of this invention are as follows: 1. The system has a simple structure and can be charged and started up by relying solely on the onshore power grid, without needing to rely on additional power sources on the offshore side, thus reducing the system construction cost and complexity.

[0021] 2. The charging start-up process is divided into two stages: uncontrollable charging and controllable charging. By using current-limiting resistors and slope control, the charging current and voltage change rate are effectively limited, avoiding damage to the equipment from large current surges and improving the safety and stability of the start-up process.

[0022] 3. The wind farm establishes a stable operating state in advance through auxiliary power supply, and the voltage and frequency matching degree on both sides is high when connected to the grid, which further reduces the grid connection inrush current and ensures a smooth transition between the wind farm and the transmission system.

[0023] 4. This system and method are specifically designed for the characteristics of direct-drive wind turbine wind farms, filling the gap in existing technologies for starting and connecting direct-drive wind farms to the grid via flexible DC transmission. It has strong adaptability and broad application prospects. Attached Figure Description

[0024] The invention will be further described below with reference to the accompanying drawings: Figure 1 This is a schematic diagram of the structure of the present invention. Detailed Implementation

[0025] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0026] The flexible DC transmission system for direct-drive wind turbine wind farms includes an onshore power grid, an onshore converter station GSMMC, an onshore transformer T1, an offshore converter station WFMMC, an offshore transformer T2, an offshore wind farm, an auxiliary power supply for the wind farm, a step-up transformer T3, a circuit breaker KG, a current-limiting resistor R, a circuit breaker KR, a circuit breaker KW, and a circuit breaker KF. The onshore power grid is connected in series with the input terminal of the onshore transformer T1, and the circuit breaker KG and the current-limiting resistor R are connected in series in the line between the onshore power grid and the onshore transformer T1. The circuit breaker KR is connected in parallel with the current-limiting resistor R to bypass the current-limiting resistor R; The output terminal of the onshore transformer T1 is connected to the AC side of the onshore converter station GSMMC, and the DC side of the onshore converter station GSMMC is connected to the DC side of the offshore converter station WFMMC through a DC line. The AC side of the offshore converter station WFMMC is connected to the input terminal of the offshore transformer T2, the output terminal of the offshore transformer T2 is connected to one end of the circuit breaker KW, and the other end of the circuit breaker KW is connected to the output terminal of the offshore wind farm. The step-up transformer T3 is connected in series in the line between the offshore wind farm and the circuit breaker KW to boost the output voltage of the offshore wind farm. The wind farm auxiliary power supply is connected in parallel with the offshore wind farm, and the circuit breaker KF is connected in series in the connection line between the wind farm auxiliary power supply and the offshore wind farm. The AC voltage amplitude and frequency of the wind farm auxiliary power supply are consistent with those of the onshore power grid. The offshore wind farm uses direct-drive wind turbines.

[0027] This system constructs a main energy transmission circuit, a charging and current limiting circuit, a wind farm startup support circuit, and core equipment configurations adapted to direct-drive wind turbines. Its working principle involves the onshore power grid converting and transmitting AC / DC power via transformers and converter stations. The current-limiting resistor R and circuit breaker KR work together to limit current during charging and reduce power consumption during normal operation via bypass. The wind farm auxiliary power supply provides stable support for wind farm startup through parallel connections. Each transformer achieves voltage level matching. The technical effect is the construction of a flexible DC transmission system with single-ended power supply, eliminating the need for additional offshore power, simplifying offshore equipment configuration, and reducing construction costs and difficulty. The modular circuit design achieves functional separation, facilitating control and maintenance. The auxiliary power supply addresses the pain point of lacking stable AC support during direct-drive wind farm startup. The matching of component parameters ensures system electrical characteristic matching, preventing startup failure or equipment damage.

[0028] Furthermore, both the onshore GSMMC and offshore WFMMC adopt a modular multilevel converter (MMC) topology, and each contains multiple sub-modules, with each sub-module having a built-in capacitor. The MMC topology achieves low harmonic voltage output through series switching of sub-modules. The sub-module capacitors serve as energy storage elements for energy storage during the charging phase and voltage stabilization during operation. The independent control of the sub-module's latched / unlocked state adapts to the phased startup strategy. The technical effect is to solve the problems of severe harmonic pollution and low control accuracy of traditional converters, improve power quality and control flexibility, adapt to the long-distance low-loss transmission requirements of offshore wind power, and the sub-module capacitors provide energy storage carriers for phased charging, ensuring voltage stability during startup. The modular structure facilitates capacity expansion and maintenance, and improves system redundancy and reliability.

[0029] Furthermore, each arm of the onshore converter station GSMMC is configured with N series-connected sub-modules, and each arm of the offshore converter station WFMMC is configured with N series-connected sub-modules. The upper and lower arms of the offshore converter station WFMMC contain a total of 2N operational sub-modules. Voltage matching at different stages is achieved through capacitor voltage division. During the controllable charging stage, the number of sub-modules engaged is halved after the WFMMC is unlocked to raise the DC bus voltage. During the rated operation stage, N sub-modules are connected in series and the rated voltage is output through stepped switching. The technical effect is to provide a hardware adaptation basis for the staged charging strategy, ensure that the voltage division law is valid during the uncontrollable charging stage, and make the charging process predictable and controllable. The WFMMC sub-module configuration realizes the function switching between multi-module voltage division during the charging stage and few-module voltage stabilization during the operation stage, taking into account both charging safety and operational stability. The unified design of N sub-modules per arm reduces the complexity of the control algorithm and improves the control accuracy.

[0030] Furthermore, the resistance value of the current-limiting resistor R is determined based on the system's rated current and the allowable charging inrush current, and is used to limit the peak current during the submodule charging process. The minimum resistance value is determined based on the system's rated current to avoid excessive power loss during normal charging, while the maximum resistance value is determined based on the allowable inrush current to ensure that the peak current does not exceed the equipment's tolerance threshold during the initial charging stage. R is applied during the charging stage to limit the inrush current, and after charging is completed, R is bypassed through KR to reduce losses. The technical effect is a precise balance between current-limiting protection and loss control, preventing damage to core components such as converter station submodules and diodes from large inrush currents, while ensuring that system efficiency is not affected during normal operation. This solves the technical problem of excessive charging current when the initial voltage of the MMC submodule capacitor is zero, improves the safety of the startup process, and the resistance value determination principle is universal and adaptable to flexible DC systems in direct-drive wind farms of different capacities.

[0031] Furthermore, the circuit breakers KG, KR, KW, and KF are all high-voltage circuit breakers. High-voltage circuit breakers possess the ability to withstand the system's rated voltage and interrupt the rated current, enabling stable operation under high-voltage conditions. They respond to control signals to achieve rapid opening and closing, meeting startup timing requirements. They also possess reliable arc-extinguishing performance to prevent arc reignition. Their technical effect is to ensure the safe operation of the system under high-voltage conditions, avoiding startup failures or operational accidents due to insufficient circuit breaker withstand voltage or breaking capacity. The rapid opening and closing characteristics provide hardware support for phased charging and precise grid connection, ensuring that each step is executed in an orderly manner according to preset logic. Reliable arc-extinguishing performance reduces electrical interference from switching operations, extends equipment lifespan, and improves system operational stability.

[0032] A flexible DC transmission control method for direct-drive wind turbine wind farms includes the following steps: Step 1: System initialization. Circuit breakers KG, KR, KW, and KF are all placed in the open state, and all sub-modules of the onshore converter station GSMMC and the offshore converter station WFMMC are in the locked state. With all circuit breakers open and sub-modules locked, the initial energized circuit of the system is eliminated, ensuring a safe startup process. Step 2: Uncontrolled charging of submodules in the flexible DC transmission system: Close circuit breaker KG and connect current-limiting resistor R. The onshore power grid charges the submodule capacitors of GSMMC and WFMMC through diode uncontrolled rectification until the submodule capacitor voltage reaches the operating voltage of the control system, completing the uncontrolled charging. Close KG and connect R. Utilize the diode uncontrolled rectification characteristics to charge the submodule capacitors of GSMMC and WFMMC from the onshore power grid until the capacitor voltage reaches the operating voltage of the control system, providing energy support for subsequent unlocking of the submodules.

[0033] Step 3: Controllable charging of WFMMC submodules: Unlock the WFMMC submodules, activate passive AC voltage control, and continuously charge the WFMMC submodule capacitors through the DC line until the WFMMC submodule capacitor voltage is equal to the GSMMC submodule capacitor voltage. At the same time, establish a stable AC voltage and frequency at the wind farm collection PCC. Unlock the WFMMC submodules again, activate passive AC voltage control, and supplement the charging of the WFMMC submodules through the DC line to make the voltages of the two submodules consistent. At the same time, establish a stable AC voltage / frequency at the PCC to provide grid support for wind farm grid connection. Step 4: Controllable charging of GSMMC submodules: Close circuit breaker KR, bypass current-limiting resistor R, unlock the GSMMC DC voltage control function with slope control, so that the submodule capacitor voltage of GSMMC rises slowly at a preset slope until the submodule capacitor voltage reaches the rated voltage Udc / N, and the DC side voltage reaches the rated value; close KR and bypass R, and through the slope-controlled DC voltage control strategy, make the submodule capacitor voltage rise slowly to the rated value to ensure that the DC side voltage meets the operating requirements; Step 5: Wind Farm Start-up and Grid Connection: The offshore wind farm is connected to the auxiliary power supply via circuit breaker KF and enters a stable operating state with the support of the auxiliary power supply. After the flexible DC transmission system completes the charging in Step 4, it sends control signals to circuit breakers KW and KF, controlling circuit breaker KF to open and circuit breaker KW to close, thus successfully connecting the offshore wind farm to the flexible DC transmission system. First, the wind farm is started and stabilized via the auxiliary power supply. After the flexible DC system completes charging, the wind farm is connected to the grid via a switching sequence control (KF opens, KW closes).

[0034] Furthermore, in step 2, during the uncontrolled charging process, the voltage applied by the onshore power grid to each arm of the GSMMC is the peak value U of the AC system valve side line voltage. Lrms When charging is complete, the capacitor voltage u of the GSMMC submodule is... sm_GSMMC1 Satisfying formula (1), the DC side voltage U dc1 Satisfying formula (2), the submodule voltage u of WFMMC usm_WFMMC1 Satisfying formula (3): Formula (1):

[0035] Formula (2):

[0036] Formula (3):

[0037] Where N represents the number of submodules in a single bridge arm of GSMMC and WFMMC.

[0038] Based on the voltage divider characteristics of the MMC bridge arm capacitors, during uncontrolled charging, each of the N sub-modules in each arm of the GSMMC withstands the peak AC system valve side line voltage ULrms, and the voltage of a single sub-module is... DC side voltage is The WFMMC has 2N sub-modules charging simultaneously on both the upper and lower arms, with the voltage of a single sub-module being ULrms / (2N). The technical effect is to quantify the voltage parameters in the uncontrollable charging stage, provide a clear voltage judgment threshold for the control system, ensure precise control in the charging stage, avoid entering the next stage too early or too late, provide a basis for hardware selection, avoid equipment damage caused by improper parameter design, and provide a benchmark for voltage rise in the controllable charging stage of the WFMMC, thereby improving control accuracy.

[0039] Furthermore, in step 3, when the WFMMC submodule is unlocked, the number of submodules put into operation is halved, causing the DC bus voltage to temporarily drop to half of the voltage at the end of step 2, and then gradually rise to the rated value during the charging process.

[0040] Based on the characteristic that the DC bus voltage of the MMC topology is proportional to the number of sub-modules engaged, the voltage dips when the number of engaged sub-modules is halved after the WFMMC is unlocked. Subsequently, the voltage recovers through continuous charging under passive AC voltage control. The technical effect is to provide a basis for predicting the dynamic characteristics of voltage in the control system. Excessive voltage dips can be avoided by designing compensation strategies or adjusting the charging current. The voltage dip and recovery rules are clearly defined, allowing the control algorithm to be designed specifically to ensure a smooth voltage recovery, avoid interruption of the startup process due to misjudgment of voltage dips, and improve the stability of the startup process.

[0041] Furthermore, in step 3, after the controllable charging is completed, the submodule capacitor voltages of both GSMMC and WFMMC satisfy formula (4): Formula (4):

[0042] Among them, u sm_GSMMC2 For the GSMMC submodule capacitor voltage, u sm_WFMMC2 This refers to the capacitor voltage of the WFMMC submodule.

[0043] After the WFMMC controlled charging phase is unlocked, the submodule continues to charge under passive AC voltage control, with the voltage starting from... Gradually increase until it matches the voltage of the GSMMC submodule. Equal voltage balance is achieved by balancing the voltage of the two sub-modules, ensuring that the electrical characteristics of the converter stations at both ends of the flexible DC system are consistent. This lays the foundation for a unified voltage boost during the subsequent controllable charging phase of the GSMMC. After voltage balancing, a stable AC voltage and frequency are established at the PCC point where the wind farm converges, meeting the grid support requirements of the wind farm and avoiding grid connection impacts caused by voltage imbalance. The endpoint criteria for controllable charging of the WFMMC are clarified, ensuring accurate timing of each step.

[0044] Furthermore, in step 5, when the offshore wind farm is connected to the flexible DC transmission system, the voltage amplitude and frequency of the offshore wind farm are kept consistent with those of the flexible DC transmission system. The opening and closing sequence of circuit breakers KF and KW is controlled to limit the grid connection inrush current. With the support of auxiliary power, the wind farm achieves parameter matching with the flexible DC system. A "break-before-close" switching operation logic is adopted to avoid circulating current impacts, and the switching time is controlled to ensure smooth switching. The technical effect is to construct an inrush current protection system from both parameter matching and timing control aspects, limiting the grid connection inrush current to within the allowable range of the equipment, avoiding damage to wind turbines, converter station equipment, and transmission lines. This solves the problem of excessive grid connection inrush current in direct-drive wind farms, improves grid connection safety and stability, and the control strategy is simple and easy to implement, requiring no complex additional equipment. It achieves inrush current suppression without increasing costs and has good engineering application value.

[0045] The detailed working process of this invention includes the following steps: S1, System initialization phase; S11. Equipment Status Preset: Set all sub-modules of the onshore converter station (GSMMC) and the offshore converter station (WFMMC) to the locked state, prohibit the switching devices in the sub-modules from being triggered and conduction, and only retain the conductivity of the anti-parallel diodes.

[0046] S12. Circuit Breaker Status Control: Disconnect all core circuit breakers, including those connecting the onshore power grid and GSMMC, those used for bypass current-limiting resistors, those connecting WFMMC and the wind farm, and those connecting auxiliary power supply and the wind farm, to ensure that the system has no initially energized circuits and avoid the risk of short-circuit faults and current surges during startup.

[0047] S13. Control system configuration: Complete the self-test of the control system, preset the voltage monitoring thresholds, control strategy parameters (such as voltage slope) and protection logic for each stage, and ensure accurate control of the subsequent charging and grid connection process.

[0048] S2, Uncontrollable charging stage of flexible DC transmission system submodule; S21. Charging circuit startup: At the initial moment, the circuit breaker connecting the onshore power grid and the GSMMC is closed, allowing the onshore power grid to connect to the GSMMC through a series current-limiting resistor. The current-limiting resistor limits the initial charging inrush current, protecting the submodule capacitors and diodes from overcurrent damage.

[0049] S22. GSMMC Submodule Charging: Since the GSMMC submodule is in a locked state, the onshore power grid charges the capacitors of each bridge arm submodule through an uncontrolled rectifier circuit formed by anti-parallel diodes within the submodule. Each bridge arm submodule is connected in series to divide the voltage, collectively bearing the peak voltage of the AC system valve side line, thus achieving stable charging of the submodule capacitors.

[0050] S23. DC side voltage establishment: As the GSMMC submodule charging progresses, the DC side bus of the flexible DC transmission system gradually establishes a stable voltage, which matches the peak value of the AC system valve side line voltage, completing the initial conversion from AC voltage to DC voltage.

[0051] S24.WFMMC Submodule Charging: The DC side voltage is transmitted to the WFMMC through the DC transmission line. The upper and lower bridge arm submodules of the WFMMC are connected in series to form a charging circuit, which jointly bears the initial DC side voltage and completes the synchronous charging of the submodule capacitors according to the voltage division law.

[0052] S25. Stage End Judgment: When the control system detects that the voltage of the GSMMC submodule, the DC side voltage, and the WFMMC submodule all reach the preset stable threshold, it determines that the uncontrollable charging stage is completed. At this time, the submodule voltage has met the working requirements of the control system and is ready to unlock controllable charging.

[0053] S3, WFMMC submodule controllable charging stage; S31. Submodule Unlocking and Voltage Sag: After uncontrolled charging is completed, the control system sends a submodule unlocking signal to the WFMMC and simultaneously activates the passive AC voltage control mode. After unlocking, the number of submodules engaged in operation on the WFMMC is halved. According to the MMC topology voltage characteristics, the DC bus voltage temporarily drops as the number of engaged submodules decreases.

[0054] S32. Voltage Equalization and AC Support Establishment: Under passive AC voltage control, the DC line continuously charges the WFMMC submodule capacitor, causing the WFMMC submodule voltage to gradually rise while the GSMMC submodule voltage remains stable. This continues until the WFMMC and GSMMC submodule voltages are completely balanced, at which point the DC bus voltage synchronously rises to the preset stable value. During this process, the WFMMC establishes a stable AC voltage and frequency at the wind farm's convergence point (PCC), providing the necessary grid support for the wind farm's grid connection.

[0055] S33. Stage End Judgment: When the control system confirms that the voltage of the sub-modules of the converter stations on both sides is balanced, and the AC voltage and frequency at the PCC are stable without fluctuation, the WFMMC controllable charging stage is completed.

[0056] S4, GSMMC submodule controllable charging stage; S41. Current-limiting resistor bypass and control unlocking: After the WFMMC completes controllable charging, the circuit breaker of the bypass current-limiting resistor is closed to remove the current-limiting resistor from the main circuit, reducing system operating losses; at the same time, the DC voltage active control function of the GSMMC is unlocked, and the voltage command value is adjusted smoothly using a preset slope control strategy to avoid voltage surges. S42. Rated Voltage Establishment: The onshore power grid continuously replenishes the capacitors of the unlocked GSMMC submodule. Under the control of the slope (such as the voltage command value rising slowly at a slope of 0.734pu~1pu), the submodule voltage is gradually fine-tuned to the rated equilibrium state, and the DC bus voltage is synchronously stabilized at the rated value. The flexible DC transmission system completes the entire charging process and is ready for grid connection of the wind farm.

[0057] S43. Stage End Judgment: When the control system detects that the voltage of the GSMMC submodule and the DC bus voltage are both stable at the rated value, the GSMMC controllable charging stage is completed.

[0058] S5, Wind farm startup and grid connection phase; S51. Wind Farm Pre-start: While the flexible DC transmission system is charging and starting, the circuit breaker connecting the auxiliary power supply and the wind farm is closed. The wind farm starts up gradually under the stable support of the auxiliary power supply and adjusts its output voltage amplitude and frequency to match the parameters of the flexible DC system and the onshore power grid.

[0059] S52. Grid Connection Sequence Control: After the flexible DC system completes all charging, the control system sends a coordinated control signal to the relevant circuit breakers and executes the "break first, close later" sequence logic: First, disconnect the circuit breaker connecting the auxiliary power supply to the wind farm to cut off the auxiliary power supply circuit; then close the circuit breaker connecting the wind farm to the WFMMC to formally connect the offshore wind farm to the flexible DC transmission system.

[0060] S53. Smooth Grid Connection Operation: Since the output parameters of the wind farm are fully matched with the flexible DC system, and the circuit breaker opening and closing sequence is precisely controlled, there is no excessive inrush current during the grid connection process. After grid connection, the electrical energy generated by the wind farm is transmitted to the onshore power grid through steps such as voltage boosting, AC-DC conversion, DC transmission, and rectification, achieving stable power transmission.

[0061] S6. System stable operation and shutdown phase; S61. Stable Operation Monitoring: After grid connection, the control system monitors core operating parameters in real time, including submodule voltages of GSMMC and WFMMC, DC bus voltage, wind farm output power, and the operating status of each circuit breaker. If submodule faults or excessive voltage fluctuations are detected, protection mechanisms are immediately triggered (such as bypassing faulty submodules or disconnecting the corresponding circuit breakers) to ensure the safe and stable operation of the system.

[0062] S62. Safety Shutdown Procedure: When the system needs to be shut down for maintenance or to stop generating electricity, first disconnect the circuit breaker connecting the wind farm and WFMMC to disconnect the wind farm from the flexible DC system; then the control system gradually locks all sub-modules of GSMMC and WFMMC, stops the AC-DC conversion of electrical energy, and the DC bus voltage gradually drops to zero; finally, disconnect the circuit breaker connecting the onshore power grid and GSMMC, and the entire system is completely de-energized and enters a safe standby state.

[0063] The embodiments described in this specification are merely examples of implementations of the inventive concept. The scope of protection of this invention should not be considered as limited to the specific forms stated in the embodiments. The scope of protection of this invention also extends to equivalent technical means that can be conceived by those skilled in the art based on the inventive concept.

Claims

1. A flexible DC transmission system for direct-drive wind turbine wind farms, characterized in that, This includes onshore power grid, onshore converter station GSMMC, onshore transformer T1, offshore converter station WFMMC, offshore transformer T2, offshore wind farm, wind farm auxiliary power supply, step-up transformer T3, circuit breaker KG, current-limiting resistor R, circuit breaker KR, circuit breaker KW, and circuit breaker KF. The onshore power grid is connected in series with the input terminal of the onshore transformer T1, and the circuit breaker KG and the current-limiting resistor R are connected in series in the line between the onshore power grid and the onshore transformer T1. The circuit breaker KR is connected in parallel with the current-limiting resistor R to bypass the current-limiting resistor R; The output terminal of the onshore transformer T1 is connected to the AC side of the onshore converter station GSMMC, and the DC side of the onshore converter station GSMMC is connected to the DC side of the offshore converter station WFMMC through a DC line. The AC side of the offshore converter station WFMMC is connected to the input terminal of the offshore transformer T2, the output terminal of the offshore transformer T2 is connected to one end of the circuit breaker KW, and the other end of the circuit breaker KW is connected to the output terminal of the offshore wind farm. The step-up transformer T3 is connected in series in the line between the offshore wind farm and the circuit breaker KW to boost the output voltage of the offshore wind farm. The wind farm auxiliary power supply is connected in parallel with the offshore wind farm, and the circuit breaker KF is connected in series in the connection line between the wind farm auxiliary power supply and the offshore wind farm. The AC voltage amplitude and frequency of the wind farm auxiliary power supply are consistent with those of the onshore power grid. The offshore wind farm uses direct-drive wind turbines.

2. The flexible DC transmission system for direct-drive wind turbine wind farms according to claim 1, characterized in that, Both the onshore converter station GSMMC and the offshore converter station WFMMC adopt a modular multilevel converter (MMC) topology and contain multiple sub-modules, each with built-in capacitors.

3. The flexible DC transmission system for direct-drive wind turbine wind farms according to claim 2, characterized in that, Each arm of the onshore converter station GSMMC is configured with N series-connected sub-modules, and each arm of the offshore converter station WFMMC is configured with N series-connected sub-modules. The upper and lower arms of the offshore converter station WFMMC contain a total of 2N operational sub-modules.

4. The flexible DC transmission system for direct-drive wind turbine wind farms according to claim 1, characterized in that, The resistance value of the current-limiting resistor R is determined based on the system's rated current and the allowable charging inrush current, and is used to limit the current peak during the submodule's charging process.

5. The flexible DC transmission system for direct-drive wind turbine wind farms according to claim 1, characterized in that, The circuit breakers KG, KR, KW, and KF are all high-voltage circuit breakers.

6. A control method for a flexible DC transmission system for a direct-drive wind turbine wind farm according to any one of claims 1-5, characterized in that, Includes the following steps: Step 1: System initialization, put circuit breakers KG, KR, KW, and KF in the open state, and put all sub-modules of the onshore converter station GSMMC and the offshore converter station WFMMC in the locked state. Step 2: Uncontrolled charging of submodules in the flexible DC transmission system: Close circuit breaker KG, connect current-limiting resistor R, and the onshore power grid charges the submodule capacitors of GSMMC and WFMMC through diode uncontrolled rectification until the submodule capacitor voltage reaches the operating voltage of the control system, thus completing the uncontrolled charging. Step 3: Controllable charging of WFMMC submodules: Unlock the WFMMC submodules, start passive AC voltage control, and continuously charge the WFMMC submodule capacitors through the DC line until the WFMMC submodule capacitor voltage is equal to the GSMMC submodule capacitor voltage. At the same time, establish a stable AC voltage and frequency at the wind farm collection PCC. Step 4: Controllable charging of GSMMC submodules: Close circuit breaker KR, bypass current limiting resistor R, unlock GSMMC DC voltage control function with slope control, so that the submodule capacitor voltage of GSMMC rises slowly at a preset slope until the submodule capacitor voltage reaches the rated voltage Udc / N and the DC side voltage reaches the rated value. Step 5: Wind farm startup and grid connection: The offshore wind farm is connected to the auxiliary power supply of the wind farm through circuit breaker KF and enters a stable operating state with the support of the auxiliary power supply; after the flexible DC transmission system completes the charging in step 4, it sends control signals to circuit breakers KW and KF to control circuit breaker KF to open and circuit breaker KW to close, and the offshore wind farm is successfully connected to the flexible DC transmission system.

7. The flexible DC transmission control method for direct-drive wind turbine wind farms according to claim 6, characterized in that, In step 2, during the uncontrolled charging process, the voltage applied by the onshore power grid to each arm of the GSMMC is the peak value U of the AC system valve side line voltage. Lrms When charging is complete, the GSMMC submodule capacitor voltage u sm_GSMMC1 Satisfying formula (1), the DC side voltage U dc1 Satisfying formula (2), the submodule voltage u of WFMMC sm_WFMMC1 Satisfying formula (3): Official (1): Official (2): Official (3): Where N represents the number of submodules in a single bridge arm of GSMMC and WFMMC.

8. The flexible DC transmission control method for direct-drive wind turbine wind farms according to claim 6, characterized in that, In step 3, when the WFMMC submodule is unlocked, the number of submodules put into operation is halved, causing the DC bus voltage to temporarily drop to half of the voltage at the end of step 2, and then gradually rise to the rated value during the charging process.

9. The flexible DC transmission control method for direct-drive wind turbine wind farms according to claim 6, characterized in that, In step 3, after the controlled charging is completed, the submodule capacitor voltages of both GSMMC and WFMMC satisfy formula (4): Official (4): Among them, u sm_GSMMC2 The voltage of the GSMMC submodule capacitor, u sm_WFMMC2 This refers to the capacitor voltage of the WFMMC submodule.

10. The flexible DC transmission control method for direct-drive wind turbine wind farms according to claim 6, characterized in that, In step 5, when the offshore wind farm is connected to the flexible DC transmission system, the voltage amplitude and frequency of the offshore wind farm are kept consistent with those of the flexible DC transmission system, and the opening and closing sequence of circuit breakers KF and KW is controlled to limit the grid connection inrush current.

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