Method and device for starting and reactive power control of a marine wind power DR transmission system

Abstract

This invention discloses a startup and reactive power control method and device for an offshore wind power DR transmission system, belonging to the field of offshore wind power transmission. The method is applicable to the host computer of an offshore wind power DR transmission system, which also includes offshore and onshore modules. The offshore module includes an offshore wind farm, a low-voltage multi-mode converter, a diode rectifier, an AC circuit breaker, and a high-speed mechanical switch. The method includes controlling the onshore module's multi-level converter to switch to buck rectification mode and the low-voltage multi-mode converter to grid-connected control mode when all switches are open, completing a black start; then controlling the multi-level converter to switch to zero DC current mode and locking the synchronous phase reference for each grid-connected wind turbine in the offshore wind farm; controlling the grid-connected wind turbine to switch to grid-connected control mode, adjusting the output voltage of the grid-connected wind turbine, closing the AC circuit breaker, and switching the low-voltage multi-mode converter to reactive power compensation mode. By implementing this invention, the problem of poor operational stability in existing technologies is solved.

Description

Technical Field

[0001] This invention relates to the field of offshore wind power transmission technology, and in particular to a method and apparatus for starting up and reactive power control of an offshore wind power DR transmission system. Background Technology

[0002] With the accelerated transformation of my country's energy structure, offshore wind power has become a core direction for renewable energy development due to its abundant wind resources, stable wind speeds, and lack of land occupation. As offshore wind power gradually expands into deeper waters and on a larger scale, flexible DC transmission technology, with its long transmission distance and flexible reactive power control, has become the mainstream technology for grid connection of offshore wind power. Traditional offshore converter stations generally use modular multilevel converters (MMCs), which have complex topologies and extremely high costs. Therefore, the solution of using diode rectifiers (DRs) to replace traditional MMCs for offshore converter stations has emerged.

[0003] However, as an uncontrolled rectifier, the DR (Diode Rectifier) ​​faces difficulties in black-starting in offshore wind power transmission scenarios. The DR's core component, the diode, only has unidirectional conductivity, making it impossible to reverse the energy flow from the onshore grid to the offshore wind farm. The system cannot obtain starting power from the onshore grid via the high-voltage DC main circuit, resulting in the offshore AC bus failing to establish initial voltage and frequency, and preventing black-starting of offshore grid-connected wind turbines. Furthermore, the DR itself lacks the ability to regulate AC side voltage and frequency; if the offshore AC grid lacks sufficient reactive power support, voltage instability can easily occur, leading to system collapse. Summary of the Invention

[0004] This invention provides a method and apparatus for starting and reactive power control of an offshore wind power DR transmission system, which can effectively solve the problem that existing offshore wind power DR transmission systems cannot simultaneously handle black start and reactive power support, resulting in poor system operation stability.

[0005] One embodiment of the present invention provides a startup and reactive power control method for an offshore wind power DR transmission system, applicable to the host computer of the offshore wind power DR transmission system. The offshore wind power DR transmission system also includes an offshore module and an onshore module. The offshore module includes an offshore wind farm, a low-voltage multimode converter, a diode rectifier, an AC circuit breaker, and a high-speed mechanical switch. The offshore wind farm includes several grid-type wind turbines. The DC side of the diode rectifier is connected to the DC main circuit. The DC side of the low-voltage multimode converter is connected to the DC main circuit through a high-speed mechanical switch. The onshore module includes a multilevel converter. The DC side of the multilevel converter is connected to the DC main circuit. The method includes: When all system switches are in the open state, the high-speed mechanical switch is closed to control the multilevel converter to switch to buck rectification mode to match the rated DC voltage of the low-voltage multimode converter; Control the low-voltage multi-mode converter to switch to grid-connected control mode so that the grid-connected wind turbine can complete black start; After the black start is completed, the control multilevel converter switches to zero DC current mode until the DC main circuit current is monitored to meet the preset arc-free interruption condition, then the control low-voltage multimode converter is locked and the high-speed mechanical switch is opened. During the transient pressure-free dead zone after the high-speed mechanical switch is disconnected, the synchronous phase reference of each grid-type fan is locked until the transient pressure-free dead zone ends. After the transient dead zone ends, the grid-type wind turbine is switched to grid control mode. The multilevel converter is controlled to raise the DC main circuit voltage to the preset voltage value, and the grid-type wind turbine is controlled to adjust the AC side output voltage so that the voltage difference between the output voltage of the diode rectifier and the current DC bus voltage meets the preset voltage difference threshold. Once the differential pressure meets the preset differential pressure threshold, the AC circuit breaker is closed to enable the diode rectifier to conduct, and the low-voltage multimode converter is switched to reactive power compensation mode.

[0006] Furthermore, the offshore module also includes: a three-phase three-winding transformer and a three-phase two-winding transformer; the AC side of the diode rectifier is connected to the offshore AC bus via the three-phase three-winding transformer and the AC circuit breaker in sequence; the AC side of the low-voltage multi-mode converter is connected to the offshore AC bus via the three-phase two-winding transformer.

[0007] Furthermore, when all switches in the system are in the open state, controlling the high-speed mechanical switch to close includes: When the system is in a state of complete shutdown, all switching devices are controlled to be in the open state; wherein, the AC circuit breaker on the AC side of the diode rectifier is in the open state; A preset closing command is issued to control the high-speed mechanical switch to close, so that the low-voltage multimode converter can be connected to the DC main circuit.

[0008] Furthermore, the grid-type wind turbine includes a grid-side converter, a phase-locked loop, and a DC link containing a DC capacitor; Controlling the low-pressure multimode converter to switch to grid-connected control mode to enable grid-connected wind turbines to complete black start includes: The low-voltage multimode converter is switched to a constant voltage and constant frequency grid control mode so that the low-voltage multimode converter establishes the initial voltage of the offshore AC bus and converts the power fed back from the onshore module into the starting power of the offshore module. Each grid-type wind turbine is controlled to charge the capacitor of the DC link through the grid-side converter based on the starting power until the DC link voltage reaches the preset power-on threshold, thus completing the power-on wake-up of the grid-type wind turbine. After the grid-type wind turbines are powered on and awakened, the phase-locked loops controlling each grid-type wind turbine track the voltage of the offshore AC bus and obtain the initial synchronization phase reference to complete the black start.

[0009] Furthermore, the grid-type wind turbine also includes a permanent magnet synchronous generator, a turbine-side converter, and a grid-side converter controller; the turbine-side converter is connected to the output terminal of the permanent magnet synchronous generator, and the output terminal of the grid-side converter is connected to the offshore AC bus; the DC link is connected between the turbine-side converter and the grid-side converter.

[0010] Furthermore, during the transient pressure-free dead zone after the high-speed mechanical switch is disconnected, the synchronization phase reference of each grid-type wind turbine is locked until the transient pressure-free dead zone ends, including: During the transient period without pressure dead zone after the high-speed mechanical switch is disconnected, the grid-type wind turbines control each grid-type wind turbine to supply power to the grid-side converter controller through the energy pre-stored in the DC capacitor in the DC link, so as to maintain the continuous operation of the offshore wind power DR transmission system. Control each grid-type wind turbine to lock the synchronous phase reference until the transient pressure-free dead zone ends.

[0011] Furthermore, after the transient pressure-free dead zone ends, the process also includes: controlling the low-voltage multimode converter to synchronize with the offshore AC bus via a phase-locked loop, and then switching to a reactive power standby state.

[0012] Furthermore, the multilevel converter of the onshore module is an onshore hybrid modular multilevel converter; the onshore hybrid modular multilevel converter includes: an AC control loop, a DC control loop, and a bridge arm circulating current suppression control loop; The onshore hybrid modular multilevel converter is used to set the DC voltage reference value to a low-voltage rated value that matches the rated DC voltage of the low-voltage multimode converter through the DC control loop when switching to buck rectification mode; reduce the DC voltage to the low-voltage rated value through the DC main circuit; lock the onshore grid phase through the AC control loop; and extract and suppress the second harmonic negative sequence circulating current of the bridge arm through the bridge arm circulating current suppression control loop.

[0013] Furthermore, the low-voltage multimode converter includes: a DC-side voltage control loop, an AC-side voltage control loop, and a harmonic compensation control loop; Controlling the low-voltage multimode converter to switch to reactive power compensation mode includes: The low-voltage multimode converter absorbs active power by controlling the DC-side voltage control loop; The AC side voltage control loop of the low-voltage multimode converter adjusts the reactive power according to the offshore AC bus voltage and switches to reactive power compensation mode. The harmonic compensation control loop of the low-voltage multimode converter extracts the characteristic harmonics generated by the diode rectifier.

[0014] As an improvement to the above solution, another embodiment of the present invention provides a start-up and reactive power control device for an offshore wind power DR transmission system, comprising: a host computer, an offshore module, and an onshore module; the offshore module includes an offshore wind farm, a low-voltage multimode converter, a diode rectifier, an AC circuit breaker, and a high-speed mechanical switch; the offshore wind farm includes several grid-type wind turbines; the DC side of the diode rectifier is connected to the DC main circuit; the DC side of the low-voltage multimode converter is connected to the DC main circuit through a high-speed mechanical switch; the onshore module includes a multilevel converter; the DC side of the multilevel converter is connected to the DC main circuit. The host computer includes: The buck rectification mode switching module is used to control the high-speed mechanical switch to close when all switches in the system are in the open state, and to control the multilevel converter to switch to buck rectification mode to match the rated DC voltage of the low-voltage multimode converter. The black start module is used to control the low-voltage multimode converter to switch to grid-connected control mode so that the grid-connected wind turbine can complete the black start. The zero DC current mode switching module is used to control the multilevel converter to switch to zero DC current mode after black start is completed, until the DC main circuit current is monitored to meet the preset arc-free interruption condition, and then control the low-voltage multimode converter to lock out and the high-speed mechanical switch to open. The transient pressure-free dead zone continuation module is used to control each grid-type wind turbine to lock the synchronous phase reference during the transient pressure-free dead zone after the high-speed mechanical switch is opened, until the transient pressure-free dead zone ends. The grid control mode switching module is used to control the grid-type wind turbine to switch to grid control mode after the transient dead zone ends, control the multilevel converter to raise the DC main circuit voltage to the preset voltage value, and control the grid-type wind turbine to adjust the AC side output voltage so that the voltage difference between the output voltage of the diode rectifier and the current DC bus voltage meets the preset voltage difference threshold. The reactive power compensation module is used to control the AC circuit breaker to close after the differential pressure meets the preset differential pressure threshold, so as to turn on the diode rectifier and at the same time control the low-voltage multi-mode converter to switch to reactive power compensation mode.

[0015] By implementing this invention, at least the following beneficial effects are achieved: This invention provides a method and apparatus for starting up and reactive power control of an offshore wind power DR transmission system. The method is applicable to the host computer of the offshore wind power DR transmission system, which also includes an offshore module and an onshore module. The offshore module includes an offshore wind farm, a low-voltage multimode converter, a diode rectifier, an AC circuit breaker, and a high-speed mechanical switch. The offshore wind farm includes several grid-type wind turbines. The DC side of the diode rectifier is connected to the DC main circuit. The DC side of the low-voltage multimode converter is connected to the DC main circuit via a high-speed mechanical switch. The onshore module includes a multilevel converter. The DC side of the multilevel converter is connected to the DC main circuit. By controlling the multilevel converter to switch to buck rectification mode, the DC main circuit is actively transmitted. The voltage drops to a value matching the rated value of the low-voltage multimode converter. With the closure of a high-speed mechanical switch, the low-voltage multimode converter is connected to the DC main circuit. This bypasses the physical limitation of single-phase conduction by the diode rectifier, establishing a reverse power transmission path between the onshore module, the DC main circuit, and the low-voltage multimode converter. This solves the problems of the diode rectifier being unable to reverse power supply and the system being unable to obtain startup power from the onshore side. Once the system enters steady-state operation after the diode rectifier is turned on, the low-voltage multimode converter is switched to reactive power compensation mode. The low-voltage multimode converter replaces the diode rectifier in undertaking the functions of voltage regulation and reactive power support for the offshore grid, thereby avoiding voltage instability and system collapse caused by reactive power deficiency on the offshore side and improving system operational stability. Attached Figure Description

[0016] Figure 1 This is a flowchart illustrating the startup and reactive power control method of an offshore wind power DR transmission system according to an embodiment of the present invention. Figure 2 This is a schematic diagram of the structure of an offshore wind power DR transmission system provided in an embodiment of the present invention; Figure 3 This is a schematic diagram of the control logic of an onshore hybrid modular multilevel converter (FH-MMC) provided in an embodiment of the present invention; Figure 4 This is a schematic diagram of the grid control logic of a marine low-pressure multimode converter during the black start phase, provided by an embodiment of the present invention. Figure 5 This is a schematic diagram of the comprehensive control logic for reactive power and specific harmonics of a marine low-pressure multimode converter during normal operation, provided by an embodiment of the present invention. Figure 6 This is a schematic diagram of a full-power converter-type grid-type wind turbine topology and its control principle for maintaining phase memory in the transient dead zone, provided by an embodiment of the present invention. Figure 7 This is a schematic diagram of the start-up and reactive power control device of an offshore wind power DR transmission system provided in an embodiment of the present invention. Detailed Implementation

[0017] 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.

[0018] See Figure 1 To address the problem of poor system stability caused by the inability of existing offshore wind power DR transmission systems to simultaneously handle black start and reactive power support, an embodiment of this invention provides a flowchart illustrating a method for starting and reactive power control in an offshore wind power DR transmission system. The method is applicable to the host computer of the offshore wind power DR transmission system, which also includes an offshore module and an onshore module. The offshore module includes an offshore wind farm, a low-voltage multimode converter, a diode rectifier, an AC circuit breaker, and a high-speed mechanical switch. The offshore wind farm includes several grid-type wind turbines. The DC side of the diode rectifier is connected to the DC main circuit. The DC side of the low-voltage multimode converter is connected to the DC main circuit via a high-speed mechanical switch. The onshore module includes a multilevel converter. The DC side of the multilevel converter is connected to the DC main circuit. Specifically, such as Figure 2 As shown, the offshore module includes: offshore wind farm 1, offshore main converter transformer 2 (three-phase three-winding transformer), auxiliary converter transformer 5 (three-phase two-winding transformer), diode rectifier (DR) 3, low-voltage multimode converter 4, high-speed mechanical switch (HSS) 7, and AC circuit breaker (DR-ACCB) 8. Offshore wind farm 1 includes offshore AC bus 12 and several grid-type wind turbines. Taking grid-type wind turbines 13 and 15 as examples, grid-type wind turbine 13 is connected to offshore AC bus 12 via AC submarine cable 14, and grid-type wind turbine 15 is connected to offshore AC bus 12 via AC submarine cable 16. The offshore module is connected to the onshore module via high-voltage direct current transmission line 6. The onshore module includes multilevel converter (FH-MMC) 9, grid-side transformer 10, onshore AC grid-connected bus 17, and onshore AC main grid 11. The three-phase six-bridge arm of the multilevel converter (FH-MMC) 9 is composed of a hybrid cascade of full-bridge submodules (FBSM) 91 and half-bridge submodules (HBSM) 92. Thanks to the ability of the full-bridge submodule 91 to output a negative level, the FH-MMC has a wide range of stepless deep voltage reduction operation capability, which allows the number of submodules of the low-voltage multimode converter 4 to be reduced to 30%-40% of that of conventional high-voltage converters, significantly reducing its requirements for deck load and cost on offshore platforms.

[0019] Specifically, such as Figure 2As shown, the topology of the onshore hybrid modular multilevel converter (FH-MMC) 9 is as follows: Figure 2 The dashed box in the lower right corner is shown. N F This represents the number of full-bridge submodules (FBSMs) cascaded in each bridge arm; N H This represents the number of half-bridge submodules (HBSMs) cascaded in each bridge arm; L arm The symbols represent the bridge arm reactor (bridge arm inductor); A, B, and C represent the three-phase terminals on the AC side; the + and - symbols represent the positive and negative busbar connection terminals on the DC side, respectively.

[0020] Specifically, DR stands for Diode Rectifier, referring to an uncontrolled rectifier device used in offshore converter stations. Its core component is a diode power device, supporting only unidirectional power transmission from the AC side to the DC side at sea. It is used to rectify the AC power from offshore wind power into DC power, which is then transmitted to the onshore power grid via DC lines. The DR transmission system is a flexible DC transmission system that uses diode rectifiers as the offshore sending-end converter unit and transmits offshore wind power to the onshore receiving-end power grid via high-voltage DC transmission lines.

[0021] Specifically, the offshore module is the set of all electrical equipment arranged on the offshore wind power platform, including the offshore wind farm, low-voltage multi-modal converter, diode rectifier, AC circuit breaker, and high-speed mechanical switch. The onshore module is the set of all electrical equipment arranged in the onshore converter station, including the multi-level converter. The offshore AC bus is the AC collection bus of the offshore wind farm, which is used to collect the output electric energy of all wind turbines, and at the same time provides an AC-side interface for the DR and the low-voltage multi-modal converter. It is the voltage and frequency reference carrier of the offshore AC system. The network-forming wind turbine is a full-power conversion type offshore wind turbine, which can actively establish the voltage and frequency of the AC system without relying on the voltage reference of the external power grid, different from the traditional grid-following wind turbine. The low-voltage multi-modal converter is a low-voltage full-controlled modular multi-level converter connected in parallel on the offshore side, which can switch different working modes at different operation stages, including the constant voltage and constant frequency network-forming mode (V / F network-forming mode) in the black start stage and the reactive power compensation mode in the steady state stage, realizing time-division multiplexing of equipment. Its rated DC voltage is significantly lower than the rated voltage of the DC main circuit. The AC circuit breaker is the DR-ACCB on the AC side of the diode rectifier, which is connected in series between the AC side of the DR and the offshore AC bus, and is used for the switching and fault isolation of the DR, and is a lightweight circuit breaker with a low breaking capacity. The high-speed mechanical switch HSS is a high-speed switching device connected in series between the DC side of the low-voltage multi-modal converter and the DC main circuit, which has the capabilities of fast switching and arc-free breaking, and is used for the switching of the DC circuit of the low-voltage multi-modal converter in the black start stage. The DC main circuit is a DC power transmission channel composed of high-voltage DC transmission lines, which connects the DC side of the offshore DR and the DC side of the onshore multi-level converter to realize the long-distance transmission of offshore wind power. The multi-level converter refers to the full-bridge - half-bridge hybrid modular multi-level converter FH-MMC, which has the capabilities of bidirectional power transmission, wide-range stepless voltage reduction, and reactive power regulation.

[0022] Preferably, the offshore module further includes: a three-phase three-winding transformer and a three-phase two-winding transformer; the AC side of the diode rectifier is connected to the offshore AC bus through the three-phase three-winding transformer and the AC circuit breaker in sequence; the AC side of the low-voltage multi-modal converter is connected to the offshore AC bus through the three-phase two-winding transformer.

[0023] Specifically, the three-phase three-winding transformer is the main offshore converter transformer, which is the converter transformer supporting the DR. It has three windings, which can suppress odd harmonics such as the 3rd and 5th harmonics generated by the DR rectification, reduce the harmonic content flowing into the offshore AC system, and at the same time realize the matching of the offshore AC bus voltage and the rated input voltage of the DR. The three-phase two-winding transformer is the auxiliary converter transformer, which is the converter transformer supporting the low-voltage multi-modal converter. It has two windings, realizes the matching of the offshore AC bus voltage and the rated AC voltage of the low-voltage multi-modal converter, and at the same time realizes electrical isolation, reducing the insulation design requirements of the low-voltage multi-modal converter.

[0024] Schematic illustration: The AC side of the DR is first connected to the low-voltage winding of a three-phase three-winding transformer. The high-voltage winding of the three-phase three-winding transformer is connected to the offshore AC bus via an AC circuit breaker, achieving electrical connection and voltage matching between the DR and the AC bus. The AC side of the low-voltage multimode converter is connected to the low-voltage winding of a three-phase two-winding transformer. The high-voltage winding of the three-phase two-winding transformer is directly connected to the offshore AC bus, achieving electrical connection and voltage matching between the low-voltage multimode converter and the AC bus. By using two transformers to achieve voltage matching between the DR and the low-voltage multimode converter respectively, the operating parameters of the two devices can be independently optimized, improving the system's operational flexibility.

[0025] Preferably, the multilevel converter of the onshore module is an onshore hybrid modular multilevel converter; the onshore hybrid modular multilevel converter includes: an AC control loop, a DC control loop, and a bridge arm circulating current suppression control loop; The onshore hybrid modular multilevel converter is used to set the DC voltage reference value to a low-voltage rated value that matches the rated DC voltage of the low-voltage multimode converter through the DC control loop when switching to buck rectification mode; reduce the DC voltage to the low-voltage rated value through the DC main circuit; lock the onshore grid phase through the AC control loop; and extract and suppress the second harmonic negative sequence circulating current of the bridge arm through the bridge arm circulating current suppression control loop.

[0026] Specifically, the onshore hybrid modular multilevel converter (FH-MMC) is the core converter equipment of the onshore receiving-end converter station. Its three-phase, six-arm configuration is composed of a hybrid cascade of full-bridge submodules (FBSM) and half-bridge submodules (HBSM), combining the advantages of low loss in half-bridge submodules and the ability to output negative levels in full-bridge submodules. It features wide-range stepless step-down, DC fault ride-through, and bidirectional power transmission capabilities. The AC control loop employs dq-axis current decoupling control to track the phase of the onshore power grid, controlling the converter's active and reactive power outputs to achieve stable grid connection with the onshore power grid. The DC control loop controls the DC voltage and current of the DC main circuit, allowing switching between buck rectification mode, zero DC current mode, and rated voltage mode by changing different reference values. The arm circulating current suppression control loop extracts the second-harmonic negative-sequence circulating current component from the arm current, suppressing circulating current through closed-loop control to reduce internal converter losses and device current stress. The second-harmonic negative-sequence circulating current is an inherent component of the second-harmonic negative-sequence current in the MMC bridge arm current. It increases device current stress and converter losses and needs to be suppressed through a dedicated control loop.

[0027] The method includes: S1. When all switches in the system are in the open state, control the high-speed mechanical switch to close, and control the multilevel converter to switch to buck rectification mode to match the rated DC voltage of the low-voltage multimode converter; Specifically, the buck rectification mode refers to the ability to output a negative level through the full-bridge submodule, which steplessly reduces the DC voltage of the DC main circuit from the rated high voltage value to a low voltage value that matches the rated DC voltage of the low-voltage multimode converter, thereby realizing the reverse energy transmission from the onshore power grid to the sea.

[0028] Indicatively, during the initial startup phase, the system is in a completely shut-down initial state, with all switching equipment, including the DR AC side circuit breaker and high-speed mechanical switch, in the open state. The host computer first issues a closing command to close the high-speed mechanical switch, connecting the DC side of the low-voltage multimode converter to the DC main circuit. Subsequently, the host computer controls the onshore multilevel converter to unlock and switch to buck rectification mode. Utilizing the negative level output capability of its full-bridge submodule, the DC voltage of the DC main circuit is stably controlled to a low voltage value that is completely consistent with the rated DC voltage of the low-voltage multimode converter, achieving voltage matching and creating conditions for onshore reverse power transmission.

[0029] Preferably, controlling the high-speed mechanical switch to close when all switches in the system are in the open state includes: When the system is in a state of complete shutdown, all switching devices are controlled to be in the open state; wherein, the AC circuit breaker on the AC side of the diode rectifier is in the open state; A preset closing command is issued to control the high-speed mechanical switch to close, so that the low-voltage multimode converter can be connected to the DC main circuit.

[0030] Specifically, the initial state of complete shutdown indicates that all wind turbines in the offshore wind farm are shut down, all converters are locked, all switchgear is in the open state, and the offshore AC system has no voltage and the DC system has no energy transmission. The preset closing command is a pre-set high-speed mechanical switch closing control command to ensure the safety and reliability of the switch closing operation.

[0031] Indicatively, after the system is powered on, the host computer first performs a system-wide equipment status inspection to confirm that all switching equipment is in the off state, especially the DR-ACCB, which must be in the open state to avoid DR mis-connection during the black start phase. After the status is confirmed to be correct, the host computer issues a preset closing command to drive the operating mechanism of the high-speed mechanical switch to perform the closing action. After the closing is completed, the host computer receives the switch position feedback signal to confirm that the HSS has been reliably closed and the DC side of the low-voltage multimode converter has been connected to the DC main circuit.

[0032] S2. Control the low-voltage multi-mode converter to switch to grid-connected control mode so that the grid-connected wind turbine can complete black start; Specifically, the grid-connected control mode can proactively establish a stable voltage amplitude and frequency for the passive offshore AC system, simulating the power supply characteristics of a synchronous generator, and providing black-start power for the wind farm. Black start refers to the process of establishing an initial voltage by reverse power transmission when the offshore wind farm is completely shut down and there is no external grid support, gradually starting the wind turbine units, and ultimately achieving normal operation of the entire wind power transmission system.

[0033] Indicatively, during the black start phase, after the DC voltage matching is completed, the host computer controls the low-voltage multi-mode converter to unlock and switch to the grid-connected control mode. It actively establishes a stable rated AC voltage and a rated frequency of 50Hz for the offshore AC bus, converting the DC power fed back from the onshore power grid into AC power as the black start power source for the offshore wind farm. After the grid-connected wind turbine detects the AC bus voltage, it completes power-on wake-up and pre-synchronization, and finally completes the black start.

[0034] Preferably, the grid-type wind turbine includes a grid-side converter, a phase-locked loop, and a DC link containing a DC capacitor; Controlling the low-pressure multimode converter to switch to grid-connected control mode to enable grid-connected wind turbines to complete black start includes: The low-voltage multimode converter is switched to a constant voltage and constant frequency grid control mode so that the low-voltage multimode converter establishes the initial voltage of the offshore AC bus and converts the power fed back from the onshore module into the starting power of the offshore module. Each grid-type wind turbine is controlled to charge the capacitor of the DC link through the grid-side converter based on the starting power until the DC link voltage reaches the preset power-on threshold, thus completing the power-on wake-up of the grid-type wind turbine. After the grid-type wind turbines are powered on and awakened, the phase-locked loops controlling each grid-type wind turbine track the voltage of the offshore AC bus and obtain the initial synchronization phase reference to complete the black start.

[0035] Specifically, the grid-side converter is a fully controlled converter connecting the DC link to the offshore AC bus, possessing bidirectional rectification and inversion capabilities. During the black start phase, uncontrolled rectification can be achieved through anti-parallel diodes to charge the DC capacitor. The phase-locked loop (PLL) is the phase tracking unit in the wind turbine control system, capable of real-time detection of the phase and frequency of the AC bus voltage, achieving synchronization between the wind turbine output voltage and the AC system, and avoiding transient impacts during grid connection. The DC link is the DC circuit connecting the turbine-side converter and the grid-side converter, with the DC support capacitor at its core, used to stabilize the DC voltage and buffer power fluctuations. The constant voltage and constant frequency grid control mode, also known as the V / F grid control mode, is the core control mode of the low-voltage multi-mode converter. Through voltage and current dual closed-loop control, it actively outputs a constant voltage amplitude (V) and frequency (F), establishing a stable voltage and frequency reference for the passive AC system. The preset power-on threshold is the minimum DC voltage required for normal startup of the wind turbine control board and auxiliary equipment, typically 80%-90% of the rated voltage of the wind turbine DC link.

[0036] Schematic diagram: The host computer controls the low-voltage multimode converter to switch to constant voltage and constant frequency grid control mode. Through dual closed-loop control of voltage outer loop and current inner loop, a stable rated AC voltage and 50Hz rated frequency are established on the offshore AC bus, converting the DC power transmitted in reverse on land into AC power as the black start power source for the wind farm. After the offshore AC bus is energized, the anti-parallel diodes of the grid-side converters of each grid-type wind turbine are turned on, charging the DC capacitors of the DC link, and the DC voltage continues to rise. When the DC link voltage reaches the preset power-on threshold, the auxiliary equipment such as the DSP control board, cooling system, and converter drive unit of the wind turbine are powered on and awakened, and the wind turbine control system is started. After the wind turbine control system is started, the control phase-locked loop tracks the voltage of the offshore AC bus in real time, obtains the phase and frequency information of the voltage, completes pre-synchronization tracking, obtains the initial synchronization phase reference of the system, and determines that the black start is complete.

[0037] By implementing this embodiment, uncontrolled rectifier charging is achieved using the anti-parallel diodes of the wind turbine grid-side converter, eliminating the need for additional auxiliary charging equipment, simplifying the wind turbine black start process, and reducing equipment costs. Through phase-locked loop pre-synchronization tracking, it is ensured that all wind turbines obtain a unified phase reference during the black start phase, laying the foundation for synchronous voltage build-up after the dead zone ends and avoiding phase conflicts among multiple turbines.

[0038] S3. After the black start is completed, control the multi-level converter to switch to zero DC current mode until the DC main circuit current is monitored to meet the preset arc-free interruption condition, then control the low-voltage multi-mode converter to lock out and the high-speed mechanical switch to open. Specifically, the zero DC current mode represents a mode in which the current in the DC main circuit is controlled to near zero, creating conditions for arc-free breaking of the high-speed mechanical switch. The preset arc-free breaking condition means that the DC current in the branch where the high-speed mechanical switch is located is close to zero. Under these conditions, the switch will not generate an arc when it breaks, thus preventing switch burn-out and improving equipment lifespan and operational safety.

[0039] Indicatively, after the black start is completed, the host computer controls the onshore multilevel converter to switch to zero DC current mode, and controls the DC current closed loop to near zero value; when the DC current is detected to meet the arc-free interruption condition, the host computer first issues a command to block the drive pulse of the low-voltage multimode converter, cuts off its AC and DC energy path, and then issues a trip command to disconnect the high-speed mechanical switch, so as to achieve arc-free interruption and avoid damage to the switching equipment.

[0040] In a preferred embodiment of the present invention, combined with Figure 6 As shown, after the high-speed mechanical switch 7 is safely disconnected, the offshore AC bus enters a transient pressure-free zone lasting between 50ms and 200ms. Figure 6 Medium-power grid-connected wind turbines include permanent magnet synchronous generators (PMSG), machine-side converters (MSC), and DC capacitors. C dc The DC link and grid-side converter (GSC) of the grid-connected wind turbine. During this dead zone, the grid-connected wind turbine makes full use of the DC capacitor ( C dc The pre-stored energy provides transient power to the GSC grid controller, maintaining the continuous operation of the control system and locking the pre-synchronous phase memory using the phase memory module. This ensures that under no-pressure transient conditions, the internal controllers of each grid-type wind turbine maintain a constant angular frequency, ensuring that when they are unlocked simultaneously after the dead zone ends, the initial phase of the output voltage of each wind turbine is highly consistent and smoothly connected to the original AC bus phase, avoiding transient impacts from multiple voltage sources in parallel.

[0041] S4. During the transient pressure-free dead zone after the high-speed mechanical switch is disconnected, control each grid-type fan to lock the synchronous phase reference until the transient pressure-free dead zone ends. Specifically, the transient zero-voltage dead zone is the transient time interval during which the offshore AC bus voltage remains zero after the high-speed mechanical switch is disconnected and before the grid-type wind turbine takes over the dominant position of the grid. The duration of this interval is 50ms-200ms.

[0042] Indicatively, after the high-speed mechanical switch is disconnected, the offshore AC bus enters a transient no-voltage dead zone. The host computer controls each grid-type wind turbine to use the energy stored in its own DC capacitor to maintain the controller operation. At the same time, it locks the synchronous phase reference tracked during the black start phase, maintains a constant angular frequency, and ensures that the output voltage phase of multiple wind turbines is consistent after the dead zone ends.

[0043] Preferably, the grid-type wind turbine further includes a permanent magnet synchronous generator, a turbine-side converter, and a grid-side converter controller; the turbine-side converter is connected to the output terminal of the permanent magnet synchronous generator, and the output terminal of the grid-side converter is connected to the offshore AC bus; the DC link is connected between the turbine-side converter and the grid-side converter.

[0044] Specifically, the permanent magnet synchronous generator (PMSG) is the power generation unit of an offshore wind turbine. It uses permanent magnet excitation, eliminating the need for excitation windings, and boasts advantages such as high efficiency, high reliability, and low maintenance costs, making it suitable for offshore wind power applications. The generator-side converter is a fully controlled converter connecting the PMSG to the DC link, used to control the generator's speed and torque to achieve maximum power point tracking. The grid-side converter controller is a DSP or FPGA controller used to execute core algorithms such as grid-connected control, phase-locked loop tracking, and phase memory.

[0045] Schematic illustration: The output terminal of the permanent magnet synchronous generator is connected to the AC side of the turbine-side converter. The DC side of the turbine-side converter is connected to the positive and negative terminals of the DC link. The positive and negative terminals of the DC link are simultaneously connected to the DC side of the grid-side converter. The AC side of the grid-side converter is connected to the offshore AC bus via the wind turbine outlet transformer. The turbine-side converter and the grid-side converter are decoupled from each other, allowing independent control of generator operation and grid-side voltage output, thus improving the flexibility and reliability of wind turbine operation.

[0046] Preferably, during the transient pressure-free dead zone after the high-speed mechanical switch is disconnected, each grid-type fan is controlled to lock the synchronous phase reference until the transient pressure-free dead zone ends, including: During the transient period without pressure dead zone after the high-speed mechanical switch is disconnected, the grid-type wind turbines control each grid-type wind turbine to supply power to the grid-side converter controller through the energy pre-stored in the DC capacitor in the DC link, so as to maintain the continuous operation of the offshore wind power DR transmission system. Control each grid-type wind turbine to lock the synchronous phase reference until the transient pressure-free dead zone ends.

[0047] Specifically, synchronous phase reference locking means that the grid-side converter controller locks the AC voltage phase obtained during the pre-synchronization phase in the black start phase through the built-in phase memory module, maintains a constant angular frequency during the dead zone, and ensures that the phase of the output voltage is consistent with the original system after the dead zone ends.

[0048] Schematic diagram: After the high-speed mechanical switch is disconnected, the offshore AC bus enters a transient no-pressure dead zone. The host computer controls the DC capacitors of the DC links of each grid-type wind turbine to discharge, providing continuous power to the grid-side converter controller and ensuring that the controller does not crash and the control logic continues to operate during the dead zone. At the same time, the host computer controls the grid-side converter controller to lock the synchronization phase reference obtained during the black start phase through the phase memory module. During the dead zone, it maintains a constant angular frequency at the rated frequency of 50Hz and continuously updates the phase angle until the transient no-pressure dead zone ends.

[0049] By implementing this embodiment, the energy stored in the DC capacitor of the wind turbine itself is used to power the controller, eliminating the need for an additional backup power supply, simplifying the system design and reducing costs. Through synchronous phase reference locking and constant angular frequency deduction, it is ensured that the initial phase of the output voltage of all wind turbines is highly consistent after the dead zone ends, and that the phase is smoothly connected with the original AC bus, avoiding phase conflicts and transient circulating current impacts from multiple voltage sources in parallel, and achieving a smooth and seamless handover of grid dominance.

[0050] S5. After the transient dead zone ends, control the grid-type fan to switch to grid control mode, control the multilevel converter to raise the DC main circuit voltage to the preset voltage value, and control the grid-type fan to adjust the AC side output voltage so that the voltage difference between the output voltage of the diode rectifier and the current DC bus voltage meets the preset voltage difference threshold. Specifically, the preset voltage value is the rated high voltage value of the DC main circuit, which is the rated DC voltage during steady-state operation of the system. The preset differential voltage threshold is the maximum allowable difference between the ideal output DC voltage of the DR and the current DC bus voltage, which is usually set to ≤2% of the rated DC voltage to ensure that the inrush current is within the allowable range when the AC circuit breaker is closed.

[0051] Schematic illustration: After the transient pressure-free dead zone ends, the host computer controls all grid-type wind turbines to unlock simultaneously, switch to grid control mode, re-establish the rated voltage and frequency of the offshore AC bus, and take over the grid control of the offshore AC system; at the same time, the host computer controls the onshore multilevel converter to switch to rated voltage control mode, quickly raising the DC main circuit voltage to the system's rated high voltage value; when the DC voltage is close to the rated value, the host computer controls the grid-type wind turbines to fine-tune the AC side output voltage amplitude, so that the difference between the ideal output DC voltage of the DR and the current DC bus voltage is less than the preset differential voltage threshold, in preparation for the DR to be put into operation.

[0052] Preferably, after the transient pressure-free dead zone ends, the method further includes: controlling the low-voltage multimode converter to complete synchronization with the offshore AC bus through a phase-locked loop, and then switching to a reactive power standby state.

[0053] Specifically, the reactive power standby state is the standby operation state of the low-voltage multi-mode converter. At this time, the converter has completed phase synchronization with the offshore AC bus, and the DC side capacitor voltage has stabilized to the rated value. It can switch to reactive power compensation mode at any time to quickly respond to the reactive power demand of the system.

[0054] Schematic illustration: After the transient dead zone ends, the grid-type wind turbines have re-established the offshore AC bus voltage. The host computer controls the phase-locked loop of the low-voltage multimode converter to start, tracking the voltage phase and frequency of the offshore AC bus in real time to achieve precise synchronization with the AC system. After synchronization, the low-voltage multimode converter is unlocked, establishing a stable DC-side capacitor voltage and switching to reactive power reserve mode, awaiting subsequent reactive power compensation mode switching commands. In the reactive power reserve mode, the low-voltage multimode converter adopts a constant DC voltage control mode to maintain a stable DC-side capacitor voltage, with the reactive power output reference value set to 0, maintaining only the minimum active power required for its own operation.

[0055] In a preferred embodiment of the present invention, when the DC bus voltage approaches its rated value, the system enters the grid connection preparation stage, thereby ensuring the equivalent DC voltage at the ideal output terminal of the DR is... U DR_dc With the current DC bus voltage U dc pressure difference Δ U It meets the preset minimum value range. The differential pressure is calculated using the following formula: In the formula, U ac This indicates the effective value of the AC line voltage of the marine diode rectifier (DR). K tr This refers to the turns ratio of the converter transformer. This is the minimum permissible differential pressure threshold.

[0056] In a preferred embodiment of the present invention, offshore wind power based on onshore FH-MMC step-down coordination is transmitted via DR by controlling the multi-mode switching of the DC control loop. The core of this method lies in the multi-mode switching strategy in the DC control loop. By setting a multi-channel selection switch, the system adapts to the needs of different operational phases. During the black start phase, the DC control external circuit selects position I, i.e., control mode I: the DC voltage reference value is set to Vdcref_blackup (low voltage rated value), matching the rated DC voltage of the offshore low voltage multimode converter, realizing deep voltage reduction of the main circuit, and providing start-up power for the offshore.

[0057] In the zero DC current stage, the DC control loop selects position II, i.e., control mode II: the DC current reference value is set to 0, creating conditions for the arc-free disconnection of the high-speed mechanical switch HSS.

[0058] During normal operation, the DC control loop selects position III, i.e., control mode III: the DC voltage reference value is switched to Udcref_N (the rated DC voltage of the high-voltage DC main circuit) to maintain the main circuit at full voltage.

[0059] In a preferred embodiment of the present invention, the AC control loop employs standard dq-axis current decoupling control, and a phase-locked loop (PLL) locks the onshore power grid phase. θ 11. The bridge arm circulating current suppression control loop extracts the second harmonic negative sequence component from the bridge arm current and outputs a circulating current suppression voltage command via a PI controller to reduce internal converter losses and device current stress. Finally, the fundamental frequency modulation wave output from the AC control loop, the DC voltage modulation wave output from the DC control loop, and the circulating current suppression command are linearly superimposed and synthesized, then sent to the valve controller. The valve controller converts the above command into trigger pulses and outputs them to... Figure 2 The power electronic switching devices (IGBTs) of each full-bridge submodule and half-bridge submodule inside the FH-MMC are used to control their actual operation.

[0060] Indicatively, in Figure 3 middle, v PCC The measured value of the three-phase AC voltage of the onshore AC grid-connected bus 17; PLL is a phase-locked loop used to extract the grid synchronization phase; θ 1 represents the fundamental synchronous phase angle of the onshore AC power grid extracted via a phase-locked loop; V dcn / 2 is half of the rated DC voltage of the land-based FH-MMC, used to restore the per-unit modulation command to the actual voltage command value; T dq-3s ( θ 1) Based on fundamental phase θ 1. Coordinate transformation from the dq rotating coordinate system to the abc stationary coordinate system.

[0061] In the AC control loop, U cavgref , U cavgpu These are the reference value and the measured per-unit value of the average capacitor voltage of the submodules in the FH-MMC, respectively. Q ref , Q pu These are the reactive power command reference value and the measured per-unit value, respectively. I sdref , I sqref These are the reference commands for the d-axis and q-axis current components of the alternating current, respectively. I sdpu ,I sqpu These are the measured per-unit values ​​of the d-axis and q-axis components of the alternating current, respectively. ω 0 represents the fundamental angular frequency of the AC power grid; L eqpu This is the per-unit equivalent reactance of FH-MMC, used for feedforward decoupling calculations of the dq axis; U sdpu , U sqpu分别为 Measured per-unit values ​​of the d-axis and q-axis components of AC grid voltage; M d , M q These are the d-axis and q-axis fundamental voltage modulation commands generated after current inner loop regulation and decoupling calculation. M a , M b , M c This is the command for the three-phase fundamental voltage modulation wave generated after coordinate inverse transformation.

[0062] In the DC control circuit, V dcref_blackup This is the reference value for DC voltage during the black start phase in control mode I (matching low voltage rating). U dcref_N This is the reference value for DC voltage during normal operation in control mode III (matching high voltage rated value). U dcpu This is the per-unit value of the measured DC bus voltage; I dcref This is the DC current reference command output by the outer loop of the DC control (in control mode II, this reference command is directly set to 0). I dcpu This is the per-unit value of the measured DC line current; V dcpu This is the measured per-unit value of DC voltage; M dc This is a DC voltage modulation command generated after DC inner loop regulation.

[0063] In the bridge arm circulating current suppression control loop, i pj , i nj For the converter j Mutually( j Measured values ​​of the upper and lower bridge arm currents (a, b, c); i diffj The differential mode current of the three-phase bridge arms (i.e., the internal circulating current component) is calculated from the currents of the upper and lower bridge arms. j =a,b,c);T 3s-dq (-2 θ 1) Based on negative sequence second harmonic phase -2 θ 1. Transformation from the stationary coordinate system abc to the rotating coordinate system dq; i 2fdpu , i 2fqpu The per-unit values ​​of the measured d-axis and q-axis components of the extracted negative-sequence second harmonic circulating current; i 2fdref , i 2fqref The d-axis and q-axis control reference commands for the negative sequence second harmonic circulating current (to suppress circulating current, both are set to 0); 2 ω 0 represents twice the fundamental angular frequency; L armpu The per-unit value of the equivalent inductance of the bridge arm reactor in the onshore FH-MMC is used for feedforward decoupling to suppress circulating current. M comd , M comq This refers to the voltage modulation commands for suppressing circulating current on the d-axis and q-axis, generated in the dq coordinate system. T dq-3s (-2 θ 1) is the inverse transformation from the negative-order second harmonic dq coordinate system to the abc stationary coordinate system; M coma , M comb , M comc This is the generated three-phase circulating current suppression voltage modulation command (compensation command).

[0064] S6. After the differential pressure meets the preset differential pressure threshold, control the AC circuit breaker to close so that the diode rectifier can be turned on, and at the same time control the low-voltage multi-mode converter to switch to reactive power compensation mode.

[0065] Specifically, the reactive power compensation mode is the steady-state operation mode of the low-voltage multi-mode converter, which can dynamically output or absorb reactive power to provide voltage support for the offshore AC system and compensate for the reactive power consumed by the DR operation.

[0066] Schematic, after the differential pressure meets the preset differential pressure threshold, the host computer issues a closing command to close the AC circuit breaker on the AC side of the DR. At this time, the voltage across the circuit breaker has achieved quasi-synchronous matching, and the closing inrush current is suppressed to an extremely low level. Subsequently, the host computer controls the grid-type wind turbine to slightly raise the AC voltage reference value, so that the diode of the DR is forward biased, and the DR naturally conducts, undertaking the task of transmitting active power from the offshore wind power. At the same time, the host computer controls the low-voltage multi-mode converter to switch to reactive power compensation mode, providing dynamic reactive power support for the offshore AC system.

[0067] Preferably, the low-voltage multimode converter includes: a DC-side voltage control loop, an AC-side voltage control loop, and a harmonic compensation control loop; Controlling the low-voltage multimode converter to switch to reactive power compensation mode includes: The low-voltage multimode converter absorbs active power by controlling the DC-side voltage control loop; The AC side voltage control loop of the low-voltage multimode converter adjusts the reactive power according to the offshore AC bus voltage and switches to reactive power compensation mode. The harmonic compensation control loop of the low-voltage multimode converter extracts the characteristic harmonics generated by the diode rectifier.

[0068] Specifically, the DC-side voltage control loop is used to stabilize the DC-side capacitor voltage of the converter, compensating for the converter's own losses by absorbing a small amount of active power, thus ensuring the normal operation of the converter. The AC-side voltage control loop is used to dynamically adjust the reactive power output of the converter based on the deviation of the offshore AC bus voltage, maintaining the stability of the offshore AC bus voltage and achieving dynamic reactive power compensation. The harmonic compensation control loop is used to extract the characteristic harmonic current generated by DR rectification, generate an inverse harmonic compensation voltage command, cancel the harmonic components, and achieve active filtering function. Characteristic harmonics are specific harmonics generated during DR rectification. For a 12-pulse DR, the characteristic harmonics are mainly the 11th and 13th harmonics, with the 11th and 13th harmonics having the highest content and being the main targets for control.

[0069] Schematic diagram: When switching to reactive power compensation mode, the DC-side voltage control loop monitors the DC-side capacitor voltage of the converter in real time, compares it with the rated value, and generates an active current command through the PI regulator. This commands the converter to absorb a small amount of active power to compensate for its own switching and conduction losses, thus maintaining the stability of the DC-side capacitor voltage. The AC-side voltage control loop monitors the offshore AC bus voltage in real time, compares it with the rated value, and generates a reactive current command through the PI regulator. This commands the converter to dynamically output or absorb reactive power to compensate for the reactive power consumed by the DR operation, maintain the stability of the offshore AC bus voltage, and switch to reactive power compensation mode. The harmonic compensation control loop monitors the current on the DR grid side in real time, extracts the characteristic harmonic components, generates the corresponding harmonic compensation voltage command, and outputs it after superimposing it with the fundamental voltage command to cancel the characteristic harmonics generated by the DR and improve power quality. Dynamic reactive power compensation can compensate for the reactive power consumed by DR commutation in real time, maintain the stability of the offshore AC bus voltage, avoid voltage instability, and improve the steady-state and transient stability of the system. Through characteristic harmonic mitigation, harmonics generated by DR rectification can be effectively suppressed, significantly improving the power quality of the offshore AC system and meeting the requirements of wind power grid connection standards.

[0070] In a preferred embodiment of the present invention, such as Figure 4As shown, during the black start phase, the low-voltage multimode converter operates as an AC voltage source, therefore its AC control loop adopts a V / F grid-type control. Unlike conventional grid-connected control, this mode directly generates the reference phase by setting a fixed 50Hz frequency through a voltage-controlled oscillator (VCO). θ 2. The reference value for the outer loop of AC voltage is set as follows: u sdref = U s_ref , u sqref =0, after the PI regulator generates an inner-loop current command, the offshore low-voltage multimode converter outputs a stable three-phase AC voltage, providing starting power for grid-connected wind turbines. Simultaneously, the DC control loop maintains the stability of the sub-module capacitor voltages of each arm. The arm circulating current suppression control loop extracts the second harmonic negative sequence component from the arm current and outputs a circulating current suppression voltage command via PI regulation to reduce internal losses and device current stress in the low-voltage multimode converter. Finally, the fundamental modulation wave output from the AC control loop, the DC voltage modulation wave output from the DC control loop, and the circulating current suppression command are linearly superimposed and synthesized, then sent to the valve controller to generate the final trigger pulses for the power electronic switching devices in the low-voltage multimode converter. (Illustratively,) Figure 4 The appearance of in Figure 3 Same control variable symbols (e.g., PI, ...) M d (etc.) all specifically refer to the control commands and parameters inside the low-voltage multimode converter.

[0071] In a preferred embodiment of the present invention, such as Figure 5 As shown, when the high-speed mechanical switch HSS is disconnected, the offshore grid-type wind farm takes over the voltage and frequency control of the offshore AC system. After entering steady-state operation, the low-voltage multimode converter is in a comprehensive reactive power and characteristic harmonic management mode. Since the diode rectifier (DR) consumes reactive power and generates a large number of harmonics during rectification, its harmonic components mainly include, but are not limited to, the 11th and 13th characteristic harmonic currents. The control logic in this mode consists of an AC control loop (fundamental compensation), a harmonic compensation loop (active filtering), and a bridge arm circulating current suppression control loop. The specific implementation is as follows: The AC control loop is responsible for maintaining the steady-state and transient stability of the offshore AC bus voltage. The reactive power outer loop is based on the AC voltage at the offshore AC bus. U ac Compared with reference value U ac_ref The deviation is processed by the PI regulator to output the fundamental reactive current command. I sqref Dynamically output or absorb reactive power Q STATCOMCombined with the charging reactive power of the AC submarine cable itself. Q cable The reactive power consumed by DR commutation Q DR It satisfies the local reactive power balance equation: In the formula, Δ Q To maintain the dynamic regulation reactive power margin required for the rated voltage of the offshore AC bus. Simultaneously, the DC capacitor voltage outer loop outputs an active current command. I sdref To maintain energy dissipation within the converter, the above commands are used to generate fundamental voltage modulation commands via the dq-axis decoupled inner loop control. M d and M q .

[0072] The harmonic compensation circuit targets specific high-order harmonics (especially the 11th and 13th characteristic harmonics) generated by the operation of the diode rectifier (DR). This embodiment extracts the current from the three-phase power grid (or the DR side). i T This is achieved by using an abc-dq rotating coordinate system transformation with a specific sampling frequency. For the h-th harmonic to be filtered out, its coordinate transformation matrix is... T abc-dq As shown in the following formula: In the formula, for h The angular frequency of the second harmonic. f The fundamental frequency, t Represents a time variable.

[0073] After the above rotational transformation, a low-pass filter (LPF) is used for filtering to extract specific harmonics. h The DC component in the next rotating coordinate system is then obtained through an inverse dq-abc transformation, yielding the three phases requiring compensation. h Subharmonic current command i Th Regarding the output current of offshore low-voltage multimode converters... i statcom Three phases were obtained using the same method. h Subharmonic current i statcomh .

[0074] In this embodiment, the harmonic compensation circuit uses multiple parallel quasi-proportional resonant controllers to achieve zero-steady-state-error tracking control of multi-frequency harmonic currents. Its transfer function... G ( s As shown in the following formula: In the formula, K p This is the proportionality coefficient; K i The integral coefficient; ω 0 represents the fundamental angular frequency; s For the Laplace transform operator; ω c The cutoff frequency (bandwidth) is used to increase the system's robustness to frequency fluctuations. hω 0 Angular frequency of a specific harmonic.

[0075] In a preferred embodiment of the present invention, a parallel quasi-proportional resonant controller group is provided for the 11th and 13th characteristic harmonic currents. It should be understood that the number of the quasi-proportional resonant controller groups and the harmonic orders they target can be flexibly increased, decreased, or adjusted according to the actual power quality requirements of the offshore power grid, in order to achieve zero-steady-state-error tracking control of harmonic currents in different frequency bands. The actual output harmonic current of the offshore low-voltage multimode converter 4 is measured. i statcomh With the generated harmonic command current i Th The subtraction results in an error signal that is fed into the parallel quasi-proportional resonant controller group to generate a corresponding harmonic compensation voltage command. This compensation command is superimposed on the fundamental modulation wave generated by the AC control circuit. By selectively filtering out the 11th and 13th characteristic harmonics, the characteristic harmonics cancel each other out on the high-voltage side of the transformer, preventing them from flowing into the offshore AC system.

[0076] In a preferred embodiment of the present invention, the system is in a fully stopped initial state, with both DR-ACCB and HSS in the open state; the host computer issues a closing command to close the HSS, connecting the DC side of the low-voltage multimode converter to the ±320kV DC main circuit; the host computer controls the onshore FH-MMC to unlock and switch to step-down rectification mode, stabilizing the DC main circuit voltage to ±100kV, matching the rated voltage of the low-voltage multimode converter; the host computer controls the low-voltage multimode converter to unlock and switch to V / F grid construction control mode, establishing a 35kV grid on the 35kV offshore AC bus. A stable voltage of kV / 50Hz converts the onshore grid-fed power into starting power for the wind farm, providing the black start power. Grid-connected wind turbines charge the DC link capacitor via anti-parallel diodes in the grid-side converter. Once the DC voltage reaches the preset 1100V power-on threshold, the turbine's DSP control board and auxiliary equipment are powered on and activated. The turbine's phase-locked loop tracks the AC bus voltage to obtain a synchronization phase reference, completing the black start. The host computer controls the onshore FH-MMC to switch to zero DC current mode, controlling the DC current to ≤20A to meet the arc-free interruption condition. The first closed... Lock the low-voltage multimode converter and then disconnect the HSS to achieve arc-free interruption. After the HSS is disconnected, the offshore AC bus enters a 100ms transient no-voltage dead zone. The host computer controls each grid-connected wind turbine to use DC capacitor energy storage to power the grid-side controller, lock the pre-synchronization phase reference, and maintain a constant 50Hz angular frequency for simulation. After the 100ms dead zone ends, the host computer controls all grid-connected wind turbines to unlock simultaneously, switch to grid-connected control mode, re-establish the 35kV / 50Hz AC bus voltage, and take over the grid-connected control. At the same time, the onshore FH-MMC is controlled to increase the DC voltage to... ±320kV rated value; when the DC voltage approaches ±320kV, the host computer controls the grid-type wind turbine to fine-tune the AC voltage so that the voltage difference between the ideal output DC voltage of DR and the DC bus voltage is ≤6.4kV (2% of the rated value), meeting the preset voltage difference threshold; the host computer issues a closing command to close DR-ACCB, and then controls the wind turbine to slightly raise the AC voltage by 0.5kV, so that the DR diode is forward biased and the DR naturally conducts, undertaking the active power transmission of offshore wind power; at the same time, the low-voltage multi-mode converter is controlled to switch to reactive power compensation mode to provide dynamic reactive power support for the system.

[0077] By implementing this embodiment, the traditional physical voltage reduction scheme of onshore starting transformers is abandoned, and the active electrical voltage reduction capability of onshore multilevel converters is directly utilized, reducing equipment investment and footprint, and lowering system complexity. The offshore low-voltage multimode converter operates at a low voltage level, and the number of sub-modules can be reduced, significantly reducing the size, weight, and cost of the offshore platform. Through the transient pressure-free dead zone phase maintenance strategy of grid-connected wind turbines, a smooth and seamless handover of grid control from the low-voltage multimode converter to the wind farm is achieved, avoiding phase conflicts and transient impacts in multi-unit parallel operation and improving system transient stability. Through the multi-mode time-division multiplexing of the low-voltage multimode converter, the same set of equipment can simultaneously realize black-start power supply and dynamic reactive power compensation functions, improving equipment utilization and reducing system costs.

[0078] This embodiment applies to the host computer of an offshore wind power DR transmission system. The offshore wind power DR transmission system also includes an offshore module and an onshore module. The offshore module includes an offshore wind farm, a low-voltage multimode converter, a diode rectifier, an AC circuit breaker, and a high-speed mechanical switch. The offshore wind farm includes an offshore AC bus and several grid-type wind turbines. The AC side of the diode rectifier is connected to the offshore AC bus via an AC circuit breaker, and the DC side is connected to the DC main circuit. The AC side of the low-voltage multimode converter is connected to the offshore AC bus, and the DC side is connected to the DC main circuit via a high-speed mechanical switch. The onshore module includes a multilevel converter. The DC side of the multilevel converter is connected to the DC main circuit. The multilevel converter is controlled to switch to buck rectification mode. The system actively reduces the DC main circuit voltage to a rated value matching the low-voltage multimode converter, and then connects the low-voltage multimode converter to the DC main circuit with the closing of a high-speed mechanical switch. This bypasses the physical limitation of single-phase conduction of the diode rectifier and establishes a reverse power transmission path between the onshore module, the DC main circuit, and the low-voltage multimode converter, solving the problems of the diode rectifier being unable to supply power in reverse and the system being unable to obtain startup power from the onshore side. Once the system enters steady-state operation after the diode rectifier is turned on, the low-voltage multimode converter is switched to reactive power compensation mode. The low-voltage multimode converter then replaces the diode rectifier in undertaking the functions of voltage regulation and reactive power support for the offshore power grid, thereby avoiding voltage instability and system collapse caused by reactive power deficiency on the offshore side and improving system operational stability.

[0079] See Figure 7This is a schematic diagram of the start-up and reactive power control device of an offshore wind power DR transmission system according to an embodiment of the present invention, including: a host computer, an offshore module, and an onshore module; the offshore module includes an offshore wind farm, a low-voltage multimode converter, a diode rectifier, an AC circuit breaker, and a high-speed mechanical switch; the offshore wind farm includes several grid-type wind turbines; the DC side of the diode rectifier is connected to the DC main circuit; the DC side of the low-voltage multimode converter is connected to the DC main circuit through the high-speed mechanical switch; the onshore module includes a multilevel converter; the DC side of the multilevel converter is connected to the DC main circuit. The host computer includes: The buck rectification mode switching module is used to control the high-speed mechanical switch to close when all switches in the system are in the open state, and to control the multilevel converter to switch to buck rectification mode to match the rated DC voltage of the low-voltage multimode converter. The black start module is used to control the low-voltage multimode converter to switch to grid-connected control mode so that the grid-connected wind turbine can complete the black start. The zero DC current mode switching module is used to control the multilevel converter to switch to zero DC current mode after black start is completed, until the DC main circuit current is monitored to meet the preset arc-free interruption condition, and then control the low-voltage multimode converter to lock out and the high-speed mechanical switch to open. The transient pressure-free dead zone continuation module is used to control each grid-type wind turbine to lock the synchronous phase reference during the transient pressure-free dead zone after the high-speed mechanical switch is opened, until the transient pressure-free dead zone ends. The grid control mode switching module is used to control the grid-type wind turbine to switch to grid control mode after the transient dead zone ends, control the multilevel converter to raise the DC main circuit voltage to the preset voltage value, and control the grid-type wind turbine to adjust the AC side output voltage so that the voltage difference between the output voltage of the diode rectifier and the current DC bus voltage meets the preset voltage difference threshold. The reactive power compensation module is used to control the AC circuit breaker to close after the differential pressure meets the preset differential pressure threshold, so as to turn on the diode rectifier and at the same time control the low-voltage multi-mode converter to switch to reactive power compensation mode.

[0080] This invention provides a start-up and reactive power control device for an offshore wind power DR transmission system, including a host computer, an offshore module, and an onshore module. The offshore module includes an offshore wind farm, a low-voltage multimode converter, a diode rectifier, an AC circuit breaker, and a high-speed mechanical switch. The offshore wind farm includes several grid-connected wind turbines. The DC side of the diode rectifier is connected to the DC main circuit. The DC side of the low-voltage multimode converter is connected to the DC main circuit via a high-speed mechanical switch. The onshore module includes a multilevel converter. The DC side of the multilevel converter is connected to the DC main circuit. By controlling the multilevel converter to switch to buck rectification mode, the DC main circuit voltage is actively reduced to match that of the low-voltage multimode converter. The rated values, combined with the high-speed mechanical switch closure, connect the low-voltage multimode converter to the DC main circuit, bypassing the physical limitation of single-phase conduction of the diode rectifier. This constructs a reverse power transmission path between the onshore module, the DC main circuit, and the low-voltage multimode converter, solving the problems of the diode rectifier being unable to supply power in reverse and the system being unable to obtain startup power from the onshore side. Once the system enters steady-state operation after the diode rectifier is turned on, the low-voltage multimode converter is switched to reactive power compensation mode. The low-voltage multimode converter replaces the diode rectifier in undertaking the functions of voltage regulation and reactive power support for the offshore power grid, thereby avoiding voltage instability and system collapse caused by reactive power deficiency on the offshore side and improving system operational stability.

[0081] The above description represents the preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications are also considered to be within the scope of protection of the present invention.

Claims

1. A method for starting and reactive power control of an offshore wind power DR transmission system, characterized in that, This is a host computer for an offshore wind power DR transmission system. The offshore wind power DR transmission system also includes an offshore module and an onshore module. The offshore module includes an offshore wind farm, a low-voltage multimode converter, a diode rectifier, an AC circuit breaker, and a high-speed mechanical switch. The offshore wind farm includes several grid-type wind turbines. The DC side of the diode rectifier is connected to the DC main circuit. The DC side of the low-voltage multimode converter is connected to the DC main circuit through a high-speed mechanical switch. The onshore module includes a multilevel converter. The DC side of the multilevel converter is connected to the DC main circuit. The method includes: When all system switches are in the open state, the high-speed mechanical switch is closed to control the multilevel converter to switch to buck rectification mode to match the rated DC voltage of the low-voltage multimode converter; Control the low-voltage multi-mode converter to switch to grid-connected control mode so that the grid-connected wind turbine can complete black start; After the black start is completed, the control multilevel converter switches to zero DC current mode until the DC main circuit current is monitored to meet the preset arc-free interruption condition, then the control low-voltage multimode converter is locked and the high-speed mechanical switch is opened. During the transient pressure-free dead zone after the high-speed mechanical switch is disconnected, control each grid-type fan to lock the synchronous phase reference until the transient pressure-free dead zone ends; After the transient dead zone ends, the grid-type wind turbine is switched to grid control mode. The multilevel converter is controlled to raise the DC main circuit voltage to the preset voltage value, and the grid-type wind turbine is controlled to adjust the AC side output voltage so that the voltage difference between the output voltage of the diode rectifier and the current DC bus voltage meets the preset voltage difference threshold. Once the differential pressure meets the preset differential pressure threshold, the AC circuit breaker is closed to enable the diode rectifier to conduct, and the low-voltage multimode converter is switched to reactive power compensation mode.

2. A method of starting and reactive power control of an offshore wind power DR transmission system according to claim 1, characterized in that, The offshore module also includes: an offshore AC bus, a three-phase three-winding transformer, and a three-phase two-winding transformer; the AC side of the diode rectifier is connected to the offshore AC bus in sequence through the three-phase three-winding transformer and the AC circuit breaker; the AC side of the low-voltage multi-mode converter is connected to the offshore AC bus through the three-phase two-winding transformer.

3. The method for starting up and reactive power control of an offshore wind power DR transmission system as described in claim 1, characterized in that, When all system switches are in the open state, control the high-speed mechanical switch to close, including: When the system is in a state of complete shutdown, all switching devices are controlled to be in the open state; wherein, the AC circuit breaker on the AC side of the diode rectifier is in the open state; A preset closing command is issued to control the high-speed mechanical switch to close, so that the low-voltage multimode converter can be connected to the DC main circuit.

4. The method for starting up and reactive power control of an offshore wind power DR transmission system as described in claim 2, characterized in that, The grid-type wind turbine includes a grid-side converter, a phase-locked loop, and a DC link containing a DC capacitor; Controlling the low-pressure multimode converter to switch to grid-connected control mode to enable grid-connected wind turbines to complete black start includes: The low-voltage multimode converter is switched to a constant voltage and constant frequency grid control mode so that the low-voltage multimode converter establishes the initial voltage of the offshore AC bus and converts the power fed back from the onshore module into the starting power of the offshore module. Each grid-type wind turbine is controlled to charge the capacitor of the DC link through the grid-side converter based on the starting power until the DC link voltage reaches the preset power-on threshold, thus completing the power-on wake-up of the grid-type wind turbine. After the grid-type wind turbines are powered on and awakened, the phase-locked loops controlling each grid-type wind turbine track the voltage of the offshore AC bus and obtain the initial synchronization phase reference to complete the black start.

5. The method for starting up and reactive power control of an offshore wind power DR transmission system as described in claim 4, characterized in that, The grid-type wind turbine also includes a permanent magnet synchronous generator, a turbine-side converter, and a grid-side converter controller; the turbine-side converter is connected to the output terminal of the permanent magnet synchronous generator, and the output terminal of the grid-side converter is connected to the offshore AC bus; the DC link is connected between the turbine-side converter and the grid-side converter.

6. The method for starting up and reactive power control of an offshore wind power DR transmission system as described in claim 5, characterized in that, During the transient pressure-free dead zone after the high-speed mechanical switch is disconnected, each grid-type wind turbine is controlled to lock the synchronous phase reference until the transient pressure-free dead zone ends, including: During the transient period without pressure dead zone after the high-speed mechanical switch is disconnected, the grid-type wind turbines control each grid-type wind turbine to supply power to the grid-side converter controller through the energy pre-stored in the DC capacitor in the DC link, so as to maintain the continuous operation of the offshore wind power DR transmission system. Control each grid-type wind turbine to lock the synchronous phase reference until the transient pressure-free dead zone ends.

7. The method for starting up and reactive power control of an offshore wind power DR transmission system as described in claim 2, characterized in that, After the transient pressure-free dead zone ends, the process also includes: controlling the low-voltage multimode converter to synchronize with the offshore AC bus via a phase-locked loop, and then switching to reactive power standby mode.

8. The method for starting up and reactive power control of an offshore wind power DR transmission system as described in claim 1, characterized in that, The onshore module's multilevel converter is an onshore hybrid modular multilevel converter; the onshore hybrid modular multilevel converter includes: an AC control loop, a DC control loop, and a bridge arm circulating current suppression control loop; The onshore hybrid modular multilevel converter is used to set the DC voltage reference value to a low-voltage rated value that matches the rated DC voltage of the low-voltage multimode converter through the DC control loop when switching to buck rectification mode; reduce the DC voltage to the low-voltage rated value through the DC main circuit; lock the onshore grid phase through the AC control loop; and extract and suppress the second harmonic negative sequence circulating current of the bridge arm through the bridge arm circulating current suppression control loop.

9. The method for starting up and reactive power control of an offshore wind power DR transmission system as described in claim 1, characterized in that, The low-voltage multimode converter includes: a DC-side voltage control loop, an AC-side voltage control loop, and a harmonic compensation control loop; Controlling the low-voltage multimode converter to switch to reactive power compensation mode includes: The low-voltage multimode converter absorbs active power by controlling the DC-side voltage control loop; The AC side voltage control loop of the low-voltage multimode converter adjusts the reactive power according to the offshore AC bus voltage and switches to reactive power compensation mode. The harmonic compensation control loop of the low-voltage multimode converter extracts the characteristic harmonics generated by the diode rectifier.

10. A start-up and reactive power control device for an offshore wind power DR transmission system, characterized in that, include: The system comprises a host computer, an offshore module, and an onshore module. The offshore module includes an offshore wind farm, a low-voltage multimode converter, a diode rectifier, an AC circuit breaker, and a high-speed mechanical switch. The offshore wind farm includes several grid-type wind turbines. The DC side of the diode rectifier is connected to the DC main circuit. The DC side of the low-voltage multimode converter is connected to the DC main circuit via a high-speed mechanical switch. The onshore module includes a multilevel converter. The DC side of the multilevel converter is connected to the DC main circuit. The host computer includes: The buck rectification mode switching module is used to control the high-speed mechanical switch to close when all switches in the system are in the open state, and to control the multilevel converter to switch to buck rectification mode to match the rated DC voltage of the low-voltage multimode converter. The black start module is used to control the low-voltage multimode converter to switch to grid-connected control mode so that the grid-connected wind turbine can complete the black start. The zero DC current mode switching module is used to control the multilevel converter to switch to zero DC current mode after black start is completed, until the DC main circuit current is monitored to meet the preset arc-free interruption condition, and then control the low-voltage multimode converter to lock out and the high-speed mechanical switch to open. The transient pressure-free dead zone continuation module is used to control each grid-type wind turbine to lock the synchronous phase reference during the transient pressure-free dead zone after the high-speed mechanical switch is opened, until the transient pressure-free dead zone ends. The grid control mode switching module is used to control the grid-type wind turbine to switch to grid control mode after the transient dead zone ends, control the multilevel converter to raise the DC main circuit voltage to the preset voltage value, and control the grid-type wind turbine to adjust the AC side output voltage so that the voltage difference between the output voltage of the diode rectifier and the current DC bus voltage meets the preset voltage difference threshold. The reactive power compensation module is used to control the AC circuit breaker to close after the differential pressure meets the preset differential pressure threshold, so as to turn on the diode rectifier and at the same time control the low-voltage multi-mode converter to switch to reactive power compensation mode.

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