A topology structure of offshore wind power grid-connected system through flexible direct-current transmission

Abstract

The present application relates to the technical field of power system planning, and discloses a topology structure of offshore wind power through flexible DC power transmission grid-connected system. In the topology structure, every two wind power generators are connected to the low-voltage side of the same multi-winding transformer, and there is no connection between the two wind power generators connected to the low-voltage side of the same multi-winding transformer; there is a connection between the two wind power generators belonging to different multi-winding transformers and located adjacently; the high-voltage side of the multi-winding transformer is connected with the sending-end converter station of the offshore wind farm; the sending-end converter station of the offshore wind farm adopts a pseudo-bipolar physical topology structure; the DC power transmission line comprises a positive DC line and a negative DC line; the receiving-end converter station of the offshore wind farm adopts a true bipolar physical topology structure; and the converter valves of the sending-end and receiving-end converter stations all adopt a modular multilevel converter structure of half-bridge sub-modules and full-bridge sub-modules in hybrid cascade. The present application balances the construction cost of the offshore converter platform and the stability of the power feeding of the onshore main network.

Description

Technical Field

[0001] This invention relates to the field of power system planning technology, specifically a topology for an offshore wind power grid connection system via flexible DC transmission. Background Technology

[0002] The construction and investment scale of offshore wind farms continues to expand, gradually moving from demonstration projects to full commercialization. As offshore wind power develops towards deeper waters and clusters, the overall topology design of offshore wind power AC / DC transmission systems has become a crucial factor affecting project economics and power supply reliability. Currently, the main challenges in the topology design of offshore wind power AC / DC transmission systems are concentrated in the following three aspects: First, on the offshore AC collection network side, it is difficult to balance economy and redundancy. Traditional AC collection networks mostly adopt single-chain or star-radial topologies. Once a single submarine cable or single-phase grounding fault occurs, it can easily cause a large-scale disconnection of wind turbines along the entire feeder. In order to improve isolation, some solutions tend to configure an independent dual-winding main transformer for each wind turbine. However, this will lead to a surge in the number of transformer devices on the offshore converter platform, greatly increasing the size, load-bearing capacity, and construction cost of the offshore platform.

[0003] Secondly, there is a trade-off between economy and reliability on the DC transmission network side. If both the sending and receiving ends adopt a true bipolar topology, although it can maintain 50% power transmission in the event of a unipolar failure, it is necessary to lay hundreds of kilometers of expensive metal return submarine cables in the deep sea, and the offshore platform needs to add independent neutral point equipment, resulting in high engineering costs. If both the sending and receiving ends adopt a pseudo-bipolar topology to save on submarine cable investment, due to the strong electrical coupling between the positive and negative poles, once a unipolar DC line failure occurs, the entire system must be locked down, resulting in a 100% instantaneous interruption of offshore wind power feed-in power. This large-capacity power surge will cause a safety impact on the onshore main grid with a high proportion of new energy sources.

[0004] Third, DC fault isolation at the converter station side is costly. Existing flexible DC converter stations mostly use conventional half-bridge modular multilevel converters, whose underlying hardware lacks the ability to self-block DC fault currents. When faced with transient faults such as DC submarine cable short circuits, they must rely heavily on expensive and bulky high-voltage DC circuit breakers to isolate the fault, further exacerbating the space constraints and investment burden on offshore platforms.

[0005] In summary, existing single-topology solutions often fall short when dealing with large-capacity transmission of offshore wind power. There is an urgent need for a new system topology with high integration at the connectivity level to fundamentally solve the challenges of reliability and economy in offshore wind power transmission systems. Summary of the Invention

[0006] To address the technical challenges of high investment in AC-side transformer equipment and difficulty in balancing cost reduction for offshore platforms with onshore grid support capabilities in current offshore wind power grid-connected systems via flexible DC transmission, this invention provides a topology for such a system.

[0007] To achieve the above objectives, the present invention provides the following technical solution: This invention discloses a topology of an offshore wind power grid-connected system via flexible DC transmission, comprising: an AC collection system for the wind farm, a sending-end converter station for the offshore wind farm, a DC transmission line, and a receiving-end converter station for the offshore wind farm. The wind farm AC collection system includes multiple multi-winding transformers and multiple wind turbine generators arranged in rows; every two wind turbine generators are connected to the low-voltage side of the same multi-winding transformer, wherein there is no connection between two wind turbine generators connected to the low-voltage side of the same multi-winding transformer; there is a connection between two wind turbine generators belonging to different multi-winding transformers but located adjacent to each other; the high-voltage side of the multi-winding transformer is connected to the sending-end converter station of the offshore wind farm; The offshore wind farm sending-end converter station adopts a pseudo-bipolar physical topology, which does not have a neutral point return structure internally, and the DC side is connected to the offshore wind farm receiving-end converter station through the DC transmission line; the DC transmission line includes a positive DC line and a negative DC line; The offshore wind farm receiving-end converter station adopts a true bipolar physical topology, including a positive converter, a negative converter, and a metal return line connecting the neutral points of the positive and negative converters. The converter valves of both the sending-end converter station and the receiving-end converter station of the offshore wind farm adopt a modular multilevel converter structure with a hybrid cascade of half-bridge sub-modules and full-bridge sub-modules.

[0008] As a further improvement to the above scheme, the multi-winding transformer is a three-winding transformer; an AC interconnection circuit breaker is provided between the AC feeder branches of two wind turbine units that belong to different multi-winding transformers and are adjacent in location; when a wind turbine unit fails, the power is transferred across units and the topology is reconfigured by opening and closing the corresponding AC interconnection circuit breaker to isolate the fault point.

[0009] As a further improvement to the above scheme, the electrical energy generated by the wind turbine is stepped up to the first voltage level by the turbine-end step-up transformer and then connected to the corresponding AC feeder branch; multiple AC feeder branches are combined and connected to the low-voltage side of the multi-winding transformer, and stepped up to the second voltage level by the multi-winding transformer and then connected in parallel to the offshore AC common bus; the offshore AC common bus is connected to the sending-end converter station of the offshore wind farm through an AC circuit breaker.

[0010] As a further improvement to the above scheme, in the offshore wind farm's sending-end converter station, the upper bridge arms of each phase unit are physically connected in series and then converge to the sending-end DC positive busbar and connected to the positive DC line, while the lower bridge arms of each phase unit are physically connected in series and then converge to the sending-end DC negative busbar and connected to the negative DC line.

[0011] As a further improvement to the above scheme, in the receiving-end converter station of the offshore wind farm, the common neutral point of the positive converter and the negative converter is connected to the metal return line, and the metal return line is grounded at the onshore receiving end.

[0012] As a further improvement to the above scheme, both the sending-end converter station and the receiving-end converter station of the offshore wind farm are equipped with a dual closed-loop control loop including an outer loop controller and an inner loop current controller. Under steady-state conditions, the offshore wind farm sending-end converter station adopts constant AC voltage and constant frequency control, and the offshore wind farm receiving-end converter station adopts constant DC voltage and constant reactive power control. The inner loop current controller integrates a positive and negative sequence current decoupling control module, which is configured to control the negative sequence current component to zero during AC-side asymmetrical faults to suppress DC-side frequency doubling voltage fluctuations.

[0013] As a further improvement to the above scheme, the topology is configured with AC / DC collaborative fault ride-through logic; when the DC transmission line experiences an inter-pole short circuit or a single-pole ground fault, the converter valve control unit shuts off the switching devices in the full-bridge submodule, and the full-bridge submodule uses its internal H-bridge physical topology to output a reverse capacitor voltage to form a reverse potential blocking, cutting off the short-circuit current fed in from the AC side.

[0014] As a further improvement to the above scheme, when a single-pole grounding fault occurs in the DC transmission line, the pole control unit of the offshore wind farm receiving-end converter station only locks the converter valve corresponding to the faulty pole, and keeps the converter valve corresponding to the non-faulty pole in an unlocked operating state; the current of the non-faulty pole forms an independent circuit through the receiving-end neutral point and the metal return line to continuously supply power. After the fault is cleared, the sending-end converter station and the receiving-end converter station of the offshore wind farm trigger adaptive restart logic to unlock the converter valve and establish DC voltage recovery steady-state power transmission.

[0015] As a further improvement to the above scheme, when a transient voltage drop occurs in the onshore AC main grid connected to the offshore wind farm receiving-end converter station, the control loop of the offshore wind farm receiving-end converter station adaptively switches to reactive voltage support mode and injects a fixed reactive current into the onshore AC main grid.

[0016] As a further improvement to the above scheme, in the modular multilevel converter structure in which half-bridge submodules and full-bridge submodules are cascaded together, each converter valve arm includes multiple half-bridge submodules, multiple full-bridge submodules and converter valve arm reactors connected in series; the AC side of the modular multilevel converter structure is also connected to an AC side tie reactor; and the full-bridge submodule is internally configured with fully controlled power electronic switching devices.

[0017] Compared with the prior art, the beneficial effects of the present invention are: 1. This invention reduces the number of transformers, load, and the need for laying metallic return cables on the offshore platform by connecting every two wind turbines to the same multi-winding transformer on the AC collection side and establishing connections between adjacent turbines on different transformers. On the DC side, it employs a heterogeneous topology of pseudo-bipolar at the offshore sending end and true bipolar at the onshore receiving end. Furthermore, it utilizes a modular multilevel converter with hybrid cascaded half-bridge and full-bridge configurations at the converter station. Simultaneously, this architecture avoids the complete power outage caused by a single-pole failure in a fully pseudo-bipolar system, balancing the construction cost of the offshore converter platform with the stability of power input from the onshore grid.

[0018] 2. This invention utilizes a topology reconfiguration mechanism by bridging AC interconnection circuit breakers between adjacent wind turbine branches with different multi-winding transformers. This mechanism enables the establishment of redundant transfer channels for non-faulty turbines when a single-phase ground fault occurs on a local feeder. This allows the power from non-faulty turbines to be continuously transmitted via adjacent healthy branches, avoiding the engineering problem of a single-point fault causing the entire feeder turbines to disconnect from the grid in conventional single-chain or star topologies.

[0019] 3. This invention utilizes the hardware topology superposition characteristics of the converter valve full-bridge submodule to shut down internal switching devices and form a reverse potential blocking wall by outputting a reverse capacitor voltage when an inter-pole short circuit or single-pole ground fault occurs in the DC line, thereby achieving forced interruption of the short-circuit current fed into the AC side. This mechanism eliminates the system's dependence on high-voltage DC circuit breakers, reducing the design margin and engineering cost of key AC / DC equipment in the entire station to cope with transient overcurrents.

[0020] 4. This invention employs a non-blocking crossover and adaptive restart strategy for unipolar faults under heterogeneous topology. In the event of a unipolar ground fault on a DC line, only the converter valve on the faulty pole at the receiving end is blocked. An independent loop is formed by the non-faulty pole, the receiving-end neutral point, and the metallic return line, enabling continuous power supply maintaining at least 50% of rated power during the fault. After the fault is cleared, the converter valve is unlocked to rebuild the voltage, smoothly restoring steady-state power transmission across the entire station and mitigating the impact of transient overcurrents on the junction temperature of the converter valve switching devices.

[0021] 5. This invention integrates a positive-negative sequence decoupling module and an adaptive reactive voltage support control at the land-based receiving end into the inner loop of the dual closed-loop control system, thereby suppressing negative sequence current and DC-side frequency-doubled voltage fluctuations during AC-side asymmetrical faults. This mechanism enables the system to suppress DC voltage overshoot during faults without the need for a centralized dissipation resistor. Attached Figure Description

[0022] Figure 1 This is a diagram of the AC system wiring topology for Comparative Scheme 1 provided in this embodiment of the invention.

[0023] Figure 2 This is a topology diagram of an AC system wiring implementation scheme according to an embodiment of the present invention.

[0024] Figure 3 This is a diagram of the AC system wiring topology for Comparative Scheme 2 provided in this embodiment of the invention.

[0025] Figure 4 This is a topology diagram of a hybrid pseudo-bipolar transmitter and a hybrid true bipolar receiver in an embodiment of the present invention.

[0026] Figure 5 This is a topology diagram of the hybrid MMC in an embodiment of the present invention.

[0027] Figure 6 This is a voltage waveform diagram of a single-phase ground fault occurring on the low-voltage side f1 in an embodiment of the present invention; the horizontal axis represents time t in seconds; the vertical axis represents voltage U in kV.

[0028] Figure 7 This is a current waveform diagram when a single-phase ground fault occurs on the low-voltage side f1 in an embodiment of the present invention; the vertical axis in the figure represents the current, in kA.

[0029] Figure 8 This is a power waveform diagram of a single-phase ground fault occurring on the low-voltage side f1 in an embodiment of the present invention; the vertical axis of the diagram represents active power (unit MW) and reactive power (unit Mvar), P_f1 is the active power when a single-phase ground fault occurs on the low-voltage side f1; Q_f1 is the reactive power when a single-phase ground fault occurs on the low-voltage side f1.

[0030] Figure 9 This is a waveform diagram of the effective voltage value when a single-phase ground fault occurs on the low-voltage side f1 in an embodiment of the present invention; Vrms_f1 in the figure is the effective voltage value when a single-phase ground fault occurs on the low-voltage side f1.

[0031] Figure 10 This is a waveform diagram of the effective current value when a single-phase ground fault occurs on the low-voltage side f1 in an embodiment of the present invention; in the figure, Irms_f1 is the effective current value when a single-phase ground fault occurs on the low-voltage side f1.

[0032] Figure 11 This is a voltage waveform diagram of f2 when a single-phase ground fault occurs on the low-voltage side f1 in an embodiment of the present invention; Vabc_f in the figure 2a Vabc_f is the A-phase voltage of f2 when a single-phase ground fault occurs on the low-voltage side f1. 2b Vabc_f represents the phase B voltage of f2 when a single-phase ground fault occurs on the low-voltage side f1. 2c This is the C-phase voltage of f2 when a single-phase ground fault occurs on the low-voltage side f1.

[0033] Figure 12 This is a current waveform diagram of f2 when a single-phase ground fault occurs on the low-voltage side f1 in an embodiment of the present invention; Iabc_f in the figure 2a This refers to the A-phase current of f2 when a single-phase ground fault occurs on the low-voltage side f1; Iabc_f 2b This refers to the B-phase current of f2 when a single-phase ground fault occurs on the low-voltage side f1; Iabc_f 2c This is the C-phase current of f2 when a single-phase ground fault occurs on the low-voltage side f1.

[0034] Figure 13 This is a power waveform diagram of f2 when a single-phase ground fault occurs on the low-voltage side f1 in an embodiment of the present invention; in the figure, P_f 2a Q_f represents the active power of f2 when a single-phase ground fault occurs on the low-voltage side f1. 2a This represents the reactive power of f2 when a single-phase ground fault occurs on the low-voltage side f1.

[0035] Figure 14 This is a waveform diagram of the effective voltage value of f2 when a single-phase ground fault occurs on the low-voltage side f1 in an embodiment of the present invention; Vrms_f2 in the figure is the effective voltage value of f2 when a single-phase ground fault occurs on the low-voltage side f1.

[0036] Figure 15 This is a waveform diagram of the effective current value of f2 when a single-phase ground fault occurs on the low-voltage side f1 in an embodiment of the present invention; in the figure, Irms_f2 is the effective current value of f2 when a single-phase ground fault occurs on the low-voltage side f1.

[0037] Figure 16 This is a voltage waveform diagram of f3 when a single-phase ground fault occurs on the low-voltage side f1 in an embodiment of the present invention; Vabc_f in the figure 3a Vabc_f is the A-phase voltage of f3 when a single-phase ground fault occurs on the low-voltage side f1. 3b This refers to the B-phase voltage of f3 when a single-phase ground fault occurs on the low-voltage side f1; Vabc_f 3c This refers to the C-phase voltage of f3 when a single-phase ground fault occurs on the low-voltage side f1.

[0038] Figure 17This is a current waveform diagram of f3 when a single-phase ground fault occurs on the low-voltage side f1 in an embodiment of the present invention; Iabc_f in the figure 3a This refers to the A-phase current of f3 when a single-phase ground fault occurs on the low-voltage side f1; Iabc_f 3b This refers to the B-phase current of f3 when a single-phase ground fault occurs on the low-voltage side f1; Iabc_f 3c This refers to the C-phase current of f3 when a single-phase ground fault occurs on the low-voltage side f1.

[0039] Figure 18 This is a power waveform diagram of f3 when a single-phase ground fault occurs on the low-voltage side f1 in an embodiment of the present invention; in the figure, P_f3 is the active power of f3 when a single-phase ground fault occurs on the low-voltage side f1; Q_f3 is the reactive power of f3 when a single-phase ground fault occurs on the low-voltage side f1.

[0040] Figure 19 This is a waveform diagram of the effective voltage value of f3 when a single-phase ground fault occurs on the low-voltage side f1 in an embodiment of the present invention; Vrms_f3 in the figure is the effective voltage value of f3 when a single-phase ground fault occurs on the low-voltage side f1.

[0041] Figure 20 The figure shows the effective current waveform of f3 when a single-phase ground fault occurs on the low-voltage side f1 in an embodiment of the present invention; Irms_f2 in the figure is the effective current of f3 when a single-phase ground fault occurs on the low-voltage side f1.

[0042] Figure 21 This is a voltage waveform diagram of f4 when a single-phase ground fault occurs on the low-voltage side f1 in an embodiment of the present invention; Vabc_f in the figure 4a Vabc_f is the phase A voltage of f4 when a single-phase ground fault occurs on the low-voltage side f1. 4b This refers to the B-phase voltage of f4 when a single-phase ground fault occurs on the low-voltage side f1; Vabc_f 4c This refers to the C-phase voltage of f4 when a single-phase ground fault occurs on the low-voltage side f1.

[0043] Figure 22 This is a current waveform diagram of f4 when a single-phase ground fault occurs on the low-voltage side f1 in an embodiment of the present invention; Iabc_f in the figure 3a This refers to the A-phase current of f4 when a single-phase ground fault occurs on the low-voltage side f1; Iabc_f 3b This refers to the B-phase current of f4 when a single-phase ground fault occurs on the low-voltage side f1; Iabc_f 3c This refers to the C-phase current of f4 when a single-phase ground fault occurs on the low-voltage side f1.

[0044] Figure 23 This is a power waveform diagram of f4 when a single-phase ground fault occurs on the low-voltage side f1 in an embodiment of the present invention; in the figure, P_f 4aQ_f represents the active power of f4 when a single-phase ground fault occurs on the low-voltage side f1. 4a This represents the reactive power of f4 when a single-phase ground fault occurs on the low-voltage side f1.

[0045] Figure 24 This is a waveform diagram of the effective voltage value of f4 when a single-phase ground fault occurs on the low-voltage side f1 in an embodiment of the present invention; Vrms_f in the figure 4a This is the effective voltage value of f4 when a single-phase ground fault occurs on the low-voltage side f1.

[0046] Figure 25 This is a schematic diagram of the structure of the offshore wind power AC / DC transmission system in an embodiment of the present invention. Detailed Implementation

[0047] 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.

[0048] I. Topology of AC System in Offshore Wind Farms This invention includes n Typhoon turbine units, and record the number of any one of the wind turbine units as... i , i =1,…, n The AC aggregation section provides a topology scheme: the AC system of the wind farm of the present invention specifically refers to a group aggregation architecture based on a three-winding transformer module.

[0049] To fully illustrate the above-mentioned three-winding transformer wiring structure adopted in this invention ( Figure 2 The significant advantages of this embodiment in terms of economy and fault tolerance Figure 1 and Figure 3 Two conventional AC aggregation cabling architectures are also listed for comparison, and the specific analysis is as follows: Comparison with Option 1 (corresponding) Figure 1 ):Include n Two multi-winding transformer modules are connected. The high-voltage side of the multi-winding transformer is connected to the sending end of the offshore wind farm, and the low-voltage side of the multi-winding transformer is connected to two wind turbine units. Adjacent wind turbine units on the low-voltage side of the same multi-winding transformer... i , i +1 uses a single busbar with dual-segment connection, meaning there is no connection between wind turbines belonging to different multi-winding transformers. This comparative scheme lacks flexible interconnection between feeders, making it impossible to transfer power across groups in the event of a single-point fault, which can easily lead to wind turbine islanding and grid disconnection, resulting in a low fault tolerance rate.

[0050] Embodiments of the present invention (corresponding) Figure 2 ):Include n / Two multi-winding transformer modules are used, and the multi-winding transformer itself is a three-winding transformer. The high-voltage side of the multi-winding transformer is connected to the sending end of the offshore wind farm, and the low-voltage side is connected to the wind turbine generators. Adjacent wind turbine generators belonging to the same multi-winding transformer are connected. i , i +1 uses a single busbar double-section connection on the low-voltage side of a multi-winding transformer group, and there is no connection between the two; these are wind turbine groups that belong to different multi-winding transformers and are physically adjacent. i +1 and wind turbine i +2 There is a connection between the two.

[0051] Comparison with Option 2 (corresponding) Figure 3 ):Include n Each transformer module has two windings. The high-voltage side of each transformer is connected to the sending end of the offshore wind farm, while the low-voltage side is connected to the corresponding individual wind turbine. i Connected, and adjacent wind turbine units i and i The connection between the low-voltage side busbars of +1. Although this comparative scheme achieves complete independence of each feeder, the large number of transformers greatly increases the size, load-bearing capacity, and initial construction cost of the offshore converter platform, resulting in poor economic efficiency.

[0052] Compared to the two comparative schemes mentioned above, the wiring architecture specifically referred to in this invention ( Figure 2 By grouping wind turbines into pairs, the amount of transformer equipment was reduced by 50%, thus improving the economic efficiency of the offshore platform. At the same time, a flexible mesh interconnection was constructed using cross-connected AC circuit breakers, which can open up power transfer channels in the event of a local fault, balancing construction costs and fault tolerance.

[0053] II. Topology Implementation Case Please see Figure 4 , Figure 4 This is a topology diagram of a hybrid pseudo-bipolar sending-end, true bipolar receiving-end, and AC system connection for an offshore wind farm (as described in the aforementioned embodiments of the present invention). Figure 4 Middle,U WF Voltage of offshore wind farm; X T1 The equivalent reactance X of the low-voltage side winding of an offshore wind farm T1 ;X T2 The equivalent reactance X of the low-voltage side winding of an offshore wind farm T1 ;X T3R is the equivalent reactance of the AC grid side of the offshore wind farm receiving end; R is the equivalent resistance of the AC grid side of the offshore wind farm receiving end; f1 is low-voltage side line 1 of the offshore wind farm; f2 is low-voltage side line 2 of the offshore wind farm; f3 is high-voltage side line of the offshore wind farm; f4 is AC side busbar of the offshore wind farm.

[0054] Please see Figure 5 , Figure 5 For a hybrid MMC topology, L in the diagram ac For AC side tie reactor; L arm For converter valve bridge arm reactor; U PA U is the voltage of the upper bridge arm of phase A of the converter; PB U is the voltage of the upper arm of phase B of the converter; PC HBSM1 is the voltage of the upper arm of phase C of the converter; HBSM1 is the first half-bridge submodule connected in series within the arm; HBSM H The Hth half-bridge submodule connected in series within the bridge arm, where H represents the total number of half-bridge submodules configured; FBSM F The bridge consists of full-bridge submodules connected in series within the bridge arm, where F represents the total number of full-bridge submodules configured; T1~T4 are fully controlled power electronic switching devices within the submodules. Through this hybrid series configuration of half-bridge and full-bridge submodules, the converter station of this invention can achieve rapid self-interception of DC fault current without the need for additional high-voltage DC circuit breakers, thus meeting the engineering requirements of low operating losses and high fault ride-through reliability for offshore wind power connected to the grid via flexible DC transmission.

[0055] III. Electromagnetic Transient Simulation Verification of Offshore Wind Power Topology To verify the economy and reliability of the true / pseudo bipolar configuration topology proposed in this invention, this embodiment... Figure 4 The transient characteristics of the pseudo-bipolar system with flexible direct transmission from low-voltage side f1 and high-voltage side f3 were simulated and verified under the condition of a permanent single-phase ground fault on low-voltage side f1.

[0056] This invention comprises, in sequence: a wind power generation and AC collection network, an offshore sending-end converter station, a DC transmission line, and an onshore receiving-end converter station. Specifically: (1) Low-voltage collection side: includes multiple doubly fed wind turbine generators. The electrical energy generated by each generator is stepped up to the first voltage level of 66kV by the generator terminal step-up transformer and then connected to the corresponding 66kV AC feeder branch.

[0057] (2) Flexible interconnection structure: AC tie circuit breakers are installed between adjacent 66kV AC feeder branches and between different wind turbine wiring groups. When a wind turbine fails, power transfer and topology reconfiguration can be achieved by opening and closing the corresponding tie circuit breakers, thus isolating the fault point.

[0058] (3) High-voltage step-up side: After the 66kV AC feeder branches are combined, they are connected to the low-voltage winding of the main collection transformer, stepped up to 515kV on the f3 side by the main transformer, and then connected in parallel to the 515kV offshore AC common bus f4 on the high-voltage side. The 515kV offshore AC common bus is connected to the offshore sending-end converter station through an AC circuit breaker. In this embodiment, the offshore sending-end converter station adopts a pseudo-bipolar topology. Specifically, the sending-end converter station includes a hybrid MMC, whose AC side is connected to the AC common bus, and whose DC side directly outputs a positive DC terminal and a negative DC terminal to maintain a symmetrical DC voltage between the positive and negative poles and ground.

[0059] (5) The ends of the positive DC submarine cable and the negative DC submarine cable are connected to the receiving-end converter station of the offshore wind farm on land. The receiving-end converter station of the offshore wind farm adopts a true bipolar topology.

[0060] To achieve low-loss power transmission in steady state and safe isolation in transient state in the above topology, this embodiment constructs a control loop and an AC / DC collaborative fault ride-through mechanism. The specific control logic and response process are as follows: (1) Configuration of converter station control system The control system of the sending-end converter station (pseudo-bipolar): The sending-end converter station adopts a control system based on... dq A dual-closed-loop control architecture with a rotating coordinate system. The outer loop control uses constant AC voltage control and constant frequency control to provide stable voltage and frequency support for the AC collection network of offshore wind farms; the inner loop current controller integrates positive and negative sequence current decoupling control links, which control the negative sequence current component to zero when there is an asymmetrical fault on the AC side, and smooth out the frequency doubling voltage fluctuations on the DC side.

[0061] The receiving-end converter station (true bipolar) control system: The receiving-end positive and negative converter valves are equipped with independent pole control units. The outer loop control dominates the constant DC voltage and constant reactive power control to ensure the stability of the onshore DC bus voltage; when a transient voltage drop occurs in the onshore AC main grid, the receiving-end control system adaptively switches to reactive voltage support mode to inject a fixed reactive current into the onshore main grid.

[0062] (2) DC-side fault ride-through (HVDC-FRT) and adaptive restart mechanism When an inter-pole short circuit or a single-pole ground fault occurs in the positive and negative DC submarine cables, the converter valve control units at both the sending and receiving ends instantaneously shut down the IGBT switching devices in the full-bridge submodule (FBSM). The full-bridge submodule utilizes its HThe bridge physical topology outputs a reverse capacitor voltage across the entire domain, forming a reverse potential blocking wall superimposed on the bottom layer of the topology, forcibly cutting off the short-circuit current fed into the AC side without relying on a high-voltage DC circuit breaker. When a single-pole (such as the positive pole) ground fault occurs, the receiving end control unit only locks the positive pole converter valve, while its negative pole converter valve remains in an open operating state. The fault current forms an independent loop through the receiving end neutral point and the metal return line, enabling the negative pole to maintain at least 50% of its rated power without lockout and continuous power supply. When the faulty pole line is cleared or the transient arc is extinguished, the sending and receiving end control circuits trigger adaptive restart logic, unlock the converter valve, and gradually establish DC voltage, smoothly restoring the steady-state power transmission of the entire station.

[0063] When a single-phase permanent ground fault occurs on the low-voltage side f1 at 6s, the waveform is as shown.

[0064] The basic parameters of the simulation model built in PSCAD / EMTDC in this embodiment are as follows: the rated transmission capacity of the system is 550MW, and the high-voltage side voltage of the AC collection side main transformer is 515kV. The single-phase grounding fault is set at t=6.0s, and the fault transition resistance is set to 0.001 ohms. Figures 6-24 This is a waveform diagram of a permanent single-phase ground fault occurring at the low-voltage side f1 of an offshore wind power grid-connected system via a flexible DC transmission line, as shown in this embodiment of the invention.

[0065] Depend on Figures 6-24 The results show that the specific heterogeneous topology defined in this invention can limit the impact of a single-point short-circuit fault to a local feeder. The entire system does not require a large-scale shutdown and exhibits adaptive restart capability after the fault is cleared, ensuring the continuous and stable operation of the high-proportion renewable energy transmission system and achieving a balance between reliability and economy.

[0066] This embodiment also provides a highly reliable offshore wind power AC / DC transmission system, such as Figure 25 As shown, it includes: Offshore wind power transmission module: Relying on a three-winding main collection transformer and an AC interconnection circuit breaker connected to the low-voltage side, it achieves the collection of power from the wind farm cluster under steady-state conditions. When a fault is detected, the redundant power transfer channel of the non-faulty wind turbines is quickly established by closing the AC interconnection circuit breaker of the adjacent branch. This module completely breaks the islanding effect of traditional single-chain or star topologies, avoids large-scale grid disconnection of wind turbines caused by single-point faults, and greatly ensures the continuity and active fault tolerance of the offshore wind power AC transmission system.

[0067] Sending-end converter station module: Utilizing a hybrid MMC pseudo-bipolar topology, this module performs low-loss conversion of offshore AC power to high-voltage DC power. Furthermore, the system eliminates the need for a neutral point return structure to minimize the load and cost on the offshore platform. In the event of extreme transient conditions such as DC line short circuits, this module instantly locks out the full-bridge submodule to output reverse capacitor voltage across the entire circuit, eliminating reliance on expensive DC circuit breakers and achieving rapid fault current interruption and self-isolation.

[0068] Receiving-end converter station module: Utilizing a hybrid MMC true bipolar topology, it forms an asymmetric heterogeneous topology with the sending-end converter station. In the event of a single-pole ground fault on the DC line, this module only locks the converter valve on the faulty pole, while the converter valves on the non-faulty poles maintain unblocked operation via an independent neutral point and metallic return line. After the fault is cleared, it coordinates with the sending end to trigger an adaptive restart and steady-state power recovery of the entire system. This module, with its optimal engineering cost and highly compact hardware configuration, ensures continuous power reception and voltage support for the onshore receiving-end main grid.

[0069] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A topology for an offshore wind power grid-connected system via flexible DC transmission, characterized in that, include: AC collection system for wind farms, sending-end converter station for offshore wind farms, DC transmission lines, and receiving-end converter station for offshore wind farms; The wind farm AC collection system includes multiple multi-winding transformers and multiple wind turbine generators arranged in rows; every two wind turbine generators are connected to the low-voltage side of the same multi-winding transformer, wherein there is no connection between the two wind turbine generators connected to the low-voltage side of the same multi-winding transformer; There is a connection between two wind turbine units that belong to different multi-winding transformers and are located adjacently; the high-voltage side of the multi-winding transformer is connected to the sending-end converter station of the offshore wind farm; The offshore wind farm sending-end converter station adopts a pseudo-bipolar physical topology, which does not have a neutral point return structure internally, and the DC side is connected to the offshore wind farm receiving-end converter station through the DC transmission line; the DC transmission line includes a positive DC line and a negative DC line; The offshore wind farm receiving-end converter station adopts a true bipolar physical topology, including a positive converter, a negative converter, and a metal return line connecting the neutral points of the positive and negative converters. The converter valves of both the sending-end converter station and the receiving-end converter station of the offshore wind farm adopt a modular multilevel converter structure with a hybrid cascade of half-bridge sub-modules and full-bridge sub-modules.

2. The topology of the offshore wind power grid-connected system via flexible DC transmission according to claim 1, characterized in that, The multi-winding transformer is a three-winding transformer; an AC interconnection circuit breaker is provided between the AC feeder branches of two wind turbine units that belong to different multi-winding transformers and are adjacent in location; when a wind turbine unit fails, the power is transferred across units and the topology is reconfigured by opening and closing the corresponding AC interconnection circuit breaker to isolate the fault point.

3. The topology of the offshore wind power grid-connected system via flexible DC transmission according to claim 2, characterized in that, The electrical energy generated by the wind turbine is stepped up to the first voltage level by the turbine-side step-up transformer and then connected to the corresponding AC feeder branch. Multiple AC feeder branches are combined and connected to the low-voltage side of the multi-winding transformer, and stepped up to the second voltage level by the multi-winding transformer before being connected in parallel to the offshore AC common bus. The offshore AC common bus is connected to the offshore wind farm's sending-end converter station through an AC circuit breaker.

4. The topology of the offshore wind power grid-connected system via flexible DC transmission according to claim 1, characterized in that, In the offshore wind farm's sending-end converter station, the upper arms of each phase unit are physically connected in series and converge to the sending-end DC positive busbar and connected to the positive DC line. The lower arms of each phase unit are physically connected in series and converge to the sending-end DC negative busbar and connected to the negative DC line.

5. The topology of the offshore wind power grid-connected system via flexible DC transmission according to claim 1, characterized in that, In the receiving-end converter station of the offshore wind farm, the common neutral point of the positive converter and the negative converter is connected to the metal return line, and the metal return line is grounded at the onshore receiving end.

6. The topology of the offshore wind power grid-connected system via flexible DC transmission according to any one of claims 1 to 5, characterized in that, Both the sending-end converter station and the receiving-end converter station of the offshore wind farm are equipped with a dual closed-loop control loop, which includes an outer loop controller and an inner loop current controller. Under steady-state conditions, the offshore wind farm sending-end converter station adopts constant AC voltage and constant frequency control, and the offshore wind farm receiving-end converter station adopts constant DC voltage and constant reactive power control. The inner loop current controller integrates a positive and negative sequence current decoupling control module, which is configured to control the negative sequence current component to zero during AC-side asymmetrical faults to suppress DC-side frequency doubling voltage fluctuations.

7. The topology of the offshore wind power grid-connected system via flexible DC transmission according to claim 6, characterized in that, The topology is configured with AC / DC collaborative fault ride-through logic; when the DC transmission line experiences an inter-pole short circuit or a single-pole ground fault, the converter valve control unit shuts off the switching devices in the full-bridge submodule, and the full-bridge submodule uses its internal H-bridge physical topology to output a reverse capacitor voltage to form a reverse potential blocking, cutting off the short-circuit current fed in from the AC side.

8. The topology of the offshore wind power grid-connected system via flexible DC transmission according to claim 7, characterized in that, When a single-pole grounding fault occurs in the DC transmission line, the pole control unit of the offshore wind farm receiving-end converter station only locks the converter valve corresponding to the faulty pole, while keeping the converter valve corresponding to the non-faulty pole in an open-loop operation state; the current of the non-faulty pole forms an independent circuit through the receiving-end neutral point and the metal return line to continuously supply power. After the fault is cleared, the sending-end converter station and the receiving-end converter station of the offshore wind farm trigger adaptive restart logic to unlock the converter valve and establish DC voltage recovery steady-state power transmission.

9. The topology of the offshore wind power grid-connected system via flexible DC transmission according to claim 6, characterized in that, When a transient voltage drop occurs in the onshore AC main grid connected to the offshore wind farm receiving-end converter station, the control loop of the offshore wind farm receiving-end converter station adaptively switches to reactive voltage support mode and injects a fixed reactive current into the onshore AC main grid.

10. The topology of the offshore wind power grid-connected system via flexible DC transmission according to claim 1, characterized in that, In the modular multilevel converter structure in which half-bridge submodules and full-bridge submodules are cascaded together, each converter valve arm includes multiple half-bridge submodules, multiple full-bridge submodules, and converter valve arm reactors connected in series; the AC side of the modular multilevel converter structure is also connected to an AC side tie reactor; and the full-bridge submodule is internally equipped with fully controlled power electronic switching devices.

Images