A true bipolar hybrid direct current transmission system based on substation interconnection for offshore wind power and a control method thereof

Abstract

This invention discloses a true bipolar hybrid DC transmission system and control method for offshore wind power based on substation interconnection, belonging to the field of offshore wind power DC transmission technology. The aforementioned true bipolar hybrid DC transmission system includes: an offshore converter system, an offshore DC interconnection line, and an onshore converter system. The offshore converter system includes: a positive offshore converter system and a negative offshore converter system. The positive and negative offshore converter systems are connected via the offshore DC interconnection line. During the black start phase, the offshore converter system performs uncontrolled pre-charging based on the onshore converter system, followed by voltage building-up to achieve grid connection of the wind turbine, and finally increases the effective voltage value to allow the diode rectifier to conduct naturally, completing the black start. By implementing this invention, the problem of severe inrush current generated during the natural conduction of the DR during black start, which reduces equipment safety, can be solved.

Description

Technical Field

[0001] This invention relates to the field of offshore wind power DC transmission technology, and in particular to an offshore wind power true bipolar hybrid DC transmission system and control method based on substation interconnection. Background Technology

[0002] With the acceleration of my country's energy transition, offshore wind power is rapidly moving towards large-scale, concentrated development in deep-sea areas. For long-distance, high-capacity offshore wind power transmission, flexible DC transmission technology based on voltage source converters (VSCs), especially modular multilevel converters (MMCs), has become the mainstream engineering solution due to its ability to power passive networks (black start) and its advantages in active and reactive power decoupling control. To achieve lightweight and compact offshore converter stations, academia and industry have proposed using diode rectifiers (DRs) to replace part or all of the MMCs. DRs utilize uncontrollable diode valve groups for natural commutation, offering technical advantages such as extremely high power density, low cost, low operating losses, and high reliability.

[0003] However, for asymmetric true bipolar systems with strong coupling, including "DR+MMC parallel connection", due to the uncontrollable characteristics of DR, the smooth black start timing logic of the system from "passive to active" lacks cross-site coordination, which can easily generate severe inrush current at the moment when DR is naturally turned on, thus reducing the safety of the equipment. Summary of the Invention

[0004] This invention provides a true bipolar hybrid DC transmission system and control method for offshore wind power based on substation interconnection. It can solve the problem that in the existing true bipolar system, due to the lack of cross-station coordination, a severe excitation inrush current will be generated at the moment of natural conduction of DR during black start, which reduces the safety of the equipment.

[0005] An embodiment of the present invention provides a true bipolar hybrid DC transmission system for offshore wind power based on substation interconnection, comprising: The system comprises an offshore converter system, an offshore DC interconnection line, and an onshore converter system. The offshore converter system includes a positive offshore converter system and a negative offshore converter system. The positive and negative offshore converter systems are connected via an offshore DC interconnection line. The negative offshore converter system includes a first modular multilevel converter and a first AC busbar. The positive offshore converter system includes a second modular multilevel converter, a second AC busbar, and a diode rectifier. The offshore converter system is used to draw power from the onshore converter system during the black start phase to perform uncontrolled pre-charging of the internal sub-module capacitors of the modular multilevel converter; after the uncontrolled pre-charging is completed, the voltage of the AC busbar is built up to achieve grid connection of the wind turbine. Specifically, when the offshore converter system is a negative-pole offshore converter system, the modular multilevel converter is the first modular multilevel converter, and the AC bus is the first AC bus; when the offshore converter system is a positive-pole offshore converter system, the modular multilevel converter is the second modular multilevel converter, and the AC bus is the second AC bus. Furthermore, during the voltage build-up operation, the positive-pole offshore converter system controls the effective voltage value of the second AC bus to be lower than the preset critical cutoff threshold until the wind turbine is connected to the grid. The offshore converter system is also used to increase the effective value of the voltage after the wind turbine is connected to the grid, so that the diode rectifier will naturally conduct when the effective value of the voltage of the corresponding AC bus is not less than the preset critical cutoff threshold, thus completing the black start.

[0006] Furthermore, the negative pole marine converter system also includes: The first offshore wind farm and the first AC convergence submarine cable; The output end of the first offshore wind farm is connected to the first AC busbar via the first AC collecting submarine cable. The AC side of the first modular multilevel converter is connected to the first AC busbar. The positive DC terminal of the DC side of the first modular multilevel converter is connected to the common point of the marine neutral line on the marine DC link, and the negative DC terminal of the DC side of the first modular multilevel converter serves as the negative DC output terminal. The first offshore wind farm is used to output active power after the wind turbines are connected to the grid; The first AC collecting submarine cable is used to integrate the active power output from the first offshore wind farm and transmit it to the first AC busbar.

[0007] Furthermore, the positive polar ocean converter system also includes: The second offshore wind farm and the second AC convergence submarine cable; The output end of the second offshore wind farm is connected to the second AC busbar via the second AC convergence submarine cable; The AC side of the diode rectifier and the AC side of the second modular multilevel converter are respectively connected to the second AC busbar; The positive DC terminal of the diode rectifier is connected to the positive DC terminal of the second modular multilevel converter to form a positive DC output terminal. The DC negative terminal of the diode rectifier and the DC negative terminal of the second modular multilevel converter are both connected to the common point of the marine neutral line on the marine DC link line. The second offshore wind farm is used to output active power after the wind turbines are connected to the grid; The second AC collecting submarine cable is used to integrate the active power output from the second offshore wind farm and transmit it to the second AC busbar.

[0008] Furthermore, the aforementioned marine converter system is also used for: Before the voltage build-up operation of the AC busbar, the AC side circuit breaker of the first modular multilevel converter is closed to connect the first offshore wind farm to the first modular multilevel converter. Close the AC side circuit breaker of the second modular multilevel converter to connect the second offshore wind farm with the second modular multilevel converter; Unlock the first modular multilevel converter and the second modular multilevel converter to generate voltage on the first AC bus and the second AC bus.

[0009] Furthermore, it also includes: true bipolar DC transmission networks; The true bipolar DC transmission network includes: a positive DC line, a neutral return line, and a negative DC line; The positive DC line is connected to the positive DC output terminal of the positive offshore converter system. The negative DC line is connected to the negative DC output terminal of the negative offshore converter system. The neutral line return line is connected to the marine neutral line common point and the onshore converter system, respectively. The negative DC line is used to transmit the active power corresponding to the first offshore wind farm to the onshore converter system. The positive DC line is used to transmit the active power corresponding to the second offshore wind farm to the onshore converter system; The neutral return line is used to provide a current loop.

[0010] Furthermore, the onshore converter system includes: Positive pole onshore modular multilevel converter, negative pole onshore modular multilevel converter, onshore AC bus and onshore AC main power grid; The DC side of the positive onshore modular multilevel converter and the DC side of the negative onshore modular multilevel converter are both connected to the neutral return line. The AC side of the positive pole onshore modular multilevel converter and the AC side of the negative pole onshore modular multilevel converter are both connected to the onshore AC main power grid through the onshore AC bus. The positive onshore modular multilevel converter is used to invert the active power transmitted by the positive DC line from DC to AC. The negative pole onshore modular multilevel converter is used to invert the active power transmitted by the negative pole DC line from DC to AC. The onshore AC bus is used to collect the active power after inversion and transmit it to the onshore AC main grid. The onshore AC main power grid is used to receive all the active power after inversion.

[0011] Furthermore, the step of performing a voltage build-up operation on the AC busbar after the uncontrolled pre-charging is completed to achieve grid connection of the wind turbine includes: Obtain the rated voltage amplitude and rated frequency of the AC busbar; Using a boost timing sequence, the voltage and frequency of the AC bus are increased at a preset slope until the voltage amplitude of the AC bus reaches the rated voltage amplitude and the frequency of the AC bus reaches the rated frequency.

[0012] Furthermore, the negative electrode marine converter system is also used for: After black start, the actual value of the first AC voltage of the first AC bus and the actual value of the first AC current of the first modular multilevel converter are obtained. Based on the rated voltage amplitude and rated frequency, a first AC voltage amplitude reference value and a first reference phase angle are generated for the first modular multilevel converter. Based on the actual value of the first AC voltage and the reference value of the first AC voltage amplitude, the first current reference value of the first modular multilevel converter is calculated. The first AC voltage reference value of the first modular multilevel converter is calculated based on the first reference phase angle, the actual value of the first AC current, and the first current reference value. Based on the first AC voltage reference value, generate the first trigger pulse for the switching device of each sub-module in the first modular multilevel converter; Based on the first trigger pulse, the switching devices of each sub-module in the first modular multilevel converter are switched on and off.

[0013] Furthermore, the positive polarization offshore converter system is also used for: After black start, the second DC output voltage of the positive offshore converter system, the second stable voltage effective value of the second AC bus, the second total active power value of the second offshore wind farm, the second AC voltage actual value of the second AC bus, and the second AC current actual value of the second modular multilevel converter are obtained. Based on the second DC output voltage, the second stable voltage RMS value, and the second total active power value, the second AC voltage amplitude reference value of the second modular multilevel converter is calculated. The second reference phase angle of the second modular multilevel converter is generated according to the rated frequency; The second current reference value of the second modular multilevel converter is calculated based on the second AC voltage amplitude reference value and the actual value of the second AC voltage. The second AC voltage reference value of the second modular multilevel converter is calculated based on the second reference phase angle, the actual value of the second AC current, and the second current reference value. The second trigger pulse is generated for the switching devices of each sub-module in the second modular multilevel converter based on the second AC voltage reference value; Based on the second trigger pulse, the switching devices of each submodule in the second modular multilevel converter are switched on and off.

[0014] Based on the above system implementation embodiments, the present invention provides a control method for a true bipolar hybrid DC transmission system for offshore wind power, applicable to the offshore converter system in any of the above-mentioned true bipolar hybrid DC transmission systems for offshore wind power based on substation interconnection. The control method includes: During the black start phase, power is drawn from the onshore converter system to perform uncontrolled pre-charging of the internal sub-module capacitors of the modular multilevel converter; after the uncontrolled pre-charging is completed, the voltage of the AC bus is built up to achieve grid connection of the wind turbine. Specifically, when the offshore converter system is a negative-pole offshore converter system, the modular multilevel converter is the first modular multilevel converter, and the AC bus is the first AC bus; when the offshore converter system is a positive-pole offshore converter system, the modular multilevel converter is the second modular multilevel converter, and the AC bus is the second AC bus. Furthermore, during the voltage build-up operation, the positive-pole offshore converter system controls the effective voltage value of the second AC bus to be lower than the preset critical cutoff threshold until the wind turbine is connected to the grid. After the wind turbine is connected to the grid, the effective value of the voltage is increased so that the diode rectifier will naturally conduct when the effective value of the voltage on the corresponding AC bus is not less than the preset critical cutoff threshold, thus completing the black start.

[0015] The embodiments of the present invention have the following beneficial effects: This invention provides a true bipolar hybrid DC transmission system and control method for offshore wind power based on substation interconnection. The offshore wind power true bipolar hybrid DC transmission system includes: an offshore converter system, an offshore DC interconnection line, and an onshore converter system; the offshore converter system includes: a positive offshore converter system and a negative offshore converter system; the positive offshore converter system and the negative offshore converter system are connected via the offshore DC interconnection line; the negative offshore converter system includes: a first modular multilevel converter and a first AC bus; the positive offshore converter system includes: a second modular multilevel converter, a second AC bus, and a diode rectifier; the offshore converter system is used to draw power from the onshore converter system during the black start phase to perform uncontrolled pre-charging of the internal sub-module capacitors of the modular multilevel converter; during the uncontrolled pre-charging... After charging is completed, the voltage of the AC bus is built up to achieve grid connection of the wind turbine. Specifically, when the offshore converter system is a negative-pole offshore converter system, the modular multilevel converter is the first modular multilevel converter, and the AC bus is the first AC bus. When the offshore converter system is a positive-pole offshore converter system, the modular multilevel converter is the second modular multilevel converter, and the AC bus is the second AC bus. During the voltage build-up process, the positive-pole offshore converter system controls the effective voltage value of the second AC bus to be lower than a preset critical cutoff threshold until the wind turbine is connected to the grid. The offshore converter system is also used to increase the effective voltage value after the wind turbine is connected to the grid, so that the diode rectifier naturally conducts when the effective voltage value of the corresponding AC bus is not less than the preset critical cutoff threshold, completing a black start. In this invention, a DC interconnection line is first used to connect the positive and negative offshore converter systems across stations, allowing the onshore converter system to simultaneously perform uncontrolled pre-charging of its internal submodule capacitors, thus providing a unified DC voltage foundation for the two offshore converter systems. Simultaneously, during voltage build-up, the effective voltage value of the second AC bus is forced to be lower than a preset critical cutoff threshold, ensuring that the diode rectifier remains reverse-cut off before the wind turbine is connected to the grid. It only naturally conducts after the wind turbine is connected, as the effective voltage value gradually increases. This conduction process is a smooth transition under load, without inrush current impact, greatly increasing the system's equipment safety. Attached Figure Description

[0016] To more clearly illustrate the technical solution of this application, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0017] Figure 1This is a schematic diagram of the structure of a true bipolar hybrid DC transmission system for offshore wind power based on substation interconnection, provided in an embodiment of the present invention.

[0018] Figure 2 This is a schematic diagram of the steady-state cooperative control strategy of a first modular multilevel converter provided in an embodiment of the present invention.

[0019] Figure 3 This is a schematic diagram of the steady-state collaborative control strategy of a second modular multilevel converter provided in an embodiment of the present invention.

[0020] Figure 4 This is a schematic diagram of the control strategy for the positive and negative onshore converters in an onshore converter system provided in an embodiment of the present invention.

[0021] Figure 5 This is a flowchart illustrating a control method for a true bipolar hybrid DC transmission system for offshore wind power, provided in an embodiment of the present invention.

[0022] Figure descriptions: 1. Negative offshore converter system; 11. First offshore wind farm; 12. First AC collection submarine cable; 13. First AC combiner bus; 14. First modular multilevel converter; 2. Positive offshore converter system; 21. Second offshore wind farm; 22. Second AC collection submarine cable; 23. Second AC combiner bus; 24. Diode rectifier; 25. Second modular multilevel converter; 3. True bipolar DC transmission network; 31. Positive DC line; 32. Neutral return line; 33. Negative DC line; 4. Onshore converter system; 41. Positive onshore modular multilevel converter; 42. Negative onshore modular multilevel converter; 43. Onshore AC bus; 44. Onshore AC main power grid; and 5. Offshore DC interconnection line. Detailed Implementation

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

[0024] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the application; the terms “comprising” and “having”, and any variations thereof, in the specification, claims, and foregoing description of the drawings are intended to cover non-exclusive inclusion.

[0025] In the description of the embodiments of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, "multiple" means two or more, unless otherwise explicitly defined.

[0026] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0027] In the description of the embodiments in this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0028] In the description of the embodiments of this application, the term "multiple" refers to two or more (including two), similarly, "multiple sets" refers to two or more (including two sets), and "multiple pieces" refers to two or more (including two pieces).

[0029] In the description of the embodiments of this application, unless otherwise expressly specified and limited, technical terms such as "installation," "connection," "joining," and "fixing" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. For those skilled in the art, the specific meaning of the above terms in the embodiments of this application can be understood according to the specific circumstances.

[0030] See Figure 1 To address the issue that existing true bipolar systems suffer from severe inrush current during black start-up due to a lack of cross-site coordination, leading to reduced equipment safety at the moment of DR natural conduction, this invention provides an embodiment of a true bipolar hybrid DC transmission system for offshore wind power based on substation interconnection, comprising: The system comprises an offshore converter system, an offshore DC interconnection line 5, and an onshore converter system 4; the offshore converter system includes a positive offshore converter system 2 and a negative offshore converter system 1; the positive offshore converter system 2 and the negative offshore converter system 1 are connected via the offshore DC interconnection line 5; the negative offshore converter system 1 includes a first modular multilevel converter 14 and a first AC busbar 13; the positive offshore converter system 2 includes a second modular multilevel converter 25, a second AC busbar 23, and a diode rectifier 24; Specifically, the diode rectifier 24 is a high-capacity 12-pulse diode rectifier, and the second modular multilevel converter 25 is a low-capacity modular multilevel converter.

[0031] The offshore converter system is used to draw power from the onshore converter system 4 during the black start phase to perform uncontrolled pre-charging of the internal sub-module capacitors of the modular multilevel converter; after the uncontrolled pre-charging is completed, the voltage of the AC busbar is built up to achieve grid connection of the wind turbine; after the wind turbine is connected to the grid, the effective value of the voltage is increased so that the diode rectifier 24 will naturally conduct when the effective value of the voltage of the corresponding AC busbar is not less than the preset critical cutoff threshold, thus completing the black start; Specifically, during the black start phase, the onshore converter system 4 is started first, so that the second modular multilevel converter 25 in the positive pole offshore converter system 2, which is located in different geographical locations, and the first modular multilevel converter 14 in the negative pole offshore converter system 1 can draw power to complete the uncontrolled pre-charging of their internal sub-module capacitors.

[0032] Specifically, when the offshore converter system is a negative offshore converter system 1, the modular multilevel converter is a first modular multilevel converter 14, and the AC bus is a first AC bus 13; when the offshore converter system is a positive offshore converter system 2, the modular multilevel converter is a second modular multilevel converter 25, and the AC bus is a second AC bus 23. Furthermore, during the voltage build-up operation, the positive offshore converter system 2 controls the effective voltage value of the second AC bus 23 to be lower than the preset critical cutoff threshold until the wind turbine is connected to the grid. The offshore converter system is also used to increase the effective value of the voltage after the wind turbine is connected to the grid, so that the diode rectifier 24 will naturally conduct when the effective value of the voltage of the corresponding AC bus is not less than the preset critical cutoff threshold, thus completing the black start.

[0033] Specifically, during the voltage build-up operation, both the first modular multilevel converter 14 and the second modular multilevel converter 25 operate in a constant AC voltage and frequency (V / f) control mode. At this time, the offshore converter systems execute a voltage boosting sequence, smoothly increasing the voltage amplitude and frequency of their respective AC busbars according to a set slope until the wind turbine grid connection conditions are met. When both offshore converter systems meet the wind turbine grid connection conditions, wind turbine grid connection begins. For the positive offshore converter system 2, during the voltage build-up process, it is also necessary to control the effective voltage value of its corresponding second AC busbar 23 to be lower than a preset critical cutoff threshold to ensure that the diode rectifier 24 can maintain reverse cutoff during the black start phase.

[0034] It should be noted that the aforementioned preset critical cutoff threshold is determined based on the second DC output voltage of the positive electrode offshore converter system 2. Therefore, the condition that the effective voltage value of the second AC bus 23 must meet during the voltage build-up phase can be expressed by the following formula: In the formula, This indicates the effective voltage value of the second AC bus 23. This represents the second DC output voltage of the positive electrode marine converter system 2.

[0035] Specifically, after voltage establishment, it indicates that the first modular multilevel converter 14 and the second modular multilevel converter 25 have established a stable offshore AC grid. At this time, the wind turbines of the first offshore wind farm 11 and the second offshore wind farm 21 can be gradually connected to the grid and output stable active power. Subsequently, as the active power increases, the second modular multilevel converter 25 can control the effective voltage value of the second AC bus 23 to gradually increase until the natural conduction condition of the diode rectifier 24 is met, causing the diode rectifier 24 to conduct naturally. After the diode rectifier 24 conducts naturally, it connects in parallel with the second modular multilevel converter 25 to jointly undertake the power transmission task of the second offshore wind farm 21, completing the black start. The natural conduction condition of the diode rectifier 24 is as follows: Preferred, from Figure 1 As can be seen, this invention can fully adapt to the geographically dispersed nature of deep-sea wind energy resources. It sets up the positive electrode offshore converter system 2, the negative electrode offshore converter system 1, and their corresponding offshore wind farms using a site-specific, independent construction mode, effectively avoiding the engineering challenges of building a single giant converter platform. By introducing the offshore DC interconnection line 5, the electrical integrity of the true bipolar system is maintained while achieving a spatially distributed layout, significantly reducing the volume, weight, and construction cost of a single converter platform, greatly improving the flexibility and feasibility of project implementation.

[0036] Preferably, the present invention utilizes the active grid-connection characteristics of the small-capacity second modular multilevel converter 25 to solve the inherent defect of the traditional pure DR scheme in being unable to establish the initial AC voltage at sea, and realizes the autonomous black start and stable grid connection of offshore wind farms constructed at separate sites.

[0037] In a preferred embodiment, the negative electrode offshore converter system 1 further includes: The first offshore wind farm 11 and the first AC convergence submarine cable 12; The output end of the first offshore wind farm 11 is connected to the first AC busbar 13 via the first AC collecting submarine cable 12; The AC side of the first modular multilevel converter 14 is connected to the first AC bus 13; The positive DC terminal of the DC side of the first modular multilevel converter 14 is connected to the common point of the marine neutral line on the marine DC connection line 5, and the negative DC terminal of the DC side of the first modular multilevel converter 14 serves as the negative DC output terminal. Specifically, the first offshore wind farm 11 is connected to the first AC combiner bus 13, and the AC side of the first modular multilevel converter 14 is connected to the first AC combiner bus 13. On the DC side, the positive DC terminal of the first modular multilevel converter 14 is connected to the common point of the offshore neutral line, and the negative DC terminal serves as the negative DC output terminal.

[0038] The first offshore wind farm 11 is used to output active power after the wind turbines are connected to the grid; The first AC busbar cable 12 is used to integrate the active power output from the first offshore wind farm 11 and transmit it to the first AC busbar.

[0039] In this preferred embodiment, the first offshore wind farm 11 and the first AC collection submarine cable 12 in the negative electrode offshore converter system 1 realize the output and collection of active power after the wind turbine is connected to the grid.

[0040] In another preferred embodiment, the positive electrode marine converter system 2 further includes: The second offshore wind farm 21 and the second AC convergence submarine cable 22; The output end of the second offshore wind farm 21 is connected to the second AC busbar 23 via the second AC collection submarine cable 22; The AC side of the diode rectifier 24 and the AC side of the second modular multilevel converter 25 are respectively connected to the second AC bus 23; The positive DC terminal of the diode rectifier 24 is connected to the positive DC terminal of the second modular multilevel converter 25 to form a positive DC output terminal; The DC negative terminal of the diode rectifier 24 and the DC negative terminal of the second modular multilevel converter 25 are both connected to the common point of the marine neutral line on the marine DC tie line 5. Specifically, the second offshore wind farm 21 is connected to the second AC busbar 23 via an AC collecting submarine cable. The AC side of the diode rectifier 24 and the AC side of the second modular multilevel converter 25 are connected in parallel to the second AC busbar 23. On the DC side, the positive DC terminal of the diode rectifier 24 is connected to the positive DC terminal of the second modular multilevel converter 25 to form a positive DC output terminal, and the negative DC terminal is connected to the common point of the offshore neutral line.

[0041] The second offshore wind farm 21 is used to output active power after the wind turbines are connected to the grid; The second AC busbar 22 is used to integrate the active power output from the second offshore wind farm 21 and transmit it to the second AC busbar.

[0042] It should be noted that the rated capacity of the second modular multilevel converter 25 is significantly smaller than the total installed capacity of the second offshore wind farm 21. It is used to provide AC voltage support and reactive power compensation, while the diode rectifier 24 undertakes the main task of active power transmission.

[0043] In this preferred embodiment, the second offshore wind farm 21 and the second AC collection submarine cable 22 in the positive offshore converter system 2 realize the output and collection of active power.

[0044] In another preferred embodiment, the marine converter system is further used for: Before performing voltage build-up operation on the AC bus, close the AC side circuit breaker of the first modular multilevel converter 14 to connect the first offshore wind farm 11 with the first modular multilevel converter 14. Close the AC side circuit breaker of the second modular multilevel converter 25 to connect the second offshore wind farm 21 with the second modular multilevel converter 25; Unlock the first modular multilevel converter 14 and the second modular multilevel converter 25 to generate voltage on the first AC bus 13 and the second AC bus 23.

[0045] Specifically, after the uncontrolled pre-charging is completed, the substation controllers inside the negative electrode offshore converter system 1 and the positive electrode offshore converter system 2 control the AC side circuit breakers corresponding to the first modular multilevel converter 14 and the second modular multilevel converter 25 to close, and at the same time control the first modular multilevel converter 14 and the second modular multilevel converter 25 to unlock.

[0046] In this preferred embodiment, before the pressure build-up operation is performed after the uncontrolled pre-charging of the offshore converter system is completed, the AC side circuit breakers corresponding to the first modular multilevel converter 14 are closed, and then the first modular multilevel converter 14 and the first modular multilevel converter 14 are unlocked.

[0047] In another preferred embodiment, it further includes: a true bipolar DC transmission network 3; Specifically, both the first modular multilevel converter 14 and the second modular multilevel converter 25 draw power from the land side through the true bipolar DC transmission network 3.

[0048] The true bipolar DC transmission network 3 includes: a positive DC line 31, a neutral return line 32, and a negative DC line 33. The positive DC line 31 is connected to the positive DC output terminal of the positive offshore converter system 2; The negative DC line 33 is connected to the negative DC output terminal of the negative offshore converter system 1. The neutral line return line 32 is connected to the marine neutral line common point and the onshore converter system 4, respectively. Specifically, the positive DC line 31 and the negative DC line 33 are respectively connected to the corresponding polarity terminals of the marine and onshore converter systems 4, and the neutral line return line 32 is connected to the common point of the marine neutral line and the onshore neutral point.

[0049] The negative DC line 33 is used to transmit the active power corresponding to the first offshore wind farm 11 to the onshore converter system 4; The positive DC line 31 is used to transmit the active power corresponding to the second offshore wind farm 21 to the onshore converter system 4; The neutral return line 32 is used to provide a current loop.

[0050] Preferably, considering that the true bipolar hybrid DC transmission system for offshore wind power proposed in this invention adopts a true bipolar main connection method, in the true bipolar DC transmission network 3, the neutral return line 32 connects the neutral point of the offshore converter system and the neutral point of the onshore converter system 4, providing independent current return paths for the positive and negative poles. When there is an imbalance in the transmitted active power between the positive offshore converter system 2 and the negative offshore converter system 1, the unbalanced DC current generated between the poles directly flows back to the onshore neutral point via the neutral return line 32, achieving complete decoupling of the positive and negative poles in terms of control and operation. This structure avoids the influence of unbalanced current on the voltage regulation of the other pole, achieving complete decoupling of the positive and negative poles in terms of control and operation.

[0051] Preferably, the structure of the true bipolar hybrid DC transmission system for offshore wind power enables fault isolation and redundant operation: when a single-pole ground fault occurs on the positive DC line 31, the positive offshore converter system 2 is locked out. At this time, the negative offshore converter system 1 transmits power to the land through the negative DC line 33, and its return current returns to the offshore side through the neutral return line 32. Due to the operating characteristics of the true bipolar structure, the overvoltage or overcurrent generated by the positive fault will not couple to the negative system, thereby ensuring that the negative offshore wind farm can continue to output full active power without interference, achieving uninterrupted power supply support at the system level.

[0052] Preferably, by employing a true bipolar topology in conjunction with the neutral return line 32, the positive and negative poles of the offshore wind power true bipolar hybrid DC transmission system possess a completely independent operating foundation at the electrical level. In the event of a DC unipolar ground fault, the faulty pole can be promptly isolated, while the healthy pole can continue to operate at rated capacity using the neutral return line 32 as a current return path. This design ensures that even under extreme operating conditions, the entire system can retain at least 50% of its active power transmission capacity, significantly improving the operational robustness of the transmission channel.

[0053] In this preferred embodiment, the positive electrode marine converter system 2 and the onshore converter system 4 are connected by a true bipolar DC transmission network 3, as are the negative electrode marine converter system 1 and the onshore converter system 4.

[0054] In another preferred embodiment, the onshore converter system 4 includes: Positive pole onshore modular multilevel converter 41, negative pole onshore modular multilevel converter 42, onshore AC bus 43 and onshore AC main grid 44; The DC side of the positive onshore modular multilevel converter 41 and the DC side of the negative onshore modular multilevel converter 42 are both connected to the neutral return line 32. The AC side of the positive onshore modular multilevel converter 41 and the AC side of the negative onshore modular multilevel converter 42 are both connected to the onshore AC main grid 44 through the onshore AC bus 43. Specifically, the DC side of the positive onshore modular multilevel converter 41 and the DC side of the negative onshore modular multilevel converter 42 are connected in series. The common connection point of their DC series connection forms the onshore neutral point and is connected to the neutral return line 32. The positive DC terminal of the positive onshore modular multilevel converter 41 is connected to the positive DC line 31, and the negative DC terminal of the negative onshore modular multilevel converter 42 is connected to the negative DC line 33. The AC sides of the positive onshore modular multilevel converter 41 and the negative onshore modular multilevel converter 42 are respectively connected to the onshore AC main grid 44.

[0055] The positive onshore modular multilevel converter 41 is used to invert the active power transmitted by the positive DC line 31 from DC to AC. The negative pole onshore modular multilevel converter 42 is used to invert the active power transmitted by the negative pole DC line 33 from DC to AC. The onshore AC bus 43 is used to collect the active power after inverter and transmit it to the onshore AC main grid 44; The onshore AC main power grid 44 is used to receive all the active power after inversion.

[0056] Preferably, during the black start phase, after starting the onshore converter system 4, the positive onshore modular multilevel converter 41 and the negative onshore modular multilevel converter 42 are unlocked, and constant DC voltage control is adopted to establish and stabilize the positive DC bus voltage and the negative DC bus voltage to their respective rated values, providing initial DC voltage support for the entire offshore wind power true dual-stage hybrid DC transmission system.

[0057] In this preferred embodiment, active power inversion and transmission are achieved through the positive pole onshore modular multilevel converter 41, the negative pole onshore modular multilevel converter 42, the onshore AC bus 43, and the onshore AC main grid 44 in the onshore converter system 4.

[0058] In another preferred embodiment, the step of performing a voltage build-up operation on the AC busbar after the uncontrolled pre-charging is completed to achieve grid connection of the wind turbine includes: Obtain the rated voltage amplitude and rated frequency of the AC busbar; Using a boost timing sequence, the voltage and frequency of the AC bus are increased at a preset slope until the voltage amplitude of the AC bus reaches the rated voltage amplitude and the frequency of the AC bus reaches the rated frequency.

[0059] Specifically, during voltage build-up, the AC side circuit breakers of the first modular multilevel converter 14 and the second modular multilevel converter 25 are closed, and the first modular multilevel converter 14 and the second modular multilevel converter 25 are unlocked. Subsequently, both the first modular multilevel converter 14 and the second modular multilevel converter 25 are operated in a constant AC voltage and frequency (V / f) control mode. The first modular multilevel converter 14 and the second modular multilevel converter 25 independently or synchronously execute the voltage boosting sequence, smoothly increasing the voltage amplitude and frequency of the corresponding first AC bus 13 and second AC bus 23 according to the set slope until the wind turbine grid connection conditions are met, at which point the wind turbine is connected to the grid.

[0060] It should be noted that the wind turbine grid connection can only begin after both the positive pole offshore converter system 2 and the negative pole offshore converter system 1 meet the conditions for wind turbine grid connection.

[0061] Preferably, the above-mentioned grid connection conditions for wind turbines are as follows: for voltage amplitude, the voltage amplitude at the grid connection point is required to be stable within ±5% of the rated voltage amplitude. For example, when the rated voltage amplitude of the first AC bus 13 or the second AC bus 23 is 66kV, the voltage amplitude needs to be maintained within the range of [62.7, 69.3]kV.

[0062] Regarding frequency, the grid connection frequency is required to be stable within ±0.2Hz of the rated frequency. For example, when the rated frequency is 50Hz, the frequency needs to be maintained within the range of [49.8, 50.2]Hz.

[0063] It should be noted that for the positive electrode offshore converter system 2, during voltage build-up, it is necessary to constantly monitor whether the effective voltage value of the second AC bus 23 is less than the preset critical cutoff threshold. If not, the control command of the second modular multilevel converter 25 needs to be adjusted to limit the effective voltage value of the second AC bus 23 from being less than the preset critical cutoff threshold, so as to ensure that the diode rectifier 24 remains in reverse cutoff state during voltage build-up. For the negative electrode offshore converter system 1, after the first modular multilevel converter 14 is unlocked, it operates in V / f control mode, that is, smoothly increases the effective voltage value of the first AC bus 13 to the corresponding rated voltage amplitude according to the preset slope, and then the offshore AC power grid corresponding to the negative electrode offshore converter system 1 is stably established.

[0064] In this preferred embodiment, after the uncontrolled pre-charging is completed, the voltage of the AC bus is built up until the conditions for wind turbine grid connection are met, and the wind turbine grid connection is started and realized.

[0065] In another preferred embodiment, the negative electrode marine converter system 1 is further used for: After black start, the actual value of the first AC voltage of the first AC bus and the actual value of the first AC current of the first modular multilevel converter 14 are obtained. Specifically, for the first modular multilevel converter 14, in the steady-state control stage after black start, an independent constant AC voltage and frequency (V / f) grid control is used for steady-state coordinated control.

[0066] Based on the rated voltage amplitude and rated frequency, a first AC voltage amplitude reference value and a first reference phase angle are generated for the first modular multilevel converter 14. Specifically, the rated voltage amplitude corresponding to the first AC bus 13 is directly used as the d-axis component reference value of the first AC voltage amplitude reference value, and the q-axis component of the first AC voltage amplitude reference value is set to 0; at the same time, the rated frequency is converted into the rated angular frequency and input to the voltage-controlled oscillator (VCO) to generate the first reference phase angle.

[0067] Specifically, based on the rated voltage amplitude and rated frequency corresponding to the first AC bus 13, a first reference phase angle and a first AC voltage amplitude reference value are generated, wherein the q-axis component of the first AC voltage amplitude reference value is 0.

[0068] Based on the actual value of the first AC voltage and the reference value of the first AC voltage amplitude, the first current reference value of the first modular multilevel converter 14 is calculated. Specifically, the difference between the actual value of the first AC voltage and the reference value of the first AC voltage amplitude is compared and input to the corresponding AC voltage outer loop PI controller to obtain the first current reference value. The first current reference value includes the d-axis current reference value and the q-axis current reference value of the first modular multilevel converter 14. The control expression for the AC voltage outer loop regulation control corresponding to the first modular multilevel converter 14 is as follows: In the formula, This represents the d-axis current reference value in the first current reference value. This represents the proportional gain of the outer loop PI controller for the d-axis voltage corresponding to the first modular multilevel converter 14. This represents the integral coefficient of the outer loop PI controller for the d-axis voltage corresponding to the first modular multilevel converter 14. The d-axis component represents the first AC voltage amplitude reference value. This represents the actual d-axis component of the actual value of the first AC voltage. This represents the q-axis current reference value in the first current reference value. This represents the proportional gain of the outer loop PI controller corresponding to the q-axis voltage of the first modular multilevel converter 14. This represents the integral coefficient of the outer loop PI controller for the q-axis voltage corresponding to the first modular multilevel converter 14. This represents the q-axis component (set to 0) in the first AC voltage amplitude reference value. This represents the actual q-axis component of the actual value of the first AC voltage.

[0069] The first AC voltage reference value of the first modular multilevel converter 14 is calculated based on the first reference phase angle, the actual value of the first AC current, and the first current reference value. Specifically, based on the first reference phase angle, the actual value of the first AC current, and the first current reference value, the first AC voltage reference value of the first modular multilevel converter 14 is generated through AC current inner loop adjustment. This first AC voltage reference value also includes the corresponding d-axis and q-axis components. The expression for the AC current inner loop control corresponding to the first modular multilevel converter 14 is as follows: In the formula, This represents the d-axis component of the first AC voltage reference value. This represents the proportional gain of the inner loop PI controller for the d-axis current corresponding to the first modular multilevel converter 14. This represents the integral coefficient of the inner loop PI controller for the d-axis current corresponding to the first modular multilevel converter 14. This represents the d-axis component of the actual value of the first alternating current. This represents the system angular frequency of the negative pole offshore converter system 1. This represents the equivalent grid-connected inductance of the first modular multilevel converter 14. This represents the q-axis component of the actual value of the first alternating current. This represents the q-axis component of the first AC voltage reference value. This represents the proportional gain of the inner loop PI controller for the q-axis current corresponding to the first modular multilevel converter 14. This represents the integral coefficient of the q-axis current inner loop PI controller corresponding to the first modular multilevel converter 14.

[0070] Based on the first AC voltage reference value, generate the first trigger pulse for the switching device of each sub-module in the first modular multilevel converter 14; Based on the first trigger pulse, the switching devices of each sub-module in the first modular multilevel converter 14 are switched on and off.

[0071] Specifically, the three-phase upper and lower arm currents of the first modular multilevel converter 14 are collected, the internal circulating current component is extracted and circulating current suppression control is performed. The generated circulating current suppression voltage component is superimposed with the first AC voltage reference value, and combined with the upper and lower arm voltages of the first modular multilevel converter 14, the d-axis modulation ratio and q-axis modulation ratio of the first modular multilevel converter 14 are generated. Then, the d-axis modulation ratio and q-axis modulation ratio of the first modular multilevel converter 14 are transformed to the abc coordinate system through dq / abc coordinate transformation, and then a first trigger pulse is generated by the modulation module. Based on the first trigger pulse, the switching devices of the sub-module are switched to provide stable voltage and frequency support for the first offshore wind farm 11, so that the negative pole offshore converter system 1 can independently undertake the active power transmission of the first offshore wind farm 11.

[0072] An illustrative diagram of the steady-state coordinated control strategy of the first modular multilevel converter 14 is shown below. Figure 2 As shown, Figure 2 "in "Indicates the first AC voltage reference value, " "This represents the d-axis component of the first AC voltage reference value." "This represents the q-axis component of the first AC voltage reference value." "This represents the actual value of the upper arm current of phase x of the first modular multilevel converter 14." "This represents the actual value of the lower arm current of phase x of the first modular multilevel converter 14, where x represents phases a, b, and c." "This indicates the d-axis modulation ratio of the first modular multilevel converter 14," "" indicates the q-axis modulation ratio of the first modular multilevel converter 14.

[0073] Specifically, the aforementioned and The current base signal obtained by real-time sampling is input to the circulating current control module inside the first modular multilevel converter 14 to extract the internal circulating current component and perform subsequent circulating current suppression adjustment.

[0074] In this preferred embodiment, the first modular multilevel converter 14 of the negative electrode offshore converter system 1 is subjected to steady-state coordinated control using independent constant AC voltage and frequency (V / f) control.

[0075] In another preferred embodiment, the positive electrode marine converter system 2 is further used for: After black start, the second DC output voltage of the positive offshore converter system 2, the second stable voltage effective value of the second AC bus 23, the second total active power value of the second offshore wind farm 21, the second AC voltage actual value of the second AC bus 23, and the second AC current actual value of the second modular multilevel converter 25 are obtained. Specifically, for the positive electrode marine converter system 2, during the steady-state control phase, an improved cooperative power distribution constant AC voltage and frequency (V / f) control is adopted.

[0076] Based on the second DC output voltage, the second stable voltage RMS value, and the second total active power value, the second AC voltage amplitude reference value of the second modular multilevel converter 25 is calculated. Specifically, the second total active power of the second wind farm satisfies the following allocation model: In the formula, This represents the second total active power value. This indicates the active power of diode rectifier 24. This indicates the active power of the second modular multilevel converter 25.

[0077] The active power expression of diode rectifier 24 is as follows: In the formula, This represents the equivalent internal resistance of diode rectifier 24.

[0078] Therefore, when the second modular multilevel converter 25 adopts constant AC voltage and frequency (V / f) control with coordinated power distribution, it calculates the active power of the second modular multilevel converter 25 based on the second total active power value, combined with the above distribution model, the active power expression of the diode rectifier 24, and the rated voltage amplitude. Then, based on this, it obtains the second AC voltage amplitude reference value by combining the preset base voltage reference value.

[0079] Specifically, the q-axis component of the second AC voltage amplitude reference value used for outer loop control is set to 0, while the d-axis component of the second AC voltage amplitude reference value is obtained by superimposing the preset base voltage reference value with the additional droop voltage compensation.

[0080] The additional droop voltage compensation is obtained by calculating the product of the active power droop control coefficient and the active power of the second modular multilevel converter 25. This compensation is then superimposed on the second AC voltage amplitude reference value to form the d-axis component of the second AC voltage amplitude reference value.

[0081] The second reference phase angle of the second modular multilevel converter 25 is generated according to the rated frequency; Specifically, a second reference phase angle is generated based on the aforementioned rated frequency and the voltage-controlled oscillator (VCO).

[0082] The second current reference value of the second modular multilevel converter 25 is calculated based on the second AC voltage amplitude reference value and the second AC voltage actual value. Specifically, based on the second AC voltage amplitude reference value and the actual value of the second AC voltage, a second current reference value for the second modular multilevel converter 25 is generated through AC voltage outer loop adjustment. This second current reference value includes d-axis and q-axis components. The AC voltage outer loop control expression corresponding to the second modular multilevel converter 25 is as follows: In the formula, This represents the d-axis component of the second current reference value. This represents the proportional gain of the outer loop PI controller for the d-axis voltage corresponding to the second modular multilevel converter 25. This represents the integral coefficient of the outer loop PI controller for the d-axis voltage corresponding to the second modular multilevel converter 25. This indicates the preset base voltage reference value. This represents the active power droop control coefficient, where " "This overall formula represents the d-axis component of the second AC voltage amplitude reference value." This represents the d-axis component of the actual value of the second AC voltage. This represents the q-axis component of the second current reference value. This represents the proportional gain of the outer loop PI controller corresponding to the q-axis voltage of the second modular multilevel converter 25. This represents the integral coefficient of the outer loop PI controller corresponding to the q-axis voltage of the second modular multilevel converter 25. Represents the Laplace operator. This represents the q-axis voltage reference value (set to 0) in the second AC voltage amplitude reference value. This represents the q-axis component of the actual value of the second AC voltage.

[0083] The second AC voltage reference value of the second modular multilevel converter 25 is calculated based on the second reference phase angle, the actual value of the second AC current, and the second current reference value. Specifically, based on the second reference phase angle, the actual value of the second AC current is transformed using the abc / dq coordinate system and combined with the second current reference value. This is then used in conjunction with the AC current inner-loop PI controller and the feedforward decoupling circuit to calculate the second AC voltage reference value. This second AC voltage reference value also includes d-axis and q-axis components. The expression for the AC current inner-loop control corresponding to the second modular multilevel converter 25 is as follows: In the formula, The d-axis component represents the second AC voltage reference value. This represents the proportional coefficient of the inner loop PI controller for the d-axis current corresponding to the second modular multilevel converter 25. This represents the integral coefficient of the inner loop PI controller for the d-axis current corresponding to the second modular multilevel converter 25. The d-axis component represents the actual value of the second alternating current. This represents the system angular frequency of the positive polarization offshore converter system 2. This represents the equivalent grid-connected inductance of the second modular multilevel converter 25. The q-axis component represents the second AC voltage reference value. This represents the proportional coefficient of the inner loop PI controller for the q-axis current corresponding to the second modular multilevel converter 25. This represents the integral coefficient of the q-axis current inner-loop PI controller corresponding to the second modular multilevel converter 25. The q-axis component represents the actual value of the second alternating current.

[0084] The second trigger pulse of each sub-module switching device in the second modular multilevel converter 25 is generated according to the second AC voltage reference value; Based on the second trigger pulse, the switching devices of each sub-module in the second modular multilevel converter 25 are switched on and off.

[0085] Specifically, the internal circulating current components of the three-phase upper and lower arm currents of the second modular multilevel converter 25 are extracted and circulating current suppression control is performed. The generated circulating current suppression voltage component is superimposed with the second AC voltage reference value, and combined with the upper and lower arm voltages of the second modular multilevel converter 25, the d-axis modulation ratio and q-axis modulation ratio of the second modular multilevel converter 25 are generated. Subsequently, based on the d-axis and q-axis modulation ratios of the second modular multilevel converter 25, a three-phase modulation wave is generated through dq / abc coordinate transformation. A specific modulation strategy (such as nearest level approximation NLM) is used to generate the second trigger pulses for the switching devices of each submodule in the upper and lower arms of the second modular multilevel converter 25. Thus, while controlling the offshore grid, the second AC voltage reference value is fine-tuned to force the large-capacity diode rectifier 24 to handle the majority of the active power.

[0086] Preferably, in the positive electrode offshore converter system 2, a large-capacity 12-pulse diode rectifier 24 is used to handle the majority of the active load, effectively replacing the expensive flexible DC converter valve of the same capacity, reducing the overall equipment investment cost, and significantly reducing the size and load requirements of the offshore converter platform. A parallel small-capacity second modular multilevel converter 25 provides the necessary AC voltage support and power regulation, achieving optimal hardware investment while maintaining the flexibility of flexible DC transmission control.

[0087] An illustrative diagram of the steady-state coordinated control strategy of the second modular multilevel converter 25 is shown below. Figure 3 As shown, Figure 3 "in "Indicates the second AC voltage reference value, " "This represents the d-axis component of the second AC voltage reference value." "This represents the q-axis component of the second AC voltage reference value." "This represents the actual value of the upper arm current of phase x of the second modular multilevel converter 25," "" represents the actual value of the lower arm current of phase x of the second modular multilevel converter 25, where x represents phases a, b, and c. "This indicates the d-axis modulation ratio of the second modular multilevel converter 25," "" indicates the q-axis modulation ratio of the second modular multilevel converter 25.

[0088] Specifically, and The current base signal obtained from real-time sampling is input to the circulating current control module inside the second modular multilevel converter 25 to extract the internal circulating current component and perform subsequent circulating current suppression adjustment.

[0089] Preferably, for the positive pole onshore modular multilevel converter 41 and the negative pole onshore modular multilevel converter 42 in the onshore converter system 4, completely symmetrical and independent constant DC voltage and constant reactive power control are adopted during the steady-state coordinated control stage. Since the control logic and mathematical model between the positive pole onshore modular multilevel converter 41 and the negative pole onshore modular multilevel converter 42 are completely consistent, the following gives their general control steps and expressions (the positive pole onshore modular multilevel converter 41 and the negative pole onshore modular multilevel converter 42 can independently execute the following control according to the physical quantities of their respective poles): First, based on the actual and reference values ​​of the DC voltage and reactive power of the corresponding pole of the onshore converter system 4, the outer loop controller generates the d-axis and q-axis reference values ​​of the AC current of the corresponding pole converter of the onshore converter system 4. The outer loop control expressions for the DC voltage and reactive power corresponding to the onshore converter system 4 are as follows: In the formula, This represents the reference value of the d-axis current for the corresponding pole converter in the onshore converter system 4. This represents the reference value of the q-axis current of the corresponding pole converter in the onshore converter system 4. This represents the proportional coefficient of the outer loop PI controller for the d-axis DC voltage of the corresponding pole converter in the onshore converter system 4. This represents the proportional gain of the outer loop PI controller for the q-axis DC voltage of the corresponding pole converter in the onshore converter system 4. This represents the integral coefficient of the outer loop PI controller for the d-axis DC voltage of the corresponding pole converter in onshore converter system 4. This represents the integral coefficient of the outer loop PI controller for the q-axis DC voltage of the corresponding pole converter in the onshore converter system 4. This indicates the reference value of the DC voltage for the corresponding pole of the onshore converter system 4. This represents the actual value of the pole-line DC voltage of the corresponding pole in onshore converter system 4. This represents the reference value of reactive power for the corresponding pole of the onshore converter system 4. This represents the actual reactive power value of the corresponding pole of the onshore converter system 4.

[0090] Subsequently, based on the actual AC voltage value at the grid connection point of onshore converter system 4, a reference phase angle for onshore converter system 4 is generated via a phase-locked loop. Then, based on the reference phase angle, the actual AC current value, and the d-axis and q-axis current reference values ​​of onshore converter system 4, an AC voltage reference value for the corresponding pole converter of onshore converter system 4 is generated via a current inner loop controller. Specifically, the AC voltage reference value for the corresponding pole converter of onshore converter system 4 includes d-axis and q-axis components. The inner loop control expression corresponding to onshore converter system 4 is as follows: In the formula, The d-axis component represents the AC voltage reference value of the corresponding pole converter in onshore converter system 4. The q-axis component represents the AC voltage reference value of the corresponding pole converter in onshore converter system 4. The d-axis component represents the actual AC voltage value at the four grid connection points of the onshore converter system. The q-axis component represents the actual AC voltage value at the four grid connection points of the onshore converter system. This represents the proportional gain of the inner loop PI controller for the d-axis current of the onshore converter system 4. This represents the proportional gain of the inner loop PI controller for the q-axis current of the onshore converter system 4. This represents the integral coefficient of the inner loop PI controller for the d-axis current corresponding to the onshore converter system 4. This represents the integral coefficient of the q-axis current inner-loop PI controller corresponding to the onshore converter system 4. The d-axis component represents the actual value of the AC current of the pole converter corresponding to pole 4 in the onshore converter system. The q-axis component represents the actual value of the AC current of the pole converter corresponding to pole 4 in the onshore converter system. This represents the system angular frequency of onshore converter system 4. This represents the equivalent grid-connected inductance of the corresponding pole converter in the onshore converter system 4.

[0091] Finally, the three-phase upper and lower arm currents of the corresponding pole converter of the onshore converter system 4 are collected, the internal circulating current components are extracted and circulating current suppression control is performed. The generated circulating current suppression voltage component is superimposed with the AC voltage reference value of the corresponding pole converter of the onshore converter system 4, and combined with the corresponding upper and lower arm voltages to generate the d-axis modulation ratio and q-axis modulation ratio of the corresponding pole converter of the onshore converter system 4. Based on the d-axis modulation ratio and q-axis modulation ratio of the corresponding pole converter of the onshore converter system 4, the modulation signal is sent to the modulation strategy module after inverse coordinate transformation to generate trigger pulses for the switching devices of each submodule in the upper and lower arms of the positive pole onshore modular multilevel converter 41 and the negative pole onshore modular multilevel converter 42. The switching devices are controlled according to these trigger pulses, thereby stabilizing the positive and negative DC voltages of the onshore converter system 4 and smoothly feeding offshore wind power into the onshore AC main grid 44.

[0092] The schematic diagram shows the control strategy flow of the positive and negative onshore converters in the onshore converter system 4. Figure 4 As shown, Figure 4 In this context, "PLL" stands for Phase-Locked Loop. "This indicates the AC grid-side voltage of the corresponding pole converter in the onshore converter system 4." "This represents the synchronization phase angle of the AC grid-side voltage extracted by the phase-locked loop." "This indicates the AC voltage reference value of the corresponding pole converter in onshore converter system 4." "This represents the d-axis component of the AC voltage reference value of the corresponding pole converter in onshore converter system 4." "This represents the q-axis component of the AC voltage reference value of the corresponding pole converter in onshore converter system 4." "This represents the actual value of the upper arm current of phase x of the pole converter corresponding to pole 4 in the onshore converter system." "This represents the actual value of the lower arm current of phase x of the pole converter corresponding to pole 4 in the onshore converter system, where x represents phases a, b, and c." "This indicates the d-axis modulation ratio of the corresponding pole converter in onshore converter system 4," "" indicates the q-axis modulation ratio of the corresponding pole converter in the onshore converter system 4.

[0093] Specifically, and The current basis signal obtained by real-time sampling is input to the circulating current control module inside the corresponding pole converter to extract the internal circulating current component and perform subsequent circulating current suppression adjustment.

[0094] In this preferred embodiment, the positive electrode marine converter system 2 achieves steady-state control through a constant AC voltage and frequency (V / f) control strategy for coordinated power distribution.

[0095] It should be noted that the device embodiments described above are merely illustrative. The modules described as separate components may or may not be physically separate, and the components shown as modules may or may not be physical modules; that is, they may be located in one place or distributed across multiple network units. Some or all of the modules can be selected to achieve the purpose of this embodiment according to actual needs. Furthermore, in the accompanying drawings of the device embodiments provided by this invention, the connection relationship between modules indicates that they have a communication connection, which can be implemented as one or more communication buses or signal lines. Those skilled in the art can understand and implement this without creative effort. The above schematic diagram is merely an example of a true bipolar hybrid DC transmission system for offshore wind power based on substation interconnection and does not constitute a limitation on a true bipolar hybrid DC transmission system for offshore wind power based on substation interconnection. It may include more or fewer components than illustrated, or combine certain components, or use different components.

[0096] Based on the above system implementation examples, the present invention provides corresponding method implementation examples; Indicative, such as Figure 5 As shown, this invention provides a control method for a true bipolar hybrid DC transmission system for offshore wind power, applicable to the offshore converter system in any of the above-mentioned true bipolar hybrid DC transmission systems for offshore wind power based on substation interconnection. The control method includes: Step S101: During the black start phase, power is drawn from the onshore converter system 4 to perform uncontrolled pre-charging of the internal sub-module capacitors of the modular multilevel converter; after the uncontrolled pre-charging is completed, the voltage of the AC busbar is built up to achieve grid connection of the wind turbine. Specifically, when the offshore converter system is a negative offshore converter system 1, the modular multilevel converter is a first modular multilevel converter 14, and the AC bus is a first AC bus 13; when the offshore converter system is a positive offshore converter system 2, the modular multilevel converter is a second modular multilevel converter 25, and the AC bus is a second AC bus 23. Furthermore, during the voltage build-up operation, the positive offshore converter system 2 controls the effective voltage value of the second AC bus 23 to be lower than the preset critical cutoff threshold until the wind turbine is connected to the grid. Specifically, during the black start phase, the offshore converter system draws power from the onshore converter system 4 on the land side to complete uncontrolled pre-charging, and then connects the wind turbine to the grid through pressure building operation.

[0097] Step S102: Increase the effective voltage value so that the diode rectifier 24 will naturally conduct when the effective voltage value of the corresponding AC bus is not less than the preset critical cutoff threshold, thus completing the black start.

[0098] Specifically, after the wind turbine is connected to the grid, the effective voltage value of the second AC bus 23 is gradually increased, which allows the diode rectifier 24 to conduct naturally, thereby realizing the black start of the system.

[0099] The above are preferred embodiments of the present invention. It should be noted that, for those skilled in the art, several improvements and modifications can be made without departing from the principle 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 true bipolar hybrid DC transmission system for offshore wind power based on substation interconnection, characterized in that, include: Offshore converter systems, offshore DC transmission lines, and onshore converter systems; The offshore converter system includes a positive offshore converter system and a negative offshore converter system; the positive offshore converter system and the negative offshore converter system are connected via an offshore DC tie line; the negative offshore converter system includes a first modular multilevel converter and a first AC busbar; the positive offshore converter system includes a second modular multilevel converter, a second AC busbar, and a diode rectifier; The offshore converter system is used to draw power from the onshore converter system during the black start phase to perform uncontrolled pre-charging of the internal sub-module capacitors of the modular multilevel converter; after the uncontrolled pre-charging is completed, the voltage of the AC busbar is built up to achieve grid connection of the wind turbine. Specifically, when the offshore converter system is a negative-pole offshore converter system, the modular multilevel converter is the first modular multilevel converter, and the AC bus is the first AC bus; when the offshore converter system is a positive-pole offshore converter system, the modular multilevel converter is the second modular multilevel converter, and the AC bus is the second AC bus. Furthermore, during the voltage build-up operation, the positive-pole offshore converter system controls the effective voltage value of the second AC bus to be lower than the preset critical cutoff threshold until the wind turbine is connected to the grid. The offshore converter system is also used to increase the effective value of the voltage after the wind turbine is connected to the grid, so that the diode rectifier will naturally conduct when the effective value of the voltage of the corresponding AC bus is not less than the preset critical cutoff threshold, thus completing the black start.

2. The true bipolar hybrid DC transmission system for offshore wind power based on substation interconnection as described in claim 1, characterized in that, The negative pole marine converter system also includes: The first offshore wind farm and the first AC convergence submarine cable; The output end of the first offshore wind farm is connected to the first AC busbar via the first AC collecting submarine cable. The AC side of the first modular multilevel converter is connected to the first AC busbar. The positive DC terminal of the DC side of the first modular multilevel converter is connected to the common point of the marine neutral line on the marine DC link, and the negative DC terminal of the DC side of the first modular multilevel converter serves as the negative DC output terminal. The first offshore wind farm is used to output active power after the wind turbines are connected to the grid; The first AC collecting submarine cable is used to integrate the active power output from the first offshore wind farm and transmit it to the first AC busbar.

3. The true bipolar hybrid DC transmission system for offshore wind power based on substation interconnection as described in claim 2, characterized in that, The positive polar ocean converter system also includes: The second offshore wind farm and the second AC convergence submarine cable; The output end of the second offshore wind farm is connected to the second AC busbar via the second AC convergence submarine cable; The AC side of the diode rectifier and the AC side of the second modular multilevel converter are respectively connected to the second AC busbar; The positive DC terminal of the diode rectifier is connected to the positive DC terminal of the second modular multilevel converter to form a positive DC output terminal. The DC negative terminal of the diode rectifier and the DC negative terminal of the second modular multilevel converter are both connected to the common point of the marine neutral line on the marine DC link line. The second offshore wind farm is used to output active power after the wind turbines are connected to the grid; The second AC collecting submarine cable is used to integrate the active power output from the second offshore wind farm and transmit it to the second AC busbar.

4. The true bipolar hybrid DC transmission system for offshore wind power based on substation interconnection as described in claim 3, characterized in that, The marine converter system is also used for: Before the voltage build-up operation of the AC busbar, the AC side circuit breaker of the first modular multilevel converter is closed to connect the first offshore wind farm to the first modular multilevel converter. Close the AC side circuit breaker of the second modular multilevel converter to connect the second offshore wind farm with the second modular multilevel converter; Unlock the first modular multilevel converter and the second modular multilevel converter to generate voltage on the first AC bus and the second AC bus.

5. A true bipolar hybrid DC transmission system for offshore wind power based on substation interconnection as described in claim 4, characterized in that, Also includes: True bipolar DC transmission network; The true bipolar DC transmission network includes: a positive DC line, a neutral return line, and a negative DC line; The positive DC line is connected to the positive DC output terminal of the positive offshore converter system. The negative DC line is connected to the negative DC output terminal of the negative offshore converter system. The neutral line return line is connected to the marine neutral line common point and the onshore converter system, respectively. The negative DC line is used to transmit the active power corresponding to the first offshore wind farm to the onshore converter system. The positive DC line is used to transmit the active power corresponding to the second offshore wind farm to the onshore converter system; The neutral return line is used to provide a current loop.

6. A true bipolar hybrid DC transmission system for offshore wind power based on substation interconnection as described in claim 5, characterized in that, The onshore converter system includes: Positive pole onshore modular multilevel converter, negative pole onshore modular multilevel converter, onshore AC bus and onshore AC main power grid; The DC side of the positive onshore modular multilevel converter and the DC side of the negative onshore modular multilevel converter are both connected to the neutral return line. The AC side of the positive pole onshore modular multilevel converter and the AC side of the negative pole onshore modular multilevel converter are both connected to the onshore AC main power grid through the onshore AC bus. The positive onshore modular multilevel converter is used to invert the active power transmitted by the positive DC line from DC to AC. The negative pole onshore modular multilevel converter is used to invert the active power transmitted by the negative pole DC line from DC to AC. The onshore AC bus is used to collect the active power after inversion and transmit it to the onshore AC main grid. The onshore AC main power grid is used to receive all the active power after inversion.

7. A true bipolar hybrid DC transmission system for offshore wind power based on substation interconnection as described in claim 6, characterized in that, The step of building up the voltage of the AC busbar after the uncontrolled pre-charging is completed to achieve grid connection of the wind turbine includes: Obtain the rated voltage amplitude and rated frequency of the AC busbar; Using a boost timing sequence, the voltage and frequency of the AC bus are increased at a preset slope until the voltage amplitude of the AC bus reaches the rated voltage amplitude and the frequency of the AC bus reaches the rated frequency.

8. A true bipolar hybrid DC transmission system for offshore wind power based on substation interconnection as described in claim 7, characterized in that, The negative electrode marine converter system is also used for: After black start, the actual value of the first AC voltage of the first AC bus and the actual value of the first AC current of the first modular multilevel converter are obtained. Based on the rated voltage amplitude and rated frequency, a first AC voltage amplitude reference value and a first reference phase angle are generated for the first modular multilevel converter. Based on the actual value of the first AC voltage and the reference value of the first AC voltage amplitude, the first current reference value of the first modular multilevel converter is calculated. The first AC voltage reference value of the first modular multilevel converter is calculated based on the first reference phase angle, the actual value of the first AC current, and the first current reference value. Based on the first AC voltage reference value, generate the first trigger pulse for the switching device of each sub-module in the first modular multilevel converter; Based on the first trigger pulse, the switching devices of each sub-module in the first modular multilevel converter are switched on and off.

9. A true bipolar hybrid DC transmission system for offshore wind power based on substation interconnection as described in claim 8, characterized in that, The positive polarization offshore converter system is also used for: After black start, the second DC output voltage of the positive offshore converter system, the second stable voltage effective value of the second AC bus, the second total active power value of the second offshore wind farm, the second AC voltage actual value of the second AC bus, and the second AC current actual value of the second modular multilevel converter are obtained. Based on the second DC output voltage, the second stable voltage RMS value, and the second total active power value, the second AC voltage amplitude reference value of the second modular multilevel converter is calculated. The second reference phase angle of the second modular multilevel converter is generated according to the rated frequency; The second current reference value of the second modular multilevel converter is calculated based on the second AC voltage amplitude reference value and the actual value of the second AC voltage. The second AC voltage reference value of the second modular multilevel converter is calculated based on the second reference phase angle, the actual value of the second AC current, and the second current reference value. The second trigger pulse is generated for the switching devices of each sub-module in the second modular multilevel converter based on the second AC voltage reference value; Based on the second trigger pulse, the switching devices of each submodule in the second modular multilevel converter are switched on and off.

10. A control method for a true bipolar hybrid DC transmission system for offshore wind power, characterized in that, Applicable to the offshore converter system in the offshore wind power true bipolar hybrid DC transmission system based on substation interconnection as described in claims 1-9; The control method includes: During the black start phase, power is drawn from the onshore converter system to perform uncontrolled pre-charging of the internal sub-module capacitors of the modular multilevel converter; after the uncontrolled pre-charging is completed, the voltage of the AC bus is built up to achieve grid connection of the wind turbine. Specifically, when the offshore converter system is a negative-pole offshore converter system, the modular multilevel converter is the first modular multilevel converter, and the AC bus is the first AC bus; when the offshore converter system is a positive-pole offshore converter system, the modular multilevel converter is the second modular multilevel converter, and the AC bus is the second AC bus. Furthermore, during the voltage build-up operation, the positive-pole offshore converter system controls the effective voltage value of the second AC bus to be lower than the preset critical cutoff threshold until the wind turbine is connected to the grid. After the wind turbine is connected to the grid, the effective value of the voltage is increased so that the diode rectifier will naturally conduct when the effective value of the voltage on the corresponding AC bus is not less than the preset critical cutoff threshold, thus completing the black start.

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