Offshore wind power low-frequency alternating current sending-out system and operation method thereof

Abstract

The invention provides an offshore wind power low-frequency alternating current sending-out system, which belongs to the technical field of offshore wind power transmission and comprises an alternating current-alternating current converter connected with a low-frequency booster station and an alternating current power grid. The AC-AC converter comprises a rectification side and an inversion side; the rectification side comprises a hybrid sub-module type modular multilevel converter valve, a diode converter valve I and a diode converter valve II; the inverter side comprises a first half-bridge module multi-level converter valve, a second half-bridge module multi-level converter valve and a third half-bridge module multi-level converter valve. The combined topology of the diode converter valve and the half-bridge module multi-level converter valve is adopted, so that the use amount of high-price power electronic devices is reduced, and the equipment investment cost is reduced; meanwhile, the main system and the subsystem multiplex the direct-current connection loop, an independent loop does not need to be additionally built, and the system construction and operation and maintenance cost is further reduced. The invention further discloses an operation method of the offshore wind power low-frequency alternating current sending-out system.

Description

Technical Field

[0001] This invention relates to the field of offshore wind power transmission technology, and more specifically to an offshore wind power low-frequency AC transmission system; the invention also relates to the operation method of such an offshore wind power low-frequency AC transmission system. Background Technology

[0002] With the acceleration of the global energy transition, deep-sea wind power has become an important direction for renewable energy development due to its abundant wind energy resources and low environmental impact. However, the long-distance, large-capacity power transmission of deep-sea wind power has always constrained its large-scale development, and traditional AC power transmission methods are no longer applicable in this scenario. On the one hand, AC transmission has inherent defects such as large capacitive charging power, significant voltage drop, and limited transmission distance, which cannot meet the ultra-long-distance power transmission needs of deep-sea wind power. On the other hand, the loss and stability of AC transmission lines in the deep-sea environment are prominent, which seriously affects the efficiency and reliability of power transmission. At present, the industry-recognized feasible deep-sea wind power transmission technology paths mainly include two categories: flexible DC power transmission and low-frequency AC power transmission, which have become the core research directions in this field. While flexible DC power transmission technology can effectively solve the distance limitation problem of AC power transmission, it also presents significant engineering and economic challenges. This technology requires the construction of large and complex converter stations at sea, which not only incurs high economic investment costs but is also affected by the harsh climate and complex geological conditions of the deep sea, making the construction and installation of converter stations extremely difficult. Subsequent operation and maintenance also face numerous challenges. The cost and efficiency of transporting maintenance personnel and equipment at sea are high, and the operational safety risks are significant. Although academia and industry have continuously explored economic improvement measures for flexible DC solutions (such as optimizing converter station topology and reducing device costs) and intelligent operation and maintenance methods (such as remote monitoring and drone inspections) in recent years, the relevant technologies are still in the research and development or pilot stage and have not yet formed mature large-scale application solutions. The actual application effects have not met expectations, making it difficult to effectively solve the economic and practical problems of deep-sea wind power transmission. Low-frequency AC power transmission technology, as another important approach, also faces pressing issues. Traditional MMC back-to-back solutions for low-frequency AC power transmission require numerous power electronic devices and auxiliary equipment, resulting in high system costs, large equipment size and weight, which is unfavorable for compact layouts on offshore platforms and lacks economic viability. While AC-AC converters without DC links simplify the topology, they suffer from complex operating mechanisms and high control difficulty. Furthermore, the lack of a DC link prevents the installation of DC energy dissipation devices, making it difficult to quickly release excess energy during system faults or power fluctuations, easily leading to overvoltage and overcurrent damage to devices and severely impacting the safe and stable operation of the system. These shortcomings have resulted in less than ideal practical engineering implementation of low-frequency AC power transmission technology, hindering its widespread application in deep-sea wind power projects. In summary, existing flexible DC and low-frequency AC power transmission solutions both have technical or economic shortcomings, making it difficult to meet the actual needs of large-scale, low-cost, and highly reliable transmission of wind power from deep-sea areas. Therefore, it is urgent to propose new technical solutions. Summary of the Invention The primary objective of this invention is to overcome the shortcomings of the aforementioned background technology and to provide a low-frequency AC transmission system for offshore wind power.

[0003] The second objective of this invention is to provide an operating method for such an offshore wind power low-frequency AC transmission system.

[0004] To achieve the aforementioned primary objective, this invention proposes a technical approach that involves setting DC taps on the inverter-side MMC converter valve and constructing a synchronization scheme in conjunction with a hybrid submodule-type modular multilevel converter valve. This invention eliminates the need for extensive additional equipment; instead, it reuses the existing topology of the main power transmission circuit to economically and efficiently build a reverse power transmission circuit. This provides the synchronization voltage required for the startup and operation of grid-connected offshore wind turbines, ensuring system reliability and economy while reducing engineering implementation difficulty. It possesses significant engineering application value and offers a novel technical solution to the challenges of transmitting wind power from deep-sea areas.

[0005] The specific technical solution of the present invention is as follows: a low-frequency AC transmission system for offshore wind power, characterized in that: it includes an AC-AC converter connecting a low-frequency booster station and an AC power grid; the AC-AC converter includes a rectifier side and an inverter side; The rectifier side includes a hybrid sub-module type modular multilevel converter valve, a diode converter valve one, and a diode converter valve two; the hybrid sub-module type modular multilevel converter valve is connected to the DC output terminals of diode converter valve one and diode converter valve two, respectively. The inverter side includes a half-bridge module multilevel converter valve one, a half-bridge module multilevel converter valve two, and a half-bridge module multilevel converter valve three; the half-bridge module multilevel converter valve one is connected to the DC output terminals of the half-bridge module multilevel converter valve two and the half-bridge module multilevel converter valve three, respectively. The hybrid sub-module modular multilevel converter valve and the half-bridge module multilevel converter valve are DC connected; the diode converter valve one and the half-bridge module multilevel converter valve two are DC connected; the diode converter valve two and the half-bridge module multilevel converter valve three are DC connected.

[0006] In the above technical solution, the hybrid sub-module type modular multilevel converter valve includes a full-bridge sub-module and a half-bridge sub-module.

[0007] In the above technical solution, the offshore wind turbine configured in the low-frequency booster station is a grid-connected offshore wind turbine.

[0008] In the above technical solution, the low-frequency booster station is connected to the rectifier side via a low-frequency AC submarine cable.

[0009] The above technical solution also includes a control system; the control system is connected to the hybrid sub-module type modular multilevel converter valve through a first valve control device, connected to the first half-bridge module multilevel converter valve through a second valve control device, and connected to the second half-bridge module multilevel converter valve and the third half-bridge module multilevel converter valve through a third valve control device.

[0010] In the above technical solution, the AC-AC converter also includes a DC energy discharge circuit; the DC energy discharge circuit is connected in parallel with the rectifier side and the inverter side.

[0011] To achieve the second objective mentioned above, the technical solution of the present invention is: an operation method for an offshore wind power low-frequency AC transmission system, characterized by comprising the following steps: Step 1, Pre-charge stage: The AC power grid pre-charges the first, second, and third multilevel converter valves of the half-bridge module via charging resistors. After pre-charging, charging continues until the sub-module capacitor voltage reaches its rated value. This provides two levels of DC voltage for the DC system. Step 2, Low-frequency side charging stage: Unlock the hybrid submodule modular multilevel converter valve and perform AC side voltage frequency control on the hybrid submodule modular multilevel converter valve. The frequency is set to the low-frequency system frequency, and the slope of the voltage control curve is set to rise slowly to complete the charging of the low-frequency AC submarine cable and the low-frequency AC transformer of the low-frequency booster station. After the low-frequency booster station power system draws power, it completes the startup of the low-frequency booster station. The control value of the AC voltage, after rectification by diode converter valve one and diode converter valve two, is less than the DC rated value of half-bridge module multilevel converter valve two and half-bridge module multilevel converter valve three. At this time, power cannot be transmitted through diode converter valve one and diode converter valve two. Step 3, Wind turbine start-up and grid connection stage: The low-frequency booster station provides synchronous voltage for grid-connected offshore wind turbines, enabling them to be started and connected to the grid in sequence. Step 4, Power Forward Feed Switching Stage: When the power of the hybrid submodule modular multilevel converter valve is reversed, adjust the control parameters of the hybrid submodule modular multilevel converter valve, increase the AC side voltage of the hybrid submodule modular multilevel converter valve, and open the power supply channels of diode converter valve one and diode converter valve two; at the same time, increase the DC side voltage of the hybrid submodule modular multilevel converter valve, block the reverse power supply channel from half-bridge module multilevel converter valve one to the hybrid submodule modular multilevel converter valve, and complete the startup of the low-frequency AC transmission system.

[0012] Compared with the prior art, the present invention has the following advantages.

[0013] 1) Improved economic efficiency: The present invention adopts a combined topology of diode converter valve and half-bridge module multilevel converter valve, which reduces the use of high-priced power electronic devices and lowers equipment investment costs; at the same time, the main and subsystems reuse DC connection circuits, eliminating the need to build additional independent circuits, further reducing system construction and maintenance costs. 2) Reduced control complexity: Each component on the inverter side of this invention adopts an independent control strategy, which simplifies the overall control logic, reduces the control difficulty, and improves the stability and reliability of the system operation. 3) Functional completeness: The dual-function design of the hybrid sub-module type modular multilevel converter valve of this invention solves the problem of synchronous voltage supply and energy transmission for grid-connected wind turbines.

[0014] 4) High system safety: The DC energy discharge circuit configured in this invention fills the gap in the traditional solution where fault energy cannot be effectively discharged, thus improving the system's safety performance. 5) Strong engineering applicability: The topology and control strategy of this invention take into account the actual working conditions of offshore wind power, are compatible with grid-connected offshore wind turbines, and have flexible component configurations that can be adjusted according to grid requirements, making them easy to promote and apply in actual engineering projects. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of the structure of the present invention.

[0016] Figure 2 This is a topology diagram of a hybrid submodule type modular multilevel converter valve.

[0017] Figure 3 This is a connection diagram of the multilevel converter valve 1, multilevel converter valve 2, and multilevel converter valve 3 of the half-bridge module.

[0018] Figure 4 This is a schematic diagram of the control system.

[0019] Figure 5 This is a schematic diagram of the power flow direction during the power return phase of the black start process.

[0020] Figure 6 This is a schematic diagram of the power flow direction during the power forward transmission stage.

[0021] Figure 7 This is a schematic diagram of a DC energy discharge circuit.

[0022] Among them, 100-AC-AC converter, 110-rectifier side, 111-hybrid submodule type modular multilevel converter valve, 1111-full bridge submodule, 1112-half bridge submodule, 112-diode converter valve one, 113-diode converter valve two, 120-inverter side, 121-half bridge module multilevel converter valve one, 122-half bridge module multilevel converter valve two, 123-half bridge module multilevel converter valve three, 130-DC energy discharge circuit, 200-low frequency booster station, 210-grid-connected offshore wind turbine, 220-low frequency AC submarine cable, 300-AC power grid, 400-control system, 410-first valve control device, 420-second valve control device, 430-third valve control device.

[0023] Figure 1-4 In the above, the hybrid sub-module type modular multilevel converter valve 111 is HF-MMC0; diode converter valve one 112 is DR1, diode converter valve two 113 is DR2; half-bridge module multilevel converter valve one 121 is H-MMC0, half-bridge module multilevel converter valve two 122 is H-MMC1, and half-bridge module multilevel converter valve three 123 is H-MMC2. Detailed Implementation

[0024] The embodiments of the present invention will be described in detail below with reference to the accompanying drawings, but these descriptions are not intended to limit the invention and are merely illustrative. The advantages of the present invention will become clearer and easier to understand through this description.

[0025] Referring to the accompanying drawings, a low-frequency AC transmission system for offshore wind power is characterized by including an AC-AC converter 100 connecting a low-frequency booster station 200 and an AC power grid 300; the AC-AC converter 100 includes a rectifier side 110 and an inverter side 120. The rectifier side 110 includes a hybrid sub-module type modular multilevel converter valve 111, a diode converter valve one 112, and a diode converter valve two 113. The hybrid sub-module type modular multilevel converter valve 111 is connected to the DC output terminals of the diode converter valve one 112 and the diode converter valve two 113, respectively. That is, the diode converter valve one 112 and the diode converter valve two 113 form the basic topology of "hybrid sub-module type modular multilevel converter valve 111 + dual diode converter valve" on the rectifier side. The diode converter valve can simplify the power transmission path and reduce energy loss due to its unidirectional conduction characteristic.

[0026] The inverter side 120 includes a half-bridge module multilevel converter valve one 121, a half-bridge module multilevel converter valve two 122, and a half-bridge module multilevel converter valve three 123. The half-bridge module multilevel converter valve one 121 is connected to the DC output terminals of the half-bridge module multilevel converter valve two 122 and the half-bridge module multilevel converter valve three 123, respectively. That is, the half-bridge module multilevel converter valve two 122 and the half-bridge module multilevel converter valve three 123 form a topology of "half-bridge module multilevel converter valve one 121 + double half-bridge converter valve" on the inverter side. The design of the half-bridge module can improve the control flexibility and operational stability of the converter valve.

[0027] To achieve efficient power transmission and coordinated system functions, the rectifier side 110 and inverter side 120 of the AC-AC converter 100 form a DC electrical connection between the main system and the subsystem: the hybrid sub-module modular multilevel converter valve 111 is DC connected to the half-bridge module multilevel converter valve one 121; the diode converter valve one 112 is DC connected to the half-bridge module multilevel converter valve two 122; and the diode converter valve two 113 is DC connected to the half-bridge module multilevel converter valve three 123. This connection method can realize the energy interaction between the main system and the subsystem without the need to build an additional independent circuit.

[0028] The hybrid submodule type modular multilevel converter valve 111 includes a full-bridge submodule 1111 and a half-bridge submodule 1112; typically, the AC input line is connected to the bridge arm reactor, and then the half-bridge or full-bridge submodules are connected in sequence. Compared with the half-bridge submodule modular multilevel scheme, this invention replaces some of the full-bridge submodules with half-bridge submodules, meeting the system requirements for fault ride-through and reduced-voltage operation of the modular multilevel converter valve.

[0029] The hybrid submodule modular multilevel converter valve 111 has dual core functions: First, during the startup and normal operation of low-frequency wind farms, the hybrid submodule modular multilevel converter valve 111 can output a stable synchronization voltage, providing the necessary voltage support for the startup and operation of grid-connected offshore wind turbines 210, thus solving the problem of difficulty in obtaining synchronization voltage in traditional solutions; Second, when the low-frequency wind farm transmits power to the AC grid 300, the hybrid submodule modular multilevel converter valve 111 serves as an energy transmission channel, realizing efficient transmission of wind power energy without the need for additional independent energy transmission components, thus simplifying the system structure.

[0030] The offshore wind turbine configured in the low-frequency booster station 200 is a grid-connected offshore wind turbine 210. Compared with grid-connected wind turbines, the grid-connected offshore wind turbine 210 is still the mainstream in engineering construction. However, this invention, through a specific topology design, can meet the operating requirements of the grid-connected offshore wind turbine 210 and expand the applicable scenarios of the system.

[0031] The low-frequency booster station 200 is connected to the rectifier side 110 via a low-frequency AC submarine cable 220.

[0032] It also includes a control system 400; the control system 400 is connected to the hybrid sub-module type modular multilevel converter valve 111 via a first valve control device 410, to the half-bridge module multilevel converter valve one 121 via a second valve control device 420, and to the half-bridge module multilevel converter valve two 122 and the half-bridge module multilevel converter valve three 123 via a third valve control device 430 respectively; the inverter side 120 in the AC-AC converter 100 of the present invention adopts an independent control mode, that is, the half-bridge module multilevel converter valve one 121 and the half-bridge module Multilevel converter valve 2 (122) and half-bridge module multilevel converter valve 3 (123) are controlled independently. This design allows each component to adjust its operating parameters according to its own functional requirements and operating mode. For example, half-bridge module multilevel converter valve 1 (121) focuses on the core control of power conversion, while half-bridge module multilevel converter valve 2 (122) and half-bridge module multilevel converter valve 3 (123) can be configured with AC connection structure or not according to the actual needs of AC power grid (such as grid capacity, load changes, etc.), further improving the adaptability of the system.

[0033] The AC-AC converter 100 also includes a DC energy discharge circuit 130; the DC energy discharge circuit 130 is connected in parallel with the rectifier side 110 and the inverter side 120; since the AC-AC converter 100 will have DC voltage during operation, in order to cope with the excess energy release demand during system faults or power fluctuations, the present invention configures a DC energy discharge circuit 130 on the AC-AC converter 100; when the system experiences abnormal conditions such as overvoltage or overcurrent, the DC energy discharge circuit 130 can quickly release excess energy, avoid damage to power electronic devices due to energy accumulation, and ensure the safe and stable operation of the system.

[0034] An operation method for a low-frequency AC transmission system for offshore wind power, characterized by the following steps: Step 1, Pre-charge stage: The AC power grid pre-charges the multilevel converter valves 121, 122, and 123 of the half-bridge module via charging resistors. After pre-charging, charging continues until the sub-module capacitor voltages are charged to their rated values. This provides two levels of DC voltage for the DC system. Step 2, Low-frequency side charging stage: Unlock the hybrid submodule modular multilevel converter valve 111, and perform AC side voltage frequency control on the hybrid submodule modular multilevel converter valve 111. The frequency is set to the low-frequency system frequency, and the slope of the voltage control curve is set to rise slowly, completing the charging of the low-frequency AC submarine cable 220 and the low-frequency AC transformer of the low-frequency booster station 200. After the power system of the low-frequency booster station 200 draws power, the low-frequency booster station 200 is started. The control value of the AC voltage, after being rectified by diode converter valve one 112 and diode converter valve two 113, is less than the DC rated value of half-bridge module multilevel converter valve two 122 and half-bridge module multilevel converter valve three 123. At this time, power cannot be transmitted through diode converter valve one 112 and diode converter valve two 113. Step 3, Wind turbine start-up and grid connection stage: The low-frequency booster station 200 provides synchronization voltage to the grid-connected offshore wind turbine 210, sequentially completing the gradual start-up and grid connection of the offshore wind turbine 210; the power flow direction during the power feedback phase of the entire black start process is as follows: Figure 5 As shown; Step 4, Power Forward Feed Switching Stage: When the power of the hybrid submodule modular multilevel converter valve 111 is reversed, the control parameters of the hybrid submodule modular multilevel converter valve 111 are adjusted to increase the AC side voltage of the hybrid submodule modular multilevel converter valve 111, opening the power supply channels of diode converter valve one 112 and diode converter valve two 113; simultaneously, the DC side voltage of the hybrid submodule modular multilevel converter valve 111 is increased to block the reverse power supply channel from half-bridge modular multilevel converter valve one 121 to the hybrid submodule modular multilevel converter valve 111, completing the startup of the low-frequency AC transmission system; the power flow direction during the forward power transmission stage is as follows: Figure 6 As shown.

[0035] All other unspecified parts belong to the prior art.

Claims

1. A low-frequency AC transmission system for offshore wind power, characterized in that: Includes an AC-AC converter (100) connecting a low-frequency booster station (200) and an AC power grid (300); the AC-AC converter (100) includes a rectifier side (110) and an inverter side (120); The rectifier side (110) includes a hybrid sub-module type modular multilevel converter valve (111), a diode converter valve one (112), and a diode converter valve two (113); the hybrid sub-module type modular multilevel converter valve (111) is connected to the DC output terminals of the diode converter valve one (112) and the diode converter valve two (113), respectively. The inverter side (120) includes a first half-bridge module multilevel converter valve (121), a second half-bridge module multilevel converter valve (122), and a third half-bridge module multilevel converter valve (123); the first half-bridge module multilevel converter valve (121) is connected to the DC output terminals of the second half-bridge module multilevel converter valve (122) and the third half-bridge module multilevel converter valve (123), respectively. The hybrid sub-module modular multilevel converter valve (111) is DC connected to the half-bridge module multilevel converter valve one (121); the diode converter valve one (112) is DC connected to the half-bridge module multilevel converter valve two (122); and the diode converter valve two (113) is DC connected to the half-bridge module multilevel converter valve three (123).

2. The offshore wind power low-frequency AC transmission system according to claim 1, characterized in that: The hybrid sub-module type modular multilevel converter valve (111) includes a full-bridge sub-module (1111) and a half-bridge sub-module (1112).

3. The offshore wind power low-frequency AC transmission system according to claim 1, characterized in that: The offshore wind turbine configured in the low-frequency booster station (200) is a grid-connected offshore wind turbine (210).

4. The offshore wind power low-frequency AC transmission system according to claim 3, characterized in that: The low-frequency booster station (200) is connected to the rectifier side (110) via a low-frequency AC submarine cable (220).

5. The offshore wind power low-frequency AC transmission system according to claim 1, characterized in that: It also includes a control system (400); the control system (400) is connected to the hybrid sub-module type modular multilevel converter valve (111) through the first valve control device (410), connected to the first half-bridge module multilevel converter valve (121) through the second valve control device (420), and connected to the second half-bridge module multilevel converter valve (122) and the third half-bridge module multilevel converter valve (123) through the third valve control device (430).

6. The offshore wind power low-frequency AC transmission system according to claim 1, characterized in that: The AC-AC converter (100) also includes a DC energy discharge circuit (130); the DC energy discharge circuit (130) is connected in parallel with the rectifier side (110) and the inverter side (120).

7. An operation method for an offshore wind power low-frequency AC transmission system, characterized in that: Includes the following steps: Step 1, Pre-charge stage: The AC power grid precharges the first (121), the second (122), and the third (123) multilevel converter valves of the half-bridge module through the charging resistor. After the precharging is completed, the charging continues to unlock and charge the sub-module capacitor voltage to the rated value, providing two levels of DC voltage for the DC system. Step 2, Low-frequency side charging stage: Unlock the hybrid sub-module type modular multilevel converter valve (111), and perform AC side voltage frequency control on the hybrid sub-module type modular multilevel converter valve (111). The frequency is set to the low frequency system frequency, and the slope of the voltage control curve is set to slowly rise, completing the charging of the low frequency AC submarine cable (220) and the low frequency AC transformer of the low frequency boost station (200). After the power system of the low frequency boost station (200) takes power, the low frequency boost station (200) is started. The control value of the AC voltage is less than the DC rated value of the half-bridge module multilevel converter valve (122) and the half-bridge module multilevel converter valve (123) after rectification by diode converter valve one (112) and diode converter valve two (113). At this time, power cannot be transmitted through diode converter valve one (112) and diode converter valve two (113). Step 3, Wind turbine start-up and grid connection stage: The low-frequency booster station (200) provides synchronous voltage for the grid-connected offshore wind turbine (210), thereby gradually starting and connecting the grid-connected offshore wind turbine (210) to the grid. Step 4, Power Forward Feed Switching Stage: When the power of the hybrid submodule modular multilevel converter valve (111) is reversed, the control parameters of the hybrid submodule modular multilevel converter valve (111) are adjusted to increase the AC side voltage of the hybrid submodule modular multilevel converter valve (111) and open the power supply channels of diode converter valve one (112) and diode converter valve two (113); at the same time, the DC side voltage of the hybrid submodule modular multilevel converter valve (111) is increased to block the reverse power supply channel from half-bridge modular multilevel converter valve one (121) to the hybrid submodule modular multilevel converter valve (111) and complete the startup of the low-frequency AC output system.

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